Role of human CYP1A1 and NAT2 in 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced mutagenicity and DNA adducts

Modifying Effect of Smoking on GSTM1 and NAT2 in Relation to the Risk of Bladder Cancer in Mongolian Population: A Case-Control Study

Objectives:

Tobacco smoking is the predominant risk factor for bladder cancer as it contains cancer-causing chemicals. However, genetic factors may play important role in response towards chemical carcinogens. In this study we aim to investigate genetic polymorphisms of glutathione S-transferase M1 (GSTM1) and N-acetyltransferase 2 (NAT2) as determinants of bladder cancer risk, independently and in combination with tobacco use in the Mongolian population.

Materials and Methods:

The current study was a hospital-based case-control study including 60 histologically confirmed bladder cancer patients and 60 cancer-free controls. PCR-RFLP assay was used to determine the presence of GSTM1 and NAT2 polymorphisms in bladder cancer patients and controls. GSTM1 and NAT2 were tested using binary logistical regression analysis with adjustment or stratification according to the smoking.

Results:

There were 46 men and 14 women diagnosed with bladder cancer, with mean age was 58±4. The controls included 37 men and 23 women with a mean age of 57±3. The frequency of GSTM1 null genotype was higher in controls (71.67%) than in bladder cancer patients (58.33%) without statistical significance (OR=0.5534; 95% CI=0.2586-1.1843), (p=0.128). The NAT2 low acetylator phenotype was more common in patients with bladder cancer (15%) than in controls (5%). Furthermore, individuals with NAT2 low acetylator phenotype had a nearly 3.35-fold increased risk to develop bladder cancer (OR=3.35; 95% CI=0.8604-13.0657), (p=0.081) while the risk was even higher when combined with null GSTM1 genotype (OR=4; 95% CI=0.4459-37.5308), (p=0.213) but there was no statistical significance. Prevalence of smoking in bladder cancer patients was higher than controls and increased significantly the risk of bladder cancer (OR=8.31; 95% CI=3.66-18.88). Smokers with GSTM1 null genotype were at 5-fold higher risk of bladder cancer (OR=5.0; 95% CI=1.55-16.16), (p=0.007) while NAT2 low acetylator phenotype increased bladder cancer risk by 20-fold (OR=20.5; 95% CI=2.33-80.86), (p=0.006).

Conclusion:

The current study shows that tobacco smokers with the NAT2 low acetylator phenotype and GSTM1 null genotype have the highest risk of bladder cancer in the Mongolian population.

Curcumin inhibits PhIP induced cytotoxicity in breast epithelial cells through multiple molecular targets

Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), found in cooked meat, is a known food carcinogen that causes several types of cancer, including breast cancer, as PhIP metabolites produce DNA adduct and DNA strand breaks. Curcumin, obtained from the rhizome of Curcuma longa, has potent anticancer activity. To date, no study has examined the interaction of PhIP with curcumin in breast epithelial cells. The present study demonstrates the mechanisms by which curcumin inhibits PhIP-induced cytotoxicity in normal breast epithelial cells (MCF-10A). Curcumin significantly inhibited PhIP-induced DNA adduct formation and DNA double stand breaks with a concomitant decrease in reactive oxygen species (ROS) production. The expression of Nrf2, FOXO targets; DNA repair genes BRCA-1, H2AFX and PARP-1; and tumor suppressor P16 was studied to evaluate the influence on these core signaling pathways. PhIP induced the expression of various antioxidant and DNA repair genes. However, co-treatment with curcumin inhibited this expression. PhIP suppressed the expression of the tumor suppressor P16 gene, whereas curcumin co-treatment increased its expression. Caspase-3 and -9 were slightly suppressed by curcumin with a consequent inhibition of cell death. These results suggest that curcumin appears to be an effective anti-PhIP food additive likely acting through multiple molecular targets.

