Reactive biliary epithelium: the product of a pluripotential stem cell compartment?

Effect of human umbilical cord blood stem cell transplantation on oval cell response in 2-AAF/CCL4 liver injury model: experimental immunohistochemical study

Background

Oval cells, specific liver progenitors, are activated in response to injury. The human umbilical cord blood (hUCB) is a possible source of transplantable hepatic progenitors and can be used in cases of severe liver injury. We detected the effect of hUCB stem cell transplantation on natural response of oval cells to injury.

Methods

Twenty-four female albino rats were randomly divided into three groups: (A) control, (B) liver injury with hepatocyte block, and (C) hUCB transplanted group. Hepatocyte block was performed by administration of 2-acetylaminofluorene (2-AAF) for 12 days. CCL4 was administrated at day 5 from experiment start. Animals were sacrificed at 9 days post CCL4 administration, and samples were collected for biochemical and histopathological analysis. Oval cell response to injury was evaluated by the percentage of oval cells in the liver tissue and frequency of cells incorporated into new ducts.

Results

Immunohistochemical analysis of oval cell response to injury was performed. There was significant deviation in the hUCB-transplanted (4.9 ± 1.4) and liver injury groups (2.4 ± 0.9) as compared to control (0.89 ± 0.4) 9 days post injury. Detection of oval cell response was dependant on OV-6 immunoreactivity. For mere localization of cells with human origin, CD34 antihuman immunoreactivity was performed. There was no significant difference in endogenous OV-6 immunoreactivity following stem cell transplantation as compared to the liver injury group.

Conclusions

In vivo transplantation of cord blood stem cells (hUCB) does not interfere with natural oval cell response to liver injury.

Cultured hepatocytes adopt progenitor characteristics and display bipotent capacity to repopulate the liver

Clinical studies have proved the therapeutic potential of hepatocyte transplantation as a promising alternative to whole organ liver transplantation in the treatment of hereditary or end-stage liver disease. However, donor shortage seriously restricts cell availability, and the lack of appropriate cell culture protocols for the storage and maintenance of donor cells constitutes a significant obstacle. The aim of this study was to stimulate mature hepatocytes in culture to multiply in vitro and track their fate on transplantation. Rat hepatocytes isolated nonenzymatically were cultured serum free for up to 10 days. They were stimulated into proliferation in the presence of growth factors and conditioned media from nonparenchymal and hepatocyte culture supernatants, as well as 10 mM lithium chloride (LiCl). Cell proliferation was assessed by determining DNA content. Additionally, the extent of cell differentiation was estimated using immunofluorescence staining of hepatic, biliary, progenitor, and mesenchymal markers and gene expression analyses. Transplantation studies were performed on the Fischer CD26-mutant rat following pretreatment with retrorsine and partial hepatectomy. Proliferating hepatocytes increasingly adopted precursor characteristics, expressing progenitor (OV6, CD133), hepatic lineage (CK18), biliary (CD49f, CK7, CK19), and mesenchymal (vimentin) markers. The supplement of LiCl further enhanced the proliferative capacity by 30%. Transplantation studies revealed extensive repopulation by large donor hepatocyte clusters. Furthermore, bile duct-like structures deriving from donor cells proved to be immunoreactive to ductular markers and formed in close proximity to endogenous bile ducts. Mature hepatocytes reveal their potential to "switch" between phenotypes, adopting progenitor characteristics during proliferation in vitro. Following transplantation, these "retrodifferentiated" cells further expanded in vivo, thereby generating bipotentially differentiated progenies (hepatocytes and bile duct-like structures). This apparent plasticity of mature hepatocytes may open new approaches for cell-based strategies to treat liver disease.

Potential applications for cell regulatory factors in liver progenitor cell therapy

Orthotopic liver transplant represent the state of the art treatment for terminal liver pathologies such as cirrhosis in adults and hemochromatosis in neonates. A limited supply of transplantable organs in relationship to the demand means that many patients will succumb to disease before an organ becomes available. One promising alternative to liver transplant is therapy based on the transplant of liver progenitor cells. These cells may be derived from the patient, expanded in vitro, and transplanted back to the diseased liver. Inborn metabolic disorders represent the most attractive target for liver progenitor cell therapy, as many of these disorders may be corrected by repopulation of only a portion of the liver by healthy cells. Another potential application for liver progenitor cell therapy is the seeding of bio-artificial liver matrix. These ex vivo bioreactors may someday be used to bridge critically ill patients to other treatments. Conferring a selective growth advantage to the progenitor cell population remains an obstacle to therapy development. Understanding the molecular signaling mechanisms and micro-environmental cues that govern liver progenitor cell phenotype may someday lead to strategies for providing this selective growth advantage. The discovery of a population of cells within the bone marrow possessing the ability to differentiate into hepatocytes may provide an easily accessible source of cells for liver therapies.

