Repair of DNA damage induced by gamma radiation and UV and gamma mimetics in virus-infected human cells

The method of chromatography of cell lysates on the columns with hydroxyapatite (HAP) and the method of ultracentrifugation of cell lysates in neutral sucrose gradient were used to study the mutagen-induced repair activity of human cells HEp-2 noninfected and chronically infected with measles and rubella viruses in order to determine the sedimentation properties of complexes containing DNA. Gamma-radiation, bleomycin, 4-nitroquinoline-1-oxide, and mitomycin C were used as DNA damaging agents. It was shown that the chronic infectious process inhibited repair of DNA damages induced by 4-nitroquinoline-1-oxide and mitomycin C and did not influence repair of DNA lesions caused by gamma-radiation and bleomycin.     

Growth-medium-dependent repair of DNA single-strand and double-strand breaks in X-irradiated Escherichia coli

The X-ray resistance of logarithmic phase cells of Escherichia coli K-12 is enhanced threefold by growth in rich medium versus minimal medium (N. J. Sargentini, W. P. Diver, and K. C. Smith, Radiat. Res. 93, 364-380, 1983). In this work, X-ray-induced DNA strand breaks were assayed by sedimentation in alkaline and neutral sucrose gradients to correlate the enhanced survival of rich-medium-grown cells with an enhanced capacity for DNA repair. While rich-medium-grown cells showed no enhanced capacity for repairing DNA single-strand breaks in buffer, i.e., fast, polA-dependent repair, they did show an enhanced capacity to repair both single-strand and double-strand breaks in growth medium, i.e., slow, recA-dependent repair. This enhanced capacity for DNA repair in rich-medium-grown cells was inhibited by rifampicin post-treatment, indicating the requirement for de novo RNA synthesis. Kinetic studies indicated that the repair of DNA double-strand breaks was a complex process. Relative to the sedimentation rate in neutral sucrose gradients of nonirradiated DNA, the sedimentation rate of X-irradiated DNA first changed from slow to very fast. Based on alkaline sucrose gradient sedimentation studies, all the strand breaks had been repaired during the formation of the very fast sedimenting DNA. With continued incubation, the sedimentation rate of the DNA on neutral sucrose gradients decreased to the normal rate.     

The formation and repair of DNA breaks in subpopulations of irradiated rat bone marrow cells

Using the method of viscosimetry of alkaline lysates it was shown that the saturable DNA repair system was present in rat bone marrow cells which was described by a complex kinetics. Isokinetic fractionation in a sucrose and dextran gradient permitted to isolate three fractions differing in the ratio between differentiated cells and cells capable of division. The estimation of the postirradiation DNA repair kinetics in these cells revealed a comparatively high level of repair in normocytes which confirmed the presence of the relationship between the level of DNA repair in cells and their proliferative potency.     

DNA damage and repair in chick embryo cells following X-irradiation in vitro as compared to mammalian cells - biochemical and physico-chemical investigations

Summary Brain cells (b-cells) and liver cells (l-cells) of the chicken embryo and thymic cells (t-cells) of the rat were X-irradiated in vitro at doses of 1.25–50 Gy. When compared to t-cells, b- and l-cells exhibited1) a lower stimulation of poly (adenosine diphosphate-ribose) transferase and unscheduled DNA synthesis following X-irradiation,2) an almost fivefold higher inhibition of semiconservative DNA synthesis,3) a less condensed chromatin,4) about fourfold higher threshold doses with regard to significant effects on nucleoid sedimentation and viscometry of alkaline cellular lysates, and5) an apparently two- to threefold lower DNA repair during a 30 min post-exposure repair period. The results suggest that the lower radiation sensitivity of chicken embryo cells is attributable to an initial mechanism of DNA repair and/or DNA protection which may be closely connected to minor chromatin compactness and higher intrinsic activities of repair enzymes.     

Length distributions of single-stranded DNA in Chinese hamster ovary cells.

