Enrichment of hepatic progenitor cells from adult mouse liver

Elaidic acid induced hepatocyte pyroptosis via autophagy-CTSB-NLRP3 pathway

Elaidic acid (EA, C18:1 trans) is a kind of principal Trans fatty acid (TFA) and is widely found in processed food. Pyroptosis is a form of programmed cell death, distinct from apoptosis and traditional necrosis. Excessive pyroptosis could induce body injury and serious inflammation. However, the effect of EA on pyroptosis has not been reported. In the study, we found that EA exposure caused liver damage and hepatocyte pyroptosis by testing GSDMD-N, Caspase 1, IL-18, and IL-1β in mice and HepG2 cells. Further exploring the mechanisms, we found that EA-induced pyroptosis depended on Cathepsin B (CTSB)-mediated NLRP3 inflammasome activation. Cell autophagy was closely related to lysosomes. Our study revealed that EA promoted hepatocyte autophagy, and activated autophagy induced lysosomal membrane permeabilization (LMP) and CTSB leakage. Inhibition of autophagy by 3-MA mitigated the CTSB leak, reduced the activation of the NLRP3 inflammasome, and then attenuated the EA-induced pyroptosis. In summary, these results indicated that EA induced hepatocyte pyroptosis via autophagy-CTSB-NLRP3 inflammasome pathway. The study revealed new insights into the toxicity mechanism of EA.

Evidence for in vitro extensive proliferation of adult hepatocytes and biliary epithelial cells

Over the last several years, a method has emerged that endows adult hepatocytes with in vitro proliferative capacity, producing chemically induced liver progenitors (CLiPs). However, there is a growing controversy regarding the origin of these cells. Here, we provide lineage tracing-based evidence that adult hepatocytes acquire proliferative capacity in vitro using rat and mouse models. Unexpectedly, we also found that the CLiP method allows biliary epithelial cells to acquire extensive proliferative capacity. Interestingly, after long-term culture, hepatocyte-derived cells (hepCLiPs) and biliary epithelial cell-derived cells (bilCLiPs) become similar in their gene expression patterns, and they both exhibit differentiation capacity to form hepatocyte-like cells. Finally, we provide evidence that hepCLiPs can repopulate injured mouse livers, reinforcing our earlier argument that CLiPs can be a cell source for liver regenerative medicine. This study advances our understanding of the origin of CLiPs and motivates the application of this technique in liver regenerative medicine.

Apigenin alleviated PA-induced pyroptosis by activating autophagy in hepatocytes

Apigenin is a kind of natural flavonoid that abundantly exists in fruits and vegetables. Pyroptosis is a new, pro-inflammatory type of programmed necrosis cell death, and the NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is the key molecule to induce pyroptosis. Excessive hepatic pyroptosis results in liver injury. In the study, we found for the first time that apigenin could alleviate palmitic acid (PA)-induced NLRP3 inflammasome activation and pyroptosis in HepG2 cells and primary mouse hepatic cells. Meanwhile, apigenin could promote the autophagy of hepatocytes. When the autophagy inhibitor chloroquine (CQ) was added, the data showed that the recovery effect of apigenin on PA-induced pyroptosis was weakened, indicating that apigenin could alleviate PA-induced pyroptosis by activating autophagy. Further mechanistic studies showed that apigenin regulated the NLRP3 inflammasome through two ways, so as to alleviate PA-induced pyroptosis. On the one hand, apigenin eliminated damaged mitochondria by activating autophagy, thereby clearing reactive oxygen species (ROS) production and inhibiting the activation of the NLRP3 inflammasome, and on the other hand, activation of autophagy could directly degrade the NLRP3 inflammasome. Our study provides a new idea and target for the use of functional factors in food to alleviate liver injury.

In Vivo Retargeting of Poly(beta aminoester) (OM-PBAE) Nanoparticles is Influenced by Protein Corona

One of the main bottlenecks in the translation of nanomedicines from research to clinics is the difficulty in designing nanoparticles actively vectorized to the target tissue, a key parameter to ensure efficacy and safety. In this group, a library of poly(beta aminoester) polymers is developed, and it is demonstrated that adding specific combinations of terminal oligopeptides (OM-PBAE), in vitro transfection is cell selective. The current study aims to actively direct the nanoparticles to the liver by the addition of a targeting molecule. To achieve this objective, retinol, successfully attached to OM-PBAE, is selected as hepatic targeting moiety. It is demonstrated that organ biodistribution is tailored, achieving the desired liver accumulation. Regarding cell type transfection, antigen presenting cells in the liver are those showing the highest transfection. Thanks to proteomics studies, organ but not cellular biodistribution can be explained by the formation of differential protein coronas. Therefore, organ biodistribution is governed by differential protein corona formed when retinol is present, while cellular biodistribution is controlled by the end oligopeptides type. In summary, this work is a proof of concept that demonstrates the versatility of these OM-PBAE nanoparticles, in terms of the modification of the biodistribution of OM-PBAE nanoparticles adding active targeting moieties.

Combination of cord blood-derived human hepatic progenitors and hepatogenic factors strongly improves recovery after acute liver injury in mice through modulation of the Wnt/β-catenin signaling

Cell therapy represents a promising alternative strategy for end-stage liver disease, and hepatic progenitors are the best candidates. The possibility to maximize the paracrine effects of transplanted cells represents a great potential benefit for cell therapy success. We studied how cell type and microenvironment modulate the Wnt/β-catenin signaling in vitro and in vivo. In vitro, the onset of hepatocyte commitment was characterized by the presence of nuclear truncated β-catenin. In vivo, we analyzed the effect of human hepatic progenitors on damage recovery and functional regeneration in a mouse model of acute liver injury, either in combination or in absence of a selected mix of hepatogenic factors. Animals injected with human hepatic progenitors and hepatogenic factors showed improved engraftment triggering the Wnt/β-catenin signaling cascade. Human hepatic progenitors expressing the human oval cell marker OV6 displayed a consistent colocalization with β-catenin and colocalized with Wnt1 main ligand of the canonical pathway. Wnt5a, on the contrary, was expressed in distinct liver cell populations. Epithelial mesenchymal transition-related markers showed enhanced expression and wider distribution, and the hepato-mesenchymal population Thy1 + CK19- was also present. Control animals injected with hepatogenic factors alone exhibited higher β-catenin, decreased Wnt5a levels, and persistent proliferation of the hepato-mesenchymal population. In conclusion, the combination of human hepatic progenitors with selected hepatogenic factors creates a positive synergy with local microenvironment, ameliorates cell engraftment, stimulates and accelerates regenerative process, and improves the rescue of hepatic function by modulating the Wnt/βcatenin signaling and activating hepato-mesenchymal population.