Cytochrome P450 mRNA expressions along with in vitro differentiation of hepatocyte precursor cells from fetal, young and old rats

Non-differentiated cells are attractive targets for cell therapy. During liver regeneration oval cells intensively proliferate and differentiate extending their metabolic activity. Hepatic cytochromes P450 (CYPs) can be linked either with metabolic activation of toxic compounds or drug metabolism. We investigated the differentiation and biotransformative potential of non-differentiated cells in primary cell cultures isolated from livers of fetuses (16-days-old), young (4-months-old) and old (20-months-old) rats. Under the conditions of experimental hepatocarcinogenesis, adult rats were fed for three weeks with CDE diet. Liver cells were cultured and precursor cells were differentiated to hepatocytes following induction with sodium butyrate (SB) or dimethyl sulphoxide (DMSO) in culture on MesenCult medium. We identified a number of cells expressing Thy-1, CD34, alpha-fetoprotein, cytokeratines--CK18 or CK19 and glutathione transferases--GSTpi or GSTalpha. In vitro differentiation of these cells, isolated from CDE-treated rats begun earlier as compared to non-treated ones. Age-dependent changes in the cell differentiation sequence, as well as CYPmRNA expression sequence accompanying precursor cells differentiation, were also observed. mRNA expression of CYP1A2, CYP2B1/2 and CYP3A1 was higher in the cells of young rats, but in the case of CYP2E1--in the cells of old rats. It was concluded that both proliferation and differentiation potential of oval cells, decreased with age.

Intestinal metaplasia in liver of rats after partial hepatectomy and treatment with acetylaminofluorene

OBJECTIVES

The liver is widely recognized for its ability to self-regenerate after damage. Hepatocyte replication is the primary source of liver restoration, although hepatic stem cells (of one kind or another) may be a secondary font, only brought into effect when primary regeneration is severely compromised.

MATERIALS AND METHODS

In experiments using small rodents, such an injury can be inflicted by surgically removing a large portion of the liver followed by treatment with hepatotoxin 2-acetylaminofluorene. Regeneration by hepatocyte replication is blocked and thus, stem cell involvement is promoted. However, other responses may be stimulated and this study describes the presence of mucinous glandular structures in the healing liver after two-thirds of its volume was removed via hepatectomy followed by treatment with 2-acetylaminofluorene.

RESULTS

Unique observation of intestinal metaplastic cells was seen under alcian blue/periodic acid-Schiff staining.

CONCLUSION

The existence of this phenotype (along with oval cells and small hepatocyte-like cells) is evidence of multipotency of progenitors involved in the hepatic healing response.

Epithelial expression of angiogenic growth factors modulate arterial vasculogenesis in human liver development

Intrahepatic bile ducts maintain a close anatomical relationship with hepatic arteries. During liver ontogenesis, the development of the hepatic artery appears to be modulated by unknown signals originating from the bile duct. Given the capability of cholangiocytes to produce angiogenic growth factors and influence peribiliary vascularization, we studied the immunohistochemical expression of vascular endothelial growth factor (VEGF), angiopoietin-1, angiopoietin-2, and their cognate receptors (VEGFR-1, VEGFR-2, Tie-2) in fetal human livers at different gestational ages and in mice characterized by defective biliary morphogenesis (Hnf6(-/-)). The results showed that throughout the different developmental stages, VEGF was expressed by developing bile ducts and angiopoietin-1 by hepatoblasts, whereas their cognate receptors were variably expressed by vascular cells according to the different maturational stages. Precursors of endothelial and mural cells expressed VEGFR-2 and Tie-2, respectively. In immature hepatic arteries, endothelial cells expressed VEGFR-1, whereas mural cells expressed both Tie-2 and Angiopoietin-2. In mature hepatic arteries, endothelial cells expressed Tie-2 along with VEGFR-1. In early postnatal Hnf6(-/-) mice, VEGF-expressing ductal plates failed to incorporate into the portal mesenchyma, resulting in severely altered arterial vasculogenesis.

CONCLUSION

The reciprocal expression of angiogenic growth factors and receptors during development supports their involvement in the cross talk between liver epithelial cells and the portal vasculature. Cholangiocytes generate a VEGF gradient that is crucial during the migratory stage, when it determines arterial vasculogenesis in their vicinity, whereas angiopoietin-1 signaling from hepatoblasts contributes to the remodeling of the hepatic artery necessary to meet the demands of the developing epithelium.

Non invasive high resolution in vivo imaging of alpha-naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity in STII medaka

A novel transparent stock of medaka (Oryzias latipes; STII), homozygous recessive for all four pigments (iridophores, xanthophores, leucophores, melanophores), permits transcutaneous, high resolution (<1 microm) imaging of internal organs and tissues in living individuals. We applied this model to in vivo investigation of alpha -naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity. Distinct phenotypic responses to ANIT involving all aspects of intrahepatic biliary passageways (IHBPs), particularly bile preductular epithelial cells (BPDECs), associated with transitional passageways between canaliculi and bile ductules, were observed. Alterations included: attenuation/dilation of bile canaliculi, bile preductular lesions, hydropic vacuolation of hepatocytes and BPDECs, mild BPDEC hypertrophy, and biliary epithelial cell (BEC) hyperplasia. Ex vivo histological, immunohistochemical, and ultrastructural studies were employed to aid in interpretation of, and verify, in vivo findings. 3D reconstructions from in vivo investigations provided quantitative morphometric and volumetric evaluation of ANIT exposed and untreated livers. The findings presented show for the first time in vivo evaluation of toxicity in the STII medaka hepatobiliary system, and, in conjunction with prior in vivo work characterizing normalcy, advance our comparative understanding of this lower vertebrate hepatobiliary system and its response to toxic insult.

