The effects of amino acids on glucose metabolism of isolated rat skeletal muscle are independent of insulin and the mTOR/S6K pathway

Abstract

Two mechanisms have been proposed for the modulation of skeletal muscle glucose metabolism by amino acids. Whereas studies on humans and cultured cells suggested acute insulin desensitization via mammalian target of rapamycin (mTOR) and its downstream target p70 S6 kinase (S6K), investigations using native specimens of rat muscle hinted at impairment of glucose oxidation by competition for mitochondrial oxidation. To better understand these seemingly contradictory findings, we explored the effects of high concentrations of mixed amino acids on fuel metabolism and S6K activity in freshly isolated specimens of rat skeletal muscle. In this setting, increasing concentrations of amino acids dose-dependently reduced the insulin-stimulated rates of CO(2) production from glucose and palmitate (decrease in glucose oxidation induced by addition of 5.5, 11, 22, and 44 mmol/l amino acids:--16 +/- 3, -25 +/- 7, -44 +/- 4, -62 +/- 4%; P < 0.02 each). This effect could not be attributed to insulin desensitization, because it was not accompanied by any reduction of insulin-stimulated glucose transport [+12 +/- 16, +17 +/- 22, +21 +/- 33, +13 +/- 12%; all nonsignificant (NS)] or glycogen synthesis (+1 +/- 6, -5 +/- 6, -9 +/- 8, +6 +/- 5%; all NS) and because it persisted without insulin stimulation. Abrogation of S6K activity by the mTOR blocker rapamycin failed to counteract amino acid-induced inhibition of glucose and palmitate oxidation, which therefore was obviously independent of mTOR/S6K signaling (decrease in glucose oxidation by addition of 44 mmol/l amino acids: without rapamycin, -60 +/- 4%; with rapamycin, -50 +/- 13%; NS). We conclude that amino acids can directly affect muscle glucose metabolism via two mechanisms, mTOR/S6K-mediated insulin desensitization and mitochondrial substrate competition, with the latter predominating in isolated rat muscle.

Bioinformatics functional analysis of let-7a, miR-34a, and miR-199a/b reveals novel insights into immune system pathways and cancer hallmarks for hepatocellular carcinoma

“…Increased level and activity of this target have been found to be induced by insulin. 99,100 Interestingly, this signaling has been demonstrated in 60% of HCC cases, which promoted cell cycle progression, proliferation, growth, and angiogenesis. [101][102][103][104][105] Regarding the relation between let-7a, miR-34a, and miR-199a/b and MTOR as a direct target in insulin signaling, it has been shown that insulin stimulation inhibits the expression of miR-199a-5p.…”

Lipophilicity as a determinant of thiazolidinedione action in vitro: findings from BLX-1002, a novel compound without affinity to PPARs

“…Fuel metabolism of native skeletal muscle was examined along procedures established at our laboratory (2,3,26). Food, but not water, was withdrawn from 6-to 8-wk-old rats for 3-4 h, before they were briefly anesthetized and killed by cervical dislocation for the preparation of two longitudinal strips of soleus muscle per leg.…”

“…Analytical methods have been outlined in more detail elsewhere (26). In short, rates of CO2 production from glucose or palmitate (referred to as glucose and palmitate oxidation) were measured in separate runs as calculated from the conversion of [ 14 C]glucose or of [ 14 C]palmitate into 14 CO2, which was trapped with a solution containing methanol and phenethylamine (1:1).…”

Insulin Promotes Glucose Consumption via Regulation of miR-99a/mTOR/PKM2 Pathway

“…These results are consistent with our observation that mTOR was inhibited by miR-99a in hepatocellular carcinoma cells (HCC). It has been demonstrated that insulin can regulate glucose metabolism by activating mTOR pathway [19], [20]. The liver is known to be important in regulating glucose metabolism in response to growth factors and insulin treatment [21], [22].…”