Regulation of glycoprotein hormone alpha-subunit gene expression.

Abstract

The appropriate, regulated expression of the glycoprotein hormone subunit genes is required to enable the biosynthesis of luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone, and chorionic gonadotropin. We have focused our attention on mechanisms mediating regulated transcription of the common alpha-subunit gene. Our studies have examined both the signaling mechanisms and the DNA elements and transcription factors that are important for alpha-subunit expression. Our initial efforts involved characterization of DNA elements of the alpha-subunit gene important for basal and GnRH-stimulated expression. Clustered point mutation analysis identified two different, unrelated sequences that play a role in alpha-subunit transcription. When tested as multiple copies on a minimal promoter, one of these elements was sufficient to permit a response to GnRH, while the other enhanced basal expression. Therefore, we designated these DNA elements as the GnRH-response element (GnRH-RE) and the pituitary glycoprotein hormone basal element (PGBE), respectively. The GnRH-RE contains a consensus binding site for the Ets family of transcription factors. As several Ets factors have been shown to mediate transcriptional responses to the mitogen-activated protein kinase (MAPK) pathway, we investigated the possibility that GnRH effects on alpha-subunit transcription may involve the MAPK cascade. We found that GnRH can indeed activate MAPK and that MAPK activation is sufficient and necessary for transcriptional activation of the alpha-subunit gene. Efforts to further characterize proteins that interact with the PGBE led to the cloning of a LIM-homeodomain transcription factor designated LH-2. Recombinant LH-2 selectively binds to the PGBE in vitro. Transfection experiments have shown that an expression vector for LH-2 can activate the alpha-subunit promoter in heterologous cells. LH-2 appears to be a component of the endogenous factors that bind to the PGBE. Thus, LH-2 appears to be an excellent candidate as a factor responsible for basal expression of the alpha-subunit gene. Overall, these studies have contributed to identification of molecular components important for regulated expression of the glycoprotein hormone alpha-subunit gene.