Cooperation of liver cells in health and disease

Liver fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins including collagen that occurs in most types of chronic liver diseases. Advanced liver fibrosis results in cirrhosis, liver failure, and portal hypertension and often requires liver transplantation. Our knowledge of the cellular and molecular mechanisms of liver fibrosis has greatly advanced. Activated hepatic stellate cells, portal fibroblasts, and myofibroblasts of bone marrow origin have been identified as major collagen-producing cells in the injured liver. These cells are activated by fibrogenic cytokines such as TGF-beta1, angiotensin II, and leptin. Reversibility of advanced liver fibrosis in patients has been recently documented, which has stimulated researchers to develop antifibrotic drugs. Emerging antifibrotic therapies are aimed at inhibiting the accumulation of fibrogenic cells and/or preventing the deposition of extracellular matrix proteins. Although many therapeutic interventions are effective in experimental models of liver fibrosis, their efficacy and safety in humans is unknown. This review summarizes recent progress in the study of the pathogenesis and diagnosis of liver fibrosis and discusses current antifibrotic strategies.

Liver fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins including collagen that occurs in most types of chronic liver diseases. Advanced liver fibrosis results in cirrhosis, liver failure, and portal hypertension and often requires liver transplantation. Our knowledge of the cellular and molecular mechanisms of liver fibrosis has greatly advanced. Activated hepatic stellate cells, portal fibroblasts, and myofibroblasts of bone marrow origin have been identified as major collagen-producing cells in the injured liver. These cells are activated by fibrogenic cytokines such as TGF-beta1, angiotensin II, and leptin. Reversibility of advanced liver fibrosis in patients has been recently documented, which has stimulated researchers to develop antifibrotic drugs. Emerging antifibrotic therapies are aimed at inhibiting the accumulation of fibrogenic cells and/or preventing the deposition of extracellular matrix proteins. Although many therapeutic interventions are effective in experimental models of liver fibrosis, their efficacy and safety in humans is unknown. This review summarizes recent progress in the study of the pathogenesis and diagnosis of liver fibrosis and discusses current antifibrotic strategies.

Lhx2-/- mice develop liver fibrosis

Liver fibrosis is a wound-healing response to chronic injury of any type and is characterized by a progressive increase in deposition of extracellular matrix (ECM) proteins, the major source of which are activated hepatic stellate cells (HSCs). Because the LIM homeobox gene Lhx2 is expressed in HSCs and liver development in Lhx2(-/-) mice is disrupted, we analyzed liver development in Lhx2(-/-) embryos in detail. Lhx2(-/-) embryos contain numerous activated HSCs and display a progressively increased deposition of the ECM proteins associated with liver fibrosis, suggesting that Lhx2 inhibits HSC activation. Transfection of Lhx2 cDNA into a human HSC line down-regulates expression of genes characteristic of activated HSCs. Moreover, the Lhx2(-/-) liver display a disrupted cellular organization and an altered gene expression pattern of the intrahepatic endodermal cells, and the increased deposition of ECM proteins precedes these abnormalities. Collectively these results show that Lhx2 negatively regulates HSC activation, and its inactivation in developing HSCs appears therefore to mimic the signals that are triggered by the wound-healing response to chronic liver injury. This study establishes a spontaneous and reproducible animal model for hepatic fibrosis and reveals that Lhx2 expression in HSCs is important for proper cellular organization and differentiation of the liver.

Thy-1 expression regulates the ability of rat lung fibroblasts to activate transforming growth factor-beta in response to fibrogenic stimuli

Distinct subpopulations of fibroblasts contribute to lung fibrosis, although the mechanisms underlying fibrogenesis in these subpopulations are not clear. Differential expression of the glycophosphatidylinositol-linked protein Thy-1 affects proliferation and myofibroblast differentiation. Lung fibroblast populations selected on the basis of Thy-1 expression by cell sorting were examined for responses to fibrogenic stimuli. Thy-1 (-) and Thy-1 (+) fibroblast populations were treated with platelet-derived growth factor-BB, interleukin-1beta, interleukin-4, or bleomycin and assessed for activation of transforming growth factor (TGF)-beta, Smad3 phosphorylation, and alpha-smooth muscle actin and fibronectin expression. Thy-1 (-) fibroblasts responded to these stimuli with increased TGF-beta activity, Smad3 phosphorylation, and expression of alpha-smooth muscle actin and fibronectin, whereas Thy-1 (+) fibroblasts resisted stimulation. The unresponsiveness of Thy-1 (+) cells is not because of defective TGF-beta signaling because both subsets respond to exogenous active TGF-beta. Rather, Thy-1 (-) fibroblasts activate latent TGF-beta in response to fibrogenic stimuli, whereas Thy-1 (+) cells fail to do so. Defective activation is common to multiple mechanisms of TGF-beta activation, including thrombospondin 1, matrix metalloproteinase, or plasmin. Thy-1 (-) lung fibroblasts transfected with Thy-1 also become resistant to fibrogenic stimulation, indicating that Thy-1 is a critical biological response modifier that protects against fibrotic progression by controlling TGF-beta activation. These studies provide a molecular basis for understanding the differential roles of fibroblast subpopulations in fibrotic lung disease through control of latent TGF-beta activation.

