Cellular effects of heparin on the production and release of tissue factor pathway inhibitor in human endothelial cells in culture

Tissue factor pathway inhibitor (TFPI), the major downregulator of procoagulant activity of the tissue factor-factor VIIa complex (TF. FVIIa), is synthesized and constitutively secreted by endothelial cells (ECs). Here we describe the in vitro effects of heparin on the cellular localization, gene expression, and release of TFPI in human ECs in culture. Both unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH; Fragmin) time-dependently induced a significant enhanced secretion of TFPI, paralleled by a redistribution and increase of TFPI on the cell surface and a decrease of intracellular TFPI. Immunogold electron microscopy showed the presence of clusters of TFPI, both on the plasmalemma proper and within cell-surface opened caveolae/enlarged caveolar profiles. Activation of FX by TF. FVIIa on ECs treated with endotoxin was inhibited by both heparins but to a higher extent by LMWH. Inhibition of protein synthesis by cycloheximide did not reduce the release of TFPI induced by heparin. Long-term incubation (48 hours) resulted in a time-dependent enhanced production of TFPI. After the first 4 to 8 hours, depletion of intracellular TFPI was observed, more significantly with UFH. Northern blot analysis of TFPI mRNA also showed a decrease of the 1.4-kb transcript after 4 hours of incubation with UFH, followed by recovery and an increase over the control level after 24 hours. Incubation of ECs with phorbol ester (PMA) significantly enhanced the secretion of TFPI and increased its activity on the cell surface, probably by preventing invagination of caveolae. Heparin-stimulated release of TFPI decreased significantly in the presence of PMA to a level that was 2. 4 times lower than the expected additive value for PMA and UFH separately. Pretreatment of ECs with PMA suppressed a subsequent response to heparin. Altogether, our results suggest that the heparin-induced release of TFPI might involve a more specific mechanism(s) than the previously hypothesized simple displacement of TFPI from the cell surface glycocalyx. We assume that the increased secretion and redistribution of cellular TFPI induced by heparins in ECs in culture can play an important role in the modulation of the anticoagulant properties of the endothelium.

Targeted deficiency or cytosolic truncation of the VE-cadherin gene in mice impairs VEGF-mediated endothelial survival and angiogenesis

Vascular endothelial cadherin, VE-cadherin, mediates adhesion between endothelial cells and may affect vascular morphogenesis via intracellular signaling, but the nature of these signals remains unknown. Here, targeted inactivation (VEC-/-) or truncation of the beta-catenin-binding cytosolic domain (VECdeltaC/deltaC) of the VE-cadherin gene was found not to affect assembly of endothelial cells in vascular plexi, but to impair their subsequent remodeling and maturation, causing lethality at 9.5 days of gestation. Deficiency or truncation of VE-cadherin induced endothelial apoptosis and abolished transmission of the endothelial survival signal by VEGF-A to Akt kinase and Bcl2 via reduced complex formation with VEGF receptor-2, beta-catenin, and phosphoinositide 3 (PI3)-kinase. Thus, VE-cadherin/ beta-catenin signaling controls endothelial survival.

Tissue-selective expression of dominant-negative proteins for the regulation of vascular smooth muscle cell proliferation

The transcription factors c-myb and c-myc are essential for vascular smooth muscle cell (VSMC) replication and are rapidly induced following mitogenic stimulation of quiescent VSMCs in vitro and in vivo following balloon catheter injury. Consequently, interference with c-myb and c-myc function provides a possible avenue for the prevention of VSMC proliferation associated with intimal hyperplasia. We have carried out studies focused on the inhibition of VSMC proliferation using dominant-negative gene constructs incorporating the DNA-binding domains of the c-myb or c-myc genes fused to the repressor domain of the Drosophila engrailed gene. Transient transfection of rat, rabbit and human vascular SMCs results in a dramatic inhibition of proliferation for at least 72 h after transfection. Furthermore, this inhibition of cellular proliferation was found to be due, at least in part, to the induction of apoptosis. Coupling expression of the chimeric dominant-negative proteins to transcriptional regulatory elements of the human vascular smooth muscle alpha-actin gene allows specific targeting of vascular smooth muscle cells.

