Elucidation of structural requirements on plasminogen activator inhibitor 1 for binding to heparin

Membrane Phospholipids and Polyphosphates as Cofactors and Binding Molecules of SERPINA12 (vaspin)

Visceral adipose tissue derived serine protease inhibitor (vaspin) is a member of the serpin family and has been shown to have beneficial effects on glucose tolerance, insulin stability as well as adipose tissue inflammation, parameters seriously affected by obesity. Some of these effects require inhibition of target proteases such as kallikrein 7(KLK7) and many studies have demonstrated vaspin-mediated activation of intracellular signaling cascades in various cells and tissues. So far, little is known about the exact mechanism how vaspin may trigger these intracellular signaling events. In this study, we investigated and characterized the interaction of vaspin with membrane lipids and polyphosphates as well as their potential regulatory effects on serpin activity using recombinant vaspin and KLK7 proteins and functional protein variants thereof. Here, we show for the first time that vaspin binds to phospholipids and polyphosphates with varying effects on KLK7 inhibition. Vaspin binds strongly to monophosphorylated phosphatidylinositol phosphates (PtdInsP) with no effect on vaspin activation. Microscale thermophoresis (MST) measurements revealed high-affinity binding to polyphosphate 45 (K_D: 466 ± 75 nM) and activation of vaspin in a heparin-like manner. Furthermore, we identified additional residues in the heparin binding site in β-sheet A by mutating five basic residues resulting in complete loss of high-affinity heparin binding. Finally, using lipid overlay assays, we show that these residues are additionally involved in PtdInsP binding. Phospholipids play a major role in membrane trafficking and signaling whereas polyphosphates are procoagulant and proinflammatory agents. The identification of phospholipids and polyphosphates as binding partners of vaspin will contribute to the understanding of vaspins involvement in membrane trafficking, signaling and beneficial effects associated with obesity.

Basic Residues of β-Sheet A Contribute to Heparin Binding and Activation of Vaspin (Serpin A12)

Many members of the serine protease inhibitor (serpin) family are activated by glycosaminoglycans (GAGs). Visceral adipose tissue-derived serpin (vaspin), serpin A12 of the serpin family, and its target protease kallikrein 7 (KLK7) are heparin-binding proteins, and inhibition of KLK7 by vaspin is accelerated by heparin. However, the nature of GAG binding to vaspin is not known. Here, we measured vaspin binding of various glycosaminoglycans and low molecular weight heparins by microscale thermophoresis and analyzed acceleration of protease inhibition by these molecules. In addition, basic residues contributing to heparin binding and heparin activation were identified by a selective labeling approach. Together, these data show that vaspin binds heparin with high affinity (K_D = 21 ± 2 nm) and that binding takes place at a basic patch on top of β-sheet A and is different from other heparin-binding serpins. Mutation of basic residues decreased heparin binding and activation of vaspin. Similarly, reactive center loop insertion into sheet A decreased heparin binding because it disturbs the basic cluster. Finally, using vaspin-overexpressing keratinocyte cells, we show that a significant part of secreted vaspin is bound in the extracellular matrix on the cell surface. Together, basic residues of central β-sheet A contribute to heparin binding and activation of vaspin. Thus, binding to GAGs in the extracellular matrix can direct and regulate vaspin interaction with target proteases or other proteins and may play an important role in the various beneficial functions of vaspin in different tissues.

Dependence on endocytic receptor binding via a minimal binding motif underlies the differential prognostic profiles of SerpinE1 and SerpinB2 in cancer

Tumor overexpression of urokinase-type plasminogen activator (uPA) and its specific inhibitor SerpinE1 (plasminogen activator inhibitor type-1) correlates with poor prognosis and increased metastatic potential. Conversely, tumor expression of uPA and another specific inhibitor, SerpinB2 (plasminogen activator inhibitor type-2), are associated with favorable outcome and relapse-free survival. It is not known how overexpression of these uPA inhibitors results in such disparate outcomes. A possible explanation may be related to the presence of a proposed low density lipoprotein receptor (LDLR)-binding motif in SerpinE1 responsible for mitogenic signaling via ERK that is absent in SerpinB2. We now show that complementation of such a LDLR-binding motif in SerpinB2 by mutagenesis of two key residues enabled high affinity binding to very LDLR (VLDLR). Furthermore, the VLDLR-binding SerpinB2 form behaved in a manner indistinguishable from SerpinE1 in terms of enhanced uPA-SerpinB2 complex endocytosis and subsequent ERK phosphorylation and cell proliferation; that is, the introduction of the LDLR-binding motif to SerpinB2 was necessary and sufficient to allow it to acquire characteristics of SerpinE1 associated with malignancy. In conclusion, this study defines the structural elements underlying the distinct interactions of SerpinE1 versus SerpinB2 with endocytic receptors and how differential VLDLR binding impacts on downstream cellular behavior. This has clear relevance to understanding the paradoxical disease outcomes associated with overexpression of these serpins in cancer.

A peptide accelerating the conversion of plasminogen activator inhibitor-1 to an inactive latent state

The serpin plasminogen activator inhibitor-1 (PAI-1) is a specific inhibitor of plasminogen activators and a potential therapeutic target in cancer and cardiovascular diseases. Accordingly, formation of a basis for development of specific PAI-1-inactivating agents is of great interest. One possible inactivation mode for PAI-1 is conversion to the inactive, so-called latent state. We have now screened a phage-displayed peptide library with PAI-1 as bait and isolated a 31-residue cysteine-rich peptide that will be referred to as paionin-4. A recombinant protein consisting of paionin-4 fused to domains 1 and 2 of the phage coat protein g3p caused a 2- to 3-fold increase in the rate of spontaneous inactivation of PAI-1. Paionin-4-D1D2 bound PAI-1 with a K(D) in the high nanomolar range. Using several biochemical and biophysical methods, we demonstrate that paionin-4-D1D2-stimulated inactivation consists of an acceleration of conversion to the latent state. As demonstrated by site-directed mutagenesis and competition with other PAI-1 ligands, the binding site for paionin-4 was localized in the loop between alpha-helix D and beta-strand 2A. We also demonstrate that a latency-inducing monoclonal antibody has an overlapping, but not identical binding site, and accelerates latency transition by another mechanism. Our results show that paionin-4 inactivates PAI-1 by a mechanism clearly different from other peptides, small organochemical compounds, or antibodies, whether they cause inactivation by stimulating latency transition or by other mechanisms, and that the loop between alpha-helix D and beta-strand 2A can be a target for PAI-1 inactivation by different types of compounds.

