Glucocorticoids inhibit somatostatin gene expression through accelerated degradation of somatostatin messenger ribonucleic acid in human thyroid medullary carcinoma (TT) cells.

We previously reported that dexamethasone (DEX) induces dose-dependent biphasic effects on steady state somatostatin (SS) messenger RNA (mRNA) levels in normal rat islet and islet SS-producing tumor cells (1027B2), characterized by stimulation at low doses and marked inhibition at high doses. The stimulatory effect is transcriptionally mediated, whereas the molecular mechanism underlying DEX-induced suppression of SS mRNA levels is unknown. In the present study, we investigated these mechanisms in human thyroid medullary carcinoma (TT) cells, which exhibit only inhibition of SS mRNA with DEX. Cultured TT cells synthesized and secreted large quantities of SS-like immunoreactivity (content, 90 ng/10(6) cells; release, 18 ng/10(6) cells/24h). DEX produced a dose-dependent reduction of both SS-like immunoreactivity secretion and SS mRNA levels, with a maximum inhibition of 60% at 10(-6) M at 48 h. In time-course studies, DEX inhibition of SS function occurred after a lag period of about 12 h, suggesting a posttranscriptional mechanism. To exclude a transcriptional effect of DEX on the SS gene, chloramphenicol acetyltransferase (CAT) activity was determined in TT cells acutely transfected with SS promoter (-750 base pairs) ligated to the receptor CAT gene. No inhibition of CAT activity occurred with DEX (10(-6) M) for 48 h. Furthermore, DEX did not influence the rate of SS gene transcription determined by nuclear run-on assay compared to approximately 2-fold stimulation by cAMP. Actinomycin D (inhibitor of mRNA synthesis) reduced the size of the SS mRNA transcript and rendered it resistant to DEX-induced degradation when coincubated with DEX, but not when it was added after a delay of 12 h, indicating that DEX destabilizes SS mRNA by an active process requiring ongoing gene transcription. Cycloheximide (inhibitor of protein synthesis) reduced SS mRNA levels to the same level as DEX, suggesting that the two agents promote SS mRNA degradation through a common pathway. We conclude that glucocorticoids inhibit steady state SS mRNA levels in TT cells. This effect is not mediated through direct transcriptional inhibition of the SS gene. It requires transcription of another gene(s) whose product(s) accelerates SS mRNA degradation.