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 reni n in juxtaglomerular cell secretory granules and accurately cleaves th e prosegment of human prorenin in vitro. It is not known whether cathe psin B can perform this function in the cell. We examined this using s ecretory granule-containing rat GH4Cl cells transfected with a human p reprorenin expression vector. When treated with secretagogue (KCl 50 m mol/L+forskolin 10 mu mol/L), these cells secrete 95% prorenin and 5% active renin into the medium, indicating little prorenin processing ac tivity. In contrast, when the cells are cotransfected with a vector th at expresses human preprocathepsin B or mouse prohormone convertase 1, secretagogue-induced secretion of active renin increased to 12% and 1 6.5%, respectively. With antisera that recognize the prosegment and re nin, prorenin and renin were identified as proteins of 47 and 43 kD, r espectively, and an antibody specific to the prosegment precipitated o nly the 47-kD species. These results do not address whether cathepsin B is the authentic renal prorenin processing enzyme. However, the resu lts do demonstrate that cathepsin B can localize to the appropriate su bcellular compartment and process prorenin to renin in GH4Cl cells and are consistent with a role for this enzyme in prorenin processing.