Heterogeneity and plasticity of hepatocyte lineage cells

Subhepatotoxic exposure to arsenic enhances lipopolysaccharide-induced liver injury in mice

Exposure to arsenic via drinking water is a serious health concern in the US. Whereas studies have identified arsenic alone as an independent risk factor for liver disease, concentrations of arsenic required to damage this organ are generally higher than found in the US water supply. The purpose of the current study was to test the hypothesis that arsenic (at subhepatotoxic doses) may also sensitize the liver to a second hepatotoxin. To test this hypothesis, the effect of chronic exposure to arsenic on liver damage caused by acute lipopolysaccharide (LPS) was determined in mice. Male C57Bl/6J mice (4-6 weeks) were exposed to arsenic (49 ppm as sodium arsenite in drinking water). After 7 months of exposure, animals were injected with LPS (10 mg/kg i.p.) and sacrificed 24 h later. Arsenic alone caused no overt hepatotoxicity, as determined by plasma enzymes and histology. In contrast, arsenic exposure dramatically enhanced liver damage caused by LPS, increasing the number and size of necroinflammatory foci. This effect of arsenic was coupled with increases in indices of oxidative stress (4-HNE adducts, depletion of GSH and methionine pools). The number of apoptotic (TUNEL) hepatocytes was similar in the LPS and arsenic/LPS groups. In contrast, arsenic pre-exposure blunted the increase in proliferating (PCNA) hepatocytes caused by LPS; this change in the balance between cell death and proliferation was coupled with a robust loss of liver weight in the arsenic/LPS compared to the LPS alone group. The impairment of proliferation after LPS caused by arsenic was also coupled with alterations in the expression of key mediators of cell cycle progression (p27, p21, CDK6 and Cyclin D1). Taken together, these results suggest that arsenic, at doses that are not overtly hepatotoxic per se, significantly enhances LPS-induced liver injury. These results further suggest that arsenic levels in the drinking water may be a risk modifier for the development of chronic liver diseases.

Experimental hepatology applied to stem cells

Transplantation is an accepted treatment today for many people suffering from organ failure. More and more patients are referred for transplant surgery, and the waiting lists are growing longer because not enough organs and tissues are donated for transplantation. This has led to several potentially viable alternatives being considered, including bio-artificial support devices, the transplantation of mature cells or stem/progenitor cells and the potential transplantation of xenogenic organs and cells [Burra P, Samuel D, Wendon J, Pietrangelo A, Gupta S. Strategies for liver support: from stem cells to xenotransplantation. J Hepatol 2004;41:1050-9]. Numerous investigators around the world are engaged in these investigations and the pace of discovery has begun to accelerate in recent years. To take stock of the achievements of recent years, the AISF sponsored a Single-Topic Conference, held in Padua on 26-27 May, 2006, with the participation of many leading investigators from various parts of Italy and Europe. This present paper summarizes the content of the Conference. Different issues were analysed, from the biology of stem cells to the possible use of gene therapy. The speakers were clinicians and scientists interested in diseases not only of the liver but also of other organs such as the kidney or heart. The fact that numerous specialties were represented helped the audience to understand the stem cell research area from different standpoints, and what research has achieved so far.

Isolation and characterization of epithelial progenitor cells from human fetal liver

AIM

Hepatic progenitor cells can serve as an alternative source of hepatocytes for the treatment of liver diseases.

METHODS

We isolated and expanded the epithelial progenitor cells (EPC) from the human fetal liver and investigated the differentiation of EPC into hepatic cells by fluorescence-activated cell sorter (FACS), real-time polymerase chain reaction (PCR), immunofluorescence assay, western blotting, and periodic acid-Schiff staining.

RESULTS

Isolated EPC possessed highly proliferative ability and subpassaged for more than 25 passages. Real-time PCR showed that EPC expressed liver epithelial markers (cytokeratin [CK]8 and CK18) and biliary-specific markers (CK7 and CK19). FACS analysis indicated that these cells were positive for CD117, CD147, CD90, CD44, human leucocyte antigen class I and CD71, but negative for CD34 and CD45. The EPCpossessed multipotential indicated by differentiating into osteoblasts and adipocytes; when subjected to the hepatic differentiation condition, EPC could be induced to hepatocyte-like cells, which expressed albumin, alpha-fetoprotein, and CK18 proteins. Two months after EPC transplantation, we observed that the grafted cells differentiated into hepatocyte-like cells and there was no observable tumor mass.

CONCLUSION

We have isolated and characterized the human fetal liver-derived EPC and these cells may serve as an ideal cell source for cell-replacement therapy of diseased livers.

Selective enrichment of hepatocytes from mouse embryonic stem cells with a culture system containing cholestatic serum

AIM

There is increasing evidence indicating that embryonic stem (ES) cells are capable of differentiating into hepatocyte-like cells in vitro. However, it is necessary to improve the differentiation efficiency so as to promote the clinical application. Here, we report an efficient culture system to support hepatocyte differentiation from ES cells by utilizing cholestatic serum.

METHODS

One week after the induction of E14 mouse ES cells into hepatocytes with sodium butyrate, cholestatic serum was added into the culture system at various concentrations and hepatocyte-like cells were induced to proliferate. The morphological and phenotypic markers of hepatocytes were characterized using light microscopy, immunocytochemistry, and RT-PCR, respectively. The function of glycogen storage of the differentiated cells was detected by Periodic acid-Schiff (PAS) reaction, and the ratio of hepatic differentiation was determined by counting the albumin and PAS-positive cells.

RESULTS

In the presence of conditional selective medium containing cholestatic serum, numerous epithelial cells resembling hepatocytes were observed. The RT-PCR analysis showed that undifferentiated ES cells did not express any hepatic-specific markers; however, in the presence of sodium butyrate and conditional selective medium containing cholestatic serum, hepatic differentiation markers were detected. Immunofluorescence staining showed that those ES-derived hepatocytes were alpha-fetoprotein, albumin, and cytokeratin 18 positive, with the ability of storing glycogen. Further determination of the hepatic differentiation ratio showed that the application of cholestatic serum efficiently enriched ES-derived hepatocyte-like cells by inducing lineage differentiation and enhancing lineage proliferation.

CONCLUSION

The conditional selective medium containing cholestatic serum is optimal to selectively enrich hepatocyte-like cells from mixed differentiated ES cells, which may provide a novel method to improve the hepatic differentiation ratio of ES cells.

Safety and efficacy profile of G-CSF therapy in patients with acute on chronic liver failure

BACKGROUND/AIM

We aimed to evaluate safety and efficacy of granulocyte-colony stimulating factor treatment in patients with acute on chronic liver failure and the effect of granulocyte-colony stimulating factor on the expression level of CXCR4, vascular endothelial growth factor receptor and very late activation antigen 4.

