Hypoxia Inducible Factor-1α Directly Induces the Expression of Receptor Activator of Nuclear Factor-κB Ligand in Periodontal Ligament Fibroblasts

During orthodontic tooth movement, local hypoxia and enhanced osteoclastogenesis are observed in the compression side of periodontal tissues. The receptor activator of nuclear factor-kappa B ligand (RANKL) is an osteoblast/stromal cell-derived factor that is essential for osteoclastogenesis. In this study, we examined the effect of hypoxia on RANKL expression in human periodontal ligament fibroblasts (PDLFs) to investigate the relationship between local hypoxia and enhanced osteoclastogenesis in the compression side of periodontal tissues. Hypoxia significantly enhanced the levels of RANKL mRNA and protein as well as hypoxia inducible factor-1 alpha (HIF-1 alpha) protein in PDLFs. Constitutively active HIF-1 alpha alone significantly increased the levels of RANKL expression in PDLFs under normoxic conditions, whereas dominant negative HIF-1 alpha blocked hypoxia-induced RANKL expression. To investigate further whether HIF-1 alpha directly regulates RANKL transcription, a luciferase reporter assay was performed using the reporter vector containing the RANKL promoter sequence. Exposure to hypoxia or overexpression of constitutively active HIF-1 alpha significantly increased RANKL promoter activity, whereas dominant negative HIF-1 alpha blocked hypoxia-induced RANKL promoter activity. Furthermore, mutations of putative HIF-1 alpha binding elements in RANKL promoter prevented hypoxia-induced RANKL promoter activity. The results of chromatin immunoprecipitation showed that hypoxia or constitutively active HIF-1 alpha increased the DNA binding of HIF-1 alpha to RANKL promoter. These results suggest that HIF-1 alpha mediates hypoxia-induced up-regulation of RANKL expression and that in compression side periodontal ligament, hypoxia enhances osteoclastogenesis, at least in part, via an increased RANKL expression in PDLFs.