Bone morphogenetic protein 2 and activin A synergistically stimulate follicle-stimulating hormone β subunit transcription

Transforming growth factor β superfamily ligands regulate pituitary FSH production and secretion. The best-described examples are the activins and inhibins, which respectively stimulate and hinder Fshb subunit transcription in gonadotrope cells. More recently, members of the bone morphogenetic protein (BMP) sub-family were shown to regulate FSH production in a manner analogous to the activins. Here, we used the murine gonadotrope cell line, LßT2, to investigate mechanisms through which BMP2 regulates the Fshb gene. Although expressed at low levels in LßT2 cells, Bmp2mRNA was readily detected in adult murine pituitary gland. Recombinant BMP2 stimulated Fshb promoter-reporter activity, although its effects were weaker than those of equimolar activin A or B. BMP4 stimulated transcription comparably with BMP2, but BMPs 6 and 7 were about tenfold less potent. Remarkably, BMP2 and activin A synergistically upregulated Fshb transcription and endogenous Fshb mRNA levels in LßT2 cells. Although functionally cooperative, the two ligands appeared to use distinct intracellular mechanisms to mediate their responses because neither ligand altered the timing or magnitude of the other's effects. Receptor overexpression analyses suggested that BMP2 may preferentially signal through complexes of the type II receptor, BMPR2, and the type I receptor, activin receptor like kinase (ALK2; Acvr1), to stimulate Fshb transcription. BMP2 rapidly activated the Smad1/5/8 intracellular signaling cascade and Smad8 overexpression potentiated BMP2's effects. In summary, BMPs regulate Fshb transcription in LßT2 cells and can amplify the already robust effects of the activins through a distinct signaling mechanism. Because BMP2 is expressed in the adult mouse pituitary, it may act as critical paracrine co-regulator of FSH synthesis by gonadotropes.