Plasminogen activator inhibitor-1 is an inhibitor of factor VII-activating protease in patients with acute respiratory distress syndrome

Factor VII-activating protease (FSAP) is a novel plasma-derived serine protease structurally homologous to tissue-type and urokinase-type plasminogen activators. We demonstrate that plasminogen activator inhibitor-1 (PAI-1), the predominant inhibitor of tissue-type and urokinase-type plasminogen activators in plasma and tissues, is an inhibitor of FSAP as well. We detected PAI-1·FSAP complexes in addition to high levels of extracellular RNA, an important FSAP cofactor, in bronchoalveolar lavage fluids from patients with acute respiratory distress syndrome. Hydrolytic activity of FSAP was inhibited by PAI-1 with a second-order inhibition rate constant (K_a) of 3.38 ± 1.12 × 10⁵m^–1·s^–1. Residue Arg³⁴⁶ was a critical recognition element on PAI-1 for interaction with FSAP. RNA, but not DNA, fragments (>400 nucleotides in length) dramatically enhanced the reactivity of PAI-1 with FSAP, and 4 μg·ml^–1 RNA increased the K_a to 1.61 ± 0.94 × 10⁶m^–1·s^–1. RNA also stabilized the active conformation of PAI-1, increasing the half-life for spontaneous conversion of active to latent PAI-1 from 48.4 ± 8 min to 114.6 ± 5 min. In contrast, little effect of DNA on PAI-1 stability was apparent. Residues Arg⁷⁶ and Lys⁸⁰ in PAI-1 were key elements mediating binding of nucleic acids to PAI-1. FSAP-driven inhibition of vascular smooth muscle cell proliferation was antagonized by PAI-1, suggesting functional consequences for the FSAP-PAI-1 interaction. These data indicate that extracellular RNA and PAI-1 can regulate FSAP activity, thereby playing a potentially important role in hemostasis and cell functions under various pathophysiological conditions, such as acute respiratory distress syndrome.

Inhibition of plasminogen activator inhibitor-1 binding to endocytosis receptors of the low-density-lipoprotein receptor family by a peptide isolated from a phage display library

The functions of the serpin PAI-1 (plasminogen activator inhibitor-1) are based on molecular interactions with its target proteases uPA and tPA (urokinase-type and tissue-type plasminogen activator respectively), with vitronectin and with endocytosis receptors of the low-density-lipoprotein family. Understanding the significance of these interactions would be facilitated by the ability to block them individually. Using phage display, we have identified the disulfide-constrained peptide motif CFGWC with affinity for natural human PAI-1. The three-dimensional structure of a peptide containing this motif (DVPCFGWCQDA) was determined by liquid-state NMR spectroscopy. A binding site in the so-called flexible joint region of PAI-1 was suggested by molecular modelling and validated through binding studies with various competitors and site-directed mutagenesis of PAI-1. The peptide with an N-terminal biotin inhibited the binding of the uPA-PAI-1 complex to the endocytosis receptors low-density-lipoprotein-receptor-related protein 1A (LRP-1A) and very-low-density-lipoprotein receptor (VLDLR) in vitro and inhibited endocytosis of the uPA-PAI-1 complex in U937 cells. We conclude that the isolated peptide represents a novel approach to pharmacological interference with the functions of PAI-1 based on inhibition of one specific molecular interaction.

Conservation of critical functional domains in murine plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 is the main physiological regulator of tissue-type plasminogen activator in normal plasma. In addition to its critical function in fibrinolysis, plasminogen activator inhibitor-1 has been implicated in roles in other physiological and pathophysiological processes. To investigate structure-function aspects of mouse plasminogen activator inhibitor-1, the recombinant protein was expressed in Escherichia coli and purified. Five variant recombinant murine proteins (R76E, Q123K, R346A, R101A, and Q123K/R101A) were also generated using site-directed mutagenesis. The variant (R346A) was found to be defective in its inhibitory activity against tissue plasminogen activator relative to its wild-type counterpart. Enzyme-linked immunosorbent assay and surface plasmon resonance experiments demonstrated reduced vitronectin-binding affinity of the (Q123K) variant (K(D) = 1800 nm) relative to the wild-type protein (K(D) = 5.4 nm). Kinetic analyses indicated that the (Q123K) variant had a slower association (k(on) = 2.92 x 10(4) m(-1) s(-1)) to, and a faster dissociation from, vitronectin (k(off) = 5.3 x 10(-2) s(-1)), (wild-type k(on) = 1.03 x 10(6) m(-1) s(-1) and k(off) = 5.27 x 10(-3) s(-1)). The Q123K/R101A variant demonstrated an even lower vitronectin-binding ability. Low density lipoprotein receptor-related protein binding was decreased for the (R76E) variant. It was also demonstrated that the plasminogen activator inhibitor-1/vitronectin complex decreased the interaction of plasminogen activator inhibitor-1 with low density lipoprotein receptor-related protein. These results indicate that the complex interactions traditionally associated with different plasminogen activator inhibitor-1 functions apply to the murine system, thus showing a commonality of subtle functions among different species and evolutionary conservation of this protein. Further, this study provides additional evidence that the human hemostasis system can be studied effectively in the mouse, which is a great asset for investigations with gene-altered mice.

Dual-site recognition of different extracellular matrix components by anti-angiogenic/neurotrophic serpin, PEDF

Pigment epithelium-derived factor (PEDF), a member of the serine protease inhibitor (serpin) superfamily, possesses anti-angiogenic and neurotrophic activities. PEDF has been reported to bind to extracellular matrix (ECM) components such as collagens and glycosaminoglycans (GAGs). In this study, to determine the binding sites for collagens and GAGs, we analyzed the interaction of recombinant mouse PEDF (rPEDF) with collagen I and heparin. By utilizing residue-specific chemical modification and site-directed mutagenesis techniques, we revealed that the acidic amino acid residues on PEDF (Asp(255), Asp(257), and Asp(299)) are critical to collagen binding, and three clustered basic amino acid residues (Arg(145), Lys(146), and Arg(148)) are necessary for heparin binding. Mapping of these residues on the crystal structure of human PEDF (Simonovic, M., Gettins, P. G. W., and Volz, K. (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 11131-11135) demonstrated that the collagen-binding site is oriented toward the opposite side of the highly basic surface where the heparin-binding site is localized. These results indicate that PEDF possesses dual binding sites for different ECM components, and this unique localization of ECM-binding sites implies that the binding to ECM components could regulate PEDF activities.

A truncated plasminogen activator inhibitor-1 protein blocks the availability of heparin-binding vascular endothelial growth factor A isoforms

We have made deletions of the porcine plasminogen activator inhibitor-1 (PAI-1) gene to obtain recombinant truncated PAI-1 proteins to examine functions of the PAI-1 isoforms. We previously reported that one recombinant truncated protein, rPAI-1(23), induces the formation of angiostatin by cleaving plasmin. The rPAI-1(23) protein is also able to bind urokinase plasminogen activator and plasminogen and then reduce the amount of plasmin that is formed. We have now prepared three different truncated rPAI-1 proteins and demonstrate that PAI-1 conformations control the release of heparin-binding vascular endothelial growth factor (VEGF) isoforms. The rPAI-1(23) isoform can regulate the functional activity of heparan sulfate-binding VEGF-A isoforms by blocking the activation of VEGF from heparan sulfate. The rPAI-1(23) conformation induced extensive apoptosis in cultured endothelial cells and thus reduced the number of proliferating cells. The rPAI-1(23) isoform inhibited migration of VEGF-stimulated sprouting from chick aortic rings by 65%, thus displaying a role in anti-angiogenic mechanisms. This insight into anti-angiogenic functions related to PAI-1 conformational changes could provide potential intervention points in angiogenesis associated with atherosclerotic plaques and cancer.

