Paracrine signals from mesenchymal cell populations govern the expansion and differentiation of human hepatic stem cells to adult liver fates

Hepatic stem cell niches

Stem cell niches are special microenvironments that maintain stem cells and control their behavior to ensure tissue homeostasis and regeneration throughout life. The liver has a high regenerative capacity that involves stem/progenitor cells when the proliferation of hepatocytes is impaired. In recent years progress has been made in the identification of potential hepatic stem cell niches. There is evidence that hepatic progenitor cells can originate from niches in the canals of Hering; in addition, the space of Disse may also serve as a stem cell niche during fetal hematopoiesis and constitute a niche for stellate cells in adults.

Hepatic stellate cells in liver development, regeneration, and cancer

Hepatic stellate cells are liver-specific mesenchymal cells that play vital roles in liver physiology and fibrogenesis. They are located in the space of Disse and maintain close interactions with sinusoidal endothelial cells and hepatic epithelial cells. It is becoming increasingly clear that hepatic stellate cells have a profound impact on the differentiation, proliferation, and morphogenesis of other hepatic cell types during liver development and regeneration. In this Review, we summarize and evaluate the recent advances in our understanding of the formation and characteristics of hepatic stellate cells, as well as their function in liver development, regeneration, and cancer. We also discuss how improved knowledge of these processes offers new perspectives for the treatment of patients with liver diseases.

Discrete nature of EpCAM+ and CD90+ cancer stem cells in human hepatocellular carcinoma

Recent evidence suggests that hepatocellular carcinoma (HCC) is organized by a subset of cells with stem cell features (cancer stem cells; CSCs). CSCs are considered a pivotal target for the eradication of cancer, and liver CSCs have been identified by the use of various stem cell markers. However, little information is known about the expression patterns and characteristics of marker-positive CSCs, hampering the development of personalized CSC-targeted therapy. Here, we show that CSC markers EpCAM and CD90 are independently expressed in liver cancer. In primary HCC, EpCAM+ and CD90+ cells resided distinctively, and gene-expression analysis of sorted cells suggested that EpCAM+ cells had features of epithelial cells, whereas CD90+ cells had those of vascular endothelial cells. Clinicopathological analysis indicated that the presence of EpCAM+ cells was associated with poorly differentiated morphology and high serum alpha-fetoprotein (AFP), whereas the presence of CD90+ cells was associated with a high incidence of distant organ metastasis. Serial xenotransplantation of EpCAM+ /CD90+ cells from primary HCCs in immune-deficient mice revealed rapid growth of EpCAM+ cells in the subcutaneous lesion and a highly metastatic capacity of CD90+ cells in the lung. In cell lines, CD90+ cells showed abundant expression of c-Kit and in vitro chemosensitivity to imatinib mesylate. Furthermore, CD90+ cells enhanced the motility of EpCAM+ cells when cocultured in vitro through the activation of transforming growth factor beta (TGF-β) signaling, whereas imatinib mesylate suppressed TGFB1 expression in CD90+ cells as well as CD90+ cell-induced motility of EpCAM+ cells.

CONCLUSION

Our data suggest the discrete nature and potential interaction of EpCAM+ and CD90+ CSCs with specific gene-expression patterns and chemosensitivity to molecular targeted therapy. The presence of distinct CSCs may determine the clinical outcome of HCC.

Sal-like protein 4 (SALL4), a stem cell biomarker in liver cancers

Liver cancers, including hepatocellular carcinomas (HCCs), cholangiocarcinomas (CCs), and fibrolamellar HCCs (FL-HCCs) are among the most common cancers worldwide and are associated with a poor prognosis. Investigations of genes important in liver cancers have focused on Sal-like protein 4 (SALL4), a member of a family of zinc finger transcription factors. It is a regulator of embryogenesis, organogenesis, pluripotency, can elicit reprogramming of somatic cells, and is a marker of stem cells. We found it expressed in normal murine hepatoblasts, normal human hepatic stem cells, hepatoblasts and biliary tree stem cells, but not in mature parenchymal cells of liver or biliary tree. It was strongly expressed in surgical specimens of human HCCs, CCs, a combined hepatocellular and cholangiocarcinoma, a FL-HCC, and in derivative, transplantable tumor lines in immune-compromised hosts. Bioinformatics analyses indicated that elevated expression of SALL4 in tumors is associated with poor survival of HCC patients. Experimental manipulation of SALL4's expression results in changes in proliferation versus differentiation in human HCC cell lines in vitro and in vivo in immune-compromised hosts. Virus-mediated gene transfer of SALL4 was used for gain- and loss-of-function analyses in the cell lines. Significant growth inhibition in vitro and in vivo, accompanied by an increase in differentiation occurred with down-regulation of SALL4. Overexpression of SALL4 resulted in increased cell proliferation in vitro, correlating with an increase in expression of cytokeratin19 (CK19), epithelial cell adhesion molecules (EpCAM), and adenosine triphosphate (ATP)-binding cassette-G2 (ABCG2).

