Cell Killing and Sensitization to Heat Shock by Hypothermic Incubation of Asynchronous and Synchronized Mouse Neuroblastoma Cells

The effect of hypothermia on cell survival and on subsequent response to hyperthermia was studied in asynchronous and synchronized Neuro-2A cells. Cell cycle progression was blocked at temperatures below 27°C. Immediately after shift to hypothermic temperatures, cells became more sensitive to hyperthermia. Development of thermosensitization was time and temperature dependent. Thermosensitization of cells by hypothermia was high at 0°C and 15°-30°C and less at 5°-10°C. Sensitization started to occur before hypothermic cell death became manifest and developed gradually. Hypothermic cell death was observed when the cells were incubated for more than 1 day at temperatures of 0°-24°C with a minimal cell death during incubation at 6°C. Thermosensitization of cells by hypothermia depended on the position of the cell in the cell cycle at the time of shift to hypothermic temperatures. Cells in late G₁ and early S phase became more thermosensitive than did cells in G₁ or late S-G₂ phase. Furthermore G1-S cells were more sensitive to prolonged hypothermia alone than were G₁ or late S-G₂ cells. In contrast, late S-G₂ cells were most sensitive to hyperthermia alone. It is concluded that the temperature- and cell cycle-dependent way of hypothermic induced cell death was similar to the thermosensitization of cells by hypothermia. But thermosensitization became manifest prior to the actual cell death, following hypothermic treatment.