Chondroprotective effects of CDK4/6 inhibition via enhanced ubiquitin-dependent degradation of JUN in synovial fibroblasts

OBJECTIVE: Targeting synovial fibroblasts (SF) using a cyclin-dependent kinase (CDK) 4/6 inhibitor (CDKI) could be a potent therapy for RA via inhibition of proliferation and MMP-3 production. This study was designed to elucidate the mechanism of chondroprotective effects on SFs by CDK 4/6 inhibition. METHODS: CDK4/6 activity was inhibited using CDKI treatment or enhanced by adenoviral gene transduction. Chondroprotective effects were evaluated using a collagen-induced arthritis model (CIA). Gene and protein expression were evaluated with quantitative PCR, ELISA and Western blotting. The binding of nuclear extracts to DNA was assessed with an electrophoresis mobility shift assay. RNA-Seq was performed to identify gene sets affected by CDKI treatment. RESULTS: CDKI attenuated cartilage destruction and MMP-3 production in CIA. In RASFs, CDKI impaired the binding of AP-1 components to DNA and inhibited the production of MMP-1 and MMP-3, which contain the AP-1 binding sequence in their promoter. CDK4/6 protected JUN from proteasome-dependent degradation by inhibiting ubiquitination. The RNA-Seq analysis identified CDKI-sensitive inflammatory genes, which were associated with the pathway of RA-associated genes, cytokine-cytokine receptor interaction and IL-17 signalling. Notably, the AP-1 motif was enriched in these genes. CONCLUSION: The mechanism of chondroprotective effects by CDK4/6 inhibition was achieved by the attenuation of AP-1 transcriptional activity via the impaired stability of JUN. Because the pharmacologic inhibition of CDK4/6 has been established as tolerable in cancer treatment, it could also be beneficial in patients with RA due to its chondroprotective and anti-inflammatory effects.