Prevention by synthetic phenolic antioxidants of 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)- or activated MeIQx-induced mutagenesis and MeIQx-induced rat hepatocarcinogenesis, and role of antioxidant activity in the prevention of carcinogenesis.

Abstract

Effects of synthetic phenolic antioxidants 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butylhydroquinone (TBHQ) and propyl gallate (PG) on 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)- or activated MeIQx-induced mutagenesis and rat hepatocarcinogenesis were compared, and the association between antioxidative activity and inhibition of carcinogenesis was examined. When the antimutagenic activity of five antioxidants against MeIQx- or activated MeIQx-induced mutagenesis was compared in the Ames assay using the Salmonella strain TA 98, HTHQ showed the greatest effect, followed by BHA, BHT, PG and TBHQ, in that order. In a rat hepatocarcinogenesis study, 6-week-old male F344 rats were given a single i.p. injection of 200 mg/kg bw of diethylnitrosamine (DEN) and starting 2 weeks later, groups of 15 animals received a diet containing 0.03% MeIQx alone, MeIQx together with each antioxidant at a dietary dose of 0.25%, each antioxidant alone, or basal diet alone for 6 weeks. Three weeks after the DEN injection, animals were subjected to 2/3 partial hepatectomy. Liver tissues obtained at partial hepatectomy were processed for the measurement of 8-hydroxydeoxyguanine (8-OHdG) and lipid peroxidation. The average number and areas of glutathione S-transferase placental form (GST-P) positive foci were increased by the treatment with MeIQx (27.2 +/- 6.5 per cm2 and 3.17 +/- 0.96 mm2/cm2, respectively). A significant decrease in these parameters was found with the simultaneous antioxidant treatment, HTHQ demonstrating the greatest effect, followed by BHA, BHT and TBHQ, and PG. Without MeIQx, a weak increase in the number of foci was observed in the BHT treatment case. Examination of 8-OHdG levels in liver DNA, as well as malondialdehyde (MDA) and 4-hydroxyalkenals, did not reveal any inter-group variation. These results indicate that antimutagenic activity of antioxidants against MeIQx roughly parallels their anticarcinogenic activity, with HTHQ as the most powerful chemopreventor, but that oxidative stress and antioxidative activity may not be responsible for MeIQx-induced hepatocarcinogenesis and its inhibition, respectively.