The alpha1-fetoprotein locus is activated by a nuclear receptor of the Drosophila FTZ-F1 family

The enzymes, regulation, and genetics of bile acid synthesis

The synthesis and excretion of bile acids comprise the major pathway of cholesterol catabolism in mammals. Synthesis provides a direct means of converting cholesterol, which is both hydrophobic and insoluble, into a water-soluble and readily excreted molecule, the bile acid. The biosynthetic steps that accomplish this transformation also confer detergent properties to the bile acid, which are exploited by the body to facilitate the secretion of cholesterol from the liver. This role in the elimination of cholesterol is counterbalanced by the ability of bile acids to solubilize dietary cholesterol and essential nutrients and to promote their delivery to the liver. The synthesis of a full complement of bile acids requires 17 enzymes. The expression of selected enzymes in the pathway is tightly regulated by nuclear hormone receptors and other transcription factors, which ensure a constant supply of bile acids in an ever changing metabolic environment. Inherited mutations that impair bile acid synthesis cause a spectrum of human disease; this ranges from liver failure in early childhood to progressive neuropathy in adults.

The role of the orphan nuclear receptor, liver receptor homologue-1, in the regulation of human corpus luteum 3beta-hydroxysteroid dehydrogenase type II

After ovulation, ovarian 3beta-hydroxysteroid dehydrogenase type II (HSD3B2) expression increases to enhance the shift of steroidogenesis toward progesterone biosynthesis. Steroidogenic factor-1 (SF-1) is a transcription factor for several genes encoding steroidogenic enzymes. However, the level of SF-1 expression decreases in the human corpus luteum (CL) after ovulation. Liver receptor homolog-1 (LRH-1) is another member of the orphan nuclear receptor family. We hypothesize that LRH-1, rather than SF-1, plays an essential role in the regulation of corpus luteum steroidogenesis. Semiquantitative RT-PCR and real-time PCR were performed to quantify the level of LRH-1 expression and correlate with HSD3B2 level. Cell transfection, mutation analysis, and EMSA were performed to examine the role of LRH-1 in the regulation of HSD3B2. LRH-1 expression was higher in CL, compared with mature ovarian follicles. Cotransfection of granulosa cells with HSD3B2 and LRH-1 resulted in a 10-fold increase of transcription. DAX-1 inhibited LRH-1-stimulated HSD3B2, which was maintained in the presence of dibutyryl cAMP. Mutation of the either of the two putative LRH-1 binding sites, which were confirmed by EMSA, in the HSD3B2 promoter decreased LRH-1 stimulation. Our findings suggest that LRH-1 is highly expressed in CL, and it plays an essential role in the regulation of HSD3B2.

Cholesterol transport and steroidogenesis by the corpus luteum

The synthesis of progesterone by the corpus luteum is essential for the establishment and maintenance of early pregnancy. Regulation of luteal steroidogenesis can be broken down into three major events; luteinization (i.e., conversion of an ovulatory follicle), luteal regression, and pregnancy induced luteal maintenance/rescue. While the factors that control these events and dictate the final steroid end products are widely varied among different species, the composition of the corpus luteum (luteinized thecal and granulosa cells) and the enzymes and proteins involved in the steroidogenic pathway are relatively similar among all species. The key factors involved in luteal steroidogenesis and several new exciting observations regarding regulation of luteal steroidogenic function are discussed in this review.

Pancreatic-duodenal homeobox 1 regulates expression of liver receptor homolog 1 during pancreas development

Liver receptor homolog 1 (LRH-1) and pancreatic-duodenal homeobox 1 (PDX-1) are coexpressed in the pancreas during mouse embryonic development. Analysis of the regulatory region of the human LRH-1 gene demonstrated the presence of three functional binding sites for PDX-1. Electrophoretic mobility shift assays and chromatin immunoprecipitation analysis showed that PDX-1 bound to the LRH-1 promoter, both in cultured cells in vitro and during pancreatic development in vivo. Retroviral expression of PDX-1 in pancreatic cells induced the transcription of LRH-1, whereas reduced PDX-1 levels by RNA interference attenuated its expression. Consistent with direct regulation of LRH-1 expression by PDX-1, PDX-1(-/-) mice expressed smaller amounts of LRH-1 mRNA in the embryonic pancreas. Taken together, our data indicate that PDX-1 controls LRH-1 expression and identify LRH-1 as a novel downstream target in the PDX-1 regulatory cascade governing pancreatic development, differentiation, and function.

Expression of LRH-1 and SF-1 in the mouse ovary: localization in different cell types correlates with differing function

Steroid biosynthesis in ovary is enhanced by the orphan nuclear receptor, steroidogenic factor-1 (SF-1); however, we reported that liver receptor homolog-1 (LRH-1), a closely related receptor to SF-1, is also expressed in mouse ovary. To further investigate the role of LRH-1 in mouse ovary, we used in situ hybridization to identify the cell types that express LRH-1 versus SF-1, and carried out functional studies to determine the role of LRH-1 in the regulation of the human (h) ovary-specific CYP19 promoter. LRH-1 expression was found to be abundant and highly restricted to cells involved in estrogen biosynthesis-granulosa cells during the estrous cycle, and in corpora lutea (CL) of pregnancy. In contrast, SF-1 was expressed most highly in C(19)-steroid-producing theca cells and interstitium, and at low levels in granulosa and luteal cells. Transfection studies using granulosa cells demonstrated that LRH-1 is a potent regulator of both basal and forskolin-induced transcription of the ovary-specific hCYP19 promoter. This activity was dependent upon two nuclear receptor half-sites within the proximal hCYP19 promoter. Based on these findings, we propose that LRH-1 plays an important role as a competence factor in regulating aromatase, and thus estrogen biosynthesis, in ovary.

Liver receptor homolog 1 controls the expression of carboxyl ester lipase

The orphan nuclear receptor liver receptor homolog 1 (LRH-1) plays a central role in cholesterol homeostasis by regulating a number of hepatic and intestinal genes critical for reverse cholesterol transport and bile acid homeostasis. Herein, we describe the identification of carboxyl ester lipase (CEL) as a novel target of LRH-1 in pancreas, a tissue in which LRH-1 is abundantly expressed. In situ hybridization and gene expression studies demonstrate that both LRH-1 and CEL are co-expressed and confined to the exocrine pancreas. LRH-1 interacts with a consensus LRH-1 response element in the human CEL promoter, which is perfectly conserved in the rat gene, and induces CEL promoter activity in cotransfection assays. As reported for other LRH-1 target genes, the nuclear receptor short heterodimer partner represses LRH-1-induced CEL promoter activity. Chromatin immunoprecipitation demonstrates that binding of LRH-1 to the CEL promoter increases histone H4 acetylation corresponding with the activation of endogenous CEL gene transcription. Our data, identifying CEL as the first pancreatic LRH-1 target gene, indicate that LRH-1 is an important player in enterohepatic cholesterol homeostasis.

Farnesoid X receptor agonists suppress hepatic apolipoprotein CIII expression

BACKGROUND & AIMS

Increased serum triglyceride levels constitute a risk factor for coronary heart disease. Apolipoprotein CIII (Apo CIII) is a determinant of serum triglyceride metabolism. In this study, we investigated whether activators of the nuclear farnesoid X receptor (FXR) modulate Apo CIII gene expression.

METHODS

The influence of bile acids and synthetic FXR activators on Apo CIII and triglyceride metabolism was studied in vivo by using FXR wild-type and FXR-deficient mice and in vitro by using human primary hepatocytes and HepG2 cells.

