Inducible nitric oxide synthase in rat hepatic lipocytes and the effect of nitric oxide on lipocyte contractility

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.

Cytokine-stimulated nitric oxide production inhibits adenylyl cyclase and cAMP-dependent secretion in cholangiocytes

BACKGROUND & AIMS

The biliary epithelium is involved both in bile production and in the inflammatory/reparative response to liver damage. Recent data indicate that inflammatory aggression to intrahepatic bile ducts results in chronic progressive cholestasis.

METHODS

To understand the effects of nitric oxide on cholangiocyte secretion and biliary tract pathophysiology we have investigated: (1) the effects of proinflammatory cytokines on NO production and expression of the inducible nitric oxide synthase (NOS2), (2) the effects of NO on cAMP-dependent secretory mechanisms, and (3) the immunohistochemical expression of NOS2 in a number of human chronic liver diseases.

RESULTS

Our results show that: (1) tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma, synergically stimulate NO production in cultured cholangiocytes through an increase in NOS2 gene and protein expression; (2) micromolar concentrations of NO inhibit forskolin-stimulated cAMP production by adenylyl cyclase (AC), cyclic adenosine monophosphate (cAMP)-dependent fluid secretion, and cAMP-dependent Cl(-) and HCO(3)(-) transport mediated by cystic fibrosis transmembrane conductance regulator (CFTR) and anion exchanger isoform 2, respectively; (3) cholestatic effects of NO and of proinflammatory cytokines are prevented by NOS-2 inhibitors and by agents (manganese(III)-tetrakis(4-benzoic acid)porphyrin [MnTBAP], urate, trolox) able to block the formation of reactive nitrogen oxide species (RNOS); (4) NOS2 expression is increased significantly in the biliary epithelium of patients with primary sclerosing cholangitis (PSC).

CONCLUSIONS

Our findings show that proinflammatory cytokines stimulate the biliary epithelium to generate NO, via NOS2 induction, and that NO causes ductular cholestasis by a RNOS-mediated inhibition of AC and of cAMP-dependent HCO(3)(-) and Cl(-) secretory mechanisms. This pathogenetic sequence may contribute to ductal cholestasis in inflammatory cholangiopathies.

Effect of leflunomide on immunological liver injury in mice

AIM: To study the effect of leflunomide on immunological liver injury (ILI) in mice.

METHODS: ILI was induced by tail vein injection of 2.5 mg Bacillus Calmette-Guerin (BCG), and 10 d later with 10 mg lipopolysaccharide (LPS) in 0.2 mL saline (BCG + LPS). The alanine aminotransferase (ALT), aspartate aminotransferase (AST), nitric oxide (NO) level in plasma and molondiadehyde (MDA), glutathione peroxidase (GSHpx) in liver homogenate were assayed by spectroscopy. The serum content of tumor necrosis factors-α (TNF-α) was determined by ELISA. Interleukin-1 (IL-1), interleukin-2 (IL-2) and Concanavalin A (ConA)-induced splenocyte proliferation response were determined by methods of ³H-infiltrated cell proliferation.

RESULTS: Leflunomide (4, 12, 36 mg·kg^-1) was found to significantly decrease the serum transaminase (ALT, AST) activity and MDA content in liver homogenate, and improve reduced GSHpx level of liver homogenate. Leflunomide (4, 12, 36 mg·kg^-1) significantly lowered TNF-α and NO level in serum, and IL-1 produced by intraperitoneal macrophages (PMF). Moreover, the decreased IL-2 production and ConA-induced splenocyte proliferation response were further inhibited.

CONCLUSION: These findings suggested that leflunomide had significant protective action on ILI in mice.

Impact of oestrogens on the progression of liver disease

As of this writing, the most common cause of hepatic fibrosis is chronic hepatitis C virus infection (HCV), the characteristic feature of which is hepatic steatosis. Hepatic steatosis leads to an increase in lipid peroxidation in hepatocytes, which in turn activates hepatic stellate cells (HSCs). HSCs are also thought to be the primary target cells for inflammatory stimuli, and produce extracellular matrix components. Based on available clinical information, chronic hepatitis C appears to progress more rapidly in men than in women, and cirrhosis is predominately a disease of men and postmenopausal women. It should be noted that estradiol (E2) is a potent endogenous antioxidant. A recent study has shown that hepatic steatosis became evident in an aromatase-deficient mouse and was diminished in animals, after treatment with E2. Our studies showed that E2 suppressed hepatic fibrosis in hepatic fibrosis models, inhibited the activation of activator protein 1 and nuclear factor-kappa B in cultured hepatocytes undergoing oxidative stress, and attenuated HSC activation in primary culture. Recently, variant oestrogen receptors (ERs) were found to be expressed to a greater extent in male patients with chronic liver disease than in female subjects. We also demonstrated decreased levels of ERs in postmenopausal women and cirrhotic patients of both genders. The actions of E2 are mediated through ER alpha and beta. HSCs have also been found to possess functional ER beta but not ER alpha. A better understanding the basic mechanisms underlying the gender-associated differences observed in the development of hepatic fibrosis would provide valuable information relative to the search for effective antifibrogenic therapies.

Effect of ischemic preconditioning on hepatic microcirculation and function in a rat model of ischemia reperfusion injury

Ischemic preconditioning (IPC) may protect the liver from ischemia reperfusion injury by nitric oxide formation. This study has investigated the effect of ischemic preconditioning on hepatic microcirculation (HM), and the relationship between nitric oxide metabolism and HM in preconditioning. Rats were allocated to 5 groups: 1. sham laparotomy; 2. 45 minutes lobar ischemia followed by 2-hour reperfusion (IR); 3. IPC with 5 minutes ischemia and 10 minutes reperfusion before IR; 4. L-arginine before IR; and 5. L-NAME + IPC before IR. HM was monitored by laser Doppler flowmeter. Liver transaminases, adenosine triphosphate, nitrites + nitrates, and guanosine 3'5'-cyclic monophosphate (cGMP) were measured. Nitric oxide synthase (NOS) distribution was studied using nicotinamide adeninine dinucleotide phosphate (NADPH) diaphorase histochemistry. At the end of reperfusion phase, in the IR group, flow in the HM recovered partially to 25.8% of baseline (P < .05 versus sham), whereas IPC improved HM to 49.5% of baseline (P < .01 versus IR). With L-arginine treatment, HM was 31.6% of baseline (NS versus IR), showing no attenuation of liver injury. In the preconditioned group treated with L-NAME, HM declined to 10.2% of baseline, suggesting not only a blockade of the preconditioning effect, but also an exacerbated liver injury. Hepatocellular injury was reduced by IPC, and L-arginine and was increased by NO inhibition with L-NAME. IPC also increased nitrate + nitrate (NOx) and cGMP concentrations. NOS detected by NADPH diaphorase staining was associated with hepatocytes and vascular endothelium, and was induced by IPC. IPC induced NOS and attenuated HM impairment and hepatocellular injury. These data strongly suggest a role for nitric oxide in IPC.

