Signal transduction for inhibition of inducible nitric oxide synthase and cyclooxygenase-2 induction by capsaicin and related analogs in macrophages

1 Although capsaicin analogs might be a potential strategy to manipulate inflammation, the mechanism is still unclear. In this study, the effects and action mechanisms of vanilloid analogs on iNOS and COX-2 expression were investigated in RAW264.7 macrophages. 2 Capsaicin and resiniferatoxin (RTX) can inhibit LPS- and IFN-gamma-mediated NO production, and iNOS protein and mRNA expression with similar IC50 values of around 10 muM. 3 Capsaicin also transcriptionally inhibited LPS- and PMA-induced COX-2 expression and PGE(2) production. However, this effect exhibited a higher potency (IC50: 0.2 muM), and RTX failed to elicit such responses at 10 muM. 4 Interestingly, we found that capsazepine, a competitive TRPV1 antagonist, did not prevent the inhibition elicited by capsaicin or RTX. Nevertheless, it mimicked vanilloids in inhibiting iNOS/NO and COX-2/PGE(2) induction with an IC50 value of 3 muM. RT-PCR and immunoblotting analysis excluded the expression of TRPV1 in RAW264.7 macrophages. 5 The DNA binding assay demonstrated the abilities of vanilloids to inhibit LPS-elicited NF-kappaB and AP-1 activation and IFN-gamma-elicited STAT1 activation. The reporter assay of AP-1 activity also supported this action. 6 The kinase assay indicated that ERK, JNK, and IKK activation by LPS were inhibited by vanilloids. 7 In conclusion, vanilloids can modulate the expression of inflammatory iNOS and COX-2 genes in macrophages through interference with upstream signalling events of LPS and IFN-gamma. These findings provide new insights into the potential benefits of the active ingredient in hot chilli peppers in inflammatory conditions.