Oxidative stress and oval cell accumulation in mice and humans with alcoholic and nonalcoholic fatty liver disease

Accumulation of hedgehog-responsive progenitors parallels alcoholic liver disease severity in mice and humans

BACKGROUND & AIMS

Improving outcomes in alcoholic liver disease (ALD) necessitates better understanding of how habitual ethanol (EtOH) consumption alters normal regenerative mechanisms within the liver. Hedgehog (Hh) pathway activation promotes expansion of progenitor populations in other tissues. We evaluated the hypothesis that chronic EtOH exposure activates Hh signaling in liver.

METHODS

Hh signaling, liver progenitors, transforming growth factor (TGF)-beta induction, and liver damage were compared in mice fed chow, high-fat diets (HF), or HF + EtOH for 4 weeks. Susceptibility to TGF-beta-mediated apoptosis was compared in Hh-responsive liver cells (eg, immature cholangiocytes and oval cells) and mature hepatocytes (which are unresponsive to Hh). Hepatic accumulation of Hh-responsive cells were compared in controls and ALD patients and correlated with a discriminant function (DF) that predicts subacute mortality.

RESULTS

Hh signaling and numbers of Hh-responsive cells were increased in HF mice and greatest in HF+EtOH mice. In both, progenitor and stromal cell populations harbored Hh-responsive cells. More ductular-type progenitors and fibrosis markers were noted in HF+EtOH mice than in HF mice. The former also expressed more TGF-beta-1. TGF-beta-1 treatment selectively promoted the viability of Hh-responsive immature liver cells and caused mature hepatocytes that survived to produce Hh ligands. Hh-responsive cells were increased in ALD patients. Lobular accumulation of Hh-responsive immature ductular cells was greater in those with a DF >32 than those with a DF <32.

CONCLUSIONS

Hh signaling is increased in ALD and may influence ALD outcomes by promoting hepatic accumulation of immature ductular cells.

Beta-catenin deletion in hepatoblasts disrupts hepatic morphogenesis and survival during mouse development

Beta-catenin, the central component of the canonical Wnt pathway, plays important roles in the processes of liver regeneration, growth, and cancer. Previously, we identified temporal expression of beta-catenin during liver development. Here, we characterize the hepatic phenotype, resulting from the successful deletion of beta-catenin in the developing hepatoblasts utilizing Foxa3-cyclization recombination and floxed-beta-catenin (exons 2 through 6) transgenic mice. Beta-catenin loss in developing livers resulted in significantly underdeveloped livers after embryonic day 12 (E12) with lethality occurring at around E17 stages. Histology revealed an overall deficient hepatocyte compartment due to (1) increased cell death due to oxidative stress and apoptosis, and (2) diminished expansion secondary to decreased cyclin-D1 and impaired proliferation. Also, the remnant hepatocytes demonstrated an immature phenotype as indicated by high nuclear to cytoplasmic ratio, poor cell polarity, absent glycogen, and decreased expression of key liver-enriched transcription factors: CCAAT-enhancer binding protein-alpha and hepatocyte nuclear factor-4alpha. A paucity of primitive bile ducts was also observed. While the stem cell assays demonstrated no intrinsic defect in hematopoiesis, distorted hepatic architecture and deficient hepatocyte compartments resulted in defective endothelial cell organization leading to overall fetal pallor.

CONCLUSION

Beta-catenin regulates multiple, critical events during the process of hepatic morphogenesis, including hepatoblast maturation, expansion, and survival, making it indispensable to survival.

Isolation and characterization of a novel population of progenitor cells from unmanipulated rat liver

Widespread use of liver transplantation in the treatment of hepatic diseases is restricted by the limited availability of donated organs. One potential solution to this problem would be isolation and propagation of liver progenitor cells or stem cells. Here, we report on the isolation of a novel progenitor cell population from unmanipulated (that is, no prior exposure to chemicals and no injury) adult rat liver. Rat liver cells were cultured following a protocol developed in our laboratory to generate a unique progenitor cell population called liver-derived progenitor cells (LDPCs). LDPCs were analyzed by fluorescence-activated cell sorting, real-time polymerase chain reaction (RT-PCR), immunostaining and microarray gene expression. LDPCs were also differentiated into hepatocytes and biliary epithelium in vitro and examined for mature hepatic markers and urea and albumin production. These analyses showed that, LDPCs expressed stem cell markers such as cluster domain (CD)45, CD34, c-kit, and Thy 1, similar to hematopoietic stem cells, as well as endodermal/hepatic markers such as hepatocyte nuclear factor (HNF)3beta, hematopoietically-expressed homeobox gene-1, c-met, and transthyretin. LDPCs were negative for OV-6, cytokeratins (CKs), albumin, and HNF1alpha. The microarray gene expression profile demonstrated that they showed some similarities to known liver progenitor/stem cells such as oval cells. In addition, LDPCs differentiated into functional hepatocytes in vitro as shown by albumin expression and urea production. In conclusion, LDPCs are a population of unique liver progenitors that can be generated from unmanipulated adult liver, which makes them potentially useful for clinical applications, especially for cell transplantation in the treatment of liver diseases.

Sonic hedgehog is an autocrine viability factor for myofibroblastic hepatic stellate cells

BACKGROUND/AIMS

Factors released during liver injury, such as platelet derived growth factor-BB (PDGF), promote accumulation of myofibroblastic hepatic stellate cells (MFB) that drive the pathogenesis of cirrhosis. The hedgehog (Hh) pathway regulates remodeling of other injured tissues. This study evaluates the hypothesis that autocrine production of Sonic hedgehog (Shh) promotes MFB growth.

METHODS

Primary rat hepatic stellate cells (HSC) were treated without or with PDGF, a pharmacologic inhibitor of PDGF-regulated kinases, adenovirus expressing activated or dominant negative AKT, or Hh signaling inhibitors. Shh production, expression of Hh inhibitors and target genes, and HSC growth were assessed.

RESULTS

HSC expressed Shh, Hh pathway components, and the Hh inhibitor, Hip. During culture Hip expression fell, Shh production increased, and Hh target gene expression was induced. Neutralizing Shh antibodies promoted apoptosis. Adding PDGF increased Shh expression and MFB growth. Both processes followed activation of AKT and were abrogated by AKT inhibitors. Adenoviral delivery of activated AKT up-regulated Shh expression, demonstrating a direct role for AKT in regulating Shh expression. Shh-neutralizing antibodies and other Hh pathway inhibitors blocked the mitogenic effects of PDGF.

CONCLUSIONS

These results identify Shh as an autocrine growth factor for MFB and suggest a role for Hh signaling in the pathogenesis of cirrhosis.

Activation of stem cells in hepatic diseases

The liver has enormous regenerative capacity. Following acute liver injury, hepatocyte division regenerates the parenchyma but, if this capacity is overwhelmed during massive or chronic liver injury, the intrinsic hepatic progenitor cells (HPCs) termed oval cells are activated. These HPCs are bipotential and can regenerate both biliary epithelia and hepatocytes. Multiple signalling pathways contribute to the complex mechanism controlling the behaviour of the HPCs. These signals are delivered primarily by the surrounding microenvironment. During liver disease, stem cells extrinsic to the liver are activated and bone-marrow-derived cells play a role in the generation of fibrosis during liver injury and its resolution. Here, we review our current understanding of the role of stem cells during liver disease and their mechanisms of activation.

Stem cells in liver regeneration and therapy

The liver has adapted to the inflow of ingested toxins by the evolutionary development of unique regenerative properties and responds to injury or tissue loss by the rapid division of mature cells. Proliferation of the parenchymal cells, i.e. hepatocytes and epithelial cells of the bile duct, is regulated by numerous cytokine/growth-factor-mediated pathways and is synchronised with extracellular matrix degradation and restoration of the vasculature. Resident hepatic stem/progenitor cells have also been identified in small numbers in normal liver and implicated in liver tissue repair. Their putative role in the physiology, pathophysiology and therapy of the liver, however, is not yet precisely known. Hepatic stem/progenitor cells also known as "oval cells" in rodents have been implicated in liver tissue repair, at a time when the capacity for hepatocyte and bile duct replication is exhausted or experimentally inhibited (facultative stem/progenitor cell pool). Although much more has to be learned about the role of stem/progenitor cells in the physiology and pathophysiology of the liver, experimental analysis of the therapeutic value of these cells has been initiated. Transplantation of hepatic stem/progenitor cells or in vivo pharmacological activation of the pool of hepatic stem cells may provide novel modalities for the therapy of liver diseases. In addition, extrahepatic stem cells (e.g. bone marrow cells) are being investigated for their contribution to liver regeneration. Hepatic progenitor cells derived from embryonic stem cells are included in this review, which also discusses future perspectives of stem cell-based therapies for liver diseases.

What fires prometheus? The link between inflammation and regeneration following chronic liver injury

Liver progenitor cells (LPCs) play a major role in the regeneration process after chronic liver damage, giving rise to hepatocytes and cholangiocytes. Thus, they provide a cell-based therapeutic alternative to organ transplant, the current treatment of choice for end-stage liver disease. In recent years, much attention has focused on unravelling the cytokines and growth factors that underlie this response. Liver regeneration following acute damage is achieved by proliferation of mature hepatocytes; yet similar cytokines, most related to the inflammatory process, are implicated in both acute and chronic liver regeneration. Thus, many recent studies represent attempts to identify LPC-specific factors. This review summarises our current understanding of LPC biology with a particular focus on the liver inflammatory response being associated with the induction of LPCs in the liver. We will describe: (i) the pathways of liver regeneration following acute and chronic damage; (ii) the similarities and differences between the two pathways; (iii) the liver inflammatory environment; (iv) the unique features of liver immunology as well as (v) the interactions between liver immune cells and LPCs. Combining data from studies on the LPC-driven regeneration process with the knowledge in the field of liver immunology will improve our understanding of the LPC response and allow us to regulate these cells in vivo and in vitro for future therapeutic strategies to treat chronic liver disease.

Hepatic accumulation of Hedgehog-reactive progenitors increases with severity of fatty liver damage in mice

Progenitors regenerate fatty livers but the mechanisms involved are uncertain. The Hedgehog pathway regulates mesendodermal progenitors and modulates mesenchymal-epithelial interactions during tissue remodeling. To determine if Hedgehog signaling increases in liver progenitors during fatty liver injury, we compared expression of Hedgehog ligands and target genes across a spectrum of injury. Leptin-deficient ob/ob mice with fatty livers and their healthy lean littermates were studied before and after exposure to the hepatotoxin, ethionine. At baseline, ob/ob mice had greater liver damage than controls. Ethionine induced liver injury in both ob/ob and lean mice, with greater injury occurring in ob/ob mice. After ethionine, the ob/ob mice developed liver atrophy and fibrosis. Liver injury increased hepatic accumulation of progenitors, including ductular cells that produced and responded to Hedgehog ligands. A dose-response relationship was demonstrated between liver injury and expansion of Hedgehog-responsive progenitors. In severely damaged, atrophic livers, nuclei in mature-appearing hepatocytes accumulated the Hedgehog-regulated mesenchymal transcription factor, Gli2 and lost expression of the liver epithelial transcription factor, hepatocyte nuclear factor 6 (HNF-6). Hepatic levels of collagen mRNA and pericellular collagen fibrils increased concomitantly. Hence, fatty liver injury increases Hedgehog activity in liver progenitors, and this might promote epithelial-mesenchymal transitions that result in liver fibrosis.

Effects of ethanol on hepatic cellular replication and cell cycle progression

Ethanol is a hepatotoxin. It appears that the liver is the target of ethanol induced toxicity primarily because it is the major site of ethanol metabolism. Metabolism of ethanol results in a number of biochemical changes that are thought to mediate the toxicity associated with ethanol abuse. These include the production of acetaldehyde and reactive oxygen species, as well as an accumulation of nicotinamide adenine dinucleotide (NADH). These biochemical changes are associated with the accumulation of fat and mitochondrial dysfunction in the liver. If these changes are severe enough they can themselves cause hepatotoxicity, or they can sensitize the liver to more severe damage by other hepatotoxins. Whether liver damage is the result of ethanol metabolism or some other hepatotoxin, recovery of the liver from damage requires replacement of cells that have been destroyed. It is now apparent that ethanol metabolism not only causes hepatotoxicity but also impairs the replication of normal hepatocytes. This impairment has been shown to occur at both the G1/S, and the G2/M transitions of the cell cycle. These impairments may be the result of activation of the checkpoint kinases, which can mediate cell cycle arrest at both of these transitions. Conversely, because ethanol metabolism results in a number of biochemical changes, there may be a number of mechanisms by which ethanol metabolism impairs cellular replication. It is the goal of this article to review the mechanisms by which ethanol metabolism mediates impairment of hepatic replication.

Microarray analyses and molecular profiling of steatosis induction in immortalized human hepatocytes

Hepatic steatosis is an important risk factor for the development of inflammation, fibrosis and impaired liver regeneration. The factors regulating lipid accumulation and driving hepatic steatosis toward inflammation, fibrosis and impaired regeneration are largely unknown. The aim of this study was to identify major alterations in gene expression occurring in steatotic hepatocytes, and to analyze how these changes impact cellular processes associated with steatosis. Microarray gene chips and RT-PCR were performed to analyze changes in gene expression induced in fatty human immortalized hepatocytes after treatment with 50 muM oleic acid for 7 days. Lipid metabolism and triglyceride accumulation in these cells was examined by Oil-Red-O staining, thin-layer chromatography (TLC) and immunofluorescence. Caspase 3 activity, BrdU incorporation and trypan blue exclusion were used to study apoptosis, proliferation and cell viability. Finally, quantitative analysis of signalling induced by insulin was performed by Western blot. Characterization of steatosis in three hepatocyte-derived cell lines indicated that the immortalized human hepatocytes (IHH) line was the most appropriate cell line for this study. Gene expression analysis showed significant alterations in the transcription of two major classes of genes involved either in cholesterol and fatty acid biosynthesis, as well as lipid export, or in apoptosis and cell proliferation. Such changes were functionally relevant, since TLC indicated that synthesis and accumulation of triglycerides were increased in steatotic cells, while synthesis of cholesterol and fatty acids were decreased. Lipid accumulation in IHH was associated with an increased apoptosis and an inhibition of cell proliferation and viability. No detectable changes in genes associated with insulin resistance were observed in steatotic cells, but signalling induced by insulin was more efficient in steatotic IHH as compared to control cells. We conclude that IHH represent a new valuable model of steatosis, not associated with insulin resistance, to study at both the genetic and functional level factors involved in the process of lipid accumulation and steatosis-associated liver injury.

Progressive fibrosis in nonalcoholic steatohepatitis: association with altered regeneration and a ductular reaction

BACKGROUND & AIMS

Portal fibrosis and linkage is a key feature of progressive disease in nonalcoholic steatohepatitis (NASH), but not simple steatosis. It is underappreciated and poorly understood. Fatty liver has impaired regeneration that induces a secondary replicative pathway using bipotential, periportal, hepatic progenitor cells (HPCs). We propose that activation of this pathway, with increased cell injury in NASH, also induces a periportal ductular reaction (DR) that could produce a profibrogenic stimulus.

METHODS

Biopsy specimens from 107 patients with nonalcoholic fatty liver disease and 11 controls were immunostained with cytokeratin-7 to quantify the DR and HPCs, and with p21 to assess hepatocyte replicative arrest. These results were correlated with clinicopathologic variables.

RESULTS

Patients with nonalcoholic fatty liver disease had expansion of HPCs, with a strong association between HPCs and the DR (r(s) = 0.582, P < .0001). In those with NASH (n = 69) there was an increased DR compared with simple steatosis, which correlated with the stage of fibrosis (r(s) = 0.510, P < .0001). The DR increased with the grade of NASH activity (r(s) = 0.478, P < .0001), grade of portal inflammation (r(s) = 0.445, P < .0001), and extent of hepatocyte replicative arrest (r(s) = 0.446, P < .0001). Replicative arrest was in turn associated with insulin resistance (r(s) = 0.450, P < .0001) and NASH activity (r(s) = 0.452, P < .0001). By multivariate analysis, the extent of DR (odds ratio [OR] = 17.9, P = .016), hepatocyte ballooning (OR = 8.1, P < .0001), and portal inflammation (OR = 3.3, P = .005) were associated independently with fibrosis.

