Palazzo della Gran Guardia
Verona, 6 dicembre 2019
Fabiana Busti
Dipartimento di Medicina, Università di Verona
Centro di Riferimento per i Disordini del Metabolismo del Ferro
www.gimferverona.org
4° Congresso Nazionale AMGe
Geriatria e Dintorni
Un viaggio di incontri
Summary
• Basi fisiopatologiche dell’omeostasi del ferro
- Il ruolo essenziale del ferro
- La necessità di una fine regolazione
• Implicazioni della carenza di ferro nell’anziano
• Dalla fisiopatologia alla pratica clinica, verso una migliore
comprensione ed un uso più razionale della terapia marziale
Andrews, Ann Rev Physiol 2007
Il ferro è un micronutriente essenziale, ma potenzialmente tossico
Trasporto ed accumulo
di O2
Produzione di energia
Funzione di enzimi e
citocromi
facile scambio elettroni
Fe3+ Fe2+
utili proprietà redox
formazione radicali liberi O2
(Fe2++ H202 Fe3+ + OH- + OH•)
Proliferazione cellulare
ed eritropoiesi
Stress ossidativo
Morte cellulare/Apoptosi
Danno di DNA, proteine
e lipidi
Normal
Fe 4 g Fe
(toxic)
HH
Cirrhosis
HCC
diabetes
heart
failure
Skin
pigmentation
hypogonadism
Fe
(anemia)
CNS dysfunction
heart
dysfunction
Muscle
weaknessRestless
legs Synd.
Pica
IRON DEFICIENCY NORMAL IRON OVERLOAD
Il ferro è un micronutriente essenziale, ma potenzialmente tossico
La maggior parte del ferro viene riciclata ogni giorno
3-4 g
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewofironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
2 g
300 mg
1 g
3-4 mg
600 mg
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewofironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
Eritrociti
Macrofagi splenici
Precursori eritroidimidollari
L’omeostasi del ferro necessita di una stretta regolazione
Pool plasmatico
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
Epatociti
Low Fe-TF
High Fe-TF
FB (7) Verona 31.05.2019
3-4 g Fe
Equilibrio mediante
regolazione di
assorbimento e riciclo
del ferro
Assenza di
meccanismi per
l’eliminazione attiva
del ferro in eccesso
20-25 mg/die
Ferro necessario
per eritropoiesi
20-25 mg/die
Ricliclo del ferro
dai GR senescenti
1-2 mg/die
Assorbimento
duodenale
Altri
utilizzatori
Perdite fisiol.
1-2 mg/die
Camaschella C, New Eng J Med 2015
The «iron economy»
Il contenuto di ferro dell’organismo deve essere finemente regolato
3-4 g
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewofironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
2 g
300 mg
1 g
3-4 mg
600 mg
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewofironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
Eritrociti
Macrofagi splenici
Precursori eritroidimidollari
L’omeostasi del ferro necessita di una stretta regolazione
Pool plasmatico
mutant zebrafish.17 Interestingly, mutations in mitoferrin result in a
clinical disorder that is very similar to erythropoietic protoporphy-
ria caused by ferrochelatase mutations.17
Although many tissues express TFR1 at low levels, relatively
few cell types are strictly dependent on the transferrin cycle for iron
uptake. Targeted disruption of the Tfr1 gene in mice demonstrated
that most tissues develop normally without Tfr1, but erythroid
precursors, early lymphoid cells, and neuroepithelial cells require
Tfr1 for differentiation.18,19 The likely role of TFR1 in erythropoi-
esis is obvious—the transferrin cycle serves to concentrate iron in
the vicinity of DMT1 to maximize iron assimilation for hemoglo-
bin production. However, it is less clear why lymphopoiesis and
neurodevelopment should require TFR1.
In the past it was assumed that iron assimilated by erythroid
precursors was incorporated into hemoglobin, remaining within the
cells until erythrocyte senescence. Recently, however, Quigley and
colleagues20 have described a heme exporter, FLVCR, which
appears to be necessary for normal erythroid development. They
hypothesize that erythroblasts need to have a pop-off valve for
extra heme to avoid its toxicity. Targeted disruption of the mouse
gene encoding FLVCR demonstrated the importance of this protein
in vivo.21 FLVCR-null mice had a failure of definitive erythropoi-
esis, resulting in fetal demise. Interestingly, the fetuses had
craniofacial and limb deformities suggestive of Diamond-Blackfan
anemia. When the FLVCR gene was inactivated after birth the
animals developed severe, macrocytic anemia, implying that heme
export is important for normal erythropoiesis.
Regulationof intracellular ironhomeostasis
Intracellular iron homeostasis is maintained, at least in part,
through a very elegant posttranscriptional regulatory mechanism.
