Bone development and remodeling depend on complex interactions between bone cells and other cells within the bone microenvironment. Balanced control of bone formative and degradative processes is normally carefully maintained in the adult skeleton, but becomes uncoupled during aging or in various pathological disease states. Systemic regulators of bone metabolism and local mediators, including matrix molecules, cytokines, leukotrienes, and other autocrine or paracrine factors, regulate the recruitment, differentiation, and function of cells participating in bone formation and turnover. Although some of these interactions are now understood, many yet remain to be elucidated, including potential communication pathways linking cells of the bone vasculature with other bone cells. The bone vasculature performs a crucial role in calcium balance, hematopoiesis, and bone development and remodeling processes, during which vascular endothelial cells may serve as both a source and target of diverse modulatory signals. The intimate physical relationship which is established during bone development and maintained thereafter between vascular endothelial cells and osteoclasts (of hematopoietic origin) enables vascular endothelial cells to potentially influence the development and/or function of the bone- resorbing osteoclast. Consistent with this theory, osteoclasts or their precursors are known to respond to factors like those made by vascular endothelial cells. Despite these observations, the influence of endothelial cells on osteoclasts has only recently begun to be directly addressed. Therefore, this proposal will investigate the hypothesis that vascular endothelial cells release factors that affect osteoclast development and/or function. In addition it is proposed that substances which increase bone loss (inflammatory cytokines, glucocorticoids) or those which reduce bone loss (estrogen) may, in part, do so through effects upon bone vascular endothelial cells which consequently alter osteoclast development or function. Isolated mature chick osteoclasts will be the target population for examining how endothelial cells affect osteoclast bone pit resorptive activities. Effects on osteoclast cytodifferentiation will be analyzed in chick marrow mononuclear cell cultures and a human preosteoclastic cell line (FLG 29.1), both capable of forming osteoclast-like giant cells in vitro. Osteoclast development will be monitored by measuring bone resorption capability, TRAP levels, and antigen expression by ELISA and cDNA probes. Mediators responsible for altered osteoclast development or activity will be identified through a combination of cytokine response assays, antibody neutralization and inhibitor studies, and purification schemes for potential novel factors. Modulation of the production or presentation of the endothelial-derived factor(s) through cell contact, or cytokine, hormonal, or glucocorticoid treatment will be assessed. Deciphering the signals by which vascular endothelial cells may affect osteoclasts, and thereby skeletal remodeling, could lead to effective preventative or therapeutic strategies to reduce the osteopenia associated with various pathologies such as inflammatory bone loss and osteoporosis.