Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0596263 (carcinogenesis)
 64,820 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

An enzyme, O6-methylguanine-DNA methyltransferase, is present in various organisms and plays an important role in repair of DNA damaged by alkylating agents. The enzyme transfers methyl groups from O6-methylguanine and other methylated moieties of the DNA to its own molecule. As a first step to construct animal models with altered levels of the enzyme activity, we cloned cDNA and genomic DNA sequences for mouse methyltransferase and elucidated their structures. The nucleotide sequence of the cDNA revealed an open reading frame comprising 211 amino acid residues. The mol. wt of mouse O6-methylguanine-DNA methyltransferase, calculated from the predicted amino acid sequence, was 22,400, and the methyltransferase protein of this size was present when the cDNA was expressed in methyltransferase-deficient human cells. The predicted amino acid sequence of the mouse methyltransferase exhibits an intense homology with those of human and bacterial counterparts. Using the cDNA as a probe, part of the mouse gene for methyltransferase was isolated. The gene consisted of at least four exons and spanned greater than 145 kb. Sequences around the exon/intron junctions for the mouse gene are almost the same as those for the human species.
Carcinogenesis 1992 Feb
PMID:Isolation and characterization of cDNA and genomic sequences for mouse O6-methylguanine-DNA methyltransferase. 137 Dec 45

O6-Methylguanine-DNA methyltransferase plays an important role in preventing tumor induction. To elucidate the significance of a highly conserved amino acid sequence of methyltransferase protein, amino acid substitutions were introduced by site-directed mutagenesis of cloned cDNA for human methyltransferase and the activity and stability of mutant forms of enzyme were examined. When cysteine-145, to which the methyl transfer occurs, was replaced by other amino acids, all of the mutants isolated showed the methyltransferase-negative phenotype. From one of the negative mutants, methyltransferase-positive revertants were isolated, all of which carried codons for cysteine. Thus the cysteine residue is essential for acceptance of the methyl group and cannot be replaced by other amino acids. Using this negative and positive selection procedure, analyses were extended to other residues near the acceptor site. At the histidine-146 site, four substitutions (phenylalanine, methionine, asparagine and glutamine) exhibited the positive phenotype but the levels of methyltransferase activity in these mutants were low. With valine-148 substitutions there were six types of positive revertants, among which mutants carrying isoleucine, cysteine and alanine showed significantly high levels of methyltransferase activity. Some mutant forms of cDNA were expressed in methyltransferase-deficient human cells, and the results obtained with Escherichia coli cells were confirmed.
Carcinogenesis 1992 May
PMID:Specific amino acid sequences required for O6-methylguanine-DNA methyltransferase activity: analyses of three residues at or near the methyl acceptor site. 158 96

We initiated this study to determine whether three structurally related bifunctional alkylating agents could induce the expression of a presumptive human DNA repair gene. The gene chosen for this study is known to encode the ribosomal phosphoprotein PO, but ironically may also share functions related to DNA repair. We now show by Northern analysis that PO is induced by L-phenylalanine mustard, 4-hydroperoxycyclophosphamide and mechlorethamine, which are DNA-damaging agents commonly used as chemotherapeutic antitumor agents. In further support of its involvement in DNA repair is the finding of a 30- to 50-fold constitutive overexpression of the PO gene in human tumor cell lines that are Mer-, cells which lack O6-methylguanine methyltransferase activity, when compared to Mer+ cell lines. This constitutively elevated level of PO in Mer- cell lines, which are thus DNA repair defective for O6-alkyguanine lesions, was not observed for other genes tested, including the human ribosomal gene S17 whose mRNA steady-state levels were uniformly the same in both Mer- and Mer+ cells. Taking these data together, it appears that increased levels of PO are somehow linked to DNA repair, and increased expression of PO may compensate for the decreased O6-methylguanine DNA methyltransferase activity in Mer- cells. Furthermore, the PO gene has also been shown to be overexpressed in colorectal tumors and polyps and the sera of some systemic lupus erythematosus patients contain antibodies against PO. The titer of the anti-PO antibodies rises significantly during lupus psychosis.
Carcinogenesis 1992 Feb
PMID:Expression of ribosomal phosphoprotein PO is induced by antitumor agents and increased in Mer- human tumor cell lines. 174 17

