Caffeine sensitization of cultured mammalian cells and human lymphocytes irradiated with gamma rays and fast neutrons: a study of relative biological effectiveness in relation to cellular repair

The sensitizing effects of caffeine were studied in baby hamster kidney (BHK-21) cells and human lymphocytes following irradiation with gamma rays and fast neutrons. Caffeine sensitization occurred only when log-phase BHK cells and mitogen-stimulated lymphocytes were exposed to the two radiations. Noncycling (confluent) cells of BHK resulted in a shouldered survival curve following gamma irradiation while a biphasic curve was obtained with the log-phase cells. Survival in the case of lymphocytes was estimated by measurement of [3H]thymidine uptake. Prestimulated lymphocytes exhibited a biphasic thymidine uptake curve while the postirradiation-stimulated lymphocytes resulted in an exponential curve. Fast neutrons reduced both shoulder and tail of survival curves representing resistant fractions of cell populations. Consequently, the relative biological effectiveness (RBE) of fast neutrons was found to be greater at survival levels corresponding to the resistant portions of the survival curves (shoulder or resistant tail). In both cell types, no reduction in RBE was observed when caffeine was present, because caffeine affected both gamma and neutron survival by the same proportion.

Repair of potentially lethal damage in yeast induced by fast neutrons

Survival curves of 3 diploid (D7) yeast strains: one wild-type, one deficient in excision of pyrimidine dimers (UV-sensitive) and one blocked in DNA double-strand-break repair (X-ray-sensitive), were compared after irradiation with cyclotron-produced fast neutrons. It was observed that both the UV-sensitive (rad3/rad3) and the X-ray-sensitive (rad52/rad52) mutants were more sensitive to neutrons than the wild-type. The role of DNA double-strand-breaks in neutron-induced cell death was further studied by comparing the relative sensitivity of the rad52/rad52 mutant to gamma-rays and fast neutrons. A comparison of the dose modification factors revealed that the deficiency in DNA double-strand-break repair did not make the yeast cells more sensitive to neutrons than to photons, which suggests that lesions of a different type may also be produced by neutrons. Survival curves obtained upon immediate plating and after delayed plating of neutron-irradiated cells showed that all 3 yeast strains were efficient in liquid holding recovery. The role of different repair pathways in cellular recovery from neutron-induced lethal damage is discussed.

Application of urine mutagenicity to monitor coal liquefaction workers

The Salmonella/microsomal assay was used to monitor workers' urine for mutagenicity as a potential indicator of human exposure to mutagens/carcinogens. Urine samples from 57 workers at a coal liquefaction pilot plant in Catlettsburg, Kentucky, were assayed for mutagenicity during work periods. Urine samples were collected twice during plant operations and once when the individuals were away from the plant for at least 48 h. In 7 individual smokers (5 operator/maintenance workers and 2 administrative staff workers) there was an indication of enhanced urine mutagenicity during work periods. Urine mutagenicity of nonsmokers from the pilot plant was significantly higher than that of an additional control group of nonsmokers from Lexington, Kentucky. While cigarette smoking was the major factor affecting urine mutagenicity, no significant mutagenicity that could be directly attributed to the pilot plant workers' environment was evident.

Mechanisms of cisplatin (cis-diamminodichloroplatinum II)-induced cytotoxicity and genotoxicity in yeast

The antitumor drug, cisplatin (cis- diamminodichloroplatinum II), dissolved in both water and phosphate-buffered saline, was studied for its genotoxic and cytotoxic effects in the yeast, Saccharomyces cerevisiae. The results showed that the drug was both recombinagenic and mutagenic in the wild-type diploid strain D7. It was observed that both cytotoxicity and genotoxicity were greatly reduced when cisplatin was dissolved in phosphate-buffered saline compared to the aqueous solution. Cell survival analyses showed that the diploid strain (D7 rad 3), deficient in excision of UV-induced pyrimidine dimers or similar adducts, was hypersensitive to cisplatin. Another diploid strain (rad 52/rad 52), blocked in the repair of DNA double-strand breaks and recombination was also hypersensitive to the drug. Mitotic gene conversion was not observed in the rad 52/rad 52 diploid after the drug treatments, while it was reduced in the excision -deficient strain. Reverse mutations occurred in the excision-deficient strain (D7 rad 3), even at low doses of cisplatin. These results are discussed in relation to the possible mechanisms of cisplatin-induced cell death and genotoxicity.