METHODS

Twenty-four patients with acute on chronic liver failure were randomised to receive standard therapy, standard therapy+granulocyte-colony stimulating factor (5 microg/kg/day for 6 days) and standard therapy+granulocyte-colony stimulating factor (15 microg/kg/day s.c. for 6 days). Data on CD34+cell mobilisation were compared to age-matched peripheral blood haematopoietic stem cell donors treated with granulocyte-colony stimulating factor. On day third of treatment, the expression level of CXCR4, vascular endothelial growth factor receptor and very late activation antigen 4 was analysed in mobilised CD34+ cells.

RESULTS

CD34 cell count increased after the second day of granulocyte-colony stimulating factor injection in both treatment groups compared to the linear increase observed in control. After the fifth day the increase was significantly higher in healthy donors versus patients with acute on chronic liver failure. A decrease in the expression of CXCR4, very late activation antigen 4 and vascular endothelial growth factor receptor compared to premobilisation values was observed. No major side effects were observed.

CONCLUSIONS

Granulocyte-colony stimulating factor treatment is able to induce CD34 mobilisation in patients with acute on chronic liver failure. The expression pattern of CXCR4, very late activation antigen 4 and vascular endothelial growth factor receptor suggests that these molecules are involved in the granulocyte-colony stimulating factor-induced stem cell mobilisation.

Stage-specific regulation of adhesion molecule expression segregates epithelial stem/progenitor cells in fetal and adult human livers

PURPOSE

Regulated expression of cell adhesion molecules could be critical in the proliferation, sequestration, and maintenance of stem/progenitor cells. Therefore, we determined fetal and adult stage-specific roles of cell adhesion in liver cell compartments.

METHODS

We performed immunostaining for the adhesion molecules, E-cadherin and Ep-CAM, associated proteins, beta-catenin and alpha-actinin, hepatobiliary markers, albumin, alpha-fetoprotein, and cytokeratin-19, and the proliferation marker, Ki-67. Expression of albumin was verified by in situ mRNA hybridization.

RESULTS

In the fetal liver, hepatoblasts showed extensive proliferation with wide expression of E-cadherin, beta-catenin, and alpha-actinin, although Ep-CAM was expressed in these cells less intensely and focally in the cell membrane to indicate weak cell adhesion. Hepatoblasts in ductal plate and bile ducts showed less proliferation and Ep-CAM was intensely expressed in these cells throughout the cell membrane, indicating strong adhesion. In some ductal plate cells, beta-catenin was additionally in the cytoplasm and nucleus, suggesting active cell signaling by adhesion molecules. In adult livers, cells were no longer proliferating and E-cadherin, beta-catenin, and alpha-actinin were expressed in hepatocytes throughout, whereas Ep-CAM was expressed in only bile duct cells. Some cells in ductal structures of the adult liver with Ep-CAM coexpressed albumin and cytokeratin-19, indicating persistence of fetal-like stem/progenitor cells.

CONCLUSIONS

Regulated expression of Ep-CAM supported proliferation in fetal hepatoblasts through weak adhesion and helped in biliary morphogenesis by promoting stronger adhesion in hepatoblasts during this process. Restriction of Ep-CAM expression to bile ducts in the adult liver presumably facilitated sequestration of stem/progenitor cells. This stage-specific and cell compartment-related regulation of adhesion molecules should be relevant for defining how liver stem/progenitor cells enter, exit, and remain in hepatic niches during both health and disease.

Murine gallbladder epithelial cells can differentiate into hepatocyte-like cells in vitro

We determined whether extrahepatic biliary epithelial cells can differentiate into cells with phenotypic features of hepatocytes. Gallbladders were removed from transgenic mice expressing hepatocyte-specific beta-galactosidase (beta-Gal) and cultured under standard conditions and under experimental conditions designed to induce differentiation into a hepatocyte-like phenotype. Gallbladder epithelial cells (GBEC) cultured under standard conditions exhibited no beta-Gal activity. beta-Gal expression was prominent in 50% of cells cultured under experimental conditions. Similar morphological changes were observed in GBEC from green fluorescent protein transgenic mice cultured under experimental conditions. These cells showed higher levels of mRNA for genes expressed in hepatocytes, but not in GBEC, including aldolase B, albumin, hepatocyte nuclear factor-4alpha, aldehyde dehydrogenase 1, and glutamine synthetase, and they synthesized bile acids. Additional functional evidence of a hepatocyte-like phenotype included LDL uptake and enhanced benzodiazepine metabolism. Connexin-32 expression was evident in murine hepatocytes and in cells cultured under experimental conditions, but not in cells cultured under standard conditions. Notch 1, 2, and 3 and Notch ligand Jagged 1 mRNAs were downregulated in these cells compared with cells cultured under standard conditions. CD34, alpha-fetoprotein, and Sca-1 mRNA were not expressed in cells cultured under standard conditions, suggesting that the hepatocyte-like cells did not arise from hematopoietic stem cells or oval cells. These results point to future avenues for investigation into the potential use of GBEC in the treatment of liver disease.

Effects of ethanol on hepatic cellular replication and cell cycle progression

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

Stem cells as a treatment for chronic liver disease and diabetes

Advances in stem cell biology and the discovery of pluripotent stem cells have made the prospect of cell therapy and tissue regeneration a clinical reality. Cell therapies hold great promise to repair, restore, replace or regenerate affected organs and may perform better than any pharmacological or mechanical device. There is an accumulating body of evidence supporting the contribution of adult stem cells, in particular those of bone marrow origin, to liver and pancreatic islet cell regeneration. In this review, we will focus on the cell therapy for the diseased liver and pancreas by adult haematopoietic stem cells, as well as their possible contribution and application to tissue regeneration. Furthermore, recent progress in the generation, culture and targeted differentiation of human haematopoietic stem cells to hepatic and pancreatic lineages will be discussed. We will also explore the possibility that stem cell technology may lead to the development of clinical modalities for human liver disease and diabetes.

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

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

Lack of increases in methylation at three CpG-rich genomic loci in non-mitotic adult tissues during aging

BACKGROUND

Cell division occurs during normal human development and aging. Despite the likely importance of cell division to human pathology, it has been difficult to infer somatic cell mitotic ages (total numbers of divisions since the zygote) because direct counting of lifetime numbers of divisions is currently impractical. Here we attempt to infer relative mitotic ages with a molecular clock hypothesis. Somatic genomes may record their mitotic ages because greater numbers of replication errors should accumulate after greater numbers of divisions. Mitotic ages will vary between cell types if they divide at different times and rates.

