Caffeine modulates phosphorylation of insulin receptor substrate-1 and impairs insulin signal transduction in rat skeletal muscle

Abstract

Caffeine decreases insulin sensitivity and insulin-stimulated glucose transport in skeletal muscle; however, the precise mechanism responsible for this deleterious effect is not understood fully. We investigated the effects of incubation with caffeine on insulin signaling in rat epitrochlearis muscle. Caffeine (≥1 mM, ≥15 min) suppressed insulin-stimulated insulin receptor substrate (IRS)-1 Tyr[612] phosphorylation in a dose- and time-dependent manner. These responses were associated with inhibition of the insulin-stimulated phosphorylation of phosphatidylinositol 3-kinase (PI3K) Tyr[458], Akt Ser[473], and glycogen synthase kinase-3β Ser9 and with inhibition of insulin-stimulated 3-O-methyl-D-glucose (3MG) transport but not with inhibition of the phosphorylation of insulin receptor-β Tyr[1158/62/63]. Furthermore, caffeine enhanced phosphorylation of IRS-1 Ser[307] and an IRS-1 Ser307 kinase, inhibitor-κB kinase (IKK)-α/β Ser[176/180]. Blockade of IKK/IRS-1 Ser[307] by caffeic acid ameliorated the caffeine-induced downregulation of IRS-1 Tyr[612]phosphorylation and 3MG transport. Caffeine also increased the phosphorylation of IRS-1 Ser789 and an IRS-1 Ser[789] kinase, 5′-AMP-activated protein kinase (AMPK). However, inhibition of IRS-1 Ser[789] and AMPK phosphorylation by dantrolene did not rescue the caffeine-induced downregulation of IRS-1 Tyr612 phosphorylation or 3MG transport. In addition, caffeine suppressed the phosphorylation of insulin-stimulated IRS-1 Ser[636/639] and upstream kinases, including the mammalian target of rapamycin and p70S6 kinase. Intravenous injection of caffeine at a physiological dose (5 mg/kg) in rats inhibited the phosphorylation of insulin-stimulated IRS-1 Tyr[612] and Akt Ser[473] in epitrochlearis muscle. Our results indicate that caffeine inhibits insulin signaling partly through the IKK/IRS-1 Ser[307] pathway, via a Ca[2+]- and AMPK-independent mechanism in skeletal muscle.