Akt: a New Activation Mechanism Cell Research (2014) 24:785-786

Akt: a new activation mechanism Cell Research (2014) 24:785-786. doi:10.1038/cr.2014.57; published online 6 May 2014

Akt phosphorylation at S473 and Akt can inactivate cell cycle inhibi- colleagues concluded that pS477/T479 T308 has been believed to be the pre- tors p27kip1 and p21cip1 to allow G1/S activates Akt and governs pS473 and requisites for its activation for years. progression [5]. However, it is unclear pT308, which are well-characterized Now, new phosphorylation event on whether dysregulated cell cycle is one and widely accepted phosphoryla- Akt is identified and can trigger Akt of the culprits for Akt hyperactivation. tion sites essential for Akt activation. activation and lead to its downstream Liu and colleagues made great strides Through structural study of Akt, they oncogenic events. and uncovered a new mechanism for suggested the possibility that pS477/ Protein Kinase B (PKB), more com- Akt activation linking two well-known T479 stabilizes Akt active conforma- monly referred to as Akt, is a serine/ cancer hallmarks together [6]. Liu et al. tion and primes pS473 by increasing its threonine-specific protein kinase. It is demonstrated that Akt activation fluctu- interaction with the mTORC2 complex. well established as a critical signaling ates across cell cycle, which resembles More importantly, phospho-mimetic node that regulates a myriad of cellular the expression pattern of cyclin A2, mutant exhibits accelerated growth in functions including proliferation, sur- the binding partner and activator for in vitro soft agar assay while phospho- vival, apoptosis and metabolism, which cyclin-dependent kinase 2 (CDK2). deficient mutant displays growth sup- are also processes important for tumori- This prompted them to further investi- pression. This result underscores the genesis [1]. Indeed, the roles of Akt in gate the relationship between Akt and role of pS477/T479 in dictating Akt cancer have been extensively scruti- cyclin A2. Indeed, depletion of cyclin oncogenic functions. nized over the years and documented A2, but not cyclin E, negatively affects Consistent with their in vitro obser- in many studies. However, more studies Akt phosphorylation and activation, vations, Liu et al. also highlighted the are still emerging to further increase which is independent of changes in cell impact of pS477/T479 on in vivo tumor our knowledge of this complex master cycle and known Akt kinases PDK1 and formation. Xenograft tumor model regulator. In a recent paper published in mTORC2, suggesting that Akt is a direct shows that phospho-mimetic mutant Nature, Liu et al. [2] have identified a substrate of cyclin A2/CDK2. drives tumor growth while phospho- new phosphorylation event on the Akt By using in vitro kinase assay and deficient mutant inhibits it. Remarkably, extreme carboxyl (C)-terminus, specifi- mass spectrometry, they further demon- not only the new phosphorylation event cally on residues S477 and T479, which strated that Akt can be phosphorylated positively correlates with pS473 in activates Akt and subsequent down- by cyclin A2/CDK2 within its extreme breast cancer patient samples, but also stream oncogenic cellular functions. C-terminus on residues S477 and T479. pS477/T479 can serve as a biomarker Akt activation is governed by a dual Interestingly, this new phosphorylation for early-stage breast cancer detection regulatory mechanism in which it is event can be triggered by different since it is frequently found in high levels first recruited to the cellular plasma upstream stimuli and different kinases. in stage II breast cancer patients. This membrane by PIP3 through a direct In addition to having cyclin A2/CDK2 finding is considered clinically signifi- interaction with the PH domain of Akt. as its kinase during cell cycle progres- cant and can be very useful in the future Once Akt arrives at the plasma mem- sion, S477 and T479 phosphorylation for developing breast cancer prevention brane, it meets two of its kinases PDK1 (pS477/T479) can also be mediated by approaches. and mTORC2 to get phosphorylated mTORC2 and DNAPK under