The Role of the Mammalian Target Of Rapamycin(mTOR) in Renal Disease

Wilfred Lieberthal* and Jerrold S. Levine†

*Department of Medicine, Stony Brook University Medical Center, Stony Brook, and Department of Medicine, NorthportVeterans Administration, Northport, New York; and †Department of Medicine, University of Illinois at Chicago, andDepartment of Medicine, Jesse Brown Veterans Administration Hospital, Chicago, Illinois

Rapamycin (also known as sirolimus) wasisolated from a soil bacterium in 1975.1,2

The discovery of rapamycin led to theidentification and cloning of mammaliantarget of rapamycin (mTOR), a serine/threonine kinase, in 1994.3,4 Rapamycin isa potent, specific inhibitor of mTOR anddoes not inhibit any kinase other thanmTOR.1,2 Because of its high specificity formTOR, rapamycin has been very useful inestablishing the role of mTOR in cell biol-ogy and in the pathogenesis of disease.3,4

Although initially isolated as an antifungalagent, rapamycin was later found to havepotent immunosuppressive effects and hasbeen used for many years as a componentof antirejection therapy for recipients oforgan transplants.1,2,5

After diffusing into cells, rapamycinforms a complex with FK506-binding

protein 12 (FKBP-12), an intracellularprotein. The rapamycin–FKBP-12 com-plex then binds and inhibits mTOR.3,4

mTOR is a component of two distinctsignaling complexes known as mTORcomplex 1 (mTORC1) and mTORC2.These complexes contain two differentscaffolding proteins, raptor and rictor,respectively (Figure 1).6 – 8 These scaf-folding proteins, by interacting with dis-tinct downstream targets, connectmTOR to different signaling pathways.As a result, mTORC1 and mTORC2 havediscrete functional roles (Figure 1).6,7,9

Activation of mTORC1 by growth fac-tors and amino acids stimulates cellgrowth (i.e., an increase in cell size andmass) and cell proliferation. Activationof mTORC2 contributes to the regula-tion of cell polarity and the actin cy-

toskeleton (Figure 1).6,7,9 The upstreamregulators of mTORC2 are as yet un-known (Figure 1).6,7,9 Rapamycin inhib-its mTORC1 by preventing the interac-tion of mTOR with raptor. Importantly,rapamycin has no effect on the activity ofmTORC2.3,6,8,10,11

UPSTREAM REGULATION OFmTORC1

mTORC1 serves as a sensor and integra-tor of the availability of multiple stimuliand factors necessary for cell growth andproliferation. These include growth fac-tors such as IGF-112,13 and EGF14,15 aswell as nutrients such as amino ac-ids,13,16,17 glucose, and oxygen.18 –20

Activation of mTORC1 begins with ac-tivation of the lipid kinase phosphatidyl-inositol 3-kinase (PI3K; Figure 1).21 PI3Kphosphorylates the membrane-associatedphospholipid phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] (PIP2) toyield PtdIns(3,4,5)P3 (PIP3).21,22 The activ-ity of PI3K is opposed by a phosphatase,phosphatase and tensin homolog on chro-mosome 10 (PTEN), which dephosphory-lates PIP3 back to PIP2.21,22

Published online ahead of print. Publication dateavailable at www.jasn.org.

Correspondence: Dr. Wilfred Lieberthal, StonyBrook Medical Center, Health Sciences Center, 16-081B, Nicholls Road, Stony Brook, NY 11794-8166;Phone: 631-444-1227; Fax: 631-444-6174; E-mail:wlieberthal@notes.cc.sunysb.edu

