title: STAT6-mediated BCL6 repression in primary mediastinal B-cell lymphoma (PMBL).
abstract: 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.
authors.affiliation: Institute of Pathology, University Ulm, Ulm, Germany.
authors.forename: Olga
authors.initials: O
authors.lastname: Ritz
authors.forename: Karolin
authors.initials: K
authors.lastname: Rommel
authors.forename: Karola
authors.initials: K
authors.lastname: Dorsch
authors.forename: Elena
authors.initials: E
authors.lastname: Kelsch
authors.forename: Julia
authors.initials: J
authors.lastname: Melzner
authors.forename: Michaela
authors.initials: M
authors.lastname: Buck
authors.forename: Karen
authors.initials: K
authors.lastname: Leroy
authors.forename: Vasiliki
authors.initials: V
authors.lastname: Papadopoulou
authors.forename: Simon
authors.initials: S
authors.lastname: Wagner
authors.forename: Ralf
authors.initials: R
authors.lastname: Marienfeld
authors.forename: Silke
authors.initials: S
authors.lastname: Brüderlein
authors.forename: Jochen K
authors.initials: JK
authors.lastname: Lennerz
authors.forename: Peter
authors.initials: P
authors.lastname: Möller
mesh_terms: D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D016393:Lymphoma, B-Cell / Q000235:genetics / Q000378:metabolism*; D008479:Mediastinal Neoplasms / Q000235:genetics / Q000378:metabolism*; D051560:Proto-Oncogene Proteins c-bcl-6; D050800:STAT6 Transcription Factor / Q000378:metabolism*; D015398:Signal Transduction; D014162:Transfection
chemical_list: C084103:BCL6 protein, human; D004268:DNA-Binding Proteins; D051560:Proto-Oncogene Proteins c-bcl-6; D050800:STAT6 Transcription Factor; C494101:STAT6 protein, human