Effects of chemoprotective agents on the metabolic activation of the carcinogenic arylamines PhIP and 4-aminobiphenyl in human and rat liver microsomes.

Carcinogenic aromatic amines, including the heterocyclic amines, may pose a significant health risk to humans. To determine the potential for chemoprotective intervention against the carcinogenicity of these arylamines and to better understand their mechanism of action, a range of agents, most of them natural dietary constituents, was examined in vitro for their ability to modulate the N-hydroxylation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 4-aminobiphenyl (ABP), an initial step in their bioactivation. Experiments were conducted with rat and human liver microsomes. The agents (diallyl sulfide, indole-3-carbinol, alpha-angelicalactone, cafestol/kahweol palmitates, cafestol, kahweol, benzylisothiocyanate, genistin, formononetin, daidzin, equol, biochanin A, Oltipraz, tannic acid, quercetin, ethoxyquin, green tea, and black tea) comprised a variety of chemical classes that included sulfur-containing compounds, antioxidants, flavonoids, phytoestrogens, diterpenes, and polyphenols. Several of these agents, quercetin, ethoxyquin, and black tea, were found to strongly inhibit PhIP N-hydroxylation in rat liver microsomes, resulting in a nearly 85-90% decrease in activity at 100 microM or 0.2%. Tannic acid and green tea, in addition to these agents, were also strong inhibitors of ABP N-hydroxylation. In human liver microsomes, each of these agents was strongly inhibitory (approx 85-95% at 100 microM or 0.02%) of PhIP and ABP N-hydroxylation. Theaflavins and polyphenols were judged to be the primary inhibiting components in the teas, the theaflavins showing the most potent effect. These results demonstrate that chemoprotective agents can inhibit the bioactivation of carcinogenic arylamines, and this is likely to be one of the mechanisms of protection.

Inhibition of PhIP mutagenicity by catechins, and by theaflavins and gallate esters.

Aqueous solutions of gallic acid, methyl gallate, catechins, theaflavins and tannic acid were tested for inhibition of the mutagenicity of PhIP in the Salmonella typhimurium TA98 assay with an S9 fraction from the liver of rats induced with alpha-naphthoflavone and phenobarbital. The IC50S were in the 80-250 microM range for the gallated catechins, theaflavins and tannic acid. No inhibition could be found with these compounds when a direct acting mutagen was used. This indicates that the anti-mutagenic properties of these phenolic compounds may be due to their inhibition of the cytochrome P-450 enzymes.

Effects of theaflavins on N-nitrosomethylbenzylamine-induced esophageal tumorigenesis.

The purpose of this experiment was to compare the inhibitory effects of the polyphenol fraction of black tea, theaflavins (TF), the polyphenol fraction of green tea, and (-)-epigallocatechin-3-gallate (EGCG) in the rat esophageal tumor model. The tea fractions were administered in the drinking water at concentrations of 360 and 1,200 ppm for two weeks before administration of the esophageal carcinogen N-nitrosomethylbenzylamine (NMBA). NMBA was administered subcutaneously in 10% dimethyl sulfoxide three times weekly for five weeks. Additional groups of rats received only vehicle and plain drinking water or vehicle and drinking water containing 1,200 ppm of each tea fraction. Twenty-five weeks after NMBA administration began, the experiment was terminated and esophagi were excised and scored for tumors. Rats that were not dosed with NMBA had no tumors. Rats treated with NMBA only had an esophageal tumor incidence of 100% and a multiplicity of 3.3 +/- 0.4 tumors/rat. The proportion of rats developing tumors was not significantly reduced by any of the four tea fractions at the concentrations tested. However, the 1,200 ppm concentrations of each tea fraction in the drinking water produced some reduction in esophageal tumor multiplicity, although only TF significantly reduced tumor multiplicity compared with rats treated with NMBA only. The rates of esophageal tumor formation were significantly reduced at 360 and 1,200 ppm by TF and EGCG.

Inhibition of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) mutagenicity by black and green tea extracts and polyphenols.

Solutions of lyophilized preparations of standard black and green tea extracts were made and tested over a range of six concentrations as inhibitors of the mutagenicity caused by the fool mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the Salmonella typhimurium TA98 assay containing S9 fraction from rats induced with alpha-naphthoflavone and phenobarbital. Extracts of both black and green tea were equally good inhibitors of mutagenicity. Purified polyphenols were prepared from tea extracts by solvent extraction. The polyphenols of black tea were more potent inhibitors of mutagenicity than the polyphenols of green tea. These findings suggest that black tea may have similar health-promoting properties to those reported previously for green tea.