Genetic analysis of the susceptibility in rats to aberrant crypt foci formation by 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine, PhIP

Identification and chromosome mapping of loci predisposing to colorectal cancer that control Wnt/beta-catenin pathway and progression of early lesions in the rat

Sporadic colorectal cancer (CRC) is a major health concern worldwide. Epidemiologic evidence suggests a polygenic predisposition to CRC, but the genes responsible remain unknown. Here, we performed genome-wide scanning of male (ACI/SegHsd x Wistar-Furth)F2 (AWF2) rats to map susceptibility genes influencing the evolution of early colorectal lesions to adenocarcinoma following 1,2-dimethylhydrazine administration. Phenotypic analysis revealed higher incidence/multiplicity and lower size of adenomas in ACI/SegHsd (ACI) and (ACI/SegHsd x Wistar-Furth)F1 (AWF1) than Wistar-Furth (WF) rats and higher incidence/multiplicity of poorly differentiated adenocarcinomas in WF than ACI rats, with intermediate values in AWF1 rats. Linkage analysis of 138 AWF2 rats identified three loci on chromosomes 4, 15 and 18 in significant linkage with lesion multiplicity that were identified as rat Colon cancer resistance (rCcr) 1, rCcr2 and rCcr3, respectively. Seven other loci on chromosomes 5, 6, 15, 17, 18 and 20 were in suggestive linkage with adenoma/adenocarcinoma multiplicity/surface area. Six of them were identified as rCcr4-9 and a locus on chromosome 5 was identified as a susceptibility locus, rCcs1. Significant interactions between rCcr3 and rCcr6, rCcr6 and rCcr8 and rCcr5 and rCcr9, and four novel epistatic loci controlling multiplicity/size of colorectal lesions were discovered. Apc, located at rCcr3, did not show functional promoter polymorphisms. However, influence of susceptibility/resistance genes on Wnt/beta-catenin pathway was shown by defective beta-catenin inactivation in WF but not in ACI and AWF1 rat adenocarcinomas. These data indicate that inheritance of predisposition to CRC depends on interplays of several genetic factors, and suggest a possible mechanism of polygenic control of CRC progression.

Significance and role of early-lesions in experimental colorectal carcinogenesis

Preneoplastic or precancerous lesions in the large bowel have attracted much attention, and aberrant crypt foci (ACF) topographically identified in the colonic mucosa have found application as effective endpoint lesions for detection of chemopreventive agents as well as carcinogenic risk assessment of environmental agents. While many ACF are regarded as hyperplastic in nature, lacking the potential lesion to give rise to neoplasia, a subset termed dysplastic ACF, or newly identified "mucin depleted foci (MDF)", and "beta-catenin accumulated crypts (BCAC)" are suggested to be more reliably related to colorectal tumorigenesis in rodents. ACF and MDF can be visualized on the surface of colonic mucosa and BCAC were recently identified by our laboratory in rodents en face in cross sections. In particular, BCAC having a similar pattern of beta-catenin gene mutation to that observed in colonic carcinomas appear to be direct precursors. This review provides a review and discussion of the relevant literature relative to early lesions in colorectal carcinogenesis.

Colonic adenocarcinomas rapidly induced by the combined treatment with 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and dextran sodium sulfate in male ICR mice possess beta-catenin gene mutations and increases immunoreactivity for beta-catenin, cyclooxygenase-2 and inducible nitric oxide synthase

Heterocyclic amines are known to be important environmental carcinogens in several organs including the colon. The aim of this study was to induce colonic epithelial malignancies within a short-term period and analyze the expression of cycooxygenase (COX)-2, inducible nitric oxide synthase (iNOS) and beta-catenin, and mutations of beta-catenin gene in induced tumors. Male Crj: CD-1 mice were given a single i.g. administration (200 mg/kg body wt) of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) or 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) followed by 2% dextran sodium sulfate (DSS) in the drinking water for a week. The expression of beta-catenin, COX-2 and iNOS was immunohistochemically assessed in colonic epithelial lesions and the beta-catenin gene mutations in colonic adenocarcinomas induced were analyzed by the single strand conformation polymorphism method, restriction enzyme fragment length polymorphism and direct sequencing. At week 16, a high incidence of colonic neoplasms with dysplastic lesions developed in mice that received PhIP and DSS, but only a few developed in those given MeIQx and DSS. Immunohistochemically, the adenocarcinomas induced were all positive for three proteins. All seven adenocarcinomas induced by PhIP and DSS have mutations. The findings suggest that DSS exerts powerful tumor-promoting effects on PhIP-initiated colon carcinogenesis in mice and this mouse model is useful for investigating environment-related colon carcinogenesis within a short-term period.

