Inhibition of myofibroblastic transformation of cultured rat hepatic stellate cells by methylxanthines and dibutyryl cAMP

Caffeine-induced inversion of prostaglandin E2 effects on hepatic stellate cell activation

BACKGROUND AND AIMS

Liver inflammation leads to the activation of hepatic stellate cells (HSCs), resulting in the development of liver fibrosis. The present study aimed to investigate the effects of prostaglandin E₂ (PGE₂), which is biosynthesized by Kupffer cells, hepatocytes, and HSCs during inflammation, on HSC activation, including its combinatory effect with caffeine.

METHODS

HSCs isolated from mice were activated by culturing in a medium supplemented with 10% fetal bovine serum for 7 days on plastic plates. The activation of HSCs was evaluated by immunofluorescence of α-smooth muscle actin in HSCs. Comprehensive gene expression analysis was performed using mRNA-sequencing to compare HSCs cultured for 1 or 7 days, with or without PGE₂, caffeine, or both.

RESULTS

PGE₂ (1 μM) facilitated the activation of HSCs but inhibited the HSC activation in the presence of caffeine (3 mM). Comprehensive gene expression analysis revealed that HSCs treated with PGE₂ in the presence of caffeine were classified in the same class as HSCs cultured for 1 day, i.e., quiescent HSCs. In contrast, PGE₂ did not exhibit an inhibitory effect on HSC activation when co-treated with any isoform-specific phosphodiesterase inhibitors. Although the adenylate cyclase inhibitor 2',5'-dideoxyadenosine suppressed the elevation of intracellular cAMP level induced by PGE₂ in the presence of caffeine, it had no effect on the inhibition of HSC activation by PGE₂ plus caffeine.

CONCLUSION

The effect of PGE₂ on HSC activation is changed from facilitatory to inhibitory when combined with caffeine, suggesting that caffeine may effectively suppress liver fibrosis during 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.

Regulating cellular cyclic adenosine monophosphate: "Sources," "sinks," and now, "tunable valves"

A number of hormones and growth factors stimulate target cells via the second messenger pathways, which in turn regulate cellular phenotypes. Cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger that facilitates numerous signal transduction pathways; its production in cells is tightly balanced by ligand-stimulated receptors that activate adenylate cyclases (ACs), that is, "source" and by phosphodiesterases (PDEs) that hydrolyze it, that is, "sinks." Because it regulates various cellular functions, including cell growth and differentiation, gene transcription and protein expression, the cAMP signaling pathway has been exploited for the treatment of numerous human diseases. Reduction in cAMP is achieved by blocking "sources"; however, elevation in cAMP is achieved by either stimulating "source" or blocking "sinks." Here we discuss an alternative paradigm for the regulation of cellular cAMP via GIV/Girdin, the prototypical member of a family of modulators of trimeric GTPases, Guanine nucleotide Exchange Modulators (GEMs). Cells upregulate or downregulate cellular levels of GIV-GEM, which modulates cellular cAMP via spatiotemporal mechanisms distinct from the two most often targeted classes of cAMP modulators, "sources" and "sinks." A network-based compartmental model for the paradigm of GEM-facilitated cAMP signaling has recently revealed that GEMs such as GIV serve much like a "tunable valve" that cells may employ to finetune cellular levels of cAMP. Because dysregulated signaling via GIV and other GEMs has been implicated in multiple disease states, GEMs constitute a hitherto untapped class of targets that could be exploited for modulating aberrant cAMP signaling in disease states. This article is categorized under: Models of Systems Properties and Processes > Mechanistic Models Biological Mechanisms > Cell Signaling.

Differentiation-inducing factor-1 prevents hepatic stellate cell activation through inhibiting GSK3β inactivation

Differentiation-inducing factor-1 (DIF-1), a morphogen produced by the cellular slime mold Dictyostelium discoideum, is a natural product that has attracted considerable attention for its antitumor properties. Here, we report a novel inhibitory effect of DIF-1 on the activation of hepatic stellate cells (HSCs) responsible for liver fibrosis. DIF-1 drastically inhibited transdifferentiation of quiescent HSCs into myofibroblastic activated HSCs in a concentration-dependent manner, thus conferring an antifibrotic effect against in the liver. Neither SQ22536, an adenylate cyclase inhibitor, nor ODQ, a guanylate cyclase inhibitor, showed any effect on the inhibition of HSC activation by DIF-1. In contrast, TWS119, a glycogen synthase kinase 3β (GSK3β) inhibitor, attenuated the inhibitory effect of DIF-1. Moreover, the level of inactive GSK3β (phosphorylated at Ser9) was significantly reduced by DIF-1. DIF-1 also inhibited nuclear translocation of β-catenin and reduced the level of non-phospho (active) β-catenin. These results suggest that DIF-1 inhibits HSC activation by disrupting the Wnt/β-catenin signaling pathway through dephosphorylation of GSK3β. We propose that DIF-1 is a possible candidate as a therapeutic agent for preventing liver fibrosis.

