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 biosynthes is of glycogen synthase under long-term control, we investigated post- transcriptional and post-translational regulation by insulin of glycog en synthase in rat hepatoma H4 cells. Glycogen synthase protein gradua lly increased in response to insulin and peaked at 6 h during a 24-h i nsulin 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 incorporati on of [S-35]-methionine into the enzyme protein during a 6-h insulin i ncubation. The amount of [S-35]-methionine incorporation into glycogen synthase was increased in response to insulin with time, and peaked a t 4 h (250% of the control). The degradation of glycogen synthase exam ined by measuring the rate of reduction of [S-35]-methionine for 6 h a fter the tracer incubation for 12 h was not affected by insulin. The r esults indicate that (a) insulin induces glycogen synthase activity by accumulating the enzyme protein due to stimulation of protein synthes is rather than inhibition of protein degradation and (b) insulin-reduc ed stability of glycogen synthase mRNA is caused by acceleration of th e translation rate.