The Protective Effect of Transplanting Liver Cells Into the Mesentery on the Rescue of Acute Liver Failure After Massive Hepatectomy

Postoperative liver failure is one of the most critical complications following extensive hepatectomy. Although transplantation of allogeneic hepatocytes is an attractive therapy for posthepatectomy liver failure, transplanting cells via the portal veins typically causes portal vein embolization. The embolization by transplanted cells would be lethal in patients who have undergone massive hepatectomy. Thus, transplant surgeons need to select extrahepatic sites as transplant sites to prevent portal vein embolization. We aimed to investigate the mechanism of how liver cells transplanted into the mesentery protect recipient rats from acute liver failure after massive hepatectomy. We induced posthepatectomy liver failure by 90% hepatectomy in rats. Liver cells harvested from rat livers were transplanted into the mesenteries of hepatectomized rats. Twenty percent of the harvested cells, which consisted of hepatocytes and nonparenchymal cells, were transplanted into each recipient. The survival rate improved significantly in the liver cell transplantation group compared to the control group 7 days after hepatectomy (69 vs. 7%). Histological findings of the transplantation site, in vivo imaging system study findings, quantitative polymerase chain reaction assays of the transplanted cells, and serum albumin measurements of transplanted Nagase analbuminemic rats showed rapid deterioration of viable transplanted cells. Although viable transplanted cells deteriorated in the transplanted site, histological findings and an adenosine-5'-triphosphate (ATP) assay showed that the transplanted cells had a protective effect on the remaining livers. These results indicated that the paracrine effects of transplanted liver cells had therapeutic effects. The same protective effects were observed in the hepatocyte transplantation group, but not in the liver nonparenchymal cell transplantation group. Therefore, this effect on the remnant liver was mainly due to the hepatocytes among the transplanted liver cells. We demonstrated that transplanted liver cells protect the remnant liver from severe damage after massive hepatectomy.

The biliary epithelium gives rise to liver progenitor cells

Severe liver diseases are characterized by expansion of liver progenitor cells (LPC), which correlates with disease severity. However, the origin and role of LPC in liver physiology and in hepatic injury remains a contentious topic. We found that ductular reaction cells in human cirrhotic livers express hepatocyte nuclear factor 1 homeobox B (HNF1β). However, HNF1β expression was not present in newly generated epithelial cell adhesion molecule (EpCAM)-positive hepatocytes. In order to investigate the role of HNF1β-expressing cells we used a tamoxifen-inducible Hnf1βCreER/R26R(Yfp/LacZ) mouse to lineage-trace Hnf1β(+) biliary duct cells and to assess their contribution to LPC expansion and hepatocyte generation. Lineage tracing demonstrated no contribution of HNF1β(+) cells to hepatocytes during liver homeostasis in healthy mice or after loss of liver mass. After acute acetaminophen or carbon tetrachloride injury no contribution of HNF1β(+) cells to hepatocyte was detected. We next assessed the contribution of Hnf1β(+) -derived cells following two liver injury models with LPC expansion, a diethoxycarbonyl-1,4-dihydro-collidin (DDC)-diet and a choline-deficient ethionine-supplemented (CDE)-diet. The contribution of Hnf1β(+) cells to liver regeneration was dependent on the liver injury model. While no contribution was observed after DDC-diet treatment, mice fed with a CDE-diet showed a small population of hepatocytes derived from Hnf1β(+) cells that were expanded to 1.86% of total hepatocytes after injury recovery. Genome-wide expression profile of Hnf1β(+) -derived cells from the DDC and CDE models indicated that no contribution of LPC to hepatocytes was associated with LPC expression of genes related to telomere maintenance, inflammation, and chemokine signaling pathways.

CONCLUSION

HNF1β(+) biliary duct cells are the origin of LPC. HNF1β(+) cells do not contribute to hepatocyte turnover in the healthy liver, but after certain liver injury, they can differentiate to hepatocytes contributing to liver regeneration.

The hepatoprotective effect of sea buckthorn (Hippophae rhamnoides) berries on induced aflatoxin B1 poisoning in chickens 1

The leaves and berries of sea buckthorn (SB; Hippophae rhamnoides; family Elaeagnaceae) are medically claimed as having phytoantioxidant, antiinflammatory, and anticancerous properties in humans. This study evaluated the hepatoprotective activity of oil from SB berries against toxicity induced by aflatoxin B1 (AFB1) in broiler chickens. The toxicity of AFB1 led to lower total serum proteins and specifically reduced albumin (P < 0.001). Serum aspartate aminotransferase increased from 191.14 ± 11.56 to 218.80 ± 13.68 (P < 0.001). When chickens were simultaneously dosed with AFB1 and an extract of SB berries, subsequent histology of the liver showed a significant reduction of necrosis and fatty formation compared with chickens treated with AFB1 alone. Immunohistochemical results indicated that COX2, Bcl-2, and p53 were highly expressed in the liver of AFB1-treated chickens and their expression was significantly reduced by SB oil supplementation. The levels of AFB1 residues in chickens livers were significantly reduced by SB oil from 460.92 ± 6.2 ng/mL in the AFB1 group to 15.59 ± 6.1 ng/mL in the AFB1 and SB oil group. These findings suggest that SB oil has a potent hepatoprotective activity, reducing the concentration of aflatoxins in liver and diminishing their adverse effects.

