The use of cultured human endothelial cells and hepatocytes as an in vitro model system to study modulation of endogenous fibrinolysis

In vitro induction of endothelial cell fibrinolytic alterations by Nigella sativa

The effect of Nigella sativa (NS) L. oil (blackseed oil) on the fibrinolytic system of the human umbilical vein (HUV) and human uterine arterial (HUA) endothelial cells (ECs) in culture was studied. Both of them showed a concentration-dependent increase in tissue-type plasminogen activator (t-PA). A maximum effect was achieved with 50 microg oil/ml conditioned medium (CM) (1.3+/-0.15ng/10(4) cells/24h vs. control 0.7+/-0.06ng/10(4) cells/24h, and 0.38+/-0.04ng/10(4) cells/24h vs. control 0.24+/-0.02ng/10(4) cells/24h, for HUVEC and HUA-EC, respectively). At 100 microg/ml, there was a significant change in the amount of t-PA antigen produced by either HUVEC or HUA-EC (1.0+/-0.1 ng/10(4) cells/24 h or 0.28+/-0.02 ng/10(4) cells/24 h) as compared to control CM from cells grown under control conditions, but still less than that recorded at 50 microg oil/ml. Plasminogen activator inhibitor-type 1 increased the CM significantly and concentration-dependently in both cells. For HUVEC, the maximum effect was achieved at a concentration of 100 microg/ml (257.7+/-8.0 ng/10(4) cells/24 h vs. control 72.7+/-3.8 ng/10(4) cells/24 h). HUA-EC showed the maximum effect at a concentration of 100 microg/ml (171.6+/-4.4 ng/10(4) cells/24 h vs, control 53.8+/-3.7 ng/10(4) cells/24 h). This study suggests a role for NS oil in modulating the balance of fibrinolysis/thrombus formation by modulating the fibrinolytic potential of endothelial cells.

The role of estrogen in cardiovascular disease

Cardiovascular disease is the number one cause of death among women, accounting for nearly 50% of female deaths. Statistics show that women on average develop cardiovascular disease 10 to 15 years later in life than men, and that the risk may increase after menopause. This observation has led to much speculation as to what physiological change(s) associated with menopause is responsible for the higher risk of atherosclerosis. Estrogen, with its potential as a cardioprotective agent and as an immunomodulator of the inflammatory response in atherosclerosis, has received the most attention. Understanding the mechanisms that lead to these differences may allow beneficial therapeutic intervention to enhance this effect in females and evoke this protection in males. This review will do the following: (1) characterize mechanisms of atherosclerosis, (2) explore the role of estrogen-replacement therapy, (3) define the effect of gender on inflammation, (4) compare and contrast the effects of estrogen and testosterone on endothelial functional, and (5) suggest mechanistic based therapeutic opportunities.

Daunorubicin attenuates tumor necrosis factor-alpha-induced biosynthesis of plasminogen activator inhibitor-1 in human umbilical vein endothelial cells

The anthracycline antibiotic daunorubicin is reported to induce apoptosis in cells by triggering ceramide generation through de novo synthesis or sphingomyelin hydrolysis. Treatment of human umbilical vein endothelial cells (HUVEC) with daunorubicin markedly decreased the mRNA expression and protein release of plasminogen activator inhibitor-1 (PAI-1). This cellular event was accompanied by a significant increase in the total ceramide content in HUVEC. On the other hand, tumor necrosis factor (TNF)-alpha treatment of HUVEC led to an increase in both PAI-1 mRNA expression and protein release, and an enhancement of total ceramide content was also observed. The stimulating effect of TNF-alpha on PAI-1 synthesis was attenuated by the pretreatment of HUVEC with daunorubicin. Interestingly, the daunorubicin-induced increase in ceramide content was blocked by addition of the potent ceramide synthase inhibitor fumonisin B(1), while the TNF-alpha-induced ceramide increase was not affected by this drug. Fumonisin B(1) treatment restored the daunorubicin-induced decrease in PAI-1 release to approximately 70% of the control, but did not affect the TNF-alpha-induced increase in PAI-1 release. Thus, these data imply the possibility that the subcellular topology of ceramide production determines its lipid mediator function in the regulation of PAI-1 synthesis in HUVEC, because both TNF-alpha and daunorubicin could increase the ceramide levels.

