Histone demethylase PHF8 protected against chondrocyte injury and alleviated posttraumatic osteoarthritis by epigenetically enhancing WWP2 expression.

Aberrant mechanical forces were considered as an important factor for osteoarthritis (OA) pathogenesis. Plant homeodomain finger-containing protein 8 (PHF8) participated in osteogenic differentiation and inflammatory progression. However, the role of PHF8 in aberrant force-related OA remains to be elucidated. In this study, a fluid shear stress (FSS) model in ATDC5 cells and an anterior cruciate ligament transection (ACLT) animal model were constructed. The results revealed the decrease of PHF8 in aberrant force-induced cartilage damage in vitro and in vivo. PHF8 overexpression alleviated the aberrant force-induced cell apoptosis, extracellular matrix degradation, and inflammation. Chromatin immunoprecipitation (ChIP) assays demonstrated that PHF8 epigenetically regulated WWP2 expression through demethylating H3K9me2 at WWP2 promoter, which was influenced by FSS treatment. C-X-C chemokine receptor type 4 (CXCR4) was identified as a potential substrate of WWP2. Co-immunoprecipitation (Co-IP) and ubiquitination experiments further demonstrated WWP2 decreased the stability of CXCR4 via the ubiquitination pathway. Subsequently, rescue experiments validated reintroduction of WWP2 significantly attenuated the effects of PHF8 deletion on FSS-induced chondrocyte injury, and CXCR4 overexpression reversed the protective effects of WWP2 overexpression on chondrocyte injury in FSS-treated ATDC5 cells. Moreover, delivery of a PHF8 adeno-associated virus (AAV) into articular cartilage remarkably ameliorated the breakdown of cartilage matrix by ACLT in mice. In conclusion, our findings highlighted the importance of PHF8/WWP2/CXCR4 signaling pathway in aberrant force-induced cartilage injury, which might provide a novel insight on future epigenetic-based treatment of posttraumatic OA.