Detoxification Of Chemical Carcinogens Research Articles

The microbiome in the activation vs. detoxification of chemical carcinogens

The microbiome in the activation vs. detoxification of chemical carcinogens

Gene Frequencies of the Human GSTT1 (Null Allele) and GSTP1 (Ile105Val) Polymorphisms among South Indian Populations

Background: Glutathione S-transferases (GSTs) are members of the phase II biotransformation enzymes that play a key role in cellular detoxification of chemical carcinogens and xenobiotics. Variations at GST genes have been reported in different human populations, and some association studies have reported increased risk for cancers and other disease end points. The present study was conducted to investigate the allele frequency variations in south Indian populations. Methods: GSTT1 null allele and GSTP1 Ile105Val polymorphisms were genotyped in two hundred and twelve subjects (aged 34 to 60 years old) belong to six populations using PCR and PCR-RFLP techniques respectively. Results: Both GSTT1 ins-del and GSTP1 Ile105Val are polymorphic in all populations. GSTP1 Ile105Val followed the Hardy-Weinberg equilibrium. The GSTT1 null allele frequencies ranged from 11.6% to 22.2% and GSTP1 Ile105Val “Val” allele frequency ranged from 20.0% to 38.2% in the study populations. HapMap data showed the highest frequency of Val105 allele in African populations followed by European populations. East Asian populations showed the lowest frequency of Val105 allele. Conclusion: The variations observed in allelic distribution of GST genes may presumably be due to the selective pressure exerted on populations of that region. In conclusion, the present study reports the frequency of GSTT1 null allele and GSTP1 Ile105Val polymorphisms in Indian populations which provides foundation for potential epidemiological and clinical studies.

4-Methylsulfanyl-3-butenyl isothiocyanate derived from glucoraphasatin is a potent inducer of rat hepatic phase II enzymes and a potential chemopreventive agent

The objective of this study was to establish whether the phytochemical glucoraphasatin, a glucosinolate present in cruciferous vegetables, and its corresponding isothiocyanate, 4-methylsulfanyl-3-butenyl isothiocyanate, up-regulate enzymes involved in the detoxification of carcinogens and are thus potential chemopreventive agents. Glucoraphasatin and myrosinase were isolated and purified from Daikon sprouts and Sinapis alba L., respectively. Glucoraphasatin (0-10μM) was incubated for 24h with precision-cut rat liver slices in the presence and absence of myrosinase, the enzyme that converts the glucosinolate to the isothiocyanate. The intact glucosinolate failed to influence the O-dealkylations of methoxy- and ethoxyresorufin or the apoprotein expression of CYP1 enzymes. Supplementation with myrosinase led to an increase in the dealkylation of methoxyresorufin, but only at the highest concentration of the glucosinolate, and CYP1A2 expression. In the absence of myrosinase, glucoraphasatin caused a marked increase in epoxide hydrolase activity at concentrations as low as 1μM paralleled by a rise in the enzyme protein expression; at the highest concentration only, a rise was also observed in glucuronosyl transferase activity, but other phase II enzyme systems were unaffected. Addition of myrosinase to the glucoraphasatin incubation maintained the rise in epoxide hydrolase and glucuronosyl transferase activities, further elevated quinone reductase and glutathione S-transferase activities, and increased total glutathione concentrations. It is concluded that at low concentrations, glucoraphasatin, either intact and/or through the formation of 4-methylsulfanyl-3-butenyl isothiocyanate, is a potent inducer of hepatic enzymes involved in the detoxification of chemical carcinogens and merits further investigation for chemopreventive activity.

