Position-specific gene expression in the liver lobule is directed by the microenvironment and not by the previous cell differentiation state

Caffeine disrupts ataxia telangiectasia mutated gene-related pathways and exacerbates acetaminophen toxicity in human fetal immortalized hepatocytes

Preterm infants are at greater risk for adverse drug effects due to hepatic immaturity. Multiple interventions during intensive care increases potential for drug interactions. In this setting, high-dose caffeine used for apnea in premature infants may increase acetaminophen toxicity by inhibiting ataxia telangiectasia mutated (ATM) gene activity during DNA damage response. To define caffeine and acetaminophen interaction, we modeled infantile prematurity in late-gestation fetal stage through human immortalized hepatocytes and liver organoids. The acute toxicity studies included assays for cell viability, mitochondrial dysfunction and ATM pathway-related DNA damage. Fetal cells expressed hepatobiliary properties, albeit with lower metabolic, synthetic and antioxidant functions than more mature hepatocytes. Acetaminophen in IC50 amount of 7.5 millimolar caused significant oxidative stress, mitochondrial membrane potential impairments, and DNA breaks requiring ATM-dependent repair. Caffeine markedly exacerbated acetaminophen toxicity by suppressing ATM activity in otherwise nontoxic 2.5 millimolar amount. Similarly, the specific ATM kinase antagonist, KU-60019, reproduced this deleterious interaction in 5 micromolar amount. Replicative stress from combined acetaminophen and caffeine toxicity depleted cells undergoing DNA synthesis in S phase and activated checkpoints for G0/G1 or G2/M restrictions. Synergistic caffeine and acetaminophen toxicity in liver organoids indicated these consequences should apply in vivo. The antioxidant, N-acetylcysteine, decreased oxidative damage, mitochondrial dysfunction and ATM pathway disruption to mitigate caffeine and acetaminophen toxicity. We concluded that hepatic DNA damage, mitochondrial impairment and growth-arrest after combined caffeine and acetaminophen toxicity will be harmful for premature infants. Whether caffeine and acetaminophen toxicity may alter outcomes in subsequently encountered hepatic disease needs consideration.

Hepatic differentiation of human pluripotent stem cells by developmental stage-related metabolomics products

Endogenous cell signals regulate tissue homeostasis and are significant for directing the fate of stem cells. During liver development, cytokines released from various cell types are critical for stem/progenitor cell differentiation and lineage expansions. To determine mechanisms in these stage-specific lineage interactions, we modeled potential effects of soluble signals derived from immortalized human fetal liver parenchymal cells on stem cells, including embryonic and induced pluripotent stem cells. For identifying lineage conversion and maturation, we utilized conventional assays of cell morphology, gene expression analysis and lineage markers. Molecular pathway analysis used functional genomics approaches. Metabolic properties were analyzed to determine the extent of hepatic differentiation. Cell transplantation studies were performed in mice with drug-induced acute liver failure to elicit benefits in hepatic support and tissue regeneration. These studies showed signals emanating from fetal liver cells induced hepatic differentiation in stem cells. Gene expression profiling and comparison of regulatory networks in immature and mature hepatocytes revealed stem cell-derived hepatocytes represented early fetal-like stage. Unexpectedly, differentiation-inducing soluble signals constituted metabolomics products and not proteins. In stem cells exposed to signals from fetal cells, mechanistic gene networks of upstream regulators decreased pluripotency, while simultaneously inducing mesenchymal and epithelial properties. The extent of metabolic and synthetic functions in stem cell-derived hepatocytes was sufficient for providing hepatic support along with promotion of tissue repair to rescue mice in acute liver failure. During this rescue, paracrine factors from transplanted cells contributed in stimulating liver regeneration. We concluded that hepatic differentiation of pluripotent stem cells with metabolomics products will be significant for developing therapies. The differentiation mechanisms involving metabolomics products could have an impact on advancing recruitment of stem/progenitor cells during tissue homeostasis.

Decellularized human placenta supports hepatic tissue and allows rescue in acute liver failure

Tissue engineering with scaffolds to form transplantable organs is of wide interest. Decellularized tissues have been tested for this purpose, although supplies of healthy donor tissues, vascular recellularization for perfusion, and tissue homeostasis in engineered organs pose challenges. We hypothesized that decellularized human placenta will be suitable for tissue engineering. The universal availability and unique structures of placenta for accommodating tissue, including presence of embedded vessels, were major attractions. We found decellularized placental vessels were reendothelialized by adjacent native cells and bridged vessel defects in rats. In addition, implantation of liver fragments containing all cell types successfully hepatized placenta with maintenance of albumin and urea synthesis, as well as hepatobiliary transport of ^99m Tc-mebrofenin, up to 3 days in vitro. After hepatized placenta containing autologous liver was transplanted into sheep, tissue units were well-perfused and self-assembled. Histological examination indicated transplanted tissue retained hepatic cord structures with characteristic hepatic organelles, such as gap junctions, and hepatic sinusoids lined by endothelial cells, Kupffer cells, and other cell types. Hepatocytes in this neo-organ expressed albumin and contained glycogen. Moreover, transplantation of hepatized placenta containing autologous tissue rescued sheep in extended partial hepatectomy-induced acute liver failure. This rescue concerned amelioration of injury and induction of regeneration in native liver. The grafted hepatized placenta was intact with healthy tissue that neither proliferated nor was otherwise altered.

CONCLUSION

The unique anatomic structure and matrix of human placenta were effective for hepatic tissue engineering. This will advance applications ranging from biological studies, drug development, and toxicology to patient therapies. (Hepatology 2018;67:1956-1969).

Application of Toxicogenomics to Toxicology: Basic Concepts in the Analysis of Microarray Data

Toxicology and the practice of pathology are rapidly evolving in the postgenomic era. Observable treatment related changes have been the hallmark of toxicology studies. Toxicogenomics is a powerful new tool that may show gene and protein changes earlier and at treatment levels below the limits of detection of traditional measures of toxicity. It may also aid in the understanding of toxic mechanisms. It is important to remember that it is only a tool and will provide meaningful results only when properly applied. As is often the case with new experimental tools, the initial utilization is driven more by the technology than application to problem solving. Toxicogenomics is interdisciplinary in nature including at a minimum, pathology, toxicology, and genomics. Most studies will require the input from the disciplines of toxicology, pathology, molecular biology, bioinformatics, biochemistry, and others depending on the types of questions being asked.

The fungicide thiabendazole causes apoptosis in rat hepatocytes

Many pharmaceutical drugs cause hepatotoxicity in humans leading to severe liver diseases, representing a serious public health issue. This study investigates the ability of the anthelmintic and antifungal drug thiabendazole to cause cell death by apoptosis and metabolic changes in primary cultures of rat hepatocytes. Thiabendazole (200-500 μM) induced apoptosis in hepatocytes after 1 to 24h, causing loss of mitochondrial membrane potential, cytochrome c release from mitochondria, Fas-associated death domain (FADD) translocation from the cytosol to membranes, and activation of caspases-3, -8 and -9. Thus, thiabendazole activated both the mitochondrial and death receptor pathways of apoptosis. Under these conditions, cell death by necrosis was not detected following exposure to thiabendazole (100-500 μM) for 24-48 h, measured by lactate dehydrogenase release and propidium iodide uptake. Furthermore, thiabendazole increased activities of cytochrome P450 (CYP) isoenzymes CYP1A and CYP2B after 24 and 48 h, determined by 7-ethoxyresorufin-O-deethylase (EROD) and 7-pentoxyresorufin-O-dealkylase (PROD) activities, respectively. An important finding is that thiabendazole can eliminate hepatocytes by apoptosis, which could be a sensitive marker for hepatic damage and cell death. This study improves understanding of the mode of cell death induced by thiabendazole, which is important given that humans and animals are exposed to this compound as a pharmaceutical agent and in an environmental context.

