Environmental factors as regulators and effectors of multistep carcinogenesis.
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This review highlights current knowledge of environmental factors in carcinogenesis and their cellular targets. The hypothesis that environmental factors influence carcinogenesis is widely supported by both epidemiological and experimental studies. The fact that only a small fraction of cancers can be attributed to germline mutations in cancer-related genes further buttresses the importance of environmental factors in carcinogenesis. Furthermore, penetrance of germline mutations may be modified by either environmental or other genetic factors. Examples of environmental factors that have been associated with increased cancer risk in the human population include chemical and physical mutagens (e.g. cigarette smoke, heterocyclic amines, asbestos and UV irradiation), infection by certain viral or bacterial pathogens, and dietary non-genotoxic constituents (e.g. macro- and micronutrients). Among molecular targets of environmental influences on carcinogenesis are somatic mutation (genetic change) and aberrant DNA methylation (epigenetic change) at the genomic level and post-translational modifications at the protein level. At both levels, changes elicited affect either the stability or the activity of key regulatory proteins, including oncoproteins and tumor suppressor proteins. Together, via multiple genetic and epigenetic lesions, environmental factors modulate important changes in the pathway of cellular carcinogenesis. (+info)
Heterocyclic aromatic amines induce DNA strand breaks and cell transformation.
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Heterocyclic aromatic amines (HAAs), formed during the cooking of foods, are known to induce tumours in rodent bioassays and may thus contribute to human cancer risk. We tested six HAAs in a morphological transformation assay and in three in vitro genotoxicity assays. The morphological transforming abilities of HAAs were tested, in the presence of rat-liver S9, in the C3H/M2 fibroblast cell line. Concentration levels of 50 microM 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (8-MeIQx), 100 microM 2-amino-3,4,8-trimethylimidazo-[4,5-f]quinoxaline (4,8-DiMeIQx), 50 microM 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 100 microM 2-amino-9H-pyrido[2,3-b]indole (AalphaC), 100 microM 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeAalphaC) and 15 microM 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induced maximum transformation potencies of 5.5, 6.6, 6.3, 5.2, 7.3 and 9.2 transformed foci per 10(4) surviving cells, respectively. Bacterial mutagenic activity was determined in the presence of rat-liver S9 using the Salmonella typhimurium reverse-mutation assay employing strain YG1019. Mutagenic potencies of 3800 revertants (revs)/ng with 8-MeIQx, 2900 revs/ng with 4,8-DiMeIQx, 3480 revs/ng with IQ, 1.6 revs/ng with AalphaC, 2.9 revs/ng with MeAalphaC and 5 revs/ng with PhIP were observed. Clastogenic activity in vitro was analysed by the micronucleus assay in metabolically competent MCL-5 cells. Dose-dependent induction of micronuclei was observed for all HAAs tested with 1-5.4% of cells containing micronuclei at 10 ng/ml. Micronucleus induction was in the order 4,8-DiMeIQx > 8-MeIQx > IQ > MeAalphaC > PhIP > AalphaC. DNA strand-breaking activity in MCL-5 cells was measured by the alkaline single cell-gel (comet) assay. The lowest effect doses for significant increases (P < or = 0.0007, Mann-Whitney test) in comet tail length (microm) were 45.5 microg/ml (200 microM) for PhIP, 90.9 microg/ml (410-510 microM) for 4,8-DiMeIQx, IQ, MeAalphaC and AalphaC, and 454.5 microg/ml (2130 microM) for 8-MeIQx. It is not yet clear which of these assays most accurately reflects the genotoxic potential to humans of compounds of this class of environmental carcinogens. (+info)
Decreased expression of glutathione S-transferase M1 in HPV16-transfected human cervical keratinocytes in culture.
