CCAAT/enhancer-binding protein alpha (C/EBPalpha) activates transcription of the human microsomal epoxide hydrolase gene (EPHX1) through the interaction with DNA-bound NF-Y

Microsomal epoxide hydrolase 1 (EPHX1): Gene, structure, function, and role in human disease

Microsomal epoxide hydrolase (EPHX1) is an evolutionarily highly conserved biotransformation enzyme for converting epoxides to diols. Notably, the enzyme is able to either detoxify or bioactivate a wide range of substrates. Mutations and polymorphic variants in the EPHX1 gene have been associated with susceptibility to several human diseases including cancer. This review summarizes the key knowledge concerning EPHX1 gene and protein structure, expression pattern and regulation, and substrate specificity. The relevance of EPHX1 for human pathology is especially discussed.

Transcription of the Human Microsomal Epoxide Hydrolase Gene (EPHX1) Is Regulated by PARP-1 and Histone H1.2. Association with Sodium-Dependent Bile Acid Transport

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in the metabolism of numerous xenobiotics as well as mediating the sodium-dependent transport of bile acids into hepatocytes. These compounds are involved in cholesterol homeostasis, lipid digestion, excretion of xenobiotics and the regulation of several nuclear receptors and signaling transduction pathways. Previous studies have demonstrated the critical role of GATA-4, a C/EBPα-NF/Y complex and an HNF-4α/CAR/RXR/PSF complex in the transcriptional regulation of the mEH gene (EPHX1). Studies also identified heterozygous mutations in human EPHX1 that resulted in a 95% decrease in mEH expression levels which was associated with a decrease in bile acid transport and severe hypercholanemia. In the present investigation we demonstrate that EPHX1 transcription is significantly inhibited by two heterozygous mutations observed in the Old Order Amish population that present numerous hypercholanemic subjects in the absence of liver damage suggesting a defect in bile acid transport into the hepatocyte. The identity of the regulatory proteins binding to these sites, established using biotinylated oligonucleotides in conjunction with mass spectrometry was shown to be poly(ADP-ribose)polymerase-1 (PARP-1) bound to the EPHX1 proximal promoter and a linker histone complex, H1.2/Aly, bound to a regulatory intron 1 site. These sites exhibited 71% homology and may represent potential nucleosome positioning domains. The high frequency of the H1.2 site polymorphism in the Amish population results in a potential genetic predisposition to hypercholanemia and in conjunction with our previous studies, further supports the critical role of mEH in mediating bile acid transport into hepatocytes.

Sp1 and Sp3 transcription factors regulate the basal expression of human microsomal epoxide hydrolase (EPHX1) through interaction with the E1b far upstream promoter

Microsomal epoxide hydrolase (mEH, EPHX1) is a critical biotransformation enzyme, catalyzing the metabolism of many xenobiotics. Human mEH is transcribed using alternative promoters. The upstream E1 promoter is active in liver while the far upstream E1b promoter drives the expression of mEH in all tissues, including liver. Although several liver-specific transcription factors have been identified in the regulation of E1 transcription, little is known regarding the mechanisms of E1b transcriptional regulation. Genome-wide mapping of DNase I hypersensitive sites revealed an open chromatin region between nucleotide -300 upstream and +400 downstream of E1b. This area coincides with a previously described promoter region responsible for maintaining high basal promoter activity. In silico analysis of this location revealed several Sp1/Sp3 binding sites. Site-directed mutagenesis of these motifs suppressed the transactivation activity of the E1b proximal promoter, indicating their importance as contributors to E1b promoter regulation. Further, E1b promoter activities were increased significantly following Sp1 and Sp3 overexpression, while Mithramycin A, a selective Sp1 inhibitor, reduced the promoter activities. EMSA studies demonstrated that Sp1 bound to two putative Sp1/Sp3 binding sites. ChIP analysis confirmed that both endogenous Sp1 and Sp3 were bound to the proximal promoter region of E1b. Knockdown of Sp1 expression using siRNA did not alter the endogenous E1b transcriptional level, while knockdown of Sp3 greatly decreased E1b expression in different human cell lines. Taken together, these results support the concept that Sp1 and Sp3 are functionally involved as transcriptional integrators regulating the basal expression of the derived mEH E1b variant transcript.

