API2-MALT1 oncoprotein promotes lymphomagenesis via unique program of substrate ubiquitination and proteolysis

Abstract

Lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) is the most common extranodal B cell tumor and accounts for 8% of non-Hodgkin’s lymphomas. Gastric MALT lymphoma is the best-studied example and is a prototypical neoplasm that occurs in the setting of chronic inflammation brought on by persistent infection or autoimmune disease. Cytogenetic abnormalities are commonly acquired during the course of disease and the most common is chromosomal translocation t(11;18)(q21;q21), which creates the API2-MALT1 fusion oncoprotein. t(11;18)-positive lymphomas can be clinically aggressive and have a higher rate of dissemination than t(11;18)-negative tumors. Many cancers, including MALT lymphomas, characteristically exhibit deregulated over-activation of cellular survival pathways, such as the nuclear factor-κB (NF-κB) pathway. Molecular characterization of API2-MALT1 has revealed it to be a potent activator of NF-κB, which is required for API2-MALT1-induced cellular transformation, however the mechanisms by which API2-MALT1 exerts these effects are only recently becoming apparent. The API2 moiety of the fusion binds tumor necrosis factor (TNF) receptor associated factor (TRAF) 2 and receptor interacting protein 1 (RIP1), two proteins essential for TNF receptor-induced NF-κB activation. By effectively mimicking ligand-bound TNF receptor, API2-MALT1 promotes TRAF2-dependent ubiquitination of RIP1, which then acts as a scaffold for nucleating and activating the canonical NF-κB machinery. Activation occurs, in part, through MALT1 moiety-dependent recruitment of TRAF6, which can directly modify NF-κB essential modulator, the principal downstream regulator of NF-κB. While the intrinsic MALT1 protease catalytic activity is dispensable for this canonical NF-κB signaling, it is critical for non-canonical NF-κB activation. In this regard, API2-MALT1 recognizes NF-κB inducing kinase (NIK), the essential upstream regulator of non-canonical NF-κB, and cleaves it to generate a stable, constitutively active fragment. Thus, API2-MALT1 harnesses multiple unique pathways to achieve deregulated NF-κB activation. Emerging data from our group and others have also detailed additional gain-of-function activities of API2-MALT1 that extend beyond NF-κB activation. Specifically, API2-MALT1 recruits and subverts multiple other signaling factors, including LIM domain and actin-binding protein 1 (LIMA1) and Smac/DIABLO. Like NIK, LIMA1 represents a unique substrate for API2-MALT1 protease activity, but unlike NIK, its cleavage sets in motion a major NF-κB-independent pathway for promoting oncogenesis. In this review, we highlight the most recent results characterizing these unique and diverse gain-of-function activities of API2-MALT1 and how they contribute to lymphomagenesis.

Keywords: Oncogene, Fusion oncoprotein, Lymphoma, Chromosomal translocation, Ubiquitination, Apoptosis, Nuclear factor-κB, Caspases

Core tip: We summarize the identification of novel API2-mucosa-associated lymphoid tissue (MALT) 1-interacting proteins that uniquely mediate the cellular effects of the fusion oncoprotein but not wild-type API2 or MALT1. API2-MALT1 recruits receptor interacting protein 1 and tumor necrosis factor (TNF) receptor associated factor 2, which normally function downstream of the TNF receptor, and utilizes these proteins to communicate unregulated canonical nuclear factor-κB (NF-κB) in a manner that does not depend on the protease activity of MALT1. Simultaneously, NF-κB inducing kinase is recruited to API2-MALT1 and is proteolytically cleaved by the MALT1 protease domain to generate a stable, non-canonical NF-κB-activating fragment. Finally, LIM domain and actin-binding protein 1 is similarly recruited and cleaved as an API2-MALT1 specific target and its cleavage mediates an NF-κB-independent mechanism of oncogenesis. Additional factors, including SMAC and BCL10, may also play key roles as API2-MALT1 binding partners and downstream signaling factors. Thus, the API2-MALT1 fusion utilizes a distinct set of protein-protein interactions to leverage multiple, divergent mechanisms and achieve potent oncogenic reprogramming of affected B cells.