Abstract
Bone-marrow derived monocyte-macrophages (BMMs) differentiate into osteoclasts by M-CSF along subsequent RANKL stimulation possibly in collaboration with many other unknown cytokines released by pre- or mature osteoblasts. The differentiation process requires receptor activator of nuclear factor kappa-B ligand (RANKL)/RANK signaling and reactive oxygen species (ROS) such as superoxide anion (O2•−). Gp91phox, a plasma membrane subunit of NADPH oxidase (Nox), is constitutively expressed in BMMs and plays a major role in superoxide anion production. In this study, we found that mice deficient in gp91phox (gp91phox−/−) showed defects in osteoclast differentiation. Femurs of these mice produced osteoclasts at about 70% of the levels seen in femurs from wild-type mice, and accordingly exhibited excessive bone density. This abnormal bone growth in the femurs of gp91phox−/− mice resulted from impaired osteoclast differentiation. In addition, gp91phox−/− mice were defective for RANKL-induced expression of nuclear factor of activated T cells c1 (NFATc1). However, H2O2 treatment compensated for gp91phox deficiency in BMMs, almost completely rescuing osteoclast differentiation. Treating wild-type BMMs with antioxidants and superoxide inhibitors resulted in a differentiation defect resembling the phenotype of gp91phox−/− BMMs. Therefore, our results demonstrate that gp91phox-derived superoxide is important for promoting efficient osteoclast differentiation by inducing NFATc1 as a downstream signaling mediator of RANK.
Highlights
Bone-marrow derived monocyte-macrophages (BMMs) differentiate into osteoclasts by macrophage colony stimulating factor (M-CSF) along subsequent receptor activator of nuclear factor kappa-B ligand (RANKL) stimulation possibly in collaboration with many other unknown cytokines released by pre- or mature osteoblasts
reactive oxygen species (ROS) are clearly linked to osteoclast differentiation, we wanted to test if gp91phox-derived superoxide participates in osteoclast differentiation and bone resorption
We compared superoxide production between osteoclast precursors derived from gp91phox−/− mice and osteoclast precursors derived from wild-type mice after stimulation with 200 ng/mL phorbol 12-myristate 13-acetate (PMA) (Fig. 1A) or 50 ng/mL RANKL (Fig. 1B) for 10 min and 30 min, respectively, by isoluminol chemiluminescence assay
Summary
Bone-marrow derived monocyte-macrophages (BMMs) differentiate into osteoclasts by M-CSF along subsequent RANKL stimulation possibly in collaboration with many other unknown cytokines released by pre- or mature osteoblasts. The differentiation process requires receptor activator of nuclear factor kappa-B ligand (RANKL)/RANK signaling and reactive oxygen species (ROS) such as superoxide anion (O2−). We found that mice deficient in gp91phox (gp91phox−/−) showed defects in osteoclast differentiation Femurs of these mice produced osteoclasts at about 70% of the levels seen in femurs from wild-type mice, and exhibited excessive bone density. Gp91phox−/− mice were defective for RANKL-induced expression of nuclear factor of activated T cells c1 (NFATc1). RANKL stimulation transiently increases intracellular ROS levels through NADPH oxidase (Nox), and ROS act as second messengers in RANKL-induced signaling pathways in osteoclast precursors, regulating their consequent differentiation into osteoclasts[7,8,9,10]. These findings indicate that Nox-derived ROS are essential intracellular mediators throughout the osteoclast differentiation process
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