HNF4 and HNF1 as well as a panel of hepatic functions are extinguished and reexpressed in parallel in chromosomally reduced rat hepatoma-human fibroblast hybrids

Regulation of hepatitis B virus expression in progenitor and differentiated cell types: evidence for negative transcriptional control in nonpermissive cells

Mechanisms regulating cell type-specific gene expression are not completely understood. We utilized hepatitis B virus (HBV) enhancer I and preS1 promoter sequences, which exhibit cell type specificity, to analyze transcriptional control in pluripotential murine embryonic stem (ES) cells, bipotential HBC-3 progenitor liver cells, mature hepatocytes, and fibroblasts. In transient transfection assays, HBV sequences were most active in primary hepatocytes, followed by HBC-3 and ES cells, and became inactive in fibroblasts. Cotransfections with HNF-3 or C/EBP plasmids increased expression of HBV sequences in hepatocytes and HBC-3 cells. However, increased HBV expression was not observed in ES cells and HBV remained inactive in fibroblasts, suggesting different transcriptional controls, which was compatible with alterations in the abundance of endogenous transcription factors. Analysis in somatic hybrid cells created from NIH 3T3 fibroblasts and Hepa1-6 mouse hepatocytes with expression of albumin and selected hepatic transcription factors showed that HBV sequences were expressed weakly but without increased expression following transfection of HNF-1, HNF-3, and C/EBP plasmids. These findings indicated that repression of HBV in nonpermissive cells involved inactivation of transcription factor activity. Expression of HBV in stem cells is relevant for mechanisms concerning viral persistence and oncogenesis, as well as analysis of hepatocytic differentiation in progenitor cells.

Alcohol-induced defects in hepatic transcytosis may be explained by impaired dynein function

Alcoholic liver disease has been clinically well described, but the molecular mechanisms leading to hepatotoxicity have not been fully elucidated. Previously, we determined that microtubules are hyperacetylated and more stable in ethanol-treated WIF-B cells, VL-17A cells, liver slices, and in livers from ethanol-fed rats. From our recent studies, we believe that these modifications can explain alcohol-induced defects in microtubule motor-dependent protein trafficking including nuclear translocation of a subset of transcription factors. Since cytoplasmic dynein/dynactin is known to mediate both microtubule-dependent translocation and basolateral to apical/canalicular transcytosis, we predicted that transcytosis is impaired in ethanol-treated hepatic cells. We monitored transcytosis of three classes of newly synthesized canalicular proteins in polarized, hepatic WIF-B cells, an emerging model system for the study of liver disease. As predicted, canalicular delivery of all proteins tested was impaired in ethanol-treated cells. Unlike in control cells, transcytosing proteins were observed in discrete sub-canalicular puncta en route to the canalicular surface that aligned along acetylated microtubules. We further determined that the stalled transcytosing proteins colocalized with dynein/dynactin in treated cells. No changes in vesicle association were observed for either dynein or dynactin in ethanol-treated cells, but significantly enhanced dynein binding to microtubules was observed. From these results, we propose that enhanced dynein binding to microtubules in ethanol-treated cells leads to decreased motor processivity resulting in vesicle stalling and in impaired canalicular delivery. Our studies also importantly indicate that modulating cellular acetylation levels with clinically tolerated deacetylase agonists may be a novel therapeutic strategy for treating alcoholic liver disease.

Adiponectin regulates expression of hepatic genes critical for glucose and lipid metabolism

The effects of adiponectin on hepatic glucose and lipid metabolism at transcriptional level are largely unknown. We profiled hepatic gene expression in adiponectin knockout (KO) and wild-type (WT) mice by RNA sequencing. Compared with WT mice, adiponectin KO mice fed a chow diet exhibited decreased mRNA expression of rate-limiting enzymes in several important glucose and lipid metabolic pathways, including glycolysis, tricarboxylic acid cycle, fatty-acid activation and synthesis, triglyceride synthesis, and cholesterol synthesis. In addition, binding of the transcription factor Hnf4a to DNAs encoding several key metabolic enzymes was reduced in KO mice, suggesting that adiponectin might regulate hepatic gene expression via Hnf4a. Phenotypically, adiponectin KO mice possessed smaller epididymal fat pads and showed reduced body weight compared with WT mice. When fed a high-fat diet, adiponectin KO mice showed significantly reduced lipid accumulation in the liver. These lipogenic defects are consistent with the down-regulation of lipogenic genes in the KO mice.

Analysis of polarized membrane traffic in hepatocytes and hepatic cell lines

The protocols described in this unit were developed to monitor membrane traffic in cultured cell monolayers that display hepatic polarity. In general, the assays are designed to visualize and/or quantitate membrane trafficking by monitoring the fates of antibodies bound to specific membrane proteins. We first describe how to infect cells with recombinant adenovirus, the preferred method for introducing exogenous genes into hepatic cells. We next provide a morphological assay to monitor basolateral to apical transcytosis. In a supporting protocol, we describe how to visualize apical recycling and/or retention. In an additional supporting protocol, we provide a semi-quantitative method to measure the relative extents of apical delivery. Finally, we describe quantitative assays to measure basolateral internalization and recycling. The methods presented in this unit provide a relatively simple, yet powerful approach to examining hepatic membrane traffic.

HNF4A is essential for specification of hepatic progenitors from human pluripotent stem cells

The availability of pluripotent stem cells offers the possibility of using such cells to model hepatic disease and development. With this in mind, we previously established a protocol that facilitates the differentiation of both human embryonic stem cells and induced pluripotent stem cells into cells that share many characteristics with hepatocytes. The use of highly defined culture conditions and the avoidance of feeder cells or embryoid bodies allowed synchronous and reproducible differentiation to occur. The differentiation towards a hepatocyte-like fate appeared to recapitulate many of the developmental stages normally associated with the formation of hepatocytes in vivo. In the current study, we addressed the feasibility of using human pluripotent stem cells to probe the molecular mechanisms underlying human hepatocyte differentiation. We demonstrate (1) that human embryonic stem cells express a number of mRNAs that characterize each stage in the differentiation process, (2) that gene expression can be efficiently depleted throughout the differentiation time course using shRNAs expressed from lentiviruses and (3) that the nuclear hormone receptor HNF4A is essential for specification of human hepatic progenitor cells by establishing the expression of the network of transcription factors that controls the onset of hepatocyte cell fate.

Ethanol selectively impairs clathrin-mediated internalization in polarized hepatic cells

Although alcoholic liver disease is clinically well-described, the molecular basis for alcohol-induced hepatotoxicity is not well understood. Previously, we determined that the clathrin-mediated internalization of asialoglycoprotein receptor was impaired in ethanol-treated WIF-B cells whereas the internalization of a glycophosphatidylinositol-anchored protein thought to be endocytosed via a caveolae/raft-mediated pathway was not changed suggesting that clathrin-mediated endocytosis is selectively impaired by ethanol. To test this possibility, we examined the internalization of a panel of proteins and compounds internalized by different mechanisms in control and ethanol-treated WIF-B cells. We determined that the internalization of markers known to be internalized via clathrin-mediated mechanisms was impaired. In contrast, the internalization of markers for caveolae/raft-mediated endocytosis, fluid phase internalization or non-vesicle-mediated uptake was not impaired in ethanol-treated cells. We further determined that clathrin heavy chain accumulated at the basolateral surface in small puncta in ethanol-treated cells while there was decreased dynamin-2 membrane association. Interestingly, the internalization of resident apical proteins that lack any known internalization signals was also disrupted by ethanol suggesting that these proteins are internalized via clathrin-mediated mechanisms. This conclusion is consistent with our findings that dominant negative dynamin-2 overexpression impaired internalization of known clathrin markers and single spanning apical residents, but not of markers of fluid phase or raft-mediated internalization. Together these results indicate that ethanol exposure selectively impairs hepatic clathrin-mediated internalization by preventing vesicle fission from the plasma membrane.

