User:Cellular Biochemistry II/Article 1

Definition
PKB/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
Nutrients 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, 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
In 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
PI3,4P 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.

Cell Survival
PKB/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
PKB/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
In 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
PKB/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.

PKB/Akt disruption
Isoform 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
As 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
As 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
It 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.