Inhibition of CDK9 alleviates osteoarthritis by suppressing inflammation and reducing chondrocyte apoptosis.

Posttraumatic osteoarthritis (PTOA) develops following joint trauma and leads to pain and functional impairment. This study aimed to investigate the role and mechanism of cyclin-dependent kinase 9 (CDK9) in PTOA progression. We established an in vitro PTOA model by treating primary rat chondrocytes with lipopolysaccharide (LPS). CDK9 expression was modulated using siRNA, overexpression plasmids, and the inhibitor flavopiridol (FLA). Inflammatory cytokines were quantified by ELISA. RT-qPCR and Western blot were used to assess CDK9 and NF-κB pathway components. Cell viability and apoptosis were measured by CCK-8 assay and flow cytometry, respectively. An in vivo PTOA rat model was generated by medial meniscectomy (MMx). Rats received intra-articular injections of FLA (1, 3.5, 7.5 mg/kg) or IL-1β. Histopathological changes were evaluated by hematoxylin-eosin (HE) and Safranin O-Fast Green staining. In vitro, FLA and si-CDK9 attenuated LPS-induced inflammation and apoptosis, enhanced cell viability, and suppressed CDK9/NF-κB activation. Conversely, CDK9 overexpression exacerbated these detrimental effects. In vivo, FLA treatment (particularly at 7.5 mg/kg) significantly inhibited CDK9 and NF-κB activation in articular cartilage, improved mechanical withdrawal threshold and locomotor activity, reduced cartilage degradation, and lowered levels of TGF-β, IL-6, TNF-α, and IL-1β. However, co-administration of IL-1β, an NF-κB activator, reversed the therapeutic effects of FLA in both models. Our findings demonstrate that CDK9 plays a critical role in PTOA pathogenesis. Inhibition of CDK9 alleviates disease progression by suppressing the NF-κB signaling pathway, highlighting its potential as a therapeutic target for PTOA.