Construction of a non-tumorigenic rat hepatocyte cell line for transplantation: reversal of hepatocyte immortalization by site-specific excision of the SV40 T antigen

Applications of bottom-up human artificial chromosomes in cell research and cell engineering

Chromosome manipulation is a useful technique in biological science. We have constructed human artificial chromosomes (HACs) based on the transfection of centromeric alphoid DNA precursors into cultured human cells. Moreover, HAC-based technology has been developed into a novel gene expression vector tool for introducing large-size genomic DNA. This technique provides natural expression, as well as stable expression without the gene silencing that often occurs with conventional vectors in mammalian cells. Here we review the properties of HACs, and issues regarding the use of HAC technology for basic and applied research.

Availability of a Magnetic Method for Hepatocyte Transplantation

INTRODUCTION

Hepatocyte transplantation is a promising alternate for the treatment of hepatic diseases. Hypothermic preservation of isolated human hepatocytes is potentially a simple and convenient strategy to provide on-demand hepatocytes in the quantity sufficient and the quality required for biotherapy. Isolated fresh hepatocytes include damaged cells that are also early apoptotic cells, which is not ideal for hepatocyte transplantation. However, this does not reflect cell viability, although it is considered that it adversely affects cell survival after transplantation. We aimed to harvest these hepatocytes and filter the apoptotic cells using a magnetic method to provide a transplantation source.

MATERIALS AND METHODS

Rat hepatocytes were isolated from caudate lobes using manual enzymatic perfusion. The hepatocyte yield was 5.3 ± 0.66 × 10⁹ cells/g of liver tissue, with a viability of 82.3 ± 3.5%. Two samples of hepatocytes were freshly isolated, one using the magnetic method, and the other without. The magnetic method was performed using DynaMag-15 Magnet, and Annexin V Antibody was used on the early apoptotic cells. We evaluated the viability and plate efficiency of the cells after 24 hours at 37°C. Hepatocytes were isolated using cell separation method, and 30 × 10⁶ cells were mixed with 1.0 mL of Dulbecco's Modified Eagle's Medium (DMEM) and directly injected into the spleen of Lewis rats (150-250 g) using 24-gauge needles. Blood samples were collected on days 0, 3, 7, and 14, and the blood albumin level was measured using enzyme-linked immunosorbent assay (ELISA):G1, control (medium injection); G2, fresh hepatocyte transplant using the magnetic method; and G3, fresh hepatocyte transplant without the magnetic method.

RESULTS

The viability was 84.9 ± 2% for fresh hepatocytes and 80.7 ± 1.2% for hepatocytes isolated using the magnetic method. The magnetic method does not damage the cells (73.5 ± 2% vs 35.2 ± 2% after 24 hours), preserving hepatocyte. The albumin level accepted significantly increased in the magnet-treated group compared with the nonmagnet group. Simultaneously, the spleen in which these hepatocytes were transplanted could be used to observe the hepatocytes; the cells were transplanted 14 days later, and the magnet-treated group had significantly higher levels of hepatocytes than the nonmagnet group.

CONCLUSION

We developed an effective technique for hepatocyte isolation for short-term preservation. As a result, we believe that transplantation not only improves the cell transplantation effect but also allows the cells to be stored efficiently using the magnetic method. These results demonstrate the usefulness of hepatocyte hypothermic preservation for cell transplantation.

Assessment of in Vitro Applicability of Reversibly Immortalized NKNT-3 Cells and Clonal Derivatives

In vitro applications of human hepatocytes, such as bioartificial livers and toxicity assays, require thoroughly testing of human cell lines prior to using them as alternative cell sources. The reversibly immortalized NKNT-3 cell line was reported to show clear in vivo functionality. Here, NKNT-3 cells were tested for their in vitro applicability. Low-passage (P2) and high-passage (P28) NKNT-3 cells and clonal derivatives were characterized for reversion of immortalization, heterogeneity, and hepatic functionality. Reversion with reduced expression of immortalizing agent could be established. However, during culturing the cells lost the capacity to be selected for completed reversion. The phenotypic instability is probably associated with heterogeneity in the culture, as clonal derivatives of P2 cells varied in morphology, growth, and reversion characteristics. The mRNA levels of genes related with hepatic differentiation increased 4–20-fold after reversion. However, the levels never exceeded 0.1% of that detected in liver and no urea production nor ammonia elimination was detected. Additionally, activities of different cytochrome P450s were limited. In conclusion, the NKNT-3 culture is heterogeneous and unstable and the in vitro functionality is relatively low. These findings emphasize that in vivo testing of hepatic cell lines is little informative for predicting their value for in vitro applications.

Intrasplenic Hepatocyte Transplantation Prolonged the Survival in Nagase Analbuminemic Rats with Liver Failure Induced by Common Bile Duct Ligation

It has already been established that hepatocyte transplantation (HTx) in animal models, such as both chemically and surgically induced acute liver failure, liver-based metabolic disease, and cirrhosis, resulted in significant improvement of liver function and survival. However, the efficacy of hepatocyte transplantation in secondary cholestatic liver disease is not well known. In this study, we transplanted hepatocytes into the spleen of Nagase analbuminemic rats (NARs) with common bile duct ligation (CBDL) to evaluate the function of transplanted hepatocytes by both of serum albumin levels and total bilirubin levels. CBDL was carried out on NARs to induce liver failure. Lewis rat hepatocytes were transplanted in NARs 7 days after CBDL. Animals, in groups of four, underwent the following interventions: group 1—intrasplenic transplantation of 30 × 106 primary Lewis rat hepatocytes in NARs with CBDL (n = 4), group 2—intrasplenic injection of 0.5 ml DMEM in NARs with CBDL (n = 4); group 3—CBDL only (n = 4); group 4—intrasplenic transplantation of 30 × 106 primary Lewis rat hepatocytes in NARs (n = 4). Both bilirubin levels and albumin levels in NARs with CBDL were significantly improved post-HTx. Animals receiving hepatocyte transplantation survived longer than animals in nontransplant control groups. This study indicates that hepatocytes can be transplanted to temporarily provide life-supporting liver-specific metabolic function and prolong the survival in recipient rats with liver failure induced by CBDL.

