Enhanced release of plasminogen activator inhibitor(s) but not of plasminogen activators by cultured rat glial cells treated with interleukin-1

Interleukin-1β enhances the angiotensin-induced expression of plasminogen activator inhibitor-1 through angiotensin receptor upregulation in human astrocytes

Plasminogen activator inhibitor-1 (PAI-1) regulates not only fibrinolysis but extracellular matrix remodeling, and angiotensin II is known to play an important role in controlling the expression of PAI-1 in astrocytes. We have studied the effect of interleukin-1beta (IL-1beta), one of major cytokines also active in the nervous system, on the angiotensin II-induced expression of PAI-1 in human astrocytes. Cultures of normal human astrocytes were stimulated with IL-1beta and angiotensin II, and the expression of mRNAs for angiotensin II type 1 receptor (AT1) and PAI-1 was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) or real-time quantitative PCR. PAI-1 protein in astrocyte-conditioned medium was measured by enzyme-linked immunosorbent assay (ELISA). IL-1beta enhanced the expression of AT1 in astrocytes in time- and concentration-dependent manners. After 24-h stimulation, 10 ng/ml IL-1beta and 10 nM angiotensin II increased the levels of PAI-1 protein in astrocyte-conditioned medium by 1.9-fold and 1.8-fold of the basal value, respectively. There was no synergistic effect when the cells were stimulated simultaneously with IL-1beta and angiotensin II. When the cells were stimulated, with angiotensin II, 16 h after the stimulation with IL-1beta, the production of PAI-1 was enhanced by 1.4-fold as compared to the cells stimulated only with IL-1beta. CV-11794, an AT1 antagonist, inhibited the enhanced PAI-1 production in response to angiotensin II. We conclude that IL-1beta increases angiotensin II-induced PAI-1 secretion by astrocytes through the induction of AT1, and the enhanced secretion of PAI-1 may modulate functions of plasminogen activators in the nervous system.

Transforming growth factor-beta and ischemic brain injury

1. Necrosis and apoptosis are the two fundamental hallmarks of neuronal death in stroke. Nevertheless, thrombolysis, by using the recombinant serine protease t-PA, remains until now the only approved treatment of stroke in man. 2. Over the last years, the cytokine termed Transforming Growth Factor-beta1 (TGF-beta1) has been found to be strongly up-regulated in the central nervous system following ischemia-induced brain damage. 3. Recent studies have shown a neuroprotective activity of TGF-beta1 against ischemia-induced neuronal death. In vitro, TGF-beta1 protects neurons against excitotoxicity by inhibiting the t-PA-potentiated NMDA-induced neuronal death through a mechanism involving the up-regulation of the type-1 plasminogen activator inhibitor (PAI-1) in astrocytes 4. In addition, TGF-beta1 has been recently characterized as an antiapoptotic factor in a model of staurosporine-induced neuronal death through a mechanism involving activation of the extracellular signal-regulated kinase 1/2 (Erk1/2) and a concomitant increase phosphorylation of the antiapoptotic protein Bad. 5. Altogether, these observations suggest that either TGF-beta signaling or TGF-beta1-modulated genes could be good targets for the development of new therapeutic strategies for stroke in man.

Smad3-dependent induction of plasminogen activator inhibitor-1 in astrocytes mediates neuroprotective activity of transforming growth factor-beta 1 against NMDA-induced necrosis

The intravenous injection of the serine protease, tissue-type plasminogen activator (t-PA), has shown to benefit stroke patients by promoting early reperfusion. However, it has recently been suggested that t-PA activity, in the cerebral parenchyma, may also potentiate excitotoxic neuronal death. The present study has dealt with the role of the t-PA inhibitor, PAI-1, in the neuroprotective activity of the cytokine TGF-beta1 and focused on the transduction pathway involved in this effect. We demonstrated that PAI-1, produced by astrocytes, mediates the neuroprotective activity of TGF-beta 1 against N-methyl-D-aspartate (NMDA) receptor-mediated excitotoxicity. This t-PA inhibitor, PAI-1, protected neurons against NMDA-induced neuronal death by modulating the NMDA-evoked calcium influx. Finally, we showed that the activation of the Smad3-dependent transduction pathway mediates the TGF-beta-induced up-regulation of PAI-1 and subsequent neuroprotection. Overall, this study underlines the critical role of the t-PA/PAI-1 axis in the regulation of glutamatergic neurotransmission.

