Proteolytically active streptococcal pyrogenic exotoxin B cleaves monocytic cell urokinase receptor and releases an active fragment of the receptor from the cell surface

Urokinase plasminogen activator (u-PA) receptor (u-PAR) is a glycosyl- phosphatidylinositol-anchored membrane protein that promotes pericellular proteolysis and cellular migration. This investigation demonstrates that u- PAR is a substrate for the proteolytically active form of streptococcal pyrogenic exotoxin B (SPE B), a potent virulence factor secreted by Streptococcus pyogenes. Treatment of U937 monocyte-like cells with SPE B decreased specific ¹²⁵I-labeled single-chain u-PA binding by up to 85%. Cysteine proteinase inhibitors neutralized SPE B without affecting the activity of phosphatidylinositol-specific phospholipase C. Due to decreased u-PA binding, SPE B-treated U937 cells expressed decreased activity against a u-PA-specific fluorogenic substrate and plasminogen. SPE B released single- chain u-PA that was noncovalently bound to U937 cells or cross-linked to cellular receptors with bis(sulfo-succinimidyl) suberate. The mass of the released u-PA-receptor complex was 100 kDa. Western blot analysis confirmed that the u-PA receptor that was cleaved by SPE B is u-PAR. After deglycosylation, the mass of SPE B-released u-PAR was 35 kDa, slightly smaller than the phosphatidylinositol-specific phospholipase C-derived form of this receptor. SPE B-released u-PAR retained the ability to bind u-PA, as determined by u-PA affinity chromatography. We conclude that SPE B may inhibit u-PA binding to monocytic cells by at least two mechanisms: (i) by decreasing the level of functional cell surface u-PAR and (ii) by releasing a soluble form of u-PAR that competes with the cellular receptor for ligand.