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...

No abstract

The transcription factor hepatocyte nuclear factor 1␤ (HNF1␤) is a tissue-specific regulator that also plays an essential role in early development of vertebrates. In humans, four heterozygous mutations in the HNF1␤ gene have been identified that lead to early onset of diabetes and severe primary renal defects. The degree and type of renal defects seem to depend on the specific mutation. We show that the frameshift mutant P328L329fsdelCCTCT associated with nephron agenesis retains its DNA-binding properties and acts as a gain-of-function mutation with increased transactivation potential in transfection experiments. Expression of this mutated factor in the Xenopus embryo leads to defective development and agenesis of the pronephros, the first kidney form of amphibians. Very similar defects are generated by overexpressing in Xenopus the wild-type HNF1␤, which is consistent with the gain-of-function property of the mutant. In contrast, introduction of the human HNF1␤ mutant R137-K161del, which is associated with a reduced number of nephrons with hypertrophy of the remaining ones and which has an impaired DNA binding, shows only a minor effect on pronephros development in Xenopus. Thus, the overexpression of both human mutants has a different effect on renal development in Xenopus, reflecting the variation in renal phenotype seen with these mutations. We conclude that mutations in human HNF1␤ can be functionally characterized in Xenopus. Our findings imply that HNF1␤ not only is an early marker of kidney development but also is functionally involved in morphogenetic events, and these processes can be investigated in lower vertebrates.