| v-akt murine thymoma viral oncogene homolog 1 | |||||||
|---|---|---|---|---|---|---|---|
| File:Akt crystal structure.jpg Crystal structure of PKB/Akt | |||||||
| Identifiers | |||||||
| Symbol | PKB/Akt | ||||||
| Alt. symbols | AKT; PKB; RAC; PRKBA; MGC99656; PKB-ALPHA; RAC-ALPHA; AKT1 | ||||||
| NCBI gene | 207 | ||||||
| UniProt | P31749 | ||||||
| |||||||
Definition
editPKB/Akt (Protein Kinase B) refers to the products of the AKT gene family. The family consists of three genes namely: PKBα/AKT1, PKBβ/AKT2 and PKBγ/AKT3. Functionally these proteins act as kinases and play a key role in signal transduction pathways originating from nutrient and growth factor signaling. Eventually, they inhibit programmed cell death and promote cell survival and growth.
PKB/Akt and Cell Survival Signaling
editNutrients and cell growth factors are the two key signals that induce cell growth and cell proliferation. In unicellular organisms such as yeast, nutrient availability is the key factor that determines the rate of proliferation. Consequently, growth is strictly coupled to cell division [1], 2], whereas in animal cells it seems that extracellular growth factors predominantly influence cell growth and division with independent but coordinated mechanisms for the two processes [1]. The signals for growth and proliferation are transduced via highly conserved pathways involving PKB/Akt. Withdrawal of growth factors from animal cells leads to autophagy and/or apoptosis (for a review see [3]). Growth factors activate their receptors at the plasma membrane such as receptor tyrosine kinases (for a review see [4]). This results in the recruitment of phosphatidylinositide 3'-OH kinase (PI3K) isoforms to the inner-surface of the plasma membrane. Membrane localized PI3Ks cause the phosphorylation of phosphoinositides in the vicinity thereby producing 3’-phosphorylated phosphoinositides, namely; phosphatidylinositol 3,4 bisphosphate (PI3,4P) and phosphatidylinositol 3,4,5 trisphosphate (PI3,4,5P) (for a review see [5]). These lipids then act as signaling intermediates that regulate activity of PKB/Akt and the signaling cascade henceforth. Lipid phosphatases such as PTEN (phosphatase and tensin homologue deleted from chromosome 10) are able to dephosphorylate these phospholipids thereby downregulating the cell survival signals and upregulating cell death [6, 7].
Structure and function of PKB/Akt
editIn animals there exist three types of PKB/Akts that are produced independently from three different genes [8]. The three forms share high sequence and structural homology [9]. They are comprised of a kinase domain that is involved in the specific phosphorylation of threonine residues in the substrate proteins (see Figure 1) [10]. Thus, this domain is key for PKB/Akt to act as a transducer of signals in a phosphorylation dependent manner. PKB/Akt phosphorylates target proteins at serine/threonine residues. Analysis of phosphorylation sites has revealed a general consensus recognition sequence of R-X-R-X-X-S/T [11]. However, only a fraction of proteins with this sequence have been confirmed in vivo as substrates of PKB/Akt. Moreover, a phosphorylation site (Thr 308) occurs in the activation-loop of the kinase domain that is important for the regulation of Akt activity itself [12]. Additionally, the N-terminus contains a pleckstrin homology (PH) domain while at the C-terminus a hydrophobic proline-rich domain is found. The former has been implicated in lipid–protein and/or protein–protein interactions important for the localization of the PKB/Akt to the plasma membrane [13]. The latter might be involved in the regulation of PKB/Akt activity as it houses the second phosphorylation site (Ser473) thought to be important for its activation.
