Phenotypic characterization of rat hepatoma cell lines and lineage-specific regulation of gene expression by differentiation agents

A PCR assay to quantify patterns of HBV transcription

Hepatitis B virus (HBV) is the prototype member of the family Hepadnaviridae and replicates via episomal copies of a covalently closed circular DNA (cccDNA) genome of approximately 3.2 kb. The chromatinization of this small viral genome, with overlapping open reading frames and regulatory elements, suggests an important role for epigenetic pathways to regulate HBV transcription. However, the host pathways that regulate HBV transcription and the temporal nature of promoter usage in infected cells are not well understood, in part due to the compact genome structure and overlapping open reading frames. To address this we developed a simple and cost-effective PCR assay to quantify the major viral RNAs and validated this technique using current state-of-art de novo HBV infection model systems. Our PCR method is three orders of magnitude more sensitive than Northern blot and requires relatively small amounts of starting material, making this an attractive tool for assessing HBV transcription.

Tumoral response and tumoral phenotypic changes in a rat model of diethylnitrosamine-induced hepatocellular carcinoma after salirasib and sorafenib administration

Background

Several intracellular signaling pathways that are deregulated during hepatocarcinogenesis might constitute potential targets for hepatocellular carcinoma (HCC) therapy. The aim of this study was to test the potential synergic antitumor effect of salirasib and sorafenib in a diethylnitrosamine (DEN)-induced HCC model in rat. The hypothesis of tumor phenotype changes during treatment was also analyzed.

Materials and methods

DEN was administered to Wistar rats during 9 weeks to induce cirrhosis and liver cancer. After tumor development, rats were treated with intraperitoneal injections of dimethyl sulfoxide (DMSO), or salirasib, and/or with oral sorafenib 5 days/week, during 4 weeks. At sacrifice, number and size of liver tumors as well as tumor burden were recorded, and all liver tumors were processed for histological and immunohistological analyses.

Results

Mortality rate was significantly higher in rats treated with salirasib and/or sorafenib than in the control group (P=0.001). Tumor burden was smaller in the treated group compared with the DMSO control group (P=0.044), but a synergistic effect of the two chemotherapies could not be observed. In 62.5% of rats (10/16) treated with salirasib and/or sorafenib, a cytokeratin-7 and -19-positive hepatocholangiocellular carcinoma (HCC/CHC) was found vs 20% (5/25) developing such phenotype in the DMSO control group (P=0.018). Ki67 immunostaining showed significantly reduced tumor cell proliferation in treated rats (P=0.001), whereas apoptosis as assessed by caspase-3 activity in cell lysate was similar in all groups.

Conclusions

The addition of sorafenib to salirasib did not seem to provide any synergistic therapeutic effect in this study. Both chemotherapeutic agents, administered alone or in combination, induced tumoral phenotypic changes in the majority of rats, a finding not associated with an increased tumor cell proliferation or decreased apoptosis. The rat model described in this work constitutes the first experimental tool generating putatively more aggressive combined HCC/CHC tumors following chemotherapy. Further work is required to better characterize this clinically relevant phenomenon.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Hepatic glycogen synthesis in the absence of glucokinase: the case of embryonic liver

Glucokinase (GK, hexokinase type IV) is required for the accumulation of glycogen in adult liver and hepatoma cells. Paradoxically, mammalian embryonic livers store glycogen successfully in the absence of GK. Here we address how mammalian embryonic livers, but not adult livers or hepatoma cells, manage to accumulate glycogen in the absence of this enzyme. Hexokinase type I or II (HKI, HKII) substitutes for GK in hepatomas and in embryonic livers. We engineered FTO2B cells, a hepatoma cell line in which GK is not expressed, to unveil the modifications required to allow them to accumulate glycogen. In the light of these results, we then examined glycogen metabolism in embryonic liver. Glycogen accumulation in FTO2B cells can be triggered through elevated expression of HKI or either of the protein phosphatase 1 regulatory subunits, namely PTG or G L. Between these two strategies to activate glycogen deposition in the absence of GK, embryonic livers choose to express massive levels of HKI and HKII. We conclude that although the GK/liver glycogen synthase tandem is ideally suited to store glycogen in liver when blood glucose is high, the substitution of HKI for GK in embryonic livers allows the HKI/liver glycogen synthase tandem to make glycogen independently of the glucose concentration in blood, although it requires huge levels of HK. Moreover, the physiological consequence of the HK isoform switch is that the embryonic liver safeguards its glycogen deposits, required as the main source of energy at birth, from maternal starvation.

