Activins (ACTs) are growth factors in the TGFbeta superfamily whose effects are antagonized by inhibin (INH) and follistatin (FS) through different mechanisms. Although these components comprise a complex interactive regulatory loop in the anterior pituitary to exert endocrine, paracrine, and autocrine regulation of follicle stimulating hormone (FSH) production, the contribution of these molecules to bone remodeling has not been previously addressed. Preliminary results leading to this application demonstrate that INH suppression of osteoblastogenesis in ex vivo marrow cultures is also associated with the inhibition of osteoclastogenesis. ACT, on the other hand, promotes both osteoclastogenesis and osteoblastogenesis. Results also show a differential effect of inhibin and follistatin on osteoblastogenesis and osteoclastogenesis: INH acts on early osteoblast progenitors to prevent their commitment to pre-osteoblasts capable of expressing alkaline phosphatase (AP). INH suppression of osteoblastogenesis is associated with decreased osteoclastogenesis. Significantly, the maintenance of the early INH effect in the presence of exogenous ACT is demonstration of uniquely INH-specific effect which is unopposed by ACT, and is a likely mediated by INH-specific binding sites. FS prevents the full differentiation of committed pre-osteoblasts expressing AP to osteoblasts producing mineralized matrix, but does not suppress osteoclast development in primary bone marrow cultures. These in vitro effects are completely consistent with the presence of ovarian inhibin in the circulation (and thus accessible to bone marrow) serving to limit ACT-stimulated bone turnover in intact mice and in cycling women. Thus, the current studies will test the hypotheses that an integrated loop, similar to that regulating pituitary FSH, exists within the bone marrow to regulate bone remodeling, whereby the stimulatory effect of locally produced ACT is modulated by endocrine-derived INH and bone marrow- derived FS. Loss of gonadal INH by gonadectomy should increase tonic ACT action in the marrow, resulting in increased bone turnover. To test these hypotheses, experiments will define the temporal expression of and sensitivity to ACT, INH, and FS which lead to stage-specific effects on bone marrow osteoblastogenesis and osteoclastogenesis; determine the extent to which these effects are targeted directly toward cells of the osteoblast lineage; and determine the physiological relevance of these agents on bone turnover in vivo. The definition of the combined and respective contributions of these hormones during mouse osteoblastogenesis will provide insight into their potentially similar roles in cycling and post-menopausal women.
