Liver stem cells: a potential source of hepatocytes for the treatment of human liver disease

KIT as a therapeutic target for non-oncological diseases

KIT is a receptor tyrosine kinase that after binding to its ligand stem cell factor activates signaling cascades linked to biological processes such as proliferation, differentiation, migration and cell survival. Based on studies performed on SCF and/or KIT mutant animals that presented anemia, sterility, and/or pigmentation disorders, KIT signaling was mainly considered to be involved in the regulation of hematopoiesis, gametogenesis, and melanogenesis. More recently, novel animal models and ameliorated cellular and molecular techniques have led to the discovery of a widen repertoire of tissue compartments and functions that are being modulated by KIT. This is the case for the lung, heart, nervous system, gastrointestinal tract, pancreas, kidney, liver, and bone. For this reason, the tyrosine kinase inhibitors that were originally developed for the treatment of hemato-oncological diseases are being currently investigated for the treatment of non-oncological disorders such as asthma, rheumatoid arthritis, and alzheimer's disease, among others. The beneficial effects of some of these tyrosine kinase inhibitors have been proven to depend on KIT inhibition. This review will focus on KIT expression and regulation in healthy and pathologic conditions other than cancer. Moreover, advances in the development of anti-KIT therapies, including tyrosine kinase inhibitors, and their application will be discussed.

Bone marrow cell-based regenerative therapy for liver cirrhosis

Bone marrow cells are capable of differentiation into liver cells. Therefore, transplantation of bone marrow cells has considerable potential as a future therapy for regeneration of damaged liver tissue. Autologous bone marrow infusion therapy has been applied to patients with liver cirrhosis, and improvement of liver function parameters has been demonstrated. In this review, we summarize clinical trials of regenerative therapy using bone marrow cells for advanced liver diseases including cirrhosis, as well as topics pertaining to basic in vitro or in vivo approaches in order to outline the essentials of this novel treatment modality.

In vitro and in vivo characteristics of hepatic oval cells modified with human hepatocyte growth factor

Hepatocyte growth factor (HGF) is a multifunctional growth factor that controls cell scattering. It has been suggested that it regulates the proliferation of hepatic oval cells (HOCs). Using a HOC line that stably expresses the human HGF gene (hHGF), we investigated the in vitro proliferation and differentiation characteristics of hHGF-modified HOCs and explored their potential capacity for intrahepatic transplantation. A modified 2-acetylaminofluorene and partial hepatectomy (2-AAF/PH) model was established to activate the proliferation of oval cells in the rat liver. HOCs were transfected with the pBLAST2-hHGF plasmid and hHGF-carrying HOCs were selected based on blasticidin resistance. The level of hHGF secretion was determined via ELISA. Cell proliferation was determined using the MTT assay. Differentiation was induced by growth factor withdrawal. A two-cuff technique was used for orthotopic liver transplantation, and HOCs or hHGF-modified HOCs were transplanted into the recipients. The levels of biochemical indicators of liver function were measured after transplantation. An HOC line stably expressing hHGF was established. The transfected line showed greater hHGF secretion than normal HOCs. The hHGF gene promoted the proliferation capability of HOCs by reducing the peak time in vitro. The hHGF-modified HOCs differentiated into hepatocytes and bile duct epithelial cells upon growth factor withdrawal in vitro. In addition, hHGF-modified HOC transplantation significantly prolonged the median survival time (MST) and improved the liver function of recipients compared to HOC transplant recipients and nontransplanted controls. Our results indicate that hHGF-modified HOCs may have valuable properties for therapeutic liver regeneration after orthotopic liver transplantation.

