Sinusoidal endothelial COX-1-derived prostanoids modulate the hepatic vascular tone of cirrhotic rat livers

Optimization of Statin-Loaded Delivery Nanoparticles for Treating Chronic Liver Diseases by Targeting Liver Sinusoidal Endothelial Cells

In this study, we developed functionalized polymeric micelles (FPMs) loaded with simvastatin (FPM-Sim) as a drug delivery system to target liver sinusoidal endothelial cells (LSECs) for preserving liver function in chronic liver disease (CLD). Polymeric micelles (PMs) were functionalized by coupling peptide ligands of LSEC membrane receptors CD32b, CD36 and ITGB3. Functionalization was confirmed via spectroscopy and electron microscopy. In vitro and in vivo FPM-Sim internalization was assessed by means of flow cytometry in LSECs, hepatocytes, Kupffer and hepatic stellate cells from healthy rats. Maximum tolerated dose assays were performed in healthy mice and efficacy studies of FPM-Sim were carried out in bile duct ligation (BDL) and thioacetamide (TAA) induction rat models of cirrhosis. Functionalization with the three peptide ligands resulted in stable formulations with a greater degree of in vivo internalization in LSECs than non-functionalized PMs. Administration of FPM-Sim in BDL rats reduced toxicity relative to free simvastatin, albeit with a moderate portal-pressure-lowering effect. In a less severe model of TAA-induced cirrhosis, treatment with FPM-CD32b-Sim nanoparticles for two weeks significantly decreased portal pressure, which was associated with a reduction in liver fibrosis, lower collagen expression as well as the stimulation of nitric oxide synthesis. In conclusion, CD32b-FPM stands out as a good nanotransporter for drug delivery, targeting LSECs, key inducers of liver injury.

The Hepatic Sinusoid in Chronic Liver Disease: The Optimal Milieu for Cancer

Simple Summary

During the development of chronic liver disease, the hepatic sinusoid undergoes major changes that further compromise the hepatic function, inducing persistent inflammation and the formation of scar tissue, together with alterations in liver hemodynamics. This diseased background may induce the formation and development of hepatocellular carcinoma (HCC), which is the most common form of primary liver cancer and a major cause of mortality. In this review, we describe the ways in which the dysregulation of hepatic sinusoidal cells—including liver sinusoidal cells, Kupffer cells, and hepatic stellate cells—may have an important role in the development of HCC. Our review summarizes all of the known sinusoidal processes in both health and disease, and possible treatments focusing on the dysregulation of the sinusoid; finally, we discuss how some of these alterations occurring during chronic injury are shared with the pathology of HCC and may contribute to its development.

Abstract

The liver sinusoids are a unique type of microvascular beds. The specialized phenotype of sinusoidal cells is essential for their communication, and for the function of all hepatic cell types, including hepatocytes. Liver sinusoidal endothelial cells (LSECs) conform the inner layer of the sinusoids, which is permeable due to the fenestrae across the cytoplasm; hepatic stellate cells (HSCs) surround LSECs, regulate the vascular tone, and synthetize the extracellular matrix, and Kupffer cells (KCs) are the liver-resident macrophages. Upon injury, the harmonic equilibrium in sinusoidal communication is disrupted, leading to phenotypic alterations that may affect the function of the whole liver if the damage persists. Understanding how the specialized sinusoidal cells work in coordination with each other in healthy livers and chronic liver disease is of the utmost importance for the discovery of new therapeutic targets and the design of novel pharmacological strategies. In this manuscript, we summarize the current knowledge on the role of sinusoidal cells and their communication both in health and chronic liver diseases, and their potential pharmacologic modulation. Finally, we discuss how alterations occurring during chronic injury may contribute to the development of hepatocellular carcinoma, which is usually developed in the background of chronic liver disease.

Role of liver sinusoidal endothelial cells in liver diseases

Liver sinusoidal endothelial cells (LSECs) form the wall of the hepatic sinusoids. Unlike other capillaries, they lack an organized basement membrane and have cytoplasm that is penetrated by open fenestrae, making the hepatic microvascular endothelium discontinuous. LSECs have essential roles in the maintenance of hepatic homeostasis, including regulation of the vascular tone, inflammation and thrombosis, and they are essential for control of the hepatic immune response. On a background of acute or chronic liver injury, LSECs modify their phenotype and negatively affect neighbouring cells and liver disease pathophysiology. This Review describes the main functions and phenotypic dysregulations of LSECs in liver diseases, specifically in the context of acute injury (ischaemia-reperfusion injury, drug-induced liver injury and bacterial and viral infection), chronic liver disease (metabolism-associated liver disease, alcoholic steatohepatitis and chronic hepatotoxic injury) and hepatocellular carcinoma, and provides a comprehensive update of the role of LSECs as therapeutic targets for liver disease. Finally, we discuss the open questions in the field of LSEC pathobiology and future avenues of research.

Interplay of cardiovascular mediators, oxidative stress and inflammation in liver disease and its complications

The liver is a crucial metabolic organ that has a key role in maintaining immune and endocrine homeostasis. Accumulating evidence suggests that chronic liver disease might promote the development of various cardiac disorders (such as arrhythmias and cardiomyopathy) and circulatory complications (including systemic, splanchnic and pulmonary complications), which can eventually culminate in clinical conditions ranging from portal and pulmonary hypertension to pulmonary, cardiac and renal failure, ascites and encephalopathy. Liver diseases can affect cardiovascular function during the early stages of disease progression. The development of cardiovascular diseases in patients with chronic liver failure is associated with increased morbidity and mortality, and cardiovascular complications can in turn affect liver function and liver disease progression. Furthermore, numerous infectious, inflammatory, metabolic and genetic diseases, as well as alcohol abuse can also influence both hepatic and cardiovascular outcomes. In this Review, we highlight how chronic liver diseases and associated cardiovascular effects can influence different organ pathologies. Furthermore, we explore the potential roles of inflammation, oxidative stress, vasoactive mediator imbalance, dysregulated endocannabinoid and autonomic nervous systems and endothelial dysfunction in mediating the complex interplay between the liver and the systemic vasculature that results in the development of the extrahepatic complications of chronic liver disease. The roles of ageing, sex, the gut microbiome and organ transplantation in this complex interplay are also discussed.

Pathophysiological role of prostanoids in coagulation of the portal venous system in liver cirrhosis

Background

Prostanoids are important regulators of platelet aggregation and thrombotic arterial diseases. Their involvement in the development of portal vein thrombosis, frequent in decompensated liver cirrhosis, is still not investigated.

Methods

Therefore, we used pro-thrombotic venous milieu generation by bare metal stent transjugular intrahepatic portosystemic shunt insertion, to study the role of prostanoids in decompensated liver cirrhosis. Here, 89 patients receiving transjugular intrahepatic portosystemic shunt insertion were included in the study, and baseline levels of thromboxane B₂, prostaglandin D₂ and prostaglandin E₂ were measured in the portal and the hepatic vein.

Results

While the hepatic vein contained higher levels of thromboxane B₂ than the portal vein, levels of prostaglandin E₂ and D₂ were higher in the portal vein (all P<0.0001). Baseline concentrations of thromboxane B₂ in the portal vein were independently associated with an increase of portal hepatic venous pressure gradient during short term follow-up, as an indirect sign of thrombogenic potential (multivariable P = 0.004). Moreover, severity of liver disease was inversely correlated with portal as well as hepatic vein levels of prostaglandin D₂ and E₂ (all P<0.0001).

Conclusions

Elevated portal venous thromboxane B₂ concentrations are possibly associated with the extent of thrombogenic potential in patients with decompensated liver cirrhosis.

Trial registration

ClinicalTrials.gov identifier: NCT03584204.

Acute-on-chronic liver disease enhances phenylephrine-induced endothelial nitric oxide release in rat mesenteric resistance arteries through enhanced PKA, PI3K/AKT and cGMP signalling pathways

Acute-on-chronic liver disease is a clinical syndrome characterized by decompensated liver fibrosis, portal hypertension and splanchnic hyperdynamic circulation. We aimed to determine whether the alpha-1 agonist phenylephrine (Phe) facilitates endothelial nitric oxide (NO) release by mesenteric resistance arteries (MRA) in rats subjected to an experimental microsurgical obstructive liver cholestasis model (LC). Sham-operated (SO) and LC rats were maintained for eight postoperative weeks. Phe-induced vasoconstriction (in the presence/absence of the NO synthase –NOS- inhibitor L-NAME) and vasodilator response to NO donor DEA-NO were analysed. Phe-induced NO release was determined in the presence/absence of either H89 (protein kinase –PK- A inhibitor) or LY 294002 (PI3K inhibitor). PKA and PKG activities, alpha-1 adrenoceptor, endothelial NOS (eNOS), PI3K, AKT and soluble guanylate cyclase (sGC) subunit expressions, as well as eNOS and AKT phosphorylation, were determined. The results show that LC blunted Phe-induced vasoconstriction, and enhanced DEA-NO-induced vasodilation. L-NAME increased the Phe-induced contraction largely in LC animals. The Phe-induced NO release was greater in MRA from LC animals. Both H89 and LY 294002 reduced NO release in LC. Alpha-1 adrenoceptor, eNOS, PI3K and AKT expressions were unchanged, but sGC subunit expression, eNOS and AKT phosphorylation and the activities of PKA and PKG were higher in MRA from LC animals. In summary, these mechanisms may help maintaining splanchnic vasodilation and hypotension observed in decompensated LC.

