Endocrine Abstracts

The glycoprotein hormone common alpha-subunit (alphaGSU) is an early endocrine marker of pituitary development and is expressed concurrently with the orphan nuclear receptor, steroidogenic factor-1 (SF-1). Our studies have examined the potential role of SF-1 in alphaGSU gene expression using two gonadotroph-derived cell lines as models of the developing pituitary.

We investigated the role of SF-1 and its binding site, the gonadotroph-specific element (GSE) in mediating human alphaGSU promoter activity by mutating the GSE to abolish SF-1 binding. This mutation, when introduced to an alphaGSU-luciferase reporter gene, halved basal promoter activity in alphaT3-1 cells and in the more differentiated LbetaT2 gonadotroph. The GSE mutant was also significantly less responsive than the wild-type promoter when stimulated with PACAP and other cAMP/PKA activators in alphaT3-1 cells or by protein kinase C (PKC)/mitogen activated protein kinase (MAPK) activation by gonadotrophin-releasing hormone (GnRH) in LbetaT2 cells. These data strongly support a role for the GSE in regulating basal and hormone-stimulated transcription of the alphaGSU gene.

SF-1-negative GH3 cells were co-transfected with human alphaGSU promoter and a wild-type SF-1 expression vector, resulting in a doubling of basal and cAMP-stimulated promoter activity. These effects were absent when a S203A mutant SF-1 expression vector was co-transfected with the alphaGSU promoter. This serine residue is a putative MAPK phosphorylation site, and these results suggest SF-1 phosphorylation is required for basal and cAMP-stimulated promoter activity. We subsequently found PACAP to stimulate MAPK activation in alphaT3-1 cells, and to enhance SF-1 phosphorylation at Ser203.

Our data shows SF-1 and the GSE regulate basal and GnRH/PACAP-stimulated alphaGSU transcription in developing gonadotroph models. Thus, SF-1 expression is not only required for the initial development of the gonadotroph lineage but also plays an important role in the regulation of alphaGSU gene expression in the more developed gonadotroph.