CONCLUSIONS

These findings suggest that an altered replication pathway in active NASH promotes a periportal DR, which in turn may provoke progressive periportal fibrogenesis.

Insulin resistance in hepatocytes and sinusoidal liver cells: mechanisms and consequences

Hepatic insulin resistance is an important underlying cause of the metabolic syndrome that manifests itself in diseases such as diabetes type II, atherosclerosis or non-alcoholic fatty liver disease (NAFLD). In this paper, we summarize comprehensively the current state of knowledge pertaining to the molecular mechanisms that lead to insulin resistance in hepatocytes and sinusoidal liver cells. In hepatocytes, the insulin resistant state is brought about by at least one, but more likely by a combination, of the following pathological alterations: hyperglycaemia and hyperinsulinaemia, formation of advanced glycation end-products, increased free fatty acids and their metabolites, oxidative stress and altered profiles of adipocytokines. Insulin resistance in hepatocytes distorts directly glucose metabolism, especially the control over glucose output into the circulation and interferes with cell survival and proliferation, while hepatic fatty acid synthesis remains stimulated by compensatory hyperinsulinaemia, resulting in steatosis. Very few studies have addressed insulin resistance in sinusoidal liver cells. These cells are not simply bystanders and passive witnesses of the changes affecting the hepatocytes. They are target cells that will respond to the pathological alterations occurring in the insulin resistant state. They are also effector cells that may exacerbate insulin resistance in hepatocytes by increasing oxidative stress and by secreting cytokines such as TNF and IL-6. Moreover, activation of sinusoidal endothelial cells, Kupffer cells and stellate cells will lead to chemo-attraction of inflammatory cells. Finally, activation of stellate cells will set in motion a fibrogenic response that paves the way to cirrhosis.

E-cadherin as a reliable cell surface marker for the identification of liver specific stem cells

Oval cells are liver-specific bipotent stem cells which accumulate in injured liver when proliferation of mature hepatocytes and/or cholangiocytes is impaired. They represent an intermediary cell type with phenotypical characteristics of both, hepatocytes and cholangiocytes. Oval cells express specific cell surface proteins allowing their identification in situ. Most of these cell surface proteins, however, are recognized by antibodies in mouse liver tissue that are not commercially available or work only on frozen sections. We show herein the unequivocal identification of oval cells in paraffin-embedded mouse liver samples based on strong E-cadherin expression different from that of hepatocytes and bile duct cells. By comparing the pattern of E-cadherin expression with that of both, A6-antigen and CD44, we suggest a tight control of E-cadherin expression depending on the differentiation stage of the progenitor cells. In human cirrhotic liver samples E-cadherin expression was found as a common feature of both, typical and atypical reactions, and, thus, can also serve as an indication of the progenitor cell compartment activation.

Analysis of liver repair mechanisms in Alagille syndrome and biliary atresia reveals a role for notch signaling

Patients with Alagille syndrome (AGS), a genetic disorder of Notch signaling, suffer from severe ductopenia and cholestasis, but progression to biliary cirrhosis is rare. Instead, in biliary atresia (BA) severe cholestasis is associated with a pronounced "ductular reaction" and rapid progression to biliary cirrhosis. Given the role of Notch in biliary development, we hypothesized that defective Notch signaling would influence the reparative mechanisms in cholestatic cholangiopathies. Thus we compared phenotype and relative abundance of the epithelial components of the hepatic reparative complex in AGS (n = 10) and BA (n = 30) using immunohistochemistry and computer-assisted morphometry. BA was characterized by an increase in reactive ductular and hepatic progenitor cells, whereas in AGS, a striking increase in intermediate hepatobiliary cells contrasted with the near absence of reactive ductular cells and hepatic progenitor cells. Hepatocellular mitoinhibition index (p21(waf1)/Ki67) was similar in AGS and BA. Fibrosis was more severe in BA, where portal septa thickness positively correlated with reactive ductular cells and hepatic progenitor cells. AGS hepatobiliary cells failed to express hepatic nuclear factor (HNF) 1beta, a biliary-specific transcription factor. These data indicate that Notch signaling plays a role in liver repair mechanisms in postnatal life: its defect results in absent reactive ductular cells and accumulation of hepatobiliary cells lacking HNF1beta, thus being unable to switch to a biliary phenotype.

Prevention of hepatic oxidative injury by Xiao-Chen-Chi-Tang in mice

The three purgative Cheng-Chi-Tang decoctions (CCTDs) including Ta-Cheng-Chi-Tang (TCCT), Xiao-Chen-Chi-Tang (XCCT), and Tiao-Wei-Chen-Chi-Tang (TWCCT) are used for treating gastrointestinal disorders, including liver diseases in traditional Chinese medicine. However, the underlying mechanisms as liver disease remedies are far from fully clarified. The objective of the study is to investigate and compare the antioxidant activity of the three purgative CCTDs in order to delineate their hepatic protective potential and mechanism. Antioxidant activity measured with the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging test indicated XCCT as the most potent preparation (IC(50) 8.94 microg/ml). In tert-butylhydroperoxide (TBH, 50mM)-induced lipid peroxidation in ICR mice liver homogenates, XCCT also showed stronger and dose-dependent inhibitory activity against TBH-induced malondialdehyde (MDA, a marker of lipid peroxidation) production (IC(50) 53.66 microg/ml). In addition, XCCT showed dose-dependent protective effect against TBH-induced cytotoxicity in normal human Chung liver cells Furthermore, in carbon tetrachloride (CCl(4))-induced acute liver injury model, mice pretreated with 0.2g/kg and 0.4 g/kg of XCCT extracts showed a decrease of 59.8 and 43.1% in serum glutamic oxaloactetic transaminase (GOT) level, 51.4 and 52% in glutamic pyruvate transaminase (GPT) level, along with a reduction of 31 and 15% in MDA level, respectively, similar to the effects exerted by silymarin. XCCT pretreated mice also showed milder necrotic changes in the microscopic picture of the liver. The results suggest that XCCT has significant antioxidant activity and hepatic protection potential.

Alcoholic liver disease

Alcohol excess is associated with a spectrum of disease ranging from simple steatosis through steatohepatitis to cirrhosis and, in some, hepatocellular carcinoma. Alcoholic steatohepatitis itself has a variable histological picture, but a constant feature is the presence of ballooning degeneration of hepatocytes. Recent studies have emphasized the importance of apoptosis as a mechanism of cell death in this condition. It is accompanied by varying degrees of perivenular, centrilobular, and pericellular fibrosis. When severe and associated with perivenular liver cell necrosis (central sclerosing hyaline necrosis), there may be precirrhotic portal hypertension. The pattern of fibrosis may initially be diffuse with little nodule formation, but in time there is frequently the development of a micronodular cirrhosis. In approximately 15% of patients with established cirrhosis, hepatocellular carcinoma develops; several precursor lesions are now recognized which can be detected histologically. Several authors have drawn attention to additional components of the spectrum of alcoholic liver disease, including vascular changes, portal tract inflammation and fibrosis, ductular reaction, and iron overload. The morphology of alcoholic liver disease can be significantly affected by abstinence; furthermore, the clinical and morphological phenotype can be significantly influenced by the presence of comorbid conditions such as nonalcoholic fatty liver disease or viral hepatitis. Biopsy appearances can provide important prognostic information in alcoholic liver disease, and this review incorporates a proposed grading and staging schema for assessment of histological severity.

