REGULATION OF CALMODULIN GENE-EXPRESSION BY INSULIN IS BOTH TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL

Previously we demonstrated that in streptozotocin-induced or spontaneo usly diabetic BB rats (BB-SDR), low-K-m cyclic AMP (cAMP), phosphodies terase (PDE), and calmodulin (CaM) are decreased. Isolated fat cells o f diabetic animals synthesized less CaM and contained reduced levels o f CaM transcripts (Solomon SS, Palazzolo MR, Green SA, Raghow R. Bioch em Biophys Res Commun 1990;168:1007-12). Treatment of diabetic animals with insulin restores CaM transcripts to normal. RNA was extracted fr om isolated hepatocytes from BB-SDR rats in primary tissue culture tre ated with insulin (from 2.8 x 10(4) to 1.4 x 10(6) mu U/ml) for 48 hou rs, was immobilized on nitrocellulose, and was sequentially hybridized with radiolabeled probes for CaM, actin, and tubulin. Insulin stimula tes steady state levels of mRNA for calmodulin > actin > tubulin. Furt hermore, decreased steady state levels of CaM mRNA in hepatocytes from diabetic animals are restored to normal levels with in vitro insulin incubation. Data from nuclear transcription run-on assays demonstrate that insulin stimulates transcription of mRNA CaM by 80%. In addition, we observed RNA degradation in the untreated diabetic but not insulin -treated liver. These data support transcriptional as well as post-tra nscriptional effects of insulin on CaM mRNA We postulate that in uncon trolled diabetes, elevations in levels of cAMP in tissue result in par t from decreased activity of the apparently co-regulated PDE and CaM a nd that PDE inactivation in diabetes results from both insulin insuffi ciency and CaM down-regulation.