Amino acids are necessary for the insulin-induced activation of mTOR/S6K1 signaling and protein synthesis in healthy and insulin resistant human skeletal muscle

Background: Amino acids (AA) activate the mammalian target of rapamycin (mTOR) signaling pathway but overactivation has a negative feedback effect on insulin signaling which may lead to insulin resistance and type 2 diabetes (T2DM). Purpose: To determine the effect of reduced AA concentrations on mTOR and insulin signaling during increased nutrient and insulin availability. Methods: Six control and six T2DM subjects were studied at baseline and following a 5 h AA lowering high energy and insulin clamp. Stable isotopic techniques in combination with femoral catheterizations were used to measure AA kinetics across the leg while muscle biopsies were used to measure mTOR and insulin signaling proteins using immunoblotting techniques. Results: AA concentrations decreased by ∼30-60% in both groups (p < 0.05). Phospho-mTOR, S6K1, eEF2, and eIF2α were unchanged in both groups following the clamp (p > 0.05). In T2DM subjects, IRS-1 serine phosphorylation was unchanged while phospho-AMPKα decreased and phospho-Akt, phospho-AS160 and glucose uptake increased following the clamp (p < 0.05). In comparison, AA concentrations were maintained in a separate group during an insulin infusion. In this group, phospho-Akt, mTOR and S6K1 (n = 4) increased. Conclusion: Amino acids are necessary for insulin-induced activation of mTOR signaling and protein synthesis in both healthy and insulin resistant skeletal muscle.