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

BACKGROUND, The current study was undertaken to investigate the influence o f wild-type or mutant p53 status on the radiosensitizing effect of paclitax el in colorectal tumor cell lines. METHODS. HCT-116 (contains wild-type p53) and MT-29 (contains mutant p53) e stablished from moderately differentiated colorectal carcinomas were used i n this study. Colony-forming assay was performed after exposure to either d ifferent radiation doses (0.5-6 gray [GY]) or paclitaxel (1-10 nM) or in co mbination. Induction of p53 and p21(waf1/cip1) by these treatments were det ermined by immunocytochemistry and Western blot analysis. RESULTS. Radiation caused an increase in nuclear p53 and p21(waf1/cip1) pro teins in HCT-116 cells, indicating that p53 functionally induced p21(waf1/c ip1). However, induction of nuclear p53 and p21(waf1/cip1) protein was not evident in MT-29 cells, suggesting that p53 was not functional in these cel ls. Survival data showed that the HCT-116 cells (survival fraction of expon entially 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 % [D-0] = 223 centigray [cGy]) were significantly sensitive to ionizing rad iation (P < 0.008) when compared with the MT-29 cells (SF2 = 0.614; D-0 = 3 51 cGy). Paclitaxel caused a higher degree of clonogenic inhibition in HCT- 116 (D-0 = 0.7 nM) than MT-29 (D-0 = 1.11 nM) cells (P < 0.06). When paclit axel and radiation were combined, an enhanced radiosensitizing effect (P < 0.05] was observed in HCT-116 cells (SF2 = 0.138; D-0 = 103 cGy), whereas i n MT-29 cells no significant radiosensitization of paclitaxel was observed (SF2 = 0.608; D-0 = 306 cGy). However, pretreatment with paclitaxel followe d by multifractionated low dose radiation (0.5- or l-Gy fractions for a tot al dose of 2 Gy) significantly enhanced the radiosensitizing effect in both HCT-116 and MT-29 cells. CONCLUSIONS. The results of the current study suggested that multifractiona ted radiation given at very low doses after exposure of cells to paclitaxel conferred a potent radiation sensitizing effect irrespective of p53 status . (C) 2000 American Cancer Society.