Radiation-induced bystander effect and adaptive response in mammalian cells

Two conflicting phenomena, bystander effect and adaptive response, are important in determining the biological responses at low doses of radiation and have the potential to impact the shape of the dose-response relationship. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, we show here that non-irradiated cells acquire mutagenesis through direct contact with cells whose nuclei have been traversed with a single alpha particle each. Pretreatment of cells with a low dose of X-rays four hours before alpha particle irradiation significantly decreased this bystander mutagenic response. Results from the present study address some of the fundamental issues regarding both the actual target and radiation dose effect and can contribute to our current understanding in radiation risk assessment.

The “Pro-drug” RibCys Decreases the Mutagenicity of High-LET Radiation in Cultured Mammalian Cells

We are carrying out studies aimed at reducing the mutagenic effects of high-LET 56Fe ions and 12C ions (56Fe ions, 143 keV/microm; 12C ions, 100 keV/microm) with certain drugs, including RibCys [2-(R,S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)-thiazolidine-4(R)-carboxylic acid]. RibCys, formed by condensation of L-cysteine with D-ribose, is designed so that the sulfhydryl amino acid L-cysteine is released intracellularly through nonenzymatic ring opening and hydrolysis leading to increased levels of glutathione (GSH). RibCys (4 or 10 mM), which was present during irradiation and for a few hours after, significantly decreased the yield of CD59- mutants induced by radiation in AL human-hamster hybrid cells. RibCys did not affect the clonogenic survival of irradiated cells, nor was it mutagenic itself. These results, together with the minimal side effects reported in mice and pigs, indicate that RibCys may be useful, perhaps even when used prophylactically, in reducing the mutation load created by high-LET radiation in astronauts or other exposed individuals.

Genotoxic damage in non-irradiated cells: contribution from the bystander effect

It has always been accepted dogma that the deleterious effects of ionising radiation such as mutagenesis and carcinogenesis are due mainly to direct damage to DNA. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, it is shown here that non-irradiated cells acquire the mutagenic phenotype through direct contact with cells whose nuclei are traversed with 2 alpha particles each. Pre-treatment of cells with lindane, a gap junction inhibitor, significantly decreased the mutant yield. Furthermore, when irradiated cells were mixed with control cells in a similar ratio as the in situ studies, no enhancement in bystander mutagenesis was detected. Our studies provide clear evidence that genotoxic damage can be induced in non-irradiated cells, and that gap junction mediated cell-cell communication plays a critical role in the bystander phenomenon.

Radiation risk to low fluences of alpha particles may be greater than we thought

Based principally on the cancer incidence found in survivors of the atomic bombs dropped in Hiroshima and Nagasaki, the International Commission on Radiation Protection (ICRP) and the United States National Council on Radiation Protection and Measurements (NCRP) have recommended that estimates of cancer risk for low dose exposure be extrapolated from higher doses by using a linear, no-threshold model. This recommendation is based on the dogma that the DNA of the nucleus is the main target for radiation-induced genotoxicity and, as fewer cells are directly damaged, the deleterious effects of radiation proportionally decline. In this paper, we used a precision microbeam to target an exact fraction (either 100% or < or =20%) of the cells in a confluent population and irradiated their nuclei with exactly one alpha particle each. We found that the frequencies of induced mutations and chromosomal changes in populations where some known fractions of nuclei were hit are consistent with non-hit cells contributing significantly to the response. In fact, irradiation of 10% of a confluent mammalian cell population with a single alpha particle per cell results in a mutant yield similar to that observed when all of the cells in the population are irradiated. This effect was significantly eliminated in cells pretreated with a 1 mM dose of octanol, which inhibits gap junction-mediated intercellular communication, or in cells carrying a dominant negative connexin 43 vector. The data imply that the relevant target for radiation mutagenesis is larger than an individual cell and suggest a need to reconsider the validity of the linear extrapolation in making risk estimates for low dose, high linear-energy-transfer (LET) radiation exposure.

Gamma-ray mutagenesis studies in a new human-hamster hybrid, A(L)CD59(+/-), which has two human chromosomes 11 but is hemizygous for the CD59 gene

Kraemer, S. M., Vannais, D. B., Kronenberg, A., Ueno, A. and Waldren, C. A. Gamma-Ray Mutagenesis Studies in a New Human-Hamster Hybrid, A(L)CD59(+/-), which has Two Human Chromosomes 11 but is Hemizygous for the CD59 Gene. Radiat. Res. 156, 10-19 (2001). We have developed a human-CHO hybrid cell line, named A(L)CD59(+/-), which has two copies of human chromosome 11 but is hemizygous for the CD59 gene and the CD59 cell surface antigen that it encodes. Our previous studies used the A(L) and A(L)C hybrids that respectively contain one or two sets of CHO chromosomes plus a single copy of human chromosome 11. The CD59 gene at 11p13.5 and the CD59 antigen encoded by it are the principal markers used in our mutagenesis studies. The hybrid A(L)CD59(+/-) contains two copies of human chromosome 11, only one of which carries the CD59 gene. The incidence of CD59 (-) mutants (formerly called S1(-)) induced by (137)Cs gamma rays is about fivefold greater in A(L)CD59(+/-) cells than in A(L) cells. Evidence is presented that this increase in mutant yield is due to the increased induction of certain classes of large chromosomal mutations that are lethal to A(L) cells but are tolerated in the A(L)CD59(+/-) hybrid. In addition, significantly more of the CD59 (-) mutants induced by (137)Cs gamma rays in A(L)CD59(+/-) cells display chromosomal instability than in A(L) cells. On the other hand, the yield of gamma-ray-induced CD59 (-) mutants in A(L)CD59(+/-) cells is half that of the A(L)C hybrid, which also tolerates very large mutations but has only one copy of human chromosome 11. We interpret the difference in mutability as evidence that repair processes involving the homologous chromosomes 11 play a role in determining mutant yields. The A(L)CD59(+/-) hybrid provides a useful new tool for quantifying mutagenesis and shedding light on mechanisms of genetic instability and mutagenesis.

The role of glutathione in the toxicity of smoke condensates from cigarettes that burn or heat tobacco

Inhalation of cigarette smoke aerosol via active smoking is associated with the development of pulmonary inflammation. The cytotoxic potential of cigarette smoke has been hypothetically related to development of pulmonary inflammation since the release of intracellular contents from dead and dying cells has been reported to induce inflammatory foci. In this study, cigarette smoke condensates (CSCs) were prepared from Kentucky 1R4F reference cigarettes and cigarettes that primarily heat tobacco (Eclipse). The two CSCs were then compared for their ability to induce killing in human-hamster A(L) hybrid cells. CSCs prepared from Eclipse were much less cytotoxic than those prepared from reference cigarettes. At 60 microg CSC/ml culture medium, survival for CSC from Eclipse cigarettes was approximately 70% compared with 1% for CSC from burned K1R4F cigarettes. The observed reduction in CSC-Eclipse cytotoxicity toward these mammalian cells is consistent with the previously published observation of a 30% decline in pulmonary white cell count and 40% reduction in visual bronchitis index in human smokers who switched to Eclipse for 2 months. Results with N-acetylcysteine and buthionine-S-R-sulfoximine indicate that glutathione markedly reduces the cytoxicity of both CSCs.

