Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0019204 (hepatocellular carcinoma)
 71,386 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have examined enzyme activities and mRNA levels corresponding to aldehyde dehydrogenase-3 genes encoding cytosolic (ALDH3c) and microsomal (ALDH3m) forms. In contrast to negligible activities in the intact mouse liver, both ALDH3c and ALDH3m enzyme activities are inducible by benzo[a]pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mouse hepatoma Hepa-1c1c7 cell cultures. Constitutive mRNA levels of ALDH3c are virtually absent, whereas those of ALDH3m are substantial; using Hepa-1 mutant lines, we show that both ALDH3c and ALDH3m are TCDD-inducible by an Ah receptor-dependent mechanism. Basal mRNA levels of ALDH3c, but not those of ALDH3m, are strikingly elevated in untreated mutant cells lacking a functional CYP1A1 enzyme; low ALDH3c basal mRNA levels can be restored by introduction of a functional murine CYP1A1 or human CYP1A2 enzyme into these mutant cells. These data suggest that the TCDD induction process is distinct from the CYP1A1/CYP1A2 metabolism-dependent repression of constitutive gene expression; we suggest that this latter property classifies the Aldh-3c gene, but not the Aldh-3m gene, as a member of the murine [Ah] battery.
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PMID:Negative regulation of the murine cytosolic aldehyde dehydrogenase-3 (Aldh-3c) gene by functional CYP1A1 and CYP1A2 proteins. 152 Mar 28

4-Ipomeanol is a pulmonary toxin in cattle and rodents that is metabolically activated by cytochromes P-450 (P-450s). P-450-mediated activation of 4-ipomeanol to DNA binding metabolites was evaluated using a vaccinia virus complementary DNA expression system and an in situ DNA-binding assay. Twelve human P-450s and two rodent P-450s were expressed in human hepatoma Hep G2 cells and examined for their abilities to metabolically activate this toxin. Three forms, designated CYP1A2, CYP3A3, and CYP3A4, were able to catalyze significant production of DNA-bound metabolites of 20-, 8-, and 5-fold, respectively, above binding catalyzed by Hep G2 cells infected with wild-type vaccinia virus. These enzymes, with highest activities, are not known to be expressed in human or rodent lung. CYP2F1 and CYP4B1, two enzymes that are expressed in lung, display only modest 3- and 2-fold respective increased abilities to metabolically activate 4-ipomeanol. Two human forms were inactive and seven other human forms showed activities ranging from 0.5- to 2-fold above control level. Surprisingly, rabbit complementary DNA-expressed CYP4B1 was the most active enzyme (180-fold above control) among all P-450s tested in producing DNA-binding metabolites from this mycotoxin. These studies demonstrate a species difference in 4-ipomeanol metabolism and suggest caution when attempting to extrapolate rodent data to humans.
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PMID:Metabolic activation of 4-ipomeanol by complementary DNA-expressed human cytochromes P-450: evidence for species-specific metabolism. 165 9

To better characterize the precise cellular distribution of CYP1A gene products in man, we have undertaken Northern-blot and in situ hybridization analyses of CYP1A expression in human liver. Using riboprobes transcribed from both CYP1A1 and CYP1A2 complementary DNAs to probe a series of Northern blots of 23 human liver messenger RNA samples, CYP1A1 expression was demonstrated in 11 samples and CYP1A2 expression was evident in 22 samples. The level of expression of both CYP1A enzymes in these livers demonstrated marked variability. The CYP1A1 and CYP1A2 riboprobes were then used for in situ hybridization localization of CYP1A1/1A2 messenger RNA sequences on paraffin-embedded, formalin-fixed human liver sections. These studies demonstrated that both CYP1A1 and CYP1A2 messenger RNAs are distributed nonuniformly across the human liver acinus, with levels highest in hepatocytes surrounding terminal hepatic venules and intercalated veins. Immunohistochemistry with an anti-rabbit CYP1A1 serum demonstrated a corresponding distribution for the translated CYP1A proteins. In situ hybridization analysis was also performed on sections of hepatocellular carcinoma, demonstrating a significant down-regulation in CYP1A expression. Functional studies using the activation of the food-derived heterocyclic amine MeIQ (2-amino-3,4-dimethylimadazo [4,5-f] quinoline) to a mutagen in the Ames test as an indicator of CYP1A expression confirmed this down-regulation. These results demonstrate heterogeneity of hepatic CYP1A expression both between individuals and in different acinar zones. This variation in expression may be of significance in assessing cell specific toxicities of various drugs and carcinogens.
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PMID:Localization of CYP1A1 and CYP1A2 messenger RNA in normal human liver and in hepatocellular carcinoma by in situ hybridization. 165 55

