Regulation of endothelin-1 synthesis by endothelin-converting enzyme-1 during wound healing

GARP on hepatic stellate cells is essential for the development of liver fibrosis

BACKGROUND & AIMS

Glycoprotein A repetitions predominant (GARP) is a membrane protein that functions as a latent TGF-β docking molecule. While the immune regulatory properties of GARP on blood cells have been studied, the function of GARP on tissue stromal cells remains unclear. Here, we investigate the role of GARP expressed on hepatic stellate cells (HSCs) in the development of liver fibrosis.

METHODS

The function of GARP on HSCs was explored in toxin-induced and metabolic liver fibrosis models, using conditional GARP-deficient mice or a newly generated inducible system for HSC-specific gene ablation. Primary mouse and human HSCs were isolated to evaluate the contribution of GARP to the activation of latent TGF-β. Moreover, cell contraction of HSCs in the context of TGF-β activation was tested in a GARP-dependent fashion.

RESULTS

Mice lacking GARP in HSCs were protected from developing liver fibrosis. Therapeutically deleting GARP on HSCs alleviated the fibrotic process in established disease. Furthermore, natural killer T cells exacerbated hepatic fibrosis by inducing GARP expression on HSCs through IL-4 production. Mechanistically, GARP facilitated fibrogenesis by activating TGF-β and enhancing endothelin-1-mediated HSC contraction. Functional GARP was expressed on human HSCs and significantly upregulated in the livers of patients with fibrosis. Lastly, deletion of GARP on HSCs did not augment inflammation or liver damage.

CONCLUSIONS

GARP expressed on HSCs drives the development of liver fibrosis via cell contraction-mediated activation of latent TGF-β. Considering that systemic blockade of TGF-β has major side effects, we highlight a therapeutic niche provided by GARP and surface-mediated TGF-β activation. Thus, our findings suggest an important role of GARP on HSCs as a promising target for the treatment of liver fibrosis.

IMPACT AND IMPLICATIONS

Liver fibrosis represents a substantial and increasing public health burden globally, for which specific treatments are not available. Glycoprotein A repetitions predominant (GARP) is a membrane protein that functions as a latent TGF-β docking molecule. Here, we show that GARP expressed on hepatic stellate cells drives the development of liver fibrosis. Our findings suggest GARP as a novel target for the treatment of fibrotic disease.

Intercellular crosstalk of liver sinusoidal endothelial cells in liver fibrosis, cirrhosis and hepatocellular carcinoma

Intercellular crosstalk among various liver cells plays an important role in liver fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Capillarization of liver sinusoidal endothelial cells (LSECs) precedes fibrosis and accumulating evidence suggests that the crosstalk between LSECs and other liver cells is critical in the development and progression of liver fibrosis. LSECs dysfunction, a key event in the progression from fibrosis to cirrhosis, and subsequently obstruction of hepatic sinuses and increased intrahepatic vascular resistance (IHVR) contribute to development of portal hypertension (PHT) and cirrhosis. More importantly, immunosuppressive tumor microenvironment (TME), which is closely related to the crosstalk between LSECs and immune liver cells like CD8+ T cells, promotes advances tumorigenesis, especially HCC. However, the connections within the crosstalk between LSECs and other liver cells during the progression from liver fibrosis to cirrhosis to HCC have yet to be discussed. In this review, we first summarize the current knowledge of how different crosstalk between LSECs and other liver cells, including hepatocytes, hepatic stellate cells (HSCs), macrophoges, immune cells in liver and extra cellular matrix (ECM) contribute to the physiological function and the progrssion from liver fibrosis to cirrhosis, or even to HCC. Then we examine current treatment strategies for LSECs crosstalk in liver fibrosis, cirrhosis and HCC.

Pentoxifylline-induced protein expression change in RAW 264.7 cells as determined by immunoprecipitation-based high performance liquid chromatography

Although pentoxifylline (PTX) was identified as a competitive non-selective phosphodiesterase inhibitor, its pharmacological effect has not been clearly elucidated. The present study explored the effect of low dose 10 μg/mL PTX (therapeutic dose) compared to high dose 300 μg/mL PTX (experimental dose) in RAW 264.7 cells through immunoprecipitation-based high performance liquid chromatography (IP-HPLC), immunohistochemistry, and western blot. 10 μg/mL PTX increased the expression of proliferation (Ki-67, PCNA, cyclin D2, cdc25A), epigenetic modification (KDM4D, PCAF, HMGB1), protein translation (DOHH, DHPS, eIF5A1), RAS signaling (KRAS, pAKT1/2/3, PI3K), NFkB signaling (NFkB, GADD45, p38), protection (HSP70, SOD1, GSTO1/2), survival (pAKT1/2/3, SP1, sirtuin 6), neuromuscular differentiation (NSEγ, myosin-1a, desmin), osteoblastic differentiation (BMP2, RUNX2, osterix), acute inflammation (TNFα, IL-1, CXCR4), innate immunity (β-defensin 1, lactoferrin, TLR-3, -4), cell-mediated immunity (CD4, CD8, CD80), while decreased the expression of ER stress (eIF2α, eIF2AK3, ATF6α), fibrosis (FGF2, CTGF, collagen 3A1), and chronic inflammation (CD68, MMP-2, -3, COX2) versus the untreated controls. The activation of proliferation by 10 μg/mL PTX was also supported by the increase of cMyc-MAX heterodimer and β-catenin-TCF1 complex in double IP-HPLC. 10 μg/mL PTX enhanced FAS-mediated apoptosis but diminished p53-mediated apoptosis, and downregulated many angiogenesis proteins (angiogenin, VEGF-A, and FLT4), but upregulated HIF1α, VEGFR2, and CMG2 reactively. Whereas, 300 μg/mL PTX consistently decreased proliferation, epigenetic modification, RAS and NFkB signaling, neuromuscular and osteoblastic differentiation, but increased apoptosis, ER stress, and fibrosis compared to 10 μg/mL PTX. These data suggest PTX has different biological effect on RWA 264.7 cells depending on the concentration of 10 μg/mL and 300 μg/mL PTX. The low dose 10 μg/mL PTX enhanced RAS/NFkB signaling, proliferation, differentiation, and inflammation, particularly, it stimulated neuromuscular and osteoblastic differentiation, innate immunity, and cell-mediated immunity, but attenuated ER stress, fibrosis, angiogenesis, and chronic inflammation, while the high dose 300 μg/mL PTX was found to alleviate the 10 μg/mL PTX-induced biological effects, resulted in the suppression of RAS/NFkB signaling, proliferation, neuromuscular and osteoblastic differentiation, and inflammation.

cAMP Signaling in Pathobiology of Alcohol Associated Liver Disease

The importance of cyclic adenosine monophosphate (cAMP) in cellular responses to extracellular signals is well established. Many years after discovery, our understanding of the intricacy of cAMP signaling has improved dramatically. Multiple layers of regulation exist to ensure the specificity of cellular cAMP signaling. Hence, disturbances in cAMP homeostasis could arise at multiple levels, from changes in G protein coupled receptors and production of cAMP to the rate of degradation by phosphodiesterases. cAMP signaling plays critical roles in metabolism, inflammation and development of fibrosis in several tissues. Alcohol-associated liver disease (ALD) is a multifactorial condition ranging from a simple steatosis to steatohepatitis and fibrosis and ultimately cirrhosis, which might lead to hepatocellular cancer. To date, there is no FDA-approved therapy for ALD. Hence, identifying the targets for the treatment of ALD is an important undertaking. Several human studies have reported the changes in cAMP homeostasis in relation to alcohol use disorders. cAMP signaling has also been extensively studied in in vitro and in vivo models of ALD. This review focuses on the role of cAMP in the pathobiology of ALD with emphasis on the therapeutic potential of targeting cAMP signaling for the treatment of various stages of ALD.

