Metabolic activation of polycyclic aromatic hydrocarbons to carcinogens by cytochromes P450 1A1 and 1B1

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitously distributed environmental chemicals. PAHs acquire carcinogenicity only after they have been activated by xenobiotic-metabolizing enzymes to highly reactive metabolites capable of attacking cellular DNA. Cytochrome P450 (CYP) enzymes are central to the metabolic activation of these PAHs to epoxide intermediates, which are converted with the aid of epoxide hydrolase to the ultimate carcinogens, diol-epoxides. Historically, CYP1A1 was believed to be the only enzyme that catalyzes activation of these procarcinogenic PAHs. However, recent studies have established that CYP1B1, a newly identified member of the CYP1 family, plays a very important role in the metabolic activation of PAHs. In CYP1B1 gene-knockout mice treated with 7,12-dimethyl-benz[a]anthracene and dibenzo[ a, l ]pyrene, decreased rates of tumor formation were observed, when compared to wild-type mice. Significantly, gene expression of CYP1A1 and 1B1 is induced by PAHs and polyhalogenated hydrocarbons such as 2,3,7,8-tetrachlorodibenzo- p -dioxin through the arylhydrocarbon receptor. Differences in the susceptibility of individuals to the adverse action of PAHs may, in part, be due to differences in the levels of expression of CYP1A1 and 1B1 and to genetic variations in the CYP1A1 and 1B1 genes.     

Resveratrol inhibits TCDD-induced expression of CYP1A1 and CYP1B1 and catechol estrogen-mediated oxidative DNA damage in cultured human mammary epithelial cells

Resveratrol (3,5,4'-trihydroxystilbene), a naturally occurring phytoalexin present in grapes and other foods, has been reported to possess chemopreventive effects as revealed by its striking inhibition of diverse cellular events associated with tumor initiation, promotion and progression. In our present study, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), when treated with the cultured human mammary epithelial (MCF-10A) cells, induced the expression of cytochrome P450 1A1 (CYP1A1) and 1B1 (CYP1B1) that are responsible for the oxidation of 17beta-estradiol to produce catechol estrogens. Resveratrol strongly inhibited the TCDD-induced aryl hydrocarbon receptor (AhR) DNA binding activity, the expression of CYP1A1 and CYP1B1 and their catalytic activities in MCF-10A cells. It also reduced the formation of 2-hydroxyestradiol and 4-hydroxyestradiol from 17beta-estradiol by recombinant human CYP1A1 and CYP1B1, respectively. Furthermore, resveratrol significantly attenuated the intracellular reactive oxygen species (ROS) formation and oxidative DNA damage as well as the cytotoxicity induced by the catechol estrogens. Our data suggest that CYP1A1- and CYP1B1-catalyzed catechol estrogen formation might play a key role in TCDD-induced oxidative damage, and resveratrol can act as a potential chemopreventive against dioxin-induced human mammary carcinogenesis by blocking the metabolic formation of the catechol estrogens and scavenging the ROS generated during their redox cycling.     

Induction of CYP1A1 gene expression in H4-II-E rat hepatoma cells by benzo[e]pyrene.

In the rat, expression of the CYP1A1 gene is closely associated with arylhydrocarbon hydroxylase (AHH) enzyme activity. AHH is an inducile enzyme activity known to play an important role in the bioactivation of polycyclic aromatic hydrocarbons (PAHs) to mutagenic and carcinogenic metabolites. PAH-induced expression of the CYP1A1 gene appears to be regulated by several trans-acting factors, including the Ah receptor and the 4S PAH-binding protein. In this study, we used the PAH isomers benzo[a]pyrene (BaP) and benzo[e]pyrene (BeP) to further evaluate the role of the 4S PAH-binding protein in induction of the CYP1A1 gene in H4-II-E rat hepatoma cells. Although BaP is believed to bind to both the Ah receptor and the 4S protein, BeP has been reported to bind exclusively to the 4S protein. The results of the study presented here indicate that BaP and BeP induce the expression of the CYP1A1 gene, as measured by ethoxyresorufin O-deethylase (EROD) activity, in a concentration-dependent manner. However, BaP is about 25 times as potent as BeP in inducing EROD activity in these cells. Slot-blot analysis of total RNA isolated from these cells indicated that BeP, BaP, and 3-methylcholanthrene increased the level of CYP1A1 mRNA expression. Sucrose-gradient analysis of BeP binding activity indicated that BeP bound with high affinity to the 4S PAH-binding protein, but not to the Ah receptor. These results suggest that the 4S protein may play a role in the PAH-induced expression of the CYP1A1 gene in rat H4-II-E cells.     