In 1987, investigators observed that conserved sequences in the
5 untranslated regions (UTRs) of both H- and L-ferritin mRNAs
were needed to control a ready but quiescent pool of ferritin mRNA
in the cell, which could quickly be mobilized to produce ferritin
protein when iron was abundant.22,23 Thermodynamic predictions
indicated that the UTR sequences could form stable RNA hairpins
with a characteristic secondary structure, termed iron responsive
(or regulatory) elements (IREs).24 Soon afterward it was shown that
cytoplasmic proteins, now known as iron regulatory proteins (IRPs,
formerly IREBPs), recognize and bind to the IREs.25-28
The 2 known IRPs share sequence homology but have distinc-
tive properties. At the time of its discovery, IRP1 was recognized to
bear strong similarity to aconitase, a mitochondrial enzyme of the
tricarboxylic acid cycle. Remarkably, IRP1 also has aconitase
activity, making it a prime candidate for a previously described
cytoplasmic aconitase.29,30 But the aconitase and IRE-binding
activities are mutually exclusive, providing a clue to a clever
regulatory switch. Similar to a number of other iron-containing
proteins, IRP1 incorporates an iron-sulfur cluster (4Fe•4S). The
iron-sulfur cluster forms when iron is abundant, but disassembles
when iron is scarce. Haile and Rouault showed that the aconitase
activity of IRP1 is present only when the iron-sulfur cluster is
complete; when it is not, IRP1 acts as an RNA binding protein,
recognizing IREs.31 IRP2, on the other hand, does not incorporate
an iron-sulfur cluster. Rather, its activity is regulated at the level of
protein stability. Under low iron conditions IRP2 accumulates, but
when iron is abundant it triggers IRP2 degradation.32-36 It is still not
entirely clear why it is necessary to have 2 IRPs, but recent observations
suggest that the 2 may respond differently over the physiologically
relevant range of oxygen tensions.37 They may also have somewhat
different target selectivity among IRE-containing mRNAs.
The ferritin IRE is located just upstream of the start codon for
protein translation. Muckenthaler and colleagues showed that IRP
binding sterically blocks recruitment of the small ribosomal
subunit to the initiation complex, thus preventing translation.38 As a
result, ferritin protein production is abrogated under low iron
circumstances when the small amount of intracellular iron is
needed for cellular functions. On the other hand, when iron is
abundant, translational repression is relieved and newly made
ferritin subunits assemble to provide iron storage capacity.
Figure1.Overviewof ironhomeostasis. The central
portion of the figure depicts the flow of iron into the body
(through the small intestine), to transferrin (Tf), to the
major site of utilization (the erythroid bone marrow), to
circulating erythrocytes, to tissue macrophages that
phagocytose senescent erythrocytes and recycle iron
(spleen), to storage in hepatocytes, and back to TF
through mobilization of iron stores. Cellular iron trans-
port is described in detail in the text and shown in
schematic form on the outside edges of this figure.
(A) Nonheme iron transport across an intestinal entero-
cyte. (B) Erythrophagocytosis and iron recycling in a
tissue macrophage. The aqua oval in the cytoplasm
represents a storage depot for ferroportin protein within
the cell. (C) Hepatocyte iron transport, with arrows
indicating that neither import nor export is well under-
stood. (D) Iron uptake through the transferrin cycle in
the erythoblast. Illustration by Kenneth Probst.
220 ANDREWS BLOOD, 15 JULY 2008 VOLUME 112, NUMBER 2
For personal use only. at SWETS INFORMATION SERVICES on July 8, 2008. www.bloodjournal.orgFrom
Epatociti
Low Fe-TF
High Fe-TF
FB (7) Verona 31.05.2019
3-4 g Fe
Camaschella C, New Eng J Med 2015
L’epcidina controlla assorbimento e riciclo del ferro
Equilibrio mediante
regolazione di
assorbimento e riciclo
del ferro
Assenza di
meccanismi per
l’eliminazione attiva
del ferro in eccesso
20-25 mg/die
Ferro necessario
per eritropoiesi
20-25 mg/die
Ricliclo del ferro
dai GR senescenti
1-2 mg/die
Assorbimento
duodenale
Altri
utilizzatori
Perdite fisiol.