Proteome-wide dysregulation by glucose-6-phosphate dehydrogenase (G6PD) reveals a novel protective role for G6PD in aflatoxin B₁-mediated cytotoxicity

Glucose-6-phosphate dehydrogenase (G6PD) is pivotal to reduced nicotinamide adenine dinucleotide phosphate (NADPH) production and cellular redox balance. Cells with G6PD deficiency are susceptible to oxidant-induced death at high oxidative stress. However, it remains unclear what precise biological processes are affected by G6PD deficiency due to altered cellular redox homeostasis, particularly at low oxidative stress. To further explore the biological role of G6PD, we generated G6PD-knockdown cell clones using lung cancer line A549. We identified proteins differentially expressed in the knockdown clones without the addition of exogenous oxidant by means of isobaric tags for relative and absolute quantification (iTRAQ) labeling coupled with multidimensional liquid chromatography-mass spectrometry (LC-MS/MS). We validated a panel of proteins that showed altered expression in G6PD-knockdown clones and were involved in metabolism of xenobiotic and glutathione (GSH) as well as energy metabolism. To determine the physiological relevancy of our findings, we investigated the functional consequence of G6PD depletion in cells treated with a prevalent xenobiotic, aflatoxin B₁(AFB₁). We found a protective role of G6PD in AFB₁-induced cytotoxicity, possibly via providing NADPH for NADPH oxidase to induce epoxide hydrolase 1 (EPHX1), a xenobiotic-metabolizing enzyme. Collectively, our findings reveal for the first time a proteome-wide dysregulation by G6PD depletion under the condition without exogenous oxidant challenge, and we suggest a novel association of G6PD activity with AFB₁-related xenobiotic metabolism.

Transcription of the human microsomal epoxide hydrolase gene (EPHX1) is regulated by an HNF-4α/CAR/RXR/PSF complex

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in the metabolism of numerous xenobiotics as well as mediating the sodium-dependent transport of bile acids into hepatocytes where they are involved in cholesterol excretion and metabolism, lipid digestion and regulating numerous signaling pathways. Previous studies have demonstrated the critical role of GATA-4 and a C/EBPα-NF/Y complex in the regulation of the mEH gene (EPHX1). In this study we show that HNF-4α and CAR/RXR also bind to the proximal promoter region and regulate EPHX1 expression. Bile acids, which inhibit the expression of HNF-4α also decrease the expression of EPHX1. Studies also established that the binding of HNF-4α was essential for the activation of EPHX1 activity by CAR suggesting the formation of a complex between these adjacent factors. The nature of this regulatory complex was further explored using a biotinylated oligonucleotide of this region in conjunction with BioMag beads and mass spectrometric analysis which demonstrated the presence of an additional inhibitory factor (PSF), confirmed by co-immunoprecipitation and ChIP analyses, which interacted with DNA-bound CAR/RXR/HNF-4α forming a 4-component regulatory complex.

Interaction of C/EBP-beta and NF-Y factors constrains activity levels of the nutritionally controlled promoter IA expressing the acetyl-CoA carboxylase-alpha gene in cattle

Background

The enzyme acetyl-CoA carboxylase-alpha (ACC-α) is rate limiting for de novo fatty acid synthesis. Among the four promoters expressing the bovine gene, promoter IA (PIA) is dominantly active in lipogenic tissues. This promoter is in principal repressed but activated under favorable nutritional conditions. Previous analyses already coarsely delineated the repressive elements on the distal promoter but did not resolve the molecular nature of the repressor. Knowledge about the molecular functioning of this repressor is fundamental to understanding the nutrition mediated regulation of PIA activity. We analyzed here the molecular mechanism calibrating PIA activity.

Results

We finely mapped the repressor binding sites in reporter gene assays and demonstrate together with Electrophoretic Mobility Shift Assays that nuclear factor-Y (NF-Y) and CCAAT/enhancer binding protein-β (C/EBPβ) each separately repress PIA activity by binding to their cognate low affinity sites, located on distal elements of the promoter. Simultaneous binding of both factors results in strongest repression. Paradoxically, over expression of NFY factors, but also - and even more so - of C/EBPβ significantly activated the promoter when bound to high affinity sites on the proximal promoter. However, co-transfection experiments revealed that NF-Y may eventually diminish the strong stimulatory effect of C/EBPβ at the proximal PIA in a dose dependent fashion. We validated by chromatin immunoprecipitation, that NF-Y and C/EBP factors may physically interact.

