Distinct mTORC1 pathways for transcription and cleavageof SREBP-1cWilliam J. Quinn III and Morris J. Birnbaum1

Division of Endocrinology, Diabetes, and Metabolism, Department of Medicine, and Institute for Diabetes, Obesity, andMetabolism, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104

In recent years, it has become in-creasingly apparent that commonpolygenic diseases generally do notresult from accumulated defects in

a single pathway but, rather, present ascomplex interactions among multiple en-vironmental and inherited factors. No-where is this more obvious than in diabetesmellitus, which is classified into at leasttwo groups, type 1 and type 2 (T2DM),that share hyperglycemia but differ in theinitiating primary immune destructionof the β-cells or insulin resistance, re-spectively. Although much literature hasemphasized the dissimilarities in the pro-gression of β-cell failure in type 1 diabetesmellitus vs. T2DM, only recently have in-vestigators focused on how the peripheralmetabolic profile of T2DM differs fromthat predicted to be the consequence of aninsulin deficiency state (i.e., simple global“insulin resistance”). As a result of thesestudies, the idea has emerged that the “in-sulin-resistant” liver, for example, remainssensitive to action of the hormone on lipidmetabolism while developing resistanceto the control of glucose metabolism, astate that has been termed “selective insulinresistance” (1). One consequence of thisrevised model has been an emphasis onexamining downstream signaling pathwaysas possible sites of insulin insensitivity aswell as potential targets for therapeutic in-tervention (2). In PNAS, Owen et al. (3)describe a unique “branch point” in thepathway by which insulin works in concertwith nutritional signals to regulate hepaticlipid synthesis positively.A characteristic abnormal pattern of

circulating lipids and triglyceride accumu-lation in the liver are hallmarks of insulin-resistant states, such as obesity, the met-abolic syndrome, and T2DM. In principle,the excess amounts of liver and plasmatriglycerides could result from increasedrates of synthesis or decreased clearance.Liver triglyceride is synthesized by the es-terification of glycerol phosphate to fattyacids either taken up from the blood-stream or newly synthesized, the latter asa result of de novo lipogenesis (DNL).Two transcription factors coordinate theexpression of genes encoding lipogeneicenzymes: carbohydrate responsive element-binding protein 1 (ChREBP1), which isregulated by metabolites of glucose, andsterol regulatory element-binding protein1c (SREBP-1c), whose activity is enhanced

by insulin and amino acids working inconcert through the mechanistic target ofrapamycin complex 1 (mTORC1) signal-ing complex (4,5). Regulation of SREBP-1c activity is particularly complex, how-ever. In the basal state, it resides as anintegral membrane protein in the endo-plasmic reticulum (ER) in association withinsulin-induced gene (Insig) protein (6).In times of high DNL, there is an increasein expression of the gene encoding theSREBP-1c precursor. In addition, the pre-cursor is escorted by SREBP-1c activatingprotein (Scap) out of the ER and into theGolgi complex, where it can be pro-teolytically processed to the mature, tran-scriptionally active form (7). Although theregulated intracellular trafficking of the pa-ralog SREBP-2, which is involved primarilyin cholesterol metabolism, is understood insome detail, the pathway by which insulinregulates processing of SREBP-1c has beenlargely unknown until recently; in PNAS,Owen et al. (3) define perhaps the mostdistal signaling event so far identifiedin the regulation of SREBP-1c processing.

SREBP-1c–dependent stimulation ofDNL appears important not only for thepathological state resulting from over-nutrition but for maintenance of the un-usual metabolic state of many cancers. Ithas been known for some time that themultiprotein complex mTORC1 residesdownstream of PI3K-Akt and functions tointegrate signals from growth and survivalfactors as well as amino acids to promotecellular growth (8). When mTORC1 rec-ognizes the right signal from either a growthfactor receptor or oncogene in combina-tion with adequate substrates, it activatesa protein translational program that drivescell growth. More recently, mTORC1 hasemerged as a central control point for moreglobal anabolic metabolism, including the

A

B

Fig. 1. Control of SREBP-1c transcription, cleavage, and stability via the AKT-mTORC1 signaling path-way. (A) During fasting or in the absence of growth factors, SREBP-1c transcription and cleavage aredepressed and nuclear SREBP-1c (nSREBP-1c) is destabilized and degraded. (B) SREBP-1c abundance,stability, and processing are increased on stimulation of the AKT-mTORC1 signaling pathway, leading toan increase in the transcription of lipogenic genes. 4EBP, 4E binding protein 1; P, phosphorylated; SCF,Skp, cullin, F-box containing complex; Ub, ubiquitin.

Author contributions: W.J.Q. and M.J.B. wrote the paper.

The authors declare no conflict of interest.

See companion article on page 16184.1To whom correspondence should be addressed. E-mail:birnbaum@mail.med.upenn.edu.

