Inhibition by caffeine of post-replication repair in Chinese hamster cells treated with 7-bromomethylbenz(a)anthracene: enhancement of toxicity, chromosome damage and inhibition of ligation of newly-synthesized DNA

Potentiation of sulphur mustard or cisplatin-induced toxicity by caffeine in Chinese hamster cells correlates with formation of DNA double-strand breaks during replication on a damaged template

Caffeine inhibited the elongation of nascent DNA and induced breaks in the template DNA of sulphur mustard-treated Chinese hamster cells. The sizes of template and nascent DNAs, as indicated by alkaline sucrose gradient sedimentation, were similar suggesting that incision of template DNA occurred opposite gaps formed in nascent DNA by the action of caffeine, forming, effectively, double-strand breaks in DNA. Double-strand break formation was demonstrated, by means of elution of labelled DNA through polycarbonate filters at neutral pH, in both sulphur mustard- and cisplatin-treated cells when they were incubated in the presence of caffeine for 24 h. Double-strand breaks were only formed in that DNA which had been replicated in the presence of caffeine after treatment with sulphur mustard or cisplatin. Non-toxic concentrations of cycloheximide abolished the potentiation by caffeine of sulphur mustard-induced toxicity to Chinese hamster cells and at the same time abolished the formation of the low molecular weight nascent DNA, and as a consequence of its inhibitory effect on DNA synthesis, and the formation of double-strand breaks in DNA. Potentiation of the lethal and clastogenic effects of genotoxic agents by caffeine is therefore due to effects on the rate and mode of DNA synthesis which lead finally to double-strand breaks in DNA.

Heat-induced inhibition of DNA synthesis in L5178Y-S cells is reversed by caffeine

Heating L5178Y cells for 15 min at 43 degrees C caused a decrease in [3H]thymidine incorporation, which could be reversed by post-treatment with 0.75 mM caffeine in an L5178Y-S (radiation-sensitive, heat-resistant) but not in an L5178Y-R (radiation-resistant, heat-sensitive) strain. The reversal was accompanied by a sparing effect of the treatment: survival of L5178Y-S cells increased by a factor of 1.5. The effect of combined (heat + caffeine) treatment of L5178Y-R cells was cumulative.

Further studies on tetracycline-induced mutation in V79 Chinese hamster cells

The interaction between ultraviolet light and tetracycline in producing cell killing and mutation has been studied in V79 Chinese hamster cells. It has been established that these agents act independently of each other. Cycloheximide altered the response to tetracycline in the fractionation experiment: when cycloheximide was not present, fractionation of TC treatment resulted in a higher mutation yield but no change in survival level; in the presence of cycloheximide, however, mutation was greatly reduced but survival increased. The results were taken to indicate that for tetracycline action to take place, de novo protein synthesis during tetracycline treatment was necessary. Caffeine had no influence on tetracycline-induced lethality or mutagenicity. This observation was considered to suggest that tetracycline did not affect cellular repair processes.

Cytotoxicity of monofunctional alkylating agents. Methyl methanesulfonate and methyl-N'-nitro-N-nitrosoguanidine have different mechanisms of toxicity for 10T1/2 cells

N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and methyl methanesulfonate (MMS) are directly active alkylating agents that methylate cellular macromolecules by SN1 and SN2 mechanisms, respectively. These two chemicals produce similar types of alkylation products in DNA and a similar level of total alkylations on a molar basis, but strikingly different proportions of alkylations of ring oxygen atoms of purines and pyrimidines. Because of this attribute, they have been used in combination to attempt to determine which types of alkylation products are responsible for mutation, transformation, and toxicity. Studies have suggested that the mutation rates produced by these and similar chemicals in cells surviving toxicity correlate well with the number of methyl adducts at the O6 position of guanine, but that cytotoxicity (reduced colony-forming efficiency) does not correlate with any single adduct or with the total level of alkylation of DNA. In this study we have investigated the cytotoxic mechanisms of MNNG and MMS in synchronized 10T1/2 cells, using colony-forming ability as a measure of toxicity. Both MNNG and MMS cause dose-dependent reduction in the ability of 10T1/2 cells to produce colonies of more than 50 cells after 2 weeks in culture. MNNG is about 100-fold more toxic than MMS on a molar basis. As indicated by the inability of cells to exclude trypan blue, MMS kills a fraction of the population of treated 10T1/2 cells after a 30-min exposure; the fraction of cells that excludes trypan blue is correlated with dose of MMS and with colony-forming efficiency. Neither the fraction of cells that is permeable to trypan blue nor the relative colony-forming efficiency is affected by the phase of the cycle when 10T1/2 cells are treated with MMS. Furthermore, MMS toxicity for 10T1/2 cells is not potentiated by caffeine, MMS treatment does not delay progress of S phase, and cells that survive acute membrane toxicity complete the cell cycle without significant delay. In contrast, MNNG treatment produces toxicity that is maximal when 10T1/2 cells are exposed during the S phase and the effect is potentiated by caffeine. MNNG treatment delays DNA replication and this delay is reversed by caffeine. In sharp contrast to 10T1/2 cells treated with MMS, MNNG-treated cells are not made permeable to trypan blue, but are blocked in their ability to proliferate. These observations indicate that MNNG and MMs kill 10T1/2 cells by drastically different mechanisms, MNNG producing toxicity mainly by preventing chromosome replication and MMS producing toxicity mainly by damaging cell membranes.

