Laxifolone A is a triterpene isolated from Euonymus laxiflorus Champ. Exposure of RAW264.7 macrophages to laxifolone A concentration-dependently suppressed lipopolysaccharide/interferon-gamma (LPS/IFN-gamma)-induced nitrite production (IC50 = 0.37 +/- 0.05 microM), inducible NO synthase (iNOS) protein, and iNOS mRNA expression. Translocation of nuclear factor-kappaB (NF-kappaB) into the nucleus with subsequent activation of iNOS gene transcription is essential in NO signaling. Western blot analysis indicated that the cytosolic NF-kappaB/p65 was obviously decreased after LPS/IFN-gamma stimulation for 30 min and this phenomenon could be reversed by laxifolone A. Similarly, a time-related NF-kappaB/p65 nuclear translocation induced by LPS/IFN-gamma was diminished in the presence of laxifolone A. However, laxifolone A failed to interfere in LPS/IFN-gamma-evoked IkappaB degradation. Our results also showed that LPS/IFN-gamma-stimulation resulted in the degradation of NF-kappaB p105, the NF-kappaB precursor, and laxifolone A treatment significantly counteracted this effect. Furthermore, laxifolone A itself was able to enhance NF-kappaB p105 protein expression. In summary, these results suggest that inhibition of NF-kappaB p105 degradation in cytoplasm may participate in the abrogation of LPS/IFN-gamma-induced NF-kappaB translocation and subsequent NO synthesis by laxifolone A.