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

Abstract

During orthodontic tooth movement, local hypoxia and enhanced osteoclastogenesis are observed in the compression side of periodontal tissues. The receptor activator of nuclear factor-κB 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-1α (HIF-1α) protein in PDLFs. Constitutively active HIF-1α alone significantly increased the levels of RANKL expression in PDLFs under normoxic conditions, whereas dominant negative HIF-1α blocked hypoxia-induced RANKL expression. To investigate further whether HIF-1α 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-1α significantly increased RANKL promoter activity, whereas dominant negative HIF-1α blocked hypoxia-induced RANKL promoter activity. Furthermore, mutations of putative HIF-1α binding elements in RANKL promoter prevented hypoxia-induced RANKL promoter activity. The results of chromatin immunoprecipitation showed that hypoxia or constitutively active HIF-1α increased the DNA binding of HIF-1α to RANKL promoter. These results suggest that HIF-1α 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.

Intermittent Hypoxia Effect on Osteoclastogenesis Stimulated by Neuroblastoma Cells

“…In addition, OPG is a decoy receptor for RANKL that regulates the functional state of RANKL [41]. A putative HIF-1α binding element resides at the RANKL promoter region [45] and induced HIF-1α was found to be associated with an increase in RANKL mRNA expression in intermittent hypoxia-exposed neuroblastoma cells. Earlier studies have shown that OPG expression level was down-regulated in hypoxic human oral squamous carcinoma cells and HIF-1α knock-down significantly increased OPG expression under hypoxic condition [46].…”

Compression and hypoxia play independent roles while having combinative effects in the osteoclastogenesis induced by periodontal ligament cells

“…Consistent with our results, HIF-1 was recently found to directly enhance RANKL expression in PDLCs, which might promote OC differentiation. 11 Moreover, our group previously found that osteogenesis of PDLCs was improved when exposed to hypoxia. 17,18 Taken together, these findings suggest that hypoxia accelerates alveolar bone remodeling by promoting osteogenic differentiation of PDLCs as well as improving the RANKL/OPG ratio, which leads to enhanced osteoclastogenesis.…”

“…9,10 Recently, HIF-1a was found to up-regulate RANKL expression in PDLCs. 11 Moreover, treatment of the inhibitor of HIF-1a suppressed the rate of tooth movement. 12 These findings illustrate that oxygen tension might be involved in osteoclastogenesis during OTM.…”

Role of the Hypoxia-Inducible Factor in Periodontal Inflammation

“…Chromatin immunoprecipitation showed that HIF-1α binds to the receptor activator of the NF-κB ligand (RANKL) promoter region, and mutations of the putative HIF-1α binding site prevented hypoxia-induced RANKL transcriptional promotion, thus suggesting that HIF-1α mediates hypoxia-induced upregulation of RANKL expression and enhanced osteoclastogenesis [95]. Furthermore, it was reported that hypoxia triggered the diferentiation of peripheral mononuclear blood cells into functional osteoclasts in a HIF-dependent manner [96].…”