Multiple pathways originate at the Fas/APO-1 (CD95) receptor: sequential involvement of phosphatidylcholine-specific phospholipase C and acidic sphingomyelinase in the propagation of the apoptotic signal.

Abstract

The early signals generated following cross‐linking of Fas/APO‐1, a transmembrane receptor whose engagement by ligand results in apoptosis induction, were investigated in human HuT78 lymphoma cells. Fas/APO‐1 cross‐linking by mAbs resulted in membrane sphingomyelin hydrolysis and ceramide generation by the action of both neutral and acidic sphingomyelinases. Activation of a phosphatidylcholine‐specific phospholipase C (PC‐PLC) was also detected which appeared to be a requirement for subsequent acidic sphingomyelinase (aSMase) activation, since PC‐PLC inhibitor D609 blocked Fas/APO‐1‐induced aSMase activation, but not Fas/APO‐1‐induced neutral sphingomyelinase (nSMase) activation. Fas/APO‐1 cross‐linking resulted also in ERK‐2 activation and in phospholipase A2 (PLA2) induction, independently of the PC‐PLC/aSMase pathway. Evidence for the existence of a pathway directly involved in apoptosis was obtained by selecting HuT78 mutant clones spontaneously expressing a newly identified death domain‐defective Fas/APO‐1 splice isoform which blocks Fas/APO‐1 apoptotic signalling in a dominant negative fashion. Fas/APO‐1 cross‐linking in these clones fails to activate PC‐PLC and aSMase, while nSMase, ERK‐2 and PLA2 activates are induced. These results strongly suggest that a PC‐PLC/aSMase pathway contributes directly to the propagation of Fas/APO‐1‐generated apoptotic signal in lymphoid cells.

Apoptosis is associated with triacylglycerol accumulation in Jurkat T-cells

“…Considering the PtdCho catabolic pathway, different phospholipases have been reported to be activated during Fas-induced apoptosis including PtdCho-specific phospholipase C (Cifone et al, 1995), phospholipase A 2 (Atsumi et al, 1998) and phospholipase D (Han et al, 1999). The increase in lipids associated with HSV-tkinduced apoptosis was attributed to the hydrolysis of PtdCho into lysophosphatidylcholine and free fatty acids via the activation of phospholipase A 2 (Hakumaki et al, 1999).…”

mtDNA‐depleted U937 cells are sensitive to TNF and Fas‐mediated cytototxicity

“…Other signalling pathways, like the activation of intracellular proteases, such as interleukin-lfl converting enzyme (ICE), recently demonstrated to be involved in Fas- [27,28] and TNF- [27] induced apoptosis, could represent the necessary cytoplasmic event to trigger cell death. The activation of an sphingomyelinase that produces the lipid mediator ceramide has been described after both Fas [29] and TNF receptor engagement [30]. The connection between membrane receptor engagement and ICE activation is still unknown, although the ceramide-activated pathway could be a common candidate for both receptors.…”

CPP32 inhibition prevents Fas‐induced ceramide generation and apoptosis in human cells

“…Sphingomyelinase and ICE-like protease activation have been implicated in several forms of apoptosis and especially in those induced by TNF and Fas [2][3][4][5]12,13,[15][16][17]31]. However, the connection between both signalling pathways had not been characterised.…”

“…Recently, the sphingolipid ceramide has ~een implicated in the apoptosis induced by stimulation of ::he Fas molecule [2,3] and of tumor necrosis factor (TNF) 55 kDa receptor [4]. In this model, upon occupancy of recep-:or, an intracellular sphingomyelinase would be activated by m unknown coupling mechanism, causing hydrolysis of ~phingomyelin and generation of ceramide [4][5][6].…”