Polymorphisms of drug‐metabolizing genes and risk of non‐Hodgkin lymphoma

Drug metabolizing genes are involved in the detoxification of chemical carcinogens. Polymorphisms in drug-metabolizing genes affect the risk of some forms of cancer. We analyzed six polymorphisms to evaluate their association with risk for non-Hodgkin lymphoma (NHL), and to examine whether smoking modifies these associations in population-based study in Korea (713 cases and 1,700 controls). The GSTP1 rs1695 AG and the combined AG/GG genotypes were associated with decreased risk of NHL (odds ratio (OR)(AG) = 0.67, 95% confidence interval (CI) = 0.55-0.82; OR(AG/GG) = 0.66, 95% CI = 0.54-0.80) and DLBCL (OR(AG) = 0.63, 95% CI = 0.49-0.82; OR(AG/GG) = 0.64, 95% CI = 0.50-0.82). For T-cell lymphoma, only the combined AG/GG genotype was associated with decreased risk (OR(AG/GG) = 0.65, 95% CI = 0.44-0.96). The CYP1A1 rs1048943 AG genotype and the combined AG/GG genotypes were associated with increased risk of NHL (OR(AG) = 1.28, 95% CI = 1.07-1.54; OR(AG/GG) = 1.26, 95% CI = 1.06-1.51) and DLBCL (OR(AG) = 1.32, 95% CI = 1.04-1.66; OR(AG/GG) = 1.30, 95% CI = 1.03-1.63), but not T-cell lymphoma. Smoking does not modify the association between these polymorphisms and NHL risk. Our data provide evidence that the GSTP1 rs1695 and the CYP1A1 rs1048943 genotypes affect the risk of NHL in Korea.

No evidence for glutathione S-transferases GSTA2, GSTM2, GSTO1, GSTO2, and GSTZ1 in breast cancer risk

Breast cancer is a complex disease and in recent years a number of breast cancer susceptibility genes have been identified, but the role of low penetrance susceptibility genes has not been completely resolved. Glutathione S-transferases (GSTs) are phase II xenobiotic metabolizing enzymes involved in the detoxification of chemical carcinogens and environmental pollutants and play an important role in cell defense mechanisms against oxidative stress. They have been in the spot light for the investigation of a potential association with breast cancer risk but so far, sparse or even no data for a potential contribution of GSTA2, GSTM2, GSTO, and GSTZ to breast cancer risk are available. We genotyped GSTA2_448_C > G (rs2180314), GSTA2_742_A > C (rs6577), GSTM2_-832_T > C (rs638820), GSTO1_-1242_G > A (rs2164624), GSTO1_419_A > C (rs4925), GSTO2_-183_A > G (rs2297235), GSTO2_342_A > G (rs156697), GSTZ1_-4378_A > G (rs1046428), and GSTZ1_94_G > A (rs3177427) by MALDI-TOF MS in the German GENICA breast cancer case-control collection of 1021 cases and 1015 controls and performed breast cancer risk association in general and with respect to the stratifications: menopausal status, family history of breast or ovarian cancer, use of oral contraceptives, use of hormone therapy, body mass index, and smoking as well as histopathological tumor characteristics including hormone receptor status, grade, histology, and node status. We did not observe any breast cancer risk associations and conclude that it is unlikely that glutathione S-transferases GSTA2, GSTM2, GSTO1, GSTO2, and GSTZ1 participate in breast cancer susceptibility.

Inducibility of cytochrome P450 1A1 and chemical carcinogenesis by benzo[ a]pyrene in AhR repressor-deficient mice

AhR repressor (AhRR) is an AhR-related bHLH-PAS transcription factor. It is known to repress AhR transcription activity in a competitive manner. To examine AhRR functions in mice, we produced AhRR-deficient mice by gene knockout. AhRR(−/−) mice were born in normal Mendelian proportions, grew well, and were fertile. AhR(−/−) mice exhibited higher levels of Cyp1a1 (Cytochrome P450 1A1) mRNA induction in the skin, stomach and spleen than wild-type mice, while expression of Cyp1a1 mRNA was not significantly altered in the liver, lung, heart or other tissues, suggesting that “super-induction” of Cyp1a1 mRNA expression in AhRR(−/−) mice occurs in a tissue specific manner. AhRR(−/−) mice displayed a delayed response to skin carcinogenesis caused by benzo[ a]pyrene. Since CYP1A1 is involved in the metabolic activation and detoxification of chemical carcinogens, these results suggest that overexpression of CYP1A1 shifts the balance of the metabolic activities in the skin of AhRR(−/−) mice in favor of the detoxification of carcinogens.

