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Molecular and Cellular Biology, March 2009, p. 1235-1248, Vol. 29, No. 5
0270-7306/09/$08.00+0 doi:10.1128/MCB.00668-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Unscheduled Akt-Triggered Activation of Cyclin-Dependent Kinase 2 as a Key Effector Mechanism of Apoptin's Anticancer Toxicity
Subbareddy Maddika ,1,2,
,
Soumya Panigrahi,1,2,
Emilia Wiechec,1,3
Sebastian Wesselborg,4
Ute Fischer,5,6
Klaus Schulze-Osthoff,5,
* and
Marek Los5,7,
Manitoba Institute of Cell Biology, CancerCare Manitoba,1 Department of Biochemistry and Medical Genetics, University of Manitoba, Winnipeg, Manitoba R3E 0V9, Canada,2 Department of Human Genetics, University of Aarhus, D-8000 Aarhus, Denmark,3 Department of Internal Medicine, University of Tübingen, D-72076 Tübingen, Germany,4 Interfaculty Institute for Biochemistry, University of Tübingen, D-72076 Tübingen, Germany,5 Institute of Molecular Medicine, University of Düsseldorf, D-40225 Düsseldorf, Germany,6 BioApplications Enterprises, Winnipeg, Manitoba R3E 0V9, Canada7
Received 24 April 2008/ Returned for modification 15 June 2008/ Accepted 10 December 2008
Apoptin, a protein from the chicken anemia virus, has attracted attention because it specifically kills tumor cells while leaving normal cells unharmed. The reason for this tumor selectivity is unclear and depends on subcellular localization, as apoptin resides in the cytoplasm of normal cells but in the nuclei of transformed cells. It was shown that nuclear localization and tumor-specific killing crucially require apoptin's phosphorylation by an as yet unknown kinase. Here we elucidate the pathway of apoptin-induced apoptosis and show that it essentially depends on abnormal phosphatidylinositol 3-kinase (PI3-kinase)/Akt activation, resulting in the activation of the cyclin-dependent kinase CDK2. Inhibitors as well as dominant-negative mutants of PI3-kinase and Akt not only inhibited CDK2 activation but also protected cells from apoptin-induced cell death. Akt activated CDK2 by direct phosphorylation as well as by the phosphorylation-induced degradation of the inhibitor p27Kip1. Importantly, we also identified CDK2 as the principal kinase that phosphorylates apoptin and is crucially required for apoptin-induced cell death. Immortalized CDK2-deficient fibroblasts and CDK2 knockdown cells were markedly protected against apoptin. Thus, our results not only decipher the pathway of apoptin-induced cell death but also provide mechanistic insights for the selective killing of tumor cells.
* Corresponding author. Mailing address: University of Tübingen, Interfaculty Institute for Biochemistry, Hoppe-Seyler-Str. 4, D-72076 Tübingen, Germany. Phone: 49-7071-2973399. Fax: 49-7071-295565. E-mail: kso{at}uni-tuebingen.de
Published ahead of print on 22 December 2008.
S.M. and S.P. contributed equally to this study.
Present address: Department of Therapeutic Radiology, Yale School of Medicine, New Haven, CT 06520-8040.
K.S.-O. and M.L. shared senior authorship of this report.
Molecular and Cellular Biology, March 2009, p. 1235-1248, Vol. 29, No. 5
0270-7306/09/$08.00+0 doi:10.1128/MCB.00668-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
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