Quercetin suppressed NADPH oxidase-derived oxidative stress via heme oxygenase-1 induction in macrophages

NADPH oxidase-derived superoxide (O₂^.-) generation and oxidative stress is usually considered as an important factor to the pathogenesis of inflammatory diseases. Quercetin, widely known for their anti-oxidant and anti-inflammatory properties in vitro and in vivo, is recently identified to induce expression of antioxidant enzyme heme oxygenase-1 (HO-1). Previous studies suggest that HO-1 induction and/or subsequent HO-1 end product generation in vitro and in vivo may suppress NADPH oxidase-derived oxidative stress. In this study, we tested whether quercetin might modulate NADPH oxidase activity in macrophages via induction of HO-1. In RAW264.7 macrophages, quercetin significantly attenuated NADPH oxidase-derived O₂^.- generation via a HO-1-dependent mechanism. Mechanistically, the protective effects of quercetin were (1) linked to increased expression of HO-1 in the presence or absence of lipopolysaccharide (LPS), (2) similar to that observed with the NADPH oxidase inhibitor apocynin, and (3) could be abolished by the specific small-interfering RNA against HO-1 expression or HO-1 activity inhibitor tin protoporphyrin. The induction of HO-1 by quercetin was associated with the nuclear accumulation of Nrf2 and downregulation of Keap1, a negative regulator of Nrf2. In addition, this flavonoid also inhibited the overproduction of nitric oxide and inflammatory cytokines in LPS-stimulated macrophages via simultaneous induction of HO-1 expression. In agreement with the observations in macrophages, pretreatment with quercetin significantly alleviated LPS-induced inflammation in mice which was concomitant with decreased NADPH oxidase activity and increased HO-1 expression. Our results suggested that quercein could modulate NADPH oxidase-derived O₂^.- production in macrophages at least partly through HO-1 induction. Suppression of NADPH oxidase-dependent oxidative stress may represent a novel mechanism underlying the anti-oxidant and anti-inflammatory properties of quercetin/HO-1 pathway.