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Unidad de Hepatología Experimental, Centro de Investigación, Hospital Universitario La Fe, 46009 Valencia, SpainDepartamento de Bioquímica y Biología Molecular, Facultad de Medicina, Universidad de Valencia, 46010 Valencia, Spain
To whom correspondence should be addressed: Unidad de Hepatología Experimental, Centro de Investigación, Hospital La Fe, Avenida de Campanar, 21, 46009 Valencia, Spain. Tel.: 34-96-197-30-48; Fax: 34-96-197-30-18;
Unidad de Hepatología Experimental, Centro de Investigación, Hospital Universitario La Fe, 46009 Valencia, SpainDepartamento de Bioquímica y Biología Molecular, Facultad de Medicina, Universidad de Valencia, 46010 Valencia, Spain
* This work was supported in part by a grant from the Ministry of Science and Technology (SAF 2003-09353) and from a European Union Integrated Research Project (PREDICTOMICS LSHB-CT-2004-504761). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3 and Tables S1 and S2. 1 Recipient of a predoctoral grant from the Consellería de Educación Ciencia y Cultura, Generalitat Valenciana (Valencian Regional Government).
Hepatocyte nuclear factor 4α (HNF4α) 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, HNF4α 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 (PGC1α, SRC1, SRC2, and PCAF) might account for the lack of function of HNF4α in HepG2 cells. Among them, PGC1α and SRC1 are the two most important HNF4α 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 PGC1α by recombinant adenoviruses led to a significant up-regulation of well characterized HNF4α-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 PGC1α was seen to be dependent on HNF4α. Finally, insulin treatment of human hepatocytes and HepG2 cells caused repression of PGC1α and a concomitant down-regulation of ApoCIII, PEPCK, AldoB, and OTC. Altogether, our results suggest that SRC1, and notably PGC1α, are key coactivators for the proper function of HNF4α in human liver and for an integrative control of multiple hepatic genes involved in metabolism and homeostasis. The down-regulation of key HNF4α coactivators could be a determinant factor for the dedifferentiation of human hepatomas.
Hepatoma cell lines and hepatocellular carcinomas (HCC)
Current research supports the notion that hepatocyte nuclear factor 4α (HNF4α) is one of the most important liver-enriched transcription factors for hepatocyte differentiation. HNF4α is a highly conserved member of the nuclear superfamily that was initially identified as a factor required for liver-specific gene expression (
). HNF4α plays critical roles not only in the specification of the hepatic phenotype during liver development but also in the transcriptional regulation of genes involved in glucose, cholesterol, fatty acids, and xenobiotic metabolism and in the synthesis of blood coagulation factors (
). Genome-scale location analysis revealed surprising results for HNF4α in hepatocytes. The number of genes that exhibit a binding of HNF4α to their regulatory regions (>1500 genes) was much larger than that observed with other typical liver-specific regulators. Notably, from the genes occupied by RNA polymerase II, 42% were also bound by HNF4α in hepatocytes (
). Therefore, HNF4α emerges as a widely acting transactivator in the liver, consistent with the observation that the expression of this constitutively active transcription factor overcomes repression of the hepatic phenotype in dedifferentiated hepatoma cells (
However, the significance of a correlation between the expressions of both HNF4α and hepatic functions is challenged by several studies showing that hepatic functions could be silent despite HNF4α being expressed. Hepatic functions were found uncoupled or dissociated from HNF4α in hepatoma cell lines, intertypic cell hybrids, and immortalized hepatocytes (
). The existence of such dissociations suggests that in some instances HNF4α could be highly expressed but not fully active.
