Review Article Role of Eukaryotic Initiation Factors...

Review ArticleRole of Eukaryotic Initiation Factors during Cellular Stress andCancer Progression

Divya Khandige Sharma Kamiko Bressler Harshil PatelNirujah Balasingam and Nehal Thakor

Department of Chemistry and Biochemistry Alberta RNA Research and Training Institute University of Lethbridge LethbridgeAB Canada T1K 3M4

Correspondence should be addressed to Nehal Thakor nthakorulethca

Received 30 October 2016 Accepted 14 November 2016

Academic Editor Hyouta Himeno

Copyright copy 2016 Divya Khandige Sharma et alThis is an open access article distributed under theCreative CommonsAttributionLicense which permits unrestricted use distribution and reproduction in anymedium provided the originalwork is properly cited

Protein synthesis can be segmented into distinct phases comprising mRNA translation initiation elongation and terminationTranslation initiation is a highly regulated and rate-limiting step of protein synthesis that requires more than 12 eukaryoticinitiation factors (eIFs) Extensive evidence shows that the transcriptome and corresponding proteome do not invariably correlatewith each other in a variety of contexts In particular translation of mRNAs specific to angiogenesis tumor development andapoptosis is altered during physiological and pathophysiological stress conditions In cancer cells the expression and functionsof eIFs are hampered resulting in the inhibition of global translation and enhancement of translation of subsets of mRNAs byalternative mechanisms A precise understanding of mechanisms involving eukaryotic initiation factors leading to differentialprotein expression can help us to design better strategies to diagnose and treat cancer The high spatial and temporal resolution oftranslation control can have an immediate effect on the microenvironment of the cell in comparison with changes in transcriptionThe dysregulation of mRNA translation mechanisms is increasingly being exploited as a target to treat cancer In this review wewill focus on this context by describing both canonical and noncanonical roles of eIFs which alter mRNA translation

1 Introduction

Regulation of protein translation is a critical step of the geneexpression process which allows cellular adaptation duringstress conditions by rapidly reprograming the proteome out-put without the requirement for changes in RNA synthesisIn conditions such as heat shock hypoxia endoplasmicreticulum (ER) stress and apoptosis an immediate changein protein levels is required stressing the importance oftranslational regulation responsible for rapid adaptationto physiological conditions [1] Transcriptome analysis isa widely accepted method for analyzing gene expressionduring stress conditions However there is an emerging bodyof evidence that shows a limited correlation between thetranscriptome and the corresponding proteome suggestingthat when it comes to translation not all transcripts aretreated equally Epidermal growth factor (EGF) treatment ofserum-starved HeLa cells resulted in only 48 differentially

expressed genes (DEGs) where a DEG represents a signif-icant change in both the transcriptome and translatome inthe same direction (homodirectionally) [2] In oppositionthe 952 of uncoupled DEGs represent a significant changein either the transcriptome or translatome or an inverserelationship between the transcriptome and translatome [2]Using parallel genome-scale measurements of mRNA andcorresponding protein levels and half-lives mRNAs werefound to explain 40 of the variability in protein levels withtranslation efficiency being the best predictor of protein levelsin mouse fibroblasts [3] Accordingly translation controlis considered to play a central role in eukaryotic geneexpression As new evidence is being uncovered scientistshave now started to appreciate the critical role of mRNAtranslation in tumor progression In a wide range of cancertypes inappropriate translation of oncogenes tumor sup-pressors and eukaryotic translation initiation factors is acritical process in cancer cell proliferation [4ndash6] Even during

Hindawi Publishing CorporationJournal of Nucleic AcidsVolume 2016 Article ID 8235121 19 pageshttpdxdoiorg10115520168235121

2 Journal of Nucleic Acids

times of stress when global levels of protein synthesis arereduced cancer cell development typically involves selectivetranslation of a specific subset of mRNAs These transcriptsencode prosurvival proteins that are translated by alternative(noncanonical) mechanisms [1 7 8] Studies continue toprovide new knowledge with regard to the developmentalcauses and possible novel treatments of various types ofcancers [4ndash7 9] Significantly much of this information canbe tied back to the paradigm of translational regulationand its critical contribution to our understanding of canceretiology

In this review we will first discuss the mechanism ofcanonical translation initiation followed by noncanonicalmechanisms that utilize RNA sequence features includingupstream open reading frames (uORFs) and internal ribo-some entry site- (IRES-) mediated translation mechanismsthe role of eukaryotic initiation factors in noncanonical trans-lation and their significance in cancer progression In addi-tion to uORFs- and IRES-mediated translation regulationother noncanonical translation mechanisms exist such asgamma interferon-activated inhibitor of translation (GAIT)complex 51015840 terminal oligopyrimidine (51015840 TOP) elements andAU-rich elements (AREs) [10 11] These mechanisms are outof the scope of this review article

Canonical Translation Initiation Eukaryotic cap-dependenttranslation initiation includes the recognition and recruit-ment of mRNA onto the small ribosomal (40S) subunitfollowed by ribosomal scanning in a 51015840ndash31015840 direction Sub-sequently the 60S large ribosomal subunit is recruitedforming the 80S initiation complex At this stage an initiatormethionyl-tRNA (met-tRNAi) is in the ribosomal peptidyl(P) site at themRNAstart codon [1 13] Canonical initiation isa complex process utilizing more than 25 proteins includinga minimum of twelve eukaryotic initiation factors (eIFs) [14]The rate of initiation varies between different mRNAs and isinfluenced by accessibility to the methylated guanosine capstructure (m7G cap) at the 51015840 terminus of the mRNA thelength and secondary structure of the 51015840 untranslated region(UTR) the sequence and secondary structure surroundingthe start codon and the poly(A) tail [15 16]

Initiation begins with the assembly of eIF4F complexcomprising eIF4E eIF4G and eIF4A onto the 51015840 m7G cap(Figure 1) eIF4E binds to the m7G cap which then interactswith the multidomain scaffold protein eIF4G and the ATP-dependent RNA helicase eIF4A The ternary complex (eIF2-GTP-met-tRNAi) associated with a 40S ribosomal subunit isthen recruited to the 51015840 end of the mRNA via a critical linkto eIF4G (captured at the cap via eIF4E) mediated by eIF3forming the 43S preinitiation complex eIF1 and eIF1A assistin stimulating recruitment of the ternary complex as wellas acting synergistically to promote continuous ribosomalscanning for AUG start codons [17] This 43S preinitiationcomplex then scans the 51015840 UTR of the mRNA with thehelp of eIF4A until an initiation codon in optimal contextis recognized [18] eIF5 and eIF5B then mediate subsequenthydrolysis of GTP to release the bound initiation factors fromthe 48S complex leaving the start codon in the ribosomal P-site with the met-tRNAi and allowing 60S ribosomal subunit

to bind [1 19] The now competent 80S initiation complexthen proceeds to translation elongation (Figure 1)

Noncanonical Translation Initiation During stress conditionssuch as hypoxia nutrient deprivation or endoplasmic retic-ulum (ER) stress alternative mechanisms that are mediatedby cis-acting sequences in specific mRNA subsets such asuORF and IRES drive the translation of stress responsemRNAs [1 6ndash8 18 20] Studies have revealed that a decentportion of human transcripts is known to contain uORFs(upstream of the initiation codon of the coding region) thatfunction as translation or mRNA stability regulators [21]Recent ribosome profiling data reveals that uORFs can existin an out-of-frame relative to the main coding sequence[22] However an overlap can also occur between uORFsand the coding sequence in which alternative translationof an upstream in-frame start codon of a gene can possiblyproduce an extended protein product [23] The mechanismof uORF-mediated translation functions primarily duringeIF2120572 phosphorylation conditions and enhances the expres-sion of proteins involved in cell-cycle regulation and apopto-sis Typical examples of uORF-mediated translation regula-tion include general control nonderepressible 4 (GCN4) theyeast transcriptional activator and activating transcriptionfactor 4 (ATF4) in mammals Normally translation initiationoccurs from the start codons located in the 51015840 UTR of mRNAwhich leads to the translation of small uORFs Additionallyreinitiation of terminating ribosomes will typically not occuron the downstream cistron thus the translation of themain coding sequence is inhibited in these conditions [24]However during stress conditions eIF2120572 phosphorylationattenuates translation of uORF sequences and allows thetranslation of main coding sequence eIF2120572 phosphorylationand reduced availability of the eIF2-GTP-tRNAi ternary com-plex favor translation reinitiation at the Gcn4p coding region(in yeast) subsequently resulting in activation of numerousgenes [25 26] Gcn4p activates these target genes by bindingto them and functioning as a transcription factor [25] Thecomplexity of RNA structure in the 51015840 UTR also plays acrucial role in uORF-mediated translation For exampletranslation of 120573-site APP-cleaving enzyme 1 (BACE1) whichis implicated in Alzheimerrsquos disease (AD) progression isregulated through uORFs However high GC content andcomplexity of the RNA secondary structure are also crucialdecisive factors for uORF-mediated translation of BACE1[27] Additionally recent genome-level studies indicate thatRNA secondary structure is negatively correlated with uORFtranslational efficiency as upstream (relative to uORFs) struc-tures restrict or even arrest ribosomal preinitiation complex(PIC) whereas structures downstream of uORFs enhancetranslation initiation of coding sequences [28]

Translation initiation mediated by IRES is another mech-anism that operates during stress conditions IRESes areRNA sequence elements that were initially discovered inthe 51015840 leader sequences of poliovirus and encephalomy-ocarditis virus genomic RNA that lack the 51015840 cap structurebut nonetheless are efficiently translated in the host cell[18 29] The viral IRES elements comprise secondary andtertiary structures that play a role in direct interactions

Journal of Nucleic Acids 3

40SAAAAAUG

60S

40S

eIF3eIF1A60S

40S

1A eIF3

eIF1eIF2

eIF2-GTP

2

40S

21A

1

eIF3

AUG AAAAmRNA

eIF4GeIF4E

eIF4A

eIF4F complexAUG AAAA

4G4A

4E

40S

2 1A

1eIF3

AAAAAUG

4G4A

4E

Scanning ATP

2 1A

1

eIF34G

4A

4E

eIF5

eIF3 1 1A

2

4G4A

4E

40SAAAAAUG

60SeIF5B GTP

40SAAAAAUG

60S

43S preinitiation complex


met-tRNAi

eIF2-GTP-met-tRNAi

+

ADP + Pi

GDP + Pi

GDP + Pi

m7G

m7G

m7G

m7G

m7G

m7G

Figure 1 An overview of eukaryotic translation initiation Most eukaryotic mRNAs contain a 51015840 m7G cap which is bound by eukaryoticinitiation factor 4F complex (eIF4E eIF4G and eIF4A)The 43S preinitiation ribosome complex which contains ternary complex (Tc) (eIF2-GTP-initiator tRNA) is recruited to the 51015840 end of mRNAs via eIF3-eIF4G interactionWith the help of eIF4A (RNA helicase) the preinitiationcomplex is thought to scan mRNA until the start codon (AUG) is found Subsequently the 48S initiation complex is formed and Tc deliverstRNA into the P-site of the ribosome Then eIF5 binds to the 48S initiation complex and induces GTPase activity of eIF2120572 Upon GTPhydrolysis all protein factors are released from the 40S ribosome subunit Subsequently eIF2120572 is recharged with GTP by ldquoGDP to GTPrdquoexchange factor eIF2B Finally eIF5B unites the 60S and 40S ribosome subunits to form the 80S initiation complex and translation elongationcommences


with the translation initiation machinery [20] Mutationsin viral IRESes such as hepatitis C virus (HCV) classicalswine fever virus (CSFV) and cricket paralysis virus (CrPV)affect the secondary and tertiary RNA structures and renderthese IRESes inactive [30] These viral IRESes are classifiedbased on structural and sequence similarities as well astheir requirement for eIFs and other protein factors fortranslation initiation [19 20] Picornavirus IRES elementsare the examples of types I and II IRESes which requireeIF4G eIF4A and eIF3 to assemble 48S initiation complex[29] Type III viral IRESes require eIF4G with HCV IRESbeing an exception [29] HCV IRES interacts with eIF3 forrecruitment of the 40S ribosomal subunit in close proximityto the start codon circumventing the requirement for the51015840 cap structure [1 19] Although IRES-mediated translationoperates independently of many canonical initiation factorsit requires RNA-binding proteins known as IRES trans-actingfactors (ITAFs)

Many cellular mRNAs are known to comprise IRESelements but they do not share structural or sequencesimilarities unlike their viral counterparts [20 31] Howeversimilar to viral IRESes cellular IRESes participate inmultipleinteractions with the canonical initiation factors and ITAFsto recruit the ribosome [1 20] In fact despite sequenceand structural dissimilarities cellular IRESes are reportedto share critical ITAFs [20] IRES elements have been iden-tified in mRNAs encoding stress response proteins (pro-and antiapoptotic) such as X-linked inhibitor of apoptosis(XIAP) cellular inhibitor of apoptosis 1 (cIAP1) B cell lym-phoma extralarge (Bcl-xL) Bcl-2 Bag-1 apoptotic protease-activating factor 1 (Apaf-1) p53 L-myc N-myc c-myc anddeath-associated protein 5 (DAP5) [1 8 32ndash34]

2 Role of Eukaryotic Initiation Factors inNoncanonical Translation

Translation initiation switches from cap-dependent to IRES-dependent mode during stress conditions such as hypoxiavascular lesions serum deprivation 120574-irradiation apoptosisgrowth arrest and angiogenesis [35] This shift is attributedto eIF2120572 phosphorylation eIF4E-BP dephosphorylationand eIF4G cleavage any of which can inhibit canonicaltranslation initiation [33 36] Although the cellular IRESelements are activated under stress conditions these IRESesdiffer in their requirement for eIFs For example the L-myc IRES requires the eIF4F complex and interaction ofboth poly(A) tail binding protein (PABP) and eIF3 witheIF4G for translation [20] On the other hand partial si-lencing assays (using the knockdown plasmid pSilencer31(si31) and hippuristanol (eIF4A inhibitor) treatment) havedemonstrated that C- and N-myc IRESes can function onlyin presence of the C-terminal domain of eIF4GI eIF4A andeIF3 This IRES does not require full-length eIF4GI or PABP[34]