Microvascular dysfunction induced by reperfusion injury and protective effect of ischemic preconditioning

Hepatic ischemia/reperfusion injury has immediate and deleterious effects on the outcome of patients after liver surgery. The precise mechanisms leading to the damage have not been completely elucidated. However, there is substantial evidence that the generation of oxygen free radicals and disturbances of the hepatic microcirculation are involved in this clinical syndrome. Microcirculatory dysfunction of the liver seems to be mediated by sinusoidal endothelial cell damage and by the imbalance of vasoconstrictor and vasodilator molecules, such as endothelin (ET), reactive oxygen species (ROS), and nitric oxide (NO). This may lead to no-reflow phenomenon with release of proinflammatory cytokines, sinusoidal plugging of neutrophils, oxidative stress, and as an ultimate consequence, hypoxic cell injury and parenchymal failure. An inducible potent endogenous mechanism against ischemia/reperfusion injury has been termed ischemic preconditioning. It has been suggested that preconditioning could inhibit the effects of different mediators involved in the microcirculatory dysfunction, including endothelin, tumor necrosis factor-alpha, and oxygen free radicals. In this review, we address the mechanisms of liver microcirculatory dysfunction and how ischemic preconditioning could help to provide new surgical and/or pharmacological strategies to protect the liver against reperfusion damage.

The relationship of hepatic tissue oxygenation with nitric oxide metabolism in ischemic preconditioning of the liver

Ischemic preconditioning (IPC) may increase the hepatic tolerance of ischemic injury during liver surgery and transplantation via nitric oxide (NO) formation. This study investigates the effect of IPC on hepatic tissue oxygenation and the role of NO stimulation and inhibition on the preconditioning effect in the rat liver. Study groups had 1) sham laparotomy; 2) 45-min lobar liver ischemia and 2-h reperfusion (IR); 3) IPC with 5-min ischemia and 10-min reperfusion before IR; 4) L-arginine before IR; and 5) Nw-Nitro-L-arginine methyl ester (L-NAME) + IPC before IR. Hepatic tissue oxygenation was monitored by near-infrared spectroscopy. Plasma alanine aminotransferase and plasma nitrite/nitrate were measured. Following IR there was significant decrease in oxyhemoglobin and cytochrome oxidase and an increase in deoxyhemoglobin (PA redox state, PL-arginine did not attenuate the impairment in hepatic tissue oxygenation after IR (P>0.05 vs IR). In contrast, inhibition of NO synthesis blocked the effect of IPC and further impaired tissue oxygenation (decreased cytochrome oxidase CuA redox state and increased deoxyhemoglobin, both PL-arginine and increased by NO blockade with L-NAME (Plasma ALT, all P< 0.05 vs IR). Hepatic tissue oxygenation correlated significantly with ALT and plasma nitrite/nitrate. Ischemic preconditioning significantly improved hepatic intra cellular oxygenation and reduced hepatocellular injury. NO stimulation reduced hepatocellular injury, whereas inhibition of nitric oxide synthesis blocked the effect of IPC and reduced tissue oxygenation and increased hepatocellular injury.

Three-dimensional examination of hepatic stellate cells in rat liver and response to endothelin-1 using confocal laser scanning microscopy

BACKGROUND AND AIM

Hepatic stellate cells (HSC) are located in the space of Disse and are considered to participate in the regulation of sinusoidal flow. The contractility of quiescent HSC in normal liver has remained controversial, unlike activated HSC in injured liver. The aim of the present study was to examine the morphological changes in quiescent HSC in response to endothelin-1 (ET-1) perfusion.

METHODS

Sections (50 micro m thick) obtained from 15 normal rat livers with or without ET-1 perfusion (1 or 400 nmol/L) were stained immunohistochemically with antiglial fibrillary acidic protein (GFAP) antibody and then examined using confocal laser scanning microscopy. For examination of HSC, hepatic lobules were divided into three anatomic regions from the portal areas to the central veins. The length of HSC cytoplasmic processes and area of the sinusoids relative to the section area, excluding portal tracts and central veins, were measured.

RESULTS

The GFAP-positive HSC were distributed relatively evenly in the hepatic lobules and those in region 2 (the area between periportal and pericentral areas) tended to have longer cytoplasmic processes. Perfusion of 1 or 400 nmol/L ET-1 for 25 min resulted in swelling of the cell bodies of GFAP-positive HSC and condensation of the intermediate filaments compared with those perfused with buffer only. Although narrowing of the sinusoidal lumen was observed in each region after perfusion with 400 nmol/L ET-1, there was no apparent shortening of the cytoplasmic processes of HSC. These findings were also confirmed quantitatively.

CONCLUSION

In the normal rat liver, quiescent HSC are not involved in the regulation of sinusoidal blood flow in response to ET-1.

Mechanical stress-dependent secretion of interleukin 6 by endothelial cells after portal vein embolization: clinical and experimental studies

BACKGROUND/AIMS

Interleukin-6 (IL-6) is an essential early signal in liver regeneration, however, little is known about what triggers IL-6 release. Changes in portal hemodynamics after portal vein embolization (PVE) may contribute to IL-6 release, leading to regeneration of non-embolized lobe.

METHODS

In 22 patients who underwent right PVE, the diameters of the left portal branches, liver volumes, and serum concentrations of IL-6, tumor necrosis factor-alpha (TNF-alpha), and hepatocyte growth factor (HGF) were measured. We then studied endothelial cells cultured on an elastic silicone membrane and subjected to continuous uni-axial stretch. Supernatant cytokine concentrations were measured.

RESULTS

The diameters of the portal branches increased by 150% after PVE. Serum IL-6 concentrations increased within 3h after PVE. The concentrations of TNF-alpha and HGF remained unchanged. The left lobe volume increased 2 weeks after PVE. The IL-6 concentrations in the supernatant of endothelial cells with stretch stress were higher than that in the non-stretched control group.

CONCLUSIONS

These findings indicate that PVE dilates the portal branches in the non-embolized lobe, exposing hepatic vasculature to stretch stress. This hemodynamic change may act as a trigger for IL-6 release from endothelial cells and contribute to the activation of regenerative cascade in the non-embolized lobes.

[Mechanisms of hepatic fibrogenesis]

Hepatic fibrosis is a scaring process leading to cirrhosis, a major complication of numerous chronic liver diseases. Hepatic stellate cells play a central role in the fibrotic process. After parenchymal or biliary injury, cytokines and growth factors allow the recruitment, proliferation, and activation, of stellate cells toward myofibroblasts, which secrete the extracellular matrix. Fibrosis, resulting from the failure of the balance between synthesis and degradation of extracellular matrix, is an evolutive and potentially reversible process. Histological examination is the main investigation to quantify fibrosis. Serological tests are warranted to allow a non invasive follow up of patients. Development of antifibrotic therapies should soon permit to slow down the evolution toward cirrhosis, limiting the needs for hepatic transplantation.

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

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

Human myofibroblastic hepatic stellate cells express Ca(2+)-activated K(+) channels that modulate the effects of endothelin-1 and nitric oxide

BACKGROUND/AIMS

High-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate the effects of vasoactive factors in contractile cells. It is unknown whether hepatic stellate cells (HSCs) contain BK(Ca) channels and what their role in the regulation of HSCs contractility is.

METHODS

The presence of BK(Ca) channels in HSCs was assessed by the patch-clamp technique. The functional role of BK(Ca) channels was investigated by measuring intracellular calcium concentration ([Ca(2+)](i)) and cell contraction in individual cells after stimulation with endothelin-1 in the presence or absence of specific modulators of BK(Ca) channels.

RESULTS

BK(Ca) channels were detected by patch-clamp in most of the activated HSCs studied. Incubation of cells with iberiotoxin, a BK(Ca) channel blocker, increased both the sustained phase of [Ca(2+)](i) elicited by endothelin-1 and the number of cells undergoing contraction, while the use of NS1619, a BK(Ca) channel opener, induced opposite effects. Stimulation of HSCs with S-nitroso-N-acetyl-penicillamine (SNAP), a nitric oxide (NO)-donor, increased the opening of BK(Ca) channels and reduced the effects of endothelin-1. Conversely, iberiotoxin abolished the inhibitory effect of SNAP on endothelin-induced [Ca(2+)](i) increase and cell contraction.