Temporal and topographic matrix metalloproteinase expression after vascular injury in mice

Temporal and topographic expression of matrix metalloproteinases (MMPs) after perivascular electric injury was studied in wild-type (WT) and urokinase-deficient (u-PA-/-) mice. Neointima formation after injury of the femoral artery was significantly reduced in u-PA-/- mice as compared to WT mice (area of 0.002+/-0.0007 mm2 versus 0.008 + 0.002 mm2 at 3 weeks after injury; p <0.001), associated with impaired cellular migration (nuclear cell counts of 44+/-5 versus 82+/-9in cross-sectional areas; p <0.001). Zymographic and/or microscopic analysis indicated that MMP expression gradually increased to reach a maximum at 1 to 2 weeks after vascular injury. In general, MMP levels were lower in u-PA-/- than in WT mice. In non-injured arteries, MMP-2 (gelatinase A) and MMP-3 (stromelysin-1) were produced mainly by adventitial fibroblasts and/or non-contractile smooth muscle cells (SMC). One week after injury, MMP-2 and MMP-3 levels were enhanced due to an increased number and size of producing cells; 2 to 3 weeks after injury, MMP-2 and MMP-3 were produced also by some contractile SMC, which stained with alpha-actin antiserum. MMP-9 (gelatinase B), MMP-12 (metalloelastase) and MMP-13 (collagenase-3) were found in macrophages located mainly in the adventitia. Immunogold electron microscopic examination revealed that MMP-2 was located predominantly in association with the cell surface of fibroblasts or SMC, while MMP-9 and MMP- 12 were located in well defined storage granules within macrophages. MMP-2, MMP-3 and MMP-13, but not MMP-9 or MMP-12, were also found extracellularly, associated with elastin-containing structures (MMP-2), with the basement membrane and occasionally with collagen fibres (MMP-3), or with proteoglycans, collagen and elastin (MMP-13). The temporal and topographic expression pattern of MMPs after vascular injury, coinciding with smooth muscle cell migration and neointima formation, thus is compatible with a role in vascular remodeling.

Function of the plasminogen/plasmin and matrix metalloproteinase systems after vascular injury in mice with targeted inactivation of fibrinolytic system genes

The matrix metalloproteinase (MMP) system, which may be activated via the plasminogen (Plg)/plasmin system, is claimed to play a role in matrix degradation and smooth muscle cell migration. To test the role of both systems, expression of fibrinolytic and gelatinolytic activity was quantified after vascular injury in mice with targeted inactivation of tissue-type Plg activator (tPA-/-), urokinase-type Plg activator (uPA-/-), or Plg (Plg-/-). Neointima formation 1 week after vascular injury was impaired in uPA-/- and Plg-/- mice compared with wild-type (WT) mice or tPA-/- mice (reduction of neointimal area to 30% and 10% of WT, respectively). Cell accumulation at the borders of the injury was significantly (P<0.01) impaired compared with that in WT mice. One week after injury of the femoral artery, tPA-mediated fibrinolytic activity in arterial sections or extracts of WT, uPA-/-, or Plg-/- mice was not altered, whereas uPA activity levels in tPA-/- and Plg-/- mice were 2- to 3-fold higher than in uninjured controls. Total levels (latent plus active) of MMP-2 (gelatinase A) were increased by 2- to 4-fold, whereas the contribution of active MMP-2 represented 38% to 63% of the total in the different genotypes. MMP-9 (gelatinase B) was not detectable in the majority of control arteries, whereas total MMP-9 levels after injury were dramatically increased (up to 50-fold above the detection limit). Active MMP-9 represented 20% to 46% of total MMP-9 in WT, tPA-/-, and uPA-/- mice but was not consistently detectable in Plg-/- mice. Similar results were obtained in carotid arteries. Thus, the unaltered ratios of active and latent MMP-2 suggest that proMMP-2 activation may occur in the absence of tPA, uPA, or Plg, whereas no active MMP-9 was detected in the absence of Plg. The data of this study confirm a role for uPA and Plg but not for tPA in smooth muscle cell migration and neointima formation after vascular injury and indicate that impairment of these phenomena may occur despite the observed increases in MMP-2 or MMP-9 levels after vascular injury.