The Therapeutic Promise of Mesenchymal Stem Cells for Liver Restoration

Hepatocyte transplantation aims to provide a functional substitution of liver tissue lost due to trauma or toxins. Chronic liver diseases are associated with inflammation, deterioration of tissue homeostasis, and deprivation of metabolic capacity. Recent advances in liver biology have focused on the pro-regenerative features of mesenchymal stem cells (MSCs). We argue that MSCs represent an attractive therapeutic option to treat liver disease. Indeed, their pleiotropic actions include the modulation of immune reactions, the stimulation of cell proliferation, and the attenuation of cell death responses. These characteristics are highly warranted add-ons to their capacity for hepatocyte differentiation. Undoubtedly, the elucidation of the regenerative mechanisms of MSCs in different liver diseases will promote their versatile and disease-specific therapeutic use.

Cell therapy for liver disease: From liver transplantation to cell factory

Work over several decades has laid solid foundations for the advancement of liver cell therapy. To date liver cell therapy in people has taken the form of hepatocyte transplantation for metabolic disorders with a hepatic basis, and for acute or chronic liver failure. Although clinical trials using various types of autologous cells have been implemented to promote liver regeneration or reduce liver fibrosis, clear evidence of therapeutic benefits have so far been lacking. Cell types that have shown efficacy in preclinical models include hepatocytes, liver sinusoidal endothelial cells, mesenchymal stem cells, endothelial progenitor cells, and macrophages. However, positive results in animal models have not always translated through to successful clinical therapies and more realistic preclinical models need to be developed. Studies defining the optimal repopulation by transplanted cells, including routes of cell transplantation, superior engraftment and proliferation of transplanted cells, as well as optimal immunosuppression regimens are required. Tissue engineering approaches to transplant cells in extrahepatic locations have also been proposed. The derivation of hepatocytes from pluripotent or reprogrammed cells raises hope that donor organ and cell shortages could be overcome in the future. Critical hurdles to be overcome include the production of hepatocytes from pluripotent cells with equal functional capacity to primary hepatocytes and long-term phenotypic stability in vivo.

Ischemic preconditioning affects long-term cell fate through DNA damage-related molecular signaling and altered proliferation

Despite the potential of ischemic preconditioning for organ protection, long-term effects in terms of molecular processes and cell fates are ill defined. We determined consequences of hepatic ischemic preconditioning in rats, including cell transplantation assays. Ischemic preconditioning induced persistent alterations; for example, after 5 days liver histology was normal, but γ-glutamyl transpeptidase expression was observed, with altered antioxidant enzyme content, lipid peroxidation, and oxidative DNA adducts. Nonetheless, ischemic preconditioning partially protected from toxic liver injury. Similarly, primary hepatocytes from donor livers preconditioned with ischemia exhibited undesirably altered antioxidant enzyme content and lipid peroxidation, but better withstood insults. However, donor hepatocytes from livers preconditioned with ischemia did not engraft better than hepatocytes from control livers. Moreover, proliferation of hepatocytes from donor livers preconditioned with ischemia decreased under liver repopulation conditions. Hepatocytes from donor livers preconditioned with ischemia showed oxidative DNA damage with expression of genes involved in MAPK signaling that impose G1/S and G2/M checkpoint restrictions, including p38 MAPK-regulated or ERK-1/2-regulated cell-cycle genes such as FOS, MAPK8, MYC, various cyclins, CDKN2A, CDKN2B, TP53, and RB1. Thus, although ischemic preconditioning allowed hepatocytes to better withstand secondary insults, accompanying DNA damage and molecular events simultaneously impaired their proliferation capacity over the long term. Mitigation of ischemic preconditioning-induced DNA damage and deleterious molecular perturbations holds promise for advancing clinical applications.

Comparable constitutive expression and activity of cytochrome P450 between the lobes of the porcine liver

Due to limited availability of human liver tissue for the study of cytochrome P450 (CYP450), porcine liver tissue has been suggested as an alternative source to prepare microsomes and hepatocytes. The porcine liver is made by four different lobes. The present study investigated the expression and activity of specific CYP450 isoforms in the four lobes, with the purpose to examine if one lobe of the porcine liver resembles the human more than others. Samples from the four major lobes were taken from female pigs and mRNA expression and activity of CYP1A, 2A, 2C, 2D, 2E and 3A determined. The results showed no differences in specific mRNA expression and activity of any of the investigated CYP450 isoforms. In conclusion, the study shows that all parts of the porcine liver are equally useful as model tissue.

Overview of hepatotoxicity

This overview presents some of the salient features of liver biology and physiology that form the basis for a variety of experimental tools that are used for the study of chemical histopathology. Prevalence and risk of heptatoxicity are examined among a subset of chemicals from the EPA IRIS database. The goal is to provide an objective assessment of the magnitude of the problem of chemically-induced hepatotoxicity and contextual background against which the diversity of responses of the liver to toxicants and the analytical methods available for their study can be appreciated.

Directly acting drugs prostacyclin or nitroglycerine and endothelin receptor blocker bosentan improve cell engraftment in rodent liver

To optimize strategies for liver-directed cell therapy, prevention of initial transplanted cell losses is particularly important for subsequent liver repopulation. After cell transplantation in hepatic sinusoids, perturbations in hepatic microcirculation along with changes in various liver cell types are among the earliest changes. Therefore, for advancing further concepts in cell engraftment we studied vascular and related events in the liver after transplanting syngeneic hepatocytes into dipeptidyl peptidase IV-deficient rats. We treated rats with vascular drugs to define whether deleterious cell transplantation-induced events could be controlled followed by improvements in transplanted cell engraftment and proliferation. We found cell transplantation altered liver gene expression related to vessel tone, inflammation, cell adhesion, thrombosis, or tissue damage/remodeling. This was due to hepatic ischemia, endothelial injury, and activation of neutrophils, Kupffer cells, and hepatic stellate cells. Treatment of rats before cell transplantation with the angiotensin converting enzyme blocker, lisinopril, or angiotensin II receptor blocker, losartan, did not improve cell engraftment. By contrast, direct-acting nitroglycerine or prostacyclin improved cell engraftment and also kinetics of liver repopulation. These drugs lowered hepatic ischemia and inflammation, whereas pretreatment of rats with the dual endothelin-1 receptor blocker, bosentan, improved cell engraftment independently of hepatic ischemia or inflammation, without improving liver repopulation. However, incubation of hepatocytes with bosentan protected cells from cytokine toxicity in vitro and produced superior cell engraftment and proliferation in vivo.

CONCLUSION

Cell transplantation-induced changes in hepatic microcirculation contributed to transplanted cell clearances from liver. Vascular drugs, such as nitroglycerine, prostacyclin, and bosentan, offer opportunities for improving cell therapy results through superior cell engraftment and liver repopulation. Ongoing clinical use of these drugs will permit rapid translation of the findings in people.

Heterogeneity of the intrahepatic portal venous blood flow: impact on hepatocyte transplantation

BACKGROUND

The poor repopulation rate of the liver by transplanted hepatocytes markedly hampers liver cell therapy, which might be due to a limited sequestration of cells within the hepatic microvasculature. We therefore present intravital fluorescence microscopic data of transplanted hepatocytes immediately after portal venous injection demonstrating their intrahepatic distribution.

