Endothelin action in rat liver. Receptors, free Ca2+ oscillations, and activation of glycogenolysis

Endothelin-1 induces lipolysis through activation of the GC/cGMP/Ca2+/ERK/CaMKIII pathway in 3T3-L1 adipocytes

Endothelin-1 (ET-1) is a potent vasoconstrictive peptide produced and secreted mainly by endothelial cells. Recent studies indicate that ET-1 can regulate lipid metabolism, which may increase the risk of insulin resistance. Our previous studies revealed that ET-1 induced lipolysis in adipocytes, but the underlying mechanisms were unclear. 3T3-L1 adipocytes were used to investigate the effect of ET-1 on lipolysis and the underlying mechanisms. Glycerol levels in the incubation medium and hormone-sensitive lipase (HSL) phosphorylation were used as indices for lipolysis. ET-1 significantly increased HSL phosphorylation and lipolysis, which were completely inhibited by ERK inhibitor (PD98059) and guanylyl cyclase (GC) inhibitor (LY83583). LY83583 reduced ET-1-induced ERK phosphorylation. A Ca2+-free medium and PLC inhibitor caused significant decreases in ET-1-induced lipolysis as well as ERK and HSL phosphorylation, and IP3 receptor activator (D-IP3) increased lipolysis. ET-1 increased cGMP production, which was not affected by depletion of extracellular Ca2+. On the other hand, LY83583 diminished the ET-1-induced Ca2+ influx. Transient receptor potential vanilloid-1 (TRPV-1) antagonist and shRNA partially inhibited ET-1-induced lipolysis. ET-1-induced lipolysis was completely suppressed by CaMKIII inhibitor (NH-125). These results indicate that ET-1 stimulates extracellular Ca2+ entry and activates the intracellular PLC/IP3/Ca2+ pathway through a cGMP-dependent pathway. The increased cytosolic Ca2+ that results from ET-1 treatment stimulates ERK and HSL phosphorylation, which subsequently induces lipolysis. ET-1 induces HSL phosphorylation and lipolysis via the GC/cGMP/Ca2+/ERK/CaMKIII signaling pathway in 3T3-L1 adipocytes.

Lignans from Schisandra chinensis ameliorate alcohol and CCl4-induced long-term liver injury and reduce hepatocellular degeneration via blocking ETBR

ETHNOPHARMACOLOGICAL RELEVANCE

Chemical hepatotoxicity, especially alcoholic liver injury (ALI), commonly occurs in young and middle-aged people who drink heavily. ALI is extremely harmful and can induce severe disease states, such as hepatitis, liver fibrosis, cirrhosis, or liver cancer, which are similar to CCl₄-induced liver disease states in animals. In recent studies, the pathological changes of hepatocytes and the hepatic stellate cell have shown a significant connection between endoplasmic reticulum (ER) stress and the development of liver pathology in patients. However, the detailed pathological mechanism needs to be further studied. Schisandra chinensis, (S. chinensis), a fruit-bearing vine used in Traditional Chinese Medicine (TCM), has been used to treat chronic or acute diseases, including liver disease. S. chinensis-derived lignans (SCDLs) in particular have been shown to alleviate liver pathological changes.

AIM OF THE STUDY

This study sought to elucidate the mechanisms underlying SCDL-mediated hepatoprotection.

MATERIALS AND METHODS

We first used in silico target prediction and computational simulation methods to identify putative lignan-binding targets relative to the hepatoprotective effect. A gene microarray analysis was performed to identify differently expressed genes that might have significance in the disease pathological process. We then used histological analyses in a mice hepatotoxicity model to test the effectiveness of SCDLs in vivo, and a hepatocellular toxicity model to analyze the candidate-compound-mediated hepatoprotection and expression states of the key targets in vitro.

RESULTS

The in silico analysis results indicated that endothelin receptor B (ETBR/EDNRB) is likely a significant node during the liver pathological change process and a promising key target for the SCDL compound schisantherin D on the hepatoprotective effect; experimental studies showed that schisantherin D alleviated the EtOH- and ET-1-induced HL-7702 cell (belongs to liver parenchymal cell lines) injury ratio, decreased the expression of ETBR, and inhibited ECMs and ET-1 secretion in LX-2 cells (one form of hepatic stellate cells). SCDLs ameliorated EtOH- and CCl₄-induced fibrosis formation in mice liver tissue. Liver tissue western blots of SCDL-treated mice showed downregulated α-SMA, ETBR, PLCβ, CHOP, Bax, and the apoptotic factors of cleaved-caspase 12, cleaved-caspase 9, and cleaved-caspase 3 hinted at an anti-apoptosis and hepatoprotective effect. The SCDL treatment also elevated serum glutathione (GSH) and reduced the serum-transforming growth factor-β1 (TGF-β1) level.

CONCLUSION

The findings indicated that SCDLs prevent hepatotoxicity via their anti-fibrotic, anti-oxidant, and anti-apoptosis properties. ETBR may be the key factor in promoting chemical hepatotoxicity.

Causality Analysis and Cell Network Modeling of Spatial Calcium Signaling Patterns in Liver Lobules

Dynamics as well as localization of Ca²⁺ transients plays a vital role in liver function under homeostatic conditions, repair, and disease. In response to circulating hormonal stimuli, hepatocytes exhibit intracellular Ca²⁺ responses that propagate through liver lobules in a wave-like fashion. Although intracellular processes that control cell autonomous Ca²⁺ spiking behavior have been studied extensively, the intra- and inter-cellular signaling factors that regulate lobular scale spatial patterns and wave-like propagation of Ca²⁺ remain to be determined. To address this need, we acquired images of cytosolic Ca²⁺ transients in 1300 hepatocytes situated across several mouse liver lobules over a period of 1600 s. We analyzed this time series data using correlation network analysis, causal network analysis, and computational modeling, to characterize the spatial distribution of heterogeneity in intracellular Ca²⁺ signaling components as well as intercellular interactions that control lobular scale Ca²⁺ waves. Our causal network analysis revealed that hepatocytes are causally linked to multiple other co-localized hepatocytes, but these influences are not necessarily aligned uni-directionally along the sinusoids. Our computational model-based analysis showed that spatial gradients of intracellular Ca²⁺ signaling components as well as intercellular molecular exchange are required for lobular scale propagation of Ca²⁺ waves. Additionally, our analysis suggested that causal influences of hepatocytes on Ca²⁺ responses of multiple neighbors lead to robustness of Ca²⁺ wave propagation through liver lobules.

Blockade of Endothelin-1 Receptor Type B Ameliorates Glucose Intolerance and Insulin Resistance in a Mouse Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is associated with insulin resistance (IR) and glucose intolerance. Elevated endothelin-1 (ET-1) levels have been observed in OSA patients and in mice exposed to intermittent hypoxia (IH). We examined whether pharmacological blockade of type A and type B ET-1 receptors (ETA and ETB) would ameliorate glucose intolerance and IR in mice exposed to IH. Subcutaneously implanted pumps delivered BQ-123 (ETA antagonist; 200 nmol/kg/day), BQ-788 (ETB antagonist; 200 nmol/kg/day) or vehicle (saline or propyleneglycol [PG]) for 14 days in C57BL6/J mice (10/group). During treatment, mice were exposed to IH (decreasing the FiO2 from 20.9% to 6%, 60/h) or intermittent air (IA). After IH or IA exposure, insulin (0.5 IU/kg) or glucose (1 mg/kg) was injected intraperitoneally and plasma glucose determined after injection and area under glucose curve (AUC) was calculated. Fourteen-day IH increased fasting glucose levels (122 ± 7 vs. 157 ± 8 mg/dL, PG: 118 ± 6 vs. 139 ± 8; both p < 0.05) and impaired glucose tolerance (AUCglucose: 19,249 ± 1105 vs. 29,124 ± 1444, PG AUCglucose: 18,066 ± 947 vs. 25,135 ± 797; both p < 0.05) in vehicle-treated animals. IH-induced impairments in glucose tolerance were partially ameliorated with BQ-788 treatment (AUCglucose: 21,969 ± 662; p < 0.05). Fourteen-day IH also induced IR (AUCglucose: 7185 ± 401 vs. 8699 ± 401; p < 0.05). Treatment with BQ-788 decreased IR under IA (AUCglucose: 5281 ± 401, p < 0.05) and reduced worsening of IR with IH (AUCglucose: 7302 ± 401, p < 0.05). There was no effect of BQ-123 on IH-induced impairments in glucose tolerance or IR. Our results suggest that ET-1 plays a role in IH-induced impairments in glucose homeostasis.