The polymorphism of mammalian aromatic hydrocarbon (Ah) responsiveness appears to be correlated with genetic differences in risk of bronchogenic carcinoma caused by cigarette smoking. The human polymorphism has been uncovered, largely as the result of corresponding genetic differences characterized first in the mouse. The murine Ah locus has been defined as the gene encoding the aromatic hydrocarbon-responsive (Ah) receptor, responsible for the inducibility of a battery of at least six genes, two of which encode P450 enzymes. The high-affinity receptor and, hence, more highly induced levels of P450, can result in greater concentrations of polycyclic aromatic reactive intermediates that form DNA adducts and, ultimately, mutation fixation (tumour initiation). The Ah receptor is also likely to participate in growth and differentiation signal transduction pathways (tumour promotion). Positive and negative control regions flanking the murine Cyp 1a-1 and human CYP1A1 (cytochrome P(1)450) genes have been identified. A DNA motif approximately 1 kb upstream of the transcription start site appears to affect the translatability of the CYP1A1 mRNA and activity of the enzyme. Expression of the CYP1A1 or CYP1A2 enzyme in mouse hepatoma Hepa-1 cells lacking endogenous CYP1A1 activity represses constitutive transcription of not only the endogenous Cyp1a-1 gene but other genes in the dioxin-inducible [Ah] battery. Human polymorphisms involving a Msp I site 450 bp downstream from the last CYP1A1 exon have been described in Japan, the Eastern Mediterranean, Norway and the USA. The '1.9 allele' is associated with an increased incidence of Kreyberg Type I bronchogenic carcinomas in Japan and has recently been correlated with a valine-to-isoleucine substitution at position 462 in the haeme-binding region. This allele is about 3 times more frequent in Japan than in Caucasians of Norway and the USA, in which no correlation has been found between this allele and lung cancer. More work is needed to clarify these findings. Isolation and sequencing of the human Ah receptor cDNA, and the subsequent screening of populations for polymorphisms, hold great promise for predicting interindividual risk of cancer caused by smoking and other environmental pollutants.
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PMID:Human AH locus polymorphism and cancer: inducibility of CYP1A1 and other genes by combustion products and dioxin. 184 73

Transcripts of the murine CYP1A1 (cytochrome P1450) mRNA are markedly elevated in mutant hepatoma cell lines that contain missense mutations in the Cyp1a-1 structural gene. This putative derepression extends to other genes in the [Ah] battery. To test whether the Cyp1a-1 gene product is involved in a mechanism of feedback regulation of transcription, we introduced expression plasmids carrying the murine wild-type Cyp1a-1 cDNA into the mutant hepatoma cells. Measurements of steady-state mRNA levels and of transcriptional rates in the transfectants reveal that expression of a functional, exogenous CYP1A1 protein is sufficient to restore the repression of the endogenous gene, as well as restore the inducibility by dioxin, and that this effect takes place primarily at the level of transcription. Similar experiments with expression plasmids that carry the human CYP1A2 cDNA indicate that the CYP1A2 protein (cytochrome P3450) can also function as a transcriptional repressor. In addition, we find that expression of the Nmo-1 [NAD(P)H:menadione oxidoreductase] gene, a third member of the [Ah] gene battery, is also repressed by the exogenous expression of either Cyp1a-1 or CYP1A2 cDNA. These results indicate that the gene product of either member of the mammalian CYP1 family has a previously unrecognized transcriptional regulatory function, which is likely to be exerted by modification of preexisting trans-acting factors. This function may help bring about a fast reprogramming of gene expression, as might be needed during detoxification of toxic foreign chemicals.
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PMID:The murine Cyp1a-1 gene negatively regulates its own transcription and that of other members of the aromatic hydrocarbon-responsive [Ah] gene battery. 208 80

