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

Abstract

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 d5-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 p70S6k Thr389 increased concomitantly with insulin, but whereas raising insulin to 30 mU/l increased the phosphorylation of mTOR Ser2448, 4E-BP1 Thr37/46, 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.

Assessment of skeletal muscle proteolysis and the regulatory response to nutrition and exercise

“…The authors hypothesized a minimal role for the ubiquitin ligases in actual proteolysis but rather changes in ubiquitin ligase expression may be due to an increased demand to remodel muscle after eccentric exercise and higher demand for energy with concentric compared to eccentric exercise. Their assertions are supported, in part by others, where changes in UPS-related gene expression appeared unrelated and did not concur with changes in 26S proteasome activity or kinetic measures of muscle protein degradation (49,69,75).…”

New strategies in sport nutrition to increase exercise performance

“…Cell based work has shown that this amino acid sensing system is incredibly sensitive with an ~7% increase in intracellular leucine leading to 50% maximal activation of mTORC1 [97]. Furthermore, only a fraction of maximal mTORC1 activity (~30%) is required to fully saturate muscle protein synthesis [98,99]. The amino acid sensing by mTORC1 also seems to be driven not by extracellular, but instead by intracellular amino acid concentrations [100].…”

“…As we mentioned earlier, carbohydrate driven increases in insulin may increase mTORC1 activity. However, when insulin is infused to supra-physiological levels mTORC1 is potently activated without a concomitant increase in muscle protein synthesis [99]. So while the data are clear that mTORC1 is required for load-induced growth [89], resistance exercise [91] and feeding [90], it is still very unclear if manipulating mTORC1 above what occurs physiologically will lead to enhanced muscle growth.…”

Exploring the Process of Energy Generation in Pathophysiology by Targeted Metabolomics: Performance of a Simple and Quantitative Method

“…This may seem paradoxical because elevated levels of branched chain amino acids are usually associated with poor health and cardiovascular disease. One explanation might be the increased endothelial proliferation and accumulation of immune cells resulting in a higher diffusion of these metabolites into the circulation, but may also indirectly point to increased breakdown in the leg muscles [36,37].…”