METHODS

Age-related increases in DNA methylation at specific CpG sites (termed "epigenetic molecular clocks") have been previously observed in mitotic human epithelium like the intestines and endometrium. These CpG rich sequences or "tags" start unmethylated and potentially changes in methylation during development and aging represent replication errors. To help distinguish between mitotic versus time-associated changes, DNA methylation tag patterns at 8-20 CpGs within three different genes, two on autosomes and one on the X-chromosome were measured by bisulfite sequencing from heart, brain, kidney and liver of autopsies from 21 individuals of different ages.

RESULTS

Levels of DNA methylation were significantly greater in adult compared to fetal or newborn tissues for two of the three examined tags. Consistent with the relative absence of cell division in these adult tissues, there were no significant increases in tag methylation after infancy.

CONCLUSION

Many somatic methylation changes at certain CpG rich regions or tags appear to represent replication errors because this methylation increases with chronological age in mitotic epithelium but not in non-mitotic organs. Tag methylation accumulates differently in different tissues, consistent with their expected genealogies and mitotic ages. Although further studies are necessary, these results suggest numbers of divisions and ancestry are at least partially recorded by epigenetic replication errors within somatic cell genomes.

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Histological and culture studies with respect to ABCG2 expression support the existence of a cancer cell hierarchy in human hepatocellular carcinoma

In this study, we examined the possible involvement of progenitor cells in the carcinogenesis of human hepatocellular carcinoma (HCC) using tissue specimens and cell lines. We used ATP-binding cassette transporter ABCG2 as a progenitor cell marker. Immunohistochemically, ABCG2(+) hepatocytes were observed in the periportal areas of the dysplastic nodule, and ABCG2(+) cancer cells were also scattered or focally clustered in HCC. We sorted the cultured HCC cells (HuH7 and PLC5) into ABCG2(+) and ABCG2(-) subpopulations and then subcultured them for 4 weeks. ABCG2(+) cells could generate ABCG2(+) and ABCG2(-) progenies during subculture, whereas ABCG2(-) cells bore only ABCG2(-) cells, suggesting that a cancer cell hierarchy with reference to ABCG2 exists in HCC cells and that ABCG2(+) cells reside at the higher rank in that hierarchy. Interestingly, other progenitor cell markers including cytokeratin 19 and alpha-fetoprotein were mainly expressed in ABCG2(+) subpopulations. Conversely, albumin expression was more intense in ABCG2(-) cells. In addition, the expression patterns of transcription factors (GATA6, CCAAT/enhancer-binding protein alpha, and CCAAT/enhancer-binding protein beta) in ABCG2(+) and ABCG2(-) cells resembled those during normal liver development. In conclusion, this study suggests that cancer cells with ABCG2 expression might play a central role in hepatocarcinogenesis and the maintenance of the cancer cell hierarchy of human HCC.

A CD133-expressing murine liver oval cell population with bilineage potential

Although oval cells are postulated to be adult liver stem cells, a well-defined phenotype of a bipotent liver stem cell remains elusive. The heterogeneity of cells within the oval cell fraction has hindered lineage potential studies. Our goal was to identify an enriched population of bipotent oval cells using a combination of flow cytometry and single cell gene expression in conjunction with lineage-specific liver injury models. Expression of cell surface markers on nonparenchymal, nonhematopoietic (CD45-) cells were characterized. Cell populations were isolated by flow cytometry for gene expression studies. 3,5-Diethoxycarbonyl-1,4-dihydrocollidine toxic injury induced cell cycling and expansion specifically in the subpopulation of oval cells in the periportal zone that express CD133. CD133+CD45- cells expressed hepatoblast and stem cell-associated genes, and single cells coexpressed both hepatocyte and cholangiocyte-associated genes, indicating bilineage potential. CD133+CD45- cells proliferated in response to liver injury. Following toxic hepatocyte damage, CD133+CD45- cells demonstrated upregulated expression of the hepatocyte gene Albumin. In contrast, toxic cholangiocyte injury resulted in upregulation of the cholangiocyte gene Ck19. After 21-28 days in culture, CD133+CD45- cells continued to generate cells of both hepatocyte and cholangiocyte lineages. Thus, CD133 expression identifies a population of oval cells in adult murine liver with the gene expression profile and function of primitive, bipotent liver stem cells. In response to lineage-specific injury, these cells demonstrate a lineage-appropriate genetic response. Disclosure of potential conflicts of interest is found at the end of this article.

Advances in signaling in vertebrate regeneration as a prelude to regenerative medicine

While all animals have evolved strategies to respond to injury and disease, their ability to functionally recover from loss of or damage to organs or appendages varies widely damage to skeletal muscle, but, unlike amphibians and fish, they fail to regenerate heart, lens, retina, or appendages. The relatively young field of regenerative medicine strives to develop therapies aimed at improving regenerative processes in humans and is predicated on >40 years of success with bone marrow transplants. Further progress will be accelerated by implementing knowledge about the molecular mechanisms that regulate regenerative processes in model organisms that naturally possess the ability to regenerate organs and/or appendages. In this review we summarize the current knowledge about the signaling pathways that regulate regeneration of amphibian and fish appendages, fish heart, and mammalian liver and skeletal muscle. While the cellular mechanisms and the cell types involved in regeneration of these systems vary widely, it is evident that shared signals are involved in tissue regeneration. Signals provided by the immune system appear to act as triggers of many regenerative processes. Subsequently, pathways that are best known for their importance in regulating embryonic development, in particular fibroblast growth factor (FGF) and Wnt/beta-catenin signaling (as well as others), are required for progenitor cell formation or activation and for cell proliferation and specification leading to tissue regrowth. Experimental activation of these pathways or interference with signals that inhibit regenerative processes can augment or even trigger regeneration in certain contexts.

Histologic characteristics and prognostic significance in small hepatocellular carcinoma with biliary differentiation: subdivision and comparison with ordinary hepatocellular carcinoma

The morphologic characteristics and biologic behavior of small liver cancers with hepatic and biliary differentiation, and their histogenesis, remain unclear. In this study, 35 cases of hepatocellular carcinoma (HCC) smaller than 3 cm in diameter with biliary differentiation were divided into 3 groups, group 1 [cytokeratin (CK) 19-negative/mucin-negative], group 2 (CK 19-positive/mucin-negative), and group 3 (CK 19-positive/mucin-positive). Sixty-one HCCs without biliary differentiation were used as controls. We compared the histologic features of these tumors and the postoperative outcomes. Three morphologic features of HCCs with biliary differentiation were respectively observed in 40% (14/35), 60% (21/35), and 42.9% (15/35) as follows: (1) cancer cells with intermediate morphology, (2) prominent inflammatory cell infiltrate, (3) desmoplastic stroma; neural cell adhesion molecule and c-kit expression were noted in 25.7%(9/35) and 8.6%(3/35), respectively. Extrahepatic tumor recurrence after surgery occurred in 0% (0/16) of group 1, 33.3% (3/9) of group 2, 40.0% (4/10) of group 3, and 8.2% (5/61) of the ordinary HCCs. The tumor-related survival of group 3 patients was worse than that of patients with ordinary HCCs, but there were no differences between the survival of group 1, or group 2 patients and those with ordinary HCCs. Our results suggest that the biliary differentiation does occur even in small HCC, and a mucin-producing cancer cells indicates aggressive tumor behavior. The combination of intermediate cancer cells, inflammatory cell infiltrate, and desmoplastic stroma is likely to be related to the biliary differentiation of HCC.

