THE EFFECTS OF DIETARY ELLAGIC ACID ON RAT HEPATIC AND ESOPHAGEAL MUCOSAL CYTOCHROMES P450 AND PHASE-II ENZYMES

Ellagic acid (EA), a naturally occurring plant polyphenol possesses br oad chemoprotective properties, Dietary EA has been shown to reduce th e incidence of N-2-fluorenylacetamide-induced hepatocarcinogenesis in rats and N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal tumo rs. In this study changes in the expression and activities of specific rat hepatic and esophageal mucosal cytochromes P450 (P450) and phase II enzymes following dietary EA treatment were investigated. Liver and esophageal mucosal microsomes and cytosol were prepared from three gr oups of Fisher 344 rats which were fed an AIN-76 diet containing no EA or 0.4 or 4.0 g/kg EA for 23 days, In the liver total P450 content de creased by up to 25% and P450 2E1-catalyzed p-nitrophenol hydroxylatio n decreased by 15%. No changes were observed in P450 1A1, 2B1 or 3A1/2 expression or activities or cytochrome b(5) activity. P450 reductase activity decreased by up to 28%. Microsomal epoxide hydrolase (mEH) ex pression decreased by up to 85% after EA treatment, but mEH activities did not change, The hepatic phase II enzymes glutathione S-transferas e (GST), NAD(P)H:quinone reductase [NAD-(P)H:QR] and UDP glucuronosylt ransferase (UDPGT) activities increased by up to 26, 17 and 75% respec tively, Assays for specific forms of GST indicated marked increases in the activities of isozymes 2-2 (190%), 4-4 (150%) and 5-5 (82%), In t he rat esophageal mucosa only P450 1A1 could be detected by Western bl ot analysis and androstendione was the only P450 metabolite of testost erone detectable. However, there were no differences in the expression of P450 1A1, the formation of androstendione or NAD(P)H:QR activities between control and EA-fed rats in the esophagus, Although there was no significant decrease in overall GST activity, as measured with 1-ch loro-2,4-dinitrobenzene (CDNB), there was a significant decrease in th e activity of the 2-2 isozyme (66% of control), In vitro incubations s howed that EA at a concentration of 100 mu M inhibited P450 2E1, 1A1 a nd 2B1 activities by 87, 55 and 18% respectively, but did not affect 3 A1/2 activity, Using standard steady-state kinetic analyses, EA was sh own to be a potent non-competitive inhibitor of both liver microsomal ethoxyresorufin O-deethylase and p-nitrophenol hydroxylase activities, with apparent K-i values of similar to 55 and 14 mu M respectively, I n conclusion, these results demonstrate that EA causes a decrease in t otal hepatic P450 with a significant effect on hepatic P450 2E1, incre ases some hepatic phase II enzyme activities [GST, NAD-(P)H:QR and UDP GT] and decreases hepatic mEH expression, It also inhibits the catalyt ic activity of some P450 isozymes in vitro. Thus the chemoprotective e ffect of EA against various chemically induced cancers may involve dec reases in the rates of metabolism of these carcinogens by phase I enzy mes, due to both direct inhibition of catalytic activity and modulatio n of gene expression, in addition to effects on the expression of phas e II enzymes, thereby enhancing the ability of the target tissues to d etoxify the reactive intermediates.