Cytokeratin 8 released by breast carcinoma cells in vitro binds plasminogen and tissue-type plasminogen activator and promotes plasminogen activation

Cell-surface activation of plasminogen may be important in diseases that involve cellular migration, including atherosclerosis and tumour invasion/metastasis. Cytokeratin 8 (CK 8) has been identified as a plasminogen-binding protein expressed on the external surfaces of hepatocytes and breast carcinoma cells [Hembrough, Vasudevan, Allietta, Glass and Gonias (1995) J. Cell Sci. 108, 1071-1082]. In this investigation, we demonstrate that a soluble form of CK 8 is released into the culture medium of breast cancer cell lines. The released CK 8 is in the form of variably sized polymers that bind plasminogen and promote the activation of [Glu1]plasminogen and [Lys78]plasminogen by single-chain tissue-type plasminogen activator (sct-PA). To assess the mechanism by which CK 8 promotes plasminogen activation, CK 8 was purified from rat hepatocytes and immobilized in microtitre plates. Immobilized CK 8 bound 125I-plasminogen and 125I-sct-PA in a specific and saturable manner. The KDs were 160 +/- 40 nM and 250 +/- 48 nM, respectively. Activation of plasminogen bound to immobilized CK 8 was accelerated compared with plasminogen in solution, as determined using a coupled-substrate fluorescence assay and SDS/PAGE. The ability of CK 8 to promote plasminogen activation may be important in the pericellular spaces surrounding breast cancer cells and at the cell surface.