Published OnlineFirst September 4, 2013; DOI: 10.1158/1078-0432.CCR-12-3869 Clinical Cancer Molecular Pathways Research Molecular Pathways: Targeting MALT1 Paracaspase Activity in Lymphoma Lorena Fontan 1,2 and Ari Melnick1,2 Abstract MALT1 mediates the activation of NF-kB in response to antigen receptor signaling. MALT1, in association with BCL10 and CARD11, functions as a scaffolding protein to activate the inhibitor of IkB kinase (IKK) complex. In addition, MALT1 is a paracaspase that targets key proteins in a feedback loop mediating termination of the NF-kB response, thus promoting activation of NF-kB signaling. Activated B-cell subtype of diffuse large B-cell lymphomas (ABC-DLBCL), which tend to be more resistant to chemotherapy, are often biologically dependent on MALT1 activity. Newly developed MALT1 small-molecule inhibitors suppress the growth of ABC-DLBCLs in vitro and in vivo. This review highlights the recent advances in the normal and disease-related functions of MALT1. Furthermore, recent progress targeting MALT1 proteolytic activity raises the possibility of deploying MALT1 inhibitors for the treatment of B-cell lymphomas and perhaps autoimmune diseases that involve increased B- or T-cell receptor signaling. Clin Cancer Res; 19(24); 1–7. Ó2013 AACR. Background phoinositide 3-kinase (PI3K), which in turn activates PDK1 and BTK, and then PLC-g2 to produce diacylglycerol (DAG) MALT1 as a critical mediator of B-cell receptor signaling 2þ The mucosa-associated lymphoid tissue lymphoma and Ca and activate PKC-b (refs. 8, 10; Fig. 1A). PKC-b translocation 1 (MALT1) gene was first identified in the then phosphorylates CARD11, promoting a conformational recurrent t(11;18)(q21;q21) in MALT lymphomas (1). change that enables interaction with BCL10 and MALT1 (9). Resulting fusion product contains the N-terminal portion Once this complex is formed, TRAF6 recruitment and poly- of cellular inhibitor of apoptosis 2 (cIAP2 or API2) and the ubiquitylation of MALT1 and BCL10 promote the binding of C-terminal portion of MALT1. MALT1 is also translocated to inhibitor of IkB kinase g (IKKg) and TAK1 (refs. 8, 11; Fig. the immunoglobulin (Ig) heavy-chain gene enhancer in 1A). Next, IKKg complexes with IKKa and IKKb, which are MALT lymphomas, leading to aberrant expression of the phosphorylated and activated by TAK1, to phosphorylate protein (2, 3). Notably, MALT1 transgenic mice develop the IkB proteins and induce their proteolytic degradation. MALT lymphomas histologically and molecularly analo- NF-kB proteins can then translocate to the nucleus, where gous to the human disease (4). MALT1 also plays a critical they activate genes involved in proliferation, apoptosis inhi- role in the activated B-cell subtype of diffuse large B-cell bition, and inflammation (ref. 8, 11; Fig. 1A). Remarkably, c- lymphoma (ABC-DLBCL; refs. 5–7), and indeed MALT1 REL nuclear translocation is dependent on MALT1 activity, transgenic mice develop an ABC-DLBCL–like disease when whereas it is dispensable for RELA activation (12–14). crossed into a p53 null background (4). Accordingly, studies of MALT1 knockout mice indicated its MALT1 forms a complex with the B-cell chronic lympho- essential role in antigen receptor–induced NF-kB activation, cytic leukemia (CLL)/lymphoma 10 (BCL10) and caspase cytokine production, and proliferation in T and B cells (15, recruitment domain family, member 11 (CARD11; ref. 8). 16). MALT1 knockout mice also exhibited impaired prolif- As part of this CARD11–BCL10–MALT1 (CBM) complex, eration of splenic B cells upon lipopolysaccharide (LPS) MALT1 transduces signals from the B-cell receptor (BCR) stimulation (15). Moreover, BCL10 interacts with IRAK1 and T-cell receptor (TCR), natural killer (NK) cell, and B cell– and transduces signaling through interaction with MALT1 activating factor receptors (9). Upon BCR engagement, a upon LPS treatment in macrophages (ref. 17; Fig. 1A). These cascade of tyrosine kinase phosphorylation activates phos- findings may implicate MALT1 in Toll-like receptor signaling; however, this remains controversial, as Ruland and collea- gues did not observe contribution of MALT1 to LPS response (16). MALT1 also signals downstream of C-type lectin family Authors' Affiliations: Departments of 1Medicine and 2Pharmacology, Weill Cornell Medical College, New York, New York and G-protein coupled receptors (GPCR) in complex with CARD9 and CARMA3, respectively (9). Corresponding Author: Ari Melnick, Weill Cornell Medical College, 1300 York Avenue, New York, NY 10065. Phone: 212-746-7643; Fax: 212-746- 8866; E-mail: [email protected] MALT1 protease activity in NF-kB signal transduction doi: 10.1158/1078-0432.CCR-12-3869 Structurally, MALT1 has a conserved dead domain, two Ó2013 American Association for Cancer