Cell source of liver functional reconstruction

Recent years, various new techniques, such as bioartificial liver system, hepatocyte transplantation and tissue engineering, were being employed to fight against the liver diseases. However, the shortage of liver cells has become a prominent problem. This review focuses on the current research progress of the liver cell differentiation source, and it also discusses the advantages and disadvantages of various hepatic stem cells and nonhepatic stem cells, expecting to provide a reference for liver cell source selection during tissue engineering, an adequate and suitable source for bioartificial liver and hepatocyte transplantation and to help to treat liver diseases.

Isolation, characterization, and differentiation to hepatocyte-like cells of nonparenchymal epithelial cells from adult human liver

Activation and proliferation of human liver progenitor cells has been observed during acute and chronic liver diseases. Our goal was to investigate the presence of these putative progenitors in the liver of patients who underwent lobectomy for various reasons but did not show any hepatic insufficiency. Hepatic lesions were evaluated by histological analysis. Nonparenchymal epithelial (NPE) cells were isolated from samples of human liver resections located at a distance from the lesion that motivated the operation and were cultured and characterized. These cells exhibited a marked proliferative potential. They did not express the classic set of stem cell/progenitor markers (Oct-4, Rex-1, alpha-fetoprotein, CD90, c-kit, and CD34) and were faintly positive for albumin. When cultured at confluence in the presence of hepatocyte growth factor and either epidermal growth factor or fibroblast growth factor-4, they entered a differentiation process toward hepatocytes. Their phenotype was quantitatively compared with that of mature human hepatocytes in primary culture. Differentiated NPE cells expressed albumin; alpha1-antitrypsin; fibrinogen; hepatobiliary markers such as cytokeratins 7, 19, and 8/18; liver-enriched transcription factors; and genes characterized by either a fetal (cytochrome P4503A7 and glutathione S-transferase pi) or a mature (tyrosine aminotransferase, tryptophan 2,3-dioxygenase, glutathione S-transferase alpha, and cytochrome P4503A4) expression pattern. NPE cells could be isolated from the liver of several patients, irrespective of the absence or presence of lesions, and differentiated toward hepatocyte-like cells with an intermediate hepatobiliary and mature/immature phenotype. These cells are likely to represent a resident progenitor population of the adult human liver, even in the absence of hepatic failure. Disclosure of potential conflicts of interest is found at the end of this article.

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

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

Cholangiocyte marker-positive and -negative fetal liver cells differ significantly in their ability to regenerate the livers of adult rats exposed to retrorsine

We have used monoclonal antibodies against cell-surface developmental epitopes in combination with micromagnetic beads to isolate phenotypically defined subpopulations of cholangiocyte marker-positive fetal liver epithelial cells (CMP-FLEC). Differentiation potential was evaluated by injecting cell isolates from dipeptidyl peptidase IV (DPPIV) positive (DPPIV+) Fischer donor rats into the spleen of partially hepatectomized, DPPIV negative (DPPIV-)Fischer host rats exposed to retrorsine. At various time points, liver tissue was harvested and cells in DPPIV+ colonies were phenotyped by immunofluorescence and histochemical protocols. Functional differentiation and liver replacement were determined by comparing donor and host hepatocyte protein expression patterns and DPPIV enzyme activity in extracts from livers of host rats receiving CMP-FLEC. Our results showed that bipotentiality was retained during differentiation and maturation of CMP-FLEC, indicating that the acquisition of ductal morphology and phenotype were not indicative of lineage commitment. CMP-FLEC transplanted into the adult rat liver lost ductal and gained hepatocyte markers, and acquired protein expression patterns in 2D gels with a close similarity (&gt;75% spot match) to host hepatocytes but differing significantly from the transplanted CMP-FLEC cell isolate (&lt;25%spot match). The average size of donor hepatocyte colonies increased with time so that by 1 year, up to 70% of the host rat liver was replaced by CMP-FLEC derived DPPIV+ hepatocytes. Depletion of CMP-FLEC from fetal liver isolates resulted in a marked decrease in adult liver colonization, suggesting that a high percentage of the hepatocyte colonies in animals receiving total fetal liver isolates are derived from CMP-FLEC.

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

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

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

AIMS

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Isolation and characterization of a stem cell population from adult human liver

Several studies suggested the presence of stem cells in the adult normal human liver; however, a population with stem cell properties has not yet been isolated. The purpose of the present study was to identify and characterize progenitor cells in normal adult human liver. By stringent conditions of liver cell cultures, we isolated and characterized a population of human liver stem cells (HLSCs). HLSCs expressed the mesenchymal stem cell markers CD29, CD73, CD44, and CD90 but not the hematopoietic stem cell markers CD34, CD45, CD117, and CD133. HLSCs were also positive for vimentin and nestin, a stem cell marker. The absence of staining for cytokeratin-19, CD117, and CD34 indicated that HLSCs were not oval stem cells. In addition, HLSCs expressed albumin, alpha-fetoprotein, and in a small percentage of cells, cytokeratin-8 and cytokeratin-18, indicating a partial commitment to hepatic cells. HLSCs differentiated in mature hepatocytes when cultured in the presence of hepatocyte growth factor and fibroblast growth factor 4, as indicated by the expression of functional cytochrome P450, albumin, and urea production. Under this condition, HLSCs downregulated alpha-fetoprotein and expressed cytokeratin-8 and cytokeratin-18. HLSCs were also able to undergo osteogenic and endothelial differentiation when cultured in the appropriated differentiation media, but they did not undergo lipogenic differentiation. Moreover, HLSCs differentiated in insulin-producing islet-like structures. In vivo, HLSCs contributed to regeneration of the liver parenchyma in severe-combined immunodeficient mice. In conclusion, we here identified a pluripotent progenitor population in adult human liver that could provide a basis for cell therapy strategies.

