The Pit-1β domain dictates active repression and alteration of histone acetylation of the proximal prolactin promoter

Abstract

A critical problem in current molecular biology is to gain a detailed understanding of the molecular mechanisms by which related transcription factor isoforms with identical DNA sequence specificity mediate distinct transcription responses. Pit-1 and Pit-1β constitute such a pair of transcription factor isoforms. Pit-1 enhances the Ras signaling pathway to the prolactin promoter, and Pit-1β represses basal prolactin promoter activity as well as Ras signaling to the prolactin promoter in pituitary cells. We have previously demonstrated that the β-domain amino acid sequence dictates the transcriptional properties of Pit-1β. Here, we show that five hydrophobic β-domain residues are required for Pit-1 isoform-specific repression of Ras signaling, and we demonstrate that sodium butyrate and trichostatin A, pharmacological inhibitors of histone deacetylation, as well as viral Ski protein, a dominant-negative inhibitor of recruitment of N-CoR/mSin3 histone deacetylase complexes, specifically reverse β isoform-specific repression of Ras signaling. Moreover, we directly demonstrate, with a chromatin immunoprecipitation assay, that the Pit-1β isoform alters the histone acetylation state of the proximal prolactin promoter. This differential analysis of Pit-1/Pit-1β isoform function provides significant insights into the structural determinants that govern how different transcription factors with identical DNA sequence specificity can display opposite effects on target gene activity.