Transplanted Human Pluripotent Stem Cell-Derived Mesenchymal Stem Cells Support Liver Regeneration in Gunn Rats.

Gunn rats bear a mutation within the uridine diphosphate glucuronosyltransferase-1a1 (Ugt1a1) gene resulting in high serum bilirubin levels as seen in Crigler-Najjar syndrome. In this study, the Gunn rat was used as an animal model for heritable liver dysfunction. Induced mesenchymal stem cells (iMSCs) derived from embryonic stem cells (H1) and induced pluripotent stem cells were transplanted into Gunn rats after partial hepatectomy. The iMSCs engrafted and survived in the liver for up to 2 months. The transplanted iMSCs differentiated into functional hepatocytes as evidenced by partially suppressed hyperbilirubinemia and expression of multiple human-specific hepatocyte markers such as albumin, hepatocyte nuclear factor 4α, UGT1A1, cytokeratin 18, bile salt export pump, multidrug resistance protein 2, Na/taurocholate-cotransporting polypeptide, and α-fetoprotein. These findings imply that transplanted human iMSCs can contribute to liver regeneration in vivo and thus represent a promising tool for the treatment of inherited liver diseases.

Isolation and characterization of vesicular and non-vesicular microRNAs circulating in sera of partially hepatectomized rats.

Circulating microRNAs are protected from degradation by their association with either vesicles or components of the RNAi machinery. Although increasing evidence indicates that cell-free microRNAs are transported in body fluids by different types of vesicles, current research mainly focuses on the characterization of exosome-associated microRNAs. However, as isolation and characterization of exosomes is challenging, it is yet unclear whether exosomes or other vesicular elements circulating in serum are the most reliable source for discovering disease-associated biomarkers. In this study, circulating microRNAs associated to the vesicular and non-vesicular fraction of sera isolated from partially hepatectomized rats were measured. Here we show that independently from their origin, levels of miR-122, miR-192, miR-194 and Let-7a are up-regulated two days after partial hepatectomy. The inflammation-associated miR-150 and miR-155 are up-regulated in the vesicular-fraction only, while the regeneration-associated miR-21 and miR-33 are up-regulated in the vesicular- and down-regulated in the non-vesicular fraction. Our study shows for the first time the modulation of non-vesicular microRNAs in animals recovering from partial hepatectomy, suggesting that, in the search for novel disease-associated biomarkers, the profiling of either vesicular or non-vesicular microRNAs may be more relevant than the analysis of microRNAs isolated from unfractionated serum.

The Role of Embryonic Stem Cell-expressed RAS (ERAS) in the Maintenance of Quiescent Hepatic Stellate Cells.

Hepatic stellate cells (HSCs) were recently identified as liver-resident mesenchymal stem cells. HSCs are activated after liver injury and involved in pivotal processes, such as liver development, immunoregulation, regeneration, and also fibrogenesis. To date, several studies have reported candidate pathways that regulate the plasticity of HSCs during physiological and pathophysiological processes. Here we analyzed the expression changes and activity of the RAS family GTPases and thereby investigated the signaling networks of quiescent HSCs versus activated HSCs. For the first time, we report that embryonic stem cell-expressed RAS (ERAS) is specifically expressed in quiescent HSCs and down-regulated during HSC activation via promoter DNA methylation. Notably, in quiescent HSCs, the high level of ERAS protein correlates with the activation of AKT, STAT3, mTORC2, and HIPPO signaling pathways and inactivation of FOXO1 and YAP. Our data strongly indicate that in quiescent HSCs, ERAS targets AKT via two distinct pathways driven by PI3Kα/δ and mTORC2, whereas in activated HSCs, RAS signaling shifts to RAF-MEK-ERK. Thus, in contrast to the reported role of ERAS in tumor cells associated with cell proliferation, our findings indicate that ERAS is important to maintain quiescence in HSCs.

Bile acids induce hepatic differentiation of mesenchymal stem cells.