Ameliorating effects of Cuscuta chinensis Lamak extract on hind‑limb ischemia, and angiogenic‑ or inflammatory associated factors in ovariectomized mice

Cuscuta chinensis Lamak (CCL) has traditionally been used in Korea to treat sexual disorders and skin problems. The aim of the present study was to investigate the effects of CCL extract on surgical injury‑induced ischemia in the hind limbs of mice. Specifically, female C57BL/6 mice were ovariectomized, and their hind‑limb vessels were ligated with surgical silk (6‑0) and excised. CCL (150 or 450 mg/kg/BW) was then administered to the mice for 3 weeks, and the blood flow rate was evaluated using a laser Doppler system at ‑7, 0, 7, 14 and 21 days following hind‑limb ischemia. The serum expression profiles of angiogenic and inflammatory mediators were measured using an antibody array, and the transcript levels were reverse transcription‑quantitative polymerase chain reaction. The rate of hind limb blood flow was normalized to non‑ischemic lesions and revealed to be markedly elevated at 14 and 21 days following ischemia when compared with the vehicle group. The density of capillaries in the hind limbs was also significantly increased following treatment with CCL in a dose‑dependent manner. In addition, the transcriptional expression of angiogenetic factors were upregulated, whereas that of inflammatory cytokines were downregulated. Finally, vascular endothelial cell migration and tube formation were evaluated in vitro using human umbilical vein endothelial cells (HUVECs) and identified to be significantly increased following treatment with CCL. Overall, the results of the present study indicate that CCL extract exhibits therapeutic potential for the treatment of hind‑limb ischemia as it promotes peripheral angiogenic and anti‑inflammatory effects in mice.

Biology of portal hypertension

Portal hypertension develops as a result of increased intrahepatic vascular resistance often caused by chronic liver disease that leads to structural distortion by fibrosis, microvascular thrombosis, dysfunction of liver sinusoidal endothelial cells (LSECs), and hepatic stellate cell (HSC) activation. While the basic mechanisms of LSEC and HSC dysregulation have been extensively studied, the role of microvascular thrombosis and platelet function in the pathogenesis of portal hypertension remains to be clearly characterized. As a secondary event, portal hypertension results in splanchnic and systemic arterial vasodilation, leading to the development of a hyperdynamic circulatory syndrome and subsequently to clinically devastating complications including gastroesophageal varices and variceal hemorrhage, hepatic encephalopathy from the formation of portosystemic shunts, ascites, and renal failure due to the hepatorenal syndrome. This review article discusses: (1) mechanisms of sinusoidal portal hypertension, focusing on HSC and LSEC biology, pathological angiogenesis, and the role of microvascular thrombosis and platelets, (2) the mesenteric vasculature in portal hypertension, and (3) future directions for vascular biology research in portal hypertension.

Hepatic Stellate Cells in Liver Fibrosis and siRNA-Based Therapy

Hepatic fibrosis is a reversible wound-healing response to either acute or chronic liver injury caused by hepatitis B or C, alcohol, and toxic agents. Hepatic fibrosis is characterized by excessive accumulation and reduced degradation of extracellular matrix (ECM). Excessive accumulation of ECM alters the hepatic architecture leading to liver fibrosis and cirrhosis. Cirrhosis results in failure of common functions of the liver. Hepatic stellate cells (HSC) play a major role in the development of liver fibrosis as HSC are the main source of the excessive production of ECM in an injured liver. RNA interference (RNAi) is a recently discovered therapeutic tool that may provide a solution to manage multiple diseases including liver fibrosis through silencing of specific gene expression in diseased cells. However, gene silencing using small interfering RNA (siRNA) is encountering many challenges in the body after systemic administration. Efficient and stable siRNA delivery to the target cells is a key issue for the development of siRNA therapeutic. For that reason, various viral and non-viral carriers for liver-targeted siRNA delivery have been developed. This review will cover the current strategies for the treatment of liver fibrosis as well as discussing non-viral approaches such as cationic polymers and lipid-based nanoparticles for targeted delivery of siRNA to the liver.

Liver sinusoidal endothelial cells in hepatic fibrosis

Capillarization, lack of liver sinusoidal endothelial cell (LSEC) fenestration, and formation of an organized basement membrane not only precedes fibrosis, but is also permissive for hepatic stellate cell activation and fibrosis. Thus, dysregulation of the LSEC phenotype is a critical step in the fibrotic process. Both a vascular endothelial growth factor (VEGF)-stimulated, nitric oxide (NO)-independent pathway and a VEGF-stimulated NO-dependent pathway are necessary to maintain the differentiated LSEC phenotype. The NO-dependent pathway is impaired in capillarization and activation of this pathway downstream from NO restores LSEC differentiation in vivo. Restoration of LSEC differentiation in vivo promotes HSC quiescence, enhances regression of fibrosis, and prevents progression of cirrhosis.

Hepatic stellate cells and liver fibrosis

Hepatic stellate cells are resident perisinusoidal cells distributed throughout the liver, with a remarkable range of functions in normal and injured liver. Derived embryologically from septum transversum mesenchyme, their precursors include submesothelial cells that invade the liver parenchyma from the hepatic capsule. In normal adult liver, their most characteristic feature is the presence of cytoplasmic perinuclear droplets that are laden with retinyl (vitamin A) esters. Normal stellate cells display several patterns of intermediate filaments expression (e.g., desmin, vimentin, and/or glial fibrillary acidic protein) suggesting that there are subpopulations within this parental cell type. In the normal liver, stellate cells participate in retinoid storage, vasoregulation through endothelial cell interactions, extracellular matrix homeostasis, drug detoxification, immunotolerance, and possibly the preservation of hepatocyte mass through secretion of mitogens including hepatocyte growth factor. During liver injury, stellate cells activate into alpha smooth muscle actin-expressing contractile myofibroblasts, which contribute to vascular distortion and increased vascular resistance, thereby promoting portal hypertension. Other features of stellate cell activation include mitogen-mediated proliferation, increased fibrogenesis driven by connective tissue growth factor, and transforming growth factor beta 1, amplified inflammation and immunoregulation, and altered matrix degradation. Evolving areas of interest in stellate cell biology seek to understand mechanisms of their clearance during fibrosis resolution by either apoptosis, senescence, or reversion, and their contribution to hepatic stem cell amplification, regeneration, and hepatocellular cancer.

