Inhibitors of urokinase and thrombin in cultured neural cells

The role of serpins in tumor cell migration

Tumor cells are characterized by uncontrolled cell growth at a primary site that is caused by genetic alterations. Tumor cells that metastasize from their primary site to distant locations are commonly referred to as malignant. Cell migration is a critical step in this process. The ability of tumor cells to migrate and invade is partly controlled by proteolytic enzymes. These enzymes are secreted by either the tumor cells themselves or adjacent cells. They represent all classes of proteases, including serine and cysteine proteases. Serine proteases, in particular urokinase plasminogen activator (uPA), initiate a proteolytic cascade that culminates in degrading components of the extracellular matrix (ECM). Some serine proteases are controlled by a superfamily of proteins known as serpins. This minireview provides an overview of serpins that are vital in regulating tumor cell migration and progressing cancer.

Matrix metalloproteinase-9 regulates tumor cell invasion through cleavage of protease nexin-1

Matrix metalloproteinase-9 (MMP-9) expression is known to enhance the invasion and metastasis of tumor cells. In previous work based on a proteomic screen, we identified the serpin protease nexin-1 (PN-1) as a potential target of MMP-9. Here, we show that PN-1 is a substrate for MMP-9 and establish a link between PN-1 degradation by MMP-9 and regulation of invasion. PN-1 levels increased in prostate carcinoma cells after downregulation of MMP-9 and in tissues of MMP-9-deficient mice, consistent with PN-1 degradation by MMP-9. We identified three MMP-9 cleavage sites in PN-1 and showed that mutations in those sites made PN-1 more resistant to MMP-9. Urokinase plasminogen activator (uPA) is inhibited by PN-1. MMP-9 augmented uPA activity in the medium of PC3-ML cells by degrading PN-1. Prostate cancer cells, overexpressing PN-1 or treated with MMP-9 shRNA, had reduced cell invasion in Matrigel. PN-1 siRNA restored uPA activity and the invasive capacity. PN-1 mutated in the serpin inhibitory domain, the reactive center loop, failed to inhibit uPA and to reduce Matrigel invasion. This study shows a novel molecular pathway in which MMP-9 regulates uPA activity and tumor cell invasion through cleavage of PN-1.

Plasticity and stabilization of neuromuscular and CNS synapses: interactions between thrombin protease signaling pathways and tissue transglutaminase

The first association of the synapse as a potential site of neurodegenerative disease burden was suggested for Alzheimer's disease (AD) almost 30 years ago. Since then protease:protease inhibitor (P:PI) systems were first linked to functional regulation of synaptogenesis and synapse withdrawal at the neuromuscular junction (NMJ) more than 20 years ago. Confirmatory evidence for the involvement of the synapse, the rate-limiting or key unit in neural function, in AD did not become clear until the beginning of the 1990s. However, over the past 15 years evidence for participation of thrombin, related serine proteases and neural PIs, homologous and even identical to those of the plasma clot cascade, has been mounting. Throughout development a balance between stabilization forces, on the one hand, and breakdown influences, on the other, becomes established at synaptic junctions, just as it does in plasma clot proteins. The formation of protease-resistant cross-links by the transglutaminase (TGase) family of enzymes may add to the stability for this balance. The TGase family includes coagulation factor XIIIA and 8 other different genes, some of which may also influence the persistence of neural connections. Synaptic location of protease-activated, G-protein-coupled receptors (PARs) for thrombin and related proteases, their serpin and Kunitz-type PIs such as protease nexin I (PNI), alpha1-antichymotrypsin (alpha-ACT), and the Kunitz protease inhibitor (KPI)-containing secreted forms of beta-amyloid protein precursor (beta-APP), along with the TGases and their putative substrates, have all been amply documented. These findings strongly add to the conclusion that these molecules participate in the eventual structural stability of synaptic connections, as they do in coagulation cascades, and focus trophic activity on surviving terminals during periods of selective contact elimination. In disease states, this imbalance is likely to be shifted in favor of destabilizing forces: increased and/or altered protease activity, enhanced PAR influence, decreased and/or altered protease inhibitor function, reduction and/or alteration in tTG expression and activity, and alteration in its substrate profile. This imbalance further initiates a cascade of events leading to inappropriate programmed cell death and may well be considered evidence of synaptic apoptosis.