RESULTS

In mice, treatment with the FXR agonist taurocholic acid strongly decreased serum triglyceride levels, an effect associated with reduced Apo CIII serum and liver messenger RNA levels. By contrast, no change was observed in FXR-deficient mice. Incubation of human primary hepatocytes and HepG2 cells with bile acids or the nonsteroidal synthetic FXR agonist GW4064 resulted in a dose-dependent down-regulation of Apo CIII gene expression. Promoter transfection experiments and mutation analysis showed that bile acid-activated FXR decrease human Apo CIII promoter activity via a negative FXR response element located in the I(4) footprint between nucleotides -739 and -704. Chromatin immunoprecipitation experiments showed that bile acid treatment led to binding of FXR/retinoid X receptor heterodimers to and displacement of HNF4alpha from this site. Bile acid treatment still repressed liver Apo CIII gene expression in hepatic HNF4alpha-deficient mice, suggesting an active rather than a competitive mechanism of Apo CIII repression by the FXR.

CONCLUSIONS

We identified bile acid and synthetic activators of the nuclear FXR as negative regulators of Apo CIII expression, an effect that may contribute to the triglyceride-decreasing action of FXR agonists.

Expression and functional analysis of liver receptor homologue 1 as a potential steroidogenic factor in rat ovary

Liver receptor homologue 1 (LRH-1) is a member of the nuclear receptor superfamily originally found in liver cells. LRH-1 participates in regulation of cholesterol metabolism and bile acid synthesis. Recent studies have shown that LRH-1 is even more highly expressed in the ovary, and LRH-1 has been implicated as a key transcriptional regulator of cytochrome P450 aromatase (P450arom) in vitro. In the present study, we investigated the spatiotemporal expression patterns of LRH-1 using in situ hybridization and immunohistochemistry in ovaries from rats with a 4-day estrous cycle, from pregnant rats, from immature rats treated with eCG to stimulate follicular development, and from eCG-treated rats that were subsequently given hCG to stimulate ovulation and luteinization. To establish a potential connection between the expression of LRH-1 and that of the steroidogenic genes in vivo, we directly compared the localization patterns of LRH-1 and P450arom transcripts in consecutive ovarian sections from these animals. LRH-1 mRNA and protein were primarily localized to granulosa cells and luteinized follicles or newly formed corpora lutea (CLs) of immature and adult rats, and the levels of expression increased during eCG-hCG-induced follicular development and ovulation. In the functional CLs of pregnant rats, a biphasic change in LRH-1 mRNA content occurred throughout the gestation process, whereas LRH-1 protein was persistently detected during the entire pregnancy. In the consecutive ovarian sections, expression of LRH-1 was approximately colocalized with that of P450arom in both tertiary and Graafian follicles and the functional CLs of pregnant rats. LRH-1 mRNA and protein expression preceded those of P450arom during early follicular development. Stage-specific expression of LRH-1 in rat granulosa and luteal cells suggests a role for LRH-1 in the regulation of ovarian function. The overlapping but distinct expression patterns of LRH-1 and P450arom circumstantially support the recent finding that LRH-1 serves as a critical upstream regulator of P450arom gene expression in ovarian cells, but LRH-1 also may be a multifunctional steroidogenic factor in ovarian physiology.

Differential expression of steroidogenic factor-1 and FTF/LRH-1 in the rodent ovary

Steroidogenic factor-1 (SF-1) (NR5A1) is an orphan nuclear receptor that plays a premier role in ovarian organogenesis. Recent studies document mRNA expression of the structurally related factor NR5A2 (FTF, LRH-1, SF-2) in the adult ovary and more specifically in granulosa cells and luteal cells but not theca cells. Conversely, SF-1 was shown to be expressed at higher levels in theca/interstitial cells. These latter observations raised the possibility that FTF/LRH-1 may control target gene expression in granulosa cells of developing follicles. Using quantitative PCR our results show that FTF/LRH-1 message is expressed at higher levels in the ovary than in liver or other tissues analyzed. We show by in situ hybridization and LacZ expression in ovaries of transgenic mice bearing an FTF-promoter-LacZ fusion gene that FTF/LRH-1 is selectively expressed in granulosa cells of rat and mouse ovaries and is not present in theca cells or interstitial cells. However, by a variety of approaches, we showed that SF-1 mRNA and protein are expressed in greater amounts than FTF/LRH-1 in granulosa cells of follicles at all stages of development. Expression of SF-1 mRNA and protein in granulosa cells was verified by in situ hybridization, immunohistochemistry of ovarian sections, and immunocytochemistry of cultured rat granulosa cells. The significance of SF-1 in regulating target gene activation was supported by EMSA. An abundant granulosa cell protein binding to the SF-1-binding motif (CCAAGGTCA) present in the aromatase promoter and an FTF/LRH-1 motif (TGTCCTTGAACA) in the alpha-fetoprotein promoter was supershifted by two SF-1-specific antibodies but not by an FTF antibody. Conversely, with the same probes, a less abundant protein/DNA complex present in liver and ovarian cell extracts was shifted by an FTF antibody but not by the SF-1 antibodies. SF-1 and FTF/LRH-1 were differentially regulated in vivo by estradiol, FSH and prolactin. Collectively these data indicate that granulosa cells of small and preovulatory follicles express both SF-1 and FTF/LRH-1 and that each orphan receptor may regulate target gene expression in these cells.

Induction of adiponectin, a fat-derived antidiabetic and antiatherogenic factor, by nuclear receptors

Adiponectin is a fat-derived hormone with antidiabetic and antiatherogenic properties. Hypoadiponectinemia seen in obesity is associated with insulin-resistant diabetes and atherosclerosis. Thiazolidinediones, peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists, have been shown to increase plasma adiponectin levels by the transcriptional induction in adipose tissues. However, the precise mechanism of such action is unknown. In this study, we have identified a functional PPAR-responsive element (PPRE) in human adiponectin promoter. PPAR-gamma/retinoid X receptor (RXR) heterodimer directly bound to the PPRE and increased the promoter activity in cells. In adipocytes, point mutation of the PPRE markedly reduced the basal transcriptional activity and completely blocked thiazolidinedione-induced transactivation of adiponectin promoter. We have also identified a responsive element of another orphan nuclear receptor, liver receptor homolog-1 (LRH-1), in adiponectin promoter. LRH-1 was expressed in 3T3-L1 cells and rat adipocytes. LRH-1 bound specifically to the identified responsive element (LRH-RE). LRH-1 augmented PPAR-gamma-induced transactivation of adiponectin promoter, and point mutation of the LRH-RE significantly decreased the basal and thiazolidinedione-induced activities of adiponectin promoter. Our results indicate that PPAR-gamma and LRH-1 play significant roles in the transcriptional activation of adiponectin gene via the PPRE and the LRH-RE in its promoter.

Nuclear receptors and the control of metabolism

The metabolic nuclear receptors act as metabolic and toxicological sensors, enabling the organism to quickly adapt to environmental changes by inducing the appropriate metabolic genes and pathways. Ligands for these metabolic receptors are compounds from dietary origin, intermediates in metabolic pathways, drugs, or other environmental factors that, unlike classical nuclear receptor ligands, are present in high concentrations. Metabolic receptors are master regulators integrating the homeostatic control of (a) energy and glucose metabolism through peroxisome proliferator-activated receptor gamma (PPARgamma); (b) fatty acid, triglyceride, and lipoprotein metabolism via PPARalpha, beta/delta, and gamma; (c) reverse cholesterol transport and cholesterol absorption through the liver X receptors (LXRs) and liver receptor homolog-1 (LRH-1); (d) bile acid metabolism through the farnesol X receptor (FXR), LXRs, LRH-1; and (e) the defense against xeno- and endobiotics by the pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR). The transcriptional control of these metabolic circuits requires coordination between these metabolic receptors and other transcription factors and coregulators. Altered signaling by this subset of receptors, either through chronic ligand excess or genetic factors, may cause an imbalance in these homeostatic circuits and contribute to the pathogenesis of common metabolic diseases such as obesity, insulin resistance and type 2 diabetes, hyperlipidemia and atherosclerosis, and gallbladder disease. Further studies should exploit the fact that many of these nuclear receptors are designed to respond to small molecules and turn them into therapeutic targets for the treatment of these disorders.

