The anticoagulant and antithrombotic mechanisms of heparin.

The molecular basis for the anticoagulant action of heparin lies in its ability to bind to and enhance the inhibitory activity of the plasma protein antithrombin against several serine proteases of the coagulation system, most importantly factors IIa (thrombin), Xa and IXa. Two major mechanisms underlie heparin's potentiation of antithrombin. The conformational changes induced by heparin binding cause both expulsion of the reactive loop and exposure of exosites of the surface of antithrombin, which bind directly to the enzyme target; and a template mechanism exists in which both inhibitor and enzyme bind to the same heparin molecule. The relative importance of these two modes of action varies between enzymes. In addition, heparin can act through other serine protease inhibitors such as heparin co-factor II, protein C inhibitor and tissue factor plasminogen inhibitor. The antithrombotic action of heparin in vivo, though dominated by anticoagulant mechanisms, is more complex, and interactions with other plasma proteins and cells play significant roles in the living vasculature.

Protein C inhibitor.

Protein C inhibitor (PCI) is a serine protease inhibitor and was originally identified as an inhibitor of activated protein C (APC). However, PCI is not specific for APC and also inhibits several proteases involved in coagulation, fibrinolysis, cancer, wound healing, and fertility. The biological function of PCI is unknown due to broad enzyme specificity, its wide tissue distribution, and the lack of a suitable animal model. This review highlights the specific roles of PCI in the areas of hemostasis and thrombosis and fertilization, and it also describes the latest information on the fascinating participation of the protein in intracellular processes, phospholipid binding, and killing of bacteria.

Heparin enhances the inhibition of factor Xa by protein C inhibitor in the presence but not in the absence of Ca2+.

Protein C inhibitor (PCI) is a versatile serine protease inhibitor with both pro- and anticoagulant and other properties. Interactions of certain ligands with PCI, including heparin, affect its specificity for proteases. In this study, heparin was found to enhance PCI inhibition of factor Xa up to 42-fold in the presence of a physiological Ca(2+) concentration, whereas no heparin-induced activation was observed in the absence of Ca(2+). These results thus show that factor Xa adds to the group of proteases whose inhibition by PCI is enhanced by heparin and that such inhibition contributes to the anticoagulant properties of PCI by a Ca(2+)-dependent mechanism.

Structural insights into the multiple functions of protein C inhibitor.

Protein C inhibitor (PCI) is a widely distributed, multifunctional member of the serpin family of protease inhibitors, and has been implicated in several physiological processes and disease states. Its inhibitory activity and specificity are regulated by binding to cofactors such as heparin, thrombomodulin and phospholipids, and it also appears to have non-inhibitory functions related to hormone and lipid binding. Just how the highly conserved serpin architecture can support the multiple diverse functions of PCI is a riddle best addressed by protein crystallography. Over the last few years we have solved the structure of PCI in its native, cleaved and protein-complexed states. They reveal a conserved serpin fold and general mechanism of protease inhibition, but with some unique features relating to inhibitory specificity/promiscuity, cofactor binding and hydrophobic ligand transport.

The multi-functional serpin, protein C inhibitor: beyond thrombosis and hemostasis.

Protein C inhibitor (PCI) is a member of the serine protease inhibitor (serpin) family. PCI was initially found to be an inhibitor of activated protein C, and later shown to be a potent inhibitor of blood coagulation and fibrinolysis such as that mediated by urokinase type-plasminogen activator. Therefore, the protein came to be known as plasminogen activator inhibitor-3. It also inhibits proteases involved in fertilization. PCI is broadly conserved, and is found in human, rhesus monkey, cow, rabbit, rat, mouse and chicken. The human PCI gene is located on chromosome 14q32.1 in a cluster of genes encoding related serpins. Sp1- and AP2-binding sites in the 5'-flanking region act as promoter and enhancer, respectively, for its expression in the liver. PCI mRNA is expressed in many organs in primates, but only in the reproductive organs in rodents. Recent studies using transgenic mice expressing the human gene have suggested that PCI is also involved in regulation of lung remodeling, tissue regeneration, vascular permeability, proteolysis in the kidney and tumor cell invasion. A protease inhibitor-independent activity of PCI, the prevention of anti-angiogenesis and metastasis of tumor cells, has also been observed. Thus, PCI is a unique multi-functional serpin playing diverse roles in the thrombosis and hemostasis in multiple organs and tissues of a variety of species.

Protein C inhibitor regulates hepatocyte growth factor activator-mediated liver regeneration in mice.

