Cardiovascular Research

mTORC1-independent Raptor prevents hepatic steatosis by stabilizing PHLPP2.

Authors: KyeongJin Kim|||Li Qiang|||Matthew S Hayden|||David P Sparling|||Nicole H Purcell|||Utpal B Pajvani

Affiliations: + Expand

Abstract

Mechanistic target of rapamycin complex 1 (mTORC1), defined by the presence of Raptor, is an evolutionarily conserved and nutrient-sensitive regulator of cellular growth and other metabolic processes. To date, all known functions of Raptor involve its scaffolding mTOR kinase with substrate. Here we report that mTORC1-independent ('free') Raptor negatively regulates hepatic Akt activity and lipogenesis. Free Raptor levels in liver decline with age and in obesity; restoration of free Raptor levels reduces liver triglyceride content, through reduced β-TrCP-mediated degradation of the Akt phosphatase, PHLPP2. Commensurately, forced PHLPP2 expression ameliorates hepatic steatosis in diet-induced obese mice. These data suggest that the balance of free and mTORC1-associated Raptor governs hepatic lipid accumulation, and uncover the potentially therapeutic role of PHLPP2 activators in non-alcoholic fatty liver disease.

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Chemical List

Adaptor Proteins, Signal Transducing|||Blood Glucose|||Insulin|||Multiprotein Complexes|||Regulatory-Associated Protein of mTOR|||Rptor protein, mouse|||Triglycerides|||beta-Transducin Repeat-Containing Proteins|||Mechanistic Target of Rapamycin Complex 1|||Oncogene Protein v-akt|||TOR Serine-Threonine Kinases|||Phosphoprotein Phosphatases