CONCLUSIONS

Activated human HSCs contain BK(Ca) channels that modulate the contractile effect of endothelin-1 and mediate the inhibitory action of NO.

Cellular distribution and function of soluble guanylyl cyclase in rat kidney and liver

Soluble guanylyl cyclase (sGC) catalyzes the biosynthesis of cGMP in response to binding of L-arginine-derived nitric oxide (NO). Functionally, the NO-sGC-cGMP signaling pathway in kidney and liver has been associated with regional hemodynamics and the regulation of glomerular parameters. The distribution of the ubiquitous sGC isoform alpha 1 beta 1 sGC was studied with a novel, highly specific antibody against the beta 1 subunit. In parallel, the presence of mRNA encoding both subunits was investigated by using in situ hybridization and reverse transcription-PCR assays. The NO-induced, sGC-dependent accumulation of cGMP in cytosolic extracts of tissues and cells was measured in vitro. Renal glomerular arterioles, including the renin-producing granular cells, mesangium, and descending vasa recta, as well as cortical and medullary interstitial fibroblasts, expressed sGC. Stimulation of isolated mesangial cells, renal fibroblasts, and hepatic Ito cells with a NO donor resulted in markedly increased cytosolic cGMP levels. This assessment of sGC expression and activity in vascular and interstitial cells of kidney and liver may have implications for understanding the role of local cGMP signaling cascades.

Effects of ethanol on L-arginine transport in rat Ito cells in relation to nitric oxide production

BACKGROUND

Nitric oxide (NO) is a potent mediator of hepatic sinusoidal hemodynamics that is synthesized in the hepatic stellate cells (Ito cells, fat-storing cells) and affects these cells. NO production may depend on the induction of inducible nitric oxide synthase and on transport of extracellular L-arginine. The precise mechanism that controls NO production in stellate cells was characterized recently.

METHODS

Kinetic analysis of L-arginine transport and reverse transcription-polymerase chain reaction for cationic amino acid transporter (CAT) were carried out by using stellate cells prepared from the male Wistar rat. The effect of ethanol on L-arginine transport and NO production of stellate cells was assessed in the presence of tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma.

RESULTS

The L-arginine transport system functioning in the hepatic stellate cells was system y+, possibly mediated by CAT-1 and CAT-2B (Km approximately 50 microM). IFN-gamma in combination with TNF-alpha induced NO production with an enhancement in CAT-2B mRNA expression and L-arginine transport, whereas L-arginine transport and NO production were suppressed by coincubated ethanol.

CONCLUSIONS

In hepatic stellate cells, ethanol has suppressive effects on NO production and extracellular L-arginine transport in the presence of TNF-alpha and IFN-gamma. The estimated Km of L-arginine transporter in hepatic stellate cells is very similar to the physiological L-arginine concentration in portal vein. Our findings may support the merit of further studies on the modulation of NO production via access to portal blood L-arginine concentration to control disturbed hepatic sinusoidal blood flow in patients with alcoholic liver disease.

Cellular and molecular basis of portal hypertension

The molecular basis of the vascular wall abnormalities that contribute to development of portal hypertension are an area of active investigation. Studies to date suggest that diminution in eNOS-derived NO production in liver contributes to this process by causing increased intrahepatic resistance. This process seems to be mediated through inhibitory posttranslational regulatory mechanisms of eNOS. Endothelin-1 signaling is also increased in the intrahepatic vasculature. The mechanisms responsible for increased ET-1 signaling include increased ET-1 production and increased ET-A receptor expression, particularly within hepatic stellate cells, although the stimulus responsible for activation of the ET-1 system remains uncertain. In the splanchnic circulation, increases in eNOS-derived NO contribute to increased portal venous inflow through transcriptional and posttranslational regulation of eNOS. Development of the porto-systemic collateral circulation characteristic of portal hypertension occurs through a combination of NO-dependent dilation of preexisting vessels and through growth factor-mediated angiogenesis and neovascularization (Fig. 3). Further studies in vascular wall biology are continuing to elucidate more clearly the molecular mechanisms of portal hypertension. The [figure: see text] mechanism by which eNOS-derived NO production is increased in the splanchnic arteriolar endothelial cell but decreased in the liver endothelial cell and the role of specific ET receptor subtypes in the mechanism of activation of the ET-1 system and its effect on contractile cells in liver cirrhosis are areas that require further investigation. Further studies are needed to determine the intrahepatic site of pressure and perfusion regulation, be it the hepatic sinusoid and its unique, specialized cell types or the endothelial and smooth muscle cells in the hepatic and portal venules. The role of more recently delineated vasoactive pathways such as urotensin-II/GPR 14 and anandamide/CB1 receptor in portal hypertension must be examined. Most importantly, future studies must focus on novel experimental therapies, using pharmacologic and genetic approaches to modulate these vascular biologic systems and thereby to ameliorate complications and symptoms relating to portal hypertension in patients with cirrhosis.

Cellular pathophysiology of portal hypertension and prospects for management with gene therapy

In summary, regulation of sinusoidal blood flow in normal and injured liver involves structural, cellular, and humoral components. Available data suggest that stellate cells, resident perisinusoidal mesenchymal cells with a histologic orientation in the sinusoid analogous to [figure: see text] vasoregulatory pericytes, modulate sinusoidal blood flow. This regulation by stellate cells is most evident in the context of liver injury but may apply also to the normal liver. The endothelin and NO systems are important in modulating stellate cell contractility, and their degree of equilibrium is significant in determining the level of local intrahepatic resistance, especially in the injured liver. Manipulation of either or both of these systems is feasible and effective in experimental models. Such findings have obvious clinical implications and are expected to set the [figure: see text] stage for novel gene therapy approaches for treatment of patients with portal hypertension.

NCX-1000, a NO-releasing derivative of ursodeoxycholic acid, selectively delivers NO to the liver and protects against development of portal hypertension

Portal hypertension resulting from increased intrahepatic resistance is a common complication of chronic liver diseases and a leading cause of death in patients with liver cirrhosis, a scarring process of the liver that includes components of both increased fibrogenesis and wound contraction. A reduced production of nitric oxide (NO) resulting from an impaired enzymatic function of endothelial NO synthase and an increased contraction of hepatic stellate cells (HSCs) have been demonstrated to contribute to high intrahepatic resistance in the cirrhotic liver. 2-(Acetyloxy) benzoic acid 3-(nitrooxymethyl) phenyl ester (NCX-1000) is a chemical entity obtained by adding an NO-releasing moiety to ursodeoxycholic acid (UDCA), a compound that is selectively metabolized by hepatocytes. In this study we have examined the effect of NCX-1000 and UDCA on liver fibrosis and portal hypertension induced by i.p. injection of carbon tetrachloride in rats. Our results demonstrated that although both treatments reduced liver collagen deposition, NCX-1000, but not UDCA, prevented ascite formation and reduced intrahepatic resistance in carbon tetrachloride-treated rats as measured by assessing portal perfusion pressure. In contrast to UDCA, NCX-1000 inhibited HSC contraction and exerted a relaxing effect similar to the NO donor S-nitroso-N-acetylpenicillamine. HSCs were able to metabolize NCX-1000 and release nitrite/nitrate in cell supernatants. In aggregate these data indicate that NCX-1000, releasing NO into the liver microcirculation, may provide a novel therapy for the treatment of patients with portal hypertension.

