Akt is an evolutionarily conserved serine/threonine kinase involved in a wide variety of cellular functions, including proliferation, cell survival, differentiation, glucose mobilization, homeostasis, cell migration, and apoptosis. Three isoforms, Akt1,Akt2, and Akt3, are expressed in mammals (Ref.1 , 2 and 3). All three isoforms of Akt share a common structure of three domains. The N-terminus of the protein is a PH (Pleckstrin Homology) domain, which interacts with membrane lipid products such as PIP2 (Phosphatidylinositol-3,4-Bisphosphate) and PIP3 (Phosphatidylinositol-3,4,5-Triphosphate) (Ref.4).
Most common Ligands activating Akt include Growth factors, Cytokines, Mitogens and Hormones. Phosphoinositol 3-Kinase (PI3K) is recruited to the phosphotyrosine residues via SH2 domains in the regulatory domain (p85), and is therefore targeted to the inner cell membrane. Binding of the p85 subunit of PI3K to the phosphorylated RTK leads to conformational changes in the catalytic domain of PI3K (p110) and consequent kinase activation. Insulin can also activate PI3K via Insulin Receptor Substrate-1 (IRS1). G-Protein-Coupled Receptor (GPCR) also activates PI3K through Guanine Nucleotide-Binding Protein-Beta (GN-Beta) and Guanine Nucleotide-Binding Protein-Gamma (GN-Gamma) subunits of G-proteins. PI3K then phosphorylates membrane bound PIP2 to generate PIP3. The binding of PIP3 anchors Akt to the plasma membrane and allows its phosphorylation and activation by Phosphoinositide-Dependent Kinase-1 (PDK1). The activity of Akt is negatively regulated by Phosphatase and Tensin Homolog (PTEN), SH2-Containing Inositol Phosphatase (SHIP) and Carboxyl-Terminal Modulator Protein (CTMP) (Ref.5, 6 and 7).
Akt inhibits apoptosis by phosphorylating the BAD component of the BAD/BclXL complex. Phosphorylated BAD binds to 14-3-3 causing dissociation of the BAD/BclXL complex and allowing cell survival. Akt activates IKK, which ultimately leads to NF-KappaB activation and cell survival. In the presence of survival factors, Akt1 phosphorylates FKHRL1, leading to the association of FKHRL1 with 14-3-3 proteins and its retention in the cytoplasm. Another notable substrate of Akt is the death protease Caspase9. Phosphorylation of Caspase9 decreases apoptosis by directly inhibiting the protease activity. Akt also activates TERT (Telomere Reverse Transcriptase), which is responsible for telomere maintenance and DNA stability. Akt can regulate several levels of Glucose metabolism. It enhances Glucose-uptake in Insulin-responsive tissues by inducing the expression of GLUT1 and GLUT3 and the translocation of GLUT4 to the plasma membrane; the GLUTs transport glucose into the cell. Akt also activates Glycogen synthesis by phosphorylating and inactivating GSK3, which leads to the activation of Glycogen Synthase and CyclinD1. Akt induces glycolysis through the phosphorylation and activation PFK2, which in turn activates PFK1. These enzymes convert Fructose-6-Phosphate into Fructose-1, 6-Bisphosphate, a key step in Glucose metabolism. Phosphorylation of TSC1 and TSC2 results in suppression of their inhibitory activity and may also target the proteins for degradation. Activation of mTOR also results in phosphorylation and inactivation of eIF4EBP (Eukaryotic Initiation Factor-4E Binding Protein), an inhibitor of the translation initiation factor eIF4E(Ref.8).
Transcription factor CREB is directly phosphorylated at Ser133 by Akt. This causes an increased affinity of CREB for its co-activator protein, CRB (Crumbs). Akt also phosphorylates AR at two serine residues, Ser210 and Ser270, which causes a decrease in AR activity on the p21 promoter. In addition to causing cell cycle progression, this also results in apoptosis inhibition in certain cell types, through other actions of AR. YAP is another transcription factor that is phosphorylated by Akt, and is of importance because it does not contain an Akt consensus sequence. Akt phosphorylates Ser127 on YAP, which causes association with 14-3-3 proteins, nuclear export and cytoplasmic localization. Akt has also been shown to phosphorylate p21 directly, on Thr145. p21 is a member of the Cip/Kip family of CDK inhibitors that arrest the cell cycle and therefore limit cell proliferation. p21 can also promote cell cycle progression, via mediating the assembly and activity of cyclin D1-CDK4/6 complexes. p27 is another cyclin-dependent kinase inhibitor, of the Kip family. 14-3-3 proteins bind phosphorylated p27 and cause active export from nucleus. Without p27 in the nucleus, the cyclin-CDK complexes form and promote cell cycle progression. Akt also phosphorylates MDM2. MDM2 is phosphorylated at many sites, only two of which have been identified. Ser166 is phosphorylated by Akt. Akt phosphorylation of MDM2 allows its entry into the nucleus where it targets p53 for degradation (Ref.9, 10 and 11).
PRAS40 is a 40 kDa substrate of AKT. Activated AKT phosphorylates PRAS40 on threonine 246, enabling PRAS40 to bind to 14-3-3. AKT and PRAS40 are components of the PI3K pathway. Akt phosphorylates, both in vitro and in vivo, the GABA(A)R, the principal receptor mediating fast inhibitory synaptic transmission in the mammalian brain. Akt- mediated phosphorylation increases the number of GABA(A)Rs on the plasma membrane surface, thereby increasing the receptor-mediated synaptic transmission in neurons. XIAP is a physiological substrate of Akt. Akt interacts with and phosphorylates XIAP at serine 87. These effects reduce XIAP degradation and the increased levels of XIAP are associated with decreased cisplatin-stimulated Caspase3 activity and programmed cell death. (Ref.12, 13 and 14).
Akt also phosphorylates Ataxin1 and modulate neurodegeration.14-3-3 protein mediates the neurotoxicity of Ataxin1 by binding to and stabilizing Ataxin1, thereby slowing its normal degradation. Akt also decreases ASK1kinase activity by phosphorylating a consensus Akt site at serine 83 of ASK1. Akt also interacts with the JIP1 (JNK Interacting Protein-1) scaffold and inhibits the ability of JIP1 to form active JNK signaling complexes Akt mediates PI3K-dependent p47Phox phosphorylation, which contributes to respiratory burst activity in human neutrophils. AKT impair Chk1 through phosphorylation, ubiquitination, and reduced nuclear localization to promote genomic instability in tumor cells. Akt and its upstream regulators are deregulated in a wide range of solid tumors and hematologic malignancies, hence the Akt pathway is considered a key determinant of biologic aggressiveness of these tumors, and a major potential target for novel anti-cancer therapies (Ref.15 ,16 and 17).