Quercetin Triggers Apoptosis of Lipopolysaccharide (LPS)-induced Osteoclasts and Inhibits Bone Resorption in RAW264.7 Cells

Aims: Quercetin, a flavonoid present in vegetables, has anti-inflammatory properties and potential inhibitory effects on bone resorption. Up to date, the effect of quercetin on lipopolysaccharide (LPS)-induced osteoclastogenesis has not yet been reported. In the current study, we evaluated the effect of quercetin on LPS-induced osteoclast apoptosis and bone resorption. Methods: RAW264.7 cells were non-treated, treated with LPS alone, or treated with both LPS and quercetin. After treatment, the number of osteoclasts, cell viability, bone resorption and osteoclast apoptosis were measured. The expressions of osteoclast-related genes including tartrate-resistant acid phosphatase (TRAP), matrix metalloproteinase-9 (MMP9) and cathepsin K (CK) were determined by real-time quantitative polymerase chain reaction (qPCR). Protein levels of receptor activator of nuclear factor-ĸB (RANK), tumor necrosis factor receptor-associated factor 6 (TRAF6), cyclooxygenase-2 (COX-2), Bax, Bcl-2 and mitogenactivated protein kinases (MAPKs) were measured using Western blotting assays. The MAPK signaling pathway was blocked by pretreatment with MAPK inhibitors. Results: LPS directly promoted osteoclast differentiation of RAW264.7 cells and upregulated the protein expression of RANK, TRAF6 and COX-2; while quercetin significantly decreased the number of LPS-induced osteoclasts in a dose-dependent manner. None of the treatments increased cytotoxicity in RAW264.7 cells. Quercetin inhibited mRNA expressions of osteoclast-related genes and protein levels of RANK, TRAF6 and COX-2 in LPS-induced mature osteoclasts. Quercetin also induced apoptosis and inhibited bone resorptive activity in LPS-induced mature osteoclasts. Furthermore, quercetin promoted the apoptotic signaling pathway including increasing the phosphorylation of p38-MAPK, c-Jun N-terminal kinases/stress-activated protein kinases