Maintenance and reversibility of active albumin secretion by adult rat hepatocytes co-cultured with another liver epithelial cell type

When adult rat hepatocytes were co-cultured with another liver epithelial cell type in a medium supplemented or not with fetal calf serum (FCS), it was found that 1. They survived for more than 2 months 2. Albumin secretion levels remained high over the whole culture period 3. Decreased secretion might be reversed 4. This protein secretion activity appeared to be dependent upon both the presence of cell-cell contacts and the production of an extracellular material. The results demonstrate for the first time long-term stabilization and reversibility of a specific function (albumin secretion) at high levels by adult hepatocytes cultured in serum-free medium and suggest that both the presence of other liver cell type(s) and the production of an extracellular matrix are needed for the maintenance of specific functions in cultured hepatocytes.

The mitogen-activated protein kinase kinase/extracellular signal-regulated kinase cascade activation is a key signalling pathway involved in the regulation of G(1) phase progression in proliferating hepatocytes

In this study, activation of the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) signalling pathway was analyzed in proliferating rat hepatocytes both in vivo after partial hepatectomy and in vitro following epidermal growth factor (EGF)-pyruvate stimulation. First, a biphasic MEK/ERK activation was evidenced in G(1) phase of hepatocytes from regenerating liver but not from sham-operated control animals. One occurred in early G(1) (30 min to 4 h), and the other occurred in mid-late G(1), peaking at around 10.5 h. Interestingly, the mid-late G(1) activation peak was located just before cyclin D1 induction in both in vivo and in vitro models. Second, the biological role of the MEK/ERK cascade activation in hepatocyte progression through the G(1)/S transition was assessed by adding a MEK inhibitor (PD 98059) to EGF-pyruvate-stimulated hepatocytes in primary culture. In the presence of MEK inhibitor, cyclin D1 mRNA accumulation was inhibited, DNA replication was totally abolished, and the MEK1 isoform was preferentially targeted by this inhibition. This effect was dose dependent and completely reversed by removing the MEK inhibitor. Furthermore, transient transfection of hepatocytes with activated MEK1 construct resulted in increased cyclin D1 mRNA accumulation. Third, a correlation between the mid-late G(1) MEK/ERK activation in hepatocytes in vivo after partial hepatectomy and the mitogen-independent proliferation capacity of these cells in vitro was established. Among hepatocytes isolated either 5, 7, 9, 12 or 15 h after partial hepatectomy, only those isolated from 12- and 15-h regenerating livers were able to replicate DNA without additional growth stimulation in vitro. In addition, PD 98059 intravenous administration in vivo, before MEK activation, was able to inhibit DNA replication in hepatocytes from regenerating livers. Taken together, these results show that (i) early induction of the MEK/ERK cascade is restricted to hepatocytes from hepatectomized animals, allowing an early distinction of primed hepatocytes from those returning to quiescence, and (ii) mid-late G(1) MEK/ERK activation is mainly associated with cyclin D1 accumulation which leads to mitogen-independent progression of hepatocytes to S phase. These results allow us to point to a growth factor dependency in mid-late G(1) phase of proliferating hepatocytes in vivo as observed in vitro in proliferating hepatocytes and argue for a crucial role of the MEK/ERK cascade signalling pathway.

Growth factor dependence of progression through G1 and S phases of adult rat hepatocytes in vitro. Evidence of a mitogen restriction point in mid-late G1

Several hepatocyte mitogens have been identified, but the signals triggering the G0/G1 transition and cell cycle progression of hepatocytes remain unknown. Using hepatocyte primary cultures, we investigated the role of epidermal growth factor/pyruvate during the entry into and progression through the G1 phase and analyzed the expression of cell cycle markers. We show that the G0/G1 transition occurs during hepatocyte isolation as evidenced by the expression of early genes such as c-fos, c-jun, and c-myc. In culture, hepatocytes progress through G1 regardless of growth factor stimulation until a restriction point (R point) in mid-late G1 beyond which they cannot complete the cell cycle without mitogenic stimulation. Changes in cell cycle gene expression were associated with progression in G1; the cyclin E mRNA level is low early in G1 but increases at the G1/S boundary, while the protein is constantly detected during cell cycle but undergoes a change of electrophoretic mobility in mid-late G1 after the R point. In addition, a drastic induction of cyclin D1 mRNA and protein, and to a lesser extent of cyclin D2 mRNA, takes place in mitogen-stimulated cells after the R point. In contrast, cyclin D3 mRNA appears early in G1, remains constant in stimulated cells, but accumulates in unstimulated arrested cells, paralleling the cyclin-dependent kinase 4 mRNA expression. These results characterize the different steps of G1 phase in hepatocytes.