Interaction of plasminogen activator inhibitor type-1 (PAI-1) with vitronectin

The serpin plasminogen activator inhibitor type 1 (PAI-1) plays an important role in physiological processes such as thrombolysis and fibrinolysis, as well as pathophysiological processes such as thrombosis, tumor invasion and metastasis. In addition to inhibiting serine proteases, mainly tissue-type (tPA) and urokinase-type (uPA) plasminogen activators, PAI-1 interacts with different components of the extracellular matrix, i.e. fibrin, heparin (Hep) and vitronectin (Vn). PAI-1 binding to Vn facilitates migration and invasion of tumor cells. The most important determinants of the Vn-binding site of PAI-1 appear to reside between amino acids 110-147, which includes alpha helix E (hE, amino acids 109-118). Ten different PAI-1 variants (mostly harboring modifications in hE) as well as wild-type PAI-1, the previously described PAI-1 mutant Q123K, and another serpin, PAI-2, were recombinantly produced in Escherichia coli containing a His(6) tag and purified by affinity chromatography. As shown in microtiter plate-based binding assays, surface plasmon resonance and thrombin inhibition experiments, all of the newly generated mutants which retained inhibitory activity against uPA still bound to Vn. Mutant A114-118, in which all amino-acids at positions 114-118 of PAI-1 were exchanged for alanine, displayed a reduced affinity to Vn as compared to wild-type PAI-1. Mutants lacking inhibitory activity towards uPA did not bind to Vn. Q123K, which inhibits uPA but does not bind to Vn, served as a control. In contrast to other active PAI-1 mutants, the inhibitory properties of A114-118 towards thrombin as well as uPA were significantly reduced in the presence of Hep. Our results demonstrate that the wild-type sequence of the region around hE in PAI-1 is not a prerequisite for binding to Vn.

Acute phase protein alpha 1-acid glycoprotein interacts with plasminogen activator inhibitor type 1 and stabilizes its inhibitory activity

alpha(1)-Acid glycoprotein, one of the major acute phase proteins, was found to interact with plasminogen activator inhibitor type 1 (PAI-1) and to stabilize its inhibitory activity toward plasminogen activators. This conclusion is based on the following observations: (a) alpha(1)-acid glycoprotein was identified to bind PAI-1 by a yeast two-hybrid system. Three of 10 positive clones identified by this method to interact with PAI-1 contained almost the entire sequence of alpha(1)-acid glycoprotein; (b) this protein formed complexes with PAI-1 that could be immunoprecipitated from both the incubation mixtures and blood plasma by specific antibodies to either PAI-1 or alpha(1)-acid glycoprotein. Such complexes could be also detected by a solid phase binding assay; and (c) the real-time bimolecular interactions monitored by surface plasmon resonance indicated that the complex of alpha(1)-acid glycoprotein with PAI-1 is less stable than that formed by vitronectin with PAI-1, but in both cases, the apparent K(D) values were in the range of strong interactions (4.51 + 1.33 and 0.58 + 0.07 nm, respectively). The on rate for binding of PAI-1 to alpha(1)-glycoprotein or vitronectin differed by 2-fold, indicating much faster complex formation by vitronectin than by alpha(1)-acid glycoprotein. On the other hand, dissociation of PAI-1 bound to vitronectin was much slower than that from the alpha(1)-acid glycoprotein, as indicated by 4-fold lower k(off) values. Furthermore, the PAI-1 activity toward urokinase-type plasminogen activator and tissue-type plasminogen activator was significantly prolonged in the presence of alpha(1)-acid glycoprotein. These observations suggest that the complex of PAI-1 with alpha(1)-acid glycoprotein can play a role as an alternative reservoir of the physiologically active form of the inhibitor, particularly during inflammation or other acute phase reactions.

A truncated plasminogen activator inhibitor-1 protein induces and inhibits angiostatin (kringles 1-3), a plasminogen cleavage product

Plasminogen activator inhibitor-1 (PAI-1) is a serpin protease inhibitor that binds plasminogen activators (uPA and tPA) at a reactive center loop located at the carboxyl-terminal amino acid residues 320-351. The loop is stretched across the top of the active PAI-1 protein maintaining the molecule in a rigid conformation. In the latent PAI-1 conformation, the reactive center loop is inserted into one of the beta sheets, thus making the reactive center loop unavailable for interaction with the plasminogen activators. We truncated porcine PAI-1 at the amino and carboxyl termini to eliminate the reactive center loop, part of a heparin binding site, and a vitronectin binding site. The region we maintained corresponds to amino acids 80-265 of mature human PAI-1 containing binding sites for vitronectin, heparin (partial), uPA, tPA, fibrin, thrombin, and the helix F region. The interaction of "inactive" PAI-1, rPAI-1(23), with plasminogen and uPA induces the formation of a proteolytic protein with angiostatin properties. Increasing amounts of rPAI-1(23) inhibit the proteolytic angiostatin fragment. Endothelial cells exposed to exogenous rPAI-1(23) exhibit reduced proliferation, reduced tube formation, and 47% apoptotic cells within 48 h. Transfected endothelial cells secreting rPAI-1(23) have a 30% reduction in proliferation, vastly reduced tube formation, and a 50% reduction in cell migration in the presence of VEGF. These two studies show that rPAI-1(23) interactions with uPA and plasminogen can inhibit plasmin by two mechanisms. In one mechanism, rPAI-1(23) cleaves plasmin to form a proteolytic angiostatin-like protein. In a second mechanism, rPAI-1(23) can bind uPA and/or plasminogen to reduce the number of uPA and plasminogen interactions, hence reducing the amount of plasmin that is produced.

Different structural requirements for plasminogen activator inhibitor 1 (PAI-1) during latency transition and proteinase inhibition as evidenced by phage-displayed hypermutated PAI-1 libraries

Plasminogen activator inhibitor type 1 (PAI-1) is a member of the serine protease inhibitor (serpin) superfamily. Its highly mobile reactive-center loop (RCL) is thought to account for both the rapid inhibition of tissue-type plasminogen activator (t-PA), and the rapid and spontaneous transition of the unstable, active form of PAI-1 into a stable, inactive (latent) conformation (t(1/2) at 37 degrees C, 2.2 hours). We determined the amino acid residues responsible for the inherent instability of PAI-1, to assess whether these properties are independent and, consequently, whether the structural basis for inhibition and latency transition is different. For that purpose, a hypermutated PAI-1 library that is displayed on phage was pre-incubated for increasing periods (20 to 72 hours) at 37 degrees C, prior to a stringent selection for rapid t-PA binding. Accordingly, four rounds of phage-display selection resulted in the isolation of a stable PAI-1 variant (st-44: t(1/2) 450 hours) with 11 amino acid mutations. Backcrossing by DNA shuffling of this stable mutant with wt PAI-1 was performed to eliminate non-contributing mutations. It was shown that the combination of mutations at positions 50, 56, 61, 70, 94, 150, 222, 223, 264 and 331 increases the half-life of PAI-1 245-fold. Furthermore, within the limits of detection the stable mutants isolated are functionally indistinguishable from wild-type PAI-1 with respect to the rate of inhibition of t-PA, cleavage by t-PA, and binding to vitronectin. These stabilizing mutations constitute largely reversions to the stable "serpin consensus sequence" and are located in areas implicated in PAI-1 stability (e.g. the vitronectin-binding domain and the proximal hinge). Collectively, our data provide evidence that the structural requirements for PAI-1 loop insertion during latency transition and target proteinase inhibition can be separated.