Hepatic microcirculation and mechanisms of portal hypertension

The liver microcirculatory milieu, mainly composed of liver sinusoidal endothelial cells (LSECs), hepatic stellate cells (HSCs) and hepatic macrophages, has an essential role in liver homeostasis, including in preserving hepatocyte function, regulating the vascular tone and controlling inflammation. Liver microcirculatory dysfunction is one of the key mechanisms that promotes the progression of chronic liver disease (also termed cirrhosis) and the development of its major clinical complication, portal hypertension. In the present Review, we describe the current knowledge of liver microcirculatory dysfunction in cirrhotic portal hypertension and appraise the preclinical models used to study the liver circulation. We also provide a comprehensive summary of the promising therapeutic options to target the liver microvasculature in cirrhosis.

Update on the management of gastrointestinal varices

Cirrhosis of liver is a major problem in the western world. Portal hypertension is a complication of cirrhosis and can lead to a myriad of pathology of which include the development of porto-systemic collaterals. Gastrointestinal varices are dilated submucosal veins, which often develop at sites near the formation of gastroesophageal collateral circulation. The incidence of varices is on the rise due to alcohol and obesity. The most significant complication of portal hypertension is life-threatening bleeding from gastrointestinal varices, which is associated with substantial morbidity and mortality. In addition, this can cause a significant burden on the health care facility. Gastrointestinal varices can happen in esophagus, stomach or ectopic varices. There has been considerable progress made in the understanding of the natural history, pathophysiology and etiology of portal hypertension. Despite the development of endoscopic and medical treatments, early mortality due to variceal bleeding remains high due to significant illness of the patient. Recurrent variceal bleed is common and in some cases, there is refractory variceal bleed. This article aims to provide a comprehensive review of the management of gastrointestinal varices with an emphasis on endoscopic interventions, strategies to handle refractory variceal bleed and newer endoscopic treatment modalities. Early treatment and improved endoscopic techniques can help in improving morbidity and mortality.

Severe steatosis induces portal hypertension by systemic arterial hyporeactivity and hepatic vasoconstrictor hyperreactivity in rats

Non-alcoholic fatty liver disease (NAFLD) has become the most prevalent chronic liver disease. The presence of portal hypertension has been demonstrated in NAFLD prior to development of inflammation or fibrosis, and is a result of extrahepatic and intrahepatic factors, principally driven by vascular dysfunction. An increased intrahepatic vascular resistance potentially contributes to progression of NAFLD via intralobular hypoxia. However, the exact mechanisms underlying vascular dysfunction in NAFLD remain unknown. This study investigates systemic hemodynamics and both aortic and intrahepatic vascular reactivity in a rat model of severe steatosis. Wistar rats were fed a methionine-choline-deficient diet, inducing steatosis, or control diet for 4 weeks. In vivo hemodynamic measurements, aortic contractility studies, and in situ liver perfusion experiments were performed. The mean arterial blood pressure was lower and portal blood pressure was higher in steatosis compared to controls. The maximal contraction force in aortic rings from steatotic rats was markedly reduced compared to controls. While blockade of nitric oxide (NO) production did not reveal any differences, cyclooxygenase (COX) blockade reduced aortic reactivity in both controls and steatosis, whereas effects were more pronounced in controls. Effects could be attributed to COX-2 iso-enzyme activity. In in situ liver perfusion experiments, exogenous NO donation or endogenous NO stimulation reduced the transhepatic pressure gradient (THPG), whereas NO synthase blockade increased the THPG only in steatosis, but not in controls. Alpha-1-adrenergic stimulation and endothelin-1 induced a significantly more pronounced increase in THPG in steatosis compared to controls. Our results demonstrate that severe steatosis, without inflammation or fibrosis, induces portal hypertension and signs of a hyperdynamic circulation, accompanied by extrahepatic arterial hyporeactivity and intrahepatic vascular hyperreactivity. The arterial hyporeactivity seems to be NO-independent, but appears to be mediated by specific COX-2-related mechanisms. Besides, the increased intrahepatic vascular resistance in steatosis appears not to be NO-related but rather to vasoconstrictor hyperreactivity.

The selective mineralocorticoid receptor antagonist eplerenone prevents decompensation of the liver in cirrhosis

BACKGROUND AND PURPOSE

The mineralocorticoid receptor (MR) contributes to fibrosis in various tissues, and MR antagonists, like eplerenone, are used to prevent fibrosis. The role of MR antagonists in hepatic fibrosis and cirrhosis is unknown. Here, we investigated the role of MRs and eplerenone in cirrhosis development.

EXPERIMENTAL APPROACH

Liver fibrosis (5 weeks) and cirrhosis, without (8 weeks) and with ascites (12 weeks), were induced by CCl₄ in rats and comprehensively analysed. The effect of eplerenone on the development of cirrhosis with ascites was assessed. MR expression, cellular and subcellular distribution and impact of hypoxia were investigated in vivo and ex vivo. Primary rat hepatocytes and cell lines were used to investigate MR trafficking and transcriptional activity mechanistically.

KEY RESULTS

In cirrhosis with ascites, MR mRNA and protein expressions were reduced in hepatocytes of hypoxic areas. While in normoxic areas MRs were mainly cytosolic, the remaining MRs in hypoxic areas were mainly localized in the nuclei, indicating activation followed by translocation and degradation. Accordingly, eplerenone treatment prevented nuclear MR translocation and the worsening of cirrhosis. Exposing hepatocytes ex vivo to hypoxia induced nuclear MR translocation and enhanced transcriptional MR activity at response elements of the NF-κB pathway.

CONCLUSIONS AND IMPLICATIONS

We showed for the first time that hypoxia leads to a pathogenetic ligand-independent activation of hepatic MRs during cirrhosis resulting in their nuclear translocation and transcriptional activation of the NF-κB pathway. Treatment with eplerenone prevented the worsening of cirrhosis by blocking this ligand-independent activation of the MR.

The anticoagulant rivaroxaban lowers portal hypertension in cirrhotic rats mainly by deactivating hepatic stellate cells

In cirrhosis, increased intrahepatic vascular resistance (IHVR) is the primary factor for portal hypertension (PH) development. Hepatic stellate cells (HSCs) play a major role increasing IHVR because, when activated, they are contractile and promote fibrogenesis. Protease-activated receptors (PARs) can activate HSCs through thrombin and factor Xa, which are known PAR agonists, and cause microthrombosis in liver microcirculation. This study investigates the effects of the oral anticoagulant, rivaroxaban (RVXB), a direct antifactor Xa, on HSC phenotype, liver fibrosis (LF), liver microthrombosis, and PH in cirrhotic rats. Hepatic and systemic hemodynamic, nitric oxide (NO) bioavailability, LF, HSC activation, and microthrombosis were evaluated in CCl₄ and thioacetamide-cirrhotic rats treated with RVXB (20 mg/kg/day) or its vehicle for 2 weeks. RVXB significantly decreased portal pressure (PP) in both models of cirrhosis without changes in portal blood flow, suggesting a reduction in IHVR. RVXB reduced oxidative stress, improved NO bioavailability, and ameliorated endothelial dysfunction. Rivaroxaban deactivated HSC, with decreased alpha-smooth muscle actin and mRNA expression of other HSC activation markers. Despite this marked improvement in HSC phenotype, no significant changes in LF were identified. RVXB markedly reduced fibrin deposition, suggesting reduced intrahepatic microthrombosis.

CONCLUSION

RVXB decreases PP in two rat models of cirrhosis. This effect is mostly associated with decreased IHVR, enhanced NO bioavailability, HSC deactivation, and reduced intrahepatic microthrombosis. Our findings suggest that RVXB deserves further evaluation as a potential treatment for cirrhotic PH. (Hepatology 2017;65:2031-2044).

