Steroids and cytokines in endometrial angiogenesis

Angiogenesis, the formation of new blood vessels from pre-existing ones, occurs physiologically in the endometrium and pathologically e.g. during tumour growth. Sex-steroid hormones affect angiogenesis in the endometrium during the menstrual cycle and might be implicated in cancer angiogenesis. Little information is presently available regarding the exact mechanisms by which these steroids exert their function on the process of both physiological and pathological angiogenesis. In this overview a survey is given of factors important for angiogenesis and of the effects of steroids on the expression of these factors. We have focused on endometrial angiogenesis, because the endometrium is unique in its cyclic growth pattern for which angiogenesis is indispensable. Stimulators and inhibitors of endometrial angiogenesis, that have been found (or suggested) to respond to ovarian steroids, are discussed These factors include vascular endothelial growth factor (VEGF), fibroblast growth factors (FGFs), tumour necrosis factor a (TNF-alpha), erythropoietin (Epo) and trombospondin-1 (TSP-1). Also, the influence of steroids on the expression of matrixdegrading proteases, in particular the plasminogen activator/plasmin system and matrixdegrading metalloproteinases (MMPs) are reviewed, because these proteases play an important role in the migration and invasion of endothelial cells during the process of angiogenesis. An insight into the effects of steroids on endometrial angiogenesis may be helpful to understand and anticipate the potential stimulatory and inhibitory effect of various steroids on angiogenesis in other tissues, in particular tumours.

Negative regulation of human fibrinogen gene expression by peroxisome proliferator-activated receptor alpha agonists via inhibition of CCAAT box/enhancer-binding protein beta

Fibrinogen is a coagulation factor and an acute phase reactant up-regulated by inflammatory cytokines, such as interleukin 6 (IL-6). Elevated plasma fibrinogen levels are associated with coronary heart diseases. Fibrates are clinically used hypolipidemic drugs that act via the nuclear receptor peroxisome proliferator-activated receptor alpha (PPAR alpha). In addition, most fibrates also reduce plasma fibrinogen levels, but the molecular mechanism is unknown. In this study, we demonstrate that fibrates decrease basal and IL-6-stimulated expression of the human fibrinogen-beta gene in human primary hepatocytes and hepatoma HepG2 cells. Fibrates diminish basal and IL-6-induced fibrinogen-beta promoter activity, and this effect is enhanced in the presence of co-transfected PPAR alpha. Site-directed mutagenesis experiments demonstrate that PPAR alpha activators decrease human fibrinogen-beta promoter activity via the CCAAT box/enhancer-binding protein (C/EBP) response element. Co-transfection of the transcriptional intermediary factor glucocorticoid receptor-interacting protein 1/transcriptional intermediary factor 2 (GRIP1/TIF2) enhances fibrinogen-beta gene transcription and alleviates the repressive effect of PPAR alpha. Co-immunoprecipitation experiments demonstrate that PPAR alpha and GRIP1/TIF2 physically interact in vivo in human liver. These data demonstrate that PPAR alpha agonists repress human fibrinogen gene expression by interference with the C/EBP beta pathway through titration of the coactivator GRIP1/TIF2. We observed that the anti-inflammatory action of PPAR alpha is not restricted to fibrinogen but also applies to other acute phase genes containing a C/EBP response element; it also occurs under conditions in which the stimulating action of IL-6 is potentiated by dexamethasone. These findings identify a novel molecular mechanism of negative gene regulation by PPAR alpha and reveal the direct implication of PPAR alpha in the modulation of the inflammatory gene response in the liver.

Adenovirus-mediated transfer of the 39 kD receptor-associated protein increases fibrinolytic capacity

BACKGROUND

The mesothelium has an important role in maintaining an adequate fibrinolytic capacity in the peritoneal cavity and thus in preventing the formation of fibrinous peritoneal adhesions by secreting the fibrinolytic enzyme tissue-type plasminogen activator (t-PA). The fibrinolytic activity of human mesothelial cells (HMCs) is counteracted by rapid uptake of t-PA via the low-density lipoprotein receptor-related protein (LRP). The 39 kD receptor-associated protein (RAP) is an inhibitor of binding of t-PA to LRP, but RAP itself is also rapidly degraded via LRP.

METHODS

Adenovirus-mediated RAP gene transfer technology was used to evaluate the effect of prolonged overexpression of RAP on t-PA accumulation in conditioned medium of HMCs under basal and inflammatory conditions.

RESULTS

Infection of HMCs with a recombinant adenovirus carrying the RAP cDNA resulted within one day in t-PA levels that were maximally twofold to threefold increased as compared with noninfected or adenovirus-beta-galactosidase-infected cells. Whereas upon prolonged incubation, t-PA levels in the conditioned medium of uninfected cells leveled off because of rapid uptake and degradation via LRP, t-PA concentrations in the medium of adenovirus-RAP-infected cells continued to increase, reaching fivefold control levels after 72 hours. The increased t-PA accumulation persisted for seven days and then slowly returned to control values over the next few weeks. In contrast, the production of a specific inhibitor of t-PA, plasminogen activator inhibitor-1 (PAI-1), was not affected by adenoviral RAP gene transfer. Northern blotting analysis showed that t-PA, PAI-1, and LRP mRNA concentrations were not changed after adenoviral infection, underlining that the elevated t-PA levels are the result of RAP-blocked uptake and degradation of t-PA rather than increased t-PA synthesis. RAP gene transfer also restored diminished fibrinolytic activity of cytokine-treated mesothelial cells.

