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

Hedgehog Signaling Pathway in Fibrosis and Targeted Therapies

Hedgehog (Hh) signaling is a well-established developmental pathway; it is crucial for early embryogenesis, cell differentiation, and damage-driven regeneration. It is being increasingly recognized that dysregulated Hh signaling is also involved in fibrotic diseases, which are characterized by excessive extracellular matrix deposition that compromises tissue architecture and function. As in-depth insights into the mechanisms of Hh signaling are obtained, its complex involvement in fibrosis is gradually being illuminated. Notably, some Hh-targeted inhibitors are currently under exploration in preclinical and clinical trials as a means to prevent fibrosis progression. In this review, we provide a concise overview of the biological mechanisms involved in Hh signaling. We summarize the latest advances in our understanding of the roles of Hh signaling in fibrogenesis across the liver, kidneys, airways, and lungs, as well as other tissues and organs, with an emphasis on both the shared features and, more critically, the distinct functional variations observed across these tissues and organs. We thus highlight the context dependence of Hh signaling, as well as discuss the current status and the challenges of Hh-targeted therapies for fibrosis.

Sonic hedgehog-heat shock protein 90β axis promotes the development of nonalcoholic steatohepatitis in mice

Sonic hedgehog (SHH) and heat shock protein 90β (HSP90β) have been implicated in nonalcoholic steatohepatitis (NASH) but their molecular mechanisms of action remain elusive. We find that HSP90β is a key SHH downstream molecule for promoting NASH process. In hepatocytes, SHH reduces HSP90β ubiquitylation through deubiquitylase USP31, thus preventing HSP90β degradation and promoting hepatic lipid synthesis. HSP90β significantly increases in NASH mouse model, leading to secretion of exosomes enriched with miR-28-5p. miR-28-5p directly targetes and decreases Rap1b levels, which in turn promotes NF-κB transcriptional activity in macrophages and stimulates the expression of inflammatory factors. Genetic deletion, pharmacological inhibition of the SHH-HSP90β axis, or delivery of miR-28-5p to macrophages in the male mice liver, impairs NASH symptomatic development. Importantly, there is a markedly higher abundance of miR-28-5p in NASH patient sera. Taken together, the SHH-HSP90β-miR-28-5p axis offers promising therapeutic targets against NASH, and serum miR-28-5p may serve as a NASH diagnostic biomarker.

Redox Biology and Liver Fibrosis

Hepatic fibrosis is a complex process that develops in chronic liver diseases. Even though the initiation and progression of fibrosis rely on the underlying etiology, mutual mechanisms can be recognized and targeted for therapeutic purposes. Irrespective of the primary cause of liver disease, persistent damage to parenchymal cells triggers the overproduction of reactive species, with the consequent disruption of redox balance. Reactive species are important mediators for the homeostasis of both hepatocytes and non-parenchymal liver cells. Indeed, other than acting as cytotoxic agents, reactive species are able to modulate specific signaling pathways that may be relevant to hepatic fibrogenesis. After a brief introduction to redox biology and the mechanisms of fibrogenesis, this review aims to summarize the current evidence of the involvement of redox-dependent pathways in liver fibrosis and focuses on possible therapeutic targets.

Enhancement of biochemical and genomic pathways through lycopene-loaded nano-liposomes: Alleviating insulin resistance, hepatic steatosis, and autophagy in obese rats with non-alcoholic fatty liver disease: Involvement of SMO, GLI-1, and PTCH-1 genes

Non-alcoholic fatty liver (NAFL) is a prevalent hepatic disorder of global significance that can give rise to severe complications. This research endeavor delves into the potential of nano-liposomal formulated Lycopene (Lip-Lyco) in averting the development of obesity and insulin resistance, both of which are major underlying factors contributing to NAFL. The investigation further scrutinizes the impact of Lip-Lyco on intricate cellular pathways within the liver tissue of rats induced with NAFL, specifically focusing on the progression of steatosis and fibrosis. To establish an obesity-NAFL model, twenty rats were subjected to a high-fat diet (HFD) for a duration of twelve weeks, after which they received an oral treatment of Lip-Lyco (10mg/kg) for an additional eight weeks. Another group of sixteen non-obese rats were subjected to treatment with or without Lip-Lyco, serving as a control for comparison. Results: The rats on a hypercaloric diet had high body mass index (BMI) and insulin resistance, reflected in disturbed serum adipokines and lipid profiles. Oxidative stress, inflammation, and apoptosis were evident in hepatic tissue, and the autophagic process in hepatocytes was inhibited. Additionally, the hedgehog pathway was activated in the liver tissue of NAFL group. Lip-Lyco was found to counteract all these aspects of NAFL pathogenesis. Lip-Lyco exhibited antioxidant, anti-inflammatory, hypoglycemic, antiapoptotic, autophagy-inducing, and Hedgehog signaling inhibitory effects. This study concludes that Lip-Lyco, a natural compound, has promising therapeutic potential in combating NAFLdisease. However, more experimental and clinical studies are required to confirm the effectiveness of lycopene in treating NAFLdisease.

Cellular heterogeneity and plasticity during NAFLD progression

Nonalcoholic fatty liver disease (NAFLD) is a progressive liver disease that can progress to nonalcoholic steatohepatitis (NASH), NASH-related cirrhosis, and hepatocellular carcinoma (HCC). NAFLD ranges from simple steatosis (or nonalcoholic fatty liver [NAFL]) to NASH as a progressive form of NAFL, which is characterized by steatosis, lobular inflammation, and hepatocellular ballooning with or without fibrosis. Because of the complex pathophysiological mechanism and the heterogeneity of NAFLD, including its wide spectrum of clinical and histological characteristics, no specific therapeutic drugs have been approved for NAFLD. The heterogeneity of NAFLD is closely associated with cellular plasticity, which describes the ability of cells to acquire new identities or change their phenotypes in response to environmental stimuli. The liver consists of parenchymal cells including hepatocytes and cholangiocytes and nonparenchymal cells including Kupffer cells, hepatic stellate cells, and endothelial cells, all of which have specialized functions. This heterogeneous cell population has cellular plasticity to adapt to environmental changes. During NAFLD progression, these cells can exert diverse and complex responses at multiple levels following exposure to a variety of stimuli, including fatty acids, inflammation, and oxidative stress. Therefore, this review provides insights into NAFLD heterogeneity by addressing the cellular plasticity and metabolic adaptation of hepatocytes, cholangiocytes, hepatic stellate cells, and Kupffer cells during NAFLD progression.

Hepatocyte and immune cell crosstalk in non-alcoholic fatty liver disease

Introduction: Nonalcoholic fatty liver disease (NAFLD) is the most widespread chronic liver disease in the world. It can evolve into nonalcoholic steatohepatitis (NASH) where inflammation and hepatocyte ballooning are key participants in the determination of this steatotic state.Areas covered: To provide a systematic overview and current understanding of the role of inflammation in NAFLD and its progression to NASH, the function of the cells involved, and the activation pathways of the innate immunity and cell death; resulting in inflammation and chronic liver disease. A PubMed search was made with relevant articles together with relevant references were included for the writing of this review.Expert opinion: Innate and adaptive immunity are the key players in the NAFLD progression; some of the markers presented during NAFLD are also known to be immunity biomarkers. All cells involved in NAFLD and NASH are known to have immunoregulatory properties and their imbalance will completely change the cytokine profile and form a pro-inflammatory microenvironment. It is necessary to fully answer the question of what initiators and metabolic imbalances are particularly important, considering sterile inflammation as the architect of the disease. Due to the shortage of elucidation of NASH progression, we discuss in this review, how inflammation is a key part of this development and we presume the targets should lead to inflammation and oxidative stress treatment.