Mechanism of plasminogen activator inhibitor-1 regulation by oncostatin M and interleukin-1 in human astrocytes

Glial cells that produce and respond to various cytokines mediate inflammatory processes in the brain. Here, we show that oncostatin M (OSM) and interleukin-1 (IL-1) regulate the expression of plasminogen activator inhibitor-1 (PAI-1) and urokinase-type plasminogen activator (uPA) in human astrocytes. Using the PAI-1 reporter constructs we show that the -58 to -51 proximal element mediates activation by both cytokines. This element is already bound by c-fos/c-jun heterodimers in unstimulated astrocytes, and treatment with cytokine strongly stimulates both expression of c-fos and binding of c-fos/c-jun heterodimers. In addition, IL-1 activates an inhibitory mechanism that down-regulates PAI-1 expression after longer exposure to this cytokine. Overexpression of dominant-negative signal transducer and activator of transcription-1 (STAT1), STAT3, STAT5 and inhibitor of nuclear factor-kappaB (IkappaB) suppressed OSM/IL-1-induced expression of the PAI-1 reporter construct. We conclude that OSM and IL-1 regulate the PAI-1 gene expression via up-regulating c-fos levels and subsequent binding of c-fos/c-jun heterodimers to the proximal element of the PAI-1 gene.

Epidermal growth factor and pro-inflammatory cytokines regulate the expression of components of plasminogen activation system in U373-MG astrocytoma cells

Cytokines and growth factors that influence both secretion of the extracellular matrix (ECM) proteins and migration of the cells decide about the final outcome of tissue remodelling. We have examined expression of the components of the plasminogen activation system in human astrocytoma U373-MG cells and found that interleukin 1beta (IL-1beta), tumour necrosis factor alpha TNF-alpha), interferon gamma (INF-gamma) and epidermal growth factor (EGF) specifically regulate the expression of tissue-type plasminogen activator (t-PA), urokinase-type plasminogen activator (u-PA), plasminogen activator inhibitor type 1 (PAI-1) and protease nexin-1 (PN-1). We conclude that EGF and IFN-gamma are new important regulators of the plasminogen activation system in astrocytoma cells and, therefore, may influence turnover of extracellular matrix and migration of cells within the brain.

Two-dimensional gel analysis of secreted proteins induced by interleukin-1 beta in rat astrocytes

Interleukin-1 beta (IL-1 beta) is a pro-inflammatory cytokine produced in the brain by endogenous microglial cells responding to injury. Levels of IL-1 beta are elevated in several neurodegenerative disorders, including Alzheimer's disease. IL-1 beta, which can act as a mitogen for astrocytes, also elicits the expression and secretion of multiple factors and paracrine 'second messengers' such as other cytokines, nerve growth factor, prostaglandins and nitric oxide that may in turn modulate neuronal and glial responses to injury. Utilizing giant, high-resolution two-dimensional gel electrophoresis, we have sought to more fully define the potential range of protein mediators that are secreted by astrocytes treated with IL-1 beta. In cultured rat astrocytes, we observe dramatic increases in the secretion of eight different protein species after 24 h of treatment with human recombinant IL-1 beta (1 U/ml). Seven of the proteins are also induced by tumor necrosis factor-alpha or basic fibroblast growth factor. Based on immunoprecipitation with specific antisera, we have identified three of these proteins as plasminogen activator inhibitor type-1, ceruloplasmin, and complement component C3. The identities of the other proteins, including the IL-1 beta-specific induction, are currently unknown. Characterization of these downstream modulators of IL-1 beta action complements gene-based approaches and will provide a better understanding of astrocyte responses to injury as well as markers for astrocyte activation in neurodegenerative diseases.