Mesenchymal stem cells (MSC) have the potential to differentiate into multiple cell lineages and their therapeutic potential has become obvious. In the liver, MSC are represented by stellate cells which have the potential to differentiate into hepatocytes after stimulation with growth factors. Since bile acids can promote liver regeneration, their influence on liver-resident and bone marrow-derived MSC was investigated. Physiological concentrations of bile acids such as tauroursodeoxycholic acid were able to initiate hepatic differentiation of MSC via the farnesoid X receptor and transmembrane G-protein-coupled bile acid receptor 5 as investigated with knockout mice. Notch, hedgehog, transforming growth factor-ß/bone morphogenic protein family and non-canonical Wnt signalling were also essential for bile acid-mediated differentiation, whereas ß-catenin-dependent Wnt signalling was able to attenuate this process. Our findings reveal bile acid-mediated signalling as an alternative way to induce hepatic differentiaion of stem cells and highlight bile acids as important signalling molecules during liver regeneration.

Bile acids and stellate cells.

Hepatic stellate cells are mainly known for their contribution to fibrogenesis in chronic liver diseases, but their identity and function in normal liver remain unclear. They were recently identified as liver-resident mesenchymal stem cells (MSCs), which can differentiate not only into adipocytes and osteocytes, but also into liver epithelial cells such as hepatocytes and bile duct cells as investigated in vitro and in vivo. During hepatic differentiation, stellate cells and other MSCs transiently develop into liver progenitor cells with epithelial characteristics before hepatocytes are established. Transplanted stellate cells from the liver and pancreas are able to contribute to liver regeneration in stem cell-based liver injury models and can also home into the bone marrow, which is in line with their classification as MSCs. There is experimental evidence that bile acids support liver regeneration and are able to activate signaling pathways in hepatic stellate cells. For this reason, it is important to analyze the influence of bile acids on developmental fate decisions of hepatic stellate cells and other MSC populations.

Hepatic stellate cells contribute to progenitor cells and liver regeneration.

Retinoid-storing hepatic stellate cells (HSCs) have recently been described as a liver-resident mesenchymal stem cell (MSC) population; however, it is not clear whether these cells contribute to liver regeneration or serve as a progenitor cell population with hepatobiliary characteristics. Here, we purified HSCs with retinoid-dependent fluorescence-activated cell sorting from eGFP-expressing rats and transplanted these GFP(+) HSCs into wild-type (WT) rats that had undergone partial hepatectomy in the presence of 2-acetylaminofluorene (2AAF) or retrorsine, both of which are injury models that favor stem cell-based liver repair. Transplanted HSCs contributed to liver regeneration in host animals by forming mesenchymal tissue, progenitor cells, hepatocytes, and cholangiocytes and elevated direct bilirubin levels in blood sera of GUNN rats, indicating recovery from the hepatic bilirubin-handling defect in these animals. Transplanted HSCs engrafted within the bone marrow (BM) of host animals, and HSC-derived cells were isolated from BM and successfully retransplanted into new hosts with injured liver. Cultured HSCs transiently adopted an expression profile similar to that of progenitor cells during differentiation into bile acid-synthesizing and -transporting hepatocytes, suggesting that stellate cells represent a source of liver progenitor cells. This concept connects seemingly contradictory studies that favor either progenitor cells or MSCs as important players in stem cell-based liver regeneration.

Hepatic stellate cells support hematopoiesis and are liver-resident mesenchymal stem cells.

