Regulation of Pancreatic Islet Cell Replication: An Inquiry into the Controversy Regarding the Effects of Steroid Hormones*

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Abstract
The controversial issue of the effects of prednisolone and 17.beta.-estradiol on replication of fetal rat pancreatic islets in culture was studied using 32P and [3H]thymidine as probes for studying DNA synthesis. DNA synthesis was not affected by the steroid hormones, as was evident from the rate of incorporation of 32P into total DNA. Decreased incorporation of [3H]thymidine into DNA found in islets treated with either of these steroids seemed to reflect an inhibitory effect of these hormones on thymidine kinase, leading to decreased phosphorylation of labeled thymidine. In additon, the hormones stimulated the activity of thymidylate synthetase, thus enhancing the endogenous synthesis of thymidine and thereby diluting the specific activity of the [3H]thymidine added to the cultured islets. Further support for a lack of inhibition of growth of islet cells treated with steroid hormones was provided by the observation that prednisolone increased uridine kinase activity and RNA biosynthesis, boh of which may participate in the growth of cells preceding mitosis and (the latter) in protein hormone biosynthesis.