Rapamycin impacts positively on longevity, despite glucose intolerance induction
Authors: Anne-Christine Piguet, Paulo J.F. Martins, Sara C. Kozma
Journal: Journal of Hepatology (2012)

COMMENTARY ON:Rapamycin-induced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity. Lamming DW, Ye L, Katajisto P, Goncalves MD, Saitoh M, Stevens DM, Davis JG, Salmon AB, Richardson A, Ahima RS, Guertin DA, Sabatini DM, Baur JA. Science. 2012 Mar 30;335(6076):1638–43. Copyright (2012). Abstract reprinted with permission from the The American Association for the Advancement of Science.http://www.ncbi.nlm.nih.gov/pubmed/22461615Abstract. Rapamycin, an inhibitor of mechanistic target of rapamycin complex 1 (mTORC1), extends the life spans of yeast, flies, and mice. Calorie restriction, which increases life span and insulin sensitivity, is proposed to function by inhibition of mTORC1, yet paradoxically, chronic administration of rapamycin substantially impairs glucose tolerance and insulin action. We demonstrate that rapamycin disrupted a second mTOR complex, mTORC2, in vivo and that mTORC2 was required for the insulin-mediated suppression of hepatic gluconeogenesis. Further, decreased mTORC1 signaling was sufficient to extend life span independently from changes in glucose homeostasis, as female mice heterozygous for both mTOR and mLST8 exhibited decreased mTORC1 activity and extended life span but had normal glucose tolerance and insulin sensitivity. Thus, mTORC2 disruption is an important mediator of the effects of rapamycin in vivo. © 2012 European Association for the Study of the Liver. Published by Elsevier Inc. All rights reservedAlthough many of us hope for a long life, this wish is undoubtedly tied to the concept of healthy life. Thus, an anticipated thinking is that health is a prerequisite for longevity. This corollary has been recently challenged in a study published in Science, where Lamming and colleagues report that rapamycin extends lifespan of mice by inhibition of mTORC1, despite impairing glucose homeostasis through the disruption of mTORC2 signaling [[1]Lamming D.W. Ye L. Katajisto P. Goncalves M.D. Saitoh M. Stevens D.M. et al.Rapamycin-induced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity.Science. 2012; 335: 1638-1643Crossref PubMed Scopus (899) Google Scholar]. The mammalian target of rapamycin (mTOR) is a protein kinase linked to two distinct multi-protein complexes: mTORC1 and mTORC2. Within these complexes, the proteins raptor and rictor are bound to mTORC1 or mTORC2, respectively, while mLST8 is associated with both mTORC1 and mTORC2. mTORC1 regulates pathways involved in cell growth, metabolism and aging, while mTORC2 is involved in cytoskeleton and insulin signaling regulation [[2]Dann S.G. Selvaraj A. Thomas G. mTOR Complex1-S6K1 signaling: at the crossroads of obesity, diabetes and cancer.Trends Mol Med. 2007; 13: 252-259Abstract Full Text Full Text PDF PubMed Scopus (403) Google Scholar]. Downstream of mTORC1, the ribosomal S6 protein kinase (S6K1) has been identified as a key player for the role of mTORC1 [[2]Dann S.G. Selvaraj A. Thomas G. mTOR Complex1-S6K1 signaling: at the crossroads of obesity, diabetes and cancer.Trends Mol Med. 2007; 13: 252-259Abstract Full Text Full Text PDF PubMed Scopus (403) Google Scholar]. Genetic attenuation of mTORC1 signaling was shown to extend lifespan in yeast, nematodes, fruit flies, as well as mice [[4]Selman C. Tullet J.M. Wieser D. Irvine E. Lingard S.J. Choudhury A.I. et al.Ribosomal protein S6 kinase 1 signaling regulates mammalian life span.Science. 2009; 326: 140-144Crossref PubMed Scopus (901) Google Scholar]. Interestingly, knocking out S6K1 led to improved insulin sensitivity [[3]Um S.H. Frigerio F. Watanabe M. Picard F. Joaquin M. Sticker M. et al.Absence of S6K1 protects against age- and diet-induced obesity while enhancing insulin sensitivity.Nature. 2004; 431: 200-205Crossref PubMed Scopus (1376) Google Scholar] and induced gene expression patterns similar to those seen in caloric restriction [[4]Selman C. Tullet J.M. Wieser D. Irvine E. Lingard S.J. Choudhury A.I. et al.Ribosomal protein S6 kinase 1 signaling regulates mammalian life span.Science. 