Ergosterol Attenuates Palmitate-Induced Tenocyte Apoptosis Through the Suppression of ER Stress and Myocyte-Derived IL-15/METRNL-Mediated Crosstalk

Ergosterol (EG), the main bioactive ingredient in <i>Wolfiporia hoelen</i>, has been shown to suppress inflammation and oxidative stress in various disease models. However, the effects of EG on endoplasmic reticulum (ER) stress and apoptosis in tenocytes in <i>in vitro</i> obesity models have not been studied. Protein expression in tenocytes was assessed by Western blotting. Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining was performed to distinguish apoptotic cells, and ELISA was conducted to quantify the protein content in the tenocyte culture medium. The autophagosomes were visualized as punctate structures via monodansylcadaverine (MDC) staining. In this study, we found that treatment with EG prevented apoptosis, collagen degradation, and ER stress and enhanced cell migration in tenocytes treated with palmitate. EG treatment augmented peroxisome proliferator-activated receptor delta (PPARδ) expression and the expression of autophagy markers. PPARδ small interfering RNA (siRNA) or 3-methyladenine, an autophagy inhibitor, abolished the effects of EG on palmitate-mediated tenocyte dysfunction. Interleukin 15 (IL-15) and meteorin-like protein (METRNL) from C2C12 myocytes treated with EG also ameliorated tenocyte injury caused by palmitate. Collectively, our findings suggest that EG exerts protective effects against palmitate-induced metabolic stress in tendinopathy by alleviating ER stress through the PPARδ/autophagy axis and enhancing IL-15- and METRNL-mediated muscle-tendon crosstalk, supporting its potential as a natural compound-based therapeutic candidate.