Conformational changes of the reactive-centre loop and beta-strand 5A accompany temperature-dependent inhibitor-substrate transition of plasminogen-activator inhibitor 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.

Generation and in vitro characterisation of inhibitory nanobodies towards plasminogen activator inhibitor 1

Plasminogen activator inhibitor 1 (PAI-1) is the principal physiological inhibitor of tissue-type plasminogen activator (t-PA) and has been identified as a risk factor in cardiovascular diseases. In order to generate nanobodies against PAI-1 to interfere with its functional properties, we constructed three nanobody libraries upon immunisation of three alpacas with three different PAI-1 variants. Three panels of nanobodies were selected against these PAI-1 variants. Evaluation of the amino acid sequence identity of the complementarity determining region-3 (CDR3) reveals 34 clusters in total. Five nanobodies (VHH-s-a98, VHH-2w-64, VHH-s-a27, VHH-s-a93 and VHH-2g-42) representing five clusters exhibit inhibition towards PAI-1 activity. VHH-s-a98 and VHH-2w-64 inhibit both glycosylated and non-glycosylated PAI-1 variants through a substrate-inducing mechanism, and bind to two different regions close to αhC and the hinge region of αhF; the profibrinolytic effect of both nanobodies was confirmed using an in vitro clot lysis assay. VHH-s-a93 may inhibit PAI-1 activity by preventing the formation of the initial PAI-1t-PA complex formation and binds to the hinge region of the reactive centre loop. Epitopes of VHH-s-a27 and VHH-2g-42 could not be deduced yet. These five nanobodies interfere with PAI-1 activity through different mechanisms and merit further evaluation for the development of future profibrinolytic therapeutics.

Functional stability of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the main inhibitor of plasminogen activators, such as tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), and a major regulator of the fibrinolytic system. PAI-1 plays a pivotal role in acute thrombotic events such as deep vein thrombosis (DVT) and myocardial infarction (MI). The biological effects of PAI-1 extend far beyond thrombosis including its critical role in fibrotic disorders, atherosclerosis, renal and pulmonary fibrosis, type-2 diabetes, and cancer. The conversion of PAI-1 from the active to the latent conformation appears to be unique among serpins in that it occurs spontaneously at a relatively rapid rate. Latency transition is believed to represent a regulatory mechanism, reducing the risk of thrombosis from a prolonged antifibrinolytic action of PAI-1. Thus, relying solely on plasma concentrations of PAI-1 without assessing its function may be misleading in interpreting the role of PAI-1 in many complex diseases. Environmental conditions, interaction with other proteins, mutations, and glycosylation are the main factors that have a significant impact on the stability of the PAI-1 structure. This review provides an overview on the current knowledge on PAI-1 especially importance of PAI-1 level and stability and highlights the potential use of PAI-1 inhibitors for treating cardiovascular disease.

Hydrogen/deuterium exchange mass spectrometry reveals specific changes in the local flexibility of plasminogen activator inhibitor 1 upon binding to the somatomedin B domain of vitronectin

The native fold of plasminogen activator inhibitor 1 (PAI-1) represents an active metastable conformation that spontaneously converts to an inactive latent form. Binding of the somatomedin B domain (SMB) of the endogenous cofactor vitronectin to PAI-1 delays the transition to the latent state and increases the thermal stability of the protein dramatically. We have used hydrogen/deuterium exchange mass spectrometry to assess the inherent structural flexibility of PAI-1 and to monitor the changes induced by SMB binding. Our data show that the PAI-1 core consisting of β-sheet B is rather protected against exchange with the solvent, while the remainder of the molecule is more dynamic. SMB binding causes a pronounced and widespread stabilization of PAI-1 that is not confined to the binding interface with SMB. We further explored the local structural flexibility in a mutationally stabilized PAI-1 variant (14-1B) as well as the effect of stabilizing antibody Mab-1 on wild-type PAI-1. The three modes of stabilizing PAI-1 (SMB, Mab-1, and the mutations in 14-1B) all cause a delayed latency transition, and this effect was accompanied by unique signatures on the flexibility of PAI-1. Reduced flexibility in the region around helices B, C, and I was seen in all three cases, which suggests an involvement of this region in mediating structural flexibility necessary for the latency transition. These data therefore add considerable depth to our current understanding of the local structural flexibility in PAI-1 and provide novel indications of regions that may affect the functional stability of PAI-1.