ER stress caused by the accumulation of unfolded pro-teins triggers the unfolded protein response (UPR) whichin turn modulates both transcription and translation of keyregulators (eg ATF4) of the cellular stress response [37] A

recent study has shown that an alternatively spliced variantof the previously discussed human ATF4 transcript (variantV1) is translated by an IRES-mediated mechanism [37]This variant also has a unique 51015840 leader sequence and isfound to be less abundant than other variants in numeroushuman tissues Interestingly these variants were shown tobe translated by different mechanisms by using luciferasereporters and modifying 51015840 leader sequences with stem-loopinsertions Chan et al found high GC content in the long andhighly structured 51015840 leader region of V1 in comparison to V2suggesting a possibility of IRES-mediated translation Testingtruncated versions with bicistronic reporter assay Chan etal found structural elements that likely interact with trans-acting cellular factors Additional tests involving inhibitionof critical canonical eIFs (eg eIF4G1) and eIF2120572 phospho-rylation indeed confirmed the IRES-mediated translation ofV1 during UPR [37]

During amino acid starvation viral infection or endo-plasmic reticulum stress several kinases are activated thatinduce eIF2120572 phosphorylation (Figure 2) This phosphory-lation in turn decreases eIF2-GTP-tRNAi ternary complexactivity resulting in suppression of cap-dependent transla-tion However several viral and cellular mRNAs are insensi-tive to this mode of translation inhibition [20] This suggeststhat mRNAs can employ alternative factors or mechanismsto recruit the eIF2-GTP-tRNAi ternary complex Examplesof cellular IRESes unaffected by eIF2120572 phosphorylation areXIAP c-myc cationic amino acid transporter-1 (cat-1) andN-myc [38]Many viral IRESes also bypass translation inhibi-tion exerted by eIF2120572 phosphorylationThe exactmechanismby which the consequences of eIF2120572 phosphorylation areavoided by the HCV IRES is not known However someIRESes (CSFV and HCV) employ eIF5B an orthologueof prokaryotic IF2 during conditions of increased eIF2120572phosphorylation [30] The eIF5B-eIF3 mediated mechanisminvolves eIF3 stimulating tRNAi binding to the 40S subunit(in the IRES40S complex) in an eIF5B-dependent mannerwhich allows for the formation of the 48S initiation complexand subsequently the translation-competent 80S ribosome[30] Like IF2 eIF5B binds and delivers initiator tRNAduring translation initiation on these IRESes [8 30] Wehave recently found that the XIAP IRES uses a similarmechanism during eIF2120572 phosphorylation conditions Thisfinding suggests that eIF5B-dependent activation of IRES-mediated XIAP mRNA translation would act as a criticalprosurvival switch in cells under stress [8] Moreover arecent publication from our lab suggests that the XIAP IRESdoes not require eIF4G eIF4E and eIF4A for initiationcomplex formation The inhibition of eIF4A activity byhippuristanol or pateamine A treatment did not affect theability of XIAP IRES to form initiation complexes suggestingeIF4A is not required for IRES-mediated translation of XIAP[8] Additionally eIF3 and PABP bind synergistically within vitro-transcribed uncapped and poly(A)-tailed XIAPIRES RNA and recruit ribosomes near the start codon [12](Figure 3) The XIAP IRES adopts a conformation that iscritical for ribosome recruitment Although cellular IRESesdo not share structural similarities in studies conducted thusfar the secondary structure is indeed important as we have


GDP

Amino acid availabilityUV irradiation

Viral infectiondsRNA

Heme levelsOsmotic shockHeat shock

ER stressHypoxia

GCN2

PKR

HRI

PERKeIF2-GDP

eIF2-GTP

GDP

Inactive eIF2-GDP-eIF2B complex

GTP

P

GDP

GTP

Selective translation Stress response

Global protein synthesis

Ternary complex formation

Translation initiationeIF2B

eIF2B120572

120572

120573

120573

120574

120574

120572

120573 120574

Figure 2 Global translation inhibition by eIF2120572 phosphorylation Several stress stimuli activate distinct protein kinases which in turnphosphorylate eIF2120572 The phosphorylation of eIF2120572 enhances the affinity of eIF2-complex (120572 120573 and 120574 subunits) for eIF2B This renderseIF2-complex inactive for the initiator tRNA delivery to the ribosome However a subset of mRNAs harboring cis-elements such as internalribosome entry site (IRES) or upstream open reading frames (uORFs) are preferentially translated during eIF2120572 phosphorylation conditionsThesemechanisms allow production of stress-related proteins GCN2 general control nonderepressible-2 PKR protein kinase R HRI heme-regulated inhibitor kinase PERK PKR-like endoplasmic reticulum kinase

40S

eIF3

eIF4A

eIF4G

PABP

eIF4EAUG

40S

eIF3

PABP

PAIP1

AUGeIF4A

ITAFs

IRES5998400 cap

5998400 cap

3998400 (AAAAAA)n

3998400 (AAAAAA)n

Figure 3 Comparison between cap-dependent and IRES-dependent translation initiation of cellular mRNAs In cap-dependent translationinitiation complex is formed on the 51015840 end of mRNA with help of several eukaryotic initiation factors (eIFs) eIF4F-complex (eIF4E eIF4Gand eIF4A) is recruited to the 51015840 m7G capThe key interaction between eIF3 and eIF4G recruits the ribosome to the 51015840 cap In IRES-mediatedtranslation initiation complex is formed in the vicinity of the start codon with help of ITAFs and eIFs For example XIAP IRES interacts witheIF3 and recruits the ribosome via eIF3-PAIP1-PABP link [12] IRES internal ribosome entry site PABP poly(A) tail binding protein PAIP1PABP interacting protein 1 40S 40S ribosomal subunit AUG start codon

found it to be required for efficient recruitment of eIF3 andthe ribosome [12]

A recent study suggests that eIF3 subunit d plays a keyrole in alternative mechanisms of translation initiation by anoncanonical mechanism of cap recognition Specifically thestudy classifies this method as a novel eIF4E-independentmechanism of translation initiation [39] Lee et al foundthat the eIF3d subunit alone provided RNase protectionto the mRNA of the early response transcription factor c-Jun by directly interacting with a mature methylated 51015840cap structure The specificity of eIF3d binding to only asubset of capped mRNAs is suggested to occur through an

ldquoRNA gaterdquo domain that regulates this novel cap-bindingfunction of eIF3d In the larger context of eIF3 specializedtranslation pathways eIF3d seems to play a critical role as acap-binding protein that helps cells regulate protein synthesiseven during times when the eIF3F complex (required forcanonical translation) is inactivated or inhibited [39]

Mitochondria play an important role in the intrinsicpathway of apoptosis which is regulated by the Bcl-2 familyof proteins For example Bax and Bac proteins activatemitochondria-dependent apoptosis whereas Bcl-xL and Bcl-2 proteins inhibit apoptosis [40] During apoptosis Bcl-xL expression is controlled by sulphated glycoprotein 2


(SGP-2) The phosphorylation levels of eIF4E and 4E-BP1are influenced by SGP-2 which in turn affects the Bcl-xL IRES-dependent translation by regulating the stability ofeIF4F complex [41] Overexpression of some protooncogenicproteins (Bcl-xL and c-myc) as implicated in numerouscancer types could be a result of elevated levels of eIF4F [41]

DAP5 (p97) eIF4G1 and eIF4G2 comprise the eIF4Gfamily and function in the formation of the translationinitiation complex DAP5 domains share homology withthe central and C-terminal region of eIF4G that interactswith eIF4A eIF3 Mnk1 and eIF2120573 but not eIF4E [42]The interaction of DAP5 with eIF2120573 and eIF4AI facilitatesthe IRES-mediated translation of various cellular mRNAsincluding those encoding pro- and antiapoptotic proteinssuch as c-Myc Bcl2 Apaf1 XIAP c-IAP1 and DAP5 itself[42 43]Moreover the cleavage of DAP5 by caspase generatesa smaller fragment p86 which also facilitates the IRES-mediated translation of various cellular mRNAs [44] Thecleavage of eIF4GI during apoptosis yields three major cleav-age products N-FAG (N-terminal Fragment of ApoptoticCleavage of eIF4G) M-FAG (Middle-FAG) and C-FAG (C-terminal FAG) the cleavage causes disassembly of the eIF4Fcomplex as the fragments no longer retain the ability to bindeIF4E eIF4A and eIF3 Surprisingly eIF4G M-FAG alonesupports the IRES-mediated translation ofmRNAs includingp97DAP5 XIAP and c-IAP1 [32]

During angiogenesis IRES expression of the potentangiogenic factor vascular endothelial growth factor(VEGF) is regulated by its interaction with eIF4E andeIF4G The direct interaction of eIF4G1 with VEGF IRESenhances its translation during breast cancer progression[45] Knockdown of eIF4GI using RNA interference in achick chorioallantoicmembrane system resulted in decreasedVEGF protein levels and a reduction in angiogenesis [45]This reduction was specific to IRES-mediated translation assilencing of eIF4GI had no drastic effect on global rates ofprotein synthesis in normoxic conditions The depletion ofeIF4GI attenuated global mRNA translation rates suggestinga bigger requirement of eIF4GI during hypoxic conditions[45] The C-terminus of eIF4GI stimulates cap-independenttranslation initiation at the 51015840 UTR of c-myc and VEGFUnder hypoxic conditions VEGF FGF-2 Bcl-2 andhypoxia-inducible factor 1120572 (HIF1120572) are all overexpresseddue to upregulation of IRES-mediated translation [46 47]This selective translation is mediated by the overexpressionof eIF4E-BPs and eIF4G and is particularly advantageousfor cancer cells as VEGF FGF-2 HIF1120572 and Bcl-2 are allsignificant factors in promoting tumor growth and survival[45 48]

3 IRES trans-Acting Factors RegulatingIRES-Mediated Translation

The ITAFs are protein factors that interact specifically withthe IRES based on the sequence and structure of the mRNAand modulate IRES-mediated translation ITAFs can act asa molecular chaperone or modify the structure of RNAto facilitate direct recruitment of the eukaryotic initiationfactors and the ribosome to form 48S initiation complex

Some of the well-characterized ITAFs are human antigen R(HuR) La autoantigen programed cell death 4 (PDCD4)polypyrimidine tract binding (PTB) protein heterogeneousnuclear ribonucleoprotein A1 (hnRNPA1) hnRNAC1C2upstream of NRAS (UnR) nuclear factor 45 (NF45) insulin-like growth factor 2-binding protein 1 (IGF2BP1) Y-boxprotein 1 (YB1) and poly(C) binding protein (PCBP) [2049 50] The levels activity and localization of these ITAFsare regulated by various signaling pathways which in turnregulate the IRES-mediated translation Hence many of theseITAFs are implicated in tumor cell survival and cancerprogression

There are two classes of ITAFs one that facilitates whilethe other represses IRES-dependent translation For exampleNF45 promotes while hnRNPA1 inhibits the IRES-mediatedtranslation of XIAP [51 52] HuR directly interacts with theXIAP and Bcl-xL IRES and modulates their translation HuRinteracts with the XIAP IRES through the RNA recognitionmotifs RRM1 and RRM2 to stimulate protein synthesisOn the contrary interaction with the Bcl-xL IRES decreasestranslation and enhances the membrane integrity of mito-chondrial promoting cell survival [53 54] La autoantigenas part of RNP complex enhances XIAP IRES-mediatedtranslation as demonstrated by in vitro and in vivo assays[55]

PDCD4 had been considered the general translationinhibitor which sequesters eIF4A and inhibits its helicaseactivity [56] However it is shown that the direct interactionof PDCD4 with the XIAP IRES is required to inhibit theIRES-mediated translation of XIAP PDCD4 in the absenceof activated S6K2 directly binds to the XIAP and Bcl-xLIRES and blocks 48S preinitiation complex formation [57]The activation of S6K2 by the fibroblast growth factor 2(FGF2) results in phosphorylation of PDCD4 and subsequentremoval by the proteasomal pathway which in turn upregu-lates IRES-mediated translation of XIAP and Bcl-xL [57]

PTB protein forms ribonucleoprotein complexes withPSF hnRNPL and other proteins to regulate the gene expres-sion stability and localization of mRNA during apoptosisThe complex formed is constantly remodeled dependingon the type of apoptotic stimulant When TRAIL activatesapoptosis PTB protein forms complex with PSF YBX1NONOp54nrb hnRNPA2B1 hnRNPC1C2 and DDX3Xand regulate the IRES activity of mRNAs involved in apop-tosis The interactions could occur in the nucleus prior tosplicing and in the cytoplasm aiding recruitment of theribosome Cytoplasmic translocation of PTB protein leads toincrease in TRAF1 p53 and p47 mRNA expression Specificmutation of cytosine to thymidine observed in domain 2of c-myc IRES derived from multiple myeloma cell linesdemonstrated enhanced interactions with PTB protein andY-box binding protein 1 This increase in protein binding iscorrelated with an elevated IRES activity of c-myc mRNA inmultiple myeloma In human melanoma single nucleotidepolymorphism at the PTB protein binding site present on p5351015840 UTR decreased IRES activity emphasizing the importanceof PTB protein as an ITAF [58ndash62]

Overexpression of hnRNPC activates XIAP IRES activitywith no effect on cap-dependent translation [63] Also


hnRNPC1C2 interaction with the p53 IRES is critical formRNA expression and thus affects transcription of proapop-totic mRNA [64 65] Moreover hnRNPC interacts withthe heptameric uridine sequence in the c-myc IRES andenhances c-myc expression only during G2M phase of cellcycle [66] hnRNPA1 which is part of hnRNP family ofproteins regulates expression of Bcl-xL and XIAP mRNAPhosphorylation at the RRM1 domain of hnRNPA1 by S6K2selectively promotes association of Bcl-xL or XIAP withhnRNPA1 and exports the RNA-protein complex into thecytoplasm hnRNPA1 interaction with the mRNA suppressesits IRES activity The suppression is relieved by sumoylationof the RRM2 domain of hnRNPA1 resulting in the decreasedaffinity of hnRNPA1 to protein and translocation into thenucleus [67]