An in vivo look at vertebrate liver architecture: three-dimensional reconstructions from medaka (Oryzias latipes)

Understanding three-dimensional (3D) hepatobiliary architecture is fundamental to elucidating structure/function relationships relevant to hepatobiliary metabolism, transport, and toxicity. To date, factual information on vertebrate liver architecture in 3 dimensions has remained limited. Applying noninvasive in vivo imaging to a living small fish animal model we elucidated, and present here, the 3D architecture of this lower vertebrate liver. Our investigations show that hepatobiliary architecture in medaka is based on a polyhedral (hexagonal) structural motif, that the intrahepatic biliary system is an interconnected network of canaliculi and bile-preductules, and that parenchymal architecture in this lower vertebrate is more related to that of the mammalian liver than previously believed. The in vivo findings presented advance our comparative 3D understanding of vertebrate liver structure/function, help clarify previous discrepancies among vertebrate liver conceptual models, and pose interesting questions regarding the "functional unit" of the vertebrate liver.

Epithelial cells with hepatobiliary phenotype: is it another stem cell candidate for healthy adult human liver?

AIM

To investigate the presence and role of liver epithelial cells in the healthy human adult liver.

METHODS

Fifteen days after human hepatocyte primary culture, epithelial like cells emerged and started proliferating. Cell colonies were isolated and subcultured for more than 160 d under specific culture conditions. Cells were analyzed for each passage using immunofluorescence, flow cytometry and reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Flow cytometry analysis demonstrated that liver epithelial cells expressed common markers for hepatic and stem cells such as CD90, CD44 and CD29 but were negative for CD34 and CD117. Using immunofluorescence we demonstrated that liver epithelial cells expressed not only immature (alpha-fetoprotein) but also differentiated hepatocyte (albumin and CK-18) and biliary markers (CK-7 and 19), whereas they were negative for OV-6. RT-PCR analysis confirmed immunofluorescence data and revealed that liver epithelial cells did not express mature hepatocyte markers such as CYP2B6, CYP3A4 and tyrosine amino-transferase. Purified liver epithelial cells were transplanted into SCID mice. One month after transplantation, albumin positive cell foci were detected in the recipient mouse parenchyma.

CONCLUSION

According to their immature and bipotential phenotype, liver epithelial cells might represent a pool of precursors in the healthy human adult liver other than oval cells.

Bone marrow-derived hepatic oval cells differentiate into hepatocytes in 2-acetylaminofluorene/partial hepatectomy-induced liver regeneration

BACKGROUND AND AIMS

The ability of the bone marrow cells to differentiate into liver, pancreas, and other tissues led to the speculation that these cells might be the source of adult stem cells found in these organs. The present study analyzed whether the bone marrow cells are a source of hepatic oval cells involved in rat liver regeneration induced by 2-acetylaminofluorene (2-AAF) and 70% partial hepatectomy (PHx).

METHODS

Three groups of mutant F344 dipeptidyl peptidase IV-deficient (DPPIV(-)) rats were required for the study. Groups A and B received the mitotic inhibitor monocrotaline, followed by male F344 (DPPIV(+)) bone marrow transplantation. Next, group A received PHx only, while group B received the 2-AAF/PHx required for the oval cell activation. The last group C was used to analyze the effects of monocrotaline on transplanted bone marrow cells. These rats underwent transplantation with bone marrow cells and were then treated with monocrotaline. Subsequently, the animals were treated with 2-AAF/PHx.

RESULTS

In group A, DPPIV(+) hepatocytes were found in the liver. Group B showed that approximately 20% of the oval cell population expressed both donor marker (DPPIV) and alpha-fetoprotein, and some differentiated into hepatocytes. In contrast, animals in group C failed to significantly induce oval cells with the donor DPPIV antigen. In addition, X/Y-chromosome analysis revealed that fusion was not contributing to differentiation of donor-derived oval cells.

CONCLUSIONS

Our results suggest that under certain physiologic conditions, a portion of hepatic stem cells might arise from the bone marrow and can differentiate into hepatocytes.

Epithelial colonies cultured from human explanted liver in subacute hepatic failure exhibit hepatocyte, biliary epithelial, and stem cell phenotypic markers

The liver in subacute hepatic failure may become enriched for hepatic progenitor cells. Liver tissue from such a patient was collagenase digested and, from the nonparenchymal cell fraction, epithelioid colonies were developed. Albumin and alpha-1-antitrypsin (AAT) were secreted for greater than 120 days from these colonies. Reverse transcription-polymerase chain reaction showed expression of markers of both hepatocyte and biliary epithelial phenotypes (cytokeratins 7, 18, and 19, albumin and AAT, hepatocyte growth factor receptor, transforming growth factor beta receptor type II, gamma-glutamyl transpeptidase, biliary glycoprotein). The cell cycle regulator p21 was also expressed. The POU domain transcription factor octamer-binding protein 4 was present in these cells, but not in RNA or cDNA prepared from adult human liver. These markers were maintained even after 165 days culture. Proliferating epithelial-like cells with combined hepatocyte- and biliary-epithelial-specific functional markers and a stem cell marker can be isolated from the nonparenchymal fraction of liver cells in subacute hepatic failure.