Relationship between lipid metabolism and Hedgehog signaling pathway

The Hedgehog (Hh) signaling pathway is highly conserved signaling pathway in cells. Steroids was found to play a vital role in Hh signaling pathway and aberrant Hh signaling was found to lead a series of disease correlate with abnormal lipid metabolism. This paper aimed to elucidate the relationship between lipid metabolism and Hedgehog signaling pathway.

Novel Interplay Between Sonic Hedgehog and Transforming Growth Factor-β1 in Human Nonalcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH) has the potential to progress to fibrosis, cirrhosis, and hepatocellular carcinoma. Upregulation of sonic hedgehog (Shh) has been documented in development of NASH through sustained cell stress. At the same time, transforming growth factor-β1 (TGF-β1), which is a central element in fibrogenic reactions in various diseases and sites, has been reported to be associated with hepatic inflammation and fibrotic reaction. To explore crosstalk between Shh and TGF-β1 in the development and progression of NASH, we investigated the expression of both these proteins in 135 human specimens of NASH, 35 fatty liver specimens, 35 specimens of alcoholic steatohepatitis with immunohistochemistry. Shh protein was expressed in the cytoplasm of ballooned hepatocytes with an ubiquitin-like pattern. In addition, a few scattered apoptotic hepatocytes in the inflammatory foci showed homogeneous cytoplasmic Shh expression. TGF-β1 protein was observed mainly in the activated hepatic stellate cells (HSCs) which were located in the inflammatory foci surrounding ballooned hepatocytes. Moreover, the mRNA levels of both Shh and TGF-β1 in the liver biopsy specimens from NASH patients was significantly increased compared with those in fatty liver patients. Statistically, there was a significant association of the expressions of Shh and TGF-β1 proteins in NASH (r=0.6, P<0.05). In addition, increased expression of Shh protein significantly parallels the severity of hepatocellular ballooning, lobular, and portal inflammatory responses and progression of fibrosis in NASH patients. Moreover, we found that much HSCs transformed into myofibroblast-like phenotype and migrated downward to HepG2 hepatocellular carcinoma cells with overexpression of Shh by transwell assay. We also observed overexpression of proteins of Shh and TGF-β1 in cultured activated HSCs with confocal microscopy. These findings strongly suggest there is interplay between Shh and TGF-β1 in hepatic inflammatory reactions. Shh secreted through damaged hepatocytes may result in activation of TGF-β1 and subsequent transformation of HSCs, which together modulate the progression of human NASH.

Liver epithelial focal adhesion kinase modulates fibrogenesis and hedgehog signaling

Focal adhesion kinase (FAK) is an important mediator of extracellular matrix-integrin mechano-signal transduction that regulates cell motility, survival, and proliferation. As such, FAK is being investigated as a potential therapeutic target for malignant and fibrotic diseases, and numerous clinical trials of FAK inhibitors are underway. The function of FAK in nonmalignant, nonmotile epithelial cells is not well understood. We previously showed that hepatocytes demonstrated activated FAK near stiff collagen tracts in fibrotic livers. In this study, we examined the role of liver epithelial FAK by inducing fibrotic liver disease in mice with liver epithelial FAK deficiency. We found that mice that lacked FAK in liver epithelial cells developed more severe liver injury and worse fibrosis as compared with controls. Increased fibrosis in liver epithelial FAK-deficient mice was linked to the activation of several profibrotic pathways, including the hedgehog/smoothened pathway. FAK-deficient hepatocytes produced increased Indian hedgehog in a manner dependent on matrix stiffness. Furthermore, expression of the hedgehog receptor, smoothened, was increased in macrophages and biliary cells of hepatocyte-specific FAK-deficient fibrotic livers. These results indicate that liver epithelial FAK has important regulatory roles in the response to liver injury and progression of fibrosis.

Liver stiffness correlates with serum osteopontin and TAZ expression in human liver cirrhosis

The pivotal role of the extracellular matrix (ECM) as both a cause and consequence of liver fibrosis is striking. However, mechanotransducer molecules and profibrogenic factors induced by liver stiffness are still unclear. The current study aimed to investigate liver stiffness and its correlation with the expression of the transcriptional coactivator with PDZ-binding motif (TAZ) and serum osteopontin (OPN) in human cirrhosis. In this case-control study, liver tissue stiffness was determined using atomic force microscopy in cirrhotic livers (n = 38) of different etiologies and in controls (n = 10). Immunohistochemical and qRT-PCR analyses were performed to analyze TAZ expression. Besides, western blotting and ELISA were performed to assess liver Indian hedgehog and serum OPN levels, respectively. Liver stiffness, TAZ expression, and hepatic gene expression and serum protein levels of OPN were significantly increased in patients with cirrhosis compared with the control groups (all P < 0.001), specifically in autoimmune- and alcohol-related cirrhosis. In cirrhotic patients, liver stiffness was significantly associated with the expression of nuclear TAZ and OPN. The correlation between matrix stiffness as a mechanical property, TAZ as a potential mechanotransducer, and OPN as a matricellular factor suggests possible effects of mechanical features of the ECM on the expression of the aforementioned profibrogenic markers, which is predominant in autoimmune- and alcohol-related cirrhosis.

Downregulation of hedgehog ligands in human simple steatosis may protect against nonalcoholic steatohepatitis: Is TAZ a crucial regulator?

The conversion of simple steatosis into nonalcoholic steatohepatitis (NASH) in patients with nonalcoholic fatty liver disease (NAFLD) has attracted many attentions in recent years. The role of the hedgehog (HH) pathway in the regulation of lipogenesis has been addressed in the literature. This study aimed to investigate the levels of the sonic hedgehog (SHH) and Indian hedgehog (IHH) ligands and the correlation of these ligands with levels of proteins involved in the transforming growth factor-β1 (TGF-β1) pathway, as well as the evaluation of the transcriptional coactivator with PDZ binding motif (TAZ) expression in human simple steatosis, NASH cirrhosis, and controls. Patients were divided into two groups: the first group consisted of patients diagnosed with simple steatosis (n = 16) and the second group included those diagnosed with NASH cirrhosis (n = 15). As a control group, 18 histologically normal liver tissues were collected in this study. The expression of the TGF-β1pathway components and SHH and IHH ligands were analyzed by means of the quantitative real-time polymerase chain reaction and western blot analyses. A significant decrease was found in the hepatic expression of the SHH, IHH, and TGF-β1 pathways along with the expression of TAZ in tissue specimens with simple steatosis in comparison with patients affected by NASH cirrhosis and controls. Also, the levels of SHH and IHH proteins were significantly correlated with the expression of proteins involved in the TGF-β1 pathway. Moreover, the expression of the HH pathway ligands was positively associated with the expression of TAZ, supporting the notion that TAZ may play a role in the activation of the HH pathway thereby regulating the expression of its ligands. It seems that in patients with NAFLD, the downregulation of the HH pathway ligands may stem from steatosis; however, at the same time, it may prevent the conversion of simple steatosis into NASH in patients with liver diseases. © 2019 IUBMB Life, 71(9):1382-1390, 2019.

