Hypoxia Exacerbates Periapical Periodontitis‐Associated Pathological Bone Loss via the Hypoxia‐Inducible Factor‐2α‐Calmodulin‐Dependent Protein Kinase <scp>IV</scp> Axis

Periapical periodontitis is one of the most common inflammatory bone destructive diseases. Epidemiological evidence suggests that hypoxia exposure, such as that resulting from high-altitude exposure or sleep apnea syndrome, may be a significant risk factor that exacerbates the disease process. However, its specific role and the underlying molecular mechanisms remain unclear. In this study, we established a mouse model of periapical periodontitis under conditions of chronic hypoxia to evaluate its impact on pathological bone loss using micro-computed tomography, histological staining, and serum cytokine analysis. Furthermore, we explored the potential molecular regulatory mechanisms using in vitro osteoclast differentiation models, adeno-associated virus-mediated in vivo gene knockdown, and cleavage under targets and tagmentation (CUT&Tag) sequencing. Our study revealed that hypoxia exposure significantly aggravated alveolar bone resorption, osteoclast activation, and systemic inflammation in the mouse model of periapical periodontitis compared to normoxia. At the molecular level, hypoxia-inducible factor-1α (HIF-1α) showed a rapid but transient increase under hypoxia, whereas HIF-2α displayed a progressive and sustained elevation throughout osteoclast differentiation. These dynamics indicate that HIF-2α plays a more prominent role than HIF-1α in mediating the hypoxia-accelerated osteoclastogenic response. In vivo, local knockdown of HIF-2α in the periapical region markedly attenuated bone destruction exacerbated by hypoxia exposure. Further mechanistic investigation, combining CUT&Tag sequencing and functional validation experiments, revealed that HIF-2α mediates its pro-osteoclastogenic function by directly binding to the promoter region of the calmodulin-dependent protein kinase IV (Camk4) gene and activating its transcription. This study unveils that hypoxia exposure, acting as a critical environmental risk factor, functions as a 'synergistic amplifier' to enhance pathological osteoclastic responses in periapical periodontitis through the HIF-2α-CAMK4 regulatory axis. The findings deepen our understanding of periapical periodontitis and suggest that targeting HIF-2α or downstream pathways may be an adjunctive therapeutic strategy for hypoxia-associated inflammatory bone loss.