In vitro proliferation and differentiation of hepatic oval cells and their potential capacity for intrahepatic transplantation

Hepatic oval cells (HOCs) are recognized as facultative liver progenitor cells that play a role in liver regeneration after acute liver injury. Here, we investigated the in vitro proliferation and differentiation characteristics of HOCs in order to explore their potential capacity for intrahepatic transplantation. Clusters or scattered HOCs were detected in the portal area and interlobular bile duct in the liver of rats subjected to the modified 2-acetylaminofluorene and partial hepatectomy method. Isolated HOCs were positive for c-kit and CD90 staining (99.8% and 88.8%, respectively), and negative for CD34 staining (3.6%) as shown by immunostaining and flow cytometric analysis. In addition, HOCs could be differentiated into hepatocytes and bile duct epithelial cells after leukemia inhibitory factor deprivation. A two-cuff technique was used for orthotopic liver transplantation, and HOCs were subsequently transplanted into recipients. Biochemical indicators of liver function were assessed 4 weeks after transplantation. HOC transplantation significantly prolonged the median survival time and improved the liver function of rats receiving HOCs compared to controls (P=0.003, Student t-test). Administration of HOCs to rats also receiving liver transplantation significantly reduced acute allograft rejection compared to control liver transplant rats 3 weeks following transplantation (rejection activity index score: control=6.3±0.9; HOC=3.5±1.5; P=0.005). These results indicate that HOCs may be useful in therapeutic liver regeneration after orthotopic liver transplantation.

Human hepatocyte growth factor (hHGF)-modified hepatic oval cells improve liver transplant survival

Despite progress in the field of immunosuppression, acute rejection is still a common postoperative complication following liver transplantation. This study aims to investigate the capacity of the human hepatocyte growth factor (hHGF) in modifying hepatic oval cells (HOCs) administered simultaneously with orthotopic liver transplantation as a means of improving graft survival. HOCs were activated and isolated using a modified 2-acetylaminofluorene/partial hepatectomy (2-AAF/PH) model in male Lewis rats. A HOC line stably expressing the HGF gene was established following stable transfection of the pBLAST2-hHGF plasmid. Our results demonstrated that hHGF-modified HOCs could efficiently differentiate into hepatocytes and bile duct epithelial cells in vitro. Administration of HOCs at the time of liver transplantation induced a wider distribution of SRY-positive donor cells in liver tissues. Administration of hHGF-HOC at the time of transplantation remarkably prolonged the median survival time and improved liver function for recipients compared to these parameters in the other treatment groups (P<0.05). Moreover, hHGF-HOC administration at the time of liver transplantation significantly suppressed elevation of interleukin-2 (IL-2), tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) levels while increasing the production of IL-10 and TGF-β1 (P<0.05). HOC or hHGF-HOC administration promoted cell proliferation, reduced cell apoptosis, and decreased liver allograft rejection rates. Furthermore, hHGF-modified HOCs more efficiently reduced acute allograft rejection (P<0.05 versus HOC transplantation only). Our results indicate that the combination of hHGF-modified HOCs with liver transplantation decreased host anti-graft immune responses resulting in a reduction of allograft rejection rates and prolonging graft survival in recipient rats. This suggests that HOC-based cell transplantation therapies can be developed as a means of treating severe liver injuries.

Activation, isolation, identification and culture of hepatic stem cells from porcine liver tissues

OBJECTIVES

  Utility of hepatic stem cells could provide a novel solution to the severe shortage of human donor livers, for treatment of liver-related diseases, due to their ability to proliferate and differentiate into functional hepatocytes. Porcine liver tissues also offer an alternative source from human donor livers. However, morphology, phenotype, successful isolation and culture of porcine hepatic stem cells still require much investigation.

MATERIALS AND METHODS

  In the present study, we performed partial hepatectomy to activate hepatic oval cells and developed a procedure utilizing enzymatic digestion and density gradient centrifugation to isolate and purify oval cells derived from porcine livers. We identified ovoid cells by their morphological characteristics and phenotypic properties, thereby providing definitive evidence for the presence of hepatic stem cells in porcine livers. Moreover, we established a culture system, using various growth factors, to provide nourishment for these cells.

RESULTS AND CONCLUSIONS

  By transmission electron microscopy, oval-shaped cells with ovoid nuclei, a high nucleus/cytoplasm ratio and few organelles were demonstrated. Flow cytometry and immunocytochemistry showed that freshly isolated oval cells expressed albumin, cytokeratin 19, alpha fetoprotein (AFP) and OV6 at high levels. Immunofluorescence revealed that porcine hepatic oval cells after culture expressed stem-cell factor, c-kit, Thy-1, CK19, OV6, and AFP. Taken together, this study provides a novel insight into morphological and phenotypic characteristics of porcine hepatic stem cells. Our ability for isolation and culturing porcine hepatic stem cells offers an abundant source of cells for transplantation and tissue engineering to help alleviate liver disease.