Human glutathione S-transferase A1, T1, M1, and P1 polymorphisms and susceptibility to prostate cancer in the Japanese population

The incidence of prostate cancer is increasing in low-risk populations such as Japanese. One of the causes of this increase is considered to be associated with the Western diet, especially the high intake of red meat and fat. Glutathione S-transferase (GST) A1, T1, M1, and P1 are phase II enzymes that are important for activation and detoxification of chemical carcinogens. In this study, 190 Japanese male patients with prostate cancer and 294 healthy controls, frequency-matched for age, were compared for frequencies of GSTA1, GSTT1, GSTM1, and GSTP1 genotypes. Among smokers, the frequency of the GSTA1*A/*B or *B/*B genotype in patients with prostate cancer (27.8%) showed a statistically significant increase compared with the control group frequency (18.2%; odds ratio [OR] =1.72; 95% CI, 1.01-2.94). In addition, the frequency of GSTT1 nondeletion genotype was associated with prostate cancer among smokers (OR =1.68; 95% CI, 1.06-2.68). The OR of carrying the combined genotyping of GSTA1*A/*B or *B/*B and GSTT1 nondeletion was 2.08 (95% CI, 1.14-3.80) with the combined genotyping of GSTA1*A/*A and GSTT1 null as a reference. On the other hand, no significant associations were observed for genotypes of GSTM1 and GSTP1 I105V. These findings suggest that the GSTA1 and GSTT1 polymorphisms are associated with prostate cancer susceptibility, especially among smokers.

Glucosinolates in the human diet. Bioavailability and implications for health

The glucosinolates are a large group of sulphur-containing glucosides found in brassica vegetables. After physical damage to the plant tissue, glucosinolates are broken down, by the endogenous enzyme myrosinase, releasing glucose and a complex variety of biologically active products. The most important and extensively studied of these compounds are the isothiocyanates. Glucosinolates can be degraded or leached from vegetable tissue during food processing, but thermal inactivation of myrosinase preserves some intact glucosinolates in cooked vegetables. Once ingested, any remaining intact glucosinolates may be broken down by plant myrosinase in the small intestine, or by bacterial myrosinase in the colon. Isothiocyanates are absorbed from the small bowel and colon, and the metabolites are detectable in human urine 2–3 h after consumption of brassica vegetables. Isothiocyanates are potent inducers of Phase II enzymes in vitro, and they have been shown to increase the metabolism and detoxification of chemical carcinogens in vitro and in animal models. Some of these compounds also inhibit mitosis and stimulate apoptosis in human tumour cells, in vitro and in vivo. This second effect raises the possibility that in addition to blocking DNA damage, isothiocyanates may selectively inhibit the growth of tumour cells even after initiation by chemical carcinogens. Epidemiological evidence supports the possibility that glucosinolate breakdown products derived from brassica vegetables may protect against human cancers, especially those of the gastrointestinal tract and lung. To define and exploit these potentially anticarcinogenic effects it is important to understand and manipulate glucosinolate chemistry and metabolism across the whole food-chain, from production and processing to consumption.

Genetic polymorphisms and metabolism of endocrine disruptors in cancer susceptibility.

Epidemiological studies have estimated that approximately 80% of all cancers are related to environmental factors. Individual cancer susceptibility can be the result of several host factors, including differences in metabolism, DNA repair, altered expression of tumor suppressor genes and proto-oncogenes, and nutritional status. Xenobiotic metabolism is the principal mechanism for maintaining homeostasis during the body's exposure to xenobiotics. The balance of xenobiotic absorption and elimination rates in metabolism can be important in the prevention of DNA damage by chemical carcinogens. Thus the ability to metabolize and eliminate xenobiotics can be considered one of the body's first protective mechanisms. Variability in individual metabolism has been related to the enzymatic polymorphisms involved in activation and detoxification of chemical carcinogens. This paper is a contemporary literature review on genetic polymorphisms involved in the metabolism of endocrine disruptors potentially related to cancer development.

P38 Mitogen-activated Protein Kinase Negatively Regulates the Induction of Phase II Drug-metabolizing Enzymes That Detoxify Carcinogens

