The strength and integrity of our bones depends on maintaining a delicate balance between bone resorption by osteoclasts and bone formation by osteoblasts. As we age or as a result of disease, this delicate balancing act becomes tipped in favor of osteoclasts so that bone resorption exceeds bone formation, rendering bones brittle and prone to fracture. A better understanding of the biology of osteoclasts and osteoblasts is providing opportunities for developing therapeutics to treat diseases of bone. Drugs that inhibit the formation or activity of osteoclasts are valuable for treating osteoporosis, Paget's disease, and inflammation of bone associated with rheumatoid arthritis or periodontal disease. Far less attention has been paid to promoting bone formation with, for example, growth factors or hormones, an approach that would be a valuable adjunct therapy for patients receiving inhibitors of bone resorption.

Bone-resorbing osteoclasts are of hemopoietic cell origin, probably of the CFU-M-derived monocytemacrophage family (1). Osteoclasts are large multinucleated giant cells that express tartrate-resistant acid phosphatase (TRAP) activity and calcitonin receptors and have the ability to form resorption pits on dentine slices (2-4). In the process of osteoclast differentiation, there is an absolute requirement for cell-cell contact between osteoclast progenitors and bone marrow stromal cells or calvaria-derived osteoblasts (5-8).We developed a mouse coculture system of hemopoietic cells and primary osteoblasts to investigate osteoclast formation in vitro. In this coculture system, several systemic and local factors were capable of inducing osteoclast-like multinucleated cell (OCL) formation (6-9). These boneresorbing factors were classified into 3 categories according to their signal transduction pathways: (a) 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2 D 3 ] induced OCL formation via 1α,25(OH) 2 D 3 receptors (VDR) present in the nuclei; (b) parathyroid hormone (PTH), PTH-related protein (PTHrP), prostaglandin E 2 (PGE 2 ), and IL-1 induced OCL formation via the A kinase system; and (c) IL-11, oncostatin M, leukemia inhibitory factor, and IL-6 in the presence of soluble IL-6 receptors, all of which transduce their signals through a signal-transducing gp130 protein, also induced OCL formation in vitro. We reported previously that the target cells of IL-6 are osteoblasts/stromal cells but that they are not osteoclast precursors in inducing osteoclast differentiation (10). Similarly, coculture experiments using VDR knockout mice and PTH/PTHrP receptor knockout mice have indicated that the signals mediated by 1α,25(OH) 2 D 3 and PTH, respectively, are also transduced into osteoblasts/stromal cells, but not into osteoclast precursors, to induce osteoclast formation (11,12). Thus, it is concluded that the signals induced by all bone-resorbing factors are transduced into osteo-blasts/stromal cells to induce osteoclast formation. Our hypothesis proposes that osteoblasts/stromal cells express a critical common mediator named osteoclast differentiation factor (ODF), a membrane-bound factor that promotes differentiation of osteoclast progenitors into osteoclasts in response to various bone-resorbing factors through a mechanism involving cell-cell contact (6, 8). IL-17 is a newly discovered T cell-derived cytokine whose role in osteoclast development has not been fully elucidated. Treatment of cocultures of mouse hemopoietic cells and primary osteoblasts with recombinant human IL-17 induced the formation of multinucleated cells, which satisfied major criteria of osteoclasts, including tartrate-resistant acid phosphatase activity, calcitonin receptors, and pit formation on dentine slices. Direct interaction between osteoclast progenitors and osteoblasts was required for IL-17-induced osteoclastogenesis, which was completely inhibited by adding indomethacin or NS398, a selective inhibitor of cyclooxgenase-2 (COX-2). Adding IL-17 incre...

Osteoblasts/stromal cells are essentially involved in osteoclast differentiation and function through cell-to-cell contact (Fig. 8). Although many attempts have been made to elucidate the mechanism of the so-called "microenvironment provided by osteoblasts/stromal cells," (5-8) it has remained an open question until OPG and its binding molecule were cloned. The serial discovery of the new members of the TNF receptor-ligand family members has confirmed the idea that osteoclast differentiation and function are regulated by osteoblasts/stromal cells. RANKL, which has also been called ODF, TRANCE, or OPGL, is a member of the TNF ligand family. Expression of RANKL mRNA in osteoblasts/stromal cells is up-regulated by osteotropic factors such as 1 alpha, 25(OH)2D3, PTH, and IL-11. Osteoclast precursors express RANK, a TNF receptor family member, recognize RANKL through cell-to-cell interaction with osteoblasts/stromal cells, and differentiate into pOCs in the presence of M-CSF. RANKL is also involved in the survival and fusion of pOCs and activation of mature osteoclasts. OPG, which has also been called OCIF or TR1, is a soluble receptor for RANKL and acts as a decoy receptor in the RANK-RANKL signaling system (Fig. 8). In conclusion, osteoblasts/stromal cells are involved in all of the processes of osteoclast development, such as differentiation, survival, fusion, and activation of osteoclasts (Fig. 8). Osteoblasts/stromal cells can now be replaced with RANKL and M-CSF in dealing with the whole life of osteoclasts. RANKL, RANK, and OPG are three key molecules that regulate osteoclast recruitment and function. Further studies on these key molecules will elucidate the molecular mechanism of the regulation of osteoclastic bone resorption. This line of studies will establish new ways to treat several metabolic bone diseases caused by abnormal osteoclast recruitment and functions such as osteopetrosis, osteoporosis, metastatic bone disease, Paget's disease, rheumatoid arthritis, and periodontal bone disease.

Estrogen prevents osteoporotic bone loss by attenuating bone resorption; however, the molecular basis for this is unknown. Here, we report a critical role for the osteoclastic estrogen receptor alpha (ERalpha) in mediating estrogen-dependent bone maintenance in female mice. We selectively ablated ERalpha in differentiated osteoclasts (ERalpha(DeltaOc/DeltaOc)) and found that ERalpha(DeltaOc/DeltaOc) females, but not males, exhibited trabecular bone loss, similar to the osteoporotic bone phenotype in postmenopausal women. Further, we show that estrogen induced apoptosis and upregulation of Fas ligand (FasL) expression in osteoclasts of the trabecular bones of WT but not ERalpha(DeltaOc/DeltaOc) mice. The expression of ERalpha was also required for the induction of apoptosis by tamoxifen and estrogen in cultured osteoclasts. Our results support a model in which estrogen regulates the life span of mature osteoclasts via the induction of the Fas/FasL system, thereby providing an explanation for the osteoprotective function of estrogen as well as SERMs.