CONCLUSION

SALL4's expression is an indicator of stem cells, a prognostic marker in liver cancers, correlates with cell and tumor growth, with resistance to 5-FU, and its suppression results in differentiation and slowed tumor growth. SALL4 is a novel therapeutic target for liver cancers.

Successful transplantation of human hepatic stem cells with restricted localization to liver using hyaluronan grafts

Cell therapies are potential alternatives to organ transplantation for liver failure or dysfunction but are compromised by inefficient engraftment, cell dispersal to ectopic sites, and emboli formation. Grafting strategies have been devised for transplantation of human hepatic stem cells (hHpSCs) embedded into a mix of soluble signals and extracellular matrix biomaterials (hyaluronans, type III collagen, laminin) found in stem cell niches. The hHpSCs maintain a stable stem cell phenotype under the graft conditions. The grafts were transplanted into the livers of immunocompromised murine hosts with and without carbon tetrachloride treatment to assess the effects of quiescent versus injured liver conditions. Grafted cells remained localized to the livers, resulting in a larger bolus of engrafted cells in the host livers under quiescent conditions and with potential for more rapid expansion under injured liver conditions. By contrast, transplantation by direct injection or via a vascular route resulted in inefficient engraftment and cell dispersal to ectopic sites. Transplantation by grafting is proposed as a preferred strategy for cell therapies for solid organs such as the liver.

Isolation of viable human hepatic progenitors from adult livers is possible even after 48 hours of cold ischemia

Liver transplantation, utilized routinely for end-stage liver disease, has been constrained by the paucity of organ donors, and is being complemented by alternative strategies such as liver cell transplantation. One of the most promising forms of liver cell transplantation is hepatic stem cell therapies, as the number of human hepatic stem cells (hHpSCs) and other early hepatic progenitor cells (HPCs) are sufficient to provide treatment for multiple patients from a single liver source. In the present study, human adult livers were exposed to cold ischemia and then processed after <24 or 48 h. Cells positive for epithelial cell adhesion molecule (EpCAM), a marker on early lineage stage HPCs, were immunoselected and counted. Approximately 100,000 EpCAM(+) cells/gram of tissue was obtained from surgical resection of livers subjected to cold ischemia up to 24 h and comparable numbers, albeit somewhat lower, were obtained from those exposed to 48 h of cold ischemia. The yields are similar to those reported from livers with minimal exposure to ischemia. When cultured on plastic dishes and in Kubota's Medium, a serum-free medium designed for early lineage stage HPCs, colonies of rapidly expanding cells formed. They were confirmed to be probable hHpSCs by their ability to survive and expand on plastic and in Kubota's Medium for months, by co-expression of EpCAM and neural cell adhesion molecule, minimal if any albumin expression, with EpCAM found throughout the cells, and no expression of alpha-fetoprotein. The yields of viable EpCAM(+) cells were surprisingly large, and the numbers from a single donor liver are sufficient to treat approximately 50-100 patients given the numbers of EpCAM(+) cells currently used in hepatic stem cell therapies. Thus, cold ischemic livers for up to 48 h are a new source of cells that might be used for liver cell therapies.

Umbilical cord blood mesenchymal stem cells protect amyloid-β42 neurotoxicity via paracrine

AIM: To understand the neuroprotective mechanism of human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) against amyloid-β42 (Aβ42) exposed rat primary neurons.

METHODS: To evaluate the neuroprotective effect of hUCB-MSCs, the cells were co-cultured with Aβ42-exposed rat primary neuronal cells in a Transwell apparatus. To assess the involvement of soluble factors released from hUCB-MSCs in neuroprotection, an antibody-based array using co-cultured media was conducted. The neuroprotective roles of the identified hUCB-MSC proteins was assessed by treating recombinant proteins or specific small interfering RNAs (siRNAs) for each candidate protein in a co-culture system.

RESULTS: The hUCB-MSCs secreted elevated levels of decorin and progranulin when co-cultured with rat primary neuronal cells exposed to Aβ42. Treatment with recombinant decorin and progranulin protected from Aβ42-neurotoxicity in vitro. In addition, siRNA-mediated knock-down of decorin and progranulin production in hUCB-MSCs reduced the anti-apoptotic effects of hUCB-MSC in the co-culture system.

CONCLUSION: Decorin and progranulin may be involved in anti-apoptotic activity of hUCB-MSCs exposed to Aβ42.

