Protease-nexin: a cellular component that links thrombin and plasminogen activator and mediates their binding to cells

The efficient catabolism of thrombin-protease nexin 1 complexes is a synergistic mechanism that requires both the LDL receptor-related protein and cell surface heparins

Protease nexin 1 (PN1) is a serine protease inhibitor (SERPIN) that acts as a suicide substrate for thrombin (Th) and urokinase-type plasminogen activator (uPA). PN1 forms 1:1 stoichiometric complexes with these proteases, which are then rapidly bound, internalized, and degraded. The low density lipoprotein receptor-related protein (LRP) is the receptor responsible for the internalization of protease-PN1 complexes. However, we found that the LRP is not significantly involved in the initial cell surface binding of thrombin-PN1, leading us to investigate what cellular component was responsible for this initial interaction. Since Th-PN1 complexes retain a high-affinity for heparin after complex formation, unlike several of the other SERPINs, we tested the possibility that cell surface heparins were involved in initial complex binding. Soluble heparin was found to be a potent inhibitor of the binding of Th-PN1 to the cell surface and greatly facilitated the dissociation of Th-PN1 complexes pre-bound in the absence of soluble heparin. To ascertain the role of cell surface heparins, further studies were done using complexes of thrombin and PN1(K7E), a variant of PN1 in which the heparin binding site was rendered non-functional. When added at equal initial concentrations of complexes, Th-PN1(K7E) was catabolized 5- to 10-fold less efficiently than Th-PN1, a direct result of the greatly diminished initial binding of the Th-PN1(K7E) complexes. These data demonstrate the sizable contribution of cell surface heparins to Thrombin-PN1 complex binding and support a model in which these heparins act to concentrate the complexes at the cell surface facilitating their subsequent LRP-dependent endocytosis.

Thrombin causes a marked delay in skeletal myogenesis that correlates with the delayed expression of myogenin and p21CIP1/WAF1

Thrombin is a multifunctional serine protease whose activity is regulated in the extravasculature by an extracellular inhibitor, protease nexin-1. Because protease nexin-1 expression has been shown to be regulated during skeletal muscle cell differentiation, we reasoned that thrombin inactivation may be an important requirement for this developmental process. To test this hypothesis, we examined the effects of thrombin on differentiating C2C12 myoblasts. We report here that myogenesis, as scored by myotube formation, is considerably delayed by thrombin. This regulation correlated with delayed expression of myogenin and p21(CIP1/WAF1), both considered critical components of the skeletal muscle cell differentiation program. Regulation occurred at the RNA level, indicating that the effect of thrombin is either transcriptional or post-transcriptional. Furthermore, we present evidence suggesting that this regulation is mediated by the thrombin receptor. Although thrombin is mitogenic for certain cell types, we found that delay of myogenesis in C2C12 cells did not involve a mitogenic signal. Taken together, these results imply that inhibition of the serine protease thrombin may be required for proper progression through the myogenic differentiation program. The data point to potentially important roles that thrombin and protease nexin-1 may play during skeletal muscle development.

Endogenous serine protease inhibitor modulates epileptic activity and hippocampal long-term potentiation

Protease nexin-1 (PN-1), a member of the serpin superfamily, controls the activity of extracellular serine proteases and is expressed in the brain. Mutant mice overexpressing PN-1 in brain under the control of the Thy-1 promoter (Thy 1/PN-1) or lacking PN-1 (PN-1-/-) were found to develop epileptic activity in vivo and in vitro. Theta burst-induced long-term potentiation (LTP) and NMDA receptor-mediated synaptic transmission in the CA1 field of hippocampal slices were augmented in Thy 1/PN-1 mice and reduced in PN-1-/- mice. Compensatory changes in GABA-mediated inhibition in Thy 1/PN-1 mice suggest that altered brain PN-1 levels lead to an imbalance between excitatory and inhibitory synaptic transmission.

Identification of a binding site in protease nexin I (PN1) required for the receptor mediated internalization of PN1-thrombin complexes

An overlapping synthetic peptide library was constructed representing most of the mature protease nexin I (PN1) sequence from the amino terminus to the reactive center. This library, along with peptides from the heparin binding domain and from the region carboxyl-terminal to the P1 residue of the cleavage site, was screened for the inhibition of 125I-thrombin (Th)-PN1 complex binding and degradation. A peptide corresponding to residues Pro47-Ile58 in the PN1 sequence was identified as a potent inhibitor of 125I-Th-PN1 complex degradation, although it did not affect binding significantly. Pro47-Ile58 was shown to competitively inhibit the low density lipoprotein receptor-related protein (LRP)/alpha2-macroglobulin receptor-mediated endocytosis of 125I-Th-PN1 complexes in mouse embryo fibroblasts. Pro47-Ile58 is an apparent transition sequence in PN1, separating sheet-6B and helix-B. The sequence of Pro47-Ile58, PHDNIVISPHGI, suggests that it forms a loop structure defined by the seven underlined amino acids bordered by proline residues at each end. These studies are the first to identify a putative binding site in a serine protease inhibitor that is required for LRP-mediated internalization.

