Preferential radiosensitization in p53-mutated human tumour cell lines by pentoxifylline-mediated disruption of the G2/M checkpoint control

PURPOSE: There is evidence that the duration of the G2/M delay following irradiation is correlated with cell survival. We studied the radiosensitizing potential of pentoxifylline (PTX) and the PTX-mediated modulation of cell-cycle progression dependent on the p53 status of various human tumour cell lines. MATERIALS AND METHODS: The cellular radiosensitivity of human MCF-7 (wild-type p53) and HT-29 (p53-defective) tumour cells, which were exposed to PTX (2 mM) immediately after gamma-irradiation was determined by colony forming assay. The influence on cell cycle progression after irradiation (6 Gy) was assessed by flow cytometric analysis using p53 wild-type MCF-7 and HPR600 cells, and p53-defective HT-29 and WiDr cells. RESULTS: Clonogenic survival assays up to 8 Gy demonstrated that p53-defective HT-29 cells (sensitizer enhancement ratio [SER]=1.54) were sensitized by PTX (2 mM) to a significantly higher degree than p53 wild-type MCF-7 (SER=1.14) cells. Exposure of irradiated (6 Gy) cells to PTX (2 mM) resulted in abrogation of the radiation-induced G2/M arrest in the p53-defective HT-29 and WiDr cells, whereas the p53 wild-type-expressing MCF-7 and HPR600 cells showed less significant impairment of the G2/M checkpoint. In HT-29 cells, the rate of transition into mitosis was even higher than in the sham-treated control cells. G2/M abrogation was accompanied by an increase of apoptosis only in HPR600 cells. CONCLUSIONS: Since PTX was less effective in cells expressing intact p53, the application of PTX suggests a promising strategy of pharmacological disruption of the G2/M checkpoint control by which preferentially radiation-resistant tumours with defective p53 function might be rendered more sensitive to ionizing radiation.