Effective activation of the proenzyme form of the urokinase‐type plasminogen activator (pro‐uPA) by the cysteine protease cathepsin L

Increased levels of both the cysteine protease, cathepsin L, and the serine protease, uPA (urokinase‐type plasminogen activator), are present in solid tumors and are correlated with malignancy. uPA is released by tumor cells as an inactive single‐chain proenzyme (pro‐uPA) which has to be activated by proteolytic cleavage. We analyzed in detail the action of the cysteine protease, cathepsin L, on recombinant human pro‐uPA. Enzymatic assays, SDS‐PAGE and Western blot analysis revealed that cathepsin L is a potent activator of pro‐uPA. As determined by N‐terminal amino acid sequence analysis, activation of pro‐uPA by cathepsin L is achieved by cleavage or the Lys 158 ‐lle 159 peptide bond, a common activation site of serine proteases such as plasmin and kallikrein. Similar to cathepsin B (Kobayashi et al., J. Biol. Chem. (1991) 266, 5147‐5152) cleavage of pro‐uPA by cathepsin L was most effective at acidic pH (molar ratio of cathepsin L to pro‐uPA of 1:2,000). Nevertheless, even at pH 7.0, pro‐uPA was activated by cathepsin L, although a 10‐fold higher concentration of cathepsin L was required. As tumor cells may produce both pro‐uPA and cathepsin L, implications for the activation of tumor cell‐derived pro‐uPA by cathepsin L may be considered. Different pathways activation of pro‐uPA in tumor tissues may coexist: (i) autocatalytic intrinsic activation of pro‐uPA; (ii) activation by serine proteases (plasmin, kallikrein. Factor XIIa); and (iii) activation by cysteine proteases (cathepsin B and L).