GOTree Machine (GOTM): a web-based platform for interpreting sets of interesting genes using Gene Ontology hierarchies

Background

Microarray and other high-throughput technologies are producing large sets of interesting genes that are difficult to analyze directly. Bioinformatics tools are needed to interpret the functional information in the gene sets.

Results

We have created a web-based tool for data analysis and data visualization for sets of genes called GOTree Machine (GOTM). This tool was originally intended to analyze sets of co-regulated genes identified from microarray analysis but is adaptable for use with other gene sets from other high-throughput analyses. GOTree Machine generates a GOTree, a tree-like structure to navigate the Gene Ontology Directed Acyclic Graph for input gene sets. This system provides user friendly data navigation and visualization. Statistical analysis helps users to identify the most important Gene Ontology categories for the input gene sets and suggests biological areas that warrant further study. GOTree Machine is available online at .

Conclusion

GOTree Machine has a broad application in functional genomic, proteomic and other high-throughput methods that generate large sets of interesting genes; its primary purpose is to help users sort for interesting patterns in gene sets.

Stages of activation of hepatic stellate cells: effects of ellagic acid, an inhibiter of liver fibrosis, on their differentiation in culture

To further explore that hepatic stellate cell (HSC) activation results in physiological protection against environmental insult, the profile of differentiation of HSC has been examined upon treatment with ellagic acid (EA), a plant-derived antioxidant that shows multiple protective effects during liver disease. Sparse rat liver cell cultures were grown in media containing EA (3, 6, 30 and 100 microg/ml) and, as controls, without EA, and inspected until day 7 in culture. The cells were double-labelled with antibodies against glial fibrillary acidic protein (GFAP) and smooth muscle alpha-actin (SMAA), marker proteins of quiescent and activated HSC, respectively. In EA-free culture conditions, the quiescent (SMAA-/GFAP+) HSC transiently acquired a semi-activated (SMAA+/GFAP+), phenotype and were further transformed into activated (SMAA+/GFAP-), pleomorphic HSC. Up to a concentration of 30 microg/ml, EA induced an early synthesis of SMAA in all HSC and inhibited their morphologic differentiation and individual growth throughout the culture period. At a concentration of 6 microg/ml, EA supported the semi-activated (SMAA+/GFAP+) phenotype of HSC throughout the culture period, whereas treatment with high EA concentrations (30 microg/ml) resulted in an early loss of GFAP expression.

IN CONCLUSION

(i) the uniform response of HSC to EA by mild activation adds functional significance to cellular features preceding the transformation of HSC to myofibroblasts; (ii) the high sensitivity of HSC to EA treatment suggests their involvement in any mechanisms of protection by this antioxidant; (iii) the maintenance of HSC morphology might be one of the factors playing a role in the prevention or slowing down of liver fibrosis; (iv) because the effects of EA are concentration- and time-dependent, an arbitrary usage of this antioxidant is a matter of potential concern; (v) the various patterns of HSC activation observed might correspond to distinct activities of these cells, which, in turn, might lead to different outcomes of liver fibrosis.

Kupffer cells promote lead nitrate-induced hepatocyte apoptosis via oxidative stress

BACKGROUND: Apoptosis and its modulation are crucial factors for the maintenance of liver health, allowing hepatocytes to die without provoking a potential harmful inflammatory response through a tightly controlled and regulated process. Since Kupffer cells play a key role in the maintenance of liver function, the aim of this study was to verify whether Kupffer cells are involved in the induction of liver apoptosis after i.v. injection of Pb(NO3)2 likely by secretion mechanisms. RESULTS: The in vivo hepatic apoptosis, induced by Pb(NO3)2 was prevented by a pre-treatment with gadolinium chloride (GdCl3), a Kupffer cells toxicant, that suppresses Kupffer cell activity and reduces to a half the apoptotic rate. In addition, in vivo Pb(NO3)2 administration deprives hepatocytes of reduced glutathione, whereas the loss of this important oxidation-preventing agent is considerably mitigated or abolished by pre-treatment with GdCl3. However, incubation of isolated hepatocytes and Kupffer cells and HepG2 cells with Pb(NO3)2 for 24 hours induced necrotic but not apoptotic cells. Apoptosis of hepatocytes and HepG2 cells was observed only after the addition of conditioned medium obtained from Kupffer cells cultured for 24 hours with Pb(NO3)2, thus indicating the secretion of soluble mediators of apoptosis by Kupffer cells. Apoptosis in the HepG2 cells was observed upon 24-hours incubation of HepG2 cells with 1 mM buthionine sulfoximine, a glutathione depleting agent, thus showing that there is an oxidative apoptogenic pathway in HepG2 cells. CONCLUSION: Pb(NO3)2 has, at most, a direct necrotic (but not apoptogenic) effect on hepatocytes and HepG2 cells, giving a clue about the regulatory role of Kupffer cells in the induction of liver apoptosis after a single Pb(NO3)2 injection without pre-treatment with GdCl3, probably via secreting soluble factors that trigger oxidative stress in target cells.