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Glutathione S-transferase (GST) M1 is a member of the GST mu family of cytosolic enzymes that have been hypothesized to catalyze the conjugation of glutathione to a large number of hydrophobic substances, including carcinogens such as polynuclear aromatic hydrocarbons present in tobacco smoke, leading to their excretion. Epidemiologic and experimental evidence suggests that the risk of cervical cancer is related to both human papillomavirus (HPV) infection and cigarette smoking. We compared the enzymatic activities and mRNA levels of GSTs in GSTM1-positive human cervical keratinocytes (HCKs) that had been transfected with HPV16 with those in the parental cells. The GSTM1 activity toward the substrate trans-stilbene oxide was 5- to 7-fold lower than in the parental cells. The relative mRNA level in HCK transfected with HPV16 E6/E7, as quantified by reverse transcriptase-polymerase chain reaction (RT-PCR) with normalization against endogenous glyceraldehyde-3-phosphate dehydrogenase (GAPDH) expression, was 6% that of the parental cells. It was 16 and 82%, respectively, in cells that were transfected with HPV16 E6 alone or HPV16 E7 alone. When quantified by competitive RT-PCR using an exogenous nuclease-resistant synthetic cyclophilin RNA transcript as control, the mRNA level in HCK transfected with HPV16 E6 was approximately 10-fold lower that that in the parental cells. It was approximately 5- to 7-fold lower in the HPV16 E7 or HPV16 E6/E7 cells. Our results suggest that viral infections, through the modulation of cellular xenobiotic-metabolizing enzymes, may play a role in the ability of cells to handle environmental carcinogens. (+info)
Histopathology and gene expression changes in rat liver during feeding of fumonisin B1, a carcinogenic mycotoxin produced by Fusarium moniliforme.
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Fumonisin B1 (FB1) is a carcinogenic mycotoxin produced by the fungus Fusarium moniliforme in corn. Feeding of FB1 to rats causes acute liver injury, chronic liver injury progressing to cirrhosis, and sometimes terminates in hepatocellular carcinoma or cholangiocarcinoma. This study describes the histolopathology and changes in gene expression in the rat liver during short-term feeding of FB1. Male Fischer rats were fed either FB1 250 mg/kg or control diet, and were killed weekly for 5 weeks. FB1 caused a predominantly zone 3 'toxic' liver injury, with hepatocyte death due to necrosis and apoptosis. Hepatocyte injury and death were mirrored by hepatic stellate cell proliferation and marked fibrosis, with progressive disturbance of architecture and formation of regenerative nodules. Despite ongoing hepatocyte mitotic activity, oval cell proliferation was noted from week 2, glutathione S-transferase pi-positive hepatic foci and nodules developed and, at later time points, oval cells were noted inside some of the 'atypical' nodules. Northern blot (mRNA) analysis of liver specimens from weeks 3 to 5 showed a progressive increase in gene expression for alpha-fetoprotein, hepatocyte growth factor, transforming growth factor alpha (TGF-alpha) and especially TGF-beta1 and c-myc. Immunostaining with LC(1-30) antibody demonstrated a progressive increase in expression of mature TGF-beta1 protein by hepatocytes over the 5 week feeding period. The overexpression of TGF-beta1 may be causally related to the prominent apoptosis and fibrosis seen with FB1-induced liver injury. Increased expression of c-myc may be involved in the cancer promoting effects of FB1. (+info)
Differential protection against benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide-induced DNA damage in HepG2 cells stably transfected with allelic variants of pi class human glutathione S-transferase.
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The pi class glutathione S-transferase (GSTP1-1), which is polymorphic in human populations, is believed to play an important role in detoxification of the ultimate carcinogen of widespread environmental pollutant benzo[a]pyrene [(+)-anti-benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide [(+)-anti-BPDE]]. The allelic variants of human GSTP1-1 (hGSTP1-1) differ in their structures by the amino acids in positions 104 (isoleucine or valine) and/or 113 (valine or alanine). Here, we have determined the protective effect of overexpression of allelic variants of hGSTP1-1, through stable transfection in HepG2 cells, against (+)-anti-BPDE-induced DNA modification. Clonal transfectants of HepG2 cells corresponding to the three allelic variants of hGSTP1-1 [(I104,A113), (V104,A113), and (V104,V113), denoted hGSTP1(IA), hGSTP1(VA), and hGSTP1(VV), respectively] with similar levels of hGSTP1 protein were identified and characterized for their GST activity and (+)-anti-BPDE-induced DNA modification. The glutathione S-transferase activity toward (+)-anti-BPDE was significantly higher (approximately 3.0-3.6-fold) in cells transfected with hGSTP1(VA) [HepG2(VA)] and hGSTP1(VV) [HepG2(VV)] compared with hGSTP1(IA) transfectant [HepG2(IA)]. The formation of (+)-anti-BPDE-DNA adducts was significantly reduced in HepG2(VA) and HepG2(VV) cells compared with cells transfected with insert-free vector (HepG2-vect). Maximum protection against (+)-anti-BPDE-induced DNA damage was afforded by the hGSTP1(VV) isoform. The results of this study indicate that the allelic variants of hGSTP1-1 significantly differ in their ability to provide protection against (+)-anti-BPDE-induced DNA damage. Thus, hGSTP1-1 polymorphism may be an important factor in differential susceptibility of individuals to tumorigenesis induced by benzo[a]pyrene. (+info)
Metabolic proficiency and benzo[a]pyrene DNA adduct formation in APCMin mouse adenomas and uninvolved mucosa.
