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 Table of Contents  
REVIEW ARTICLE
Year : 2016  |  Volume : 7  |  Issue : 1  |  Page : 1-9

Natural coagulation inhibitors in liver diseases


Department of Physiology, Coagulation Research Laboratory, College of Medicine, King Khalid University Hospital, King Saud University, Riyadh 11461, Saudi Arabia

Date of Web Publication25-Apr-2016

Correspondence Address:
Abeer Khalid Al-Ghumlas
Department of Physiology, Coagulation Research Laboratory, College of Medicine, King Khalid University Hospital, King Saud University, Riyadh 11461
Saudi Arabia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1658-5127.181114

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  Abstract 

The hemostatic derangements accompanying liver disease are complex and affect all aspects of hemostasis. Depressed levels of the major natural anticoagulants (NAs): Antithrombin (AT), protein C (PC), protein S (PS), and tissue factor pathway inhibitor (TFPI) were reported in advanced liver disease and this reduction correlated with the severity of liver disease. Recent evidence suggests that changes in the blood levels of NAs particularly, PS and PC, were found to be more sensitive to hepatocyte dysfunction than the conventional coagulation tests prothrombin time and activated partial thromboplastin time. Depressed levels of PS and PC were found even in the mildest forms of liver disease such as chronic viral hepatitis and its carrier state when the other coagulation tests and routine liver function tests were normal. This topic did not receive enough coverage and was not described in many recent publications. The current review provides an overview of the current understanding of the pathophysiology of the major anticoagulants; AT, PC, PS, and TFPI with a particular focus on their fluctuations in different types of liver disease. It also discusses the emerging important roles of these NAs as sensitive markers of liver disease.

Keywords: Antithrombin, chronic hepatitis, hepatitis B carriers, liver cirrhosis, liver diseases, natural anticoagulants, protein C, protein S, tissue factor pathway inhibitor


How to cite this article:
Al-Ghumlas AK. Natural coagulation inhibitors in liver diseases. J Appl Hematol 2016;7:1-9

How to cite this URL:
Al-Ghumlas AK. Natural coagulation inhibitors in liver diseases. J Appl Hematol [serial online] 2016 [cited 2020 Apr 1];7:1-9. Available from: http://www.jahjournal.org/text.asp?2016/7/1/1/181114


  Introduction Top


The normal process of coagulation begins when a vessel is injured, and platelets adhere to the vascular endothelium at the site of injury, a process mediated by Von Willebrand factor. Platelets then release procoagulant substances that attract other platelets, which aggregate together and form a platelet plug. The clotting cascade proceeds through the complicated intrinsic, extrinsic, and common pathways to generate thrombin. Thrombin will act on fibrinogen to convert it to fibrin, producing the hemostatic plug, and ultimately stabilizing the clot [Figure 1].[1]
Figure 1: Schematic representation of the coagulation system with sites of action of antithrombin (indicated by broken arrows). Coagulation factors are represented by roman numbers. TF = Tissue factor; PL = Platelet phospholipid; AT = Antithrombin

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The generation of this explosive procoagulant response is essential to prevent excessive hemorrhage from a damaged blood vessel. Conversely, it is also essential that this process is prevented from disseminating widely beyond the site of injury. It is obvious that coagulation is precisely balanced through the activation of procoagulant and anticoagulant factors. Thus, several natural anticoagulant (NA) mechanisms exist to downregulate every step of the procoagulant response to prevent excessive clotting and restrict it only at the site of the vascular injury. The most important well-identified NAs include Antithrombin (AT), protein C (PC), protein S (PS), and tissue factor pathway inhibitor (TFPI).[2]

During the past decade, considerable effort has been made to investigate the nature and function of these NAs. These studies have significantly improved our understanding of the control of the hemostatic system.