The distribution of lengths of single-strand DNA in Chinese hamster ovary cells in the G₁ phase of the cell cycle has been observed for various conditions of cell lysis and incubation of the lysates. The method of analysis was band sedimentation through a self-generating density gradient, a technique developed originally for the analytical ultracentrifuge, but modified here for the preparative ultracentrifuge so that measurements of sedimentation coefficients could be made under conditions that minimize shearing of the single-stranded DNA. The effect of rotor speed dependence of the sedimentation coefficient is considered in developing the relation between the sedimentation coefficient and molecular weight for this technique.Special precautions were taken to ensure that complete separation of long single strands took place upon alkaline denaturation, to preclude the possibility of anomalous sedimentation due to interstrand entanglement. Bromodeoxyuridine was incorporated into the DNA in the last round of replication. Advantage was taken of the increased sensitivity to ultraviolet irradiation for the production of single-strand breaks in DNA strands substituted with bromodeoxyuridine. After irradiation the bromodeoxyuridine-substituted strand could be completely separated from the complementary strand in alkaline sedimentation profiles without any apparent breakage in the unsubstituted strand.The conditions of lysis, chosen to minimize the degradation of DNA in the lysates, included lysis at pH 9.3 with Pronase and lysis at high pH (10.8 and 12.0). Sedimentation analysis was performed at various time intervals after incubation at 4 °C or 37 °C. Lysis and incubation at pH 12.0 produced a continuous single-strand breakdown of the DNA in the lysate. Analysis of the sedimentation profiles indicates that these alkaline-induced breaks are randomly distributed. However, lysis and incubation at pH 10.8 and at pH 9.3 with Pronase produced stable sedimentation profiles with number-average molecular weights of 1.7 × 10⁸ and 6.0 × 10⁷, respectively. Analysis of the single-strand DNA sedimentation profiles for these lysates indicates that the distribution of lengths of single-stranded DNA is non-random, i.e. that the distributions may represent regular subunits of chromosomal DNA structure. Suggestive evidence is presented that the approximately 60-μm units are structurally alternated in the two chains. The possible origin of the discontinuities between the subunits is also discussed.     

Absence of swiveling at sites of intercalator-induced protein-associated deoxyribonucleic acid strand breaks in mammalian cell nucleoids

The sedimentation of DNA-nuclear protein complexes in 1.9 M salt-neutral sucrose gradients (nucleoid sedimentation) was used to examine the effects of the DNA intercalator 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) on mouse leukemia cell DNA. Mild detergent cell lysis and neutral pH make nucleoid sedimentation an extremely gentle, but sensitive, method to detect DNA scission. DNA breaks reduce the compaction of nucleoids and slow their sedimentation. Nucleoids from m-AMSA-treated cells sedimented as did those from untreated cells, indicating no detectable m-AMSA-dependent alterations in compaction despite an apparent underlying DNA break frequency of approximately 3 per 10(6) nucleotides, as measured by alkaline elution with proteinase. Mild proteinase digestion of cell lysates prior to nucleoid sedimentation unmasked some, but not all, of the underlying breaks. The frequency of DNA-protein cross-links in nucleoids from cells treated with m-AMSA was comparable to the single-strand break frequency produced by m-AMSA in whole cells. These results indicate that m-AMSA-induced DNA-protein cross-links conceal DNA breaks so as to prevent swiveling around the breaks within the nucleoids. This unique sort of DNA scission is consistent with the involvement of topoisomerases in the DNA breaks elicited by intercalators in mammalian cells.     

