Preferential role of calcium in the regulation of prolactin gene transcription by thyrotropin-releasing hormone in GH3 pituitary cells

TRH induces two separate events in pituitary PRL cells. It increases the release of stored PRL and enhances the rate of PRL gene transcription, which results in an increased steady state concentration of PRL messenger RNA (mRNA) and a concomitant augmentation of PRL production. The mechanisms underlying the release process involve the activation of phosphatidylinositol turnover which generates inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. In order to determine whether these intracellular messengers also mediate the stimulation of PRL gene expression by TRH, we have correlated the level of receptor occupancy with the rate of gene transcription and investigated the action of drugs which increase cytosolic calcium or activate protein kinase C. We have determined that sustained stimulation of transcription requires the persistent occupancy of a limited number of TRH receptor sites and that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), calcium ionophores (A23187, ionomycin), and the calcium channel agonist BAY K 8644 enhance PRL gene transcription. However, TPA is less potent and ionomycin requires a low concentration of TPA to fully mimic TRH action, whereas BAY K 8644 alone displays the same potency as TRH. The effects of BAY K 8644 and TRH are not additive and thus suggest that the influx of calcium plays a predominant role in the regulation of PRL gene transcription by TRH.