Quercetin inhibits LPS-induced delay in spontaneous apoptosis and activation of neutrophils

Objective: To investigate the effects of quercetin, an herbal flavonoid, on LPS-induced delay in spontaneous apoptosis, adhesion molecules (CD62L, CD11b/CD18) expression of neutrophils, and superoxide (O-2(-)) generation by LPS-primed fMLP-induced human neutrophils. Methods: Neutrophils were incubated in the presence or absence of lipopolysaccharide (LPS) at a final concentration of 1 mu g/ml for 24 hours. Some wells with neutrophils were pre-treated with quecetin at the final concentration ranging from 0-100 mu M for 30 min and then 1 mu g/ml LPS was added into the cultures for 24 hours. The apoptosis of neutrophils was evaluated by flowcytometry analysis of propidum iodide (PI)-staining of the nuclei and annexin V staning of phosphatidylserine (PS) in the cell membrane. Agarose gel electrophoresis of low molecular weight DNA was performed to analyze DNA fragmentation. The effects of quercetin on adhesion molecules were detected by using flowcytometry analysis. The generation of O-2(-) by LPS-primed fMLP-induced neutrophils was determined by reduced cytochrome c assay. Results: LPS markedly inhibited the spontaneous apoptosis of neutrophils, but the inhibitory effect was abrogated after the pre-treatment of neutrophils with quercetin (similar to 40 mu M) for 30 min. Quercetin (40 mu M) also prevented LPS-induced down-regulation of CD62L expression, up-regulation of CD11b/CD18 expression, and O-2(-) generation by fMLP-induced neutrophils. Conclusion: As one of the pro-inflammatory factors, LPS aggravates inflammation through priming neutrophils to synthesize/release cytotoxic contents and prolonging functional lifespan of neutrophils by delaying the spontaneous apoptosis. Thus, our data suggest to us that quercetin might decrease the susceptibility of neutrophils to pro-inflammatory factors (e.g. LPS), which could partially explain the anti-inflammatory mechanisms of quercetin.