ABSTRACT Diffuse large B cell lymphoma (DLBCL), which represents 30% to 40% of newly diagnosed lymphomas, comprises two main molecular subtypes: activated B cell-like (ABC) and germinal center B cell-like (GCB). ABC DLBCL is more aggressive and less curable. More than 50% of patients with ABC DLBCL are refractory to or relapse from current frontline immunochemotherapy. Clinical use of ibrutinib, a selective inhibitor for Bruton tyrosine kinase (BTK) in the B cell receptor (BCR) signaling pathway, has achieved an initial response rate of 30%-40% in refractory/relapsed ABC DLBCL. Primary and acquired drug resistance, however, are still significant and impact the long-term survival of more than 60% of these patients. Therefore, understanding and targeting ibrutinib resistance mechanisms is an unmet clinical need. The BCR and JAK1/STAT3 signaling pathways are essential for the survival and proliferation of ABC DLBCL cells. We discovered that EGR1 is a converged downstream target of both pathways in ABC DLBCL and the level of EGR1 expression is elevated in ABC DLBCL compared with normal human tonsils and lymph nodes. We revealed novel mechanisms of EGR1 in transcriptional activation and repression of target genes with strong translational impact in treating aggressive lymphoma. EGR1 mediates transcriptional activation through the p300/H3K27ac/BRD4 axis to induce MYC expression and activate MYC target genes. Synergistic inhibition of cell growth was observed between EGR1 shRNA and AZD5153, a novel BRD4 inhibitor that is currently under clinical investigation. On the other hand, EGR1 mediates transcriptional repression of the type I interferon pathway genes, expression of which otherwise causes cancer cell death. Consistently, EGR1 knockdown by shRNA synergizes with the type I interferon inducer lenalidomide in growth inhibition of ABC DLBCL cells in vitro and in a xenograft mouse model. Using newly derived, ibrutinib-resistant ABC DLBCL cell lines, we demonstrated that EGR1 is among the most highly expressed genes relative to ibrutinib-sensitive parental cells, and co-targeting of BRD4 and interferon signaling inhibits growth of ibrutinib-resistant cells in vitro and in vivo. Based on these discoveries, the central hypothesis is that EGR1 is a unique oncogenic driver orchestrating multiple important signaling pathways and represents therapeutic vulnerability in patients with ABC DLBCL, especially for those with ibrutinib resistance. To test our hypothesis, we will pursue the following specific aims: (1) Elucidate effects of EGR1 on oncogenesis; (2) Establish the role of EGR1 in ibrutinib resistance; and (3) Co-target EGR1 downstream BRD4 and type I interferon signaling to overcome drug resistance in DLBCL. Dissecting the transcriptional activation and repression modules of EGR1 in DLBCL tumorigenesis and ibrutinib resistance is essential because the novel mechanistic insights will provide a molecular basis for developing the most effective therapeutic ...