Multipotent stem/progenitor cells in the human foetal biliary tree

BACKGROUND & AIMS

Biliary tree, liver, and pancreas share a common embryological origin. We previously demonstrated the presence of stem/progenitor cells of endodermal origin in the adult human extrahepatic biliary tree. This study evaluated the human foetal biliary trees as sources of stem/progenitor cells of multiple endodermal-derived mature fates.

METHODS

Human foetal intrahepatic and extrahepatic biliary tree tissues and isolated cells were tested for cytoplasmic and surface markers of stem cells and committed progenitors, as well as endodermal transcription factors requisite for a liver versus pancreatic fate. In vitro and in vivo experiments were conducted to evaluate the potential mature fates of differentiation.

RESULTS

Foetal biliary tree cells proliferated clonogenically for more than 1 month on plastic in a serum-free Kubota medium. After culture expansion, cells exhibited multipotency and could be restricted to certain lineages under defined microenvironments, including hepatocytes, cholangiocytes, and pancreatic islet cells. Transplantation of foetal biliary tree cells into the livers of immunodeficient mice resulted in effective engraftment and differentiation into mature hepatocytes and cholangiocytes.

CONCLUSIONS

Foetal biliary trees contain multipotent stem/progenitor cells comparable with those in adults. These cells can be easily expanded and induced in vitro to differentiate into liver and pancreatic mature fates, and engrafted and differentiated into mature cells when transplanted in vivo. These findings further characterise the development of these stem/progenitor cell populations from foetuses to adults, which are thought to contribute to liver and pancreas organogenesis throughout life.

Organotypic liver culture models: meeting current challenges in toxicity testing

Prediction of chemical-induced hepatotoxicity in humans from in vitro data continues to be a significant challenge for the pharmaceutical and chemical industries. Generally, conventional in vitro hepatic model systems (i.e. 2-D static monocultures of primary or immortalized hepatocytes) are limited by their inability to maintain histotypic and phenotypic characteristics over time in culture, including stable expression of clearance and bioactivation pathways, as well as complex adaptive responses to chemical exposure. These systems are less than ideal for longer-term toxicity evaluations and elucidation of key cellular and molecular events involved in primary and secondary adaptation to chemical exposure, or for identification of important mediators of inflammation, proliferation and apoptosis. Progress in implementing a more effective strategy for in vitro-in vivo extrapolation and human risk assessment depends on significant advances in tissue culture technology and increasing their level of biological complexity. This article describes the current and ongoing need for more relevant, organotypic in vitro surrogate systems of human liver and recent efforts to recreate the multicellular architecture and hemodynamic properties of the liver using novel culture platforms. As these systems become more widely used for chemical and drug toxicity testing, there will be a corresponding need to establish standardized testing conditions, endpoint analyses and acceptance criteria. In the future, a balanced approach between sample throughput and biological relevance should provide better in vitro tools that are complementary with animal testing and assist in conducting more predictive human risk assessment.

Hyaluronan-supplemented buffers preserve adhesion mechanisms facilitating cryopreservation of human hepatic stem/progenitor cells

The supply of human hepatic stem cells (hHpSCs) and other hepatic progenitors has been constrained by the limited availability of liver tissues from surgical resections, the rejected organs from organ donation programs, and the need to use cells immediately. To facilitate accessibility to these precious tissue resources, we have established an effective method for serum-free cryopreservation of the cells, allowing them to be stockpiled and stored for use as an off-the-shelf product for experimental or clinical programs. The method involves use of buffers, some serum-free, designed for cryopreservation and further supplemented with hyaluronans (HA) that preserve adhesion mechanisms facilitating postthaw culturing of the cells and preservation of functions. Multiple cryopreservation buffers were found to yield high viabilities (80-90%) of cells on thawing of the progenitor cells. Serum-free CS10 supplemented with 0.05% hyaluronan proved the most effective, both in terms of viabilities of cells on thawing and in yielding cell attachment and formation of expanding colonies of cells that stably maintain the stem/progenitor cell phenotype. Buffers to which 0.05 or 0.1% HAs were added showed cells postthaw to be phenotypically stable as stem/progenitors, as well as having a high efficiency of attachment and expansion in culture. Success correlated with improved expression of adhesion molecules, particularly CD44, the hyaluronan receptor, E-cadherin, β4 integrin in hHpSCs, and β1 integrins in hepatoblasts. The improved methods in cryopreservation offer more efficient strategies for stem cell banking in both research and potential therapy applications.