Copyright � 2009 by the American Society ofNephrology

ABSTRACTThe mammalian target of rapamycin (mTOR) is a serine/threonine kinase that playsa pivotal role in mediating cell size and mass, proliferation, and survival. mTOR hasalso emerged as an important modulator of several forms of renal disease. mTORis activated after acute kidney injury and contributes to renal regeneration andrepair. Inhibition of mTOR with rapamycin delays recovery of renal function afteracute kidney injury. Activation of mTOR within the kidney also occurs in animalmodels of diabetic nephropathy and other causes of progressive kidney disease.Rapamycin ameliorates several key mechanisms believed to mediate changesassociated with the progressive loss of GFR in chronic kidney disease. Theseinclude glomerular hypertrophy, intrarenal inflammation, and interstitial fibrosis.mTOR also plays an important role in mediating cyst formation and enlargement inautosomal dominant polycystic kidney disease. Inhibition of mTOR by rapamycin orone of its analogues represents a potentially novel treatment for autosomal dom-inant polycystic kidney disease. Finally, inhibitors of mTOR improve survival inpatients with metastatic renal cell carcinoma.

J Am Soc Nephrol 20: 2493–2502, 2009. doi: 10.1681/ASN.2008111186

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PIP3 activates a 3-phosphoinositide-dependent kinase-1, which in turn acti-vates Akt (also known as protein kinaseB) by phosphorylating amino acid resi-due Thr308.3,21 Akt then activatesmTORC1 through a cascade of down-stream intermediates.3,13,23,24 These in-clude the tuberous sclerosis complex(TSC), a dimer composed of TSC1(hamartin) and TSC2 (tuberin), andRheb, a Ras family GTPase that directlyactivates mTOR.3,13,25 TSC2 has GTPase-activating protein activity, which inhibitsRheb activity, whereas TSC1 binds toand is necessary for TSC2 function.3,25–28

Akt activates mTOR by phosphorylatingand inhibiting TSC2, thereby releasing

Rheb from the inhibitory effects ofTSC.3,4,29,30

Akt, in addition to being phosphory-lated by 3-phosphoinositide-dependentkinase-1 at Thr308, can be phosphory-lated by mTORC2 at Ser473.31–33 How-ever, the phosphorylation of AKT bymTORC2 is not necessary for the activa-tion mTORC1 but instead is necessaryfor Akt to phosphorylate a distinct subsetof downstream targets that promote cellsurvival.32–34

Growth factors and amino acids acti-vate Akt and mTOR through PI3K (Fig-ure 1). Growth factors use the class IAPI3K,21 whereas amino acids stimulatethe class III PI3K.17 A deficiency of glu-

cose and/or oxygen, by reducing cell en-ergy stores, inhibits mTOR through aPI3K-independent pathway. A fall in cellATP stores is a potent stimulus for theactivation of 5�-adenosine monophos-phate–activated protein kinase (AMPK),a serine/threonine kinase18 –20 that inhib-its mTOR by phosphorylating and acti-vating TSC2.3,24,35

Downstream Effectors of mTORC1Cell division results in the formation oftwo daughter cells that are approxi-mately half the size of a normal cell. Be-fore reentering the cell cycle, these cellsmust double in size (mass). mTORC1promotes efficient and sustained cellularproliferation by tightly coupling in-creases in cell size with progressionthrough G0/1 of the cell cycle.3,4,36 –39

These effects of mTORC1 are mediatedby stimulating the translation of mRNAand the synthesis of proteins necessaryfor mediating increases in cell size andfor promoting progression of the earlystages (G0/1) of the cell cycle (Figure1).3,4,36 –39 The translation of mRNA in-volves an initiation phase and an elonga-tion phase. The initiation phase mediatesthe association of ribosomal subunitswith mRNA, whereas amino acids are se-quentially added to the nascent peptidechain during the elongation phase inaccordance with mRNA-determinedcodon sequences.39,40

The best characterized downstreamtargets of mTORC1 are the 4E-bindingproteins (4EBP) and the 70-kD ribo-somal S6 kinases (p70S6K).3,39 The 4EBPare a family of translation repressor pro-teins. When unphosphorylated, theseproteins bind to and inhibit the activityof the eukaryotic translation initiationfactor 4E (eIF4E).38,39 Activation ofmTOR leads to the phosphorylation of4EBP.38,39 Once phosphorylated, 4EBPcan no longer bind to and inhibit eIF4E,which then becomes free to initiatemRNA translation.3,4,38,39 mTOR alsophosphorylates and activates p70S6K, akinase that enhances the translation andsynthesis of proteins essential to ribo-somal function and the elongation phaseof translation.41 Thus, phosphorylationof the 4EBP and p70S6K by mTOR acts

