AKT/PKB Signaling: Navigating the Network

Abstract

The Ser and Thr kinase AKT, also known as protein kinase B (PKB), was discovered 25 years ago and has been the focus of tens of thousands of studies in diverse fields of biology and medicine. There have been many advances in our knowledge of the upstream regulatory inputs into AKT, key multifunctional downstream signaling nodes (GSK3, FoxO, mTORC1), which greatly expand the functional repertoire of AKT, and the complex circuitry of this dynamically branching and looping signaling network that is ubiquitous to nearly every cell in our body. Mouse and human genetic studies have also revealed physiological roles for the AKT network in nearly every organ system. Our comprehension of AKT regulation and functions is particularly important given the consequences of AKT dysfunction in diverse pathological settings, including developmental and overgrowth syndromes, cancer, cardiovascular disease, insulin resistance and type 2 diabetes, inflammatory and autoimmune disorders, and neurological disorders. There has also been much progress in developing AKT-selective small molecule inhibitors. Improved understanding of the molecular wiring of the AKT signaling network continues to make an impact that cuts across most disciplines of the biomedical sciences.

Figure 1. Molecular mechanisms of Akt regulation

A. Stimulation of RTKs or GPCRs leads to activation of PI3K, leading to PIP₃ production at the plasma membrane. Cytosolic inactive AKT is recruited to the membrane and engages PIP₃ through PH domain binding. This leads to phosphorylation of T308 and S473 by PDK1 and mTORC2, respectively, resulting in full activation. Signal termination is achieved by the PIP₃ phosphatase PTEN, and the PP2A and PHLPP protein phosphatases. A separate endomembrane pool of active AKT likely exists that is activated through engagement of PI3,4P₂ through the action of the SHIP phosphatase, and terminated by INPP4B. B. The modular structure of AKT1 with position of PTMs color coded for phosphorylation (pSer/pThr/pTyr), acetylation (Lys-Ac), ubiquitylation (Lys-Ub), methylation (Lys-Me), hydroxylation (Pro-OH), glycosylation (O-GlcNac) and SUMOylation (Lys-SUMO).

Figure 1. Molecular mechanisms of Akt regulation

A. Stimulation of RTKs or GPCRs leads to activation of PI3K, leading to PIP₃ production at the plasma membrane. Cytosolic inactive AKT is recruited to the membrane and engages PIP₃ through PH domain binding. This leads to phosphorylation of T308 and S473 by PDK1 and mTORC2, respectively, resulting in full activation. Signal termination is achieved by the PIP₃ phosphatase PTEN, and the PP2A and PHLPP protein phosphatases. A separate endomembrane pool of active AKT likely exists that is activated through engagement of PI3,4P₂ through the action of the SHIP phosphatase, and terminated by INPP4B. B. The modular structure of AKT1 with position of PTMs color coded for phosphorylation (pSer/pThr/pTyr), acetylation (Lys-Ac), ubiquitylation (Lys-Ub), methylation (Lys-Me), hydroxylation (Pro-OH), glycosylation (O-GlcNac) and SUMOylation (Lys-SUMO).

Figure 2. Substrates and functions of the Akt signaling network

Akt phosphorylates downstream substrates involved in the regulation of diverse cellular functions, including multifunctional substrates. A partial list of known substrates is shown. P indicates phosphorylation, with red and green denoting inhibitory and activating regulation, respectively.

Figure 3. GSK3 regulation and substrate phosphorylation

GSK3 recognizes and phosphorylates substrates that are previously phosphorylated by a priming kinase. A partial list of known GSK3 substrates is shown. Akt’s phosphorylation of GSK3 inactivates it by blocking its access to primed substrates.

Figure 4. Akt-mediated regulation and transcriptional targets of FoxO family members

A. Schematic of three FoxO family members, with the three conserved Akt phosphorylation sites denoted relative to the DNA-binding domain (DBD), nuclear localization sequence (NLS) and nuclear export sequence (NES). B. Akt-mediated phosphorylation of FoxO leads to its binding and cytosolic sequestration by 14-3-3 proteins, thereby attenuating the expression of its gene targets, a partial list of which is shown.

Figure 5. Regulation of mTORC1 via the TSC complex and downstream functions of mTORC1

A. Schematic of the TSC complex components, their regions of association (dashed lines), and Akt phosphorylation sites on TSC2. Conserved domains of unknown function and GAP, coiled-coil, and TBC domains of the components are shown. B. Model of signal integration by growth factors and amino acids for regulation of mTORC1. The Rag heterodimer interacts with the Ragulator and V-ATPase at the lysosomal surface, and amino acids promote mTORC1 binding to this complex. The TSC complex maintains Rheb in the GDP-bound state. Growth factor-stimulated Akt phosphorylates TSC2, resulting in dissociation from the lysosomal surface, allowing Rheb to become GTP loaded and activate mTORC1. C. The PI3K-mTOR signaling pathway, depicting downstream functions and feedback regulation.

Figure 5. Regulation of mTORC1 via the TSC complex and downstream functions of mTORC1

A. Schematic of the TSC complex components, their regions of association (dashed lines), and Akt phosphorylation sites on TSC2. Conserved domains of unknown function and GAP, coiled-coil, and TBC domains of the components are shown. B. Model of signal integration by growth factors and amino acids for regulation of mTORC1. The Rag heterodimer interacts with the Ragulator and V-ATPase at the lysosomal surface, and amino acids promote mTORC1 binding to this complex. The TSC complex maintains Rheb in the GDP-bound state. Growth factor-stimulated Akt phosphorylates TSC2, resulting in dissociation from the lysosomal surface, allowing Rheb to become GTP loaded and activate mTORC1. C. The PI3K-mTOR signaling pathway, depicting downstream functions and feedback regulation.

Figure 6. Signaling crosstalk and redundancy in the AKT network

A. Several points of cross-regulation exist between the PI3K-Akt pathway and both the RAS-ERK and AMPK pathways, leading to both reciprocal pathway regulation and convergent regulation of downstream processes. B. Various AGC family kinases can redundantly phosphorylate overlapping sites on key downstream substrates of AKT, thereby altering the regulatory input into these targets.