Study of ultraviolet radiation and genotoxic effects of natural sunlight in relation to skin cancer in Saudi Arabia

Sunlight-related biological effects such as skin cancer, sunburning, and synthesis of vitamin D in the body have been found to be very low in people in the midregion of Saudi Arabia. The present studies were undertaken to measure the sunburning-carcinogenic ultraviolet light (UV) radiation (UV-B) in natural sunlight in the city of Riyadh (25 degrees north latitude). The average noontime incidence of UV-B, measured with a sunburn UV-meter, was found to be between 1.9 and 3.4 sun units/hr in the months of March to May. Concomitantly with the UV measurements, a bioassay using a wild-type and an excision repair-deficient diploid strain of the yeast, Saccharomyces cerevisiae, was carried out to detect both lethal and genotoxic effects (mutation and mitotic gene conversion) of the sunlight. Exposure of the yeast cells to sunlight 30 to 180 min resulted in a significant level of cell death and a dose-dependent induction of mutations and mitotic gene conversion. The use of a Mylar filter cutting off virtually all of the wavelengths below 312 nm greatly reduced the lethal and genotoxic effects of sunlight. The results of UV measurements and biological studies suggest that an appreciable amount of potentially carcinogenic short UV wavelengths is present in sunlight in the Riyadh area. Therefore, factors other than the lack of biologically significant UV radiation in sunlight appear more likely to be responsible for the reduced incidence of sunburning and skin cancer in this geographical area.

Parameters affecting the mutagenic activity of cigarette smokers' urine

Cigarette smoke is a complex chemical mixture which displays a number of biological activities including mutagenesis. The absorption of smoke components by the lungs results in their systemic distribution, which then leads to the presence of mutagens in the urine. The detection of mutagenic substances in the urine by short-term bioassays has been suggested for use in monitoring human exposure to various carcinogenic/mutagenic substances. In this connection, we felt that further studies on the mutagenic activity present in cigarette smokers' urine were needed. In the present studies, the dose-response relationship between the number of cigarettes smoked versus urine mutagenic activity and the persistence of mutagens in the urine after the cessation of smoking have been examined. The studies were carried out on a group of smokers and non-smokers in Lexington, Kentucky. Although a dose-response relationship (urine mutagenicity versus the number of cigarettes smoked per day) was not observed in the group of smokers assayed, a linear dose-response relationship was detected in an individual smoker who smoked a varying number of cigarettes over eight successive days. In another smoker who ceased smoking, mutagens were detected in the urine for one week. The urine mutagenicity data from a group of smokers and non-smokers should prove useful in extending this technique to the monitoring of other environmental exposure.

Co-mutagenic effects of 2-aminoanthracene and cigarette smoke condensate on smoker's urine in the Ames Salmonella assay system

Cigarette smoke condensate (CSC) and cigarette smoker's urine (concentrated) (UC) were tested alone and in combination with direct and indirect mutagens for histidine reversion in the Ames Salmonella assay system. While both CSC and smoker's urine showed some mutagenic activity upon metabolic activation with S9-mix, each of them in combination with the aromatic amine 2-aminoanthracene (2AA) exhibited a synergistic effect on mutagenicity. Such a synergistic effect was not found when these agents were combined with the direct mutagens, ethylmethane sulfonate (EMS) and methylmethane sulfonate (MMS), or the indirect mutagens, benzo[a]pyrene (BP) and 9,10-dimethyl-1,2-benzanthracene (DMBA), tested in this study, nor was the synergistic effect observed when 2AA was tested with urine from a non-smoker. The results, thus, reflected a specificity of the co-mutagenic action of factor(s) in cigarette smoke or smoker's urine and 2AA. The significance of co-mutagens in carcinogenesis has been discussed and the importance of investigating co-mutagenesis particularly in the case of suspected human exposure to multiple environmental agents has been emphasized.

