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)

The possibility that carcinogens may affect methylase-mediated methylation of replicating DNA was investigated. A system eminently suitable for this purpose is liver regenerating after partial hepatectomy, as one injection of dimethylnitrosamine (DMN) given during the ensuing period of increased DNA synthesis induces hepatocellular carcinoma. Methylation of DNA by DNA methylase normally occurs only in proportion to DNA synthesis. Therefore simultaneous measurements were made of synthesis (incorporation of [14C]adenine into DNA adenine, or of d[5-3H]cytidine into DNA cytosine), and of methylation (incorporation of [methyl-3H]methionine into 5-methylcytosine of DNA) in liver regenerating after partial hepatectomy. After treatment with DMN, the ratio of methylation: synthesis remained within the normal range. Methyl methanesulphonate (MMS), a compound which damages DNA in regenerating liver in a similar but not identical way to DMN and which does not induce tumors in liver even when given after partial hepatectomy, caused an increase in methylation in relation to synthesis. These experiments therefore do not support the view that altered DNA methylase activity is involved in carcinogenesis.
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PMID:Effect of a single treatment with the alkylating carcinogens dimethylnitrosamine and methyl methanesulphonate on liver regenerating after partial hepatectomy. IV. Effect on methylase-mediated methylation of DNA. 47 54

O6-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein which plays an important role in chemotherapy, mutagenesis, and carcinogenesis. The specific activity of MGMT in female rat liver can be induced by approximately 20-fold by treatment of the rats with gamma-irradiation. Maximum response occurred 48 h after 15 Gy irradiation. MGMT levels in male rats were induced by only 3-fold. MGMT activity was also induced by irradiation of rat hepatoma H4IIE cells with a 3-fold increase noted after treatment with 3 Gy. Northern analysis and nuclear run-on assays indicated that the induction of MGMT was regulated at the transcriptional level. The radiation-mediated increase in MGMT was blocked by H7, a protein kinase inhibitor, but not by H89, an inhibitor of protein kinase A. Hydroxyl radicals may play a role in the induction mechanism since dimethyl sulfoxide, a radical scavenger, blocked the radiation-mediated increase in MGMT. MGMT activity was also increased by treatment of the cells with H2O2, in accordance with the involvement of activated oxygen species in the induction of MGMT. Finally, the addition of cycloheximide, an inhibitor of protein synthesis, prior to but not after irradiation, abolished the increase in MGMT activity.
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PMID:Irradiation-induced expression of O6-methylguanine-DNA methyltransferase in mammalian cells. 137 30

DNA repair capacity is likely to be a critical factor in mutagenesis and carcinogenesis, as well as for the response to some cytostatics. We have studied inter- and intra-individual variation in the activities of O6-methylguanine--DNA methyltransferase (O6-MT) and uracil--DNA glycosylase (UDG) in 35 placentae from smokers and non-smokers. The maximum interindividual variation in the activities of O6-MT and UDG were 8.3- and 7.7-fold, respectively. The corresponding intraindividual variations were 2.7- and 3.3-fold. Generally, a high level of O6-MT activity was accompanied by a high O6-MT mRNA level, but no such correlation was seen for UDG. These results were not due to degradation of the enzymes or mRNAs after delivery. No correlation between the activities of O6-MT and UDG was observed, indicating that they are differentially regulated. A 1.4-fold (P < or = 0.05) higher activity of O6-MT was observed in smokers as compared to non-smokers, indicating a small, but statistically significant difference. No significant difference was observed for UDG. Our results demonstrate that DNA repair capacities vary largely between different individuals, and that environmental factors may modulate the expression of DNA repair enzymes.
Carcinogenesis 1992 Oct
PMID:Expression of O6-methylguanine--DNA methyltransferase and uracil--DNA glycosylase in human placentae from smokers and non-smokers. 142 36