Protease nexin 1 in the murine kidney: glomerular localization and up-regulation in glomerulopathies

Protease nexin 1 (PN-1), a potent serpin-class antiprotease, is thought to be synthesized in the murine kidney. However, neither the cellular localization of PN-1 synthesis nor its role has yet been defined. To address these questions, we determined by in situ hybridizations RNase protection assay and immunoblotting, the sites of PN-1 mRNA accumulation in normal mouse kidneys and the modulation of PN-1 expression in several pathological conditions. In normal kidneys, PN-1 mRNA was detected primarily in glomeruli, most likely in mesangial cells. The glomerular expression of PN-1 was substantially enhanced not only in lupus-like glomerulonephritis (induced by IgG3 monoclonal rheumatoid factors or occurring spontaneously in lupus-prone mice), but also in mild glomerular lesions associated with intracapillary thrombi induced by IgG3 anti-trinitrophenyl monoclonal antibodies. In contrast, no modulation of PN-1 mRNA levels was observed during the course of lipopolysaccharide-induced acute tubular necrosis. A constitutive PN-1 gene expression and its up-regulation during glomerular injury suggest a possible role for PN-1 in glomerular biology. In view of its high inhibitory activity towards thrombin, mesangial PN-1 may be involved in the control of glomerular coagulation following initial glomerular injuries.

Protease nexin I (PNI) in mouse brain is expressed from the same gene as in seminal vesicle

Protease nexin I (PNI), a serine protease inhibitor (serpin), is the most potent tissue inhibitor of thrombin. In the nervous system, PNI has been shown to participate in processes related to synaptic plasticity and neuronal survival. We assigned the human gene for PNI (P17) to chromosome 2q33-35, and to syntenic regions in mouse chromosome 1. Others showed that a similar serpin was expressed in mouse seminal vesicle, which presented the possibility of a "duplicate" gene. The data also raised controversy over the quantity of PNI mRNA expressed in the brain vs peripheral tissues, such as seminal vesicle. In order to further our investigations of PNI regulation and its influence on neuronal survival and neuroprotection, it was necessary to confirm whether the nexin observed in mouse brain samples was identical to the published protease nexin I sequences. To accomplish this, we performed DNA sequence analysis of cDNAs made from RNAs isolated from mouse forebrain and hindbrain as well as from seminal vesicle. These confirmed the identity of the mouse PNI gene (SPI4) in brain and peripheral tissues. Furthermore, Northern hybridization studies indicated that the PNI message is present at lower levels in the adult brain compared to the adult seminal vesicle. Western immunoblotting showed no differences between brain and seminal vesicle PNI proteins. The PNI cDNAs generated will serve as useful probes for the continued characterization of the serpin:protease balance as it relates to nerve cell function.

Prostaglandin E2 regulates production of plasminogen activator isoenzymes, urokinase receptor, and plasminogen activator inhibitor-1 in primary cultures of rat calvarial osteoblasts

The bone resorbing agent, prostaglandin E2 (PGE2), was found to alter several components of the plasminogen activator (PA)/plasmin pathway in primary cultures of rat neonatal osteoblast-like cells. The mRNA and activities of both urokinase-type PA (uPA) and tissue-type PA (tPA) were enhanced by PGE2 treatment. The presence of mRNA for the uPA receptor (uPAR) has been demonstrated in these cells and steady-state levels shown to be greatly enhanced, the response being rapid and sustained for at least 24 hours. mRNA for plasminogen activator inhibitor 1 (PAI-1) was modulated in a biphasic manner, with inhibition of the constitutive level apparent at 4 hours of treatment and stimulation apparent at 12 hours and longer, while PAI-1 protein, measured by an ELISA assay for rat PAI-1, was diminished over this period. Neither PAI-2 mRNA nor mRNA for the broad spectrum protease inhibitor, protease nexin-1 (PN-1), was found to be modulated by PGE2. Therefore, PGE2 is likely to stimulate cell surface proteolytic activity, since uPA mRNA and cell-associated activity were elevated, as was mRNA for the cellular receptor for uPA. Although it was not possible to measure uPAR number and affinity it seems likely that elevated uPAR mRNA would translate into increased uPARs which would localize the increased uPA activity to the pericellular region. tPA mRNA and activity were also increased transiently with the activity inhibited with prolonged incubations, apparently by PAI-1. Elevation of tPA mRNA and activity may result in elevated activity within the extracellular matrix as tPA has been reported to associate with several matrix proteins. Thus the early effect of PGE2 would be to promote proteolysis, both pericellularly and in the extracellular matrix. The inhibition of PAI-1 mRNA and protein, which would contribute to the elevation of activity, is due to PGE2, but the later stimulatory effect on PAI-1 mRNA may be due to feedback regulation by transforming growth factor beta (TGF beta), secreted by osteoblasts and activated by elevated levels of PA.

