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

Targeting PAI-1 in Cardiovascular Disease: Structural Insights Into PAI-1 Functionality and Inhibition

Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor (serpin) superfamily with antiprotease activity, is the main physiological inhibitor of tissue-type (tPA) and urokinase-type (uPA) plasminogen activators (PAs). Apart from being crucially involved in fibrinolysis and wound healing, PAI-1 plays a pivotal role in various acute and chronic pathophysiological processes, including cardiovascular disease, tissue fibrosis, cancer, and age-related diseases. In the prospect of treating the broad range of PAI-1-related pathologies, many efforts have been devoted to developing PAI-1 inhibitors. The use of these inhibitors, including low molecular weight molecules, peptides, antibodies, and antibody fragments, in various animal disease models has provided ample evidence of their beneficial effect in vivo and moved forward some of these inhibitors in clinical trials. However, none of these inhibitors is currently approved for therapeutic use in humans, mainly due to selectivity and toxicity issues. Furthermore, the conformational plasticity of PAI-1, which is unique among serpins, poses a real challenge in the identification and development of PAI-1 inhibitors. This review will provide an overview of the structural insights into PAI-1 functionality and modulation thereof and will highlight diverse approaches to inhibit PAI-1 activity.

Remarkable stabilization of plasminogen activator inhibitor 1 in a "molecular sandwich" complex

Plasminogen activator inhibitor 1 (PAI-1) levels are elevated in a number of life-threatening conditions and often correlate with unfavorable outcomes. Spontaneous inactivation due to active to latent transition limits PAI-1 activity in vivo. While endogenous vitronectin (Vn) stabilizes PAI-1 by 1.5-2.0-fold, further stabilization occurs in a "molecular sandwich" complex (MSC) in which a ligand that restricts the exposed reactive center loop is bound to PAI-1/Vn. The effects of S195A two-chain urokinase (tcuPA) and Vn on inactivation of wild-type (wt) glycosylated (Gl-PAI-1), nonglycosylated (rPAI-1), and nonglycosylated Q123K PAI-1 (lacks Vn binding) forms were studied. S195A tcuPA decreased the rate constant (kL) for spontaneous inactivation at 37 °C for rPAI-1, Q123K, and Gl-PAI-1 by 6.7-, 3.4-, and 7.8-fold, respectively, and both S195A tcuPA and Vn by 66.7-, 5.5-, and 103.3-fold, respectively. Analysis of the temperature dependences of kL revealed a synergistic increase in the Gibbs free activation energy for spontaneous inactivation of wt Gl-PAI-1 and rPAI-1 in MSC from 99.8 and 96.1 to 111.3 and 107.0 kJ/mol, respectively, due to an increase in the activation enthalpy and a decrease in the activation entropy. Anti-PAI-1 monoclonal antibodies (mAbs) competing with proteinase also stabilize PAI-1/Vn. The rate of inhibition of target proteinases by MSCs, with a stoichiometry close to unity, was limited by the dissociation (k = 10(-4) to 10(-3) s(-1)) of S195A tcuPA or mAb. The stabilization of PAI-1 in MSCs in vivo may potentiate uncontrolled thrombosis or extravascular fibrin deposition, suggesting a new paradigm for using PAI-1 inhibitors and novel potential targets for therapy.

Characterization of a site on PAI-1 that binds to vitronectin outside of the somatomedin B domain

Vitronectin and plasminogen activator inhibitor-1 (PAI-1) are proteins that interact in the circulatory system and pericellular region to regulate fibrinolysis, cell adhesion, and migration. The interactions between the two proteins have been attributed primarily to binding of the somatomedin B (SMB) domain, which comprises the N-terminal 44 residues of vitronectin, to the flexible joint region of PAI-1, including residues Arg-103, Met-112, and Gln-125 of PAI-1. A strategy for deletion mutagenesis that removes the SMB domain demonstrates that this mutant form of vitronectin retains PAI-1 binding (Schar, C. R., Blouse, G. E., Minor, K. M., and Peterson, C. B. (2008) J. Biol. Chem. 283, 10297-10309). In the current study, the complementary binding site on PAI-1 was mapped by testing for the ability of a battery of PAI-1 mutants to bind to the engineered vitronectin lacking the SMB domain. This approach identified a second, separate site for interaction between vitronectin and PAI-1. The binding of PAI-1 to this site was defined by a set of mutations in PAI-1 distinct from the mutations that disrupt binding to the SMB domain. Using the mutations in PAI-1 to map the second site suggested interactions between alpha-helices D and E in PAI-1 and a site in vitronectin outside of the SMB domain. The affinity of this second interaction exhibited a K(D) value approximately 100-fold higher than that of the PAI-1-somatomedin B interaction. In contrast to the PAI-1-somatomedin B binding, the second interaction had almost the same affinity for active and latent PAI-1. We hypothesize that, together, the two sites form an extended binding area that may promote assembly of higher order vitronectin-PAI-1 complexes.

