NPM-RAR Binding To Tradd Selectively Blocks TNF-Induced Apoptosis While Allowing TNF Activation Of JNK

The t(5;17)(q35;q21) variant of acute promyelocytic leukemia fuses nucleophosmin (NPM) to the retinoic acid receptor alpha (RARA). Expression of the NPM-RAR fusion protein leads to a leukemic phenotype similar to that caused by the t(15;17)(q22;q21) PML-RAR fusion. Using a proteomic approach to identify proteins that bind NPM-RAR, we previously demonstrated that TRADD, amongst other proteins, is a distinct binding partner for NPM-RAR. TRADD serves as an adapter protein that mediates TNF-receptor-mediated activation of the caspase, NFkB, and JNK pathways. We reported that NPM-RAR inhibits TNF-activation of caspase 8 and caspase 3, with subsequent impairment of TNF-induced apoptosis. Extending these findings, we assayed activation of JNK kinase in NPM-RAR-expressing subclones of HELA and U937 cells. We find that TNF activated JNK kinase in NPM-RAR HELA and U937 cells to a similar extent as nontransfected parental controls. This indicates that NPM-RAR binding to TRADD selectively inhibits TNF’s ability to activate caspases, without impairing its ability to activate JNK. In this regard, NPM-RAR effects on TRADD is similar to the EBV oncogenic viral protein LMP1, which binds to the death domain of TRADD, to selectively inhibit caspase activity, while still allowing TNF-mediated activation of survival pathways. Our studies identify a novel mechanism through which NPM-RAR may impact leukemogenesis.