Computational Modeling of Spatiotemporal Ca(2+) Signal Propagation Along Hepatocyte Cords

OBJECTIVE

The purpose of this study is to model the dynamics of lobular Ca(2+) wave propagation induced by an extracellular stimulus, and to analyze the effect of spatially systematic variations in cell-intrinsic signaling parameters on sinusoidal Ca(2+) response.

METHODS

We developed a computational model of lobular scale Ca(2+) signaling that accounts for receptor- mediated initiation of cell-intrinsic Ca(2+) signal in hepatocytes and its propagation to neighboring hepatocytes through gap junction-mediated molecular exchange.

RESULTS

Analysis of the simulations showed that a pericentral-to-periportal spatial gradient in hormone sensitivity and/or rates of IP3 synthesis underlies the Ca(2+) wave propagation. We simulated specific cases corresponding to localized disruptions in the graded pattern of these parameters along a hepatic sinusoid. Simulations incorporating locally altered parameters exhibited Ca(2+) waves that do not propagate throughout the hepatic plate. Increased gap junction coupling restored normal Ca(2+) wave propagation when hepatocytes with low Ca(2+) signaling ability were localized in the midlobular or the pericentral region.

CONCLUSION

Multiple spatial patterns in intracellular signaling parameters can lead to Ca(2+) wave propagation that is consistent with the experimentally observed spatial patterns of Ca(2+) dynamics. Based on simulations and analysis, we predict that increased gap junction-mediated intercellular coupling can induce robust Ca(2+) signals in otherwise poorly responsive hepatocytes, at least partly restoring the sinusoidally oriented Ca (2+) waves.

SIGNIFICANCE

Our bottom-up model of agonist-evoked spatial Ca(2+) patterns can be integrated with detailed descriptions of liver histology to study Ca(2+) regulation at the tissue level.

Endogenous ghrelin released in response to endothelin stimulates growth hormone secretion in cattle

The purpose of this study was to evaluate whether circulating ghrelin and growth hormone (GH) concentrations in cattle are regulated by endothelin-1 (ET-1), endothelin-3 (ET-3), and secretin. Six Holstein steers (242+/-1 d old, 280.5+/-4.4 kg body weight [BW]; mean+/-SEM) were allocated randomly in an incomplete Latin square design to receive each of 4 treatment compounds (vehicle, ET-1, ET-3, and secretin) with 1-d intervals between successive treatments. The treatment compounds were injected intravenously via a catheter inserted into the external jugular vein of each steer. Blood was sampled from the indwelling catheter at -30, -15, 0, 5, 10, 15, 20, 30, 45, 60, 90, 120, 150, and 180 min. Plasma ghrelin and GH responses to the treatment compounds were measured by a double-antibody radioimmunoassay system. Data were analyzed by using a MIXED procedure of SAS, version 9.1. Plasma acyl ghrelin, total ghrelin, and GH concentrations were increased by both ET-1 and ET-3 injection (ET-1 injection: 311+/-15 pg/mL vs 245+/-15 pg/mL, 2.4+/-0.2 ng/mL vs 1.61+/-0.05 ng/mL, 4.73+/-0.92 ng/mL vs 1.17+/-0.09 ng/mL for acyl ghrelin, total ghrelin, and GH, respectively; ET-3 injection: 337+/-27 pg/mL vs 245+/-15 pg/mL, 2.6+/-0.1 ng/mL vs 1.61+/-0.05 ng/mL, 5.56+/-0.97 ng/mL vs 1.17+/-0.09 ng/mL for acyl ghrelin, total ghrelin, and GH, respectively; P<0.01). Ghrelin and GH concentrations were not changed by secretin injection throughout the experimental periods. These results indicate that ET-1 and ET-3 stimulate ghrelin and GH secretion in cattle and demonstrate for the first time that endogenous ghrelin released in response to endothelin injection stimulates GH secretion in vivo in cattle.

Transcriptional regulation of vascular bone morphogenetic protein by endothelin receptors in early autoimmune diabetes mellitus

Endothelin (ET) and bone morphogenic proteins (BMP) have been implicated in the development of micro- and macrovascular complications of type 2 diabetes mellitus due to atherosclerosis. This study investigated vascular BMP-expression during early development of experimental autoimmune diabetes mellitus and whether ET(A) receptors are involved in its regulation, using the selective ET(A) receptor antagonist BSF461314. Specificity of BSF461314 was confirmed through ET-mediated p44/42 mitogen-activated protein kinase (ERK1/2) phosphorylation experiments. For animal studies, non-obese diabetic (NOD) and control mice at 16 weeks of age were treated with BSF461314 for 6 weeks. Plasma glucose levels were measured before and after treatment and vascular gene expression of BMP-2, BMP-7, and BMP-type II receptor was determined in the aorta by quantitative real-time polymerase chain reaction analysis. At the beginning of the study in all animals, plasma glucose levels were within the normal range. After 6 weeks gene expression of vascular BMP-2, BMP-7 and BMP-type II receptor was almost doubled in NOD mice compared with non-diabetic controls (p < 0.05). Concomitant treatment with BSF461314 significantly reduced expression of all BMPs and lowered plasma glucose levels in NOD mice close to controls (all p < 0.05 versus untreated). In conclusion, vascular BMP-2, BMP-7, and BMP-type II receptor expression is upregulated in early stages of autoimmune diabetes mellitus. The data further indicate that ET(A) receptors inhibit diabetes-associated activation of vascular BMPs and regulate plasma glucose levels suggesting that ET(A) receptors might provide a new therapeutic target to interfere with the early development of atherosclerosis in patients with type 1 diabetes mellitus.

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.

Ursodeoxycholic acid inhibits endothelin-1 production in human vascular endothelial cells

Endothelin-1 is known to be implicated in the pathogenesis of hepatobiliary diseases such as cirrhosis, especially in portal hypertension. This study aimed to investigate the effects of ursodeoxycholic acid on endothelin-1 production in human endothelial cells. The effects of ursodeoxycholic acid and its conjugates (tauroursodeoxycholic and glycoursodeoxycholic acids) on endothelin-1 production as well as nitric oxide (NO) in human umbilical vein endothelial cells (HUVECs) were examined. The production of endothelin-1 and nitric oxide in culture medium was measured using enzyme-linked immunosorbent assay (ELISA) and the Griess method, respectively. Endothelin-1 and endothelial nitric oxide synthase (eNOS) mRNA expression were investigated by real-time quantitative reverse transcriptase/polymerase chain reaction (RT-PCR). Ursodeoxycholic acid (30-1000 microM) inhibited endothelin-1 production in a concentration-dependent manner, and ursodeoxycholic acid at concentrations higher than 300 microM increased nitric oxide production in culture medium. The conjugates of ursodeoxycholic acid also increased nitric oxide production and decreased endothelin-1 production, which was less effective than ursodeoxycholic acid. N-nitro-L-arginine-mythel-ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, suppressed the ursodeoxycholic acid-induced nitric oxide production, but it did not antagonize the inhibitory effects of ursodeoxycholic acid on endothelin-1 production. Ursodeoxycholic acid also induced a concentration-dependent decrease in endothelin-1 mRNA expression without significant changes in eNOS mRNA expression. These results provide novel evidence that ursodeoxycholic acid inhibits endothelin-1 production in human endothelial cells, but nitric oxide is not responsible for the inhibitory effect of ursodeoxycholic acid on endothelin-1. Thus, ursodeoxycholic acid therapy may prevent the development of several pathogenesis such as portal hypertension observed in patients with cirrhosis due to the improvement of endothelial function.