Chemokine-mediated robust augmentation of liver engraftment: a novel approach

Effective repopulation of the liver is essential for successful clinical hepatocyte transplantation. The objective was to improve repopulation of the liver with human hepatocytes using chemokines. We used flow cytometry and immunohistochemistry assays to identify commonly expressed chemokine receptors on human fetal and adult hepatocytes. The migratory capacity of the cells to various chemokines was tested. For in vivo studies, we used a nude mouse model of partial hepatectomy followed by intraparenchymal injections of chemokine ligands at various concentrations. Human fetal liver cells transformed with human telomerase reverse transcriptase were used for intrasplenic cell transplantation. Repopulation and functionality were assessed 4 weeks after transplantation. The receptor CXCR3 was commonly expressed on both fetal and adult hepatocytes. Both cell types migrated efficiently toward corresponding CXC chemokine ligands 9, 10, and 11. In vivo, animals injected with recombinant chemokines showed the highest cell engraftment compared with controls (p<.05). The engrafted cells expressed several human hepatic markers such as cytokeratin 8 and 18 and albumin as well as transferrin, UGT1A1, hepatocyte nuclear factor (1α, 1β, and 4α), cytochrome CYP3A1, CCAAT/enhancer binding protein (α and β), and human albumin compared with controls. No inflammatory cells were detected in the livers at 4 weeks after transplantation. The improved repopulation of transplanted cells is likely a function of the chemokines to mediate cell homing and retention in the injured liver and might be an attractive strategy to augment repopulation of transplanted hepatocytes in vivo.

Strategies for immortalization of primary hepatocytes

The liver has the unique capacity to regenerate in response to a damaging event. Liver regeneration is hereby largely driven by hepatocyte proliferation, which in turn relies on cell cycling. The hepatocyte cell cycle is a complex process that is tightly regulated by several well-established mechanisms. In vitro, isolated hepatocytes do not longer retain this proliferative capacity. However, in vitro cell growth can be boosted by immortalization of hepatocytes. Well-defined immortalization genes can be artificially overexpressed in hepatocytes or the cells can be conditionally immortalized leading to controlled cell proliferation. This paper discusses the current immortalization techniques and provides a state-of-the-art overview of the actually available immortalized hepatocyte-derived cell lines and their applications.

Effects of edaravone, a radical scavenger, on hepatocyte transplantation

BACKGROUND

Hepatocyte transplantation (HTx) has yielded significant improvements in liver function and survival in experimentally induced acute liver failure and liver-based metabolic disease. However, transplantation is inefficient, and it is thought that transplanted hepatocytes have a shortened lifespan because of inflammation involving excess nitric oxide (NO). The present study aimed to clarify whether edaravone, a free radical scavenger used to treat ischemic stroke, could reduce ischemic changes in hepatocyte-transplanted livers.

METHODS

Edaravone (3 mg/kg) was administered intravenously 24 h before HTx to Nagase analbuminemic rats (NARs). Hepatocytes were isolated, and 30 × 10(6) cells were injected in a 1.0-ml volume directly into the spleens of NARs. All experimental groups studied received FK506 to control rejection. Animals in Group A received medium-only; Group B received HTx only; and Group C received HTx and edaravone. Forty-eight hours after transplantation, the hepatocytes from animals were isolated and analyzed for staining with propidium iodide- and annexin-V using flow cytometry. Liver sections were also studied by immunostaining for albumin, and TUNEL. Peripheral blood serum albumin levels were measured on post-transplant days 0, 3, 5, 7, 10 and 14 using ELISA.

RESULTS

The edaravone-treated animals demonstrated an increased number of engrafted donor hepatocytes in the liver. The edaravone-treated liver sections also contained fewer TUNEL-positive cells and animals that received edaravone had higher serum albumin levels post-transplantation. Hepatocytes were also found to have increased in numbers 2 weeks following treatment with edaravone.

CONCLUSIONS

Edaravone administration during HTx can suppress apoptosis near the transplanted cells, increasing engraftment. These studies indicate its potential usefulness for future clinical application.

Hepatic differentiation of human embryonic stem cells on microcarriers

Translation of stem cell research to industrial and clinical settings mostly requires large quantities of cells, especially those involving large organs such as the liver. A scalable reactor system is desirable to ensure a reliable supply of sufficient quantities of differentiated cells. To increase the culture efficiency in bioreactor system, high surface to volume ratio needs to be achieved. We employed a microcarrier culture system for the expansion of undifferentiated human embryonic stem cells (hESCs) as well as for directed differentiation of these cells to hepatocyte-like cells. Cells in single cell suspension were attached to the bead surface in even distribution and were expanded to 1×10(6)cells/ml within 2 days of hESC culture with maintenance of the level of pluripotency markers. Directed differentiation into hepatocyte-like cells on microcarriers, both in static culture and stirred bioreactors, induced similar levels of hepatocyte-like cell differentiation as observed with cells cultured in conventional tissue culture plates. The cells expressed both immature and mature hepatocyte-lineage genes and proteins such as asialoglycoprotein receptor-1 (ASGPR-1) and albumin. Differentiated cells exhibited functional characteristics such as secretion of albumin and urea, and CYP3A4 activity could be detected. Microcarriers thus offer the potential for large-scale expansion and differentiation of hESCs induced hepatocyte-like cells in a more controllable bioreactor environment.

