Cell-Specific Transcriptional Regulation of Follicle-Stimulating Hormone- by Activin and Gonadotropin-Releasing Hormone in the L T2 Pituitary Gonadotrope Cell Model

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 LβT2 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 α- and β-subunits of LH, but was thought not to express FSH. Using RT-PCR, we show that these cells synthesize FSH β- subunit messenger RNA, which is induced by activin and inhibited by follistatin. Furthermore, in transient transfections an ovine FSHβ 5′-regulatory region (5.5 kb) confers LβT2 cell-specific expression to a reporter gene compared with other pituitary and nonpituitary cell lines. This FSHβ regulatory region responds to activin specifically in LβT2 cells, an effect that is blocked by follistatin. The LHβ, α-subunit, and GnRH receptor regulatory regions are induced by activin and blocked by follistatin. Furthermore, LβT2 cells express the components of the activin system, and addition of follistatin alone reduces FSHβ gene expression, demonstrating that an endogenous activin autocrine loop regulates FSH in these cells. In addition, GnRH stimulates both the FSHβ and LHβ regulatory regions, specifically in LβT2 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, LβT2 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.