Enhancement of postreplication repair in Chinese hamster cells

Filling gaps in translesion DNA synthesis in human cells

During DNA replication, forks may encounter unrepaired lesions that hamper DNA synthesis. Cells have universal strategies to promote damage bypass allowing cells to survive. DNA damage tolerance can be performed upon template switch or by specialized DNA polymerases, known as translesion (TLS) polymerases. Human cells count on more than eleven TLS polymerases and this work reviews the functions of some of these enzymes: Rev1, Pol η, Pol ι, Pol κ, Pol θ and Pol ζ. The mechanisms of damage bypass vary according to the lesion, as well as to the TLS polymerases available, and may occur directly at the fork during replication. Alternatively, the lesion may be skipped, leaving a single-stranded DNA gap that will be replicated later. Details of the participation of these enzymes are revised for the replication of damaged template. TLS polymerases also have functions in other cellular processes. These include involvement in somatic hypermutation in immunoglobulin genes, direct participation in recombination and repair processes, and contributing to replicating noncanonical DNA structures. The importance of DNA damage replication to cell survival is supported by recent discoveries that certain genes encoding TLS polymerases are induced in response to DNA damaging agents, protecting cells from a subsequent challenge to DNA replication. We retrace the findings on these genotoxic (adaptive) responses of human cells and show the common aspects with the SOS responses in bacteria. Paradoxically, although TLS of DNA damage is normally an error prone mechanism, in general it protects from carcinogenesis, as evidenced by increased tumorigenesis in xeroderma pigmentosum variant patients, who are deficient in Pol η. As these TLS polymerases also promote cell survival, they constitute an important mechanism by which cancer cells acquire resistance to genotoxic chemotherapy. Therefore, the TLS polymerases are new potential targets for improving therapy against tumors.

Eukaryotic Translesion DNA Synthesis on the Leading and Lagging Strands: Unique Detours around the Same Obstacle

During S-phase, minor DNA damage may be overcome by DNA damage tolerance (DDT) pathways that bypass such obstacles, postponing repair of the offending damage to complete the cell cycle and maintain cell survival. In translesion DNA synthesis (TLS), specialized DNA polymerases replicate the damaged DNA, allowing stringent DNA synthesis by a replicative polymerase to resume beyond the offending damage. Dysregulation of this DDT pathway in human cells leads to increased mutation rates that may contribute to the onset of cancer. Furthermore, TLS affords human cancer cells the ability to counteract chemotherapeutic agents that elicit cell death by damaging DNA in actively replicating cells. Currently, it is unclear how this critical pathway unfolds, in particular, where and when TLS occurs on each template strand. Given the semidiscontinuous nature of DNA replication, it is likely that TLS on the leading and lagging strand templates is unique for each strand. Since the discovery of DDT in the late 1960s, most studies on TLS in eukaryotes have focused on DNA lesions resulting from ultraviolet (UV) radiation exposure. In this review, we revisit these and other related studies to dissect the step-by-step intricacies of this complex process, provide our current understanding of TLS on leading and lagging strand templates, and propose testable hypotheses to gain further insights.

Kinetics of DNA Repair in Ultraviolet-Irradiated and N-acetoxy-2-acetylaminofluorene-Treated Mammalian Cells

Repair kinetics after saturating doses of ultraviolet radiation (UV), N-acetoxy-2-acetylaminofluorene (AAAF), and combinations of both agents were studied in human fibroblasts proficient and deficient in excision repair, and in Chinese hamster cells (V-79) deficient in excision repair. Three techniques were used: unscheduled DNA synthesis, photolysis of DNA repaired in the presence of bromodeoxyuridine (BrdUrd), and measurements of sites sensitive to a UV-endonuclease. The repair rate appears to be approximately constant during the first few hours after treatment. Later there is a decrease with time; the magnitude of the decrease depends on the cell line. Our data show that the decrease in repair observed in repair-deficient cells treated with combinations of both agents as compared to separate treatments is due neither to the cytotoxic effects of the agents used, nor to a shutoff of the repair system by the high concentrations of AAAF employed.

