Targeted Disruption of the Gene Encoding Hepatocyte Nuclear Factor 3γ Results in Reduced Transcription of Hepatocyte-Specific Genes

Abstract

The winged helix transcription factor hepatocyte nuclear factor 3␥ (HNF3␥) is expressed in embryonic endoderm and its derivatives liver, pancreas, stomach, and intestine, as well as in testis and ovary. We have generated mice carrying an Hnf3g-lacZ fusion which deletes most of the HNF3␥ coding sequence as well as 5.5 kb of 3 flanking region. Mice homozygous for the mutation are fertile, develop normally, and show no morphological defects. The mild phenotype change of the Hnf3g ؊/؊ mice can be explained in part by an upregulation of HNF3␣ and HNF3␤ in the liver of the mutant animals. Analysis of steady-state mRNA levels as well as transcription rates showed that levels of expression of several HNF3 target genes (phosphoenolpyruvate carboxykinase, transferrin, tyrosine aminotransferase) were reduced by 50 to 70%, indicating that HNF3␥ is an important activator of these genes in vivo.Transcription factors control gene expression in adult liver and hepatoma cell lines. These factors contain various structural motifs to allow for high-specificity DNA binding; among them are the divergent homeodomain proteins hepatocyte nuclear factor 1␣ (HNF1␣) and HNF1␤; the winged helix proteins HNF3␣, -␤, and -␥; the leucine zipper proteins C/EBP␣ and -␤; the orphan nuclear receptor HNF4; and the D site binding protein (DBP) (reviewed in references 5 and 32). Liver-specific gene expression cannot be achieved through the action of any individual transcription factor, because none of these factors is expressed exclusively in liver. When cis-regulatory elements of liver-specific genes were analyzed in detail, binding sites for multiple transcription factors were found. Therefore, the combinatorial action of the regulatory proteins most likely produces the stringency of hepatic gene expression.The HNF3 proteins were discovered by their ability to bind to the promoters of the genes encoding ␣1-antitrypsin (␣1-AT) and transthyretin (TTR) (8). Cloning of the cDNAs identified three HNF3 genes (HNF3␣, -␤, and -␥) in mammals (14, 15). The HNF3 genes are closely related to the Drosophila melanogaster gene forkhead, which is essential for the proper formation of the foregut and hindgut in flies (30). Therefore, it has been suggested that the HNF3 genes function in mammalian liver and gut development (15). This hypothesis is supported by the observation that the HNF3 genes are expressed very early during the formation of definite endoderm, from which liver and gut are derived (3,19,27).During formation of the definite endoderm, HNF3␤ is activated first, followed by HNF3␣, and finally HNF3␥ (3,19,27). The three HNF3 genes have different anterior boundaries of expression in the definite endoderm, suggesting that they are involved in the regionalization of the primitive gut tube. In addition, HNF3␤ is expressed in the node, and HNF3␣ and HNF3␤ are both expressed in the notochord and floorplate (3,19,27). HNF3␤ is absolutely required for notochord and floorplate formation, because these structures are missing in embryos homozygous for a targeted n...

Embryonic development of the liver†

“…This is due either to functional redundancy between individual FoxA and GATA factor proteins or an essential role for some of these factors prior to the onset of hepatic development. [61][62][63][64][65][66][67][68][69][70] For example, development of Gata6 Ϫ/Ϫ mouse embryos is blocked before hepatic specification due to an indispensable requirement for GATA6 in extraembryonic endoderm differentiation. 63,65 This early embryonic lethality has, however, recently been circumvented by providing Gata6 Ϫ/Ϫ embryos with a wild-type extra-embryonic endoderm via tetraploid embryo complementation.…”

“…73,81,128,129 These include the hepatocyte nuclear factors HNF1␣ and ␤, c/EBP␣, HNF4␣, and FoxA, which act in combination to control aspects of hepatocyte differentiation and liver function. 67,73,[130][131][132][133][134][135][136][137][138] Of these transcriptional regulators, the nuclear hormone receptor HNF4␣ appears to be particularly potent in controlling hepatocyte differentiation. 133,134,139 Hnf4 knockout embryos die before the onset of liver development due to a crucial role for this factor in regulating extra-embryonic endoderm function.…”

Transcriptional regulation of liver development

“…These include HNF-1␣ and -␤ (vHNF-1), c/EBP family, HNF-3␣, -␤ and -␥, HNF-4␣, and more recently HNF-6 (Cereghini, 1996; Lemaigre et al, 1996;Samadani and Costa, 1996). Gene-disruption studies in mice have revealed that many of these transcription factors have crucial roles in regulating liver function and metabolism in adult or newborn mice (Wang et al, 1995;Pontoglio et al, 1996;Croniger et al, 1997;Tanaka et al, 1997;Kaestner et al, 1998;Lee et al, 1998;Kaestner et al, 1999;Shih et al, 1999). Somewhat surprisingly, however, liver development is relatively unaffected in the majority of these mutant mouse strains.…”

Does chasing selected ‘Fox’ to the nucleus prevent diabetes?

“…They are also inactivated by insulin, similar to Foxa2 [13,20]. Increased longevity has been reported in Caenorhabditis elegans that are deficient for insulin signaling cascades and mice with white-adiposetissue-specific ablation of the insulin receptor [19]. Caloric restriction also causes similar increased longevity through Sir2/SIRT1-mediated deacetylation and the subsequent activation of the Foxo transcription factors, inducing genes involved in cell growth arrest and anti-oxidative stress (Figure 1) [20].…”

“…Foxa1 is a potent transactivator mediating glucagon gene expression [16,17], whereas Foxa3 regulates glucose homeostasis during a prolonged fast through the maintenance of hepatic Glut2 and gluconeogenic gene expression [18,19].…”