A Single Pitx1 Binding Site Is Essential for Activity of the LHb Promoter in Transgenic Mice

Reproduction depends on regulated expression of the LHb gene. Tandem copies of regulatory elements that bind early growth response protein 1 (Egr-1) and steroidogenic factor 1 (SF-1) are located in the proximal region of the LHb promoter and make essential contributions to its activity as well as mediate responsiveness to GnRH. Located between these tandem elements is a single site capable of binding the homeodomain protein Pitx1. From studies that employ overexpression paradigms performed in heterologous cell lines, it appears that Egr-1, SF-1, and Pitx1 interact cooperatively through a mechanism that does not require the binding of Pitx1 to its site. Since the physiological ramifications of these overexpression studies remain unclear, we reassessed the requirement for a Pitx1 element in the promoter of the LHb gene using homologous cell lines and transgenic mice, both of which obviate the need for overexpression of transcription factors. Our analysis indicated a striking requirement for the Pitx1 regulatory element. When assayed by transient transfection using a gonadotrope-derived cell line (LbT2), an LHb promoter construct harboring a mutant Pitx1 element displayed attenuated transcriptional activity but retained responsiveness to GnRH. In contrast, analysis of wild-type and mutant expression vectors in transgenic mice indicated that LHb promoter activity is completely dependent on the presence of a functional Pitx1 binding site. Indeed, the dependence on an intact Pitx1 binding site in transgenic mice is so strict that responsiveness to GnRH is also lost, suggesting that the mutant promoter is inactive. Collectively, our data reinforce the concept that activity of the LHb promoter is determined, in part, through highly cooperative interactions between SF-1, Egr-1, and Pitx1. While Egr-1 can be regarded as a key downstream effector of GnRH, and Pitx1 as a critical partner that activates SF-1, our data firmly establish that the Pitx1 element plays a vital role in permitting these functions to occur in vivo. (Molecular Endocrinology 15: 734–746, 2001)