Research. Ig-like domains, and a caspase-like paracaspase domain www.aacrjournals.org OF1 Downloaded from clincancerres.aacrjournals.org on September 29, 2021. © 2013 American Association for Cancer Research. Published OnlineFirst September 4, 2013; DOI: 10.1158/1078-0432.CCR-12-3869 Fontan and Melnick A * IgM CD79A/B TLR PI3K ∗ BTK PLC-γ2 PDK1 SYK MYD88 Figure 1. B-cell receptor IP3 DAG IRAK induced activation of NF-kB through the CBM complex. A, ↑ 2+ MALT1 (highlighted in orange) is Ca a mediator of NF-kB signaling in PKC B lymphocytes. BCR stimulation leads to a cascade of tyrosine phosphorylations that activate SYK and then ∗ CARD11 PI3K. This activates PDK1 and ∗ A20 BTK, which subsequently promotes PLCg activation and þ DAG and Ca2 production, to finally activate PKC. PKC TRAF6 ub BCL10 ub phosphorylates CARD11, MALT1 TAK1 which allows formation of the ub CBM complex. TRAF6 and TAK1 are then recruited to the IKKγ CBM complex and activate the IKKβ IKKα IKK signalosome through polyubiquitination and phosphorylation to finally ∗ Nuclear translocation engage canonical NF-kB Proteasome IκB REL and gene expression signaling. Red and bold with asterisk denote frequently mutated genes in ABC-DLBCL. B B, targets of the paracaspase Paracaspase activity: targets and effects activity of MALT1 and API2- MALT1 and effects of their cleavage. In red, inhibitory A20 BCL10 actions; in green, activating actions. Reduced MALT1 Integrin-mediated deubiquitinylation adhesion MALT1 API2-MALT1 CYLD Regnase-1 NIK Increased JNK Increase mRNA stability and AP-1 activity IL-6, IL-2, c-REL Noncanonical RELB NF-κB Reduced DNA binding © 2013 American Association for Cancer Research OF2 Clin Cancer Res; 19(24) December 15, 2013 Clinical Cancer Research Downloaded from clincancerres.aacrjournals.org on September 29, 2021. © 2013 American Association for Cancer Research. Published OnlineFirst September 4, 2013; DOI: 10.1158/1078-0432.CCR-12-3869 MALT1 Paracaspase Targeting (18). The paracaspase domain was first predicted by struc- Fontan and colleagues identified novel MALT1 protease tural similarity, but its protease activity and cleavage targets inhibitors by screening small-molecule libraries using an in remained elusive for years until the identification of MALT1 vitro active form of MALT1 (13). The most biologically substrates tumor necrosis factor a-induced protein 3 potent inhibitor identified, "MI-2," exhibited irreversible (TNFAIP3/A20; ref. 19) and BCL10 (20). Other identified and specific binding to MALT1 and suppressed its protease targets include cylindromatosis (CYLD; ref. 21), v-rel reti- function in vitro and in vivo. MI-2 induced nuclear depletion culoendotheliosis viraloncogene homologB (RELB;ref.22), of c-REL and suppressed NF-kB activity (13). Most notably, and regnase-1 (23). Notably, fusion of MALT1 to API2 leads MI-2 was nontoxic to mice and displayed potent and to ectopic cleavage of NF-kB–inducing kinase (NIK; specific activity against ABC-DLBCL cells in vitro and xeno- ref. 24; Fig. 1B). A20 and CYLD are deubiquitylases that transplanted in vivo (13). The compound was also specif- cleave Lys-63–linked polyubiquitin chains (25). A20 deu- ically effective against primary human non-GCB DLBCLs ex biquitylates MALT1, decreasing its stability and attenuating vivo (13). Hence, MI-2 may represent a potentially clinically the B-cell response (26). CYLD decreases c-jun-NH2-kinase useful MALT1 inhibitor. (JNK) activity, thereby inhibiting activator protein-1 (AP-1; Finally, monoubiquitylation of MALT1 on Lys644 acti- ref. 21). RELB cleavage by MALT1 enhanced RELA and c-REL vates the protease function of MALT1. Expression of a DNA binding (22). Regnase-1 is an RNase that specifically nonubiquitylatable MALT1(K644R) mutant reduced sur- targets and degrades mRNAs implicated in the inflamma- vival of ABC-DLBCL cell lines (35). Targeting the ubiquitin tory response such as interleukin (IL)-2, IL-6, and c-REL, ligase responsible for this activation, currently unknown, thus attenuating signaling downstream of the TCR (23). might also disrupt MALT1 activity. Collectively, MALT1 cleavage of its substrate proteins enhances and prolongs NF-kB signaling downstream of the Clinical context for translation of MALT1-targeted BCR and/or TCR (19–24). therapy Biologic dependency on BCR signaling is a central feature of several types of B-cell neoplasms (36), and its inhibition – Clinical Translational Advances has been proposed as a strategy to treat lymphomas. A MALT1 protease activity inhibition in ABC-DLBCL number of BCR pathway inhibitors are in development DLBCLs are a heterogeneous group of diseases. Among (Fig. 2). Among these, ibrutinib, an irreversible BTK inhib- them, the ABC subtype, characterized by constitutive NF-kB itor, is showing