Thrombin signal transduction of the fibrinolytic system in human adult venous endothelium in vitro

Thrombin can regulate the-fibrinolytic system by increasing the endothelial production of both tissue plasminogen activator (t-PA) and plasminogen activator inhibitor type-1 (PAI-1). The thrombin receptor transducts signals through the GTP-binding protein system, the classical pathway being the Galpha q-protein. The purpose of the present study was to examine the roles of Galpha i-protein and tyrosine kinases in the thrombin signal transduction of t-PA and PAI-1 production from human adult vein endothelial cells (HAVEC). t-PA and PAI-1 antigen were analysed in conditioned medium from cultured HAVEC after 16 h incubation. Data are expressed as percentages of basal release (100%), means +/- 95% confidence intervals. Thrombin increased t-PA and PAI-1 production (234 +/- 42% and 211 +/- 42%, respectively). Pertussis toxin (PTX) (inhibiting Galpha i-pathway) reduced basal PAI-1 (66 +/- 8%), but had only a weak influence on basal t-PA production. Pertussis toxin and genistein (inhibiting tyrosine kinase) significantly reduced the thrombin induction of both t-PA and PAI-1 (PTX: 142 +/- 23% and 146 +/- 19%, respectively, genistein: 156 +/- 42% and 76 +/- 24%, respectively). The present study demonstrated that thrombin can increase the production of t-PA and PAI-1 by transducting signals through the Galpha i and tyrosine kinase pathway, in addition to the Galpha q/protein kinase C pathway as has been found previously.

Changes in arterial expression of fibrinolytic system proteins in atherogenesis

Plasminogen activators (PAs) and their inhibitor, plasminogen activator inhibitor type-1 (PAI-1), have been implicated in modulation of luminal fibrinolysis and mural proteolysis contributing to atherogenesis. Expression of PAs/PAI-1 (normalized to extracted tissue protein) was delineated by assays of conditioned media and of extracts from walls of human arterial segments in culture. Arterial specimens (n = 39 from 26 subjects) were divided into four groups: normal (n = 14), fatty streak (n = 6), moderate atherosclerosis (mural thickening with < 70% lumen obstruction, n = 5), and severe atherosclerosis (mural thickening with > 70% lumen obstruction, n = 14). Paired samples from the same individual comprising a normal arterial segment and an atherosclerotic segment were evaluated also. A fourfold molar excess in PAI-1:t-PA was seen in conditioned media from samples with any evidence of atherosclerosis compared with normal specimens (normal 21 +/- 4, diseased 82 +/- 21, P < or = .05). Compared with normal pairs, the tissue content of PAI-1 (ng) was increased in fatty streak lesions (n = 3, normal 35 +/- 12, fatty streak 50 +/- 8, P < or = .05); stable to decreased in moderate atherosclerosis (n = 3, normal 34 +/- 3, moderate 22 +/- 7, P = .16); and increased in severe atherosclerosis (n = 6, normal 48 +/- 9, severe 85 +/- 19, P < or = .05). The tissue content of PAs (ng), though not increased in fatty streak lesions, was elevated in moderately and severely atherosclerotic segments (normal 0.7 +/- 0.2, moderate 1.6 +/- 0.1; normal 0.8 +/- 0.3, severe 2.1 +/- 0.3, P < or = .05 for each comparison). Atherogenesis is associated with decreased luminal fibrinolytic capacity that may exacerbate thrombosis. Decreased mural proteolysis in early atherogenesis may exacerbate matrix accumulation. Increased mural proteolysis later is associated with, and may potentiate, smooth muscle cell migration and proliferation.

