Oncogenic CARMA1 couples NF-κB and β-Catenin signaling in diffuse large B cell lymphomas

Constitutive activation of the anti-apoptotic NF-κB signaling pathway is a hallmark of the activated B-cell-like (ABC) subtype of diffuse large B-cell lymphomas (DLBCL) that is characterized by adverse survival. Recurrent oncogenic mutations are found in the scaffold protein CARMA1 (CARD11) that connects B-cell receptor (BCR) signaling to the canonical NF-κB pathway. We asked how far additional downstream processes are activated and contribute to the oncogenic potential of DLBCL-derived CARMA1 mutants. To this end, we expressed oncogenic CARMA1 mutants in the NF-κB negative DLBCL lymphoma cell line BJAB. By a proteomic approach we identified recruitment of β-Catenin and its destruction complex consisting of APC, AXIN1, CK1α and GSK3β to oncogenic CARMA1. Recruitment of the β-Catenin destruction complex was independent of CARMA1-BCL10-MALT1 (CBM) complex formation or constitutive NF-κB activation and promoted the stabilization of β-Catenin. Elevated β-Catenin expression was detected in cell lines and biopsies from ABC DLBCL that rely on chronic BCR signaling. Increased β-Catenin amounts alone were not sufficient to induce classical WNT target gene signatures, but could augment TCF/LEF dependent transcriptional activation in response to WNT signaling. In conjunction with NF-κB, β-Catenin enhanced expression of immune suppressive IL-10 and repressed anti-tumoral CCL3, indicating that β-Catenin may induce a favorable tumor microenvironment. Thus, parallel activation of NF-κB and β-Catenin signaling by gain-of-function mutations in CARMA1 can augment WNT stimulation and is required for maintaining high expression of distinct NF-κB target genes and can thereby trigger cell intrinsic and extrinsic processes that promote DLBCL lymphomagenesis. CARMA1 mutants were expressed in BJAB, an NF-κB negative GCB DLBCL lymphoma cell line. Gene expression induced by GCB DLBCL derived CARMA1 L225LI mutant was compared with the empty vector (mock) control, CARMA1 WT, CARMA1 point mutant R35A and with CARMA1 double mutant R35A/L225LI.