Effects of dobutamine and dopexamine on hepatic micro- and macrocirculation during experimental endotoxemia: an intravital microscopic study in the rat

OBJECTIVE

To test the effects of dobutamine and dopexamine on hepatic portal and sinusoidal blood flow in a model of normodynamic endotoxemia.

DESIGN

Randomized, controlled trial.

SETTING

Experimental laboratory.

SUBJECTS

Male Wistar rats (250-350 g).

INTERVENTIONS

A total of 40 male Wistar rats were randomized into four groups: a control group, which only received Ringer's solution; an endotoxin group, which received a continuous infusion of 2 mg/kg body weight (bw)/hr of endotoxin; a dobutamine group, which received endotoxin and a continuous infusion of dobutamine (3 microg/kg bw/min); and a dopexamine group, which received endotoxin and dopexamine (2 microg/kg bw/min). The experimental period was 120 min.

MEASUREMENTS AND MAIN RESULTS

Mean arterial blood pressure (MAP), heart rate (HR), and cardiac output (CO) were detected. Portal blood flow was measured using an ultrasonic flow probe positioned around the portal vein, and sinusoidal blood flow was detected in the left liver lobe using intravital microscopy. All detected variables remained stable in the control group. In the endotoxin group, HR increased significantly and MAP decreased significantly from 111 +/- 10 mm Hg to 95 +/- 8 mm Hg at 120 mins, whereas CO remained unchanged. Both in the dobutamine and the dopexamine group HR increased and MAP decreased more than in the endotoxin group. CO increased in both groups significantly. Portal blood flow (23 +/- 4 mL/min to 16 +/- 3 mL/min) and sinusoidal blood flow (38.6 +/- 2.5 to 22.8 +/- 1.2 10(3) microm(3)/sec) decreased significantly in the endotoxin group. In the dobutamine and the dopexamine group portal and sinusoidal blood flow remained at baseline values.

CONCLUSIONS

In our model of endotoxemia, dobutamine and dopexamine preserved systemic and hepatic blood flow. These preservations of hepatic blood flow during endotoxemia could portend beneficial effects but need to be studied further.

Effects of ethanol on L-arginine transport in rat Ito cells in relation to nitric oxide production

BACKGROUND

Nitric oxide (NO) is a potent mediator of hepatic sinusoidal hemodynamics that is synthesized in the hepatic stellate cells (Ito cells, fat-storing cells) and affects these cells. NO production may depend on the induction of inducible nitric oxide synthase and on transport of extracellular L-arginine. The precise mechanism that controls NO production in stellate cells was characterized recently.

METHODS

Kinetic analysis of L-arginine transport and reverse transcription-polymerase chain reaction for cationic amino acid transporter (CAT) were carried out by using stellate cells prepared from the male Wistar rat. The effect of ethanol on L-arginine transport and NO production of stellate cells was assessed in the presence of tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma.

RESULTS

The L-arginine transport system functioning in the hepatic stellate cells was system y+, possibly mediated by CAT-1 and CAT-2B (Km approximately 50 microM). IFN-gamma in combination with TNF-alpha induced NO production with an enhancement in CAT-2B mRNA expression and L-arginine transport, whereas L-arginine transport and NO production were suppressed by coincubated ethanol.

CONCLUSIONS

In hepatic stellate cells, ethanol has suppressive effects on NO production and extracellular L-arginine transport in the presence of TNF-alpha and IFN-gamma. The estimated Km of L-arginine transporter in hepatic stellate cells is very similar to the physiological L-arginine concentration in portal vein. Our findings may support the merit of further studies on the modulation of NO production via access to portal blood L-arginine concentration to control disturbed hepatic sinusoidal blood flow in patients with alcoholic liver disease.

Regulation of hepatic eNOS by caveolin and calmodulin after bile duct ligation in rats

In carbon tetrachloride-induced liver cirrhosis, diminution of hepatic endothelial nitric oxide synthase (eNOS) activity may contribute to impaired hepatic vasodilation and portal hypertension. The mechanisms responsible for these events remain unknown; however, a role for the NOS-associated proteins caveolin and calmodulin has been postulated. The purpose of this study is to characterize the expression and cellular localization of the NOS inhibitory protein caveolin-1 in normal rat liver and to then examine the role of caveolin in conjunction with calmodulin in regulation of NOS activity in cholestatic portal hypertension. In normal liver, caveolin protein is expressed preferentially in nonparenchymal cells compared with hepatocytes as assessed by Western blot analysis of isolated cell preparations. Additionally, within the nonparenchymal cell populations, caveolin expression is detected within both liver endothelial cells and hepatic stellate cells. Next, studies were performed 4 wk after bile duct ligation (BDL), a model of portal hypertension characterized by prominent cholestasis, as evidenced by a significant increase in serum cholesterol in BDL animals. After BDL, caveolin protein levels from detergent-soluble liver lysates are significantly increased as assessed by Western blot analysis. Immunoperoxidase staining demonstrates that this increase is most prominent within sinusoids and venules. Additionally, caveolin-1 upregulation is associated with a significant reduction in NOS catalytic activity in BDL liver lysates, an event that is corrected with provision of excess calmodulin, a protein that competitively binds eNOS from caveolin. We conclude that, in cholestatic portal hypertension, caveolin may negatively regulate NOS activity in a manner that is reversible by excess calmodulin.

Nitric oxide and the liver

In recent years, the role of nitric oxide (NO) in the pathogenesis of liver disease and its complications has been extensively studied. There remain, however, many areas of controversy. In particular, the effect of NO on vascular function in the systemic circulation and the hepatic microcirculation has received the greatest attention. It has been proposed on the one hand that increased NO synthesis is responsible for the development of the hyperdynamic circulation in cirrhosis, while decreased production of NO within the hepatic microcirculation may be important in the development of parenchymal tissue damage and the onset of portal hypertension. The purpose of this review is to examine the available data concerning the role of NO in liver disease and to discuss some of the controversies and contradictions that surround it.

Estrogen upregulates nitric oxide synthase expression in cultured rat hepatic sinusoidal endothelial cells

BACKGROUND/AIMS

Estrogen receptor (ER) is present in vascular endothelial cells and estrogen promotes nitric oxide (NO) synthesis, which relaxes smooth muscle cells. It is also speculated that NO is synthesized by estrogen in hepatic sinusoidal endothelial cells (SECs). Here we investigated the localization of ER and endothelial cell nitric oxide synthase (ecNOS), and determined 17beta-estradiol (E2)-induced ecNOS expression in normal rat SECs.

METHODS

Cultured SECs were used. Fluorescence intensities of ecNOS were measured by immunofluorescence using a confocal laser-scanning microscope. E2 was added (100 pg/ml) to the culture medium, and the expressions of ecNOS mRNA and protein were analyzed by reverse-transcription polymerase chain reaction and Western blotting. NO production in cultured SECs was examined using diaminofluorescein-2 diacetate as a fluorescent indicator for NO.

RESULTS

Immunolocalization of ER and ecNOS in normal liver was demonstrated in endothelial cells lining the hepatic sinusoids. ER and ecNOS were localized in the nuclei and cytoplasm of cultured SECs, respectively. The mRNA expression of ecNOS in cultured SECs was increased after 6 h, and the protein expression of ecNOS was increased 24 h after E2 stimulation. The fluorescence intensity of NO in cultured SECs was increased by E2 stimulation compared with untreated control cells.

CONCLUSIONS

These results suggested that ER is present in SECs, and estrogen upregulates NO production in SECs. E2 may be involved in the regulation of the hepatic sinusoidal microcirculation.

