Sun, Yue

Introduction: Interferon regulatory factor 4 (IRF4) is a transcription factor involved in B cell activation and differentiation into plasma cells, as well as B-cell lineage neoplasia. Diverse genetic abnormalities and dysregulated expression of IRF4 have been documented in various hematologic malignancies, including multiple myeloma (MM). IRF4 is also identified as a major downstream target of CRBN/IKZF to regulate MYC expression, representing a primary mechanism underlying anti-MM action of IMiDs. However, it remains unclear how the expression and function of IRF4 are regulated in MM cells. Materials and Methods: DNAseq, whole exome RNAseq (WES), and label-free analyses of quantitative proteomics and post-translational modifications (PTMs) mapping were conducted to examine genetic aberrations, transcriptional dysregulation, or protein and PTM alterations in primary patient samples and drug-naive vs -resistant cell lines. Flow cytometry, qPCR, and Western blot analysis were utilized to monitor apoptosis or mRNA and protein levels of target genes. Stable transfection with wild-type or mutated IRF4 was carried out to evaluate its function. Analysis of the MM genome-wide GEP databases (R2: Genomics Analysis and Visualization Platform) was performed to validate the clinical significance of IRF4 and its target genes in MM patients. Results: MM cells that acquired bortezomib (btz) resistance (PS-R) exhibited a marked increase in protein levels of multiple key components of the canonical and non-canonical NF-κB signaling cascades, in association with increased NF-κB transcriptional activity. Pharmacological inhibition of NF-κB using several compounds (e.g., Bay 11-7082, parthenolide, triptolide) significantly increased btz sensitivity in PS-R cells and their parental U266 cells, as well as other MM cell lines. These events were accompanied by down-regulation of various genes, including NF-κB-dependent genes and those whose relationship with NF-κB remains uncertain. The latter included IRF4, MYC, MCL1, BIRC2/cIAP1, SQSTM1/p62, among others. Of note, overexpression of wild-type IRF4 sharply reduced sensitivity of drug-naive cells to btz alone or in combination with NF-κB inhibitors. However, IRF4 overexpression did not prevent inactivation of NF-κB and down-regulation of the NF-κB-dependent genes (e.g., Bcl-xL, TNFAIP3/A20), suggesting that IRF4 acts downstream of NF-κB. Surprisingly, IRF4 overexpression failed to restore expression of MYC, a well-established target of IRF4, while largely reversed expression of cIAP1 and Mcl-1 at both mRNA and protein levels. Moreover, inhibitors of either IAP (birinapant/TL32711) or Mcl-1 (S63845) overcame btz-resistance conferred by IRF4 overexpression. DNAseq unveiled multiple IRF4 SNPs located in introns and exons (both 3'-UTR and coding region) of chromosome 6. WES revealed that IRF4 was highly expressed in primary bone marrow and extramedullary samples of MM patients (2~10 folds over normal donors). But no difference at mRNA and protein levels of IRF4 was observed between drug-naive and -resistant cell lines. The global PTM mapping identified two lysine residues (K59 and K399) that were highly ubiquitinated in PS-R cells (p = 0.0447 and p = 0.0004, vs U266 cells). Interestingly, ectopic expression of IRF4 mutants (e.g., K59N, K123R) lost the cytoprotective capability of their wild-type counterpart, in association with failure to reverse down-regulation of cIAP1 and Mcl-1. This observation argues that these point mutations are loss-of-function, which might explain better prognosis of patients carrying IRF4 mutations (including K59N and K123R) than wild-type IRF4 as reported previously. Last, the MM GEP database analysis validated the correlation between IRF4 and Mcl-1 or cIAP expression, as well as the adverse effect of their dysregulation on overall survival of MM patients. Conclusion: IRF4 is highly expressed in MM cells, likely via an NF-κB-dependent mechanism. IRF4 acts downstream of the NF-κB signaling pathways, which in turn directly targets cIAP1 and Mcl-1, rather than MYC, to confer drug resistance. IRF4 could be mutated in MM patients, while the point mutations, at least at certain lysine sites, seem to be loss-of-function and thus predict better outcome. Together, these findings suggest that the oncogenic function of IRF4 might be governed at genetic, transcriptional, and PTM levels in MM cells. Disclosures Kumar: Takeda: Research Funding; Celgene: Consultancy, Research Funding; Janssen: Consultancy, Research Funding.

Abstract1q21 gain is a common cytogenetic abnormality featuring high‐risk multiple myeloma (HRMM). However, the molecular mechanism underlying the adverse prognostic effect of 1q21 gain remains largely unclear. Here, we report that ARNT/HIF‐1β, a 1q21 gene, is highly expressed in HRMM and induced by microenvironmental hypoxia, which confers drug resistance and correlates with inferior outcome. Analysis of the gene expression profile database revealed that ARNT expression was upregulated in MM and increased with disease progression or in HRMM subtypes (particularly 1q21 gain), while correlated to shorter overall survival. In a cohort of 40 MM patients, qPCR further validated that ARNT expression was higher in MM patients than normal donors. MM cells carrying 1q21 gain or acquired drug resistance displayed a robust increase in HIF‐1β protein level. Hypoxia induced HIF‐1β expression via a NF‐κB‐dependent process. Notably, HIF‐1β overexpression impaired bortezomib sensitivity, whereas shRNA knockdown of ARNT reversed hypoxia‐mediated drug resistance. Together, these findings suggest that ARNT/HIF‐1β might represent a novel biomarker for risk stratification and prognosis of HRMM patients, as well as a potential therapeutic target for overcoming 1q21 gain‐ or microenvironment‐mediated and acquired drug resistance in MM.