Bone remodeling is regulated by local factors that regulate bone-forming osteoblasts and boneresorbing osteoclasts, in addition to hormonal activity. Recent studies have shown that reactive oxygen species (ROS) act as an intracellular signal mediator for osteoclast differentiation. However the role of ROS on osteoblast differentiation is poorly understood. Here, we investigated the impact of ROS on osteoblastic differentiation of MC3T3-E1 cells. Osteogenic induction resulted in notable enhancement of mineralization and expression of osteogenic marker gene alkaline phosphatase, which were accompanied by an increase in ROS production. Additionally, we found that mitochondrial morphology dynamically changed from tubular reticulum to fragmented structures during the differentiation, suggesting that mitochondrial morphological transition is a novel osteoblast differentiation index. The antioxidant N-acetyl cysteine prevented not only ROS production but also mineralization and mitochondrial fragmentation. It is therefore suggested that the ROS-dependent signaling pathways play a role in osteoblast differentiation accompanied by mitochondrial morphological transition.

Bone is a dynamic organ that undergoes continuous remodeling while maintaining a balance between bone formation and resorbtion. Osteoblasts, which synthesize and mineralize new bone, and osteoclasts, which resorb bone, act in concert to maintain bone homeostasis. Bone mass density is maintained constant under the control of multiple systemic and local factors such as sex hormones, parathyroid hormone, growth hormone, and proinflammatory cytokines (21, 22, 25). Imbalanced functions of these two activities are involved in various types of bone diseases such as osteoporosis and vascular calcification, which are the major age-related diseases. To understand the pathogenic mechanism of these dis-