Hepatic fibrosis. Pathogenesis and principles of therapy

There has been great progress made in our understanding of the cellular mechanisms of hepatic fibrosis. The recognition that the hepatic stellate cell, (formerly know as lipocyte, Ito, or fat-storing cell), played a central role in the fibrotic response was key to our understanding. Stellate cells undergo a process known as activation, in response to any insult. Activation is a broad phenotypic response, characterized by distinct functional changes in proliferation, fibrogenesis, contractility, cytokine secretion, and matrix degradation. Insights gained into the molecular regulations of stellate cell activation may lead to new antifibrotic therapies, which may reduce morbidity and mortality in patients with chronic liver injury.

Prooxidant and antioxidant functions of nitric oxide in liver toxicity

In response to tissue damage and inflammation induced by a variety of xenobiotics including acetaminophen, carbon tetrachloride, ethanol, galactosamine, and endotoxin, as well as disease states such as viral hepatitis, and postischemic and regenerative injury, the liver produces large quantities of nitric oxide. Indeed, nearly all cell types in the liver including hepatocytes, Kupffer cells, stellate cells, and endothelial cells have the capacity to generate nitric oxide. Thus, these cells, as well as infiltrating leukocytes, may indirectly augment tissue injury. In many models of liver damage, nitric oxide and its oxidation products such as peroxynitrite contribute to the injury process by directly damaging the tissue or by initiating additional immunologic reactions that result in damage. In some models, nitric oxide donors or peroxynitrite can mimic the cytotoxic actions of liver toxins. Moreover, agents that prevent the generation of nitric oxide or antioxidants that bind reactive nitrogen intermediates, or knockout mice with reduced capacity to produce nitric oxide, are protected from xenobiotic-induced tissue injury. In contrast, there have been reports that blocking nitric oxide production enhances xenobiotic-induced tissue injury. This has led to the concept that nitric oxide either inactivates proteins critical for xenobiotic-induced tissue injury or acts as an antioxidant, reducing cellular levels of cytotoxic reactive oxygen intermediates. Whether or not nitric oxide or secondary oxidants generated from nitric oxide act as mediators of tissue injury or protect against toxicity is likely to depend on the precise targets of these reactive nitrogen intermediates, as well as levels of superoxide anion present and the extent to which tissue injury is mediated by reactive oxygen intermediates. In addition, as toxicity is a complex process involving a variety of cell types and many soluble mediators, the contribution of each of these factors must be taken into account when considering the role of nitric oxide as a determinant of tissue injury.

Apoptosis in dissociation between DNA synthesis and cellular functions of activated hepatic stellate cells--a study with carbon tetrachloride-induced rat liver injury

It is widely believed that DNA synthesis and expressions of smooth muscle alpha actin and TGF-beta are all together increased in activated hepatic stellate cells both in vitro and in vivo. Our previous reports disclosed that these increases did not always coexist under experimental conditions. Liver necrosis was induced in rats by oral administration of carbon tetrachloride. Hepatic stellate cells were isolated from these rats 2 days later. When these cells were cultured on plastic dishes for 3 days, they showed marked DNA synthesis and smooth muscle alpha actin and TGF-beta mRNA expressions assessed by (3)H-thymidine incorporation and Northern blotting, respectively. In the cells further cultured for 7 days, the DNA synthesis was decreased, whereas both smooth muscle alpha actin and TGF-beta mRNA expressions were increased, compared to the cells cultured for 3 days. The cells cultured for 10 days showed apoptotic nuclei positive for nick-end labeling, and DNA extracted from the cells revealed laddering patterns on agarose gels by electrophoresis. Apoptotic nuclei were also immunohistochemically found in stellate cells in the liver of rats 4 days after the intoxication. We conclude that apoptosis developed in activated hepatic stellate cells both in vitro and in vivo, and this may contribute to the discrepancy between DNA synthesis and cellular functions of the cells.

Human hepatic stellate cells secrete adrenomedullin: potential autocrine factor in the regulation of cell contractility

BACKGROUND/AIMS

Hepatic stellate cells (HSCs) are perisinusoidal pericytes which have receptors for vasoactive factors, such as endothelin-1, which can regulate cell contractility in an autocrine manner. It is unknown whether human HSCs have receptors for and are able to synthesize the vasodilator peptide adrenomedullin (ADM), a peptide produced by most contractile cells.

METHODS AND RESULTS

Stimulation of HSCs with ADM resulted in a dose-dependent raise in cAMP concentration (radioimmunoassay) and markedly blunted the endothelin-induced increase in [Ca2+]i and cell contraction, as assessed in cells loaded with fura-2 using a morphometric method. The existence of the receptor CRLR for ADM and their associated proteins RAMP-1 and RAMP-2 was demonstrated by reverse transcriptase-polymerase chain reaction (RT-PCR). Moreover, activated human HSCs spontaneously secreted ADM in the culture medium in a time-dependent manner. ADM secretion was markedly enhanced by tumour necrosis factor-alpha and interleukin-1beta. Specific mRNA for ADM (RT-PCR and Northern blot) was detected in HSCs and increased after incubation of cells with cytokines.

CONCLUSIONS

Human HSCs have functional receptors for ADM, the stimulation of which blunts the contractile effect of endothelin-1. Cultured human HSCs secrete ADM in baseline conditions. This secretion is markedly increased by cytokines. These results suggest that ADM can regulate HSCs' contractility in an autocrine manner.

Levels of circulating endothelin-1 and nitrates/nitrites in patients with virus-related hepatocellular carcinoma

A balance between endothelins (ET) and nitric oxide (NO) might interfere with liver haemodynamics and disease progression in various liver diseases. Increased levels of endothelin 1 (ET-1) and nitrites and nitrates (NOx, the end products of NO metabolism) have been reported in hepatocellular carcinoma (HCC), but the balance has not been studied. The purpose of this study was to assess the ratio of NOx to ET-1 in patients with virus-related hepatocellular carcinoma and to investigate its correlation with the extent of the disease. Eighteen patients with virus-related HCC (six Okuda stage I, six Okuda stage II and six Okuda stage III) were included in the study and were compared with 22 patients with viral cirrhosis (14 decompensated, eight compensated) and seven normal controls. ET-1 was measured with an ELISA assay and NOx with a modification of the Griess reaction. Patients with virus-related HCC had the highest levels of circulating ET-1 and NOx (13.24 +/- 0.82 pg/ml and 112.28 +/- 18.56 micromol/l) compared to compensated cirrhosis (9.47 +/- 0.50 pg/ml, P < 0.004 and 54.47 +/- 2.36 micromol/l, P < 0.01), decompensated cirrhosis (9.57 +/- 0.32 pg/ml, P < 0.001 and 90.20 +/- 11.23 micromol/l, NS) and normal controls (8.84 +/- 0.61 pg/ml, P < 0.001 and 51.17 +/- 6.18 micromol/l, P < 0.01). There was a significant increase of ET-1 and NOx at HCC stage III compared to HCC stages I and II, cirhotics and controls. HCC stage III patients also had a NOx/ET-1 ratio that was higher than HCC stages I and II patients, normal controls and patients with compensated cirrhosis. Virus-related HCC patients have high levels of circulating ET-1, compared to compensated or decompensated cirrhosis. Highest levels of ET-1 are produced in Okuda III tumours. NOx are also increased but only in Okuda stage III tumours. The NOx/ET-1 ratio is increased in virus-related HCC and DC. This increase may account for the known increase in tumour blood flow.