METHODS

Male Wistar rats were transplanted with freshly isolated, rhodamine 123 labelled, primary rat hepatocytes. Cells (10(6) in 0.5 ml) were slowly injected via a catheter in the V. lienalis over 2 min. Their distribution in the left lateral liver lobe was visualized simultaneously as well as over the following 30 min by intravital fluorescence microscopy. In a second set of animals green fluorescent microspheres exhibiting a size of 15 μm were injected and observed identically. For further analyses of portal venous blood flow distribution sodium fluorescein was injected via the V. lienalis as well as via the V. jugularis.

RESULTS

In vivo imaging allowed the clear detection and observation of hepatocytes flowing into the liver and forming microemboli, which are trapped particularly in small distal portal branches. To a minor extent they were trapped as solitary cells in the periportal zone of sinusoids. Most interestingly, the distribution of cells within the liver was highly heterogeneous, as wide areas of acini were found free of transplanted cells after portal venous injection, while neighbouring areas showed disproportionately high hepatocyte occurrence. To further investigate this phenomenon sodium fluorescein was injected via the V. lienalis instead and an identical heterogeneous distribution pattern with clear anatomical borders defining highly, semi, and non-portal venous perfused liver acini could be observed. In contrast, systemic injection of sodium fluorescein via the V. jugularis in the same animals resulted in a homogenous dispersion within the liver.

CONCLUSION

Using in vivo fluorescence microscopy and exclusive portal venous injection of a fluorescent dye, we provide evidence for the existence of liver areas, differentially supplied by portal venous blood. As a consequence, hepatocytes transplanted via the portal tract are very heterogeneously distributed within the liver. This observation forces us to reconsider our current knowledge on (i) monitoring engrafted cells, (ii) the optimal hepatocyte number to be transplanted, (iii) portal hypertension after cell injection, and last but not least (iv) the optimal transplantation route. Moreover, the established model for in vivo visualization of transplanted hepatocytes allows development of new therapeutic strategies facilitating an improved engraftment of cells.

Implication of hepatic stem cells in functional liver repopulation

The liver has an enormous potential to restore the parenchymal tissue loss due to injury. This is accomplished by the proliferation of either the hepatocytes or liver progenitor cells in cases where massive damage prohibits hepatocytes from entering the proliferative response. Under debate is still whether hepatic stem cells are involved in liver tissue maintenance and regeneration or even whether they exist at all. The definition of an adult tissue-resident stem cell comprises basic functional stem cell criteria like the potential of self-renewal, multipotent, i.e. at least bipotent differentiation capacity and serial transplantability featuring the ability of functional tissue repopulation. The relationship between a progenitor and its progeny should exemplify the lineage commitment from the putative stem cell to the differentiated cell. This is mainly assessed by lineage tracing and immunohistochemical identification of markers specific to progenitors and their descendants. Flow cytometry approaches revealed that the liver stem cell population in animals is likely to be heterogeneous giving rise to progeny with different molecular signatures, depending on the stimulus to activate the putative stem cell compartment. The stem cell criteria are met by a variety of cells identified in the fetal and adult liver both under normal and injury conditions. It is the purpose of this review to verify hepatic stem cell candidates in the light of the stem cell definition criteria mentioned. Also from this point of view adult stem cells from non-hepatic tissues such as bone marrow, umbilical cord blood or adipose tissue, have the potential to differentiate into cells featuring functional hepatocyte characteristics. This has great impact because it opens the possibility of generating hepatocyte-like cells from adult stem cells in a sufficient amount and quality for their therapeutical application to treat end-stage liver diseases by stem cell-based hepatocytes in place of whole organ transplantation.

Mesenchymal stem cell-derived hepatocytes for functional liver replacement

Mesenchymal stem cells represent an alternate cell source to substitute for primary hepatocytes in hepatocyte transplantation because of their multiple differentiation potential and nearly unlimited availability. They may differentiate into hepatocyte-like cells in vitro and maintain specific hepatocyte functions also after transplantation into the regenerating livers of mice or rats both under injury and non-injury conditions. Depending on the underlying liver disease their mode of action is either to replace the diseased liver tissue or to support liver regeneration through their anti-inflammatory and anti-apoptotic as well as their pro-proliferative action.

Immunological tolerance of human hepatocyte xenograft induced by adenovirus vector-mediated CTLA4Ig gene transfer

BACKGROUND

Systemic administration of CTLA4Ig has been applied in inducing immunological tolerance of hepatocyte implants, but has potential for systemic immune inhibition. This study was designed to induce hepatocyte immunological tolerance by locally expressing CTLA4Ig in an attempt to improve the effectiveness of cell transplantation.

METHODS

A normal human liver cell line (L02) was transfected with adenovirus vector containing the CTLA4Ig gene (Ad-CTLA4Ig-EGFP) in vitro, and the expression of CTLA4Ig by transfected cells was assessed by fluorescent imaging and immunocytochemical staining. Transfected cells then were injected into the spleen of Sprague-Dawley rats, the survival of cells was determined by immunohistochemistry, and the immune status was examined through CD4+ and CD69+ T cell-counts and ELISA detection of IL-2 in peripheral blood.

RESULTS

L02 cells expressed CTLA4Ig in the cytoplasm for >4 weeks. Surviving L02 cells were observed in the experimental group at 3 and 4 weeks post-transplantation, while none was detected in the control group. Furthermore, the percentages of CD4+ and CD4+CD69+ T cells in the CTLA4-transfected group were 24.5% and 45.1%, markedly lower than those in the control group at 4 weeks post-transplantation (P<0.01). Furthermore, the IL-2 level was also lower in the CTLA4-transfected group than in the control group.

CONCLUSION

Adenovirus-mediated CTLA4Ig gene transfer into human hepatocytes has the potential to become an effective method of inducing immunological tolerance in hepatocyte transplantation.

Spontaneous origin from human embryonic stem cells of liver cells displaying conjoint meso-endodermal phenotype with hepatic functions

Understanding the identity of lineage-specific cells arising during manipulations of stem cells is necessary for developing their potential applications. For instance, replacement of crucial functions in organ failure by transplantation of suitable stem-cell-derived cells will be applicable to numerous disorders, but requires insights into the origin, function and fate of specific cell populations. We studied mechanisms by which the identity of differentiated cells arising from stem cells could be verified in the context of natural liver-specific stem cells and whether such differentiated cells could be effective for supporting the liver following cell therapy in a mouse model of drug-induced acute liver failure. By comparing the identity of naturally occurring fetal human liver stem cells, we found that cells arising in cultures of human embryonic stem cells (hESCs) recapitulated an early fetal stage of liver cells, which was characterized by conjoint meso-endoderm properties. Despite this fetal stage, hESC-derived cells could provide liver support with appropriate metabolic and ammonia-fixation functions, as well as cytoprotection, such that mice were rescued from acute liver failure. Therefore, spontaneous or induced differentiation of human embryonic stem cells along the hepatic endoderm will require transition through fetal-like stages. This offers opportunities to prospectively identify whether suitable cells have been generated through manipulation of stem cells for cell therapy and other applications.