Growthfactors

Aminoacids

Glucose and/orO2 deprivation

ATPdepletion

AMP:ATPratio

PI3K

PIP2

PTEN

AMPK

PIP3 PDK1T308 S473

AKT mTOR

mTORRaptor

Rapamycin(sirolimus)

p70S6K

“mTOR-C2”

Regulation ofcell polarity andthe cytoskeleton

?

TSC

Rheb GTPase

4EBPs

Rictor

“mTOR-C1”

Cell growth and proliferation

Figure 1. mTOR is a component of two major intracellular signaling complexes:mTORC1 and mTORC2. These complexes contain two different scaffolding proteins(raptor and rictor, respectively) that “connect” them to different downstream targets. Asa result, these complexes have different functional roles. mTORC1 stimulates cell growthand proliferation, whereas mTORC2 regulates cell polarity and the cytoskeleton.mTORC1 is activated by growth factors and amino acids, which activate PI3K, a lipidkinase. PI3K then phosphorylates PIP2 to yield PIP3. PIP3 phosphorylates and activates Aktthrough an intermediary kinase, 3-phosphoinositide-dependent kinase-1 (PDK1). Theactivity of PI3K is negatively regulated by PTEN, a phosphatase that dephosphorylatesPIP3 back to PIP2. Once activated, Akt phosphorylates and inhibits the TSC. TSC nega-tively regulates mTORC1 by inhibiting Rheb, a small cytoplasmic GTPase and activator ofmTORC1. AMPK, which is activated by any form of cell stress that decreases cell ATPstores and increases the AMP-ATP ratio, inhibits mTOR by phosphorylating and activat-ing TSC2. Akt is also phosphorylated by mTORC2, although phosphorylation of Akt bymTORC2 is not necessary for the activation mTORC1. Once activated, mTORC1 phos-phorylates p70S6K and 4EBP, leading to the translation of mRNAs and the synthesis ofproteins necessary for cell growth and cell-cycle progression. The upstream modulatorsof mTORC2 are as yet unknown.

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in concert to promote the initiationand elongation phases of mRNA trans-lation.3,39 More detailed informationregarding the regulation of mRNAtranslation by mTOR can be found inother review articles.38,41,42 In this re-view, we focus exclusively on the role ofmTORC1 in renal disease and use thegeneric term “mTOR” to refer to theeffects of mTORC1.

ROLE OF mTOR IN ACUTEKIDNEY INJURY

Complete restoration of renal morphol-ogy and function can occur after acutekidney injury (AKI) induced by ischemicor toxic injury.43,44 Renal regenerationafter acute tubular injury depends, inpart, on the ability of the remaining, via-ble tubular cells to proliferate and restorethe injured tubular epithelium.43– 46 Thewidely recognized immunosuppressiveproperties of rapamycin are due largelyto inhibition of mTOR-mediated prolif-eration and clonal expansion of Tcells.5,47 However, mTOR is a ubiquitouskinase, and its inhibition by rapamycinalso blocks the proliferation of virtuallyall cells types, including cells within thekidney.2,48

Our group has demonstrated thatmTOR plays an important role in medi-ating the process of regeneration and re-covery after experimental AKI.48,49

mTOR activity is low or absent in thenormal kidney but increases markedlyafter ischemia-reperfusion injury.48 Inaddition, inhibition of mTOR by rapa-mycin delays renal recovery and repair.48