Effects of endothelin-1 on hepatic stellate cell proliferation, collagen synthesis and secretion, intracellular free calcium concentration

AIM: To explore the effects of endothelin-1 (ET-1) on hepatic stellate cells (HSCs) DNA uptake, DNA synthesis, collagen synthesis and secretion, inward whole-cell calcium concentration ([Ca²⁺]_i) as well as the blocking effect of verapamil on ET-1-stimulated release of inward calcium (Ca²⁺) of HSC in vitro.

METHODS: Rat hepatic stellate cells (HSCs) were isolated and cultivated. ³H-TdR and ³H-proline incorporation used for testing DNA uptake and synthesis, collagen synthesis and secretion of HSCs cultured in vitro; Fluorescent calcium indicator Fura-2/AM was used to measure [Ca²⁺]_i inward HSCs.

RESULTS: ET-1 at the concentration of 5 × 10^-8 mol/L, caused significant increase both in HSC DNA synthesis (2247 ± 344 cpm, P < 0.05) and DNA uptake (P < 0.05) when compared with the control group. ET-1 could also increase collagen synthesis (P < 0.05 vs control group) and collagen secretion (P < 0.05 vs control group). Besides, inward HSC [Ca²⁺] _i reached a peak concentration (422 ± 98 mol/L, P < 0.001) at 2 min and then went down slowly to165 ± 51 mol/L (P < 0.01) at 25 min from resting state (39 ± 4 mol/L) after treated with ET-1. Verapamil (5 mol/L) blocked ET-1-activated [Ca²⁺]_i inward HSCs compared with control group (P < 0.05). Fura-2/AM loaded HSC was suspended in no Ca²⁺ buffer containing 1 mol/L EGTA, 5 min later, 10^-8 mol/L of ET-1 was added, [Ca²⁺]_i inward HSCs rose from resting state to peak 399 ± 123 mol/L, then began to come down by the time of 20 min. It could also raise [Ca²⁺]_i inward HSCs even without Ca²⁺ in extracellular fluid, and had a remarkable dose-effect relationship (P < 0.05). Meanwhile, verapamil could restrain the action of ET-1 (P < 0.05).

CONCLUSION: Actions of ET-1 on collagen metabolism of HSCs may depend on the transportation of inward whole-cell calcium.

Endothelin receptor antagonist TAK-044 arrests and reverses the development of carbon tetrachloride induced cirrhosis in rats

BACKGROUND AND AIMS

Hepatic concentrations of the powerful vasoconstrictor and fibrogen endothelin 1 (ET-1) and its receptors increase in human and experimental cirrhosis, suggesting a major role for ET-1 in the pathology of chronic liver disease. We investigated whether ET-1 receptor antagonism, after the development of fibrosis and cirrhosis, arrests/reverses the progression of chronic liver disease.

METHODS

Chronic liver injury was induced in rats by carbon tetrachloride (CCl(4)) treatment (0.15 ml/kg intraperitoneally twice a week) in conjunction with phenobarbital in drinking water (0.4 g/l) for four (group 1: fibrosis) and eight (group 2: cirrhosis) weeks. Rat were then treated concurrently with the ET-1 receptor antagonist TAK-044 (10 mg/kg/day) and CCl(4)/phenobarbital for a further four weeks.

RESULTS

Histopathological examination revealed significant arrest of progression to cirrhosis in group 1 and reversal of cirrhosis in group 2 rats. TAK-044 treatment caused significant amelioration of portal hypertension, systemic hypotension, and liver injury (reduced activities of serum aspartate aminotransferase, alanine aminotransferase, and lactate dehydrogenase), and improved hepatic synthetic capacity (increased serum albumin concentration) in both groups of rats relative to vehicle treated rats. TAK-044 treatment reduced collagen synthesis, as evidenced by decreased hepatic hydroxyproline content, mRNA expression of collagen-alpha type I, and tissue inhibitors of matrix metalloproteinases 1 and 2, and mRNA and protein expression of a potent fibrogenic cytokine, transforming growth factor beta1.

CONCLUSIONS

The results emphasise the role of ET-1 in the development of cirrhosis and strongly suggest that blockade of its actions can be a rational therapy for chronic liver disease and its complications.

Thapsigargin and EGTA inhibit endothelin-1-induced glucose transport

We have previously demonstrated that ET-1 may enhance glucose transport in 3T3-L1 adipocytes, secondarily to its stimulatory effect on GLUT1 gene expression by a mitogen-activated protein kinase (MAPK)-dependent pathway. In the present study, we further tested the involvement of Ca(2+) in glucose uptake in response to ET-1. Among a variety of Ca(2+)-related agents tested, EGTA and thapsigargin were found to suppress both the glucose uptake and intracellular Ca(2+) mobilization induced by ET-1, as determined by Fura-2 analysis. However, a phospholipase C inhibitor, U73122, also eliminated the intracellular calcium mobilization induced by ET-1, but had no effect on ET-1-stimulated glucose uptake. The finding that neither EGTA nor thapsigargin had any influence on ET-1-induced MAPK activation implies that some mechanism downstream of MAPK activation is involved. Further investigation showed that both agents exerted global inhibitory effects on protein and RNA syntheses. Since both thapsigargin and EGTA may deplete endoplasmic reticulum (ER) Ca(2+) stores, our results suggest that (1) ET-1-induced glucose transport is independent of ET-1's effect on Ca(2+) mobilization and (2) depletion of ER Ca(2+) stores per se may interfere with ET-1's effect on GLUT1 expression.

Endothelin and vascular function in liver disease

The endothelins are a group of three related peptides with two receptor subtypes, ET(A) and ET(B). Following the discovery of endothelin 1 as a potent vasoconstrictor, there has been intense interest in the role of endothelin on vascular function in liver disease. Speculation on the role of endothelin in the pathogenesis of acute renal failure, including hepatorenal syndrome, has also been speculated.

Chronic endothelin-1 treatment leads to insulin resistance in vivo

We determined whether chronic endothelin-1 (ET-1) treatment could lead to in vivo insulin resistance. Like insulin, ET-1 acutely stimulated glucose transport in isolated soleus muscle strips of WKY rats. ET-1 pretreatment (1 h) decreased insulin-stimulated glucose transport in muscle strips (-23%). Both ET-1-mediated effects were generated through ET(A) receptors, because a specific ET(A) receptor antagonist (BQ610) blocked these effects of ET-1. Osmotic minipumps were used to treat normal rats with ET-1 for 5 days. Subsequent hyperinsulinemic-euglycemic clamps showed that ET-1 treatment led to an approximately 30% decrease in insulin-stimulated glucose disposal rates in male and female rats. In addition, ex vivo study of soleus muscle strips showed decreased glucose transport into muscle from ET-1-treated animals. With respect to insulin signaling, chronic in vivo ET-1 treatment led to a 30-40% decrease in IRS-I protein content, IRS-I-associated p110(alpha), and AKT activation. In summary, 1) in vitro ET-1 pretreatment leads to decreased insulin-stimulated glucose transport in skeletal muscle strips; 2) chronic ET-1 administration in vivo leads to whole-body insulin resistance, with decreased skeletal muscle glucose transport and impaired insulin signaling; and 3) elevated ET-1 levels may be a cause of insulin resistance in certain pathophysiologic states.

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.

Hepatic stellate cells: role in microcirculation and pathophysiology of portal hypertension

Accumulating evidence suggests that stellate cells are involved in the regulation of the liver microcirculation and portal hypertension. Activated hepatic stellate cells have the necessary machinery to contract or relax in response to a number of vasoactive substances. Because stellate cells play a role in both fibrosis and portal hypertension, they are currently regarded as therapeutic targets to prevent and treat the complications of chronic liver disease.

