Analysis of the DNA-binding and activation properties of the human transcription factor AP-2

Regulation of the murine alpha(2)(I) collagen promoter by retinoic acid and retinoid X receptors

Retinoic acid decreases collagen production by hepatic stellate cells. This study investigated the effects of retinoic acid receptor beta (RARbeta) and retinoid X receptor alpha (RXRalpha) on the regulation of the alpha(2)(I) collagen promoter. Retinoic acid and the RARbeta and RXRalpha expression vectors suppressed the promoter in transfected stellate cells with maximal suppression obtained when combined. Mutation of the retinoic acid response element (RARE) at -879 to -874 (site 1) enhanced promoter activity and diminished but did not eliminate the suppression by RARbeta and RXRalpha. Mutation of another RARE site (site 2), at -930 to -911, resulted in low activity that was inhibited by retinoic acid. Mutation of the AP-2-binding site enhanced promoter activity that was inhibited by retinoic acid. This study shows that the suppressive effect of retinoic acid on the promoter is maximal with a combination of RARbeta and RXRalpha and occurs at more than one RARE site. The effect of retinoic acid is not mediated by AP-2.

Transcription factor AP-2 is an essential and direct regulator of epidermal development in Xenopus

Expression of the Xenopus homolog of the mammalian transcription factor AP-2alpha (XAP-2) is activated throughout the animal hemisphere shortly after the midblastula transition, and becomes restricted to prospective epidermis by the end of gastrulation, under the control of BMP signal modulation. Elevated expression in the future neural crest region begins at this time. Ectopic expression of XAP-2 can restore transcription of epidermal genes in neuralized ectoderm, both in ectodermal explants and in the intact embryo. Likewise, loss of XAP-2 function, accomplished by injection of antisense oligonucleotides or by overexpression of antimorphic XAP-2 derivatives, leads to loss of epidermal and gain of neural gene expression. These treatments also result in gastrulation failure. Thus, AP-2 is a critical regulator of ectodermal determination that is required for normal epidermal development and morphogenesis in the frog embryo.

Ectopic expression of AP-2alpha transcription factor in the lens disrupts fiber cell differentiation

AP-2alpha is a developmentally important transcription factor which has been implicated in the regulation of cell growth, programmed cell death, and differentiation. To investigate the specific function of AP-2alpha in differentiation of the lens, AP-2alpha was expressed in the differentiating lens fiber cells under control of the alphaA-crystallin promoter. Normally, AP-2alpha is selectively expressed in lens epithelial cells and expression terminates at the lens equator, where epithelial cells terminally differentiate into fiber cells. Ectopic expression of the AP-2alpha gene in the fiber cell compartment resulted in bilateral cataracts and microphthalmia in mice by 2 weeks of age. Histological evaluation of embryonic and adult transgenic lenses revealed a significant reduction in lens size and anterior shifting of the transitional zone. Two aspects of fiber cell differentiation were also blocked, including the migration of newly formed fiber cells and an inhibition in fiber cell denucleation. Correlated with these defects were expanded expression of E-cadherin in the lens transitional zone and reduced expression of the fiber cell-specific protein MIP (major intrinsic protein). Together, these data demonstrate that AP-2alpha acts as a negative regulator of terminal fiber cell differentiation through the regulation of genes involved in cell adhesion and migration.

Exencephaly in a subset of animals heterozygous for AP-2alpha mutation

BACKGROUND

Transcription factor AP-2alpha has been implicated as a cell-type-specific regulator of gene expression during vertebrate embryogenesis based on its expression pattern in neural crest cells, ectoderm, and the nervous system in mouse, chick, and frog embryos. AP-2alpha is prominently expressed in cranial neural crest cells, a population of cells migrating from the lateral margins of the neural folds during closure of the neural tube in E (embryonic day of development) 8-9 mouse embryos. Homozygous AP-2alpha mutant mice die perinatally with cranio-abdominoschisis, full facial clefting, and defects in cranial ganglia and sensory organs.

METHODS

Mice heterozygous for the AP-2alpha mutation on a 129/Sv strain were crossed with wildtype mice from the strain 129/Ola. The resulting embryos were genotyped, examined and used for histological analysis.

RESULTS

A subset of animals heterozygous for the AP-2alpha mutation develop a midbrain exencephaly after the mutation was crossed for one generation in the 129/Ola mouse strain. Up to 14% of the animals show a failure of the cranial neural folds to close resulting in a partial exencephaly, all of them being heterozygous for the mutation. The affected animals show reduced rostrocaudal dimensions of the skull and malformations of the bones of the cranial vault. The neural tube defects vary from pure midbrain exencephaly to a forebrain/midbrain exencephaly where the proliferating neural tissue covers the eyes completely.

CONCLUSIONS

The results support a role of AP-2alpha in the etiology of exencephalic disorders. The phenotype observed might be due to a downregulation of the remaining allele suggesting the presence of an upstream modifier gene.

Up-regulation of acid sphingomyelinase during retinoic acid-induced myeloid differentiation of NB4, a human acute promyelocytic leukemia cell line

All-trans-retinoic acid (ATRA) induces myeloid differentiation of a human promyelocytic leukemia cell line, NB4, but does not affect its subclone NB4/RA harboring a point-mutated ligand-binding domain (AF2) in retinoic acid receptor alpha (RARalpha) gene. We found that ATRA induced the 4-fold elevation of acid sphingomyelinase (ASMase) activity 24 h after treatment in NB4 cells, but not in NB4/RA cells. ATRA did not affect neutral sphingomyelinase activity in either NB4 or NB4/RA. Upon treatment with ATRA, ceramide, the product of an ASMase reaction, accumulated in NB4 cells. Northern blot analysis showed a marked elevation of the ASMase mRNA 8 h after ATRA treatment, reaching a plateau at 24 h. Regulation of ASMase gene expression was studied by a promoter analysis using luciferase reporter assay. The 5'-upstream flanking region of human ASMase gene (-519/+300) conjugated with the luciferase gene was introduced into COS-7 cells. Luciferase activity in transformed cells markedly increased in response to ATRA stimulation when the wild type RARalpha or the PML/RARalpha hybrid protein was co-expressed. Deletion experiments revealed that a short sequence at the 5'-end (-519/-485) was indispensable for the ATRA response. Within this short region, two retinoic acid-responsive element-like motifs (TGCCCG and TCTCCT) and one AP2-like motif (CCCTTCCC) were identified. Deletion and base-substitution experiments showed that all three motifs are required for the full expression induced by ATRA. Electrophoresis mobility shift assays with the nuclear extract of ATRA-treated NB4 cells showed that proteins were bound specifically to the probe being mediated by all three motifs in the promoter sequence.

Molecular cloning and functional analysis of the human Na(+)/H(+) exchanger NHE3 promoter

Na(+)/H(+) exchanger (NHE) isoforms NHE2 and NHE3, colocalized to the brush border membrane of the epithelial cells, exhibit differences in their pattern of tissue expression and regulation by various molecular signals. To investigate the mechanisms involved in regulation of NHE3 gene expression, the human NHE3 promoter region was cloned and characterized. Primer extension experiments located the transcription start site to a position 116 nucleotides upstream from the translation start codon. The 5'-flanking region lacked a CCAAT box but contained a TATA-like sequence. Nucleotide sequencing of the 5'-flanking region revealed the presence of a number of cis elements including Sp1, AP-2, MZF-1, CdxA, Cdx-2, steroid and nonsteroid hormone receptor half sites, and a phorbol 12-myristate 13-acetate-response element. Transient transfection experiments using C2/bbe cell line defined a maximal promoter activity in -95/+5 region. The regulatory response elements clustered within this region include a potential transcription factor IID (TF IID), a CACCC, two Sp1, and two AP-2 motifs. Deletion of a fragment containing the AP-2 and Sp1 motifs resulted in a drastic decrease in promoter activity. In gel mobility shift assays, an oligonucleotide spanning from -78 to -56 bp bound a recombinant AP-2, and the corresponding binding activity in nuclear extracts was supershifted with anti-AP2alpha antibody. Our studies suggest that the NHE3 expression is regulated by a combination of cis elements and their cognate transcription factors that include the AP-2 and Sp1 family members.

