Interferon regulatory factor 4 mediates non-enzymatic IRE1 dependency in multiple myeloma cells [ChIP-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 identify IRF4-dependent loci, we performed ChIP-seq analysis in AMO1 cells proficient vs deficient of IRF4. Towards this, IRF4 (Antibody: #4964, CST) or RNAPII-pS2 (Antibody: ab5095, Abcam) were immunoprecipitated in AMO1 shIRF4 cells (conditionally expressing shRNAs against IRF4 upon doxycycline treatment) in the presence or absence of Doxycycline (Dox, 0.2 ug/mL) for 24 h.