Functional domains of the nuclear receptor hepatocyte nuclear factor 4

Novel Genetic Variations and Haplotypes of Hepatocyte Nuclear Factor 4α(HNF4A) Pound in Japanese Type II Diabetic Patients

Thirty-nine single nucleotide variations, including 16 novel ones, were found in the 5' promoter region, all of the exons and their surrounding introns of HNF4A in 74 Japanese type II diabetic patients. The following novel variations were identified (based on the amino acid numbering of splicing variant 2): -208G>C in the 5' promoter region; 1154C>T (A385V) and 1193T>C (M398T) in the coding exons; 1580G>A, 1852G>T, 2180C>T, 2190G>A, and 2362_2380delAAGAATGGTGTGGGAGAGG in the 3'-untranslated region, and IVS1+231G>A, IVS2-83C>T, IVS3+50C>T, IVS3-54delC, IVS5+173_176delTTAG, IVS5-181_-180delAT, IVS8-106A>G, and IVS9-151A>C in the introns. The allele frequencies were 0.311 for 2362_2380delAAGAATGGTGTGGGAGAGG, 0.054 for 1580G>A, 0.047 for 1852G>T, 0.020 for IVS1+231G>A, 0.014 for IVS9-151A>C, and 0.007 for the other 11 variations. In addition, one known nonsynonymous single nucleotide polymorphism, 416C>T (T139I), was detected at a 0.007 frequency. Based on the linkage disequilibrium profiles, the region analyzed was divided into three blocks. Haplotype analysis determined/inferred 10, 16, and 12 haplotypes for block 1, 2, and 3, respectively. Our results on HNF4A variations and haplotypes would be useful for pharmacogenetic studies in Japanese.

Intestinal apolipoprotein A-IV gene transcription is controlled by two hormone-responsive elements: a role for hepatic nuclear factor-4 isoforms

In the small intestine, the expression of the apolipoprotein (apo) C-III and A-IV genes is restricted to the enterocytes of the villi. We have previously shown that, in transgenic mice, specific expression of the human apo C-III requires a hormone-responsive element (HRE) located in the distal region of the human apoA-IV promoter. This HRE binds the hepatic nuclear factors (HNF)-4alpha and gamma. Here, intraduodenal injections in mice and infections of human enterocytic Caco-2/TC7 cells with an adenovirus expressing a dominant-negative form of HNF-4alpha repress the expression of the apoA-IV gene, demonstrating that HNF-4 controls the apoA-IV gene expression in enterocytes. We show that HNF-4alpha and gamma functionally interact with a second HRE present in the proximal region of the human apoA-IV promoter. New sets of transgenic mice expressing mutated forms of the promoter, combined with the human apo C-III enhancer, demonstrate that, whereas a single HRE is sufficient to reproduce the physiological cephalo-caudal gradient of apoA-IV gene expression, both HREs are required for expression that is restricted to villi. The combination of multiple HREs may specifically recruit regulatory complexes associating HNF-4 and either coactivators in villi or corepressors in crypts.

The functional Thr130Ile and Val255Met polymorphisms of the hepatocyte nuclear factor-4alpha (HNF4A): gene associations with type 2 diabetes or altered beta-cell function among Danes

HNF4A encodes an orphan nuclear receptor that plays crucial roles in regulating hepatic gluconeogenesis and insulin secretion. The aim of the present study was to examine two rare missense polymorphisms of HNF4A, Thr130Ile and Val255Met, for altered function and for association with type 2 diabetes (T2D). We have examined these polymorphisms 1) by in vitro transactivation studies and 2) by genotyping the variants in 1409 T2D patients and in 4726 glucose-tolerant Danish white subjects. When tested in COS7 cells, both the Thr130Ile and the Val255Met variants showed a significant decrease in transactivation activity compared with wild-type (73% of wild-type, P = 0.02, and 76%, P = 0.04, respectively). The Thr130Ile variant had a significantly increased carrier frequency among T2D patients compared with glucose-tolerant subjects [odds ratio, 1.26 (1.01-1.57); P = 0.04]. The rare Val255Met polymorphism had a similar frequency among T2D patients and glucose-tolerant subjects. Heterozygous glucose-tolerant carriers of the variant showed, however, decreased levels of fasting serum C-peptide (76%; P = 0.03) and decreased fasting serum triglyceride (58%; P = 0.02). In conclusion, The Thr130Ile and the Val255Met polymorphisms decrease the transcriptional activity of HNF4A, and the Thr130Ile polymorphism associates with T2D, whereas the Val255Met variant associates with a decrease in fasting serum C-peptide.

Distinct amino acid residues may be involved in coactivator and ligand interactions in hepatocyte nuclear factor-4alpha

Hepatocyte nuclear factor-4 (HNF-4) is a transcription factor of the nuclear hormone receptor superfamily that is constitutively active without the addition of exogenous ligand. Crystallographic analysis of the HNF-4alpha and HNF-4gamma ligand binding domains (LBDs) demonstrated the presence of endogenous ligands that may act as structural cofactors for HNF-4. It was also proposed by crystallographic studies that a combination of ligand and coactivator might be required to lock the receptor in its active state. We previously showed that mutations in amino acid residues Ser-181 and Met-182 in H3, Leu-219 and Leu-220 and Arg-226 in H5, Ileu-338 in H10, and Ileu-346 in H11, which line the LBD pocket in HNF-4alpha and come in contact with the ligand, impair its transactivation potential. In the present study, physical and functional interaction assays were utilized with two different coactivators, PGC-1 and SRC-3, to address the role of coactivators in HNF-4 function. We show that the integrity of the hinge (D) domain of HNF-4alpha and the activation function (AF)-2 activation domain region are critical for coactivation. Surprisingly, a different mode of coactivation is observed among the LBD point mutants that lack transcriptional activity. In particular, coactivation is maintained in mutants Ser-181, Arg-226, and Ile-346 but is abolished in mutants Met-182, Leu-219, and Ile-338. Physical interactions confirm this pattern of activation, implying that distinct amino acid residues may be involved in coactivator and ligand interactions, although some residues may be critical for both functions. Our results provide evidence and expand predictions based on the crystallographic data as to the role of coactivators in HNF-4alpha constitutive transcriptional activity.

Functional characterization of the HNF4α isoform (HNF4α8) expressed in pancreatic β-cells

Mutations in the hepatocyte nuclear factor (HNF) 4alpha gene cause a form of maturity-onset diabetes of the young (MODY1), which is a monogenic form of type 2 diabetes characterized by impaired insulin secretion by pancreatic beta-cells. HNF4alpha is a transcription factor expressed in the liver, kidney, intestine, and pancreatic islet. Multiple splice variants of the HNF4alpha gene have been identified and an isoform of HNF4alpha8, an N-terminal splice variant, is expressed in pancreatic beta-cells. However, expression levels of HNF4alpha protein in pancreatic beta-cells and the transcriptional activity of HNF4alpha8 are not yet understood. In the present study, we investigated the expression of HNF4alpha in beta-cells and examined its functional properties. Western blotting and immunohistochemical analysis revealed that the expression of HNF4alpha protein in pancreatic islets and INS-1 cells was much lower than in the liver. A reporter gene assay showed that the transactivation potential of HNF4alpha8 was significantly weaker than that of HNF4alpha2, which is a major isoform in the liver, suggesting that the total level of HNF4alpha activity is very weak in pancreatic beta-cells. We also showed that the N-terminal A/B region of HNF4alpha8 possessed no activation function and C-terminal F region negatively regulated the transcriptional activity of HNF4alpha8. The information presented here would be helpful for the better understanding of MODY1/HNF4alpha diabetes.

Role of regulatory F-domain in hepatocyte nuclear factor-4alpha ligand specificity

The F-domain of rat HNF-4alpha1 has a crucial impact on the ligand binding affinity, ligand specificity and secondary structure of HNF-4alpha. (i) Fluorescent binding assays indicate that wild-type, full-length HNF-4alpha (amino acids 1-455) has high affinity (Kd=0.06-12 nm) for long chain fatty acyl-CoAs (LCFA-CoA) and low affinity (Kd=58-296 nm) for unesterified long chain fatty acids (LCFAs). LCFA-CoA binding was due to close molecular interaction as shown by fluorescence resonance energy transfer (FRET) from full-length HNF-4alpha tryptophan (FRET donor) to bound cis-parinaroyl-CoA (FRET acceptor), which yielded an intermolecular distance of 33 A, although no FRET to cis-parinaric acid was detected. (ii) Deleting the N-terminal A-D-domains, comprising the AF1 and DNA binding functions, only slightly affected affinities for LCFA-CoAs (Kd=0.9-4 nm) and LCFAs (Kd=93-581 nm). (iii) Further deletion of the F-domain robustly reduced affinities for LCFA-CoA and reversed ligand specificity (i.e. high affinity for LCFAs (Kd=1.5-32 nm) and low affinity for LCFA-CoAs (Kd=54-302 nm)). No FRET from HNF-4alpha-E (amino acids 132-370) tryptophan (FRET donor) to bound cis-parinaroyl-CoA (FRET acceptor) was detected, whereas an intermolecular distance of 28 A was calculated from FRET between HNF-4alpha-E and cis-parinaric acid. (iv) Circular dichroism showed that LCFA-CoA, but not LCFA, altered the secondary structure of HNF-4alpha only when the F-domain was present. (v) cis-Parinaric acid bound to HNF-4alpha with intact F-domain was readily displaceable by S-hexadecyl-CoA, a nonhydrolyzable thioether analogue of LCFA-CoAs. Truncation of the F-domain significantly decreased cis-parinaric acid displacement. Hence, the C-terminal F-domain of HNF-4alpha regulated ligand affinity, ligand specificity, and ligand-induced conformational change of HNF-4alpha. Thus, characteristics of F-domain-truncated mutants may not reflect the properties of full-length HNF-4alpha.