CONCLUSIONS

Adenovirus-mediated transfer of the RAP gene provides an efficient way of transiently increasing the fibrinolytic capacity of mesothelial cells.

Aberrant fibrin formation and cross-linking of fibrinogen Nieuwegein, a variant with a shortened Aalpha-chain, alters endothelial capillary tube formation

A congenital dysfibrinogenemia, fibrinogen(Nieuwegein), was discovered in a young man without any thromboembolic complications or bleeding. A homozygous insertion of a single nucleotide (C) in codon Aalpha 453 (Pro) introduced a stop codon at position 454, which resulted in the deletion of the carboxyl-terminal segment Aalpha 454-610. The ensuing unpaired cysteine at Aalpha 442 generated fibrinogen-albumin complexes of different molecular weights. The molecular abnormalities of fibrinogen(Nieuwegein) led to a delayed clotting and a fibrin network with a low turbidity. Electron microscopy confirmed that thin fibrin bundles were organized in a fine network. The use of fibrinogen(Nieuwegein)-derived fibrin (fibrin(Nieuwegein)) in an in vitro angiogenesis model resulted in a strong reduction of tube formation. The ingrowth of human microvascular endothelial cells (hMVEC) was independent of alpha(v)beta(3), indicating that the reduced ingrowth is not due to the absence of the RGD-adhesion site at position Aalpha 572-574. Rather, the altered structure of fibrin(Nieuwegein) is the cause, since partial normalization of the fibrin network by lowering the pH during polymerization resulted in an increased tube formation. Whereas factor XIIIa further decreased the ingrowth of hMVEC in fibrin(Nieuwegein), tissue transglutaminase (TG), which is released in areas of vessel injury, did not. This is in line with the absence of the cross-linking site for TG in the alpha-chains of fibrinogen(Nieuwegein). In conclusion, this newly discovered congenital dysfibrinogenemia has a delayed clotting time and leads to the formation of an altered fibrin structure, which could not be cross-linked by TG and which is less supportive for ingrowth of endothelial cells.

Short-term effect of surgical trauma on rat peritoneal fibrinolytic activity and its role in adhesion formation

BACKGROUND

Fibrin deposition, the primary step in the formation of post-surgical adhesions, is the result of a disbalance between the fibrin-forming and the fibrin-dissolving capacity of the peritoneum. Literature data suggest a transient reduction in local plasminogen activator activity after peritoneal trauma, which results in a reduction of fibrinolysis and permits deposited fibrin to become organized into fibrous, permanent adhesions. In the present study, the fibrinolytic parameters tissue-type plasminogen activator (tPA; antigen and activity) and plasminogen activator inhibitor type-1 (PAI-1; antigen and activity) were measured in peritoneal fluid, in peritoneal biopsies and in plasma to establish the time course of changes in fibrinolytic activity.

DESIGN

A standardized peritoneal adhesion model in the rat.

OUTCOME MEASURES

Analysis, over a 72-h period following surgical trauma. of the main fibrinolytic parameters in peritoneal lavage, in biopsies of damaged and undamaged peritoneum, and in plasma, and determination of fibrin and fibrin(ogen)-degradation products in peritoneal lavage fluid.

RESULTS

At all time intervals, tPA antigen was found to be about six-fold increased in peritoneal lavage after surgical trauma. This significant rise in tPA antigen was accompanied by a large increase in its main inhibitor PAI-1, resulting in tPA activity levels similar to, or slightly higher than, those found in control animals. tPA activity was lowest at 4 h and increased thereafter. Also in biopsies from damaged peritoneum, tPA antigen was significantly increased. Tissue tPA activity was also lowest at 4 h, after which it increased, significantly so at 24 and 72 h. Similar, though smaller, changes were seen in the biopsies from undamaged areas of the peritoneal wall in operated rats. PAI-1 (antigen and activity) was not detected in peritoneal biopsies. Fibrin-related material (especially fibrin monomer/fibrinogen, an indicator of forming fibrin) in peritoneal fluid was slightly increased at 4 h, and abundantly present at 16 and 24 h, returning to control levels at 72 h. Fibrin degradation products were always present. From 2 h onward, adhesions were found.

CONCLUSIONS

In contrast to the view that adhesions are formed as a result of a reduced fibrinolytic activity, our results demonstrate that tPA activity remained unchanged or slightly increased after surgical trauma, and point to increased fibrin formation rather than diminished fibrinolytic activity as the main cause of fibrin deposition after peritoneal trauma. Therapies directed at prevention of adhesion formation should therefore aim at avoiding massive fibrin production and at promoting fibrinolytic activity during the early period after trauma.

Use of fibrinolytic agents in the prevention of postoperative adhesion formation

OBJECTIVE

To review the events leading to the formation of adhesions, to describe the development of fibrinolytic agents, to review more than a century of research on the use of fibrinolytic agents in adhesion prevention, and to look at future aspects of adhesion prevention.