Gene transfer of recombinant endothelial nitric oxide synthase to liver in vivo and in vitro

Endothelial nitric oxide synthase (eNOS)-derived nitric oxide (NO) contributes to hepatic vascular homeostasis. The aim of this study was to examine whether delivery of an adenoviral vector encoding eNOS gene to liver affects vasomotor function in vivo and the mechanism of NO production in vitro. Rats were administered adenoviruses encoding beta-galactosidase (AdCMVLacZ) or eNOS (AdCMVeNOS) via tail vein injection and studied 1 wk later. In animals transduced with AdCMVLacZ, beta-galactosidase activity was increased in the liver, most prominently in hepatocytes. In AdCMVeNOS-transduced animals, eNOS protein levels and catalytic activity were significantly increased. Overexpression of eNOS diminished baseline perfusion pressure and constriction in response to the alpha(1)-agonist methoxamine in the perfused liver. Transduction of cultured hepatocytes with AdCMVeNOS resulted in the targeting of recombinant eNOS to a perinuclear distribution and binding with the NOS-activating protein heat shock protein 90. These events were associated with increased ionomycin-stimulated NO release. In summary, this is the first study to demonstrate successful delivery of the recombinant eNOS gene to liver in vivo and in vitro with ensuing NO production.

Effects of exogenous endothelin-1 application on liver perfusion in native and transplanted porcine livers

PURPOSE

This study was designed to assess and differentiate the impact of progressivly increasing portal venous endothelin-1 (ET) plasma concentrations on hepatic micro- and macroperfusion of native porcine livers (Group A) and liver grafts after experimental transplantation (Group B).

METHODS

A standardized gradual increment in systemic ET plasma concentration (0-58 pg/ml) was induced by continuous ET-1 infusion into the portal vein in both groups (A: n = 10, B: n = 10). Control animals received only saline (n = 5, each group). Hepatic microcirculation (HMC) was quantified by thermodiffusion electrodes, hepatic artery flow (HAF), and portal venous flow (PVF) by Doppler flowmetry.

RESULTS

No changes in ET or perfusion parameters were observed in controls. The mean ET level after orthotopic liver transplantation (OLT) in Group B was elevated (baseline: 3.8 +/- 2.4 pg/ml) compared with Group A (2.8 +/- 1.9 pg/ml). With rising ET levels HAF decreased progressively in Group A from 205 +/- 97 (baseline) to 160 +/- 72 ml/min, and in Group B from 161 +/- 87 to 146 +/- 68 ml/min. PVF decreased in Group A from 722 +/- 253 to 370 +/- 198 ml/min, and in Group B from 846 +/- 263 to 417 +/- 203 ml/min. Baseline HMC in Group A was 86 +/- 15 and decreased significantly to 29 +/- 9 ml/100 g/min, and baseline MC in Group B was 90 +/- 22 and decreased to 44 +/- 32 ml/100 g/min. No significant alteration in systemic circulation was noted at the ET concentrations investigated.

CONCLUSIONS

Significant impairment of hepatic micro- and macrocirculation was detected after induction of systemic ET levels above 9.4 pg/ml both in native and in transplanted livers. Disturbance of HMC was caused predominantly by reduction of portal venous flow, while the effect of ET on HAF was less pronounced. Characteristics of flow impairment in transplanted and native livers were analogous after short cold ischemic graft storage (6 h).

Nitrovasodilators inhibit platelet-derived growth factor-induced proliferation and migration of activated human hepatic stellate cells

BACKGROUND & AIMS

Nitrovasodilators have been proposed for the treatment of portal hypertension alone or in combination with beta-blockers. In addition to their vasodilatory properties, nitric oxide (NO) donors may exert direct antifibrogenic properties. We evaluated the effect of nitroglycerin (NTG) and S-nitroso-N-acetyl penicillamine (SNAP) on the mitogenic and chemotactic properties of platelet-derived growth factor (PDGF)-BB and the modulation of the relative intracellular signaling pathways in fully activated human hepatic stellate cells (HSCs), a cell type that plays an active role in liver fibrogenesis and portal hypertension.

METHODS & RESULTS

Both NTG and SNAP induced a dose-dependent decrease in PDGF-induced DNA synthesis and cell migration, which was associated with a decrease in PDGF-induced intracellular Ca(2+) increase and extracellular signal-regulated kinase (ERK) activity. These effects were not related to activation of the classic soluble guanylate cyclase (sGC)/guanosine 3',5'-cyclic monophosphate pathway; accordingly, Western blot analysis of HSC lysates revealed the absence of the alpha(1)beta(1) ubiquitous subunits of sGC, whereas they were detectable in quiescent HSCs, freshly isolated from normal human liver. Conversely, both NTG and SNAP induced a more than 10-20-fold increase in prostaglandin E(2) in cell supernatants within 1 minute, associated with an increase in intracellular adenosine 3',5'-cyclic monophosphate levels. Accordingly, the inhibitory effects of NO donors on PDGF action and signaling were eliminated after preincubation with ibuprofen.

CONCLUSIONS

These results suggest that NO donors may exert a direct antifibrogenic action by inhibiting proliferation, motility, and contractility of HSCs in addition to a reduction of fibrillar extracellular matrix accumulation.

The cell and molecular biology of hepatic fibrogenesis. Clinical and therapeutic implications

Much has been learned in the past 2 decades about the cellular and molecular mechanisms underlying hepatic fibrogenesis and about potential therapeutic approaches in patients with liver disease. The central event in fibrogenesis seems to be the activation of hepatic stellate cells. Stellate cell activation is characterized by several important features, including enhanced matrix synthesis and a prominent contractile phenotype, processes that probably contribute to the physical distortion and dysfunction of the liver in advanced disease. It is important to emphasize that the factors controlling activation are multifactorial and complex. The extracellular matrix is a dynamic, active constituent of the fibrogenic response and undergoes active remodeling, including synthesis and degradation. Effective therapy for hepatic fibrogenesis will probably also be multifactorial, based on the basic mechanisms underlying the fibrogenic process. The most effective therapies will probably be directed at the stellate cell. Approaches that address matrix remodeling (i.e., by enhancing matrix degradation or by inhibiting factors that prevent matrix breakdown) may be effective.

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

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

Enhanced expression of vsmNOS mRNA in glomeruli from rats with unilateral ureteral obstruction

BACKGROUND

The vasodilatory/cytotoxic gas, nitric oxide (NO), is associated with an alteration in glomerular hemodynamics seen after the induction of ureteral ligation. As yet the type of nitric oxide synthase (NOS) protein involved in the mechanism has not been clearly established in the unilateral ureteral obstruction (UUO) model.

METHODS

Using reverse transcription (RT)-polymerase chain reaction (PCR), the expression and localization of vascular smooth muscle-derived nitric oxide synthase (vsmNOS) mRNA were examined in glomeruli from sham-operated control (SOC) rats and rats with UUO of three hours duration. Moreover, the effect of endogenous angiotensin II on the expression of vsmNOS mRNA in glomeruli was explored using SOC rats and rats with UUO that were pretreated or not with enalapril, an angiotensin-converting enzyme inhibitor.

RESULTS

The expression of vsmNOS mRNA was significantly greater in glomeruli of rats with UUO than in those of SOC rats. In rats with UUO, the expression of vsmNOS mRNA was substantially increased in glomeruli of the obstructed kidney (OK) compared to the contralateral, nonobstructed kidney (CLK). Suppression of angiotensin II production in vivo with enalapril restored the expression of vsmNOS mRNA in glomeruli of the CLK and OK from rats with UUO to levels comparable to that seen in glomeruli from SOC rats. In addition, the in situ RT-PCR analysis, a novel method for mRNA identification in cells and tissue, revealed that vsmNOS mRNA was expressed in the cytoplasm of glomerular mesangial and epithelial cells in SOC rats and rats with UUO.