The status of plasminogen activator inhibitor-1 as a therapeutic target

Plasminogen activator inhibitor-1 (PAI-1) is the major physiological inhibitor of tissue-type plasminogen activator (tPA). An increase in the plasma concentration of PAI-1 has been proposed as a risk factor in thrombotic disease and elevated PAI-1 is associated with a poor prognosis in a variety of cancers. These observations have led to numerous studies addressing the physiological and pathophysiological role of PAI-1 and to the proposal that manipulation of PAI-1 activity presents a new therapeutic target. Recent experimental studies with anti-PAI-1 antibodies and low molecular weight inhibitors have demonstrated efficacy in both arterial and venous thrombosis models. These studies have confirmed the potential clinical benefit of reducing PAI-1 activity. As it is now possible to manipulate PAI-1 activity in vivo, future studies should be aimed at confirming the importance of PAI-1 as a major therapeutic target.

Construction of a plasminogen activator inhibitor-1 variant without measurable affinity to vitronectin but otherwise normal

Vitronectin (VN) and plasminogen activator inhibitor-1 (PAI-1) have important functional interactions: VN stabilises the protease inhibitory activity of PAI-1 and PAI-1 inhibits binding of adhesion receptors to VN. Having previously mapped the PAI-1 binding area for VN, we have now constructed a PAI-1 variant, R103A-M112A-Q125A, without measurable affinity to VN, but with full protease inhibitory activity and endocytosis receptor binding. As a tool for evaluating the physiological and pathophysiological functions of the PAI-1-VN interaction, our new variant is far superior to the previously widely used PAI-1 variant Q125K, which we have found possesses an only about 10-fold reduced affinity to VN.

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 (Vn): mapping the binding sites on PAI-1 and Vn

The serpin plasminogen activator inhibitor type-1 (PAI-1), as the primary physiological inhibitor of both urokinase-type (uPA) and tissue-type (tPA) plasminogen activator, plays an important role in the regulation of the fibrinolytic system as well as in extracellular remodeling in both physiological and pathophysiological processes. In plasma as well as in the extracellular matrix PAI-1 binds to vitronectin (Vn), an interaction that affects the function of both proteins. As PAl-1/Vn interaction has a significant regulatory function in fibrinolysis, thrombolysis, and cell adhesion in cancer spread, there is a strong interest in defining the binding sites on PAI-1 and Vn as the basis of a rational design of novel drugs that may modulate PAI-1/Vn-mediated effects. In this minireview, we give an overview on the approaches to define the Vn binding site of PAI-1 and vice versa. Although in the case of PAI-1 the region around alpha-helix E and alpha-helix F of PAI-1 has been demonstrated to be important for its interaction with Vn, the precise location of the Vn-binding region has not completely been resolved. The major high-affinity PAI-1 binding region of Vn is localized within the N-terminal somatomedin B (SMB) domain of Vn. There are indications for at least one other low-affinity PAI-1 binding site in the C-terminal region of Vn, which seems to be involved in the formation of larger PAI-1/Vn complexes.

The vitronectin binding area of plasminogen activator inhibitor-1, mapped by mutagenesis and protection against an inactivating organochemical ligand

A distinguishing feature of serpins is their ability to undergo a conformational change consisting in insertion of the reactive centre loop (RCL) into beta-sheet A. In the serpin plasminogen activator inhibitor-1 (PAI-1), RCL movements are regulated by vitronectin, having a previously poorly defined binding site lateral to PAI-1's beta-sheet A. Using a novel strategy, based on identification of amino acid residues necessary for vitronectin protection of PAI-1 against inactivation by 4,4'-dianilino-1,1'-bisnaphthyl-5,5'-disulfonic acid, we have defined a vitronectin binding surface spanning 10 residues between alpha-helix F, beta-strand 2A, and alpha-helix E. Our results contribute to elucidating the unique serpin conformational change.

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.

The molecular basis for anti-proteolytic and non-proteolytic functions of plasminogen activator inhibitor type-1: roles of the reactive centre loop, the shutter region, the flexible joint region and the small serpin fragment

The serine proteinase inhibitor plasminogen activator inhibitor type-1 (PAI-1) is the primary physiological inhibitor of the tissue-type and the urokinase-type plasminogen activator (tPA and uPA, respectively) and as such an important regulator of proteolytic events taking place in the circulation and in the extracellular matrix. Moreover, a few non-proteolytic functions have been ascribed to PAI-1, mediated by its interaction with vitronectin or the interaction between the uPA-PAI-1 complex bound to the uPA receptor and members of the low density lipoprotein receptor family. PAI-1 belongs to the serpin family, characterised by an unusual conformational flexibility, which governs its molecular interactions. In this review we describe the anti-proteolytic and non-proteolytic functions of PAI-1 from both a biological and a biochemical point of view. We will relate the various biological roles of PAI-1 to its biochemistry in general and to the different conformations of PAI-1 in particular. We put emphasis on the intramolecular rearrangements of PAI-1 that are required for its antiproteolytic as well as its non-proteolytic functions.

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.