Sedimentation Analysis of DNA from Irradiated and Unirradiated L Cells

DNA, released from unirradiated mouse L-cells gently lysed in a thin layer of 2% sucrose on top of an alkaline sucrose gradient, was found to sediment in a narrow band with a sedimentation coefficient of about 500S. Exposure of cells to increasing doses of X-rays (89-712 rads) continuously reduced the DNA sedimentation velocity until, after about 890 rads, the DNA appeared in a narrow peak with a sedimentation coefficient of approximately 180S. As the dose given to cells was increased beyond 890 rads, the sedimentation coefficient of the DNA released continued to decrease and the sedimentation profiles now broadened in a manner consistent with the random production of single-strand breaks in the DNA. The DNA released from unirradiated cells (500S) is thought to be loosely aggregated and only partially single stranded. It is presumed that cells exposed to low doses of radiation release DNA with marked reductions in sedimentation coefficient because single-strand breaks produced in the DNA aid the alkaline denaturation process. By using the system to be described, it has been possible to demonstrate DNA repair (rejoining of X-ray-induced single-strand breaks) during postirradiation incubation of cells given doses as low as 400 rads.     

DNA-protein cross-linking by ultraviolet radiation in normal human and xeroderma pigmentosum fibroblasts.

DNA-protein cross-linking by ultraviolet radiation was measured in human fibroblasts by an adaptation of the method of DNA alkaline elution. To measure cross-linking, a controlled frequency of DNA single-strand breaks was introduced by exposing the cells to a low dose of X-ray at 0 degrees C prior to analysis by alkaline elution. The effect of prior exposure of the cells to ultraviolet radiation was to reduce the rate and/or extent of DNA elution from X-irradiated cells. This effect was attributed to DNA-protein cross-linking, since the effect was reversed by treatment of the cell lysates with proteinase-K. Cross-linking in normal human fibroblasts occurred immediately after ultraviolet irradiation, prior to the appearance of DNA single-strand breaks due to excision repair. Upon incubation of normal cells after exposure, to ultraviolet radiation, the cross-linking was partially repaired. In xeroderma pigmentosum cells, cross-links appeared as in normal cells, but there was no repair. Instead, the extent of cross-linking appeared to increase upon incubation after ultraviolet irradiation.     

The relationship between molecular and cellular repair from potentially lethal damage induced by N-methyl-N'-nitro-N-nitrosoguanidine in Chinese hamster V79 cells

The relationship between molecular and cellular repair from potentially lethal damage (PLD) induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was investigated in exponentially growing V79 Chinese hamster cells. We compared the repair processes by an alkaline sucrose sedimentation analysis and a colony formation assay. MNNG-treated cells were exposed to the conditioned medium (CM) from density-inhibited plateau-phase V79 cell cultures, as a post-treatment for the induction of PLD repair. When MNNG-treated cells were postincubated in CM, cell survival continuously increased for 18 h, and during this period, DNA replication was substantially suppressed. CM did not inhibit the rejoining of the single-strand breaks of parental DNA. Rather, parental DNA fragments sedimented more rapidly when postincubated in CM than in fresh medium. These data indicate that cellular recovery from MNNG-induced PLD increases in proportion to the resealing of MNNG-induced single-strand breaks of DNA during the suppression of DNA replication, suggesting that excision repair is involved in the PLD repair process.     

Structural studies of a membrane-bound acetylcholine receptor from Torpedo californica

