Preferential induction of guanine deletion at 5'-GGGA-3' in rat mammary glands by 2-amino- 1-methyl-6-phenylimidazo[4,5-b]pyridine.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is known to induce a characteristic mutation, G deletion at the 5'-GGGA-3' site, preferentially in the lacI transgene of the colonic mucosa of Big Blue((R)) rats (BBR) and mice and specifically in the Apc gene of rat colon tumors. In this study, lacI mutations of the mammary glands in PhIP-treated rats were investigated. Six-week-old female (BBRxSprague-Dawley)F(1) rats were administered 10 gavages of 65 mg/kg/day PhIP. Mammary ducts were collected from the macroscopically normal mammary tissue of PhIP-treated and untreated rats at 56-69 weeks of age by collagenase treatment. The mutant frequencies were 25 +/- 2.1x10(-6) in control rats and 323 +/- 44x10(-6) in the PhIP-treated rats. By sequencing 40 and 177 mutants in the control and PhIP-treated groups, respectively, 34 and 149 mutations were considered independent mutations. In the control group, G:C-->A:T transitions at CpG sites dominated and no G:C deletions were detected. In the PhIP-treated group, G:C-->T:A transversions were most frequent (43%), followed by single base pair deletions of G:C (21%). A total of nine deletions were at 5'-GGGA-3' sites, accounting for 29% of the G:C deletions and 6% of the 149 total mutations. Clusters of more than three mutations at one nucleotide position were observed at 12 positions and two were G deletions at 5'-GGGA-3' sites. Comparison of the PhIP-induced mutations in the mammary glands with those previously reported in the colon revealed that G:C-->T:A transversions occurred at a significantly higher frequency in the mammary glands and that G:C deletions occurred at a significantly lower frequency. However, the signature mutation, G deletion at the 5'-GGGA-3' site, was commonly observed in both tissues.     

Mutagenicity of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the mammary gland of Big Blue rats on high-and low-fat diets

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a food-borne mutagen and mammary gland carcinogen in female rats. A high-fat diet has been shown to increase the incidence of PhIP-induced mammary gland tumors. The current study used Big Blue rats harboring the lambda lacI mutational reporter transgene, to address whether the promotional effect of a high-fat diet is mediated via modulation in mammary gland mutagenesis. Big Blue rats were given 10 doses of PhIP (75 mg/kg, p.o.) and placed on defined low-fat (5% corn oil) or high-fat (23.5% corn oil) diet for 6 weeks prior to collecting mammary glands. The lacI mutant frequency (mean +/- standard error, n = 3 rats) was 231 +/- 15 (x10(-6)) and 193 +/- 12 (x10(-6)) in the low-and high-fat group, respectively. Values were increased 12-fold over control but were not significantly different between the two diets. In a parallel study, diet did not alter the mutant frequency induced by 7,12-dimethylbenz[a]anthracene (DMBA) (125 mg/kg, p.o.) in the mammary gland. The findings suggest that the promotion by the high-fat diet is not mediated via an increase in mutations. Consistent with the high potency of DMBA as a mammary carcinogen, the mutant frequency was 20-30% higher with DMBA than with PhIP. Sixty-nine and 56 PhIP-induced lacI mutants were sequenced from the low-and high-fat diet groups, respectively. While the percentage of various types of mutations was identical between the diet groups, some difference in the distribution of mutations along the lacI gene was observed. The mutation spectrum in the mammary gland from rats on both diets was consistent with the formation of PhIP-guanine adducts which were detected by a (32)P-post-labeling assay. Guanine base substitutions accounted for approximately 85% of all mutations irrespective of diet. Single base pair deletions at guanine occurred in 11-17% of mutants. G:C to T:A transversions were the predominant base substitution mutation accounting for 35-43% of all mutations. The majority of all guanine mutations (74%) occurred at guanine bases adjacent to another G:C pair. Five out of 125 (4%) mutations involved a guanine deletion in the 5'-GGGA-3' sequence, a PhIP signature mutation reported previously. Twelve out of 125 (10%) mutations involved the guanine base in the sequence 5'-CAG(Purine)-3' (Pu). The findings from these studies suggest that 5'-CAG(Pu)-3' is an additional characteristic target site for PhIP-guanine adduct-induced mutations in vivo in the mammary gland.     