Human umbilical cord mesenchymal stem cells improve liver function and ascites in decompensated liver cirrhosis patients

Decompensated liver cirrhosis (LC), a life-threatening complication of chronic liver disease, is one of the major indications for liver transplantation. Recently, mesenchymal stem cell (MSC) transfusion has been shown to lead to the regression of liver fibrosis in mice and humans. This study examined the safety and efficacy of umbilical cord-derived MSC (UC-MSC) in patients with decompensated LC. A total of 45 chronic hepatitis B patients with decompensated LC, including 30 patients receiving UC-MSC transfusion, and 15 patients receiving saline as the control, were recruited; clinical parameters were detected during a 1-year follow-up period. No significant side-effects and complications were observed in either group. There was a significant reduction in the volume of ascites in patients treated with UC-MSC transfusion compared with controls (P < 0.05). UC-MSC therapy also significantly improved liver function, as indicated by the increase of serum albumin levels, decrease in total serum bilirubin levels, and decrease in the sodium model for end-stage liver disease scores. UC-MSC transfusion is clinically safe and could improve liver function and reduce ascites in patients with decompensated LC. UC-MSC transfusion, therefore, might present a novel therapeutic approach for patients with decompensated LC.

The biliary tree--a reservoir of multipotent stem cells

The biliary tree is composed of intrahepatic and extrahepatic bile ducts, lined by mature epithelial cells called cholangiocytes, and contains peribiliary glands deep within the duct walls. Branch points, such as the cystic duct, perihilar and periampullar regions, contain high numbers of these glands. Peribiliary glands contain multipotent stem cells, which self-replicate and can differentiate into hepatocytes, cholangiocytes or pancreatic islets, depending on the microenvironment. Similar cells-presumably committed progenitor cells-are found in the gallbladder (which lacks peribiliary glands). The stem and progenitor cell characteristics indicate a common embryological origin for the liver, biliary tree and pancreas, which has implications for regenerative medicine as well as the pathophysiology and oncogenesis of midgut organs. This Perspectives article describes a hypothetical model of cell lineages starting in the duodenum and extending to the liver and pancreas, and thought to contribute to ongoing organogenesis throughout life.

Can case study approaches speed implementation of the NRC report: "toxicity testing in the 21st century: a vision and a strategy?"

The 2007 report "Toxicity Testing in the 21st Century: A Vision and a Strategy" argued for a change in toxicity testing for environmental agents and discussed federal funding mechanisms that could be used to support this transformation within the USA. The new approach would test for in vitro perturbations of toxicity pathways using human cells with high-throughput testing platforms. The NRC report proposed a deliberate timeline, spanning about 20 years, to implement a wholesale replacement of current in-life toxicity test approaches focused on apical responses with in vitro assays. One approach to accelerating implementation is to focus on well-studied prototype compounds with known toxicity pathway targets. Through a series of carefully executed case studies with four or five pathway prototypes, the various steps required for implementation of an in vitro toxicity pathway approach to risk assessment could be developed and refined. In this article, we discuss alternative approaches for implementation and also outline advantages of a case study approach and the manner in which the case studies could be pursued using current methodologies. A case study approach would be complementary to recently proposed efforts to map the human toxome, while representing a significant extension toward more formal risk assessment compared to the profiling and prioritization approaches inherent in programs such as the EPA's ToxCast effort.

Abnormal WT1 expression in human fetuses with bilateral renal agenesis and cardiac malformations

BACKGROUND

Bilateral renal agenesis has multiple etiologies. Animal models have provided useful information on possible causes of this condition, but its etiology in humans is less clear. We recently described autopsy findings of two human fetuses with bilateral renal agenesis and abnormal expression of WT1 (Wilms tumor 1) in liver mesothelium.

METHODS

We have identified 14 additional fetuses with bilateral renal agenesis from autopsies performed in our institution over the past 10 years and subjected archival liver biopsy specimens from these cases to immunohistochemistry for WT1, as well as α-smooth muscle actin (α-SMA) and desmin to assess liver mesenchymal abnormalities.

RESULTS

Six of seven fetuses with combined bilateral renal agenesis and cardiac anomalies showed abnormalities of WT1 expression in liver mesothelial cells, which was not seen in other fetuses with bilateral renal agenesis. Except in one case, the fetuses with renal agenesis and cardiac defects also showed liver mesenchymal anomalies (assessed by increased α-SMA expression), which was not present in other renal agenesis fetuses.

CONCLUSIONS

WT1 is widely expressed in mesothelial cells during development, and we hypothesized that some of the defects are caused by abnormal function of mesenchyme derived from mesothelial cells, similar to the mesothelium-derived defects proposed in animal models. The methods we used are available to many laboratories and can be applied to archival paraffin tissue blocks. We suggest that future similar studies could help to expand the understanding of renal agenesis in humans and could help to subclassify this condition. This would be useful in patient management and counseling.

