Hepatic progenitor cell resistance to TGF-beta1's proliferative and apoptotic effects

Chemical approaches targeting the hurdles of hepatocyte transplantation: mechanisms, applications, and advances

Hepatocyte transplantation (HTx) has been a novel cell-based therapy for severe liver diseases, as the donor livers for orthotopic liver transplantation are of great shortage. However, HTx has been confronted with two main hurdles: limited high-quality hepatocyte sources and low cell engraftment and repopulation rate. To cope with, researchers have investigated on various strategies, including small molecule drugs with unique advantages. Small molecules are promising chemical tools to modulate cell fate and function for generating high quality hepatocyte sources. In addition, endothelial barrier, immune responses, and low proliferative efficiency of donor hepatocytes mainly contributes to low cell engraftment and repopulation rate. Interfering these biological processes with small molecules is beneficial for improving cell engraftment and repopulation. In this review, we will discuss the applications and advances of small molecules in modulating cell differentiation and reprogramming for hepatocyte resources and in improving cell engraftment and repopulation as well as its underlying mechanisms.

"Small Hepatocytes" in the Liver

Mature hepatocytes (MHs) in an adult rodent liver are categorized into the following three subpopulations based on their proliferative capability: type I cells (MH-I), which are committed progenitor cells that possess a high growth capability and basal hepatocytic functions; type II cells (MH-II), which possess a limited proliferative capability; and type III cells (MH-III), which lose the ability to divide (replicative senescence) and reach the final differentiated state. These subpopulations may explain the liver's development and growth after birth. Generally, small-sized hepatocytes emerge in mammal livers. The cells are characterized by being morphologically identical to hepatocytes except for their size, which is substantially smaller than that of ordinary MHs. We initially discovered small hepatocytes (SHs) in the primary culture of rat hepatocytes. We believe that SHs are derived from MH-I and play a role as hepatocytic progenitors to supply MHs. The population of MH-I (SHs) is distributed in the whole lobules, a part of which possesses a self-renewal capability, and decreases with age. Conversely, injured livers of experimental models and clinical cases showed the emergence of SHs. Studies demonstrate the involvement of SHs in liver regeneration. SHs that appeared in the injured livers are not a pure population but a mixture of two distinct origins, MH-derived and hepatic-stem-cell-derived cells. The predominant cell-derived SHs depend on the proliferative capability of the remaining MHs after the injury. This review will focus on the SHs that appeared in the liver and discuss the significance of SHs in liver regeneration.

Environmental Ligands of the Aryl Hydrocarbon Receptor and Their Effects in Models of Adult Liver Progenitor Cells

The toxicity of environmental and dietary ligands of the aryl hydrocarbon receptor (AhR) in mature liver parenchymal cells is well appreciated, while considerably less attention has been paid to their impact on cell populations exhibiting phenotypic features of liver progenitor cells. Here, we discuss the results suggesting that the consequences of the AhR activation in the cellular models derived from bipotent liver progenitors could markedly differ from those in hepatocytes. In contact-inhibited liver progenitor cells, the AhR agonists induce a range of effects potentially linked with tumor promotion. They can stimulate cell cycle progression/proliferation and deregulate cell-to-cell communication, which is associated with downregulation of proteins forming gap junctions, adherens junctions, and desmosomes (such as connexin 43, E-cadherin, β-catenin, and plakoglobin), as well as with reduced cell adhesion and inhibition of intercellular communication. At the same time, toxic AhR ligands may affect the activity of the signaling pathways contributing to regulation of liver progenitor cell activation and/or differentiation, such as downregulation of Wnt/β-catenin and TGF-β signaling, or upregulation of transcriptional targets of YAP/TAZ, the effectors of Hippo signaling pathway. These data illustrate the need to better understand the potential role of liver progenitors in the AhR-mediated liver carcinogenesis and tumor promotion.

Proliferation of rat small hepatocytes requires follistatin expression

Small hepatocytes (SHs) are a subpopulation of hepatocytes that have high growth potential in culture and can differentiate into mature hepatocytes (MHs). The activin (Act)/follistatin (Fst) system critically contributes to homeostasis of cell growth in the normal liver. ActA and ActB consist of two disulfide-linked Inhibin (Inh)β subunits, InhβA and InhβB, respectively. Fst binds to Act and blocks its bioactivity. In the present study we carried out the experiments to clarify how Fst regulates the proliferation of SHs. The gene expression was analyzed using DNA microarray analysis, reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR, and protein expression was examined by western blots, immunocytochemistry, and enzyme-linked immunosorbent assay. RT-PCR showed that Fst expression was high in SHs and low in MHs. Although the ActA expression was opposite to that of Fst, ActB expression was high in SHs and low in MHs and increased with time in culture. Fst protein was detected in the cytoplasm of SHs and secreted into the culture medium. ActB protein was also secreted into the medium. Although the exogenous administration of ActA and ActB apparently suppressed the proliferation of SHs, apoptosis of SHs was not induced by treatment with ActA or ActB. On the other hand, Fst treatment did not affect the colony formation of SHs but prevented the inhibitory effect of ActA. Neutralization by the anti-Fst antibody resulted in the suppression of DNA synthesis in SHs, and small hairpin RNA against Fst suppressed the expansion of SH colonies. In conclusion, Fst expression is necessary for the proliferation of SHs.

