Interaction of tissue-type plasminogen activator and plasminogen activator inhibitor 1 on the surface of endothelial cells

The site of the reaction between plasminogen activators and plasminogen activator inhibitor 1 (PAI-1) was investigated in cultures of human umbilical vein endothelial cells. In conditioned medium from endothelial cells, two forms of a plasminogen activator-specific inhibitor can be demonstrated: an active form that readily binds to and inhibits plasminogen activators and an immunologically related quiescent form which has no anti-activator activity but which can be activated by denaturation. In conditioned medium, only a few percent of PAI-1 is the active form. However, the addition of increasing concentrations of tissue-type plasminogen activator (t-PA) or urokinase to confluent endothelial cells produced a saturable (3.0 pmol/5 x 10(5) cells), dose-dependent increase of the activator-PAI-1 complex in the conditioned medium even in the presence of actinomycin D or cycloheximide. This resulted also in a dose-dependent decrease of the residual PAI activity measured by reverse fibrin autography both in the conditioned medium and cell extracts. Short-time exposure of endothelial cells to a large amount of t-PA caused almost complete depletion of all cell-associated PAI activity. Although there was no detectable PAI activity even after activation of PAI by denaturants or antigen in the culture medium at 4 degrees C without the addition of t-PA, the addition of t-PA at 4 degrees C not only resulted in the formation of 70% of the amount of the t-PA.PAI complex in conditioned medium at 37 degrees C, but also induced PAI-1 antigen in a time and dose-dependent manner in the conditioned medium. Moreover, 125I-labeled t-PA immobilized on Sepharose added directly to endothelial cells formed a complex with PAI-1 in a dose-dependent manner. On the other hand, no detectable complex was formed with PAI-1 when Sepharose-immobilized 125I-labeled t-PA was added to endothelial cells under conditions in which the added t-PA could not contact the cells directly but other proteins could pass freely by the use of a Transwell. All these results suggest that a "storage pool" on the surface of endothelial cells or the extracellular matrix produced by endothelial cells contains almost all the active PAI-1, and reaction between PA and PAI-1 mainly occurs on the endothelial cell membranes, resulting in a decrease of the conversion of active PAI-1 to the quiescent form.     

Mechanism of action of angiostatic steroids: Suppression of plasminogen activator activity via stimulation of plasminogen activator inhibitor synthesis

Recently, a novel class of angiostatic steroids which block angiogenesis in several systems has been described. Since the elaboration of proteases is believed to be an important component of angiogenesis, we tested whether these steroids blocked the fibrinolytic response of endothelial cells to the angiogenic protein, basic fibroblast growth factor [bFGF]). Cultured bovine aortic endothelial (BAE) cells were incubated with bFGF and/or medroxyprogesterone acetate (MPA), an angio-static steroid which has been shown to inhibit vascularization, collagenolysis, and tumor growth. When bFGF (3 ng/ml) was added to confluent monolayers of BAE cells, plasminogen activator (PA) activity in the medium was increased threefold. In contrast, MPA at 10^?6 M, 10^?7 M, 10^?8 M, and 10^?9 M decreased PA levels in the medium by 83%, 83%, 75%, and 39%, respectively. The stimulation of PA levels in BAE cells by bFGF (3 ng/ml) was abrogated by the presence of 10^?6 M MPA. This decrease in PA activity was found to be mediated by a significant increase in plasminogen activator inhibitor type-1 (PAI-1) production. MPA, therefore, negated one of the important enzymatic activities associated with the angiogenic process. In contrast to the decreased levels of secreted PA in cultures exposed simultaneously to MPA and bFGF, cell-associated PA levels remained high, consistent with earlier observations indicating that PAI-1 does not inhibit cell-associated PA. Thus, angiostatic steroids may exert their inhibitory effects on angiogenesis by increasing the synthesis of PAI-1. This, in turn, inhibits PA activity and, therefore, plasmin generation, which is essential for the invasive aspect of angiogenesis. © 1993 Wiley-Liss, Inc.     