Role of stem cell factor and granulocyte colony-stimulating factor in remodeling during liver regeneration

Functional pluripotent characteristics have been observed in specific subpopulations of hepatic cells that express some of the known cholangiocyte markers. Although evidence indicates that specific cytokines, granulocyte macrophage colony-stimulating factors (GM-CSFs), and stem cell factors (SCFs) may be candidate treatments for liver injury, the role of these cytokines in intrahepatic biliary epithelium remodeling is unknown. Thus, our aim was to characterize the specific cytokines that regulate the remodeling potentials of cholangiocytes after 70% partial hepatectomy (PH). The expression of the cytokines and their downstream signaling molecules was studied in rats after 70% PH by immunoblotting and in small and large murine cholangiocyte cultures (SMCCs and LMCCs) by immunocytochemistry and real-time polymerase chain reaction (PCR). There was a significant, stable increase in SCF and GM-CSF levels until 7 days after PH. Real-time PCR analysis revealed significant increases of key remodeling molecules, such as S100 calcium-binding protein A4 (S100A4) and miR-181b, after SCF plus GM-CSF administration in SMCCs. SMCCs produced significant amounts of soluble and bound SCFs and GM-CSFs in response to transforming growth factor-beta (TGF-β). When SMCCs were incubated with TGF-β plus anti-SCF+GM-CSF antibodies, there was a significant decrease in S100A4 expression. Furthermore, treatment of SMCCs with SCF+GM-CSF significantly increased matrix metalloproteinases (MMP-2 and MMP-9) messenger RNA as well as miR-181b expression, along with a reduction of metalloproteinase inhibitor 3. Levels of MMP-2, MMP-9, and miR-181b were also up-regulated in rat liver and isolated cholangiocytes after PH.

CONCLUSION

Our data suggest that altered expression of SCF+GM-CSF after PH can contribute to biliary remodeling (e.g., post-transplantation) by functional deregulation of the activity of key signaling intermediates involved in cell expansion and multipotent differentiation.

Stem cells in liver diseases and cancer: recent advances on the path to new therapies

Stem cells have potential for therapy of liver diseases, but may also be involved in the formation of liver cancer. Recently, the American Association for the Study of Liver Diseases Henry M. and Lillian Stratton Basic Research Single Topic Conference "Stem Cells in Liver Diseases and Cancer: Discovery and Promise" brought together a diverse group of investigators to define the status of research on stem cells and cancer stem cells in the liver and identify problems and solutions on the path to clinical translation. This report summarizes the outcomes of the conference and provides an update on recent research advances. Progress in liver stem cell research includes isolation of primary liver progenitor cells (LPCs), directed hepatocyte differentiation of primary LPCs and pluripotent stem cells, findings of transdifferentiation, disease-specific considerations for establishing a therapeutically effective cell mass, and disease modeling in cell culture. Tumor-initiating stem-like cells (TISCs) that emerge during chronic liver injury share the expression of signaling pathways, including those organized around transforming growth factor beta and β-catenin, and surface markers with normal LPCs. Recent investigations of the role of TISCs in hepatocellular carcinoma have provided insight into the transcriptional and post-transcriptional regulation of hepatocarcinogenesis. Targeted chemotherapies for TISC are in development as a means to overcome cellular resistance and mechanisms driving disease progression in liver cancer.

Engineering of implantable liver tissues

In this chapter, from the engineering point of view, we introduce the results from our group and related research on three typical configurations of engineered liver tissues; cell sheet-based tissues, sheet-like macroporous scaffold-based tissues, and tissues based on special scaffolds that comprise a flow channel network. The former two do not necessitate in vitro prevascularization and are thus promising in actual human clinical trials for liver diseases that can be recovered by relatively smaller tissue mass. The third approach can implant a much larger mass but is still not yet feasible. In all cases, oxygen supply is the key engineering factor. For the first configuration, direct oxygen supply using an oxygen-permeable polydimethylsiloxane membrane enables various liver cells to exhibit distinct behaviors, complete double layers of mature hepatocytes and fibroblasts, spontaneous thick tissue formation of hepatocarcinoma cells and fetal hepatocytes. Actual oxygen concentration at the cell level can be strictly controlled in this culture system. Using this property, we found that initially low then subsequently high oxygen concentrations were favorable to growth and maturation of fetal cells. For the second configuration, combination of poly-L: -lactic acid 3D scaffolds and appropriate growth factor cocktails provides a suitable microenvironment for the maturation of cells in vitro but the cell growth is limited to a certain distance from the inner surfaces of the macropores. However, implantation to the mesentery leaves of animals allows the cells again to proliferate and pack the remaining spaces of the macroporous structure, suggesting the high feasibility of 3D culture of hepatocyte progenitors for liver tissue-based therapies. For the third configuration, we proposed a design criterion concerning the dimensions of flow channels based on oxygen diffusion and consumption around the channel. Due to the current limitation in the resolution of 3D microfabrication processes, final cell densities were less than one-tenth of those of in vivo liver tissues; cells preferentially grew along the surfaces of the channels and this fact suggested the necessity of improved 3D fabrication technologies with higher resolution. In any case, suitable oxygen supply, meeting the cellular demand at physiological concentrations, was the most important factor that should be considered in engineering liver tissues. This enables cells to utilize aerobic respiration that produces almost 20 times more ATP from the same glucose consumption than anaerobic respiration (glycolysis). This also allows the cells to exhibit their maximum reorganization capability that cannot be observed in conventional anaerobic conditions.

