Aucubin from Eucommiae Cortex Alleviates Tendinopathy via an Estrogen Receptor β-Mediated Mechanism

Background: Tendinopathy remains a prevalent musculoskeletal disorder with limited disease-modifying pharmacotherapy. This study aimed to identify a reparative agent from the traditional medicinal herb Eucommiae Cortex and elucidate its mechanism of action. Methods: A bioactive fraction was first identified through a bioactivity-guided strategy using tenocyte cytoprotection and migration assays, then characterized by UHPLC-HRMS/MS. Its major constituent, aucubin (AU), which mirrors the fraction's key pharmacological activities, was evaluated both in vitro and in vivo. In H₂O₂-injured tenocytes, AU's effects on viability, apoptosis, oxidative stress (ROS, MDA, SOD) and inflammation (IL-1β, TNF-α) were assessed, with specific focus on estrogen receptor (ER) pathway involvement using pharmacological tools (17β-estradiol and (R, R)-THC). In a collagenase-induced Achilles tendinopathy model using male SD rats, AU's therapeutic efficacy was evaluated via multimodal assessment: ultrasonography, histopathology (H&E, Masson's trichrome, Sirius red), TEM, immunohistochemistry, and biochemical analysis of tissue markers. Results: AU effectively attenuated H₂O₂-induced tenocyte injury by enhancing viability, reducing apoptosis, and mitigating oxidative/inflammatory stress. These effects were mimicked by 17β-estradiol and reversed by the selective ERβ antagonist (R, R)-THC, indicating ERβ dependence. In vivo, AU treatment promoted structural and functional recovery, improved collagen maturity (increased Col I/Col III ratio and fibril diameter), suppressed matrix degradation (MMP-3, MMP-13) and apoptosis, and reduced oxidative stress and inflammation in tendon tissue. Conclusions: This study identifies aucubin as a novel phytoestrogenic compound from Eucommiae Cortex that promotes tendon repair through an ERβ-mediated mechanism. These findings position ERβ activation as a promising therapeutic strategy for tendinopathy and highlight AU as a promising lead compound for further development.