Conclusion

The proximal promoter segment of PIA appears to be principally in an active state, since even minute concentrations of both, NF-Y and C/EBPβ factors can saturate the high affinity activator sites. Higher factor concentrations will saturate the low affinity repressive sites on the distal promoter resulting in reduced and calibrated promoter activity. Based on measurements of the mRNA concentrations of those factors in different tissues we propose that the interplay of both factors may set tissue-specific limits for PIA activity.

ATXN8 -62 G/A promoter polymorphism and risk of Taiwanese Parkinson's disease

BACKGROUND AND PURPOSE

We recently reported a novel -62 G/A polymorphism within ataxin 8 (ATXN8) gene promoter region, with -62 G displaying significantly higher luciferase activity compared with -62 A. Phenotypic variability in spinocerebellar ataxia type 8 (SCA8) has been suggested, and large SCA8 repeats were found in patients with Parkinson's disease (PD). We aimed to investigate the association of ATXN8 -62 G/A polymorphism with the risk of Taiwanese PD, and identify the trans-acting factor modulating the ATXN8 promoter activity.

METHODS

A case-control study in a cohort of 569 PD cases and 547 ethnically matched controls was conducted by polymerase chain reaction (PCR) and restriction enzyme analysis. The trans-acting factor binding to the ATXN8 promoter was examined by chromatin immunoprecipitation (ChIP)-PCR assay, cDNA co-transfection and luciferase reporter assay.

RESULTS

When genotype distribution was calculated by comparing the rare AA genotype with the GG + GA genotypes (recessive model), a significant difference was found (P = 0.035, 1 df). Individuals carrying AA genotype exhibited a decreased risk of developing PD (odds ratio: 0.73; 95% CI: 0.55-0.98, P = 0.035). After stratification by age, individuals over 60 years of age carrying AA genotype demonstrated a further decrease in the risk of developing PD (odds ratio: 0.64; 95% CI: 0.43-0.96, P = 0.030). ChIP-PCR and cDNA over-expression revealed that CCAAT/enhancer-binding protein alpha binds to the ATXN8 proximal promoter to upregulate ATXN8 expression in neuroblastoma SK-N-SH cells.

CONCLUSIONS

Our data suggest that ATXN8 -62 G/A polymorphism plays a role in Taiwanese PD susceptibility.

Role of the CCAAT-binding protein NFY in SCA17 pathogenesis

Spinocerebellar ataxia 17 (SCA17) is caused by expansion of the polyglutamine (polyQ) tract in human TATA-box binding protein (TBP) that is ubiquitously expressed in both central nervous system and peripheral tissues. The spectrum of SCA17 clinical presentation is broad. The precise pathogenic mechanism in SCA17 remains unclear. Previously proteomics study using a cellular model of SCA17 has revealed reduced expression of heat shock 70 kDa protein 5 (HSPA5) and heat shock 70 kDa protein 8 (HSPA8), suggesting that impaired protein folding may contribute to the cell dysfunction of SCA17 (Lee et al., 2009). In lymphoblastoid cells, HSPA5 and HSPA8 expression levels in cells with mutant TBP were also significantly lower than that of the control cells (Chen et al., 2010). As nuclear transcription factor Y (NFY) has been reported to regulate HSPA5 transcription, we focused on if NFY activity and HSPA5 expression in SCA17 cells are altered. Here, we show that TBP interacts with NFY subunit A (NFYA) in HEK-293 cells and NFYA incorporated into mutant TBP aggregates. In both HEK-293 and SH-SY5Y cells expressing TBP/Q(61~79), the level of soluble NFYA was significantly reduced. In vitro binding assay revealed that the interaction between TBP and NFYA is direct. HSPA5 luciferase reporter assay and endogenous HSPA5 expression analysis in NFYA cDNA and siRNA transfection cells further clarified the important role of NFYA in regulating HSPA5 transcription. In SCA17 cells, HSPA5 promoter activity was activated as a compensatory response before aggregate formation. NFYA dysfunction was indicated in SCA17 cells as HSPA5 promoter activity reduced along with TBP aggregate formation. Because essential roles of HSPA5 in protection from neuronal apoptosis have been shown in a mouse model, NFYA could be a target of mutant TBP in SCA17.