BACKGROUND/AIMS: Hematopoiesis can occur in the liver, when the bone marrow fails to provide an adequate environment for hematopoietic stem cells. Hepatic stellate cells possess characteristics of stem/progenitor cells, but their contribution to hematopoiesis is not known thus far. METHODS: Isolated hepatic stellate cells from rats were characterized with respect to molecular markers of bone marrow mesenchymal stem cells (MSC) and treated with adipocyte or osteocyte differentiation media. Stellate cells of rats were further co-cultured with murine stem cell antigen-1(+) hematopoietic stem cells selected by magnetic cell sorting. The expression of murine hematopoietic stem cell markers was analyzed by mouse specific quantitative PCR during co-culture. Hepatic stellate cells from eGFP(+) rats were transplanted into lethally irradiated wild type rats. RESULTS: Desmin-expressing stellate cells were associated with hematopoietic sites in the fetal rat liver. Hepatic stellate cells expressed MSC markers and were able to differentiate into adipocytes and osteocytes in vitro. Stellate cells supported hematopoietic stem/progenitor cells during co-culture similar to bone marrow MSC, but failed to differentiate into blood cell lineages after transplantation. CONCLUSION: Hepatic stellate cells are liver-resident MSC and can fulfill typical functions of bone marrow MSC such as the differentiation into adipocytes or osteocytes and support of hematopoiesis.

Stellate cells from rat pancreas are stem cells and can contribute to liver regeneration.

The identity of pancreatic stem/progenitor cells is still under discussion. They were suggested to derive from the pancreatic ductal epithelium and/or islets. Here we report that rat pancreatic stellate cells (PSC), which are thought to contribute to pancreatic fibrosis, have stem cell characteristics. PSC reside in islets and between acini and display a gene expression pattern similar to umbilical cord blood stem cells and mesenchymal stem cells. Cytokine treatment of isolated PSC induced the expression of typical hepatocyte markers. The PSC-derived hepatocyte-like cells expressed endodermal proteins such as bile salt export pump along with the mesodermal protein vimentin. The transplantation of culture-activated PSC from enhanced green fluorescent protein-expressing rats into wild type rats after partial hepatectomy in the presence of 2-acetylaminofluorene revealed that PSC were able to reconstitute large areas of the host liver through differentiation into hepatocytes and cholangiocytes. This developmental fate of transplanted PSC was confirmed by fluorescence in situ hybridization of chromosome Y after gender-mismatched transplantation of male PSC into female rats. Transplanted PSC displayed long-lasting survival, whereas muscle fibroblasts were unable to integrate into the host liver. The differentiation potential of PSC was further verified by the transplantation of clonally expanded PSC. PSC clones maintained the expression of stellate cell and stem cell markers and preserved their differentiation potential, which indicated self-renewal potential of PSC. These findings demonstrate that PSC have stem cell characteristics and can contribute to the regeneration of injured organs through differentiation across tissue boundaries.

The epigenetic regulation of stem cell factors in hepatic stellate cells.

The epigenetic regulation by DNA methylation is an important mechanism to control the expression of stem cell factors as demonstrated in tumor cells. It was recently shown that hepatic stellate cells (HSC) express stem/progenitor cell factors and have a differentiation potential. The aim of this work was to investigate if the expression of stem cell markers is regulated by DNA methylation during activation of rat HSC. It was found that CD133, Notch1, and Notch3 are regulated via DNA methylation in HSC, whereas Nestin shows no DNA methylation in HSC and other undifferentiated cells such as embryonic stem cells and umbilical cord blood stem cells from rats. In contrast to this, DNA methylation controls Nestin expression in differentiated cells like hepatocytes and the hepatoma cell line H4IIE. Demethylation by 5-Aza-2-deoxycytidine was sufficient to induce Nestin in H4IIE cells. In quiescent stellate cells and embryonic stem cells, the Nestin expression was suppressed by histone H3 methylation at lysine 9, which is another epigenetic mechanism. Apart from the known induction of Nestin in cultured HSC, this intermediate filament protein was also induced after partial hepatectomy, indicating activation of HSC during liver regeneration. Taken together, this study demonstrates for the first time that the expression of stem cell-associated factors such as CD133, Notch1, and Notch3 is controlled by DNA methylation in HSC. The regulation of Nestin by DNA methylation seems to be restricted to differentiated cells, whereas undifferentiated cells use different epigenetic mechanisms such as histone H3 methylation to control Nestin expression.

The niche of stellate cells within rat liver.

