Muscle-Specific Deletion of Rictor Impairs Insulin-Stimulated Glucose Transport and Enhances Basal Glycogen Synthase Activity

Abstract

Rictor is an essential component of mTOR (mammalian target of rapamycin) complex 2 (mTORC2), a kinase complex that phosphorylates Akt at Ser473 upon activation of phosphatidylinositol 3-kinase (PI-3 kinase). Since little is known about the role of either rictor or mTORC2 in PI-3 kinase-mediated physiological processes in adult animals, we generated muscle-specific rictor knockout mice. Muscle from male rictor knockout mice exhibited decreased insulin-stimulated glucose uptake, and the mice showed glucose intolerance. In muscle lacking rictor, the phosphorylation of Akt at Ser473 was reduced dramatically in response to insulin. Furthermore, insulin-stimulated phosphorylation of the Akt substrate AS160 at Thr642 was reduced in rictor knockout muscle, indicating a defect in insulin signaling to stimulate glucose transport. However, the phosphorylation of Akt at Thr308 was normal and sufficient to mediate the phosphorylation of glycogen synthase kinase 3 (GSK-3). Basal glycogen synthase activity in muscle lacking rictor was increased to that of insulin-stimulated controls. Consistent with this, we observed a decrease in basal levels of phosphorylated glycogen synthase at a GSK-3/protein phosphatase 1 (PP1)-regulated site in rictor knockout muscle. This change in glycogen synthase phosphorylation was associated with an increase in the catalytic activity of glycogen-associated PP1 but not increased GSK-3 inactivation. Thus, rictor in muscle tissue contributes to glucose homeostasis by positively regulating insulin-stimulated glucose uptake and negatively regulating basal glycogen synthase activity.

Rictor is a novel target of p70 S6 kinase-1

“…This finding indicates that Rictor plays a major role in the regulation of the SGK1 signaling cascade, which has been suggested to be a primary effector of mTORC2's function in yeast (Kamada et al, 2005;Aronova et al, 2008) and Caenorhabditis elegans (Jones et al, 2009;Soukas et al, 2009), but suggests that it may be somewhat less critical in the regulation of the activity of Akt/PKB. It is noteworthy that in muscle-specific, Rictor-knockout mice, Akt/PKB phosphorylation at S473 is decreased, while phosphorylation of FOXO3A at T32 and FOXO1 at T24 is maintained (Bentzinger et al, 2008;Kumar et al, 2008). …”

Gene expression analysis of mTOR pathway: association with human longevity

“…Rictor, an important component of mTORC2, contributes to glucose homeostasis in murine muscle tissue (Kumar et al, 2008), TORC2 C. elegans mutants show modulation of lifespan by affecting feeding and metabolism of different diets (Soukas et al, 2009), TORC2 is a mediator of proliferative and survival signals in cancer cells (Fang et al, 2012) and a study in mouse embryonic fibroblasts showed that mTORC2 regulates the TLR-mediated inflammatory response via FoxO1 (Brown et al, 2011). In addition, PRR5L, which is part of mTORC2, has been suggested to play a role in apoptosis in HeLa cells, but whether it is pro-apoptotic or antiapoptotic remains unclear (Thedieck et al, 2007).…”

mTOR-Dependent Cell Survival Mechanisms

“…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.…”

“…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).…”