Gene Expression of CYP1A1 and its Possible Clinical Application in Thyroid Cancer Cases

Background: Thyroid cancer is the most common endocrine malignancy, and exact causes remain unknown. The role of CYP450 1A1 (CYP1A1) in cancer initiation and progression has been investigated. The aim of this work was to analyze, for the first time, CYP1A1 gene expression and its relationship with several clinicopathological factors in Mexican patients diagnosed with thyroid cancer. Materials and Methods: Realtime PCR analysis was conducted on 32 sets of thyroid tumors and benign pathologies. Expression levels were tested for correlations with clinical and pathological data. All statistical analysis were performed using GraphPad Prism version 3.0 software. Results: We found that female gender was associated with thyroid cancer risk (P<0.05). A positive relationship was identified between CYP1A1 mRNA levels and the presence of chronic disease, alcohol use, tumor size, metastasis and an advanced clinical stage (P<0.05). Conclusions: The results suggest that CYP1A1 gene expression could be used as a marker for thyroid cancer.     

Relationship between CYP1A1 genetic polymorphisms and renal cancer in China

Aim: To study the potential role of cytochrome P450, family 1, subfamily A, polypeptide 1 (CYP1A1) polymorphisms in the risk of renal cell cancer in Chinese. Methods: A total of 181 pathologically-proven renal cancers and 350 controls from the second Xiangya Hospital in Changsha were collected during the period from May 2007 to December 2010. CYP1A1 genetic polymorphisms were genotyped using PCR-RFLP. Unconditional logistic regression analysis was performed to analyze their relationship with risk of RCC. Results: Individuals with Val/Val genotypes had a significantly increased risk of RCC compared those with CYP1A1 IIe/IIe (OR=1.69, 95%CI=1.03-2.85). We also found CYP1A1 Wt/Vt and Vt/Vt to confer a significantly greater risk than CYP1A1 Wt/Wt (Wt/Vt: OR=2.14, 95%CI=1.24-3.45; Vt/Vt: OR=1.78, 95%CI=1.31-3.96). In smokers, a high increase risk of RCC was observed in those with CYP1A1 Val allele and Vt allele (Val allele: OR=2.13, 95%CI=1.40-2.57; Vt allele: OR=3.75, 95%CI=2.43-6.79), but no other significant interactions were found. Conclusion: Our study found suggestive evidence that CYP1A1 polymorphisms may play an important role in the etiology of RCC. Cigarette smoking may increase the susceptibility to RCC carcinogenesis in individuals with a high-risk genotype.     

Interactions between CYP1A1 polymorphisms and exposure to environmental tobacco smoke in the modulation of lymphocyte bulky DNA adducts and chromosomal aberrations

CYP1A1 plays an important role in the metabolic activation of polycyclic aromatic hydrocarbons (PAH), carcinogenic components of air pollution. The influence of CYP1A1 genotype (*2A, *2B and *4) on the levels of lymphocyte bulky DNA adducts and the frequency of cells with aberrant chromosomes was assessed in 194 non-smoking subjects in whom recent exposure to environmental tobacco smoke (ETS) and airborne particulate-associated PAH were measured during two consecutive seasons (winter and summer). While CYP1A1*4 had no consistent effect on either biomarker of genetic damage, the levels of both biomarkers responded in a parallel fashion to changes in exposure/CYP1A1*2A genotype combinations during both seasons. Specifically, the levels of both biomarkers were increased in carriers of at least one CYP1A1*2A allele, as compared with CYP1A1*1 homozygotes, in subjects with ETS exposures >0.8 h/day during the previous 4 days and mean personal exposure to benzo[a]pyrene <0.9 ng/m3 during the previous 24 h (all P < 0.05). Outside these exposure limits the differential effect in CYP1A1*2A variants was lost. Although the numbers of subjects with the CYP1A1*2B polymorphism was small, the same trend appeared to be followed in this case. These effects are interpreted as resulting from differential induction of CYP1A1 expression in CYP1A1*2A and CYP1A1*2A/*2B carriers by components of ETS-polluted air at levels of exposure readily suffered by large segments of the general population and suggest that subjects with these genotypes may have increased susceptibility to the genotoxic effects of ETS.     