Shedding light on proteins, nucleic acids, cells, humans and fish

I was trained as a physicist in graduate school. Hence, when I decided to go into the field of biophysics, it was natural that I concentrated on the effects of light on relatively simple biological systems, such as proteins. The wavelengths absorbed by the amino acid subunits of proteins are in the ultraviolet (UV). The wavelengths that affect the biological activities, the action spectra, also are in the UV, but are not necessarily parallel to the absorption spectra. Understanding these differences led me to investigate the action spectra for affecting nucleic acids, and the effects of UV on viruses and cells. The latter studies led me to the discovery of the important molecular nature of the damages affecting DNA (cyclobutane pyrimidine dimers) and to the discovery of nucleotide excision repair. Individuals with the genetic disease xeroderma pigmentosum (XP) are extraordinarily sensitive to sunlight-induced skin cancer. The finding, by James Cleaver, that their skin cells were defective in DNA repair strongly suggested that DNA damage was a key step in carcinogenesis. Such information was important for estimating the wavelengths in sunlight responsible for human skin cancer and for predicting the effects of ozone depletion on the incidence of non-melanoma skin cancer. It took experiments with backcross hybrid fish to call attention to the probable role of the longer UV wavelengths not absorbed by DNA in the induction of melanoma. These reflections trace the biophysicist's path from molecules to melanoma.

Isolation and characterization of Chinese hamster cell line resistant to monofunctional alkylating agents

A clonal derivative of a Chinese hamster Don D-6 cell line resistant to methyl methane sulfonate (MMS) has been isolated following mutagenesis by ethyl methane sulfonate (EMS). The clone, designated as MMSr-1, exhibited high resistance to killing by the monofunctional alkylating agents MMS and EMS. This characteristic had not been acquired by a transient adaptation to the alkylating agents but was found to be a stable heritable trait. MMSr-1 was more sensitive to high-molecular-weight chemicals, such as colchicine and puromycin, than Don D-6. Both MMSr-1 and its parental cells showed the same ability to take up radioactive MMS. The resistance of MMSr-1 appears not to be due to altered uptake of MMS. The resistance was accompanied by low chromosomal aberration and sister chromatid exchange (SCE) induction but not by mutability. Protein synthesis inhibitors such as cycloheximide and puromycin reduced the resistance to the same level as that in Don D-6. SCE induction by MMS in this clone was not antagonized by the protein synthesis inhibitors, whereas mutagenesis was reversed to the normal parental cell level by these inhibitors. Aphidicolin, a DNA-synthesis inhibitor, exhibited no such effects. These results suggest that MMSr-1 might have modified repair capacity, which can be normalized by treatment with the protein-synthesis inhibitors, for lethal DNA damage by monofunctional alkylating agents, and that SCE formation by the alkylating agents is closely correlated with chromosomal aberration and cell lethality.

Biologic interactions between smoking and occupational exposures

Cigarette smoking is a major cause of cancer and lung disease in the U.S. population. The biological processes that underlie the response of the lung to cigarette smoke are important considerations for designing analyses of the effects of occupational exposures. Interactions between cigarette smoking and occupational exposures may occur through a combined effect on the mechanism of disease production, through an effect on the dose of the toxic substances that reach the target issue, or through an effect on the response of the lung to the toxic agents. Disease due to occupational exposures can occur in a similar pattern in both smokers and nonsmokers; however, as more complex interactions are examined, different responses to the same occupational exposure may be identified for smokers and nonsmokers. It is only through the successful intermingling of biologic information with epidemiologic data that these interactions can be fully examined.

Induction of urokinase-type plasminogen activator by UV light in human fetal fibroblasts is mediated through a UV-induced secreted protein

Plasminogen activator was previously shown to be induced by UV light in human cells with low capacity to repair UV-induced DNA lesions. We now show that in human fetal fibroblasts UV light enhanced the two mRNA species coding for the urokinase-type plasminogen activator (uPA) and the tissue-type plasminogen activator, but immunological analysis revealed exclusively uPA activity. Several independent and complementary experiments indicated that induction of uPA was mediated, apparently entirely, through a UV-induced, secreted protein (UVIS) in the growth medium of irradiated cells. First, elevation of uPA mRNA after irradiation was severely blocked by cycloheximide. Second, replacement of conditioned medium in irradiated cells while the rate of plasminogen activator induction was maximal rapidly and completely stopped any further increase in uPA activity. Third, addition of the same removed conditioned medium to nonirradiated cells mimicked UV light in enhancing the level of uPA activity as well as that of uPA mRNA. Fourth, UVIS activity was completely lost by treating the conditioned medium with trypsin but not with nucleases. Kinetic measurements indicated that the accumulation of UVIS rather than the induction of uPA by UVIS conferred the rate-limiting step in the overall process of uPA induction. Both UV light and UVIS acted synergistically with inhibitors of DNA repair for uPA induction. Based on these results, a model is proposed implicating relaxation of DNA torsional stress of an as yet undefined DNA sequence(s) in the induction of UVIS, which is then responsible for activation of the uPA gene.