Identification of a target site in plasminogen activator inhibitor-1 that allows neutralization of its inhibitor properties concomitant with an allosteric up-regulation of its antiadhesive properties

The serpin plasminogen activator inhibitor-1 (PAI-1) has a dual function: 1) it plays an important role as a direct inhibitor of the plasminogen activation system, and 2) its interaction with the adhesive glycoprotein vitronectin suggests a role in tissue remodeling and metastasis, independent from its proteinase inhibitory properties. Unique to this serpin is the close association between its conformational and functional properties. Indeed, PAI-1 can occur in an active and a latent conformation, but both functions are exclusively present in the active conformation. We report here the epitope localization and functional effects of a monoclonal antibody (MA-124K1) that inhibits rat PAI-1 activity and simultaneously increases the binding of inactive PAI-1 to vitronectin (the affinity constant of PAI-1 for vitronectin is 2 x 10(7) m(-1) in the absence of MA-124K1 and 160 x 10(7) m(-1) in the presence of MA-124K1). To the best of our knowledge, this is the first monoclonal antibody dissociating the proteinase inhibitory properties from the vitronectin binding properties in PAI-1. Mutation of Glu(212) and/or Glu(220) in rat PAI-1 to Ala results in a strongly reduced affinity or absence of binding to MA-124K1. The three-dimensional structure of PAI-1 reveals that these residues constitute a conformational epitope close to the reactive-site loop and compatible with the effect of MA-124K1 on the inhibitory properties of PAI-1. However, the vitronectin binding site is localized at the opposite site of the molecule, indicating that the effect of MA-124K1 involves an allosteric modulation of the vitronectin binding site. Cell culture experiments revealed a significant reduction of cell attachment and migration in the presence of MA-124K1, providing evidence for the functional relevance of this antibody-mediated up-regulation of the vitronectin binding properties of PAI-1. In conclusion, a novel mechanism for interference with PAI-1 functions has been identified and is of importance in the modulation of cell migration and related events (e.g. tumor metastasis).

Antithrombotic and fibrinolytic system of human endothelial cells seeded on PTFE: the effects of surface modification of PTFE by ammonia plasma treatment and ECM protein coatings

The aim of this study was to determine the effects of ECM protein coatings and surface modification of PTFE on the ability of seeded human endothelial cells (EC) to secrete prostacyclin (PGI2), plasminogen inhibitor-1 (PAI-1) and tissue plasminogen activator (t-PA). PTFE surfaces were modified by a novel surface modification technique based on ammonia plasma. Fibronectin, collagen type-1 and gelatin-coated ammonia plasma modified PTFE and unmodified PTFE surfaces were employed and compared in this study. All ammonia plasma modified surfaces showed similar secretions of PGI2 compared to non-modified PTFE surfaces. With the exception of gelatin-coated modified PTFE, seeded EC seeded on all modified PTFE showed lower levels of PAI-1 secretion compared to those seeded on unmodified PTFE. The specific activity of t-PA secreted by EC seeded on ammonia plasma modified and fibronectin coated modified PTFE showed increases of 100 and 30%, respectively, when compared to their unmodified counterparts. Our studies show that EC seeded on modified PTFE have ability to secrete PGI2 that modulates the early phase of thrombus formation. Furthermore, superior t-PA profile, along with lower levels of PAl-1 suggest that ammonia plasma modification and use of appropriate ECM proteins can modulate antithrombotic and fibrinolytic properties of in vitro endothelialized vascular prostheses. Accordingly, these surfaces may be suitable to further develop protocols and other strategies for arterial and venous reconstruction.

A regulatory hydrophobic area in the flexible joint region of plasminogen activator inhibitor-1, defined with fluorescent activity-neutralizing ligands. Ligand-induced serpin polymerization

We have characterized the neutralization of the inhibitory activity of the serpin plasminogen activator inhibitor-1 (PAI-1) by a number of structurally distinct organochemicals, including compounds with environment-sensitive spectroscopic properties. In contrast to latent and reactive center-cleaved PAI-1 and PAI-1 in complex with urokinase-type plasminogen activator (uPA), active PAI-1 strongly increased the fluorescence of the PAI-1-neutralizing compounds 1-anilinonaphthalene-8-sulfonic acid and 4,4'-dianilino-1,1'-bisnaphthyl-5,5'-disulfonic acid. The fluorescence increase could be competed by all tested nonfluorescent neutralizers, indicating that all neutralizers bind to a common hydrophobic area preferentially accessible in active PAI-1. Activity neutralization proceeded through two consecutive steps as follows: first step is conversion to forms displaying substrate behavior toward uPA, and second step is to forms inert to uPA. With some neutralizers, the second step was associated with PAI-1 polymerization. Vitronectin reduced the susceptibility to the neutralizers. Changes in sensitivity to activity neutralization by point mutations were compatible with the various neutralizers having overlapping, but not identical, binding sites in the region around alpha-helices D and E and beta-strand 1A, known to act as a flexible joint when beta-sheet A opens and the reactive center loop inserts as beta-strand 4A during reaction with target proteinases. The defined binding area may be a target for development of compounds for neutralizing PAI-1 in cancer and cardiovascular diseases.

Regulation of plasminogen activator inhibitor 1 expression by interaction of epidermal growth factor with progestin during decidualization of human endometrial stromal cells

OBJECTIVE

During human pregnancy implantation, trophoblasts invade maternal blood vessels in a process that risks hemorrhage. Previous studies have demonstrated enhanced expression of type 1 plasminogen activator inhibitor, the primary inhibitor of fibrinolysis, during progestin-induced decidualization of estradiol-primed human endometrial stromal cells in vivo and in vitro. Decidual cell-expressed plasminogen activator inhibitor 1 is appropriately positioned to avert implantational hemorrhage. Because of the absence of estrogen or progesterone response elements from the plasminogen activator inhibitor 1 gene promoter, I posited that epidermal growth factor mediates these steroid effects and that expression of epidermal growth factor receptor in human endometrial stromal cells is under ovarian steroid control.