Underexpressed Coactivators PGC1α AND SRC1 Impair Hepatocyte Nuclear Factor 4α Function and Promote Dedifferentiation in Human Hepatoma Cells

Hepatocyte nuclear factor 4alpha (HNF4alpha) plays critical roles during liver development and in the transcriptional regulation of many hepatic genes in adult liver. Here we have demonstrated that in human hepatoma HepG2 cells, HNF4alpha is expressed at levels as high as in human liver but its activity on target genes is very low or absent. We have discovered that the low expression of key coactivators (PGC1alpha, SRC1, SRC2, and PCAF) might account for the lack of function of HNF4alpha in HepG2 cells. Among them, PGC1alpha and SRC1 are the two most important HNF4alpha coactivators as revealed by reporter assays with an Apo-CIII promoter construct. Moreover, the expression of these two coactivators was found to be down-regulated in all human hepatomas investigated. Overexpression of SRC1 and PGC1alpha by recombinant adenoviruses led to a significant up-regulation of well characterized HNF4alpha-dependent genes (ApoCIII, ApoAV, PEPCK, AldoB, OTC, and CYP7A1) and forced HepG2 cells toward a more differentiated phenotype as demonstrated by increased ureogenic rate. The positive effect of PGC1alpha was seen to be dependent on HNF4alpha. Finally, insulin treatment of human hepatocytes and HepG2 cells caused repression of PGC1alpha and a concomitant down-regulation of ApoCIII, PEPCK, AldoB, and OTC. Altogether, our results suggest that SRC1, and notably PGC1alpha, are key coactivators for the proper function of HNF4alpha in human liver and for an integrative control of multiple hepatic genes involved in metabolism and homeostasis. The down-regulation of key HNF4alpha coactivators could be a determinant factor for the dedifferentiation of human hepatomas.

How to induce non-polarized cells of hepatic origin to express typical hepatocyte polarity: generation of new highly polarized cell models with developed and functional bile canaliculi

Few in vitro models expressing complex hepatocyte polarity are available. We used the unpolarized rat Fao cell line to isolate the polarized WIF-B line. These complex rat-human hybrid cells form functional simple bile canaliculi. To obtain Fao-derived polarized models with a simpler chromosome content and developed bile canaliculi, we employed two approaches. Partial success was achieved with monochromosomal hybrids. As shown by the immunolocalization of apical, basolateral, and tight-junctional proteins, monochromosomal hybrid 11-3 cells were polarized. They formed simple functional bile canaliculi and transiently expressed the typical polarity of simple epithelial cells. One subclone blocked in this polarity state was isolated. A more robust approach was provided by spheroid culture, a three-dimensional system that strengthens cell-cell contacts. Transient spheroid culture induced irreversible polarization of Fao cells. This induction occurred in most spheroids (approximately 1% of the cells). From populations enriched in stably polarized cells, we generated new polarized cell models, designated Can. Can 3-1 cells formed simple functional bile canaliculi when plated at high density. Regardless of plating density, Can 9 and Can 10 cells formed long tubular branched canaliculi competent for vectorial transport of organic anions and bile acids, and involving several dozen adjacent cells. Thus, we have generated new cell models stably expressing typical hepatocyte polarity. Among these models, Can 9 and Can 10 are the first capable of forming functional, highly developed bile canaliculi similar to those formed in vivo.

Dissociation of the Hepatic Phenotype from HNF4 and HNF1α Expression

Dedifferentiated cells have served as tools to understand the molecular consequences of the loss of tissue-specific pathways. Here we report the characterization of one of these cell lines, M29, which lacks the liver-enriched HNF4-HNF1α pathway, in order to determine if this class of variant cell lines could provide additional information regarding requirements for tissue-type expression. We report that although the liver-specific α1-antitrypsin (α1AT) gene remains silent despite reactivation of the HNF4/HNF1α pathway in the M29 cells, the frequency of activation of an integrated α1AT-APRT transgene is increased 1000-fold in response to these transcription factors. The human α1AT locus (introduced via chromosome transfer) also remained silent on these cells, despite HNF4 and HNF1α expression. Results from cell fusion experiments suggest that the defect in the M29 cells is recessive. Results suggest that the M29 cells contain a defect that represses liver gene expression despite the presence of the HNF4/HNF1α pathway.

Expression and subcellular localization of aquaporin water channels in the polarized hepatocyte cell line, WIF-B

Background

Recent data suggest that canalicular bile secretion involves selective expression and coordinated regulation of aquaporins (AQPs), a family of water channels proteins. In order to further characterize the role of AQPs in this process, an in vitro cell system with retained polarity and expression of AQPs and relevant solute transporters involved in bile formation is highly desirable. The WIF-B cell line is a highly differentiated and polarized rat hepatoma/human fibroblast hybrid, which forms abundant bile canalicular structures. This cell line has been reported to be a good in vitro model for studying hepatocyte polarity.

Results

Using RT-PCR, immunoblotting and confocal immunofluorescence, we showed that WIF-B cells express the aquaporin water channels that facilitate the osmotically driven water movements in the liver, i.e. AQP8, AQP9, and AQP0; as well as the key solute transporters involved in the generation of canalicular osmotic gradients, i.e., the bile salt export pump Bsep, the organic anion transporter Mrp2 and the chloride bicarbonate exchanger AE2. The subcellular localization of the AQPs and the solute transporters in WIF-B cells was similar to that in freshly isolated rat hepatocytes and in intact liver. Immunofluorescent costaining studies showed intracellular colocalization of AQP8 and AE2, suggesting the possibility that these transporters are expressed in the same population of pericanalicular vesicles.

Conclusion

The hepatocyte cell line WIF-B retains the expression and subcellular localization of aquaporin water channels as well as key solute transporters for canalicular bile secretion. Thus, these cells can work as a valuable tool for regulatory and mechanistic studies of the biology of bile formation.

Progression of HCC in mice is associated with a downregulation in the expression of hepatocyte nuclear factors

Hepatocyte nuclear factors (HNF) play a critical role in development of the liver. Their roles during liver tumorigenesis and progression of hepatocellular carcinomas (HCC) are, however, poorly understood. To address the role of HNFs in tumor progression, we generated a new experimental model in which a highly differentiated slow-growing transplantable mouse HCC (sgHCC) rapidly gives rise in vivo to a highly invasive fast-growing dedifferentiated variant (fgHCC). This in vivo model has allowed us to investigate the fundamental mechanisms underlying HCC progression. A complete loss of cell polarity, a decrease in cell-cell and cell-extracellular matrix (ECM) adhesion, elevation of telomerase activity, and extinction of liver-specific gene expression accompanies tumor progression. Moreover, cells isolated from fgHCCs acquired the ability to proliferate rapidly in culture. These alterations were coupled with a reduced expression of several liver transcription factors including HNF4, a factor essential for hepatocyte differentiation. Forced re-expression of HNF4alpha1 in cultured fgHCC cells reversed the progressive phenotype and induced fgHCC cells to re-establish an epithelium and reform cell-ECM contacts. Moreover, fgHCC cells that expressed HNF4alpha1 also re-established expression of the profile of liver transcription factors and hepatic genes that are associated with a differentiated hepatocyte phenotype. Importantly, re-expression of HNF4alpha1 in fgHCC reduced the proliferation rate in vitro and diminished tumor formation in congenic recipient mice. In conclusion, loss of HNF4 expression is an important determinant of HCC progression. Forced expression of this factor can promote reversion of tumors toward a less invasive highly differentiated slow-growing phenotype.

Mechanisms controlling early development of the liver

Over the last decade significant advances have been made in our understanding of the molecular mechanisms that control early aspects of mammalian liver development. Studies using tissue explant cultures and molecular biology techniques as well as the analysis of transgenic and knockout mice have identified signaling molecules and transcription factors that are necessary for the onset of hepatogenesis. This review presents an overview of these studies and discusses the role of individual factors during hepatic development.

