Abstract IA17: Targeting downstream effectors of growth factor signaling

Transmembrane growth factor receptors mediate signaling through multiple intracellular pathways. In breast cancer cells, the type I insulin-like growth factor receptor (IGF1R) has been implicated in the malignant phenotype. However, clinical trials with anti-IGF1R antibodies have been disappointing, in part, due to adaptive feedback pathways stimulated when IGF1R is blocked. To determine whether IGF1R inhibition could be enhanced by disrupting other pathways, we evaluated gene expression induced by receptor activation. In previous work, we found that xCT (SLC7A11) mRNA expression was increased by IGF-I in estrogen receptor (ER) positive breast cancer cell lines (MCF-7, T47D, and ZR-75-1) in an insulin receptor substrate-1 (IRS-1) dependent manner. xCT encodes the functional subunit of the heterodimeric plasma membrane transport system xC- critical for the cellular uptake of cystine to generate glutathione to modulate cellular redox control. IGF-I increased xC- transporter expression and function to control cellular redox levels. In MCF-7 cells, IGF-I-stimulated monolayer and anchorage-independent growth was suppressed by infecting cells with xCT shRNA or by treating cells with the xC- chemical inhibitor sulfasalazine (SASP). Anchorage-independent growth assays showed that disruption of xC- function by SASP sensitized cellular response of MCF-7 cells to anti-IGF-IR inhibitors (monoclonal antibody huEM164 and tyrosine kinase inhibitor NVP-AEW-541). IGF1R also activates PI3K/Akt/mTORC1 signaling to affect ER phosphorylation and mRNA cap dependent translation. In tamoxifen resistant cells, IGF1R is lost yet PI3K signaling is maintained. Since mRNA cap dependent translation is increased by PI3K signaling, we determined if inhibition of the eIF4F translation pathway would affect endocrine responsive and tamoxifen resistant cells. The eIF4F translation pathway is activated by IGF1R in wild-type cells and hyperactive in tamoxifen resistant MCF-7L (TamR) breast cancer cells. Targeting the eIF4E subunit of the eIF4F complex through its degradation using an antisense oligonucleotide (ASO) or via sequestration using a mutant 4E-BP1 inhibited the proliferation and colony formation of parental and TamR cells. Use of these agents also resulted in cell cycle arrest and induction of apoptosis in TamR cells. Finally, pharmacologic inhibition of the eIF4E-eIF4G interaction also decreased the proliferation and anchorage dependent colony formation in TamR cells. Taken together, these data show that IGF1R activation stimulates multiple downstream effectors important for breast cancer cell biology. Inhibition of selected downstream signaling molecules is likely to have synergy with anti-IGF-IR drugs. Citation Format: Yuzhe Yang, Dedra Fagan, Lynsey Fettig Anderson, Kelly LaPara, Xihong Zhang, Aleksandra Ochnik, Jie Ying Chan, Heather Beckwith, Douglas Yee. Targeting downstream effectors of growth factor signaling. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr IA17.