Aberrant crypt foci and beta-catenin accumulated crypts; significance and roles for colorectal carcinogenesis

Preneoplastic or precancerous lesions in the large bowel have been characterized in terms of morphology and histochemical phenotype. However, the detailed histogenesis and relation of particular lesions to malignancies has not yet to be unequivocally clarified. Aberrant crypt foci (ACF), identified in whole-mount preparations of colonic mucosa in rodents and also recognized in human colon, are now frequently used as effective surrogate biomarkers for experimentally detection of chemopreventive agents against colorectal cancers, but the preneoplastic or precancerous nature of ACF in rodents and humans still remains inconclusive. Relatively recently, early appearing beta-catenin accumulated crypts (BCAC) have been described in en face preparations of colonic mucosa in rodents which differ from ACF in many features. BCAC are suggested to be premalignant rather than preneoplastic. The pathological significance of both lesions, including their advantages and disadvantages as surrogate end points for large bowel neoplasms, and roles in colorectal carcinogenesis are discussed here.

Heterocyclic amines: Mutagens/carcinogens produced during cooking of meat and fish

Research leading to the discovery of a series of mutagenic and carcinogenic heterocyclic amines (HCAs) was inspired by the idea that smoke produced during cooking of food, especially meat or fish, might be carcinogenic. More than ten kinds of HCAs, actually produced by cooking or heating of meat or fish, have now been isolated and their structures determined, most being previously unregistered compounds. They are highly mutagenic towards Salmonella typhimurium in the presence of S9 mix and are also mutagenic in vitro and in vivo toward mammalian cells. HCAs have now been chemically synthesized in quantity and subjected to long-term animal testing. When HCAs were fed in the diet, rodents developed cancers in many organs, including the colon, breast and prostate, and one HCA produced hepatomas in monkeys. The lesions exhibited alteration in genes including Apc, beta-catenin and Ha-ras, and these changes provide clues to the induction mechanisms. The HCAs are oxidized to hydroxyamino derivatives by cytochrome P450s, and further converted to ester forms by acetyltransferase and sulfotransferase. Eventually, they produce DNA adducts through the formation of N-C bonds at guanine bases. There are HCA-sensitive and resistant strains of rodents and a search for the responsible genes is now under way. While the content of HCAs in dishes consumed in ordinary life is low and not sufficient in itself to explain human cancer, the coexistence of many other mutagens/carcinogens of either autobiotic or xenobiotic type and the possibility that HCAs induce genomic instability and heightened sensitivity to tumor promoters suggest that avoidance of exposure to HCAs or reduction of HCAs' biological effects as far as possible are to be highly recommended. Usage of microwave ovens for cooking and supplementation of the diet, for example with soy-isoflavones, which have been found to suppress the occurrence of HCA-induced breast cancers, should be encouraged. Advice to the general public about how to reduce the carcinogenic load imposed by HCAs would be an important contribution to cancer prevention.

Differential gene expression profiles in colon epithelium of two rat strains with distinct susceptibility to colon carcinogenesis after exposure to PhIP in combination with dietary high fat

Colon cancers develop through accumulation of multiple genetic and epigenetic alterations in colon epithelial cells, and the environment of the genetically altered epithelial cells may also have a substantial impact on their further development to cancer. In the present study, groups of 6-week-old F344 and ACI male rats, the former strain being susceptible to colon carcinogenesis induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and the latter being relatively resistant, were subjected to a long-term carcinogenesis experiment using our intermittent feeding protocol of PhIP in combination with a high-fat diet, which serves as a relevant risk factor that promotes the development of colon cancers. Animals were sacrificed at 60 weeks, and global gene expression analyses of normal parts of colon epithelial tissues were conducted using a high-density oligonucleotide microarray to elucidate the differential gene expression profile (environment) in normal colonic regions between F344 and ACI strains. Of 8799 entries on the RatU34A array, 74 genes exhibited 3-fold or greater variation. A subset of genes encoding ribosomal RNAs and proteins were highly preferentially expressed in the F344 strain. In addition, genes encoding fatty acid binding proteins and the peroxisome membrane protein 70 appeared up-regulated in the susceptible F344 strain. In the ACI strain, a mismatch repair gene, Msh2, was preferentially expressed, at approximately 20-fold the F344 level, along with a gene encoding a detoxification enzyme, catechol-O-methyltransferase. The combined effects of the repertoire of these differentially expressed genes in normal colon epithelial tissues may account for the distinct susceptibilities of F344 and ACI strains to colon carcinogenesis.