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

O6-Methylguanine-DNA methyltransferase (MGMT) is responsible for removal of O6-alkylguanine from DNA induced by alkylating mutagens/carcinogens. To analyze the involvement of O6-alkylguanine in the generation and MGMT in avoidance of various genotoxic effects of alkylating agents, we transfected Chinese hamster ovary (CHO) cells that lack MGMT activity with human MGMT cDNA cloned into a mammalian expression vector (pSV2MGMT). A high proportion (60-80%) of transfectants selected for a cotransfected neo gene survived treatment with high doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-hydroxyethyl-N-chloroethylnitrosourea (HeCNU). Parallel transfections with an expression vector containing the bacterial ada gene (pSV2ada) showed the human MGMT to be more effective than the ada expression vector in mediating alkylation resistance. Various clonal CHO cell lines have been established stably transfected with the human MGMT cDNA. The transfectants expressed human MGMT at levels ranging from 8600 to 210,000 molecules per cell. The high MGMT expressors became strongly resistant to the killing effects of MNNG, HeCNU, N-methyl-N-nitrosourea (MNU) and, to a significant lesser degree, methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS). No killing resistance was observed to N-ethyl-N-nitrosourea (ENU), though the MGMT and ada transfectants showed reduction in mutation frequency induced by this agent. Protection from mutation induction by MGMT (and ada) expression was also demonstrated for MNNG. The transfectants were also protected from the sister chromatid exchange (SCE) inducing and, to a lesser degree, clastogenic effect of MNNG and MNU, and slightly to EMS and MMS. Again no protection was observed towards ENU. Correlations between MGMT activity and resistance to a given end point suggest that, for MNNG, O6-methylguanine is the preponderant toxic, mutagenic and SCE inducing lesion. About 90% of MNNG (and MNU) induced SCEs and nearly all of the MNNG-induced gene mutations seem to be due to this adduct. For alkylation-induced chromosomal aberrations, however, and for cell killing and SCEs induced by MMS, EMS and ENU, other lesions than O6-alkylguanine appear to be of major importance. The data strongly support the view that O6-methylguanine is a genotoxic lesion and MGMT a function decisively involved in avoidance of genotoxic effects in cells exposed to MNNG and related compounds. They indicate also that it is important to take into account the property and mode of action of any given alkylating agent in assessing the protective role of MGMT against alkylation-induced genotoxicity.
Carcinogenesis 1991 Oct
PMID:Transfection and expression of human O6-methylguanine-DNA methyltransferase (MGMT) cDNA in Chinese hamster cells: the role of MGMT in protection against the genotoxic effects of alkylating agents. 165 27

O6-Methylguanine-DNA methyltransferase (O6-MT) has been described as a DNA repair enzyme that reverses alkylation damage at the O6 position of guanine in DNA. We demonstrate that the concentration of this protein decreases immediately prior to DNA synthesis in cultured chick hepatocytes. If intracellular levels are experimentally depleted by treatment of cultures with O6-methylguanine, DNA synthesis occurs as an associated resultant. This effect is dose dependent and can be followed by discernible morphological changes of organoids in culture. Increased and altered growth caused by O6-methylguanine was quantified and was also found to be dose dependent. Therefore, O6-MT may play a role in the regulation of DNA synthesis.
Carcinogenesis 1992 Jan
PMID:Relationship between the depletion of O6-methylguanine-DNA methyltransferase by O6-methylguanine and the stimulation of DNA synthesis and growth of cultured chick hepatocytes. 173 70