STUDY DESIGN

Confluent human endometrial stromal cells were exposed to vehicle control or to either estradiol (10(-8) mol/L) or medroxyprogesterone acetate (10(-7) mol/L), or both, with or without growth factors. After 40 hours the cultures were analyzed for plasminogen activator inhibitor 1 protein and messenger ribonucleic acid expressions. Immunostaining for epidermal growth factor receptor was carried out in sections of cycling and gestational endometrial tissues.

RESULTS

In the absence of steroids, epidermal growth factor did not alter plasminogen activator inhibitor 1 expression. In the absence of epidermal growth factor, estradiol and medroxyprogesterone acetate enhanced human endometrial stromal cell-secreted plasminogen activator inhibitor 1 protein levels 8-fold (n = 12; P <.001), whereas estradiol alone had no effect. Marked synergistic increases in plasminogen activator inhibitor 1 levels were elicited when epidermal growth factor was added with estradiol and medroxyprogesterone acetate (n = 12; 65-fold; P <.0001). Both transforming growth factor alpha and epidermal growth factor, which act through epidermal growth factor receptor, increased steady-state plasminogen activator inhibitor 1 messenger ribonucleic acid levels several-fold when added with estradiol and medroxyprogesterone acetate. In contrast, transforming growth factor beta, which does not activate epidermal growth factor receptor, did not elevate plasminogen activator inhibitor 1 messenger ribonucleic acid or protein levels whether added alone or with estradiol and medroxyprogesterone acetate. In correspondence with these in vitro observations, immunostaining for epidermal growth factor receptor was increased in human endometrial stromal cells undergoing decidualization in sections of secretory phase and first-trimester endometrial tissue.

CONCLUSIONS

Taken together, these in vitro and in vivo results indicate that both epidermal growth factor and progesterone receptors are required for maximal plasminogen activator inhibitor 1 expression by human endometrial stromal cells.

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.

Epitope mapping for four monoclonal antibodies against human plasminogen activator inhibitor type-1: implications for antibody-mediated PAI-1-neutralization and vitronectin-binding

The inhibitory mechanism of serine proteinase inhibitors of the serpin family is based on their unique conformational flexibility. The formation of a stable proteinase-serpin complex implies insertion of the reactive centre loop of the serpin into the large central beta-sheet A and a shift in the relative positions of two groups of secondary structure elements, the smaller one including alpha-helix F. In order to elucidate this mechanism, we have used phage-display and alanine scanning mutagenesis to map the epitopes for four monoclonal antibodies against alpha-helix F and its flanking region in the serpin plasminogen activator inhibitor-1 (PAI-1). One of these is known to inhibit the reaction between PAI-1 and its target proteinases, an effect that is potentiated by vitronectin, a physiological carrier protein for PAI-1. When combined with the effects these antibodies have on PAI-1 activity, our epitope mapping points to the mobility of amino-acid residues in alpha-helix F and the loop connecting alpha-helix F and beta-strand 3A as being important for the inhibitory function of PAI-1. Although all antibodies reduced the affinity of PAI-1 for vitronectin, the potentiating effect of vitronectin on antibody-induced PAI-1 neutralization is based on formation of a ternary complex between antibody, PAI-1 and vitronectin, in which PAI-1 is maintained in a state behaving as a substrate for plasminogen activators. These results thus provide new details about serpin conformational changes and the regulation of PAI-1 by vitronectin and contribute to the necessary basis for rational design of drugs neutralizing PAI-1 in cancer and cardiovascular diseases.

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.

New insights into the size and stoichiometry of the plasminogen activator inhibitor type-1.vitronectin complex

Plasminogen activator inhibitor-type 1 (PAI-1) is the primary inhibitor of endogenous plasminogen activators that generate plasmin in the vicinity of a thrombus to initiate thrombolysis, or in the pericellular region of cells to facilitate migration and/or tissue remodeling. It has been shown that the physiologically relevant form of PAI-1 is in a complex with the abundant plasma glycoprotein, vitronectin. The interaction between vitronectin and PAI-1 is important for stabilizing the inhibitor in a reactive conformation. Although the complex is clearly significant, information is vague regarding the composition of the complex and consequences of its formation on the distribution and activity of vitronectin in vivo. Most studies have assumed a 1:1 interaction between the two proteins, but this has not been demonstrated experimentally and is a matter of some controversy since more than one PAI-1-binding site has been proposed within the sequence of vitronectin. To address this issue, competition studies using monoclonal antibodies specific for separate epitopes confirmed that the two distinct PAI-1-binding sites present on vitronectin can be occupied simultaneously. Analytical ultracentrifugation was used also for a rigorous analysis of the composition and sizes of complexes formed from purified vitronectin and PAI-1. The predominant associating species observed was high in molecular weight (M(r) approximately 320,000), demonstrating that self-association of vitronectin occurs upon interaction with PAI-1. Moreover, the size of this higher order complex indicates that two molecules of PAI-1 bind per vitronectin molecule. Binding of PAI-1 to vitronectin and association into higher order complexes is proposed to facilitate interaction with macromolecules on surfaces.

