Extracellular matrix modulates sensitivity of hepatocytes to fibroblastoid dedifferentiation and transforming growth factor beta-induced apoptosis

Recent progress in understanding pathogenesis and liver pathology in biliary atresia

Biliary atresia is an infantile destructive inflammatory cholangiopathy that causes obliteration of both intrahepatic and extrahepatic bile ducts and eventually liver cirrhosis. So far, the exact etiology and pathogenesis of biliary atresia remain unclear, and possible etiologies include congenital and genetic factors, infection, inflammation, immune reaction, maternal factors, and vascular factors. Immunoinflammatory theory has been accepted by most researchers, which is supported by liver pathological changes. This review focuses on the recent progress in understanding pathogenesis and liver pathology in biliary atresia.

Differentiation and selection of hepatocyte precursors in suspension spheroid culture of transgenic murine embryonic stem cells

Embryonic stem cell-derived hepatocyte precursor cells represent a promising model for clinical transplantations to diseased livers, as well as for establishment of in vitro systems for drug metabolism and toxicology investigations. This study aimed to establish an in vitro culture system for scalable generation of hepatic progenitor cells. We used stable transgenic clones of murine embryonic stem cells possessing a reporter/selection vector, in which the enhanced green fluorescent protein- and puromycin N-acetyltransferase-coding genes are driven by a common alpha-fetoprotein gene promoter. This allowed for "live" monitoring and puromycin selection of the desired differentiating cell type possessing the activated alpha-fetoprotein gene. A rotary culture system was established, sequentially yielding initially partially selected hepatocyte lineage-committed cells, and finally, a highly purified cell population maintained as a dynamic suspension spheroid culture, which progressively developed the hepatic gene expression phenotype. The latter was confirmed by quantitative RT-PCR analysis, which showed a progressive up-regulation of hepatic genes during spheroid culture, indicating development of a mixed hepatocyte precursor-/fetal hepatocyte-like cell population. Adherent spheroids gave rise to advanced differentiated hepatocyte-like cells expressing hepatic proteins such as albumin, alpha-1-antitrypsin, cytokeratin 18, E-cadherin, and liver-specific organic anion transporter 1, as demonstrated by fluorescent immunostaining. A fraction of adherent cells was capable of glycogen storage and of reversible up-take of indocyanine green, demonstrating their hepatocyte-like functionality. Moreover, after transplantation of spheroids into the mouse liver, the spheroid-derived cells integrated into recipient. These results demonstrate that large-scale hepatocyte precursor-/hepatocyte-like cultures can be established for use in clinical trials, as well as in in vitro screening assays.

The microenvironment in hepatocyte regeneration and function in rats with advanced cirrhosis

In advanced cirrhosis, impaired function is caused by intrinsic damage to the native liver cells and from the abnormal microenvironment in which the cells reside. The extent to which each plays a role in liver failure and regeneration is unknown. To examine this issue, hepatocytes from cirrhotic and age-matched control rats were isolated, characterized, and transplanted into the livers of noncirrhotic hosts whose livers permit extensive repopulation with donor cells. Primary hepatocytes derived from livers with advanced cirrhosis and compensated function maintained metabolic activity and the ability to secrete liver-specific proteins, whereas hepatocytes derived from cirrhotic livers with decompensated function failed to maintain metabolic or secretory activity. Telomere studies and transcriptomic analysis of hepatocytes recovered from progressively worsening cirrhotic livers suggest that hepatocytes from irreversibly failing livers show signs of replicative senescence and express genes that simultaneously drive both proliferation and apoptosis, with a later effect on metabolism, all under the control of a central cluster of regulatory genes, including nuclear factor κB and hepatocyte nuclear factor 4α. Cells from cirrhotic and control livers engrafted equally well, but those from animals with cirrhosis and failing livers showed little initial evidence of proliferative capacity or function. Both, however, recovered more than 2 months after transplantation, indicating that either mature hepatocytes or a subpopulation of adult stem cells are capable of full recovery in severe cirrhosis.

CONCLUSION

Transplantation studies indicate that the state of the host microenvironment is critical to the regenerative potential of hepatocytes, and that a change in the extracellular matrix can lead to regeneration and restoration of function by cells derived from livers with end-stage organ failure.

Role of chemokine receptor CXCR7 in bladder cancer progression

Bladder cancer is one of the most common tumors of the genitourinary tract; however, the molecular events underlying growth and invasion of the tumor remain unclear. Here, role of the CXCR7 receptor in bladder cancer was further explored. CXCR7 protein expression was examined using high-density tissue microarrays. Expression of CXCR7 showed strong epithelial staining that correlated with bladder cancer progression. In vitro and in vivo studies in bladder cancer cell lines suggested that alterations in CXCR7 expression were associated with the activities of proliferation, apoptosis, migration, invasion, angiogenesis and tumor growth. Moreover, CXCR7 expression was able to regulate expression of the proangiogenic factors IL-8 or VEGF, which may involve in the regulation of tumor angiogenesis. Finally, we found that signaling by the CXCR7 in bladder cancer cells activates AKT, ERK and STAT3 pathways. The AKT and ERK pathways may reciprocally regulate, which are responsible for in vitro and in vivo epithelial to mesenchymal transition (EMT) process of bladder cancer. Simultaneously targeting the two pathways by using U0126 and LY294002 inhibitors or using CCX733, a selective CXCR7 antagonist drastically reduced CXCR7-induced EMT process. Taken together, our data show for the first time that CXCR7 plays a role in the development of bladder cancer. Targeting CXCR7 or its downstream-activated AKT and ERK pathways may prove beneficial to prevent metastasis and provide a more effective therapeutic strategy for bladder cancer.

