Targeting the unfolded protein response for cancer therapy: mitigating tumor adaptation and immune suppression

There are significant stress factors within the tumor microenvironment (TME), such as hypoxia, oxidative stress, and nutrient deprivation. These disrupt endoplasmic reticulum (ER) function in cancer cells, as well as the infiltrating immune cells, leading to activation of the unfolded protein response (UPR) signaling, which the tumor uses to mitigate stress and survive. There are three canonical UPR pathways that are regulated by respective ER-resident transmembrane sensors: inositol-requiring protein 1α (IRE1α), PKR-like ER kinase (PERK), and activating transcription factor 6 (ATF6); activation of these pathways results in expression of cognate transcription factors that regulate gene expression to mitigate ER stress. Persistent UPR activation in the TME has been linked to aberrant tumor growth, progression, metastasis, angiogenesis, and therapy resistance in different cancer types. In addition, modulation of UPR activity significantly impacts immune cell function at different levels further impacting its role on the TME. Therefore, there is now significant interest to design novel therapies that target the UPR to kill cancer cells and simultaneously enhance protective anti-tumor immunity. Here we summarize recent findings as to how targeting UPR signaling can induce tumor regression and at the same time galvanize the immune response. We discuss the potential of integrating UPR targeting with other therapies, such as immune checkpoint inhibition, highlighting emerging strategies to improve therapeutic efficacy and overcome resistance. These recent insights underscore the importance of UPR as a novel therapeutic target for cancer treatment.