Activation of PKB/Akt
editPI3,4P and PI3,4,5P once produced at the inner leaflet of the membrane recruit PKB/Akt by directly binding to its PH domain [15]. This relocalization to the plasma membrane brings PKB/Akt in the vicinity of regulatory kinases. Additionally, phospholipid binding of PKB/Akt is thought to impose conformational changes in PKB/Akt, exposing its two main phosphorylation sites and therefore making it accessible to the regulatory kinases [16]. PDK1(3-phophoinositide-dependent protein kinase) phosphorylates Thr308 [17] and stabilizes the activation loop in an active conformation. PDK1 possesses a PH domain that binds with high affinity to the PI3,4,5P and is localized on the inner side of the plasma membrane [17]. Unlike PKB/Akt, PDK1 is constitutively active, but its activity might be upregulated upon direct binding to phosphoinositides [17]. Phosphorylation at Ser473 is required for maximal activation of PKB/Akt [16]. Furthermore, the activity of PDK2 which phosphorylates PKB/Akt on Ser473 has been proposed for many years. Several candidates have been suggested to function as PDK2, including PDK1 itself [18]. It was shown that PDK1 interacts with a small C-terminal fragment of protein kinase C-related kinase-2 (PRK2), that was called PDK1-interacting fragment (PIF), that converts PDK1 into an enzyme that can phosphorylate both Thr308 and Ser473 [18]. PKB/Akt has been suggested to autophosphorylate itself at Ser473 [19]. Furthermore, integrin-linked kinase (ILK), mitogen-activated protein kinase-activated protein kinase 2 (MAPKAP-K2) and mammalian target of rapamycin complex 2 (mTORC2) [20, 21] have been identified as PDK2 kinases for PKB/Akt Ser473 phosphorylation.
Ca2+/calmodulin-dependent kinase can also activate PKB/Akt, independent of PI3K [22].
Once activated PKB/Akt detaches from the plasma membrane and translocates to the cytoplasm and nucleus where it phosphorylates its targets and promotes cell survival. The majority of its targets are key regulators of apoptosis, cell growth and glucose metabolism and glycogen synthesis.
Physiological Role of PKB/Akt
editCell Survival
editPKB/Akt downregulates several proapoptotic B-cell lymphoma 2 (Bcl-2) proteins. Most of them are Bcl-2 homology domain 3-only (BH3-only) proteins whose function is to bind to prosurvival Bcl-2 family members and inactivate them. Their inhibition through phosphorylation by PKB/Akt prevents the conversion of procaspase-9 to caspase-9 thereby blocking a crucial step in the mitochondrial apoptosis pathway [23]. The downregulation of the Bcl-2-associated death promoter (BAD), a BH3-only protein, is mediated by direct phosphorylation of the protein, which is bound by 14-3-3 proteins, rendering it inactive. PKB/Akt can also phosphorylate and inhibit forkhead box (FOX) transcription factors such as FOXO1, FOXO3a and FOXO4 [24-28]. They induce the expression of proapoptotic proteins such as the BH3-only protein BIM and the Fas ligand (FasL), a signal molecule that causes the activation of caspase-8. PKB/Akt also phosphorylates the mouse double minute 2 (MDM2) protein, which subsequently translocates to the nucleus and inhibits the transcription factor p53 that induces expression of the proapoptotic Bcl-2 family members Bax, Puma and Noxa [29]. PKB/Akt also upregulates the prosurvival myeloid cell leukemia sequence 1 (MCL-1) protein by inactivating the inhibiting glycogen synthase kinase 3 (GSK3) [30, 31]. PKB/Akt may also support cell survival by activating IKKa, a subunit of the IκB kinase (IKK) that activates NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), a promotor for prosurvival Bcl-2 family members Bfl-1/A1 and caspase inhibitors c-IAP1 and c-IAP2 [32].
Cell Growth and Proliferation
editPKB/Akt promotes cell growth and proliferation by upregulating the mammalian target of rapamycin complex 1 (mTORC1). mTORC1 activates the S6 Kinase 1 (S6K1) and inhibits the eukaryotic translation initiation factor 4E binding protein 1 (4E-BP). Both processes activate ribosomes, increasing protein translation. PKB/Akt activates mTORC1 by phosphorylating and inactivating the tuberous sclerosis protein 2 (TSC2) [33-35]. This prevents TSC2 to form a complex with TSC1 that serves as a GTPase. Without its GTPase-activity, the Rheb (Ras homolog enriched in brain) protein can remain in its active GTP bound form and activate mTORC1. PKB/Akt also phosphorylates another inhibitor of mTORC1 named PRAS40 (proline-rich Akt substrate of 40 kDa), which is then inactivated by binding to 14-3-3 proteins [36-38].
