Protein kinase B

The Akt proteins possess a protein domain known as a PH domain, or pleckstrin homology domain, named after pleckstrin, the protein in which it was first discovered. This domain binds to phosphoinositides with high affinity. In the case of the PH domain of the Akt proteins, it binds either PIP₃ (phosphatidylinositol (3,4,5)-trisphosphate, PtdIns(3,4,5)P₃) or PIP₂ (phosphatidylinositol (3,4)-bisphosphate, PtdIns(3,4)P₂).[11] This is useful for control of cellular signaling because the di-phosphorylated phosphoinositide PIP₂ is only phosphorylated by the family of enzymes, PI 3-kinases (phosphoinositide 3-kinase or PI3-K), and only upon receipt of chemical messengers which tell the cell to begin the growth process. For example, PI 3-kinases may be activated by a G protein coupled receptor or receptor tyrosine kinase such as the insulin receptor. Once activated, PI 3-kinase phosphorylates PIP₂ to form PIP₃.

Once correctly positioned at the membrane via binding of PIP3, Akt can then be phosphorylated by its activating kinases, phosphoinositide-dependent kinase-1 (PDPK1 at threonine 308 in Akt1 and threonine 309 in Akt2) and the mammalian target of rapamycin complex 2 (mTORC2 at serine 473 (Akt1) and 474 (Akt2)) which is found at high levels in the fed state,[12][13] first by mTORC2. mTORC2 therefore functionally acts as the long-sought PDK2 molecule, although other molecules, including integrin-linked kinase (ILK) and mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK2) can also serve as PDK2. Phosphorylation by mTORC2 stimulates the subsequent phosphorylation of Akt isoforms by PDPK1.

Activated Akt isoforms can then go on to activate or deactivate their myriad substrates (e.g. mTOR) via their kinase activity.

Besides being a downstream effector of PI 3-kinases, Akt isoforms can also be activated in a PI 3-kinase-independent manner.[14] ACK1 or TNK2, a non-receptor tyrosine kinase, phosphorylates Akt at its tyrosine 176 residue, leading to its activation in PI 3-kinase-independent manner.[14] Studies have suggested that cAMP-elevating agents could also activate Akt through protein kinase A (PKA) in the presence of insulin.[15]

Akt can be O-GlcNAcylated by OGT. O-GlcNAcylation of Akt is associated with a decrease in T308 phosphorylation.[16]

Akt1 is normally phosphorylated at position T450 in the turn motif when Akt1 is translated. If Akt1 is not phosphorylated at this position, Akt1 does not fold in the right way. The T450-non-phosphorylated misfolded Akt1 is ubiquitinated and degraded by the proteasome. Akt1 is also phosphorylated at T308 and S473 during IGF-1 response, and the resulting polyphosphorylated Akt is ubiquitinated partly by E3 ligase NEDD4. Most of the ubiquitinated-phosphorylated-Akt1 is degraded by the proteasome, while a small amount of phosphorylated-Akt1 translocates to the nucleus in a ubiquitination-dependent way to phosphorylate its substrate. A cancer-derived mutant Akt1 (E17K) is more readily ubiquitinated and phosphorylated than the wild type Akt1. The ubiquitinated-phosphorylated-Akt1 (E17K) translocates more efficiently to the nucleus than the wild type Akt1. This mechanism may contribute to E17K-Akt1-induced cancer in humans.[17]

PI3K-dependent Akt1 activation can be regulated through the tumor suppressor PTEN, which works essentially as the opposite of PI3K mentioned above.[18] PTEN acts as a phosphatase to dephosphorylate PIP3 back to PIP2. This removes the membrane-localization factor from the Akt signaling pathway. Without this localization, the rate of Akt1 activation decreases significantly, as do all of the downstream pathways that depend on Akt1 for activation.

PIP3 can also be de-phosphorylated at the "5" position by the SHIP family of inositol phosphatases, SHIP1 and SHIP2. These poly-phosphate inositol phosphatases dephosphorylate PIP3 to form PIP2.

The phosphatases in the PHLPP family, PHLPP1 and PHLPP2 have been shown to directly de-phosphorylate, and therefore inactivate, distinct Akt isoforms. PHLPP2 dephosphorylates Akt1 and Akt3, whereas PHLPP1 is specific for Akt2 and Akt3.

Overview of signal transduction pathways involved in apoptosis.

