Chicken ovalbumin upstream promoter-transcription factor, hepatocyte nuclear factor 3, and CCAAT/enhancer binding protein control the far-upstream enhancer of the rat alpha-fetoprotein gene

Manipulation of OCT4 Levels in Human Embryonic Stem Cells Results in Induction of Differential Cell Types

To fully understand self-renewal and pluripotency and their regulation in human embryonic stem cells (hESCs), it is necessary to generate genetically modified cells and analyze the consequences of elevated and reduced expression of genes. Genes expressed in hESCs using plasmid vectors, however, are subject to silencing. Moreover, hESCs have a low plating efficiency when dissociated to single cells, making creation of subcloned lines inefficient. In addition to overexpression experiments, it is important to perform loss-of-function studies, which can be achieved rapidly using RNA interference (RNAi). We report stable long-term expression of enhanced green fluorescent protein (eGFP) in hESCs using a lentiviral vector, and establishment of an eGFP-expressing subline (RG6) using manual dissection. To demonstrate the efficacy of RNAi in hESCs, an RNAi expression vector was used to achieve reduced expression of eGFP in hESCs. To evaluate the role of OCT4 in the regulation of hESC self-renewal and differentiation, a vector expressing a hairpin RNA targeting endogenous expression of OCT4 was constructed. In a novel experiment in hESCs, the OCT4 cDNA sequence was cloned into an expression vector to allow for the transient upregulation of OCT4 in hESCs. The ability to manipulate levels of OCT4 above and below enodogenous levels allows the determination of OCT4 function in hESCs. Specifically, reduced expression of OCT4 in hESCs promoted upregulation of markers indicative of mesoderm and endoderm differentiation, and elevated levels of OCT4 in hESCs promoted upregulation of markers indicative of endoderm derivatives. Thus, both upregulation and downregulation of Oct4 in hESCs results in differentiation, but with patterns distinct from parallel experiments in mice.

Pericentral activity of alpha-fetoprotein enhancer 3 and glutamine synthetase upstream enhancer in the adult liver are regulated by β-catenin in mice

We previously showed that mouse alpha-fetoprotein (AFP) enhancer 3 activity is highly restricted to pericentral hepatocytes in the adult liver. Here, using transgenic mice, we show that the upstream enhancer of the rat glutamine synthetase gene is also active, specifically in pericentral regions. Activity of both enhancers is lost in the absence of β-catenin, a key regulator of zonal gene expression in the adult liver. Both enhancers contain a single, highly conserved T-cell factor/lymphoid enhancer factor binding site that is required for responsiveness to β-catenin. We also show that endogenous AFP messenger RNA levels in the perinatal liver are lower when β-catenin is reduced.

CONCLUSION

These data identify the first distinct zonally active regulatory regions required for β-catenin responsiveness in the adult liver, and suggest that postnatal AFP repression and the establishment of zonal regulation are controlled, at least in part, by the same factors.

Pathophysiologic role of hepatocyte nuclear factor 6

Hepatocyte nuclear factor 6 (HNF6) is one of liver-enriched transcription factors. HNF6 utilizes the bipartite onecut-homeodomain sequence to localize the HNF6 protein to the nuclear compartment and binds to specific DNA sequences of numerous target gene promoters. HNF6 regulates an intricate network and mediates complex biological processes that are best known in the liver and pancreas. The function of HNF6 is correlated to cell proliferation, cell cycle regulation, cell differentiation and organogenesis, cell migration and cell-matrix adhesion, glucose metabolism, bile homeostasis, inflammation and so on. HNF6 controls the transcription of its target genes in different ways. The details of the regulatory pathways and their mechanisms are still under investigation. Future study will explore HNF6 novel functions associated with apoptosis, oncogenesis, and modulation of the inflammatory response. This review highlights recent progression pertaining to the pathophysiologic role of HNF6 and summarizes the potential mechanisms in preclinical animal models. HNF6-mediated pathways represent attractive therapeutic targets for the treatment of the relative diseases such as cholestasis.

Zhx2 and Zbtb20: novel regulators of postnatal alpha-fetoprotein repression and their potential role in gene reactivation during liver cancer

The mouse alpha-fetoprotein (AFP) gene is abundantly expressed in the fetal liver, normally silent in the adult liver but is frequently reactivated in hepatocellular carcinoma. The basis for AFP expression in the fetal liver has been studied extensively. However, the basis for AFP reactivation during hepatocarcinogenesis is not well understood. Two novel factors that control postnatal AFP repression, Zhx2 and Zbtb20, were recently identified. Here, we review the transcription factors that regulate AFP in the fetal liver, as well as Zhx2 and Zbtb20, and raise the possibility that the loss of these postnatal repressors may be involved in AFP reactivation in liver cancer.