Cellullar Metabolism
editIn response to insulin, PKB/Akt promotes the uptake of glucose in adipocytes and skeletal muscle cells by phosphorylating AS160 (Akt substrate of 160 kDa) [39, 40]. This GTPase activates several Rab proteins [41], mediating the fusion of vesicles containing glucose transporter 4 (GLUT4) with the plasma membrane [42, 43]. PKB/Akt also seems to upregulate the activity of hexokinases which convert glucose to glucose-6-phosphate, a substrate for both glycolysis and glycogen production. PKB/Akt can promote glycolysis through upregulation of glycolytic enzymes, and it can promote glycogen production by inactivating GSK3, preventing it from inhibiting glycogen synthase [44-46]. GSK3 also inhibits sterol regulatory element binding proteins (SREBPs). These transcription factors promote the synthesis of cholesterol and fatty acids [47].
Angiogenesis
editPKB/Akt promotes angiogenesis by promoting the survival, growth and proliferation of endothelial cells in response to vascular endothelial growth factor (VEGF). Furthermore, it activates the endothelial nitric oxide synthase (eNOS), which produces the angiogenic factor NO [48, 49], and the hypoxia-inducible factor alpha (HIF1a), which is a transcription factor that induces the expression of additional VEGF, serving as positive reinforcement for angiogenesis.
Figure 3 summarizes the targets of PKB/Akt and its physiological roles.
Role of PKB/Akt in disease
editPKB/Akt disruption
editIsoform specific knockout of the AKT gene in mouse germ lines have uncovered specific physiological functions of the three AKT genes. Mice lacking individual AKT isoforms are viable and show relatively subtle but distinct phenotypes, whereas combined disruption of AKT1/AKT3 or AKT1/AKT2 causes embryonic and neonatal lethality, respectively. AKT1-deficient mice show defects in apoptosis induction and growth. Mice in which AKT2 is disrupted show defects in the capacity of insulin to reduce the glucose level in the blood and AKT3-null mice have defects in brain development (reviewed in [51]). The viability of the single knockouts and the lethality of the double knockouts indicate that the isoforms can compensate for each other. However, the distinct phenotypes of the individual knockout mice suggest that the three gene products still have unique functions.
PKB/Akt and cancer
editAs PKB/Akt is known to play a crucial role in cell survival and cell cycle control, it has become a prime target in the search for cancer-related genes. It has been shown that the PTEN/PI3K/Akt pathway is altered in many various human cancers. Negative regulators of PI3K activation are implicated as tumour suppressor genes. For example, PTEN is mutated or deleted in various human malignancies, such as breast cancer and glioblastoma (reviewed in [52]). It also has been shown that PKB/Akt is overexpressed and constitutively active in many human cancers: Akt1 is upregulated in primary gastric adenocarcinoma [53], Akt2 amplification was reported in pancreatic, ovarian, and breast cancer [54, 55] and Akt3 was found amplified in estrogen receptor-deficient breast cancer and in androgen-independent prostate cancer cell lines [56]. There are various molecular mechanisms that may contribute to the activation of the PI3K/Akt pathway in human cancer. PKB/Akt activation may result from PI3K activation due to autocrine or paracrine stimulation of receptor tyrosine kinases [57] or overexpression of growth factor receptors. Other mechanisms include constitutive activation of of the PKB/Akt signal transduction pathway due to mutant receptors [58].
PKB/Akt and diabetes
editAs PKB/Akt is involved in many of the metabolic actions of insulin, it is reasonable to assume that PKB/Akt activity has implications for diabetes. A reduction of insulin-stimulated PKB/Akt kinase activity was reported in skeletal muscle of non-insulin-dependent diabetes patients [59]. Furthermore, diabetes-prone mice exhibit elevated activity of GSK3, which is negatively regulated by PKB/Akt (see above) [60].
