The TNF-family molecule, Receptor Activator of Nuclear factor κ B Ligand (RANKL) is known as a key regulator for bone remodeling, and is essential for the development and activation of osteoclasts. In this study, we examined the regulation of RANKL in primary human bone marrow adipocytes and the relationship between bone marrow adipocytes and bone metabolism. RANKL expression and the RANKL/osteoprotegerin (OPG) mRNA ratio in marrow adipocytes increased following dexamethasone treatment. In co-cultures of human osteoclast precursors and bone marrow adipocytes with dexamethasone, osteoclast precursors differentiated to TRAP-positive multinuclear cells. Moreover, the ability of bone resorption was confirmed in co-culture in flasks coated with calcium phosphate film. Osteoclast precursor differentiation and bone resorption were blocked by RANKL antibody pretreatment. TRAP-positive multinuclear cells did not form in coculture without cell-to-cell contact conditions. We conclude that primary human bone marrow adipocytes have the ability to promote osteoclast differentiation and activities, similar to osteoblasts and other RANKL-expressing cells.

There are various hypotheses about the function of bone marrow adipocytes, which are present in large quantities in the choke marrow space. Some hold that bone marrow adipocytes function as 1) a spacer between hematopoietic cells and bone tissue, 2) a fat metabolism organ, 3) an energy storehouse, or 4) support cells of hematopoiesis and bone metabolism (14). In contrast, it is clear that subcutaneous and visceral fats are not only simple spacers, but also energy storehouses. Subcutaneous and visceral fats also serve as metabolic organs secreting physiologically active substances named adipokines, which are associated with hypertension and metabolic syndrome