Serine protease inhibitors: novel therapeutic targets for stroke?

Although the thrombolytic activity of tissue-type plasminogen activator (t-PA) may be beneficial in the acute treatment of stroke, recent studies have suggested that this serine protease could also play a critical role in determining the extent of neuronal death after injury to the central nervous system (CNS). This hypothesis is based on several experimental results: t-PA-deficient mice are resistant to excitotoxic neuronal death induced by the intrahippocampal injection of kainate; the infarct volume induced by occlusion of the middle cerebral artery is reduced in t-PA knockout mice; and the intravenous injection of t-PA can under certain circumstances potentiate the infarct volume in animals subjected to middle cerebral artery occlusion. In the CNS, the serine proteases have been identified to occur both in neurons and glial cells. Their enzymatic activity regulates the balance between the accumulation and the degradation of the extracellular matrix. They are involved in many physiologic functions, ranging from synaptic outgrowth during perinatal development to plasticity in adults. For instance, thrombin and t-PA are known to modulate neurite outgrowth and tissue remodeling in the early stages of development. In the adult brain, t-PA may contribute to the late phase of long-term potentiation and to the subsequent synaptic growth in the hippocampal mossy fiber pathway. This balance between the degradation and accumulation of the extracellular matrix may also be integral to various pathologic processes involved in acute brain injury. For example, compounds that modulate the activity of serine proteases exhibit neuroprotective activity. Based on the above, numerous studies have focused on the production and modulation of the endogenously produced serine protease inhibitors, termed serpins, such as type 1 plasminogen activator inhibitor, neuroserpin, and protease nexin-1. In the present review, we will discuss the need to distinguish between the potentially neurotoxic effects of t-PA and its beneficial effect on reperfusion. We will present data supporting the idea that the modulation of serine protease activity may represent a novel and efficient strategy for the treatment of acute cerebral injury in humans.

Plasminogen enhances the secretion of plasminogen activator inhibitor-1 from cultured rat astrocytes

To investigate the physiological significance of microglia-derived plasminogen (PGn) in the central nervous system, we determined the effects of rat PGn on the secretion of plasminogen activator inhibitor (PAI) in cultured rat astrocytes by reverse zymography and Western blotting. The cultured astrocytes normally produce only a limited amount of PAI-1. After stimulation with PGn, however, the amount of active PAI-1 in the astrocyte-conditioned medium was significantly increased in a time-dependent manner. PGn was also proved to activate p38 MAP kinase and c-Jun N-terminal kinase in these astrocytes. Taken together, these results suggest that microglia-derived PGn increases the secretion of PAI-1 in astrocytes through the activation of MAP kinases, and that enhanced PAI-1 regulates various physiological phenomena including tissue remodeling, neuronal plasticity and neurotoxicity.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Axonal regeneration

Axons damaged in a peripheral nerve are often able to regenerate from the site of injury along the degenerate distal segment of the nerve to reform functional synapses. Schwann cells play a central role in this process. However, in the adult mammalian central nervous system, from which Schwann cells are absent, axonal regeneration does not progress to allow functional recovery. This is due to inhibitors of axonal growth produced by both oligodendrocytes and astrocytes and also to the decreased ability of adult neurons to extend axons during regeneration compared to embryonic neurons during development. However once provided with a substrate conducive to axonal growth, such as a peripheral nerve graft, many central neurons are able to regenerate axons over long distances. Over the past year this response has been utilised in experimental models to produce a degree of behavioural recovery.