Perspective
editIt is now well established that the three PKB/Akt isoforms play a crucial role in multiple pathways controlling cell survival and glucose metabolism. The major task is to uncover the mechanism by which PKB/Akt executes these multiple functions, since PKB/Akt may be an excellent target for drug development. Specific inhibition of PKB/Akt kinases might be used for cancer therapy, whereas activators might be useful for diabetes treatment and degenerative diseases resulting from increased cell death.
See Also
editApoptosis
Cell growth
Phosphoinositide 3-kinase
References
edit1. Grewal, S.S. and B.A. Edgar, Controlling cell division in yeast and animals: does size matter? J Biol, 2003. 2(1): p. 5.
2. Rupes, I., Checking cell size in yeast. Trends Genet, 2002. 18(9): p. 479-85.
3. Lockshin, R.A. and Z. Zakeri, Apoptosis, autophagy, and more. Int J Biochem Cell Biol, 2004. 36(12): p. 2405-19.
4. Segal, R.A. and M.E. Greenberg, Intracellular signaling pathways activated by neurotrophic factors. Annu Rev Neurosci, 1996. 19: p. 463-89.
5. Rameh, L.E. and L.C. Cantley, The role of phosphoinositide 3-kinase lipid products in cell function. J Biol Chem, 1999. 274(13): p. 8347-50.
6. Gu, J., M. Tamura, and K.M. Yamada, Tumor suppressor PTEN inhibits integrin- and growth factor-mediated mitogen-activated protein (MAP) kinase signaling pathways. J Cell Biol, 1998. 143(5): p. 1375-83.
7. Myers, M.P., et al., The lipid phosphatase activity of PTEN is critical for its tumor supressor function. Proc Natl Acad Sci U S A, 1998. 95(23): p. 13513-8.
8. Bellacosa, A., et al., Structure, expression and chromosomal mapping of c-akt: relationship to v-akt and its implications. Oncogene, 1993. 8(3): p. 745-54.
9. Vanhaesebroeck, B. and D.R. Alessi, The PI3K-PDK1 connection: more than just a road to PKB. Biochem J, 2000. 346 Pt 3: p. 561-76.
10. Bellacosa, A., et al., A retroviral oncogene, akt, encoding a serine-threonine kinase containing an SH2-like region. Science, 1991. 254(5029): p. 274-7.
11. Alessi, D.R., et al., Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase Balpha. Curr Biol, 1997. 7(4): p. 261-9.
12. Alessi, D.R., et al., Mechanism of activation of protein kinase B by insulin and IGF-1. EMBO J, 1996. 15(23): p. 6541-51.
13. Mayer, B.J., et al., A putative modular domain present in diverse signaling proteins. Cell, 1993. 73(4): p. 629-30.
14. Hanada, M., J. Feng, and B.A. Hemmings, Structure, regulation and function of PKB/AKT--a major therapeutic target. Biochim Biophys Acta, 2004. 1697(1-2): p. 3-16.
15. Franke, T.F., et al., Direct regulation of the Akt proto-oncogene product by phosphatidylinositol-3,4-bisphosphate. Science, 1997. 275(5300): p. 665-8.
16. Bellacosa, A., et al., Akt activation by growth factors is a multiple-step process: the role of the PH domain. Oncogene, 1998. 17(3): p. 313-25.
17. Currie, R.A., et al., Role of phosphatidylinositol 3,4,5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1. Biochem J, 1999. 337 ( Pt 3): p. 575-83.
18. Balendran, A., et al., PDK1 acquires PDK2 activity in the presence of a synthetic peptide derived from the carboxyl terminus of PRK2. Curr Biol, 1999. 9(8): p. 393-404.
19. Toker, A. and A.C. Newton, Akt/protein kinase B is regulated by autophosphorylation at the hypothetical PDK-2 site. J Biol Chem, 2000. 275(12): p. 8271-4.
