Gene transfer of the neuronal NO synthase isoform to cirrhotic rat liver ameliorates portal hypertension

The future treatment of portal hypertension

Increased understanding of the hyperdynamic circulation syndrome has resulted in novel therapeutic approaches, some of which have already reached clinical practice. Central to the hyperdynamic circulation syndrome is an imbalance between the increase in different vasodilators (foremost among which is nitric oxide) and the compensatory increase in vasoconstrictors--usually accompanied by a blunted response. This chapter discusses the role of endothelin in the pathogenesis of the syndrome and in future treatment approaches. A relatively new area of research in this field is the role of infection and inflammation in the initiation and maintenance of the hyperdynamic circulation syndrome. The use of antibiotics in the setting of acute variceal bleeding is standard practice. Studies have suggested that chronic manipulation of the intestinal flora could have beneficial effects in the treatment of portal hypertension. The bile salts are another novel and interesting target. Although their vasoactive properties have been known for some time, recent data demonstrate that their effects could be central in the pathogenesis of the hyperdynamic circulation syndrome, and that manipulation of the composition of the bile acid pool could be a therapeutic approach to portal hypertension. Finally, hypoxia and angiogenesis play a role in the development of portal hypertension and the formation of collaterals. This role needs to be further defined but it appears likely that this phenomenon is yet another target for therapeutic intervention.

Hemodynamic and antifibrotic effects of a selective liver nitric oxide donor V-PYRRO/NO in bile duct ligated rats

AIM: To assess whether a liver specific nitric oxide (NO) donor (V-PYRRO/NO) would prevent the development of portal hypertension and liver fibrosis in rats with bile duct ligation (BDL).

METHODS: Treatment (placebo or V-PYRRO/NO 0.53 μmol/kg per hour) was administered i.v. to rats 2 d before BDL (D-2) and maintained until the day of hemodynamic measurement (D26). Intra-hepatic NO level was estimated by measuring liver cGMP level. Effects of V-PYRRO/NO on liver fibrosis and lipid peroxidation were also assessed.

RESULTS: Compared to placebo treatment, V-PYRRO/NO improved splanchnic hemodynamics in BDL rats: portal pressure was significantly reduced by 27% (P < 0.0001) and collateral circulation development was almost completely blocked (splenorenal shunt blood flow by 74%, P = 0.007). Moreover, V-PYRRO/NO significantly prevented liver fibrosis development in BDL rats (by 30% in hepatic hydroxyproline content and 31% in the area of fibrosis, P < 0.0001 respectively), this effect being probably due to a decrease in lipid peroxidation by 44% in the hepatic malondialdehyde level (P = 0.007). Interestingly, we observed a significant and expected increase in liver cGMP, without any systemic hemodynamic effects (mean arterial pressure, vascular systemic resistance and cardiac output) in both sham-operated and BDL rats treated with V-PYRRO/NO. This result is in accordance with studies on V-PYRRO/NO metabolism showing a specific release of NO in the liver.

CONCLUSION: Continuous administrations of V-PYRRO/NO in BDL rats improved liver fibrosis and splanchnic hemodynamics without any noxious systemic hemo-dynamic effects.

Intrahepatic upregulation of RhoA and Rho-kinase signalling contributes to increased hepatic vascular resistance in rats with secondary biliary cirrhosis

BACKGROUND AND AIMS

Portal hypertension in cirrhosis is mediated in part by increased intrahepatic resistance, reflecting an increased sensitivity of the hepatic microvasculature to vasoconstrictors. Activation of the RhoA/Rho-kinase pathway is essential for contraction of vascular smooth muscle. The aim of this study was to investigate RhoA/Rho-kinase mediated regulation of the intrahepatic vascular tone in cirrhotic rats.

METHODS

Cirrhosis was induced by bile duct ligation (BDL). Hepatic RhoA and Rho-kinase expressions were studied by real time reverse transcription polymerase chain reaction and western blot analysis. Hepatic Rho-kinase activity in rat and human livers was assessed as phosphorylation of the Rho-kinase substrate moesin. The effect of the Rho-kinase inhibitor Y-27632 on hepatic perfusion pressure was measured in livers perfused at constant flow. The in vivo effect of intravenous application of Y-27632 was studied by haemodynamic measurements.

RESULTS

Hepatic expressions of RhoA and Rho-kinase were increased at mRNA and protein level in BDL rats. Intrahepatic moesin phosphorylation was increased in livers from cirrhotic rats and patients with alcohol induced cirrhosis. Y-27632 reduced the basal perfusion pressure of in situ perfused livers in BDL rats but not in sham operated rats. Y-27632 reduced the sensitivity to methoxamine in isolated perfused livers in sham operated rats more than in BDL rats. In vivo, Y-27632 reduced portal pressure to a greater extent in BDL rats than in sham operated rats. Intrahepatic vascular resistance was decreased in response to bolus injection of Y-27632 in BDL rats but not in sham operated rats.

CONCLUSIONS

Upregulation of RhoA and Rho-kinase contributes to increased intrahepatic resistance in cirrhotic rats and to an increased sensitivity of cirrhotic livers to vasoconstrictors.

Reversal of fibrosis: no longer a pipe dream?

The reversal of liver fibrosis is not a new phenomenon. Treatment of the primary disease remains the most effective strategy, but new approaches to promote resolution of fibrosis are being built on the foundations that were provided by research into the basic mechanisms of fibrogenesis. A return to normal hepatic architecture from advanced fibrosis is achievable in some cases, and cirrhosis itself may be partly remediable.

