Retsat promotes synovial mesenchymal stem cell senescence and aggravates osteoarthritis by inhibiting the PPARγ/RXR heterodimerization pathway

Osteoarthritis (OA), a prevalent degenerative joint disorder, lacks curative conventional therapies. Stem cell therapy is being subjected to increasing attention in the realm of OA treatment. This study aimed to investigate the metabolic changes in senescent synovial mesenchymal stem cells (SMSCs) and the role and molecular mechanisms of the retinol metabolism pathway enzyme retinol saturase (Retsat) in SMSCs in the regulation of OA. Metabolomic profiling compared young and aged OA-free patient-derived SMSCs, identifying trans-13,14-dihydroxyretinol as a key discriminatory metabolite. Retsat expression was modulated using siRNA interference, followed by functional assessments in vitro, including cell proliferation, senescence, inflammation, and in vivo using a murine OA model. Metabolomics revealed significant differences between young and elderly SMSCs. Trans-13,14-dihydroxyretinol, a Retsat substrate, was the primary differential metabolite (Fold Change = 14.585, P = 0.012), highlighting altered retinol metabolism. Silencing Retsat promoted proliferation (mean difference 77.67, 95% confidence interval: [67.33, 88.00]) and inhibited senescence (-35.20, [-45.30, -25.10]) and inflammation in senescent SMSCs. Moreover, Retsat inhibited PPARγ/RXR pathway activation; conversely, PPARγ/RXR activation enhanced the viability-promoting effects of Retsat silencing (44.00 [26.77, 61.23]). In vivo, Retsat-silenced senescent SMSCs ameliorated OA progression in middle-aged mice, significantly reducing OARSI scores, inflammation and the levels of senescence marker p21 (cartilage -0.103 [-0.162, -0.045] synovial -0.087, [-0.141, -0.033]). Furthermore, Retsat silencing promoted proliferation in senescent bone marrow mesenchymal stem cells (BMSCs). Silencing Retsat improved cartilage repair in OA by activating the PPARγ/RXR heterodimer pathway and enhancing the activity of senescent SMSCs.