Oxygen radicals generated during anoxia followed by reoxygenation reduce the synthesis of tissue-type plasminogen activator and plasminogen activator inhibitor-1 in human endothelial cell culture

The effect of anoxia and reoxygenation on the synthesis and secretion of tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor-1 (PAI-1) was studied in primary cultures of human umbilical vein endothelial cells. Sublethal anoxia, determined by trypan blue dye exclusion and lactate dehydrogenase release, was produced by cell culture under a 95% N2, 5% CO2 atmosphere for 2-24 h and was followed by reoxygenation with 95% air, 5% CO2 for 24 or 48 h. Anoxia did not alter the levels of mRNA for t-PA or PAI-1 in the cells or the secretion of t-PA or PAI-1 into the medium. At 24 h, t-PA secreted into conditioned medium was 7.0 +/- 1.4 ng/2 x 10(6) cells (n = 9) and PAI-1 was 300 +/- 13 IU/2 x 10(6) cells (n = 9), whereas the content of t-PA mRNA was 2.2 pg/micrograms of RNA and PAI-1 mRNA was 180 pg/micrograms of RNA. During reoxygenation, however, t-PA antigen and PAI-1 activity as well as mRNA for PAI-1 decreased proportionally to the duration of anoxia, to reach 27 +/- 1.0, 49 +/- 2.0, and 47 +/- 14% of control values, respectively, within 24 h of anoxia. t-PA mRNA also decreased significantly during reoxygenation following anoxia, but the extent could not be accurately quantitated. Addition, during anoxia, of a 200 micrograms/ml concentration of the superoxide anion radical scavenger superoxide dismutase or of a 5 mM concentration of the iron chelator deferoxamine mesylate prevented the subsequent decrease of t-PA antigen during reoxygenation; addition of these compounds during reoxygenation had no effect. Superoxide dismutase, but not deferoxamine mesylate, when added during anoxia prevented the subsequent decrease in PAI-1 activity. These studies suggest that the marked alteration of endothelial cell fibrinolysis during anoxia followed by reoxygenation is most likely mediated by a mechanism dependent on oxygen radicals. Impaired endothelial cell fibrinolysis may contribute to the pathophysiology of ischemia/reperfusion injury.     

Different mechanisms contribute to simultaneous inhibition of urokinase and tissue-type plasminogen activators by glucocorticoids in human ovarian carcinoma cells

Previous studies from our laboratory have demonstrated that OVCA 433 human ovarian carcinoma cells are glucocorticoid responsive by several criteria and contain high affinity, saturable, steroid-specific glucocorticoid receptors. These cells secrete both mammalian plasminogen activators (PAs), urokinase (uPA) and tissue-type PA (tPA). Treatment of OVCA 433 cells with 1 x 10(-7) M dexamethasone (Dex) for 4 days led to 77% and 83% reductions in the extracellular activities of uPA and tPA, respectively, released into serum-free conditioned medium during a 1-h period. Dex treatment led to a 71% decrease in the rate of extracellular uPA antigen accumulation, as determined by enzyme-linked immunosorbent assay, as well as a 73% reduction in steady state uPA mRNA levels. In contrast, Dex treatment led to only a 42% decrease in the rate of extracellular tPA antigen accumulation and a 48% decrease in tPA mRNA levels; such decreases were insufficient to account for the 83% reduction in tPA activity. Thus, while Dex-induced decreases in uPA antigen and mRNA levels accounted for all but 6% of the decrease in uPA activity, a large discrepancy existed between the magnitudes of decreased tPA activity and decreased tPA antigen and mRNA levels. OVCA 433 cells produce both PAI-1 and PAI-2, two specific PA inhibitors. Treatment of cells with 1 x 10(-7) M Dex for 4 days led to a 3.3-fold increase in the rate of extracellular PAI-1 accumulation, with little or no effect on PAI-2 accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of the fibrinolytic response of cultured human vascular endothelium by extracellularly generated oxygen radicals.

