Generation of the serine proteinase plasmin from the extracellular zymogen plasminogen can be catalyzed by either of two other serine proteinases, the urokinase- and tissue-type plasminogen activators (uPA and tPA). The plasminogen activation system also includes the serpins PAI-1 and PAI-2, and the uPA receptor (uPAR). Many findings, gathered over several decades, strongly suggest an important and causal role for uPA-catalyzed plasmin generation in cancer cell invasion through the extracellular matrix. Recent evidence suggests that the uPA system is also involved in cancer cell-directed tissue remodeling. Moreover, the system also supports cell migration and invasion by plasmin-independent mechanisms, including multiple interactions between uPA, uPAR, PAI-1, extracellular matrix proteins, integrins, endocytosis receptors, and growth factors. These interactions seem to allow temporal and spatial reorganizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. The increased knowledge about the plasminogen activation system may allow utilization of its components as targets for anti-invasive therapy.

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 environmentsensitive 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 ␣-helices D and E and ␤-strand 1A, known to act as a flexible joint when ␤-sheet A opens and the reactive center loop inserts as ␤-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.Plasminogen activator inhibitor-1 (PAI-1) 1 is a fast and specific inhibitor of the serine proteinases urokinase-type (uPA) and tissuetype plasminogen activator (tPA) and, as such, an important regulator of extracellular proteolysis in turn over of extracellular matrix and in fibrinolysis (for reviews see Refs. 1 and 2). PAI-1 binds with high affinity to vitronectin (for reviews see Refs. 3 and 4) and may regulate cell migration and adhesion by inhibition of vitronectin binding of integrins and the uPA receptor (5-10). The PAI-1 level in malignant tumors is one of the most informative biochemical markers of a poor prognosis (for reviews see Refs. 11 and 12), and PAI-1 seems to be causally involved in tumor invasion and angiogenesis (13). A high PAI-1 level in blood plasma is a risk factor for ischemic cardiovascular disease and venous thromboembolism (for review see Ref. 14). PAI-1 is therefore a potential target for both anti-cancer and anti-thrombotic therapy.PAI-1 belongs to the serpin superfamily. Serpins are composed of 3 ␤-sheets and 9 ␣-helices. Serpins and their target proteinases form stable complexes by interaction of the active site of the proteinases with the reactive center peptide bond (P 1 -P 1 Ј) in the solvent-exposed, ϳ20-amino acid long peptide loop, the reactive center loop (RCL) (for reviews see Refs. 2 and 15-17). There is both structural and biochemical evidence that complex formation is associated with the P 1 -P 1 Ј bond being cle...

The amyloid precursor protein (APP) plays a central role in Alzheimer's disease, but its actions in normal development are not well understood. Here, a tagged APP ectodomain was used to identify extracellular binding partners in developing chick brain. Prominent binding sites were seen in the olfactory bulb and on retinal axons growing into the optic tectum. Co-precipitation from these tissues and tandem mass spectrometry led to the identification of two associated proteins: contactin 4 and NgCAM. In vitro binding studies revealed direct interactions among multiple members of the APP and contactin protein families. Levels of the APP processing fragment, CTF␣, were modulated by both contactin 4 and NgCAM. In the developing retinotectal system, APP, contactin 4 and NgCAM are expressed in the retina and tectum in suitable locations to interact. Functional assays revealed regulatory effects of both APP and contactin 4 on NgCAM-dependent growth of cultured retinal axons, demonstrating specific functional interactions among these proteins. These studies identify novel binding and functional interactions among proteins of the APP, contactin and L1CAM families, with general implications for mechanisms of APP action in neural development and disease.

We have analysed the susceptibility of latent, active, reactive-centre-cleaved and plasminogen-activator-complexed type-I plasniinogen-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 P-sheet A. Proteinase-hypersensitive sites were clustered in three regions. Firstly, susceptibility was observed in the region around a-helix E, p-strand IA, and the flanking loops, which are believed to form tlexible joints during movements of p-sheet A. Secondly, hypersensitive sites were observed in the loop between a-helix I and p-strand 5A. Thirdly, the gate region, encompassing ,&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 PAT-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 reactivecentre-cleaved forms produced under different solvent conditions. The digestion pattern of plasminogenactivator-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 /?-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 P-sheet A and during the inhibitory reaction of serpins with their target proteinases.

Most known members of the serpin superfamily are serine proteinase inhibitors. Serpins are therefore important regulators of blood coagulation, complement activation, fibrinolysis, and turnover of extracellular matrix. Serpins form SDS-resistant complexes of 1:1 stoichiometry with their target proteinases by reaction of their P1-P1' peptide bond with the active site of the proteinases. The nature of the interactions responsible for the high stability of the complexes is a controversial issue. We subjected the complex between the serine proteinase urokinase-type plasminogen activator (uPA) and the serpin plasminogen activator inhibitor-1 (PAI-1) to proteolytic digestion under nondenaturing conditions. The complex could be degraded to a fragment containing two disulfide-linked peptides from uPA, one of which included the active site Ser, while PAI-1 was left undegraded. By further proteolytic digestion after denaturation and reduction, it was also possible to degrade the PAI-1 moiety, and we isolated a fragment containing 10 amino acids from uPA, encompassing the active site Ser, and 6 amino acids from PAI-1, including the P1 Arg. Characterization of the fragment gave results fully in agreement with the hypothesis that it contained an ester bond between the hydroxyl group of the active site Ser and the carboxyl group of the P1 Arg. These data for the first time provide direct evidence that serine proteinases are entrapped at an acyl intermediate stage in serine proteinase-serpin complexes.