Message of nexin 1, a serine protease inhibitor, is accumulated in the follicular papilla during anagen of the hair cycle

A group of specialized mesenchymal cells located at the root of the mammalian hair follicle, known as the follicular or dermal papillary cells, are involved in regulating the hair cycle, during which keratinocytes of the lower follicle undergo proliferation, degeneration and regrowth. Using the arbitrarily primed-PCR approach, we have identified a 1.3 kb messenger RNA that is present in large quantities in cultured rat follicular papillary cells, but not in skin fibroblasts. This mRNA encodes nexin 1, a potent protease inhibitor that can inactivate several growth-modulating serine proteases including thrombin, urokinase and tissue plasminogen activator. In situ hybridization showed that nexin 1 message is accumulated in the follicular papilla cells of anagen follicles, but is undetectable in keratinocytes or other skin mesenchymal cells. In addition, nexin 1 message level varies widely among several immortalized rat vibrissa papillary cell lines, and these levels correlate well with the reported abilities of these cell lines to support in vivo follicular reconstitution. These results suggest a possible role of nexin 1 in regulating hair follicular growth.

Purification of a novel serpin-like protein from bovine brain

We purified a novel serine proteinase inhibitor (serpin)-like protein from the bovine brain and named it B-43 from its molecular mass, 43 kDa. A cleaved peptide from B-43 was copurified with the native B-43. Partial amino acid sequencing of the purified B-43 showed that this protein was homologous to glia-derived nexin/protease nexin-1 (GDN/PN-1), plasminogen activator inhibitor 2, leukocyte elastase inhibitor (LEI) and placental thrombin inhibitor (PTI) among the serpins. Although B-43 had a similar amino acid composition to these serpins, the biochemical features of B-43 were different from them. B-43 did not form sodium dodecyl sulfate (SDS)-resistant serpin-proteinase complexes with thrombin, urokinase, pancreatic elastase and plasmin, suggesting that these proteinases were not the targets of B-43. In contrast to GDN/PN-1, B-43 did not have an affinity for heparin. B-43, having different biochemical properties from GDN/PN-1, appears to be an additional serpin expressed in the brain.

Utilization of transgenic mice in the study of matrix degrading proteinases and their inhibitors

The extracellular matrix (ECM) acts as both a structural scaffold and an informational medium. Its dynamic status is determined by cells that secrete its constituent molecules and, in most cases, also secrete enzymes that catalyze degradation of these molecules. A stasis between ECM degrading enzymes and their inhibitors maintains the integrity of the matrix. While controlled ECM remodelling is fundamental to several normal processes, uncontrolled disruption underlies diverse pathological conditions. Transgenic mice with specific modulations or a total lack of expression of certain metalloproteinases, serine proteinases or their inhibitors have been generated to elucidate endogenous expression patterns, identify regulatory elements of these genes, and study the physiological consequences of their deregulated expression. With these models we enhance our understanding of the role of proteinases and their inhibitors in diverse normal processes and pathologies including mammary gland development, hemostasis, emphysema and cancer.

The urokinase/urokinase-receptor system and cancer invasion

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

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

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

Plasminogen activator inhibitor-1 in brain tumors: relation to malignancy and necrosis

Tumor necrosis is a common feature of malignant neoplasms. The pathogenesis of tumor necrosis remains poorly documented. Recent evidence has shown a correlation between the presence of tumor necrosis and low content of tissue plasminogen activator in brain tumors and significantly higher levels of plasminogen activator inhibitor-1 (PAI-1) in human glioblastomas. We subjected fresh brain tumor tissue samples (n = 197) to an enzyme-linked immunosorbent assay to determine PAI-1 content. The results were correlated with the presence of edma and necrosis on imaging studies. The samples studied were from normal brain (n = 10), low-grade gliomas (n = 26), meningiomas (n = 47), acoustic neuromas (n = 18), glioblastomas (n = 45), metastases (n = 45), and areas of tumor necrosis (n = 6). The benign tumor samples (n = 96) had 3.5 times less PAI-1 than did the malignant tumors (n = 101). Tumor necrosis samples contained 3.8 times more PAI-1 than did the nonnecrotic malignant tumor samples (P < 0.000001). The benign meningioma samples showed a similar ratio compared with their malignant counterparts (0.35 versus 1.59 ng/mg, respectively, P = 0.0004). Regression analysis results showed a strong correlation between PAI-1 and necrosis (r = 0.47, P < 0.0000028) and, to a lesser extent, brain edema (r = 0.26, P = 0.001). A negative correlation between PAI-1 and tissue plasminogen activator levels almost reached statistical significance (P = 0.07). There was no correlation between PAI-1 content and the tumor size, duration of symptoms, or the sex or age of the patients. The results of this study indicate that malignant transformation is associated with a significant increase in PAlI1 and that PAI-1 may play an integral role in the pathogenesis of tissue necrosis, perhaps via the inhibition of tissue plasminogen activator and the promotion of microthrombosis.