HNF-4-dependent induction of apolipoprotein A-IV gene transcription by an apical supply of lipid micelles in intestinal cells

Apolipoprotein (apo) A-IV, a component of triglyceride-rich lipoproteins secreted by the small intestine, has been shown to play an important role in the control of lipid homeostasis. Numerous studies have described the induction of apoA-IV gene expression by lipids, but the molecular mechanisms involved in this process remain unknown. In this study, we have demonstrated that a lipid bolus induced transcription of the apoA-IV gene in transgenic mice and that the regulatory region of the apoA-IV gene, composed of the apoC-III enhancer and the apoA-IV promoter (eC3-A4), was responsible for this induction. In enterocyte Caco-2/TC7 cells, a permanent supply of lipids at the basal pole induced expression of the apoA-IV gene both at the transcriptional level and through mRNA stabilization. ApoA-IV gene transcription and protein secretion were further induced by an apical supply of complex lipid micelles mimicking the composition of duodenal micelles, and this effect was not reproduced by apical delivery of different combinations of micelle components. Only induction of the apoA-IV gene by lipid micelles involved the participation of hepatic nuclear factor (HNF)-4, as demonstrated using a dominant negative form of this transcription factor. Accordingly, lipid micelles increased the DNA binding activity of HNF-4 on the eC3-A4 region. These results emphasize the importance of physiological delivery of dietary lipids on apoA-IV gene expression and the implication of HNF-4 in this regulation.

Transcriptional regulation of human microsomal triglyceride transfer protein by hepatocyte nuclear factor-4alpha

Microsomal triglyceride transfer protein (MTP) catalyzes the assembly of triglyceride (TG)-rich apolipoprotein B-containing liver (e.g., VLDL) and intestinal (e.g., chylomicron) lipoproteins. The human MTP gene promoter is reported here to associate in vivo with endogenous hepatocyte nuclear factor-4alpha (HNF-4alpha) and to be transactivated or transsuppressed by overexpressed or by dominant negative HNF-4alpha, respectively. Human MTP (hMTP) transactivation by HNF-4alpha is accounted for by the concerted activity of distal (-83/-70) and proximal (-50/-38) direct repeat 1 elements of the hMTP promoter that bind HNF-4alpha. Transactivation by HNF-4alpha is specifically antagonized by chicken ovalbumin upstream promoter. Transcriptional activation of hMTP by HNF-4alpha is mediated by HNF-4alpha domains engaged in ligand binding and ligand-driven transactivation and is further complemented by HNF-4alpha/HNF-1alpha synergism that involves the HNF-4alpha activation function 1 (AF-1) domain. hMTP transactivation by HNF-4alpha is specifically inhibited by beta,beta-tetramethyl-hexadecanedioic acid acting as an HNF-4alpha antagonist ligand. hMTP transactivation by HNF-4alpha may account for the activation or inhibition of MTP expression and the production of TG-rich lipoproteins by agonist (e.g., saturated fatty acids) or antagonist [e.g., (n-3) PUFA, hypolipidemic fibrates, or Methyl-substituted dicarboxylic acid (Medica) compounds] HNF-4alpha ligands.

Nitric oxide and TGF-beta1 inhibit HNF-4alpha function in HEPG2 cells

This study analyzes if the profibrogenic factors nitric oxide and transforming growth factor-beta1 (TGF-beta1) affect hepatocyte nuclear factor-4alpha (HNF-4alpha) function. For this purpose, HepG2 cells were treated with TGF-beta1 or with a nitric oxide donor to determine mRNA levels of coagulation factor VII and HNF-4alpha. Treatment effect on factor VII gene promoter was assessed by chloramphenicol acetyl-transferase assays in cells transfected with the pFVII-CAT plasmid. HNF-4alpha binding and protein levels were determined by gel shift assays and Western blot. TGF-beta1 and nitric oxide downregulated factor VII mRNA levels by inhibiting its gene promoter activity. This inhibition is caused by a decrease in the DNA binding of HNF-4alpha. TGF-beta1 induces degradation of HNF-4alpha in the proteasome while nitric oxide provokes nitrosylation of cysteine residues in this factor. TGF-beta1 and nitric oxide inhibit HNF-4alpha activity. These findings may explain the loss of liver functions that occurs during fibrosis progression.

Bile acid reduces the secretion of very low density lipoprotein by repressing microsomal triglyceride transfer protein gene expression mediated by hepatocyte nuclear factor-4

Microsomal triglyceride transfer protein (MTP) is involved in the transfer of triglycerides, cholesterol esters, and phospholipids to newly synthesized apolipoprotein (apo) B. It is therefore essential for lipoprotein synthesis and secretion in the liver and the small intestine. Although several recent experiments have revealed the transcriptional regulation of the MTP gene, little has been revealed to date about hepatocyte nuclear factor-4 (HNF-4)-dependent regulation. We here report that the human MTP gene promoter contains a pair of functional responsive elements for HNF-4 and HNF-1, the latter of which is another target gene of HNF-4. Chromatin immunoprecipitation assays provide evidence that endogenous HNF-4 and HNF-1 can bind these elements in chromatin. In Hep G2 cells overexpression of either a dominant negative form of HNF-4 or small interfering RNAs (siRNAs) against HNF-4 dramatically reduces the activities of both the wild type and the HNF-4 site mutant MTP promoter. This suggests that HNF-4 regulates MTP gene expression either directly or indirectly through elevated HNF-1 levels. When Hep G2 cells were cultured with chenodeoxycholic acid (CDCA), a ligand for the farnesoid X receptor (FXR), mRNA levels for MTP and apo B were reduced because of increased expression of the factor small heterodimer partner (SHP), which factor suppresses HNF-4 activities. Chenodeoxycholic acid, but not a synthetic FXR ligand, attenuated expression of HNF-4, bringing about a further suppression of MTP gene expression. Over time the intracellular MTP protein levels and apo B secretion in the culture medium significantly declined. These results indicate that two nuclear receptors, HNF-4 and FXR, are closely involved in MTP gene expression, and the results provide evidence for a novel interaction between bile acids and lipoprotein metabolism.

Liver fatty acid-binding protein colocalizes with peroxisome proliferator activated receptor alpha and enhances ligand distribution to nuclei of living cells

Although it is hypothesized that long-chain fatty acyl CoAs (LCFA-CoAs) and long-chain fatty acids (LCFAs) regulate transcription in the nucleus, little is known regarding factors that determine the distribution of these ligands to nuclei of living cells. Immunofluorescence colocalization showed that liver fatty acid-binding protein (L-FABP; binds LCFA-CoA as well as LCFA) significantly colocalized with PPARalpha in nuclei of transfected L-cell fibroblasts. Colocalization with a DNA binding dye (SYTO59) revealed that, within the nucleus of control L-cells, the nonhydrolyzable fluorescent LCFA-CoA (BODIPY-C16-S-S-CoA) was distributed primarily in a punctate pattern throughout the nucleoplasm, while nonmetabolizable fluorescent LCFAs (BODIPY-C16 and BODIPY-C12) were localized primarily near the nuclear envelope membranes. L-FABP overexpression selectively increased the targeting of BODIPY-C16-S-S-CoA by 1.9- and 2.7-fold into the nuclear membrane and nucleoplasm, respectively. L-FABP also increased the targeting of fluorescent LCFAs (especially long-chain-length BODIPY-C16) by 1.7-fold to the nuclear membrane and 7.4-fold into the nucleoplasm. A cis-parinaric acid displacement assay showed that L-FABP bound BODIPY-C12 and BODIPY-C16 with K(i)s of 10.1 +/- 2.5 and 20.7 +/- 1.5 nM, respectively, in the same range as naturally occurring LCFAs. Finally, solid-phase extraction and HPLC analysis revealed that, depending on the fatty acid content of the culture medium, L-FABP expression also increased the cellular LCFA-CoA pool size and altered the LCFA-CoA acyl chain composition. Thus, L-FABP may function as a carrier for selectively enhancing the distribution of LCFA-CoA, as well as LCFA, to nuclei for potential interaction with nuclear receptors.