Eternity and functionality – rational access to physiologically relevant cell lines

In the first 50 years of cell culture, the development of new cell lines was mainly based on trial and error. Due to the understanding of the molecular networks of aging, senescence, proliferation, and adaption by mutation, the generation of new cell lines with physiologic properties has become more systematic. This endeavor has been supported by the availability of new technological achievements and increasing knowledge about the biology of cell differentiation and cell-cell communication. Here, we review some promising developments that are contributing toward this goal. These include molecular tools frequently used for the immortalization process. In addition to these broadly acting immortalization regimens, we focus on the developments of cell type-specific immortalization and on the methodologies of how to control the growth of newly established cell lines.

Pluripotent stem cell-derived hepatocytes: potential and challenges in pharmacology

The liver is a fascinating organ and performs a wide range of functions necessary for life. Because the hepatocyte is the major functional cell type found in the liver, it is important that we better understand its role in health and disease. Functional hepatocytes have been derived from many sources, including human stem cell populations. These models offer new opportunities to further our understanding of human liver biology from diverse genotypes and, in the future, to facilitate the development of novel medicines or cell-based therapies. This review discusses limitations in current cell-based models and the advantages offered by pluripotent stem cell-derived hepatocytes.

Review of the history and current status of cell-transplant approaches for the management of neuropathic pain

Treatment of sensory neuropathies, whether inherited or caused by trauma, the progress of diabetes, or other disease states, are among the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord would be the logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the transplant of cells or a cell line to treat human disease. The history of the research and development of useful cell-transplant-based approaches offers an understanding of the advantages and problems associated with these technologies, but as an adjuvant or replacement for current pharmacological treatments, cell therapy is a likely near future clinical tool for improved health care.

Immortalized mesenchymal stem cells: an alternative to primary mesenchymal stem cells in neuronal differentiation and neuroregeneration associated studies

Background

Mesenchymal stem cells (MSCs) can be induced to differentiate into neuronal cells under appropriate cellular conditions and transplanted in brain injury and neurodegenerative diseases animal models for neuroregeneration studies. In contrast to the embryonic stem cells (ESCs), MSCs are easily subject to aging and senescence because of their finite ability of self-renewal. MSCs senescence seriously affected theirs application prospects as a promising tool for cell-based regenerative medicine and tissue engineering. In the present study, we established a reversible immortalized mesenchymal stem cells (IMSCs) line by using SSR#69 retrovirus expressing simian virus 40 large T (SV40T) antigen as an alternative to primary MSCs.

Methods

The retroviral vector SSR#69 expressing simian virus 40 large T (SV40T) antigen was used to construct IMSCs. IMSCs were identified by flow cytometry to detect cell surface makers. To investigate proliferation and differentiation potential of IMSCs, cell growth curve determination and mesodermal trilineage differentiation tests were performed. Neuronal differentiation characteristics of IMSCs were detected in vitro. Before IMSCs transplantation, we excluded its tumorigenicity in nude mice firstly. The Morris water maze tests and shuttle box tests were performed five weeks after HIBD models received cells transplantation therapy.

Results

In this study, reversible IMSCs were constructed successfully and had the similar morphology and cell surface makers as primary MSCs. IMSCs possessed better ability of proliferation and anti-senescence compared with primary MSCs, while maintained multilineage differentiation capacity. Neural-like cells derived from IMSCs had similar expressions of neural-specific genes, protein expression patterns and resting membrane potential (RMP) compared with their counterparts derived from primary MSCs. There was no bump formation in nude mice subcutaneously injected with IMSCs. IMSCs played same role as primary MSCs to improve learning ability and spatial memory of HIBD rats.

Conclusions

IMSCs not only retain their features of primary MSCs but also possess the ability of high proliferation and anti-senescence. IMSCs can definitely be induced to differentiate into neuronal cells in vitro and take the place of primary MSCs for cell transplantation therapy without tumorigenesis in vivo. The stable cell line is particularly useful and valuable as an alternative to MSCs in neuronal differentiation and neuroregeneration associated studies.

Porcine hepatocyte isolation and reversible immortalization mediated by retroviral transfer and site-specific recombination

AIM: To develop a hepatocyte cell line, we immortalized primary porcine hepatocytes with a retroviral vector SSR#69 containing the Simian Virus 40 T antigen (SV40Tag).

METHODS: We first established a method of porcine hepatocyte isolation with a modified four-step retrograde perfusion technique. Then the porcine hepatocytes were immortalized with retroviral vector SSR#69 expressing SV40T and hygromycin-resistance genes flanked by paired loxP recombination targets. SV40T cDNA in the expanded cells was subsequently excised by Cre/LoxP site-specific recombination.

RESULTS: The resultant hepatocytes with high viability (97%) were successfully immortalized with retroviral vector SSR#69. One of the immortalized clones showed the typical morphological appearance, TJPH-1, and was selected by clone rings and expanded in culture. After excision of the SV40T gene with Cre-recombinase, cells stopped growing. The population of reverted cells exhibited the characteristics of differentiated hepatocytes.

CONCLUSION: In conclusion, we herein describe a modified method of hepatocyte isolation and subsequently established a porcine hepatocyte cell line mediated by retroviral transfer and site-specific recombination.