A serine protease inhibitor, protease nexin I, rescues motoneurons from naturally occurring and axotomy-induced cell death

Protease nexin I (PNI) is a member of the family of serine protease inhibitors (serpins) that have been shown to promote neurite outgrowth in vitro from different neuronal cell types. These include neuroblastoma cells, hippocampal neurons, and sympathetic neurons. Free PNI protein is markedly decreased in various anatomical brain regions, including hippocampus, of patients with Alzheimer disease. Here, we report that PNI rescued spinal motoneurons during the period of naturally occurring (programmed) cell death in the chicken in a dose-dependent fashion. Furthermore, PNI prevented axotomy-induced spinal motoneuron death in the neonatal mouse. The survival effect of PNI on motoneurons during the period of programmed cell death was not associated with increased intramuscular nerve branching. PNI also significantly increased the nuclear size of motoneurons during the period of programmed cell death and prevented axotomy-induced atrophy of surviving motoneurons. These results are consistent with the possible role of PNI as a neurotrophic agent. They also support the idea that serine proteases or, more precisely, the balance of proteases and serpins may be involved in regulating the fate of neuronal cells during development.

Protease nexin-1, a potent thrombin inhibitor, is reduced around cerebral blood vessels in Alzheimer's disease

The clotting protease thrombin might contribute to the pathophysiology of central nervous system (CNS) injury and certain diseases by its ability to retract processes on neurons and astrocytes and to stimulate astrocyte proliferation. Protease nexin-1 (PN-1) is a 43 kDa thrombin inhibitor found predominantly in the brain where much of it resides around capillaries and large blood vessels. This location of PN-1 prompted the hypothesis that it may play a protective role against extravasated thrombin released following cerebrovascular injury or under certain pathological conditions. Recent studies indicated that the levels of PN-1 are markedly reduced in the postmortem brains of patients with Alzheimer's disease (AD). It was suggested that this reduction in PN-1 levels was due to the sequestration of PN-1 by extravasated thrombin. In the present study we examined the specific nature of this reduction by immunohistochemical staining of sections from control and AD brains using PN-1 specific antibodies. We show that the levels of PN-1 immunoreactivity around blood vessels and the number of blood vessels exhibiting PN-1 immunoreactivity were markedly reduced in the brains of patients with AD compared to age-matched controls; this reduction was reflected by a decrease in the levels of PN-1 activity and PN-1 protein. Thus an imbalance between PN-1 and thrombin may be a contributing factor in the pathology of AD.

Proteolytic action of thrombin is required for electrical activity-dependent synapse reduction

Molecular mechanisms of activity-dependent synapse reduction were studied in an in vitro mammalian neuromuscular preparation. Synapse reduction in this model is activity-dependent and is substantially reduced by the broad-spectrum protease inhibitor, leupeptin, suggesting the role of activity-dependent proteolytic action in the process. Our present experiments show that a potent and specific thrombin inhibitor, hirudin, at nanomolar concentration completely blocked the activity-dependent synapse reduction. Furthermore, a naturally occurring serine protease inhibitor, protease nexin I (PNI), which closely colocalizes with acetylcholine receptors at the neuromuscular junction, inhibited the synapse reduction at the same low concentration. In contrast, neither cystatin, a cysteine protease inhibitor, nor aprotinin, a serine protease inhibitor that does not inhibit thrombin, blocked the synapse reduction. Similarly, neither of the inhibitors of the calcium-activated proteases calpain I and II prevented the reduction of synapses. These results strongly suggest that serine proteolytic action by thrombin or thrombin-like molecules is required for synapse reduction in our in vitro model of the mammalian neuromuscular junction.

Plasminogen activator inhibitor-1 in the pathogenesis of delayed radiation damage in rat spinal cord in vivo

The pathophysiology of radiation-induced damage to the central nervous system (CNS) is poorly understood. Preliminary data suggest that fibrinolytic inhibitors are involved in the development of necrosis. In this study, cervical spinal cord irradiation was studied in 90 rats by measuring plasminogen activator inhibitor (PAI)-1 on Days 2, 7, 30, 60, 90, 120, 130, or 145 after irradiation. Paralysis due to radiation necrosis developed in all animals kept alive for 140 to 150 days. Assay of PAI-1 was by Western blot, enzyme-linked immunosorbent assay (ELISA), and complex formation with 125I-labeled urokinase. No PAI-1 was detected in normal spinal cord tissue or in irradiated spinal cord up to Day 90. However, PAI-1 was detected at Day 120 and was marked by elevated ELISA levels at the time of paralysis. Western blot showed detectable PAI-1 (51 kD) at Day 120 and very significant levels at the time of paralysis. Complex formation with 125I-labeled urokinase was also detected at Day 120 with similar results. Immunohistochemical studies showed that PAI-1 was highly concentrated within and immediately adjacent to zones of necrosis at 145 days and was absent in normal tissue. This study adds considerable weight to the proposal that PAI-1 is closely associated with the pathogenesis of CNS radiation necrosis.