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

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

Hepatic targeting and biodistribution of human fetal liver stem/progenitor cells and adult hepatocytes in mice

Tracking stem/progenitor cells through noninvasive imaging is a helpful means of assessing the targeting of transplanted cells to specific organs. We performed in vitro and in vivo studies wherein adult human hepatocytes and human fetal liver stem/progenitor cells were labeled with indium-111 ((111)In)-oxine and technetium-99m ((99m)Tc)-Ultratag or (99m)Tc-Ceretec. The labeling efficiency and viability of cells was analyzed in vitro, and organ biodistribution of cells was analyzed in vivo after transplantation in xenotolerant nonobese diabetic/severe combined immunodeficiency mice through intrasplenic or intraportal routes. We found that adult hepatocytes and fetal liver stem/progenitor cells incorporated (111)In but not (99m)Tc labels. After radiolabeling, cell viability was unchanged. Transplanted adult hepatocytes or fetal liver stem/progenitor cells were targeted to the liver more effectively by the intraportal rather than the intrasplenic route. Transplanted cells were retained in the liver after intraportal injection and in the liver and spleen after intrasplenic injection, without translocations into pulmonary or systemic circulations. Compared with fetal liver stem/progenitor cells, fewer adult hepatocytes were retained in the spleen after intrasplenic transplantation. The distribution of transplanted cells in organs was substantiated by genetic assays, including polymerase chain reaction amplification of DNA sequences from a primate-specific Charcot-Marie-Tooth element, and in situ hybridization for primate alphoid satellite sequences ubiquitous in all centromeres.

CONCLUSION

(111)In labeling of human fetal liver stem/progenitor cells and adult hepatocytes was effective for noninvasive localization of transplanted cells. This should facilitate continued development of cell therapies through further animal and clinical studies.

Hepatocyte transplantation in animal models

More than 30 years after the first hepatocyte transplant to treat the Gunn rat, the animal model for Crigler-Najjar syndrome, there are still a number of impediments to hepatocyte transplantation. Numerous animal models are still used in work aimed at improving hepatocyte engraftment and/or long-term function. Although other cell sources, particularly hepatic and extrahepatic stem cells, are being explored, adult hepatocytes remain the cells of choice for the treatment of liver diseases by cell therapy. In recent years, diverse approaches have been developed in various animal models to enhance hepatocyte transduction and amplification in vitro and cell engraftment and functionality in vivo. They have led to significant progress in hepatocyte transplantation for the treatment of patients with metabolic diseases and for bridging patients with acute injury until their own livers regenerate. This review presents and considers the results of this work with a special emphasis on procedures that might be clinically applicable.

Construction of a biological tissue model based on a single-cell model: a computer simulation of metabolic heterogeneity in the liver lobule

An enormous body of information has been obtained by molecular and cellular biology in the last half century. However, even these powerful approaches are not adequate when it comes to higher-level biological structures, such as tissues, organs, and individual organisms, because of the complexities involved. Thus, accumulation of data at the higher levels supports and broadens the context for that obtained on the molecular and cellular levels. Under such auspices, an attempt to elucidate mesoscopic and macroscopic subjects based on plentiful nanoscopic and microscopic data is of great potential value. On the other hand, fully realistic simulation is impracticable because of the extensive cost entailed and enormous amount of data required. Abstraction and modeling that balance the dual requirements of prediction accuracy and manageable calculation cost are of great importance for systems biology. We have constructed an ammonia metabolism model of the hepatic lobule, a histological component of the liver, based on a single-hepatocyte model that consists of the biochemical kinetics of enzymes and transporters. To bring the calculation cost within reason, the porto-central axis, which is an elemental structure of the lobule, is defined as the systems biological unit of the liver, and is accordingly modeled. A model including both histological structure and position-specific gene expression of major enzymes largely represents the physiological dynamics of the hepatic lobule in nature. In addition, heterogeneous gene expression is suggested to have evolved to optimize the energy efficiency of ammonia detoxification at the macroscopic level, implying that approaches like this may elucidate how properties at the molecular and cellular levels, such as regulated gene expression, modify higher-level phenomena of multicellular tissue, organs, and organisms.

Hepatocyte transplantation and drug-induced perturbations in liver cell compartments

The potential for organ damage after using drugs or chemicals is a critical issue in medicine. To delineate mechanisms of drug-induced hepatic injury, we used transplanted cells as reporters in dipeptidyl peptidase IV-deficient mice. These mice were given phenytoin and rifampicin for 3 days, after which monocrotaline was given followed 1 day later by intrasplenic transplantation of healthy C57BL/6 mouse hepatocytes. We examined endothelial and hepatic damage by serologic or tissue studies and assessed changes in transplanted cell engraftment and liver repopulation by histochemical staining for dipeptidyl peptidase IV. Monocrotaline caused denudation of the hepatic sinusoidal endothelium and increased serum hyaluronic acid levels, along with superior transplanted cell engraftment. Together, phenytoin, rifampicin, and monocrotaline caused further endothelial damage, reflected by greater improvement in cell engraftment. Phenytoin, rifampicin, and monocrotaline produced injury in hepatocytes that was not apparent after conventional tissue studies. This led to transplanted cell proliferation and extensive liver repopulation over several weeks, which was more efficient in males compared with females, including greater induction by phenytoin and rifampicin of cytochrome P450 3A4 isoform that converts monocrotaline to toxic intermediates. Through this and other possible mechanisms, monocrotaline-induced injury in the endothelial compartment was retargeted to simultaneously involve hepatocytes over the long term. Moreover, after this hepatic injury, native liver cells were more susceptible to additional pro-oxidant injury through thyroid hormone, which accelerated the kinetics of liver repopulation.

CONCLUSION

Transplanted reporter cells will be useful for obtaining insights into homeostatic mechanisms involving liver cell compartments, whereas targeted injury in hepatic endothelial and parenchymal cells with suitable drugs will also help advance liver cell therapy.

Zonal expression of hepatocytic marker enzymes during liver repopulation

Hepatocytes are metabolically specialised cells displaying distinctive gene expression patterns within the liver lobule. Here, we investigate whether pre-cultured adult rat hepatocytes adopt periportal and pericentral enzyme expression following their transplantation into the regenerating rat liver. Isolated primary rat hepatocytes, representing a mixture of both periportal and pericentral origin, lost expression of carbamoyl phosphate synthetase I (CPS I) and cytochrome P450 subtype 2B1 (CYP2B1) in culture as shown by immunofluorescence and Western blot analysis. Accordingly, urea synthesis and CYP2B1 enzyme activity decreased. Hepatocytes from DPPIV (CD26) wild type rats were cultured for 4 and 7 days, and then transplanted into the livers of CD26 deficient rats following prior treatment with retrorsine and partial hepatectomy to drive selective donor cell proliferation. CD26 positive donor cells engrafted in the periportal regions and grew in clusters expanding into the parenchyma as time proceeded. Ten weeks after transplantation, cells derived from donors surrounding the portal veins expressed CPS I, but not CYP2B1. The reverse was true for CD26 positive cells in close proximity to the central veins displaying immunoreactivity to CYP2B1, but no longer to CPS I. Hepatocytes lose their specific marker enzyme expression in culture. After transplantation, donor hepatocytes proliferate in the host parenchyma whilst acquiring the position-specific enzyme expression of the surrounding periportal and pericentral host hepatocytes. These results indicate the high degree of plasticity of gene expression in hepatocytes subjected to a change in microenvironment.

