Hypoxia enhances osteoblast differentiation via osteoprotegerin upregulation

During orthodontic tooth movement (OTM), mechanical stress (MS) induces alveolar bone remodeling via osteogenesis on the tension side and osteoclastogenesis on the compression side. Changes in blood flow alter low-oxygen conditions on the compression side of the MS loading force. Piezos have recently been identified as mechanosensitive cation channels that are activated in osteoblasts during OTM; however, many aspects remain unclear regarding the relationship between expression of mechanoreceptors in osteoblasts during OTM and bone remodeling. Moreover, studies on the expression RANKL and OPG during osteoclastogenesis have focused on the periodontal ligament; however, the expression of RANKL and OPG in osteoblasts remains unclear. Expression of mechanoreceptors in hypoxia and compression using MC3T3-E1 cells mouse pre-osteoblasts, and changes in the expression of the osteogenesis-related molecules RANKL and OPG, using a CCK8 assay, a luciferase assay, quantitative RT-PCR, and Western blot analysis, were examined. The proliferation rate of MC3T3-E1 cells was minimally affected under hypoxic conditions and cobalt chloride (Co) treatment under pseudo-hypoxic conditions. Hypoxia and Co treatment induced the expression of hypoxia-related genes. Hypoxia also modestly upregulated the expression of osteogenesis-related molecules. Hypoxia transiently upregulated OPG, resulting in decrease the RANKL/OPG ratio, followed by upregulation of RANKL, whereas compression markedly increased this ratio via upregulation of RANKL. MC3T3-E1 cells expressed Piezo1 and TRPV4 as mechanosensors, and the expression of Piezo1 was markedly increased by hypoxia. Hypoxia enhances differentiation of osteoblasts via upregulation of OPG, and compression enhances osteoclast differentiation via upregulation of RANKL.