Peripheral markers and diagnostic criteria in Alzheimer's disease: critical evaluations

This review analyzes recent developments in diagnostic criteria and peripheral markers used clinically in the definitive diagnosis of Alzheimer's disease (AD), comparing past and current views, together with a discussion of their shortcoming and difficulties of implementation. Consideration is given to studies on the presence of amyloid substances outside the central nervous system: in cerebrospinal fluid, in plasma, in primary cultures, and in continuous cultures of cell lines of neuronal and glial origin. We discuss alterations of cholinesterases and noradrenaline in red blood cells (RBC) in AD and, with relation to the infectious theory, the presence of spirochaetes in patients. The activities of the enzymes leading to the formation of amyloid substances and those reflecting more general alterations of metabolic processes are considered, both in respect to their role in the pathogenesis of the neurodegenerative disorders of AD and of their potential use as markers. Enzymatic changes have been studied comparing AD patients with non AD controls as well as with AD relatives: proteases and their inhibitors; plasminogen activators; transketolases; increases in the activity of Cu-Zn superoxide dismutase in AD patients' RBC, serum, fibroblasts and cortical neurons, pointing to alterations in oxidative processes; and apolipoprotein E epsilon 4 allele, linked to late-onset AD and familial cases. This review presents reasons why the involvement of peripheral markers in AD should advance from hypothesis to accepted fact.

Tacrine alters the secretion of the beta-amyloid precursor protein in cell lines

The characteristic features of Alzheimer's disease (AD) include a high density of beta-amyloid-containing plaques in the cerebral cortex and the loss of basal forebrain cholinergic neurons. Amyloid beta-protein (A beta, Mr. approximately 4.5 kDa) is derived from a family of large (Mr. approximately 110-140 kDa) beta-amyloid precursor proteins (APP) which are integral membrane glycoproteins consisting of a large extracytoplasmic domain, a transmembrane domain, and a short cytoplasmic tail. Secreted derivatives of APP lacking the cytoplasmic tail, transmembrane domain, and a small portion of the extracellular domain are generated by the proteolytic processing of full length APP by a family of proteolytic enzymes known as APP secretases. Using cell cultures, we investigated the possibility that APP processing can be regulated by a centrally active cholinesterase inhibitor, tacrine, which has recently been shown to improve memory and cognitive functions in patients with AD. We analyzed the level of APP in glial, fibroblast, pheochromocytoma (PC12), and neuroblastoma cells by immunoblotting cell lysates and conditioned media. Normal levels of secretion of soluble APP derivatives by cells into conditioned media were severely inhibited by treating cells with tacrine. A similar decrease after treatment with tacrine was observed when neuroblastoma and PC12 cells were pretreated with either growth factors, phorbol ester, or retinoic acid. To determine whether the effect of tacrine on APP levels was specific or a more general phenomenon affecting other proteins, we measured the level of heat shock protein-70 (HSP-70) and another secretory protein, protease nexin-1 (PN-1). Tacrine treatment did not alter the level of HSP-70 in cell extracts and tacrine affected mildly the secretion of PN-1. Thus, the processing of HSP and PN-1, unlike APP, was not severely affected by treating cells with tacrine. Our results suggest that tacrine may inhibit an acetylcholinesterase-associated proteolytic activity involved in the secretion of APP, which results in less secretion of soluble APP into the conditioned media from tacrine treated cells. These results demonstrate that tacrine regulates APP secretion in cell cultures and suggest the possibility that tacrine therapy of Alzheimer's disease may, in the longer term, have effects on the process of A beta deposition.

Alzheimer disease: an imbalance of proteolytic regulation?

Alzheimer disease is typified by the accumulation of protein and neuronal death. We propose that neuronal death creates a proteolytic imbalance that generates the pathological lesions. Our hypothesis explains the morphology and topographic distribution of neuritic plaques and neurofibrillary tangles.

Glia-derived nexin/protease nexin-1 is expressed by a subset of neurons in the rat brain

Glia-derived nexin/protease nexin-1 (GDN/PN-1) is a serine protease inhibitor that is secreted by glial cells and fibroblasts in culture. In the adult mammalian nervous system it has been shown to be expressed in the olfactory system and by some glial cells in response to neuronal injury. In situ hybridization and immunocytochemical studies were performed to identify the structures expressing GDN/PN-1 in the developing and adult rat brain. In contrast to a transient widespread expression during pre- and postnatal development, some brain structures constitutively express GDN/PN-1. These include the olfactory nerve layer of the olfactory bulb, basal forebrain, striatum, pyramidal neurons of layer V in the cortex, thalamic nuclei, pars compacta of the substantia nigra, inferior and superior colliculi, and deep cerebellar nuclei. All of these parts, excluding the olfactory nerve layer, are characterized by a high neuronal cell density. Neurons in these regions were immunoreactive for GDN/PN-1. Furthermore GDN/PN-1 expression in cell lines showed that the active protein was synthesized and secreted from B104 but not from NB2a neuroblastoma cells. Although GDN/PN-1 has only been reported to be synthesized by glia, the results presented here demonstrate that in addition, a subset of neurons express this protease inhibitor.

