Insulin stimulates protein synthesis of glycogen synthase in rat hepatoma H4 cells associated with acceleration of translation rate

To assess the regulatory mechanism of insulin action on the biosynthesis of glycogen synthase under long-term control, we investigated post-transcriptional and post-translational regulation by insulin of glycogen synthase in rat hepatoma H4 cells. Glycogen synthase protein gradually increased in response to insulin and peaked at 6 h during a 24-h insulin incubation (185% of the control), corresponding with the second peak of glycogen synthase activation measured by the activity ratio (low glucose 6-P/high glucose 6-P). The effect of insulin on synthesis of glycogen synthase protein was assessed by measuring the incorporation of [S-35]-methionine into the enzyme protein during a 6-h insulin incubation. The amount of [S-35]-methionine incorporation into glycogen synthase was increased in response to insulin with time, and peaked at 4 h (250% of the control). The degradation of glycogen synthase examined by measuring the rate of reduction of [S-35]-methionine for 6 h after the tracer incubation for 12 h was not affected by insulin. The results indicate that (a) insulin induces glycogen synthase activity by accumulating the enzyme protein due to stimulation of protein synthesis rather than inhibition of protein degradation and (b) insulin-reduced stability of glycogen synthase mRNA is caused by acceleration of the translation rate.