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)

The inducibility of two DNA repair proteins, the O6-methylguanine-DNA-methyltransferase (MGMT) and the N3-methyladenine-DNA-glycosylase (ANPG), was studied by measuring the protein activities and the transcription of the MGMT and ANPG genes in a human hepatoma cell line (LICH cells). The two protein activities are enhanced after treatment with a variety of DNA-damaging agents. They are maximum 72 hr after the inducing treatments and remain elevated for about 120 hr. This induction is abolished when the cells are grown in the presence of protein or RNA synthesis inhibitors. Northern blot analysis shows that the DNA-damaging agents increase to different extents the transcription of the MGMT or ANPG genes. The transferase activity is also increased by DNA damage in a human glioblastoma cell line (T98G cells), but is not significantly modified in human normal fibroblasts, suggesting that this repair activity enhancement might occur preferentially in transformed cells, as we have previously shown for cells of rat origin. Therefore, these increased repair activities may play an important role in removing the lethal N3-methyladenine residues, the promutagenic O6-methylguanine lesions, and the potentially lethal chloroethyl adducts formed by the nitrosoureas used in cancer chemotherapy more efficiently from the cellular DNA.
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PMID:Induction of O6-methylguanine-DNA-methyltransferase and N3-methyladenine-DNA-glycosylase in human cells exposed to DNA-damaging agents. 846 46

Plasma membranes from liver of control rats or from chemical-induced hepatoma were prepared. The basal activity of adenylate cyclase was increased significantly in the rat plasma membranes of DEN-induced hepatoma compared to normal tissue. The glucagon-induced response on the cellular effector systems via guanine nucleotide-binding regulatory proteins (G proteins) was inhibited in hepatoma plasma membranes. These findings suggest that in hepatoma membranes, unlike normal hepatic membranes, the response to hormonal stimuli through regulatory G proteins results in a loss of response to glucagon, as well as to GTP plus glucagon or to GTP gamma S. However, the activating effects of forskolin, which catalyses the formation of cyclic AMP from ATP acting on the catalytic subunit, were to some extent retained. The methyltransferase-I behaved in the opposite direction to the adenylate cyclase, showing a decreased activity in hepatoma plasma membranes compared to control membranes. In contrast, the activity of the ecto-5'-nucleotidase was significantly increased in hepatoma. These enzymatic changes have been found to influence the membrane fluidity and to be responsible for the ultrastructural modifications of hepatoma plasma membranes which are induced by chemical carcinogens.
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PMID:Enzymatic, biophysical and ultrastructural changes of plasma membranes in chemical-induced rat hepatoma. 856 46

Chronic dietary methyl deficiency in F344 rats was used as an in vivo mammalian model in which to evaluate the gene-specific alterations in DNA methylation patterns during multistage hepatocarcinogenesis. Using bisulfite mapping, the site-specific methylation profile within exons 6-7 of the 53 gene was determined in control liver, preneoplastic nodules (after 36 weeks of folate/methyl deficiency) and in hepatocellular carcinoma (after 54 weeks of deficiency). A progressive loss of methyl groups was observed at most CpG sites on both coding and non-coding strands during the first 36 weeks of folate/methyl deficiency, with the greatest loss occurring on the coding strand. When the same sequence was evaluated in tumor DNA after 54 weeks of deficiency, the majority of cytosines were unexpectedly found to have become remethylated. CpG sites that had previously lost methyl groups on both strands during preneoplasia as well as CpG sites that had been constitutively non-methylated, had undergone de novo methylation in tumor DNA. Maintenance methyltransferase and de novo methyltransferase activity in nuclear extracts were assessed using hemimethylated and non-methylated DNA substrates, respectively. In tumor, de novo methyltransferase capacity was increased approximately 4-fold relative to control or preneoplastic liver and associated with a relative increase in both p53 and genome-wide methylation density. In the preneoplastic nodules, the level p53 mRNA was increased and associated with hypomethylation in the coding region of the gene, whereas in tumor tissue, p53 mRNA was decreased and associated with relative hypermethylation. Taken together, these results provide additional insights into the dysregulation and instability in DNA methylation that accompanies the transition to tumor.
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PMID:Alterations in hepatic p53 gene methylation patterns during tumor progression with folate/methyl deficiency in the rat. 909 76