Receptor-independent role of urokinase-type plasminogen activator in pericellular plasmin and matrix metalloproteinase proteolysis during vascular wound healing in mice

It has been proposed that the urokinase receptor (u-PAR) is essential for the various biological roles of urokinase-type plasminogen activator (u-PA) in vivo, and that smooth muscle cells require u-PA for migration during arterial neointima formation. The present study was undertaken to evaluate the role of u-PAR during this process in mice with targeted disruption of the u-PAR gene (u-PAR-/-). Surprisingly, u-PAR deficiency did not affect arterial neointima formation, neointimal cell accumulation, or migration of smooth muscle cells. Indeed, topographic analysis of arterial wound healing after electric injury revealed that u-PAR-/- smooth muscle cells, originating from the uninjured borders, migrated over a similar distance and at a similar rate into the necrotic center of the wound as wild-type (u-PAR+/+) smooth muscle cells. In addition, u-PAR deficiency did not impair migration of wounded cultured smooth muscle cells in vitro. There were no genotypic differences in reendothelialization of the vascular wound. The minimal role of u-PAR in smooth muscle cell migration was not because of absent expression, since wild-type smooth muscle cells expressed u-PAR mRNA and functional receptor in vitro and in vivo. Pericellular plasmin proteolysis, evaluated by degradation of 125I-labeled fibrin and activation of zymogen matrix metalloproteinases, was similar for u-PAR-/- and u-PAR+/+ cells. Immunoelectron microscopy of injured arteries in vivo revealed that u-PA was bound on the cell surface of u-PAR+/+ cells, whereas it was present in the pericellular space around u-PAR-/- cells. Taken together, these results suggest that binding of u-PA to u-PAR is not required to provide sufficient pericellular u-PA-mediated plasmin proteolysis to allow cellular migration into a vascular wound.

Urokinase-generated plasmin activates matrix metalloproteinases during aneurysm formation

The molecular mechanisms predisposing to atherosclerotic aneurysm formation remain undefined. Nevertheless, rupture of aortic aneurysms is a major cause of death in Western societies, with few available treatments and poor long-term prognosis. Indirect evidence suggests that matrix metalloproteinases (MMPs) and plasminogen activators (PAs) are involved in its pathogenesis. MMPs are secreted as inactive zymogens (pro-MMPs), requiring activation in the extracellular compartment. Plasmin, generated from the zymogen plasminogen by tissue-type plasminogen activator (t-PA) or urokinase-type plasminogen activator (u-PA; refs 14,15), has been proposed as a possible activator in vitro, but evidence for such a role in vivo is lacking. Analysis of atherosclerotic aorta in mice with a deficiency of apoliprotein E (Apoe-/-; ref. 18), singly or combined with a deficiency of t-PA (Apoe-/-:Plat-/-) or of u-PA (Apoe-/-:Plau-/-; ref. 19), indicated that deficiency of u-PA protected against media destruction and aneurysm formation, probably by means of reduced plasmin-dependent activation of pro-MMPs. This genetic evidence suggests that plasmin is a pathophysiologically significant activator of pro-MMPs in vivo and may have implications for the design of therapeutic strategies to prevent aortic-wall destruction by controlling Plau gene function.

Inhibitory role of plasminogen activator inhibitor-1 in arterial wound healing and neointima formation: a gene targeting and gene transfer study in mice

BACKGROUND

Plasminogen-deficient mice display impaired vascular wound healing and reduced arterial neointima formation after arterial injury, suggesting that inhibition of plasmin generation might reduce arterial neointima formation. Therefore, we studied the consequences of plasminogen activator inhibitor-1 (PAI-1) gene inactivation and adenoviral PAI-1 gene transfer on arterial neointima formation.

METHODS AND RESULTS

Neointima formation was evaluated in PAI-1-deficient (PAI-1(-/-)) mice with perivascular electric or transluminal mechanical injury. PAI-1 deficiency improved vascular wound healing in both models: the cross-sectional neointimal area was 0.001+/-0.001 mm2 in PAI-1(+/+) and 0.016+/-0.008 mm2 in PAI-1(-/-) mice within 1 week after electric injury (P<.02) and 0.055+/-0.008 mm2 in PAI-1(+/+) and 0.126+/-0.006 mm2 in PAI-1(-/-) mice within 3 weeks after mechanical injury (P<.001). Proliferation of smooth muscle cells was not affected by PAI-1 deficiency. Topographic analysis of arterial wound healing after electric injury revealed that PAI-1(-/-) smooth muscle cells, originating from the uninjured borders, more rapidly migrated into the necrotic center of the arterial wound than wild-type smooth muscle cells. On the basis of immunostaining, PAI-1 expression was markedly upregulated during vascular wound healing. There were no genotypic differences in reendothelialization of the vascular wound. When PAI-1(-/-) mice were intravenously injected with replication-defective adenovirus expressing human PAI-1 (AdCMVPAI-1), plasma PAI-1 antigen levels increased in a dose-dependent fashion up to to 61+/-8 microg/mL with 2x10(9) plaque-forming units (pfu) virus. Luminal stenosis was 35+/-13% in control AdRR5-treated (2x10(9) pfu) and suppressed to 5+/-5% in AdCMVPAI-1-treated (6x10(8) pfu) PAI-1(-/-) mice (P<.002).