The role of glucose, serum, and three-dimensional cell culture on the metabolism of bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) have become a critical addition to all facets of tissue engineering. While most in vitro research has focused on their behavior in two-dimensional culture, relatively little is known about the cells' behavior in three-dimensional culture, especially with regard to their metabolic state. To evaluate MSC metabolism during twodimensional culture, murine bone marrow-derived MSCs were cultured for one week using twelve different medium compositions, varying in both glucose and fetal bovine serum (FBS) concentrations. The results indicate that glucose concentration was the more important factor in sustaining cell growth and viability. To evaluate metabolic state during three-dimensional culture, MSCs were cultured for one week using two different medium compositions and two different concentrations of collagen gel matrix. The medium compositions only varied in glucose concentration. The results indicate that glucose and extracellular matrix were significant factors in the metabolic response of the cells. However, cells cultured in low density collagen exhibited considerable cell death, likely because of physical contraction of the collagen hydrogel which was not observed in the higher density collagen. These findings will be useful to the development of in vitro cell culture models that properly mimic in vivo physiological processes.

Tissue engineering of human bladder

There are a number of conditions of the bladder that can lead to loss of function. Many of these require reconstructive procedures. However, current techniques may lead to a number of complications. Replacement of bladder tissues with functionally equivalent ones created in the laboratory could improve the outcome of reconstructive surgery. A review of the literature was conducted using PubMed to identify studies that provide evidence that tissue engineering techniques may be useful in the development of alternatives to current methods of bladder reconstruction. A number of animal studies and several clinical experiences show that it is possible to reconstruct the bladder using tissues and neo-organs produced in the laboratory. Materials that could be used to create functionally equivalent urologic tissues in the laboratory, especially non-autologous cells that have the potential to reject have many technical limitations. Current research suggests that the use of biomaterial-based, bladder-shaped scaffolds seeded with autologous urothelial and smooth muscle cells is currently the best option for bladder tissue engineering. Further research to develop novel biomaterials and cell sources, as well as information gained from developmental biology, signal transduction studies and studies of the wound healing response would be beneficial.

Stem/progenitor cells in liver injury repair and regeneration

Morbidity and mortality from cirrhosis is increasing rapidly in the world. Currently, orthotopic liver transplantation is the only definitive therapeutic option. However, its clinical use is limited, because of poor long-term graft survival, donor organ shortage and high costs associated with the procedure. Stem cell replacement strategies are therefore being investigated as an attractive alternative approach to liver repair and regeneration. In this review we discuss recent preclinical and clinical investigations that explore the therapeutic potential of stem cells in repair of liver injuries. Several types of stem cells. including embryonic stem cells, haematopoietic stem cells and mesenchymal stem cells, can be induced to differentiate into hepatocyte-like cells by defined culture conditions in vitro. Stem cell transplantation has been shown to significantly improve liver function and increase animal survival in experimentally-induced liver-injury models. Moreover, several pilot clinical studies have reported encouraging therapeutic effects in patients treated with stem cells. Although there remain many unresolved issues, the available data support the notion that stem cell technology may lead to the development of effective clinical modalities for human liver diseases.

Advance in hepatic stem cells and liver injury

The liver diseases remain major causes of death all over the world. Although orthotopic liver transplantation is an effective treatment for end-stage liver diseases. However, shortage of healthy livers for transplantation worldwide have urgently limited the use of liver transplantation for acute and chronic liver diseases. Stem cells play an important role in the concert of liver regeneration. Hepatic stem cells have been shown experimentally to participate in liver proliferation. Furthermore, it has been postulated that hepatic stem cells are able to transdifferentiate into both hepatocytes and bole duct cells. These data indicate a possible role and therapeutic potential of hepatic stem cells in liver diseases. In this paper, we reviewed the application of stem cells in liver diseases.

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

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

Expression of endoderm stem cell markers: evidence for the presence of adult stem cells in human thyroid glands

OBJECTIVE

Adult stem cells have been detected in several human tissues. The object of this study was to investigate whether they also occur in the human thyroid gland.