Transcriptional regulation of the human manganese superoxide dismutase gene: the role of specificity protein 1 (Sp1) and activating protein-2 (AP-2)

Manganese superoxide dismutase (MnSOD) plays an important role in regulating cellular redox conditions. Expression of MnSOD has been shown to protect against damage by oxidative stress and to suppress the malignant phenotype of human cancer cells. We have previously cloned the human MnSOD (SOD2) gene and analysed its 5' proximal promoter, which has been characterized by a lack of a TATA or CAAT box and the presence of multiple GC boxes. To define further the molecular mechanisms for the regulation of MnSOD expression, multiple transcription factor-binding motifs containing overlapping specificity protein 1 (Sp1)- and activator protein (AP)-2-binding sites were identified by DNase I footprinting analysis. Functional studies in three cell lines with different levels of Sp1 and AP-2 proteins suggested that the cellular levels of these proteins may differentially regulate transcription via GC-binding motifs in the human SOD2 promoter. Co-transfection of an Sp1 expression vector resulted in an increase in the transcription of the promoter-driven reporter gene. In contrast, co-transfection of the AP-2 expression vector caused a decrease in transcription. Direct mutagenesis analysis of Sp1- and AP-2-binding sites showed that Sp1 is essential for transcription of the human SOD2 gene, whereas AP-2 plays a negative role in the transcription. Immunoprecipitation of Sp1 and AP-2 proteins demonstrated that Sp1 interacts with AP-2 in vivo. Two-hybrid analysis revealed that interaction between Sp1 and AP-2 plays both a positive and negative role in the transcription of the reporter gene in vivo. Taken together, our data indicate that AP-2 down-regulates transcription of the human SOD2 gene via its interaction with Sp1 within the promoter region. These findings, coupled with our previous observation that several cancer cell lines have mutations in the promoter region of the human MnSOD gene, which lead to an increase in an AP-2-binding site and a decrease in the promoter activity, signal the importance of understanding the promoter structure and the regulation of the human SOD2 gene by Sp1 and AP-2.

GC box-binding transcription factors control the neuronal specific transcription of the cyclin-dependent kinase 5 regulator p35

Cyclin-dependent kinase 5 (cdk5)/p35 kinase activity is highest in post-mitotic neurons of the central nervous system and is critical for development and function of the brain. The neuronal specific activity of the cdk5/p35 kinase is achieved through the regulated expression of p35 mRNA. We have identified a small 200-bp fragment of the p35 promoter that is sufficient for high levels of neuronal specific expression. Mutational analysis of this TATA-less promoter has identified a 17-bp GC-rich element, present twice, that is both required for promoter activity and sufficient for neuronal specific transcription. A GC box within the 17-bp element is critical for both promoter activity and protein-DNA complex formation. The related transcription factors Sp1, Sp3, and Sp4 constitute most of the GC box DNA binding activity in neurons. We have found that both the relative contribution of the Sp family proteins to GC box binding and the transcriptional activity of these proteins is regulated during neuronal differentiation. Thus, our data show that the GC box-binding Sp proteins contribute to the regulation of p35 expression in neurons, suggesting changes in the Sp transcription factors level and activity may contribute to cell type-specific expression of many genes in the central nervous system.

Dominant negative interference of transcription factor AP-2 causes inhibition of ErbB-3 expression and suppresses malignant cell growth

ErbB-3 (HER3) is a member of the epidermal growth factor receptor family. Increasing evidence suggests that elevated expression of ErbB-3 is important for malignancy. In this study, we found that elevated levels of ErbB-3 expression did not occur in the absence of AP-2gamma in a panel of human mammary epithelial and fibroblasts cell lines. In contrast, there was no association between the expression of AP-2alpha or AP-2beta and the level of ErbB-3, or between AP-2alpha and AP-2gamma double positivity and ErbB-3 expression. In co-transfection experiments, exogenous expression of AP-2gamma robustly activated ErbB-3 promoter activity. Moreover, expression of a dominant negative AP-2 protein, AP-2delta (deleted residues 31-117), not only repressed the ErbB-3 promoter activity but also suppressed endogenous ErbB-3 transcription in the ErbB-3 overexpressing cell line MRC-5VA. Overexpression of AP-2A resulted in a decreased proliferation rate and inhibitin of colony formation. Taken together, these data strongly support a role for the AP-2 gene family, in particular, AP-2gamma, in the control of ErbB-3 expression. Interference with the function of transcription factor AP-2 might provide a potential strategy for modulation of the malignant phenotype.

Cloning and characterization of a novel human transcription factor AP-2 beta like gene (TFAP2BL1)

The AP-2 transcription factor has been shown to play an important role in development, morphogenesis, apoptosis, cell-cycle control and has also been implicated in mammary oncogenesis. Here we report the cloning and characterization of a novel human transcription factor AP-2 like gene (TFAP2BL1), which is located on human chromosome 6p12.1-21.1. The TFAP2BL1 cDNA is 2076 base pairs in length, encoding a 452-amino acid polypeptide related to human Ap-2protein. TFAP2BL1 gene has significantly high homology to transcription factor AP-2 gene of human, mouse, chicken, sheep, fruit fly, and C. elegans at amino acid level. RT-PCR analysis shows its relatively high expression level in adult thymus, prostate, small intestine, skeletal muscle, placenta, brain, and testis tissues.

Modulation of HIV-1 enhancer activity and virus production by cAMP

The effect of cAMP on the transcriptional activity of the HIV-1 long terminal repeat/enhancer was investigated and compared to the effect of cAMP on virus replication. In culture cAMP repressed virus replication in vivo using different cell types. Transient transfection studies with HIV-1 enhancer-derived luciferase reporter gene constructs identified the minimal DNA sequence mediating the negative regulatory effect of cAMP on HIV-1 transcription. A single nuclear factor kappaB element from the HIV-1 enhancer mediates the repressive effect on transcription. AP-2 is not involved in cAMP repression. Stable transfection of Jurkat T cells with the co-activators CREB binding protein (CBP) and p300 completely abolished the cAMP repressive effect, supporting the hypothesis that elevation of intracellular cAMP increases phosphorylation of CREB, which then competes with phosphorylated p65 and Ets-1 for limiting amounts of CBP/p300 thereby mediating the observed repressive effect on transcription. These findings suggest an important role of cAMP on HIV-1 transcription.

Cloning and characterization of a novel mouse AP-2 transcription factor, AP-2delta, with unique DNA binding and transactivation properties

AP-2 transcription factors are sequence-specific DNA-binding proteins expressed in neural crest and other tissues during mammalian development. Three mammalian genes, AP-2alpha, AP-2beta, and AP-2gamma, have been reported previously. A partial predicted AP-2 gene was identified in tandem with AP-2beta on human chromosome 6p12-p21.1. The orthologous mouse gene, which we named Ap-2delta, was identified from a fetal mouse head cDNA library. Northern analysis revealed two transcripts in embryonic and newborn mouse brain, with markedly higher steady-state levels in the former. The predicted Ap-2delta protein comprised 452 amino acids and was highly similar to other AP-2 proteins across the DNA-binding and dimerization domains. Ap-2delta formed homodimers and heterodimers in vitro, bound an optimized AP-2 consensus DNA sequence, and transactivated gene expression in eukaryotic cells. Ap-2delta dimers bound poorly to an AP-2 binding sequence from the human metallothionein IIa promoter in vitro, revealing a sequence specificity not previously observed among other AP-2 proteins. The PY motif and critical residues in the transactivation domain, which are highly conserved in the AP-2 family and believed necessary for transactivation, were divergent in Ap-2delta. The unique protein sequence and functional features of Ap-2delta suggest mechanisms, besides tissue-specific AP-2 gene expression, for specific control of target gene activation.