Multipotent stem/progenitor cells in human biliary tree give rise to hepatocytes, cholangiocytes, and pancreatic islets

Multipotent stem/progenitors are present in peribiliary glands of extrahepatic biliary trees from humans of all ages and in high numbers in hepato-pancreatic common duct, cystic duct, and hilum. They express endodermal transcription factors (e.g., Sox9, SOX17, FOXA2, PDX1, HES1, NGN3, PROX1) intranuclearly, stem/progenitor surface markers (EpCAM, NCAM, CD133, CXCR4), and sometimes weakly adult liver, bile duct, and pancreatic genes (albumin, cystic fibrosis transmembrane conductance regulator [CFTR], and insulin). They clonogenically expand on plastic and in serum-free medium, tailored for endodermal progenitors, remaining phenotypically stable as undifferentiated cells for months with a cell division initially every ≈36 hours and slowing to one every 2-3 days. Transfer into distinct culture conditions, each comprised of a specific mix of hormones and matrix components, yields either cords of hepatocytes (express albumin, CYP3A4, and transferrin), branching ducts of cholangiocytes (expressing anion exchanger-2-AE2 and CFTR), or regulatable C-peptide secreting neoislet-like clusters (expressing glucagon, insulin) and accompanied by changes in gene expression correlating with the adult fate. Transplantation into quiescent livers of immunocompromised mice results in functional human hepatocytes and cholangiocytes, whereas if into fat pads of streptozocin-induced diabetic mice, results in functional islets secreting glucose-regulatable human C-peptide.

CONCLUSION

The phenotypes and availability from all age donors suggest that these stem/progenitors have considerable potential for regenerative therapies of liver, bile duct, and pancreatic diseases including diabetes.

In situ labeling and magnetic resonance imaging of transplanted human hepatic stem cells

PURPOSE

The purpose is to address the problem in magnetic resonance imaging (MRI) of contrast agent dilution.

PROCEDURES

In situ magnetic labeling of cells and MRI were used to assess distribution and growth of human hepatic stem cells (hHpSCs) transplanted into severe combined immunodeficiency (SCID)/non-obese diabetic (NOD) mice. It was done with commercially available magnetic microbeads coupled to an antibody to a surface antigen, epithelial cell adhesion molecule (EpCAM), uniquely expressed in the liver by hepatic progenitors.

RESULTS

We validated the microbead connection to cells and related MRI data to optical microscopy observations in order to develop a means to quantitatively estimate cell numbers in the aggregates detected. Cell counts of hHpSCs at different times post-transplantation revealed quantifiable evidence of cell engraftment and expansion.

CONCLUSIONS

This magnetic labeling methodology can be used with any antibody coupled to a magnetic particle to target any surface antigen that distinguishes transplanted cells from host cells, thus facilitating studies that define methods and strategies for clinical cell therapy programs.

Regulation of hepatic stem/progenitor phenotype by microenvironment stiffness in hydrogel models of the human liver stem cell niche

Human livers have maturational lineages of cells within liver acini, beginning periportally in stem cell niches, the canals of Hering, and ending in polyploid hepatocytes pericentrally and cholangiocytes in bile ducts. Hepatic stem cells (hHpSCs) in vivo are partnered with mesenchymal precursors to endothelia (angioblasts) and stellate cells, and reside in regulated microenvironments, stem cell niches, containing hyaluronans (HA). The in vivo hHpSC niche is modeled in vitro by growing hHpSC in two-dimensional (2D) cultures on plastic. We investigated effects of 3D microenvironments, mimicking the liver's stem cell niche, on these hHpSCs by embedding them in HA-based hydrogels prepared with Kubota's Medium (KM), a serum-free medium tailored for endodermal stem/progenitors. The KM-HA hydrogels mimicked the niches, matched diffusivity of culture medium, exhibited shear thinning and perfect elasticity under mechanical loading, and had predictable stiffness depending on their chemistry. KM-HA hydrogels, which supported cell attachment, survival and expansion of hHpSC colonies, induced transition of hHpSC colonies towards stable heterogeneous populations of hepatic progenitors depending on KM-HA hydrogel stiffness, as shown by both their gene and protein expression profile. These acquired phenotypes did not show morphological evidence of fibrotic responses. In conclusion, this study shows that the mechanical properties of the microenvironment can regulate differentiation in endodermal stem cell populations.