Expression of CYP1A1, CYP1B1 and MnSOD in a panel of human cancer cell lines

The expression of P450 enzymes and antioxidative enzymes in tumour tissue can have a major impact on the responsiveness of tumours to cancer chemotherapeutic drugs, therefore such information may be very precious when experiments are designed. The compressive information, concerning the expression of drug metabolism enzymes or antioxidative enzymes is still lacking, therefore in this study the expression of CYP1A1, CYP1B1 and mitochondrial superoxide dismutase MnSOD (both mRNA and protein) in a panel of eight commonly used cancer cell lines, representing four tumour tissues was assayed. In the study two ovarian cancer cell lines A2780 and SKOV-3, two colorectal cancer LOVO and DLD-1, two breast cancer derived MCF-7 and MDA-MB-231 and two cervical cancer cell lines HeLa and C33A were employed. The relatively high expression of all assayed enzymes was shown in MDA-MB-231 breast cancer cells, lack of cancer cell specific CYP1B1 protein was discovered in LOVO colorectal cells. In order to test possible correlation between expression of CYP1A1, CYP1B1 and MnSOD and modulators of their activity, cytotoxicity of resveratrol and its promising hydroxylated analogue 3,3′,4,4′,5,5′-trans-hexahydroxystilbene against cell lines used in experiment was assayed. The relatively high correlation was found between IC₅₀ values calculated for 3,3′,4,4′,5,5′-trans-hexahydroxystilbene and expression of MnSOD (r = 0.6562).

The position of imidazopyridine and metabolic activation are pivotal factors in the antimutagenic activity of novel imidazo[1,2-a]pyridine derivatives

The antimutagenic activity of eight novel imidazo[1,2-a]pyridine derivatives (I-VIII) against sodium azide (NaN3) and benzo[a]pyrene (B[a]P) was evaluated using the Salmonella reverse mutation assay. At non-toxic concentrations (12.5-50 µM), imidazopyridines I, II, III, and V with a terminal imidazopyridine group were mutagenic, while derivatives VII and VIII with a central imidazopyridine group were not mutagenic. Compounds IV, VII, and VIII exerted a moderate antimutagenic activity against NaN3 under pre-exposure conditions, and a strong activity (>40%) against B[a]P in the presence of S9 under both pre- and co-exposure conditions and mostly independent on the dose. Imidazopyridines possibly inhibited the microsomal-dependent activation of B[a]P. The demethylated derivative VII was the most active antimutagen. All imidazopyridines had a low to moderate antioxidant activity. The antibacterial activity of imidazopyridines was sporadic and moderate probably due to the failure of bacteria to convert imidazopyridines into active metabolites. The position of imidazopyridine was a pivotal factor in the mutagenic/antimutagenic activity. The strong antimutagenic compounds were dicationic planar compounds with a centered imidazo[1,2-a]pyridine spacer. With LD50 of 60 mg/kg in mice for both derivatives VII and VIII, it is safe to investigate the anticancer activity of these derivatives in animal models.

Epigenetic influences in the aetiology of cancers arising from breast and prostate: a hypothesised transgenerational evolution in chromatin accessibility

Epidemiological studies have consistently supported the notion that environmental and/or dietary factors play a central role in the aetiology of cancers of the breast and prostate. However, for more than five decades investigators have failed to identify a single cause-and-effect factor, which could be implicated; identification of a causative entity would allow the implementation of an intervention strategy in at-risk populations. This suggests a more complex pathoaetiology for these cancer sites, compared to others. When one examines the increases or decreases in incidence of specific cancers amongst migrant populations, it is notable that disease arising in colon or stomach requires one or at most two generations to exhibit a change in incidence to match that of high-incidence regions, whereas for breast or prostate cancer, at least three generations are required. This generational threshold could suggest a requirement for nonmutation-driven epigenetic alterations in the F0/F1 generations (parental/offspring adopting a more westernized lifestyle), which then predisposes the inherited genome of subsequent generations to mutagenic/genotoxic alterations leading to the development of sporadic cancer in these target sites. As such, individual susceptibility to carcinogen insult would not be based per se on polymorphisms in activating/detoxifying/repair enzymes, but on elevated accessibility of crucial target genes (e.g., oncogenes, tumour suppressor genes) or hotspots therein to mutation events. This could be termed a genomic susceptibility organizational structure (SOS). Several exposures including alcohol and heavy metals are epigens (i.e., modifiers of the epigenome), whereas others are mutagenic/genotoxic, for example, heterocyclic aromatic amines; humans are continuously and variously exposed to mixtures of these agents. Within such a transgenerational multistage model of cancer development, determining the interaction between epigenetic modification to generate a genomic SOS and genotoxic insult will facilitate a new level of understanding in the aetiology of cancer.