20. Rane, M.J., et al., Heat shock protein 27 controls apoptosis by regulating Akt activation. J Biol Chem, 2003. 278(30): p. 27828-35.
21. Hresko, R.C. and M. Mueckler, mTOR.RICTOR is the Ser473 kinase for Akt/protein kinase B in 3T3-L1 adipocytes. J Biol Chem, 2005. 280(49): p. 40406-16.
22. Yano, S., H. Tokumitsu, and T.R. Soderling, Calcium promotes cell survival through CaM-K kinase activation of the protein-kinase-B pathway. Nature, 1998. 396(6711): p. 584-7.
23. Cardone, M.H., et al., Regulation of cell death protease caspase-9 by phosphorylation. Science, 1998. 282(5392): p. 1318-21.
24. Biggs, W.H., 3rd, et al., Protein kinase B/Akt-mediated phosphorylation promotes nuclear exclusion of the winged helix transcription factor FKHR1. Proc Natl Acad Sci U S A, 1999. 96(13): p. 7421-6.
25. Brunet, A., et al., Akt promotes cell survival by phosphorylating and inhibiting a Forkhead transcription factor. Cell, 1999. 96(6): p. 857-68.
26. Kops, G.J., et al., Direct control of the Forkhead transcription factor AFX by protein kinase B. Nature, 1999. 398(6728): p. 630-4.
27. Rena, G., et al., Phosphorylation of the transcription factor forkhead family member FKHR by protein kinase B. J Biol Chem, 1999. 274(24): p. 17179-83.
28. Tang, E.D., et al., Negative regulation of the forkhead transcription factor FKHR by Akt. J Biol Chem, 1999. 274(24): p. 16741-6.
29. Villunger, A., et al., p53- and drug-induced apoptotic responses mediated by BH3-only proteins puma and noxa. Science, 2003. 302(5647): p. 1036-8.
30. Cross, D.A., et al., Inhibition of glycogen synthase kinase-3 by insulin mediated by protein kinase B. Nature, 1995. 378(6559): p. 785-9.
31. Maurer, U., et al., Glycogen synthase kinase-3 regulates mitochondrial outer membrane permeabilization and apoptosis by destabilization of MCL-1. Mol Cell, 2006. 21(6): p. 749-60.
32. Zong, W.X., et al., The prosurvival Bcl-2 homolog Bfl-1/A1 is a direct transcriptional target of NF-kappaB that blocks TNFalpha-induced apoptosis. Genes Dev, 1999. 13(4): p. 382-7.
33. Inoki, K., et al., TSC2 is phosphorylated and inhibited by Akt and suppresses mTOR signalling. Nat Cell Biol, 2002. 4(9): p. 648-57.
34. Manning, B.D., et al., Identification of the tuberous sclerosis complex-2 tumor suppressor gene product tuberin as a target of the phosphoinositide 3-kinase/akt pathway. Mol Cell, 2002. 10(1): p. 151-62.
35. Potter, C.J., L.G. Pedraza, and T. Xu, Akt regulates growth by directly phosphorylating Tsc2. Nat Cell Biol, 2002. 4(9): p. 658-65.
36. Kovacina, K.S., et al., Identification of a proline-rich Akt substrate as a 14-3-3 binding partner. J Biol Chem, 2003. 278(12): p. 10189-94.
37. Sancak, Y., et al., PRAS40 is an insulin-regulated inhibitor of the mTORC1 protein kinase. Mol Cell, 2007. 25(6): p. 903-15.
38. Vander Haar, E., et al., Insulin signalling to mTOR mediated by the Akt/PKB substrate PRAS40. Nat Cell Biol, 2007. 9(3): p. 316-23.
39. Eguez, L., et al., Full intracellular retention of GLUT4 requires AS160 Rab GTPase activating protein. Cell Metab, 2005. 2(4): p. 263-72.
40. Sano, H., et al., Insulin-stimulated phosphorylation of a Rab GTPase-activating protein regulates GLUT4 translocation. J Biol Chem, 2003. 278(17): p. 14599-602.