Identification of a major heparin-binding site in kallistatin

Kallistatin is a heparin-binding serine proteinase inhibitor (serpin), which specifically inhibits human tissue kallikrein by forming a covalent complex. The inhibitory activity of kallistatin is blocked upon its binding to heparin. In this study we attempted to locate the heparin-binding site of kallistatin using synthetic peptides derived from its surface regions and by site-directed mutagenesis of basic residues in these surface regions. Two synthetic peptides, containing clusters of positive-charged residues, one derived from the F helix and the other from the region encompassing the H helix and C2 sheet of kallistatin, were used to assess their heparin binding activity. Competition assay analysis showed that the peptide derived from the H helix and C2 sheet displayed higher and specific heparin binding activity. The basic residues in both regions were substituted to generate three kallistatin double mutants K187A/K188A (mutations in the F helix) and K307A/R308A and K312A/K313A (mutations in the region between the H helix and C2 sheet), using a kallistatin P1Arg variant as a scaffold. Analysis of these mutants by heparin-affinity chromatography showed that the heparin binding capacity of the variant K187A/K188A was not altered, whereas the binding capacity of K307A/R308A and K312A/K313A mutants was markedly reduced. Titration analysis with heparin showed that the K312A/K313A mutant has the highest dissociation constant. Like kallistatin, the binding activity of K187A/K188A to tissue kallikrein was blocked by heparin, whereas K307A/R308A and K312A/K313A retained significant binding and inhibitory activities in the presence of heparin. These results indicate that the basic residues, particularly Lys(312)-Lys(313), in the region between the H helix and C2 sheet of kallistatin, comprise a major heparin-binding site responsible for its heparin-suppressed tissue kallikrein binding.

A positively charged loop on the surface of kallistatin functions to enhance tissue kallikrein inhibition by acting as a secondary binding site for kallikrein

Kallistatin is a serine proteinase inhibitor (serpin) that specifically inhibits tissue kallikrein. The inhibitory activity of kallistatin is abolished upon heparin binding. The loop between the H helix and C2 sheet of kallistatin containing clusters of basic amino acid residues has been identified as a heparin-binding site. In this study, we investigated the role of the basic residues in this region in tissue kallikrein inhibition. Kallistatin mutants containing double Ala substitutions for these basic residues displayed a 70-80% reduction of association rate constants, indicating the importance of these basic residues in tissue kallikrein inhibition. A synthetic peptide derived from the sequence between the H helix and C2 sheet of kallistatin was shown to suppress the kallistatin-kallikrein interaction through competition for tissue kallikrein binding. To further evaluate the function of this loop, we used alpha1-antitrypsin, a non-heparin-binding serpin and slow tissue kallikrein inhibitor as a scaffold to engineer kallikrein inhibitors. An alpha1-antitrypsin chimera harboring the P3-P2' residues and a sequence homologous to the positively charged region between the H helix and C2 sheet of kallistatin acquired heparin-suppressed inhibitory activity toward tissue kallikrein and exhibited an inhibitory activity 20-fold higher than that of the other chimera, which contained only kallistatin's P3-P2' sequence, and 2300-fold higher than that of wild-type alpha1-antitrypsin. The alpha1-antitrypsin chimera with inhibitory characteristics similar to those of kallistatin demonstrates that the loop between the H helix and C2 sheet of kallistatin is crucial in tissue kallikrein inhibition, and this functional loop can be used as a module to enhance the inhibitory activity of a serpin toward tissue kallikrein. In conclusion, our results indicate that a positively charged loop between the H helix and C2 sheet of a serpin can accelerate the association of a serpin with tissue kallikrein by acting as a secondary binding site.

Structures of active and latent PAI-1: a possible stabilizing role for chloride ions

Serpins exhibit a range of physiological roles and can contribute to certain disease states dependent on their various conformations. Understanding the mechanisms of the large-scale conformational reorganizations of serpins may lead to a better understanding of their roles in various cardiovascular diseases. We have studied the serpin, plasminogen activator inhibitor 1 (PAI-1), in both the active and the latent state and found that anionic halide ions may play a role in the active-to-latent structural transition. Crystallographic analysis of a stable mutant form of active PAI-1 identified an anion-binding site between the central beta-sheet and a small surface domain. A chloride ion was modeled in this site, and its identity was confirmed by soaking crystals in a bromide-containing solution and calculating a crystallographic difference map. The anion thus located forms a 4-fold ligated linchpin that tethers the surface domain to the central beta-sheet into which the reactive center loop must insert during the active-to-latent transition. Timecourse experiments measuring active PAI-1 stability in the presence of various halide ions showed a clear trend for stabilization of the active form with F(-) > Cl(-) > Br(-) >> I(-). We propose that the "stickiness" of this pin (i.e., the electronegativity of the anion) contributes to the energetics of the active-to-latent transition in the PAI-1 serpin.

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.

Epitope mapping of monoclonal antibodies directed to PAI-1 using PAI-1/PAI-2 chimera and PAI-1-derived synthetic peptides

Plasminogen activator inhibitor type-1 is a key regulatory protein of the fibrinolytic system that is involved in a variety of physiological and pathophysiological processes. A panel of 14 monoclonal antibodies directed against plasminogen activator inhibitor type-1 was analyzed regarding epitope specificity on plasminogen activator inhibitor type-1. For this purpose, chimera consisting of plasminogen activator inhibitor type-1 and another plasminogen activator inhibitor, plasminogen activator inhibitor type-2, with different portions of the respective wild-type proteins, were generated and plasminogen activator inhibitor type-1-derived 20-mer and 10-mer linear peptides were synthesized. Nine of the 14 monoclonal antibodies recognized an epitope located in the region between amino acid 76-188 of plasminogen activator inhibitor type-1, which encompasses the binding sites for vitronectin, heparin, and part of the fibrin binding region. Of these nine monoclonal antibodies, six reacted with a quadruple plasminogen activator inhibitor type-1 mutant (N152H, K156T, Q321L, M356I), and seven detected a plasminogen activator inhibitor type-1 deletion mutant (DeltaF111-H114). Two of the remaining five monoclonal antibodies recognized epitopes located between amino acid 209-227 and amino acid 352-371, respectively, while the other three antibodies reacted with wild-type plasminogen activator inhibitor type-1, only. Additional experiments revealed that two of the 14 mAbs neutralized and one monoclonal antibodies increased plasminogen activator inhibitor type-1 activity toward urokinase-type plasminogen activator, one of its target proteases.

