Cell-specific transcriptional regulation of follicle-stimulating hormone-beta by activin and gonadotropin-releasing hormone in the LbetaT2 pituitary gonadotrope cell model.

Abstract

FSH is secreted by gonadotropes of the anterior pituitary and plays a crucial role in mammalian reproduction. However, little is known about FSH gene regulation due to the lack of a gonadotrope cell line that synthesizes FSH. The LbetaT2 mouse pituitary cell line, isolated by targeted tumorigenesis in transgenic mice, has the characteristics of a mature gonadotrope, including expression of GnRH receptor, steroidogenic factor 1, and both the alpha- and beta-subunits of LH, but was thought not to express FSH. Using RT-PCR, we show that these cells synthesize FSH beta- subunit messenger RNA, which is induced by activin and inhibited by follistatin. Furthermore, in transient transfections an ovine FSHbeta 5'-regulatory region (5.5 kb) confers LbetaT2 cell-specific expression to a reporter gene compared with other pituitary and nonpituitary cell lines. This FSHbeta regulatory region responds to activin specifically in LbetaT2 cells, an effect that is blocked by follistatin. The LHbeta, alpha-subunit, and GnRH receptor regulatory regions are induced by activin and blocked by follistatin. Furthermore, LbetaT2 cells express the components of the activin system, and addition of follistatin alone reduces FSHbeta gene expression, demonstrating that an endogenous activin autocrine loop regulates FSH in these cells. In addition, GnRH stimulates both the FSHbeta and LHbeta regulatory regions, specifically in LbetaT2 cells. Surprisingly, GnRH induction is reduced by follistatin, suggesting its dependence on endogenous activin. As the mouse GnRH receptor promoter is inhibited by follistatin, reduction of GnRH receptor levels might be one mechanism by which follistatin interferes with GnRH induction of gonadotropin genes. In summary, LbetaT2 cells exhibit the characteristics of fully differentiated gonadotropes, including the expression of LH, FSH, GnRH receptor, and components of the activin/follistatin system, as well as display the appropriate responses to activin and GNRH: