Hepatic stem cells: from inside and outside the liver?

Functional characterization of CD49f+ hepatic stem/progenitor cells in adult mice liver

Hepatic Stem/progenitor cells (HSPCs) have gained a large amount of interest for treating acute liver disease. However, the isolation and identification of HSPCs are unclear due to the lack of cell-specific surface markers. To isolate adult HSPCs, we used cell surface-marking antibodies, including CD49f and Sca-1. Two subsets of putative HSPCs, Lin^-CD45^-Sca-1^-CD49f⁺ (CD49f⁺) and Lin^-CD45^-Sca-1⁺CD49f^- (Sca-1⁺) cells, were isolated from adult mice liver by flow cytometry. Robust proliferative activity and clonogenic activity were found in both CD49f⁺ and Sca-1⁺ cells through colony-forming tests and cell cycle analyses. Immunofluorescence staining revealed that CD49f⁺ cells expressed ALB and CK-19 while Sca-1⁺ cells expressed only ALB, indicating that CD49f⁺ cells were bipotential and capable of differentiating into hepatocyte and cholangiocyte. Consequently, PAS stain showed that differentiated CD49f⁺ and Sca-1⁺ cells synthesised glycogen, indicating they could differentiate into functional hepatocytes. mRNA expression profile indicated that both CD49f⁺ and Sca-1⁺ cells showed differential expression of genes that are associated with liver progenitor function such as Sox9 and EpCam. Moreover, two subsets of putative HSPCs were activated by DDC and we found that their abundance and proliferation increased with age. In summary, we hypothesized that CD49f⁺ cells were a type of potential HSPCs and may be utilised for clinical stem cell therapy.

The Cancer Stem Cell in Hepatocellular Carcinoma

The recognition of intra-tumoral cellular heterogeneity has given way to the concept of the cancer stem cell (CSC). According to this concept, CSCs are able to self-renew and differentiate into all of the cancer cell lineages present within the tumor, placing the CSC at the top of a hierarchical tree. The observation that these cells—in contrast to bulk tumor cells—are able to exclusively initiate new tumors, initiate metastatic spread and resist chemotherapy implies that CSCs are solely responsible for tumor recurrence and should be therapeutically targeted. Toward this end, dissecting and understanding the biology of CSCs should translate into new clinical therapeutic approaches. In this article, we review the CSC concept in cancer, with a special focus on hepatocellular carcinoma.

Participation of liver stem cells in cholangiocarcinogenesis after aflatoxin B1 exposure of glutathione S-transferase A3 knockout mice

Aflatoxin B1, arguably the most potent human carcinogen, induces liver cancer in humans, rats, trout, ducks, and so on, but adult mice are totally resistant. This resistance is because of a detoxifying enzyme, mouse glutathione S-transferase A3, which binds to and inactivates aflatoxin B1 epoxide, preventing the epoxide from binding to DNA and causing mutations. Glutathione S-transferase A3 or its analog has not been detected in any of the sensitive species, including humans. The generation of a glutathione S-transferase A3 knockout (represented as KO or -/-) mice has allowed us to study the induction of liver cancer in mice by aflatoxin B1. In contrast to the induction of hepatocellular carcinomas in other species, aflatoxin B1 induces cholangiocarcinomas in GSTA3-/- mice. In other species and in knockout mice, the induction of liver cancer is preceded by extensive proliferation of small oval cells, providing additional evidence that oval cells are bipolar stem cells and may give rise to either hepatocellular carcinoma or cholangiocarcinoma depending on the nature of the hepatocarcinogen and the species of animal. The recent development of mouse oval cell lines in our laboratory from aflatoxin B1-treated GSTA3-/- mice should provide a new venue for study of the properties and potential of putative mouse liver stem cells.

Circulating CD34+ hematopoietic stem/progenitor cells paralleled with level of viremia in patients chronically infected with hepatitis B virus

Introduction: Liver regeneration is a heterogeneous process involving proliferation of different cell types in response to injury. Bone marrow derived stem cells may be involved in this process, by making contribution to parenchymal restoration and cellular replacement. We aimed to investigate the correlation between level of circulating mobilized CD34+ hematopoietic stem progenitor cells (HSPCs) and viremia level in patients chronically infected with hepatitis B virus (HBV).

Methods: Blood samples were prospectively collected for assessing percentage and absolute counts of circulating CD34+ HSPCs and viral load level using flow cytometry and RT-PCR respectively. Patients with chronic hepatitis B (CHB) (n = 30), Entecavir (ETV) treated subjects (n = 30) and 20 age and gender matched healthy controls were enrolled in this study. Results were expressed as mean ± SD.

Results and discussion: A significant increase in circulating CD34+ HSPCs level was observed in CHB patients (5 ± 3.1, 324 ± 195 × 10³/ml) as compared to ETV treated subjects (0.57 ± 0.27,1022 ± 325) and healthy controls (0.53 ± 0.37, 694 ± 254, P < 0.001) in regards to percentage and absolute counts respectively. Levels of CD34+ HSPCs strongly and positively correlated with HBV DNA viral load levels in CHB patients (r² = 0.8417, 0.649, P < 0.001).Thus, in chronic liver disorders (CHB), when reduced regenerative capacity of hepatocytes is reached, BMSCs mobilization occurs and their level increases in peripheral blood. The level of circulating CD34+ cells in peripheral blood of CHB patients paralleled with the hepatitis B viral load.

Prospects for Adult Stem Cells in the Treatment of Liver Diseases

Hepatocytes constitute the main bulk of the liver and perform several essential functions. After injury, the hepatocytes have a remarkable capacity to regenerate and restore functionality. However, in some cases, the endogenous hepatocytes cannot replicate or restore the function, and liver transplantation, which is not exempt of complications, is required. Stem cells offer in theory the possibility of generating unlimited supply of hepatocytes in vitro due to their capacity to self-renew and differentiate when given the right cues. Stem cells isolated from an array of tissues have been investigated for their capacity to differentiate into hepatocyte-like cells in vitro and are employed in rescue experiments in vivo. Adult stem cells have gained in attractiveness over embryonic stem cells for liver cell therapy due to their origin, multipotentiality, and the possibility of autologous transplantation. This review deals with the promise and limitations of adult stem cells in clinically restoring liver functionality.

Generation of functional hepatocyte-like cells from human deciduous periodontal ligament stem cells

Human deciduous periodontal ligament stem cells have been introduced for as an easily accessible source of stem cells from dental origin. Although recent studies have revealed the ability of these stem cells in multipotential attribute, their efficiency of hepatic lineage differentiation has not been addressed so far. The aim of this study is to investigate hepatic lineage fate competence of periodontal ligament stem cells through direct media induction. Differentiation of periodontal ligament stem cells into hepatocyte-like cells was conducted by the exposure of two phase media induction. First phase was performed in the presence of hepatocyte growth factors to induce a definitive endoderm formation. In the subsequent phase, the cells were treated with oncostatin M and dexamethosone followed by insulin and transferrin to generate hepatocyte-like cells. Hepatic-related characters of the generated hepatocyte-like cells were determined at both mRNA and protein level followed by functional assays. Foremost changes observed in the generation of hepatocyte-like cells were the morphological features in which these cells were transformed from fibroblastic shape to polygonal shape. Temporal expression of hepatic markers ranging from early endodermal up to late markers were detected in the hepatocyte-like cells. Crucial hepatic markers such as glycogen storage, albumin, and urea secretion were also shown. These findings exhibited the ability of periodontal ligament stem cells of dental origin to be directed into hepatic lineage fate. These cells can be regarded as an alternative autologous source in the usage of stem cell-based treatment for liver diseases.

Cancer Stem Cell and Gastrointestinal Cancer: Current Status, Targeted Therapy and Future Implications

The cancer stem cells (CSCs) are biologically distinct subset of rare cancer cells with inherent ability of self-renewal, de-differentiation, and capacity to initiate and maintain malignant tumor growth. Studies have further reported that CSCs prime cancer recurrence and therapy resistance. Therefore, targeting CSCs to inhibit cancer progression has become an attractive anti-cancer therapeutical strategy. Recent technical advances have provided a greater appreciation of the multistep nature of the oncogenesis and also clarified that CSC concept is not universally applicable. Irrespective, the role of the CSCs in gastrointestinal (GI) cancers, responsible for the most cancer-associated death, has been widely accepted and appreciated. However, despite the tremendous progress made in the last decade in developing markers to identify CSCs, and assays to assess tumorigenic function of CSCs, it remains an area of active investigation. In current article, we review findings related to the role and identification of CSCs in GI-cancers and discuss the crucial pathways involved in regulating CSCs populations’ development and drug resistance, and use of the tumoroid culture to test novel CSCs-targeted cancer therapies.

Identification and characterization of mesenchymal-epithelial progenitor-like cells in normal and injured rat liver

In normal rat liver, thymocyte antigen 1 (Thy1) is expressed in fibroblasts/myofibroblasts and in some blood progenitor cells. Thy1-expressing cells also accumulate in the liver during impaired liver regeneration. The origin and nature of these cells are not well understood. By using RT-PCR analysis and immunofluorescence microscopy, we describe the presence of rare Thy1(+) cells in the liver lobule of normal animals, occasionally forming small collections of up to 20 cells. These cells constitute a small portion (1.7% to 1.8%) of nonparenchymal cells and reveal a mixed mesenchymal-epithelial phenotype, expressing E-cadherin, cytokeratin 18, and desmin. The most potent mitogens for mesenchymal-epithelial Thy1(+) cells in vitro are the inflammatory cytokines interferon γ, IL-1, and platelet-derived growth factor-BB, which are not produced by Thy1(+) cells. Thy1(+) cells express all typical mesenchymal stem cell and hepatic progenitor cell markers and produce growth factor and cytokine mRNA (Hgf, Il6, Tgfa, and Tweak) for proteins that maintain oval cell growth and differentiation. Under appropriate conditions, mesenchymal-epithelial cells differentiate in vitro into hepatocyte-like cells. In this study, we show that the adult rat liver harbors a small pool of endogenous mesenchymal-epithelial cells not recognized previously. In the quiescent state, these cells express both mesenchymal and epithelial cell markers. They behave like hepatic stem cells/progenitors with dual phenotype, exhibiting high plasticity and long-lasting proliferative activity.

Adult hepatocytes are generated by self-duplication rather than stem cell differentiation

The liver is thought to utilize facultative stem cells, also known as "oval cells" or "atypical ductal cells" (ADCs), for regeneration following various types of injury. However, this notion has been based largely on in vitro studies and transplantation models; where lineage tracing has been used, results have been conflicting and effect sizes have been small. Here, we used genetic and nucleoside analog-based tools to mark and track the origin and contribution of various cell populations to liver regeneration in vivo following several ADC-inducing insults. We report that, contrary to prevailing stem-cell-based models of regeneration, virtually all new hepatocytes come from preexisting hepatocytes.

Hepatic progenitor cells in canine and feline medicine: potential for regenerative strategies

New curative therapies for severe liver disease are urgently needed in both the human and veterinary clinic. It is important to find new treatment modalities which aim to compensate for the loss of parenchymal tissue and to repopulate the liver with healthy hepatocytes. A prime focus in regenerative medicine of the liver is the use of adult liver stem cells, or hepatic progenitor cells (HPCs), for functional recovery of liver disease. This review describes recent developments in HPC research in dog and cat and compares these findings to experimental rodent studies and human pathology. Specifically, the role of HPCs in liver regeneration, key components of the HPC niche, and HPC activation in specific types of canine and feline liver disease will be reviewed. Finally, the potential applications of HPCs in regenerative medicine of the liver are discussed and a potential role is suggested for dogs as first target species for HPC-based trials.

Human bone marrow mesenchymal stem cell-derived hepatocytes improve the mouse liver after acute acetaminophen intoxication by preventing progress of injury

Mesenchymal stem cells from human bone marrow (hMSC) have the potential to differentiate into hepatocyte-like cells in vitro and continue to maintain important hepatocyte functions in vivo after transplantation into host mouse livers. Here, hMSC were differentiated into hepatocyte-like cells in vitro (hMSC-HC) and transplanted into livers of immunodeficient Pfp/Rag2⁻/⁻ mice treated with a sublethal dose of acetaminophen (APAP) to induce acute liver injury. APAP induced a time- and dose-dependent damage of perivenous areas of the liver lobule. Serum levels of aspartate aminotransferase (AST) increased to similar levels irrespective of hMSC-HC transplantation. Yet, hMSC-HC resided in the damaged perivenous areas of the liver lobules short-term preventing apoptosis and thus progress of organ destruction. Disturbance of metabolic protein expression was lower in the livers receiving hMSC-HC. Seven weeks after APAP treatment, hepatic injury had completely recovered in groups both with and without hMSC-HC. Clusters of transplanted cells appeared predominantly in the periportal portion of the liver lobule and secreted human albumin featuring a prominent quality of differentiated hepatocytes. Thus, hMSC-HC attenuated the inflammatory response and supported liver regeneration after acute injury induced by acetaminophen. They hence may serve as a novel source of hepatocyte-like cells suitable for cell therapy of acute liver diseases.

Partial hepatectomy in rats results in immediate down-regulation of p27Kip1 in residual liver tissue by transcriptional and post-translational processes

PURPOSE

The cyclin-dependent kinase (Cdk) inhibitor p27Kip1 may be involved in regulating re-entry of residual hepatocytes into the cell cycle upon loss of liver tissue by partial hepatectomy (PH). As yet, changes in Kip1 expression during the initial period following PH are not well-characterized. We investigated immediate changes in Kip1 mRNA and protein levels as well as changes in Kip1 phosphorylation in liver tissue within the relevant time window between surgery and the onset of DNA synthesis at 10-12 h.

METHODS

We used real-time PCR, quantitative Western blotting, and immune histochemistry on tissue samples of adult rats obtained during or between 2 and 10 h after surgical removal of two thirds of the liver to analyze Kip1 mRNA or protein levels, respectively, or to quantify nuclear expression of Kip1.

RESULTS

Kip1 mRNA was down-regulated within 4 h after PH by 60% and remained unchanged thereafter up to 10 h. With a lag phase of 2-3 h, Kip1-protein was down-regulated to a level of 40% of the control. The level of Thr187-phosphorylated Kip1 started to increase at 4 h and reached a maximum level at 8-10 h after PH. Kip1 immunoreactivity was observed in 30% of the hepatocytes before PH. Within 6-8 h after PH, more than half of the hepatocytes lost nuclear Kip1 signals. Kip1-specific micro-RNAs (miRNA221, miRNA222) were not changed upon PH.

CONCLUSIONS

A portion of hepatocytes in adult rats constitutively express Kip1 and down-regulate Kip1 immediately upon PH. This response involves transcriptional processes (loss of Kip1 mRNA) as well as accelerated degradation of existing protein (increase in pThr187-phosphorylation mediating polyubiquitinylation and proteasomal degradation of Kip1). Kip1 down-regulation occurs precisely within the intervall between surgery and onset of DNA synthesis which supports the hypothesis that it mediates activation of G0/0S-phase Cdk/cyclin-complexes and re-entry of hepatocytes into the cell cycle.

New advances of DNA methylation in liver fibrosis, with special emphasis on the crosstalk between microRNAs and DNA methylation machinery

Epigenetics refers to the study of heritable changes in the pattern of gene expression that is controlled by a mechanism specifically not due to changes the primary DNA sequence. Well-known epigenetic mechanisms include DNA methylation, post-translational histone modifications and RNA-based mechanisms including those controlled by small non-coding RNAs (miRNAs). Recent studies have shown that epigenetic modifications orchestrate the hepatic stellate cell (HSC) activation and liver fibrosis. In this review we focus on the aberrant methylation of CpG island promoters of select genes is the prominent epigenetic mechanism to effectively silence gene transcription facilitating HSC activation and liver fibrosis. Furthermore, we also discuss epigenetic dysregulation of tumor-suppressor miRNA genes by promoter DNA methylation and the interaction of DNA methylation with miRNAs involved in the regulation of HSC activation and liver fibrosis. Recent advances in epigenetics alterations in the pathogenesis of liver fibrosis and their possible use as new therapeutic targets and biomarkers.

Recent advances in cancer stem cell research for cholangiocarcinoma

Cancer stem cells have been identified as cells with the capacity to self-renew and differentiate into multiple lineages of human malignancies. Cholangiocarcinoma is one of the most difficult intra-abdominal malignancies that can be treated using a surgical approach. Chemotherapy in addition to surgery is necessary to improve patient survival. However, its clinical benefit is limited, and, to date, no other effective anticancer drug is available for this disease. Several reports have shown the existence of cholangiocarcinoma stem cells. Cell surface antigens such as CD133, CD24, EpCAM, CD44, and others have been used to isolate cholangiocarcinoma stem cells. In general, enhanced expression of these markers in resected specimens of cholangiocarcinoma was associated with malignant potential. Distinct and specific pathways are expected to be present in cancer stem cells compared to other cancer cells that have no stem cell properties. To date, reports showing possible signaling pathways in cholangiocarcinoma stem cells are limited. More research is anticipated. Targeting therapies for surface molecular markers or specific signaling pathways of cholangiocarcinoma stem cells may be important in order to change the clinical outcome of patients with this disease. However, no clinical trial has been performed so far. This review will focus on the markers and signaling pathways used to define cholangiocarcinoma stem cells. A novel therapeutic approach of targeting cholangiocarcinoma stem cells will also be discussed.

Complement 5a stimulates hepatic stellate cells in vitro, and is increased in the plasma of patients with chronic hepatitis B

Complement 5a (C5a) is a critical modulator of liver immunity. In this study, we investigated the role of C5a and its receptor in liver fibrosis in patients with hepatitis B virus infection. We found that plasma C5a concentration was significantly increased in patients with chronic hepatitis B, particularly in those patients with higher grade and stage scores. Further analysis indicated that the increased C5a concentration was positively correlated with clinical parameters reflecting liver fibrosis severity, including type IV collagen and procollagen type III N-terminal peptide. Our in vitro data indicated that the C5a receptor is highly expressed in hepatic stellate cells (HSCs). Addition of C5a significantly activated HSCs and up-regulated α-smooth muscle actin, hyaluronic acid and type IV collagen expression. Also, addition of C5a could inhibit the spontaneous and soluble tumour necrosis factor-related apoptosis-inducing ligand-induced apoptosis of HSCs. These findings highlight the potential role of C5a in the regulation of liver fibrosis.

