AKT has been shown to be important for G S

AKT1 has been shown to be important for G1-S checkpoint transition and proliferation, whereas AKT2 regulates cell-cycle exit through its interaction with p21 (Héron-Milhavet et al., 2006). In a recent study in triple negative breast cancer, AKT3, rather than AKT1 activity was most important for cellular proliferation (Chin, Yoshida, et al., 2014), suggesting a degree of context specificity for these findings. Interestingly, it has been observed that although activating mutations in AKT1 result in accelerated tumourigenesis, metastatic dissemination is decreased due to inhibitory effects of AKT1 on tumour cell invasion and migration (Chin and Toker, 2010, Hutchinson et al., 2004). By contrast, AKT2 activity promotes tumour invasion and metastatic dissemination in mouse breast cancer models (Dillon et al., 2009, Maroulakou et al., 2007). In addition, it has recently been shown that INPP4B inhibition of AKT2 is important for metastatic spread of follicular thyroid cancer (Chew et al., 2015). A number of different AKT1/2 substrates have been described that may account for the differential phenotype of cellular migration, which play a role in epithelial-mesenchymal transition (EMT), including the NFAT transcription factor (Yoeli-Lerner et al., 2005), palladin (Chin & Toker, 2010), the integrin β subunit (Arboleda et al., 2003) and the miR-200 family of micro-RNAs (Iliopoulos et al., 2009). There are likely to be many more (Clark & Toker, 2014) and the relative importance of each of these is likely to depend on tumour subtype, as well as perhaps tumour stage and genetic background. An example of this is that PTEN deficient tumour Regorafenib have recently been shown to be dependent, specifically on AKT2 for survival (Chin, Yuan, Balk and Toker, 2014). This was shown to be true for prostate cancer, breast cancer and glioblastoma PTEN deficient cells lines and was at least in part, mediated through AKT2 dependent upregulation p21 (Chin, Yuan, et al., 2014). Several lines of evidence suggest that AKT does not need to be fully activated in order to phosphorylate all substrates. This suggests that different AKT substrates and therefore different cellular functions of AKT depend on varying levels of AKT activation. For example AKT Ser473 phosphorylation appears to be dispensable for phosphorylation of the AKT targets TSC2 and GSK3, as well as the TORC1 effectors, S6K and 4E-BP1, but not for FOXO1/3a (Jacinto et al., 2006). Interestingly, in patient non-small cell lung cancer samples, AKT Thr308 correlates with phosphorylation of AKT substrates PRAS40, TSC2 and TBC1D4, whereas AKT Ser473 phosphorylation does not (Vincent et al., 2011). This raises an important point clinically, when trying to determine whether or not a tumour might be dependent on AKT signalling for survival, suggesting the AKT Thr308 is a more suitable biomarker of AKT activation than AKT Ser473. Relative levels of the negative regulators of the PI3K/AKT pathway will also ultimately affect amplitude of AKT activation. For example, the PHLPP isoforms -1 and -2 have been shown to preferentially hydrolyse Thr308 on AKT2 and AKT3 respectively (Brognard, Sierecki, Gao, & Newton, 2007). Cellular location and post-translational modification of AKT isoforms also contribute to specificity within the pathway (Clark & Toker, 2014) and untangling all of these factors within the context of cancer is proving to be a considerable challenge.
Feedback loops within the PI3K/AKT signalling pathway The PI3K/AKT signalling pathway is far from linear with multiple negative feedback loops fine-tuning overall activation levels (Carracedo & Pandolfi, 2008). The activity of the S6 kinases (S6K1/2), which are phosphorylation targets of mTORC1 plays a pivotal role in negatively regulating AKT activity. IRS-1 (insulin receptor substrate-1) is an adaptor protein required for binding of the class IA PI3Ks with insulin and insulin growth factor (IGF) receptors. Phosphorylation of IRS-1 by S6K1 inhibits its interaction with insulin and IGF receptors and also promotes proteasomal degradation of IRS-1 (Harrington, Findlay, & Lamb, 2005), thereby dampening PI3K signalling and reducing AKT activity. In addition to its effects on AKT activity via upstream inhibition of PI3K signalling, the mTORC1 complex also inhibits AKT by up-regulating PHLPP1 translation (Liu, Stevens, & Gao, 2011), the phosphatase responsible for dephosphorylating AKT pSer473 (Fig. 3).