Ly6C(+) monocytes are extrahepatic precursors of hepatic stellate cells in the injured liver of mice

OBJECTIVE

We previously reported that hepatic stellate cells (HpSCs) are of hematopoietic origin in liver injury. However, the immediate precursors of HpSCs remain unknown. This study was conducted to elucidate whether terminally differentiated blood cells can differentiate into HpSCs.

MATERIALS AND METHODS

We adoptively transferred a variety of cells isolated from enhanced green fluorescent protein (EGFP)-transgenic mice into carbon tetrachloride (CCl(4))-treated nontransgenic mice twice weekly for 2 weeks. We examined the presence of EGFP(+) HpSCs in the injured liver using immunofluorescence analysis.

RESULTS

Monocytes, neutrophils, eosinophils, B cells, or T cells from EGFP mice were transferred into CCl(4)-treated mice. Thirty percent of EGFP(+) cells in the livers of mice given Ly6C(high)c-kit(-) monocytes were negative for CD45, but were positive for glial fibrillary acidic protein, desmin, CD146, ADAMTS13, and α-smooth muscle actin, well-known markers of HpSCs. EGFP(+)CD45(-) cells were predominantly positive for glial fibrillary acidic protein. Although 48% of EGFP(+) cells were positive for procollagen type I, half of them were CD45(-). In the livers of mice given neutrophils, eosinophils, B cells, or T cells, all of the EGFP(+) cells were CD45(+). The majority of EGFP(+) cells in the nonparenchymal cell fraction purified from the livers of mice given Ly6C(high)c-kit(-) monocytes contained lipid droplets and were positive for glial fibrillary acidic protein, desmin, ADAMTS13, and procollagen type I. When Ly6C(+) monocyte-depleted peripheral blood total nucleated cells were adoptively transferred into CCl(4)-treated mice, we found no EGFP(+)CD45(-) cells in the liver.

CONCLUSIONS

These results suggest that Ly6C(+) monocytes can become HpSCs in the injured liver.

Novel therapeutic strategies for targeting liver cancer stem cells

The cancer stem cell (CSC) hypothesis was first proposed over 40 years ago. Advances in CSC isolation were first achieved in hematological malignancies, with the first CSC demonstrated in acute myeloid leukemia. However, using similar strategies and technologies, and taking advantage of available surface markers, CSCs have been more recently demonstrated in a growing range of epithelial and other solid organ malignancies, suggesting that the majority of malignancies are dependent on such a compartment.

Primary liver cancer consists predominantly of hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (ICC). It is believed that hepatic progenitor cells (HPCs) could be the origin of some HCCs and ICCs. Furthermore, stem cell activators such as Wnt/β-catenin, TGF-β, Notch and Hedgehog signaling pathways also expedite tumorigenesis, and these pathways could serve as molecular targets to assist in designing cancer prevention strategies. Recent studies indicate that additional factors such as EpCAM, Lin28 or miR-181 may also contribute to HCC progression by targeting HCC CSCs. Various therapeutic drugs that directly modulate CSCs have been examined in vivo and in vitro. However, CSCs clearly have a complex pathogenesis, with a considerable crosstalk and redundancy in signaling pathways, and hence targeting single molecules or pathways may have a limited benefit for treatment. Many of the key signaling molecules are shared by both CSCs and normal stem cells, which add further challenges for designing molecularly targeted strategies specific to CSCs but sparing normal stem cells to avoid side effects. In addition to the direct control of CSCs, many other factors that are needed for the maintenance of CSCs, such as angiogenesis, vasculogenesis, invasion and migration, hypoxia, immune evasion, multiple drug resistance, and radioresistance, should be taken into consideration when designing therapeutic strategies for HCC.

Here we provide a brief review of molecular signaling in liver CSCs and present insights into new therapeutic strategies for targeting liver CSCs.