We investigated the differential repair of DNA lesions induced by bifunctional mitomycin C, monofunctional decarbamoyl mitomycin C and ultraviolet irradiation in normal human, Xeroderma pigmentosum and Fanconi's anemia cells using assays for the survival of clone-forming ability, alkaline sucrose sedimentation and hydroxyapatite chromatography of DNA. Four FA cell lines exhibited about 5 to 15 times higher sensitivity to MC killing, despite normal resistance to u.v. and DMC, than did normal human cells. The XP cells, however, were highly sensitive to u.v. and DMC killings due to their deficiency in excision repair, but the cells unexpectedly had an almost normal capacity for surviving MC and repairing the MC interstrand cross-links.In experiments to determine the sedimentation velocity of the DNA in alkaline sucrose gradients, normal and XP cells showed evidence for single-strand cutting following MC treatment. The sedimentation velocity of the DNA covalently cross-linked by MC in an FA strain was 2.5 times faster than that of the untreated control, and remained unaltered during post-incubation due to the lack of half-excision⁴ of cross-links. However, FA cells, but not XP cells, had the normal ability to incise DNA with the DMC monoadducts. Hydroxyapatite chromatography revealed the reversibly bihelical property of MC cross-linked DNA after denaturation. Normal and XP cells lost such reversibility during post-MC incubation as the result of cross-link removal with first-order kinetics (half-life = 2 h). The three FA lines studied exhibited two- to eightfold reduced rates of cross-link removal than normal and XP cells, indicating a difference in the repair deficiency of the FA strain. Thus we have been led to conclude that FA cells may have different levels of deficiency in half-excision repair of interstrand cross-links induced by MC, despite having normal mechanisms for repair of u.v.-induced pyrimidine dimers and DMC monoadducts, and vice versa in XP cells.     

Repair of neocarzinostatin-induced deoxyribonucleic acid damage in human lymphoblastoid cells: possible involvement of apurinic/apyrimidinic sites as intermediates

Neocarzinostatin (NCS) induces repair in a xeroderma pigmentosum lymphoblastoid line deficient in the ability to repair DNA damage induced with (acetoxyacetyl-amino)fluorene. Repair was demonstrated by the induction of repair synthesis and by the disappearance of NCS-induced single-strand breaks and/or alkaline-labile sites in DNA. Estimation of NCS-induced repair patch size, based on the density shift induced in DNA by extensive shear after incubation of treated cells in medium with bromodeoxyuridine or by calculation from the extent of restoration of DNA sedimentation profiles in alkaline sucrose gradients and the amount of repair synthesis measured by the BND cellulose method, indicated that only a few nucleotides were inserted per repaired region. NCS-treated bacteriophage T7 DNA requires incubation with alkaline phosphatase to make it a substrate for DNA polymerase I. NCS-reacted T7 DNA, even after phosphatase treatment, is not a substrate for a DNA polymerase alpha obtained from human lymphoma cells. NCS-treated T7 DNA did serve as a substrate for the DNA polymerase alpha when incubated with an apurinic/apyrimidinic (AP) endonuclease with associated 5'-3'-exonuclease activity. The results suggest that NCS-induced AP sites could be intermediates for the in vivo repair synthesis.     

Further characterization of a cell-free system for measuring replicative and repair DNA synthesis with cultured human fibroblasts and evidence for the involvement of DNA polymerase alpha in DNA repair.

DNA repair synthesis can be specifically measured in osmotically opened, confluent cultured human fibroblasts after exposure to DNA damaging agents such that both induction and mediation of DNA repair synthesis can take place in this cell-free system. Alternatively, by utilizing osmotically shocked, log phase cells and altering the DNA precursors, pH and ionic strength, replicative DNA synthesis can be specifically monitored. Autoradiographic studies show that virtually all of the nuclei from the lysates of the confluent, UV-iradiated cells are lightly labeled in the fashion characteristic of DNA repair. By contrast, only a fraction of nuclei is labeled in a population of unperturbed, opened log phase cells and the labeling is heavy and characteristic of replicative synthesis. Furthermore, equilibrium density gradient sedimentation shows that DNA synthesis in lysates of log-phase cells is semiconservative, whereas that with UV-irradiated cells is repair synthesis. This open cell system has been used to study the enzymology of DNA repair. Thus, dideoxythymidine triphosphate, a specific inhibitor of DNA polymerases beta and gamma, does not inhibit either replicative or repair synthesis. By contrast, aphidicolin, a specific inhibitor of DNA polymerase alpha, inhibits DNA repair and replicative synthesis in both intact and permeabilized cells. Finally, phage T4 UV-exonuclease stimulates repair synthesis, but only when phage T4 UV-endonuclease is also added to the UV-irradiated nuclei.     