Role of cAMP and phosphodiesterase signaling in liver health and disease

Liver disease is a significant health problem worldwide with mortality reaching around 2 million deaths a year. Non-alcoholic fatty liver disease (NAFLD) and alcoholic liver disease (ALD) are the major causes of chronic liver disease. Pathologically, NAFLD and ALD share similar patterns of hepatic disorders ranging from simple steatosis to steatohepatitis, fibrosis and cirrhosis. It is becoming increasingly important to identify new pharmacological targets, given that there is no FDA-approved therapy yet for either NAFLD or ALD. Since the evolution of liver diseases is a multifactorial process, several mechanisms involving parenchymal and non-parenchymal hepatic cells contribute to the initiation and progression of liver pathologies. Moreover, certain protective molecular pathways become repressed during liver injury including signaling pathways such as the cyclic adenosine monophosphate (cAMP) pathway. cAMP, a key second messenger molecule, regulates various cellular functions including lipid metabolism, inflammation, cell differentiation and injury by affecting gene/protein expression and function. This review addresses the current understanding of the role of cAMP metabolism and consequent cAMP signaling pathway(s) in the context of liver health and disease. The cAMP pathway is extremely sophisticated and complex with specific cellular functions dictated by numerous factors such abundance, localization and degradation by phosphodiesterases (PDEs). Furthermore, because of the distinct yet divergent roles of both of its effector molecules, the cAMP pathway is extensively targeted in liver injury to modify its role from physiological to therapeutic, depending on the hepatic condition. This review also examines the behavior of the cAMP-dependent pathway in NAFLD, ALD and in other liver diseases and focuses on PDE inhibition as an excellent therapeutic target in these conditions.

Caffeine Suppresses the Activation of Hepatic Stellate Cells cAMP-Independently by Antagonizing Adenosine Receptors

During liver injury, hepatic stellate cells (HSCs) are activated by various cytokines and transdifferentiated into myofibroblast-like activated HSCs, which produce collagen, a major source of liver fibrosis. Therefore, the suppression of HSC activation is regarded as a therapeutic target for liver fibrosis. Several epidemiological reports have revealed that caffeine intake decreases the risk of liver disease. In this study, therefore, we investigated the effect of caffeine on the activation of primary HSCs isolated from mice. Caffeine suppressed the activation of HSC in a concentration-dependent manner. BAPTA-AM, an intracellular Ca²⁺ chelator, had no effect on the caffeine-induced suppression of HSC activation. None of the isoform-selective inhibitors of phosphodiesterase1 to 5 affected changes in the morphology of HSC during activation, whereas CGS-15943, an adenosine receptor antagonist, inhibited them. Caffeine had no effect on intracellular cAMP level or on the phosphorylation of extracellular signal-regulated kinase (ERK)1/2. In contrast, caffeine significantly decreased the phosphorylation of Akt1. These results suggest that caffeine inhibits HSC activation by antagonizing adenosine receptors, leading to Akt1 signaling activation.

Cilostazol attenuates hepatic stellate cell activation and protects mice against carbon tetrachloride-induced liver fibrosis

AIM

Liver fibrosis is a common pathway leading to cirrhosis. Cilostazol, a clinically available oral phosphodiesterase-3 inhibitor, has been shown to have antifibrotic potential in experimental non-alcoholic fatty liver disease. However, the detailed mechanisms of the antifibrotic effect and its efficacy in a different experimental model remain elusive.

METHODS

Male C57BL/6J mice were assigned to five groups: mice fed a normal diet (groups 1 and 2); 0.1% or 0.3% cilostazol-containing diet (groups 3 and 4, respectively); and 0.125% clopidogrel-containing diet (group 5). Two weeks after feeding, groups 2-5 were intraperitoneally administered carbon tetrachloride (CCl4 ) twice a week for 6 weeks, while group 1 was treated with the vehicle alone. To investigate the effects of cilostazol on hepatic cells, in vitro studies were conducted using primary hepatic stellate cells (HSC), Kupffer cells and hepatocytes with cilostazol supplementation.

RESULTS

Sirius red staining revealed that groups 3 and 4 exhibited a lesser fibrotic area (2.49 ± 0.43% and 2.31 ± 0.30%, respectively) than group 2 (3.17 ± 0.67%, P < 0.05 and P < 0.001, respectively). In vitro studies showed cilostazol dose-dependently suppressed HSC activation (assessed by morphological change, cell proliferation, and the expression of HSC activation markers), suggesting the therapeutic effect of cilostazol is mediated by its direct action on HSC.

CONCLUSION

Cilostazol could alleviate CCl4 -induced hepatic fibrogenesis in vivo, presumably due, at least partly, to its direct effect to suppress HSC activation. Given its clinical availability and safety, it may be a novel therapeutic intervention for chronic liver diseases.

