Estrogen deficiency causes bone loss, which can be prevented by estrogen replacement therapy. Using a recently developed technique for isolation of highly purified mammalian osteoclasts, we showed that 17 β-estradiol (E2) was able to directly inhibit osteoclastic bone resorption. At concentrations effective for inhibiting bone resorption, E2 also directly induced osteoclast apoptosis in a dose- and time-dependent manner. ICI164,384 and tamoxifen, as pure and partial antagonists, respectively, completely or partially blocked the effect of E2 on both inhibition of osteoclastic bone resorption and induction of osteoclast apoptosis. These data suggest that the protective effects of estrogen against postmenopausal osteoporosis are mediated in part by the direct induction of apoptosis of the bone-resorbing osteoclasts by an estrogen receptor– mediated mechanism.

IntroductionMultiple myeloma (MM) almost exclusively develops in the bone marrow and generates devastating bone destruction by osteoclasts (OCs) recruited around MM cells. A marked stimulation of osteoclastic bone resorption causes debilitating clinical symptoms, including intractable bone pain, disabling multiple fractures, and hypercalcemia. The severity of bone disease correlates with the tumor burden and is one of the major parameters in widely used Durie and Salmon clinical staging system. It is of note that the aggressive features of MM bone lesions have significantly contributed to its poor prognosis despite the recent development of intensive chemotherapeutic regimens. 1,2 Therefore, elucidation of the molecular mechanism of bone destruction and tumor progression is essential for the development of effective therapies to improve survival as well as quality of life of patients with MM.Interactions between receptor activator of nuclear factorkappaB (RANK) expressed on the surface of the OC lineage cells and RANK ligand expressed on stromal cells play a key role in the development and activation of OCs, whereas osteoprotegerin, a decoy receptor for RANK ligand secreted from stromal cells, inhibits RANK ligand-RANK signaling. 3-8 MM cells stimulate osteoclastogenesis by triggering a coordinated increase in RANK ligand and decrease in osteoprotegerin in bone marrow (BM) stromal cells. [9][10][11] We and others have demonstrated that osteoclastogenic CC chemokines macrophage inflammatory protein 1␣ (MIP-1␣) and MIP-1␤ are secreted from most of MM cells and play a critical role in the development of MM bone lesions. [12][13][14][15][16][17][18] These chemokines act on MM cells in an autocrine/paracrine fashion and enhance MM cell adhesion to stromal cells through activation of integrins, including very late antigen-4 (VLA-4). The interaction between MM and stromal cells then induces RANK ligand expression by stromal cells, leading to OC differentiation and activation. 12 Almost exclusive development of MM in the BM suggests that the BM microenvironment supports MM cell growth and survival. Among cell components in the BM, roles of stromal cells in MM cell growth and survival have been extensively studied. When cocultured with MM cells, stromal cells are stimulated to produce interleukin 6 (IL-6), which promotes proliferation of MM cells and prevents them from apoptosis induced by anticancer agents. [19][20][21] Other than stromal cells, OCs induced by MM cells are among major cellular components of the BM microenvironment. Administration of inhibitors of osteoclast activity, including bisphosphonates, RANK-Fc, and osteoprotegerin, not only prevented MMinduced bone destruction but also interfered with tumor progression in animal models of MM. 9,22-25 Repeated administration of bisphosphonates has also been reported to reduce the tumor burden without chemotherapy in a portion of patients with MM. 26 These observations raise a possibility that an interaction between OCs and MM Materials and methods ChemicalsThe f...