Novel treatment options for portal hypertension

Portal hypertension is most frequently associated with cirrhosis and is a major driver for associated complications, such as variceal bleeding, ascites or hepatic encephalopathy. As such, clinically significant portal hypertension forms the prelude to decompensation and impacts significantly on the prognosis of patients with liver cirrhosis. At present, non-selective β-blockers, vasopressin analogues and somatostatin analogues are the mainstay of treatment but these strategies are far from satisfactory and only target splanchnic hyperemia. In contrast, safe and reliable strategies to reduce the increased intrahepatic resistance in cirrhotic patients still represent a pending issue. In recent years, several preclinical and clinical trials have focused on this latter component and other therapeutic avenues. In this review, we highlight novel data in this context and address potentially interesting therapeutic options for the future.

How to Face Chronic Liver Disease: The Sinusoidal Perspective

Liver microcirculation plays an essential role in the progression and aggravation of chronic liver disease. Hepatic sinusoid environment, mainly composed by hepatocytes, liver sinusoidal endothelial cells, Kupffer cells, and hepatic stellate cells, intimately cooperate to maintain global liver function and specific phenotype of each cell type. However, continuous liver injury significantly deregulates liver cells protective phenotype, leading to parenchymal and non-parenchymal dysfunction. Recent data have enlightened the molecular processes that mediate hepatic microcirculatory injury, and consequently, opened the possibility to develop new therapeutic strategies to ameliorate liver circulation and viability. The present review summarizes the main cellular components of the hepatic sinusoid, to afterward focus on non-parenchymal cells phenotype deregulation due to chronic injury, in the specific clinical context of liver cirrhosis and derived portal hypertension. Finally, we herein detail new therapies developed at the bench-side with high potential to be translated to the bedside.

Contribution of Cyclooxygenase End Products and Oxidative Stress to Intrahepatic Endothelial Dysfunction in Early Non-Alcoholic Fatty Liver Disease

Introduction

Metabolic syndrome induces endothelial dysfunction, a surrogate marker of cardiovascular disease. In parallel, metabolic syndrome is frequently associated with non-alcoholic fatty liver disease (NAFLD), which may progress to cirrhosis. The aim of the present study was to evaluate intrahepatic endothelial dysfunction related to cyclooxygenase end products and oxidative stress as possible mechanisms involved in the pathophysiology of NAFLD.

Materials and Methods

Sprague-Dawley rats were fed standard diet (control-diet, CD) or high-fat-diet (HFD) for 6 weeks. Metabolic syndrome was assessed by recording arterial pressure, lipids, glycemia and rat body weight. Splanchnic hemodynamics were measured, and endothelial dysfunction was evaluated using concentration-effect curves to acetylcholine. Response was assessed with either vehicle, L-N^G-Nitroarginine (L-NNA), indomethacin, tempol, or a thromboxane receptor antagonist, SQ 29548. We quantified inflammation, fibrosis, oxidative stress, nitric oxide (NO) bioavailability and thromboxane B₂ levels.

Results

HFD rats exhibited metabolic syndrome together with the presence of NAFLD. Compared to control-diet livers, HFD livers showed increased hepatic vascular resistance unrelated to inflammation or fibrosis, but with decreased NO activity and increased oxidative stress. Endothelial dysfunction was observed in HFD livers compared with CD rats and improved after cyclooxygenase inhibition or tempol pre-incubation. However, pre-incubation with SQ 29548 did not modify acetylcholine response.

Conclusions

Our study provides evidence that endothelial dysfunction at an early stage of NAFLD is associated with reduced NO bioavailability together with increased cyclooxygenase end products and oxidative stress, which suggests that both pathways are involved in the pathophysiology and may be worth exploring as therapeutic targets to prevent progression of the disease.

Pathophysiology and a Rational Basis of Therapy

Portal hypertension is a common complication of chronic liver disease. Its relevance comes from the fact that it determines most complications leading to death or liver transplantation in patients with cirrhosis of the liver: bleeding from esophageal or gastric varices, ascites and renal dysfunction, sepsis and hepatic encephalopathy. Portal hypertension results from increased resistance to portal blood flow through the cirrhotic liver. This is caused by two mechanisms: (1) distortion of the liver vascular architecture due to the liver disease causing structural abnormalities (nodule formation, remodeling of liver sinusoids, fibrosis, angiogenesis and vascular occlusion), and (2) increased hepatic vascular tone due to sinusoidal endothelial dysfunction, which results in a defective production of endogenous vasodilators, mainly nitric oxide (NO), and increased production of vasoconstrictors (thromboxane A2, cysteinyl leukotrienes, angiotensin II, endothelins and an activated adrenergic system). Hepatic endothelial dysfunction occurs early in the course of chronic liver disease as a consequence of inflammation and oxidative stress, and determines loss of the normal phenotype of liver sinusoidal endothelial cells (LSECs) that become proliferative, prothrombotic, proinflammatory and vasoconstrictor. The cross-talk between LSECs and hepatic stellate cells (HSCs) induces activation of the latter, which in turn proliferate, migrate and increase collagen deposition around the sinusoids, contributing to fibrogenesis, architectural disruption and angiogenesis, which further increase the hepatic vascular resistance and worsen liver failure by interfering with the blood perfusion of the liver parenchyma. An additional factor further worsening portal hypertension is an increased blood flow through the portal system due to splanchnic vasodilatation. This is an adaptive response to decreased effective hepatocyte perfusion, and is maximal once portal pressure has increased sufficiently to promote the development of intrahepatic shunts and portal-systemic collaterals, including varices, through which portal blood flow bypasses the liver. In human portal hypertension collateralization and hyperdynamic circulation start at a portal pressure gradient >10 mm Hg. Rational therapy for portal hypertension aims at correcting these pathophysiological abnormalities: liver injury, fibrogenesis, increased hepatic vascular tone and splanchnic vasodilatation. Continuing liver injury may be counteracted specifically by etiological treatments (the best example being the direct-acting antivirals for hepatitis C viral infection), while architectural disruption and fibrosis can be ameliorated by a variety of antifibrotic drugs and antiangiogenic strategies. Several drugs in this category are currently under investigation in phase II-III randomized controlled trials. Sinusoidal endothelial dysfunction is ameliorated by statins as well as by other drugs increasing NO availability. It is of note that simvastatin has already been proven to be clinically effective in two randomized controlled trials. Splanchnic hyperemia can be counteracted by nonselective β-blockers (NSBBs), vasopressin analogs and somatostatin analogs, drugs that until recently were the only available treatments for portal hypertension, but that are not very effective in the initial stages of cirrhosis. There is experimental and clinical evidence indicating that a more effective reduction of portal pressure is obtained by combining agents acting on these different pathways. It is likely that the treatment of portal hypertension will evolve to use etiological treatments together with antifibrotic agents and/or drugs improving sinusoidal endothelial function in the initial stages of cirrhosis (preprimary prophylaxis), while NSBBs will be added in advanced stages of the disease.

Arachidonic acid metabolites and endothelial dysfunction of portal hypertension

Increased resistance to portal flow and increased portal inflow due to mesenteric vasodilatation represent the main factors causing portal hypertension in cirrhosis. Endothelial cell dysfunction, defined as an imbalance between the synthesis, release, and effect of endothelial mediators of vascular tone, inflammation, thrombosis, and angiogenesis, plays a major role in the increase of resistance in portal circulation, in the decrease in the mesenteric one, in the development of collateral circulation. Reduced response to vasodilators in liver sinusoids and increased response in the mesenteric arterioles, and, viceversa, increased response to vasoconstrictors in the portal-sinusoidal circulation and decreased response in the mesenteric arterioles are also relevant to the pathophysiology of portal hypertension. Arachidonic acid (AA) metabolites through the three pathways, cyclooxygenase (COX), lipoxygenase, and cytochrome P450 monooxygenase and epoxygenase, are involved in endothelial dysfunction of portal hypertension. Increased thromboxane-A2 production by liver sinusoidal endothelial cells (LSECs) via increased COX-1 activity/expression, increased leukotriens, increased epoxyeicosatrienoic acids (EETs) (dilators of the peripheral arterial circulation, but vasoconstrictors of the portal-sinusoidal circulation), represent a major component in the increased portal resistance, in the decreased portal response to vasodilators and in the hyper-response to vasoconstrictors. Increased prostacyclin (PGI2) via COX-1 and COX-2 overexpression, and increased EETs/heme-oxygenase-1/K channels/gap junctions (endothelial derived hyperpolarizing factor system) play a major role in mesenteric vasodilatation, hyporeactivity to vasoconstrictors, and hyper-response to vasodilators. EETs, mediators of liver regeneration after hepatectomy and of angiogenesis, may play a role in the development of regenerative nodules and collateral circulation, through stimulation of vascular endothelial growth factor (VEGF) inside the liver and in the portal circulation. Pharmacological manipulation of AA metabolites may be beneficial for cirrhotic portal hypertension.

