Regulation of humeral fracture healing by miR-21 through SMAD7-mediated activation of Wnt/β-catenin signaling.

MicroRNAs (miRNAs) are key regulators of bone regeneration. We investigated the role of miR-21 in humeral fracture (HF) healing, focusing on its interaction with the SMAD7/Wnt/β-catenin signaling axis. Serum and tissue levels of miR-21 were analyzed in HF patients and healthy controls using qRT-PCR. A rat HF model was established, and agomiR-21 was administered to evaluate its effect on fracture healing. Histological analysis using hematoxylin and eosin (H&E) staining was performed to assess bone tissue morphology and fracture healing. In vitro, MC3T3-E1 osteoblasts were transfected with miR-21 mimics to assess cell viability (CCK-8), proliferation (EdU), apoptosis (flow cytometry), migration (Transwell assay), and osteogenic differentiation (Western blot). The downstream target of miR-21 was identified using bioinformatics and validated via luciferase reporter and RNA immunoprecipitation assays. Rescue experiments involving SMAD7 overexpression and Wnt/β-catenin inhibitors (DKK1) were conducted to explore the mechanism of action. Serum analysis revealed reduced β-catenin and phosphorylated GSK3β (p-GSK3β) levels in humeral fracture (HF) patients compared to healthy controls, indicating suppressed Wnt/β-catenin signaling. In MC3T3-E1 cells, miR-21 overexpression enhanced osteogenic markers and increased total β-catenin, active β-catenin, and p-GSK3β levels, confirming activation of the Wnt/β-catenin pathway. Dual-luciferase and expression analyses demonstrated that miR-21 directly targets SMAD7, a known Wnt pathway inhibitor. Co-transfection with SMAD7 reversed the effects of miR-21 on β-catenin activation, ALP activity, and osteogenic gene expression. Additionally, miR-21 knockdown impaired osteogenesis, further supporting its regulatory role. Importantly, in vivo, agomiR-21 treatment enhanced osteoblast activity and significantly promoted bone healing in a rat fracture model, reinforcing the therapeutic potential of miR-21-mediated SMAD7 suppression and Wnt/β-catenin pathway activation in bone regeneration. miR-21 promotes bone regeneration after humeral fracture by enhancing osteoblast function and activating Wnt/β-catenin signaling through direct inhibition of SMAD7.