Disassociation between the effects of amino acids and insulin on signaling, ubiquitin ligases, and protein turnover in human muscle

We determined the effects of intravenous infusion of amino acids (AA) at serum insulin of 5, 30, 72, and 167 mU/l on anabolic signaling, expression of ubiquitin-proteasome components, and protein turnover in muscles of healthy young men. Tripling AA availability at 5 mU/l insulin doubled incorporation of [1-¹³C]leucine [i.e., muscle protein synthesis (MPS), P < 0.01] without affecting the rate of leg protein breakdown (LPB; appearance of d ₅-phenylalanine). While keeping AA availability constant, increasing insulin to 30 mU/l halved LPB (P < 0.05) without further inhibition at higher doses, whereas rates of MPS were identical to that at 5 mU/l insulin. The phosphorylation of PKB Ser⁴⁷³ and p70^S6k Thr³⁸⁹ increased concomitantly with insulin, but whereas raising insulin to 30 mU/l increased the phosphorylation of mTOR Ser²⁴⁴⁸, 4E-BP1 Thr ^37/46, or GSK3β Ser⁹ and decreased that of eEF2 Thr⁵⁶, higher insulin doses to 72 and 167 mU/l did not augment these latter responses. MAFbx and proteasome C2 subunit proteins declined as insulin increased, with MuRF-1 expression largely unchanged. Thus increasing AA and insulin availability causes changes in anabolic signaling and amounts of enzymes of the ubiquitin-proteasome pathway, which cannot be easily reconciled with observed effects on MPS or LPB.