Unr acts as either a positive or a negative regulator ofapoptosis depending on the cell type Deleting unr in theembryonic stem cells resulted in decreased expression ofp53 Gadd45g and caspase-3 impairing apoptosis signalingpathway in response to gamma irradiation whereas whenunr expression was partially silenced induction of apoptosiswas observed [68] Unr interaction with the Apaf1 IRESopens the stem-loop structure of IRES enabling PTB proteinbinding PTBproten andunr act synergistically as chaperonesto create a single-stranded region for the recruitment of48S complex [69] PCBP like unr unwinds Bag-I IRES andfacilitates landing of the ribosome Besides the mentionedpositive regulator ITAFs such as hnRNPC1 and nucleolincan negatively regulate p53 expression [49] In conclusionthe levels and localization of ITAFs in the cell are criticalfor regulation of gene expression During stress conditionsproteins are modified mostly by phosphorylation reactionsthat trigger nuclear to cytoplasmic localization andmodulateIRES-mediated translation

4 Role of Eukaryotic Initiation Factors inCanonical Translation

41 eIF2 eIF2 is a heterotrimeric protein composed of 120572 120573and 120574 subunits [70] eIF2 is a required element of the ternarycomplex that delivers met-tRNAi to the 40S ribosomal P-sitein translation initiation [71] eIF2 exists in a GDP- or GTP-bound configuration which has a critical role in translationalcontrol during stress [71] During translation initiation eIF2bound GTP hydrolysis is induced by eIF5 releasing eIF2-GDP in the inactive form eIF2B catalyzes the exchange ofGDP toGTPwhich is necessary for reformation of the ternarycomplex [70] Under stress conditions such as an excess ofunfolded proteins in the endoplasmic reticulum or aminoacid starvation 120572 subunit of eIF2 is phosphorylated at serine51 by one of four members of the eIF2120572 kinase family [72 73]Phosphorylation of eIF2120572 sequesters eIF2B locking eIF2 andeIF2B in an inactive complex [71] Inhibition of active eIF2-GTP regeneration results in decreased ternary complex thusinhibiting overall translation (Figure 2) Furthermore eIF2120572phosphorylation has a role in suppression of tumorigenesisdemonstrated by the ability of protein kinase RNA-activated(PKR) to promote malignant transformation of NIH 3T3

cells [70 74] The transformation mechanism inhibits eIF2120572phosphorylation by decreasing the activity of upstream targetPKR potentially through the formation of inactive PKRheterodimers [70 74] Decreased levels of phosphorylatedeIF2120572 were found in osteosarcoma tumors while increasedPKR levels and associated phosphorylated eIF2120572 levels werecorrelated with tumor cell differentiation [74 75] Expressionlevels of phosphorylated eIF2120572 serve as a marker for deter-mining the prognosis of non-small lung cancer (NSCLC)patients [76]

ER stress is closely associated with solid tumor progres-sion having implications in cancer proliferation and apopto-sis Downstream mediators and targets of ER stress includeactivating transcription factor 6 inositol-requiring enzyme1 (IRE1) and protein kinase RNA-like ER kinase (PERK)which is an upstream activator of eIF2120572 phosphorylation[77 78] ER stress is induced in chronic myeloid leukemia(CML) which activates PERK and eIF2120572 phosphorylation[79] Phosphorylation of eIF2120572 supports CML progressionwith a prosurvival role shown in the inhibition of PERKwhich prevents eIF2120572 phosphorylation [79] This in turnallows sensitization of CML cells to imatinib and decreasestheir proliferative abilities [79] Insulin-like growth factorbinding protein-5 (IGFBP-5) and protein family memberIGFBP-3 upregulate expression of growth arrest and DNAdamage-inducible protein 34 (GADD34) which assemblesan eIF2120572 dephosphorylation complex enabling regenera-tion of active eIF2 critical to ternary complex formation[80] GADD34rsquos dephosphorylation activity is required torecommence protein synthesis after periods of global trans-lation inhibition However translation of uORF-containingprooncogenic protein ATF4 is upregulated by eIF2120572 phos-phorylation promoting osteogenesis and osteoblast differen-tiation [80] In ovarian cancer cells autophagy and activationof the PERKeIF2120572 pathway attenuate and protect cancer cellsfrom metformin-induced apoptosis [81] In contrast proteinObg-like ATPase 1 (OLA1) inhibits protein synthesis andpromotes integrated stress response without utilizing eIF2120572phosphorylation [82] OLA1 is a GTPase that binds to eIF2preventing ternary complex formation [82] In vivo OLA1-knockdown inhibits the mainly prosurvival integrated stressresponse (ISR) pathway in cancer cells which is responsiblefor restoring cellular homeostasis in response to physiologicalchanges as well as intrinsic stresses such as ER stress [82 83]Inhibition of the ISR pathway results in attenuated CCAAT-enhancer-binding protein homologous protein (CHOP) lev-els and promotion of tumor growth and metastasis throughcell proliferation [82] CHOP expression is triggered byunfolded protein accumulation in the endoplasmic reticulum(ER) [84] CHOP induces apoptosis during prolonged stressor stress responsemalfunction through the formation of het-erodimers with other CEBP family members [84] Therebyattenuation of CHOP results in inhibition of apoptosis instress conditions

Besides phosphorylation of eIF2120572 increased levels ofeIF2120572 expression are detected in tumor samples in bron-chioloalveolar carcinomas of the lung Hodgkinrsquos lym-phoma gastrointestinal carcinomas malignant melanomaandmelanocytic neoplasms [70 85] eIF2120572 levels were highly


Table 1 Differential expression of eIF3 subunits in human cancers and notable eIF3-protein interactions

eIF3 Subunit Expression Cancer associations Protein interactions

eIF3a uarr Breast cervix esophagus lung amp gastric [89 90] ribonucleotide reductase M2 [91] RAR120572 [92]Raf-1 [93]

eIF3b uarr Breast bladder amp prostate [90] mTORS6K1 [94] ICP27 [95] DDX3 [96] NREP[97]

eIF3c uarrColon [98] meningioma [99] amp testicularseminomas [100]

mTORS6K1 [94] MAPK6 [101] CDK2 [102]TARDBP [103] schwannomin [99]

eIF3d uarr Colon [104 105] gastric [70] amp mesothelioma [70] hTDAG51 [106] VPg [107]eIF3e darr Breast amp lung [89 90] S6K1 [94] DDX3 [96] Rpn5 [108]eIF3f darr Breast colon melanoma amp pancreas [90 109] mTOR [110] S6K1 [111] HnRNP K [109]eIF3h uarr Breast colon liver amp prostate [89 90] Acetylated HIV-1 IN [112] MGMT [113]

expressed along with eIF4E in the germinal centers ofreactive follicles when examined in several types of non-Hodgkinrsquos lymphoma [86] Comparably eIF2120572 and eIF4Ewere present in the nuclei and cytoplasm of brain tumor cellswith a higher concentration of eIF2120572 in the nuclei of gastroin-testinal cancer tumor cells [87 88] Differential expressions ofeIF2120572may relate to abnormal protein synthesis furthering itsrole in tumorigenesis [85] As themain effector of both globaltranslation and translation of specific subsets of mRNAseIF2 continually shows potential for cancer therapeutics andtreatments

42 eIF3 Eukaryotic initiation factor 3 (eIF3) is a 13-subunitcomplex of 800 kilodaltons required for translation initia-tion through interactions with the 40S ribosomal subunitmRNA and other eIFs necessary for the formation ofcompetent translation initiation complexes [19 114] eIF3along with eIF1A and plausibly eIF5 associates with theternary complex and the 40S ribosomal subunit to formthe 43S preinitiation complex eIF3 enhances the stability of43S preinitiation complex through eIF3-eIF4G interaction[1 19 89] Primarily eIF3j regulates eIF3 interaction withthe mRNA-binding cleft on the 40S subunit by inhibitingmRNA entry and confirming met-tRNAi is present in theP-site [115ndash117] Interestingly beyond the protein synthesisrelated functions of the eIF3 complex dysregulation of eIF3subunits has been implicated in several types of cancers[70 90] Variations in the levels and activity of eIF3 subunitsare a result of upstream signaling molecules such as proteinkinases involving phosphorylation of eIF3 subunits Forexample the mammalian target of rapamycin complex 1(mTORC1) a key protein kinase involved in the regulation ofprotein synthesis facilitates interaction between PAIP1 andeIF3 [111] The eIF3g subunit directly interacts with PAIP1in an RNA-independent manner which enhances PAIP1-mediated translation stimulation in vivo [111 118] Stimu-lation of mTORC1 (eg by amino acids) phosphorylatesS6K which ultimately stimulates interaction between eIF3and PAIP1 This PAIP1-eIF3 interaction is also proposedto stabilize the conformation of circularized mRNA bystimulating the eIF4G-PABP interaction [111 118] Moreover

mTOR inhibition by rapamycin and PP242 (inhibitor ofmTORC1 and mTORC2) significantly decreased S6K1 phos-phorylation and subsequently diminished this PAIP1-eIF3interaction [111 118] Evidence for the direct association ofmTOR and S6K1 with eIF3 points to probable effects ontranslation Specifically studies have shown that the eIF3complex as found on the translation preinitiation complex(eIF3-PIC) functions as a scaffolding structure which isassociated with mTOR by mitogenhormone stimulationwhereas S6K1 dissociates from this complex upon similarstimulation [94] Immunoprecipitation experiments in theimmortalized human embryonic kidney cells (HEK293ET)have shown that S6K1 associates with eIF3b eIF3c eIF3eand eIF3f and mTOR coimmunoprecipitates with eIF3c [94111] HEK293T cells are derived from the original HEK293Ecell lineage and have been modified to allow for transienttransfection of vectors containing the SV40 origin of replica-tion [119] Interestingly in nutrient-deprived conditions theS6K1-eIF3 association is observed whereas the addition ofamino acids diminishes this direct interaction [94] Notablywith insulin treatment S6K1 dissociates from the eIF3-PIC whereas mTOR is associated with this complex Thisinsulin stimulation results in an increase in cap-dependenttranslation suggesting the mTOR-eIF3-PIC association andthe subsequent series of phosphorylation events are criticalfor efficient protein synthesis [94]

In addition to the critical function of the eIF3 complex intranslation initiation many eIF3 subunits have been shownto be involved in a diverse set of cellular processes includingapoptosis oncogenesis and cellular growth and proliferation[89 100 109 114 120ndash122] Upregulation of eIF3a eIF3beIF3c eIF3d eIF3e eIF3h and eIF3i along with reducedlevels of eIF3e and eIF3f has been observed in several cancers(Table 1) Most notably increased levels of the largest eIF3subunit eIF3a have been found in breast cervix esopha-gus lung and stomach cancers [89 90] The mechanismby which upregulation of eIF3a promotes the malignantphenotype in lung cancer (H1299) and breast cancer (MCF7)cells involves enhancing DNA synthesis for maintaining cellproliferation During the S phase of the cell-cycle eIF3aupregulates the translational expression of ribonucleotidereductase M2 which in turn maintains high levels of DNA


synthesis [89 91] When eIF3a is downregulated by usingantisense eIF3a cDNA ribonucleotide reductase M2 levels(and subsequently DNA synthesis) are significantly reducedFurthermore independently downregulating ribonucleotidereductase M2 expression levels has shown to reduce theextent of malignancy in human cancer cells [91] Thesefindings delineate the mechanism of how high expressionlevels of eIF3a maintain cell proliferation in cancer cellsAdditionally eIF3a is also known to enhance phagocytosisduring apoptosis by facilitating the association betweenapoptotic cells and macrophages [122]

Overexpression of eIF3b has been observed in breastbladder and prostate cancers however the specific mecha-nism through which upregulated eIF3b promotes the cancerstate is still unclear [90] Increased eIF3c transcript levelshave been found in human testicular seminomas [100] aswell as increased eIF3c gene expression in colon cancer cells[98] Interestingly eIF3c also directly binds to the neurofibro-matosis 2 (NF2) tumor suppressor protein schwannomin inSTS26T schwannoma cells [99] Schwannomin is thought toemploy its tumor suppressive functions by binding eIF3c andinhibiting eIF3c-mediated cell proliferation possibly due tothe role of eIF3c during protein translation initiation Addi-tionally in meningiomas which have significantly reducedlevels of schwannomin eIF3c is upregulated suggestinga role of eIF3c in tumor growth and proliferation [99]Furthermore overexpression of eIF3c or eIF3h resulted in theenhanced translation of cell proliferation mRNAs encodinggrowth-regulating cyclin D1 c-Myc fibroblast growth factor2 and ornithine decarboxylase (ODC) [100] ODC serves as amarker for cell proliferation and functions as an oncoprotein[123]This enhancement of translation rates for these proteinsand subsequent production ofmalignant phenotypesmay notbe a direct consequence of the overexpression of a single eIF3subunit since enhanced levels of other eIF3 subunits (a bc f h and j) were also noted Thus the overexpression ofeIF3a eIF3b or eIF3c subunits stimulates the expression ofother eIF3 subunits that further supports the translationalcomponents necessary for faster cancer cell growth [100]

Like other eIF3 subunits eIF3d is also involved in proteinsynthesis and has been shown to be upregulated in gastriccancer and mesothelioma [70 104] Studies using lentivirus-mediated RNA interference to knockdown eIF3d in the colon(HCT116) and non-small cell lung cancer cells (NSCLCmdashA549 and 95D) showed significantly reduced cell prolifera-tion (induced apoptosis) and inhibited colony formation dueto induced cell-cycle arrest in the G2M phases Notably inHCT116 cells eIF3d knockdown resulted in phosphorylationof AMPK120572 Bad PRAS40 SAPK (stress-activated proteinkinase)JNK GSK3120573 and PARP [poly(ADP-ribose) poly-merase] cleavage Phosphorylation of Bad a proapoptoticprotein induces apoptosis while phosphorylation of GSK3120573promotes reovirus-induced apoptosis Furthermore PARPcleavage is typically used as a signal for apoptosis induction[104] eIF3d knockdown inNSCLC cells resulted in decreasedphosphorylation and thus inhibition of AKT HSP27 andSAPKJNK (involved in cellular growth and cancer progres-sion pathways)This data supports the crucial role of eIF3d incell proliferation and cancer growth [104 105]