Advances in cell sources of hepatocytes for bioartificial liver

BACKGROUND

Orthotopic liver transplantation (OLT) is the most effective therapy for liver failure. However, OLT is severely limited by the shortage of liver donors. Bioartificial liver (BAL) shows great potential as an alternative therapy for liver failure. In recent years, progress has been made in BAL regarding genetically engineered cell lines, immortalized human hepatocytes, methods for preserving the phenotype of primary human hepatocytes, and other functional hepatocytes derived from stem cells.

DATA SOURCES

A systematic search of PubMed and ISI Web of Science was performed to identify relevant studies in English language literature using the key words such as liver failure, bioartificial liver, hepatocyte, stem cells, differentiation, and immortalization. More than 200 articles related to the cell sources of hepatocyte in BAL were systematically reviewed.

RESULTS

Methods for preserving the phenotype of primary human hepatocytes have been successfully developed. Many genetically engineered cell lines and immortalized human hepatocytes have also been established. Among these cell lines, the incorporation of BAL with GS-HepG2 cells or alginate-encapsulated HepG2 cells could prolong the survival time and improve pathophysiological parameters in an animal model of liver failure. The cBAL111 cells were evaluated using the AMC-BAL bioreactor, which could eliminate ammonia and lidocaine, and produce albumin. Importantly, BAL loading with HepLi-4 cells could significantly improve the blood biochemical parameters, and prolong the survival time in pigs with liver failure. Other functional hepatocytes differentiated from stem cells, such as human liver progenitor cells, have been successfully achieved.

CONCLUSIONS

Aside from genetically modified liver cell lines and immortalized human hepatocytes, other functional hepatocytes derived from stem cells show great potential as cell sources for BAL. BAL with safe and effective liver cells may be achieved for clinical liver failure in the near future.

Adult human liver mesenchymal stem/progenitor cells participate in mouse liver regeneration after hepatectomy

The advances in stem cell science have promoted research on their use in liver regenerative medicine. Beyond the demonstration of their ability to display metabolic functions in vitro, candidate cells should demonstrate achievable in situ differentiation and ability to participate to liver repopulation. In this work, we studied the in vivo behavior of adult liver mesenchymal stem/progenitor cells (ADHLSCs) after transplantation into immunodeficient mice. The kinetics of engraftment and in situ hepatogenic differentiation were analyzed. Response of transplanted ADHLSCs to regenerative stimulus was also evaluated. Nondifferentiated ADHLSCs were intrasplenically transplanted into SCID mice. Efficiency of transplantation was evaluated at the level of engraftment and in situ differentiation using immunohistochemistry, in situ hybridization, and RT-PCR. After bromodeoxyuridine (BrdU) implantation, proliferation of transplanted ADHLSCs in response to 20% hepatectomy was assessed using immunohistochemistry. We demonstrated that ADHLSC engraftment in the SCID mouse liver was low but remained stable up to 60 days posttransplantation, when albumin (ALB) immunopositive ADHLSCs were still detected and organized as clusters. Coexpression of ornithine transcarbamylase (OTC) demonstrated ADHLSC in situ differentiation mostly near the hepatic portal vein. After 20% hepatectomy on 1 month transplanted mice, the percentage of BrdU and human ALB immunopositive ADHLSCs increased from 3 to 28 days post-BrdU implantation to reach 31.3 ± 5.4% of the total analyzed human cells. In the current study, we demonstrate that transplanted ADHLSCs are able to differentiate in the non preconditioned SCID mouse liver mainly in the periportal area. In response to partial hepatectomy, integrated ADHLSCs proliferate and participate to recipient mouse liver regeneration.

Implication of hepatic stem cells in functional liver repopulation

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

Hepatocarcinogenesis in non-cirrhotic liver is associated with a reduced number of clonal hepatocellular patches in non-tumorous liver parenchyma

We investigated circumscribed cell proliferations in healthy livers in comparison to non-cirrhotic livers bearing hepatocellular carcinoma. Using histochemical staining for cytochrome c oxidase, the fourth complex of the respiratory chain, we visualized patch-forming descendents of regeneratively active liver cells. The clonal nature of these patches was verified by laser-capture microdissection and Sanger sequencing of the enzyme's core subunits in patches carrying marker mutations on the mtDNA. We demonstrate a highly significant increase of the patch size and also a highly significant increase in the number of patches carrying marker mutations between hepatocellular carcinoma-free and -bearing livers. Thus, the carcinoma-bearing livers accumulated more genetic damage on mtDNA than the control group. Furthermore, for the first time, we present evidence in hepatocellular carcinoma-bearing non-cirrhotic livers of a significantly reduced pool of regeneratively active liver cells that are genetically and functionally altered. The analogy to ageing-related changes is suggestive of premature ageing of stem cells in non-cirrhotic hepatocellular carcinoma-bearing liver as an early step to hepatocarcinogenesis.

Participation of liver progenitor cells in liver regeneration: lack of evidence in the AAF/PH rat model

When hepatocyte proliferation is impaired, liver progenitor cells (LPC) are activated to participate in liver regeneration. We used the 2-acetaminofluorene/partial hepatectomy (AAF/PH) model to evaluate the contribution of LPC to liver cell replacement and function restoration. Fischer rats subjected to AAF/PH (or PH alone) were investigated 7, 10 and 14 days post-hepatectomy. Liver mass recovery (LMR) was estimated, and the liver mass to body weight ratio calculated. We used serum albumin and bilirubin levels, and liver albumin mRNA levels to assess the liver function. LPC expansion was analyzed by cytokeratin 19 (CK19), glutathione S-transferase protein (GSTp) immunohistochemistry and by CK19, CD133, transforming growth factor-β1 and hepatocyte growth factor mRNA expression in livers. Cell proliferation was evaluated by Ki67 and BrdU immunostaining. Compared with PH alone where LMR was ∼100% 14 days post-PH, LMR was defective in AAF/PH rats (64.1±15.5%, P=0.0004). LPC expansion was scarce in PH livers (0.5±0.4% of CK19(+) area), but significant in AAF/PH livers (8.5±7.2% of CK19(+)), and inversely correlated to LMR (r(2)=0.63, P<0.0001). A quarter of AAF/PH animals presented liver failure (low serum albumin and high serum bilirubin) 14 days post-PH. Compared with animals with preserved function, this was associated with a lower LMR (50±6.8 vs 74.6±9.4%, P=0.0005), a decreased liver to body weight ratio (2±0.3 vs 3.5±0.6%, P=0.001), and a larger LPC expansion such as proliferating Ki67(+) LPC covered 17.4±4.2% of the liver parenchyma vs 3.1±1.5%, (P<0.0001). Amongst those, rare LPC with an intermediate hepatocyte-like phenotype were seen. Also, less than 2% of hepatocytes were engaged into the cell cycle (Ki67(+)), while more numerous (∼25% of hepatocytes) in the livers with preserved function. These observations suggest that, in this model, the efficient recovery of the liver function was ensured rather by the proliferation of mature hepatocytes than by the LPC expansion and differentiation into hepatocytes.