Src family kinases regulate renal epithelial dedifferentiation through activation of EGFR/PI3K signaling

Dedifferentiation, a process by which differentiated cells become mesenchymal-like proliferating cells, is the first step in renal epithelium repair and occurs in vivo after acute kidney injury and in vitro in primary culture. However, the underlying mechanism remains poorly understood. In this report, we studied the signaling events that mediate dedifferentiation of proximal renal tubular cells (RPTC) in primary culture. RPTC dedifferentiation characterized by increased expression of vimentin concurrent with decreased expression of cytokeratin-18 was observed at 24 h after the initial plating of freshly isolated proximal tubules and persisted for 72 h. At 96 h, RPTC started to redifferentiate as revealed by reciprocal expression of cytokeratin-18 and vimentin and completed at 120 h. Phosphorylation levels of Src, epidermal growth factor receptor (EGFR), AKT (a target of phosphoinositide-3-kinase (PI3K)), and ERK1/2 were increased in the early time course of culture (<72 h). Inhibition of Src family kinases (SFKs) with PP1 blocked EGFR, AKT, and ERK1/2 phosphorylation, as well as RPTC dedifferentiation. Inhibition of EGFR with AG1478 also blocked AKT and ERK1/2 phosphorylation and RPTC dedifferentiation. Although inactivation of the PI3K/AKT pathway with LY294002 inhibited RPTC dedifferentiation, blocking the ERK1/2 pathway with U0126 did not show such an effect. Moreover, inhibition of SFKs, EGFR, PI3K/AKT, but not ERK1/2 pathways abrogated RPTC outgrowth and SFK inhibition decreased RPTC proliferation and migration. These findings demonstrate a critical role of SFKs in mediating RPTC dedifferentiation through activation of the EGFR/PI3K signaling pathway.

Wnt/β-catenin signaling is critical for dedifferentiation of aged epidermal cells in vivo and in vitro

Aged epidermal cells have the capacity to dedifferentiate into stem cell-like cells. However, the signals that regulate the dedifferentiation of aged epidermal cells remain unclear. Here, we provide evidence that Wnt/β-catenin is critical for aged epidermal cell dedifferentiation in vivo and in vitro. Some aged epidermal cells in human ultrathin epidermal sheets lacking basal stem cells transplanted onto wounds dedifferentiated into stem cell-like cells that were positive for CK19 and β1 integrin but negative for CK10. In addition, Wnt/β-catenin pathway was activated during this process. There was increased expression of Wnt-1, Wnt-4, Wnt-7a, β-catenin, cyclin D1, and c-myc. Secreted frizzled-related protein 1, a Wnt/β-catenin pathway inhibitor, blocked dedifferentiation in vivo. Then, the activator, a highly specific glycogen synthase kinase (GSK)-3β inhibitor, of Wnt/β-catenin pathway was added to the culture medium of aged epidermal cells. Surprisingly, we found that the activator induced higher expression of CK19, β1 integrin, Oct4, and Nanog proteins. The induced aged epidermal cells exhibited high colony-forming efficiency, long-term proliferative potential and could regenerate a skin equivalent (as do epidermal stem cells). These results suggested that activation of Wnt/β-catenin pathway induced the dedifferentiation of aged epidermal cells, which suggest a new approach to generate epidermal stem cell-like cells.

The MAPK MEK1/2-ERK1/2 Pathway and Its Implication in Hepatocyte Cell Cycle Control

Primary cultures of hepatocytes are powerful models in studying the sequence of events that are necessary for cell progression from a G0-like state to S phase. The models mimic the physiological process of hepatic regeneration after liver injury or partial hepatectomy. Many reports suggest that the mitogen-activated protein kinase (MAPK) ERK1/2 can support hepatocyte proliferation in vitro and in vivo and the MEK/ERK cascade acts as an essential element in hepatocyte responses induced by the EGF. Moreover, its disregulation has been associated with the promotion of tumor cell growth of a variety of tumors, including hepatocellular carcinoma. Whereas the strict specificity of action of ERK1 and ERK2 is still debated, the MAPKs may have specific biological functions under certain contexts and according to the differentiation status of the cells, notably hepatocytes. In this paper, we will focus on MEK1/2-ERK1/2 activations and roles in normal rodent hepatocytes in vitro and in vivo after partial hepatectomy and in human hepatocarcinoma cells. The possible specificity of ERK1 and ERK2 in normal and transformed hepatocyte will be discussed in regard to other differentiated and undifferentiated cellular models.

Liver fibrosis protects mice from acute hepatocellular injury

BACKGROUND & AIMS

Development of fibrosis is part of the pathophysiologic process of chronic liver disease. Although it is considered deleterious, it also represents a form of tissue repair. Deposition of extracellular matrix changes the cellular environment of the liver; we investigated whether it increases resistance to noxious stimuli and the role of changes in intracellular signaling to hepatocytes in mediating this effect.

