Abstract 1213: Nuclear export of E2F7 in squamous cell carcinoma in an actionable event that reverses resistance to anthracyclines
Authors: Ponce Alba Natalia Saenz , Landesman Yosef , Kashyap Trinayan , Guminski Alexander , Gannon Orla , Saunders Nicholas 
Published: July 2017

Abstract
Objectives and Background: Squamous cell carcinoma (SCC) of the skin or head and neck region is a major cause of cancer death in the western world. Patient with advanced SCC have few therapeutic options and SCC remains resistant to current treatments. Overcoming drug resistance in SCC would significantly change patient outcomes and treatment strategies. We previously showed that SCCs express the activating transcription factor E2F1 and the inhibitory transcription factor E2F7, which are mutually antagonistic. We showed that E2F1/7 controlled the sensitivity to anthracyclines in SCC cells via E2F-dependent regulation of sphingosine kinase 1 (Sphk1) and its product sphingosine-1-phosphate (S1P). In addition, we showed that S1P treatment of cells induces profound anthracycline resistance. However, it was consistently difficult to reconcile the mutual overexpression of E2F1 and E2F7 with a seemingly pro-E2F1 environment within the SCC cells (i.e. pro-proliferation, differentiation-suppressive and pro-survival). In this study we have examined the role of nuclear export pathways in determining nuclear E2F activation in SCC.
Methods: We have interrogated human tissue microarrays of normal stratified epithelia and SCC of the skin or head and neck tumours from patient samples. To validate our in vivo findings we used a suite of SCC cell lines and examine the nuclear and cytoplasmic localization of E2F1 and E2F7 by immunofluorescence and western blotting. We deployed inhibitors of the Exportin 1 (XPO1) pathway and have used siRNA against XPO1 to confirm a functional role of XPO1 in the nuclear export of E2F7. We have used inhibitors of XPO1 in vitro and in vivo to examine their ability to enhance the cytotoxic responses to anthracyclines.
Results & Conclusions: We report that E2F7 is selectively localized to the cytoplasm of SCC cells and tumors but is exclusively nuclear in normal cells and tissues. In contrast, E2F1 is almost exclusively localized to the nucleus in normal and SCC cells and tissues. The localization of E2F7 to the cytoplasm could be reversed using inhibitors of the nuclear export protein XPO1 and siRNA against XPO1. XPO1-dependent nuclear export of E2F7 shows that this pathway is selectively activated in SCC. The inactivation of XPO1 is now possible using selinexor, a Selective Inhibitor of Nuclear Export (SINE) compound currently in advanced clinical trials to treat solid and hematological malignancies. We show that treatment of SCC cells with selinexor reverses anthracycline resistance. Significantly, we show that a combination of selinexor + doxorubicin in vivo induces profound anticancer activity.
Significance: Our studies show that i) E2F7 is selectively mislocalized in SCC, ii) E2F7 nuclear export is XPO1-dependent, iii) dysregulation of XPO1 causes derepression of S1P-mediated anthracycline resistance and iv) anthracycline resistance is reversed with inhibitors of XPO1 in vivo.
Citation Format: Alba Natalia Saenz Ponce, Yosef Landesman, Trinayan Kashyap, Alexander Guminski, Orla Gannon, Nicholas Saunders. Nuclear export of E2F7 in squamous cell carcinoma in an actionable event that reverses resistance to anthracyclines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1213. doi:10.1158/1538-7445.AM2017-1213