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Protein kinase B
Protein kinase B (PKB), also known as Akt, is the collective name of a set of three serine/threonine-specific protein kinases that play key roles in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription, and cell migration.
There are three different genes that encode isoforms of protein kinase B. These three genes are referred to as AKT1, AKT2, and AKT3 and encode the RAC alpha, beta, and gamma serine/threonine protein kinases respectively. The terms PKB and Akt may refer to the products of all three genes collectively, but sometimes are used to refer to PKB alpha and Akt1 alone.[citation needed]
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. A mouse model with complete deletion of the Akt1 gene 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. Akt1 is also a positive regulator of cell migration. 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. Akt2 promotes cell migration as well. 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.
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) cancers.
The name Akt does not refer to its function. The "Ak" in Akt refers to the AKR mouse strain that develops spontaneous thymic lymphomas. The "t" stands for 'thymoma'; the letter was added when a transforming retrovirus was isolated from the Ak mouse strain, which was termed "Akt-8". The authors state, "Stock A Strain k AKR mouse originally inbred in the laboratory of Dr. C. P. Rhoads by K. B. Rhoads at the Rockefeller Institute." When the oncogene encoded in this virus was discovered, it was termed v-Akt. Thus, the more recently identified human analogs were named accordingly.
Akt1 is involved in the PI3K/AKT/mTOR pathway and other signaling pathways.
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). 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.
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Protein kinase B
Protein kinase B (PKB), also known as Akt, is the collective name of a set of three serine/threonine-specific protein kinases that play key roles in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription, and cell migration.
There are three different genes that encode isoforms of protein kinase B. These three genes are referred to as AKT1, AKT2, and AKT3 and encode the RAC alpha, beta, and gamma serine/threonine protein kinases respectively. The terms PKB and Akt may refer to the products of all three genes collectively, but sometimes are used to refer to PKB alpha and Akt1 alone.[citation needed]
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. A mouse model with complete deletion of the Akt1 gene 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. Akt1 is also a positive regulator of cell migration. 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. Akt2 promotes cell migration as well. 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.
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) cancers.
The name Akt does not refer to its function. The "Ak" in Akt refers to the AKR mouse strain that develops spontaneous thymic lymphomas. The "t" stands for 'thymoma'; the letter was added when a transforming retrovirus was isolated from the Ak mouse strain, which was termed "Akt-8". The authors state, "Stock A Strain k AKR mouse originally inbred in the laboratory of Dr. C. P. Rhoads by K. B. Rhoads at the Rockefeller Institute." When the oncogene encoded in this virus was discovered, it was termed v-Akt. Thus, the more recently identified human analogs were named accordingly.
Akt1 is involved in the PI3K/AKT/mTOR pathway and other signaling pathways.
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). 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.