Clinical and experimental data indicate that activated oxygen species interfere with vascular endothelial cell function. Here, the impact of extracellular oxidant injury on the fibrinolytic response of cultured human umbilical vein endothelial (HUVE) cells was investigated at the protein and mRNA levels. Xanthine (50 microM) and xanthine oxidase (100 milliunits), which produces the superoxide anion radical (O2-) and hydrogen peroxide (H2O2), was used to sublethally injure HUVE cells. Following a 15-min exposure, washed cells were incubated for up to 24 h in serum-free culture medium. Tissue-type plasminogen activator (t-PA) antigen, plasminogen activator inhibitor-1 (PAI-1) antigen, and PAI-1 activity were determined in 1.25 ml of conditioned medium and t-PA and PAI-1 mRNA in the cell extracts of 2 x 10(6) HUVE cells. Control cells secreted 3.9 +/- 1.3 ng/ml (mean +/- S.D., n = 12) within 24 h. Treatment with xanthine/xanthine oxidase for 15 min induced a 2.8 +/- 0.4-fold increase (n = 12, p less than 0.05) of t-PA antigen secretion after 24 h. The t-PA antigen was recovered predominantly in complex with PAI-1. The oxidant injury caused a 3.0 +/- 0.8-fold increase (n = 9, p less than 0.05) in t-PA mRNA within 2 h. Total protein synthesis was unaltered by xanthine/xanthine oxidase. The oxidant scavengers superoxide dismutase and catalase, in combination, abolished the effect of xanthine/xanthine oxidase on t-PA secretion and t-PA mRNA synthesis. Xanthine/xanthine oxidase treatment of HUVE cells did not affect the PAI-1 secretion in conditioned medium nor the PAI-1 mRNA levels in cell extracts. Thus extracellular oxidant injury induces t-PA but not PAI-1 synthesis in HUVE cells.     

The THP-1 cell line is a urokinase-secreting mononuclear phagocyte with a novel defect in the production of plasminogen activator inhibitor-2

Mononuclear phagocytes regulate the generation of plasmin by secreting urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor-2 (PAI-2). We investigated the production of plasminogen activator (PA) and PA inhibitor by the human monocytic leukemia cell line, THP-1. Similar to U937 monoblast-like cells and peripheral blood monocytes (PBM), THP-1 cells produce a PA that is specifically neutralized by anti-uPA antibody and comigrates with human high molecular mass uPA (54 kDa) on casein-plasminogen zymogaphy. PA activity could be dissociated from intact THP-1 cells by brief treatment with a weak acid-glycine buffer, indicating that the uPA is secreted and bound to receptors on the plasma membrane. Regulation of uPA proceeds normally in THP-1 cells, with cell-associated PA activity increasing from 77 +/- 20 to 163 +/- 26 and 325 +/- 30 mPU/10(6) cells in response to PMA and LPS, respectively; parallel increases in steady state levels of uPA mRNA were observed. In contrast to normal expression of uPA activity, functional PAI-2 could not be demonstrated in either the conditioned media or cell lysates of THP-1 under basal or stimulated conditions. Both U937 and PBM secrete low levels of PA inhibitor activity that increase substantially in response to stimulation with PMA and LPS. Immunoreactive PAI-2, measured by ELISA, was undetectable in THP-1 lysates or conditioned medium, but was consistently present in U937 and PBM, paralleling the presence of PA inhibitor activity. THP-1 cells express low levels of an abnormally sized mRNA for PAI-2 and demonstrate a regulatory defect whereby steady state levels of PAI-2 mRNA are markedly reduced upon stimulation with PMA or LPS. By contrast, U937 and PBM respond to identical stimulation with increases in PAI-2 mRNA. We conclude that THP-1 cells express a structurally abnormal species of PAI-2 mRNA, with complete loss of inhibitory activity as well as altered function of PMA- and LPS-responsive regulatory elements.     

Synthesis and secretion of plasminogen activator inhibitor 1 by human endothelial cells in vitro. Effect of active site mutagenized tissue-type plasminogen activator

