Targeting the Akt Kinase to Modulate Survival, Invasiveness and Drug Resistance of Cancer Cells

The deregulation of oncogenic signaling pathways which provide survival advantages to tumor cells is mediated by multiple cellular networks. Among them, the PI3K-Akt-mTOR axis, in particular the serine/threonine kinase Akt, is recognized as a key player. The kinase is hyperactivated due to a variety of mechanisms including loss of PTEN, mutations in the PI3K catalytic subunit, receptor tyrosine kinase and Ras activation. Indeed, inappropriate activation of the Akt kinase is a common event in human tumors and Akt appears to be a critical player in cell survival that may also account for the therapeutic resistance and the invasive phenotype of tumors. Inhibition of Akt signalling results in apoptosis and growth inhibition of tumour cells with elevated Akt activity. A functional role in drug resistance is supported by evidence that tumor cells with acquired resistance to antitumor agents may display increased Akt activation and that treatment with molecularly targeted agents can activate feed-back loops involving Akt. This serine/threonine kinase may therefore represent an amenable target for modulation of sensitivity to compounds with different molecular features due to its pleiotropic role in cell survival. Different types of Akt inhibitors [i.e., ATP mimetics and pleckstrin-homology (PH) domain binders] have been generated and some of them have reached the clinical setting. The present review focuses on the i) mechanisms implicating Akt in increased survival and invasive potential of tumor cells of different tumor types and ii) on the development of Akt inhibitors as modulators of drug resistance.