C-terminal AKT Phosphorylation by CDK2–Cyclin A or mTOR Activates AKT

The AKT kinase plays a major role in the regulation of cellular proliferation, metabolism, and survival. Hyperactivation of AKT via S473 and T308 phosphorylation has previously been linked with carcinogenesis; however, it remains unclear whether additional molecular mechanisms contribute to AKT activation. Using single live-cell imaging and synchronization experiments, Liu and colleagues found that AKT activity and phosphorylation fluctuate across the cell cycle in a pattern that closely mirrors expression of cyclin A2, an activator of cyclin-dependent kinase 2 (CDK2). Inhibition of CDK2 and cyclin A2, but not cyclin E or cyclin A1, reduced AKT phosphorylation independently of changes in the cell cycle or known AKT regulators, suggesting that AKT is a direct CDK2–cyclin A2 substrate. Indeed, in vitro kinase assays revealed that CDK2–cyclin A2 phosphorylated AKT within the carboxy (C)-terminus at residues S477 and T479. Similar to S473 phosphorylation, which is regulated by multiple upstream signaling pathways, either Cdk2/Cyclin A or mTOR could phosphorylate S477/T479 under different physiologic conditions, such as cell-cycle progression or growth stimulation, respectively. Alanine substitution at S477/T479 inhibited phosphorylation of the canonical S473 activation site, whereas phosphomimetic mutations increased S473 phosphorylation, raising the possibility that S477/T479 either primes S473 for mTOR-mediated phosphorylation or locks AKT in an active conformation. Analysis of breast tumor samples revealed that AKT1 pS477/pT479 levels were correlated with cyclin A2 expression, indicating that phosphorylation at these residues may mediate AKT oncogenicity. In line with these findings, expression of the phosphomimetic S477/T479 AKT1 mutant rescued in vivo tumor formation and proliferation defects in cells depleted of AKT1 or cyclin A2 to a greater extent than wild-type AKT1. C-terminal AKT phosphorylation by CDK2–cyclin A2 may thus be required for the oncogenic activity of AKT, suggesting that cyclin A2 overexpression may drive cancer by promoting AKT-dependent proliferation and survival.

Liu P, Begley M, Michowski W, Inuzuka H, Ginzberg M, Gao D, et al. Cell-cycle-regulated activation of Akt kinase by phosphorylation at its carboxyl terminus. Nature 2014 Mar 9 [Epub ahead of print].