In the present study, we show that Fas receptor ligation or cellular treatment with synthetic C 6 -ceramide results in activation or phosphorylation, respectively, of the small G-protein Rac1, Jun N-terminal kinase (JNK)/ p38 kinases (p38-K), and the transcription factor GADD153. A signaling cascade from the Fas receptor via ceramide, Ras, Rac1, and JNK/p38-K to GADD153 is demonstrated employing transfection of transdominant inhibitory N17Ras, N17Rac1, c-Jun, or treatment with a specific p38-K inhibitor. The critical function of this signaling cascade is indicated by prevention of Fas-or C 6 -ceramide-induced apoptosis after inhibition of Ras, Rac1, or JNK/p38-K.

ABSTRACT

Fas/CD95/Apo-I has been shown to stimulate a variety of molecules including several members of the caspase family and the acidic sphingomyelinase (Martin and Green 1995;Gulbins et al, 1995). Here, we demonstrate that Fas receptortriggered activation of the acidic sphingomyelinase, consumption of sphingomyelin, release of ceramide, and subsequent activation of JNK and p38-K are regulated by caspases. Inhibition of caspases by Ac-YVAD-chloromethylketone or transient CrmA transfection prevented stimulation of acidic sphingomyelinase, release of ceramide and activation of JNK and p38-K upon Fas-receptor crosslinking. Likewise, Fas triggered apoptosis was almost completely blocked by Ac-YVAD-chloromethylketone or CrmA mediated inhibition of caspases. The results suggest a new signalling cascade from the Fas receptor via caspases to acidic sphingomyelinase, ceramide and JNK/p38-K.