Endothelin receptor remodeling induces the portal venous hyper-response to endothelin-1 following endotoxin pretreatment

The purpose of this study was to determine the changes in endothelin (ET) receptor subtype expression and their functional significance after endotoxin pretreatment. Rats were pretreated with lipopolysaccharide (LPS) or sterile saline (control). After 24 h, liver samples were homogenized and competitive receptor binding assays were performed. There was no significant difference in ET receptor binding affinity between the control and LPS groups. However, the receptor subtype density showed a significant increase in ET(B) receptors in LPS-treated rats, whereas the amount of ET(A) receptors was almost identical between the two groups. In control, almost all ET receptors (95%) were displaced by using combined ET(A) antagonist (BQ-610) and ET(B) agonist (IRL-1620) as competitors, whereas only 80% (P < 0.05 versus control) was displaced in LPS group, raising the possibility of novel type of ET receptor expression. An infusion of ET(B) agonist (Sarafotoxin 6c, S6c) through portal vein in isolated perfused livers produced the same pressure response in both LPS and control groups; however, the portal pressure increase in response to the ET-1, which binds all ET receptors, was significantly potentiated in LPS-treated rats compared to controls. We conclude that altered regulation of ET receptors, in particular, the appearance of ET binding capacity that is not displaced by ET(A) or ET(B) competitors, may explain the hyper-response of the portal venous system to ET-1 during endotoxemia.

Tissue- and time-dependent effects of endothelin-1 on insulin-stimulated glucose uptake

Hyperendothelinaemia is associated with various insulin-resistant states, e.g., diabetes, obesity and heart failure, but whether endothelin-1 (ET-1) has a direct effect on insulin-mediated glucose uptake is unclear because the interpretation of in vivo metabolic studies is complicated by ET-1 effects on muscle blood flow and insulin secretion. This study investigated the effects of ET-1 (1-10 nM) on basal and insulin-stimulated 2-deoxy-D-[3H]glucose (2-DOG) uptake in cultured L6 myoblasts and 3T3-adipocytes. RT-PCR analysis showed that both cell types express ET(A) but not ET(B) receptors. ET-1 had no effect on basal (non-insulin-mediated) glucose transport, but there was evidence of a tissue- and time-dependent inhibitory effect of ET-1 on insulin-stimulated glucose uptake. Specifically, ET-1 10 nM had a transient (0.5 h) inhibitory effect on glucose uptake in 3T3 cells (C(I-150) [dose of insulin required to increase glucose uptake by 50%, relative to control 100%] increased from 89 +/- 14 nM to 270 +/- 12 nM at 30 mins, P < 0.05) but no effect on insulin sensitivity in L6 myoblasts (C(I-150) was 56 +/- 14 nM [control], 43 +/- 14 [30 mins] and 26 +/- 16 [2 h]). In conclusion, the inhibitory effect of ET-1 on insulin-stimulated glucose uptake is transient and occurs in 3T3-L1 adipocytes but not skeletal muscle-derived cells, perhaps reflecting tissue differences in ET(A)-receptor signaling. It is therefore unlikely that chronic hyperendothelinaemia has a direct insulin-antagonist effect contributing to peripheral (ie muscle/fat) insulin resistance in vivo.

Endothelin-1 increases glucose transporter glut1 mRNA accumulation in 3T3-L1 adipocytes by a mitogen-activated protein kinase-dependent pathway

The mechanism of enhancing glucose transport by prolonged endothelin-1 (ET-1) treatment of 3T3-L1 adipocytes was examined. Western and Northern blot analyses indicated that ET-1 increased the amount of both GLUT1 protein and mRNA. The degradation rate of GLUT1 mRNA as measured in the presence of actinomycin D, nevertheless, was not significantly altered by ET-1. Whereas various inhibitors for distinct signalling pathways were tested, only the mitogen-activated protein kinase (MAPK) kinase inhibitor, PD98059, was found to decrease significantly the enhancing effect of ET-1. Similar extent of inhibition was observed in cells pretreated with pertussis toxin (PT). Immunoblot analysis revealed that ET-1 may stimulate a transient phosphorylation of p42/p44 MAPK and both PT and PD98059 inhibited this stimulation. In addition, the effect of ET-1 on GLUT1 mRNA accumulation was inhibited by PD98059 and cycloheximide, implying that a trans-activation was involved. Taken together, these results suggest that ET-1 may induce GLUT1 gene expression by a MAPK-dependent mechanism.

ATTENUATION OF ISCHEMIA AND REPERFUSION INJURY OF CANINE LIVERS BY INHIBITION OF TYPE II PHOSPHOLIPASE A2 WITH LY3297221

BACKGROUND

Membrane phospholipid breakdown, caused by ischemia and reperfusion (I/R) of the liver, releases free fatty acids including arachidonic acids and lysophospholipids, which serve as precursors of various inflammatory lipid derivatives. Phospholipase A2 (PLA2) is a key enzyme that initiates this reaction. In this study, we tested our hypothesis that a type II PLA2 inhibitor, LY329722, could attenuate hepatic I/R injury caused by a 2-hr total hepatic vascular exclusion (THVE) in dogs.

METHODS

Eighteen beagle dogs, subjected to a 2-hr THVE, were divided into three groups. Group 1 (n=6) was untreated and served as a control group. LY329722 was administered to animals in group 2 (n=6) intravenously (0.2 mg x kg(-1) x hr(-1)) for 60 min before ischemia, and to animals in group 3 (n=6) for 60 min starting 15 min before reperfusion (0.2 mg x kg(-1) x hr(-1)). Animal survival, systemic and splanchnic hemodynamics, hepatic tissue blood flow, liver functions, energy metabolism, hepatic venous thromboxane B2 and endothelin-1 levels, phospholipid levels and tumor necrosis factor-a mRNA expression in liver tissue, and histopathologic findings were evaluated.

RESULTS

Two-week animal survival was 33% (two of six) in group 1, and 100% (six of six) in groups 2 and 3. LY329722 improved systemic and splanchnic hemodynamics, hepatic tissue blood flow, and energy metabolism, reduced liver enzyme, thromboxane B2, and endothelin-1 release, prevented hepatic phospholipid degradation and tumor necrosis factor-alpha mRNA expression, and lessened histopathologic damage and the number of neutrophil infiltrating into the liver tissue.

CONCLUSION

The present study demonstrated that a type II PLA2 inhibitor, LY329722, attenuated hepatic I/R injury caused by a 2-hr THVE model in dogs.

Plasma endothelin-1 levels after stem cell transplantation

Acute renal failure and veno-occlusive disease of the liver are serious complications following stem cell transplantation (SCT) and contribute to the non-relapse mortality associated with this procedure. Endothelins, a family of vasoconstrictor peptides, may be involved in the pathogenesis of a variety of renal and hepatic diseases, including CsA-associated hypertension and the hepatorenal syndrome. In order to study the relevance of endothelins to SCT-related liver and kidney dysfunction, we determined endothelin-1 (ET-1) levels in plasma samples obtained from 65 patients (38 autologous, 27 allogeneic) 7 days before and 7, 14 and 28 days after SCT. A steady increase in plasma ET-1 was observed after SCT (5.36 pg/ml, 95% CI 4.30-6.43 on day +28 vs 3.82 pg/ml, 95% CI 3.21-4.43 on day -7; P = 0.020). No differences in ET-1 levels existed between autologous and allogeneic SCT recipients at any of the time points studied (P = 0.561). In addition, no significant differences were observed among patients with renal dysfunction vs those without (P = 0.187), nor in patient groups with or without hepatic dysfunction (P = 0.075). In conclusion, even though plasma ET-1 levels showed a steady increase following SCT, no correlation could be found with development of SCT-related kidney or liver dysfunction.

Enhanced synthesis and reduced metabolism of endothelin-1 (ET-1) by hepatocytes--an important mechanism of increased endogenous levels of ET-1 in liver cirrhosis

BACKGROUND/AIMS

Hepatic concentration of endothelin-1 (ET-1) is increased in human and experimental liver cirrhosis. Because of its potent actions in the liver, ET-1 has been suggested to play an important role in the pathophysiology of cirrhosis. Since hepatocytes are the major cell type to metabolize ET-1, we investigated whether their reduced capacity to degrade ET-1 is a mechanism of its elevated levels in cirrhosis.