Pure and mosaic clones--a reflection of differences in mechanisms of mutagenesis by different agents in Saccharomyces cerevisiae

The induction of pure and mosaic clones has been studied in haploid G1 cells of Saccharomyces cerevisiae. Following treatments with ultraviolet light, methyl methanesulfonate, ethyl methanesulfonate, nitrous acid, and N-methyl-N'-nitro-N-nitrosoguanidine, the relative proportions of pure mutant clones varied from 25 to 100% at comparable survival levels. Ultraviolet light and methyl methanesulfonate produced mainly pure mutant clones, whereas ethyl methanesulfonate and nitrous acid produced mainly mosaics at 59 to 100% survival levels. The ratio of pure to mosaic clones induced by nitrosoguanidine fell between these two classes. These results are consistent with a classification of mutagens on the basis of repair and replication-dependent mechanisms of mutagenesis in other organisms. Agents having actions similar to ultraviolet light may produce mainly pure clones through a pre-replicative process involving an error-prone DNA repair process. Others may produce mainly mosaic mutants due to the different nature of DNA lesions which may require a replication-dependent process for fixation of mutations. Preliminary data from combined treatments of mutagens belonging to two different classes (i.e. ultraviolet light and nitrous acid) suggest the possibility of an interaction between these agents, resulting in a higher proportion of pure clones, possibly due to an inducible process. Studies of induced frequencies of pure and mosaic clones may be useful in the characterization of mutagens with functional differences.

Effect of tumor promoters on ultraviolet light-induced mutation and mitotic recombination in Saccharomyces cerevisiae

Recently, it has been suggested that mitotic recombination is involved in tumor promotion. On this basis, one might expect tumor promoters to be recombinagenic. D7 is a diploid strain of yeast in which both mutation and mitotic recombination can be measured. We have used this strain to assay the known tumor promoters, iodoacetate, anthralin, and 12-O-tetradecanoylphorbol-13-acetate, and the cocarcinogen, catechol, for mutagenicity, recombinagenicity, and the ability to enhance ultraviolet light (UV)-induced genetic events. In the absence of preirradiation with UV, iodoacetate was found to be recombinagenic whereas catechol was mutagenic; however, in both cases, the effects were small. Iodoacetate, anthralin, and catechol potentiated UV-induced mitotic crossing-over, aberrant colony formation, and mutation, while catechol also increased UV-induced gene conversion. We were unable to detect any mutagenic or recombinagenic effect of 12-O-tetradecanoyl-phorbol-13-acetate in either whole cells or spheroplasts. Our results do not indicate any consistent correlation between tumor-promoting activity and the ability of an agent to induce mitotic recombination in yeast. However, the ability to potentiate UV-induced mutation and mitotic recombination may reflect the cocarcinogenic activity of certain promoters.

Recombinagenic and mutagenic effects of sunlamp (UV-B) irradiation in Saccharomyces cerevisiae

Genetic effects of pyrex-filtered sunlamp irradiation (primarily UV-B, lambda approximately 280-320 nm which is present in natural sunlight) were studied in diploid and haploid yeast strains designed to monitor the incidence of mitotic crossing over, mitotic gene conversion and mutations. Exposure to UV-B was found to be very effective in inducing all three types of genetic endpoints. These effects could be observed even after UV-B treatments resulting in no cellular inactivation. Comparative studies using filtered sunlamp radiation and germicidal UV radiation (lambda approximately 254 nm) indicated that, at an equivalent survival level (including exposures causing little or no cell death), UV-B induced more gene convertants and that the kinetics of mutations induced by the two lights are demonstrably different.