BACKGROUND: We recently reported that human protein C inhibitor (PCI), a major inhibitor of activated protein C (APC), inhibits hepatocyte growth factor activator (HGFA) by forming HGFA-PCI complexes in vitro. In this study, we evaluated whether PCI regulates HGFA-mediated liver regeneration in a human PCI gene transgenic (hPCI-Tg) mouse model. METHODS AND RESULTS: After partial hepatectomy in hPCI-Tg and wild-type (WT) mice, the degree of liver regeneration, protein and mRNA expression of HGFA, proHGF activation, plasma levels of PCI and HGFA-PCI complex, and other markers were evaluated in the remnant liver. We also evaluated the effect of anti-human PCI antibody on liver regeneration, which significantly decreased in hPCI-Tg mice compared to WT mice. HGFA mRNA levels in naive and remnant livers after hepatectomy were the same in both WT and hPCI-Tg mice; however, plasma HGFA levels and HGF activation in the liver were lower in hPCI-Tg than in WT mice. There was no difference in plasma levels of transaminases and inflammatory cytokines. However, sinusoidal congestion and bleeding were detected and the serum hyaluronic acid level was elevated in hPCI-Tg mice, indicating that human PCI aggravates sinusoidal injury by inhibiting the cytoprotective effect of APC. APC decreased thrombin-induced IL-6 production in isolated hepatic nonparenchymal cells (NPCs) in vitro. This impaired liver regeneration was reversed by anti-human PCI antibody treatment in vivo. CONCLUSION: PCI regulates liver regeneration after hepatectomy by forming an HGFA-PCI complex and aggravates hepatic NPC injury by inhibiting the cytoprotective effect of APC. Anti-PCI antibody treatment may be a novel therapy for improving liver regeneration.

Protein C inhibitor inhibits breast cancer cell growth, metastasis and angiogenesis independently of its protease inhibitory activity.

Protein C inhibitor (PCI) regulates the anticoagulant protein C pathway and also inhibits urinary plasminogen activator (uPA), a mediator of tumor cell invasion. In the present study, we evaluated the effect of human PCI and its inactive derivatives on tumor growth and metastasis of human breast cancer (MDA-231) cells, and on angiogenesis in vivo. The invasiveness of MDA-231 cells was inhibited by recombinant intact PCI, but not by reactive site-modified PCI (R354APCI) or by the N-terminal fragment of protease-cleaved PCI (NTPCI). The in vitro invasiveness of MDA-231 cells expressing intact PCI (MDA-PCI) was significantly decreased as compared to MDA-231 cells expressing R354APCI (MDA-R354APCI) or NTPCI (MDA-NTPCI). Further, in vivo growth and metastatic potential of MDA-PCI, MDA-R354APCI and MDA-NTPCI cells in severe combined immunodeficient (SCID) mice were significantly decreased as compared to MDA-Mock cells. Angiogenesis was also significantly decreased in Matrigel implant containing MDA-PCI, MDA-R354APCI or MDA-NTPCI cells as compared to that containing MDA-Mock cells. In vivo angiogenesis in rat cornea and in vitro tube formation were also inhibited by recombinant intact PCI, R354APCI and NTPCI. Furthermore, the anti-angiogenic activity of PCI was strong as cleaved antithrombin (AT), and slightly stronger than that of plasminogen activator inhibitor (PAI)-1 and pigment epithelium-derived factor (PEDF). Overall, this study showed that, in addition to a reactive site-dependent mechanism, PCI may also regulate tumor growth and metastasis independently of its protease inhibitory activity by inhibiting angiogenesis.

A long-term "memory" of HIF induction in response to chronic mild decreased oxygen after oxygen normalization.

BACKGROUND: Endothelial dysfunction (ED) is functionally characterized by decreased vasorelaxation, increased thrombosis, increased inflammation, and altered angiogenic potential, has been intimately associated with the progression and severity of cardiovascular disease. Patients with compromised cardiac function oftentimes have a state of chronic mild decreased oxygen at the level of the vasculature and organs, which has been shown to exacerbate ED. Hypoxia inducible factor (HIF) is a transcription factor complex shown to be the master regulator of the cellular response to decreased oxygen levels and many HIF target genes have been shown to be associated with ED. METHODS: Human endothelial and aortic smooth muscle cells were exposed either to A) normoxia (21% O2) for three weeks, or to B) mild decreased oxygen (15% O2) for three weeks to mimic blood oxygen levels in patients with heart failure, or to C) mild decreased oxygen for two weeks followed by one week of normoxia ("memory" treatment). Levels of HIF signaling genes (HIF-1alpha, HIF-2alpha, VEGF, BNIP3, GLUT-1, PAI-1 and iNOS) were measured both at the protein and mRNA levels. RESULTS: It was found that chronic exposure to mild decreased oxygen resulted in significantly increased HIF signaling. There was also a "memory" of HIF-1alpha and HIF target gene induction when oxygen levels were normalized for one week, and this "memory" could be interrupted by adding a small molecule HIF inhibitor to the last week of normalized oxygen. Finally, levels of ubiquitylated HIF-1alpha were reduced in response to chronic mild decreased oxygen and were not full restored after oxygen normalization. CONCLUSION: These data suggest that HIF signaling may be contributing to the pathogenesis of endothelial dysfunction and that normalization of oxygen levels may not be enough to reduce vascular stress.

