Synergy between Activin A and Gonadotropin-Releasing Hormone in Transcriptional Activation of the Rat Follicle-Stimulating Hormone-β Gene

Both activin and GnRH can independently stimulate expression of the FSHβ subunit gene. In this study, we used the gonadotrope-derived LβT2 cell line to investigate the potential interaction between activin and GnRH in regulating the transcriptional activity of the rat FSHβ gene promoter. Activin A and GnRH synergistically enhanced rat FSHβ transcriptional activity. Overexpression of SMAD3 (mediator of decapentaplegic-related protein 3), but not of SMAD2, increased transcriptional activation of the rat (r) FSHβ gene promoter, which was further enhanced by the combined overexpression of SMAD3 and 4 (3+4). The stimulatory effects of SMAD3 overexpression were localized to −472/−256 of the rFSHβ gene promoter, and activin- and GnRH-responsive proteins were shown to bind to region −284/−252. Sequence analysis identified a consensus palindromic SMAD-binding site at −266/−259 of the rFSHβ gene promoter. Mutation of two bases located in the center of this palindrome effectively abrogated SMAD4 binding, markedly reduced SMAD3 and 3+4 stimulation of the rFSHβ gene promoter, and significantly decreased the synergistic enhancement of promoter activity by both activin A and GnRH, and SMAD3 and GnRH. Blockade of the MAPK-signaling pathway did not significantly affect the response to combined stimulation with activin and GnRH. In contrast, interference with SMAD3 signaling caused a significant reduction in activin and GnRH synergy. The results indicate that SMAD3 plays an important role in the synergistic effects of activin and GnRH and demonstrate that this synergy is mediated by a palindromic cis-element located at −266/−259 of the rFSHβ gene promoter.