Comparative study of transplantation of hepatocytes at various differentiation stages into mice with lethal liver damage

Hepatocyte transplantation utilizing induced pluripotent stem cells (iPSCs) or embryonic stem cells (ESCs) has been expected to provide an alternative to liver transplantation. However, it remains uncertain precisely which cell type is the best suited for cell transplantation. In particular, it is unclear whether mature hepatocytes, which have sufficient liver function, or immature hepatic progenitor cells, which have a higher proliferative capacity, will provide a better outcome. The main objective of this study was to investigate the therapeutic efficacy of the transplantation of hepatocytes at various differentiation stages. We utilized transgenic mice that expressed diphtheria toxin (DT) receptors under the control of an albumin enhancer/promoter. ESC-derived endodermal cells, fetal hepatocytes, and adult hepatocytes were transplanted into these mice with experimentally induced lethal acute liver injury caused by DT administration. The transplanted cells were marked by enhanced green fluorescent protein. We evaluated their effects on survival. At 35 days after transplantation, the survival rate of the adult hepatocyte-transplanted group (8/20, 40%) was significantly improved in comparison to that of the sham-operated group (2/25, 8%), the fetal hepatocyte-transplanted group (1/20, 5%), and the ESC-derived endodermal cell-transplanted group (0/21, 0%). The adult hepatocytes proliferated in the recipient livers and replaced a large part of their parenchyma. The transplantation of adult hepatocytes for acute liver failure significantly improved the survival rate in comparison to that of transplantation of immature cells, thus suggesting that ESCs and iPSCs should be differentiated into mature hepatocytes before cell transplantation for acute liver failure.

Efficient human fetal liver cell isolation protocol based on vascular perfusion for liver cell-based therapy and case report on cell transplantation

Although hepatic cell transplantation (CT) holds the promise of bridging patients with end-stage chronic liver failure to whole liver transplantation, suitable cell populations are under debate. In addition to hepatic cells, mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) are being considered as alternative cell sources for initial clinical cell work. Fetal liver (FL) tissue contains potential progenitors for all these cell lineages. Based on the collagenase incubation of tissue fragments, traditional isolation techniques yield only a fraction of the number of available cells. We report a 5-step method in which a portal vein in situ perfusion technique is used for tissue from the late second trimester. This method results in the high viabilities known for adult liver vascular perfusion, addresses the low cell yields of conventional digestion methods, and reduces the exposure of the tissue to collagenase 4-fold. We used donated tissue from gestational weeks 18 to 22, which yielded 1.8 ± 0.7 × 10(9) cells with an average viability of 78%. Because HSC transplantation and MSC transplantation are of interest for the treatment of hepatic failure, we phenotypically confirmed that in addition to hepatic progenitors, the resulting cell preparation contained cells expressing typical MSC and HSC markers. The percentage of FL cells expressing proliferation markers was 45 times greater than the percentage of adult hepatocytes expressing these markers and was comparable to the percentage of immortalized HepG2 liver hepatocellular carcinoma cells; this indicated the strong proliferative capacity of fetal cells. We report a case of human FL CT with the described liver cell population for clinical end-stage chronic liver failure. The patient's Model for End-Stage Liver Disease (MELD) score improved from 15 to 10 within the first 18 months of observation. In conclusion, this human FL cell isolation protocol may be of interest for further clinical translation work on the development of liver cell-based therapies.

Hydrogen sulfide inhibits hypoxia- but not anoxia-induced hypoxia-inducible factor 1 activation in a von hippel-lindau- and mitochondria-dependent manner

AIMS

In addition to nitric oxide and carbon monoxide, hydrogen sulfide (H(2)S) is an endogenously synthesized gaseous molecule that acts as an important signaling molecule in the living body. Transcription factor hypoxia-inducible factor 1 (HIF-1) is known to respond to intracellular reduced oxygen (O(2)) availability, which is regulated by an elaborate balance between O(2) supply and demand. However, the effect of H(2)S on HIF-1 activity under hypoxic conditions is largely unknown in mammalian cells. In this study, we tried to elucidate the effect of H(2)S on hypoxia-induced HIF-1 activation adopting cultured cells and mice.

RESULTS

The H(2)S donors sodium hydrosulfide and sodium sulfide in pharmacological concentrations reversibly reduced cellular O(2) consumption and inhibited hypoxia- but not anoxia-induced HIF-1α protein accumulation and expression of genes downstream of HIF-1 in established cell lines. H(2)S did not affect HIF-1 activation induced by the HIF-α hydroxylases inhibitors desferrioxamine or CoCl(2). Experimental evidence adopting von Hippel-Lindau (VHL)- or mitochondria-deficient cells indicated that H(2)S did not affect neosynthesis of HIF-1α protein but destabilized HIF-1α in a VHL- and mitochondria-dependent manner. We also demonstrate that exogenously administered H(2)S inhibited HIF-1-dependent gene expression in mice.

INNOVATION

For the first time, we show that H(2)S modulates intracellular O(2) homeostasis and regulates activation of HIF-1 and the subsequent gene expression induced by hypoxia by using an in vitro system with established cell lines and an in vivo system in mice.

CONCLUSIONS

We demonstrate that H(2)S inhibits hypoxia-induced HIF-1 activation in a VHL- and mitochondria-dependent manner.

Alpha-fetoprotein-producing pancreatic cancer cells possess cancer stem cell characteristics

We aimed to demonstrate the existence of cancer stem cells in human pancreatic cancer, and to clarify that they are alpha-fetoprotein (AFP) producing cells. Six cell lines derived from human pancreatic cancers were examined, and AsPC-1 and PANC-1 were noted to express AFP. Single cell culture assays and xenotransplantation revealed that the AFP-producing cells had the capacity for self-renewal and differentiation, and that these cells were tumorigenic. Furthermore, they were resistant to anti-cancer agents. The ABCA12 transporter was expressed in the AFP-producing cells at a level more than twice as high as that in the non-AFP-producing cells. The AFP-producing cells were shown to be putative pancreatic cancer stem cells. Furthermore, the expression of ABCA12 appears to be associated with drug resistance.