Expression of the alpha7 isoform of hepatocyte nuclear factor (HNF) 4 is activated by HNF6/OC-2 and HNF1 and repressed by HNF4alpha1 in the liver

The hepatocyte nuclear factor (HNF) 4alpha gene possesses two promoters, proximal P1 and distal P2, whose use results in HNF4alpha1 and HNF4alpha7 transcripts, respectively. Both isoforms are expressed in the embryonic liver, whereas HNF4alpha1 is almost exclusively in the adult liver. A 516-bp fragment, encompassing a DNase I-hypersensitive site associated with P2 activity that is still retained in adult liver, contains functional HNF1 and HNF6 binding sites and confers full promoter activity in transient transfections. We demonstrate a critical role of the Onecut factors in P2 regulation using site-directed mutagenesis and embryos doubly deficient for HNF6 and OC-2 that show reduced hepatic HNF4alpha7 transcript levels. Transient transgenesis showed that a 4-kb promoter region is sufficient to drive expression of a reporter gene in the stomach, intestine, and pancreas, but not the liver, for which additional activating sequences may be required. Quantitative PCR analysis revealed that throughout liver development HNF4alpha7 transcripts are lower than those of HNF4alpha1. HNF4alpha1 represses P2 activity in transfection assays and as deduced from an increase in P2-derived transcript levels in recombinant mice in which HNF4alpha1 has been deleted and replaced by HNF4alpha7. We conclude that although HNF6/OC-2 and perhaps HNF1 activate the P2 promoter in the embryo, increasing HNF4alpha1 expression throughout development causes a switch to essentially exclusive P1 promoter activity in the adult liver.

Hepatocyte nuclear factor 4alpha enhances the hepatocyte nuclear factor 1alpha-mediated activation of transcription

Hepatocyte Nuclear Factor 1alpha (HNF1alpha) and Hepatocyte Nuclear Factor 4alpha (HNF4alpha) are two liver-enriched transcription factors coexpressed in specific tissues where they play a crucial role through their involvement in a complex cross-regulatory network. HNF1alpha down regulates HNF4alpha-mediated activation of transcription via a direct protein-protein interaction. Here we show that HNF4alpha enhances the transcriptional activity of HNF1alpha in a DNA binding independent manner, thus indicating that it behaves as a HNF1alpha coactivator. Using mutations in the ligand binding domain (LBD) of HNF4alpha, we confirmed the involvement of the Activation Function 2 module and demonstrated the requirement of the integrity of the LBD for the interaction with HNF1alpha. Moreover, we show that HNF4alpha cooperates with p300 to achieve the highest HNF1alpha-mediated transcription rates. Our findings highlight a new way by which HNF4alpha can regulate gene expression and extend our knowledge of the complexity of the transcriptional network involving HNF4alpha and HNF1alpha.

Critical role of residues defining the ligand binding pocket in hepatocyte nuclear factor-4alpha

Hepatocyte nuclear factor-4alpha (HNF-4alpha), a member of the nuclear receptor superfamily, is a crucial regulator of a large number of genes involved in glucose, cholesterol, and fatty acid metabolism. Unlike other members of the superfamily, HNF-4alpha activates transcription in the absence of exogenously added ligand. Recently published crystallographic data show that fatty acids are endogenous ligands for HNF-4. Transcriptional analysis of point mutations of the residues that are located in helices H3, H5, H10, and H11, which have been shown to come in contact with the ligand, resulted in a dramatic decrease in activity, without affecting DNA binding and dimerization. Our results show the importance of residues Ser-181, Met-182 in H3, Leu-219, Leu-220 and Arg-226 in H5, Ile-338 in H10, and Ile-346 in H11 that line the ligand-binding domain pocket in HNF-4alpha and impair its transactivation potential. Structural modeling reveals that the mutations do not cause any large scale structural alterations, and the observed loss in transactivation can be attributed to local changes, demonstrating that these residues play a significant role in maintaining the structural integrity of the HNF-4alpha ligand binding pocket.

Critical role of charged residues in helix 7 of the ligand binding domain in Hepatocyte Nuclear Factor 4alpha dimerisation and transcriptional activity

Hepatocyte Nuclear Factor 4alpha (HNF4alpha, NR2A1) is central to hepatocyte and pancreatic beta-cell functions. Along with retinoid X receptor alpha (RXRalpha), HNF4alpha belongs to the nuclear receptor subfamily 2 (NR2), characterised by a conserved arginyl residue and a glutamate residue insert in helix 7 (H7) of the ligand binding domain (LBD). Crystallographic studies indicate that R348 and E352 residues in RXRalpha H7 are involved in charge-driven interactions that improve dimerisation. Consistent with these findings, we showed that removing the charge of the corresponding residues in HNF4alpha H7, R258 and E262, impaired dimerisation in solution. Moreover, our results provide a new concept according to which helices of the HNF4alpha LBD dimerisation interface contribute differently to dimerisation required for DNA binding; unlike H9 and H10, H7 is not involved in DNA binding. Substitutions of E262 decreased the repression of HNF4alpha transcriptional activity by a dominant-negative HNF4alpha mutant, highlighting the importance of this residue for dimerisation in the cell context. The E262 insert is crucial for HNF4alpha function since its deletion abolished HNF4alpha transcriptional activity and coactivator recruitment. The glutamate residue insert and the conserved arginyl residue in H7 most probably represent a signature of the NR2 subfamily of nuclear receptors.

Contribution of the Hormone-Response Elements of the Proximal ApoA-I Promoter, ApoCIII Enhancer, and C/EBP Binding Site of the Proximal ApoA-I Promoter to the Hepatic and Intestinal Expression of the ApoA-I and ApoCIII Genes in Transgenic Mice

We have generated and studied the pattern of expression of transgenic mouse lines carrying the human apoA-I and apoCIII gene cluster mutated at different sites. In two lines, we have either mutated the hormone-response element (HRE) of element G of the apoCIII enhancer or the C/EBP binding site of the proximal apoA-I promoter. In a third line, we have mutated the two HREs of the apoA-I promoter and the HRE of the apoCIII enhancer. Mutations in the HRE of element G reduced the hepatic and intestinal expressions of the reporter chloramphenicol acetyltransferase (CAT) gene (which substituted the apoCIII gene) to 4 and 13% of the wild-type (WT) control, whereas the hepatic and intestinal expressions of the apoA-I gene were reduced to 92 and 25% of the WT control, respectively. A mutation in the C/EBP site increased the hepatic and intestinal expressions of the apoA-I gene approximately 1.25- and 1.6-fold, respectively, and did not affect the expression of the CAT gene. The mutation in the three HNF-4 binding sites of the apoA-I promoter/apoCIII enhancer nearly abolished the expression of apoA-I and the reporter CAT gene in all tissues. These findings establish the importance of the HREs for the hepatic and intestinal expressions of the apoA-I and apoCIII genes and suggest that C/EBP does not play a central role in the expression of the apoA-I gene.

In vitro transcriptional induction of the human apolipoprotein A-II gene by glucose

Type 2 diabetic patients present high triglyceride and low HDL levels, significant determinants for the risk of atherosclerosis. Transgenic mice overproducing human apolipoprotein (apo)A-II, one of the two major apos of HDLs, display the same lipid disorders. Here, we investigated the possible regulation of apoA-II gene expression by glucose. In primary rat hepatocytes and in HepG2 cells, the transcription of the human apoA-II gene was upregulated by glucose. This response was mediated by a hormone-responsive element within the enhancer of the apoA-II promoter and was dependent on hepatocyte nuclear factor-4alpha. Accordingly, in transgenic mice, the human apoA-II gene is stimulated by a high-carbohydrate diet after fasting and at weaning. By contrast, the apoA-II mRNA level is not modified in streptozotocin-induced diabetic rats. In transgenic mice overexpressing the human apoA-II gene, plasma human apoA-II concentration was positively correlated with blood glucose levels. These mice displayed a marked delay in plasma glucose tolerance as compared with control mice. We hypothesize that the following pathogenic pathway might occur in the course of type 2 diabetes: increased apoA-II level causes a rise in plasma triglyceride level and glucose intolerance, resulting in hyperglycemia, which in turn might further increase apoA-II gene transcription.