In breast cancer cells reduction of eIF3e expression byRNAi induces EMT (epithelial-to-mesenchymal transition)suggesting a role of eIF3e in breast cancer metastasis Thisstudy and others [108 124] suggest that eIF3e normallyfunctions as a tumor suppressor since the reduction of itsexpression results in enhanced mRNA stability and expres-sion of the transcription factors and EMT regulators Snail1and Zeb2 This suggests that the loss of eIF3e directly resultsin cancer progression and metastasis as EMT is inducedin breast cancer cells [124] Conversely another report hasshown that eIF3e functions as an oncogene [125] In thisstudy the knockdown of eIF3e using RNAi in U2OS andMDA-MB-231 resulted in a reduction in protein levels ofBcl-xL (antiapoptotic protein) and urokinase-type plasmino-gen activator (PLAU) but an increase in MAD2L1 (mitoticcheckpoint component) Overabundant Bcl-xL protein levelsare associated with chemoresistance in cancers while PLAUfunctions in promoting metastasis in tumors InterestinglyBcl-xL mRNA associates directly with the eIF3 complexin an eIF3e-dependent manner as determined by RNA IPFollowing eIF3e knockdown the specific changes in proteinlevels of the mentioned eIF3e targets without any changes inglobal protein synthesis suggest that eIF3e specifically regu-lates translation of mRNAs involved in tumorigenesis [125]Furthermore eIF3e gene silencing using siRNA in glioblas-toma results in cell-cycle arrest in the G1 phase decreasescell proliferation and induces both caspase-dependent andcaspase-independent apoptosis [126]

Furthermore a recent study suggests the role of eIF3d-eIF3e module within the eIF3 complex that regulates thetranslation of specific mRNAs involved in maintainingmetabolic pathways that are likely disrupted in cancer cells[127] Critical components of the mitochondrial electrontransport chain were downregulated in both yeast and mam-malian cells (nontumorigenic MCF-10A and nontumori-genicMCF7) that were eIF3e-depleted using siRNA whereasglucose metabolism and amino acid biosynthesis processeswere upregulated Their findings suggest that depletion ofeIF3e triggers a metabolic switch that increases dependenceon glycolysis as respiratory deficiencies alongside increasedsensitivity to oxidative stress are also observed when eIF3dis depleted in addition to eIF3e knockdown Essentiallythis data suggests that the novel function of eIF3d-eIF3ein maintaining mitochondrial respiration components andserving to adjust metabolic pathways may help us betterunderstand how the cancer-promoting properties of the eIF3complex emerge [127]

Unlike eIF3e eIF3f is consistently shown to function asa tumor suppressor in pancreatic cancer [109] Endogenouslevels of both eIF3f mRNA and protein levels are reduced inpancreatic cancer cells [128] whereas eIF3f-overexpressingNIH3T3 cells have shown reduced cell proliferation andinduced apoptosis [100] Likewise eIF3f knockdown in nor-mal human pancreatic epithelial cells has shown an increasein cell proliferation and increased resistance to apoptosis[109] By utilizing a bicistronic luciferase report system itwas shown that eIF3f normally inhibits both cap-dependentand cap-independent (ie IRES) mechanisms of transla-tion initiation Furthermore one mechanism of translation


inhibition likely involves eIF3f-mediated rRNA degradationby a direct eIF3f-hnRNP K (RNA-binding protein) inter-action [109] Specifically when eIF3f is present it bindsto hnRNP K preventing it from binding to rRNA whichsubsequently is degraded and the translation is reduced Incancer cells the loss of eIF3f results in an increased bindingof hnRNP K to rRNA reducing rRNA degradation andpossibly favoring oncogenesis through increased translation[109]These findings suggest that eIF3f functions as a negativeregulator of cell growth due to the naturally reduced levels ofeIF3f contributing to cancer development and overexpressionresulting in apoptosis [128]

Despite the vast quantity of correlative evidences betweeneIF3 subunit expression levels and observed cancer pheno-types it has been difficult to establish a clear mechanism ofhow cancer progression is directly affected by eIF3 expres-sion Understanding the interactions of the eIF3 subunitswith antiapoptotic proteins cellular growth and prolifera-tion proteins and oncogenic proteins may provide a betterunderstanding of how eIF3 subunits contribute to supportingor inhibiting the oncogenic state A functional mechanismwill most likely involve aspects of translation initiationpreinitiation complex formation and other processes suchas the recruitment of the preinitiation complex to the cellularmRNA and ribosome scanning all of which are mediatedto some degree by a subset of eIF3 subunits Additionallystudies depicting eIF3 as both an activator (for the pro-tooncogene c-Jun) and repressor (for the negative regulatorof cell proliferation BTG1) of cap-dependent translationmediated by binding to specific RNA structural elements[114] illustrate the diversity of functions carried out by theeIF3 complex through interactions with proteins and nucleicacids Future studies will need to focus on determining thesignaling pathways that are involved in regulation of eIF3 andthe consequences of eIF3-directed therapeutics for humancancers This information will assist in the development ofnovel therapeutics that target eIF3 subunits in order to treatcancers and possibly other human diseases

43 eIF4F Complex eIF4E interacts with the mRNA capstructure and eIF4G for efficient translation The interactionwith eIF4G is inhibited by binding of eIF4E-binding protein(4E-BP) to eIF4E [129] eIF4G and 4E-BP are known tocompete for a common binding site on eIF4EThe eIF4E and4E-BP interaction is highly regulated via a phosphorylationreaction and acts as a primary target for hindering translationinitiation during stress conditions [130] Activation of themTOR pathway phosphorylates 4E-BP and enhances cap-binding efficiency of eIF4E In contrast inhibition of themTOR pathway by amino acid starvation dephosphorylates4E-BP resulting in increased association between 4E-BP andeIF4E thus repressing cap-dependent translation Overex-pression of 4E-BP leads to a decrease in the mRNA andprotein levels of cyclin D1 and an increase in p27 a cell-cycleregulatory protein that promotes cell-cycle arrest in breastcancer cell lines Additionally eIF4E has been demonstratedto have distinct nuclear and cytoplasmic roles It localizes tothe nuclear bodies by interacting with the eIF4E-transporter

protein (4E-T) In the nuclear bodies it remains associatedwith promyelocytic leukemia proteins that share a commonbinding site with 4E-T and 4E-BPThus an increase in 4E-BPmay affect the mRNA transport function of eIF4E from thenucleus to the cytoplasm resulting in differential expressionof cyclin D1 Also dysregulation of 4E-BP phosphorylationis correlated with poor prognosis in lung cancer breastcancer melanoma cervical carcinoma and astrocytoma [131132] eIF4E is phosphorylated at Ser209 by p38 mitogen-activated protein kinase (MAPK) p38MAPKphosphorylatesMnk a serinethreonine kinase and enablesMnk interactionwith eIF4G and phosphorylation of eIF4E [133] Inductionwith transforming growth factor 120573 (TGF120573) increases eIF4Ephosphorylation alongside mesenchymal markers such asN-cadherin fibronectin and vimentin as a result of thenoncanonical signaling pathway As a downstream targetof p38 Mnk1 is activated Mnk1 phosphorylates eIF4E tospecifically translate mRNAs transcribed by the canonicalSMAD pathway This eventually leads to the upregulationof SNAIL and matrix metalloproteinase 3 promoting cellinvasion and metastasis [134] Phosphorylation of eIF4E isnot an absolute requirement for translation but is observedto increase the rate of translation initiation [129] Genome-wide studies of translating mRNA have indicated that eIF4Ephosphorylation is necessary for synthesizing proteins essen-tial for tumorigenesis and the levels of eIF4E are critical forantiapoptotic protein expression [135ndash137]

In nasopharyngeal carcinoma the latent membrane pro-tein 1 enhances transcription of many oncogenes such asVEGF c-Myc and matrix metalloproteinases (MMP) eIF4Epromoter activity is enhanced by c-Myc and as a feedbackmechanism eIF4E increases translation of these oncogenes[138] Further overexpression of eIF4E leads to increasedexpression of a subset of proteins influencing angiogenesisand tumor progression (VEGF and fibroblast growth factor-2 (FGF-2)) growth stimulation (platelet-derived growth fac-tor) prosurvival (Bcl-2 and Bcl-xL) cell-cycle progression(c-myc cyclin D1 and ornithine decarboxylase) epithelial-to-mesenchymal transition (SNAIL andMMP) and invasion(integrin 1205731) [134 137 139ndash142] Besides the role of eIF4E asa cap-binding protein it is reported to stimulate the helicaseactivity of eIF4A and aid translation of mRNAs comprisinglong structured 51015840 UTRs [143]

A subset of transcripts harbors a 12-nucleotide motif(CGG)4 called the G-quadruplex in their 51015840 UTRs Someexamples ofmRNAs including such structures areVEGF Bcl-2 and NRASThese mRNAs are sensitive to silvestrol a drugthat inhibits the helicase activity of eIF4A thus indicatingthe role of eIF4A discrete frommTOR-dependent translationregulation [144ndash146] Ribosome footprinting and polysomeprofiling experiments have detected more than 250 genesincluding MYC MDM2 CDK6 and AFR6 that are affectedby silvestrol treatment making it a promising drug to treatcancer [146 147] In the case of breast cancer eIF4A augmentsexpression of an oncoprotein mucin 1 Mucin 1 forms a com-plex with EGFR and activates the PI3 K-Akt-mTOR pathway[148] Akt and p70 S6K along with activation of MEK-ERKsignaling promote ubiquitination and degradation of PDCD4[149] This favors translation as the MA-3 protein binding


domain of PDCD4 is known to interact with eIF4A N-terminal domain and inhibit its helicase activity Activation ofthe helicase activity stimulates the autoinductive pathway andincreases levels of oncogenic proteins [56 148 150] FurtherPDCD4 inhibits eIF4A-eIF4G interaction by interacting witheIF4A as well as eIF4G Colocalization of eIF4A and PDCD4in the cytoplasm inhibits eIF4A helicase activity [151 152]eIF4A also interacts with the PAIP1 domain containingsequence similarity to eIF4G and promotes circularization ofthemRNA affecting protein expression involved in apoptosis[153]

As mentioned earlier eIF4G interacts with eIF4E eIF4Aand eIF3 to form the preinitiation complex eIF4G consists oftwobinding sites for eIF4Aand a single binding site for eIF4EeIF3 PABP Mnk1 and RNA which interact independentlyof each other [154] eIF4G interacts with PABP for effectivecircularization of the mRNA Consequently it increasesthe cap-binding activity of eIF4F and joining of the 60Sribosomal subunit to the 40S subunit [155ndash157] Interaction ofeIF4G with eIF4E enhances eIF4E-mediated cap recognitionand the cap-binding activity of eIF4F complex [130] eIF4Eis associated with positive regulator HOXA9 in the nucleusThis promotes eIF4E-mediated transport ofmRNAs from thenucleus to the cytoplasm eIF4G has a higher affinity towardseIF4E in comparison to HOXA9 Therefore HOXA9 isdisplaced from eIF4E in the cytoplasm to initiate translation[141]

During translation eIF4G and eIF3 interactions bringthe ternary complex onto the mRNA and stabilize the 43Spreinitiation complexThese interactions are mediated by the3c 3d and 3e subunits of eIF3 complex [154] Activation ofmTOR by insulin amino acids or growth factors influencesthe direct interaction between mTOR and eIF3 and increaseseIF4G-eIF3 interaction The effect on interactions is mostlikely mediated by phosphorylation reactions [110] Thesecritical interactions are extensively exploited as drug targetsin cancer treatment [4]

44 eIF5 eIF5 is a 49 kDa protein in mammals and 46 kDain Saccharomyces cerevisiae [158 159] eIF5 interacts withthe 40S initiation complex to mediate hydrolysis of eIF2-bound GTP [159] This step is critical to initiation complexformation as the release of multiple eIFs including eIF3eIF4E and eIF2-GDP is necessary for the recruitment of60S ribosomal subunit [158] In higher eukaryotes includinghumans eIF5 has a 9-residue C-terminal tail that can bindto the eIF5B-CTD subdomain [160] This is opposed tothe binding of the eIF1A C-terminus to eIF5B-CTD whichis a relatively weak interaction in both humans and yeast[160] The interaction between eIF1A and eIF5B coordinatesrecruitment and release of one another in S cerevisiae How-ever in humans eIF1AeIF5B interaction facilitates subunitjoining and recruitment is coordinated separately througheIF5eIF5B interaction [160] eIF5 is a downstream targetof casein kinase 2 (CK2) which phosphorylates eIF5 at themajor sites Ser389 and Ser390 [161] CK2has a significant rolein cell proliferation and both CK2 and eIF5 have critical rolesin cell-cycle control and progression [161] CK2 is necessary

for theG1 andG2Mphase transitions in yeast [162]TheCK2enzymatic activity also increases and phosphorylates eIF53 hours after serum stimulation of cell-cycle arrested (G0)human embryonic kidney HEK-293 cells and normal humanfetal lung fibroblasts TIG-7 cells suggesting a role for CK2and eIF5 in promoting cell proliferation [162] In TIG-7 cellseIF5 was associated with CK2When CK2 levels were highestand when eIF5 mutants were unable to be phosphorylated byCK2 there was a decrease in growth rate mature translationinitiation complex formation and expression of cell-cycle-regulated proteins [161] Nuclear CK2120572 (catalytic subunit)localization is a sign of poor prognosis in prostate cancerand gastric carcinoma [163ndash165] CK2s phosphorylation tar-gets include deleted in breast cancer 1 (DBC1) eIF5 andendothelin-converting enzyme-1c (ECE-1c) which promotecancer cell invasion and progression [163ndash165] Depletion ofeIF5 in S cerevisiae resulted in the inhibition of cell growthand a decrease in the rate of in vivo protein synthesis [159]In yeast eIF5 is able to mimic the effect of eIF2120572 phospho-rylation acting as a translational inhibitor and promotingtranslation of prooncogenic protein GCN4 (yeast equivalentof ATF4) [166 167]When overexpressed in S cerevisiae eIF5increases the levels of aN eIF2eIF5 complex which preventseIF2B interaction and subsequently prevents ternary complexformation [167] eIF5-mimic protein (5MP) is a partialmimicand competition of eIF5 function Human 5MP1 protein wasfound to interact with human eIF2s 120573 subunit similarly toeIF5 eIF2B120576 and Kra [168] Furthermore in vitro eIF2120573demonstrated mutually exclusive interactions with eIF5 and5MP1 suggesting 5MP1 as a competitive inhibitor of eIF5[168] 5MP promotes expression of GADD34 (a downstreamtarget of ATF4) in Tribolium castaneum [169] Further5MP binds eIF2 to inhibit general translation and whenoverexpressed it promotes ATF4 expression in fibrosarcoma[166 169] ATF4 is expressed in hypoxic- and nutrient-deprived tumor regions with functions in development pro-moting metabolic homeostasis and cancer cell proliferation[170] eIF5 demonstrates critical roles in cell-cycle regula-tion and cell proliferation with specific oncogenic proteininteractions