Relaxin modulates human and rat hepatic myofibroblast function and ameliorates portal hypertension in vivo

Active myofibroblast (MF) contraction contributes significantly to the increased intrahepatic vascular resistance that is the primary cause of portal hypertension (PHT) in cirrhosis. We sought proof of concept for direct therapeutic targeting of the dynamic component of PHT and markers of MF activation using short-term administration of the peptide hormone relaxin (RLN). We defined the portal hypotensive effect in rat models of sinusoidal PHT and the expression, activity, and function of the RLN-receptor signaling axis in human liver MFs. The effects of RLN were studied after 8 and 16 weeks carbon tetrachloride intoxication, following bile duct ligation, and in tissue culture models. Hemodynamic changes were analyzed by direct cannulation, perivascular flowprobe, indocyanine green imaging, and functional magnetic resonance imaging. Serum and hepatic nitric oxide (NO) levels were determined by immunoassay. Hepatic inflammation was assessed by histology and serum markers and fibrosis by collagen proportionate area. Gene expression was analyzed by quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and western blotting and hepatic stellate cell (HSC)-MF contractility by gel contraction assay. Increased expression of RLN receptor (RXFP1) was shown in HSC-MFs and fibrotic liver diseases in both rats and humans. RLN induced a selective and significant reduction in portal pressure in pathologically distinct PHT models, through augmentation of intrahepatic NO signaling and a dramatic reduction in contractile filament expression in HSC-MFs. Critical for translation, RLN did not induce systemic hypotension even in advanced cirrhosis models. Portal blood flow and hepatic oxygenation were increased by RLN in early cirrhosis. Treatment of human HSC-MFs with RLN inhibited contractility and induced an antifibrogenic phenotype in an RXFP1-dependent manner.

CONCLUSION

We identified RXFP1 as a potential new therapeutic target for PHT and MF activation status.

A novel small compound that promotes nuclear translocation of YB-1 ameliorates experimental hepatic fibrosis in mice

Transforming growth factor-β (TGF-β) is considered to be a major factor contributing to liver fibrosis. We have previously shown that nuclear translocation of YB-1 antagonizes the TGF-β/Smad3 signaling in regulating collagen gene expression. More recently, we have demonstrated that the novel small compound HSc025 promotes nuclear translocation of YB-1, resulting in the improvement of skin and pulmonary fibrosis. Here, we presented evidence as to the mechanism by which HSc025 stimulates nuclear translocation of YB-1 and the pharmacological effects of HSc025 on a murine model of hepatic fibrosis. A proteomics approach and binding assays using HSc025-immobilized resin showed that HSc025 binds to the amino acid sequence within the C-tail region of YB-1. In addition, immunoprecipitation experiments and glutathione S-transferase pulldown assays identified poly(A)-binding protein (PABP) as one of the cytoplasmic anchor proteins of YB-1. HSc025 directly binds to YB-1 and interrupts its interaction with PABP, resulting in accelerated nuclear translocation of YB-1. Transfection of cells with PABP siRNA promoted nuclear translocation of YB-1 and subsequently inhibited basal and TGF-β-stimulated collagen gene expression. Moreover, HSc025 significantly suppressed collagen gene expression in cultured activated hepatic stellate cells. Oral administration of HSc025 to mice with carbon tetrachloride-induced hepatic fibrosis improved liver injury as well as the degree of hepatic fibrosis. Altogether, the results provide a novel insight into therapy for organ fibrosis using YB-1 modulators.

Relaxin receptors in hepatic stellate cells and cirrhotic liver

The polypeptide hormone relaxin has antifibrotic effects on a number of tissues, including the liver. Central to the progression of hepatic fibrosis is the transdifferentiation of hepatic stellate cells (HSC) from a quiescent state to an activated, myofibroblastic phenotype that secretes fibrillar collagen. Relaxin inhibits markers of HSC activation, but relaxin receptor expression in the liver is unclear. The purpose of this study was to determine the expression of the relaxin receptors LGR7 and LGR8 in activated HSC. Production of cAMP was induced by treatment of HSC with relaxin, or the relaxin-related peptides InsL3 or relaxin-3, selective activators of LGR8 and LGR7, respectively. Quiescent HSC expressed low levels of LGR7 but not LGR8. During progression to the activated phenotype, expression of both receptors increased markedly. Immunocytochemistry confirmed the presence of both receptors in activated HSC. In normal rat liver, LGR7, but not LGR8, was expressed at low levels. In cirrhotic liver, expression of both receptors significantly increased. Neither receptor was detectable in normal liver by immunohistochemistry, but both LGR7 and LGR8 were readily detectable in cirrhosis. These results were confirmed in human cirrhotic tissue, with the additional finding of occasional perisinusoidal LGR7 immunoreactivity in non-cirrhotic tissue. In conclusion, the expression of LGR7 and LGR8 is increased with activation of HSC in culture. Cirrhosis also caused increased expression of both receptors. Therefore, agents that stimulate LGR8 and LGR7 may be therapeutically useful to limit the activation of hepatic stellate cells in liver injury.

