DISTINCT ADAPTER PROTEINS MEDIATE ACID VERSUS NEUTRAL SPHINGOMYELINASE ACTIVATION THROUGH THE P55 RECEPTOR FOR TUMOR-NECROSIS-FACTOR

Ceramide, generated by the enzymatic. function of sphingomyelinases (S Mases) has emerged as an important signaling pathway transducing diver se biological effects of various cytokine receptors. The 55-kDa recept or for tumor necrosis factor (TNF-R55) activates two types of SMases t hrough distinct cytoplasmic domains. The death domain that is responsi ble for the initiation of the apoptotic pathway also signals for the a ctivation of an acid SMase (A-SMase). The adapter protein TRADD binds to TNF-R55 in a ligand-dependent manner and serves as an anchor for th e subsequent recruitment of other proteins into the signaling complex that directly lead to cell death or nuclear factor-kappa B (NF-kappa B ) induction, Notably, the two pro-apoptotic adapter proteins TRADD and FADD are also involved in the activation of A-SMase. In contrast, the NF-kappa B-inducing adapters TRAF2 and RIP do not signal for A-SMase. Thus, activation of A-SMase ap pears to belong to signals leading to TNF-induced cell death. A second signaling domain (NSD) is located ups tream of the death domain and directly links the TNF-R55 to the activa tion of a neutral SMase (N-SMase). A novel adapter protein, FAN, has b een identified that specifically binds to the NSD, FAN contains five W D repeats at its carboxy terminus, while it shows significant sequence homology with the mouse beige protein and its human homolog, the CHS protein, in the center portion of the protein. Overexpression of full- length FAN enhanced N-SMase activity in TNF-treated cells, whereas tru ncated mutants of FAN produced dominant negative effects. FAN, however , did not interfere with any of the TNF responses signaled for by the death domain, Taken together, our data suggest that distinct cytoplasm ic domains of TNF-R55 initiate independent signaling pathways by bindi ng different adapter proteins.