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.

Differential effects of histone deacetylase inhibitors in tumor and normal cells-what is the toxicological relevance?

Histone deacetylase (HDAC) inhibitors target key steps of tumor development: They inhibit proliferation, induce differentiation and/or apoptosis, and exhibit potent antimetastatic and antiangiogenic properties in transformed cells in vitro and in vivo. Preliminary studies in animal models have revealed a relatively high tumor selectivity of HDAC inhibitors, strenghtening their promising potential in cancer chemotherapy. Until now, preclinical in vitro research has almost exclusively been performed in cancer cell lines and oncogene-transformed cells. However, as cell proliferation and apoptosis are essential for normal tissue and organ homeostasis, it is important to investigate how HDAC inhibitors influence the regulation of and interplay between proliferation, differentiation, and apoptosis in primary cells as well. This review highlights the discrepancies in molecular events triggered by trichostatin A, the reference compound of hydroxamic acid-containing HDAC inhibitors, in hepatoma cells and primary hepatocytes (which are key targets for drug-induced toxicity). The implications of these differential outcomes in both cell types are discussed with respect to both toxicology and drug development. In view of the future use of HDAC inhibitors as cytostatic drugs, it is highly recommended to include both tumor cells and their healthy counterparts in preclinical developmental studies. Screening the toxicological properties of compounds early in their development process, using a battery of different cell types, will enable researchers to discard those compounds bearing undesirable adverse activity before entering into expensive clinical trials. This will not only reduce the risk for harmful exposure of patients but also save time and money.

Efficacy of a polyurethane foam/spheroid artificial liver by using human hepatoblastoma cell line (Hep G2)

We invesigated the availability of human hepatoblastoma cell line (Hep G2), compared with human primary hepatocytes (HH) and porcine primary hepatocytes (PH), as a cell source for the hybrid artificial liver support system (HALSS) by using polyurethane foam (PUF). All three kinds of hepatocytes spontaneously formed spherical multicellular aggregates (spheroids) of 100-200 microm diameter in the pores of PUF within 3 days of culture. In a PUF stationary culture, Hep G2 spheroids recovered the ammonia removal activity that was lost in monolayer culture, although the removal for each unit cell number was about one tenth that of HH spheroids and about one eighth of PH spheroids. The synthesis activities of albumin and fibrinogen of each unit cell number of Hep G2 were also upregulated by PUF spheroid culture, and were about twice as high as in monolayer culture. The albumin secretion activity of Hep G2 spheroids was almost the same as that of PH spheroids. HH scarcely secreted these proteins in this experiment, probably because they were cultured in a serum-free medium. In the PUF module in a circulation culture, HH had high ammonia removal and low synthesis activities similar to stationary culture. Hep G2 proliferated to a high cell density, such as about 4.8 x 10(7) cells/cm3-module at 10 days of culture. Although Hep G2 spheroids had low ammonia removal activity in each cell, the removal rate in the PUF module was almost the same as for PH at 7 days of culture because of the high cell density culture by cell proliferation. The albumin secretion rate by Hep G2 in the PUF module also increased with cell proliferation and was about 10 times higher than the initial for the rate for PH at 7 days of culture. These results suggest that Hep G2 is a potential cell source PUF-HALSS.

The role of liver X receptor-alpha in the fatty acid regulation of hepatic gene expression

