Cell-cycle-regulated activation of Akt kinase by phosphorylation at its carboxyl terminus

Abstract

Akt, also known as protein kinase B, plays key roles in cell proliferation, survival and metabolism. Akt hyperactivation contributes to many pathophysiological conditions, including human cancers1–3, and is closely associated with poor prognosis and chemo- or radio-therapeutic resistance4. Phosphorylation of Akt at S473 (ref. 5) and T308 (ref. 6) activates Akt. However, it remains unclear whether further mechanisms account for full Akt activation, and whether Akt hyperactivation is linked to misregulated cell cycle progression, another cancer hallmark7. Here we report that Akt activity fluctuates across the cell cycle, mirroring cyclin A expression. Mechanistically, phosphorylation of S477 and T479 at the Akt extreme carboxy terminus by cyclin-dependent kinase 2 (Cdk2)/cyclin A or mTORC2, under distinct physiological conditions, promotes Akt activation through facilitating, or functionally compensating for, S473 phosphorylation. Furthermore, deletion of the cyclin A2 allele in the mouse olfactory bulb leads to reduced S477/T479 phosphorylation and elevated cellular apoptosis. Notably, cyclin A2-deletion-induced cellular apoptosis in mouse embryonic stem cells is partly rescued by S477D/T479E-Akt1, supporting a physiological role for cyclin A2 in governing Akt activation. Together, the results of our study show Akt S477/T479 phosphorylation to be an essential layer of the Akt activation mechanism to regulate its physiological functions, thereby providing a new mechanistic link between aberrant cell cycle progression and Akt hyperactivation in cancer.

Phosphorylation of Akt at the C-terminal tail triggers Akt Activation

“…This notion was further supported by the observation that Akt1-S477D/T479E partially rescued cellular survival in part through compensating for the loss of Cyclin A2 in mouse embryonic stem cells. 6 Taken together, these data indicate that Cyclin A2/Cdk2-mediated Akt1-S477/T479 phosphorylation triggers Akt activation and plays critical roles under both physiological and pathological conditions. Intriguingly, we found that similar to Akt-S473 phosphorylation that has been shown to be mediated by multiple kinases including mTORC2, DNAPK, IKK, and TBK1; Akt1-pS477/pT479 could also be governed by multiple kinases under various physiological settings.…”

“…6 We further showed that Cdk2/Cyclin A directly interacted with Akt via its Cyclin A-binding motifs (RxL) and phosphorylated the Akt1 extreme C-terminal residues S477 and T479 during cell cycle transition to promote Akt-S473 and T308 phosphorylation for full Akt activation. Consistently, inactivation of Cdk2/Cyclin A led to reduced Akt1-pS477/pT479, pS473, and pT308, while accumulated Cyclin A2 levels resulted in elevated Akt phosphorylation and subsequent activation.…”

“…Specifically, Cdk2/Cyclin A and mTORC2 could phosphorylate the Akt tail under cell cycle progression or growth stimulation conditions, respectively. 6 Thus, Akt1-pS477/pT479 might serve as an antenna for Akt to receive various inputs dependent on the availability of upstream regulators to timely and spatially activate Akt (Fig. 1).…”

RasGAP Shields Akt from Deactivating Phosphatases in Fibroblast Growth Factor Signaling but Loses This Ability Once Cleaved by Caspase-3

“…Akt phosphorylation on both the 473 and 308 sites was reported to be critical for Akt activity (50). Although Akt phosphorylation on Thr-308 alone may be sufficient to activate Akt in some instances (51,52), stronger Akt activity can be generated by additional phosphorylation on Ser-473 (14) and possibly other sites (53). In our heterologous system, however, Akt phosphorylation on serine 473 appears dispensable for the proliferative response downstream of FGFR activation because inhibition of FGF1-stimulated Akt Ser-473 phosphorylation with anti-Sin1 antibodies did not result in the blockage of G 2 /M transition.…”

Applications of targeted proteomics in systems biology and translational medicine

“…Moreover, a protein can exist with different variant sequences due to splicing or mutations, and be subject to different PTMs at different sites, resulting in a vast number of theoretical combination of PTMs, known as “mod‐forms”, see for example the histone‐code 18. Even for a protein as well studied as Akt, a recent study found PTMs affecting a new layer of activation mechanism in cell cycle progression 27. Therefore, while there are compelling reasons to study protein networks of the cell, their analysis challenges current algorithmic and technical capabilities.…”