Epithelial-to-mesenchymal transition (EMT) promotes both tumor progression and drug resistance, yet few vulnerabilities of this state have been identifi ed. Using selective small molecules as cellular probes, we show that induction of EMT greatly sensitizes cells to agents that perturb endoplasmic reticulum (ER) function. This sensitivity to ER perturbations is caused by the synthesis and secretion of large quantities of extracellular matrix (ECM) proteins by EMT cells. Consistent with their increased secretory output, EMT cells display a branched ER morphology and constitutively activate the PERK-eIF2α axis of the unfolded protein response (UPR). Protein kinase RNA-like ER kinase (PERK) activation is also required for EMT cells to invade and metastasize. In human tumor tissues, EMT gene expression correlates strongly with both ECM and PERK-eIF2α genes, but not with other branches of the UPR. Taken together, our fi ndings identify a novel vulnerability of EMT cells, and demonstrate that the PERK branch of the UPR is required for their malignancy. SIGNIFICANCE:EMT drives tumor metastasis and drug resistance, highlighting the need for therapies that target this malignant subpopulation. Our fi ndings identify a previously unrecognized vulnerability of cancer cells that have undergone an EMT: sensitivity to ER stress. We also fi nd that PERK-eIF2α signaling, which is required to maintain ER homeostasis, is also indispensable for EMT cells to invade and metastasize. Cancer Discov; 4(6);

The relationship between mutated proteins and the cancer stem cell population is unclear. Glioblastoma tumors frequently express EGFRvIII, an EGFR variant that arises via gene rearrangement and amplification. However, expression of EGFRvIII is restricted despite the prevalence of the alteration. Here we show that EGFRvIII is highly co-expressed with CD133 and that EGFRvIII+/CD133+ defines the population of cancer stem cells with the highest degree of self-renewal and tumor initiating ability. EGFRvIII+ cells are associated with other stem/progenitor markers while markers of differentiation are found in EGFRvIII− cells. EGFRvIII expression is lost in standard cell culture but its expression is maintained in tumor sphere culture, and cultured cells also retain the EGFRvIII+/CD133+ co-expression and self-renewal and tumor initiating abilities. Elimination of the EGFRvIII+/CD133+ population using a bispecific antibody reduced tumorigenicity of implanted tumor cells better than any reagent directed against a single epitope. This work demonstrates that a mutated oncogene can have CSC specific expression and be used to specifically target this population.