A key element for a correct nuclear function is a balanced, physiologic level of coregulators. It is well known that HNF4α interacts with coactivators and corepressors through its activation function domains. Full activity is achieved through the interaction of HNF4α homodimers with DNA and coactivators. Various studies have shown that HNF4α interacts strongly with the p160 family coactivators (SRC1, 2, and 3) (
In the present study, we have demonstrated that HNF4α is highly expressed but not fully active in the human hepatoma HepG2. This lack of function can be accounted for by the low levels of the coactivators SRC1 and, notably, PGC1α, which after re-expression caused a marked improvement of the HNF4α function and its target genes and enhanced the hepatic phenotype significantly. Expression analysis in several human hepatomas also suggests that the down-regulation of PGC1α and SRC1 could be an important mechanism involved in hepatocyte dedifferentiation and progression of HCC.
Cell Culture—Human hepatoma cells (HepG2, Hep3B, Mz-Hep-1, and Chang Liver) were plated in Ham's F-12/Leibovitz L-15 (1/1, v/v) supplemented with 6% fetal calf serum and cultured to 70–80% confluence. Human hepatoma BC2 cells were cultured in a mixture of 75% minimal essential medium and 25% Medium 199, supplemented with 10% fetal bovine serum, 1 mg/ml bovine serum albumin, 0.7 μm insulin and hydrocortisone hemisuccinate, and maintained at confluence for 3 weeks. HeLa (human cervix carcinoma) and 293 cells (AdE1A-transformed human embryonic kidney) were grown in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum and maintained as monolayer cultures. Culture medium for 293 cells was also supplemented with 3.5 g/liter of glucose. Human hepatocytes were isolated from liver biopsies (1–3 g) of patients undergoing liver surgery after informed consent. None of the patients habitually consumed alcohol or other drugs. A total of five liver biopsies (two male and three female of ages ranging from 26 to 65 years) were used. Hepatocytes were isolated using a two-step perfusion technique (
) and seeded on plates coated with fibronectin (3.6 μg/cm2) at a density of 8 × 104 cells/cm2. The culture medium was Ham's F-12/Leibovitz L-15 (1/1, v/v) supplemented with 2% newborn calf serum, 5 mm glucose, 0.2% bovine serum albumin, and 10–8m insulin. The medium was changed 1 h later to remove unattached hepatocytes. After 24 h, the culture medium was changed to serum-free medium containing 10–8m dexamethasone. Cultures were routinely supplemented with 50 units/ml penicillin and 50 μg/ml streptomycin.
Development of Adenoviral Vectors—A recombinant adenovirus was prepared for the expression of human HNF4α as follows: HNF4α2 cDNA was released from the expression vector pMT2-HNF4B (Dr. Talianidis) by EcoRI digestion and subcloned into the EcoRI site of the adenoviral shuttle vector pAC/CMVpLpA. This plasmid was cotransfected with pJM17, containing the full-length adenovirus-5 genome (dl309), into 293 cells by calcium phosphate/DNA coprecipitation. Homologous recombination between adenovirus sequences in the shuttle vector pAC/CMVpLpA and in the pJM17 plasmid generates a genome of a packable size in which most of the adenovirus early region 1 is lacking, thus rendering the recombinant virus replication defective (
A recombinant adenovirus for the coactivator SRC1 was prepared by using the AdEasy™ adenoviral vector system (Stratagene). SRC1 cDNA was released from pCR3.1-SRC1a (Dr. O'Malley) by ApaI digestion, subcloned into the pSPORT vector (Invitrogen), and ligated into the BglII and KpnI sites of the adenoviral pShuttle-CMV vector (Stratagene). To generate recombinant adenovirus, the linearized plasmid (PmeI digestion) was transferred into BJ 5183 cells that contained the pretransferred Ad-Easy-1 vector. Colonies containing the correct recombinant adenovirus were identified using restriction enzymes and PCR with insert-specific primers. The recombinant adenovirus DNA was then linearized by PacI and transfected into human embryonic kidney 293 cells by the calcium phosphate precipitation method. After several days of culture, infected 293 cells were collected and subjected to three cycles of freezing/thawing. The generation of a high titer adenovirus stock was performed as described (
). The Ad-PGC-1 vector contains, in tandem, the green fluorescent protein gene and the PGC1α cDNA (containing FLAG and HA epitope tags) downstream of separate cytomegalovirus promoters.