Regulatory phases of early liver development: paradigms of organogenesis

Genetic analysis, embryonic tissue explantation and in vivo chromatin studies have together identified the distinct regulatory steps that are necessary for the development of endoderm into a bud of liver tissue and, subsequently, into an organ. In this review, I discuss the acquisition of competence to express liver-specific genes by the endoderm, the control of early hepatic growth, the coordination of hepatic and vascular development and the cell differentiation that is necessary to generate a functioning liver. The regulatory mechanisms that underlie these phases are common to the development of many organ systems and might be recapitulated or disrupted during stem-cell differentiation and adult tissue pathogenesis.

Extinction of expression of the genes encoding haematopoietic cell-restricted transcription factors in T-lymphoma x fibroblast cell hybrids

We previously reported that expression of the T-cell receptor (TCR) alpha and lck genes is extinguished in hybrids between mouse T-lymphoma EL4 cells and mouse fibroblast B82 cells. In the present study, we found that the activities of the TCRalpha minimum enhancer and the lck promoter monitored by the luciferase or chloramphenicol acetyltransferase (CAT) assays were markedly inhibited in the hybrids. Expression of the TCF-1, LEF-1, GATA-3, Ikaros, c-myb and Fli-1 genes, which encode the haematopoietic cell-restricted transcription factors that appear to be responsible for the activities of the enhancer and the promoter, was fully extinguished or markedly suppressed in the hybrids. On the other hand, expression of the transcription factor genes observed in both parental cells, such as the AML1 and c-ets-1 genes, and that of the genes encoding ubiquitously expressed transcription factors, such as the E2A, CREB and c-ets-2 genes, was not significantly suppressed in the hybrids. These results suggest that the genes encoding haematopoietic cell-restricted transcription factors are targets for negative regulation in fibroblastic background and that the repression of these genes may consequently lead to suppression of the promoter and/or enhancer activities of several T-cell-specific structural genes in T-lymphoma x fibroblast cell hybrids.

Identification of retinoic acid-responsive elements on the HNF1alpha and HNF4alpha genes

Hepatocyte nuclear factor 1alpha (HNF1alpha) and HNF4alpha are liver-selective transcription factors and are essential for hepatocyte differentiation. This study demonstrates that HNF1alpha as well as HNF4alpha genes contain a direct repeat with a space of one nucleotide (DR1)-retinoic acid (RA) response element that can be bound and regulated by RA and retinoid x receptor alpha (RXRalpha) complex. Transient transfection experiments showed that RA increased the promoter activity of the HNF1alpha and HNF4alpha genes in Hep3B cells. Overexpression of RXRalpha further enhanced the activities of both genes. Two putative RXRalpha binding sites on the HNF1alpha (-295 to -276) and HNF4alpha (-418 to -399) genes have been characterized. By transient transfection, both sites positively responded to RA, and overexpression of RXRalpha in Hep3B cells increased the regulatory effect. Gel mobility shift assay demonstrated that these two DR-1 sites could be bound by RXRalpha specifically. These data suggest that the differentiation effect of RA on hepatocyte may be due to direct interaction of RXRalpha with the RA-responsive elements on the HNF1alpha and HNF4alpha genes.

Fatty acyl-CoAs inhibit retinoic acid-induced apoptosis in Hep3B cells

Retinoic acid (RA) induces apoptosis in Hep3B human hepatoma cells. 9-Cis-RA (c-RA) had a similar effect as all-trans-RA (t-RA) in inducing cell death in Hep3B cells. RA-induced Hep3B-cell death was associated with inhibited expression of the hepatocyte nuclear factor 4 (HNF-4) gene. Palmitoyl-CoA ((C16:0)-CoA), the reported HNF-4 ligand, prevented RA-induced apoptosis. The effect of (C16:0)-CoA was specific, since palmitic acid and co-enzyme A had no effect in preventing RA-induced apoptosis. Bovine serum albumin (BSA) also prevented RA-induced apoptosis. However, in contrast to BSA, which induced cell growth, (C16:0)-CoA alone had no effect on cell growth. Investigating the possible role of HNF-4 in apoptosis, the reported HNF-4 antagonist (C18:0)-CoA was employed, and it also prevented RA-induced apoptosis. By transient transfection, overexpression of HNF-4 did not prevent RA-induced apoptosis. The induction and prevention of apoptosis caused by RA and (C16:0)-CoA were associated, respectively with the induction and inhibition of the expression of transforming growth factor beta (TGFbeta), which is known to play a role in apoptosis. Furthermore, RA and (C16:0)-CoA can regulate AP-1, which is a key regulator of the TGFbeta gene. Our data indicate that fatty acyl-CoAs can prevent RA-induced apoptosis and that TGFbeta, rather than HNF-4, may play a role in these regulatory processes. Our data also suggest that (C16:0)-CoA and (C18:0)-CoA are not the agonist and antagonist for HNF4, respectively in the Hep3B cell system.

Effects of growth and differentiation factors on the epithelial-mesenchymal transition in cultured neonatal rat hepatocytes

BACKGROUND/AIMS

Loss of specific differentiation markers, adoption of a migrating morphology and progressive replacement of the cytokeratin network by vimentin intermediate filaments characterize the epithelial-mesenchymal transition of cultured neonatal rat hepatocytes. In a previous study (Hepatology 1997; 25: 598-606), we reported that this process can be differentially regulated by EGF and DMSO, two agents that affect hepatocyte growth and differentiation. The aim of the present study was to determine if growth activation or differential gene expression could explain the differences in EMT observed between these two factors.

METHODS

We compared the effects of EGF, HGF, TGF-beta1 and DMSO on growth, proto-oncogene expression, epithelial-mesenchymal transition markers and expression of liver transcription factors in cultured neonatal rat hepatocytes using thymidine incorporation, Northern blotting and Western blotting analysis.

RESULTS

When TGF-beta1 or DMSO was added to the cultures supplemented with EGF and HGF, the mitogenic activity induced by these factors was inhibited. DMSO down-regulated c-myc and c-fos expression. mRNA levels of some liver-specific genes such as albumin, or liver-enriched transcription factors such as C/EBPdelta, HNF-4 and HNF-1beta were slightly different in cultures supplemented with DMSO or TGF-beta1. However, no differences were found when DMSO or TGF-beta1 was added to the cultures supplemented with EGF. Western blotting analysis showed that TGF-beta1 decreased cytokeratin and increased vimentin levels, while DMSO decreased both cytokeratin and vimentin. When DMSO or TGF-beta1 was added in combination with EGF or HGF, both factors maintained the increase in albumin and cytokeratin induced by the growth factors although DMSO, but not TGF-beta1, inhibited vimentin expression.

CONCLUSIONS

Activation of vimentin expression produced in cultures supplemented with the mitogenic factors (EGF and HGF) is independent of the activation of cell growth, because DMSO but not TGF-beta1 can abolish vimentin synthesis, although both inhibited growth. Moreover, the vimentin expression in these cultures seems to be independent of the mRNA levels of transcription factors associated with the differentiated liver phenotype.

The structural and functional polarity of the hepatic human/rat hybrid WIF-B is a stable and dominant trait

WIF-B cells were generated previously to obtain a good in vitro model expressing the structural and functional polarity of hepatocytes. Here we tested the stability and the strength of the WIF-B polarized phenotype. WIF-B cells stayed polarized and formed functional bile canaliculi even after 3 months in culture or after injection in nude mice and culture of the resulting tumors. WIF-B was subcloned and 10,000 colonies were examined; all (except for 3) were composed of bile canaliculi forming cells. Some subclones were characterized; the polarized ones presented the same properties and karyotype as the WIF-B cells; the 3 unpolarized subclones had a lower level of E-cadherin and different karyotypes. WIF-B cells were fused with their nonpolarized hepatic parental cells. The polarity state of the resulting FWIF hybrids was studied from day 11 to day 38 after fusion, by immunolocalization of hepatocyte domain-specific plasma membrane proteins. Most FWIF colonies (>80%) were composed of polarized cells. Soon after fusion these cells were exclusively polarized as simple epithelial cells. The percent of colonies containing cells expressing the typical hepatocyte polarity increased with time and reached 80% at day 38. This result confirms the two-step polarization process previously described for WIF-B. Chromosomally complete FWIF hybrids were examined several months after fusion. As shown by the study of bile acid transport and by confocal analysis of the localization of membrane domain markers, FWIF cells expressed a functional and fully polarized hepatic phenotype. In conclusion, polarity is a stable and dominant trait of WIF-B.

