Downregulation of Wip-1 phosphatase expression in MCF-7 breast cancer cells enhances doxorubicin-induced apoptosis through p53-mediated transcriptional activation of Bax

Cancer Biol Ther. 2009 Mar 15;8(6):555-63. doi: 10.4161/cbt.8.6.7742. Epub 2009 Mar 15.


The human breast cancer cell line MCF-7 carries an amplified PPM1 D/Wip-1 gene and over expresses Wip-1 phosphatase protein. MCF-7 cells also harbor a wild type p53 gene. We established stable isogenic lines (MCF-Sp53 clones) which exhibit decreased levels of p53 protein. We show that although the PPM1 D gene is amplified in MCF-7 cells it is still expressed in a p53-dependent manner. Stable isogenic cell lines derived from MCF-7 cells (designated MCF-clones) were also established in which Wip-1 expression is significantly decreased by a plasmid-based PPM1D antisense RNA. Decreasing Wip-1 expression sensitized MCF-clones to doxorubicin-induced apoptosis. The enhanced apoptotic response was correlated with increased phosphorylation of N-terrninal p53-Ser15 and -Ser46 and increased expression of the pro-apoptotic Bax gene at both the mRNA and protein level. The enhanced apoptotic response was blocked by Bax-siRNA knock down suggesting that the increased response was a result of increased Bax protein expression. Moreover, reporter gene assays using the Waf-1 and Bax promoters to drive a luciferase gene revealed that luciferase activity driven by the Bax promoter was enhanced in MCF-clones while luciferase activity driven by the Waf-1 promoter was decreased relative to parental MCF-7 cells. The study reveals a novel molecular mechanism involving Wip-1 phosphatase, p53 phosphorylation and an enhanced apoptotic response mediated by transcriptional activation of the pro-apoptotic Bax gene. W. Edward Mercer Ph.D., who made great contributions to this paper, passed away on Thursday, October 30, 2008, after a brief illness. This paper is in memorial to his honorable attitude toward science and education.

MeSH terms

  • Antibiotics, Antineoplastic / pharmacology
  • Apoptosis / drug effects*
  • Apoptosis / genetics
  • Base Sequence
  • Breast Neoplasms / genetics*
  • Breast Neoplasms / metabolism*
  • Cell Line, Tumor
  • Doxorubicin / pharmacology*
  • Female
  • Gene Expression Regulation, Neoplastic / drug effects
  • Gene Silencing
  • Genes, Reporter
  • Humans
  • Phosphoprotein Phosphatases / genetics*
  • Phosphorylation
  • Promoter Regions, Genetic
  • Protein Phosphatase 2C
  • RNA Interference
  • Transcriptional Activation
  • Tumor Suppressor Protein p53 / metabolism*
  • bcl-2-Associated X Protein / genetics*


  • Antibiotics, Antineoplastic
  • Tumor Suppressor Protein p53
  • bcl-2-Associated X Protein
  • Doxorubicin
  • PPM1D protein, human
  • Phosphoprotein Phosphatases
  • Protein Phosphatase 2C