A novel hepatocytic transcription factor that binds the alpha-fetoprotein promoter-linked coupling element

NK2 homeobox gene cluster: Functions and roles in human diseases

NK2 genes (NKX2 gene cluster in humans) encode for homeodomain-containing transcription factors that are conserved along the phylogeny. According to the most detailed classifications, vertebrate NKX2 genes are classified into two distinct families, NK2.1 and NK2.2. The former is constituted by NKX2-1 and NKX2-4 genes, which are homologous to the Drosophila scro gene; the latter includes NKX2-2 and NKX2-8 genes, which are homologous to the Drosophila vnd gene. Conservation of these genes is not only related to molecular structure and expression, but also to biological functions. In Drosophila and vertebrates, NK2 genes share roles in the development of ventral regions of the central nervous system. In vertebrates, NKX2 genes have a relevant role in the development of several other organs such as the thyroid, lung, and pancreas. Loss-of-function mutations in NKX2-1 and NKX2-2 are the monogenic cause of the brain-lung-thyroid syndrome and neonatal diabetes, respectively. Alterations in NKX2-4 and NKX2-8 genes may play a role in multifactorial diseases, autism spectrum disorder, and neural tube defects, respectively. NKX2-1, NKX2-2, and NKX2-8 are expressed in various cancer types as either oncogenes or tumor suppressor genes. Several data indicate that evaluation of their expression in tumors has diagnostic and/or prognostic value.

Telomerase inhibition decreases alpha-fetoprotein expression and secretion by hepatocellular carcinoma cell lines: in vitro and in vivo study

Alpha-fetoprotein (AFP) is a diagnostic marker for hepatocellular carcinoma (HCC). A direct relationship between poor prognosis and the concentration of serum AFP has been observed. Telomerase, an enzyme that stabilizes the telomere length, is expressed by 90% of HCC. The aim of this study was to investigate the effect of telomerase inhibition on AFP secretion and the involvement of the PI3K/Akt/mTOR signaling pathway. Proliferation and viability tests were performed using tetrazolium salt. Apoptosis was determined through the Annexin V assay using flow cytometry. The concentrations of AFP were measured using ELISA kits. The AFP mRNA expression was evaluated using RT-PCR, and cell migration was evaluated using a Boyden chamber assay. The in vivo effect of costunolide on AFP production was tested in NSG mice. Telomerase inhibition by costunolide and BIBR 1532 at 5 and 10 μM decreased AFP mRNA expression and protein secretion by HepG2/C3A cells. The same pattern was obtained with cells treated with hTERT siRNA. This treatment exhibited no apoptotic effect. The AFP mRNA expression and protein secretion by PLC/PRF/5 was decreased after treatment with BIBR1532 at 10 μM. In contrast, no effect was obtained for PLC/PRF/5 cells treated with costunolide at 5 or 10 μM. Inhibition of the PI3K/Akt/mTOR signaling pathway decreased the AFP concentration. In contrast, the MAPK/ERK pathway appeared to not be involved in HepG2/C3A cells, whereas ERK inhibition decreased the AFP concentration in PLC/PRF/5 cells. Modulation of the AFP concentration was also obtained after the inhibition or activation of PKC. Costunolide (30 mg/kg) significantly decreased the AFP serum concentration of NSG mice bearing HepG2/C3A cells. Both the inhibition of telomerase and the inhibition of the PI3K/Akt/mTOR signaling pathway decreased the AFP production of HepG2/C3A and PLC/PRF/5 cells, suggesting a relationship between telomerase and AFP expression through the PI3K/Akt/mTOR pathway.

Quercetin modulates NF-kappa B and AP-1/JNK pathways to induce cell death in human hepatoma cells

Quercetin, a dietary flavonoid, has been shown to possess anticarcinogenic properties, but the precise molecular mechanisms of action are not thoroughly elucidated. The aim of this study was to investigate the regulatory effect of quercetin (50 microM) on two main transcription factors (NF-kappa B and AP-1) related to survival/proliferation pathways in a human hepatoma cell line (HepG2) over time. Quercetin induced a significant time-dependent inactivation of the NF-kappa B pathway consistent with a downregulation of the NF-kappa B binding activity (from 15 min onward). These features were in concert with a time-dependent activation (starting at 15 min and maintained up to 18 h) of the AP-1/JNK pathway, which played an important role in the control of the cell death induced by the flavonoid and contributed to the regulation of survival/proliferation (AKT, ERK) and death (caspase-3, p38, unbalance of Bcl-2 proapoptotic and antiapoptotic proteins) signals. These data suggest that NF-kappa B and AP-1 play a main role in the tight regulation of survival/proliferation pathways exerted by quercetin and that the sustained JNK/AP-1 activation and inhibition of NF-kappa B provoked by the flavonoid induced HepG2 death.