The mouse alpha-albumin (afamin) promoter is differentially regulated by hepatocyte nuclear factor 1α and hepatocyte nuclear factor 1β

Alpha-albumin (AFM), a member of the albumin gene family that also includes albumin, alpha-fetoprotein, and vitamin D-binding protein, is expressed predominantly in the liver and activated at birth. Here, we identify two hepatocyte nuclear factor 1 (HNF1)-binding sites in the AFM promoter that are highly conserved in different mammals. These two sites bind HNF1α and HNF1β. The distal site (centered at -132, relative to AFM exon 1) is more important than proximal site (centered at -58), based on HNF1 binding and mutational analysis in transfected cells. Our data indicate that HNF1α is a more potent activator of AFM promoter than is HNF1β. However, HNF1β can act in a dominant manner to inhibit HNF1α-dependent transactivation of the AFM promoter when both proteins are expressed together. This suggests that the differential timing with which the albumin family genes are activated in the liver may be influenced by their responsiveness to HNF1α and HNF1β. Our comparison of HNF1-binding sites in the promoters in the albumin family of genes indicates that the primordial albumin-like gene contained two HNF1 sites; one of these sites was lost from the albumin promoter, but both sites still are present in other members of this gene family.

Regulatory role of CCAAT/enhancer binding protein-beta in the production of plasma proteins in yolk sac tumor

Yolk sac tumor (endodermal sinus tumor) is a malignant germ cell tumor characterized by AFP production, in which histologic foci similar to hepatocellular carcinoma occasionally coexist. We assumed a possible contribution of CCAAT/enhancer binding protein (C/EBP)-beta, a transcription factor implicated in the regulation of plasma proteins in the liver, to the regulation of AFP production and to the expression of other plasma proteins in yolk sac tumor cells because our immunohistochemical analysis revealed nuclear expression of C/EBP-beta in human yolk sac tumors. Overexpression of C/EBP-beta in a rat yolk sac tumor cell line, AT-2-TC, increased production of AFP and other plasma proteins, including albumin, alpha-1-antitrypsin, hepatoglobin, and transferrin. Liver-enriched transcription factors, including hepatocyte nuclear factors (HNF)-1alpha, -1 beta, and -4, were also induced. The induction of this protein expression was only evident in xenografts, where C/EBP-beta was phosphorylated and the activating isoform of C/EBP-beta was relatively predominant. These results indicate that C/EBP-beta plays a role in the production of plasma proteins of yolk sac tumors.

Striking differences between the mouse and the human alpha-fetoprotein enhancers

The alpha-fetoprotein (AFP) gene is expressed abundantly in the fetal liver and transcriptionally repressed in the adult liver, but can be reactivated during liver regeneration and in liver tumors. Previous studies identified three enhancers, E1, E2, and E3, upstream of the mouse and rat Afp genes and a single enhancer upstream of the human gene. We have compared the sequences upstream of the rodent and primate AFP genes. Our analysis demonstrates that the previously identified human enhancer is the counterpart to mouse E2. This comparison also reveals that a functional primate counterpart to the rodent E1 is absent due to a deletion that removes the core region of this enhancer. Furthermore, our studies identify a novel human enhancer corresponding to rodent E3. Despite the overall similarity of E3 between human and mouse, we found differences in transcription factor binding sites between these species. A C/EBP binding site is conserved but two other motifs in rodent E3, one that binds orphan nuclear receptors and a second that binds FoxA proteins, are not conserved in humans. The human counterpart to the rodent FoxA site can bind COUP-TF factors. Despite the overall sequence similarity in E3 between mice and humans, the difference in factor binding sites in E3, as well as the absence of E1 in primates, indicates that different mechanisms regulate AFP transcription in these different species.

Inhibition of human m-epoxide hydrolase gene expression in a case of hypercholanemia

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in carcinogen metabolism and is also able to mediate the sodium-dependent uptake of bile acids into hepatocytes. Studies have identified a subject (S-1) with extremely elevated serum bile salt levels in the absence of observable hepatocellular injury, suggesting a defect in bile acid uptake. In this individual, mEH protein and mEH mRNA levels were reduced by approximately 95% and 85%, respectively, whereas the expression and amino acid sequence of another bile acid transport protein (NTCP) was unaffected. Sequence analysis of the mEH gene (EPHX1) revealed a point mutation at an upstream HNF-3 site (allele I) and in intron 1 (allele II), which resulted in a significant decrease in EPHX1 promoter activity in transient transfection assays. Gel shift assays using a radiolabeled oligonucleotide from each region resulted in specific transcription factor binding patterns, which were altered in the presence of the mutation. These studies demonstrate that the expression of mEH is greatly reduced in a patient with hypercholanemia, suggesting that mEH participates in sodium-dependent bile acid uptake in human liver where its absence may contribute to the etiology of this disease.