Separate origins of hepatitis B virus surface antigen-negative foci and hepatocellular carcinomas in transgenic HBsAg (alb/psx) mice

We have examined the development and transgene expression in liver lesions of transgenic mice bearing the hepatitis B surface antigen (HBsAg) gene of hepatitis B virus under the control of the albumin promoter (alb/psx) to study liver regeneration and hepatocellular carcinoma (HCC) associated with hepatitis B virus infection. Storage of the HBsAg in the endoplasmic reticulum precedes loss of liver cells and regenerative hyperplastic nodules that do not express HBsAg. Histological analysis indicated that HBsAg-negative foci and nodules arose from liver progenitor cells in the portal zone and lacked mRNA expression. Genomic DNA from eight of nine HBsAg-negative laser capture-excised liver foci showed loss of part of the alb/psx gene, whereas no loss of the actin gene was observed. The alb/psx DNA was intact in adjacent HBsAg-positive tissue. Sequencing of polymerase chain reaction products suggested that alterations in the HBsAg transgene in HBsAg-negative foci occurred via large-scale deletions as opposed to single-site mutations. Southern blot analysis of HCC from 2-year-old transgenic HBsAg mice, however, revealed an intact alb/psx gene. Thus, HBsAg-negative progenitor cells with deletions in the transgene appear to be responsible for compensatory regeneration of the liver, whereas HCCs arise from clonal expansion of hepatocytes with intact alb/psx transgenes.

Differentiation of rat bone marrow stem cells in liver after partial hepatectomy

AIM: To investigate the differentiation of rat bone marrow stem cells in liver after partial hepatectomy.

METHODS: Bone marrow cells were collected from the tibia of rat with partial hepatectomy, the medial and left hepatic lobes were excised. The bone marrow stem cells (Thy⁺CD3^-CD45RA^- cells) were enriched from the bone marrow cells by depleting red cells and fluorescence-activated cell sorting. The sorted bone marrow stem cells were labeled by PKH26-GL in vitro and autotransplanted by portal vein injection. After 2 wk, the transplanted bone marrow stem cells in liver were examined by the immunohistochemistry of albumin (hepatocyte-specific marker).

RESULTS: The bone marrow stem cells (Thy⁺CD3^-CD45RA^- cells) accounted for 2.8% of bone marrow cells without red cells. The labeling rate of 10 μM PKH26-GL on sorted bone marrow stem cells was about 95%. There were sporadic PKH26-GL-labeled cells among hepatocytes in liver tissue section, and some of the cells expressed albumin.

CONCLUSION: Rat bone marrow stem cells can differentiate into hepatocytes in regenerative environment and may participate in liver regeneration after partial hepatectomy.

Zonal hierarchy of differentiation markers and nestin expression during oval cell mediated rat liver regeneration

Oval cells constitute a heterogeneous population of proliferating progenitors found in rat livers following carcinogenic treatment (2-acetylaminofluorene and 70% hepatectomy). The aim of this study was to investigate the cellular pattern of various differentiation and cell type markers in this model of liver regeneration. Immunophenotypic characterisation revealed at least two subtypes emerging from the portal field. First, a population of oval cells formed duct-like structures and expressed bile duct (CD49f) as well as hepatocytic markers (alpha-foetoprotein, CD26). Second, a population of non-ductular oval cells was detected between and distally from the ductules expressing the neural marker nestin and the haematopoietic marker Thy1. Following oval cell isolation, a subset of the nestin-positive cells was shown to co-express hepatocytic and epithelial markers (albumin, CD26, pancytokeratin) and could be clearly distinguished from anti-desmin reactive hepatic stellate cells. The gene expression profiles (RT-PCR) of isolated oval cells and oval cell liver tissue were found to be similar to foetal liver (ED14). The present results suggest that the two oval cell populations are organised in a zonal hierarchy with a marker gradient from the inner (displaying hepatocytic and biliary markers) to the outer zone (showing hepatocytic and extrahepatic progenitor markers) of the proliferating progeny clusters.

Origin of hepatocellular carcinoma: role of stem cells

The question of whether hepatocellular carcinoma (HCC) arises from the differentiation block of stem cells or dedifferentiation of mature cells remains controversial. Recently, researchers suggested that HCC may originate from the transdifferentiation of bone marrow cells. Interestingly, there are four levels of cells in the hepatic stem cell lineage: bone marrow cells, hepato-pancreas stem cells, oval cells and hepatocytes. Hematopoietic stem cells and the liver are known to have a close relationship in early development. Bone marrow stem cells could differentiate into oval cells, which could differentiate into hepatocytes and duct cells. The development of pancreatic and liver buds in embryogenesis suggests the existence of a common progenitor cell to both the pancreas and liver. Cellular events during hepatocarcinogenesis illustrate that HCC may arise from cells at various stages of differentiation in the hepatic stem cell lineage.

Characterization of two distinct liver progenitor cell subpopulations of hematopoietic and hepatic origins

Despite extensive studies, the hematopoietic versus hepatic origin of liver progenitor oval cells remains controversial. The aim of this study was to determine the origin of such cells after liver injury and to establish an oval cell line. Rat liver injury was induced by subcutaneous insertion of 2-AAF pellets for 7 days with subsequent injection of CCl(4). Livers were removed 9 to 13 days post-CCl(4) treatment. Immunohistochemistry was performed using anti-c-kit, OV6, Thy1, CK19, AFP, vWF and Rab3b. Isolated non-parenchymal cells were grown on mouse embryonic fibroblast, and their gene expression profile was characterized by RT-PCR. We identified a subpopulation of OV6/CK19/Rab3b-expressing cells that was activated in the periportal region of traumatized livers. We also characterized a second subpopulation that expressed the HSCs marker c-kit but not Thy1. Although we successfully isolated both cell types, OV6/CK19/Rab3b(+) cells fail to propagate while c-kit(+)-HSCs appeared to proliferate for up to 7 weeks. Cells formed clusters which expressed c-kit, Thy1 and albumin. Our results indicate that a bona fide oval progenitor cell population resides within the liver and is distinct from c-kit(+)-HSCs. Oval cells require the hepatic niche to proliferate, while cells mobilized from the circulation proliferate and transdifferentiate into hepatocytes without evidence of cell fusion.

Establishment and characterization of primary canine hepatocellular carcinoma cell lines producing alpha-fetoprotein

A primary cultured cell line named CHKS was established from a hepatocellular carcinoma (HCC) of a dog showing a high level of serum alpha-fetoprotein (AFP). CHKS secreted a 66 KDD AFP into the growth medium regardless of the presence or absence of fetal bovine serum (FBS). Cloning CHKS with limiting dilution produced 4 clones, CHKS-1, -2, -3, and -4, which secreted 826, 471, 70, and less than 10 ng/ml, respectively, of AFP into the culture medium. In culture, these cell lines were similar in morphology and proliferation pattern to epithelial cells and positive to periodic acid-Schiff (PAS) staining. The presence of mRNA for canine albumin was demonstrated by nested PCR. The doubling times of the clone cell lines were 21, 45, 36, and 35 h, saturation densities 34, 18, 22, and 24 x 10(4)/cm(2), and plating efficiencies 18, 45, 46, and 45%, respectively. Chromosome analysis of these cell lines showed near triploidy. These results show that CHKS and its clones have hepatic cell functions and are useful for carcinogenetic and clinical studies of canine HCC.

