The paradox of the unfolded protein response in cancer

The endoplasmic reticulum (ER) is an elaborate organelle that is essential for cellular function and survival. Conditions that interfere with ER functioning can lead to the accumulation of unfolded proteins, which are detected by transmembrane sensors that then initiate the unfolded protein response (UPR) to restore ER proteostasis. If the adaptive response fails, apoptotic cell death ensues. Many studies have focused on how this failure initiates apoptosis, particularly because ER stress-induced apoptosis is implicated in the pathophysiology of several diseases, including cancer. Whether the UPR inhibits tumour growth or protects tumour cells by facilitating their adaptation to stressful conditions within the tumour microenvironment is unknown, and dissection of the UPR network will likely provide answers to this question. In this review, we aim to elucidate the paradoxical role of the UPR in apoptosis and cancer.