RESULTS

A better understanding of the pathogenesis of adhesion formation has resulted in the use of fibrinolytic agents in their prevention. Fibrinolytic agents promote fibrinolytic activity during the early period after peritoneal trauma during which an increased formation of fibrin is seen in combination with a deficiency of endogenous fibrinolytic activity. Initially, chemical attacks on fibrin (fibrolysin and hypertonic glucose), foreign digestive ferments (pepsin, trypsin, and papain), and stimulation of intraperitoneal leukocytosis (amniotic fluid) were used. Development of new thrombolytic agents was soon followed by experiments in animal adhesion models and clinical studies to examine their antiadhesion properties. Plasmin preparations (plasmin, actase, and fibrinolysin) and plasmin activators (streptokinase, urokinase, and tissue-type plasminogen activator) were found to be efficacious in preventing adhesion formation in the greater part of reviewed animal and clinical studies.

CONCLUSION(S)

From the current literature, it can be concluded that postoperative intraperitoneal administration of thrombolytic agents can significantly decrease adhesion formation. Given the large number of experimental studies in animals, future studies should focus on the clinical use of fibrinolytic agents in the prevention of postsurgical adhesion formation.

Cardiovascular disease risk and hormone replacement therapy (HRT): a review based on randomised, controlled studies in postmenopausal women

Epidemiological data suggest that the use of oestrogen replacement therapy (ERT) and combined oestrogen/progestagen replacement therapy (HRT) in healthy postmenopausal women is associated with a decreased risk of cardiovascular events. In sharp contrast, the HERS study, a secondary prevention trial in postmenopausal women with established coronary heart disease, did not show a favourable effect, with a trend towards an increased risk of cardiovascular disease in the first year of treatment. This paper provides an overview of randomised, controlled trials (RCTs) in postmenopausal women published in the literature and discusses possible explanations for the contrast between data from the epidemiological studies and the results of the HERS study. ERT and HRT are associated with: 1) an improved lipid profile; and 2) a decrease in homocysteine and endothelin levels. Data on factor VII and fibrinogen were not consistent. There were insufficient data on the effects on blood pressure, glucose metabolism, vasomotor regulation, arterial stiffness, thrombomodulin, adhesion molecules, and clotting and fibrinolysis, as well as on the effects of route of administration and the role of progestagens. Finally, endothelium-dependent vasodilatation appears to increase with ERT, but the effects of HRT are less clear This paucity of controlled data indicates that, although ERT and HRT improve surrogate measures of risk of atherothrombosis, adverse effects of ERT and HRT on biological mechanisms related to risk of atherothrombosis can by no means be excluded.

Oral, but not transdermal, administration of estrogens lowers tissue-type plasminogen activator levels in humans without affecting endothelial synthesis

Oral estrogen administration decreases plasma levels of tissue-type plasminogen activator (tPA), which may be explained by a decrease in endothelial tPA synthesis, an increase in its hepatic clearance, or both. In the present study, we determined (1) differences between oral (ie, via the liver) ethinyl estradiol and transdermal (ie, systemic) 17beta-estradiol administration on plasma antigen levels of tPA and plasminogen activator inhibitor type-1 before and after 4 months of hormone administration and (2) effects on endothelial tPA synthesis, by measuring the local increase in plasma tPA during venous occlusion of the upper extremity. Thirty transsexual males (median age 32 years, range 20 to 44 years ) were randomly assigned to either oral ethinyl estradiol (n=15) or transdermal 17beta-estradiol (n=15); both treatments included the antiandrogen cyproterone acetate (CA). Ten males were treated with CA alone. Seventeen transsexual females (median age 27 years, range 18 to 37 years) were treated with intramuscular testosterone esters. Only oral ethinyl estradiol plus CA but neither transdermal 17beta-estradiol plus CA, nor oral CA, nor parenteral testosterone lowered plasma tPA and plasminogen activator inhibitor-1 (P<0.001 for both). tPA release during venous occlusion was not affected by oral ethinyl estradiol plus CA in males (P=0.52) or by parenteral testosterone in females (P=0.89). These data are consistent with a previous observation, in rodents, that the decrease in tPA after oral estrogen administration can be explained by an increase in hepatic tPA clearance, leaving endothelial tPA synthesis unchanged, and suggest that these mechanisms also explain the decrease in tPA in humans.

Relationship between visceral fat and PAI-1 in overweight men and women before and after weight loss

This study was aimed at evaluating the relationship between visceral fat accumulation and plasma plasminogen activator inhibitor-1 (PAI-1) levels in healthy, obese men and women undergoing weight loss therapy. The subjects, 25 men and 25 premenopausal women, aged between 26 and 49 years, with an initial body mass index between 28 and 38 kg/m2, received a controlled diet for 13 weeks providing a 4.2 MJ/day energy deficit. Magnetic resonance imaging was used to measure visceral and subcutaneous abdominal fat. Our results show that before weight loss visceral fat was significantly correlated with PAI-1 in men (r = 0.45; p<0.05), but not in women (r = -0.15; ns). The association between visceral fat and PAI-1 in men remained significant after adjustment for age and total fat mass, and multiple linear regression analysis showed a significant independent contribution of visceral fat to plasma PAI-1 levels. Both visceral fat areas and PAI-1 levels decreased significantly with weight loss in both men and women. Changes in visceral fat area were related to changes in PAI-1 in women (r = -0.43; p = 0.05) but not in men (r = -0.01; ns); however, this association in women disappeared after adjustment for total fat mass. We conclude that there is a relationship between visceral fat and PAI-1 in obese men but not in obese women, and that PAI-1 levels decrease substantially (52%) by weight loss, but this change is not related to changes in visceral fat mass per se.

