Regulation of Cathpsin L by the Serpin Protein C Inhibitor.

Protein C inhibitor (PCI) is a plasma serine protease inhibitor (serpin) that regulates several serine proteases in coagulation and fibrinolysis including thrombin and activated protein C (APC). The physiological role of PCI, however, remains under investigation since PCI both inhibits and promotes thrombin generation. The cysteine protease, cathepsin L, has been shown to play a role in many physiological processes including cardiovascular disease, atherosclerosis, blood vessel remodeling, and tumor cell invasion. Recently, several serpins have been described to inhibit both serine and cysteine proteases and they are termed “cross-class” inhibitors. The goal of this project was to determine if PCI inhibits cathepsin L activity and if so, does this inhibition process mimic the mechanism of serine proteases. Previous studies have shown that the prototypical serpin, antithrombin (AT), inhibits the cysteine proteases papain and cathepsin L. We found that PCI is a more efficient inhibitor of cathepsin L than AT with an inhibition rate (k₂) of 1.5 × 10⁶ M⁻¹min⁻¹ compared to 5.2 × 10⁴ M⁻¹min⁻¹ for AT. Also, PCI is a more efficient inhibitor of cathepsin L than either thrombin or APC whose inhibition rates are 5.7 × 10⁵ M⁻¹min⁻¹ and 3.4 × 10⁴ M⁻¹min⁻¹, respectively. In contrast to AT, PCI does not inhibit papain. Thrombin inhibition by AT and PCI is accelerated in the presence of glycosaminoglycans such as heparin and heparan sulfate. The inhibition of cathepsin L by PCI is not accelerated in the presence of heparin suggesting either that cathepsin L does not bind heparin or that heparin is not required for accelerated inhibition of cysteine proteases. Interestingly, a reactive site P1 mutant (R354A) of PCI does not inhibit thrombin but does inhibit cathepsin L at rates comparable to wild-type PCI. This implies that the P1 residue of PCI does not determine specificity for inhibition of cathepsin L unlike for thrombin and APC. We believe that the specificity is primarily determined by the hydrophobic Phe residue located at the P2 position since other serpins that inhibit cathepsin L contain either a Phe or Val at the P2 position. Mutating the P14 residue (T341R; mutation in the hinge region) of PCI results in the conversion of PCI from an inhibitor to a substrate. As expected, the PCI-P14 mutant does not inhibit either thrombin or cathepsin L. Another characteristic of the serpin inhibition mechanism is formation of a bi-molecular SDS stable complex. We found that wild-type PCI and PCI-P1 mutant both form a stable complex with cathepsin L under non-reducing conditions. Lastly, the wild-type PCI-cathepsin L interaction has a stoichiometry of inhibition (SI) value of 1.6. This indicates that PCI is an effective and possibly a physiologically relevant inhibitor of cathepsin L. Regulating cathepsin L by serpins like PCI may be a novel and new pathway of regulation of hemostasis-thrombosis, cardiovascular and metastatic diseases.