Correlation between non-repairable DNA lesions and fixation of cell damage by hypertonic solutions in Chinese hamster cells

DNA damage evaluated by gammaH2AX foci formation by a selective group of chemical/physical stressors

It has been reported that the phosphorylated form of histone variant H2AX (gammaH2AX) plays an important role in the recruitment of DNA repair and checkpoint proteins to sites of DNA damage, particularly at double strand breaks (DSBs). Using gammaH2AX foci formation as an indicator for DNA damage, several chemicals/stress factors were chosen to assess their ability to induce gammaH2AX foci in a 24h time frame in a human amnion FL cell line. Two direct-acting genotoxins, methyl methanesulfonate (MMS) and N-ethyl-N-nitrosourea (ENU), can induce gammaH2AX foci formation in a time- and dose-dependent manner. Similarly, an indirect-acting genotoxin, benzo[a]pyrene (BP), also induced the formation of gammaH2AX foci in a time- and dose-dependent manner. Another indirect genotoxin, 2-acetyl-aminofluorene (AAF), did not induce gammaH2AX foci formation in FL cells; however, AAF can induce gammaH2AX foci formation in Chinese hamster CHL cells. Neutral comet assays also revealed the induction of DNA strand breaks by these agents. In contrast, epigenetic carcinogens azathioprine and cyclosporine A, as well as non-carcinogen dimethyl sulfoxide, did not induce gammaH2AX foci formation in FL cells. In addition, heat shock and hypertonic saline did not induce gammaH2AX foci. Cell survival analyses indicated that the induction of gammaH2AX is not correlated with the cytotoxic effects of these agents/factors. Taken together, these results suggest that gammaH2AX foci formation could be used for evaluating DNA damage; however, the different cell types used may play an important role in determining gammaH2AX foci formation induced by a specific agent.

DNA-PK is responsible for enhanced phosphorylation of histone H2AX under hypertonic conditions

Exposure of cells to hypertonic medium after X-irradiation results in a 3-4-fold increase in the phosphorylation of histone H2AX (gammaH2AX) at sites of radiation-induced DNA double-strand breaks. This increase was previously associated with salt-induced radiosensitization and inhibition of repair of DNA double-strand breaks. To examine possible mechanisms for the increase in foci size, chemical inhibitors of kinase and phosphatase activity and cell lines deficient in ATM and DNA-PK, two kinases known to phosphorylate H2AX, were examined. H2AX kinase and phosphatase activity were maintained in the presence of high salt. ATM mutant HT144 melanoma cells showed the expected 3-4-fold increase in H2AX phosphorylation in the presence of 0.5M Na(+). However, DNA-PKcs deficient M059J cells failed to respond to hypertonic treatment and M059J Fus1 cells corrected for this deficiency showed the expected increase in foci size. Although the active phosphoform of ATM, phosphoserine-1981, increased after irradiation, the level was unaffected by the addition of 0.5M Na(+). Instead, 0.5M Na(+) caused a partial redistribution of serine-1981-ATM to perinuclear regions. Hypertonic medium added after irradiation was effective in inhibiting rejoining of the radiation-induced double-strand breaks even in DNA-PK deficient M059J cells. We suggest that hypertonic treatment following irradiation inhibits double-strand break rejoining that in turn maintains DNA-PK activity at the site of the break, enhancing the size of the gammaH2AX foci.

Hypertonic saline enhances expression of phosphorylated histone H2AX after irradiation

Phosphorylation of histone H2AX at serine 139 occurs at sites surrounding DNA double-strand breaks, producing discrete spots called "foci" that are visible with a microscope after antibody staining. This modification is believed to create changes in chromatin structure and assemble various repair proteins at sites of DNA damage. To examine the role of chromatin structure, human SiHa cells were exposed to hypertonic salt solutions that are known to condense chromatin and sensitize cells to chromosome damage and killing by ionizing radiation. Postirradiation incubation in 0.5 M Na(+) increased gammaH2AX expression about fourfold as measured by flow cytometry and immunoblotting, and loss of gammaH2AX was inhibited in the presence of high salt. Focus size rather than the number of radiation-induced gammaH2AX foci was also increased about fourfold. When high-salt treatment was delayed for 1 h after irradiation, effects on focus size and retention were reduced. The increase in focus size was associated with a decrease in the rate of rejoining of double-strand breaks as measured using the neutral comet assay. We conclude that gammaH2AX expression after irradiation is sensitive to salt-induced changes in chromatin structure during focus formation, and that a large focus size may be an indication of a reduced ability to repair DNA damage.

