Mechanistic approach of contrasting modifying effects of caffeine on carcinogenesis in the rat colon and mammary gland induced with 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine
Introduction
Since Sugimura and co-workers’ first found that many highly mutagenic compounds, named heterocyclic amines (HCAs), are produced during cooking of meats and fish [1], ten HCAs have been shown to induce malignant tumors in rats and/or mice, some also being carcinogenic in primates [2]. Among them, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is most abundant HCA in cooked food, and confirmed to be present in human urine [3], [4], [5], [6]. In our laboratory, PhIP has been demonstrated to induce mammary, colon and prostate carcinomas in rats [7], [8], [9]. These target organs correspond with the major three organs in which malignant tumors are most common in Western countries, where consumption of meat is high. Thus, PhIP may play an important role in cancer development in man.
PhIP, like many carcinogens, requires metabolic activation in order to exert its genotoxicity and this is highly dependent upon cytochrome P450 (CYP) 1A2 to be N-hydroxy-PhIP [10], [11]. Further metabolism is through O-acetylation, catalyzed by N-acetyltransferase (NAT), but the N-acetoxy ester derivative form is unstable and spontaneously converted to a reactive electrophilic arylnitrenium ion. This ultimate metabolite is able to bind covalently to DNA and form adducts that may cause mutations and lead to induction of cancer [12]. It is thus possible that chemicals influencing induction of phase I or II enzymes can modulate PhIP-induced carcinogenesis.
Caffeine, one of the most commonly consumed compounds contained in many drinks, especially coffee and tea, is known to have a broad range of biochemical and physiological activities [13]. Several reports have documented effects of caffeine on carcinogenesis at various organ sites. Inhibition has been shown in the lung of rats and mice treated with a nicotine-derived carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone [14], [15], in the skin of mice exposed to ultraviolet light [16] and in the stomach of rats given N-methyl-N′-nitro-N-nitrosoguanidine and sodium chloride [17]. However, with initiation/promotion protocols using mammary gland specific carcinogens such as N-methyl-N-nitrosourea and dimethylbenz(a)anthracene, no inhibition was apparent [18], [19]. Since caffeine is also a hepatic CYP1A2 inducer in rats [20], it would be expected to modify PhIP metabolism and carcinogenesis. In the Ames test and a micronucleus assay with Hep-G2 cells, caffeine was found to inhibit PhIP-induced mutagenesis [21], [22]. In contrast, co-administration of caffeine and PhIP to F344 rats resulted in a significant increase of colonic aberrant crypt foci and CYP1A2 expression [23]. Interestingly, in our previous study, the simultaneous administration of PhIP and caffeine to female F344 rats for 54 weeks resulted in significant reduction of mammary gland tumor formation but colon carcinogenesis was clearly enhanced [24]. This contradictory result regarding caffeine modification of carcinogenesis may have implications with respect to human disease, because of the possible exposure to both caffeine and PhIP on a daily basis.
In the present study, in an attempt to elucidate the reasons why caffeine acts differently in the mammary glands and colon of PhIP-treated rats, we investigated its influence on PhIP-DNA adduct formation, cell proliferation, apoptosis and the expression of relevant metabolic enzymes, as well as examples involved in DNA repair.
Section snippets
Chemicals and animals
PhIP hydrochloride was obtained from the NARD Institute (Osaka, Japan) and caffeine anhydrous from Wako Pure Chemical Industries (Osaka, Japan). A total of 25 female F344 rats, 5-week-old, were purchased from Charles River Japan (Atsugi, Japan) and housed three or four to a plastic cage on hard-wood chips, in an air-conditioned room at 22±2 °C and 50% humidity with a 12/12 h light/dark cycle. They were given commercial pellet basal diet (Oriental MF; Oriental Yeast, Tokyo, Japan) and tap water
Results
The final body weights in the group treated with PhIP alone were significantly lower than those in the non-PhIP treated group (Table 1). The relative liver weights of rats treated caffeine were higher than the non-caffeine treated group values. The relative uterus weights were not significantly affected by PhIP and/or caffeine treatment. Serum levels of estrogen showed no apparent inter-group differences (data not shown).
Histological examination did not show evidence of hyperproliferation,
Discussion
PhIP is a carcinogenic heterocyclic amine to which man may be exposed through the daily diet [1], [3], [6]. Significant is the fact that it induces three major malignancies, cancers of the breast, colon and prostate in rats [7], [9], pointing to a role as a human carcinogen. Preventing PhIP carcinogenicity may therefore be an important issue for human health.
In the present study an increase of PhIP-DNA adduct formation with caffeine treatment was noted in the colon mucosa but without any change
Acknowledgements
This work was supported in part by Grants-Aid for Cancer Research from the Ministry of Education, Culture, Sports, Science and Technology and the Ministry of Health, Labour and Welfare, a Grant-in-aid from the Ministry of Health, Labour and Welfare for the Second Term Comprehensive 10-Year Strategy for Cancer Control, Japan, and a grant from the Society for Promotion of Toxicological Pathology of Nagoya, Japan.
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