METHODS

Primary cultures of mouse hepatocytes were exposed to type I collagen (COL1); cell injury was assessed by morphologic and biochemical criteria. The expression of Bcl-2 family members was evaluated by immunoblot analyses. Activation of extracellular signal-regulated kinase (ERK) was assessed using phospho-specific antibodies. Liver fibrosis was induced by repeated administration of thioacetamide or carbon tetrachloride to mice; mice were then exposed to Fas antibodies.

RESULTS

Hepatocytes exposed to COL1 were more resistant to a variety of hepatotoxins, in a dose-dependent manner, and had lower levels of Bad, Bid, and Bax proapoptotic proteins compared with control hepatocytes. Activation of ERK1/2 was stronger and quicker in hepatocytes exposed to COL1. The MEK1/2 inhibitors U0126 and PD98059 reversed the protective effects of COL1 and the decrease in proapoptotic proteins. Hepatocytes isolated from ERK1(-/-) mice were insensitive to the protective effect of COL1. Fibrotic livers from wild-type mice had high levels of phospho-ERK1 and were resistant to Fas-induced cell death. ERK1(-/-) mice lost this effect.

CONCLUSIONS

Production of COL1 during liver fibrosis induces a hepatoprotective response that is mediated by activation of ERK1 signaling.

Regulation of TGF-β1-Induced Pro-Apoptotic Signaling by Growth Factor Receptors and Extracellular Matrix Receptor Integrins in the Liver

Hepatocellular carcinoma (HCC) often arises from chronically diseased livers. Persistent liver inflammation causes the accumulation of excessive extracellular matrix (ECM) proteins and impairs the liver function, finally leading to the development of HCC. A pleiotropic cytokine, transforming growth factor (TGF)-β1, plays critical roles throughout the process of fibrogenesis and hepatocarcinogenesis. In the liver, TGF-β1 inhibits the proliferation of hepatocytes and stimulates the production of ECM from hepatic stellate cells (HSCs) to maintain tissue homeostasis. During disease progression, both growth factors/cytokines and the ECM alter the TGF-β1 signals by modifying the phosphorylation of Smad proteins at their C-terminal and linker regions. TGF-β1 stimulates the expression of integrins, cellular receptors for ECM, along with an increase in ECM accumulation. The activation of integrins by the ECM modulates the response to TGF-β1 in hepatic cells, resulting in their resistance to TGF-β1-induced growth suppression in hepatocytes and the sustained production of ECM proteins in activated HSCs/myofibroblasts. Both growth factor receptors and integrins modify the expression and/or functions of the downstream effectors of TGF-β1, resulting in the escape of hepatocytes from TGF-β1-induced apoptosis. Recent studies have revealed that the alterations of Smad phosphorylation that occur as the results of the crosstalk between TGF-β1, growth factors and integrins could change the nature of TGF-β1 signals from tumor suppression to promotion. Therefore, the modification of Smad phosphorylation could be an attractive target for the prevention and/or treatment of HCC.

TGF-β in progression of liver disease

Transforming growth factor-β (TGF-β) is a central regulator in chronic liver disease contributing to all stages of disease progression from initial liver injury through inflammation and fibrosis to cirrhosis and hepatocellular carcinoma. Liver-damage-induced levels of active TGF-β enhance hepatocyte destruction and mediate hepatic stellate cell and fibroblast activation resulting in a wound-healing response, including myofibroblast generation and extracellular matrix deposition. Being recognised as a major profibrogenic cytokine, the targeting of the TGF-β signalling pathway has been explored with respect to the inhibition of liver disease progression. Whereas interference with TGF-β signalling in various short-term animal models has provided promising results, liver disease progression in humans is a process of decades with different phases in which TGF-β or its targeting might have both beneficial and adverse outcomes. Based on recent literature, we summarise the cell-type-directed double-edged role of TGF-β in various liver disease stages. We emphasise that, in order to achieve therapeutic effects, we need to target TGF-β signalling in the right cell type at the right time.

Distinct role of endocytosis for Smad and non-Smad TGF-β signaling regulation in hepatocytes

BACKGROUND & AIMS

In injured liver, TGF-β affects all hepatic cell types and participates in wound healing and fibrogenesis. TGF-β downstream signaling is highly complex and cell type dependent, involving Smad and non-Smad signaling cascades thus requiring tight regulation. Endocytosis has gained relevance as important mechanism to control signaling initiation and termination. In this study, we investigated endocytic mechanisms for TGF-β mediated Smad and non-Smad signaling in hepatocytes.

METHODS

Endocytosis in hepatocytes was elucidated using chemical inhibitors, RNAi, viral gene transfer and caveolin-1-/- mice. TGF-β signaling was monitored by Western blot, reporter assays and gene expression analysis.

RESULTS

In hepatocytes, Smad activation is to a large degree accomplished AP-2 complex dependent on the hepatocyte surface without the necessity of clathrin coated pit formation or an endocytic step. In contrast, non-Smad/AKT pathway activation required functional dynamin mediated endocytosis and the presence of caveolin-1, an essential protein for caveolae formation. Furthermore, these two TGF-β signaling initiation platforms discriminate distinct signaling routes that integrate at the transcriptional level as shown for TGF-β target genes, Id1, Smad7, and CTGF. Endocytosis inhibition increased canonical Smad signaling and culminated in a superinduction of Id1 and Smad7 expression, whereas caveolin-1 mediated AKT pathway activation was required for maximal CTGF induction.

