Accelerated reversal of carbon tetrachloride-induced cirrhosis in rats by the endothelin receptor antagonist TAK-044

Differential recruitment of monocyte-derived macrophages in control and stellate cell-depleted mice during recurrent carbon tetrachloride-induced acute liver injury

Liver depleted of hepatic stellate cells (HSCs) is resistant to ischemia/reperfusion-, concanavalin A-, and acetaminophen-induced acute injury. Whether HSCs regulate carbon tetrachloride (CCl4 )-induced acute liver injury is not known. CCl4 treatment damages pericentral hepatocytes that express CCl4 -metabolizing Cyp2E1 and activates HSCs. We investigated whether HSC-depletion in mice transgenic for thymidine kinase under the glial fibrillary acidic protein promoter (GFAP-TK-Tg) confers resistance to injury and inflammation due to CCl4 rechallenge. GFAP-TK-Tg or wild type (WT) mice were administered 0.16 ml/kg CCl4 (3× at 3 days intervals), then 40 μg/g/day ganciclovir for 10 days. The treatment depletes ~70%-75% HSCs from GFAP-TK-Tg but not WT mice while the liver recovers from earlier CCl4 -induced injury. Mice were then administered CCl4 , and liver injury and inflammation were determined at 24 h. HSC-depleted and HSC-sufficient mice showed similar CCl4 -induced hepatocyte necrosis and oxidative stress. However, increase in F4/80+ macrophages, but not CD68+ cells, was greater in CCl4 rechallenged HSC-depleted compared to HSC-sufficient mice. Expression of tumor necrosis factor-α (TNF-α), CCL2, and CXCL1 increased similarly, whereas increase in interleukin-6 (IL6), IL1β, and IL10 expression was higher in CCl4 rechallenged HSC-depleted compared to HSC-sufficient mice. CCl4 rechallenge of HSC-sufficient mice rapidly activated HSCs causing significant fibrosis with increased expression of Col1a1, transforming growth factor β1 (TGFβ1), tissue inhibitors of metalloproteinases 1 (TIMP1); increase in TIPM1 was much lower and metalloproteinases 13 (MMP13) greater in CCl4 rechallenged HSC-depleted mice. Interestingly, hepatic recruitment of both profibrogenic (Ly6Chi ) and antifibrogenic restorative (Ly6Clo ) macrophages, and neutrophils was significantly greater in CCl4 rechallenged HSC-depleted mice. These data suggest that CCl4 directly damages hepatocytes but HSCs regulate inflammation. Rapid fibrogenesis in CCl4 rechallenged HSC-sufficient mice recovered from earlier injury indicates that even transiently activated HSCs that had reverted to the quiescent phenotype remain primed to become reactivated.

Functional Immune Anatomy of the Liver-As an Allograft

The liver is an immunoregulatory organ in which a tolerogenic microenvironment mitigates the relative "strength" of local immune responses. Paradoxically, necro-inflammatory diseases create the need for most liver transplants. Treatment of hepatitis B virus, hepatitis C virus, and acute T cell-mediated rejection have redirected focus on long-term allograft structural integrity. Understanding of insults should enable decades of morbidity-free survival after liver replacement because of these tolerogenic properties. Studies of long-term survivors show low-grade chronic inflammatory, fibrotic, and microvascular lesions, likely related to some combination of environment insults (i.e. abnormal physiology), donor-specific antibodies, and T cell-mediated immunity. The resultant conundrum is familiar in transplantation: adequate immunosuppression produces chronic toxicities, while lightened immunosuppression leads to sensitization, immunological injury, and structural deterioration. The "balance" is more favorable for liver than other solid organ allografts. This occurs because of unique hepatic immune physiology and provides unintended benefits for allografts by modulating various afferent and efferent limbs of allogenic immune responses. This review is intended to provide a better understanding of liver immune microanatomy and physiology and thereby (a) the potential structural consequences of low-level, including allo-antibody-mediated injury; and (b) how liver allografts modulate immune reactions. Special attention is given to the microvasculature and hepatic mononuclear phagocytic system.

Regulator of G-protein signaling-5 is a marker of hepatic stellate cells and expression mediates response to liver injury

Liver fibrosis is mediated by hepatic stellate cells (HSCs), which respond to a variety of cytokine and growth factors to moderate the response to injury and create extracellular matrix at the site of injury. G-protein coupled receptor (GPCR)-mediated signaling, via endothelin-1 (ET-1) and angiotensin II (AngII), increases HSC contraction, migration and fibrogenesis. Regulator of G-protein signaling-5 (RGS5), an inhibitor of vasoactive GPCR agonists, functions to control GPCR-mediated contraction and hypertrophy in pericytes and smooth muscle cells (SMCs). Therefore we hypothesized that RGS5 controls GPCR signaling in activated HSCs in the context of liver injury. In this study, we localize RGS5 to the HSCs and demonstrate that Rgs5 expression is regulated during carbon tetrachloride (CCl₄)-induced acute and chronic liver injury in Rgs5^LacZ/LacZ reporter mice. Furthermore, CCl₄ treated RGS5-null mice develop increased hepatocyte damage and fibrosis in response to CCl₄ and have increased expression of markers of HSC activation. Knockdown of Rgs5 enhances ET-1-mediated signaling in HSCs in vitro. Taken together, we demonstrate that RGS5 is a critical regulator of GPCR signaling in HSCs and regulates HSC activation and fibrogenesis in liver injury.

