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

Mario Maira,Catherine Couture,Gwendal Le Martelot,Anne-Marie Pulichino,Steve Bilodeau,Jacques Drouin

doi:10.1074/jbc.m305626200

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.

Highlights

Tpit is a member of the T-box family of transcription factors
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
We have demonstrated that nuclear receptors (NRs) of the NGFI-B subfamily (Nur factors) are important mediators of corticotropin-releasing hormone (CRH) action on POMC, and at least part of the CRH effects are mediated through their binding element in the promoter, the NurRE (16 –18)

Summary

Introduction

Tpit ( known as Tbx19) is a member of the T-box family of transcription factors. These transcriptional regulators play critical roles in development, and the prototypical and founding member of the family is Brachyury or T [1,2,3]. Tpit activates POMC gene transcription in cooperation with the homeoprotein Pitx, and both transcription factors bind DNA as monomers at contiguous sites on the POMC promoter, within the Tpit/Pitx regulatory element (Tpit/Pitx-RE) [4]. We have demonstrated that nuclear receptors (NRs) of the NGFI-B subfamily (Nur factors) are important mediators of CRH action on POMC, and at least part of the CRH effects are mediated through their binding element in the promoter, the NurRE (16 –18). In adrenal-derived Y1 cells adrenocorticotropin treatment leads to dephosphorylation of Ser316 and unmasking of DNA binding activity, with a concomitant hyperphosphorylation of its N terminus that results in increased transcriptional activity [21, 22]. We have demonstrated that CRH treatment of AtT-20 cells leads to dephosphorylation of Ser316, playing a permissive role for subsequent NGFI-B dimer binding and co-activator recruitment [18]. SRC-2 has been shown to function as a co-repressor for the glucocorticoid receptor GR [27], and different domains of SRC-2 are implicated in coactivator versus co-repressor activities [28]

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