Pituitary homeobox 1 (Pitx1) stimulates rat LHβ gene expression via two functional DNA-regulatory regions

Abstract

Luteinizing hormone (LH) plays a central role in the reproductive axis, stimulating both gonadal steroid biosynthesis and the development of mature gametes. Over the past decade, significant progress has been made in characterizing the transcription factors and associated DNA-regulatory sites which mediate expression of the LH beta-subunit gene (LHbeta). One of these factors, pituitary homeobox 1 (Pitx1), has been shown to stimulate LHbeta gene promoter activity, both alone and in synergy with the orphan nuclear receptor, steroidogenic factor-1 (SF-1), and the early growth response gene 1 (Egr-1). Prior reports have attributed the Pitx1 response to a cis-element located at position -101 in the rat LHbeta gene promoter. While investigating the role of Pitx1 in regulating rat LHbeta gene expression, we observed a small, but significant, residual Pitx1 response despite mutation or deletion of this site. In the studies presented here, we identify the presence of a second functional Pitx1 region spanning positions -73 to -52 in the rat LHbeta gene promoter. Based on electrophoretic mobility shift assay, Pitx1 binds to both the initially described 5'Pitx1 site as well as this putative 3'Pitx1 region. In transient transfection analysis, mutation of the LHbeta-3'Pitx1 site significantly blunted Pitx1 responsiveness, with elimination of the Pitx1 response in a construct containing mutations in both Pitx1 cis-elements. We also analyzed the importance of each of these Pitx1 sites for providing functional synergy with SF-1 and with Egr-1. We observed a markedly decreased synergistic response with mutation of the 5'Pitx1 site with further loss following mutation of the 3'Pitx1 site. In contrast, functional interaction between Pitx1 and Egr-1 persisted with mutation of both Pitx1 regions. We conclude that Pitx1 stimulates the rat LHbeta gene promoter via two Pitx1 DNA-regulatory regions. These results further our understanding of the molecular mechanisms that regulate expression of this critical reproductive gene promoter.