METHODS

The expression of ET-1 receptors, ET-1 and endothelin converting enzyme (ECE), and metabolism of ET-1 and ECE activity were compared in hepatocytes isolated from control and carbon tetrachloride-induced cirrhotic rats.

RESULTS

ET-1 receptor density and receptor-mediated internalization of ET-1 were significantly increased in cirrhotic hepatocytes relative to the control cells. However, compared to control hepatocytes, metabolism of ET-1 by the cirrhotic cells was reduced significantly. Interestingly, hepatocytes were found to contain preproET-1 mRNA, ECE-1 mRNA and ET-1. PreproET-1 mRNA and ET-1 levels were increased in cirrhotic hepatocytes but their ECE mRNA and ECE activity were not altered.

CONCLUSIONS

These results provide the first evidence that hepatocytes have the ability to synthesize ET-1 and demonstrate that decreased metabolism and enhanced synthesis, of ET-1 in hepatocytes are an important mechanism of its elevated levels in cirrhosis.

Effects of endothelin-1 and nitric oxide on glucokinase activity in isolated rat hepatocytes

To test the hypothesis that endothelin-1 (ET-1) and nitric oxide (NO) influence glucokinase (GK) activity in an opposite manner, we evaluated the effects of ET-1, L-NAME, an inhibitor of NO synthase, and L-arginine, a substrate for NO synthase, on GK activity and glycogen content in isolated rat hepatocytes. Moreover, to understand the receptor involved in the process, the effects of BQ 788, a specific antagonist of ETB receptor, and PD 142893, an antagonist of ETA-ETB receptors, were also evaluated. GK activity, cyclic guanosine monophosphate (cGMP), and glycogen intracellular content were measured on isolated hepatocytes, while glucose levels and NO as NO2-/NO3- were determined in the medium. High ET-1 levels induced a 20% decrease of NO2-/NO3- levels and cGMP intracellular content, followed by a 49% reduction of GK activity and a 15% decrease of glycogen. In parallel, a 10% increase of glucose in the medium was observed. In the presence of L-NAME, GK activity and glycogen levels showed analogous decrements as observed with ET-1. Also in this case, a significant decrease of the intracellular content of cGMP was observed. No synergistic effects of ET-1 and L-NAME were observed. L-Arginine was able to counteract the inhibitory effect of ET-1 on cGMP and GK activity. Glycogen content was slightly but not significantly reduced, and under those conditions, a significant decrease of glucose in the medium was observed. When hepatocytes were incubated with ET-1 plus BQ 788 or ET-1 plus PD 142893, GK activity was unchanged. Interestingly, no changes were observed in NO2-/NO3- levels and the intracellular content of cGMP was not modified when the antagonists of ET-1 receptors were added to the medium. In conclusion, the present study shows that the NO pathway seems to be an important regulator of GK activity and glycogen content through cGMP activity. In addition, ET-1 seems to be not active per se, but its activity seems mediated by a simultaneous decrease of NO levels.

Kinetics of endothelin-1 binding in the dog liver microcirculation in vivo

Endothelin-1 (ET-1) is a 21-amino acid peptide produced by vascular endothelial cells that acts as a potent constrictor of hepatic sinusoids. Hepatic binding of tracer (125)I-labeled ET-1 was investigated in anesthetized dogs with the multiple-indicator dilution technique with simultaneous measurements of unlabeled immunoreactive ET-1 plasma levels. Despite 80% binding to albumin, tracer (125)I-ET-1 was avidly extracted by the liver, with only 15 +/- 6% of the peptide surviving passage through the organ. Exchange of ET-1 between plasma and binding sites, probably located on the surface of liver cells, was quantitatively described by a barrier-limited, space-distributed variable transit time model. Reversible and irreversible parallel binding sites were found. Reversible and irreversible plasma clearances of unbound (125)I-ET-1 were 0.084 +/- 0.033 ml. s(-1). g liver(-1) and 0.17 +/- 0.09 ml. s(-1). g liver(-1), respectively, and the dissociation rate constant for reversible binding was 0.24 +/- 0.12 s(-1). The specific ET(A) receptor antagonist BMS-182874 did not modify binding to either site. The nonspecific ET(A)/ET(B) antagonist LU-224332 dose-dependently reduced irreversible binding only. ET-1 levels in the hepatic vein were significantly lower than in the portal vein but were not different from those in the hepatic artery. The ratio between hepatic vein and portal vein levels (0.64 +/- 0.31) was considerably higher than survival fractions, suggesting a substantial simultaneous release of newly synthesized or stored ET-1 by the liver. These results demonstrate both substantial clearance and production of ET-1 by the intact liver. Hepatic ET-1 clearance is mediated by the ET(B) receptor, with the presence of reversible, nonspecific ET-1 binding at the liver surface

Receptor subtype mediating the action of circulating endothelin on glucose metabolism and hemodynamics in perfused rat liver

The subtype of endothelin receptor that mediates metabolic and hemodynamic effects of circulating endothelin was explored using perfused rat liver. Infusion of endothelin (ET)-1 or ET-3 into the portal vein at a concentration of 0.3 nM increased glucose and lactate output and decreased perfusion flow, although ET-3 was less effective than ET-1. The metabolic effects of ET-1 were observed even under costant-flow perfusion. Infusion of either sarafotoxin S6b or S6c, an ET(A)- or ET(B)-receptor agonist, mimicked the actions of ET-1 to an equal extent. The flow reduction and glucose production induced by ET-1 were partly attenuated by the ET(A)-receptor antagonist BQ485. By contrast, ET(B)-receptor antagonist BQ788 enhanced glucose production caused by ET-1 and ET-3 without affecting the hemodynamic change. The effects of ET-1 and ET-3 were almost totally inhibited by the combination of BQ485 and BQ788. These results suggest that both ET(A) and ET(B) receptors are involved in the metabolic and hemodynamic effects of circulating endothelin in rat liver, while the ET(A)-receptor-mediated action appears to be dominant.

Biological effects of C-type natriuretic peptide in human myofibroblastic hepatic stellate cells

During chronic liver diseases, hepatic stellate cells (HSC) acquire a myofibroblastic phenotype, proliferate, and synthetize fibrosis components. Myofibroblastic HSC (mHSC) also participate to the regulation of intrahepatic blood flow, because of their contractile properties. Here, we examined whether human mHSC express natriuretic peptide receptors (NPR). Only NPR-B mRNA was identified, which was functional as demonstrated in binding studies and by increased cGMP levels in response to C-type natriuretic peptide (CNP). CNP inhibited mHSC proliferation, an effect blocked by the protein kinase G inhibitor 8-(4 chlorophenylthio)-cGMP and by the NPR antagonist HS-142-1 and reproduced by analogs of cGMP. Growth inhibition was associated with a reduction of extracellular signal-regulated kinase and c-Jun N-terminal kinase and with a blockade of AP-1 DNA binding. CNP and cGMP analogs also blunted mHSC contraction elicited by thrombin, by suppressing calcium influx. The relaxing properties of CNP were mediated by a blockade of store-operated calcium channels, as demonstrated using a calcium-free/calcium readdition protocol. These results constitute the first evidence for a hepatic effect of CNP and identify mHSC as a target cell. Activation of NPR-B by CNP in human mHSC leads to inhibition of both growth and contraction. These data suggest that during chronic liver diseases, CNP may counteract both liver fibrogenesis and associated portal hypertension.

Down-regulation of endothelin receptors by transforming growth factor beta1 in hepatic stellate cells

BACKGROUND/AIMS

Hepatic endothelin-1 (ET-1) receptor density as well as the levels of both ET-1 and transforming growth factor beta1 (TGF-beta1) increase in liver cirrhosis. Considering their potent contractile (ET-1) and fibrogenic (TGF-beta1) actions on myofibroblastic stellate cells found in the fibrotic/cirrhotic liver, we aimed to investigate the effects of TGF-beta1 on ET-1 receptors and ET-1 synthesis in these cells.

