Influence of interferons on the repair of UV-damaged DNA

The capacity for nucleotide excision repair of a normal (WISH) and three tumour (MCF-7, HeLa, Namalva) cell lines treated with human recombinant interferons (hrIFN-alpha and hrIFN-gamma) was compared by the host cell reactivation assay. The cells were transfected with in vitro UV-damaged plasmid DNA (pEGFP-N1). The repair capacity was determined by measuring the fluorescence intensity of the expressed marker protein in total cell lysates. The correlation between the interferon-induced NO content and the suppressive effect of interferons on DNA repair was shown. The decrease of repair activity and NO induction by hrIFN-alpha were greatest in WISH, followed by MCF-7, Namalva and HeLa cells, whereas hrIFN-gamma was the best NO inducer and inhibitor for the repair of Namalva, followed by WISH, MCF-7 and HeLa cells. Our data clearly show that the two types of interferon have a strong inhibitory effect on the repair of UV-damaged DNA and this effect is cell type-dependent.     

Optimization of protective properties of interferons in human cells exposed to mutagens estimated by the DNA repair activity

Protective properties of human interferons against physical and chemical mutagens have been described earlier. This work was aimed at detecting an optimum of protective action of interferons in human fibroblasts using two criteria: the number of single-strand DNA breaks formed and the index of DNA repair synthesis. The protective ability of interferon was shown to be expressed starting after 4 h of cells' pretreatment and proceeding through 40 h in experiments with N-methyl-N-nitro-N-nitrosoquanidin. The phenomenon of stimulation of DNA repair synthesis in human cells pretreated with interferon proceeded even after replating cells during 8 h in the experiments with UV irradiation.     

Modification of human DNA polymerase alpha by N-acetylimidazole

The modification of tyrosine residues of the human placenta DNA-polymerase alpha by N-acetylimidazole was investigated. The poly(dT)-template and the r(pA)10-primer a each added separately or simultaneously do not influence the rate of enzyme inactivation. In the presence of poly(dT)-r(pA)10 no effect of dCTP and dTTP (noncomplementary to template) and of dAMP and dADP (complementary to template) on the rate and the level of the enzyme inactivation was found. However dATP revealed practically complete protection. Orthophosphate, pyrophosphate each taken separately do not influence the rate of enzyme inactivation with this reagent. The presence of dADP with either ortho- or pyrophosphate, or dAMP with the one of these ligands leads to half protective action in comparison with dATP. Imidazolides of phosphonoacetic acid and 5'-adenylyl++ 1(phosphonoacetic acid) do not inactivate DNA-polymerase alpha from human placenta and the Klenov fragment of DNA-polymerase I from E. coli. All data obtained allow to suggest that the tyrosine residue in the dNTP binding site of DNA-polymerase reveals stacking with the nucleotide only if dNTP is complementary to the template.     

Induction of unique mRNAs by human interferons

Treatment of human fibroblast cells with human interferon (INF-alpha, IFN-beta, or IFN-gamma) resulted in the accumulation of at least four newly synthesized mRNAs. The mRNAs code for proteins having molecular weights of 56,000, 57,000, 62,000, and 68,000 when characterized in a wheat germ cell-free translation system. A direct relationship was observed between the amount of IFN used and the degree of both the accumulation of the induced mRNAs and the development of an antiviral state. In the case of IFN-alpha or IFN-beta, time course studies indicated that the induced mRNAs appeared as early as 40 min, accumulated for 2 h, then remained ribosome bound for up to 16 h. The ability of fibroblast cells to develop an antiviral state always coincided directly with both the appearance and the level of accumulation of the induced mRNAs. Further mRNA synthesis beyond 2 h had a minimal effect on the development of an antiviral state. Human IFN-gamma also induced the synthesis of the same four mRNAs but required higher interferon titers and a longer incubation time. In addition, IFN-gamma induced a disproportionate amount of the mRNA coding for the 68,000 molecular weight protein and three new mRNAs not detected in cells treated with IFN-alpha or IFN-beta. Mouse interferon induces the original four mRNAs in human cells but to a far lesser extent. This correlated with the inability of these cells to develop much resistance to viral infection.     

Trapping of human DNA topoisomerase I by DNA structures mimicking intermediates of DNA repair

In this report we show that human DNA Topoisomerase I (Top1) forms DNA-protein adducts with nicked and gapped DNA structures lacking a conventional Top1 cleavage site. The radioactively labeled crosslinking products were identified by SDS-gel electrophoresis. The chemical structure of the groups at 5' or 3' end of the nick does not have an effect on the formation of these covalent adducts. Therefore, all kinds of nicks, either directly induced by ionizing radiation or reactive oxygen species or indirectly induced in the course of base excision repair (BER) are targets for Top1 that competes with BER proteins and other nick-sensors. Top1-DNA covalent adducts formed in cells exposed to DNA damaging agents can promote genetic instability.     