Liver X receptors (LXR) alpha and beta play an important role in regulating the expression of genes involved in hepatic bile and fatty acid synthesis, glucose metabolism, as well as sterol efflux. Studies with human embryonic kidney 293 cells indicate that unsaturated fatty acids interfere with oxysterols binding to LXR and antagonize oxysterol-induced LXRalpha activity. In this report, we evaluated the effects of unsaturated fatty acids on LXR-regulated hepatic gene expression. The LXR agonist, T1317, induced mRNAs encoding sterol regulatory element-binding protein 1c (SREBP-1c) and two SREBP-1c-regulated lipogenic genes, e.g. fatty-acid synthase and the S14 protein in primary hepatocytes. Treatment of hepatocytes with eicosapentaenoic acid (20:5n-3) suppressed these mRNAs in the absence and presence of T1317. The cis-regulatory elements targeted by T1317 were not required for fatty-acid suppression of FAS or S14 promoter activity. In contrast to SREBP-1-regulated lipogenic genes, 20:5n-3 had no effect on the T1317 induction of ABCG5 or ABCG8 in the rat hepatoma cell line, FTO-2B. These two genes require LXR but not SREBP-1c for their expression. Feeding rats a diet supplemented with fish oil suppressed hepatic SREBP-1c-regulated genes and induced PPARalpha-regulated genes but had no effect on the LXR-regulated transcripts, CYP7A1, ABCG5, or ABCG8. Transfection studies, using either full-length hLXRalpha or a chimera containing only the LXRalpha ligand binding domain, indicate that a wide array of unsaturated fatty acids had little effect on LXRalpha activity in primary hepatocytes or FTO-2B. These studies suggest that LXRalpha is not a target for unsaturated fatty acid regulation in primary rat hepatocytes or in liver. Thus, oxysterol/LXR-mediated regulation of transcripts involved in bile acid synthesis or sterol efflux appear insensitive to dietary unsaturated fatty acids. The unsaturated fatty acid suppression of SREBP-1 and its targeted lipogenic genes is independent of LXRalpha

Expression of HNFs and C/EBP alpha is correlated with immunocytochemical differentiation of cell lines derived from human hepatocellular carcinomas, hepatoblastomas and immortalized hepatocytes

Objective assessment of the differentiation grade of hepatocellular carcinomas (HCCs) is important for evaluation of the pathological diagnosis, prognosis and therapeutic treatment. Differentiation of hepatocytes is reflected by their expression of hepatic functional proteins in the mouse embryo, and liver-enriched transcription factors (LETFs) have been shown to regulate hepatic functional genes strictly. Previous reports demonstrated that the level of LETF expression is altered in HCC or preneoplastic nodules compared with noncancerous tissues. Therefore, LETF expression levels might be useful as a measure of HCC maturation. In this study, to clarify the correlation between the expression of LETFs and the differentiation grade of HCCs, we performed a quantitative analysis of the mRNA expressions of HNFs and C/EBP alpha using real-time reverse-transcription PCR and immunocytochemical analysis for hepatic functional proteins in twelve cell lines. Furthermore, we examined orthotopic transplantations of the HCC cell lines in C.B-17/Icrj-scid/scid mice and characterized the histologic and cytologic differentiation of the tumors that developed. Our results showed that comprehensive expressions of HNF-3beta, HNF-4 alpha, HNF-1 alpha, and C/EBP alpha were specific to HCCs with well-differentiated function and morphology. Furthermore, among these four transcription factors, HNF-4 alpha and HNF-1 alpha expressions showed synchronism and had a close relation with HCC differentiation. These in vitro results were confirmed in tumors developed in SCID mice in vivo. These findings suggested that HNF-4 alpha and HNF-1 alpha are useful markers to assess the degree of HCC differentiation, which we suggest could be evaluated objectively by the quantitative analysis of HNFs and C/EBP alpha in HCCs.

Liver glycogen synthase but not the muscle isoform differentiates between glucose 6-phosphate produced by glucokinase or hexokinase

Using adenovirus-mediated gene transfer into FTO-2B cells, a rat hepatoma cell line, we have overexpressed hexokinase I (HK I), glucokinase (GK), liver glycogen synthase (LGS), muscle glycogen synthase (MGS), and combinations of each of the two glucose-phosphorylating enzymes with each one of the GS isoforms. FTO-2B cells do not synthesize glycogen even when incubated with high doses of glucose. Adenovirus-induced overexpression of HK I and/or LGS, two enzymes endogenously expressed by these cells, did not produce a significant increase in the levels of active GS and the total glycogen content. In contrast, GK overexpression led to the glucose-dependent activation of endogenous or overexpressed LGS and to the accumulation of glycogen. Similarly overexpressed MGS was efficiently activated by the glucose-6-phosphate (Glc-6-P) produced by either endogenous or overexpressed HK I and by overexpressed GK. These results indicate the existence of at least two pools of Glc-6-P in the cell, one of them is accessible to both isoforms of GS and is replenished by the action of GK, whereas LGS is excluded from the cellular compartment where the Glc-6-P produced by HK I is directed. These findings are interpreted in terms of the metabolic role that the two pairs of enzymes, HK I-MGS in the muscle and GK-LGS in the hepatocyte, perform in their respective tissues.

