Effect of moderate-intensity treadmill exercise on bone quality in adult rats and the underlying mechanisms

Background: While exercise is widely recognized to enhance bone mass, the underlying molecular and cellular mechanisms remain incompletely elucidated. The present study was designed to investigate the effect of moderate-intensity exercise on skeletal quality, morphological characteristics, and metabolic homeostasis in adult Sprague-Dawley (SD) rats. Methods: An animal treadmill exercise model was established using 16-week-old male specific pathogen-free SD rats. Following the 6-week moderate-intensity exercise, bilateral tibiae were harvested for analysis. Body indices of obesity and glucose metabolism were evaluated after the training period. Micro-computed tomography scanning was used to assess three-dimensional bone microarchitecture, while hematoxylin and eosin staining was employed to characterize bone morphological features. RNA sequence and reverse transcription-quantitative PCR identified and validated differentially expressed mRNAs in bone, with bioinformatics analyses predicting their target signaling pathways. Additionally, immunohistochemical staining quantified protein expression of osteogenic, osteoclastic, key pathway and mechanosensitive markers. The apoptosis rate was determined by TdT-mediated dUTP nick end labeling staining. Serum bone turnover markers and inflammatory cytokines were measured after exercise intervention via ELISA kits. Results: Compared with the Control group, exercised rats showed decreasing trends in body weight, Lee’s index, and BMI, with a tendency toward increased tibial length. No significant difference was observed in fasting blood glucose between two groups. Micro-CT analysis revealed that exercise significantly increased trabecular bone volume fraction (BV/TV) and trabecular number (Tb.N). At the molecular level, exercise upregulated osteogenic proteins (COL1, OCN) and serum P1NP, while downregulating bone resorption markers (Sclerostin, RANKL, TRAP) and serum CTX. Further, differentially expressed genes were detected, and functional enrichment analyses revealed that the p53/Caspase-3 pathway and inflammatory factors were significantly down-regulated in the Exercise group. Apoptosis rate was significantly decreased after exercise in bone tissue. Treadmill training also resulted in increased protein expression of Piezo1. Conclusions: Moderate-intensity treadmill exercise enhanced tibial cancellous bone microstructure and bone quality in adult SD rats by improving the balance of bone remodeling via p53/Caspase-3 signaling pathway. Furthermore, the alteration of p53 signaling pathway may be driven by the activation of mechanosensitive markers. These findings provide mechanistic insights into exercise-induced improvement of bone quality. Clinical trial number: Not applicable.