Enhancement of cell growth by human interferons

The effects of human interferons (HuIFN) on the human osteosarcoma cells were examined. HuIFN-alpha, -beta and -gamma enhanced dose-dependently the cell growth. There was no difference in the degree of the enhancement of the cell growth among HuIFN-alpha, -beta and -gamma. The higher the cell density was, the lower the degree of the enhancement of the cell growth. When HuIFN-gamma was neutralized with anti-HuIFN-gamma, the enhancement of cell growth was not found.     

DNA repair and human pathology

Disorders in the DNA repair in the human lymphocytes isolated from patients with Marfan's syndrome, homocystinuria, schizophrenia, and gout have been found. In this investigation criteria used estimating the DNA repair were the following: host cell reactivation (vaccinia virus reactivation) and its mutagenesis, DNA repair synthesis, resynthesis of DNA breakages. Lymphoblastoid interferon was used as a modulator of DNA repair activity. Pretreatment of normal human cells with interferon stimulated all steps of DNA repair. In human cells with disorders, interferon stimulated DNA repair (XP) in some cases but failed in others.     

DNA binding and nucleotide flipping by the human DNA repair protein AGT

O(6)-alkylguanine-DNA alkyltransferase (AGT), or O(6)-methylguanine-DNA methyltransferase (MGMT), prevents mutations and apoptosis resulting from alkylation damage to guanines. AGT irreversibly transfers the alkyl lesion to an active site cysteine in a stoichiometric, direct damage reversal pathway. AGT expression therefore elicits tumor resistance to alkylating chemotherapies, and AGT inhibitors are in clinical trials. We report here structures of human AGT in complex with double-stranded DNA containing the biological substrate O(6)-methylguanine or crosslinked to the mechanistic inhibitor N(1),O(6)-ethanoxanthosine. The prototypical DNA major groove-binding helix-turn-helix (HTH) motif mediates unprecedented minor groove DNA binding. This binding architecture has advantages for DNA repair and nucleotide flipping, and provides a paradigm for HTH interactions in sequence-independent DNA-binding proteins like RecQ and BRCA2. Structural and biochemical results further support an unpredicted role for Tyr114 in nucleotide flipping through phosphate rotation and an efficient kinetic mechanism for locating alkylated bases.     

Inhibition of DNA repair by deoxyadenosine in resting human lymphocytes

Profound lymphopenia is characteristic of immunodeficient children who lack adenosine deaminase (ADA). When ADA is inactive, deoxyadenosine (dAdo) is phosphorylated by immature T lymphoblasts and inhibits cell division. However, dAdo also causes the slow accumulation of DNA strand breaks in nondividing, mature human peripheral blood lymphocytes. To explore the basis for this phenomenon, we have assessed the effects of dAdo and other deoxynucleosides on the repair of gamma-radiation induced DNA strand breaks in resting normal lymphocyte cultures. As measured by a sensitive DNA unwinding assay, most DNA strand breaks were rejoined within 2 hr after exposure of lymphocytes to 500 rad. In medium supplemented with deoxycoformycin, a tight binding ADA inhibitor, dAdo retarded DNA rejoining in a dose and time dependent manner. The inhibition required dAdo phosphorylation. Over an 8-hr period, 10 microM dAdo gradually rendered peripheral blood lymphocytes incompetent for DNA repair. Among several other compounds tested, 2-chlorodeoxyadenosine, an ADA resistant dAdo congener with anti-leukemic and immunosuppressive activity, was the most powerful inhibitor of DNA repair, exerting significant activity at concentrations as low as 100 nM. Both dAdo and 2-chlorodeoxyadenosine blocked unscheduled DNA synthesis in irradiated resting lymphocytes, as measured by [3H]thymidine uptake. On the basis of this and other data, we suggest that quiescent peripheral blood lymphocytes break and rejoin DNA at a slow and balanced rate. The accumulation of dATP progressively retards the DNA repair process and thereby fosters the time-dependent accretion of DNA strand breaks. By inhibiting DNA repair, dAdo, 2-chlorodeoxyadenosine and related compounds may substantially potentiate the toxicity of DNA damaging agents to normal and malignant lymphocytes.     

The effect of natural and recombinant leukocyte interferons on DNA repair and the formation of chromosome aberrations induced by N-methyl-N'-nitro-N-nitrosoguanidine

The anticlastogenic action of natural leukocyte and recombinant (alpha 2) interferons was studied in human lymphocyte cultures treated with N-methyl-N'-nitro-N-nitrosoguanidine. The criteria of cell viability, proliferation, chromosome aberrations, frequency of micronucleus formation, formation and repair of DNA breaks were used for estimation of interferons activity. Reduction of the induced chromosomal aberrations was obtained in cells pretreated with interferons. The protective effect of natural leukocytic interferon was more expressed as compared with the effect of recombinant (alpha 2) interferon. The natural interferon was also more efficient than the recombinant one in DNA breaks formation and repair.     

