Amino acid and insulin signaling via the mTOR/p70 S6 kinase pathway - A negative feedback mechanism leading to insulin resistance in skeletal muscle cells

Amino acids have emerged as potent modulators of the mTOR/p70 S6 kinase pat hway. The involvement of this pathway in the regulation of insulin-stimulat ed glucose transport was investigated in the present study. Acute exposure (I h) to a balanced mixture of amino acids reduced insulin-stimulated gluco se transport by as much as 55% in L6 muscle cells. The effect of amino acid s was fully prevented by the specific mTOR inhibitor rapamycin. Time course analysis of insulin receptor substrate 1 (IRS-1)-associated phosphatidylin ositol (PI) 3-kinase activity revealed that incubation with amino acids spe eds up its time-dependent deactivation, leading to a dramatic suppression ( -70%) of its activity after 30 min of insulin stimulation as compared with its maximal activation (5 min of stimulation). This accelerated deactivatio n of PI 3-kinase activity in amino acid-treated cells was associated with a concomitant and sustained increase in the phosphorylation of p70 S6 kinase . In marked contrast, inhibition of mTOR by rapamycin maintained PI 3-kinas e maximally activated for up to 30 min. The marked inhibition of insulin-me diated PI 3-kinase activity by amino acids was linked to a rapamycin-sensit ive increase in serine/threonine phosphorylation of IRS-1 and a decreased b inding of the p85 subunit of PI 3-kinase to IRS-1. Furthermore, amino acids were required for the degradation of IRS-1 during long term insulin treatm ent. These results identify the mT0R/p70 S6 kinase signaling pathway as a n ovel modulator of insulin-stimulated glucose transport in skeletal muscle c ells.