METHODS

Stellate cells isolated from rat liver by enzymatic digestion were cultured and subjected to TGF-beta1 treatment. Cellular ET-1 receptors and ET-1 released in the medium were determined.

RESULTS

TGF-beta1 treatment produced time- and dose-dependent decrease in ET-1 binding sites, but did not affect the affinity of the receptors for ET-1. TGF-beta1 also stimulated the release of ET-1 from stellate cells. The extent of TGF-beta1-induced inhibition of [125I]ET-1 binding was much greater for ETB subtype (73+/-18% inhibition), which comprised a major portion (78+/-12%) of the total ET-1 receptors, than for ETA subtype (35+/-11% inhibition). The mRNA expression of the ET-1 receptors also was reduced by TGF-beta1 treatment. TGF-beta1-induced reduction in ET-1 receptor density was coupled to the inhibition of ET-1-stimulated release of [3Hlarachidonic acid from the prelabeled cells. The effects of TGF-beta1 were inhibited by a TGF-beta1 neutralizing monoclonal antibody.

CONCLUSIONS

These results suggest that the TGF-beta1-induced decrease in ET-1 receptor density may be an important mechanism in limiting the pathologic actions of ET-1 on stellate cells in chronic liver disease.

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.

Endothelin-1 inhibits secretin-stimulated ductal secretion by interacting with ETA receptors on large cholangiocytes

We studied the expression of endothelin-1 (ET-1) receptors (ETA and ETB) and the effects of ET-1 on cholangiocyte secretion. The effects of ET-1 on cholangiocyte secretion were assessed in normal and bile duct-ligated (BDL) rats by measuring 1) basal and secretin-induced choleresis in vivo, 2) secretin receptor gene expression and cAMP levels in small and large cholangiocytes, and 3) luminal expansion in response to secretin in intrahepatic bile duct units (IBDU). ETA and ETB receptors were expressed by small and large cholangiocytes. ET-1 had no effect on basal bile flow or bicarbonate secretion in normal or BDL rats but decreased secretin-induced bicarbonate-rich choleresis in BDL rats. ET-1 decreased secretin receptor gene expression and secretin-stimulated cAMP synthesis in large cholangiocytes and secretin-induced luminal expansion in IBDU from normal or BDL rats. The inhibitory effects of ET-1 on secretin-induced cAMP synthesis and luminal duct expansion were blocked by specific inhibitors of the ETA (BQ-610) receptor. ET-1 inhibits secretin-induced ductal secretion by decreasing secretin receptor and cAMP synthesis, two important determinants of ductal secretion.

Varying very low-density lipoprotein secretion of rat hepatocytes by altering cellular levels of calcium and the activity of protein kinase C

BACKGROUND

Calcium antagonists lower plasma levels of lipoproteins and suppress hepatic very low-density lipoprotein (VLDL) secretion. Similar effects have been observed with the calcium ionophore A23187. We studied further the effect of calcium on VLDL metabolism.

METHODS

Hepatocytes from male Wistar rats were isolated and cultured in the presence or absence of calcium-mobilizing hormones, or compounds that either stimulate or inhibit the activity of protein kinase C. Secreted VLDL (d < 1.006 g mL-1) was isolated by centrifugation (145,000 x g), and lipids and apolipoprotein B were analysed.

RESULTS

VLDL secretion reached maximum in hepatocytes cultured in medium containing calcium 0.8-2.4 mmolL-1. Depleting the cells of calcium by incubating in calcium-free medium or by treating the cells with the Ca(2+)-ATPase inhibitor thapsigargin (5 x 10-7 molL-1) suppressed lipid secretion to less than 15% of control, and this was accompanied by an increase in cellular levels of triacylglycerol. Calcium loading (medium calcium > 2.4 mmolL-1) suppressed both lipoprotein secretion and cellular levels of lipids, suggesting a reduced overall rate of lipid synthesis. At an extracellular calcium concentration of 0.8 mmolL-1, angiotensin II, vasopressin, endothelin-1 (10(-7) molL-1) or phenylephrine (10(-4) molL-1) suppressed VLDL secretion (maximum to 37% of control), and elevated medium calcium attenuated this effect. The protein kinase C inhibitor chelerythrine (5 x 10(-5) molL-1) and the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) (10(-6) molL-1), suppressed VLDL secretion to 18% and 60% of control, respectively, whereas the protein kinase C-inactive 4 alpha-PMA was without an effect. No effect on ketogenesis was observed by these compounds, indicating that suppressed lipid secretion was not due to an enhanced oxidation of lipids.

CONCLUSIONS

Hepatic VLDL secretion can be related to changes in hepatocyte levels of calcium and the activity of protein kinase C.

Endothelin 1 aggravates acute liver injury in perfused livers of rats after treatment with D-galactosamine

The effects of endothelin 1 (ET-1) on hemodynamics and acute liver damage were studied using perfused livers of rats treated with D-galactosamine. In control liver perfused in situ with constant pressure, infusion of ET-1 into the portal vein at a concentration of 0.1 nmol/L decreased the flow rate without a significant leakage of lactate dehydrogenase (LDH) or aspartate transaminase (AST) into the effluent. In contrast, in similarly perfused liver 24 hours after treatment with D-galactosamine (800 mg/kg intraperitoneally), ET-1 caused rapid and remarkable increases in the leakage of LDH and AST from the liver accompanied by the reduction of perfusion flow to the extent similar to that observed in control livers. In addition, ET-1 decreased oxygen uptake and bile secretion in galactosamine-treated livers. The potentiating effects of ET-1 on enzyme leakage were also observed under constant flow conditions. Moreover, infusion of the thromboxane A2 analogue at a concentration of 10 nmol/L decreased the flow rate markedly, yet the rapid increases in enzyme leakage were not observed. Infusion of ET-3 induced the responses of flow reduction and the potentiation of rapid enzyme leakage similar to those obtained with ET-1. Neither the endothelin A-receptor antagonist BQ485 nor the endothelin B-receptor antagonist BQ788 could inhibit the acute liver damage caused by ET-1; instead they exaggerated its effects. The combination of both antagonists together, however, almost completely suppressed the flow reduction and the potentiation of enzyme leakage caused by ET-1. These results indicate that ET-1 is capable of aggravating acute liver damage not merely through reduction of the flow rate but through direct action on liver cells. They also suggest that both the endothelin A and endothelin B receptors are involved in this action of ET-1.

Endothelin-1 is synthesized and inhibits cyclic adenosine monophosphate- dependent anion secretion by an autocrine/paracrine mechanism in gallbladder epithelial cells

Ion and fluid transport across the biliary epithelium contributes to bile secretion. Since endothelin (ET)-1 affects ion transport activities and is released by human gallbladder- derived biliary epithelial cells in primary culture, we examined the expression of ET peptides and ET receptors and the influence of ET-1 on ion transport in this epithelium ex vivo. In freshly isolated gallbladder epithelial cells, preproET-1, -2, and -3 mRNAs were detected by reverse transcription PCR and ET-1 isopeptide was identified by chromatography. The cells also displayed ET receptor mRNAs and high-affinity binding sites for ET-1, mostly of the ETB type. Electrogenic anion secretion across intact gallbladder mucosa was stimulated by forskolin, secretin, and exogenous ATP, as assessed by short-circuit current (Isc) increases in Ussing-type chambers. ET-1 inhibited forskolin- and secretin-induced changes in Isc, without affecting baseline Isc or ATP-induced changes. Accordingly, ET-1 significantly reduced the accumulation of intracellular cAMP elicited by forskolin and secretin in the epithelial cells, and this effect was abolished by pertussis toxin. This is the first evidence that ET-1 is synthesized and inhibits, via a Gi protein-coupled receptor, cAMP-dependent anion secretion in human gallbladder epithelium, indicating a role in the control of bile secretion by an autocrine/paracrine mechanism.

Is captopril-induced improvement of insulin sensitivity mediated via endothelin?