Relationship between oxidative stress levels and activation state on a hepatic stellate cell line

BACKGROUND/AIMS

Oxidative stress plays an important role in liver fibrosis. Under pathological conditions, hepatic stellate cells (HSC) undergo an activation process, developing a myofibroblast-like phenotype from the lipocyte phenotype. In this study, we determined the levels of oxidative stress and proliferation in different activation states of an experimental model of mouse HSC, the GRX cell line. These cells can be induced in vitro to display a more activated state or a quiescent phenotype.

METHODS/RESULTS

We observed increased oxidative damage and higher levels of reactive oxygen species, measured by thiobarbituric acid reactive species and 2',7'-dichlorofluorescein diacetate, respectively, and diminished catalase activity in activated cells. Activation decreased proliferation and increased the number of cells in G2/M. Antioxidants N-acetylcysteine and Trolox varied in their capacity to correct the oxidative stress and proliferation status.

CONCLUSIONS

The differences in physiological functions of stellate cell phenotypes suggest a relationship between oxidative stress levels and activation state.

Effect of S-adenosyl-L-methionine on the activation, proliferation and contraction of hepatic stellate cells

Inhibition of hepatic stellate cell activation is an important clinical aspect for the control of liver inflammation, fibrosis and cirrhosis. S-adenosyl-L-methionine (SAM), an intermediate product of L-methionine metabolism, is a precursor of glutathione and an endogenous methyl donor. Although the hepato-protective action of SAM has been reported in several animal models, the effect of SAM on the function of hepatic stellate cells has not been elucidated. Using a primary-culture model of hepatic stellate cells, we found that SAM blunts the activation process as indicated by the suppression of expression of collagen alpha1(I) and smooth muscle alpha-actin. SAM also hampers the DNA synthesis of hepatic stellate cells stimulated with a dimer of platelet-derived growth factor-B via the inhibition of phosphorylation of PDGF receptor-beta and down-stream signaling pathways. SAM additionally inhibits the contraction of hepatic stellate cells by disturbing the formation of F-actin stress fibers and phosphorylated myosin light chains. Thus, SAM regulates the activation of hepatic stellate cells and may clinically contribute to therapy targeted at human liver fibrosis.

Exogenous nucleosides modulate the expression of rat liver extracellular matrix genes in single cultures of primary hepatocytes and a liver stellate cell line and in their co-culture

BACKGROUND & AIMS

We have previously reported the antifibrotic effect of dietary nucleotides in cirrhotic rats. In this work, we used primary rat hepatocytes, a liver stellate cell line (CFSC-2G) and co-cultures of both cell types to investigate the effects of exogenous nucleosides on the gene expression of various extracellular matrix components and on markers of liver function, and to ascertain whether the effects found in vivo are due to CFSC-2G, hepatocytes, or are the consequence of cell-cell interactions.

RESULTS

Nucleosides enhanced fibronectin, laminin, and alpha1(I) procollagen levels in CFSC-2G and hepatocytes, as well as collagen synthesis and secretion in CFSC-2G. In contrast, nucleosides lowered fibronectin, laminin and alpha1(I) procollagen levels, and decreased collagen synthesis in co-cultures. Matrix metalloproteinase-13 content and collagen secretion increased in co-cultures incubated with nucleosides. Albumin increased in hepatocytes and co-cultures incubated in the presence of nucleosides.

CONCLUSIONS

Nucleosides modulate the production of extracellular matrix in single cultures of hepatocytes and of CFSC-2G, and in co-cultures. This effect seems to be regulated at the translational level. The opposite behavior of single cultures and co-cultures is probably due to the fact that the latter model reproduces many of the physical and functional relationships observed in vivo between hepatocytes and stellate cells.

Hepatic stellate cell activation in vitro: cell cycle arrest at G2/M and modification of cell motility

Hepatic fibrosis is a common response to chronic liver injury and is characterized by increased production of extracellular matrix components, whose major part is produced by hepatic stellate cells activated by inflammatory mediators to proliferate and migrate into the injured regions. GRX cells are a model of hepatic stellate cells characterized as myofibroblasts by morphological and biochemical criteria. We have recently shown that they respond to inflammatory mediators and cytokines present in the concanavalin A-activated spleen cell supernatant (SCS) by quantitative changes in the expression of intermediate filaments. The present study investigated the effects of SCS and TNF-alpha on the GRX cell proliferation and on the organization of the actin cytoskeleton. SCS and TNF-alpha diminished the culture cell density, with an increase of cell [(3)H]thymidine incorporation and of cellular protein content, indicating an arrest in the G2/M phase of the cell cycle, which was reversible 48 h after removal of SCS. This effect was abrogated by dibutiryl-cAMP. Actin cytoskeleton reorganization was observed after 24 h treatment, indicating increased cell motility. Our results suggest that inflammation-dependent activation of stellate cells occurs in ordered interaction and coordination of proinflammatory agents. The increase of cAMP levels activates the conversion of lipocytes into myofibroblasts and increases the number of cells that can participate in repair. Since cAMP retains cells in the G1 phase, cytokines of the TNF-alpha group are required for cell proliferation inducing the entry into the S phase. The progression through the G2/M checkpoint is mediated again by increased cAMP levels.