Gamma interferon production by hepatic NK T cells during Escherichia coli infection is resistant to the inhibitory effects of oxidative stress

The reductive-oxidative status of tissues regulates the expression of many inflammatory genes that are induced during gram-negative bacterial infections. The cytokine gamma interferon (IFN-gamma) is a potent stimulus for host inflammatory gene expression, and oxidative stress has been shown to inhibit its production in mice challenged with Escherichia coli bacteria. The objective of the present study was to characterize the cells that produced IFN-gamma in a mouse bacterial peritonitis model and determine the effects of oxidative stress on their activation. The liver contained large numbers of IFN-gamma-expressing lymphocytes following challenge with viable E. coli bacteria. The surface phenotypes of IFN-gamma-expressing hepatic lymphocytes were those of natural killer (NK) cells (NK1.1(+) CD3(-)), conventional T cells (NK1.1(-) CD3(+)), and NK T cells (NK1.1(+) CD3(+)). Treating mice with diethyl maleate to deplete tissue thiols significantly impaired IFN-gamma production by NK cells, conventional T cells, and CD1d-restricted NK T cells in response to E. coli challenge. However, IFN-gamma expression by a subset of NK T cells, which did not bind alpha-galactosylceramide-CD1d tetramers, was resistant to the inhibitory effects of tissue oxidative stress. Stress-resistant IFN-gamma-expressing cells were also predominantly CD8(+) and bore gamma delta T-cell antigen receptors. The residual IFN-gamma response by NK T cells may explain previous reports of hepatic gene expression following gram-negative bacterial challenge in thiol-depleted mice. The finding also demonstrates that innate immune cells differ significantly in their responses to altered tissue redox status.

Innate and acquired immunity intersect in a global view of the acute-phase response

Recognition of stereotypic chemical patterns by sentinel cells of the innate immune system provokes a transient deviation from homeostasis, the acute-phase response (APR). Although APR effectors have been identified individually, the complexity of the response suggested that emergent properties would be uncovered by a more comprehensive examination. Our global assessment revealed that approximately 7% of genes in the mouse are mobilized in the hepatic APR to endotoxin. Extensive metabolic adjustments include suppression of pathways for cholesterol, fatty acid, and phospholipid synthesis. Increased expression of genes for innate defense was accompanied by coordinate induction of the MHC class I antigen presentation machinery, illustrating an intersection between innate and adaptive immunity.

Effects of pentoxifylline on the hepatic content of TGF-β1 and collagen in Schistosomiasis japonica mice with liver fibrosis

AIM: To study the effects of pentoxifylline (PTX) on the content of hepatic TGF-β1, type I and type III collagen in schistosomiasis japonica mice with liver fibrosis and its mechanism of anti-fibrosis.

METHODS: Forty mice with schistosomiasis were divided into four groups: one group as control without any treatment, other three were treated with Praziquantel 500 mg/(kg·d)for 2 d, high dose PTX 360 mg/(kg·d) for 8 wk, and low dose PTX 180 mg/(kg·d) for 8 wk respectively. Immunohistochemical technique and multimedia color pathographic analysis system were applied to observe the content change of hepatic TGF-β1, type I and type III collagen in schistosomiasis japonica mice with liver fibrosis before and after PTX treatment.

RESULTS: Effects of PTX on the content change of hepatic TGF-β1, type I and type III collagen in schistosomiasis japonica mice with liver fibrosis were related to the dosage of PTX, high dose PTX treated group could significantly reduce the content of TGF-β1 (0.709 ± 0.111), type I (0.644 ± 0.108) and type III (0.654 ± 0.152) collagen compared with those of control group (0.883 ± 0.140, 0.771 ± 0.156, 0.822 ± 0.129) with statistical significance (P < 0.05). Low dose PTX could also reduce the hepatic content of TGF-β1 (0.752 ± 0.152), type I (0.733 ± 0.117) and type III (0.788 ± 0.147) collagen, but without statistical significance (P > 0.05). Both high dose and low dose PTX groups have significant differences on the content of TGF-β1, type I and type III collagen (P < 0.05, P < 0.05, P < 0.01, respectively).

CONCLUSION: High dose of PTX treatment could reduce the content of hepatic TGF-β1, type I and type III collagen significantly in schistosomiasis japonica mice with liver fibrosis, and thus plays its role of antifibrosis.