Inhibitory effect of the small heterodimer partner on hepatocyte nuclear factor-4 mediates bile acid-induced repression of the human angiotensinogen gene

Bile acids function as transcriptional regulators for the genes important in bile acid synthesis and cholesterol homeostasis. In this study, we identified angiotensinogen (ANG), the precursor of vasoactive octapeptide angiotensin II, as a novel target gene of bile acids. In human ANG transgenic mice, administration of cholic acid resulted in the down-regulation of human ANG gene expression in the liver. ANG gene expression in HepG2 cells was also repressed by chenodeoxycholic acid. Because the expression of small heterodimer partner (SHP) mRNA was induced by chenodeoxycholic acid in HepG2 cells, we analyzed the effects of SHP on the human ANG promoter. Promoter mutation analysis demonstrated that SHP repressed human ANG promoter activity through the element, which has been previously determined as a binding site for hepatocyte nuclear factor-4 (HNF-4). SHP repressed human ANG promoter activity only when the HNF-4 expression vector was cotransfected in HeLa cells. Furthermore, we found that SHP bound to the HNF-4 N-terminal region including the DNA-binding domain and activation function-1 and that SHP prevented HNF-4 from binding to the human ANG promoter. These results suggest that bile acids negatively regulate the human ANG gene through the inhibitory effect of SHP on HNF-4.

Activation functions 1 and 2 of nuclear receptors: molecular strategies for transcriptional activation

Nuclear receptors (NRs) comprise a family of ligand inducible transcription factors. To achieve transcriptional activation of target genes, DNA-bound NRs directly recruit general transcription factors (GTFs) to the preinitiation complex or bind intermediary factors, so-called coactivators. These coactivators often constitute subunits of larger multiprotein complexes that act at several functional levels, such as chromatin remodeling, enzymatic modification of histone tails, or modulation of the preinitiation complex via interactions with RNA polymerase II and GTFs. The binding of NR to coactivators is often mediated through one of its activation domains. Many NRs have at least two activation domains, the ligand-independent activation function (AF)-1, which resides in the N-terminal domain, and the ligand-dependent AF-2, which is localized in the C-terminal domain. In this review, we summarize and discuss current knowledge regarding the molecular mechanisms of AF-1- and AF-2-mediated gene activation, focusing on AF-1 and AF-2 conformation and coactivator binding.

Hepatic scavenger receptor class B, type I is stimulated by peroxisome proliferator-activated receptor gamma and hepatocyte nuclear factor 4alpha

Excessive cellular cholesterol is transported to the liver by a pathway called 'reverse cholesterol transport.' Scavenger receptor class B, type I (SR-BI) mediates cholesterol uptake in the liver. Polyunsaturated fatty acids, known to activate peroxisome proliferator-activated receptor (PPAR), have been reported to increase hepatic cholesterol uptake. We found in the present study that PPARgamma induces expression of SR-BI in rat hepatocytes, liver endothelial cells, and Kupffer cells. In contrast, PPARalpha increased SR-BI levels only in hepatocytes and liver endothelial cells. PPARgamma/RXR binds to a response element between -459 and -472 bp in the human SR-BI promoter. Furthermore, hepatocyte nuclear factor 4alpha (HNF4alpha) was found to enhance PPARgamma-mediated SR-BI transcription. Thiazolidinedione (TZD)-activated PPARgamma/RXR increased hepatic SR-BI levels, which may lead to increased hepatic cholesterol uptake and less accumulation of lipids in peripheral tissues. The present results are in agreement with previous reports, indicating that specific PPARgamma-agonists (such as TZDs) protect against atherosclerosis.

ARP1 interacts with the 5' flanking region of the coagulation factor VII gene

Factor (F)VII plays a critical role in initiation of coagulation. Several segments within the 5' flanking region of the FVII gene were previously demonstrated to recognize hepatic nuclear proteins, but few have been identified. To identify a regulatory protein binding the nuclear hormone response region (-237 to -200) of the FVII 5' flanking region and demonstrate that the interaction is functional. Electrophoretic mobility shift assays and mutation analysis showed that ARP1, an orphan nuclear hormone receptor, interacted with two regions of the FVII 5' flanking region, the hepatic nuclear factor 4 binding region (-77 to -47) and the nuclear hormone response region (-237 to -200). Transfection experiments demonstrated that reporter gene expression was decreased from vectors including the nuclear hormone response segment compared with that containing only the minimal promoter between positions -109 and +1, and that ARP1 also repressed expression through an interaction with the minimal promoter. These data indicate a role for ARP1 in transcriptional modulation of the FVII gene.

Hepatocyte nuclear factor-4 is a novel downstream target of insulin via FKHR as a signal-regulated transcriptional inhibitor

Previous studies have shown that FKHR, a member of the forkhead family of transcription factors, acts as a DNA binding-independent cofactor of nuclear receptors, including estrogen, retinoid, and thyroid hormone receptors, in addition to the original function as a DNA binding transcription factor that redistributes from the nucleus to the cytoplasm by insulin-induced phosphorylation. Here, we demonstrated the physical interaction of FKHR with hepatocyte nuclear factor (HNF)-4, a member of steroid/thyroid nuclear receptor superfamily, and the repression of HNF-4 transactivation by FKHR. FKHR interacted with the DNA binding domain of HNF-4 and inhibited HNF-4 binding to the cognate DNA. Furthermore, the binding affinity of HNF-4 with phosphorylated FKHR significantly decreased in comparison to that with unphosphorylated FKHR. Therefore, a phosphorylation of FKHR by insulin followed by its dissociation from HNF-4 and the redistribution of FKHR from the nucleus to the cytoplasm would expect to induce the transcriptional activation of HNF-4 by facilitating to the access of HNF-4 to its DNA element. Indeed, most intriguingly, insulin stimulation reversed the repression of HNF-4 transcriptional activity by phosphorylation-sensitive (wild-type) FKHR, but not by phosphorylation-deficient FKHR. These results suggest that insulin regulates the transcriptional activity of HNF-4 via FKHR as a signal-regulated transcriptional inhibitor.

Serine/threonine phosphorylation regulates HNF-4alpha-dependent redox-mediated iNOS expression in hepatocytes

Nitric oxide (NO), endogenously synthesized by inducible NO synthase (iNOS), serves antioxidant and antiapoptotic functions in settings characterized by oxidative stress and proinflammatory cytokines such as sepsis and shock. However, the redox-sensitive mechanisms regulating hepatocyte expression of iNOS are largely unknown. In interleukin-1beta (IL-1beta)-stimulated hepatocytes exposed to superoxide, we demonstrate that hepatocyte nuclear factor-4alpha (HNF-4alpha) acts as an activator of redox-associated hepatocyte iNOS expression at the level of protein, mRNA, and promoter activation. In the absence of HNF-4alpha, this redox-mediated enhancement is ablated. HNF-4alpha functional activity is associated with a unique serine/threonine kinase-mediated phosphorylation pattern. This suggests that a redox-sensitive kinase pathway targets HNF-4alpha to augment hepatocyte iNOS expression. Previous studies have not addressed a redox-dependent kinase signaling pathway that targets HNF-4alpha and enhances hepatocyte iNOS gene transcription. A unique pattern of phosphorylation determines HNF-4alpha activity as a trans-activator of IL-1beta-mediated hepatocyte iNOS expression in the presence of oxidative stress.

T130I mutation in HNF-4alpha gene is a loss-of-function mutation in hepatocytes and is associated with late-onset Type 2 diabetes mellitus in Japanese subjects

AIMS/HYPOTHESIS

Mutations in hepatocyte nuclear factor (HNF)-4alpha gene cause a form of maturity-onset diabetes of the young (MODY1). The T130I mutation is a rare missense mutation, which affects a conserved amino acid in a DNA binding domain. This mutation can be found in the general population, so this variant alone does not cause MODY. However, its significance in the development of late-onset Type 2 diabetes is not known.

METHODS

We screened 423 unrelated Japanese patients with late-onset Type 2 diabetes and 354 unrelated non-diabetic control subjects for the T130I mutation in the HNF-4alpha gene. The transactivation ability of T130I-HNF-4alpha was assessed using reporter gene assay.

RESULTS

The frequency of the T130I mutation was higher in Type 2 diabetic patients ( p=0.015, odds ratio 4.3, 95%CI 1.24-14.98) than control subjects. The serum HDL-cholesterol concentration was lower in Type 2 diabetic patients with the T130I mutation compared with those without this mutation ( p=0.006). Reporter gene analysis showed that T130I-HNF-4alpha transcriptional activity was not impaired compared with wild-type HNF-4alpha in Hela and MIN6 cells, but it was reduced in HepG2 and primary cultured mouse hepatocytes (27-78% of wild type, p<0.05).

CONCLUSION/INTERPRETATION

Our findings suggest that T130I-HNF-4alpha is a loss-of-function mutation in hepatocytes and that this mutation is associated with late-onset Type 2 diabetes in Japanese subjects. The T130I mutation in the HNF-4alpha gene might be involved in the development of Type 2 diabetes in the Japanese population.

