c-Jun N-terminal kinase – c-Jun pathway transactivates Bim to promote osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disorder. Previous studies have shown abnormally increased apoptosis of chondrocytes in patients and animal models of OA. TNF-α and nitric oxide have been reported to induce chondrocyte ageing; however, the mechanism of chondrocyte apoptosis induced by IL-1β has remained unclear. The aim of this study is to identify the role of the c-Jun N-terminal kinase (JNK) – c-Jun pathway in regulating induction of Bim, and its implication in chondrocyte apoptosis. This study showed that Bim is upregulated in chondrocytes obtained from the articular cartilage of OA patients and in cultured mouse chondrocytes treated with IL-1β. Upregulation of Bim was found to be critical for chondrocyte apoptosis induced by IL-1β, as revealed by the genetic knockdown of Bim, wherein apoptosis was greatly reduced in the chondrocytes. Moreover, activation of the JNK–c-Jun pathway was observed under IL-1β treatment, as indicated by the increased expression levels of c-Jun protein. Suppression of the JNK–c-Jun pathway, using chemical inhibitors and RNA interference, inhibited the Bim upregulation induced by IL-1β. These findings suggest that the JNK–c-Jun pathway is involved in the upregulation of Bim during OA and that the JNK–c-Jun–Bim pathway is vital for chondrocyte apoptosis.