Colorectal cancer and genetic polymorphisms of CYP1A1, GSTM1 and GSTT1: a case-control study in the Grampian region of Scotland

Cytochrome P-450 CYP1A1 is involved in the metabolism of polycyclic aromatic hydrocarbons (PAHs) that are derived from meat intake and tobacco smoking. Expression of the CYP1A1 gene is induced by compounds present in cruciferous vegetables. The glutathione S-transferases play a central role in the detoxification of carcinogens, including PAHs. We investigated the association between colorectal cancer and three variants (CYP1A1*2A, CYP1A1*2C, CYP1A1*4) of the CYP1A1 gene, and homozygosity for the null deletion of the GSTM1 and GSTT1 genes, and the joint effects of these genotypes and smoking, meat intake and intake of green leafy vegetables in a population-based study of 264 cases and 408 controls in Northeast Scotland. There was an inverse association with the CYP1A1*4 (m4) variant (OR 0.3, 95% CI 0.13-0.70). The OR for the CYP1A1*2C (m2) variant was 1.3 (95% CI 0.59-2.91), which is similar to a combined estimate for previous studies (OR 1.2, 95% CI 0.95-1.41). We observed no association with the CYP1A1*2A (m1) variant, or the GSTM1 and GSTT1 polymorphisms. Significant interactions between all 3 CYP1A1 variants and meat intake, and between the m1 and m2 variants and intake of green leafy vegetables, were observed. There was no evidence of interaction between CYP1A1 and smoking, and no evidence of interaction between the GSTM1 or GSTT1 polymorphisms and smoking, meat intake, green leafy vegetable intake, CYP1A1 variants or each other.     

Polymorphisms in the CYP1A1 gene are associated with prostate cancer risk

CYP1A1 is likely to play an important role in the etiology of CaP through its function in activating environmental procarcinogens and catalyzing the oxidative metabolites of estrogens. To test the hypothesis that genetic polymorphisms in the CYP1A1 gene may be associated with the risk for CaP, we compared the allele, genotype and haplotype frequencies of 3 SNPs (3801T>C, 2455A>G and 2453C>A) of CYP1A1 among 159 HPC probands, 245 sporadic CaP cases and 222 unaffected men. Two SNPs (3801T>C and 2455A>G) were each individually associated with CaP risk when the allele and genotype frequencies were compared between CaP patients and unaffected controls. Furthermore, a combined SNP analysis using a haplotype approach revealed an even stronger association in Caucasians. Specifically, 4 major haplotypes (T-A-C, C-A-C, C-G-C and T-A-A) accounted for 99.8% of all observed haplotypes. These 4 haplotypes correspond to the previously described nomenclature (CYP1A1*1A, CYP1A1*2A, CYP1A1*2B and CYP1A1*4). The frequencies of these 4 haplotypes were significantly different among CaP patients and controls. The haplotype T-A-C (CYP1A1*1A) was significantly associated with increased risk for CaP, and the haplotype C-A-C (CYP1A1*2A) was significantly associated with decreased risk for CaP. These findings suggest that genetic polymorphisms in CYP1A1 may modify the risk for CaP.     

Lung cancer risk and CYP1A1 genotype in African Americans

The role of CYP1A1 genotype in lung cancer risk was assessed in African Americans through a case control study. The complete CYP1A1 genotype, including the frequency of all three polymorphisms (Msp1 [CYP1A1*2], exon 7 [CYP1A1*3] and African American specific [CYP1A1*4]) was determined by PCR on 307 controls and 105 cases of lung cancer among African Americans. We have confirmed our earlier observation of a significant increased risk (odds ratio = 2.8, 95% CI = 1.3-6.5) for lung adenocarcinoma among people with the *4 polymorphism, although we did not observe any association of this polymorphism with overall lung cancer risk. As previously reported, we found that lung adenocarcinoma patients with the *4 RFLP smoked significantly less than patients without this polymorphism, suggesting an important role in cancer risk of low exposure levels to cigarette smoke in subjects carrying susceptibility polymorphisms. There was no association with the other two polymorphisms and lung cancer in this population. When we examined lung cancer risk as a function of composite genotype, taking into account all three polymorphisms simultaneously in each subject, our preliminary data suggested an association of one rare genotype (homozygous Msp1, heterozygous exon 7 or *2/*2*3) with overall lung cancer risk (OR = 8.4, 95% CI = 1.6-43.2).      

