Transfer of human genes conferring resistance to methylating mutagens, but not to UV irradiation and cross-linking agents, into Chinese hamster ovary cells

Overexpression of enzymes that repair endogenous damage to DNA

A significant contribution to human mutagenesis and carcinogenesis may come from DNA damage of endogenous, rather than exogenous, origin. Efficient repair mechanisms have evolved to cope with this. The main repair pathway involved in repair of endogenous damage is DNA base excision repair. In addition, an important contribution is given by O6-alkylguanine DNA alkyltranferase, that repairs specifically the miscoding base O6-alkylguanine. In recent years, several attempts have been carried out to enhance the efficiency of repair of endogenous damage by overexpressing in mammalian cells single enzymatic activities. In some cases (e.g. O6-alkylguanine DNA alkyltransferase or yeast AP endonuclease) this approach has been successful in improving cellular protection from endogenous and exogenous mutagens, while overexpression of other enzymatic activities (e.g. alkyl N-purine glycosylase or DNA polymerase beta) were detrimental and even produced a genome instability phenotype. The reasons for these different outcomes are analyzed and alternative enzymatic activities whose overexpression may improve the efficiency of repair of endogenous damage in human cells are proposed.

Transgenic systems in studies on genotoxicity of alkylating agents: critical lesions, thresholds and defense mechanisms

Transgenic systems, both cell lines and mice with gain or loss of function, are being used in order to modulate the expression of DNA repair proteins, thus allowing to assess their contribution to the defense against genotoxic mutagens and carcinogens. In this review, questions have been addressed concerning the use of transgenic systems in elucidating critical primary DNA lesions, their conversion into genotoxic endpoints, low-dose effects, and the relative contribution of individual cellular functions in defense. It has been shown that the repair protein alkyltransferase (MGMT) is decisive for protection against methylating and chloroethylating compounds. Protection pertains also to tumor formation, as revealed by the response of MGMT transgenic and knockout mice. Overexpression of genes involved in base excision repair (N-methylpurine-DNA glycosylase, apurinic endonuclease, DNA polymerase beta) is in most cases not beneficial in increasing the protection level, whereas their down-modulation or inactivation increases cellular sensitivity. This indicates that non-repaired base N-alkylation lesions and/or repair intermediates possess genotoxic potential. Modulation of mismatch repair and poly(ADP)ribosyl transferase has also been shown to affect the cellular response to alkylating agents. Furthermore, the role of Fos, Jun and p53 in cellular defense against alkylating mutagens is discussed.

The ras-related small GTP-binding protein RhoB is immediate-early inducible by DNA damaging treatments

The low molecular weight GTP-binding proteins RhoA, RhoB, and RhoC are characterized as specific substrates for the ADP-ribosyltransferase C3 from Clostridium botulinum and are supposed to be involved in the organization of the microfilamental network and transformation. rhoB is known to be immediate-early inducible by growth factors and protein-tyrosine kinases. Since increasing evidence indicates overlapping of growth factor- and UV-induced signal pathways, we studied the effect of UV light and other genotoxic agents on early rhoB transcription. Within 30 min after UV irradiation of NIH3T3 cells, the amount of rhoB mRNA increased 3-4-fold. Elevated rhoB mRNA was accompanied by an increase in RhoB protein, as detected by C3-mediated [32P]ADP-ribosylation. The transcription inhibitor actinomycin D prevented the UV-induced increase in rhoB mRNA and proved rhoB mRNA to be unstable with a half-life of approximately 20 min. Transcriptional activation of rhoB by UV light was confirmed by run-on analysis. The increase in rhoB mRNA after UV irradiation was prevented by inhibitors of protein kinase A (H9) and C (H7, Gö18). The tyrosine kinase inhibitor genistein did not affect UV induction of rhoB. In addition to UV, N-methyl-N-nitrosourea and the cytostatic drug cisplatin evoked rhoB response. Cycloheximide was likewise effective in increasing the amount of rhoB mRNA, whereas Bt2cAMP, 12-O-tetradecanoylphorbol-13-acetate, and retinoic acid were without effect. Prior down-regulation of signaling by 12-O-tetradecanoylphorbol-13-acetate and serum pretreatment reduced UV-stimulated rhoB expression. The data indicate that rhoB represents a novel DNA damage-inducible function involved in early steps of signal transduction upon genotoxic stress.

DNA damage tolerance, mismatch repair and genome instability

DNA mismatch repair is an important pathway of mutation avoidance. It also contributes to the cytotoxic effects of some kinds of DNA damage, and cells defective in mismatch repair are resistant, or tolerant, to the presence of some normally cytotoxic base analogues in their DNA. The absence of a particular mismatch binding function from some mammalian cells confers resistance to the base analogues O6-methylguanine and 6-thioguanine in DNA. Cells also acquire a spontaneous mutator phenotype as a consequence of this defect. Impaired mismatch binding can cause an instability in DNA microsatellite regions that comprise repeated dinucleotides. Microsatellite DNA instability is common in familial and sporadic colon carcinomas as well as in a number of other tumours. Several independent lines of investigation have identified defects in mismatch repair proteins that are causally related to these cancers.

