Hypoxia-mediated mitochondrial stress in RAW264.7 cells induces osteoclast-like TRAP-positive cells.

Previously we showed that mitochondrial dysfunction induced by mitochondrial DNA depletion or treatment with electron transport chain inhibitors triggers a stress signaling involving activation of calcineurin and Ca2+-responsive factors. In this study we show that exposure of RAW 264.7 cells to hypoxia, causing increased reactive oxygen species (ROS) production and disruption of mitochondrial transmembrane potential, also induced a similar stress signaling. Hypoxia caused increased [Ca2+]c, activation of cytosolic calcineurin and induced expression of Ryanodine Receptor 2 (RyR2) gene. Prolonged hypoxia (5% O2 for 5-6 days) also induced the expression of calcitonin receptor at high levels, and those of cathepsin K, and tartarate-resistant alkaline phosphatase (TRAP) at low-moderate levels in macrophage cells. Addition of RANKL had an additive effect suggesting different mechanisms of activation. Consistent with this possibility, prolonged hypoxia induced the formation of TRAP-positive osteoclast-like cells suggesting the occurrence of an autocrine mechanism for osteoclastogenesis.