  Physiology and Biochemistry of Antithrombin, Protein C, Protein S, and Tissue Factor Pathway Inhibitor Top


Antithrombin

It has long been suggested that the anticoagulant action of heparin depends on a plasma factor which was eventually isolated in 1968 and was given the names: Heparin cofactor and AT III. Today, the protein is known as AT.[3] AT is a glycoprotein with a molecular weight of 58,000 daltons that does not require Vitamin K for its synthesis. AT is synthesized mainly by hepatocytes but also by endothelial cells.[4]

AT controls the coagulation process by inactivating FIXa, FXa, and thrombin, with its main targets being thrombin and FXa. It can also inhibit FXIa, XIIa, and VIIa. AT activity is accelerated at least 2000-fold by heparin,[5] which acts as a cofactor for AT.[6]

Adequate levels of AT are important for the maintenance of the hemostatic balance. This is obvious since individuals with an inherited deficiency of AT have increased the tendency of thromboembolism.[3] The normal reference range of AT activity in normal pooled plasma in Saudi population has been established to be 76–130%.[7]

Protein C

In 1976, a new Vitamin K-dependent glycoprotein was isolated from bovine plasma by Stenflo.[8] Later, it was shown that it was a zymogen of a serine protease and that its activated form expressed anticoagulant properties.[9] PC is produced in the liver and it is synthesized as a single polypeptide chain made of 461 amino acids that circulates in the blood as a zymogen which, upon activation, becomes a powerful NA.[10] While synthesis predominantly occurs in the liver, PC has also been identified in the epididymis, kidney, lung, brain, and male reproductive tissue.[11]

The PC anticoagulant pathway plays a critical role in the regulation of the clotting system as it limits the coagulation response to injury.[12] Thrombin generated at the sites of vascular injury expresses a number of procoagulant functions: (i) It converts the procofactors FV and FVIII into their active forms FVa and FVIIIa, respectively (ii) thrombin activates and aggregates platelets, and (iii) converts fibrinogen into an insoluble fibrin network.[13] The activation of FV and FVIII by thrombin is a crucial step in generating sufficient thrombin. However, if this process is left uncontrolled, it will lead to excessive fibrin and thrombus formation. Therefore, free thrombin in the vicinity of endothelial cells can be neutralized by plasma AT to prevent the propagation of the thrombus, but it may also bind to thrombomodulin which is a membrane-bound receptor for thrombin, expressed on normal endothelium.[14] The thrombin-thrombomodulin complex then activates PC on the surface of endothelial cells to its active form “activated PC” (APC).[15] APC in the presence of its cofactor PS inactivates FVa and FVIIIa, thus preventing further formation of thrombin [Figure 2]. These events are enhanced by the presence of calcium ions and phospholipids.[11] It is obvious that as thrombin activates the PC anticoagulant pathway when it binds to thrombomodulin, its function changes from a procoagulant to an anticoagulant enzyme.[16],[17] In addition to thrombin downregulation, APC stimulates fibrinolysis by suppressing the activation of thrombin-activatable fibrinolytic inhibitor (TAFI).[12]
Figure 2: Activation of PC. PC = Protein C; APC = Activated protein C; PS = Protein S; VIIa = Activated factor VII; VIIIa = Activated factor VIIIa; PAI-1 = Plasminogen activator inhibitor type-1

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The importance of APC as an NA is evident from the fact that heterozygous PC-deficient individuals are at high risk for thrombophlebitis, deep vein thrombosis, or pulmonary embolism.[14] On the other hand, homozygous PC deficiency causes fatal massive disseminated intravascular coagulation or purpura fulminans unless treated by PC replacement therapy.[18]

In addition to its fundamental role in the inhibition of the coagulation system, APC possesses anti-inflammatory as well as cytoprotective activities. APC plays an important role in the modulation of inflammatory and immune responses, wound healing, and apoptosis.[19],[20],[21] Anti-inflammatory functions of PC include its ability to inhibit proinflammatory cytokines release from monocytes.[11] More recent data support the critical involvement of the PC pathway in the pathogenesis of several human conditions, including sepsis, inflammatory bowel disease, airway inflammation, rheumatoid arthritis, and chronic vascular inflammation.[11],[22]

Protein S

PS is another Vitamin K-dependent glycoprotein which is synthesized as a single-chain glycoprotein with 635 amino acids [9] and multiple biologic functions.[23]

PS is synthesized and secreted mainly by hepatocytes; other cells such as megakaryocytes, endothelial cells, Leydig and  Sertoli cells More Details, osteoblasts, dendritic cells, T-cells, vascular smooth muscle cells, and tumor cells also produce and secrete PS.[24] In these cells, PS has no anticoagulant function but functions to activate a unique family of receptors; protein–tyrosine kinase.[23]