Levels of transcription factors AP-2alpha and AP-2beta in the brainstem are correlated to monoamine turnover in the rat forebrain

The transcription factors AP-2alpha and AP-2beta are implicated to play an important role during embryonic development of different parts of the brain, and in targeted regulation of gene expression in the adult brain. Several monoaminergic genes have binding sites for AP-2 in regulatory regions. We have in the present study, analysed the association between AP-2 levels in the brainstem of rats (n=9) and monoamine levels in the frontal cortex, septum and hippocampus. Several regionally specific correlations were found between AP-2alpha and AP-2beta and specific monoamines in the rat forebrain. The data support our notion that the transcription factor AP-2 family is involved in the regulation of the monoaminergic systems and, therefore, might be involved in the pathophysiology of neuropsychiatric disorders.

Plant bZIP G-box binding factors. Modular structure and activation mechanisms

In this review we sum-up the knowledge about bZIP G-box binding factors (GBFs), which possess an N-terminal, proline-rich domain. The GBF has been one of the most extensively studied transcription factor family. Based on protein sequence homology with yeast and animal basic leucine-zipper (bZIP) transcription factors, bioinformatic studies have identified their main structural domains (proline-rich, basic and leucine-zipper), which have been further functionally characterized by in vitro and in vivo experiments. Recent reports have led to the discovery of other GBF-specific short amino-acid sequences that may take part in the regulation of gene expression by post-transcriptional modifications or interaction with other proteins such as bZIP enhancing factors or plant 14-3-3-like proteins. We identified a GBF region, called the 'multifunctional mosaic region', that may be implicated in cytoplasmic retention, translocation to the nucleus and regulation of transcription. We also identified many conserved protein motifs that suggest a modular structure for GBFs. At the whole plant level, GBFs have been shown to be involved in developmental and physiological processes in response to major cues such as light or hormones. Nevertheless, it remains difficult to assign a physiological role to a particular GBF protein modular structure. Finally, bringing together these different aspects of GBF studies we propose a model describing the puzzling transduction pathway involving GBFs from cytoplasmic events of signal transduction to the regulation of gene expression in the nucleus.

Redistribution of transcription factor AP-2alpha in differentiating cultured human epidermal cells

Expression of the transcription factor AP-2alpha was examined in cultured human epidermal cells. Levels of AP-2alpha mRNA increased substantially after the cultures reached confluence, similar to the expression pattern of the differentiation markers involucrin and keratinocyte transglutaminase. The level of AP-2alpha protein in nuclear extracts declined markedly after confluence, however, along with its ability to form complexes with oligonucleotides containing the AP-2 response element. In contrast, the levels of AP-2alpha protein in cytoplasmic extracts increased dramatically after confluence, but these extracts had low DNA binding activity. Supershift experiments with specific antisera detected only AP-2alpha and not the beta or gamma isoforms. Examination of its localization by confocal microscopy revealed that AP-2alpha was primarily in the nucleus of basal cells and largely cytoplasmic in the most superficial cells. Localization was a dynamic phenomenon in that changing the medium resulted in accumulation of this transcription factor in the nucleus after several hours. Overall, the data indicate that AP-2alpha transcriptional activity is regulated in a differentiation-dependent manner in cultured keratinocytes and that this occurs by relocalization of the protein. Nuclear localization of the AP-2alpha protein in basal cells permits its accessibility to response elements in gene promoters, whereas sequestration in the cytoplasm as the differentiation program progresses curtails its transcriptional activity. This regulatory scheme may provide keratinocytes with the ability to restore AP-2 transcriptional activity rapidly by redistribution to the nucleus after receiving an appropriate growth signal, such as a medium change.

Biological activity of RE-1 silencing transcription factor (REST) towards distinct transcriptional activators

The zinc finger protein RE-1 silencing transcription factor (REST) is a transcriptional repressor that represses neuronal genes in non-neuronal tissues. We have analyzed the ability of REST and the REST mutants, RESTDeltaN and RESTDeltaC lacking either the N-terminal or C-terminal repression domains of REST, to inhibit transcription mediated by distinct transcriptional activator proteins. For this purpose we have designed an activator specific assay where transcription is activated as a result of only one distinct activation domain. In addition, binding sites for REST were inserted in the 5'-untranslated region or at a distant position downstream of the polyadenylation signal. The results show that REST or the REST mutants containing only one repression domain were able to block transcriptional activation mediated by the transcriptional activation domains derived from p53, AP2, Egr-1, and GAL4. Moreover, REST, as well as the REST mutants, blocked the activity of the phosphorylation-dependent activation domain of Elk1. However, the activity of the activation domain derived from cAMP response element binding protein 2 (CREB2), was not inhibited by REST, RESTDeltaN or RESTDeltaC, suggesting that REST is able to distinguish between distinct transcriptional activation domains. Additionally, the activator specific assay, together with a positive-dominant mutant of REST that activated instead of repressed transcription, was used in titration experiments to show that REST has transcriptional repression and no transcriptional activation properties when bound to the 5'-untranslated region of a gene.

Cloning and characterization of the pseudonajatoxin b precursor

An Australian common brown snake, Pseudonaja textilis, is known to contain highly lethal neurotoxins. Among them, a long-chain alpha-neurotoxin, pseudonajatoxin b, has been identified. In this report, while presenting evidence for the presence of at least four such long-chain alpha-neurotoxins in the venom of P. textilis, we describe the characteristics of both the mRNA and the gene responsible for the synthesis of these neurotoxins. A precursor toxin synthesized from the gene has been identified as being capable of producing the isoforms possibly by post-translational modifications at its C-terminal end. Recombinant toxins corresponding to the precursor and its product have been found to possess similar binding affinities for muscular acetylcholine receptors (IC(50)=3x10(-8) M) and a lethality, LD(50), of 0.15 microg/g in mice.

Genetic mechanisms of behavior--don't forget about the transcription factors

Major changes in psychiatric phenotypes due to genetic factors are seldom the result of single gene polymorphisms, but more often the result of several genetic mechanisms. In this millennium article we discuss the notion that the expression of numerous candidate genes could be regulated by the same transcription factors, and that polymorphisms in transcription factor genes might explain some phenotypes. We describe recent results of studies on the biological marker thrombocyte monoamine oxidase (trbc MAO) and the transcription factor AP-2beta. Low levels of trbc MAO is associated with temperamental characteristics such as sensation seeking and impulsiveness, and the enzyme is genetically regulated by specific transcriptional mechanisms. Transcription factor AP-2beta is important for the development of midbrain structures and AP-2beta has several binding sites in the regulatory regions of genes encoding key proteins in the monoamine transmitter systems. We have recently shown AP-2beta to be linked to personality, binge-eating disorder, treatment with antidepressant drugs, and also to trbc MAO. Regardless of whether transcriptions factors, such as AP-2beta, regulate the expression of eg, the number of monoamine neurons or a variety of candidate genes within the monoamine systems, or both, we would like to emphasize the role of transcription factors, besides polymorphisms in monoaminergic candidate genes, when explaining inter-individual differences in temperament and psychiatric vulnerability.

Transcription factor AP-2alpha is preferentially cleaved by caspase 6 and degraded by proteasome during tumor necrosis factor alpha-induced apoptosis in breast cancer cells

Several reports have linked activating protein 2alpha (AP-2alpha) to apoptosis, leading us to hypothesize that AP-2alpha is a substrate for caspases. We tested this hypothesis by examining the effects of tumor necrosis factor alpha (TNF-alpha) on the expression of AP-2 in breast cancer cells. Here, we provide evidence that TNF-alpha downregulates AP-2alpha and AP-2gamma expression posttranscriptionally during TNF-alpha-induced apoptosis. Both a general caspase antagonist (zVADfmk) and a caspase 6-preferred antagonist (zVEIDfmk) inhibited TNF-alpha-induced apoptosis and AP-2alpha downregulation. In vivo tests showed that AP-2alpha was cleaved by caspases ahead of the DNA fragmentation phase of apoptosis. Recombinant caspase 6 cleaved AP-2alpha preferentially, although caspases 1 and 3 also cleaved it, albeit at 50-fold or higher concentrations. Activated caspase 6 was detected in TNF-alpha-treated cells, thus confirming its involvement in AP-2alpha cleavage. All three caspases cleaved AP-2alpha at asp(19) of the sequence asp-arg-his-asp (DRHD(19)). Mutating D(19) to A(19) abrogated AP-2alpha cleavage by all three caspases. TNF-alpha-induced cleavage of AP-2alpha in vivo led to AP-2alpha degradation and loss of DNA-binding activity, both of which were prevented by pretreatment with zVEIDfmk. AP-2alpha degradation but not cleavage was inhibited in vivo by PS-431 (a proteasome antagonist), suggesting that AP-2alpha is degraded subsequent to cleavage by caspase 6 or caspase 6-like enzymes. Cells transfected with green fluorescent protein-tagged mutant AP-2alpha are resistant to TNF-alpha-induced apoptosis, further demonstrating the link between caspase-mediated cleavage of AP-2alpha and apoptosis. This is the first report to demonstrate that degradation of AP-2alpha is a critical event in TNF-alpha-induced apoptosis. Since the DRHD sequence in vertebrate AP-2 is widely conserved, its cleavage by caspases may represent an important mechanism for regulating cell survival, proliferation, differentiation, and apoptosis.