Expression of fibrinolytic genes in tissues from human atherosclerotic aneurysms and from obese mice

The disturbances in the balance of pro- and antifibrinolytic activity, as observed in AAA and obesity, respectively, have considerable potential for influencing both intra- and extravascular fibrinolytic events and may be causally related to the development of vascular disease. For example, the wall of the aortic atherosclerotic aneurysm seems to host an uneven distribution and imbalanced expression of the various components of the fibrinolytic system. The sites of increased proteolytic activity may contribute to localized neovascularization and promote the rapid breakdown of ECM components, which result in mural weakening and eventual rupture of untreated aortic aneurysms. On the other hand, the disturbance of the normal hemostatic balance observed in obesity appears to result from the elevated expression of PAI-1 by the adipose tissue. Our data strongly suggest that the adipocyte is one of the primary cells in the adipose tissue capable of expressing PAI-1 both in obesity, and in response to cytokines and hormones like TNF-alpha and insulin. Since both TNF-alpha and insulin are known to increase in obesity, the elevated levels of PAI-1 observed in the plasma of obese individuals may result from TNF-alpha and/or insulin induction of PAI-1 in the adipose tissue itself.

Effects of hormone-replacement therapy on fibrinolysis in postmenopausal women

BACKGROUND

Plasma levels of plasminogen-activator inhibitor type 1 (PAI-1), an essential inhibitor of fibrinolysis in humans, increase in women after menopause, and this may contribute to the risk of cardiovascular disease. We studied the effects of hormone-replacement therapy on PAI-1 levels.

METHODS

In a randomized, crossover study, we investigated the effects of oral conjugated estrogen (0.625 mg per day) in 30 postmenopausal women and transdermal estradiol (0.1 mg per day) in 20 postmenopausal women, either alone or in combination with medroxyprogesterone acetate (2.5 mg daily) for one month, on plasma PAI-1 antigen levels. Degradation products of cross-linked fibrin (D-dimer) were measured in serum as an index of fibrinolysis.

RESULTS

PAI-1 levels were inversely associated with D-dimer levels at base line (r= -0.540, P=0.002). Conjugated estrogen, both alone and in combination with medroxyprogesterone acetate, reduced mean (+/-SD) plasma levels of PAI-1 from 32+/-34 ng per milliliter to 14+/-10 ng per milliliter (P<0.001) and from 31+/-29 ng per milliliter to 15+/-11 ng per milliliter (P=0.003), respectively; there was a significant inverse correlation between pretreatment PAI-1 levels and the degree of reduction in these levels during therapy (r= -0.631, P<0.001 for conjugated estrogen; r = -0.507, P=0.004 for combined therapy). The degree of reduction in PAI-1 levels was associated with increases in D-dimer levels both when conjugated estrogen was given alone (r= -0.572, P=0.001) and when combined hormone therapy was given (r= -0.541, P=0.002). Transdermal estradiol caused no significant changes in PAI-1 levels from base-line values.

CONCLUSIONS

Conjugated estrogen, alone or combined with progestin therapy, reduced PAI-1 levels by approximately 50 percent in postmenopausal women and was associated with enhanced systemic fibrinolysis. These findings may partly explain the protective effect of hormone-replacement therapy with respect to coronary artery disease.

Complex intracellular signal transduction regulates tissue plasminogen activator (t-PA) and plasminogen activator inhibitor type-1 (PAI-1) synthesis in cultured human umbilical vein endothelium

Endothelial cells are central in fibrinolysis because of their high production of both activators (t-PA, uPA) and inhibitors (PAI-1). The t-PA and PAI-1 synthesis could be regulated by signals transduction at several cellular levels. The purpose of this in vitro study, on cultured endothelial cells, was to explore the receptor/second messenger regulation of the t-PA and PAI-1 synthesis. Quiescent confluent human umbilical vein endothelial cells, cultured in passage 1, were exposed to different test substances. Samples from the conditioned medium were collected after 16 and 24 h and analysed for t-PA and PAI-1 antigen. All data presented were related to the data from control dishes (= 100%), in the same experiment. The results from the present study (mean +/- 95% confidence interval) demonstrated the following. (1) Forskolin, with a documented direct cAMP-inducing effect, decreased the basal PAI-1 production to 61 +/- 15%, and Na-nitroprusside, with a documented cGMP-inducing effect, increased the basal PAI-1 production to 141 +/- 38% without affecting the basal t-PA production. The surface receptor agonists isoprenalin or ephedrine, which indirectly affect adenylate cyclase, had no effect on t-PA or PAI-1 production. (2) Phorbolester (PMA), which directly activates proteinkinase C (PKC), increased the basal t-PA and PAI-1 production to 350 +/- 71%, and 163 +/- 35% respectively. (3) Thrombin, but not endothelin-1 (ET-1), increased the basal t-PA and PAI-1 production to 195 +/- 34% and 136 +/- 18%, respectively, indicating an PKC-mediated thrombin effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of notoginsenoside R1 on the synthesis of tissue-type plasminogen activator and plasminogen activator inhibitor-1 in cultured human umbilical vein endothelial cells

