Fat Cell–Specific Ablation of Rictor in Mice Impairs Insulin-Regulated Fat Cell and Whole-Body Glucose and Lipid Metabolism

Abstract

OBJECTIVERictor is an essential component of mammalian target of rapamycin (mTOR) complex (mTORC) 2, a kinase that phosphorylates and activates Akt, an insulin signaling intermediary that regulates glucose and lipid metabolism in adipose tissue, skeletal muscle, and liver. To determine the physiological role of rictor/mTORC2 in insulin signaling and action in fat cells, we developed fat cell–specific rictor knockout (FRic−/−) mice.RESEARCH DESIGN AND METHODSInsulin signaling and glucose and lipid metabolism were studied in FRic−/− fat cells. In vivo glucose metabolism was evaluated by hyperinsulinemic-euglycemic clamp.RESULTSLoss of rictor in fat cells prevents insulin-stimulated phosphorylation of Akt at S473, which, in turn, impairs the phosphorylation of downstream targets such as FoxO3a at T32 and AS160 at T642. However, glycogen synthase kinase-3β phosphorylation at S9 is not affected. The signaling defects in FRic−/− fat cells lead to impaired insulin-stimulated GLUT4 translocation to the plasma membrane and decreased glucose transport. Furthermore, rictor-null fat cells are unable to suppress lipolysis in response to insulin, leading to elevated circulating free fatty acids and glycerol. These metabolic perturbations are likely to account for defects observed at the whole-body level of FRic−/− mice, including glucose intolerance, marked hyperinsulinemia, insulin resistance in skeletal muscle and liver, and hepatic steatosis.CONCLUSIONSRictor/mTORC2 in fat cells plays an important role in whole-body energy homeostasis by mediating signaling necessary for the regulation of glucose and lipid metabolism in fat cells.

Anti-diabetic activity of stigmasterol from soybean oil by targeting the GLUT4 glucose transporter

“…This screening system has been established according to our previous study [21,22]. To be brief, in cellular GLUTs storage vesicles (GSV), IRAP and GLUT4 displayed a strong co-localization reported by many researchers [23,24]. Thus, detecting the IRAP can indirectly reflect the situation of GLUT4 [22].…”

mTOR-Dependent Cell Survival Mechanisms

“…The reason for this discrepancy remains unclear but may reflect a difference between acute knockdown and chronic knockout experiments, or the existence of a compensatory mechanism. Although phosphorylation at both T308 and S473 is required for maximal AKT activity in vitro, it appears that T308 phosphorylation alone empowers AKT with enough activity to phosphorylate many of its substrates in cultured cells or in tissues (Alessi et al 1996;Guertin et al 2006;Jacinto et al 2006;Yang et al 2006;Bentzinger et al 2008;Kumar et al 2008Kumar et al , 2010Cybulski et al 2009;Gu et al 2011).…”

“…However, despite most models placing mTORC2 upstream of the AKT-TSC2-mTORC1 signaling axis, evidence that this connection is important in vivo is lacking. In fact, it appears that losing mTORC2 activity has minimal effects on mTORC1 signaling in many cell types (Guertin et al 2006;Jacinto et al 2006;Shiota et al 2006;Bentzinger et al 2008;Kumar et al 2008Kumar et al , 2010Cybulski et al 2009;Gu et al 2011). Interestingly, mTORC2 may regulate autophagy independently of mT ORC1 via the AKT-FoxO3a axis (Mammucari et al 2007).…”

“…Although the exact cause of lethality is unknown, increased cell death was not readily apparent in the knockout embryos. To gain further insight into the tissue-specific functions of mTORC2, conditional knockout models of rictor have been developed, including skeletal muscle, white adipose tissue, and pancreatic b cells (Bentzinger et al 2008;Kumar et al 2008Kumar et al , 2010Cybulski et al 2009;Gu et al 2011). Although some metabolic defects are reported, there is no indication from these studies that mTORC2 loss-at least under otherwise normal physiological conditions-results in increased apoptosis.…”

mTORC2 promotes type I insulin-like growth factor receptor and insulin receptor activation through the tyrosine kinase activity of mTOR

“…While mTORC1 acts as an effector downstream of IGF/insulin signaling and elicits feedback inhibition of IRS1 and stabilization of Grb10 [18,19], mTORC2 may positively regulate IGF signaling by phosphorylating IGF-II mRNA-binding protein to promote IGF-II translation [20,21]. In addition, mTORC2 disruption in mice leads to insulin resistance [22,23]. mTORC2 also negatively feeds back to IRS1 via Fbw8-mediated degradation [24].…”