Activation of the Bone Morphogenetic Protein Signaling Pathway Induces Inhibin βB-Subunit mRNA and Secreted Inhibin B Levels in Cultured Human Granulosa-Luteal Cells

Abstract

During the human menstrual cycle the circulating levels of inhibin B, a dimer of inhibin α- and βB-subunits, fluctuate in a fashion distinct from that of inhibin A, the α-βA-subunit dimer. This suggests that human inhibin subunits are each regulated in a distinct manner in human ovarian granulosa cells by endocrine and local factors. We have previously shown using cultures of human granulosa-luteal (hGL) cells that gonadotropins stimulate the steady state mRNA levels of inhibin α- and βA-subunits, but not those of the βB-subunit, which, on the other hand, are up-regulated by, for instance, activin and TGFβ. We recently identified the TGFβ gene family member bone morphogenetic protein-3 (BMP-3) as a granulosa cell-derived growth factor, but whether BMP-3 or other structurally related BMPs regulate human granulosa cell inhibin production is not known. We show here that hGL cells express mRNAs for distinct serine/threonine kinase receptors (BMP-RIA and BMP-RII) and Smad signaling proteins (Smad1, Smad4, and Smad5) involved in the mediation of cellular effects of BMPs. Subsequently, we determined in hGL cell cultures the effects of distinct members of the BMP family previously found to be expressed in mammalian ovaries. Recombinant BMP-2 induces potently in a time- and concentration-dependent manner the expression of the inhibin βB-subunit mRNAs in hGL cells without affecting the levels of α- or βA-subunit mRNAs. BMP-6 has a similar, but weaker, effect than BMP-2, whereas BMP-3 and its close homolog, BMP-3b (also known as growth differentiation factor-10) had no effect on inhibin subunit mRNA expression. hCG treatment of hGL cells was previously shown to abolish the stimulatory effect of activin on βB-subunit mRNA levels, and here hCG is also shown to suppress the effect of BMP-2. Furthermore, BMP-2 stimulates hGL cell secreted dimeric inhibin B levels in a concentration-dependent manner. Depending on the experiment, maximal increases in inhibin B levels of 6- to 28-fold above basal levels were detected during a 72-h culture period. We conclude that activation of the BMP-signaling pathway in hGL cells stimulates inhibin βB-subunit mRNA levels and leads at the protein level to a dramatic stimulation of secreted inhibin B dimers. Our results are consistent with the suggestion that in addition to the distinct activin- and TGFβ-activated signaling pathways, the BMP-activated pathway is likely to be implicated in the complex regulation of inhibins in the human ovary.