Effects of gemfibrozil and ciprofibrate on plasma levels of tissue-type plasminogen activator, plasminogen activator inhibitor-1 and fibrinogen in hyperlipidaemic patients

Evaluation of fibrate treatment in humans has focused primarily on its anti-lipidaemic effects. A potentially favourable haemostasis-modulating activity of fibrates has also been recognized but the data are not consistent. We sought to learn more about this variability by examining the effects of gemfibrozil and ciprofibrate on plasma levels of tissue-type plasminogen activator (t-PA), plasminogen activator inhibitor-1 (PAI-1) and fibrinogen in primary hyperlipidaemic patients after six and twelve weeks of treatment using different assay systems for PAI-1 and fibrinogen. Although both fibrates effectively lowered triglyceride and cholesterol levels, no effect on the elevated baseline antigen levels of t-PA and PAI-1 was observed after fibrate treatment. However, both fibrates influenced plasma fibrinogen levels, albeit in a different way. Fibrinogen antigen levels were elevated by 17.6% (p <0.05) and 24.3% (p <0.001) with gemfibrozil after six and twelve weeks, respectively, whereas with ciprofibrate there was no effect. Using a Clauss functional assay with either a mechanical end point or a turbidity-based end point, no significant change in fibrinogen levels was seen after six weeks of gemfibrozil treatment. However, after twelve weeks, gemfibrozil enhanced functional fibrinogen levels by 7.2% (p <0.05) as assessed by the Clauss mechanical assay, but decreased functional fibrinogen levels by 12.5% (p <0.0001) when a Clauss assay based on turbidity was used. After six or twelve weeks of ciprofibrate treatment, functional fibrinogen levels were decreased by 10.1% (p <0.001) and 10.5% (p <0.0001), respectively on the basis of Clauss mechanical and by 14.2% (p <0.001) and 28.2% (p <0.0001), respectively with the Clauss turbidimetric assay. A remarkable and consistent finding with both fibrates was the decrease in functionality of fibrinogen as assessed by the ratio of functional fibrinogen (determined by either of the two Clauss assays) to fibrinogen antigen. Taken together, our results indicate that at least part of the variability in the effects of fibrates on haemostatic parameters can be explained by intrinsic differences between various fibrates, by differences in treatment period and/or by the different outcomes of various assay systems. Interestingly, the two fibrates tested both reduced the functionality of fibrinogen.

Involvement of calcium and G proteins in the acute release of tissue-type plasminogen activator and von Willebrand factor from cultured human endothelial cells

In this study, we investigated the role of Ca2+ and G proteins in thrombin-induced acute release (regulated secretion) of tissue-type plasminogen activator (TPA) and von Willebrand factor (vWF), using a previously described system of primary human umbilical vein endothelial cells (HUVECs). The acute release of TPA and vWF, as induced by alpha-thrombin, was almost zero after chelation of Ca2+i, showing that an increase in [Ca2+]i was required. It did not matter whether the increase in [Ca2+]i came from an intracellular or extracellular Ca2+ source. Thrombin-induced release of TPA and vWF already started at low [Ca2+]i, around 100 nmol/L. Half-maximal release was found at a [Ca2+]i, of 261 nmol/L for TPA and at 222 nmol/L for vWF. The Ca2+ signal was transduced to calmodulin, as calmodulin inhibitors inhibited TPA and vWF release. The Ca2+ ionophore ionomycin dose dependently released vWF; half-maximal vWF release occurred at a [Ca2+]i of 311 nmol/L. In contrast, no TPA release was found at all below a [Ca2+]i of 500 nmol/L. Thus, below 500 nmol/L [Ca2+]i, an increase in [Ca2+]i alone was sufficient to induce vWF release but not sufficient to induce TPA release. Protein kinase C did not appear to be involved in TPA or vWF release, as neither an activator nor an inhibitor of protein kinase C significantly influenced release. Inhibition of phospholipase A2 also did not reduce thrombin-induced TPA and vWF release. The involvement of G proteins was studied by using both saponin-permeabilized and intact cells. GDP-beta-S, which inhibits heterotrimeric and small G proteins, significantly inhibited thrombin-induced vWF and TPA release from permeabilized cells. AlF-4, which activates heterotrimeric G proteins, induced TPA and vWF release in both intact and permeabilized HUVECs. Preincubation of HUVECs with pertussis toxin significantly inhibited thrombin-induced vWF release, due to inhibition of thrombin-induced Ca2+ influx. Pertussis toxin did not affect ionomycin-induced release. The inhibitory effect of pertussis toxin was less obvious in thrombin-induced TPA release, because it was counterbalanced by a positive effect of the toxin on TPA release. Thus, both inhibitory and stimulatory (pertussis toxin-sensitive) G proteins were involved in TPA release. Therefore, thrombin-induced acute release of TPA and vWF differed in two respects. First, below a [Ca2+]i of 500 nmol/L, an increase in Ca2+ was sufficient for vWF release but not for TPA release. Second, pertussis toxin-sensitive G proteins were differentially involved in acute TPA and vWF release.