1-2 mg/die
HEP-(atic) CIDIN (antimicrobial)
Hepcidin Ferroportin
Epcidina è l’ormone regolatore chiave dell’omeostasi marziale
• piccolo (25 aa) peptide prevalentemente prodotto dal fegato
• defensin-like (peptidi dell’immunità innata con attività antimicrobica)
• interagisce col suo recettore (ferroportina, l’unico esportatore cellulare di ferro,
ubiquitario ma altamente espresso in cellule duodenali, macrofagi ed epatociti)
Kushner JP, Blood 2010
FPN localized on cell membrane
Iron export
FPN internalized and degraded
control
Il ruolo principale di epcidina è controllare l’espressione di FPN
Il legame con epcidina determina internalizzazione di FPN e la sua degradazione
Nemeth E, Science 2004
FPN internalized and degraded
Iron export blocked
+ 1 g/ml hepcidin
Il ruolo principale di epcidina è controllare l’espressione di FPN
Il legame con epcidina determina internalizzazione di FPN e la sua degradazione
Nemeth E, Science 2004
FPN localized on cell membrane
Iron export
control
Milza
Fegato
Duodeno
Hepcidin
Fpn
Fpn
Fpn
Plasma
Fe-Tf
Midollo osseo e
altri utilizzatori
di ferro
L’asse epcidina-ferroportina regola il flusso di ferro nel plasma
I livelli di epcidina influenzano la distribuzione del ferro nell’organismo
Fpn
Macrofago
Fpn
Enterocita
Epcidina è regolata da molteplici fattori
Girelli D et al., Blood 2016
Prodotta in risposta all’ del Fe, per regolare negativamente la sideremia
EPCIDINA
Summary
• Basi fisiopatologiche dell’omeostasi del ferro
- Il ruolo essenziale del ferro
- La necessità di una fine regolazione
• Implicazioni della carenza di ferro nell’anziano
• Dalla fisiopatologia alla pratica clinica, verso una migliore
comprensione ed un uso più razionale della terapia marziale
Impatto clinico della carenza di ferro (ID) nell’anziano
• Alta prevalenza (10-40%), causa frequente di anemia nell’anziano
• Cut-off diagnostici più elevati: ferritina <45-50 ng/ml
• Complicanze: decadimento cognitivo, depressione, rischio di caduta, malattie CV,
ospedalizzazione, morte
• Associata ad outcomes avversi anche in assenza di anemia
• Trattamento migliora outcomes
Busti F et al., Front Pharmacol 2014
Girelli D et al, HemaSphere 2018
Perdite GI alte
Non infrequentemente carenza di ferro assoluta e funzionale coesistono
La carenza di ferro può essere «assoluta» (AID) e/o «funzionale (FID)
Malassorbimento
Perdite GI basse
Inadeguata assunzione
Scompenso cardiaco
Malattie renali croniche
Disordini autoimmuni
± fattori iatrogeni (es. PPI, antitrombotici/anticoagulanti)
=depositi di ferro “bloccati"
Infezioni
AID =depositi di ferro esauriti XFID
EPCIDINA
Summary
• Basi fisiopatologiche dell’omeostasi del ferro
- Il ruolo essenziale del ferro
- La necessità di una fine regolazione
• Implicazioni della carenza di ferro nell’anziano
• Dalla fisiopatologia alla pratica clinica, verso una migliore
comprensione ed un uso più razionale della terapia marziale
La terapia orale è semplice, facilmente accessibile e sicura
Modificato da Veuthey T et al., Front Pharmacol 2014
Lume
intestinale
Sangue
• Per molto tempo
considerata terapia di 1°
scelta nei soggetti con
IDA lieve-moderata
(Hb >8 g/dL)
• Sali di ferro e
complessi
polisaccaridici, in
forma ferrosa (2+) o
ferrica (3+)
Assorbimento attraverso la via del
ferro non-eme (scarsamente
efficiente), mediante trasportatore
specializzato (DMT1)
Assorbimento favorito da ambiente
acido (potenzialmente ↑ da
vitamina C 250-500 mg/die, ↓ da
PPI)
• I preparati maggiormente utilizzati sono il ferro
gluconato, il ferro fumarato e il ferro solfato
Attenzione al
contenuto di
ferro elementare
(80-100 mg)
Modificato da Parmanand BA et al, J Nutr Biochem, 2019
Ferro orale
80-90% ferro non
assorbito raggiunge
il colon
Tolkien et al., 2015
Danno direttoNausea, vomito, dolore
addominale, diarrea o stipsi
nel 30-70% dei soggetti
Modificazioni microbioma
intestinale ↓ Lactobacilli e Bifidobacteria
↑Enterobacteria
= Tempi di assunzione
prolungata (almeno 3-6
mesi) per ottenere non
solo normalizzazione
dell’Hb, ma anche
replezione dei depositi
(ferritina >100 ng/ml)
Solo 10-20% viene
assorbito