In vitro differentiation of mouse ES cells into hepatocytes with coagulation factors VIII and IX expression profiles

Coagulation factors II, V, VII, VIII, IX and X are produced by hepatocytes. So factors VIII and IX deficiencies, which result in hemophilia A and B, have the potential to respond to cellular replacement therapy. Embryonic stem (ES) cells provide a unique source for therapeutic applications. Here, E14 mouse ES cells have been induced into hepatocytes in vitro. Morphology revealed that ES-derived hepatic-like cells were round or polyhedral shaped with distinct boundary of individual cells, and some arranged in trabeculae. These cells expressed endodermal- or liver-specific mRNA--transthyretin (TTR), alpha1-anti-trypsin (AAT), alpha-fetoprotein (AFP), albumin (ALB), glucose-6-phoshpatase (G6P) and tyrosine aminotransferase (TAT). Approximately (85.1 +/- 0.5)% of the ES-derived cells was stained positive green with ICG uptake. These cells were also stained magenta as a result of PAS reaction. In this paper, expression of coagulation factors VIII and IX mRNA in the ES-derived cells is documented. Therefore, ES cells might be developed as substitute donor cells for the therapy of coagulation factor deficiencies.

The expression of mesenchymal, neural and haematopoietic stem cell markers in adult hepatocytes proliferating in vitro

BACKGROUND/AIMS

Cultured adult hepatocytes may be stimulated into clonal expansion. We raise the question whether adult hepatocytes proliferating in vitro recapitulate the early process of hepatic development.

METHODS

A non-enzymatic method was used to isolate hepatocytes free of contamination with non-parenchymal cells. Hepatocytes were stimulated into proliferation in the presence of mitogens and conditioned media from non-parenchymal cell and hepatocyte culture supernatants. Immunofluorescence methods and PCR analysis were used to demonstrate immunophenotypical characteristics and gene expression profiles similar to those of progenitor cells.

RESULTS

Rapid growth occurred during the first 7 days of culture. Cells continued to express hepatic markers (phosphoenolpyruvate carboxykinase, cytokeratin 18, transferrin and dipeptidylpeptidase IV), but the gap junction protein connexin 32 was down-regulated. In the early stage of proliferation, cells started to express biliary and extrahepatic progenitor markers (cytokeratin 19, CD49b, CD49f, nestin, vimentin, Thy1 and c-kit), followed by cytokeratin 7, connexin 43, and neural cell adhesion molecule. Co-expression of the epithelial liver progenitor marker alpha-foetoprotein with either nestin (neural marker) or Thy1 (mesenchymal marker) was also demonstrated.

CONCLUSIONS

Mature hepatocytes reveal their potential to regain a spectrum of progenitor markers from different germ layers, suggesting enormous plasticity and differentiation potential of adult liver cells.

Analysis of the functional integrity of cryopreserved human liver cells including xenografting in immunodeficient mice to address suitability for clinical applications

BACKGROUND

The availability of well-characterized human liver cell populations that can be frozen and thawed will be critical for cell therapy. We addressed whether human hepatocytes can recover after cryopreservation and engraft in immunodeficient mice.

METHODS

We isolated cells from discarded human livers and studied the properties of cryopreserved cells. The viability of thawed cells was established with multiple in vitro assays, including analysis of liver gene expression, ureagenesis, cytochrome P450 activity, and growth factor-induced cell proliferation. The fate of transplanted cells was analysed in immunodeficient NOD-SCID mice.

RESULTS

After thawing, the viability of human hepatocytes exceeded 60%. Cells attached to culture dishes, proliferated following growth factor stimulation and exhibited liver-specific functions. After transplantation in NOD-SCID mice, cells engrafted in the peritoneal cavity, a heterologous site, as well as the liver itself, retained hepatic function and proliferated in response to liver injury. Transplanted hepatocytes were integrated in the liver parenchyma. Occasionally, transplanted cells were integrated in bile ducts.

CONCLUSIONS

Cryopreserved human liver cell showed the ability to retain functional integrity and to reconstitute both hepatic and biliary lineages in mice. These studies offer suitable paradigms aimed at characterizing liver cells prior to transplantation in people.

The engineering of tissues using progenitor cells

The "engineering" of a tissue implies that it can be constructed by assembling the necessary components. However, tissues are formed through an evolving, interactive process, not through a collection of parts. This chapter focuses on the biology of the progenitor cell, the native precursor to new tissue, and its role in neogenesis, or the de novo generation of functional tissue. We present a working hypothesis for the generation of parenchymal cell populations and use this hypothesis as a basis for analysis of three parenchymal populations, epidermal cells, hepatocytes of the liver, and pancreatic islets, with a view toward what impact this information will have on the development of cell therapies. By comparing developmental processes, response to injury and disease, and behavior in vitro, we conclude that the adult progenitor cell retains the potential for substantial growth and organ neogenesis and that its biological properties make it the cell of first choice for the engineering of tissues.

A population of c-Kit(low)(CD45/TER119)- hepatic cell progenitors of 11-day postcoitus mouse embryo liver reconstitutes cell-depleted liver organoids

Embryo liver morphogenesis takes place after gastrulation and starts with a ventral foregut evagination that reacts to factor signaling from both cardiac mesoderm and septum transversum mesenchyme. Current knowledge of the progenitor stem cell populations involved in this early embryo liver development is scarce. We describe here a population of 11-day postcoitus c-Kit(low)(CD45/TER119)- liver progenitors that selectively expressed hepatospecific genes and proteins in vivo, was self-maintained in vitro by long-term proliferation, and simultaneously differentiated into functional hepatocytes and bile duct cells. Purified c-Kit(low)(CD45/TER119)- liver cells cocultured with cell-depleted fetal liver fragments engrafted and repopulated the hepatic cell compartments of the latter organoids, suggesting that they may include the embryonic stem cells responsible for liver development.

Therapeutic potential of hepatocyte transplantation

Liver repopulation with transplanted cells offers unique opportunities for treating a variety of diseases and for studies of fundamental mechanisms in cell biology. Our understanding of the basis of liver repopulation has come from studies of transplanted cells in animal models. A variety of studies established that transplanted hepatocytes as well as stem/progenitor cells survive, engraft, and function in the liver. Transplanted cells survive life-long, although cells do not proliferate in the normal liver. On the other hand, the liver is repopulated extensively when diseases or other injuries afflict native hepatocytes but spare transplanted cells. The identification of ways to repopulate the liver with transplanted cells has greatly reinvigorated the field of liver cell therapy. The confluence of insights in stem/progenitor cells, transplantation immunology, cryobiology, and liver repopulation in specific models of human diseases indicates that the field of liver cell therapy will begin to reap the promised fruit in the near future.

Hepatocyte transplantation

Repopulation of the liver with transplanted cells holds significant promise for developing novel therapies. The liver is a most suitable target for treating a variety of genetic, metabolic and acquired diseases. Liver disease, such as chronic viral hepatitis, constitutes an enormous burden worldwide. Advancing liver cell therapy requires insights into mechanisms of cell engraftment and proliferation, as well as unique requirements of specific diseases for correction by cell transplantation. This review highlights recent developments in the area of hepatocyte transplantation. Aspects concerning modulation of cell engraftment, regulation of gene expression and proliferation of transplanted cells are discussed. Other issues concern the current status of clinical applications of hepatocyte transplantation, as well as novel sources of cells that could benefit cell therapy in the future. The general conclusion is that cell therapy has become more practical in recent years and insights into how the normal liver and the diseased liver can be repopulated will offer effective ways to treat many disorders in the near future.

