Multiple hepatic functions of endothelin-1: physiopathological relevance

Hepatic and Plasma Endothelin-1 in Dogs with Chronic Hepatitis

BACKGROUND

Endothelin (ET)-1 is a 21-amino-acid peptide with potent vasoactive properties, which increases intrahepatic resistance in patients with chronic hepatitis (CH) or cirrhosis. ET-1 concentrations have not been investigated in dogs with CH.

HYPOTHESIS/OBJECTIVES

This study compared hepatic and plasma ET-1 levels in healthy dogs and in dogs with CH, and examined the relationship between the plasma ET-1 level and portal vein pressure in dogs with CH.

ANIMALS

Fourteen healthy dogs and twenty dogs with CH were used in this study.

METHODS

Prospective case-control study. Hepatic ET-1 mRNA expression was determined by real-time reverse transcription polymerase chain reaction, and hepatic and plasma ET-1 levels were assessed using ELISA. Splenic pulp pressure (SPP), as an indicator of portal vein pressure, was measured laparoscopically.

RESULTS

Hepatic ET-1 mRNA levels were 3.7 times higher in dogs with CH than in healthy dogs (P = .008). The median hepatic and plasma ET-1 protein levels were significantly higher in dogs with CH than in healthy dogs (13.20 pg/mg wet liver vs. 3.42 pg/mg wet liver, P = .004, and 0.99 pg/mL vs. 0.71 pg/mL, P = .013, respectively). Moreover, there was a weak but significant correlation between plasma ET-1 level and SPP in dogs with CH (P = .036; r_s = 0.53).

CONCLUSIONS AND CLINICAL IMPORTANCE

The results indicate that ET-1 might play an important role in the pathogenesis of portal hypertension caused by CH.

Increased endothelin receptor B and G protein coupled kinase-2 in the mesentery of portal hypertensive rats

AIM: To elucidate the mechanisms of mesenteric vasodilation in portal hypertension (PHT), with a focus on endothelin signaling.

METHODS: PHT was induced in rats by common bile duct ligation (CBDL). Portal pressure (PP) was measured directly via catheters placed in the portal vein tract. The level of endothelin-1 (ET-1) in the mesenteric circulation was determined by radioimmunoassay, and the expression of the endothelin A receptor (ETAR) and endothelin B receptor (ETBR) was assessed by immunofluorescence and Western blot. Additionally, expression of G protein coupled kinase-2 (GRK2) and β-arrestin 2, which influence endothelin receptor sensitivity, were also studied by Western blot.

RESULTS: PP of CBDL rats increased significantly (11.89 ± 1.38 mmHg vs 16.34 ± 1.63 mmHg). ET-1 expression decreased in the mesenteric circulation 2 and 4 wk after CBDL. ET-1 levels in the systemic circulation of CBDL rats were increased at 2 wk and decreased at 4 wk. There was no change in ETAR expression in response to CBDL; however, increased expression of ETBR in the endothelial cells of mesenteric arterioles and capillaries was observed. In sham-operated rats, ETBR was mainly expressed in the CD31⁺ endothelial cells of the arterioles. With development of PHT, in addition to the endothelial cells, ETBR expression was noticeably detectable in the SMA⁺ smooth muscle cells of arterioles and in the CD31⁺ capillaries. Following CBDL, increased expression of GRK2 was also found in mesenteric tissue, though there was no change in the level of β-arrestin 2.

CONCLUSION: Decreased levels of ET-1 and increased ETBR expression in the mesenteric circulation following CBDL in rats may underlie mesenteric vasodilation in individuals with PHT. Mechanistically, increased GRK2 expression may lead to desensitization of ETAR, as well as other vasoconstrictors, promoting this vasodilatory effect.

Endothelin receptors: what's new and what do we need to know?