Maintenance of genomic integrity in mammalian kidney cells exposed to hyperosmotic stress

Changes in environmental salinity/osmolality impose an osmotic stress upon cells because, if left uncompensated, such changes will alter the conserved intracellular ionic milieu and macromolecular density, for which cell metabolism in most extant cells has been optimized. Cell responses to osmotic stress include rapid posttranslational and slower transcriptional events for the compensatory regulation of cell volume, intracellular electrolyte concentrations, and protein stability/activity. Changes in external osmolality are perceived by osmosensors that control the activation of signal transduction pathways giving rise to the above responses. We have recently shown that the targets of such pathways include cell cycle-regulatory and DNA damage-inducible genes (reviewed in Kültz, D., 2000. Environmental stressors and gene responses, Elsevier, Amsterdam. pp 157-179). Moreover, recent evidence suggests that hyperosmotic stress causes chromosomal aberrations and DNA double-strand breaks in mammalian cells. We propose that the modulation of cell cycle checkpoints and the preservation of genomic integrity are important aspects of cellular osmoprotection and as essential for cellular osmotic stress resistance as the capacity for cell volume regulation and maintaining inorganic ion homeostasis and protein stability/activity.

Hyperosmolality in the form of elevated NaCl but not urea causes DNA damage in murine kidney cells

This study demonstrates, by using neutral comet assay and pulsed field gel electrophoresis, that hyperosmotic stress causes DNA damage in the form of double strand breaks (dsb). Different solutes increase the rate of DNA dsb to different degrees at identical strengths of hyperosmolality. Hyperosmolality in the form of elevated NaCl (HNa) is most potent in this regard, whereas hyperosmolality in the form of elevated urea (HU) does not cause DNA dsb. The amount of DNA dsb increases significantly as early as 15 min after the onset of HNa. By using neutral comet and DNA ladder assays, we show that this rapid induction of DNA damage is not attributable to apoptosis. We demonstrate that renal inner medullary cells are able to efficiently repair hyperosmotic DNA damage within 48 h after exposure to hyperosmolality. DNA repair correlates with cell survival and is repressed by 25 microM LY294002, an inhibitor of DNA-activated protein kinases. These results strongly suggest that the hyperosmotic stress resistance of renal inner medullary cells is based not only on adaptations that protect cellular proteins from osmotic damage but, in addition, on adaptations that compensate DNA damage and maintain genomic integrity.

Hypertonic treatment inhibits radiation-induced nuclear translocation of the Ku proteins G22p1 (Ku70) and Xrcc5 (Ku80) in rat fibroblasts

The effects of X irradiation and hypertonic treatment with 0.5 M NaCl on the subcellular localization of the Ku proteins G22p1 (also known as Ku70) and Xrcc5 (also known as Ku80) in rat fibroblasts with normal radiosensitivity were examined using confocal laser microscopy and immunoblotting. Although these proteins were observed mainly in the nuclei of human fibroblasts, approximately 80% of the intensities of immunofluorescence from both G22p1 and Xrcc5 was observed in the cytoplasm of rat fibroblasts. When the rat cells were X-irradiated with 4 Gy, the intensities of the fluorescence derived from G22p1 and Xrcc5 in the nuclei increased from 20% to 50% of the total cellular fluorescence intensity at 20 min postirradiation. No significant differences were observed between the total intensities of the cellular fluorescence from the proteins in unirradiated and irradiated rat fibroblasts. The results showed that the proteins were translocated from the cytoplasm to the nucleus in the rat cells after X irradiation. The nuclear translocation of the proteins from the cytoplasm was inhibited by hypertonic treatment of the cells with 0.5 M NaCl for 20 min, which inhibits the fast repair process of potentially lethal damage (PLD). When the rat cells were treated with 0.5 M NaCl immediately after X irradiation, the repair of DNA DSBs was inhibited. The surviving fraction was approximately 60% of that of irradiated cells that were not treated with 0.5 M NaCl. The surviving fraction increased with incubation time in the growth medium before treatment with NaCl. The proportions of the intensities of fluorescence from G22p1 in the nuclei of X-irradiated cells also increased from 20% to 50% with increasing interval between X irradiation and treatment with NaCl. These results suggest that nuclear translocation of G22p1 and Xrcc5 is important for the fast repair process of PLD in rat cells.