D-glucose increases the synthesis of tissue-type plasminogen activator (t-PA) in human peritoneal mesothelial cells

Physical and chemical irritation of the peritoneum through glucose-based hyperosmolar dialysis solutions results in a nonbacterial serositis with fibrinous exudation. Thereby, human peritoneal mesothelial cells (HMC) play an important role in maintaining the balance between the peritoneal generation and degradation of fibrin by expressing the fibrinolytic enzyme tissue-type plasminogen activator (t-PA) as well as the specific plasminogen activator inhibitor-1 (PAI-1). In this study, we analyzed the effect of D-glucose and metabolically inert monosaccharides on the synthesis of t-PA and PAI-1 in cultured HMC. Incubation of HMC with D-glucose or the metabolically inert monosaccharides mannitol and L-glucose (5-90 mM) resulted in a time- and concentration-dependent increase in t-PA mRNA expression and antigen secretion without affecting PAI-1 synthesis. A similar effect was evident when HMC were first exposed sequentially to pooled spent peritoneal dialysis effluent for up to 4 hours, and subsequently incubated for 20 hours in control medium. The stimulating effect of high D-glucose on t-PA expression in HMC was prevented by treating the cells with different protein kinase C (PKC) inhibitors (Ro 31-8220, Gö 6976), but could not be mimicked by the PKC-activating phorbol ester PMA, indicating that this effect of high glucose is dependent on PKC activity, but not mediated through PKC activation. Also, using specific inhibitors (PD 98059, SB 203580) and activators (PMA, anisomycin, IL-1alpha) of the major routes of the mitogen-activated protein kinases (MAPKs) cascade, we found no evidence for a role of this cascade in regulating t-PA expression in HMC. We conclude that hyperosmolarity induces t-PA (but not PAI-1) in HMC via a regulatory mechanism that requires active PKC, but that does not involve a major pathway in the MAPK cascade.

Increased clearance explains lower plasma levels of tissue-type plasminogen activator by estradiol: evidence for potently enhanced mannose receptor expression in mice

Several clinical studies have demonstrated an inverse relationship between circulating levels of estrogen and tissue-type plasminogen activator (t-PA). The present study was designed to test the hypothesis that estrogens lower plasma levels of t-PA by increasing its clearance from the bloodstream. 17alpha-Ethinyl estradiol (EE) treatment resulted in a significant increase in the clearance rate of recombinant human t-PA in mice (0.46 mL/min in treated mice v 0. 32 mL/min in controls; P <.01). The clearance of endogenous, bradykinin-released t-PA in rats was also significantly increased after EE treatment (area under the curve [AUC], 24.9 ng/mL. min in treated animals v 31.9 ng/mL. min in controls; P <.05). Two distinct t-PA clearance systems exist in vivo: the low-density lipoprotein receptor-related protein (LRP) on liver parenchymal cells and the mannose receptor on mainly liver endothelial cells. Inhibition of LRP by intravenous injection of receptor-associated protein (RAP) as a recombinant fusion protein with Salmonella japonicum glutathione S-transferase (GST) significantly retarded t-PA clearance in control mice (from 0.41 to 0.25 mL/min; n = 5, P <.001) and EE-treated mice (from 0.66 to 0.35 mL/min; n = 5, P <.005), but did not eliminate the difference in clearance capacity between the 2 experimental groups. Similar results were obtained in mice in which LRP was inhibited via overexpression of the RAP gene in liver by adenoviral gene transduction. In contrast, administration of mannan, a mannose receptor antagonist, resulted in identical clearances (0.22 mL/min in controls and 0.24 mL/min in EE-treated mice). Northern blot analysis showed a 6-fold increase in mannose receptor mRNA expression in the nonparenchymal liver cells of EE-treated mice, whereas the parenchymal LRP mRNA levels remained unchanged. These findings were confirmed at the protein level by ligand blotting and Western blotting analysis. Our results demonstrate that EE treatment results in increased plasma clearance rate of t-PA via induction of the mannose receptor and could explain for the inverse relationship between estrogen status and plasma t-PA concentrations as observed in humans.

Hormone replacement therapy

Hormone replacement therapy is increasingly being used for purposes unrelated to the alleviation of menopausal symptoms, such as the prevention of osteoporosis and cardiovascular disease. Clinical trials, however, suggest that the one drug/many purposes concept may be too optimistic. The availability of new estrogen-like compounds and the discovery of a second estrogen receptor have opened new possibilities for more specific drug development.