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

O6-Methylguanine-DNA methyltransferase (O6-MT) is a DNA repair protein that reverses alkylation damage at the O6 position of guanine. In the process, O6-MT undergoes suicide inactivation. To determine if this enzyme might be regulated by pregnancy-associated hormones we measured changes in the level of O6-MT in isolated mouse mammary epithelial cell homogenates during different reproductive states. These were pregnancy, ectopic pituitary transplantation, proestrus/estrus and diestrus. O6-MT levels were found to be similar in mice in proestrus/estrus (0.95 fmol/micrograms DNA) as compared to diestrus (0.94 fmol/micrograms DNA) and also mixed populations of virgin mice (1.09 fmol/micrograms DNA). A mean for all virgin mice (0.97 fmol/micrograms DNA) was used as a comparative index. O6-MT decreased 2-fold during pregnancy in mammary epithelial cells to a mean value of 0.45 fmol/micrograms DNA (P less than 0.05). A smaller decrease (0.65 fmol/micrograms DNA; P less than 0.01) in mammary epithelial cells was found at 3 weeks following pituitary isograft. The repair capacity of mammary epithelial cells to liver was compared by measurements made in liver homogenates from the same mice and are approximately 3-fold higher in liver from virgin mice (3.2 fmol/micrograms DNA) than mammary gland. Liver levels of O6-MT increased in pregnant (5.3 fmol/micrograms DNA) and pituitary transplanted (3.9 fmol/micrograms DNA) mice, and were 5- and 4-fold higher than the concentration in virgin mammary epithelial cells respectively.
Carcinogenesis 1991 Oct
PMID:Cellular levels of O6-methylguanine-DNA methyltransferase in mammary epithelial cells and liver from virgin, pregnant and pituitary grafted mice. 193 59

To assess the biological role of DNA methylation at the O6 position of guanine (O6MeG) a human cell line was created that contains a regulatable gene of the O6MeG-DNA methyltransferase (MT), a repair activity that removes O6MeG adducts from the DNA. MT-deficient HeLa MR cells were transformed with an SV40-based expression vector in which the bacterial MT gene ada was put under the control of a glucocorticoid-inducible MMTV promoter. In response to dexamethasone (Dex), pSV MTV ada cells actively accumulated MT protein to reach a constant level after 10-12 h of approximately 15,000 MT molecules per cell. Co-induction by Dex and 12-O-tetradecanoylphorbol-13-acetate (TPA) further accelerated this synthesis approximately 2-fold and, as a result, higher final MT levels were achieved. The inducers were added to exponentially growing cells either before or at the time of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) exposure and the kinetics of MT synthesis was studied. MNNG affected in a dose-dependent manner (i) the loss of the pre-existing MT activity; (ii) the lag before newly synthesized MT appeared; (iii) the final level of MT accumulated by the cells; and (iv) to a lesser extent the rate of MT synthesis. In cells with a down-regulated MT gene (no inducer) even small MNNG doses lead to an irreversible loss of the pre-existing MT activity, i.e. to incomplete repair, whereas an up-regulated MT gene supported the restoration of a pool of active MT molecules in the cells, i.e. an O6MeG repair that has gone to completion. Hence, effective O6MeG repair relies not only on the pre-existing MT level, but depends to an even greater extent on the state of expression of the MT gene. The activity of the MT gene also correlated with cell survival, which confirms our earlier finding that O6MeG adducts are cytotoxic for the cell.
Carcinogenesis 1990 Jan
PMID:Effect of O6-methylguanine-DNA methyltransferase gene activity on repair in human cells transformed by a regulatable ada gene. 229 24

In a previous communication, we proposed that N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced cytotoxicity in an O6-methylguanine (O6-MeG)-DNA methyltransferase-deficient (MT-) HeLa cell mutant was mainly the consequence of DNA strand breaks resulting from the failure to remove O6-MeG lesions. If MNNG-induced cytotoxicity, DNA strand breaks and O6-MeG lesions are related, there should be a corresponding relationship of these properties in MNNG-resistant clones derived from the MT- strain. A study of such revertants indicated that they were a heterogeneous group with increased repair of DNA strand breaks and O6-MeG lesions and increased resistance to the cytotoxic effects of MNNG. These observations support the hypothesis relating O6-MeG, DNA strand breaks and cytotoxicity. The relationship of these 'revertants' to the MT- and wild-type strains is discussed.
Carcinogenesis 1990 May
PMID:O6-methylguanine (O6-MeG) and cytotoxicity: reversion analysis involving an N-methyl-N'-nitro-N-nitrosoguanidine-sensitive, O6-MeG-DNA methyltransferase-deficient HeLa cell mutant. 233 6


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