Abstract B01: Multilevel interference with receptor-PI3K-mTORC1 signaling is key mechanism for anticancer activity of fatty acid synthase inhibitors
Authors: Wagner Renate , Pröstling Katharina , Veigel Daniel , Stübiger Gerald , Grusch Michael , Weghuber Julian , Singer Christian , Karlic Heidrun , Colomer Ramón , Benhamú Bellinda , López-Rodríguez María Luz , Hegardt Fausto , García Jordi , Serra Dolors , Valent Peter , Grunt Thomas 
Published: May 2016

Abstract
The metabolic oncogene fatty acid synthase (FASN) is overexpressed in 80% of ovarian cancers (OC) and indicates poor prognosis. Exposure of OC to inhibitors of FASN elicits a complex stress response that interferes with receptor-PI3K-mTORC1 signaling (briefly designated ‘PI3K pathway’). Here we demonstrate that FASN inhibitors capitalize on multiple mechanisms to interfere with the PI3K pathway, and that silencing this cascade is crucial for the anticancer action of the drugs. Data were obtained using thin-layer chromatography, ELISA, Western blotting, quantitative micropatterning and growth assays, respectively. Exposure of OC cells (SKOV3, OVCAR-3) to FASN inhibitors (C75, G28UCM) causes lipid redistribution toward storage lipids, whereas membrane lipid rafts and signaling lipids are diminished, which significantly impairs EGF receptor/ErbB/HER function and expression. A severe depletion of phosphatidylinositol (3,4,5) trisphosphate (PIP3), which represents the crucial product of PI3K action, is associated with drug-dependent silencing of AKT. Moreover, FASN blockers rapidly stimulate expression of the stress response gene REDD1 (RTP801/Dig2/DDIT4) followed by slow activation of the energy sensor AMPK. Induction of these mTORC1 upstream repressors has been found to block downstream phosphorylation of ribosomal S6 protein. Moreover, long-term stress imposed by persistent FASN blockade leads to accelerated degradation of signaling proteins. Interestingly, concurrent targeting of the PI3K pathway using the dual PI3K/mTOR blocker NVP-BEZ235 does not aggravate the FASN anticancer drug effects. However, forced expression of constitutive active AKT counteracts FASN inhibitor-mediated mTORC1 silencing and abrogates growth arrest. Our data thus suggest that FASN inhibitors by themselves can efficiently eliminate PI3K downstream activity. This mechanism appears crucial for the anticancer effect, which cannot be augmented by co-exposure to drugs that target the very same pathway. On the other hand, silencing PI3K signaling by FASN inhibitors was found to release a negative feedback loop toward MAPK. Thus, FASN drug-mediated PI3K silencing is associated with cross-activation of ERK1/2. Accordingly, co-treatment with the MEK inhibitor selumetinib (AZD6244) significantly improves the anticancer action of FASN inhibitors, whereas introduction of constitutive active MEK does not alter FASN drug-induced growth inhibition. Collectively these data demonstrate that FASN inhibitors utilize a whole panel of different mechanisms to abrogate receptor-PI3K-mTORC1 signaling, which represents at least one of the crucial mechanisms of anticancer action of these compounds.
Supported by Medical Scientific Fund of the Mayor of the City of Vienna & ‘Initiative Krebsforschung’ of the Medical University Vienna, Austria.
Citation Format: Renate Wagner, Katharina Pröstling, Daniel Veigel, Gerald Stübiger, Michael Grusch, Julian Weghuber, Christian Singer, Heidrun Karlic, Ramón Colomer, Bellinda Benhamú, María Luz López-Rodríguez, Fausto Hegardt, Jordi García, Dolors Serra, Peter Valent, Thomas Grunt. Multilevel interference with receptor-PI3K-mTORC1 signaling is key mechanism for anticancer activity of fatty acid synthase inhibitors. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr B01.