Adult Hepatocytes Are Hedgehog-Responsive Cells in the Setting of Liver Injury: Evidence for Smoothened-Mediated Activation of NF-κB/Epidermal Growth Factor Receptor/Akt in Hepatocytes that Counteract Fas-Induced Apoptosis

Although hedgehog (Hh) signaling pathway is inactive in adult healthy liver, it becomes activated during acute and chronic liver injury and, thus, modulates the reparative process and disease progression. We developed a novel mouse model with liver-specific knockout of Smoothened (Smo LKO), and animals were subjected to Fas-induced liver injury in vivo. Results showed that Smo deletion in hepatocytes enhances Fas-induced liver injury. Activation of Hh signaling in hepatocytes in the setting of Fas-induced injury was indicated by the fact that Jo2 treatment enhanced hepatic expression of Ptch1, Smo, and its downstream target Gli1 in control but not Smo LKO mice. Primary hepatocytes from control mice showed increased Hh signaling activation in response to Jo2 treatment in vitro. On the other hand, the Smo KO hepatocytes were devoid of Hh activation and were more susceptible to Jo2-induced apoptosis. The levels of NF-κB and related signaling molecules, including epidermal growth factor receptor and Akt, were lower in Smo KO livers/hepatocytes than in control livers/hepatocytes. Accordingly, hydrodynamic gene delivery of active NK-κB prevented Jo2-induced liver injury in the Smo LKO mice. Our findings provide important evidence that adult hepatocytes become responsive to Hh signaling through up-regulation of Smo in the setting of Fas-induced liver injury and that such alteration leads to activation of NF-κB/epidermal growth factor receptor/Akt, which counteracts Fas-induced hepatocyte apoptosis.

Nonalcoholic Steatohepatitis: Histopathology Basics Within a Broader Context

Nonalcoholic fatty liver disease (NAFLD) is a major health concern and the prevalence continues to increase in many industrialized and developing countries around the world. NAFLD affects adults and children. NAFLD-related cirrhosis is expected to become the top indication for liver transplantation in the near future, and the incidence of NAFLD-related hepatocellular carcinoma is also increasing. Nonalcoholic steatohepatitis is the more severe form of NAFLD. The pathogenesis of NALFD/nonalcoholic steatohepatitis is complex and new concepts continue to evolve. The diagnosis and categorization of nonalcoholic steatohepatitis currently rests on hepatopathologists. Accurate morphologic interpretation is important for therapeutic, prognostic, and investigational purposes.

The hedgehog pathway in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of obesity-associated liver diseases and it has become the major cause of cirrhosis in the Western world. The high prevalence of NAFLD-associated advanced liver disease reflects both the high prevalence of obesity-related fatty liver (hepatic steatosis) and the lack of specific treatments to prevent hepatic steatosis from progressing to more serious forms of liver damage, including nonalcoholic steatohepatitis (NASH), cirrhosis, and primary liver cancer. The pathogenesis of NAFLD is complex, and not fully understood. However, compelling evidence demonstrates that dysregulation of the hedgehog (Hh) pathway is involved in both the pathogenesis of hepatic steatosis and the progression from hepatic steatosis to more serious forms of liver damage. Inhibiting hedgehog signaling enhances hepatic steatosis, a condition which seldom results in liver-related morbidity or mortality. In contrast, excessive Hh pathway activation promotes development of NASH, cirrhosis, and primary liver cancer, the major causes of liver-related deaths. Thus, suppressing excessive Hh pathway activity is a potential approach to prevent progressive liver damage in NAFLD. Various pharmacologic agents that inhibit Hh signaling are available and approved for cancer therapeutics; more are being developed to optimize the benefits and minimize the risks of inhibiting this pathway. In this review we will describe the Hh pathway, summarize the evidence for its role in NAFLD evolution, and discuss the potential role for Hh pathway inhibitors as therapies to prevent NASH, cirrhosis and liver cancer.

Hedgehog signalling in liver pathophysiology

Liver disease remains a leading cause of mortality worldwide despite recent successes in the field of viral hepatitis, because increases in alcohol consumption and obesity are fuelling an epidemic of chronic fatty liver disease for which there are currently no effective medical therapies. About 20% of individuals with chronic liver injury ultimately develop end-stage liver disease due to cirrhosis. Hence, treatments to prevent and reverse cirrhosis in individuals with ongoing liver injury are desperately needed. The development of successful treatments requires an improved understanding of the mechanisms controlling liver disease progression. The liver responds to diverse insults with a conserved wound healing response, suggesting that it might be generally beneficial to optimise pathways that are crucial for effective liver repair. The Hedgehog pathway has emerged as a potential target based on compelling preclinical and clinical data, which demonstrate that it critically regulates the liver's response to injury. Herein, we will summarise evidence of the Hedgehog pathway's role in liver disease and discuss how modulating pathway activity might be applied to improve liver disease outcomes.

Pathophysiology of Nonalcoholic Fatty Liver Disease/Nonalcoholic Steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of liver disorders ranging from hepatic steatosis to nonalcoholic steatohepatitis (NASH) and ultimately may lead to cirrhosis. Hepatic steatosis or fatty liver is defined as increased accumulation of lipids in hepatocytes and results from increased production or reduced clearance of hepatic triglycerides or fatty acids. Fatty liver can progress to NASH in a significant proportion of subjects. NASH is a necroinflammatory liver disease governed by multiple pathways that are not completely elucidated. This review describes the main mechanisms that have been reported to contribute to the pathophysiology of NAFLD and NASH.

The diversity and plasticity of adult hepatic progenitor cells and their niche

The liver is a unique organ for homoeostasis with regenerative capacities. Hepatocytes possess a remarkable capacity to proliferate upon injury; however, in more severe scenarios liver regeneration is believed to arise from at least one, if not several facultative hepatic progenitor cell compartments. Newly identified pericentral stem/progenitor cells residing around the central vein is responsible for maintaining hepatocyte homoeostasis in the uninjured liver. In addition, hepatic progenitor cells have been reported to contribute to liver fibrosis and cancers. What drives liver homoeostasis, regeneration and diseases is determined by the physiological and pathological conditions, and especially the hepatic progenitor cell niches which influence the fate of hepatic progenitor cells. The hepatic progenitor cell niches are special microenvironments consisting of different cell types, releasing growth factors and cytokines and receiving signals, as well as the extracellular matrix (ECM) scaffold. The hepatic progenitor cell niches maintain and regulate stem cells to ensure organ homoeostasis and regeneration. In recent studies, more evidence has been shown that hepatic cells such as hepatocytes, cholangiocytes or myofibroblasts can be induced to be oval cell-like state through transitions under some circumstance, those transitional cell types as potential liver-resident progenitor cells play important roles in liver pathophysiology. In this review, we describe and update recent advances in the diversity and plasticity of hepatic progenitor cell and their niches and discuss evidence supporting their roles in liver homoeostasis, regeneration, fibrosis and cancers.

Forskolin, a hedgehog signalling inhibitor, attenuates carbon tetrachloride-induced liver fibrosis in rats

BACKGROUND AND PURPOSE

Liver fibrosis is one of the leading causes of morbidity and mortality worldwide with very limited therapeutic options. Given the pivotal role of activated hepatic stellate cells in liver fibrosis, attention has been directed towards the signalling pathways underlying their activation and fibrogenic functions. Recently, the hedgehog (Hh) signalling pathway has been identified as a potentially important therapeutic target in liver fibrosis. The present study was designed to explore the antifibrotic effects of the potent Hh signalling inhibitor, forskolin, and the possible molecular mechanisms underlying these effects.