Cellular osmoregulation: beyond ion transport and cell volume

All cells are characterized by the expression of osmoregulatory mechanisms, although the degree of this expression is highly variable in different cell types even within a single organism. Cellular osmoregulatory mechanisms constitute a conserved set of adaptations that offset antagonistic effects of altered extracellular osmolality/environmental salinity on cell integrity and function. Cellular osmoregulation includes the regulation of cell volume and ion transport but it does not stop there. We know that organic osmolyte concentration, protein structure, cell turnover, and other cellular parameters are osmoregulated as well. In this brief review two important aspects of cellular osmoregulation are emphasized: 1) maintenance of genomic integrity, and 2) the central role of protein phosphorylation. Novel insight into these two aspects of cellular osmoregulation is illustrated based on two cell models, mammalian kidney inner medullary cells and teleost gill epithelial cells. Both cell types are highly hypertonicity stress-resistant and, therefore, well suited for the investigation of osmoregulatory mechanisms. Damage to the genome is discussed as a newly discovered aspect of hypertonic threat to cells and recent insights on how mammalian kidney cells deal with such threat are presented. Furthermore, the importance of protein phosphorylation as a core mechanism of osmosensory signal transduction is emphasized. In this regard, the potential roles of the 14-3-3 family of phospho-protein adaptor molecules for cellular osmoregulation are highlighted primarily based on work with fish gill epithelial cells. These examples were chosen for the reader to appreciate the numerous and highly specific interactions between stressor-specific and non-specific pathways that form an extensive cellular signaling network giving rise to adaptive compensation of hypertonicity. Furthermore, the example of 14-3-3 proteins illustrates that a single protein may participate in several pathways that are non-specific with regard to the type of stress and, at the same time, in stress-specific pathways to promote cell integrity and function during hypertonicity.

Deficiency in fast repair process of potentially lethal damage induced by X-irradiation in fibroblasts derived from LEC strain rats

The time course for the repair of PLD in LEC and WKAH rat cells irradiated at 5 Gy was examined. In the case of WKAH rat cells, the surviving fraction increased with increasing incubation times after X-irradiation. When hypertonic treatment was performed at each incubation time with 0.5 M NaCl for 20 min, increase in the surviving fractions was not shown. In contrast, no significant recovery of the surviving fraction in LEC rat cells was observed after incubation of irradiated cells with or without 0.5 M NaCl for 20 min. On dose-survival curves, hypertonic treatment with 0.5 M NaCl enhanced radiosensitivity of WKAH rat cells, but not LEC rat cells. Although the surviving fraction of the cells from backcross mice with normal radiosensitivity reduced by treatment with 0.5 M NaCl, the survival fraction was not affected in the cells from backcross mice with higher radiosensitivity by treatment with 0.5 M NaCl. When the cells were X-irradiated and incubated with or without 0.225 M NaCl, the radiosensitivities of LEC and WKAH rat cells treated with 0.225 M NaCl for 4 h were approximately two-fold higher than those of untreated cells. Treatment with caffeine also reduced the surviving fractions of both X-irradiated LEC and WKAH rat cells, compared with those of untreated cells. These results indicated that the slow repair of PLD occurred in LEC rat cells but not the fast repair of PLD.