Maximal PAI-1 inhibition in vivo requires neutralizing antibodies that recognize and inhibit glycosylated PAI-1

Plasminogen activator inhibitor-1 (PAI-1) regulates the activity of t-PA and u-PA and is an important inhibitor of the plasminogen activator system. Elevated PAI-1 levels have been implicated in the pathogenesis of several diseases. Prior to the evaluation of PAI-1 inhibitors in humans, there is a strong need to study the effect of PAI-1 inhibition in mouse models. In the current study, four monoclonal antibodies previously reported to inhibit recombinant PAI-1 in vitro, were evaluated in an LPS-induced endotoxemia model in mice. Both MA-33H1F7 and MA-MP2D2 exerted a strong PAI-1 inhibitory effect, whereas for MA-H4B3 and MA-124K1 no reduced PAI-1 activity was observed in vivo. Importantly, the lack of PAI-1 inhibition observed for MA-124K1 and MA-H4B3 in vivo corresponded with the absence of inhibition toward glycosylated mouse PAI-1 in vitro. Three potential N-glycosylation sites were predicted for mouse PAI-1 (i.e. N209, N265 and N329). Electrophoretic mobility analysis of glycosylation knock-out mutants before and after deglycosylation indicates the presence of glycan chains at position N265. These data demonstrate that an inhibitory effect toward glycosylated PAI-1 is a prerequisite for efficient PAI-1 inhibition in mice. Our data also suggest that PAI-1 inhibitors for use in humans must preferably be screened on glycosylated PAI-1 and not on recombinant non-glycosylated PAI-1.

Development of inhibitors of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serine protease inhibitor super family (serpin) and is the primary inhibitor of both the tissue-type (tPA) and urokinase-type (uPA) plasminogen activators. PAI-1 has been implicated in a wide range of pathological processes where it may play a direct role in a variety of diseases. These observations have made PAI-1 an attractive target for small molecule drug development. However, PAI-1's structural plasticity and its capacity to interact with multiple ligands have made the identification and development of such small molecule PAI-1 inactivating agents challenging. In the following pages, we discuss the difficulties associated with screening for small molecule inactivators of PAI-1, in particular, and of serpins, in general. We discuss strategies for high-throughput screening (HTS) of chemical and natural product libraries, and validation steps necessary to confirm identified hits. Finally, we describe steps essential to confirm specificity of active compounds, and strategies to examine potential mechanisms of compound action.

Inhibition of chymotrypsin- and subtilisin-like serine proteases with Tk-serpin from hyperthermophilic archaeon Thermococcus kodakaraensis

A serpin homologue (Tk-serpin) from the hyperthermophilic archaeon Thermococcus kodakaraensis was overproduced in E. coli, purified, and characterized. Tk-serpin irreversibly inhibits Tk-subtilisin (TKS) from the same organism with the second-order association rate constants (k(ass)) of 5.2×10³ M⁻¹ s⁻¹ at 40°C and 3.1×10⁵ M⁻¹ s⁻¹ at 80°C, indicating that Tk-serpin inhibits TKS more strongly at 80°C than at 40°C. It also irreversibly inhibits chymotrypsin, subtilisin Carlsberg, and proteinase K at 40°C with the k(ass) values comparable to that for TKS at 80°C. Casein zymography showed that Tk-serpin inhibits these proteases by forming a SDS-resistant complex, which is typical to inhibitory serpins. The ratio of moles of Tk-serpin needed to inhibit 1 mol of protease (stoichiometry of inhibition, SI) varies from 40 to 80 at 20°C, but decreases to the minimum values of 3-7 as the temperature increases. The inhibitory activities of Tk-serpin for these proteases increase as the stabilities of these proteases decrease, suggesting that a flexibility of the active-site of protease is one of the determinants for susceptibility of protease to inhibition by Tk-serpin. This report showed for the first time that Tk-serpin inhibits both chymotrypsin- and subtilisin-like serine proteases and its inhibitory activity increases as the temperature increases up to 100°C.