Receptors are at the heart of how a molecule transmits a signal to a cell. Two receptor classes for endothelin (ET) are recognized, the ET(A) and ET(B) receptors. Intriguing questions have arisen in the field of ET receptor pharmacology, physiology, and function. For example, a host of pharmacological studies support the interaction of the ET(A) and ET(B) receptor in tissues (veins, arteries, bronchus, arterioles, esophagus), but yet few have been able to demonstrate direct ET(A)/ET(B) receptor interaction. Have we modeled this interaction wrong? Do we have a truly selective ET(A) receptor agonist such that we could selectively stimulate this important receptor? What can we learn from the recent phylogenic studies of the ET receptor family? Have we adequately addressed the number of biological molecules with which ET can interact to exert a biological effect? Recent mass spectrometry studies in our laboratory suggest that ET-1 interacts with other hereto unrecognized proteins. Biased ligands (ligands at the same receptor that elicit distinct signaling responses) have been discovered for other receptors. Do these exist for ET receptors and can we take advantage of this possibility in drug design? These and other questions will be posed in this minireview on topics on ET receptors.

Superimposed coagulopathic conditions in cirrhosis: infection and endogenous heparinoids, renal failure, and endothelial dysfunction

In this article, the authors discuss three pathophysiologic mechanisms that influence the coagulation system in patients who have liver disease. First, bacterial infections may play an important role in the cause of variceal bleeding in patients who have liver cirrhosis, affecting coagulation through multiple pathways. One of the pathways through which this occurs is dependent on endogenous heparinoids, on which the authors focus in this article. Secondly, the authors discuss renal failure, a condition that is frequently encountered in patients who have liver cirrhosis. Finally, they review dysfunction of the endothelial system. The role of markers of endothelial function in cirrhotic patients, such as von Willebrand factor and endothelin-1, is discussed.

Imbalance between expression of endothelin receptors A and B in terminal liver cirrhosis due to hepatitis C viral infection: immunohistochemical study of autopsy cases

BACKGROUND AND AIM

Expression of endothelin receptors in terminal liver cirrhosis is not well investigated. The aim of this study was to investigate the expression of the endothelin type A receptor (ETAR) and endothelin type B receptor (ETBR) immunohistochemically using paraffin-embedded tissue sections from patents with terminal liver cirrhosis (TLC), non-terminal liver cirrhosis (NTLC) and non-cirrhotic liver fibrosis (NCLF) caused by hepatitis C viral infection.

METHODS

Liver tissue sections from 38 autopsy cases, including 12 cases of NCLF (mild, moderate or severe liver fibrosis), 11 cases of NTLC and 15 cases of TLC, were stained using anti-ETAR and anti-ETBR antibodies after antigen retrieval. Double staining using antibodies to alpha-smooth muscle actin (ASMA) was also performed.

RESULTS

There were significantly fewer ETBR-positive cells in TLC compared with NTLC and NCLF. Numbers of ASMA-positive stellate cells expressing ETBR were also significantly lower in TLC. Therefore, the ETAR/ETBR ratio of sinusoidal cells is significantly higher in TLC than in NTLC and NCLF. ASMA-positive stellate cells showed similar evidence of ETAR and ETBR expression.

CONCLUSIONS

There is a difference in ETAR and ETBR expression among TLC, NTLC and NCLF: the ETAR/ETBR ratio is increased in TLC due to a relative decrease in ETBR expression. This finding may be useful for the diagnosis of TLC with regard to circulatory disturbances in the liver.

Pathophysiology and clinical practice analysis on endothelin system and portal hypertension

Portal hypertension (PHT) is a common clinical syndrome which leads to various severe, even lethal complications. The concentration of endothelin-1 (ET-1) in plasma is increased both in human body and PHT animal model. The effect of ET-1 depends on the kind of tissue and the expression of ET-1 receptor in this tissue. However, the expression of ET-1 receptor is not identical even in the same tissue at different PHT phases. This review aims to give an update on the endothelin syetem in PHT and elucidate a potential novel strategy.

Vagally mediated cholestatic and choleretic effects of centrally applied Endothelin-1 through ETA receptors