Establishment of transgenic mice carrying the gene of human nuclear receptor NR5A2 (hB1F)

AIM: Human hepatitis B virus enhancer II B1 binding factor (hB1F) was cloned and characterized as a novel member of the Ftz-F1 (NR5A) nuclear receptor subfamily. Although progresses have recently been made, its biological function remains largely unidentified. The aim of this study was to establish an hB1F transgenic mouse model to promote the functional study of hB1F.

METHODS: Transgene fragments were microinjected into fertilized eggs of mice. The manipulated embryos were transferred into the oviducts of pseudopregnant female mice. The offsprings were identified by PCR and Southern blot analysis. Transgene expression was analyzed with RT-PCR and Western blot analysis. Transgenic founder mice were used to establish transgenic mouse lineages. The F1 and F2 mice were identified by PCR analysis.

RESULTS: Seven mice were identified as carrying copies of transgene. RT-PCR and Western blotting results showed that the transgene was expressed in heart, liver, lung, kidney and stomach in one of the transgenic mouse lineages. Genetic analysis of the transgenic mice demonstrated that the transgene was integrated into the chromosome at a single site, and was transmitted stably.

CONCLUSION: In this study we established an hB1F transgenic mouse model, which will facilitate the investigation of the biological function of hB1F in vivo.

Structural basis for ligand-independent activation of the orphan nuclear receptor LRH-1

The orphan nuclear receptors SF-1 and LRH-1 are constitutively active, but it remains uncertain whether their activation is hormone dependent. We report the crystal structure of the LRH-1 ligand binding domain to 2.4 A resolution and find the receptor to be a monomer that adopts an active conformation with a large but empty hydrophobic pocket. Adding bulky side chains into this pocket resulted in full or greater activity suggesting that, while LRH-1 could accommodate potential ligands, these are dispensable for basal activity. Constitutive LRH-1 activity appears to be conferred by a distinct structural element consisting of an extended helix 2 that provides an additional layer to the canonical LBD fold. Mutating the conserved arginine in helix 2 reduced LRH-1 receptor activity and coregulator recruitment, consistent with the partial loss-of-function phenotype exhibited by an analogous SF-1 human mutant. These findings illustrate an alternative structural strategy for nuclear receptor stabilization in the absence of ligand binding.

Fushi tarazu factor-1 mRNA and protein is expressed in steroidogenic and cholesterol metabolising tissues during different life stages in Arctic char (Salvelinus alpinus)

Fushi tarazu factor-1 (FTZ-F1) genes belong to the nuclear receptor family 5A (NR5A). The distribution pattern of NR5A genes in teleosts suggests that they control functions separate to, or in addition to, those of other vertebrates. In mammals NR5A1 genes, including steroidogenic factor-1 (SF-1), are primarily involved in steroidogenesis. NR5A2 contain the alpha-fetoprotein transcription factor (FTF) genes, which protect mammalian embryos against maternal estrogens, and are involved in cholesterol transfer and metabolism. In this study we have analysed the expression of two Arctic char FTZ-F1 forms belonging to the NR5A2 group. The expression starts during early development and the transcripts are present in embryonic liver/pancreas and gonadal regions. The genes are up-regulated during embryogenesis as the embryo develops towards hatch, as shown by increased mRNA and protein levels. In adult Arctic char the FTZ-F1 forms are primarily located to tissues involved in steroidogenesis as well as cholesterol metabolism. Thus, a division of NR5A into SF-1 (NR5A1) and FTF (NR5A2) specific functions does not appear to have occurred in teleosts.

Synergistic expression of Ad4BP/SF-1 and cytochrome P-450 aromatase (ovarian type) in the ovary of Nile tilapia, Oreochromis niloticus, during vitellogenesis suggests transcriptional interaction

Involvement of Ad4BP/SF-1 in the ovarian cytochrome P-450 aromatase (oP450arom) gene expression was investigated using ovarian follicles of the Nile tilapia, possessing an average 14-day spawning cycle. The promoter region (5' flanking region) of oP450arom gene cloned from tilapia contains two Ad4 binding sites. Subsequently, a cDNA encoding Ad4BP/SF-1 was cloned from the ovarian follicles. It is expressed in gonadal tissues, brain, and kidney. Oligonucleotide probes containing putative orphan nuclear receptor binding motifs (derived from promoter region of the aromatase gene) formed complexes with in vitro-translated Ad4BP/SF-1 and nuclear extracts of tilapia ovarian (midvitellogenic) follicles, indicating that Ad4BP/SF-1 is one of the transcriptional regulators for aromatase gene expression. Northern blot analysis revealed that the expression of both oP450arom and Ad4BP/SF-1 increased in parallel with ovarian growth from Day 0 to Day 5 after spawning and declined sharply from Day 8 to Day 11. On the day of spawning (Day 14), the expression of both correlates became undetectable. In vitro incubation of post vitellogenic full-grown immature follicles (corresponding to Day 11 after spawning) with hCG purged both oP450arom and Ad4BP/SF-1 messenger RNA transcripts at 18 h. Conversely, in vitro incubation of late vitellogenic follicles (corresponding to Day 8 after spawning) with hCG retained Ad4BP/SF-1 messenger RNA transcripts more or less steadily and up-regulated oP450arom. Ad4BP/SF-1 probably acts as a transcriptional modulator to implement the paradoxical actions of gonadotropins on oP450arom gene.

Genetic background of cholesterol gallstone disease

Cholesterol gallstone formation is a multifactorial process involving a multitude of metabolic pathways. The primary pathogenic factor is hypersecretion of free cholesterol into bile. For people living in the Western Hemisphere, this is almost a normal condition, certainly in the elderly, which explains the very high incidence of gallstone disease. It is probably because the multifactorial background genes responsible for the high incidence have not yet been identified, despite the fact that genetic factors clearly play a role. Analysis of the many pathways involved in biliary cholesterol secretion reveals many potential candidates and considering the progress in unraveling the regulatory mechanisms of the responsible genes, identification of the primary gallstone genes will be successful in the near future.

LXXLL-related motifs in Dax-1 have target specificity for the orphan nuclear receptors Ad4BP/SF-1 and LRH-1

The orphan receptor Ad4BP/SF-1 (NR5A1) is a constitutive activator, and its activity is repressed by another orphan receptor, Dax-1 (NR0B1). In the present study, we investigated the molecular mechanisms underlying this repression by Dax-1. Yeast two-hybrid and transient-transfection assays confirmed the necessity of three LXXLL-related motifs in Dax-1 for interaction with and repression of Ad4BP/SF-1. In vitro pull-down experiments confirmed that Dax-1 interacts with Ad4BP/SF-1 and also with LRH-1 (NR5A2). The target specificity of the LXXLL-related motifs was indicated by the observations that Ad4BP/SF-1, ERalpha (NR3A1), LRH-1, ERR2 (NR3B2), and fly FTZ-F1 (NR5A3) interacted through their ligand binding domains with all the LXXLL-related motifs in Dax-1 whereas HNF4 (NR2A1) and RORalpha (NR1F1) did not. Transcriptional activities of the receptors whose DNA binding domains (DBDs) were replaced by the GAL4 DBD were repressed by Dax-1 to various levels, which correlated with the strength of interaction. Amino acid substitutions revealed that Ad4BP/SF-1 and LRH-1 preferentially interact with L(+1)XXLL-related motifs containing serine, tyrosine, serine, and threonine at positions -2, +2, +3, and +6, respectively. Taken together, our results indicate that the specificities of LXXLL-related motifs in Dax-1 based on their amino acid sequences play an important role in regulation of orphan receptors.

