Fas ligand-induced c-Jun kinase activation in lymphoid cells requires extensive receptor aggregation but is independent of DAXX, and Fas-mediated cell death does not involve DAXX, RIP, or RAIDD.

Jun kinase signaling can be elicited by death receptor activation, but the mechanism and significance of this event are still unclear. It has been reported that cross-linking Abs to Fas trigger c-Jun N-terminal kinase (JNK) signaling via caspase-mediated activation of MEKK1 (JNK kinase kinase), elevation of ceramide levels or by recruitment of death domain associated protein (DAXX) to Fas. The effect of physiological ligand for Fas on JNK signaling was never investigated, although evidence is accumulating that Fas ligand is able to induce cellular responses distinct from those evoked by Ab-mediated cross-linking of Fas. Therefore, we investigated the effect of Fas ligand on JNK signaling. Like its ability to induce cell death, Fas ligand reliably activated JNK only upon extensive aggregation of the receptor. Although this was partially dependent on caspase activation, DAXX was not required. DAXX and other death receptor-associated proteins, which have been reported to bind directly or indirectly to Fas, such as receptor interacting protein (RIP) and RIP-associated ICH-1/CED-3-homologous protein with a death domain (RAIDD), were shown to be dispensable for Fas ligand-induced apoptosis.