Fas ligand is present in tumors of the Ewing's sarcoma family and is cleaved into a soluble form by a metalloproteinase

Fas ligand (FasL) exists in transmembrane and soluble forms and induces apoptosis on cross-linking with the Fas receptor. We evaluated the biological significance of Fast and Fas in 61 tumor tissues and 9 cell lines of the Ewing's sarcoma family of tumors (ESFT). Fast was present in 62.5% and Fas in 79.4% of primary ESFT, Metastatic tumors had higher expression of Fast (95%), suggesting association with a metastatic phenotype. Fast was detected in the cytoplasm and membrane of ESFT cells by immunofluorescence. Western blotting revealed transmembrane and soluble Fast in cytosolic extracts and soluble Fast in conditioned media. Both transmembrane and soluble Fast induced apoptosis of Fas-sensitive Jurkat cells in co-culture experiments with ESFT cells or their media. Treatment with phenanthroline and the synthetic metalloproteinase inhibitor BB-3103 reduced the levels of soluble Fast in the media, suggesting that in ESFT, Fast is processed by a metalloproteinase and released in the extracellular milieu. The released soluble Fast may serve to attack cells of the immune system and/or interfere with the binding of transmembrane Fast with Fas, and results in down-regulation of transmembrane Fast. Synthetic metalloproteinase inhibitors may modify the ratio of transmembrane to soluble FasL.