Long-term co-cultures of adult human hepatocytes with rat liver epithelial cells: modulation of albumin secretion and accumulation of extracellular material

High yields of viable human hepatocytes were obtained by enzymatic perfusion of the left hepatic lobe of kidney donors and cultured alone or with an epithelial cell line derived from rat liver. In conventional cultures, human hepatocytes did not survive more than 2 to 3 weeks and by Day 8 decreased their ability to secrete albumin. When co-cultured, they survived for more than 2 months and secreted high levels of albumin even in a serum-free medium. This long-term survival appeared to correlate with production of an extracellular material which is rich in Type III collagen. In vitro phenotypic alterations of parenchymal cells were reversed by addition of rat liver cells and were characterized by recovery of cuboidal morphology, increased albumin secretion and a shift from Type I to Type III collagen deposition. Rat liver epithelial cells could not be replaced by nonhepatic epithelial cells. These observations suggest that when adult human hepatocytes are maintained in a culture which closely resembles their in vivo environment, they are capable of continuing to actively express specific cell functions.

Dependence of hepatocyte-specific gene expression on cell-cell interactions in primary culture

In co-culture with non-parenchymal liver epithelial cells, rat hepatocytes show a marked increase in albumin and total protein synthesis when compared with cells maintained as pure populations in which an early decline in albumin secretion takes place. Analysis of the relative amounts of different mRNA sequences, determined by hybridization, indicated that the increase in protein synthesis resulted essentially from an increased level of the corresponding mRNAs. In addition, when cell-cell contacts were established between the two cell types several days after the seeding of hepatocytes, the stimulation of albumin secretion was similarly observed with a significant increase of the corresponding mRNA on days 10-14 of culture. Transcriptional assays, in which isolated nuclei were used for the study of RNA synthesis, showed that liver-specific gene transcription was significantly increased and maintained for at least 2 weeks. These results demonstrate for the first time long-term stabilization and reversibility of various specific mRNAs at high levels by adult hepatocytes in primary culture. They suggest that establishment of cell-cell contacts between hepatocytes and liver epithelial cells are essential for the maintenance of a high rate of transcription of the liver-specific genes.

A plasma membrane protein is involved in cell contact-mediated regulation of tissue-specific genes in adult hepatocytes

We have identified the liver-regulating protein (LRP), a cell surface protein involved in the maintenance of hepatocyte differentiation when cocultured with rat liver epithelial cells (RLEC). LRP was defined by immunoreactivity to a monoclonal antibody (mAb L8) prepared from RLEC. mAb L8 specifically detected two polypeptides of 85 and 73 kD in immunoprecipitation of both hepatocyte- and RLEC-iodinated plasma membranes. The involvement of these polypeptides, which are integral membrane proteins, in cell interaction-mediated regulation of hepatocytes was assessed by evaluating the perturbing effects of the antibody on cocultures with RLEC. Several parameters characteristic of differentiated hepatocytes were studied, such as liver-specific and house-keeping gene expression, cytoskeletal organization and deposition of extracellular matrix (ECM). An early cytoskeletal disturbance was evidenced and a marked alteration of hepatocyte functional capacity was observed in the presence of the antibody, together with a loss of ECM deposition. By contrast, cell-cell aggregation or cell adhesion to various extracellular matrix components were not affected. These findings suggest that LRP is distinct from an extracellular matrix receptor. The fact that early addition of mAb L8 during cell contact establishment was necessary to be effective may indicate that LRP is a novel plasma membrane protein that plays an early pivotal role in the coordinated metabolic changes which lead to the differentiated phenotype of mature hepatocytes.