PS, which lacks enzymatic activity, functions primarily as an anticoagulant cofactor to the APC in the inactivation of activated factors V (FVa) and activated factor VIII (FVIIIa). After binding to phospholipid surfaces, PS helps to localize APC in proximity to FVa and FVIIIa inducing the inactivation of FVa and FVIIIa by 20- and 5-fold, respectively, thereby regulating thrombin generation.[25] More recently, it was reported that PS has also APC-independent activity. This APC-independent anticoagulant activity is mediated by PS that acts as a cofactor of TFPI and specifically stimulates the inhibition of FXa.[26]

PS circulates in the plasma in two forms: Bound and free forms. Sixty percent of plasma PS bound to C4b-binding protein (C4BP) forming an inactive complex which is greatly enhanced by calcium ions. C4BP is an acute-phase protein that controls complement system activation and functions as negative regulator of PS.[27] The remaining (40%) of PS is free (free PS) and represents its active form.[25] Free PS participates in anticoagulation as a cofactor for APC while the complexed form loses its APC cofactor function.[28] Therefore, total PS referred to both the bound and free forms. The importance of the anticoagulant role of PS is evident from the consequences of its deficiency, which is associated with increased risk of venous thromboembolism and fetal loss.[29] Individuals with heterozygous PS deficiency, most commonly present with deep venous thrombosis, pulmonary embolism, and superficial thrombophlebitis. Homozygous PS deficiencies are rarely reported and usually associated with severe purpura fulminans in the neonatal period.[25]

PS plasma levels are also influenced by hormonal status;[30] PS levels are lower in healthy women than in men.[31],[32] Furthermore, PS levels are lower in premenopausal women than in postmenopausal women.[31],[33],[34] Total PS is very low in the newborn, the total antigen levels being 36% of normal adult values.[7] Nonetheless, the levels rise to the adult normal range by the age of 6 months.[35] Similarly, C4BP levels are very low in neonate, but free PS levels are normal.[4] Lower levels of PS are also found during pregnancy and in those taking oral contraceptives. The levels of PS decrease during treatment with oral anticoagulants since PS is a Vitamin K-dependent protein. Acquired PS deficiency is also associated with nephrotic syndrome.[36]

In addition to its role as a key Vitamin K-dependent NA protein, PS is involved in multiple biological processes including phagocytosis of apoptotic cells, cell survival, activation of innate immunity, vessel integrity, angiogenesis, local invasion, and metastasis.[23]

Tissue Factor Pathway Inhibitor

TFPI is a physiologically significant NA [37] that regulates the activity of the TF-FVIIa complex, thus controlling the generation of thrombin and ultimately, fibrin.[38] There are two types of TFPI: TFPI-1 and TFPI-2; the former is responsible for the regulation of the extrinsic pathway of coagulation.[39] The site of synthesis of TFPI is the endothelium.[40] It is present in the body in three pools: (i) Bound to endothelium which constitute the majority (80–85%) of TFPI in the body,[39],[40] (ii) 15–20% of TFPI circulates in plasma, of which 80% bound to lipoproteins and the rest is the free form, and (iii) sequestered in platelets which constitute around 3% of the total TFPI.[39],[41] The importance of TFPI as a natural coagulation inhibitor is reflected by the role TFPI plays in several diseases. TFPI is involved in the pathogenesis of a variety of diseases including atherosclerosis, arterial thrombosis, coronary restenosis following arterial intervention, stroke and ischemia-reperfusion injury, deep venous thrombosis, antiphospholipid antibody syndrome, acute lung injury, malignancy particularly the metastasis of tumor cell, chronic renal failure, nephrotic syndrome, crescentic glomerulonephritis, and thrombosis associated with paroxysmal nocturnal hemoglobinuria.[42]


  Liver Disease and Hemostasis Top


The liver plays a central role in hemostasis as it is the site of synthesis of the majority of the coagulation factors and inhibitors as well as some of the proteins involved in the fibrinolytic system. Consequently, the hemostatic changes that accompany liver disease (whether acute or chronic) are complex and affect all aspects of the hemostatic system.[43] In brief, the typical hemostatic profile of patients with advanced liver disease includes reduced levels of coagulation factors and inhibitors, reduced levels of fibrinolytic proteins, increased plasma levels of coagulation factor VIII and vWF, thrombocytopenia, and platelet function defects.[43],[44],[45] Differences in the hemostatic changes associated with liver disease differ according to the etiology.[46] In contrast to Western countries where alcoholic liver disease is the most common cause of chronic liver disease (CLD), in Saudi Arabia, viral hepatitis is the predominant cause of liver diseases.

