Secreted cathepsin L generates endostatin from collagen XVIII

Endostatin, an inhibitor of angiogenesis and tumor growth, was identified originally in conditioned media of murine hemangioendothelioma (EOMA) cells. N-terminal amino acid Sequencing demonstrated that it corresponds to a fragment of basement membrane Collagen XVIII. Here we report that Cathepsin L is secreted by EOMA cells and is responsible for the generation of endostatin with the predicted N-terminus, while metalloproteases produce larger fragments in a parallel processing pathway. Efficient endostatin generation requires a moderately acidic pH similar to the pericellular milieu of tumors. The secretion of Cathepsin L by a tumor cell line of endothelial origin suggests that this Cathepsin may play a role in angiogenesis. We propose that cleavage within Collagen XVIII's protease-sensitive region evolved to regulate excessive proteolysis in conditions of induced angiogenesis.