The effects of recombinant tissue-type plasminogen activator (rt-PA) and of an inactive mutant of rt-PA, obtained by mutagenesis of the active site Ser478 to Ala (rt-PA-Ala478), on the synthesis and secretion of plasminogen activator inhibitor-1 (PAI-1) by human umbilical vein endothelial cells (HUVEC) in culture were studied. Under base-line conditions, PAI-1 antigen secretion was 4.3 +/- 1.0 micrograms (mean +/- S.D., n = 8) per 10(6) cells in 24 h. This PAI-1 had a low specific activity (6,000 +/- 1,600 units/mg) and Mr of 50,000, which was not altered by addition of rt-PA. In HUVEC cultured with 2 micrograms/ml rt-PA-Ala478, PAI-1 antigen secretion was 2.1 +/- 0.8 micrograms (n = 5) per 10(6) cells in 24 h with a specific activity of 120,000 +/- 42,000 units/mg and Mr of 50,000. Addition of rt-PA to this conditioned medium resulted in generation of three main components: 16% migrated as an Mr 106,000 rt-PA.PAI-1 complex, 16% as an Mr 81,000 degraded rt-PA.PAI-1 complex and the remainder as an Mr 45,000 degradation product of PAI-1. HUVEC cultured with 2 micrograms/ml rt-PA secreted 3.9 +/- 0.6 micrograms (n = 8) PAI-1 antigen per 10(6) cells within 24 h, of which 20-50% occurred as intact or degraded complexes with t-PA (Mr 106,000 and 81,000) and the rest as an inactive Mr 45,000 degradation product of PAI-1. PAI-1 mRNA levels, determined by Northern blot analysis and expressed relative to beta-actin mRNA levels, were very similar for HUVEC cultured in the absence or the presence of rt-PA or rt-PA-Ala478. It is concluded that PAI-1 is secreted by HUVEC in culture in fully active form which spontaneously inactivates. PAI-1 can be stabilized by addition of rt-PA-Ala478 to the culture medium, resulting in a 20-fold increase in specific activity. Interaction of rt-PA with active PAI-1 produces both t-PA.PAI-1 complex and an inactive degradation product of PAI-1.     

Effects of cellular transformation on expression of plasminogen activator inhibitors 1 and 2. Evidence for independent regulation

Expression of plasminogen activators (PA) has been reported to be associated with invasive tumor growth and increased metastatic ability. In order to delineate changes in PA and PA inhibitor (PAI) expression that accompany cellular transformation, we studied oncogene-containing variants of the Rat-1 cell line. We report here that transfection of the oncogenes v-src, erbB, c-myc, v-myc, N-myc, and EJras into these cells does not result in detectable PA activity in conditioned media or cell extracts. In addition, Northern blot analysis fails to demonstrate urokinase mRNA in Rat-1 cells or transfectants. Moreover, cells transformed by EJras and v-src but not other oncogenes secrete an active placental-type PAI, PAI-2. Using inducible EJras constructs, we find that increased PAI-2 gene expression is detectable within 6-12 h after treatment with the inducing agent. Peak expression of PAI-2 mRNA is increased 10-15-fold over base line, and high levels are maintained for at least 72 h. In contrast to the results with PAI-2, secretion of endothelial-type PAI-1 into conditioned media is sharply down-regulated by several oncogenes. Thus, we have found that PAI-1 and PAI-2 are independently regulated in transformed variants of Rat-1 cells. The specific induction of PAI-2 in cells transformed by oncogenic ras and src suggests that this protease inhibitor may have a previously unsuspected role in malignancy.     

Platelet-derived growth factor and transforming growth factor-beta regulate plasminogen activator inhibitor-1 synthesis in vascular smooth muscle cells

Bovine vascular smooth muscle cells (SMC) were examined for production of plasminogen activator inhibitor-1 (PAI-1) which may play a key role in regulating the fibrinolytic system. Growth-arrested SMC released active PAI (101 arbitrary units (AU)/10(6) cells/24 h) and a latent form of PAI (880 AU/10(6) cells/24 h) into the conditioned medium (CM). The levels of PAI were significant since 880 AU of PAI could inhibit approximately 1 microgram of tissue plasminogen activator. The extracellular matrix of SMC also contained PAI activity; however, the level was 17-fold less than that observed in the CM. SMC-PAI was a rapid inhibitor of tissue plasminogen activator (kass greater than 10(7) M-1 S-1) and was identified as a 45-kDa protein immunologically related to endothelial cell PAI-1. PAI-1 comprised 20 and 30%, respectively, of the newly synthesized protein detected in the CM and extracellular matrix of SMC. The SMC growth modulators, platelet-derived growth factor and transforming growth factor-beta, induced PAI-1 activity and protein synthesis by 2- and 3-fold, respectively, in a dose- and time-dependent manner. The increases in PAI-1 activity and protein synthesis were ascribed to elevated levels of PAI-1 mRNA as judged by Northern blot analysis of total RNA prepared from control and platelet-derived growth factor- and transforming growth factor-beta-treated cells. Increases in PAI-1 mRNA levels were evident 1 h after growth factor treatment and were maximal after 4 h. PAI-1 mRNA levels were unaffected by cycloheximide treatment. The results indicate that SMC synthesize and release PAI-1 which could regulate the normal fibrinolytic environment of the arterial wall. During atherosclerosis or after vascular injury increases in platelet-derived or locally produced mitogens may stimulate further PAI-1 synthesis and generate a prothrombotic state.     

