BAP, a rat liver protein that activates transcription through a promoter element with similarity to the USF/MLTF binding site

Inhibitor of the tissue-specific transcription factor HNF4, a potential regulator in early Xenopus development

Hepatocyte nuclear factor 4alpha (HNF4alpha) is an orphan receptor of the nuclear receptor superfamily and expressed in vertebrates as a tissue-specific transcription factor in liver, kidney, intestine, stomach, and pancreas. It also plays a crucial role in early embryonic development and has been identified as a maternal component in the Xenopus egg. We now report on an activity present in Xenopus embryos that inhibits the DNA binding of HNF4. This HNF4 inhibitor copurifies with a 25-kDa protein under nondenaturing conditions but can be separated from this protein by sodium dodecyl sulfate treatment. Protease treatment of the inhibitor results in a core fragment of about 5 kDa that retains full inhibitory activity. The activity of the HNF4 inhibitor can also be monitored in the absence of DNA, as it alters the mobility of the HNF4 protein in native polyacrylamide gels and the accessibility of antibodies. Comparing the activity of the HNF4 inhibitor with acyl coenzyme A's, recently proposed to be ligands of HNF4, we observe a more stringent specificity for the HNF4 inhibitor activity. Using deletion constructs of the HNF4 protein, we could show that the potential ligand-binding domain of HNF4 is not required, and thus the HNF4 inhibitor does not represent a classical ligand as defined for the nuclear receptor superfamily. Based on our previous finding that maternal HNF4 is abundantly present in Xenopus embryos but the target gene HNF1alpha is only marginally expressed, we propose that the HNF4 inhibitor functions in the embryo to restrict the activity of the maternal HNF4 proteins.

p53 suppresses cytokine induced, Stat5 mediated activation of transcription

p53 is a tumor suppressor which exerts its function through the regulation of genes mediating cell cycle arrest and the induction of apoptosis. Cellular survival and proliferation can be positively regulated through the action of cytokines. These signals act through the activation of cell surface receptors, and the phosphorylation of intracellular signaling components, e.g. members of the Stat family (signal transducers and activators of transcription). The signaling effects of p53 and the cytokine receptors on the cellular phenotype are counteracting. We investigated the influence of p53 on the transactivation potential of Stat5. p53 repressed the prolactin induction of the Stat5 mediated transcription of the beta-casein promoter-luciferase reporter gene, but did not affect IFN-gamma induced, Stat1 dependent transcription of the IRF-1 promoter. The inhibition was not due to a decrease in the cellular concentration of Stat5 or to interference with its specific DNA binding activity. No repression of the basal transcriptional activity of the beta-casein promoter was observed. p53 mutants defective in their DNA binding or oligomerization functions had only weak inhibitory effects, but a mutant of p53 in the transactivation domain, efficiently repressed Stat5 dependent induction. The repressive function of p53 on Stat5 activity is independent of the amino-terminal transactivation domain, but requires a functional DNA binding domain and the carboxyl-terminal domain. Our experiments show that p53 counteracts Stat5 mediated cytokine induction of gene transcription. The effect is specific for Stat5 and independent of p53 induced apoptosis.

Regulation of the vitellogenin gene B1 promoter after transfer into hepatocytes in primary cultures

The estrogen-dependent and tissue-specific regulation of the Xenopus laevis vitellogenin gene B1 promoter has been studied by lipid-mediated DNA transfer into Xenopus hepatocytes in primary culture. Hepatocytes achieve an efficient hormonal control of this promoter through a functional interaction between the estrogen responsive elements and a promoter proximal region upstream of the TATA box, which is characterized by a high density of binding sites for the transcription factors CTF/NF-1, C/EBP and HNF3. DNA accessibility to restriction enzymes within the chromosomal copy of the vitellogenin gene B1 promoter shows that the estrogen responsive unit and the promoter proximal region are sensitive to digestion in uninduced and estrogen-induced hepatocytes but not in erythrocyte nuclei. Together, these findings support the notion that chromatin configuration as well as the interplay of promoter elements mediate proper hormone-dependent and tissue-specific expression of the B1 vitellogenin gene.

