Effects of naturally occurring flavonoids on nitric oxide production in the macrophage cell line RAW 264.7 and their structure-activity relationships

Flavonoids affect the inflammatory process of the mammalian system and poss ess antiinflammatory as well as immunomodulatory activities in vitro and in vivo. Since nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS) is one of the inflammatory mediators, the effects of various natural ly occurring flavonoids on NO production in LPS activated RAW 264.7 cells w ere evaluated in vitro. Flavonoids such as apigenin, wogonin, luteolin, tec torigenin, and quercetin inhibited NO production, as measured by nitrite fo rmation at 10-100 mu M. The most active among 26 flavonoid derivatives test ed were apigenin, wogonin, and luteolin, having IC50 values of 23, 17, and 27 mu M, respectively, while AMT, a synthetic selective iNOS inhibitor, had an IC50 value of 0.09 mu M. In contrast, flavanones, such as naringenin, a nd flavonoid glycosides, such as apiin, did not demonstrate significant inh ibition up to 100 mu M. These results clearly indicated that a C-2,3 double bond might be important, and that the potency of inhibition depended upon the substitution patterns of the flavonoid molecules. The inhibitory activi ty of flavonoids was not due to direct inhibition of iNOS enzyme activity b ecause they did not reasonably inhibit iNOS activity, as measured by [H-3]c itrulline formation from [H-3]arginine, up to 100 mu M. In contrast, wogoni n and luteolin concentration-dependently reduced iNOS enzyme expression, wh en measured by western blotting, at 10-100 mu M. All these results clearly demonstrated that certain flavonoids inhibit NO production in lipopolysacch aride-activated RAW 264.7 cells, and their inhibitory activity might be due to reduction of iNOS enzyme expression. (C) 1999 Elsevier Science Inc.