Culture of newborn monkey liver epithelial progenitor cells in chemical defined serum-free medium

Studies with hepatic progenitor cells from non-human primates would allow better understanding of their human counterparts. In this study, rhesus monkey liver epithelial progenitor cells (mLEPCs) were derived from a small piece of newborn livers in chemical defined serum-free medium. Digested hepatic cells were treated in Ca(2+)-containing medium to form cell aggregates. Two types of cell aggregates were generated: elongated spindle cells and polygonal epithelial cells. Elongated spindle cells were expressed as vimentin and brachyury, and they were disappeared within 5 d in our cultures. The remaining type consisted of small polygonal epithelial cells that expressed cytokeratin 7 (CK7), CK8, CK18, nestin, CD49f, and E-cad, the markers of hepatic stem cells, but were negative for alpha-fetoprotein, albumin, and CK19. They can proliferate and be passaged, if on laminin or rat tail collagen gel, to initiate colonies. When cultured with dexamethasone and oncostatin M, the expression of mature hepatocyte markers, such as alpha-1-antitrypsin, intracytoplasmic glycogen storage, indocyanine green uptake, and lipid droplet generation, were induced in differentiated cells. If transferred onto mouse embryonic fibroblasts feeders, they gave rise to CK19-positive cholangiocytes with formation of doughnut-like structure. Thus, mLEPCs with bipotency were derived from newborn monkey liver and may serve as a preclinical model for assessment of cell therapy in humans.

Identification and location of label retaining cells in mouse liver

BACKGROUND

In most somatic tissues, adult stem cells are crucial for the maintenance of tissue homeostasis under normal physiological states and during recovery from injuries. Label retaining cell (LRC) assay remains the well-known method to identify possible somatic stem/progenitor cells and their location both in situ and in vivo.

METHODS

Here, BrdU was used to tag the possible hepatic stem/progenitor cells (HSPCs) in newborn pups, followed by a trace period of up to 23 months. Additionally, we report a method to rapidly kill proliferating cells in adult liver tissue, and activate and label (KAL) surviving possible HSPCs.

RESULTS

We found that LRCs definitively exist in the liver tissues of adult mice, that LRCs express cell cycle proteins cyclind3 and cdk6, but do not express sca-1 or c-kit, and that LRCs locate primarily in the periportal and pericentral regions. Moreover, the number of these LRCs remains nearly constant during the lifespan of the mice. After injury induced by 5-fluorouracil, we observed that the activation of possible HSPCs tagged by the BrdU label was almost completely inhibited at day 4. The cellular kinetics of repair of BrdU-tagged HSPCs were different every 12 h between day 3 and day 4. Moreover, HSPCs still retained labels and located definitively in the periportal region after a prolonged chase.

CONCLUSIONS

The LRC method together with our novel KAL method reported here may be used to identify and locate possible HSPCs.

Proliferation and differentiation potential of mouse adult hepatic progenitor cells cultured in vitro

This study aimed to isolate the stem cells or progenitors, if exist, from normal adult mouse liver and investigate their potential of proliferation and differentiation. Hepatocytes were isolated by modified two-step liver perfusion method and centrifugation, and then cultured in modified serumcontaining DMEM for observation more than 60 days. Immunofluorescence technique was applied to check the hepatocytes and to examine the formation of colonies with albumin, alpha-fetoprotein (AFP) and cytokeratin 19 (CK19). Results showed that some hepatocytes that were strongly positive for hepatocyte specific markers albumin on Day 1 in culture, could be activated at Days 2-3, followed by rapid proliferation and formation of colonies. The colonies could expand continually for more than 60 days. On Day 5, all the cells in the colony expressed hepatic stem cell (HSC) markers AFP. With the time of culture, some cells in colonies lost ability to divide at Days 13-15, and differentiated into cells which had a large cytoplasm and some two nuclei, similar to the appearance of mature hepatocytes morphologically. These differentiated cells demonstrated strong expression of albumin. Around Day 30, some big cells appeared in colonies and expressed bile duct cell marker CK19. Therefore, this subpopulation of mouse hepatocytes could acquire some characteristics of immature hepatocytes and showed the profile of hepatic progenitor cells with a high proliferating ability and bi-potential of differentiation. They were isolated from normal adult mouse, hence, named adult hepatic progenitor cells (AHPCs). Mouse AHPCs may be used as an HSC model for hepatocytes transplantation and hepatopathy study.

Hepatic stem cells

Early studies in hepatocyte turnover and liver regeneration showed that the parenchymal cell, the hepatocyte, was the primary and only cell involved in tissue renewal. However, new studies of liver regeneration, hepatocarcinogenesis, liver transplantation, and various cell lines have shown that a variety of cell types participate in maintaining hepatocyte number and mass and question the dogma of the previous hierarchy of hepatocyte differentiation in vitro and in vivo.

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.

Biology of the adult hepatic progenitor cell: "ghosts in the machine"

This chapter reviews some of the basic biological principles governing adult progenitor cells of the liver and the mechanisms by which they operate. If scientists were better able to understand the conditions that govern stem cell mechanics in the liver, it may be possible to apply that understanding in a clinical setting for use in the treatment or cure of human pathologies. This chapter gives a basic introduction to hepatic progenitor cell biology and explores what is known about progenitor cell-mediated liver regeneration. We also discuss the putative stem cell niche in the liver, as well as the signaling pathways involved in stem cell regulation. Finally, the isolation and clinical application of stem cells to human diseases is reviewed, along with the current thoughts on the relationship between stem cells and cancer.

Isolation and characterization of adult human liver progenitors from ischemic liver tissue derived from therapeutic hepatectomies

Recent evidence suggests that progenitor cells in adult tissues and embryonic stem cells share a high resistance to hypoxia and ischemic stress. To study the ischemic resistance of adult liver progenitors, we characterized remaining viable cells in human liver tissue after cold ischemic treatment for 24-168 h, applied to the tissue before cell isolation. In vitro cultures of isolated cells showed a rapid decline of the number of different cell types with increasing ischemia length. After all ischemic periods, liver progenitor-like cells could be observed. The comparably small cells exhibited a low cytoplasm-to-nucleus ratio, formed densely packed colonies, and showed a hepatobiliary marker profile. The cells expressed epithelial cell adhesion molecule, epithelial-specific (CK8/18) and biliary-specific (CK7/19) cytokeratins, albumin, alpha-1-antitrypsin, cytochrome-P450 enzymes, as well as weak levels of hepatocyte nuclear factor-4 and gamma-glutamyl transferase, but not alpha-fetoprotein or Thy-1. In vitro survival and expansion was facilitated by coculture with mouse embryonic fibroblasts. Hepatic progenitor-like cells exhibit a high resistance to ischemic stress and can be isolated from human liver tissue after up to 7 days of ischemia. Ischemic liver tissue from various sources, thought to be unsuitable for cell isolation, may be considered as a prospective source of hepatic progenitor cells.