High quality structure of cleaved PAI-1-stab

Here we report the crystal structure of a stablilized plasminogen activator inhibitor-1 variant (PAI-1-N150H-K154T-Q301P-Q319L-M354I (PAI-1-stab)) that shows a cleavage within the reactive centre loop. The new structure is of superior quality compared to the previously determined structure of the cleaved PAI-1-A335P mutant. We present a detailed comparison of the two structures and also compare them with the structure of the active PAI-1-stab. The structural data give important insights into the working mechanism of PAI-1 and also explain the role of various stabilizing mutations.

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.

The length of the reactive center loop modulates the latency transition of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serine protease inhibitor (serpin) protein family, which has a common tertiary structure consisting of three beta-sheets and several alpha-helices. Despite the similarity of its structure with those of other serpins, PAI-1 is unique in its conformational lability, which allows the conversion of the metastable active form to a more stable latent conformation under physiological conditions. For the conformational conversion to occur, the reactive center loop (RCL) of PAI-1 must be mobilized and inserted into the major beta-sheet, A sheet. In an effort to understand how the structural conversion is regulated in this conformationally labile serpin, we modulated the length of the RCL of PAI-1. We show that releasing the constraint on the RCL by extension of the loop facilitates a conformational transition of PAI-1 to a stable state. Biochemical data strongly suggest that the stabilization of the transformed conformation is owing to the insertion of the RCL into A beta-sheet, as in the known latent form. In contrast, reducing the loop length drastically retards the conformational change. The results clearly show that the constraint on the RCL is a factor that regulates the conformational transition of PAI-1.

Biochemical mechanism of action of a diketopiperazine inactivator of plasminogen activator inhibitor-1

XR5118 [(3 Z,6 Z )-6-benzylidine-3-(5-(2-dimethylaminoethyl-thio-))-2-(thienyl)methylene-2,5-dipiperazinedione hydrochloride] can inactivate the anti-proteolytic activity of the serpin plasminogen activator inhibitor-1 (PAI-1), a potential therapeutic target in cancer and cardiovascular diseases. Serpins inhibit their target proteases by the P(1) residue of their reactive centre loop (RCL) forming an ester bond with the active-site serine residue of the protease, followed by insertion of the RCL into the serpin's large central beta-sheet A. In the present study, we show that the RCL of XR5118-inactivated PAI-1 is inert to reaction with its target proteases and has a decreased susceptibility to non-target proteases, in spite of a generally increased proteolytic susceptibility of specific peptide bonds elsewhere in PAI-1. The properties of XR5118-inactivated PAI-1 were different from those of the so-called latent form of PAI-1. Alanine substitution of several individual residues decreased the susceptibility of PAI-1 to XR5118. The localization of these residues in the three-dimensional structure of PAI-1 suggested that the XR5118-induced inactivating conformational change requires mobility of alpha-helix F, situated above beta-sheet A, and is in agreement with the hypothesis that XR5118 binds laterally to beta-sheet A. These results improve our understanding of the unique conformational flexibility of serpins and the biochemical basis for using PAI-1 as a therapeutic target.

Distal hinge of plasminogen activator inhibitor-1 involves its latency transition and specificities toward serine proteases

BACKGROUND

The plasminogen activator inhibitor-1 (PAI-1) spontaneously converts from an inhibitory into a latent form. Specificity of PAI-1 is mainly determined by its reactive site (Arg346-Met347), which interacts with serine residue of tissue-type plasminogen activator (tPA) with concomitant formation of SDS-stable complex. Other sites may also play roles in determining the specificity of PAI-1 toward serine proteases.