2009; 326: 140-144Crossref PubMed Scopus (901) Google Scholar]. For several years, caloric restriction has been known to increase lifespan in mammals [[5]Anderson R.M. Weindruch R. Metabolic reprogramming, caloric restriction and aging.Trends Endocrinol Metab. 2010; 21: 134-141Abstract Full Text Full Text PDF PubMed Scopus (201) Google Scholar]. Caloric restriction reduced mTORC1 signaling but also improved glucose tolerance as well as insulin sensitivity and these effects have been shown to positively influence longevity [[5]Anderson R.M. Weindruch R. Metabolic reprogramming, caloric restriction and aging.Trends Endocrinol Metab. 2010; 21: 134-141Abstract Full Text Full Text PDF PubMed Scopus (201) Google Scholar]. Along the line that implies the role of mTORC1 in longevity, the activity of the mTORC1/S6K1 signaling pathway increases with aging, while S6K1 deletion leads to increased lifespan and resistance to age-related pathologies [[4]Selman C. Tullet J.M. Wieser D. Irvine E. Lingard S.J. Choudhury A.I. et al.Ribosomal protein S6 kinase 1 signaling regulates mammalian life span.Science. 2009; 326: 140-144Crossref PubMed Scopus (901) Google Scholar]. On the other hand, mTORC2 is largely insensitive to rapamycin in an acute setting; however, it can be inhibited by chronic exposure to rapamycin [[6]Sarbassov D.D. Ali S.M. Sengupta S. Sheen J.H. Hsu P.P. Bagley A.F. et al.Prolonged rapamycin treatment inhibits mTORC2 assembly and Akt/PKB.Mol Cell. 2006; 22: 159-168Abstract Full Text Full Text PDF PubMed Scopus (2207) Google Scholar]. It was demonstrated that chronic treatment with rapamycin caused glucose intolerance in humans and rodents [7Houde V.P. Brule S. Festuccia W.T. Blanchard P.G. Bellmann K. Deshaies Y. et al.Chronic rapamycin treatment causes glucose intolerance and hyperlipidemia by upregulating hepatic gluconeogenesis and impairing lipid deposition in adipose tissue.Diabetes. 2010; 59: 1338-1348Crossref PubMed Scopus (341) Google Scholar, 8Fraenkel M. Ketzinel-Gilad M. Ariav Y. Pappo O. Karaca M. Castel J. et al.mTOR Inhibition by rapamycin prevents beta-cell adaptation to hyperglycemia and exacerbates the metabolic state in type 2 diabetes.Diabetes. 2008; 57: 945-957Crossref PubMed Scopus (318) Google Scholar]. One therefore would predict that the deleterious effects of chronic rapamycin treatment on glucose homeostasis would hamper the life-extending effects of mTORC1 inhibition. However, mice fed rapamycin late in life had their median and maximal lifespan extended [[9]Harrison D.E. Strong R. Sharp Z.D. Nelson J.F. Astle C.M. Flurkey K. et al.Rapamycin fed late in life extends lifespan in genetically heterogeneous mice.Nature. 2009; 460: 392-395Crossref PubMed Scopus (2768) Google Scholar]. Having this paradox in mind, Lamming and colleagues [[10]Bluher M. Kahn B.B. Kahn C.R. Extended longevity in mice lacking the insulin receptor in adipose tissue.Science. 2003; 299: 572-574Crossref PubMed Scopus (1071) Google Scholar] verified whether chronic rapamycin treatment, at a dose used to increase lifespan, would induce glucose intolerance and increased gluconeogenesis. They demonstrated in vivo that rapamycin not only disrupted the mTORC1 complex in muscle, adipose and liver tissues, but also affected mTORC2 assembling in the same tissues. This was further confirmed by finding reduced phosphorylation of the mTORC2 substrates PKCα-S657, AKT-S473, and NDRG1-T346 in all tissues studied upon refeeding of mice or insulin stimulation [[1]Lamming D.W. Ye L. Katajisto P. Goncalves M.D. Saitoh M. Stevens D.M. et al.Rapamycin-induced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity.Science. 2012; 335: 1638-1643Crossref PubMed Scopus (899) Google Scholar]. Using hyperinsulinemic-euglycemic clamps, the authors showed that rapamycin specifically affected the efficiency of insulin to suppress hepatic glucose production [[1]Lamming D.W. Ye L. Katajisto P. Goncalves M.D. Saitoh M. Stevens D.M. et al.Rapamycin-induced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity.Science. 