The HNF-4/HNF-1alpha transactivation cascade regulates gene activity and chromatin structure of the human serine protease inhibitor gene cluster at 14q32.1

Hepatocyte-specific expression of the alpha1-antitrypsin (alpha1AT) gene requires the activities of two liver-enriched transactivators, hepatocyte nuclear factors 1alpha and 4 (HNF-1alpha and HNF-4). The alpha1AT gene maps to a region of human chromosome 14q32.1 that includes a related serine protease inhibitor (serpin) gene encoding corticosteroid-binding globulin (CBG), and the chromatin organization of this approximately 130-kb region, as defined by DNase I-hypersensitive sites, has been described. Microcell transfer of human chromosome 14 from fibroblasts to rat hepatoma cells results in activation of alpha1AT and CBG transcription and chromatin reorganization of the entire locus. To assess the roles of HNF-1alpha and HNF-4 in gene activation and chromatin remodeling, we transferred human chromosome 14 from fibroblasts to rat hepatoma cell variants that are deficient in expression of HNF-1alpha and HNF-4. The variant cells failed to activate either alpha1AT or CBG transcription, and chromatin remodeling failed to occur. However, alpha1AT and CBG transcription could be rescued by transfecting the cells with expression plasmids encoding HNF-1alpha or HNF-4. In these transfectants, the chromatin structure of the entire alpha1AT/CBG locus was reorganized to an expressing cell-typical state. Thus, HNF-1alpha and HNF-4 control both chromatin structure and gene activity of two cell-specific genes within the serpin gene cluster at 14q32.1.

Regulation of alpha1-antitrypsin gene expression in human intestinal epithelial cell line caco-2 by HNF-1alpha and HNF-4

There is still relatively limited information about mechanisms of gene expression in enterocytes and mechanisms by which gene expression is regulated during enterocyte differentiation. Using the human intestinal epithelial cell line Caco-2, which spontaneously differentiates from a cryptlike to a villouslike enterocyte, we have previously shown that there is a marked increase in transcription of the well-characterized alpha1-antitrypsin (alpha1-AT) gene during enterocyte differentiation. In this study we examined the possibility of identifying the cis-acting elements and trans-acting DNA-binding proteins responsible for expression of the alpha1-AT gene in Caco-2 cells during differentiation. Footprint analysis and electrophoretic mobility shift assays showed that hepatocyte nuclear factor-1alpha (HNF-1alpha), HNF-1beta, and HNF-4 from nuclear extracts of Caco-2 cells specifically bound to two regions in the proximal promoter of the alpha1-AT gene. Cotransfection studies showed that HNF-1alpha and HNF-4 had a synergistic effect on alpha1-AT gene expression. RNA blot analysis showed that HNF-1alpha and HNF-4 mRNA levels and electrophoretic mobility shift assays showed that HNF-1alpha binding activity increase coordinately with alpha1-AT mRNA levels during differentiation of Caco-2 cells. Finally, overexpression of antisense ribozymes for HNF-1alpha in Caco-2 cells resulted in a selective decrease in endogenous alpha1-AT gene expression. Together, these results provide evidence that HNF-1alpha and HNF-4 play a role in the mechanism by which the alpha1-AT gene is upregulated during enterocyte differentiation in the model Caco-2 cell system.

Expression and localization of hepatocyte domain-specific plasma membrane proteins in hepatoma × fibroblast hybrids and in hepatoma dedifferentiated variants

We have studied two aspects of the plasma membrane of hepatocytes, highly differentiated epithelial cells that exhibit a particular and complex polarity. Using a genetic approach, we have distinguished between the expression/regulation of proteins specific for all three hepatocyte membrane domains and their organization into discrete domains. For this analysis we used a panel of previously isolated cell clones, derived from the differentiated rat hepatoma line H4IIEC3, and that present different expression patterns for liver-specific genes. This panel was composed of (1) differentiated clones, (2) chromosomally reduced hepatoma-fibroblast hybrids characterized by a pleiotropic extinction/reexpression of liver-specific genes and (3) dedifferentiated variant and revertant clones. The expression of 16 hepatocyte membrane polarity markers was studied by western blotting and immunolocalization. Even though cells of differentiated clones express all of these polarity markers, they are not polarized, and are therefore suitable for studying the regulation of plasma membrane protein expression, and for identifying gene products implicated in the establishment of membrane polarity. In hepatoma-fibroblast hybrids the expression of four markers, three apical (dipeptidylpeptidase IV, alkaline phosphodiesterase B10 and polymeric IgA receptor) and one lateral (E-cadherin), is down-regulated in extinguished clones and restored in reexpressing subclones, as previously reported for liver-specific functions. The dipeptidylpeptidase IV mRNA was undetectable or strongly reduced in extinguished hybrids, but expressed at a robust level in some of the reexpressing clones. Concerning the dedifferentiated variants, each has its own pattern of membrane marker expression (loss of expression of three to six markers), that differs from that of extinguished hybrids. Revertant cells express all of the membrane markers examined. Among all of these hepatoma derivatives, only cells of reexpressing hybrids are polarized, and form bile canaliculi-like structures, with spherical and even, for one clone, long tubular and branched forms. All apical markers examined are confined in these canalicular structures, whereas the other markers are excluded from them, and present on the rest of the membrane (basolateral markers) or at the cell-cell contacts (lateral markers). Cells of reexpressing hybrids also express simple epithelial polarity. Thus the expression of only a few hepatocyte-domain-specific plasma membrane proteins is subject to down-regulation, as is the case for liver-specific genes so far studied, and the expression of polarity markers and the formation of poles are dissociable events.

Phenotypic effects of the forced expression of HNF4 and HNF1alpha are conditioned by properties of the recipient cell

Tagged versions of HNF4 or HNF1alpha cDNAs in expression vectors have been introduced by transient and stable transfection into three cell lines of hepatic origin that all fail to express these two liver-enriched transcription factors and hepatic functions. C2 and H5 cells are dedifferentiated rat hepatoma variants and WIF12-E cells are human fibroblast-rat hepatoma hybrids with a reduced complement of human chromosomes. Transfectants were analyzed for the expression state of the endogenous genes coding for these transcription factors and for hepatic functions. Each cell line showed a different response to the forced expression of the transcription factors. In C2 cells, no measurable effect was observed, either upon transitory or stable expression. H5 cells reexpressed the endogenous HNF4 gene only upon transient HNF1alpha transfection, and the endogenous HNF1alpha gene only in stable HNF4 transfectants. WIF12-E cells responded to the forced transient or stable expression of either HNF1alpha or HNF4 by cross-activation of the corresponding endogenous gene. In addition, the stable transfectants reexpress HNF3alpha and C/EBPalpha, as well as all of the hepatic functions examined. Hybrid cells similar to WIF12-E had previously been observed to show pleiotropic reexpression of the hepatic phenotype in parallel with loss of human chromosome 2. For the stable WIF12-E transfectants, it was verified that reexpression of the hepatic phenotype was not due to loss of human chromosome 2. The demonstration of reciprocal cross-regulation between HNF4 and HNF1alpha in transient as well as stable transfectants implies that direct effects are involved.