Association of CYP3A5*3 and CYP1A1*2C Polymorphism with Development of Acute Myeloid Leukemia in Egyptian Patients

Aim: Cytochrome P450 (CYP) enzyme catalyzes the phase I metabolism reaction which metabolize endogenous and exogenous DNA-reactive chemical compounds and xenobiotics which could induce genotoxicity and increase the risk for leukemia. We aimed to detect frequency of CYP3A5*3 and CYP1A1*2C polymorphisms in Egyptian acute myeloid leukemia (AML) patients and to determine role of allele’s variants as a risk factor for developing leukemia. Patients and Methods: A case-control study was conducted on seventy acute myeloid leukemia patients and thirty control subjects. Samples were analyzed for prevalence of CYP3A5*3 and CYP1A1*2C polymorphisms using PCR - restriction fragment length polymorphism method. Results: CYP3A5*3 polymorphism (3/3) and (1/3) genotype were significantly elevated in AML group compared to control group (p=0.002). However, no statistical significant differences were found between patients and control group as regard CYP1A1*2C polymorphism. Conclusion: Our results suggest that Egyptians carrying CYP3A5*3 polymorphism might have an increased risk of AML emphasizing the significance of effective phase I detoxification in carcinogenesis.     

CYP1A1 and GSTM1 genetic polymorphisms and lung cancer risk in Caucasian non-smokers: a pooled analysis

Polymorphisms for genes encoding the metabolic enzymes cytochrome P450 1A1 (CYP1A1) and glutathione S-transferase M1 (GSTM1) might contribute to the variability in individual susceptibility to lung cancer. The role of CYP1A1 and GSTM1 in lung carcinogenesis might be more important at low levels of exposure to carcinogens. Non-smokers represent a population at low exposure, however, they are often overlooked because of the small number of cases. We therefore conducted a pooled analysis of 14 case-control studies on lung cancer in Caucasian non-smokers with comparable information on genetic polymorphisms included in the International Collaborative Study on Genetic Susceptibility to Environmental Carcinogens. We pooled the raw data from a total of 302 cases and 1631 controls with random effects models. We also evaluated the possibility of inclusion bias and conducted influence analyses. The odds ratio (OR) of lung cancer for the variant CYP1A1 Ile(462)Val polymorphism (Ile/Val, Val/Val) was 2.99 [95% confidence interval (95%CI) 1.51-5.91]; this effect was stronger on lung adenocarcinoma (OR 4.85, 95%CI 2.03-11.6). After excluding outlying or imprecise studies, we did not observe a significant effect of the CYP1A1 MspI (T(3801)C) polymorphism or GSTM1 null genotype (OR 1.20, 95%CI 0.89-1.63). Furthermore, our analyses suggested a combined effect of the CYP1A1 Ile(462)Val polymorphism and GSTM1 null genotype. The OR for the combination of the CYP1A1 Ile(462)Val variant and GSTM1 null genotype was 4.67 (95%CI 2.00-10.9) compared with the concurrent presence of the CYP1A1 wild-type and GSTM1 non-null genotype. We did not observe a modification of the effect of the GSTM1 null genotype according to exposure to environmental tobacco smoke and urban/rural residence. Our study therefore suggests that the CYP1A1 Ile(462)Val variant allele might play a role in lung carcinogenesis among non-smokers, possibly in combination with the GSTM1 null genotype.     

Mouse lung CYP1A1 catalyzes the metabolic activation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) carcinogenesis is initiated by N(2)-hydroxylation, mediated by several cytochromes P450, including CYP1A1. However, the role of CYP1A1 in PhIP metabolic activation in vivo is unclear. In this study, Cyp1a1-null and wild-type (WT) mice were used to investigate the potential role of CYP1A1 in PhIP metabolic activation in vivo. PhIP N(2)-hydroxylation was actively catalyzed by lung homogenates of WT mice, at a rate of 14.9 +/- 5.0 pmol/min/g tissue, but <1 pmol/min/g tissue in stomach and small intestine, and almost undetectable in mammary gland and colon. PhIP N(2)-hydroxylation catalyzed by lung homogenates of Cyp1a1-null mice was approximately 10-fold lower than that of WT mice. In contrast, PhIP N(2)-hydroxylation activity in lung homogenates of Cyp1a2-null versus WT mice was not decreased. Pretreatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin increased lung Cyp1a1 mRNA and lung homogenate PhIP N(2)-hydroxylase activity approximately 50-fold in WT mice, where the activity was substantially inhibited (70%) by monoclonal antibodies against CYP1A1. In vivo, 30 min after oral treatment with PhIP, PhIP levels in lung were similar to those in liver. After a single dose of 0.1 mg/kg [(14)C]PhIP, lung PhIP-DNA adduct levels in Cyp1a1-null mice, but not in Cyp1a2-null mice, were significantly lower (P = 0.0028) than in WT mice. These results reveal that mouse lung has basal and inducible PhIP N(2)-hydroxylase activity predominantly catalyzed by CYP1A1. Because of the high inducibility of human CYP1A1, especially in cigarette smokers, the role of lung CYP1A1 in PhIP carcinogenesis should be considered. (237 words).     