Hedgehog signaling maintains resident hepatic progenitors throughout life

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

Hepatoblastomas and liver development: a study of cytokeratin immunoexpression in twenty-nine hepatoblastomas

Hepatoblastomas (HBs) recapitulate liver development. It is possible that HBs result from malignant transformation of hepatic precursor cells, and they may reflect a blockage in normal development. Here we study the expression of cytokeratins (CKs) in order to delineate the immunoprofile and relationship with liver development, as well as vimentin and alphafetoprotein (AFP), of HBs. Immunohistochemistry was performed in a tissue microarray (TMA) containing representative areas of 18 HBs (fetal and/or embryonal and/or mesenchymal); we also reviewed 11 cases not included in the TMA. No cases stained for CKs 1, 5/6, 7, 10, 13, 15, 16, 20, and 34betaE12. CK8 stained 73.07% of fetal, 50% of embryonal, and 18% of mesenchymal areas. CK18 stained 100% of epithelial areas. CK19 staining was intense and diffuse in 100% of embryonal samples, but it was weaker in fetal areas (66.66%). AE1 stained epithelial areas in all cases, and it stained 29.41% of mesenchymal areas. AE3 stained 84.61% of embryonal and 60% of fetal components. AE1/AE3 showed stronger staining in embryonal (100%) than in fetal areas (76.92%). Vimentin staining was strong in embryonal (66.66%) and mesenchymal (84.61%) components but weak in fetal areas (8%). Alphafetoprotein was positive in only 20% of fetal and 70% of embryonal areas. Our results support the hypothesis that immunoexpression of HBs follows the stages of normal liver development. Embryonal areas look less differentiated, expressing vimentin and biliary epithelium CKs, whereas fetal areas display a more developed phenotype, similar to that of mature hepatocytes. These data aid in understanding the ontogenesis of HBs and may be used in histopathological diagnosis.

Stem cells in the lung parenchyma and prospects for lung injury therapy

Until recently, it was thought that only embryonic stem cells were pluripotent and that adult stem cells were restricted in their differentiative and regenerative potential to become the tissues in which they reside. However, the discovery that adult stem cells in one tissue can contribute to the formation of other tissues, especially after injury or cell damage, implies that stem cells have developmental plasticity. For example, haematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) from bone marrow can be used to regenerate diverse tissues at distant sites, including the lung. This article reviews the character of stem cells in the lung parenchyma and focuses on the potential uses of adult stem cells in research of lung injury and lung disease therapies.

Hepatocellular carcinoma in a liver-cell adenoma within a non-cirrhotic liver

Liver-cell adenomas are benign lesions of the liver occurring predominantly in young women. Hepatocellular carcinomas in most of the cases arise in a cirrhotic liver during the fifth or sixth decade. We describe the case of a 40-year-old woman in whom work-up for epigastric pain revealed a peptic ulcer and a large hepatic mass. Tests for chronic liver diseases were negative. Imaging findings and biopsy specimens of the tumour were inconclusive. The tumour was surgically removed and a hepatocellular carcinoma arising within a liver-cell adenoma in a non-cirrhotic liver was found. Malignant transformation of liver-cell adenoma has only been reported in a few case reports. Mechanisms of transformation remain unclear. The imaging findings as well as histological features are presented in detail and the literature is discussed.

Hematopoietic stem cell trafficking in liver injury

Bone marrow (BM) hematopoietic stem cells (HSCs) have been shown to facilitate regeneration in multiple nonhematopoietic tissues by either generating epithelial cells or altering the inflammatory response. Depending on injury type, the predominant mechanism of epithelial lineage regeneration occurs by spontaneous cell fusion or transdifferentiation. Irrespective of the mechanism, mobilization from the BM is a prerequisite. Mechanisms by which HSCs mobilize into damaged organs are currently under scrutiny. Murine and human studies have shown that the chemokine SDF-1 and its receptor CXCR4 participate in the mobilization of HSCs from BM and in the migration of HSCs to injured liver. SDF-1 is a potent HSC chemoattractant and is produced by the liver. Production is increased during liver injury leading to increased HSC migration to the liver, a finding diminished by neutralizing anti-CXCR4 antibodies. Additional factors have been implicated in the control of hepatic migration of HSCs such as IL-8, hepatocyte growth factor, and MMP-9. Matriceal remodeling is an essential component in HSC engraftment, and MMP-9 expression is increased in liver injury. This review focuses on the complex interaction of chemokines, adhesion molecules, and extracellular matrix factors required for successful migration and engraftment of HSCs into the liver.

Gene expression analysis of human hepatocellular carcinoma by using full-length cDNA library

Hepatocellular carcinoma (HCC) is a leading cause of death worldwide. Hepatitis B virus (HBV) or hepatitis C virus (HCV) infection has been shown to cause hepatic carcinogenesis. A total 58,251 of cDNA clones of full-length cDNA libraries of HBV and HCV-infected HCC and their surrounding non-tumor tissues, respectively, were sequenced and analyzed by blasting against GENEBANK maintained by NCBI. About 180 and 279 of genes were shown an obviously increased and decreased expression patterns between HCC tissue and its adjacent non-tumor tissue. The candidate genes consisted of the genes encoded liver specific metabolism enzymes, secretory functional proteins, proteases and their inhibitors, protein chaperon, cell cycle components, apoptosis-related proteins, transcriptional factors, and DNA binding proteins. Several genes were further investigated by using real-time PCR to confirm the gene expression levels in at least 24 pairs of HCC tissues and adjacent non-tumor tissues. The results showed that genes encoded reticulon 4, RGS-1, antiplasmin, and kallikrein B were down-regulated with the average of 2.8, 8.5, 3.2, and 10.5-fold, respectively. Our results provide crucial candidate genes to develop clinical diagnosis and gene therapy of HCC.

Prometheus' challenge: molecular, cellular and systemic aspects of liver regeneration

The fascinating aspect of the liver is the capacity to regenerate after injury or resection. A variety of genes, cytokines, growth factors, and cells are involved in liver regeneration. The exact mechanism of regeneration and the interaction between cells and cytokines are not fully understood. There seems to exist a sequence of stages that result in liver regeneration, while at the same time inhibitors control the size of the regenerated liver. It has been proven that hepatocyte growth factor, transforming growth factor, epidermal growth factor, tumor necrosis factor-alpha, interleukins -1 and -6 are the main growth and promoter factors secreted after hepatic injury, partial hepatectomy and after a sequence of different and complex reactions to activate transcription factors, mainly nuclear factor kappaB and signal transduction and activator of transcription-3, affects specific genes to promote liver regeneration. Unraveling the complex processes of liver regeneration may provide novel strategies in the management of patients with end-stage liver disease. In particular, inducing liver regeneration should reduce morbidity for the donor and increase faster recovery for the liver transplantation recipient.

