G(i) alpha 2- and G(o) alpha-mediated signaling in the Pit-1-dependent inhibition of the prolactin gene promoter. Control of transcription by dopamine D2 receptors.

Dopaminergic signaling in pituitary lactotrophs is dependent on coupling of D2 receptors to several inhibitory G-protein subtypes, resulting in the activation of multiple signaling pathways. In prolactin-secreting GH4 cells that express cloned D2 receptors, dopamine selectively inhibits the activity of the prolactin gene promoter, a response mediated in part by the pituitary transcription factor Pit-1. Transfected gain-of-function mutants of the G alpha subtypes, Gi alpha 2 (Q205L) and G(o) alpha (Q205L), mimic the promoter-specific and Pit-1-dependent inhibition by dopamine. Whereas the activated Gi alpha 2 subtype suppresses cAMP levels, the G(o) alpha mutant does not, demonstrating a cAMP-independent pathway in the inhibition of the prolactin gene. This alternate pathway could involve other regulators, possibly calcium. Interestingly, in Ltk- cells in which cloned D2 receptors modestly suppress cAMP, but elevate [Ca2+]i, the activity of the prolactin promoter is enhanced rather than inhibited by dopamine. The response is promoter-specific, dependent on Pit-1, and completely blocked by low concentrations of EGTA, consistent with a calcium-regulated pathway. Last, in GH4 cells, the absence of additivity between Gi alpha 2 and G(o) alpha mutants suggests a convergent mechanism in the reduction of prolactin promoter activity, in which either signaling pathway may be sufficient for maximum inhibition. This apparent redundancy in inhibitory control mechanisms may be of physiological importance for maintaining efficient tonic suppression of prolactin synthesis.