Domain-dependent modulation of insulin-induced AS160 phosphorylation and glucose uptake by Ca2+/calmodulin-dependent protein kinase II in L6 myotubes

Abstract

In skeletal muscle, the molecular mechanisms by which insulin stimulates glucose transport remains incompletely understood. Our study investigated the cellular dynamics of intracellular Ca2+ mobilisation and Ca2+/calmodulin-dependent protein kinase II (CaMKII) activation on insulin-induced skeletal muscle glucose transport. L6 myotubes were treated without or with insulin [100nM] for 15min and subsequently monitored for glucose uptake using isotope-labelled 2-deoxyglucose (I-2DOG), intracellular Ca2+ (Cai2+) release using Fluo-4AM and protein phosphorylation using Western blotting. Acute exposure of myotubes to insulin increased both Akt substrate-160kDa (AS160) phosphorylation and I-2DOG uptake. Insulin concurrently increased Cai2+ and activated CaMKII. Exposing myotubes to either BAPTA/AM to sequester Cai2+ or KN-93 to inhibit CaMKII activity, decreased insulin-induced glucose uptake without affecting AS160 phosphorylation. On the other hand, blocking either calmodulin or the autoregulatory domain of CaMKII blocked the effect of insulin on both AS160 phosphorylation and glucose transport. Likewise, genetic knockdown of CaMKII in myotubes using siRNA completely abolished insulin-mediated glucose uptake. These results illustrate impairments in Cai2+ mobilisation and CaMKII activation are sufficient to negatively influence insulin-dependent glucose transport by L6 myotubes. Additionally, our results show for the first time that Cai2+ and domain-dependent CaMKII signalling differentially affect insulin-induced AS160 phosphorylation, and establish that Ca2+ and CaMKII are components of the insulin signalling pathway in L6 myotubes.