Stress factors affecting expression of O6-methylguanine-DNA methyltransferase mRNA in rat hepatoma cells

O6-Methylguanine-DNA methyltransferase (MGMT) is decisively involved in protecting mammalian cells against genotoxic effects of alkylating carcinogens. We analysed regulation of MGMT expression after exposing rat hepatoma H4IIE cells to various 'stress' factors. Treatments that damage DNA such as alkylation, hydrogen peroxide, ultraviolet or X-ray exposure, as well as restriction enzymes introduced into cells by electroporation or arrest of replication by hydroxyurea significantly induced MGMT mRNA (2.5 to 5-fold). Slight induction (up to 2.5-fold) was observed after heat shock or cadmium/zinc treatment. No or only a very weak induction (less than 1.5-fold) was observed after treatment with 6-thioguanine, 5-azacytidine, transfection of methylated DNA, depletion of MGMT by feeding with O6-methylguanine or O6-benzylguanine, serum starvation and feeding of starved cells, cAMP, TPA and dexamethasone treatment. Inhibitors of protein kinases, H8 and H9, induced MGMT mRNA. On the other hand, an inhibitor of phosphatases (sodium vanadate) prevented induction of MGMT by N-methyl-N'-nitro-N-nitrosoguanidine. The data indicate that DNA breaks are an ultimate signal for MGMT mRNA induction and that protein phosphorylation is involved in regulating MGMT expression.

Caffeine-induced reduction of the survival of gamma-irradiated HeLa cells and the reversal of the caffeine effect by Escherichia coli RecA protein

It is confirmed that survival of gamma-irradiated HeLa cells is decreased by post-treatment with caffeine. The caffeine effect is believed to be the result of an inhibition of the repair of gamma-ray-induced DNA damage. In this work we show that the caffeine-induced reduction of the survival of gamma-irradiated HeLa cells is reversed when Escherichia coli RecA protein is introduced into the cells with the aid of liposomes.

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.

DNA repair and chromosomal stability in the alkylating agent-hypersensitive Chinese hamster cell line 27-1

27-1 is a mutant of Chinese hamster ovary cells (CHO cells) that is hypersensitive to the toxic effects of ultraviolet light, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and other monofunctional alkylating agents. We show here that the enhanced MNNG sensitivity of these cells is not due to alterations in the amount of DNA methylation products introduced nor by a defect in the first step of removal of the main alkylation products 7-methylguanine and 3-methyladenine. However, these mutant cells perform more DNA repair synthesis after treatment with MNNG than normal CHO-9 cells. This observation might indicate a possible defect of a ligase involved in sealing DNA repair patches. 27-1 cells did not show elevated frequencies of sister-chromatid exchange and chromosomal aberration induced by MNNG. The data show that MNNG-induced cell killing is not necessarily related to increased chromosomal instability.

Cytogenetical characterisation of Chinese hamster 43-3B transferants with the amplified or non-amplified human DNA repair gene ERCC-1

A comparative study on the biological responses to different mutagens (UV, 4NQO, MMC, MMS and EMS) was made on CHO wild-type cells (CHO-9), its UV-hypersensitive mutant 43-3B, and 2 types of its transferants, i.e., one containing a few copies of the human repair gene ERCC-1 and the other having more than 100 copies of ERCC-1 (due to gene amplification). Cell survival, chromosomal aberrations and SCEs were used as biological end-points. The spontaneous frequency of chromosomal aberrations in the transferants was less than found in 43-3B mutant cells, but still 2-3 times higher than in wild-type CHO cells. The spontaneous frequency of SCEs in the transferants was less than in 43-3B and similar to that of wild-type cells. The induction of SCEs by all tested agents in transferants was similar to that found in CHO-9 cells, while the mutant is known to respond with higher frequencies. ERCC-1 also bestowed resistance to MMS and EMS on the mutant to induction of chromosomal aberrations and cell killing to levels comparable with those of the wild-type strain. On the other hand ERCC-1 could not completely regain the repair proficiency against cell killing and induction of chromosomal aberrations by UV or MMC to the wild-type level. These results suggest that the ERCC-1 corrects the repair defect in CHO mutant cells, but it is unable to rectify fully the defect; probable reasons for this are discussed. However, amplified transferants (having more than 100 copies of the ERCC-1 gene) restored the impaired repair function in 43-3B to UV-, MMC- or 4NQO-induced DNA damage better than non-amplified transferants with a few copies of the ERCC-1. This difference may be due to the high amount of gene product involved in the excision repair process in the amplified cells.

A promising genomic transfectant into Xeroderma pigmentosum group A with highly amplified mouse DNA and intermediate UV resistance turns revertant

Following transfection of genomic mouse DNA into an SV40 transformed fibroblast cell line from a patient with Xeroderma pigmentosum (complementation group A, XPA), a single UV resistant cell clone was isolated out of a total of 10(4) independent transfectants. The recipient XPA cell line has as yet not produced spontaneous revertants among 2.2 x 10(8) cells. The isolated cell clone contains 50-70 kb of mouse sequences which are heavily amplified (500-fold), and has acquired both intermediate resistance to UV killing and intermediate unscheduled DNA synthesis (UDS) capacity. By continued passage without selective pressure, cells were generated, which had lost both the dominant marker gene and repetitive mouse sequences. Single colonies of these cells were still intermediately resistant to UV suggesting that either undetected unique mouse DNA had segregated from the bulk of repetitive DNA, or, more likely, that the initially isolated transfectant was a spontaneous revertant. This documents that a persuasive clone isolated can still be a false positive (spontaneous revertant) and that an extremely laborious approach may lead into a dead end.

Ataxia telangiectasia resists gene cloning: an account of parameters determining gene transfer into human recipient cells

A subclone of an SV40-transformed fibroblast cell line from a patient with Ataxia telangiectasia (AT) with a relatively high rate of DNA uptake was isolated. However, more than 65000 independent genomic transfectants (using wild-type human DNA) did not contain the functional AT gene. This number represents the statistical distribution of an amount of DNA equivalent to more than three times the haploid human genome. The transfectants were screened by an X ray selection protocol that could rescue a single wild-type cell out of a population of 10(6) AT cells. This suggests a reversion frequency for AT of below 10(-8). The DNA uptake into human cells is compared with that into NIH3T3 cells and future possibilities for the isolation of human repair genes are discussed.