Despite the hemostatic disturbances in patient with liver disease, those patients appear to be in hemostatic balance with adequate hemostatic function.[46],[47],[48] These new insights in the understanding of the coagulopathy of liver disease stem from the fact that low plasma levels of procoagulant factors are rebalanced by the concomitant decrease of naturally occurring anticoagulants.[49] Other additional important contribution to coagulation rebalance is provided by the increase of factor VIII [47] and Von Willebrand factor along with reduced levels of ADAMTS 13 that enhances platelet adhesion.[50] There is also an increase in the FVIII: PC ratio which indicates marked reduction in the ability of the PC to undertake its inhibitory action on active FVIII, so-called resistance to APC. However, still this complex rebalance in both the procoagulants and anticoagulants cannot be reflected accurately by measurement of the conventional coagulation screening test, the prothrombin time (PT), and activated partial thromboplastin time (APTT).[50],[51] These tests are misleading to the clinician and may prompt inappropriate and risky therapies with little real benefit to the patient.[50]

Accordingly, the longstanding belief that the hemostatic changes in liver disease predispose the patients to a bleeding tendency, is no longer supported by data from both clinical and laboratory studies. Recent researches showed that transfusional and nontransfusional hemostatic medications are of little value as adjuvants to control bleeding in advanced liver disease.[48] These new insights have brought about a reduction in the use of blood products in patients undergoing liver transplantation. Currently, many centers avoid preoperative correction of the laboratory coagulopathy in patients undergoining liver transplantation.[52] In addition, routine laboratory coagulation tests, as mentioned earlier, do not fully reflect the underlying hemostatic changes and are poor predictors of bleeding or thrombosis in liver diseases.[45],[47],[53],[54] In fact, the majority of coagulation tests are aimed at assessing only procoagulant factors and are insensitive to low plasma levels of the anticoagulants; AT, PC, or PS in patients with normal PT or APTT and thus do not reveal the possible compensatory effects within the system.[55]

The hemorrhagic tendency that accompanies liver diseases particularly in cirrhotic patients results frequently from noncoagulopathic reasons [55] and should be explained by the superimposed conditions. Portal hypertension with related hemodynamic alterations, endothelial dysfunction, bacterial infection, hepatorenal syndrome, and thrombocytopenia, which are common in cirrhosis,[56] are much more critical than hypocoagubality in causing the bleeding tendency.[57],[58]

CLD should now be regarded as a condition associated with bleeding as well as thrombotic tendency. This is based on direct evidence that plasma from cirrhotic patients have normal or increased thrombin generation as compared to healthy individuals when thrombin generation is measured in the presence of thrombomodulin.[47] In line with this notion, a recent study has shown that the risk of venous thromboembolism is 2-fold higher in patients with liver disease than in controls, and the resulting clinical manifestations include portal-vein thrombosis, peripheral vein thrombosis, and pulmonary embolism. The occurrence of thrombotic complications in cirrhosis is consistent with the recent findings that these patients have a procoagulant imbalance related to the impairment of the thrombomodulin/PC anticoagulant pathway combined with very high plasma levels of factor VIII.[48],[59] This may result in a prothrombotic state or contribute to the hypercoagulable complications that predispose to thrombosis in liver disease.[60]

Despite intense studies of the hemostatic system in CLDs, knowledge about NAs has only recently begun to expand in the area of hemostasis in liver disease. Limited studies characterized their changes in different spectrum of liver diseases such as viral hepatitis and the carrier states. In the following sections, we review their reported changes in various types of liver diseases.


  Natural Anticoagulants in Liver Diseases Top


Antithrombin

Deficiency of the main inhibitor of thrombin, AT occurs in a variety of liver diseases including acute and CLDs.[49], 53, [61],[62],[63] This reduction is attributed to decreased production by the dysfunctional hepatocyte and/or increased consumption by thrombin.