An alternative splice variant of the tissue specific transcription factor HNF4alpha predominates in undifferentiated murine cell types

The transcription factor hepatocyte nuclear factor 4alpha (HNF4alpha) is a tissue specific transcription factor mainly expressed in the liver, kidney, intestine and the endocrine pancreas, but is also an essential regulator for early embryonic events. Based on its protein structure HNF4alpha is classified as an orphan member of the nuclear receptor superfamily. Comparing HNF4alpha transcription factors in the differentiated and dedifferentiated murine hepatocyte cell line MHSV-12 we identified in dedifferentiated cells the novel splice variant HNF4alpha7. This variant is characterized by an alternative first exon and has a lower transactivation potential in transient transfection assays using HNF4 dependent reporter genes. HNF4alpha7 mRNA and the corresponding protein are expressed in the undifferentiated pluripotent embryonal carcinoma cell line F9, whereas HNF4alpha1 only appears after differentiation of F9 cells to visceral endoderm. HNF4alpha7 mRNA is also found in totipotent embryonic stem cells. However, the function of HNF4alpha7 seems not to be restricted to embryonic cells as the HNF4alpha7 mRNA is also present in adult tissues, most notably the stomach. All these features suggest that the presence of distinct splice variants of HNF4alpha modulates the activity of HNF4alphain a cell type specific way.

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.

Developmental expression of the maternal protein XDCoH, the dimerization cofactor of the homeoprotein LFB1 (HNF1)

The tissue-specific transcription factors LFB1 (HNF1) and LFB3 (vHNF1) mainly expressed in liver, kidney and intestine are homeoproteins that interact with the regulatory element HP1. The HP1 sequence constitutes one of the most important cis-acting elements in liver-specifically expressed genes, while its function in other cell types containing LFB1 and LFB3 is not fully understood. In mammals, LFB1 activity is modulated by DCoH, a cofactor that stimulates the LFB1 transactivation significantly. Using the rat cDNA probe, we cloned the corresponding Xenopus sequence XDCoH, encoding a 104 amino acid protein, that is 85% identical to the rat protein. XDCoH enhances the LFB1-dependent transactivation potential in transfection experiments and interacts in vitro directly with LFB1 and its variant form LFB3. The protein is detectable in liver and kidney extracts of adult frogs and in small amounts also in lung and stomach, organs expressing LFB1 and/or LFB3 protein as well. To investigate the possible involvement of XDCoH in Xenopus development, we analyzed its temporal and spatial expression pattern during early embryogenesis. XDCoH is a maternal factor, although LFB1 is absent in the egg. In early cleavage stages, the protein is detectable in the cytoplasm of each blastomere and enters the nuclei of the cells as early as the zygotic transcription in the Xenopus embryo starts. The amount of XDCoH increases dramatically following neurulation, when the formation of liver, pronephros and other organs takes place. Whole-mount immunostaining demonstrates that, in the developing larvae, XDCoH is localized in the nuclei of the hepatocytes, the gut cells and the pronephric cells, tissues of mesodermal and endodermal origin known to contain LFB1 and LFB3. Surprisingly it is also present in the pigmented epithelium surrounding the eye of the embryo, which is derived from the anterior part of the ectodermal neural plates and lacks LFB1. The tissue distribution of XDCoH during embryogenesis suggests that XDCoH is involved in determination and differentiation of various unrelated cell types. It seems likely that XDCoH interaction is not only essential for the function of LFB1 and LFB3 but also for certain other transcription factors.

Elements and factors involved in tissue-specific and embryonic expression of the liver transcription factor LFB1 in Xenopus laevis

LFB1 (HNF1) is a tissue-specific transcription factor found in the livers, stomachs, intestines, and kidneys of vertebrates. By analyzing the promoter of the Xenopus LFB1 gene, we identified potential autoregulation by LFB1 and regulation by HNF4, a transcription factor with a tissue distribution similar to that of LFB1. Injection of LFB1 promoter-chloramphenicol acetyltransferase constructs into Xenopus eggs revealed embryonic activation that is restricted to the region of the developing larvae expressing endogeneous LFB1. Proper embryonic activation was also observed with a rat LFB1 promoter. Deletion analysis of the Xenopus and rat promoters revealed that in both promoters embryonic activation is absolutely dependnet on the presence of an element that contains CCNCTCTC as the core consensus sequence. Since this element is recognized by the maternal factor OZ-1 previously described by N. Ovsenek, A. M. Zorn, and P. A. Krieg (Development 115:649-655, 1992), we might have identified the main constituents of a hierarchy that leads via LFB1 to the activation of tissue-specific genes during embryogenesis.