Differences in metabolism and isomerization of all-trans-retinoic acid and 9-cis-retinoic acid between human endothelial cells and hepatocytes

Retinoic acid stimulates the expression of tissue-type plasminogen activator (t-PA) in vascular endothelial cells in vitro and enhances t-PA levels in plasma and tissues in vivo. Compared with the in vivo situation, high retinoic acid concentrations are required to induce optimally t-PA expression in vitro. These findings led us to study retinoic acid metabolism in cultured human endothelial cells. For comparison, these studies were also performed in the human hepatoma cell line, HepG2, and key experiments were repeated with human primary hepatocytes. Both hepatocyte cultures gave very similar results. Human endothelial cells were shown to possess an active retinoic acid metabolizing capacity, which is quantitatively comparable to that of hepatocytes, but different from that of hepatocytes in several qualitative aspects. Our results demonstrate that all-trans-retinoic acid is quickly metabolized by both endothelial cells and hepatocytes. All-trans-retinoic acid induces its own metabolism in endothelial cells but not in hepatocytes. 9-cis-Retinoic acid is degraded slowly by endothelial cells, whereas hepatocytes metabolize 9-cis-retinoic acid very quickly. Furthermore, our data show that hepatocytes, but not endothelial cells, detectably isomerise all-trans-retinoic acid to 9-cis-retinoic acid and vice versa. In both endothelial cells and hepatocytes all-trans-retinoic acid metabolism was inhibitable by the cytochrome P-450 inhibitors liarozole (10 microM) and ketoconazole (10 microM), albeit to different extents and with different specificities. In the presence of the most potent retinoic acid metabolism inhibitor in endothelial cells, liarozole, at least 10-fold lower all-trans-retinoic acid concentrations were required than in the absence of the inhibitor to obtain the same induction of t-PA. In conclusion, our results clearly demonstrate that all-trans-retinoic acid and 9-cis retinoic acid are actively but differently metabolized and isomerised by human endothelial cells and hepatocytes. The rapid metabolism of retinoic acid explains the relatively high concentrations of retinoic acid required to induce t-PA in cultured endothelial cells.

Cell-type specific DNA-protein interactions at the tissue-type plasminogen activator promoter in human endothelial and HeLa cells in vivo and in vitro

Tissue-type plasminogen activator (t-PA) gene expression in human endothelial cells and HeLa cells is stimulated by the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) at the level of transcription. To study the mechanism of transcriptional regulation, we have characterized a segment of the t-PA gene extending from -135 to +100 by in vivo footprinting analysis [dimethyl sulphate (DMS) method] and gel mobility shift assay. In vivo footprinting analysis revealed changes in cleavage pattern in five distinct promoter elements in both endothelial cells and HeLa cells, including a PMA-responsive element (TRE), a CTF/NF-1 binding site and three GC-boxes, and an altered cleavage pattern of the TRE and CTF/NF-1 element after PMA treatment of HeLa cells. Although endothelial cells and HeLa cells differed in the exact G residues protected by nuclear proteins,in vitro bandshift analysis showed that nuclear protein binding to the t-PA promoter was qualitatively and quantitatively very similar in both cell types, except for the TRE. Protein binding to the TRE under non- stimulated conditions was much higher in human endothelial cells than in HeLa cells, and this TRE-bound protein showed a lower dissociation rate in the endothelial cells than in HeLa cells. In endothelial cells, the proteins bound to the TRE consisted mainly of the AP-1 family members JunD and Fra-2, while in HeLa cells predominantly JunD, FosB and Fra-2 were bound. The proteins bound to the other protected promoter elements were identified as SP-1 (GC-box II and III) and CTF/NF-1 (CTF/NF-1 binding site). After PMA treatment of the cells, AP-1 and SP-1 binding was increased two-fold in endothelial cell nuclear extracts and >20-fold in HeLa nuclear extracts. In the endothelial cells, all Jun and Fos forms (c-Jun, JunB, JunD, c-Fos, FosB, Fra-1 and Fra-2) were part of the AP-1 complex after PMA induction. In HeLa cells, the complex consisted predominantly of c-Jun and the Fos family members FosB and Fra-2. In the light of previous studies involving mutational analysis of the human and murine t-PA promoter our results underline an important role of the five identified promoter regions in basal and PMA-stimulated t-PA gene expression in intact human endothelial cells and HeLa cells. The small differences in DMS protection pattern and differences in the individual AP-1 components bound in endothelial cells and HeLa cells point to subtle cell-type specific differences in t-PA gene regulation.