DESIGN

The expression of the stem cell marker Oct- 4 and the early endodermal markers GATA-4 and HNF4alpha was analyzed in histologic slides and cultured cells derived from goiters, in the FRTL5 cell line, and the HTh74, HTC, C643, and XTC133 thyroid carcinoma cell lines.

MAIN OUTCOME

Stem cell markers were detectable in all primary cultures whereas in the differentiated FRTL5 cell line no expression was observed. Expression of stem cell marker mRNA was not affected by thyrotropin (TSH) stimulation and did not decrease when cells underwent several passages. Immunostaining of cultured cells and of histologic slides of goitrous tissues showed only single cells that were immunoreactive for Oct-4, GATA-4, and HNF4a. Expression of Oct-4 but not of endodermal marker GATA-4 was also detectable in some thyroid carcinoma cell lines. Fluorescence-activated cell sorter (FACS) analysis demonstrated cell populations that were positive for either Oct-4, GATA-4, or HNF4alpha but negative for thyroglobulin. When these putative, FACS-sorted stem cell populations were further analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR), expression of all stem cell markers and of Pax8 but not of thyroglobulin mRNA was detectable.

CONCLUSIONS

These data provide evidence for the presence of adult stem and precursor cells of endodermal origin in the human thyroid gland.

Transdifferentiation of bioencapsulated bone marrow cells into hepatocyte-like cells in the 90% hepatectomized rat model

Under specific conditions, bone marrow cells can transdifferentiate into a variety of cell types including hepatocytes. In this study, bioencapsulated bone marrow cells were transplanted intraperitoneally into 90% hepatectomized rats. We then followed the transdifferentiation of the bone marrow cells and the effect of this on liver regeneration in this liver failure model. Bone marrow cells isolated from Wistar rats were bioencapsulated using alginate-polylysine-alginate method. These bioencapsulated bone marrow cells were transplanted intraperitoneally into 90% hepatectomized Wistar rats. Blood chemistry, HGF, liver weight, and survival of the recipient rats were evaluated. Histology and immunocytochemistry were used to analyze the bioencapsulated cells before and 14 days after transplantation. Unlike free bone marrow cells, transplantation of bioencapsulated bone marrow cells improved the survival of 90% hepatectomized rats and improved the blood chemistry with an efficacy similar to that of bioencapsulated hepatocytes or free hepatocytes transplantation. Some bioencapsulated bone marrow cells expressed hepatocytes markers of cytokeratins 8, cytokeratins 18, albumin, and AFP after 2 weeks of transplantation. These results suggest that syngeneic bioencapsulated bone marrow cells can transdifferentiate into hepatocyte-like cells in the peritoneal cavity of 90% hepatectomized rats and increased the survival rates of these rats. In conclusion, these findings suggest the potential for a new alternative to hepatocyte transplantation for cellular therapy of acute liver failure.

Serum osteopontin levels in patients with acute liver dysfunction

OBJECTIVE

Fulminant hepatic failure (FHF) is a clinical syndrome of sudden and severe liver dysfunction accompanied by encephalopathy in a previously healthy person. In FHF, hepatocytes are severely damaged and ordinary liver regeneration is impaired. We demonstrated that the expression of osteopontin (OPN), a multifunctional cytokine, was up-regulated in mouse oval cell (a stem-cell progenitor) induction models.

MATERIAL AND METHODS

Based on this finding, serum OPN levels were examined in 43 patients with FHF and in 45 patients with acute self-limited hepatitis (AH). To determine the cellular source of OPN, the expression of OPN was studied in a liver specimen derived from an FHF patient.

RESULTS

The mean OPN level of patients with FHF was 2.80+/-0.48 ng/ml (log(10), +/-SD), which was significantly higher than that of the patients with AH (2.42+/-0.39 ng/ml) (p=0.003, unpaired t-test). Patients with elevated serum OPN levels had a significantly poorer prognosis than patients whose serum OPN levels were not elevated. In the FHF patient's liver, OPN protein was expressed not only in inflammatory cells but also in regenerating hepatocytes and bile ductular structures.