Measuring the spectrum of mutation induced by nitrogen ions and protons in the human-hamster hybrid cell line A(L)C

Astronauts can be exposed to charged particles, including protons, alpha particles and heavier ions, during space flights. Therefore, studying the biological effectiveness of these sparsely and densely ionizing radiations is important to understanding the potential health effects for astronauts. We evaluated the mutagenic effectiveness of sparsely ionizing 55 MeV protons and densely ionizing 32 MeV/nucleon nitrogen ions using cells of two human-hamster cell lines, A(L) and A(L)C. We have previously characterized a spectrum of mutations, including megabase deletions, in human chromosome 11, the sole human chromosome in the human-hamster hybrid cell lines A(L)C and A(L). CD59(-) mutants have lost expression of a human cell surface antigen encoded by the CD59 gene located at 11p13. Deletion of genes located on the tip of the short arm of 11 (11p15.5) is lethal to the A(L) hybrid, so that CD59 mutants that lose the entire chromosome 11 die and escape detection. In contrast, deletion of the 11p15.5 region is not lethal in the hybrid A(L)C, allowing for the detection of chromosome loss or other chromosomal mutations involving 11p15.5. The 55 MeV protons and 32 MeV/nucleon nitrogen ions were each about 10 times more mutagenic per unit dose at the CD59 locus in A(L)C cells than in A(L) cells. In the case of nitrogen ions, the mutations observed in A(L)C cells were predominantly due to chromosome loss events or 11p deletions, often containing a breakpoint in the pericentromeric region. The increase in the CD59(-) mutant fraction for A(L)C cells exposed to protons was associated with either translocation of portions of 11q onto a hamster chromosome, or discontinuous or "skipping" mutations. We demonstrate here that A(L)C cells are a powerful tool that will aid in the understanding of the mutagenic effects of different types of ionizing radiation.

Comparison of repair of DNA double-strand breaks in identical sequences in primary human fibroblast and immortal hamster-human hybrid cells harboring a single copy of human chromosome 11

We have optimized a pulsed-field gel electrophoresis assay that measures induction and repair of double-strand breaks (DSBs) in specific regions of the genome (Löbrich et al., Proc. Natl. Acad. Sci. USA 92, 12050-12054, 1995). The increased sensitivity resulting from these improvements makes it possible to analyze the size distribution of broken DNA molecules immediately after the introduction of DSBs and after repair incubation. This analysis shows that the distribution of broken DNA pieces after exposure to sparsely ionizing radiation is consistent with the distribution expected from randomly induced DSBs. It is apparent from the distribution of rejoined DNA pieces after repair incubation that DNA ends continue to rejoin between 3 and 24 h postirradiation and that some of these rejoining events are in fact misrejoining events, since novel restriction fragments both larger and smaller than the original fragment are generated after repair. This improved assay was also used to study the kinetics of DSB rejoining and the extent of misrejoining in identical DNA sequences in human GM38 cells and human-hamster hybrid A(L) cells containing a single human chromosome 11. Despite the numerous differences between these cells, which include species and tissue of origin, levels of TP53, expression of telomerase, and the presence or absence of a homologous chromosome for the restriction fragments examined, the kinetics of rejoining of radiation-induced DSBs and the extent of misrejoining were similar in the two cell lines when studied in the G(1) phase of the cell cycle. Furthermore, DSBs were removed from the single-copy human chromosome in the hamster A(L) cells with similar kinetics and misrejoining frequency as at a locus on this hybrid's CHO chromosomes.

Vanillin (3-methoxy-4-hydroxybenzaldehyde) inhibits mutation induced by hydrogen peroxide, N-methyl-N-nitrosoguanidine and mitomycin C but not (137)Cs gamma-radiation at the CD59 locus in human-hamster hybrid A(L) cells

We have investigated the ability of the naturally occurring plant essence vanillin (3-methoxy-4-hydroxybenzaldehyde) to inhibit mutation at the CD59 locus on human chromosome 11 by hydrogen peroxide, N-methyl-N-nitrosoguanidine, mitomycin C and (137)Cs gamma-radiation in human-hamster hybrid A(L) cells. Previous studies using vanillin have suggested that it can inhibit chromosome aberrations induced by hydrogen peroxide and mitomycin C, as well as inhibiting X-ray- and UV-induced mutations at the hprt locus. Other studies with vanillin have shown that it can increase both the toxicity and mutagenicity of ethyl methane sulfonate and increase the induction of sister chromatid exchange by mitomycin C and a variety of other mutagens. The increased sensitivity of the A(L) assay, which is due in part to its ability to detect both small (single locus) and large (multilocus) genetic damage, allows us to measure the effect of vanillin at low doses of mutagen. Vanillin is shown, in these studies, to inhibit mutation induced by hydrogen peroxide, N-methyl-N-nitrosoguanidine and mitomycin C, as well as to enhance the toxicity of these agents. Vanillin had no effect on either toxicity or mutation induced by (137)Cs gamma-radiation. The vanillin-induced potentiation of H(2)O(2) toxicity is shown not to involve inhibition of catalase or glutathione peroxidase. These results show that vanillin is able to inhibit mutation at the CD59 locus and modify toxicity in a mutagen-specific manner. Possible mechanisms to explain the action of vanillin include inhibition of a DNA repair process that leads to the death of potential mutants or enhancement of DNA repair pathways that protect from mutation but create lethal DNA lesions during the repair process.

Induction of a bystander mutagenic effect of alpha particles in mammalian cells

Ever since the discovery of X-rays was made by Röntgen more than a hundred years ago, it has always been accepted that the deleterious effects of ionizing radiation such as mutation and carcinogenesis are attributable mainly to direct damage to DNA. Although evidence based on microdosimetric estimation in support of a bystander effect appears to be consistent, direct proof of such extranuclear/extracellular effects are limited. Using a precision charged particle microbeam, we show here that irradiation of 20% of randomly selected A(L) cells with 20 alpha particles each results in a mutant fraction that is 3-fold higher than expected, assuming no bystander modulation effect. Furthermore, analysis by multiplex PCR shows that the types of mutants induced are significantly different from those of spontaneous origin. Pretreatment of cells with the radical scavenger DMSO had no effect on the mutagenic incidence. In contrast, cells pretreated with a 40 microM dose of lindane, which inhibits cell-cell communication, significantly decreased the mutant yield. The doses of DMSO and lindane used in these experiments are nontoxic and nonmutagenic. We further examined the mutagenic yield when 5-10% of randomly selected cells were irradiated with 20 alpha particles each. Results showed, likewise, a higher mutant yield than expected assuming no bystander effects. Our studies provide clear evidence that irradiated cells can induce a bystander mutagenic response in neighboring cells not directly traversed by alpha particles and that cell-cell communication process play a critical role in mediating the bystander phenomenon.