Studies on the mechanism of fibrate-inhibited expression of plasminogen activator inhibitor-1 in cultured hepatocytes from cynomolgus monkey

Fibrates are widely used drugs in hyperlipidemic disorders. In addition to lowering serum triglyceride levels, fibrates have also been shown to reduce elevated plasma plasminogen activator inhibitor-1 (PAI-1) levels in vivo. We demonstrate that fibrates suppress PAI-1 synthesis in cultured cynomolgus monkey hepatocytes in a concentration-dependent way (0.1 to 1.0 mmol/L) and independent of their lipid-lowering effect. Different fibrates showed different potency in suppressing PAI-1 production: gemfibrozil and clofibric acid, at a concentration of 1 mmol/L, reduced PAI-1 synthesis over 24 hours to 52 +/- 20% and 60 +/- 5%, while clofibrate and bezafibrate lowered PAI-1 synthesis to only 86 +/- 17% and 84 +/- 15% of control values, respectively. These changes in PAI-1 production by fibrates correlated with changes in PAI-1 mRNA levels and were also visible at the level of gene transcription. Fibrates did not lower basal PAI-1 synthesis but attenuated an acceleration of PAI-1 production during culture. The suppressing effect of fibrates on PAI-1 synthesis could not be mimicked with activators or inhibitors of protein kinase C (PKC). Furthermore, fibrates did not inhibit the increase in PAI-1 synthesis induced by epidermal growth factor or transforming growth factor-beta. These results make mechanisms involving PKC modulation or growth factor receptor inactivation as a mode of action of fibrates unlikely. The suppressing effect of fibrates on PAI-1 synthesis could involve the nuclear receptor peroxisome proliferator-activated receptor (PPAR) and its heterodimeric partner, the retinoid X receptor (RXR). The alpha forms of PPAR and RXR were both found to be expressed in cynomolgus monkey hepatocytes. the ligand for RXR alpha, 9-cis retinoic acid, suppressed PAI-1 synthesis to the same extent as gemfibrozil, while a combination of gemfibrozil and 9-cis retinoic acid had no more effect on PAI-1 synthesis than any of these compounds alone at optimal concentrations. In conclusion, fibrates downregulate an induced PAI-1 production in cynomolgus monkey hepatocytes independent of a decrease in triglyceride levels. A possible involvement of PPAR alpha/RXR alpha in this down-regulation is discussed.

Modulation of procoagulant and fibrinolytic system components of mesothelial cells by inflammatory mediators

