Differential effect of benzamide on NAD+ content and the frequency of chromatid aberrations in X-irradiated L5178Y-R and L5178Y-S cells

Intrinsic radiation sensitivity: cellular signaling is the key

The concept that the balance between DNA damage and repair determines intrinsic radiation sensitivity has dominated radiobiology for several decades. There is undeniably a cause- effect relationship between radiation-induced molecular alterations in the genomic DNA and cellular consequences. In the last decade, however, it has become obvious that the chromatin context affects the fate of damaged DNA and that cellular signaling is an important factor in defining intrinsic radiation sensitivity. Damaged DNA is the site of signal generation; however, alternative signaling at the plasma membrane is triggered: Reactive oxygen species (ROS) inactivate phosphatases and consequently cause activation of kinases localized at the plasma membrane; this includes ligand-independent activation of receptor kinases. Cells with an apparently functional DNA repair system may show increased radiation sensitivity due to deficiencies in specific kinases essential for repair activation and checkpoint control. Other signals that determine intrinsic radiosensitivity may affect proneness to apoptosis, the balance between DNA damage fixation and repair, and the translocation of proteins participating in the response to ionizing radiation. Interplay between the various signals decides the extent to which the repair of radiation-inflicted damage is supported or limited; in some cell types, this includes DNA-damage-independent processes guided by plasma membrane-generated signaling. Cellular signaling in the context of specific subcellular structures is the key to understanding how the molecular effects of radiation are expressed as biological consequences in various cell types. A systems approach should bring us closer to this end.

L5178Y sublines: a look back from 40 years. Part 2: response to ionizing radiation

The aim was to review and summarize the results of 40 years of study concerning the response to ionizing radiation of the pair of L5178Y (LY) sublines, LY-R and LY-S, that differ in sensitivity to various DNA-damaging agents, among them X- and gamma-rays. The reviewed data indicate the key importance of DNA damage repair and fixation for the ultimate fate of the irradiated LY cell. The cause of slow double-strand break (DSB) repair in LY-S cells is not identified, but a defect in non-homologous end-joining (NHEJ) would explain most features of the cellular response of LY-S cells to irradiation, as compared with repair-competent LY-R cells. The most prominent features are the very high radiosensitivity of G1 cells, extensive poly(ADP-ribose)-dependent damage fixation, long G2 arrest, considerable chromosomal damage seen as premature chromatin condensation (PCC) fragments and aberrations in metaphase cells. The main cause of radiosensitivity difference between LY sublines is in DNA repair/damage fixation ability. At the level of damage corresponding to a comparable lethal effect, the type of death differs between LY sublines; LY-S cells die in considerably greater proportion by apoptosis than LY-R cells, whereas the latter die in greater proportion by necrosis. This observation is consistent with differential expression of proteins that are pro- or anti-apoptotic. The prominent role of poly(ADP-ribosylation) in the response of LY-S cells apparently is connected with damage fixation, but is in contrast to other cell lines hypersensitive to X- or gamma-radiation with DSB repair defects.

Differential DNA double strand break fixation dependence on poly(ADP-ribosylation) in L5178Y and CHO cells

PURPOSE

To investigate the role of poly(ADP-ribosylation) in DNA double-strand break repair and fixation in murine lymphoma L5178Y (LY) sublines, LY-R and LY-S, and a pair of Chinese hamster ovary lines: wild-type and mutant xrs6 cells, that have differences in repair competence and degree of radiosensitization with poly(ADP-ribosylation) inhibitors.

MATERIALS AND METHODS

Cells (asynchronous, logarithmic phase) were pre-incubated with 2 mM aminobenzamide at 37 or 25 degrees C, X-irradiated with 10 Gy and allowed to repair DNA breaks for 15, 60 and 120 min at 37 or 25 degrees C. The remaining double-strand break were estimated by the neutral comet assay.

RESULTS

At 37 degrees C, no effect of AB treatment on the repair kinetics was observed either in xrs6 or Chinese hamster ovary (wild-type) cells. In contrast, aminobenzamide decreased the repair of double-strand break in the LY-S line but not the LY-R line, in agreement with the previously observed radiosensitization of LY cells by poly(ADP-ribosylation) inhibition. However, double-strand break rejoining in the repair competent cell lines, Chinese hamster ovary and LY-R, also was affected by aminobenzamide when the post-irradiation incubation was carried out at 25 degrees C. Analysis of these results together with earlier data on LY-S cells have been interpreted in terms of Radford's model of radiation damage fixation.

CONCLUSION

The reported results indicate that poly(ADP-ribosylation) can be an important modulator of the conversion of DNA damage to lethal events.

Ca2+ mobilization is related to the lethal effect of X-irradiation in L5178Y cells

Radiation-resistant L5178Y-R (LY-R) and radiation-sensitive L5178Y-S (LY-S) murine lymphoma cells were X-irradiated and their free Ca2+ concentration was examined with Fura-2 in Ca2(+)-free salt solution. The release of free Ca2+ from intracellular stores was linear between 10 and 60 min after irradiation (1-5 Gy X-rays) and was higher in LY-S than in LY-R cells. Pre-treatment with 2 mM benzamide (Bz) further increased the concentration of free Ca2+ in LY-S cells but not in LY-R cells. In contrast with LY-R cells, LY-S cells had previously been found to be radiosensitized by continuous 2 mM Bz treatment (Szumiel et al. 1984b). Thus, there was a parallel effect of Bz on survival and on the increase in free Ca2+ concentration in LY cells. Moreover, the rates of Ca2+ release after irradiation with 1-5 Gy of X-rays with or without Bz treatment were inversely related to the respective surviving fractions of LY cells.