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The recurrent translocation t(11;18)(q21;q21) associated with mucosa-associated lymphoid tissue (MALT) lymphoma results in the expression of an API2µMALT1 fusion protein that constitutively activates NF-κB. The first baculovirus lAP repeat (BIR) domain of API2 and the C terminus of MALT1, which contains its caspase-like domain, are present in all reported fusion variants and interact with TRAF2 and TRAF6, respectively, suggesting their contribution to NF-κB signaling by API2·MALT1. Also, the involvement of BCL10 has been suggested via binding to BIR1 of API2 and via its interaction with the immunoglobulin domains of MALT1, present in half of the fusion variants. However, conflicting reports exist concerning their roles in API2·MALT1-induced NF-κB signaling. In this report, streptavidin pulldowns of biotinylated API2·MALT1 fusion variants showed that none of the fusion variants interacted with endogenous BCL10; its role in NF-κB signaling by API2·MALT1 was further questioned by RNA interference experiments. In contrast, TRAF6 was essential for NF-κB activation by all fusion variants, and we identified a novel TRAF6 binding site in the second immunoglobulin domain of MALT1, which enhanced NF-κB activation when present in the fusion protein. Furthermore, inclusion of both immunoglobulin domains in API2·MALT1 further enhanced NF-κB signaling via intramolecular TRAF6 activation. Finally, binding of TRAF2 to BIR1 contributed to NF-κB activation by API2·MALT1, although additional mechanisms involving BIR1-mediated raft association are also important. Taken together, these data reveal distinct mechanisms of NF-κB activation by the different API2·MALT1 fusion variants with an essential role for TRAF6. [ABSTRACT FROM AUTHOR]

Two general pathways for cell death have been defined, apoptosis and necrosis. Previous studies in Jurkat cells have demonstrated that the Fas-associated death domain (FADD) is required for Fas-mediated signaling to apoptosis and necrosis. Here we developed L929rTA cell lines that allow Tet-on inducible expression and FK506-binding protein (FKBP)-mediated dimerization of FADD, FADD-death effector domain (FADD-DED), or FADD-death domain (FADD-DD). We show that expression and dimerization of FADD heads to necrosis. However, pretreatment of the cells with the Hsp90 inhibitor geldanamycin, which leads to proteasome-mediated degradation of receptor interacting protein 1 (RIP1), reverts FKBP-FADD-induced necrosis to apoptosis. Expression and dimerization of FADD-DD mediates necrotic cell death. We found that FADD-DD is able to bind RIP1, another protein necessary for Fasmediated necrosis. Expression and dimerization of FADD-DED initiates apoptosis. Remarkably, in the presence of caspase inhibitors, FADD-DED mediates necrotic cell death. Coimmunoprecipitation studies revealed that FADD-DED in the absence procaspase-8 C/A is also capable of recruiting RIP1. However, when procaspase-8 C/A and RIP1 are expressed simultaneously, FADD-DED preferentially recruits procaspase-8 C/A. [ABSTRACT FROM AUTHOR]

Analyzes whether the geldanamycin (GA)-induced inhibition of HSP90 function cold block tumor necrosis factor receptor-1 (TNFR1)-induced necrosis in L929 fibrosarcoma cells. Demonstration that inhibition of the function of HSP90 leads to a switch from TFR1 necrotic cell death to apoptotic cell death; Finding that GA pretreatment does not affect the expression levels of proteins resulting in the abolishment of TNF-mediated activation of NF-kB, JNK, p38MAPK and necrosis.

The recurrent translocation t(11;18)(q21;q21) associated with mucosa-associated lymphoid tissue (MALT) lymphoma results in the expression of an API2.MALT1 fusion protein that constitutively activates NF-kappaB. The first baculovirus IAP repeat (BIR) domain of API2 and the C terminus of MALT1, which contains its caspase-like domain, are present in all reported fusion variants and interact with TRAF2 and TRAF6, respectively, suggesting their contribution to NF-kappaB signaling by API2.MALT1. Also, the involvement of BCL10 has been suggested via binding to BIR1 of API2 and via its interaction with the immunoglobulin domains of MALT1, present in half of the fusion variants. However, conflicting reports exist concerning their roles in API2.MALT1-induced NF-kappaB signaling. In this report, streptavidin pulldowns of biotinylated API2.MALT1 fusion variants showed that none of the fusion variants interacted with endogenous BCL10; its role in NF-kappaB signaling by API2.MALT1 was further questioned by RNA interference experiments. In contrast, TRAF6 was essential for NF-kappaB activation by all fusion variants, and we identified a novel TRAF6 binding site in the second immunoglobulin domain of MALT1, which enhanced NF-kappaB activation when present in the fusion protein. Furthermore, inclusion of both immunoglobulin domains in API2.MALT1 further enhanced NF-kappaB signaling via intramolecular TRAF6 activation. Finally, binding of TRAF2 to BIR1 contributed to NF-kappaB activation by API2.MALT1, although additional mechanisms involving BIR1-mediated raft association are also important. Taken together, these data reveal distinct mechanisms of NF-kappaB activation by the different API2.MALT1 fusion variants with an essential role for TRAF6.