This effect of rapamycin is due to thedual effects of inhibition of proliferationand induction of apoptosis of tubularcells.48,49 Notably, although rapamycindelays recovery of glomerular filtrationrate after AKI by approximately 2 to 3 d,full recovery of renal function still ulti-mately occurs, despite continued treat-ment with rapamycin.49 Studies with cul-tured renal tubular cells suggest that thetransient nature of the effect of rapamy-cin on the process of recovery after AKI isdue, at least in part, to an acquired resis-tance of renal tubular cells to the effects

of rapamycin on cell growth and prolif-eration.49

Because mTOR is activated by growthfactors and amino acids and is inhibitedby ATP depletion (Figure 1), mTOR ac-tivity is probably highly suppressed dur-ing the ischemic period of ischemia/reperfusion injury, when the availabilityof growth factors, amino acids, and cellATP all are likely to be reduced. Themechanisms responsible for the markedactivation of mTOR during the reperfu-sion period after ischemia/reperfusioninjury remain to be elucidated.48,49 How-ever, we speculate this represents a re-bound phenomenon occurring in re-sponse to the sudden availability ofgrowth factors, amino acids, and cellularATP after a period of profound defi-ciency.

The clinical relevance of our findingsin rats became evident when later studiesof human renal transplant recipientsshowed that rapamycin causes and/orexacerbates delayed graft function.50 –55

Delayed graft function, which is due toischemic tubular injury during the peri-transplantation period, is an importantclinical problem, occurring in approxi-mately 30% of cadaveric and 10% of liv-ing-related renal transplants. Since rec-ognition of the adverse effects ofrapamycin on recovery from delayedgraft function, it has become routine todelay administration of rapamycin untilthe transplanted kidney is functional andtemporarily to discontinue administra-tion of rapamycin in patients who haverenal transplants and develop AKI.50 –55

ROLE OF mTOR IN CHRONICKIDNEY DISEASE

Substantial and persuasive evidence ex-ists that the mTOR pathway plays an im-portant role in the mechanisms underly-ing the progression of chronic kidneydisease (CKD) caused by diabetes andother causes. This evidence has beenobtained from a number of studies us-ing animal models, in which inhibitionof mTOR by rapamycin markedly ame-liorates the interstitial inflammation,

fibrosis, and loss of renal function as-sociated with CKD.

Diabetic NephropathyDiabetic nephropathy (DN) is the lead-ing cause of ESRD in the United Statesand Western Europe.56,57 Its characteris-tic morphologic changes include glo-merular hypertrophy, basement mem-brane thickening, and the accumulationof mesangial matrix.56 –58 In addition,DN is associated with progressive tubu-lointerstitial injury, inflammation, andfibrosis.56 –58

Renal enlargement, one of the firststructural changes in DN, is due to thehypertrophy of existing glomerular andtubular cells rather than to cellular pro-liferation.56,57,59,60 A number of studieshave shown that activation of mTORplays a pivotal role in physiologic andpathologic forms of hypertrophy in thekidney and other organs, including therenal hypertrophy characteristic ofDN.11,61 The pathogenic importance ofglomerular hypertrophy is that it maycontribute to podocyte injury and theprogressive loss of renal function in DNand in other forms of CKD.59,60 In addi-tion to its role in glomerular hypertro-phy, mTOR-dependent changes increasethe synthesis of matrix proteins thatcontributes to basement membranethickening and the accumulation ofmesangial matrix characteristic ofDN.40,62,63 Activation of mTOR in dia-betes is due, at least in part, to the ef-fects of hyperglycemia. Hyperglycemiaincreases mTOR activity by the com-bined effects of Akt activation andAMPK inhibition (Figure 2).40,62,64 – 69

The importance of mTOR in mediat-ing the renal changes associated with DNhas been demonstrated in vivo by severalstudies evaluating the effect of rapamy-cin on the course of DN in rats withstreptozotocin-induced diabetes.70 –72

This model of DN is associated with earlyactivation of mTOR within the kid-ney.70 –72 Rapamycin not only reducesmTOR activity in this model, but alsoameliorates the glomerular changescharacteristic of DN, including hyper-trophy, basement membrane thickening,and mesangial matrix accumula-