EXPERIMENTAL APPROACH

Male Sprague-Dawley rats were treated with either CCl4 and/or forskolin for 6 consecutive weeks. Serum hepatotoxicity markers were determined, and histopathological evaluation was performed. Hepatic fibrosis was assessed by measuring α-SMA expression and collagen deposition by Masson's trichrome staining and hydroxyproline content. The effects of forskolin on oxidative stress markers (GSH, GPx, lipid peroxides), inflammatory markers (NF-κB, TNF-α, COX-2, IL-1β), TGF-β1 and Hh signalling markers (Ptch-1, Smo, Gli-2) were also assessed.

KEY RESULTS

Hepatic fibrosis induced by CCl4 was significantly reduced by forskolin, as indicated by decreased α-SMA expression and collagen deposition. Forskolin co-treatment significantly attenuated oxidative stress and inflammation, reduced TGF-β1 levels and down-regulated mRNA expression of Ptch-1, Smo and Gli-2 through cAMP-dependent PKA activation.

CONCLUSION AND IMPLICATIONS

In our model, forskolin exerted promising antifibrotic effects which could be partly attributed to its antioxidant and anti-inflammatory effects, as well as to its inhibition of Hh signalling, mediated by cAMP-dependent activation of PKA.

MicroRNAs in liver fibrosis: Focusing on the interaction with hedgehog signaling

Liver fibrosis is a repair process in response to damage in the liver; however, severe and chronic injury promotes the accumulation of fibrous matrix, destroying the normal functions and architecture of liver. Hepatic stellate cells (HSCs) are quiescent in normal livers, but in damaged livers, they transdifferentiate into myofibroblastic HSCs, which produce extracellular matrix proteins. Hedgehog (Hh) signaling orchestrates tissue reconstruction in damaged livers and contributes to liver fibrogenesis by regulating HSC activation. MicroRNAs (miRNAs), endogenous small non-coding RNAs interfering with RNA post-transcriptionally, regulate various cellular processes in healthy organisms. The dysregulation of miRNAs is closely associated with diseases, including liver diseases. Thus, miRNAs are good targets in the diagnosis and treatment of various diseases, including liver fibrosis; however, the regulatory mechanisms of miRNAs that interact with Hh signaling in liver fibrosis remain unclear. We review growing evidence showing the association of miRNAs with Hh signaling. Recent studies suggest that Hh-regulating miRNAs induce inactivation of HSCs, leading to decreased hepatic fibrosis. Although miRNA-delivery systems and further knowledge of interacting miRNAs with Hh signaling need to be improved for the clinical usage of miRNAs, recent findings indicate that the miRNAs regulating Hh signaling are promising therapeutic agents for treating liver fibrosis.

Role of Hedgehog Signaling Pathway in NASH

Non-alcoholic fatty liver disease (NAFLD) is the number one cause of chronic liver disease in the Western world. Although only a minority of patients will ultimately develop end-stage liver disease, it is not yet possible to efficiently predict who will progress and, most importantly, effective treatments are still unavailable. Better understanding of the pathophysiology of this disease is necessary to improve the clinical management of NAFLD patients. Epidemiological data indicate that NAFLD prognosis is determined by an individual's response to lipotoxic injury, rather than either the severity of exposure to lipotoxins, or the intensity of liver injury. The liver responds to injury with a synchronized wound-healing response. When this response is abnormal, it leads to pathological scarring, resulting in progressive fibrosis and cirrhosis, rather than repair. The hedgehog pathway is a crucial player in the wound-healing response. In this review, we summarize the pre-clinical and clinical evidence, which demonstrate the role of hedgehog pathway dysregulation in NAFLD pathogenesis, and the preliminary data that place the hedgehog pathway as a potential target for the treatment of this disease.

Accumulation of duct cells with activated YAP parallels fibrosis progression in non-alcoholic fatty liver disease

BACKGROUND & AIMS

Mechanisms that regulate regeneration of injured livers are complex. YAP, a stem cell associated factor, controls liver growth in healthy adult mice. Increasing nuclear localization of YAP triggers accumulation of reactive-appearing ductular cells (YAP+RDC) with liver progenitor capabilities. The significance of YAP activation, and mechanisms involved, are unknown in diseased livers. We evaluated the hypothesis that YAP is more activated in injured livers that are scarring than in those that are regenerating effectively.

METHODS

Immunohistochemistry and qRT-PCR analysis were used to localize and quantify changes in YAP and RDC in 52 patients with non-alcoholic fatty liver disease (NAFLD) and two mouse models of diet-induced non-alcoholic steatohepatitis (NASH). Results were correlated with liver disease severity, metabolic risk factors, and factors proven to control NAFLD progression.

RESULTS

YAP increased in NAFLD where it mainly localized in nuclei of RDC that expressed progenitor markers. Accumulation of YAP+RDC paralleled the severity of hepatocyte injury and accumulation of Sonic hedgehog, but not steatosis or metabolic risk factors. YAP+RDC expressed osteopontin, a Shh-regulated fibrogenic factor. Myofibroblast accumulation, fibrosis, and numbers of YAP+RDC strongly correlated. In murine NASH models, atrophic fibrotic livers contained significantly more YAP+RDC than livers with less severe NASH.

CONCLUSION

YAP+RDC promote scarring, rather than effective regeneration, during NASH.

Reduced lipoapoptosis, hedgehog pathway activation and fibrosis in caspase-2 deficient mice with non-alcoholic steatohepatitis

OBJECTIVE

Caspase-2 is an initiator caspase involved in multiple apoptotic pathways, particularly in response to specific intracellular stressors (eg, DNA damage, ER stress). We recently reported that caspase-2 was pivotal for the induction of cell death triggered by excessive intracellular accumulation of long-chain fatty acids, a response known as lipoapoptosis. The liver is particularly susceptible to lipid-induced damage, explaining the pandemic status of non-alcoholic fatty liver disease (NAFLD). Progression from NAFLD to non-alcoholic steatohepatitis (NASH) results, in part, from hepatocyte apoptosis and consequential paracrine-mediated fibrogenesis. We evaluated the hypothesis that caspase-2 promotes NASH-related cirrhosis.

DESIGN

Caspase-2 was localised in liver biopsies from patients with NASH. Its expression was evaluated in different mouse models of NASH, and outcomes of diet-induced NASH were compared in wild-type (WT) and caspase-2-deficient mice. Lipotoxicity was modelled in vitro using hepatocytes derived from WT and caspase-2-deficient mice.

RESULTS

We showed that caspase-2 is integral to the pathogenesis of NASH-related cirrhosis. Caspase-2 is localised in injured hepatocytes and its expression was markedly upregulated in patients and animal models of NASH. During lipotoxic stress, caspase-2 deficiency reduced apoptosis, inhibited induction of profibrogenic hedgehog target genes in mice and blocked production of hedgehog ligands in cultured hepatocytes.

CONCLUSIONS

These data point to a critical role for caspase-2 in lipid-induced hepatocyte apoptosis in vivo for the production of apoptosis-associated fibrogenic factors and in the progression of lipid-induced liver fibrosis. This raises the intriguing possibility that caspase-2 may be a promising therapeutic target to prevent progression to NASH.