Ascorbic acid improves the intrahepatic endothelial dysfunction of patients with cirrhosis and portal hypertension

Patients with cirrhosis show intrahepatic endothelial dysfunction, characterized by an impaired flow-dependent vasorelaxation. This alteration is responsible for the marked postprandial increase in portal pressure and is attributed to an insufficient release of nitric oxide (NO). Ascorbic acid reverts endothelial dysfunction in other vascular disorders, via the increase of NO bioavailability through the neutralization of superoxide anions, thus preventing the scavenging of NO by superoxide. This study examined whether acute ascorbic acid administration might improve endothelial dysfunction in cirrhosis. Thirty-seven portal hypertensive patients with cirrhosis had measurements of hepatic and systemic hemodynamics, ascorbic acid, and malondialdehyde (MDA). Patients were randomly allocated to receive ascorbic acid (3 g, intravenously, n = 15) or placebo (n = 12) followed by a liquid meal. A third group received ascorbic acid followed by a sham meal (n = 10). Measurements were repeated after 30 minutes (hepatic venous pressure gradient at 15 and 30 minutes). Patients with cirrhosis had significantly lower ascorbic acid levels and higher MDA than healthy controls. Ascorbic acid significantly reduced MDA levels and markedly attenuated the postprandial increase in the hepatic venous pressure gradient (4% +/- 7% vs. 18% +/- 10% in placebo at 30 minutes, P < .001). Ascorbic acid followed by sham meal did not modify hepatic or systemic hemodynamics. In conclusion, patients with cirrhosis exhibited intrahepatic endothelial dysfunction, associated with decreased levels of ascorbic acid and increased levels of MDA. Ascorbic acid improved intrahepatic endothelial dysfunction, blunting the postprandial increase in portal pressure. These results encourage the performance of further studies testing antioxidants as adjunctive therapy in the treatment of portal hypertension.

Emerging therapies for liver fibrosis

Liver fibrosis occurs as a result of a wide range of injurious processes and in its end-stage results in cirrhosis. This gross disruption of liver architecture is associated with impaired hepatic function, portal hypertension and significant resultant morbidity and mortality. Indeed, liver fibrosis and cirrhosis represent a major worldwide healthcare burden. Recent progress in liver transplantation, the management of portal hypertension and the treatment of chronic viral hepatitis have had an important impact. However, these approaches are not without their limitations - in particular, issues regarding organ availability for transplantation - and serve to highlight the urgent requirement to influence pharmacologically the underlying fibrotic process in many patients. Liver fibrosis has been shown to be a bidirectional process and increasing data from laboratory and clinical studies reveal that even advanced fibrosis and cirrhosis are potentially reversible. Exploration of the molecular mechanisms underlying this bi-directionality will lead to char acterisation of the essential attributes of an antifibrotic therapy. In this review, these mechanisms are highlighted and the growing number of emerging antifibrotic agents discussed.

Antifibrotic agents for liver disease

Complications from chronic hepatitis C (HCV) and recurrent HCV post-transplant are responsible for significant morbidity and mortality in the United States and Europe. Current antiviral therapies are at best, effective in up to 50% of patients in the pre-transplant setting, and in the post-transplant setting are associated with more limited efficacy and increased toxicity. With this reduced efficacy of antiviral strategies in the post-transplant setting, new approaches are urgently needed. Substantial progress has been made in understanding the pathogenesis of hepatic fibrosis over the last 20 years, which has yielded potential new therapeutic targets. The prospect of antifibrotic therapies is nearing reality in order to reduce progression to cirrhosis, thereby reducing morbidity, mortality and the need for re-transplantation. Current and evolving approaches primarily target the activated hepatic stellate cells, which are the main source of extracellular matrix, along with related fibrogenic cell types. Key issues yet to be clarified include the optimal duration of antifibrotic therapies, endpoints of clinical trials, indications in clinical practice and whether combination therapies might yield synergistic activity.

Portal hypertension: from pathophysiology to clinical practice

Portal hypertension (PHT) is responsible for the more severe and often lethal complications of cirrhosis such as bleeding oesophageal varices, ascites, renal dysfunction and hepatic encephalopathy. Because of the combined impact of these complications, PHT remains the most important cause of morbidity and mortality in patients with cirrhosis. Over the years, it has become clear that a decrease in portal pressure is not only protective against the risk of variceal (re)bleeding but is also associated with a lower long-term risk of developing complications and an improved long-term survival. A milestone in therapy was the introduction of non-selective beta-blockers for the prevention of bleeding and rebleeding of gastro-esophageal varices. However, in practice, less than half the patients under beta-blockade are protected from these risks, supporting the overall demand for innovation and expansion of our therapeutic armamentarium. Recent advances in the knowledge of the pathophysiology of cirrhotic PHT have directed future therapy towards the increased intrahepatic vascular resistance, which, in part, is determined by an increased hepatic vascular tone. This increased vasculogenic component provides the rationale for the potential use of therapies aimed at increasing intrahepatic vasorelaxing capacity via gene therapy, liver-selective nitric oxide donors and statines on the one hand, and at antagonizing excessive intrahepatic vasoconstrictor force through the use of endothelin antagonists, angiotensin blockers, alpha(1) adrenergic antagonists or combined alpha(1)- and non-selective beta-blockers or somatostatin analogues on the other. The focus of this review is to give an update on the pathophysiology of PHT in order to elucidate these potential novel strategies subsequently.

Research progress of vasculopathy in portal hypertension

Portal hypertension, one of the vascular diseases, not only has lesions in liver, but also changes in vascular structures and functions of extrahepatic portal system, systemic system and pulmonary circulation. The pathological changes of vasculopathy in portal hypertension include remodeling of arterialized visceral veins, intimal injury of visceral veins and destruction of contractile structure in visceral arterial wall. The mechanisms of vasculopathy in portal hypertension may be attributed to the changes of hemodynamics in portal system, immune response, gene modulation, vasoactive substances, and intrahepatic blood flow resistance. Portal hypertension can cause visceral hyperdynamic circulation, and the development and progression of visceral vasculopathy, while visceral vasculopathy can promote the development and progression of portal hypertension and visceral hyperdynamic circulation in turn. The aforementioned three factors interact in the pathogenesis of hepatic cirrhosis-induced portal hypertension and are involved in hemorrhage due to varicose vein rupture.

A crucial role for GRK2 in regulation of endothelial cell nitric oxide synthase function in portal hypertension

Nitric oxide (NO) production by endothelial cell nitric oxide synthase (eNOS) in sinusoidal endothelial cells is reduced in the injured liver and leads to intrahepatic portal hypertension. We sought to understand the mechanism underlying defective eNOS function. Phosphorylation of the serine-threonine kinase Akt, which activates eNOS, was substantially reduced in sinusoidal endothelial cells from injured livers. Overexpression of Akt in vivo restored phosphorylation of Akt and production of NO and reduced portal pressure in portal hypertensive rats. We found that Akt physically interacts with G-protein-coupled receptor kinase-2 (GRK2), and that this interaction inhibits Akt activity. Furthermore, GRK2 expression increased in sinusoidal endothelial cells from portal hypertensive rats and knockdown of GRK2 restored Akt phosphorylation and NO production, and normalized portal pressure. Finally, after liver injury, GRK2-deficient mice developed less severe portal hypertension than control mice. Thus, an important mechanism underlying impaired activity of eNOS in injured sinusoidal endothelial cells is defective phosphorylation of Akt caused by overexpression of GRK2 after injury.