The dietary charred meat carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine acts as both a tumor initiator and promoter in the rat ventral prostate

Exposure of Fisher344 rats to 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a heterocyclic amine in cooked meat, causes cancer in the rat ventral prostate, while sparing the dorsolateral and anterior lobes. Uncovering the molecular mechanisms of the lobe specificity of PhIP-induced rat prostate cancer may provide clues to the pathogenesis of human prostate cancer, which is also lobe selective. We examined the prostate and other organs for mutation frequencies using transgenic Fisher344 rats (Big Blue rats) after PhIP treatment. After PhIP treatment for as early as 4 weeks, the colon, spleen, seminal vesicles, and all lobes of the prostate had significantly elevated mutation frequencies compared with the saline-treated control group, and the differences became even greater after 8 weeks. G:C --> T:A transversions were the predominant type of mutation. After 8 weeks of treatment with PhIP, the Ki-67 index was increased (P < 0.001) in the ventral prostate, but not in the dorsolateral or anterior prostate. An increase in the number of stromal mast cells and macrophages was seen in the ventral prostate, but not in the other prostatic lobes. The apoptotic index also increased in the ventral lobe only. The increased proliferation and cell death in response to PhIP indicates that in addition to PhIP acting as an "initiator" of cancer, PhIP is also acting like an organ- and lobe-specific tumor "promoter." The prostate lobe-specific infiltration of mast cells and macrophages in response to PhIP suggests a potential new mechanism by which this dietary compound can increase cancer risk-by prompting inflammation.     

Specificity of mutations induced by the food-associated heterocyclic amine 2-amino-1-methyl-6-phenylimidazo-[4,5-b]-pyridine in colon cancer cell lines defective in mismatch repair

Recently, we have shown a hypermutable response to the food-associated heterocyclic amine 2-amino-1-methyl-6-phenylimidazo-[4,5-b]-pyridine (PhIP) in human cells defective in mismatch repair (MMR). These findings suggest that exogenous compounds such as PhIP may play an important role in the generation of tumors in MMR-defective individuals. The specificity of mutations induced by PhIP exposure at the endogenous HPRT locus was determined in cell lines defective in MMR to better understand the mutagenic effects of PhIP in MMR-defective individuals and to gain insight into the molecular mechanism of carcinogenesis induced by PhIP. Eighty-six induced HPRT mutants from two different cell lines were isolated and sequenced after exposure to 10 microM PhIP. Nineteen (22%) of these mutants contained G:C to T:A transversion mutations, consistent with the promutagenic adduct of PhIP at the C8 position of guanine miscoding with adenine. This level of PhIP-induced G:C to T:A transversions was approximately 4.5-fold higher than spontaneous G:C to T:A frequencies. Additionally, a hotspot for mutation was observed in a run of six guanines in HPRT exon 3, where a total of 23 (27%) of all PhIP-induced mutations occurred. These mutations consisted of transversions, transitions, and frameshift mutations. The increase in mutant frequency at this run of guanines corresponded to a 24-fold elevation above the spontaneous frequency in one cell line and a 3.3-fold increase in the other. These data suggest that PhIP may increase the risk of human carcinogenesis mediated by MMR by increasing mutations at runs of guanine residues. PhIP may thereby promote tumorigenesis by mutating growth-regulating genes that contain runs of guanines in their coding sequences, such as BAX, the insulin-like growth factor II receptor IGFIIR, and even the mismatch repair gene hMSH6.     