The role of Endothelin-1 (ET-1) in the central nervous system is not fully understood yet although several studies strongly support its neuromodulatory role. A high density of endothelin receptors is present in the dorsal vagal complex that is the major site for the regulation of the digestive function. Therefore in the present study we sought to establish the role of ET-1 in the central regulation of bile secretion in the rat. Intracerebroventricular ET-1 injection exhibited opposite behaviors on spontaneous bile secretion according to the dose administered. Lower doses of ET-1 (1 fM) increased bile flow and bicarbonate excretion whereas higher doses (1 nM) decreased bile flow and bile acid output. Both the choleretic and the cholestatic effects of ET-1 were abolished in animals pretreated with icv BQ-610 (selective ETA antagonist) but not with BQ-788 (selective ETB antagonist). In addition, truncal vagotomy but not adrenergic blockade abolished ET-1 effects on bile secretion. Brain nitric oxide was not involved in ET-1 response since L-NAME pretreatment failed to affect ET-1 actions on the liver. Portal venous pressure was increased by centrally administered ET-1 being the magnitude of the increase similar with low and high doses of the peptide. These results show that centrally applied ET-1 modified different bile flow fractions independent of hemodynamic changes. Lower doses of ET-1 increased bile acid independent flow whereas higher doses decreased bile acid dependent flow. Vagal pathways through the activation of apparently distinct ETA receptors mediated the cholestatic as well as the choleretic effects induced by ET-1. Present findings show that ET-1 participates in the central regulation of bile secretion in the rat and give further insights into the complexity of brain-liver interaction.

Hepatic fibrosis: molecular mechanisms and drug targets

Liver fibrosis is the common response to chronic liver injury, ultimately leading to cirrhosis and its complications, portal hypertension, liver failure, and hepatocellular carcinoma. Efficient and well-tolerated antifibrotic drugs are currently lacking, and current treatment of hepatic fibrosis is limited to withdrawal of the noxious agent. Efforts over the past decade have mainly focused on fibrogenic cells generating the scarring response, although promising data on inhibition of parenchymal injury and/or reduction of liver inflammation have also been obtained. A large number of approaches have been validated in culture studies and in animal models, and several clinical trials are underway or anticipated for a growing number of molecules. This review highlights recent advances in the molecular mechanisms of liver fibrosis and discusses mechanistically based strategies that have recently emerged.

The Incidence of self-prescribed oral complementary and alternative medicine use by patients with gastrointestinal diseases

GOALS

To assess the incidence of oral complementary and alternative medicine (CAM) usage by gastroenterology patients at a single university center and compare against controls.

BACKGROUND

The public awareness and usage of CAM have increased. The use of CAM has been described in patients with functional bowel disorders; however, their role in patients with gastrointestinal disease is less clear.

STUDY

Patients attending luminal gastroenterology clinics and customers at local supermarkets completed a 30-point, structured questionnaire assessing their use of CAM.

RESULTS

A total of 1,409 subjects were recruited. The incidence of CAM use was 49.5% for inflammatory bowel disease, 50.9% for irritable bowel syndrome, 20% for general gastrointestinal diseases, and 27% for controls. Pearson's chi(2) tests showed that patients with inflammatory bowel disease (IBD) or irritable bowel syndrome were more likely to use CAM than controls (P < 0.001). Binary logistic regression analysis showed that females were more likely to take CAM than men (P < 0.05).

CONCLUSIONS

The percentage of CAM users among patients with IBD is similar to those with a functional diagnosis. Increasing numbers of IBD patients are using CAM in addition to conventional therapy. Awareness of this may prevent adverse CAM and conventional drug interactions.

Expression of the hepatic endothelin system in human cirrhotic livers

It is considered that endothelin-1 participates in the development of liver cirrhosis and it has been recognized that every component of the endothelin system is upregulated in cirrhotic livers. However, the expression pattern of this system, including interaction between its components, is not fully understood in human livers. In this study, the expression pattern of the endothelin system was examined. Immunohistochemical analysis for endothelin-1, endothelin receptors and endothelin-converting enzyme was performed in 16 cirrhotic and 17 normal human liver tissues. Peptides, proteins, and RNAs extracted from the livers were also investigated using quantitative assays for the components of the hepatic endothelin system. Hepatic endothelin-1 levels were significantly higher in cirrhotic livers (0.084 +/- 0.052 pg/mg wet liver) than in normal livers (0.041 +/- 0.032 pg/mg; p < 0.01), and were closely related to the severity of liver fibrosis and portal hypertension. Immunoreactivity for endothelin-1, endothelin receptors, and endothelin-converting enzyme was detected mainly in fibrous areas and in the hepatic vasculature, and was enhanced in cirrhosis. Although there was a negative correlation between the expression of receptor mRNA and the hepatic endothelin-1 level, the amounts of the mRNAs were greater in cirrhotic livers than in normal livers. However, expression of endothelin-converting enzyme in cirrhotic livers was increased at the protein level but was relatively reduced at the mRNA level. These findings suggest that the hepatic endothelin system is activated in human cirrhotic livers in association with worsening of the disease, but that the regulation of the components of this system in this disorder is complex.

