Abstract 3025: Ligand-independent EphA2 signaling drives an amoeboid phenotype that promotes melanoma brain metastasis development
Authors: Zhang Chao , Smalley Inna , Sharma Ritin , Emmons Michael , Messina Jane , Koomen John , Smalley Keiran 
Published: August 2018

Abstract
The majority of melanoma patients treated with BRAF inhibitors (BRAFi) ultimately develop resistance and fail on therapy. It has previously been shown that BRAFi resistance is highly correlated with the adoption of a strongly invasive phenotype through ligand-independent EphA2 signaling.
In this study, we used comprehensive mass spectrometry-based proteomic approaches to delineate the global signaling changes associated with the aggressive phenotype driven by ligand-independent EphA2 signaling. Preliminary data have identified new signaling adaptations associated with a mesenchymal-to-amoeboid transition (MAT) phenotype, which was confirmed in 3D collagen cultures. Functional experiments demonstrated that the amoeboid phenotype promoted cell migration and invasion, which was mediated through the interaction of EphA2 and CDC42. These findings were confirmed by expressing a constitutively activated form of CDC42. BRAFi resistant cell lines also exhibited the MAT phenotype and were more invasive compared to their treatment-naïve counterparts, in line with their dependence upon ligand-independent EphA2 signaling following BRAFi selection pressure.
To further demonstrate the metastatic potential of these amoeboid cells driven by ligand-independent EpHA2 signaling in vivo, we performed intracardiac injections in mice with cell lines expressing either EpHA2 S897A (inactive) or S897E (constitutively active phosphomimetic). Interestingly, we found a preferential homing of EphA2 S897E cells to the brain, but no difference in metastasis to other organs including the lung and liver. An analysis of brain metastasis specimens from patients failing BRAF and BRAF/MEK inhibitor therapy showed strong staining for the amoeboid phenotype marker EphA2.
To investigate whether the amoeboid phenotype may confer a survival advantage in circulation, we carried out shear stress assays. These experiments demonstrated that the amoeboid phenotype driven by ligand-independent EphA2 signaling promoted the survival of melanoma cells under shear stress. Cell attachment assays, trans-endothelial invasion assays and vascular permeability assays show that these cells are better suited to attach to and permeate an endothelial monolayer. We further show that inhibiting PI3K reversed the amoeboid phenotype and limited the EphA2-driven invasive capacity.
In summary, we show for the first time that BRAFi resistance is associated with the adoption of an MAT phenotype that increases the metastatic seeding of melanoma cells to the brain, which can be reversed through inhibition of PI3K signaling.
Citation Format: Chao Zhang, Inna Smalley, Ritin Sharma, Michael Emmons, Jane Messina, John Koomen, Keiran Smalley. Ligand-independent EphA2 signaling drives an amoeboid phenotype that promotes melanoma brain metastasis development [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3025.