Cell lines and primary hepatocytes were infected with recombinant adenoviruses for 120 min at a m.o.i. (multiplicity of infection) ranging from 1 to 40 plaque-forming units/cell. Thereafter, cells were washed and fresh medium added. 48 h post-transfection, cells were analyzed or directly frozen in liquid N2.
Transfection and Reporter Gene Assays—The chimeric luciferase reporter construct containing three in-tandem copies of a HNF4α response element for human ApoCIII in front of a TK promoter (pGL3-B-3xApoCIII-TK-LUC) (
) and its control reporter vector (pGL3-B-TK-LUC) were kindly provided by Dr. Talianidis. The expression vectors for transcription factors and coactivators were the following: pMT2-HNF4B (Dr. Talianidis), pcDNA3-HA-hPGC1 (Dr. Kralli), pCR3.1-SRC1a (Dr. O'Malley), pCMX-FLAG-PCAF (Dr. Talianidis), pSG5-GRIPI (Dr. Stallcup), pSG5-TIF-II (Dr. Gronemeyer), pCMX-ACTR (Dr. Evans).
Plasmid DNAs were purified with Qiagen Maxiprep kit columns (Qiagen) and quantified by A260 and fluorescence using PicoGreen® (Molecular Probes). The day before transfection, cells were plated in 35-mm dishes with 1.5 ml of Dulbecco's modified Eagle's medium/Nut F12 (Invitrogen) supplemented with 6% newborn calf serum, 50 units/ml penicillin, and 50 μg/ml streptomycin. Firefly luciferase expression constructs (pGL3-B-3xApoCIII-TK-LUC and pGL3-B-TK-LUC) (0.5 μg) were transfected with varying amounts of expression plasmids (0.2–3.0 μg) by the calcium phosphate precipitation method as indicated in the figures. The total amount of expression vector was kept constant by adding empty expression vector. In parallel, 0.08 μg of pRL-CMV (a plasmid expressing Renilla reniformis luciferase under the cytomegalovirus immediate early enhancer/promoter) was cotransfected to correct variations in transfection efficiency. Calcium phosphate/DNA coprecipitates were added directly to each culture, and cells were incubated for an additional 48 h. Luciferase activities were assayed using the Dual-Luciferase® reporter kit (Promega).
Quantification of mRNA Levels—Total cellular RNA was extracted with the RNeasy Total RNA kit (Qiagen), and contaminating genomic DNA was removed by incubation with DNase I Amplification Grade (Invitrogen). RNA (1 μg) was reverse transcribed as described (
). Diluted cDNA (3 μl) was amplified with a rapid thermal cycler (LightCycler Instrument; Roche Diagnostics) in 15 μl of LightCycler DNA Master SYBR Green I (Roche Applied Science), 5 mm MgCl2, and 0.3 μm of each oligonucleotide. We designed specific primer sets for 18 different cDNAs including liver genes, transcription factors, and coactivators (supplemental Table S1). Whenever possible, primer sequences were chosen to span exon boundaries. In parallel, we always analyzed the mRNA concentration of the human housekeeping porphobilinogen deaminase (hydroxymethylbilane synthase) as an internal control for normalization (supplemental Table S1). A stable expression of the housekeeping porphobilinogen deaminase gene was validated by comparison with TATA box-binding protein expression as a second constitutive control gene (Human TBP Primer Set; Invitrogen). We found that the expression ratio of these two internal control genes was practically constant in the different tissues and cells investigated. Moreover, human porphobilinogen deaminase and TATA box-binding protein do not harbor pseudogenes and show genomic stability in cancer (
). PCR amplicons were confirmed to be specific by size (agarose gel electrophoresis) and melting curve analysis. After denaturing for 30 s at 95 °C, amplification was performed in 40 cycles of 1 s at 94°C, 5 s at 62 °C, and 15–20 s at 72 °C. The real-time monitoring of the PCR reaction and the precise quantification of the products in the exponential phase of the amplification were performed with the LightCycler quantification software according to the manufacturer's recommendations. Reproducibility of the measurements was assessed by conducting triplicate reactions.