Suppression of hepatitis B virus core promoter by the nuclear orphan receptor TR4

The TR4 orphan receptor is a member of the nuclear receptor superfamily that modulates gene expression via binding to the AGGTCA direct repeat hormone response element. Here we report a functional study of TR4 interaction with the core promoter of the hepatitis B virus (HBV). The electrophoretic mobility shift assay shows that TR4 can bind to the direct repeat 1 sequence element (AGGTTAAAGGTCT, nucleotide coordinates 1757-1769, TR4RE-HBV) on the HBV core promoter. TR4 also can enhance the activity of a synthetic luciferase reporter linked with four copies of TR4RE-HBV in either liver HepG2 or non-liver H1299 cells in a dose-dependent manner. Surprisingly, TR4 represses the activity of a luciferase reporter containing the entire HBV genome sequences. Moreover, mutation of this TR4RE-HBV site in the HBV core promoter diminishes the TR4 suppression effect. This TR4-induced suppression of HBV core promoter activity is further confirmed by primer extension analysis of the HBV core RNAs, showing that TR4 represses both pre-core and core mRNAs. Further dissection of this repressive mechanism indicates that TR4 may suppress the HBV core promoter activity via repressing HNF4alpha-mediated transactivation by protein-protein interactions without inhibition of HNF4alpha DNA binding. Furthermore, our results indicate that the N- and C-terminal regions of TR4 protein are required for TR4-HNF4alpha interaction. It is possible that TR4-HNF4alpha interaction may block the HNF4alpha function that results in the suppression of HBV gene expression. Together, these results demonstrate that TR4 can serve as a negative modulator in the transcriptional regulation of HBV core gene expression.

Mechanism of a transcriptional cross talk between transforming growth factor-beta-regulated Smad3 and Smad4 proteins and orphan nuclear receptor hepatocyte nuclear factor-4

We have shown previously that the transforming growth factor-beta (TGFbeta)-regulated Sma-Mad (Smad) protein 3 and Smad4 proteins transactivate the apolipoprotein C-III promoter in hepatic cells via a hormone response element that binds the nuclear receptor hepatocyte nuclear factor 4 (HNF-4). In the present study, we show that Smad3 and Smad4 but not Smad2 physically interact with HNF-4 via their Mad homology 1 domains both in vitro and in vivo. The synergistic transactivation of target promoters by Smads and HNF-4 was shown to depend on the specific promoter context and did not require an intact beta-hairpin/DNA binding domain of the Smads. Using glutathione S-transferase interaction assays, we established that two regions of HNF-4, the N-terminal activation function 1 (AF-1) domain (aa 1-24) and the C-terminal F domain (aa 388-455) can mediate physical Smad3/HNF-4 interactions in vitro. In vivo, Smad3 and Smad4 proteins enhanced the transactivation function of various GAL4-HNF-4 fusion proteins via the AF-1 and the adjacent DNA binding domain, whereas a single tyrosine to alanine substitution in AF-1 abolished coactivation by Smads. The findings suggest that the transcriptional cross talk between the TGFbeta-regulated Smads and HNF-4 is mediated by specific functional domains in the two types of transcription factors. Furthermore, the specificity of this interaction for certain target promoters may play an important role in various hepatocyte functions, which are regulated by TGFbeta and the Smads.

Developmentally regulated N-terminal variants of the nuclear receptor hepatocyte nuclear factor 4alpha mediate multiple interactions through coactivator and corepressor-histone deacetylase complexes

To understand the mechanisms governing the regulation of nuclear receptor (NR) function, we compared the parameters of activation and repression of two isoforms of the orphan receptor hepatocyte nuclear factor (HNF) 4alpha. HNF4alpha7 and HNF4alpha1 differ only in their N-terminal domains, and their expression in the liver is regulated developmentally. We show that the N-terminal activation function (AF)-1 of HNF4alpha1 possesses significant activity that can be enhanced through interaction with glucocorticoid receptor-interacting protein 1 (GRIP-1) and cAMP response element-binding protein-binding protein (CBP). In striking contrast, HNF4alpha7 possesses no measurable AF-1, implying major functional differences between the isoforms. Indeed, although HNF4alpha1 and HNF4alpha7 are able to interact via AF-2 with GRIP-1, p300, and silencing mediator for retinoid and thyroid receptors (SMRT), only HNF4alpha1 interacts in a synergistic fashion with GRIP-1 and p300. Although both isoforms interact physically and functionally with SMRT, the repression of HNF4alpha7 is less robust than that of HNF4alpha1, which may be caused by an increased ability of the latter to recruit histone deacetylase (HDAC) activity to target promoters. Moreover, association of SMRT with HDACs enhanced recruitment of HNF4alpha1 but not of HNF4alpha7. These observations suggest that NR isoform-specific association with SMRT could affect activity of the SMRT complex, implying that selection of HDAC partners is a novel point of regulation for NR activity. Possible physiological consequences of the multiple interactions with these coregulators are discussed.

TRAP/SMCC/mediator-dependent transcriptional activation from DNA and chromatin templates by orphan nuclear receptor hepatocyte nuclear factor 4

The orphan nuclear receptor hepatocyte nuclear factor 4 (HNF-4) regulates the expression of many liver-specific genes both during development and in the adult animal. Towards understanding the molecular mechanisms by which HNF-4 functions, we have established in vitro transcription systems that faithfully recapitulate HNF-4 activity. Here we have focused on the coactivator requirements for HNF-4, especially for the multicomponent TRAP/SMCC/Mediator complex that has emerged as the central regulatory module of the transcription apparatus. Using a system that has been reconstituted from purified transcription factors, as well as one consisting of unfractionated nuclear extract from which TRAP/SMCC/Mediator has been depleted by specific antibodies, we demonstrate a strong dependence of HNF-4 function on this coactivator. Importantly, we further show a TRAP/SMCC/Mediator-dependence for HNF-4 transcriptional activation from chromatin templates. The latter involves cooperation with the histone acetyltransferase-containing coactivator p300, in accord with a synergistic mode of action of the two divergent coactivators. We also show that HNF-4 and TRAP/SMCC/Mediator can interact physically. This interaction likely involves primary HNF-4 activation function 2 (AF-2)-dependent interactions with the TRAP220 subunit of TRAP/SMCC/Mediator and secondary (AF-2-independent) interactions with TRAP170/RGR1. Finally, recruitment experiments using immobilized templates strongly suggest that the functional consequences of the physical interaction probably are manifested at a postrecruitment step in the activation pathway.

Ligand specificity and conformational dependence of the hepatic nuclear factor-4alpha (HNF-4alpha )

Hepatic nuclear factor-4alpha (HNF-4alpha) controls the expression of genes encoding proteins involved in lipid and carbohydrate metabolism. Fatty acyl-CoA thioesters have recently been proposed to be naturally occurring ligands of HNF-4alpha and to regulate its transcriptional activity as function of their chain length and degree of unsaturation (Hertz, R., Magenheim, J., Berman, I., and Bar-Tana, J. (1998) Nature 392, 512-516). However, the apparent low affinities (microm K(d) values) obtained with a radiolabeled fatty acyl-CoA ligand binding assay raised questions regarding the physiological significance of this finding. Furthermore, it is not known whether interaction with fatty acyl-CoA alters the structure of HNF-4alpha. These issues were examined using rat recombinant HNF-4alpha ligand-binding domain (HNF-4alphaLBD) in conjunction with photon counting fluorescence and circular dichroism. First, fluorescence resonance energy transfer between HNF-4alphaLBD tryptophan (Trp) and cis-parinaroyl-CoA yielded an intermolecular distance of <or=42 A, thus pointing to direct molecular interaction rather than nonspecific coaggregation. Second, quenching of HNF-4alphaLBD intrinsic Trp fluorescence by fatty acyl-CoAs (e.g. pamitoyl-, stearoyl-, linoleoyl-, and arachidonoyl-CoAs) yielded a single binding site with K(d) values of 1.6-4.0 nm. These affinities were 2-3 orders of magnitude higher than those previously derived by radiolabeled fatty acyl-CoA ligand binding assay. Third, binding of fatty acyl-CoAs was specific as the binding affinities of the respective free fatty acids or free CoA (K(d) values of 421-742 nm) were significantly lower. Fourth, circular dichroism demonstrated that the HNF-4alphaLBD secondary structure was significantly and differentially altered by fatty acyl-CoA binding. The opposite effects of saturated versus polyunsaturated fatty acyl-CoAs on HNF-4alpha LBD secondary structure correlated with their opposite regulatory effects on HNF-4alpha function. Fifth, the CoA thioesters of some hypolipidemic peroxisome proliferators bind with high affinity (K(d) values as low as 2.6 nm) to HNF-4alpha LBD, thus indicating that HNF-4alpha may serve as target for these drugs. In summary, these data demonstrate for the first time high affinity binding to HNF-4alpha of fatty and xenobiotic acyl-CoAs in the physiological range, resulting in significantly altered HNF-4alpha conformation.