Dominant-negative transcription factor AP-2 augments SB-2 melanoma tumor growth in vivo

We have previously demonstrated that the transition of melanoma to the metastatic phenotype is associated with a loss of expression of the transcription factor AP-2. To further investigate the role of AP-2 in the progression of human melanoma, we attempted to inactivate AP-2 in primary cutaneous SB-2 melanoma cells by using a dominant-negative AP-2, or AP-2B, gene. AP-2B is an alternatively spliced AP-2 variant capable of inhibiting AP-2 trans-activator function. Stable transfection of primary cutaneous melanoma SB-2 cells with the dominant-negative AP-2B gene was confirmed by RT--PCR and Northern blot analyses. Electromobility shift assay using nuclear extracts from these cell lines demonstrated decreased functional binding of AP-2B-transfected cells to the AP-2 consensus binding sequence compared with neo-transfected controls. In addition, CAT activity driven by a construct containing the AP-2 consensus binding sequence was downregulated in the AP-2B transfected cells, indicating AP-2 activity was quenched in the transfected cells. Orthotopic (subcutaneous) injection of the dominant-negative (AP-2B)-transfected cell lines into nude mice increased their tumorigenicity compared to control neo-transfected cells. The AP-2B-transfected cells displayed an increase in MMP-2 expression (by Northern blot) and MMP-2 activity (by zymography), which resulted in an increase in invasiveness through Matrigel-coated filters. The AP-2B-transfected tumors also displayed an increase in MMP-2 expression, microvessel density, and angiogenesis in vivo. These results demonstrate that inactivation of AP-2 contributes to the progression of melanoma, at least partially via deregulation of the MMP-2 gene.

Two CCAAT boxes in a novel inverted repeat motif are required for Hlx homeobox gene expression

Hlx is a homeobox transcription factor gene required for normal intestinal and hepatic growth in development. We previously found high sequence identity and 17 conserved consensus cis-regulatory/transcription factor binding elements in the mouse and human Hlx 5' regions. A 594 bp sequence in the Hlx 5' region possessing the same activity in driving luciferase expression as larger Hlx 5' sequences had three segments each necessary but not sufficient for luciferase expression in NIH 3T3 cells (which express Hlx). Nine of the conserved putative regulatory elements are positioned within these segments, including two CCAAT boxes on opposite strands within a conserved 44 bp inverted repeat sequence. To test the hypothesis that these elements are required for promoter activity, we compared the reporter expression activity of segments containing mutations of these elements with activity of the parent Hlx promoter sequence. We found that mutation of either CCAAT box or a conserved AP-2 site resulted in a significant decrease in promoter activity. Restoration of the inverted repeat with complementary mutations of both CCAAT boxes did not restore activity. Further, mutation of other portions of the inverted repeat did not affect promoter activity. Mutation of other elements had no effect on promoter activity.

Transcription factor AP-2 functions as a repressor that contributes to the liver-specific expression of serum amyloid A1 gene

We previously identified transcription factor AP-2 as the nuclear factor that interacts with the tissue-specific repressor element in the rat serum amyloid A1 (SAA1) promoter. In this report, we provide evidence for a second AP-2-binding site and show that both AP-2 sites participate in mediating the transcription repression of SAA1 promoter. This proximal AP-2 site overlaps with the NFkappaB-binding site known to be essential for SAA1 promoter activity. Protein binding competition experiments demonstrated that AP-2 and NFkappaB binding to these overlapping sites were mutually exclusive. Furthermore, the addition of AP-2 easily displaced prebound NFkappaB, whereas NFkappaB could not displace AP-2. These results thus suggest that one mechanism by which AP-2 negatively regulates SAA1 promoter activity may be by antagonizing the function of NFkappaB. Consistent with a repression function, transient expression of AP-2 in HepG2 cells inhibited conditioned medium-induced SAA1 promoter activation. This inhibition was dependent on functional AP-2-binding sites, since mutation of AP-2-binding sites abolished inhibitory effects of AP-2 in HepG2 cells as well as resulted in derepression of the SAA1 promoter in HeLa cells. In addition to SAA1, we found that several other liver gene promoters also contain putative AP-2-binding sites. Some of these sequences could specifically inhibit AP-2.DNA complex formation, and for the human complement C3 promoter, overexpression of AP-2 also could repress its cytokine-mediated activation. Finally, stable expression of AP-2 in hepatoma cells significantly reduced the expression of endogenous SAA, albumin, and alpha-fetoprotein genes. Taken together, our results suggest that AP-2 may function as a transcription repressor to inhibit the expression of not only SAA1 gene but also other liver genes in nonhepatic cells.

The retinoblastoma-related Rb2/p130 gene is an effector downstream of AP-2 during neural differentiation

Rb2/p130, a member of the Retinoblastoma family of growth and tumour suppressor genes, is extensively implicated in the control of cell cycle and differentiation. The minimal promoter region of Rb2/p130 in T98G human glioblastoma cells was identified and its analysis revealed the presence of a KER1 palindromic sequence able to bind the transcription factor AP-2, a regulatory protein that plays a crucial role in ectodermal differentiation. This KER1 site interacted in vitro with AP-2, and AP-2 overexpression increased Rb2/p130 transcription and translation. We also found that rat PC12 pheochromocytoma cells, when induced to differentiate by NGF, displayed an increase of AP-2 protein levels and of Rb2/p130 transcription and protein levels. AP-2-transfected PC12 cells displayed enhanced transcription and translation of Rb2/p130 and of the cdk inhibitor p21(WAF1/CIP1), a gene known to be under the control of AP-2, but unable by itself to elicit PC12 differentiation. Overexpression of either AP-2 or Rb2/p130 elicited per se cell differentiation in the absence of NGF, while coexpression of AP-2B, a negative regulator of AP-2 transcriptional activity, inhibited only AP-2-induced differentiation. Altogether, these results indicate that Rb2/p130 is a critical effector of AP-2 in sustaining ectodermal differentiation.

Novel activator protein-2alpha splice-variants function as transactivators of the ovine placental lactogen gene

Activator protein-2 (AP-2) has been implicated as a transactivator of the human and ovine placental lactogen (oPL) genes. Transcriptional enhancement through an AP-2 cis-acting element has been described for other genes expressed in the placenta, but the AP-2 isoform enhancing expression is species dependent. Transactivation of the oPL minimal promoter (-124 bp to +16 bp) by AP-2 was confirmed by mutational analysis in transiently transfected human choriocarcinoma cells (BeWo). AP-2alpha was localized in ovine chorionic epithelial cells by immunohistochemistry and a 3-kb transcript was identified by Northern hybridization. Four nearly full-length AP-2 cDNAs were isolated from an ovine placenta cDNA library. Nucleotide sequencing these cDNAs revealed that the AP-2 mRNA expressed in the ovine placenta shares identity with human AP-2alpha, but variations in the predicted N-terminus were observed, and three unique AP-2alpha splice-variants were identified. Expression of AP-2alpha variants in HepG2 cells, devoid of endogenous AP-2, indicates that enhancement through the AP-2 element in the oPL gene minimal promoter was variant dependent. RNA transcripts for all of the ovine AP-2alpha splice-variants were confirmed in ovine placenta by RT-PCR, and homologs for two variants were found in human placenta. However, only one AP-2alpha transcript, which shares identity to Xenopus AP-2alpha, was expressed in BeWo cells. Immunoblot analysis confirmed AP-2alpha variants in ovine chorionic binucleate cell nuclear extracts, one of which migrates similar to the AP-2alpha variant identified in BeWo cell nuclear extracts. These data indicate the presence of new mammalian AP-2alpha splice-variants that augment transactivation of the oPL gene in ovine chorionic binucleate cells.

