Protein kinase B

AKT1
Ribbon Representation of crystal structure of Akt-1-inhibitor complexes.
Identifiers
Symbol	AKT1
Entrez	207
HUGO	391
OMIM	164730
RefSeq	NM_005163
UniProt	P31749
Other data
Locus	Chr. 14 q32.32-32.33

AKT2
Crystal structure of Akt-2-inhibitor complexes.
Identifiers
Symbol	AKT2
Entrez	208
HUGO	392
OMIM	164731
RefSeq	NM_001626
UniProt	P31751
Other data
Locus	Chr. 19 q13.1-13.2

AKT3
Identifiers
Symbol	AKT3
Entrez	10000
HUGO	393
OMIM	611223
RefSeq	NM_181690
UniProt	Q9Y243
Other data
Locus	Chr. 1 q43-44

Protein kinase B (PKB), also known as Akt, is a serine/threonine-specific protein kinase that plays a key role in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription and cell migration.

Family members

Akt1 is involved in cellular survival pathways, by inhibiting apoptotic processes. Akt1 is also able to induce protein synthesis pathways, and is therefore a key signaling protein in the cellular pathways that lead to skeletal muscle hypertrophy, and general tissue growth. Mouse model with complete deletion of Akt1 manifests growth retardation and increased spontaneous apoptosis in tissues such as testes and thymus. Since it can block apoptosis, and thereby promote cell survival, Akt1 has been implicated as a major factor in many types of cancer. Akt (now also called Akt1) was originally identified as the oncogene in the transforming retrovirus, AKT8.

Akt2 is an important signaling molecule in the insulin signaling pathway. It is required to induce glucose transport. In a mouse which is null for Akt1 but normal for Akt2, glucose homeostasis is unperturbed, but the animals are smaller, consistent with a role for Akt1 in growth. In contrast, mice which do not have Akt2, but have normal Akt1, have mild growth deficiency and display a diabetic phenotype (insulin resistance), again consistent with the idea that Akt2 is more specific for the insulin receptor signaling pathway. Akt isoforms are overexpressed in a variety of human tumors, and, at the genomic level, are amplified in gastric adenocarcinomas (Akt1), ovarian (Akt2), pancreatic (Akt2) and breast (Akt2) cancer.

The role of Akt3 is less clear, though it appears to be predominantly expressed in the brain. It has been reported that mice lacking Akt3 have small brains.

Name

The name Akt does not refer to its function. The "Ak" in Akt was a temporary classification name for a mouse bred and maintained by Jacob Furth that developed spontaneous thymic lymphomas. The "t" stands for 'thymoma'; the letter was added when a transforming retrovirus was isolated from the Ak strain, which was termed "Akt-8". When the oncogene encoded in this virus was discovered, it was termed v-Akt. Thus, the later identified human analogues were named accordingly.

Regulation

Akt[1] is involved in the PI3K/AKT/mTOR pathway and other signaling pathways.

Binding phospholipids

Akt possesses 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 Akt, it binds either PIP₃ (phosphatidylinositol (3,4,5)-trisphosphate, PtdIns(3,4,5)P₃) or PIP₂ (phosphatidylinositol (3,4)-bisphosphate, PtdIns(3,4)P₂). 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.

Phosphorylation

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) and the mammalian target of rapamycin complex 2 (mTORC2 at serine 473), 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 by PDPK1.

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

Besides being a downstream effector of PI 3-kinases, Akt can also be activated in a PI 3-kinase-independent manner. 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. Studies have suggested that cAMP-elevating agents could also activate Akt through protein kinase A (PKA) in the presence of insulin.

Ubiquitination

Akt is normally phosphorylated at position T450 in the turn motif when Akt is translated. If Akt is not phosphorylated at this position, Akt does not fold in the right way. The T450-non-phosphorylated misfolded Akt is ubiquitinated and degraded by the proteasome. Akt 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-Akt is degraded by the proteasome, while a small amount of phosphorylated-Akt translocates to the nucleus in a ubiquitination-dependent way to phosphorylate its substrate. A cancer-derived mutant Akt (E17K) is more readily ubiquitinated and phosphorylated than the wild type Akt. The ubiquitinated-phosphorylated-Akt (E17K) translocates more efficiently to the nucleus than the wild type Akt. This mechanism may contribute to E17K-Akt-induced cancer in humans.

Lipid phosphatases and PIP3

PI3K-dependent Akt activation can be regulated through the tumor suppressor PTEN, which works essentially as the opposite of PI3K mentioned above. 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 Akt activation decreases significantly, as do all of the downstream pathways that depend on Akt 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.

Protein phosphatases

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 Akt 2 and Akt3.

Function

Akt regulates cellular survival and metabolism by binding and regulating many downstream effectors, e.g. Nuclear Factor-κB, Bcl-2 family proteins and murine double minute 2 (MDM2).

Cell survival

Overview of signal transduction pathways involved in apoptosis.

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

Cell Cycle

Akt is known to play a role in the cell cycle. Under various circumstances, activation of Akt was shown to overcome cell cycle arrest in G1 and G2 phases. Moreover, activated Akt 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.

Metabolism

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. Still unknown role in HCV induced steatosis.

Angiogenesis

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.

Clinical relevance

Akt is associated with tumor cell survival, proliferation, and invasiveness. The activation of Akt is also one of the most frequent alterations observed in human cancer and tumor cells. Tumor cells that have constantly active Akt may depend on Akt for survival. Therefore, understanding Akt and its pathways is important for the creation of better therapies to treat cancer and tumor cells. A mosaic-activating mutation (c. 49G→A, p.Glu17Lys) in AKT1 is associated with the Proteus Syndrome, which causes overgrowth of skin, connective tissue, brain and other tissues.

AKT Inhibitors

Because of the Akt functions above, Akt inhibitors may treat cancers such as neuroblastoma. Some Akt inhibitors have undergone clinical trials. In 2007 VQD-002 had a phase I trial. In 2010 Perifosine reached phase II. but it failed phase III in 2012.

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

AKT 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 AKT, 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.

MK-2206 has reported phase 1 results.

In 2013 AZD5363 reported phase I results re solid tumors.

A new type of Akt inhibitor has been discovered.

Decreased AKT Can Cause Deleterious Effects

AKT 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.

See also

• Akt/PKB signaling pathway
• Discovery and development of mTOR inhibitors
• PI3K/AKT/mTOR pathway
• Phospho Akt
• Akt inhibitor
• PTEN

External links

• Proto-Oncogene Proteins c-akt at the US National Library of Medicine Medical Subject Headings (MeSH)