Bone Abstracts

Osteocytes may function as mechanotransducers by regulating local osteoclastogenesis. Reduced availability of oxygen, i.e. hypoxia could occur during disuse, bone development and fracture. Receptor activator of nuclear factor-κB ligand (RANKL) is an osteoblast/stromal cell derived essential factor for osteoclastogenesis. Hypoxia induced osteoclastogenesis via increased RANKL expression in osteoblasts was demonstrated. Hypoxic regulation of gene expression generally involves activation of the hypoxia-inducible factor (HIF) transcription pathway. In the present study, we investigated whether hypoxia regulates RANKL expression in murine osteocytes and HIF-1α mediates hypoxia-induced RANKL expression by transactivating RANKL promoter to elucidate the role of osteocyte in osteoclastogenesis in the context of hypoxic condition.

Expression levels of RANKL mRNA and protein as well as hypoxia inducible factor-1α (HIF-1α) protein significantly increased in hypoxic condition in MLO-Y4s. Constitutively active HIF-1α alone significantly increased the levels of RANKL expression in MLO-Y4s under normoxic conditions, whereas dominant negative HIF-1α blocked hypoxia-induced RANKL expression. To further explore if HIF-1α directly regulates RANKL transcription, a luciferase reporter assay was conducted. Hypoxia significantly increased RANKL promoter activity, whereas mutations of putative HIF-1α binding elements in RANKL promoter prevented this hypoxia-induced RANKL promoter activity in MLO-Y4s.

These results suggest that HIF-1α mediates hypoxia-induced up-regulation of RANKL expression and that in osteocytes in mechanically unloaded bone, hypoxia enhances osteoclastogenesis, at least in part, via an increased RANKL expression in osteocytes.

Disclosure: The authors declared no competing interests.