Effect of chronic ethanol administration on hepatic eNOS activity and its association with caveolin-1 and calmodulin in female rats

Although chronic and excessive alcohol consumption is associated with liver disease, the mechanism of alcoholic liver injury is still not clear. Whether reduced hepatic production of nitric oxide, which is evident in models of liver injury, is associated with alcohol-induced liver injury has not been investigated. We measured nitric oxide synthase (NOS) activity in the liver of pair-fed rats receiving liquid diet with or without alcohol [3% (vol/vol)] for 12 wk. Compared with control rats, hepatic NOS activity was significantly reduced in alcohol-treated rats along with the evidence of liver injury. Interestingly, there was no difference in the hepatic expression of endothelial NOS (eNOS) between ethanol-fed and pair-fed rats. We then tested the hypothesis that an imbalance between the binding of eNOS with inhibitory and stimulatory proteins may underlie the reduced activity of eNOS because eNOS catalytic activity is regulated partly through dynamic interactions with the inhibitory protein caveolin-1 and the stimulatory protein calmodulin. We found that hepatic caveolin-1 was markedly increased in alcohol-treated rats compared with control rats, whereas calmodulin remained unaltered. The binding of caveolin-1 and calmodulin with eNOS was increased and decreased, respectively, in alcohol-treated rats. Our results suggest that chronic alcohol intake attenuates hepatic eNOS activity by increasing the expression of the inhibitory protein caveolin-1 and enhancing its binding with eNOS.

Targeted treatments for cirrhosis

The causes of hepatic scarring (fibrosis) are protean but, unchecked, all result in a common fate--the development of cirrhosis--with gross disruption of the normal liver architecture. Subsequent liver cell dysfunction and portal hypertension give rise to major systemic complications and premature death. Cirrhosis and its sequelae represent a huge, and global, healthcare burden. The success of liver transplantation and the development of efficacious antiviral regimens for hepatitis B and C should not be underestimated, but they also serve to highlight our current inability to manipulate the underlying fibrotic process in many patients with liver disease. Moreover, transplantation as a treatment is limited by organ availability, among other factors. The development of antifibrotic therapies is urgently needed and for this we require a mechanistic and evidence-based approach. Accumulating data from clinical and laboratory studies demonstrate that even advanced fibrosis and cirrhosis are potentially reversible. The hepatic stellate cells have been identified as the pivotal effector cells orchestrating the fibrotic process and, furthermore, reversibility appears to hinge upon their elimination. This review draws on recent scientific advances, and highlights emerging therapeutic interventions in liver fibrosis.

rAAV-mediated stable expression of heme oxygenase-1 in stellate cells: a new approach to attenuate liver fibrosis in rats

Liver fibrosis is the consequence of activation of hepatic stellate cells mediated by persistent or recurrent liver injury, where oxidative stress or inflammatory response resulting from immune cells and cytokines are involved. Targeting of hepatic stellate cells could be an important strategy for the therapy of liver fibrosis. In this study, we showed a tropism of recombinant adeno-associated virus (rAAV, serotype 2) with high efficiency in transduction of a homeostatic gene, heme oxygenase-1 (HO-1), to activated stellate cells. The binding of rAAVs to stellate cells increased significantly after serum-stimulated activation compared with quiescent status. Portal injection of rAAVs to normal or carbon tetrachloride (CCl(4))-induced liver fibrosis showed a distinct distribution of rAAV binding. The majority of injected rAAVs bound to the cells in fibrotic areas that were associated with higher expression levels of fibroblast growth factor receptor-1alpha at 2 hours after administration. Isolation of different types of cells from CCl(4)-induced fibrotic livers showed predominant expression of transgene in stellate cells after rAAV/HO-1 administration on day 3 and remained stable for 12 weeks. In addition, HO-1-transduced stellate cells showed reduced transcript levels of type 1 collagen and impaired proliferative ability compared with controls. With this approach, the severity of established micronodular cirrhosis was markedly reduced. In conclusion, these findings suggest a new approach for the treatment of liver fibrosis using adeno-associated virus-mediated gene transfer.

Chronic administration of aminoguanidine reduces vascular nitric oxide production and attenuates liver damage in bile duct-ligated rats

BACKGROUND

Nitric oxide (NO) has been implicated in the pathogenesis of liver cirrhosis. This study investigated the activity of nitric oxide synthase (NOS) in cirrhosis induced by bile duct-ligation (BDL) with NOS inhibitors.

METHOD

Three days after operation, rats were randomized to receive aminoguanidine (AG, 25 mg/kg/day) or L-N(G)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg/day) for 21 days.

RESULTS

Vascular NO production, which was increased in BDL cirrhotic rats, was reduced by 75% with AG but not L-NAME chronic administration. AG treatment attenuated liver damage, while L-NAME aggravated it. AG significantly suppressed inducible NOS (iNOS) expression in aorta of BDL rats at both mRNA and protein level, but much less efficient in reducing it in liver. In contrast, endothelial NOS (eNOS) expression was not markedly affected. Calcium-independent NOS activity, which was dramatically increased in aorta of BDL rats, was abolished by AG treatment. In liver, however, both calcium-dependent and -independent NOS activity were increased by AG treatment.

CONCLUSION

Chronic administration of AG could reduce systemic NO levels as well as suppress iNOS expression and activity in aorta of BDL rats. It also improved liver function, possibly because of its ability to increase hepatic NOS activity, and to correct the systemic hemodynamic disorders by decreasing vascular NO production.

