Regulation of calmodulin gene expression by insulin is both transcriptional and post-transcriptional

Previously we demonstrated that in streptozotocin-induced or spontaneously diabetic BB rats (BB-SDR), low-Km cyclic AMP (cAMP), phosphodiesterase (PDE), and calmodulin (CaM) are decreased. Isolated fat cells of diabetic animals synthesized less CaM and contained reduced levels of CaM transcripts (Solomon SS, Palazzolo MR, Green SA, Raghow R. Biochem Biophys Res Commun 1990; 168: 1007-12). Treatment of diabetic animals with insulin restores CaM transcripts to normal. RNA was extracted from isolated hepatocytes from BB-SDR rats in primary tissue culture treated with insulin (from 2.8 x 10(4) to 1.4 x 10(6) microU/ml) for 48 hours, was immobilized on nitrocellulose, and was sequentially hybridized with radiolabeled probes for CaM, actin, and tubulin. Insulin stimulates steady state levels of mRNA for calmodulin > actin > tubulin. Furthermore, 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-transcriptional effects of insulin on CaM mRNA. We postulate that in uncontrolled diabetes, elevations in levels of cAMP in tissue result in part from decreased activity of the apparently co-regulated PDE and CaM and that PDE inactivation in diabetes results from both insulin insufficiency and CaM down-regulation.