Antigen S1, encoded by the MIC1 gene, is characterized as an epitope of human CD59, enabling measurement of mutagen-induced intragenic deletions in the AL cell system

S1 cell membrane antigen is encoded by the MIC1 gene on human chromosome 11. This antigen has been widely used as a marker for studies in gene mapping or in analysis of mutagen-induced gene deletions/mutations, which utilized the human-hamster hybrid cell-line, AL-J1, carrying human chromosome 11. Evidence is presented here which identifies S1 as an epitope of CD59, a cell membrane complement inhibiting protein. E7.1 monoclonal antibody, specific for the S1 determinant, was found to react strongly with membrane CD59 in Western blotting, and to bind to purified, urinary form of CD59 in ELISAs. Cell membrane expression of S1 on various cell lines always correlated with that of CD59 when examined by immunofluorescent staining. In addition, E7.1 antibody inhibited the complement regulatory function of CD59. Identification of S1 protein as CD59 has increased the scope of the AL cell system by enabling analysis of intragenic mutations, and multiplex PCR analysis of mutated cells is described, showing variable loss of CD59 exons.

Targeted cytoplasmic irradiation with alpha particles induces mutations in mammalian cells

Ever since x-rays were shown to induce mutation in Drosophila more than 70 years ago, prevailing dogma considered the genotoxic effects of ionizing radiation, such as mutations and carcinogenesis, as being due mostly to direct damage to the nucleus. Although there was indication that alpha particle traversal through cellular cytoplasm was innocuous, the full impact remained unknown. The availability of the microbeam at the Radiological Research Accelerator Facility of Columbia University made it possible to target and irradiate the cytoplasm of individual cells in a highly localized spatial region. By using dual fluorochrome dyes (Hoechst and Nile Red) to locate nucleus and cellular cytoplasm, respectively, thereby avoiding inadvertent traversal of nuclei, we show here that cytoplasmic irradiation is mutagenic at the CD59 (S1) locus of human-hamster hybrid (AL) cells, while inflicting minimal cytotoxicity. The principal class of mutations induced are similar to those of spontaneous origin and are entirely different from those of nuclear irradiation. Furthermore, experiments with radical scavenger and inhibitor of intracellular glutathione indicated that the mutagenicity of cytoplasmic irradiation depends on generation of reactive oxygen species. These findings suggest that cytoplasm is an important target for genotoxic effects of ionizing radiation, particularly radon, the second leading cause of lung cancer in the United States. In addition, cytoplasmic traversal by alpha particles may be more dangerous than nuclear traversal, because the mutagenicity is accomplished by little or no killing of the target cells.

Mutant yields and mutational spectra of the heterocyclic amines MeIQ and PhIP at the S1 locus of human-hamster AL cells with activation by chick embryo liver (CELC) co-cultures

Cooking meat and fish at high temperature creates heterocyclic amines (HA) including 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Several HA are mutagens in the Ames' S9/Salmonella assay. While PhIP is a substantial Ames' test mutagen, it is 1000-fold less active than the extraordinarily potent MeIQ. In contrast, MeIQ is significantly less mutagenic than PhIP in several mammalian cell assays, especially in repair-deficient Chinese hamster ovary (CHO) cells. HA are suspect human carcinogens on the basis of (i) epidemiological evidence, (ii) induction of tumors in rodents and monkeys, (iii) DNA adduct formation and (iv) mutagenic capacity. In this study, MeIQ and PhIP were significant mutagens at the S1 locus of co-cultivated human/hamster hybrid AL cells following metabolic activation by beta-napthoflavone (betaNF)-induced chick embryonic liver cultures (CELC). MeIQ was more mutagenic than PhIP in the CELC+AL cell assay. The mutant response curves increase with dose and then plateau (PhIP), or decrease (MeIQ). The inflections in these response curves coincide with dose-dependent decreases in cytochrome CYP1A1 activity. Molecular analysis of S1- mutants indicates that a substantial fraction, >65%, of the mutations induced by PhIP are deletions of 4.2 to 133 (Mbp); half are larger than 21 Mbp. Mutations induced by MeIQ were smaller, most (56%) being less than 5.7 Mbp. When appropriate metabolic activation is combined with a target locus, which can detect both small and large chromosomal mutations, both MeIQ and PhIP are significant mutagens and clastogens in repair proficient mammalian cells.

Chromosomal mutations and chromosome loss measured in a new human-hamster hybrid cell line, ALC: studies with colcemid, ultraviolet irradiation, and 137Cs gamma-rays

Small mutations, megabase deletions, and aneuploidy are involved in carcinogenesis and genetic defects, so it is important to be able to quantify these mutations and understand mechanisms of their creation. We have previously quantified a spectrum of mutations, including megabase deletions, in human chromosome 11, the sole human chromosome in a hamster-human hybrid cell line AL. S1- mutants have lost expression of a human cell surface antigen, S1, which is encoded by the M1C1 gene at 11p13 so that mutants can be detected via a complement-mediated cytotoxicity assay in which S1+ cells are killed and S1- cells survive. But loss of genes located on the tip of the short arm of 11 (11p15.5) is lethal to the AL hybrid, so that mutants that have lost the entire chromosome 11 die and escape detection. To circumvent this, we fused AL with Chinese hamster ovary (CHO) cells to produce a new hybrid, ALC, in which the requirement for maintaining 11p15.5 is relieved, allowing us to detect mutations events involving loss of 11p15.5. We evaluated the usefulness of this hybrid by conducting mutagenesis studies with colcemid, 137Cs gamma-radiation and UV 254 nm light. Colcemid induced 1000 more S1- mutants per unit dose in ALC than in AL; the increase for UV 254 nm light was only two-fold; and the increase for 137Cs gamma-rays was 12-fold. The increase in S1- mutant fraction in ALC cells treated with colcemid and 137Cs gamma-rays were largely due to chromosome loss and 11p deletions often containing a breakpoint within the centromeric region.

