UMLS:C0019204 - FACTA Search

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Query: UMLS:C0019204 (hepatocellular carcinoma)
71,386 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

In mouse hepatoma Hepa-1c1c7 cultures, polycyclic aromatic compounds such as benzol[a]pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) activate the Cyp1a-1 (cytochrome P(1)450) and Nmo-1[NAD(P)H:menadione-oxidoreductase] genes, two members of the aromatic hydrocarbon (Ah)-responsive gene battery. Mevinolin is known to inhibit 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase (EC 1.1.1.34), the rate-limiting step in cholesterol biosynthesis. We show here that in the absence of TCDD, mevinolin markedly increases Cyp1a-1 transcription, CYP1A1 mRNA and protein levels and enzyme activity, and NMO1 mRNA concentrations. Addition of mevalonate, the product of HMG-CoA reductase activity, fails to reverse the effects of mevinolin. In fact, when used at high concentrations, mevalonate activates Cyp1a-1 transcription. Mevinolin-induced Cyp1a-1 gene activation: (1) occurs independently of the lipid content of the growth medium, (2) is not suppressed by adding 25-hydroxycholesterol, which blocks MHG-CoA reductase activity, and (3) requires a functional Ah receptor and unimpaired nuclear translocation of the receptor. It is possible that an unknown metabolite (or metabolites) of mevinolin activates Cyp1a-1 expression and that high concentrations of mevalonate act via the same mechanism. Using chimaeric plasmids that contain different lengths of Cyp1a-1 5' flanking regions fused to the bacterial neomycin (neo) gene, we find that the mevinolin effect on Cyp1a-1 induction requires the 5' flanking sequences between -1647 and -824, which are also needed for TCDD induction. Mevinolin, however, is not a ligand for the Ah receptor. Gel mobility shift assays revealed that Cyp1a-1 activation caused by mevinolin does not involve the ligand-dependent formation of a functional Ah receptor-dependent DNA-binding complex, but instead appears to be correlated with release of a putative repressor from its cognate DNA site. Our results suggest that the basel level of Cyp1a-1 transcription is maintained by an unknown negative regulatory factor. We propose that Cyp1a-1 transcriptional activation can result not only from induction by polycyclic aromatic compounds but also from derepression by mevinolin, independent of HMG-CoA reductase inhibition.
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PMID:Transcriptional derepression of the murine Cyp1a-1 gene by mevinolin. 131 Dec 72

Transcriptional activation of the murine Cyp1a-1 (cytochrome P(1)450) gene by inducers such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (dioxin) requires the aromatic hydrocarbon (Ah) receptor and the interaction of an inducer-receptor complex with one or more of the Ah-responsive elements (AhREs) located about 1 kb upstream from the transcriptional initiation site. We find that treatment of mouse hepatoma Hepa-1 cells with 2-aminopurine, an inhibitor of protein kinase activity, inhibits CYP1A1 mRNA induction by TCDD as well as the concomitant increase in CYP1A1 enzyme activity. Formation of DNA-protein complexes between the Ah receptor and its AhRE target is also inhibited by 2-aminopurine, as determined by gel mobility shift assays. Phosphorylation is required for the formation of Ah receptor-specific complexes, since in vitro dephosphorylation of nuclear extracts from TCDD-treated Hepa-1 cells abolishes the capacity of the Ah receptor to form specific complexes with its cognate AhRE sequences. To determine whether any one of several known protein kinases was involved in the transcriptional regulation of the Cyp1a-1 gene, we treated Hepa-1 cells with nine other protein kinase inhibitors prior to induction with TCDD; nuclear extracts from these cells were analyzed for their capacity to form specific DNA-protein complexes. Only extracts from cells treated with staurosporine, a protein kinase C inhibitor, were unable to form these complexes. In addition, staurosporine completely inhibited CYP1A1 mRNA induction by TCDD. Depletion of protein kinase C by prolonged treatment with phorbol ester led to the complete suppression of CYP1A1 mRNA induction by TCDD. We conclude that (i) phosphorylation is necessary for the formation of a transcriptional complex and for transcriptional activation of the Cyp1a-1 gene; (ii) the phosphorylation site(s) exists on at least one of the proteins constituting the transcriptional complex, possibly the Ah receptor itself; and (iii) the enzyme responsible for the phosphorylation is likely to be protein kinase C.
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PMID:Dioxin-dependent activation of murine Cyp1a-1 gene transcription requires protein kinase C-dependent phosphorylation. 131 72

DNA-protein interactions before and after transcriptional activation of the carcinogen- and dioxin-inducible enhancer of the murine CYP1A1 gene were detected in vivo by treatment with dimethyl sulfate followed by ligation-mediated, polymerase chain reaction-aided genomic sequencing. Following 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment of mouse Hepa-1 hepatoma cells, evidence of protein binding was detected at the sequence 5' CACGCNA/T 3' within two previously defined xenobiotic response elements (XREs). The observed XRE footprint was similar to that previously identified by in vitro methylation protection footprints and attributed to the binding protein for 2,3,7,8-tetrachlorodibenzo-p-dioxin the Ah receptor. No XRE footprinting was observed in Hepa-1 mutant cells possessing a defective Ah receptor. Unexpectedly, evidence of protein binding was also detected at a G-rich DNA sequence immediately adjacent to one of the XREs. Footprinting of the G-rich sequence element, like that of XRE1 and XRE2, was dependent on the presence of a functional Ah receptor. The Ah receptor is therefore able to bind to its own DNA target sites in vivo and is also required for the binding of a second factor to the G-rich element.
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PMID:Dioxin- and Ah receptor-dependent protein binding to xenobiotic responsive elements and G-rich DNA studied by in vivo footprinting. 131 25