45 eIF5A eIF5A is a 17 kDa protein that is activated by post-translational hypusination and functions to mediate cell pro-liferation apoptosis and inflammatory response [171 172]Hypusination is unique to eIF5A and is a posttranslationalenzymaticmodification that involves two sequential enzymesand the substrate spermidine [173] Deoxyhypusine synthase(DHS) catalyzes NAD-dependent cleavage and transfer of anaminobutyl moiety of spermidine to the 120576-amino group of aconserved lysine of eIF5A [173] The resulting intermediateresidue deoxyhypusine is hydroxylated by deoxyhypusinehydroxylase (DOHH) yielding a hypusine residue and anactive eIF5A [173] New treatments for chronic myeloidleukemia utilize hypusination inhibitors to deactivate eIF5Acreating a target in response to imatinib resistance towardsthe BCR-ABL tyrosine kinase [171] Inhibition of eIF5Aresults in an antiproliferative effect on BCR-ABL positive-and negative-leukemia cell lines in vitro [171] eIF5A is


overexpressed in murine pancreatic intraepithelial neoplasia(PanIN) and in human pancreatic ductal adenocarcinoma(PDAC) [174] Pharmacological inhibitors N(1)-guanyl-17-diamineoheptane (GC7) and ciclopirox olamine (CPX) areable to inhibit DHS and DOHH respectively which furtherinhibits eIF5A hypusination and results in eIF5A geneticknockdown [174] Genetic knockdown of eIF5A inhibitedPDAC cell growth in vitro and orthotopic tumor formationin vivo potentially through pseudopodium-enriched atypicalkinase 1 (PEAK1) which is essential to PDAC tumor growthmetastasis and gemcitabine resistance [174] In melan-a (amurine melanocyte cell line) and Tm5 (a murine melanomacell line derived from melan-a) GC7 was used to inhibiteIF5A [175] More pronounced DNA fragmentation wasobserved in Tm5 cells and decreased viability was observedin both cell lines [175] Additionally treatment with GC7was tested on melanoma growth in C57BL6 mice and foundto inhibit further tumor growth although it did not inducevolume reduction of established tumors [175] eIF5Arsquos isoformeIF5A2 is upregulated in various cancer types includinghepatocellular carcinoma ovarian carcinoma and colorectalcarcinoma (CRC) [176ndash178] eIF5A2 overexpression in cancercells is correlated to prognosis factors of tumor metastasisand venous infiltration [176 177] In ovarian carcinomaoverexpression of eIF5A2 was detected in 7 cystadenomas30 borderline tumors and 53 invasive carcinomas asopposed to normal expression in normal ovaries [177] InCRC eIF5A2 upregulates metastasis-associated protein 1(MTA1) by increasing the enrichment of regulator gene c-mycon MTA1s promoter [178] This both increases cell motilityand invasion in vitro and results in tumor metastasis induc-ing epithelial-mesenchymal transition and further promotingCRC aggressiveness [178] In hepatocellular carcinoma cellsin vivo eIF5A2 suppression attenuates tumorigenic prop-erties [176 177] eIF5As unique activation by hypusinationcreates a desirable mechanism of regulation and knockdownwithin various cancer lines

46 eIF5B eIF5B is a 175 kDaprotein that promotes 60S ribo-some subunit joining and pre-40S subunit proofreadingand can indirectly support the tRNA-Met119894 association withthe ribosome in translation initiation [179ndash181] In serum-starved THP1 cells elevated eIF5B levels resulted in increasedeIF5B complexes with tRNA-Met119894 as well as increasedphosphorylation of eIF2120572 [180] These eIF5B complexes areformed during times of attenuated global translation insteadof promoting translation of specific stress-related mRNAs[180] eIF5B is antagonistic of G0 and G0-like states witheIF5B overexpression promoting maturation and cell deatheIF5B depletion is associated with increased phosphor-Cdc2which is a marker for immaturity [180] This suggests acritical role for eIF5B in the regulation of cell-cycle transitions[180] eIF5B interacts with DEAD-box RNA helicase Vasa(Vas) with reduction of Vas-eIF5B interaction in Drosophilacausing female sterility reduced Gurken (Grk) protein levelsnearly complete loss of germ cell formation and reduction ofsomatic posterior patterning [182] eIF5B interactionwithVasis necessary for early Drosophila development through the

progression of oogenesis and pole plasm assembly suggestingeIF5B as a potential method to regulate Vas and subsequentlyGrk [182] In poliovirus (PV) and coxsackie B virus (CBV)infection of cultured cells eIF5B is proteolytically cleavedsuggesting that eIF5B cleavage is involved in the translationinhibitionwithin enterovirus-infected cells [183] Infection ofenteroviruses PV and CBV begins with inhibition of host celltranslation followed by IRES-directed enterovirus proteinsynthesis with eIF5B cleavage beginning at 3 hours afterinfection during the attenuation of host cell protein synthesis[183 184] eIF5B eIF5 and eIF5A all contain links intohallmark stages and mechanisms of cancer proliferation andmanipulation of these proteins offers a potential regulationpoint of gene expression regulation

5 Conclusions

Mechanisms utilizing elements of protein translation specif-ically in the rate-limiting step of initiation offer potentialmethods to diagnose and treat cancer Translational regula-tion is capable of efficiently altering specific protein levelsin physiological stress conditions that are typical in can-cer Translation is mediated by eukaryotic initiation factors(eIFs) which have varying roles in regulating the rate ofinitiation as described in this reviewThe critical role of eIF2120572phosphorylation has been classified in the integrated stressresponse with the requirement of eIF4F complex for cap-binding and efficient translation through eIF4E and eIF4GOther mechanisms include eIF3 subunit interactions withS6K1 and mTOR and eIF5Arsquos necessary activation throughposttranslational hypusination important to the mediationof cell proliferation apoptosis and inflammatory responseVarying levels of eIFs in various cancer lines and stages alongwith the mechanistic background enforce the use of eIFsto regulate gene expression in cancer During cellular stressinduced by cancer noncanonical translation utilizing uORFelements or IRES elements drives translation of specific stressresponse and adaptation proteins Proteins such as ATF4 andGCN4 have critical roles in the integrated stress responseand help determine whether cell proliferation ensues TheeIFs required for IRES-dependent translation are specificto the mRNA in question with a subset of cellular IRESesnot requiring eIF4G and eIF4A while L-myc requires thefull eIF4F complex and PABP Both facilitation and inhibi-tion of IRES-dependent translation are mediated by ITAFsIRES-dependent translation which specifically favors about10 percent of cellular mRNAs under cellular stress offerspotential gene regulation targets including various eIFs andITAFs with critical implications for cancer treatment Themechanisms underlying both canonical and noncanonicaltranslation and the proteins responsible for the processes arecritical in treating the dysregulated gene expression in cancerHowever these studies are often limited in terms of whatone can extract from them since the observed phenotypicalchanges in nearly all levels of complexity starting fromthe cellular level to the whole organism level tend to becomplicated by several other protein-protein and possiblyprotein-nucleic acid interactions Future studies will need to


continue building on the present data by taking mechanis-tic approaches in elucidating the signaling pathways tran-scriptomic translatomic and proteomic profiles of patientsamples and model systems to determine the appropriatemethodology to target the function of specific regulators (egproteins like initiation factors) to produce effective noveltherapeutics that have intrinsically high specificity Currentlythere are several drugs and antisense oligonucleotides beingtested against the initiation factors to increase mortality ofcancer cells Initiation factors being a common elementacross various types of translation hold great potential tobe tested for RNA-based therapeutics as well as chemicalcompound-based therapeutics to treat cancer

Competing Interests

The authors declare that they have no competing interests

Authorsrsquo Contributions

Divya Khandige Sharma Kamiko Bressler and Harshil Patelcontributed equally

Acknowledgments

The authors thank Drs Tyson Graber (Children Hospitalof Eastern Ontario Apoptosis Research Centre Ottawa)and Joseph Ross (University of Lethbridge) for criticallyreading the manuscript Anand Krishnan Nambisan for hiscontributions towards designing the figures and Universityof Lethbridge and Campus Alberta Innovates Program forfunding Divya Khandige Sharma and Nirujah Balasingamare the recipients of the University of Lethbridge Schoolof Graduate Studies Scholarships Kamiko Bressler receivedChinook Summer Research Award (CSRA) (2016) HarshilPatel received CSRA-2015 and Natural Sciences and Engi-neering Research Council-Undergraduate Student ResearchAwards (NSERC-USRA) (2016) Anand Krishnan Nambisanwas an MITACS Globalink intern (2016) and he technicallyassisted Divya Khandige Sharma Kamiko Bressler andHarshil Patel in preparing Adobe Fireworks figures

References

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[2] T Tebaldi A Re G Viero et al ldquoWidespread uncoupling be-tween transcriptome and translatome variations after a stimulusin mammalian cellsrdquo BMC Genomics vol 13 article 220 2012

[3] B Schwanhuusser D Busse N Li et al ldquoGlobal quantificationof mammalian gene expression controlrdquo Nature vol 473 no7347 pp 337ndash342 2011

[4] M Bhat N Robichaud L Hulea N Sonenberg J Pelletier andI Topisirovic ldquoTargeting the translation machinery in cancerrdquoNature Reviews Drug Discovery vol 14 no 4 pp 261ndash278 2015

[5] J Kong and P Lasko ldquoTranslational control in cellular anddevelopmental processesrdquo Nature Reviews Genetics vol 13 no6 pp 383ndash394 2012

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[7] D Silvera S C Formenti and R J Schneider ldquoTranslationalcontrol in cancerrdquoNature Reviews Cancer vol 10 no 4 pp 254ndash266 2010

[8] N Thakor and M Holcik ldquoIRES-mediated translation of cel-lular messenger RNA operates in eIF2120572-independent mannerduring stressrdquoNucleic Acids Research vol 40 no 2 pp 541ndash5522012

[9] G Leprivier B Rotblat D Khan E Jan and P H SorensenldquoStress-mediated translational control in cancer cellsrdquo Biochim-ica et BiophysicaActa (BBA)mdashGeneRegulatoryMechanisms vol1849 no 7 pp 845ndash860 2015

[10] B D Fonseca C Zakaria J-J Jia et al ldquoLa-related protein1 (LARP1) represses terminal oligopyrimidine (TOP) mRNAtranslation downstream of mTOR complex 1 (mTORC1)rdquo TheJournal of Biological Chemistry vol 290 no 26 pp 15996ndash16020 2015

[11] S Marquez-Jurado A Nogales S Zuniga L Enjuanes andF Almazan ldquoIdentification of a gamma interferon-activatedinhibitor of translation-like RNA motif at the 31015840 end of thetransmissible gastroenteritis coronavirus genome modulatinginnate immune responserdquomBio vol 6 no 2 Article ID e001052015

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[13] T Preiss and M W Hentze ldquoStarting the protein synthesismachine eukaryotic translation initiationrdquo BioEssays vol 25no 12 pp 1201ndash1211 2003

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[31] L Balvay R Soto Rifo E P Ricci D Decimo and T OhlmannldquoStructural and functional diversity of viral IRESesrdquo Biochimicaet Biophysica Acta (BBA)mdashGene Regulatory Mechanisms vol1789 no 9-10 pp 542ndash557 2009

[32] S Henis-Korenblit G Shani T Sines L Marash G Shohatand A Kimchi ldquoThe caspase-cleaved DAP5 protein supportsinternal ribosome entry site-mediated translation of deathproteinsrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 99 no 8 pp 5400ndash5405 2002

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[35] M Stoneley S A Chappell C L Jopling M Dickens MMacFarlane and A E Willis ldquoc-Myc protein synthesis isinitiated from the internal ribosome entry segment duringapoptosisrdquo Molecular and Cellular Biology vol 20 no 4 pp1162ndash1169 2000

[36] A Howard and A N Rogers ldquoRole of translation initiationfactor 4G in lifespan regulation and age-related healthrdquo AgeingResearch Reviews vol 13 no 1 pp 115ndash124 2014

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[38] A A Komar and M Hatzoglou ldquoInternal ribosome entry sitesin cellular mRNAs mystery of their existencerdquo The Journal ofBiological Chemistry vol 280 no 25 pp 23425ndash23428 2005

[39] A S Lee P J Kranzusch J A Doudna and J H Cate ldquoeIF3d isan mRNA cap-binding protein that is required for specializedtranslation initiationrdquo Nature vol 536 no 7614 pp 96ndash992016

