Receptor activator of NF-kappa B and osteoprotegerin expression by human microvascular endothelial cells, regulation by inflammatory cytokines, and role in human osteoclastogenesis

The receptor activator of NF-kappaB (RANKL) is the essen tial signal requir ed for full osteoclast (OC) development, activation, and survival. RANKL is highly expressed in areas of trabecular bone remodeling and inflammatory b one loss, is increased on marrow stromal cells or osteoblasts by osteotropi c hormones or cytokines, and is neutralized by osteoprotegerin (OPG), a sol uble decoy receptor also crucial for preventing arterial calcification. Vas cular endothelial cells (VEC) are critically involved in bone development a nd remodeling and influence OC recruitment, formation, and activity. Althou gh OCs develop and function in close association with bone VEC and sinusoid s, signals mediating their interactions are not well known. Here, we show f or the first time that human microvascular endothelial cells (HMVEC) expres s transcripts for both RANKL and OPG; inflammatory cytokines tumor necrosis factor-alpha and interleukin-1 alpha elevate RANKL and OPG expression 5-40 -fold in HMVEC (with an early OPG peak that declines as RANKL rises), and R ANKL protein increases on the surface of tumor necrosis factor-alpha -activ ated HMVEC. Cytokine-activated HMVEC promoted the formation, fusion, and bo ne resorption of OCs formed in co-cultures with circulating human monocytic precursors via a RANKL-mediated mechanism fully antagonized by exogenous O PG. Furthermore, paraffin sections of human osteoporotic fractured bone exh ibited increased RANKL immunostaining in vivo on VEC located near resorbing OCs in regions undergoing active bone turnover. Therefore, cytokine-activa ted VEC may contribute to inflammatory-mediated bone loss via regulated pro duction of RANKL and OPG. VEC-derived OPG may also serve as an autocrine si gnal to inhibit blood vessel calcification.