Endothelin-converting enzyme-1 gene ablation attenuates pulmonary fibrosis via CGRP-cAMP/EPAC1 pathway

Endothelin-1 (ET-1) has been shown to be involved in human pulmonary fibrosis. However, recent clinical trials targeting the ET-1 pathway with ET-1 receptor antagonists failed to achieve beneficial outcomes. Another strategy opposing the actions of ET-1 involves the inhibition of endothelin-converting enzyme-1 (ECE-1). We hypothesize that ECE-1 inhibition exerts beneficial effects on pulmonary fibrosis. Pulmonary fibrosis was induced by instilling bleomycin intratracheally into ECE-1 heterozygous knockout mice (ECE-1(+/-)) and their wild-type control mice (ECE-1(+/+)). Lung inflammation and fibrosis were assessed on Days 7, 14, and 28 after bleomycin instillation. The activity of ECE-1 and the concentrations of its related peptides, ET-1, bradykinin, atrial natriuretic peptide (ANP), and calcitonin gene-related peptide (CGRP), were determined. ECE-1(+/-) mice demonstrated less lung inflammation and limited fibrosis compared with control mice. ECE-1 activity was half-reduced in ECE-1(+/-) mice, and this activity also altered ET-1 and CGRP concentrations, but not concentrations of bradykinin and ANP. ET-1 concentrations were found to be lower in ECE-1(+/-) mice after the development of fibrosis, in contrast to the unaltered concentrations during inflammation. Reduced ECE-1 activity resulted in higher CGRP concentrations, which altered the pathological functionality of the lung, indicating the activation of the CGRP pathway involving cyclic adenosine monophosphate (cAMP)/exchange protein directly activated by cAMP and cAMP/protein kinase A in ECE-1(+/-) mice. Bleomycin instillation on Day 14 induced the accumulation of M2 macrophages expressing CGRP receptors in ECE-1(+/-) mice. Our results emphasize that the in vivo ECE-1-mediated degradation of CGRP promotes the transition from lung inflammation to fibrosis. Further, our study identified M2 macrophages as the target cells of CGRP action during this transition.

Inhibition of hepatic stellate cell activation and liver fibrosis by fat-specific protein 27

In order to explore the effects of fat-specific protein 27 (Fsp27) on regulation of hepatic stellate cell (HSC) activation and liver fibrosis. HSCs were isolated from rat liver tissues and cultivated in vitro for gene expression and lentivirus infection. CCK-8 cell viability assay, immunofluorescence staining, qRT-PCR, and western blot assays were used to assess phenotypic changes and gene expression in HSCs. The rat liver fibrosis model was produced by intraperitoneal injection of carbon tetrachloride for assessing the effects of Fsp27 in the rat liver. Gene expression was then detected by immunohistochemistry and ELISA assays. The results of the study showed that Fsp27 was constitutively expressed in primary quiescent HSCs, but was absent in activated HSCs. Ectopic expression of Fsp27 significantly inhibited HSC proliferation and activation, as well as expression of α-smooth muscle actin. Fsp27 expression also significantly reduced collagen I production and matrix metalloproteinases 2 protein levels, and to a lesser degree, reduced tissue inhibitors of metalloproteinases 1 expression. In vivo data showed that ectopic expression of Fsp27 protein significantly reduced levels of hydroxyproline in liver tissue, and decreased serum levels of collagen III and hyaluronic acid, which in turn, suppressed liver fibrosis in rats. From these findings, it can be concluded that Fsp27 expression suppressed HSC activation in vitro and liver fibrogenesis in vivo. Further studies are needed to explore whether expression of Fsp27 can be selected as a potential novel strategy for anti-fibrotic therapy against liver fibrosis.

Pathogenesis of liver fibrosis

Liver fibrosis is a major cause of morbidity and mortality worldwide due to chronic viral hepatitis and, more recently, from fatty liver disease associated with obesity. Hepatic stellate cell activation represents a critical event in fibrosis because these cells become the primary source of extracellular matrix in liver upon injury. Use of cell-culture and animal models has expanded our understanding of the mechanisms underlying stellate cell activation and has shed new light on genetic regulation, the contribution of immune signaling, and the potential reversibility of the disease. As pathways of fibrogenesis are increasingly clarified, the key challenge will be translating new advances into the development of antifibrotic therapies for patients with chronic liver disease.

Mechanisms of hepatic fibrogenesis

Multiple etiologies of liver disease lead to liver fibrosis through integrated signaling networks that regulate the deposition of extracellular matrix. This cascade of responses drives the activation of hepatic stellate cells (HSCs) into a myofibroblast-like phenotype that is contractile, proliferative and fibrogenic. Collagen and other extracellular matrix (ECM) components are deposited as the liver generates a wound-healing response to encapsulate injury. Sustained fibrogenesis leads to cirrhosis, characterized by a distortion of the liver parenchyma and vascular architecture. Uncovering the intricate mechanisms that underlie liver fibrogenesis forms the basis for efforts to develop targeted therapies to reverse the fibrotic response and improve the outcomes of patients with chronic liver disease.

Mechanisms of hepatic fibrogenesis

Multiple etiologies of liver disease lead to liver fibrosis through integrated signaling networks that regulate the deposition of extracellular matrix. This cascade of responses drives the activation of hepatic stellate cells (HSCs) into a myofibroblast-like phenotype that is contractile, proliferative and fibrogenic. Collagen and other extracellular matrix (ECM) components are deposited as the liver generates a wound-healing response to encapsulate injury. Sustained fibrogenesis leads to cirrhosis, characterized by a distortion of the liver parenchyma and vascular architecture. Uncovering the intricate mechanisms that underlie liver fibrogenesis forms the basis for efforts to develop targeted therapies to reverse the fibrotic response and improve the outcomes of patients with chronic liver disease.

Mechanisms of hepatic fibrogenesis

Multiple etiologies of liver disease lead to liver fibrosis through integrated signaling networks that regulate the deposition of extracellular matrix. This cascade of responses drives the activation of hepatic stellate cells (HSCs) into a myofibroblast-like phenotype that is contractile, proliferative and fibrogenic. Collagen and other extracellular matrix (ECM) components are deposited as the liver generates a wound-healing response to encapsulate injury. Sustained fibrogenesis leads to cirrhosis, characterized by a distortion of the liver parenchyma and vascular architecture. Uncovering the intricate mechanisms that underlie liver fibrogenesis forms the basis for efforts to develop targeted therapies to reverse the fibrotic response and improve the outcomes of patients with chronic liver disease.

Molecular mechanism of hepatic stellate cell activation and antifibrotic therapeutic strategies

Activation of hepatic stellate cells (HSCs) is the dominant event in liver fibrosis. The early events in the organization of HSC activation have been termed initiation. Initiation encompasses rapid changes in gene expression and phenotype that render the cells responsive to cytokines and other local stimuli. Cellular responses following initiation are termed perpetuation, which encompasses those cellular events that amplify the activated phenotype through enhanced growth factor expression and responsiveness. Multiple cells and cytokines play a part in the regulation of HSC activation. HSC activation consists of discrete phenotype responses, mainly proliferation, contractility, fibrogenesis, matrix degradation, chemotaxis and retinoid loss. Currently, antifibrotic therapeutic strategies include inhibition of HSC proliferation or stimulation of HSC apoptosis, downregulation of collagen production or promotion of its degradation, administration of cytokines, and infusion of mesenchymal stem cells. In this review, we summarize the latest advances in our understanding of the mechanisms of HSC activation and possible antifibrotic therapeutic strategies.