Hepatocyte nuclear factor-6 stimulates transcription of the alpha-fetoprotein gene and synergizes with the retinoic-acid-receptor-related orphan receptor alpha-4

The rat alpha-fetoprotein ( afp ) gene is controlled by three enhancers whose function depends on their interaction with liver-enriched transcription factors. The afp enhancer III, located at -6 kb, is composed of three regions that act in synergy. Two of these regions, called s1 and s2, contain a putative binding site for hepatocyte nuclear factor-6 (HNF-6). This factor is the prototype of the ONECUT family of cut-homoeodomain proteins and is a known regulator of liver gene expression in adults and during development. We show here that the two splicing isoforms of HNF-6 bind to a site in the s1 region and in the s2 region. The core sequence of the s1 site corresponds to none of the known HNF-6 binding sites. Nevertheless, the binding properties of the s1 site are identical with those of the s2 site and of previously characterized HNF-6 binding sequences. The HNF-6 consensus should therefore be rewritten as DRRTCVATND. Binding of HNF-6 to the s1 and s2 sites requires both the cut and the homoeo domains, is co-operative and induces DNA bending. HNF-6 strongly stimulates the activity of the afp enhancer III in transient transfection experiments. This effect requires the stereo-specific alignment of the two HNF-6 sites. Moreover, HNF-6 stimulates the enhancer in synergy with the retinoic-acid-receptor-related orphan receptor alpha (RORalpha), which binds to a neighbouring site in the s1 region. Thus expression of the afp gene requires functional interactions between HNF-6 molecules and between HNF-6 and RORalpha.

The mouse alpha-fetoprotein promoter is repressed in HepG2 hepatoma cells by hepatocyte nuclear factor-3 (FOXA)

The mouse alpha-fetoprotein gene is expressed at high levels in the fetal liver and is transcriptionally silenced at birth. The repression is governed, at least in part, by the 250 base pair (bp) AFP promoter. We show here that the AFP promoter is dramatically repressed by HNF3 in HepG2 hepatoma cells. This repression is governed by the region between -205 and -150. Furthermore, this fragment can confer HNF3-mediated repression on a heterologous promoter. The repression is abolished by a mutation that is centered at -165. EMSA analyses using in vivo and in vitro synthesized proteins indicate that HNF3 proteins do not bind DNA from the -205 to -150 region. We propose that HNF3 represses AFP promoter activity through indirect mechanisms that modulate the binding or activity of a liver-enriched factor that interacts with the -165 region of the AFP promoter.

Alpha-fetoprotein structure and function: relevance to isoforms, epitopes, and conformational variants

Mammalian alpha-fetoprotein (AFP) is classified as a member of the albuminoid gene superfamily consisting of albumin, AFP, vitamin D (Gc) protein, and alpha-albumin. Molecular variants of AFP have long been reported in the biomedical literature. Early studies identified isoelectric pH isoforms and lectin-binding variants of AFP, which differed in their physicochemical properties, but not in amino acid composition. Genetic variants of AFP, differing in mRNA kilobase length, were later extensively described in rodent models during fetal/perinatal stages, carcinogenesis, and organ regeneration. With the advent of monoclonal antibodies in the early 1980s, multiple antigenic epitopes on native AFP were detected and categorized, culminating in the identification of six to seven major epitopes. During this period, various AFP-binding proteins and receptors were reported to inhibit certain AFP immunoreactions. Concomittantly, human and rodent AFP were cloned and the amino acid sequences of the translated proteins were divulged. Once the amino acid composition of the AFP molecule was known, enzymatic fragments could be identified and synthetic peptide segments synthesized. Following discovery of the molten globule form in 1981, the existence of transitory, intermediate forms of AFP were acknowledged and their physiological significance was realized. In the present review, the various isoforms and variants of AFP are discussed in light of their potential biological relevance.

Modulation of the far-upstream enhancer of the rat alpha-fetoprotein gene by members of the ROR alpha, Rev-erb alpha, and Rev-erb beta groups of monomeric orphan nuclear receptors

Expression of the oncodevelopmental alpha-fetoprotein (AFP) gene is tightly regulated and occurs in the yolk sac, fetal liver and intestine, and cancerous liver cells. Transcription of the AFP gene is under the control of three enhancers that are very tissue specific. We have shown that the most upstream of these enhancers, located at -6 kb, works through the combined action of liver-enriched factors and nuclear receptors that bind to three regions of this DNA regulatory element. This study showed that orphan nuclear receptors of the ROR alpha, Re-verb alpha, and Rev-erb beta groups can bind as monomers with high affinity and specificity to an evolutionarily conserved AGGTCA motif in the functionally important region 1 of this AFP enhancer. Transient transfection experiments performed with human HepG2 hepatoma cells showed that overproduction of ROR alpha 4 stimulated the activity of the AFP enhancer in a dose-dependent manner, while that of Rev-erb alpha and Rev-erb beta had the opposite effect. These effects were highly specific and required the integrity of the AGGTCA motif. The action of these nuclear receptors also occurred in the context of the entire 7-kb regulatory region of the rat AFP gene. These results suggest that altering the amounts or activities of these orphan receptors in cells of hepatic or endodermal origin could modulate AFP gene expression in response to a variety of developmental or carcinogenic stimuli.

