Synergistic cytotoxic effects of glucocorticoid steroids and local anesthetics on human rotator cuff fibroblasts: protective role of N-Acetylcysteine

PURPOSE: Steroid injections are commonly used for the treatment of tendinopathy, often in combination with local anesthetics. However, both agents may exert cytotoxic effects on tendon cells. This study aimed to investigate the synergistic cytotoxic effects of glucocorticoid steroids and local anesthetics on human rotator cuff fibroblasts (RCFs) and to evaluate the potential protective role of N-acetylcysteine (NAC), an antioxidant. METHODS AND MATERIALS: Human RCFs were treated with individual glucocorticoid steroids or local anesthetics, their combinations, and NAC pretreatment. Cell viability was assessed using MTT and Live/Dead assays. Apoptosis was evaluated by Annexin V/propidium iodide staining and TUNEL assays. Intracellular reactive oxygen species (ROS) generation was measured using the DCF-DA assay, and the expression of apoptosis-related proteins was analyzed by Western blotting. RESULTS: Both glucocorticoid steroids and local anesthetics reduced cell viability in a time- and concentration-dependent manner. Their combined administration produced a synergistic increase in cytotoxicity, intracellular ROS generation, and apoptotic markers, including cleaved caspase-3 and poly (ADP-ribose) polymerase (PARP). Among the combinations tested, methylprednisolone plus lidocaine induced the greatest cytotoxicity, whereas combinations containing ropivacaine showed the lowest toxicity. NAC pretreatment significantly attenuated cytotoxicity, ROS generation, and apoptosis. CONCLUSIONS: Glucocorticoid steroids and local anesthetics synergistically induce apoptosis in human rotator cuff fibroblasts through oxidative stress. Pretreatment with NAC effectively attenuates these cytotoxic effects, suggesting that NAC may serve as a promising protective strategy to improve the safety of steroid and local anesthetic injections in the treatment of tendinopathy.