Small epithelial cells in human liver cirrhosis exhibit features of hepatic stem-like cells: immunohistochemical, electron microscopic and immunoelectron microscopic findings

AIMS

To investigate whether cells with features similar to those of the oval cells of rodents and the small epithelial cells (SEC) recently described in certain human liver diseases, i.e. hepatic progenitor cells, also occur in human liver cirrhosis.

METHODS AND RESULTS

Surgical specimens from 35 cases of hepatitis B virus-positive cirrhosis (30 cases containing hepatocellular carcinoma) were investigated by immunohistochemical staining for cytokeratin 7 and albumin. Electron microscopic investigations, and immunoelectron microscopic investigations using the same antibodies and a double-labelling technique were performed in 15 and seven cases, respectively. SEC were observed in proliferated bile ductules, at the margins of regenerating nodules and in the fibrous septa in all cases of cirrhosis. The SEC were morphologically similar to the SEC described previously, and to the oval cells seen in experimental hepatocarcinogenesis. They were characterized by their small size, oval shape, scanty electron-dense or electron-lucent cytoplasm, a high nucleo-cytoplasmic ratio, tonofilaments and intercellular junctions. Immunoelectron microscopy revealed that the SEC co-expressed cytokeratin 7 and albumin. Both relatively undifferentiated SEC and SEC with morphological and immunophenotypical signs of differentiation towards biliary epithelial cells and hepatocytes were found.

CONCLUSIONS

SEC that exhibit morphological and immunophenotypical features of the SEC seen in certain other liver diseases are found in cirrhosis. These findings further support the hypothesis that a bipotent hepatic stem cell that may give rise to biliary epithelial cells and hepatocytes exists in the human liver.

Stem cells in adult tissues

In recent years the concept of a stem cell has evolved to encompass the hypotheses that stem cells exist within many adult tissues, and that a common 'interchangeable' progenitor cell may exist within the bone marrow capable of regenerating and repairing tissues throughout the body. As more knowledge is gained about stem cells, their potential roles in disease processes, including the development and progression of cancer, have moved to the forefront. The underlying hypothesis of this review is that cell fate is determined by a combination of intrinsic and extrinsic factors; growth and differentiation are regulated through intracellular integration of a multitude of signals initiated by internal and external stimuli. The development of successful stem cell based therapies may depend on experimental approaches that consider both the intrinsic and extrinsic factors that control cell fate.

Neoductular progenitor cells regenerate hepatocytes in severely damaged liver: a comparative ultrastructural study

In severely injured liver, stem cells give rise to progeny that tend to replace lost hepatocytes. Neoductular reaction appears as an inherent stage of liver reconstruction following severe damage caused by different pathological mechanisms. Few ultrastructural types of progenitor cells have been described, and some molecular phenotypes of progenitor stages have been characterized, but the details of the differentiation process are largely unknown. We prepared for light and electron microscopy examination human liver from biopsies of patients with chronic active hepatitis, and rat liver with allyl alcohol-induced periportal necrosis. We found that progenitor neoductular cells acquire the hepatocytic polarity pattern during a multi-step process apparently involving cell migration and dissolution of neoductular basement membrane. An intermediate stage with "mixed" ductular and hepatocytic polarity was described.

Hepatic stem cells: a review

The existence of a liver stem cell population has only gained credence recently, following the results of animal experiments. These cells are thought to reside in the terminal bile ductules (canals of Hering). Hepatocyte division is responsible for liver regeneration after most causes of injury. However, stem cells may contribute to hepatocyte regeneration, or even take over this role if the liver injury is severe and associated with an impairment of hepatocyte proliferation as in cirrhosis or submassive/massive necrosis, due to drugs, toxins or viruses. "Oval" cells are the descendants of the stem cells and are found in the portal and periportal regions in experimental animals within days of the liver injury. These cells proliferate to form narrow ductules, which may stain positively for biliary cytokeratins CK 19, and radiate out into the damaged parenchyma. Both in vitro and in vivo animal studies now suggest that oval cells can differentiate into bile ductular cells or hepatocytes to allow repopulation of the injured liver. As the oval cells differentiate into hepatocytes they may show positive staining for pyruvate kinase isoenzyme L-PK, albumin and alpha-fetoprotein. There is also growing evidence that bone marrow stem cells may contribute to liver regeneration. The possible involvement of hepatic stem cells in the development of dysplastic nodules, hepatocellular carcinoma and cholangiocarcinoma has been suggested but remains highly controversial. Oval cell isolation and culture techniques, together with stem cell transplantation strategies, may in the future provide novel treatments for individuals with inherited and acquired hepatic disorders.