Among other Chinese herb drugs, Panax notoginseng is used to treat cardiovascular diseases. To elucidate any possible effects of this drug on the hemostatic system in vitro, we analyzed the influence of one of its major active constituents on fibrinolytic parameters of cultured human umbilical vein endothelial cells (HUVECs). When confluent cultures of HUVECs (passages 2 to 3) were conditioned with purified notoginsenoside R1 (NR1), a dose- (0.01 to 100 micrograms NR1/mL) and time-dependent increase in tissue-type plasminogen activator (TPA) synthesis was observed, which was significant from 0.1 microgram NR1/mL and from 6 hours of incubation with 100 micrograms NR1/mL on. TPA antigen increased from 3.9 +/- 0.2 ng per 10(5) cells per 24 hours to 8.0 +/- 0.5 ng per 10(5) cells per 24 hours on addition of 100 micrograms NR1/mL. In contrast, no change in urokinase-type plasminogen activator and plasminogen activator inhibitor-1 (PAI-1) antigen synthesis was seen. There was also no effect of NR1 on PAI-1 deposition in the extracellular matrix. As judged from fibrin autography and reverse fibrin autography, TPA activity and TPA-PAI-1 complexes reached a maximal stimulation of more than threefold and twofold, respectively, at a concentration of 100 micrograms NR1/mL in conditioned media. On the contrary, NR1 induced a more than fivefold decrease in PAI-1 activity at the same concentration of NR1 in conditioned media. On Northern blot analysis of RNA obtained from NR1-stimulated and control HUVECs, NR1 induced a significant increase in TPA mRNA (192% of control value at 100 micrograms NR1/mL) while PAI-1 mRNA remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Augmentation of synthesis of plasminogen activator inhibitor type-1 in arterial endothelial cells by glucose and its implications for local fibrinolysis

Because of the frequent occurrence of premature cardiovascular disease in patients with non-insulin-dependent, type II diabetes mellitus (NIDDM), the attenuated fibrinolytic activity of plasma from type II diabetic patients with increased concentrations of plasminogen activator inhibitor type-1 (PAI-1), and the fact that insulin stimulates synthesis of PAI-1 by human hepatic cells in vitro, we and others have hypothesized that accelerated vascular disease in type II diabetes may result in part from impaired fibrinolysis secondary to excessive elaboration of PAI-1 stimulated by insulin. Alternatively, the hyperglycemia associated with type II diabetes could influence the synthesis and secretion of PAI-1 directly. The present study was performed to determine whether PAI-1 secretion is or is not sensitive to the prevailing concentration of glucose in the conditioned medium of endothelial and liver cells, which are thought to be the major sources of circulating PAI-1 in vivo. Confluent cells were exposed to 0, 2.8, 5.6, 11.1, or 22.2 mmol/L (0, 50, 100, 200, or 400 mg/dL) glucose in medium without serum and subsequently to media with or without insulin (7.3 nmol/L). Secretion of PAI-1 by highly differentiated human hepatoma (Hep G2) cells did not increase as a function of increasing concentrations of glucose, whether or not insulin was present. In contrast, with pig aortic endothelial cells, the secretion of PAI-1 increased significantly with extracellular glucose with or without insulin. The increases in PAI-1 were specific (as shown by metabolic labeling experiments) and not attributable to osmotic effects (as shown by replacement of glucose by sorbitol).(ABSTRACT TRUNCATED AT 250 WORDS)