Mechanism in the sequential control of cell morphology and S phase entry by epidermal growth factor involves distinct MEK/ERK activations

Cell shape plays a role in cell growth, differentiation, and death. Herein, we used the hepatocyte, a normal, highly differentiated cell characterized by a long G1 phase, to understand the mechanisms that link cell shape to growth. First, evidence was provided that the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) cascade is a key transduction pathway controlling the hepatocyte morphology. MEK2/ERK2 activation in early G1 phase did not lead to cell proliferation but induced cell shape spreading and demonstration was provided that this MAPK-dependent spreading was required for reaching G1/S transition and DNA replication. Moreover, epidermal growth factor (EGF) was found to control this morphogenic signal in addition to its mitogenic effect. Thus, blockade of cell spreading by cytochalasin D or PD98059 treatment resulted in inhibition of EGF-dependent DNA replication. Our data led us to assess the first third of G1, is exclusively devoted to the growth factor-dependent morphogenic events, whereas the mitogenic signal occurred at only approximately mid-G1 phase. Moreover, these two growth factor-related sequential signaling events involved successively activation of MEK2-ERK2 and then MEK1/2-ERK1/2 isoforms. In addition, we demonstrated that inhibition of extracellular matrix receptor, such as integrin beta1 subunit, leads to cell arrest in G1, whereas EGF was found to up-regulated integrin beta1 and fibronectin in a MEK-ERK-dependent manner. This process in relation to cytoskeletal reorganization could induce hepatocyte spreading, making them permissive for DNA replication. Our results provide new insight into the mechanisms by which a growth factor can temporally control dual morphogenic and mitogenic signals during the G1 phase.

Antiproliferative and apoptotic effects of O-Trensox, a new synthetic iron chelator, on differentiated human hepatoma cell lines

We investigated the effects of a new iron chelator, O-Trensox (TRX), compared with desferrioxamine (DFO), on proliferation and apoptosis in cultures of the human hepatoblastoma HepG2 and hepatocarcinoma HBG cell lines. Our results show that TRX decreased DNA synthesis in a time- and dose-dependent manner and with a higher efficiency than DFO. Mitotic index was also strongly decreased by TRX and, unexpectedly, DFO inhibited mitotic activity to the same extent as TRX, thus there is a discrepancy between the slight reduction in DNA synthesis and a large decrease in mitotic index after DFO treatment. In addition, we found that TRX induced accumulation of cells in the G(1) and G(2) phases of the cell cycle whereas DFO arrested cells in G(1) and during progression through S phase. These data suggest that the partial inhibition of DNA replication observed after exposure to DFO may be due to a lower efficiency of metal chelation and/or that it does not inhibit the G(1)/S transition but arrests cells in late S phase. The effects of both TRX and DFO on DNA synthesis and mitotic index were reversible after removing the chelators from the culture medium. An apoptotic effect of TRX was strongly suggested by analysis of DNA content by flow cytometry, nuclear fragmentation and DNA degradation in oligonucleosomes and confirmed by the induction of a high level of caspase 3-like activity. TRX induced apoptosis in a dose- and time-dependent manner in proliferating HepG2 cells. In HBG cells, TRX induced apoptosis in proliferating and confluent cells arrested in the G(1) phase of the cell cycle, demonstrating that inhibition of proliferation and induction of apoptosis occurred independently. DFO induced DNA alterations only at concentrations >100 microM and without induction of caspase 3-like activity, indicating that DFO is not a strong inducer of apoptosis. Addition of Fe or Zn to the culture medium during TRX treatment led to a complete restoration of proliferation rate and inhibition of apoptosis, demonstrating that Fe/Zn-saturated TRX was not toxic in the absence of metal depletion. These data show that TRX, at concentrations of 20-50 microM, strongly inhibits cell proliferation and induces apoptosis in proliferating and non-proliferating HepG2 and HBG cells, respectively.

