title: 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 p21(waf1/cip1) by these treatments were determined by immunocytochemistry and Western blot analysis.
  
  
  RESULTS
  Radiation caused an increase in nuclear p53 and p21(waf1/cip1) proteins in HCT-116 cells, indicating that p53 functionally induced p21(waf1/cip1). However, induction of nuclear p53 and p21(waf1/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 [SF(2)] = 0.383; dose required to reduce the fraction of cells to 37% [D(0)] = 223 centigray [cGy]) were significantly sensitive to ionizing radiation (P < 0.008) when compared with the HT-29 cells (SF(2) = 0.614; D(0) = 351 cGy). Paclitaxel caused a higher degree of clonogenic inhibition in HCT-116 (D(0) = 0.7 nM) than HT-29 (D(0) = 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 (SF(2) = 0.138; D(0) = 103 cGy), whereas in HT-29 cells no significant radiosensitization of paclitaxel was observed (SF(2) = 0.608; D(0) = 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.
authors.affiliation: Department of Radiation Medicine, College of Medicine, University of Kentucky, Lexington, Kentucky, USA.
authors.forename: D
authors.initials: D
authors.lastname: Chendil
authors.forename: R
authors.initials: R
authors.lastname: Oakes
authors.forename: R A
authors.initials: RA
authors.lastname: Alcock
authors.forename: N
authors.initials: N
authors.lastname: Patel
authors.forename: C
authors.initials: C
authors.lastname: Mayhew
authors.forename: M
authors.initials: M
authors.lastname: Mohiuddin
authors.forename: V S
authors.initials: VS
authors.lastname: Gallicchio
authors.forename: M M
authors.initials: MM
authors.lastname: Ahmed
mesh_terms: D000972:Antineoplastic Agents, Phytogenic / Q000627:therapeutic use*; D015179:Colorectal Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000378:metabolism; D019583:Dose Fractionation, Radiation; D016158:Genes, p53; D019073:HT29 Cells; D006801:Humans; D009154:Mutation; D017239:Paclitaxel / Q000627:therapeutic use*; D011838:Radiation-Sensitizing Agents / Q000627:therapeutic use*; D014407:Tumor Cells, Cultured; D014410:Tumor Stem Cell Assay; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: D000972:Antineoplastic Agents, Phytogenic; C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D011838:Radiation-Sensitizing Agents; D016159:Tumor Suppressor Protein p53; D017239:Paclitaxel