Liver receptor homolog 1 controls the expression of the scavenger receptor class B type I

The scavenger receptor class B type I (SR-BI), which mediates selective cellular cholesterol uptake from high-density lipoproteins (HDLs), plays a key role in reverse cholesterol transport. The orphan nuclear receptor liver receptor homolog 1 (LRH-1) and SR-BI are co-expressed in liver and ovary, suggesting that LRH-1 might control the expression of SR-BI in these tissues. LRH-1 induces human and mouse SR-BI promoter activity by binding to an LRH-1 response element in the promoter. Retroviral expression of LRH-1 robustly induces SR-BI, an effect associated with histone H3 acetylation on the SR-BI promoter. The decrease in SR-BI mRNA levels in livers of LRH-1(+/-) animals provides in vivo evidence that LRH-1 regulates SR-BI expression. Our data demonstrate that SR-BI is an LRH-1 target gene and underscore the pivotal role of LRH-1 in reverse cholesterol transport.

Suppression of cholesterol 7alpha-hydroxylase transcription and bile acid synthesis by an alpha1-antitrypsin peptide via interaction with alpha1-fetoprotein transcription factor

alpha1-Antitrypsin (alpha1-AT) is a serum protease inhibitor that is synthesized mainly in the liver, and its rate of synthesis markedly increases in response to inflammation. This increase in alpha1-AT synthesis results in an increase in peptides, like its carboxyl-terminal C-36 peptide (C-36), resulting from alpha1-AT cleavage by proteases. Atherosclerosis is a form of chronic inflammation, and one of the risk factors is elevated plasma cholesterol levels. Because of the correlation between atherosclerosis, plasma cholesterol content, inflammation, and alpha1-AT rate of synthesis, we investigated the effect of the C-36 serpin peptide on hepatic bile acid biosynthesis. We discovered that C-36 is a powerful and specific transcriptional down-regulator of bile acid synthesis in primary rat hepatocytes, through inhibition of the cholesterol 7alpha-hydroxylase/CYP7A1 (7alpha-hydroxylase) promoter. Mice injected with the C-36 peptide also showed a decrease in 7alpha-hydroxylase mRNA. A mutated but very similar peptide did not have any effect on 7alpha-hydroxylase mRNA or its promoter. The sterol 12alpha-hydroxylase/CYP8B1 (12alpha-hydroxylase) promoter is also down-regulated by the C-36 peptide in HepG2 cells but not by the mutated peptide. The DNA element involved in the C-36-mediated regulation of 7alpha- and 12alpha-hydroxylase promoters mapped to the alpha1-fetoprotein transcription factor (FTF) site in both promoters. The C-36 peptide prevented binding of FTF to its target DNA recognition site by direct interaction with FTF. We hypothesize that the C-36 peptide specifically interacts with FTF and induces a conformational change that results in loss of its DNA binding ability, which results in suppression of 7alpha- and 12alpha-hydroxylase transcription. These results suggest that peptides derived from specific serum proteins may alter hepatic gene expression in a highly specific manner.

A functionally conserved member of the FTZ-F1 nuclear receptor family from Schistosoma mansoni

The fushi tarazu factor 1 (FTZ-F1) nuclear receptor subfamily comprises orphan receptors with crucial roles in development and sexual differentiation in vertebrates and invertebrates. We describe the structure and functional properties of an FTZ-F1 from the platyhelminth parasite of humans, Schistosoma mansoni, the first receptor from this family to be characterized in a Lophotrochozoan. It contains a well conserved DNA-binding domain (55-63% identity to other family members) and a poorly conserved ligand-binding domain (20% identity to that of zebrafish FF1a). However, both the ligand domain signature sequence and the activation function 2-activation domain (AF2-AD) are perfectly conserved. Phylogenetic analysis confirmed that SmFTZ-F1 is a member of nuclear receptor subfamily 5, but that it clustered with the Drosophila receptor DHR39 and has consequently been named NR5B1. The gene showed a complex structure with 10 exons and an overall size of 18.4 kb. Two major transcripts were detected, involving alternative promoter usage and splicing of the two 5' exons, but which encoded identical proteins. SmFTZ-F1 mRNA is expressed at all life-cycle stages with the highest amounts in the larval forms (miracidia, sporocysts and cercariae). However, expression of the protein showed a different pattern; low in miracidia and higher in adult male worms. The protein bound the same monomeric response element as mammalian SF-1 (SF-1 response element, SFRE) and competition experiments with mutant SFREs showed that its specificity was identical. Moreover, SmFTZ-F1 transactivated reporter gene transcription from SFRE similarly to SF-1. This functional conservation argues for a conserved biological role of the FTZ-F1 nuclear receptor family throughout the metazoa.

Expression and Regulation of Fushi Tarazu Factor-1 and Steroidogenic Genes During Reproduction in Arctic Char (Salvelinus alpinus)1

Teleost fushi tarazu factor-1 (FTZ-F1) is a potential regulator of steroidogenesis. The present study shows sex-specific regulation of Arctic char fushi tarazu factor-1 (acFF1) and steroidogenic genes during reproductive maturation and in response to hormone treatment. A link between gonadal expression of acFF1, steroidogenic acute regulatory protein (StAR), and cytochrome P450-11A (CYP11A), was observed in the reproductive maturation process, as elevated acFF1 mRNA and protein levels preceded increased StAR and CYP11A transcription. Sex-specific differences were observed as estrogen treatment resulted in down-regulated levels of acFF1 mRNA in testis and male head kidney, whereas no significant effect was observed in females. 11-Ketotestosterone (11-KT) down-regulated CYP11A and 3beta-hydroxysteroid dehydrogenase (3betaHSD) in head kidney and up-regulated CYP11A in testis. StAR remained unaffected by hormone treatment. This suggests that acFF1 is controlled by 17beta-estradiol, whereas the effects on CYP11A and 3betaHSD are mediated by 11-KT. Coexpression of acFF1, StAR, and CYP11A was observed in head kidney, in addition to gonads, indicating correlation between these steroidogenic genes. StAR and acFF1 were also coexpressed in liver, suggesting a potential role in cholesterol metabolism. Although these results indicate conserved steroidogenic functions for FTZ-F1 among vertebrates, they also raise the question of additional roles for FTZ-F1 in teleosts.

Regulation of alpha-fetoprotein expression by Nkx2.8

The alpha-fetoprotein (AFP) gene is an important model of developmental gene silencing and neoplastic gene reactivation. Nkx2.8 is a divergent homeodomain factor originally cloned through its binding to the promoter-coupling element (PCE), a regulatory region upstream of the AFP promoter that mediates stimulation by distant enhancers. Nkx2.8 is the only developmentally regulated factor that has been associated with AFP gene expression. Fetoprotein transcription factor, an orphan nuclear receptor, has also been shown to bind the PCE but is not developmentally regulated. The binding specificities of both families of transcription factor were determined, and overlapping sites for each were defined in the PCE. After modification of nuclear extract and gel shift analysis procedures, Nkx2.8 was identified in six AFP-positive cell lines. Transient-transfection analysis did not show transcriptional stimulation by Nkx2.8 or other active NK2 factors, which only interfered with gene expression. However, two sets of analysis demonstrated the relationship of Nkx2.8 to AFP expression: chromatin immunoprecipitation demonstrated that Nkx2.8 bound to the active AFP promoter, and antisense inhibition of Nkx2.8 mRNA translation selectively reduced expression of both the endogenous human AFP gene and transfected reporters containing the rat AFP promoter.

