Protease-catalyzed conversion of insulin-like growth factor-1 and interleukin-6 into high-molecular-mass species through the sequential action of hematopoietic surface-associated cathepsin G and gamma-glutamyl transpeptidase-related activities.

Interleukin-6 (IL-6) and insulin-like growth-factor-1 (IGF-1) are cytokines produced by a variety of cells that act on a wide range of tissues, influencing cell growth and differentiation. Purified plasma membranes from human U937 monoblastic cells produced in vitro dimeric species of IL-6- and IGF-1-derived peptides through the sequential actions of surface-associated enzymes cathepsin G and transpeptidase activities. Cathepsin G degraded native unglycosylated IL-6 and IGF-1 molecules into 8-kDa and 7-kDa peptides respectively. Subsequent dimerisation of these intermediate forms into 16-kDa IL-6- and 14-kDa IGF-1-derived peptides was inhibited by acivicin and glutathione which are specific inhibitors of the standard cell-surface gamma-glutamyl transpeptidase (gamma-GT). However U937 plasma membranes, cleared of gamma-GT activity by immunoprecipitation with anti-gamma-GT and adsorption on protein-G-Sepharose, were still able to convert the intermediate forms of IL-6 and IGF-1 into dimers. Together, these observations indicate that the transpeptidase involved in the formation of the dimeric species of IL-6 and IGF-1 was related to, but distinct from, standard cell-surface gamma-GT. Cells of all hematopoietic lineages expressed gamma-GT-related activity. In contrast to the 16-kDa IL-6-derived peptide that did not retain growth-stimulating activity, the 14-kDa IGF-1 peptide was at least equipotent with native IGF-1 in the BALB/c 3T3 fibroblast DNA synthesis response. The N/O-glycosylated IL-6 was clearly as sensitive to cathepsin-G- and gamma-GT-related activities as the unglycosylated IL-6 from Escherichia coli, thus indicating that the sugar chains did not protect the cleavage sites of the two proteases on the IL-6 molecule. Our in vitro findings raise the possibility that similar proteases participate in the regulation of the catabolism of IL-6 and IGF-1 in vivo.