Endothelin and hepatic wound healing

Liver wound healing is a coordinated response to injury caused by infections (hepatitis) or toxins (alcohol) or other processes where activation of hepatic stellate cells are a central component. During stellate cell activation, a major phenotypic transformation occurs which leads to increased production of increased extracellular matrix proteins and smooth muscle α-actin the results is organ dysfunction due to gross architectural disruption and impaired blood flow. Endothelin-1 (ET-1) is produced in increased amounts and the cellular source of ET-1 shifts from endothelial cells to stellate cells during liver injury thus setting a feedback loop which accentuates further activation, stellate cell proliferation, and production of extracellular matrix proteins. Therapy directed at intervening the ET-1 signaling pathway has significant therapeutic potential in patients with liver disease.

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.

Oxidative stress in the development of liver cirrhosis: a comparison of two different experimental models

BACKGROUND/AIMS

Oxidative stress has been implicated in liver cirrhosis. Carbon tetrachloride and thioacetamide are the most widely used models to develop cirrhosis in rats and the present study compares oxidative stress in the liver induced by these compounds at different stages of cirrhosis development.

METHODS

Twice-weekly intragastric or intraperitoneal administration of carbon tetrachloride or thioacetamide, respectively, produced liver cirrhosis after 3 months. Histology, serum markers and hepatic hydroxy proline content confirmed the cirrhosis.

RESULTS

An increase in oxidative stress parameters was seen in mitochondria, peroxisomes and microsomes from the liver after carbon tetrachloride or thioacetamide treatment. Oxidative stress was more severe in carbon tetrachloride treated animals than thioacetamide. Mild oxidative stress was evident at 1 and 2 months of treatment and a significant increase was seen by 3 months of treatment with either compound. By this time, frank liver cirrhosis was also observed.

CONCLUSIONS

These results suggest that evidence of oxygen free radicals is also found early in the development of fibrosis and cirrhosis in both models.

Honokiol protects against carbon tetrachloride induced liver damage in the rat

This study aims to investigate the possible hepato-protective effects of honokiol against liver damage and cirrhosis induced by carbon tetrachloride (CCl(4)) in the rat. Rats were treated acutely, or chronically with CCl(4) at 5 day intervals (0.06 mL/100 g body weight, administered as 50% vol/vol solution in liquid paraffin) by gavage, in combination with phenobarbitone in drinking water (0.5 g/L for 7 days prior to, and during CCl(4) treatment) to induce liver damage. Some were also co-treated with 0.1 mg/kg or 0.03 mg/kg honokiol (i.p.) or with appropriate vehicle. In vivo measurement of the liver sinusoidal area was performed using confocal microscopy following i.v. fluorescein isothiocyanate (FITC) dextran. Liver histology and function tests were performed, and liver and body weights were measured. Confocal microscopy showed that acute and chronic CCl(4) treatment significantly reduced the sinusoidal area. Honokiol (0.1 mg/kg, but not 0.03 mg/kg) partially reversed the decrease in the sinusoidal area after acute or chronic treatments with CCl(4). Acute and chronic CCl(4) treatment produced significant histological liver damage. Honokiol (0.1 mg/kg) significantly reduced the histological damage caused by chronic treatment. Chronic treatment with CCl(4) caused a significant increase in the bilirubin level that was not observed following the high dose of honokiol (0.1 mg/kg). In conclusion, this study showed that honokiol exhibits potent hepato-protective effects in rats treated with CCl(4).