Induction of urokinase-type plasminogen activator by UV light in human fetal fibroblasts is mediated through a UV-induced secreted protein

Plasminogen activator was previously shown to be induced by UV light in human cells with low capacity to repair UV-induced DNA lesions. We now show that in human fetal fibroblasts UV light enhanced the two mRNA species coding for the urokinase-type plasminogen activator (uPA) and the tissue-type plasminogen activator, but immunological analysis revealed exclusively uPA activity. Several independent and complementary experiments indicated that induction of uPA was mediated, apparently entirely, through a UV-induced, secreted protein (UVIS) in the growth medium of irradiated cells. First, elevation of uPA mRNA after irradiation was severely blocked by cycloheximide. Second, replacement of conditioned medium in irradiated cells while the rate of plasminogen activator induction was maximal rapidly and completely stopped any further increase in uPA activity. Third, addition of the same removed conditioned medium to nonirradiated cells mimicked UV light in enhancing the level of uPA activity as well as that of uPA mRNA. Fourth, UVIS activity was completely lost by treating the conditioned medium with trypsin but not with nucleases. Kinetic measurements indicated that the accumulation of UVIS rather than the induction of uPA by UVIS conferred the rate-limiting step in the overall process of uPA induction. Both UV light and UVIS acted synergistically with inhibitors of DNA repair for uPA induction. Based on these results, a model is proposed implicating relaxation of DNA torsional stress of an as yet undefined DNA sequence(s) in the induction of UVIS, which is then responsible for activation of the uPA gene.

Carcinogenicity of some anticancer drugs--a survey

A list of 37 anticancer drugs with reported carcinogenicity or co-carcinogenicity has been compiled. These drugs include 16 alkylating agents, eight antimetabolites, four antibiotics, four hormones, one alkaloid, and four miscellaneous drugs. The reported carcinogenicity of these drugs in model systems and their possible mechanisms of action are reviewed together with structure-carcinogenicity relationships. These drugs have been reported to cause cancers in different sites in various test animals. Most of these drugs react with DNA, and the ultimate reactive forms of the drugs are electrophiles.

Exposure of nondividing populations of primary human fibroblasts to UV (254 nm) radiation induces a transient enhancement in capacity to repair potentially lethal cellular damage

Nondividing (arrested) populations of primary human fibroblasts from normal individuals exposed to an initial dose (1.5 or 3 J X m-2) of far-UV (254 nm) radiation and then incubated in medium containing low (0.5%) serum develop enhanced resistance to inactivation of cloning efficiency by a second (challenge) dose of UV. The resistance develops within 2-4 days, after which there is a decline. Resistance develops to a higher degree and more rapidly (1-2 days) in cells derived from patients with the variant form of xeroderma pigmentosum. Excision-deficient cells from xeroderma pigmentosum complementation group A individuals also develop UV resistance after a lower (0.2 J X m-2) exposure to UV. Enhanced UV resistance does not develop in UV-irradiated cell populations incubated with the protein synthesis inhibitor cycloheximide (5 microM). These observations are consistent with the interpretation that exposure of human fibroblasts to low doses of UV induces synthesis of a protein involved in a metabolic pathway that transiently enhances the capacity of cells to repair potentially lethal damage resulting from a subsequent dose of UV.