Hepatic stem/progenitor cells and stem-cell transplantation for the treatment of liver disease

Allogeneic liver transplantation is still the only effective treatment available to patients with liver failure. However, because there is a serious shortage of liver donors, an alternative therapeutic approach is needed. Transplantation of mature hepatocytes has been evaluated in clinical trials, but the long-term efficacy remains unclear and the paucity of donor cells limits this strategy. Stem-cell transplantation is a more promising alternative approach. Several studies have provided information about the mechanism underlying the proliferation and differentiation of hepatic stem/progenitor cells. Moreover, in experimental models of liver disease, transplantation of hepatic stem/progenitor cells or hepatocyte-like cells derived from multipotent stem cells led to donor cell-mediated repopulation of the liver and improved survival rates. However, before stem-cell transplantation can be applied in the clinic to treat liver failure in humans, it will be necessary to overcome several difficulties associated with the technique.

In vitro differentiation of liver progenitor cells derived from healthy dog livers

Naturally occurring liver disease in dogs resemble human liver disease in great detail; including the activation of liver progenitor cells (LPC) in acute and chronic liver disease. The aim of the present study was to isolate, culture, and characterize progenitor cells derived from healthy mature dog livers. A nonparenchymal cell fraction enriched with small hepatocytes was isolated and cultured in Hepatozyme-serum-free media (SFM) to stimulate the growth of colony-forming small epithelial cells. After 2 weeks of culturing, clonal expansion of keratin 7 (K7) immunopositive small cells with a large nucleus/cytoplasm ratio emerged in the hepatocyte monolayer. These colonies expressed genes of several hepatocyte (CYP1A1, ALB, and KRT18), cholangiocyte/LPC (KRT7 and KRT19), and progenitor cell markers (alpha-fetoprotein, CD44, prominin1, KIT, THY1, and neural cell adhesion molecule 1), indicating their immature and bipotential nature. Gene-expression profiles indicated a more pronounced hepatic differentiation in Hepatozyme-SFM compared to William's Medium E (WME). Furthermore, colony-forming cells differentiated toward intermediate hepatocyte-like cells with a more pronounced membranous K7 immunostaining. In conclusion, colony-forming small epithelial cells in long-term canine liver cell cultures express LPC markers and have differentiating capacities. These cells may therefore be considered as progenitor cells of the liver.

Isolation, in vitro cultivation and characterisation of foetal liver cells

Hepatocyte transplantation has recently become an efficient clinical method in the treatment of patients with metabolic liver diseases. The shortage of donor cells remains an obstacle to treat more patients. Foetal liver tissues may therefore be useful as an alternative source of generating functional hepatocytes after in vitro culture and maturation.

Stem cells in liver regeneration and therapy

The liver has adapted to the inflow of ingested toxins by the evolutionary development of unique regenerative properties and responds to injury or tissue loss by the rapid division of mature cells. Proliferation of the parenchymal cells, i.e. hepatocytes and epithelial cells of the bile duct, is regulated by numerous cytokine/growth-factor-mediated pathways and is synchronised with extracellular matrix degradation and restoration of the vasculature. Resident hepatic stem/progenitor cells have also been identified in small numbers in normal liver and implicated in liver tissue repair. Their putative role in the physiology, pathophysiology and therapy of the liver, however, is not yet precisely known. Hepatic stem/progenitor cells also known as "oval cells" in rodents have been implicated in liver tissue repair, at a time when the capacity for hepatocyte and bile duct replication is exhausted or experimentally inhibited (facultative stem/progenitor cell pool). Although much more has to be learned about the role of stem/progenitor cells in the physiology and pathophysiology of the liver, experimental analysis of the therapeutic value of these cells has been initiated. Transplantation of hepatic stem/progenitor cells or in vivo pharmacological activation of the pool of hepatic stem cells may provide novel modalities for the therapy of liver diseases. In addition, extrahepatic stem cells (e.g. bone marrow cells) are being investigated for their contribution to liver regeneration. Hepatic progenitor cells derived from embryonic stem cells are included in this review, which also discusses future perspectives of stem cell-based therapies for liver diseases.

Adult rat hepatic bipotent progenitor cells remain dormant even after extensive hepatectomy

S It remains unknown whether the normal adult liver contains bipotent stem/progenitor cells, and if it does, then what are the circumstances under which they proliferate. The aim of this study was to clarify whether the normal adult liver contains hepatic stem/progenitor cells, and if it does, will they be activated by extensive hepatectomy? Adult rat liver cells were isolated and cultured at a low-density, and the colony-forming assay was performed to evaluate the cell proliferative capacity. Immunocytochemistry and reverse transcription-polymerase chain reaction were used to investigate the multilineage differentiation capability. The rate of colony formation by cells from the normal liver and those from the regenerating liver after partial hepatectomy (PH) were compared to determine whether progenitor cell proliferation might be activated by PH. Only a few epithelial colonies (0.043+/-0.009% of nonparenchymal cells) continued to proliferate for more than 1 month. Reverse transcription-polymerase chain reaction and immnocytochemistry showed that these progenitor colonies expressed both hepatocyte and cholangiocyte markers. The proportion of progenitor cells that formed bipotential colonies did not differ significantly between the cells obtained from the normal and PH livers. Adult normal liver contains bipotent hepatic progenitor cells, but they are scarcely activated even after extensive hepatectomy.

Putative human liver progenitor cells in explanted liver

BACKGROUND/AIMS

Hepatocyte progenitors have frequently been cultured from rodents but reports from human liver are rare.

METHODS

Non-parenchymal cell fraction isolated from 19 explant livers (removed at orthotopic liver transplantation for acute or chronic liver disease) and histologically normal human liver was cultured.

RESULTS

Proliferating epithelioid colonies were identifiable after 2-3 weeks culture as a very rare event (<1 per million cells plated) expressing mRNAs and protein antigens of mixed hepatocytic/biliary phenotype. Colony survival could be prolonged by transduction of the catalytic sub-unit of telomerase. Hepatocyte growth factor, epidermal growth factor and oncostatin M did not further enhance hepatocytic differentiation. The expression of markers associated with hepatocyte precursor status was investigated by flow cytometry. Cells expressing the stem cell-associated markers CD133 and CD117 were identified at low frequency. The proportion of cells expressing the integrin CD49f was higher in diseased liver than in normal liver, but the proportion expressing the hepatocyte growth factor receptor c-met was lower. Successful enrichment of plated populations for progenitors was not achieved.

