Reduced Cardiolipin Synthesis Protects Against the Development of Obesity and Insulin Resistance

Abstract

s / Can J Diabetes 37 (2013) S13eS84 S63 Methods: Eight healthy men (21.4 1.3 year, 21.5 0.3 kg/m2) underwent a 2-stage (2 3h) pancreatic clamp, with octreotide suppression and basal glucagon and hGH replacement. Glycemia was clamped at w5.5 mmol/L and branched-chain amino acids w350 mmol/L. Insulin was infused to maintain postabsorptive then raised to postprandial levels. Glucose turnover was measured using 3-3H-glucose. Vastus lateralis muscle biopsies were taken before and at 30 and 120 min during hyperinsulinemia. Primary skeletal myoblasts were isolated, cultured and differentiated into myotubes. Results: Hyperinsulinemia at 30 min (589.2 37.0 pmol/L) was maintained (150 to 180 min, 551.2 23.2 pmol/L). Glucose disposal increased from 2.73 0.08 mg/kg/min at postabsorptive to 9.54 0.62 120 min (P<0.00001) hyperinsulinemia. In biopsies, AktSer473 phosphorylation was maximal within 30 min and sustained at 120 min hyperinsulinemia. Grb10Ser476 phosphorylation was not different at 30 min (2.36 0.91 vs. 2.14 0.68; n1⁄44), but doubled by 120 min (4.16 1.14; P1⁄40.067). siRNA-mediated silencing reduced Grb10 protein abundance by 54-94%. The insulin receptor abundance was 2.5-fold higher in Grb10-depleted myotubes. Knockdown of Grb10 increased both basal and insulin-stimulated phosphorylation of IRS1Tyr632 and AktThr308. Acute inhibition of mTORC1 by rapamycin eliminated basal and insulin-stimulated Grb10Ser476 phosphorylation, while enhancing insulin-stimulated phosphorylation of AktThr308. Conclusions: Grb10 negatively regulates insulin signaling in part by modulating its receptor abundance. Insulin-stimulated Grb10 phosphorylation by mTORC1 is a negative feedback mechanism of PI3K/Akt signaling in human skeletal muscle.