Liver sinusoidal endothelial cells (LSECs) function and NAFLD; NO-based therapy targeted to the liver

Liver sinusoidal endothelial cells (LSECs) present unique, highly specialised endothelial cells in the body. Unlike the structure and function of typical, vascular endothelial cells, LSECs are comprised of fenestrations, display high endocytic capacity and play a prominent role in maintaining overall liver homeostasis. LSEC dysfunction has been regarded as a key event in multiple liver disorders; however, its role and diagnostic, prognostic and therapeutic significance in nonalcoholic fatty liver disease (NAFLD) is still neglected. The purpose of this review is to provide an overview of the importance of LSECs in NAFLD. Attention is focused on the LSECs-mediated NO-dependent mechanisms in NAFLD development. We briefly describe the unique, highly specialised phenotype of LSECs and consequences of LSEC dysfunction on function of hepatic stellate cells (HSC) and hepatocytes. The potential efficacy of liver selective NO donors against liver steatosis and novel treatment approaches to modulate LSECs-driven liver pathology including NAFLD are also highlighted.

New cellular and molecular targets for the treatment of portal hypertension

Portal hypertension (PH) is a common complication of chronic liver disease, and it determines most complications leading to death or liver transplantation in patients with liver cirrhosis. PH results from increased resistance to portal blood flow through the cirrhotic liver. This is caused by two mechanisms: (a) distortion of the liver vascular architecture and (b) hepatic microvascular dysfunction. Increment in hepatic resistance is latterly accompanied by splanchnic vasodilation, which further aggravates PH. Hepatic microvascular dysfunction occurs early in the course of chronic liver disease as a consequence of inflammation and oxidative stress and determines loss of the normal phenotype of liver sinusoidal endothelial cells (LSEC). The cross-talk between LSEC and hepatic stellate cells induces activation of the latter, which in turn proliferate, migrate and increase collagen deposition around the sinusoids, contributing to fibrogenesis, architectural disruption and angiogenesis. Therapy for PH aims at correcting these pathophysiological abnormalities: liver injury, fibrogenesis, increased hepatic vascular tone and splanchnic vasodilatation. Continuing liver injury may be counteracted specifically by etiological treatments, while architectural disruption and fibrosis can be ameliorated by a variety of anti-fibrogenic drugs and anti-angiogenic strategies. Sinusoidal endothelial dysfunction is ameliorated by statins and other drugs increasing NO availability. Splanchnic hyperemia can be counteracted by non-selective beta-blockers (NSBBs), vasopressin analogs and somatostatin analogs. Future treatment of portal hypertension will evolve to use etiological treatments together with anti-fibrotic agents and/or drugs improving microvascular function in initial stages of cirrhosis (pre-primary prophylaxis), while NSBBs will be added in advanced stages of the disease.

Endothelial dysfunction in cirrhosis: Role of inflammation and oxidative stress

This review describes the recent developments in the pathobiology of endothelial dysfunction (ED) in the context of cirrhosis with portal hypertension and defines novel strategies and potential targets for therapy. ED has prognostic implications by predicting unfavourable early hepatic events and mortality in patients with portal hypertension and advanced liver diseases. ED characterised by an impaired bioactivity of nitric oxide (NO) within the hepatic circulation and is mainly due to decreased bioavailability of NO and accelerated degradation of NO with reactive oxygen species. Furthermore, elevated inflammatory markers also inhibit NO synthesis and causes ED in cirrhotic liver. Therefore, improvement of NO availability in the hepatic circulation can be beneficial for the improvement of endothelial dysfunction and associated portal hypertension in patients with cirrhosis. Furthermore, therapeutic agents that are identified in increasing NO bioavailability through improvement of hepatic endothelial nitric oxide synthase (eNOS) activity and reduction in hepatic asymmetric dimethylarginine, an endogenous modulator of eNOS and a key mediator of elevated intrahepatic vascular tone in cirrhosis would be interesting therapeutic approaches in patients with endothelial dysfunction and portal hypertension in advanced liver diseases.

Vascular pathobiology in chronic liver disease and cirrhosis - current status and future directions

Chronic liver disease is associated with remarkable alterations in the intra- and extrahepatic vasculature. Because of these changes, the fields of liver vasculature and portal hypertension have recently become closely integrated within the broader vascular biology discipline. As developments in vascular biology have evolved, a deeper understanding of vascular processes has led to a better understanding of the mechanisms of the dynamic vascular changes associated with portal hypertension and chronic liver disease. In this context, hepatic vascular cells, such as sinusoidal endothelial cells and pericyte-like hepatic stellate cells, are closely associated with one another, where they have paracrine and autocrine effects on each other and themselves. These cells play important roles in the pathogenesis of liver fibrosis/cirrhosis and portal hypertension. Further, a variety of signaling pathways have recently come to light. These include growth factor pathways involving cytokines such as transforming growth factor β, platelet derived growth factor, and others as well as a variety of vasoactive peptides and other molecules. An early and consistent feature of liver injury is the development of an increase in intra-hepatic resistance; this is associated with changes in hepatic vascular cells and their signaling pathway that cause portal hypertension. A critical concept is that this process aggregates signals to the extrahepatic circulation, causing derangement in this system's cells and signaling pathways, which ultimately leads to the collateral vessel formation and arterial vasodilation in the splanchnic and systemic circulation, which by virtue of the hydraulic derivation of Ohm's law (pressure = resistance × flow), worsens portal hypertension. This review provides a detailed review of the current status and future direction of the basic biology of portal hypertension with a focus on the physiology, pathophysiology, and signaling of cells within the liver, as well as those in the mesenteric vascular circulation. Translational implications of recent research and the future directions that it points to are also highlighted.

Inflammation and portal hypertension - the undiscovered country

Portal hypertension has traditionally been viewed as a progressive process, involving ultrastructural changes including fibrosis, nodule formation, and vascular thrombosis, leading to increased intrahepatic resistance to flow. However, it is increasingly recognized that a significant component of this vascular resistance results from a dynamic process, regulated by complex interactions between the injured hepatocyte, the sinusoidal endothelial cell, the Kupffer cell and the hepatic stellate cell, which impact on sinusoidal calibre. Recent findings suggest these haemodynamic findings are most marked in patients with superimposed inflammation. The precise mechanisms for vascular dysfunction in cirrhosis with superimposed inflammation remain to be fully elucidated but several studies over the past decade have started to generate the hypothesis that inflammation may be a key mediator of the pathogenesis and severity of portal hypertension in this context. This review provides a comprehensive overview of the biological mechanisms for inflammation playing a key role in the severity of portal hypertension, and illustrates potential novel therapies that act by modifying these processes.

Role of gap junctions modulating hepatic vascular tone in cirrhosis

BACKGROUND & AIMS

Gap junctions are formed by connexins (Cx), a family of proteins that couple endothelial and smooth muscle cells in systemic vessels. In this context, Cx allow the transmission of signals modulating vascular tone. Recently, vascular Cx have been observed in liver cells implicated in liver blood flow regulation. Here, we investigated the role of Cx in the regulation of intrahepatic vascular tone in cirrhosis.

METHODS

Livers of Sprague-Dawley control and cirrhotic (common bile duct ligation-CBDL and CCl4 ) rats were perfused, and concentration-effect curves in response to acetylcholine (ACh) precontracted with methoxamine were obtained in the presence of the specific Cx inhibitor 18-alpha-glycyrrhetinic acid or vehicle. Cx expression was assessed by immunofluorescence, western blot and reverse-transcription polymerase chain reaction in liver tissue, hepatic stellate cells, sinusoidal endothelial cells and hepatocytes isolated from control and cirrhotic rat livers. Cx protein expression was also determined in cirrhotic human tissue.

RESULTS

Gap junction blockade markedly attenuated relaxation of hepatic vasculature in response to ACh in control (maximal relaxation, -55 ± 10.5% vs. -95.3 ± 10% with vehicle; P < 0.01) and CBDL rats (50.9 ± 18.5% vs. -18.7 ± 5.5% with vehicle; P = 0.01). Livers from CBDL rats and patients with cirrhosis exhibited Cx overexpression. By contrast, CCl4 -cirrhotic rats did not show attenuated relaxation of hepatic vasculature after blockade and Cx expression was significantly lower than in controls.

CONCLUSIONS

Gap junctions may contribute to modulating portal pressure and intrahepatic vascular relaxation. Liver gap junctions may represent a new therapeutic target in cirrhotic portal hypertension.