The binding of unfractionated heparin and low molecular weight heparin to thrombin-activated human endothelial cells

The binding of unfractionated heparin to endothelium is thought to be responsible for the rapid and saturable phase of unfractionated heparin clearance. Thrombin can induce endothelial cells to express and/or secrete a number of heparin binding proteins that have the potential to increase the binding of unfractionated heparin and to a lesser extent the binding of low molecular weight heparin. To explore this possibility, we examined the binding of unfractionated heparin and low molecular weight heparin to thrombin-activated endothelial cells. Cultured human umbilical vein endothelial cells were used to determine the binding of 125I-labeled unfractionated heparin and low molecular weight heparin to untreated and to thrombin-activated cells. After thrombin treatment, we obtained a time-dependent increase in the binding of radio-labeled unfractionated heparin. In contrast, there was much less binding of low molecular weight heparin, and a time-dependent increase was not apparent. After 30, 45, and 60 minutes of thrombin treatment, the binding of unfractionated heparin was significantly higher than that of low molecular weight heparin. The increase in binding of unfractionated heparin to thrombin-activated cells also was demonstrated using fluorescently labeled unfractionated heparin followed by fluorescence microscopy. The average fluorescence intensity of thrombin-treated cells increased by 44% when compared with resting cells. The present results indicate that thrombin can increase the binding of unfractionated heparin to human umbilical vein endothelial cells. Thus, an activated endothelium may contribute to the variability of the anticoagulant response to unfractionated heparin. In contrast, the binding of low molecular weight heparin is much less affected, which may account for its better bioavailability and longer half-life.

Role of exosites 1 and 2 in thrombin reaction with plasminogen activator inhibitor-1 in the absence and presence of cofactors

The cofactors heparin, vitronectin (VN), and thrombomodulin (TM) modulate the reactivity of alpha-thrombin with plasminogen activator inhibitor (PAI-1). While heparin and VN accelerate the reaction by approximately 2 orders of magnitude, TM protects alpha-thrombin from rapid inactivation by PAI-1 in the presence of VN. To understand how these cofactors function, we studied the kinetics of PAI-1 inactivation of alpha-thrombin, the exosite 1 variant gamma-thrombin, the exosite 2 mutant R93,97,101A thrombin, and recombinant meizothrombin in both the absence and presence of these cofactors. Heparin and VN accelerated the second-order association rate constant [k(2) = (7.9 +/- 0.5) x 10(2) M(-)(1) s(-)(1)] of alpha-thrombin with PAI-1 approximately 200- and approximately 240-fold, respectively. The k(2) value for gamma-thrombin [(7.9 +/- 0.7) x 10(1) M(-)(1) s(-)(1)] was impaired 10-fold, but was enhanced by heparin and VN approximately 280- and approximately 75-fold, respectively. Similar to inactivation of gamma-thrombin, PAI-1 inactivation of alpha-thrombin in complex with the epidermal growth factor-like domains 4-6 of TM (TM4-6) was impaired approximately 10-fold. The exosite 2 mutant R93,97,101A thrombin, which was previously shown not to bind heparin, and meizothrombin, in which exosite 2 is masked, reacted with PAI-1 at similar rates in both the absence and presence of heparin [k(2) = (1.3-1.5) x 10(3) M(-)(1) s(-)(1) for R93,97,101A thrombin and k(2) = (3.6-5.1) x 10(2) M(-)(1) s(-)(1) for meizothrombin]. Unlike heparin, however, VN enhanced the k(2) of R93,97,101A thrombin and meizothrombin inactivation approximately 80- and approximately 30-fold, respectively. Continuous kinetic analysis as well as competition kinetic studies in the presence of S195A thrombin suggested that the accelerating effect of VN or heparin occurs primarily by lowering the dissociation constant (K(d)) for formation of a noncovalent, Michaelis-type complex. Analysis of these results suggest that (1) heparin binds to exosite 2 of alpha-thrombin to accelerate the reaction by a template mechanism, (2) VN accelerates PAI-1 inactivation of alpha-thrombin by lowering the K(d) for initial complex formation by an unknown mechanism that does not require binding to either exosite 1 or exosite 2 of alpha-thrombin, (3) alpha-thrombin may have a binding site for PAI-1 within or near exosite 1, and (4) TM occupancy of exosite 1 partially accounts for the protection of thrombin from rapid inactivation by PAI-1 in the presence of vitronectin.

Regulation of cell adhesion by PAI-1

Type I plasminogen activator inhibitor (PAI-1) is the primary inhibitor of tissue- and urokinase-type plasminogen activators. It circulates in plasma complexed with vitronectin (VN), the primary PAI-1 binding protein. The somatomedin B (SMB) domain of VN contains both the high affinity PAI-1 binding site and the specific site for urokinase plasminogen activator receptor (uPAR). PAI-1 is able to regulate uPAR-mediated cell adhesion by competing with uPAR for VN binding. Binding of PAI-1 to SMD may also affect integrin-mediated cell adhesion to VN by hindering integrin binding to the RGD sequence adjacent to the uPAR binding site.

Role of blood clot formation on early edema development after experimental intracerebral hemorrhage

BACKGROUND AND PURPOSE

Blood "toxicity" is hypothesized to induce edema and brain tissue injury following intracerebral hemorrhage (ICH). Lobar ICH in pigs produces rapidly developing, marked perihematomal edema (>10% increase in water content) associated with clot-derived plasma protein accumulation. Coagulation cascade activation and, specifically, thrombin itself contribute to edema development during the first 24 hours after gray matter ICH in rats. In the present study, we sought to determine whether blood clot formation is necessary for edema development by comparing intracerebral infusions of heparinized and unheparinized blood in pig (white matter) and in rat (gray matter). We also examined heparin's effect on thrombin-induced gray matter edema.

METHODS

In pigs, we infused autologous blood (with or without heparin) into the cerebral white matter to produce lobar hematomas and froze the brains in situ at 1, 4, or 24 hours after ICH. We determined hematomal and perihematomal edema volumes on coronal sections by computer-assisted morphometry. In rats, we infused either blood or thrombin (with or without heparin) into the basal ganglia and measured water, sodium, and potassium contents at 24 hours after ICH.

RESULTS

In pigs, unheparinized blood induced rapid (at 1 hour) and prolonged (24 hours) perihematomal edema (average volume, 1.29+/-0. 20 mL; n=6). No perihematomal edema was present following heparinized blood infusions (n=6). In rats, unheparinized blood produced significantly greater edema than heparinized blood infusions. As with whole blood, thrombin-induced gray matter edema at 24 hours was significantly reduced by coinjection of heparin.

CONCLUSIONS

After ICH, blood clot formation is required for rapid and prolonged edema development in perihematomal white and gray matter. Thrombin also contributes to prolonged edema in gray matter.