Fibrates suppress fibrinogen gene expression in rodents via activation of the peroxisome proliferator-activated receptor-alpha

Plasma fibrinogen levels have been identified as an important risk factor for cardiovascular diseases. Among the few compounds known to lower circulating fibrinogen levels in humans are certain fibrates. We have studied the regulation of fibrinogen gene expression by fibrates in rodents. Treatment of adult male rats with fenofibrate (0.5% [wt/wt] in the diet) for 7 days decreased hepatic Aalpha-, Bbeta-, and gamma-chain mRNA levels to 52% +/- 7%, 46% +/- 8%, and 81% +/- 19% of control values, respectively. In parallel, plasma fibrinogen concentrations were decreased to 63% +/- 7% of controls. The suppression of fibrinogen expression was dose-dependent and was already evident after 1 day at the highest dose of fenofibrate tested (0.5% [wt/wt]). Nuclear run-on experiments showed that the decrease in fibrinogen expression after fenofibrate occurred at the transcriptional level, as exemplified for the gene for the Aalpha-chain. Other fibrates tested showed similar effects on fibrinogen expression and transcription. The effect of fibrates is specific for peroxisome proliferator-activated receptor-alpha (PPARalpha) because a high-affinity ligand for PPARgamma, the thiazolidinedione BRL 49653, lowered triglyceride levels, but was unable to suppress fibrinogen expression. Direct evidence for the involvement of PPARalpha in the suppression of fibrinogen by fibrates was obtained using PPARalpha-null (-/-) mice. Compared with (+/+) mice, plasma fibrinogen levels in (-/-) mice were significantly higher (3.20 +/- 0.48 v 2.67 +/- 0.42 g/L). Also, hepatic fibrinogen Aalpha-chain mRNA levels were 25% +/- 11% higher in the (-/-) mice. On treatment with 0.2% (wt/wt) fenofibrate, a significant decrease in plasma fibrinogen to 77% +/- 10% of control levels and in hepatic fibrinogen Aalpha-chain mRNA levels to 65% +/- 12% of control levels was seen in (+/+) mice, but not in (-/-) mice. These studies show that PPARalpha regulates basal levels of plasma fibrinogen and establish that fibrate-suppressed expression of fibrinogen in rodents is mediated through PPARalpha.

Relationship between Visceral Fat and PAI-1 in Overweight Men and Women before and after Weight Loss

This study was aimed at evaluating the relationship between visceral fat accumulation and plasma plasminogen activator inhibitor-1 (PAI-1) levels in healthy, obese men and women undergoing weight loss therapy. The subjects, 25 men and 25 premenopausal women, aged between 26 and 49 years, with an initial body mass index between 28 and 38 kg/m2, received a controlled diet for 13 weeks providing a 4.2 MJ/day energy deficit. Magnetic resonance imaging was used to measure visceral and subcutaneous abdominal fat. Our results show that before weight loss visceral fat was significantly correlated with PAI-1 in men (r = 0.45; p <0.05), but not in women (r = -0.15; ns). The association between visceral fat and PAI-1 in men remained significant after adjustment for age and total fat mass, and multiple linear regression analysis showed a significant independent contribution of visceral fat to plasma PAI-1 levels. Both visceral fat areas and PAI-1 levels decreased significantly with weight loss in both men and women. Changes in visceral fat area were related to changes in PAI-1 in women (r = -0.43; p = 0.05) but not in men (r = -0.01; ns); however, this association in women disappeared after adjustment for total fat mass. We conclude that there is a relationship between visceral fat and PAI-1 in obese men but not in obese women, and that PAI-1 levels decrease substantially (52%) by weight loss, but this change is not related to changes in visceral fat mass per se.

On the role of c-Jun in the induction of PAI-1 gene expression by phorbol ester, serum, and IL-1alpha in HepG2 cells

We have characterized the regulation of plasminogen activator inhibitor-1 (PAI-1) gene expression by phorbol 12-myristate 13-acetate (PMA), serum, and interleukin-1alpha (IL-1alpha) in the human hepatoma cell line HepG2. PMA, serum, and IL-1alpha induced a rapid and transient 28-fold (PMA), 9-fold (serum), and 23-fold (IL-1alpha) increase in PAI-1 mRNA, peaking after approximately 4 hours. These inductions of PAI-1 mRNA accumulation were reduced by pretreatment of the HepG2 cells with the protein tyrosine kinase inhibitor genistein. Conversely, stimulation of tyrosine phosphorylation by sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, caused an increase in PAI-1 mRNA levels. The effects of PMA, serum, and IL-1alpha on PAI-1 mRNA expression have been compared with their ability to modulate the expression of a chloramphenicol acetyltransferase (CAT) reporter plasmid, which was under control of the -489 to +75 region of the PAI-1 promoter, and stably transfected into HepG2 cells. This region of the PAI-1 promoter was previously found to contain a tetradecanoyl phorbol acetate-response element (TRE; between -58 and -50) necessary for PMA responsiveness and with a high affinity for c-Jun homodimers. Whereas incubation of these transfected HepG2 cells with PMA and serum showed an induction profile of CAT mRNA similar to that of PAI-1 mRNA, hardly any induction of CAT mRNA was found with IL-1alpha. In line with these findings, IL-1alpha poorly induced c-Jun homodimer binding to the PAI-1 TRE in gel mobility-shift assays. Pretreatment of HepG2 cells with the protein kinase C inhibitor Ro 31-8220 or the mitogen-activated protein kinase kinase (MAPKK)1,2 activity blocker PD98059 selectively suppressed the induction of PAI-1 (and CAT) expression by PMA, but not that by IL-1alpha. In contrast, the protein tyrosine kinase inhibitor herbimycin A blocked PAI-1 mRNA induction by IL-1 alpha only. We propose 2 separate PAI-1 inductory pathways for PMA and IL-1alpha in HepG2, both involving protein tyrosine kinase activation; the serum-induced signaling pathway may (partially) overlap with the PMA-activated protein kinase C/mitogen-activated protein kinase kinase pathway, leading to c-Jun homodimer binding to the PAI-1 TRE.