Increased hepatic resistance: a new target in the pharmacologic therapy of portal hypertension

Increased resistance to portal blood flow is the primary factor in the pathophysiology of portal hypertension, and is mainly determined by the morphologic changes occurring in chronic liver diseases. This is aggravated by an increased hepatic vascular tone, which results from an insufficient hepatic bioavailability of nitric oxide (NO) and an increased production of circulating and local vasoconstrictors (angiotensin, endothelin, cysteinyl-leukotrienes, and thromboxane, among others). This dynamic and reversible component provides the rationale for the use of therapies aimed at decreasing portal pressure by reducing the vascular tone. Among them, systemic and liver-selective NO donors, statins, and gene therapy with adenovirus encoding NO synthases have been used to increase NO availability with promising results. Other attempts have been the blockade of the effect of vasoconstrictors, using anti alpha-adrenergic agents and renin-angiotensin system blockers. Some of these pharmacologic approaches have already been incorporated into clinical practice while others are still under investigation.

Synaptic and vascular associations of neurons containing cyclooxygenase-2 and nitric oxide synthase in rat somatosensory cortex

Cyclooxygenase-2 (COX-2) is a rate-limiting enzyme for prostanoid synthesis that is present in cortical pyramidal neurons and highly implicated in control of cerebral blood flow during neural activity. We examined the electron microscopic localization of COX-2 and neuronal nitric oxide synthase (nNOS), a functionally related enzyme, in the somatosensory cortex of rat brain to determine the relevant functional sites. COX-2 immunoreactivity was detected in significantly more somatodendritic than axonal profiles, while nNOS was more often seen in axon terminals. The dendritic COX-2 was localized to endomembranes near synaptic inputs from axon terminals, some of which contained nNOS. Conversely, COX-2 terminals formed asymmetric, excitatory-type synapses with dendrites containing nNOS. The dendritic and axonal profiles containing COX-2 as well as those containing nNOS were minimally separated from penetrating arterioles and capillaries by filamentous glial processes. The perivascular COX-2 labeled terminals were among those that also formed axo-dendritic synapses, suggesting that the release of prostanoids and/or excitatory transmitters from a single terminal may simultaneously affect neuronal activity and cerebral blood flow. Thus, COX-2 has a compartmental distribution in somatosensory cortical neurons consistent with the local neuronal synthesis of prostanoids that are involved in neurovascular coupling and whose actions are modulated by nitric oxide.

Regulation of hepatic stellate cell activation and growth by transcription factor myocyte enhancer factor 2

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) undergo activation during the development of liver fibrosis. Transcriptional regulation plays a key role in this process. We studied the role of transcription factor myocyte enhancer factor 2 (MEF2) during HSC activation.

METHODS

Culture of HSCs isolated from rat liver on plastic dishes and HSC-T6 on a basement membrane-like matrix were used as models of HSC activation and deactivation, respectively. The expression and activity of MEF2 were correlated with HSC activation. The roles of MEF2 during HSC activation were assessed in vitro and in vivo by animal models of fibrosis.

RESULTS

Early induction of MEF2 messenger RNA and protein accompanied culture-induced HSC activation. This was associated with enhanced MEF2 DNA binding and transactivation activity. p38 mitogen-activated protein kinase but not extracellular signal-regulated kinase pathway was required for increased MEF2 activity during HSC activation. Increased MEF2 protein also correlated with fibrosis in vivo. Reversal of HSC activation was paralleled by a marked decrease in MEF2 protein and activity. Functionally, enhancing MEF2 significantly increased the expression of alpha-smooth muscle actin (alpha-SMA), activated collagen I promoter activity, and stimulated HSC proliferation. MEF2 interference RNA significantly inhibited expression of alpha-SMA, collagen alpha1(I), and proliferating cell nuclear antigen.

CONCLUSIONS

The studies provide the first evidence for the presence of MEF2 in the liver and show that MEF2 regulates multiple aspects of HSC activation. These studies show a novel role of MEF2 as a key nuclear mediator that may participate in the pathologic process of liver fibrogenesis in vivo.

Dominant-negative soluble PDGF-beta receptor inhibits hepatic stellate cell activation and attenuates liver fibrosis

Hepatic fibrogenesis is a consequence of hepatic stellate cells that become activated and transdifferentiate into a myofibroblastic phenotype with the ability to proliferate and synthesize large quantities of extracellular matrix components. In this process, platelet-derived growth factor (PDGF) is the most potent stimulus for hepatic stellate cell proliferation and migration, and is overexpressed during active hepatic fibrogenesis. This cytokine binds to the PDGF receptor type beta, activates Ras and sequentially propagates the stimulatory signal sequentially via phosphorylation of Raf-1, MEK and the extracellular-signal regulated kinases ERK1/ERK2. Hepatic injury is associated with both increased autocrine PDGF signaling and upregulation of PDGF receptor. In this study, we report that a dominant-negative soluble PDGF-beta receptor consisting of a chimeric IgG containing the extracellular portion of the PDGF receptor type beta blocks HSC activation and attenuates fibrogenesis induced by ligation of the common bile duct in rats. In culture-activated hepatic stellate cells, the soluble receptor blocks phosphorylation of endogenous PDGF receptor, phosphorylation of the ERK1/EKR2 signal and reduces proliferative activities of HSC. In vivo, both the delivery of the purified soluble PDGF antagonist and the administration of adenoviruses expressing the artificial transgene were able to reduce significantly the expression of collagen and alpha-smooth muscle actin. Our results demonstrate that PDGF plays a critical role in the progression and initiation of experimental liver fibrogenesis, and suggest that early anti-PDGF intervention should have a therapeutical impact on the treatment of liver fibrogenesis.

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Vasodilator mRNA levels are increased in the livers of portal hypertensive NO-synthase 3-deficient mice

BACKGROUND/AIMS

Nitric oxide synthase (NOS) 3-deficient (NOS-3 KO) mice have an increased systemic arterial pressure but develop portal hypertension to the same extent as wildtype (WT) mice. We hypothesized that other vasodilators in the portal circulation compensate for the lack in NOS-3 activity. We used quantitative PCR as a screening method to identify mediators that possibly compensate for NOS-3 in NOS-3 KO mice.

METHODS

Mean arterial pressure (MAP) and portal venous pressure (PVP) were measured in the anaesthetized animal. mRNA levels in whole liver tissue were determined by quantitative RT-PCR.