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

Hepatoma cell lines can be characterized by their expression of hepatocyte- and biliary-specific genes and by their response to differentiating agents in a lineage-dependent manner. These characteristics can be used to map the maturational lineage position of the cell lines. Tissue-specific gene expression and regulation by heparin, dimethylsulfoxide (DMSO), and sodium butyrate (SB) were examined in three rat hepatoma cell lines and two rat liver epithelial cell lines. Based on antigenic profiles and gene expression in serum-supplemented medium, the hepatoma cell lines could be organized in distinct categories of hepatic differentiation. All three hepatomas expressed the following five genes: gamma-glutamyl transpeptidase (GGT), glutathione-S-transferase pi (Yp), glutamine synthetase, and alpha 5 and beta 1 integrin. Cell line H4AzC2 also expressed alpha-fetoprotein (AFP), albumin. IGF II receptor, and the biliary/oval cell antigens OC.2 and OC.3, a phenotype characteristic of fetal hepatocytes. FTO-2B cells lacked AFP, OC.2, and OC.3 but expressed albumin and IGF II receptor in addition to the five commonly expressed genes, consistent with a more hepatocyte-like phenotype. Cell line H5D-7 expressed neither albumin nor the IGF II receptor, but did express OC.2, OC.3, and alpha 3 integrin in addition to the five commonly expressed genes, characteristic of biliary epithelial cells. Regulation of gene expression by heparin, DMSO, and SB was examined in cells cultured in hormonally defined medium. The patterns of regulation of AFP, albumin, GGT, and Yp were dependent upon the state of differentiation of the cell. FTO-2B cells regulated genes in a manner similar to that of E16 fetal hepatocytes, H4AzC2 regulated genes characteristic of both hepatocytic and biliary lineages, and H5D.7 regulated only biliary genes. Suppression of GGT by DMSO was uniformly observed. The three cell lines expressed equal amounts of HNF-4, but FTO-2B cells expressed more HNF-3 beta and less HNF-3 alpha, while the reverse was true of H4AzC2 and H5D.7 cells.

The polarized hepatic human/rat hybrid WIF 12-1 and WIF-B cells communicate efficiently in vitro via connexin 32-constituted gap junctions

Gap junction intercellular communication (GJIC) plays an essential role in the control of growth, differentiation, and functions of different tissues. The expression of connexins (Cxs), the structural proteins of gap junctions, is developmentally regulated and tissue-specific. In vivo hepatocytes express Cx32 and Cx26. Most currently available in vitro hepatic cell systems express Cx43 instead of the expected Cxs. This work analyzes the GJIC competence and Cx expression of the highly differentiated and polarized hepatoma-derived hybrid cell lines, WIF 12-1 and WIF-B. It shows (using two dye transfer assays) that both lines communicate efficiently and that the acquisition of GJIC competence precedes the formation of bile canaliculi. Interestingly, these cells communicate via Cx32 expression, whereas Cx26 and Cx43 are not expressed, as demonstrated by Western and Northern blotting, immunocytochemistry, and confocal microscopy. The human fibroblast W138 parent communicates via Cx43, whereas the rat hepatoma parent Fao and the subclone WIF 12-1 TGdelta, that has lost the human X chromosome, do not communicate, the expression of Cx32 being restricted to the mRNA in these two lines. The GJIC competence of WIF cells could thus result from the activation of the human X chromosome-linked Cx32 gene.

Hepatocyte nuclear factor 4 provokes expression of epithelial marker genes, acting as a morphogen in dedifferentiated hepatoma cells

We have recently shown that stable expression of an epitope-tagged cDNA of the hepatocyte- enriched transcription factor, hepatocyte nuclear factor (HNF)4, in dedifferentiated rat hepatoma H5 cells is sufficient to provoke reexpression of a set of hepatocyte marker genes. Here, we demonstrate that the effects of HNF4 expression extend to the reestablishment of differentiated epithelial cell morphology and simple epithelial polarity. The acquisition of epithelial morphology occurs in two steps. First, expression of HNF4 results in reexpression of cytokeratin proteins and partial reestablishment of E-cadherin production. Only the transfectants are competent to respond to the synthetic glucocorticoid dexamethasone, which induces the second step of morphogenesis, including formation of the junctional complex and expression of a polarized cell phenotype. Cell fusion experiments revealed that the transfectant cells, which show only partial restoration of E-cadherin expression, produce an extinguisher that is capable of acting in trans to downregulate the E-cadherin gene of well-differentiated hepatoma cells. Bypass of this repression by stable expression of E-cadherin in H5 cells is sufficient to establish some epithelial cell characteristics, implying that the morphogenic potential of HNF4 in hepatic cells acts via activation of the E-cadherin gene. Thus, HNF4 seems to integrate the genetic programs of liver-specific gene expression and epithelial morphogenesis.

Efficient in vitro vectorial transport of a fluorescent conjugated bile acid analogue by polarized hepatic hybrid WIF-B and WIF-B9 cells

Efficient transport of bile acids, a typical characteristic of hepatocytes, is partially lost in most hepatoma cell lines and in normal hepatocytes after some days in culture. We have tested whether the polarized rat hepatoma-human fibroblast hybrid WIF (hybrids between W138 and Fao cells) cells previously obtained by our group were able to perform vectorial transport of the fluorescent bile acid derivative cholylglycylamidofluorescein (CGamF) towards the bile canaliculi (BC). Four different WIF clones were analyzed. All were well polarized, as shown by the formation of spherical and even tubular BC-like structures and by the restricted localization at the BC, visualized by immunofluorescence, of the apical membrane marker HA4, a possible bile acid carrier. WIF-B and its subclone WIF-B9 were found to accumulate CGamF in 65% to 75% of their BC. This transport was time, temperature, and partly sodium dependent and was inhibited by coincubation with the parental natural bile salt cholylglycine. Dinitrophenyl glutathione, a substrate of the canalicular multispecific organic anion transporter, did not inhibit CGamF canalicular secretion, whereas it greatly impaired the canalicular secretion of a non-bile acid organic anion, fluorescein, generated intracellularly from fluorescein diacetate. Confocal microscopy confirmed the presence of CGamF in the cytoplasm, supporting a transcellular route from medium to BC. In contrast, two other polarized clones exhibited a poor ability (WIF 12-6) or no ability (WIF12-1 TGdelta) to vectorially transport CGamE In conclusion, WIF-B and WIF-B9 exhibit not only structural but also functional polarity, at least as far as vectorial organic anion transport is concerned.

HNF-6 is expressed in endoderm derivatives and nervous system of the mouse embryo and participates to the cross-regulatory network of liver-enriched transcription factors

Hepatocyte nuclear factor-6 (HNF-6) is a liver-enriched transcription factor that contains a single cut domain and a novel type of homeodomain. Here we have studied the developmental expression pattern of HNF-6 in the mouse. In situ hybridization experiments showed that HNF-6 mRNA is detected in the liver at embryonic day (E) 9, at the onset of liver differentiation. HNF-6 mRNA disappeared transiently from the liver between E12.5 and E15. In transfection experiments HNF-6 stimulated the expression of HNF-4 and of HNF-3 beta, two transcription factors known to be involved in liver development and differentiation. HNF-6 was detected in the pancreas from E10.5 onward, where it was restricted to the exocrine cells. HNF-6 was also detected in the developing nervous system. Both the brain and the spinal cord started to express HNF-6 at E9-9.5 in postmitotic neuroblasts. Later on, HNF-6 was restricted to brain nuclei, to the retina, to the ventral horn of the spinal cord, and to dorsal root ganglia. Our observations that HNF-6 contributes to the control of the expression of transcription factors and is expressed at early stages of liver, pancreas, and neuronal differentiation suggest that HNF-6 regulates several developmental programs.