Human peritoneal mesothelial cells (HMC) play a critical role in maintaining the intraperitoneal balance between fibrinolysis and coagulation by expressing the fibrinolytic enzyme tissue-type plasminogen activator (t-PA) as well as a specific plasminogen activator inhibitor, PAI-1, and the procoagulant protein tissue factor (TF). Of three compounds known to stimulate t-PA synthesis in cultured human endothelial cells, i.e., retinoic acid, the protein kinase C activator 4 beta-phorbol 12-myristate 13-acetate (PMA), and sodium butyrate, only butyrate (1 mM) caused about a threefold increase in t-PA synthesis and mRNA expression in HMC after 24 h of incubation, without markedly affecting PAI-1 synthesis. PMA (10 nM) induced a threefold increase in urokinase-type plasminogen activator (u-PA) mRNA, but u-PA antigen levels in the HMC conditioned media remained below the detection level (0.5 ng/ml), possibly as a result of rapid uptake and degradation by the u-PA receptor. The u-PA receptor mRNA levels were about fivefold enhanced above control levels after PMA treatment of the cells. An increase in intracellular adenosine 3',5'-cyclic monophosphate levels by forskolin (10 microM) diminished t-PA and PAI-1 levels 43 and 17%, respectively. Among the inflammatory mediators tested [tumor necrosis factor-alpha (TNF-alpha), interleukin-1 alpha, and bacterial lipopolysaccharide], TNF-alpha (10-1,000 U/ml) showed the strongest procoagulant effects. We found that the isoflavone compound genistein (25 micrograms/ml) prevented the TNF-alpha-induced expression of PAI-1 and TF while also slightly counteracting the decrease in t-PA synthesis. The protein kinase C inhibitor R0-318220 (3 microM) only moderately opposed the TNF-alpha-induced changes in t-PA and PAI-1 synthesis but completely prevented the induction of TF mRNA. In summary, our results demonstrate that t-PA synthesis in HMC is relatively insensitive to pharmacological stimulation. To restore the balance between fibrinolysis and coagulation under inflammatory conditions, attempts to interfere with the TNF-alpha-signaling pathway were more successful.

Role of c-Jun and proximal phorbol 12-myristate-13-acetate-(PMA)-responsive elements in the regulation of basal and PMA-stimulated plasminogen-activator inhibitor-1 gene expression in HepG2

Experiments were designed to clarify the role of c-Jun/c-Fos and of putative phorbol 12-myristate-13-acetate-(PMA)-responsive elements (TREs) in the induction of plasminogen-activator inhibitor 1 (PAI-1) gene transcription in the human hepatoma cell line HepG2 by activators of protein kinase C (PKC). Treatment of HepG2 cells with the phorbol ester PMA or serum rapidly and transiently increased c-Jun and c-Fos mRNA and protein levels prior to PAI-1 induction. This induction of PAI-1 gene transcription was found to be dependent on ongoing protein synthesis. An essential role of c-Jun and c-Fos in basal and PMA-stimulated transcription of the PAI-1 gene is demonstrated by our finding that antisense c-jun and c-fos oligodeoxynucleotides both strongly reduced basal and PMA-stimulated PAI-1 synthesis. Since it has already been shown that two TREs between positions -58 and -50 and between -79 and -72 of the PAI-1 promoter are essential for basal and PMA-induced PAI-1 promoter activity ([16]), we examined binding of nuclear proteins to these elements. The protein-binding activity to the TRE between positions -79 and -72 shows very strong PMA induction of an unknown factor, which is not related to c-Jun or c-Fos. The TRE binding between positions -58 and -50 forms two complexes, both containing c-Jun protein. The faster migrating complex primarily contains c-Jun homodimers. The amount of the faster migrating complex is enhanced more than 30-fold in PMA-treated cells, due to a strongly increased binding of c-Jun homodimers and, to a minor extent, to binding of c-Jun/c-Fos heterodimers. Dissociation experiments suggest that the c-Jun/c-Fos heterodimers bind with much lower affinity compared to binding of c-Jun homodimers. Together with the finding that both antisense c-jun and antisense c-fos oligodeoxynucleotides reduced the amount of c-Jun homodimer, we conclude that binding of c-Jun homodimer to the TRE at positions -58 to -50 is important in the basal activity and PMA activation of the PAI-1 promoter in HepG2 cells.

Stimulation of tissue-type plasminogen activator expression by retinoic acid in human endothelial cells requires retinoic acid receptor beta 2 induction

We previously showed the involvement of retinoic acid receptor alpha (RAR alpha) in the induction of tissue-type plasminogen activator (t-PA) synthesis by RA in human umbilical vein endothelial cells (HUVECs). However, the rather slow onset of this induction of t-PA synthesis suggested an indirect role of RAR alpha. Here, we show that the protein synthesis inhibitor, cycloheximide completely blocks the induction of t-PA by RA, which points to the need of an intermediary protein in t-PA stimulation. This intermediary protein is likely to be RAR beta 2 on the basis of the following findings: (1) the induction of RAR beta by RA exactly precedes that of t-PA; (2) HUVECs with elevated RAR beta mRNA levels show an undelayed t-PA induction on stimulation with RA, and this response can be almost completely inhibited with an RAR antagonist; and (3) an antisense oligodeoxynucleotide against the translation initiation site of RAR beta 2 mRNA greatly reduces the t-PA induction by RA. Thus, induction of t-PA by RA in HUVECs involves a 2-step mechanism requiring induction of RAR beta 2 via RAR alpha, followed by induction of t-PA synthesis via RAR beta 2. Each of these steps is shown to have a different activation profile with RA and 9 cis RA.