U.V. enhanced reactivation of U.V.-and gamma-irradiated adenovirus in Cockayne syndrome and Xeroderma pigmentosum fibroblasts

U.V.-enhanced reactivation (UVER) of both U.V.-irradiated and gamma-irradiated human adenovirus type 2 (Ad 2) was examined following the infection of a variety of Cockayne Syndrome (CS) and Xeroderma pigmentosum (XP) fibroblast strains which had been pre-irradiated with U.V. light. U.V.-irradiated or non-irradiated fibroblasts were infected with either non-irradiated or irradiated Ad2, and at 48 hours after infection cells were examined for the presence of viral structural antigens (Vag) using immunofluorescent staining. Normal levels of UVER (i.e. 2-4 fold) of U.V.- and of gamma-irradiated Ad 2 were detected in 2 CS strains (CS IBE and CS 3BE), 2 XP complementation group A strains (XP 12BE and XP 25RO), and 2 XP complementation group D strains (XP 5BE and XP 6BE), although the U.V. doses to these mutant cells which resulted in peak UVER values (0 . 2 Jm-2 for XP 25RO, 0 . 14 Jm-2 for XP 12BE, 0 . 8 Jm-2 for XP 5BE and XP 6BE, and 1 . 6-5 . 0 Jm-2 for CS 1BE and CS 3BE) were considerably lower than those yielding peak UVER in normal strains (10-15 Jm-2). XP variant strains (XP 4BE and XP 115LO), however, showed substantially lower levels of UVER than normal strains.

Comparison between protracted and conventional dose rates of irradiation on the growth of the Bp8 mouse ascites sarcoma

Four days after inoculation, in vivo growing Bp8 mouse ascites sarcoma was irradiated with 1.75, 2.5 and 5 Gy at a dose rate of 0.0185 Gy/min. The results were compared with those obtained previously at a dose rate of 1.33 Gy/min. Using flow-cytometry and the mitotic index, the total number of tumour cells and the proportion of cells in the various parts of the cell cycle were determined up to 10 days after irradiation. In addition, from sequential analyses of the total number of cells in the various compartments of the cell cycle, the flow of cells through cell cycle was calculated. The total number of tumour cells at the plateau phase of growth, the mitotic index and flow calculations indicated that protracted irradiation was more effective than short-time irradiation following 2.5 Gy; no differences were found following 1.75 Gy, and a lesser effect following 5 Gy. While the cells during protracted irradiation were effectively blocked in G2 and in split-dose experiments a good repair capacity has been demonstrated, redistribution of cells in the cell cycle can apparently not explain the results. Induction of repair, dependent both on dose rate and on total dose, may explain the higher efficiency of protracted irradiation.

Dissociated occurrence of single-gene mutation and oncogenic transformation in C3H 10T1/2 cells exposed to ultraviolet light and caffeine

We examined the relationship of cytotoxicity, mutagenesis, and malignant transformation by measuring in parallel clonogenic survival, mutation to ouabain resistance, and malignant transformation in cultured C3H mouse 10T1/2 cells. Exposure to caffeine alone for 48 hours was cytotoxic and induced transformation in a dose-dependent manner. However, this same treatment did not induce any detectable ouabain-resistant mutants. When caffeine was present for 48 hours immediately following UV irradiation, alkaline sucrose gradient sedimentation of DNA showed that postreplication repair was inhibited. This inhibition of repair was correlated with reduced survival and inhibition of mutation induction, but the transformation frequencies were either unaltered or potentiated, depending on the UV dose and caffeine concentration. Thus, these experiments demonstrate that gene mutation and malignant transformation in 10T1/2 cells can be dissociated. We suggest that the mechanism of transformation of 10T1/2 cells is nonmutagenic in nature.

Chemical carcinogenesis -- the price for DNA - repair?

This essay examines the possibility of merging the mutation theory of cancer with the hypothesis that cancer is a change in the state of the differentiation of cells. It is suggested that during normal development DNA rearrangements occur, concerning genes which code for differentiation specific cell communication proteins. These proteins are responsible for the proper functioning of growth control in a multicellular organism. DNA-damaging agents - mutagens - induce DNA repair enzymes, some of which may catalyse illegitimate genome rearrangements, thus leading to a change of the balance between growth and differentiation. A cell with a selective advantage may arise and become the origin of a tumor.