Bile acid regulation of gene expression: roles of nuclear hormone receptors

Bile acids derived from cholesterol and oxysterols derived from cholesterol and bile acid synthesis pathways are signaling molecules that regulate cholesterol homeostasis in mammals. Many nuclear receptors play pivotal roles in the regulation of bile acid and cholesterol metabolism. Bile acids activate the farnesoid X receptor (FXR) to inhibit transcription of the gene for cholesterol 7alpha-hydroxylase, and stimulate excretion and transport of bile acids. Therefore, FXR is a bile acid sensor that protects liver from accumulation of toxic bile acids and xenobiotics. Oxysterols activate the liver orphan receptors (LXR) to induce cholesterol 7alpha-hydroxylase and ATP-binding cassette family of transporters and thus promote reverse cholesterol transport from the peripheral tissues to the liver for degradation to bile acids. LXR also induces the sterol response element binding protein-1c that regulates lipogenesis. Therefore, FXR and LXR play critical roles in coordinate control of bile acid, cholesterol, and triglyceride metabolism to maintain lipid homeostasis. Nuclear receptors and bile acid/oxysterol-regulated genes are potential targets for developing drug therapies for lowering serum cholesterol and triglycerides and treating cardiovascular and liver diseases.

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.

On the mechanism of bile acid inhibition of rat sterol 12alpha-hydroxylase gene (CYP8B1) transcription: roles of alpha-fetoprotein transcription factor and hepatocyte nuclear factor 4alpha

The sterol 12alpha-hydroxylase (CYP8B1) is a key enzyme of the bile acid biosynthetic pathway. It regulates the composition of bile acids in bile, i.e. ratio between cholic acid (CA) and chenodeoxycholic acid (CDCA). In similarity with cholesterol 7alpha-hydroxylase (CYP7A1), this enzyme is subjected to a negative feedback regulation by bile acids. It has been recently reported that bile acid-activated farnesoid X receptor (FXR) induces the small heterodimer partner (SHP) that interacts with alpha-fetoprotein transcription factor (FTF) and down-regulates CYP7A1 transcription. We studied whether the same mechanism also regulated rat CYP8B1 gene transcription. Feeding rats with CDCA caused a 40-50% decrease of CYP8B1 and hepatocyte nuclear factor 4alpha (HNF4alpha) mRNA expression levels. This was associated with an increase in FTF mRNA expression, but SHP mRNA expression was not altered. Electrophoretic mobility shift assay (EMSA) and transient transfection assay of promoter/reporter genes coupled to mutagenesis analysis identified a putative bile acid response element (BARE) that has an HNF4alpha binding site embedded in two overlapping FTF binding sites. Mutation of the HNF4alpha binding site markedly reduced basal promoter activity and its repression by bile acids. Cotransfection with FTF strongly repressed CYP8B1 transcription. Interestingly, HNF4alpha could overcome the inhibitory effects of FTF and bile acids. We conclude that FTF and HNF4alpha not only play critical roles on CYP8B1 gene transcription, but also mediate bile acid feedback inhibition. This study reveals a novel mechanism by which bile acids inhibit rat CYP8B1 gene transcription by inducing FTF and inhibiting HNF4alpha expression.

Liver receptor homologue-1 (LRH-1) regulates expression of aromatase in preadipocytes

Estrogen biosynthesis from C(19) steroids is catalyzed by aromatase cytochrome P450. Aromatase is expressed in breast adipose tissue through the use of a distal, cytokine-responsive promoter (promoter I.4). Breast tumors, however, secrete soluble factors that stimulate aromatase expression through an alternative proximal promoter, promoter II. In other estrogenic tissues such as ovaries, transcription from promoter II requires the presence of the Ftz-F1 homologue steroidogenic factor-1 (SF-1); adipose tissue, however, does not express SF-1. We have explored the hypothesis that in adipose tissue, an alternative Ftz-F1 family member, liver receptor homologue-1 (LRH-1), substitutes for SF-1 in driving transcription from promoter II. In transient transfection assays using 3T3-L1 preadipocytes, promoter II reporter constructs were modestly (2-3-fold) stimulated by either treatment with activators of protein kinases A or C (PKA/C) or by cotransfection with LRH-1. In combination, these treatments synergistically activated promoter II (>30-fold). Induction by LRH-1 (but not by PKA/C) required an AGGTCA motif at -130 base pairs, to which LRH-1 bound in gel shift assays. Activity of GAL4-LRH-1 fusion proteins was not altered by activators of PKA or PKC. Quantitative real-time PCR revealed that LRH-1 (but not SF-1) is expressed in the preadipocyte fraction of human adipose tissue at levels comparable with that of liver. Differentiation of cultured human preadipocytes into mature adipocytes was associated with a time-dependent induction of peroxisome proliferator-activated receptor-gamma (PPARgamma), and rapid loss of LRH-1 and aromatase expression. We conclude that LRH-1 is a preadipocyte-specific nuclear receptor that regulates expression of aromatase in adipose tissue. Alterations in LRH-1 expression and/or activity in adipose tissue could therefore have considerable effects on local estrogen production and breast cancer development.

Bile acid-activated nuclear receptor FXR suppresses apolipoprotein A-I transcription via a negative FXR response element

Serum levels of HDL are inversely correlated with the risk of coronary heart disease. The anti-atherogenic effect of HDL is partially mediated by its major protein constituent apoA-I. In this study, we identify bile acids that are activators of the nuclear receptor farnesoid X receptor (FXR) as negative regulators of human apoA-I expression. Intrahepatocellular accumulation of bile acids, as seen in patients with progressive familial intrahepatic cholestasis and biliary atresia, was associated with diminished apoA-I serum levels. In human apoA-I transgenic mice, treatment with the FXR agonist taurocholic acid strongly decreased serum concentrations and liver mRNA levels of human apoA-I, which was associated with reduced serum HDL levels. Incubation of human primary hepatocytes and hepatoblastoma HepG2 cells with bile acids resulted in a dose-dependent downregulation of apoA-I expression. Promoter mutation analysis and gel-shift experiments in HepG2 cells demonstrated that bile acid-activated FXR decreases human apoA-I promoter activity by a negative FXR response element mapped to the C site. FXR bound this site and repressed transcription in a manner independent of retinoid X receptor. The nonsteroidal synthetic FXR agonist GW4064 likewise decreased apoA-I mRNA levels and promoter activity in HepG2 cells.

Differential effects of sterol regulatory binding proteins 1 and 2 on sterol 12 alpha-hydroxylase. SREBP-2 suppresses the sterol 12 alpha-hydroxylase promoter

The most important pathway for the catabolism and excretion of cholesterol in mammals is the formation of bile acids. Improper regulation of this pathway has implications for atherosclerosis, cholesterol gallstone formation, and some lipid storage diseases. Sterol 12 alpha-hydroxylase (12 alpha-hydroxylase) is required for cholic acid biosynthesis. The alpha(1)-fetoprotein transcription factor FTF is crucial for the expression and the bile acid-mediated down-regulation of 12 alpha-hydroxylase. Cholesterol, on the other hand, down-regulates expression of the 12 alpha-hydroxylase gene. In this study, we show that the two sterol regulatory binding proteins (SREBPs) have opposite effects on the 12 alpha-hydroxylase promoter. SREBP-1 activated the 12 alpha-hydroxylase promoter, as it does with many other cholesterol-regulated genes. In contrast, SREBP-2 suppressed 12 alpha-hydroxylase promoter activity. SREBP-1 mediates the cholesterol-down-regulation of 12 alpha-hydroxylase promoter by binding to two inverted sterol regulatory elements found approximately 300 nucleotides from the transcriptional initiation site. SREBP-2 mediated suppression of 12 alpha-hydroxylase without binding to its promoter. Data are presented suggesting that SREBP-2 suppresses the 12 alpha-hydroxylase promoter by interacting with FTF. This is the first report of a promoter responding oppositely to two members of the SREBP family of transcription factors. These studies provide a novel function and mode of action of a SREBP protein.