Epicatechin induces NF-κB, activator protein-1 (AP-1) and nuclear transcription factor erythroid 2p45-related factor-2 (Nrf2) via phosphatidylinositol-3-kinase/protein kinase B (PI3K/AKT) and extracellular regulated kinase (ERK) signalling in HepG2 cells

The dietary flavonoid epicatechin has been reported to exhibit a wide range of biological activities. The objective of the present study was to investigate the time-dependent regulation by epicatechin on the activity of the main transcription factors (NF-κB, activator protein-1 (AP-1) and nuclear transcription factor erythroid 2p45-related factor (Nrf2)) related to antioxidant defence and survival and proliferation pathways in HepG2 cells. Treatment of cells with 10 μm-epicatechin induced the NF-κB pathway in a time-dependent manner characterised by increased levels of IκB kinase (IKK) and phosphorylated inhibitor of κB subunit-α (p-IκBα) and proteolytic degradation of IκB, which was consistent with an up-regulation of the NF-κB-binding activity. Time-dependent activation of the AP-1 pathway, in concert with enhanced c-Jun nuclear levels and induction of Nrf2 translocation and phosphorylation were also demonstrated. Additionally, epicatechin-induced NF-κB and Nrf2 were connected to reactive oxygen species intracellular levels and to the activation of cell survival and proliferation pathways, being phosphatidylinositol-3-kinase/protein kinase B (PI3K/AKT) and extracellular regulated kinase (ERK) associated to Nrf2 modulation and ERK to NF-κB induction. These data suggest that the epicatechin-induced survival effect occurs by the induction of redox-sensitive transcription factors through a tight regulation of survival and proliferation pathways.

Characterization of Distant Enhancers and Promoters in the Albumin-α-Fetoprotein Locus during Active and Silenced Expression

The albumin and alpha-fetoprotein genes are adjacent and express closely related serum proteins. Both genes are strongly expressed in fetal liver, primarily through activation by distant enhancers, but the AFP gene selectively undergoes developmental silencing. We used chromatin immunoprecipitation to study enhancers and promoters during active and silenced gene expression. In adult phenotype cells, the silenced AFP gene was actively repressed at the promoter and two proximal enhancers, characterized by the absence of coactivators and acetylated histone 4, and the presence of corepressors and K9-methylated histone 3. Specific transcription factors, TBP, and RNA polymerase II were all detected on both active and silenced genes, indicating that both states were actively regulated. Surprisingly, promoter-specific factors were also detected on enhancers, especially with reduced chromatin shearing. Under these conditions, an enhancer-specific factor was also detected on the albumin promoter. Association of promoter- and enhancer-specific factors was confirmed by sequential immunoprecipitation. Because no binding was detected on intervening segments, these promoter-enhancer associations suggest looping.

Methods for transcription factor separation

Recent advances in the separation of transcription factors (TFs) are reviewed in this article. An overview of the transcription factor families and their structure is discussed and a computer analysis of their sequences reveals that while they do not differ from other proteins in molecular mass or isoelectric pH, they do differ from other proteins in the abundance of certain amino acids. The chromatographic and electrophoretic methods which have been successfully used for purification and analysis are discussed and recent advances in stationary and mobile phase composition is discussed.

Developmental regulation of galactokinase in suckling mouse liver by the Egr-1 transcription factor

The numerous changes in metabolic pathways that accompany liver development entail associated changes in gene expression. Egr-1 is a zinc-finger transcription factor that regulates genes involved in cellular growth, differentiation, stress response, and apoptosis in many cell types. Egr-1 is induced in liver regeneration in rodents, but its role in normal hepatocyte function has not been characterized. We examined the developmental expression of Egr-1 in mouse liver and found that its expression increased during the suckling period. In screening the sequences of the genes involved in lactose assimilation, we found that the galactokinase gene Glk contains four potential Egr-1 binding sites in its proximal promoter. A minimal promoter of 155 nucleotides encompassing the four Egr-1 sites exhibited activity in hepatoma cell lines by transient transfection assays. Moreover, co-transfection of an Egr-1 expression plasmid increased promoter activity. Finally, mutations introduced into three of the four Egr-1 binding sites decreased activity, whereas mutation of the remaining site increased promoter activity. These data tie Egr-1 and galactokinase together in a developmentally regulated chain to prepare the neonate for suckling.

