SYNTHESIS AND SECRETION OF PROCATHEPSIN-B AND CYSTATIN-C BY HUMAN BRONCHIAL EPITHELIAL-CELLS IN-VITRO - MODULATION OF CATHEPSIN-B ACTIVITY BY NEUTROPHIL ELASTASE

Procathepsin B and cystatin C are found in human lung secretions. We i nvestigated the capacity of human bronchial epithelial cells to synthe size and secrete these proteins. Immunoprecipitation of [S-35]methioni ne-labeled proteins from cultured bronchial epithelial cell lysates, f ollowed by denaturing gel electrophoresis and autoradiography, showed the presence of newly synthesized procathepsin B of M(r) 42,000; no ma ture form was detected. Cathepsin B in conditioned medium from epithel ial cells was tagged with nzyloxycarbonyl-I-125-tyrosyl-alanine-diazom ethane before and after treatment of the medium with neutrophil elasta se. Control medium again showed a predominant form of cathepsin B with a M(r) of 42,000, but upon treatment with neutrophil elastase this pr otein was converted to a M(r) of 38,000, similar to the active form pr eviously found in lung secretions, and cathepsin B activity was genera ted. The medium also contained the cathepsin B inhibitor, cystatin C, but cystatins A, B, S, SN, SA, and kininogen were not detected. After removal of cystatin C from the medium, elastase was still required to activate procathepsin B. These results suggest that bronchial epitheli al cells are a source of procathepsin B and cystatin C in lung secreti ons. Cleavage both of cystatin C and procathepsin B by neutrophil elas tase is essential for the generation of cathepsin B activity in the me dium. (C) 1995 Academic Press, Inc.