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Reference List

Ford E. S., Giles W. H. & Dietz W. H. Prevalence of the metabolic syndrome among US adults: findings from the third National Health and Nutrition Examination Survey. JAMA 287, 356–359 (2002) .|||Villanova N. et al. Endothelial dysfunction and cardiovascular risk profile in nonalcoholic fatty liver disease. Hepatology 42, 473–480 (2005) .|||Dowman J. K., Armstrong M. J., Tomlinson J. W. & Newsome P. N. Current therapeutic strategies in non-alcoholic fatty liver disease. Diabetes Obes. Metab. 13, 692–702 (2011) .|||Slawik M. & Vidal-Puig A. J. Lipotoxicity, overnutrition and energy metabolism in aging. Ageing Res. Rev. 5, 144–164 (2006) .|||Howell J. J. & Manning B. D. mTOR couples cellular nutrient sensing to organismal metabolic homeostasis. Trends Endocrinol. Metab. 22, 94–102 (2011) .|||Peterson T. R. et al. mTOR complex 1 regulates lipin 1 localization to control the SREBP pathway. Cell 146, 408–420 (2011) .|||Porstmann T. et al. SREBP activity is regulated by mTORC1 and contributes to Akt-dependent cell growth. Cell Metab. 8, 224–236 (2008) .|||Brown N. F., Stefanovic-Racic M., Sipula I. J. & Perdomo G. The mammalian target of rapamycin regulates lipid metabolism in primary cultures of rat hepatocytes. Metabolism 56, 1500–1507 (2007) .|||Yecies J. L. et al. Akt stimulates hepatic SREBP1c and lipogenesis through parallel mTORC1-dependent and independent pathways. Cell Metab. 14, 21–32 (2011) .|||Duvel K. et al. Activation of a metabolic gene regulatory network downstream of mTOR complex 1. Molecular cell 39, 171–183 (2010) .|||Yabe D., Brown M. S. & Goldstein J. L. Insig-2, a second endoplasmic reticulum protein that binds SCAP and blocks export of sterol regulatory element-binding proteins. Proc. Natl Acad. Sci. USA 99, 12753–12758 (2002) .|||Yabe D., Komuro R., Liang G., Goldstein J. L. & Brown M. S. Liver-specific mRNA for Insig-2 down-regulated by insulin: implications for fatty acid synthesis. Proc. Natl Acad. Sci. USA 100, 3155–3160 (2003) .|||Li S., Brown M. S. & Goldstein J. L. Bifurcation of insulin signaling pathway in rat liver: mTORC1 required for stimulation of lipogenesis, but not inhibition of gluconeogenesis. Proc. Natl Acad. Sci. USA 107, 3441–3446 (2010) .|||Taniguchi C. M., Emanuelli B. & Kahn C. R. Critical nodes in signalling pathways: insights into insulin action. Nat. Rev. Mol. Cell Biol. 7, 85–96 (2006) .|||Lu M. et al. Insulin regulates liver metabolism in vivo in the absence of hepatic Akt and Foxo1. Nat. Med. 18, 388–395 (2012) .|||Lin H. V. & Accili D. Hormonal regulation of hepatic glucose production in health and disease. Cell Metab. 14, 9–19 (2011) .|||Haas J. T. et al. Hepatic insulin signaling is required for obesity-dependent expression of SREBP-1c mRNA but not for feeding-dependent expression. Cell Metab. 15, 873–884 (2012) .|||Stephens L. et al. Protein kinase B kinases that mediate phosphatidylinositol 3,4,5-trisphosphate-dependent activation of protein kinase B. Science 279, 710–714 (1998) .|||Leslie N. R., Biondi R. M. & Alessi D. R. Phosphoinositide-regulated kinases and phosphoinositide phosphatases. Chem. Rev. 101, 2365–2380 (2001) .|||Yuan M., Pino E., Wu L., Kacergis M. & Soukas A. A. Identification of Akt-independent regulation of hepatic lipogenesis by mammalian target of rapamycin (mTOR) complex 2. J. Biol. Chem. 287, 29579–29588 (2012) .|||Hagiwara A. et al. Hepatic mTORC2 Activates Glycolysis and Lipogenesis through Akt, Glucokinase, and SREBP1c. Cell Metab. 15, 725–738 (2012) .|||Mora A., Lipina C., Tronche F., Sutherland C. & Alessi D. R. Deficiency of PDK1 in liver results in glucose intolerance, impairment of insulin-regulated gene expression and liver failure. Biochem. J. 385, 639–648 (2005) .|||Pajvani U. B. et al. Inhibition of Notch uncouples Akt activation from hepatic lipid accumulation by decreasing mTorc1 stability. Nat. Med. 19, 1054–1060 (2013) .|||Kim D. H. et al. mTOR interacts with raptor to form a nutrient-sensitive complex that signals to the cell growth machinery. Cell 110, 163–175 (2002) .|||Gao T., Furnari F. & Newton A. C. PHLPP: a phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth. Mol. Cell 18, 13–24 (2005) .|||Brognard J., Sierecki E., Gao T. & Newton A. C. PHLPP and a second isoform, PHLPP2, differentially attenuate the amplitude of Akt signaling by regulating distinct Akt isoforms. Mol. Cell 25, 917–931 (2007) .|||Menon S. et al. Spatial control of the TSC complex integrates insulin and nutrient regulation of mTORC1 at the lysosome. Cell 156, 771–785 (2014) .|||Kim S. et al. Amino acid signaling to mTOR mediated by inositol polyphosphate multikinase. Cell Metab. 13, 215–221 (2011) .|||Sengupta S., Peterson T. R., Laplante M., Oh S. & Sabatini D. M. mTORC1 controls fasting-induced ketogenesis and its modulation by ageing. Nature 468, 1100–1104 (2010) .