Transforming growth factor-beta activates both pro-apoptotic and survival signals in fetal rat hepatocytes

Transforming growth factor-beta (TGF-beta) induces apoptosis in fetal rat hepatocytes. However, a subpopulation of these cells survives concomitant with changes in morphology and phenotype, reminiscent of an epithelial mesenchymal transition (EMT) [Exp. Cell Res. 252 (1999) 281-291]. In this work, we have isolated the subpopulation that survives to TGF-beta-induced apoptosis, showing that these cells maintain the response to TGF-beta in terms of Smads activation and growth inhibition. Analyses of the intracellular signals that could impair the apoptotic effects of TGF-beta have indicated that the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway is activated in these resistant cells. Experiments in fetal rat hepatocytes have shown that TGF-beta is able to transiently activate PI 3-K/Akt by a mechanism independent of protein synthesis but dependent on a tyrosine kinase activity. Pro-apoptotic signals, such as oxidative stress and caspases, contribute to the loss of Akt at later times. Inhibiting PI 3-K sensitizes fetal hepatocytes (FH) to the apoptosis induced by TGF-beta and causes spontaneous death in the resistant cells. In conclusion, TGF-beta, as it is known for other cytokines, could be inducing pro-apoptotic and survival signals in hepatocytes, the balance among them will decide cell fate.

The mouse alpha-fetoprotein promoter is repressed in HepG2 hepatoma cells by hepatocyte nuclear factor-3 (FOXA)

The mouse alpha-fetoprotein gene is expressed at high levels in the fetal liver and is transcriptionally silenced at birth. The repression is governed, at least in part, by the 250 base pair (bp) AFP promoter. We show here that the AFP promoter is dramatically repressed by HNF3 in HepG2 hepatoma cells. This repression is governed by the region between -205 and -150. Furthermore, this fragment can confer HNF3-mediated repression on a heterologous promoter. The repression is abolished by a mutation that is centered at -165. EMSA analyses using in vivo and in vitro synthesized proteins indicate that HNF3 proteins do not bind DNA from the -205 to -150 region. We propose that HNF3 represses AFP promoter activity through indirect mechanisms that modulate the binding or activity of a liver-enriched factor that interacts with the -165 region of the AFP promoter.

Distant enhancers stimulate the albumin promoter through complex proximal binding sites

The albumin-alpha-fetoprotein locus epitomizes the main features of transcriptional regulation of fetal and adult hepatocyte-specific genes: developmentally regulated promoters and strong distant enhancers. Full enhancer activity required only a proximal albumin-promoter region containing the TATA box, hepatic nuclear factor 1 (HNF1), and nuclear factor Y (NF-Y) sites. Deletion of the HNF1 site abrogated enhancer and promoter activity, whereas methylation of the site reduced all activity by about 3-fold. Deletion of the NF-Y site attenuated activity by about half, but much of the activity could be replaced by juxtaposition of an upstream region (designated distal element IV). Gel shift and competition analysis demonstrated that binding of architectural factors overlapped NF-Y binding. Moreover, a mutation that eliminated NF-Y binding but only minimally perturbed the surrounding region did not affect enhancer function. In plasmids with a second promoter, the enhancers simultaneously stimulated both albumin and alpha-fetoprotein promoters with minimal competition, but surprisingly some mutations in the albumin promoter attenuated expression from both promoters, whereas another uncoupled their expression. With single promoters, the function of the proximal promoter region was controlled by three parameters in the following hierarchy: HNF1 binding > local architecture > NF-Y binding, but integrated two-promoter function had a much greater dependence on NF-Y.