Cholangiocytic apoptosis in chronic ductopenic rejection

The significance of cholangiocytic apoptosis as a mechanism of ductopenia in liver rejection remains controversial. In a previous study, the presence but not the extent of ductal apoptosis was assessed by electron microscopy. Other previously published studies using an in situ hybridization method (in situ end labeling) produced conflicting results (no apoptosis v massive apoptosis). We studied 47 liver needle biopsies from 8 patients with chronic ductopenic rejection confirmed by pathologic examination of the failed grafts. These biopsies were performed because of graft dysfunction, during a period of several months before retransplantation, and they showed cholangiocytic injury with progressive ductal paucity. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick-end labeling (TUNEL) was used to detect apoptosis (tissue digestion with proteinase K 20 microg/mL for 20 minutes). The interlobular bile ducts did not show labeling, even in lymphocytic cholangitis with obvious epithelial injury. However, there was minimal staining of ductular nuclei. Lymphocytic nuclei were also labeled. Apoptosis was not detectable in the vanishing interlobular bile ducts, even when more representative samples were studied and a more sensitive method was used. Unless apoptosis of cholangiocytes is an exceptionally rapid process escaping detection by conventional methods, ductopenia results mainly from ordinary, nonprogrammed cholangiocytic death. Apoptosis could still be involved in the pathogenesis of ductopenia by depleting cholangiocytic precursors, generally presumed to reside in ductules. This is a possible mechanism suggested by the following: (1) the established role of apoptosis in the homeostatic control of immature/progenitor cells, (2) the paucity of ductular proliferation in chronic rejection, (3) the previously reported decrease of ductular bcl-2 expression in rejection, and (4) the sporadic ductular TUNEL labeling seen in this study.

Is heterozygous alpha-1-antitrypsin deficiency type PIZ a risk factor for primary liver carcinoma?

BACKGROUND

It is well known that homozygotes with alpha-1-antitrypsin deficiency type PiZ are associated with an increased risk of chronic liver disease and liver carcinoma. The aim of this study was to determine whether heterozygous PiZ status is a risk factor for liver carcinoma development.

METHODS

Three hundred seventeen consecutive primary liver carcinomas and the tumor-bearing liver tissue (tumor series) from adult patients were screened immunohistochemically for hepatocellular PiZ deposits. Liver specimens from 1663 consecutive adult patients (biopsy series) and liver tissue from 1030 consecutive adult autopsies (autopsy series) served as controls. The zygosity status of alpha-1-antitrypsin was verified by analysis of single strand conformational polymorphism and by sequencing DNA extracted from paraffin embedded tissue.

RESULTS

The PiZ frequency in the tumor series (5.99%) was significantly higher than in the biopsy series (3.43%) or the autopsy series (1.84%). Cholangiocarcinomas and/or combined hepatocholangiocarcinomas were seen significantly more frequently in PiZ-associated liver carcinomas (57.9%) than in non-PiZ-associated carcinomas (27.2%). Cirrhosis was found in only 3 of the 19 PiZ-associated carcinomas. The remaining 16 livers showed varying stages of fibrosis or normal tissue. All nine cases with PiZ-associated liver carcinoma suitable for genetic analysis showed heterozygous PiZ mutations.

CONCLUSIONS

Heterozygotes of type PiZ are associated with an increased risk of primary liver carcinoma. PiZ-associated carcinoma may develop in noncirrhotic liver tissue and without concurrent liver disease, and is frequently characterized by cholangiocellular differentiation. The site specific antibody ATZ11 is a reliable morphologic tool for detecting PiZ individuals.

Unique epithelial cell production of hepatocyte growth factor/scatter factor by putative precancerous intestinal metaplasias and associated "intestinal-type" biliary cancer chemically induced in rat liver

Recently, we observed that Met, the receptor for hepatocyte growth factor/scatter factor (HGF/SF), is overexpressed in epithelial cells of both early-appearing intestinal metaplastic glands in precancerous hepatic cholangiofibrotic tissue and neoplastic glands in later developed intestinal-type of cholangiocarcinoma originated from the furan rat model of cholangiocarcinogenesis when compared with normal and hyperplastic intrahepatic biliary epithelia. We now show that HGF/SF is also aberrantly expressed in a manner closely paralleling that of its receptor in the neoplastic epithelial cells of furan-induced rat cholangiocarcinomas and in a majority of metaplastic epithelial cells within earlier formed precancerous hepatic cholangiofibrotic tissue. Using in situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR), we further showed specific expression of HGF/SF messenger RNA (mRNA) in a novel rat cholangiocarcinoma epithelial cell line overexpressing Met. This cholangiocarcinoma cell line, termed C611B, was established from tumorigenic cells isolated from a furan-induced transplantable tumor. Moreover, we detected by in situ hybridization strong expression of HGF/SF mRNA transcripts in the cancerous epithelial glands of cholangiocarcinoma developed in recipient rats after in vivo cell transplantation of C611B cells. In contrast, mRNA transcripts and protein immunoreactivity for this cytokine were not detected in hepatocytes and biliary epithelial cells in adult normal rat liver nor in rat hyperplastic intrahepatic biliary epithelium. Our results clearly show that HGF/SF becomes aberrantly expressed in cholangiocarcinoma epithelium and in putative precancerous intestinal metaplastic epithelium induced in the liver of furan-treated rats.