Mosaic tetrasomy 8p: molecular cytogenetic confirmation and measurement of glutathione reductase and tissue plasminogen activator levels

We report the case of a 5-year-old girl with severe developmental disabilities, skeletal anomalies, hypotonia, rectal atresia, malrotation of the intestine, horseshoe kidney, vesicoureteric reflux, and minor facial anomalies. Conventional cytogenetic techniques suggested that she had a mosaic 46,XX/47,XX,+i(8p) constitution, and the identity of the isochromosome was confirmed by in situ hybridization and chromosome painting. Polymorphic DNA markers are consistent with the i(8p) having arisen as the result of a segregation error and centromere misdivision at the second maternal meiotic division. The i(8p) was seen in 17/25 (68%) lymphocytes at the age of one month but had declined to 31/100 (31%) cells by the age of 5 years. At this time the i(8p) was seen in 30/68 (44%) cultured skin fibroblasts. The proposita had an approximately twofold increase in red cell glutathione reductase activity but a normal level of tissue-plasminogen activator. These enzyme results are consistent with the known localisation of the glutathione reductase gene on the short arm of chromosome 8 but suggest that the tissue-plasminogen activator gene may map outside this region.

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.

Tissue-type plasminogen activator, type 1 plasminogen activator inhibitor and their complex in plasma with disseminated intravascular coagulation

The levels of tissue-type plasminogen activator (t-PA), type 1 plasminogen activator inhibitor (PAI-1), and t-PA/PAI-1 complex antigens were analyzed in the plasma of disseminated intravascular coagulation (DIC) patients and healthy controls. Other fibrinolytic parameters such as the levels of plasminogen, alpha 2-antiplasmin (alpha 2-AP), plasmin/alpha 2-AP (PAP), and D-dimer were also estimated to clarify the fibrinolytic states in these plasmas. The antigens of t-PA, PAI-1, and t-PA/PAI-1 complex were found to increase from 8.5 +/- 4.3, 54.4 +/- 21.2, and 8.6 +/- 3.5 ng/ml in normal plasma to 36.4 +/- 25.1, 106.8 +/- 54.7, and 46.6 +/- 34.5 ng/ml in DIC plasma, respectively. The molar ratio of total t-PA to total PAI-1 was 1:6 and 1:3 in normal plasma and DIC plasma, respectively, indicating an enhanced fibrinolytic state in the DIC plasma. The DIC plasma revealed a significant consumption of plasminogen (62.1 +/- 27.8%), and alpha 2-AP (63.7 +/- 25.3%) and an increase in PAP (2.6 +/- 2.7 micrograms/ml) and D-dimer (3.9 +/- 10.7 micrograms/ml). These results suggest that the production and secretion of t-PA and PAI-1 from endothelial cells were enhanced in DIC, resulting in an increased t-PA/PAI-1 complex with dominant fibrinolytic activity.

Direct effects of gemfibrozil on the fibrinolytic system. Diminution of synthesis of plasminogen activator inhibitor type 1

BACKGROUND

Platelet-associated epidermal growth factor (EGF) and transforming growth factor-beta (TGF-beta) can augment synthesis of plasminogen activator inhibitor type 1 (PAI-1). Accordingly, exacerbation of atherogenesis may accompany release of platelet-associated growth factors (or mitogens) occurring in association with occult, repetitive thrombosis and thrombolysis. In the Helsinki primary prevention trial, gemfibrozil decreased coronary events but did so essentially only in initially hypertriglyceridemic subjects. Such subjects are known to exhibit high concentrations of PAI-1 in plasma.

METHODS AND RESULTS

To determine whether pharmacological concentrations of gemfibrozil directly affect PAI-1 synthesis, we characterized its effects on a human hepatoma cell line (Hep G2) in vitro. Gemfibrozil decreased basal PAI-1 secretion by 43% and attenuated the augmentation of PAI-1 synthesis over 24 hours induced by EGF and TGF-beta by 37% and 39% without altering overall protein synthesis. Furthermore, it blocked the EGF and TGF-beta-induced increases in PAI-1 mRNA over 6 hours by 65% and 60%. Increases in plasma PAI activity induced by infusion of purified growth factors or by autologous platelet lysates in rabbits were inhibited by gemfibrozil by more than 50%.