Fibrinolytic activity of human mesothelial cells is counteracted by rapid uptake of tissue-type plasminogen activator

BACKGROUND

Human mesothelial cells (HMCs) have an important role in maintaining an adequately functioning fibrinolytic system in the peritoneal cavity by secreting the fibrinolytic enzymes tissue-type and urokinase-type plasminogen activator (t-PA and u-PA), as well as a specific PA inhibitor, PA inhibitor type 1 (PAI-1). In this study, we investigated whether the fibrinolytic capacity of HMCs is further counterbalanced by rapid uptake of t-PA and u-PA from the medium.

METHODS

Cultured HMCs were used to study the uptake and degradation of radiolabeled t-PA and u-PA in the absence or presence of an inhibitor of cellular protein degradation, chloroquine, and of specific receptor antagonists. Northern blotting and ligand-blotting techniques were applied to demonstrate the presence of specific receptors for binding of t-PA and u-PA.

RESULTS

At 37 degreesC, HMCs rapidly internalized and degraded 125I-t-PA and 125I-u-PA, which could be inhibited by an excess of unlabeled t-PA and u-PA, respectively, and by the lysosomotropic agent chloroquine. Northern blot analysis showed the expression of low-density lipoprotein (LDL) receptor-related protein (LRP), very low density lipoprotein (VLDL) receptor, and u-PA receptor. The addition of recombinant 39 kDa receptor-associated protein (RAP; an inhibitor of LRP and VLDL receptor) almost completely blocked the degradation of t-PA and partly that of u-PA. RAP ligand blotting demonstrated predominantly the presence of LRP, suggesting a major role for the LRP in mediating uptake and degradation of t-PA in HMCs. Endocytosis of u-PA occurs via two different pathways. After binding to u-PA receptor, a RAP-inhibitable and a non-RAP-inhibitable route for u-PA degradation was demonstrated. Tumor necrosis factor alpha (TNFalpha) diminished the fibrinolytic activity of HMCs by decreasing t-PA and increasing PAI-1 synthesis. The fall in t-PA levels could be counteracted by inhibiting t-PA degradation by either RAP or chloroquine. Interestingly, chloroquine also quenched the TNFalpha-induced changes in t-PA and PAI-1 mRNA levels. Using TNFalpha mutants and agonistic or blocking monoclonal antibodies specific for the TNF receptors p55 and p75, we found evidence that chloroquine interfered with the activation of the TNF receptor p55 and/or its intracellular signaling route.

CONCLUSIONS

Receptor-mediated endocytosis plays a crucial role in regulating the fibrinolytic capacity of HMCs by its participation in the degradation of t-PA and u-PA, and in the TNFalpha-induced decrease in t-PA and the increase in PAI-1 expression.

Fibrate-modulated expression of fibrinogen, plasminogen activator inhibitor-1 and apolipoprotein A-I in cultured cynomolgus monkey hepatocytes -- role of the peroxisome proliferator-activated receptor-alpha

Fibrates are used to lower plasma triglycerides and cholesterol levels in hyperlipidemic patients. In addition, fibrates have been found to alter the plasma concentrations of fibrinogen, plasminogen activator inhibitor-1 (PAI-1) and apolipoprotein A-I (apo A-I). We have investigated the in vitro effects of fibrates on fibrinogen, PAI-1 and apo A-I synthesis and the underlying regulatory mechanisms in primary monkey hepatocytes. We show that fibrates time- and dose-dependently increase fibrinogen and apo A-I expression and decrease PAI-1 expression in cultured cynomolgus monkey hepatocytes, the effects demonstrating different potency for different fibrates. After three consecutive periods of 24 h the most effective fibrate. ciprofibrate (at 1 mmol/l), increased fibrinogen and apo A-I synthesis to 356% and 322% of control levels, respectively. Maximum inhibition of PAI-1 synthesis was about 50% of control levels and was reached by 1 mmol/l gemfibrozil or ciprofibrate after 48 h. A ligand for the retinoid-X-receptor (RXR), 9-cis retinoic acid, and specific activators of the peroxisome proliferator-activated receptor-alpha (PPARalpha), Wy14,643 and ETYA, influenced fibrinogen, PAI-1 and apo A-I expression in a similar fashion, suggesting a role for the PPARalpha/RXRalpha heterodimer in the regulation of these genes. When comparing the effects of the various compounds on PPARalpha transactivation activity as determined in a PPARalpha-sensitive reporter gene system and the ability of the compounds to affect fibrinogen, PAI-1 and apo A-I antigen production, a good correlation (r=0.80; p <0.01) between PPARalpha transactivation and fibrinogen expression was found. Apo A-I expression correlated only weakly with PPARalpha transactivation activity (r=0.47; p=0.24), whereas such a correlation was absent for PAI-1 (r=0.03; p=0.95). These results strongly suggest an involvement of PPARalpha in the regulation of fibrinogen gene expression.

