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Molecular and Cellular Biology, November 2006, p. 8267-8280, Vol. 26, No. 22
0270-7306/06/$08.00+0     doi:10.1128/MCB.00201-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Only Akt1 Is Required for Proliferation, while Akt2 Promotes Cell Cycle Exit through p21 Binding{triangledown} ,{dagger}

Lisa Héron-Milhavet,1 Celine Franckhauser,1 Vanessa Rana,1 Cyril Berthenet,1 Daniel Fisher,3 Brian A. Hemmings,2 Anne Fernandez,1 and Ned J. C. Lamb1*

Cell Biology Unit, Institut de Génétique Humaine, CNRS UPR1142, 141 rue de la Cardonille, 34396 Montpellier Cedex 5, France,1 Friedrich Miescher Institute, Maulbeerstrasse 66, CH 4002 Basel, Switzerland ,2 IGMM, CNRS UMR5535, Montpellier, France3

Received 3 February 2006/ Returned for modification 1 May 2006/ Accepted 28 August 2006

Protein kinase B (PKB/Akt) is an important modulator of insulin signaling, cell proliferation, and survival. Using small interfering RNA duplexes in nontransformed mammalian cells, we show that only Akt1 is essential for cell proliferation, while Akt2 promotes cell cycle exit. Silencing Akt1 resulted in decreased cyclin A levels and inhibition of S-phase entry, effects not seen with Akt2 knockdown and specifically rescued by microinjection of Akt1, not Akt2. In differentiating myoblasts, Akt2 knockout prevented myoblasts from exiting the cell cycle and showed sustained cyclin A expression. In contrast, overexpression of Akt2 reduced cyclin A and hindered cell cycle progression in M-G1 with increased nuclear p21. p21 is a major target in the differential effects of Akt isoforms, with endogenous Akt2 and not Akt1 binding p21 in the nucleus and increasing its level. Accordingly, Akt2 knockdown cells, and not Akt1 knockdown cells, showed reduced levels of p21. A specific Akt2/p21 interaction can be reproduced in vitro, and the Akt2 binding site on p21 is similar to that in cyclin A spanning T145 to T155, since (i) prior incubation with cyclin A prevents Akt2 binding, (ii) T145 phosphorylation on p21 by Akt1 prevents Akt2 binding, and (iii) binding Akt2 prevents phosphorylation of p21 by Akt1. These data show that specific interaction of the Akt2 isoform with p21 is key to its negative effect on normal cell cycle progression.

* Corresponding author. Mailing address: Cell Biology Unit, Institut de Génétique Humaine, CNRS UPR1142, 141 rue de la Cardonille, 34396 Montpellier Cedex 5, France. Phone and fax: 33 499 61 9966. E-mail: Ned.Lamb{at}acrux.igh.cnrs.fr.

{triangledown} Published ahead of print on 18 September 2006.

{dagger} Supplemental material for this article may be found at http://mcb.asm.org/.

Molecular and Cellular Biology, November 2006, p. 8267-8280, Vol. 26, No. 22
0270-7306/06/$08.00+0     doi:10.1128/MCB.00201-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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