Ellagic acid: a potent naturally occurring inhibitor of benzo[a]pyrene metabolism and its subsequent glucuronidation, sulfation and covalent binding to DNA in cultured BALB/C mouse keratinocytes

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Abstract
The metabolism of [ 3 H]benzo[a]pyrene (BP) by cultured primary keratinocytes prepared from BALB/C mouse epidermis was found to be largely inhibited by the dietary plant phenol, ellagic acid. Varying concentrations of ellagic acid added to the keratinocyte cultures resulted in a dosedependent inhibition of the cytochrome P-450-dependent monooxygenases aryl hydrocarbon hydroxylase (AHH) and 7-ethoxycoumarin-0-deethylase (ECD). The major organic solvent-extractable metabolites found intracellularly in the cultured cells were trans -7,8-dihydro-7,8-dihydroxybenzo[a]-pyrene (BP-7,8-diol) and 3-hydroxybenzo[a]pyrene (3-OH-BP), although small amounts of 9-hydroxybenzo[a]pyrene, quinones and trans -9,10-dihydro-9,10-dihydroxybenzo[a]-pyrene (BP-9,10-diol) were also present. The major organic solvent-extractable metabolites found in the extracellular culture medium were BP-7,8-diol and BP-9,10-diol, with smaller quantities of unconjugated phenols and quinones. The major intracellular and extracellular water-soluble metabolites of BP were conjugated with glucuronide (primarily 3-OH-BP and several BP-quinones), and to a lesser extent with sulfate (primarily BP-7,8-diol). Both intracellular and extracellular metabolism of organic solvent-extractable and water-soluble conjugates was significantly inhibited by ellagic acid in a dose-dependent manner. The intracellular enzyme-mediated binding of BP to mouse keratinocyte DNA was also largely inhibited in a dose-dependent fashion by ellagic acid. Our results indicate that cultured primary mouse keratinocytes offer a useful model system for studying factors affecting the metabolic activation and detoxification of polycyclic aromatic hydrocarbon carcinogens in the epidermis, and that polyphenolic compounds such as ellagic acid may prove useful in modulating the risk of cutaneous cancer that results from exposure to these environmental chemicals.