RESULTS

NOS-3 KO mice had a significantly higher mean arterial pressure than WT mice, but portal venous pressure did not differ. Bile duct ligation (BDL) induced a drop in MAP and a rise in PVP in both groups. Bile duct ligation induced a significant increase in mRNA levels of the cannabinoid receptor (CB)-1, adrenomedullin and NOS-2 in the liver of NOS-3 KO and WT mice. Nitric oxide synthase-1 and NOS-3 mRNA levels were elevated in BDL WT mice compared with sham-operated WT mice. Higher mRNA levels of CB-1, NOS-1 and the adrenomedullin receptor were found in sham-operated NOS-3 KO mice compared with sham-operated WT mice.

CONCLUSIONS

We used quantitative PCR as a screening method to identify vasodilative mediators that might be involved in the compensation for the lack of NOS-3 activity in NOS-3 KO mice. Elevated mRNA levels in sham-operated NOS-3 KO mice compared with sham-operated WT mice were demonstrated for CB-1, NOS-1 and the adrenomedullin receptor.

Nitric oxide in gastrointestinal health and disease

Nitric oxide is an intracellular and intercellular messenger with important functions in a number of physiologic and pathobiologic processes within gastroenterology and hepatology, including gastrointestinal tract motility, mucosal function, inflammatory responses, gastrointestinal malignancy, and blood flow regulation. Since the broad review of this topic in Gastroenterology more than 10 years ago, a number of advances have been made in the area of NO biology and its relevance to the gastrointestinal system. The aim of this review is to focus on our expanded understanding of the role NO plays in human gastrointestinal and hepatic physiology and disease processes by drawing on data from relevant in vitro and animal models as well as observational human studies.

Simvastatin enhances hepatic nitric oxide production and decreases the hepatic vascular tone in patients with cirrhosis

BACKGROUND & AIMS

In cirrhosis, an insufficient release of nitric oxide contributes to increased hepatic resistance and portal pressure and enhances the postprandial increase in portal pressure. We hypothesized that simvastatin, which enhances Akt-dependent endothelial nitric oxide synthase phosphorylation, may increase hepatic nitric oxide release and decrease hepatic resistance in patients with cirrhosis and portal hypertension.

METHODS

In protocol 1, 13 patients had measurements of the hepatic venous pressure gradient, hepatic blood flow, mean arterial pressure, cardiac output, and nitric oxide products before and 30 and 60 minutes after 40 mg of simvastatin. In protocol 2, 17 patients were randomized to receive placebo or simvastatin (40 mg) 12 hours and 1 hour before the study. After baseline measurements of the hepatic venous pressure gradient, hepatic blood flow, and nitric oxide products, a standard liquid meal was given, and measurements were repeated at 15, 30, and 45 minutes.

RESULTS

In protocol 1, acute simvastatin did not modify the hepatic venous pressure gradient but increased the hepatic blood flow (21% +/- 13% at 30 minutes; P = 0.01) and decreased hepatic sinusoidal resistance by 14% +/- 11% (P = 0.04). Nitric oxide product levels significantly increased in hepatic venous blood (from 31.4 +/- 12.3 nmol. mL(-1) to 35.8 +/- 10.7 nmol. mL(-1); P = 0.04), but not in peripheral blood. Systemic hemodynamics were not modified. In protocol 2, simvastatin pretreatment significantly attenuated the postprandial increase in hepatic venous pressure gradient (mean peak increase, 10% +/- 9% vs. 21% +/- 6% in placebo; P = 0.01). Hepatic blood flow increased similarly in the 2 groups. Hepatic nitric oxide products increased in the simvastatin group but not in the placebo group.

CONCLUSIONS

Simvastatin administration increases the hepatosplanchnic output of nitric oxide products and decreases hepatic resistance in patients with cirrhosis.

Construction and identification of polycistron adenoviral expression vector PCA13/FasL-IRES-iNOS

OBJECTIVE

To construct the Fas ligand (FasL) and inducible nitric oxide synthase (iNOS) coexpressed PCA13 plasmid used for packing of adenovirus, so as to observe the immunoprotective effect of FasL on the adenovirus vector.

METHODS

By way of the internal ribosome entry site (IRES), gene engineering techniques such as preparation and transformation of competent cells, plasmid extraction, agarose gel electrophoresis and restriction enzymolysis were used for the construction of the polycistron adenoviral expression vector PCA13/FasL-IRES-iNOS, which could coexpress FasL and the iNOS gene after multisubcloning steps.

RESULTS

FasL and iNOS connected with the IRES were successfully cloned to the PCA13 plasmid and verified by enzymolysis (600 bp FasL, 1000 bp IRES and 4000 bp iNOS) and the gene sequence was concordant with the gene bank.

CONCLUSIONS

The polycistron adenoviral expression vector PCA13/FasL-IRES-iNOS was successfully constructed.

CSRP2, TIMP-1, and SM22alpha promoter fragments direct hepatic stellate cell-specific transgene expression in vitro, but not in vivo

BACKGROUND/AIMS

The activation of hepatic stellate cells (HSC) and their transdifferentiation into myofibroblasts (MFB) is the key step for development of liver fibrosis. Over the past several years, significant progress has been made in the understanding of the critical pathways involved incells undergoing activation. Cellular activation in the course of transdifferentiation involves, among other biochemical modifications, functionally relevant changes in the control of gene expression. These include the up-regulation of transcription factors, different extracellular matrix proteins, cell adhesion molecules, smooth muscle specific genes, and proteins involved in matrix remodelling, or cytoskeletal organization. The corresponding regulatory elements of these genes have afforded us the opportunity to express transgenes with antifibrotic potential in a cell type- and/or transdifferentiation-dependent manner.

METHODS

In the present study, we have tested several promoters for their ability to mediate cell-specific expression, including those for CSRP2, SM22alpha, and TIMP-1 (CSRP2, gene encoding the LIM domain protein CRP2; SM22alpha, smooth muscle-specific gene encoding a 22-kDa protein; TIMP-1, gene encoding the tissue inhibitor of metalloproteinases-1), which in liver are specifically expressed in HSC or become strongly activated during the acute remodelling into MFB. We constructed adenoviral reporter vectors in which relevant portions of the promoters were fused to the green fluorescent protein.

RESULTS AND CONCLUSION

Our experiments demonstrate that each of these promoters is sufficient to achieve strong or partially selective expression in vitro but none is able to direct a specific or inducible expression of transgenes in HSC/MFB in vivo.