Differences in susceptibility to chemical carcinogenesis between rodent strains and species have been linked to variations in genetically-determined mixed function oxidase activities. In order to verify whether such variations also determine the susceptibility of individual animals of the same strain to a chemical carcinogen, outbred male Wistar rats were administered diethylnitrosamine (DEN) (1, 2, or 3 mg/kg) five times a week for 20 weeks. The relationship was examined between the outcome (i.e., presence or absence of liver tumors, and latency period) and the hepatic activities of mixed function oxidases and conjugating enzymes, as well as of O6-methylguanine-DNA-methyltransferase, measured before the carcinogen treatment. In addition, the metabolic profiles of two model drugs, antipyrine and disopyramide, in the urine were analyzed and correlated with the carcinogen susceptibility. The length of the latency period of hepatocellular tumors in individual rats was negatively related to the activities of hepatic dimethylnitrosamine N-demethylase, aryl hydrocarbon hydroxylase and epoxide hydrolase and positively related to the amount of microsomal protein. Consistent relationships between the other 10 measured parameters and the susceptibility to DEN-induced carcinogenesis were not detected. Long-term treatment with DEN slightly decreased the proportion of metabolism of antipyrine into norantipyrine, and increased the share of 4-hydroxyantipyrine; a decrease in the metabolism of disopyramide to N-deisopropyldisopyramide was also detected. It is concluded that the pattern of cytochrome P-450 isoenzymes is related to differences in individual susceptibility to nitrosamine-induced carcinogenesis. The relationship was most marked at low dose levels, which are the levels at which nitrosamine exposures of humans are known to occur.
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PMID:Cytochrome P-450 isozyme pattern is related to individual susceptibility to diethylnitrosamine-induced liver cancer in rats. 184 44

Chinese hamster ovary cells with no detectable (less than 200 molecules/cell) O6-methylguanine-DNA methyltransferase (EC 2.1.1.63) were transfected with human cell DNA and pSV2neo plasmid by electroporation. Two stable transformant clones, GC-1 and GC-2, containing 4 X 10(4) and 4-6 X 10(3) methyltransferase molecules/cell respectively were isolated by successive screening in the presence of G418 and 2-chloroethyl-N-nitrosourea (CNU). Only three or four copies of pSV2neo DNA and no repetitive human DNA sequence were detected in these isolates. Secondary transfection of parent cells with GC-1 DNA yielded several clones containing 2-10 X 10(3) methyltransferase molecules/cell. The rate of removal of O6-methylguanine in GC-1, GC-2 and parent cells in vivo reflected their methyltransferase levels, while the N-methylpurines were removed at similar rates in all three cell lines. The differential sensitivity of these cells to several alkylating agents, namely CNU, N-methyl-N-nitrosourea, N-methyl-N'-nitro-N-nitrosoguanidine and methyl-methane sulfonate (MMS), known to yield different proportions of O6-alkylguanine among the alkyl adducts in DNA, varied widely. The largest and smallest differences in toxic response were observed with CNU and MMS respectively. These cell lines showed no difference in sensitivity to the DNA cross-linking agent psoralen. These data strongly suggest that alkylating agents produce two classes of lethal lesions, one of which is O6-alkylguanine. Induction of mutations at the hypoxanthine-phosphoribosyltransferase locus in these cells lines suggests that, regardless of its relative yield, O6-methylguanine is the major mutagenic lesion for all alkylating agents.
Carcinogenesis 1991 Jan
PMID:The role of O6-alkylguanine in cell killing and mutagenesis in Chinese hamster ovary cells. 198 86