Pathophysiology of portal hypertension

Portal hypertension is a major complication of liver disease that results from a variety of pathologic conditions that increase the resistance to the portal blood flow into the liver. As portal hypertension develops, the formation of collateral vessels and arterial vasodilation progresses, which results in increased blood flow to the portal circulation. Hyperdynamic circulatory syndrome develops, leading to esophageal varices or ascites. This article summarizes the factors that increase (1) intrahepatic vascular resistance and (2) the blood flow in the splanchnic and systemic circulations in liver cirrhosis. In addition, the future directions of basic/clinical research in portal hypertension are discussed.

Terutroban, a TP-receptor antagonist, reduces portal pressure in cirrhotic rats

Increased production of vasoconstrictive prostanoids, such as thromboxane A2 (TXA2 ), contributes to endothelial dysfunction and increased hepatic vascular tone in cirrhosis. TXA2 induces vasoconstriction by way of activation of the thromboxane-A2 /prostaglandin-endoperoxide (TP) receptor. This study investigated whether terutroban, a specific TP receptor blocker, decreases hepatic vascular tone and portal pressure in rats with cirrhosis due to carbon tetrachloride (CCl4 ) or bile duct ligation (BDL). Hepatic and systemic hemodynamics, endothelial dysfunction, liver fibrosis, hepatic Rho-kinase activity (a marker of hepatic stellate cell contraction), and the endothelial nitric oxide synthase (eNOS) signaling pathway were measured in CCl4 and BDL cirrhotic rats treated with terutroban (30 mg/kg/day) or its vehicle for 2 weeks. Terutroban reduced portal pressure in both models without producing significant changes in portal blood flow, suggesting a reduction in hepatic vascular resistance. Terutroban did not significantly change arterial pressure in CCl4 -cirrhotic rats but decreased it significantly in BDL-cirrhotic rats. In livers from CCl4 and BDL-cirrhotic terutroban-treated rats, endothelial dysfunction was improved and Rho-kinase activity was significantly reduced. In CCl4 -cirrhotic rats, terutroban reduced liver fibrosis and decreased alpha smooth muscle actin (α-SMA), collagen-I, and transforming growth factor beta messenger RNA (mRNA) expression without significant changes in the eNOS pathway. In contrast, no change in liver fibrosis was observed in BDL-cirrhotic rats but an increase in the eNOS pathway.

CONCLUSION

Our data indicate that TP-receptor blockade with terutroban decreases portal pressure in cirrhosis. This effect is due to decreased hepatic resistance, which in CCl4 -cirrhotic rats was linked to decreased hepatic fibrosis, but not in BDL rats, in which the main mediator appeared to be an enhanced eNOS-dependent vasodilatation, which was not liver-selective, as it was associated with decreased arterial pressure. The potential use of terutroban for portal hypertension requires further investigation.

Resveratrol improves intrahepatic endothelial dysfunction and reduces hepatic fibrosis and portal pressure in cirrhotic rats

BACKGROUND & AIMS

Resveratrol, a polyphenol found in a variety of fruits, exerts a wide range of beneficial effects on the endothelium, regulates multiple vasoactive substances and decreases oxidative stress, factors involved in the pathophysiology of portal hypertension. Our study aimed at evaluating the effects of resveratrol on hepatic and systemic hemodynamics, hepatic endothelial dysfunction, and hepatic fibrosis in CCl₄ cirrhotic rats.

METHODS

Resveratrol (10 and 20 mg/kg/day) or its vehicle was administered to cirrhotic rats for two weeks and hepatic and systemic hemodynamics were measured. Moreover, we evaluated endothelial function by dose-relaxation curves to acetylcholine, hepatic NO bioavailability and TXA2 production. We also evaluated liver fibrosis by Sirius Red staining of liver sections, collagen-1, NFκB, TGFβ mRNA expression, and desmin and α-smooth muscle actin (α-SMA) protein expression, as a surrogate of hepatic stellate cell activation.

RESULTS

Resveratrol administration significantly decreased portal pressure compared to vehicle (12.1 ± 0.9 vs. 14.3 ± 2.2 mmHg; p <0.05) without significant changes in systemic hemodynamics. Reduction in portal pressure was associated with an improved vasodilatory response to acetylcholine, with decreased TXA2 production, increased endothelial NO, and with a significant reduction in liver fibrosis. The decrease in hepatic fibrosis was associated with a reduced collagen-1, TGFβ, NFκB mRNA expression and desmin and α-SMA protein expression.

CONCLUSIONS

Resveratrol administration reduces portal pressure, hepatic stellate cell activation and liver fibrosis, and improves hepatic endothelial dysfunction in cirrhotic rats, suggesting it may be a useful dietary supplement in the treatment of portal hypertension in patients with cirrhosis.

Interaction between NO and COX pathways modulating hepatic endothelial cells from control and cirrhotic rats

Reduced intrahepatic nitric oxide (NO) bioavailability and increased cyclooxygenase-1 (COX-1)-derived vasoconstrictor prostanoids modulate the hepatic vascular tone in cirrhosis. We aimed at investigating the reciprocal interactions between NO and COX in the hepatic endothelium of control and cirrhotic rats. NO bioavailability (DAF-FM-DA staining), superoxide (O₂⁻) content (DHE staining), prostanoid production (PGI₂ and TXA₂ by enzyme immunoassays) as well as COX expression (Western Blot), were determined in hepatic endothelial cells (HEC) from control and cirrhotic rats submitted to different experimental conditions: COX activation, COX inhibition, NO activation and NO inhibition. In control and cirrhotic HEC, COX activation with arachidonic acid reduced NO bioavailability and increased O₂⁻ levels. These effects were abolished by pre-treating HEC with the COX inhibitor indomethacin. In control, but not in cirrhotic HEC, scavenging of O₂⁻ by superoxide dismutase (SOD) incubation partially restored the decrease in NO bioavailability promoted by COX activation. NO supplementation produced a significant and parallel reduction in PGI₂ and TXA₂ production in control HEC, whereas it only reduced TXA₂ production in cirrhotic HEC. By contrast, in control and cirrhotic HEC, NO inhibition did not modify COX expression or activity. Our results demonstrate that NO and COX systems are closely interrelated in HEC. This is especially relevant in cirrhotic HEC where COX inhibition increases NO bioavailability and NO supplementation induces a reduction in TXA₂. These strategies may have beneficial effects ameliorating the vasoconstrictor/vasodilator imbalance of the intrahepatic circulation of cirrhotic livers.

The transcription factor KLF2 mediates hepatic endothelial protection and paracrine endothelial-stellate cell deactivation induced by statins

BACKGROUND & AIMS

Statins improve hepatic endothelial function and liver fibrosis in experimental models of cirrhosis, thus they have been proposed as therapeutic options to ameliorate portal hypertension syndrome. The transcription factor Kruppel-like factor 2 (KLF2) may be induced by statins in liver sinusoidal endothelial cells (SEC), orchestrating an efficient vasoprotective response. The present study aimed at characterizing whether KLF2 mediates statins-derived hepatic protection.

METHODS

Expression of KLF2 and its vasoprotective target genes was determined in SEC freshly isolated from control or CCl(4)-cirrhotic rats treated with four different statins (atorvastatin, mevastatin, simvastatin, and lovastatin), in the presence of mevalonate (or vehicle), under static or controlled shear stress conditions. KLF2-derived vasoprotective transcriptional programs were analyzed in SEC transfected with siRNA for KLF2 or siRNA-control, and incubated with simvastatin. Paracrine effects of SEC highly-expressing KLF2 on the activation status of rat and human hepatic stellate cells (HSC) were evaluated.

RESULTS

Statins administration to SEC induced significant upregulation of KLF2 expression. KLF2 upregulation was observed after 6h of treatment and was accompanied by induction of its vasoprotective programs. Simvastatin vasoprotection was inhibited in the presence of mevalonate, and was magnified in cells cultured under physiological shear stress conditions. Statin-dependent induction of vasoprotective genes was not observed when KLF2 expression was muted with siRNA. SEC overexpressing KLF2 induced quiescence of HSC through a KLF2-nitric oxide-guanylate cyclase-mediated paracrine mechanism.

CONCLUSIONS

Upregulation of hepatic endothelial KLF2-derived transcriptional programs by statins confers vasoprotection and stellate cells deactivation, reinforcing the therapeutic potential of these drugs for liver diseases that course with endothelial dysfunction.

Increased number and function of endothelial progenitor cells stimulate angiogenesis by resident liver sinusoidal endothelial cells (SECs) in cirrhosis through paracrine factors

BACKGROUND & AIMS

Recent studies have shown a pathological role of angiogenesis in the progression of chronic liver diseases (CLDs). The present study focused on numbers and angiogenic functions of circulating endothelial progenitor cells (EPCs) in patients with cirrhosis.

