Role of irisin in effects of chronic exercise on muscle and bone in ovariectomized mice

Introduction Exercise is beneficial for the prevention and treatment of osteoporosis. Skeletal muscle affects other tissues via myokines, the release of which is regulated by acute exercise. However, the effects of chronic exercise on myokines linking muscle to bone have not been fully elucidated. Therefore, we investigated the effects of chronic exercise on bone and myokines using ovariectomized (OVX) mice. Materials and methods Treadmill exercise with moderate intensity was performed for 8 weeks after OVX or sham surgery. We measured bone mineral density (BMD) at the femurs and tibias of mice by quantitative computed tomography and myokine mRNA levels in the gastrocnemius and soleus muscles. Results Treadmill exercise ameliorated decreases in trabecular and cortical BMD in the femurs of OVX mice. Irisin is a proteolytic product of fibronectin type III domain-containing 5 (Fndc5). Among the myokines examined, treadmill exercise increased irisin protein and Fndc5 mRNA levels in the gastrocnemius and soleus muscles of sham and OVX mice. Treadmill exercise increased peroxisome proliferator-activated receptor γ coactivator-1α mRNA levels in the gastrocnemius muscles of mice. Fndc5 mRNA levels in the gastrocnemius muscles positively correlated with trabecular BMD, but not with cortical BMD, at the femurs and tibias of mice in simple regression analyses. Conclusions We demonstrated that chronic exercise elevated irisin expression in the gastrocnemius and soleus muscles of estrogen-deficient mice. Irisin might be related to increases in trabecular BMD in mice; however, further studies are needed to clarify the involvement of irisin in the effects of chronic exercise on muscle/bone interactions.