growth This study presents compelling evi- on T308 and S473, respectively, for its factor stimulation and DNA damaging dence to reveal a new mechanism for full activation [3, 4]. One way that Akt conditions, respectively, as shown by Akt activation and how Akt exerts its affects cell proliferation is through regu- their in vitro kinase assays. To address oncogenic functions (Figure 1). Nev- lating cell cycle progression. Activated the relevancy of this modification event, ertheless, there are still questions that form of Akt can modulate the functions they generated phospho-deficient and remained to be addressed. First, since of many substrates responsible for phospho-mimetic mutants of Akt S477/ cyclin A2 is not solely localized in the cell-cycle progression. For example, T479. By using these mutants, Liu and cellular membrane, then where does npg 786 Akt achieve this new phosphorylation event? Akt is known to be recruited to the cellular plasma membrane by PIP3 to be further activated by its kinases PDK1 and mTORC2. Since Liu and colleagues showed that pS477/T479 not only governs pS473, but also pT308, the substrate site of PDK1, then one must wonder how the new phosphorylation event affects pT308. Does it affect Akt translocation to be phosphorylated by PDK1 at the plasma membrane? Another possibility is that pS477/T479 happens after Akt membrane recruit- ment, and simply increases the binding affinity between Akt and PDK1, similar to its effect on pS473 and mTORC2. Finally, ubiquitination of Akt on the PH domain by E3 ligases upon growth factor stimulations translocates Akt to the plasma membrane for activation Figure 1 Akt phosphorylation triggers its activation. Liu et al. report that during cell and downstream biological functions cycle progression, under growth factor stimulation and DNA damage, Akt can be such as glycolysis and tumorigenesis [7, phosphorylated by cyclin A2/CDK2, mTORC2 and DNAPK. This new phosphorylation governs pS473 and pT308 to reach Akt activation. To fully understand how this 8]. It will also be interesting to see the modification event triggers Akt activation, we should further pursue studies that can crosstalk between these two important address whether it affects Akt membrane translocation, Akt ubiquitination as well as Akt modification events. The answers through what mechanism it affects pT308. to these questions will certainly allow us to gain a more complete perspective on the modes of Akt activation. Hui-Kuan Lin1, 2, 3, 4 ture 2014; 508:541-545. Akt, being the central convergence 3 Vivanco I, Sawyers CL. Nat Rev Cancer 2002; 2:489-501. node, participates in a wide range of 1Department of Molecular and Cellular Oncology, 4 Sarbassov DD, Guertin DA, Ali SM, et al. influential cellular signaling pathways. The University of Texas MD Anderson Cancer Science 2005; 307:1098-1101. Center, Houston, TX 77030, USA; 2The University Because of its pivotal role in cancer 5 Xu N, Lao Y, Zhang Y, et al. J Oncol 2012; of Texas Graduate School of Biomedical Sciences development, how it gets activated has 2012:951724. at Houston, Houston, TX 77030, USA; 3Gradu- 6 Hanahan D, Weinberg RA. Cell 2011; been a prevalent area of study. Liu and ate Institute of Basic Medical Science, China 144:646-674. colleagues have provided convincing Medical University, Taichung 404; 4Department 7 Chan CH, Li CF, Yang WL, et al. Cell 2012; of Biotechnology, Asia University, Taichung 404; evidence in their study that pS477/T479 149:1098-1111. 5Department of Primary Care Health Sciences, is a new phosphorylation event to trig- 8 Yang WL, Wang J, Chan CH, et al. Science Nuffield Department of Clinical Medicine, Uni- 2009; 325:1134-1138. ger Akt activation and is important for versity of Oxford, Oxford OX3 9DU, UK its potency in driving tumorigenesis. Correspondence: Hui-Kuan Lin Without a doubt, this article is an es- E-mail: [email protected] sential stepping-stone for us to fully understand Akt, this intricate master References regulator. 1 Manning BD, Cantley LC. Cell 2007; 129:1261-1274. Yuan Gao1, 2, Asad Moten1, 5, 2 Liu P, Begley M, Michowski W, et al. Na-

Cell Research | Vol 24 No 7 | July 2014