METHODS

Circulating EPCs were counted by flow-cytometry, and correlated with different parameters of liver disease. They were cultured in patients and controls to compare colony-formation, proliferation and tube formation. Interactions of EPCs with hepatic stellate cells (HSCs) and sinusoidal endothelial cells (SECs) were examined by indirect and direct co-cultures in presence of EPCs and EPC-conditioned medium, respectively. ELISA and inhibition assays were performed to assess the role of EPC-derived angiogenic factors.

RESULTS

The number of circulating EPCs was substantially higher in cirrhotic patients compared to controls (p<0.05), and showed good correlation with hepatic disease severity. Functional assays revealed that colonies and proliferation of EPCs were significantly increased in patients compared to controls (p<0.05). Direct and indirect co-cultures of patients' EPCs showed an increase in tube formation by SECs as compared to that observed with control EPCs (p<0.05). There was, however, no tube formation in HSC-EPC co-cultures. Levels of PDGF-BB and VEGF were substantially increased in patients' EPC media and inhibition of these factors by neutralizing antibodies led to a significant reduction in SECs proliferation.

CONCLUSIONS

Mobilization and proliferation of EPCs are significantly enhanced in cirrhotic patients in comparison to controls. EPCs may play an important paracrine role in liver angiogenesis by stimulating resident SECs in cirrhosis.

Endothelial dysfunction in the regulation of cirrhosis and portal hypertension

Portal hypertension is caused by an increased intrahepatic resistance, a major consequence of cirrhosis. Endothelial dysfunction in liver sinusoidal endothelial cells (LSECs) decreases the production of vasodilators, such as nitric oxide, and favours vasoconstriction. This contributes to an increased vascular resistance in the intrahepatic/sinusoidal microcirculation and develops portal hypertension. Portal hypertension, in turn, causes endothelial dysfunction in the extrahepatic, i.e. splanchnic and systemic, circulation. Unlike dysfunction in LSECs, endothelial dysfunction in the splanchnic and systemic circulation causes overproduction of vasodilator molecules, leading to arterial vasodilation. In addition, portal hypertension leads to the formation of portosystemic collateral vessels. Both arterial vasodilation and portosystemic collateral vessel formation exacerbate portal hypertension by increasing the blood flow through the portal vein. Pathological consequences, such as oesophageal varices and ascites, result. While the sequence of pathological vascular events in cirrhosis and portal hypertension has been elucidated, the underlying cellular and molecular mechanisms causing endothelial dysfunctions are not yet fully understood. This review article summarizes the current cellular and molecular studies on endothelial dysfunctions found during the development of cirrhosis and portal hypertension with a focus on the intra- and extrahepatic circulations. The article ends by discussing the future directions of the study for endothelial dysfunction.

In vivo liver endocytosis followed by purification of liver cells by liver perfusion

The liver is the metabolic center of the mammalian body and serves as a filter for the blood. The basic architecture of the liver is illustrated in figure 1 in which more than 85% of the liver mass is composed of hepatocytes and the remaining 15% of the cellular mass is composed of Kupffer cells (KCs), stellate cells (HSCs), and sinusoidal endothelial cells (SECs). SECs form the blood vessel walls within the liver and contain specialized morphology called fenestrae within in the cytoplasm. Fenestration of the cytoplasm is the appearance of holes (˜100 μm) within the cells so that the SECs act as a sieve in which most chylomicrons, chylomicron remnants and macromolecules, but not cells, pass through to the hepatocytes and HSCs ¹ (Fig. 1). Due to the lack of a basement membrane, the gap between the SECs and hepatocytes form the Space of Disse. HSCs occupy this space and play a prominent role in regulation and response to injury, storage of retinoic acid and immunoregulation of the liver ².

SECs are among the most endocytically active cells of the body displaying an array of scavenger receptors on their cell surface ³. These include SR-A, Stabilin-1 and Stabilin-2. Generally, small colloidal particles less than 230 nm and macromolecules in buffer phase are taken up by SECs, whereas, large particles and cellular debris is endocytosed (phagocytosed) by KCs ⁴. Thus, the bulk clearance of extracellular material such as the glycosaminoglycans from blood is largely dependent on the health and endocytic functions of SECs ^5,6. For example, an increase in blood hyaluronan levels is indicative of liver disease ranging from mild to more severe forms ⁷.

With the exception of one report ⁸, there are no immortalized SEC cell lines in existence. Even this immortalized cell line is de-differentiated in that it does not express scavenger receptors that are present on primary SECs (our data, not shown). All cell biological studies must be performed on primary cells obtained freshly from the animal. Unfortunately, SECs dedifferentiate under standard culture conditions and must be used within 1 or 2 days upon isolation from the animal. Differentiation of SECs is marked by the expression of Stabilin-2 or HARE receptor ⁹ , CD31, and the presence of cytoplasmic fenestration ¹. Differentiation of SECs can be extended by the addition of VEGF in culture media or by culturing cells in hepatocyte conditioned medium ^10,11.

In this report, we will demonstrate the endocytic activity of SECs in the intact organ using radio-labeled heparin for hyaluronan for the SEC-specific Stabilin-2 receptor. We will then purify hepatocytes and SECs from the perfused liver to measure endocytosis.

Tempol administration, a superoxide dismutase mimetic, reduces hepatic vascular resistance and portal pressure in cirrhotic rats

BACKGROUND & AIMS

Increased superoxide in cirrhotic livers, by reducing nitric oxide bioavailability, contributes to increase intrahepatic vascular resistance to portal blood flow and as a consequence portal pressure. We aimed to evaluate whether a strategy directed to reduce superoxide using tempol, a small membrane permeable SOD-mimetic, is able to modulate intrahepatic nitric oxide content and reduce portal pressure in cirrhotic rats.

METHODS

Superoxide and nitric oxide were evaluated in control sinusoidal endothelial cells (SEC) pre-treated with the pro-oxidant diethyldithiocarbamate (DDC) and in CCl(4)-cirrhotic rat livers treated with tempol or vehicle. Mean arterial pressure, portal pressure, and portal blood flow were measured in control and cirrhotic rats treated with tempol (180μmol/kg/h; via ileocholic vein) or vehicle. In a subset of animals, hemodynamic measurements were performed after NO-inhibition with l-NAME.

RESULTS

Tempol reduced superoxide content and increased NO both in SEC and cirrhotic livers. In cirrhotic rats, but not in controls, tempol significantly reduced portal pressure, and increased portal blood flow, which most likely reflects a reduction in intrahepatic vascular resistance. Tempol significantly reduced mean arterial pressure. l-NAME prevented all these effects.

CONCLUSIONS

Tempol reduces superoxide, increases nitric oxide, and reduces portal pressure in sinusoidal endothelial cells and in cirrhotic livers. These results confirm that oxidative stress has a role in the pathogenesis of portal hypertension and supports the use of antioxidants in its treatment. However, when considering the use of antioxidants as additional therapy to treat portal hypertension, the potential to produce deleterious effects on systemic hemodynamics needs to be carefully evaluated.

The hepatic microcirculation: mechanistic contributions and therapeutic targets in liver injury and repair

The complex functions of the liver in biosynthesis, metabolism, clearance, and host defense are tightly dependent on an adequate microcirculation. To guarantee hepatic homeostasis, this requires not only a sufficient nutritive perfusion and oxygen supply, but also a balanced vasomotor control and an appropriate cell-cell communication. Deteriorations of the hepatic homeostasis, as observed in ischemia/reperfusion, cold preservation and transplantation, septic organ failure, and hepatic resection-induced hyperperfusion, are associated with a high morbidity and mortality. During the last two decades, experimental studies have demonstrated that microcirculatory disorders are determinants for organ failure in these disease states. Disorders include 1) a dysregulation of the vasomotor control with a deterioration of the endothelin-nitric oxide balance, an arterial and sinusoidal constriction, and a shutdown of the microcirculation as well as 2) an overwhelming inflammatory response with microvascular leukocyte accumulation, platelet adherence, and Kupffer cell activation. Within the sequelae of events, proinflammatory mediators, such as reactive oxygen species and tumor necrosis factor-alpha, are the key players, causing the microvascular dysfunction and perfusion failure. This review covers the morphological and functional characterization of the hepatic microcirculation, the mechanistic contributions in surgical disease states, and the therapeutic targets to attenuate tissue injury and organ dysfunction. It also indicates future directions to translate the knowledge achieved from experimental studies into clinical practice. By this, the use of the recently introduced techniques to monitor the hepatic microcirculation in humans, such as near-infrared spectroscopy or orthogonal polarized spectral imaging, may allow an early initiation of treatment, which should benefit the final outcome of these critically ill patients.