|||Yip C. K., Murata K., Walz T., Sabatini D. M. & Kang S. A. Structure of the human mTOR complex I and its implications for rapamycin inhibition. Mol. Cell 38, 768–774 (2010) .|||Jain A. et al. Stoichiometry and assembly of mTOR complexes revealed by single-molecule pulldown. Proc. Natl Acad. Sci. USA 111, 17833–17838 (2014) .|||Kim D. H. et al. GbetaL, a positive regulator of the rapamycin-sensitive pathway required for the nutrient-sensitive interaction between raptor and mTOR. Mol. Cell 11, 895–904 (2003) .|||Cook J. R. et al. A mutant allele encoding DNA binding-deficient FoxO1 differentially regulates hepatic glucose and lipid metabolism. Diabetes 64, 1951–1965 (2015) .|||Wang J. et al. Effects of adenovirus-mediated liver-selective overexpression of protein tyrosine phosphatase-1b on insulin sensitivity in vivo. Diabetes Obes. Metab. 3, 367–380 (2001) .|||Facchinetti V. et al. The mammalian target of rapamycin complex 2 controls folding and stability of Akt and protein kinase C. EMBO J. 27, 1932–1943 (2008) .|||Ikenoue T., Inoki K., Yang Q., Zhou X. & Guan K. L. Essential function of TORC2 in PKC and Akt turn motif phosphorylation, maturation and signalling. EMBO J. 27, 1919–1931 (2008) .|||Sarbassov D. D., Guertin D. A., Ali S. M. & Sabatini D. M. Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex. Science 307, 1098–1101 (2005) .|||Li X., Liu J. & Gao T. beta-TrCP-mediated ubiquitination and degradation of PHLPP1 are negatively regulated by Akt. Mol. Cell. Biol. 29, 6192–6205 (2009) .|||Calvisi D. F. et al. Increased lipogenesis, induced by AKT-mTORC1-RPS6 signaling, promotes development of human hepatocellular carcinoma. Gastroenterology 140, 1071–1083 (2011) .|||Li L. et al. SCD1 Expression is dispensable for hepatocarcinogenesis induced by AKT and Ras oncogenes in mice. PLoS ONE 8, e75104 (2013) .|||Ono H. et al. Hepatic Akt activation induces marked hypoglycemia, hepatomegaly, and hypertriglyceridemia with sterol regulatory element binding protein involvement. Diabetes 52, 2905–2913 (2003) .|||Kaizuka T. et al. Tti1 and Tel2 are critical factors in mammalian target of rapamycin complex assembly. J. Biol. Chem. 285, 20109–20116 (2010) .|||Michael M. D. et al. Loss of insulin signaling in hepatocytes leads to severe insulin resistance and progressive hepatic dysfunction. Mol. Cell 6, 87–97 (2000) .|||Dong X. C. et al. Inactivation of hepatic Foxo1 by insulin signaling is required for adaptive nutrient homeostasis and endocrine growth regulation. Cell Metab. 8, 65–76 (2008) .|||Haeusler R. A. et al. Integrated control of hepatic lipogenesis versus glucose production requires FoxO transcription factors. Nat. Commun. 5, 5190 (2014) .|||Newton A. C. & Trotman L. C. Turning off AKT: PHLPP as a drug target. Annu. Rev. Pharmacol. Toxicol. 54, 537–558 (2014) .|||Brognard J., Niederst M., Reyes G., Warfel N. & Newton A. C. Common polymorphism in the phosphatase PHLPP2 results in reduced regulation of Akt and protein kinase C. J. Biol. Chem. 284, 15215–15223 (2009) .|||Bradley E. W., Carpio L. R. & Westendorf J. J. Histone deacetylase 3 suppression increases PH domain and leucine-rich repeat phosphatase (Phlpp)1 expression in chondrocytes to suppress Akt signaling and matrix secretion. J. Biol. Chem. 288, 9572–9582 (2013) .|||Gao M. H., Miyanohara A., Feramisco J. R. & Tang T. Activation of PH-domain leucine-rich protein phosphatase 2 (PHLPP2) by agonist stimulation in cardiac myocytes expressing adenylyl cyclase type 6. Biochem. Biophys. Res. Commun. 384, 193–198 (2009) .|||Andreozzi F. et al. Increased levels of the Akt-specific phosphatase PH domain leucine-rich repeat protein phosphatase (PHLPP)-1 in obese participants are associated with insulin resistance. Diabetologia 54, 1879–1887 (2011) .|||Cozzone D. et al. Isoform-specific defects of insulin stimulation of Akt/protein kinase B (PKB) in skeletal muscle cells from type 2 diabetic patients. Diabetologia 51, 512–521 (2008) .|||Mishra N. et al. Efficient hepatic delivery of drugs: novel strategies and their significance. Biomed. Res. Int. 2013, 382184 (2013) .|||Folch J., Lees M. & Sloane Stanley G. H. A simple method for the isolation and purification of total lipides from animal tissues. J. Biol. Chem. 226, 497–509 (1957) .|||Zhang Y. L. et al. Aberrant hepatic expression of PPARgamma2 stimulates hepatic lipogenesis in a mouse model of obesity, insulin resistance, dyslipidemia, and hepatic steatosis. J. Biol. Chem. 281, 37603–37615 (2006) .|||Miyamoto S. et al. PHLPP-1 negatively regulates Akt activity and survival in the heart. Circ. Res. 107, 476–484 (2010) .|||Kim K., Pyo S. & Um S. H. S6 kinase 2 deficiency enhances ketone body production and increases peroxisome proliferator-activated receptor alpha activity in the liver. Hepatology 55, 1727–1737 (2012) .