Endogenous antithrombin associated with microvascular endothelium. Quantitative analysis in perfused rat hearts

A recirculating Langendorff heart preparation is used to characterize the endogenous antithrombin associated reversibly with murine vascular endothelium. Rat hearts are perfused clear of blood and then recirculated with a physiological salt solution. Addition of heparin educes antithrombin activity continuously into the perfusate during 6 min of recirculation. This process contrasts with a more rapid equilibration of the system as assessed by displacement of [125I]thrombin with hirudin or with a heparin-antithrombin mixture. Perfusion of washed hearts with [125I]factor Xa, which evidences no significant binding to the coronary endothelium, identifies a minor fraction of the endogenous antithrombin that reacts immediately with factor Xa, i.e., at a rate indicative of heparin enhancement. This rapid-reacting antithrombin is not reproducibly detected with [125I]thrombin, which binds preferentially to thrombomodulin in this system. The amount of antithrombin reacting rapidly with factor Xa is too low to detect as a burst of antithrombin activity eluted into the perfusate when the hearts are perfused with heparin. It is concluded that the murine myocardial microvasculature harbors at least two pools of antithrombin, the minor of which is in an activated configuration characteristic of association with heparin. The major pool is in a more slowly accessible compartment or configuration.

Serine proteases and their serpin inhibitors in Alzheimer's disease

Our article documents recent studies in the proteolytic processing of the Alzheimer's beta-amyloid protein precursor (beta-APP), as well as the role of thrombin and its potent inhibitor, protease nexin I in Alzheimer's disease (AD). Since synapse loss correlates best with cognitive decline in AD, we also present in detail, our model of synapse formation and elimination, reviewing recent findings related to the subject as well as our own original data. Recent exciting findings concerning the involvement of thrombin-like activity in synapse elimination, which we feel to be important in neural plasticity are also discussed.

Thrombin in inflammation and healing: relevance to rheumatoid arthritis

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The anion-binding exosite is critical for the high affinity binding of thrombin to the human thrombin receptor

The thrombin receptor has been shown to be a novel member of the family of G-protein coupled receptors (Vu, T.-K. H., Hung, D.T., Wheaton, V.I., and Coughlin, S.R. (1991) Cell 64, 1057-1068). This receptor appears to be activated through a thrombin-mediated proteolytic mechanism which exposes a "tethered ligand" responsible for receptor activation. In order to investigate the initial interactions of thrombin with this receptor, we have constructed cell lines which express high levels of the human thrombin receptor and studied the binding of various forms of thrombin to the cell surface. Analysis of transfected cells with thrombin receptor monoclonal antibodies identified a particular cell line (clone #5-18) which displayed > 150,000 thrombin receptors per cell. Clone #5-18 appeared to express functional receptors since treatment with thrombin resulted in both a 15-20 fold increase of cytoplasmic phosphoinositide levels and a comparable shift in the EC50 of thrombin-mediated calcium mobilization when compared to non-transfected CHO cells. Binding of 125I-alpha-thrombin to clone #5-18 did not reach equilibrium at 37 degrees C. However, direct binding studies of 125I-alpha-, 125I-diisopropylphospho (DIP)-alpha-, and 125I-beta-thrombin to clone #5-18 demonstrated that binding at 4 degrees C was saturable and reversible for each ligand. Analysis of the binding data revealed Kd's of 0.8 nM, 0.7 nM and 9.7 nM for 125I-alpha-, 125I-DIP-alpha- and 125I-beta-thrombin respectively. Association of 125I-alpha-, DIP-alpha, and beta-thrombin could be competed by unlabelled alpha- and DIP-alpha-thrombin. Unlabelled beta-thrombin, which has a modified anion-binding exosite, was a poor competitor for 125I-alpha- and 125I-DIP-alpha-thrombin, but did compete for 125I-beta-thrombin. In addition, the hirudin54-65 peptide competed at submicromolar concentrations for the binding of alpha- and DIP-alpha-thrombin, but not for beta-thrombin. This peptide binds specifically at the anion-binding exosite of alpha-thrombin and has been shown to have a lower affinity for beta-thrombin. These results demonstrate directly a high affinity interaction between thrombin and its receptor, and suggest that an important component is the high affinity association of the thrombin receptor with the anion-binding exosite of thrombin.

Urokinase-type plasminogen activator and its receptor: new targets for anti-metastatic therapy?

Urokinase-type plasminogen activator (uPA) and its receptor are instrumental in cell invasion and metastasis; their high levels of expression in human tumours correlates with a high risk of recurrence. uPA has a pleiotropic effect on cell migration and spreading in vivo and in vitro through the activation of plasminogen or other protein factors at the cell surface or in the extracellular matrix. Three specific inhibitors, with different tissue-specificities and regulatory properties, modulate cell-surface exposure of uPA activity. Overall, uPA is at the centre of a complex system affecting cell movement and invasiveness, and inhibition of uPA is now a goal of anti-metastatic therapy. The role of uPA and its inhibition are discussed in this review by Francesca Fazioli and Francesco Blasi.

