Regulation of skeletal muscle metabolism by physiologically relevant adipokine mixtures

In recent years, the adipose tissue has emerged as an important regulator of endocrine function. Many studies have demonstrated that adipokines can independently regulate skeletal omuscle metabolism. Although it is important to study the isolated effects of each adipokine, their ultimate in vivo actions are influenced by the presence of other adipokines. To address this, we have designed a coculture system of primary rat adipocytes and L6 skeletal muscle cells to examine the cross-talk between muscle and fat. Under acute conditions, adipocyte coculture can stimulate glucose uptake in L6 cells and induce phosphorylation of IRS1(Y612), Akt(T308 & S473) and AMPK(T172) resulting in an increase of GLUT4 localization to the plasma membrane. Temporal analysis of these effects revealed that increase in glucose uptake can be maintained under chronic conditions and this was attributed to an increase in GLUT1 but not GLUT4 expression. Analysis of coculture media revealed that adiponectin is the primary mediator of this effect since siRNA knockdown of adiponectin receptor-2 in L6 cells abolished the increase in glucose uptake and phosphorylation of IRS1, Akt and AMPK. We investigated the effect of diabetes on this cross-talk by coculturing L6 cells with streptozotocin(STZ)-induced diabetic rat adipocytes. The profile of adiponectin in coculture media showed that STZ rat adipocytes secreted lower levels of HMW adiponectin compared to wild-type adipocytes. This correlated with decreased phosphorylation of IRS-1, Akt and AMPK. We also observed that coculture acutely stimulated a transient increase in fatty acid uptake and CD36 translocation to the plasma membrane. Our coculture studies demonstrate the importance of physiologically relevant adipokine mixtures in mediating positive cross-talk between adipose tissue and skeletal muscle.