Comparative analysis of O6-methylguanine methyltransferase activity and cellular sensitivity to alkylating agents in cell strains derived from a variety of animal species

Quantification of O6-methylguanine-DNA methyltransferase mRNA in human brain tumors

O6-Methylguanine-DNA methyltransferase (MGMT) is strongly involved in drug resistance mechanism of tumor cells to chloroethylnitrosoureas (CENUs), because it removes and repairs CENU-induced O6-alkylguanine-DNA by accepting the alkyl group at a cysteine moiety. MGMT activity and MGMT mRNA expression are good indicators for detection of sensitive cells or resistant cells to CENUs. In the present study, we applied a non-radioactive reverse transcription-polymerase chain reaction (RT-PCR) method on quantitative measurement of MGMT mRNA expression. Estimated levels of MGMT mRNA expression determined by this RT-PCR method were consistent with the actual doses of MGMT mRNA. This relationship was noted at a wide range from 10 fg to 10 pg. The relative expression levels of MGMT mRNA estimated from kinetic analysis correlated well with MGMT activity determined using 3H-methyl-nitrosourea-treated DNA substrate in brain tumor cells (P<0.001 with a correlation coefficient of 0.997). The RT-PCR method facilitated quantitative measurements in even a small amount of biopsy specimens obtained by stereotactic brain surgery.

Differential inactivation of O6-methylguanine-DNA methyltransferase activity by O6-arylmethylguanines

Activity of O6-methylguanine-DNA methyltransferase (MGMT) is well related with drug resistance of tumor cells to chloroethylnitrosoureas (CENUs), because MGMT removes CENU-induced O6-alkylguanines in DNA by accepting the alkyl group at a cysteine moiety. Inactivation of MGMT is a feasible way to overcome MGMT-related resistance of tumor cells to CENUs. O6-Benzylguanine is known to be a strong depleter of MGMT. We previously reported the potentiation effect of O6-arylmethylguanine derivatives on cytotoxicity of 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU), a CENU, for HeLa S3 cells. In this study, we tested the activity of these O6-arylmethylguanine derivatives as depleters of MGMT using HeLa S3 cell-free extract. The O6-arylmethylguanine derivatives tested were O6-benzylguanine (1), O6-(4-, 3-, and 2-fluorobenzyl)guanines (2-4), O6-(4, 3-, and 2-trifluoromethylbenzyl)guanines (5-7), O6-(4-, 3-, and 2-pyridylmethyl)guanines (8-10), and O6-(2- and 1-naphtylmethyl)guanines (11,12). Among these, compounds 1-3, 5, 8, 9 and 11 showed a strong MGMT depletion activity, whereas compounds 4, 6, 7, 10 and 12 were inactive. These inactive compounds, except for 6, have a substituent at the alpha position of the benzyl group (4, 7, 12) or are an alpha nitrogen analogue of 1 (10). There was a good relation (r = -0.856, p < 0.001) between the MGMT depletion activity of O6-arylmethylguanine derivatives and their potentiation activity of ACNU cytotoxicity. These results suggest that the alpha position of the benzyl group plays an important role in the interaction of O6-arylmethylguanine derivatives with MGMT to result in the inactivation of MGMT. Potent MGMT inactivators (1-3, 5, 8, 9, 11) sensitize tumor cells to CENU chemotherapy.

Cloning of a marsupial DNA photolyase gene and the lack of related nucleotide sequences in placental mammals

Photoreactivating enzyme, DNA photolyase, reduces lethal, mutagenic and carcinogenic effects of ultraviolet light (UV) by catalyzing near UV or visible light-dependent repair of cyclobutane pyrimidine dimers (CPDs) in DNA. The enzyme activity has been detected in a wide variety of organisms ranging from bacteria to nonplacental mammals. However, the evidence for photoreactivation in placental mammals, including humans, is controversial. As a first step to identify the presence and activity of the gene in mammalian species, we isolated a cDNA clone of this gene from a marsupial, the South American opossum Monodelphis domestica. Photolyase activity was expressed in Escherichia coli from the cDNA which is predicted to encode a polypeptide of 470 amino acid residues. The deduced amino acid sequence of this protein is strikingly similar to those of photolyases from two metazoans; the opossum photolyase shares 59% and 63% sequence identity with the Drosophila melanogaster and goldfish Carassius auratus enzymes, respectively. However, no closely related nucleotide sequence was detected in higher mammals and a homologous transcript was undetectable in a number of human tissues. These results strongly suggest that humans, as well as other placental mammals, lack the photolyase gene.