Liver cirrhosis, whether alcoholic or nonalcoholic, is associated with significantly reduced levels of AT; the reduction correlates with the severity of cirrhosis with lowest levels seen in Child C patients.[43],[54],[59],[61],[64] Patients with fulminant hepatic failure (FHF) also demonstrate reduced level of AT. However, the reduced level did not correlate with survival in these patients.[65]

Protein C and Protein S

Many studies have demonstrated depressed levels of these proteins in different forms of the liver disease including both alcoholic and nonalcoholic liver cirrhosis.[53],[61],[63],[64],[66] The low levels of both functional activity and antigen were attributed to reduced synthesis and/or increased consumption.[43]

In liver cirrhosis, the drop in these anticoagulants correlates with the degree of liver function impairment as assessed by the Child–Pugh score, with the lowest level seen in Child's C patients.[54],[66],[67] PC was more affected than PS, which was explained by the fact that PS is synthesized by tissues other than the liver, such as endothelial cells.[68] Interestingly, successful, liver transplantation results in normalization of PC levels.[4] In addition, PC has a relatively short half-life (6–8 h), and thus, among the coagulation proteins synthesized in the liver, it was found to be the first to decrease in liver disease.[35] Furthermore, recent liver researches and studies of fibrogenesis, consider PC deficiency in addition to other factors; factor V Leiden mutation and increased expression of factor VIII, to be associated with accelerated progression to cirrhosis in chronic hepatitis C infection. Therefore, it was promised that interference with the coagulation cascade may reduce hepatic fibrosis.[69]

Depressed levels of PC and PS were also reported in FHF [53],[70] where both components of free and total PS were affected; however, the drop was more pronounced for total PS as compared to free PS.[70] It is possible that the decrease in the levels of total and free PS to be due, in part, to a decrease in the synthesis of both PS as well as its carrier protein, C4BP.[61]


  Could the Natural Anticoagulants (Antithrombin, Protein C, and Protein S) Tests Be Useful as Liver Function Tests? Top


For long time, coagulation tests remained a cornerstone in the assessment, prognosis as well as the prediction of response to treatment of liver disease.[71] The most commonly employed tests were PT, APT, and international normalized ratio. However, these tests measure only procoagulant factors and are insensitive to plasma levels of the anticoagulants and congenital deficiencies of AT, PC, or PS present with normal PT or APTT. The lack of comprehensive test which helps in the clinical assessment of the risk of bleeding or clotting triggered attempts to explore the possible use of other hemostatic variables. NAs (AT, PC, and PS) are synthetic products of the liver and in advanced liver disease, their blood levels show marked reduction. Thus, we attempted to explore the concept further and see not only how sensitive these tests are as liver function tests (LFTs), but also whether these hemostatic tests are abnormal in liver diseases when conventional biochemical tests are within normal limits.[61] The laboratory techniques used for measuring these NAs are relatively sensitive and easy to perform, especially with the availability of improved and more precise assay kits.

We probed these hemostatic factors in a wide range of liver diseases using both cross-sectional and a follow-up studies. The studied liver affections ranged from symptomless viral carriers, through to chronic as well as acute liver disease, in addition to liver cirrhosis and hepatocellular carcinoma (HCC), i.e., an approach that offers liver functions ranging from normal to very severe derangement.

It is noteworthy that in the first study,[61] we found significant reduction of both total and free PS in Hepatitis B (HB) carrier group (symptomless viral carriers) at a time when other routine LFTs, routine coagulation screening tests (PT and APTT) as well as other NAs, AT, and PC, did not show significant drop. In fact, we were the first to report such changes in PS level in HB carriers. Since these patients have normal LFTs and are unlikely to have significant liver fibrosis, we assumed that this reduction in PS may be related to mild subclinical liver inflammation.[61] Indeed, this suggestion was proven later by undertaken more detailed statistical analysis. Moreover, a significant reduction in PS was also reported in the milder form of liver diseases such as chronic viral hepatitis that are with mild inflammation and no fibrosis as documented by ultrasound. The percentages of abnormal levels of total and free PS were 54% and 58%, respectively, in chronic hepatitis, and 67% in HB carriers.