microRNA-27b suppresses mouse MSC migration to the liver by targeting SDF-1αin vitro

The SDF-1/CXCR4 axis is critical for inducing stem cell mobilization into the circulation, for homing stem cells to the site of injury, and for stem cell participation in the regeneration of liver tissue. In this study, we have gained insight into the molecular mechanisms involved in regulating the expression of SDF-1α by miRNAs. Using microarray and bioinformatics approaches, we identified six miRNAs with differential expression in damaged liver tissue (21 days after liver injury) compared to normal C57BL/6 murine liver tissue and further confirmed these observations by qPCR; miR-23a, which was identified by other researchers, was also included for comparative purposes. We found that miR-23a, miR-27a and miR-27b expression was significantly lower in the damaged liver than in the normal liver (p<0.05). We further confirmed that miR-27b could directly interact with the 3'UTR of SDF-1α to suppress SDF-1α protein expression using a luciferase reporter assay and Western blot analysis. In addition, we found that the over-expression of miR-27b significantly reduced the directional migration of primary cultured CRCX4-positive murine mesenchymal stem cells (mMSCs) in vitro using a transwell assay. These results suggest that miR-27b may be a unique signature of the stem cell niche in the damaged mouse liver and that mir-27b can suppress the directional migration of mMSCs by down-regulating SDF-1α expression by binding directly to the SDF-1α 3'UTR.

In vitro generation of myofibroblasts-like cells from liver epithelial progenitor cells of rhesus monkey (Macaca mulatta)

The origin of the myofibroblast, the primary effector cell of liver fibrosis, is still elusive. Here, we report that fluorescence-activated cell sorting purified E-cad + rhesus monkey liver epithelial progenitor cells (mLEPCs) may serve as a potential source for liver myofibroblasts. Adult mLEPCs colonies were cultured in medium containing 2 ng/ml transforming growth factor β (TGF-β) and 10% fetal bovine serum (FBS) to induce differentiation. Phenotypic changes of cells were analyzed by morphological observation, immunostaining, and reverse transcription-polymerase chain reaction (RT-PCR). After cultured with TGF-β and FBS, some cells in adult mLEPCs colonies converted to fibroblasts-like cells. Immunostaining showed that fibroblasts-like cells had acquired the expression of mesenchymal cell marker vimentin but lost the expression of epithelial cell marker CK8. Fibroblasts-like cells were maintained in culture for up to 40 passages. RT-PCR analysis revealed that fibroblasts-like cells had acquired the expression of mesenchymal genes (snail, PAI-1, and collagen I) and lost the expression of epithelial specific genes (E-cad, ZO-1, CK18, and occludin). In addition, more than 60% of fibroblasts-like cells expressed myofibroblastic-related proteins such as αSMA, vimentin, and N-cad, which were not presented in mLEPCs. Furthermore, increased cell motility was also detected in these fibroblasts-like cells by time-lapse video observation. Our results demonstrate that hepatic epithelial progenitor cells, mLEPCs, transform to myofibroblast-like cells via epithelial-mesenchymal transition. This finding will facilitate understanding of the origin of myofibroblasts in liver fibrosis.

Growth factor- and cytokine-driven pathways governing liver stemness and differentiation

Liver is unique in its capacity to regenerate in response to injury or tissue loss. Hepatocytes and other liver cells are able to proliferate and repopulate the liver. However, when this response is impaired, the contribution of hepatic progenitors becomes very relevant. Here, we present an update of recent studies on growth factors and cytokine-driven intracellular pathways that govern liver stem/progenitor cell expansion and differentiation, and the relevance of these signals in liver development, regeneration and carcinogenesis. Tyrosine kinase receptor signaling, in particular, c-Met, epidermal growth factor receptors or fibroblast growth factor receptors, contribute to proliferation, survival and differentiation of liver stem/progenitor cells. Different evidence suggests a dual role for the transforming growth factor (TGF)-β signaling pathway in liver stemness and differentiation. On the one hand, TGF-β mediates progression of differentiation from a progenitor stage, but on the other hand, it contributes to the expansion of liver stem cells. Hedgehog family ligands are necessary to promote hepatoblast proliferation but need to be shut off to permit subsequent hepatoblast differentiation. In the same line, the Wnt family and β-catenin/T-cell factor pathway is clearly involved in the maintenance of liver stemness phenotype, and its repression is necessary for liver differentiation during development. Collectively, data indicate that liver stem/progenitor cells follow their own rules and regulations. The same signals that are essential for their activation, expansion and differentiation are good candidates to contribute, under adequate conditions, to the paradigm of transformation from a pro-regenerative to a pro-tumorigenic role. From a clinical perspective, this is a fundamental issue for liver stem/progenitor cell-based therapies.