Curcumin regulates cell fate and metabolism by inhibiting hedgehog signaling in hepatic stellate cells

Accumulating evidence indicates that Hedgehog (Hh) signaling becomes activated in chronic liver injury and plays a role in the pathogenesis of hepatic fibrosis. Hepatic stellate cells (HSCs) are Hh-responsive cells and activation of the Hh pathway promotes transdifferentiation of HSCs into myofibroblasts. Targeting Hh signaling may be a novel therapeutic strategy for treatment of liver fibrosis. We previously reported that curcumin has potent antifibrotic effects in vivo and in vitro, but the underlying mechanisms are not fully elucidated. This study shows that curcumin downregulated Patched and Smoothened, two key elements in Hh signaling, but restored Hhip expression in rat liver with carbon tetrachloride-induced fibrosis and in cultured HSCs. Curcumin also halted the nuclear translocation, DNA binding, and transcription activity of Gli1. Moreover, the Hh signaling inhibitor cyclopamine, like curcumin, arrested the cell cycle, induced mitochondrial apoptosis, reduced fibrotic gene expression, restored lipid accumulation, and inhibited invasion and migration in HSCs. However, curcumin's effects on cell fate and fibrogenic properties of HSCs were abolished by the Hh pathway agonist SAG. Furthermore, curcumin and cyclopamine decreased intracellular levels of adenosine triphosphate and lactate, and inhibited the expression and/or function of several key molecules controlling glycolysis. However, SAG abrogated the curcumin effects on these parameters of glycolysis. Animal data also showed that curcumin downregulated glycolysis-regulatory proteins in rat fibrotic liver. These aggregated data therefore indicate that curcumin modulated cell fate and metabolism by disrupting the Hh pathway in HSCs, providing novel molecular insights into curcumin reduction of HSC activation.

Mouse models of diet-induced nonalcoholic steatohepatitis reproduce the heterogeneity of the human disease

BACKGROUND AND AIMS

Non-alcoholic steatohepatitis (NASH), the potentially progressive form of nonalcoholic fatty liver disease (NAFLD), is the pandemic liver disease of our time. Although there are several animal models of NASH, consensus regarding the optimal model is lacking. We aimed to compare features of NASH in the two most widely-used mouse models: methionine-choline deficient (MCD) diet and Western diet.

METHODS

Mice were fed standard chow, MCD diet for 8 weeks, or Western diet (45% energy from fat, predominantly saturated fat, with 0.2% cholesterol, plus drinking water supplemented with fructose and glucose) for 16 weeks. Liver pathology and metabolic profile were compared.

RESULTS

The metabolic profile associated with human NASH was better mimicked by Western diet. Although hepatic steatosis (i.e., triglyceride accumulation) was also more severe, liver non-esterified fatty acid content was lower than in the MCD diet group. NASH was also less severe and less reproducible in the Western diet model, as evidenced by less liver cell death/apoptosis, inflammation, ductular reaction, and fibrosis. Various mechanisms implicated in human NASH pathogenesis/progression were also less robust in the Western diet model, including oxidative stress, ER stress, autophagy deregulation, and hedgehog pathway activation.

CONCLUSION

Feeding mice a Western diet models metabolic perturbations that are common in humans with mild NASH, whereas administration of a MCD diet better models the pathobiological mechanisms that cause human NAFLD to progress to advanced NASH.

The Protective Effect of Resveratrol on Concanavalin-A-Induced Acute Hepatic Injury in Mice

Pharmacologic Relevance. Resveratrol, an antioxidant derived from grapes, has been reported to modulate the inflammatory process. In this study, we investigated the effects of resveratrol and its mechanism of protection on concanavalin-A- (ConA-) induced liver injury in mice. Materials and Methods. Acute autoimmune hepatitis was induced by ConA (20 mg/kg) in Balb/C mice; mice were treated with resveratrol (10, 20, and 30 mg/kg) daily by oral gavage for fourteen days prior to a single intravenous injection of ConA. Eight hours after injection, histologic grading, proinflammatory cytokine levels, and hedgehog pathway activity were determined. Results. After ConA injection, the cytokines IL-2, IL-6, and TNF-α were increased, and Sonic hedgehog (Shh), Glioblastoma- (Gli-) 1, and Patched (Ptc) levels significantly increased. Pretreatment with resveratrol ameliorated the pathologic effects of ConA-induced autoimmune hepatitis and significantly inhibited IL-2, IL-6, TNF-α, Shh, Gli-1, and Ptc. The effects of resveratrol on the hedgehog pathway were studied by western blotting and immunohistochemistry. Resveratrol decreased Shh expression, possibly by inhibiting Shh expression in order to reduce Gli-1 and Ptc expression. Conclusion. Resveratrol protects against ConA-induced autoimmune hepatitis by decreasing cytokines expression in mice. The decreases seen in Gli-1 and Ptc may correlate with the amelioration of hedgehog pathway activity.

Analysis of glial fibrillary acidic protein (GFAP)-expressing ductular cells in a rat liver cirrhosis model induced by repeated injections of thioacetamide (TAA)

Glial fibrillary acidic protein (GFAP), a type III intermediate filament protein, is expressed in hepatic stellate cells (HSCs), the principal fibrogenic cell type in the liver. Further, GFAP could be a marker for hepatic progenitor cells (HPCs). In this study, the participation of GFAP-expressing cells in HPC expansion/ductular reaction was investigated in a rat model of liver cirrhosis. Six-week-old male F344 rats were injected intraperitoneally with thioacetamide (100mg/kg BW, twice a week) and examined at post-first injection weeks 5, 10, 15, 20 and 25. Fibrosis-related proliferation of ductular cells was observed as demonstrated by CK19 immunostaining. Some of these cells were stained with GFAP. No co-staining was observed between CK19 and α-smooth muscle actin (α-SMA; myofibroblast marker). There were proliferating ductular cells stained with α-fetoprotein or β-catenin; the ductular reaction was related to increased expression of hepatocarcinogenesis-related factors (Wnt2, Wnt4 and glypican-3). These results for the first time show the participation of GFAP-positive HPCs in ductular reaction in a chemically induced rodent model. Though the ductular cells were chaperoned by myofibroblasts, they show no direct evidence for epithelial to mesenchymal transition. These findings shed new light in understanding the roles of GFAP-expressing HPCs in liver cirrhosis and provide further evidence of interaction between newly-formed bile ductules and HSCs, suggesting that both cells could be in the common lineage of HPCs.

An overview of hedgehog signaling in fibrosis

The Hedgehog (Hh) signaling pathway plays a key role during embryogenesis and tissue regeneration. Recently, studies revealed that overactivated Hh signaling leads to fibrogenesis in many types of tissues. The activation of Hh signaling is involved in the epithelial-mesenchymal transition and excessive extracellular matrix deposition. Blockade of Hh signaling abolishes the induction of the epithelial-mesenchymal transition and ameliorates tissue fibrosis. Therefore, new therapeutic targets to alleviate fibrosis based on the Hh signaling have attracted a great deal of attention. This is a new strategy for treating fibrosis and other related diseases. In this review, we discuss the crucial role of Hh signaling in fibrogenesis to provide a better understanding of their relationship and to encourage the study of novel targeted therapies.