In contrast, to AT level, a significant reduction in PC occurs also in the milder form of liver disease; chronic viral hepatitis. Since PC level was affected in patients with chronic viral hepatitis and liver cirrhosis, who were having fibrosis, but its level was normal in the apparently normal HB carriers who were having normal LFTs, we suggested that PC could be a useful marker of fibrosis.

We also found reduction in the levels of AT, PC and PS in the severe forms of liver disease (acute hepatitis, liver cirrhosis, and HCC), indicating strongly the significant derangement of the hepatocyte synthetic function. This is supported further by the positive correlations between the levels of these parameters and other liver synthetic products, specially albumin, fibrinogen, and plasminogen, in acute hepatitis and liver cirrhosis.[61]

Another interesting observation in this study was that the reduction in PS levels in all the groups of patients with liver disease was more in the total PS than the free PS. The reduction in PS level was attributed to reduced synthesis by the liver. However, we also suggested another possibility; the decrease in the levels of total and free PS to be due, in part, to a decrease in the synthesis of both PS and C4BP. This explanation is because changes in the circulating levels of total and free PS cannot be interpreted without testing the levels of its binding protein, the “C4BP.” C4BP, which is synthesized in the liver, is a carrier protein that forms a reversible inactive complex with 60% of plasma PS.[27] Many studies that addressed the changes affecting PS in liver disease did not undertake simultaneous measurement of C4BP. In an early study in which C4BP was measured, its level was found to be depressed in hepatic cirrhosis.[72]

This explanation stems from in vitro studies which have demonstrated a direct impact of the proinflammatory cytokines (interleukin-6 [IL-6] and tumor necrosis factor-alpha [TNF-α]) on C4BP and PS. In a human hepatoma cell line, IL-6 upregulated the production of C4BP.[27] Similar findings were reported with respect to PS. Several in vitro studies done on human hepatoma cell lines have also demonstrated an increase in the expression of PS in response to IL-6.[73],[74] On the other hand, TNF-α had no effect on PS expression by hepatocytes; however, it inhibited the IL-6 mediated regulation of PS synthesis by these cells.[73],[74] Similarly, in vitro studies on hepatocytes and sinusoidal endothelial cells (SECs) isolated from normal and cirrhotic rats, showed that IL-6 increased PS production by hepatocytes, whereas TNF-α decreased PS production by SECs from both cirrhotic and normal rats.[75]

On these bases, we concluded that the most sensitive markers of hepatocyte synthetic malfunction are total and free PS as their levels are affected in all grades of hepatocyte malfunction, particularly the mildest liver disease such as HB carriers, at a time when other routine LFTs, as well as coagulation tests, did not show significant abnormalities.[61] Additional studies were recommended to establish the role of PS carrier protein; C4BP in different types of acute and CLDs.

We also looked into the possible utility of NAs in reflecting the response of treatment to antiviral; Interferon alpha (IFN-α) in chronic viral hepatitis.[76] Test samples were collected before the commencement of therapy and 3 and 6 months later in 21 patients with chronic viral hepatitis B and C. Of the measured levels of NAs, only total PS exhibited an increase at 3 and 6 months after the commencement of IFN therapy. The levels of other NAs hardly changed. This finding as well as the results of the first study [61] led us to conclude that the level of total PS level is a good marker of both hepatocyte synthetic function and hepatocyte response to IFN-α therapy. If these observations could be confirmed in larger studies, then PS deserves the consideration to be included among LFTs.[76]

To confirm the potential utility of these tests in the diagnosis and prognosis in liver diseases, we extended the findings of our previous studies [61],[76] and evaluated the NAs and C4BP levels in different types and stages of liver diseases that are clinically and biochemically well defined. Furthermore, clinical diagnosis was supported by more precise virological evaluation of hepatitis B virus DNA and hepatitis C virus RNA using polymerase chain reaction-based assays. On this basis, we also took hepatitis B inactive carrier state to represent the mildest form of liver disease with normal LFTs, no identifiable evidence of inflammation and with no evidence of significant fibrosis. Next, in severity were patients with active hepatitis B and C as well as patients with nonalcoholic steatohepatitis (NASH). Finally, patients with liver cirrhosis represent the advanced stage of hepatocellular dysfunction.[77] This study confirmed that the reduction in the levels of PS, PC, and AT displayed gradual reduction that correlates with the severity of the disease from HB inactive carrier state to liver cirrhosis.