Metabolism of neuropeptide Y and calcitonin gene-related peptide by cultivated neurons and glial cells

Neuropeptide Y and calcitonin gene-related peptide are abundant neuropeptides in the mammalian central and peripheral nervous systems. Their enzymatic degradation by cultivated neurons, astrocytes, and microglia, as well as by purified urokinase-type plasminogen activator, plasmin, thrombin, and trypsin, was investigated in an in vitro approach to elucidate the role of matrix-degrading serine proteinases for inactivation of neuropeptides, especially those of higher amino acid chain length, in the brain. Astrocytes were almost unable to catabolize the peptides. Cultivated neurons and microglia digested neuropeptide Y through cleavage after Arg19, Arg25, Arg33, and Arg35, calcitonin gene-related peptide was cleaved after Arg11 and Arg18. The same cleavage pattern was observed, when neuropeptide Y and calcitonin gene-related peptide were degraded by purified urokinase-type plasminogen activator, plasmin, thrombin, and trypsin. For further characterization of the neuropeptide-degrading serine proteinase activities from cell cultures, urokinase-type plasminogen activator was identified on microglia by immunostaining, whereas tissue-type plasminogen activator mRNA occurred in neurons and astrocytes, but not in microglia. The data are consistent with the possibility that the neuropeptide-degrading serine proteinase activity on neurons and microglia is due to a mixture of plasmin and plasminogen activator activities.

Interleukin-1β inhibits luteinizing hormone-induced plasminogen activator activity in rat preovulatory folliclesin vitro

The effects of interleukin-1β (IL-1β) and tumour necrosis factor-α (TNFα) on ovulation-associated plasminogen activator (PA) activity were investigated using preovulatory follicles excised 48h after equine chorionic gonadotrophin (16IU)-priming of immature rats. Follicles were incubated for 6 and 14h with a single dose of LH (1 μg/ml) only, or various cytokine doses in the presence or absence of LH. PA activity in follicular homogenates was determined by a radioactively labelled fibrin-coated plate method and secreted levels of the ovulatory mediators progesterone (P) and prostaglandin E (PGE) were measured by radioimmunoassay. LH induced timedependent rises in PA (2.5-fold over control at 6h and fourfold over control at 14h), while IL-1β and TNFα alone had no effect over either time period. LH and cytokine coincubations over 14h revealed that IL-1β dosedependently inhibited the LH-induced increase in PA activity, up to 85%. The effects of TNFα on LH-induced PA activity were not significant. Both IL-1β and TNFα increased P and PGE secretion time- and dose-dependently. In summary, IL-1β dose-dependently inhibits the LH-induced increase in PA activity in rat preovulatory folliclesin vitro while, as with TNFα, increasing P and PGE synthesis. This study, shows that the earlier reported pro-ovulatory action of IL-1β is not likely to be mediated by activation of the PA-system and suggests that IL-1β may mediate a regulatory loop controlling the extent and distribution of LH-induced PA activity in rat preovulatory follicles.

Astrocytes and axon regeneration in the central nervous system

The failure of axons to regenerate in the central nervous system is mainly due to inhibition by the environment, made up of astrocytes and oligodendrocytes, which surrounds regions of damage. Both cell types are inhibitory to axon regeneration, and it seems likely that each will have to be neutralised before significant axon regeneration is achieved. Axons regenerate over the surface of astrocytes grown in normal monolayer culture but not through three-dimensional astrocyte cultures. Astrocyte cell lines have been created, some of which resemble embryonic astrocytes and form a loose tissue with extensive extracellular space which permits axon regeneration, and others which model astrocytes in the damaged brain having little extracellular space and much extracellular matrix material. There is no correlation between the inhibitory effect on axons and the expression of cell adhesion molecules, proteases, protease inhibitors, and a variety of extracellular matrix molecules. However, extracellular matrix produced by inhibitory cell lines is inhibitory to axon regeneration, while that produced by permissive cell lines is not. This difference depends on the production of a chondroitinase-sensitive proteoglycan which can block the neurite-inducing effects of laminin so that treatment of inhibitory extracellular matrix with chondroitinase renders it more permissive to axon regeneration.