Dai-kenchu-to attenuates rat sinusoidal obstruction syndrome by inhibiting the accumulation of neutrophils in the liver

BACKGROUND AND AIM

Sinusoidal obstruction syndrome (SOS) is drug-induced liver injury that occurs in patients who receive hematopoietic cell transplantation and oxaliplatin-contained chemotherapy. The aim of study was to investigate the pharmacological treatment of SOS using a traditional Japanese medicine, Dai-kenchu-to (DKT).

METHODS

Male Sprague-Dawley rats were treated with monocrotaline (MCT) to induce SOS. The rats were divided into three groups: control, MCT and MCT+DKT groups. In the MCT+DKT group, DKT was gavaged at 12 h after MCT treatment and given every 12 h until the end of the protocol. The rats of MCT group were treated with water instead of DKT. At 48 h after MCT treatment, blood and liver samples were collected.

RESULTS

In the MCT+DKT group, the macroscopic and histological findings revealed liver congestion, sinusoidal alteration and the destruction of sinusoidal lining, which were comparable with those of the MCT group. However, the area of hepatic necrosis and serum AST levels significantly decreased in the MCT+DKT group compared with those of the MCT group. Treatment with DKT resulted in the reduction of neutrophil accumulation, myeloperoxidase activity and the expression of cytokine-induced neutrophil chemoattractant (CINC) and intracellular adhesion molecule-1 (ICAM-1) mRNA in the liver compared with those of the MCT group. Treatment with processed ginger, one of the ingredients in DKT, resulted in similar effects to those shown by DKT.

CONCLUSIONS

Dai-kenchu-to attenuates MCT-induced liver injury by preventing neutrophil-induced liver injury through blockage of upregulation of CINC and ICAM-1 mRNA level.

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

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

The management of portal hypertension: rational basis, available treatments and future options

Variceal bleeding is the last step in a chain of events initiated by an increase in portal pressure, followed by the development and progressive dilation of varices until these finally rupture and bleed. This sequence of events might be prevented - and reversed - by achieving a sufficient decrease in portal pressure. A different approach is the use of local endoscopic treatments at the varices. This article reviews the rationale for the management of patients with cirrhosis and portal hypertension, the current recommendations for the prevention and treatment of variceal bleeding, and outlines the unsolved issues and the perspectives for the future opened by new research developments.

Increased oxidative stress in cirrhotic rat livers: A potential mechanism contributing to reduced nitric oxide bioavailability

In cirrhotic livers, decreased nitric oxide (NO) bioavailability is a major factor increasing intrahepatic vascular tone. In several vascular disorders, an increase in superoxide (O(2) (-)) has been shown to contribute to reduced NO bioavailability through its reaction with NO to form peroxynitrite. This study was aimed to test the hypothesis that, in cirrhotic livers, increased O(2) (-), by reacting with NO, reduces NO bioavailability. In control and cirrhotic rat livers, NO bioavailability was evaluated by the measurement of cyclic guanosine monophosphate in liver tissue and by 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM-DA) fluorescence in isolated sinusoidal endothelial cells (SEC); the O(2) (-) content was determined by dihydroethidium staining in fresh liver sections. In addition, the role of endothelial nitric oxide synthase (eNOS), xanthine oxidase (XO), and cyclooxygenase (COX) as possible sources of O(2) (-) and the role of superoxide dismutase (SOD) enzymatic activity as an O(2) (-) scavenger were determined in liver homogenates. Protein-nitrotyrosination, a marker of the NO-O(2) (-) reaction, was evaluated in liver homogenates. Furthermore, in control SEC and bovine aortic endothelial cells, NO modulation by O(2) (-) was evaluated. Cirrhotic livers exhibited increased O(2) (-) levels. This was due, at least in part, to increased production by COX and XO but not eNOS and to reduced scavenging by SOD. Increased O(2) (-) was associated with a significant reduction in NO bioavailability and increased nitrotyrosinated proteins. In endothelial cells, an inverse relationship between O(2) (-) levels and NO bioavailability was observed.

CONCLUSION

Our data show that oxidative stress may contribute to reduced NO bioavailability in cirrhotic livers, supporting the evaluation of O(2) (-) reduction as a potential mechanism to restore NO content.

Large-conductance calcium-activated potassium channels modulate vascular tone in experimental cirrhosis

BACKGROUND

Large-conductance calcium-activated potassium (BK(Ca)) channels regulate vascular tone in different vascular systems. Moreover, activated hepatic stellate cells (HSC) contain BK(Ca) channels. The aim of this study was to evaluate the role of BK(Ca) channels in the regulation of vascular tone in control (CT) and carbon tetrachloride-cirrhotic (CH) rat livers.

METHODS

Changes in intrahepatic vascular resistance were assessed by evaluating the portal perfusion pressure (PP) response to methoxamine (Mtx) in the presence of Iberiotoxin (Ibtx; a BK(Ca) channel blocker), NS1619 (a BK(Ca) channel opener), Ibtx plus the nitric oxide (NO) synthase inhibitor, N(G)-nitro-L-arginine (L-NNA) or L-NNA alone. In addition, in CH livers, PP dose-response curves to the NO donor, S-nitroso-N-acetyl-D,L-penicillamine (SNAP), were performed after pre-incubation with Ibtx or its vehicle. BK(Ca) mRNA expression was assessed in liver homogenates, and BK(Ca) protein expression in HSC isolated from CT and CH livers.

RESULTS

In CH livers, Ibtx significantly increased baseline PP and exacerbated the PP response to Mtx. Conversely, NS1619 induced a mild nonsignificant decrease of baseline PP and attenuated the hyperresponse to Mtx. CH livers exhibited an upregulation of both mRNA and protein of the alpha-subunit of BK(Ca).

CONCLUSION

Large-conductance calcium-activated potassium channels are overexpressed in CH livers and might represent a compensatory mechanism modulating the increased hepatic vascular tone of cirrhosis.

Hemodynamics in the isolated cirrhotic liver

Increased intrahepatic resistance is the initial event to the increased portal pressure and development portal hypertension in cirrhosis. Narrowing of the sinusoids due to anatomic changes is the main component of the increased intrahepatic resistance. However, a dynamic component is also involved in the increased vascular tone in cirrhosis. The imbalance between the hyperresponsiveness and overproduction of vasoconstrictors (mainly endothelin-1 and cyclooxygenase-derived prostaglandins) and the hyporesponsiveness and impaired production of vasodilators [mainly nitric oxide (NO)] are the mechanisms responsible of the increased vascular tone in the sinusoidal/postsinusoidal area. In contrast, the vascular resistance in the hepatic artery, which is determined in the presinusoidal area, is decreased due to increased vasodilators (NO and adenosine). This suggests different availabilities of NO in the intrahepatic circulation with preserved production in the presinusoidal area and impaired production in the sinusoidal/postsinusoidal area.

Vascular deterioration in cirrhosis: the big picture

Cirrhosis is characterized by marked abnormalities in the hepatic circulation. Functionally, there is an increased vascular tone and impaired flow-mediated vasorelaxation, whereas anatomically there is sinusoidal remodeling and capillarization, angiogenesis, venous thrombosis, and vascular distortion, all contributing to increase hepatic vascular resistance and portal hypertension. However, vascular changes are not limited to the liver, but are also present in the splanchnic organs, heart, lungs, kidney, brain, and skin. Advances in the knowledge of the mechanisms of these abnormalities have disclosed new targets for therapy and ultimately improved survival.

Vasoactive factors and hemodynamic mechanisms in the pathophysiology of portal hypertension in cirrhosis

Portal hypertension is primarily caused by the increase in resistance to portal outflow and secondly by an increase in splanchnic blood flow, which worsens and maintains the increased portal pressure. Increased portal inflow plays a role in the hyperdynamic circulatory syndrome, a characteristic feature of portal hypertensive patients. Almost all the known vasoactive systems/substances are activated in portal hypertension, but most authors stress the pathogenetic role of endothelial factors, such as COX-derivatives, nitric oxide, carbon monoxide. Endothelial dysfunction is differentially involved in different vascular beds and consists in alteration in response both to vasodilators and to vasoconstrictors. Understanding the pathogenesis of portal hypertension could be of great utility in preventing and curing the complications of portal hypertension, such as esophageal varices, hepatic encephalopathy, ascites.

Screening for genes regulated by gamma aminobutyric acid in hepatic stellate cells using suppression subtractive hybridization

AIM: To screen for genes regulated by gamma aminobutyric acid (GABA) in hepatic stellate cells (HSC) using suppression subtractive hybridization (SSH), and to investigate the biological function of GABA in the liver.