It is worthwhile adding that in our previous studies [77] we found that PS and not AT or PC levels to be significantly reduced in HB carriers when compared to normal controls, NASH, and active hepatitis. As these patients are unlikely to have any significant liver fibrosis, this reduction in PS levels was attributed to mild subclinical liver inflammation and/or the influence of low levels of clinically insignificant inflammatory cytokines. In our univariate and multivariate analysis when HB carriers (representing no fibrosis) were compared with chronic hepatitis patients, NASH and liver cirrhosis, the strongest associated factor that emerged was PS, whereas when HB carriers were compared with cirrhotic patients, PC was the strongest factor. Similarly, when HB and NASH groups were individually compared for PC levels, no significant difference was detected.

It is clear from the in vitro evidence described above, that cytokines have a direct effect on the fluctuations of both PS and its binding protein C4BP. However, in our study there was no significant difference in the level of C4BP among liver disease groups, even when compared to controls.[77] Thus, the reduction in PS level in patients with liver disease was attributed mainly to a decrease in PS production by the malfunctioning hepatocytes but not to its carrier protein, C4BP.

We, therefore, hypothesize that both PS and PC are sensitive markers of liver disease; PS is a sensitive marker of liver inflammation, whereas PC is sensitive marker for liver fibrosis, as it is least affected by liver inflammation.

The last relevant issue is whether the administration of Vitamin K to patients in different stages of liver disease would have any effect on the circulating level of the Vitamin K-dependent proteins; PC, PS and FVII. In the latest study from our laboratory, the patient groups tested included: HB carriers, chronic hepatitis, liver cirrhosis, and HCC. PC, total and free PS were measured at baseline and 72 h after Vitamin K administration. The results showed progressive decrement in PC, and PS in addition to fibrinogen and FVII, across the study groups rather than an expected increase. Vitamin K therapy does not cause significant improvements in the measured blood levels of the hemostatic factors, particularly PC, and PS. Hence, Vitamin K administration does not seem to be of any benefit to patients with liver disease.[78]


  Tissue Factor Pathway Inhibitor in Liver Diseases Top


Some investigators showed that the level of TFPI was decreased in patients with liver disease particularly in the advanced stages such as liver cirrhosis. Others reported normal or even increased levels of TFPI in patients with liver cirrhosis.[66],[79] Unlike other NAs, the fluctuation in the changes of TFPI level in liver disease can be attributed to the fact that TFPI is synthesized by endothelial cells and not hepatocytes. With no information on the degree of activation of coagulation and involvement of tissue factor, it is difficult to find an explanation for TFPI fluctuations.


  Conclusion Top


This review has summarized our current knowledge on the pathophysiology of NAs (AT, PC, PS, and TFPI) in liver diseases. These NAs reflect more accurately the hepatocyte synthetic function than the PT, APTT and the routine LFTs. There is evidence to show that their blood levels are reduced in a wide spectrum of liver disease, progressively from mild to severe form of liver disease. Among these NAs, PS was found to be the most sensitive marker in detecting hepatocyte malfunction as it was abnormal when other routine LFTs, coagulation, and other NAs tests were normal. Furthermore, the reduction in the levels of PS was more prominent in inflammatory liver disease, while PC was suggested to be used as a marker of fibrosis. It seems likely that the consideration of these NAs will open the door to new insights into the understanding of the role of these proteins in liver diseases. Further detailed studies of these anticoagulants in a broad spectrum of liver diseases and in large numbers of patients will be needed to confirm these preliminary findings. It is recommended to correlate these markers (PS and PC) with more precise determination of the degree of inflammation and fibrosis as evaluated by liver biopsy, and the noninvasive markers of fibrosis such as the APRI score,[80] fibrotest,[81] or fibroscan.[82] It is also worthwhile to probe the prognostic value of these markers (PC and PS) in end-stage liver disease, acute fulminant hepatitis, and also as monitors of response to antiviral therapy.

Financial Support and Sponsorship

Nil.

Conflicts of Interest

There are no conflicts of interest.

 
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