[40] A Shamas-Din J Kale B Leber and D W Andrews ldquoMech-anisms of action of Bcl-2 family proteinsrdquo Cold Spring HarborPerspectives in Biology vol 5 no 4 Article ID a008714 2013

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[42] L Marash N Liberman S Henis-Korenblit et al ldquoDAP5promotes cap-independent translation of Bcl-2 and CDK1 tofacilitate cell survival during mitosisrdquo Molecular Cell vol 30no 4 pp 447ndash459 2008

[43] N Liberman V Gandin Y V Svitkin et al ldquoDAP5 associateswith eIF2120573 and eIF4AI to promote internal ribosome entry sitedriven translationrdquo Nucleic Acids Research vol 43 no 7 pp3764ndash3775 2015

[44] L Marash and A Kimchi ldquoDAP5 and IRES-mediated transla-tion during programmed cell deathrdquo Cell Death and Differenti-ation vol 12 no 6 pp 554ndash562 2005

[45] J P Crew S Fuggle R Bicknell D W Cranston A DeBenedetti and A L Harris ldquoEukaryotic initiation factor-4E in superficial and muscle invasive bladder cancer and itscorrelation with vascular endothelial growth factor expressionand tumour progressionrdquo British Journal of Cancer vol 82 no1 pp 161ndash166 2000

[46] G Akiri D Nahari Y Finkelstein S-Y Le O Elroy-Stein andB-Z Levi ldquoRegulation of vascular endothelial growth factor(VEGF) expression is mediated by internal initiation of transla-tion and alternative initiation of transcriptionrdquo Oncogene vol17 no 2 pp 227ndash236 1998

[47] F Morfoisse E Renaud F Hantelys A Prats and B Garmy-Susini ldquoRole of hypoxia and vascular endothelial growth factorsin lymphangiogenesisrdquo Molecular amp Cellular Oncology vol 1no 1 Article ID e29907 2014

[48] I Stein A Itin P Einat R Skaliter Z Grossman and E KeshetldquoTranslation of vascular endothelial growth factor mRNA byinternal ribosome entry implications for translation underhypoxiardquoMolecular and Cellular Biology vol 18 no 6 pp 3112ndash3119 1998

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[50] K A SpriggsM Bushell S AMitchell and A EWillis ldquoInter-nal ribosome entry segment-mediated translation during apop-tosis the role of IRES-trans-acting factorsrdquo Cell Death andDifferentiation vol 12 no 6 pp 585ndash591 2005

[51] M D Faye T E Graber P Liu et al ldquoNucleotide compositionof cellular internal ribosome entry sites defines dependenceon NF45 and predicts a posttranscriptional mitotic regulonrdquoMolecular and Cellular Biology vol 33 no 2 pp 307ndash318 2013

[52] S M Lewis A Veyrier N Hosszu Ungureanu S Bonnal SVaguer and M Holcik ldquoSubcellular relocalization of a trans-acting factor regulates XIAP IRES-dependent translationrdquoMolecular Biology of the Cell vol 18 no 4 pp 1302ndash1311 2007

[53] D Durie M Hatzoglou P Chakraborty and M Holcik ldquoHuRcontrols mitochondrial morphology through the regulation ofBclxL translationrdquo Translation vol 1 no 1 2013

[54] D Durie SM Lewis U LiwakM KisilewiczM Gorospe andMHolcik ldquoRNA-binding proteinHuRmediates cytoprotection


through stimulation of XIAP translationrdquoOncogene vol 30 no12 pp 1460ndash1469 2011

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[56] C Suzuki R G Garces K A Edmonds S Hiller S G Hybertsand A M Wagner ldquoPDCD4 inhibits translation initiation bybinding to eIF4A using both its MA3 domainsrdquo Proceedings ofthe National Academy of Sciences of the United States of Americavol 105 no 9 pp 3274ndash3279 2008

[57] U Liwak N Thakor L E Jordan et al ldquoTumor suppressorPDCD4 represses internal ribosome entry site-mediated trans-lation of antiapoptotic proteins and is regulated by S6 kinase2rdquoMolecular and Cellular Biology vol 32 no 10 pp 1818ndash18292012

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[65] R Grover A Sharathchandra A Ponnuswamy D Khan andS Das ldquoEffect of mutations on the p53 IRES RNA structureimplications for de-regulation of the synthesis of p53 isoformsrdquoRNA Biology vol 8 no 1 pp 132ndash142 2011

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[74] O Donze R Jagus A E Koromilas J W B Hershey and NSonenberg ldquoAbrogation of translation initiation factor eIF-2phosphorylation causes malignant transformation of NIH 3T3cellsrdquoThe EMBO Journal vol 14 no 15 pp 3828ndash3834 1995

[75] F Wimbauer C Yang K L Shogren et al ldquoRegulation ofinterferon pathway in 2-methoxyestradiol-treated osteosar-coma cellsrdquo BMC Cancer vol 12 article 93 2012

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[77] S B Cullinan and J ADiehl ldquoCoordination of ER and oxidativestress signaling the PERKNrf2 signaling pathwayrdquo Interna-tional Journal of Biochemistry and Cell Biology vol 38 no 3pp 317ndash332 2006

[78] A M Gorman S J M Healy R Jager and A Samali ldquoStressmanagement at the ER regulators of ER stress-induced apopto-sisrdquo Pharmacology andTherapeutics vol 134 no 3 pp 306ndash3162012

[79] M Kusio-Kobialka P Podszywalow-Bartnicka P Peidis et alldquoThe PERK-eIF2120572 phosphorylation arm is a pro-survival path-way of BCR-ABL signaling and confers resistance to imatinibtreatment in chronic myeloid leukemia cellsrdquo Cell Cycle vol 11no 21 pp 4069ndash4078 2012

[80] A Saito K Ochiai S Kondo et al ldquoEndoplasmic reticulumstress response mediated by the PERK-eIF2120572-ATF4 pathway isinvolved in osteoblast differentiation induced by BMP2rdquo TheJournal of Biological Chemistry vol 286 no 6 pp 4809ndash48182011

[81] H S Moon B Kim H Gwak et al ldquoAutophagy and pro-tein kinase RNA-like endoplasmic reticulum kinase (PERK)eukaryotic initiation factor 2 120572 kinase (eIF2120572) pathway pro-tect ovarian cancer cells from metformin-induced apoptosisrdquoMolecular Carcinogenesis vol 55 no 4 pp 346ndash356 2016

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[86] S Wang I B Rosenwald M J Hutzler et al ldquoExpression ofthe eukaryotic translation initiation factors 4E and 2120572 in non-Hodgkinrsquos lymphomasrdquoTheAmerican Journal of Pathology vol155 no 1 pp 247ndash255 1999

[87] M V T Lobo M E Martın M I Perez et al ldquoLevels phospho-rylation status and cellular localization of translational factoreIF2 in gastrointestinal carcinomasrdquoHistochemical Journal vol32 no 3 pp 139ndash150 2000

[88] S TejadaMV T LoboMGarcıa-Villanueva et al ldquoEukaryoticinitiation factors (eIF) 2120572 and 4E expression localization andphosphorylation in brain tumorsrdquo Journal ofHistochemistry andCytochemistry vol 57 no 5 pp 503ndash512 2009

[89] Z Dong and J-T Zhang ldquoInitiation factor eIF3 and regulationof mRNA translation cell growth and cancerrdquo Critical Reviewsin OncologyHematology vol 59 no 3 pp 169ndash180 2006

[90] J W B Hershey ldquoThe role of eIF3 and its individual subunits incancerrdquo Biochimica et Biophysica Acta (BBA)mdashGene RegulatoryMechanisms vol 1849 no 7 pp 792ndash800 2015

[91] Z Dong L H Liu B Han R Pincheira and J-T Zhang ldquoRoleof eIF3 p170 in controlling synthesis of ribonucleotide reductaseM2 and cell growthrdquo Oncogene vol 23 no 21 pp 3790ndash38012004

[92] N Chen B Onisko and J L Napoli ldquoThe nuclear transcriptionfactor RAR120572 associates with neuronal RNA granules andsuppresses translationrdquoThe Journal of Biological Chemistry vol283 no 30 pp 20841ndash20847 2008

[93] T-R Xu R-F Lu D Romano et al ldquoEukaryotic translationinitiation factor 3 subunit a regulates the extracellular signal-regulated kinase pathwayrdquo Molecular and Cellular Biology vol32 no 1 pp 88ndash95 2012

[94] M K Holz B A Ballif S P Gygi and J Blenis ldquomTORand S6K1 mediate assembly of the translation preinitiationcomplex through dynamic protein interchange and orderedphosphorylation eventsrdquo Cell vol 123 no 4 pp 569ndash580 2005

[95] E C Fontaine-Rodriguez T J Taylor M Olesky and D MKnipe ldquoProteomics of herpes simplex virus infected cell protein27 association with translation initiation factorsrdquo Virology vol330 no 2 pp 487ndash492 2004

[96] C-S Lee A P Dias M Jedrychowski A H Patel J L Hsuand R Reed ldquoHuman DDX3 functions in translation andinteracts with the translation initiation factor eIF3rdquo NucleicAcids Research vol 36 no 14 pp 4708ndash4718 2008

[97] M M Yue K Lv S C Meredith J L Martindale M Gorospeand L Schuger ldquoNovel RNA-binding protein P311 bindsEukaryotic translation initiation factor 3 subunit B (eIF3b) topromote translation of transforming growth factor 1205731-3 (TGF-1205731-3)rdquo The Journal of Biological Chemistry vol 289 no 49 pp33971ndash33983 2014

[98] N Song YWang X-D Gu Z-Y Chen and L-B Shi ldquoEffect ofsiRNA-mediated knockdown of eIF3c gene on survival of coloncancer cellsrdquo Journal of Zhejiang University SCIENCE B vol 14no 6 pp 451ndash459 2013

[99] D R ScolesWH Yong Y Qin KWawrowsky and SM PulstldquoSchwannomin inhibits tumorigenesis through direct interac-tion with the eukaryotic initiation factor subunit c (eIF3c)rdquoHuman Molecular Genetics vol 15 no 7 pp 1059ndash1070 2006

[100] L Zhang X Pan and J W B Hershey ldquoIndividual overex-pression of five subunits of human translation initiation factoreIF3 promotesmalignant transformation of immortal fibroblastcellsrdquo The Journal of Biological Chemistry vol 282 no 8 pp5790ndash5800 2007

[101] A Vinayagam U Stelzl R Foulle et al ldquoA directed proteininteraction network for investigating intracellular signal trans-ductionrdquo Science Signaling vol 4 no 189 article rs8 2011

[102] I Neganova F Vilella S P Atkinson et al ldquoAn important rolefor CDK2 in G1 to S checkpoint activation and DNA damageresponse in human embryonic stem cellsrdquo STEM CELLS vol29 no 4 pp 651ndash659 2011

[103] B D Freibaum R K Chitta A A High and J P TaylorldquoGlobal analysis of TDP-43 interacting proteins reveals strongassociation with RNA splicing and translation machineryrdquoJournal of Proteome Research vol 9 no 2 pp 1104ndash1120 2010

[104] X Yu B Zheng and R Chai ldquoLentivirus-mediated knockdownof eukaryotic translation initiation factor 3 subunit D inhibitsproliferation of HCT116 colon cancer cellsrdquo Bioscience Reportsvol 34 no 6 Article ID e00161 2014

[105] Z Lin L Xiong and Q Lin ldquoKnockdown of eIF3d inhibits cellproliferation through G2M phase arrest in non-small cell lungcancerrdquoMedical Oncology vol 32 no 7 article 183 2015

[106] T Hinz S Flindt A Marx O Janssen and D KabelitzldquoInhibition of protein synthesis by the T cell receptor-induciblehuman TDAG51 gene productrdquo Cellular Signalling vol 13 no5 pp 345ndash352 2001

[107] K F Daughenbaugh C S Fraser J W B Hershey and M EHardy ldquoThe genome-linked protein VPg of the Norwalk virusbinds eIF3 suggesting its role in translation initiation complexrecruitmentrdquoThe EMBO Journal vol 22 no 11 pp 2852ndash28592003

[108] J Suo S J Snider G B Mills et al ldquoInt6 regulates both pro-teasomal degradation and translation initiation and is criticalfor proper formation of acini by humanmammary epitheliumrdquoOncogene vol 30 no 6 pp 724ndash736 2011

[109] F Wen R Zhou A Shen A Choi D Uribe and J Shi ldquoThetumor suppressive role of eIF3f and its function in translationinhibition and rRNA degradationrdquo PLoS ONE vol 7 no 3Article ID e34194 2012

[110] T E Harris A Chi J Shabanowitz D F Hunt R E Rhoadsand J C Lawrence Jr ldquomTOR-dependent stimulation of theassociation of eIF4G and eIF3 by insulinrdquo The EMBO Journalvol 25 no 8 pp 1659ndash1668 2006

[111] Y Martineau X Wang T Alain et al ldquoControl of Paip1-eukayrotic translation initiation factor 3 interaction by aminoacids through S6 kinaserdquo Molecular and Cellular Biology vol34 no 6 pp 1046ndash1053 2014

[112] A Allouch and A Cereseto ldquoIdentification of cellular factorsbinding to acetylated HIV-1 integraserdquoAmino Acids vol 41 no5 pp 1137ndash1145 2011

[113] S K Niture C E Doneanu C S Velu N I Bailey andK S Srivenugopal ldquoProteomic analysis of human O6-meth-ylguanine-DNA methyltransferase by affinity chromatographyand tandem mass spectrometryrdquo Biochemical and BiophysicalResearch Communications vol 337 no 4 pp 1176ndash1184 2005

[114] A S Y Lee P J Kranzusch and J H D Cate ldquoeIF3 targetscell-proliferation messenger RNAs for translational activationor repressionrdquo Nature vol 522 no 7554 pp 111ndash114 2015

