Kava blocks 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced lung tumorigenesis in association with reducing O6-methylguanine DNA adduct in A/J mice

Abstract

We previously reported the chemopreventive potential of kava against 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)- and benzo(a)pyrene (BaP)-induced lung tumorigenesis in A/J mice during the initiation and postinitiation stages. In this study, we investigated the tumorigenesis-stage specificity of kava, the potential active compounds, and the underlying mechanisms in NNK-induced lung tumorigenesis in A/J mice. In the first experiment, NNK-treated mice were given diets containing kava at a dose of 5 mg/g of diet during different periods. Kava treatments covering the initiation stage reduced the multiplicity of lung adenomas by approximately 99%. A minimum effective dose is yet to be defined because kava at two lower dosages (2.5 and 1.25 mg/g of diet) were equally effective as 5 mg/g of diet in completely inhibiting lung adenoma formation. Daily gavage of kava (one before, during, and after NNK treatment) completely blocked lung adenoma formation as well. Kavalactone-enriched fraction B fully recapitulated kava's chemopreventive efficacy, whereas kavalactone-free fractions A and C were much less effective. Mechanistically, kava and fraction B reduced NNK-induced DNA damage in lung tissues with a unique and preferential reduction in O(6)-methylguanine (O(6)-mG), the highly tumorigenic DNA damage by NNK, correlating and predictive of efficacy on blocking lung adenoma formation. Taken together, these results demonstrate the outstanding efficacy of kava in preventing NNK-induced lung tumorigenesis in A/J mice with high selectivity for the initiation stage in association with the reduction of O(6)-mG adduct in DNA. They also establish the knowledge basis for the identification of the active compound(s) in kava.

Figure 1

Characterization of the effect of kava and kava fractions on DNA adducts induced by NNK in the lung of A/J mice (n = 3 each group): * p < 0.05; ** p < 0.01; *** p < 0.001. A. The amount of DNA adducts at different time points after NNK treatment; NNK alone:; NNK + kava:). B. Relative amount of DNA adducts in NNK + kava treatment group at different time points after NNK treatment (the amount with kava treatment normalized to that induced by NNK alone at the same time point). C. The amount of DNA adducts with different kava fraction treatment 24 h after NNK treatment. D. Relative amount of DNA adducts by different kava fractions normalized to that induced by NNK alone at the 24 h time point.

Figure 2

Two metabolic activation pathways of NNK leading to different methylated- vs. 4–(3-pyridyl)-4-oxobut-1-yl (pob)-DNA adducts. Solid blocks indicate measured reduction in different classes of DNA adducts, as exemplified by O⁶-mG vs. 7-pobG, O²-pobdT, and O⁶-pobdG. Dashed blocks indicate hypothetical points of action by kava chemicals or their metabolites: (i) to differentially inhibit cytochrome p450 isoform-mediated NNK metabolic activation or (ii) directly react with NNK methylene hydroxylation metabolites as chemical traps.