Potent antimutagenic activity of white tea in comparison with green tea in the Salmonella assay.

There is growing interest in the potential health benefits of tea, including the antimutagenic properties. Four varieties of white tea, which represent the least processed form of tea, were shown to have marked antimutagenic activity in the Salmonella assay, particularly in the presence of S9. The most active of these teas, Exotica China white tea, was significantly more effective than Premium green tea (Dragonwell special grade) against 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and four other heterocyclic amine mutagens, namely 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethyl-3H-imidazo[4,5-f]quinoxaline (4,8-DiMeIQx), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2). Mechanism studies were performed using rat liver S9 in assays for methoxyresorufin O-demethylase (MROD), a marker for the enzyme cytochrome P4501A2 that activates heterocyclic amines, as well as Salmonella assays with the direct-acting mutagen 2-hydroxyamino-3-methylimidazo[4,5-f]quinoline (N-hydroxy-IQ). White tea at low concentrations in the assay inhibited MROD activity, and attenuated the mutagenic activity of N-hydroxy-IQ in the absence of S9. Nine of the major constituents found in green tea also were detected in white tea, including high levels of epigallocatechin-3-gallate (EGCG) and several other polyphenols. When these major constituents were mixed to produce "artificial" teas, according to their relative levels in white and green teas, the complete tea exhibited higher antimutagenic potency compared with the corresponding artificial tea. The results suggest that the greater inhibitory potency of white versus green tea in the Salmonella assay might be related to the relative levels of the nine major constituents, perhaps acting synergistically with other (minor) constituents, to inhibit mutagen activation as well as "scavenging" the reactive intermediate(s).

Colonic cell proliferation, apoptosis and aberrant crypt foci development in rats given 2-amino-3-methylimidaz.

Sodium-copper chlorophyllin (CHL) inhibits the formation of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)- and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced colonic aberrant crypt foci (ACF) and tumours in the F344 rat when it is given simultaneously with either carcinogen. However, CHL reportedly increased the incidence of dimethylhydrazine (DMH)-induced colon tumours in the same species when administered post-initiation. In the present study, rats were given IQ (130 mg/kg body weight, by oral gavages on alternating days) for 2 weeks, starting in experiment week 3, and one week after the final IQ dose rats received CHL treatment until the study was terminated at 16 weeks. Compared with animals given carcinogen alone, the mean number of IQ-induced ACF per colon was reduced significantly by 1% (w/v) CHL in the drinking water (P < 0.05), whereas 0.1% and 0.01% CHL had no effect. These CHL concentrations increased in a dose-related manner both the terminal deoxynucleotidyl transferase-mediated nick end labelling (TUNEL) and bromodeoxyuridine (BrdU) labelling indices in the distal colon. However, the lowest concentration tested, 0.001% CHL, increased the mean number of IQ-induced ACF per colon (P < 0.05), and increased the BrdU labelling index without a concomitant change in TUNEL. These studies indicated that 0.001% CHL promoted IQ-ACF due to deregulation of the homeostatic balance between cell birth and apoptosis in the colonic mucosa, whereas higher concentrations of CHL had either no effect or protected against IQ-induced ACF by causing dose-related increases in the overall rate of cell turnover in the colon.

Differential effects of dihalogenated and trihalogenated acetates in the liver of B6C3F1 mice.

Haloacetates are produced in the chlorination of drinking water in the range 10--100 microg l(-1). As bromide concentrations increase, brominated haloacetates such as bromodichloroacetate (BDCA), bromochloroacetate (BCA) and dibromoacetate (DBA) appear at higher concentrations than the chlorinated haloacetates: dichloroacetate (DCA) or trichloroacetate (TCA). Both DCA and TCA differ in their hepatic effects; TCA produces peroxisome proliferation as measured by increases in cyanide-insensitive acyl CoA oxidase activity, whereas DCA increases glycogen concentrations. In order to determine whether the brominated haloacetates DBA, BCA and BDCA resemble DCA or TCA more closely, mice were administered DBA, BCA and BDCA in the drinking water at concentrations of 0.2--3 g l(-1). Both BCA and DBA caused liver glycogen accumulation to a similar degree as DCA (12 weeks). The accumulation of glycogen occurred in cells scattered throughout the acinus in a pattern very similar to that observed in control mice. In contrast, TCA and low concentrations of BDCA (0.3 g l(-1)) reduced liver glycogen content, especially in the central lobular region. The high concentration of BDCA (3 g l(-1)) produced a pattern of glycogen distribution similar to that in DCA-treated and control mice. This effect with a high concentration of BDCA may be attributable to the metabolism of BDCA to DCA. All dihaloacetates reduced serum insulin levels. Conversely, trihaloacetates had no significant effects on serum insulin levels. Dibromoacetate was the only brominated haloacetate that consistently increased acyl-CoA oxidase activity and rates of cell replication in the liver. These results further distinguish the effects of the dihaloacetates from those of peroxisome proliferators like TCA.