Cellular bases for the lipid-related aspects of preeclampsia

Lipid accumulation - in hepatoytes (both subclinically and in acute fatty liver of pregnancy), in the endothelium of placental vessels ("acute atherosis"), and in the bloodstream - has been well established to be a consequence of preeclampsia. Hyperlipidemia (specifically hypertriglyceridemia) has been demonstrated to be a risk factor for the development of preeclampsia. These lipid-related aspects of preeclampsia may appear unrelated, but all are here demonstrated to provide evidence for a causative role for endothelin in the etiology of preeclampsia. Evidence for the potential of endothelin to cause lipid accumulation in hepatocytes and in endothelial cells, by means of activating G protein cascades in these cells, is presented. The capacity of typical free fatty acid constituents of triglycerides to "drive" interacellular G protein cascade-related events is also discussed - which, in this scheme, offers a plausible explanation for hypertriglyceridemia's role as a risk factor for developing preeclampsia. Additional evidence is provided which substantiates endothelin's capacity to cause most of the observed aberrations known to occur in preeclampsia.

Regulation of endothelin-A receptor sensitivity by cyclic adenosine monophosphate in rat hepatic stellate cells

Sensitization of the endothelin-A receptor (ET(A)) occurs during HSC transdifferentiation, but the underlying mechanisms remained unclear. Sensitization of ET(A) was studied in quiescent and activated hepatic stellate cells (HSC) at the levels of receptor phosphorylation, localization, endothelin (ET)-1-induced Ca(2+) signals, and cell contraction. The endothelin-1 (ET-1) concentrations required to obtain an ET(A)-mediated Ca(2+) signal in 50% of HSC cultured for 1 to 2 or 10 days were approximately 1.2 and 0.012 nmol/L, respectively. This transdifferentiation-dependent sensitization of ET(A) was accompanied by receptor translocation to the plasma membrane. Cyclic AMP rapidly desensitized ET(A) in activated HSC and shifted their ET-1 responsiveness from picomolar to nanomolar concentrations with respect to Ca(2+) signals and HSC contraction. ET(A) desensitization also occurred in response to prostaglandin E(2), adenosine, or ET(B) stimulation. Desensitization by cAMP in activated HSC was accompanied by an increased Ser/Thr phosphorylation of ET(A) and their rapid internalization. Quiescent HSC exhibited Ser/Thr phosphorylation of the ET(A) protein, which was not affected by cAMP. In conclusion, the ET(A) response in HSC is regulated by protein kinase A (PKA)-dependent receptor phosphorylation and internalization. This may explain the transdifferentiation-dependent sensitization of HSC towards ET-1 and its reversal by cAMP and ET(B) activation.

Endothelin receptor A blockade ameliorates hypothermic ischemia-reperfusion-related microhemodynamic disturbances during liver transplantation in the rat

BACKGROUND

The objective of this study was to investigate the effect of graft treatment with specific endothelin receptor antagonists (ET(A) and ET(B)) on the microhemodynamic disturbances which occur following ischemia/reperfusion injury during orthotopic liver transplantation (OLT) in the rat.

MATERIALS AND METHODS

OLT was performed in male Sprague-Dawley rats. An ET(A) receptor antagonist (BQ-610; 0.3 mg/kg) or ET(B) receptor antagonist IRL-1038 (20 nmol/kg) was administered intraportally into liver grafts in vitro at the beginning of 2- and 6-h cold storage (4 degrees C) using physiological saline. Sham-operated animals served as controls (Cont). Seven groups were studied: Cont; vehicle-2 h (saline treated); ET(B) antagonist-2 h; ET(A) antagonist-2 h; vehicle-6 h; ET(A) antagonist-6 h; and ET(B) antagonist-6 h. At 1 h after graft implantation, the liver microcirculation was investigated by intravital fluorescence microscopy.