Mutagenic effects of a single and an exact number of alpha particles in mammalian cells

One of the main uncertainties in risk estimation for environmental radon exposure using lung cancer data from underground miners is the extrapolation from high- to low-dose exposure where multiple traversal is extremely rare. The biological effects of a single alpha particle are currently unknown. Using the recently available microbeam source at the Radiological Research Accelerator Facility at Columbia University, we examined the frequencies and molecular spectrum of S1- mutants induced in human-hamster hybrid (A(L)) cells by either a single or an exact number of alpha particles. Exponentially growing cells were stained briefly with a nontoxic concentration of Hoechst dye for image analysis, and the location of individual cells was computer-monitored. The nucleus of each cell was irradiated with either 1,2,4, or 8 alpha particles at a linear energy transfer of 90 keV/microm consistent with the energy spectrum of domestic radon exposure. Although single-particle traversal was only slightly cytotoxic to A(L) cells (survival fraction approximately 0.82), it was highly mutagenic, and the induced mutant fraction averaged 110 mutants per 10(5) survivors. In addition, both toxicity and mutant induction were dose-dependent. Multiplex PCR analysis of mutant DNA showed that the proportion of mutants with multilocus deletions increased with the number of particle traversals. These data provide direct evidence that a single a particle traversing a nucleus will have a high probability of resulting in a mutation and highlight the need for radiation protection at low doses.

Expression of human O6-methyl guanine methyl transferase (MGMT) in post replication repair (PRR) deficient CHO-UV-1 cells: compensation for hypersensitivity to methylating and ethylating agents but not to mitomycin C

The cDNA for human MGMT was transfected into and expressed in CHO cells and the post-replication repair deficient mutant CHO-UV-1 cell, both of which are devoid of endogenous MGMT activity. Expression of MGMT activity was demonstrated by measurement of activity and by immunoblot analysis. The mutant phenotype of UV-1 is characterized by extreme hypersensitivity to killing by methylating and ethylating agents as well as the antitumor antibiotic mitomycin C (MMC). MGMT expression conferred equivalent, supra-normal levels of resistance to killing by MNNG (N-methyl-N'-nitro-nitrosoguanidine) or EMS (ethyl methanesulfonate) on CHO and UV-1, but had no effect on the lethality of MMC. So, even though a mutated gene other than MGMT is known to underlie the pleiotropic phenotype of UV-1, expression of MGMT compensates for part of this phenotype. This result indicates that attempts to concordance map and clone the gene(s) responsible for the UV-1 phenotype can be complicated when using MNNG selection due to compensation by the MGMT gene. These results also indicate that the post-replication repair deficient phenotype characterized in CHO-UV-1 cells, will be masked in cells normally expressing MGMT due to MGMT-mediated resistance to methylating and ethylating agents.

A low, adaptive dose of gamma-rays reduced the number and altered the spectrum of S1- mutants in human-hamster hybrid AL cells

We examined the effects of a low, adaptive dose of 137Cs-gamma-irradiation (0.04 Gy) on the number and kinds of mutants induced in AL human-hamster hybrid cells by a later challenge dose of 4 Gy. The yield of S1- mutants was significantly less (by 53%) after exposure to both the adaptive and challenge doses compared to the challenge dose alone. The yield of hprt- mutants was similarly decreased. Incubation with cycloheximide (CX) or 3-aminobenzamide largely negated the decrease in mutant yield. The adaptive dose did not perturb the cell cycle, was not cytotoxic, and did not of itself increase the mutant yield above background. The adaptive dose did, however, alter the spectrum of S1- mutants from populations exposed only to the adaptive dose, as well as affecting the spectrum of S1- mutants generated by the challenge dose. The major change in both cases was a significant increase in the proportion of complex mutations compared to small mutations and simple deletions.

Expression of human NAD(P)H: quinone oxidoreductase (DT-diaphorase) in Chinese hamster ovary cells: effect on the toxicity of antitumor quinones

Previous studies have indicated that NAD(P)H: quinone oxidoreductase [DT-diaphorase (NQO1)] plays an important role in the bioreductive activation of quinone-containing antitumor agents. Although these studies demonstrated that purified NQO1 can reduce these compounds in vitro, the importance of NQO1 in the intracellular activation of quinone-containing antitumor agents remains controversial. In our study, we transfected human NQO1 into Chinese hamster ovary cells that do not normally express NQO1 activity and obtained stable clones that expressed NQO1 activity of 19-3527 nmol of 2,6-dichlorophenolindophenol reduced/min/mg of protein. The level of NQO1 expression correlated with an increased killing by streptonigrin, EO9 (3-hydroxymethyl-5-aziridinyl-1-methyl-2-(1H-indole-4,7-dione)-propen ol), and 2,5-diaziridinyl-3,6-dimethyl-1,4-benzoquinone, but mitomycin C sensitivity was independent of this activity. NQO1 expression also led to a slight decrease in the sensitivity of cells to menadione. Our data demonstrate that compounds that are efficient substrates for NQO1 in vitro are also bioactivated in cultured mammalian cells when they are transfected with human NQO1. These results are consistent with the relative abilities of mitomycin C, streptonigrin, EO9, and 2,5-diaziridinyl-3,6-dimethyl-1,4-benzoquinone to serve as substrates for bioreduction by human NQO1, and show that NQO1 levels are not necessarily predictive in terms of sensitivity to mitomycin C.

Cellular and molecular analysis of mutagenesis induced by charged particles of defined linear energy transfer

Mutation induction by charged particles of defined linear energy transfer (LET) and gamma rays was scored using human-hamster hybrid AL cells. The LET values for charged particles accelerated at the Radiological Research Accelerator Facility ranged from 10 keV/microm protons to 150 keV/microm 4He ions. The induced mutant fractions at both the S1 and HGPRT loci were dependent on the dose and LET. In addition, for each dose examined, the mutant yield at the S1 locus was 30-60 fold higher than at the corresponding HGPRT locus. To determine whether the mutation spectrum was comparably dependent on dose and LET, independent S1- and HGPRT- mutants induced by 150 keV/microm 4He ions and gamma rays were isolated, and their DNA was analyzed by both Southern blotting and multiplex PCR methods. While the majority of radiation-induced mutants showed deletions of varying sizes, the relative percentage of large deletions was found to be related to both the dose and LET of the radiation examined. Using a mutation system that can detect multilocus changes, results of the present study show that radiation-induced chromosomal loss can be in the millions of base pairs.