Activities, localizations, and roles of serine proteases and their inhibitors in human brain tumor progression

The plasminogen activation system consists of plasminogen activators and their inhibitors, serine proteases, and serpins. The proteases and inhibitors regulate a variety of processes in tissue morphogenesis, differentiation, cell migration, and cancer cell invasiveness and metastasis. One of the plasminogen activators, urokinase-type plasminogen activator (uPA), binds to a specific surface and provides a localized cell surface proteolytic activity required for the destruction of extracellular matrix, which is a vital step in tumor cell invasion. The proteolytic activity of uPA is modulated by its cell surface receptor, as well as by plasminogen activator inhibitor type-1 (PAI-1) and, to a lesser degree, by other inhibitors. The role of plasminogen activators and their inhibitors in cancer invasion can be demonstrated in the development and progression of malignant brain tumors. Our findings indicate that uPA and PAI-1 expression are dramatically upregulated in malignant brain tumors in parallel with the histological progression of the tumors. The results suggest that these molecules may contribute to tumor invasion in addition to their significant role in angiogenesis. An evaluation of the plasminogen activation system could add diagnostic and prognostic significance to the evaluation of individual patients.

Modulation of fibrinolysis by ionizing radiation

Radiation-induced damage to the central nervous system is believed to be targeted to glial or endothelial cells or both, although the pathophysiology of this process is still poorly understood. A series of experiments were, therefore, conducted, including irradiation to primary rat astrocytes (in vitro) and rat spinal cords (in vivo). The levels of plasminogen activators (uPA and tPA) and their inhibitors (PNI and PAI-1) were determined by fibrin zymography, ELISA, amidolytic activity assay, complex formation, and Western blot analysis. Fibrin zymography revealed the presence of M(r) 48,000 (uPA) and M(r) 68,000 (tPA) lytic bands that were increased in irradiated samples. Three- to four-fold higher levels of tPA and 8- to 10-fold higher levels of uPA were detected in irradiated samples. Western blot analysis confirmed the presence of a 51-kDa band (PAI-1) in irradiated samples. PAI-1 is undetectable in nonirradiated spinal cord. Serum-free medium and cell and spinal cord extracts of nonirradiated samples showed a 43-kDa band (PNI), the intensity of which is decreased in irradiated samples. Four- to five-fold decreased levels of PNI were detected in irradiated serum-free media and cell extracts, but no levels of PNI were detected in irradiated spinal cord extracts. This study provides additional information regarding the proposed roles of plasminogen activators and their inhibitors in the development of CNS damage after irradiation.

Co-distribution of protease nexin-1 and protease nexin-2 in brains of non-human primates

The protease nexins are protease inhibitors which regulate key blood coagulation proteases and which appear to be involved in certain physiological and pathological processes in the brain. Protease nexin-1 (PN-1), a potent inhibitor of thrombin, can regulate processes on cultured neurons and astrocytes. Protease nexin-2 (PN-2), a potent inhibitor of coagulation factor XIa, is identical to the secreted form of the Alzheimer's amyloid beta-protein precursor. In the present studies, PN-1 and PN-2 were analyzed in different tissues of monkey using monoclonal antibodies for either quantitative immunoblotting or specific [125I]protease-binding assays. PN-1 was detected only in brain. PN-2 was most abundant in brain, followed by testis and to a lesser extent kidney. Other tissues examined including spinal cord, heart, pancreas, spleen, liver, lung and muscle were essentially devoid of both PN-1 and PN-2. Within the brain, the levels of PN-1 and PN-2 were highest in the parietal cortex and lowest in the cerebellum and brainstem. The thalamus and striatum contained intermediate amounts of both proteins. Aged Cebus monkey cerebral cortical tissue contained slightly lower levels of PN-1 than did the middle-aged or young monkey tissue. The co-distribution of PN-1 and PN-2 in brain, their relative abundance in brain cortex, and previous studies on their functions suggest that in the brain they may participate in the regulation of blood coagulation and cell growth and differentiation.

Increased levels of plasminogen activator inhibitor-1 (PAI-1) in human brain tumors

Considerable interest in the roles of serine proteases and serine protease inhibitors (serpins) in regulating physiologic and pathologic tissue remodeling has led to studies that indicate their critical participation in development and diseases of the brain. Plasminogen activator inhibitor-1 (PAI-1) is the most significant regulator of fibrinolysis in plasma, but little is known of the levels or activities of this important serpin in normal brain and brain tumors. For this reason, we estimated qualitative and quantitative levels of PAI-1 in normal human brain and various brain tumors. Western-blot results indicated that a 51 kDa band recognized with polyclonal anti-PAI-1 was more prominently in metastatic and glioblastoma than in meningiomas and low-grade gliomas; normal human brain lacked any detectable band. Reverse zymography also showed high levels of PAI-1 in malignant brain tumors. The complex formation with 125I-urokinase demonstrated that PAI-1 complex levels were increased in metastatic and glioblastoma when compared with low-grade gliomas and meningiomas. Since PAI-1 acts as a modulator of fibrinolysis, a better understanding of the balance between serine proteases and PAI-1 is likely to enhance our knowledge of brain tumor biology.