Progress and future challenges in stem cell-derived liver technologies

The emergence of regenerative medicine has led to significant advances in the identification and understanding of human stem cells and adult progenitor cells. Both cell populations exhibit plasticity and theoretically offer a potential source of somatic cells in large numbers. Such a resource has an important role to play in the understanding of human development, in modeling human disease and drug toxicity, and in the generation of somatic cells in large numbers for cell-based therapies. Presently, liver transplantation is the only effective treatment for end-stage liver disease. Although this procedure can be carried out with high levels of success, the routine transplant of livers is severely limited by organ donor availability. As a result, attention has focused on the ability to restore liver mass and function by alternative approaches ranging from the bioartificial device to transplantation of human hepatocytes. In this review we will focus on the generation of human hepatic endoderm from different stem/progenitor cell populations with a view to its utility in regenerative medicine.

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.

Review of hepatocyte transplantation

BACKGROUND

Hepatocyte transplantation is a promising treatment for several liver diseases and can also be used as a "bridge" to liver transplantation in cases of liver failure. Although the first animal experiments with this technique began in 1967, it was first applied in humans only in 1992. Unfortunately, unequivocal evidence of transplanted human hepatocyte function has been obtained in only one patient with Crigler-Najjar syndrome type I and, even then, the amount of bilirubin-UDP-glucuronosyltransferase enzyme activity derived from the transplanted cells was not sufficient to eliminate the patient's eventual need for organ transplantation.

METHODS

A literature review was carried out using MEDLINE and library searches.

RESULTS

This review considers the following: (1) alternatives or bridges to orthotopic liver transplantation (OLT); (2) solutions to the shortage of organs-the shortage of organ donors has impeded the development of human hepatocyte transplantation, and immortalized hepatocytes in particular could provide an unlimited supply of transplantable cells in a nearly future; (3) future directions. We review these efforts along with hepatocyte transplantation over the last 13 years.

Cellular Transplantation for Liver Diseases

Presently, the orthotropic liver transplantation (OLT) is still the most effective therapeutic for patients with acute or chronic hepatic failure. However, due to the shortage of donor livers, the number of patients benefited from this approach is limited. Therefore, some alternative modalities have been paid attention for restoring the liver function. The cell transplantation is one of the promising modalities to realize this purpose. The types of cells used in the cell transplantation include syngeneic hepatocytes, allogeneic hepatocytes, immortalized hepatocytes, and stem cells derived heptocytes. The stem cells, especially the adult stem cells from bone marrow, are shown as a promising cell source for liver repopulation. The mesenchymal bone marrow stem cells and embryonic stem cells can be induced to differentiate into the hepatic lineage and might be used in the cell transplantation for liver diseases. Compared to OLT, the advantages of cell-based therapy for liver disease are, but not limited to, less invasive, less expensive, easy manipulated, easy expansion of cells in vitro. Cells can be stored in a cell bank for future use. Though most of the current studies are experimental and animal based, the cellular therapy for liver disease is expected to be an effective alternative in clinical settings in near future.

Immortalized hepatocytes using human artificial chromosome

The shortage of organ donors has impeded the development of human hepatocyte transplantation. Immortalized hepatocytes could provide an unlimited supply of transplantable cells. To determine whether immortalized hepatocytes could provide global metabolic support in end-stage liver disease, rat hepatocyte clones were developed by transduction with the gene encoding the Simian virus 40 T antigen (SVT) using the human artificial minichromosome (HAC). The SVLT sequence was excised by FRT recombination. Following HAC infusion, the transduced hepatocytes express SVT, blasticidine resistance (BS), and the PGK promoter TK gene. Forty-six cell clones were obtained and at least partially characterized, as previously described, for albumin, alpha-1-antitrypsin, glucose-6-phosphatase (G6Pase), dipeptidylpeptidase 4 (Dpp4), gamma-glutamyltransferase 1 (Ggt), SVT, and beta-actin expression using RT-PCR. Clones were also assessed for albumin secretion into the culture medium using ELISA. All of the cell line secreted approximately 10 mg/dl of albumin, which is equivalent to the amount secreted by primary hepatocytes. In further experiments, this cell line will be used for transplantable cells or artificial organ using HAC. These results represent an important step toward the development of immortalized hepatocytes.

Establishment of Immortalized Human Hepatocytes by Introduction of HPV16 E6/E7 and hTERT as Cell Sources for Liver Cell-Based Therapy

For future cell-based therapies for liver diseases, the shortage of cell sources must be resolved. Immortalized human hepatocytes are expected to be among the new sources. In addition to telomerase activation by the introduction of human telomerase reverse transcriptase (hTERT), inactivation of the p16/RB pathway and/or p53 by E6/E7 of human papillomavirus type 16 (HPV16) has been shown to be useful for efficient immortalization of several human cell types. Here we report the immortalization of human hepatocytes by the introduction of HPV16 E6/E7 and hTERT. Human adult hepatocytes were lentivirally transduced with HPV16 E6/E7 and hTERT. Two human immortalized hepatocyte cell lines were established and were named HHE6E7T-1 and HHE6E7T-2. Those cells proliferated in culture beyond 200 population doublings (PDs). Albumin synthesis and expression of liver-enriched genes were confirmed, but gradually decreased as passages progressed. Karyotype analysis showed that HHE6E7T-1 cells remained near diploid but that HHE6E7T-2 cells showed severe aneuploidy at 150 PDs. Subcutaneous injection of these cells into severe combined immunodeficiency (SCID) mice did not induce tumor development. Intrasplenic transplantation of dedifferentiated HHE6E7T-1 cells over 200 PDs significantly improved the survival of acetaminophen-induced acute liver failure SCID mice. In conclusion, we successfully established immortalized human hepatocytes that retain the characteristics of differentiated hepatocytes. We also showed the reduction of hepatocyte-specific functions in long-term culture. However, the results of intrasplenic transplantation to SCID mice with acetaminophen-induced acute liver failure showed the possibility of HHE6E7T-1 serving as a cell source for hepatocyte transplantation.