Type 1 plasminogen activator inhibitor binds to fibrin via vitronectin

Type 1 plasminogen activator inhibitor (PAI-1), the primary inhibitor of tissue-type plasminogen activator (t-PA), circulates as a complex with the abundant plasma glycoprotein, vitronectin. This interaction stabilizes the inhibitor in its active conformation In this report, the effects of vitronectin on the interactions of PAI-1 with fibrin clots were studied. Confocal microscopic imaging of platelet-poor plasma clots reveals that essentially all fibrin-associated PAI-1 colocalizes with fibrin-bound vitronectin. Moreover, formation of platelet-poor plasma clots in the presence of polyclonal antibodies specific for vitronectin attenuated the inhibitory effects of PAI-1 on t-PA-mediated fibrinolysis. Addition of vitronectin during clot formation markedly potentiates PAI-1-mediated inhibition of lysis of (125)I-labeled fibrin clots by t-PA. This effect is dependent on direct binding interactions of vitronectin with fibrin. There is no significant effect of fibrin-associated vitronectin on fibrinolysis in the absence of PAI-1. The binding of PAI-1 to fibrin clots formed in the absence of vitronectin was characterized by a low affinity (K(d) approximately 3.5 micrometer) and rapid loss of PAI-1 inhibitory activity over time. In contrast, a high affinity and stabilization of PAI-1 activity characterized the cooperative binding of PAI-1 to fibrin formed in the presence of vitronectin. These findings indicate that plasma PAI-1.vitronectin complexes can be localized to the surface of fibrin clots; by this localization, they may modulate fibrinolysis and clot reorganization.

Heritable coagulopathies in pregnancy

Heritable coagulopathies are leading causes of maternal thromboembolism and are associated with an increased risk of maternal and perinatal morbidity and mortality. The most common of these disorders are antithrombin III deficiency, protein C deficiency, protein S deficiency, activated protein C resistance resulting from the factor V Leiden mutation, elevated prothrombin activity associated with a mutation in the prothrombin gene, and hyperhomocystinemia. The maternal risk of a thromboembolic episode is increased by a factor of eight in the presence of any of these heritable states. In addition, the relative risk for a stillbirth in the presence of one of these disorders is 3.6. These conditions are also associated with intrauterine growth retardation and preeclampsia. Proper management of heritable coagulopathies during pregnancy is essential to reduce the risk of these serious sequelae. Patients with newly diagnosed deep-vein thromboses or pulmonary emboli should be treated with therapeutic levels of unfractionated or low molecular weight heparin, followed by subsequent prophylactic heparin therapy. All patients with a history of thromboembolism before pregnancy or evidence of any of these coagulopathies may be offered prophylactic therapy with low molecular weight heparin. Patients with antithrombin III deficiency should receive full therapeutic heparin therapy for the entire pregnancy, irrespective of their thromboembolic history. Postpartum therapy with either heparin or warfarin is required in all cases.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians

LEARNING OBJECTIVES

After completion of this article, the reader will be able to describe the various heritable coagulopathies that can complicate pregnancy, to state the potential adverse effects of heritable coagulopathies in pregnancy, and to explain the management of heritable coagulopathies during pregnancy.

Antithrombotic activity of a monoclonal antibody inducing the substrate form of plasminogen activator inhibitor type 1 in rat models of venous and arterial thrombosis

1. Elevated plasminogen activator inhibitor 1 (PAI-1) is a risk factor for thrombosis, and inhibitors of the interaction between PAI-1 and tissue plasminogen activator (t-PA) have antithrombotic and prothrombolytic activity in animals. We describe the antithrombotic effects in the rat of a monoclonal antibody (MA33H1) which converts PAI-1 to a non-inhibitory substrate. 2. The activity of MA33H1 against rat PAI-1 was confirmed using two-chain t-PA and a chromogenic substrate. MA33H1 was evaluated in rat venous (thromboplastin + stasis in the abdominal vena cava) and arterial (electric current applied to a carotid artery) thrombosis models. The effects on tail-transection bleeding time were studied. 3. MA33H1 at 100 ng ml(-1) inhibited both human (44.1%) and rat PAI-1 (49.7%). This effect was concentration-dependent. Its effect on human PAI-1 was not significantly inhibited by 1 microg ml(-1) fibrin or a approximately 7 fold molar excess of vitronectin (1 nM). Inhibition of rat PAI-1 was unchanged by fibrin, but vitronectin reduced inhibition from 0.5 nM. 4. In the venous thrombosis model, MA33H1 significantly reduced thrombus weights by 38 and 58.6% at 50 and 100 microg kg(-1) min(-1) i.v. respectively. This effect was inhibited by tranexamic acid. In the arterial model, MA33H1 significantly increased the delay to occlusive thrombus formation by 58 and 142% at 50 and 100 microg kg(-1) min(-1) i.v., and did not affect bleeding time at 300 microg kg(-1) min(-1) i.v. 5. Thus, a monoclonal antibody which transforms PAI-1 to a t-PA substrate prevents thrombus formation in the rat with no effect on bleeding time at a higher dose.