Extraction of Nuclear Proteins and Immunoblotting—Nuclear extracts from cultured cells were prepared as described (
) and electrophoresed in an SDS-polyacrylamide gel (20 μg of protein/lane). Proteins were transferred to polyvinylidene fluoride membranes (Immobilon; Millipore), and sheets were incubated with a goat polyclonal antibody raised against a carboxyl-terminal epitope of human HNF4α (Santa Cruz Biotechnology). After washing, blots were developed with horseradish peroxidase-labeled IgG using an Enhanced Chemiluminescence kit (Amersham Biosciences). Equal loading was verified by Coomassie Blue staining of the membrane blots.
Chromatin Immunoprecipitation (ChIP) Assay and RNAPol-ChIP—Cells were treated with 1% formaldehyde in phosphate-buffered saline buffer under gentle agitation for 10 min at room temperature in order to cross-link transcription factors to DNA. Thereafter, cells were collected by centrifugation, washed, resuspended in lysis buffer, and sonicated on ice for 6 × 10-s steps at 75% output in a Branson Sonicator. Cross-linking and sonication of chromatin from human liver tissue (750 mg) was carried out following a partially different protocol (
). Sonicated samples were centrifuged to clear supernatants. DNA content was carefully measured by fluorescence with PicoGreen dye (Molecular Probes) and properly diluted to obtain an equivalent amount of DNA in all samples (input DNA). For immunoprecipitation, two different antibodies for HNF4α (sc-6556 and sc-8987; Santa Cruz Biotechnology) and a specific antibody against the RPB1 subunit of RNApol II (sc-899) were used. The immunofractionation of protein-DNA complexes was performed by the addition of 10 μg/ml of specific antibodies with incubation at 4 °C overnight on a 360° rotator (antibody-bound DNA fraction). For each cell preparation, an additional mock immunoprecipitation with rabbit preimmune IgG (sc-2027; Santa Cruz Biotechnology) was performed (background DNA fraction). The immunocomplexes were affinity absorbed with 10 mg of protein A/G-Sepharose (prewashed with lysis buffer for 4 h at 4°C under gentle rotation) and collected by centrifugation (6500 × g, 1 min). The antibody-bound and background DNA fractions were washed as described (
). The cross-links were reversed by heating the samples at 65 °C overnight. The DNA from bound, background, and input fractions was purified, diluted (1/10 bound and background fractions, 1/400 input fraction), and subjected to quantitative real-time PCR with a LightCycler instrument. Amplification was real-time monitored and allowed to proceed in the exponential phase until fluorescent signal from input samples reached a significant value. Amplified DNA was then analyzed by agarose gel electrophoresis. Amplification and quantification of ApoCIII gene sequences (–740 and –80-bp 5′-flanking regions, and exons 3 and 4) among the pull of DNA was performed with specific primers flanking these regions (supplemental Table S2). The detection of RNA polymerase II within the coding region of the ApoCIII gene allows the quantification of the actual transcriptional rate (
). To ensure reproducibility, immunoprecipitation and PCR analysis were performed in duplicate from different liver tissue samples, cultured hepatocytes, and cell lines.
Ureagenesis—The ureogenic rate was assessed in HepG2 cells incubated with 3 mm ammonium chloride by measuring the appearance of urea in the culture medium. Urea concentration was determined by the diacetylmonoxime method (
HNF4α Levels in Different Hepatic and Non-hepatic Cell Models and in Human Liver—HNF4α expression levels in cultured human hepatocytes and hepatoma cell lines (HepG2, Hep3B, and BC2) were similar to those of liver tissue as assessed by RT-PCR (Fig. 1A) and immunoblotting analysis (Fig. 1B). Among the several hepatomas analyzed, the highest HNF4α expression level was found in the widely used cell line HepG2. The expression of HNF4α was, however, very low or absent in non-hepatic cell lines (i.e. 293 and HeLa) and in the more dedifferentiated human hepatoma Mz-Hep-1 (Fig. 1).