NF-Y and the transcriptional activation of CCAAT promoters

The CCAAT box promoter element and NF-Y, the transcription factor (TF) that binds to it, were among the first cis-elements and trans-acting factors identified; their interplay is required for transcriptional activation of a sizeable number of eukaryotic genes. NF-Y consists of three evolutionarily conserved subunits: a dimer of NF-YB and NF-YC which closely resembles a histone, and the "innovative" NF-YA. In this review, we will provide an update on the functional and biological features that make NF-Y a fundamental link between chromatin and transcription. The last 25 years have witnessed a spectacular increase in our knowledge of how genes are regulated: from the identification of cis-acting sequences in promoters and enhancers, and the biochemical characterization of the corresponding TFs, to the merging of chromatin studies with the investigation of enzymatic machines that regulate epigenetic states. Originally identified and studied in yeast and mammals, NF-Y - also termed CBF and CP1 - is composed of three subunits, NF-YA, NF-YB and NF-YC. The complex recognizes the CCAAT pentanucleotide and specific flanking nucleotides with high specificity (Dorn et al., 1997; Hatamochi et al., 1988; Hooft van Huijsduijnen et al, 1987; Kim & Sheffery, 1990). A compelling set of bioinformatics studies clarified that the NF-Y preferred binding site is one of the most frequent promoter elements (Suzuki et al., 2001, 2004; Elkon et al., 2003; Mariño-Ramírez et al., 2004; FitzGerald et al., 2004; Linhart et al., 2005; Zhu et al., 2005; Lee et al., 2007; Abnizova et al., 2007; Grskovic et al., 2007; Halperin et al., 2009; Häkkinen et al., 2011). The same consensus, as determined by mutagenesis and SELEX studies (Bi et al., 1997), was also retrieved in ChIP-on-chip analysis (Testa et al., 2005; Ceribelli et al., 2006; Ceribelli et al., 2008; Reed et al., 2008). Additional structural features of the CCAAT box - position, orientation, presence of multiple Transcriptional Start Sites - were previously reviewed (Dolfini et al., 2009) and will not be considered in detail here.

A novel mechanism involving coordinated regulation of nuclear levels and acetylation of NF-YA and Bcl6 activates RGS4 transcription

Neuronally enriched RGS4 plays a critical role attenuating G protein signaling in brain, although the mechanisms regulating RGS4 expression are unknown. Here we describe a novel mechanism for transcriptional activation of RGS4 in neuron-like PC6 cells, where RGS4 is markedly induced during confluence-induced growth arrest. Transcriptional activation of RGS4 in confluent PC6 cells was accompanied by impaired G(i/o)-dependent MAPK activation. In the human RGS4 gene promoter, we identified three phylogenetically conserved cis-elements: an inverted CCAAT box element (ICE), a cAMP response element, and a B-cell lymphoma 6 (Bcl6)-binding site. The ICE and the cAMP response element mediate activation, and the Bcl6 site mediates repression of RGS4 transcription. Activation of RGS4 transcription in confluent PC6 cells is accompanied by increases in NF-YA and C/EBPβ and decreases in Bcl6 levels in the nucleus. Increases in NF-YA and C/EBPβ lead to their increased binding to the RGS4 promoter in vivo, and dominant negative forms of these proteins repressed RGS4 promoter activity. Acetylation of NF-YA and Bcl6 were increased in postconfluent cells. Trichostatin A stimulation of RGS4 promoter activity, accompanied by increased binding of NF-YA and decreased binding of Bcl6 to the promoter, was abolished by mutation of the ICE and enhanced by mutation of the Bcl6 site. These findings demonstrate a dynamic and coordinated regulation of nuclear levels and acetylation status of trans-acting factors critical in determining the off/on state of the RGS4 promoter.

Characterization of the human PANK2 promoter

Pantothenate kinase 2 (PANK2) is an essential regulatory enzyme in coenzyme A biosynthesis. PANK2 mutations cause pantothenate kinase-associated neurodegeneration (PKAN), which leads to pigmentary retinopathy, progressive dystonia and other abnormalities. Two nearly identical PANK2 isoforms have been described: short PANK2 and mature PANK2, which are processed from a precursor isoform. Since the biological relevance of these isoforms remains unclear, we sought to explore their transcriptional regulation. Here we show that their regulation is distinct and describe a promoter for the short isoform of PANK2. Moreover, we identify potential regulators of PANK2 expression, including NF-Y, FOXN4 and the human heterogeneous nuclear ribonucleoprotein A/B family. These findings validate expression of the short PANK2 isoform and enable predictions about potentially deleterious sequence variants in the regulatory region of this human disease gene.