CONCLUSION

Although there is clear histological evidence of hepatocyte precursors in human explant livers, predictable culture of such cells with differentiation toward mature hepatocyte phenotype remains elusive.

In vitro differentiation of hepatic progenitor cells from mouse embryonic stem cells induced by sodium butyrate

Recently it was shown that embryonic stem (ES) cells could differentiate into hepatocytes both in vitro and in vivo, however, prospective hepatic progenitor cells have not yet been isolated and characterized from ES cells. Here we presented a novel 4-step procedure for the differentiation of mouse ES cells into hepatic progenitor cells and then hepatocytes. The differentiated hepatocytes were identified by morphological, biochemical, and functional analyses. The hepatic progenitor cells were isolated from the cultures after the withdrawal of sodium butyrate, which was characterized by scant cytoplasm, ovoid nuclei, the ability of rapid proliferation, expression of a series of hepatic progenitor cell markers, and the potential of differentiation into hepatocytes and bile duct-like cells under the proper conditions that favor hepatocyte and bile epithelial differentiation. The differentiation of hepatocytes from hepatic progenitor cells was characterized by a number of hepatic cell markers including albumin secretion, upregulated transcription of glucose-6-phosphatase and tyrosine aminotransferase, and functional phenotypes such as glycogen storage. The results from our experiments demonstrated that ES cells could differentiate into a novel bipotential hepatic progenitor cell and mature into hepatocytes with typical morphological, phenotypic and functional characteristics, which provides an useful model for the studies of key events during early liver development and a potential source of transplantable cells for cell-replacement therapies.

Two populations of Thy1-positive mesenchymal cells regulate in vitro maturation of hepatic progenitor cells

We previously reported that the in vitro maturation of CD49f(+)Thy1(-)CD45(-) (CD49f positive) fetal hepatic progenitor cells (HPCs) is supported by Thy1-positive mesenchymal cells derived from the fetal liver. These mesenchymal cell preparations contain two populations, one of a cuboidal shape and the other spindle shaped in morphology. In this study, we determined that the mucin-type transmembrane glycoprotein gp38 could distinguish cuboidal cells from spindle cells by immunocytochemistry. RT-PCR analysis revealed differences between isolated CD49f(+/-)Thy1(+)gp38(+)CD45(-) (gp38 positive) cells and CD49f(+/-)Thy1(+)gp38(-)CD45(-) (gp38 negative) cells, whereas both cells expressed mesenchymal cell markers. The coculture with gp38-positive cells promoted the maturation of CD49f-positive HPCs, which was estimated by positivity for periodic acid-Schiff (PAS) staining, whereas the coculture with gp38-negative cells maintained CD49f-positive HPCs negative for PAS staining. The expression of mature hepatocyte markers, such as tyrosine aminotransferase, tryptophan-2,3-dioxygenase, and glucose-6-phosphatase, were upregulated on HPCs by coculture with gp38-positive cells. Furthermore, transmission electron microscopy revealed the acquisition of mature hepatocyte features by HPCs cocultured with gp38-positive cells. This effect on maturation of HPCs was inhibited by the addition of conditioned medium derived from gp38-negative cells. By contrast, the upregulation of bromodeoxyuridine incorporation by HPCs demonstrated the proliferative effect of coculture with gp38-negative cells. In conclusion, these results suggest that in vitro maturation of HPCs promoted by gp38-positive cells may be opposed by an inhibitory effect of gp38-negative cells, which likely maintain the immature, proliferative state of HPCs.

Long-term culture of postnatal mouse hepatic stem/progenitor cells and their relative developmental hierarchy

Few studies on the long-term culture of postnatal mouse hepatic stem/progenitor cells have been reported. We successfully adapted a serum-free culture system that we employed previously to expand fetal mouse hepatic stem/progenitor cells and maintained them in culture over long periods. The expanded postnatal cells contained immature alpha-fetoprotein-positive cells along with hepatocytic and cholangiocytic lineage-committed cells. These cells expressed CD49f but not CD45, CD34, Thy-1, c-kit, CD31, or flk-1, and oncostatin M induced their differentiation. This heterogeneous population contained side population (SP) cells, which express the ATP-binding cassette transporter ABCG2, and sca-1+ cells. As mice aged, the frequency of SP and sca-1+ cells decreased along with the ability of cultured cells to expand. Approximately 20%-40% of the SP cells expressed sca-1, but only a few sca-1+ cells were also SP cells. Analysis of colonies derived from single SP or sca-1+ cells revealed that, although both cells had dual differentiation potential and self-renewal ability, SP cells formed colonies more efficiently and gave rise to SP and sca-1+ cells, whereas sca-1+ cells generated only sca-1+ progeny. Thus, SP cells are more characteristic of stem cells than are sca-1+ cells. In regenerating livers, ABCG2+ cells and sca-1+ cells were detected around or in the portal area (the putative hepatic stem cell niche). The expanded cells share many features of fetal hepatic stem/progenitor cells or oval cells and may be useful in determining the mechanisms whereby hepatic stem cells self-renew and differentiate.

Production of infectious hepatitis C virus by well-differentiated, growth-arrested human hepatoma-derived cells

Dimethyl sulfoxide (DMSO) has been shown to induce the differentiation of primary hepatocytes in vitro. When actively dividing poorly differentiated human hepatoma-derived (Huh7) cells were cultured in the presence of 1% DMSO, cells became cytologically differentiated and transitioned into a nondividing state, characterized by the induction of hepatocyte-specific genes. Moreover, these cells were highly permissive for acute hepatitis C virus (HCV) infection, and persistent long term infection of these cultures could also be achieved. As HCV naturally replicates in highly differentiated nondividing human hepatocytes, this system may more accurately mimic the conditions under which HCV replicates in vivo than previous models using poorly differentiated rapidly dividing hepatoma cells.