The endothelin/nitric oxide balance determines small-for-size liver injury after reduced-size rat liver transplantation

Small-for-size (SFS) liver graft injury is probably related to microcirculatory disorders due to an imbalance of vasoconstricting, e.g. endothelin (ET)-1, and vasorelaxing mediators, e.g. nitric oxide (NO). We studied the role of ET-1/NO balance and the effect of an endothelin A receptor (ETAR) antagonist on SFS injury after liver resection and reduced-size liver transplantation (RSLT). One hundred twenty-six Lewis rats were divided into five groups: (I) 70% liver resection, (II) 70% liver resection treated with the ETAR antagonist LU 135252 (1 mg/kg b.w. i.v.), (III) RSLT (30% residual liver volume), (IV) RSLT treated with the ETAR antagonist, (V) sham operation. Liver microcirculation was measured by intravital microscopy. ET-1, ETAR, endothelial NO-synthase (eNOS), activation of Kupffer cells (KCs) and parenchymal injury were studied by immunohistology. Survival and liver function were followed up to 14 days. RSLT led to increased ET-1, ETAR and decreased eNOS protein expression, accompanied by activation of KC, reduced perfusion rate, vasoconstriction and elevated sinusoidal blood flow, as well as hepatocellular damage, impaired liver function and impaired survival. ETAR blockade (groups II + IV) improved the ET-1/NO balance, attenuated microcirculatory disorders and improved hepatocellular apoptosis and liver function. Microcirculatory disorders related to an ET-1/NO imbalance may contribute to SFS liver injury. Maintenance of ET-1/NO balance by blocking ETAR reduces SFS injury by protecting liver microcirculation, thus reducing hepatocellular damage.

Amelioration of microcirculatory damage by an endothelin A receptor antagonist in a rat model of reversible acute liver failure

BACKGROUND/AIMS

Hepatocellular damage in acute liver failure (ALF) is aggravated by proinflammatory and cytotoxic mediators released from sinusoidal-lining cells. We studied a selective endothelin A receptor (ETAR) antagonist for its potential influence on the microcirculation in the setting of ALF.

METHODS

Seventy Wistar rats were divided into five groups: (I) induction of ALF by a 70% liver resection combined with injection of 400 microg/kg endotoxin, (II) ALF treated with the ETAR antagonist LU 135252 (1 mg/kg b.w. i.v.), (III) sham operation, (IV) injection of endotoxin, (V) 70% liver resection. Liver microcirculation was measured by intravital microscopy. Parenchymal injury, growth fractions, endothelin (ET)-1 and ETAR were studied by histology and immunohistology. Survival, liver function, and morphology were followed up to 14 days.

RESULTS

100% mortality, impaired liver function, widespread endothelial lesions, highest ET-1 and ETAR levels, a decreased perfusion rate, reduced sinusoidal diameter, as well as an increase in both leukocyte-endothelium interactions and sinusoidal blood flow were observed after induction of ALF. ETAR antagonist-treated rats showed decreased ET-1 and ETAR levels as well as improved microcirculatory function, morphology, liver function, and 85% survival.

CONCLUSIONS

Microcirculatory disturbances correlate with liver dysfunction in ALF. ETAR blockade represents a new therapeutic approach to ALF by reducing microcirculatory lesions and their sequelae.

Superoxide-induced apoptosis of activated rat hepatic stellate cells

BACKGROUND/AIMS

During liver injury, reactive oxygen species (ROS) are produced by the resident macrophages (Kupffer cells) and infiltrating blood cells such as neutrophils. ROS cause transformation of desmin-positive quiescent hepatic stellate cells (HSCs) into the proliferating activated phenotype that expresses alpha-smooth muscle actin (alpha-SMA). The highly fibrogenic and contractile activated HSCs (aHSCs) produce various cytokines and growth factors, and play important role in the pathophysiology of chronic liver disease. However, apoptotic aHSCs are also observed during active fibrogenesis in the injured liver. Therefore, we investigated the mechanisms of apoptosis of aHSCs in relation to ROS.

METHODS

HSCs, isolated from normal rat liver, were activated in culture and effects of superoxide were determined between subcultures 3 and 5.

RESULTS

Treatment with superoxide caused apoptosis of aHSCs as determined by flow cytometry, TUNEL assay and DNA laddering analysis. The mechanisms of superoxide-induced apoptosis involved release of cytochrome c, increased Bax expression, increased caspase-3 activity, and hydrolysis of polyADP-ribose polymerase. Superoxide also increased the expression of antiapoptotic Bcl-xL and nuclear translocation of NFkappaB. Caspase-3 inhibitor (DEVD-fmk) and antioxidants (N-acetylcysteine, vitamin E and superoxide dismutase) inhibited superoxide-induced apoptosis.

CONCLUSIONS

Superoxide-induced apoptosis of aHSCs may be a novel mechanism of limiting chronic fibrotic liver injury.

Pathological comparison of hepatic-fibrosis models induced by porcine serum and carbon tetrachloride in rats

AIM: To compare liver pathological changes between two different hepatic-fibrosis models induced by porcine serum and carbon tetrachloride in rats.

METHODS: Hepatic fibrosis was induced in rats by biweekly intraperitoneal injection of porcine serum (0.5 mL) or 400 mL/L carbon tetrachloride (2 mL/kg) for 7 weeks. Five rats were randomly selected from the survivals in each group. Liver tissue was removed, fixed, sliced and stained with hematoxylin and esosin for routine light-microscopy, Masson trichrome for collagen, Jame's double ammoniated siver solution for reticulin, and then subjected to semiquantitative evaluation by pathological image analyzer. Another portion of liver tissue was prepared for electron microscopy.