Enhancing effect of O6-alkylguanine derivatives on chloroethylnitrosourea cytotoxicity toward tumor cells

O6-Alkylguanine derivatives are well known as chemical modulators of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT). Depletion of the enzyme by these derivatives leads to increase sensitivity of tumor cells to chloroethylnitrosoureas. We tested the effect of O6-methylguanine, O6-benzylguanine, O6-(p-methylbenzyl)guanine, O6-(p-chlorobenzyl)guanine, O6-(p-methoxybenzyl)guanine, O6-methylhypoxanthine and O6-benzylhypoxanthine on the sensitivity of tumor cell lines to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3- nitrosourea hydrochloride (ACNU) using a colorimetric cytotoxicity assay. The sensitivity of MGMT-proficient tumor cells including HeLA S3, C6-1, C6-2/ACNU, U-138 MG and U-373 MG cells was greatly enhanced by 2 hr pretreatment of 10-100 microM O6-benzylguanine, O6-(p-methylbenzyl)guanine and O6-(p-chlorobenzyl)guanine, but not by O6-methylguanine or O6-methylhypoxanthine. O6-(p-methylbenzyl)guanine moderately sensitized the 2 cell lines, HeLa S3 and C6-1, tested in our study to ACNU cytotoxicity. O6-Benzylhypoxanthine at the high concentration (100 microM) sensitized, to some extent, 3 MGMT-proficient cell lines. Lesser degrees of enhancement by the O6-benzylguanine derivatives were noted in MGMT-deficient tumor cells. Biological effects of O6-alkylguanine derivatives on enhancing ACNU cytotoxicity of tumor cells suggest that the exocyclic 2-amino and O6-benzyl groups in O6-benzylguanine skeleton are both essential for the inhibition of MGMT activity.

Influence of O6-methylguanine-DNA methyltransferase activity on chloroethylnitrosourea chemotherapy in brain tumors

Chloroethylnitrosoureas (CENUs) alkylate DNA at specific sites and inhibit DNA replication in tumor cells. O6-Alkylguanine moieties resulting from alkylation of guanine bases are thought to be one of most lethal adducts in living cells. Effectiveness of CENUs is known to relate well with an enzymic activity of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT), which recognizes and removes O6-alkylguanine. To improve therapeutic results of CENUs, we have measured MGMT activity of human brain tumors and studied the relationship between MGMT activity and clinical responsiveness to I-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU). Thirty-seven patients with brain tumors were entered into the study. The neoplasms included gliomas, non-glial tumors, and brain metastases. The MGMT activity of gliomas was significantly lower than that of non-glial tumors and brain metastases. No significant difference in the enzyme activity was noted between low- and high-grade gliomas. Out of the 22 gliomas 5 tumors indicated a value below 60 fmol/mg, suggestive of a methyl excision repair minus (Mer-) tumor. Two out of 3 evaluable patients with a Mer- tumor responded well to post-operative ACNU adjuvant chemotherapy. Our results suggest that brain tumors include a certain percentage of Mer- phenotype tumors, and that CENUs such as ACNU should be applied selectively on tumors with a low MGMT activity in order to increase the therapeutic effectiveness.

Interrelationship between O6-alkylguanine-DNA alkyltransferase activity and susceptibility to chloroethylnitrosoureas in several glioma cell lines

In order to study the dynamic relationship in glioma cells between O6-alkylguanine-DNA alkyltransferase (AGT) activity and resistance to the cytotoxic effect of chloroethylnitrosoureas (CENUs), we investigated the changes in sensitivity to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) after modulation of AGT activity. In ACNU-resistant rat glioma cell lines (9LR1, 9LR3, and 9LR12) and a human glioma cell (HNG-1), O6-methylguanine enhanced cytotoxicity to ACNU following a depletion of AGT activity. But no enhancement of cytotoxicity was seen in an ACNU-sensitive rat glioma cell line (9L). In the 9L and 9LR12 cells, equivalently sublethal doses of ACNU similarly depleted AGT activity but the regeneration rates of this repair protein were different. In the case of a 7-day pretreatment with human recombinant interferon-beta (HuIFN-beta), although it could modulate AGT activity in HNG-1 cells, no definite influence on cellular sensitivity to CENUs was observed. However, a 50-day pretreatment with HuIFN-beta conferred resistance to CENUs on them despite its effect to reduce AGT activity. Thus, diversity was seen in the relation between AGT activity and resistance to CENUs when AGT activity was modulated by HuIFN-beta. The results of this study suggest that AGT activity is one of factors affecting cellular sensitivity to CENUs but that alternative mechanisms of tolerance may be induced depending upon some environmental effects.