CONCLUSIONS

Endocytosis is critical for TGF-β signaling regulation in hepatocytes and determines gene expression signature and (patho)physiological outcome.

Epithelial-mesenchymal transition in chronic liver disease: fibrogenesis or escape from death?

The possibility that epithelial-mesenchymal transition (EMT) could contribute to hepatic fibrogenesis in chronic liver diseases as reported in other organs, particularly the kidney, reinforced the concept that activated hepatic stellate cells were not the only key players in the hepatic fibrogenic process and that other cell types, either hepatic (i.e. portal fibroblast) or extrahepatic (bone marrow-derived cells and circulating fibrocytes) could contribute to this process. The possibility of the rapid mobilization of a large amount of fibrogenic cells by EMT after liver tissue injury made this phenomenon a relevant and suitable target for anti-fibrogenic strategies. Following an initial enthusiasm for the discovery of this novel pathway in fibrogenesis and the publication of a several highly quoted papers, more recent research has started to cast serious doubts upon the real relevance of this phenomenon in human fibrogenetic disorders. The debate on the authenticity of EMT or at least on its real contribution to the fibrogenic process has become very animated, sometimes reaching levels of "religious" integralism. The overall result is a general confusion on the meaning and on the definition of several key aspects. The aim of this article is to analyze and discuss the evidence supporting or confuting this possibility in order to reach reasonable and useful conclusions.

Reversal of hepatic fibrosis: pathophysiological basis of antifibrotic therapies

Chronic liver injuries of different etiologies eventually lead to fibrosis, a scarring process associated with increased and altered deposition of extracellular matrix in the liver. Progression of fibrosis has a major worldwide clinical impact due to the high number of patients affected by chronic liver disease which can lead to severe complications, expensive treatment, a possible need for liver transplantation, and death. Liver fibrogenesis is characterized by activation of hepatic stellate cells and other extracellular matrix producing cells. Liver fibrosis may regress following specific therapeutic interventions. Other than removing agents causing chronic liver damage, no antifibrotic drug is currently available in clinical practice. The extent of liver fibrosis is variable between individuals, even after controlling for exogenous factors. Thus, host genetic factors are considered to play an important role in the process of liver scarring. Until recently it was believed that this process was irreversible. However, emerging experimental and clinical evidence is starting to show that even cirrhosis in its early stages is potentially reversible.

Reversal of hepatic fibrosis: pathophysiological basis of antifibrotic therapies

Chronic liver injuries of different etiologies eventually lead to fibrosis, a scarring process associated with increased and altered deposition of extracellular matrix in the liver. Progression of fibrosis has a major worldwide clinical impact due to the high number of patients affected by chronic liver disease which can lead to severe complications, expensive treatment, a possible need for liver transplantation, and death. Liver fibrogenesis is characterized by activation of hepatic stellate cells and other extracellular matrix producing cells. Liver fibrosis may regress following specific therapeutic interventions. Other than removing agents causing chronic liver damage, no antifibrotic drug is currently available in clinical practice. The extent of liver fibrosis is variable between individuals, even after controlling for exogenous factors. Thus, host genetic factors are considered to play an important role in the process of liver scarring. Until recently it was believed that this process was irreversible. However, emerging experimental and clinical evidence is starting to show that even cirrhosis in its early stages is potentially reversible.

Matrix stiffness modulates proliferation, chemotherapeutic response, and dormancy in hepatocellular carcinoma cells

There is increasing evidence that the physical environment is a critical mediator of tumor behavior. Hepatocellular carcinoma (HCC) develops within an altered biomechanical environment, and increasing matrix stiffness is a strong predictor of HCC development. The aim of this study was to establish whether changes in matrix stiffness, which are characteristic of inflammation and fibrosis, regulate HCC cell proliferation and chemotherapeutic response. Using an in vitro system of "mechanically tunable" matrix-coated polyacrylamide gels, matrix stiffness was modeled across a pathophysiologically relevant range, corresponding to values encountered in normal and fibrotic livers. Increasing matrix stiffness was found to promote HCC cell proliferation. The proliferative index (assessed by Ki67 staining) of Huh7 and HepG2 cells was 2.7-fold and 12.2-fold higher, respectively, when the cells were cultured on stiff (12 kPa) versus soft (1 kPa) supports. This was associated with stiffness-dependent regulation of basal and hepatocyte growth factor-stimulated mitogenic signaling through extracellular signal-regulated kinase, protein kinase B (PKB/Akt), and signal transducer and activator of transcription 3. β1-Integrin and focal adhesion kinase were found to modulate stiffness-dependent HCC cell proliferation. Following treatment with cisplatin, we observed reduced apoptosis in HCC cells cultured on stiff versus soft (physiological) supports. Interestingly, however, surviving cells from soft supports had significantly higher clonogenic capacity than surviving cells from a stiff microenvironment. This was associated with enhanced expression of cancer stem cell markers, including clusters of differentiation 44 (CD44), CD133, c-kit, cysteine-X-cysteine receptor 4, octamer-4 (CXCR4), and NANOG.