Genetic Polymorphisms of the Human Cytochrome P450 1A1 (CYP1A1) and Cervical Cancer Susceptibility among Northeast Thai Women

Background: CYP1A1 is an enzyme in phase I of the cytochrome P450 (CYP) superfamily, and plays a key role in detoxification of carcinogens. Host genetic predisposition in the CYP1A1 may be associated with an increased susceptibility to cervical cancer.The study aimed to evaluate four common polymorphisms of the CYP1A1 and cervical cancer susceptibility among Northeast Thai women. Methods: A case-control study was conducted involving 204 patients with squamous cell cervical cancer (SCCA) and 204 age-matched healthy controls. DNA was extracted from peripheral blood leucocytes. CYP1A1 m1, m3, and m4 genotypes were detected using PCR-RFLP, whereas the CYP1A1 m2 genotype was investigated using real-time PCR. Haplotype analysis was performed using PHASE algorithm version 2.1.1. Results: CYP1A1 m3 was monomorphic. Association between the common CYP1A1 polymorphisms, m1 and m2, and cervical cancer risk was not observed (p>0.05), nor was any association found between the m1–m2–m4 haplotype and cervical cancer risk (p>0.05). Interestingly, the CA genotype of CYP1A1 m4 was observed in 30.88% of the cervical cancer patients but was absent in healthy controls. Conclusion: Our results demonstrated a possible involvement of the CYP1A1 m4 polymorphism but no other common polymorphisms (viz., m1, m2, and m3) in the risk for cervical cancer.This finding may be useful when screening for risk of cervical cancer among Northeast Thai women.     

Differential metabolism of benzo[a]pyrene and benzo[a]pyrene-7,8-dihydrodiol by human CYP1A1 variants

Cytochrome P450 1A1 (CYP1A1) plays a key role in the metabolism of carcinogens, such as benzo[a]pyrene (B[a]P) and metabolites to ultimate carcinogens. Three human allelic variants, namely wild-type (CYP1A1.1), CYP1A1.2 (I462V) and CYP1A1.4 (T461N), were coexpressed by coinfection of baculovirus-infected insect cells with human NADPH-P450 reductase. These recombinant enzymes (in microsomal membranes) were used to analyze whether CYP1A1 polymorphisms affect catalytic activities towards B[a]P and B[a]P-7,8-dihydrodiol. The complete spectrum of phase I metabolites, including the tetrahydrotetrols resulting from hydrolysis of the ultimate carcinogen, B[a]P-7,8-dihydrodiol-9,10-epoxide, was examined by HPLC. Wild-type enzyme showed the highest total metabolism of B[a]P, CYP1A1.2 was approximately 50%, and CYP1A1.4 approximately 70%. Km values for all metabolites with CYP1A1.2 were generally significantly lower than with wild-type enzyme (e.g. B[a]P-7,8-diol formation: 13.8 microM for wild-type, 3.5 microM for CYP1A1.2 and 7.7 microM for CYP1A1.4). Addition of epoxide hydrolase markedly increases the relative diol-to-phenol activities by all three variants. However, CYP1A1.4 exhibits the greatest efficiency to produce diol species. Each variant produced the diol epoxides from B[a]P-7,8-dihydrodiol. CYP1A1.1 exhibited with 10.4 pmol/min/pmol CYP1A1 the greatest total rate for 7,8-diol metabolites followed by CYP1A1.2 (7.2 pmol/min/pmol CYP1A1) and CYP1A1.4 (5.5 pmol/min/pmol CYP1A1). All enzyme variants produced about three times more diol epoxide 2-derived metabolites than diol epoxide 1-derived ones, whereby both rare allelic variants exhibited statistically significantly increased formation of diol epoxide 2. This study showed that the three CYP1A1 variants had different enzyme kinetics properties to produce both the diol metabolites from B[a]P and the ultimate mutagenic species diol epoxide 2 from B[a]P-7,8-dihydrodiol, which must be considered in the evaluation of individual susceptibility to cancer.