Regulation of the tyrosine hydroxylase and dopamine beta-hydroxylase genes by the transcription factor AP-2

The retinoic acid-inducible and developmentally regulated transcription factor AP-2 plays an important role during development. In adult mammals, AP-2 is expressed in both neural and non-neural tissues. However, the function of AP-2 in different neuronal phenotypes is poorly understood. In this study, transcriptional regulation of tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) genes by AP-2 was investigated. AP-2 binding sites were identified in the upstream regions of both genes. Electrophoretic mobility shift assays (EMSA) and DNase I footprinting analyses indicate that the AP-2 interaction with these motifs is more prominent in catecholaminergic SK-N-BE(2)C and CATH.a than in non-catecholaminergic HeLa and HepG2 cell lines. Exogenous expression of AP-2 robustly transactivated TH and DBH promoter activities in non-catecholaminergic cell lines. While AP-2 regulates the DBH promoter activity via a single site, transactivation of the TH promoter by AP-2 appears to require multiple sites. In support of this, mutation of multiple AP-2 binding sites but not that of single site diminished the basal promoter activity of the TH gene in cell lines that express TH and abolished transactivation by exogenous AP-2 expression in cell lines that do not express TH. In contrast, mutation of a single AP-2 binding site of the DBH gene completely abolished transactivation by AP-2. Double-label immunohistochemistry showed that AP-2 is coexpressed with TH in noradrenergic and adrenergic neurons in both the central and peripheral nervous systems of adult rodents. Numerous non-catecholaminergic cell groups within the spinal cord, medulla, cerebellum, and pons also express AP-2. The concentration of AP-2 in dorsomedial locations along the neuraxis suggests a regionally specific role for this transcription factor in the regulation of neuronal function. Based on these findings we propose that AP-2 may coregulate TH and DBH gene expression and thus participate in expression/maintenance of neurotransmitter phenotypes in (nor)adrenergic neurons and neuroendocrine cells.

Regulatory roles of AP-2 transcription factors in vertebrate development, apoptosis and cell-cycle control

AP-2 transcription factors represent a family of three closely related and evolutionarily conserved sequence-specific DNA-binding proteins, AP-2alpha, -beta and -gamma. Subsequent studies have identified spatially and temporally regulated embryonic expression patterns in a number of different tissues including neural crest derivatives, neural, epidermal and urogenital tissues. Here, we review the current understanding of developmental defects in AP-2-deficient mice and consider regulatory functions of AP-2 in control of apoptosis, cell cycle, and gene expression. Recently, the first inherited human disorder, Char syndrome, was identified to be caused by AP-2beta missense mutations. In light of the manifold and essential functions of AP-2 proteins in cell growth, differentiation and programmed death, mutations or changes in precisely programmed expression patterns are likely to contribute to other congenital malformations or neoplastic diseases.

The minimal promoter of the human alpha 3 nicotinic receptor subunit gene. Molecular and functional characterization

The minimal promoter of the human alpha(3) nicotinic receptor subunit gene has been mapped to a region of 60 base pairs and found to contain two Sp1 sites, one of which is essential for promoter activity. DNase footprinting has revealed the presence of another region of interaction with nuclear factors (named F2) immediately downstream of the Sp1 sites. This region has been found to be functional since it is capable of stimulating the minimal promoter. The F2 protection is completely and specifically competed by an AP2 consensus oligonucleotide that has been proved to bind AP2alpha exclusively. However, the AP2alpha recombinant protein was unable to bind the F2 region directly, thus suggesting that AP2alpha may participate in F2 protection by protein-protein interactions with other nuclear factors. The minimal promoter has been shown to be stimulated by two additional regions, one located downstream of F2 and the other upstream of the minimal promoter itself. In neuronal cells, the combined stimulatory activities of these three regions have synergistic effects, whereas in non-neuronal cells, there is a negative interference between the upstream and downstream regions. These opposite transcriptional effects may account for at least part of the neuro-specific expression profile of the alpha(3) gene.

TAZ: a novel transcriptional co-activator regulated by interactions with 14-3-3 and PDZ domain proteins

The highly conserved and ubiquitously expressed 14-3-3 proteins regulate differentiation, cell cycle progression and apoptosis by binding intracellular phosphoproteins involved in signal transduction. By screening in vitro translated cDNA pools for the ability to bind 14-3-3, we identified a novel transcriptional co-activator, TAZ (transcriptional co-activator with PDZ-binding motif) as a 14-3-3-binding molecule. TAZ shares homology with Yes-associated protein (YAP), contains a WW domain and functions as a transcriptional co-activator by binding to the PPXY motif present on transcription factors. 14-3-3 binding requires TAZ phosphorylation on a single serine residue, resulting in the inhibition of TAZ transcriptional co-activation through 14-3-3-mediated nuclear export. The C-terminus of TAZ contains a highly conserved PDZ-binding motif that localizes TAZ into discrete nuclear foci and is essential for TAZ-stimulated gene transcription. TAZ uses this same motif to bind the PDZ domain-containing protein NHERF-2, a molecule that tethers plasma membrane ion channels and receptors to cytoskeletal actin. TAZ may link events at the plasma membrane and cytoskeleton to nuclear transcription in a manner that can be regulated by 14-3-3.

Tau promoter confers neuronal specificity and binds Sp1 and AP-2

Tau, a microtubule-associated protein, is encoded by a single gene, whose expression is primarily neuronal. In this work, we defined an 80-bp region of the tau promoter that confers tau protein with neuronal expression. This fragment works in conjunction with an endogenous initiation region to activate neuronal precursor-specific transcription of the tau promoter and works independently of this initiation region to confer nerve growth factor inducibility. Furthermore, this 80-bp fragment binds both Sp1 and AP-2 proteins. DNase I foot-print analysis revealed a third protein binding region at the center of this 80-bp fragment in neuronal cells. Mutation within any of these three protein binding sites decreases transcriptional activation of the tau gene. Comprehension of the interactions that occur between cis- and trans-regulatory elements of the tau promoter is important to understand the regulation of tau expression during normal development and changes that may occur in many cases of dementia, including Alzheimer's disease.

Characterization of the activation domains of AP-2 family transcription factors

Despite sequence variation, all AP-2 isotypes are capable of activating transcription, which indicates a functional conservation. We used this property to gain a unique insight into the structure and function of the activation motifs of AP-2 family transcription factors. We have precisely localized the activation motif of human AP-2 alpha to amino acids 52-108. Our experiments indicate that similar sequence of amino acids in all AP-2 isotypes except Drosophila AP-2 alpha harbor their activation motifs. Within this sequence, fewer than 36 residues are critical for transcription activation. Our comparison studies and site-directed mutagenic analyses show that these critical amino acids are strategically placed within this sequence. These residues are interspersed with nonessential and influential residues that vary in composition and length, indicating a structural flexibility. The Drosophila AP-2 alpha has its partly conserved activation motif in an extended region about twice the length of other AP-2 isotypes. Our results reveal essential elements of the amino acid composition of activators in general and shed new light on the mechanism of transcription activation.

