Fas antigen and two tumor necrosis factor receptors (TNFR), p55 and p75, are implicated in the triggering of cell death upon stimulation by natural ligands and specific monoclonal antibodies. However, the relative efficiency of each receptor, the mechanisms that regulate their function and the signaling pathways they employ, remain to be elucidated. In this study, fusion proteins, composed of the extracellular domain of CD40 and the intracellular and transmembrane domains of Fas, TNFRp55 and TNFRp75, were stably expressed in a human melanoma cell line that is deficient in Fas and TNFR expression. Transfectants were stimulated by a soluble recombinant form of the CD40 ligand gp39, and the effect on cell viability determined. Engagement of all three fusion proteins by the gp39 ligand induced lethal signals, but the rate at which cell death occurred was distinct. Fas-derived signals were observed to have the most rapid effect, killing most cells within hours of stimulation, whereas TNFRp55- and TNFRp75-associated signals resulted in cell death within 2-3 d after engagement by ligand. It is interesting to note that optimal cell killing by all three fusion proteins was dependent on a critical, low to intermediate, cell surface expression level. High levels of fusion protein expression, on the other hand, were associated with inhibition of cell death. Our results provide a model to study Fas and TNFR-mediated cell death and suggest a novel mechanism for the regulation of death signals triggered by members of the TNFR family.

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