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Abstract

AXL Receptor Tyrosine Kinase (RTK) inhibition presents a promising therapeutic strategy for aggressive tumor subtypes, as AXL signaling is upregulated in many cancers resistant to first line treatments. Furthermore, the AXL ligand Growth Arrest-Specific gene 6 (GAS6) has recently been linked to cancer drug resistance. Here, we established that challenging conditions, such as serum deprivation, divide AXL overexpressing tumor cell lines into non-self-sustaining and self-sustaining subtypes in 3D spheroid culture. Self-sustaining cells are characterized by excessive GAS6 secretion and TAM-PDK-RSK-mTOR pathway activation. In 3D spheroid culture, the activation of the TAM-PDK-RSK-mTOR pathway proves crucial following treatment with AXL/MET inhibitor BMS777607, when the self-sustaining tumor cells react with TAM-RSK hyper-activation and enhanced SRC-AKT-mTOR signaling. Thus, bidirectional activated mTOR leads to enhanced proliferation and counteracts the drug effect. mTOR activation is accompanied by an enhanced AXL expression and hyper-phosphorylation following 24 hours of treatment with BMS777607. Thereby, we elucidate a double role of AXL which can be assigned to RSK-mTOR as well as SRC-AKT-mTOR pathway activation, specifically through AXL Y779 phosphorylation. This phosphosite fuels the resistance mechanism in 3D spheroids, alongside further SRC dependent EGFR Y1173 and/or MET Y1349 phosphorylation which is defined by the cell-specific addiction. In conclusion, self-sustenance in cancer cells is based on a signaling synergy, individually balanced between GAS6 TAM dependent PDK-RSK-mTOR survival pathway and the AXLY779/EGFR/MET driven SRC-mTOR pathway. Molecular Oncology (2017) 2017 The Authors. Published by FEBS Press and John Wiley & Sons Ltd.