Heavy ion mutagenesis: linear energy transfer effects and genetic linkage

We have characterized a series of 69 independent mutants at the endogenous hprt locus of human TK6 lymphoblasts and over 200 independent S1-deficient mutants of the human x hamster hybrid cell line AL arising spontaneously or following low-fluence exposures to densely ionizing Fe ions (600 MeV/amu, linear energy transfer = 190 keV/microns). We find that large deletions are common. The entire hprt gene (> 44 kb) was missing in 19/39 Fe-induced mutants, while only 2/30 spontaneous mutants lost the entire hprt coding sequence. When the gene of interest (S1 locus = M1C1 gene) is located on a nonessential human chromosome 11, multilocus deletions of several million base pairs are observed frequently. The S1 mutation frequency is more than 50-fold greater than the frequency of hprt mutants in the same cells. Taken together, these results suggest that low-fluence exposures to Fe ions are often cytotoxic due to their ability to create multilocus deletions that may often include the loss of essential genes. In addition, the tumorigenic potential of these HZE heavy ions may be due to the high potential for loss of tumor suppressor genes. The relative insensitivity of the hprt locus to mutation is likely due to tight linkage to a gene that is required for viability.

Mutant quantity and quality in mammalian cells (AL) exposed to cesium-137 gamma radiation: effect of caffeine

We examined the effect of caffeine (1,3,7-trimethylxanthine) on the quantity and quality of mutations in cultured mammalian AL human-hamster hybrid cells exposed to 137Cs gamma radiation. At a dose (1.5 mg/ml for 16 h) that reduced the plating efficiency (PE) by 20%, caffeine was not itself a significant mutagen, but it increased by approximately twofold the slope of the dose-response curve for induction of S1- mutants by 137Cs gamma radiation. Molecular analysis of 235 S1- mutants using a series of DNA probes mapped to the human chromosome 11 in the AL hybrid cells revealed that 73 to 85% of the mutations in unexposed cells and in cells treated with caffeine alone, 137Cs gamma rays alone or 137Cs gamma rays plus caffeine were large deletions involving millions of base pairs of DNA. Most of these deletions were contiguous with the region of the MIC1 gene at 11p13 that encodes the S1 cell surface antigen. In other mutants that had suffered multiple marker loss, the deletions were intermittent along chromosome 11. These "complex" mutations were rare for 137Cs gamma irradiation (1/63 = 1.5%) but relatively prevalent (23-50%) for other exposure conditions. Thus caffeine appears to alter both the quantity and quality of mutations induced by 137Cs gamma irradiation.

Molecular analysis of mutagenesis by high LET radiation

Mutation induction by high linear energy transfer [LET] alpha particles and gamma-rays was scored in the human hamster hybrid [AL] cells. Southern blotting technique was used to analyse the molecular changes in the DNA from both the HGPRT- and S1- mutants. Dose dependent mutagenesis in the AL cells irradiated with the charged particles was higher by almost 20 fold at the S1 than the corresponding HGPRT locus. Southern analysis of the mutants induced by the high LET particles showed mostly multilocus deletion at both the HGPRT and S1 genes.

Megabase pair deletions in mutant mammalian cells following exposure to amsacrine, an inhibitor of DNA topoisomerase II

Amsacrine, [4'-(9-acridinylamino)-methanesulfon-m-auisidide], belongs to the class of cancer chemotherapeutic agents that target DNA topoisomerase II. We show that, over its cytotoxic range, amsacrine is a potent mutagen of the S1 phenotype in the AL (human x hamster) hybrid cell line. By contrast, amsacrine induction of the HPRT- phenotype in AL cells is at least two decades less frequent and is not concentration dependent. Such differential mutation frequencies are hypothesized to reflect the concomitant loss of essential genes neighboring the hprt locus. It may be that some amsacrine cytotoxicity is due to the inactivation of essential genes by large deletions. The AL mutation system is well suited for the detection and mapping of mutations which are large deletions because its MIC1 locus, which controls the expression of the selectable cell surface antigen S1, is on a single human chromosome. This human chromosome 11 is in addition to the genome of the Chinese hamster ovary cell and is basically nonessential. Since there are no sister human chromosomes in AL cells, deletions which extend beyond the MIC1 locus may be conveniently and unambiguously mapped. We have detected the presence or absence of 9 different chromosome 11 markers in 48 S1- mutants cloned from amsacrine-treated cultures. We find that almost all (92%) of the mutants have deletions of at least 1.5-2 megabase pairs in length. The distribution of marker loss frequencies flanking the MIC1 locus does not appear symmetric with respect to distance from that locus. We speculate that amsacrine-induced deletions are mediated by a series of subunit exchanges between overlapping topoisomerase II dimers at the bases of replicons or larger chromosomal structures such as replicon clusters or chromosome minibands.

Caffeine enhanced measurement of mutagenesis by low levels of gamma-irradiation in human lymphocytes

The well-known action of caffeine in synergizing mutagenesis (including chromosome aberrations) of agents like ionizing radiation by inhibition of cellular repair processes has been incorporated into a rapid procedure for detection of mutagenicity with high sensitivity. Effects of 5-10 rads of gamma-irradiation, which approximate the human lifetime dose accumulation from background radiation, can be detected in a two-day procedure using an immortalized human WBC culture. Chromosomally visible lesions are scored on cells incubated for 2 h after irradiation in the presence and absence of 1.0 mg/ml of caffeine. An eightfold amplification of scorable lesions is achieved over the action of radiation alone. This approach provides a closer approximation to absolute mutagenicity unmitigated by repair processes, which can vary in different situations. It is proposed that mutagenesis testing of this kind, using caffeine or other repair-inhibitory agents, be employed to identify mutagens in their effective concentrations to which human populations may be exposed; to detect agents such as caffeine that may synergize mutagenic actions and pose epidemiologic threats; and to discover effective anti-mutagens. Information derived from the use of such procedures may help prevent cancer and newly acquired genetic disease.

Chrysotile fiber is a strong mutagen in mammalian cells

Although chrysotile asbestos is a proven human carcinogen, several studies have concluded that these fibers are not mutagenic to cultured mammalian cells. We show here, on the other hand, that when tested using the AL cell system that detects both intragenic and multilocus mutations, chrysotile is indeed mutagenic and comparable in strength to that of gamma-rays. Southern analysis of the induced mutants shows that the majority contains large deletions ranging in size from a few thousand to several million base pairs. Results of our study demonstrate that, while chrysotile may be less durable in vivo than the amphibole fibers such as crocidolites and amosites, it can effectively create genetic damage involved in the cancer process.

