Pulsed Electromagnetic Field Promotes Bone Anabolism in Postmenopausal Osteoporosis through the miR-6976/BMP/Smad4 Axis

Background. Insufficient bone formation is the key reason for the imbalance of bone metabolism and one of the main mechanisms for the occurrence and deterioration of postmenopausal osteoporosis (PMOP). Accumulating evidence has demonstrated that pulsed electromagnetic field (PEMF), as a physiotherapy, can treat osteoporosis by promoting osteogenic differentiation in osteoblasts. However, little is known about its mechanisms. Methods. In vivo, ovariectomized mice were administered PEMF for 4 weeks, and skeletal analysis was conducted. In vitro, hydrogen peroxide-treated mouse osteoblast precursor cells with or without PEMF intervention were subjected to osteogenic differentiation testing and miRNA microarrays. The potential target miRNAs were validated, followed by gene expression assays to further clarify their regulatory relationships with target pathways. Results. We found that PEMF reduced bone loss in ovariectomized mice and promoted osteogenic differentiation of hydrogen peroxide-treated osteoblast precursor cells via downregulation of miR-6976-5p. Mechanistically, reduced miR-6976-5p enhanced the nuclear transport of phosphorylated Smad1/5/9 by upregulating Smad4, thereby activating the BMP/Smad pathway. Additionally, the administration of miR-6976-5p inhibitors successfully promoted osteogenic differentiation in vitro, and its antagomirs protected bone mass in vivo. miR-6976-5p mimics and agomirs acted in the opposite way. Conclusion. These results provide evidence that PEMF alleviates estrogen deficiency-induced bone loss by activating osteoblastic progenitor cells and maintaining their osteogenic differentiation and shed light on the mechanisms involved, which may provide a potential option for the clinical application of PEMF in PMOP.