CONCLUSIONS

Our current study indicates that serum OPN levels increased in patients with FHF and that OPN might play an important role in liver regeneration due to activation of hepatic stem cells.

Defined Conditions for Development of Functional Hepatic Cells from Human Embryonic Stem Cells

Human embryonic stem (hES) cells provide an important means to evaluate specific soluble and cell-bound stimuli that regulate development of specific cell lineages. Here, we examined specific cytokines and extracellular matrix (ECM) proteins that support differentiation of hES cells to hepatocytes. Tests of several different conditions determined that addition of fibroblast growth factor (FGF)-4 and hepatocyte growth factor in completely serum-free cultures of hES cell-derived embryoid bodies subsequently allowed to attach to type I collagen-coated dishes led to maximal differentiation into cells, not only with the morphologic and phenotypic characteristics of hepatocytes but also the functional characteristics. Expression of common hepatic transcription factors including HNF-3beta, HNF-1, and GATA-4 were all significantly induced under these conditions. Hepatocyte function was demonstrated by multiple complementary criteria: production of urea and albumin, phenobarbital-induced cytochrome P450 expression, and uptake of indocyanine green. These hES cell-derived hepatocytes will serve as a resource to understand normal human hepatocyte development and for applications such as cell replacement therapies and screening of pharmacologic drugs.

Detection and clinical significance of bone marrow micrometastases in patients undergoing liver transplantation for hepatocellular carcinoma

BACKGROUND

Existing methods of selecting patients with hepatocellular carcinoma (HCC) for liver transplantation rely on computed tomography and magnetic resonance imaging, which fail to detect vascular invasion or micrometastases. Vascular invasion cannot be assessed pretransplant because of the risks of tumor biopsy. Pretransplant detection of micrometastases in bone marrow is an alternative method that may identify patients at risk of recurrence and permit better organ allocation. The authors' aim was to develop an accurate method of detecting hepatocellular carcinoma micrometastases that may have a clinical role in pretransplant assessment.

METHODS

Iliac crest bone marrow was sampled from 18 patients with HCC (before liver transplantation) and 14 controls (cirrhosis, 9; hematologic disorders, 5). Mononuclear bone marrow fractions were evaluated for the presence of micrometastases by immunocytochemistry (ICC) using Hep Par-1 (HPICC) and Glypican-3 (GPICC), and by reverse-transcriptase (RT) polymerase chain reaction using two primer pairs targeting albumin (Alb1RT and Alb2RT) and one pair targeting alpha-fetoprotein (AFPRT). Each marker was compared in terms of (1) specificity and (2) positive and negative predictive values for tumor recurrence.

RESULTS

The specificity of each marker for detecting HCC micrometastases were as follows: HPICC, 93%; GPICC, 92%; Alb1RT, 33%; Alb2RT, 59%; and AFPRT, 0%. The positive (negative) predictive values for HPICC, GPICC, and Alb2RT for posttransplant tumor recurrence were 42% (100%), 40% (73%), and 0% (43%), respectively. HPICC-positive micrometastases were identified in all patients with postoperative tumor recurrence (n=5) and in 75% of patients with microscopic vascular invasion.

CONCLUSIONS

Detection of HCC micrometastases in bone marrow of potential liver transplant candidates is a potentially useful technique that may provide important prognostic information without the need for preoperative tumor biopsy. The authors' preliminary data indicate that HPICC is a promising immunocytochemical marker for HCC micrometastases, and larger studies are needed to confirm its clinical role.

Differentiation of rat bone marrow cells cultured on artificial basement membrane containing extracellular matrix into a liver cell lineage

BACKGROUND/AIMS

Bone marrow (BM) cells have been shown to be capable of differentiating into a liver cell lineage in vitro. However, their differentiation and proliferation is poor, and the cell characteristics are poorly understood.

METHODS

We cultured rat BM cells on an artificial basement membrane containing extracellular matrix (ECM) with hepatocyte growth factor (HGF). The expression of mRNA for liver-specific genes was analyzed by reverse transcription PCR. The expression of albumin and Musashi-1 by cultured cells was analyzed using a fluorescence-activated cell sorter (FACS). The proportions of albumin-positive cells when culture was performed with different concentrations of HGF were analyzed by FACS.

