Defining a new class of transcriptional complex: ErbB2 acts as coactivator of Stat3 inducing proliferation in breast tumors

El receptor con actividad de tirosina quinasa ErbB2 y la proteína transductora de señales y activadora de la transcripción 3 (Stat3) juegan un rol importante en el desarrollo del cáncer de mama. Distintas evidencias sugieren la existencia de interacciones cruzadas entre ErbB2 y Stat3. Sin embargo, los mecanismos moleculares que subyacen a esta interacción en tumores mamarios permanecen poco estudiados. En este trabajo identificamos un nuevo mecanismo de interacción entre ErbB2 y Stat3 que involucra la translocación nuclear de ErbB2 y la formación de un complejo en el que ErbB2 actúa como coactivador transcripcional de Stat3. Mostramos que la formación de este complejo es inducida tanto por el ligando de los receptores ErbBs, heregulina, como por los progestágenos a través del receptor de progesterona (PR). Demostramos también que la función de ErbB2 como coactivador de Stat3 promueve la activación del promotor de ciclina D1. Cuando la formación del complejo Stat3/ErbB2 es inducida por progestágenos, encontramos al PR co-reclutado en el promotor de ciclina D1, revelando un nuevo mecanismo de acción genómico no clásico del PR. Demostramos que la presencia de ErbB2 en el núcleo celular ejerce un rol fundamental en la proliferación in vitro e in vivo en tumores mamarios. Estos hallazgos revelan una posible intervención terapéutica nueva en tumores de mama que sobreexpresan ErbB2, mediante la inhibición de la translocación nuclear de ErbB2, dado que esta estrategia es efectiva en células tumorales resistentes a las terapias anti ErbB2 convencionales.ErbB2 tyrosine kinase receptor and Signal Transducer and Activator of Transcription 3 (Stat3) have been unraveled as two major players in breast cancer growth. In spite of all the accumulating evidence suggesting a crosstalk between ErbB2 and Stat3, the molecular mechanisms underlying ErbB2 and Stat3 interaction in breast tumor remain poorly explored. In this work, we identified a new mechanism of ErbB2 and Stat3 interaction, which involves ErbB2 nuclear translocation and the assembly of a complex in which ErbB2 acts as a transcriptional coactivator of Stat3. We showed that the assembly of this complex is induced by heregulin, a ligand of the ErbBs receptors, as well as by ligand bound progesterone receptor (PR). We also highlighted that ErbB2 function as Stat3 coactivator drives cyclin D1 promoter activation. When the assembly of Stat3/ErbB2 complex is induced by progestins, we found PR recruitment together with Stat3 and ErbB2 to the cyclin D1 promoter, unraveling a new nonclassical PR genomic mechanism. We found that the nuclear Stat3/ErbB-2 transcriptional complex plays a key role in in vitro and in vivo proliferation of breast tumors. Our findings reveal a novel therapeutic intervention in ErbB2-overexpressing breast tumors, by inhibition of ErbB2 nuclear translocation, since this strategy has proven effective in tumor cells which are resistant to conventional anti ErbB2 therapies.Fil:Béguelin, Wendy. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

EZH2 and BCL6 Cooperate to Assemble CBX8-BCOR Complex to Repress Bivalent Promoters, Mediate Germinal Center Formation and Lymphomagenesis

The EZH2 histone methyltransferase mediates the humoral immune response and drives lymphomagenesis through formation of bivalent chromatin domains at critical germinal center (GC) B cell promoters. Herein we show that the actions of EZH2 in driving GC formation and lymphoma precursor lesions require site-specific binding by the BCL6 transcriptional repressor and the presence of a non-canonical PRC1-BCOR-CBX8 complex. The chromodomain protein CBX8 is induced in GC B cells, binds to H3K27me3 at bivalent promoters, and is required for stable association of the complex and the resulting histone modifications. Moreover, oncogenic BCL6 and EZH2 cooperate to accelerate diffuse large B cell lymphoma (DLBCL) development and combinatorial targeting of these repressors results in enhanced anti-lymphoma activity in DLBCLs

TET2 Deficiency Causes Germinal Center Hyperplasia, Impairs Plasma Cell Differentiation, and Promotes B-cell Lymphomagenesis.

TET2 somatic mutations occur in ∼10% of diffuse large B-cell lymphomas (DLBCL) but are of unknown significance. Herein, we show that TET2 is required for the humoral immune response and is a DLBCL tumor suppressor. TET2 loss of function disrupts transit of B cells through germinal centers (GC), causing GC hyperplasia, impaired class switch recombination, blockade of plasma cell differentiation, and a preneoplastic phenotype. TET2 loss was linked to focal loss of enhancer hydroxymethylation and transcriptional repression of genes that mediate GC exit, such as PRDM1. Notably, these enhancers and genes are also repressed in CREBBP-mutant DLBCLs. Accordingly, TET2 mutation in patients yields a CREBBP-mutant gene-expression signature, CREBBP and TET2 mutations are generally mutually exclusive, and hydroxymethylation loss caused by TET2 deficiency impairs enhancer H3K27 acetylation. Hence, TET2 plays a critical role in the GC reaction, and its loss of function results in lymphomagenesis through failure to activate genes linked to GC exit signals. SIGNIFICANCE: We show that TET2 is required for exit of the GC, B-cell differentiation, and is a tumor suppressor for mature B cells. Loss of TET2 phenocopies CREBBP somatic mutation. These results advocate for sequencing TET2 in patients with lymphoma and for the testing of epigenetic therapies to treat these tumors