beta-Catenin mutation in rat colon tumors initiated by 1,2-dimethylhydrazine and 2-amino-3-methylimidazo[4,5-f]quinoline, and the effect of post-initiation treatment with chlorophyllin and indole-3-carbinol.

Carcinogens 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 1,2-dimethylhydrazine (DMH) induce colon tumors in the rat that contain mutations in beta-catenin, but the pattern of mutation differs from that found in human colon cancers. In both species, mutations affect the glycogen synthase kinase-3beta consensus region of beta-catenin, but whereas they directly substitute critical Ser/Thr phosphorylation sites in human colon cancers, the majority of mutations cluster around Ser33 in the rat tumors. Two dietary phytochemicals, chlorophyllin and indole-3-carbinol, given post-initiation, shifted the pattern of beta-catenin mutations in rat colon tumors induced by IQ and DMH. Specifically, 17/39 (44%) of the beta-catenin mutations in groups given carcinogen plus modulator were in codons 37, 41 and 45, and substituted critical Ser/Thr residues directly, as seen in human colon cancers. None of the tumors from groups given carcinogen alone had mutations in these codons. Interestingly, many of the mutations that substituted critical Ser/Thr residues in beta-catenin were from a single group given DMH and 0.001% chlorophyllin, in which a statistically significant increase in colon tumor multiplicity was observed compared with the group given DMH only. These tumors had marked over-expression of cyclin D1, c-myc and c-jun mRNA and c-Myc and c-Jun proteins were strongly elevated compared with tumors containing wild-type beta-catenin. The results indicate that the pattern of beta-catenin mutations in rat colon tumors can be influenced by exposure to dietary phytochemicals administered post-initiation, and that the mechanism might involve the altered expression of beta-catenin/Tcf/Lef target genes.

Post-initiation effects of chlorophyllin and indole-3-carbinol in rats given 1,2-dimethylhydrazine or 2-amino-3-methyl- imidazo.

Chlorophyllin (CHL) is a water-soluble derivative of chlorophyll, the ubiquitous pigment in green and leafy vegetables, whereas indole-3-carbinol (I3C) is present in cruciferous vegetables such as cabbage, broccoli and cauliflower. In rats initiated with 1,2-dimethylhydrazine (DMH), CHL and I3C reportedly promoted or enhanced the incidence of colon tumors when they were administered after, or during and after the carcinogen exposure, respectively. The same compounds given post-initiation inhibited the formation of colonic aberrant crypts induced by heterocyclic amines, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), but tumor suppression was not examined in the latter studies. In the present investigation, male F344 rats were treated with IQ or DMH during the first 5 weeks of a 1 year study; IQ was given in the diet (0.03%), whereas DMH was administered once a week by s.c. injection (20 mg/kg body wt). Beginning 1 week after the last dose of IQ or DMH until sacrifice, rats received 0.001, 0.01 or 0.1% (w/v) CHL in the drinking water or 0.001, 0.01 or 0.1% I3C in the diet. Compared with controls given carcinogen alone, 0.1% I3C treatment suppressed the multiplicity of IQ-induced colon tumors, and CHL inhibited in a dose-related manner the incidence of IQ-induced liver tumors. However, 0.001% CHL increased significantly the multiplicity of DMH-induced colon tumors while having no effect on the colon tumors induced by IQ. These results indicate that both the choice of carcinogen as well as the dose of the tumor modulator can be important determinants of the events that occur during post-initiation exposure to CHL or I3C. Based on the present findings and data in the literature, it is possible for CHL and I3C to act as tumor promoters or anticarcinogens, depending upon the test species, initiating agent and exposure protocol.