UNLABELLED: It is well-accepted that hepatic stellate cells (HSCs) can develop into myofibroblast-like cells that synthesize extracellular matrix proteins and contribute to liver fibrosis. Recently, molecular markers of stem/progenitor cells were discovered in HSCs of rats. Moreover, the cells displayed the capacity to differentiate and to participate in liver regeneration. In addition, stellate cells possess signaling pathways important for maintenance of stemness and cell differentiation such as hedgehog and beta-catenin-dependent Wnt signaling. All these properties are congruently found in stem/progenitor cells. Stem cells require a special microenvironment, the so-called stem cell niche, to maintain their characteristics. Thus, we investigated if the space of Disse, where stellate cells reside in the liver innervated by the sympathetic nervous system and surrounded by sinusoidal endothelial cells and parenchymal cells, exhibits similarities with known stem cell niches. The present study describes the niche of stellate cells within the liver of rats that is composed of sinusoidal endothelial cells, which release stromal cell-derived factor-1 to attract stellate cells via the cysteine-X-cysteine receptor 4, basal lamina proteins (laminin and collagen type IV), and parenchymal cells, which synthesize beta-catenin-dependent Wnt ligands and Jagged1. CONCLUSION: The space of Disse shows analogies to typical stem cell niches comprising of basal lamina components, sympathetic innervation, and adjacent cells that constitute a milieu by paracrine factors and direct physical interactions to retain HSCs at this site and to influence their cellular fate. The space of Disse serves as a niche of stellate cells, which is a novel function of this unique organ structure.

Hepatic and pancreatic stellate cells in focus.

Stellate cells are vitamin A-storing cells of liver and pancreas and have been described in all vertebrates ranging from lampreys (primitive fish) to humans, demonstrating their major importance. This cell type is thought to contribute to fibrosis, a condition characterized by an excess deposition of extracellular matrix proteins. Recently, the expression of stem/progenitor cell markers, such as CD133 (prominin-1) and Oct4, was discovered in hepatic stellate cells (HSCs) of rats. Moreover, HSCs possess signaling pathways important for maintenance of stemness and cell differentiation, such as hedgehog, beta-catenin-dependent Wnt, and Notch signaling, and are resistant to CD95-mediated apoptosis. In analogy to a stem cell niche, some characteristics of quiescent HSC are maintained by aid of a special microenvironment located in the space of Dissé. Finally, stellate cells display a differentiation potential as investigated in vitro and in vivo. Collectively all these properties are congruently found in stem/progenitor cells and support the concept that stellate cells are undifferentiated cells, which might play an important role in liver regeneration. The present review highlights findings related to this novel aspect of stellate cell biology.

Canonical Wnt signaling maintains the quiescent stage of hepatic stellate cells.

It is well known that hepatic stellate cells (HSC) develop into cells, which are thought to contribute to liver fibrogenesis. Recent data suggest that HSC are progenitor cells with the capacity to differentiate into cells of endothelial and hepatocyte lineages. The present study shows that beta-catenin-dependent canonical Wnt signaling is active in freshly isolated HSC of rats. Mimicking of the canonical Wnt pathway in cultured HSC by TWS119, an inhibitor of the glycogen synthase kinase 3beta, led to reduced beta-catenin phosphorylation, induced nuclear translocation of beta-catenin, elevated glutamine synthetase production, impeded synthesis of alpha-smooth muscle actin and Wnt5a, but promoted the expression of glial fibrillary acidic protein, Wnt10b, and paired-like homeodomain transcription factor 2c. In addition, canonical Wnt signaling lowered DNA synthesis and hindered HSC from entering the cell cycle. The findings demonstrate that beta-catenin-dependent Wnt signaling maintains the quiescent state of HSC and, similar to stem and progenitor cells, influences their developmental fate.

CD133+ hepatic stellate cells are progenitor cells.

