Metabolic activation pathway for the formation of DNA adducts of the carcinogen 2-amino-l-methyl-6-phenyUmidazo[4,5-b]pyridine (PhIP) in rat extrahepatic tissues

Abstract

The food-borne mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induces tumors in colon of male rats and has been implicated in the etiology of human cancers, particularly colorectal cancer. This study was conducted to examine: (1) the biliary and/or circulatory transport of N-hydroxy-PhIP and its N-glucuronides, N-sulfonyloxy-PhIP and N-acetoxy-PhIP; (2) their role as proximate and ultimate carcinogenic metabolites of PhIP; (3) the potential role of glutathione in modulating PhIP-DNA adduct formation. PhIP-DNA adducts, measured by the 32P-postlabeling method, were highest in the pancreas (361 adducts/10(8) nucleotides or 100%), followed by colon (56%), lung (28%), heart (27%) and liver (2%), at 24 h after a single oral dose of PhIP (220 mumol/kg) to male rats. In each tissue examined, we observed two major adducts, each of which accounted for 35-45% of the total, and one minor adduct, which represented about 10-20% of the total. One of the major adducts was identified as N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6-phenylimidazo[4,5- b]pyridine by chromatographic comparisons with an authentic standard. The major urinary metabolites of PhIP in these rats were 4'-hydroxy-PhIP and its glucuronide and sulfate conjugates, followed by N-hydroxy-PhIP N3-glucuronide, N-hydroxy-PhIP N2-glucuronide and unchanged PhIP. In bile duct-ligated rats, the urinary excretion of the N-OH-PhIP N3-glucuronide was increased two-fold, but there was no effect on PhIP-DNA adduct formation in the colon, heart, lung, pancreas or liver. 2,6-Dichloro-4-nitrophenol, which strongly inhibits arylsulfo-transferase-mediated DNA binding in vivo, had no effect on PhIP-DNA adduct levels in liver or in extrahepatic tissues. Pretreatment of rats with buthionine sulfoximine, which results in hepatic glutathione depletion, caused a five-fold increase in adduct formation in the liver. Intravenous administration (10 mumol/kg) of N-hydroxy-PhIP and N-acetoxy-PhIP each led to high levels of PhIP-DNA adducts in each of the extrahepatic tissues examined. Adduct levels ranged from two- to six-fold higher (for N-hydroxy-PhIP) and four- to 28-fold higher (for N-acetoxy-PhIP) as compared to that after an i.v. dose of the parent compound, indicating that these two bioactivated derivatives of PhIP are sufficiently stable to be transported through the circulation to extrahepatic tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of rat pulmonary carcinogen-metabolising enzyme systems by the isothiocyanates erucin and sulforaphane

“…These carcinogens are indirect-acting so that in order to elicit their carcinogenicity they must first be metabolically converted to DNA-binding species, and this process is largely catalysed by cytochromes P450 [27]. Such bioactivation can take place in situ, but there is also experimental evidence to support the view that reactive intermediates generated in the liver, the principal site of bioactivation, may be transported to extrahepatic tissues although the underlying mechanism(s) remain elusive [28,29]. A number of studies have linked pulmonary CYP1 activity to human cancer incidence, denoting the importance of in situ metabolism.…”

Meat and Meat-Mutagen Intake and Pancreatic Cancer Risk in the NIH-AARP Cohort

“…Our findings are further supported by rodent studies that have shown DiMeIQx (48) and PhIP (49-51) to enhance pancreatic carcinogenesis. PhIP DNA adducts are formed at high levels in the pancreas of animals (52,53) and have been detected with higher intensity in pancreatic tissue of cancer patients compared with noncancer controls (54,55). Polymorphisms in genes that are involved in HCA metabolism have also been associated with pancreatic cancer (56,57).…”

Chemoprevention of heterocyclic amine‐induced mammary carcinogenesis in rats

“…Unexpectedly, caffeine treatment was found to significantly increase the incidence of colon adenomas. Other investigators have demonstrated that caffeine can enhance PhIP induction of colonic aberrant crypt foci [Tsuda et al, 1999], possibly by influencing levels of CYP1A2 or N-acetyltransferase in colonic tissue, or biliary transport of conjugated N-hydroxy-PhIP, which is then deconjugated by the intes- tinal flora [Kaderlik et al, 1994]. Epidemiological studies investigating a possible relationship between caffeine consumption and breast [Boyle et al, 1984;Phelps and Phelps, 1988] and colon [Slattery et al, 1990[Slattery et al, , 1999 cancers in humans have yielded inconstant and inconclusive results.…”