Inhibition by white tea of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced colonic aberrant crypts in the F344 rat.

There is growing interest in the potential health benefits of tea, including the anticarcinogenic properties. We report here that white tea, the least processed form of tea, is a potent inhibitor of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced colonic aberrant crypts in the rat. Male Fischer 344 rats were treated for 8 wk with white tea (2% wt/vol) or drinking water alone, and on alternating days in experimental Weeks 3 and 4 the animals were given PhIP (150 mg/kg body wt p.o.) or vehicle alone. At the end of the study there were 5.65 +/- 0.81 and 1.31 +/- 0.27 (SD) aberrant crypt foci per colon in groups given PhIP and PhIP + white tea, respectively (n = 12, P < 0.05). No changes were detected in N-acetyltransferase or arylsulfotransferase activities compared with controls, but there was marked induction of ethoxyresorufin O-deethylase, methoxyresorufin O-demethylase, and UDP-glucuronosyltransferase after treatment with white tea. Western blot revealed corresponding increases in cytochrome P-450 1A1 and 1A2 proteins. Enzyme assays and Western blot also revealed induction of glutathione S-transferase by white tea. There was less parent compound and 4'-hydroxy-PhIP but more PhIP-4'-O-glucuronide and PhIP-4'-O-sulfate in the urine from rats given PhIP + white tea than in urine from animals given carcinogen + drinking water. The results indicate that white tea inhibits PhIP-induced aberrant crypt foci by altering the expression of carcinogen-metabolizing enzymes, such that there is increased ring hydroxylation at the 4' position coupled with enhanced phase 2 conjugation.

In vivo MRI measurements of tumor growth induced by dichloroacetate: implications for mode of action.

Dichloroacetate (DCA) is an important by-product of the chlorination of drinking water that produces liver cancer in rodents. Assessment of the risk that results from concentrations that occur in drinking water will be dependent upon the mode of action held responsible for these tumors. A study by Stauber and Bull [Stauber, A.J. and Bull, R. J (1997) Differences in phenotype and cell replicative behavior of hepatic tumors inducted by dichloroacetate (DCA) and trichloroacetate (TCA). Toxicol. Appl. Pharmacol. 144, 235-246] in mice treated with DCA demonstrated a lesion distribution that was skewed towards many small, altered foci of cells that are assumed to be precursor lesions [EPA, (1996). U.S. Environmental Protection Agency: Proposed Guidelines for carcinogen risk assessment; notice. Fed. Reg. 61, pp. 17960-10811]. The present study was designed to determine the extent to which the tumorigenic effects of DCA could be explained by its effect on tumor growth rates (i.e. tumor promoting activity). In vivo magnetic resonance imaging (MRI) allowed accurate determination of growth rates of individual lesions in mice that had been treated with DCA in drinking water at 2 g/l. Out of thirty treated mice, ten were found to have hepatic tumors detectable by MRI at 48 weeks of treatment. These tumor-bearing animals were assigned to two groups matched on the size of lesions observed by in vivo MR1. Treatment with DCA continued in one group of five mice and was stopped in the other. For both groups, tumor growth rates were determined by measuring changes in size of all lesions greater than 1 mm(3) in volume during a 14-day period. Removal of DCA treatment resulted in growth rates that could not be distinguished from zero across all lesion sizes represented in the sample. These data are in agreement with previous observations of DCAs effects on replication rates within tumors (Stauber and Bull, (1997)). Tumor growth rates observed in animals maintained on treatment decreased with lesion volume in a manner that is consistent with a stochastic Gompertz birth-death process proposed by Tan [Tan, W.Y. (1986) A stochastic Gompertz birth-death process. Stat. Prob. Lett. 4, 25-28]. Parameters of this model obtained by fitting measured growth rates were used to predict the lesion-size distribution expected after one year of DCA treatment. The shape of the predicted lesion-size distribution was similar to that observed by Stauber and Bull (Stauber and Bull, (1997)) in mice sacrificed after 40 weeks of DCA treatment. We conclude that the effects of DCA on the division and/or death rates of spontaneously initiated cells can account for the predominance of small lesions in DCA-treated animals.

