Complex Mechanisms Underlying Impaired Activation of Cdk4 and Cdk2 in Replicative Senescence: Roles of p16, p21, and Cyclin D1

• Published in: Experimental cell research Vol. 253; no. 2; pp. 503 - 510
• Main Authors: Morisaki, Hirobumi, Ando, Akikazu, Nagata, Yoshiho, Pereira-Smith, Olivia, Smith, James R., Ikeda, Kyoji, Nakanishi, Makoto
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15.12.1999
• Subjects: Blood Proteins - pharmacology; CAK; CDC2-CDC28 Kinases; Cdk inhibitor; Cdk2; Cdk4; Cell Cycle Proteins; Cell Division - drug effects; Cell Division - physiology; Cells, Cultured; Cellular Senescence - physiology; cyclin D1; Cyclin D1 - metabolism; Cyclin D1 - pharmacology; cyclin E; Cyclin E - metabolism; Cyclin-Dependent Kinase 2; Cyclin-Dependent Kinase 4; Cyclin-Dependent Kinase Inhibitor p16 - metabolism; Cyclin-Dependent Kinase Inhibitor p21; Cyclin-Dependent Kinase Inhibitor p27; Cyclin-Dependent Kinases - metabolism; Cyclins - metabolism; DNA - biosynthesis; Enzyme Activation - drug effects; Enzyme Activation - physiology; Fibroblasts - cytology; Fibroblasts - enzymology; Humans; Microtubule-Associated Proteins - metabolism; Protein-Serine-Threonine Kinases - metabolism; Protein-Serine-Threonine Kinases - pharmacology; Proto-Oncogene Proteins; Resting Phase, Cell Cycle - physiology; senescence; Skin - cytology; Tumor Suppressor Proteins; senescence; cyclin E; Cdk4; Cdk inhibitor; Cdk2; cyclin D1; CAK
• ISSN: 0014-4827; 1090-2422
• DOI: 10.1006/excr.1999.4698

Numerous changes in gene expression are known to occur during replicative senescence, including changes in genes involved in the cell cycle control. In the present study, we have found a severe impairment in the activation of Cdk2 and Cdk4 in response to mitogens in senescent human fibroblasts and determined the molecular basis for this. Although Cdk4 protein was constitutively expressed in senescent cells at the same level as in early-passage young cells, it was found to be complexed with a distinct set of Cdk inhibitors. Cdk4 derived from early passage quiescent cells was effectively activated by incubation with cyclin D1 and Cdk-activating kinase (CAK) in vitro, whereas Cdk4 from senescent cells was not. Cdk2 protein was dramatically decreased in senescent cells and complexed primarily with cyclin D1 and p21. This cyclin D1-bound Cdk2 was not activated by CAK either in vivo or in vitro, implicating cyclin D1 as an inhibitor of Cdk2 activation. Thus, one of the underlying molecular events involved in replicative senescence is the impaired activation of Cdk4 and Cdk2 due to increased binding of p16 to Cdk4 and increased association of Cdk2 with cyclin D1 and p21.