Involvement of FAN in TNF-induced apoptosis

TNF-alpha is a pleiotropic cytokine activating several signaling path ways initiated at distinct intracellular domains of the TNF receptors. Although the C-terminal region is believed to be responsible for apoptosis induction , the functions of more membrane-proximal domains, including the domain tha t couples to neutral sphingomyelinase activation, are not yet fully elucida ted. The roles of this region and of the associated adapter protein FAN (fa ctor associated with neutral SMase activation) in the cytotoxic response to TNF have been investigated. We have now shown that stable expression in hu man fibroblasts of a dominant negative form of FAN abrogates TNF-induced ce ramide generation from sphingomyelin hydrolysis and reduces caspase process ing, thus markedly inhibiting TNF-triggered apoptosis. However, the cytotox ic responses to daunorubicin and exogenous ceramide remain unaltered, as do the TNF-induced p42/p44 MAPK activation and CD54 expression. Fibroblasts f rom FAN-knockout mice also proved to be resistant to TNF toxicity. These fi ndings highlight the previously unrecognized role of the adapter protein FA N in signaling cell death induction by TNF.