Potency of dietary indole-3-carbinol as a promoter of aflatoxin B1-initiated hepatocarcinogenesis: results from a 9000 animal tumor study.

Indole-3-carbinol (I3C), a metabolite of glucobrassicin found in cruciferous vegetables, is documented as acting as a modulator of carcinogenesis and, depending on timing and dose of administration, it may promote hepatocarcinogenesis in some animal models. In this study we demonstrate that, when given post-initiation, dietary I3C promotes aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in the rainbow trout model at levels as low as 500 p.p.m. Trout embryos (approximately 9000) were initiated with 0, 25, 50, 100, 175 or 250 p.p.b. AFB1 by a 30 min immersion. Experimental diets containing 0, 250, 500, 750, 1000 or 1250 p.p.m. I3C were administered starting at 3 months and fish were sampled for liver tumors at 11-13 months. Promotion at the level of tumor incidence was statistically significant for all dietary levels, except 250 p.p.m. Relative potency for promotion markedly increased at dietary levels >750 p.p.m. We propose that more than one mechanism could be involved in promotion and that both estrogenic and Ah receptor-mediated pathways could be active. The estrogenicity of I3C, measured as its ability to induce vitellogenin (an estrogen biomarker in oviparous vertebrates) was evident at the lowest dietary level (250 p.p.m.), whereas CYPIA (a P450 isozyme induced through the Ah receptor pathway) was not induced until dietary levels of 1000 p.p.m. Therefore, at lower dietary levels, promotion by I3C in this model could be explained by estrogenic activities of I3C acid derivatives, as it is known that estrogens promote hepatocarcinogenesis in trout. Much stronger promotion was observed at high dietary I3C levels (1000 and 1250 p.p.m.), at which levels both CYP1A and vitellogenin were induced.

Effects of dichloroacetate on glycogen metabolism in B6C3F1 mice.

Dichloroacetate (DCA) is a by-product of drinking water chlorination. Administration of DCA in drinking water results in accumulation of glycogen in the liver of B6C3F1 mice. To investigate the processes affecting liver glycogen accumulation, male B6C3F1 mice were administered DCA in drinking water at levels varying from 0.1 to 3 g/l for up to 8 weeks. Liver glycogen synthase (GS) and glycogen phosphorylase (GP) activities, liver glycogen content, serum glucose and insulin levels were analyzed. To determine whether effects were primary or attributable to increased glycogen synthesis, some mice were fasted and administered a glucose challenge (20 min before sacrifice). DCA treatments in drinking water caused glycogen accumulation in a dose-dependent manner. The DCA treatment in drinking water suppressed the activity ratio of GS measured in mice sacrificed at 9:00 AM, but not at 3:00 AM. However, net glycogen synthesis after glucose challenge was increased with DCA treatments for 1-2 weeks duration, but the effect was no longer observed at 8 weeks. Degradation of glycogen by fasting decreased progressively as the treatment period was increased, and no longer occurred at 8 weeks. A shift of the liver glycogen-iodine spectrum from DCA-treated mice was observed relative to that of control mice, suggesting a change in the physical form of glycogen. These data suggest that DCA-induced glycogen accumulation at high doses is related to decreases in the degradation rate. When DCA was administered by single intraperitoneal (i.p.) injection to naïve mice at doses of 2-200 mg/kg at the time of glucose challenge, a biphasic response was observed. Doses of 10-25 mg/kg increased both plasma glucose and insulin concentrations. In contrast, very high i.p. doses of DCA (> 75 mg/kg) produced progressive decreases in serum glucose and glycogen deposition in the liver. Since the blood levels of DCA produced by these higher i.p. doses were significantly higher than observed with drinking water treatment, we conclude that apparent differences with data of previous investigations is related to substantial differences in systemic dose and/or dose-time relations.

Modulation of aflatoxin-B1 hepatocarcinogenesis in trout by dehydroepiandrosterone: initiation/post-initiation and latency effects.

