Autophagy regulates hypoxia‐induced osteoclastogenesis through the HIF‐1α/BNIP3 signaling pathway

<jats:title>Abstract</jats:title><jats:p>Previous studies have implicated that hypoxic stress could enhance osteoclast differentiation; however, the underlying mechanism remains poorly understood. Autophagy is a dynamic lysosomal degradation process that has emerged as an important regulator under hypoxic environment. In the present study, we demonstrate for the first time that autophagy regulates hypoxia‐induced osteoclastogenesis in vitro. We found that exposure of RAW264.7 cells to hypoxia (0.2% oxygen) resulted in enhanced osteoclast differentiation, accompanied by the observation of several specific features of autophagy, including appearance of membranous vacuoles, formation of acidic vesicular organelles, cleavage and recruitment of microtubule‐associated protein 1 light chain 3 (LC3) to autophagosomes, increase in autophagic flux, as well as up‐regulation of autophagy‐related gene (Atg) expression. Moreover, suppression of autophagy with DN‐Atg5<jats:sup>K130R</jats:sup> or 3‐methyladenine (3‐MA) significantly attenuated the osteoclast differentiation under hypoxic conditions, indicating the functional significance of autophagy in hypoxia‐induced osteoclastogenesis. The data also showed that the activation of autophagy under hypoxic conditions was caused by up‐regulated expression of hypoxia‐inducible factor‐1α (HIF‐1α)‐dependent Bcl‐2 adenovirus E1a 19 kDa interacting protein 3 (BNIP3). Importantly, knockdown of HIF‐1α or BNIP3 obviously abrogated hypoxia‐induced autophagy activation and osteoclastogenesis enhancement. Collectively, our results highlight the fact that autophagy is a pivotal regulator for hypoxia‐induced osteoclast differentiation, which may provide new insight into the pathological processes of osteoclastogenesis under hypoxic stress and help develop new therapeutic strategies for abnormal osteoclastogenesis. J. Cell. Physiol. 227: 639–648, 2012. © 2011 Wiley Periodicals, Inc.</jats:p>