Using the mouse hepatoma Hepa-1c1c7 c37 mutant cell line that exhibits negligible benzo[a]pyrene hydroxylase (Cyp1a1) and acetanilide 4-hydroxylase (Cyp1a2) enzyme activities, we developed stable transfectants of plasmids containing the murine Cyp1a1 (cytochrome P(1)450) and the human CYP1A2 (P(3)450) cDNAs. We show that the assay measuring metabolism of ethoxyfluorescein ethyl ester (EFEE) was invaluable in screening large numbers of individual cell lines for high Cyp1a1 enzyme activity. Nine different plasmid constructs containing various combinations of promoter and enhancer sequences were compared, including: the Drosophila heat shock promoter, the mouse mammary tumor virus long terminal repeat (MMTV LTR) carrying the glucocorticoid-responsive element (GRE), enhancer sequences from simian virus 40 (SV40) and herpes simplex virus type 1 (HSV-1), and the aromatic hydrocarbon-responsive domain (AhRD) of the murine Cyp1a1 gene. Interestingly, only those constructs containing the AhRD produced high levels of Cyp1a1 enzyme activity. In contrast, high levels of CYP1A2 activity were obtained with plasmids carrying the HSV-1 enhancer, as well as the AhRD. These studies suggest that the AhRD, which responds to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), provides a post-transcriptional signal necessary for the induction of functional Cyp1a1 enzyme activity. Although untransfected c37 cells exhibit markedly elevated levels of endogenous Cyp1a1 mRNA, the expression of exogenous Cyp1a1 or CYP1A2 enzyme activity in these cells decreases the concentration of this endogenous Cyp1a1 mRNA to negligible levels and restores Cyp1a1 mRNA inducibility by TCDD; these data indicate that the functional product of either the Cyp1a1 gene or the CYP1A2 gene might have a role in an autoregulatory loop controlling the constitutive expression of the Cyp1a1 gene. The cell lines described herein should be valuable in assessing the contribution of these two P450 enzymes to the processes of cytotoxicity, mutagenesis, and carcinogenesis.
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PMID:Stable expression of mouse Cyp1a1 and human CYP1A2 cDNAs transfected into mouse hepatoma cells lacking detectable P450 enzyme activity. 220 99

The oxidation of O6-benzylguanine, an inactivator of O6-alkylguanine-DNA alkyltransferase, was examined using human liver cytosol, microsomes, and several P450 isoforms. Incubation of O6-benzylguanine with human liver cytosol resulted in the formation of O6-benzyl-8-oxoguanine, which was inhibited by menadione, a potent inhibitor of aldehyde oxidase. Inhibition by allopurinol, a xanthine oxidase inhibitor, was less dramatic. Oxidation of O6-benzylguanine also occurred with pooled human liver microsomes and was inhibited by both furafylline and troleandomycin, selective inhibitors of CYP1A2 and CYP3A4, respectively. Human P450s CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2E1, and CYP3A4 expressed in Hep G2 hepatoma cells using vaccinia virus vectors were incubated with 10 or 200 microM O6-benzylguanine. At 10 microM, O6-benzylguanine was oxidized primarily by CYP1A2 and to a lesser extent by CYP3A4. However, an appreciable increase in CYP3A4 contribution was noted at 200 microM. CYP1A2 exhibited a more than 200-fold higher relative catalytic activity (Vmax/Km) compared with CYP3A4. Therefore, at therapeutically relevant concentrations of O6-benzylguanine, CYP1A2 could be primarily involved in its oxidation since it shows a much lower Km value (1.3 microM) than CYP3A4 (52.2 microM) and cytosol (81.5 microM). However, one would expect interindividual variation in the extent of oxidation of O6-benzylguanine depending on the levels of aldehyde oxidase, CYP1A2, and CYP3A4.
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PMID:Human liver oxidative metabolism of O6-benzylguanine. 750 88

The promutagenic and procarcinogenic heterocyclic amines (HAs) found in cooked meats are N-hydroxylated by microsomal cytochrome P450 enzymes as the first step in their metabolic activation. In cynomolgus monkeys, one of the HAs, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), has been shown to be a potent hepatocarcinogen. However, the structurally similar HA 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) lacks this potency to induce hepatocellular carcinoma in monkeys. Liver microsomes from cynomolgus monkeys show a striking substrate specificity for the metabolic activation of IQ and MeIQx, the former being a far better substrate for N-hydroxylation. Western blot analysis showed that cynomolgus monkey hepatic microsomes constitutively express P450s immunologically related to the human CYP3A, CYP2C, and low levels of CYP1A1. For comparison, Western blot analysis of rat, human and patas monkey microsomes was also carried out. Treatment of cynomolgus monkeys with rifampicin induced hepatic cytochromes P450 related to human CYP3A4 and CYP2C9/10 without inducing CYP1A1 or CYP1A2. Immunoblot analysis also showed that chronic exposure of cynomolgus monkeys to IQ induced hepatic microsomal cytochrome CYP1A1 and CYP1A2, similarly but lesser in magnitude to that observed with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCCD) induction. Using the Ames Salmonella mutagenicity assay, we examined the effect of the inducers on the mutagenic activation (i.e. N-hydroxylation) of IQ and MeIQx by cynomolgus monkey hepatic microsomes. We also examined the mutagenic activation of these HAs by rat, human and patas monkey liver microsomes. Microsomes from cynomolgus monkeys treated with rifampicin showed a 3-fold increase in the mutagenic activation of IQ but showed no increase in the mutagenic activation of MeIQx. Since cytochromes P4503A and/or P4502C are constitutively expressed in cynomolgus monkey hepatic microsomes, and upon induction with rifampicin are associated with an increased metabolic activation of IQ but not MeIQx, it appears that CYP3A and/or CYP2C are the isoform(s) showing the selective substrate specificity in the metabolic activation of IQ over MeIQx. Treatment of monkeys with TCDD significantly increased the mutagenic activation of both IQ and MeIQx, concomitant with an induction of CYP1A isozymes. Thus, it appears that TCDD-inducible CYP1A enzymes N-hydroxylate both substrates without selectivity. Together, these findings suggest that CYP3A and CYP2C are the principal isoforms in the cynomolgus monkey, associated with the metabolic activation implicated in the induction of hepatocarcinogenicity by IQ. Furthermore, the poor metabolic activation of MeIQx by CYP3A and CYP2C, coupled with low constitutive levels of CYP1A isozymes, provide a metabolic explanation for the low hepatocarcinogenic potency of MeIQx in cynomolgus monkeys.
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PMID:Cytochromes P450 in cynomolgus monkeys mutagenically activate 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) but not 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx). 761 88

