Activating Transcription Factor 4 Modulates TGFβ-Induced Aggressiveness in Triple-Negative Breast Cancer via SMAD2/3/4 and mTORC2 Signaling

Abstract

<b>Purpose:</b> On the basis of the identified stress-independent cellular functions of activating transcription factor 4 (ATF4), we reported enhanced ATF4 levels in MCF10A cells treated with TGFβ1. <i>ATF4</i> is overexpressed in patients with triple-negative breast cancer (TNBC), but its impact on patient survival and the underlying mechanisms remain unknown. We aimed to determine <i>ATF4</i> effects on patients with breast cancer survival and TNBC aggressiveness, and the relationships between TGFβ and ATF4. Defining the signaling pathways may help us identify a cell signaling-tailored gene signature.<b>Experimental Design:</b> Patient survival data were determined by Kaplan-Meier analysis. Relationship between TGFβ and ATF4, their effects on aggressiveness (tumor proliferation, metastasis, and stemness), and the underlying pathways were analyzed in three TNBC cell lines and <i>in vivo</i> using patient-derived xenografts (PDX).<b>Results:</b> <i>ATF4</i> overexpression correlated with TNBC patient survival decrease and a SMAD-dependent crosstalk between ATF4 and TGFβ was identified. <i>ATF4</i> expression inhibition reduced migration, invasiveness, mammosphere-forming efficiency, proliferation, epithelial-mesenchymal transition, and antiapoptotic and stemness marker levels. In PDX models, <i>ATF4</i> silencing decreased metastases, tumor growth, and relapse after chemotherapy. ATF4 was shown to be active downstream of SMAD2/3/4 and mTORC2, regulating TGFβ/SMAD and mTOR/RAC1-RHOA pathways independently of stress. We defined an eight-gene signature with prognostic potential, altered in 45% of 2,509 patients with breast cancer.<b>Conclusions:</b> ATF4 may represent a valuable prognostic biomarker and therapeutic target in patients with TNBC, and we identified a cell signaling pathway-based gene signature that may contribute to the development of combinatorial targeted therapies for breast cancer. <i>Clin Cancer Res; 24(22); 5697-709. ©2018 AACR</i>.