[1426] Constitutively Active STAT6 Represses BCL6 in Primary Mediastinal B-Cell Lymphoma

Jochen K Lennerz, Karolin Rommel, Karola Dorsch, Elena Kelsch, Julia Melzner, Michaela Buck, Karen Leroy, Silke Bruderlein, Peter Moller, Olga Ritz. Univerity of Ulm, Ulm, Germany; Institute for Research in Biomedicine, Bellinzona, Switzerland; University Paris East Creteil, Paris, France

Background: Targeting of the key lymphomagenesis factor BCL6 in cellular and murine models of diffuse-large B-cell lymphoma (DLBCL) appears effective. Despite substantial work on BCL6 in a variety of lymphomas, the function of BCL6 in primary mediastinal B-cell lymphoma (PMBL), a clinically distinct DLBCL subtype, is unknown.
Design: The physiological function of BCL6 was analyzed in all available PMBL cell models using knock-down strategies in combination with cellular assays. Molecularly, PMBL is characterized by constitutively active STAT6 and a possible regulatory interaction between pSTAT6 and BCL6 was assessed using DNA-binding assays as well as chromatin immunoprecipitation (ChIP). Colocalization of pSTAT6 and BCL6 was analyzed by double-immunofluorescence (IF).
Results: We found that PMBL is partially dependent but not addicted to BCL6 expression. Notably, BCL6 expression is highly heterogeneous in all cellular models of PMBCL. We argued that increasing the fraction of BCL6-positive cells via IL-4/IL-13 treatment might increase the level of BCL6-expressing cells and, therefore, raise BCL6-dependence. Prior murine studies showed that activation of STAT6 by cytokine-signaling resulted in BCL6 upregulation. Although IL-4/IL-13 treatment resulted in increased phosphorylated (p)STAT6 levels in all human PMBL cell lines, unexpectedly, we found drastically reduced BCL6 protein levels. Examination of a regulatory interaction demonstrated in vivo and in vitro binding of STAT6 to the proximal promoter of BCL6. STAT6 depletion and ectopic expression of constitutively active STAT6 confirmed that pSTAT6 is sufficient for transcriptional repression of BCL6. Co-localization studies in all cell lines as well as ten primary patient samples demonstrated pSTAT6- and BCL6 staining in mutually exclusive subsets as a visual hallmark of the repression mechanism.
Conclusions: Collectively, our data show that constitutively active STAT6 transcriptionally represses BCL6 in PMBL. The delineated repression mechanism may prevent addiction to one single oncogenic pathway (i.e. BCL6) in PMBL.

Category: Hematopathology

Tuesday, March 5, 2013 9:30 AM

Poster Session III # 168, Tuesday Morning


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