Recombinant adenovirus vectors represent an efficient means of transferring genes into many different organs. The first-generation E1-deleted vector genome remains episomal and, in the absence of host immunity, persists long-term in quiescent tissues such as the liver. The mechanism(s) which allows for persistence has not been established; however, vector DNA replication may be important because replication has been shown to occur in tissue culture systems. We have utilized a site-specific methylation strategy to monitor the replicative fate of E1-deleted adenovirus vectors in vitro and in vivo. Methylation-marked adenovirus vectors were produced by the addition of a methyl group onto the N6 position of the adenine base of XhoI sites, CTCGAG, by propagation of vectors in 293 cells expressing the XhoI isoschizomer PaeR7 methyltransferase. The methylation did not affect vector production or transgene expression but did prevent cleavage by XhoI. Loss of methylation through viral replication restores XhoI cleavage and was observed by Southern analysis in a wide variety of, but not all, cell culture systems studied, including hepatoma and mouse and macaque primary hepatocyte cultures. In contrast, following liver-directed gene transfer of methylated vector in C57BL/6 mice, adenovirus vector DNA was not cleaved by XhoI and therefore did not replicate, even after a period of 3 weeks. Although replication may occur in some tissues, these results show that stabilization of the vector within the target tissue prior to clearance by host immunity is not dependent upon replication of the vector, demonstrating that the input transduced DNA genomes were the persistent molecules. This information will be useful for the design of optimal adenovirus vectors and perhaps nonviral episomal vectors for clinical gene therapy.
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PMID:Persistence of recombinant adenovirus in vivo is not dependent on vector DNA replication. 934 56

In the process of investigating basic questions about the function of the enzyme phosphatidyl-ethanolamine N-methyltransferase (PEMT), we have made several unexpected findings. PEMT catalyzes the conversion of phosphatidylethanolamine to phosphatidylcholine in hepatocytes. A novel isoform, PEMT2, has been cloned, expressed and localized to a mitochondria-associated membrane in rat liver. Expression of PEMT2 in cultured rat hepatoma cells decreased the rate of cell division. Mechanistic studies suggest that the slower growth of transfected hepatoma cells is due to down regulation of CTP:phosphocholine cytidylyltransferase and the CDP-choline pathway for phosphatidylcholine biosynthesis. A role for PEMT2 in the regulation of hepatocyte cell division is also indicated by PEMT2 down-regulation in regenerating rat liver. Another unexpected finding was the discovery that PEMT2 is a liver specific tumor suppressor. Also surprising was the finding that expression of PEMT2 does not rescue mutant Chinese hamster ovary cells that have a temperature sensitive defect in the CDP-choline pathway even though the levels of phosphatidylcholine are returned to normal. Thus, curiosity-driven research has resulted in unexpected findings about PEMT and its function in liver.
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PMID:Phosphatidylethanolamine N-methyltransferase: unexpected findings from curiosity-driven research. 938 67

Messenger RNA differential display was used to study liver-gene expression in paired tumor and non-tumor tissues from hepatocellular carcinoma (HCC) patients. mRNA differential display and Northern-blot analyses showed that a 0.8-kb cDNA fragment was diminished or absent from the tumorous tissues of 7 HCC patients. The cDNA fragment was sequenced and found to have 98.7% nucleotide sequence homology with human glycine-N-methyltransferase cDNA (GNMT). In addition, there was no detectable level of GNMT expression in 4 human HCC cell lines, SK-Hep1, Hep 3B, HuH-7 and HA22T, examined by Northern-blot assay. A full-length GNMT cDNA clone-9-1-2 was obtained by screening a Taiwanese liver cDNA library. In comparison with the GNMT cDNA sequence reported elsewhere, clone 9-1-2 had 4 nucleotide differences resulting in 1 amino-acid change. Immunohistochemical staining with rabbit anti-recombinant GNMT serum showed that GNMT protein almost completely disappeared in liver-cancer cells, while it was abundant in the non-tumorous liver cells. Down-regulation of GNMT gene expression may be involved in the pathogenesis of liver cancer.
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PMID:Characterization of glycine-N-methyltransferase-gene expression in human hepatocellular carcinoma. 949 50

Despite being widely hypothesized, the actual contribution of choline as a methyl source for phosphatidylethanolamine (PE) methylation has never been demonstrated, mainly due to the inability of conventional methods to distinguish the products from that of the CDP-choline pathway. Using a novel combination of stable-isotope labeling and tandem mass spectrometry, we demonstrated for the first time that choline contributed to phosphatidylcholine (PC) synthesis both as an intact choline moiety via the CDP-choline pathway and as a methyl donor via PE methylation pathway. When hepatocytes were labeled with d(9)-choline containing three deuterium atoms on each of the three methyl groups, d(3)-PC and d(6)-PC were detected, indicating that newly synthesized PC contained one or more individually mobilized methyl groups from d(9)-choline. The synthesis of d(3)-PC and d(6)-PC was sensitive to the general methylation inhibitor 3-deazaadenosine and were specific products of PE methylation using choline as a one-carbon donor. While the contribution to the CDP-choline pathway remained intact in hepatocarcinoma cells, contribution of choline to PE methylation was completely disrupted. In addition to a previously identified lack of PE methyltransferase, hepatocarcinoma cells were found to lack the abilities to oxidize choline to betaine and to donate the methyl group from betaine to homocysteine, whereas the usage of exogenous methionine as a methyl group donor was normal. The failure to use choline as a methyl source in hepatocarcinoma cells may contribute to methionine dependence, a widely observed aberration of one-carbon metabolism in malignancy.
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PMID:Disruption of choline methyl group donation for phosphatidylethanolamine methylation in hepatocarcinoma cells. 1186 70