Hepatitis B viral transactivator HBx alleviates p53-mediated repression of alpha-fetoprotein gene expression

Chronic infection with hepatitis B virus (HBV) is associated with development of hepatocellular carcinoma (HCC). The exact mechanism by which chronic infection with HBV contributes to onset of HCC is unknown. However, previous studies have implicated the HBV transactivator protein, HBx, in progression of HCC through its ability to bind the human tumor suppressor protein, p53. In this study, we have examined the ability of HBx to modify p53 regulation of the HCC tumor marker gene, alpha-fetoprotein (AFP). By utilizing in vitro chromatin assembly of DNA templates prior to transcription analysis, we have demonstrated that HBx functionally disrupts p53-mediated repression of AFP transcription through protein-protein interaction. HBx modification of p53 gene regulation is both tissue-specific and dependent upon the p53 binding element. Our data suggest that the mechanism by which HBx alleviates p53 repression of AFP transcription is through an association with DNA-bound p53, resulting in a loss of p53 interaction with liver-specific transcriptional co-repressors.

Retinoic acid mediates down-regulation of the alpha-fetoprotein gene through decreased expression of hepatocyte nuclear factors

alpha-Fetoprotein (AFP), a protein highly induced during fetal liver development, is down-regulated by retinoids in the human hepatoma cell line Hep3B, in contrast to up-regulation observed in other cell types. Previously, we have documented that such up-regulation involves direct effects through cis-retinoid X receptor-binding sites in the AFP enhancer. In this report, we show a distinctive effect of all-trans-retinoic acid (RA) in Hep3B cells. RA caused a marked decrease in AFP transcripts. Deletion analysis of the upstream regulatory region of the AFP gene revealed that cis-acting sites required for down-regulation resided near the promoter. Gel mobility shift assays for factors binding to key elements in the AFP promoter region demonstrated that hepatocyte nuclear factor (HNF) 1 binding was diminished in nuclear extracts from RA-treated cells. In addition, HNF4, which is not known to bind to the AFP promoter but does regulate HNF1, was also diminished. The levels of HNF1 and HNF4 mRNA were also decreased following RA treatment. AFP promoter-chloramphenicol acetyltransferase transient transfection assays demonstrated that the level of HNF1 had a direct impact on basal transcription as well as RA-mediated down-regulation of the AFP gene, and that co-transfection of HNF1 and HNF4, but not transfection of either factor alone, reversed the RA-mediated inhibition. Taken together these data point to an interaction among the RA, HNF1, and HNF4 signals, which is reflected in decreased expression of AFP.

In situ hybridization with 33P-labeled RNA probes for determination of cellular expression patterns of liver transcription factors in mouse embryos

Murine hepatocyte nuclear factor-3beta (HNF-3beta) protein is a member of a large family of developmentally regulated transcription factors that share homology in the winged helix/fork head DNA binding domain and that participate in embryonic pattern formation. HNF-3beta also mediates cell-specific transcription of genes important for the function of hepatocytes, intestinal and bronchiolar epithelium, and pancreatic acinar cells. We have previously identified a hepatocyte and pancreatic cut-homeodomain transcription factor, HNF-6, which is required for HNF-3beta promoter activity. In this study, we used in situ hybridization studies of stage-specific embryos to demonstrate that HNF-6 and its target gene, HNF-3beta, are coexpressed in the foregut endoderm and in the pancreatic and hepatic diverticulum. More detailed analysis of HNF-6 and HNF-3beta's developmental expression patterns provides evidence of colocalization in hepatocytes, intestinal epithelium, and pancreatic ductal epithelium and exocrine acinar cells. In support of the role of HNF-6 in regulating HNF-3beta expression in developing hepatocytes, their liver expression levels are both transiently reduced between 14 and 15 days of gestation. At day 18 of gestation and in adult pancreas, HNF-6 and HNF-3beta transcripts remain colocalized in the exocrine acinar cells, but their expression patterns diverge in endocrine cells. HNF-3beta expression is restricted to the endocrine cells of the islets of Langerhans, whereas the ductal epithelium expresses HNF-6. We discuss these expression patterns with respect to specification of hepatocytes and differentiation of the endocrine and exocrine pancreas.

