Ectopic expression of the CDK inhibitor p21Cip1 enhances deregulated E2F activity and increases cancer cell-specific cytotoxic gene expression mediated by the ARF tumor suppressor promoter

Biochem Biophys Res Commun. 2017 Jan 29;483(1):107-114. doi: 10.1016/j.bbrc.2016.12.185. Epub 2016 Dec 29.


In cancer treatment, specifically targeting cancer cells is important for optimal therapeutic efficacy. One strategy is to utilize a cancer specific promoter to express a cytotoxic gene or a viral gene required for replication. In this approach, the therapeutic window is dependent on the relative promoter activity in cancer cells versus normal cells. Therefore, a promoter with optimal cancer cell-specificity should be used. The tumor suppressor ARF promoter, which specifically responds to deregulated E2F activity, is a potent candidate. Defects in the RB pathway resulting in deregulated E2F activity are observed in almost all cancers. Furthermore, the ARF promoter exhibits greater cancer cell specificity than the E2F1 promoter and consequently, adenovirus expressing HSV-TK under the control of the ARF promoter (Ad-ARF-TK) has more selective cytotoxicity in cancer cells than the analogous E2F1 construct. Ideally, cancer specific gene expression driven by the ARF promoter could be enhanced for optimal therapeutic efficacy, with minimal side effects. We show here that ectopic expression of the CDK inhibitor p21Cip1 enhanced deregulated E2F activity and pro-apoptotic E2F target gene expression in cancer cells. Moreover, ectopic expression of p21Cip1 augmented cancer specific cytotoxicity of Ad-ARF-TK, and apoptosis induced by p21Cip1 was dependent on deregulated E2F activity. These results suggest that p21Cip1 specifically enhances deregulated E2F activity and that a combination of the CDK inhibitor with Ad-ARF-TK could be effectively employed for cancer therapy.

Keywords: ARF; Cancer specific; E2F; Oncolytic virotherapy; Suicide gene therapy; pRB.

MeSH terms

  • Apoptosis
  • Cell Line, Tumor
  • Cyclin-Dependent Kinase Inhibitor p16
  • Cyclin-Dependent Kinase Inhibitor p18 / genetics
  • Cyclin-Dependent Kinase Inhibitor p18 / metabolism
  • Cyclin-Dependent Kinase Inhibitor p21 / genetics*
  • Cyclin-Dependent Kinase Inhibitor p21 / metabolism*
  • E2F Transcription Factors / genetics*
  • E2F Transcription Factors / metabolism*
  • E2F1 Transcription Factor / genetics
  • E2F1 Transcription Factor / metabolism
  • Ectopic Gene Expression
  • Gene Expression Regulation, Neoplastic
  • Genes, p16*
  • Humans
  • Neoplasms / genetics
  • Neoplasms / metabolism
  • Neoplasms / pathology
  • Promoter Regions, Genetic


  • CDKN1A protein, human
  • CDKN2A protein, human
  • Cyclin-Dependent Kinase Inhibitor p16
  • Cyclin-Dependent Kinase Inhibitor p18
  • Cyclin-Dependent Kinase Inhibitor p21
  • E2F Transcription Factors
  • E2F1 Transcription Factor
  • E2F1 protein, human