Functional effects of single amino acid substitutions in the region of Phe113 to Asp138 in the plasminogen activator inhibitor 1 molecule

Thirteen amino acid substitutions have been introduced within the stretch Phe113 to Asp138 in the plasminogen activator inhibitor 1 (PAI-1) molecule by site-directed mutagenesis. The different proteins and wild-type (wt) PAI-1 have been overexpressed in Escherichia coli and purified by chromatography on heparin-Sepharose and on anhydrotrypsin-agarose. The PAI-1 variants have been characterized by their reactivity with tissue plasminogen activator (tPA), interactions with vitronectin or heparin, and stability. Most PAI-1 variants, except for Asp125-->Lys, Phe126-->Ser and Arg133-->Asp, displayed a high spontaneous inhibitory activity towards tPA, which did not change greatly on reactivation with 4 M guanidinium chloride, followed by dialysis at pH 5.5. The variants Asp125-->Lys and Arg133-->Asp became much more active after reactivation and they were also more rapidly transformed to inactive forms (t12 22-31 min) at physiological pH and temperature than the other variants. However, in the presence of vitronectin they were both almost equally stable (t12 2.3 h) as wtPAI-1 (t12 3.0 h). The mutant Glu130-->Lys showed an increased stability, both in the absence and in the presence of vitronectin compared with wtPAI-1. Nevertheless a similar affinity between all the active PAI-1 variants and vitronectin was observed. Further, all mutants, including the three mutants with low activity, were to a large extent adsorbed on anhydrotrypsin-agarose and were eluted in a similar fashion. In accordance with these data, the three variants with a low activity were all to a large extent cleaved as a result of their reaction with tPA, suggesting that they occurred predominantly in the substrate conformation. Our results do not support the presence of a binding site for vitronectin in this part of the molecule, but rather that it might be involved in controlling the active PAI-1 to substrate transition. Partly, this region of the PAI-1 molecule (Arg115 to Arg118) seems also to be involved in the binding of heparin to PAI-1.

Binding of urokinase-type plasminogen activator-plasminogen activator inhibitor-1 complex to the endocytosis receptors alpha2-macroglobulin receptor/low-density lipoprotein receptor-related protein and very-low-density lipoprotein receptor involves basic residues in the inhibitor

The complex of the type-1 plasminogen activator inhibitor (PAI-1) and its target proteinases, the urokinase and tissue-type plasminogen activators (uPA and tPA), but not the free components, bind with high affinity to the endocytosis receptors alpha2-macroglobulin receptor/low-density lipoprotein receptor-related protein (alpha2MR/LRP) and very-low-density lipoprotein receptor (VLDLR). To characterize the molecular interaction between the complexes and the receptors, alanine codons were introduced into the human PAI-1 cDNA to replace the four basic residues, Arg-78, Lys-82, Arg-120 and Lys-124, as double mutations. The purified recombinant mutant proteins, rPAI-1/R78A-K124A and rPAI-1/K82A-R120A, produced by the yeast Pichia pastoris, were indistinghuisable from wild-type recombinant and natural human PAI-1 with respect to inhibitory activity against uPA, stability of SDS-resistant complexes with uPA, and vitronectin binding. Radiolabelled mutant uPA.PAI-1 complexes bound with a 10- to 20-fold, and 3- to 7-fold reduced affinity to purified alpha2MR/LRP and VLDLR respectively. alpha2MR/LRP-mediated endocytosis of the mutant complexes by COS-1 cells was reduced to 48 and 38% of the level of endocytosis of wild-type PAI-1. Binding of the mutant complexes to the uPA receptor was not affected. These findings suggest that the binding mode of the uPA.PAI-1 complex to both alpha2MR/LRP and VLDLR is similar. The four residues are surface exposed in the region defined by alpha-helix D and beta-strand 1A in the serine protease inhibitor (serpin) structure. Our study represents the first identification of residues in a surface region implicated in molecular recognition of protease.serpin complexes by endocytosis receptors of the low-density lipoprotein receptor family.

Type-1 plasminogen-activator inhibitor -- conformational differences between latent, active, reactive-centre-cleaved and plasminogen-activator-complexed forms, as probed by proteolytic susceptibility

We have analysed the susceptibility of latent, active, reactive-centre-cleaved and plasminogen-activator-complexed type-1 plasminogen-activator inhibitor (PAI-1) to the non-target proteinases trypsin, endoproteinase Asp-N, proteinase K and subtilisin. This analysis has allowed us to detect conformational differences between the different forms of PAI-1 outside the reactive-centre loop and beta-sheet A. Proteinase-hypersensitive sites were clustered in three regions. Firstly, susceptibility was observed in the region around alpha-helix E, beta-strand 1A, and the flanking loops, which are believed to form flexible joints during movements of beta-sheet A. Secondly, hypersensitive sites were observed in the loop between alpha-helix I and beta-strand 5A. Thirdly, the gate region, encompassing beta-strands 3C and 4C, was highly susceptible to trypsin in latent PAI-1, but not in the other conformations. The digestion patterns differed among all four forms of PAI-1, indicating that each represents a unique conformation. The differential proteolytic susceptibility of the flexible-joint region may be coupled to the differential affinity to vitronectin, binding in the same region. The analysis also allowed detection of conformational differences between reactive-centre-cleaved forms produced under different solvent conditions. The digestion pattern of plasminogen-activator-complexed PAI-1 was different from that of active PAI-1, but indistinguishable from that of one of the reactive-centre-cleaved forms, as the complexed and this particular cleaved PAI-1 were completely resistant to all the non-target proteinases tested. This observation is in agreement with the notion that complex formation involves reactive-centre cleavage and a large degree of insertion of the reactive-centre loop into beta-sheet A. Our analysis has allowed the identification of some flexible regions that appear to be implicated in the conformational changes during the movements of beta-sheet A and during the inhibitory reaction of serpins with their target proteinases.