Molecular analysis of the human thrombin-activatable fibrinolysis inhibitor gene promoter

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a carboxypeptidase B-like pro-enzyme that, once activated, attenuates fibrinolysis. Little is presently known of the factors that regulate expression of CPB2, the gene encoding TAFI. This study identified 10 potential transcription factor binding sites (denoted A-J) within the proximal promoter region of CPB2, spanning nucleotides -425 to +21; two of these represent previously-described binding sites for CCAAT/enhancer binding protein and glucocorticoid receptor. We identified additional transcription factors that bind within the proximal CPB2 promoter, namely, nuclear factor-Y (NF-Y) and hepatocyte nuclear factor-1alpha (HNF-1alpha). Binding of NF-Y to the region between nucleotides -76 to -59 (Site B) is important for basal CPB2 promoter activity; NF-Y may be a key factor for the recruitment of the transcriptional machinery to the TAFI gene promoter. HNF-1alpha binds at the interface between Sites C and B. Transient transfections of CPB2 promoter-reporter constructs showed that HNF-1alpha binding is essential for the activity of this promoter in HepG2 cells, indicating that HNF-1alpha is involved in the liver-specific expression of CPB2.

CCAAT/enhancer-binding protein delta (C/EBPdelta) maintains amelogenin expression in the absence of C/EBPalpha in vivo

C/EBPalpha is implicated to regulate mouse amelogenin gene expression during tooth enamel formation in vitro. Because enamel formation occurs during postnatal development and C/EBPalpha-deficient mice die at birth, we used the Cre/loxP recombination system to characterize amelogenin expression in C/EBPalpha conditional knock-out mice. Mice carrying the Cre transgene under the control of the human keratin-14 promoter show robust Cre expression in the ameloblast cell lineage. Mating between mice bearing the floxed C/EBPalpha allele with keratin-14-Cre mice generate C/EBPalpha conditional knock-out mice. Real-time PCR analysis shows that removal of one C/EBPalpha allele from the molar enamel epithelial organ of 3-day postnatal mice results in dramatic decrease in endogenous C/EBPalpha mRNA levels and coordinately altered amelogenin mRNA abundance. Conditional deletion of both C/EBPalpha alleles further diminishes C/EBPalpha mRNA levels; however, rather than ablating amelogenin expression, we observe wild-type amelogenin mRNA abundance levels. We examined C/EBPbeta and nuclear factor YA expression, two transcription factors that had previously been shown to modestly participate in amelogenin expression, in vitro but found no significant changes in either of their mRNA abundance levels comparing conditional knock-out mice with wild-type counterparts. Although the abundance of C/EBPdelta is also unchanged in C/EBPalpha conditional knock-out mice, in vitro we find that C/EBPdelta activates the mouse amelogenin promoter and synergistically cooperates with nuclear factor Y, suggesting that C/EBPdelta can functionally substitute for C/EBPalpha to produce an enamel matrix competent to direct biomineralization.

The bidirectional promoter of two genes for the mitochondrial translational apparatus in mouse is regulated by an array of CCAAT boxes interacting with the transcription factor NF-Y

The genes for mitoribosomal protein S12 (Mrps12) and mitochondrial seryl-tRNA ligase (Sarsm and Sars2) are oppositely transcribed from a conserved promoter region of <200 bp in both human and mouse. Using a dual reporter vector we identified an array of 4 CCAAT box elements required for efficient transcription of the two genes in cultured mouse 3T3 cells, and for enforcing directionality in favour of Mrps12. Electrophoretic mobility shift assay (EMSA) and in vivo footprinting confirmed the importance of these promoter elements as sites of protein-binding, and EMSA supershift and chromatin immunoprecipitation (ChIP) assays identified NF-Y as the key transcription factor involved, revealing a common pattern of protein–DNA interactions in all tissues tested (liver, brain, heart, kidney and 3T3 cells). The inherently bidirectional activity of NF-Y makes it an especially suitable factor to govern promoters of this class, whose expression is linked to cell proliferation.