Gastrointestinal stem cells. III. Emergent themes of liver stem cell biology: niche, quiescence, self-renewal, and plasticity

This essay will address areas of liver stem/progenitor cell studies in which consensus has emerged and in which controversy still prevails over consensus, but it will also highlight important themes that inevitably should be a focus of liver stem/progenitor cell investigations in coming years. Thus concepts regarding cell plasticity, the existence of a physiological/anatomic stem cell niche, and whether intrahepatic liver stem/progenitor cells comprise true stem cells or progenitor cells (or both) will be approached in some detail.

Ectopic activity of fibroblast growth factor receptor 1 in hepatocytes accelerates hepatocarcinogenesis by driving proliferation and vascular endothelial growth factor-induced angiogenesis

Fibroblast growth factor (FGF) signaling mediates cell-to-cell communication in development and organ homeostasis in adults. Of the four FGF receptor (FGFR) tyrosine kinases, only FGFR4 is expressed in mature hepatocytes. Although FGFR1 is expressed by hepatic cell progenitors and adult nonparenchymal cells, ectopic expression is commonly observed in hepatoma cells. Here, we determined whether ectopic FGFR1 is a cause or consequence of hepatocellular carcinoma by targeting a constitutively active human FGFR1 to mouse hepatocytes. Livers of transgenic mice exhibited accelerated regeneration after partial hepatectomy but no signs of neoplastic or preneoplastic abnormalities for up to 18 months. However, in diethylnitrosamine-treated mice, the chronic FGFR1 activity promoted an incidence of 44% adenomas at 4 months and 38% hepatocellular carcinoma at 8 months. No adenoma or hepatocellular carcinoma was observed in diethylnitrosamine-treated wild-type (WT) livers at 4 or 8 months, respectively. At 10 and 12 months, tumor-bearing livers in transgenic mice were twice the size of those in WT animals. Isolated hepatoma cells from the transgenic tumors exhibited a growth advantage in culture. Advanced hepatocellular carcinoma in the transgenic livers exhibited a reduced rate of necrosis. This was accompanied by a mean microvessel density of 2.7 times that of WT tumors and a markedly higher level of vascular endothelial growth factor. In cooperation with an initiator, the persistent activity of ectopic FGFR1 in hepatocytes is a strong promoter of hepatocellular carcinoma by driving cell proliferation at early stages and promoting neoangiogenesis at late stages of progression.

Liver stem cells and prospects for liver reconstitution by transplanted cells

Although it was proposed almost 60 years ago that the adult mammalian liver contains hepatic stem cells, this issue remains controversial. Part of the problem is that no specific marker gene unique to the adult hepatic stem cell has yet been identified, and regeneration of the liver after acute injury is achieved through proliferation of adult hepatocytes and does not require activation or proliferation of stem cells. Also, there are differences in the expected properties of stem versus progenitor cells, and we attempt to use specific criteria to distinguish between these cell types. We review the evidence for each of these cell types in the adult versus embryonic/fetal liver, where tissue-specific stem cells are known to exist and to be involved in organ development. This review is limited to studies directed toward identification of hepatic epithelial stem cells and does not address the controversial issue of whether stem cells derived from the bone marrow have hepatocytic potential, a topic that has been covered extensively in other recent reviews.

Cell Growth and Differentiation of Different Hepatic Cells Isolated From Fetal Rat Liver in Vitro

Stem cells are interesting candidates as a new source for cell/organ culture or cell transplantation concepts. So far it is believed that the hepatoblast is the common progenitor cell during fetal liver development. In previous studies two distinct fractions of liver cells were found during development: cells co-expressing Thy1 and CK-18 (cytokeratin-18) and cells expressing CK-18 only. In this study we cultured Thy1-positive and Thy1-negative hepatic progenitors isolated from collagenase digested fetal rat livers after depletion of OX43/OX44-positive hematopoietic cells. The cells were cultured on a collagen-I matrix in a medium containing epidermal growth factor, insulin, and fetal calf serum. Thy1-positive cells isolated from ED16, ED18, or ED20 livers showed significantly enhanced cell growth compared with Thy1-negative cells during the culture period. Both cell types showed expression of the liver-specific genes CK-18, albumin and alpha-feto-protein at the beginning of the culture period, as assessed by reverse-transcription polymerase chain reaction and immunocytochemistry. The growth of Thy1-positive cells was significantly higher when compared with Thy1-negative cells and declined with maturation of the liver. The data suggest a stem cell-like growth potential of Thy1-positive fetal hepatic cells. Thus, these cells might be useful for concepts of cell-based therapies. However, further efforts must be undertaken to define the biological, ethical, and legal aspects of using fetal cells.

Cellular aspects of liver regeneration

This paper has the objective to analyze the cellular aspects of liver regeneration (LR). Upon damage in this organ, the regenerative capacity of hepatocyte is sufficiently able to reestablish the parenchyma as a whole. Taking into account the regenerative capacity of hepatocyte, the need of a progenitor or a liver trunk cell was not obvious. Nowadays it is well-established that precursor cells take part in the liver regenerative process. The liver trunk cell, oval cell, acts as a bypotential precursor, contributing for the liver restoration, mainly when the hepatocytes are unable to proliferate. Another precursor, trunk cell of hematopoetic origin (HSC), takes part in the regenerative process, originating cells of the hepatocitic lineage and colangiocytes, as well as the oval cell. The way the trans-differentiation takes place is not established yet. A number of studies must be undertaken in order to clarify questions, such as the possible occurrence of cellular fusion process between the HSC and the hepatic cells and the possibility of application as a new therapeutic procedure in the treatment of diseases associated with insufficiency of this noble organ.

