Nur77 Prevents Osteoporosis by Inhibiting the NF-kappa B Signalling Pathway and Osteoclast Differentiation

Inflammation is a major risk factor for osteoporosis, and reducing inflammatory levels is important for the prevention of osteoporosis. Although nuclear receptor 77 (Nur77) protects against inflammation in a variety of diseases, its role in osteoporosis is unknown. Therefore, the main purpose of this study was to investigate the osteoprotective and anti-inflammatory effects of Nur77. The microCT and haematoxylin and eosin staining results indicated that knockout of Nur77 accelerated femoral bone loss in mice. The enzyme-linked immunosorbent assay (ELISA) results showed that knockout of Nur77 increased the serum levels of hsCRP and IL-6. The expression levels of NF-kappa B, IL-6, TNF-alpha and osteoclastogenesis factors (TRAP, NFATC1, Car2, Ctsk) in the femurs of Nur77 knockout mice were increased significantly. Furthermore, in vitro, shNur77 promoted the differentiation of RAW264.7 cells into osteoclasts by activating NF-kappa B, which was confirmed by PDTC treatment. Mechanistically, Nur77 inhibited osteoclast differentiation by inducing I kappa B-alpha and suppressing IKK-beta. In RAW264.7 cells, overexpression of Nur77 alleviated inflammation induced by siI kappa B-alpha, while siIKK-beta alleviated inflammation induced by shNur77. Consistent with the in vivo studies, we found that compared with control group, older adults with high serum hsCRP levels were more likely to suffer from osteoporosis (OR = 1.76, p < 0.001). Our data suggest that Nur77 suppresses osteoclast differentiation by inhibiting the NF-kappa B signalling pathway, strongly supporting the notion that Nur77 has the potential to prevent and treat osteoporosis.