The maturity-onset diabetes of the young (MODY1) transcription factor HNF4alpha regulates expression of genes required for glucose transport and metabolism

Hepatocyte nuclear factor 4alpha (HNF4alpha) plays a critical role in regulating the expression of many genes essential for normal functioning of liver, gut, kidney, and pancreatic islets. A nonsense mutation (Q268X) in exon 7 of the HNF4alpha gene is responsible for an autosomal dominant, early-onset form of non-insulin-dependent diabetes mellitus (maturity-onset diabetes of the young; gene named MODY1). Although this mutation is predicted to delete 187 C-terminal amino acids of the HNF4alpha protein the molecular mechanism by which it causes diabetes is unknown. To address this, we first studied the functional properties of the MODY1 mutant protein. We show that it has lost its transcriptional transactivation activity, fails to dimerize and bind DNA, implying that the MODY1 phenotype is because of a loss of HNF4alpha function. The effect of loss of function on HNF4alpha target gene expression was investigated further in embryonic stem cells, which are amenable to genetic manipulation and can be induced to form visceral endoderm. Because the visceral endoderm shares many properties with the liver and pancreatic beta-cells, including expression of genes for glucose transport and metabolism, it offers an ideal system to investigate HNF4-dependent gene regulation in glucose homeostasis. By exploiting this system we have identified several genes encoding components of the glucose-dependent insulin secretion pathway whose expression is dependent upon HNF4alpha. These include glucose transporter 2, and the glycolytic enzymes aldolase B and glyceraldehyde-3-phosphate dehydrogenase, and liver pyruvate kinase. In addition we have found that expression of the fatty acid binding proteins and cellular retinol binding protein also are down-regulated in the absence of HNF4alpha. These data provide direct evidence that HNF4alpha is critical for regulating glucose transport and glycolysis and in doing so is crucial for maintaining glucose homeostasis.

mRNA expression of HNF-4 isoforms and of HNF-1alpha/HNF-1beta variants and differentiation of human cell lines that mimic highly specialized phenotypes of intestinal epithelium

The mRNA expression of HNF-4 isoforms and the ratio of HNF-1alpha/HNF-1beta variants in cell lines representing highly specialized phenotypes of human intestinal epithelium were studied by RT-PCR. A strong rise in expression of HNF-4 isoforms alpha2, alpha4 and gamma correlates with commitment into highly differentiated enterocyte-like phenotype of Caco-2 cells which best mimic enterocytes, whereas only isoform alpha4 expression is high in the less differentiated HT-29 G- cells. These increased expressions are not encountered in the highly differentiated mucous-secreting HT-29 MTX cells. Differentiation into highly specialized enterocyte-like Caco-2 cells and mucous-secreting HT-29 MTX cells is accompanied by a moderate rise in HNF-1 without change in the ratio of its variants. Our data corroborate those of Spath et al. (Mol. Cell. Biol., 1997, 17, 1913) in hepatoma cells and suggest that HNF-4 isoforms alpha2, alpha4 and gamma play a major role in the differentiation of enterocytes.

Hepatocyte nuclear factor-4 prevents silencing of hepatocyte nuclear factor-1 expression in hepatoma x fibroblast cell hybrids

Hepatocyte nuclear factors-1alpha (HNF1alpha) and -4 (HNF4) are components of a liver-enriched transcription activation pathway which is thought to play a critical role in hepatocyte-specific gene expression, including activation of alpha1-antitrypsin gene expression. HNF1alpha, HNF4 and alpha1-antitrypsin (alpha1AT) genes are extinguished in hepatoma/fibroblast somatic cell hybrids, suggesting that fibroblasts contain a repressor-like activity. To determine the molecular basis for silencing of these genes in cell hybrids, ectopic expression of HNF1alpha and HNF4 was used. Results show that constitutive expression of HNF4 prevents extinction of HNF1alpha gene expression in hepatoma/fibroblast hybrids. In contrast, forced HNF1alpha expression failed to prevent extinction of the HNF4 locus in cell hybrids. Likewise, the alpha1AT gene remained silent in the presence of both HNF1alpha and HNF4. These results suggest that extinction of HNF1alpha is a simple lack-of-activation phenotype, whereas extinction of HNF4 andalpha1AT loci is more complex, perhaps involving negative regulation.

Selective loss of the hepatic phenotype due to the absence of a transcriptional activation pathway

Liver-enriched trans-acting factors hepatocyte nuclear factor-1 alpha (HNF1 alpha) and -4 (HNF4) are components of a transcriptional activation pathway that is thought to play a major role in hepatic gene activation. We previously described the isolation and characterization of distinct classes of hepatoma variants which lack the HNF4-->HNF1 alpha pathway (1). In order to determine the influence of the HNF4-->HNF1 alpha pathway on hepatic gene expression, genetic rescue experiments were done using hepatoma variant line H11 as a model system. Results suggest that this pathway is required for basal expression of a number of endogenous hepatocyte-specific genes. Complementation groups were established by fusion of H11 cells with other variant lines. Lastly, introduction of human chromosome 20 (containing the HNF4 locus) or randomly-marked human chromosomes into H11 cells failed to rescue the hepatic phenotype, suggesting that what appears to be a 'simple' defect may involve multiple genetic loci.

Hepatocyte nuclear factor 4 expression overcomes repression of the hepatic phenotype in dedifferentiated hepatoma cells

The capacity of the liver-enriched transcription factor hepatocyte nuclear factor 4 (HNF4) to direct redifferentiation of dedifferentiated rat hepatoma cells was investigated by stable transfection of epitope-tagged HNF4 cDNA into H5 variant cells. HNF4-producing cells expressed the previously silent HNF1 gene and showed activation of some hepatic functions, including alpha1-antitrypsin, beta-fibrinogen, and transthyretin, but not of the endogenous HNF4 gene. Expression of the other hepatocyte-enriched transcription factors was not modified. Treatment of the HNF4tag-expressing cells with dexamethasone induced expression of the transgene by 10-fold, resulting in enhanced expression of target genes of both glucocorticoid hormones and HNF4. The set of activated hepatic genes was extended by treatment of cells with the demethylating agent 5-azacytidine followed by selection in dexamethasone-containing glucose-free medium. Some of the colonies that developed reexpressed the entire set of hepatic functions tested. Fusion of HNF4tag-producing H5 cells with well-differentiated Fao cells showed that only those hybrids which maintained expression of HNF4tag were protected from complete extinction, including that of the Fao HNF4 gene. Thus, H5 cells must produce an extinguisher of the HNF4 gene. In addition, this result implies that HNF4 itself, or its target HNF1, is a positive regulator of HNF4. In conclusion, HNF4tag expression overcomes repression of the hepatic phenotype of the H5 cell without abolishing its potential to extinguish an active genome. Taken together, these results predict that expression of HNF4 should be sufficient to establish heritable expression of many parameters of the hepatic differentiated state.

trans-Acting factors, detoxication enzymes and hepatitis B virus replication in a novel set of human hepatoma cell lines

A panel of four novel human hepatoma cell lines was isolated from a single tumor from a male individual. BC1, B16 and B16A2 lines were well differentiated, while cells of the B9 line were only poorly differentiated, being essentially negative for the functions analyzed. These cell lines have been surveyed for expression of a large set of plasma proteins, accumulation of liver-specific mRNAs and DNA-binding activity of ubiquitous and liver-enriched transcription factors. BC1 cells expressed the highest levels of albumin mRNA, whereas B16 and B16A2 cells accumulated the largest amounts of haptoglobin mRNA. In addition, B16 and B16A2 cells were unique in that they expressed CYP2E1 mRNA, a species absent from the available human liver cells, including HepG2 hepatoma cells, and 3-methylcholanthrene-inducible CYP1A2 mRNA. The activities of genes encoding transcription factors were evidenced in all four cell lines which expressed mRNAs for nuclear factor interleukin 6 and hepatocyte nuclear factor 1 (HNF) together with the DNA-binding activity of NFY and AP1 nuclear proteins. Strikingly, HNF-1 and HNF-4-like DNA-binding activities were restricted to BC1, B16 and B16A2 cells, supporting the idea of the potential role of these (or closely related) factors in the maintenance and/or in the establishment of the differentiated phenotype. B9 cells contained variant HNF1-like DNA-binding activity, similar to dedifferentiated rat hepatoma cells of the H5 line. CCAAT/enhancer-binding protein and HNF-3-like activities were found in all cell lines, although at a lower level and/or activity in B9 cells. Finally, transfection experiments of plasmids containing the whole hepatitis-B virus genome demonstrated that B16 cells, but not B9 cells, were able to support hepatitis-B virus replication and virion production, in agreement with the notion that HNF-1 activity is necessary for viral replication. We believe that the specific complement of transcription factors expressed in the differentiated BC1, B16 and B16A2 cells, and in the poorly differentiated B9 cells, will allow studies on the regulation of hepatic gene expression in these human lines, and will also aid the analysis of xenobiotic metabolism and the biology of hepatitis-B virus replication.

