Abstract 4376: Beyond AKT: Critical pathways for PI3K-dependent transformation

Abstract Genetic alterations activating the PI3K/AKT pathway have been identified in most human cancers, including those originating in the thyroid gland, particularly follicular thyroid cancer (FTC) and anaplastic thyroid cancer (ATC). However, although PI3K and AKT represent relevant targets for therapeutic purposes, the number of essential roles played by this pathway in normal cells poses the problem of reaching therapeutic efficacy without harming healthy tissues. Ideally, the toxicity associated with wide-range inhibitors could be reduced by targeting only molecules directly involved in the transformation process. The PDK1 kinase is a master regulator of PI3K downstream effectors belonging to the AGC kinases family, such as AKT and S6K1. Additional AGC kinases are controlled by PDK1 independently of PI3K activity. If PDK1-controlled AGC kinases contribute to PI3K-dependent transformation, they would clearly represent novel important therapeutic targets. PDK1 possesses a substrate-docking site, the “PIF pocket”, required for the phosphorylation of most AGC kinases (including S6K, PKC, SGK, and RSK) but not of AKT. The L155E mutation in PDK1disrupts the PIF pocket, without affecting AKT activation. This differential mechanism of substrate selection allows us to clearly define the relative contribution of AKT and other PDK1 targets to the neoplastic transformation process. Using in vivo as well as ex vivo and in vitro genetic and pharmacological approaches, we have shown that Akt activation is not sufficient to transform thyroid epithelial cells. Mice in which the PI3K pathway is constitutively activated in the thyroid epithelium through loss of Pten show thyroid hyperplasia at birth that progresses to invasive and metastatic follicular carcinoma. While Pdk1 deletion, as expected, completely rescue the phenotype observed in Pten−/- thyroids, we found that the simple impairment of the PIF-pocket is able to completely abrogate neoplastic transformation, despite AKT and mTOR being constitutively activated. We show that members of two families of AGC kinases, SGK (1and 3) and RSK (1-3) are essential components of this PDK1-dependent pathway required for PI3K-dependent transformation. Genetic and pharmacological SGK and RSK inhibition strongly reduces cell proliferation in Pten−/− as well as Pik3ca mutant cell lines, both in 2d and in 3D systems, with SGK1 and RSK3 being the most critical isoforms. Furthermore, genetic inhibition of SGK and RSK drastically impairs tumor growth in syngeneic allograft and metastasis models. Taken together, our data identify two new signaling cascades that are essential for neoplastic transformation of thyroid epithelial cells, and thus novel strong candidates for drug development. Citation Format: Arturo Orlacchio, Antonio Di Cristofano. Beyond AKT: Critical pathways for PI3K-dependent transformation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4376.