Regulation of glucose transport and glycogen synthesis in L6 muscle cells during oxidative stress - Evidence for cross-talk between the insulin and SAPK2/p38 mitogen-activated protein kinase signaling pathways

We have investigated the cellular mechanisms that participate in reducing insulin sensitivity in response to increased oxidant stress in skeletal muscle. Measurement of glucose transport and glycogen synthesis in L6 myotubes showed that insulin stimulated both processes, by 2- and 5-fold, respectively. Acute (30 min) exposure of muscle cells to hydrogen peroxide (H2O2) blocked the hormonal activation of both these processes. Immunoblot analyses of cell lysates prepared after an acute oxidant challenge using phospho-specific antibodies against c-Jun N-terminal kinase (JNK), p38, protein kinase B (PKB), and p42 and p44 mitogen-activated protein (MAP) kinases established that H2O2 induced a dose dependent activation of all five protein kinases, In vitro kinase analyses revealed that 1 mM H2O2 stimulated the activity of JNK by similar to 8-fold, MAP-KAP-K2 (the downstream target of p38 MAP kinase) by similar to 12-fold and that of PKB by up to 34-fold. PKB activation was associated with a concomitant inactivation of glycogen synthase kinase-3, Stimulation of the p38 pathway, but not that of JNK, was blocked by SE 202190 or 8B203580, while that of p42/p44 MAP kinases and PKB was inhibited by PD 98059 and wortmannin respectively. However, of the kinases assayed, only p38 MAP kinase was activated at H2O2 concentrations (50 mu M) that caused an inhibition of insulin-stimulated glucose transport and glycogen synthesis. Strikingly, inhibiting the activation of p38 MAP kinase using either SE 202190 or SE 203580 prevented the loss in insulin-stimulated glucose transport, but not that of glycogen synthesis, by oxidative stress, Our data indicate that activation of the p38 MAP kinase pathway plays a central role in the oxidant-induced inhibition of insulin-regulated glucose transport, and unveils an important biochemical link between the classical stress-activated and insulin signaling pathways in skeletal muscle.