Induction of plasminogen activator by UV light in normal and xeroderma pigmentosum fibroblasts

Normal and DNA repair-deficient human fibroblasts have been used to study induction of plasminogen activator (PA) by DNA damage. UV light induced the synthesis of PA in skin fibroblasts of all types of xeroderma pigmentosum (XP) in XP heterozygotes and in human amniotic cells. Enzyme induction was, however, not observed in fibroblasts of normal adults. In classical XP, which are deficient in excision repair, PA synthesis occurred in a narrow range of low-UV fluences. In such strains, the level of enzyme produced was correlated with the extent of repair deficiency. UV fluences required for PA induction in XP variants and XP heterozygotes were at least 10 times those inducing enzyme synthesis in excision-deficient XP. Maximum enzyme induction occurred 48 hr after irradiation, and the highest levels of enzyme produced were 15-20 times those of PA baseline levels. Electrophoretic analysis showed that UV irradiation enhances the synthesis of the Mr 60,000 human urokinase-type PA, which is present in low amounts in untreated cells. Our results suggest that PA induction in human cells is caused by unrepaired DNA damage and represents a eukaryotic SOS-like function. In addition, PA induction may provide a sensitive assay for detection of cellular DNA repair deficiencies and identification of XP heterozygotes.

Mechanisms of tolerance to DNA lesions in mammalian cells

In recent years it has become clear that different pathways are involved in the process of removing lesions from DNA. In spite of a continuous surveillance of the genetic integrity by repair enzymes, quite often lesions are not eliminated before the portion of the genome where they have been inserted is used for DNA replication or transcription. Actually, the number of unexcised lesions a cell can tolerate without significantly losing its capacity to reproduce is surprising. As an example, human fibroblasts from certain patients with the genetic disease xeroderma pigmentosum (XP)† are virtually unable to excise pyrimidine dimers, the major DNA lesion produced by short-wavelength UV light.

Simian virus 40 as a probe for studying inducible repair functions in mammalian cells

We describe the use of Simian Virus 40 (SV40) as a molecular probe for studying the cellular functions induced in cultured monkey kidney cells in response to DNA damaging agents. (a) Ultraviolet (UV) irradiation of SV40-infected cells inhibits viral DNA replication. Replication forks are blocked by the first pyrimidine dimer encountered. In some cases, a single-strand break seems to occur at the level of the dimer inhibiting the fork of replication. This break, which can be visualized by electron microscopy studies, might be the first step in an excision repair pathway. (b) Treatment of monkey kidney cells with acetoxy-acetyl-aminofluorene or UV light before infection with UV-irradiated SV40 induces a mutagenic replication mode, as shown by an increase of the mutation frequency of thermosensitive SV40 mutants. (c) A possible recombination assay using various SV40 mutants infecting the same cell is proposed and discussed.

Postreplication repair and the susceptibility of Chinese hamster cells to cytotoxic and mutagenic effects of alkylating agents

A cell variant (VR-43) resistant to the cytotoxic effect of N-methyl-N'-nitro-N-nitrosoguanidine and N-methyl-N'-nitro-N-nitrosourea was isolated from Chinese hamster V79 cells for use in studies of the relationship among cell survival, mutagenesis, and DNA repair by alkylating agents. Resistance to ouabain or 6-thioguanine was used as the genetic marker. After treatment with N-methyl-N'-nitro-N-nitrosoguanidine, the VR-43 cells exhibited mutation frequencies that were lower, on a dose basis, than those of the wild-type V79 cells. However, when analyzed at equicytotoxic doses, the VR-43 cells were more mutable than the V79 cells. No difference in cell survival or mutagenicity could be observed after treatment with other mutagens such as N-ethyl-N'-nitro-N-nitrosoguanidine, ethyl methanesulfonate, or x-rays. Postreplication repair was analyzed by determination of the molecular weight of the newly synthesized DNA by alkaline sucrose gradients. After treatment with N-methyl-N'-nitro-N-nitrosoguanidine, the VR-43 cells exhibited an enhanced postreplication repair relative to the V79 cells. No such enhancement was found after N-ethyl-N'-nitro-N-nitrosoguanidine or ethyl methanesulfonate treatment. Based on these results we propose that, after treatment of these and presumably other mammalian cells with some methylating mutagens, postreplication repair can cope with DNA lesions responsible for cytotoxicity and, to a lesser degree, with lesions responsible for mutagenicity.