CONCLUSIONS

An increase in vsmNOS mRNA expression in glomeruli of the CLK and OK from rats with UUO may be mediated by increased action of endogenous angiotensin II that occurs after the onset of ureteral obstruction. Enhanced expression of vsmNOS mRNA in glomeruli of the OK compared to the CLK may be due to differences in levels of angiotensin II acting on the two kidneys in vivo. Additionally, the expression of vsmNOS mRNA in glomeruli originates in mesangial and epithelial cells in SOC rats and rats with UUO.

Effect of endotoxemia on hepatic portal and sinusoidal blood flow in rats

A decrease in liver blood flow leads to dysfunction of hepatocytes and Kupffer cells, with subsequent local and systemic liberation of proinflammatory mediators that may maintain systemic inflammatory response syndrome (SIRS) and may lead to multiple organ dysfunction syndrome (MODS). There is only limited knowledge on the hepatic micro- and macrocirculation during sepsis or endotoxemia. Therefore, the aim of our study was to investigate alterations in hepatic portal blood flow (PBF) and sinusoidal blood flow (SBF) during endotoxemia. In male Wistar rats endotoxemia was induced by continuous infusion of 2 mg/kg/h lipopolysaccharides from Escherichia coli 026:B6 immediately after baseline measurements (n = 8). The control group (n = 8) received an equivalent volume of Ringer's solution. Mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), PBF, and SBF were measured at baseline and 60 and 120 min after induction of endotoxemia. PBF was measured using an ultrasonic flow probe that was positioned around the portal vein. SBF was detected by in vivo videomicroscopy of the left liver lobe. In the LPS group MAP decreased, but CO remained at baseline values. During endotoxemia PBF decreased significantly from 23 +/- 3 to 15 +/- 4 mL/min (60 min) and 16 +/- 3 mL/min (120 min). SBF also significantly decreased to 68.5% (60 min) and 57.1% (120 min) of baseline value. Our results demonstrate that during early endotoxemia hepatic macro- and microcirculatory perfusion is significantly decreased despite unchanged CO. This early reduction of hepatic perfusion might be caused by an increased hepatic vessel resistance as a consequence of liberation of vasoconstrictive mediators or/and by a decrease in intestinal perfusion.

Hypoxic stimulation of vascular endothelial growth factor expression in activated rat hepatic stellate cells

The tissue repair response to hypoxic stimuli during wound healing includes enhanced production of angiogenic factors, such as vascular endothelial growth factor (VEGF). Hepatic stellate cells are oxygen-sensing cells, capable of producing VEGF. We hypothesized that hypoxia-stimulated signaling in activated stellate cells mediate VEGF secretion during liver injury. The specific aim was to evaluate the effect of hypoxia on the gene expression of VEGF in HSC-T6 cells, an immortalized rat hepatic stellate cell line, and in rat primary cultures of stellate cells. Hypoxic induction of VEGF mRNA was dose- and time-dependent. The hypoxic stimulation of VEGF messenger RNA (mRNA) correlated with the secretion of VEGF protein in conditioned media by hypoxic T6 cells. S-Nitroso-N-acetyl-D, L-penicillamine (SNAP), a nitric oxide (NO) donor, and desferrioxamine (DFx) and cobalt chloride, mimics of cellular hypoxia, similarly stimulated VEGF mRNA expression and secretion. Four previously described splice variants of the VEGF mRNA (VEGF-120, 144, 164, 188) were detected in both normoxic- or hypoxic-activated stellate cells. There was differential expression of the VEGF receptors, Flt-1 and Flk-1, in hypoxic T6 cells. Hypoxic conditions selectively stimulated Flt-1 mRNA expression, whereas Flk-1 mRNA remained unchanged. Hypoxic induction of VEGF was also demonstrated in primary stellate cell cultures and after in vivo injury. Hypoxia stimulates cell signaling in stellate cells, culminating in the rapid induction of VEGF and Flt-1 mRNA expression and VEGF secretion. The hypoxic induction of VEGF is mimicked by NO and may be of mechanistic importance in the pathogenesis of hepatic wound healing and hepatocarcinogenesis.

Myofibroblasts: paracrine cells important in health and disease

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

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

Liver fibrogenesis and the role of hepatic stellate cells: new insights and prospects for therapy

Hepatic fibrosis is a wound-healing response to chronic liver injury, which if persistent leads to cirrhosis and liver failure. Exciting progress has been made in understanding the mechanisms of hepatic fibrosis. Major advances include: (i) characterization of the components of extracellular matrix (ECM) in normal and fibrotic liver; (ii) identification of hepatic stellate cells as the primary source of ECM in liver fibrosis; (iii) elucidation of key cytokines, their cellular sources, modes of regulation, and signalling pathways involved in liver fibrogenesis; (iv) characterization of key matrix proteases and their inhibitors; (v) identification of apoptotic mediators in stellate cells and exploration of their roles during the resolution of liver injury. These advances have helped delineate a more comprehensive picture of liver fibrosis in which the central event is the activation of stellate cells, a transformation from quiescent vitamin A-rich cells to proliferative, fibrogenic and contractile myofibroblasts. The progress in understanding fibrogenic mechanisms brings the development of effective therapies closer to reality. In the future, targeting of stellate cells and fibrogenic mediators will be a mainstay of antifibrotic therapy. Points of therapeutic intervention may include: (i) removing the injurious stimuli; (ii) suppressing hepatic inflammation; (iii) down-regulating stellate cell activation; and (iv) promoting matrix degradation. The future prospects for effective antifibrotic treatment are more promising than ever for the millions of patients with chronic liver disease worldwide.

Myofibroblasts. I. Paracrine cells important in health and disease

Myofibroblasts are a unique group of smooth-muscle-like fibroblasts that have a similar appearance and function regardless of their tissue of residence. Through the secretion of inflammatory and anti-inflammatory cytokines, chemokines, growth factors, both lipid and gaseous inflammatory mediators, as well as extracellular matrix proteins and proteases, they play an important role in organogenesis and oncogenesis, inflammation, repair, and fibrosis in most organs and tissues. Platelet-derived growth factor (PDGF) and stem cell factor are two secreted proteins responsible for differentiating myofibroblasts from embryological stem cells. These and other growth factors cause proliferation of myofibroblasts, and myofibroblast secretion of extracellular matrix (ECM) molecules and various cytokines and growth factors causes mobility, proliferation, and differentiation of epithelial or parenchymal cells. Repeated cycles of injury and repair lead to organ or tissue fibrosis through secretion of ECM by the myofibroblasts. Transforming growth factor-beta and the PDGF family of growth factors are the key factors in the fibrotic response. Because of their ubiquitous presence in all tissues, myofibroblasts play important roles in various organ diseases and perhaps in multisystem diseases as well.