RESULTS

In vehicle-treated livers, the hepatic microcirculation was markedly impaired compared with the Cont as manifested by a reduced lobular perfusion index, increased incidence of sinusoidal nonperfusion, elevated leukocyte adhesion in sinusoids and terminal hepatic venules, and increased hepatic venous resistance (23-fold; 6-h group). In addition, plasma liver enzymes were significantly elevated in the vehicle treated groups. Alterations to all these parameters were markedly reduced in the ET(A) receptor antagonist-treated liver grafts although there was still evidence of hepatic injury. The ET(B) receptor antagonist had little effect on the I/R-induced changes to the hepatic microcirculation.

CONCLUSIONS

Our results indicate that the ET(A) antagonism ameliorates hypothermic I/R-related microhemodynamic disturbances during OLT in the rat, suggesting that application of an ET(A) antagonist to liver grafts may have therapeutic potential in human liver transplantation.

Enhanced expression of endothelin B receptor at protein and gene levels in human cirrhotic liver

Endothelin (ET) has been implicated in the regulation of hepatic microcirculation and development of portal hypertension. This study examined the localization of ETA receptor (ETAR) and ETB receptor (ETBR) in cirrhotic liver tissues from patients with hepatocellular carcinoma with hepatitis C-related cirrhosis, and normal liver samples from patients with metastatic liver carcinoma. Anti-ETAR and ETBR antibodies were used for immunohistochemistry and Western blot. Immunoelectron microscopy was conducted using immunoglobulin-gold and silver staining. For in situ hybridization (ISH), human ETAR and ETBR peptide nucleic acid probes were used with the catalyzed signal amplification system. In normal liver tissue, immunohistochemistry revealed that ETBR was predominantly expressed on hepatic sinusoidal lining cells, particularly on sinusoidal endothelial (SECs) and hepatic stellate cells (HSCs), and ETAR was scantily expressed. These findings were confirmed by Western blot and ISH. In cirrhotic liver tissue, overexpression of ETBR was demonstrated by Western blot and ISH. Morphometric analysis showed significant increase of ETBR expression on HSCs and SECs in cirrhotic liver, particularly on HSCs. ETAR expression was increased but remained low. Enhanced ETBR expression in cirrhosis may intensify the effect of endothelin on HSCs and increase hepatic microvascular tone.

Altered endothelin receptor expression in prehepatic portal hypertension predisposes the liver to microcirculatory dysfunction in rats

BACKGROUND/AIMS

Endothelin (ET) is one of the most active vascular regulators in the liver. It is unknown how partial portal vein ligation (PPVL) induced prehepatic portal hypertension influences the response of the liver to ET and its agonists. Therefore, this study was conducted to determine the expression of ET receptors and its functional significance after PPVL.

METHODS

Competitive receptor binding study and semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) were performed using liver homogenates after 2 weeks of PPVL or sham operation in rats. Hepatic microcirculation was evaluated in vivo using intravital microscopy.

RESULTS

Although there was no significant difference in dissociation constant (Kd) and total amount of receptors (Bmax) between sham and PPVL, the proportion of ET(B) receptor was significantly increased in PPVL. RT-PCR analysis confirmed the up-regulation of ET(B) receptors demonstrated by the competitive receptor binding assay. In the functional study, infusion of ET(B) agonist (IRL 1620) in a low dosage did not change the hepatic microcirculation in sham but strongly constricted the sinusoids leading to a reduction of sinusoidal perfusion in PPVL.

CONCLUSIONS

These results suggest that prehepatic portal hypertension may predispose the hepatic microcirculation to dysregulation in stress conditions where ET is upregulated.

Enhanced expression of endothelin receptor subtypes in cirrhotic rat liver

BACKGROUND/AIMS

A number of vasoactive substances have been implicated as potential mediators of intrahepatic portal hypertension. Endothelin (ET)-1 has been suggested to be involved in the regulation of hepatic microcirculation and development of portal hypertension. The aim of this study was to clarify the localization of two subtypes of ET receptors, ET A (ETAR) and B receptors (ETBR), in normal rat liver, and how the receptor expressions are altered in CCl4-induced cirrhotic rat liver.