Inhibition of markers of hepatic stellate cell activation by the hormone relaxin

Hepatic fibrosis results from excess extracellular matrix produced primarily by hepatic stellate cells (HSC). In response to injury, HSC differentiate to a myofibroblastic phenotype expressing smooth muscle actin and fibrillar collagens. Relaxin is a polypeptide hormone shown to have antifibrotic effects in fibrosis models. In this study, activated HSC from rat liver were treated with relaxin to determine if relaxin can reverse markers of HSC activation. Relaxin treatment resulted in a decrease in the expression of smooth muscle actin, but had no effect on cell proliferation rate. The levels of total collagen and type I collagen were reduced, while the synthesis of new collagen was inhibited. Furthermore, relaxin caused an increase in the expression and secretion of rodent interstitial collagenase (MMP-13), but there was no effect on the gelatinases MMP-2 or MMP-9. Relaxin also increased secretion of TIMP-1 and TIMP-2. The effective concentration of relaxin to induce these effects was consistent with action through the relaxin receptor. In conclusion, relaxin reversed markers of the activated phenotype of HSC including the production of fibrillar collagen. At the same time, the activity of a fibrillar collagenase was increased. These data suggest that relaxin not only inhibits HSC properties that contribute to the progression of hepatic fibrosis, but also promotes the clearance of fibrillar collagen. Therefore, relaxin may be a useful approach in the treatment of hepatic fibrosis.

Aloe emodin suppresses myofibroblastic differentiation of rat hepatic stellate cells in primary culture

We have studied the inhibitory effect of aloe emodin on hepatic stellate cells activation and proliferation, as these cells play a key role in the pathogenesis of hepatic fibrosis. Rat hepatic stellate cells were activated by contact with plastic dishes, resulting in their transformation into myofibroblast-like cells. Primary hepatic stellate cells were exposed to aloe emodin (1-10 microg/ml). Possible cytotoxic effects were measured on stellate cells and hepatocytes using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The effects of aloe emodin on production of type I collagen and smooth muscle cell alpha-actin were examined at the same concentration, by quantitative immunoprecipitation. Antiproliferative effects were examined by bromodeoxyuridine incorporation. Aloe emodin at 10 microg/ml restored the morphological changes characteristic of activated primary stellate cells, reduced DNA synthesis to 95% of control hepatic stellate cells at 10 microg/ml without affecting cell viability, and inhibited type I collagen production and smooth muscle alpha-actin expression by 86.77% and 99%, respectively, which suggest that aloe emodin is a potent inhibitor of stellate cell transformation.

Prevention of rat hepatic fibrosis by the protease inhibitor, camostat mesilate, via reduced generation of active TGF-beta

BACKGROUND & AIMS

Proteolytic release and activation of latent transforming growth factor beta (TGF-beta) by the hepatic stellate cells (HSCs) are key events for pathogenesis of hepatic fibrosis, and protease inhibitors suppress TGF-beta generation by cultured HSCs, suggesting their potential use as antifibrogenic agents. We explored this idea using camostat mesilate, a serine protease inhibitor, to determine its effects and mechanisms of action in vivo.

METHODS

Camostat mesilate was either added to cultured rat HSCs or administered orally to rats during porcine serum treatment, followed by overexpression of urokinase. We measured cellular and hepatic levels of plasmin, TGF-beta, TGF-beta activity, activated HSC markers (increased cell number, morphologic change, and expression of both alpha-smooth muscle actin and collagen(alpha2)[I]), and fibrosis (Azan-staining and quantification of hydroxyproline content).

RESULTS

Camostat mesilate (500 micromol/L) inhibited generation of TGF-beta by suppressing plasmin activity and reduced the activity of TGF-beta, which blocked in vitro activation of HSCs. In the in vivo model, camostat mesilate (1-2 mg/g of diet) markedly attenuated an increase in hepatic plasmin and TGF-beta levels, HSC activation, and hepatic fibrosis without apparent systemic or local side effects, all of which were reverted by restoration of hepatic plasmin activity.

CONCLUSIONS

Camostat mesilate prevents porcine serum-induced rat hepatic fibrosis via a profound reduction in TGF-beta generation.