Visceral fat accumulation is an important determinant of PAI-1 levels in young, nonobese men and women: modulation by cross-sex hormone administration

Increased plasminogen activator inhibitor type-1 (PAI-1) levels, leading to impaired fibrinolysis, are associated with increased visceral fat in middle-aged and obese subjects. It is unknown, however, whether this association is independent of other disturbances clustered in the insulin resistance syndrome. We analyzed this association in young, nonobese transsexual men and women before and after administration of cross-sex steroids, which potentially influence many elements of the insulin resistance syndrome, including PAI-1 levels and visceral fat accumulation. We assessed the visceral fat area (by MRI); total body fat; insulin sensitivity (with a glucose clamp technique); and plasma levels of PAI-1, insulin, and triglycerides in young (<37 years old), nonobese (body mass index <28 kg/m2), healthy men (n=18) and women (n=15) before and after 12 months of cross-sex hormone administration. Men were treated with ethinyl estradiol 100 microgram/d plus cyproterone acetate 100 mg/d, and women were treated with testosterone esters 250 mg IM every 2 weeks. At baseline, only visceral fat area was significantly correlated with plasma PAI-1 levels in both men (r=0.57, P=0.03) and women (r=0.59, P=0.03). In multivariate linear regression analysis, this association was independent of total body fat, insulin sensitivity, and plasma levels of triglycerides and insulin. After 12 months of cross-sex hormone administration, the plasma PAI-1 levels were no longer correlated with visceral fat (which had increased). We conclude that in young, nonobese men and women, visceral fat area is an important determinant of plasma PAI-1 levels. After cross-sex hormone administration, this association was no longer demonstrable.

Retinoids increase human apo C-III expression at the transcriptional level via the retinoid X receptor. Contribution to the hypertriglyceridemic action of retinoids

Hypertriglyceridemia is a metabolic complication of retinoid therapy. In this study, we analyzed whether retinoids increase the expression of apo C-III, an antagonist of plasma triglyceride catabolism. In men, isotretinoin treatment (80 mg/d; 5 d) resulted in elevated plasma apo C-III, but not apo E concentrations. In human hepatoma HepG2 cells, retinoids increased apo C-III mRNA and protein production. Transient transfection experiments indicated that retinoids increase apo C-III expression at the transcriptional level. This increased apo C-III transcription is mediated by the retinoid X receptor (RXR), since LG1069 (4-[1-(5,6,7,8-tetrahydro-3,5,5,8, 8-pentamethyl-2-naphtalenyl)ethenyl]benzoic acid), a RXR-specific agonist, but not TTNPB ((E)- 4-[2-(5,6,7,8-tetrahydro-5,5,8, 8-tetramethyl-2-naphtalenyl)propenyl]benzoic acid), a retinoic acid receptor (RAR)-specific agonist, induced apo C-III mRNA in HepG2 cells and primary human hepatocytes. Mutagenesis experiments localized the retinoid responsiveness to a cis-element consisting of two imperfect AGGTCA sequences spaced by one oligonucleotide (DR-1), within the previously identified C3P footprint site. Cotransfection assays showed that RXR, but not RAR, activates apo C-III transcription through this element either as a homo- or as a heterodimer with the peroxisome proliferator-activated receptor. Thus, apo C-III is a target gene for retinoids acting via RXR. Increased apo C-III expression may contribute to the hypertriglyceridemia and atherogenic lipoprotein profile observed after retinoid therapy.

Effect of steroid hormones and retinoids on the formation of capillary-like tubular structures of human microvascular endothelial cells in fibrin matrices is related to urokinase expression