Tissue-specific formation and short-term persistence of alkylated DNA bases have been studied immunocytochemically in Syrian hamsters and rats killed 3-48 h after a single s.c. or oral dose of N-nitrosobis(2-oxopropyl)amine (BOP). Antisera specific for O6-(m)ethylguanine and for 7-(m)ethylguanine were used. Strong nuclear staining, indicative of a high level of DNA alkylation, was observed at all time points in the intra- and interlobular duct cells and in the centroacinar cells of the hamster pancreas, the main target organ of BOP-induced carcinogenesis. Acinar cells were weakly stained for up to 24 h. In the liver, nuclear staining was strong in all cell types, and more pronounced in the periportal than in the central venous area. Both O6-alkylguanine and 7-alkylguanine preferentially disappeared from the centrilobular area of the liver which is in agreement with the high O6-methyltransferase activity of liver and the unusually high levels of 7-methylguanine DNA glycosylase activity in hamster tissues. Strong staining was observed throughout the experiment in the tubular cells of the renal cortex and in bronchiolar Clara and alveolar type II cells of the lung. The staining intensity of the cells of the thyroid follicles and of the columnar epithelial cells of the colon was moderate. In the rat, nuclear staining was strong in the nasal cavity (Bowman glands), the epithelium lining the thyroid follicles, the lung, liver and in the fibroblasts of the ureter intima and adventitia. The epithelial cell nuclei of the colon and ureter were moderately stained. In the pancreas, staining was weak in acinar, duct and islet cells; no acinar staining remained at 48 h. In the liver, nuclear staining was strong all over the lobule. O6-Alkylguanine was preferentially removed from the centrilobular area. The renal tubular cells were only weakly stained. From the present study we can conclude that--with the exception of hamster kidney and rat liver--high levels of DNA alkylation and stability of the alkylated products were related to a high tumor incidence.
Carcinogenesis 1991 Apr
PMID:Cell specific DNA alkylation in target and non-target organs of N-nitrosobis(2-oxopropyl)amine-induced carcinogenesis in hamster and rat. 201 23

O6-Methylguanine-DNA-methyltransferase (OMMT) is a DNA repair protein that plays an important role in chemotherapy, mutagenesis and carcinogenesis. The sp. act. of OMMT in rat liver can be induced by approximately 12- to 20-fold by treatment of the rats with ionizing radiation. The effects of dose and time were investigated in this study. We have found that OMMT sp. act. can be increased, although to a lower extent, in kidney, spleen and brain in addition to liver. However, the sp. act. of OMMT in lung was reduced by irradiation. OMMT has been purified from the livers of irradiated rats by solubilization in high-salt-containing buffer, ammonium sulfate precipitation and a series of column chromatographic steps, including phenyl-Sepharose, heparin-agarose, double-stranded DNA-cellulose and FPLC. A 3000-fold enrichment of OMMT was achieved from the induced liver preparations. However, with regard to the sp. act. of this protein in normal rat liver, the fold purification was approximately 35,000. After methylation, OMMT during the course of its action exhibited a mol. wt of 28 kd under SDS-PAGE conditions.
Carcinogenesis 1990 Jul
PMID:Induction and purification of O6-methylguanine-DNA-methyltransferase from rat liver. 219 15

