Insulin and exercise decrease glycogen synthase kinase-3 activity by different mechanisms in rat skeletal muscle.

Glycogen synthase activity is increased in response to insulin and exercise in skeletal muscle. Part of the mechanism by which insulin stimulates glycogen synthesis may involve phosphorylation and activation of Akt, serine phosphorylation and deactivation of glycogen synthase kinase-3 (GSK-3), leading to dephosphorylation and activation of glycogen synthase. To study Akt and GSK-3 regulation in muscle, time course experiments on the effects of insulin injection and treadmill running exercise were performed in hindlimb skeletal muscle from male rats. Both insulin and exercise increased glycogen synthase activity (%I-form) by 2-3-fold over basal. Insulin stimulation significantly increased Akt phosphorylation and activity, whereas exercise had no effect. The time course of the insulin-stimulated increase in Akt was closely matched by GSK-3alpha Ser(21) phosphorylation and a 40-60% decrease in GSK-3alpha and GSK-3beta activity. Exercise also deactivated GSK-3alpha and beta activity by 40-60%. However, in contrast to the effects of insulin, there was no change in Ser(21) phosphorylation in response to exercise. Tyrosine dephosphorylation of GSK-3, another putative mechanism for GSK-3 deactivation, did not occur with insulin or exercise. These data suggest the following: 1) GSK-3 is constitutively active and tyrosine phosphorylated under basal conditions in skeletal muscle, 2) both exercise and insulin are effective regulators of GSK-3 activity in vivo, 3) the insulin-induced deactivation of GSK-3 occurs in response to increased Akt activity and GSK-3 serine phosphorylation, and 4) there is an Akt-independent mechanism for deactivation of GSK-3 in skeletal muscle.