Plasminogen activator system and vascular disease.

Vascular diseases, such as atherosclerosis, thromboembolic disorders and stroke, in addition to surgical procedures such as restenosis, all share the plasminogen activator system as a central component in the pathogenesis of vascular injury. Since the development of plasminogen deficient mice our knowledge of the effects of this proteolytic system in cardiovascular disease has vastly increased. The plasminogen activator system plays a key role in vascular homeostasis and constitutes a critical response mechanism to cardiovascular injury. The central components of the PA system are the proteolytic activators, urokinase-plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA), plasminogen (plg) and its degradation product, plasmin, together with the major inhibitors of this system, plasminogen activator inhibitor-1 and -2 (PAI-1, PAI-2). An extensive network of additional proteases, inhibitors, receptors and modulators directly associate and are influenced by the PA system, the largest group being the Matrix Metalloproteinases (MMPs) and their respective inhibitors the Tissue inhibitors of MMPs (TIMPS).

Characterization of a novel human protein C inhibitor (PCI) gene transgenic mouse useful for studying the role of PCI in physiological and pathological conditions.

In humans, protein C inhibitor (PCI) is expressed in various tissues and present in many body fluids including plasma and seminal fluid. In rodents, PCI is expressed in reproductive organs only and is absent in plasma. In this study, we characterized the tissue expression and physiological role of PCI in novel human PCI gene transgenic (TG) mice. Northern blot and immunohistochemical analyses demonstrated that human PCI is expressed in liver hepatocytes, renal epithelial cells as well as heart, brain and reproductive organs of the TG mice. This PCI tissue distribution is similar to that found in humans. PCI in plasma of TG mice showed the same immunological and functional properties as human plasma PCI. Next, we evaluated the effect of PCI on coagulation, inflammation and tissue damage in lipopolysaccharide-treated TG mice. The results suggested that PCI efficiently inhibits not only the anticoagulant and anti-inflammatory activities of exogenously injected human activated protein C (APC) but also that of endogenously produced APC in mice with endotoxemia. These findings suggest that PCI exerts a procoagulant and proinflammatory effect by inhibiting APC. We believe our results also show how useful these TG mice may be for assessing the therapeutic effect of human APC in vivo and for evaluating the role of PCI in human physiological and pathological conditions.

Protein C inhibitor (plasminogen activator inhibitor-3) and the risk of venous thrombosis.

Protein C inhibitor (PCI), also known as plasminogen activator inhibitor-3, is a serine proteinase inhibitor that can inhibit enzymes in blood coagulation, fibrinolysis and fertility. The role of PCI in regulating the blood coagulation mechanism is not known, as it can inhibit both procoagulant (thrombin, factor Xa, factor XIa) and anticoagulant (activated protein C, thrombin-thrombomodulin, urokinase) enzymes. To determine the relevance of this inhibitor in thrombosis, PCI levels were assessed in the Leiden Thrombophilia Study, a case-control study of venous thrombosis in 473 patients with a first deep-vein thrombosis and 474 age- and sex-matched control subjects. PCI levels above the 95th percentile of the controls (136.1%) increased the risk 1.6-fold compared with PCI levels below the 95th percentile (95% confidence interval 0.9-2.8). There was a gradual increase in risk of thrombosis with further increasing levels of PCI. Adjustment for a number of possible confounders led to a reduction of the risk estimates associated with PCI. However, it is unclear whether adjustment for such factors in the risk models is justified. These results indicate that high levels of PCI may constitute a mild risk factor for venous thrombosis.

Role of each Asn-linked glycan in the anticoagulant activity of human protein C inhibitor.

The N-glycosylation site mutants of human protein C inhibitor (PCI; N230S, N243Q, N319Q, N230S/N243Q, and N230S/N319Q) were prepared by amino acid replacement of the asparagine residue with a serine or glutamine residue using site-directed mutagenesis and expressed in the baculovirus/insect cell expression system. To examine the importance of each Asn-linked glycan in the activity of PCI, we compared wtPCI with the mutants of N-glycosylation site(s) in terms of the procoagulant protease-inhibitory and anticoagulant activities. The inhibitory activities of N230S, N319Q, and N230S/N319Q toward human thrombin and plasma kallikrein were significantly increased compared with wtPCI, but those of N243Q and N230S/N243Q were reduced. The inhibitory activity of N230S toward human plasma coagulation was significantly increased compared with wtPCI, and that of N230S/N319Q was also significantly increased compared with N319Q. Furthermore, the procoagulant protease-inhibitory and anticoagulant activities of N230S/N319Q (glycosylated on Asn243 only) compared favorably with those of N230S, and both of the mutants possessed highest activities in the purified mutants. These results suggest that the Asn243-linked glycan in PCI molecule possesses critical roles for its anticoagulant activity in the circulation, and the Asn230-linked glycan down-regulates the activity of PCI.

