Phenolics: Blocking agents for heterocyclic amine-induced carcinogenesis

Chemopreventive effects of synthetic and naturally occurring antioxidants o n heterocyclic amine (HCA)-induced rat carcinogenesis and mechanisms of inh ibition were assessed. In a medium-term liver bioassay, combined treatment with 0.03% 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and synthe tic antioxidants such as 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), buty lated hydroxyanisole (BHA), butylated hydroxutoluene (BHT), tert-butylhydro quinone (TBHQ) or propyl gallate, each at a dose of 0.25%, inhibited develo pment of preneoplastic glutathione S-transferase placental form (GST-P) pos itive foci as compared with MeIQx alone, after initiation with diethylnitro samine (DEN). Of these antioxidants, HTHQ showed the greatest activity. 8-H ydroxydeoxyguanosine (8-OHdG), a marker for DNA damage induced by active ox ygen species, and malonedialdehyde and 4-hydroxynonenal levels were not lar gely influenced by the treatment with MeIQx or antioxidants, either alone o r in combination. In the same medium-term liver bioassay, effects of some n aturally occurring antioxidants, such as green tea catechins (GTC), hesperi din, chlorogenic acid, quercetin, rutin, curcumin, daidzin, ferulic acid an d genistein were also examined. Of these antioxidants, only GTC tended to i nhibit GST-P positive foci development, while quercetin, rutin, curcumin, d aidzin, ferulic acid and genistein all exerted significant enhancing effect s. Examination of HTHQ influence in a medium term liver bioassay with HCA G lu-P-1, in which the experimental period was extended for up to 26 weeks, a lso demonstrated a significant decrease in the incidence of liver tumours t o 40% in the group treated with 0.5% HTHQ and 0.03% 2-amino-6-methyldipyrid o[1,2-a:3',2'-d]imidazole (Glu-P-1) as compared with the Glu-P-1 alone valu e of 89%. Effects of HTHQ on colon carcinogenesis induced by 2-amino-1-meth yl-6-phenylimidazo[4,5-b] pyridine (PhIP) were evaluated in a two-stage col on carcinogenesis model using 1,2-dimethylhydrazine (DMH) as an initiator. At week 36, the multiplicity of colon tumours induced by 0.02% PhIP after D MH initiation (9.1 +/- 6.2/rat) was dose-dependently decreased by the combi ned treatment with 0.5% HTHQ (3.6 +/- 1.8, P < 0.001) and 0.125% HTHQ (6.2 +/- 3.2, not significant). Similarly, the incidence of mammary carcinomas i n female F344 rats induced by oral administration of 0.02% PhIP (40%) for 5 2 weeks was significantly decreased by simultaneous treatment with 0.5% HTH Q (5%). alpha-Tocopherol and chlorophyllin only reduced the multiplicity of carcinomas. Analysis of the influence of HTHQ on metabolic activation of G lu-P-1 or PhIP after incubation with rat S9 mixture and NADPH by HPLC, reve aled that each major metabolite was strongly reduced by the addition of HTH Q. Immunohistochemically detected PhIP-DNA adduct positive nuclei in the co lon induced by continuous oral treatment with 0.02% PhIP for 2 weeks decrea sed by the combined treatment with 0.5 or 0.125% HTHQ. These results indica te that synthetic antioxidant HTHQ is a very strong chemopreventor of heter ocyclic amine (HCA)-induced carcinogenesis and that depressed metabolic act ivation rather than antioxidant activity is responsible for the observed ef fect. (C) 1999 Elsevier Science Ltd. All rights reserved.