CONCLUSION

Increasing matrix stiffness promotes proliferation and chemotherapeutic resistance, whereas a soft environment induces reversible cellular dormancy and stem cell characteristics in HCC. This has implications for both the treatment of primary HCC and the prevention of tumor outgrowth from disseminated tumor cells. (HEPATOLOGY 2011;).

Evidence for mesenchymal-epithelial transition associated with mouse hepatic stem cell differentiation

Mesenchymal−epithelial transition events are related to embryonic development, tissue construction, and wound healing. Stem cells are involved in all of these processes, at least in part. However, the direct evidence of mesenchymal−epithelial transition associated with stem cells is unclear. To determine whether mesenchymal−epithelial transition occurs in liver development and/or the differentiation process of hepatic stem cells in vitro, we analyzed a variety of murine liver tissues from embryonic day 11.5 to adults and the colonies derived from hepatic stem/progenitor cells isolated with flow cytometry. The results of gene expression, immunohistochemistry and Western blot showed that as liver develops, the expression of epithelial markers such as Cytokeratin18 and E-cadherin increase, while expression of mesenchymal markers such as vimentin and N-cadherin decreased. On the other hand, in freshly isolated hepatic stem cells, the majority of cells (65.0%) co-express epithelial and mesenchymal markers; this proportion is significantly higher than observed in hematopoietic cells, non-hematopoietic cells and non-stem cell fractions. Likewise, in stem cell-derived colonies cultured over time, upregulation of epithelial genes (Cytokeratin-18 and E-cadherin) occurred simultaneously with downregulation of mesenchymal genes (vimentin and Snail1). Furthermore, in the fetal liver, vimentin-positive cells in the non-hematopoietic fraction had distinct proliferative activity and expressed early the hepatic lineage marker alpha-fetoprotein.

Ductal plates in hepatic ductular reactions. Hypothesis and implications. III. Implications for liver pathology

This article discusses on the basis of the ductal plate hypothesis the implication of the concept for several liver abnormalities. The occurrence of ductal plates (DP) during liver growth in childhood would explain the paraportal and parenchymal localizations of von Meyenburg complexes in postnatally developed parts of the liver, and their higher incidence in adulthood versus childhood. It partly clarifies the lack of postnatal intrahepatic bile duct development in Alagille syndrome and the reduced number of portal tracts in this disease. Ductular reactions (DRs) in DP configuration are the predominant type of progenitor cell reaction in fulminant necro-inflammatory liver disease, when lack of sufficient parenchymal regeneration results in liver failure. The concept of dissecting DRs explains the micronodular pattern of advanced biliary and alcoholic cirrhosis. The concept explains the DP patterns of bile ducts in several cases of biliary atresia, with implications for diagnosis and prognosis. The hypothesis also has an impact on concepts about stem/progenitor cells and their niche.

Transcription factors ETF, E2F, and SP-1 are involved in cytokine-independent proliferation of murine hepatocytes

The cellular basis of liver regeneration has been intensely investigated for many years. However, the mechanisms initiating hepatocyte "plasticity" and priming for proliferation are not yet fully clear. We investigated alterations in gene expression patterns during the first 72 hours of C57BL/6N mouse hepatocyte culture on collagen monolayers (CM), which display a high basal frequency of proliferation in the absence of cytokines. Although many metabolic genes were down-regulated, genes related to mitogen-activated protein kinase (MAPK) signaling and cell cycle were up-regulated. The latter genes showed an overrepresentation of transcription factor binding sites (TFBS) for ETF (TEA domain family member 2), E2F1 (E2F transcription factor 1), and SP-1 (Sp1 transcription factor) (P < 0.001), all depending on MAPK signaling. Time-dependent increase of ERK1/2 phosphorylation occurred during the first 48 hours (and beyond) in the absence of cytokines, accompanied by an enhanced bromodeoxyuridine labeling index of 20%. The MEK inhibitor PD98059 blunted these effects indicating MAPK signaling as major trigger for this cytokine-independent proliferative response. In line with these in vitro findings, liver tissue of mice challenged with CCl(4) displayed hepatocytes with intense p-ERK1/2 staining and nuclear SP-1 and E2F1 expression. Furthermore, differentially expressed genes in mice after partial hepatectomy contained overrepresented TFBS for ETF, E2F1, and SP-1 and displayed increased expression of E2F1.

CONCLUSION

Cultivation of murine hepatocytes on CM primes cells for proliferation through cytokine-independent activation of MAPK signaling. The transcription factors ETF, E2F1, and SP-1 seem to play a pronounced role in mediating proliferation-dependent differential gene expression. Similar events, but on a shorter time-scale, occur very early after liver damage in vivo.

Augmenter of liver regeneration (ALR) protects human hepatocytes against apoptosis

Augmenter of liver regeneration (ALR) is known to support liver regeneration and to stimulate proliferation of hepatocytes. However, it is not known if ALR exerts anti-apoptotic effects in human hepatocytes and whether this protective effect is cell type specific. This is relevant, because compounds that protect the liver against apoptosis without undesired effects, such as protection of metastatic tumour cells, would be appreciated in several clinical settings. Primary human hepatocytes (phH) and organotypic cancer cell lines were exposed to different concentrations of apoptosis inducers (ethanol, TRAIL, anti-Apo, TGF-β, actinomycin D) and cultured with or without recombinant human ALR (rhALR). Apoptosis was evaluated by the release of cytochrome c from mitochondria and by FACS with propidium iodide (PI) staining. ALR significantly decreased apoptosis induced by ethanol, TRAIL, anti-Apo, TGF-β and actinomycin D. Further, the anti-apoptotic effect of ALR was observed in primary human hepatocytes and in HepG2 cells but not in bronchial (BC1), colonic (SW480), gastric (GC1) and pancreatic (L3.6PL) cell lines. Therefore, the hepatotrophic growth factor ALR acts in a liver specific manner with regards to both its mitogenic and its anti-apoptotic effect. Unlike the growth factors HGF and EGF, rhALR acts in a liver specific manner. Therefore, ALR is a promising candidate for further evaluation as a possible hepatoprotective factor in clinical settings.

