Interferon regulatory factor 4 mediates non-enzymatic IRE1 dependency in multiple myeloma cells [RNA-Seq]

Multiple Myeloma (MM) arises through oncogenic transformation of immunoglobulin-secreting plasma cells. MM often co-opts the central endoplasmic-reticulum (ER)-stress mitigator, inositol-requiring enzyme 1 (IRE1), to sustain malignant growth. While certain MMs require enzymatic IRE1-dependent activation of the transcription factor XBP1s, others display a nonenzymatic IRE1 dependency that is not yet mechanistically understood. Here we identify interferon regulatory factor 4 (IRF4), which stimulates genes that promote immune-cell proliferation, as a key conduit for IRE1's nonenzymatic control of cell-cycle progression in MM. IRE1 silencing increased inhibitory S114/S270 phosphorylation on IRF4, disrupting IRF4's chromatin-binding and transcriptional activity. IRF4 knockdown recapitulated, whereas IRF4 repletion reversed the anti-proliferative phenotype of IRE1 silencing. Furthermore, phospho-deficient, but not phospho-mimetic, IRF4 mutants rescued proliferation under IRE1 silencing. Functional studies revealed that IRF4 engages the E2F1 and CDC25A genes and promotes CDK2 activation to drive cell-cycle progression. Our results advance mechanistic understanding of IRE1 and IRF4 in MM. Overall design: To follow the consequences of IRF4 silencing, we performed an RNA sequencing transcriptomic analysis of AMO1 and KMS27 human multiple myeloma cells subjected to IRF4 vs. NTC (non-targeting control) knockdown. AMO1 shIRF4 cl.1, cl. 4-3 or shNTC cells were harvested in biologic triplicates at 0 (Mock), 8, 16 and 24 h after treatment with Doxycycline (Dox, 0.2 µg/ml). KMS27 shIRF4 Cl. 20 or shNTC cells were harvested in biologic triplicates at 0 (Mock), 20 and 30 h after treatment with Doxycycline (Dox, 0.2 µg/ml).