Suppression of inducible nitric oxide synthase expression in RAW 264.7 macrophages by two beta-carboline alkaloids extracted from Melia azedarach

We investigated the mechanism of suppression of inducible nitric oxide synt hase (iNOS) by two beta -carboline alkaloids isolated from Melia azedarach, 4,8-dimethoxy-1-vinyl-beta -carboline (compound 1, C-1) and 4-methoxy-1-vi nyl-beta -carboline (compound 2, C-2). iNOS activity in a cell-free extract of lipopolysaccharide/interferon-gamma -stimulated RAW 264.7 cells was fou nd to be markedly increased, and this increase was prevented by C-1 and C-2 , accompanied by the parallel reduction in nitrite accumulation in culture medium. However, C-1 and C-2 had no further effect on the iNOS activity pre pared from fully lipopolysaccharide/interferon-gamma -stimulated RAW 264.7 cells. Treatment with C-1 or C-2 decreased the levels of iNOS protein and m RNA in a concentration-dependent manner. In addition, prostaglandin E-2 pro duction, cyclooxygenase-2 protein and DNA binding of nuclear factor-kappaB (NF-kappaB) in lipopolysaccharide-stimulated RAW 264.7 cells were reduced b y these compounds. These results indicate that C-1 and C-2 primarily inhibi t iNOS and cyclooxygenase-2 activities via the suppression of de novo synth esis of these two enzymes, and that the inhibition of iNOS expression may b e associated with the inhibition of NF-kappaB activation. Taken together, t he results suggest that suppression of iNOS and cyclooxygenase-2 induction by lipopolysaccharide is responsible for the anti-inflammatory activity of these alkaloids through selective inhibition of the expression of genes, wh ich play important roles in inflammatory signaling pathways. (C) 2000 Elsev ier Science B.V. All rights reserved.