Primary structure, chromosomal mapping, expression and transcriptional activity of murine hepatocyte nuclear factor 4gamma

We demonstrate the presence of a new member of the orphan nuclear receptor hepatocyte nuclear factor 4 (HNF4) subfamily in mouse which is genetically distinct from the previously characterized mouse HNF4alpha gene. The new member of the HNF4 subfamily shows highest amino acid identity, similar tissue distribution and syntenous chromosomal localization to the recently described human HNF4gamma (NR2A2), we therefore classify it as mouse HNF4gamma (mHNF4gamma). A combination of RT-PCR and immunohistochemical analysis showed expression of mHNF4gamma mRNA and protein in the endocrine pancreas, testes, kidney and gut. By co-transfection experiments, we show that mHNF4gamma is able to activate transcription, acting through binding sites that have been previously characterized as HNF4alpha binding sites. The presence of HNFgamma in human and mouse implies that a complex transcriptional network exists in higher vertebrates involving a number of HNF4 members with overlapping yet distinct function and tissue distribution.

HNF4beta, a new gene of the HNF4 family with distinct activation and expression profiles in oogenesis and embryogenesis of Xenopus laevis

The transcription factor hepatocyte nuclear factor 4 (HNF4) is an orphan member of the nuclear receptor superfamily expressed in mammals in liver, kidney, and the digestive tract. Recently, we isolated the Xenopus homolog of mammalian HNF4 and revealed that it is not only a tissue-specific transcription factor but also a maternal component of the Xenopus egg and distributed within an animal-to-vegetal gradient. We speculate that this gradient cooperates with the vegetally localized embryonic induction factor activin A to activate expression of HNF1alpha, a tissue-specific transcription factor with an expression pattern overlapping that of HNF4. We have now identified a second Xenopus HNF4 gene, which is more distantly related to mammalian HNF4 than the previously isolated gene. This new gene was named HNF4beta to distinguish it from the known HNF4 gene, which is now called HNF4alpha. By reverse transcription-PCR, we detected within the 5' untranslated region of HNF4beta two splice variants (HNF4beta2 and HNF4beta3) with additional exons, which seem to affect RNA stability. HNF4beta is a functional transcription factor acting sequence specifically on HNF4 binding sites known for HNF4alpha, but it seems to have a lower DNA binding activity and is a weaker transactivator than the alpha isoform. Furthermore, the two factors differ with respect to tissue distribution in adult frogs: whereas HNF4alpha is expressed in liver and kidney, HNF4beta is expressed in addition in stomach, intestine, lung, ovary, and testis. Both factors are maternal proteins and present at constant levels throughout embryogenesis. However, using reverse transcription-PCR, we found the RNA levels to change substantially: whereas HNF4alpha is expressed early during oogenesis and is absent in the egg, HNF4beta is first detected in the latest stage of oogenesis, and transcripts are present in the egg and early cleavage stages. Furthermore, zygotic HNF4alpha transcripts appear in early gastrula and accumulate during further embryogenesis, whereas HNF4beta mRNA transiently appears during gastrulation before it accumulates again at the tail bud stage. All of these distinct characteristics of the newly identified HNF4 protein imply that the alpha and beta isoform have different functions in development and in adult tissues.

Human renal cell carcinogenesis is accompanied by a coordinate loss of the tissue specific transcription factors HNF4 alpha and HNF1 alpha

Human renal cell carcinogenesis is usually accompanied by dedifferentiation processes including the loss of expression of tissue specifically expressed genes. Based on the hypothesis that these dedifferentiation processes might be attributed to a functional change in tissue specific transcription factors, we have analyzed the expression and function of the tissue specific transcription factor HNF4 alpha in human renal cell carcinomas. By Western blot analysis and gel retardation assay using HNF4 alpha specific antibodies, we observed that in most cases the amount as well as the binding activity of HNF4 is reduced in the tumor samples compared to the corresponding normal tissues. Furthermore, we found a clear correlation between the HNF4 alpha binding activity and the amount of another transcription factor (HNF1 alpha), which is thought to be transcriptionally activated by HNF4 alpha. We therefore speculate that disruption of the HNF4 alpha/HNF1 alpha pathway of kidney specific gene expression might be an important molecular mechanism in renal cell carcinogenesis.