Comments on the history and importance of aromatic and heterocyclic amines in public health

The carcinogenic risk of aromatic amines in humans was first discovered when a physician related the occurrence of urinary bladder cancer to the occupation of his patients. They were employed in the dyestuff industry, chronically exposed to large amounts of intermediate arylamines. Laboratory investigations disclosed that rats and mice administered specific azo dyes arylamines or derivatives developed cancer, primarily in the liver. Also, at that time, a possible pesticide, 2-aminofluorene, was tested for chronic toxicity, revealing that it rapidly induced cancers in several organs of rodents. This led to investigations on the mode of action of this class of chemicals, including their metabolic conversion. Biochemical activation to more reactive N-hydroxy compounds was found to occur, mostly in the liver, through what is now known as the cytochrome p450 enzyme systems, and also through prostaglandin synthetases. There were species differences. Guinea pigs were resistant to carcinogenesis because of the low titer of the necessary activating enzymes. In target tissues, a second essential reaction was necessary, namely acylation or sulfate ester formation. The reactive compounds produced display attributes of genotoxicity in appropriate test systems. Interest in this class of compounds increased when of Sugimura and colleagues discovered the formation of mutagens at the surface of cooked meat or fish, that were identified as heterocyclic amines (HCAs). These compounds undergo the same type of activation reactions, as do other arylamines. Epidemiological data suggest that meat eaters may have a higher risk of breast and colon cancer. HCAs induced cancer in rats in these organs and also in the prostate and the pancreas. In addition, there is some evidence that they affect the vascular system. The formation of HCAs during cooking can be decreased by natural and synthetic antioxidants, by tryptophan or proline, or by removing the essential creatine through brief microwave cooking prior to frying or broiling. The amounts of HCAs in cooked foods are small, but other components in diet such as omega-6-polyunsaturated oils have powerful promoting effects in target organs of HCAs. On the other hand, the action of HCAs may be decreased by foods containing antioxidants, such as vegetables, soy, and tea. Some constituents in foods also induce phase II enzymes that detoxify reactive HCA metabolites. Additional mechanisms involved decreased growth of neoplasms by intake of protective foods. Possibly, the carcinogenic effect of HCAs is accompanied by the presence of reactive oxygen species (ROS), which are also inhibited by antioxidants. World-wide, there have been many contributors to knowledge in this field. Adequate information may permit now to adjust lifestyle and lower the risk of human disease stemming from this entire class of aryl and HCA.

A rat colon cancer model induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, PhIP

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is one of the most abundant heterocyclic amines contained in cooked meat and fish, and induces aberrant crypt foci (ACF), putative preneoplastic lesions of the colon, and colon cancers in male rats when administered orally. As has been reported previously, F344 rats are susceptible to induction of ACF by PhIP, while ACI rats being relatively resistant. Approximately one-fourth of ACF induced by PhIP in F344 rats are dysplastic; exhibiting lesions with structural distortion of the crypt, decrease of goblet cells, nuclear stratification and enlargement of nuclei. Dysplastic ACF demonstrate beta-catenin accumulation, mainly in the cytoplasm, and increased cell proliferation in crypts. These dysplastic ACF are, therefore, strongly considered to be putative preneoplastic lesions of the colon.A genetic trait affecting the susceptibility to colon carcinogenesis in F344 rats was mapped to chromosome 16, between D16Rat17 and D16Wox3, using the number of ACF as a surrogate biomarker for colon carcinogenesis. Since the number of dysplastic lesions is well correlated with the total number of ACF, being approximately one-fourth of the total ACF as described above in F344 rats and will be described elsewhere in ACI rats, the gene involved in the susceptibility to ACF induction may possibly be partly responsible for the susceptibility to colon carcinogenesis by PhIP. We, thus, tentatively referred the name of the candidate susceptibility gene on rat chromosome 16 as susceptibility to colon tumor (Sct). In the present study, the colonic lesions induced by PhIP were well refined histologically and genetically, and the multi-step profiles of colon cancer development by PhIP were well characterized and revealed to be similar to the multi-step model of colon carcinogenesis in humans. The PhIP-induced colon cancer model in rats, thus contributes as a relevant tool to elucidate genetic factors responsible for susceptibility to colon carcinogenesis in human. Other unknown genetic or epigenetic alterations, which are essential for the development of early lesions of colon carcinogenesis, could also be clarified using this system.