CONCLUSIONS

By affecting cellular migration, PAI-1 plays an inhibitory role in vascular wound healing and arterial neointima formation after injury, and adenoviral PAI-1 gene transfer reduces arterial neointima formation in mice.

Urokinase but not tissue plasminogen activator mediates arterial neointima formation in mice

To define the role of the plasminogen activators (PAs) tissue PA (t-PA) and urokinase PA (u-PA) in vascular wound healing, neointima formation and reendothelialization were evaluated after electric or mechanical arterial injury in mice with a single or combined deficiency of t-PA (t-PA-/-) and/or u-PA (u-PA-/-). In both models, neointima formation and neointimal cell accumulation were reduced in u-PA-/- and in t-PA-/-/u-PA-/- arteries but not in t-PA-/- arteries. The electric injury model was used to characterize the underlying cellular mechanisms. Topographic analysis of vascular wound healing in electrically injured wild-type and t-PA-/- arteries revealed a similar degree of migration of smooth muscle cells from the noninjured borders into the necrotic center. In contrast, in u-PA-/- and t-PA-/-/u-PA-/- arteries, smooth muscle cells accumulated at the uninjured borders but failed to migrate into the necrotic center. Cultured u-PA-/- but not t-PA-/- smooth muscle cells also failed to migrate in vitro after scrape wounding. Proliferation of smooth muscle cells was not affected by PA deficiency. Reendothelialization after electric injury was similar in all genotypes. In situ analysis revealed markedly elevated u-PA zymographic activity, mRNA, and immunoreactivity in smooth muscle cells, endothelial cells, and leukocytes within 1 week after injury, eg, when cells migrated into the wound. Thus, u-PA plays a significant role in vascular wound healing and arterial neointima formation after injury, most likely by affecting cellular migration.

Tissue factor pathway inhibitor in endothelial cells colocalizes with glycolipid microdomains/caveolae. Regulatory mechanism(s) of the anticoagulant properties of the endothelium

Tissue factor pathway inhibitor (TFPI), the main downregulator of the procoagulant activity of tissue factor.factor VIIa complex, locates in human endothelial cells (EC) in culture as well-defined clusters uniformly distributed both on the cell surface and intracellularly. We here demonstrate by immunofluorescence that TFPI colocalizes in EC with caveolin, urokinase-type plasminogen activator receptor, and glycosphingolipids. The localization of TFPI in caveolae in resting endothelium is proved by double immunogold electron microscopy for TFPI and caveolin. After ultracentrifugation of rat lung or EC homogenates through density gradients of Nycodenz, TFPI was highly enriched at densities of 1.05 to 1.08 g/mL, together with caveolin and alkaline phosphatase. By ELISA, more than half of the cellular TFPI was detected in Triton X-100-insoluble extracts of EC. TFPI incorporates [1-3H]ethanolamine and is cleaved from the cell surface by phosphatidylinositol-phospholipase C, indicating a specific glycosylphosphatidylinositol-anchorage mechanism for TFPI in the plasma membrane. Clustering of TFPI and its localization in caveolae are dependent on the presence of cholesterol in the membrane. Agonist-induced stimulation of EC caused marked changes of distribution for both TFPI and caveolin at subcellular level, with subsequent increase of the cell surface-associated inhibitory activity toward tissue factor.factor VIIa. Our findings suggest that, beside their function in transcytosis, potocytosis, cell surface proteolysis, and regulation of signal transduction, caveolae also play a direct role in the regulation of EC anticoagulant properties.