Human hepatic stem cell and maturational liver lineage biology

Livers are comprised of maturational lineages of cells beginning extrahepatically in the hepato-pancreatic common duct near the duodenum and intrahepatically in zone 1 by the portal triads. The extrahepatic stem cell niches are the peribiliary glands deep within the walls of the bile ducts; those intrahepatically are the canals of Hering in postnatal livers and that derive from ductal plates in fetal livers. Intrahepatically, there are at least eight maturational lineage stages from the stem cells in zone 1 (periportal), through the midacinar region (zone 2), to the most mature cells and apoptotic cells found pericentrally in zone 3. Those found in the biliary tree are still being defined. Parenchymal cells are closely associated with lineages of mesenchymal cells, and their maturation is coordinated. Each lineage stage consists of parenchymal and mesenchymal cell partners distinguishable by their morphology, ploidy, antigens, biochemical traits, gene expression, and ability to divide. They are governed by changes in chromatin (e.g., methylation), gradients of paracrine signals (soluble factors and insoluble extracellular matrix components), mechanical forces, and feedback loop signals derived from late lineage cells. Feedback loop signals, secreted by late lineage stage cells into bile, flow back to the periportal area and regulate the stem cells and other early lineage stage cells in mechanisms dictating the size of the liver mass. Recognition of maturational lineage biology and its regulation by these multiple mechanisms offers new understandings of liver biology, pathologies, and strategies for regenerative medicine and treatment of liver cancers.

Ductal plates in hepatic ductular reactions. Hypothesis and implications. III. Implications for liver pathology

This article discusses on the basis of the ductal plate hypothesis the implication of the concept for several liver abnormalities. The occurrence of ductal plates (DP) during liver growth in childhood would explain the paraportal and parenchymal localizations of von Meyenburg complexes in postnatally developed parts of the liver, and their higher incidence in adulthood versus childhood. It partly clarifies the lack of postnatal intrahepatic bile duct development in Alagille syndrome and the reduced number of portal tracts in this disease. Ductular reactions (DRs) in DP configuration are the predominant type of progenitor cell reaction in fulminant necro-inflammatory liver disease, when lack of sufficient parenchymal regeneration results in liver failure. The concept of dissecting DRs explains the micronodular pattern of advanced biliary and alcoholic cirrhosis. The concept explains the DP patterns of bile ducts in several cases of biliary atresia, with implications for diagnosis and prognosis. The hypothesis also has an impact on concepts about stem/progenitor cells and their niche.

New concepts in liver regeneration

The unique ability of the liver to regenerate itself has fascinated biologists for years and has made it the prototype for mammalian organ regeneration. Harnessing this process has great potential benefit in the treatment of liver failure and has been the focus of intense research over the past 50 years. Not only will detailed understanding of cell proliferation in response to injury be applicable to other dysfunction of organs, it may also shed light on how cancer develops in a cirrhotic liver, in which there is intense pressure on cells to regenerate. Advances in molecular techniques over the past few decades have led to the identification of many regulatory intermediates, and pushed us onto the verge of an explosive era in regenerative medicine. To date, more than 10 clinical trials have been reported in which augmented regeneration using progenitor cell therapy has been attempted in human patients. This review traces the path that has been taken over the last few decades in the study of liver regeneration, highlights new concepts in the field, and discusses the challenges that still stand between us and clinical therapy.

Lineage restriction of human hepatic stem cells to mature fates is made efficient by tissue-specific biomatrix scaffolds

Current protocols for differentiation of stem cells make use of multiple treatments of soluble signals and/or matrix factors and result typically in partial differentiation to mature cells with under- or overexpression of adult tissue-specific genes. We developed a strategy for rapid and efficient differentiation of stem cells using substrata of biomatrix scaffolds, tissue-specific extracts enriched in extracellular matrix, and associated growth factors and cytokines, in combination with a serum-free, hormonally defined medium (HDM) tailored for the adult cell type of interest. Biomatrix scaffolds were prepared by a novel, four-step perfusion decellularization protocol using conditions designed to keep all collagen types insoluble. The scaffolds maintained native histology, patent vasculatures, and ≈1% of the tissue's proteins but >95% of its collagens, most of the tissue's collagen-associated matrix components, and physiological levels of matrix-bound growth factors and cytokines. Collagens increased from almost undetectable levels to >15% of the scaffold's proteins with the remainder including laminins, fibronectins, elastin, nidogen/entactin, proteoglycans, and matrix-bound cytokines and growth factors in patterns that correlate with histology. Human hepatic stem cells (hHpSCs), seeded onto liver biomatrix scaffolds and in an HDM tailored for adult liver cells, lost stem cell markers and differentiated to mature, functional parenchymal cells in ≈1 week, remaining viable and with stable mature cell phenotypes for more than 8 weeks.

CONCLUSION

Biomatrix scaffolds can be used for biological and pharmaceutical studies of lineage-restricted stem cells, for maintenance of mature cells, and, in the future, for implantable, vascularized engineered tissues or organs.