Strong antiproliferative effects of baicalein in cultured rat hepatic stellate cells

Recently, antifibrogenetic effects of Sho-saiko-to, a traditional herbal medicine in Japan, have been shown in experimental hepatic fibrosis, and flavonoids in Sho-saiko-to are suspected as active ingredients. Thus, we evaluated the effects of baicalein, a major flavonoid in Sho-saiko-to, on proliferation and protein synthesis in cultured rat hepatic stellate cells. Baicalein decreased [3H]thymidine incorporation in cells stimulated with platelet-derived growth factor-B subunit homodimer (PDGF-BB) in a concentration-dependent manner (approximate ED50<10 microM, P<0.0001), and the decrease observed with 10 microM baicalein was greater than those observed with 5 microM retinol or 500 microM 3-isobutyl-1-methylxanthine (IBMX). Baicalein consistently decreased [3H]thymidine incorporation and cell number in cells stimulated with fetal calf serum (ED50<10 microM, P<0.0001), and moderately suppressed [3H]leucine and [3H]proline incorporation (P<0.0001). These results demonstrate the strong antiproliferative effect of baicalein in hepatic stellate cells, showing the possibility of baicalein as an antifibrogenetic drug for hepatic fibrosis.

OPC-13013, a cyclic nucleotide phosphodiesterase type III, inhibitor, inhibits cell proliferation and transdifferentiation of cultured rat hepatic stellate cells

Activated hepatic stellate cells (HSC; lipocytes; Ito cells) proliferate and are responsible for extracellular matrix synthesis during hepatic fibrogenesis. During activation, HSC undergo transdifferentiation into myofibroblasts expressing alpha-smooth muscle actin (alpha-SMA). Adenosine 3', 5'-cyclic monophosphate (cyclic AMP) is an ubiquitous intracellular signaling molecule, and is upregulated by the activation of adenylate cyclase and downregulated via hydrolysis by cyclic nucleotide phosphodiesterases (PDEs). Recently, increased intracellular cyclic AMP has been shown to inhibit HSC activation. The aim of the current study was to determine the effects of inhibition of PDEs on cell proliferation and transdifferentiation in cultured rat HSC. Cell proliferation was determined by [3H]thymidine incorporation, and Western blot analysis was performed for detection of alpha-SMA, a phenotypic marker of transdifferentiation into myofibroblast. When the cells were exposed to 3-isobutyl-1-methylxanthine (IBMX; 50-1000 microM), a nonselective PDE inhibitor, serum-stimulated [3H]thymidine incorporation was suppressed in a dose-dependent manner with a maximum inhibition of 66% at a concentration of 500 microM OPC-13013 (1-60 microM), a selective PDE III isoenzyme inhibitor, induced a dose-dependent inhibitory effect on serum-stimulated DNA synthesis that reached a maximum inhibition of 95% at a concentration of 60 microM, while neither 8-methoxymethyl-3-isobutyl-1-methylxanthine (8-MMX), a PDE I isoenzyme inhibitor, nor Ro-20-1724, a PDE IV isoenzyme inhibitor, had an inhibitory effect. Western blot analysis revealed that IBMX or OPC-13013 decreased alpha-SMA expression, while other selective PDE isoenzyme inhibitors did not have a suppressive effect. IBMX, OPC-13013 or Ro-20-1724, but not 8-MMX augmented forskolin-induced increase in intracellular cyclic AMP levels although cyclic AMP levels were not affected by treatment with any of these PDE inhibitors alone. These data indicate that inhibition of PDEs, especially PDE III isoenzyme, can produce an inhibitory effect on HSC activation. The PDE III isoenzyme may contribute to the regulation of HSC activation during fibrogenesis. In addition, OPC-13013 may have the potential to inhibit initiation and progression of hepatic fibrosis by interfering with HSC activation.

Expression of cyclins D1, D2 and E correlates with proliferation of rat stellate cells in culture

BACKGROUND/AIMS

Regulation of cell cycle progression of cultured rat stellate cells was studied.

METHODS

DNA synthesis was determined by the uptake of [3H]thymidine or 5-bromo-2'-deoxyuridine. Cell cycle distribution was analyzed using FACScan of cellular DNA stained with propidium iodide. Expression of cyclins and cyclin-dependent kinase 4 was evaluated by Western and Northern blotting.

RESULTS

DNA synthesis of primary-cultured stellate cells was found to accelerate 48 h after plating. Cell cycle analysis revealed that more than 93% of the cells were in G0/G1 phase during the first 48 h after plating. The cell population in S phase abruptly increased to about 16% 72 h after culture and shifted to G2/M phase thereafter. The level of proteins and mRNAs for cyclins D1, D2 and E started to increase 48 h after culture with a concomitant expression of platelet-derived growth factor receptor beta, while the level of cyclin-dependent kinase 4 and its mRNA remained unchanged. On the other hand, stellate cells remained in G1 phase when they were cultured in the presence of 3-isobutyl-1-methylxanthine or dibutyryl cAMP after plating. Attenuation of the expression of cyclins D1, D2 and E and platelet-derived growth factor receptor beta, but not cyclin-dependent kinase 4 was found in stellate cells cultured with these agents. Further analysis revealed that LY294002, a selective inhibitor of phosphatidylinositol 3-kinase, suppressed DNA synthesis and cyclin D1 expression in a dose-dependent manner without affecting platelet-derived growth factor receptor beta expression.