Alteration of gamma-ray-induced chromosome aberration by 0.5 M NaCl in Chinese hamster cells

Hypertonic treatment (0.5 M NaCl in phosphate-buffered saline, pH 7.2) at 37 degrees C for 20 min slightly delayed the mitotic frequency for non-irradiated cells in G1 and G2 phases. The mitotic frequency for irradiated cells in G2 was delayed by hypertonic treatment, and that in G1 was slightly delayed by hypertonic treatment. Hypertonic treatment in non-irradiated cells did not induce any chromosomal or chromatid aberrations in either G1 or G2. Chromosomal aberrations caused by gamma-irradiation were slightly enhanced by hypertonic treatment, and chromatid aberrations were markedly enhanced by hypertonic treatment. The enhancement ratio of gamma-irradiation-induced chromatid breaks and exchanges was 1.4 and 3.0, respectively. This cell cycle dependency of chromosome aberrations induced by postirradiation hypertonic treatment was the same as that of cell survival. These findings suggested that hypertonic treatment modifies the rejoining of DNA strand breaks in G2, but slightly modifies that in G1.

Postirradiation exposure to hypotonic saline shows normal damage processing in radiation-sensitive cell lines

Three pairs of cell lines (one human and two Chinese hamster ovary (CHO) cell lines) each comprising a cell line with a normal radiation response and a radiation-sensitive mutant, were evaluated for recovery of potentially lethal damage (PLDR) and recovery of sublethal damage (SLDR). In all cases, the normal cell lines (GM1522, human; AA8-4 and K1, CHO) exhibited capacity for PLDR and SLDR was also normal in the two CHO lines. For the mutants (GM3395, human AT; V3 and 5-11, CHO) there was no ability for PLDR and SLDR was also absent in the two CHO cell lines. Postirradiation exposure to hypotonic NaCl solutions resulted in fixation of radiation damage in all the cell lines. This form of damage is repaired if left unperturbed after irradiation. This shows that cells have a large capacity for repair of this form of damage which accounts for much greater changes in survival than those observed in conventional PLDR experiments. These data show that the mutant cell lines retained their capacity to repair the damage which was susceptible to postirradiation fixation by anisotonic salt solutions. In addition, initial (i.e. preirradiation) DNA polymerase activities were measured in the four CHO cell lines; they were not correlated to radiation sensitivity.

Sublethal damage, potentially lethal damage, and chromosomal aberrations in mammalian cells exposed to ionizing radiations

Sublethal and potentially lethal damage are abstract terms that originated and were defined without reference to molecular and subcellular entities in which such radiation damage was registered. The establishment of a cause-and-effect relationship between chromosome fragment loss and cell killing by ionizing radiations, along with a substantial body of knowledge in radiation cytogenetics, allows a definition of these terms in a context where hypotheses become testable and progress toward a better understanding of these phenomena is more likely. Accordingly, the simplest hypothesis which best fits the observations is as follows. Most aberrations are exchange types requiring an interaction to form the exchange between two broken regions of a chromosome or chromosomes, that is, a break-pair. Very few single breaks fail to rejoin or restitute, so the vast majority are sublethal. Any such sublethal break may become a potentially lethal break-pair if another sublethal break occurs within some range where it is possible for the two to interact. The proportion of break-pairs in which a mis-repair event results in a lethal acentric fragment-producing exchange, can be altered depending on treatment conditions. Such conditions change the balance between "PLD repair" and "PLD fixation." Studies on the control of radiosensitivity have focused on differences in repair processes, but large differences in radiation response may just as well occur with identical repair processes in operation but with different conditions of fixation.

Significance and measurement of DNA double strand breaks in mammalian cells

Techniques for measuring DNA double strand breaks in mammalian cells are being used increasingly by researchers studying both physiological processes, such as recombination, replication, and apoptosis, as well as pathological processes, such as clastogenesis induced by ionizing radiation, chemotherapeutic drugs, and chemical toxicants. In this review we evaluate commonly used assays for measuring DNA double strand breaks, focusing on neutral filter elution and pulsed field gel electrophoresis, and explore the advantages and limitations of applying these techniques to problems of current interest in carcinogenesis and genetic toxicology.