Regulation of gonadotropins by inhibin and activin

Inhibin, activin, and follistatin were first identified as gonadal hormones that could exert selective effects on follicle-stimulating hormone (FSH) secretion without affecting luteinizing hormone (LH). Although the primary source of inhibin remains the gonad, both activin and follistatin are produced in extragonadal tissues and can exert effects on FSH through an autocrine-paracrine mechanism. These proteins can effect the regulation of the gonadotropins at many levels. First, activin can directly stimulate FSH biosynthesis and release from the gonadotrope cells of the pituitary gland. Second, activin up-regulates gonadotropin-releasing hormone receptor (GnRHR) gene expression, leading to alterations in the synthesis and release of both gonadotropins in response to GnRH. Third, activin can stimulate GnRH release from GnRH neurons in the hypothalamus and thereby affect FSH and LH secretion. Both inhibin and follistatin can negatively regulate these effects by preventing activin binding to the activin receptor at the cell membrane and blocking activation of downstream signal transduction pathways. This review concentrates on the mechanisms through which inhibin, activin, and follistatin regulate the gonadotropins. We discuss the expression of inhibin/activin subunits and receptors throughout the hypothalamus and pituitary and their role in the regulation of FSH and LH. The mechanisms of inhibin and activin signaling are also reported, with particular attention to developments in our understanding of inhibin receptor action and activin-induced transcriptional regulation of the FSHbeta gene promoter. Finally, we present recent findings that other members of the transforming growth factor beta superfamily may also play an important role in transcriptional regulation of the pituitary gonadotropins.