Involvement of retinoic acid receptor alpha in the stimulation of tissue-type plasminogen-activator gene expression in human endothelial cells

Retinoids stimulate tissue-type plasminogen-activator (t-PA) gene expression in human endothelial cells, and are likely to do so by binding to one or more nuclear retinoid receptors. The present study was initiated to identify the retinoid receptor(s) involved in this process. Expression and regulation of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) were analyzed by Northern-blot analysis of total or poly(A)-rich RNA prepared from cultured human umbilical vein endothelial cells (HUVEC). Prior to any exposure to retinoids, HUVEC express two transcripts for RAR-alpha (3.6 kb and 2.8 kb), and low levels of transcripts for RAR-beta (3.4 kb and 3.2 kb) and RAR-gamma (3.3 kb and 3.1 kb). Two RXR subtypes were identified, RXR-alpha (4.8 kb) and, at a much lower concentration, RXR-beta (2.4 kb); no evidence for the presence of RXR-gamma was found. Furthermore, HUVEC express cellular retinol-binding protein I (CRBP-I) and cellular retinoic-acid-binding protein I (CRABP-I) mRNA. Exposure of HUVEC to 1 microM retinoic acid or the retinobenzoic acid, Ch55, led to the induction of the two RAR-beta mRNAs, RXR-alpha mRNA and CRBP-I mRNA, whereas the expression of the other receptor and CRABP-I transcripts did not change appreciably. Using retinoid analogues that bind preferentially to one of the RAR or RXR subtypes, we found evidence that RAR-alpha is involved in the retinoid-induced t-PA expression in HUVEC. This conclusion was strengthened by experiments in which blocking of RAR-alpha with a specific RAR-alpha antagonist, Ro 41-5253, was demonstrated to suppress the induction of t-PA by retinoids.

Stimulation of tissue-type plasminogen activator gene expression by sodium butyrate and trichostatin A in human endothelial cells involves histone acetylation

We have previously shown that the pleiotropic agent sodium butyrate strongly stimulates tissue-type plasminogen activator (t-PA) expression in human umbilical vein endothelial cells (HUVEC). Here we provide the following evidence that the butyrate-induced t-PA expression in HUVEC involves histone H4 acetylation. (1) t-PA induction by butyrate occurs at the transcriptional level and does not require new protein synthesis, indicating a direct effect. (2) t-PA induction by butyrate can be fully mimicked by a specific, structurally unrelated, histone deacetylase inhibitor, trichostatin A. (3) At optimally stimulatory conditions, a combination of butyrate and trichostatin A does not enhance t-PA production more than each of the compounds alone, indicating that both compounds act through a common regulatory mechanism. (4) Induction of t-PA transcription by butyrate and trichostatin A was found to be preceded by histone H4 acetylation; at suboptimal inducing concentrations of butyrate and trichostatin A, the degree of acetylation of histone H4 caused by each agent was similarly reduced. These results are consistent with a role for histone H4 acetylation in t-PA induction by butyrate in HUVEC.

Studies on the acute release of tissue-type plasminogen activator from human endothelial cells in vitro and in rats in vivo: evidence for a dynamic storage pool