Influence of medium composition and culture conditions on glutathione S-transferase activity in adult rat hepatocytes during culture

Glutathione S-transferase (GST) activity was measured in adult rat hepatocytes during either pure culture or coculture with another rat liver cell type in various media. Addition of nicotinamide, selenium, or dimethylsulfoxide, deprivation of cyst(e)ine and the use of two complex media were tested. Whatever the conditions used, after a constant decrease during the first 24 h, GST remained active over the whole culture period (1-2 wk). However, various patterns were observed: GST activity either remained relatively stable to approximately 50% of the initial value or showed a moderate or strong increase. The highest values were found in pure hepatocyte cultures maintained in the presence of nicotinamide or dimethylsulfoxide. Similar changes were observed using 1-chloro-2,4-dinitrobenzene or 1,2-dichloro-4-nitrobenzene as substrates for GST. Addition of 10(-4) M indomethacin resulted in 37 to 60% inhibition of enzyme activity. Thus, these results demonstrate that GST remained expressed during culture but its levels markedly varied depending on the medium composition and type and age of culture.

Expression and induction of a large set of drug-metabolizing enzymes by the highly differentiated human hepatoma cell line BC2

The BC2 cell line derived from the human hepatocarcinoma, HGB, undergoes a spontaneous sharp differentiation process in culture as it becomes confluent, remains stably differentiated for several weeks, and may return to proliferation thereafter under appropriate density conditions. The relevance of the line as an hepatic model has been evaluated. Cells synthesize a large number of plasma proteins, and rates of glycogen and urea synthesis increase with time of confluency and become sensitive to insulin, reflecting the process of differentiation. Differentiated BC2 cells express the most relevant cytochrome P-450 (CYP) isozyme activities (CYP1A1/2, 2A6, 2B6, 2C9, 2E1, and 3A4) and conjugating enzymes (glutathione S-transferase and UDP-glucuronyltransferase) and also respond to model inducers. Methylcholanthrene induced an increase in CYP1A1/2 enzyme activity (eightfold), phenobarbital induced CYP2B6 activity (1.7-fold), and dexamethasone induced CYP3A4 activity (fivefold). In parallel, expression of the most relevant liver-enriched transcription factors, HNF-4, HNF-1, C/EBP-alpha and C/EBP-beta mRNAs, was significantly increased in differentiated cultures. This increase was largest in HNF-1 and HNF-4, which supports the idea that a redifferentiation process towards the hepatic phenotype takes place. BC2 is an hepatic cell line that is able to express most hepatic functions, especially the drug-biotransformation function, far more efficiently than any previously described human hepatoma cell line.

Effects of administration of the chemoprotective agent oltipraz on CYP1A and CYP2B in rat liver and rat hepatocytes in culture

The success of oltipraz (OPZ) [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione] as a chemoprotective agent against aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in the rat is thought to depend principally on its ability to enhance detoxication by inducing phase II enzymes, especially glutathione transferases. However, in primary cultures of human hepatocytes, we recently demonstrated that OPZ also has an important inhibitory effect on the major cytochromes P450 (CYPs) of human hepatic AFB1 metabolism. This has prompted a detailed study of the effect of OPZ on some CYPs involved in metabolism of AFB1 in the rat. Primary cultures of rat hepatocytes behaved similarly to human hepatocytes and responded to OPZ by inhibition of ethoxyresorufin-O-deethylase (EROD) and pentoxyresorufin-O-depentylase (PROD) activities mainly associated, respectively, with CYP1A and CYP2B. A time-course shows that this inhibition is largely reversible, with EROD and PROD activities reaching a minimum at 12 h and tending towards control values within 24 h. As is to be expected, the incubation of isolated microsomes with OPZ also inhibits CYP1A and 2B. The effect of OPZ on CYP1A is not a phenomenon limited to cells in culture, but also occurs in vivo. Using the whole animal, we were able to demonstrate that OPZ also transiently inhibited CYP1A activity in a rat given caffeine, by measuring the amounts of methylxanthines found in the serum. However, microsomes isolated from rats, that had been treated with OPZ in vivo, show no such inhibition, presumably because, since OPZ is a reversible inhibitor, it dissociates and is lost during the course of conventional procedures of microsomal preparation. This explains some earlier failures in studies of isolated microsomes to observe the inhibition of CYPs by OPZ. In addition to inhibiting their enzymatic activity, OPZ is also an inducer of CYP1A and 2B as shown by the increased levels of their mRNAs and of caffeine metabolism in vivo after 24 h or more. It is concluded that the mechanism of chemoprotection by OPZ, of toxic chemical metabolism in the rat, is complex and involves competitive inhibition of activation succeeded by induction of the enzymes of both activation and detoxication.