Protein C inhibitor regulates the thrombin-thrombomodulin complex in the up- and down regulation of TAFI activation.

Thrombin Activatable Fibrinolysis Inhibitor (TAFI) is a carboxy-peptidase B-like proenzyme that after activation by thrombin down regulates fibrinolysis. Thrombomodulin (TM) stimulates the activation of both TAFI and protein C whereas activated protein C (APC) inhibits the activation of TAFI by down regulating thrombin generation. Recently, protein C inhibitor (PCI) was identified as a potent inhibitor of thrombin bound to TM and it can thereby regulate the balance between TAFI activation, and inhibition of TAFI activation by APC. Both in a purified system and in plasma, activation of TAFI and protein C by [Ia-TM could be inhibited by PCI. Previously we found in plasma that at low concentrations (approximately 1 nM), TM predominantly stimulated the activation of TAFI whereas at higher concentrations of TM (approximately 10 nM) the activation of protein C resulted in inhibition of the activation of TAFI. In agreement with this. PCI inhibited the activation of TAFI at 1 nM TM whereas at 10 nM TM PCI inhibited the activation of protein C resulting in an increase in the activation of TAFI. This suggests that PCI can up regulate TAFI activation by inhibiting the protein C activation. PCI may therefore be an important regulator in the balance between coagulation and fibrinolysis by differentially inhibiting the activation of TAFI and of protein C. The local TM concentration plays an important role in the outcome of this process.

Protein C inhibitor (PCI).

PCI is a non-specific serpin that inhibits several proteases of the coagulation and fibrinolytic systems as well as plasma- and tissue kallikreins and the sperm protease acrosin. The precise physiological role of PCI has not been defined yet. Heparin stimulates most PCI/protease reactions, but interferes with the tissue kallikrein/PCI-interaction. Thereby heparin not only regulates PCI-activity but also its specificity in systems containing two or more of its target proteases. This effect is not restricted to heparin, but is also seen with other glycosaminoglycans (GAGs) and large, negatively charged molecules. PCI also binds to GAGs present on the surface of epithelial kidney cells, and GAGs isolated from these cells have a similar effect on PCI activity as heparin. Studies analyzing the role of PCI as an acrosin inhibitor revealed that endogenous PCI is immunocytochemically localized to disrupted acrosomal membranes of morphologically abnormal sperms, while intact sperms are negative for PCI-antigen. In a mouse in vitro fertilization model human PCI inhibited sperm/egg binding and decreased the fertilization rate. Northern blotting of human and mouse mRNA using human and mouse PCI-cDNA probes revealed that in the mouse PCI is exclusively synthesized in the genital tract (testis, seminal vesicle, ovary), while in humans PCI is additionally synthesized in many other organs (e.g., liver, pancreas, heart). Therefore PCI might regulate enzymes involved in fertilization (e.g. acrosin) in both species. Other proteases (e.g., tissue kallikrein) are possibly regulated in a species specific manner by different inhibitors.

Inhibition of acrosin by protein C inhibitor and localization of protein C inhibitor to spermatozoa.

Protein C inhibitor (PCI) is synthesized by cells throughout the male reproductive tract and is present in high concentrations (220 micrograms/ml) in seminal plasma. Seminal plasma as well as the acrosome of spermatozoa are rich in possible target proteases for PCI. We analyzed the interaction of PCI with acrosin, a serine protease stored in its zymogen form in the acrosome of spermatozoa. Purified human PCI inhibited the amidolytic activity of purified boar acrosin with an apparent second-order rate constant of 3.7 x 10(4) M-1.s-1. Inhibition was paralleled by the degradation of PCI from its 57- to its 54-kDa form. Human PCI also inhibited the amidolytic activity of activated human sperm extracts and formed complexes with acrosin as determined by an enzyme-linked immunosorbent assay. Immunocytochemistry revealed that morphologically abnormal spermatozoa stained for PCI antigen, whereas morphologically normal spermatozoa were negative. In immunoelectron microscopy, PCI was exclusively localized in the immediate vicinity of disrupted acrosomal membranes of sperm heads. These data suggest that PCI might function as a scavenger of prematurely activated acrosin, thereby protecting intact surrounding cells and seminal plasma proteins from possible proteolytic damage.