Abstract
Type 2 diabetes is characterized by reduced muscle glycogen synthesis. The key enzyme in this process, glycogen synthase (GS), is activated via proximal insulin signaling, but the exact molecular events remain unknown. Previously, we demonstrated that phosphorylation of Thr³&sup0;&sup8; on Akt (p-Akt-Thr³&sup0;&sup8;), Akt2 activity, and GS activity in muscle were positively associated with insulin sensitivity. Here, in the same study population, we determined the influence of several upstream elements in the canonical PI3K signaling on muscle GS activation. One-hundred eighty-one nondiabetic twins were examined with the euglycemic hyperinsulinemic clamp combined with excision of muscle biopsies. Insulin signaling was evaluated at the levels of the insulin receptor, IRS-1-associated PI3K (IRS-1-PI3K), Akt, and GS employing activity assays and phosphospecific Western blotting. The insulin-stimulated GS activity was positively associated with p-Akt-Thr³&sup0;&sup8; (P = 0.01) and Akt2 activity (P = 0.04) but not p-Akt-Ser&sup4;&sup7;³ or IRS-1-PI3K activity. Furthermore, p-Akt-Thr³&sup0;&sup8; and Akt2 activity were negatively associated with NH&sub2;-terminal GS phosphorylation (P = 0.001 for both), which in turn was negatively associated with insulin-stimulated GS activity (P < 0.001). We found no association between COOH-terminal GS phosphorylation and Akt or GS activity. Employing whole body Akt2-knockout mice, we validated the necessity for Akt2 in insulin-mediated GS activation. However, since insulin did not affect NH&sub2;-terminal phosphorylation in mice, we could not use this model to validate the observed association between GS NH&sub2;-terminal phosphorylation and Akt activity in humans. In conclusion, our study suggests that although COOH-terminal dephosphorylation is likely necessary for GS activation, Akt2-dependent NH&sub2;-terminal dephosphorylation may be the site for "fine-tuning" insulin-mediated GS activation in humans.