2-Amino-3,4-dimethylimidazo[4,5-f]quinoline induces and inhibits cytochrome P450 from the IA subfamily in chick and rat hepatocytes

Several heterocyclic amines, found in cooked food, are powerful mutagens in the Ames Salmonella mutagenicity test system. One of these, 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) is one of the most mutagenic chemicals tested in this assay. In primary cultures of chick and rat hepatocytes, MeIQ, by itself, induced cytochrome P450 from the IA subfamily but was a weak inducer compared to 3-methylcholanthrene. However, in both chick and rat hepatocytes in culture, MeIQ decreased the amount of 3-methylcholanthrene-induced ethoxyresorufin deethylase activity, which is catalyzed by cytochrome P450 IA. The protein moiety of cytochrome P450 IA was decreased at MeIQ concentrations of 2.5 micrograms/ml or greater in chick hepatocytes and 25 micrograms/ml in rat hepatocytes. In hepatic microsomes from methylcholanthrene-treated chicks and rats, MeIQ was a competitive inhibitor of both ethoxyresorufin deethylase activity, a reaction catalyzed mainly by rodent cytochrome P450 IA1, and uroporphyrinogen oxidation, a reaction catalyzed by rodent P450 IA2. In cultured chick hepatocytes, MeIQ also decreased cytochrome P450-mediated oxidation of uroporphyrinogen by intact cells. The ability of MeIQ to inhibit as well as to induce cytochrome P450s of the IA subfamily may be important in assessing the mutagenic and carcinogenic effects of MeIQ in mammals.

The hypersensitivity of the Chinese hamster ovary variant BL-10 to bleomycin killing is due to a lack of glutathione S-transferase-alpha activity

As a means to understand the fundamental mechanisms of bleomycin cell killing, we previously isolated 19 bleomycin-sensitive mutants which represent at least six genetically distinct complementation groups (T.D. Stamato, B. Peters, P. Patil, N. Denko, R. Weinstein, and A. Giaccia. Cancer Res., 47: 1588-1592, 1987). One class of mutants represented by the cell line BL-10 displays only hypersensitivity to killing by bleomycin in both acute (16 h) and chronic treatments but no sensitivity to killing by other DNA-damaging agents. Complementation studies between this mutant and human fibroblasts suggested that the human gene which corrects the defect of BL-10 rested on human chromosome 6. It has been reported that the gene for human glutathione S-transferase (GST) alpha also resides on chromosome 6. Measurements of selenium-independent peroxidase (alpha-GST + glutathione peroxidase) activity in wild-type Chinese hamster ovary (CHO) cells, using cumene hydrogen peroxide as a substrate, gave a value of 112 nmol of glutathione oxidized/min/mg protein compared with 88.1 nmol of glutathione oxidized/min/mg protein for BL-10. Measurement of the selenium-dependent peroxidase activity, using H2O2 as a substrate, resulted in 65.9 nmol of reduced glutathione oxidized/min/mg protein in CHO and 81.5 nmol of reduced glutathione oxidized/min/mg protein for BL-10. In other words, BL-10 cells did not exhibit a difference in their ability to metabolize both substrates in contrast to CHO cells. This indicates that BL-10 possesses little alpha-GST activity. Transfection of BL-10 cells with a mammalian expression vector containing the alpha-GST gene increases the survival of BL-10 to bleomycin and does not increase the bleomycin resistance of two other bleomycin mutants which lie in different genetic complementation groups. These data strongly implicate a role for alpha-GST in the resistance of cells to bleomycin.

Loss of the intrinsic heat resistance of human cells and changes in Mr 70,000 heat shock protein expression in human x hamster hybrids

Since mammalian cells vary widely in their intrinsic thermoresistance, we have investigated the genetic basis underlying this phenomenon in human and rodent cell lines. Typically, human cells are considerably more resistant to killing by heat than rodent cell lines. To determine whether the heat-resistant phenotype is dominant or recessive and to locate the chromosome(s) bearing determinants for heat resistance, we have prepared hybrids of heat-resistant human HT1080 cells and heat-sensitive Chinese hamster ovary (CHO) cells to test their response to heat. For both mass hybrid cultures and individual clones, the heat response of the hybrids was similar to that of the CHO parent. Analysis by in situ hybridization revealed the presence of five to 20 human chromosomes per cell in the mass hybrids and four to eight intact chromosomes plus some fragments in individual clones isolated from the hybrid cell population. A similar result was obtained using a different human cell line, AG1522. These data suggest that heat resistance is a recessive trait. Consistent with this conclusion are the results from a study of a fusion of HT1080 to a CHO mutant, BL-10, which was found to be hypersensitive to heat-induced killing. These hybrids had a normal CHO heat response and not the more heat-resistant phenotype of HT1080 cells. Two hybrid clones, H2 and H4, from the HT1080/BL-10 fusion were studied in more detail. Both clones possess similar amounts of Mr 70,000 heat shock protein (HSP70), despite the fact that H4 contains three human chromosomes (Nos. 6, 14, and 21) which carry HSP70 genes while H2 contains only one (chromosome 6). Both hybrid cell lines have the same response to heat. Although we found a wide range of sensitivities to heat, all cell lines contained a similar amount of constitutive HSP70, suggesting that HSP70 levels per se are not the critical determinant of intrinsic heat resistance.

Measurement of radiation-induced DNA double-strand breaks by pulsed-field gel electrophoresis

We have examined the use of pulsed-field gel electrophoresis (PFGE) to measure DNA double-strand breaks induced in CHO cells by ionizing radiation. The PFGE assay provides a simple method for the measurement of DNA double-strand breaks for doses as low as 3-4 Gy ionizing radiation, and appears applicable for the measurement of damage produced by any agent producing double-strand breaks. The conditions of transverse alternating field electrophoresis determined both the sensitivity of the assay and the ability to resolve DNA fragments with different sizes. For example, with 0.8% agarose and a 1-min pulse time at 250 V for 18 h of electrophoresis, 0.39% of the DNA per gray migrated into the gel, and only molecules less than 1500 kb could be resolved. With 0.56% agarose and a 60-min pulse time at 40 V for 6 days of electrophoresis, 0.55-0.90% of the DNA per gray migrated into the gel, and molecules between 1500 and 7000 kb could be resolved.

