Gene Regulation and Suppression of Type I Interferon Signaling By STAT3 in Diffuse Large B-Cell Lymphoma

The Janus kinas (JAK)/signal transducer and activator of transcription (STAT) pathway is central to signaling by receptors of diverse cytokines, growth factors and related molecules, which is critical for normal hematopoiesis and immune response. However, this signaling pathway is deregulated in several B cell lymphomas. In activated B-cell-like diffuse large B cell lymphoma (ABC DLBCL), autocrine production of IL-6 and IL-10 can result from high NF-κB activity caused by a variety of genetic alterations, such as mutations in MYD88 and CARD11. Our recent work has demonstrated that JAK1 is an active kinase downstream of IL-6 and IL-10 signaling that activates STAT3. Interestingly, JAK1 can function in the nucleus to regulate gene expression through phosphorylating histone H3 on tyrosine 41. There are nearly 3,000 JAK1 epigenetic target genes including MYD88, IRF4 and MYC, of which expression is essential for cancer cell survival and proliferation.

Here we have investigated the canonical gene regulatory mechanism by STAT3 in ABC DLBCL cells. We performed genome-wide analysis and identified 2,251 STAT3 direct target genes, all of which involve B cell activation, survival, proliferation, differentiation and migration. Whole transcriptome profiling revealed that STAT3 acts as both a transcriptional activator and suppressor, with a comparable number of up- and down-regulated genes. STAT3 regulates multiple oncogenic signaling pathways, including NF-κB, cell cycle checkpoint, PI3K/AKT/mTORC1 and STAT3 itself. These results suggest that STAT3 is involved in the positive feedback regulation of IL-6/IL-10 signaling and shows crosstalk with NF-κB signaling pathways in ABC DLBCL cells. In addition, STAT3 negatively regulates the lethal type I interferon signaling pathway by inhibiting expression of IRF7, IRF9, STAT1 and STAT2, all of which are critical transcription factors in the type I interferon pathway. Inhibition of STAT3 activity by ruxolitinib synergizes with the type I interferon inducer lenalidomide in growth inhibition of ABC DLBCL cells in vitro . In an ABC DLBCL xenograft mouse model, combination treatment using ruxolitinib and lenalidomide completely inhibits the tumor growth during the period of treatment.

In summary, we demonstrate that STAT3 is a critical transcriptional regulator of ABC DLBCL. Gene regulation by STAT3 in ABC DLBCL accentuates survival signaling pathways while dampening the lethal type I interferon pathway. Knowledge of these STAT3 regulated genes has led to our demonstration that a small molecule inhibitor in the JAK1-STAT3 signaling pathway synergizes with lenalidomide, suggesting a new therapeutic strategy for ABC DLBCL, a subtype that is particularly difficult to treat and has poor prognosis.