Divergent transforming growth factor-beta signaling in hepatic stellate cells after liver injury: functional effects on ECE-1 regulation

In liver wound healing, transforming growth factor-beta (TGF-beta) plays a critical role in stellate cell activation as well as signaling cascades in the fibrogenic response to injury. We postulate that the TGF-beta-dependent downstream signaling pathway may vary according to the mechanism of stellate cell activation; this study was undertaken to ascertain whether the downstream signaling pathways mediated by TGF-beta vary in different liver injury models. We measured Smad3 and MAP kinase activation after isolating stellate cells from rat livers injured by either bile duct ligation (BDL) or repeated carbon tetrachloride (CCl(4)) administration. Phospho-Smad3 was dramatically up-regulated in stellate cells after CCl(4) injury, but not after BDL-induced injury. TGF-beta signaling in stellate cells activated after BDL was mediated prominently through ERK activation, whereas activation induced by CCl(4) injury or culture led to a cross-signaling mechanism involving both Smad3 and p38. The divergent Smad signaling pathways observed appeared to be attributable to the differential regulation of the early growth response gene-1 (Egr-1), an apparent negative transcriptional factor for Smad3 in our system. In addition, inhibition of ERK activation in stellate cells from BDL-injured liver led to a decrease in expression of endothelin-converting enzyme-1, a critical regulator of endothelin-1. We speculate that TGF-beta signaling proceeds through differential signaling pathways depending on the mechanism of liver injury that leads to stellate cell activation.

Pathophysiology and clinical practice analysis on endothelin system and portal hypertension

Portal hypertension (PHT) is a common clinical syndrome which leads to various severe, even lethal complications. The concentration of endothelin-1 (ET-1) in plasma is increased both in human body and PHT animal model. The effect of ET-1 depends on the kind of tissue and the expression of ET-1 receptor in this tissue. However, the expression of ET-1 receptor is not identical even in the same tissue at different PHT phases. This review aims to give an update on the endothelin syetem in PHT and elucidate a potential novel strategy.

The serum endothelin-1 level in steatosis and NASH, and its relation with severity of liver fibrosis

Endothelin-1 (ET-1) is known to play an important role in hepatic fibrosis. ET-1 is also a mediator that is elevated in conditions such as insulin resistance, hyperglycemia, oxidative stress, and endothelial cell dysfunction. In this study, we investigated whether ET-1 has a role in determining the severity of liver fibrosis in NASH. Also, the relation between ALT levels, obesity, diabetes, and AST/ALT ratio and fibrosis and ET-1 level was sought. A total of 92 patients were enrolled in the study. The patients were categorized into three groups: group 1, patients with elevated transaminase levels who were diagnosed as NASH by liver biopsy (n=40); group II, patients with only hepatosteatosis determined by biopsy but having elevated transaminase levels (n=12); and group III, patients with hepatosteatosis observed by ultrasonography, having normal transaminase levels (n=40). The serum ET-1 level was measured by an appropriate ELISA kit for all patients. Mean serum ET-1 level was statistically significantly higher in the NASH group compared to the other two groups (15.56+/-4.63 vs 6.75+/-2.46 and 5.74+/-2.34 micromol/L; P < 0.01). Mean serum ET-1 levels in NASH patients with grade I, grade II, and grade IV fibrosis were 14.06+/-0.92, 17.70+/-2.32, and 20.40+/-1.40 micromol/L, respectively. None of the patients were identified as grade III fibrosis. It was found that the serum ET-1 level showed a statistically significant increase as fibrosis severity increased in NASH patients (P < 0.05). In conclusion, the serum ET-1 level is higher in NASH patients compared to patients having only steatosis. There appears to be a correlation between severity of fibrosis and serum ET-1 level in NASH patients. It has been found that NASH patients having a twofold increase in their ALT levels had higher ET-1 levels and a more severe grade of fibrosis.

Lack of Endothelial Nitric Oxide Synthase Promotes Endothelin-Induced Hypertension: Lessons from Endothelin-1 Transgenic/Endothelial Nitric Oxide Synthase Knockout Mice

Endothelin-1 (ET-1) is one of the most potent biologic vasoconstrictors. Nevertheless, transgenic mice that overexpress ET-1 exhibit normal BP. It was hypothesized that vascular effects of ET-1 may be antagonized by an increase of the endothelial counterpart of ET-1, nitric oxide (NO), which is produced by the endothelial NO synthase (eNOS). Therefore, cross-bred animals of ET transgenic mice (ET+/+) and eNOS knockout (eNOS-/-) mice and were generated, and BP and endothelial function were evaluated in these animals. Endothelium-dependent and -independent vascular function was assessed as relaxation/contraction of isolated preconstricted aortic rings. The tissue ET and NO system was determined in aortic rings by quantitative real-time PCR and Western blotting. Systolic BP was similar in ET+/+ and wild-type (WT) mice but was significantly elevated in eNOS-/- mice (117 +/- 4 mmHg versus 94 +/- 6 mmHg in WT mice; P < 0.001) and even more elevated in ET+/+ eNOS-/- cross-bred mice (130 +/- 4 mmHg; P < 0.05 versus eNOS-/-). Maximum endothelium-dependent relaxation was enhanced in ET+/+ mice (103 +/- 6 versus 87 +/- 4% of preconstriction in WT littermates; P < 0.05) and was completely blunted in eNOS-/- (-3 +/- 4%) and ET+/+ eNOS-/- mice (-4 +/- 4%), respectively. Endothelium-independent relaxation was comparable among all groups. Quantitative real-time PCR as well as Western blotting revealed an upregulation of the aortic ET(A) and ET(B) receptors in ET+/+ eNOS-/-, whereas eNOS was absent in aortic rings of eNOS-/- and ET+/+ eNOS-/- mice. ET-1 aortic tissue concentrations were similar in WT mice and ET+/+ eNOS-/- mice most probably as a result of an enhanced clearance of ET-1 by the upregulated ET(B) receptor. These data show for the first time that in transgenic mice that overexpress human ET-1, additional knockout of eNOS results in a further enhancement of BP as compared with eNOS-/- mice. The human ET+/+ eNOS-/- mice therefore represent a novel model of hypertension as a result of an imbalance between the vascular ET-1 and NO systems.

New therapies in hepatitis C virus and chronic liver disease: antifibrotics

Fibrotic liver disease occurs after any of the various forms of injury to the liver. Fibrosis is a critical factor leading to hepatic dysfunction and portal hypertension and its complications. The fibrogenic cascade is complex but leads to accumulation of extracellular matrix proteins, followed by nodular fibrosis, tissue contraction, and alteration in blood flow. A critical concept emerging is that activation of effector cells, which produce extracellular matrix, underlies the fibrogenic process. The aggregate data has not only helped lead to an understanding of the pathophysiologic basis of hepatic fibrogenesis, but it has also provided an important context with which to base novel antifibrotic therapy.