NCX-1000, a nitric oxide-releasing derivative of ursodeoxycholic acid, ameliorates portal hypertension and lowers norepinephrine-induced intrahepatic resistance in the isolated and perfused rat liver

BACKGROUND/AIMS

We studied whether acute administration of NCX-1000, a nitric oxide (NO)-releasing derivative of ursodeoxycholic acid (UDCA), to animals with established liver cirrhosis decreases intrahepatic resistance and modulates hepatic vascular hypereactivity to norepinephrine (NE).

METHODS

Four-week bile duct ligated (BDL) cirrhotic and control, sham-operated, rats were treated orally with 28 mg/kg per day NCX-1000 or 15 mg/kg per day UDCA for 5 days. Isolated normal and cirrhotic livers were perfused with NE, from 10 nM to 30 microM, in a recirculating system.

RESULTS

NCX-1000 administration to BDL cirrhotic rats decreased portal pressure (P<0.01) without affecting mean arterial pressure and heart rate. In the isolated perfused liver system, administration of NE resulted in a dose-dependent increase of intrahepatic resistance. Vasoconstriction caused by 30 microM NE was reduced by 60% in animals treated with NCX-1000 (P<0.001), while UDCA was uneffective. The same portal pressure lowering effect was documented in cirrhotic and sham operated rats. Administration of NCX-1000 to BDL and sham operated rats resulted in a similar increase of nitrite/nitrate and cGMP concentrations in the liver.

CONCLUSIONS

By selectively delivering NO to the liver, NCX-1000 increases cGMP concentrations and effectively counteracts the effect of endogenous vasoconstrictors on the hepatic vascular tone.

Adenoviral expression of a transforming growth factor-beta1 antisense mRNA is effective in preventing liver fibrosis in bile-duct ligated rats

Background

Transforming growth factor-β (TGF-β) is a key mediator in establishing liver fibrosis. Therefore, TGF-β as a causative agent may serve as a primary target for antifibrotic gene therapy approaches. We have previously shown that the adenoviral delivery of a transgene constitutively expressing a TGF-β1 antisense mRNA blocks TGF-β synthesis in culture-activated hepatic stellate cells and effectively abolishes ongoing fibrogenesis in vitro.

Methods

Ligature of the common bile duct was used to induce liver fibrosis in rats. The effect of the TGF-β1 antisense on fibrogenesis was analyzed in this model of liver injury.

Results

In the present study, we demonstrate that the adenoviral vector directs the synthesis of mRNA quantities that are approximately 8000-fold more abundant than endogenous TGF-β1 mRNA. In experimentally injured rat livers induced by ligature of the common bile duct, a model for persistent fibrogenesis and cirrhosis, administration of the adenoviral vector abrogates TGF-β-enhanced production of collagen and α-smooth muscle actin. Furthermore, the number of cells positive for α-smooth muscle actin resulting from active recruitment of activated hepatic stellate cells around the bile ductular structures was significantly reduced in animals after application of Ad5-CMV-AS-TGF-β1. However, the observed elevated serum levels of aspartate aminotransferase, alanine aminotransferase, and bilirubin induced in this obstructive liver injury model were not significantly altered in the presence of the TGF-β antagonist.

Conclusion

Taken together, our data provides in vivo evidence that the delivery of TGF-β1 antisense mRNA specifically abolishes the diverse effects of direct TGF-β function in ongoing liver fibrogenesis. Therefore, we conclude that the expressed transgene is therapeutically useful for inhibition of TGF-β effects in diverse applications, ranging from clarification of TGF-β function in the course of liver injury to the development of novel gene therapeutic approaches.

Influence of caveolin on constitutively activated recombinant eNOS: insights into eNOS dysfunction in BDL rat liver

Diminished endothelial nitric oxide (NO) synthase (eNOS)-derived NO production from the hepatic vascular endothelium contributes to hepatic vasoconstriction in portal hypertension. The aim of this study was to examine the mechanism of this process by testing the influence of a constitutively active form of eNOS (S1179DeNOS) in both primary and propagated liver cells in vitro and in the sham and bile duct ligated (BDL) rat liver in vivo, using an adenoviral vector encoding green fluorescent protein (AdGFP) and S1179DeNOS (AdS1179DeNOS). AdS1179DeNOS transduction augmented basal and agonist-stimulated NO generation in nonparenchymal liver cells. Sham rats transduced in vivo with AdS1179DeNOS evidenced a decreased pressor response to incremental doses of the vasoconstrictor methoxamine compared with sham rats transduced with AdGFP. However, BDL rats transduced with AdS1179DeNOS did not display improved vasodilatory responses as evidenced by similar flow-dependent pressure increases to that observed in BDL rats transduced with AdGFP, despite similar levels of viral transgene expression. We next examined the influence of the eNOS inhibitory protein caveolin on S1179DeNOS dysfunction in cirrhotic liver. Immunogold electron microscopic analysis of caveolin in BDL liver demonstrated prominent expression not only in liver endothelial cells, but also in hepatic stellate cells. In vitro studies in the LX2 hepatic stellate cell line demonstrate that caveolin precipitates recombinant S1179DeNOS in LX2 cells, that recombinant S1179DeNOS coprecipitates caveolin, and that binding is enhanced in the presence of overexpression of caveolin. Furthermore, caveolin overexpression inhibits recombinant S1179DeNOS activity. These studies indicate that recombinant S1179DeNOS protein functions appropriately in normal liver cells and tissue but evidences dysfunction in the cirrhotic rat liver and that caveolin expression and inhibition in BDL nonparenchymal cells, including hepatic stellate cells, may account for this dysfunction.

Transduction of the liver with activated Akt normalizes portal pressure in cirrhotic rats

BACKGROUND & AIMS

Portal hypertension in cirrhosis is secondary to an increase in hepatic resistance that occurs mainly through collagen deposition. However, recent evidence points to a major contribution by other factors, such as an intrahepatic reduction in nitric oxide production. Akt is a major activator of the endothelial nitric oxide synthase (eNOS) enzyme, but its potential role in intrahepatic resistance in cirrhosis is unknown. For this reason the aims of the present study were to determine whether there is an impaired Akt activation in cirrhotic livers and how this phenomenon relates to the decrease in NO production associated with portal hypertension.