Differential responses of log and stationary phase human fibroblasts to inhibition of DNA repair by aphidicolin

The tetracyclic diterpenoid, aphidicolin, is an effective inhibitor of DNA repair in human cells following ultraviolet irradiation. This inhibition is very efficient in confluent resting cells but not in rapidly cycling cells as measured by (1) analysis of DNA single-strand breaks by alkaline sucrose sedimentation, (2) chromatographic analysis of pyrimidine-dimer removal, and (3) repair replication using CsCl density centrifugation. The inhibition is reversed by deoxycytidine or thymidine but not by deoxyadenosine or deoxyguanosine during the repair period. The data suggest that differences in deoxynucleoside triphosphate pools between cycling and confluent resting cells determine the different efficacies of the agent in these two situations.     

Chemical radiosensitization by misonidazole: production and repair of DNA single-strand breaks in Yoshida ascites tumor cells.

Formation of strand-breaks in DNA and its repair in Yoshida ascites tumor cells exposed to gamma radiation (100-400 Gy) in presence and absence of misonidazole (10 mM) were studied. The methodology involved pre-labelling of cellular DNA by 3H-thymidine during cell proliferation in rats, irradiation of cells in vitro and analysing sedimentation profile of DNA by ultracentrifugation in alkaline sucrose density gradients. Irradiation under euoxic conditions resulted in formation of about 1.5 times greater number of strand breaks as compared to those formed during irradiation under hypoxic conditions. Misonidazole (10 mM) by its presence along with the cells during irradiation under hypoxic conditions caused a 3-fold increase in the number of single strand breaks, but under euoxic conditions of irradiation the presence of misonidazole did not enhance the strand break formation. Incubation of cells irradiated in absence of misonidazole for 1 hr in tissue culture medium at 37 degrees C resulted in repair of substantial fraction of the strand breaks while there was no repair of the DNA strand breaks in cells irradiated in the presence of the chemical.     

Post replication repair in an excision defective strain of Escherichia coli following treatment with cis dichlorodiammineplatinum(II)

The size of the DNA synthetized after treatment of an excision defective E. coli strain with cis-dichlorodiammineplatinum(II) (cis-PDD) was examinated using sedimentation in alkaline sucrose gradients. DNA synthetized during a 10 minutes pulse after treatment with cis-PDD sediments with a molecular weight lower than control DNA from untreated cells. Post treatment incubation of the cells leads to an increase in the sedimentation rate of this DNA which approaches that of normal DNA. This last process is partially abolished in a uvr B5 rec B21 double mutant.These results suggest that single strand breaks or gaps are produced during treatment and are filled in during further reincubation as part of a post replication repair process.     

Application of alkaline sucrose gradient sedimentation to the study of DNA damage and its repair in mammalian cells treated with methylmethanesulfonate and 4-nitroquinoline-1-oxide.

KB cells and L cells were treated with methylmethanesulfonate (MMS) or 4-nitroquinoline-1-oxide (4 NQO) and the resulting damage to DNA and its repair were examined by sedimentation in an alkaline sucrose gradient. The sedimentation profiles obtained were found to be the resultant of a complex interrelationship between drug dosage, duration of the lysis period and the repair capacity of the cells. A systematic study of these variables was made which led to a plausible and useful interpretation of the sedimentation profiles. Both drugs produce two kinds of DNA modifications which show up as a single-strand breaks but affect the sedimentation profile in characteristic ways. One of these modifications which is quite alkali-labile can be studied using a 30-min lysis period. The other modification is less alkali-labile and can be studied using a long lysis period. Both KB cells and L cells can repair the former type of damage but only KB cells can repair the latter type of damage.     