In vitro migratory potential of rat quiescent hepatic stellate cells and its augmentation by cell activation

In liver injury, hepatic stellate cells are considered to depart from the sinusoidal wall and accumulate in the necrotic lesion through migration and proliferation. In this study, we investigated the migratory capacity of quiescent stellate cells in vitro and analyzed the relationship with proliferative response. Freshly isolated stellate cells that were seeded in the upper chamber of Cell Culture Insert (Becton Dickenson, Franklin Lakes, NJ) started to migrate to the lower chamber at 1 day and increased in migration index to 19% at 2 days. Cells in the lower chamber were stretched in shape with many lipid droplets and showed quiescent properties, i.e., negative expression of alpha-smooth muscle actin (alpha-SMA) or platelet-derived growth factor receptor-beta (PDGFR-beta). Migratory capacity in quiescent cells was also shown in the Matrigel-coated insert. Matrix metalloproteinase-2 (MMP-2) messenger RNA expression was low just after isolation, but was enhanced as migration became prominent. Migrating cells further showed higher proliferative activity than resting ones. The presence of PDGF/BB and Kupffer cells accelerated stellate cell migration by the chemotactic mechanism and concurrently augmented proliferation, whereas that of dexamethasone and interferon-gamma (IFN-gamma) attenuated migration as a result of general suppression effects. Compared with quiescent ones, alpha-SMA and PDGFR-beta-positive activated stellate cells obtained by 14-day culture exhibited more rapid and prominent migration, being regulated by mediators in a similar manner as described previously. These data indicate that quiescent stellate cells undergo migration, which is linked to proliferation and enhanced by PDGF/BB and Kupffer cells, suggesting the involvement of this function in the initial phase of development of postnecrotic fibrosis.

Hepatic and splanchnic nitric oxide activity in patients with cirrhosis

BACKGROUND

In animal models of cirrhosis, altered activity of nitric oxide (NO) has been implicated in the pathogenesis of increased intrahepatic portal vascular resistance and abnormal mesenteric vasodilatation.

AIMS

To investigate NO activity in the liver and splanchnic vascular bed of patients with cirrhosis.

METHODS

Activity of the calcium dependent constitutive and calcium independent inducible isoforms of NO synthase (cNOS and iNOS, respectively) was assayed biochemically in biopsy specimens of liver and a vascular portion of the greater omentum (representative of mesenteric vasculature) obtained from patients with cirrhosis undergoing liver transplantation (n=14) and non-cirrhotic control patients undergoing liver resection for metastases (n=9). The concentration of NO metabolites (NO2 + NO3) in portal and peripheral venous plasma was measured.

RESULTS

The activity of cNOS was lower in cirrhotic compared with non-cirrhotic subjects for both liver and omentum. Hepatic and omental iNOS activities did not differ significantly between the two groups. Portal (NO2 + NO3) was threefold higher in cirrhotic than non-cirrhotic patients, but no differences were observed in systemic venous samples from the two groups.

CONCLUSIONS

The activity of cNOS is diminished in the cirrhotic human liver. The resultant decrease in constitutive NO release may promote an increase in the intrahepatic portal vascular resistance. Elevated portal venous (NO2 + NO3) indicates enhanced splanchnic vascular release of NO in cirrhotic patients, but the absence of increased NOS activity in the mesenteric vasculature suggests differential regulation of NO synthesis within the splanchnic vascular bed.

Activated stellate (Ito) cells possess voltage-activated calcium current

We previously reported stellate (Ito) cells possess voltage-activated Ca2+ current. The activation of stellate cells has been indicated to contribute to liver fibrosis and the regulation of hepatic hemodynamics. The aim of this study was to investigate the relationship between voltage-activated Ca2+ current and activation of stellate cells. Voltage-activated Ca2+ current in stellate cells isolated from rats were studied using whole-cell patch clamp technique. L-type voltage-activated Ca2+ current was hardly detected in stellate cells cultured for less than 9 days. Ca2+ current was detected 12.5 and 69% of cells at the 10th and 14th day of culture, respectively. BrdU incorporation indicated cell proliferation was recognized over 50% of cells at the 3rd and 5th day of culture, respectively, then decreased significantly in a time-dependent manner. On the other hand, the expression of alpha-smooth muscle actin indicated cell activation increased from 7th day of culture and collagen type I mRNA appeared remarkably in cells cultured for more than 10 days. In this study, we concluded L-type voltage-activated Ca2+ current was recognized in activated stellate (myofibroblast-like) cells.

Diet-induced protection against lipopolysaccharide includes increased hepatic NO production

The host response to Gram-negative infection includes the elaboration of numerous proinflammatory agents, including tumor necrosis factor alpha (TNFalpha) and nitric oxide (NO). A component of the hepatic response to infection is an elevation in serum lipids, the so-called "lipemia of sepsis," which results from the increased production of triglyceride (TG)-rich lipoproteins by the liver. We have postulated that these lipoproteins are components of a nonadaptive, innate immune response to endotoxin [lipopolysaccharide (LPS)] and have previously demonstrated the capacity of TG-rich lipoproteins to protect against endotoxicity in rodent models of sepsis. Herein we report the capacity of a high-fructose diet to protect against LPS, most likely by inducing high circulating levels of endogenous TG-rich lipoproteins. The protective phenotype included the increased production of NO by hepatic endothelial cells. Rats, made hypertriglyceridemic by fructose feeding, experienced decreased LPS-induced mortality (P < 0.03) and systemic TNFalpha levels (P < 0.05) as compared with normolipidemic (chow-fed) controls. The increased survival was associated with elevated levels of inducible NO synthase (NOS2) mRNA levels and NO production (82 +/- 26 vs 3 +/- 3 nmol nitrite/10(6) cells, P < 0.001) by hepatic endothelial cells. Nonselective NOS inhibitors reversed the protective phenotype in vivo and readily decreased NO production by cultured endothelial cells from hypertriglyceridemic rats in vitro. This study suggests that a high-fructose diet can protect against endotoxicity in part through induction of endogenous TG-rich lipoproteins and hepatic endothelial cell NO production. This is the first report of diet-induced hyperlipoproteinemia and subsequent protection against endotoxemia.

Characterization of cationic amino acid transporter and its gene expression in rat hepatic stellate cells in relation to nitric oxide production

BACKGROUND/AIMS

Nitric oxide is a potent mediator of hepatic sinusoidal hemodynamics and affects hepatic stellate cells (Ito cells, fat-storing cells). Although nitric oxide production may depend on the induction of inducible nitric oxide synthase and on transport of extracellular L-arginine, the precise mechanisms controlling nitric oxide production in stellate cells have not been well characterized.

METHODS

Using stellate cells prepared from the male Wistar rat, kinetic analysis of L-arginine transport and reverse transcription-polymerase chain reaction for cationic amino acid transporter were carried out. The effect of tumor necrosis factor-alpha and interferon-gamma on L-arginine transport, mRNA expression of cationic amino acid transporter and inducible nitric oxide synthase, and nitric oxide production of stellate cells was assessed.

RESULTS

The L-arginine transport system functioning in the transformed hepatic stellate cells was system y+, possibly mediated by cationic amino acid transporter-1 and cationic amino acid transporter-2B (Km approximately 50 microM). Tumor necrosis factor-alpha enhanced cationic amino acid transporter-2B mRNA expression and L-arginine transport, whereas cationic amino acid transporter-1 mRNA expression remained unchanged. Interferon-gamma induced the expression of inducible nitric oxide synthase mRNA without obvious changes in L-arginine transport. Interferon-gamma in combination with tumor necrosis factor-alpha induced nitric oxide production with an enhancement in cationic amino acid transporter-2B mRNA expression, inducible nitric oxide synthase mRNA expression, and L-arginine transport, while extracellular L-lysine competitively inhibited this nitric oxide production.

CONCLUSIONS

In transformed hepatic stellate cells, tumor necrosis factor-alpha and interferon-gamma have a crucial role in nitric oxide production, and extracellular L-arginine transport and inducible nitric oxide synthase expression are regulated in a differential cytokine-specific manner. As the estimated Km of L-arginine transporter in transformed hepatic stellate cells is very similar to the physiological L-arginine concentration in portal vein, we assume that increased portal L-arginine concentration may easily affect sinusoidal blood flow through enhancement of autocrine nitric oxide production in transformed hepatic stellate cells of diseased liver.

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

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