Properties of mer- HeLa cells sensitive or resistant to the cytotoxic effects of MNU; effects on DNA synthesis and of post treatment with caffeine

A line of HeLa cells was shown to be particularly sensitive to N-methyl-N-nitrosurea (MNU) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), but not to a variety of other cytotoxic agents. A resistant line (designated HeLa/A22), was derived by treating HeLa cells repeatedly with MNU. Both the sensitive (HeLa) and resistant (HeLa/A22) cells have a mer- phenotype based both on their reduced rates of loss of O6-methylguanine (O6-MeG) from DNA and their low levels of the enzyme O6-methylguanine methyltransferase (MT). HeLa cells are therefore sensitive to unrepaired O6-MeG in DNA while the HeLa/A22 cells are resistant to unexcised O6-MeG and thus the A22 cells have the mer-rem+ phenotype. MNU produced an immediate dose-dependent inhibition of DNA synthesis in cultures of both sensitive and resistant cells which increased with time until about 4 h after treatment. DNA synthesis then recovered to near control rates in both sensitive and resistant cells before then exhibiting a progressive decrease after about 24 h. DNA synthesis was more depressed at these late times after treatment in cultures of sensitive cells than in those of similarly-treated resistant cells. DNA synthesis remained depressed in sensitive cells but recovered 3 days after treatment in resistant cells. Post treatment incubation of MNU-treated HeLa cells with caffeine did not increase the toxic action of MNU. In contrast, post treatment of the resistant HeLa/A22 cells with caffeine resulted in a dramatic increase in the toxic effects of a higher equitoxic dose of MNU. The depressed rate of DNA synthesis observed in both cell lines after high doses of MNU was partially reversed by post treatment with caffeine in both sensitive and resistant cells. These observations can be interpreted in terms of the effects of caffeine on DNA replication in treated cells.

Immunocytochemical analysis of in vivo DNA modification

In the past decades a large number of DNA adducts induced in the intact animal by alkylating agents have been identified. The formation and repair of these adducts are important determinants, not only of mutagenesis, tumor initiation and DNA-mediated toxicity but probably also of tumor progression. Most studies on in vivo DNA modification have been performed on isolated bulk DNA. More recently, methods have been developed to study the distribution of DNA adducts at the level of either the individual gene or the individual cell. This paper reviews immunocytochemical methods to study the formation and repair of DNA adducts and other DNA modifications at the level of the individual cell. DNA modifications induced by alkylating agents and a variety of other agents including ultraviolet radiation, aromatic amines, polycyclic aromatic hydrocarbons and platinum anti-cancer drugs will be discussed. Up to now, immunocytochemical analysis of in vivo modified DNA has largely concentrated on experimental animals. These studies have revealed striking heterogeneities with regard to formation and/or repair of DNA adducts in tissues from rat, hamster and mouse. Immunocytochemical adduct analysis can be used to identify in a convenient, fast and detailed way cell types, cell stages and sites in which biological effects of the adducts might be expressed. More recently, immunocytochemical analysis of DNA adducts also proved to be feasible on in situ exposed human samples. A number of existing and potential applications in the field of chemical carcinogenesis, experimental chemotherapy and molecular epidemiology are discussed.

Linkage between O6-methylguanine-DNA methyltransferase (O6-MT) activity and cellular resistance to antitumour nitrosoureas in cultured rat brain tumour cell strains

We have examined O6-methylguanine-DNA methyltransferase (O6-MT) activity of rat brain tumour cell strains with reference to cellular resistance to antitumour nitrosoureas, 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (nimustine, ACNU) and methyl-6-[3-(2-chloroethyl)-3-nitrosoureido]-6-deoxy-alpha-D-glucopyrano side (ramustine, MCNU). The values of O6-MT activity were 52 and 160 fmol/mg protein extract in 9L and C6 rat brain tumour cells, respectively; while HeLa S3 cells, as a methyl excision repair positive (Mer+) cell strain, revealed a rather high value of 488 fmol/mg. 9L cells indicative of a low O6-MT activity showed 13 microM for ACNU and 18 microM for MCNU at a 10% survival dose (SD10), determined by a clonogenic cell assay as an index of cellular resistance. In contrast to this, C6 cells revealed a SD10 value of 67 microM and 36 microM for ACNU and MCNU, respectively, indicating higher resistance than 9L cells. HeLa S3 cells showed the highest SD10 value as follows: 84 microM for ACNU and 73 microM for MCNU. The relationship between the O6-MT activity and the cellular resistance was almost linear, with relatively resistant cell lines exhibiting the higher levels of the O6-MT activity. This correlation between the O6-MT activity and the cellular resistance to nitrosoureas as ACNU and MCNU was not observed among other antitumour drugs, which included bleomycin (BLM), neocarzinostatin (NCS), cis-diamminedichloroplatinum (II) (CDDP), and etoposide (VP-16) in clinical use for brain tumour chemotherapy. This indicates that O6-MT activity can be an indicator of cellular resistance to antitumour nitrosoureas in the chemotherapy of brain tumours.

