Regulation of growth hormone-releasing hormone receptor messenger ribonucleic acid expression by glucocorticoids in MtT-S cells and in the pituitary gland of fetal rats

Regulation of GH-releasing hormone receptor (GHRH-R) messenger RNA (mRNA) e xpression was studied, with the ribonuclease protection assay, in the fetal rat pituitary gland and in MtT-S clonal cells. GHRH-R mRNA was first detec ted on embryonic day (E)19 and increased rapidly thereafter, to reach a max imum at E21. Incubation of E17 or E18 pituitaries with 50 nM dexamethasone (DEX), a synthetic glucocorticoid, induced GHRH-R mRNA expression, suggesti ng that glucocorticoids play a pivotal role in the developmental expression of this mRNA. In E19 pituitaries, 24 h treatment with DEX increased GHRH-R mRNA by 60%, and GH mRNA by 76%, but did not affect pit-1 mRNA level, sugg esting that the effect of DEX is specific for expressions of GH mRNA and GH RH-R mRNA. The accumulation of GHRH-R mRNA by DEX was time dependent, and i t was slightly enhanced by the protein synthesis inhibitor, puromycin (100 mu M). In MtT-S cells (a pituitary cell line established from an estrogen-induced tumor), DEX induced GHRH-R mRNA expression within 2 h in a dose-dependent m anner. This induction was augmented by pu romycin (100 mu M) or cycloheximi de (3.5 mu M). However, the RNA synthesis inhibitor Actinomycin D (1 mu M) completely inhibited GHRH-R mRNA accumulation in response to either DEX or DEX plus puromycin, suggesting that glucocorticoids induce GHRH-R mRNA main ly through stimulation of mRNA transcription. These results suggest: that GHRH-R mRNA accumulation in the fetal pituitary gland of rats normally occurs at E19, probably because of the direct actio n of glucocorticoids on the pituitary gland, to stimulate GHRH-R mRNA. tran scription; and that the expression of glucocorticoid receptors is an import ant event in GH cell development in rats. Accordingly, immunocytochemical r esults suggest an increase in glucocorticoid receptors in immature GH cells between E17 and E18. The present results also imply that MtT-S cells may b e a good model in which to further study the molecular mechanisms of the re gulation of GHRH-R gene expression.