Protease production by cultured microglia: substrate gel analysis and immobilized matrix degradation

The production of collagen-degrading proteases by cultured neonatal rat microglia was examined using an immobilized fibronectin-gelatin matrix coupled to a fluorescent marker and by substrate gel analysis. When microglia were plated onto the surface of the matrix and incubated under resting (nonstimulated) conditions, a small but visible amount of immobilized matrix was degraded. Treatment with lipopolysaccharide (LPS) or interleukin-1 (IL-1) significantly increased the number of microglia demonstrating substrate degradation. Substrate-SDS polyacrylamide gel electrophoresis of samples of supernatants from untreated cultured microglia indicated the presence of a 72 and a 92 kD metalloproteinase with characteristics corresponding to collagenases. Supernatants from untreated astrocyte cultures were shown to have primarily a 72 kD metalloproteinase. Proteinase activity increased on stimulation of the microglia with LPS and IL-1 in a dose-dependent fashion. These results indicate that cultured microglia release active proteases capable of degrading the extracellular matrix in a localized region. The production of proteases by activated microglia may have important physiological and pathophysiological consequences within the restricted extracellular matrix of the CNS.

Molecular profile of reactive astrocytes--implications for their role in neurologic disease

The central nervous system responds to diverse neurologic injuries with a vigorous activation of astrocytes. While this phenomenon is found in many different species, its function is obscure. Understanding the molecular profile characteristic of reactive astrocytes should help define their function. The purpose of this review is to provide a summary of molecules whose levels of expression differentiate activated from resting astrocytes and to use the molecular profile of reactive astrocytes as the basis for speculations on the functions of these cells. At present, reactive astrocytosis is defined primarily as an increase in the number and size of cells expressing glial fibrillary acidic protein. In vivo, this increase in glial fibrillary acidic protein-positive cells reflects predominantly phenotypic changes of resident astroglia rather than migration or proliferation of such cells. Upon activation, astrocytes upmodulate the expression of a large number of molecules. From this molecular profile it becomes apparent that reactive astrocytes may benefit the injured nervous system by participating in diverse biological processes. For example, upregulation of proteases and protease inhibitors could help remodel the extracellular matrix, regulate the concentration of different proteins in the neuropil and clear up debris from degenerating cells. Cytokines are key mediators of immunity and inflammation and could play a critical role in the regulation of the blood-central nervous system interface. Neurotrophic factors, transporter molecules and enzymes involved in the metabolism of excitotoxic amino acids or in the antioxidant pathway may help protect neurons and other brain cells by controlling neurotoxin levels and contributing to homeostasis within the central nervous system. Therefore, an impairment of astroglial performance has the potential to exacerbate neuronal dysfunction. Based on the synopsis of studies presented, a number of issues become apparent that deserve a more extensive analysis. Among them are the relative contribution of microglia and astrocytes to early wound repair, the characterization of astroglial subpopulations, the specificity of the astroglial response in different diseases as well as the analysis of reactive astrocytes with techniques that can resolve fast physiologic processes. Differences between reactive astrocytes in vivo and primary astrocytes in culture are discussed and underline the need for the development and exploitation of models that will allow the analysis of reactive astrocytes in the intact organism.

Transforming growth factor beta as a neuronoglial signal during peripheral nervous system response to injury

In contrast to the central nervous system (CNS), the peripheral nervous system (PNS) displays an important regenerative ability which is dependent, at least in part, on Schwann cell properties. The mechanisms which stimulate Schwann cells to adapt their behavior after a lesion to generate adequate conditions for PNS regeneration remain unknown. In this work, we report that adult rat dorsal root ganglion (DRG) neurons are able, after a lesion performed in vivo or when they are dissociated and cultured in vitro, to synthesize transforming growth factor beta (TGF beta), a pleiotropic growth factor implicated in wound healing processes and in carcinogenesis. This TGF beta is tentatively identified as the beta-1 isoform. Adult rat DRG neurons release a biologically active form of TGF beta which is able to elicit multiple Schwann cell responses including a stimulation to proliferate. Moreover, purified TGF beta-1 produces a Schwann cell morphology alteration and decreases the secretion of tissue-type plasminogen activator (tPA) and enhances the secretion of plasminogen activator inhibitor (PAI) by Schwann cells. This generates conditions which are thought to favor a successful neuritic regrowth. Furthermore, purified TGF beta-1 stimulates type IV collagen mRNA expression in Schwann cells. This subtype of collagen is associated with the process of myelinization. Finally, TGF beta-1 decreases nerve growth factor (NGF) mRNA expression by Schwann cells, an effect which could participate in the maintenance of a distoproximal NGF gradient during nerve regeneration. We propose that neuronal TGF beta plays an essential role as a neuronoglial signal that modulates the response of Schwann cells to injury and participates in the successful regeneration processes observed in the PNS.