METHODS: The hepatic stellate cell line HSC-T6 was co-cultured with 10 μmol/L GABA for 24 h, after which mRNA was extracted and reverse transcribed into cDNA. HSC-T6 cells co-cultured with phosphate buffer saline (PBS) were used as controls. mRNAs upregulated by GABA were identified by SSH. Thirty one randomly selected clones were sequenced and analyzed bioinformatically.

RESULTS: Fifteen genes were found to be significantly up-regulated, including genes involved in DNA synthesis, apoptosis, mitochondrial function and tumor suppression. These results showed that GABA might promote HSC-T6 cell proliferation and inhibit apoptosis.

CONCLUSION: SSH technology successfully enabled the identification of genes that are differentially expressed in the presence and absence of GABA, demonstrating that GABA can affect the gene expression profiles of HSCs.

Enhanced vasoconstrictor prostanoid production by sinusoidal endothelial cells increases portal perfusion pressure in cirrhotic rat livers

BACKGROUND/AIMS

Cyclooxygenase-1 (COX-1) is overexpressed in sinusoidal endothelial cells (SEC) of cirrhotic rat livers, and through an enhanced production of vasoconstrictor prostanoids contributes to increase intrahepatic resistance. Our study was aimed at investigating the role of enhanced AA bioavailability modulating the hepatic vascular tone of cirrhotic livers and identifying which prostanoid is involved.

METHODS

SEC isolated from control and cirrhotic rat livers were incubated with AA, methoxamine or vehicle. TXA(2) was quantified. In addition, portal perfusion pressure (PP) response curves to AA were performed in rat livers pre-incubated with vehicle, SC-560 (COX-1 inhibitor), Furegrelate (inhibitor of TXA(2) synthesis) and SQ-29548 (PGH(2)/TXA(2) receptor blocker). cPLA2 activity was determined in control and cirrhotic livers.

RESULTS

AA and methoxamine incubation promoted a significant increase in TXA(2) release by Cirrhotic-SEC, but not in Control-SEC. AA produced a dose-dependent increase in the PP, associated with increased TXA(2) release. These responses were significantly greater in cirrhotic livers. COX-1 inhibition and PGH(2)/TXA(2) receptor blockade, but not TXA(2) synthase inhibition, markedly attenuated the PP response to AA of cirrhotic livers. Additionally, cirrhotic livers exhibited significantly increased cPLA2 activity.

CONCLUSIONS

An enhanced production of vasoconstrictor prostanoids, probably PGH(2), by SEC contributes to increase vascular tone of cirrhotic livers.

Vascular endothelial dysfunction in cirrhosis

Endothelial dysfunction is regarded as an early key event in multiple diseases. The assessment of vascular nitric oxide (NO) level is an indicative of endothelial dysfunction. In liver cirrhosis, on one hand, endothelial dysfunction is known as impaired endothelium-dependent relaxation in the liver microcirculation and contributes to increased intra-hepatic vascular resistance, leading to portal hypertension. On the other, increased production of vasodilator molecules mainly NO contributes to increased endothelium-dependent relaxation in the arteries of the systemic and splanchnic circulation. The aims of this review are to summarize and discuss: (1) unique characteristics of sinusoidal endothelial cell (SECs) and SEC dysfunctions in cirrhosis, and (2) endothelial dysfunctions in the arterial splanchnic and systemic circulation in cirrhosis with portal hypertension.

Identification and functional characterization of the hepatic stellate cell CD38 cell surface molecule

The activation of hepatic stellate cells (HSCs) is a critical event in hepatic fibrosis, because these cells are the main producers of extracellular matrix proteins in the liver and contribute to the modulation of inflammatory responses via the secretion of several cytokines and the expression of adhesion molecules. The goal of the present study was to characterize cell surface proteins that regulate HSC activation. To this end, a panel of monoclonal antibodies (mAbs) was generated. mAb 14.27 recognized a protein of 45 kd that was highly expressed on HSCs. Affinity purification of this protein followed by sequencing revealed that protein to be CD38. We subsequently demonstrated that CD38 was constitutively expressed by HSCs and that its expression increased after in vitro and in vivo activation. mAb 14.27 induced an increase in cytosolic Ca2+ levels in HSCs, showing that it functions as an agonistic antibody. Moreover, the effects mediated by the CD38 mAb included induction of the proinflammatory cytokine interleukin-6 and up-regulation of the adhesion molecules intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and neural cell adhesion molecule. Collectively, our data suggest that CD38 can act as a regulator of HSC activation and effector functions.

Nitroflurbiprofen, a nitric oxide-releasing cyclooxygenase inhibitor, improves cirrhotic portal hypertension in rats

BACKGROUND & AIMS

We studied whether administration of nitroflurbiprofen (HCT-1026), a cyclooxygenase inhibitor with nitric oxide (NO)-donating properties, modulates the increased intrahepatic vascular tone in portal hypertensive cirrhotic rats.

METHODS

In vivo hemodynamic measurements (n = 8/condition) and evaluation of the increased intrahepatic resistance by in situ perfusion (n = 5/condition) were performed in rats with thioacetamide-induced cirrhosis that received either nitroflurbiprofen (45 mg/kg), flurbiprofen (30 mg/kg, equimolar concentration to nitroflurbiprofen), or vehicle by intraperitoneal injection 24 hours and 1 hour prior to the measurements. Additionally, we evaluated the effect of acute administration of both drugs (250 micromol/L) on the intrahepatic vascular tone in the in situ perfused cirrhotic rat liver (endothelial dysfunction and hyperresponsiveness to methoxamine) and on hepatic stellate cell contraction in vitro. Typical systemic adverse effects of nonsteroidal anti-inflammatory drugs, such as gastrointestinal ulceration, renal insufficiency, and hepatotoxicity, were actively explored.

RESULTS

In vivo, nitroflurbiprofen and flurbiprofen equally decreased portal pressure (8 +/- 0.8 and 8.4 +/- 0.1 mm Hg, respectively, vs 11.8 +/- 0.6 mm Hg) and reduced the total intrahepatic vascular resistance. Systemic hypotension was not aggravated in the different treatment groups (P = .291). In the perfused cirrhotic liver, both drugs improved endothelial dysfunction and hyperresponsiveness. This was associated with a decreased hepatic thromboxane A(2)-production and an increased intrahepatic nitrate/nitrite level. In vitro, nitroflurbiprofen, more than flurbiprofen, decreased hepatic stellate cells contraction. Flurbiprofen-treated rats showed severe gastrointestinal ulcerations (bleeding in 3/8 rats) and nefrotoxicity, which was not observed in nitroflurbiprofen-treated cirrhotic rats.

CONCLUSIONS

Treatment with nitroflurbiprofen, an NO-releasing cyclooxygenase inhibitor, improves portal hypertension without major adverse effects in thioacetamide-induced cirrhotic rats by attenuating intrahepatic vascular resistance, endothelial dysfunction, and hepatic hyperreactivity to vasoconstrictors.

The role of the cylooxygenase pathway in nociception and pain

Cycloxygenase (COX) pathways have long been targeted for the treatment of inflammatory pain, initially through the use of NSAIDs. With the demonstration of two major COX isoforms, COX-1 and COX-2, involved in the production of prostanoids, but with different distribution and regulation, selective COX-2 inhibitors have been developed. This review covers factors influencing COX enzyme activity, the role of their products in the development and maintenance of pain and discusses recent safety concerns of COX-2 inhibitors.

Effect of verapamil on nitric oxide synthase in a portal vein-ligated rat model: Role of prostaglandin

AIM: To investigate the effects of verapamil on nitric oxide (NO) synthesis in a portal vein-ligated rat model.

METHODS: Systemic and splanchnic hemodynamics were measured by radiolabeled microspheres in portal hypertensive rats after acute administration of verapamil (2 mg/kg) on chronic treatment with N^w–nitro-L-arginine (NNA)(80 mg/kg) and/or indomethacin (2 mg/kg) .

RESULTS: Verapamil (2 mg/kg) caused a marked fall in both arterial pressure and cardiac output accompanied by an insignificant change in the portal pressure and no change in portal venous inflow. This result suggested that verapamil did not cause a reduction in portal vascular resistance of portal hypertensive rats, which was similar between N^w- nitro–L-arginine-treated and indomethacin-treated groups.

CONCLUSION: In portal hypertensive rats pretreated with NNA and/or indomethacin, acute verapamil administration can not reduce the portal pressure, suggesting that NO and prostaglandin play an important role in the pathogenesis of splanchnic arterial vasodilation in portal hypertension.