Phenotype and growth behavior of residual β-catenin-positive hepatocytes in livers of β-catenin-deficient mice

Signaling through the Wnt/β-catenin pathway is a crucial determinant of hepatic zonal gene expression, liver development, regeneration, and tumorigenesis. Transgenic mice with hepatocyte-specific knockout of Ctnnb1 (encoding β-catenin) have proven their usefulness in elucidating these processes. We now found that a small number of hepatocytes escape the Cre-mediated gene knockout in that mouse model. The remaining β-catenin-positive hepatocytes showed approximately 25% higher cell volumes compared to the β-catenin-negative cells and exhibited a marker protein expression profile similar to that of normal perivenous hepatocytes or hepatoma cells with mutationally activated β-catenin. Surprisingly, the expression pattern was observed independent of the cell's position within the liver lobule, suggesting a malfunction of physiological periportal repression of perivenously expressed genes in β-catenin-deficient liver. Clusters of β-catenin-expressing hepatocytes lacked expression of the gap junction proteins Connexin 26 and 32. Nonetheless, β-catenin-positive hepatocytes had no striking proliferative advantage, but started to grow out on treatment with phenobarbital, a tumor-promoting agent known to facilitate the formation of mouse liver adenoma with activating mutations of Ctnnb1. Progressive re-population of Ctnnb1 knockout livers with wild-type hepatocytes was seen in aged mice with a pre-cirrhotic phenotype. In these large clusters of β-catenin-expressing hepatocytes, perivenous-specific gene expression was re-established. In summary, our data demonstrate that the zone-specificity of a hepatocyte's gene expression profile is dependent on the presence of β-catenin, and that β-catenin provides a proliferative advantage to hepatocytes when promoted with phenobarbital, or in a pre-cirrhotic environment.

Evaluation of an in vitro toxicogenetic mouse model for hepatotoxicity

Numerous studies support the fact that a genetically diverse mouse population may be useful as an animal model to understand and predict toxicity in humans. We hypothesized that cultures of hepatocytes obtained from a large panel of inbred mouse strains can produce data indicative of inter-individual differences in in vivo responses to hepato-toxicants. In order to test this hypothesis and establish whether in vitro studies using cultured hepatocytes from genetically distinct mouse strains are feasible, we aimed to determine whether viable cells may be isolated from different mouse inbred strains, evaluate the reproducibility of cell yield, viability and functionality over subsequent isolations, and assess the utility of the model for toxicity screening. Hepatocytes were isolated from 15 strains of mice (A/J, B6C3F1, BALB/cJ, C3H/HeJ, C57BL/6J, CAST/EiJ, DBA/2J, FVB/NJ, BALB/cByJ, AKR/J, MRL/MpJ, NOD/LtJ, NZW/LacJ, PWD/PhJ and WSB/EiJ males) and cultured for up to 7 days in traditional 2-dimensional culture. Cells from B6C3F1, C57BL/6J, and NOD/LtJ strains were treated with acetaminophen, WY-14,643 or rifampin and concentration-response effects on viability and function were established. Our data suggest that high yield and viability can be achieved across a panel of strains. Cell function and expression of key liver-specific genes of hepatocytes isolated from different strains and cultured under standardized conditions are comparable. Strain-specific responses to toxicant exposure have been observed in cultured hepatocytes and these experiments open new opportunities for further developments of in vitro models of hepatotoxicity in a genetically diverse population.

The mechanisms underlying fibroblast apoptosis regulated by growth factors during wound healing

While investigating the mechanisms underlying cell death during wound healing processes, we uncovered the pro-apoptotic effects of basic fibroblast growth factor (bFGF) on granulation tissue fibroblasts following pretreatment with transforming growth factor (TGF)-beta1 in vitro. bFGF induced caspase-3 activation and apoptosis in TGF-beta1-pretreated granulation tissue-derived fibroblasts (GF-1) following bFGF treatment for 48 and 96 h. In contrast, fibroblasts that had been treated in the same manner and that originated from the uninjured dermis did not display apoptosis, indicating that the mechanisms underlying apoptosis events in fibroblasts that originate from normal dermal and wound tissues differ. In this process, we also found that bFGF inhibited Akt phosphorylation at serine 473 and induced a rapid loss of phosphorylation of focal adhesion kinase (FAK) at tyrosine 397 in pretreated GF-1 cells, an event that coincided with the dissociation of phosphorylated FAK from the focal adhesions. Therefore, inhibition of survival signals relayed via the disrupted focal adhesion structures and inactivated Akt following bFGF treatment may lead to apoptosis in GF-1 cells pretreated with TGF-beta1. Pretreatment of GF-1 with TGF-beta1 followed by the addition of bFGF resulted in significantly greater inhibition of phosphorylation of Akt and FAK compared to treatment with TGF-beta1 or bFGF alone. The combinatorial treatment also led to proteolysis of FAK and inhibition of FAK and Akt protein expression in GF-1 cells. These findings demonstrated a significant role for the two cytokines in apoptosis of granulation tissue fibroblasts during wound healing. In vivo studies also confirmed a marked decline in phosphorylation and protein expression of Akt and FAK in bFGF-injected skin wounds. These results led to the hypothesis that temporal activation of TGF-beta1 and bFGF at the injury site promotes apoptosis in granulation tissue fibroblasts, an event that is critical for the termination of proliferative granulation tissue formation.