Characterization of mutations induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in the colon of gpt delta transgenic mouse: novel G:C deletions beside runs of identical bases

Mutations induced by one of the typical dietary mutagens/carcinogens, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), were characterized using gpt delta transgenic mice. This transgenic mouse model has two selection methods to efficiently detect different types of mutations, i.e. 6-thioguanine selection for point mutations and Spi(-) selection for deletions. The mice were fed with a diet containing 400 p.p.m. PhIP for 13 weeks and gpt and Spi(-) mutations were analyzed from the colon, where the highest mutant frequencies were detected. Concerning the types of gpt mutations from PhIP-treated mice, 81% were single base pair substitutions and G:C-->T:A transversions predominated; single base pair deletions at G:C base pairs were also observed. In untreated mice G:C-->A:T transitions predominated and >80% of these events involved 5'-CpG-3' sites. Concerning Spi(-) mutants from PhIP-treated mice, 76% were G:C base pair deletions and more than half of these events occurred in monotonic G or C run sequences. Interestingly, a novel type of frameshift motif, i.e. G:C base pair deletions beside run sequences, was observed. The most frequently observed mutation in this class was the 5'-TTTTTTG-3'-->5'-TTTTTT-3' event. These results suggest that PhIP induces point mutations, such as base substitutions and single base pair deletions, rather than larger deletions in vivo and that run sequences may play an important role in PhIP-induced G:C base pair deletions.     

Agreement of mutational characteristics of heterocyclic amines in lacI of the Big Blue mouse with those in tumor related genes in rodents

The mutational spectra of carcinogenic heterocyclic amines (HCAs), 2- amino-3,4-dimethylimidazo[4,5-b]quinoline (MeIQ), 2-amino-1-methyl-6- phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-9H-pyrido[2,3-b]indole (A alphaC) were studied in the colon of Big Blue mice. In 90, 115 and 105 lacI mutants from mice fed 300 p.p.m. MeIQ, 400 p.p.m. PhIP and 800 p.p.m. A alphaC, respectively, 92, 115 and 105 mutations were identified. G:C-->T:A transversions predominated with these HCAs. Mutational hot spots for base-substitution mutations caused by MeIQ, PhIP and A alphaC were in distinct sequence contexts; at 5'-GC-3', in runs of guanine and in 5'-CGT-3', respectively. Further, 30 of 115 (26%) PhIP-induced mutations were G:C base pair deletions, and eight of these deletions were in 5'-GGGA-3'. The mutational characteristics of MeIQ in the lacI gene coincided well with those in the Ha-ras gene of MeIQ-induced mouse forestomach tumors and rat Zymbal gland tumors. The characteristic single-base deletion induced by PhIP in the lacI gene also coincided well with those in the Apc gene of PhIP-induced rat colon tumors. These results suggest that the mutational characteristics of each chemical are conserved across different genes in different species.      

Diallyl sulfide inhibits PhIP-induced DNA strand breaks in normal human breast epithelial cells

Heterocyclic amines (HCAs) are formed when meat products such as beef, chicken, pork and fish are cooked at high temperatures. The most abundant HCA found in the human diet is 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP). PhIP causes mammary carcinomas in female rats and mice, and is associated with an increased risk of developing colon, breast, and prostate cancer in humans. PhIP is metabolized by cytochrome P-450s producing N-OH-PhIP. The N-OH-PhIP can be esterified by phase II enzymes forming an arylnitrenium ion that binds to DNA causing adducts. Furthermore, N-OH-PhIP may be reduced by cytochrome b5 reductase producing superoxide anions and hydroxyl radicals causing DNA strand breaks. Diallyl sulfide (DAS) has been shown to prevent cancer in several animal models, presumably by metabolic modulation. We hypothesize that PhIP produces reactive oxygen species causing DNA strand breaks and that DAS will inhibit the formation of PhIP induced DNA strand breaks. To test this hypothesis we treated normal breast epithelial (MCF-10A) cells with PhIP, DAS and a combination of PhIP and DAS. The detection of lipid peroxides was used as a surrogate for ROS. Lipid peroxides were detected using a PeroxiDetect kit (Sigma). PhIP increased the production of lipid peroxides and DAS decreased the PhIP-induced peroxidation by 47%. To determine if PhIP causes DNA strand breaks in MCF-10A cells, cells were treated for 3, 6, 9, and 24 h with PhIP (100 microM), DAS (100 microM) and a combination of PhIP (100 microM) and DAS (100 microM). DNA strand breaks were evaluated using the Comet assay. PhIP produced DNA strand breaks in a dose- and time-dependent fashion. We have shown that DAS inhibits PhIP-induced DNA strand breaks by inhibiting the production of reactive oxygen species. Therefore, we propose that DAS can prevent PhIP-induced breast cancer.     