Proliferation of hepatic lineage cells of normal C57BL and interleukin-6 knockout mice after cocaine-induced periportal injury

The cellular response to periportal liver injury, induced by phenobarbital feeding and cocaine injection, is used to compare the restitutive proliferation of hepatocytes, cholangiocytes, and oval cells in the livers of normal control to those of interleukin-6 (IL-6) knockout mice. After this injury hepatocytes in noninjured middle and central zones start to proliferate first, followed by proliferation of cholangiocytes and intraportal oval cells. Proliferation of all cell types peaks at 2 days, but oval cells continue to proliferate and differentiate through days 4 and 6 as they reconstitute the necrotic zone. By day 10, the injured zone is completely repaired, and no dividing cells remain. During the first 3 to 4 days after injury, the number of proliferating hepatocytes, cholangiocytes, and sinusoidal cells is lower in IL-6 knockout mice than in normal mice, whereas the number of dividing oval cells is higher. However, overall repair of the injury is accomplished in the same time period in both groups. During repair of the periportal zone, oval cells acquire differentiation markers of hepatocytes as they cross the zone of injury. In conclusion, the phenobarbital/cocaine injury model is useful to study restitutive proliferation of mouse liver cell lineages. The proliferative response in IL-6 knockout mice shows that IL-6 is not required for proliferation of liver cells; timely repair of liver injury occurs in both normal and IL-6 knockout mice. Increased proliferation of oval cells in IL-6 knockout mice may compensate for the lower proliferation of other liver cell types.

Fractionation of rat hepatocyte subpopulations with varying metabolic potential, proliferative capacity, and retroviral gene transfer efficiency

The liver contains hepatocytes with varying ploidy and gene expression. To isolate cells on the basis of ploidy for analyzing mechanisms concerning cell proliferation and differentiation, we used Percoll gradients to separate F344 rat hepatocyte subpopulations. Specific fractions were enriched in polyploid (H2 fraction) or diploid (H3 and H4 fractions) hepatocytes containing glycogen and glucose-6-phosphatase. H4 cells were relatively smaller with greater nuclear/cytoplasmic ratios, less complex cytoplasm, and higher serum albumin or ceruloplasmin biosynthetic rates. H2 fraction cells were larger with lesser nuclear/cytoplasmic ratio, more complex cytoplasm, and more cytochrome P450 activity. Phenotypic marking showed that H4 cells originated in zone one and H2 cells in zones two or three of the liver lobule. H4 cells showed much greater mitogenic responsiveness to human hepatocyte growth factor. Retroviral gene transfer, which requires both viral receptors and cellular DNA synthesis, was significantly more efficient in H4 cells. The findings indicated that small diploid and large polyploid hepatocytes show unique biological differences. The ability to isolate hepatocytes of varying maturity is relevant for mechanisms concerning liver growth control and hepatic gene expression.

Isolation from human fetal liver of cells co-expressing CD34 haematopoietic stem cell and CAM 5.2 pancytokeratin markers

BACKGROUND/AIMS

Ductal plate and bile duct cells in developing human liver express haematopoietic stem cell markers, such as c-kit and CD34, in association with cytokeratin markers CAM 5.2 and CK 18. The identification of such ductal plate cells as likely progenitors for both bile duct epithelial cells and hepatocytes and their possible reappearance as oval cells in the regenerating liver have generated much interest in their pluripotential capacities. This study aimed to isolate cells from human fetal liver that co-express haematopoietic stem cell and epithelial cell markers.

METHODS

Human fetal liver was harvested following legal termination of pregnancy at week 14-22. CD34+ mononuclear cells were isolated from liver cell suspensions with immunomagnetic beads. Immunofluorescent staining, using anticytokeratin CAM 5.2 against CK 8 and 18, was performed on permeabilised CD34+ cells for flow cytometry and fluorescent microscopy. CD34+ cells were also stained for other stem cell markers (HLA-DR, c-kit) and committed haematopoietic cell markers (CD33, CD38).

RESULTS

Approximately 0.9% (range 0.07-4.0%) of the mononuclear cells isolated were CD34+ cells. The number of mononuclear cells isolated correlated with fetal liver weight (r=0.508). About 3-8% of these CD34+ cells stained positively for CAM 5.2. In addition, CD34+ cells were positive for HLA-DR, but only a small percentage was positive for c-kit. Staining for the committed haematological markers, CD33 and CD38, was consistently negative.

CONCLUSIONS

This study describes an immunoaffinity method for the enrichment from human fetal liver of cells that co-express haematopoietic stem cell and epithelial cell markers. Such cellular subsets may correspond to pluripotential ductal plate and bile duct cells.

Hepatic OV-6 expression in human liver disease and rat experiments: evidence for hepatic progenitor cells in man

BACKGROUND/AIMS/METHODS

Since in rat experiments, activation of progenitor cells is seen in conditions associated with hepatocyte injury or inhibited replication, we compared the activation and fate of human putative progenitor cells in regenerating liver versus chronic cholestatic disease, using immunohistochemistry, rat oval cell marker OV6 and a panel of bile ductular cell markers. We compared the results with different rat models: the choline-deficient acetylaminofluorene (CDAAF)- and alpha-naphthylisothiocyanate (ANIT)-model, using immunohistochemistry and electron microscopy.