15974–15975 | PNAS | October 2, 2012 | vol. 109 | no. 40 www.pnas.org/cgi/doi/10.1073/pnas.1214113109

stimulation of lipogenesis. Recent parallelstudies in cancer cells (9) and normal (10)insulin-responsive organs, such as the liver(5), have demonstrated that mTORC1 isrequired for the increase in SREBP-1 ex-pression and stimulation of its target genes(Fig. 1); whether activation of mTORC1is sufficient to promote lipogenesis viaSREBP-1c is still unclear. Recently, lipin 1,a phosphatidic acid phosphatase, has beenadvanced as a link between mTORC1 anSREBP-1c action by controlling the con-centration of nuclear SREBP-1c (11).In contrast to gene expression, illumi-

nating the role of mTORC1 and itsdownstream targets in the control ofSREBP-1c processing has been muchmore challenging; in some studies, it hasnot even been clear that processing is reg-ulated independent of gene expression.Study of the pathways controlling SREBP-1c processing has always been confoundedby concomitant changes in SREBP-1cprecursor protein levels that inevitablyoccur following engagement of insulinsignaling (12). It is virtually impossible toquantify the rate of generation of thenuclear form when the amount of pre-cursor is also changing. In addition, phos-phorylation by glycogen synthase kinase3 (GSK3) (13) or cyclin-dependent kinase8 (CDK8) (14) of a classic phosphodegronmotif in SREBP-1c induces its degrada-tion, further complicating the interpre-tation of experiments on SREBP-1processing. Owen et al. (3) now overcomethese technical limitations by generatinga transgene for human SREBP-1c, suchthat the protein is expressed exclusively inthe liver, driven by a promoter that main-tains the precursor at constant levels in-dependent of changes in nutritional stateor pharmacological manipulation of up-stream signaling pathways. The additionalclever twist that Owen et al. (3) provideis producing the transgene in rats ratherthan the much more conventional rodentmodel, mice. This choice of animal was

inspired by the generally recognized butpoorly understood (and even more sparselypublished) observation that isolated mousehepatocytes are relatively insensitiveto the actions of insulin, particularly inregard to regulation of SREBP-1c. Usingthis genetically modified rat, Owen et al.(3) show clearly that insulin does indeedincrease the levels of the nuclear tran-scriptionally active form of SREBP-1c andthat this is likely due to enhanced pro-teolytic cleavage rather than protein sta-bilization. Moreover, they go on todemonstrate that, like the regulation ofSREBP-1c gene expression, the mTORC1complex is absolutely necessary for hor-mone-dependent activation of SREBP1cprocessing. Further, using pharmacologi-cal reagents, Owen et al. (3) ask which ofthe many signaling pathways downstreamof mTORC1 conveys this message. In-hibition of p70 ribosomal S6 protein kinase(S6K) blocked the insulin-dependent in-crease in the nuclear form of SREBP-1c (3)but not increased expression of the endog-enous gene (12). These data reveal a bi-furcation in the regulation of SREBP-1c,distal to mTORC1, such that one armcontrols SREBP-1c processing and theother regulates gene expression. Still to beuncovered are the mechanisms by whichS6K leads to SREBP-1c cleavage. In reality,S6K has always been a bit of a mystery inregard to its physiological roles, becausemany of its known substrates do not appearto play a role in the regulation of its ca-nonical target function, protein translation.Perhaps these results from Owen et al. (3)point to an unrecognized function for S6Kin the control of membrane protein traf-ficking or in the retention of ER proteins.What implications can be drawn from

the discovery of a distinct signaling path-way in the control of SREBP-1c expressionand trafficking? For one, it raises thepossibility that other mTORC1-regulatedfactors might be important for lipid syn-thesis. One perplexing mTORC1 target is

hypoxia-inducible factor (HIF)-1α, whichhas been best characterized in the contextof its role as a master transcriptionalregulator of the response to reduced oxy-gen (15). However, HIF-1α is also fre-quently activated in an mTORC1 mannerin cancer, presumably as part of the met-abolic adaptions that allow continuousgrowth in unfavorable conditions. HIF di-rectly stimulates the transcription of py-ruvate dehydrogenase kinase (16, 17),which phosphorylates and inhibits pyru-vate dehydrogenase (PDH), favoring gly-colysis and lactate production over the citricacid cycle and the generation of lipogenicprecursors. Because the synthesis of fattyacids from glucose depends on flux throughPDH, it is difficult to understand why a co-ordinated lipogenic signal by mTORC1would include inhibition of this enzyme,which is typically stimulated by insulin. It ispossible to generate fatty acids from gluta-mate by reductive carboxylation in the liver,adipose tissue, and some tumors by a path-way not requiring PDH (18–20), but thequantitative contribution of this pathway tonormal and pathological lipid accumulationhas not been defined. Interestingly, mouselivers, which have constitutive accumulationof HIF-2 by virtue of tissue-specific deletionof von Hippel–Lindau protein, developsteatosis, consistent with HIF-2 beinga positive regulator of triglyceride accumu-lation (21).The other interesting implication of

branched signaling cascades is the possibil-ities for the development of drugs that tar-get only a subset of the biological effects ofinsulin. As noted above, this approach hasbecome very appealing since the realizationthat not all insulin-signaling pathways areaffected during the insulin-resistant state.The work of Owen et al. (3) will inevitablylead to strategies to block different com-ponents of the activation of lipogenic geneexpression in a selective manner, perhapsalleviating the dyslipidemia of T2DM witha minimum of unwanted adverse effects.

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