Reversible transfection of human melanocytes mediated by Cre/loxP site-specific recombination system and SV40 large T antigen

OBJECTIVE

To study the reversible transfection of human melanocytes mediated by simian virus 40 large T antigen (SV40LTAg) and Cre/loxP site-specific recombination system.

METHODS

The reconstructed SV40LTAg-EGFP-neo-loxP vector was transfected into primary cultured human melanocytes with Sofast(TM) transfection reagent and the positive cells were selected using G418. After expanding culture of these positive cell clones, the expression of SV40LTAg was detected by polymerase chain reaction (PCR), reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescent method. After that, these positive cells were infected by virus supernatant of Cre-ER(T2) retrovirus vector and Cre recombinase was induced to act by tamoxifen. On the 6th and 10th day after Cre recombinase acting, the expression of SV40LTAg was detected using the same methods as above, and cell tumorigenicity was studied using soft agar assay, athymic mouse study and karyotype analysis. On 10th day after tamoxifen treatment, cell biological characters were identified with immunofluorescent staining and transmission electron microscopy. Then these cells were transplanted into vitiligo animal model to observe their melanogenesis ability in vivo.

RESULTS

The genome DNA and total RNA were isolated from the positive cells transfected by SV40LTAg (designated as MCT) and specific 288 bp fragment was amplificated using PCR and RT-PCR methods. The results of immunofluorescence confirmed the expression of SV40LTAg in cell nucleus. On the 6th day after tamoxifen treatment in infected cells by Cre-ER(T2) retrovirus vector (designated as MCT-Cre), there could be detected SV40LTAg expression, but on 10th day, there could not be detected SV40LTAg expression in cells. These results showed that the excised efficiency of Cre recombinase increased along with time prolongation, and would obtain complete recombination efficiency. The identification of MCT-Cre cell biological characters showed that these cells had normal parent-cell-like cell phenotype and no tumorigenicity in vitro. The pigmentation started in 4 weeks and formed black macula in 3 months after grafting. The pathological results showed that there had been significant melanocytes and melanin accumulation in epidermis and some hair follicle in transplanted area, which confirmed that MCT-Cre had melanogenesis function in vivo.

CONCLUSION

Human melanocytes could be mediated by reversible transfection by SV40LTAg and Cre/loxP site-specific recombination system, which had stable parent-cell-like phenotypic characters and no tumorigenicity in vitro; moreover, these cells still had melanogenesis function in vivo.

Direct differentiation of human embryonic stem cells to hepatocyte-like cells exhibiting functional activities

The utilization of human hepatocytes for biomedical research, drug discovery, and treatment of liver diseases is hindered by the limited availability of donated livers and the variability of their derived hepatocytes. Human embryonic stem cells (hESCs) are pluripotent and provide a unique, unlimited resource for human hepatocytes. However, differentiation of hESCs to hepatocytes remains a challenge. We have developed a multistage procedure by which hESCs can be directly differentiated to hepatocyte-like cells without embryoid body formation and the requirement of sodium butyrate. The hESC-derived hepatocyte-like cells (HLCs) exhibited characteristic hepatocyte morphology, expressed hepatocyte markers, including alpha-fetoprotein, albumin, and hepatocyte nuclear factor 4alpha, and possessed hepatocyte-specific activities, such as p450 metabolism, albumin production, glycogen storage, and uptake and excretion of indocyanine green. Hepatocyte growth factor was found to play a positive role in promoting hepatocyte differentiation. Our differentiation system has shown that hESCs can be differentiated to hepatocyte-like cells capable of executing a range of hepatocyte functions. Therefore, it presents a proof-of-principle of potential applications of using the hESC-derived hepatocytes. Additionally, the hESC-derived HLCs provide a unique model to study the mechanisms involved in human hepatocyte differentiation and liver function.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

Hepatocyte transplantation: studies in preclinical models

Transplantation of allogeneic or genetically modified autologous hepatocytes may be an alternative to whole-liver transplantation for the treatment of hereditary metabolic liver diseases. Human hepatocytes have already been transplanted in patients, demonstrating the safety and feasibility of both approaches. Although a few cases of allogeneic transplantation have resulted in long-term engraftment and function, only a partial and transient correction of the disease was achieved. This may partly result from a lack of proliferation of transplanted cells. In rodents, transplanted hepatocytes do not proliferate in adult quiescent livers and repopulate recipient livers only when they display a proliferative advantage over resident hepatocytes. Most of these models are not transposable to humans, however. Our aim is to develop preclinical approaches to hepatocyte transplantation in nonhuman primates. We have defined a strategy that increases the engraftment efficiency of transplanted hepatocytes by inducing their proliferation together with that of resident hepatocytes. We have also immortalized simian fetal hepatic progenitor cells and shown that these cells do not proliferate in situ after transplantation into the livers of immunodeficient mice. By contrast early human hepatoblasts repopulate mouse livers more efficiently. However, if we consider the number of cells to be transplanted (one to several billion), the means of expanding and differentiating stem or progenitor cells other than hepatocytes will have to be determined prior to envisaging treating patients.