Upregulation of connexin43 gap junctions between smooth muscle cells after balloon catheter injury in the rat carotid artery

Phenotypic transformation of smooth muscle cells (SMCs) to the synthetic state in vitro and in human coronary atherosclerosis is reported to be associated with upregulation of connexin43 gap junctions. To determine whether cellular interactions mediated by gap junctions participate in the phenotypic transformation of SMCs in arterial injury and disease in general and to establish the spatial and temporal pattern of any such change in relation to neointimal development, we investigated SMC connexin43 gap junction expression during vascular healing in the rat carotid artery after balloon catheter injury. Quantitative immunoconfocal microscopy was applied to localize and to quantify connexin43 gap junctions 1, 3, 9, and 14 days after injury. Parallel studies were conducted by electron microscopy (direct morphological demonstration of SMC gap junctions) and immunoconfocal microscopy (localization of altered actin expression). Synthetic-state SMCs in the neointima (first apparent from 9 days postinjury) revealed abundant expression of gap junctions, with levels of immunodetectable connexin43 threefold greater than those of medial cells. However, the first detectable changes were found in the media, before neointimal formation; at 1 to 3 days postinjury, an increase in SMC gap junction expression was apparent in the innermost (subluminal) zone, the major site from which the cells subsequently found in the neointima are recruited. We conclude that upregulation of connexin43 gap junctions is intimately linked to SMC phenotypic transition and that interactions mediated by gap junctions may be a hitherto unrecognized contributor to the cellular mechanisms underlying the vascular response to injury.

An endothelial storage granule for tissue-type plasminogen activator

In previous studies we have shown that, after stimulation by a receptor ligand such as thrombin, tissue-type plasminogen activator (tPA) and von Willebrand factor (vWf) will be acutely released from human umbilical vein endothelial cells (HUVEC). However, the mechanisms involved in the secretion of these two proteins differ in some respects, suggesting that the two proteins may be stored in different secretory granules. By density gradient centrifugation of rat lung homogenates, a particle was identified that contained nearly all tPA activity and antigen. This particle had an average density of 1.11-1.12 g/ml, both in Nycodenz density gradients and in sucrose density gradients. A similar density distribution of tPA was found for a rat endothelial cell line and for HUVEC. After thrombin stimulation of HUVEC to induce tPA secretion, the amount of tPA present in high-density fractions decreased, concomitant with the release of tPA into the culture medium and a shift in the density distribution of P-selectin. vWf, known to be stored in Weibel-Palade bodies, showed an identical distribution to tPA in Nycodenz gradients. In contrast, the distribution in sucrose gradients of vWf from both rat and human lung was very different from that of tPA, suggesting that tPA and vWf were not present in the same particle. Using double-immunofluorescence staining of HUVEC, tPA- and vWf-containing particles showed a different distribution by confocal microscopy. The distribution of tPA also differed from the distribution of tissue factor pathway inhibitor, endothelin-1, and caveolin. By immunoelectronmicroscopy, immunoreactive tPA could be demonstrated in small vesicles morphologically different from the larger Weibel-Palade bodies. It is concluded that tPA in endothelial cells is stored in a not-previously-described, small and dense (d = 1.11-1.12 g/ml) vesicle, which is different from a Weibel-Palade body.

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.

High efficiency reporter gene transfection of vascular tissue in vitro and in vivo using a cationic lipid-DNA complex

Efficient transfection conditions for a number of human, rat and rabbit primary cells and established lines of vascular origin have been determined using a complex of a commercially available cationic lipid transfection agent (Tfx-50) and luciferase reporter plasmid constructs. The optimised conditions have also been successfully applied to rabbit carotid arteries in vivo and a series of human arteries in vitro. The most critical factors influencing the efficiency of gene transfection with this protocol are: DNA concentration; ratio of lipid reagent to DNA; transfection time and the presence or absence of serum. Immunohistochemical analysis shows that a high percentage of cells (approximately 30-80% dependent on lineage) were transfected under optimal conditions with minimal toxicity effects. Similar analyses performed on undamaged rabbit carotid vessels transfected in vivo and human arteries transfected in vitro show high-efficiency transfer and strong expression of the luciferase vector as demonstrated by reporter gene expression. The optimisation of gene transfer into vascular cells with this cationic lipid complex will be valuable for molecular studies of genes implicated in cardiovascular diseases and as a possible method of gene delivery with therapeutic intent.