The role of stem cells in physiology, pathophysiology, and therapy of the liver

The objectives of the present review is to update readers with the rapidly changing concepts in liver stem cell biology and related clinical applications. The liver has adapted to the inflow of ingested toxins by the evolutionary development of unique regenerative properties and responds to injury or tissue loss by rapid division of the mature cells, hepatocytes, and bile duct epithelial cells. Proliferation of the parenchymal cells is regulated by numerous cytokine/growth factor-mediated pathways and is timely synchronized with extracellular matrix degradation and the restoration of the vasculature. The putative role of stem cells in physiology, pathophysiology, and therapy is not yet precisely known but currently is under intensive investigation. Resident hepatic stem/ progenitor cells have been identified in small numbers and implicated in liver tissue repair, when hepatocyte and bile duct replication capacity is exhausted or experimentally inhibited. Several independent reports have suggested that bone marrow cells can give rise to different hepatic epithelial cells types, including hepatic stem cells, hepatocytes, and bile duct epithelium. These observations have resulted in the hypothesis that extrahepatic stem cells, specifically bone marrow-derived stem cells, are an important source for liver epithelial cell replacement, particularly during chronic injury. Most of published data, however, now suggest that they do not play a relevant role in replacement of epithelial cells in any known form of hepatic injury. In vitro differentiation protocols for various adult extrahepatic stem cells might eventually provide valuable sources of cells for transplantation and therapy. Amniotic epithelial stem cells, fetal liver progenitor cells as well as embryonic stem cells currently emerge as alternative stem cell sources and open new possibilities for cellular therapies of liver disease.

Steatosis as a cofactor in other liver diseases: hepatitis C virus, alcohol, hemochromatosis, and others

As obesity prevalence rises, there is evidence that fatty liver disease can act synergistically with other chronic liver diseases to aggravate parenchymal injury. This is characterized best in chronic hepatitis C, where steatosis is caused by viral and metabolic effects. There is evidence that steatosis and its metabolic abnormalities also exacerbate other diseases, such as alcoholic liver disease, hemochromatosis, and, possibly, drug-induced liver disease. The pathogenesis seems related to increased susceptibility of steatotic hepatocytes to apoptosis, enhanced oxidative injury, and altered hepatocytic regeneration. Data suggest that active management of obesity may improve liver injury and decrease the progression of fibrosis in patients who have other chronic liver diseases.

Liver repair by intra- and extrahepatic progenitors

Despite its remarkable capacity for endogenous regeneration, the mammalian liver is vulnerable to a number of chronic or acute conditions that exceed or circumvent the proliferative capabilities of its mature cell complement. Bipotential hepatic progenitors, or "oval cells," have been shown to contribute to organ regeneration under such circumstances, both in human patients and in animal models. These progenitors are attractive agents for cell therapy, but have thus far proven challenging to isolate and manipulate. New reports indicating that transplanted bone marrow cells (BMCs) can also generate hepatocytes and contribute to liver repair have attracted considerable attention, because these cells are familiar and accessible to both clinicians and scientists. Recently, the issue of whether nuclear transfer (via cell fusion between donor BMC and recipient hepatocyte) or previously unrecognized differentiation potential (i.e., plasticity/transdifferentiation of BMC) is the primary origin of donor-derived hepatocytes has generated considerable controversy. In the liver, most evidence supports cell fusion as the key agent in the reversal of hepatopathology. However, regardless of their origin, the frequency of hepatocyte correction events is low. As is the case for the delivery of intrahepatic progenitors, substantial improvements in the understanding of this process will be needed before clinical application becomes practical.

Proliferating cholangiocytes: a neuroendocrine compartment in the diseased liver

In the last 15 years, the intrahepatic biliary tree has become the object of extensive studies, which highlighted the extraordinary biologic properties of cholangiocytes involved in bile formation, proliferation, injury repair, fibrosis, angiogenesis, and regulation of blood flow. Proliferation is a "typical" property of cholangiocytes and is key as a mechanism of repair responsible for maintaining the integrity of the biliary tree. Cholangiocyte proliferation occurs virtually in all pathologic conditions of liver injury where it is associated with inflammation, regeneration, and repair, thus conditioning the evolution of liver damage. Interestingly, proliferating cholangiocytes acquire the phenotype of neuroendocrine cells, and secrete different cytokines, growth factors, neuropeptides, and hormones, which represent potential mechanisms for cross talk with other liver cells. Many studies suggest the generation of a neuroendocrine compartment in the injured liver, mostly constituted by cells with cholangiocyte features, which functionally conditions the progression of liver disease. These insights on cholangiocyte pathophysiology will provide new potential strategies for the management of chronic liver diseases. The purpose of this review is to summarize the recent findings on the mechanisms regulating cholangiocyte proliferation and the significance of the neuroendocrine regulation of cholangiocyte biology.

Liver stem cells and molecular signaling pathways in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most lethal cancers. Surgical intervention is the only curative option, with only a small fraction of patients being eligible. Conventional chemotherapy and radiotherapy have not been effective in treating this disease, thus leaving patients with an extremely poor prognosis. In viral, alcoholic, and other chronic hepatitis, it has been shown that there is an activation of the progenitor/stem cell population, which has been found to reside in the canals of Hering. In fact, the degree of inflammation and the disease stage have been correlated with the degree of activation. Dysregulation of key regulatory signaling pathways such as transforming growth factor-beta/transforming growth factor-beta receptor (TGF-beta/TBR), insulin-like growth factor/IGF-1 receptor (IGF/IGF-1R), hepatocyte growth factor (HGF/MET), Wnt/beta-catenin/FZD, and transforming growth factor-alpha/epidermal growth factor receptor (TGF-alpha/EGFR) in this progenitor/stem cell population could give rise to HCC. Further understanding of these key signaling pathways and the molecular and genetic alterations associated with HCC could provide major advances in new therapeutic and diagnostic modalities.

Histological assessment of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is an important complication of the metabolic syndrome, which is becoming an increasingly common cause of chronic liver disease. Histological changes typically mainly affect perivenular regions of the liver parenchyma and include an overlapping spectrum of steatosis, steatohepatitis and persinusoidal or pericellular fibrosis, in some cases leading to cirrhosis. Once cirrhosis has developed, typical hepatocellular changes are often no longer conspicuous, leading to such cases being mistakenly diagnosed as 'cryptogenic'. Portal inflammation, ductular reaction and periportal fibrosis can also be seen as part of the morphological spectrum of NAFLD, particularly in the paediatric population. Hepatocellular carcinoma has also been described as a complication of NAFLD-associated cirrhosis. NAFLD is also an important cofactor in other chronic liver diseases, especially hepatitis C. Histological assessments have an important role to play in the diagnosis and management of NAFLD. These include making the potentially important distinction between simple steatosis and steatohepatitis and providing pointers to the aetiology, including cases where a dual pathology exists. A number of systems have been devised for grading and staging the severity of fatty liver disease. These require further evaluation, but have a potentially important role to play in determining prognosis and monitoring therapeutic responses.

Defective hepatic regeneration after partial hepatectomy in leptin-deficient mice is not rescued by exogenous leptin

Liver regeneration after partial hepatectomy (PH) is impaired in leptin-deficient ob/ob mice. Here, we tested whether exogenous leptin and/or correction of the obese phenotype (by food restriction or long-term leptin administration) would rescue hepatocyte proliferation and whether the hepatic progenitor cell compartment was activated in leptin-deficient ob/ob livers after PH. Because of the high mortality following 70% PH to ob/ob mice, we performed a less extensive (55%) resection. Compared to lean mice, liver regeneration after 55% PH was deeply impaired and delayed in ob/ob mice. Administration of exogenous leptin to ob/ob mice at doses that restored circulating leptin levels during the surgery and postsurgery period or for 3 weeks prior to the surgical procedure did not rescue defective liver regeneration. Moreover, correction of obesity, metabolic syndrome and hepatic steatosis by prolonged administration of leptin or food restriction (with or without leptin replacement at the time of PH) did not improve liver regeneration in ob/ob mice. The hepatic progenitor cell compartment was increased in ob/ob mice. However, after PH, the number of progenitor cells decreased and signs of proliferation were absent from this cell compartment. In this study, we have conclusively shown that neither leptin replacement nor amelioration of the metabolic syndrome, obese phenotype and hepatic steatosis, with or without restitution of normal circulating levels of leptin, was able to restore replicative competence to ob/ob livers after PH. Thus, leptin does not directly signal to liver cells to promote hepatocyte proliferation, and the obese phenotype is not solely responsible for impaired regeneration.