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.
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PMID:Induction of CYP1A1 gene expression in H4-II-E rat hepatoma cells by benzo[e]pyrene. 131 59

We have analyzed dioxin-inducible, Ah receptor-dependent changes in protein-DNA interactions at the CYP1A1 transcriptional promoter in intact mouse hepatoma cells. Our findings indicate that in uninduced cells, the promoter is inaccessible to its cognate binding proteins, which are known to be expressed constitutively. Dioxin induces, in Ah receptor-dependent fashion, an increase in promoter accessibility, which occurs rapidly and does not require ongoing transcription of the CYP1A1 gene. The change in promoter accessibility is not due to an altered pattern of cytosine methylation at the promoter; it probably reflects a 2,3,7,8-tetrachlorodibenzo-p-dioxin- induced change in the chromatin structure. These findings provide new insight into the mechanism of dioxin action and contribute to a better understanding of the regulation of inducible gene transcription in mammalian cells.
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PMID:Mechanism of dioxin action: Ah receptor-mediated increase in promoter accessibility in vivo. 131 73

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces CYP1A1 gene expression as determined by increased CYP1A1 mRNA levels and ethoxyresorufin O-deethylase (EROD) activity in mouse Hepa 1c1c7, rat hepatoma H-4II E and human Hep G2 cancer cell lines. In contrast, treatment of these cell lines with either alpha-naphthoflavone (alpha NF) or 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) at concentrations as high as 10(-6) M resulted in only minimal induction of CYP1A1 mRNA levels or EROD activity. Cotreatment of the cells with 10(-9) M TCDD plus different concentrations (10(-8)-10(-6) M) of MCDF or alpha NF resulted in a concentration-dependent decrease in TCDD-induced CYP1A1 mRNA levels and EROD activity in the three cell lines. Moreover, using 10(-9) M [3H]TCDD, it was shown that the alpha NF- and MCDF-mediated antagonism of TCDD-induced CYP1A1 gene expression was paralleled by a decrease in levels of the nuclear [3H]TCDD-Ah receptor complex as determined by velocity sedimentation analysis of the nuclear extracts. The binding of nuclear extracts from the treated cells to a synthetic consensus dioxin responsive element (DRE) (a 26-mer) was determined by gel retardation studies using 32P-DRE. In cells treated with 10(-9) M TCDD or TCDD plus 10(-8)-10(-6) M alpha NF, the concentration-dependent decrease in TCDD-induced CYP1A1 gene expression by alpha NF was also paralleled by decreased levels of a retarded band associated with the nuclear Ah receptor-DRE complex. In contrast, the results of the gel shift assay of nuclear extracts treated with 10(-9) M TCDD or TCDD plus 10(-8)-10(-6) M MCDF indicated that there were relatively high levels of nuclear MCDF-Ah receptor complex in the cells co-treated with TCDD plus the antagonist but this was not accompanied by induced CYP1A1 gene expression. The results suggest that alpha NF and possibly MCDF compete with TCDD for cytosolic Ah receptor binding sites; however, MCDF may also inhibit the induction response by competing for and/or partially inactivating genomic binding sites.
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PMID:Mechanism of action of aryl hydrocarbon receptor antagonists: inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced CYP1A1 gene expression. 132 56

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

Mouse hepatoma Hepa 1c1c7 cells and nonresponsive mutants have been extensively used as models for investigating the molecular mechanism of induction of CYP1A1 gene transcription by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Incubation of cytosolic [3H]TCDD-aryl hydrocarbon (Ah) receptor from wild-type Hepa 1c1c7 cells for 16 h at 4 degrees C in 0.4 M KCl resulted in the formation of transformed liganded receptor which exhibited increased binding affinity on DNA-Sepharose columns. The elution properties of the peak with the highest DNA binding affinity were similar to the elution profiles of the nuclear receptor complex isolated from wild-type cells. TAOc1BPrcl (class I) nonresponsive mutant cells were characterized by relatively low levels of the cytosolic and nuclear Ah receptor complex. The BPrcl (class II) variant cell line contained levels of cytosolic receptor which were comparable to those observed in the wild-type cells; however, significantly reduced levels of nuclear receptor complex were observed in the class II variant cell line. Incubation of the nuclear or transformed liganded cytosolic Ah receptor from wild-type cells with a consensus 32P-labeled dioxin responsive element (DRE) in a gel shift assay gave a retarded band associated with the receptor-DRE complex. Incubation of the cytosolic receptor complex from the class I and II mutant cells for 16 h at 4 degrees C in 0.4 M KCl or for 2 h at 20 degrees C did not yield complexes with increasing binding affinities on DNA-Sepharose columns. Moreover, incubation of these complexes with 32P-labeled DRE did not give a retarded band in a gel shift assay. However, coincubation of the liganded class II mutant cytosol with cytosol from class I cells resulted in transformation of the liganded receptor and this was confirmed in both the DNA-Sepharose and gel retardation assays. These results suggest that the failure of class II mutant cells to respond to TCDD is due to a defect in the factors responsible for transformation of the cytosolic receptor complex.
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PMID:DNA binding properties of the Ah receptor in wild-type and variant mouse hepatoma cells. 165 77

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

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