Dual mechanisms for repression of the monomeric orphan receptor liver receptor homologous protein-1 by the orphan small heterodimer partner

The orphan nuclear hormone receptor liver receptor homologous protein-1 (LRH-1; NR5A2, also known as FTF), an unusual receptor that binds DNA as a monomer, is an essential regulator of expression of a rate-limiting enzyme in bile acid formation, cholesterol 7-alpha-hydroxylase. In a classic negative feedback loop that is a crucial component of the complex regulation of cholesterol metabolism, cholesterol 7-alpha-hydroxylase expression is decreased when bile acid levels are high. This repression is thought to be based on the bile acid-dependent induction of expression of the orphan receptor small heterodimer partner (SHP) NR0B2, which inhibits the activity of LRH-1. We have explored the molecular basis for this important regulatory effect by characterizing the mechanisms by which mouse and human SHP inhibit LRH-1-mediated transactivation. Both SHP proteins specifically interact with the AF-2 transactivation domain of LRH-1 both in vivo and in vitro. This domain is a common target for coactivator interaction, and the SHP proteins can compete with p160 coactivators for binding to LRH-1. In addition to the N-terminal receptor interaction domain, SHP includes a C-terminal domain with autonomous repression function. Neither a deletion nor a point mutation specifically affecting this domain blocked the ability to interact with LRH-1 to compete for coactivator binding or to repress LRH-1 transactivation. However, the relative ability of these mutants to inhibit LRH-1-mediated transactivation was markedly decreased. We conclude that the proposed central role of SHP in cholesterol metabolism is based on a two-step mechanism that is dependent on both coactivator competition and direct transcriptional repression.

HBV integrants of hepatocellular carcinoma cell lines contain an active enhancer

Hepatitis B virus (HBV) infection is a major risk factor worldwide for the development of hepatocellular carcinoma (HCC). Integrated HBV DNA fragments, often highly rearranged, are frequently detected in HCC. In woodchuck, the viral enhancer plays a central role in hepatocarcinogenesis, but in humans the mechanism of HBV oncogenesis has not been established. In this study we investigated the status of the viral enhancer in two human HCC cell lines, Hep3B and PLC/PRF/5 each containing one or more integrated HBV DNA fragments. Active enhancer was defined by virtue of its protein occupancy as determined by genomic in vivo DMS footprinting. In PLC/PRF/5 cells, the HBV DNA was integrated in a cellular gene at chromosome 11q13, at a locus reported to be amplified in many tumors. We show here that in both cell lines, the integrated HBV DNA fragments contain an active enhancer-I. In particular, the occupation of the two previously defined basic enhancer elements, E and EP, was prominent. While in both cell lines the same protein binds to the EP elements, the E element, however, is occupied in a cell-line specific manner. In PLC/PRF/5 but not Hep3B, the prominent binding of an undefined protein was detected. Our data suggest that this protein is likely to be the fetoprotein transcription factor (FTF). The finding that enhancer sequences are conserved and functional in different cell lines suggests a selection pressure for their long-term maintenance. We therefore propose that the HBV enhancer-I might play a role in hepatocellular carcinogenesis.

Suppression of sterol 12alpha-hydroxylase transcription by the short heterodimer partner: insights into the repression mechanism

Cholesterol conversion to bile acids is subject to a feedback regulatory mechanism by which bile acids down-regulate their own synthesis. This regulation occurs at the level of transcription of several genes encoding enzymes in the bile acid biosynthetic pathway. One of these enzymes is sterol 12alpha-hydroxylase/CYP8B1 (12alpha-hydroxylase), the specific enzyme required for cholic acid synthesis. The levels of this enzyme determine the ratio of cholic acid to chenodeoxycholic acid and thus the hydrophobicity of the circulating bile acid pool. Previous studies from this laboratory showed that fetoprotein transcription factor (FTF) is required for 12alpha-hydroxylase promoter activity and bile acid-mediated regulation. Here, we report that the short heterodimer partner (SHP) suppresses 12alpha-hydroxylase promoter activity via an interaction with FTF. Hepatic nuclear factor-4 (HNF-4) binds and activates the 12alpha-hydroxylase promoter and is required for 12alpha-hydroxylase promoter activity. Although HNF-4 interacts with SHP, it is not involved in SHP-mediated suppression of 12alpha-hydroxylase promoter activity. FTF and not HNF-4 is the factor involved in regulation of 12alpha-hydroxylase promoter activity by bile acids through its interaction with SHP. Finally, interaction of SHP with FTF displaces FTF binding to its sites within the 12alpha-hydroxylase promoter. These results provide insights into the mechanism of action of bile acid-mediated regulation of sterol 12alpha-hydroxylase transcription.

Glucocorticoid stimulates primate but inhibits rodent alpha-fetoprotein gene promoter

Glucocorticoids inhibit rodent alpha-fetoprotein (AFP) gene activity but stimulate expression of the human homologue. Like human, activity of the AFP promoter from other primates was stimulated by the synthetic glucocorticoid dexamethasone (Dex) in various cell lines. A glucocorticoid responsive element (GRE) is located within 180 bp upstream of the transcription initiation site of all AFP genes examined. Comparative analysis of the GRE in the two different groups of promoters revealed a common 3' hexamer, 5'-TGTCCT-3', but the 5' hexamers were different. This difference converts the rodent GRE to a DR-1 motif. DR-1 is a binding site for members of the nuclear receptor superfamily including the orphan receptor hepatocyte nuclear factor-4 (HNF-4). The presence of DR-1 in the rodent but not human may underlie the opposite actions of Dex on the AFP promoter. We tested this hypothesis using a transient transfection assay. In hepatoma cells that expressed GR and HNF-4, reporter-activity was inhibited by Dex. The same construct in nonhepatoma cells was strongly induced by over expression of HNF-4 and the induced activity was inhibited by Dex. The findings show that Dex induction of human AFP is mediated by a GRE. But Dex repression of the rodent promoter requires a DR-1 motif that interacts with GR and HNF-4.

Expression of FTZ-F1alpha in frog testicular cells

Fushi tarazu transcription factor-1 (FTZ-F1), a member of a nuclear hormone receptor superfamily, is a transcriptional regulator for fushi tarazu gene expression in Drosophila (Ueda et al., '90). We have cloned a homologue (rrFTZ-F1alpha) of the FTZ-F1 gene of the frog Rana rugosa. The gene, in frogs, has been shown to have high expression level in the testis (Nakajima et al., 2000). In this study, the RT-PCR analysis showed that the FTZ-F1alpha mRNA level in adult frogs did not change throughout the year, even during hibernation. However, when immunohistological studies using the anti-rrFTZ-F1alpha antibody were employed to examine which testicular cells expressed this gene, Sertoli cells were found to produce rrFTZ-F1alpha in the two seasons: the breeding season (from March through May) and the pre-hibernating season (from October through November). Interstitial cells, however, did it in only the breeding season (from April through May). Taken together, the results suggest that the rrFTZ-F1alpha expression is regulated at the post-transcriptional step, and that the rrFTZ-F1alpha may play an important role(s) in the seasonal activities of Sertoli and interstitial cells in the frog testis. J. Exp. Zool. 290:182-189, 2001.

Alpha-fetoprotein structure and function: relevance to isoforms, epitopes, and conformational variants

Mammalian alpha-fetoprotein (AFP) is classified as a member of the albuminoid gene superfamily consisting of albumin, AFP, vitamin D (Gc) protein, and alpha-albumin. Molecular variants of AFP have long been reported in the biomedical literature. Early studies identified isoelectric pH isoforms and lectin-binding variants of AFP, which differed in their physicochemical properties, but not in amino acid composition. Genetic variants of AFP, differing in mRNA kilobase length, were later extensively described in rodent models during fetal/perinatal stages, carcinogenesis, and organ regeneration. With the advent of monoclonal antibodies in the early 1980s, multiple antigenic epitopes on native AFP were detected and categorized, culminating in the identification of six to seven major epitopes. During this period, various AFP-binding proteins and receptors were reported to inhibit certain AFP immunoreactions. Concomittantly, human and rodent AFP were cloned and the amino acid sequences of the translated proteins were divulged. Once the amino acid composition of the AFP molecule was known, enzymatic fragments could be identified and synthetic peptide segments synthesized. Following discovery of the molten globule form in 1981, the existence of transitory, intermediate forms of AFP were acknowledged and their physiological significance was realized. In the present review, the various isoforms and variants of AFP are discussed in light of their potential biological relevance.

