The T-box Factor Tpit Recruits SRC/p160 Co-activators and Mediates Hormone Action

Abstract

Tpit (Tbx19) is a transcription factor belonging to the T-box family, and it is essential for late differentiation of pituitary pro-opiomelanocortin (POMC)-expressing corticotroph and melanotroph cells. Tpit is also required, both in humans and mice, for cell-specific expression of the POMC gene in cooperation with the homeoprotein Pitx1. Despite their important roles as developmental regulators, the molecular mechanisms underpinning the functions of T-box factors in general, and of Tpit in particular, are still poorly defined. We now report that Tpit functions as an activator of transcription by recruiting SRC/p160 co-activators to its cognate DNA target in the POMC promoter, the Tpit/Pitx-RE. We also show that Tpit is a mediator of hormone signaling and that the Tpit/Pitx-RE is responsive to signals elicited by hypothalamic corticotropin-releasing hormone. These signals are mediated by the cAMP-dependent protein kinase and mitogen-activated protein kinase pathways, and activation of cAMP-dependent protein kinase also enhances Tpit and SRC-dependent transcription. We have previously shown that corticotropin-releasing hormone action is also exerted at the POMC promoter through the orphan nuclear receptor NGFI-B and its recruitment of SRC co-activators. Given that Tpit exhibits transcriptional synergy with NGFI-B, our results suggest that Tpit, along with NGFI-B and SRC-2, is part of a transcription regulatory complex assembled on the POMC promoter in response to hormonal stimulation.