[115] C S Fraser K E Berry J W B Hershey and J A DoudnaldquoeIF3j is located in the decoding center of the human 40s


ribosomal subunitrdquo Molecular Cell vol 26 no 6 pp 811ndash8192007

[116] C S Fraser J Y LeeG LMayeurM Bushell J ADoudna andJ W B Hershey ldquoThe j-subunit of human translation initiationfactor eIF3 is required for the stable binding of eIF3 and itssubcomplexes to 40 S ribosomal subunits in vitrordquoThe Journalof Biological Chemistry vol 279 no 10 pp 8946ndash8956 2004

[117] K H Nielsen L Valasek C Sykes A Jivotovskaya and A GHinnebusch ldquoInteraction of the RNP1motif in PRT1withHCR1promotes 40S binding of eukaryotic initiation factor 3 in yeastrdquoMolecular and Cellular Biology vol 26 no 8 pp 2984ndash29982006

[118] Y Martineau M C Derry X Wang et al ldquoPoly(A)-bindingprotein-interacting protein 1 binds to eukaryotic translationinitiation factor 3 to stimulate translationrdquo Molecular andCellular Biology vol 28 no 21 pp 6658ndash6667 2008

[119] Y-C Lin M Boone L Meuris et al ldquoGenome dynamics ofthe human embryonic kidney 293 lineage in response to cellbiology manipulationsrdquo Nature Communications vol 5 article4767 2014

[120] R Marchione D Laurin L Liguori M P Leibovitch S ALeibovitch and J Lenormand ldquoMD11-mediated delivery ofrecombinant eIF3f inducesmelanoma and colorectal carcinomacell deathrdquo Molecular TherapymdashMethods amp Clinical Develop-ment vol 2 article 14056 2015

[121] R Marchione S A Leibovitch and J-L Lenormand ldquoThetranslational factor eIF3f the ambivalent eIF3 subunitrdquoCellularand Molecular Life Sciences vol 70 no 19 pp 3603ndash3616 2013

[122] Y Nakai A Shiratsuchi J Manaka et al ldquoExternalization andrecognition by macrophages of large subunit of eukaryotictranslation initiation factor 3 in apoptotic cellsrdquo ExperimentalCell Research vol 309 no 1 pp 137ndash148 2005

[123] A Shayovits and U Bachrach ldquoOrnithine decarboxylase anindicator for growth of NIH 3T3 fibroblasts and their c-Ha-ras transformantsrdquo Biochimica et Biophysica Acta (BBA)mdashMolecular Cell Research vol 1267 no 2-3 pp 107ndash114 1995

[124] L D Gillis and S M Lewis ldquoDecreased eIF3eInt6 expressioncauses epithelial-to-mesenchymal transition in breast epithelialcellsrdquo Oncogene vol 32 no 31 pp 3598ndash3605 2013

[125] M Grzmil T Rzymski M Milani et al ldquoAn oncogenic role ofeIF3eINT6 in human breast cancerrdquo Oncogene vol 29 no 28pp 4080ndash4089 2010

[126] J Sesen A Cammas S J Scotland et al ldquoInt6eIF3e is essentialfor proliferation and survival of human glioblastoma cellsrdquoInternational Journal of Molecular Sciences vol 15 no 2 pp2172ndash2190 2014

[127] M Shah D Su J S Scheliga et al ldquoA transcript-specificeIF3 complex mediates global translational control of energymetabolismrdquo Cell Reports vol 16 no 7 pp 1891ndash1902 2016

[128] A Doldan A Chandramouli R Shanas et al ldquoLoss of theeukaryotic initiation factor 3f in pancreatic cancerrdquo MolecularCarcinogenesis vol 47 no 3 pp 235ndash244 2008

[129] J Zuberek A Wyslouch-Cieszynska A Niedzwiecka et alldquoPhosphorylation of eIF4E attenuates its interaction withmRNA 51015840 cap analogs by electrostatic repulsion intein-mediat-ed protein ligation strategy to obtain phosphorylated proteinrdquoRNA vol 9 no 1 pp 52ndash61 2003

[130] N Sonenberg and A G Hinnebusch ldquoRegulation of translationinitiation in eukaryotes mechanisms and biological targetsrdquoCell vol 136 no 4 pp 731ndash745 2009

[131] H Jiang J Coleman R Miskimins and W K MiskiminsldquoExpression of constitutively active 4EBP-1 enhances p271198701198941199011expression and inhibits proliferation of MCF7 breast cancercellsrdquo Cancer Cell International vol 3 article 2 2003

[132] Y Qu R Zhao H Wang et al ldquoPhosphorylated 4EBP1 isassociatedwith tumor progression and poor prognosis in Xp112translocation renal cell carcinomardquo Scientific Reports vol 6Article ID 23594 2016

[133] M Shveygert C Kaiser S S Bradrick and M Gromeier ldquoReg-ulation of eukaryotic initiation factor 4E (eIF4E) phospho-rylation by mitogen-activated protein kinase occurs throughmodulation ofMnk1-eIF4G interactionrdquoMolecular andCellularBiology vol 30 no 21 pp 5160ndash5167 2010

[134] N Robichaud S VDel Rincon BHuor et al ldquoPhosphorylationof eIF4E promotes EMT andmetastasis via translational controlof SNAIL andMMP-3rdquoOncogene vol 34 no 16 pp 2032ndash20422015

[135] L Furic L Rong O Larsson et al ldquoeIF4E phosphorylationpromotes tumorigenesis and is associated with prostate cancerprogressionrdquo Proceedings of the National Academy of Sciences ofthe United States of America vol 107 no 32 pp 14134ndash141392010

[136] B A Jacobson S C Thumma J Jay-Dixon et al ldquoTargetingeukaryotic translation in mesothelioma cells with an eIF4E-specific antisense oligonucleotiderdquo PLoS ONE vol 8 no 11Article ID e81669 2013

[137] Y Mamane E Petroulakis L Rong K Yoshida L W Ler andN Sonenberg ldquoeIF4Emdashfrom translation to transformationrdquoOncogene vol 23 no 18 pp 3172ndash3179 2004

[138] Y Zhao T-Y Pang Y Wang et al ldquoLMP1 stimulates the tran-scription of eIF4E to promote the proliferation migration andinvasion of human nasopharyngeal carcinomardquo FEBS Journalvol 281 no 13 pp 3004ndash3018 2014

[139] L Decarlo CMestel M-H Barcellos-Hoff and R J SchneiderldquoEukaryotic translation initiation factor 4E is a feed-forwardtranslational coactivator of transforming growth factor 120573 earlyprotransforming events in breast epithelial cellsrdquoMolecular andCellular Biology vol 35 no 15 pp 2597ndash2609 2015

[140] F Pettersson S V Del Rincon A Emond et al ldquoGenetic andpharmacologic inhibition of eIF4E reduces breast cancer cellmigration invasion and metastasisrdquo Cancer Research vol 75no 6 pp 1102ndash1112 2015

[141] I Topisirovic A Kentsis J M Perez M L Guzman C TJordan and K L B Borden ldquoEukaryotic translation initiationfactor 4E activity is modulated by HOXA9 at multiple levelsrdquoMolecular andCellular Biology vol 25 no 3 pp 1100ndash1112 2005

[142] A De Benedetti and A L Harris ldquoeIF4E expression in tumorsits possible role in progression of malignanciesrdquo The Interna-tional Journal of Biochemistry amp Cell Biology vol 31 no 1 pp59ndash72 1999

[143] K Feoktistova E Tuvshintogs A Do and C S Fraser ldquoHumaneIF4E promotes mRNA restructuring by stimulating eIF4Ahelicase activityrdquo Proceedings of the National Academy of Sci-ences of the United States of America vol 110 no 33 pp 13339ndash13344 2013

[144] S Kumari A Bugaut J L Huppert and S BalasubramanianldquoAn RNA G-quadruplex in the 51015840 UTR of the NRAS proto-oncogene modulates translationrdquoNature Chemical Biology vol3 no 4 pp 218ndash221 2007

[145] R Shahid A Bugaut and S Balasubramanian ldquoThe BCL-2 51015840untranslated region contains an RNA G-quadruplex-forming


motif that modulates protein expressionrdquo Biochemistry vol 49no 38 pp 8300ndash8306 2010

[146] A L Wolfe K Singh Y Zhong et al ldquoRNA G-quadruplexescause eIF4A-dependent oncogene translation in cancerrdquoNature vol 513 no 7516 pp 65ndash70 2014

[147] C A Rubio BWeisburdMHolderfield et al ldquoTranscriptome-wide characterization of the eIF4A signature highlights plastic-ity in translation regulationrdquo Genome Biology vol 15 no 10article no 476 2014

[148] C Jin H Rajabi C M Rodrigo J A Porco and D KufeldquoTargeting the eIF4A RNA helicase blocks translation of theMUC1-C oncoproteinrdquo Oncogene vol 32 no 17 pp 2179ndash21882013

[149] T Schmid A P Jansen A R Baker G Hegamyer J P Haganand N H Colburn ldquoTranslation inhibitor Pdcd4 is targeted fordegradation during tumor promotionrdquoCancer Research vol 68no 5 pp 1254ndash1260 2008

[150] O Fehler P Singh A Haas et al ldquoAn evolutionarily con-served interaction of tumor suppressor protein Pdcd4 with thepoly(A)-binding protein contributes to translation suppressionby Pdcd4rdquoNucleic Acids Research vol 42 no 17 pp 11107ndash111182014

[151] P SchutzM BumannA EOberholzer et al ldquoCrystal structureof the yeast eIF4A-eIF4G complex an RNA-helicase controlledby proteinmdashprotein interactionsrdquo Proceedings of the NationalAcademy of Sciences of the United States of America vol 105 no28 pp 9564ndash9569 2008

[152] H-S Yang A P Jansen A A Komar et al ldquoThe transformationsuppressor Pdcd4 is a novel eukaryotic translation initiationfactor 4A binding protein that inhibits translationrdquo Molecularand Cellular Biology vol 23 no 1 pp 26ndash37 2003

[153] A W B Craig A Haghighat A T K Yu and N SonenbergldquoInteraction of polyadenylate-binding protein with the eIF4Ghomologue PAIP enhances translationrdquo Nature vol 392 no6675 pp 520ndash523 1998

[154] N Villa A Do J W B Hershey and C S Fraser ldquoHumaneukaryotic initiation factor 4G (eIF4G) protein binds to eIF3c-d and -e to promote mRNA recruitment to the ribosomerdquoJournal of Biological Chemistry vol 288 no 46 pp 32932ndash32940 2013

[155] J-Y Lu N Bergman N Sadri and R J Schneider ldquoAssemblyof AUF1 with eIF4G-poly(A) binding protein complex suggestsa translation function in AU-rich mRNA decayrdquo RNA vol 12no 5 pp 883ndash893 2006

[156] A Kahvejian Y V Svitkin R Sukarieh M-N MrsquoBoutchouand N Sonenberg ldquoMammalian poly(A)-binding protein is aeukaryotic translation initiation factor which acts via multiplemechanismsrdquoGenes and Development vol 19 no 1 pp 104ndash1132005

[157] M Wakiyama H Imataka and N Sonenberg ldquoInteraction ofelF4G with poly(A)-binding protein stimulates translation andis critical for Xenopus oocyte maturationrdquo Current Biology vol10 no 18 pp 1147ndash1150 2000

[158] S Das and U Maitra ldquoMutational analysis of mammaliantranslation initiation factor 5 (eIF5) role of interaction betweenthe 120573 subunit of eIF2 and eIF5 in eIF5 function in vitro and invivordquo Molecular and Cellular Biology vol 20 no 11 pp 3942ndash3950 2000

[159] T Maiti and U Maitra ldquoCharacterization of translation initia-tion factor 5 (eIF5) from Saccharomyces cerevisiae Functionalhomology with mammalian eIF5 and the effect of depletion of

eIF5 on protein synthesis in vivo and in vitrordquo The Journal ofBiological Chemistry vol 272 no 29 pp 18333ndash18340 1997

[160] A Marintchev and G Wagner ldquoTranslation initiation struc-tures mechanisms and evolutionrdquo Quarterly Reviews of Bio-physics vol 37 no 3-4 pp 197ndash284 2004

[161] M K Homma and Y Homma ldquoCell cycle and activation ofCK2rdquoMolecular and Cellular Biochemistry vol 316 no 1-2 pp49ndash55 2008

[162] M K Homma I Wada T Suzuki J Yamaki E G KrebsandYHomma ldquoCK2phosphorylation of eukaryotic translationinitiation factor 5 potentiates cell cycle progressionrdquoProceedingsof the National Academy of Sciences of the United States ofAmerica vol 102 no 43 pp 15688ndash15693 2005

[163] J S Bae S-H Park K M Kim et al ldquoCK2120572 phosphorylatesDBC1 and is involved in the progression of gastric carcinomaand predicts poor survival of gastric carcinoma patientsrdquoInternational Journal of Cancer vol 136 no 4 pp 797ndash809 2015

[164] M Laramas D Pasquier O Filhol F Ringeisen J-L Descotesand C Cochet ldquoNuclear localization of protein kinase CK2catalytic subunit (CK2120572) is associated with poor prognosticfactors in human prostate cancerrdquo European Journal of Cancervol 43 no 5 pp 928ndash934 2007

[165] I Niechi E Silva P Cabello et al ldquoColon cancer cell inva-sion is promoted by protein kinase CK2 through increase ofendothelin-converting enzyme-1c protein stabilityrdquoOncotargetvol 6 no 40 pp 42749ndash42760 2015

[166] C Kozel BThompson S Hustak et al ldquoOverexpression of eIF5or its protein mimic 5MP perturbs eIF2 function and inducesATF4 translation through delayed re-initiationrdquo Nucleic AcidsResearch vol 44 no 18 pp 8704ndash8713 2016

[167] C R Singh B Lee T Udagawa et al ldquoAn eIF5eIF2 complexantagonizes guanine nucleotide exchange by eIF2B duringtranslation initiationrdquo EMBO Journal vol 25 no 19 pp 4537ndash4546 2006

[168] C R Singh RWatanabe D Zhou et al ldquoMechanisms of trans-lational regulation by a human eIF5-mimic proteinrdquo NucleicAcids Research vol 39 no 19 pp 8314ndash8328 2011

