Protein kinase B or AKT (PKB) is a serine/threonine kinase. In mammals it is comprised of 3 highly homologous isoforms PKBα (Akt1), PKBβ (Akt2), and PKBγ (Akt3) (Manning and Cantley 2007). AKT signalling is activated in response to a variety of hormones, growth factors and extracellular matrix (ECM) components. The serine/threonine kinase AKT is involved in the regulation of a number of cellular processes including cell growth, proliferation, metabolism and cell survival.
AKT Signalling Activation
AKTs are activated by receptor tyrosine kinases (RTKs). RTKs activate phosphatidylinositol 3-kinase (PI3K) through tyrosine phosphorylation of adaptor proteins such IRS-1. Activated PI3K converts phosphatidylinositol 4, 5-bisphosphate (PIP2) into phosphatidylinositol 3, 4, 5-tri trisphosphate (PIP3). Formation of PIP3 is essential for AKT/PKB translocation to the plasma membrane. Following translocation to the plasma membrane AKT/PKB is phosphorylated and activated by Pyruvate Dehydrogenase Kinase 1 (PDK1). PDK1 phosphorylates AKT at Thr308, located on the activation loop (Manning and Cantley 2007).
Upon activation AKT regulates a wide variety of biological processes. AKT can phosphorylate and inhibit Bcl-2 homology domain 3 (BH3)-only protein BAD which inhibits the pro-apoptotic effect of BAD, thus promoting cell survival. AKT phosphorylation of CREB and NF-κB results in the increase of transcription of anti-apoptotic Bcl-2 family members.
Cellular processes regulated by AKT
AKT modulates the function of numerous substrates involved in the regulation of cell survival, cell cycle progression and cellular growth
AKT/PKB in prosurvival and antiapoptotic mechanisms
Studies have highlighted that AKT/PKB is essential for cell survival. Its activity can be triggered in response to a variety of hormones, growth factors and extracellular components. This can be seen in dominant negative alleles for PKB/Akt which the reduce the ability of growth factors and other stimuli to maintain cell survival (Nicholson and Anderson 2002) . Overexpression of AKT/PKB however, has been shown to rescue cells from apoptosis induced by various stress signals (Nicholson and Anderson 2002).
It is likely that there are multiple mechainisms by which AKT/PKB can promote cell survival. For instance decreasing the transcription of death genes, through phosphorylation of forkhead family transcription factors such as FKHR. Phosphorylation of FKHR promotes sequestration by 14-3-3 proteins in the cytoplasm (Nicholson and Anderson 2002). Another mechanism of AKT/PKB includes enhanced transcription of survival genes via activation of NF-κB and CREB transcription factors. Phosphorylation and inactivation of the proapoptotic protein BAD can also be noted. Maintenance of mitochondrial integrity is also an essential component of prosurvival/antiapptotic mechanisms, this is achieve by AKT/PKB through activation of hexokinase (Nicholson and Anderson 2002).
AKT/PKB and cell cycle progression
AKT/PKB also plays a role in cell cycle progression.The first direct evidence supporting this came from a study showing that overexpression of PKBβ /Akt2 accelerated cell cycle progression and caused transformation in murine fibroblasts (Cheng et al. 1997). AKT can phosphorylate GSK3, c-myc and c-jun, allowing for the G1 to S cell cycle progression (Wei et al. 2005, Yeh et al. 2013).Transition of cells through the G1/S checkpoint is regulated by retinoblastoma protein (pRB). Retinoblastoma protein suppresses the transcription of genes required for G1/S transition. pRB is inactivated by phosphorylation,which is carried out by cyclin-dependent kinases (CDKs). CDKs require cyclins for activation and are negatively regulated by CDKIs, cyclin dependent inhibitors. P21 is one of the most well known CDKIs (Assosian and Schwartz 2001).
Overexpression of cyclin D1 has been found in many human cancers, particularly breast cancer, where the protein is up-regulated in at least 50% of all cases (Nicholson and Anderson 2002, Bartkova et al. 1994). Increased cyclin D1 expression leads to shorter cell cycle times, driving tumour progression. Cyclin D1 expression is controlled via multiple coordinated signals that alter gene transcription, mRNA translation, and protein stability. Accumulating evidence suggests that PKB/Akt can regulate cyclin D1 at each of these levels (Nicholson and Anderson 2002, Takuwa et al. 1999)
AKT & MST2
AKT has also been demonstrated to regulate MST2, suggesting possible crosstalk between the AKT pathway and the MST2 pathway. MST2 is phosphorylated by AKT on Thr117 and Thr384 residues. The mutations of these sites result in the disruption of the MST2-Raf-1 complex which promotes apoptosis. This results in an increase in MST2 interaction with RASSF1A (Romano et al. 2010). MST2 depletion was shown to increase the phosphorylation and activity of AKT (Cinar et al. 2007).
AKT inhibits YAP1
AKT phosphorylates the co-transcription factor YAP1 on the Ser127 residues and this sequesters YAP1 in the cytoplasm reducing the induction of the pro-apoptotic protein BAX (Basu et al. 2003). Interestingly, YAP activation leads to an increase in miR-29 that results in the decreased expression of the tumour suppressor PTEN (Tumaneng et al. 2012).
AKT and Cancer
AKT has been identified as a key regulator of cellular processes such as apoptosis, proliferation, differentiation and metabolism. Therefore, it is not surprising that disruption of normal AKT signalling has been identified to play an important role in cancer progression. Amplification of AKT can be noted in many human cancers such as breast, oesophageal, ovarian and pancreatic cancer. With such over-expression likely being associated with high grade aggressive tumours (Nicholson and Anderson 2002). Components of the AKT signalling pathway such as PI3K are also known to contribute to cancer development. For instance the PI3K pathway may confer resistance to cell death in oesophageal dysplastic cells. Novel therapeutic strategies could be developed to target the PI3K pathway alone or in combination with other oesophageal treatment (Li et al. 2014).
As a result of the role AKT signalling plays in cell survival and proliferation the AKT signalling pathway is an attractive therapeutic target. The ultimate aim of such targeting – prevention of tumour progression through direct or indirect induction of apoptosis. However, chemoresistance remains a major problem. For instance, ovarian cancer cell lines with either constitutive AKT1 activity or AKT2 gene amplification are highly resistant to paclitaxel, in contrast to cells with low AKT levels (Page et al., 2000, Altomare and Testa 2005).
Targeting of the PI3K/Akt/mTOR pathway has been proposed to overcome therapeutic resistance in acute myelogenous leukaemia (AML). Selective inhibition of upstream receptor kinases as well as PI3K, PDK1, AKT, and mTOR kinases may also prove beneficial. Caution should be exerted with such interventions as it has been suggested that pharmaceutical inhibition of AKT may impact glucose metabolism, as a result of the essential role of AKT plays in insulin signaling and maintenance of glucose homeostasis (Whiteman et al., 2002).