Human hepatocyte nuclear factor 4 isoforms are encoded by distinct and differentially expressed genes

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 tissue-specifically 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 alpha 2, as well as a previously unknown splice variant of this protein, which we called HNF alpha 4. More importantly, we also cloned a novel HNF4 subtype (HNF4 gamma) derived from a different gene and showed that the genes encoding HNF 4 alpha and HNF4 gamma are located on human chromosomes 20 and 8, respectively. Northern (RNA) blot analysis revealed that HNF4 GAMMA is expressed in the kidney, pancreas, small intestine, testis, and colon but not in the liver, while HNF4 alpha RNA was found in all of these tissues. By cotransfection experiments in C2 and HeLa cells, we showed that HNF4 gamma is significantly less active than HNF4 alpha 2 and that the novel HNF4 alpha splice variant HNF4 alpha 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.

Estrogen-inducible derivatives of hepatocyte nuclear factor-4, hepatocyte nuclear factor-3 and liver factor B1 are differently affected by pure and partial antiestrogens

The tissue-specific transcription factors of the hepatocyte nuclear factor-4 (HNF4), hepatocyte nuclear factor-3 (HNF3), and liver factor B1 (LFB1) families are thought to play a role in the development of internal organs and in the tissue-specific expression of many distinct genes. We have now constructed derivatives of these proteins by introducing the hormone-binding domain of the estrogen receptor and show that in transient transfections these chimeric proteins act as estrogen-inducible transcription factors with the DNA sequence specificity of the original factors. These chimeric transcription factors are differently affected by the partial estrogen antagonist 4-hydroxytamoxifen and the pure antiestrogen N-n-butyl-11-(3,17-dihydroxy-estra-1,3,5(10)-trien- 7 alpha-yl)N-methyl-undecamide (ICI 164384); 4-hydroxytamoxifen activates, at least partially, all the chimeric factors and the estrogen receptor, while ICI 164384 surprisingly activates the transcription factors derived from HNF3 and LFB1 and inhibits only the estrogen receptor and the HNF4 derivative. Together with the DNA-sequence-binding specificity, the different response to estrogen and anti-estrogens makes our estrogen receptor fusion proteins useful tools for the investigation of the roles of HNF4, HNF3 and LFB1 in gene expression, differentiation and developmental processes.

Liver specific transcription factors of the HNF3-, C/EBP- and LFB1-families interact with the A-activator binding site

The A-activator binding site (AABS), present in the Xenopus A2 vitellogenin gene and several mammalian liver specifically expressed genes, interacts with different liver specific transcription factors including LFB1- and C/EBP-isobinders. We have now isolated some additional proteins interacting with AABS and show that they are HNF3-isobinders. The interactions between AABS and members of the HNF3 family are confirmed by binding studies using bacterially made HNF3-alpha protein. Thus a short DNA module of 24 bp is able to bind proteins of three different families of liver specific transcription factors. Competition experiments in the cell free in vitro transcription show that AABS dependent transcriptional activation is mediated by transcription factors belonging to at least two different families, the C/EBP- and the HNF3-isobinders. Being able to mediate the action of several distinct transactivators, AABS may thus be a prototype for a novel kind of tissue specific promoter modules with unique regulatory capacities.

Cloning and expression of a human voltage-gated potassium channel. A novel member of the RCK potassium channel family

We have isolated and characterized a human cDNA (HBK2) that is homologous to novel member (RCK2) of the K+ channel RCK gene family expressed in rat brain. RCK2 mRNA was detected predominantly in midbrain areas and brainstem. The primary sequences of the HBK2/RCK2 K+ channel proteins exhibit major differences to other members of the RCK gene family. The bend region between segments S1 and S2 is unusually long and does not contain the N-glycosylation site commonly found in this region. They might be O-glycosylated instead. Functional characterization of the HBK2/RCK2 K+ channels in Xenopus laevis oocytes following micro-injection in in vitro transcribed HBK2 or RCK2 cRNA showed that the HBK2/RCK2 proteins form voltage-gated K+ channels with novel functional and pharmacological properties. These channels are different to RCK1, RCK3, RCK4 and RCK5 K+ channels.