Genetic and biochemical characterization of the CHO-UV-1 mutant defective in postreplication recovery of DNA

The CHO-UV-1 mutant, a Chinese hamster ovary cell with defective postreplication recovery of DNA, is 2- to 4-fold more sensitive than its wild-type counterpart (CHO-77256) to the lethal effects of ethylating agents and UV radiation; it is also hypersensitive (10- to 20-fold) to some DNA-methylating and -cross-linking agents. We studied the CHO-UV-1 mutant further to define its phenotype in terms of DNA damage induction and repair, methyltransferase activity, and effects of caffeine on mutational and lethal responses. Both wild-type and CHO-UV-1 cells incurred similar levels and types of damage when exposed to UV radiation, N-methyl-N'-nitro-N-nitrosoguanidine, or N-methyl-N-nitrosourea. The rate and extent of repair of Micrococcus luteus endonuclease-sensitive sites after UV irradiation or treatment with N-methyl-N'-nitro-N-nitrosoguanidine were also equivalent in these two cell types. Twenty % of the initial endonuclease-sensitive sites induced in either cell line remained at 18 h after UV irradiation; approximately 8% of the sites after N-methyl-N'-nitro-N-nitrosoguanidine exposure were present in both parental and CHO-UV-1 cells after a 17-h repair period. Moreover, the ability of CHO-UV-1 to resynthesize and ligate DNA during excision repair was similar to that of its parent. Neither CHO-UV-1 nor CHO-77256 had appreciable levels of O6-methylguanine-DNA methyltransferase activity which ameliorates the cytotoxicity of alkylating agents. Caffeine, a known inhibitor of postreplication repair, decreased the frequency of mutation induction at the hypoxanthine-guanine phosphoribosyltransferase locus by 40-55% in CHO-77256 but not in CHO-UV-1. These results rule out defective excision repair as a factor in the hypersensitivity of the CHO-UV-1 mutant to DNA-damaging agents. Hence, this cell line appears to derive from a mutation affecting nonexcision repair processes and should be useful in clarifying the mechanism(s) of postreplication recovery of DNA in mammalian cells.

Mutation induction and relative biological effectiveness of neutrons in mammalian cells. Experimental observations

The induction of mutation by graded doses of monoenergetic neutrons was examined using the human-hamster hybrid cell system. The AL cells, formed by fusion of human fibroblasts with the gly- A mutant of the Chinese hamster ovary cells, contain the standard set of hamster chromosomes plus a single human chromosome, number 11. These cells contain specific human cell surface antigens that render them sensitive to killing by specific antisera in the presence of complement. Mutant AL cells that have lost the surface markers, however, would survive and give rise to scorable colonies. The cells were irradiated with neutrons produced at the Radiological Research Accelerator Facility of Columbia University. Doses corresponding to low, moderate, and high cytotoxicities and in energies ranging from 0.33 to 14 MeV were used. Neutrons induced a dose-dependent cytotoxicity and mutation frequency in the AL cells. Over the range of doses examined, it was found that the mutagenesis induced by neutrons was energy-dependent and the frequencies were a curvilinear function of dose for both the a1 and a2 antigenic loci examined. In comparison to gamma rays, the relative biological effectiveness (RBE) for cell lethality at the 10% survival level ranged from 5.2 for 0.33 MeV to 1.8 for 14 MeV neutrons. The RBE for mutation induction at the a1 locus, however, ranged from 30 for 0.33 MeV to 4.2 for 14 MeV neutrons at or around the lowest levels of effect examined. Results of the present study demonstrated that neutrons, when measured under conditions which permit detection of a spectrum of gene and chromosomal mutations, in fact, are more efficient mutagens than previously thought.

Aberrant DNA repair and enhanced mutagenesis following mutagen treatment of Chinese hamster Ade-C cells in a state of purine deprivation

Ade-C is a Chinese hamster ovary cell line auxotrophic for purines because of a mutation in the de novo synthetic pathway. We now show that, in the absence of exogenous hypoxanthine, replicative DNA synthesis is rapidly shut down. Various aspects of DNA repair have been studied in purine-starved cells. Incision, the first step of excision repair of UV damage, appears normal, as do the later steps, repair synthesis (demonstrated following chemical damage as well as UV-irradiation) and ligation. However, removal of UV-induced pyrimidine dimers is not detected, and it seems that the repair that occurs is aberrant. This behaviour is associated with an increase in cell killing by UV light, and a several-fold increase in the frequency of mutations induced by UV.

Excessive chromosome fragility and abundance of sister-chromatid exchanges induced by UV in an Indian muntjac cell line defective in postreplication (daughter strand) repair

Two UV-hypersensitive animal cell mutants defective in postreplication recovery (daughter strand synthesis) display quite different patterns of induced sister-chromatid exchange (SCE). One, an SV40-transformed Indian muntjac cell (SVM), shows extremely high frequencies of SCE after UV; induced exchanges can be measured after UV doses as low as 0.01 J/m2. This cell also displays exaggerated levels of induced and spontaneous chromosome aberrations. By contrast SCE rates in the Chinese hamster cell mutant, UV-1, are essentially normal. In both SVM and UV-1, however, there is a clear correlation between the cell density and spontaneous frequencies of SCE, a feature which could be related to the observed density-dependent rate of DNA maturation.

Effects of caffeine on pyrimidine biosynthesis and 5-phosphoribosyl 1-pyrophosphate metabolism in Chinese hamster cells

Caffeine, at doses which enhance killing by UV light, inhibits the biosynthesis of pyrimidines in Chinese hamster ovary cells (K1) in culture. This inhibition was measured as a decrease in [14C]UTP and [14C]CTP accumulation after a 3-h incubation with [14C]aspartate or [14C]orotate and a similar decrease in Urd-A cells (which lack the first three enzymes of the pathway) using [14C]orotate as substrate. There was no such inhibition in Urd-C cells (which lack the last two enzymes) using [14C]aspartate as substrate and measuring accumulation of orotate. There is some inhibition of the fifth enzyme of the pathway, orotate phosphoribosyltransferase by caffeine in vitro and this is most striking at low 5-phosphoribosyl 1-pyrophosphate concentrations. The level of 5-phosphoribosyl 1-pyrophosphate is decreased in Chinese hamster ovary K1 cells by about 20% after 3 h and by about 70% after 16 h in the presence of caffeine. It is suggested that inhibition of pyrimidine biosynthesis by caffeine over a 16-h period may be due mainly to decreased intracellular 5-phosphoribosyl 1-pyrophosphate levels but that in the decrease in pyrimidine accumulation over 3 h, direct inhibition of orotate phosphoribosyltransferase by caffeine may also play a role.

Chinese hamster ovary mutant UV-1 is hypomutable and defective in a postreplication recovery process

CHO-UV-1 is a mutant of the Chinese hamster cell CHO-K1 hypersensitive to killing by ultraviolet light but with normal resistance to X-ray. It is also hypersensitive to killing by ethyl methane sulfonate. Hybrid clones formed bu fusing UV-1 and Chinese hamster lung cells display the normal ultraviolet resistance of the latter. The sensitive phenotype behaves, therefore, in a genetically recessive manner. Ultraviolet sensitivity of UV-1 is not associated with a deficiency in excision repair. Alkaline sucrose gradient sedimentation analysis of nascent DNA from ultraviolet-irradiated cells reveals that UV-1 is, however, markedly deficient in postreplication recovery. Furthermore, UV-1 has a lower rate of induced mutation to 6-thioguanine resistance than does the parental cell when treated with ultraviolet light or ethyl methane sulfonate. These results suggest that the phenotype of UV-1 is due to a mutation in a form of postreplication recovery which in normal cells is error prone.