RESULTS

In very early stages of human liver regeneration, putative progenitor cells in the vicinity of portal tracts were immunoreactive for OV6, CK7, CK19 and chrom-A. In later stages of regeneration and in chronic cholestasis, reactive bile ductules (immunoreactive for OV6, CK7, CK19, chrom-A, NCAM) and intermediate hepatocyte-like cells (immunoreactive for OV6, CK7, chrom-A), became apparent, suggesting bidirectional differentiation of the putative progenitor cells. In regenerating human liver, intermediate hepatocyte-like cells became more numerous with time and extended far into the lobule. In advanced cholestasis, intermediate hepatocyte-like cells were less numerous and formed periportal rosettes and small clusters. In the CDAAF rat model (associated with inhibited hepatocyte replication), but not in the ANIT model, gradual differentiation of oval cells into hepatocytes was seen after stopping the diet.

CONCLUSIONS

Our results in human liver suggest that reactive ductules and intermediate hepatocyte-like cells originate at least partly from activation and differentiation of "progenitor cells". In regeneration after submassive necrosis, in analogy with what is seen in rat models, differentiation towards hepatocytes is more pronounced than in chronic cholestasis.

The normal adult human liver biopsy: a quantitative reference standard

In assessing adult human liver histology, questions remain concerning the normal number of portal tracts and bile ducts in a liver biopsy. We therefore reviewed liver biopsies obtained with use of a percutaneous Menghini cutting needle (14G, internal diameter 1.6 mm), from 16 patients undergoing liver biopsy for screening procedures (age 49 +/- 14 years, +/-SD) and found to be normal by histological examination. The average aggregate length of the liver tissue was 1.8 +/- 0.8 cm (area of 16.4 +/- 10.7 mm2), representing 7 +/- 3 tissue fragments. Portal triads containing at least one profile each of a portal vein, hepatic artery, and interlobular bile duct numbered 11 +/- 6 per biopsy (range 3-23). Portal dyads, which did not contain one of these profiles, usually the portal vein, numbered 8 +/- 5 (range 1-18). On a per-specimen basis, 38% of portal tracts did not contain a portal vein, 7% did not contain a bile duct, and 9% did not contain a hepatic artery. Because of multiplicity of profiles within portal tracts, however, the average number of profiles per portal tract was 6 +/- 5 (range 2-35). Notably, on average there were 2.3 +/- 2.2 interlobular bile ducts per portal tract, compared to 2.6 +/- 2.3 hepatic arteries and 0.7 +/- 0.7 portal veins. The average minimum external diameter of interlobular bile ducts was 13 +/- 4 microm, of hepatic arteries 12 +/- 5 microm, and of portal veins 35 +/- 25 microm. Bile ducts greater than 30 microm in diameter were rare, only one each in two biopsies were observed. In contrast, probable canals of Hering were occasionally evident at the periphery of portal tracts (6 +/- 6 per biopsy) and within the lobular parenchyma as strings of cuboidal cells (5 +/- 5 per biopsy). We conclude that, although multiplicity of profiles is normal, portal dyads are almost as common as portal triads in normal peripheral liver tissue. On average, there are two interlobular bile ducts, two hepatic arteries, and one portal vein per portal tract, with 6 full portal triads per linear cm of tissue obtained by external Menghini biopsy technique with use of a 14G needle, equivalent to 0.8 +/- 0.5 portal triads per mm2. By serving as a reference standard for adult human liver histology, these findings may assist in the histopathological assessment of liver biopsies, particularly those performed for disease conditions featuring loss of intrahepatic bile ducts.

Primary liver tumour of intermediate (hepatocyte-bile duct cell) phenotype: a progenitor cell tumour?

A 57-year-old female patient presented with painless obstructive jaundice and mild mesogastric pain; she was in good general condition on admission. Abdominal ultrasonography revealed diffuse tumoral invasion of the liver, suggesting diffuse metastases. A liver biopsy showed a tumour with a trabecular growth pattern, composed of uniform relatively small cells, very suggestive of an endocrine carcinoma. Additional immunohistochemical stains, however, did not show any endocrine differentiation, but showed positivity for both hepatocyte-type cytokeratins (cytokeratin 8 and 18) and bile duct-type cytokeratins (cytokeratin 7 and 19). In addition, parathyroid hormone-related peptide, shown to be a good marker for cholangiocarcinoma, was immunoreactive. Electron microscopy revealed tumour cells with an intermediate phenotype: the cells clearly showed hepatocyte features on one hand and bile duct cell features on the other hand. Nine days after admission, the patient died due to liver failure and hepatic encephalopathy. Autopsy excluded another primary tumour site. Overall, this tumour was a primary liver tumour with an intermediate phenotype and with a very rapid clinical course. The intermediate (between hepatocyte and bile duct cell) phenotype suggests an immature progenitor cell origin, which is concordant with a rapid clinical course. This type of tumour has not been described previously and provides additional evidence for the existence of progenitor cells in human liver.

