18-β-glycyrrhetinic acid facilitates nuclear-mitochondrial communications to alleviate oxidative stress through HMGB1-cGAS-Mul1 axis in tendinopathy.

BACKGROUND: Tendinopathy is a prevalent orthopaedic condition characterized by disrupted tendon homeostasis, with oxidative stress being a key contributing mechanism. Although the natural compound 18-β-Glycyrrhetinic acid (GA) exhibits antioxidant properties and is a therapeutic candidate for tendinopathy, its precise molecular mechanism remains unclear. This study aimed to elucidate how GA alleviates tendinopathy, with a focus on its role in regulating the HMGB1-cGAS-STING axis and NLRP3 inflammasome activation in the context of oxidative stress. METHODS: We employed single-cell RNA sequencing (scRNA-seq) of clinical samples, proteomics of animal tissues, and comprehensive pharmacological assays to investigate the mechanisms of tendinopathy. Furthermore, the rat tendinopathy model and H(2)O(2)-induced oxidative stress model of tendon stem cells (TSCs) were used to validate the protective effects of GA. RESULTS: We found that GA significantly reduced oxidative stress and subsequent inflammation, thereby mitigating collagen disruption in rats with tendinopathy. Notably, scRNA-seq revealed that the proportion of TSCs increased significantly during tendinopathy, which were particularly susceptible to reactive oxygen species (ROS). TSCs from oxidative damage and inhibited activation of the NLRP3 inflammasome by suppressing the cGAS-STING pathway. Mechanistically, GA promoted cGAS degradation by enhancing its interaction with the mitochondrial E3 ubiquitin ligase Mul1. This effect was mediated through high-mobility group box 1 (HMGB1), as GA disrupted the HMGB1-cGAS interaction. Specifically, GA induced methylation of HMGB1 at lysine 43, a modification essential for its activity. This methylation was catalyzed by the methyltransferase DOT1L, which was upregulated and directly bound by GA. Collectively, GA alleviates tendinopathy by targeting the DOT1L-HMGB1-cGAS axis to resolve oxidative stress and inflammation. CONCLUSION: Collectively, our findings provide new insights into how oxidative stress accelerates tendinopathy progression. Moreover, they delineate the mechanism by which GA in mitigates oxidative damage and inflammation in TSCs by inhibiting the co-localization of HMGB1 and cGAS. Overall, this study offers scientific support for further developing GA as a promising therapeutic agent for tendinopathy treatment.