Cloning and characterization of the human factor XI gene promoter: transcription factor hepatocyte nuclear factor 4alpha (HNF-4alpha ) is required for hepatocyte-specific expression of factor XI

Factor XI is the zymogen of a plasma protease produced primarily in liver that is required for normal blood coagulation. We cloned approximately 2600 base pairs of the human factor XI gene upstream of exon one, identified transcription start sites, and conducted a functional analysis. Luciferase reporter assays demonstrate that the 381 base pairs upstream of exon one are sufficient for maximum promoter activity in HepG2 hepatocellular carcinoma cells. The removal of 19 base pairs between -381 and -363 results in a nearly complete loss of promoter activity. This region contains the sequence ACTTTG, a motif required for binding of the transcription factor hepatocyte nuclear factor 4alpha (HNF-4alpha) to the promoters of several genes. Gel mobility shift assays using HepG2 or rat hepatocyte nuclear extract confirm HNF-4alpha binds between bp -375 and -360. Scrambling the ACTTTG motif completely abolishes promoter activity in luciferase assays. The factor XI promoter functions poorly when transfected into HeLa carcinoma cells, and gel mobility shift experiments with HeLa nuclear extracts demonstrate no HNF-4alpha binding to the ACTTTG sequence. When a rat HNF-4alpha expression construct is co-transfected into HeLa cells, factor XI promoter activity is enhanced approximately 10-fold. We conclude that HNF-4alpha is required for hepatocyte-specific expression of factor XI.

Two initiator-like elements are required for the combined activation of the human apolipoprotein C-III promoter by upstream stimulatory factor and hepatic nuclear factor-4

Human apoC-III (-890/+24) promoter activity is strongly activated by hepatic nuclear factor (HNF)-4 through its binding to the proximal (-87/-72) element B. This site overlaps the binding site for an activity that we identified as the ubiquitously expressed upstream stimulatory factor (USF) (Ribeiro, A., Pastier, D., Kardassis, D., Chambaz, J., and Cardot, P. (1999) J. Biol. Chem. 274, 1216-1225). In the present study, we characterized the relationship between USF and HNF-4 in the activation of human apoC-III transcription. Although USF and HNF-4 binding to element B is mutually exclusive, co-transfection experiments in HepG2 cells surprisingly showed a combined effect of USF and HNF-4 in the transactivation of the (-890/+24) apoC-III promoter. This effect only requires the proximal region (-99/+24) of the apoC-III promoter and depends neither on USF binding to its cognate site in element B nor on a USF-dependent facilitation of HNF-4 binding to its site. By contrast, we found by electrophoretic mobility shift assay and footprinting analysis two USF low affinity binding sites, located within the proximal promoter at positions -58/-31 (element II) and -19/-4 (element I), which are homologous to initiator-like element sequence. Co-transfection experiments in HepG2 cells show that a mutation in element II reduces 2-fold the USF transactivation effect on the proximal promoter of apoC-III and that a mutation in element I inhibits the combined effect of USF and HNF-4. In conclusion, these initiator-like elements are directly involved in the transactivation of the apoC-III promoter by USF and are necessary to the combined effect between USF and HNF-4 for the apoC-III transcription.

Thyroid hormone receptor interacting protein 3 (trip3) is a novel coactivator of hepatocyte nuclear factor-4alpha

Mutations of the hepatocyte nuclear factor-4alpha (HNF-4alpha) gene are associated with a subtype of maturity-onset diabetes of the young (MODY1) that is characterized by impaired insulin secretion in response to a glucose load. HNF-4alpha, which is a transcription factor expressed in pancreatic beta-cells, plays an important role in regulating the expression of genes involved in glucose metabolism. Thus, cofactors that interact with HNF-4alpha and modify its transcriptional activity might also play an important role in regulating the metabolic pathways in pancreatic beta-cells, and the genes of such cofactors are plausible candidate genes for MODY. In the present study, we showed, using a yeast two-hybrid screening assay, that thyroid hormone receptor interacting protein 3 (Trip3) interacted with HNF-4alpha, and their interaction was confirmed by the glutathione S-transferase pull-down assay. Human Trip3 cDNA contained an open reading frame for a protein of 155 amino acids, and the gene was expressed in both pancreatic islets and MIN6 cells. Cotransfection experiments indicated that Trip3 could enhance (two- to threefold) the transcription activity of HNF-4alpha in COS-7 cells and MIN6 cells. These results suggest that Trip3 is a coactivator of HNF-4alpha. Mutation screening revealed that variation of the Trip3 gene is not a common cause of MODY/early-onset type 2 diabetes in Japanese individuals. Trip3 may play an important role in glucose metabolism by regulating the transcription activity of HNF-4alpha.

Hepatocyte nuclear factor-4alpha mediates redox sensitivity of inducible nitric-oxide synthase gene transcription

The underlying redox-sensitive mechanisms that regulate hepatocyte expression of inducible nitric-oxide synthase (iNOS) and its antioxidant functions are largely unknown. We have demonstrated previously that oxidative stress induced by benzenetriol-mediated superoxide production increases interleukin-1beta-induced iNOS protein synthesis, steady state iNOS mRNA expression, NO production, iNOS gene transcription, and trans-activation of the iNOS promoter in primary cultures of rat hepatocytes. In this study, we extend these studies by establishing the sequence specificity and binding of nuclear protein to the previously described 15-base cis-regulatory element of the rat hepatocyte iNOS promoter, isolating and identifying the cis-regulatory element transcription factor as hepatocyte nuclear factor-4alpha (HNF-4alpha), and confirming the functional role of HNF-4alpha in mediating redox-sensitive iNOS promoter trans-activation. In addition, we demonstrate that binding of HNF-4alpha to the transcriptional coactivator, PC4, in the presence of oxidative stress and interleukin-1beta stimulation is essential for increased iNOS promoter activity in this setting. Our results indicate that HNF-4alpha is the transcription factor that mediates redox regulation of hepatocyte iNOS gene transcription.

Direct physical interactions between HNF-4 and Sp1 mediate synergistic transactivation of the apolipoprotein CIII promoter

We have investigated the mechanism of functional cooperativity between specificity protein 1 (Sp1) and hepatocyte nuclear factor-4 (HNF-4) on the human apolipoprotein CIII (apoCIII) promoter. Cotransfections in Drosophila SL2 cells that lack endogenous Sp1 or Sp1-related activities showed that HNF-4 and Sp1 synergistically transactivate the -890/+24 apoCIII promoter up to 150-fold. Synergistic transactivation required the HNF-4 binding site of the apoCIII enhancer. Deletion of part of the Ser/Thr-rich and Gln-rich domain or the C-terminal domain of Sp1 decreased, and deletion of residues 501-610 of Sp1 increased, the functional cooperativity between Sp1 and HNF-4. Physical interactions between the two factors were demonstrated by glutathione S-transferase pull-down and co-immunoprecipitation assays. The amino terminal domain of both factors and the carboxy terminal domain of Sp1 contribute to these interactions. Antagonism between HNF-4 and Sp1 was demonstrated on homopolymeric promoters containing multiple binding sites for either factor, suggesting that the synergism between the two factors occurs only when both factors are bound simultaneously to the DNA. The observed physical interactions between Sp1 and HNF-4 in the context of the apoCIII promoter may explain in part their in vitro and in vivo synergism in the transcriptional activation of the apolipoprotein A-I/apoCIII/apolipoprotein A-IV gene cluster.

Differential regulation of the orphan nuclear receptor small heterodimer partner (SHP) gene promoter by orphan nuclear receptor ERR isoforms

The orphan nuclear receptor small heterodimer partner (SHP; NR0B2) interacts with a wide array of nuclear receptors and represses their transcriptional activity. SHP expression is regulated by several other members of the nuclear receptor superfamily, including the orphan receptors SF-1 and LRH-1, and the bile acid receptor FXR. We have found that the SHP promoter is also activated by the estrogen receptor-related receptor gamma (ERRgamma) but not the related ERRalpha and ERRbeta isoforms. SHP and ERRgamma mRNAs are coexpressed in several tissues, including pancreas, kidney, and heart, confirming the potential relevance of this transactivation. ERRgamma transactivation is dependent on only one of five previously characterized DNA-binding sites for SF-1, and this element differs from previously reported ERR response elements. However, treatment with the histone deacetylase inhibitor trichostatin A significantly increased ERRalpha and ERRbeta activity on this element indicating that the lack of activity of ERRalpha and -beta may depend on their association with co-repressor in vivo. Furthermore, using protease sensitivity assays on DNA bound receptors it was demonstrated that DNA sequence of different response elements may cause allosteric modulation of ERR proteins, which in turn may be responsible for the differential activities of these receptors on different response elements. SHP inhibits ERRgamma transactivation and physically interacts with all three members of ERR subfamily, as demonstrated by both yeast two-hybrid and biochemical assays. As with other SHP targets, this interaction is dependent on the AF-2 coactivator-binding site of ERRgamma and the previously described N-terminal receptor interaction domain of SHP. Several recently described SHP mutations associated with moderate obesity in humans block the inhibition of ERRgamma activity. Overall, these results identify a new autoregulatory loop controlling SHP gene expression and significantly extend the potential functional roles of the three ERRs.