Non-alcoholic fatty liver disease: what the clinician needs to know

Non-alcoholic fatty liver disease (NAFLD) is the most frequent cause of liver disease in the Western world. Furthermore, it is increasing worldwide, paralleling the obesity pandemic. Though highly frequent, only about one fifth of affected subjects are at risk of developing the progressive form of the disease, non-alcoholic steatohepatitis with fibrosis. Even in the latter, liver disease is slowly progressive, though, since it is so prevalent, it is already the third cause of liver transplantation in the United States, and it is predicted to get to the top of the ranking in few years. Of relevance, fatty liver is also associated with increased overall mortality and particularly increased cardiovascular mortality. The literature and amount of published papers on NAFLD is increasing as fast as its prevalence, which makes it difficult to keep updated in this topic. This review aims to summarize the latest knowledge on NAFLD, in order to help clinicians understanding its pathogenesis and advances on diagnosis and treatment.

Molecular mechanisms of stem cell therapy in alcoholic liver disease

Alcoholic liver disease affects a great number of people worldwide. With limited therapeutic options, stem cell therapy offers significant potential for these patients. To date, a limited number of clinical trials have produced transient clinical responses to cell therapy in patients with alcoholic liver disease. Stem cell therapy to reorganize the postnatal liver is an important theme and mission for patients with chronic liver disorders including alcoholic liver injury. We therefore should redevelop the evidence of cell-based liver regeneration therapy, focusing on targets (disease, patient's status and hepatic function), materials (cells, cytokines and genes), and methodology (stem cell types and their derived microparticles, transplantation route, implantation technology and tissue engineering). In this review, we summarize the recent findings regarding the experimental and clinical use of mesenchymal and liver stem cells, focusing mainly on the treatment of alcoholic liver disorders and their relevance in the field of regenerative medicine, and advances on the role of microvesicles and exosomes in this process. We discuss new advances in stem cell therapy from liver regeneration to liver re-organization, which is involved in the recent progress of on-going clinical trials, basic research in stem cell therapy and liver regeneration, and updated exosomes/microvesicles recovery/repairing technology.

Alcohol activates the hedgehog pathway and induces related procarcinogenic processes in the alcohol-preferring rat model of hepatocarcinogenesis

BACKGROUND

Alcohol consumption promotes hepatocellular carcinoma (HCC). The responsible mechanisms are not well understood. Hepatocarcinogenesis increases with age and is enhanced by factors that impose a demand for liver regeneration. Because alcohol is hepatotoxic, habitual alcohol ingestion evokes a recurrent demand for hepatic regeneration. The alcohol-preferring (P) rat model mimics the level of alcohol consumption by humans who habitually abuse alcohol. Previously, we showed that habitual heavy alcohol ingestion amplified age-related hepatocarcinogenesis in P rats, with over 80% of alcohol-consuming P rats developing HCCs after 18 months of alcohol exposure, compared with only 5% of water-drinking controls.

METHODS

Herein, we used quantitative real-time PCR and quantitative immunocytochemistry to compare liver tissues from alcohol-consuming P rats and water-fed P rat controls after 6, 12, or 18 months of drinking. We aimed to identify potential mechanisms that might underlie the differences in liver cancer formation and hypothesized that chronic alcohol ingestion would activate Hedgehog (HH), a regenerative signaling pathway that is overactivated in HCC.

RESULTS

Chronic alcohol ingestion amplified age-related degenerative changes in hepatocytes, but did not cause appreciable liver inflammation or fibrosis even after 18 months of heavy drinking. HH signaling was also enhanced by alcohol exposure, as evidenced by increased levels of mRNAs encoding HH ligands, HH-regulated transcription factors, and HH target genes. Immunocytochemistry confirmed increased alcohol-related accumulation of HH ligand-producing cells and HH-responsive target cells. HH-related regenerative responses were also induced in alcohol-exposed rats. Three of these processes (i.e., deregulated progenitor expansion, the reverse Warburg effect, and epithelial-to-mesenchymal transitions) are known to promote cancer growth in other tissues.

CONCLUSIONS

Alcohol-related changes in Hedgehog signaling and resultant deregulation of liver cell replacement might promote hepatocarcinogenesis.

NAFLD, NASH and liver cancer

NAFLD affects a large proportion of the US population and its incidence and prevalence are increasing to epidemic proportions around the world. As with other liver diseases that cause cirrhosis, NAFLD increases the risk of liver cancer, a disease with poor outcomes and limited therapeutic options. The incidences of hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma are also rising, and HCC is now the leading cause of obesity-related cancer deaths in middle-aged men in the USA. In this Review, we summarize the correlations between liver cancer and NAFLD-related cirrhosis, and the role of the metabolic syndrome in the development of liver cancer from diverse aetiologies, including HCV-mediated cirrhosis. Recent advances in understanding the progression of NAFLD to HCC from preclinical models will also be discussed. Targeted genetic manipulation of certain metabolic or stress-response pathways, including one-carbon metabolism, NF-κB, PTEN and microRNAs, has been valuable in elucidating the pathways that regulate carcinogenesis in NAFLD. Although tremendous advances have occurred in the identification of diagnostic and therapeutic opportunities to reduce the progression of NAFLD, considerable gaps in our knowledge remain with regard to the mechanisms by which NAFLD and its risk factors promote liver cancer.

Transcriptomic and epigenetic changes in early liver steatosis associated to obesity: effect of dietary methyl donor supplementation

Non-alcoholic fatty liver disease is a primary hepatic manifestation of obesity and an important adverse metabolic syndrome trait. Animal models of diet-induced obesity promote liver fat accumulation putatively associated with alterations in epigenetic profile. Dietary methyl donor-supplementation may protect against this disturbance during early developmental stages affecting the molecular basis of gene regulation. The aim of this study was to investigate the transcriptomic and epigenetic mechanisms implicated in liver fat accumulation as a result of an obesogenic diet and the putative preventive role of dietary methyl donors. Forty-eight male Wistar rats were assigned into four dietary groups for 8 weeks; control, control methyl-donor-supplemented with a dietary cocktail containing betaine, choline, vitamin B12 and folic acid, high-fat-sucrose and high-fat-sucrose methyl-donor-supplemented. Liver fat accumulation induced by a HFS diet was prevented by methyl donor supplementation in HFS-fed animals. A liver mRNA microarray, subsequently validated by real time-qPCR, showed modifications in some biologically relevant genes involved in obesity development and lipid metabolism (Lepr, Srebf2, Agpat3 and Esr1). Liver global DNA methylation was decreased by methyl donor supplementation in control-fed animals. Methylation levels of specific CpG sites from Srebf2, Agpat3 and Esr1 promoter regions showed changes due to the obesogenic diet and the supplementation with methyl donors. Interestingly, Srebf2 CpG23_24 methylation levels (-167 bp and -156 bp with respect to the transcriptional start site) correlated with HDLc plasma levels, whereas Esr1 CpG14 (-2623 bp) methylation levels were associated with body and liver weights and fat content. Furthermore HFS diet-induced liver fat accumulation was prevented by methyl donor supplementation. In conclusion, both obesogenic diet and methyl donor supplementation modified the mRNA hepatic profile as well as the methylation of specific gene promoters and total DNA.