Effects of caffeine on purine metabolism and ultraviolet light-induced lethality in cultured mammalian cells

Caffeine, at doses which enhance the killing action of ultraviolet light, inhibits both de novo synthesis and the utilization of exogenous purines in cultured CHO-K1, a Chinese hamster ovary cell line. The decrease in synthesis was measured as inhibition by caffeine of the accumulation of phosphoribosylformylglycineamide or of phosphoribosylaminoimidazolecarboxamide, the fourth and ninth intermediates, respectively, in the de novo biosynthetic pathway. The effect is dose dependent, with a caffeine concentration of 7.5 mM producing a 90% reduction in 15 min. Interference with utilization of exogenous purines was seen as a substantial decrease in the conversion of [14C]hypoxanthine, [14C]adenine, or [14C]guanine into their respective di- and triphosphates in the presence of caffeine. Purine deprivation either by starvation of purine-requiring mutants or by treatment of parental cells with methylmercaptopurine ribonucleoside, a known inhibitor of purine synthesis, results in a partial sensitization to killing by ultraviolet light which can be maximized by the addition of caffeine. Thus, one of the ways by which antimetabolites and caffeine act to enhance ultraviolet light killing may be by interference with the supply of purine nucleotides needed for repair.

Isolation of UV-sensitive variants of CHO-K1 by nylon cloth replica plating

Techinques are described which permit the identification and isolation of UV-sensitive variants from mutagenized populations of Chinese hamster ovary (CHO) cells. Identification is based on the observation that within two days after receiving a dose of approximately 240 ergs/mm2 of UV irradiation most of the cells in a colony of CHO detach from the surface of a plastic tissue culture dish. At a lower dose of UV, which does not kill or detach a significant number of parental cells, UV-sensitive colonies are killed and become detached. Thus a clear plaque is produced in a lawn of unirradiated parental cells, marking the site occupied by a sensitive colony. Live cells from such sensitive colonies have been recovered from a nylon cloth replica prepared prior to irradiation and characterized. One UV-sensitive variant (CHO-UV-1) is indistinguishable from parental cells in X-ray resistance, chromosome number, generation time, and duration of the phases of the cell cycle. For UV irradiation the hit number (-n), shoulder width (Dq), and mean lethal dose (Do) for the variant are 2.8, 21 ergs/mm2, and 21 ergs/mm2, respectively, as compared to 2.6, 36 ergs/mm2, and 45 ergs/mm2 for CHO-K1 cells. These values have not changed for a period of eight months in culture.

Changes in the organization of chromosomes during the cell cycle: response to ultraviolet light

Fusion between mitotic and interphase cells results in the premature condensation of the interphase chromosomes into a morphology related to the position in the cell cycle at the time of fusion. These prematurely condensed chromosomes (PCC) have been used in conjunction with u.v. irradiation to examine the interphase chromosome condensation cycle of HeLa cells. The following observations have been made: (I) There is a progressive decondensation of the chromosomes during G1 which is accentuated by u.v. irradiation: (2) The chromosomes become more resistant to u.v.-induced decondensation during G2 and mitosis. (3) There is a close correlation between the degree of chromosome decondensation and the amount of unscheduled DNA synthesis induced by u.v. irradiation during G1 and mitosis: (4) Hydroxyurea enhances the ability of u.v. irradiation to promote the decondensation of chromosomes during G1, G2 and mitosis. Hydroxyurea also potentiates the lethal action of u.v. irradiation during mitosis and G1. These data are discussed in relation to the suggestion that chromosomes undergo a progressive decondensation during G1 and condensation during G2.

Analysis of interphase chromosome damage by means of premature chromosome condensation after X- and ultraviolet-irradiation

Sendai virus-mediated fusion between mitotic and interphase mammalian cells causes the rapid condensation of the interphase chromosomes into distinct structures, a process termed premature chromosome condensation. This phenomenon has been used to assess the immediate action of x-rays and ultraviolet light on the chromosomes of HeLa cells irradiated in the G1 phase of the life cycle. X-irradiation produces fragmented chromosomes; but even the most finely chopped fragments retain the condensed morphology characteristic of the premature chromosome condensation of unirradiated G1 cells. For doses up to about 1800 rads, the increase in the number of fragments is linearly related to the dose. One mean lethal dose (about 100 rads) yields a net increase of 10-15 fragments per G1 cell, which is considerably larger than previous estimates based on scoring of mitotic chromosomes. Incubation of irradiated cells produces a rapid (within 2 hr) reduction in the number of fragments, indicative of a rejoining process. The decrease in the number of pieces is not accompanied by unscheduled DNA synthesis detectable by radioautography. G1 chromosomes of cells irradiated with UV light in G1 phase are not appreciably fragmented but are elongated and attenuated so that they resemble the premature-chromosome-condensation chromosomes of unirradiated S-phase cells. Both the degree of "S-like" character attained by the G1 chromosomes in a cell and the percentage of the cell population displaying the G1 --> S transition increase with the dose and incubation time after irradiation. Thus, in contrast to the immediate manifestation of damage from x-rays, the maximum induction of the "S-like" state does not occur until about 2 hr after irradiation. The "S-like" chromosomes are capable of unscheduled DNA synthesis. We suggest that the difference in chromosome morphology found after UV- and x-irradiation underlies the reason why the former, but not the latter, induces unscheduled DNA synthesis in G1 cells.

Membrane dynamics in the action of dibutyryl adenosine 3':5'-cyclic monophosphate and testosterone on mammalian cells

The first manifestation of reverse transformation, in which a permanent epithelial-like cell is reversibly transformed into a fibroblast-like cell by dibutyryl adenosine cyclic 3':5'-monophosphate, is the disappearance of a set of violently extending and retracting knobbed structures on the cell membrane with consequent tranquilization of the membrane. The cell then elongates to assume a fibroblast-like morphology. Permanently fibroblast-like cells, which have smooth membranes, can be changed into knobbed epithelial-like cells by the addition of either colcemid or cytochalasin B, agents that disorganize one or the other component, respectively, of the microtubular-microfibrillar system. Permanently smoothmembraned, epithelial-like cells also become knobbed by such treatment. All of these knobbed epithelial-like cells can be converted into smooth-membraned fibroblast-like cells by dibutyryl cAMP alone or with the aid of supporting molecules like testosterone. It is proposed that these are general phenomena and that the knobbed epithelial-like forms and the more highly differentiated smooth-membraned fibroblast-like cells represent extreme situations with respect to the degree of organization of the microtubular-microfibrillar system inside the cell.

Mammalian cell growth proteins. I. Growth stimulation of fetuin

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