Tissue-specific splicing of Omi stress-regulated endoprotease leads to an inactive protease with a modified PDZ motif

Omi is a human serine protease whose catalytic domain is homologous to a bacterial heat shock endoprotease (HtrA), a protein indispensable to the survival of bacteria at elevated temperatures. Omi is expressed ubiquitously, and its protein product is predominantly localized in the endoplasmic reticulum of mammalian cells. Here we present the genomic structure of Omi, consisting of eight exons located on human chromosome 2p12-p13. Furthermore, we describe an alternatively splice form of Omi (D-Omi) that is expressed predominantly in the kidney, colon, and thyroid. D-Omi lacks peptide sequence encoded by two exons (exons III and VII). The absence of exon VII leads to a protein with a modified PDZ domain unable to interact with a known partner, the Mxi2 protein. The absence of exon III affects the catalytic domain and leads to a protein with no detectable protease activity. Our studies suggest that D-Omi may have a unique role in the normal function of kidney, colon, and thyroid.

Enhanced expression of the leukotriene C(4) synthase due to overactive transcription of an allelic variant associated with aspirin-intolerant asthma

Aspirin-intolerant asthma (AIA), a distinct clinical syndrome affecting about 10% of adult asthmatics, appears to be unusually dependent on cysteine leukotriene (cys-LT) overproduction by pulmonary eosinophils. The gene coding for leukotriene (LT) C(4) synthase (LTC(4)S), the enzyme controlling cys-LT biosynthesis, exists as two common alleles distinguished by an A to C transversion at a site 444 nucleotides upstream of the translation start. We tested the hypothesis that this single nucleotide polymorphism (SNP) affects binding of transcription factors and influences the transcription rate, predisposing to AIA. Gel shift assay studies revealed that the (-444)C allele, conferring an activator protein-2 binding sequence, is an additional target for a transcription factor of histone H4 consensus. Introduction of the H4TF-2 decoy oligonucleotide into LTC(4)S-positive, differentiated HL-60 cells decreased accumulation of LTC(4) to 68%. Transfection of COS-7 with promoter construct increased expression of beta-galactosidase reporter for the (-444)C variant. The (-444)C allelic frequency was significantly higher in AIA patients (n = 76) as compared with matched aspirin-tolerant asthmatics (n = 110) and healthy controls (n = 75). Patients with AIA had also upregulated LTC(4)S messenger RNA expression in peripheral blood eosinophils. An inhaled provocation test with lysine-aspirin led to an increase in urinary output of LTE(4), which reached statistical significance only in carriers of the (-444)C allele. Our results suggest that a transcription factor, present in dividing and bone marrow resident progenitors of eosinophils, triggers LTC(4)S transcription in carriers of a common (-444)C allele due to binding with the histone H4 promoter element of the gene. Genetic predisposition to cys-LT pathway upregulation, a hallmark of AIA, can be related to overactive expression of the LTC(4)S (-444)C allele.

Identification of amino acid residues of transcription factor AP-2 involved in DNA binding

AP-2 is a cell-type specific, developmentally regulated transcription factor which has been described as a critical regulator of gene expression during vertebrate development and embryogenesis. Although the overall domains of this factor necessary for their activity have been identified, the exact identity of AP-2 amino acid residues responsible for its interaction with the DNA structure has not yet been described. Here, we describe the identification of a region of AP-2 which was protected by an oligonucleotide probe containing its binding site from trypsin digestion, monitored by peptide mapping by MALDI-TOF mass spectrometry. Furthermore, we analyzed the relative in vitro DNA-binding activity, the stimulatory potency on the AP-2-dependent APOE promoter, as well as the ability to inhibit the effect of the wild-type protein of each one of a set of single-site substitution AP-2 mutants spanning the identified region. Taken together, our data clearly demonstrate that the region between amino acid residues 252-260 of AP-2 is essential for its DNA-binding activity. Particularly, the individual substitution in any of the residues 253, 254, 255, 257 or 260 is sufficient for completely abolishing the interaction with DNA and the stimulation of APOE promoter activity. These results indicate a crucial role of this region in the formation of an active DNA-binding domain and strongly suggest that these residues provide direct contacts with the DNA structure at the AP-2 binding site.

Tau promoter activity in neuronally differentiated P19 cells

Tau proteins are encoded by a single gene which is regulated by a unique promoter. The proximal 196 base pairs of the tau 5' flanking region confers tau protein with neuronal specific expression and nerve growth factor inducibility. We tested tau promoter activity in neuronally differentiated embryonal carcinoma cells, the P19 mouse blastoderm cell line. In these experiments, we examined the temporal expression pattern of the tau promoter and compared it to other viral and cellular promoters. Tau promoter activity increases significantly with differentiation, specifically during neurite initiation. In addition, tau promoter activity in neuronally differentiated P19 cells was significantly greater than all five of the other neuronal or non neuronal promoters tested. All other promoters displayed low levels of promoter activity throughout retinoic acid induced neuronal differentiation of P19 cells. Taken together, our results suggest that the tau promoter is a good choice for ectopic expression of exogenous genes in P19 cells, which serves as a differentiating neuronal model system.

RINX(VSX1), a novel homeobox gene expressed in the inner nuclear layer of the adult retina

The locus control region (LCR) of the human red and green visual pigment genes is critical for the formation of functional red and green cones in the retina. A 37-bp core of the LCR is perfectly conserved among mammals and binds specific retinal nuclear proteins. Here, we employed a yeast one-hybrid screen of an adult retinal cDNA library to clone and characterize these proteins. We identified clones encoding homeodomain (HD) transcription factors Pax6, Rx, and Chx10 and a novel paired-like HD protein, RINX. In the adult retina, RINX is exclusively expressed in a subset of cells (likely to be bipolar cells) of the retinal inner nuclear layer (INL). RINX is closely related to Chx10, which is also exclusively expressed in the INL of the adult retina and is critical for retinal development. The RINX gene is expressed in two classes of mRNA. One class encodes proteins that lack either part of or all of the HD, but retain the transcriptional activation domain. The RINX gene maps to chromosome 20p11.2 to which no retinal disease has been assigned. In conclusion, the LCR contains two adjacent motifs that are targets for binding of HD proteins that may specify the development and differentiation of cone photoreceptors and a subset of INL bipolar cells. Mutations in the related human CHX10 gene cause microphthalmia in a subset of families, and, therefore, the RINX gene is a candidate for this phenotype in another subset of patients. Since the RINX gene is likely an ortholog of the goldfish Vsx1 gene, it has been named VSX1 by the Human Gene Nomenclature Committee.

Transcriptional regulation of gene expression of sec6, a component of mammalian exocyst complex at the synapse

Sec6, an essential component of the mammalian brain exocyst complex, is believed to function in synapse formation and synaptic plasticity. During neuronal development, the expression of the Sec6 gene correlates temporally with neurite outgrowth and synaptogenesis. To understand the mechanisms that regulate the Sec6 gene expression, we have cloned and characterized the 5'-terminal region of the murine Sec6 gene. We have shown that the 5'-untranslated region of the murine Sec6 gene is encoded by two exons that are separated by a 1560-bp intron. Primer extension analysis demonstrates that Sec6 gene transcription is initiated from a unique site. The Sec6 promoter is embedded in a CpG island and lacks canonical TATA or CAAT boxes. Sequence analysis of the 5'-flanking region and the first intron reveals the presence of a number of binding sites for transcription factors AP-1, AP-2, AP-4, ATF, C/EBPbeta, GATA-1, Oct 1, SP1, STAT, and NRSF. Transfection experiments using Sec6-luciferase fusion genes demonstrate that the 5'-flanking sequence functions as a strong promoter in neuronal but not in nonneuronal cells. Deletion analysis reveals the presence of a core promoter between nucleotide position -139 and +53, and two enhancer and four silencer elements within the 5'-flanking region and the first intron sequence. These results indicate that neuronal expression of the Sec6 gene involves a relatively specific core promoter and interplay between multiple positive and negative regulatory elements.

