Increased phosphorylation of Akt substrate of 160 kDa (AS160) in rat skeletal muscle in response to insulin or contractile activity

Abstract

In 3T3-L1 adipocytes, insulin-stimulated GLUT4 translocation requires phosphorylation of the protein designated Akt substrate of 160 kDa (AS160). Both insulin and contractions activate Akt in skeletal muscle. Therefore, we assessed the effects in skeletal muscle of each stimulus on phosphorylation of proteins, including AS160, on the Akt phosphomotif. Isolated rat epitrochlearis muscles were incubated with insulin (for time course and dose response), stimulated to contract, or incubated with 5-aminoimidazole-4-carboxamide-1- β-D-ribofuranoside (AICAR) and used to assess the following: serine-phosphorylation of Akt (P-Akt), immunoreactivity with an antibody recognizing the Akt phosphomotif (α-phosplio-[Ser/Thr] Akt substrate [PAS]), and PAS immunoreactivity of samples immunoprecipitated with anti-AS160. P-Akt peaked at 5 min of insulin, and PAS immunoreactivity subsequently peaked for proteins of 250 kDa (10 min) and 160 kDa (15 min). P-Akt, PAS-160, and PAS-250 increased significantly with 0.6 nmol/l insulin. Contractile activity led to increased P-Akt and PAS immunoreactivity of proteins of 160 and 250 kDa. The 160-kDa protein was confirmed to be AS160 based on elevated PAS immunoreactivity in AS160 immunoprecipitates. Wortmannin inhibited insulin (120 nmol/l) and contraction effects on AS160 phosphorylation. Incubation with AICAR caused increased phosphorylation of AMP-activated protein kinase and AS160 but not Akt. Our working hypothesis is that phosphorylation of these putative Akt substrates is important for some of the insulin and contraction bioeffects.