THE EFFECTS OF STREPTOZOTOCIN-INDUCED DIABETES AND INSULIN SUPPLEMENTATION ON EXPRESSION OF THE GLYCOGEN-PHOSPHORYLASE GENE IN RAT-LIVER

We have previously observed that the chronic effects of streptozotocin -induced diabetes cause a decrease in the total hepatic glycogen phosp horylase activity with a corresponding reduction in the phosphorylase protein levels. These effects were normalized by insulin administratio n to diabetic rats. There was no change in the total glycogen synthase activity as a result of diabetes or insulin supplementation. These re sults are extended to examine the effects of diabetes and insulin admi nistration to diabetic animals on the expression of phosphorylase and glycogen synthase enzymes. The expression (i.e. mRNA levels) of phosph orylase was down-regulated (45% of normal levels) in diabetic livers, and this was normalized by insulin supplementation to diabetic animals . Diabetes or insulin supplementation to diabetic rats showed no effec t on the transcription rate of phosphorylase. As expected, diabetes (o r insulin administration to diabetic animals) did not cause any altera tion in the mRNA levels or in the transcription rate of hepatic glycog en synthase. The stability of phosphorylase mRNA was then examined usi ng hepatocytes prepared from normal and diabetic rats. Diabetes caused a decrease in the half-life of phosphorylase mRNA from 14 h in normal hepatocytes to 6.5 h in diabetic hepatocyte. Insulin supplementation to the medium of diabetic hepatocytes increased the half-life of phosp horylase mRNA to a level comparable with normal values. This study ind icates that the chronic effect of insulin on the activation of the tot al hepatic phosphorylase activity (and protein) is mediated through th e stabilization of its mRNA levels.