Plasticity of hepatic cell differentiation: bipotential adult mouse liver clonal cell lines competent to differentiate in vitro and in vivo

In fetal liver, bipotential hepatoblasts differentiate into hepatocytes and bile duct cells (cholangiocytes). The persistence of such progenitor cells in adult mouse liver is still debated. In damaged liver of adult murine animals, when hepatocyte proliferation is compromised, bipotential oval cells emerge, probably from bile ducts, proliferate, and differentiate to regenerate the liver. However, treatment to elicit oval cell proliferation is not necessary to obtain bipotential stem cells from adult mouse liver. Here, we have isolated bipotential clonal cell lines from healthy liver of 8-10-week-old C57BL/6 mice. Primary cultures established from hepatocyte-enriched suspensions were characterized by time-lapse image acquisition, immunocytology, and RNA transcript analysis. Although hepatocytes dedifferentiated with loss of apical polarity and other hepatocyte markers, they rapidly activated expression of bile duct/oval cell markers. Reversibility of these processes was achieved in part by culture under dilute Matrigel or by aging of confluent cultures. Cell lines were obtained at high frequency from mass cultures, from isolated colonies, and by primary cloning of the hepatocyte-enriched suspension. Cells of the clonal cell lines do not grow in soft agar and are nontumorigenic, and they express cytokeratin 19, A6 antigen, and alpha6 integrin, as well as a large panel of hepatocyte functions. Furthermore, they can participate in liver regeneration in albumin-urokinase-type plasminogen activator/severe combined immune-deficient mice, where they differentiate in clusters of hepatocytes and occasionally bile ducts. These results demonstrate the existence, in normal adult mouse liver, of a significant pool of clonogenic cells that are (or can become) bipotential.

Hepatic Stem Cells: In Search of

The field of stem cell biology has exploded with the study of a wide range of cellular populations involving endodermal, mesenchymal, and ectodermal organs. One area of extensive study has included the identification of hepatic stem and progenitor cell subpopulations. Liver stem cells provide insights into the potential pathways involving liver regeneration that are independent of mature hepatocytes. Hepatic progenitor cells are either bipotent or multipotent and capable of multiple rounds of replication. They have been identified in fetal as well as adult liver. Various injury models have been used to expand this cellular compartment. The nomenclature, origin, and function of the hepatic progenitor cell populations are areas of ongoing debate. In this review, we will discuss the different definitions and functions of hepatic progenitor cells as well as the current research efforts examining their therapeutic potential.

Suppressive effect on hepatocyte differentiation of hepatitis C virus core protein

The influence of hepatitis C virus (HCV) protein(s) on cellular differentiation remains to be clarified. Using murine normal liver epithelial cells, we investigated whether HCV core protein affects differentiation into hepatocytes. Mock and HCV core-expressing cells were stimulated with oncostatin M (OSM) and dexamethasone, and the degree of differentiation was evaluated by measuring the expression of albumin and tyrosine aminotransferase (TAT). Lower amounts after stimulation were found in HCV core-expressing cells than in mock cells. Phosphorylation of the signal transducer and activator transcription factor 3 (STAT3) was prevented by the HCV core under OSM stimulation. Reporter gene assay revealed that the HCV core/Janus kinase (JAK) interaction directly suppressed the OSM-dependent JAK-STAT signal transduction. Furthermore, expression of OSM receptor beta (OSMRbeta) after stimulation was prevented by the HCV core. In conclusion, the HCV core may suppress differentiation into hepatocytes via inhibition of the JAK-STAT pathway and OSMRbeta expression.

Hedgehog signaling maintains resident hepatic progenitors throughout life

Hedgehog signaling through its receptor, Patched, activates transcription of genes, including Patched, that regulate the fate of various progenitors. Although Hedgehog signaling is required for endodermal commitment and hepatogenesis, the possibility that it regulates liver turnover in adults had not been considered because mature liver epithelial cells lack Hedgehog signaling. Herein, we show that this pathway is essential throughout life for maintaining hepatic progenitors. Patched-expressing cells have been identified among endodermally lineage-restricted, murine embryonic stem cells as well as in livers of fetal and adult Ptc-lacZ mice. An adult-derived, murine hepatic progenitor cell line expresses Patched, and Hedgehog-responsive cells exist in stem cell compartments of fetal and adult human livers. In both species, manipulation of Hedgehog activity influences hepatic progenitor cell survival. Therefore, Hedgehog signaling is conserved in hepatic progenitors from fetal development through adulthood and may be a new therapeutic target in patients with liver damage.

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

The success of hepatocellular therapies using stem or progenitor cell populations is dependent upon multiple factors including the donor cell, microenvironment, and etiology of the liver injury. The following experiments investigated the impact of TGF-beta1 on a previously described population of hepatic progenitor cells (HPC). The majority of the hepatic progenitor cells were resistant to endogenously produced TGF-beta1's proapoptotic and anti-proliferative effects unlike more well-differentiated cellular populations (e.g., mature hepatocytes). Surprisingly, in vitro TGF-beta1 supplementation significantly inhibited de novo hepatic progenitor cell colony formation possibly via an indirect mechanism(s). Therefore despite the HPC's direct resistance to supplemental TGF-beta1, this cytokine's inhibitory effect on colony formation could have a potential negative impact on the use of these cells as a therapy for patients with liver disease.

Oncostatin M stimulates proliferation and functions of mouse fetal liver cells in three-dimensional cultures

In order to develop a tissue engineered bioartificial liver (BAL), long-term three-dimensional (3-D) culture of fetal liver cells (FLCs) utilizing porous polymer as a scaffold was performed for up to 1 month. The effects of the basal medium and supplementation with oncostatin M (OSM) on the proliferation and differentiation of mouse FLCs were examined in both 3-D culture and conventional monolayer dish culture. Compared with monolayer culture, cell numbers and hepatic function of FLCs were better maintained by 3-D culture. When two kinds of basal media were tested in this study, Williams' medium E (WE) was superior to minimum essential medium alpha (alphaMEM) in expressing hepatic function of FLCs in both 3-D and monolayer cultures, although higher cell densities were obtained with alphaMEM. OSM potently stimulated both cell growth and metabolic activity, especially in 3-D culture. When WE supplemented with OSM was used for 3-D culture, albumin secretion by FLCs increased dramatically after day 5, and a high level of secretion was maintained until the end of culture. During a period of over 1 month, no decrease of albumin secretion was observed. In conclusion, this 3-D culture method was expected to be one of the realistic attempts to develop a tissue engineered BAL.