RESULTS

To understand more about the role of distal hinge for PAI-1 specificities towards serine proteases and for its conformational transition, wild type PAI-1 and its mutants were expressed in baculovirus system. WtPAI-1 was found to be about 12 fold more active than the fibrosarcoma PAI-1. Single site mutants within the Asp355-Arg356-Pro357 segment of PAI-1 yield guanidine activatable inhibitors (a) that can still form SDS stable complexes with tPA and urokinase plasminogen activator (uPA), and (b) that have inhibition rate constants towards plasminogen activators which resemble those of the fibrosarcoma inhibitor. More importantly, latency conversion rate of these mutants was found to be approximately 3-4 fold faster than that of wtPAI-1. We also tested if Glu351 is important for serine protease specificity. The functional stability of wtPAI-1, Glu351Ala, Glu351Arg was about 18 +/- 5, 90 +/- 8 and 14 +/- 3 minutes, respectively, which correlated well with both their corresponding specific activities (84 +/- 15 U/ug, 112 +/- 18 U/ug and 68 +/- 9 U/ug, respectively) and amount of SDS-stable complex formed with tPA after denatured by Guanidine-HCl and dialyzed against 50 mM sodium acetate at 4 degrees C. The second-order rate constants of inhibition for uPA, plasmin and thrombin by Glu351Ala and Glu351Arg were increased about 2-10 folds compared to wtPAI-1, but there was no change for tPA.

CONCLUSION

The Asp355-Pro357 segment and Glu351 in distal hinge are involved in maintaining the inhibitory conformation of PAI-1. Glu351 is a specificity determinant of PAI-1 toward uPA, plasmin and thrombin, but not for tPA.

Plasminogen activator inhibitor-1 polymers, induced by inactivating amphipathic organochemical ligands

Negatively charged organochemical inactivators of the anti-proteolytic activity of plasminogen activator inhibitor-1 (PAI-1) convert it to inactive polymers. As investigated by native gel electrophoresis, the size of the PAI-1 polymers ranged from dimers to multimers of more than 20 units. As compared with native PAI-1, the polymers exhibited an increased resistance to temperature-induced unfolding. Polymerization was associated with specific changes in patterns of digestion with non-target proteases. During incubation with urokinase-type plasminogen activator, the polymers were slowly converted to reactive centre-cleaved monomers, indicating substrate behaviour of the terminal PAI-1 molecules in the polymers. A quadruple mutant of PAI-1 with a retarded rate of latency transition also had a retarded rate of polymerization. Studying a number of serpins by native gel electrophoresis, ligand-induced polymerization was observed only with PAI-1 and heparin cofactor II, which were also able to copolymerize. On the basis of these results, we suggest that the binding of ligands in a specific region of PAI-1 leads to so-called loop-sheet polymerization, in which the reactive centre loop of one molecule binds to beta-sheet A in another molecule. Induction of serpin polymerization by small organochemical ligands is a novel finding and is of protein chemical interest in relation to pathological protein polymerization in general.

Mapping of the epitope of a monoclonal antibody protecting plasminogen activator inhibitor-1 against inactivating agents

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serpin family of serine proteinase inhibitors. Serpins inhibit their target proteinases by an ester bond being formed between the active site serine of the proteinase and the P1 residue of the reactive centre loop (RCL) of the serpin, followed by insertion of the RCL into beta-sheet A of the serpin. Concomitantly, there are conformational changes in the flexible joint region lateral to beta-sheet A. We have now, by site-directed mutagenesis, mapped the epitope for a monoclonal antibody, which protects the inhibitory activity of PAI-1 against inactivation by a variety of agents acting on beta-sheet A and the flexible joint region. Curiously, the epitope is localized in alpha-helix C and the loop connecting alpha-helix I and beta-strand 5A, on the side of PAI-1 opposite to beta-sheet A and distantly from the flexible joint region. By a combination of site-directed mutagenesis and antibody protection against an inactivating organochemical ligand, we were able to identify a residue involved in conferring the antibody-induced conformational change from the epitope to the rest of the molecule. We have thus provided evidence for communication between secondary structural elements not previously known to interact in serpins.

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.

Acyl-enzyme complexes between tissue-type plasminogen activator and neuroserpin are short-lived in vitro