The mouse fetoprotein transcription factor (FTF) gene promoter is regulated by three GATA elements with tandem E box and Nkx motifs, and FTF in turn activates the Hnf3beta, Hnf4alpha, and Hnf1alpha gene promoters

Fetoprotein transcription factor (FTF) is an orphan nuclear receptor that activates the alpha(1)-fetoprotein gene during early liver developmental growth. Here we sought to define better the position of FTF in transcriptional cascades leading to hepatic differentiation. The mouse FTF gene was isolated and assigned to chromosome 1 band E4 (one mFTF pseudogene was also found). Exon/intron mapping shows an mFTF gene structure similar to that of its close homologue SF1, with two more N-terminal exons in the mFTF gene; exon mapping also delimits several FTF mRNA 5'- and 3'-splice variants. The mFTF transcription initiation site was located in adult liver at 238 nucleotides from the first translation initiator codon, with six canonical GATA, E box, and Nkx motifs clustered between -50/-140 base pairs (bp) from the cap site; DNA/protein binding assays also pinpointed an HNF4-binding element at +36 bp and an FTF-binding element at -257 bp. Transfection assays and point mutations showed that the mFTF promoter is activated by GATA, HNF4alpha, FTF, Nkx, and basic helix-loop-helix factors, with marked cooperativity between GATA and HNF4alpha. A tandem GATA/E box activatory motif in the proximal mFTF promoter is strikingly similar to a composite motif coactivated by differentiation inducers in the hematopoietic lineage; a tandem GATA-Nkx motif in the distal mFTF promoter is also similar to a composite motif transducing differentiation signals from transforming growth factor-beta-like receptors in the cardiogenic lineage. Three genes encoding transcription factors critical to early hepatic differentiation, Hnf3beta, Hnf4alpha, and Hnf1alpha, each contain dual FTF-binding elements in their proximal promoters, and all three promoters are activated by FTF in transfection assays. Direct DNA binding action and cooperativity was demonstrated between FTF and HNF3beta on the Hnf3beta promoter and between FTF and HNF4alpha on the Hnf1alpha promoter. These combined results suggest that FTF is an early intermediary between endodermal specification signals and downstream genes that establish and amplify the hepatic phenotype.

Glucocorticoid-induced DNA demethylation and gene memory during development

Glucocorticoid hormones were found to regulate DNA demethylation within a key enhancer of the rat liver-specific tyrosine aminotransferase (Tat) gene. Genomic footprinting analysis shows that the glucocorticoid receptor uses local DNA demethylation as one of several steps to recruit transcription factors in hepatoma cells. Demethylation occurs within 2-3 days following rapid (< 1 h) chromatin remodeling and recruitment of a first transcription factor, HNF-3. Upon demethylation, two additional transcription factors are recruited when chromatin is remodeled. In contrast to chromatin remodeling, the demethylation is stable following hormone withdrawal. As a stronger subsequent glucocorticoid response is observed, demethylation appears to provide memory of the first stimulation. During development, this demethylation occurs before birth, at a stage where the Tat gene is not yet inducible, and it could thus prepare the enhancer for subsequent stimulation by hypoglycemia at birth. In vitro cultures of fetal hepatocytes recapitulate the regulation analyzed in hepatoma cells. There fore, demethylation appears to contribute to the fine-tuning of the enhancer and to the memorization of a regulatory event during development.

Intestinal bile acid transport: biology, physiology, and pathophysiology

Intestinal reabsorption of bile salts plays a crucial role in human health and disease. This process is primarily localized to the terminal ileum and is mediated by a 48-kd sodium-dependent bile acid cotransporter (SLC10A2 = ASBT). ASBT is also expressed in renal tubule cells, cholangiocytes, and the gallbladder. Exon skipping leads to a truncated version of ASBT, which sorts to the basolateral surface and mediates efflux of bile salts. Inherited mutation of ASBT leads to congenital diarrhea secondary to bile acid malabsorption. Partial inhibition of ASBT may be useful in the treatment of hypercholesterolemia and intrahepatic cholestasis. During normal development in the rat ileum, ASBT undergoes a biphasic pattern of expression with a prenatal onset, postnatal repression, and reinduction at the time of weaning. The bile acid responsiveness of the ASBT gene is not clear and may be dependent on both the experimental model used and the species being investigated. Future studies of the transcriptional and posttranscriptional regulation of the ASBT gene and analysis of ASBT knockout mice will provide further insight into the biology, physiology, and pathophysiology of intestinal bile acid transport.

The role of orphan nuclear receptors in the regulation of cholesterol homeostasis

Cholesterol balance is maintained by a series of regulatory pathways that control the acquisition of cholesterol from endogenous and exogenous sources and the elimination of cholesterol, facilitated by its conversion to bile acids. Over the past decade, investigators have discovered that a family of membrane-bound transcription factors, sterol regulatory element-binding proteins (SREBPs), mediate the end-product repression of key enzymes of cholesterol biosynthesis. Recently orphan members of another family of transcription factors, the nuclear hormone receptors, have been found to regulate key pathways in bile acid metabolism, thereby controlling cholesterol elimination. The study of these orphan nuclear receptors suggests their potential as targets for new drug therapies.

Developmental expression patterns of FTZ-F1 homologues in zebrafish (Danio rerio)

The fushi tarazu factor 1 (FTZ-F1) gene family constitutes a subgroup of orphan nuclear receptors which can be divided into two groups (LRH/FTF- and SF-1/Ad4BP-like) based on sequence homology, function, and tissue distribution. Analysis of zebrafish FTZ-F1 homologues (zFF1 and ff1b) during embryogenesis indicated distinct expression patterns for both genes. Besides the previously observed expression in pituitary/hypothalamus and mandibular arch, zFF1 transcripts were also detected in domains corresponding to the pronephric duct, somites, liver, and hindbrain. Additionally, ff1b transcripts were detected at other developmental stages than earlier documented. Comparative sequence analysis showed that zFF1 exhibited higher sequence similarity to the LRH/FTF group than the SF-1/Ad4BP group, whereas ff1b was indistinguishable between the groups. These observations, coupled with obtained expression patterns, indicate that zebrafish FTZ-F1 homologues exhibit characteristics that are indicative of both LRH/FTF- and SF-1/Ad4BP-like genes.

FTZ-Factor1 and Fushi tarazu interact via conserved nuclear receptor and coactivator motifs

To activate transcription, most nuclear receptor proteins require coactivators that bind to their ligand-binding domains (LBDs). The Drosophila FTZ-Factor1 (FTZ-F1) protein is a conserved member of the nuclear receptor superfamily, but was previously thought to lack an AF2 motif, a motif that is required for ligand and coactivator binding. Here we show that FTZ-F1 does have an AF2 motif and that it is required to bind a coactivator, the homeodomain-containing protein Fushi tarazu (FTZ). We also show that FTZ contains an AF2-interacting nuclear receptor box, the first to be found in a homeodomain protein. Both interaction motifs are shown to be necessary for physical interactions in vitro and for functional interactions in developing embryos. These unexpected findings have important implications for the conserved homologs of the two proteins.