DNA repair after ultraviolet irradiation of ICR 2A frog cells. Pyrimidine dimers are long acting blocks to nascent DNA synthesis

The ability of ICR 2A frog cells to repair DNA damage induced by ultraviolet irradiation was examined. These cells are capable of photoreactivation but are nearly totally deficient in excision repair. They have the ability to convert the small molecule weight DNA made after irradiation into large molecules but do not show an enhancement in this process when the UV dose is delivered in two separate exposures separated by a 3- or 24-h incubation. Total DNA synthesis is depressed and low molecular weight DNA continues to be synthesized during pulse-labeling as long as 48 h after irradiation. The effects of pyrimidine dimer removal through exposure of UV irradiated cells to photoreactivating light indicate that dimers act as the critical lesions blocking DNA synthesis.

The early and late modes of DNA replication in ultraviolet irradiated Syrian hamster embryo cells

The nature of DNA replication in UV irradiated Syrian hamster embryo cells (HEC) was investigated by measuring the size distribution of nascent daughter strand DNA. During the early mode nascent strands are made in smaller pieces than in nonirradiated cells. The late mode begins when nascent strands recover to normal size. This was observed in HEC 5 h post-UV. When the late mode is operational, nascent strands elongate to parental size in greater than 2 h, whereas less than 3 h are required during early mode function. Evidence from split dose experiments demonstrates that the recovery of the size of nascent strands is not due to enhanced gap filling. Furthermore, pyrimidine dimers are probably recognized differently by the replication complex during early and late mode DNA synthesis. The late mode of replication could account for the ability of HEC to survive UV irradiation even though they are inefficient in both excision and postreplication repair.

Effect of uncouplers on radiosensitivity and mutagenicity in x-irradiated mammalian cells

The number of x-irradiated mammalian cells surviving is markedly increased when the cells are incubated with an uncoupler of oxidative phosphorylation prior to or immediately after irradiation. This increase is greater in plateau-phase cells than in exponentially growing cells. The increase in survival is related to the potency of the uncouplers, which do not modify the effective x-ray dose. The influence of uncouplers on survival is related to an increase of repair and semiconservative DNA synthesis. The mutation frequency (8-azaguanine-resistant mutants) is significantly higher in irradiated cells treated with uncouplers than in untreated cells. These results suggest the existence of an error-prone repair process in mammalian cells.

Deoxyribonucleic Acid Repair in Bacillus subtilis : Development of Competent Cells into a Tester for Carcinogens

The development of competent transformed Bacillus subtilis into a tester system for carcinogens is described. Precocious or noninduced activation of SOS functions occur in competent cells. Thus, lower doses or concentrations of SOS inducing agents are needed to cause cell death due to indigenous prophage activation and lysis of bacteria. The two known defective prophages in B. subtilis enhance the sensitivity of competent cells to the carcinogens ultraviolet light, mitomycin C, and methyl methanesulfonate. However, these same cells have no enhanced sensitivity for the non-carcinogenic ethyl methanesulfonate or for nalidixic acid. Therefore, competent B. subtilis appear to be a sensitive tester for carcinogens.

In vivo replication of hepatic deoxyribonucleic acid of rats treated with dimethylnitrosamine: presence of dimethylnitrosamine-induced O6-methylguanine, N7-methylguanine, and N3-methyladenine in the replicated hybrid deoxyribonucleic acid

Experiments were designed to determine whether some chemical lesions such as O6-methylguanine, N7-methylguanine, and N3-methyladenine induced in rat liver DNA by the hepatocarcinogen dimethylnitrosamine permit replication in vivo. For this purpose, [14C]dimethylnitrosamine was administered to methylate the parental strand of liver DNA. Four hours later, a time period when the carcinogen cannot be detected in either the liver or the blood, rats were subjected to partial hepatectomy in order to induce DNA replication. During the S phase, 5-bromo-2-deoxyuridine was administered to render the newly made strands heavy. The rebanded, hybrid, hepatic DNA of density 1.714 g/cm3 and greater was pooled from the neutral cesium chloride gradient, dialyzed, and lyophilized. The hybrid DNA was then treated with S1 nuclease to digest any single-stranded regions. The results obtained indicated the presence of O6-methylguanine, N7-methylguanine, and N3-methyladenine in S1 nuclease resistant, hybrid DNA. The results are interpreted to indicate that these chemical lesions permitted in vivo DNA replication.

