A nonenzymatic dependency on inositol-requiring enzyme 1 controls cancer cell cycle progression and tumor growth

Endoplasmic-reticulum resident inositol-requiring enzyme 1a (IRE1) supports protein homeostasis via its cytoplasmic kinase-RNase module. Known cancer dependency on IRE1 entails its enzymatic activation of the transcription factor XBP1s and of regulated RNA decay. We discovered that some cancer cells surprisingly require IRE1 but not its enzymatic activity. IRE1 knockdown but not enzymatic IRE1 inhibition or XBP1 disruption attenuated cell cycle progression and tumor growth. IRE1 silencing led to activation of TP53 and CDKN1A/p21 in conjunction with increased DNA damage and chromosome instability, while decreasing heterochromatin as well as DNA and histone H3K9me3 methylation. Immunoelectron microscopy detected endogenous IRE1 at the nuclear envelope. Thus, cancer cells co-opt IRE1 either enzymatically or nonenzymatically, which has significant implications for IRE1's biological role and therapeutic targeting. Overall design: To follow the consequences of IRE1 silencing, we performed an RNA sequencing transcriptomic analysis of AMO1 human multiple myeloma cells subjected to IRE1 vs. XBP1 knockdown. AMO1 shIRE1 cl.1, cl.3 or shXBP1 cl.1, cl.18 cells were harvested in biologic triplicates at 0, 24, 48 and 72 h after treatment with Doxycycline (Dox, 0.2 µg/ml).