Biliary epithelial senescence and plasticity in acute cellular rejection

Biliary epithelial cells (BEC) are important targets in some liver diseases, including acute allograft rejection. Although some injured BEC die, many can survive in function compromised states of senescence or phenotypic de-differentiation. This study was performed to examine changes in the phenotype of BEC during acute liver allograft rejection and the mechanism driving these changes. Liver allograft sections showed a positive correlation (p < 0.0013) between increasing T cell mediated acute rejection and the number of BEC expressing the senescence marker p21(WAF1/Cip) or the mesenchymal marker S100A4. This was modeled in vitro by examination of primary or immortalized BEC after acute oxidative stress. During the first 48 h, the expression of p21(WAF1/Cip) was increased transiently before returning to baseline. After this time BEC showed increased expression of mesenchymal proteins with a decrease in epithelial markers. Analysis of TGF-β expression at mRNA and protein levels also showed a rapid increase in TGF-β2 (p < 0.006) following oxidative stress. The epithelial de-differentiation observed in vitro was abrogated by pharmacological blockade of the ALK-5 component of the TGF-β receptor. These data suggest that stress induced production of TGF-β2 by BEC can modify liver allograft function by enhancing the de-differentiation of local epithelial cells.

Mechanisms of disease progression in NASH: new paradigms

The incidence of nonalcoholic fatty liver disease is increasing at an astonishing rate in the US population. Although only a small proportion of these patients develop steatohepatitis (NASH), those who do have a greater likelihood of developing end-stage liver disease and complications. Research on liver fibrosis and NASH progression shows that hedgehog (Hh) is reactivated after liver injury to assist in liver repair and regeneration. When the process of tissue repair and regeneration is prolonged or when Hh ligand and related genes are aberrantly regulated and excessive, tissue repair goes awry and NASH progresses to cirrhosis and hepatocellular carcinoma.

Controlling the stem cell compartment and regeneration in vivo: the role of pluripotency pathways

Since the realization that embryonic stem cells are maintained in a pluripotent state through the interplay of a number of key signal transduction pathways, it is becoming increasingly clear that stemness and pluripotency are defined by the complex molecular convergence of these pathways. Perhaps this has most clearly been demonstrated by the capacity to induce pluripotency in differentiated cell types, so termed iPS cells. We are therefore building an understanding of how cells may be maintained in a pluripotent state, and how we may manipulate cells to drive them between committed and pluripotent compartments. However, it is less clear how cells normally pass in and out of the stem cell compartment under normal and diseased physiological states in vivo, and indeed, how important these pathways are in these settings. It is also clear that there is a potential "dark side" to manipulating the stem cell compartment, as deregulation of somatic stem cells is being increasingly implicated in carcinogenesis and the generation of "cancer stem cells." This review explores these relationships, with a particular focus on the role played by key molecular regulators of stemness in tissue repair, and the possibility that a better understanding of this control may open the door to novel repair strategies in vivo. The successful development of such strategies has the potential to replace or augment intervention-based strategies (cell replacement therapies), although it is clear they must be developed with a full understanding of how such approaches might also influence tumorigenesis.

Up-regulation of Hedgehog pathway is associated with cellular permissiveness for hepatitis C virus replication

Studies of the hepatitis C virus (HCV) life-cycle rely heavily on Huh7.5 cells, but the reasons why these cells are exceptionally permissive for HCV replication are not clear. Based on recent clinical observations, we hypothesized that the Hedgehog (Hh) pathway, which has not been previously associated with HCV replication, may be involved in the Huh7.5 phenotype of increased permissiveness. We tested this hypothesis by comparing levels of a variety of Hh-related cellular markers in Huh7.5 cells with the parental Huh7 cells, which are far less permissive. Here we demonstrate that Huh7.5 cells, when compared with Huh7 cells, have substantially decreased expression of epithelial markers, increased levels of mesenchymal markers, and markedly up-regulated Hh pathway activity: Shh, >100-fold, Gli1, >30-fold, Ptc, 2-fold. In Huh7.5 cells, we found that cyclopamine, an Hh pathway antagonist, reduced HCV RNA levels by 50% compared with vehicle and inactive isomer controls. Moreover, in Huh7 cells treatment with recombinant Shh ligand and SAG, both Hh pathway agonists, stimulated HCV replication by 2-fold and 4-fold, respectively. These effects were observed with both viral infections and a subgenomic replicon. Finally, we demonstrated that GDC-0449 decreased HCV RNA levels in a dose-response manner.

CONCLUSION

We have identified a relationship between HCV and Hh signaling where up-regulated pathway activity during infection promotes an environment conducive to replication. Given that Hh activity is very low in most hepatocytes, these findings may serve to further shift the model of HCV liver infection from modest widespread replication in hepatocytes to one where a subset of cells support high-level replication. These findings also introduce Hh pathway inhibitors as potential anti-HCV therapeutics.

Hedgehog signaling antagonist promotes regression of both liver fibrosis and hepatocellular carcinoma in a murine model of primary liver cancer

OBJECTIVE

Chronic fibrosing liver injury is a major risk factor for hepatocarcinogenesis in humans. Mice with targeted deletion of Mdr2 (the murine ortholog of MDR3) develop chronic fibrosing liver injury. Hepatocellular carcinoma (HCC) emerges spontaneously in such mice by 50-60 weeks of age, providing a model of fibrosis-associated hepatocarcinogenesis. We used Mdr2(-/-) mice to investigate the hypothesis that activation of the hedgehog (Hh) signaling pathway promotes development of both liver fibrosis and HCC.

METHODS

Hepatic injury and fibrosis, Hh pathway activation, and liver progenitor populations were compared in Mdr2(-/-) mice and age-matched wild type controls. A dose finding experiment with the Hh signaling antagonist GDC-0449 was performed to optimize Hh pathway inhibition. Mice were then treated with GDC-0449 or vehicle for 9 days, and effects on liver fibrosis and tumor burden were assessed by immunohistochemistry, qRT-PCR, Western blot, and magnetic resonance imaging.

RESULTS

Unlike controls, Mdr2(-/-) mice consistently expressed Hh ligands and progressively accumulated Hh-responsive liver myofibroblasts and progenitors with age. Treatment of aged Mdr2-deficient mice with GDC-0449 significantly inhibited hepatic Hh activity, decreased liver myofibroblasts and progenitors, reduced liver fibrosis, promoted regression of intra-hepatic HCCs, and decreased the number of metastatic HCC without increasing mortality.

CONCLUSIONS

Hh pathway activation promotes liver fibrosis and hepatocarcinogenesis, and inhibiting Hh signaling safely reverses both processes even when fibrosis and HCC are advanced.

Steatotic liver: a suitable source for the isolation of hepatic progenitor cells

BACKGROUND

Alternative and/or complementary sources of cells such as hepatic progenitor cells (HPC) are under investigation for hepatic cell therapy purposes. Steatotic livers are those most commonly rejected for clinical transplantation and are also unsuitable for good quality hepatocyte isolation.

AIM

Taken together these two facts, our aim was to investigate whether they could represent a suitable source for the isolation of progenitor cells.

METHODS

Rats fed for 7 weeks with methionine-choline deficient diets showing proved steatotic signs (i.e. increase in hepatic lipids; macrovesicular steatosis) and steatotic and normal human liver samples were used to study the expression of HPC markers and to isolate these cells.

RESULTS

In the liver of the steatotic rats there was a significant increase in HPC (known as oval cells in rodents) markers such as Thy-1, epithelial cell adhesion molecule (EpCAM) and OV-6 (2-, 3- and 5-fold increase respectively). Additionally, there was an increase in the yield of isolated oval cells compared to control rats. Similarly, studies using human livers clearly confirmed an increase in the expression of HPC markers in the steatotic tissue and a significant rise in the number of isolated progenitor cells (EpCAM+, Thy-1+, OV-6+) (10, 12 and 11.6 × 10(4)  cells/g of tissue respectively).

