Crosslinking of the transmembrane receptor CD95/Fas leads to activation of a signaling cascade resulting in apoptosis. c-FLIP is a recently described protein that potently inhibits Fas-mediated apoptosis and has been shown to be a key factor in germinal center B cell survival. Because Hodgkin and Reed-Sternberg cells in classical Hodgkin's disease (cHD) are also resistant to Fas-mediated apoptosis we studied the role of c-FLIP in classical HD. High levels of c-FLIP protein were identified in two Fas-resistant Hodgkin-derived cell lines. In contrast to other tumor cells, inhibition of protein synthesis by cycloheximide did not lead to down-regulation of c-FLIP protein in these HD cell lines. Furthermore, Fas-mediated apoptosis was only partially restored suggesting that normal regulation of c-FLIP was disrupted. The in vivo relevance of these findings was supported by demonstration of significant c-FLIP expression by immunohistochemistry in 18 of 19 evaluable cases of primary HD. Taken together, c-FLIP is constitutively expressed in HD and may therefore be a major mechanism responsible for Fas-resistance in HD.
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