HNF4α Function Is Impaired in Human Hepatoma HepG2 Cells—The expression of many hepatic genes has an absolute dependence on HNF4α. Data obtained from HNF4α null mice demonstrated that this transcription factor is indispensable for the constitutive expression of key hepatic genes such as apolipoproteins (A, B, and C families), L-FABP, PEPCK, AldoB, OTC, and CYP7A1 (
). A comparative analysis of eight well characterized HNF4α target genes in different cell types revealed high expression levels in cultured human hepatocytes and null or very low levels in HepG2 cells (supplemental Fig. S1). Among the eight mRNA measured, we specifically found that ApoCIII, AldoB, PEPCK, and OTC were essentially not expressed in HepG2 cells, whereas ApoAII, ApoAV, CYP7A1, and L-FABP showed levels of ∼20% of human liver (supplemental Fig. S1). The expression profile in HepG2 cells was closer to that of non-hepatic cell lines (293 and HeLa).
To gain a better understanding of the discrepancy between high HNF4α levels and the low or null expression of target genes in HepG2, we performed chromatin immunoprecipitation assays and analyzed the occupancy of two different binding sites in the human ApoCIII gene by HNF4α (Fig. 2A). In parallel, we performed RNAPol-chromatin immunoprecipitation to measure the binding of RNApol-II to the promoter and the transcription through ApoCIII coding regions (exons 3 and 4) (Fig. 2B). We confirmed an appropriate binding of HNF4α to the –740 and –80-bp elements in human liver samples, as well as RNApol-II binding to the promoter and active transcription through exons 3 and 4. Similar results were found in cultured human hepatocytes (data not shown). However, in the human hepatoma HepG2, binding of HNF4α to the –80-bp element was substantially decreased (Fig. 2A) and RNApol-II occupancy at the promoter and coding regions was almost undetectable (Fig. 2B). As a negative control, we also analyzed non-hepatic HeLa cells where HNF4α and ApoCIII are not expressed. Our results suggest that binding and transactivation by HNF4α is impaired in HepG2 cells.
Possible Mechanisms Underlying the Dysfunction of HNF4α in Hepatoma Cells—HNF4α exists in several isoforms, all of which are capable of binding to the same regulatory elements but with different transactivating properties. An imbalanced expression of HNF4α isoforms could explain why HNF4α is non-operative in HepG2. However, this apparently is not the case; the major HNF4α splicing variants (α1, α2, α3, and α7) had similar expression levels in HepG2 and in human hepatocytes (data not shown).
Another mechanism causing HNF4α dysfunction in hepatoma cells could be an increase of negative factors that block HNF4α activity. The small heterodimeric partner, which lacks a DNA binding domain and exhibits inhibiting interactions with HNF4α, might be involved. Similarly, the chicken ovalbumin upstream promoter-transcription factors could act as transcription repressors for several nuclear s, including HNF4α. However, the expression levels of these negative factors in HepG2 cells do not differ substantially from those in cultured hepatocytes or human liver (data not shown), suggesting that other mechanisms might be involved.
Another possible explanation for the loss of function of endogenously expressed HNF4α could be that HepG2 cells either lack the essential coactivators needed for proper functionality or have increased levels of corepressors. We measured seven coactivators in cultured cells and liver tissue and found that four of them (SRC1, SRC2, PGC1α, and PCAF) were down-regulated in HepG2 cells to 10–30% of the human liver levels (supplemental Fig. S2A). Other important coactivators, SRC3, CBP, and P300, did not change. We also measured two corepressors (nuclear corepressor (NcoR) and silencing mediator of retinoic acid and thyroid hormone (SMRT)) and found that their expression levels in HepG2 cells were similar to those of liver or cultured hepatocytes (supplemental Fig. S2A). Thus, the low level of coactivators (SRC1, SRC2, PGC1α, and PCAF) could be a cause of HNF4α dysfunction in HepG2 cells.
