Characterization and determination of the relative abundance of two types of insulin-like growth factor binding protein-5 proteases that are secreted by human fibroblasts.

We previously reported that human fibroblasts secrete a protease into their conditioned medium that cleaves insulin-like growth factor binding protein-5 (IGFBP-5) into non-IGF-I binding fragments. Because the protease activity in the fibroblast medium has characteristics of both serine and metalloproteases, the activity was purified and analyzed to determine whether it retained serine or metalloprotease properties. The protease was purified by heparin Sepharose affinity chromatography followed by alpha1 antichymotrypsin affinity or gelatin agarose chromatography. The heparin Sepharose purified material degraded IGFBP-5 into 22-, 17-, and 16-kDa fragments. Amino acid sequencing showed that the 22-kDa fragment contained the amino-terminus of the protein. The protease activity in the fibroblast conditioned medium that was purified by heparin Sepharose was inhibited by both serine and metalloprotease inhibitors. To attempt to separate these activities, the heparin Sepharose purified activity was further purified by gelatin agarose chromatography. The IGFBP-5 protease activity that did not bind to gelatin agarose was inhibited by serine protease inhibitors, such as 3,4 dichloroisocoumain (3,4 DCI), whereas tissue inhibitor metalloprotease-1 (TIMP-1) had minimal activity. When this same pool of protease activity that had been eluted from heparin Sepharose was applied to an alpha1 antichymotrypsin peptide affinity column, the protease activity that bound to the column was inhibited by 3,4 dichloroisocoumain, but was not inhibited by TIMP-1. In contrast, the activity that did not adhere to this column was inhibited by TIMP-1. IGFBP-5 zymography showed that the Mr estimate of the protease that was inhibited by serine protease inhibitors was 92 kDa, whereas gelatin zymography showed that the metalloproteases had Mr estimates of 72, 69, and 55 kDa. When the protease activity in the crude conditioned medium was analyzed by zymography, almost all of the detectable protease had an Mr estimate of 92 kDa, suggesting that the metalloproteases that were detected in the partially purified fractions were inactive in the medium. In summary, fibroblasts secrete a 92-kDa protease that cleaves IGFBP-5 into 22-, 17-, and 16-kDa fragments. The protease inhibitor specificity results, chromatographic characteristics, and zymographic analyses suggest that this is a serine protease. Although metalloproteases are secreted by these cells, the 92-kDa serine protease is the predominate form of activity in the conditioned medium that cleaves IGFBP-5.