2012; 335: 1638-1643Crossref PubMed Scopus (899) Google Scholar]. By conditionally deleting either raptor or rictor in the mouse liver, or by using livers from rictor knockout animals, the authors established that disruption of mTORC2 signaling, but not mTORC1, impaired glucose tolerance and induced hepatic gluconeogenesis by increased hepatic glucose production [[1]Lamming D.W. Ye L. Katajisto P. Goncalves M.D. Saitoh M. Stevens D.M. et al.Rapamycin-induced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity.Science. 2012; 335: 1638-1643Crossref PubMed Scopus (899) Google Scholar]. This effect was not further affected by rapamycin treatment. However, livers from raptor knockout mice had normal glucose homeostasis and remained rapamycin sensitive. Together these observations suggested that chronic rapamycin treatment at a dose used to extend lifespan, impaired hepatic glucose homeostasis by induction of gluconeogenesis, an effect mediated by mTORC2 disruption. However, can the paradoxical effects of mTORC1 and mTORC2 inhibition be dissociated? To test that, Lamming and colleagues generated several mice carrying only one allele of different components of the mTORC1 (Mtor+/−, Raptor+/−, Mlst8+/−) or double-mutants (Mtor+/−Raptor+/− and Mtor+/−Mlst8+/− mice). Similar to what described for S6K1 deletion [[4]Selman C. Tullet J.M. Wieser D. Irvine E. Lingard S.J. Choudhury A.I. et al.Ribosomal protein S6 kinase 1 signaling regulates mammalian life span.Science. 2009; 326: 140-144Crossref PubMed Scopus (901) Google Scholar], the female Mtor+/−Mlst8+/− mice showed an increased lifespan and this was accompanied by normal glucose tolerance. Interestingly, although both mTOR and mlst8 are present in both complexes, hepatic mTORC1 signalling was reduced in Mtor+/−Mlst8+/− female mice, whereas mTORC2 signalling was not affected. This corresponded to a lower raptor–mTOR ratio compared to rictor–mTOR ratio, bringing the authors to hypothesize a recruitment of mTOR and mlst8 to mTORC2 when protein amounts were limited. The data from Lamming and colleagues strongly suggest that selective inhibition of mTORC1 or mTORC2 signaling results in lifespan extension and glucose homeostasis impairment, respectively (Fig. 1). However, it is unclear whether rapamycin treatment extends lifespan by postponing death from age-related pathologies and/or by retarding other cellular mechanisms of aging. For instance, while Harrison et al. [[9]Harrison D.E. Strong R. Sharp Z.D. Nelson J.F. Astle C.M. Flurkey K. et al.Rapamycin fed late in life extends lifespan in genetically heterogeneous mice.Nature. 2009; 460: 392-395Crossref PubMed Scopus (2768) Google Scholar] showed that mice fed rapamycin live longer, they did not report on the mouse glucose-homeostasis status, their body weight or food intake. However, more recent studies have shown that feeding or intraperitonealy injections of rapamycin reduce body weight and food intake of rodents [[6]Sarbassov D.D. Ali S.M. Sengupta S. Sheen J.H. Hsu P.P. Bagley A.F. et al.Prolonged rapamycin treatment inhibits mTORC2 assembly and Akt/PKB.Mol Cell. 2006; 22: 159-168Abstract Full Text Full Text PDF PubMed Scopus (2207) Google Scholar]. Conversely, while Lamming et al. reported a clear effect of rapamycin treatment on glucose homeostasis, the changes in body weight and lifespan under the rapamycin-induced insulin resistance were not determined. Indeed, female Mtor+/−Mlst8+/− mice, which lived longer, had apparently normal glucose tolerance. All these data together suggest that the ability of mTORC1 inhibition to extend lifespan may involve maintenance of glucose homeostasis. Accordingly, it has been shown that fat-specific insulin receptor knockout mice have reduced fat mass without caloric restriction, which is associated with increased longevity; possibly through improvement of insulin signaling in the remaining body tissues [[10]Bluher M. Kahn B.B. Kahn C.R. Extended longevity in mice lacking the insulin receptor in adipose tissue.Science. 2003; 299: 572-574Crossref PubMed Scopus (1071) Google Scholar]. The determination of whether the effects of rapamycin treatment on longevity require changes in body weight and composition, food intake, and glucose homeostasis would help gain insight into the mechanism by which mTOR complexes influence lifespan. The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.