The p160 steroid coactivator gene family contains three homologous members (SRC1, 2, and 3) that operate as transcriptional coactivators for nuclear s and other transcription factors. It has been shown that these coactivators have an overlapping activity, and it may be speculated that a decrease in one SRC form (e.g. SRC1 or 2) could be compensated by an increase of the expression of the other family members (e.g. SRC3). However, absolute quantification of mRNA levels demonstrated that SRC1 and SRC2 are the most abundant forms in human liver and cultured hepatocytes and their decrease in HepG2 cells cannot be compensated by SRC3 expression levels (supplemental Fig. S2B).
Relevance of Coactivators for HNF4α-mediated Activity in Reporter Gene Assay—In HepG2 cells, the luciferase (LUC) activity of a construct containing three copies of a HNF4α response element (pGL3-B-3xApoCIII-TK-LUC) was no different from that of a control construct lacking HNF4α binding sites (pGL3-B-TK-LUC), which supports a lack of function of endogenous HNF4α in hepatoma cells (supplemental Fig. S3A). This is reinforced by the fact that the transfection of HNF4α caused a modest dose-dependent increase in luciferase activity, which suggests that other missing factors may limit the response (supplemental Fig. S3A). The transfection of coactivators SRC1 and, notably, PGC1α caused a significant increase in the 3xApoCIII-TK-LUC reporter activity in HepG2 cells (2.3- and 25.0-fold, respectively; supplemental Fig. S3A), whereas a much lower effect was noted in HeLa cells lacking endogenous HNF4α (1.1- and 3.3-fold increase, respectively; supplemental Fig. S3A). Other coactivators (SRC2, SRC3, and PCAF) produced no substantial change in promoter activity. This experimental evidence suggests that endogenous HNF4α in HepG2 cells is functional but its activity is limited by the low concentration of SRC1 and PGC1α. This possibility was further demonstrated by cotransfection experiments. We found a synergistic effect of HNF4α and PGC1α in HeLa cells, where the cotransfection of both factors was needed to largely improve the HNF4-dependent reporter gene activity (26.0-fold increase; supplemental Fig. S3B). However, cotransfection of both factors in HepG2 cells caused a very similar response to that obtained with the sole transfection of PGC1α (supplemental Fig. S3B), likely because in HepG2 cells the coactivation by exogenous PGC1α is attained via endogenous HNF4α.
Down-regulation of PGC1α and SRC1 Is a Common Event in Human Hepatomas—We measured the mRNA levels of PGC1α and SRC1 in five different human hepatoma cell lines derived from hepatocellular carcinomas, and we compared them with liver levels. We found that PGC1α was consistently down-regulated in all hepatomas with levels of ∼20% of human liver (Fig. 3A). SRC1 mRNA expression was also consistently lower in all hepatoma cell lines, although in this case the relative levels were between 20–50% of human liver (Fig. 3B). These results suggest that underexpressed coactivators could be a common feature of hepatomas and hepatocellular carcinomas.
Adenovirus-mediated Re-expression of PGC1α and SRC1 Reactivates HNF4α Target Genes in Hepatoma Cells—Adenoviral-mediated transfection of PGC1α in HepG2 cells caused a dose-dependent increase in most of the HNF4α target genes, ApoCIII, ApoAV, AldoB, PEPCK, OTC, and CYP7A1 (Fig. 4A). On the contrary, adenoviral transfection of SRC1 had a less broad effect on HNF4α target genes, where a significantly increased expression was observed only in CYP7A1 and PEPCK mRNAs (Fig. 4B). A parallel transfection with the control adenoviral vector did not modify the expression of these genes. The effect observed with Ad-PGC1α or Ad-SRC1 in HepG2 cells did not significantly improve via the cotransfection with an adenoviral vector for HNF4α (data not shown). Therefore, we may postulate that the re-expression of PGC1α and SRC1 restores HNF4α activity in human hepatoma cells, which in turn leads to a strong up-regulation of multiple hepatic-specific genes.
