Intracellular triggering of Fas, independently of FasL, as a new mechanism of antitumor ether lipid-induced apoptosis

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Abstract
Antitumor ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3; edelfosine) induces apoptosis in cancer cells, sparing normal cells. We have found that the apoptotic action of ET-18-OCH3 required drug uptake and Fas in the target cell. Failure to accomplish one of these requirements prevents cell killing by the ether lipid. In human lymphoid leukemic cells, ET-18-OCH3 does not promote Fas or FasL expression and ET-18-OCH3-induced apoptosis is not inhibited by pre-incubation with an anti-Fas blocking antibody that abrogates cell killing mediated by Fas/FasL interactions. ET-18-OCH3-resistant normal human Fas-positive fibroblasts do not incorporate ET-18-OCH3, but undergo apoptosis upon ET-18-OCH3 microinjection. Murine fibroblasts L929 and L929-Fas, stably transfected with human Fas cDNA, do not incorporate ET-18-OCH3 and are resistant to its action when added exogenously. Microinjection of ET-18-OCH3 induces apoptosis in L929-Fas cells, but not in wild-type L929 cells. Confocal laser scanning microscopy shows that ET-18-OCH3 induces Fas clustering and capping during triggering of ET-18-OCH3-induced apoptosis. Microinjection-induced apoptosis and Fas clustering are specific for the molecular structure of ET-18-OCH3. Our data indicate that ET-18-OCH3 induces apoptosis via Fas after the ether lipid is inside the cell, and this Fas activation is independent of the interaction of Fas with its natural ligand FasL. This explains the selective action of ET-18-OCH3 on tumors since only cancer cells incorporate sufficient amounts of the drug. Int. J. Cancer 85:674–682, 2000.

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