Impact of age on liver regeneration response to injury after partial hepatectomy in a rat model

BACKGROUND

Liver resection is a feasible treatment for multiple liver diseases. There is no evidence about the impact of age on liver regeneration.

OBJECTIVE

To assess the effect of age on liver regeneration in an experimental in vivo animal model of 70%-partial hepatectomy.

METHODS

Forty young (Y) and old (O) Wistar male rats (n = 80) were distributed into four groups [controls (C), sham operated (SO), hepatectomy 6 h (H6), and 48 h (H48)]. Different morphometric and biochemical factors, oxidative and nitrosative stress, lipid peroxidation, cytokines kinetics, and histopathologic tissular parameters were determined.

RESULTS

Early postoperative mortality was higher in aged rats (P = 0.049). Morphometric determinations, liver regeneration index, and total volume weight were favorable to young rats. Serum transaminase levels were higher in aged rats. Parameters of necrosis (measured by histopathologic injury [HI: 0-I-II-III]), regeneration (measured by bromodeoxyuridine-BrdU incorporation) and apoptosis (determined by the TDT-mediated dUTP nick end labeling-TUNEL) were well-synchronized in young rats. Parameters of oxidative stress such as reduced (GSH), oxidized (GSSG) glutathione and lipid peroxidation (measured by hepatic malondialdehyde -MDA-) were lower in young animals throughout the studied period. Nitrosative stress measured by nitric oxide (NO) end-products was higher in late stages in resected old rats. Pro-inflammatory cytokines (TNF- α) reached higher and earlier levels in aged rats while pro-regenerative cytokines (IL-6) were significantly higher in early stages for young rats and in late stages for aged rats. The levels of TGF-β were higher in young rats.

CONCLUSION

Liver regeneration is delayed and reduced in aged animals submitted to liver resection.

Liver fibrosis: mechanisms of immune-mediated liver injury

Liver fibrosis and its end-stage consequence, cirrhosis, represent the final common pathway of virtually all chronic liver diseases. Research into hepatic stellate cell activation, imbalance of the extracellular matrix synthesis and degradation and the contribution of cytokines and chemokines has further elucidated the mechanisms underlying fibrosis. Furthermore, clarification of changes in host adaptive and innate immune systems has accelerated our understanding of the association between liver inflammation and fibrosis. Continued elucidation of the mechanisms of hepatic fibrosis has provided a comprehensive model of fibrosis progression and regression. This review summarizes the current concepts of improvements that have been made in the field of fibrosis.

Biomedical and social contributions to sustainability

Over the past two or three centuries, biomedical advances have provided methods to prevent and treat infectious diseases. These changes have greatly reduced human suffering and enhanced sustainability by allowing people to live longer and healthier lives. The challenge for the coming centuries will be to ensure that these longer, healthier lives are also more productive lives. We must build on the gains of the past by translating new discoveries in regenerative medicine into therapies for degenerative and genetic diseases. Stem cells may be used to identify drugs that prevent the development of symptoms or to replace cells that have either died or lost their physiological function. In the case of genetic diseases, it may be possible to correct the genetic error. While most conditions that might be treated in these ways are common to all communities, some are more prevalent in specific races. Provision of these and other benefits depends not only on attainment of the research objectives, but also upon our ability to make treatment opportunities available throughout both developed and developing communities. The long history of researching and treating infectious diseases shows that it may take many decades to reap the full benefit of the new biological understanding.

Differentiation of bone marrow-derived mesenchymal stem cells into hepatocyte-like cells on nanofibers and their transplantation into a carbon tetrachloride-induced liver fibrosis model

There are limited data available on the effect of a physicochemical microenvironment on mesenchymal stem cell (MSC) differentiation and repopulation of the liver. Therefore, in this study nanofibers have been used to better differentiate and maintain the function and engraftment of differentiating MSCs both in vitro and in vivo. Mouse MSCs were differentiated into early (day 18) and late (day 36) hepatocyte-like cells (HLCs) in the presence or absence of ultraweb nanofibers (nano(+) and nano(-)) and their transplantation for recovery in mice with CCl(4) induced hepatic fibrosis was investigated. In the nano(+) group, hepatocyte markers-ALB and HNF4α- were elevated in a time-dependent manner; however, those were similar levels or slightly decreased in the nano(-) group from day 18 to 36. Ultrastructural studies of the differentiated cells revealed some similarities to hepatocytes. Urea production, secretion of albumin and α-fetoprotein, and metabolic activity of the CYP450 enzymes were significantly increased within in vitro differentiated HLCs on nanofibers at day 36. MSCs, early and late HLCs in both nano(-) and nano(+) culture conditions that were transplanted by an intravenous route caused a decrease in liver fibrosis when engrafted in the recipient liver and were able to differentiate into functional hepatocytes (ALB(+)), except for late HLCs in the nano(-) group. Late HLCs transplanted in the nano(+) group were more effective in rescuing liver failure, enhancing serum ALB, homing transplanted cells and undergoing functional engraftment than the other groups. These results showed that topographic properties of nanofibers enhance differentiation of HLCs from MSCs and maintain their function in long-term culture, which has implications for cell therapies.

