The frequency window effect of sinusoidal electromagnetic fields in promoting osteogenic differentiation and bone formation involves extension of osteoblastic primary cilia and activation of protein kinase A

Abstract

Electromagnetic fields (EMFs) have emerged as a versatile means for osteoporosis treatment and prevention. However, its optimal application parameters are still elusive. Here, we optimized the frequency parameter first by cell culture screening and then by animal experiment validation. Osteoblasts isolated from newborn rats (ROBs) were exposed 90\u2009min/day to 1.8\u2009mT SEMFs at different frequencies (ranging from 10 to 100\u2009Hz, interval of 10\u2009Hz). SEMFs of 1.8\u2009mT inhibited ROB proliferation at 30, 40, 50, 60\u2009Hz, but increased proliferation at 10, 70, 80\u2009Hz. SEMFs of 10, 50, and 70\u2009Hz promoted ROB osteogenic differentiation and mineralization as shown by alkaline phosphatase (ALP) activity, calcium content, and osteogenesis‐related molecule expression analyses, with 50\u2009Hz showing greater effects than 10 and 70\u2009Hz. Treatment of young rats with 1.8\u2009mT SEMFs at 10, 50, or 100\u2009Hz for 2 months significantly increased whole‐body bone mineral density (BMD) and femur microarchitecture, with the 50\u2009Hz group showing the greatest effect. Furthermore, 1.8\u2009mT SEMFs extended primary cilia lengths of ROBs and increased protein kinase A (PKA) activation also in a frequency‐dependent manner, again with 50\u2009Hz SEMFs showing the greatest effect. Pretreatment of ROBs with the PKA inhibitor KT5720 abolished the effects of SEMFs to increase primary cilia length and promote osteogenic differentiation/mineralization. These results indicate that 1.8\u2009mT SEMFs have a frequency window effect in promoting osteogenic differentiation/mineralization in ROBs and bone formation in growing rats, which involve osteoblast primary cilia length extension and PKA activation.