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 apo ptosis 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 Ew ing'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 i n the cytoplasm and membrane of ESFT cells by immunofluorescence. Western b lotting revealed transmembrane and soluble Fast in cytosolic extracts and s oluble Fast in conditioned media. Both transmembrane and soluble Fast induc ed apoptosis of Fas-sensitive Jurkat cells in co-culture experiments with E SFT cells or their media. Treatment with phenanthroline and the synthetic m etalloproteinase inhibitor BB-3103 reduced the levels of soluble Fast in th e media, suggesting that in ESFT, Fast is processed by a metalloproteinase and released in the extracellular milieu. The released soluble Fast may ser ve to attack cells of the immune system and/or interfere with the binding o f transmembrane Fast with Fas, and results in down-regulation of transmembr ane Fast. Synthetic metalloproteinase inhibitors may modify the ratio of tr ansmembrane to soluble FasL.