Microglia isolated from rat brain secrete a urokinase-type plasminogen activator

In a previous study, we found particular proteases which degrade myelin basic protein (MBP) in a conditioned medium of cultured rat brain microglia. The MBP degrading activity in microglial-conditioned medium (Mic-CM) increased markedly in the presence of plasminogen. By Sephadex G-150 column chromatography, plasminogen-dependent MBP degrading activity was eluted at the position of about 47 kDa and 28 kDa. Furthermore slight plasminogen-dependent protease activity in the presence of fibrin (tissue plasminogen activator activity) was detected at a molecular weight of about 68 kDa. The two molecular forms (47 kDa and 28 kDa) of plasminogen-dependent protease were demonstrated by casein-zymography, and it was suggested that they were urokinase type-plasminogen activators (uPA). This suggestion was confirmed by immunoblotting using anti-uPA antiserum. The unique 28 kDa type was considered to be produced from the 47 kDa form by limited proteolysis. Secretion of PA from microglia was demonstrated by cell zymography. In contrast, significant secretion of plasminogen activator inhibitor could not be detected in the Mic-CM. In addition, lipopolysaccharide significantly decreased the secretion of PA from microglia, while interleukin-1 and basic fibroblast growth factor enhanced the secretion.

Modulation of plasminogen activator and plasminogen activator inhibitor expression in the human U373 glioblastoma/astrocytoma cell line by inflammatory mediators

The human U373 glioblastoma/astrocytoma cell line was found to constitutively produce and secrete a plasminogen activator and a plasminogen activator inhibitor. The plasminogen activator was identified as urokinase based on apparent molecular weight, immunoblotting with anti-urokinase antibodies, and Northern blotting with a human urokinase cDNA probe. The inhibitor secreted by U373 cells was found to be related to the PAI-1 molecule based on reactivity with anti-human PAI-1 antibodies, apparent molecular weight, and Northern blot analysis with a human PAI-1 cDNA probe. The expression of both urokinase and the PAI-1-like molecule by U373 cells could be modulated by phorbol myristate acetate or by inflammatory mediators such as interferon-gamma and interleukin-1. In the case of interleukin-1, the alpha form exhibited no detectable effect while the beta form not only elevated inhibitor levels, it also appeared to induce the production of tissue plasminogen activator. Thus, in these cells interleukin-1 beta induces alterations in PA and PAI expression and interleukin-1 alpha does not, even though the two forms are reported to utilize the same cellular receptor.

Regulation of plasminogen activators and type-1 plasminogen activator inhibitor by cyclic AMP and phorbol ester in rat astrocytes

Two plasminogen activators (PAs): tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), as well as the type-1 plasminogen activator inhibitor (PAI-1) are synthesized and secreted by rat astrocytes. Preliminary studies suggest that PA activity plays a role in astrocyte development and differentiation. We have examined the regulation of the PA system by the cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) in purified rat astrocyte cultures. PKA activity was increased by exposing cultured astrocytes to forskolin or dibutyryl cyclic AMP, whereas PKC activity was stimulated with phorbol-12-myristate 13-acetate (PMA). Activation of both second-messenger pathways produced a time- and dose-dependent increase in the total PA activity. However, based on SDS-PAGE/zymography we found that forskolin increased t-PA activity and reduced u-PA activity, whereas PMA treatment caused a significant increase in u-PA activity without altering t-PA activity. Reverse zymography analysis revealed that astrocyte PAI-1 activity is decreased by forskolin and increased by PMA. Together, these results demonstrate that the components of the PA system in rat astrocytes are independently and reciprocally regulated by PKA and PKC. Our findings raise the possibility that the plasminogen activator system could be involved in some of the actions of growth factors and/or neuromodulators that modulate PKC or PKA in astrocytes.

