RESUMEN
BACKGROUND/AIMS: Quercetin, a flavonoid found in onions and other vegetables, has potential inhibitory effects on bone resorption in vivo and in vitro. In our previous study, we found that quercetin treatment reversed lipopolysaccharide (LPS)-induced inhibition of osteoblast differentiation through the mitogen-activated protein kinase (MAPK) pathway in MC3T3-E1 cells. In this study, we investigated the underlying mechanisms of pretreatment with quercetin on apoptosis and the inhibition of osteoblast differentiation in MC3T3-E1 cells induced by LPS. METHODS: MC3T3-E1 osteoblasts were treated with quercetin for 2 h; cells were then incubated with LPS in the presence of quercetin for the indicated times. Cell viability was measured using the Cell Counting Kit-8 (CCK-8) assay, and cell apoptosis was evaluated using Hoechst 33258 staining. The mRNA expression levels of osteoblast-specific genes, Bax and caspase-3 were determined by real-time quantitative polymerase chain reaction (qPCR). Protein levels of osteoblast-specific genes, caspase-3, Bax, cytochrome c, Bcl-2, Bcl-XL, phosphorylated MAPKs and Wnt/ß-catenin were measured using Western blot assays. The MAPK and Wnt/ß-catenin signalling pathways were blocked prior to pretreatment with quercetin. RESULTS: Pretreatment with quercetin significantly restored LPS-suppressed bone mineralization and the mRNA and protein expression levels of osteoblast-specific genes such as Osterix (OSX), runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP) and osteocalcin (OCN) in a dose-dependent manner. Pretreatment with quercetin also inhibited osteoblast apoptosis, significantly restored the down-regulated expression of Bcl-2 and Bcl-XL and decreased the upregulated expression of caspase-3, Bax, and cytochrome c in MC3T3-E1 cells induced by LPS. Furthermore, pretreatment with quercetin not only decreased the abundance of phosphorylated p38 MAPK and increased the abundance of phosphorylated extracellular signal regulated kinase (ERK), but also triggered the Wnt/ß-catenin pathway through enhancing expression of Wnt3 and ß-catenin. Pretreatment with MAPK inhibitors or the Wnt/ß-catenin inhibitor XAV939 blocked the protective effects of quercetin against LPS-induced apoptosis and the inhibition of osteoblast differentiation. CONCLUSIONS: Our findings suggest that pretreatment with quercetin may be a potential drug for preventing abnormal human bone loss induced by LPS in bacteria-induced bone diseases.