To further test whether the effects caused by PGC1α were mediated through HNF4α, we carried out transfection experiments in Mz-Hep-1, a hepatoma cell line that lacks endogenous HNF4α (see Fig. 1). Preliminary transfection experiments with Ad-HNF4α and Ad-GFP showed that a dose of 16 m.o.i. was sufficient for a high expression level (Fig. 5A) in nearly 100% of the cultured cells (Fig. 5B). Although separate transfections of PGC1α and HNF4α did not cause a significant increase in HNF4α target genes, the cotransfection of both factors triggered a dramatic increase (Fig. 5C). ApoCIII, AldoB, PEPCK, and OTC mRNA concentrations rose from marginal levels to reach expression levels comparable with those observed in adult livers (Fig. 5C). For instance, ApoCIII mRNA in Mz-Hep-1 cells transfected with HNF4α plus PGC1α increased from levels of around the detection limit (>36 cycles) to 15–20% of those detected in human liver (>130-fold increase).
Finally, we investigated whether a re-expression of PGC1α in human hepatoma HepG2 cells could improve liver-specific metabolic functions associated with the differentiated phenotype. We transfected HepG2 cells with Ad-PGC1α, and 48 h post-transfection the rate of urea synthesis was determined in cultures incubated with 3 mm ammonia for 2 h. Transfection of HepG2 cells with Ad-PGC1α at 4 and 32 m.o.i. led to an increase in the ureogenic rate of 2.2- and 4.0-fold, respectively (Table 1). These data demonstrate that activation of hepatic genes by PGC1α in hepatoma cells leads to an improvement of hepatic metabolic functions associated with a differentiated hepatic phenotype.
TABLE 1Effect of PGC1α on the ureogenic rate in HepG2 cells
Insulin Represses PGC1α and Causes Down-regulation of HNF4α Target Genes in Cultured Human Hepatocytes and HepG2 Cells—Our results point to PGC1α as one of the most important coactivators for the basal expression of HNF4α target genes in the liver. However, PGC1α is also modulated during the feeding-fasting cycle by stimuli such as glucagon or insulin. It can therefore be suggested that physiologic modulation of PGC1α will simultaneously influence the expression of multiple HNF4α target genes. To investigate this possibility, we treated human hepatocytes and hepatoma HepG2 cells with insulin and measured the expression levels of both PGC1α and HNF4α target genes. Cells were cultured in serum- and hormone-free medium and treated with insulin for 12 h. Results were coincident in both cell systems (Fig. 6, A and B). Insulin caused a 35–55% decrease in PGC1α mRNA and a concomitant 25–65% decrease in ApoCIII, OTC, AldoB, and PEPCK mRNAs (Fig. 6). Transfection of HepG2 cells with Ad-PGC1α (16 m.o.i.) increased basal expression levels of target genes and prevented repression by insulin (data not shown). Altogether, our results reinforce the notion that PGC1α plays a significant role in the transcriptional regulation of HNF4α-dependent genes in human liver cells.
The regulation of gene transcription has generally been thought to occur via changes in amounts or activities of transcription factors. However, it is now quite clear that a substantial component of gene control is directed by coactivators acting as the primary targets of differentiation or physiological signals. The down-regulation of a few coactivators can change the activity of multiple transcription factors and facilitate the progress of distinct biological programs (
). Indeed, we have shown that a lower intracellular level of coactivators (SRC1, SRC2, PGC1α, and PCAF) in human hepatoma cells is associated with a deficient expression of hepatic genes. We have also demonstrated that the coactivator PGC1α plays a prominent role in sustaining the basal expression of multiple distinctive hepatic genes, suggesting that this factor could have a particular relevance in the maintenance of the differentiated adult hepatic phenotype. The importance of PGC1α in other programs of differentiation such as chondrogenesis has been demonstrated (
PGC1α shows a specific expression pattern restricted to tissues that have a high energy demand such as heart, brown adipose tissue, and skeletal muscle, where PGC1α expression is induced in response to stimuli such as cold or physical exercise (
). This supports the notion that PGC1α can also be an important coactivator for the constitutive expression of many hepatic genes in the absence of inducible stimuli.