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tion.70 –72 Importantly, these effects ofrapamycin were also associated with a re-duction in albuminuria.70 –72

Rapamycin markedly inhibited theinflux of inflammatory cells, predomi-nantly lymphocytes and macrophages,associated with DN.70 –72 This effect islikely attributable to rapamycin-inducedinhibition of the proliferation and clonalexpansion of B and T lymphocytes.3,4

Rapamycin also ameliorates the releaseof proinflammatory cytokines and che-mokines within the kidney, such asmonocyte chemoattractant protein-1,RANTES, IL-8, and fractaline, which ex-acerbate the inflammatory process in DN(Figure 3).70 –72

Interstitial fibrosis is another impor-tant feature of DN. While the precisepathways and mediators responsible forthe pro-fibrotic effects of mTOR in DNand other forms of CKD have not beenfully elucidated, several potential mech-anisms can be envisioned (Figure 3).mTOR stimulates the proliferation of fi-broblasts and their synthesis of colla-gen.73–75 In addition, mTOR increasesthe expression of profibrotic cytokines,such as TGF-�1 and connective tissuegrowth factor, which play important

roles in mediating the tubulointerstitialchanges associated with DN and otherforms of progressive CKD.70 –72 Finally,mTOR may play a role in epithelial-to-mesenchymal transition (EMT). EMT isa process in which tubular epithelial cellsundergo a phenotypic change into fibro-blasts. These fibroblasts then migratethrough the tubular basement mem-brane and into the interstitium.76 – 80 Arole for mTOR in EMT is suggested bystudies showing that rapamycin inhibitsEMT in some animal models of studies ofDN and other forms of CKD.72,81,82

Role of mTOR in NondiabeticForms of CKDRapamycin also slows the progressionof renal fibrosis and delays the onset ofrenal failure in experimental models ofCKD due to causes other than diabetes.Many of the same events that promoteprogression of diabetic CKD are alsoimportant in nondiabetic forms ofCKD. These include glomerular hyper-trophy, an increased expression ofproinflammatory and profibrotic cyto-kines, infiltration of the interstitium byinflammatory cells, and renal fibro-sis.76,80,83

In a rat model of progressive membra-nous nephropathy (Heymann nephritis),inhibition of mTOR with rapamycin ame-liorated glomerular hypertrophy, de-creased the renal expression of proinflam-matory and profibrotic cytokines, andretarded the development of tubulointer-stitial inflammation and fibrosis.84 Similarfindings were reported in animal models ofCKD from other causes, including reducedrenal mass from five-sixths nephrecto-my,82 renal obstruction after ureteral liga-tion,81,85 and chronic mesangioprolifera-tive glomerulonephritis induced by anti-Thy1 antibody.73,86

In summary, mTOR plays an impor-tant role in the progression of CKD in avariety of animal models. Available evi-dence suggests a hypothetical schema toexplain the beneficial effects of rapamy-cin on the progression of CKD (Figure3). This model highlights five major ef-fects of rapamycin: Reduction of glomer-ular hypertrophy, decreased productionof proinflammatory and profibrotic cy-tokines, amelioration of interstitial in-flammation, reduced fibroblast prolifer-ation, and inhibition of EMT. Despitethe appeal of this relatively simple model,however, additional research is needed toelucidate more fully the connection be-tween mTOR and progressive CKD.

mTOR AND AUTOSOMALDOMINANT POLYCYSTIC KIDNEYDISEASE

Autosomal dominant polycystic kidneydisease (ADPKD) is one of the mostcommon human monogenic diseases,with an incidence of 1:400 to 1:1000.58 Itis characterized by the development andgradual enlargement of multiple fluid-filled cysts within both kidneys. Thesecysts encroach on and destroy normaladjacent nephrons.58 ADPKD typicallypresents clinically in the second or thirddecade of life with hypertension, hema-turia, and/or proteinuria, followed byprogressive renal failure. Although therate of progression to ESRD is highlyvariable, most patients with ADPKDreach ESRD by the fifth decade of life.