Transforming growth factor-beta (TGF-beta) and EGF promote cord-like structures that indicate terminal differentiation of fetal hepatocytes in primary culture

When fetal hepatocytes were cultured in the presence of transforming growth factor-beta (TGF-beta 1) and epidermal growth factor (EGF), some morphological changes were observed. Under these conditions, cells migrated, from typical clusters that hepatocytes adopt in culture, to form elongated, cord-like structures similar to the hepatic acinus organization. Immunocytochemical analysis of these cells revealed high levels of albumin and cytokeratin 18, phenotypic markers of parenchymal hepatocytes. Although some of the cells in the cord-like structures presented a cortical ring distribution of F-actin filaments, the cord also presented thick peripheral bundles and cells of the tips showed thin stress fibers oriented to the cell edges, typical of a migratory phenotype. In addition to these morphological effects, flow cytometric analysis of the cells revealed a larger size, granularity and intracellular lipid content (as a parameter related to liver metabolic function), in TGF-beta + EGF-treated hepatocytes. Western blot analysis of the albumin levels revealed that both expression and secretion of albumin were increased in EGF + TGF-beta-treated cells. Finally, all these changes were coincident with an enhancement in the DNA-binding activity for hepatocyte nuclear factors (HNF1, HNF3, and HNF4), as revealed in gel-shift experiments with nuclear extracts. We conclude that a cooperative action between TGF-beta and EGF might modulate terminal maturation of fetal hepatocytes.

Phorbol esters down-regulate alpha-fetoprotein gene expression without affecting growth in fetal hepatocytes in primary culture

The effects of phorbol esters (phorbol-12,13-dibutyrate, PDB) on alpha-fetoprotein expression and cell growth were assayed by using fetal hepatocytes in primary culture. PDB acts synergistically with epidermal growth factor (EGF) to specifically decrease alpha-fetoprotein (AFP) mRNA levels, without affecting the expression of other genes of the same family, such as albumin and Vitamin D-binding protein (DBP). This effect is PDB-dose dependent, maximal effects being at 10 ng/ml. The implication of protein kinase C (PKC) in this effect seems clear since bisindolylmaleimide (BIS), a specific PKC inhibitor, completely blocks the PDB effect on AFP expression. Nuclear run-on experiments show that the decrease in AFP mRNA levels is mainly due to an inhibition in the transcription rate of the gene. Determination of PKC activities shows that fetal hepatocytes contain mainly Ca(2+)-independent isoenzymes, which patterns of activation was not modified by EGF plus PDB treatment with respect to PDB treatment. We have found that MAPK and JNK activities, c-jun and c-fos mRNA levels and AP-1 binding activity are notably increased when cells are incubated with both EGF and PDB, PDB does not stimulate growth of fetal hepatocytes, measured either as [3H]-thymidine incorporation into DNA or by cell cycle analysis using flow cytometry. All these results suggest that activation of PKC may affect liver gene expression rather than cell growth in fetal hepatocytes.

A novel nk-2-related transcription factor associated with human fetal liver and hepatocellular carcinoma

A novel cDNA was partially isolated from a HepG2 cell expression library by screening with the promoter-linked coupling element (PCE), a site from the alpha-fetoprotein (AFP) gene promoter. The remainder of the cDNA was cloned from fetal liver RNA using random amplification of cDNA ends. The cDNA encodes a 239-amino acid peptide with domains closely related to the Drosophila factor nk-2. The new factor is the eighth vertebrate factor related to nk-2, hence nkx-2.8. Northern blot and reverse transcriptase polymerase chain reaction analysis demonstrated mRNA in HepG2, two other AFP-expressing human cell lines, and human fetal liver. Transcripts were not detected in adult liver. Cell-free translation produced DNA binding activity that gel shifted a PCE oligonucleotide. Cotransfection of nkx-2.8 expression and PCE reporter plasmids into HeLa cells demonstrated transcriptional activation; NH2-terminal deletion eliminated this activity. Cotransfection into AFP-producing hepatocytic cells repressed AFP reporter expression, suggesting that endogenous activity was already present in these cells. In contrast, cotransfection into an AFP-negative hepatocytic line produced moderate activation of the AFP gene. The cardiac developmental factor nkx-2.5 could substitute for nkx-2.8 in all transfection assays, whereas another related factor, thyroid transcription factor 1, showed a more limited range of substitution. Although the studies have yet to establish definitively that nkx-2.8 is the AFP gene regulator PCF, the two factors share a common DNA binding site, gel shift behavior, migration on SDS-acrylamide gels, and cellular distribution. Moreover, the nk-2-related genes are developmental regulators, and nkx-2.8 is the first such factor associated with liver development.

