Mantle cell lymphoma (MCL) is a mature B-cell lymphoma characterized by poor clinical outcome. Recent studies revealed the importance of B-cell receptor (BCR) signaling in maintaining MCL survival. However, it remains unclear which role MALT1, an essential component of the CARD11-BCL10-MALT1 complex that links BCR signaling to the NF-κB pathway, plays in the biology of MCL. Here we show that a subset of MCLs is addicted to MALT1, as its inhibition by either RNA or pharmacologic interference induced cytotoxicity both in vitro and in vivo. Gene expression profiling following MALT1 inhibition demonstrated that MALT1 controls an MYC-driven gene expression network predominantly through increasing MYC protein stability. Thus, our analyses identify a previously unappreciated regulatory mechanism of MYC expression. Investigating primary mouse splenocytes, we could demonstrate that MALT1-induced MYC regulation is not restricted to MCL, but represents a common mechanism. MYC itself is pivotal for MCL survival because its downregulation and pharmacologic inhibition induced cytotoxicity in all MCL models. Collectively, these results provide a strong mechanistic rationale to investigate the therapeutic efficacy of targeting the MALT1-MYC axis in MCL patients.

The Carma1–Bcl10–Malt1 (CBM) complex bridges T‐cell receptor (TCR) signalling to the canonical IκB kinase (IKK)/NF‐κB pathway. NF‐κB activation is triggered by PKCθ‐dependent phosphorylation of Carma1 after TCR/CD28 co‐stimulation. PKCθ‐phosphorylated Carma1 was suggested to function as a molecular scaffold that recruits preassembled Bcl10–Malt1 complexes to the membrane. We have identified the serine–threonine protein phosphatase PP2A regulatory subunit Aα (PPP2R1A) as a novel interaction partner of Carma1. PPP2R1A is associated with Carma1 in resting as well as activated T cells in the context of the active CBM complex. By siRNA‐mediated knockdown and in vitro dephosphorylation, we demonstrate that PP2A removes PKCθ‐dependent phosphorylation of Ser645 in Carma1, and show that maintenance of this phosphorylation is correlated with increased T‐cell activation. As a result of PP2A inactivation, we find that enhanced Carma1 S645 phosphorylation augments CBM complex formation, NF‐κB activation and IL‐2 or IFN‐γ production after stimulation of Jurkat T cells or murine Th1 cells. Thus, our data define PP2A‐mediated dephosphorylation of Carma1 as a critical step to limit T‐cell activation and effector cytokine production.

The ubiquitin ligase TRAF6 is a key regulator of canonical IκB kinase (IKK)/NF-κB signaling in response to interleukin-1 (IL-1) stimulation. Here, we identified the deubiquitinating enzyme YOD1 (OTUD2) as a novel interactor of TRAF6 in human cells. YOD1 binds to the C-terminal TRAF homology domain of TRAF6 that also serves as the interaction surface for the adaptor p62/Sequestosome-1, which is required for IL-1 signaling to NF-κB. We show that YOD1 competes with p62 for TRAF6 association and abolishes the sequestration of TRAF6 to cytosolic p62 aggregates by a non-catalytic mechanism. YOD1 associates with TRAF6 in unstimulated cells but is released upon IL-1β stimulation, thereby facilitating TRAF6 auto-ubiquitination as well as NEMO/IKKγ substrate ubiquitination. Further, IL-1 triggered IKK/NF-κB signaling and induction of target genes is decreased by YOD1 overexpression and augmented after YOD1 depletion. Hence, our data define that YOD1 antagonizes TRAF6/p62-dependent IL-1 signaling to NF-κB.DOI: http://dx.doi.org/10.7554/eLife.22416.001

Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (Malt1) is a promising therapeutic target for the treatment of activated B cell-like diffuse large B cell lymphoma (ABC-DLBCL). Several research groups have reported on the development of Malt1 inhibitors and activity-based probes for in vitro and in situ monitoring and modulating Malt1 activity. In this paper, we report on two activity-based Malt1 probes (6 and 7) and a focused library of 19 new Malt1 inhibitors. Our peptide-based probe 6 labels Malt1 in an activity-based manner. In contrast, probe 7, derived from the known covalent inhibitor MI-2, labels both wild type and catalytically inactive Cys to Ala mutant Malt1, suggesting that MI-2 inhibits Malt1 by reacting with a nucleophilic residue other than the active site cysteine. Furthermore, two of our inhibitors (9, apparent IC50 3.0μM, and 13, apparent IC50 2.1μM) show good inhibitory activity against Malt1 and outperform MI-2 (apparent IC50 7.8μM) in our competitive activity-based protein profiling assay.

BackgroundThe CARMA1-BCL10-MALT1 (CBM) complex bridges T cell receptor (TCR) signaling to the canonical IκB kinase (IKK)/NF-κB pathway. The CBM complex constitutes a signaling cluster of more than 1 Mio Dalton. Little is known about factors that facilitate the rapid assembly and maintenance of this dynamic higher order complex.FindingsHere, we report the novel interaction of the aryl hydrocarbon receptor (AHR) interacting protein (AIP) and the molecular scaffold protein CARMA1. In T cells, transient binding of CARMA1 and AIP enhanced formation of the CBM complex. Thereby, AIP promoted optimal IKK/NF-κB signaling and IL-2 production in response to TCR/CD28 co-stimulation.ConclusionsOur data demonstrate that AIP acts as a positive regulator of NF-κB signaling upon T cell activation.