Insulin-like growth factor binding protein-5 binds to plasminogen activator inhibitor-I

Insulin-like growth factor binding protein-5 (IGFBP-5) has been shown to bind to the extracellular matrix (ECM) of both fibroblasts and smooth muscle cells. The ECM-IGFBP-5 interaction is mediated in part by binding to heparan sulfate containing proteoglycans. Because proteoglycans may not be the only components of ECM that bind to IGFBP-5, we have determined its ability to bind to other ECM proteins. When a partially purified mixture of the proteins that were present in fibroblast conditioned medium was purified by IGFBP-5 affinity chromatography, a 55-kDa protein was eluted. Amino acid sequencing of the amino terminal 28 amino acids showed that it was human plasminogen activator inhibitor-1 (PAI-1). To determine if this interaction was specific, purified human PAI-1 was incubated with IGFBP-5 and the IGFBP-5/PAI-1 complex immunoprecipitated with anti-PAI-1 antiserum. When the precipitate was analyzed by immunoblotting using anti-IGFBP-5 antiserum, the intensity of the IGFBP-5 band was substantially increased compared with controls that did not contain human PAI-1. A synthetic IGFBP-5 peptide that contained the amino acid sequence between positions 201 and 218 inhibited IGFBP-5/PAI-1 interaction. Coincubation of IGFBP-5 mutants that contained substitutions for specific basic residues located between positions 201 and 218 with PAI-1 indicated that some of these amino acids were important for binding. Two mutants that contained neutral substitutions for specific basic amino acids within the glycosaminoglycan binding domain had reduced binding to PAI-1. In contrast, three other mutants that also had substitutions for charged residues in the same region had no reduction in binding. Heparin and heparan sulfate inhibited the IGFBP-5/PAI-1 interaction; however, several other glycosaminoglycans had no effect. PAI-1 was determined to be an important ECM component for binding because approximately 27% of total ECM binding could be inhibited with anti-PAI-1 antiserum. Competitive binding studies with unlabeled IGFBP-5 showed that the dissociation constant of PAI-1 for IGFBP-5 was 9.1 x 10(-8) M. In summary, IGFBP-5 binds specifically to plasminogen activator inhibitor-1. Because this is present in the extracellular matrix of several cell types, it may be one of the important binding components of ECM. PAI-1 binding partially protects IGFBP-5 from proteolysis, suggesting that it is one of the ECM components that is involved in mediating this effect.

Differential modulation of cell adhesion by interaction between adhesive and counter-adhesive proteins: characterization of the binding of vitronectin to osteonectin (BM40, SPARC)

Heparin-binding forms of vitronectin, a multifunctional adhesive glycoprotein, are associated with the extracellular matrix (ECM) at different locations in the body and serve to promote cell adhesion and the regulation of pericellular proteolysis at sites of angiogenesis. In the present study we characterized the interactions of vitronectin with the counter-adhesive protein osteonectin (also termed SPARC or BM40). Osteonectin and vitronectin were both found associated with the ECM of cultured endothelial cells and were localized in vessel wall sections of kidney tissue. In vitro, the heparin-binding multimeric isoform of vitronectin bound to immobilized osteonectin in a saturable manner with half-maximal binding at 30-40 nM. Preincubation of plasma vitronectin with plasminogen activator inhibitor 1 (PAI-1), which provoked multimer formation, induced the binding of vitronectin to osteonectin. Binding was optimal at physiological ionic strength, and binary complexes were stabilized by tissue transglutaminase-mediated cross-linking. In a concentration-dependent fashion, PAI-1, CaCl2, heparin and heparan sulphate, but not other glycosaminoglycans, interfered with the binding of vitronectin to osteonectin. Using vitronectin-derived synthetic peptides as well as mutant forms of recombinant osteonectin, we found that the heparin-binding region of vitronectin interacted with the C-terminal region of osteonectin that contains a high-affinity Ca2+-binding site with counter-adhesive properties. Adhesion of cultured endothelial cells was partly abrogated by osteonectin and was correspondingly reversed by vitronectin in a concentration-dependent manner. These results indicate that specific interactions between vitronectin and osteonectin modulate cell adhesion and might thereby regulate endothelial cell function during angiogenesis.