CONCLUSIONS

Beneficial effects of gemfibrozil in reducing coronary events in hypertriglyceridemic patients may depend, in part, on potentiation of fibrinolysis by direct diminution of synthesis of endogenous PAI-1.

IL-1-induced procoagulant and fibrinolytic activities of human endothelium grown on carbodiimide crosslinked proteins

We examined the regulation of procoagulant activity and the production of fibrinolytic components by human vascular endothelium grown on coating membranes of gelatin, pure or mixed with albumin, crosslinked by carbodiimide ((G)C, (AG)C) in comparison with plastic culture dishes. Confluent monolayers were stimulated by human recombinant interleukin (IL-1 beta) and responses in terms of tissue factors like procoagulant activity, tissue plasminogen activator (tPA) and plasminogen activator inhibitor (PAI-1) were followed for up to 72 h. Procoagulant activity of cell extracts displayed similar patterns whatever the substratum tested. Quantitative immunological assays revealed a 2-fold increase in tPA antigen released from monolayers grown on (G)C and on (AG)C compared to cells grown on plastic. Exposure of monolayers to IL-1 beta reduced the secretion of tPA antigen which still reached higher values on coated than on uncoated substratum. We found that the quasi-totality of tPA formed stable complexes with PAI-1, thereby suppressing measurable fibrinolytic activity. IL-1 beta stimulated the release of PAI-1 antigen quantified by immunoassay and the kinetics of secretion were comparable on both coated and uncoated substratum.

Role of protein kinase C and cyclic adenosine monophosphate in the regulation of tissue-type plasminogen activator, plasminogen activator inhibitor-1, and platelet-derived growth factor mRNA levels in human endothelial cells. Possible involvement of proto-oncogenes c-jun and c-fos

Activation of protein kinase C leads to a strong induction of tissue-type plasminogen activator (t-PA) expression in endothelial cells. Using endothelial cells from human umbilical vein (HUVECs) and human aorta (HAECs), we have studied this regulation of t-PA and its inhibitor, plasminogen activator inhibitor-1 (PAI-1), at the mRNA level and have compared their induction with the expression of platelet-derived growth factors A and B (PDGF-A and PDGF-B) and the proto-oncogenes c-jun and c-fos. Treatment of HUVECs with exogenous bacterial phospholipase C or the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol led to a threefold and a twofold increase, respectively, in t-PA concentrations in 24-hour-conditioned medium. Similarly, the more stable protein kinase C activator 4 beta-phorbol-12-myristate-13-acetate (PMA) caused about a 10-fold increase in t-PA antigen levels. This effect of PMA is maximal between 8 and 16 hours at a concentration of 10 nM and is fully accounted for by parallel increases in t-PA mRNA levels. An increase in intracellular cyclic adenosine monophosphate levels by forskolin (10 microM) slightly diminished t-PA expression but further enhanced the PMA-induced increases in t-PA synthesis and mRNA levels by at least twofold. PMA also enhanced the mRNA levels of two other important endothelium-expressed genes, PDGF-A and PDGF-B, with a time profile similar to that of t-PA, with peak values about fivefold higher than control values. Forskolin did not further stimulate this PMA-induced PDGF expression in HUVECs, which suggests a regulatory mechanism different from that of t-PA. Qualitatively very similar induction patterns of t-PA, PDGF-A, and PDGF-B were seen with HAECs. In contrast to t-PA and PDGF, PAI-1 mRNA and antigen levels increased only slightly after PMA treatment of HUVECs or HAECs; forskolin alone or in combination with PMA diminished the expression of PAI-1. The induction of t-PA mRNA by PMA was dependent on protein synthesis and was preceded by a strong transient increase in c-jun and c-fos mRNA levels; the induction of c-fos but not of c-jun was potentiated by forskolin. Because the products of these two proto-oncogenes form dimeric complexes for which specific binding sites are present in the t-PA promoter region, they may mediate the protein kinase C-dependent increase in t-PA gene expression, including the stimulating action of cyclic adenosine monophosphate.