The serine protease tissue-type plasminogen activator (t-PA) initiates the fibrinolytic protease cascade and plays a significant role in motor learning, memory, and neuronal cell death induced by excitotoxin and ischemia. In the fibrinolytic system, the serpin PAI-1 negatively regulates the enzymatic activity of both single-chain and two-chain t-PA (sct-PA and tct-PA). In the central nervous system, neuroserpin (NSP) is a serpin thought to regulate t-PA enzymatic activity. We report that although both sct-PA and tct-PA rapidly form acyl-enzyme complexes with NSP in vitro, the interactions are short-lived, rapidly progressing to complete cleavage of NSP and regeneration of fully active enzyme. All NSP molecules appear to transit through the detectable acyl-enzyme intermediate and progress to completion of cleavage; no subpopulation that functions as a pure substrate was detected. Likewise, all molecules were reactive, with no evidence of a latent subpopulation. The interactions between NSP and t-PA were distinct from those between plasmin and NSP, wherein the same peptide bond was cleaved but there was no evidence of a detectable plasmin-NSP acyl-enzyme complex. The interactions between t-PA and NSP contrast with the formation of long-lived, physiologically irreversible acyl-enzyme complexes between t-PA and PAI-1, suggesting that the physiologic effect of t-PA-NSP interactions may be more complex than previously thought.

The role of beta-strand 5A of plasminogen activator inhibitor-1 in regulation of its latency transition and inhibitory activity by vitronectin

Plasminogen activator inhibitor-1 (PAI-1) is a potential target for anti-thrombotic and anti-cancer therapy. It circulates in plasma in a complex with vitronectin (VN). We have studied biochemical mechanisms for PAI-1 neutralisation and its modulation by VN, using site-directed mutagenesis and limited proteolysis. We demonstrate that VN, besides delaying conversion of PAI-1 to the inactive latent form, also protects PAI-1 against cold- and detergent-induced substrate behaviour and counteracts conversion of PAI-1 to inert forms by certain amphipathic organochemical compounds. VN protection against cold- and detergent-induced substrate behaviour is associated with inhibition of the proteolytic susceptibility of beta-strand 5A. Alanine substitution of a lysine residue placed centrally in beta-strand 5A implied a VN-induced acceleration of latency transition, instead of the normal delay. This substitution not only protects PAI-1 against neutralisation, but also counteracts VN-induced protection against neutralisation. We conclude that beta-strand 5A plays a crucial role in VN-regulation of PAI-1 activity.

Relationship between protein structure and methionine oxidation in recombinant human alpha 1-antitrypsin

alpha 1-Antitrypsin is a metastable and conformationally flexible protein that belongs to the serpin family of protease inhibitors. Although it is known that methionine oxidation in the protein's active site results in a loss of biological activity, there is little specific knowledge regarding the reactivity of each of the protein's methionine residues. In this study, we have used peptide mapping to study the oxidation kinetics of each of alpha 1-antitrypsin's methionines in alpha 1-AT((C232S)) as well as M351L and M358V mutants. These kinetic studies establish that Met1, Met226, Met242, Met351, and Met358 are reactive with hydrogen peroxide at neutral pH and that each reactive methionine is oxidized in a bimolecular, rather than coupled, mechanism. Analysis of Met226, Met351, and Met358 oxidation provides insights regarding the structure of alpha 1-antitrypsin's active site that allow us to relate conformation to experimentally observed reactivity. The relationship between solution pH and methionine oxidation was also examined to evaluate methionine reactivity under conditions that perturb the native structure. Methionine oxidation data show that at pH 5, global conformational changes occur that alter the oxidation susceptibility of each of alpha 1-antitrypsin's 10 methionine residues. Between pH 6 and 9, however, more localized conformational changes occur that affect primarily the reactivity of Met242. In sum, this work provides a detailed analysis of methionine oxidation in alpha 1-antitrypsin and offers new insights into the protein's solution structure.

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.

Importance of the amino-acid composition of the shutter region of plasminogen activator inhibitor-1 for its transitions to latent and substrate forms

The serpins are of general protein chemical interest due to their ability to undergo a large conformational change consisting of the insertion of the reactive centre loop (RCL), which becomes strand 4, into the central beta sheet A. To make space for the incoming RCL, the 'shutter region' opens by the beta strands 3A and 5A sliding apart over the underlying alpha helix B. Loop insertion occurs during the formation of complexes of serpins with their target serine proteinases and during latency transition. This type of loop insertion is unique to plasminogen activator inhibitor-1 (PAI-1). We report here that amino-acid substitutions in a buried cluster of three residues forming a hydrogen bonding network in the shutter region drastically accelerate PAI-1 latency transition; that the rate was in all cases normalized by the PAI-1 binding protein vitronectin; and that substitution of an adjacent beta strand 5A Lys residue, believed to anchor beta strand 5A to other secondary structural elements, had differential effects on the rates of latency transition in the absence and the presence of vitronectin, respectively. An overlapping, but not identical set of substitutions resulted in an increased tendency to substrate behaviour of PAI-1 at reaction with its target proteinases. These findings show that vitronectin regulates the movements of the RCL through conformational changes of the shutter region and beta strand 5A, are in agreement with RCL insertion proceeding by different routes during latency transition and complex formation, and contribute to the biochemical basis for the potential use of PAI-1 as a therapeutic target in cancer and cardiovascular diseases.