The activation function-1 of hepatocyte nuclear factor-4 is an acidic activator that mediates interactions through bulky hydrophobic residues

The hepatocyte nuclear factor-4 (HNF-4) contains two transcription activation domains. One domain, activation function-1 (AF-1), consists of the extreme N-terminal 24 amino acids and functions as a constitutive autonomous activator of transcription. This short transactivator belongs to the class of acidic activators, and it is predicted to adopt an amphipathic alpha-helical structure. Transcriptional analysis of sequential point mutations of the negatively charged residues (Asp and Glu) revealed a stepwise decrease in activity, while mutation of all acidic residues resulted in complete loss of transcriptional activity. Mutations of aromatic and hydrophobic amino acids surrounding the negatively charged residues had a much more profound effect than mutations of acidic amino acids, since even a single mutation of these residues resulted in a dramatic decrease in transactivation, thus demonstrating the importance of hydrophobic residues in AF-1 activity. Like other acidic activators, the AF-1 of HNF-4 binds the transcription factor IIB and the TATA-binding protein directly in vitro. In addition, the cAMP-response-element-binding-protein, a transcriptional adapter involved in the transactivation of a plethora of transcription factors, interacts with the AF-1 of HNF-4 and co-operates in the process of transactivation by HNF-4. The different protein targets of AF-1 suggest that the AF-1 of HNF-4 may be involved in recruiting both general transcription factors and chromatin remodelling proteins during activation of gene expression.

Maturity-onset diabetes of the young Type 1 (MODY1)-associated mutations R154X and E276Q in hepatocyte nuclear factor 4alpha (HNF4alpha) gene impair recruitment of p300, a key transcriptional co-activator

Hepatocyte nuclear factor 4alpha (HNF4alpha) is a nuclear receptor involved in glucose homeostasis and is required for normal beta-cell function. Mutations in the HNF4alpha gene are associated with maturity-onset diabetes of the young type 1. E276Q and R154X mutations were previously shown to impair intrinsic transcriptional activity (without exogenously supplied co-activators) of HNF4alpha. Given that transcriptional partners of HNF4alpha modulate its intrinsic transcriptional activity and play crucial roles in HNF4alpha function, we investigated the effects of these mutations on potentiation of HNF4alpha activity by p300, a key co-activator for HNF4alpha. We show here that loss of HNF4alpha function by both mutations is increased through impaired physical interaction and functional cooperation between HNF4alpha and p300. Impairment of p300-mediated potentiation of HNF4alpha transcriptional activity is of particular importance for the E276Q mutant since its intrinsic transcriptional activity is moderately affected. Together with previous results obtained with chicken ovalbumin upstream promoter-transcription factor II, our results highlight that impairment of recruitment of transcriptional partners represents an important mechanism leading to abnormal HNF4alpha function resulting from the MODY1 E276Q mutation. The impaired potentiations of HNF4alpha activity were observed on the promoter of HNF1alpha, a transcription factor involved in a transcriptional network and required for beta-cell function. Given its involvement in a regulatory signaling cascade, loss of HNF4alpha function may cause reduced beta-cell function secondary to defective HNF1alpha expression. Our results also shed light on a better structure-function relationship of HNF4alpha and on p300 sequences involved in the interaction with HNF4alpha.

Hepatocyte nuclear factor 4alpha (nuclear receptor 2A1) is essential for maintenance of hepatic gene expression and lipid homeostasis

The numerous functions of the liver are controlled primarily at the transcriptional level by the concerted actions of a limited number of hepatocyte-enriched transcription factors (hepatocyte nuclear factor 1alpha [HNF1alpha], -1beta, -3alpha, -3beta, -3gamma, -4alpha, and -6 and members of the c/ebp family). Of these, only HNF4alpha (nuclear receptor 2A1) and HNF1alpha appear to be correlated with the differentiated phenotype of cultured hepatoma cells. HNF1alpha-null mice are viable, indicating that this factor is not an absolute requirement for the formation of an active hepatic parenchyma. In contrast, HNF4alpha-null mice die during embryogenesis. Moreover, recent in vitro experiments using tetraploid aggregation suggest that HNF4alpha is indispensable for hepatocyte differentiation. However, the function of HNF4alpha in the maintenance of hepatocyte differentiation and function is less well understood. To address the function of HNF4alpha in the mature hepatocyte, a conditional gene knockout was produced using the Cre-loxP system. Mice lacking hepatic HNF4alpha expression accumulated lipid in the liver and exhibited greatly reduced serum cholesterol and triglyceride levels and increased serum bile acid concentrations. The observed phenotypes may be explained by (i) a selective disruption of very-low-density lipoprotein secretion due to decreased expression of genes encoding apolipoprotein B and microsomal triglyceride transfer protein, (ii) an increase in hepatic cholesterol uptake due to increased expression of the major high-density lipoprotein receptor, scavenger receptor BI, and (iii) a decrease in bile acid uptake to the liver due to down-regulation of the major basolateral bile acid transporters sodium taurocholate cotransporter protein and organic anion transporter protein 1. These data indicate that HNF4alpha is central to the maintenance of hepatocyte differentiation and is a major in vivo regulator of genes involved in the control of lipid homeostasis.

SMAD proteins transactivate the human ApoCIII promoter by interacting physically and functionally with hepatocyte nuclear factor 4

Cotransfection of HepG2 cells with SMADs established that SMAD3 and SMAD3-SMAD4 transactivated (15-70-fold) the -890/+24 apoCIII promoter and shorter promoter segments, whereas cotransfection with a dominant negative SMAD4 mutant repressed the apoCIII promoter activity by 50%, suggesting that SMAD proteins participate in apoCIII gene regulation. Transactivation required the presence of a hormone response element, despite the fact that SMADs could not bind directly to it. Cotransfection of SMAD3-SMAD4 along with hepatocyte nuclear factor-4 resulted in a strong synergistic transactivation of the -890/+24 apoCIII promoter, proximal promoter segments, or synthetic promoters containing either the apoCIII enhancer or the proximal apoCIII hormone response element. Inhibition of endogenous hepatocyte nuclear factor-4 synthesis by an antisense ribozyme construct reduced the constitutive activity of the apoCIII promoter in HepG2 cells to 10% and abolished the SMAD-mediated transactivation. Co-immunoprecipitation and GST pull-down assays provided evidence for physical interactions between SMAD3, SMAD4, and hepatic nuclear factor-4. Our findings indicate that transforming growth factor beta and its signal transducer SMAD proteins can modulate gene transcription by novel mechanisms that involve their physical and functional interaction with hepatocyte nuclear factor-4, suggesting that SMAD proteins may play an important role in apolipoprotein gene expression and lipoprotein metabolism.

A new mutation in the HNF4 binding region of the factor VII promoter in a patient with severe factor VII deficiency

Investigation of the molecular basis of a severe factor VII (fVII) deficiency revealed compound heterozygosity in the fVII gene. On the paternal allele the patient had 3 structural gene abnormalities frequently associated with fVII deficiency. A new mutation, a C to T transition at position −55 relative to the translational start site, was found on the maternal allele. The study demonstrates that this mutation partially impeded binding of the transcriptional activator, hepatic nuclear factor 4, to the fVII promoter while greatly reducing reporter gene expression in hepatic cells.

A new mutation in the HNF4 binding region of the factor VII promoter in a patient with severe factor VII deficiency

Investigation of the molecular basis of a severe factor VII (fVII) deficiency revealed compound heterozygosity in the fVII gene. On the paternal allele the patient had 3 structural gene abnormalities frequently associated with fVII deficiency. A new mutation, a C to T transition at position −55 relative to the translational start site, was found on the maternal allele. The study demonstrates that this mutation partially impeded binding of the transcriptional activator, hepatic nuclear factor 4, to the fVII promoter while greatly reducing reporter gene expression in hepatic cells.