The process of acute release of tissue-type plasminogen activator (tPA) is important in locally speeding up fibrinolysis. Using a sensitive enzyme-linked immunosorbent assay for tPA, we investigated the acute release of tPA from cultured human umbilical vein endothelial cells. The addition of thrombin (0.003 to 3 NIH U/mL) caused the dose-dependent release of noncomplexed, enzymatically active tPA into the medium. The amount of tPA released into the medium by thrombin was similar to the difference in the amounts of tPA present in extracts from thrombin-treated cells and control cells. The process of acute release of tPA was complete in 1 minute, whereas the concomitant release of von Willebrand factor into the medium was slightly slower (maximum after 3 minutes). By increasing (c.q. decreasing) tPA synthesis, it was found that the amount of tPA constitutively secreted, the amount acutely released, and the amount in cell extracts were increased (c.q. decreased) to the same extent. The same relation was found in vivo. When rats were pretreated with cholera toxin or retinoic acid to increase tPA synthesis, plasma levels of tPA were increased, whereas acute release of tPA, as induced by bradykinin, was increased to the same extent. Acutely released tPA and constitutively secreted tPA were liberated from different pathways in human umbilical vein endothelial cells; tPA had, relative to the in vivo situation, a short residence time in the acutely releasable pathway.

Tumor necrosis factor alpha induces endothelial galactosyl transferase activity and verocytotoxin receptors. Role of specific tumor necrosis factor receptors and protein kinase C

Infections with verocytotoxin (VT) producing Escherichia coli have been strongly implicated in the epidemic form of hemolytic uremic syndrome (HUS). Endothelial damage plays a central role in the pathogenesis of HUS. In vitro studies have shown that VT can damage endothelial cells after interaction with its cellular receptor globotriaosylceramide (GbOse3cer). Cytokines, such as tumor necrosis factor alpha (TNF alpha) and interleukin-1 (IL-1) can potentiate the toxic effect of VT by inducing a protein-synthesis dependent increase in VT receptors on endothelial cells. In this study, the mechanisms underlying the increase in endothelial VT receptors induced by TNF alpha were studied in more detail. To investigate which proteins were involved in this induction, endothelial cells were incubated with and without TNF alpha in the presence of 14C-galactose or 14C-glucose. Thin-layer chromatography (TLC) analysis of the glycolipid extracts of these cells demonstrated a markedly enhanced incorporation of 14C-galactose in GbOse3cer and other galactose-containing glycolipids, suggesting that TNF alpha enhanced galactosyl-transferase activity. To examine the role of the two recently cloned TNF-receptors (TNFR-p75 and TNFR-p55) in the TNF alpha-induced increase in GbOse3cer in human endothelial cells, cells were incubated with TNF alpha, the TNFR-p55 selective R32W-S86T-TNF alpha-mutant, or the TNFR-p75 selective D143N-A145R-TNF alpha-mutant. The effect of TNF alpha activation, determined by binding-experiments with 125I-VT-1, could be largely, but not completely mimicked by R32W-S86T-TNF alpha. Although incubation of cells with D143N-A145R-TNF alpha did not show an increase in VT-1 binding, the monoclonal antibody utr-1, which prevents binding to TNFR-p75, decreased the TNF alpha-induced VT-1 binding. Activation of protein kinase C (PKC) by phorbol ester increases the expression of VT-1 receptors; this effect was prevented by the PKC inhibitor Ro31-8220 and by homologous desensitization by pretreatment with phorbol ester. In contrast, the presence of the protein kinase inhibitor Ro31-8220 or desensitization of PKC activity reduced the TNF alpha-induced increase in VT-1 receptors maximally by 50% and 24%, respectively. Comparable reductions in overall protein synthesis and the synthesis of E-selectin and plasminogen activator inhibitor-1 (PAI-1) were observed. This suggests an effect on general protein synthesis rather than a specific effect of PKC in the signal transduction pathway, by which TNF alpha induces VT-1 receptors. Our results indicate that TNF alpha can increase the VT-1 receptors on endothelial cells by inducing galactosyl-transferase activity, that this action of TNF alpha mainly occurs via the TNFR-p55; and that PKC activation increases expression of VT-1 receptors by a separate mechanism that acts additively to the TNF alpha-induced increase in VT-1 receptors.

