What do diet-induced changes in phase I and II enzymes tell us about prevention from exposure to heterocyclic amines?

Well-done cooked protein containing foods derived from muscle can contain 1–200 parts per billion of mutagenic/carcinogenic heterocyclic amines. The most abundant of these compounds, 1-methyl-6-phenyl-1H-imidazo[4,5-b]pyridin-2-amine (PhIP), has recently been classified by the National Toxicology Program (NTP) to be ‘‘a reasonably anticipated human carcinogen.’’ This is in part because of the extensive human epidemiology data associated with these heterocyclic amines (1). In .30 human epidemiology studies done to correlate cancer incidence and well-done meat consumption, 80% of them show a positive correlation.This includes cancer sites inbreast, colon, rectum, esophagus, larynx, lung,pancreas, and prostate.The relative risks range from 2 to 3 in the breast to 8 in the colon and prostate. Human exposure to these carcinogens predominantly occurs by consumption of well-done cooked meats. Once ingested, these compounds are rapidly metabolized and distributed throughout the body, where they can cause mutations in multiple tissues. Studies have shown that the internal dose of these compounds can be reduced by binding up the heterocyclic amines by other foods in the diet (2). Artificial digestion of muscle foods and subsequent human studies, where total urinary metabolites were examined after consumption of heterocyclic amines from grilled chicken, show that foods traditionally high in fiber such as pasta, prunes, and beans may be able to bind to the carcinogens, thus reducing the internal dose of these carcinogens when these foods are consumed in the diet at the same time. Numerous laboratories have shown that heterocyclic amines are first metabolized by cytochrome P4501A2 to bioactive N-hydroxy intermediates that can then be either detoxified by Phase II conjugation enzymes such as UDP-glucuronosyltransferase (UGT) or further activated by other Phase II enzymes (i.e., acetyltransferase, sulfotransferase). These reactive enzymatic products presumably act as leaving groups. The reactive free radicals then bind almost exclusively to the C-8 of guanine, causing DNA adducts, mutations, chromosomal abnormalities, and cancer in multiple tissues. Human differences in response to the internal exposure to these compounds most likely depends on 2 major factors: 1) individual genotypic differences in the activities of the Phase I and II enzymes and 2) the dietary modulation of the enzyme levels by natural inducers or inhibitors found in our food. An example of individual differences that might promote DNA damage can best be illustrated by those related to UGTs. Polymorphic expression of several UGT isoforms has been reported. These polymorphisms have been implicated as risk factors for several diseases, including cancer (3). The most notable polymorphisms are variants in the UGT1A1 gene that result in significant downregulation of UGT1A1 activity. The most common polymorphism is characterized by an allelic variant in the UGT1A1 gene that contains an additional (TA) dinucleotide repeat in the ‘‘A(TA)nTAA’’ box region of the promoter (4). Wild-type UGT1A1 activity is associated with 6 repeats, whereas the variant allele contains 7 TA repeats (UGT1A1*28). This variance results in significant downregulation of UGT1A1 activity and occurs in 7–10% of the general population. It is also the polymorphism associated with Gilbert’s syndrome, which is a benign form of hyperbilirubinemia that results from a reduced capacity to glucuronidate bilirubin. There is evidence to suggest that individuals with Gilbert’s syndrome may be at greater risk from exposure to heterocyclic amines that are conjugated by UGT1A1 because their ability to detoxify these compounds would be diminished (5). For example, a recent study has reported a correlation between the UGT1A1*28 polymorphism and a decreased ability to glucuronidate and detoxify N-hydroxy-PhIP in human liver microsomes (6). Subjects possessing the UGT1A1*28 allelic variant (which contains (TA)7 repeats) showed significant decreases in UGT1A1 protein expression and N-hydroxy-PhIP glucuronidation activity in liver microsomes when compared with subjects having the wild-type UGT1A1*1 genotype. 1 Published in a supplement to The Journal of Nutrition. Presented as part of the conference ‘‘The Use and Misuse of Biomarkers as Indicators of Cancer Risk Reduction Following Dietary Manipulation’’ held July 12–13, 2005 in Bethesda, MD. This conference was sponsored by the Center for Food Safety and Applied Nutrition (CFSAN), Food and Drug Administration (FDA), Department of Health and Human Services (DHHS); the Office of Dietary Supplements (ODS), NIH, DHHS; and the Division of Cancer Prevention (DCP), National Cancer Institute, NIH, DHHS. Guest Editors for the supplement publication were Harold E. Seifried, National Cancer Institute, NIH; and Claudine Kavanaugh, CFSAN, FDA. Guest editor disclosure: H.E. Seifried, no relationships to disclose; C. Kavanaugh, no relationships to disclose. 2 This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, and supported by NCI Grant CA55861. The contents are solely the responsibility of the authors. 3 Author disclosure: no relationships to disclose. 4 Abbreviations used: NTP, National Toxicology Program; PAH, polycyclic aromatic hydrocarbon PhIP, 1-Methyl-6-phenyl-1H-imidazo[4,5-b]pyridin-2-amine; UGT, UDP-glucuronosyltransferase. * To whom correspondence should be addressed. E-mail: [email protected].