Evidence for epithelial-mesenchymal transitions in adult liver cells

Both myofibroblastic hepatic stellate cells (HSC) and hepatic epithelial progenitors accumulate in damaged livers. In some injured organs, the ability to distinguish between fibroblastic and epithelial cells is sometimes difficult because cells undergo epithelial-mesenchymal transitions (EMT). During EMT, cells coexpress epithelial and mesenchymal cell markers. To determine whether EMT occurs in adult liver cells, we analyzed the expression profile of primary HSC, two HSC lines, and hepatic epithelial progenitors. As expected, all HSC expressed HSC markers. Surprisingly, these markers were also expressed by epithelial progenitors. In addition, one HSC line expressed typical epithelial progenitor mRNAs, and these epithelial markers were inducible in the second HSC line. In normal and damaged livers, small ductular-type cells stained positive for an HSC marker. In conclusion, HSC and hepatic epithelial progenitors both coexpress epithelial and mesenchymal markers, providing evidence that EMT occurs in adult liver cells.

The clinicopathological and prognostic relevance of cytokeratin 7 and 19 expression in hepatocellular carcinoma. A possible progenitor cell origin

AIMS

Cytokeratin (CK) 7 and CK19 expression, present in hepatic progenitor cells (HPCs) and in cholangiocytes but not in normal hepatocytes, has been reported in some hepatocellular carcinomas (HCCs); however, the incidence and relevance of this expression in HCC in Caucasians is not known. Therefore, our aim was to study the occurrence and clinicopathological characteristics of HCC expressing CK7 and/or CK19 in 109 Caucasian patients.

METHODS AND RESULTS

The expression of hepatocellular differentiation markers (Hepar, canalicular polyclonal carcinoembryonic antigen), biliary/progenitor cell markers (CK7, CK19), alpha-fetoprotein (AFP), p53 and beta-catenin in HCC was semiquantitatively assessed by immunohistochemistry. Of 109 HCCs, 78 were CK7-/CK19- (72%), 13 CK7+/CK19- (12%), seven CK7-/CK19+ (6%), 11 CK7+/CK19+ (10%). CK19 expression was significantly associated with elevated serum AFP (400 ng/ml) (P = 0.023), tumour AFP expression (P < 0.0001), presence in serum of anti-hepatitis B core (P = 0.016), less fibrosis in non-neoplastic parenchyma (P = 0.009) and less nuclear beta-catenin expression (P = 0.021). CK7 expression was significantly associated with elevated serum bilirubin (> 2 mg/dl) (P = 0.0005) and less nuclear beta-catenin expression (P = 0.003). HCC expressing CK19 had a higher rate of recurrence (P = 0.009, hazard ratio 12.5, n = 31) after liver transplantation compared with CK19- tumours.

CONCLUSIONS

In our series, 28% of HCCs contained cells expressing CK7 and/or CK19. They potentially derive from HPCs. The higher recurrence rate of CK19+ HCC after transplantation suggests a worse prognosis for these HCCs compared with CK19- HCC.

Interleukin-15 increases hepatic regenerative activity

BACKGROUND/AIMS

Interleukin-15 (IL-15) is expressed in many organs. It generally inhibits apoptosis and increases cellular proliferation and differentiation. However, IL-15's roles in liver are unknown. We aimed to determine if IL-15 influences hepatic integrity and regenerative activity.

METHODS

Expression of IL-15 and its receptors was evaluated in several liver injury models, primary hepatocytes, and two liver cell lines. Effects of IL-15 on viability, proliferation, and apoptosis were assessed in cultured liver cells, and also in the livers of healthy mice.

RESULTS

IL-15 and its receptors are expressed constitutively in healthy livers, and ligand expression is induced in injured livers. Cultured primary hepatocytes and liver cell lines express IL-15 and its receptors. Administration of IL-15 has minimal effects on cultured liver cells, but significantly up-regulates oval cell accumulation, cyclin mRNA expression, and mature hepatocyte replication in healthy mice. These effects are associated with focal hepatic inflammation and increased expression of TNF-alpha and IFN-gamma, but not with increased cell death or aminotransferase release.

CONCLUSIONS

IL-15 expression increases during liver injury and IL-15 treatment induces a wound healing-type response in healthy adult mice. These findings suggest that IL-15 may contribute to regenerative activity in damaged liver.

Liver stem cells and their implication in hepatocellular and cholangiocarcinoma

In the liver, several cell types have the longevity that is needed to be the cell of origin of a cancer: hepatocytes, cholangiocytes and progenitor cells. The latter are located in the most peripheral branches of the biliary tree, the ductules and canals of Hering. The most important risk factors for liver cancer are chronic viral hepatitis B and C and alcoholic and non-alcoholic steatohepatitis. In these and other chronic liver diseases, progenitor cell activation is seen, rendering them a target cell population for carcinogenesis. The degree of activation is positively correlated with the inflammatory activity and the stage of the disease. Recently, it has been shown that in the cirrhotic stage of most chronic liver diseases, the hepatocytes become senescent owing to telomere shortening. This makes it even more plausible that at least part of the hepatocellular carcinomas originate from a progenitor cell. Hepatocellular carcinomas expressing progenitor cell/ductular markers like cytokeratin 19 have a more aggressive clinical course. It is therefore important to recognize this entity.

Pathogenesis of chronic liver injury and hepatocellular carcinoma in alpha-1-antitrypsin deficiency

Alpha-1-antitrypsin (AT) deficiency is the most common genetic cause of liver disease in children. In addition to chronic liver inflammation and injury, it has a predilection to cause hepatocellular carcinoma later in life. The deficiency is caused by a mutant protein, ATZ, which is retained in the endoplasmic reticulum (ER) in a polymerized form rather than secreted into the blood in its monomeric form. The histologic hallmark of the disease is ATZ-containing globules in some, but not all, hepatocytes. Liver injury results from a gain-of-toxic function mechanism in which mutant ATZ retained in the ER initiates a series of pathologic events, but little is known about the mechanism by which this leads to carcinogenesis. Several recent observations from my laboratory have led to a novel hypothetical paradigm for carcinogenesis in AT deficiency in which globule-containing hepatocytes are "sick," relatively growth suppressed, but also elaborating trans-acting regenerative signals. These signals are received and transduced by globule-devoid hepatocytes, which, because they are younger and have a lesser load of accumulated ATZ, have a selective proliferative advantage. Chronic regeneration in the presence of tissue injury leads to adenomas and ultimately carcinomas. Aspects of this hypothetical paradigm may also explain the proclivity for hepatocarcinogenesis in other chronic liver diseases, including other genetic diseases, viral hepatitis, and nonalcoholic steatohepatitis.

Hedgehog signaling maintains resident hepatic progenitors throughout life

Hedgehog signaling through its receptor, Patched, activates transcription of genes, including Patched, that regulate the fate of various progenitors. Although Hedgehog signaling is required for endodermal commitment and hepatogenesis, the possibility that it regulates liver turnover in adults had not been considered because mature liver epithelial cells lack Hedgehog signaling. Herein, we show that this pathway is essential throughout life for maintaining hepatic progenitors. Patched-expressing cells have been identified among endodermally lineage-restricted, murine embryonic stem cells as well as in livers of fetal and adult Ptc-lacZ mice. An adult-derived, murine hepatic progenitor cell line expresses Patched, and Hedgehog-responsive cells exist in stem cell compartments of fetal and adult human livers. In both species, manipulation of Hedgehog activity influences hepatic progenitor cell survival. Therefore, Hedgehog signaling is conserved in hepatic progenitors from fetal development through adulthood and may be a new therapeutic target in patients with liver damage.