Sequence requirements for Afr-2 regulation of alpha-fetoprotein gene expression during liver regeneration

Alpha-fetoprotein (AFP) gene expression occurs in the yolk sac, fetal liver and gut, and in the adult liver during regeneration and tumorigenesis. Polymorphism at a single genetic locus, Afr-2 (formerly known as Rif) between inbred mouse strains C3H/He and C57B1/6, results in different levels of AFP expression during liver regeneration. We examined AFP, histone H3, and albumin gene expression during liver regeneration and found that the strain-specific variance in AFP gene expression could not be attributed to a difference in the numbers of dividing cells. Experiments with transgenic mice revealed sequences required for Afr-2 regulation included 172 bp between -1010 and -838 bp and 118 bp immediately upstream of the AFP transcriptional start site-the same regions required for induction during liver regeneration. This suggests that the Afr-2 phenotype may stem from an allelic difference in a gene regulating gene expression during liver regeneration.

Apoptosis induced by transforming growth factor-beta in fetal hepatocyte primary cultures: involvement of reactive oxygen intermediates

Transforming growth factor-beta (TGF-beta), a growth regulator of fetal hepatocytes in primary culture, also regulates death of these cells. Dose-response analysis showed that the TGF-beta concentration needed to induce hepatocyte death (2.5 ng/ml) was 5 times that needed to inhibit growth in these cells (0.5 ng/ml). In response to TGF-beta, hepatocytes induced DNA fragmentation and the appearance of nuclei with a DNA content lower than 2C (diploid content), typical of a programmed cell death model. TGF-beta-induced apoptosis in fetal hepatocytes was preceded by an induction of reactive oxygen species production and a decrease in the glutathione intracellular content, indicating that this factor induces oxidative stress in fetal hepatocytes. Studies performed to analyze levels of c-fos mRNA, a gene whose expression is modulated by redox state, demonstrated that only high, apoptotic concentrations of TGF-beta (2.5 ng/ml) produced an increase in the mRNA levels of this gene, the level of induction being similar to that found when cells were incubated in the presence of tert-butyl hydroperoxide. Gel mobility shift assays showed that the c-fos-induced expression was coincident with an increase in AP-1 activity. Finally, cell death induced by TGF-beta in fetal hepatocytes was partially blocked by radical scavengers, which decreased the percentage of apoptotic cells, whereas these agents did not modify the growth-inhibitory effect elicited by TGF-beta in these cells. In summary, the results presented in this paper provide evidence for the involvement of an oxidative process in the apoptosis elicited by TGF-beta in fetal hepatocytes.

A novel cis-acting element in a liver cytochrome P450 3A gene confers synergistic induction by glucocorticoids plus antiglucocorticoids

The induction by dexamethasone of rat liver CYP3A1 differs from classical glucocorticoid gene regulation in part because both glucocorticoids and antiglucocorticoids such as pregnenolone 16 alpha-carbonitrile (PCN) induce CYP3A1 through transcriptional gene activation. In the present study, we transiently expressed in primary cultures of rat hepatocytes plasmids consisting of CYP3A1 5'-flanking sequences fused to a chloramphenicol acetyltransferase reporter plasmid. Deletional analysis identified a 78-base pair (bp) element located approximately 135 bp upstream of the transcriptional start site that was inducible by treatment of the cultures with dexamethasone or PCN and was induced synergistically by dexamethasone plus PCN. Nuclear extract from control rat liver protected two regions within the 78-bp sequence against digestion with DNase I. The same two regions were protected when nuclear extracts from dexamethasone-treated animals were used. Analysis of both of the "footprints" (FP1 and FP2) failed to reveal a classical sequence for the glucocorticoid-responsive element. A 33-bp element that includes FP1 sequences inserted into the chloramphenicol acetyltransferase reporter plasmid and transiently expressed in rat hepatocytes conferred a profile of dexamethasone and PCN induction similar to that of the 78-bp element. However, an Escherichia coli expressed glucocorticoid receptor protein failed to protect sequences within FP1 in DNase I footprinting experiments and failed to change its mobility in gel shift assays. Moreover, as judged by the gel shift assay, the specific protein binding to this fragment was the same whether nuclear extracts from the liver of untreated or dexamethasone-treated rats were used. We conclude that the activation of CYP3A1 gene transcription by glucocorticoids may involve proteins already bound to the controlling element in the CYP3A1 gene through a mechanism in which GR in the presence of hormone does not bind directly to CYP3A1 DNA.