Sequence and expression analysis of bovine pigment epithelium-derived factor

PEDF, a member of the serpin superfamily of proteins related through their highly conserved folded conformation, has neurotrophic properties, including promotion of neurite-outgrowth and neuronal survival. Previously, we have purified and characterized PEDF protein from extracellular matrixes of bovine eyes. Here, we show the cDNA sequence and expression analysis of bovine PEDF. Northern analysis of RNA from bovine retinal pigment epithelium (RPE) and neural retina using a human PEDF cDNA fragment reveals expression of the PEDF gene only for RPE. Sequence analysis of a cDNA clone isolated from bovine RPE predicts a polypeptide of 416 amino acid residues that shares 88.6% and 85% amino acid identity with human and mouse PEDF, respectively. It has an N-terminal signal peptide, a consensus glycosylation site and homology with serpins including the conserved residues required for maintaining the serpin tertiary structure. Cell-free expression of the bovine PEDF cDNA by in vitro transcription and translation yields a precursor polypeptide of 45,000-Mr that immunoprecipitates with an antibody to human PEDF. Expression analysis in stably transfected baby hamster kidney cells shows that the recombinant bovine protein is secreted to the culture media as a mature 50,000-Mr protein, which induces neurite-outgrowth on retinoblastoma cells, like the naturally-occurring PEDF protein. Thus, the bovine PEDF cDNA isolated here codes for a functional soluble secreted PEDF glycoprotein.

Specificity of serine proteinase/serpin complex binding to very-low-density lipoprotein receptor and alpha2-macroglobulin receptor/low-density-lipoprotein-receptor-related protein

Very-low-density lipoprotein receptor (VLDLR) and alpha2-macroglobulin receptor/low-density-lipoprotein-receptor-related protein (alpha2MR/LRP) are multifunctional endocytosis receptors of the low-density lipoprotein receptor family. Both have been shown to mediate endocytosis and degradation of complex between plasminogen activators and type-1 plasminogen-activator inhibitor (PAI-1) by cultured cells. We have now studied the specificity of binding and endocytosis by VLDLR and alpha2MR/LRP among a variety of serine proteinase/serpin complexes, including various combinations of the serine proteinases urokinase-type and tissue-type plasminogen activators, plasmin, thrombin, human leukocyte elastase, cathepsin G, and plasma kallikrein with the serpins PAI-1, horse leukocyte elastase inhibitor, protein C inhibitor, C1-inhibitor, alpha2-antiplasmin, alpha1-proteinase inhibitor, alpha1-antichymotrypsin, protease nexin-1, heparin cofactor II, and antithrombin III. Binding was estimated with radiolabelled ligands in ligand blotting analysis and microtiter well assays. Endocytosis was estimated by measuring receptor-associated protein (RAP)-sensitive degradation of radiolabelled complexes by Chinese hamster ovary cells transfected with VLDLR cDNA and by COS-1 cells, which have a high endogenous expression of alpha2MR/LRP. We found that the receptors bind with high affinity to some, but not all, combinations of plasminogen activators and thrombin with PAI-1, protease nexin-1, protein C inhibitor, and antithrombin III, while complexes of many serine proteinases with their primary inhibitor, i.e. plasmin/alpha2-antiplasmin complex, do not bind, or bind with a very low affinity. Both the serine proteinase and the serpin moieties contribute to the binding specificity. The binding specificities of VLDLR and alpha2MR/LRP are overlapping, but not identical. The results suggest that VLDLR and alpha2MR/LRP have different biological functions by having different binding specificities as well as by being expressed by different cell types.

Identification of the antithrombin III heparin binding site

The heparin binding site of the anticoagulant protein antithrombin III (ATIII) has been defined at high resolution by alanine scanning mutagenesis of 17 basic residues previously thought to interact with the cofactor based on chemical modification experiments, analysis of naturally occurring dysfunctional antithrombins, and proximity to helix D. The baculovirus expression system employed for this study produces antithrombin which is highly similar to plasma ATIII in its inhibition of thrombin and factor Xa and which resembles the naturally occurring beta-ATIII isoform in its interactions with high affinity heparin and pentasaccharide (Ersdal-Badju, E., Lu, A., Peng, X., Picard, V., Zendehrouh, P., Turk, B., Björk, I., Olson, S. T., and Bock, S. C. (1995) Biochem. J. 310, 323-330). Relative heparin affinities of basic-to-Ala substitution mutants were determined by NaCl gradient elution from heparin columns. The data show that only a subset of the previously implicated basic residues are critical for binding to heparin. The key heparin binding residues, Lys-11, Arg-13, Arg-24, Arg-47, Lys-125, Arg-129, and Arg-145, line a 50-A long channel on the surface of ATIII. Comparisons of binding residue positions in the structure of P14-inserted ATIII and models of native antithrombin, derived from the structures of native ovalbumin and native antichymotrypsin, suggest that heparin may activate antithrombin by breaking salt bridges that stabilize its native conformation. Specifically, heparin release of intramolecular helix D-sheet B salt bridges may facilitate s123AhDEF movement and generation of an activated species that is conformationally primed for reactive loop uptake by central beta-sheet A and for inhibitory complex formation. In addition to providing a structural explanation for the conformational change observed upon heparin binding to antithrombin III, differences in the affinities of native, heparin-bound, complexed, and cleaved ATIII molecules for heparin can be explained based on the identified binding site and suggest why heparin functions catalytically and is released from antithrombin upon inhibitory complex formation.

Rational design of complex formation between plasminogen activator inhibitor-1 and its target proteinases

Considerable progress in understanding the mechanism of inhibition of proteinases by serpins has been obtained from different biochemical studies. These studies reveal that stable serpin/proteinase complex formation involves insertion of the reactive-site loop of the serpin and occurs at the acyl-enzyme stage. Even though no three-dimensional structure of a serpin/proteinase complex is resolved, structural information is available on some of the individual compounds. Molecular modeling techniques combined with recently acquired biochemical/biophysical data were used to provide insight into the stable complex formation between plasminogen activator inhibitor-1 (PAI-1) and the target proteinases: tissue-type plasminogen activator, urokinase-type plasminogen activator, and thrombin. This study reveals that PAI-1 initially interacts with its target proteinase when its reactive-site loop is solvent exposed and thereby accessible for the proteinase. Stable complex formation, however, involves the insertion of the reactive-site loop up to P7 and results in a tight binding geometry between PAI-1 and its target proteinase. The influence of different biologically relevant molecules on PAI-1/proteinase complex formation and the differences in inhibition rate constants observed for the different proteinases can be explained from these models.

The inhibition of human factor Xa by plasminogen activator inhibitor type 1 in the presence of calcium ion, and its enhancement by heparin and vitronectin

Plasminogen activator inhibitor type 1 (PAI-1), a member of serine proteinase inhibitor superfamily, is known to inhibit thrombin in the presence of either heparin or vitronectin. We analyzed possible inhibitory activity of PAI-1 on human factor Xa. PAI-1 inhibited factor Xa in the presence of calcium ion (Ca2+), whereas no inhibition was observed in the absence of Ca2+. Half maximal enhancement by Ca2+ was obtained at 0.8 mM. An equimolar complex formation between factor Xa and PAI-1 in the presence of Ca2+ was observed by SDS polyacrylamide gel electrophoresis. Both unfractionated heparin and vitronectin enhanced the inhibition only in the presence of Ca2+. Apparent second-order rate constant (ki) for the inhibition of factor Xa by PAI-1 at 5 mM Ca2+ was 1.6 x 10(4) M-1 s-1, and was enhanced 3-fold by 2 u/ml of heparin (4.6 x 10(4) M-1 s-1) and 10-fold by 100 nM vitronectin (1.6 x 10(5) M-1 s-1), respectively. The interaction between Ca(2+)-bound factor Xa and PAI-1 could be important from the view of PAI-1 neutralization and enhancement of fibrinolysis.