Fetal liver hepatic progenitors are supportive stromal cells for hematopoietic stem cells

Previously we showed that the ~2% of fetal liver cells reactive with an anti-CD3epsilon monoclonal antibody support ex vivo expansion of both fetal liver and bone marrow hematopoietic stem cells (HSCs); these cells express two proteins important for HSC ex vivo expansion, IGF2, and angiopoietin-like 3. Here we show that these cells do not express any CD3 protein and are not T cells; rather, we purified these HSC-supportive stromal cells based on the surface phenotype of SCF(+)DLK(+). Competitive repopulating experiments show that SCF(+)DLK(+) cells support the maintenance of HSCs in ex vivo culture. These are the principal fetal liver cells that express not only angiopoietin-like 3 and IGF2, but also SCF and thrombopoietin, two other growth factors important for HSC expansion. They are also the principal fetal liver cells that express CXCL12, a factor required for HSC homing, and also alpha-fetoprotein (AFP), indicating that they are fetal hepatic stem or progenitor cells. Immunocytochemistry shows that >93% of the SCF(+) cells express DLK and Angptl3, and a portion of SCF(+) cells also expresses CXCL12. Thus SCF(+)DLK(+) cells are a highly homogenous population that express a complete set of factors for HSC expansion and are likely the primary stromal cells that support HSC expansion in the fetal liver.

Human Fetal Liver-Derived Stem Cell Transplantation as Supportive Modality in the Management of End-Stage Decompensated Liver Cirrhosis

Liver transplantation is the only existing modality for treating decompensated liver cirrhosis. Several factors, such as nonavailability of donors, combined with operative risks, complications associated with rejection, usage of immunosuppressive agents, and cost intensiveness, make this strategy available to only a few people. With a tremendous upsurge in the mortality rate of patients with liver disorders worldwide, there is a need to search for an alternative therapeutic tool that can combat the above limitations and serve as a supportive therapy in the management of liver diseases. Cell therapy using human fetal liver-derived stem cells can provide great potential to conservatively manage end-stage liver diseases. Therefore, the present investigation aimed to study and prove the safety and efficacy of human fetal liver-derived stem cell transplantation in patients with end-stage liver cirrhosis. Twenty-five patients with liver cirrhosis of different etiologies were infused with human fetal liver-derived stem cells (EpCAM+ve) labeled with Tc-HMPAO through hepatic artery. Our high throughput analysis using flow cytometry, RT-PCR, and cellular characterization exemplifies fetal liver cells with their high proliferation rate could be the best source for rejuvenating the diseased liver. Further, no episodes related to hepatic encephalopathy recurred in any of the subjects following hepatic stem cell transplantation. There was marked clinical improvement observed in terms of all clinical and biochemical parameters. Further, there was decrease in mean MELD score ( p < 0.01) observed in 6 months follow-up in all patients. Therapy using human fetal liver stem/progenitor cells offers a potentially supportive modality to organ transplantation in the management of liver diseases.

Architectural and immunohistochemical characterization of biliary ductules in normal human liver

The canals of Hering or biliary ductules have been described to connect the bile canaliculi with the interlobular bile ducts, and thus forming the distal part of the biliary tree. Studies in the last two decades suggested that the cells constructing these ductules could behave as hepatic progenitor cells. The canals of Hering are confined to the periportal space in the rat, while they have been reported to spread beyond the limiting plate in human liver. The distribution of the distal biliary ductules in normal human hepatic tissue has been investigated in our recent experiments. We could demonstrate the presence of interlobular connective tissue septa in a rudimentary form in healthy livers. The canals of Hering run in these septa in line with the terminal branches of the portal vein and hepatic arteries. This arrangement develops in the postnatal period but regresses after early childhood. The canals of Hering can be identified by the unique epithelial membrane antigen (EMA)-/CD56+/CD133+ immunophenotype. The canals of Hering leave the periportal space and spread into the liver parenchyma along rudimentary interlobular septa outlining the hepatic lobules. Our observations refine the original architectural description of the intraparenchymal portion of the canals of Hering in the human liver. The distinct immunophenotype supports their unique biological function.

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

The quest for liver progenitor cells: a practical point of view

Many chronic liver diseases can lead to hepatic dysfunction with organ failure. At present, orthotopic liver transplantation represents the benchmark therapy of terminal liver disease. However this practice is limited by shortage of donor grafts, the need for lifelong immunosuppression and very demanding state-of-the-art surgery. For this reason, new therapies have been developed to restore liver function, primarily in the form of hepatocyte transplantation and artificial liver support devices. While already offered in very specialized centers, both of these modalities still remain experimental. Recently, liver progenitor cells have shown great promise for cell therapy, and consequently they have attracted a lot of attention as an alternative or supportive tool for liver transplantation. These liver progenitor cells are quiescent in the healthy liver and become activated in certain liver diseases in which the regenerative capacity of mature hepatocytes and/or cholangiocytes is impaired. Although reports describing liver progenitor cells are numerous, they have not led to a consensus on the identity of the liver progenitor cell. In this review, we will discuss some of the characteristics of these cells and the different ways that have been used to obtain these from rodents. We will also highlight the challenges that researchers are facing in their quest to identify and use liver progenitor cells.

Comparative study of technique to obtain stem cells from bone marrow collection between the iliac crest and the femoral epiphysis in rabbits

PURPOSE: To assess the technique for the collection of rabbit bone marrow stem cells from different regions to be used as an experimental model in regenerative medicine. METHODS: Thirty rabbits were allocated into 2 groups: GROUP A, n=8, animals that underwent bone marrow blood (BMB) harvesting from the iliac crest; and GROUP B: including 22 rabbits that underwent BMB harvesting from the femur epiphysis. After harvesting, mononuclear cells were isolated by density gradient centrifugation (Ficoll - Histopaque). The number of mononuclear cells per ml was counted in a Neubauer chamber and cell viability was checked through Tripan Blue method. RESULTS: Harvesting from the iliac crest yielded an average of 1 ml of BMB and 3,6.10(6) cells/ml over 1 hour of surgery, whereas an average of 3ml of BMB and 11,79.10(6) cells./ml were obtained in 30 min from the femur epiphysis with a reduced animal death rate. CONCLUSION: The analysis for the obtention of a larger number of mononuclear cells/ml from rabbit bone marrow blood was more satisfactory in the femur epiphysis than in the iliac crest.

