Biochemical alteration of membrane-associated IL-6 RI (80-kDa) in adherentmacrophages and vascular endothelium

The potential biochemical mechanisms that mediate the 'shedding' of soluble IL-6 RI (80-kDa) receptor fragments in populations of adherent macrophages were explored. Stimulated macrophages displayed proportional increases in both the expression of membrane-associated IL-6 RI (80-kDa) and the release of soluble receptor fragments. The use of protease inhibitors implicated t hiol/cysteine and carboxyl/aspartate proteases in this process. Cathepsin-D depleted membrane-associated IL-6 RI (80-kDa) complexes and generated solu ble receptor fragments. A carboxyl/aspartate protease from activated macrop hages isolated utilizing pepstatin-A affinity chromatography, was also foun d to affect membrane-associated IL-6 RI (80-kDa) complexes and generate sol uble receptor fragments. Most likely, this fraction corresponded to catheps in-D based upon its origin, pepstatin-A binding avidity, Hb-PAGE zymography , and hydrolysis of an enzyme-specific substrate. We conclude that cathepsi n-D can generate soluble fragments of IL-6 RI (80-kDa) expressed by both ma crophages and vascular endothelium. (C) 2001 Elsevier Science Ltd. All righ ts reserved.