Correlated increase of the expression of the c-ras genes in chemically induced hepatocarcinomas

The expression of the c-Ha-ras, the c-Ki-ras and the N-ras genes was measured by the dot blot technique in rat liver tumors induced by a short diethylnitrosamine (DENA) treatment and in the surrounding liver cells. A 2 to 25 times higher level of transcript was found as well in the surrounding cells, as in the tumor cells, as compared to the level in hepatocytes. In addition the increase of expression was parallel for the three ras genes. We conclude that this enhanced expression can be attributed to an epigenetic mechanism and it can, in certain cases, be dissociated from cell proliferation.

Identification of a novel Skp2-like mammalian protein containing F-box and leucine-rich repeats

The F-box protein Skp2 is important for S phase entry and binds to Skp1 and the cyclin A-Cdk2 complex. Here we report the cloning, analysis of genomic organization and characterization of a novel gene product related to Skp2 named FBL2. The human FBL2 gene was found to be a highly interrupted gene of at least 126.6 kb located on chromosome 17 in close proximity to the TRAP220 gene in a head-to-tail orientation. The predicted protein contains an F-box and six perfect C-terminal leucine-rich repeats. Similar to Skp2, this protein interacts with Skp1 and deletion of the F-box inhibits this association. However, in contrast to Skp2, FBL2 was detected in non-proliferating hepatocytes and its expression increased in growth-arrested liver epithelial cells. In addition, FBL2 was localized primarily in the cytoplasm concentrated around the nucleus. Overall, our data indicate that although FBL2 shares strong structural homology with Skp2 as well as having a similar ability to associate with Skp1, these proteins likely play distinct roles and target different substrates to the SCF complex.

Long-term maintenance of taurocholate uptake by adult rat hepatocytes co-cultured with a liver epithelial cell line

Taurocholate (TC) uptake by adult rat hepatocytes co-cultured with other rat liver epithelial cells (RLEC) was studied comparatively to hepatocytes in primary culture. Cells were cultured on Petri dishes for desired times prior to measuring their ability to transport TC. TC uptake was linear for 150 sec in both culture conditions. In hepatocytes cultured alone, the initial rate of TC uptake at an extracellular concentration of 100 microM was 0.19 +/- 0.02 nmole per min per 10(6) cells after 48 hr of culture and decreased by 75% after 4 to 6 days. In hepatocytes co-cultured with RLEC, the rate of uptake at 48 hr (0.31 +/- 0.01 nmole per min per 10(6) cells) was significantly higher than in hepatocytes cultured alone (p less than 0.01); in addition, TC uptake remained stable at an average rate of 0.17 +/- 0.01 nmole per min per 10(6) cells for up to 56 days. No detectable uptake was found in RLEC cultured alone. TC uptake exhibited both saturable (Vmax = 0.30 +/- 0.03 nmole per min per 10(6) cells and Km = 42.6 +/- 4.4 microM) and nonsaturable components. These kinetic parameters were similar to those previously reported in isolated hepatocytes and in short-term cultured hepatocytes. TC uptake exhibited sodium dependence and was significantly reduced when extracellular sodium was replaced by lithium and sucrose, or in the presence of 1 mM ouabain. After 18 days of co-culture, TC uptake had qualitatively the same characteristics as at 48 hr, with a saturable and a nonsaturable component.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Regulation of the major detoxication functions by phenobarbital and 3-methylcholanthrene in co-cultures of rat hepatocytes and liver epithelial cells