Treatment of acetaminophen-induced acute liver failure in the mouse with conditionally immortalized human hepatocytes

BACKGROUND/AIMS

Liver failure is a life threatening condition currently treated by palliative measures and, when applicable, organ transplantation. The use of a bioartificial organ capable of fulfilling the main functions of the liver would represent an attractive alternative. However, the shortage of suitable donor cells, and their limited growth ability have impeded the development of this strategy. We investigated whether lentiviral vectors allow for conditional immortalization of human hepatocytes and whether these immortalized hepatocytes could reverse lethal acute liver failure.

METHODS

We exposed primary human hepatocytes to Cre-excisable lentiviral vectors coding for SV40T Antigen, telomerase, and/or Bmi-1 and tested the functionality of the resulting cell lines. Therapeutic potential of immortalized hepatocytes were tested in a murine model of acetaminophen-induced hepatic injury.

RESULTS

The immortalized hepatocytes grew continuously yet were non-tumorigenic, stopped proliferating when exposed to Cre recombinase, and conserved defining properties of primary hepatocytes, including the ability to secrete liver-specific proteins and to detoxify drugs. The implantation of encapsulated immortalized human hepatocytes rescued mice from lethal doses of acetaminophen.

CONCLUSIONS

Lentiviral vectors represent tools of choice for immortalization of non-dividing primary cells, and lentivirally immortalized human hepatocytes are promising reagents for cell-based therapy of acute liver failure.

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Establishment of highly differentiated immortalized human hepatocyte line with simian virus 40 large tumor antigen for liver based cell therapy

Acute liver failure and metabolic liver disorder animal models have demonstrated that hepatocytes transplanted into the liver or spleen survive and participate in the liver repopulation process, and recent studies have documented the usefulness of hepatocyte transplantation in humans. However, despite the promising cell therapy, there are still many restrictions, such as the shortage of donor human livers and the limited lifespan and the functional insufficiency of primary cultured hepatocytes. The immortalized and highly differentiated human hepatocyte could provide an unlimited supply of transplantable cells. In this study, we established an efficient and highly differentiated immortalized human hepatocyte line for bioartificial liver and hepatocyte transplantation research. Hepatocytes isolated from the liver of a 25 year old, brain dead male were transfected with pcDNA3.1 (-) recombinant plasmid containing the genes encoding simian virus 40 (SV40) large tumor antigen. One of the hepatocyte clones, HepLL, displayed highly differentiated liver functions with immortalized characteristics and was selected with a 700-300 microg/ml of G418 technique in 42 days. To characterize this immortalized cell line for cell therapy in the near future, HepLL cells were studied with immunohistochemistry, reverse transcription-polymerase chain reactions, immunoblotting, and tumorigenicity tests. The results revealed that HepLL cells displayed morphologic characteristics of liver parenchymal cells, secreted albumin, synthesized urea and glycogen, and expressed liver enriched functional markers, but there were no tumorigenic qualities after transplantation into severe combined immunodeficiency mice. Thus this immortalized human hepatocyte line is expected to be a useful tool for studying the functions of differentiated human hepatocyte and a promising strategy to resolve the shortages of donor organs and the limits of primary human hepatocyte for transplantation and bioartificial liver support systems.

Transcriptionally regulated immortalization overcomes side effects of temperature-sensitive SV40 large T antigen

The temperature-sensitive mutant of the SV40 virus large T antigen (TAg) tsA58 is frequently employed for the conditional immortalization of primary cells. By increasing the temperature to 39 degrees C, the activity of the mutant TAg is reduced and the status of such cells may then resemble more closely that of primary cells. As an alternative, we used a novel immortalization vector with a tetracycline-regulated expression of the wild-type TAg. This enabled us to investigate the effects of the immortalizing gene expression and of temperature shifts independently of each other. Even for wild-type TAg-derived cell lines the elevated temperatures led to various clone-dependent phenotypes. This suggests that in freshly established cell lines temperature-sensitive growth phenotypes can arise spontaneously and independently of a temperature-sensitive immortalizing gene. Similar effects were observed with spontaneously immortalized cells. On the other hand, not all of the ts-TAg-derived cell lines were proliferation arrested at the non-permissive temperature. Therefore, the assumption that temperature-sensitive growth is solely due to the ts-TAg must be verified for each ts-TAg-derived cell line individually. This complexity could be avoided by using the autoregulatory immortalization vector expressing the wild-type TAg.

Useful cell lines derived from the adrenal medulla

Five approaches for the preparation of adrenal chromaffin cell lines have been developed. Initially, continuous chromaffin lines were derived from spontaneous pheochromocytoma tumors of the medulla, either from murine or human sources, such as the rat PC12 cell line and the human KNA and KAT45 cell lines. Over the last few decades, more sophisticated molecular methods have allowed for induced tumorigenesis and targeted oncogenesis in vivo, where isolation of specific populations of mouse cell lines of endocrine origin have resulted in model cells to examine a variety of regulatory pathways in the chromaffin phenotype. As well, conditional immortalization with retroviral infection of chromaffin precursors has provided homogeneous and expandable chromaffin cells for transplant studies in animal models of pain. This same strategy of immortalization with conditionally expressed oncogenes has been expanded recently to create the first disimmortalizable chromaffin cells, with an excisable oncogenic cassette, as might be envisioned for the creation of human chromaffin cell lines. Eventually, as we increase our understanding of regulating the phenotypic fate of chromaffin cells in vitro, stem or progenitor adrenal medullary cell lines will be derived as an alternative source for expansion and clinical use.

