Direct modulation of osteoblastic activity with estrogen

Estrogens play an important but poorly understood role in the maintenance of skeletal mass. Whereas the mechanisms of estrogen action on bone may be complex, the finding that osteoblasts express estrogen receptors suggests that this class of hormones exerts direct effects on bone cells. To understand how estrogens regulate osteoblastic function, the physiologically active estrogen metabolite 17 beta-estradiol was tested to determine its effects on the well characterized murine osteoblastic cell-line MC3T3-E1. Experiments were designed to identify the effects of estrogen on osteoblastic activities associated with both the formation and the resorption of bone. Estrogen treatment coordinately increased DNA content and alkaline phosphatase activity in MC3T3-E1 cells 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. The effect was also persistent, since alkaline phosphatase activity remained elevated for several days after withdrawal of the hormone. Estrogen increased the levels of messenger RNA for alkaline phosphatase and type-I collagen as well, and these effects also persisted after removal 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 osteotropic agents known to stimulate the resorption of bone: parathyroid hormone, prostaglandin E₂, and the beta-adrenergic agonist isoproterenol. Thus, estrogen promoted the expression of traits associated with the formation of bone while reducing cellular responsiveness to hormones that may trigger the resorption of bone. These effects on osteoblastic cells in vitro are consistent with demonstrated anabolic and anticatabolic effects of the hormone on bone in vivo. CLINICAL RELEVANCE: Estrogen deficiency is a recognized cause of postmenopausal loss of bone, and the administration of estrogen to perimenopausal women prevents 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 enhance their osteogenic activities but also attenuate resorption-stimulating pathways.