In the present study, we analysed the expression of monooxygenase activities and mRNAs associated with cytochrome P-450 (CYP), including CYP1A1/2, CYP2B1/2, CYP2C6, CYP2E1, CYP3A1/2, glutathione transferase alpha (GST alpha), aldehyde dehydrogenase and epoxide hydrolase in co-cultures of primary rat hepatocytes and rat liver epithelial cells. We observed that pentoxyresorufin O-deethylation activity was well maintained and ethoxyresorufin O-deethylation activity gradually decreased during co-culture time. In addition, we showed that phenobarbital and 3-methylcholanthrene treatments resulted in a significant increase of these activities. Two general patterns of accumulation of liver-specific mRNAs were observed. CYP1A1/2, CYP2B1/2, CYP3A1/2, GST alpha, aldehyde dehydrogenase and epoxide hydrolase mRNAs were maintained at a stable level, whereas CYP2C6 and CYP2E1 mRNAs showed a continuous decline. In addition, we observed a strong increase of CYP1A1/2 (13.6-fold) and GST alpha (3.9-fold) mRNA expression in 3-methylcholanthrene-treated co-cultures and induction of CYP2B1/2 (19-fold), CYP2C6 (10-fold), CYP3A1/2 (11.2-fold), GST alpha (9-fold), aldehyde dehydrogenase (6-fold) and epoxide hydrolase (5-fold) mRNA expression in phenobarbital-treated co-cultures. Furthermore, we demonstrated that liver-specific gene expression was restricted to hepatocytes, with the notable exception of epoxide hydrolase and CYP2E1 which were expressed in both cell types during the co-culture, as shown by the selective recovery of both hepatocytes and rat liver epithelial cells. Finally, to investigate whether co-cultures could be used to study the molecular mechanisms regulating CYP transcription, we performed transfection of hepatocytes, before the establishment of the co-culture, with large CYP2B1 (3.9 kb) or CYP2B2 (4.5 kb) promoter chloramphenicol acetyltransferase constructs or with a construct containing a 163-bp DNA sequence element reported to confer phenobarbital responsiveness. A 2-3-fold increase over the basal level of chloramphenicol acetyltransferase activity was observed in phenobarbital-treated co-cultures transfected with the phenobarbital-responsive element construct, although phenobarbital had no effect on large CYP2B1 or CYP2B2 promoter fragments. Our results demonstrate that the co-culture system provides a good tool for studying drug metabolism, and shows promise as a new tool for analysing transcriptional regulation under the influence of xenobiotics within primary hepatocytes.

Constitutive expression of functional P-glycoprotein in rat hepatoma cells

P-glycoprotein is a plasma-membrane glycoprotein involved in multidrug resistance. P-glycoprotein overexpression has been demonstrated to occur in tumor cells after cytotoxic drug exposure, but also in some cancers including hepatocellular carcinomas before any chemotherapeutic treatment. In order to better analyze this constitutive type of tumoral drug resistance, we have investigated P-glycoprotein expression and function in rat liver tumors induced experimentally by administration of diethylnitrosamine and in two cell clones derived from one of these tumors designated as RHC1 and RHC2. High levels of P-glycoprotein mRNAs were found in both liver tumor samples and the two hepatoma cell clones as assessed by Northern blotting; both RHC1 and RHC2 cells displayed altered liver functions commonly observed in rat hepatoma cells, particularly the decreased expression of albumin and overexpression of the fetal glutathione S-transferase 7-7. The use of specific multidrug resistance (mdr) probes revealed a major induction of the mdr1 gene in liver tumor samples while RHC1 and RHC2 cells expressed both mdr1 and mdr3 genes without displaying a major alteration in the number of mdr gene copies as assessed by Southern blotting. High amounts of P-glycoprotein were also demonstrated in RHC1 and RHC2 cells by Western blotting. These cells were strongly resistant to doxorubicin and vinblastine, two anticancer drugs transported by P-glycoprotein. Doxorubicin intracellular retention was low in RHC1 and RHC2 cells, but was strongly enhanced in the presence of verapamil, a known modulator agent of P-glycoprotein; low retention appeared to occur via a drug efflux mechanism, indicating that P-glycoprotein was fully active. These results show that rat hepatoma cells can display elevated levels of functional P-glycoprotein without any prior cytotoxic drug selection and suggest that these cells represent a useful model for analyzing P-glycoprotein regulation in intrinsically clinical drug-resistant cancers.