Reduced 4-aminobiphenyl-induced liver tumorigenicity but not DNA damage in arylamine N-acetyltransferase null mice

The aromatic amine 4-aminobiphenyl (ABP) is a liver procarcinogen in mice, requiring enzymatic bioactivation to exert its tumorigenic effect. To assess the role of arylamine N-acetyltransferase (NAT)-dependent acetylation capacity in the risk for ABP-induced liver tumors, we compared 1-year liver tumor incidence following the postnatal exposure of wild-type and NAT-deficient Nat1/2(-/-) mice to ABP. At an ABP exposure of 1200 nmol, male Nat1/2(-/-) mice had a liver tumor incidence of 36% compared to 69% in wild-type males, and at 600 nmol there was a complete absence of tumors compared to 60% in wild-type mice. Only one female wild-type mouse had a tumor using this exposure protocol. However, levels of N-deoxyguanosin-8-yl-ABP-DNA adducts did not correlate with either the strain or sex differences in tumor incidence. These results suggest that female sex and NAT deficiency reduce risk for ABP-induced liver tumors, but by mechanisms unrelated to differences in DNA-damaging events.

Codominant expression of N-acetylation and O-acetylation activities catalyzed by N-acetyltransferase 2 in human hepatocytes

Human populations exhibit genetic polymorphism in N-acetylation capacity, catalyzed by N-acetyltransferase 2 (NAT2). We investigated the relationship between NAT2 acetylator genotype and phenotype in cryopreserved human hepatocytes. NAT2 genotypes determined in 256 human samples were assigned as rapid (two rapid alleles), intermediate (one rapid and one slow allele), or slow (two slow alleles) acetylator phenotypes based on functional characterization of the NAT2 alleles reported previously in recombinant expression systems. A robust and significant relationship was observed between deduced NAT2 phenotype (rapid, intermediate, or slow) and N-acetyltransferase activity toward sulfamethazine (p < 0.0001) and 4-aminobiphenyl (p < 0.0001) and for O-acetyltransferase-catalyzed metabolic activation of N-hydroxy-4-aminobiphenyl (p < 0.0001), N-hydroxy-2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (p < 0.01), and N-hydroxy-2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (p < 0.0001). NAT2-specific protein levels also significantly associated with the rapid, intermediate, and slow NAT2 acetylator phenotypes (p < 0.0001). As a negative control, p-aminobenzoic acid (an N-acetyltransferase 1-selective substrate) N-acetyltransferase activities from the same samples did not correlate with the three NAT2 acetylator phenotypes (p > 0.05). These results clearly document codominant expression of human NAT2 alleles resulting in rapid, intermediate, and slow acetylator phenotypes. The three phenotypes reflect levels of NAT2 protein catalyzing both N- and O-acetylation. Our results suggest a significant role of NAT2 acetylation polymorphism in arylamine-induced cancers and are consistent with differential cancer risk and/or drug efficacy/toxicity in intermediate compared with rapid or slow NAT2 acetylator phenotypes.

Effect of rapid human N-acetyltransferase 2 haplotype on DNA damage and mutagenesis induced by 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx)

Heterocyclic amines such as 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx) are dietary carcinogens generated when meats are cooked well-done. Bioactivation includes N-hydroxylation catalyzed by cytochrome P4501A2 (CYP1A2) followed by O-acetylation catalyzed by N-acetyltransferase 2 (NAT2). Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human CYP1A2 and either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles were treated with IQ or MeIQx to examine the effect of NAT2 genetic polymorphism on IQ- or MeIQx-induced DNA adducts and mutagenesis. MeIQx and IQ both induced decreases in cell survival and significantly (p<0.001) greater number of endogenous hypoxanthine phosphoribosyl transferase (hprt) mutants in the CYP1A2/NAT2*4 than the CYP1A2/NAT2*5B cell line. IQ- and MeIQx-induced hprt mutant cDNAs were sequenced and over 85% of the mutations were single-base substitutions with the remainder exon deletions likely caused by splice-site mutations. For the single-base substitutions, over 85% were at G:C base pairs. Deoxyguanosine (dG)-C8-IQ and dG-C8-MeIQx adducts were significantly (p<0.001) greater in the CYP1A2/NAT2*4 than the CYP1A2/NAT2*5B cell line. DNA adduct levels correlated very highly with hprt mutants for both IQ and MeIQx. These results suggest substantially increased risk for IQ- and MeIQx-induced DNA damage and mutagenesis in rapid NAT2 acetylators.

