Oncogenic Cooperation between EGR1 and BRD4 in Diffuse Large B Cell Lymphoma

Diffuse large B cell lymphoma (DLBCL) grows rapidly and represents 30% to 40% of newly diagnosed lymphomas, and comprises two main molecular subtypes: activated B cell-like (ABC) and germinal center B cell-like (GCB). ABC DLBCL is more aggressive with a current cure rate of only ~40%. The B cell antigen receptor (BCR) signaling pathway is known for its critical role in the survival and proliferation of ABC DLBCL cells. Our recent studies revealed that IL6- and IL10-mediated JAK1/STAT3 activation enhances the survival and proliferation of ABC DLBCL cells. Furthermore, we discovered that EGR1, a transcription factor known to be a downstream effector of BCR signaling, is also a downstream target of JAK1 signaling in DLBCL. While EGR1 has been characterized as a tumor suppressor in leukemia, the role of EGR1 in human lymphoma remains unclear.

Initial immunohistochemical analysis of 102 DLBCL cases revealed that not only is the level of EGR1 elevated in DLBCL as compared to normal human tonsils, lymph nodes and mantle cell lymphomas, but its level is higher in ABC DLBCL (non-GCB) than GCB DLBCL, indicating functional significance of EGR1 in DLBCL. In addition, we found that 7 out of 8 ABC DLBCL cell lines are sensitive to EGR1 knockdown, whereas only half of 8 GCB DLBCL cell lines are affected. Our genomics studies discovered dual functions of EGR1 in regulating three signaling pathways. EGR1 mediates transcriptional activation associated with two hallmark oncogenic pathways, JAK1/STAT3 and BCR signaling. Our EGR1 ChIP-seq results revealed a large overlap of EGR1 genomic occupancy with that of BRD4, a highly expressed bromo and extraterminal (BET) protein in DLBCL known to induce MYC expression and activation of MYC target genes. This agrees with the RNA-seq results showing that multiple oncogenic pathways including MYC are activated by EGR1 in ABC DLBCL. The synergistic inhibition of cell growth was observed between EGR1 shRNA and JQ1, an inhibitor of BRD4. Surprisingly, the RNA-seq data and pathway analyses also revealed EGR1-mediated suppression of the type I interferon pathway, which otherwise causes cancer cell death. The majority of the interferon pathway genes are up-regulated upon EGR1 knockdown. EGR1-modulated suppression of the interferon pathway appears to be functionally significant as 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.

EGR1 is known to bind the promoter of p300 and activate p300 expression. P300 catalyzes H3K27 acetylation (H3K27ac), an active histone mark that recruits BRD4 for transcription of MYC and MYC pathway genes, among others. Indeed, immunoblot analysis revealed that EGR1 knockdown reduces H3K27ac expression in ABC DLBCL cells, leading to reduced expression of MYC and MYC target genes based on H3K27ac ChIP qPCR analysis. The results provide a molecular basis for oncogenic cooperation between EGR1 and BRD4 in ABC DLBCL.

In summary, our study demonstrates that EGR1 is an oncogenic driver orchestrating multiple important signaling pathways. Notably, EGR1 creates therapeutic vulnerability in ABC DLBCL because both EGR1-mediated transcriptional activation and repression provide growth advantage to the cancer cells.