Reduction in formation of 2-amino-1-methyl-6-phenylimidazo[4,5- b]pyridine (PhIP)-induced aberrant crypt foci in the rat colon by docosahexaenoic acid (DHA)

Docosahexaenoic acid (DHA), a major component of fish oil, suppresses the formation and growth of aberrant crypt foci induced by 1,2- dimethylhydrazine and azoxymethane. In the present study we examined the effects of intragastric gavage administration of DHA on the yield of rat colonic aberrant crypt foci due to treatment with a heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), which induces colon cancer in male F344 rats and is considered to be a possible human colon carcinogen. Male F344 rats were given a standard diet (AIN-76A) and received 10 doses of PhIP (75 mg/kg body wt, by intragastric intubation, on days 1-5 and 8-12) with or without intragastric application of 1 ml DHA 4 h prior to each carcinogen treatment, followed by further DHA dosing. The numbers of PhIP-induced aberrant crypt foci per colon after 4 and 12 weeks DHA administration were significantly reduced to 47 and 38% respectively of the values obtained when PhIP alone was used. The mean number of aberrant crypts per focus was also decreased by DHA treatment. At week 4 the PhIP-DNA adduct levels in the colon of rats from the PhIP+DHA group were approximately two thirds of the PhIP group value. The results thus suggest that DHA exerts a preventive effect on PhIP-induced colon carcinogenesis.      

The cooked meat carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine activates the extracellular signal regulated kinase mitogen-activated protein kinase pathway

During the cooking of meat, mutagenic and carcinogenic heterocyclic amines are formed, the most abundant of which, 2-amino-1-methyl-6-phenylimidazo[4-5-b]pyridine (PhIP), induces tumors of the prostate, colon, and mammary gland in rats. Humans consuming cooked meat are exposed to PhIP on a daily basis, yet few studies have assessed the effects of PhIP at dietary relevant concentrations. In addition to its genotoxic properties, recent studies have shown that PhIP can activate estrogen receptor-mediated signaling pathways at doses that are similar to those that may be present in the body following consumption of a cooked meat meal. In the present study, we examined whether such doses of PhIP can affect estrogen receptor-independent signal transduction via the mitogen-activated protein kinase (MAPK) extracellular signal-related kinase (ERK) pathway to influence proliferation and migration in the human mammary epithelial cell line MCF10A and the prostate cancer cell line PC-3. At doses shown to have a proliferative effect on MCF10A cells (10(-11)-10(-7) mol/L), PhIP induced a rapid, transient increase in phosphorylation of both MAPK/ERK kinase 1/2 and ERKs. Inhibition of this pathway significantly reduced the PhIP-induced proliferation of MCF10A cells and the migration of PC-3 cells. The data presented here show that levels of PhIP that approximate to human dietary exposure stimulate cellular signaling pathways and result in increased growth and migration, processes linked to the promotion and progression of neoplastic disease. These findings provide strong evidence that PhIP acts as a tumor initiator and promoter and that dietary exposure to this compound could contribute to carcinogenesis in humans.     

The food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine activates S-phase checkpoint and apoptosis, and induces gene mutation in human lymphoblastoid TK6 cells