Purification and characterization of natural and recombinant human plasminogen activator inhibitor-1 (PAI-1)

Human plasminogen activator inhibitor-1 (PAI-1) was purified from the conditioned medium of endotoxin-stimulated umbilical vein endothelial cell cultures by combinations of zinc-chelate-Sepharose chromatography, gel filtration on Sephacryl S-300 and immunoadsorption on an insolubilized murine monoclonal antibody (MA-7D4). The final product was obtained with a recovery of approximately 20% from conditioned medium containing about 3 micrograms/ml PAI-1. The yield of PAI-1 was 15-100 micrograms/umbilical cord, depending on the culture and harvest conditions. SDS gel electrophoresis revealed a main band with Mr = 46,000 both under reducing and non-reducing conditions. On gel filtration on Sephacryl S-300, however, the material was separated in two fractions, one eluting at the void volume, which contains active PAI-1, and one with Mr = 46,000 containing inactive material that could be reactivated with 12 M urea. SDS gel electrophoresis of the isolated high-Mr fraction revealed several bands including a main 46,000-Mr component, which reacted with anti-(PAI-1) antibodies on immunoblotting and neutralized tissue-type plasminogen activator (t-PA). The active high-Mr fraction and the reactivated low-Mr fraction of PAI-1 inhibited t-PA very rapidly with an apparent second-order rate constant of (1.5-4) x 10(7) M-1 s-1. The cDNA of endothelial cell PAI-1 was cloned and expressed in Chinese hamster ovary cells. The translation product, purified from conditioned medium of transfected cells, also revealed a high-Mr and a low-Mr fraction on gel filtration, which were indistinguishable from the natural proteins by physicochemical, immunochemical and functional analysis. On reduced SDS gel electrophoresis, the high-Mr fraction was separated into the Mr-46,000 low-Mr PAI-1 and two other components with Mr 65,000 and one barely entering the gel. When reactivated low-Mr PAI-1 was added to plasma, PAI activity and PAI-1 antigen eluted with an apparent Mr greater than or equal to 300,000 on gel filtration, indicating that active PAI-1 complexes with one or more binding proteins in plasma.     

Lipopolysaccharide inhibits activation of latent transforming growth factor-β in bovine endothelial cells

The activation of latent transforming growth factor-β (TGF-β) by vascular endothelial cells (ECs) is regulated by cellular plasminogen activator (PA)/plasmin, transglutaminase (TGase), and latent TGF-β levels. Because lipopolysaccharide (LPS) has been reported to reduce EC surface plasmin levels by increasing the production of the inhibitor of PA, PA inhibitor-1 (PAI-1), we have tested whether LPS might suppress latent TGF-β activation in ECs using two different systems, namely, bovine aortic ECs (BAECs) cocultured with smooth muscle cells (SMCs) and BAECs treated with retinol. BAECs were either cocultured with SMCs after treatment with 15 ng/ml LPS or were treated with 2 μM retinol and/or 10 ng/ml LPS, and the expression of PA, surface plasmin, TGase, and the amounts of active and latent TGF-β secreted into the culture modium were measured. The downregulation of surface PA/plasmin levels with LPS was accompanied by a profound decline of both TGase and latent TGF-β expression as well as the suppression of surface activation of latent TGF-β. The effect was dependent on the concentration of LPS and on treatment time. The formation of TGF-β did not occur in cells maintained in LPS-contaminated culture medium. © 1995 Wiley-Liss, Inc.     

Human neutrophils promote angiogenesis by a paracrine feedforward mechanism involving endothelial interleukin-8