Reversible conversion of epithelial and mesenchymal phenotypes in SV40 large T antigen-immortalized rat liver cell lines

EMT (epithelial–mesenchymal transition) is a key process in the development of liver fibrosis. This process is also essential for liver morphogenesis in embryonic development. To study the cellular and molecular basis of EMT, we established two phenotypically different SV40 large T antigen-immortalized cell lines from rat hepatocytes. The first cell line, which had an epithelial morphology and was established in DMEM (Dulbecco’s modified Eagle’s medium)/Ham’s F-12 (DF)-based medium (RL/DF cells), expressed CK18 (cytokeratin 18), a marker of parenchymal hepatocytes. The other, a mesenchymal-like cell line established in DMEM-based medium (RL/DMEM cells), expressed αSMA (α-smooth muscle actin), a marker of hepatic myofibroblasts. Epithelial RL/DF cells underwent phenotypic changes, such as increased expression of αSMA, when the culture medium was switched to DMEM-based medium. In contrast, mesenchymal RL/DMEM cells were induced to express the epithelial marker CK18 with a concomitant decrease in αSMA expression when the culture medium was replaced with DF-based medium. These cell lines may provide novel in vitro models for the study of the conversion between epithelial and mesenchymal phenotypes during EMT in liver fibrosis and morphogenesis.

Evidence for epithelial-mesenchymal transition in adult human pancreatic exocrine cells

It has been shown that adult pancreatic ductal cells can dedifferentiate and act as pancreatic progenitors. Dedifferentiation of epithelial cells is often associated with the epithelial-mesenchymal transition (EMT). In this study, we investigated the occurrence of EMT in adult human exocrine pancreatic cells both in vitro and in vivo. Cells of exocrine fraction isolated from the pancreas of brain-dead donors were first cultured in suspension for eight days. This led to the formation of spheroids, composed of a principal population of cells with duct-like phenotype. When cultivated in tissue culture-treated flasks, spheroid cells exhibited a proliferative capacity and coexpressed epithelial (cytokeratin7 and cytokeratin19) and mesenchymal (vimentin and alpha-smooth muscle actin) markers as well as marker of progenitor pancreatic cells (pancreatic duodenal homeobox factor-1) and surface markers of mesenchymal stem cells. The switch from E-cadherin to N-cadherin associated with Snail1 expression suggested that these cells underwent EMT. In addition, we showed coexpression of epithelial and mesenchymal markers in ductal cells of one normal adult pancreas and three type 2 diabetic pancreases. Some of the vimentin-positive cells were found to coexpress glucagon or amylase. These results point to the occurrence of EMT, which may take place on dedifferentiation of ductal cells during the regeneration or renewal of human pancreatic tissues.

Histone deacetylase 1 and 2 differentially regulate apoptosis by opposing effects on extracellular signal-regulated kinase 1/2

Histone deacetylases (HDACs) are epigenetic regulators that are important for the control of various pathophysiological events. We found that HDAC inhibitors completely abolished transforming growth factor-β1 (TGF-β1)-induced apoptosis in AML-12 and primary mouse hepatocytes. Expression of a dominant-negative mutant of HDAC1 or downregulation of HDAC1 by RNAi both suppressed TGF-β1-induced apoptosis. In addition, overexpression of HDAC1 enhanced TGF-β1-induced apoptosis, and the rescue of HDAC1 expression in HDAC1 RNAi cells restored the apoptotic response of cells to TGF-β1. These data indicate that HDAC1 functions as a proapoptotic factor in TGF-β1-induced apoptosis. In contrast, downregulation of HDAC2 by RNAi increased spontaneous apoptosis and markedly enhanced TGF-β1-induced apoptosis, suggesting that HDAC2 has a reciprocal role in controlling cell survival. Furthermore, inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) by MEK1 inhibitor PD98059 or expression of a kinase-dead mutant of MEK1 restored the apoptotic response to TGF-β1 in HDAC1 RNAi cells. Strikingly, HDAC2 RNAi caused an inhibition of ERK1/2, and the spontaneous apoptosis can be abolished by reactivation of ERK1/2. Taken together, our data demonstrate that HDAC1 and 2 reciprocally affect cell viability by differential regulation of ERK1/2; these observations provide insight into the roles and potential mechanisms of HDAC1 and 2 in apoptosis.

Immortalized epithelial cells derived from human colon biopsies express stem cell markers and differentiate in vitro

BACKGROUND & AIMS

Long-term propagation of human colonic epithelial cells (HCEC) of adult origin has been a challenge; currently used HCEC lines are of malignant origin and/or contain multiple cytogenetic changes. We sought to immortalize human colon biopsy-derived cells expressing stem cell markers and retaining multilineage epithelial differentiation capability.