Regeneration of multilineage skin epithelia by differentiated keratinocytes

Although homeostasis of rapidly renewing tissues like skin epithelia is maintained by stem cells, the committed progeny of stem cells in the basal layer of epidermis retain regenerative potential and are capable of forming epidermis in response to environmental cues. It is not clear, however, at what point within the epidermal lineage keratinocytes lose this regenerative potential. In this study, we examined the extent of tissue formation by post-mitotic differentiated keratinocytes. We show that cultures of mouse keratinocytes that were, by all measures, differentiated were able to reform a self-renewing, hair-bearing skin when transplanted onto suitable sites in vivo. Genetic labeling and lineage-tracing studies in combination with an involucrin-driven Cre/lox reporter system confirmed that transplanted differentiated keratinocytes were indeed the source of the regenerated skin. More importantly, analysis of early stages of skin regeneration showed hallmarks of dedifferentiation of transplanted differentiated keratinocytes. These data indicate that commitment to differentiation does not prohibit cells from re-entering the cell cycle, de-differentiating, and acquiring "stemness". These findings suggest that epidermis can use different strategies for homeostasis and tissue regeneration.

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.

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.

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Gallbladder epithelial cells that engraft in mouse liver can differentiate into hepatocyte-like cells

We tested the hypothesis that well-differentiated gallbladder epithelial cells (GBECs) are capable of engrafting and surviving in murine liver and acquire phenotypic characteristics of hepatocytes. GBECs isolated from transgenic mice that constitutively express green fluorescent protein (GFP) were either cultured before transplantation or transplanted immediately following isolation. Recipient mice with severe-combined immunodeficiency underwent retrorsine treatment and either partial hepatectomy before transplantation or carbon tetrachloride treatment following transplantation. From 1 to 4 months following transplantation, the livers of recipient mice contained discrete colonies of GFP(+) cells. Most GFP(+) cells surrounded vesicles, were epithelial cell-like in morphology, and expressed the biliary epithelial markers cytokeratin 19 and carbonic anhydrase IV. Subpopulations of GFP(+) cells resembled hepatocytes morphologically and expressed the hepatocyte-specific markers connexin-32 and hepatic nuclear factor-4alpha, but not cytokeratin 19 or carbonic anhydrase IV. At 4 months, cells in GFP(+) colonies were not actively proliferating as determined by proliferating cell nuclear antigen expression. Thus, GBECs are capable of engrafting and surviving in damaged mouse livers, and some can differentiate into cells with hepatocyte-like features. These findings suggest that environmental cues in the recipient liver are sufficient to allow a subpopulation of donor GBECs to differentiate into hepatocyte-like cells in the absence of exogenous transcriptional reprogramming. GBECs might be used as donor cells in a cell transplantation approach for the treatment of liver disease.

p75 Neurotrophin receptor signaling regulates hepatic myofibroblast proliferation and apoptosis in recovery from rodent liver fibrosis

Hepatic myofibroblast apoptosis is critical to resolution of liver fibrosis. We show that human hepatic myofibroblasts co-express p75(NTR) (p75 neurotrophin receptor) and sortilin, thus facilitating differential responses to mature and pro nerve growth factor (proNGF). Although mature NGF is proapoptotic, proNGF protects human hepatic myofibroblasts from apoptosis. Moreover, in recovery from experimental liver fibrosis, the decrease in proNGF parallels loss of hepatic myofibroblasts by apoptosis. Macrophage-derived matrix metalloproteinase 7 (MMP7) cleaves proNGF in a concentration-dependent manner, and its expression in the liver coincides with falling proNGF levels. To define the dominant effect of p75(NTR)-mediated events in experimental liver fibrosis, we have used a mouse lacking the p75(NTR) ligand-binding domain but expressing the intracellular domain. We show that absence of p75(NTR) ligand-mediated signals leads to significantly retarded architectural resolution and reduced hepatic myofibroblast loss by apoptosis. Lack of the ligand-competent p75(NTR) limits hepatocyte and oval cell proliferative capacity in vivo without preventing hepatic stellate cell transdifferentiation.

CONCLUSION

NGF species have a differential effect on hepatic myofibroblast survival. Our data suggest that cleavage of proNGF by MMP7 during the early phase of recovery from liver fibrosis alters the pro/mature NGF balance to facilitate hepatic myofibroblast loss. Whereas fibrosis develops in the absence of p75(NTR) signaling, the dominant effects of loss of p75(NTR) ligand-mediated events are the retardation of liver fibrosis resolution via regulation of hepatic myofibroblast proliferation and apoptosis, and the reduction of hepatocyte and oval cell proliferation.

Stem cells in liver regeneration, fibrosis and cancer: the good, the bad and the ugly

The worldwide shortage of donor livers to transplant end stage liver disease patients has prompted the search for alternative cell therapies for intractable liver diseases, such as acute liver failure, cirrhosis and hepatocellular carcinoma (HCC). Under normal circumstances the liver undergoes a low rate of hepatocyte 'wear and tear' renewal, but can mount a brisk regenerative response to the acute loss of two-thirds or more of the parenchymal mass. A body of evidence favours placement of a stem cell niche in the periportal regions, although the identity of such stem cells in rodents and man is far from clear. In animal models of liver disease, adopting strategies to provide a selective advantage for transplanted hepatocytes has proved highly effective in repopulating recipient livers, but the poor success of today's hepatocyte transplants can be attributed to the lack of a clinically applicable procedure to force a similar repopulation of the human liver. The activation of bipotential hepatic progenitor cells (HPCs) is clearly vital for survival in many cases of acute liver failure, and the signals that promote such reactions are being elucidated. Bone marrow cells (BMCs) make, at best, a trivial contribution to hepatocyte replacement after damage, but other BMCs contribute to the hepatic collagen-producing cell population, resulting in fibrotic disease; paradoxically, BMC transplantation may help alleviate established fibrotic disease. HCC may have its origins in either hepatocytes or HPCs, and HCCs, like other solid tumours appear to be sustained by a minority population of cancer stem cells.

Transplantation of urokinase-type plasminogen activator gene-modified bone marrow-derived liver stem cells reduces liver fibrosis in rats

BACKGROUND

Bone marrow-derived liver stem cells (BDLSCs) are very robust cells that can differentiate into liver epithelial cells. These stem cells are promising targets for gene therapy treatment of liver diseases. Liver fibrosis results from chronic liver damage characterized by an accumulation of extracellular matrix (ECM) and levels of urokinase-type plasminogen activator (uPA) play an important role in ECM degradation. In the present study, we investigated the therapeutic effects of uPA gene-modified BDLSC transplantation on carbon tetrachloride (CCl4)-induced liver fibrosis in rats.

