Fiber type‐specific regulation of the Akt substrate of 160 kDa (AS160) in mouse skeletal muscle

Abstract

AS160 phosphorylation has recently emerged as a critical regulatory step in contraction- and insulin-stimulated glucose uptake in skeletal muscle. Mice express a long and short AS160 splice-variant differing by a single exon. Using PCR we determined that skeletal muscle expresses 17-fold more long form than short form AS160 mRNA. With cell free assays we found that two putative AS160 kinases in skeletal muscle, AMP-activated protein kinase (AMPK) and Akt, directly phosphorylate the AS160 long form. To compare fiber type-specific AS160 regulation, we used soleus (SOL, rich in type I fibers), gastrocnemius (GAS, both type II and I), and tibialis anterior (TA, predominantly type II) muscles. Relative AS160 protein expression was greatest in SOL (2.3 ± 0.08) compared to GAS (1.0 ± 0.03) and TA (0.45 ± 0.04). In contrast, AS160 phosphorylation in response to in vivo treatment with AICAR (AMPK activator) or insulin was greatest in TA [AICAR: TA (1.8 ± 0.06); GAS (1.3 ± 0.03); SOL (1.09 ± 0.17), Insulin: TA (2.1 ± 0.2); GAS (1.9 ± 0.3); SOL (1.8 ± 0.3)]. Thus, type II fibers have an increased ratio of AS160 phosphorylation to AS160 protein expression. We conclude that the AS160 long form is the predominant splice-variant expressed in skeletal muscle, and that AMPK and Akt are AS160 kinases. Fiber-type is a key determinant of AS160 phosphorylation in skeletal muscle. Support: NIH F32 DK07851, R01DK25336, R01AR42238