The magnitude and time-dependence of the apoptotic response of normal and malignant cells subjected to ionizing radiation versus hyperthermia

Purpose: The purpose of the study was to examine the optimal time of exposure and dose of heat and ionizing radiation that results in the killing of human cancer cells in vitro via apoptosis vs. necrosis. Materials and methods: Human mammary carcinoma, colorectal carcinoma and normal bovine capillary endothelial (BCE) cell lines were subjected to 20 Gy ionizing radiation and 6, 12, 24, and 72 h later assessed for apoptosis using detection of apoptotic bodies and caspase assays. Necrosis was detected by loss of cells from the surface and lactate dehydrogenase (LDH) release. The colorectal carcinoma cells were subjected to hyperthermia using temperatures ranging from 39 - 44°C for 5, 15 or 45 min. exposures and at varying times post-treatment, apoptosis and necrosis were measured. Results: In response to ionizing radiation, none of the cells underwent necrosis and some cell types apoptosed 24 and 72 h posttreatment. The colorectal cancer cells exhibited a steady increase of apoptosis at 6, 12, and 24 h. When these cells were exposed to 40°C for 5 min, caspases increased within 6 h and a significant fraction (50%) of cells apoptosed. If the time of exposure to 40°C was increased to 15 or 45 min, 80% and 100% of the dying cells apoptosed, respectively. A temperature of 39°C did not cause cell death even after 45 min exposures. If heat was elevated to 42 or 44°C, increased necrosis was observed with a corresponding decrease in apoptosis. Conclusions: These studies reveal time and temperature dependent in vitro cell responses to ionizing radiation and water-bath hyperthermia.