DIRECT MODULATION OF OSTEOBLASTIC ACTIVITY WITH ESTROGEN

Estrogens play an important but poorly understood role in the maintena nce of skeletal mass. Whereas the mechanisms of estrogen action on bon e may be complex, the finding that osteoblasts express estrogen recept ors suggests that this class of hormones exerts direct effects on bone cells. To understand how estrogens regulate osteoblastic function, th e physiologically active estrogen metabolite 17 beta-estradiol was tes ted to determine its effects on the well characterized murine osteobla stic cell-line MC3T3-E1. Experiments were designed to identify the eff ects of estrogen on osteoblastic activities associated with both the f ormation and the resorption of bone. Estrogen treatment coordinately i ncreased DNA content and alkaline phosphatase activity in MC3T3-E1 cel ls as much as twofold. The stimulatory effect on alkaline phosphatase was stereospecific, dose-dependent between 0.1 and ten nanomolar, and dependent on the time in culture when the hormone was administered. Th e effect was also persistent, since alkaline phosphatase activity rema ined elevated for several days after withdrawal of the hormone. Estrog en increased the levels of messenger RNA for alkaline phosphatase and type-I collagen as well, and these effects also persisted after remova l of the hormone. The levels of messenger RNA for osteopontin, another bone-matrix protein, were only slightly affected by estrogen. Finally , estrogen inhibited the activation of adenylate cyclase by three oste otropic agents known to stimulate the resorption of bone: parathyroid hormone, prostaglandin E(2), and the beta adrenergic agonist isoproter enol. Thus, estrogen promoted the expression of traits associated with the formation of bone while reducing cellular responsiveness to hormo nes that may trigger the resorption of bone. These effects on osteobla stic cells in vitro are consistent with demonstrated anabolic and anti catabolic effects of the hormone on bone in vivo. CLINICAL RELEVANCE: Estrogen deficiency is a recognized cause of postmenopausal loss of bo ne, and the administration of estrogen to perimenopausal women prevent s or delays the onset of osteoporosis. Well characterized osteoblastic cell-lines such as MC3T3-E1 enable investigators to assess mechanisms of estrogen action at the cellular level. The findings in this study show that, while estrogen may regulate osseous metabolism by multiple mechanisms, its direct effects on osteoblastic cells may not only enha nce their osteogenic activities but also attenuate resorption-stimulat ing pathways.