Endothelin and endothelin receptor antagonism in portopulmonary hypertension

Portopulmonary hypertension (PPHT) is a rare but devastating complication in patients with portal hypertension, characterized by pulmonary arterial obliterative disease with a concomitant rise in pulmonary vascular resistance. A broad body of evidence has accumulated, indicating that endothelin (ET) peptides and their cognate receptors are causally involved in the pathophysiology of pulmonary arterial hypertension (PAH) owing to different aetiologies, including PPHT. In addition, the ET system may be involved in hepatic fibrotic remodelling and portal hypertension. Several experimental models have provided evidence that ET receptor antagonism may have therapeutic potential in PPHT. Initial experience has accumulated during the last 2 years, suggesting that targeting the ET system may have beneficial effects in the clinical setting. In these studies, the orally active, dual ET receptor antagonist bosentan improved pulmonary haemodynamics and functional capacity. These effects were sustained and occurred in the absence of adverse events. If these observations can be corroborated by controlled clinical trials, bosentan would offer several advantages over available therapies, which have major drawbacks owing to their invasive and demanding mode of application.

Hepatic fibrosis, stellate cells, and portal hypertension

Hepatic fibrogenesis is the common result of injury to the liver. It is believed to be a critical factor that leads to hepatic dysfunction and may be important in portal hypertension. The fibrogenic response is a complex process in which accumulation of extracellular matrix proteins, tissue contraction, and alteration in blood flow are prominent. A critical event in fibrogenesis is activation of resident perisinusoidal cells that are termed "hepatic stellate cells". Stellate cell activation is characterized by many important phenotypes, including enhanced extracellular matrix synthesis and prominent contractility. Given the central role of stellate cell activation in hepatic fibrogenesis (and portal hypertension), effective therapy for hepatic fibrogenesis is most likely will be directed at this event.

Increased immunoreactivities against endothelin-converting enzyme-1 and monocyte chemotactic protein-1 in hepatic stellate cells of rat fibrous liver induced by thioacetamide

The progression of rat liver fibrosis induced by intraperitoneal administration of thioacetamide (TAA) was evaluated by immunocytochemistry using anti-alpha-smooth muscle actin (alpha-SMA), antiendothelin-converting enzyme (ECE)-1, and anti-monocyte chemotactic protein (MCP)-1 antibodies. The fibrous septal spaces gradually increased after administration of TAA, and pseudolobules were established in the 7-week TAA-treated groups. Immunoreactivities against alpha-SMA were not detected in hepatic stellate cells (HSCs) of the control group without TAA treatment, although they were observed in the HSCs around the fibrous septal spaces in all TAA-treated groups, indicating that activation of HSCs occurs during the establishment of pseudolobules. Immunoreactivities against ECE-1 and MCP-1 were seen in such HSCs of the TAA-treated groups, but few or no immunoreactivities were detected in the HSCs of the control group. The most significant increase in the ECE-1 immunoreactivities was detected in the 1-week TAA-treated group, whereas that in MCP-1 was observed in the 7-week TAA-treated group. The present immunocytochemistry indicated a difference in the accelerated expression period between immunoreactivities against ECE-1 and MCP-1 in the HSCs during the progression of TAA-induced liver fibrosis, suggesting that ECE-1 is involved in the early phase of liver fibrosis and that MCP-1 plays a role during the later phase.

Endothelin-1-dependent leptin induction in gastric mucosal inflammatory responses to Helicobacter pylori lipopolysaccharide

Leptin, a multifunctional hormone that regulates food intake and energy expenditure, has emerged recently as an important modulator of gastric mucosal responses to Helicobacter pylori infection. We applied the animal model of H. pylori LPS-induced gastritis to investigate the role of endothelin-1 (ET-1) in the mucosal leptin production. We show that the histologic pattern of inflammation reached a maximum on the fourth day following the LPS and was reflected in a marked increase in the mucosal level of ET-1 and leptin. Therapeutic administration of phosphoramidon, an inhibitor of ECE-1 activity, led to a 61.2% decline in the mucosal ET-1 level and a 64.1% reduction in leptin, while the severity of mucosal inflammatory involvement increased by 28.6%. A drop in the level of leptin and the increase in severity of the inflammatory involvement elicited by the LPS was also attained in the presence of ET(A) receptor antagonist BQ610, but not the ET(B) receptor antagonist BQ788. Moreover, administration of ERK inhibitor, PD98059, in the presence of ET(B) receptor antagonist, but not the ET(A) receptor antagonist, caused reduction in the mucosal leptin level. Our findings are the first to implicate ET-1 as a key factor in up-regulation of gastric mucosal leptin-associated H. pylori infection. We also show that the effect of ET-1 on leptin production is a consequence of ET(A) receptor activation.

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

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

Endothelin-1-dependent up-regulation of leptin production in gastric mucosal injury by indomethacin

Leptin, a multifunctional hormone that regulates food intake and metabolic and endocrine responses, has emerged recently as an important modulatory factor in gastric mucosal resistance to injury. In this study, we applied the animal model of gastric mucosal injury caused by indomethacin to investigate the role of endothelin-1 (ET-1) in the mucosal leptin production. Using groups of rats subjected to intragastric administration of indomethacin (at 0-60 mg/kg), we show that gastric mucosal damage reached a maximum 4 h following the drug, and was accompanied by a marked elevation (up to 3.5-fold) in the mucosal leptin level, up to 4-fold enhancement in the expression of endothelin-converting enzyme-1 (ECE-1) activity and up to 4.5-fold increase in ET-1 generation. Pretreatment with phosphoramidon, an inhibitor of ECE-1 activity, not only led to a decline in ECE-1 and ET-1 generation, but also produced a dose-dependent reduction in the mucosal level of leptin and the extent of mucosal damage caused by indomethacin. This effect of phosphoramidon, however, was subject to suppression by the exogenous ET-1 administration. Moreover, a marked drop in the mucosal level of leptin and the reduction in the severity of mucosal damage was attained following pretreatment with ET(A) receptor antagonist BQ610, but not by ET(B) receptor antagonist BQ788. The results implicate ET-1 as a key factor in the regulation of leptin production associated with gastric mucosal response to injury, and show that the stimulatory effect of ET-1 on leptin production occurs via ET(A) receptor activation.

Up-regulation in endothelin-1 by Helicobacter pylori lipopolysaccharide interferes with gastric mucin synthesis via epidermal growth factor receptor transactivation

OBJECTIVE

Endothelin-1 (ET-1), a key mediator of inflammatory processes associated with bacterial infection, is a 21-amino acid peptide produced from a biologically inactive big ET-1 by the action of endothelin-converting enzyme-1 (ECE-1) that acts through G protein-coupled ET(A) and ET(B) receptors. Here we report on the role of ET-1 in the mediation of the detrimental influence of Helicobacter pylori on the synthesis of gastric mucin.

