Abstract 2158: PDK1-dependent activation of RSK is an absolute requirement for PI3K oncogenic activity in the thyroid gland

Abstract

Abstract Aberrant activation of the phosphatidylinositol-3 kinase (PI3K)/Akt pathway plays a key role in thyroid tumorigenesis, particularly in follicular thyroid cancer (FTC) and anaplastic thyroid cancer (ATC). Accordingly, conditional deletion of Pten in the mouse thyroid epithelium induces constitutive activation of the PI3K/AKT pathway, and causes thyroid hyperplasia at birth that progresses to invasive and metastatic follicular carcinoma. This phenotype is dramatically accelerated by simultaneous deletion of Trp53, leading to the development of ATC, or activation of Kras, leading to poorly differentiated carcinomas. The PDK1 kinase plays a central role in the PI3K signaling cascade, acting as 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 independent of PI3K activity. 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 PDK1 mutant disrupts the PIF pocket and impairs AGC kinase activation, without any effect on AKT activation. Thanks to this differential mechanism of substrate activation, we are defining the relative contribution of AKT and other PDK1 targets to the neoplastic transformation process in Pten-/- thyrocytes. Surprisingly, in vivo disruption of the PIF-pocket completely abrogates neoplastic transformation of Pten-/- thyroids, even in the face of AKT constitutive activation. This finding supports the hypothesis that the linear PI3K-AKT pathway is not sufficient to drive thyroid transformation and that concomitant activation of additional PDK1-dependent pathways is absolutely required. We now show that RSK family members (RSK1-3) are essential components of this PDK1-dependent accessory pathway. Remarkably, RSK3 levels in the thyroid are increased in the absence of Pten, and RSK activity in thyroid cells is abrogated when PDK1 PIF-pocket is mutated. Genetic and pharmacological inhibition of RSK drastically impairs [Pten,Trp53]-/- ATC cell proliferation both in vitro and in vivo, in syngeneic allograft models. Taken together, our data identify a new signaling cascade that is essential for neoplastic transformation of thyroid epithelial cells, and thus a new critical candidate for drug development. Citation Format: Arturo Orlacchio, Antonio Di Cristofano. PDK1-dependent activation of RSK is an absolute requirement for PI3K oncogenic activity in the thyroid gland. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2158. doi:10.1158/1538-7445.AM2015-2158