Background/Purpose: Hypoxia is a major feature of the inflamed rheumatoid arthritis synovial membrane and promotes osteoclasts formation in vitro. We aimed to investigate the molecular mechanisms by which hypoxia contribute to bone destruction in the presence of local inflammation.

Methods: Osteoblast-like cells were cultured in normoxic (21% O₂) or hypoxic (1% O₂) conditions with or without tumor necrosis factor (TNF)-α. Receptor activator of the NF-kB ligand (RANKL) and osteoprotegerin (OPG) were detected by rtPCR, Western blot and ELISA. siRNA deletion of HIF-1a, HIF-2a and VHL was performed in osteoblasts. Interaction between hyocia inducible factor (HIF) and RANKL was investigated by promoter chromatin immunoprecipitation (ChIP). Chemical hypoxia using dimethyloxalylglycine (DMOG) and TNFα were tested on osteoclast formation from peripheral blood mononuclear cells of RA patients and on osteologic bone discs. Statistical analysis was performed using one-way ANOVA.

Results: Exposure of osteoblasts to hypoxia resulted in a significant increase in RANKL mRNA and cellular protein expression and a concomitant decrease of soluble OPG. Small interfering RNA against HIF-2α but not HIF-1α was able to abolish hypoxia effect on RANKL expression. Chromatin immunoprecipiation assay confirmed the direct interaction between HIF-2α with at least one hypoxia responsive element (HRE) in the RANKL promoter. Presence of TNFα had an additive effect with hypoxia to increase RANKL expression. Hypoxia mimicking by DMOG demonstrated an additional direct effect on osteoclastogenesis with an additive effect of hypoxia and TNFα to promote osteoclastogenesis and bone resorption in vitro. 

Conclusion: Hypoxia promotes HIF-2a dependent RANKL up-regulation and osteoclastogenesis and has an additive effect with pro inflammatory cytokines to promote bone destruction.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/evidence-for-an-additive-effect-of-tumor-necrosis-factor-alpha-and-hypoxia-to-promote-bone-destruction-in-arthritis/