Binding of urokinase to plasminogen activator inhibitor type-1 mediates cell adhesion and spreading

Urokinase plasminogen activator and its receptor are both found at the surface of the cell membrane in many cell types. The plasminogen activator inhibitor type-1 (PAI-1) is often associated with the extracellular matrix. The spatial localization of these three molecules could account for their involvement in cell adhesion and/or migration. We have shown previously that the urokinase receptor mediates mechanical force transmission across the cell surface to the cytoskeleton. Here we investigated whether immobilized plasminogen activator inhibitor type 1 (PAI-1) could regulate cell spreading and cytoskeleton reorganization. Serum deprived human myogenic cells were plated in serum free medium onto bacteriologic dishes precoated with different extracellular matrix ligands (fibronectin, vitronectin, or type 1 collagen) or PAI-1 at increasing concentrations. The number of adherent cells and their projected area were quantitated after 3 hours of plating. PAI-1 promoted cell adhesion and spreading in a dose dependent manner. Addition of antibodies to PAI-1 inhibited the adhesion on PAI-1 coated dishes in a dose dependent way. The PAI-1 mediated cell adhesion required the presence of urokinase at the cell surface. Removal of the glycosylphosphatidylinositol (GPI)-linked proteins abolished cell adhesion on PAI-1 dish, suggesting its dependence on the presence of the urokinase receptor, a GPI-linked receptor. Furthermore, addition of antibodies against alpha v beta3 integrin completely inhibited cell adhesion on PAI-1, suggesting that alpha v beta3 might be the transmembrane molecule that physically connects the complex of PAI-1, urokinase, and urokinase receptor to the cytoskeleton. Visualization of spread cells stained for filamentous actin with confocal microscopy showed a dose-dependent increase of filopodia on PAI-1 coated dishes and cytoskeletal reorganization, suggesting a migratory profile. These data indicate that PAI-1 plays a direct role in dynamic cell adhesion particularly at the leading edge, where increased levels of urokinase plasminogen activator (uPA) and its receptor (uPAR) are localized in migrating cells. Immobilized PAI-1 could therefore serve to bridge the cell surface with the extracellular matrix via the formation of a multimolecular complex that includes alpha v beta3 integrins in myogenic cells.

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.

Characterization of the binding of different conformational forms of plasminogen activator inhibitor-1 to vitronectin. Implications for the regulation of pericellular proteolysis

Plasminogen activator inhibitor type 1 (PAI-1), the primary physiologic inhibitor of plasminogen activation, is associated with the adhesive glycoprotein vitronectin (Vn) in plasma and the extracellular matrix. In this study we examined the binding of different conformational forms of PAI-1 to both native and urea-purified vitronectin using a solid-phase binding assay. These results demonstrate that active PAI-1 binds to urea-purified Vn with approximately 6-fold higher affinity than to native Vn. In contrast, inactive forms of PAI-1 (latent, elastase-cleaved, synthetic reactive center loop peptide-annealed, or complexed to plasminogen activators) display greatly reduced affinities for both forms of adsorbed Vn, with relative affinities reduced by more than 2 orders of magnitude. Structurally, these inactive conformations all differ from active PAI-1 by insertion of an additional strand into beta-sheet A, suggesting that it is the rearrangement of sheet A that results in reduced Vn affinity. This is supported by the observation that PAI-1 associated with beta-anhydrotrypsin, which does not undergo rearrangement of beta-sheet A, shows no such decrease in affinity, whereas PAI-1 complexed to beta-trypsin, which does undergo sheet A rearrangement, displays reduced affinity for Vn similar to PAI-1.plasminogen activator complexes. Together these data demonstrate that the interaction between PAI-1 and Vn depends on the conformational state of both proteins and suggest that the Vn binding site on PAI-1 is sensitive to structural changes associated with loss of inhibitory activity.

The use of fluorescent probes to characterize conformational changes in the interaction between vitronectin and plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of tissue-type plasminogen activator and urokinase, is known to convert readily to a latent form by insertion of the reactive center loop into a central beta-sheet. Interaction with vitronectin stabilizes PAI-1 and decreases the rate of conversion to the latent form, but conformational effects of vitronectin on the reactive center loop of PAI-1 have not been documented. Mutant forms of PAI-1 were designed with a cysteine substitution at either position P1' or P9 of the reactive center loop. Labeling of the unique cysteine with a sulfhydryl-reactive fluorophore provides a probe that is sensitive to vitronectin binding. Results indicate that the scissile P1-P1' bond of PAI-1 is more solvent exposed upon interaction with vitronectin, whereas the N-terminal portion of the reactive loop does not experience a significant change in its environment. These results were complemented by labeling vitronectin with an arginine-specific coumarin probe which compromises heparin binding but does not interfere with PAI-1 binding to the protein. Dissociation constants of approximately 100 nM are calculated for the vitronectin/PAI-1 interaction from titrations using both fluorescent probes. Furthermore, experiments in which PAI-1 failed to compete with heparin for binding to vitronectin argue for separate binding sites for the two ligands on vitronectin.

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.

Divining the serpin inhibition mechanism: a suicide substrate 'springe'?

The most important of diverse serpin functions is serine-protease inhibition. In contrast to the 'standard-mechanism' inhibitors, inhibitory serpins use a mechanism that involves unusual flexibility, and cofactor and receptor interactions. The principal feature is a refolding step, during which a disordered or helical strand is inserted into the center of a beta sheet. This transition, which is essential for inhibition, can be induced by heating, proteolytic cleavage of the serpin, or complexation with the proteinase target; analogous transitions can be induced by peptide complexation or aggregation. Although it is difficult to determine the details of this mechanism, information derived from crystal structures and other experiments has stimulated drug design efforts with wide-ranging potential applications.