Esther C. Shiu

Similarities in Cellular Inactivation by Hyperthermia or by Ethanol

The question of how cells are inactivated by mild hyperthermia (41 to 45/sup 0/C) is difficult to resolve because heat stresses most (if not all) cellular organelles. As a result, studies showing correlations between cell death resulting from heat exposure and damage to specific subcellular units tend to be inconclusive. We have looked for a heat analog, i.e., another agent whose mode of cell inactivation closely resembles that of heat. Our hypothesis is that information about the mechanism of cell inactivation closely resembles that of heat. Our hypothesis is that information about the mechanism of cell inactivation by this agent should aid in identifying the targets for heat inactivation. Here we report results that we interpret as showing that ethanol may be such an analog. Exposure of cells to heat or ethanol induces tolerance to heat, ethanol, or adriamycin. The cytotoxicity of both ethanol and heat is enhanced by low pH and cysteamine; it is reduced by deuterium oxide. Finally, treating cells either before or after x irradiation with either agent enhances cell killing.

Increase in Radiation-Induced HPRT Gene Mutation Frequency after Nonthermal Exposure to Nonionizing 60 Hz Electromagnetic Fields

It is widely accepted that moderate levels of nonionizing electric or magnetic fields, for example 50/60 Hz magnetic fields of about 1 mT, are not mutagenic. However, it is not known whether such fields can enhance the action of known mutagens. To explore this question, a stringent experimental protocol, which included blinding and systematic negative controls, was implemented, minimizing the possibility of observer bias or experimental artifacts. As a model system, we chose to measure mutation frequencies induced by 2 Gy gamma rays in the redox-sensitive hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene in Chinese hamster ovary cells. We tested whether a 12-h exposure to a 60 Hz sinusoidally oscillating magnetic-flux density (Brms = 0.7 mT) could affect the mutagenic effects of ionizing radiation on the HPRT gene locus. We determined that the magnetic-field exposure induced an approximate 1.8-fold increase in HPRT mutation frequency. Additional experiments at Brms = 0.23 and 0.47 mT revealed that the effect was reduced at lower flux densities. The field exposure did not enhance radiation-induced cytotoxicity or mutation frequencies in cells not exposed to ionizing radiation. These results suggest that moderate-strength, oscillating magnetic fields may act as an enhancer of mutagenesis in mammalian cells.

DNA Damage Does Not Appear to be a Trigger for Thermotolerance in Mammalian Cells

The hypothesis that DNA damage is the trigger for thermotolerance in mammalian cells was tested in Chinese hamster ovary cells by looking for evidence of thermotolerance after ionizing radiation or ultraviolet light exposure. As previous studies have demonstrated that relatively non-toxic radiation exposures do not induce thermotolerance in mammalian cells (Li et al. 1976), higher doses, comparable to those used in yeast to induce thermotolerance (Mitchel and Morrison 1984), were tested in this study. Doses of this magnitude are lethal to mammalian cells, thereby precluding the use of clonogenic survival as an endpoint. We therefore used three alternative assays which are indicators of the subsequent development of thermotolerance. These were; (a) heat-induced inhibition of total protein synthesis, (b) heat-induced uptake of dansyl lysine, and (c) synthesis of heat shock proteins. Only total protein synthesis revealed evidence of a small degree of thermotolerance which occurred immediately after ionizing radiation exposure. By 4 h postirradiation the tolerance, as measured by this assay, was no longer evident. No evidence of thermotolerance was seen following UV exposure. In addition, when a large radiation dose was given either immediately before or after a heat treatment used to induce thermotolerance, there was no alteration in the level of heat-induced tolerance, despite the extensive number of DNA stand breaks caused by the radiation. Our data therefore suggest that, in mammalian cells, the type of DNA damage caused by ionizing radiation is not the trigger for the induction of thermotolerance.