Previously, we demonstrated that dehydroepiandrosterone (DHEA) enhances aflatoxin B1 (AFB1) hepatocarcinogenesis in trout when administered following AFB1 exposure. This paper examines the effect of DHEA on tumor latency and the comparative potency for DHEA to modulate AFB1 carcinogenesis when administered prior to and concurrent with AFB1 compared with a post-initiation exposure. Trout were initiated by a 30 min water bath exposure to 10 p.p.b. AFB1. At 3 months post-initiation, animals were started on either control diet or a diet containing 444 p.p.m. dehydroepiandrosterone (DHEA). Fifty trout per treatment were sampled prior to the start of experimental diets, and then at monthly intervals for the next 7 months and examined for the presence of tumors. Tumors were not detected in initiated controls until 7 months after initiation. In initiated trout fed DHEA, the first tumor was detected 5 months after initiation (after just 2 months of dietary DHEA). At 6 months post-initiation, 20% of the AFB1-initiated trout fed DHEA had tumors, while no tumors were visible in either AFB1-initiated controls or noninitiated trout fed DHEA. A second experiment was designed to determine if the enhancing effect of DHEA on AFB1 carcinogenesis is dependent on the time of DHEA administration relative to the time of AFB1 exposure, and if DHEA could be chemopreventive if administered prior to and concurrent with AFB1. Trout were fed one of two levels of DHEA (888 or 1776 p.p.m.) either prior to and during a 4-week initiation period of dietary AFB1 administration, or for 8 weeks following initiation with AFB1. At 9 months after initiation the livers were examined for tumors. Neither exposure protocol provided protection towards AFB1 hepatocarcinogenesis. The strongest enhancement occurred when DHEA was fed during the post-initiation period. Levels of p53 and p34cdc2 were decreased by DHEA treatment, indicating that DHEA may act through alterations in cell-cycle control.

Modulation of N-methyl-N'-nitro-nitrosoguanidine multiorgan carcinogenesis by dehydroepiandrosterone in rainbow trout.

Dehydroepiandrosterone (DHEA) and its sulfate conjugate are the major circulating steroids in human plasma. Low levels of these adrenal steroids are associated with a number of human diseases including certain cancers. In animal studies, DHEA is chemopreventive toward both spontaneous and chemically induced cancers. A potential concern for long-term usage of DHEA in humans is the finding that DHEA is hepatocarcinogenic in rats. The human health risk has been thought to be minimal, however, as the mechanism of DHEA hepatocarcinogenesis is assumed to be due to its properties as a peroxisome proliferator, a class of compounds to which humans are relatively insensitive. Recently, we have found DHEA to be a potent promoter of aflatoxin B1-initiation as well as a complete hepatocarcinogen in the rainbow trout, a species which is also insensitive to peroxisome proliferators. In order to determine the initiator- and tissue-specificity of DHEA promotion, we examined the effects of DHEA on N-methyl-N'-nitro-nitrosoguanidine (MNNG)-initiated carcinogenesis. Trout fry were initiated by a bath exposure (30 min at 35 ppm) to MNNG and then fed DHEA at levels of 0, 55, 111, 222, 444, or 888 ppm for 7 months. DHEA increased liver tumor incidence, multiplicity, and size in a dose-dependent manner. The liver tumor incidence ranged from 0 in the MNNG-initiated controls to 99% in initiated trout fed 888 ppm DHEA. The latter represents a potential synergistic interaction in liver between MNNG and DHEA, as tumor incidence in sham-initiated trout fed this level of DHEA was 41%. The kidney tumor incidence was also enhanced two- and threefold over initiated controls by 111 and 888 ppm DHEA, respectively. In contrast, the total number of stomach and swim bladder tumors was reduced by DHEA treatment. This study demonstrates differential effects of DHEA on MNNG-initiated carcinogenesis in liver, kidney, stomach, and swim bladder.

Comparison of the enhancing effects of dehydroepiandrosterone with the structural analog 16 alpha-fluoro-5-androsten-17-one on aflatoxin B1 hepatocarcinogenesis in rainbow trout.

