STAT6-mediated BCL6 repression in primary mediastinal B-cell lymphoma (PMBL)

• Published in: Oncotarget 2013-07, Vol.4 (7), p.1093-1102
• Main Authors: Ritz, Olga, Rommel, Karolin, Dorsch, Karola, Kelsch, Elena, Melzner, Julia, Buck, Michaela, Leroy, Karen, Papadopoulou, Vasiliki, Wagner, Simon, Marienfeld, Ralf, Brüderlein, Silke, Lennerz, Jochen K, Möller, Peter
• Format: Article
• Language: English
• Subjects: Cell Line, Tumor; DNA-Binding Proteins - genetics; DNA-Binding Proteins - metabolism; Gene Expression Regulation, Neoplastic; Humans; Lymphoma, B-Cell - genetics; Lymphoma, B-Cell - metabolism; Mediastinal Neoplasms - genetics; Mediastinal Neoplasms - metabolism; Proto-Oncogene Proteins c-bcl-6; Research Papers; Signal Transduction; STAT6 Transcription Factor - metabolism; Transfection
• ISSN: 1949-2553; 1949-2553
• DOI: 10.18632/oncotarget.1149

Primary mediastinal B-cell lymphoma (PMBL) is characterized by aberrant activation of JAK/STAT-signaling resulting in constitutive presence of phosphorylated STAT6 (pSTAT6). In primary PMBL samples pSTAT6 is only expressed in a sub-population of lymphoma cells in a pattern that is reminiscent of that of the BCL6 oncogene. Double-fluorescence staining was carried out to determine the association between these two proteins in ten primary PMBL cases and three available PMBL cell line models. Surprisingly, only a minute fraction of double-positive nuclei was observed, while each sample contained considerable fractions of single-positive pSTAT6 and BCL6 nuclei. The intratumoral coexistence of BCL6+/pSTAT6- and BCL6-/pSTAT6+ subpopulations suggests a negative interaction between these factors. In silico screening of the STAT6 /BCL6 promoters for DNA consensus binding sites identified five STAT-binding-sites in the BCL6 promoter. We confirmed STAT6 binding to the BCL6 promoter in vitro and in vivo by band shift / super shift assays and chromatin immunoprecipitations. Using BCL6 luciferase reporter assays, depletion of STAT6 by siRNA, and ectopic overexpression of a constitutive active STAT6 mutant, we proved that pSTAT6 is sufficient to transcriptionally repress BCL6. Recently developed small molecule inhibitors 79-6 and TG101348 that increases BCL6 target gene expression and decreases pSTAT6 levels, respectively, demonstrate that a combined targeting results in additive efficacy regarding their negative effect on cell viability. The delineated pSTAT6-mediated molecular repression mechanism links JAK/STAT to BCL6-signaling in PMBL and may carry therapeutic potential.