41. Miinea, C.P., et al., AS160, the Akt substrate regulating GLUT4 translocation, has a functional Rab GTPase-activating protein domain. Biochem J, 2005. 391(Pt 1): p. 87-93.
42. Calera, M.R., et al., Insulin increases the association of Akt-2 with Glut4-containing vesicles. J Biol Chem, 1998. 273(13): p. 7201-4.
43. Kohn, A.D., et al., Expression of a constitutively active Akt Ser/Thr kinase in 3T3-L1 adipocytes stimulates glucose uptake and glucose transporter 4 translocation. J Biol Chem, 1996. 271(49): p. 31372-8.
44. Lum, J.J., et al., The transcription factor HIF-1alpha plays a critical role in the growth factor-dependent regulation of both aerobic and anaerobic glycolysis. Genes Dev, 2007. 21(9): p. 1037-49.
45. Majumder, P.K., et al., mTOR inhibition reverses Akt-dependent prostate intraepithelial neoplasia through regulation of apoptotic and HIF-1-dependent pathways. Nat Med, 2004. 10(6): p. 594-601.
46. Semenza, G.L., et al., Transcriptional regulation of genes encoding glycolytic enzymes by hypoxia-inducible factor 1. J Biol Chem, 1994. 269(38): p. 23757-63.
47. Sundqvist, A., et al., Control of lipid metabolism by phosphorylation-dependent degradation of the SREBP family of transcription factors by SCF(Fbw7). Cell Metab, 2005. 1(6): p. 379-91.
48. Dimmeler, S., et al., Activation of nitric oxide synthase in endothelial cells by Akt-dependent phosphorylation. Nature, 1999. 399(6736): p. 601-5.
49. Fulton, D., et al., Regulation of endothelium-derived nitric oxide production by the protein kinase Akt. Nature, 1999. 399(6736): p. 597-601.
50. Manning, B.D. and L.C. Cantley, AKT/PKB signaling: navigating downstream. Cell, 2007. 129(7): p. 1261-74.
51. Dummler, B. and B.A. Hemmings, Physiological roles of PKB/Akt isoforms in development and disease. Biochem Soc Trans, 2007. 35(Pt 2): p. 231-5.
52. Di Cristofano, A. and P.P. Pandolfi, The multiple roles of PTEN in tumor suppression. Cell, 2000. 100(4): p. 387-90.
53. Staal, S.P., Molecular cloning of the akt oncogene and its human homologues AKT1 and AKT2: amplification of AKT1 in a primary human gastric adenocarcinoma. Proc Natl Acad Sci U S A, 1987. 84(14): p. 5034-7.
54. Bellacosa, A., et al., Molecular alterations of the AKT2 oncogene in ovarian and breast carcinomas. Int J Cancer, 1995. 64(4): p. 280-5.
55. Miwa, W., et al., Isolation of DNA sequences amplified at chromosome 19q13.1-q13.2 including the AKT2 locus in human pancreatic cancer. Biochem Biophys Res Commun, 1996. 225(3): p. 968-74.
56. Nakatani, K., et al., Up-regulation of Akt3 in estrogen receptor-deficient breast cancers and androgen-independent prostate cancer lines. J Biol Chem, 1999. 274(31): p. 21528-32.
57. Yuan, Z.Q., et al., Frequent activation of AKT2 and induction of apoptosis by inhibition of phosphoinositide-3-OH kinase/Akt pathway in human ovarian cancer. Oncogene, 2000. 19(19): p. 2324-30.
58. Sordella, R., et al., Gefitinib-sensitizing EGFR mutations in lung cancer activate anti-apoptotic pathways. Science, 2004. 305(5687): p. 1163-7.
59. Krook, A., et al., Insulin-stimulated Akt kinase activity is reduced in skeletal muscle from NIDDM subjects. Diabetes, 1998. 47(8): p. 1281-6.
60. Rondinone, C.M., et al., Impaired glucose transport and protein kinase B activation by insulin, but not okadaic acid, in adipocytes from subjects with Type II diabetes mellitus. Diabetologia, 1999. 42(7): p. 819-25.