Akt2 influences glycogen synthase activity in human skeletal muscle through regulation of NH₂-terminal (sites 2 + 2a) phosphorylation

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 Thr³⁰⁸ on Akt (p-Akt-Thr³⁰⁸), 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-Thr³⁰⁸ (P = 0.01) and Akt2 activity (P = 0.04) but not p-Akt-Ser⁴⁷³ or IRS-1-PI3K activity. Furthermore, p-Akt-Thr³⁰⁸ and Akt2 activity were negatively associated with NH₂-terminal GS phosphorylation (P = 0.001 for both), which in turn was negatively associated with insulin-stimulated GS activity (P