Type 1 plasminogen activator inhibitor induces multimerization of plasma vitronectin. A suggested mechanism for the generation of the tissue form of vitronectin in vivo

The conformation and degree of multimerization of vitronectin (Vn) appears to be of critical importance for its functions, but little is known about the underlying mechanisms that control Vn multimerization. We report that Vn secreted by cultured hepatoma cells is present as a mixture of monomeric and multimeric forms. A single protein of Mr 45,000 co-purified with hepatoma cell-derived Vn, which was immunologically identified as type 1 plasminogen activator inhibitor (PAI-1). The possibility that PAI-1 may modulate Vn multimerization was investigated. The addition of active PAI-1 to unfractionated plasma containing Vn monomers resulted in the formation of covalently and noncovalently associated Vn multimers and expression of conformationally sensitive epitopes. In contrast, inactive forms of PAI-1 did not efficiently induce Vn multimerization and conformational change. Gel filtration analysis revealed that Vn remained multimeric after dissociation from PAI-1. Vn multimers were also assembled using purified monomeric Vn and PAI-1, suggesting that a plasma cofactor was not required to induce Vn multimerization. This study provides insights into physiological mechanism responsible for the generation of homomultimeric Vn, a multimeric form of Vn that is not in complex with other proteins and which expresses a functional repertoire distinct from that of plasma Vn.

Selective screening of a large phage display library of plasminogen activator inhibitor 1 mutants to localize interaction sites with either thrombin or the variable region 1 of tissue-type plasminogen activator

Phage display technology has been exploited to study in detail the interaction between plasminogen activator inhibitor 1 (PAI-1) and either thrombin or an essential positively charged "loop" of tissue-type plasminogen activator (t-PA), denoted variable region 1 (VR1). For this purpose, a PAI-1 mutant phage library was used that served as a reservoir of PAI-1 proteins potentially deficient in the interaction with either VR1 or thrombin. A stringent two-step selection procedure was developed. (i) A negative selection was performed by incubating the pComb3/PAI-1 mutant library with an excess of a thrombin mutant with its VR1 domain substituted with that of t-PA (thrombin-VR1). (ii) The remaining phages were complexed with t-PA (positive selection) and selected by panning with an immobilized anti-t-PA monoclonal antibody. Four consecutive panning rounds yielded an enrichment of pComb3/PAI-1 mutant phages of approximately 50-fold. Sequence analysis of 16 different cDNAs, encoding PAI-1 mutants that are hampered in the binding to thrombin-VR1, revealed the following mutations. Four independent variants share a mutation of the P4' residue (Glu350 --> Lys). Nine independent PAI-1 variants share a substitution of P1' (Met347 --> Lys), whereas three others share a P2 substitution (Ala345 --> Asp). Kinetic analysis of representative PAI-1 mutants provides evidence that the P4' residue is essential for the interaction with the VR1 domain, consistent with the data of Madison et al. (Madison, E.L., Goldsmith, E.J., Gething, M.J., Sambrook, J.F., and Gerard, R.D. (1990) J. Biol. Chem. 265, 21423-21426), whereas the P1' and P2 residues confer thrombin specificity. Concordant with the design of the selection procedure, mutants were obtained that inhibit thrombin-VR1 at least 100-fold slower than wild-type PAI-1, identifying residues that are central to the interaction with either thrombin or VR1. This study demonstrates that phage technology can be used to analyze large numbers of mutants defective in their interaction with other (domains of) proteins, provided an adequate selection scheme is devised.

Substitution of specific amino acids in insulin-like growth factor (IGF) binding protein 5 alters heparin binding and its change in affinity for IGF-I response to heparin

Heparin binding to insulin-like growth factor (IGF)-binding protein 5 (IGFBP-5) leads to a 17-fold decrease in its affinity for IGF-I, and a region that contains several basic amino acids (Arg201-Arg218) may be involved in this affinity shift. In the present study, mutagenesis was used to analyze the effect of substitutions for basic amino acids in the Arg201-Arg218 region of IGFBP-5 on heparin-binding and the heparin-induced affinity shift. Nine mutant forms were prepared. Their association constants (Ka) for IGF-I were similar to native IGFBP-5. When 10 microg/ml of heparin was added, the Ka of native IGFBP-5 decreased 17-fold, and the Ka of the K134A/R136A mutant decreased 16-fold. In contrast, substitutions for specific basic amino acids in the Arg2O1-Arg218 region decrease the affinity shift to 1.1-3.2-fold. Lys 211 was especially important. When a mutant containing that single substitution was tested, heparin caused only a 2.5-fold reduction in IGF-I affinity. Affinity cross-linking studies showed that heparin was equipotent in inhibiting the formation of 125I-IGF-I-K134A/Rl36A mutant complexes compared to native IGFBP-5. In contrast, heparin had minimal effects on the formation of complexes between 125I-IGF-I and the other mutants. The heparin-binding activity of each mutant was determined. Four mutants, R201A/K202N, K202A/K206A/R207A, R201A/K202N/K206N/K208N, and K211N/R214A/K217A/R218A, had reduced heparin binding compared to native IGFBP-5. The other five mutants, including the K21IN mutant, showed no change in heparin binding. The four mutants with reduced heparin binding could be dissociated from heparin-Sepharose with much lower NaCl concentrations, indicating that they had reduced affinity. These findings suggest that Arg201 Lys202, LysS206, and Arg214 are important for heparin binding. In contrast, LyS211 is not important for the binding of IGFBP-5 to heparin, but substitution for it reduced the heparin-induced affinity shift.

The urokinase/urokinase-receptor system and cancer invasion

u-PA binds with high affinity to its specific GPI-anchored receptor on the cell surface. The binding has at least two important consequences: (1) it enhances the rate of plasminogen activation on the cell surface; and (2) it focuses the u-PA proteolytic activity at the leading front of migrating cells. Several recent findings suggest that surface-bound u-PA is essential for the invasive ability of tumour cells, even if a picture is emerging indicating a concerted action with other proteases, like collagenases and cathepsin B (Kobayashi et al, 1992; Ossowski, 1992; Schmitt et al, 1992; (Danø et al, 1994). In some tumours, e.g. colon, breast and skin cancer, in situ hybridization studies have given an insight into the u-PA/u-PAR tumour biology showing a complex interplay between stromal and cancer cells Danø et al, 1994). u-PA, u-PAR, and PAI-1 tumour content are now well established prognostic factor in breast cancer. This body of knowledge could be used for theurapeutic purposes. For example, a large study with 671 patients has allowed the identification of node-negative patients which, according to their u-PA levels, would need adjuvant therapy (Foekens et al, 1992). Many other tumours, especially colorectal cancer, expect a direct clinical evaluation of u-PA, u-PAR and serpins as prognostic factors.

