Dual targeting of SREBP2 and ERRα by carnosic acid suppresses RANKL-mediated osteoclastogenesis and prevents ovariectomy-induced bone loss

Osteoporosis develops because of impaired bone formation and/or excessive bone resorption. Several pharmacological treatment of osteoporosis has been developed; however, new treatments are still necessary. Cholesterol and estrogen receptor-related receptor alpha (ERR alpha) promote osteoclasts formation, survival, and cellular fusion and thus become high risk factors of osteoporosis. In this study, we identified that carnosic acid (CA) suppressed bone loss by dual-targeting of sterol regulatory element-binding protein 2 (SREBP2, a major regulator that regulates cholesterol synthesis) and ERR alpha. Mechanistically, CA reduced nuclear localization of mature SREBP2 and suppressed de novo biogenesis of cholesterol. CA subsequently decreased the interaction between ERR alpha and peroxisome proliferator-activated receptor gamma coactivator 1-beta (PGC1 beta), resulting in decreased the transcription activity of ERR alpha and its target genes expression. Meanwhile, CA directly bound to the ligand-binding domain of ERR alpha and significantly promoted its ubiquitination and proteasomal degradation. Subsequently, STUB1 was identified as the E3 ligase of ERR alpha. The lysine residues (K51 and K68) are essential for ubiquitination and proteasomal degradation of ERR alpha by CA. In conclusion, CA dually targets SREBP2 and ERR alpha, thus inhibits the RANKL-induced osteoclast formation and improves OVX-induced bone loss. CA may serve as a lead compound for pharmacological control of osteoporosis.