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Tumour formation may involve interactions between genetic factors and environmental carcinogens. Adenoma formation in APCMin/+ mice is associated homozygous adenomatous polyposis coli (APC) gene mutation, but the effects on carcinogen susceptibility are unknown. This study tests the hypothesis that APCMin/+ adenoma formation is accompanied by changes in metabolic proficiency and carcinogen susceptibility. Cytochrome P450 (CYP)1A1/1A2, glutathione S-transferase (GST)alpha, mu and pi classes and DNA adduct formation were assayed in adenomas and uninvolved mucosa from APCMin/+ mice, before and after benzo[a]pyrene (B[a]P) treatment. In untreated adenomas and mucosa, CYP1A1/1A2 and B[a]P-DNA adducts were undetected but GSTalpha, mu and pi class enzymes were constitutively expressed. In adenomas, B[a]P only induced CYP1A1/1A2 to low level while GSTalpha and pi class enzymes were unaffected. A GST mu band which was absent from mucosa, was induced in adenomas. In mucosa, B[a]P induced CYP1A1/1A2 and GSTalpha and pi, to high levels. B[a]P-DNA adduct levels were 56 +/- 15/10(8) nucleotides (median +/- SE) in adenomas versus 89 +/- 19/10(8) nucleotides in mucosa (P < 0.0001). APCMin adenomas show reduced bioactivation capacity and sustain less DNA damage from B[a]P exposure, than APCMin uninvolved mucosa. These properties could influence mutagenesis and subsequent neoplastic transformation of adenomas. (+info)
Cytosine methylation in a CpG sequence leads to enhanced reactivity with Benzo[a]pyrene diol epoxide that correlates with a conformational change.
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Benzo[a]pyrene (B[a]P) is a widespread environmental carcinogen that must be activated by cellular metabolism to a diol epoxide form (BPDE) before it reacts with DNA. It has recently been shown that BPDE preferentially modifies the guanine in methylated 5'-CpG-3' sequences in the human p53 gene, providing one explanation for why these sites are mutational hot spots. Using purified duplex oligonucleotides containing identical methylated and unmethylated CpG sequences, we show here that BPDE preferentially modified the guanine in hemimethylated or fully methylated CpG sequences, producing between 3- and 8-fold more modification at this site. Analysis of this reaction using shorter duplex oligonucleotides indicated that it was the level of the (+)-trans isomer that was specifically increased. To determine if there were conformational differences between the methylated and unmethylated B[a]P-modified DNA sequences that may be responsible for this enhanced reactivity, a native polyacrylamide gel electrophoresis analysis was carried out using DNA containing isomerically pure B[a]P-DNA adducts. These experiments showed that each adduct resulted in an altered gel mobility in duplex DNA but that only the presence of a (+)-trans isomer and a methylated C 5' to the adduct resulted in a significant gel mobility shift compared with the unmethylated case. (+info)
Mutagenic activation of environmental carcinogens by microsomes of gastric mucosa with intestinal metaplasia.
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Coexpression of cytochrome P450 monooxygenases (CYPs) and reductase was found in human gastric mucosa with intestinal metaplasia. Immunohistochemistry showed reactivity to P450 reductase in metaplastic epithelial cells and in pyloric gland cells in glands showing intestinal metaplasia. These cells exhibit NADPH-diaphorase activity. Reverse transcription-PCR analysis and Western blotting showed that CYP1A1 and CYP1A2 were expressed in specimens with intestinal metaplasia. Tissue distribution of CYP1A1 coincided with that of P450 reductase. However, immunoreactivity to CYP1A2 protein was localized only in the pyloric gland cells near the intestinal metaplastic gland. Salmonella typhimurium mutagen assay definitively revealed that microsomes prepared from gastric mucosa with intestinal metaplasia, in particular in the pyloric gland, functionally activated benzo(a)pyrene and 2-amino-3-methylimidazo [4,5-f]quinoline. These results indicate that carcinogen activation by CYP enzymes expressed in the gastric mucosa may contribute to carcinogenesis of the stomach. (+info)