The mutagenic heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-blpyridine (PhIP) is formed at parts per billion levels when meat is cooked. It is efficiently absorbed from cooked food and extensively activated to its genotoxic N-hydroxy derivative by human cytochrome P4501A enzymes. It is also a rodent carcinogen. To better understand the genetic toxicity of PhIP, we have examined its effect on the cell cycle and gene mutation frequency using human lymphoblastoid cells (TK6) as a model. Because TK6 cells are unable to activate PhIP, we have cultured the cells in the presence of irradiated Chinese hamster XEMh1A2-MZ cells that have been genetically engineered to express human CYP1A2. Asynchronized TK6 cells were harvested at various times after treatment with PhIP (1.25-10 microg/ml), fixed and stained with propidium iodide for the examination of cell cycle by fluorescence-activated flow cytometry. After 20 h of PhIP treatment, a slight S-phase delay of the cell cycle was observed. Normal cell cycle recovered after the cells were washed and further cultured in the absence of PhIP for 5 days. However, PhIP treatment for 40 h induced a more pronounced S-phase arrest that was accompanied by a decrease in the level of cyclin A, an S-phase cyclin. This was followed by the appearance of a sub-G1 population (indicative of apoptotic cell death), range from 13 to 54% with PhIP concentrations from 1.25 to 10 microg/ml, compared with 5% in the vehicle control. A concomitant increase of mutation frequency at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus, assessed by colony formation assay in the presence of 6-thioguanine, was detected after 40 h-range, 16 to 45 x 10(-6) compared with 12 x 10(-6) in cultures without PhIP. In G1-enriched cell populations (synchronized culture), although PhIP induced S-phase delay, the induction of sub-G1 cells was substantially decreased. Our studies show that in TK6 cells, PhIP activates S-phase checkpoint, yet eludes G1 and G2-M checkpoints, and is accompanied by increased apoptosis and gene mutation. If treatment with PhIP induces similar cellular reactions in vivo, then activation of S-phase checkpoint with avoidance of G1 and G2-M checkpoints could be important factors in PhIP-induced genetic damage and neoplastic disease.     

Organ-dependent Modifying Effects of Caffeine, and Two Naturally Occurring Antioxidants α-Tocopherol and n-Tritriacontane-16,18-dione, on 2-Amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)-induced Mammary and Colonic Carcinogenesis in Female F344 Rats

Modifying effects of caffeine, α-tocopherol, and n -tritriacontane-16,18-dione (TTAD) on 2-amino-1-methyl-6-phenylimidazo[4,5- b ]pyridine (PhIP)-induced mammary and colonic carcinogenesis were investigated in female F344 rats. Groups of 20 rats, 6 weeks old, were given 0.02% PhIP (in diet) alone, or together with 0.1% caffeine (in drinking water), 0.5%α-tocopherol (in diet) or 0.1% TTAD (in diet) for up to 54 weeks. Groups of 10 females receiving basal diet or one of the test chemicals without PhIP supplementation were also maintained. The final combined incidences (adenomas plus adenocarcinomas) and multiplicity (No./rat) of mammary adenomas and adenocarcinomas were significantly lowered in the PhIP plus caffeine group (10%, 0.10) as compared to the PhIP alone value (40%, 0.50). Incidences of mammary tumors in the PhIP plus α-tocopherol or TTAD groups tended to be decreased while their multiplicities were significantly lowered. With regard to colon tumor development, on the other hand, rats given PhIP plus caffeine exhibited an elevated incidence (75% versus 15% in the control), whereas α-tocopherol and TTAD had no effect. Surprisingly, metabolic activation of PhIP was inhibited by addition of caffeine in an in vitro assay. The results indicate that caffeine exerts a potent chemopreventive action against PhIP-induced mammary carcinogenesis, but acts as a co-carcinogen for PhIP-induced colonic carcinogenesis.     

Genomic imbalance in rat mammary gland carcinomas induced by 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine

2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP), a compound found in cooked meat, is a mammary gland carcinogen in female Sprague-Dawley rats. PhIP-induced rat mammary gland carcinomas were examined for mutations in several genes (exons) known to regulate cell growth and apoptosis, including p53 (4-8), p21(Waf1) (coding region), Apc (14, 15), B-catenin (3), E-cadherin (9,13,15), Bcl-x (coding region), Bax (3), IGFIIR (28), and TGFBIIR (3). DNA from 30 carcinomas was examined by single-strand conformation polymorphism analysis, but no mutations were detected in these genes or gene regions. DNA from carcinomas and matching normal tissue were further screened for allelic imbalance by using a polymerase chain reaction-based approach with primers to known microsatellite regions located throughout the rat genome. Of 53 markers examined, 12 revealed allelic imbalance. Microsatellite instability (MSI) was detected at two markers, one on chromosome 4 and one on chromosome 6. Sixty-five percent and 96% of all carcinomas examined (N=23) showed MSI at these loci on chromosomes 4 and 6, respectively, supporting the notion that MSI plays a role in PhIP-induced mammary carcinogenesis. Loss of heterozygosity (LOH), an indication of a possible tumor suppressor gene, was observed at 10 markers distributed on chromosomes 3, 10, 11, 14, and X. The frequency of LOH at these markers was 75-94%, supporting that the regions of allelic imbalance were largely similar for the PhIP-induced carcinomas examined in this study. When PhIP-induced carcinomas from rats placed on high-fat and low-fat diet were compared, no unique regions of allelic imbalance or statistical differences in the frequency of allelic imbalance were observed. Therefore, the high-fat diet, known to be a promoter of PhIP-induced rat mammary carcinogenesis, did not appear to influence allelic imbalance in the carcinomas. Interestingly, 7,12-dimethylbenz[a]-anthracene-induced mammary carcinomas did not show allelic imbalance at 11 of the 12 loci that showed allelic imbalance in PhIP-induced carcinomas. These findings suggest that distinct chemical carcinogens induce different patterns of allelic imbalance during rat mammary carcinogenesis. Since several of the known genes involved in carcinogenesis did not harbor mutations in PhIP-induced carcinomas, further studies are needed to clarify the critical genes involved in PhIP-induced mammary carcinogenesis and to determine whether regions of LOH harbor potentially novel tumor suppressor genes involved in this disease.     

Oxazepam is mutagenic in vivo in Big Blue transgenic mice.

Although oxazepam (Serax), a widely used benzodiazepine anxiolytic, does not induce gene mutations in vitro or chromosomal aberrations in vivo, it was found to be a hepatocarcinogen in a 2 year bioassay in B6C3F1 mice. Thus, it was of interest to determine whether this carcinogen is mutagenic in vivo. Male B6C3F1 Big Blue transgenic mice were fed 2500 p.p.m. oxazepam or control diet alone for 180 days and killed on the next day. The mutant frequency (MF) of lacI in control mice was 5.02 +/- 2.4x10(5), whereas the MF in the oxazepam-treated mice was 9.17 +/- 4.82x10(-5), a significant increase (P < 0.05). Correction of the mutant frequency of lacI from the oxazepam-treated mice for clonality resulted in a decrease in the mean mutant frequency to 8.15 +/- 2. 54x10(-5). Although the mutant frequency difference was small, sequencing of a random collection of the mutants from each oxazepam-exposed mouse showed a significant difference (P < 0.015) in the mutation spectrum compared with that from control mice. In the oxazepam-exposed mice, an increase in G:C-->T:A and G:C-->C:G transversions and a concomitant decrease in G:C-->A:T transitions were observed. Clonal expansion of mutations at guanines in 5'-CpG-3' sequencing contexts at three sites was noted. It is postulated that some of the mutations found in the oxazepam-derived spectrum were due to oxidative damage elicited by induction of CYP2B isozymes as the result of chronic oxazepam administration. This study demonstrates that the in vivo Big Blue transgenic rodent mutation assay can detect mutations derived from a carcinogen that did not induce gene mutations in vitro or micronuclei in mouse bone marrow. Moreover, the sequencing of the recovered mutants can distinguish between the mutation spectrum from treated mice compared with that from control mice, thereby confirming the genotoxic consequences.     