Transcriptional control of SV40 T-antigen expression allows a complete reversion of immortalization

Conditional proliferation of mouse embryo fibroblasts was achieved with a novel autoregulatory vector for Tet-dependent expression of the SV40 T-antigen. The majority of cell clones that were isolated under induced conditions showed strict regulation of cell growth. Status switches were found to be fully reversible and highly reproducible with respect to gene expression characteristics. A consequence of T-antigen expression is a significant deregulation of >400 genes. Deinduced cells turn to rest in G0/G1 phase and exhibit a senescent phenotype. The cells are not oncogenic and no evidence for transformation was found after several months of cultivation. Conditional immortalization allows diverse studies including those on cellular activities without the influence of the immortalizing gene(s), senescence as well as secondary effects from T-antigen expression.

Generation of hepatocyte-like cells from human embryonic stem cells

Use of human hepatocytes for therapeutic and drug discovery applications is hampered by limited tissue source and the inability of hepatocytes to proliferate and maintain function long term in vitro. Human embryonic stem (hES) cells are immortal and pluripotent and may provide a cell source for functional human hepatocytes. We report here that hES cells can be induced to differentiate into hepatocyte-like cells. Treatment with sodium butyrate induced hepatic differentiation as well as significant cell death, resulting in approximately 10-15% yield of a homogeneous population of cells. The differentiated cells have morphological features similar to that of primary hepatocytes and 70-80% of the cells express liver-associated proteins (albumin, alpha-1-antitrypsin, cytokeratin 8 and 18), accumulate glycogen, have inducible cytochrome P450 activity, and do not express alpha-fetoprotein. Because of the inherent proliferative capacity of hES cells, these cells may provide a reliable source of normal human hepatocytes for research and transplantation.

Engineering liver therapies for the future

Treatment of liver disease has been greatly improved by the advent and evolution of liver transplantation. However, as demand for donor organs continues to increase beyond their availability, the need for alternative liver therapies is clear. Several approaches including extracorporeal devices, cell transplantation, and tissue-engineered constructs have been proposed as potential adjuncts or even replacements for transplantation. Simultaneously, experience from the liver biology community have provided valuable insight into tissue morphogenesis and in vitro stabilization of the hepatocyte phenotype. The next generation of cellular therapies must therefore consider incorporating cell sources and cellular microenvironments that provide both a large population of cells and strategies to maintain liver-specific functions over extended time frames. As cell-based therapies evolve, their success will require contribution from many diverse disciplines including regenerative medicine, developmental biology, and transplant medicine.

Treatment of liver failure in rats with end-stage cirrhosis by transplantation of immortalized hepatocytes

The shortage of organ donors has impeded the development of human hepatocyte transplantation. Immortalized hepatocytes could provide an unlimited supply of transplantable cells. To determine whether immortalized hepatocytes could provide global metabolic support in end-stage liver disease, 35 immortalized rat hepatocyte clones were developed by transduction with the gene encoding the simian virus 40 T antigen (SV40Tag). The SV40Tag sequence and a suicide gene, herpes simplex virus thymidine kinase (HSV-tk), were flanked by loxP sequences so that they could be excised by Cre/lox recombination. When transplanted into the spleens of portacaval-shunted rats, 3 of the 35 immortalized hepatocyte clones prevented the development of hyperammonemia-induced hepatic encephalopathy. The protection was reversed by treatment with ganciclovir, which kills HSV-tk-expressing cells. Transplantation of alginate-encapsulated, immortalized hepatocytes into the spleens of cirrhotic rats resulted in significant improvement in prothrombin time, serum albumin and bilirubin levels, hepatic encephalopathy score, and duration of survival. The metabolic support provided by the immortalized cells equaled that observed after transplantation of primary rat hepatocytes. In conclusion, immortalized hepatocytes can function as well as primary hepatocytes following transplantation and can be engineered to contain safeguards that could make them clinically useful. Further investigation is warranted regarding the mechanisms of loss of mass or function of the transplanted hepatocytes over time and how the relatively few engrafted hepatocytes can ameliorate liver decompensation in cirrhosis.

Immortalized chromaffin cells disimmortalized with Cre/lox site-directed recombination for use in cell therapy for pain after partial nerve injury

To prepare immortalized adrenal chromaffin cells for eventual clinical use, the immortalizing oncogene must be removed. We have utilized a Cre-mediated excision of a loxP-flanked Tag sequence to test whether immortalized chromaffin cells could be disimmortalized by this method. Cultures of embryonic rat adrenal cells were immortalized with the tsA-TN retroviral vector encoding the loxP-flanked temperature-sensitive allele of SV40 large T antigen (tsA-TN) and a positive/negative neo/HSV-TK sequence for selection with either G418 or gancyclovir, respectively. These cells were then infected with the 1710-CrePR1 bicistronic retroviral vector coding for a form of Cre modulatable by the synthetic steroid RU486. These immortalized loxTsTag/CrePR1/RAD cells expressed immunoreactivities (ir) for all the catecholamine enzymes: tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DbetaH), and phenylethanolamine-N-methyltransferase (PNMT). After initial incubation at 37 degrees C with RU486 for 3 days, followed by the addition of gancyclovir for 7 days, Tag-ir was not detectable in most of the surviving chromaffin cells, compared to 100% expression in immortalized loxTsTag/CreR1/RAD cells not treated with RU486 and gancyclovir. The expression of TH, DbetaH, and PNMT was increased after disimmortalization and the ability of disimmortalized cells to synthesize norepinephrine was also significantly increased compared to immortalized cells. When both types of chromaffin cells were transplanted in a model of neuropathic pain and partial nerve injury, both cell grafts were equally able to reverse the behavioral hypersensitivity induced by the injury. The use of Cre/lox site-directed disimmortalization of chromaffin cells that are able to deliver neuroactive molecules offers a novel approach to cell therapy.

