Senescence of human fibroblasts induced by oncogenic Raf

  1. Jiyue Zhu1,3,
  2. Douglas Woods2,3,
  3. Martin McMahon2,4, and
  4. J. Michael Bishop1
  1. 1Department of Microbiology and Immunology and G.W. Hooper Foundation, University of California at San Francisco (UCSF), California 94143-0552 USA; 2Department of Cell Signaling, DNAX Research Institute, Palo Alto, California 94304-1104 USA

Abstract

The oncogenes RAS and RAF came to view as agents of neoplastic transformation. However, in normal cells, these genes can have effects that run counter to oncogenic transformation, such as arrest of the cell division cycle, induction of cell differentiation, and apoptosis. Recent work has demonstrated that RAS elicits proliferative arrest and senescence in normal mouse and human fibroblasts. Because the Raf/MEK/MAP kinase signaling cascade is a key effector of signaling from Ras proteins, we examined the ability of conditionally active forms of Raf-1 to elicit cell cycle arrest and senescence in human cells. Activation of Raf-1 in nonimmortalized human lung fibroblasts (IMR-90) led to the prompt and irreversible arrest of cellular proliferation and the premature onset of senescence. Concomitant with the onset of cell cycle arrest, we observed the induction of the cyclin-dependent kinase (CDK) inhibitors p21Cip1 and p16Ink4a. Ablation of p53 and p21Cip1 expression by use of the E6 oncoprotein of HPV16 demonstrated that expression of these proteins was not required for Raf-induced cell cycle arrest or senescence. Furthermore, cell cycle arrest and senescence were elicited in IMR-90 cells by the ectopic expression of p16Ink4a alone. Pharmacological inhibition of the Raf/MEK/MAP kinase cascade prevented Raf from inducing p16Ink4a and also prevented Raf-induced senescence. We conclude that the kinase cascade initiated by Raf can regulate the expression of p16Ink4a and the proliferative arrest and senescence that follows. Induction of senescence may provide a defense against neoplastic transformation when the MAP kinase signaling cascade is inappropriately active.

Keywords

Footnotes

  • 3 These authors contributed equally to this work.

  • 4 Corresponding author. Present address: Cancer Research Institute, UCSF Cancer Center, San Francisco, California 94145 USA.

  • E-MAIL mcmahon{at}cc.ucsf.edu; FAX (415) 502-3179.

    • Received June 8, 1998.
    • Accepted August 5, 1998.
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