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

Abstract

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-G 1 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.

Akt2, but not Akt1, is required for cell survival by inhibiting activation of JNK and p38 after UV irradiation

“…It has been reported that the Tcl1 oncoprotein interacts with Akt1, but not with Akt2 (Pekarsky et al, 2000). More recently, Heron-Milhavet et al (2006) demonstrated that Akt isoforms regulate p21 function in opposite manners. Whereas Akt1 phosphorylates p21, inhibiting its function and thereby promoting cellular proliferation, Akt2 binds to p21 and prevents phosphorylation by Akt1.…”

PKB-mediated PHF20 phosphorylation on Ser291 is required for p53 function in DNA damage

“…Furthermore, PKBα directly phosphorylates and inhibits p21, thereby blocking cell-cycle progression [18]. Recently PKB was shown to activate and phosphorylate transcription factor Twist-1, resulting in the inhibition of p53 induction in response to DNA damage in MCF-7 cells [19].…”

Promiscuous affairs of PKB/AKT isoforms in metabolism

“…Since the identification of PKB as a serine/threonine kinase, almost 20 years ago, PKB isoforms were studied intensively and are now considered as major regulators of elementary cellular process, such as proliferation, survival, cell growth and energy metabolism (Bozulic et al, 2008, Contreras-Ferrat et al, 2010, Haga et al, 2005, Heron-Milhavet et al, 2006. Consequently, PKB isoforms play pivotal roles in physiology and their deregulation engenders diseases, such as cancer, neurodegeneration and metabolic disorders (Altomare andTesta, 2005, Zhao andTownsend, 2009).…”