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

Abstract

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-γ-mediated NO production, and iNOS protein and mRNA expression with similar IC 50 values of around 10 μM. 3. Capsaicin also transcriptionally inhibited LPS- and PMA-induced COX-2 expression and PGE 2 production. However, this effect exhibited a higher potency (IC 50: 0.2 μM), and RTX failed to elicit such responses at 10 μM. 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 IC 50 value of 3 μM. 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-κB and AP-1 activation and IFN-γ- 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-γ. These findings provide new insights into the potential benefits of the active ingredient in hot chilli peppers in inflammatory conditions.