Neovascularization by sprouting angiogenesis is critical for inflammation-mediated tissue remodeling and wound healing. We report here that human polymorphonuclear neutrophils (PMN) stimulated for 1 h with 100 nM N-formyl-methionyl-leucyl-phenylalanine (fMLP) released a proangiogenic entity that induced sprouting of capillary-like structures in an in vitro angiogenesis assay. The effect was comparable to the response obtained on stimulation with 100 ng/ml basic FGF. The PMN-mediated response was inhibited by neutralizing antibodies against VEGF or IL-8. As measured by ELISA technique, we found that fMLP-activated PMN (5 x 10(6)/ml) released 78 pg/ml IL-8 and 39 pg/ml VEGF within 1 h after stimulation. IL-8 release was blocked by actinomycin D or cycloheximide, but the inhibitors had no effect on VEGF release, suggesting that IL-8 secretion required de novo synthesis whereas VEGF was secreted from preformed stores. Accordingly, RT-PCR analysis revealed that IL-8 mRNA was upregulated on PMN stimulation, whereas the expression of VEGF mRNA was not affected. Moreover, supernatant derived from activated PMN induced upregulation of endothelial IL-8 mRNA expression, suggesting that release of VEGF and IL-8 from activated PMN may activate a paracrine feedforward mechanism involving endothelial IL-8. Moreover, VEGF-induced upregulation of endothelial IL-8 expression as well as sprouting of capillary-like structures was inhibited by a neutralizing anti-IL-8 antibody. These findings suggest that bacteria-derived tripeptides stimulate human PMN to release VEGF and IL-8, which activate endothelial cells and induce angiogenesis by a paracrine feedforward mechanism involving endothelial IL-8 upregulation.     

Vascular endothelial growth factor (VEGF) induces plasminogen activators and plasminogen activator inhibitor-1 in microvascular endothelial cells.

Extracellular proteolysis is believed to be an essential component of the angiogenic process. The effects of VEGF, a recently described angiogenic factor, were assessed on PA activity and PA and PAI-1 mRNA levels in microvascular endothelial cells. u-PA and t-PA activity were increased by VEGF in a dose-dependent manner, with maximal induction at 30 ng/ml. u-PA and t-PA mRNAs were increased 7.5- and 8-fold respectively after 15 hours, and PAI-1 mRNA 4.5-fold after 4 hours exposure to VEGF. At equimolar concentrations (0.5 nM), VEGF was a more potent inducer of t-PA mRNA than bFGF, while bFGF was a more potent inducer of u-PA and PAI-1 mRNAs. In addition, VEGF induced u-PA and PAI-1 mRNAs with kinetics similar to those previously demonstrated for bFGF. These results demonstrate the regulation of PA and PAI-1 production by VEGF in microvascular endothelial cells and are in accord with the hypothesis that extracellular proteolysis, appropriately balanced by protease inhibitors, is required for normal capillary morphogenesis.     

Plasminogen activator activity in differentiating leukemia cells

Plasminogen activator (PA) activity of human promyelocytic leukemia cell line HL-60 was assayed by following the conversion of plasminogen to plasmin and the plasmin-mediated hydrolysis of 14C-labeled globin. When HL-60 cells were induced to differentiate into macrophages by 12-O-tetradecanoyl-phorbol-13-acetate (TPA), cell-associated PA activity and secretion of PA into the conditioned medium increased profoundly. PA activity increased earlier and as a result of lower concentrations of TPA than the ability of the cells to adhere. Exposure to 10(-6)M dexamethasone did not prevent TPA-induced adherence and produced a slight inhibition of cellular PA activity. These findings imply that TPA-induced differentiation of HL-60 cells to macrophage-like cells is associated with induction of PA activity.     

Thrombin increases proliferation and decreases fibrinolytic activity of kidney glomerular epithelial cells.

Human glomerular epithelial cells (GECs) in culture synthesize single-chain, urokinase-type plasminogen activator (SC-uPA), tissue-type plasminogen activator (t-PA), and plasminogen activator inhibitor 1 (PAI-1) and possess specific membrane-binding sites for u-PA. Using purified 125I-alpha thrombin, we demonstrate here the presence of two populations of specific binding sites for thrombin on GECs (1.Kd = 4.3 +/- 1.0 x 10(-10) M, 5.4 +/- 1.4 x 10(4) M sites per cell, 2. Kd = 1.6 +/- 0.5 x 10(-8) M, 7.9 +/- 1.8 x 10(5) sites per cell). Purified human alpha thrombin promoted the proliferation of GECs and induced a time- and dose-dependent increase of SC-uPA, t-PA, and PAI-1 antigens released by GECs. Thrombin-mediated increase in antigen was paralleled by an increase in the levels of corresponding u-PA and PAI-1 messenger RNA. In contrast, thrombin decreased u-PA activity in conditioned medium. This discrepancy between u-PA antigen and u-PA activity was explained by a limited proteolysis of SC-uPA by thrombin, leading to a two-chain form detected by immunoblotting and that could not be activated by plasmin. Thrombin also decreased the number of u-PA binding sites on GECs (p less than 0.05) without changing receptor affinity. Hirudin inhibited the binding and the cellular effects of thrombin, whereas thrombin inactivated by diisopropylfluorophosphate had no effect, indicating that both membrane binding and catalytic activity of thrombin were required. We conclude that thrombin, through specific membrane receptors, stimulates proliferation of GECs and decreases the fibrinolytic activity of GECs both at the cell surface and in the conditioned medium. These results suggest that thrombin could be involved in the pathogenesis of extracapillary proliferation and persistency of fibrin deposits in crescentic glomerulonephritis.     