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.

Conformational studies of plasminogen activator inhibitor type 1 by fluorescence spectroscopy. Analysis of the reactive centre of inhibitory and substrate forms, and of their respective reactive-centre cleaved forms

The inhibitors that belong to the serpin family are suicide inhibitors that control the major proteolytic cascades in eucaryotes. Recent data suggest that serpin inhibition involves reactive centre cleavage followed by loop insertion, whereby the covalently linked protease is translocated away from the initial docking site. However under certain circumstances, serpins can also be cleaved like a substrate by target proteases. In this report we have studied the conformation of the reactive centre of plasminogen activator inhibitor type 1 (PAI-1) mutants with inhibitory and substrate properties. The polarized steady-state and time-resolved fluorescence anisotropies were determined for BODIPY(R) probes attached to the P1' and P3 positions of the substrate and active forms of PAI-1. The fluorescence data suggest an extended orientational freedom of the probe in the reactive centre of the substrate form as compared to the active form, revealing that the conformation of the reactive centres differ. The intramolecular distance between the P1' and P3 residues in reactive centre cleaved inhibitory and substrate mutants of PAI-1, were determined by using the donor-donor energy migration (DDEM) method. The distances found were 57+/-4 A and 63+/-3 A, respectively, which is comparable to the distance obtained between the same residues when PAI-1 is in complex with urokinase-type plasminogen activator (uPA). Following reactive centre cleavage, our data suggest that the core of the inhibitory and substrate forms possesses an inherited ability of fully inserting the reactive centre loop into beta-sheet A. In the inhibitory forms of PAI-1 forming serpin-protease complexes, this ability leads to a translocation of the cognate protease from one pole of the inhibitor to the opposite one.

Partitioning of serpin-proteinase reactions between stable inhibition and substrate cleavage is regulated by the rate of serpin reactive center loop insertion into beta-sheet A

The serpin family of serine proteinase inhibitors is a mechanistically unique class of naturally occurring proteinase inhibitors that trap target enzymes as stable covalent acyl-enzyme complexes. This mechanism appears to require both cleavage of the serpin reactive center loop (RCL) by the proteinase and a significant conformational change in the serpin structure involving rapid insertion of the RCL into the center of an existing beta-sheet, serpin beta-sheet A. The present study demonstrates that partitioning between inhibitor and substrate modes of reaction can be altered by varying either the rates of RCL insertion or deacylation using a library of serpin RCL mutants substituted in the critical P(14) hinge residue and three different proteinases. We further correlate the changes in partitioning with the actual rates of RCL insertion for several of the variants upon reaction with the different proteinases as determined by fluorescence spectroscopy of specific RCL-labeled inhibitor mutants. These data demonstrate that the serpin mechanism follows a branched pathway, and that the formation of a stable inhibited complex is dependent upon both the rate of the RCL conformational change and the rate of enzyme deacylation.

Determination of the Complex between Urokinase and Its Type-1 Inhibitor in Plasma from Healthy Donors and Breast Cancer Patients

Background: The complex between urokinase (uPA) and its type-1 inhibitor (PAI-1) is formed exclusively from the active forms of these components; thus, the complex concentration in a biological sample may reflect the ongoing degree of plasminogen activation. Our aim was to establish an ELISA for specific quantification of the uPA:PAI-1 complex in plasma of healthy donors and breast cancer patients.

Methods: A kinetic sandwich format immunoassay was developed, validated, and applied to plasma from 19 advanced-stage breast cancer patients, 39 age-matched healthy women, and 31 men.