Bleomycin-induced DNA repair synthesis in permeable human fibroblasts: mediation of long-patch and short-patch repair by distinct DNA polymerases

Treatment of permeable human fibroblasts with bleomycin elicits DNA repair synthesis that is only partially sensitive to aphidicolin, an inhibitor of mammalian DNA polymerases alpha and delta. Inhibition of long-patch repair synthesis by omission of the three unlabeled deoxyribonucleoside triphosphates (dNTPs) selectively eliminates the aphidicolin-sensitive component. The majority of this residual aphidicolin-resistant repair synthesis is contained in ligated patches as revealed by resistance to exonuclease III. Determination of repair patch length by bromodeoxyuridine-induced density shift under conditions where essentially all of the repair synthesis is sensitive or resistant to aphidicolin yielded values of approximately 20 and 4 nucleotides per patch, respectively. On the basis of these data and the relative sensitivity of bleomycin-induced repair synthesis to N2-(p-n-butylphenyl)-2'-deoxyguanosine 5'-triphosphate (BuPdGTP), 2',3'-dideoxythymidine 5'-triphosphate (ddTTP), and N-ethylmaleimide (NEM), long-patch repair is attributed to DNA polymerase delta and short-patch repair to DNA polymerase beta.     

Suppression of UV-induced apoptosis by the human DNA repair protein XPG

The severe xeroderma pigmentosum/Cockayne syndrome (XP/CS) syndrome is caused by mutations in the XPB, XPD and XPG genes that encode the helicase subunits of TFIIH and the 3' endonuclease of nucleotide excision repair (NER). Because XPB and XPD have been implicated in p53-mediated apoptosis, we examined the possible involvement of XPG in this process. After ultraviolet light (UV) irradiation, primary fibroblasts of XP complementation group G (XP-G) individuals with CS enter apoptosis more readily than other NER-deficient cells, but this is unlinked to unrepaired damage. These XP-G/CS cells accumulate p53 post-UV but they fail to accumulate the 90/92 kDa isoforms of Mdm2 and their cellular distribution of Mdm2 is impaired. Apoptosis levels revert to wild type, Mdm2 90/92 kDa isoforms accumulate, and Mdm2 regains its normal post-UV nuclear location in transduced XP-G/CS cells expressing wild-type XPG, but not an XPG catalytic site mutant. These results suggest that XPG suppresses UV-induced apoptosis and that this suppression, most simply, requires its endonuclease function.     

Untangling the relationships between DNA repair pathways by silencing more than 20 DNA repair genes in human stable clones

Much effort has long been devoted to unraveling the coordinated cellular response to genotoxic insults. In view of the difficulty of obtaining human biological samples of homogeneous origin, I have established a set of stable human clones where one DNA repair gene has been stably silenced by means of RNA interference. I used pEBVsiRNA plasmids that greatly enhance long-term gene silencing in human cells. My older clones reached >500 days in culture. Knock-down HeLa clones maintained a gene silencing phenotype for an extended period in culture, demonstrating that I was able to mimic cells from cancer-prone syndromes. I have silenced >20 genes acting as sensors/transducers (ATM, ATR, Rad50, NBS1, MRE11, PARG and KIN17), or of different DNA repair pathways. In HeLa cells, I have switched off the expression of genes involved in nucleotide excision repair (XPA, XPC, hHR23A, hHR23B, CSA and CSB), nonhomologous end-joining (DNA-PKcs, XRCC4 and Ligase IV), homologous recombination repair (Rad51 and Rad54), or base excision repair (Ogg1 and Ligase III). These cells displayed the expected DNA repair phenotype. We could envisage untangling the complex network between the different DNA repair pathways. In this study, no viral vehicles, with their attendant ethical and safety concerns, were used.     

DNA repair replication by soluble extracts from human lymphoid cell lines

A system is described in which extracts from human cells can perform repair replication on DNA damaged by ultraviolet light or chemical carcinogens. Whole cell extracts from lymphoid cell lines are incubated with damaged plasmid DNA circles at 30 degrees C in the presence of ATP and the four deoxynucleoside triphosphates. Repair synthesis is monitored by the incorporation of alpha-32P-dATP into closed circular plasmid molecules. Analysis of the time course of the reaction suggests that the slowest step in repair is incision, rather than polymerization or ligation. The size of repair patches inserted into ultraviolet-irradiated DNA during a reaction was estimated by substitution of thymidine triphosphate with 5-bromodeoxyuridine triphosphate and sedimentation in alkaline cesium chloride gradients. Patches with heterogeneous sizes of less than 120 bases were observed.     

Comparison of the antiviral action of different human interferons against DNA and RNA viruses

Abstract Five different interferon preparations were compared for their antiviral activity against Herpes simplex virus type 1 (HSV-1) and several RNA viruses. The interferons used were: interferon α from human buffy coats, interferon β from human fibroblasts, interferon γ from human lymphocytes after stimulation with phytohemagglutinin (PHA), lymphoblastoid interferon from Namalva cells IFN-α (Ly) and cloned α ₂ interferon produced by Escherichia coli containing the human gene for interferon α ₂. All preparations were able to protect monolayers of HeLa cells against HSV-1 infection when low multiplicities were used. The five IFN preparations were also tested against encephalomyocarditis (EMC) virus, poliovirus and vesicular stomatitis virus (VSV).