Differences between human slow N-acetyltransferase 2 alleles in levels of 4-aminobiphenyl-induced DNA adducts and mutations

Aromatic amines such as 4-aminobiphenyl (ABP) require biotransformation to exert their carcinogenic effects. Genetic polymorphisms in biotransformation enzymes such as N-acetyltransferase 2 (NAT2) may modify cancer risk following exposure. Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human cytochrome P4501A1 (CYP1A1) and a single copy of either NAT2*4 (rapid acetylator), NAT2*5B (common Caucasian slow acetylator), or NAT2*7B (common Asian slow acetylator) alleles (haplotypes) were treated with ABP to test the effect of NAT2 polymorphisms on DNA adduct formation and mutagenesis. ABP N-acetyltransferase catalytic activities were detectable only in cell lines transfected with NAT2 and were highest in cells transfected with NAT2*4, lower in cells transfected with NAT2*7B, and lowest in cells transfected with NAT2*5B. Following ABP treatment, N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP) was the primary adduct formed. Cells transfected with both CYP1A1 and NAT2*4 showed the highest concentration-dependent cytotoxicity, hypoxanthine phosphoribosyl transferase (hprt) mutants, and dG-C8-ABP adducts. Cells transfected with CYP1A1 and NAT2*7B showed lower levels of cytotoxicity, hprt mutagenesis, and dG-C8-ABP adducts. Cells transfected with CYP1A1 only or cells transfected with both CYP1A1 and NAT2*5B did not induce cytotoxicity, hprt mutagenesis or dG-C8-ABP adducts. ABP-DNA adduct levels correlated very highly (r>0.96) with ABP-induced hprt mutant levels following each treatment. The results of the present study suggest that investigations of NAT2 genotype or phenotype associations with disease or toxicity could be more precise and reproducible if heterogeneity within the "slow" NAT2 acetylator phenotype is considered and incorporated into the study design.

Effect of N-acetyltransferase 2 polymorphism on tumor target tissue DNA adduct levels in rapid and slow acetylator congenic rats administered 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine or 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline

2-Amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are suspected human carcinogens generated in well done meats. After N-hydroxylation, they are O-acetylated by N-acetyltransferase 2 (NAT2) to electrophiles that form DNA adducts. dG-C8-MeIQx and dG-C8-PhIP adducts have been identified in human tissues. In the female rat, administration of PhIP leads to mammary and colon tumors, whereas MeIQx induces liver tumors. Both humans and rats exhibit NAT2 genetic polymorphism yielding rapid and slow acetylator phenotypes. Because O-acetylation is an activation pathway, we hypothesized that MeIQx- and PhIP-induced DNA damage would be greater in tumor target tissues and higher in rapid than slow NAT2 acetylators. Adult female rapid and slow acetylator rats congenic at the Nat2 locus received a single dose of 25 mg/kg MeIQx or 50 mg/kg PhIP by gavage, and tissue DNA was isolated after 24 h. Deoxyribonucleoside adducts were identified and quantified by capillary liquid chromatography-tandem mass spectrometry using isotope dilution methods with deuterated internal standards. Major adducts were those bound to the C8 position of deoxyguanosine. dG-C8-PhIP DNA adducts were highest in colon, lowest in liver and did not significantly differ between rapid and slow acetylator congenic rats in any tissue tested. In contrast, dG-C8-MeIQx adducts were highest in liver and significantly (p < 0.001) higher in rapid acetylator liver than in slow acetylator liver. Our results are consistent with the tumor target specificity of PhIP and MeIQx and with increased susceptibility to MeIQx-induced liver tumors in rapid NAT2 acetylators.