The DEAD box protein DP103 is a regulator of steroidogenic factor-1

The nuclear receptor steroidogenic factor-1 (SF-1) is essential for development of the gonads, adrenal gland, and the ventromedial hypothalamic nucleus. It also regulates the expression of pivotal steroidogenic enzymes and other important proteins in the reproductive system. We sought to elucidate the mechanisms that govern the transcriptional activity of SF-1. We demonstrate here that a previously uncharacterized domain, located C-terminal to the DNA binding domain of SF-1, exhibits transcriptional repression function. Point mutations in this domain markedly potentiate the transcriptional activity of native SF-1. Using an SF-1 region that spans this proximal repression domain as bait in a yeast two-hybrid system, we cloned an SF-1 interacting protein that is homologous to human DP103, a member of the DEAD box family of putative RNA helicases. DP103 directly interacts with the proximal repression domain of SF-1, and mutations in this domain abrogate its interaction with DP103. DP103 is expressed predominantly in the testis and is also expressed at a lower level in other steroidogenic and nonsteroidogenic tissues. Functionally, DP103 exhibits a native transcriptional repression function that localizes to the C-terminal region of the protein and represses the activity of wild-type, but not mutant, SF-1. Together, the physical and functional interaction of DP103 with a previously unrecognized repression domain within SF-1 represents a novel mechanism for regulation of SF-1 activity.

Expression and regulation of transcripts encoding two members of the NR5A nuclear receptor subfamily of orphan nuclear receptors, steroidogenic factor-1 and NR5A2, in equine ovarian cells during the ovulatory process

Steroidogenic factor-1 (SF-1, NR5A1a) is a member of the NR5A nuclear receptor subfamily and has been implicated as a key transcriptional regulator of all ovarian steroidogenic genes in vitro. To establish links between the expression of SF-1 and that of the steroidogenic genes in vivo, the objectives of this study were to clone equine SF-1 and examine the regulation of its messenger RNA (mRNA) in follicular cells during human CG (hCG)-induced ovulation. The equine SF-1 primary transcript was cloned by a combination of RT-PCR techniques. Results showed that the transcript was composed of a 5'-untranslated region (UTR) of 161 bp, an open reading frame (ORF) of 1386 bp that encodes a highly-conserved 461-amino acid protein, and a 3'-UTR of 518 bp. The cloning of SF-1 also led to the unexpected and serendipitous isolation of the highly-related orphan nuclear receptor NR5A2, which was shown to include a 5'-UTR of 243 bp, an ORF of 1488 bp, and a 3'-UTR of 1358 bp. The NR5A2 ORF encodes a 495-amino acid protein that is 60% identical to SF-1, including 99%-similar DNA-binding domains. Northern blot analysis revealed that SF-1 and NR5A2 were expressed in all major steroidogenic tissues, with the exception that NR5A2 was not present in the adrenal. Interestingly, NR5A2 was found to be, by far, the major NR5A subfamily member expressed in the preovulatory follicle and the corpus luteum. Using a semiquantitative RT-PCR/Southern blotting approach, the regulation of SF-1 and NR5A2 mRNAs in vivo was studied in equine follicular cells obtained from preovulatory follicles isolated between 0 and 39 h post hCG. Results showed that the theca interna was the predominant site of SF-1 mRNA expression in the follicle, and that hCG caused a significant decrease in SF-1 levels between 12-39 h in theca interna and between 24-39 h post hCG in granulosa cells (P < 0.05). In contrast, the granulosa cell layer was the predominant, if not the sole, site of NR5A2 mRNA expression in the follicle. Importantly, NR5A2 was much more highly expressed in granulosa cells than SF-1. The administration of hCG caused a significant decrease in NR5A2 transcripts in granulosa cells at 30, 36, and 39 h post hCG (P < 0.05). Thus, this study is the first to report the concomitant regulation of SF-1 in theca interna and granulosa cells throughout the ovulation/luteinization process, and to demonstrate the novel expression and hormonal regulation of NR5A2 in ovarian cells. Based on the marked expression of NR5A2 in equine granulosa and luteal cells and on mounting evidence of a functional redundancy between SF-1 and NR5A2 in other species, it is proposed that NR5A2 may play a key role in the regulation of gonadal steroidogenic gene expression.

Significant down-regulation of alpha-albumin in human hepatoma and its implication

The potential association of alpha-albumin (ALF) with hepatocellular carcinoma (HCC) was investigated. Expression of ALF was significantly reduced in HCC tumor tissue as compared with the paired peritumor tissue from 16 patients and in four HCC cell lines as compared with normal hepatocytes. ALF mRNA was also down-expressed in circulating HCC cells compared to circulating normal hepatocytes. The proliferation of Hep3B cells was inhibited by over-expression of ALF. Taken together, ALF is significantly down-regulated in HCC, and this might facilitate the proliferation of HCC. Thus, detection of ALF mRNA, in addition to that of alpha-fetoprotein (AFP) mRNA, might help to distinguish normal or malignant hepatocytes in peripheral blood.

FTZ-F1alpha is expressed in the developing gonad of frogs

Fushi tarazu transcription factor-1 (FTZ-F1), a member of the nuclear hormone receptor superfamily, is a regulator for fushi tarazu gene expression in Drosophila. Its expression pattern during organogenesis in vertebrates, however, is not known yet. In this study, we cloned a frog FTZ-F1 homologue (rrFTZ-F1alpha) and analyzed its expression and localization during gonadal development of the frog Rana rugosa. Cloned rrFTZ-F1alpha cDNA encoded a protein of 501 amino acids including the regions I-III and FTZ-F1 box that are evolutionally conserved in the FTZ-F1 superfamily. rrFTZ-F1alpha shared high similarity at the amino acid level with mouse LRH-1 (76%), human FTF (92%), chicken OR2.0 (92%), Xenopus laevis FF1rA (94%) and zebrafish FF1A (82%). Northern blot analysis showed that the rrFTZ-F1alpha mRNA at a size of 7.4 kb was the most prominent in the testis among various tissues of adult frogs examined. The RT-PCR analysis revealed that the expression of rrFTZ-F1alpha was weak in the gonad of tadpoles before stage XVI, but it became stronger in the testis of froglets at stage XXV and much higher in the testis of frogs 2 months after metamorphosis. In addition, in situ hybridization analysis revealed that the rrFTZ-F1alpha gene was transcribed in germ cells except for sperm in the testis, and in oocytes at stage A in the ovary of frogs 2 months after metamorphosis. Together, these results suggest that FTZ-F1alpha probably plays an important role in differentiation of germ cells in the gonad of frogs in both sexes.

Elevated levels of hepatocyte nuclear factor 3beta in mouse hepatocytes influence expression of genes involved in bile acid and glucose homeostasis

The winged helix transcription factor, hepatocyte nuclear factor-3beta (HNF-3beta), mediates the hepatocyte-specific transcription of numerous genes important for liver function. However, the in vivo role of HNF-3beta in regulating these genes remains unknown because homozygous null HNF3beta mouse embryos die in utero prior to liver formation. In order to examine the regulatory function of HNF-3beta, we created transgenic mice in which the -3-kb transthyretin promoter functions to increase hepatocyte expression of the rat HNF-3beta protein. Postnatal transgenic mice exhibit growth retardation, depletion of hepatocyte glycogen storage, and elevated levels of bile acids in serum. The retarded growth phenotype is likely due to a 20-fold increase in hepatic expression of insulin-like growth factor binding protein 1 (IGFBP-1), which results in elevated levels in serum of IGFBP-1 and limits the biological availability of IGFs required for postnatal growth. The defects in glycogen storage and serum bile acids coincide with diminished postnatal expression of hepatocyte genes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase and glycogen synthase) and sinusoidal bile acid uptake (Ntcp), respectively. These changes in gene transcription may result from the disruptive effect of HNF-3beta on the hepatic expression of the endogenous mouse HNF-3alpha,-3beta, -3gamma, and -6 transcription factors. Furthermore, adult transgenic livers lack expression of the canalicular phospholipid transporter, mdr2, which is consistent with ultrastructure evidence of damage to transgenic hepatocytes and bile canaliculi. These transgenic studies represent the first in vivo demonstration that the HNF-3beta transcriptional network regulates expression of hepatocyte-specific genes required for bile acid and glucose homeostasis, as well as postnatal growth.