HIF-1α induces hypoxic apoptosis of MLO-Y4 osteocytes via JNK/caspase-3 pathway and the apoptotic-osteocyte-mediated osteoclastogenesis in vitro

Apoptotic osteocytes were found in the hypoxic bone microenvironment in osteoporosis, osteotomy, orthodontic tooth movement and periodontitis, and played a key role in bone remolding and the differentiation of osteoclasts. Hypoxia inducible factor-1 alpha(HIF-1 alpha), as a transcription factor under hypoxic conditions, has been confirmed to participate in cell apoptosis. However, the effect of HIF-1 alpha on osteocytes apoptosis and the osteocyte-mediated osteoclast formation remains elusive. Here, we hypothesized that HIF-1 alpha was involved in osteocytes apoptosis. Our results showed that CoCl2 increased the MLO-Y4 cells apoptosis by upregulating the proapoptotic gene expression of caspase-3. Moreover, siRNA-mediated knockdown of HIF-1 alpha decreased the phosphorylation by JNK and the activation of caspase-3 to inhibit the cell apoptosis in MLO-Y4. Furthermore, SP600125, an inhibitor of JNK, reversed CoCl2-induced the increased apoptosis of MLO-Y4 cells in term of reducing the expression of caspase-3. These findings revealed that HIF-1 alpha served as a pro-apoptotic factor in the apoptosis of MLO-Y4 cells cultured with CoCl2, by activating the JNK/caspase-3 signaling pathway. Besides, the osteocyte-mediated osteoclastogenesis was reduced with the decline of the expression of HIF-1 alpha and caspase-3 in MLO-Y4 cells. Our study provided an idea for a more comprehensive understanding of HIF-1 alpha and the process of bone remodeling.