Angiogenesis, the formation of new capillary blood vessels, is a feature of a variety of pathological processes. To study the effects of a specific group of hormones (all ligands of the steroid/retinoid/thyroid hormone receptor superfamily) on the angiogenic process in humans, we have used a model system in which human microvascular endothelial cells from foreskin (hMVEC) are cultured on top of a human fibrin matrix in the presence of basic fibroblast growth factor and tumor necrosis factor-alpha. This model mimics the in vivo situation where fibrin appears to be a common component of the matrix present at sites of chronic inflammation and tumor stroma. Our results show that testosterone and dexamethasone are strong inhibitors and all-trans retinoic acid (at-RA) and 9-cis retinoic acid (9-cis RA) are potent stimulators of the formation of capillary-like tubular structures. These effects are mediated by their respective nuclear hormone receptors as demonstrated by the use of specific synthetic receptor agonists and antagonists. 17beta-estradiol, progesterone, and 1,25-dihydroxyvitamin D3 did not affect or only weakly affected in vitro angiogenesis, which may be related to the lack of significant nuclear receptor expression. Although hMVEC express both thyroid hormone receptors alpha and beta, no effect of thyroid hormone on tube formation was found. The effects of testosterone, dexamethasone, at-RA, and 9-cis RA on tube formation were accompanied by parallel changes in urokinase-type plasminogen activator (u-PA) expression, at both mRNA and antigen levels. Exogenous suppletion of the medium with single chain u-PA enhances tube formation in our in vitro model, whereas quenching of u-PA activity (but not of tissue-type plasminogen activator activity) or of u-PA binding to u-PA receptor by specific antibodies suppressed basal and retinoid-stimulated tube formation. Moreover, addition of scu-PA to testosterone- or dexamethasone-treated hMVEC restored the suppressed angiogenic activity for a substantial part. Aprotinin, an inhibitor of plasmin activity, completely inhibited tube formation, indicating that the proteolytic properties of the u-PA/u-PA receptor complex are crucial in this process. Our results show that steroid hormones (testosterone and dexamethasone) and retinoids have strong, but opposite effects on tube formation in a human in vitro model reflecting pathological angiogenesis in the presence of fibrin and inflammatory mediators. These effects can be explained by hormone-receptor-mediated changes in u-PA expression, resulting in enhanced local proteolytic capacity of the u-PA/u-PA receptor complex.

The effects of sex steroids on plasma levels of marker proteins of endothelial cell functioning

We studied thirteen male-to-female (M-->F) and ten female-to-male (F-->M) transsexuals who, for four months, received cross-sex treatment with, respectively, ethinylestradiol and cyproterone acetate, and with testosterone esters. We assessed the effects of treatment on plasma levels of tissue-type plasminogen activator (tPA), von Willebrand factor (vWF), vWF-propeptide (vWF:AgII) and big-endothelin-1 (big-ET-1), four proteins that are markers of endothelial cell functioning. We also measured urokinase-type PA (uPA) and plasminogen activator inhibitor-type 1 (PAI-1), which may not be endothelium-derived but share major clearance pathways with tPA. In M-->F transsexuals, mean plasma levels of tPA (minus 4.4 ng/ml), big-ET-1 (minus 0.8 pg/ml), uPA (minus 0.5 ng/ml) and PAI-1 (minus 26 ng/ml) decreased (all Ps < or =0.02). The level of vWF increased (plus 24%; P = 0.005), while vWF: AgII did not change (P = 0.49). In F-->M transsexuals, levels of big-ET-1 increased (plus 0.4 pg/ml; P = 0.02), while tPA, uPA and PAI-1 did not change (all Ps >0.25). In this group vWF decreased (minus 14%; P = 0.06), but vWF:AgII did not change (P = 0.38). Estrogens and androgens have clear effects on plasma levels of endothelial marker proteins. The mechanisms behind these effects are complex and appear to involve both altered secretion (big-ET-1) and processing and/or clearance (vWF and possibly tPA). Therefore, effects of hormones on the levels of endothelial marker proteins do not necessarily reflect changes in endothelial cell functioning, at least with regard to changes in vWF level associated with the oral administration of high doses of ethinylestradiol and cyproterone acetate to healthy men and the parenteral administration of testosterone to healthy women.

Mechanism of the slow induction of apolipoprotein A-I synthesis by retinoids in cynomolgus hepatocytes: involvement of retinoic acid and retinoid X receptors

We showed previously that retinoids stimulate apolipoprotein A-I (apoA-I) synthesis in cultured cynomolgus hepatocytes only after a 24-h lag phase. Here we report on the biochemical background of the slow response, the requirement for high retinoic acid concentrations, and the involvement of different retinoid receptors. The time course of the effect of 10 microM all-trans retinoic acid (at-RA) on apoA-I mRNA levels and protein secretion were comparable, i.e., minor increases were observed after a 24-h incubation and mRNA levels were increased 2.2- and 3.5-fold after 48 h and 72 h, respectively. In contrast, apoA-I gene transcription was already increased (2.6-fold) after a 4-h incubation with 10 microM at-RA. At-RA disappeared rapidly from the cultures: after 2 h of incubation 40% of the added amount was left and after 24 h only 2%. RAR beta mRNA and gene expression were increased after incubation with 10 microM at-RA, whereas RAR alpha and RXR alpha mRNA levels and expression remained unchanged. No transcriptional activity and mRNA for other retinoid receptors were detectable. Both RAR-selective (TTNPB) and RXR-selective (3-methyl-TTNEB) agonists induced apoA-I synthesis at 1 and 10 microM. These results show that i) the slow increase in apoA-I secretion is caused by a slow increase of its mRNA level; ii) the apoA-I gene transcription in cynomolgus hepatocytes is induced rapidly by retinoids; iii) the added at-RA disappeared rapidly from the cultures, explaining the necessity for high initial concentrations; iv) RLR alpha and/or RAR beta and RXR alpha are involved in the activation of apoA-I expression by retinoids.