During the pathogenesis of human hepatocellular carcinoma (HCC), the CpG island encompassing the pi-class glutathione S-transferase gene (GSTP1) becomes hypermethylated. Repression of transcription accompanying CpG island hypermethylation has been proposed to be mediated by methyl-CpG binding domain (MBD) proteins. We report here that inhibition of transcription from hypermethylated GSTP1 promoters in Hep3B HCC cells, which fail to express GSTP1 mRNA or GSTP1 polypeptides, appears to be mediated by MBD2. Treatment of Hep3B cells with 5-azadeoxycytidine (5-aza-dC), a methyltransferase inhibitor, activated GSTP1 expression, whereas treatment with trichostatin A, a histone deacetylase inhibitor, had little effect. To more precisely assess the contribution of the pattern of GSTP1 CpG island methylation on GSTP1 mRNA expression, Hep3B cells were treated for 72 h with 5-aza-dC and then subjected to limiting dilution cloning. Bisulfite sequencing was used to map the methylation patterns of the GSTP1 promoter region in GSTP1-expressing and -non-expressing clones. In the clone that expressed GSTP1 mRNA determined by Northern blot analysis and quantitative reverse transcriptase (RT)-PCR, widespread demethylation of at least one GSTP1 allele was evident. Chromatin immunoprecipitation experiments revealed the presence of MBD2, but not Sp1, at the GSTP1 promoter in Hep3B cells. In contrast, Sp1 was detected at the GSTP1 promoter in a GSTP1-expressing Hep3B 5-aza-dC subclone. To test whether MBD2 might be responsible for the inhibition of GSTP1 transcription from hypermethylated GSTP1 promoters, siRNAs were used to reduce MBD2 polypeptide levels in Hep3B cells. SssI-catalyzed methylation of GSTP1 promoter sequences resulted in diminished luciferase reporter activity after transfection into Hep3B cells. However, when hypermethylated GSTP1 promoter sequences were transfected into Hep3B cells that had been treated with siRNA-targeting MBD2 mRNA, no repression of luciferase reporter expression was evident. These findings implicate MBD2 in the repression of GSTP1 expression associated with GSTP1 CpG island hypermethylation in HCC cells.
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PMID:Methyl-CpG binding domain protein 2 represses transcription from hypermethylated pi-class glutathione S-transferase gene promoters in hepatocellular carcinoma cells. 1196 Sep 94

Mild hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Homocysteine, a non-protein amino acid, is formed from S-adenosylhomocysteine and partially secreted into plasma. A potential source for homocysteine is methylation of the lipid phosphatidylethanolamine to phosphatidylcholine by phosphatidylethanolamine N-methyltransferase in the liver. We show that mice that lack phosphatidylethanolamine N-methyltransferase have plasma levels of homocysteine that are approximately 50% of those in wild-type mice. Hepatocytes isolated from methyltransferase-deficient mice secrete approximately 50% less homocysteine. Rat hepatoma cells transfected with phosphatidylethanolamine N-methyltransferase secrete more homocysteine than wild-type cells. Thus, phosphatidylethanolamine N-methyltransferase is an important source of plasma homocysteine and a potential therapeutic target for hyperhomocysteinemia.
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PMID:Plasma homocysteine is regulated by phospholipid methylation. 1248 59

Genetic ablation of phosphatidylethanolamine N-methyltransferase (PEMT) in mice causes a 50% reduction in plasma homocysteine (Hcy) levels. Because hyperhomocysteinemia is an independent risk factor for cardiovascular disease, resolution of the molecular basis for this reduction is of significant clinical interest. The PEMT pathway is a metabolically channeled process localized to the endoplasmic reticulum (ER). To assess the importance of PEMT localization for Hcy homeostasis, we identified and ablated the minimal ER targeting motif. Mutagenesis of a conserved, C-terminal lysine residue (197) relocalized the enzyme to the Golgi, demonstrating that Lys-197 is essential for targeting PEMT to the ER. To evaluate the functional significance of PEMT localization, hepatoma cell lines were generated that stably expressed either ER- or Golgi-localized PEMT only. Intriguingly, stable expression of PEMT in either the ER or the Golgi caused increased Hcy secretion. Moreover, PEMT-mediated Hcy secretion correlated with the methyltransferase activity of the enzyme, independently of subcellular localization. Thus, our data suggest that Hcy homeostasis is regulated concomitantly with PEMT activity but independently of PEMT localization.
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PMID:Localization-independent regulation of homocysteine secretion by phosphatidylethanolamine N-methyltransferase. 1592 61


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