In the liver, PGC1α is induced in response to fasting and insulin deficiency and in isolated hepatocytes by cAMP, glucagon, and glucocorticoids, leading to the activation of all key enzymes of gluconeogenesis and the increase of hepatic glucose production (
). Our results have also shown that insulin treatment of human hepatic cells causes a parallel down-regulation of PGC1α and several HNF4 target genes. These results reinforce the idea that PGC1α plays a significant role in sustaining transcription by HNF4α in human liver cells but also emphasize the importance of PGC1α as a wide-ranging integrating coactivator in response to feeding-fasting stimuli.
In the human hepatoma cell line HepG2, we analyzed eight liver genes that have been well characterized as HNF4α-dependent genes (
). The expression levels of these genes in HepG2 cells were consistently much lower than in human liver tissue. Transfection of the HNF4α coactivator PGC1α significantly improved the transcription of most of the genes analyzed (six of eight). Some of these genes were well characterized targets of PGC1α (e.g. PEPCK and ApoAV), but others (e.g. AldoB and OTC) have for the first time been described as bona fide targets of PGC1α in this study. Our results do not support the previous notion that PGC1α and HNF4α have a significant activating effect only on the gluconeogenic genes PEPCK and glucose-6-phosphatase in the liver (
). We have shown that PGC1α and HNF4α also play an important role in the activation of genes for apolipoproteins, ureagenesis, and bile acids synthesis, which coincides with other studies showing a more general role for PGC1α in association with HNF4α (
), also demonstrate that not all HNF4α-dependent genes are coactivated by PGC1α (i.e. L-FABP and ApoAII).
The gain-of-function studies described herein provide convincing evidence that PGC1α is a key coactivator for sustaining the expression of multiple HNF4-dependent genes in human hepatoma cells. The relevance of these findings to the in vivo situation in humans cannot be addressed, but mice with a targeted deletion of PGC1α have been established and characterized (
). However, a comprehensive analysis of HNF4α target genes in PGC1α–/– mice has not been shown.
The re-expression of SRC1 in hepatoma cells had a more limited impact on liver genes than PGC1α. SRC1-mediated activation was restricted to PEPCK and CYP7A1, which is in agreement with previous studies (
). Interestingly, we observed a marked activation of these genes by the sole transfection of SRC1 without any hormonal or physiological stimulation, suggesting that a lower expression of SRC1 in hepatoma cells could also contribute to the down-regulation of key liver genes involved in glucose and bile acid metabolism.
Besides the constitutive activation of signal transduction pathways that promote cell growth and survival, one of the most critical steps in the pathogenesis of HCC is dedifferentiation and alteration of liver function. Recent studies comparing expression profiles in HCC and noncancerous liver revealed a down-regulation of typical hepatic genes such as those coding for key enzymes involved in gluconeogenesis, glycogen synthesis, amino acid and lipid metabolism (
). In the present work, analyses of several hepatoma cell lines have shown that PGC1α and SRC1 are consistently down-regulated, which would explain the lack of correlation between the high levels of HNF4α and the low expression of many HNF4 target genes. Moreover, the strong dependence found between coactivator levels and hepatic gene expression prompts us to propose that a significant down-regulation of coactivators could not only switch off the regulatory balance required for the maintenance of the adult hepatic phenotype but also promote a program of cell dedifferentiation with clinical significance in the pathogenesis of HCC.
We thank C. Corchero and E. Belenchon for expert technical assistance.