METHODS

Liver specimens were examined immunohistochemically after reacting with anti-ETAR and anti-ETBR rabbit polyclonal antibodies. Immunogold staining was also performed using the same antibodies, and examined under light and electron microscopy.

RESULTS

In normal rat liver, immunohistochemistry revealed expression of ETAR and ETBR on the hepatic sinusoidal lining cells. By immunogold electron microscopy, electron-dense gold particles indicating the presence of ETARs were localized mainly on hepatic stellate cells (HSCs) and to a lesser extent on sinusoidal endothelial cells (SECs), while ETBRs were expressed equally intensely on HSCs and SECs. In cirrhotic animals, both ETAR and ETBR increased significantly on HSCs, while there were no significant increases in either receptor on SECs.

CONCLUSIONS

In the normal state, HSCs possess both ETARs and ETBRs, while SECs mainly possess ETBRs. In cirrhosis, endothelins may exert more intense effects on HSCs via the enhanced ETARs and ETBRs, causing an increase in hepatic sinusoidal microvascular tone.

Production and release of endothelin-1 from the gut and spleen in portal hypertension due to cirrhosis

This study was aimed to evaluate the source of endothelin-1 (ET-1) in cirrhotic patients. ET-1 is implicated in the pathogenesis of portal hypertension. However, the mechanism and source for increased plasma ET-1 in cirrhotic patients are still obscure. Plasma ET-1 levels in systemic (SV), superior mesenteric (SMV), and splenic venous (SPV) blood were measured in 23 patients with cirrhosis and 8 controls with normal liver. Fourteen removed spleens were immunohistochemically studied for ET-1, CD34, CD68, and CD20. In situ hybridization was done to localize ET-1 messenger RNA (mRNA). In cirrhosis, ET-1 levels in both SMV and SPV were higher than in SV. ET-1 in SV and SPV were significantly higher in cirrhotic patients than in control patients. Three groups of cells in the spleen expressed both protein and mRNA of ET-1: endothelial cells in the sinus, which were also stained for CD34; cells in the germinal center; and cells in the marginal zone of lymphoid sheaths and follicles, which were also stained for CD20 but not for CD34 and CD68. The ET-1 concentration released from the spleen was in parallel with the grade of ET-1 expression in the spleen. The spleen is one of the major sites of ET-1 release in cirrhotic patients. Endothelial cells of the splenic sinus and possibly B lymphocytes in the germinal center and marginal zone of lymphoid sheaths and follicles seem to be the sites of ET-1 production in the spleen.

Functional significance of endothelin B receptors in mediating sinusoidal and extrasinusoidal effects of endothelins in the intact rat liver

Endothelins (ET) are important regulators of the hepatic microcirculation that act through different receptor subtypes. We investigated functional significance of ET(B) receptors in mediating microhemodynamic effects of ETs in normal and endotoxin (lipopolysaccharide [LPS])-primed rat liver. LPS priming (Escherichia coli O26:B6; 1 mg. kg(-1)) selectively increased ET(B) mRNA and led to a shift in available receptors to the ET(B) subtype. IRL 1620 (an ET(B) agonist) increased portal pressure in a dose-dependent manner, and the increase in ET(B) expression was associated with prolonged portal pressor response in isolated livers. However, lactate dehydrogenase (LDH) release was attenuated and sinusoidal blood flow was better maintained upon ET(B) stimulation in vivo. In isolated livers, portal constriction as well as release of LDH, were substantially increased in the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase (NOS). In vivo microscopic assessment of sinusoidal perfusion during ET(B) stimulation revealed a disruption of the flow pattern including frequent reversal of the flow direction without significant sinusoid constriction. Sinusoidal flow decreased even further after discontinuation of IRL 1620. Both effects were mediated at extrasinusoidal sites that probably included postsinusoidal sites. However, after pretreatment with L-NAME, IRL 1620 evoked a significant sinusoidal constriction that colocalized with the body of the stellate cell. We propose that ET(B1)-induced NOS activity attenuates ET(B2) (and presumably ET(A))-mediated portal pressor response and stellate cell constriction. Transcriptional activation of the ET(B) gene may have a permissive effect on liver blood flow and protect against hepatocellular damage under pathophysiological conditions associated with endotoxemia.