An O4-ethylthymine-specific antibody and immunoslot blot assay were used to identify an enzymatic activity in human tissue and cell extracts, specific for removing O4-ethylthymine from ethylated DNA in vitro. The assay allowed the quantitation of activity in the range of 2 fmol O4-ethylthymine removed per mg cell-free protein extract. The specific activities in human brain, kidney and liver tissue extracts ranged from 5.2 to 26.0 with mean levels of 12.8, 9.9 and 12.3 fmol/mg protein respectively. The activity in extracts prepared from cultured kidney and liver epithelial cells ranged from 8.4 to 16.0 fmol/mg protein, exhibiting mean levels of 10.3 and 13.5 fmol/mg protein respectively. The similar levels of repairing O4-ethylthymine in human liver, kidney and brain contrast the organ-specific activity of the O6-alkylguanine DNA-methyltransferase. The repair activity for O4-ethylthymine was not inhibited by preincubation of the extracts with either O4-methylthymine or O6-methylguanine, indicating that the loss of O4-ethylthymine was not mediated by an alkyltransferase. However, there was no detectable activity in extracts prepared from the GM006 mer- cell line lacking O6-alkylguanine DNA-methyltransferase. These data suggest that the activity for repairing O4-ethylthymine may be due to a protein distinct from the O6-alkylguanine DNA-methyltransferase. The low, but significant, level of repair activity specific for O4-ethylthymine identified in human tissue and cell extracts is consistent with the slow, but active, repair of this adduct in vivo.
Carcinogenesis 1990 Aug
PMID:A human DNA repair activity specific for O4-ethylthymine: identification and partial characterization. 238 29

The mutagenic and cytotoxic effects of N-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU) were compared in two isogenic Chinese hamster ovary (CHO) cell lines differing for the expression of the repair function for O6-methylguanine (O6-meGua), the O6-methyl-DNA-methyltransferase (MT). Survival and ouabain resistance (ouar) mutation frequency were similar in the two cell lines after treatment with ENU while both effects were strongly reduced in the MT-proficient (MT+) CHO cells after exposure to MNU. The slow repair kinetics of O6-ethylguanine (O6-etGua) when compared to O6-meGua, i.e. 25% versus 88% removal at 20 h after treatment, could still account for the similar mutational curves reported in the two cell lines after ENU treatment. The number of ENU-induced sister chromatid exchanges (SCE) was slightly reduced in the MT+ as compared to MT-deficient CHO cells suggesting a role for O6-etGua in SCE formation. Comparison of survival after exposure to ENU and MNU showed that, at similar levels of O6-alkylguanine on DNA, the ethyl- is more tolerated than the methyl-adduct. These data focus the attention on the importance of DNA damage processing in the cytotoxic response to alkylating agents.
Carcinogenesis 1989 Jul
PMID:O6-methyltransferase-deficient and -proficient CHO cells differ in their responses to ethyl- and methyl-nitrosourea-induced DNA alkylation. 273 22

Exposure to phenobarbital (PB) (0.05% in drinking water) markedly increased the rate of repair of O6-methylguanine (O6-MeG) from the hepatic DNA of rats given N-nitrosodimethylamine (2 mg/kg). No effect of comparable magnitude was seen for the repair of O4-methylthymine. During 21 weeks of exposure to PB the increased repair of O6-MeG exhibited a biphasic response and was maximal at approximately 3 weeks of treatment. Although this increased repair was readily observed when direct measurements were made of the loss of O6-MeG from hepatic DNA in vivo, no corresponding increased level of methyltransferase activity was detected in cell-free liver extracts, indicating that the methyltransferase protein was induced in a relatively limited population of cells. Immunohistochemical procedures have been used to demonstrate the formation of O6-MeG in, and its repair from, the DNA of hepatocytes in the centrilobular region of the liver lobule. Comparison with published data, for changes in the level of asialoglycoprotein receptors [Evarts et al. (1985) Carcinogenesis, 6, 1767-1773] and for the induction of cytochrome P450 [Schwartz et al. (1987) Carcinogenesis, 8, 1355-1357] in hepatocytes during PB administration, indicate that PB is acting at membrane sites in a relatively limited population of cells associated with the central vein. These observations show that the methyltransferase activity responsible for the repair of the major promutagenic base O6-MeG can be induced by a membrane active agent, without recourse to the genotoxic action of initiators and toxins, or the induction of restorative hyperplasia, previously employed for this purpose.
Carcinogenesis 1988 Nov
PMID:Phenobarbital: a non-genotoxic agent which induces the repair of O6-methylguanine from hepatic DNA. 318 Mar 41


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