Hepatocyte nuclear factor 4alpha regulates the expression of pancreatic beta -cell genes implicated in glucose metabolism and nutrient-induced insulin secretion

Mutations in the HNF4alpha gene are associated with the subtype 1 of maturity-onset diabetes of the young (MODY1), which is characterized by impaired insulin secretory response to glucose in pancreatic beta-cells. Hepatocyte nuclear factor 4alpha (HNF4alpha) is a transcription factor critical for liver development and hepatocyte-specific gene expression. However, the role of HNF4alpha in the regulation of pancreatic beta-cell gene expression and its correlation with metabolism secretion coupling have not been previously investigated. The tetracycline-inducible system was employed to achieve tightly controlled expression of both wild type (WT) and dominant-negative mutant (DN) of HNF4alpha in INS-1 cells. The induction of WT-HNF4alpha resulted in a left shift in glucose-stimulated insulin secretion, whereas DN-HNF4alpha selectively impaired nutrient-stimulated insulin release. Induction of DN-HNF4alpha also caused defective mitochondrial function substantiated by reduced [(14)C]pyruvate oxidation, attenuated substrate-evoked mitochondrial membrane hyperpolarization, and blunted nutrient-generated cellular ATP production. Quantitative evaluation of HNF4alpha-regulated pancreatic beta-cell gene expression revealed altered mRNA levels of insulin, glucose transporter-2, L-pyruvate kinase, aldolase B, 2-oxoglutarate dehydrogenase E1 subunit, and mitochondrial uncoupling protein-2. The patterns of HNF4alpha-regulated gene expression are strikingly similar to that of its downstream transcription factor HNF1alpha. Indeed, HNF4alpha changed the HNF1alpha mRNA levels and HNF1alpha promoter luciferase activity through altered HNF4alpha binding. These results demonstrate the importance of HNF4alpha in beta-cell metabolism-secretion coupling.

Analysis of protein dimerization and ligand binding of orphan receptor HNF4alpha

Hepatocyte nuclear factor 4alpha (HNF4alpha) (NR2A1), an orphan member of the nuclear receptor superfamily, binds DNA exclusively as a homodimer even though it is very similar in amino acid sequence to retinoid X receptor alpha (RXRalpha), which heterodimerizes readily with other receptors. Here, experimental analysis of residues involved in protein dimerization and studies on a reported ligand for HNF4alpha are combined with a structural model of the HNF4alpha ligand-binding domain (LBD) (residues 137 to 384). When K300 (in helix 9) and E327 (in helix 10) of HNF4alpha1 were converted to the analogous residues in RXRalpha (E390 and K417, respectively) the resulting construct did not heterodimerize with the wild-type HNF4alpha, although it was still able to form homodimers and bind DNA. Furthermore, the double mutant did not heterodimerize with RXR or RAR but was still able to dimerize in solution with an HNF4alpha construct truncated at amino acid residue 268. This suggests that the charge compatibility between helices 9 and 10 is necessary, but not sufficient, to determine dimerization partners, and that additional residues in the HNF4alpha LBD are also important in dimerization. The structural model of the HNF4alpha LBD and an amino acid sequence alignment of helices 9 and 10 in various HNF4 and other receptor genes indicates that a K(X)(26)E motif can be used to identify HNF4 genes from other organisms and that a (E/D(X)(26-29)K/R) motif can be used to predict heterodimerization of many, but not all, receptors with RXR. In vitro analysis of another HNF4alpha mutant construct indicates that helix 10 also plays a structural role in the conformational integrity of HNF4alpha. The structural model and experimental analysis indicate that fatty acyl CoA thioesters, the proposed HNF4alpha ligands, are not good candidates for a traditional ligand for HNF4alpha. Finally, these results provide insight into the mechanism of action of naturally occurring mutations in the human HNF4alpha gene found in patients with maturity onset diabetes of the young 1 (MODY1).

Functional properties of the R154X HNF-4alpha protein generated by a mutation associated with maturity-onset diabetes of the young, type 1

Mutations in the hepatocyte nuclear factor 4alpha (HNF-4alpha) gene are associated with one form of maturity-onset diabetes of the young (MODY1). The R154X mutation generates a protein lacking the E-domain which is required for normal HNF-4alpha functions. Since pancreatic beta-cell dysfunction is a feature of MODY1 patients, we compared the functional properties of the R154X mutant in insulin-secreting pancreatic beta-cells and non-beta-cells. The R154X mutation did not affect nuclear localisation in beta-cells and non-beta-cells. However, it did lead to a greater impairment of HNF-4a function in beta-cells compared to non-beta-cells, including a complete loss of transactivation activity and a dominant-negative behaviour. .

Naturally occurring mutations in the human HNF4alpha gene impair the function of the transcription factor to a varying degree

The hepatocyte nuclear factor (HNF)4alpha, a member of the nuclear receptor superfamily, regulates genes that play a critical role in embryogenesis and metabolism. Recent studies have shown that mutations in the human HNF4alpha gene cause a rare form of type 2 diabetes, maturity onset diabetes of the young (MODY1). To investigate the properties of these naturally occurring HNF4alpha mutations we analysed five MODY1 mutations (R154X, R127W, V255M, Q268X and E276Q) and one other mutation (D69A), which we found in HepG2 hepatoma cells. Activation of reporter genes in transfection assays and DNA binding studies showed that the MODY1-associated mutations result in a variable reduction in function, whereas the D69A mutation showed an increased activity on some promoters. None of the MODY mutants acted in a dominant negative manner, thus excluding inactivation of the wild-type factor as a critical event in MODY development. A MODY3-associated mutation in the HNF1alpha gene, a well-known target gene of HNF4alpha, results in a dramatic loss of the HNF4 binding site in the promoter, indicating that mutations in the HNF4alpha gene might cause MODY through impaired HNF1alpha gene function. Based on these data we propose a two-hit model for MODY development.

The orphan nuclear receptor SHP inhibits hepatocyte nuclear factor 4 and retinoid X receptor transactivation: two mechanisms for repression

The orphan nuclear hormone receptor SHP interacts with a number of other nuclear hormone receptors and inhibits their transcriptional activity. Several mechanisms have been suggested to account for this inhibition. Here we show that SHP inhibits transactivation by the orphan receptor hepatocyte nuclear factor 4 (HNF-4) and the retinoid X receptor (RXR) by at least two mechanisms. SHP interacts with the same HNF-4 surface recognized by transcriptional coactivators and competes with them for binding in vivo. The minimal SHP sequences previously found to be required for interaction with other receptors are sufficient for interaction with HNF-4, although deletion results indicate that additional C-terminal sequences are necessary for full binding and coactivator competition. These additional sequences include those associated with direct transcriptional repressor activity of SHP. SHP also competes with coactivators for binding to ligand-activated RXR, and based on the ligand-dependent interaction with other nuclear receptors, it is likely that coactivator competition is a general feature of SHP-mediated repression. The minimal receptor interaction domain of SHP is sufficient for full interaction with RXR, as previously described. This domain is also sufficient for full coactivator competition. Functionally, however, full inhibition of RXR transactivation requires the presence of the C-terminal repressor domain, with only weak inhibition associated with this receptor interaction domain. Overall, these results suggest that SHP represses nuclear hormone receptor-mediated transactivation via two separate steps: first by competition with coactivators and then by direct effects of its transcriptional repressor function.

Modulation of transcriptional activation and coactivator interaction by a splicing variation in the F domain of nuclear receptor hepatocyte nuclear factor 4alpha1

Transcription factors, such as nuclear receptors, often exist in various forms that are generated by highly conserved splicing events. Whereas the functional significance of these splicing variants is often not known, it is known that nuclear receptors activate transcription through interaction with coactivators. The parameters, other than ligands, that might modulate those interactions, however, are not well characterized, nor is the role of splicing variants. In this study, transient transfection, yeast two-hybrid, and GST pulldown assays are used to show not only that nuclear receptor hepatocyte nuclear factor 4 alpha1 (HNF4alpha1, NR2A1) interacts with GRIP1, and other coactivators, in the absence of ligand but also that the uncommonly large F domain in the C terminus of the receptor inhibits that interaction. In vitro, the F domain was found to obscure an AF-2-independent binding site for GRIP1 that did not map to nuclear receptor boxes II or III. The results also show that a natural splicing variant containing a 10-amino-acid insert in the middle of the F domain (HNF4alpha2) abrogates that inhibition in vivo and in vitro. A series of protease digestion assays indicates that there may be structural differences between HNF4alpha1 and HNF4alpha2 in the F domain as well as in the ligand binding domain (LBD). The data also suggest that there is a direct physical contact between the F domain and the LBD of HNF4alpha1 and -alpha2 and that that contact is different in the HNF4alpha1 and HNF4alpha2 isoforms. Finally, we propose a model in which the F domain of HNF4alpha1 acts as a negative regulatory region for transactivation and in which the alpha2 insert ameliorates the negative effect of the F domain. A conserved repressor sequence in the F domains of HNF4alpha1 and -alpha2 suggests that this model may be relevant to other nuclear receptors as well.