Low dose fractionated radiation enhances the radiosensitization effect of paclitaxel in colorectal tumor cells with mutant p53

Abstract

BACKGROUND The current study was undertaken to investigate the influence of wild-type or mutant p53 status on the radiosensitizing effect of paclitaxel in colorectal tumor cell lines. METHODS HCT-116 (contains wild-type p53) and HT-29 (contains mutant p53) established from moderately differentiated colorectal carcinomas were used in this study. Colony-forming assay was performed after exposure to either different radiation doses (0.5–6 gray [Gy]) or paclitaxel (1–10 nM) or in combination. Induction of p53 and p21waf1/cip1 by these treatments were determined by immunocytochemistry and Western blot analysis. RESULTS Radiation caused an increase in nuclear p53 and p21waf1/cip1 proteins in HCT-116 cells, indicating that p53 functionally induced p21waf1/cip1. However, induction of nuclear p53 and p21waf1/cip1 protein was not evident in HT-29 cells, suggesting that p53 was not functional in these cells. Survival data showed that the HCT-116 cells (survival fraction of exponentially growing cells that were irradiated at the clinically relevant dose of 2 Gy [SF2] = 0.383; dose required to reduce the fraction of cells to 37% [D0] = 223 centigray [cGy]) were significantly sensitive to ionizing radiation (P < 0.008) when compared with the HT-29 cells (SF2 = 0.614; D0 = 351 cGy). Paclitaxel caused a higher degree of clonogenic inhibition in HCT-116 (D0 = 0.7 nM) than HT-29 (D0 = 1.11 nM) cells (P < 0.06). When paclitaxel and radiation were combined, an enhanced radiosensitizing effect (P < 0.05) was observed in HCT-116 cells (SF2 = 0.138; D0 = 103 cGy), whereas in HT-29 cells no significant radiosensitization of paclitaxel was observed (SF2 = 0.608; D0 = 306 cGy). However, pretreatment with paclitaxel followed by multifractionated low dose radiation (0.5- or 1-Gy fractions for a total dose of 2 Gy) significantly enhanced the radiosensitizing effect in both HCT-116 and HT-29 cells. CONCLUSIONS The results of the current study suggested that multifractionated radiation given at very low doses after exposure of cells to paclitaxel conferred a potent radiation sensitizing effect irrespective of p53 status. Cancer 2000;89:1893–900. © 2000 American Cancer Society.