The rate of formation of styrene glycol from styrene was compared in human, rat, and mouse liver microsomes. At a low styrene concentration (0.085 mM), the rates decreased in the order, mouse (2.43 +/- 0.29 nmol/(mg of protein.min)) > rat (1.07 +/- 0.20) > human (0.73 +/- 0.45); at a high concentration (1.85 mM), the order was rat (4.21 +/- 0.72) > mouse (2.72 +/- 0.11) > human (1.91 +/- 0.84). Kinetic analysis indicated the presence of at least two forms of styrene-metabolizing cytochrome P450s with different Km values in human liver microsomes. Styrene was also metabolized in human lung microsomes: the rate of styrene glycol formation was higher in the lung microsomes from smokers than in those from current nonsmokers. The P450 forms responsible for transforming styrene to styrene glycol were determined by analyzing cDNA-expressed individual P450 forms produced in cultured hepatoma G2 cells by recombinant vaccinia viruses. Of the 12 human P450 forms studied, CYP2B6 and CYP2E1 existing in human liver and/or lungs and CYP2F1 in human lungs were the most active in the forming of styrene glycol, followed by CYP1A2 and CYP2C8. Human CYP3A3, CYP3A4, CYP3A5, and CYP4B1 also catalyzed the metabolism but were much less active. CYP2A6, CYP2C9, and CYP2D6 had only a little detectable activity. CYP1A2, CYP2B6, CYP2C8, CYP2E1, and CYP3A4/3A3 were expressed in human liver microsomes, and CYP2C8 was expressed in human lung microsomes, although the expression of CYP2F1 and CYP4B1 could not be investigated. These data indicate that several human hepatic and/or pulmonary P450 forms are capable of metabolizing styrene, albeit at different rates.
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PMID:Styrene metabolism by cDNA-expressed human hepatic and pulmonary cytochromes P450. 769 48

The gene for cytochrome P4501A2 is constitutively expressed in the liver of vertebrates and shows induced expression when an organism is exposed to polycyclic aromatic hydrocarbons and halogenated hydrocarbons. To identify DNA elements regulating transcription of the human CYP1A2 gene, transient transfection experiments were conducted in the human hepatoma cell line HepG2. Dissection of the 5'-flanking portion of the CYP1A2 gene identified two regions that contributed to the overall induction by 3-methylcholanthrene. One region located at -2532/-2423 contains an xenobiotic-responsive element-like sequence, termed X1, that binds a nuclear 2,3,7,8-tetrachlorodibenzo-p-dioxin-inducible protein in HepG2 and wild type mouse Hepa-1 cells, but not in the Ah receptor nuclear translocation defective mouse C- mutant c4 cells. In addition, deletion of this region of the CYP1A2 gene reduces the 3-methylcholanthrene (3-MC)-initiated induction of chloramphenicol acetyltransferase activity in both promoter- and enhancer-specific constructs. The second responsive region is located at -2259/-1987. This region of the gene contains a second xenobiotic-responsive element-like element, but this element does not associate with the nuclear Ah receptor. However, there does exist several potential AP1 binding sites and a conserved TATA box. A DNA fragment from -2259/-1970 that contains these elements was shown to function as an efficient eukaryotic promoter, in addition to supporting 3-MC-induced promoter activity. These results suggest that Ah receptor-specific and promoter-specific elements regulate the expression of the human CYP1A2 gene.
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PMID:The human CYP1A2 gene and induction by 3-methylcholanthrene. A region of DNA that supports AH-receptor binding and promoter-specific induction. 812 57


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