Differentiation of apurinic/apyrimidinic sites and single-strand breaks in DNA by formamide- and alkaline-sucrose gradient sedimentation

The excision repair of DNA damaged by physical or chemical agents may produce either apurinic/apyrimidinic (AP) sites or single-strand breaks (SSB) in the DNA. Alkaline-sucrose gradient sedimentation and alkaline elution, techniques generally used for the study of DNA repair which depend upon high pH to denature the DNA, cannot differentiate between these possibilities. A simple method for the quantitative measurement of SSB in DNA which leaves any AP sites intact is presented. This method relies upon the separation by size of the fragments resulting from the denaturation of the DNA under neutral conditions by sedimentation through gradients of sucrose in formamide. By combining the use of both formamide- and alkaline-sucrose sedimentation methods, we can quantify both AP sites and SSB in DNA.     

Chemical oncogenesis in cultured mouse embryo cells in relation to the cell cycle.

Using the C3H/10T 1/2 CL8 line of mouse embryo fibroblasts and three different methods of obtaining cell cycle synchrony, namely arginine or isoleucine deficiency and release from postconfluence inhibition of growth, a sensitive phase for oncogenic transformation induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) has been found. This sensitive phase is located somewhere between the G1/S boundary and a point 4 hr prior to this marker. Methylation of cellular macromolecules by tritiated MNNG is not cycle-dependent in cells synchronized by arginine deficiency. The capacity of cells to repair DNA single strand breaks produced by MNNG was examined by alkaline sucrose sedimentation analysis in cells synchronized by arginine deficiency and treated with MNNG during phases of the cell cycle sensitive and insensitive to oncogenic transformation. Whereas DNA repair was found to be equally rapid in cells treated just before S phase (I), or just after commencement of DNA synthesis (III), transformation was maximal in I. By contrast, cells treated when blocked by arginine deficiency (II) repaired DNA slowly and were not sensitive to malignant transformation. Cells in I and II, which repaired DNA at very different rates, were equally sensitive to MNNG-induced lethality, while cells in III, which repaired DNA at the same rate as cells in I, suffered greater lethality. Thus, in this system it was concluded that there was no direct correlation between DNA repair, as measured by alkaline sucrose sedimentation analysis of prelabeled DNA, and malignant transformation or lethality produced by MNNG. In preliminary experiments malignant transformation induced by cytosine arabinoside (1-beta-D-arabinofuranosylcytosine, ara-C) has been found to occur mainly in S phase, indicating that diverse chemical oncogens may have different sites of action, or that activation of chemical oncogens is cell cycle-specific for some agents.     

Choloroquine inhibition of repair of DNA damage induced in mammalian cells by methyl methanesulfonate

Chloroquine (ClQ) inhibited the repair of DNA damage produced in cultured rat liver cells by methyl methanesulfonate (MMS). MMS caused fragmentation of single-strand DNA in alkaline sucrose gradients. Repair of the damage was followed by observing the restoration of the normal sedimentation pattern at intervals after treatment. Repair was significant by 7 h and nearly complete at 24 h. Addition of ClQ during the repair peiod markedly reduced the rate of repair. Also, ClQ increased the lethality of MMS, which could be due to the inhibition of repair. ClQ was found to inhibit protein synthesis, but the effect on repair is probably not due entirely to this action since comparable inhibition of protein synthesis by cycloheximide produced a lesser degree of delay in repair.     

Thermal enhancement of DNA damage by an alkylating agent in human cells

Human skin cells were incubated at various temperatures during and after treatment with methyl methanesulfonate and the number of single-strand breaks introduced into the cellular DNA then estimated by alkaline sucrose sedimentation. Elevation of temperature above 37° greatly enhanced damage to the DNA caused by methyl methanesulfonate. Inactivation of an essential step in the repair of DNA was indicated by the observation that rejoining of breaks in the DNA was halted above a critical temperature (about 41.5°). Enhancement of damage to DNA increased with temperature, especially above 42°. Similar results were obtained for Chinese hamster cells. A correlation of these results with cell viability is discussed.