4-methylcoumarin derivatives inhibit human neutrophil oxidative metabolism and elastase activity

Increased neutrophil activation significantly contributes to the tissue damage in inflammatory illnesses; this phenomenon has motivated the search for new compounds to modulate their effector functions. Coumarins are natural products that are widely consumed in the human diet. We have evaluated the antioxidant and immunomodulator potential of five 4-methylcoumarin derivatives. We found that the 4-methylcoumarin derivatives inhibited the generation of reactive oxygen species by human neutrophils triggered by serum-opsonized zymosan or phorbol-12-myristate-13-acetate; this inhibition occurred in a concentration-dependent manner, as revealed by lucigenin- and luminol-enhanced chemiluminescence assays. Cytotoxicity did not mediate this inhibitory effect. The 7,8-dihydroxy-4-methylcoumarin suppressed the neutrophil oxidative metabolism more effectively than the 6,7- and 5,7-dihydroxy-4-methylcoumarins, but the 5,7- and 7,8-diacetoxy-4-methylcoumarins were less effective than their hydroxylated counterparts. An analysis of the biochemical pathways suggested that the 6,7- and 7,8-dihydroxy-4-methylcoumarins inhibit the protein kinase C-mediated signaling pathway, but 5,7-dihydroxy-4-methylcoumarin, as well as 5,7- and 7,8-diacetoxy-4-methylcoumarins do not significantly interfere in this pathway of the activation of the human neutrophil oxidative metabolism. The 4-methylcoumarin derivatives bearing the catechol group suppressed the elastase and myeloperoxidase activity and reduced the 1,1-diphenyl-2-picrylhydrazyl free radical the most strongly. Interestingly, the 5,7-dihydroxy-4-methylcoumarin scavenged hypochlorous acid more effectively than the o-dihydroxy-substituted 4-methylcoumarin derivatives, and the diacetoxylated 4-methylcoumarin derivatives scavenged hypochlorous acid as effectively as the 7,8-dihydroxy-4-methylcoumarin. The significant influence of small structural modifications in the inhibitory potential of 4-methylcoumarin derivatives on the effector functions of neutrophil makes them interesting candidates to develop new drugs for the treatment of inflammatory diseases mediated by increased neutrophil activation.