Abstract 5881: Identification of resistance mechanisms to IGF-IR targeting in triple negative breast cancer
Authors: Tsui Jennifer , Vaniotis George , Fernandez Maria Celia , Brodt Pnina 
Published: July 2017

The triple negative subtypes of breast cancer (TNBC) are associated with poor prognosis. Unlike HER2+ and hormone receptor-positive BC, TNBC do not respond to targeted therapy and chemotherapy remains the primary treatment option. There is therefore an unmet need to develop effective therapy for TNBC. The insulin-like growth factor 1 (IGF-I) axis plays a critical role in BC progression by conveying survival and growth signals. Our laboratory reported on the production of a soluble fusion protein comprised of the extracellular domain of human IGF-IR fused to the Fc portion of human IgG (the IGF-Trap). The IGF-Trap reduces the bioavailability of circulating and locally produced IGF-I, thereby limiting tumor growth. When human TNBC MDA-MB-231 cells were xenotransplanted into nude mice and treated with the IGF-Trap, we observed variability in the response as it ranged from complete tumor regression to disease stabilization and tumor progression in some mice. This suggested that MDA-MB-231 cells are heterogeneous in respect to their sensitivity to IGF-IR signaling blockade. The aim of the present study was to identify resistance mechanisms that allow the cells to progress in the face of IGF-IR signaling blockade by the IGF-Trap.

We first analyzed the tyrosine kinase receptor profile of these cells and confirmed by PCR that in addition to IGF-IR, they express epidermal growth factor receptor (EGFR), c-Met, and fibroblast growth factor receptor 1 (FGFR1). They also produce IGF-I, EGF and relatively high level of FGF1 that could provide potential autocrine signaling to compensate for IGF signaling blockade. Using limiting dilution cloning, we isolated MDA-MB-231 cells with a range of IGF-IR expression levels, as confirmed by qPCR and Western blotting. We found that clones with higher basal IGF-IR activation levels, independently of expression levels had increased sensitivity to IGF-Trap treatment in the presence of serum, identifying them as IGF-addicted clonal subpopulations. Furthermore, an IGF-Trap resistant population selected from MDA-MB-231 cells by prolonged exposure to the IGF-Trap had an increased proliferation rate in the presence of the IGF-Trap as compared to unselected cells, as assessed by MTT and showed higher p-EGFR and p-ERK levels, suggesting that prolonged IGF-Trap treatment enriched an IGF-I-independent population with increased aggressiveness.

Collectively these results showed that MDA-MB-231 cells are heterogeneous in respect to IGF-IR expression levels and that IGF-Trap sensitivity correlated with constitutive IGF-IR activation levels. Moreover, IGF-Trap resistance in these cells was associated with increased EGFR signaling and proliferation. Further interrogation of the gene expression profile of these cells will define a “resistance signature” with potential relevance to personalized treatment with IGF-targeting drugs.

Supported by the CIHR and Mitacs.

Citation Format: Jennifer Tsui, George Vaniotis, Maria Celia Fernandez, Pnina Brodt. Identification of resistance mechanisms to IGF-IR targeting in triple negative breast cancer [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 5881. doi:10.1158/1538-7445.AM2017-5881