Akt kinases can promote growth factor-mediated cell survival both directly and indirectly. BAD is a pro-apoptotic protein of the Bcl-2 family. Akt1 can phosphorylate BAD on Ser136,[20] which makes BAD dissociate from the Bcl-2/Bcl-X complex and lose the pro-apoptotic function.[21] Akt1 can also activate NF-κB via regulating IκB kinase (IKK), thus result in transcription of pro-survival genes.[22]

The Akt isoforms are known to play a role in the cell cycle. Under various circumstances, activation of Akt1 was shown to overcome cell cycle arrest in G1[23] and G2[24] phases. Moreover, activated Akt1 may enable proliferation and survival of cells that have sustained a potentially mutagenic impact and, therefore, may contribute to acquisition of mutations in other genes.

Akt2 is required for the insulin-induced translocation of glucose transporter 4 (GLUT4) to the plasma membrane. Glycogen synthase kinase 3 (GSK-3) could be inhibited upon phosphorylation by Akt, which results in increase of glycogen synthesis. GSK3 is also involved in Wnt signaling cascade, so Akt might be also implicated in the Wnt pathway. Its role in HCV induced steatosis is unknown.

Akt1 regulates TFEB, a master controller of lysosomal biogenesis,[25] by direct phosphorylation at serine 467.[26] Phosphorylated TFEB is excluded from the nucleus and less active.[26] Pharmacological inhibition of Akt promotes nuclear translocation of TFEB, lysosomal biogenesis and autophagy.[26]

Akt1 has also been implicated in angiogenesis and tumor development. Although deficiency of Akt1 in mice inhibited physiological angiogenesis, it enhanced pathological angiogenesis and tumor growth associated with matrix abnormalities in skin and blood vessels.[27][28]

Akt inhibitors may treat cancers such as neuroblastoma. Some Akt inhibitors have undergone clinical trials. In 2007 VQD-002 had a phase I trial.[31] In 2010 Perifosine reached phase II.[32] but it failed phase III in 2012.

Miltefosine is approved for leishmaniasis and under investigation for other indications including HIV.

Akt1 is now thought to be the "key" for cell entry by HSV-1 and HSV-2 (herpes virus: oral and genital, respectively). Intracellular calcium release by the cell allows for entry by the herpes virus; the virus activates Akt1, which in turn causes the release of calcium. Treating the cells with Akt inhibitors before virus exposure leads to a significantly lower rate of infection.[33]

MK-2206 reported phase 1 results for advanced solid tumors in 2011,[34] and subsequently has undergone numerous phase II studies for a wide variety of cancer types.[35]

In 2013 AZD5363 reported phase I results regarding solid tumors.[36] with a study of AZD5363 with olaparib reporting in 2016.[37]

Ipatasertib is in phase II trials for breast cancer.[38]

Akt isoform activation is associated with many malignancies; however, a research group from Massachusetts General Hospital and Harvard University unexpectedly observed a converse role for Akt and one of its downstream effector FOXOs in acute myeloid leukemia (AML). They claimed that low levels of Akt activity associated with elevated levels of FOXOs are required to maintain the function and immature state of leukemia-initiating cells (LICs). FOXOs are active, implying reduced Akt activity, in ~40% of AML patient samples regardless of genetic subtype; and either activation of Akt or compound deletion of FoxO1/3/4 reduced leukemic cell growth in a mouse model.[39]

Two studies show that Akt1 is involved in Juvenile Granulosa Cell tumors (JGCT). In-frame duplications in the pleckstrin-homology domain (PHD) of the protein were found in more than 60% of JGCTs occurring in girls under 15 years of age. The JGCTs without duplications carried point mutations affecting highly conserved residues. The mutated proteins carrying the duplications displayed a non-wild-type subcellular distribution, with a marked enrichment at the plasma membrane. This led to a striking degree of Akt1 activation demonstrated by a strong phosphorylation level and corroborated by reporter assays.[40]

Analysis by RNA-Seq pinpointed a series of differentially expressed genes, involved in cytokine and hormone signaling and cell division-related processes. Further analyses pointed to a possible dedifferentiation process and suggested that most of the transcriptomic dysregulations might be mediated by a limited set of transcription factors perturbed by Akt1 activation. These results incriminate somatic mutations of Akt1 as major probably driver events in the pathogenesis of JGCTs.[41]