METHODS

BDLSCs were obtained from the bone marrow of cholestatic rats. These stem cells were selected and proliferated in medium containing 5% cholestatic serum. BDLSCs transfected with adenovirus-mediated human urokinase-plasminogen activator were transplanted into rats with CCl(4)-induced hepatic fibrosis. Liver function and the area of hepatic fibrosis were correlated with the development and prognosis of hepatic fibrosis.

RESULTS

Hepatocyte-like colony-forming units were formed by bone marrow cells after 2 weeks in culture. In the uPA gene-modified BDLSC group, the areas of hepatic fibrosis were smaller and liver function was markedly ameliorated compared to controls. The expression of alpha-smooth muscle actin protein, transforming growth factor-beta1 protein and collagen types I and III mRNA were downregulated. By contrast, the levels of matrix metalloproteinases-2, -3 and -9 mRNA, hepatic growth factor mRNA and proliferating cell nuclear antigen protein increased.

CONCLUSIONS

Transplantation of uPA gene-modified BDLSCs may suppress hepatic fibrosis and ameliorate liver function.

Recent progress on tissue-resident adult stem cell biology and their therapeutic implications

Recent progress in the field of the stem cell research has given new hopes to treat and even cure diverse degenerative disorders and incurable diseases in human. Particularly, the identification of a rare population of adult stem cells in the most tissues/organs in human has emerged as an attractive source of multipotent stem/progenitor cells for cell replacement-based therapies and tissue engineering in regenerative medicine. The tissue-resident adult stem/progenitor cells offer the possibility to stimulate their in vivo differentiation or to use their ex vivo expanded progenies for cell replacement-based therapies with multiple applications in human. Among the human diseases that could be treated by the stem cell-based therapies, there are hematopoietic and immune disorders, multiple degenerative disorders, such as Parkinson's and Alzheimer's diseases, type 1 or 2 diabetes mellitus as well as eye, liver, lung, skin and cardiovascular disorders and aggressive and metastatic cancers. In addition, the genetically-modified adult stem/progenitor cells could also be used as delivery system for expressing the therapeutic molecules in specific damaged areas of different tissues. Recent advances in cancer stem/progenitor cell research also offer the possibility to targeting these undifferentiated and malignant cells that provide critical functions in cancer initiation and progression and disease relapse for treating the patients diagnosed with the advanced and metastatic cancers which remain incurable in the clinics with the current therapies.

Stem cells for end stage liver disease: How far have we got?

End stage liver disease (ESLD) is a health problem worldwide. Liver transplantation is currently the only effective therapy, but its many drawbacks include a shortage of donors, operative damage, risk of rejection and in some cases recidivism of the pre-transplant disease. These factors account for the recent growing interest in regenerative medicine. Experiments have sought to identify an optimal source of stem cells, sufficient to generate large amounts of hepatocytes to be used in bioartificial livers or injected in vivo to repair the diseased organ. This update aims to give non-stem cell specialists an overview of the results obtained to date in this fascinating field of biomedical research.

Liver fibrosis

Liver damage leads to an inflammatory response and to the activation and proliferation of mesenchymal cell populations within the liver which remodel the extracellular matrix as part of an orchestrated wound-healing response. Chronic damage results in a progressive accumulation of scarring proteins (fibrosis) that, with increasing severity, alters tissue structure and function, leading to cirrhosis and liver failure. Efforts to modulate the fibrogenesis process have focused on understanding the biology of the heterogeneous liver fibroblast populations. The fibroblasts are derived from sources within and out with the liver. Fibroblasts expressing alpha-smooth muscle actin (myofibroblasts) may be derived from the transdifferentiation of quiescent hepatic stellate cells. Other fibroblasts emerge from the portal tracts within the liver. At least a proportion of these cells in diseased liver originate from the bone marrow. In addition, fibrogenic fibroblasts may also be generated through liver epithelial (hepatocyte and biliary epithelial cell)-mesenchymal transition. Whatever their origin, it is clear that fibrogenic fibroblast activity is sensitive to (and may be active in) the cytokine and chemokine profiles of liver-resident leucocytes such as macrophages. They may also be a component driving the regeneration of tissue. Understanding the complex intercellular interactions regulating liver fibrogenesis is of increasing importance in view of predicted increases in chronic liver disease and the current paucity of effective therapies.

Differentially expressed genes in a porcine adult hepatic stem-like cell line and their expression in developing and regenerating liver

To identify differentially expressed genes in adult hepatic stem cells, we performed suppression-subtractive hybridization (SSH) between adult porcine hepatic stem-like cells (HSLCs) and hepatocytes, and the expression of selected genes was assessed in porcine fetal livers and regenerating liver in an 80% hepatectomy model. SSH and subsequent differential screening selected 39 clones that were expressed differentially in HSLCs, including six known genes, 10 unknown genes, one unidentified gene and some chimeric fragments. Four of these genes showed significantly higher expression in HSLCs than in mature hepatocytes: anti-leukoproteinase, matrix Gla protein, amyloid-beta precursor protein (APP) and dickkopf-3 (DKK-3). Among them, the mRNA expression of APP and DKK-3 was significantly higher in fifth GW fetal liver than in seventh and thirteenth GW fetal and adult livers, unlike the expression patterns of alpha-fetoprotein (AFP) or albumin. These mRNAs were detected in the parenchyma of fifth GW fetal liver, whereas in normal adult liver possible expression was limited to the periportal area. On the other hand, immunohistochemistry, Masson's trichrome staining and silver impregnation demonstrated APP and DKK-3 proteins in fifth GW fetal liver in which intralobular bile ducts and hepatic plates had not completely developed. DKK-3 and AFP mRNAs were upregulated on the seventh day (7D) after 80% hepatectomy. In the liver tissue, DKK-3 and AFP proteins were detected in mesenchymal cells in the periportal area and parenchyma, respectively. These data for DKK-3 expression in adult livers suggest the possible presence of adult HSLCs in the periportal area. The pattern of histological staining suggested that 7D liver was in the process of regeneration, showing a character similar to the fifth GW fetal liver. It is speculated that DKK-3 is upregulated in immature and developing livers, and has possible involvement in hepatic differentiation and liver regeneration.