Increased MUC6 apomucin expression is a characteristic of reactive biliary epithelium in chronic viral hepatitis

The significance of bile ductular proliferation in the progression of various hepatobiliary diseases remains unclear. Increased expression of MUC6 apomucin, a major gastric mucin, has previously been noticed in proliferating bile ductules in chronic viral hepatitis. The purpose of the present study was to characterize MUC6 apomucin and mRNA expression in 35 histologically 'normal livers', 47 livers with chronic viral hepatitis, 28 with primary biliary cirrhosis, and seven with extrahepatic biliary obstruction. MUC6 protein was expressed focally in cytoplasm and/or on the luminal surface of septal and interlobular bile ducts in normal and diseased livers. Bile ductules in normal livers rarely expressed MUC6 protein. The MUC6 expression intensified and spread in proliferating bile ductules and small bile ducts in chronic viral hepatitis and to a lesser degree in other diseases. In the former, the extent and degree of MUC6 expression paralleled the degree of active necroinflammation. MUC6 mRNA expression resembled MUC6 protein expression in proliferating bile ductules and intralobular small biliary cells, suggesting increased transcription and synthesis of MUC6. In conclusion, proliferating bile ductular cells express MUC6 apomucin in diseased liver, especially in chronic viral hepatitis with active necroinflammation. This secreted mucin may act as a cytoprotective agent and represent a phenotype of reactive biliary epithelium in chronic viral hepatitis.

Liver carcinoma in PiZ alpha-1-antitrypsin deficiency

One hundred sixty-four consecutive cases of primary liver carcinoma were evaluated for tumor type, (i.e., hepatocellular carcinoma [HCC], cholangiocarcinoma [CC], and combined hepatocholangiocarcinoma [CHCC]), and for signs of alpha-1-antitrypsin deficiency (AATD) in the surrounding liver tissue. Hepatocellular globular alpha-1-antitrypsin deposits, as detected by a monoclonal antibody to the mutant PiZ alpha-1-antitrypsin (AAT), were seen in 13 cases (7.9%). With regard to tumor type, 4 of 111 HCC cases (3.5%), but 4 of 37 CC cases (10.5%), and even 5 of 16 CHCC cases (30%) were positive for this antitrypsin variant. In all but 1 of 13 cases of alpha-1-antitrypsin deficiency, the carcinoma developed in noncirrhotic liver tissue of elderly people (mean age, 62.9 years). In three patients, a heterozygous state of ATT (PiMZ) could be revealed using isoelectric focusing or direct genetic analysis. We conclude from our findings that CHCC and CC especially might be associated with PiZ alpha-1-antitrypsin deficiency. Primary liver carcinoma might develop even in a heterozygote state of PiZ alpha-1-antitrypsin deficiency without concurrent liver disease. Furthermore, liver cirrhosis is not a precondition for these tumors.

Comparison of liver progenitor cells in human atypical ductular reactions with those seen in experimental models of liver injury

The ultrastructural characteristics of liver progenitor cell types of human atypical ductular reactions seen in chronic cholestasis, in regenerating human liver after submassive necrosis, in alcoholic liver disease, and in focal nodular hyperplasia are compared with liver progenitor cell types seen during experimental cholangiocarcinogenesis in hamsters; during hepatocarcinogenesis in rats; and in response to periportal liver injury induced by allyl alcohol in rats. Three types of progenitor cells have been identified in human atypical ductular reactions: type I: primitive, has an oval shape, marginal chromatin, few cellular organelles, rare tonofilaments, and forms desmosomal junctions with adjacent liver cells; type II: bile duct-like, is located within ducts, has few organelles, and forms lateral membrane interdigitations with other duct-like cells; and type III: hepatocyte-like, is located in hepatic cords, forms a bile canaliculus, has tight junctions with other hepatocyte-like cells, prominent mitochondria and rough endoplasmic reticulum, and some have lysosomes and a poorly developed Golgi apparatus. Each type is seen during cholangiocarcinogenesis in hamsters, but the most prominent cell type is type II, duct-like. A more primitive cell type ("type 0 cell"), as well as type I cells, are seen in the intraportal zone of the liver within 1 to 2 days after carcinogen exposure or periportal injury in the rat, but both type II and type III are seen later as the progenitor cells expand into the liver lobule. After allyl alcohol injury, type 0 cells precede the appearance of type I and type III cells, but most of the cells that span the periportal necrotic zone are type III hepatocyte-like cells showing different degrees of hepatocytic differentiation. Some type II cells are also seen, but these are essentially limited to ducts. It is concluded that there is a primitive stem cell type in the liver (type 0) that may differentiate directly into type I and then into type II, duct-like or or type III hepatocyte-like cells. The terms oval cell, transitional hepatocyte, biliary hepatocyte, hepatocyte-like cell, atypical ductular cell, neocholangiole, etc., are used to describe these cells. Although these terms are useful as general descriptive terms for liver precursor cells at the light microscopic level, the cells included in these descriptive categories may be very different from one another biologically and ultrastructurally.