Inactivation of p53 and of pRb protects human colorectal carcinoma cells against hyperthermia-induced cytotoxicity and apoptosis

Cell-cycle checkpoints are thought to govern the cellular response to exter nal stimuli. The involvement of the p53 tumour-suppressor protein and the r etinoblastoma protein (pRb) in the cell-cycle checkpoint in G1 phase is wel l established. However, little is known about the importance of these G1 ch eckpoint regulators in hyperthermia-induced cytotoxicity. Such information is relevant because of the clinical application of hyperthermia in combinat ion with chemotherapy or with radiotherapy. The effects of p53 or pRb inact ivation were studied in a well-established isogenic system using the human colorectal carcinoma cell line (RKO). The cells were treated with clinicall y relevant heat doses (60 min at 40-43 degrees C). Cell survival, cell-cycl e redistribution and induction of apoptosis were investigated. Survival of the p53-inactivated transfectants was higher than that of the wild-type p53 cells. The pRb-inactivated transfectants showed an intermediate sensitivit y to hyperthermia. All transfectants showed G2 arrest after hyperthermia an d the appearance of a sub-G1 population. The induction of apoptosis was inh ibited in p53-inactivated and pRb-inactivated transfectants. These results suggest that p53 and/or pRb status may be an important determinant of the c linical response to hyperthermia.