Involvement of activin in the regulation of bone metabolism.

Osteogenic activities of activin, a member of TGF-beta superfamily, have been shown in both in vivo and in vitro studies. Local injection of activin promoted fracture healing in rat fibula fracture models. Since both activin and its receptor are expressed during fracture healing, activin would be involved in the healing process via autocrine and/or paracrine mode of action. Activin was abundantly stored also in normal bone matrix, presumably produced by osteoblasts in the process of normal bone formation. It was observed that activin was released in the culture of neonatal mouse calvaria, and the release was strongly coupled with bone resorption. Thus, activin could be involved in the regulation of bone remodeling as one of coupling factors, as was suggested for TGF-ss. Systemic administration of activin in aged ovariectomized rats, in which bone mass decreases due to uncoupling between bone resorption and formation, increased both bone mass and mechanical strength of vertebral bodies. These findings suggest physiological roles of activin in the regulation of bone formation, and further, its possible usefulness for the therapy of fracture and osteoporosis.