The majority of cases of ADPKD are

Hyperglycemia

PI3K

Akt AMPK

Cell mass Matrix synthesis

Glomerularhypertrophy

GBMthickening

Mesangialexpansion

mTOR

Figure 2. Hyperglycemia stimulates mTOR. mTOR is activated within the kidney in DN.Hyperglycemia activates PI3K and Akt and inhibits AMPK. The activation of Akt andinhibition of AMPK lead to activation of mTORC1. Activation of mTORC1 contributes tothe renal changes characteristic of DN, including glomerular hypertrophy, glomerularbasement membrane (GBM) thickening, and the accumulation of mesangial matrix.

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due to mutations in either the PKD1 orthe PKD2 genes,87– 89 which cause abnor-malities of polycystin 1 (PC1) and PC2proteins, respectively.87– 89 Mutations inPKD1 account for �85% of cases ofADPKD.87– 89 The pathogenesis of cystdevelopment in ADPKD is a process ofgrowing complexity. A number of ab-normalities have been described in thephenotype of tubular cells lining the cystsin ADPKD. These include increased pro-liferation, increased apoptosis, abnor-malities of protein sorting and polarity,and disorganization of the underlyingextracellular matrix.90 –92

Current management of ADPKD is

limited to the treatment of the complica-tions of hypertension and renal failure93;however, ongoing elucidation of themechanisms underlying cyst formationhas led to the development of severalpromising new therapies. These includenonpeptide vasopressin 2 receptor an-tagonists,94 –97 inhibitors of the receptorsfor EGF and vascular endothelial growthfactor (VEGF),93,98 and inhibitors of c-myc expression.99 –101 All of these modal-ities inhibit disease progression in exper-imental models of PKD.92–99

A role for mTOR in the pathogenesisof ADPKD was first suggested in studiesby Edelstein and colleagues,102 who dem-

onstrated that rapamycin slowed cystformation in the Han:SPRD rat model ofPKD. These findings have been con-firmed by other investigators in the samemodel.103 Subsequently, Shillingford etal.104 provided substantial evidence thatmTOR contributes to the pathogenesisof human ADPKD. These investigatorsdemonstrated the cytoplasmic tail ofPC1 interacts with and inhibits mTOR.They also found that loss of function ofPC1 in ADPKD leads to the marked acti-vation of mTOR within the epithelialcells of renal cysts in both mouse and hu-man cystic disease.104 In addition, theyfound that inhibition of mTOR withrapamycin not only slowed cyst enlarge-ment in murine models of PKD but alsoslowed the increase in size of native kid-neys of humans with ADPKD who hadreceived a renal transplant.104 There is asyet no information regarding the role, ifany, of mTOR in the pathogenesis ofADPKD as a result of mutations in thePKD2 gene.

Evidence that mTOR plays an impor-tant role in the pathogenesis of ADPKDhas spurred several studies of humansthat examine the effect of rapamycin onthe progression of ADPKD.105,106 Threeclinical trials are currently under way,one at the Cleveland Clinic (a Phase I/IIstudy of 30 patients), one at the MarioNegri Institute in Milan (a Phase II studyof 16 patients), and one at Zurich Uni-versity (a Phase II study of 100 pa-tients).106

mTOR AND RENAL CELLCARCINOMA

Renal cell carcinoma (RCC) accountsfor 2 to 3% of all adult malignan-cies.107–109 Surgery is curative in ap-proximately one third of patients withRCC in whom the tumor is localized tothe kidney. However, approximately30% of all patients with RCC have me-tastases at the time of presentation andan additional 30% of patients developmetastatic disease during the follow-upperiod.110 Metastatic RCC has a poorprognosis because it is highly resistantto conventional forms of chemother-

mTOR

Increasedcell mass

Increasedcell proliferation

Glomerularhypertrophy

Lymphocyteproliferation

Fibroblastproliferation

Proteinuria

Tubular cell“activation”