Mutagenicity and recombinogenicity of daunomycin in Saccharomyces cerevisiae

The antineoplastic antibiotic daunomycin was tested for mutagenic and recombinogenic effects in haploid and diploid strains of Saccharomyces cerevisiae. This antibiotic failed to induce mutations to lysine and histidine independence in the haploid strain, XV 185-14C, carrying the auxotrophic markers lysl-1 and hisl-7. On the contrary, daunomycin did induce aberrant colonies, including reciprocal products of mitotic crossing over in the diploid strain D7. The results suggest that while daunomycin may be recombinogenic, its mutagenic effects, if any, are either highly specific or negligible. The significance of these results in evaluating the mutagenic/carcinogenic potential of antitumor agents is discussed.

Mutagenic and recombinogenic effects of the antitumor antibiotic anthramycin

Anthramycin, one of the pyrrolo(1,4)benzodiazepine antibiotics with potent antitumor activity, was tested for its effects on a number of genetic parameters. The results show that this antibiotic is nonmutagenic in the Ames strains of Salmonella typhimurium while mutagenic in only one and antimutagenic in the rest of the genes tested in the eukaryotic organism Saccharomyces cerevisiae. The antibiotic is, however, a potent recombinogen inasmuch as it induced mitotic crossing over, mitotic gene conversion, and possibly other chromosomal alterations in a diploid strain of S. cerevisiae. These studies emphasize the need for a battery of test systems including eukaryotic organisms to detect the genetic activity of certain antitumor drugs. The importance of considering data distinguishing between highly mutagenic and poorly mutagenic cancer chemotherapeutic agents is also discussed.

Genetic activity of bleomycin: differential effects on mitotic recombination and mutations in yeast

The genetic effects of the antitumor antibiotic, bleomycin were studied in different strains of Saccharomyces cerevisiae. It was observed that the drug induced a high frequency of mitotic recombination and gene conversion. In contrast, it produced only a few mutations from adenine independence to adenine dependence and histidine dependence (a missense mutant) to histidine independence. In the strains carrying ochre-suppressible auxotrophic markers, no prototrophs were induced by this drug. The results indicating the specific activity of bleomycin are discussed and, in this connection, the usefulness of yeast as a test organism in mutagenicity screening is emphasized.

Caffeine enhancement of radiation killing in different strains of Saccharomyces cerevisiae

Haploid and diploid wild type strains, and three classes of radiation-sensitive mutants of Saccharomyces cerevisiae were tested for enhancement of UV-inactivation by caffeine in growth medium. In addition, the sensitizing effect of caffeine was studied in a haploid and a diploid wild type strain after gamma-irradiation. The drug sensitized the UV-irradiated cells of all strains except those reported to be only slightly UV-sensitive but highly sensitive to ionizing radiation. After gamma-irradiation, no caffeine-enhancement of killing was observed in stationary phase cells of either the haploid or the diploid strain. However, log-phase cells of both strains were partially sensitized. The results of both sets of experiments suggested that caffeine interferes with a recombinational repair occurring in cells in S or G2 phase.

Induction of mutations by chemicals and gamma rays in mutants of yeast refractory to UV-mutagenesis

Radiation-sensitive mutants of Schizosaccharomyces pombe, known to be refractory to UV-mutagenesis, were tested for mutability caused by treatments with chemicals and gamma rays. One such mutant (rad3) was studied over a wide range of UV doses to compare the kinetics of its mutational response to that of the wild type. All such comparisons were carried out using a forward mutation system. Data show that, unlike UV, the chemical mutagens as well as gamma rays produced mutations (although at reduced frequency), in the strains of S. pombe tested, indicating the existence of an additional mechanism(s) for chemical and gamma ray induced mutations. These observations are discussed as these relate to the pathways for repair of mutational damage in yeast.

UV-induced lethal sectoring and pure mutant clones in yeast

The induction of lethal sectoring and pure mutant clones by ultraviolet light has been studied in a homogeneous G1 population of Saccharomyces cerevisiae grown in a normal growth medium. At the lowest UV dose of 250 ergs, which corresponds to a shoulder in the survival curve, all mutants appeared as pure clones. At higher doses the frequency of mosaic mutants progressively increased. These results indicate a relationship between the highest frequency of complete mutants and the maximum repair activity. In addition, the frequency of lethal sectoring at all doses tested was too low to account for the origin of pure mutant clones.