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Skeletal muscle atrophy is caused by disruption in the homeostatic balance of muscle degeneration and regeneration under various pathophysiological conditions. We have previously reported that iron accumulation induces skeletal muscle atrophy via a ubiquitin ligase–dependent pathway. However, the potential effect of iron accumulation on muscle regeneration remains unclear. To examine the effect of iron accumulation on myogenesis, we used a mouse model with cardiotoxin (CTX)‐induced muscle regeneration in vivo and C2C12 mouse myoblast cells in vitro. In mice with iron overload, the skeletal muscles exhibited increased oxidative stress and decreased expression of satellite cell markers. Following CTX‐induced muscle injury, these mice also displayed delayed muscle regeneration with a decrease in the size of regenerating myofibers, reduced expression of myoblast differentiation markers, and decreased phosphorylation of MAPK signaling pathways. In vitro, iron overload also suppressed the differentiation of C2C12 myoblast cells but the suppression could be reversed by superoxide scavenging using tempol. Excess iron inhibits myogenesis via oxidative stress, leading to an imbalance in skeletal muscle homeostasis. —Ikeda, Y., Satoh, A., Horinouchi, Y., Hamano, H., Watanabe, H., Imao, M., Imanishi, M., Zamami, Y., Takechi, K., Izawa‐Ishizawa, Y., Miyamoto, L., Hirayama, T., Nagasawa, H., Ishizawa, K., Aihara, K.‐I., Tsuchiya, K., Tamaki, T. Iron accumulation causes impaired myogenesis correlated with MAPK signaling pathway inhibition by oxidative stress. FASEB J. 33, 9551–9564 (2019). http://www.fasebj.org

Low serum bilirubin levels are associated with the risk of cardiovascular diseases including peripheral artery disease. Bilirubin is known to exert its property such as antioxidant effect or the enhancement of flow-mediated vasodilation, however, bilirubin action on angiogenesis remains unclear. To investigate the molecular mechanism of bilirubin on angiogenic effect, we first employed C57BL/6J mice with unilateral hindlimb ischemia surgery and divided the mice into two groups (vehicle-treated group and bilirubin-treated group). The analysis of laser speckle blood flow demonstrated the enhancement of blood flow recovery in response to ischemia of mice with bilirubin treatment. The density of capillaries was significantly higher in ischemic-adductor muscles of bilirubin-treated mice. The phosphorylated levels of endothelial nitric oxide synthase (eNOS) and Akt were increased in ischemic skeletal muscles of mice with bilirubin treatment compared with vehicle treatment. In in vitro experiments by using human aortic endothelial cells, bilirubin augmented eNOS and Akt phosphorylation, cell proliferation, cell migration and tube formation. These bilirubin actions on endothelial cell activation were inhibited by LY294002, a phosphatidylinositol 3-kinase inhibitor. In conclusion, bilirubin promotes angiogenesis through endothelial cells activation via Akt-eNOS-dependent manner.

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