Phase II drug-metabolizing enzymes, such as glutathione S-transferase and quinone reductase, play an important role in the detoxification of chemical carcinogens. The induction of these detoxifying enzymes by a variety of agents occurs at the transcriptional level and is regulated by a cis-acting element, called the antioxidant response element (ARE) or electrophile-response element. In this study, we identified a signaling kinase pathway that negatively regulates ARE-mediated gene expression. Treatment of human hepatoma HepG2 and murine hepatoma Hepa1c1c7 cells with tert-butylhydroquinone (tBHQ) stimulated the activity of p38, a member of mitogen-activated protein kinase family. Inhibition of p38 activation by its inhibitor, SB203580, enhanced the induction of quinone reductase activity and the activation of ARE reporter gene by tBHQ. In contrast, SB202474, a negative analog of SB203580, had little effect. Consistent with this result, interfering with the p38 kinase pathway by overexpression of a dominant-negative mutant of p38 or MKK3, an immediate upstream regulator of p38, potentiated the activation of the ARE reporter gene by tBHQ, whereas the wild types of p38 and MKK3 diminished such activation. In addition, inhibition of p38 activity augmented the induction of ARE reporter gene activity by tert-butylhydroxyanisole, sulforaphane, and beta-naphthoflavone. Thus, p38 kinase pathway functions as a negative regulator in the ARE-mediated induction of phase II detoxifying enzymes.

A case-control study of cytochrome P450 1A1, glutathione S-transferase M1, cigarette smoking and lung cancer susceptibility (Massachusetts, United States)

Cytochrome P450 1A1 (CYP1A1) and glutathione S-transferase M1 (GSTM1) genetic polymorphisms are involved in the activation and detoxification of chemical carcinogens found in tobacco smoke; thus they may influence host susceptibility to lung cancer. In this study at Massachusetts General Hospital (Boston, MA, USA) of 416 cases and 446 controls (mostly White) we evaluated the association between the CYP1A1 MspI and GSTM1 polymorphisms and lung cancer risk, and their interaction with cigarette smoke. The CYP1A1 MspI heterozygous genotype was present in 18 percent of cases and 16 percent of controls, and one percent of cases and controls were CYP1A1 MspI homozygous variant. The GSTM1 null genotype was detected in 54 percent of cases and 52 percent of controls. After adjusting for age, gender, pack-years of smoking, and years since quitting smoking, while neither the CYP1A1 MspI heterozygous genotype alone nor the GSTM1 null genotype alone were associated with a significant increase in lung cancer risk, having both genetic traits was associated with a twofold increase in risk (95 percent confidence interval [CI] = 1.0-3.4). Our data did not provide enough evidence for a substantial modification of the effect of pack-years on lung cancer risk by the CYP1A1 MspI and GSTM1 genotypes. However, limitations of our study preclude a conclusion about this potential interaction.

Formation of 3-(glutathion-S-yl)-N-methyl-4-aminoazobenzene and inhibition of aminoazo dye-nucleic acid binding in vitro by reaction of glutathione with metabolically-generated N-methyl-4-aminoazobenzene- N-sulfate

The reaction of glutathione (GSH) with metabolically-formed N-methyl-4-aminoazobenzene- N-sulfate (MAB- N-sulfate), a presumed ultimate carcinogenic metabolite of N, N-dimethyl-4-aminoazobenzene (DAB), was investigated using a hepatic sulfotransferase incubation mixture containing GSH and the proximate carcinogen, N-hydroxy- N-methyl-4-aminoazobenzene ( N-HO-MAB). Under these conditions, 6–16% of the MAB- N-sulfate formed could be trapped as an aminoazo dye-GSH adduct. Upon subsequent purification, the adduct was shown to be chromatographically and spectrally identical to 3-(glutathion- S-yl)- N-methyl-4-aminoazobenzene (3-GS-MAB), a known biliary metabolite of DAB and a product of the reaction of the synthetic ultimate carcinogen, N-benzoyloxy- N-methyl-4-aminoazobenzene( N-BzO-MAB), with GSH. Neither 2′- nor 4′-GS-MAB, both products of the latter reaction, were detected in the sulfotransferase incubation mixture. GSH- S-transferases did not appear to be involved in the reaction of MAB- N-sulfate or N-BzO-MAB with GSH. The addition of triethyltin, a potent GSH- S-transferase inhibitor, had no effect on the yield of 3-GS-MAB in (N-HO-MAB sulfotransferase)-GSH incubations; and the addition of cytosol or purified GSH transferases A and B to a (N-BzO-MAB)-GSH reaction mixture did not increase the amount of 3-GS-MAB formed. GSH was shown to inhibit only partially the covalent binding of [ 3H]-MAB- N-sulfate to DNA and rRNA. At 10 and 100 mM GSH, the sulfotransferase-mediated binding of [ 3H] N-HO-MAB to both nucleic acids was reduced by 30% and 70%, respectively. The role of GSH in the detoxification of chemical carcinogens is discussed.