Generation of glycogen- and albumin-producing hepatocyte-like cells from embryonic stem cells

We present a novel two-step protocol for the differentiation of embryonic stem (ES) cells into the hepatic lineage. Differentiated hepatocyte-like cells express genes and proteins characteristic for endodermal and hepatic cells and acquire a functional hepatic phenotype as demonstrated by albumin secretion and glycogen storage. During differentiation, alpha-fetoprotein, albumin, transthyretin, alpha-1-antitrypsin, cytochrome P450 subunits 2b9 and 2b13 and tyrosine aminotransferase transcripts are upregulated. Quantitative RT-PCR data revealed a fetal hepatic phenotype corresponding to day 13-14 of liver development. Terminally differentiated hepatocyte-like cells show a bi-nucleated, cuboidal morphology labeled by albumin, alpha-1-antitrypsin, liver amylase, dipeptidyl peptidase IV, c-met and cytokeratin 18. ES-derived intermediate cell types transiently and partially co-express nestin with albumin and alpha-fetoprotein, respectively, but not cytokeratin 19. This finding suggests an ES-derived potential hepatic progenitor cell type, which is partially nestin-, albumin- and alpha-fetoprotein-positive, but cytokeratin 19-negative.

Hepatocyte progenitors in man and in rodents--multiple pathways, multiple candidates

In severe injury, liver-cell progenitors may play a role in recovery, proliferating, and subsequently differentiating into mature liver cells. Identifying these progenitors has major therapeutic potential for ex vivo pharmaceutical testing, bioartificial liver support, tissue engineering and gene therapy protocols. Potential liver-cell progenitors have been identified from bone marrow, peripheral blood, cord blood, foetal liver, adult liver and embryonic stem cells. Differences and similarities are found among cells isolated from rodents and humans. This review will discuss identifying markers and differentiation potential in in vitro and in vivo models of these putative progenitors in both humans and rodents.

Adenoviral-mediated efficiency expression of enhanced green fluorescence protein in adult liver stem cells of rats

AIM: To investigate the feasibility of adenoviral-mediated exogenous gene expression in adult liver stem cells of rats and to establish a cell line that stably and efficiently express enhanced green fluorescence protein (EGFP).

METHODS: A pAd-CMV-EGFP vector under the control of CMV promoter was constructed by homologous recombination in E.coilBJ 5 183, and the recombinant virus was Packaged in HEK 293 cell line. Hepatic adult stem cells cultured in vitro were infected with recombinant adenovirus. Expression of EGFP was observed by fluorescent microscopy and infection efficiency was analyzed. Adult liver stem cells were further cultured to estabilish a cell line that stably and efficiently expressed EGFP through cloning culture and the biological characteristics of the cell line were observed and analyzed by fluorescence microscopy, immunocytochemistry and differentiation-inducing experiment.

RESULTS: Adenovirus vector of pAd-CMV-EGFP was constructed and high titer recombinant virus were produced successfully. EGFP, mediated by adenovirus, could be transfected into hepatic adult stem cells with a high efficiency (about 40-70%). After cloning culture, WB-EGFP cell line was established, and it could stably express EGFP in 8-9 generations. Furthermore, biological characteristics such as marker of stem cells, proliferation speed and differentiation capability had not been affected.

CONCLUSION: Target gene can be efficiently transfected into hepatic adult stem cells through adeno-vector system. EGFP can be stably and long-term expressed in transfected cells and their offspring. It can serve as a tracker in the research of stem cells.

Inhibition of mouse hepatocyte apoptosis via anti-Fas ribozyme

AIM: To investigate the effects of anti-Fas ribozyme on Fas expression and apoptosis in primary cultured mouse hepatocytes.

METHODS: Mouse hepatocytes were isolated by using collagenase irrigation. A hammerhead ribozyme targeting the Fas mRNA was constructed, and transfected into mouse hepatocytes via Effectene. Then Fas expression in mouse hepatocytes was detected by RT-PCR and western blotting. After being treated with anti-Fas antibody (JO₂), hepatocytes viability was measured with MTT assay. Caspase-3 proteolytic activity was detected, and cell apoptosis was measured according to Annexin V-FITC apoptosis detection kit.

RESULTS: Fas expressed in primary mouse hepatocytes. Fas expression in hepatocytes transfected with anti-Fas ribozyme was decreased remarkably and correlated with the resistance to Fas-mediated apoptosis as determined by flow cytometry and caspase-3 proteolytic activity.

CONCLUSION: Anti-Fas ribozyme can remarkably decrease the Fas expression in mouse hepatocytes, thus inhibit Fas-mediated apoptosis in hepatocytes. It is suggested that anti-Fas ribozyme could significantly increase the resistance of transplanted hepatocytes to apoptosis and improve the survival of transplanted hepatocytes.

Differentiation of adult bone marrow CD45-CD117- stem cells into hepatocyte in vitro

AIM: To observe whether the CD117-CD45- bone marrow cell can differentiate into the cells with characters of hepatocyte by the stimulation of hepatocyte growth factor (HGF)and fibroblast growth factor-4 (FGF4), and to explore the acting mechanism of the growth factors.

METHODS: Bone marrow cells were from the stern or ilium of 4 healthy volunteers between 4-40 years old. CD117-CD45- bone marrow cells separated by magnetic cell sorting method, cultured in DMEM medium with FGF4, HGF, FGF4+ HGF or no growth factor were divided into four groups: groups A, B, C, and D accordingly. Cells were collected on d 0, 7, 14, 21, and 28 for detecting the hepatocyte markers: AFP, CK18, albumin by immunocytochemistry, glycogen by PAS staining and c-met, FGFR2 mRNA by RT-PCR.

RESULTS: The specific markers of hepatocyte were positive in groups A, B and C but not in group D. C-met and FGFR2 mRNA expressed at a low level in the cells newly isolated or cultured in the medium without growth factor, but at increasing level after cutured with growth factor.

CONCLUSION: HGF and FGF4 can induce CD45-CD117- differentiate into hepatocyte-like cells. The effect of growth factor on cell differentiation is probably by the positive regulation between the growth factor and its receptor.