Ectopic expression of luteinizing hormone-releasing hormone and peripherin in the respiratory epithelium of mice lacking transcription factor AP-2alpha

The vertebrate transcription factor activator protein-2 (AP-2alpha) is involved in craniofacial morphogenesis. In the nasal placode AP-2alpha expression delineates presumptive respiratory epithelia from olfactory epithelia, with AP-2alpha expression restricted to the anterior region of the respiratory epithelium (absent from the olfactory epithelium) at later stages. To address the role AP-2alpha plays in differentiation of cell groups in the nasal placode, the spatiotemporal expression pattern of four markers normally associated with olfactory epithelial structures was analyzed in mice lacking AP-2alpha. These markers were the intermediate filament protein peripherin, the neuropeptide luteinizing hormone-releasing hormone (LHRH), the neural cell adhesion molecule (NCAM) and the olfactory transcription factor Olf-1. Development of cells expressing these markers was similar in both genotypes until embryonic day 12.5 (E12.5), indicating that the main olfactory epithelium and olfactory pit formation was normal. At E13.5 in mutant mice, ectopic LHRH neurons and peripherin axons were detected in respiratory epithelial areas, areas devoid of Olf-1 and NCAM staining. Over the next few days, an increase in total nasal LHRH neurons occurred. The increase in nasal LHRH neurons could be accounted for by LHRH neurons arising and migrating out of respiratory epithelial regions on peripherin-positive fibers. These results indicate that AP-2alpha is not essential for the separation of the olfactory and respiratory epithelium from the nasal placode and is consistent with AP-2alpha preventing recapitulation of developmental programs within the respiratory epithelium that lead to expression of LHRH and peripherin phenotypes.

Mutations in TFAP2B cause Char syndrome, a familial form of patent ductus arteriosus

Char syndrome is an autosomal dominant trait characterized by patent ductus arteriosus, facial dysmorphism and hand anomalies. Using a positional candidacy strategy, we mapped TFAP2B, encoding a transcription factor expressed in neural crest cells, to the Char syndrome critical region and identified missense mutations altering conserved residues in two affected families. Mutant TFAP2B proteins dimerized properly in vitro, but showed abnormal binding to TFAP2 target sequence. Dimerization of both mutants with normal TFAP2B adversely affected transactivation, demonstrating a dominant-negative mechanism. Our work shows that TFAP2B has a role in ductal, facial and limb development and suggests that Char syndrome results from derangement of neural-crest-cell derivatives.

Interaction of Ap1, Ap2, and Sp1 with the regulatory regions of the human pro-alpha1(I) collagen gene

In the pro-alpha1(I) collagen gene a number of cis-regulatory elements, which interact with a variety of trans-acting factors, are present in the promoter and first intron. We have undertaken a comprehensive study of Sp1, Ap1, and Ap2 binding in the region spanning -442 to +1697 nt. DNase I footprinting analysis revealed these factors bind with varying affinities to some of the potential sites: Sp1 binds to 16 of 34 potential sites, Ap2 binds to 22 of 40 potential binding sites, and Ap1 binds to its only potential site. The Sp1 sites were mostly clustered in the intron region, while the Ap2 sites were clustered in the promoter region. Transmission electron microscopic analysis of DNA-protein complexes not only confirmed these results, but also clearly showed that heterologous and/or homologous protein-protein interactions between Sp1 and/or Ap2 bring the promoter and intron in contact with each other, with the resulting looping out of the intervening DNA. This strongly suggests that the DNA-looping model is an explanation for the orientation preference of the enhancing element in the first intron as these interactions possibly create an optimum environment for the binding of the rest of the transcriptional machinery.

A polymorphic region in the human transcription factor AP-2beta gene is associated with specific personality traits

Transcription factor AP-2beta is implicated in playing an important role during embryonic development of different parts of the brain, eg, midbrain, hindbrain, spinal cord, dorsal and cranial root ganglia.1,2 The gene encoding AP-2beta contains a polymorphic region which includes a tetranucleotide repeat of [CAAA] four or five times, located in intron 2 between nucleotides 12593 and 12612.3 Since the midbrain contains structures important for variables such as mood and personality, we have investigated if the AP-2beta genotype is associated with personality traits estimated by the Karolinska Scales of Personality (KSP). Identification of transcription factor genes as candidate genes in psychiatric disorders is a novel approach to further elucidate the genetic factors that, together with environmental factors, are involved in the expression of specific psychiatric phenotypes. The AP-2beta genotype and KSP scores were determined for 137 Caucasian volunteers (73 females and 64 males). The personality traits muscular tension, guilt, somatic anxiety, psychastenia and indirect aggression were significantly associated with the specific AP-2beta genotype, albeit with significant difference between genders. Based on this result the human AP-2beta gene seems to be an important candidate gene for personality disorders. Moreover, the present results suggest that the structure of the intron 2 region of the AP-2beta gene is one factor that contributes to development of the constitutional component of specific personality traits.

Role of AP2 consensus sites in regulation of rat Npt2 (sodium-phosphate cotransporter) promoter

Expression of the Npt2 gene, encoding the type II sodium-dependent phosphate cotransporter, is restricted to renal proximal tubule epithelium. We have isolated a 4,740-bp fragment of the 5'-flanking sequence of the rat Npt2 gene, identified the transcription initiation site, and demonstrated that this 5'-flanking sequence drives luciferase-reporter gene expression, following transfection in the proximal tubule cell-derived opossum kidney (OK) cell line but not in unrelated cell lines. Analysis of the promoter sequence revealed the presence of 10 consensus binding motifs for the AP2 transcription factor. Transient transfection assays revealed an important effect of the number of tandemly repeated AP2 sites in enhancing promoter activity. The promoter sequence also revealed a pair of inverted repeats enclosing 1,324 bp of intervening sequence and containing 8 of the total 10 AP2 consensus sites in the promoter sequence. Deletion or reversal of orientation of the distal inverted repeat resulted in marked enhancement of promoter activity. Electrophoretic mobility shift analysis revealed a distinct pattern of transcription factor binding to oligonucleotides containing AP2 sites, using nuclear extracts from OK cells, compared with unrelated cell lines. Taken together, these results suggest an important role for AP2 consensus binding sites in regulating Npt2 gene expression and suggest a mechanism of regulation mediated by the interaction of inverted repeats enclosing these sites.

AP-2 family members regulate basal and cAMP-induced expression of human chorionic gonadotropin

The AP-2 family of transcriptional regulator proteins has three members, alpha, beta and gamma. AP-2alpha and gamma are expressed in placenta and in the human trophoblast cell line JEG-3. AP-2 has been shown to regulate expression of the placental human chorionic gonado-tropin (hCG) alpha- and beta-subunit genes, however, previous work did not distinguish between the family members. Tryptic peptides of the AP-2 protein complexes purified from JEG-3 cells by oligo-affinity chromatography using the hCGalpha AP-2 site match the amino acid sequence of AP-2gamma. The fact that AP-2gamma is present at significant levels and binds the hCGalpha trophoblast-specific element suggests that AP-2gamma is at least part of the binding complex in vivo and plays a role in regulating hCG expression. We show that mutation of each of four AP-2 binding sites within the hCGbeta promoter decreases expression in transfection assays, demonstrating that all four sites are required for maximal expression in JEG-3 cells. Furthermore, we find differences in regulation of the family members: AP-2alpha mRNA levels increase in response to cAMP while AP-2gamma mRNA levels do not. The demonstrated importance of the AP-2 sites in controlling hCGalpha and beta expression and the likely involvement of more than one family member suggest that a balance in AP-2 proteins is involved in coordinate regulation of these genes. Moreover, many placenta-restricted genes are regulated by AP-2 proteins, thus members of this family may play an important overall role in placenta-specific expression.

The rat Mist1 gene: structure and promoter characterization

Transcription factors of the basic Helix-Loop-Helix (bHLH) protein family play key roles in several developmental processes. Mist1 belongs to this group of proteins and shares several properties with the other family members. For example, Mist1 is capable of dimerization with the ubiquitously expressed E2A bHLH proteins and exhibits a strong DNA-binding activity to the core E-box sequence. Using in-situ hybridization and Northern blot hybridization, Mist1 mRNA has been detected in a variety of embryonic and adult rodent tissues. To understand the molecular mechanisms involved in the expression of the gene, we have cloned the rat Mist1 gene and analyzed 2.5 kb of its 5' flanking region. The Mist1 gene spans over 5 kilobases and is composed of two exons separated by a unique intron. The entire coding region is localized in the second exon. Sequence analysis of the promoter region indicated an absence of TATA-box or CAAT-box sequence, but several consensus Sp1-binding sites were present near the transcription start site. Deletion analysis of the promoter region identified a 272 bp proximal fragment to be sufficient to drive expression of a reporter gene in NIH3T3 fibroblasts. Subsequent deletion of potential Sp1 sites results in a marked decrease in promoter activity. Electrophoretic mobility shift assays revealed that Sp1 binds to two different regions in the proximal promoter, a typical Sp1 site located at (-38; -33) and a G/C-rich region between (-67; -62). These data suggest that the basal expression of this TATA-less gene might be driven by general transcription factors, such as Sp1.