Hepatic stellate cells (HSC) play an important role in the development of liver fibrosis. Here, we report that HSC express the stem/progenitor cell marker CD133 and exhibit properties of progenitor cells. CD133+ HSC of rats were selected by specific antibodies and magnetic cell sorting. Selected cells displayed typical markers of HSC, endothelial progenitor cells (EPC), and monocytes. In cell culture, CD133+ HSC transformed into alpha-smooth muscle actin positive myofibroblast-like cells, whereas application of cytokines known to facilitate EPC differentiation into endothelial cells led to the formation of branched tube-like structures and induced expression of the endothelial cell markers endothelial nitric oxide synthase and vascular-endothelial cadherin. Moreover, cytokines that guide stem cells to develop hepatocytes led to the appearance of rotund cells and expression of the hepatocyte markers alpha-fetoprotein and albumin. It is concluded that CD133+ HSC are a not yet recognized progenitor cell compartment with characteristics of early EPC. Their potential to differentiate into endothelial or hepatocyte lineages suggests important functions of CD133+ HSC during liver regeneration.

Expression patterns of PDGF-A, -B, -C and -D and the PDGF-receptors alpha and beta in activated rat hepatic stellate cells (HSC).

The platelet-derived growth factor (PDGF) family, which regulates many physiological and pathophysiological processes has recently been enlarged by two new members, the isoforms PDGF-C and -D. Little is known about the expression levels of these new members in hepatic fibrosis. We therefore investigated by quantitative real time PCR (Taqman) the mRNA expression profiles of all four PDGF isoforms in transdifferentiating primary cultured hepatic stellate cells (HSC), an in vitro model system of hepatic fibrogenesis, either with or without stimulation of the cells with PDGF-BB or TGF-beta1. All four isoforms were expressed in HSC transdifferentiating to myofibroblast-like cells (MFB) albeit with different profiles: while PDGF-A mRNA exhibited minor fluctuations only, PDGF-B was rapidly down-regulated. In contrast, both PDGF-C and -D mRNA were strongly induced: PDGF-C up to 5 fold from day 2 to day 8 and PDGF-D up to 8 fold from day 2 to day 5 of culture. Presence of PDGF-DD in activated HSC was confirmed at the protein level by immunocytochemistry. Stimulation of HSC and MFB with PDGF-BB led to down-regulation of the new isoforms, whereas TGF-beta1 upregulated PDGF-A only. We further show that PDGF receptor-beta (PDGFR-beta) mRNA was rapidly upregulated within the first day of culture and was constantly expressed from day 2 on while the expression profile of PDGFR-alpha mRNA was very similar to that of PDGF-A during transdifferentiation. Given the dramatic changes in PDGF-C and -D expression, which may compensate for down-regulation of PDGF-B, we hypothesize that the new PDGF isoforms may fulfil specific functions in hepatic fibrogenesis.

Characterization of intracellular pathways leading to coinduction of thrombospondin-1 and TGF-beta1 expression in rat hepatic stellate cells.

Accumulating evidence has identified Thrombospondin (TSP)-1 as important activator of latent TGF-beta. Since little is known about signal transduction pathways regulating TSP expression in liver, we investigated cytokine-mediated upregulation of TSP-1 and TGF-beta1 in primary rat hepatic stellate cells (HSC). PDGF-BB and TNF-a rapidly coinduce mRNA levels of TSP-1 and TGF-beta1. Interestingly, blockade of basal Erk activity by synthetic Erk-binding peptides also leads to strong induction of both mRNA transcripts in non-stimulated cells. We show that PDGF-BB induces TSP-1 and TGF-beta1 via the src kinase pathway whereas TNF-a utilizes the MAPK/Erk pathway. However, especially TSP-1 induction by both cytokines involves a pathway, which depends to a certain extent on PI3 kinase activity. In summary the data illustrate specific pathways activated by PDGF-BB and TNF-a in HSC giving new insights into the tightly controlled mechanisms regulating TSP-1 and TGF-beta1 expression in these cells.