CD133+ liver cancer stem cells from methionine adenosyl transferase 1A-deficient mice demonstrate resistance to transforming growth factor (TGF)-beta-induced apoptosis

Methionine adenosyltransferase (MAT) is an essential enzyme required for S-adenosylmethionine biosynthesis. Hepatic MAT activity falls during chronic liver injury, and mice lacking Mat1a develop spontaneous hepatocellular carcinoma by 18 months. We have previously demonstrated that CD133(+)CD45(-) oval cells isolated from 16-month-old Mat1a(-/-) mice represent a liver cancer stem cell population. The transforming growth factor beta (TGF-beta) pathway constitutes a central signaling network in proliferation, apoptosis, and tumorigenesis. In this study, we tested the response of tumorigenic liver stem cells to TGF-beta. CD133(+)CD45(-) oval cells were isolated from premalignant 16-month-old Mat1a(-/-) mice by flow cytometry and expanded as five clone lines derived from a single cell. All clone lines demonstrated expression of both hepatocyte and cholangiocyte markers and maintained a small population (0.5% to 2%) of CD133(+) cells in vitro, and three of five clone lines produced tumors. Although TGF-beta1 inhibited cell growth equally in CD133(-) and CD133(+) cells from each clone line, the CD133(+) population demonstrated significant resistance to TGF-beta-induced apoptosis compared with CD133(-) cells. Furthermore, CD133(+) cells demonstrated a substantial increase in mitogen-activated protein kinase (MAPK) pathway activation, as demonstrated by phosphorylated extracellular signal-regulated kinase levels before and after TGF-beta stimulation. MAPK inhibition using mitogen-activated protein kinase kinase 1 (MEK1) inhibitor PD98059 led to a significant increase in TGF-beta-induced apoptosis in CD133(+) cells. Conversely, a constitutively active form of MEK1 blocked the apoptotic effects of TGF-beta in CD133(-) cells.

CONCLUSION

CD133(+) liver cancer stem cells exhibit relative resistance to TGF-beta-induced apoptosis. One mechanism of resistance to TGF-beta-induced apoptosis in CD133(+) cancer stem cells is an activated mitogen-activated protein kinase/extracellular signal-regulated kinase pathway.

Transforming growth factor beta controls the directional migration of hepatocyte cohorts by modulating their adhesion to fibronectin

Transforming growth factor beta (TGF-beta) has a strong impact on liver development and physiopathology, exercised through its pleiotropic effects on growth, differentiation, survival, and migration. When exposed to TGF-beta, the mhAT3F cells, immortalized, highly differentiated hepatocytes, maintained their epithelial morphology and underwent dramatic alterations of adhesion, leading to partial or complete detachment from a culture plate, followed by readhesion and spreading. These alterations of adhesive behavior were caused by sequential changes in expression of the alpha5beta1 integrin and of its ligand, the fibronectin. The altered specificity of anchorage to the extracellular matrix gave rise to changes in cells' collective motility: cohorts adhering to fibronectin maintained a persistent, directional motility, with ezrin-rich pathfinder cells protruding from the tips of the cohorts. The absence of adhesion to fibronectin prevented the appearance of polarized pathfinders and lead to random, oscillatory motility. Our data suggest a novel role for TGF-beta in the control of collective migration of epithelial cohorts.

In search of the best candidate for regeneration of ischemic tissues: are embryonic/fetal stem cells more advantageous than adult counterparts?

Human stem cells and progenitor cells from the bone marrow have been proposed for the regeneration of ischemic cardiac tissues. Early clinical trials indicate that infusion of autologous bone-marrow cells into the infarcted heart enhances ventricular function, albeit the long-term benefit remains to be ascertained. Alternatively, angiogenic growth factors could be used to stimulate the recruitment of vascular progenitor cells into tissues in need of regeneration. Unfortunately, in atherosclerotic patients, the curative potential of autologous stem cells might be impoverished by underlying disease and associated risk factors. Thus, research is focusing on the use of embryonic stem cells which are capable of unlimited self-renewal and have the potential to give rise to all tissue types in the body. Ethical problems and technical hurdles may limit the immediate application of embryonic stem cells. In the meanwhile, fetal hematopoietic stem cells,which have been routinely used to reconstitute the hematopoietic system in man, could represent an alternative, owing to their juvenile phenotype and ability to differentiate into vascular endothelial, muscular, and neuronal cell lineages. With progresses in stem cell expansion, the blood of a single cord could be sufficient to transplant an adult. These observations raise the exciting possibility of using fetal cells as a new way to speed up the healing of damaged tissues.