Hormonal Regulation of the Thyrotropin β-Subunit Gene by Phosphorylation of the Pituitary-Specific Transcription Factor Pit-1

The pituitary-specific transcription factor Pit-1 is a cell-specific activator of prolactin and growth hormone gene transcription in the anterior pituitary. Pit-1 has also been shown to mediate both thyrotropin-releasing hormone (TRH) and cAMP stimulation of the prolactin and thyrotropin β-subunit (TSHβ) genes. The molecular mechanism by which Pit-1 mediates these stimulatory effects remains unclear. At least three Pit-1-binding elements within the TSHβ gene mediate responsiveness to TRH and cAMP. The present studies were designed to test the hypothesis that phosphorylation is an important modulator of Pit-1 interaction with the TSHβ gene. TSHβ elements bind less well to nonphosphorylated Pit-1 than to phosphorylated Pit-1 and are weak activators of gene expression, unlike high-affinity Pit-1 binding sites in the prolactin and growth hormone genes. Phosphorylation by protein kinase A or C enhances Pit-1 binding to TSHβ elements 3- to 8-fold. Conversely, phosphorylation generally reduces binding of Pit-1 to elements within the prolactin and growth hormone genes. A variation within the consensus sequence for Pit-1 binding in TSHβ gene elements [(A/T)(A/T)AATNCAT in the TSHβ gene versus A(A/T)(A/T)TATNCAT in the prolactin and growth hormone genes] could explain these differences. These elements may limit basal activation of the TSHβ gene by binding less well to nonphosphorylated Pit-1 while conferring hormonal stimulation through enhanced binding of phosphorylated Pit-1.