The majority of type 1 plasminogen activator inhibitor associated with cultured human endothelial cells is located under the cells and is accessible to solution-phase tissue-type plasminogen activator

The interactions between exogenously added tissue-type plasminogen activator (t-PA) and the active form of type 1 plasminogen activator inhibitor (PAI-1) produced by and present in cultured human umbilical vein endothelial cells (HUVECs) were investigated. Immunoblotting analysis of the conditioned media obtained from monolayers of HUVECs treated with increasing concentrations of t-PA (less than or equal to 10 micrograms/ml) revealed a dose-dependent formation of both t-PA/PAI-1 complexes, and of a 42,000-Mr cleaved or modified form of the inhibitor. Immunoradiometric assays indicated that t-PA treatment resulted in a fourfold increase in PAI-1 antigen present in the conditioned media. This increase did not result from the release of PAI-1 from intracellular stores, but rather reflected a t-PA-dependent decrease in the PAI-1 content of the Triton X-100 insoluble extracellular matrix (ECM). Although the rate of t-PA-mediated release of PAI-1 was increased by the removal of the monolayer, similar quantities of PAI-1 were removed in the presence or absence of the cells. These results suggest that the cells only represent a semipermeable barrier between ECM-associated PAI-1 and exogenous t-PA. Treatment of HUVECs with t-PA (1 microgram/ml, 2 h) to deplete the ECM of PAI-1 did not affect the subsequent rate of PAI-1 production and deposition into the ECM. Immunogold electron microscopy of HUVECs not only confirmed the location of PAI-1 primarily in the region between the culture substratum and ventral cell surface but failed to demonstrate significant (less than 1%) PAI-1 on the cell surface. Thus, the majority of PAI-1 associated with cultured HUVEC monolayers is present under the cells in the ECM and is accessible to solution-phase t-PA.     

大鼠卵巢细胞分泌纤溶酶原激活因子抑制因子(PAI-1)的激素调节

哺乳动物卵巢合成两类纤溶酶原激活因子(PA),即组织型PA(tPA)和尿激酶型PA(uPA)。大鼠卵巢除主要合成tPA外,还分泌一种纤溶酶激活因子的抑制因子(PAI-1)。在促性腺激素作用下,卵巢tPA和PAI-1两种基因的协调表达是导致滤泡破裂的原因。本实验进一步证实,卵巢中的PAI-1主要由膜-间质细胞分泌,可能作为一种屏障限制颗粒细胞tPA分泌到滤泡外间质。当排卵来临时,两种细胞所分泌的tPA和PAI-1相互作用后仍发现有大量tPA活性。这可能是引起排卵的主要原因。因为成熟的卵丘细胞除分泌高量tPA外,还分泌大量PAI-1,在人工授精中两者有可能作为鉴定卵子优劣的可靠指标。     

大鼠卵巢细胞分泌纤溶酶原激活因子抑制因子（PAI—1）的...

Rat ovarian cells produce not only plasminogen activator (tPA) but also plasminogen activator inhibitor type 1 (PAI-1), and their coordinated geneexpression induced by gonadotropins are thought to be responsible for follicular rupture. In this study, it was demonstrated that (1) theca-interstitial compartment synthesizes the majority of PAI-1 activity in the ovary before ovulation, the follicular wall may therefore serve as a specific barrier to prevent the secretion of PA into the extrafollicular compartment; (2) Granulosa cells contribute only small amount of ovarian PAI-1 activity, but synthesize most of tissue-type plasminogen activator activity involved in the process leading to ovulation: (3) Since only matured cumulus-oocyte complexes secrete high level of tPA and PAI-1, both tPA and PAI-1 activity in the conditioned medium may be used as reliable markers for evaluating oocyte quality for in vitro fertilization.