In vitro differentiation of WB-F344 rat liver epithelial cells into the biliary lineage

Differentiation of hepatic precursor cells in the biliary lineage has rarely been investigated, owing to the lack of convenient in vitro models. In this study, we used sodium butyrate and culture on Matrigel to promote differentiation of WB-F344 rat liver epithelial cells along the biliary phenotype. This differentiation was assessed by following the expression of phenotypic markers at the protein or mRNA level. Sodium butyrate induced cytokeratin 19 expression and gamma-glutamyltranspeptidase activity, together with a large increase in gamma-glutamyltranspeptidase mRNA IV, a transcript expressed at high levels in biliary cells. We also observed an increase in aquaporin-1 and beta4 integrin mRNAs, encoding two proteins expressed in adult biliary cells. Culture on Matrigel increased cytokeratin 19, gamma-glutamyltranspeptidase, and BDS7 expression in WB-F344 cells which still expressed aquaporin-1 and beta4 integrin. These results show that WB-F344 cells are able to differentiate in vitro along the biliary pathway, making them a candidate model for analyzing the molecular events associated with the hepatoblast-biliary cell transition.

Hepatic progenitors and strategies for liver cell therapies

Liver cell therapies, including liver cell transplantation and bioartificial livers, are being developed as alternatives to whole liver transplantation for some patients with severe liver dysfunction. Hepatic progenitors are proposed as ideal cells for use in these liver cell therapies given their ability to expand extensively, differentiate into all mature liver cells, have minimal immunogenicity, be cryopreservable, and reconstitute liver tissue when transplanted. We summarize our ongoing efforts to develop clinical programs of hepatic progenitor cell therapies with a focus on hepatic stem cell biology and strategies that have emerged in analyzing that biology.

Soft, porous poly(D,L-lactide-co-glycotide) microcarriers designed for ex vivo studies and for transplantation of adherent cell types including progenitors

Our laboratory is undertaking tissue engineering of liver using enriched liver progenitor cells. We report here our ongoing study to design biodegradable and biocompatible three-dimensional substratum supports of both natural and synthetic polymeric materials suitable to the adhesion, growth, and differentiation of adult and progenitor liver cells for their transplantation, and for the development of a bioartificial liver assist device. Porous biocompatible and biodegradable microcarriers of diameter 20-40 microm and 100-300 microm were prepared from (alpha-hydroxy) acid family of polymers. Human hepatoma cell line HepG2 and adult rodent liver cells were found to attach to collagen-coated surface of poly(D,L-lactide-co-glycotide) microcarriers. HepG2 cells attached to the degradable microcarriers remained viable and underwent growth expansion, forming three-dimensional cell-degradable microcarrier colonies in culture. These cell-degradable microcarrier colonies may undergo further growth expansion, thus providing a viable approach for three-dimensional organogenesis of tissue.

The in vitro differentiating capacity of nonparenchymal epithelial cells derived from adult porcine livers

Specific nonparenchymal epithelial cell (NPEC) clusters derived from normal adult porcine livers demonstrate a characteristic developmental pattern in the presence of other types of nonparenchymal cells in vitro. This pattern includes scattering, colonial growth, and an emergence of duct-like structures (DLSs) in the colonies. It has been confirmed that 96% of the scattered cell clusters in these cultures develop into colonies containing DLSs. In this study, we examine the differentiation of NPEC clusters using the scattered formation as a marker of the DLS-emerged colonies. We report that the NPECs expressed albumin, alpha-fetoprotein, transferrin, cytokeratin (CK) 18, CK7, and c-met, but not alpha-1-antitrypsin (AAT), at the scattering stage. In addition, at the same stage, NPECs expressed oval-cell-related markers such as OV6, but not biliary epithelial cell (BEC) markers such as gamma-glutamyltransferase, CK19, and CK14. At the DLS emerging stage, hepatocyte markers, including AAT, were detectable in the cells either at the periphery of colonies or in the cells surrounded by the DLSs. On the other hand, the cells constituting DLSs expressed BEC markers, suggesting a bile duct nature of the DLSs. Furthermore, the cells in the colonies possessed an ultrastructural appearance of differentiated hepatocytes and BECs. These results suggest that certain NPECs are bipotent, and that, in culture, they mimic hepatoblast development in vivo.