Inhibition of nitric oxide production by quercetin in endotoxin/cytokine-stimulated microglia.

Aims: Flavonoids possess several biological and pharmacological activities. Quercetin, a naturally occurring flavonoid, has been shown to down-regulate inflammatory responses and provide neuroprotection. However, the mechanisms underlying the anti-inflammatory properties of quercetin are poorly understood. In the present study, we investigated the modulatory effect of quercetin against neuroinflammation.

Main Methods: We herein describe a potential regulatory mechanism by which quercetin suppresses nitric oxide (NO) production by lipopolysaccharide (LPS)/interferon-gamma (IFN-gamma)-stimulated BV-2 microglial cells. The underlying regulatory cascades were approached by biochemical and pharmacological strategies.

Key Findings: Quercetin produced an inhibitory effect on inducible nitric oxide synthase (iNOS) expression and NO production. Biochemical studies revealed that the anti-inflammatory effect of quercetin was accompanied by the down-regulation of extracellular signal-regulated kinase, c-Jun N-terminal kinase, p38, Akt, Src, Janus kinase-1, Tyk2, signal transducer and activator of transcription-1, and NF-kappaB. In addition, quercetin scavenged free radicals and produced inhibitory effects on serine/threonine and tyrosine phosphatase activities. Intriguingly, the accumulation of lipid rafts, which is the critical step for signaling, was disrupted by quercetin.

Significance: The data indicate that the anti-inflammatory action of quercetin may be attributable to its raft disrupting and anti-oxidant effects. These distinct mechanisms work in synergy to down-regulate iNOS expression and NO production.