Involvement of sphingosine in mitochondria-dependent Fas-induced apoptosisof type II Jurkat T cells

Exposure to anti-Fas antibody in Jurkat cells (type II cells), which are ch aracterized by a weak caspase-8 activation at the death-inducing signaling complex (DISC), induced a biphasic increase in ceramide levels. The early g eneration of ceramide preceded transient activation of acidic ceramidase an d subsequent production of sphingosine, followed by cytochrome c release, a ctivation of caspases-2, -3, -6, -7, -8, and -9, Bid cleavage, and a later sustained ceramide accumulation. The caspase inhibitor benzyloxycarbonyl-Va l-Ala-Asp-fluoromethyl ketone inhibited early increases of ceramide and sph ingosine, whereas overexpression of Bcl-x(L), had no effect, and both preve nted the later sustained ceramide accumulation. Exogenous sphingosine, as w ell as cell-permeable C-2-ceramide, induced cytochrome c release from mitoc hondria in a caspase-independent fashion leading to activation of caspase-9 and executioner caspases and, surprisingly, activation of the initiator ca spase-8 and processing of its substrate Bid. These effects were also comple tely abolished by Bcl-x(L) overexpression. Our results suggest that sphingo sine might also be involved in the mitochondria-mediated pathway of Fas-ind uced cell death in type II cells.