Vitronectin effects on recombinant plasminogen activator inhibitor-1: structural and functional analysis

PAI-1 stability: the role of histidine residues

The role of the 13 histidine residues in plasminogen activator inhibitor 1 (PAI-1) for the stability of the molecule was studied by replacing these residues by threonine, using site-directed mutagenesis. The generated mutants were expressed in Escherichia coli, purified and characterized. All variants had a normal activity and formed stable complexes with tissue-type plasminogen activator. Most of these PAI-1 variants displayed a similar pH-dependency in stability as wild-type PAI-1, with increased half-lives at lower pH. However, the variant His364Thr had a half-life of about 50 min at 37 degrees C and had almost completely lost its pH-dependency. Therefore, our data suggest that His(364), in the COOH-terminal end of the molecule might be responsible for the pH-dependent stability of PAI-1.

The role of His(143) for the pH-dependent stability of plasminogen activator inhibitor-1

Using site-directed mutagenesis, His(143) on the alpha-helix F of PAI-1 was substituted by Lys, Asp, Phe and Thr, respectively. The generated single-site changed plasminogen activator inhibitor-1 (PAI-1) mutants were expressed in Escherichia coli and purified by heparin-Sepharose and anhydrotrypsin agarose chromatographies. When compared with wild-type (wtPAI-1), the PAI-1 mutants His143Asp and His143Phe had shorter half-lives at pH 7.5 (1.1 and 1.4 h, respectively, vs. 2 h for wtPAI-1). They also exhibited less pH dependency of their stability, with half-lives at pH 5.5 of 2.5 and 2.2 h, respectively, vs. 18 h for wtPAI-1. However, the PAI-1 mutants His143Lys and His143Thr had similar properties as wtPAI-1 in this respect. In conclusion, our results suggest that His(143) in one way or another might be involved in the pH-dependent stability of PAI-1. However, it seems that the protonation of this particular residue is of less importance. The PAI-1 mutants His143Asp and His143Phe only displayed about 20% of the specific activity obtained for wtPAI-1, because they, to a large extent, act as substrates for tissue-type plasminogen activator.

Cathepsin D-like aspartyl protease activity mediates the degradation of tissue-type plasminogen activator/plasminogen activator inhibitor-1 complexes in human monocytes

Plasminogen activator inhibitor-1 (PAI-1) is the most important inhibitor of tissue-type plasminogen activator (t-PA) in plasma and plays a major role in the regulation of fibrinolysis. Plasma t-PA/PAI-1 complexes are cleared via a receptor-dependent mechanism in hepatocytes, while the fate of complexes formed in the extracellular matrix and in thrombi is less well understood. In this study, the degradation of t-PA/PAI-1 complexes by monocytes was examined. THP-1 monocytoid cells and freshly isolated human monocytes internalize and degrade [125I]t-PA/PAI-1 complexes at rates of 11.4 +/- 5.9 (mean +/- S.D.) and 44.6 +/- 6.3 ng/10(6) cells/h, respectively. Degradation is blocked by receptor-associated protein (RAP), indicating a member of the low density lipoprotein (LDL) receptor family is involved in the uptake/degradation of t-PA/PAI-1 complexes by monocytes. Degradation of t-PA/PAI-1 complexes is also inhibited by chloroquine and by pepstatin A, suggesting that a lysosomal aspartyl protease is likely involved. SDS-PAGE and Western blotting demonstrated that the purified lysosomal aspartyl protease, cathepsin D, is capable of digesting t-PA (t1/2 15 min), active PAI-1 (t1/2 2 h), and t-PA/PAI-1 complex (t1/2 30 min). Cathepsin D sequentially cleaves PAI-1 after hydrophobic amino acids, yielding lower molecular weight fragments. PAI-1 conformation influences the degradative efficiency of cathepsin D, with vitronectin-bound PAI-1 and latent PAI-1 exhibiting resistance to proteolysis and > 10-fold prolongation in t1/2. These data provide evidence that t-PA/PAI-1 complexes are internalized by human monocytes via a member of the low density lipoprotein (LDL) receptor family, and identifies cathepsin D-like aspartyl protease activity as largely responsible for the degradation of these complexes. Furthermore, vitronectin-bound PAI-1 and latent PAI-1 are relatively resistant to degradation by cathepsin D, which may be of importance in complex physiological environments.