Cathepsin B is a prorenin processing enzyme.

Conversion of prorenin to renin results from proteolytic cleavage of a 43-amino-acid prorenin prosegment in renal juxtaglomerular cells. The enzyme that performs this processing is not known. Of several enzymes proposed, cathepsin B is a candidate because it colocalizes with renin in juxtaglomerular cell secretory granules and accurately cleaves the prosegment of human prorenin in vitro. It is not known whether cathepsin B can perform this function in the cell. We examined this using secretory granule-containing rat GH4C1 cells transfected with a human preprorenin expression vector. When treated with secretagogue (KCl 50 mmol/L + forskolin 10 micromol/L), these cells secrete 95% prorenin and 5% active renin into the medium, indicating little prorenin processing activity. In contrast, when the cells are cotransfected with a vector that expresses human preprocathepsin B or mouse prohormone convertase 1, secretagogue-induced secretion of active renin increased to 12% and 16.5%, respectively. With antisera that recognize the prosegment and renin, prorenin and renin were identified as proteins of 47 and 43 kD, respectively, and an antibody specific to the prosegment precipitated only the 47-kD species. These results do not address whether cathepsin B is the authentic renal prorenin processing enzyme. However, the results do demonstrate that cathepsin B can localize to the appropriate subcellular compartment and process prorenin to renin in GH4C1 cells and are consistent with a role for this enzyme in prorenin processing.