The effect of chemical induction and bile acids on phase I and II biotransformation enzymes in the multistep process of colon carcinogenesis
The study objectives were twofold. One, to determine if modulation of colon phase I and II biotransformation enzymes altered the colon's susceptibility to cancer. Two, to delineate the effects of experimental protocol on bile acid mediated colon carcinogenesis. Four studies were conducted. In general, a multi injection azoxymethane (AOM) (2, 15 mg/kg BW, s.c., injections given 1 week apart) colon carcinogenesis model in Sprague Dawley rats provided the study framework. NADPH-cytochrome P450 reductase (CYC), glutathione S-transferase (GST) and UDP-glucuronyltransferase (UGT) enzyme activities were determined in the colon. Number and growth features of aberrant crypt foci (ACF) and tumor incidence were used as biological end points. Colonic proliferation and apoptotic bodies, biological risk markers of the disease process, were assessed. Phenobarbital (PB) and 3-methylcholanthrene (MC), classical inducers of phase I and II enzymes, were used in the induction study. MC induction had a long lasting effect on colon enzymes and this treatment group had fewer ACF than PB or positive control (AOM and saline treated) animals. In contrast, the effect of PB induction on colon enzymes was of short duration and this treatment produced an ACF outcome similar to the positive control animals. These observations suggest prolonged, heightened enzyme activity was protective against AOM induced colon cancer. The effect of bile acids on biotransformation enzymes was examined in a 5 week feeding trial. The enzyme response to bile acids was specific, CYC activity increased and GST activity was inhibited by bile acids. This study provided the framework to test the effect of bile acid inhibition of biotransformation enzymes in colon carcinogenesis. Bile acid mediated colon cancer was examined in two studies. To determine cholic acid's (CHA) role in the early or post initiation phases of cancer, CHA (0.2% by weight) was fed in conjunction with the carcinogen AOM or delayed until two weeks after the second AOM injection. CHA affected the early rather than the post initiation stages of colon cancer. Introduction of CHA in c njunction with AOM promoted ACF with advanced growth features and tumor outcome. Delaying CHA introduction produced an ACF and tumor outcome similar to control. The second study examined the role of deoxycholic (DCA) and lithocholic acid (LCA) in the post initiation phases of cancer. Diets containing 0.2% by weight DCA, LCA and high fat (HF) diets (20%) (positive control) were introduced two weeks after the second AOM injection. HF diets provided the most stimulatory environment, enhancing the number and growth of ACF and adenomatous lesions at both weeks 12 and 20. LCA also enhanced ACF outcome, but a time lag in its response was observed. DCA produced a response similarto the control group. HF and LCA affect the post initiation phases of colon cancer, while DCA does not. Bile acid inhibition of colonic GST and UGT was associated with increase in ACF parameters and colonic tumor outcome, suggesting in vivo inhibition of GST and UGT increased AOM toxicity. In conclusion, bile acid inhibition of colonic biotransformation enzymes plays a role in colon carcinogenesis, and future studies involved in the identification and evaluation of potential modulators of colon carcinogenesis must consider the importance of experimental protocol in their assessments.