Apple flavonoids inhibit growth of HT29 human colon cancer cells and modulate expression of genes involved in the biotransformation of xenobiotics

Flavonoids from fruits and vegetables probably reduce risks of diseases associated with oxidative stress, including cancer. Apples contain significant amounts of flavonoids with antioxidative potential. The objectives of this study were to investigate such compounds for properties associated with reduction of cancer risks. We report herein that apple flavonoids from an apple extract (AE) inhibit colon cancer cell growth and significantly modulate expression of genes related to xenobiotic metabolism. HT29 cells were treated with AE at concentrations delivering 5-50 microM of one of the major ingredients, phloridzin ("phloridzin-equivalents," Ph.E), to the cell culture medium, with a synthetic flavonoid mixture mimicking the composition of the AE or with 5-100 microM individual flavonoids. HT29 cell growth was inhibited by the complex extract and by the mixture. HT29 cells were treated with nontoxic doses of the AE (30 microM, Ph.E) and after 24 h total RNA was isolated to elucidate patterns of gene expression using a human cDNA-microarray (SuperArray) spotted with 96 genes of drug metabolism. Treatment with AE resulted in an upregulation of several genes (GSTP1, GSSTT2, MGST2, CYCP4F3, CHST5, CHST6, and CHST7) and downregulation of EPHX1, in comparison to the medium controls. The enhanced transcriptional activity of GSTP1 and GSTT2 genes was confirmed with real-time qRT-PCR. On the basis of the pattern of differential gene expression found here, we conclude that apple flavonoids modulate toxicological defense against colon cancer risk factors. In addition to the inhibition of tumor cell proliferation, this could be a mechanism of cancer risk reduction.

Analysis of folylpoly-gamma-glutamate synthetase gene expression in human B-precursor ALL and T-lineage ALL cells

BACKGROUND

Expression of folylpoly-gamma-glutamate synthetase (FPGS) gene is two- to three-fold higher in B-precursor ALL (Bp- ALL) than in T-lineage ALL (T-ALL) and correlates with intracellular accumulation of methotrexate (MTX) polyglutamates and lymphoblast sensitivity to MTX. In this report, we investigated the molecular regulatory mechanisms directing FPGS gene expression in Bp-ALL and T-ALL cells.

METHODS

To determine FPGS transcription rate in Bp-ALL and T-ALL we used nuclear run-on assays. 5'-RACE was used to uncover potential regulatory regions involved in the lineage differences. We developed a luciferase reporter gene assay to investigate FPGS promoter/enhancer activity. To further characterize the FPGS proximal promoter, we determined the role of the putative transcription binding sites NFY and E-box on FPGS expression using luciferase reporter gene assays with substitution mutants and EMSA.

RESULTS

FPGS transcription initiation rate was 1.6-fold higher in NALM6 vs. CCRF-CEM cells indicating that differences in transcription rate led to the observed lineage differences in FPGS expression between Bp-ALL and T-ALL blasts. Two major transcripts encoding the mitochondrial/cytosolic and cytosolic isoforms were detected in Bp-ALL (NALM6 and REH) whereas in T-ALL (CCRF-CEM) cells only the mitochondrial/cytosolic transcript was detected. In all DNA fragments examined for promoter/enhancer activity, we measured significantly lower luciferase activity in NALM6 vs. CCRF-CEM cells, suggesting the need for additional yet unidentified regulatory elements in Bp-ALL. Finally, we determined that the putative transcription factor binding site NFY, but not E-box, plays a role in FPGS transcription in both Bp- and T-lineage.

CONCLUSION

We demonstrated that the minimal FPGS promoter region previously described in CCRF-CEM is not sufficient to effectively drive FPGS transcription in NALM6 cells, suggesting that different regulatory elements are required for FPGS gene expression in Bp-cells. Our data indicate that the control of FPGS expression in human hematopoietic cells is complex and involves lineage-specific differences in regulatory elements, transcription initiation rates, and mRNA processing. Understanding the lineage-specific mechanisms of FPGS expression should lead to improved therapeutic strategies aimed at overcoming MTX resistance or inducing apoptosis in leukemic cells.