Kupffer cells participate in early clearance of syngeneic hepatocytes transplanted in the rat liver

BACKGROUND & AIMS

Kupffer cells are activated shortly after deposition of hepatocytes in liver sinusoids, with clearance of a significant fraction of transplanted cells, especially when cells are entrapped in portal spaces. We determined whether perturbation of Kupffer cells would improve transplanted cell engraftment.

METHODS

Dipeptidyl peptidase IV-deficient rats were used as recipients of syngeneic Fischer 344 rat hepatocytes. Kupffer cell function was analyzed by measuring phagocytic activity with carbon particle or (99m)Tc-sulfur colloid incorporation. Transplanted cell survival and integration in the liver parenchyma was determined by histochemical analysis of tissues. Transplanted cell proliferation was analyzed in rats conditioned with retrorsine and partial hepatectomy.

RESULTS

Gadolinium chloride significantly impaired Kupffer cell function, especially in periportal areas, where transplanted cells were localized. Transplanted cell survival increased by approximately 2-fold in animals treated with gadolinium chloride 24 hours before cell transplantation. In gadolinium-treated rats, more transplanted cells were observed in portal vein radicles, as well as in liver sinusoids, albeit integration of cells in the liver parenchyma was slower in gadolinium-treated rats and cells separated from other hepatocytes in portal vein radicles that failed to exhibit bile canalicular reconstitution. Finally, hepatocyte transplantation in rats primed with retrorsine and partial hepatectomy showed accelerated kinetics of liver repopulation in animals pretreated with gadolinium chloride.

CONCLUSIONS

Perturbation of Kupffer cell activity will benefit liver repopulation with cells and further analysis of clinically suitable approaches to exploit this mechanism will be appropriate.

Cell transplantation after oxidative hepatic preconditioning with radiation and ischemia-reperfusion leads to extensive liver repopulation

The inability of transplanted cells to proliferate in the normal liver hampers cell therapy. We considered that oxidative hepatic DNA damage would impair the survival of native cells and promote proliferation in transplanted cells. Dipeptidyl peptidase-deficient F344 rats were preconditioned with whole liver radiation and warm ischemia-reperfusion followed by intrasplenic transplantation of syngeneic F344 rat hepatocytes. The preconditioning was well tolerated, although serum aminotransferase levels rose transiently and hepatic injury was observed histologically, along with decreased catalase activity and 8-hydroxy adducts of guanine, indicating oxidative DNA damage. Transplanted cells did not proliferate in the liver over 3 months in control animals and animals preconditioned with ischemia-reperfusion alone. Animals treated with radiation alone showed some transplanted cell proliferation. In contrast, the liver of animals preconditioned with radiation plus ischemia-reperfusion was replaced virtually completely over 3 months. Transplanted cells integrated in the liver parenchyma and liver architecture were preserved normally. These findings offer a paradigm for repopulating the liver with transplanted cells. Progressive loss of cells experiencing oxidative DNA damage after radiation and ischemia-reperfusion injury could be of significance for epithelial renewal in additional organs.

Isolation of human progenitor liver epithelial cells with extensive replication capacity and differentiation into mature hepatocytes

The liver can regenerate itself through the progenitor cells it harbors. Here we demonstrate isolation of epithelial progenitor/stem cells from the fetal human liver, which contains a large number of hepatoblasts. Progenitor liver cells displayed clonogenic capacity, expressed genes observed in hepatocytes, bile duct cells and oval cells, and incorporated genes transferred by adenoviral or lentiviral vectors. Under culture conditions,progenitor cells proliferated for several months, with each cell undergoing more than forty divisions, but they retained normal karyotypes. Progenitor cells differentiated into mature hepatocytes in mice with severe combined immunodeficiency, both when in an ectopic location and when in the liver itself. Cells integrated in the liver parenchyma and proliferated following liver injury. An abundance of progenitor cells in the fetal human liver is consistent with models indicating depletion of progenitor/stem cells during aging and maturation of organs. The studies indicate that isolation of progenitor cells from fetal organs will be appropriate for establishing novel systems to investigate basic mechanisms and for cell and gene therapy.

Cyclophosphamide disrupts hepatic sinusoidal endothelium and improves transplanted cell engraftment in rat liver

To determine whether disruption of the hepatic sinusoidal endothelium will facilitate engraftment of transplanted cells, we treated Fischer 344 (F344) rats lacking dipeptidyl peptidase IV (DPPIV) activity with cyclophosphamide (CP). Electron microscopy showed endothelial injury within 6 hours following CP, and, after 24 and 48 hours, the endothelium was disrupted in most hepatic sinusoids. CP did not affect Kupffer cell function. Similarly, CP had no obvious effects on hepatocytes. Intrasplenic transplantation of F344 rat hepatocytes followed by their localization with DPPIV histochemistry showed 3- to 5-fold increases in the number of transplanted cells in CP-treated animals. Transplanted cells integrated in the liver parenchyma more rapidly in CP-treated animals, and hybrid bile canaliculi developed even 1 day after cell transplantation, which was not observed in control animals. To demonstrate whether improved cell engraftment translated into superior liver repopulation, recipient animals were conditioned with retrorsine and two-thirds partial hepatectomy (PH), which induces transplanted cell proliferation. CP treatment of these animals before cell transplantation significantly increased the number and size of transplanted cell foci. In conclusion, disruption of the hepatic sinusoidal endothelium was associated with accelerated entry and integration of transplanted cells in the liver parenchyma. These results provide insights into hepatocyte engraftment in the liver and will help in optimizing liver-directed cell therapy.

Hepatic sinusoidal vasodilators improve transplanted cell engraftment and ameliorate microcirculatory perturbations in the liver

After transplantation, hepatocytes entering liver sinusoids are engrafted, whereas cells entrapped in portal spaces are cleared. We studied whether hepatic sinusoidal dilatation will increase the entry of transplanted cells in the liver lobule, improve cell engraftment, and decrease microcirculatory perturbations. F344 rat hepatocytes were transplanted intrasplenically into syngeneic dipeptidyl peptidase IV (DPPIV)-deficient rats. Animals were treated with adrenergic receptor blockers (phentolamine, labetalol), a calcium channel blocker (nifedipine), and splanchnic vasodilators (nitroglycerine, calcitonin gene-related peptide [CGRP], glucagon). Transplanted cells were localized by histochemistry. The hepatic microcirculation was studied with in vivo videomicroscopy. Changes in cell translocations were analyzed by injection of (99m)Tc-labeled hepatocytes. Pretreatment with phentolamine and nitroglycerine increased transplanted cell entry in liver sinusoids, whereas labetalol, nifedipine, CGRP, and glucagon were ineffective. Increased deposition of transplanted cells in sinusoids resulted in greater cell engraftment. In vivo microscopy showed disruption of sinusoidal blood flow immediately after cell transplantation with circulatory restoration requiring more than 12 to 24 hours after cell transplantation. However, in nitroglycerine-treated animals, sinusoidal blood flow was perturbed less. Nitroglycerine did not meaningfully increase intrapulmonary cell translocations. In conclusion, these findings indicate that hepatic sinusoidal capacitance is regulated by alpha-adrenergic- and nitroglycerine-responsive elements. Sinusoidal vasodilatation benefited intrahepatic distribution of transplanted cells and restored hepatic microcirculation after cell transplantation. This shall facilitate optimization of clinical cell transplantation and offers novel ways to investigate vascular mechanisms regulating hepatic sinusoidal reactivity.