Angiotensin-converting enzyme (ACE) inhibitors have been reported to improve insulin sensitivity during either short-term or long-term administration. Recent studies indicate that endothelin-1 (ET-1) has potent glycogenolytic effects in rat hepatocytes and may cause insulin resistance in rat adipocytes. In addition, ET may also have a role in stimulation of the hypothalamic-pituitary-adrenal axis. To test the hypothesis that part of the effect of captopril in enhancing insulin sensitivity may be mediated via ET and/or by glucocorticoids, we measured 24-h urinary excretion of ET and free cortisol before and after short-term treatment with captopril. The 24-h urinary immunoreactive endothelin (IR-ET) excretion decreased significantly (p < 0.05) from 65 +/- 4 ng at baseline to 42 +/- 3 ng after captopril treatment, whereas no significant change in the 24-h urinary free cortisol excretion was observed. Moreover, no significant change in the 24-h urinary IR-ET and free cortisol excretions was noted in the placebo-treated group. We speculate that ACE inhibitors may exert their effect on insulin sensitivity not only by blocking the renin-angiotensin and kinin systems but also by inhibiting production and/or release of ET.

Endothelin-stimulated nitric oxide production in the isolated Kupffer cell

Endothelin (ET) is a potent peptide mediator exhibiting a wide variety of effects in both the parenchymal and nonparenchymal hepatic cells. In the Kupffer cell, ET activates several transmembrane signaling pathways to generate numerous second messengers including the phospholipase C-generated products inositol-1,4,5-trisphosphate and diacylglycerol and the cyclooxygenase product prostaglandin E2 via specific ETB-type receptors. In addition to these findings, we have now demonstrated that endothelin stimulates the production of nitric oxide (NO) in the Kupffer cell in a time- and concentration-dependent manner. Western blot analysis indicates that ET-stimulated NO production occurs though activation of the inducible form of the nitric oxide synthase enzyme. These findings have important implications as the stimulation of NO production by ET may be part of the physiological response to inflammation or infection. Elevated levels of ET and NO have been found to be associated with numerous hepatic pathophysiological conditions that may contribute to derangements in the vascular system seen in these conditions.

Attenuation of ischemic liver injury by monoclonal anti-endothelin antibody, AwETN40

BACKGROUND

Enhanced production of endothelin-1 (ET-1), vasoconstrictive 21 amino acids produced by endothelial cells during ischemia and after reperfusion of the liver, is known to cause sinusoidal constriction and microcirculatory disturbances, which lead to severe tissue damage. Using a 2-hour hepatic vascular exclusion model in dogs, we tested our hypothesis that neutralization of ET-1 by monoclonal anti-ET-1 and anti-ET-2 antibody (AwETN40) abates vascular dysfunction and ameliorates ischemia/reperfusion injury of the liver.

STUDY DESIGN

After skeletonization, the liver was made totally ischemic by cross-clamping the portal vein, the hepatic artery, and the vena cava (above and below the liver). Veno-venous bypass was used to decompress splanchnic and inferior systemic congestion. AwETN40, 5 mg/kg, was administered intravenously 10 minutes before ischemia (treatment group, n = 5). Nontreated animals were used as controls (control group, n = 10). Animal survival, hepatic tissue blood flow, liver function tests, total bile acid, high-energy phosphate, ET-1 levels, and liver histopathology were studied.

RESULTS

Treatment with AwETN40 improved 2-week animal survival from 30% to 100%. Hepatic tissue blood flow after reperfusion was significantly higher in the treatment group. The treatment significantly attenuated liver enzyme release, total bile acid, and changes in adenine nucleotides. Immunoreactive ET-1 levels in the hepatic venous blood of the control group showed a significant increase and remained high for up to 24 hours after reperfusion. Histopathologic alterations were significantly lessened in the treatment group.

CONCLUSIONS

These results indicate that ET-1 is involved in ischemia/reperfusion injury of the liver, which can be ameliorated by the monoclonal anti-ET-1 and anti-ET-2 antibody AwETN40.

Hepatoprotective effect of a nonselective endothelin receptor antagonist (TAK-044) in the transplanted liver

This study was designed to investigate whether or not a novel nonselective endothelin A/B (ETA/ETB) receptor antagonist (TAK-044) provides hepatoprotection during porcine liver transplantation. The grafts were stored in chilled Euro-Collins solution and recirculated following reflush with lactated Ringer's with (TAK group) or without (control group) TAK-044 (10 mg/kg). Intracellular (cytoplasma, mitochondria, and nucleus) calcium (Ca) concentrations were measured in the hepatic biopsy materials obtained serially at varying time point from donor laparotomy to recipient closure using an electron probe X-ray microanalyzer. Liver function tests also were determined. The cold and warm ischemia times of the grafts were comparable between the two groups. The peak endothelin-1 T-1) concentration after recirculation was significantly higher in the TAK group than in the control group (129 +/- 30 pg/ml vs 26 +/- 6.5 pg/ml). However, release of liver enzymes, increases in total bile acid, and deterioration of indocyanine green retention rate were significantly suppressed in the TAK group. In the control group, the intracellular Ca concentrations, especially in the mitochondrial fraction, were elevated markedly following recirculation of the hepatic arterial flow. In the TAK group, this effect was suppressed. Thus, the supplementary use of the nonselective ETA/ETB receptor antagonist TAK-044 via a rinse route may alleviate an early postreperfusion microcirculatory disturbance of the liver grafts without adverse effects by the increased ET-1 on the systemic circulation.

Endothelin-1 induces liver vasoconstriction through both ETA and ETB receptors

BACKGROUND/AIMS

We investigated which endothelin receptors mediate the vasoconstrictive effects of endothelin-1 on liver circulation.

METHODS

An isolated perfused rat liver model in recirculation was used.

RESULTS

The perfusion of 10(-10) M endothelin-1 had no significant influence on the liver flow, whereas 10(-9) M endothelin-1 induced significant vasoconstriction, with flow dropping from 3.20 +/- 0.34 to 1.48 +/- 0.28 ml. min-1.g-1 liver tissue (p < 0.01 vs controls). The liver flow was interrupted following the perfusion of 10(-8) M endothelin-1. Sarafatoxin C and BQ 3020, two agonists of ETB receptor, had vasoconstrictive effects in this model. Sarafatoxin C decreased the liver flow in a dose-dependent manner, from 3.32 +/- 0.21 to 2.18 +/- 0.20, 1.60 +/- 0.09, and 1.01 +/- 0.06 ml.min-1. g-1, respectively, with 10(-9) M, 10(-8) M, and 10(-7) M. While BQ 123, an antagonist of ETA receptor, or BQ 788, an antagonist of ETB receptor, partially reversed the effect of 10(-9) M endothelin-1, the simultaneous administration of BQ 123 and BQ 788 completely reversed these effects.

CONCLUSIONS

These results indicate that the vasoconstrictive effects of endothelin-1 on the liver circulation are mediated through both ETA and ETB receptors.

Endothelin-1 levels in portal venous blood in relation to hepatic tissue microcirculation disturbance and hepatic cell injury after ischemia/reperfusion

This study was conducted to clarify the role of endothelin-1 in the portal vein after hepatic ischemia/ reperfusion and to ascertain whether it is related to hepatic microcirculation disturbance. Using a canine ischemic liver model, the portal and systemic endothelin-1 levels were measured before ischemia, then after 1 h and 2 h of reperfusion, and comparatively evaluated with the serum levels of GOT and lactic dehydrogenase (LDH). As an indicator of liver tissue microcirculation, tissue blood flow volume (TBF) was also measured in the site subjected to ischemia. The animals were divided into: group 1, which received ischemia for 30 min; group 2, which received ischemia for 60 min; and group 3, which received a sequence repeated four times of 15 min ischemia and 10 min reperfusion. The portal endothelin-1 level became significantly elevated after reperfusion compared to that before ischemia in all groups, being significantly higher in group 2 than in the other groups. The systemic endothelin-1 level also increased after reperfusion; significantly in group 2. The portal endothelin-1 level was generally higher than the systemic level, which again was statistically significant in group 2. After 2 h of reperfusion, a significant positive correlation was found between the portal endothelin-I level and serum LDH, whereas a significant negative correlation was found between the portal endothelin-1 level and TBF. The finding that the portal endothelin-1 level became elevated after hepatic ischemia/reperfusion suggests that it probably plays an essential role in hepatic ischemia/ reperfusion injury by adversely influencing tissue microcirculation.