Progenitor cells in liver regeneration: molecular responses controlling their activation and expansion

Although normally quiescent, the adult mammalian liver possesses a great capacity to regenerate after different types of injuries in order to restore the lost liver mass and ensure maintenance of the multiple liver functions. Major players in the regeneration process are mature residual cells, including hepatocytes, cholangiocytes and stromal cells. However, if the regenerative capacity of mature cells is impaired by liver-damaging agents, hepatic progenitor cells are activated and expand into the liver parenchyma. Upon transit amplification, the progenitor cells may generate new hepatocytes and biliary cells to restore liver homeostasis. In recent years, hepatic progenitor cells have been the subject of increasing interest due to their therapeutic potential in numerous liver diseases as alternative or supportive/complementary tools to liver transplantation. While the first investigations on hepatic progenitor cells have focused on their origin and phenotypic characterization, recent attention has focused on the influence of the hepatic microenvironment on their activation and proliferation. This microenvironment comprises the extracellular matrix, epithelial and non-epithelial resident liver cells, and recruited inflammatory cells as well as the variety of growth-modulating molecules produced and/or harboured by these elements. The cellular and molecular responses to different regenerative stimuli seem to depend on the injury inflicted and consequently on the molecular microenvironment created in the liver by a certain insult. This review will focus on molecular responses controlling activation and expansion of the hepatic progenitor cell niche, emphasizing similarities and differences in the microenvironments orchestrating regeneration by recruitment of progenitor cell populations or by replication of mature cells.

Claudin-4 differentiates biliary tract cancers from hepatocellular carcinomas

The recently identified claudins are dominant components of tight junctions, responsible for cell adhesion, polarity and paracellular permeability. Certain claudins have been shown to have relevance in tumor development, with some of them, especially claudin-4, even suggested as future therapeutic target. The aim of the present study was to analyze the expression of claudin-4 in the biliary tree, biliary tract cancers and hepatocellular carcinomas. A total of 107 cases were studied: 53 biliary tract cancers, 50 hepatocellular carcinomas, 10 normal liver and 10 normal extrahepatic biliary duct samples. Immunohistochemical analysis was performed on conventional specimens and on tissue microarrays as well. Claudin-4 was further investigated by Western blot analysis and real-time RT-PCR. Intense membranous immunolabeling was found for claudin-4 in all biliary tract cancers unrelated to the primary site of origin, namely intrahepatic, extrahepatic or gallbladder cancers. Normal biliary epithelium showed weak positivity for claudin-4. In contrast, normal hepatocytes and tumor cells of hepatocellular carcinomas did not express claudin-4. The results of Western immunoblot analysis and real-time RT-PCR were in correlation with the immunohistochemical findings. Cytokeratins, as CK7 (92%) and CK19 (83%) were mostly positive in biliary tract cancers, however, one-third of hepatocellular carcinomas also expressed CK7 (34%). HSA antibody (HepPar1) reacted with the majority of hepatocellular carcinomas (86%), while being positive in a low percentage of the biliary tract cancers (8%). In conclusion, this is the first report of a significantly increased claudin-4 expression in biliary tract cancers, which represents a novel feature of tumors of biliary tract origin. Claudin-4 expression seems to be a useful marker in differentiating biliary tract cancers from hepatocellular carcinomas and could well become a potential diagnostic tool.

The sources of parenchymal regeneration after chronic hepatocellular liver injury in mice

After liver injury, parenchymal regeneration occurs through hepatocyte replication. However, during regenerative stress, oval cells (OCs) and small hepatocyte like progenitor cells (SHPCs) contribute to the process. We systematically studied the intra-hepatic and extra-hepatic sources of liver cell replacement in the hepatitis B surface antigen (HBsAg-tg) mouse model of chronic liver injury. Female HBsAg-tg mice received a bone marrow (BM) transplant from male HBsAg-negative mice, and half of these animals received retrorsine to block indigenous hepatocyte proliferation. Livers were examined 3 and 6 months post-BM transplantation for evidence of BM-derived hepatocytes, OCs, and SHPCs. In animals that did not receive retrorsine, parenchymal regeneration occurred through hepatocyte replication, and the BM very rarely contributed to hepatocyte regeneration. In mice receiving retrorsine, 4.8% of hepatocytes were Y chromosome positive at 3 months, but this was frequently attributable to cell fusion between indigenous hepatocytes and donor BM, and their frequency decreased to 1.6% by 6 months, as florid OC reactions and nodules of SHPCs developed. By analyzing serial sections and reconstructing a 3-dimensional map, continuous streams of OCs could be seen that surrounded and entered deep into the nodules of SHPCs, connecting directly with SHPCs, suggesting a conversion of OCs into SHPCs. In conclusion, during regenerative stress, the contribution to parenchymal regeneration from the BM is minor and frequently attributable to cell fusion. OCs and SHPCs are of intrinsic hepatic origin, and OCs can form SHPC nodules.

Chronic liver disease is triggered by taurine transporter knockout in the mouse

Taurine is an abundant organic osmolyte with antioxidant and immunomodulatory properties. Its role in the pathogenesis of chronic liver disease is unknown. The liver phenotype was studied in taurine transporter knockout (taut-/-) mice. Hepatic taurine levels were ~21, 15 and 6 mumol/g liver wet weight in adult wild-type, heterozygous (taut+/-) and homozygous (taut-/-) mice, respectively. Immunoelectronmicroscopy revealed an almost complete depletion of taurine in Kupffer and sinusoidal endothelial cells, but not in parenchymal cells of (taut-/-) mice. Compared with wild-type mice, (taut-/-) and (taut+/-) mice developed moderate unspecific hepatitis and liver fibrosis with increased frequency of neoplastic lesions beyond 1 year of age. Liver disease in (taut-/-) mice was characterized by hepatocyte apoptosis, activation of the CD95 system, elevated plasma TNF-alpha levels, hepatic stellate cell and oval cell proliferation, and severe mitochondrial abnormalities in liver parenchymal cells. Mitochondrial dysfunction was suggested by a significantly lower respiratory control ratio in isolated mitochondria from (taut-/-) mice. Taut knockout had no effect on taurine-conjugated bile acids in bile; however, the relative amount of cholate-conjugates acid was decreased at the expense of 7-keto-cholate-conjugates. In conclusion, taurine deficiency due to defective taurine transport triggers chronic liver disease, which may involve mitochondrial dysfunction.

Effects of compound from Chinese medicine on oxidation and anti-oxidation system of rat liver with steatohepatitis induced by high-fat diets

AIM: To explore the action of oxygen stress and lipid peroxidation in the establishment of steatohepatitis and the action mechanisms of compound from Chinese medicine (CCM) in the treatment of steatohepatitis in rats fed by high-fat diets.

METHODS: Sixty male Sprague Dawley rats were randomly assigned into 6 groups. The rats fed by normal food served as the controls. The rats in model groups (Ⅰ, Ⅱ) were fed with high-fat diets, and those in medication Ⅰ and Ⅱ group were given CCM 9 and 13 wk after high-fat diets feeding. The rats in diet therapeutic group were fed with normal food 13 wk after high-fat diet feeding. The animal in control, model Ⅰ, and medication Ⅰ group were killed at 12 wk, and the rest ones were killed at 16 wk. Blood sample were collected for the detection of serum aspartate transaminase (AST), alanine aminotransferase (ALT), and glutathione perioxidase (GSH-PX), and liver tissues were obtained for the detection of malondialdehyde (MDA) contents, superoxide dismutase (SOD) activity, total antioxidative capacity (T-AOC), nitric exide (NO), inducible nitric oxide synthase (iNOS) and glutathione (GSH) levels. The histological changes were observed under light microscope.

RESULTS: Pure fatty liver was formed in the model rats at 12, and fatty hepatitis was established at 16 wk. In comparison with those of normal rats, the contents of MDA were increased (6.45±1.07, 8.38±1.32 µmol/g vs 5.08±0.91 µmol/g, P <0.01) but anti-oxide SOD (171±14, 148±26 kNU/gt vs 198±25 kNU/gt, P <0.05 and P <0.01), GSH (40.8±5.1, 35.0±9.0 mg/g vs 48.5±7.6 mg/g, P <0.05 and P <0.01), GSH-PX (11.2±1.5, 10.3±1.8 µmol/L vs 16.4±3.7 µmol/L, P <0.01) contents were decreased significantly. The severity degree of hepatic fatty degeneration aggravated with the prolonging of the high-fat diet given time. In comparison with those of model rats, the markers such as MDA, SOD, GSH, and GSH-PX etc. were all significantly improved in the medication rats (P <0.05 or P <0.01), and the degree of fatty degeneration was also alleviated. Light or moderate fatty degeneration was observed in diet therapeutic rats, and the relative markers were not notably different from those of model group.

