Cdk4 disruption renders primary mouse cells resistant to oncogenic transformation, leading to Arf/p53-independent senescence (original) (raw)

  1. Xianghong Zou1,
  2. Dipankar Ray1,
  3. Aileen Aziyu1,
  4. Konstantin Christov2,
  5. Alexander D. Boiko1,3,
  6. Andrei V. Gudkov1,3, and
  7. Hiroaki Kiyokawa1,4
  8. Departments of 1Molecular Genetics and 2Surgical Oncology, University of Illinois College of Medicine, Chicago, Illinois 60607, USA; 3Department of Molecular Biology, Lerner Research Institute, the Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA

Abstract

A large number of human cancers display alterations in the_Ink4a/cyclin D/Cdk4_ genetic pathway, suggesting that activation of Cdk4 plays an important role in oncogenesis. Here we report that_Cdk4-null mouse embryonic fibroblasts are resistant to transformation in response to Ras activation with dominant-negative (DN) p53 expression or in the Ink4a/Arf_-null background, judged by foci formation, anchorage-independent growth, and tumorigenesis in athymic mice. Cdk4_-null fibroblasts proliferate at normal rates during early passages. Whereas_Cdk4 +/+ Ink4a/Arf −/− cells are immortal in culture,Cdk4 −/− Ink4a/Arf −/− cells undergo senescence during continuous culture, as do wild-type cells. Activated Ras also induces premature senescence in_Cdk4 −/− Ink4a/Arf −/− cells and_Cdk4 −/− cells with DNp53 expression. Thus, Cdk4 deficiency causes senescence in a unique Arf/p53-independent manner, which accounts for the loss of transformation potential._Cdk4_-null cells express high levels of p21Cip1/Waf1with increased protein stability. Suppression of p21Cip1/Waf1by small interfering RNA (siRNA), as well as expression of HPV-E7 oncoprotein, restores immortalization and Ras-mediated transformation in Cdk4 −/− Ink4a/Arf −/− cells and Cdk4 −/− cells with DNp53 expression. Therefore, Cdk4 is essential for immortalization, and suppression of Cdk4 could be a prospective strategy to recruit cells with inactive Arf/p53 pathway to senescence.

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