Inhibitory control of prolactin and Pit-1 gene promoters by dopamine. Dual signaling pathways required for D2 receptor-regulated expression of the prolactin gene.

Transcription of the prolactin gene is suppressed by dopaminergic activation of D2 receptors in pituitary lactotrophs. The mechanism of signal transduction at the nuclear level and the cell surface was examined in the dopamine-responsive GH4ZR7 cell line. Dopamine treatment caused a 40-50% decrease in endogenous prolactin mRNA that was specifically blocked by addition of (-)-sulpiride. To define dopamine-responsive elements, plasmids containing 5'-regulatory regions of the prolactin gene fused to the coding sequences for luciferase were transiently or stably transfected into GH4ZR7 cells. Chimeric transcripts initiated at the authentic transcription start site were regulated in a promoter-selective manner; dopamine or the agonist bromocryptine inhibited prolactin promoter (position -422) activity by 70%, but had no regulatory effects on other cellular or viral promoters. A shorter prolactin promoter (position -78) or a prolactin TATAA box linked to heterologous binding sites for transcription factor Pit-1 was sufficient to confer dopamine inhibition (40%). In addition to the prolactin promoter, we found that dopamine inhibited transcriptional activity of the Pit-1 promoter (positions -258 to +8) by 60%. Surprisingly, deletion of two cAMP response elements in the Pit-1 promoter only partially eliminated dopamine responsiveness. These data suggest that sequences in the Pit-1 promoter between positions -92 and +8, which include an autoregulatory Pit-1-binding site and the TATAA box, are sufficient for negative regulation. In this study, we also examined the signal transduction pathways that link D2 receptor activation and the inhibition of prolactin gene transcription. We found, as suggested in earlier studies, that a dopamine-dependent decrease in cAMP may be important for mediating negative regulation of transcription. However, high extracellular K+ concentrations that prevent dopamine effects on membrane potential and [Ca2+]i, but not cAMP levels, completely blocked dopamine regulation of the prolactin promoter. This suggests that two distinct signaling pathways initiated at D2 receptors may be required for transcriptional regulation of the prolactin gene.