CONCLUSION: Oxygen stress and lipid peroxidation play a key role in the establishment of nonalcoholic steatohepatitis in rats, and the compound of Chinese medince has therapeutic effect on nonalcoholic steatohepatitis by anti-lipid peroxidation.

Role of inflammation in nonalcoholic steatohepatitis

PURPOSE OF REVIEW

Products of hepatic macrophages and lymphocytes are acknowledged regulators of liver injury and repair. Recent studies have identified inflammatory modulators from sources within and outside the liver that are critical to the pathogenesis and progression of chronic liver diseases, including nonalcoholic fatty liver disease. This review will focus on these developments to clarify how inflammatory mediators from adipose tissue and the liver interact to mediate the pathogenesis of nonalcoholic fatty liver disease.

RECENT FINDINGS

Hepatic steatosis and steatohepatitis are extremely prevalent in obese individuals with the metabolic syndrome. The metabolic syndrome results from abnormal production of various adipose-derived and liver-derived factors that modulate energy substrate flux to coordinate tissue anabolism and catabolism. Individuals with the metabolic syndrome produce a relative excess of proinflammatory factors. Some factors inhibit hepatic fat disposal and promote lipid accumulation within hepatocytes. The latter induces sustained hepatic generation of proinflammatory cytokines, particularly when the hepatic innate immune system becomes Th-1 polarized. Although chronic inflammation induces production of various profibrogenic factors, progression to latter stages of nonalcoholic fatty liver disease is relatively unusual in individuals with the metabolic syndrome. This may reflect requirements for additional factors that become abundant only in individuals who have additional defects in hepatic innate immunity.

SUMMARY

Obesity and the metabolic syndrome represent chronic inflammatory states and are associated with nonalcoholic fatty liver disease. Liver injury that ensues is dictated by metabolic and immunomodulatory factors that are produced by adipose tissue and within the liver.

Increased lipopolysaccharide sensitivity in alcoholic fatty livers is independent of leptin deficiency and toll-like receptor 4 (TLR4) or TLR2 mRNA expression

BACKGROUND

Both alcoholic (AFL) and nonalcoholic (NAFL) fatty livers show increased sensitivity to endotoxin-induced injury. Lipopolysaccharide (LPS) is recognized by toll-like receptor 4 (TLR4), whereas lipopeptide triggers TLR2 to induce common downstream activation of nuclear factor (NF)-kappaB and pro-inflammatory pathways that are activated in AFL and NAFL.

METHODS

Serum alanine aminotransferase (ALT), tumor necrosis factor (TNF)-alpha, and interleukin (IL)-6 levels; hepatic NF-kappaB activity; and expression of TLR2, TLR4, inducible nitric oxide synthase (iNOS), and heme oxygenase (HO)-1 mRNAs were investigated in lean and leptin-deficient ob/ob mice after LPS challenge in combination with acute or chronic alcohol feeding.

RESULTS

Increased LPS sensitivity in AFL and NAFL was characterized by elevated serum TNF-alpha and IL-6 induction. However, there was no difference in TLR2 and TLR4 mRNA levels between lean and ob/ob livers at baseline and after acute or chronic alcohol treatment. LPS increased TLR2, but not TLR4, mRNA levels in all groups. Chronic alcohol feeding and LPS increased serum ALT and TNF-alpha levels in lean but not in ob/ob mice compared with pair-fed controls. Hepatic NF-kappaB activation was increased in both ob/ob and lean mice after chronic alcohol feeding compared with pair-fed controls. Expression of iNOS, an inducer of oxidative stress, and HO-1, a cytoprotective protein, were higher in ob/ob compared with lean mice after chronic alcohol feeding. However, LPS-induced HO-1, but not iNOS, expression was attenuated in ob/ob compared with lean mice.

CONCLUSION

These results imply that the increased sensitivity of AFL to LPS occurs without up-regulation of TLR2 or TLR4 genes and may be related to an imbalance of pro-inflammatory/oxidative and cytoprotective mechanisms.

[Which stem cells for adult liver?]

While hepatocytes can be considered conceptually as unipotent stem cells, the presence of true stem or progenitor cells within adult livers has been largely debated. It is now accepted that the atypical ductular reaction observed in livers with sub-massive hepatitis represents the proliferation of hepatic progenitor cells similar to rat oval cells and able to differentiate towards the biliary and the hepatocytic lineage through intermediate progeny. In the normal liver, the identification of progenitor cells with a panel of markers including c-kit, CD34, Ov6, CK7, CK19, chromogranine A, CD56 remains difficult because these cells are very few and most of the markers are not specific. These progenitor cells could be located either within the canals of Hering or in periductular situation or both. Mechanisms leading to the activation and the proliferation of hepatic progenitor cells are still largely unknown: they involve growth factors as the stem cell factor, ligand of c-kit, cytokines, chemokines as SDF1 a and vagal or sympathetic innervatioñ. Other potential stem cells for liver could be hematopoietic stem cells from bone marrow. First publications have showed that hematopoietic stem cells were able to differentiate into hepatocytes and cholangiocytes and to yield high level engraftment of injured livers. However it appears now that this phenomenon is minimal or even absent in physiological and usual pathological conditions. It does occur in extreme experimental conditions either by true transdifferentiation or cell fusion. The shared property of stem cells and tumor cells to proliferate endlessly, rises the question of the potential role of progenitor cells in liver carcinogenesis. In a number of animal models of hepatocarcinogenesis, tumors originate from oval cells. The identification of progenitor cells close to murine oval cells in the human liver raises the hypothesis of a potential role of these cells in the development of human liver tumors. Liver progenitor cells have been identified morphologically and phenotypically in dysplastic foci of cirrhotic livers and hepatocellular adenomas. More generally speaking, typical hepatocellular carcinomas and cholangiocarcinomas are at the two ends of a spectrum which includes transitional-type tumors intermediate between hepatocellular carcinoma and cholangiocarcinoma and combined hepato-cellular cholangiocarcinoma; these intermediate and combined types can be more easily explained as deriving from progenitor cells. Despite the difficulties, the doubts and the potential dangers, new experimental modalities to obtain efficient repopulation of the liver from bone marrow stem cells are currently under study: exogenous administration of cytokines and chemokines involved in cell homing and differentiation or development of selective pressure strategies. Other cell types as intra-hepatic progenitor cells, bone marrow multipotent adult progenitor cells (MAPCs) or fetal hepatocytes could be alternative sources for liver cell therapy. Thus, progressing knowledge about stem cells in adult liver would allow to better understand mechanisms of hepatic homeostasia and regeneration and would open the way to cell-based therapy for liver diseases.

Steatosis: co-factor in other liver diseases

The prevalence of fatty liver is rising in association with the global increase in obesity and type 2 diabetes. In the past, simple steatosis was regarded as benign, but the presence of another liver disease may provide a synergistic combination of steatosis, cellular adaptation, and oxidative damage that aggravates liver injury. In this review, a major focus is on the role of steatosis as a co-factor in chronic hepatitis C (HCV), where the mechanisms promoting fibrosis and the effect of weight reduction in minimizing liver injury have been most widely studied. Steatosis, obesity, and associated metabolic factors may also modulate the response to alcohol- and drug-induced liver disease and may be risk factors for the development of hepatocellular cancer. The pathogenesis of injury in obesity-related fatty liver disease involves a number of pathways, which are currently under investigation. Enhanced oxidative stress, increased susceptibility to apoptosis, and a dysregulated response to cellular injury have been implicated, and other components of the metabolic syndrome such as hyperinsulinemia and hyperglycemia are likely to have a role. Fibrosis also may be increased as a by-product of altered hepatocyte regeneration and activation of bipotential hepatic progenitor cells. In conclusion, active management of obesity and a reduction in steatosis may improve liver injury and decrease the progression of fibrosis.