RESULTS: Liver tissue in rats receiving pig serum displayed narrow fibrotic sepatae including obvious collagen deposition and more mesenchymal constituents, and collagen was confined to the septae. Hepatocellular injury is rare. In contrast, in the livers from rats treated with CCl₄, the fibrotic septae were coarse, more cellular and diffuse. The parenchymal damage was more severe and extensive. The fatty metamorphosis of hepatocytes was evident. Semiquantitative analyses suggested that quantity of collagenic fibers and reticular fibers in CCl₄ group were significantly higher than those in porcine serum group.

CONCLUSION: Hepatic fibrosis in rats induced by CCl₄ is more severe than that induced by porcine serum.

Kupffer cells are a major source of increased platelet activating factor in the CCl4-induced cirrhotic rat liver

BACKGROUND/AIMS

Endothelin-1 (ET-1) stimulates the synthesis of platelet-activating factor (PAF) by Kupffer cells in vitro. Hepatic concentrations of both ET-1 (a potent vasoconstrictor) and PAF (a mediator of hepatic vasoconstriction and the cirrhotic hyperdynamic state) increase in cirrhosis. The aim of this study was to determine if the responsiveness of Kupffer cells to produce PAF upon ET-1 challenge is modified by cirrhosis.

METHODS

Kupffer cells, isolated from the livers of control and CCl(4)-induced cirrhotic rats, were placed in serum-free medium after overnight culture. PAF and ET-1 receptors, ET-1-induced PAF synthesis, and PAF- and ET-1-induced prostaglandin E(2) (PGE(2)) synthesis were determined 24 h later.

RESULTS

Both basal and ET-1-stimulated PAF synthesis was increased in cirrhotic Kupffer cells as indicated by increased cell-associated and released PAF. Cirrhotic Kupffer cells also had elevated densities of functional receptors for both PAF and ET-1 (exclusively ET(B)), as measured by ligand binding, mRNA expression of the respective receptors, and ligand-stimulated PGE(2) synthesis.

CONCLUSIONS

Cirrhosis sensitizes Kupffer cells to both ET-1 and PAF by elevating their respective receptor levels. Since both mediators individually cause portal hypertension, an increase in ET-1-stimulated PAF synthesis in Kupffer cells will exacerbate the hepatic and extrahepatic complications of cirrhosis.

Prevention of cultured rat stellate cell transformation and endothelin-B receptor upregulation by retinoic acid

1 Physiologically, perisinusoidal hepatic stellate cells (HSC) are quiescent and store retinoids. During liver injury and in cell culture, HSC transform into proliferating myofibroblast-like cells that express alpha-smooth muscle actin (alpha-sma) and produce excessive amounts of extracellular matrix. During transformation (also known as activation), HSC are depleted of the retinoid stores, and their expression of the endothelin-1 (ET-1) system is increased. ET-1 causes contraction of transformed HSC and is implicated in their proliferation and fibrogenic activity. In order to understand the association between retinoids, ET-1 and the activation of HSC, we investigated the effect of 13-cis-retinoic acid on the transformation of cultured HSC and the expression of ET-1 system. 2 HSC derived from normal rat liver were maintained for 10-12 days in a medium supplemented with 5% serum and containing 2.5 micro M retinoic acid without or with 50 nM ET-1 (ETA+ETB agonist) or sarafotoxin S6c (ETB agonist). In another set of experiments, cells treated for 10-12 days with vehicle (ethanol) or retinoic acid were challenged with ET-1 or sarafotoxin S6c, and various determinations were made at 24 h. 3 Retinoic acid inhibited transformation and proliferation of HSC as assessed by morphological characteristics, expression of alpha-sma, bromodeoxyuridine incorporation and cell count. Retinoic acid also prevented upregulation of ETB receptors without affecting ET-1 or ETA expression. Total protein synthesis ([(3)H]leucine incorporation), collagen alpha types I mRNA expression and collagen synthesis ([(3)H]proline incorporation) were lower in retinoic acid-treated cells. Although ET-1-treated cells were morphologically similar to the control cells, their expression of alpha-smooth muscle actin was significantly inhibited. The presence of retinoic acid in the medium during treatment with ET-1 caused further reduction in the expression of alpha-smooth muscle actin. ET-1 and sarafotoxin S6c stimulated total protein synthesis in vehicle- and retinoic acid-treated cells, but collagen synthesis only in the latter. 4 These results showing prevention of HSC activation and negative regulation of ETB receptor expression in them by retinoic acid may have important pathophysiologic implications.