METHODS

Cirrhosis was induced in rats by carbon tetrachloride inhalation. Protein abundance and phosphorylation status of Akt and eNOS were examined by Western blotting. The role of Akt in the liver of cirrhotic rats was investigated through infection with adenoviruses encoding either beta-galactosidase (beta-gal) or constitutively active Akt (myr-Akt).

RESULTS

The liver of cirrhotic animals showed a significant reduction in Akt and eNOS phosphorylation. Adenoviral delivery of myr-Akt restored eNOS phosphorylation and increased the intrahepatic concentration of guanosine 3',5'-cyclic monophosphate. These events were associated with normalization in portal pressure and a significant increase in mean arterial pressure after 3 days of adenoviral infection. In contrast, transduction of livers with beta-gal did not produce any change in these hemodynamic parameters.

CONCLUSIONS

myr-Akt gene therapy restored Akt activation and NO production in the cirrhotic liver, suggesting that this therapy may be useful for the treatment of portal hypertension.

Effects of ET-1 and NO on hepatic hemodynamics at various stages of isolated perfused cirrhotic liver in rats

AIM

To investigate the effects of ET-1, NO on hepatic hemodynamics in isolated perfused rat liver at various stages of liver cirrhosis (LC).

METHODS

LC was induced by an intraperitoneal injection of CCL₄ combined with ethanol as drinking water. According to time points of CCL₄ injection, and combined with histopathological changes of liver and ascites, the isolated perfusion of liver was performed at a constant flow rate to determine the modulating effects of ET-1 and NO in the ends of 9th week (E-LC) and 14^th week (L-LC) after injected CCL₄.

RESULTS

After perfusion of L-NAME into the portal vein, there were no significant changes in the perfused pressure of portal vein (PP) and the hepatic venous pressure (Phv) of the L-LC group, the E-LC group and control group (P>0.05). After perfusion of ET-1, the PP of each group increased significantly (P<0.01). The elevated ranges of PP of the L-LC group was more than that of the E-LC group (P<0.01), both of which were higher than that of the control group (P<0.01). Compared with the ET-1 groups, the PP of the control group, the E-LC group and the L-LC group increased significantly (P<0.05) after perfusion of ET-1+L-NAME. There were no significant differences between the elevated ranges of PP of the L-LC and that of the E-LC group (P>0.05), both of which were more than that of the control group (P<0.01).

CONCLUSION

ET-1 plays a key role in elevating intra-hepatic resistance, facilitating synthesis of NO, which grow stronger in LC. With the development of LC, the compensation of NO decreases further. It is considered that antagonist of ET receptor and NO provider can increase synthesis of NO and be thus used in treatment of the high pressure of portal vein.

Nitric oxide and portal hypertension: a nitric oxide-releasing derivative of ursodeoxycholic acid that selectively releases nitric oxide in the liver

Portal hypertension, a common consequence of chronic liver diseases, is directly responsible for most complications of cirrhosis. In liver microcirculation, nitric oxide is considered a major fine tuner of vascular tone by counterbalancing vasoconstrictors (sympathetic nervous activity, the renin-angiotensin system, and endothelin-1) in normal and cirrhotic livers. The deficiency of endothelial nitric oxide release is a key factor in the hemodynamic abnormalities associated with the dynamic component of portal hypertension. Conventional nitric oxide donors release nitric oxide into the blood stream, causing systemic hypotension and progression of vasodilatory syndrome in cirrhotic patients. NCX1000 is a nitric oxide-releasing derivative of ursodeoxycholic acid-derived compounds, being capable of selectively releasing nitric oxide into the liver circulation. Administration of NCX1000 to portal hypertensive rats decreases intrahepatic resistance providing a novel therapy for the treatment of portal hypertension.

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Low doses of isosorbide mononitrate attenuate the postprandial increase in portal pressure in patients with cirrhosis

Postprandial hyperemia is associated with a significant increase in portal pressure in cirrhosis, which may contribute to progressive dilation and rupture of gastroesophageal varices. In cirrhosis, an insufficient hepatic production of nitric oxide (NO) may impair the expected hepatic vasodilatory response to increased blood flow, further exaggerating the postprandial increase in portal pressure. This study was aimed at investigating whether low doses of an oral NO donor might counteract the postprandial peak in portal pressure. Twenty-three portal hypertensive cirrhotics, 8 of them under propranolol therapy, were randomized to receive orally 5-isosorbide mononitrate (ISMN; 10 mg; n = 11) or placebo (n = 12) and a standard liquid meal 15 minutes later. Hepatic venous pressure gradient (HVPG), mean arterial pressure (MAP), and hepatic blood flow (HBF) were measured at baseline and 15, 30, and 45 minutes after a meal. ISMN significantly attenuated the postprandial increase in portal pressure as compared with placebo (peak HVPG increase: 2.4 +/- 1.4 mm Hg vs. 5.2 +/- 2.1 mm Hg, P =.002). Percentual increases in HBF were similar in both groups. MAP decreased slightly in ISMN group (-7.5% +/-.5%; P <.01 vs. baseline). These effects were also observed in patients on chronic propranolol therapy. In conclusion, hepatic NO supplementation by low doses of ISMN effectively reduces the postprandial increase of portal pressure in cirrhosis, with only a mild effect on arterial pressure. The same was observed in patients receiving propranolol. Our results suggest that therapeutic strategies based on selective hepatic NO delivery may improve the treatment of portal hypertension.

Treatment of hepatic fibrosis: almost there

Hepatic fibrosis is the scarring response of the liver to chronic liver injury; when fibrosis progresses to cirrhosis, morbid complications can develop. Available therapies for many chronic liver diseases are ineffective, with liver transplantation as the only option, though the supply of donor organs is inadequate to meet the growing demand. Novel approaches that attack the scarring response are therefore urgently needed. Optimism in this effort is fueled by major insights into the pathogenesis of fibrosis and by accumulating evidence that even cirrhosis is reversible in many patients. Most evolving antifibrotic therapies will be aimed at inhibiting the activated hepatic stellate cell, which is responsible for the fibrotic response to injury. This review describes the ways in which insights into the cellular basis of hepatic fibrosis are leading to realistic strategies for antifibrotic treatment that may revolutionize the management of patients with chronic liver disease.