RESULTS

On culture day 21, polygonal cells proliferated and formed cell colonies. These cells expressed mRNA for all the liver-specific genes analyzed, and showed heterogeneous differentiation, some cells expressing albumin, others expressing Musashi-1. Albumin-positive differentiated cells were large and rich in intracellular structures, while Musashi-1-positive undifferentiated cells had the opposite characteristics. Culturing cells with higher concentrations of HGF induced an increased proportion of albumin-positive cells.

CONCLUSIONS

The results suggest that cell culture on an ECM with a high concentration of HGF increases the extent to which BM cells differentiate into a liver cell lineage and proliferate in vitro.

Hepatic tissue engineering

Fulminant hepatic failure (FHF) attributes to rising medical cost and accounts for many deaths each year in the United States. Currently, the only solution is organ transplantation. Due to increasing donor organ shortage, many in need of transplantation continue to remain on the waiting list. Liver Assist Devices (LADs) are being used to temporarily sustain liver function and bridge the period between FHF and transplantation. Hepatic Tissue Engineering is a step toward alleviating the need for donor organs; yet many challenges must be overcome including scaffold choice, cell source and immunological barriers. Bioreactors have aided in hepatocyte survival and have proven to sustain viable cells for several weeks. Achieving the necessary functions required for hepatic replacement is aided by the incorporation of growth factors and mitogens many that now can be bound to the polymer scaffold and released in a timely manner. Utilizing concepts such as MicroElectroMechanical systems (MEMs) technology, our laboratory is able to mimic the natural vasculature of the liver and sustain functional and viable hepatocytes. Expanding and improving upon this platform technology, advancements made will continue toward the development of a fully functioning and implantable liver.

Influence of serum from liver-damaged rats on differentiation tendency of bone marrow-derived stem cells

AIM: Recent studies in both rodents and humans indicated that bone marrow (BM)-derived stem cells were able to home to the liver after they were damaged and demonstrated plasticity in becoming hepatocytes. However, the question remains as to how these stem cells are activated and led to the liver and where the signals initiating the mechanisms of activation and differentiation of stem cells originate. The aim of this study was to investigate the influence of serum from liver-damaged rats on differentiation tendency of bone marrow-derived stem cells.

METHODS: Serum samples were collected from rats treated with a 2-acetylaminofluorene (2-AAF) /carbon tetrachloride (CCl4) program for varying time points and then used as stimulators of cultured BM stem cells. Expression of M₂- and L-type isozymes of rat pyruvate kinase, albumin as well as integrin-β 1 were then examined by reverse transcription polymerase chain reaction (RT-PCR) to estimate the differentiation state of BM stem cells.

RESULTS: Expression of M₂-type isozyme of pyruvate kinase (M2-PK), a marker of immature hepatocytes, was detected in each group stimulated with experimental serum, but not in controls including mature hepatocytes, BM stem cells without serum stimulation, and BM stem cells stimulated with normal control serum. As a marker expressed in the development of liver, the expression signal of integrin-β 1 was also detectable in each group stimulated with experimental serum. However, expression of L-type isozyme of pyruvate kinase (L-PK) and albumin, marker molecules of mature hepatocytes, was not detected in groups stimulated with experimental serum.

CONCLUSION: Under the influence of serum from rats with liver failure, BM stem cells begin to differentiate along a direction to hepatocyte lineage and to possess some features of immature hepatocytes.

Participation of bone marrow derived cells in cutaneous wound healing

Bone marrow has long been known to be a source of stem cells capable of regeneration of the hematopoeitic system. Recent reports, however, have indicated that bone marrow might also contain early stem cells that can differentiate into other organ tissues such as skin. While these studies have illustrated that bone marrow stem cells could find their way to the skin, they have not addressed the dynamics of how bone marrow stem cells might participate in the homeostatis and regeneration of skin. In this report we followed green fluorescent protein (GFP) labeled bone marrow transplanted into non-GFP mice in order to determine the participation of bone marrow stem cells in cutaneous wounds. Our results indicate that there are a significant number of bone marrow cells that traffic through both wounded and non-wounded skin. Wounding stimulated the engraftment of bone marrow cells to the skin and induced bone marrow derived cells to incorporate into and differentiate into non-hematopoietic skin structures. This report thus illustrates that bone marrow might be a valuable source of stem cells for the skin and possibly other organs. Wounding could be a stimulus for bone marrow derived stem cells to travel to organs and aid in the regeneration of damaged tissue.