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine induces a higher number of aberrant crypt foci in Fischer 344 (rapid) than in Wistar Kyoto (slow) acetylator inbred rats.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the most abundant heterocyclic amine carcinogen in the human diet and is a colon carcinogen in the rat. N-Acetyltransferase-2 (NAT2) catalyzes the conversion of PhIP and other heterocyclic amines to a DNA-reactive form. NAT2 has a polymorphic distribution in humans and other mammals, including rats. The rapid NAT2 genotype has been shown to be associated with increased colorectal cancer risk in some, but not all, human epidemiological studies. This investigation was designed to study the role of acetylator genotype in PhIP-induced colon carcinogenesis using aberrant crypt foci (ACF) as an intermediate biomarker. Five-week-old male, rapid-acetylator Fischer 344 (F344) rats and slow-acetylator Wistar-Kyoto (WKY) rats were fed the semipurified AIN76A diet with 0.01% PhIP, 0.04% PhIP, or no PhIP (control) for 8 weeks. PhIP induced ACF in both rapid- and slow-acetylator rats; 0.04% PhIP induced more ACF than 0.01% PhIP. There was no difference in the number of ACF between rapid- and slow-acetylator rats that were fed 0.01% PhIP. However, 0.04% PhIP induced 2-fold higher ACF and a greater dose-dependent increase in PhIP-induced ACF in the rapid-acetylator F344 rats compared with the slow-acetylator WKY rats. The results support human epidemiological studies showing higher risk for colorectal cancer in rapid acetylators who frequently consume meat that is very well done.     

Effect of dietary fat on codon 12 and 13 Ha-ras gene mutations in 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine–induced rat mammary gland tumors

Activating mutations in and expression of the Ha-ras gene were examined in benign and malignant female Sprague-Dawley rat mammary gland tumors induced by the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and promoted by a diet high in polyunsaturated fat. Ha-ras mutations were detected in codons 12 and 13 by selective polymerase chain reaction amplification of mutated sequences and nucleotide sequencing. The percentage of Ha-ras mutations in carcinomas from PhIP-treated rats was significantly higher in rats on a low-fat diet than in rats on a high-fat diet (82% (nine of 11) vs 26% (seven of 27), respectively, P < 0.01). In addition, whereas 56% of the carcinomas with Ha-ras mutations from rats on a low-fat diet carried double Ha-ras mutations, none of the carcinomas from rats on a high-fat diet had double mutations. Ha-ras mutations were also detected in benign tumors (largely adenomas) induced by PhIP in rats on different diets; two of eight and three of four benign tumors examined from rats on low-fat and high-fat diets, respectively, had Ha-ras mutations, suggesting that activating Ha-ras mutations alone are not sufficient for PhIP-induced tumors to become malignant. No differences were observed in the level of Ha-ras mRNA expression in the different groups. In our animal model, a high-fat diet increased the incidence and percentage of malignant PhIP-induced mammary gland tumors yet decreased the percentage of carcinomas showing Ha-ras mutations. Thus, the complement of genetic alterations associated with PhIP-induced mammary gland carcinogenesis is probably altered by the level of dietary fat. Mol. Carcinog. 20:348–354, 1997. © 1997 Wiley-Liss, Inc.     

Experimental chemical carcinogenesis in the stomach and colon

Experimental chemical carcinogenesis in the digestive tract is reviewed, mainly on the basis of information obtained in the laboratories of the National Cancer Center Research Institute. It is generally accepted that cancer is the outcome of DNA damage, resulting in mutation, loss, amplification and recombination of genes. Gastric cancer is no exception. It was shown very early that cancer of the glandular stomach can be produced in rats by administration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), a widely used mutagen. However, this depends on the genotype. Whereas the ACI rat is susceptible to MNNG, the Buffalo rat is resistant and this is a dominantly inherited trait. Genes responsible for the sensitivity to gastric cancer induction are at present under investigation by linkage analysis of rat genome markers. With regard to cancer in humans, our finding that cooked proteinaceous foods can give rise to a series of heterocyclic amines (HCAs) is of major significance. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), one of the most abundant, causes colon cancers in male rats, whereas in females it induces breast cancers. The colon cancers induced by PhIP feature a deletion of G as represented by 5-GGGA-3-->5-GGA-3 in the Apc gene, resulting in a truncated Apc molecule. Microsatellite mutations have also been found in PhIP-induced colon tumors, as in human hereditary non-polyposis colorectal cancer cases. Similarly to the case of gastric cancer production by MNNG, there is a genetic component and F344 rats are more susceptible to PhIP colon carcinogenesis than the ACI/N strain and the gene responsible is being sought. Since carcinogenesis proceeds with accumulation of genetic alteration, often involving genomic instability, exposure to any kind of carcinogenic substances, either xeno- or autobiotics, needs to be reduced as far as possible, taking account of inconvenience at the individual and socio-economical levels.      