Plasminogen activator inhibitor 1, the primary regulator of fibrinolysis, in normal human cerebrospinal fluid

The occurrence of type-1 plasminogen activator inhibitor (PAI-1) in human cerebrospinal fluid (CSF) has not previously been reported. As a member of the serpin superfamily of serine protease inhibitors and an acute phase response component, PAI-1 has powerful potential roles in nervous system homeostasis. We have detected this serpin antigen using a polyclonal anti-PAI-1 antibody in normal human CSF. In Western blotting, PAI-1 in several CSF samples appears as a two-band antigen of Mr = 54 and 35 kDa, presumably the intact and proteolytic fragment, respectively. In vitro complex formation studies confirm that the 54 kDa form of PAI-1 interacts with 125I-urokinase after activation with SDS, but the 35 kDa form does not. Quantification of total PAI-1 antigen in 18 normal human CSF samples by ELISA reveals a mean value of 1.0 +/- 0.07 (SEM) micrograms/dL, indicating that a relatively low concentration of the inhibitor occurs in normal human CSF. This information should now allow comparison of PAI-1 levels and activity in various neurologic disorders.

Glycosaminoglycans and the regulation of blood coagulation

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Polarized secretion of plasminogen activators by epithelial cell monolayers

We have investigated the synthesis and the polarized secretion of plasminogen activators (PAs) in three epithelial cell lines (FRT, derived from rat thyroid; MDCK, from canine kidney, and CaCo-2, from human intestine) grown on filters, in bicameral systems. Confluency and acquisition of functional polarity were assessed by measuring transepithelial resistance and by showing polarized secretion of endogenous proteins. By zymography, before and after immunoprecipitation with specific antibodies, we found that FRT cells synthesized tissue plasminogen activator (tPA) and that tPA activity was mostly confined to the apical cell compartment. MDCK and CaCo-2 cells, instead, synthesized urokinase-type plasminogen activator (uPA). In MDCK cells the uPA activity was found predominantly in the apical cell compartment while in CaCo-2 cells it was mostly basolateral.

The plasminogen-plasmin system in malignancy

The study of the plasminogen-plasmin system has, in the past, contributed much to the understanding of fibrinolysis and thrombolysis. Attention is now focused on the role of the components of this system in many biologic functions. Findings of uPA, its receptor and its inhibitor in many tumor tissues and tumor cell lines, strongly implicate their involvement in tumor invasion, tumor cell proliferation and metastasis. The characteristics of the plasminogen activators, the uPA receptor and the plasminogen activator inhibitors as well as their expression and regulation in tumors and tumor cell lines are reviewed.

Regulation of neuronal migration and neuritogenesis by distinct surface proteases. Relative contribution of plasmin and a thrombin-like protease

The relative contribution of two neuronal surface proteases, plasmin and a protease with thrombin-like specificity, on NB2a/dl neuroblastoma migration and neuritogenesis were examined. Exogenous plasmin induced cell body rounding and increased cell migration, but did not prevent or reverse neurite outgrowth. Inhibition of endogenous plasmin by its specific inhibitor, aprotinin, suppressed migration but did not induce neuritogenesis. Removal or inhibition of the thrombin-like protease by serum deprivation or hirudin addition, respectively, induced neurite outgrowth, as shown in our previous studies, but did not suppress migration. By contrast, trypsin induced simultaneous cell rounding and neurite retraction. These findings indicated that plasmin may regulate cell migration, while the thrombin-like protease may regulate facets of neurite outgrowth. Although unable to induce de novo neuritogenesis, plasmin inhibition potentiated the otherwise transient neurites induced by simultaneous inhibition of the thrombin-like protease. Since cultured neuronal cells migrate primarily in the direction of newly elaborated neurites, this finding is interpreted to indicate that cessation of neuronal migration by plasmin inhibition enhances net neurite outgrowth by inhibition of the putative thrombin-like protease.

Thrombin as a multifunctional protein: induction of cell adhesion and proliferation

The serine protease thrombin (E.C.3.4.21.5) is well recognized for its central role in hemostasis. In addition, thrombin is unique among the enzymes participating in the clotting cascade, by virtue of its cell activation effects induced via the enzymatic pocket or via functional domains located throughout the molecule. In this review, we elaborate on "nonhemostatic" activities of thrombin among which are interactions with vessel wall components. These activities include promotion of cellular adhesion and induction of smooth muscle cell proliferation. Thrombin can exert these effects when it is in a fluid phase and when it is immobilized to extracellular matrix.