[169] H Hiraishi J Oatman S L Haller et al ldquoEssential role of eIF5-mimic protein in animal development is linked to control ofATF4 expressionrdquo Nucleic Acids Research vol 42 no 16 pp10321ndash10330 2014

[170] D C Singleton and A L Harris ldquoTargeting the ATF4 pathwayin cancer therapyrdquo Expert Opinion on Therapeutic Targets vol16 no 12 pp 1189ndash1202 2012

[171] S Balabanov A Gontarewicz P Ziegler et al ldquoHypusination ofeukaryotic initiation factor 5A(eIF5A) a novel therapeutic tar-get in BCR-ABL-positive leukemias identified by a proteomicsapproachrdquo Blood vol 109 no 4 pp 1701ndash1711 2007

[172] B Maier T Ogihara A P Trace et al ldquoThe unique hypusinemodification of eIF5A promotes islet 120573 cell inflammation anddysfunction in micerdquo Journal of Clinical Investigation vol 120no 6 pp 2156ndash2170 2010

[173] B Belda-Palazon M A Nohales J L Rambla et al ldquoBio-chemical quantitation of the eIF5Ahypusination inArabidopsisthaliana uncovers ABA-dependent regulationrdquo Frontiers inPlant Science vol 5 article no 202 2014

[174] K Fujimura T Wright J Strnadel et al ldquoA hypusine-eIF5A-PEAK1 switch regulates the pathogenesis of pancreatic cancerrdquoCancer Research vol 74 no 22 pp 6671ndash6681 2014

[175] M G Jasiulionis A D Luchessi A G Moreira et al ldquoInhi-bition of eukaryotic translation initiation factor 5A (eIF5A)


hypusination impairs melanoma growthrdquoCell Biochemistry andFunction vol 25 no 1 pp 109ndash114 2007

[176] F H Shek S Fatima and N P Lee ldquoImplications of the use ofeukaryotic translation initiation factor 5A (eIF5A) for prognosisand treatment of hepatocellular carcinomardquo International Jour-nal of Hepatology vol 2012 Article ID 760928 6 pages 2012

[177] G-F Yang D Xie J-H Liu et al ldquoExpression and amplificationof eIF-5A2 in human epithelial ovarian tumors and overexpres-sion of EIF-5A2 is a new independent predictor of outcomein patients with ovarian carcinomardquo Gynecologic Oncology vol112 no 2 pp 314ndash318 2009

[178] W Zhu M-Y Cai Z-T Tong et al ldquoOverexpression ofEIF5A2 promotes colorectal carcinoma cell aggressivenessby upregulating MTA1 through C-myc to induce epithelial-mesenchymaltransitionrdquo Gut vol 61 no 4 pp 562ndash575 2012

[179] J H Lee T V Pestovat B-S Shin C Cao S K Choi andT E Dever ldquoInitiation factor eIF5B catalyzes second GTP-dependent step in eukaryotic translation initiationrdquo Proceedingsof the National Academy of Sciences of the United States ofAmerica vol 99 no 26 pp 16689ndash16694 2002

[180] S Lee S S Truesdell S I A Bukhari et al ldquoUpregulationof eIF5B controls cell-cycle arrest and specific developmentalstagesrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 111 no 41 pp E4315ndashE4322 2014

[181] T V Pestova I B Lomakin J H Lee S K Choi T E Deverand C U T Hellen ldquoThe joining of ribosomal subunits ineukaryotes requires eIF5Brdquo Nature vol 403 no 6767 pp 332ndash335 2000

[182] O Johnstone and P Lasko ldquoInteraction with eIF5B is essentialfor Vasa function during developmentrdquo Development vol 131no 17 pp 4167ndash4178 2004

[183] S de Breyne J M Bonderoff KM Chumakov R E Lloyd andC U T Hellen ldquoCleavage of eukaryotic initiation factor eIF5Bby enterovirus 3C proteasesrdquo Virology vol 378 no 1 pp 118ndash122 2008

[184] J Szeberenyi ldquoProblem-solving test the mechanism of polio-virus-induced block of host protein synthesis in human cellsrdquoBiochemistry andMolecular Biology Education vol 42 no 3 pp270ndash273 2014

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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PeptidesInternational Journal of


Hindawi Publishing Corporation httpwwwhindawicom

International Journal of

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Zoology


Molecular Biology International

GenomicsInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


BioinformaticsAdvances in

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Signal TransductionJournal of


BioMed Research International

Evolutionary BiologyInternational Journal of



Biochemistry Research International

ArchaeaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Genetics Research International


Advances in

Virolog y

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Nucleic AcidsJournal of

Volume 2014

Stem CellsInternational



Enzyme Research



Microbiology


times of stress when global levels of protein synthesis arereduced cancer cell development typically involves selectivetranslation of a specific subset of mRNAs These transcriptsencode prosurvival proteins that are translated by alternative(noncanonical) mechanisms [1 7 8] Studies continue toprovide new knowledge with regard to the developmentalcauses and possible novel treatments of various types ofcancers [4ndash7 9] Significantly much of this information canbe tied back to the paradigm of translational regulationand its critical contribution to our understanding of canceretiology

In this review we will first discuss the mechanism ofcanonical translation initiation followed by noncanonicalmechanisms that utilize RNA sequence features includingupstream open reading frames (uORFs) and internal ribo-some entry site- (IRES-) mediated translation mechanismsthe role of eukaryotic initiation factors in noncanonical trans-lation and their significance in cancer progression In addi-tion to uORFs- and IRES-mediated translation regulationother noncanonical translation mechanisms exist such asgamma interferon-activated inhibitor of translation (GAIT)complex 51015840 terminal oligopyrimidine (51015840 TOP) elements andAU-rich elements (AREs) [10 11] These mechanisms are outof the scope of this review article

Canonical Translation Initiation Eukaryotic cap-dependenttranslation initiation includes the recognition and recruit-ment of mRNA onto the small ribosomal (40S) subunitfollowed by ribosomal scanning in a 51015840ndash31015840 direction Sub-sequently the 60S large ribosomal subunit is recruitedforming the 80S initiation complex At this stage an initiatormethionyl-tRNA (met-tRNAi) is in the ribosomal peptidyl(P) site at themRNAstart codon [1 13] Canonical initiation isa complex process utilizing more than 25 proteins includinga minimum of twelve eukaryotic initiation factors (eIFs) [14]The rate of initiation varies between different mRNAs and isinfluenced by accessibility to the methylated guanosine capstructure (m7G cap) at the 51015840 terminus of the mRNA thelength and secondary structure of the 51015840 untranslated region(UTR) the sequence and secondary structure surroundingthe start codon and the poly(A) tail [15 16]

Initiation begins with the assembly of eIF4F complexcomprising eIF4E eIF4G and eIF4A onto the 51015840 m7G cap(Figure 1) eIF4E binds to the m7G cap which then interactswith the multidomain scaffold protein eIF4G and the ATP-dependent RNA helicase eIF4A The ternary complex (eIF2-GTP-met-tRNAi) associated with a 40S ribosomal subunit isthen recruited to the 51015840 end of the mRNA via a critical linkto eIF4G (captured at the cap via eIF4E) mediated by eIF3forming the 43S preinitiation complex eIF1 and eIF1A assistin stimulating recruitment of the ternary complex as wellas acting synergistically to promote continuous ribosomalscanning for AUG start codons [17] This 43S preinitiationcomplex then scans the 51015840 UTR of the mRNA with thehelp of eIF4A until an initiation codon in optimal contextis recognized [18] eIF5 and eIF5B then mediate subsequenthydrolysis of GTP to release the bound initiation factors fromthe 48S complex leaving the start codon in the ribosomal P-site with the met-tRNAi and allowing 60S ribosomal subunit

to bind [1 19] The now competent 80S initiation complexthen proceeds to translation elongation (Figure 1)

Noncanonical Translation Initiation During stress conditionssuch as hypoxia nutrient deprivation or endoplasmic retic-ulum (ER) stress alternative mechanisms that are mediatedby cis-acting sequences in specific mRNA subsets such asuORF and IRES drive the translation of stress responsemRNAs [1 6ndash8 18 20] Studies have revealed that a decentportion of human transcripts is known to contain uORFs(upstream of the initiation codon of the coding region) thatfunction as translation or mRNA stability regulators [21]Recent ribosome profiling data reveals that uORFs can existin an out-of-frame relative to the main coding sequence[22] However an overlap can also occur between uORFsand the coding sequence in which alternative translationof an upstream in-frame start codon of a gene can possiblyproduce an extended protein product [23] The mechanismof uORF-mediated translation functions primarily duringeIF2120572 phosphorylation conditions and enhances the expres-sion of proteins involved in cell-cycle regulation and apopto-sis Typical examples of uORF-mediated translation regula-tion include general control nonderepressible 4 (GCN4) theyeast transcriptional activator and activating transcriptionfactor 4 (ATF4) in mammals Normally translation initiationoccurs from the start codons located in the 51015840 UTR of mRNAwhich leads to the translation of small uORFs Additionallyreinitiation of terminating ribosomes will typically not occuron the downstream cistron thus the translation of themain coding sequence is inhibited in these conditions [24]However during stress conditions eIF2120572 phosphorylationattenuates translation of uORF sequences and allows thetranslation of main coding sequence eIF2120572 phosphorylationand reduced availability of the eIF2-GTP-tRNAi ternary com-plex favor translation reinitiation at the Gcn4p coding region(in yeast) subsequently resulting in activation of numerousgenes [25 26] Gcn4p activates these target genes by bindingto them and functioning as a transcription factor [25] Thecomplexity of RNA structure in the 51015840 UTR also plays acrucial role in uORF-mediated translation For exampletranslation of 120573-site APP-cleaving enzyme 1 (BACE1) whichis implicated in Alzheimerrsquos disease (AD) progression isregulated through uORFs However high GC content andcomplexity of the RNA secondary structure are also crucialdecisive factors for uORF-mediated translation of BACE1[27] Additionally recent genome-level studies indicate thatRNA secondary structure is negatively correlated with uORFtranslational efficiency as upstream (relative to uORFs) struc-tures restrict or even arrest ribosomal preinitiation complex(PIC) whereas structures downstream of uORFs enhancetranslation initiation of coding sequences [28]

Translation initiation mediated by IRES is another mech-anism that operates during stress conditions IRESes areRNA sequence elements that were initially discovered inthe 51015840 leader sequences of poliovirus and encephalomy-ocarditis virus genomic RNA that lack the 51015840 cap structurebut nonetheless are efficiently translated in the host cell[18 29] The viral IRES elements comprise secondary andtertiary structures that play a role in direct interactions


40SAAAAAUG

60S

40S

eIF3eIF1A60S

40S

1A eIF3

eIF1eIF2

eIF2-GTP

2

40S

21A

1

eIF3

AUG AAAAmRNA

eIF4GeIF4E

eIF4A


4G4A

4E

40S

2 1A

1eIF3

AAAAAUG

4G4A

4E

Scanning ATP

2 1A

1

eIF34G

4A

4E

eIF5

eIF3 1 1A

2

4G4A

4E

40SAAAAAUG

60SeIF5B GTP

40SAAAAAUG

60S



met-tRNAi

eIF2-GTP-met-tRNAi

+

ADP + Pi

GDP + Pi

GDP + Pi

m7G

m7G

m7G

m7G

m7G

m7G











GDP




ER stressHypoxia

GCN2

PKR

HRI

PERKeIF2-GDP

eIF2-GTP

GDP


GTP

P

GDP

GTP





eIF2B120572

120572

120573

120573

120574

120574

120572

120573 120574


40S

eIF3

eIF4A

eIF4G

PABP

eIF4EAUG

40S

eIF3

PABP

PAIP1

AUGeIF4A

ITAFs

IRES5998400 cap

5998400 cap

3998400 (AAAAAA)n

3998400 (AAAAAA)n






























































5 Conclusions




Competing Interests




Acknowledgments


References











































































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology


40SAAAAAUG

60S

40S

eIF3eIF1A60S

40S

1A eIF3

eIF1eIF2

eIF2-GTP

2

40S

21A

1

eIF3

AUG AAAAmRNA

eIF4GeIF4E

eIF4A


4G4A

4E

40S

2 1A

1eIF3

AAAAAUG

4G4A

4E

Scanning ATP

2 1A

1

eIF34G

4A

4E

eIF5

eIF3 1 1A

2

4G4A

4E

40SAAAAAUG

60SeIF5B GTP

40SAAAAAUG

60S



met-tRNAi

eIF2-GTP-met-tRNAi

+

ADP + Pi

GDP + Pi

GDP + Pi

m7G

m7G

m7G

m7G

m7G

m7G











GDP




ER stressHypoxia

GCN2

PKR

HRI

PERKeIF2-GDP

eIF2-GTP

GDP


GTP

P

GDP

GTP





eIF2B120572

120572

120573

120573

120574

120574

120572

120573 120574


40S

eIF3

eIF4A

eIF4G

PABP

eIF4EAUG

40S

eIF3

PABP

PAIP1

AUGeIF4A

ITAFs

IRES5998400 cap

5998400 cap

3998400 (AAAAAA)n

3998400 (AAAAAA)n






























































5 Conclusions




Competing Interests




Acknowledgments


References











































































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology










GDP




ER stressHypoxia

GCN2

PKR

HRI

PERKeIF2-GDP

eIF2-GTP

GDP


GTP

P

GDP

GTP





eIF2B120572

120572

120573

120573

120574

120574

120572

120573 120574


40S

eIF3

eIF4A

eIF4G

PABP

eIF4EAUG

40S

eIF3

PABP

PAIP1

AUGeIF4A

ITAFs

IRES5998400 cap

5998400 cap

3998400 (AAAAAA)n

3998400 (AAAAAA)n






























































5 Conclusions




Competing Interests




Acknowledgments


References











































































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

























































5 Conclusions




Competing Interests




Acknowledgments


References











































































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology



Competing Interests




Acknowledgments


References











































































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology





















































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

Review Article Role of Eukaryotic Initiation Factors...

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Transcript of Review Article Role of Eukaryotic Initiation Factors...