Binding of pigment epithelium-derived factor (PEDF) to retinoblastoma cells and cerebellar granule neurons. Evidence for a PEDF receptor

Pigment epithelium-derived factor (PEDF) has neuronal differentiation and survival activity on retinoblastoma and cerebellar granule (CG) cells. Here, we investigated the presence of PEDF receptors on retinoblastoma Y-79 and CG cells. PEDF radiolabeled with (l25)I remained biologically active and was used for radioligand binding analysis. The binding was saturable and specific to a single class of receptors on both cells and with similar affinities (K(d) = 1.7-3.6 nM, B(max) = 0.5-2.7 x 10(5) sites/Y-79 cell; and K(d) = 3.2 nM, B(max) = 1.1 x 10(3) sites/CG cell). A polyclonal antiserum to PEDF, previously shown to block the PEDF neurotrophic activity, prevented the (125)I-PEDF binding. We designed two peptides from a region previously shown to confer the neurotrophic property to human PEDF, synthetic peptides 34-mer (positions 44-77) and 44-mer (positions 78-121). Only peptide 44-mer competed for the binding to Y-79 cell receptors (EC(50) = 5 nM) and exhibited neuronal differentiating activity. PEDF affinity column chromatography of membrane proteins from both cell types revealed a PEDF-binding protein of approximately 80 kDa. These results are the first demonstration of a PEDF-binding protein with characteristics of a PEDF receptor and suggest that the region comprising amino acid positions 78-121 of PEDF might be involved in ligand-receptor interactions.

Differentially expressed genes in hippocampal cell cultures in response to an excitotoxic insult by quinolinic acid

The NMDA-type glutamate receptor agonist quinolinic acid (QA), which causes tissue lesions in the rat brain as well as cell loss in neuronal cultures, is widely used in models of glutamate excitotoxicity. The aim of this study was to evaluate the alterations in gene expression in a primary hippocampal cell culture after exposure to QA. By means of differential mRNA display, we were able to pinpoint as many as 23 bands which appeared to be upregulated after a 6-h treatment with quinolinic acid. The differential expression of 13 cDNAs could be confirmed by dot blot and/or Northern analysis. Of the cDNAs, the p112 regulatory subunit of the 26S proteasome, a PDGF-associated protein and the glia-derived protease nexin PN-1 could be identified. The results provide emphasis to the participation of proteolysis and protease inhibition in neurodegenerative processes.

Plasminogen activators potentiate thrombin-induced brain injury

BACKGROUND AND PURPOSE

Evidence suggests that cerebral edema following intracerebral hemorrhage (i.c.h.) results from a mass effect in combination with neurotoxic injury from clot-derived substrates such as thrombin. Thrombolytics can compete for thrombin inhibitors endogenous to the brain. This study examines the effect of intracerebral infusion of thrombolytics, tissue plasminogen activator (tPA), and urokinase (uPA), individually and in combination with thrombin.

METHODS

Various 100 microL solutions were stereotactically infused into the right basal ganglia of adult male rats. Animals were euthanized 24 hours later, and brain sections were taken for measurement of water, sodium, and potassium content.

RESULTS

Regardless of dose, when infused independently tPA (2 micrograms) and uPA (2000 and 5000 Plough units) failed to produce any significant tissue edema compared with vehicle control tissues. However, when either thrombolytic was infused concomitantly with thrombin (1 or 5 U), brain water, sodium, and potassium content all demonstrated a potentiation of thrombin-induced brain injury (P < 0.05). In addition, animal deaths were significantly greater than expected in animals receiving a combination of tPA (2 micrograms) and thrombin (5 U) compared with either drug alone (P < 0.001).

CONCLUSIONS

This study indicates that brain edema caused by thrombin can be greatly amplified by the presence of plasminogen activators, perhaps because the latter compete for naturally occurring thrombin inhibitors. In the context of ICH, our results suggest that the use of tPA or uPA to lyse clotted blood in brain parenchyma may promote edema formation in surrounding tissue.

The role of thrombin-like (serine) proteases in the development, plasticity and pathology of the nervous system

There is increasing evidence suggesting that members of the serine protease family, including thrombin, chymotrypsin, urokinase plasminogen activator, and kallikrein, may play a role in normal development and/or pathology of the nervous system. Serine proteases and their cognate inhibitors have been shown to be increased in the neural parenchyma and cerebrospinal fluid following injury to the blood brain barrier. Zymogen precursors of thrombin and thrombin-like proteases as well as their receptors have also been localized in several distinct regions of the developing or adult brain. Thrombin-like proteases have been shown to exert deleterious effects, including neurite retraction and death, on different neuronal and non-neuronal cell populations in vitro. These effects appear to be mediated through cell surface receptors and can be prevented or reversed with specific serine protease inhibitors (serpins). Furthermore, we have recently shown that treatment with protease nexin-1 (a serpin that inhibits thrombin-like proteases) promotes the survival and growth of spinal motoneurons during the period of programmed cell death and following injury. Taken together, these observations suggest that thrombin-like proteases play a deleterious role, whereas serpins promote the development and maintenance of neuronal cells. Thus, changes in the balance between serine proteases and their cognate inhibitors may lead to pathological states similar to those associated with some neurodegenerative diseases such as Alzheimer's disease. The present review summarizes the current state of research involving such serine proteases and speculates on the possible role of these thrombin-like proteases in the development, plasticity and pathology of the nervous system.