CONCLUSIONS

Induction of G1 cyclins may play crucial roles in cell cycle transition of cultured stellate cells from G1 to S. Expression of platelet-derived growth factor receptor beta and activation of phosphatidylinositol 3-kinase may be involved in the process.

Effects of Sho-saiko-to, a Japanese herbal medicine, on hepatic fibrosis in rats

It has been shown that lipid peroxidation is associated with hepatic fibrosis and stellate cell activation. Sho-saiko-to (TJ-9) is an herbal medicine, which is commonly used to treat chronic hepatitis in Japan, although the mechanism by which TJ-9 protects against hepatic fibrosis is not known. As a result, we assayed the preventive and therapeutic effects of TJ-9 on experimental hepatic fibrosis, induced in rats by dimethylnitrosamine (DMN) or pig serum (PS), and on rat stellate cells and hepatocytes in primary culture, and assessed the antioxidative activities and the active components of TJ-9. Male Wistar rats were given a single intraperitoneal injection of 40 mg/kg DMN or 0.5 mL PS twice weekly for 10 weeks. In each model, rats were fed a basal diet throughout, or the same diet, which also contained 1.5% TJ-9, for 2 weeks before treatment or for the last 2 weeks of treatment. TJ-9 suppressed the induction of hepatic fibrosis, increased hepatic retinoids, and reduced the hepatic levels of collagen and malondialdehyde (MDA), a production of lipid peroxidation. Immunohistochemical examination showed that TJ-9 reduced the deposition of type I collagen and the number of alpha-smooth muscle actin (alpha-SMA) positive-stellate cells in the liver and inhibited, not only lipid peroxidation in cultured rat hepatocytes that were undergoing oxidative stress, but also the production of type I collagen, alpha-SMA expression, cell proliferation, and oxidative burst in cultured rat stellate cells. In addition, TJ-9 inhibited Fe2+/adenosine 5'-diphosphate-induced lipid peroxidation in rat liver mitochondria in a dose-dependent manner and showed radical scavenging activity. Among the active components of TJ-9, baicalin and baicalein were found to be mainly responsible for the antioxidative activity. These findings suggest that Sho-saiko-to (TJ-9) functions as a potent antifibrosuppressant by inhibition of lipid peroxidation in hepatocytes and stellate cells in vivo.

Inhibitory effects of the herbal medicine Sho-saiko-to (TJ-9) on cell proliferation and procollagen gene expressions in cultured rat hepatic stellate cells

BACKGROUND/AIMS

It is of extreme importance to prevent liver fibrosis and subsequent progression to liver cirrhosis. The aim of our study was to elucidate in vitro whether Sho-saiko-to exerted inhibitory effects on hepatic stellate cells.

METHODS

Hepatic stellate cells were isolated from male Wistar rats. Water-soluble ingredients of Sho-saiko-to were obtained at concentrations of 10, 100, 250, 500 and 1000 microg/ml. Morphological transformation was observed under a phase-contrast microscope. Flow cytometric analysis was performed on day 4 after culture to evaluate the potential to proliferate of the stellate cells by analyzing cell cycles. Northern blot analysis was carried out on day 3 after culture to determine the expressions of type I and type III procollagen mRNAs.

RESULTS

(i) Sho-saiko-to 500 and 1000 microg/ml inhibited morphological transformation of the stellate cells to myofibroblast-like cells. (ii) Sho-saiko-to 500 and 1000 microg/ml significantly (p<0.0001) accumulated the cells in the G0/G1 phase (118.8+/-0.7%, 119.2+/-0.5%, respectively as compared with control) and significantly (p<0.0001) decreased cell numbers subsequently in G2/M phase (47.5+/-8.1%, 48.9+/-2.0%, respectively). (iii) Sho-saiko-to 500 and 1000 microg/ml also significantly (p<0.05 or p<0.0001) suppressed procollagen mRNA expression of type I to 51.5+/-6.4%, 34.9+/-3.7%, respectively, and type III to 51.3+/-12.3%, 46.7+/-11.4%, respectively.

CONCLUSIONS

We have clarified the inhibitory effects of Sho-saiko-to on hepatic stellate cells in vitro. Sho-saiko-to could be a potent inhibitor in the pathogenesis of liver fibrosis.