METHODS

We isolated and cultured cells from biopsy samples of 2 patients undergoing routine screening colonoscopy. Cells were immortalized by expression of the nononcogenic proteins cyclin-dependent kinase 4 (Cdk4) and the catalytic component of human telomerase (hTERT) and maintained for more than 1 year in culture.

RESULTS

The actively proliferating HCECs expressed the mesenchymal markers vimentin and alpha-smooth muscle actin. Upon growth arrest, cells assumed a cuboidal shape, decreased their mesenchymal features, and expressed markers of colonic epithelial cells such as cytokeratin 18, zonula occludens-1, mucins-1 and -2, antigen A33, and dipeptidyl peptidase 4. Immortalized cells expressed stem cell markers that included LGR5, BMI1, CD29, and CD44. When placed in Matrigel in the absence of a mesenchymal feeder layer, individual cells divided and formed self-organizing, cyst-like structures; a subset of cells exhibited mucin-2 or polarized villin staining.

CONCLUSIONS

We established immortalized HCECs that are capable of self-renewal and multilineage differentiation. These cells should serve as valuable reagents for studying colon stem cell biology, differentiation, and pathogenesis.

Epithelial-mesenchymal transition in hepatocellular carcinoma

The transition of epithelial cells to a mesenchymal phenotype is of paramount relevance for embryonic development and adult wound healing. During the past decade, the epithelial-mesenchymal transition (EMT) has been increasingly recognized to occur during the progression of various carcinomas such as hepatocellular carcinoma (HCC). Here, we focus on EMT in both experimental liver models and human HCC, emphasizing the underlying molecular mechanisms which show partial recurrence of embryonic programs such as TGF-beta and Wnt/ beta-catenin signaling, including collaboration with hepatitis viruses. We further discuss the differentiation repertoire of malignant hepatocytes with respect to the potential acquisition of stemness, and the involvement of the mesenchymal to epithelial transition, the reversal of EMT, in cancer dissemination and metastatic colonization. The strong evidence for EMT in HCC patients demands novel strategies in pathological assessments and therapeutic concepts to efficiently combat HCC progression.

The cell biology of hepatitis C virus

Hepatitis C virus infects 3% of the world's population and has a variable disease course with potentially sever outcomes, liver failure and hepatocellular carcinoma. The influence of HCV the biology of infected hepatocytes is now just becoming known. This review will focus on effect of HCV on host cells.

Reversible manipulation of apoptosis sensitivity in cultured hepatocytes by matrix-mediated manipulation of signaling activities

Hepatocytes in culture are a valuable tool to investigate mechanisms involved in the response of the liver to cytokines. However, it is well established that hepatocytes cultured as monolayers on dried stiff collagen dedifferentiate, loosing specialized liver functions. In contrast, softer matrix systems like gelled collagen help to preserve these structural and functional features. We show that the de-differentiation process induced in conventional dry collagen is a reversible consequence of a specific signaling network constellation triggered by the extracellular matrix that results in apoptosis resistance. A dried stiff collagen activates Akt and ERK1/2 pathways that results in apoptosis resistance. In contrast to stiff collagen, a soft collagen gel does not activate these pathways keeping the hepatocytes in a state where they remain sensitive to TGF-beta-induced apoptosis. Finally, we show that matrix-induced apoptosis resistance is reversible by re-plating cells from dried stiff to soft gel collagen. Practical consequences of these observations are that differentiated functions of hepatocytes, such as metabolism, endocytosis, and apoptosis, should be studied in hepatocyte sandwiches. On the other hand, proliferation and regeneration associated signaling can better be studied in hepatocytes cultured on collagen monolayers. In this chapter we focus on mechanisms that influence apoptosis sensitivity in cultured mouse hepatocytes.

Modeling system states in liver cells: survival, apoptosis and their modifications in response to viral infection

BACKGROUND

The decision pro- or contra apoptosis is complex, involves a number of different inputs, and is central for the homeostasis of an individual cell as well as for the maintenance and regeneration of the complete organism.

RESULTS

This study centers on Fas ligand (FasL)-mediated apoptosis, and a complex and internally strongly linked network is assembled around the central FasL-mediated apoptosis cascade. Different bioinformatical techniques are employed and different crosstalk possibilities including the integrin pathway are considered. This network is translated into a Boolean network (74 nodes, 108 edges). System stability is dynamically sampled and investigated using the software SQUAD. Testing a number of alternative crosstalk possibilities and networks we find that there are four stable system states, two states comprising cell survival and two states describing apoptosis by the intrinsic and the extrinsic pathways, respectively. The model is validated by comparing it to experimental data from kinetics of cytochrome c release and caspase activation in wildtype and Bid knockout cells grown on different substrates. Pathophysiological modifications such as input from cytomegalovirus proteins M36 and M45 again produces output behavior that well agrees with experimental data.

CONCLUSION

A network model for apoptosis and crosstalk in hepatocytes shows four different system states and reproduces a number of different conditions around apoptosis including effects of different growth substrates and viral infections. It produces semi-quantitative predictions on the activity of individual nodes, agreeing with experimental data. The model (SBML format) and all data are available for further predictions and development.