Position-dependent activity of alpha -fetoprotein enhancer element III in the adult liver is due to negative regulation

alpha-Fetoprotein (AFP) transcription is activated early in hepatogenesis, but is dramatically repressed within several weeks after birth. AFP regulation is governed by multiple elements including three enhancers termed EI, EII, and EIII. All three AFP enhancers continue to be active in the adult liver, where EI and EII exhibit high levels of activity in pericentral hepatocytes with a gradual reduction in activity in a pericentral-periportal direction. In contrast to these two enhancers, EIII activity is highly restricted to a layer of cells surrounding the central veins. To test models that could account for position-dependent EIII activity in the adult liver, we have analyzed transgenes in which AFP enhancers EII and EIII were linked together. Our results indicate that the activity of EIII is dominant over that of EII, indicating that EIII is a potent negative regulatory element in all hepatocytes except those encircling the central veins. We have localized this negative activity to a 340-bp fragment. This suggests that enhancer III may be involved in postnatal AFP repression.

A winged-helix family member is involved in a steroid hormone-triggered regulatory circuit

A common theme emerging in eukaryotic gene regulation is that maximal gene induction requires several transcription factors acting in concert to regulate the activation of critical genes. Increasingly, nuclear receptors play key roles in orchestrating this regulation, often by integrating additional signaling pathways, through complex regulatory elements known as hormone response units. The ovalbumin gene contains one such unit, known as the steroid-dependent regulatory element. The binding of the chicken ovalbumin induced regulatory protein-I (Chirp-I) to this element occurs only in response to treatment with estrogen and glucocorticoid. Evidence presented herein demonstrates that Chirp-I has many features in common with the winged-helix (W-H) family of transcription factors. The binding sites for Chirp-I and for the W-H proteins have similar sequence recognition requirements. Northern blots establish that members of the W-H family are expressed in oviduct. Most convincing, the Chirp-I complex interacts with two different antibodies specific to W-H family members. The culmination of this work supports the hypothesis that Chirp-I is a member of the W-H family, and it lends credence to the idea that W-H proteins are essential components of some steroid hormone regulatory circuits.

Growth and differentiation of rat hepatocytes: changes in transcription factors HNF-3, HNF-4, STAT-3, and STAT-5

The liver enriched transcription factors HNF-3 and HNF-4 are known to play major roles in development and differentiation of hepatocytes. STAT-3 and STAT-5 are signaling peptides activated by a variety of cytokines and growth factors including HGF and EGF. Their role in hepatocyte growth and differentiation is yet to be determined. We examined protein expression and DNA binding activities of these transcription factors in a hepatocyte culture system in which the hepatocytes first de-differentiate and proliferate. Overlaying proliferating hepatocytes with EHS-matrix led to an increase in HNF-4 protein and DNA-binding activity. STAT-5 DNA binding activity was only slightly effected by EHS-matrix. HNF-3 and STAT-3 DNA-binding activities were reduced in the presence of EHS-matrix. This is consistent with the role of HNF-3 as the major initiating transcription factor involved in embryonic liver development and suggests, that STAT-3 might also play a role in growth and differentiation of hepatocytes.

Glucocorticoids are insufficient for neonatal gene induction in the liver

Glucocorticoids and their receptor (GR) play a key role in perinatal gene induction. In the liver, the GR is essential for the neonatal induction of a number of genes, including that coding for tyrosine aminotransferase (TAT). To assess the function of the GR in the perinatal period, we have compared the activity of two types of glucocorticoid responsive elements in transgenic mice; one is the Tat gene glucocorticoid-responsive unit (GRU), an assembly of numerous binding sites for transcription factors, including the GR; the other is a simple dimer of high-affinity GR binding sites (GREs). Both elements confer strong glucocorticoid response in the adult liver. However, only the Tat GRUs are able to promote neonatal induction; the GRE dimer is unresponsive. Because this dimer is responsive to glucocorticoid administration in the neonate, the absence of neonatal induction is not due to the inactivity of the GR at this stage. At birth, the neonate has to withstand a brief period of starvation and hypoglycemia, a nutritional and hormonal situation that resembles fasting in the adult. In transgenic mice, the responses at birth and after fasting in the adult are similar: the Tat GRUs but not the dimeric GREs are activated. Our results show that, in rodents, glucocorticoids are not sufficient for neonatal gene induction in the liver and support the conclusion that the hypoglycemia at birth is the main trigger for expression.