Survival and mutation of Chinese hamster cells after ultraviolet irradiation and caffeine treatment

UV-irradiated Chinese hamster cells on post-irradiation treatment with caffeine in growth medium for 24 h gave rise to biphasic UV-survival curves. At caffeine concentrations between 0.001 and 0.1 mM, control and caffeine-grown cells had similar survival curves initially from 0 to 30 J/m2. At fluences greater than 30 J/m2, there was effectively only little further killing of caffeine-grown cells beyond that observed at 30 J/m2. At concentrations of caffeine greater than 0.5 mM, there was a gradual sensitization in the early part of the survival curve with increasing caffeine concentrations; but at fluences greater than 30 J/m2, the slopes in the survival curves decreased very much. It has been proposed that the initial sensitization observed at low UV fluences is due to the suppression of post-replication repair by caffeine. After high fluences of UV exposures in these excision-deficient cells, in the presence of caffeine, the possibility of an induced 'SOS'-like repair process has been suggested. This suggestion was supported by the observation that caffeine increased the yield of the UV-induced 8-azaguanine-resistant mutants only for the cell population exposed to UV fluences greater than 30 J/m2.

The effects of bifunctional alkylating agents on DNA synthesis in sensitive and resistant Yoshida cells

The effects of the alkylating agents methylene dimethanesulphonate (MDMS), sulphur mustard (SM) and methyl methanesulphonate (MMS) on DNA synthesis have been investigated in Yoshida sarcoma cell lines sensitive (YS) and resistant (YR) to these agents. Measurement of overall DNA synthesis by [3H]thymidine uptake into DNA indicated that both MDMS and SM, but not MMS, induced a greater depression of DNA synthesis in YS cells than in YR cells. Analysis of the sizes of newly synthesised strands of DNA by pulse labelling and alkaline sucrose gradient sedimentation revealed that MDMS inhibited the initation of new replicons and also reduced the size of new DNA strands. The relative extents of inhibition of replicon initation in MDMS-treated YS and YR cells were similar and reduced by caffeine while YR cells were comparatively less sensitive to the effect of MDMS on the sizes of DNA molecules synthesised. Similarly, YR cells apparently synthesised longer DNA strands following SM treatment compared to YS cells suggesting an association between the size of DNA molecules synthesized and the sensitivity of Yoshida cells to bifunctional alkylating agents.

Failure of caffeine to influence induced mutation frequencies and the independence of cell killing and mutation induction in V79 Chinese hamster cells

Using V79 Chinese hamster cells and a replating assay, no effect of caffeine post-treatment on spontaneous or UV- or EMS-induced mutation frequencies to 8-azaguanine resistance was demonstrable. However, considerable potentiation of cell killing was observed. Previous reports that caffeine enhances induced mutation frequencies are explained by an artefact in the situ method used; a similar artefact may also explain the cumulative in situ mutation dose-response curves. Furthermore, the relationship between mutation induction and dose has been shown to be qualitatively distinct from that between cell killing and dose. These differences suggest that cell killing and mutation induction are mediated via independent mechanisms and that pre-mutational lesions may be qualitatively distinct from pre-lethal lesions.

