Conditioned medium of estrogen-treated osteoblasts inhibits osteoclast maturation and function in vitro

The increase of bone resorption and reduction of bone mass in postmenopausal women can be prevented by treatment with estrogen. Although it is well established that estrogen treatment normalizes the increased bone turnover, the mechanism by which estrogen exerts its protective influence at the cellular and molecular level in bone remains elusive. It has been shown that osteoblasts are involved in osteoclast development and osteoclastic bone resorption. In this work we examine the effect of estrogen (E2) on osteoclast-mediated bone resorption via the medium conditioned by osteoblast cultures. The conditioned medium collected from osteoblast cultures without (CM) or with 0.1 nmol/L 17beta-estradiol (E-CM) was mixed in a 1:1 ratio with fresh osteoclast culture medium. Osteoclasts were isolated from the bone marrow of 3-day-old NMRI mice and cultured on bovine bone slices. The total number of multinucleated tartrate-resistant alkaline phosphatase (TRAP)-positive cells in cultures with CM and E-CM was similar to that of cells incubated in control medium. However, the number of osteoclasts containing more than three nuclei was significantly smaller in the cultures containing E-CM. The total area of resorption was only slightly decreased in cultures containing CM, but was markedly inhibited in cultures with E-CM. In osteoblast cultures, the production of interleukin (IL)-1 and IL-6, but not of TNF-alpha, was reduced by 0.1 nmol/L E2. Our data suggest that E2 treatment of osteoblasts decreases the production of factor(s) that induces osteoclast differentiation to multinucleated cells with a higher capacity for bone resorption.