Four proteins synthesized in response to deoxyribonucleic acid damage in Micrococcus radiodurans

Four proteins, alpha beta, gamma, and delta, preferentially synthesized in ultraviolet light-treated cells of Micrococcus radiodurans, were characterized in terms of their molecular weights and isoelectric points. Within the sublethal-dose range, the differential rate of synthesis for these proteins increased linearly with the inducing UV dose. The degree of induction reached 100-fold, and the most abundant protein beta, amounted to approximately 2% of the total newly synthesized protein after irradiation. Damage caused by ionizing radiation or by treatment with mitomycin C also provoked the synthesis of the four proteins. The proportions between the individual proteins, however, varied strikingly with the damaging agent. In contrast to treatments which introduced damage in the cellular deoxyribonucleic acid, the mere arrest of deoxyribonucleic acid replication, caused by nalidixic acid or by starvation for thymine, failed to elicit the synthesis of either protein. Repair of deoxyribonucleic acid damage requires that a number of versatile and efficient processes by employed. It is proposed that the induced proteins participate in deoxyribonucleic acid repair in M. radiodurans. Mechanisms are discussed which would allow a differentiated cellular response to damages of sufficiently distinctive nature.

Ultraviolet light induction of diphtheria toxin-resistant mutations in normal and DNA repair-deficient human and Chinese hamster fibroblasts

The role of unrepaired DNA lesions in the production of mutations is suspected of contributing to the initiation phase of carcinogenesis. Since the molecular basis of mutagenesis is not understood in eukaryotic cells, development of new genetic markers for quantitative in vitro measurement of mutations for mammalian cells is needed. Furthermore, mammalian cells, genetically deficient for various DNA repair enzymes, will be needed to study the role of unrepaired DNA lesions in mutagenesis. The results in this report relate to preliminary attempts (1) to characterize the diphtheria toxin resistance marker as a useful quantitative genetic marker in human cells and (2) to isolate and characterize various DNA repair-deficient Chinese hamster cells.

Pretreatment with acetylaminofluorene enhances the repair of O6-methylguanine in DNA

Epidemiological and experimental evidence suggests that the interplay of environmental factors may be particularly relevant to the induction of cancer in man. We have investigated the possible interrelationships, at the level of DNA repair, of two chemically different hepatocarcinogens administered to rats. Animals were pretreated for several weeks by inclusion of 2-acetylaminofluorene (AAF) in the diet and a study was made of the capacity of liver to repair lesions subsequently introduced into DNA by a pulse of dimethylnitrosamine (DMN). Of particular interest was the repair of adducts at the O6 position of guanine as this product has been implicated as a critical reaction site for carcinogenesis by the monofunctional alkylating agents. We show here that the capacity of liver to repair O6-methylguanine in DNA is enhanced by prolonged exposure of rats to the chemically unrelated agent, AAF.

Postreplication repair: questions of its definition and possible alteration in xeroderma pigmentosum cell strains

DNA synthesis in normal cells and in excision-defective and variant xeroderma pigmentosum cells was investigated after irradiation with ultraviolet light. The sizes of DNA synthesized during brief pulses of [3H]thymidine 1-2 hr after irradiation were decreased, the xeroderma pigmentosum variant showing the smallest molecular weight. Once synthesized, however, labeled DNA increased in size at the same rat as control in all cell strains, and the rate was relatively insensitive to caffeine. After 2-3 hr, labeled DNA in each cell type reached a maximum size that was less than that in control cells, indicating the presence of long-lived blocks to DNA chain growth. This kind of experiment (pulse-chase) has in the past been used to investigate a repair process believed to be associated with the bypass of damaged sites in parental DNA: postreplication repair. We present an alternative model that does not involve a specific postreplication repair mechanism, but involve a specific postreplication repair mechanism, but involves normal chain elongation and termination mechanisms in which we conceive that dimers and other damaged sites act as well-or-nothing blocks to the progress of replication forks. No evidence could be found for any inducible process that enhanced the bypass of damaged sites.

