Human Hepatocyte Nuclear Factor 4 Isoforms Are Encoded by Distinct and Differentially Expressed Genes

Abstract

Hepatocyte nuclear factor 4 (HNF4) was first identified as a DNA binding activity in rat liver nuclear extracts. Protein purification had then led to the cDNA cloning of rat HNF4, which was found to be an orphan member of the nuclear receptor superfamily. Binding sites for this factor were identified in many tissuespecifically expressed genes, and the protein was found to be essential for early embryonic development in the mouse. We have now isolated cDNAs encoding the human homolog of the rat and mouse HNF4 splice variant HNF4␣2, as well as a previously unknown splice variant of this protein, which we called HNF4␣4. More importantly, we also cloned a novel HNF4 subtype (HNF4␥) derived from a different gene and showed that the genes encoding HNF4␣ and HNF4␥ are located on human chromosomes 20 and 8, respectively. Northern (RNA) blot analysis revealed that HNF4␥ is expressed in the kidney, pancreas, small intestine, testis, and colon but not in the liver, while HNF4␣ RNA was found in all of these tissues. By cotransfection experiments in C2 and HeLa cells, we showed that HNF4␥ is significantly less active than HNF4␣2 and that the novel HNF4␣ splice variant HNF4␣4 has no detectable transactivation potential. Therefore, the differential expression of distinct HNF4 proteins may play a key role in the differential transcriptional regulation of HNF4-dependent genes.While intensive research during the last decade has established that tissue-specific gene expression is regulated to a large extent at the level of transcription, it has also become clear that most if not all of the tissue-specific transcription factors involved in this regulation are not restricted to a single tissue. Furthermore, the molecular cloning of these factors has shown that they are usually encoded by gene families with a multitude of subtypes and splice variants, and it is believed that it is the intra-and interfamily interplay of the cell-type-specific subset of these transcription factors that determines the identity of a certain tissue.Analysis of the regulatory regions of liver-specifically expressed genes, for example, has led to the identification of three gene families of transcription factors (i.e., the C/EBP, hepatocyte nuclear factor 1 [HNF1], and HNF3 families) and of another factor (HNF4) for which up to now splice variants but no subtypes derived from different genes had been identified (for reviews, see references 13, 19, and 25). All of these transcription factors originally considered to be liver specific are now known to be expressed also in other organs, but their combined expression is a unique feature of the liver (24).The factor HNF4 was originally identified in rat liver nuclear extracts as a protein binding to a DNA element of the transthyretin promoter (5). Protein purification and cDNA cloning revealed that HNF4 is an orphan member of the nuclear receptor superfamily with a zinc finger DNA binding domain and a putative ligand binding domain (20). Binding sites for HNF4 have been found in the regulatory regions of many ge...

A transcription factor regulatory circuit in differentiated pancreatic cells

“…4A, and not shown). Hnf4␥ is a nuclear receptor structurally related to hnf4␣ but expressed at only very low levels in liver (26,27). Hnf4␥ mRNA is enriched in islet cells within the pancreas, but is present and exhibits hnf1␣ dependence in both endocrine and exocrine compartments (Fig.…”

Identification of the Flavonoid Luteolin as a Repressor of the Transcription Factor Hepatocyte Nuclear Factor 4α

“…HNF4␣ is expressed at high levels in the liver and expressed at a lesser degree in the intestine, kidney, and pancreas (2). The HNF4␣ homodimer binds to the HNF4␣-responsive element in DNA, consisting of a direct repeat of the core sequence AGGTCA, separated by either one or two nucleotides, referred to as direct repeat 1 (DR-1) or direct repeat 2 (DR-2), respectively.…”

CREB-binding Protein Is a Transcriptional Coactivator for Hepatocyte Nuclear Factor-4 and Enhances Apolipoprotein Gene Expression

“…Originally identified by its importance in the regulation of liver-specific genes, it is also expressed in the pancreas, kidney, stomach, skin, and intestine (1-3). There are currently three family members HNF-4␣, -␤, and -␥ (4,5), and seven splice variants of HNF-4␣ have been identified from human, rat, and mouse cDNAs (6 -11). HNF-4 is a crucial regulator of several metabolic pathways, including those for glucose and lipid homeostasis.…”