title: Role of the p53/p21 system in the response of human colon carcinoma cells to Doxorubicin.
abstract: BACKGROUND
  Colon adenocarcinomas are refractory to a number of widely used anticancer agents. Multifactorial mechanisms have been implicated in this intrinsically resistant phenotype, including deregulation of cell death pathways. In this regard, the p53 protein has a well established role in the control of tumor cell response to DNA damaging agents; however, the relationship between p53-driven genes and drug sensitivity remains controversial. The present study investigates the role of the p53/p21 system in the response of human colon carcinoma cells to treatment with the cytotoxic agent doxorubicin (DOX) and the possibility to modify the therapeutic index of DOX by modulation of p53 and/or p21 protein levels.
  
  
  METHODS
  The relationship between p53 and p21 protein levels and the cytotoxic effect of DOX was investigated, by MTT assay and western blot analysis, in HCT116 (p53-positive) and HT29 (p53-negative) colon cancer cells. We then assessed the effects of DOX in two isogenic cell lines derived from HCT116 by abrogating the expression and/or function of p53 and p21 (HCT116-E6 and HCT116 p21-/-, respectively). Finally, we evaluated the effect of pre-treatment with the piperidine nitroxide Tempol (TPL), an agent that was reported to induce p21 expression irrespective of p53 status, on the cytotoxicity of DOX in the four cell lines. Comparisons of IC50 values and apoptotic cell percentages were performed by ANOVA and Bonferroni's test for independent samples. C.I. calculations were performed by the combination Index method.
  
  
  RESULTS
  Our results indicate that, in the colon carcinoma cell lines tested, sensitivity to DOX is associated with p21 upregulation upon drug exposure, and DOX cytotoxicity is potentiated by pre-treatment with TPL, but only in those cell lines in which p21 can be upregulated.
  
  
  CONCLUSIONS
  p21 induction may significantly contribute to the response of colon adenocarcinomas cells to DOX treatment; and small molecules that can exploit p53-independent pathways for p21 induction, such as TPL, may find a place in chemotherapeutic protocols for the clinical management of colorectal cancer, where p53 function is often lost, due to genetic or epigenetic defects or to post-transcriptional inactivating mechanisms.
authors.affiliation: Dept. of Structural and Functional Biology, Section of Pharmacology, University of Insubria, Via A, da Giussano 10, 21052 Busto Arsizio (VA), Italy. raffaella.ravizza@uninsubria.it <raffaella.ravizza@uninsubria.it>
authors.forename: Raffaella
authors.initials: R
authors.lastname: Ravizza
authors.forename: Marzia B
authors.initials: MB
authors.lastname: Gariboldi
authors.forename: Laura
authors.initials: L
authors.lastname: Passarelli
authors.forename: Elena
authors.initials: E
authors.lastname: Monti
mesh_terms: D000704:Analysis of Variance; D000903:Antibiotics, Antineoplastic / Q000627:therapeutic use*; D017209:Apoptosis; D018797:Cell Cycle Proteins / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D003110:Colonic Neoplasms / Q000188:drug therapy* / Q000235:genetics / Q000378:metabolism; D003497:Cyclic N-Oxides / Q000627:therapeutic use; D050759:Cyclin-Dependent Kinase Inhibitor p21; D004305:Dose-Response Relationship, Drug; D004317:Doxorubicin / Q000627:therapeutic use*; D019008:Drug Resistance, Neoplasm / Q000235:genetics; D004357:Drug Synergism; D019073:HT29 Cells; D006801:Humans; D009363:Neoplasm Proteins / Q000378:metabolism*; D011500:Protein Synthesis Inhibitors / Q000627:therapeutic use; D013113:Spin Labels; D014162:Transfection / Q000379:methods; D016159:Tumor Suppressor Protein p53 / Q000502:physiology*
chemical_list: D000903:Antibiotics, Antineoplastic; C493956:CDKN1A protein, human; D018797:Cell Cycle Proteins; D003497:Cyclic N-Oxides; D050759:Cyclin-Dependent Kinase Inhibitor p21; D009363:Neoplasm Proteins; D011500:Protein Synthesis Inhibitors; D013113:Spin Labels; D016159:Tumor Suppressor Protein p53; D004317:Doxorubicin; C001803:tempol