Production of thrombin and antithrombin III by brain and astroglial cell cultures

There is increasing evidence that some proteases and protease inhibitors are produced within the central nervous system. It has been proposed that the balance between these two classes of proteins may be an important modulator of brain cell growth and differentiation. Here we report that antithrombin III (ATIII) is produced in brain and primary astroglial cultures. In addition, we show that human astroglial cultures contain prothrombin mRNA, and secrete a thrombin-like protein that makes complexes with antithrombin III.

Modulation of proteolytic activity during neuritogenesis in the PC12 nerve cell: differential control of plasminogen activator and plasminogen activator inhibitor activities by nerve growth factor and dibutyryl-cyclic AMP

Extracellular proteolysis is considered to be required during neuritic outgrowth to control the adhesiveness between the growing neurite membrane and extracellular matrix proteins. In this work, PC12 nerve cells were used to study the modulation of proteolytic activity during neuronal differentiation. PC12 cells were found to contain and release a 70-75-kDa tissue-type plasminogen activator (tPA) and a much less abundant 48-kDa urokinase-type plasminogen activator. A plasminogen activator inhibitor (PAI) activity with molecular sizes of 54 and 58 kDa was also detected in PC12 cell conditioned medium and formed high-molecular-mass complexes with released tPA. Release of PAI activity was dependent on treatment with nerve growth factor (NGF), whereas tPA synthesis and release were under control of a cyclic AMP-dependent mechanism and increased on treatment with dibutyryl-cyclic AMP [(But)2cAMP] or cholera toxin. Simultaneous treatment with NGF and (But)2cAMP resulted in increases of both tPA and PAI release and enhancement of tPA-PAI complex formation. The resulting plasminogen activator activity in conditioned medium was high in (But)2cAMP-treated cultures with short neuritic outgrowth but remained low in NGF- or NGF plus (But)2cAMP-treated cultures, where neurite extension was, respectively, large and very large. These results suggest that excess proteolytic activity may be detrimental to neuritic outgrowth and that not only PAI release but also tPA-PAI complex formation is associated with production of large and stable neuritic outgrowth. This can be understood as an involvement of PAI in the protection against neurite-destabilizing proteolytic activity.

Immunoregulators in the nervous system

The nervous system, through the production of neuroregulators (neurotransmitters, neuromodulators and neuropeptides) can regulate specific immune system functions, while the immune system, through the production of immunoregulators (immunomodulators and immunopeptides) can regulate specific nervous system functions. This indicates a reciprocal communication between the nervous and immune systems. The presence of immunoregulators in the brain and cerebrospinal fluid is the result of local synthesis--by intrinsic and blood-derived macrophages, activated T-lymphocytes that cross the blood-brain barrier, endothelial cells of the cerebrovasculature, microglia, astrocytes, and neuronal components--and/or uptake from the peripheral blood through the blood-brain barrier (in specific cases) and circumventricular organs. Acute and chronic pathological processes (infection, inflammation, immunological reactions, malignancy, necrosis) stimulate the synthesis and release of immunoregulators in various cell systems. These immunoregulators have pivotal roles in the coordination of the host defense mechanisms and repair, and induce a series of immunological, endocrinological, metabolical and neurological responses. This review summarizes studies concerning immunoregulators--such as interleukins, tumor necrosis factor, interferons, transforming growth factors, thymic peptides, tuftsin, platelet activating factor, neuro-immunoregulators--in the nervous system. It also describes the monitoring of immunoregulators by the central nervous system (CNS) as part of the regulatory factors that induce neurological manifestations (e.g., fever, somnolence, appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic pathological processes.