Recovery of DNA synthesis after ultraviolet irradiation of xeroderma pigmentosum cells depends on excision repair and is blocked by caffeine

Normal human and xeroderma pigmentosum (XP, excision-defective group A) cells (both SV40-transformed) pulse-labeled with [(3)H]thymidine at various times after irradiation with ultraviolet light showed a decline and recovery of both the molecular weights of newly synthesized DNA and the rates of synthesis per cell. At the same ultraviolet dose, both molecular weights and rates of synthesis were inhibited more in XP than in normal cells. This indicates that excision repair plays a role in minimizing the inhibition of chain growth, possibly by excision of dimers ahead of the growing point. The ability to synthesize normal-sized DNA recovered more rapidly than rates of synthesis in normal cells, but both parameters recovered in phase in XP cells. During recovery in normal cells there are therefore fewer actively replicating clusters of replicons because the single-strand breaks involved in the excision of dimers inhibit replicon initiation. XP cells have few excision repair events and therefore fewer breaks to interfere with initiation, but chain growth is blocked by unexcised dimers. In both cell types recovery of the ability to synthesize normal-sized DNA was prevented by growing cells in caffeine after irradiation, possibly because of competition between the DNA binding properties of caffeine and replication proteins. Our observations imply that excision repair and semiconservative replication interact strongly in irradiated cells to produce a complex spectrum of changes in DNA replication which may be confused with parts of alternative systems such as post-replication repair.

Response of plateau-phase mouse embryo fibroblasts to ultra-violet light

Clonogenic survival was measured in plateau-phase cultures of the 10T1/2 mouse cell line exposed to 254 nm ultra-violet light. The survival curve was found to be biphasic, Do for the two components being 37 and 1191 erg/mm2 respectively. This extreme resistance at higher doses can only be partly accounted for by the increased cytoplasmic absorption of U.V.L. due to an increased thickness of plateau-phase cells. When the cultures were held for 24 hours in plateau phase in conditioned medium after irradiation, recovery yielding a 1.4-fold enhancement of survival was found at higher doses. This recovery process was inhibited by neither caffeine nor cycloheximide. When caffeine was given for 48 hours after sub-culture, the effect on survival was also negligible. We propose that this plateau-phase recovery process is associated with excision repair of DNA adducts induced by U.V.L. Delayed sub-culturing favours the excision mode of repair and renders the post-replication mode less necessary.

Effects of cis-platinum(II) diamminedichloride on survival and the rate of DNA synthesis in synchronously growing Chinese hamster V79-379A cells in the absence and presence of caffeine inhibited post-replication repair; evidence for an inducible repair mechanism

The mechanism by which cis-Platinum(II)diamminedichloride, (cis-Pt(II)), inhibited DNA synthesis was investigated using synchronously growing Chinese hamster V79-379A cells. The progression of G1 phase treated cells into S phase was not inhibited. However the rate of DNA synthesis throughout the S phase was depressed in a dose dependent manner compared to a control. The peak rate of DNA synthesis was delayed and the duration of the S phase was extended resulting in a dose dependent delay in the timing of mitosis in the first cell cycle after treatment. The delay in the timing of mitosis after the second S phase in treated cells was comparable to the delay observed after the first cycle. Post treatment incubation of cells in the presence of a non toxic concentration of caffeine resulted in an alteration in the pattern of DNA synthesis, leading to a reversal of the dose dependent delay in the timing of the peak rate of DNA synthesis to a position corresponding to the control. Caffeine potentiated the toxicity of cis-Pt(II) only if it was added during the first cell cycle after treatment. The patterns of DNA synthesis in synchronous cultures of Chinese hamster V79-379A cells after cis-Pt(II) treatment appeared to be similar to those previously demonstrated for difunctional alkylating agents. The results were discussed in relation to an existing model in which it was postulated that a DNA replication dependent repair mechanism enables cells to replicate DNA using a cis-Pt(II) damaged DNA template. This mechanism has been shown in Chinese hamster V79-379A cells to be sensitive to non toxic concentrations of caffeine resulting in an inhibition of the rate of ligation of nascent DNA which is followed by an increase in chromosome aberrations and cell death. The results presented support the proposed model and emphasise the importance of this replication repair mechanism in enabling cells to tolerate unexcised damage to their genome. In addition we propose that cis-Pt(II) damage stimulates the induction of a system which is involved in enhancing so called post replication repair in Chinese hamster V79 cells and that this induction might be inhibited by the presence of caffeine.