Transcriptional regulation of the ovine follicle-stimulating hormone-beta gene by activin and gonadotropin-releasing hormone (GnRH): involvement of two proximal activator protein-1 sites for GnRH stimulation.

Previous studies from our laboratory demonstrated that a transgene consisting of the promoter for the ovine FSH beta-subunit gene and a luciferase reporter (wt-oFSHbetaLuc) was expressed and regulated like the FSHbeta gene in vivo and in vitro. In the present study pituitary cultures were prepared from these transgenic mice as well as mice carrying mutated oFSHbetaLuc lacking two functional activator protein-1 (AP-1) sites at -120 and -83 bp (mut-oFSHbetaLuc). These AP-1 sites were reported necessary for induction of oFSHbetaLuc by GnRH in a HeLa cell system. To examine the importance of the two AP-1 sites in mediating GnRH and activin effects in primary gonadotropes, pituitary cultures derived from transgenic mice were pretreated with follistatin to remove activin or activin-like factors present in the cultures. Follistatin lowered luciferase expression in cultures carrying both wt-oFSHbetaLuc and mut-oFSHbetaLuc transgenes by 74-86%, and subsequent addition of activin induced luciferase expression of both wt- and mut-transgenes by 4- to 14-fold within 4 h, suggesting that these AP-1 sites are not involved in activin stimulation of FSHbeta gene transcription. When GnRH was added along with activin, the wt-oFSHbetaLuc transgene was induced 200% compared with activin alone, but this effect was not observed with the mut-oFSHbetaLuc transgene. These data confirmed the HeLa cell studies showing that GnRH signals through two AP-1 sites to increase oFSHbeta transcription in gonadotropes. However, as the mutation of both AP-1 sites had no apparent effect on the expression and regulation of the transgene in vivo (basal, castration, GnRH down-regulation, cycle stage, and GnRH immunoneutralization), it appears that these AP-1 sites have little influence over the major effect of GnRH observed in vivo. These data also showed that activin plays a major role in transcriptional regulation of the FSHbeta gene, and the oFSHbeta promoter contains the activin response element(s) that is as yet undefined.