EYA1 regulates CIITA phosphorylation to promote EYA1–CIITA–Runx2 complex formation and extracellular matrix integrity in osteoarthritis

BACKGROUND: A demethylase ALKBH5 stabilizes Runx2 mRNA in chondrocytes via YTHDF1, thereby increasing MMP and ADAMTS expression, causing ECM degradation. However, the transcriptional co-regulators of Runx2 remain unidentified. METHODS: OA was induced in mice via injection of sodium monoiodoacetate (MIA). The histological architecture and proteoglycan content of the articular cartilage were evaluated using H&E and Safranin O-Fast Green staining, while subchondral bone microarchitecture was assessed by micro-computed tomography. Circulating proinflammatory cytokine levels were determined by ELISA. RESULTS: Immunoprecipitation and mass spectrometry analyses of articular cartilage from MIA-induced OA mice identified the transcriptional coactivator EYA1 (Eyes Absent Homolog 1) and CIITA (Class II Major Histocompatibility Complex Transactivator) as novel Runx2-interacting proteins. In vitro and in vivo experiments demonstrated that EYA1, CIITA, and Runx2 form a functional EYA1-CIITA-Runx2 complex that binds to the promoters of MMP and ADAMTS genes and activates their expression. Silencing any member of the complex significantly reduced transcription of these matrix-degrading enzymes. Furthermore, we identified EYA1 as a phosphatase that dephosphorylates CIITA at serine 782, a phosphorylation site targeted by Pim3 (provirus integration site for Moloney murine leukemia virus 3) under inflammatory conditions. CONCLUSION: Our findings uncover a previously unrecognized transcriptional mechanism in OA pathogenesis, in which EYA1 promotes ECM degradation by regulating CIITA dephosphorylation and assembling a transcriptional complex with CIITA and Runx2. Targeting this pathway may provide a promising therapeutic strategy for OA.