IRE1α disruption in triple-negative breast cancer cooperates with antiangiogenic therapy by reversing ER stress adaptation and remodeling the tumor microenvironment

Cancer cells exploit the unfolded protein response (UPR) to mitigate endoplasmic reticulum (ER) stress caused by cellular oncogene activation and a hostile tumor microenvironment (TME). The key UPR sensor IRE1a resides in the ER and deploys a cytoplasmic kinase-endoribonuclease module to activate the transcription factor XBP1s, which facilitates ER-mediated protein folding. Studies of triple-negative breast cancer (TNBC)-a highly aggressive malignancy with a dismal posttreatment prognosis- implicate XBP1s in promoting tumor vascularization and progression. However, it remains unknown whether IRE1a adapts the ER in TNBC cells and modulates their TME, and whether IRE1a inhibition can enhance antiangiogenic therapy-previously found to be ineffective in patients with TNBC. To gauge IRE1a function, we defined an XBP1s-dependent gene signature, which revealed significant IRE1a pathway activation in multiple solid cancers, including TNBC. IRE1a knockout inTNBC cells markedly reversed substantial ultrastructural expansion of their ER upon growth in vivo. IRE1a disruption also led to significant remodeling of the cellular TME, increasing pericyte numbers while decreasing cancerassociated fibroblasts and myeloid-derived suppressor cells. Pharmacologic IRE1a kinase inhibition strongly attenuated growth of cell line-based and patient-derived TNBC xenografts in mice and synergized with anti-VEGFA treatment to cause tumor stasis or regression. Thus, TNBC cells critically rely on IRE1a to adapt their ER to in vivo stress and to adjust the TME to facilitate malignant growth. TNBC reliance on IRE1a is an important vulnerability that can be uniquely exploited in combination with antiangiogenic therapy as a promising new biologic approach to combat this lethal disease.