NF-Y and CCAAT/enhancer-binding protein alpha synergistically activate the mouse amelogenin gene

Amelogenin is the major protein component of the forming enamel matrix. In situ hybridization revealed a periodicity for amelogenin mRNA hybridization signals ranging from low to high transcript abundance on serial sections of developing mouse teeth. This in vivo observation led us to examine the amelogenin promoter for the activity of transcription factor(s) that account for this expression aspect of the regulation for the amelogenin gene. We have previously shown that CCAAT/enhancer-binding protein alpha (C/EBPalpha) is a potent transactivator of the mouse X-chromosomal amelogenin gene acting at the C/EBPalpha cis-element located in the -70/+52 minimal promoter. The minimal promoter contains a reversed CCAAT box (-58/-54) that is four base pairs downstream from the C/EBPalpha binding site. Similar to the C/EBPalpha binding site, the integrity of the reversed CCAAT box is also required for maintaining the activity of the basal promoter. We therefore focused on transcription factors that interact with the reversed CCAAT box. Using electrophoretic mobility shift assays we demonstrated that NF-Y was directly bound to this reversed CCAAT site. Co-transfection of C/EBPalpha and NF-Y synergistically increased the promoter activity. In contrast, increased expression of NF-Y alone had only marginal effects on the promoter. A dominant-negative DNA binding-deficient NF-Y mutant (NF-YAm29) dramatically decreased the promoter activity both in the absence or presence of exogenous expression of C/EBPalpha. We identified protein-protein interactions between C/EBPalpha and NF-Y by a co-immunoprecipitation analysis. These results suggest that C/EBPalpha and NF-Y synergistically activate the mouse amelogenin gene and can contribute to its physiological regulation during amelogenesis.

Transcription of brain creatine kinase in U87-MG glioblastoma is modulated by factor AP2

Our previous studies established in U87-MG glioblastoma cells that elevated cAMP increased transcription of the endogenous as well as a transiently-transfected brain creatine kinase (CKB) gene, despite the absence of a cAMP response element (CRE) in the CKB proximal promoter. This report employed transfection to show that the transcription of CKB in U87 cells is induced by transcription factor AP2alpha, which is known to be activated by cAMP. Dominant-negative forms of AP2alpha not only prevented the AP2alpha-mediated activation of CKB but also blocked the cAMP-mediated increase in CKB transcription caused by forskolin treatment. The mutation of the four potential AP2 elements within the CKB proximal promoter showed that induction of CKB by AP2 was mediated principally through the AP2 element located at -50 bp in the promoter. Electromobility shift assays revealed a protein in U87 nuclear extracts that bound to a consensus AP2alpha element as well as to the (-50) AP2 element in CKB. Interestingly, the CKB (-50) AP2 element contains GCCAATGGG which also bound NF-Y, the CCAAT-binding protein, suggesting that interplay between AP2 and NF-Y may modulate CKB transcription. This is the first report of a role for AP2 in the regulation of CKB transcription and of an AP2 element within which an NF-Y site is located.

Hepatic CCAAT/enhancer binding protein alpha mediates induction of lipogenesis and regulation of glucose homeostasis in leptin-deficient mice

CCAAT/enhancer binding protein alpha (C/EBP alpha) is a critical factor in glucose metabolism in the neonate as revealed by conventional C/EBP alpha-null mice that do not survive beyond the first day after birth because of severe hypoglycemia and a deficiency in hepatic glycogen accumulation. To elucidate the function of C/EBP alpha in leptin-deficient mouse (ob/ob) liver, a C/EBP alpha-liver null mouse on an ob/ob background (ob/ob-C/EBP alpha/Cre(+)) was produced using a floxed C/EBP alpha allele and Cre recombinase under control of the albumin promoter (AlbCre). The C/EBP alpha-deficient liver in ob/ob mice had significantly decreased triglyceride content compared with equivalent mice lacking the AlbCre transgene (ob/ob-C/EBP alpha/Cre(-)). Expression of genes involved in lipogenesis including fatty acid synthase, acetyl-coenzyme A carboxylase, stearoyl-coenzyme A desaturase 1 and ATP-citrate lyase dramatically decreased in ob/ob-C/EBP alpha/Cre(+) mouse liver. Induction of these lipogenic genes by a high-carbohydrate diet caused an exacerbation in the development of fatty liver and an increase in liver size, hepatic triglyceride, and cholesterol contents in ob/ob-C/EBP alpha/Cre(-) mice but not in ob/ob-C/EBP alpha/Cre(+) mice. Deficiency in hepatic C/EBP alpha expression caused an exacerbation of hyperglycemia because of decreased insulin secretion. Taken together, these results indicate that hepatic C/EBP alpha plays a critical role in the acceleration of lipogenesis in ob/ob mice and in glucose homeostasis by the indirect regulation of insulin secretion.