Liver repopulation and long-term function of rat small hepatocyte transplantation as an alternative cell source for hepatocyte transplantation

Hepatocyte transplantation (HT) is an attractive therapeutic modality for liver disease as an alternative for liver organ transplantation. Primary fresh hepatocytes (FHs) are the exclusive cell source that has been used for clinical HT. However, the use of FHs is limited due to a shortage of donor cells. Small hepatocytes (SHs) are hepatic progenitor cells and can be isolated not only from rodents but also from humans. SHs can proliferate in vitro and express liver functions, although conventional hepatocytes lose them within a short period after culture. SH functions in vivo have never been studied. We therefore investigated HT using SHs to evaluate cell engraftment and function compared to HT using FHs. The donor cell number in the SH group was smaller than that in the FH group at HT. The cell engraftment in the SH group was smaller in the liver and larger in the spleen than in the FH group. The cell engraftment in the liver increased after HT; however, that in the spleen decreased after HT in both groups. HT using SHs supported the serum albumin level in the NAR experiment as well as that using FH, and albumin mRNA was detectable in the recipients' tissues at 12 weeks after HT. In conclusion, HT using SHs showed hepatic repopulation similar to that using FHs. This suggests that both SHs and FHs can repopulate the liver as if they were hepatic stem cells. In addition, HT using SHs supported liver functions such as albumin correction at the same level as that using FHs. These observations strongly support the idea that SHs could be an alternative to primary FHs as a novel cell source for future HT.

Partial hepatectomy induces mobilization of a unique population of haematopoietic progenitor cells in human healthy liver donors

BACKGROUND/AIMS

Recent studies indicate that after transplantation, circulating bone marrow-derived stem cells migrate into the liver and contribute to liver regeneration. Whether such cells are actively recruited from the bone marrow for liver repair remains to be determined. In this regard, we investigated whether liver resection leads to a release of stem cell marker-positive (+) cells into the peripheral circulation.

METHODS

Peripheral blood samples from 11 living liver donors were analyzed by flow cytometry one day before and 12h after partial hepatectomy (PH) using antibodies against CD133, CD34, CD45, CD14, c-kit, bcrp-1. Immunomagnetic separation was performed to select CD133+ cells for functional assays in vitro.

RESULTS

A significant increase in the percentage of CD133+ cells could be demonstrated in all donors studied. Unexpectedly, virtually all CD133+ cells coexpressed CD45 and CD14, whereas only a small subset expressed CD34. No significant staining was observed for c-kit and bcrp-1. In culture, immunoselected CD133+ cells displayed characteristics of myelomonocytic precursors. In addition, enriched CD133+ generated an adherent cell population that expressed CK8, alpha-fetoprotein, and human albumin.

CONCLUSIONS

PH induces mobilization of a distinct population of myelomonocytic progenitor cells, which have hepatic differentiation potential in vitro, and might play a role in liver regeneration after PH in humans.

Quantitative evaluation of long-term liver repopulation and the reconstitution of bile ductules after hepatocellular transplantation

AIM

The treatment of liver disease is severely limited by a shortage of donor livers. In trying to address this growing problem, hepatocellular transplantation (HTx) has received much attention as an alternative to whole organ transplant. However, the expansion of transplanted cells is at low level, and the reconstitution of functional liver tissue is limited by this cellular property. We set up an animal model to better understand cell dose effect and the kinetics of liver repopulation following HTx.

METHODS

Dipeptidyl peptidase IV (DPPIV)-deficient rats treated with retrorsine and subjected to partial hepatectomy were infused with DPPIV-positive hepatocytes. Rats were injected with varying numbers of donor hepatocytes down to 100 cells low, and liver repopulation was examined at different time points up to 20 mo long. Repopulation was assessed by computer-aided quantitative detection.

RESULTS

Transplanted hepatocytes underwent multiple rounds of proliferation and stably repopulated the injured livers after 20 mo and at all cell doses. Transplanted cells divided 14 times within the 3-mo time period following infusion, and the liver repopulation reached a plateau between 3 and 20 mo. Approximately 90% replacement occurred. Donor-derived cells also reconstituted the bile ductules of the recipients.

CONCLUSION

The ability of transplanted hepatocytes to fully reconstitute injured livers strongly supports further investigation into the clinical potential of HTx. Additionally, the observation that transplanted hepatocytes also form components of the biliary system suggests that these cells may have bi-potential property of the stem cells.

Liver inflammation and cytokine production, but not acute phase protein synthesis, accompany the adult liver progenitor (oval) cell response to chronic liver injury

Oval cells are facultative liver progenitor cells, which are invoked during chronic liver injury in order to replenish damaged hepatocytes and bile duct cells. Previous studies have observed inflammation and cytokine production in the liver during chronic injury. Further, it has been proposed that inflammatory growth factors may mediate the proliferation of oval cells during disease progression. We have undertaken a detailed examination of inflammation and cytokine production during a time course of liver injury and repair, invoked by feeding mice a choline-deficient, ethionine-supplemented (CDE) diet. We show that immediately following initial liver injury, B220-expressing leucocytes transiently infiltrate the liver. This inflammatory response occurred immediately before oval cell numbers began to expand in the liver, suggesting that the two events may be linked. Two waves of liver cytokine production were observed during the CDE time course. The first occurred shortly following commencement of the diet, suggesting that it may represent a hepatic acute phase response. However, examination of acute phase marker expression in CDE-fed mice did not support this hypothesis. The second wave of cytokine expression correlated with the expansion of oval cell numbers in the liver, suggesting that these factors may mediate oval cell proliferation. No inflammatory signalling was detected following withdrawal of the injury stimulus. In summary, our results document a close correlation between inflammation, cytokine production and the expansion of oval cells in the liver during experimental chronic injury.

Dual effects of adenovirus-mediated thrombopoietin gene transfer on hepatic oval cell proliferation and platelet counts

Thrombopoietin (TPO) is the growth factor for megakaryocytes and platelets, however, it also acts as a potent regulator of stem cell proliferation. To examine the significance of TPO expression in proliferation of hepatic oval cells, the effect of adenovirus-mediated TPO gene transfer into livers of the Solt-Farber model, which mimics the condition where liver regeneration is impaired, was examined. Hepatic TPO mRNA peaked its expression at 2 days after gene transduction and then gradually decreased. The peripheral platelet number began to increase at 4 days (P<0.05) and reached its plateau at 9 days (P<0.01). Oval cells expressed c-Mpl, a receptor for TPO as well as immature hematopoietic and hepatocytic surface markers such as CD34 and AFP. The proliferating cell nuclear antigen-positive oval cells in rats into which adenovirus-TPO gene was transferred at 7 and 9 days were significantly greater than those in adenovirus-LacZ gene transferred (P<0.05, each), and the total numbers of oval cells in the adenovirus-TPO gene transferred at 9 and 13 days were also significantly greater than those in adenovirus-LacZ gene transferred (P<0.05, each). Expression of SCF protein was increased at 4, 7, and 9 days by TPO gene administration and that of c-Kit was increased at 4 and 7 days. These data suggest that adenovirus-mediated TPO gene transfer stimulated oval cell proliferation in liver as well as increasing peripheral platelet counts, emphasizing the significance of the TPO/c-Mpl system in proliferation of hepatic oval cells.

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