Emerging Roles for AKT Isoform Preference in Cancer Progression Pathways

The phosphoinositol-3 kinase (PI3K)-AKT pathway is one of the most mutated in human cancers, predominantly associated with the loss of the signaling antagonist, PTEN, and to lesser extents, with gain-of-function mutations in PIK3CA (encoding PI3K-p110α) and AKT1. In addition, most oncogenic driver pathways activate PI3K/AKT signaling. Nonetheless, drugs targeting PI3K or AKT have fared poorly against solid tumors in clinical trials as monotherapies, yet some have shown efficacy when combined with inhibitors of other oncogenic drivers, such as receptor tyrosine kinases or nuclear hormone receptors. There is growing evidence that AKT isoforms, AKT1, AKT2, and AKT3, have different, often distinct roles in either promoting or suppressing specific parameters of oncogenic progression, yet few if any isoform-preferred substrates have been characterized. This review will describe recent data showing that the differential activation of AKT isoforms is mediated by complex interplays between PTEN, PI3K isoforms and upstream tyrosine kinases, and that the efficacy of PI3K/AKT inhibitors will likely depend on the successful targeting of specific AKT isoforms and their preferred pathways.