Mapping of human non-muscle type cofilin (CFL1) to chromosome 11q13 and muscle-type cofilin (CFL2) to chromosome 14

Cofilin is a widely-distributed, intracellular, actin binding protein which is involved in the translocation of actin-cofilin complex from cytoplasm to nucleus. We have cloned a non-muscle-type cofilin (CFL1) from a human promyelocytic cDNA library and mapped this to human chromosome 11 by PCR amplification of 3' untranslated sequence in a panel of rodent-human somatic cell hybrids, and to the interval 11q12-q13.2 in a chromosome 11 somatic cell hybrid mapping panel. Confirmation of regional localisation to 11q13 has been obtained by fluorescent in situ hybridisation of genomic cosmid clones, by demonstration of the presence of both SEA (the human homologue of avian retrovirus proviral tyrosine kinase, 11q13) and CFL1 in some of these clones and by close linkage of CFL1 to SEA in a panel of high-dose irradiation hybrids. We have identified human muscle-type cofilin sequences by comparison of human expressed sequence tags with M-type cofilins of other species and we have mapped the human M-type cofilin, CFL2, to chromosome 14.

Extinction of peroxisomal functions in hepatoma cell-fibroblast hybrids

Although peroxisomes are ubiquitous, differences in the number of organelles and in the expression of associated metabolic activities are observed, depending on the cell type. To investigate the control of peroxisomal activity in connection with cell differentiation, we constructed hybrids between two types of cells whose histogenetic origins dictate significant differences in peroxisomal activities: hepatoma cells and fibroblasts, with high and low expression, respectively, of peroxisomal functions. In these hybrids, extinction of the elevated activities that characterize liver cells is observed, in parallel with the well-documented extinction of differentiated functions. This suggests the existence in fibroblasts of a negative trans-acting regulation.

Transcriptional hierarchy in Xenopus embryogenesis: HNF4 a maternal factor involved in the developmental activation of the gene encoding the tissue specific transcription factor HNF1 alpha (LFB1)

The tissue specific transcription factor HNF1 alpha (LFB1) expressed in liver, kidney, stomach and gut gets transcriptionally activated in Xenopus shortly after zygotic transcription starts. By microinjection into fertilized Xenopus eggs, a HNF1 alpha promoter fragment is activated in the middle part of developing larvae, reflecting the activation pattern of the endogenous HNF1 alpha gene. Mutational analysis of the HNF1 alpha promoter shows that HNF1 and HNF4 binding sites are essential for proper embryonic regulation. Since by injecting HNF4 mRNA into fertilized eggs the endogenous HNF1 alpha gene is activated ectopically and HNF4 is present as a maternal protein within an animal to vegetal gradient in the embryo, we assume that HNF4 initiates a transcriptional hierarchy involved in determination of different cell fates.

Chromosomal localisation of genes coding for human and mouse liver cytosolic cysteine dioxygenase

A panel of 22 hybrids was tested for the presence of the gene coding for human cysteine dioxygenase (CDO) by using human specific oligonucleotide primers in the polymerase chain reaction. Detection of human CDO completely correlated with the presence of human chromosome 5. A human total genome cosmid library was screened with a PCR product from the coding region of human CDO cDNA and the two positive clones identified were used in fluorescent in situ hybridisation (FISH) analysis on metaphase chromosome spreads. Fluorescent signals were seen on chromosome 5q22-23. Interspecific backcross mapping in the mouse indicated that Cdo, the mouse homologue of CDO, is situated in the central region of mouse chromosome 18 which shares a region of homology with human chromosome 5.

The class I alcohol dehydrogenase gene is glucocorticoid-responsive in the rat hepatoma microcell hybrid cell line, 11-3

Expression of the class I alcohol dehydrogenase (ADH) gene in the rat hepatoma microcell hybrid cell line, 11-3, was examined. The steady-state level of ADH mRNA in 11-3 was approximately 2-fold higher than that or rat liver and Fao, the parental cell line of 11-3. Removal of steroid hormones by activated charcoal from the serum in which 11-3 cells were maintained resulted in a significant decrease in the level of ADH transcript. Dexamethasone at a concentration of 1 muM increased the ADH mRNA content in 11-3 in a time-dependent fashion, up to 48 hr after its addition to cells that had first been deprived of steroid hormones. In addition, levels of ADH transcript in cells treated with dexamethasone increased in a dose-dependent manner, and the concentration of dexamethasone required to achieve half-maximal activation was 5 nM. By using the techniques of reverse transcription and polymerase chain reaction, and by taking advantage of a restriction polymorphism present between the rat and mouse ADH cDNA, we found that 11-3 contained both the rat and mouse class I ADH transcripts, although the rat sequence accounted for the great majority. Moreover, levels of both rat and mouse class I ADH transcripts increased in a similarly time-dependent manner in cells treated with dexamethasone. These results indicate that expression of class I ADH gene in 11-3 is high and is regulated by glucocorticoids, making the cell line an excellent model for the in vitro study of ADH expression.

Transcription factor and liver-specific mRNA expression in facultative epithelial progenitor cells of liver and pancreas

The pattern of mRNA expression for liver-specific proteins and liver-enriched transcription factors was studied in two models of facultative gut epithelial progenitor cells activation: D-galactosamine (GalN)-induced liver injury and dietary copper depletion leading to pancreatic acinar atrophy. After 5 weeks of copper deficiency (CuD), pancreatic acini of Fischer 344 rats underwent atrophy, associated with intense proliferation of small duct-like cells with oval-shaped nuclei. These cells resemble morphologically epithelial progenitor cells of the liver that proliferate after GalN administration. Activated pancreatic epithelial cells express mRNAs for liver-specific genes normally expressed in fetal liver, including alpha-fetoprotein, albumin, alpha-1 antitrypsin, glucose-6-phosphatase, and others, but not genes that are turned on after birth such as serine dehydratase, tyrosine aminotransferase, and multidrug resistance gene-1b. They express mRNAs for liver-enriched transcription factors including HNF-1 alpha, HNF-3 beta and gamma, HNF-4, and members of the CCAAT-enhancer binding protein (C/EBP) family. The only mRNA for a liver-enriched transcription factor not detected in the pancreas of CuD animals was HNF-3 alpha. Expression of HNF-3 alpha, beta, and gamma, and C/EBP-beta mRNA was highly activated in proliferating liver epithelial cells on days 2 and 3 after GalN injury. Increased expression of C/EBP-delta was observed first in the liver on day 1 after GalN administration and in the pancreas at 4 weeks after initiating CuD. We suggest that C/EBP-delta could be involved in the initial activation of epithelial progenitor cells and that HNF-3 alpha, beta, and gamma, and C/EBP-beta might participate in their maturation. We conclude further that pancreatic epithelial progenitor cells undertake differentiation through the hepatocyte lineage but cannot complete the differentiation program within the pancreatic milieu.

Tissue-specific regulation of mouse hepatocyte nuclear factor 4 expression

Hepatocyte nuclear factor 4 (HNF-4) is a liver-enriched transcription factor and a member of the steroid hormone receptor superfamily. HNF-4 is required for the hepatoma-specific expression of HNF-1 alpha, another liver-enriched transcription factor, suggesting the early participation of HNF-4 in development. To prepare for further study of HNF-4 in development, the tissue-specific expression of the mouse HNF-4 gene was studied by analyzing the promoter region for required DNA elements. DNase-hypersensitive sites in the gene in liver and kidney tissues were found in regions both distal and proximal to the RNA start that were absent in tissues in which HNF-4 expression did not occur. By use of reporter constructs in transient-transfection assays and with transgenic mice, a region sufficient to drive liver-specific expression of HNF-4 was identified. While an HNF-1 binding site between bp -98 and -68 played an important role in the hepatoma-specific promoter activity of HNF-4 in transient-transfection assays, it was not sufficient for the liver-specific expression of a reporter gene in transgenic mice. Distal enhancer elements indicated by the presence of DNase I-hypersensitive sites at kb -5.5 and -6.5, while not functional in transient-transfection assays, were required for the correct expression of the mouse HNF-4 gene in animals.

