Yuxiong Feng

UNLABELLED: Epithelial-to-mesenchymal transition (EMT) promotes both tumor progression and drug resistance, yet few vulnerabilities of this state have been identified. 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 findings 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 findings identify a previously unrecognized vulnerability of cancer cells that have undergone an EMT: sensitivity to ER stress. We also find that PERK-eIF2α signaling, which is required to maintain ER homeostasis, is also indispensable for EMT cells to invade and metastasize.

PERK signaling is required for cancer invasion and there is interest in targeting this pathway for therapy. Unfortunately, chemical inhibitors of PERK's kinase activity cause on-target side effects that have precluded their further development. One strategy for resolving this difficulty would be to target downstream components of the pathway that specifically mediate PERK's pro-invasive and metastatic functions. Here we identify the transcription factor CREB3L1 as an essential mediator of PERK's pro-metastatic functions in breast cancer. CREB3L1 acts downstream of PERK, specifically in the mesenchymal subtype of triple-negative tumors, and its inhibition by genetic or pharmacological methods suppresses cancer cell invasion and metastasis. In patients with this tumor subtype, CREB3L1 expression is predictive of distant metastasis. These findings establish CREB3L1 as a key downstream mediator of PERK-driven metastasis and a druggable target for breast cancer therapy.

Malignant carcinomas that recur following therapy are typically de-differentiated and multidrug resistant (MDR). De-differentiated cancer cells acquire MDR by up-regulating reactive oxygen species (ROS)-scavenging enzymes and drug efflux pumps, but how these genes are up-regulated in response to de-differentiation is not known. Here, we examine this question by using global transcriptional profiling to identify ROS-induced genes that are already up-regulated in de-differentiated cells, even in the absence of oxidative damage. Using this approach, we found that the Nrf2 transcription factor, which is the master regulator of cellular responses to oxidative stress, is preactivated in de-differentiated cells. In de-differentiated cells, Nrf2 is not activated by oxidation but rather through a noncanonical mechanism involving its phosphorylation by the ER membrane kinase PERK. In contrast, differentiated cells require oxidative damage to activate Nrf2. Constitutive PERK-Nrf2 signaling protects de-differentiated cells from chemotherapy by reducing ROS levels and increasing drug efflux. These findings are validated in therapy-resistant basal breast cancer cell lines and animal models, where inhibition of the PERK-Nrf2 signaling axis reversed the MDR of de-differentiated cancer cells. Additionally, analysis of patient tumor datasets showed that a PERK pathway signature correlates strongly with chemotherapy resistance, tumor grade, and overall survival. Collectively, these results indicate that de-differentiated cells up-regulate MDR genes via PERK-Nrf2 signaling and suggest that targeting this pathway could sensitize drug-resistant cells to chemotherapy.