Effects of an Igf1 gene null mutation on mouse reproduction

Both sexes of adult mice homozygous for a targeted mutation of the Igf1 gene, encoding insulin-like growth factor 1, are infertile dwarfs (approximately 30% of normal size). The testes are reduced in size less than expected from the degree of dwarfism but sustain spermatogenesis only at 18% of the normal level. The epididymides are overall nearly allometric to the reduced body weight, but the distal regions of the duct, vas deferens, seminal vesicles, and prostate are vestigial. Despite the mutational impact on the epididymis, capacitated sperm are able to fertilize wild type eggs in vitro. It is hypothesized that the infertility of male mutants is caused by failure of androgenization resulting in absence of mating behavior, due to drastically reduced levels of serum testosterone (18% of normal). This hormonal deficiency was correlated with an ultrastructural analysis of mutant Leydig cells revealing a significant developmental delay, while assays in organ culture showed that the basal and LH-stimulated production of testosterone by testicular parenchyma is reduced in comparison with wild type controls. The female mutants fail to ovulate even after administration of gonadotropins, which is apparently the primary cause of their infertility, and possess an infantile uterus that exhibits a dramatic hypoplasia especially in the myometrium. The phenotypic manifestations of the mutation were correlated with the localization of transcripts for insulin-like growth factor I and its cognate receptor in wild type reproductive tissues by in situ hybridization.

Prothrombin cleavage by human vascular smooth muscle cells: a potential alternative pathway to the coagulation cascade

Thrombin is a potent mitogen for human vascular smooth muscle cells (HVSMC) and its enzymatic activity is required for this function. The present study demonstrates that prothrombin is also mitogenic for HVSMC due to the generation of enzymatically active thrombin which occurs upon incubation of prothrombin with the cells. Analysis by SDS-PAGE, immunoblotting, and amino acid sequencing revealed that prothrombin incubated with HVSMC undergoes limited proteolysis. Prethrombin 1 was formed through cleavage at R155-S156. Cleavage at R271-T272 generated fragment 1.2 and prethrombin 2 whilst cleavage at R284-T285 yielded truncated prothrombin 2 (prethrombin 2'). However, cleavage at R320-I321 which, during prothrombin activation produces two-chain alpha-thrombin, was not detectable. Studies on HVSMC-conditioned medium revealed that a similar pattern of prothrombin cleavage occurred by a cell-secreted factor(s). Amidolytic activity analysis indicated that 1-3% catalytically active thrombin-like activity was generated upon incubation of prothrombin with HVSMC-conditioned medium. By treating conditioned medium with various classes of proteinase inhibitors or hirudin, it was determined that prothrombin is cleaved by a cell-derived serine proteinase-like factor(s) at R271-S272 and by alpha-thrombin at R155-S156 and R284-T285. Antibodies neutralising the activity of either urokinase, tissue plasminogen activator, or factor Xa failed to alter the prothrombin cleaving activity of conditioned medium. This activity which may catalyse an alternative pathway for the generation of thrombin, was eluted from a gel filtration column as a single peak with apparent molecular mass of 30-40 kDa.

Thrombin receptor activating peptide (TRAP) stimulates mitogenesis, c-fos and PDGF-A gene expression in human vascular smooth muscle cells

The synthetic peptide SFLLRNPNDKYEPF, identical in sequence to the new amino-terminus of the thrombin receptor generated following cleavage of thrombin, acts a thrombin receptor agonist/activating peptide (TRAP). In this study, Northern blot analysis showed that cultured human vascular smooth muscle cells (HVSMC) express a thrombin receptor transcript. TRAP, in contrast to thrombin was shown to be a weak mitogen for HVSMC. A combination of TRAP and enzymatically-inactivated thrombin (PPACK-thrombin) which provides receptor occupancy, did not potentiate TRAP-induced mitogenesis, indicating that TRAP and PPACK-thrombin do not reproduce the mitogenic effect of enzymatically-active thrombin. Both thrombin and TRAP, induced the expression of c-fos and the PDGF-A gene in a pertussis toxin (PTX)-insensitive manner. Examination of thrombin and TRAP-treated cells by immunofluorescence staining followed by computer assisted image analysis revealed that thrombin and to a lesser extent TRAP induced PDGF-AA protein expression. Antibodies to PDGF-AA partially inhibited thrombin but not TRAP-induced mitogenesis in HVSMC. This study indicates that in addition to the common signalling pathways utilised by thrombin and TRAP, enzymatically-active thrombin activates other signalling pathways and hence TRAP does not mimic fully the biological effect of thrombin on HVSMC.