Caffeine inhibits excision of 7-bromomethylbenz (a) anthracene-DNA adducts from exponentially growing but not from stationary phase Chinese hamster cells

Excision of 7-bromomethylbenz(a)anthracene (7-BMBA)-DNA adducts from exponentially growing cultures of Chinese hamster V79-379A cells followed logarithmic kinetics with a half of approximately 20 hrs. Post-treatment incubation in the presence of a sub-toxic concentration of caffeine markedly reduced this loss. Caffeine brought about a concomitant increase in overall DNA synthetic rate in treated exponential cultures. Excision in stationary, non-DNA-replicating cultures, was slower and caffeine did not affect this reduced rate of excision. These findings lend support to a previous proposition that the caffeine-induced inhibition of elongation of nascent DNA on a template containing chemical lesions results in an interference with the excision repair mechanism that removes these lesions.

Excision of hydrocarbon-DNA adducts and consequent cell survival in normal and repair defective human cells

Survival curves for normal human cells and xeroderma pigmentosum variant cells (XP4BE) after ultraviolet radiation were indistinguishable. In comparison, cells from xeroderma pigmentosum complementation group A (XP12BE) were very sensitive to ultraviolet radiation. Complementation group C (XP2BE) cells were almost as sensitive as group A cells. These survival phenomena parallel to known unscheduled DNA synthesis responses of these cells to ultraviolet radiation, which, compared with normal cells, are: XP4BE, 100%; XP2BE, 20%; XP12BE, 2%. The relative capacities of these cells to excise 7-bromomethylbenz[a]anthracene-DNA adducts and to survive treatment with the carcinogen were similar to the responses to ultraviolet irradiation, except that the XP2BE cell line both excised and survived this damage far better than anticipated from its response to ultraviolet irradiation. Moreover, whilst in the normal cells and variant cells the ratio of hydrocarbon-adenine adduct to hydrocarbon-guanine adduct remaining in DNA decreased notably with excision, this ratio did not change significantly with excision in the XP2BE cell line. The relationship between greater excision capacity and increased cell survival in the experiments with the chemical carcinogen indicates that the unexcised damage is responsible for the cell-killing action of this agent. The different relative repair and survival responses of these cell lines to ultraviolet irradiation on the one hand, and to 7-bromomethylbenz[a]anthracene chemical carcinogen treatment on the other, indicate that in at least one of these cell lines (XP2BE), and possibly in all the lines, different cellular mechanisms are involved in the repair of DNA damage resulting from ultraviolet irradiation and that resulting from the chemical carcinogen treatment.

Increased sensitivity of UV-repair-deficient human cells to DNA bound platinum products which unlike thymine dimers are not recognized by an endonuclease extracted from Micrococcus luteus

We have studied the response of human cells in culture to cis platinum[II] diammine dichloride (cis Pt[II]) induced DNA damage. The survival data, measured as a function of cis Pt[II] dose were similar in a normal cell line (Human foetal lung) compared to a UV-sensitive, thymine dimer excision repair-deficient cell line (Xeroderma pigmentosum). However, there was a marked difference between the two cell lines when binding to DNA was plotted against dose of cis Pt[II] given for 1 h. When these findings were expressed as cell survival versus binding to DNA, a 4.1--fold difference between the slopes of the survival curves for the two cell lines was obtained. These findings are consistent with the notion that normal cells are able to excise cis Pt[II] induced damage from their genome and thus increase their ability to survive as compared to excision-deficient cells. An endonuclease preparation from Micrococcus luteus is able to recognise UV damage in DNA, but did not recognise cis Pt[II] induced damage. These results possibly indicate differences in the pathways of repair of damage caused by the two agents.

Caffeine-inhibited DNA repair in 7-bromomethylbenz (a)-anthracene-treated Chinease hamster cells: formation of breaks in parental DNA and inhibition of ligation of nascent DNA as a mechanism for enhancement of lethality and chromosome damage

The cytotoxic and clastogenic effects of 7-bromomethylbenz(a)anthracene (7-BMBA) are potentiated by post-treatment incubation of cells in the presence of an non-toxic concentration of caffeine. Under these conditions caffeine inhibits the rate of ligation of newly-synthesised DNA and induces breaks in the template strand of DNA. It is proposed that endonucleolytic attack occurs at the site of lesions in the template strand of DNA and that a later step(s) of excision-repair is (are) inhibited by the presence of caffeine-induced 'gap' in the nascent DNA opposite these lesions.