Transdifferentiation-dependent expression of alpha-SMA in hepatic stellate cells does not involve TGF-beta pathways leading to coinduction of collagen type I and thrombospondin-2.

Hepatic stellate cells (HSC) cultured on plastic spontaneously transdifferentiate to a myofibroblast-like cell type (MFB). This model system of hepatic fibrogenesis is characterized by phenotypic changes of the cells and increased matrix synthesis. Here, we analyzed if transdifferentiation-dependent induction of ECM components, e.g., collagen type I and thrombospondin-2 (TSP-2), and phenotypic changes are coregulated events and if both processes are mediated via TGF-beta pathway(s). Blocking the TGF-beta-dependent p38 MAPK pathway in HSC with the specific inhibitor SB203580 strongly reduces collagen I and TSP-2 mRNA expression without inhibiting upregulation of the typical MFB-marker, alpha-smooth-muscle actin (alpha-SMA). Similarly, interference with the Smad2/3/4 pathway using dexamethasone also heavily decreased expression of collagen type I and TSP-2 whereas transdifferentiation of HSC to the typical morphology of MFB with loss of fat droplets and increasing alpha-SMA was unchanged. Further, p38 MAPK mediated induction of collagen I and TSP-2 expression by TGF-beta1 was still achieved in the presence of dexamethasone, showing that dexamethasone does not block p38 while it delays Smad2 phosphorylation and antagonizes stimulation of a Smad3/Smad4 dependent TGF-beta reporter construct. Interestingly, in contrast to SB203580 and dexamethasone, overexpression of the TGF-beta antagonist Smad7 reduced ECM expression and simultaneously inhibited morphologic transdifferentiation, indicating that Smad7 fulfills additional features in HSC. In conclusion, our data show that phenotypic changes of transdifferentiating HSC and induction of matrix synthesis are independent processes, the latter being stimulated by both, Smad dependent and MAPK dependent TGF-beta signaling.

Expression of cytosolic and membrane associated tissue transglutaminase in rat hepatic stellate cells and its upregulation during transdifferentiation to myofibroblasts in culture.

Transdifferentiation of hepatic stellate cells (HSC) to collagen producing myofibroblasts (MFB) is a principal event in liver fibrogenesis. In our studies we investigated if tissue transglutaminase (tTG) from these cell types may play a role in liver fibrosis. Separation of cytosol and membrane components showed membrane associated tTG and during transdifferentiation an upregulation of total tTG on mRNA and protein level was found, but no modulation during stimulation with TGF-beta1. In HSC and fully differentiated MFB a significant amount of the total tTG synthesised during transdifferentiation is found to be membrane-associated whereas the remaining portion is cytosol-associated and only very little is found within the extracellular matrix (ECM). The data implicate that tTG in this cell type seems to play an important role in liver fibrogenesis.

PDGF-BB induces expression of LTBP-1 but not TGF-beta1 in a rat cirrhotic fat storing cell line.

TGF-beta, a profibrogenic cytokine is predominantly secreted as a latent molecule complexed with one of the latent TGF-beta binding proteins (LTBP). Due to the proposed functions of LTBP-1 and -3 in regulating TGF-beta-bioavailability and -activity, we investigated the effects of PDGF-BB and TGF-beta1 on their expression levels in Cirrhotic fat storing cells (CFSC). CFSC basally express LTBP-1 and -3 and TGF-beta1. LTBP-1 colocalizes with LAP and the cells secrete some active TGF-beta1. Promoter studies showed no strong induction of the LTBP-1 promoters after stimulation, although mRNA and protein levels were increased by PDGF-BB treatment without affecting TGF-beta1 expression. Vice versa, TGF-beta1 treatment did not alter LTBP-1 expression while an autocrine induction was found. Our data indicate that LTBP-1 but not TGF-beta1 is induced by PDGF-BB and that TGF-beta1 autoinduction does not affect the expression of LTBP-beta1. This divergent regulation may represent an important mechanism for modulation of TGF-beta bioavailability.