Regulation of stellate cell proliferation by lipopolysaccharide: Role of endogenous nitric oxide

We studied the effect of lipopolysaccharide (LPS) on the proliferation of culture-stimulated rat stellate cells. DNA synthesis as determined by [³ H]-thymidine incorporation was significantly suppressed by up to 52% compared with the control culture in the presence of LPS (> 5 ng/mL). Such an inhibitory effect of LPS was dramatically augmented in the presence of interferon-γ (IFNγ). Lipopolysaccharide alone or in combination with IFNγ activated transcription factors AP-1 and NF-κB, and elicited nitric oxide (NO) production by stellate cells by inducing NO synthase. Inhibition of NO production by the addition of L-arginine antagonists to the culture, partially cancelled such an inhibitory effect of LPS and/or IFNγ on DNA synthesis without affecting the activation of AP-1 and NF-κB and the NO synthase level. The cellular level of cyclic guanosine monophosphate (cGMP) increased in response to LPS and IFNγ, and dibutyryl cGMP or 8-bromo-cGMP inhibited the incorporation of [³ H]-thymidine in a dose-dependent manner. These results indicate that LPS is potent in modulating stellate cell proliferation by some NO- and cGMP-dependent mechanism.

Effect of antioxidants, resveratrol, quercetin, and N-acetylcysteine, on the functions of cultured rat hepatic stellate cells and Kupffer cells

Effects of antioxidants, resveratrol, quercetin, and N-acetylcysteine (NAC) on the functions of cultured rat hepatic stellate cells and Kupffer cells were studied. These compounds dose-dependently suppressed serum-dependent proliferation of stellate cells as determined by [3H]thymidine and 5-bromo-2'-deoxyuridine uptake. Expression of smooth muscle alpha-actin was suppressed by a high dose of resveratrol and quercetin. These phenolic compounds also suppressed inositol phosphate metabolism, tyrosine phosphorylation, and mitogen-activated protein (MAP) kinase activation in platelet-derived growth factor/BB-stimulated stellate cells. Moreover, the phenolic compounds selectively reduced the level of cell cycle protein cyclin D1 in stellate cells. Thus, resveratrol and quercetin might inhibit stellate cell activation by perturbing signal transduction pathway and cell cycle protein expression, whereas mechanism of potent antiproliferative effect of NAC remains to be elucidated. On the other hand, kinetic analysis showed that production of nitric oxide (NO) and tumor necrosis factor alpha (TNF-alpha) by lipopolysaccharide-stimulated Kupffer cells was strongly inhibited by resveratrol and quercetin but not by NAC. Although expression of messenger RNAs for inducible NO synthase and TNF-alpha was not affected by the phenolic compounds, cellular levels of inducible NO synthase and TNF-alpha secretion were suppressed significantly, indicating the posttranscriptional process of generating these proteins might be affected predominantly by these phenolic compounds. Thus, NAC and these phenolic compounds may have therapeutic potential against liver injury by regulating functions of hepatic stellate cells and Kupffer cells.

Pharmacological aspects of pentoxifylline with emphasis on its inhibitory actions on hepatic fibrogenesis

1. Pentoxifylline (PTX), a derivative of the methylxanthine theobromine, has been used for many years in the treatment of peripheral vascular diseases. Increased red blood cell flexibility, reduction of blood viscosity, and decreased potential of platelet aggregation are the basic actions of PTX, resulting in therapeutic benefits due to improved microcirculation and tissue oxygenation. 2. PTX's generally accepted mechanism of action is the inhibition of phosphodiesterases, leading to increased intracellular levels of cyclic adenosine monophosphate (cAMP). 3. A number of studies have shown PTX's effects on the cytokine network. The most relevant clinical results are the therapeutic benefits of PTX in attenuating the effects of tumor necrosis factor-alpha (TNF-alpha) in conditions such as septic shock. 4. PTX also has been found to exert antifibrogenic actions, using cultured fibroblasts or animal models of fibrosis, including liver fibrosis. 5. In hepatic stellate cell culture PTX has been shown to inhibit the basic reactions of liver fibrogenesis, being effective on cytokines and growth factors relevant in fibrogenesis of the liver, too. 6. Therefore, PTX might be an effective drug with few side effects in the treatment of liver fibrosis. Further clinical studies have to be done to establish the real therapeutic benefits of PTX in liver fibrosis and cirrhosis.

Regulation by cAMP of STAT1 activation in hepatic stellate cells

Previously we reported that dibutyryl cAMP and phosphodiesterase inhibitor methylxanthines block rat stellate cell proliferation. To analyze the underlying mechanism, modulation by these agents of platelet-derived growth factor (PDGF)/BB-stimulating signal pathway was studied. Without reducing STAT1 protein level, these agents were found to attenuate STAT1 activation in stellate cells stimulated with PDGF/BB as revealed by an electrophoretic mobility shift assay. Inhibitory effect started 12 h after exposure of the cells to these agents at concentrations of more than 100 microM. These agents had no effects on DNA binding activity of STAT1 that had already been activated. Treatment with these agents failed to affect the function of PDGF receptors except for partial attenuation of phospholipase C activation under PDGF/BB stimulation. The present results indicate that inhibition of STAT1 activation may be one of factors involved in the cAMP-dependent stellate cell growth arrest.