Pro-inflammatory andpro-fibrotic cytokines

Progressive lossof GFR

EMTInterstitial

inflammation

Interstitialfibrosis

Tubularinjury/atrophy

Figure 3. Activation of mTORC1 in CKD plays a major role in mediating the glomerularhypertrophy associated with the loss of functioning nephrons in CKD. The increasedintracapillary pressures and flows associated with adaptive glomerular hypertrophy ulti-mately lead to podocyte injury and proteinuria. Increased mTORC1 activity in CKD alsopromotes interstitial inflammation by promoting the proliferation of lymphocytes andinterstitial fibroblasts. Fibroblasts produce the connective tissue necessary for interstitialfibrosis. mTORC1 activation also leads indirectly to the release of proinflammatory andprofibrotic cytokines, which are derived from multiple sources, including tubular cells(activated by proteinuria), inflammatory cells, and fibroblasts. These cytokines result in apositive-feedback loop that increases inflammation and fibrosis. Some cytokines are alsonecessary for EMT, which further promotes interstitial fibrosis. Injury to tubules caused byinflammatory cells promotes tubular injury, atrophy, and “dropout.” In addition, thedeposition of fibrous tissue within the renal interstitium reduces local blood flow, causingareas of hypoxia that lead to further loss of functioning nephrons.

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apy. Until recently, high-dosage IL-2was the only approved therapy forRCC111. However, novel therapeuticapproaches for metastatic RCC thattarget angiogenesis have been devel-oped.108,111–113

Interest in targeting angiogenesis arosefrom the observed link between inactiva-tion of the von Hippel-Lindau (VHL) tu-mor suppressor gene and predisposition toRCC. Malignant transformation of RCC isdriven by loss-of-function mutations ofthe VHL gene, leading to increased expres-sion of hypoxia-inducible factor (HIF), atranscription factor that plays a central rolein the adaptation to hypoxia (Figure4).108,112–115 In general, tumor growth ischaracterized by an initial phase of rapid

proliferation, which then slows as malig-nant cells outstrip their blood supply andbecome hypoxic.116 Adaptations to hyp-oxia, which include alterations in cell me-tabolism and neovascularization (angio-genesis), are necessary for continuedtumor growth. HIF, which is an importantstimulus to angiogenesis, consists of twosubunits: HIF-1� and HIF-1�. Expressionof HIF-1� is tightly coordinated with oxy-gen availability through the ubiquitin-pro-teosome pathway.117

The VHL product controls HIF ex-pression in response to oxygen availabil-ity.117 During normoxia, HIF-1� bindsto VHL, which marks HIF-1� for ubiq-uitin-mediated degradation. Under hy-poxic conditions, there is reduced bind-

ing of HIF-1� to VHL and consequentlydecreased degradation such that cellularlevels of HIF-1� increase. Loss-of-func-tion mutations of VHL lead to increasedHIF-mediated expression of VEGF,PDGF-�, and TGF-�, all of which stim-ulate tumor angiogenesis and prolifera-tion (Figure 4).111 Several novel thera-peutic agents, such as sunitinib andsorafenib, have been developed for thetreatment of metastatic RCC.107–109,118

These agents inhibit VEGF and PDGF-me-diated angiogenesis by inhibiting their ty-rosine kinase receptors (VEGF-R andPDGF-R, respectively; Figure 4).107–109,118

In addition, mTOR plays a critical rolein the pathogenesis of RCC. Genetic muta-tions that lead to constitutive increases inmTOR activity increase the incidence ofmetastatic RCC. For example, loss-of-function mutations of PTEN, a negativeregulator of mTOR through the PI3K/Aktpathway (Figure 1), are found in approxi-mately 5% of patients with RCC (Figure4).119 Also, in patients with tuberous scle-rosis, loss-of-function mutations ofgenes encoding either TSC1 or TSC2lead to the inactivation of the TSC, anegative regulator of mTOR (Figure 1).Patients with tuberous sclerosis arepredisposed to the development ofRCC.120,121