MATERIAL AND METHODS

Rat gastric mucosal cells were exposed to H. pylori key virulence factor, lipopolysaccharide (LPS).

RESULTS

The LPS inhibitory effect on gastric mucin synthesis was accompanied by a marked increase in ET-1 generation and enhancement in ECE-1 activity. Inhibition of ECE-1 with phosphoramidon not only led to the impedance of LPS-induced ET-1 generation, but also countered the detrimental effect of LPS on mucin synthesis. Moreover, the LPS inhibitory effect on mucin synthesis was blocked by ET(A) receptor antagonist BQ610, but not by ET(B) receptor antagonist BQ788. Furthermore, the LPS-induced suppression in gastric mucin synthesis was countered in a concentration-dependent fashion by PD153035 (81.7%), a specific inhibitor of epidermal growth factor receptor (EGFR) kinase as well as PP2 (69.8%), a selective inhibitor of tyrosine kinase Src responsible for ligand-independent EGFR transactivation.

CONCLUSIONS

Our findings are the first to show that the detrimental effect of H. pylori on gastric mucin synthesis is intimately linked to the events associated with ECE-1 up-regulation, enhancement in ET-1 production, and G protein-coupled ET(A) receptor activation that triggers the EGFR transactivation.

Role of Leptin in Modulation of Porphyromonas gingivalis Lipopolysaccharide-induced Up-regulation of Endothelin-1 in Salivary Gland Acinar Cells

Leptin, a pleiotropic cytokine that regulates food intake and metabolic and endocrine responses, has emerged recently as an important regulator of mucosal inflammatory responses to bacterial infection. In this study, we report that in sublingual salivary gland acinar cells leptin plays a role in the suppression of up-regulation in endothelin-1 (ET-1), induced by the LPS of a periodontopathic bacterium P. gingivalis. We show that P. gingivalisLPS detrimental effect on salivary mucin synthesis, associated with up-regulation (3.9-fold) in ET-1 generation and the enhancement (3.2-fold) in endothelin-converting enzyme-1 (ECE-1) activity, was subject to a dose-dependent suppression by leptin. The impedance by leptin of the LPS inhibitory effect on mucin synthesis was blocked by wortmannin, an inhibitor of PI3K, as well as by ERK inhibitor, PD98059. However, while the blockade of ERK led also to amplification in the impedance by leptin of the LPS-induced expression of ECE-1 and ET-1, the effect was not observed in the presence of wortmannin. The findings are the first to demonstrate that leptin counters the pathological consequences of P. gingivalisinfection on the synthesis of salivary mucin through the involvement in signaling events of PI3K and ERK pathways. We also show that the ERK cascade represents a critical signaling target for leptin in the LPS-induced up-regulation in ET-1.

Molecular mechanisms of alcohol-induced hepatic fibrosis

Alcohol abuse is a major cause of liver fibrosis and cirrhosis in developed countries. Before alcoholic liver fibrosis becomes evident, the liver undergoes several stages of alcoholic liver disease including steatosis and steatohepatitis. Although the main mechanisms of fibrogenesis are independent of the etiology of liver injury, alcoholic liver fibrosis is distinctively characterized by a pronounced inflammatory response due to elevated gut-derived endotoxin plasma levels, an augmented generation of oxidative stress with pericentral hepatic hypoxia and the formation of cell-toxic and profibrogenic ethanol metabolites (e.g. acetaldehyde or lipid oxidation products). These factors, based on a complex network of cytokine actions, together result in increased hepatocellular damage and activation of hepatic stellate cells, the key cell type of liver fibrogenesis. Although to date removal of the causative agent, i.e. alcohol, still represents the most effective intervention to prevent the manifestation of alcoholic liver disease, sophisticated molecular approaches are underway, aiming to specifically blunt profibrogenic signaling pathways in liver cells or specifically induce cell death in activated hepatic stellate cells to decrease the scarring of the liver.

Expression of the hepatic endothelin system in human cirrhotic livers

It is considered that endothelin-1 participates in the development of liver cirrhosis and it has been recognized that every component of the endothelin system is upregulated in cirrhotic livers. However, the expression pattern of this system, including interaction between its components, is not fully understood in human livers. In this study, the expression pattern of the endothelin system was examined. Immunohistochemical analysis for endothelin-1, endothelin receptors and endothelin-converting enzyme was performed in 16 cirrhotic and 17 normal human liver tissues. Peptides, proteins, and RNAs extracted from the livers were also investigated using quantitative assays for the components of the hepatic endothelin system. Hepatic endothelin-1 levels were significantly higher in cirrhotic livers (0.084 +/- 0.052 pg/mg wet liver) than in normal livers (0.041 +/- 0.032 pg/mg; p < 0.01), and were closely related to the severity of liver fibrosis and portal hypertension. Immunoreactivity for endothelin-1, endothelin receptors, and endothelin-converting enzyme was detected mainly in fibrous areas and in the hepatic vasculature, and was enhanced in cirrhosis. Although there was a negative correlation between the expression of receptor mRNA and the hepatic endothelin-1 level, the amounts of the mRNAs were greater in cirrhotic livers than in normal livers. However, expression of endothelin-converting enzyme in cirrhotic livers was increased at the protein level but was relatively reduced at the mRNA level. These findings suggest that the hepatic endothelin system is activated in human cirrhotic livers in association with worsening of the disease, but that the regulation of the components of this system in this disorder is complex.

Regulation of ERK/JNK/p70S6K in two rat models of liver injury and fibrosis

BACKGROUND/AIMS

The regulation of three major intracellular signalling protein kinases was investigated in two models of liver injury leading to hepatic fibrosis, dimethylnitrosamine administration (DMN) and bile duct ligation (BDL).

METHODS

Extracellular signal-regulated kinases (ERK)1/2, c-Jun terminal kinase (JNK) and p70S6-kinase (p70(S6K)) were studied in vivo in the whole liver, in liver sections and in isolated hepatocytes, cholangiocytes and hepatic stellate cells (HSC).

RESULTS

In the whole liver, activation of these kinases occurred with a different kinetic pattern in both models of liver injury. By immunohistochemistry and Western blot in isolated cells, phosphorylated kinases were detected in proliferating cells (i.e. hepatocytes and cholangiocytes after DMN and BDL, respectively), in addition to stellate-like elements. ERK1/2, JNK and p70(S6K) activation was associated with hepatocytes proliferation after DMN, while JNK activation was not associated with cholangiocytes proliferation after BDL. In HSC isolated from injured livers, protein kinases were differentially activated after BDL and DMN. Kinases activation in HSC in vivo preceded cell proliferation and alpha-smooth muscle actin appearance, a marker of HSC transformation in myofibroblast-like cells, and collagen deposition.

CONCLUSIONS

Our findings indicate that these kinases are coordinately regulated during liver regeneration and suggest that their modulation could be considered as a future therapeutic approach in the management of liver damage.

Production and effects of endothelin-1 in rat pancreatic stellate cells

INTRODUCTION

Proliferation and matrix synthesis of activated pancreatic stellate cells (PSCs) participate in the development of chronic pancreatitis. Besides other substances, endothelin-1 (ET-1) may influence the activation process of PSCs. Until now, ET-1 has not been studied in this particular cell type.

