FAS AND TUMOR-NECROSIS-FACTOR RECEPTOR-MEDIATED CELL-DEATH - SIMILARITIES AND DISTINCTIONS

Fas antigen and two tumor necrosis factor receptors (TNFR), p55 and p7 5, are implicated in the triggering of cell death upon stimulation by natural ligands and specific monoclonal antibodies. However, the relat ive efficiency of each receptor, the mechanisms that regulate their fu nction 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, TNFRp 55 and TNFRp75, were stably expressed in a human melanoma cell line th at is deficient in Fas and TNFR expression. Transfectants were stimula ted by a soluble recombinant form of the CD40 ligand gp39, and the eff ect on cell viability determined. Engagement of all three fusion prote ins by the gp39 ligand induced lethal signals, but the rate at which c ell death occurred was distinct. Fas-derived signals were observed to have the most rapid effect, killing most cells within hours of stimula tion, whereas TNFRp55- and TNFRp75-associated signals resulted in cell death within 2-3 d after engagement by ligand. It is interesting to n ote that optimal cell killing by all three fusion proteins was depende nt on a critical, low to intermediate, cell surface expression level. High levels of fusion protein expression, on the other hand, were asso ciated with inhibition of cell death. Our results provide a model to s tudy Fas and TNFR-mediated cell death and suggest a novel mechanism fo r the regulation of death signals triggered by members of the TNFR fam ily.