CONCLUSIONS

These data suggest that steatotic livers, discarded for orthotopic liver transplantation and hepatocyte isolation, could be a suitable source for large scale isolation of HPC which might be potential candidates in liver cell therapy.

Increased production of sonic hedgehog by ballooned hepatocytes

Ballooned hepatocytes distinguish non-alcoholic steatohepatitis (NASH) from steatosis. Such cells contain dilated endoplasmic reticulum and ubiquitin aggregates, characteristics of endoplasmic reticulum stress. Hepatocyte ballooning increases the risk for fibrosis in NASH, suggesting that ballooned hepatocytes release pro-fibrogenic factors. Hedgehog ligands function as pro-fibrogenic factors in liver diseases, but mechanisms for hedgehog ligand production remain poorly understood. We evaluated the hypothesis that endoplasmic reticulum stress induces hepatocyte production of hedgehog ligands that provide paracrine pro-fibrogenic signals to neighbouring cells. In livers from NASH patients, keratin 8/18 and ubiquitin staining demonstrated enlarged, keratin 8/18-negative/ubiquitin-positive hepatocytes (ballooned hepatocytes) that were positive for Sonic hedgehog. In order to model endoplasmic reticulum stress in vitro, primary mouse hepatocytes were treated with tunicamycin. Compared to vehicle, tunicamycin significantly increased Sonic hedgehog and Indian hedgehog expression. Furthermore, conditioned medium from tunicamycin-treated hepatocytes increased Gli-luciferase reporter activity 14-fold more than conditioned medium from vehicle-treated hepatocytes. Cyclopamine (hedgehog signalling inhibitor) abrogated the effect of conditioned medium from tunicamycin-treated hepatocytes, verifying that soluble hepatocyte-derived factors activate hedgehog signalling. Ballooned hepatocytes in NASH patients did not express the hedgehog target gene, Gli2, α-smooth muscle actin or vimentin, but were surrounded by Gli2-positive stromal cells expressing these myofibroblast markers. Trichrome staining demonstrated the accumulation of ballooned hepatocytes in areas of matrix deposition, and numbers of Sonic hedgehog-positive hepatocytes correlated with the degree of ballooning and fibrosis stage. Hepatocytes undergoing endoplasmic reticiulum stress generate hedgehog ligands which act as paracrine pro-fibrogenic factors for hedgehog-responsive stromal cells. These results help to explain why fibrosis stage correlates with hepatocyte ballooning in NASH.

Hedgehog signaling in the liver

Reactivation of Hedgehog (Hh), a morphogenic signaling pathway that controls progenitor cell fate and tissue construction during embryogenesis occurs during many types of liver injury in adult. The net effects of activating the Hedgehog pathway include expansion of liver progenitor populations to promote liver regeneration, but also hepatic accumulation of inflammatory cells, liver fibrogenesis, and vascular remodeling. All of these latter responses are known to be involved in the pathogenesis of cirrhosis. In addition, Hh signaling may play a role in primary liver cancers, such as cholangiocarcinoma and hepatocellular carcinoma. Study of Hedgehog signaling in liver cells is in its infancy. Additional research in this area is justified given growing experimental and clinical data supporting a role for the pathway in regulating outcomes of liver injury.

New concepts in liver regeneration

The unique ability of the liver to regenerate itself has fascinated biologists for years and has made it the prototype for mammalian organ regeneration. Harnessing this process has great potential benefit in the treatment of liver failure and has been the focus of intense research over the past 50 years. Not only will detailed understanding of cell proliferation in response to injury be applicable to other dysfunction of organs, it may also shed light on how cancer develops in a cirrhotic liver, in which there is intense pressure on cells to regenerate. Advances in molecular techniques over the past few decades have led to the identification of many regulatory intermediates, and pushed us onto the verge of an explosive era in regenerative medicine. To date, more than 10 clinical trials have been reported in which augmented regeneration using progenitor cell therapy has been attempted in human patients. This review traces the path that has been taken over the last few decades in the study of liver regeneration, highlights new concepts in the field, and discusses the challenges that still stand between us and clinical therapy.

Cancer stem cells: repair gone awry?

Because cell turnover occurs in all adult organs, stem/progenitor cells within the stem-cell niche of each tissue must be appropriately mobilized and differentiated to maintain normal organ structure and function. Tissue injury increases the demands on this process, and thus may unmask defective regulation of pathways, such as Hedgehog (Hh), that modulate progenitor cell fate. Hh pathway dysregulation has been demonstrated in many types of cancer, including pancreatic and liver cancers, in which defective Hh signaling has been linked to outgrowth of Hh-responsive cancer stem-initiating cells and stromal elements. Hence, the Hh pathway might be a therapeutic target in such tumors.

Growth factor- and cytokine-driven pathways governing liver stemness and differentiation

Liver is unique in its capacity to regenerate in response to injury or tissue loss. Hepatocytes and other liver cells are able to proliferate and repopulate the liver. However, when this response is impaired, the contribution of hepatic progenitors becomes very relevant. Here, we present an update of recent studies on growth factors and cytokine-driven intracellular pathways that govern liver stem/progenitor cell expansion and differentiation, and the relevance of these signals in liver development, regeneration and carcinogenesis. Tyrosine kinase receptor signaling, in particular, c-Met, epidermal growth factor receptors or fibroblast growth factor receptors, contribute to proliferation, survival and differentiation of liver stem/progenitor cells. Different evidence suggests a dual role for the transforming growth factor (TGF)-β signaling pathway in liver stemness and differentiation. On the one hand, TGF-β mediates progression of differentiation from a progenitor stage, but on the other hand, it contributes to the expansion of liver stem cells. Hedgehog family ligands are necessary to promote hepatoblast proliferation but need to be shut off to permit subsequent hepatoblast differentiation. In the same line, the Wnt family and β-catenin/T-cell factor pathway is clearly involved in the maintenance of liver stemness phenotype, and its repression is necessary for liver differentiation during development. Collectively, data indicate that liver stem/progenitor cells follow their own rules and regulations. The same signals that are essential for their activation, expansion and differentiation are good candidates to contribute, under adequate conditions, to the paradigm of transformation from a pro-regenerative to a pro-tumorigenic role. From a clinical perspective, this is a fundamental issue for liver stem/progenitor cell-based therapies.

The role of Hedgehog signaling in fibrogenic liver repair

Repair of adult liver, like many tissues, involves the coordinated response of a number of different cell types. In adult livers, fibroblastic cells, ductular cells, inflammatory cells, and progenitor cells contribute to this process. Our studies demonstrate that the fates of such cells are dictated, at least in part, by Hedgehog, a fetal morphogenic pathway that was once thought to be active mainly during embryogenesis. Studies of injured adult human and rodent livers demonstrate that injury-related activation of the Hedgehog pathway modulates several important aspects of repair, including the growth of hepatic progenitor populations, hepatic accumulation of myofibroblasts, repair-related inflammatory responses, vascular remodeling, liver fibrosis and hepatocarcinogenesis. These findings identify the Hedgehog pathway as a potentially important target for biomarker development and therapeutic manipulation, and emphasize the need for further research to advance knowledge about how this pathway is regulated by and interacts with other signals that regulate adult liver repair.