Hepatocyte nuclear factor 4 (HNF-4), a highly conserved member of the steroid hormone receptor superfamily critical for development and liver-specific gene expression, is very similar to another superfamily member, retinoid X receptor ␣ (RXR␣), in overall amino acid sequence and DNA binding specificity. Since RXR␣ is known to heterodimerize with many other nuclear receptors, the formation of heterodimers between HNF-4 and RXR␣ was examined. With the electrophoretic mobility shift assay, coimmunoprecipitation, and transient transfection assays, it is shown that, unlike other nuclear receptors, HNF-4 does not form heterodimers with RXR␣ either in the presence or in the absence of DNA. We also show that in vitro-translated HNF-4 does not form heterodimeric complexes on DNA with a number of other receptors, including RXR␤, RXR␥, retinoic acid receptor ␣, or thyroid hormone receptor ␣. To investigate the hypothesis that the lack of heterodimerization between HNF-4 and RXR␣ is due to a strong homodimerization activity of HNF-4, glycerol gradient sedimentation and kinetic analysis were used to show that HNF-4 is in fact a stable homodimer in solution. Finally, immunohistochemistry is used to show that the HNF-4 protein is found exclusively in the nuclei in both HepG2 cells, which express endogenous HNF-4, and transfected COS cells, which overexpress HNF-4. These findings lead us to propose that HNF-4 defines a new subclass of nuclear receptors which reside primarily in the nucleus and which bind DNA and regulate transcription as homodimers.Hepatocyte nuclear factor 4 (HNF-4) is a positive-acting transcription factor which is expressed very early in embryo development and is essential to liver development and function (reviewed in references 84 and 85). Mouse HNF-4 mRNA appears in the primary endoderm of implanting blastocysts at embryonic day 4.5 and in the liver and gut primordia at day 8.5 (20), while mice deficient in HNF-4 do not survive past day 9 postcoitus (12). HNF-4 has also been proposed to be responsible for the final commitment for cells to differentiate into hepatocytes (68). In adult rodents, HNF-4 is located primarily in the liver, kidney, and intestine, and in insects HNF-4 is found in the equivalent tissues (86, 95). HNF-4 is known to activate a wide variety of essential genes, including those involved in cholesterol, fatty acid, and glucose metabolism; blood coagulation; detoxification mechanisms; hepatitis B virus infections; and liver differentiation (reviewed in references 84 and 85).HNF-4 is a member of the superfamily of ligand-dependent transcription factors, which includes the steroid hormone receptors, thyroid hormone receptor (TR), vitamin A receptor, and vitamin D receptor (VDR), as well as a large number of receptors for which ligands have not yet been identified, the so-called orphan receptors (reviewed in references 56, 72, 73, and 88). All receptors are characterized by two conserved domains: the zinc finger region, which mediates DNA binding, and a large hydrophobic domain which mediate...

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.

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Recent studies have shown that mutations in the hepatocyte nuclear factor (HNF)-4alpha gene give rise to maturity-onset diabetes of the young, type 1 (MODY1). HNF-4, an orphan member of the nuclear receptor superfamily, contains a DNA-binding domain (DBD) and a putative ligand-binding domain (LBD) that can act independently of each other. The first MODY1 mutation identified creates a stop codon at amino acid 268 in the LBD of HNF-4 (Q268X) that leaves the DBD intact, suggesting that the mutant protein may retain some of the properties of the wild-type protein. To determine the functional properties of this mutant, we constructed HNF4.Q268X and tested it in vitro and in vivo for DNA binding, protein dimerization, and transactivation activity. Results of an electrophoretic mobility shift assay showed that HNF4.Q268X neither binds DNA alone nor binds it as a dimer with wild-type HNF-4 (HNF4.wt). In contrast, a co-immunoprecipitation assay showed that HNF4.Q268X is capable of dimerizing in solution with HNF4.wt. Transient transfection assays, however, indicated that HNF4.Q268X does not affect transactivation by HNF4.wt in vivo, supporting the argument against a dominant negative effect. Additional results suggest that the lack of a dominant negative effect could be due to a striking differential subcellular localization of the HNF4.Q268X protein: HNF4.Q268X could be extracted from transfected cells only when treated with SDS. Taken together, our results suggest that the MODY1 phenotype is due to a loss of functional HNF-4 protein that is aggravated in tissues that express relatively low amounts of HNF-4, such as pancreas.