Caspase 8 expression and signaling in Fas injury-resistant human fetal astrocytes

Fas (APO-1/CD95) is a cell surface receptor initially identified in lymphoi d cells, but more recently detected in the central nervous system under pat hological, usually inflammatory, conditions. In most Fas expressing cells, triggering of Fas by its ligand or by antagonistic antibodies leads to apop tosis. Human fetal astrocytes (HFA) constitutively express Fas yet are resi stant to cell death following Fas ligation. In the current study, using dis sociated cultures of human fetal central nervous system- derived cells, we attempted to identify a basis for HFA resistance to Fas-mediated injury. We compared the components of the Fas signaling pathway of HFA to those of tw o human cell lines susceptible to Fas-mediated injury, U251 glioma and Jurk at T-cells. We found that HFA did not express caspase 8 (FLICE), the caspas e primarily activated on Fas signaling. Although we could induce caspase 8 in HFA with the inflammatory cytokines IFN gamma and TNF alpha, HFA remaine d resistant to Fas-mediated injury. Addition of inflammatory cytokines to t he extracellular milieu also increased FLIP mRNA (FLICE inhibitory protein) . Furthermore, upon triggering of cytokine-treated cells with FasL, we obse rved upregulation of the cleavage product of FLIP (p43-FLIP) previously sho wn to associate with the DISC and to block caspase 8 recruitment, thereby i nhibiting Fas-mediated death. Our findings indicate that caspase 8 and its regulators play a central role in determining the response to Fas ligation of HFA and support a role for Fas signaling in the developing central nervo us system other than related to cytotoxicity. (C) 2001 Wiley-Liss, Inc.