AIMS

To characterize PSCs in rat pancreas with respect to expression of ET(A)-receptors, production of ET-1, and physiological effects induced by ET-1 during PSC activation.

METHODOLOGY

Immunocytochemical and ELISA techniques and cDNA microarray analysis were used. Physiologic effects were characterized by single cell measurements of free cytosolic Ca2+-concentration and of PSC contractility on collagen lattices.

RESULTS

Activation of PSCs in vitro, as assessed by alpha-smooth muscle actin expression, was accompanied by the de novo expression of ET(A)-receptors and synthesis of ET-1 mRNA and protein. Cytosolic Ca2+-concentration was increased upon ET-1 stimulation in activated but not in quiescent PSCs. Contractility of activated PSCs was significantly reduced by the selective ET(A)-receptor antagonist BQ123 but not by the ET(B)-receptor antagonist IRL-1038.

CONCLUSIONS

The results suggest that ET-1 may act as a paracrine and autocrine factor for activated PSCs and may mediate contractions of activated, but not quiescent, PSCs.

Cell and molecular regulation of endothelin-1 production during hepatic wound healing

During hepatic wound healing, activation of key effectors of the wounding response known as stellate cells leads to a multitude of pathological processes, including increased production of endothelin-1 (ET-1). This latter process has been linked to enhanced expression of endothelin-converting enzyme-1 (ECE-1, the enzyme that converts precursor ET-1 to the mature peptide) in activated stellate cells. Herein, we demonstrate up-regulation of 56- and 62-kDa ECE-1 3'-untranslated region (UTR) mRNA binding proteins in stellate cells after liver injury and stellate cell activation. Binding of these proteins was localized to a CC-rich region in the proximal ECE-1 3' UTR base pairs (the 56-kDa protein) and to a region between 60 and 193 base pairs in the ECE-1 3' UTR mRNA (62 kDa). A functional role for the 3' UTR mRNA/protein interaction was established in a series of reporter assays. Additionally, transforming growth factor-beta1, a cytokine integral to wound healing, stimulated ET-1 production. This effect was due to ECE-1 mRNA stabilization and increased ECE-1 expression in stellate cells, which in turn was a result of de novo synthesis of the identified 56- and 62-kDa ECE-1 3' UTR mRNA binding proteins. These data indicate that liver injury and the hepatic wound healing response lead to ECE-1 mRNA stabilization in stellate cells via binding of 56- and 62-kDa proteins, which in turn are regulated by transforming growth factor-beta. The possibility that the same or similar regulatory events are present in other forms of wound healing is raised.

Molecular genetics of microvascular disease in diabetic retinopathy

Diabetic retinopathy is a sight-threatening complication of the retinal microvasculature. While important environmental factors have been clearly identified as influencing its development, increasing evidence suggests that diabetic retinopathy has a genetic component. A variety of studies have explored associations between candidate genes and frequency and severity of retinopathy. Overall, this review has found that the majority of candidate genes studied exhibit weak or no association with retinopathy status, and where associations have been detected these results have not been replicated in multiple populations. This may reflect inaccurate case definition, small subject numbers and possibly inadequate markers for genetic studies.

Heterodimerization of endothelin-converting enzyme-1 isoforms regulates the subcellular distribution of this metalloprotease

Endothelin-converting enzyme (ECE) is a membrane metalloprotease that generates endothelin from its direct precursor big endothelin. Four isoforms of ECE-1 are produced from a single gene through the use of alternate promoters. These isoforms share the same extracellular catalytic domain and contain unique cytosolic tails, which results in their specific subcellular targeting. We investigated the distribution of ECE-1 isoforms in transfected AtT-20 neuroendocrine cells. Whereas ECE-1a and 1c were present at the plasma membrane, ECE-1b and ECE-1d were retained inside the cells. We found that both intracellular isoforms were concentrated in the endosomal system: ECE-1d in recycling endosomes, and ECE-1b in late endosomes/multivesicular bodies. Leucine-based motifs were involved in the intracellular retention of these isoforms, and the targeting of ECE-1b to the degradation pathway required an additional signal in the N terminus. The concentration of ECE-1 isoforms in the endosomal system suggested new functions for these enzymes. Potential novel functions include redistribution of other isoforms through direct interaction. We have showed that ECE-1 isoforms could heterodimerize, and that in such heterodimers the ECE-1b targeting signal was dominant. Interaction of a plasma membrane isoform with ECE-1b resulted in its intracellular localization and decreased its extracellular activity. These data demonstrated that the targeting signals specific for ECE-1b constitute a regulatory domain per se that could modulate the localization and the activity of other isoforms.

Effects of endothelins on hepatic stellate cell synthesis of endothelin-1 during hepatic wound healing

Endothelin-1 production is increased after liver injury and the subsequent wounding response. Further, endothelin-1 has prominent effects on hepatic stellate cells (key effectors of the hepatic wounding response), including on collagen synthesis, proliferation, and expression of smooth muscle proteins. We tested the hypothesis that endothelins (ETs) may regulate endothelin-1 production during hepatic wounding, and have investigated potential mechanisms underlying this process. Studies were performed on isolated stellate cells from normal and injured livers; in addition, potential autocrine effects of ET were assessed in vivo using an ET receptor antagonist in a model of liver injury. In stellate cells isolated from either normal or injured rat livers, ET receptor stimulation with endothelin-3 or sarafotoxin S6C (preferential ET(B) agonists) caused a dose-dependent increase in endothelin-1 production. Additionally, administration of a mixed ET antagonist in vivo during injury led to reduced stellate cell production of endothelin-1. The mechanism by which ETs stimulated endothelin-1 in this system appeared to be through upregulation of ET converting enzyme-1 (which converts precursor ET to mature peptide), rather than by modulation of precursor endothelin-1. We conclude that during liver injury and wound healing, stellate cell endothelin-1 production is, at least partially, stimulated by ETs via autocrine mechanisms that occur at the level of ET converting enzyme-1.

The mechanism of hyperpigmentation in seborrhoeic keratosis involves the high expression of endothelin-converting enzyme-1alpha and TNF-alpha, which stimulate secretion of endothelin 1

BACKGROUND

Seborrhoeic keratosis (SK) is a benign epidermal tumour with increased pigmentation. We have recently demonstrated that increased secretion of endothelin (ET)-1, a strong keratinocyte-derived mitogen and melanogen for human melanocytes, is intrinsically involved in the hyperpigmentation mechanism of SK.

OBJECTIVES

To examine whether the increased ET secretion results from cytokines that induce ET production and/or from differences in the processing of ET that lead to its final active, secreted form.

METHODS

We used immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to determine whether ET-inducing enzymes and/or cytokines are also highly expressed in SK.