Genistein reduces tumor necrosis factor alpha-induced plasminogen activator inhibitor-1 transcription but not urokinase expression in human endothelial cells

The plasminogen activator inhibitor PAI-1 is markedly elevated in vivo and in vitro upon exposure to the inflammatory mediators tumor necrosis factor alpha (TNF alpha), interleukin-1 (IL-1), and bacterial lipopolysaccharide. Here we report that the isoflavone compound genistein prevents the increase in synthesis of PAI-1 induced by these inflammatory mediators in human endothelial cells in vitro, and partially reduces the basal PAI-1 production by these cells. These effects of genistein were accompanied by a decrease in PAI-1 mRNA and in a suppression of the PAI-1 transcription rate as shown by run-on assay. A specific action of genistein, probably by inhibiting a tyrosine protein kinase, is likely, because the structural genistein analogue daidzein, which has a low tyrosine protein kinase inhibitor activity, did not inhibit PAI-1 synthesis. Vanadate, a tyrosine protein phosphatase inhibitor, increased PAI-1 production. The effect of genistein on PAI-1 synthesis was rather selective. Herbimycin A also reduced PAI-1 synthesis, but several other tyrosine protein kinase inhibitors, namely tyrphostin A47, methyl-2,5-dihydroxy-cinnamate, and compound 5, were unable to do so. All these tyrosine protein kinase inhibitors reduced basic fibroblast growth factor (b-FGF)-induced [3H]thymidine incorporation in endothelial cells. This indicates that the effect of genistein on PAI-1 transcription proceeds independently of its effect on mitogenesis. In contrast to TNF-alpha-induced PAI-1 production, the transcription and synthesis of urokinase-type plasminogen activator (u-PA) was not inhibited by genistein. A TNF-alpha-mutant (Trp32Thr86TNF alpha) that specifically recognizes the 55-kD TNF-receptor, mimicked the effects of TNF alpha on both PAI-1 and u-PA. Because genistein affected PAI-1, but not u-PA induced by this mutant, involvement of different TNF-receptors cannot underlie the difference in the effects of genistein on PAI-1 and u-PA synthesis. Because genistein also inhibited PAI-1 induction by thrombin and IL-4, it is likely that genistein does not act on a TNF alpha-receptor-coupled protein kinase but on the signal transduction pathway enhancing PAI-1 transcription. Our results suggest that the TNF alpha-induced signal transduction pathway of PAI-1 transcription involves a genistein-sensitive step that is not involved in the induction of u-PA by TNF alpha. Given the limited sensitivity to several other tyrosine protein kinase inhibitors, this genistein-sensitive step may be a potential target for pharmacologic intervention to reduce elevated plasma PAI-1 levels.

Regulation of endothelial cell t-PA synthesis and release

The fibrinolytic activity of blood is to a large extent determined by the plasma level of tissue-type plasminogen activator (t-PA). Changes in the plasma level of t-PA are mainly achieved by the endothelium by two mechanisms: (a) a rapid, short-term release of t-PA, which occurs within minutes (acute release, regulated secretion) and (b) a long-term change in the rate of synthesis and constitutive secretion of t-PA. The rapid t-PA release response of the endothelium upon stimulation may play an important role in the dissolution of fibrin at the initial event of fibrin formation, and hence in the prevention of thrombus formation. The rate of constitutive t-PA secretion is an important determinant of the actual levels of t-PA under basal and stimulated conditions. Our insight into the regulation of the synthesis and release of t-PA has extended markedly in the last decade and still adapts by the continuous stream of new experimental data. In this review we summarize our present knowledge about the factors, including intracellular signalling pathways, promoter elements and transcription factors involved in the modulation of t-PA gene expression, with particular reference to the regulation in human endothelial cells. We also discuss the mechanisms underlying acute release of t-PA from the endothelium, and provide evidence that the regulated and constitutive secretion of t-PA are interrelated in several aspects.

A sensitive ELISA for human tissue-type plasminogen activator applicable to the study of acute release from cultured human endothelial cells

The development of a highly sensitive, enzyme-linked immunosorbent assay (ELISA) for tissue-type plasminogen activator (tPA) is described. The use of a biotin-avidin system resulted in a detection limit of 10 pg of tPA per ml, which is 50 to 150 times more sensitive than commercially-available ELISAs. Free tPA and tPA complexed to PA inhibitor type-1 were detected with equal efficiency. The ELISA proved suitable for measuring tPA antigen in conditioned media and in cell extracts. The intra-assay coefficient of variation varied in six different experiments from 1.9% to 3.4% in the range 0 to 250 pg of tPA antigen per ml. The inter-assay coefficient of variation was 7% (n = 6). The ELISA was used to study the acute release of tPA from human umbilical vein endothelial cells in vitro. Upon addition of thrombin (1 NIH U/ml) to endothelial cells, tPA was rapidly released into the medium, the highest release occurring during the first minute. Concomitantly, the tPA concentration in the cell extracts decreased. Evidence is presented that tPA is released from an intracellular source.