Differentiation of putative hepatic stem cells derived from adult rats into mature hepatocytes in the presence of epidermal growth factor and hepatocyte growth factor

Oval cells, putative hepatic stem cells, can differentiate into a wide range of cell types including hepatocytes, bile epithelial cells, pancreatic cells and intestinal epithelial cells. In this study, we used different growth factor combinations to induce oval cells to differentiate into mature hepatocytes. We isolated and purified oval cells utilizing selective enzymatic digestion and density gradient centrifugation. Oval cells were identified by their morphological characteristics and the strong expressions of OV-6, albumin, cytokeratin (CK)-19 and CK-7. Using a 2-step induction protocol, we demonstrated that oval cells first changed into small hepatocytes, then differentiated into mature hepatocytes. Small hepatocytes were distinguished from oval cells by their morphological features (e.g. round shape and nuclei) and the lack of CK-19 mRNA expression. Mature hepatocytes were identified by their ultrastructural traits and their expressions of albumin, CK-18, tyrosine aminotransferase (TAT), and alpha-1-antitrypsin (alpha-1-AT). Differentiated cells acquired the functional attributes of hepatocytes in that they secreted albumin and synthesized urea at a high level throughout differentiation. Oval cells can thus differentiate into cells with the morphological, phenotypic and functional characteristics of hepatocytes. This 2-step induction procedure could provide an abundant source of hepatocytes for cell transplantation and tissue engineering.

Enrichment of hepatic progenitor cells from adult mouse liver

Hepatic progenitor cells (HPCs) have been characterized in several drug-treated rodent models and in the fetal liver; however, their properties have not been fully clarified in the normal adult liver, presumably because of their relatively small population and the existence of mature hepatocytes. In an attempt to resolve this issue, we developed a new enrichment system for HPCs using their cell aggregate formation properties. Nonparenchymal cells (NPCs) derived from enzymatically digested liver cells in normal adult mouse liver were treated in a hypoxic 2-hour suspension culture under constant shaking. This procedure resulted in cell aggregate formation and almost complete elimination of mature hepatocytes. Cell aggregates were formed only in Ca(2+)-containing medium, suggesting cadherin-dependent cell-cell adhesion. In these cell aggregates, 95% consisted of vascular endothelial cells that expressed VE-cadherin. The remaining 5% consisted of rapidly proliferating, small epithelial cells that expressed alpha-fetoprotein (AFP), E-cadherin, and albumin but not cytokeratin 19 (CK19), alpha-smooth muscle actin, or VE-cadherin. These results are consistent with an immature hepatic cell phenotype. When these immature hepatic cells were cultured with 10(-7) mol/L dexamethasone and 1% dimethyl sulfoxide, the de novo expression of mature hepatocyte markers such as tryptophan-2,3-dioxygenase (TO) was induced concomitantly with the induction of morphologic characteristics such as mitochondria- and peroxisome-rich cytoplasm and bile canaliculi formation. In conclusion, our methodology allows the enrichment of immature hepatic cells from the normal adult mouse. These cells are capable of growth and maturation along the hepatocyte lineage, indicating that these cells are HPCs.