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.

Interaction of endothelin-1 with cloned bovine ETA receptors: biochemical parameters and functional consequences

This paper defines the properties of interaction of endothelin-1 (Et-1) with cloned bovine ETA receptors. The Kd value of Et-1/ETA receptor complexes was estimated in membrane preparations to 20 pM using kinetic experiments and saturation experiments performed under quasi equilibrium conditions. Competition experiments yield a wide range of apparent Kd(Et-1) values from 20 pM to 1 nM which were in fact measures of the receptor concentrations rather than of Kd values. This resulted from the fact that complex second-order rate kinetics rather than pseudo-first-order kinetics control the association of Et-1 to its receptor when the receptor concentration is larger than Kd(Et-1). Et-1 induced a production of inositol phosphates with an apparent affinity of 2.3 nM, 100 times higher than the Kd(Et-1) value determined previously. Numerical simulation suggested that under time-limited conditions, sub-nanomolar rather than picomolar concentrations of Et-1 are necessary to occupy an important fraction of picomolar sites. It is concluded that bovine ETA receptors have a single affinity state for Et-1 (Kd = 20 pM) and that this affinity state can account for nanomolar actions of Et-1 in intact cells. It is suggested that the sensitivity of a preparation to Et-1 is a cell property rather than a receptor property. It is also suggested that the main advantage of high-affinity Et-1 binding is to promote autocrine actions rather than a high potency of the peptide.

Endothelins in chronic liver disease

This review describes recent progress in the accumulation of knowledge about the endothelins (ETs), a family of vasoactive 21-amino acid polypeptides, in chronic liver disease. Particular prominence is given to the dynamics of ET-1 and ET-3 and their possible relation to the disturbed circulation and neurohumoral dysregulation found in cirrhosis. Recent studies have shown that the ET system is highly activated in most cirrhotic patients. Circulating ET-1 and ET-3 levels have a positive relation to the severity of the disease and fluid retention, with the highest values recorded in patients with functional renal failure. Studies on liver biopsies have revealed synthesis of ET-1 in hepatic endothelial and other cells, and recent investigations have identified the hepatosplanchnic system as a major source of ET-1 and ET-3 spillover into the circulation, with a direct relation to portal venous hypertension. In addition, marked associations with disturbance of systemic haemodynamics and with abnormal distribution of blood volume have been reported. Although the pathophysiological importance of the ET system in chronic liver disease is not completely understood, similarities to other vasopressive and antinatriuretic regulatory systems (i.e. the sympathetic nervous system, renin-angiotensin-aldosterone and vasopressin) are apparent, with respect to kinetics and haemodynamic dysregulation. Cirrhosis seems to be a pathophysiological condition with indications of the occurrence of ETs, not only as local modulators, but also as a system with potential importance for systemic regulation.

Altered endothelin homeostasis in patients undergoing liver transplantation

The liver is a major site of synthesis, clearance, and actions of the powerful vasoactive peptide endothelin-1 (ET-1). We investigated the role of the liver in ET-1 homeostasis by comparing circulating and hepatic ET-1 levels and hepatic ET receptors in patients undergoing orthotopic liver transplantation (OLTx) for end-stage liver disease (ESLD) with those in patients undergoing liver resection for focal lesions with otherwise normal hepatic synthetic function. Central venous and radial arterial blood was drawn immediately after induction of anesthesia (point I), 10 minutes before beginning of resection or the anhepatic stage (point II), and 30 minutes after completion of resection or reperfusion of the grafted liver (point III). Portal and hepatic venous blood was drawn at points II and III. Plasma ET-1 levels were higher in ESLD patients than in resection patients. Plasma ET-1 levels rose both during resection and transplantation; the increase in ET-1 was more pronounced during transplantation. In ESLD patients, hepatic venous ET-1 was higher than portal venous ET-1, suggesting reduced clearance and/or enhanced synthesis of the peptide in the cirrhotic liver. Conversely, hepatic venous ET-1 was lower than portal venous ET-1 in resection patients at all time points and at point III in the ESLD patients. Hepatic concentration of ET-1 was greater and the capacity of the liver to catabolize ET-1 was reduced in ESLD patients as compared to the resection patients. Further, hepatic ET receptor density was higher in ESLD than in resection patients. These results suggest that the cirrhotic liver may contribute to elevated plasma ET-1 in ESLD. Considering its potent hemodynamic and metabolic effects in the liver, increased hepatic ET-1 and ET receptors and plasma ET-1 could play a role in the pathophysiology of liver disease and perioperative complications of OLTx.

Role of the paracrine liver endothelin system in the pathogenesis of CCl4-induced liver injury

This study analyzed if the paracrine liver endothelin system participates in the pathogenesis of CCl4-induced hepatotoxicity. Wistar Kyoto rats were divided into four groups: a bosentan (mixed endothelin ETA and ETB receptor antagonist) treated group with CCl4 intoxication, a vehicle treated group with CCl4 intoxication, a nontreated control group and a bosentan treated control group. Hepatotoxicity was assessed by determination of alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH) followed by histopathological examinations. Tissue endothelin-1 concentrations and expression of endothelin receptor subtypes were analyzed. The tissue levels of endothelin-1 in the liver of rats with CCl4 intoxication were significantly higher than those in normal rats. Scatchard analysis revealed no differences in the density and binding constant of endothelin ETA and ETB receptor between rats with CCl4 intoxication and controls. Bosentan treatment of rats undergoing CCl4 inhalation resulted in a significant protection against elevation of ALT, AST, LDH and bilirubin. Histopathological examination of live sections for necrotic, swollen and lipid-laden cells revealed findings that were in agreement with the serum enzyme data. In conclusion, this study showed that the paracrine endothelin system is involved in the pathogenesis of CCl4-induced hepatotoxicity and that the blockade of the stimulated liver endothelin systems reduces CCl4-induced liver injury.

Inhibition of cyclic AMP accumulation by endothelin is pertussis toxin sensitive and calcium independent in isolated adult feline cardiac myocytes

The aims of this study were to determine whether endothelin-1 (ET-1), a positive inotropic agent, altered the production of cyclic AMP (cAMP) in adult feline cardiac myocytes and to characterize the effect with respect to G-protein-coupling and calcium regulation of adenylyl cyclase. ET-1 inhibited basal and/or stimulated cAMP accumulation in the intact cardiac myocyte and in membrane preparations in a dose-dependent manner. In intact cells, maximal inhibition of forskolin-stimulated cAMP accumulation was 90-95% with an EC50 of 5 x 10(-10) M. Inhibition of isoproterenol-stimulated cAMP was biphasic with maximal inhibition of 70% observed by 10(-11)M; at higher doses inhibition was not consistently observed. The inhibitory response to ET-1 occurred in the absence or presence of isobutylmethylxanthine suggesting that activation of cAMP phosphodiesterases was not the means for reducing cAMP levels. Prior exposure of cardiac myocytes to 100ng/ml pertussis toxin blocked the inhibitory action of ET-1, indicating that this response is mediated through the involvement of a pertussis toxin-sensitive G-protein such as Gi. Studies carried out in the absence of extracellular calcium and under conditions of cell-loading with the intracellular calcium chelator, 1,2-bis-(2-aminophenoxy)-ethane-N,N,N'N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM), suggest that the mechanism by which ET-1 inhibits cAMP accumulation is not calcium-dependent. Thus, inhibition of cAMP accumulation by ET-1 appears to be mediated through a pertussis toxin sensitive protein rather than by activation of phosphodiesterases or calcium inhibition of cardiac forms of adenylyl cyclase. Though unlikely to play a role in the positive inotropic effect of ET-1, transduction of ET-1 responses through Gi suggests another means for regulation of growth in these adult cardiac myocytes.