Thrombin induces the redistribution and acute release of tissue factor pathway inhibitor from specific granules within human endothelial cells in culture

Tissue factor pathway inhibitor (TFPI) is a vascular anticoagulant that regulates the tissue (TF)-dependent pathway of coagulation. The majority of intravascular TFPI is thought to be noncovalently bound to the vessel wall. Our immunolocalization studies in cultures of human umbilical vein endothelial cells (HUVEC) and immortalized EA.hy926 cells that TFPI is located in well-defined granules evenly spread over the cell surface and with apical polarization within the cytoplasm. These granules are smaller than and distinct from Weibel-Palade bodies. Upon treatment of cultured cells with low concentrations of thrombin (0.01 to 1 NIH U/mL), a marked redistribution of TFPI, occurred with patching in focal points and increased exposure of both TFPI antigen and anticoagulant activity on the surface of the stimulated endothelial cells. This redistribution was paralleled by an acute release of TFPI in the cell medium. EA.hy926 cells responded more readily to thrombin stimulation than HUVECs. The process was inhibited by both hirudin and anti-thrombin receptor antibody. Our findings demonstrate a novel mechanism by which thrombin may exert a negative feedback control on blood coagulation. Therefore, this pathway can be physiological importance in controlling TF-mediated thrombin generation.

A novel L23-related gene 40 kb downstream of the imprinted H19 gene is biallelically expressed in mid-fetal and adult human tissues

The closely linked IGF2 and H19 genes on human chromosome 11p15.5 are monoallelically expressed as a result of genomic imprinting and show altered expression in Wilms' tumors (WTs). To map regional imprinting we have sought to isolate additional human genes close to IGF2/H19 and to characterize their allelic expression patterns. Here we report a novel gene, provisionally named L23MRP [L23 (mitochondrial)-related protein], which is oriented 'tail-to-tail' with H19 and is transcribed to within 40 kb of the last H19 exon. L23MRP is expressed biallelically in many mid-fetal and adult human tissues. This gene is also expressed at normal levels in WTs which have lost expression of H19 either via loss of the maternal chromosome 11p15.5 or via an epigenetic pathway involving site-specific DNA hypermethylation. These data indicate that, at least in post-embryonic stages, L23MRP is functionally insulated from the IGF2/H19 imprinted domain.

Plasminogen activator expression in human atherosclerotic lesions

The plasminogen activator (PA) system may participate in the pathogenesis of atherosclerosis by modulating the turnover of intimal fibrin and extracellular matrix deposits and by contributing to intimal cell migration. We present an analysis of tissue-type PA (tPA) and urokinase-type PA (uPA) expression at three levels: mRNA by in situ hybridization, antigen by immunohistochemistry, and enzymatic activity by histoenzymology and zymography. For PA colocalization with cellular or matrix components, we used double immunofluorescence labeling in conjunction with confocal microscopy. In normal arteries, tPA antigen and mRNA were detected in endothelial cells and smooth muscle cells (SMCs). In atherosclerotic arteries, tPA antigen and mRNA were increased in intimal SMCs and in macrophage-derived foam cells of fibro-fatty lesions. Part of the tPA was detected in the extracellular space and colocalized with fibrin deposits. uPA antigen and mRNA were detected in association with the intimal macrophages and SMCs. A particularly high uPA expression was noted on macrophages localized on the rims of the necrotic core. Moreover, using a novel histoenzymological assay as well as classic zymography, we revealed uPA-dependent lytic activity in the advanced lesions, whereas in normal arteries, only tPA-dependent activity was detected, mainly over the vasa vasorum. Also, strong tPA and uPA staining was detected in neomicrovessels of the plaques, suggesting that PAs may play a role in plaque angiogenesis. Our results suggest a local dynamic process of PA-dependent proteolysis in lesion areas that is associated with macrophages and SMCs. A better comprehension of these proteolytic mechanisms in advanced atherosclerotic plaques may provide the basis for therapeutic approaches for plaque stabilization.