Sister-chromatid exchanges (SCEs), cell survival and mutation in HeLa s3 cells with different sensitivity to alkylating agents; evidence that SCE induction and cell survival or mutation induction are dissociable

We previously isolated N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-resistant cells, MR from HeLa S3 Mer- cells. In the present study, we have isolated 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU)-resistant cells, ACr. The MR cells had only a little O6-methylguanine-DNA methyltransferase (MT) activity, while the ACr cells had increased MT activity and also became resistant to the cytotoxic effect of MNNG. We compared the induction of sister-chromatid exchanges (SCEs), cell survival and mutation in these HeLa S3 cells with different sensitivity to MNNG. The ACr cells were much more resistant than the parental HeLa S3 Mer- cells to cytotoxicity, mutagenicity and SCE induction by MNNG, showing a positive correlation between SCE induction and cell killing or mutation. In contrast, this positive relationship was not observed between HeLa S3 Mer- and MR cells. These results suggest that O6-methylguanine (O6-MeG) is involved in the induction of the biological effects of MNNG such as cytotoxicity, mutagenicity and SCEs, and also indicate that SCE induction does not always correlate with cell killing and mutation.

Chromosomal sensitivity of lymphocytes from individuals with therapy-related acute nonlymphocytic leukemia

A small fraction of those individuals exposed to cytotoxic chemotherapy or radiation for the treatment of a primary malignant disease will develop a second malignancy some time later. Although exposure to the cytotoxic agents is believed to be the causative factor, the reason only certain individuals develop the second malignancy is unknown. Some studies have suggested that these individuals might be predisposed to cancer because of an inherent sensitivity to the alkylating agents used in cancer therapy. We have reported that these individuals with therapy-related acute nonlymphocytic leukemia (t-ANLL) have reduced endogenous levels of the repair protein O6-alkylguanine alkyltransferase (AGT). To further investigate the etiology of this disease, alkylation-induced sister-chromatid exchange (SCE) formation in individuals who developed second malignancies, was compared to other patient groups and normal controls. Peripheral blood lymphocytes from patients with (1) t-ANLL, (2) primary forms of acute nonlymphocytic leukemia (ANLL de novo), (3) patients with primary malignancies at risk of developing secondary disease, and (4) unexposed, healthy controls were treated in vitro with N-methyl-N'-nitro-nitrosoguanidine or mitomycin C. Baseline and mutagen-induced frequencies of SCEs were determined. These studies failed to detect any increased sensitivity in those patients who developed second malignancies as compared to controls or patients with de novo forms of the same disease. Also, no correlation between sensitivity to the alkylating agent N-methyl-N'-nitro-nitrosoguanidine and endogenous levels of the AGT repair protein was found. These results suggest that t-ANLL patients are not sensitive to SCE induction by either MNNG or MMC.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of the c-Ha-ras-1 proto-oncogene by methylation in vitro with alpha-acetoxy-N-nitrosodimethylamine

alpha-Acetoxy-N-nitrosodimethylamine, an activated derivative of the carcinogen N-nitrosodimethylamine, methylated in vitro a plasmid containing the human c-Ha-ras-1 proto-oncogene, resulting in the generation of a transforming oncogene, assayed by transfection into NIH 3T3 cells. The resulting transformed cells were tumorigenic and metastatic in immune-deprived mice. Further transfection using tumor DNA led to the formation of three secondary NIH 3T3 transformants. DNA from these secondary transformants contained human ras gene sequences. Two of the three secondary transformants contained G----A mutations at guanine 35 in codon 12, and the third secondary transformant retained the wild-type sequence at codons 12, and 61. For the latter, the activating mutation was not determined. These results demonstrate that a simple methylating agent can activate a normal human ras proto-oncogene to a transforming oncogene.