USP15-dependent modulation of TGF-β/Smad2/3 signaling: implications for chondrocyte matrix degradation and autophagy in osteoarthritis.

The dysregulation of extracellular matrix (ECM) metabolism and autophagy in chondrocytes is central to osteoarthritis (OA) pathogenesis, yet the molecular mechanisms remain poorly defined. Ubiquitin-specific protease 15 (USP15), a key modulator of TGF-β/Smad2/3 signaling through TβR-I stabilization, may play a critical role in maintaining chondrocyte homeostasis. We conducted in vitro and ex vivo experiments using OA patient-derived cartilage samples and chondrocyte models to test the hypotheses that (1) USP15 deficiency exacerbates OA progression by impairing TGF-β/Smad2/3 signaling, leading to ECM degradation and autophagy dysfunction; and (2) restoring USP15 expression activates this pathway to mitigate cartilage degeneration, an effect reversible by TGF-β/Smad inhibition. Lentiviral-mediated USP15 knockdown or overexpression, combined with pharmacological inhibition assays, revealed that reduced USP15 levels in OA lesions correlated with disease severity (Kellgren-Lawrence grading) and triggered ECM catabolism (downregulated Col-II, MMP-13), suppressed autophagy (reduced LC3-II, accumulated p62), and increased apoptosis. Conversely, USP15 overexpression stabilized TβR-I, enhanced TGF-β/Smad2/3 signaling activity, increased Smad2/3 protein expression and promoted Smad2/3 phosphorylation(p < 0.01), and restored ECM synthesis and autophagic flux. These protective effects were abolished by the TGF-β receptor kinase inhibitor SIS3 (p < 0.001), confirming pathway dependency. Our findings establish USP15 as a pivotal regulator of chondrocyte homeostasis via the TβR-I/Smad axis, offering novel therapeutic targets for OA intervention.