Fibrosis correlates with a ductular reaction in hepatitis C: roles of impaired replication, progenitor cells and steatosis

The mechanisms for progressive fibrosis and exacerbation by steatosis in patients with chronic hepatitis C (HCV) are still unknown. We hypothesized that proliferative blockade in HCV-infected and steatotic hepatocytes results in the default activation of hepatic progenitor cells (HPC), capable of differentiating into both biliary and hepatocyte lineages, and that the resultant ductular reaction promotes portal fibrosis. To study this concept, 115 liver biopsy specimens from subjects with HCV were scored for steatosis, inflammation, and fibrosis. Biliary epithelium and HPC were decorated by cytokeratin 7 immunoperoxidase, and the replicative state of hepatocytes was assessed by p21 and Ki-67 immunohistochemistry. A ductular reaction at the portal interface was common. There was a highly significant correlation between the area of ductular reaction and fibrosis stage (r = 0.453, P < .0001), which remained independently associated after multivariate analysis. HPC numbers also correlated with fibrosis (r = 0.544, P < .0001) and the ductular area (r = 0.624, P < .0001). Moreover, steatosis correlated with greater HPC proliferation (r = 0.372, P = .0004) and ductular reaction (r = 0.374, P < .0001) but was not an obligate feature. Impaired hepatocyte replication by p21 expression was independently associated with HPC expansion (P = .002) and increased with the body mass index (P < .001) and lobular inflammation (P = .005). In conclusion, the strong correlation between portal fibrosis and a periportal ductular reaction with HPC expansion, the exacerbation by steatosis, and the associations with impaired hepatocyte replication suggest that an altered regeneration pathway drives the ductular reaction. We believe this triggers fibrosis at the portal tract interface. This may be a stereotyped response of importance in other chronic liver diseases.

Sympathetic nervous system regulation of liver repair

This chapter reviews recent evidence that the sympathetic nervous system (SNS) regulates liver repair by modulating the phenotypes of hepatic stellate cells (HSCs), the liver's principal fibrogenic cells, and hepatic epithelial progenitors, i.e., oval cells. SNS nerve fibers touch HSCs and these cells express adrenoceptors, suggesting that HSCs may be targets for SNS neurotransmitters. HSCs also contain catecholamine biosynthetic enzymes, release norepinephrine (NE), and are growth-inhibited by adrenoceptor antagonists. In addition, HSCs from mice with reduced levels of NE grow poorly in culture and exhibit inhibited activation during liver injury. Finally, growth and injury-related fibrogenic responses are rescued by adrenoceptor agonists. Thus, certain SNS inhibitors (SNSIs) protect experimental animals from cirrhosis. Conversely, SNSIs enhance the hepatic accumulation of oval cells (OCs) in injured livers. This response is associated with improved liver injury. Because SNSIs do not affect the expression of cytokines, growth factors, or growth factor receptors that are known to regulate OCs, and OCs express adrenoceptors, it is conceivable that catecholamines influence OCs by direct interaction with OC adrenoceptors. Given evidence that the SNS regulates the viability and activation of HSCs and OCs differentially, SNSIs may be novel therapies to improve the repair of damaged livers.

Increased oxidative stress in obesity and its impact on metabolic syndrome

Obesity is a principal causative factor in the development of metabolic syndrome. Here we report that increased oxidative stress in accumulated fat is an important pathogenic mechanism of obesity-associated metabolic syndrome. Fat accumulation correlated with systemic oxidative stress in humans and mice. Production of ROS increased selectively in adipose tissue of obese mice, accompanied by augmented expression of NADPH oxidase and decreased expression of antioxidative enzymes. In cultured adipocytes, elevated levels of fatty acids increased oxidative stress via NADPH oxidase activation, and oxidative stress caused dysregulated production of adipocytokines (fat-derived hormones), including adiponectin, plasminogen activator inhibitor-1, IL-6, and monocyte chemotactic protein-1. Finally, in obese mice, treatment with NADPH oxidase inhibitor reduced ROS production in adipose tissue, attenuated the dysregulation of adipocytokines, and improved diabetes, hyperlipidemia, and hepatic steatosis. Collectively, our results suggest that increased oxidative stress in accumulated fat is an early instigator of metabolic syndrome and that the redox state in adipose tissue is a potentially useful therapeutic target for obesity-associated metabolic syndrome.

Increased oxidative stress in obesity and its impact on metabolic syndrome

Obesity is a principal causative factor in the development of metabolic syndrome. Here we report that increased oxidative stress in accumulated fat is an important pathogenic mechanism of obesity-associated metabolic syndrome. Fat accumulation correlated with systemic oxidative stress in humans and mice. Production of ROS increased selectively in adipose tissue of obese mice, accompanied by augmented expression of NADPH oxidase and decreased expression of antioxidative enzymes. In cultured adipocytes, elevated levels of fatty acids increased oxidative stress via NADPH oxidase activation, and oxidative stress caused dysregulated production of adipocytokines (fat-derived hormones), including adiponectin, plasminogen activator inhibitor-1, IL-6, and monocyte chemotactic protein-1. Finally, in obese mice, treatment with NADPH oxidase inhibitor reduced ROS production in adipose tissue, attenuated the dysregulation of adipocytokines, and improved diabetes, hyperlipidemia, and hepatic steatosis. Collectively, our results suggest that increased oxidative stress in accumulated fat is an early instigator of metabolic syndrome and that the redox state in adipose tissue is a potentially useful therapeutic target for obesity-associated metabolic syndrome.

Increased oxidative stress in obesity and its impact on metabolic syndrome

Obesity is a principal causative factor in the development of metabolic syndrome. Here we report that increased oxidative stress in accumulated fat is an important pathogenic mechanism of obesity-associated metabolic syndrome. Fat accumulation correlated with systemic oxidative stress in humans and mice. Production of ROS increased selectively in adipose tissue of obese mice, accompanied by augmented expression of NADPH oxidase and decreased expression of antioxidative enzymes. In cultured adipocytes, elevated levels of fatty acids increased oxidative stress via NADPH oxidase activation, and oxidative stress caused dysregulated production of adipocytokines (fat-derived hormones), including adiponectin, plasminogen activator inhibitor-1, IL-6, and monocyte chemotactic protein-1. Finally, in obese mice, treatment with NADPH oxidase inhibitor reduced ROS production in adipose tissue, attenuated the dysregulation of adipocytokines, and improved diabetes, hyperlipidemia, and hepatic steatosis. Collectively, our results suggest that increased oxidative stress in accumulated fat is an early instigator of metabolic syndrome and that the redox state in adipose tissue is a potentially useful therapeutic target for obesity-associated metabolic syndrome.

Increased oxidative stress in obesity and its impact on metabolic syndrome

Obesity is a principal causative factor in the development of metabolic syndrome. Here we report that increased oxidative stress in accumulated fat is an important pathogenic mechanism of obesity-associated metabolic syndrome. Fat accumulation correlated with systemic oxidative stress in humans and mice. Production of ROS increased selectively in adipose tissue of obese mice, accompanied by augmented expression of NADPH oxidase and decreased expression of antioxidative enzymes. In cultured adipocytes, elevated levels of fatty acids increased oxidative stress via NADPH oxidase activation, and oxidative stress caused dysregulated production of adipocytokines (fat-derived hormones), including adiponectin, plasminogen activator inhibitor-1, IL-6, and monocyte chemotactic protein-1. Finally, in obese mice, treatment with NADPH oxidase inhibitor reduced ROS production in adipose tissue, attenuated the dysregulation of adipocytokines, and improved diabetes, hyperlipidemia, and hepatic steatosis. Collectively, our results suggest that increased oxidative stress in accumulated fat is an early instigator of metabolic syndrome and that the redox state in adipose tissue is a potentially useful therapeutic target for obesity-associated metabolic syndrome.

Recent events in alcoholic liver disease V. effects of ethanol on liver regeneration

Liver regeneration is necessary to recover from alcoholic liver injury. Herein, we review evidence that ethanol interferes with liver regeneration. Briefly, alcoholic fatty livers demonstrate increased rates of hepatocyte death. The latter provides a regenerative stimulus. However, unlike mature hepatocytes in healthy adult livers, most surviving mature hepatocytes in alcoholic fatty livers cannot replicate. Therefore, less mature cells (progenitors) must differentiate to replace dead hepatocytes. Little is known about the general mechanisms that modulate the differentiation of liver progenitors in adults. Delineation of these mechanisms and clarification of how ethanol influences them might suggest new therapies for alcoholic liver disease.