Nitric oxide in liver transplantation: pathobiology and clinical implications

The gaseous molecule nitric oxide is involved in a variety of liver transplant-relevant processes, including ischemia-reperfusion injury, acute cellular rejection, and circulatory changes characteristic of advanced liver disease. This review article focuses on new advances relating to the role of nitric oxide in these syndromes with an emphasis on pathobiology and potential clinical implications.

Nitric oxide and portal hypertension

In liver cirrhosis, an increase in hepatic resistance is the initial phenomenon leading to portal hypertension. This is primarily due to the structural distortion of the intrahepatic microcirculation caused by cirrhosis. However, similar to other vascular conditions, architectural changes in the liver are associated with a deficient nitric oxide (NO) production, which results in an increased vascular tone with a further increase in hepatic resistance and portal pressure. New therapeutic strategies are being developed to selectively provide the liver with NO, overcoming the deleterious effects of systemic vasodilators. On the other hand, a strikingly opposite process occurs in splanchnic arterial circulation, where NO production is increased. This results in splanchnic vasodilatation and subsequent increase in portal inflow, which contributes to portal hypertension. Systemic blockade of NO in portal hypertension attenuates the hyperdynamic circulation, but its effects increasing hepatic resistance may offset the benefit of reducing portal inflow, thus preventing an effective reduction of portal pressure. Moreover, it cannot be ruled out that NO blockade may have a deleterious action on cirrhosis progression, which raises caution about their use in patients with cirrhosis.

In vivo gene transfer of endothelial nitric oxide synthase decreases portal pressure in anaesthetised carbon tetrachloride cirrhotic rats

BACKGROUND

Portal hypertension in cirrhosis results from enhanced intrahepatic resistance to an augmented inflow. The former is partly due to an imbalance between intrahepatic vasoconstriction and vasodilatation. Enhanced endothelin-1 and decreased activity of hepatic constitutive endothelial nitric oxide synthase (NOS 3) was reported in carbon tetrachloride (CCl(4)) cirrhotic rat liver.

AIMS

To study whether an increase in hepatic NOS 3 could be obtained in the CCl(4) cirrhotic rat liver by in vivo cDNA transfer and to investigate a possible effect on portal pressure.

METHODS

Hepatic NOS 3 immunohistochemistry and western blotting were used to measure the amount of NOS 3 protein. Recombinant adenovirus, carrying cDNA encoding human NOS 3, was injected into the portal vein of CCl(4) cirrhotic rats. Cirrhotic controls received carrier buffer, naked adenovirus, or adenovirus carrying the lac Z gene.

RESULTS

NOS 3 immunoreactivity and amount of protein (western blotting) were significantly decreased in CCl(4) cirrhotic livers. Following cDNA transfer, NOS 3 expression and the amount of protein were partially restored. Portal pressure was 11.4 (1.6) mm Hg in untreated cirrhotic (n=9) and 11.8 (0.6) in lac Z transfected (n=4) cirrhotic rats but was reduced to 7.8 (1.0) mm Hg (n=9) five days after NOS 3 cDNA transfer. No changes were observed in systemic haemodynamics, in liver tests or urinary nitrates, or in NOS 3 expression in lung or kidney, indicating a highly selective transfer.

CONCLUSIONS

NOS 3 cDNA transfer to cirrhotic rat liver is feasible and the increase in hepatic NOS 3 leads to a marked decrease in portal hypertension without systemic effects. These data indicate a major haemodynamic role of intrahepatic NOS 3 in the pathogenesis of portal hypertension in CCl(4) cirrhosis.

Differential expression and localization of nitric oxide synthases in cirrhotic livers of bile duct-ligated rats

Increased vascular nitric oxide (NO) production has been implicated in the pathogenesis of the hyperdynamic circulation in liver cirrhosis. This study investigated the expression of three isoforms of NO synthase (NOS) in rat cirrhotic livers. Cirrhosis was induced by chronic bile duct ligation (BDL). NOS enzyme activity was assessed by L-citrulline generation. Competitive RT-PCR was performed to detect the mRNA levels of NOS. In situ hybridization was done to localize NOS mRNA. Protein expression of NOS was evaluated by Western blotting and immunohistochemistry. The L-citrulline assay showed that constitutive NOS (cNOS) enzymatic activity was decreased, while inducible NOS (iNOS) activity was increased in BDL livers. Both endothelial NOS (eNOS) and neuronal NOS (nNOS) mRNA were detected in BDL and sham rats, but with enhanced expression in BDL rats. eNOS protein was redistributed with less expression in sinusoidal endothelial cells, but the total levels in liver were not changed. nNOS was induced in hepatocytes of BDL rats, in contrast to only a weak signal observed around some blood vessels in sham livers. Intense mRNA and protein expression of iNOS was induced in livers of BDL rats and was localized in hepatocytes, with no or a negligible amount in control livers. In conclusion, iNOS was induced in cirrhotic liver with its activity increased. In contrast, cNOS activity was impaired, regardless of unchanged eNOS protein levels and enhanced nNOS expression. These results suggest that all three types of NOS have a role in cirrhosis, but their expression and regulation are different.

Nitric oxide synthase gene therapy for cardiovascular disease

Gene therapy refers to the transfer of specific genes to the host tissue to intervene in a disease process, with resultant alleviation of the symptoms of a particular disease. Cardiovascular gene transfer is not only a powerful technique for studying the function of specific genes in cardiovascular biology and pathobiology, but also a novel and promising strategy for treating cardiovascular diseases. Since the mid-1990s, nitric oxide synthase (NOS), the enzyme that catalyzes the formation of nitric oxide (NO) from L-arginine, has received considerable attention as a potential candidate for cardiovascular gene therapy, because NO exerts critical and diverse functions in the cardiovascular system, and abnormalities in NO biology are apparent in a number of cardiovascular disease processes including cerebral vasospasm, atherosclerosis, postangioplasty restenosis, transplant vasculopathy, hypertension, diabetes mellitus, impotence and delayed wound healing. There are three NOS isoforms, i.e., endothelial (eNOS), neuronal (nNOS) and inducible (iNOS). All three NOS isoforms have been used in cardiovascular gene transfer studies with encouraging results. This review will discuss the rationale of NOS gene therapy in different cardiovascular disease settings and summarize the results of experimental NOS gene therapy from various animal models of cardiovascular disease to date.