Recent studies demonstrate roles for osteoprotegerin (OPG) in both skeletal and extra-skeletal tissues. Although its role in preventing osteoclast (OC) formation and activity is well documented, emerging evidence suggests a role of OPG in endothelial cell survival and the prevention of arterial calcification. In this communication, we show that vascular endothelial cells in situ, and human umbilical vein endothelial cells (HUVEC) in vitro, express abundant OPG. In HUVEC, OPG co-localizes with P-selectin and von Willebrand factor (vWF), within the Weibel-Palade bodies (WPB). Treatment of HUVEC with the pro-inflammatory cytokines, tumor necrosis factor (TNF)-alpha and IL-1beta, resulted in mobilization from the WPBs and subsequent secretion of OPG protein into the culture supernatant. Furthermore, TNF-alpha treatment of HUVEC resulted in a sustained increase in OPG mRNA levels and protein secretion over the 24-h treatment period. Reciprocal immunoprecipitation experiments revealed that while not associated with P-Selectin, OPG is physically complexed with vWF both within the WPB and following secretion from endothelial cells. Interestingly, this association was also identified in human peripheral blood plasma. In addition to its interaction with vWF, we show that OPG also binds with high avidity to the vWF reductase, thrombospondin (TSP-1), raising the intriguing possibility that OPG may provide a link between TSP-1 and vWF. In summary, the intracellular localization of OPG in HUVEC, in association with vWF, together with its rapid and sustained secretory response to inflammatory stimuli, strongly support a modulatory role in vascular injury, inflammation and hemostasis.

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Histone deacetylase inhibitors (HDACi) suppress cancer cell growth, inflammation, and bone resorption. The aim of this study was to determine the effect of inhibitors of different HDAC classes on human osteoclast activity in vitro. Human osteoclasts generated from blood mononuclear cells stimulated with receptor activator of nuclear factor kappa B (RANK) ligand were treated with a novel compound targeting classes I and II HDACs (1179.4b), MS-275 (targets class I HDACs), 2664.12 (targets class II HDACs), or suberoylanilide hydroxamic acid (SAHA; targets classes I and II HDACs). Osteoclast differentiation was assessed by expression of tartrate resistant acid phosphatase and resorption of dentine. Expression of mRNA encoding for osteoclast genes including RANK, calcitonin receptor (CTR), c-Fos, tumur necrosis factor (TNF) receptor associated factor (TRAF)6, nuclear factor of activated T cells (NFATc1), interferon-β, TNF-like weak inducer of apoptosis (TWEAK), and osteoclast-associated receptor (OSCAR) were assessed. Expression of HDACs 1-10 during osteoclast development was also assessed. 1179.4b significantly reduced osteoclast activity (IC(50) < 0.16 nM). MS-275 (IC(50) 54.4 nM) and 2664.12 (IC(50) > 100 nM) were markedly less effective. A combination of MS-275 and 2664.12 inhibited osteoclast activity similar to 1179.4b (IC(50) 0.35 nM). SAHA was shown to suppress osteoclast activity (IC(50) 12 nM). 1179.4b significantly (P < 0.05) reduced NFATc1, CTR, and OSCAR expression during the later stages of osteoclast development. Class I HDAC 8 and Class II HDAC5 were both elevated (P < 0.05) during osteoclast development. Results suggest that inhibition of both classes I and II HDACs may be required to suppress human osteoclastic bone resorption in vitro.

OPG expression on macrophage type synovial lining cells as well as endothelial cells is deficient in RA patients with active synovitis, in contrast to that seen in spondyloarthropathy patients with active synovitis. This deficiency in OPG expression in the inflamed joint of RA patients may be important in the development of radiologically defined joint erosions.

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