Early cell transplantation in LEC rats modeling Wilson's disease eliminates hepatic copper with reversal of liver disease

BACKGROUND & AIMS

The Long-Evans Cinnamon (LEC) rat is an excellent model of Wilson's disease with impaired copper excretion, hypoceruloplasminemia, and copper toxicosis. We hypothesized that early hepatocyte transplantation would improve copper excretion and liver disease in Wilson's disease.

METHODS

Normal syngeneic Long-Evans Agouti rat hepatocytes were transplanted intrasplenically into 2-week-old LEC rats. Untreated LEC pups were controls. Liver repopulation was shown by changes in serum ceruloplasmin, hepatic atp7b messenger RNA, and bile and liver copper levels. Histologic analysis of the liver was performed.

RESULTS

Significant copper accumulation and liver disease were observed in 5-month-old LEC rats, with occasional treated rats showing increased bile copper excretion. The liver was repopulated extensively in 10 of 14 treated LEC rats (71%) 20 months after cell transplantation. In these 10 rats, hepatic copper content was virtually normal in 6 rats (53 +/- 12 microg/g liver) and substantially less in 4 others (270 +/- 35 microg/g) compared with elevated liver copper levels in untreated LEC rats (1090 +/- 253 microg/g) (P < 0.001). Changes in serum ceruloplasmin levels, bile copper excretion capacity, and liver histology were in concordance with decreases in liver copper levels.

CONCLUSIONS

Transplanted cells proliferated subsequent to the onset of liver injury, and the liver was repopulated over an extended period. Cell transplantation eventually restored copper homeostasis and reversed liver disease without hepatic preconditioning in LEC rats.

The effect of CD28/B7 blockade on alloreactive T and B cells after liver cell transplantation

BACKGROUND

Hepatocyte cell lines are beginning to be developed as universal donors for isolated liver cell transplantation, which is a less invasive method than orthotopic liver transplantation for treatment of metabolic liver disease. The immune response to isolated liver cell transplantation and its modification by costimulatory blockade are as yet not well delineated.

METHODS

Adenovirus expressing CTLA4Ig was used to study blockade of the costimulatory CD28/B7 pathway in murine models of hepatocyte transplantation, and the effects on alloreactive T and B cells were studied.

RESULTS

CTLA4Ig delayed rejection of subcutaneously administered C57L-derived murine hepatoma cells in CBA/J recipients for >50 days. Activation and cytokine secretion by allospecific CD4+ and CD8+ T cells were initially blocked by CTLA4Ig; delayed rejection was associated with tumor infiltration by CD8+ T cells that did not secrete interferon-gamma. CTLA4Ig failed to block transplant rejection in primed mice, indicating that memory effector T cells were resistant to its action. In contrast, CTLA4Ig suppressed both naive and memory alloreactive B cells. High levels of CTLA4Ig mediated acceptance of hepatoma cells delivered directly into the spleen. However, isolated primary C57BL/6 mouse hepatocytes delivered into the spleen were rejected with only moderately delayed kinetics.

CONCLUSIONS

Transplant antigenicity, transplant site, and CTLA4Ig dose all affected the survival of transplanted liver cells. CD8+ T cells are significant mediators of hepatocyte transplant rejection and are relatively resistant to costimulatory blockade with CTLA4Ig. Strategies to specifically antagonize CD8+ T cells or to modulate MHC class I expression in association with costimulatory blockade by CTLA4Ig may enhance the clinical feasibility of transplanting allogeneic hepatocytes.

Correction of liver disease following transplantation of normal rat hepatocytes into Long-Evans Cinnamon rats modeling Wilson's disease

To establish the efficacy of cell therapy in Wilson's disease, we used the Long-Evans Cinnamon (LEC) rat model with atp7b gene mutation and copper toxicosis. Several groups of LEC rats were established, including animals pretreated with retrorsine to exacerbate copper toxicosis and inhibit proliferation in native hepatocytes followed by partial hepatectomy to promote liver repopulation. Hepatocytes from normal, syngeneic LEA rats were transplanted intrasplenically. Animal survival, biliary copper excretion, and hepatic copper were determined. The magnitude of liver repopulation was demonstrated by measuring serum ceruloplasmin and hepatic atp7b mRNA. Long-term survival in LEC rats treated with retrorsine, partial hepatectomy, and cell transplantation was up to 90%, whereas fewer than 10% of animals pretreated with retrorsine, without cell therapy, survived, P < 0.001. Liver repopulation occurred gradually after cell transplantation, ranging from <25% at 6 weeks, 26 to 40% at 4 months, and 74 to 100% at 6 months or beyond. Liver repopulation restored biliary copper excretion capacity and lowered liver copper levels. Remarkably, liver histology was completely normal in LEC rats with extensive liver repopulation, compared with widespread megalocytosis, apoptosis, oval cell proliferation, and cholangiofibrosis in untreated animals. These data indicate that liver repopulation with functionally intact cells can reverse pathophysiological perturbations and cure Wilson's disease.

Transplanted reporter cells help in defining onset of hepatocyte proliferation during the life of F344 rats

Transplanted hepatocytes integrate in the liver parenchyma and exhibit gene expression patterns that are similar to adjacent host hepatocytes. To determine the fate of genetically marked hepatocytes in the context of hepatocellular proliferation throughout the rodent life span, we transplanted Fischer 344 (F344) rat hepatocytes into syngeneic dipeptidyl peptidase IV-deficient rats. The proliferative activity in transplanted hepatocytes was studied in animals ranging in age from a few days to 2 yr. Transplanted hepatocytes proliferated during liver development between 1 and 6 wk of age, each dividing an estimated two to five times. DNA synthesis in occasional cells was demonstrated by localizing bromodeoxyuridine incorporation. There was no evidence for transplanted cell proliferation between 6 wk and 1 yr of age. Subsequently, transplanted cells proliferated again, with increased sizes of transplanted cell clusters at 18 and 24 mo of age. The proliferative activity of transplanted cells was greater in rats entering senescence compared with during postnatal liver development. In old rats, some liver lobules were composed entirely of transplanted cells. We conclude that hepatocyte proliferation in the livers of very young and old F344 rats is regulated in a temporally determined, biphasic manner. The findings will be relevant to mechanisms concerning liver development, senescence, and oncogenesis, as well as to cell and gene therapy.

Effect of phenobarbital on intralobular expression of CYP2B1/2 in livers of rats: difference in the expression between single and repetitive administrations

Phenobarbital (PB) was shown to induce the major PB-inducible cytochrome P450 (CYP) isoforms, CYP2B1/2, in perivenular hepatocytes by a single injection, and in midzonal and periportal hepatocytes in addition to perivenular hepatocytes by injections of the same dosage once a day for 3 days in rat livers. The present study was undertaken to determine whether the spread of enzyme induction to midzonal and periportal hepatocytes is caused by the increase in total dose of the drug by repetitive injections or by the repetitive injections of the drug themselves. Male adult rats were administered PB by a single injection (80 mg/kg) or repetitive injections (20 mg/kg once a day for 4 days; a total dose of 80 mg/kg), and the molar content of CYP2B1/2 was measured by quantitative immunohistochemistry in the cytoplasm of perivenular, midzonal, and periportal hepatocytes. In addition, the molar content of total CYP in the cytoplasm was measured by microphotometry, and the expression of CYP2B2 mRNA was examined by in situ hybridization. When animals received the single injection, the isoforms and CYP2B2 mRNA increased markedly in perivenular hepatocytes, increased somewhat in midzonal hepatocytes, and remained unchanged in periportal hepatocytes. If animals received the repetitive injections, however, although the isoforms and the mRNA increased markedly in perivenular hepatocytes, they also increased markedly in midzonal hepatocytes and somewhat in periportal hepatocytes. These findings demonstrated that the enlargement of the sublobular area in which induction of the isoforms occurred was caused by the repetitive injections of PB themselves.