AP-2 transcription factors in the regulation of ERBB2 gene transcription by oestrogen

Transcription of the ERBB2 oncogene is repressed by oestrogen in human breast cancer cells. We show that a 218 bp fragment of the human ERBB2 gene promoter is responsive to oestrogen in transient transfection in ZR75.1 and SKBR.3 cells when the oestrogen receptor is expressed. Deletion analysis of this fragment shows that a sequence located at the 5' end, which is known to mediate ERBB2 overexpression in breast cancer, is also responsible for the oestrogen response. This sequence binds AP-2 transcription factors and appears functionally identical to an element of the oestrogen-dependent enhancer described in the first intron of human ERBB2. We observed that oestrogen treatment down-regulates expression of AP-2 proteins but does not affect the DNA binding activity of AP-2. Constitutive expression of AP-2beta or AP-2gamma, but not AP-2alpha, abrogates the estrogenic repression. Our results demonstrate that AP-2 transcription factors are implicated in the oestrogenic regulation of ERBB2 gene expression and suggest a complex interplay involving the different AP-2 isoforms and other unidentified factors.

Crystal structure of a transcriptionally active Smad4 fragment

BACKGROUND

Smad4 functions as a common mediator of transforming growth factor beta (TGF-beta) signaling by forming complexes with the phosphorylated state of pathway-restricted SMAD proteins that act in specific signaling pathways to activate transcription. SMAD proteins comprise two domains, the MH1 and MH2 domain, separated by a linker region. The transcriptional activity and synergistic effect of Smad4 require a stretch of proline-rich sequence, the SMAD-activation domain (SAD), located N-terminal of the MH2 domain. To understand how the SAD contributes to Smad4 function, the crystal structure of a fragment including the SAD and MH2 domain (S4AF) was determined.

RESULTS

The structure of the S4AF trimer reveals novel features important for Smad4 function. A Smad4-specific sequence insertion within the MH2 domain interacts with the C-terminal tail to form a structural extension from the core. This extension (the TOWER) contains a solvent-accessible glutamine-rich helix. The SAD reinforces the TOWER and the structural core through interactions; two residues involved in these interactions are targets of tumorigenic mutation. The solvent-accessible proline residues of the SAD are located on the same face as the glutamine-rich helix of the TOWER, forming a potential transcription activation surface. A tandem sulfate-ion-binding site was identified within the subunit interface, which may interact with the phosphorylated C-terminal sequence of pathway-restricted SMAD proteins.

CONCLUSIONS

The structure suggests that the SAD provides transcriptional capability by reinforcing the structural core and coordinating with the TOWER to present the proline-rich and glutamine-rich surfaces for interaction with transcription partners. The sulfate-ion-binding sites are potential 'receptors' for the phosphorylated sequence of pathway-restricted SMAD proteins in forming a heteromeric complex. The structure thus provides a new model that can be tested using biochemical and cellular approaches.

Transcriptional regulation of 5-aminolevulinate synthase by phenobarbital and cAMP-dependent protein kinase

5-Aminolevulinate synthase (ALA-S) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of the heme biosynthesis. There are two ALA-S isozymes encoded by distinct genes. One gene encodes an isozyme that is expressed exclusively in erythroid cells, and the other gene encodes a housekeeping isozyme that is apparently expressed in all tissues. In this report we examine the mechanisms by which phenobarbital and cAMP regulate housekeeping ALA-S expression. We have determined that cAMP and phenobarbital effects are additive and the combined action is necessary to observe the cAMP effect on ALA-S mRNA in rat hepatocytes. The role of the cAMP-dependent protein kinase (PKA) has been examined. A synergism effect on ALA-S mRNA induction is observed in rat hepatocytes treated with pairs of selective analogs by each PKA cAMP binding sites. A 870-bp fragment of ALA-S 5'-flanking region is able to provide cAMP and phenobarbital stimulation to chloramphenicol O-acetyltranferase fusion vectors in transiently transfected HepG2 cells. ALA-S promoter activity is induced by cotransfection with an expression vector containing the catalytic subunit of PKA. Furthermore, cotransfection with a dominant negative mutant of the PKA regulatory subunit impairs the cAMP analog-mediated increase, but the phenobarbital-mediated induction is not modified. Our data suggest that the transcription factor cAMP-response element binding protein (CREB) is probably involved in PKA induction of ALA-S gene expression. Finally, heme addition greatly decreases the basal and phenobarbital or cAMP analog-mediated induction of ALA-S promoter activity. The present work provides evidence that cAMP, through PKA-mediated CREB phosphorylation, and phenobarbital induce ALA-S expression at the transcriptional level, while heme represses it.

Characterization of the human very-long-chain acyl-CoA dehydrogenase gene promoter region: a role for activator protein 2

Very-long-chain acyl-CoA dehydrogenase (VLCAD) is one of a family of nuclear-encoded enzymes that catalyze the initial step in mitochondrial fatty acid beta-oxidation (FAO). Previous studies have indicated that two other members of the AD gene family (medium-chain AD and long-chain AD) are controlled at the transcriptional level by nuclear hormone receptors. In this study, we have cloned and characterized the human VLCAD gene promoter region to identify cis-acting elements involved in its transcriptional control. VLCAD gene promoter-luciferase reporter (VLCAD-Luc) constructs were found to be transcriptionally active in a variety of mammalian cell lines and in primary rat cardiomyocytes when driven by varying lengths of the VLCAD promoter region. Removal of a 20-bp DNA segment of the proximal VLCAD gene promoter markedly reduced the transcriptional activity of VLCAD-Luc constructs. Gel mobility shift assays identified a DNA-binding activity in nuclear extracts prepared from human hepatoma G2 cells that interacted with the 20-bp regulatory region. Competition studies revealed that this DNA-binding activity could be abolished by a molar excess of unlabeled specific oligonucleotide as well as a DNA fragment containing an activator protein 2 (AP-2)-binding site but not by an unrelated nonspecific DNA fragment. These results provide an initial characterization of the human VLCAD gene promoter, identify AP-2 as a candidate activator of VLCAD gene transcription, and suggest that VLCAD gene transcription may be regulated by pathways distinct from that of other AD genes.

Spatiotemporal expression pattern of keratins in skin of AP-2alpha-deficient mice

Transcription factor AP-2alpha has been implicated as being a cell-type-specific regulator of gene expression during vertebrate embryogenesis based on its expression pattern in neural crest cells, ectoderm, and the nervous system in mouse and frog embryos. In mice, AP-2alpha is expressed in surface ectoderm beginning at the single cell layer state around E8.75. AP-2alpha-deficient mice, derived by targeted mutagenesis, display a severe ventral closure defect resulting in cranio-abdominoschisis and a hypoplasia of the cranial ganglia. This study analyzed the effect of a targeted disruption of the AP-2alpha gene on the architecture and the expression of intermediate filaments in skin. We analyzed skin samples from newborn mice and found no difference in either the morphology of the skin or the amount of intermediate filaments expressed. This suggests that despite the results from other analyses, loss of transcription factor AP-2alpha does not affect the expression of intermediate filaments in the skin of newborn animals. We found an altered spatial distribution of intermediate filament expression in the single layered cranial ectoderm during days 9-12 of gestation leading to an evenly distributed expression of keratin 5 and 15 in the mutants. Furthermore, the mutants lack a ring of ectodermal cells highly positive for keratin 15 in the area where lens induction occurs, indicating a defect in the inductive interactions underlying eye formation.