Originally, leptin was described as a product of adipocytes that acts on the hypothalamus to regulate appetite. However, subsequently, it has been shown that leptin receptors are distributed widely and that leptin has diverse functions, including promotion of hemopoietic and osteoblastic differentiation. It has been recognized for some time that both serum leptin and bone mass are correlated positively to body fat mass and, recently, we have shown a direct positive relationship between serum leptin and bone mass in nonobese women. We now report that leptin inhibits osteoclast generation in cultures of human peripheral blood mononuclear cells

Vitamin D acts through the immature osteoblast to stimulate osteoclastogenesis. Transgenic elevation of VDR in mature osteoblasts was found to inhibit osteoclastogenesis associated with an altered OPG response. This inhibition was confined to cancellous bone. This study indicates that vitamin D-mediated osteoclastogenesis is regulated locally by OPG production in the mature osteoblast.Introduction: Vitamin D stimulates osteoclastogenesis acting through its nuclear receptor (VDR) in immature osteoblast/stromal cells. This mobilization of calcium stores does not occur in a random manner, with bone preferentially removed from cancellous bone. The process whereby the systemic, humoral regulator is targeted to a particular region of the skeleton is unclear. Materials and Methods: Bone resorption was assessed in mice with vitamin D receptor transgenically elevated in mature osteoblasts (OSVDR). Vitamin D-mediated osteoclastogenesis was examined in vitro using OSVDR osteoblasts and osteoblastic RANKL: osteoprotegerin (OPG) examined in vivo and in vitro after vitamin D treatment. Results: Vitamin D-mediated osteoclastogenesis was reduced in OSVDR mice on chow and calciumrestricted diets, with effects confined to cancellous bone. OSVDR osteoblasts had a reduced capacity to support osteoclastogenesis in culture. The vitamin D-mediated reduction in OPG expression was reduced in OSVDR osteoblasts in vivo and in vitro, resulting in a reduced RANKL/OPG ratio in OSVDR compared with wildtype, after exposure to vitamin D. Conclusions: Mature osteoblasts play an inhibitory role in bone resorption, with active vitamin D metabolites acting through the VDR to increase OPG. This inhibition is less active in cancellous bone, effectively targeting this region for resorption after the systemic release of activated vitamin D metabolites.

Human osteoclasts can be efficiently generated in vitro from cord blood mononuclear cells and derived CFU-GM colonies. However, CFU-M colonies are poorly osteoclastogenic. Short-term (2-48 h) treatment with GM-CSF stimulates osteoclast formation by proliferating precursors, whereas longer exposure favors dendritic cell formation.Introduction: Osteoclasts (OC) differentiate from cells of the myelomonocytic lineage under the influence of macrophage-colony stimulating factor (M-CSF) and RANKL. However, cells of this lineage can also differentiate to macrophages and dendritic cells (DC) depending on the cytokine environment. The aims of this study were to develop an efficient human osteoclastogenesis model and to investigate the roles of granulocyte macrophage-colony stimulating factor (GM-CSF) and M-CSF in human OC differentiation. Materials and Methods: A human osteoclastogenesis model, using as precursors colony forming unit-granulocyte macrophage (CFU-GM) colonies generated from umbilical cord mononuclear cells cultured in methylcellulose with GM-CSF, interleukin (IL)-3 and stem cell factor (SCF), has been developed. CFU-GM, colony forming unitmacrophage (CFU-M), or mixed colonies were cultured on dentine with soluble RANKL (sRANKL) and human M-CSF with and without GM-CSF. Major endpoints were OC number, dentine resorption, and CD1a ϩ DC clusters. Results: Osteoclast generation from CFU-GM and mixed colonies treated with M-CSF and sRANKL for 7-14 days was highly efficient, but CFU-M colonies were poorly osteoclastogenic under these conditions. Pretreatment of precursors with M-CSF for 7 or 14 days maintained the precursor pool, but OCs were smaller and resorption was reduced. The effect of GM-CSF treatment was biphasic, depending on the timing and duration of exposure. Short-term treatment (2-48 h) at the beginning of the culture stimulated cell proliferation and enhanced OC formation up to 100%, independent of sRANKL. Longer-term GM-CSF treatment in the presence of sRANKL, however, inhibited OC generation with the formation of extensive CD1a ϩ DC clusters, accompanied by downregulation of c-Fos mRNA. Delaying the addition of GM-CSF resulted in progressively less inhibition of osteoclastogenesis. Conclusions: Human CFU-GM, but not CFU-M, progenitors have high osteoclastogenic potential. GM-CSF plays an important role in osteoclastogenesis and has a biphasic effect: Short-term treatment potentiates OC differentiation by proliferating precursors, but persistent exposure favors DC formation.