Inhibitors of mTOR show substan-tial promise for the treatment of pa-tients with metastatic RCC and under-lie the importance of mTOR in thepathogenesis of RCC. Temsirolimus,an analogue of rapamycin that is ad-ministered intravenously, has been ap-proved by the Food and Drug Ad-ministration for the treatment ofmetastatic RCC.111,122 In addition, pre-liminary data from Phase II studies sug-gest that everolimus, an oral derivative ofrapamycin, is beneficial in some patientswith metastatic RCC.107–109,111

Part of the therapeutic benefit ofmTOR inhibition in RCC is likely due toinhibition of cell growth and prolifera-tion (Figure 4). However, mTOR in-creases the expression of HIF by mecha-nisms that remain to be elucidated. Partof the efficacy of mTOR inhibitors inRCC is related to a reduction in HIF-me-diated angiogenesis (Figure 4).121,123

Loss-of-functionmutations

mTOR(constitutively active)

HIF-1α

PDGFα

Angiogenesis andcell proliferation

TNFαVEGF

Tumor growth

• PTEN• TSC

SirolimusTemsirolimus

Everolimus

Figure 4. An increase in the constitutive activity of mTORC1 promotes the growth andinvasiveness of many aggressive tumors, including metastatic RCC. This is due to severalmechanisms. First, increased activity of mTORC1 promotes cell growth and proliferation.In addition, mTORC1 increases the cellular levels of HIF-1�, which in turn stimulates theproduction of proangiogenic factors such as VEGF, PDGF-�, and TNF-�. Increasedangiogenesis is necessary to meet the increasing oxygen requirements of growingtumors. In some patients with RCC, activation of mTORC1 is mediated by loss-of-functionmutations of the phosphatase PTEN, which negatively regulates mTORC1 through theupstream PI3K/Akt pathway. Also, patients with tuberous sclerosis are predisposed to thedevelopment of RCC because of loss-of-function mutations of the TSC, which interfereswith the negative regulation of mTORC1 activity by TSC.

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CONCLUSIONS

The discovery of mTOR has led to the elu-cidation of a complex signaling pathwaythat plays a pivotal role in cell growth, pro-liferation, and survival. Numerous studieshave recognized that mTOR plays an im-portant role in many renal diseases. Activa-tion of mTOR plays an important role inmediating renal recovery and repair afterAKI, and inhibition of mTOR delays renalrecovery after AKI.

Interestingly, whereas activation ofmTOR plays an adaptive role in AKI, itsactivation in many other renal diseaseshas been shown to be deleterious. Pro-vocative studies in animal models sug-gested that the mTOR signaling pathwayis activated in diabetic and in nondia-betic forms of CKD and that rapamycinand its analogues ameliorate the progres-sion of renal failure in these models. Inaddition, the inappropriate activation ofmTOR in ADPKD contributes to theprogression of renal failure by increasingthe formation and enlargement of cysts.Finally, mTOR is an important patho-genic factor in some patients with RCC.

Our increased understanding of thecomplex mechanisms that regulate themTOR pathway has led, during a relativelyshort period of time, to the development ofnovel forms of treatment for a number ofrenal diseases. Rapamycin has been usedfor many years as an immunosuppressantin recipients of renal transplants. More re-cently, inhibition of mTOR, using ana-logues of rapamycin, benefit patients withmetastatic RCC. The potential therapeuticrole of mTOR inhibition in patients withADPKD is being evaluated in ongoing clin-ical trials. Clinical studies are also neededto determine whether inhibitors of mTORslow progression of renal failure in DN andother forms of CKD in humans for whichno specific forms of therapy are currentlyavailable.

ACKNOWLEDGMENTS

This work was supported by a Veterans Ad-

ministration Merit Award (W.L.) and a GRIP

Renal Innovations Program Award from

Genzyme, Inc. (J.S.L.).

DISCLOSURESNone.

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