Endothelin-1 stimulation of endothelial nitric oxide synthase in the pathogenesis of hepatopulmonary syndrome

Biliary cirrhosis in the rat triggers intrapulmonary vasodilatation and gas exchange abnormalities that characterize the hepatopulmonary syndrome. This vasodilatation correlates with increased levels of pulmonary microcirculatory endothelial nitric oxide synthase (eNOS) and hepatic and plasma endothelin-1 (ET-1). Prehepatic portal hypertension induced by portal vein ligation (PVL) does not cause similar changes, suggesting that ET-1 in cirrhosis may modulate pulmonary eNOS and vascular tone. We assessed whether ET-1 altered eNOS expression and nitric oxide production in bovine pulmonary artery endothelial cells (BPAECs) and if a 2-wk low-level intravenous ET-1 infusion in PVL animals modulated pulmonary eNOS levels, microcirculatory tone, and gas exchange. ET-1 caused a 2.5-fold increase in eNOS protein in BPAECs, inhibitable with an endothelin B receptor antagonist, and an increase in eNOS mRNA and nitrite production. ET-1 infusion in PVL animals caused increased pulmonary eNOS levels, intrapulmonary vasodilatation, and gas exchange abnormalities without increasing pulmonary arterial pressure. ET-1 produced during hepatic injury may contribute to the hepatopulmonary syndrome by modulating eNOS and inducing pulmonary microcicrulatory vasodilatation.

Activation of rat hepatic stellate cells in culture is associated with increased sensitivity to endothelin 1

The effect of endothelin (ET) 1 on intracellular Ca2+ transients in cultured rat hepatic stellate cells (HSCs) during transformation was studied by use of single-cell fluorescence. Regardless of the duration of HSC culture, ET-1 caused a BQ-123-sensitive but IRL-1038-insensitive elevation of [Ca2+]i, indicating the involvement of ETA but not ETB receptors. HSCs in early culture ("quiescent HSCs") were mildly responsive to ET-1: the ET-1 concentration required to obtain a [Ca2+]i transient in 50% of the cells (RC50) was 7 nmol/L, and all cells responded to ET-1 concentrations above 40 nmol/L. With culture time, -smooth muscle actin (-SMA) expression increased, as did the ET-1 sensitivity of cells, resulting in a shift of the RC50 value from 7 nmol/L to 13 pmol/L within 8 days. Independent of the duration of culture, ET-1 sensitivity was higher in -SMA-expressing cells. On the other hand, sensitivity of HSCs to produce a [Ca2+]i response to extracellular uridin 5'-triphosphate (UTP) or phenylephrine did not change during the activation process. There was no difference between quiescent and activated HSCs with respect to the sharing of intracellular Ca2+ stores, which could be mobilized by ET-1, UTP, and phenylephrine, respectively. The data suggest three conclusions. (1) A marked increase in ET-1 sensitivity of HSCs during the activation process suggests a potentiation of autocrine/paracrine stimulation. (2) HSCs are susceptible to -adrenergic and purinergic stimulation, but sensitivity to phenylephrine and UTP is not affected during the transformation process. (3) The ET-1-mobilizable Ca2+ store is contained in and is smaller than the Ca2+ pool, which is mobilized by phenylephrine or UTP.

Platelet-derived growth factor-BB and thrombin generate positive and negative signals for human hepatic stellate cell proliferation. Role of a prostaglandin/cyclic AMP pathway and cross-talk with endothelin receptors

Proliferation of myofibroblastic hepatic stellate cells (HSC) in response to growth factors is essential for the development of liver fibrosis. We have reported that prostaglandins (PG) and cyclic AMP (cAMP) inhibit growth of human HSC. This PG/cAMP pathway transduces the endothelin (ET) B-mediated antiproliferative effect of endothelin-1 (ET-1) and up-regulates ETB receptors. Here, we show that platelet-derived growth factor (PDGF)-BB and thrombin, although mitogenic, generate growth inhibitory PGE2 in myofibroblastic human HSC. The two peptides elicit early PGE2 and cAMP synthesis, and also promote delayed induction of cyclooxygenase (COX)-2. Both early and delayed production of PGE2 counteract the mitogenic effect of PDGF-BB and thrombin because: (i) pretreatment with the COX inhibitor ibuprofen markedly enhances the mitogenic effect of both peptides; (ii) blocking early synthesis of PGE2 greatly enhances extracellular signal-regulated kinase (ERK) activation by both growth factors; (iii) enhancement of DNA synthesis by ibuprofen is only lost when the inhibitor is added after COX-2 induction has occurred. Finally, PDGF-BB and thrombin raise ETB receptors through the PG pathway. Thus, ibuprofen blunts growth factor-induced increase in ETB receptors. Up-regulation of the growth inhibitory ETB receptors by both mitogens may enhance the antiproliferative effect of ET-1 and thereby establish a negative feedback of their mitogenic effect. Our results shed light on novel growth inhibitory signals evoked by two mitogenic growth factors expressed during liver injury.