Organ-dependent modifying effects of caffeine, and two naturally occurring antioxidants alpha-tocopherol and n-tritriacontane-16,18-dione, on 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced mammary and colonic carcinogenesis in female F344 rats.

Modifying effects of caffeine, alpha-tocopherol, and n-tritriacontane-16,18-dione (TTAD) on 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced mammary and colonic carcinogenesis were investigated in female F344 rats. Groups of 20 rats, 6 weeks old, were given 0.02% PhIP (in diet) alone, or together with 0.1% caffeine (in drinking water), 0.5% alpha-tocopherol (in diet) or 0.1% TTAD (in diet) for up to 54 weeks. Groups of 10 females receiving basal diet or one of the test chemicals without PhIP supplementation were also maintained. The final combined incidences (adenomas plus adenocarcinomas) and multiplicity (No./rat) of mammary adenomas and adenocarcinomas were significantly lowered in the PhIP plus caffeine group (10%, 0.10) as compared to the PhIP alone value (40%, (1.50). Incidences of mammary tumors in the PhIP plus alpha-tocopherol or TTAD groups tended to be decreased while their multiplicities were significantly lowered. With regard to colon tumor development, on the other hand, rats given PhIP plus caffeine exhibited an elevated incidence (75% versus 15% in the control), whereas alpha-tocopherol and TTAD had no effect. Surprisingly, metabolic activation of PhIP was inhibited by addition of caffeine in an in vitro assay. The results indicate that caffeine exerts a potent chemopreventive action against PhIP-induced mammary carcinogenesis, but acts as a co-carcinogen for PhIP-induced colonic carcinogenesis.     

Induction of aberrant crypt foci in DNA mismatch repair-deficient mice by the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)

Disruption of the DNA mismatch repair (MMR) pathway results in elevated mutation rates, inappropriate survival of cells bearing DNA damage, and increased cancer risk. Relatively little is known about the impact of environmentally relevant carcinogens on cancer risk in individuals with MMR-deficiency. We evaluated the effect of MMR status (Mlh1(+/+) versus Mlh1(-/-)) on the carcinogenic potential of the cooked-meat mutagen, 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) in mice. PhIP exposure did not obviously increase lymphoma or small intestinal tumorigenesis in either Mlh1-deficient or -proficient mice. In contrast, the frequency of aberrant crypt foci (ACF), a preneoplastic biomarker for colon tumorigenesis, was increased by PhIP, and the increase due to PhIP was significantly greater in Mlh1(-/-) versus wild-type littermates. This apparent heightened susceptibility to induction of ACF parallels the previously reported hypermutability of Mlh1-deficient mice to PhIP and is consistent with the hypothesis that MMR-deficiency would increase the likelihood of PhIP-induced carcinogenic mutations. Further evaluation of the risk that consumption of heterocyclic amines may impart to MMR-deficient individuals therefore is warranted.     

Diallyl sulfide inhibits PhIP-induced cell death via the inhibition of DNA strand breaks in normal human breast epithelial cells

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is associated with mammary carcinomas in animals and humans. PhIP is metabolized by CYP 1A1/1A2 and cytochrome b5 reductase, producing free radicals causing DNA strand breaks. Diallyl sulfide (DAS) prevents cancer in animals. We hypothesized that DAS will attenuate PhIP-induced DNA strand breaks and cell death. To test this hypothesis, we treated MCF-10A cells with PhIP, DAS and PhIP/DAS for 24, 48 and 72 h. DAS inhibited the PhIP-induced DNA strand breaks by 22% after 48 h and the strand breaks were completely inhibited at 72 h. PhIP reduced cell viability at each time point. However, DAS only attenuated this reduction in cell viability by 56% at 72 h. N-OH PhIP inhibited cell viability by 26% at 72 h. DAS completely attenuated this reduction in cell viability and may prevent PhIP-induced breast cancer via alterations in DNA damage and cell viability.