RESULTS

RT-PCR of mRNAs encoding interleukin (IL)-1alpha, tumour necrosis factor (TNF)-alpha and endothelin-converting enzyme (ECE)-1alpha demonstrated that there is an increased expression of TNF-alpha and ECE-1alpha mRNAs in SK, whereas the IL-1alpha transcript is rather downregulated in SK compared with that in perilesional normal epidermis. In parallel, immunohistochemical analysis of SK revealed marked immunostaining for TNF-alpha in basaloid cells at lower levels of the epidermis and in basal cells, and for ECE-1alpha in most basaloid and basal cells in comparison with their weak staining throughout the epidermis in perilesional normal controls. In contrast, immunostaining for IL-1alpha was almost negative in SK relative to distinctive staining throughout the epidermis in the perilesional normal controls.

CONCLUSIONS

These findings suggest that the increased secretion of ET-1 leading to enhanced pigmentation in SK results from the co-ordinated increased expression of TNF-alpha and ECE-1alpha.

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.

Characterization of a stellate cell activation-associated protein (STAP) with peroxidase activity found in rat hepatic stellate cells

A proteome approach for the molecular analysis of the activation of rat stellate cell, a liver-specific pericyte, led to the discovery of a novel protein named STAP (stellate cell activation-associated protein). We cloned STAP cDNA. STAP is a cytoplasmic protein with molecular weight of 21,496 and shows about 40% amino acid sequence homology with myoglobin. STAP was dramatically induced in in vivo activated stellate cells isolated from fibrotic liver and in stellate cells undergoing in vitro activation during primary culture. This induction was seen together with that of other activation-associated molecules, such as smooth muscle alpha-actin, PDGF receptor-beta, and neural cell adhesion molecule. The expression of STAP protein and mRNA was augmented time dependently in thioacetamide-induced fibrotic liver. Immunoelectron microscopy and proteome analysis detected STAP in stellate cells but not in other hepatic constituent cells. Biochemical characterization of recombinant rat STAP revealed that STAP is a heme protein exhibiting peroxidase activity toward hydrogen peroxide and linoleic acid hydroperoxide. These results indicate that STAP is a novel endogenous peroxidase catabolizing hydrogen peroxide and lipid hydroperoxides, both of which have been reported to trigger stellate cell activation and consequently promote progression of liver fibrosis. STAP could thus play a role as an antifibrotic scavenger of peroxides in the liver.

Fibroblast matrix gene expression and connective tissue remodeling: role of endothelin-1

This study examines endothelin-induced modulation of extracellular matrix synthesis and remodeling by fibroblasts, and its potential role in the pathogenesis of systemic sclerosis (scleroderma). Endothelin-1 promoted fibroblast synthesis of collagen types I and III, but not fibronectin, by a mechanism dependent upon both ETA and ETB receptors. Conversely, endothelin-1 inhibited both protein expression of matrix metalloproteinase 1 and zymographic activity exclusively via ETA receptors. A dual regulatory role for endothelin-1 in transcriptional regulation was suggested by the ability of endothelin-1 to enhance steady-state levels of collagen mRNA and activate the proalpha2(I) collagen (Col1a2) promoter, but in contrast to reduce matrix metalloproteinase 1 transcript expression and suppress transcription of a human matrix metalloproteinase 1 promoter reporter construct in transient transfection assays. Although endothelin-1 significantly enhanced remodeling of three-dimensional collagen lattices populated by normal fibroblasts, this was not observed for lattices populated by systemic sclerosis fibroblasts. Promotion of matrix remodeling was dependent upon ETA receptor expression and was blocked by specific inhibitors of tyrosine kinases or protein kinase C. Reverse transcriptase polymerase chain reaction, S1 nuclease, and functional cell surface binding studies showed that normal and systemic sclerosis fibroblasts express both ETA and ETB receptors (predominantly ETA), but that ETA receptor mRNA levels and ETA binding sites on fibroblasts cultured from systemic sclerosis skin biopsies are reduced by almost 50%. Endothelin-1 is thus able to induce a fibrogenic phenotype in normal fibroblasts that is similar to that of lesional systemic sclerosis fibroblasts. Moreover, reduced responsiveness to exogenous endothelin-1 in systemic sclerosis suggests that downstream pathways may have already been activated in vivo. These data further implicate dysregulated endothelin-receptor pathways in fibroblasts in the pathogenesis of connective tissue fibrosis.

Effect of aldosterone on collagen steady state levels in primary and subcultured rat hepatic stellate cells

BACKGROUND/AIMS

Activation of the renin-angiotensin-aldosterone system can lead to collagen accumulation and reactive myocardial fibrosis. This study aims at evaluating the effect of aldosterone on extracellular matrix synthesis by rat hepatic stellate cells.

METHODS

Cultured cells were treated with different concentrations of aldosterone (10(-6)-10(-10) M) and metabolically labeled with 35S-methionine/35S-cysteine. Procollagen types I, III and IV, laminin and fibronectin were specifically immunoprecipitated and quantified by phosphor imaging. Using the reverse transcription-polymerase chain reaction, we investigated the expression of the mineralocorticoid receptor in hepatic stellate cells.

RESULTS

Quantitation showed that 10(-6) M aldosterone induced procollagen type I synthesis significantly, whereas procollagen type IV expression was significantly affected by 10(-9) and 10(-10) M aldosterone, both in primary hepatic stellate cells. RT-PCR experiments clearly demonstrated a lack of expression of the mineralocorticoid receptor in hepatic stellate cells.

CONCLUSION

We demonstrated that aldosterone altered moderately procollagen type I and IV synthesis by primary hepatic stellate cells, but not by activated stellate cells which are the principal cellular sources of extracellular matrix proteins in chronic liver disease. Moreover, hepatic stellate cells do not express the mineralocorticoid receptor, suggesting that the observed modest changes of extracellular matrix synthesis are probably due to mineralocorticoid receptor unrelated mechanisms.

Overexpression of endothelin-1 in bile duct ligated rats: correlation with activation of hepatic stellate cells and portal pressure

BACKGROUND/AIMS

Hepatic stellate cells (HSC) are involved in the pathogenesis of liver fibrosis; although ET-1 is increased in cirrhosis, its pathophysiological role in fibrogenesis and portal hypertension remains controversial. The aim of this study was to investigate splanchnic hemodynamics and to correlate them with changes in ET-1 expression and HSC activation in bile duct ligated (BDL) rats.

METHODS/RESULTS

Expression of the ET-1 gene was increased early as measured by quantitative reverse transcriptase-polymerase chain reaction (6-fold 3 days after BDL) whereas ET-1 peptide measured by RIA increased significantly only in the late phase (30-fold at 28 days). There was a linear correlation between portal pressure and the amount of ET-1 in the portal vein (r = 0.66; P = 0.003), as well as between ET-1 and the volume fraction of myofibroblasts (r = 0.80, P < 10(-7)) as assessed by morphometry and immunohistochemical staining using alpha-smooth muscle actin.

CONCLUSIONS

During chronic liver injury activation of HSCs and of preproET-1 mRNA is accentuated in the early phase after BDL. The late increase in ET-1 peptide may indicate that this peptide is only secondarily involved in HSC activation. The correlation between ET-1 in portal vein and portal pressure suggests that ET-1 may play an important role in the development of portal hypertension.

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

BACKGROUND/AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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