Results: The assay detection limit was <2 ng/L, and the detection of complex in plasma was validated using immunoabsorption, competition, and recovery tests. Eighteen cancer patients had a measurable complex concentration (median, 68 ng/L; range, <16 to 8700 ng/L), whereas for healthy females and males the median signal values were below the detection limit (median, <16 ng/L; range, <16 to 200 ng/L; P <0.0001). For patient plasma, a comparison with total uPA and PAI-1 showed that the complex represented a variable, minor fraction of the uPA and PAI-1 concentrations of each sample.

Conclusion: The reported ELISA enables detection of the uPA:PAI-1 complex in blood and, therefore, the evaluation of the complex as a prognostic marker in cancer.

Engineering of conformations of plasminogen activator inhibitor-1. A crucial role of beta-strand 5A residues in the transition of active form to latent and substrate forms

The serpin (serine proteinase inhibitor) family is of general protein chemical interest because of its ability to undergo large conformational changes, in which the surface-exposed reactive centre loop (RCL) is inserted as strand 4 in the large central beta-sheet A. Loop insertion is an integral part of the inhibitory mechanism and also takes place at conversion of serpins to the latent state, occurring spontaneously only in plasminogen activator inhibitor-1 (PAI-1). We have investigated the importance of beta-strand 5A residues for the activity and latency transition of PAI-1. An approximately fourfold increase in the rate of latency transition resulted from His-substitution of Gln324 (position 334 in the alpha(1)-proteinase inhibitor template numbering), which interacts with the underlying alpha-helix B. The side chains of Gln321 and Lys325 (template residues 331 and 335, respectively) form hydrogen bonds to the peptide backbone of a loop connecting alpha-helix F and beta-strand 3A. While substitution with Ala of Glu321 had only minor effects on the properties of PAI-1, substitution with Ala of Lys325 led to stabilization of the inhibitory activity at incubation conditions leading to conversion of wild-type PAI-1 to a substrate form, and to an anomalous reaction towards a monoclonal antibody, which induced a delay in the latency transition of the mutant, but not wild-type PAI-1. We conclude that the anchoring of beta-strand 5A plays a crucial role in loop insertion. These findings provide new information about the mechanism of an important example of protein conformational changes.

Topology of the stable serpin-protease complexes revealed by an autoantibody that fails to react with the monomeric conformers of antithrombin

Solving the structure of the stable complex between a serine protease inhibitor (serpin) and its target has been a long standing goal. We describe herein the characterization of a monoclonal antibody that selectively recognizes antithrombin in complex with either thrombin, factor Xa, or a synthetic peptide corresponding to residues P14 to P9 of the serpin's reactive center loop (RCL, ultimately cleaved between the P1 and P'1 residues). Accordingly, this antibody reacts with none of the monomeric conformers of antithrombin (native, latent, and RCL-cleaved) and does not recognize heparin-activated antithrombin or antithrombin bound to a non-catalytic mutant of thrombin (S195A, in which the serine of the charge stabilizing system has been swapped for alanine). The neoepitope encompasses the motif DAFHK, located in native antithrombin on strand 4 of beta-sheet A, which becomes strand 5 of beta-sheet A in the RCL-cleaved and latent conformers. The inferences on the structure of the antithrombin-protease stable complex are that either a major remodeling of antithrombin accompanies the final elaboration of the complex or that, within the complex, at the most residues P14 to P6 of the RCL are inserted into beta-sheet A. These conclusions limit drastically the possible locations of the defeated protease within the complex.

Interactions of serine proteinases with pNiXa, a serpin of Xenopus oocytes and embryos

In a previous study, kinetic assays showed that pNiXa, an Ni(II)-binding serpin of Xenopus oocytes and embryos, strongly inhibits bovine chymotrypsin, weakly inhibits porcine elastase, and does not inhibit bovine trypsin. In this study, analyses by SDS-PAGE and gelatin zymography showed that an SDS-resistant complex is formed upon the interaction of pNiXa with bovine chymotrypsin. No such pNiXa-enzyme complex was detected after pNiXa interactions with porcine elastase, bovine trypsin, or human cathepsin G. The major products of pNiXa cleavage by the four proteinases were partially sequenced by Edman degradation. The cleavage products were also tested by immunoblotting with an antibody to the His-cluster of pNiXa, and by radio-blotting with 63Ni(II). These assays showed that chymotrypsin and elastase cleave pNiXa at the P1-P1 (Thr-Lys) peptide bond near the C-terminus, while trypsin and cathepsin G cleave pNiXa at specific peptide bonds near the N-terminus, within an interesting 26-residue segment, rich in Lys and Gln, that separates the His-cluster of pNiXa from the rest of the molecule. The segment lacks homology to other serpins, but resembles a domain of Xenopus POU3 transcription factor. This study identifies the specific sites for interactions of four serine proteinases with pNiXa, indicates that pNiXa inhibition of chymotrypsin involves a serpin-like mechanism, and shows that 63Ni(II)-binds to the His-cluster of pNiXa.