Dehydroepiandrosterone (DHEA) is an adrenal steroid with chemoprotective effects against a wide variety of conditions including cancer, obesity, diabetes, and cardiovascular disease. However, DHEA is also a carcinogen in laboratory animals, possibly through its function as a precursor of sex steroids or peroxisome proliferation. The structural analog 16 alpha-fluoro-5-androsten-17-one (8354) has been reported to have enhanced chemopreventive activity without the steroid precursor and peroxisome proliferating effects of DHEA. This study compares DHEA and 8354 in rainbow trout, a species that is resistant to peroxisome proliferation but is highly susceptible to the carcinogenic and tumor enhancing effects of DHEA. Trout were exposed as fry to aflatoxin B1 (AFB1) or given a sham exposure, then were fed diets containing 444 ppm DHEA or 8354 for 6 months. Postinitiation treatment with DHEA significantly increased liver tumor incidence, multiplicity, and size compared to initiated controls. The analog 8354 slightly increased tumor incidence (p = 0.06) but had no effect on multiplicity or size. Six percent of trout treated with DHEA alone developed tumors, whereas no tumors occurred in noninitiated trout fed control or 8354-containing diets. Serum levels of androstenedione were elevated by DHEA (48-fold) or 8354 (6-fold) treatment. Serum beta-estradiol titers were increased in DHEA- but not 8354-treated trout. Vitellogenin was induced significantly by either DHEA (434-fold) or 8354 (21-fold). Peroxisomal beta-oxidation was not increased by either compound and catalase activity was decreased in DHEA-treated animals. Both steroids were potent inhibitors in vitro of trout liver glucose-6-phosphate dehydrogenase with IC50s of 24 and 0.5 microM for DHEA and 8354, respectively. This research suggests that in trout the tumor enhancing effects of DHEA may be due to its function as a sex steroid precursor and are unrelated to peroxisome proliferation. These carcinogenic properties are reduced in the analog 8354 which has been advocated as an alternative to DHEA for chemoprevention.

Dehydroepiandrosterone is a complete hepatocarcinogen and potent tumor promoter in the absence of peroxisome proliferation in rainbow trout.

Dehydroepiandrosterone (DHEA), fed for 30 weeks to rainbow trout after initiation with the hepatocarcinogen aflatoxin B1 (AFB1), produced a dose-dependent enhancement of carcinogenesis as measured by increased tumor incidence, multiplicity and size. Significant enhancement was observed at 222 p.p.m., which corresponds to a daily dosage one-half that previously administered to humans in clinical trials. DHEA was also capable of acting as a complete carcinogen in this model, producing liver tumors at doses as low as 222-444 p.p.m. Tumors isolated from trout treated with DHEA alone contained mutations in Ki-ras, primarily codon 12[1] G-->A transitions, providing the first suggestive evidence that DHEA could be a genotoxic carcinogen. The carcinogenicity of DHEA in trout is independent of peroxisome proliferation, as measurements of peroxisomal beta-oxidation and catalase activity support previous observations that trout, like humans, are weak responders to peroxisome proliferators.

Dietary hydrogen peroxide enhances hepatocarcinogenesis in trout: correlation with 8-hydroxy-2'-deoxyguanosine levels in liver DNA.

The tumor-enhancing effect of hydrogen peroxide (H2O2) in N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-initiated rainbow trout hepatocarcinogenesis was investigated and correlated with the levels of the mutagenic DNA adduct 8-hydroxy-2'-deoxyguanosine (oh8dG). In addition, the protective role of vitamin E was examined in relation to tumor enhancement and oh8dG levels in liver DNA. Trout were fed diets containing two levels of vitamin E (1000 or 20 mg/kg wet wt), each of which were made up to contain three levels of H2O2 (0, 600 or 3000 p.p.m.). Dietary vitamin E levels had no significant effect on tumor incidence or levels of oh8dG in liver DNA. On the other hand, dietary H2O2 enhanced liver tumors in a dose-dependent manner. Liver tumor incidence correlated significantly with the mean level of liver DNA oh8dG content (r = 0.87). We conclude that the H2O2 tumor-enhancing effect coincides with higher levels of oh8dG in the trout liver genome. Thus, rainbow trout may be a useful model for the study of the relationship of oh8dG levels in vivo to enhancement or promotion of carcinogenesis and its modulation by dietary enhancers and inhibitors of oxidative stress.