Human glioma U-251 cells contain type 1 plasminogen activator inhibitor in a rapidly releasable form

Because recent information suggests that the localized deposition of protease inhibitors is one mechanism by which cells regulate pericellular proteolysis during tissue invasion, the distribution of type 1 plasminogen activator inhibitor (PA1-1) associated with the invasive human glioma cell line U-251 was investigated. Direct and reverse fibrin zymography indicated the presence of urokinase-like plasminogen activator (u-PA) and PAI-1 in U-251 conditioned media and cell lysates. PA1-1 antigen was detected immunologically in cytoplasmic granules present within cellular processes of U-251 cells and these organelles could be isolated on Percoll density gradients in a high density band. In contrast, u-PA activity and another secreted protein, amyloid beta-protein precursor, were only present in the low density region of the gradients. Functional analysis of PAI-1 in the granules contained within the high density fractions revealed the presence of active PAI-1. Incubation of U-251 cells with the secretagogue, 8-bromoadenosine 3':5'-cyclic monophosphate, resulted in a 3-fold increase in the release of PAI-1 in the media conditioned by these cells. These data suggest that the human glioma cell line U-251 contains PAI-1 in a rapidly releasable form, which may provide another mechanism by which these tumors could regulate proteolytic activity in a localized manner.

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.

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.

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.

Enhancement of the synthesis and secretion of nerve growth factor in primary cultures of glial cells by proteases: a possible involvement of thrombin

Newborn rat brain astrocytes cultured in vitro in a chemically defined medium are shown to secrete enhanced levels of nerve growth factor (NGF) when they are exposed to various types of proteases. Proteolytic enzymes such as alpha-thrombin or collagenase induce a continuous, dose-dependent enhancement of the levels of cell-secreted NGF. Incubation of astrocytes for a 24-h period with 300 ng/ml of alpha-thrombin (approximately 9 nM, or 1 U/ml) results in an increase of the levels of cell-secreted NGF by a factor of three- to fourfold, and at doses 10 times higher, stimulation by a factor of up to four- to fivefold was observed. This phenomenon reflects an enhancement of the cellular pool of NGF mRNA, already noticeable after 3 h of treatment, which is preceded by a temporary activation of protooncogenes encoding transcription factors of the AP-1 family, such as c-fos, c-jun or junB. Trypsin, plasmin, alpha-chymotrypsin, or elastase also enhanced, to different extents, the levels of cell-secreted NGF. However, unlike alpha-thrombin or collagenase, these enzymes cause, above a critical concentration, an extensive cell detachment from the solid support, and this is accompanied by a decrease of their activity on the production of NGF, so that their dose-response curves are bell shaped. Stimulation was maximal at those concentrations that cause a limited loosening of the cell-substratum interactions, as evidenced by a retraction of some cell processes after 24 h of treatment. Studies of the effect of alpha-thrombin indicate that the proteolytic activity itself is required to enhance the production of NGF by astrocytes. Inactivation of alpha-thrombin with D-phenyl-alanyl-L-propyl-L-arginine chloromethyl ketone, phenylmethylsulfonyl fluoride, antithrombin III, or hirudin results in a marked decrease of the stimulatory effect. Furthermore, the prolonged presence of alpha-thrombin is required to elicit a maximal effect on the levels of extracellular NGF, which was observed after 48 h of treatment. It is known that some effects of alpha-thrombin require binding to the cell surface. We found that gamma-thrombin, which still has some proteolytic activity but has lost its ability to bind to the cell surface, is almost as potent as alpha-thrombin in promoting the release of NGF. It is concluded that the effect of thrombin on NGF synthesis is essentially mediated by its proteolytic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycosaminoglycans and the regulation of blood coagulation

Images

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.

Synthesis and release of neuroactive substances by glial cells

Glia contain, synthesize, or release more than 20 neuroactive compounds including neuropeptides, amino acid transmitters, eicosanoids, steroids, and growth factors. The stimuli that elicit release differ among compounds but include neuropeptides, neurotransmitters, receptor agonists, and elevated external [K+]. The mechanisms of release are poorly understood in most cases. Many of the neuroactive compounds are localized in discrete subpopulations of glia. Thus, glia are equipped to send as well as receive chemical messages and appear to be present as classes of cells with differing abilities to communicate chemically. It is possible that glia are as diverse as neurons in their functional characteristics.