Controlled expansion of human endothelial cell populations by Cre-loxP-based reversible immortalization

Endothelial cells (ECs) play multiple physiological functions and are central to many pathological processes. Various biological studies as well as cell and gene therapy applications would benefit substantially from a procedure that would result in the expansion in culture of large numbers of highly differentiated human ECs. Here, we report the amplification in vitro of human EC populations, which occurred during the first phase of reversible immortalization resulting from the retroviral transfer of an oncogene that was subsequently excised by Cre-loxP-mediated site-specific recombination. Human umbilical vein endothelial cells (HUVECs) and human liver sinusoidal endothelial cells (HLSECs) were transduced with a retroviral vector that expresses the simian virus 40 large T (SV40T) gene flanked by positive and negative selectable markers and a pair of loxP recombination targets. Transduced HUVECs and HLSECs yielded clones with greatly extended life spans, referred to as HNNT-1 and HNNT-2 cells, respectively. HNNT-1 and HNNT-2 cells showed morphological characteristics of ECs and were maintained in culture up to population doubling level (PDL) 80 for HNNT-1 and PDL 65 for HNNT-2 cells. HNNT-1 and HNNT-2 cells were not tumorigenic when transplanted into severe combined immunodeficiency mice and were sensitive to ganciclovir as well as G418. Both cell clones expressed EC markers, which include factor VIII, VEGF receptors (Flt-1 and KDR/Flk-1), and CD34, and endocytosed acetylated low-density lipoproteins. Formation of capillary-like structures in a Matrigel assay was observed with HNNT-1 and HNNT-2 cells until at least PDL 50. Complete elimination of the transferred SV40T gene was achieved in virtually 100% of HNNT-1 and HNNT-2 cells after infection with a recombinant adenovirus expressing the Cre recombinase fused to a nuclear localization signal and subsequent selection with G418. Reverted cells maintained their differentiated EC phenotype. This study extends the utility of the reversible immortalization procedure and provides a means to expand primary human ECs of various sources for basic studies and possible cell and gene therapies.

Construction of a differentiated human hepatocyte cell line expressing the herpes simplex virus-thymidine kinase gene

Transient support using a hybrid artificial liver (HAL) device is a promising treatment for the patients with acute liver failure. Primary human hepatocytes are an ideal source for HAL therapy; however, the number of human livers available for hepatocyte isolation is limited by competition for use in whole organ transplantation. To overcome this problem, we previously established a highly differentiated human fetal hepatocyte cell line OUMS-29. Considering the potential risk when using these genetically engineered cells in humans, additional safeguards should be added to make the cells more clinically useful. In this work, the herpes simplex virus thymidine kinase (HSVtk) gene was retrovirally introduced into OUMS-29 cells. One of the HSVtk-expressed clones, OUMS-29/thymidine kinase (TK), grew in chemically defined serum free medium and expressed the genes of albumin, asialoglycoprotein receptor, glutamine synthetase, glutathione-S-transferase pi, and blood coagulation factor X. In vitro sensitivity of the cells to ganciclovir was evaluated. Intrasplenic transplantation of 50 x 10(6) OUMS-29/TK cells prolonged the survival of 90% hepatectomized rats compared with medium injection alone (control). In the present study, we have established highly differentiated immortalized human hepatocytes with tight regulation. The cells may be clinically useful for HAL treatment.

A reversibly immortalized human hepatocyte cell line as a source of hepatocyte-based biological support

The application of hepatocyte transplantation (HTX) is increasingly envisioned for temporary metabolic support during acute liver failure and provision of specific liver functions in inherited liver-based metabolic diseases. Compared with whole liver transplantation, HTX is a technically simple procedure and hepatocytes can be cryopreserved for future use. A major limitation of this form of therapy in humans is the worldwide shortage of human livers for isolating an adequate number of transplantable human hepatocyes when needed. Furthermore, the numbers of donor livers available for hepatocyte isolation is limited by competition for their use in whole organ transplantation. Considering the cost of hepatocyte isolation and the need for immediate preparation of consistent and functional cells, it is unlikely that human hepatocytes can be obtained on such a scale to treat a large number of patients with falling liver functions. The utilization of xenogenic hepatocytes will result in additional concerns regarding transmission of infectious pathogens and immunological and physiological incompatibilities between animals and humans. An attractive alternative to primary human hepatocytes is the use of tightly regulated human hepatocyte cell lines. Such cell lines can provide the advantages of unlimited availability, sterility and uniformity. We describe here methods for creating transplantable human hepatocyte cell lines using currently available cell cultures and gene transfer technology.

Novel strategies for immortalization of human hepatocytes

Normal somatic cells have a finite life span due in part to their inability to maintain telomere length and chromosome stability. Immortalization strategies based on recent advances in telomere biology and aging research have led to the creation of genetically stable, nontumorigenic immortalized cell lines. Reversible immortalization, using the Cre-lox recombination and excision system, has been developed for the expansion of primary cells for cell based clinical therapies. Immortalized human hepatocyte cell lines with differentiated liver functions would find broad applications in biomedical research, especially for pharmacology and toxicology, artificial liver support, and hepatocyte transplantation. The biological basis of these new immortalization methods and their application to human hepatocytes is reviewed.