Gli effetti avversi, soprattutto GI, compromettono la compliance alla terapia
Ferro orale
Molteplici fattori contribuiscono alla refrattarietà alla terapia orale
• Sospensione prematura trattamento
• Mancata compliance
• Gastriti atrofica, autoimmune, HP-correlata
• Chirurgia bariatrica
• Malattia celiaca (o altro malassorbimento)
• (Iron Refractory Iron Deficiency Anemia)
• Condizioni infiammatorie, anche
«subcliniche» (es. scompenso cardiaco,
CKD)
• Perdite troppo abbondanti
Mancata compliance
Inadeguata acidità gastrica
Mucosa danneggiata
Refrattarietà alla terapia orale =
incremento di Hb < 1 g/dl dopo 3
settimane di trattamento
Tf plasmatica
Fpn
Enterocita
xBlocco assorbimento
causato da ↑ epcidina
50 donne pre-menopausa
ID (ferritina < 20 ng/ml), non anemiche
Dose crescenti (40-60-80-120-240 mg)
Ferro per os per 2 giorni consecutivi
La terapia con ferro influenza i livelli di epcidina
EPCIDINA
Replete iron stores
1. Incremento acuto di epcidina
persistente per 48 ore
+ 9 h picco
epcidina
+ 4 h
picco TSAT
+24 h
+48 h
normalizzazione
epcidina
50 donne pre-menopausa
ID (ferritina < 20 ng/ml), non anemiche
Dose crescenti (40-60-80-120-240 mg)
Ferro per os per 2 giorni consecutivi
TSAT= saturazione
transferrina
Il picco di epcidina indotto dalla terapia con ferro ne riduce l’assorbimento
1. Incremento acuto di epcidina
persistente per 48 ore
2. Ridotto assorbimento della
seconda dose (-35-45%)
2° Dose 60 mg
Assorbimento -36%
50 donne pre-menopausa
ID (ferritina < 20 ng/ml), non anemiche
Dose crescenti (40-60-80-120-240 mg)
Ferro per os per 2 giorni consecutivi
Il picco di epcidina indotto dalla terapia con ferro ne riduce l’assorbimento
Tutti i giorni per
14 giorni
Dì alterni per
28 giornip
Frazione ferro assorbita (%) 16.3 21.8 0.0013
Quantità ferro assorbita (mg) 131 mg 175 mg 0.0010
Ferritina 13.8→28.3 13.8→23.6 0.058
Frequenza effetti avversi GI → 33% più bassa gg alterni (p=0.57)
20 donne
ID, non anemiche
Ferro orale 60 mg
Somministrazione per 14 giorni consecutivi
oppure per 28 giorni a dì alterni
La somministrazione a dì alterni sembra ottimizzare l’assorbimento di ferro
Necessità di
conferma in
pazienti
anemici
Mancano dati
sugli esiti a
lungo termine
Ferritina umana
1947: Fe Saccaridico
1954: Fe-Destrano (HMW)
1991: Fe-Destrano (LMW)
1999: Fe-Gluconato
2000: Fe-Sucrosio
>2009: Ferumoxytol
Fe-isomaltoside
Fe-carboximaltosio
“Preparati di
III generazione”
8-2
5 n
m
Fe3+
oxyhydroxide(CORE)
Carboidrati (GUSCIO)
Caratteristica unica di
ciascun composto, ne
influenza:
• Immunogenicità
• Stabilità
Moderate risk
High risk
Very low risk
1932: FeOH
Modificato da Girelli D et al., Int J Hematol 2018
I più recenti preparati di ferro EV sono efficaci e sicuri
Ferro EV <1:200000
AEs gravi
Trasfusioni 1:21000
Avni T et al., Mayo Clin Proc 2015
3° generazione
FCM
Si basa su
peso e
livelli di Hb
Max 1
g/settimana
Hb Peso
g/dl <35 kg 35-70 kg >70 kg
<10 500 mg 1500 mg 2000 mg
10-14 500 mg 1000 mg 1500 mg
>14 500 mg 500 mg 500 mg
Determinazione della dose (schema semplificato)
Modificato da Girelli D et al., Int J Hematol 2018
La farmacocinetica del ferro EV è diversa da quella del ferro orale
Terapia orale Terapia EV
Integrità
mucosa
gastro-
duodenale
Assorbimento
bloccato da
epcidina
Va direttamente
in circolo,
captata dai
macrofagi
Meno
influenzata dai
livelli di
epcidina
Modificato da Girelli D et al.,
Int J Hematol 2018
Principali indicazioni alla terapia marziale EV
Modificato da Girelli D et al., Int J Hematol 2018
Take home messages:
• La carenza di ferro nell’anziano è un problema rilevante
che non andrebbe trascurato
• Terapia orale presenta molteplici limitazioni, che
potrebbero essere superate da schemi di trattamento più
«fisiologici» e meglio tollerati (dì alterni?)
• I «moderni» preparati di ferro EV permettono correzione dell’ID in 1-2 dosi
con ottima tollerabilità ed efficacia (anche in condizioni infiammatorie)
• Reazioni avverse al ferro EV sono estremamente rare e solo eccezionalmente
pericolose (molto inferiori a quelle trasfusionali)
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