Differentiation of bone marrow cells into cells that express liver-specific genes in vitro: implication of the Notch signals in differentiation

Bone marrow (BM) stem cells have been shown to differentiate into liver cells. It remains difficult to sort and culture BM stem cells, and the gene expression of liver-specific proteins in these cells has not been fully investigated. We used a negative selective magnetic cell separation system to obtain stem cell-enriched BM cells. The cells obtained were cultured with hepatocytes or with hepatocyte growth factor (HGF), and the differentiation of BM cells into cells expressing liver-specific genes, hepatocyte nuclear factor (HNF) 1alpha, cytokeratin (CK) 8, alpha-fetoprotein (AFP), and albumin was investigated by the reverse transcription-polymerase chain reaction. We also investigated the gene expressions of Notch receptor-1 (Notch-1) and its ligand Jagged-1 in BM cell differentiation. Sorted BM cells showed positive for Sca-1 (Ataxin-1) by immunofluorescence staining. Fluorescence activated cell sorter analysis showed that 32.6% of sorted BM cells had a high level of expression of the hematopoietic stem cell marker CD90 (Thy-1). When cultured with hepatocytes, these cells expressed the liver-specific genes HNF1alpha and CK8 on culture day 3, AFP and albumin on culture day 7. When cultured with HGF (20ng/ml), the cells expressed HNF1alpha on day 3 and CK8 on day 7. Gene expressions of Notch-1 and Jagged-1 were detected in cultured BM cells on day 3. These results suggest that the negative selective magnetic cell separation system is useful for the rapid preparation of stem cell-enriched BM cells, and that the Notch signaling pathway plays a role in BM cell differentiation into a hepatocyte lineage in vitro.

Biological characteristics of rat hepatic oval cells

AIM

To observe the morphological parameters, phenotypes and evolution of hepatic oval cells in rats.

METHODS

Rat models for hepatic oval cell proliferation were established, and cell image analysis and immunohistochemical staining of hepatic oval cells were performed on the tissue sections.

RESULTS

Hepatic oval cells were characterized by small size, oval shape and ovoid nuclei. The plasma of the cells was positive for cytokeratin (CK) 19, OV6, alfa-fetal protein (AFP) and vimentin staining, and negative for leucocyte common antigen (LCA) staining. The nuclei were positive for proliferating cell nuclear antigen (PCNA). Some hepatocyte-like cells were discerned around the portal tracts and these cells were positive for CK19 and OV6 staining.

CONCLUSION

Hepatic oval cells differ from hepatocytes morphologically and express the phenotypes of ductular cells and hepatocytes. Hepatocyte-like cells are most likely the intermediate transition cells between hepatic oval cells and hepatocytes.

A selective tropism of transfused oval cells for liver

AIM: To explore the biological behaviors of hepatic oval cells after transfused into the circulation of experimental animals.

METHODS: Oval cells from male SD rat were transfused into the circulation of a female rat which were treated by a 2-AAF/CCl₄ program, through caudal vein. Sex-determining gene sry which located on Y chromosome was examined by PCR and in situ hybridization technique in liver, kidney and spleen of the experimental animals, respectively.

RESULTS: The results of the cell-transplant experiment showed that the sry gene was detectable only in the liver but not in spleen and kidney of the experimental rats, and no signals could be detected in the control animals. It can be also morphologically proved that some exogenous cells had migrated into the parenchyma of the liver and settled there.

CONCLUSION: The result means that there are exogenous cells located in the liver of the experimental animal and the localization is specific to the liver. This indicates that some "signal molecules" must exist in the circulation of the rats treated by 2-AAF/CCl₄. These "signal molecules" might play an important role in specific localization and differentiation of transfused oval cells.

Extracorporeal support of the failing liver

Successes in machine-based extracorporeal support for different organ functions stimulated research in the field of liver support approximately 50 years ago. Initial failure to improve outcome using detoxification methods like dialysis, blood and plasma exchange, or plasmapheresis over sorbents fueled interest in biologic liver support concepts using bioreactors or combined methods. New device configurations, technical improvement of existing detoxification methods, and the refinement in cell culture techniques led to a boost in research on biologic and nonbiologic approaches. Currently, many systems are in the preclinical phase or have entered clinical studies. A number of completed clinical trials have reported a favorable therapeutic impact of the most advanced solutions on the course and outcome of liver failure. Often, findings must be reconfirmed. However, current knowledge suggests that extracorporeal liver support can successfully stabilize liver function, improve the clinical condition of patients, and considerably improve survival in certain subgroups of patients with fulminant hepatic failure and acute decompensation of chronic hepatic failure. Although the initial focus of liver support methods was bridging to liver transplantation, bridging to recovery of organ function and treatment of intractable pruritus are now valuable indications.