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Molecular and Cellular Biology, April 2004, p. 2808-2819, Vol. 24, No. 7
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.7.2808-2819.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Cellular Senescence Requires CDK5 Repression of Rac1 Activity

Kamilah Alexander, Hai-Su Yang,^, and Philip W. Hinds^*

Department of Pathology, Harvard Medical School, Boston, Massachusetts 02115

Received 1 August 2003/ Returned for modification 5 September 2003/ Accepted 22 December 2003

Cellular senescence is a tumor-suppressive process characterized by an irreversible cell cycle exit, a unique morphology, and expression of senescence-associated ß-galactosidase (SA-ß-Gal). We report here a role for CDK5 in induction of senescent cytoskeletal changes. CDK5 activation is upregulated in senescing cells. The increased activity of CDK5 further reduces GTPase Rac1 activity and Pak activation. The repression of the activity of the GTPase Rac1 by CDK5 is required for expression of the senescent phenotype. CDK5 regulation of Rac1 activity is necessary for actin polymerization accompanying senescent morphology in response to expression of pRb, activated Ras, or continuous passage. Inhibition of CDK5 attenuates SA-ß-Gal expression and blocks actin polymerization. These results point to a unique, nonneuronal role for CDK5 in regulation of Rac1 activity in senescence, illuminating the mechanisms underlying induction of senescence and the senescent shape change.

K.A. and H.-S.Y. contributed equally to this work.

Present address: Molecular Oncology Research Institute, Tufts-New England Medical Center, Boston, MA 02111.

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