Molecular Mechanisms of Curdlan-Induced Suppression of NFATc1 Expression in Osteoclasts

Osteoclasts derived from hematopoietic stem cells express immunoreceptors on their cell surface. Previously, we showed that the β-glucan curdlan suppressed osteoclastogenesis via binding to dectin-1, a pattern recognition receptor. Curdlan negatively regulates osteoclast differentiation and bone resorption capacity by suppressing the expression of nuclear factor of activated T cells 1 (NFATc1), a master factor for osteoclast differentiation, in a dectin-1-dependent manner; however, the mechanism involved in this process has not yet been fully elucidated. In this study, we aimed to elucidate the molecular mechanism involved in the suppression of RANKL-induced osteoclast differentiation by curdlan. Real-time RT-qPCR results showed that curdlan suppressed the expression of NFATc1 in cells of the osteoclast progenitor cell line RAW264.7 overexpressing dectin-1 (d-RAW cells), without altering the expression of negative regulators. Therefore, we examined the effect of curdlan on the NF-κB pathway, which is important for the induction of NFATc1 expression. Western blot analysis results showed that curdlan addition suppressed RANKL-induced NF-κB activation in the vector control line (c-RAW) cells with low expression of dectin-1, in d-RAW cells, and the parental RAW264.7 (RAW) cells. The results of tartrate-resistant alkaline phosphatase staining and real-time RT-qPCR showed that curdlan addition suppressed osteoclast differentiation in RAW cells, suggesting the presence of a dectin-1-independent modification system. Finally, we focused on the complement receptor 3 (CR3), which binds β-glucan, and revealed that blocking the binding of β-glucan to the CD11b molecule, a component of CR3, by neutralizing antibody, recovered the suppression of IκBα degradation by curdlan. These results suggest that the suppression of osteoclast differentiation by curdlan involves not only the dectin-1-dependent pathway but also the negative regulation of NFATc1 via modification of the NF-κB pathway via CR3 recognition. The results of this study may aid to establish treatment methods for metabolic bone diseases and inflammatory bone destruction and to clarify their pathogenesis.