Transplanted hepatocytes engraft, survive, and proliferate in the liver of rats with carbon tetrachloride-induced cirrhosis

Repopulation of the cirrhotic liver with disease-resistant hepatocytes could offer novel therapies, as well as systems for biological studies. Establishing whether transplanted hepatocytes can engraft, survive, and proliferate in the cirrhotic liver is a critical demonstration. Dipeptidyl peptidase IV-deficient F344 rats were used to localize transplanted hepatocytes isolated from the liver of syngeneic normal F344 rats. Cirrhosis was induced by administration of carbon tetrachloride with phenobarbitone and these drugs were withdrawn prior to cell transplantation. Cirrhotic rats showed characteristic hepatic histology, as well as significant portosystemic shunting. When hepatocytes were transplanted via the spleen, cells were distributed immediately in periportal areas, fibrous septa, and regenerative nodules of the cirrhotic liver. Although some transplanted cells translocated into pulmonary capillaries, this was not deleterious. At 1 week, transplanted cells were fully integrated in the liver parenchyma, along with expression of glucose-6-phosphatase and glycogen as reporters of hepatic function. Transplanted cells proliferated in the liver of cirrhotic animals and survived indefinitely. At 1 year, transplanted hepatocytes formed large clusters containing several-fold more cells than normal control animals, which was in agreement with increased cell turnover in the cirrhotic rat liver. The findings indicate that the cirrhotic liver can be repopulated with functionally intact hepatocytes that are capable of proliferating. Liver repopulation using disease-resistant hepatocytes will be applicable in chronic conditions, such as viral hepatitis or Wilson's disease.

Comprehensive gene expression profile of a normal human liver

To investigate the gene expression profile of a normal human liver, we performed serial analysis of gene expression (SAGE), which allows the quantitative and simultaneous analysis of thousands of genes expressed in tissue. Polyadenylated RNA was obtained from a bulk normal human liver sample and SAGE was performed. Reverse transcriptase-polymerase chain reaction (RT-PCR) was also performed in each of 3 different normal liver samples to evaluate the validity of the profile in each individual. A total of 30,982 tags were sequenced, 8,596 of which were unique. The genes highly expressed in the normal liver were those encoding plasma proteins (>21.8% of total transcripts), cytoplasmic proteins (>8.6%), enzymes (>4.8%), protease inhibitors (>1.7%), complements (>1.1%), and coagulation factors (>0.75%). About 13.9% of all transcripts encoded genes not reported in GenBank thus far. This study identifies candidate genes to be examined in relation to various human liver diseases, including viral hepatitis, liver cirrhosis, and hepatocellular carcinoma.

VI. Liver repopulation systems and study of pathophysiological mechanisms in animals

The ability to localize transplanted hepatocytes in the liver offers exciting new opportunities. Transplanted hepatocytes enter liver plates, form hybrid plasma membrane structures with adjacent hepatocytes, express liver genes correctly, and survive indefinitely. The transplanted cell mass is regulated, such that cell proliferation is limited in the normal adult liver, whereas the liver is repopulated extensively when proliferation rates in transplanted and host hepatocytes become dissociated or host hepatocytes are ablated selectively. Transplanted hepatocytes are susceptible to hepatitis viruses. These aspects of transplanted hepatocyte biology indicate that liver repopulation systems can help address questions concerning pathophysiological mechanisms.

Liver repopulation with hepatocyte transplantation: new avenues for gene and cell therapy

Liver-directed gene therapy is appropriate for many conditions. Recent work established that liver repopulation with transplanted cells can be effective in treating genetic disorders. Although hepatocytes express therapeutic genes with considerable efficiency, correction of genetic disorders is constrained by limitations in permanent gene transfer into hepatocytes and repopulation of the liver with transplanted cells. Adenoviral vectors are highly efficient for hepatic gene transfer but the onset of deleterious host immune responses against adenoviral vectors, along with clearance of transduced hepatocytes have caused problems. Nonetheless, recent work concerning engraftment and proliferation of transplanted hepatocytes in the liver has provided significant new information, which should refocus interest in hepatocyte-based therapies. Moreover, hepatocyte transplantation systems offer creative tools for defining critical mechanisms in gene regulation and survival of transduced cells.

Rapid clearance of syngeneic transplanted hepatocytes following transduction with E-1-deleted adenovirus indicates early host immune responses and offers novel ways for studying viral vector, target cell and host interactions

To distinguish between transduced cell clearance and transgene regulation following adenoviral gene transfer, we infected F344 rat hepatocytes with an E-1-deleted adenovirus (Ad beta gal) and studied cell survival in the liver of dipeptidyl peptidase IV-deficient (DPPIV-) F344 rats. Transplanted cells were localized with histochemical staining for DPPIV and transgene expression localized with staining for beta-galactosidase (lacZ). The transgene was expressed in 90-100% hepatocytes without impairment in cell viability in vitro, although transplanted cells were cleared mostly within 1 day by infiltrates containing activated macrophages, CD4+ or CD8+ lymphocytes, and phagocytes. When Ad beta gal-transduced hepatocytes were transplanted repeatedly at 14-day intervals, transplanted cells were cleared rapidly each time. LacZ expression following Ad beta gal administration to intact animals was associated with apoptosis and unscheduled DNA synthesis in the liver. To determine whether adenoviral antigen expression activated consequential MHC-restricted liver injury, we transplanted Ad beta gal-hepatocytes followed subsequently by transplantation of nontransduced hepatocytes. Transplanted cells expressing Ad beta gal were rapidly cleared as before, whereas nontransduced hepatocytes engrafted with progressive liver repopulation. The findings indicated that adenovirally transduced cells are cleared early in the host liver. Use of ex vivo strategies will facilitate analysis of modified adenoviral vectors in the context of immunoregulatory, cellular and viral mechanisms.

Partial hepatectomy-induced polyploidy attenuates hepatocyte replication and activates cell aging events

In understanding mechanisms of liver repopulation with transplanted hepatocytes, we studied the consequences of hepatic polyploidization in the two-thirds partial hepatectomy model of liver regeneration. Liver repopulation studies using genetically marked rodent hepatocytes showed that the number of previously transplanted hepatocytes did not increase in the liver with subsequential partial hepatectomy. In contrast, recipients undergoing partial hepatectomy before cells were transplanted showed proliferation in transplanted hepatocytes, with kinetics of DNA synthesis differing in transplanted and host hepatocytes. Also, partial hepatectomy caused multiple changes in the rat liver, including accumulation of polyploid hepatocytes along with prolonged depletion of diploid hepatocytes, as well as increased senescence-associated beta-galactosidase and p21 expression. Remnant hepatocytes in the partially hepatectomized liver showed increased autofluorescence and cytoplasmic complexity on flow cytometry, which are associated with lipofuscin accumulation during cell aging, and underwent apoptosis more frequently. Moreover, hepatocytes from the partially hepatectomized liver showed attenuated proliferative capacity in cell culture. These findings were compatible with decreased proliferative potential of hepatocytes experiencing partial hepatectomy compared with hepatocytes from the unperturbed liver. Attenuation of proliferative capacity and other changes in hepatocytes experiencing partial hepatectomy offer novel perspectives concerning liver regeneration in the context of cell ploidy.