Characterization of the mouse HNF-4 gene and its expression during mouse embryogenesis

Hepatocyte nuclear factor 4 (HNF-4) is a member of the nuclear receptor gene superfamily with unknown ligand. It has been assumed to play an important role in the regulation of gene expression in the liver. Here, we report the cloning and characterization of the mouse HNF-4 gene, as well as its expression during embryogenesis. The HNF-4 protein is encoded by ten exons. The gene structure is unique in the steroid receptor superfamily in that the second zinc finger is encoded by two exons. HNF-4 mRNA is expressed in a limited number of mouse adult tissues: liver, kidney, intestine, stomach and skin. HNF-4 could play an important role in the formation and function of visceral yolk sac and in the development of the liver and kidney since its mRNA, as determined by in situ hybridization, appears upon primary differentiation of these organs. As a first step in the study of the regulatory elements of the HNF-4 gene, we mapped the transcription start site and carried out DNase I hypersensitive site (HS) analysis over a region of approximately 22kb upstream of the gene. The complexity of the HSs suggests that multiple elements might contribute to the transcriptional regulation of the HNF-4 gene.

Tissue-specific regulation of mouse hepatocyte nuclear factor 4 expression

Hepatocyte nuclear factor 4 (HNF-4) is a liver-enriched transcription factor and a member of the steroid hormone receptor superfamily. HNF-4 is required for the hepatoma-specific expression of HNF-1 alpha, another liver-enriched transcription factor, suggesting the early participation of HNF-4 in development. To prepare for further study of HNF-4 in development, the tissue-specific expression of the mouse HNF-4 gene was studied by analyzing the promoter region for required DNA elements. DNase-hypersensitive sites in the gene in liver and kidney tissues were found in regions both distal and proximal to the RNA start that were absent in tissues in which HNF-4 expression did not occur. By use of reporter constructs in transient-transfection assays and with transgenic mice, a region sufficient to drive liver-specific expression of HNF-4 was identified. While an HNF-1 binding site between bp -98 and -68 played an important role in the hepatoma-specific promoter activity of HNF-4 in transient-transfection assays, it was not sufficient for the liver-specific expression of a reporter gene in transgenic mice. Distal enhancer elements indicated by the presence of DNase I-hypersensitive sites at kb -5.5 and -6.5, while not functional in transient-transfection assays, were required for the correct expression of the mouse HNF-4 gene in animals.

Cloning and sequencing of cDNAs encoding the human hepatocyte nuclear factor 4 indicate the presence of two isoforms in human liver

Hepatocyte nuclear factor 4 (HNF-4) is a key transcription factor involved in the specific expression of many genes in liver and intestine. Sequences of cDNAs coding for HNF-4 have been established in rat and Drosophila melanogaster. Rat HNF-4 exhibits two isoforms which probably result from differential splicing. We have isolated HNF-4 cDNAs from an adult human cDNA library. Sequence analysis revealed that two HNF-4 isoforms are also present in human liver. The complete sequence of the longest human isoform has been established and compared to the rat HNF-4 amino-acid sequences.

Expression of hepatic transcription factors during liver development and oval cell differentiation

The oval cells are thought to be the progeny of a liver stem cell compartment and strong evidence now exists indicating that these cells can participate in liver regeneration by differentiating into different hepatic lineages. To better understand the regulation of this process we have studied the expression of liver-enriched transcriptional factors (HNF1 alpha and HNF1 beta, HNF3 alpha, HNF3 beta, and HNF3 gamma, HNF4, C/EBP, C/EBP beta, and DBP) in an experimental model of oval cell proliferation and differentiation and compared the expression of these factors to that observed during late stages of hepatic ontogenesis. The steady-state mRNA levels of four (HNF1 alpha, HNF3 alpha, HNF4, and C/EBP beta) "liver-enriched" transcriptional factors gradually decrease during the late period of embryonic liver development while three factors (HNF1 beta, HNF3 beta, and DBP) increase. In the normal adult rat liver the expression of all the transcription factors are restricted to the hepatocytes. However, during early stages of oval cell proliferation both small and large bile ducts start to express HNF1 alpha and HNF1 beta, HNF3 gamma, C/EBP, and DBP but not HNF4. At the later stages all of these factors are also highly expressed in the proliferating oval cells. Expression of HNF4 is first observed when the oval cells differentiate morphologically and functionally into hepatocytes and form basophilic foci. At that time the expression of some of the other factors is also further increased. Based on these data we suggest that the upregulation of the "establishment" factors (HNF1 and -3) may be an important step in oval cell activation. The high levels of these factors in the oval cells and embryonic hepatoblasts further substantiates the similarity between the two cell compartments. Furthermore, the data suggest that HNF4 may be responsible for the final commitment of a small portion of the oval cells to differentiate into hepatocytes which form the basophilic foci and eventually regenerate the liver parenchyma.

Tissue-specific extinguisher loci in the human genome: a screening study based on random marking and transfer of human chromosomes

Expression of many liver-specific genes is extinguished when cultured hepatoma cells are fused with fibroblasts, but liver genes can be reexpressed in hybrid segregants that have lost fibroblast chromosomes. To map extinguisher loci involved in this process, hepatoma microcell hybrids retaining single fibroblast chromosomes have been employed. Two different, transdominant loci that affect liver gene expression have been defined in this way. To determine whether other monochromosomal extinction phenotypes could be observed, we inserted a selectable marker into many human chromosomal sites and transferred the marked human chromosomes into rat hepatoma recipient cells by microcell fusion. Nearly 200 microcell hybrid clones were isolated and screened for expression of liver-specific mRNAs. Most liver transcripts continued to be expressed. However, PEPCK mRNA was extinguished in 12 hybrid clones. Some of these hybrids contained human TSE1, the previously characterized extinguisher locus on chromosome 17, but others contained a novel extinguishing function that mapped to human chromosome 14. The implications of these findings are discussed.

An improved polarized rat hepatoma hybrid cell line. Generation and comparison with its hepatoma relatives and hepatocytes in vivo

Studies of hepatocyte polarity, an important property of liver epithelial cells, have been hampered by the lack of valid in vitro models. We report here that a new polarized hepatoma-derived hybrid cell line, called WIF-B, has improved characteristics to those of its parent, WIF12-1. This latter line originated from the fusion of non-polarized rat hepatoma Fao cells with human fibroblasts (WI-38) and selection for a polarized phenotype. We generated the WIF-B line by growing WIF12-1 cells as unattached aggregates for three weeks and selecting for survivors. Karyotype analysis showed a broad chromosome pattern in the initial WIF-B population, but this pattern stabilized after a few passages. The growth and phenotypic properties of these cells were quite different from those of their polarized WIF12-1 parent. WIF-B cells attained a 4-fold higher maximal density in monolayer culture, survived at this density for > 5 days rather than 1 day, and exhibited two to three times more apical structures during this period (80 to 95%). We compared several parameters of liver differentiation in the WIF-B cells with those of a related hybrid clone, WIF12-E, which is extinguished for most liver-specific functions, and with the common hepatoma parent, Fao. By immunoblot analysis, the levels of expression of eight plasma membrane proteins were higher in the WIF-B cells than in either of the other two cell lines and ranged from 10 to 200% of those in vivo. Two plasma membrane proteins were not detected in WIF12-E cells. By immunofluorescence, the apical membrane proteins in WIF-B displayed different cellular localizations than in either of the other two cell lines. In WIF-B cells, apical proteins were confined to a plasma membrane region that we have identified as the apical domain by several criteria (Ihrke, G., Neufeld, E.D., Meads, T., Shanks, M.R., Cassio, D., Laurent, M., Schroer, T.A., Pagano, R. E. and Hubbard, A. L. J. Cell Biol., 123, 1761–1765). The same molecules were distributed over the entire plasma membrane of Fao and WIF12-E cells and also (for Fao cells) in intracellular punctate structures that did not colocalize with the majority of structures containing a secretory protein, albumin. Our results indicate that the WIF-B cells are more highly differentiated than any of their ancestors (Fao or WIF12-1 cells) and thus, are promising candidates for in vitro studies of hepatocyte polarity.