Elements and factors involved in tissue-specific and embryonic expression of the liver transcription factor LFB1 in Xenopus laevis

LFB1 (HNF1) is a tissue-specific transcription factor found in the livers, stomachs, intestines, and kidneys of vertebrates. By analyzing the promoter of the Xenopus LFB1 gene, we identified potential autoregulation by LFB1 and regulation by HNF4, a transcription factor with a tissue distribution similar to that of LFB1. Injection of LFB1 promoter-chloramphenicol acetyltransferase constructs into Xenopus eggs revealed embryonic activation that is restricted to the region of the developing larvae expressing endogeneous LFB1. Proper embryonic activation was also observed with a rat LFB1 promoter. Deletion analysis of the Xenopus and rat promoters revealed that in both promoters embryonic activation is absolutely dependnet on the presence of an element that contains CCNCTCTC as the core consensus sequence. Since this element is recognized by the maternal factor OZ-1 previously described by N. Ovsenek, A. M. Zorn, and P. A. Krieg (Development 115:649-655, 1992), we might have identified the main constituents of a hierarchy that leads via LFB1 to the activation of tissue-specific genes during embryogenesis.

Developmental regulation and tissue distribution of the liver transcription factor LFB1 (HNF1) in Xenopus laevis

The transcription factor LFB1 (HNF1) was initially identified as a regulator of liver-specific gene expression in mammals. It interacts with the promoter element HP1, which is functionally conserved between mammals and amphibians, suggesting that a homologous factor, XLFB1, also exists in Xenopus laevis. To study the role of LFB1 in early development, we isolated two groups of cDNAs coding for this factor from a Xenopus liver cDNA library by using a rat LFB1 cDNA probe. A comparison of the primary structures of the Xenopus and mammalian proteins shows that the myosin-like dimerization helix, the POU-A-related domain, the homeo-domain-related region, and the serine/threonine-rich activation domain are conserved between X. laevis and mammals, suggesting that all these features typical for LFB1 are essential for function. Using monoclonal antibodies, we demonstrate that XLFB1 is present not only in the liver but also in the stomach, intestine, colon, and kidney. In an analysis of the expression of XLFB1 in the developing Xenopus embryo, XLFB1 transcripts appear at the gastrula stage. The XLFB1 protein can be identified in regions of the embryo in which the liver diverticulum, stomach, gut, and pronephros are localized. The early appearance of XLFB1 expression during embryogenesis suggests that the tissue-specific transcription factor XLFB1 is involved in the determination and/or differentiation of specific cell types during organogenesis.

Developmental regulation and tissue distribution of the liver transcription factor LFB1 (HNF1) in Xenopus laevis

The transcription factor LFB1 (HNF1) was initially identified as a regulator of liver-specific gene expression in mammals. It interacts with the promoter element HP1, which is functionally conserved between mammals and amphibians, suggesting that a homologous factor, XLFB1, also exists in Xenopus laevis. To study the role of LFB1 in early development, we isolated two groups of cDNAs coding for this factor from a Xenopus liver cDNA library by using a rat LFB1 cDNA probe. A comparison of the primary structures of the Xenopus and mammalian proteins shows that the myosin-like dimerization helix, the POU-A-related domain, the homeo-domain-related region, and the serine/threonine-rich activation domain are conserved between X. laevis and mammals, suggesting that all these features typical for LFB1 are essential for function. Using monoclonal antibodies, we demonstrate that XLFB1 is present not only in the liver but also in the stomach, intestine, colon, and kidney. In an analysis of the expression of XLFB1 in the developing Xenopus embryo, XLFB1 transcripts appear at the gastrula stage. The XLFB1 protein can be identified in regions of the embryo in which the liver diverticulum, stomach, gut, and pronephros are localized. The early appearance of XLFB1 expression during embryogenesis suggests that the tissue-specific transcription factor XLFB1 is involved in the determination and/or differentiation of specific cell types during organogenesis.

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.