Progression through G1 and S phases of adult rat hepatocytes

Regenerating liver, hepatocyte primary cultures and differentiated hepatoma cell lines are widely used to study the proliferation/differentiation/apoptosis equilibrium in liver. In hepatocytes, priming factors (TNF alpha, IL6) target G0/G1 transition while growth factors (HGF, EGF, TGF alpha) control a mid-late G1 restriction point. A characteristic pattern of cdk/cyclin expression is observed in hepatocytes, presumably related to their ability to proliferate a limited number of times and to undergo a reversible differentiation. Interestingly, cell-cell interactions between hepatocytes and liver biliary cells in co-cultures, result in a cell cycle arrest in mid G1 of hepatocytes which are insensitive to mitogens. Apoptosis exists in hepatocytes but is still poorly documented. However, hepatoma cell lines stimulated by TGF beta undergo cell death in a p53-independent pathway. In conclusion, the interplay of growth and apoptosis regulators and cell-cell interactions control the proliferation/differentiation/apoptosis balance which is a specific feature of hepatocytes.

Isozyme differentiation of aldolase and pyruvate kinase in fetal, regenerating, preneoplastic, and malignant rat hepatocytes during culture

Aldolase and pyruvate kinase isozymes were investigated in cultured hepatocytes from fetal, regenerating, and 2-acetyl-aminofluorene-fed rat liver as well as in some epithelial liver cell lines. Our results show that: (a) cell proliferation and prolonged expression of specific isozymes were found only in cultured hepatocytes from 17-day old fetuses; (b) the fetal type of pyruvate kinase expressed in regenerating and carcinogen-treated liver was temporarily lost only in cultured hepatocytes from regenerating liver; (c) the adult type of aldolase and pyruvate kinase was absent in one epithelial cell line derived from a carcinogen-treated liver and in the hepatoma tissue cell (HTC) line but was found in the Faza clone of the Reuber H35 cell line during the 50 first passages in vitro; and (d) the isozyme pattern of pyruvate kinase was always more strongly shifted than that of aldolase. The observations suggest that: (a) hepatocytes from carcinogen-treated liver exhibit the same lack of ability to proliferate in primary culture as normal adult hepatocytes; (b) adult hepatocytes can produce fetal isozymes without prior cell division; (c) pyruvate kinase is a stronger marker of dedifferentiation (retrodifferentiation) than aldolase; and (d) regulatory processes of isozyme expression are different during ontogenesis, regeneration, and hepatocarcinogenesis.

Modulation of human fetal hepatocyte survival and differentiation by interactions with a rat liver epithelial cell line

Fetal human hepatocytes were isolated by collagenase digestion of liver fragments and cultured either alone or mixed with rat liver epithelial cells. Whereas they did not survive more than 2-3 weeks and showed rapid morphologic and functional alterations in conventional culture, fetal hepatocytes survived and retained or reverted to a globular morphology for several weeks and showed active albumin secretion for at least 13 days when cultured with rat liver cells. Increased levels of secreted albumin correlated with deposition of an insoluble extracellular material containing fibronectin and type III collagen located principally between the two cell types and around parenchymal cells. These observations show that fetal human hepatocytes are able to interact in vitro with another epithelial liver cell type obtained from a divergent species and that these cell-cell interactions influence both hepatocyte survival and expression of albumin.

Skp2 induction and phosphorylation is associated with the late G1 phase of proliferating rat hepatocytes

The changes in phosphoproteins purified with the affinity peptide p9CKShs1 were analyzed from extracts of regenerating rat livers in order to define some G1 and G1/S regulations characteristic of mature hepatocytes stimulated to proliferate. We observed a 47 kDa phosphoprotein that occurred first at the end of G1 before peaking in the S phase. P47 was also found to be phosphorylated in late G1 in primary hepatocyte cultures stimulated with mitogens. P47 was still phosphorylated in extracts depleted of Cdc2, but to a lesser extent after Cdk2 depletion. This phosphoprotein was identified as Skp2. (i) P47 shared the same electrophoretic mobility than Skp2, a cell cycle protein essential for S phase entry in human fibroblasts; (ii) Skp2, like P47, started to be expressed and was highly phosphorylated during the G1/S transition of hepatocytes stimulated to proliferate in vivo and in vitro; (iii) P47 was specifically immunoprecipitated by an antibody directed against Skp2. In addition, cyclin A/Cdk2 complexes from regenerating liver clearly interacted with Skp2. This is the first demonstration that Skp2 is induced and phosphorylated in the late G1 and S phase of hepatocytes in vivo in regenerating liver as well as in vitro in mitogen-stimulated hepatocytes.