Identification of tissue-type plasminogen activator-specific plasminogen activator inhibitor-1 mutants. Evidence that second sites of interaction contribute to target specificity

Plasminogen activator inhibitor-1 (PAI-1) is the primary inhibitor of the plasminogen activators (PAs), tissue-type plasminogen activator (tPA), and urokinase-type plasminogen activator (uPA). A library of PAI-1 mutants containing substitutions at the P1 and P1' positions was screened for functional activity against tPA and thrombin. Several PAI-1 variants that were inactive against uPA in a previous study (Sherman, P. M., Lawrence, D. A., Yang, A. Y., Vandenberg, E. T., Paielli, D., Olson, S. T., Shore, J. D., and Ginsburg, D. (1992) J. Biol. Chem. 267, 7588-7595) had significant inhibitory activity toward tPA. This set of tPA-specific PAI-1 mutants contained a wide range of amino acid substitutions at P1 including Asn, Gln, His, Ser, Thr, Leu, Met, and all the aromatic amino acids. This group of mutants also demonstrated a spectrum of substitutions at P1'. Kinetic analyses of selected variants identified P1Tyr and P1His as the most efficient tPA-specific inhibitors, with second-order rate constants (ki) of 4.0 x 10(5) M-1s-1 and 3.6 x 10(5) M-1s-1, respectively. Additional PA-specific PAI-1 variants containing substitutions at P3 through P1' were constructed. P3Tyr-P2Ser-P1Lys-P1'Trp and P3Tyr-P2Ser-P1Tyr-P1'Met had ki values of 1.7 x 10(6) M-1s-1 and 2.5 x 10(6) M-1s-1 against tPA, respectively, but both were inactive against uPA. In contrast, P2Arg-P1Lys-P1'Ala inhibited uPA 74-fold more rapidly than tPA. The mutant PAI-1 library was also screened for inhibitory activity toward thrombin in the presence and absence of the cofactor heparin. While wild-type PAI-1 and several P1Arg variants inhibited thrombin in the absence of heparin, a number of variants were thrombin inhibitors only in the presence of heparin. These results demonstrate the importance of the reactive center residues in determining PAI-1 target specificity and suggest that second sites of interaction between inhibitors and proteases can also contribute to target specificity. Finally, the PA-specific mutants described here should provide novel reagents for dissecting the physiological role of PAI-1 both in vitro and in vivo.

Plasminogen activation by human keratinocytes: molecular pathways and cell-biological consequences

Keratinocytes are the major cellular constituent of stratified epithelia. Defects in these epithelia are re-epithelialized by keratinocytes migrating from the edge of the defect into the wound. The cells form a monolayer with subsequent differentiation into a multilayered epithelium. It is thought that plasminogen activation by migrating keratinocytes is an important event during re-epithelialization. In the present report we summarize the studies on plasminogen activation by human keratinocytes in vitro and in vivo. Under the aspect of pericellular proteolysis the discussion is focused on the molecular mechanisms of plasminogen activation at the keratinocyte surface and on the cell-biological consequences of pericellular plasmin formation. We describe a cell surface-associated pathway of plasminogen activation which crucially depends on cell surface receptors for (pro)-uPA and plasmin(ogen). uPA bound to its receptor converts cell-bound plasminogen into the active protease plasmin. Compared to plasminogen activation in solution, activation at the keratinocyte cell surface is accelerated by a factor of approx. 7-10, and the plasmin generated and bound at the cell surface is protected against its specific inhibitor alpha 2-antiplasmin. Plasmin thus provided in the pericellular space leads to detachment of cultured keratinocytes from the growth substratum. Plasmin interferes with the adhesion of keratinocytes to fibrin, but not with the adhesion to collagen type I. By demonstrating that keratinocytes of the epithelial outgrowth in healing skin wounds express uPA and the uPA-R and that plasmin(ogen) is colocalized with uPA and/or uPA-R, indirect evidence is provided that this pathway may be operative in vivo. In view of previous findings that plasminogen activation is also observed under certain pathologic conditions in the epidermis, we conclude that plasminogen activation by keratinocytes is rather related to tissue damage and subsequent repair mechanisms than to a specific pathologic situation.

Identification of a novel heparin binding domain in RHAMM and evidence that it modifies HA mediated locomotion of ras-transformed cells

We have previously reported that the hyaluronan (HA) receptor RHAMM (Receptor for HA Mediated Motility) [Turley et al., 1991] contains two HA binding motifs located within a 35 amino acid region of its C-terminus end [Yang et al., 1993] and that HA stimulation of the motility of ras-transformed fibroblasts is mediated via its interaction with RHAMM. Here we show that RHAMM also contains binding sites for heparin (HP) and that interaction of HP with these sites can regulate the locomotion of ras-transformed fibroblasts. At low concentrations (0.01 mg/ml), HP inhibited HA-induced locomotion of ras-transformed cells in a manner independent of RHAMM. At higher, but still physiological concentrations (0.1 mg/ml), HP alone stimulated cell locomotion and this stimulation appeared to be RHAMM-dependent as it was blocked by anti-RHAMM antibodies. Other related glycosaminoglycans such as chondroitin sulfate and dermatin sulfate had no effect on cell motility. In ligand blotting assays, GST-RHAMM fusion protein was shown to bind biotin-labelled HP and this binding was displaceable with unlabelled HP. In similar ligand binding analyses conducted with truncations of RHAMM fusion protein, the HP binding region was found to be localized in the same 35 amino acid segment of RHAMM that contains the two HA binding domains. Synthetic peptides corresponding to these HA binding domains were retained on and bound effectively to an HP-Sepharose affinity column. Fusion proteins generated by linkage of these peptides to the non-HP binding amino terminus of RHAMM conferred HP binding capacity to the genetically engineered proteins. Conversely, deletion of the HA binding domains of RHAMM resulted in fusion proteins devoid of HP binding activity. The relative affinities of RHAMM for HA and HP, as determined by competition and transblot assays as well as quantification of binding at various salt concentrations, indicated that RHAMM had lower affinity for HP than that for HA. These results demonstrate the existence of a new HP binding motif that has biological relevance to cell locomotion.

Determination of the vitronectin binding site on plasminogen activator inhibitor 1 (PAI-1)

Vitronectin is the carrier protein of plasminogen activator inhibitor 1 (PAI-1). We used a well-characterized panel of anti-human PAI-1 monoclonal antibodies (MoAbs) to localize the vitronectin-binding site on PAI-1. By employing a direct vitronectin/PAI-1 binding assay and two vitronectin-dependent inhibition assays, we demonstrate that the anti-PAI-1 MoAbs CLB-5, CLB-10, CLB-2C8 and I1, directed against different epitopes in the region between amino acids 110 and 145, prevent the interaction of PAI-1 with vitronectin. We conclude that the region between amino acids 110 and 145 of PAI-1 harbours an important determinant for the interaction with vitronectin.