Effect of isotretinoin on endogenous tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor 1 (PAI-1) in humans

The effect of isotretinoin on fibrinolysis was investigated in 10 healthy, male volunteers in a randomized, double-blind, crossover-designed study. Isotretinoin (40 mg) was administered in the morning and in the evening for 5 days. t-PA, u-PA and PAI-1 antigen and activity in plasma were measured every morning at 9 a.m. on days 1 to 4 and every 3 hours over 24 hours on day 5. Isotretinoin treatment had no significant stimulatory effect on endogenous t-PA antigen and activity in morning plasma samples nor on their circadian variation. Also, u-PA antigen levels did not change after isotretinoin treatment. Mean PAI-1 antigen and PAI activity in 9 a.m. plasma samples were non-significantly higher during isotretinoin than during placebo treatment. After treatment with isotretinoin a significant rise of fasting triglyceride plasma levels was observed as compared to placebo. The study shows that isotretinoin has no clinically significant effect on endogenous fibrinolysis.

Triazolobenzodiazepines: a new class of stimulators of tissue-type plasminogen activator synthesis in human endothelial cells

In our search for compounds that can stimulate endogenous fibrinolysis, we have found that certain triazolobenzodiazepines enhance the production of tissue-type plasminogen activator (t-PA) by vascular endothelial cells maintained in vitro, with no or even a lowering effect on plasminogen activator inhibitor type-1 (PAI-1) production. The most active compounds tested, U-34599, U-46195 and U-51477, were studied in more detail and showed a time- and dose-dependent increase in the production of t-PA by human umbilical vein endothelial cells. At optimal stimulatory concentrations (about 10 microM), the three compounds stimulated t-PA expression about 2-fold after 24 hr and maximally about 4-fold after 48 hr of incubation; this maximal increase in t-PA synthesis was sustained at prolonged incubations of 72 or 96 hr. The triazolobenzodiazepine effects on t-PA production were accompanied by parallel increases in t-PA mRNA levels, without marked changes in PAI-1 or glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA concentrations. Numerous analogues of the three lead compounds were then tested to determine the relationship between benzodiazepine structure and the ability to stimulate t-PA production. No positive correlation was found between the ability of the various triazolobenzodiazepines to stimulate t-PA production and their affinity for the benzodiazepine receptor. In agreement with this, no specific binding of [3H]flunitrazepam, a specific ligand for benzodiazepine receptors, to endothelial cell membrane preparations was observed. Thus, it is unlikely that the triazolobenzodiazepines act through central-type benzodiazepine receptors to stimulate t-PA production. Similarly, no evidence was found for the presence of peripheral-type benzodiazepine receptors on endothelial cell membranes. The ability of the benzodiazepines to stimulate t-PA production, however, appeared to be related to their platelet-activating factor (PAF) antagonist activity. Despite this finding, several non-benzodiazepine PAF antagonists did not stimulate t-PA production. While the precise mechanism of action is not yet clear, selected benzodiazepine analogues possessing PAF antagonist activity stimulate the production of t-PA by endothelial cells in vitro.

Modulation of tissue-type plasminogen activator by retinoids in rat plasma and tissues

In this study the effect of vitamin A status and retinoid treatment on the activity of tissue-type plasminogen activator (TPA) and its inhibitor, plasminogen activator inhibitor-type 1 (PAI-1), in plasma and in several tissues was investigated in BN/BiRij rats. Hypervitaminosis A and retinoic acid treatment increased plasma TPA activity by approximately 50%, but PAI-1 activity was not affected. The effect of retinyl palmitate treatment on plasma TPA activity was already significant after 5 days and continued for at least up to 8 wk. Isotretinoin treatment affected neither plasma TPA nor PAI-1 activity. In plasma of vitamin A-deficient rats, TPA activity was decreased by a factor of three, whereas PAI-1 activity was increased twofold. Modulation of plasma TPA activity by vitamin A status and retinoic acid treatment was associated with similar changes in tissue TPA activity. This suggests that the changes in plasma TPA activities might be the result of changes in tissue TPA synthesis.