Abstract B121: Ligand-independent AXL diversification of ErbB signaling and ErbB-targeted drug response mitigation in triple negative breast carcinoma cells

The relationship between drug resistance, changes in signaling, and emergence of an invasive phenotype is well appreciated, but the underlying mechanisms are not well understood. Recently, activation of alternative RTKs as an important resistance process has been identified, although underlying mechanisms are not well understood. Amplification of the targeted signal can also confer resistance, particularly in vivo, where access to the tumor site by therapeutics may be limited. AXL is an RTK linked to secondary resistance in a variety of solid cancers, including those of the breast, lung, glioma, prostate, and colon. Though AXL-mediated resistance is well appreciated, the most effective means of targeting the receptor and basic mechanisms of activation have not been elucidated. Using ligand-blocking antibodies, siRNA-mediated ligand knockdown, and ligand binding-incompetent mutants, we show ligand-independent activation of AXL is present in a subset of cell lines and that this form of activation is actively regulated by ectodomain interactions. With this in mind, we hypothesized ligand-independent mechanisms of activation may be predominant in particular contexts. Using cross-linking coimmunoprecipitation assays, we determined that AXL associated with EGFR, other ErbB receptor family members, MET, and PDGFR but not IGF1R or INSR. Activation of EGFR transactivated AXL, and this ligand-independent AXL activity diversified EGFR-induced signaling into additional downstream pathways beyond those triggered by EGFR alone. AXL-mediated signaling diversification amplified a subset of downstream signaling and was required for EGF-elicited motility responses in AXL-positive TNBC (triple-negative breast cancer) cells. This alternative mechanism of receptor activation may limit the utility of ligand-blocking therapeutic approaches and indicate against therapy withdrawal after acquired resistance. Inhibiting ligand-independent activation, in contrast, may provide more specific efficacy on cancer cells. Moreover, sub-additive interaction between EGFR- and AXL-targeted inhibitors across all AXL-positive TNBC cell lines may indicate that increased abundance of EGFR principally yields transactivation-mediated signaling. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B121. Citation Format: Aaron S. Meyer, Frank B. Gertler, Douglas A. Lauffenburger. Ligand-independent AXL diversification of ErbB signaling and ErbB-targeted drug response mitigation in triple negative breast carcinoma cells. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B121.