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Altered protease activity is considered important for tumour invasion and metastasis, processes in which the cysteine proteases cathepsin B and L are involved. Their natural inhibitor cystatin C is a secreted protein, suggesting that it functions to control extracellular protease activity. | ỊFEBS Journal Internalization of cystatin C in human cell lines Ulf Ekstrom1 Hanna Wallin1 Julia Lorenzo2 Bo Holmqvist3 Magnus Abrahamson1 and Francesc X. Aviles2 1 Department of Laboratory Medicine Lund University Sweden 2 Institut de Biotecnologia i de Biomedicina Universidad Autonoma de Barcelona Spain 3 Department of ClinicalSciences Lund University Sweden Keywords cancer cysteine proteases internalization protease inhibitors uptake Correspondence M. Abrahamson Department of Laboratory Medicine Division of ClinicalChemistry and Pharmacology Lund University University Hospital SE-221 85 Lund Sweden Fax 46 46 130064 Tel 46 46 173445 E-mail magnus.abrahamson@med.lu.se Website http www.klinkem.lu.se E abrahamson Received 2 April2008 revised 26 June 2008 accepted 17 July 2008 doi 10.1111 j.1742-4658.2008.06600.x Altered protease activity is considered important for tumour invasion and metastasis processes in which the cysteine proteases cathepsin B and L are involved. Their natural inhibitor cystatin C is a secreted protein suggesting that it functions to control extracellular protease activity. Because cystatins added to cell cultures can inhibit polio herpes simplex and coronavirus replication which are intracellular processes the internalization and intracellular regulation of cysteine proteases by cystatin C should be considered. The extension mechanism and biological importance of this hypothetical process are unknown. We investigated whether internalization of cystatin C occurs in a set of human cell lines. Demonstrated by flow cytometry and confocal microscopy A-431 MCF-7 MDA-MB-453 MDA-MB-468 and Capan-1 cells internalized fluorophore-conjugated cystatin C when exposed to physiological concentrations 1 pM . During cystatin C incubation intracellular cystatin C increased after 5 min and accumulated for at least 6 h reaching four to six times the baseline level. Western blotting showed that the internalized inhibitor was not degraded. It was functionally .