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.

The P6-P2 region of serpins is critical for proteinase inhibition and complex stability

Two of the prototypic serpins are alpha1-proteinase inhibitor and ovalbumin. alpha1-Proteinase inhibitor is a rapid inhibitor of a number of proteinases and undergoes the characteristic serpin conformational change on cleavage within the reactive center loop, whereas ovalbumin is noninhibitory and does not undergo the conformational change. To investigate if residues from P12 to P2 in the reactive center loop of ovalbumin are intrinsically incapable of being in an inhibitory serpin, we have made chimeric alpha1-proteinase inhibitor variants containing residues P12-P7, P6-P2, or P12-P2 of ovalbumin and determined their inhibitory properties with trypsin and human neutrophil elastase. With the P12-P7 and P6-P2 variants, the steps before and after the fork of the branched suicide-substrate pathway were affected as reflected by changes in rates and stoichiometries of inhibition with both proteinases. The P12-P2 variant showed that those effects were nonadditive, with exclusive substrate behavior for elastase and only residual inhibitory activity against trypsin. The properties of the variants were consistent with them obeying the suicide-substrate mechanism characteristic of serpins. Enzyme activity was regenerated from complexes formed with the P6-P2 variant faster than with wild-type indicating that the rate of turnover of the complex was increased. Based on proteinase susceptibility in the reactive center loops of the P6-P2 and P12-P2 variants, and on an increase in heat stability of the cleaved P12-P2 variant, it was concluded that the variants had undergone complete loop insertion on cleavage. These results show that the reactive center loop residues P12-P2 of ovalbumin can be present in inhibitory serpins although decreasing the inhibitory properties. These data also demonstrate that the residues in the P6-P2 region of serpins are critical for rapid inhibition of proteinases and formation of stable serpin-proteinase complexes.

Plasminogen activator inhibitor-1 represses integrin- and vitronectin-mediated cell migration independently of its function as an inhibitor of plasminogen activation

Cell migration involves the integrins, their extracellular matrix ligands, and pericellular proteolytic enzyme systems. We have studied the role of plasminogen activator inhibitor-1 (PAI-1) in cell migration, using human amnion WISH cells and human epidermoid carcinoma HEp-2 cells in an assay measuring migration from microcarrier beads and a modified Boyden-chamber assay. Active, but not latent or reactive center-cleaved, PAI-1 inhibited migration. A PAI-1 mutant without ability to inhibit plasminogen activation was as active as wild-type PAI-1 as a migration inhibitor, showing that inhibition of plasminogen activation was not involved. PAI-1 specifically interfered with intergrin- and vitronectin-mediated migration: Migration onto vitronectin-coated but not onto fibronectin-coated surfaces was inhibited by PAI-1, a cyclic RGD peptide inhibited migration, and both cell lines expressed vitronectin-binding alpha v-integrins. In addition, active PAI-1, but not latent or reactive center-cleaved PAI-1, inhibited vitronectin binding to integrins in an in vitro binding assay, without affecting binding of fibronectin. Monoclonal antibodies against the urokinase receptor, another vitronectin binding protein, did not affect cell migration in the beads assay, while some inhibitory effect was observed in the Boyden-chamber assay. We conclude that PAI-1, independently of its role as a proteinase inhibitor, inhibits cell migration by competing for vitronectin binding to integrins, while the interference of PAI-1 with binding of vitronectin to the urokinase receptor may play a secondary role. These data define a novel function for the serpin PAI-1, enabling it to regulate cell migration over vitronectin-rich extracellular matrix in the body.