Endothelin A-receptor blockade worsens endotoxin-induced hepatic microcirculatory changes and necrosis

BACKGROUND & AIMS

Endothelin 1 is considered to be an important regulator of sinusoidal blood flow and increases during endotoxemia. The purpose of this study was to investigate the role of endothelin 1 in hepatic microcirculation, oxygen transport, and liver injury during endotoxemia.

METHODS

Male Sprague-Dawley rats were continuously infused with 2.5 mL/h of saline, 0.8 mg . kg-1 . h-1 of lipopolysaccharide (LPS), 3 mg . kg-1 . h-1 of BQ-485, an endothelin A-receptor antagonist, or LPS plus BQ-485 for 7 hours.

RESULTS

BQ-485 infusion had no significant effect on hepatic microcirculation and liver injury. LPS increased the plasma levels of aspartate aminotransferase (AST) and total bilirubin and decreased the hepatic adenosine triphosphate (ATP) level and bile flow rate. LPS + BQ-485 infusion further increased the plasma levels of AST and total bilirubin and decreased the bile flow rate and the hepatic ATP level. Dual-spot microspectroscopy revealed mild decreases in sinusoidal erythrocyte velocity and oxygen transport in the LPS group and profound decreases in these parameters in the LPS + BQ-485 group. Histological examinations revealed massive necrotic changes in the pericentral regions of the LPS + BQ-485 group.

CONCLUSIONS

These results suggest that blockade of endothelin A receptors disturbs hepatic microcirculation and oxygen transport and aggravates the necrotic injury induced by endotoxin.

Growth inhibitory properties of endothelin-1 in activated human hepatic stellate cells: a cyclic adenosine monophosphate-mediated pathway. Inhibition of both extracellular signal-regulated kinase and c-Jun kinase and upregulation of endothelin B receptors

During chronic liver diseases, hepatic stellate cells (HSC) acquire an activated myofibroblast-like phenotype, proliferate, and synthetize fibrosis components. We have shown that endothelin-1 (ET-1) inhibits the proliferation of activated human HSC via endothelin B (ETB) receptors. We now investigate the transduction pathway involved in the growth inhibitory effect of ET-1 in activated HSC. Endothelin-1 and the ETB receptor agonist, sarafotoxin-S6C, increased synthesis of PGI2 and PGE2, leading to elevation of cAMP. The cyclooxygenase inhibitor ibuprofen and the adenylyl cyclase inhibitor SQ22536 both blunted the growth inhibitory effect of ET-1. Analysis of early steps associated with growth inhibition indicated that: (a) similar to ET-1, forskolin decreased c-jun mRNA induction without affecting c-fos and krox 24 mRNA expression; (b) ET-1, sarafotoxin-S6C, as well as forskolin, reduced activation of both c-Jun kinase and extracellular signal-regulated kinase. Finally, forskolin, PGI2, and PGE2 raised by fivefold the number of ET binding sites after 6 h, and increased the proportion of ETB receptors from 50% in control cells to 80% in treated cells. In conclusion, ET-1 inhibits proliferation of activated HSC via ETB receptors, through a prostaglandin/cAMP pathway that leads to inhibition of both extracellular signal-regulated kinase and c-Jun kinase activities. Upregulation of ETB receptors by prostaglandin/cAMP raises the possibility of a positive feedback loop that would amplify the growth inhibitory response. These results suggest that ET-1 and agents that increase cAMP might be of interest to limit proliferation of activated HSC during chronic liver diseases.