Transcription factors RFX1/EF-C and ATF-1 associate with the adenovirus E1A-responsive element of the human proliferating cell nuclear antigen promoter

A novel application of E1A in combination therapy with EGFR-TKI treatment in breast cancer

Epidermal growth factor receptor (EGFR) is commonly overexpressed in breast cancer and is associated with poor clinical outcomes; however, an increasing number of patients have shown a poor effective response to EGFR tyrosine kinase inhibitors (EGFR-TKI). Here, we found that AXL expression was positively correlated with poor progression in breast cancer patients. Suppression of AXL by an anti-tumor protein, E1A, enhanced EGFR-TKI (gefitinib, erlotinib and lapatinib) sensitization, resulting in significant inhibition of tumor growth in breast cancer cells. Additionally, AXL overexpression dramatically impaired E1A-mediated EGFR-TKI sensitization. These findings show that downregulation of AXL expression by E1A contributes to sensitization to EGFR-TKI in breast cancer, suggesting that combinatorial therapy of AXL inhibitors or E1A gene therapy with EGFR-TKI may be a potential therapeutic strategy for treatment of breast cancer patients.

YY1-binding sites provide central switch functions in the PARP-1 gene expression network

Evidence is presented for the involvement of the interplay between transcription factor Yin Yang 1 (YY1) and poly(ADP-ribose) polymerase-1 (PARP-1) in the regulation of mouse PARP-1 gene (muPARP-1) promoter activity. We identified potential YY1 binding motifs (BM) at seven positions in the muPARP-1 core-promoter (-574/+200). Binding of YY1 was observed by the electrophoretic supershift assay using anti-YY1 antibody and linearized or supercoiled forms of plasmids bearing the core promoter, as well as with 30 bp oligonucleotide probes containing the individual YY1 binding motifs and four muPARP-1 promoter fragments. We detected YY1 binding to BM1 (-587/-558), BM4 (-348/-319) and a very prominent association with BM7 (+86/+115). Inspection of BM7 reveals overlap of the muPARP-1 translation start site with the Kozak sequence and YY1 and PARP-1 recognition sites. Site-directed mutagenesis of the YY1 and PARP-1 core motifs eliminated protein binding and showed that YY1 mediates PARP-1 binding next to the Kozak sequence. Transfection experiments with a reporter gene under the control of the muPARP-1 promoter revealed that YY1 binding to BM1 and BM4 independently repressed the promoter. Mutations at these sites prevented YY1 binding, allowing for increased reporter gene activity. In PARP-1 knockout cells subjected to PARP-1 overexpression, effects similar to YY1 became apparent; over expression of YY1 and PARP-1 revealed their synergistic action. Together with our previous findings these results expand the PARP-1 autoregulatory loop principle by YY1 actions, implying rigid limitation of muPARP-1 expression. The joint actions of PARP-1 and YY1 emerge as important contributions to cell homeostasis.

The oncogenic role of Yin Yang 1

Yin Yang 1 (YY1) is a transcription factor with diverse and complex biological functions. YY1 either activates or represses gene transcription, depending on the stimuli received by the cells and its association with other cellular factors. Since its discovery, a biological role for YY1 in tumor development and progression has been suggested because of its regulatory activities toward multiple cancer-related proteins and signaling pathways and its overexpression in most cancers. In this review, we primarily focus on YY1 studies in cancer research, including the regulation of YY1 as a transcription factor, its activities independent of its DNA binding ability, the functions of its associated proteins, and mechanisms regulating YY1 expression and activities. We also discuss the correlation of YY1 expression with clinical outcomes of cancer patients and its target potential in cancer therapy. Although there is not a complete consensus about the role of YY1 in cancers based on its activities of regulating oncogene and tumor suppressor expression, most of the currently available evidence supports a proliferative or oncogenic role of YY1 in tumorigenesis.

Two estrogen response element sequences near the PCNA gene are not responsible for its estrogen-enhanced expression in MCF7 cells

Background

The proliferating cell nuclear antigen (PCNA) is an essential component of DNA replication, cell cycle regulation, and epigenetic inheritance. High expression of PCNA is associated with poor prognosis in patients with breast cancer. The 5′-region of the PCNA gene contains two computationally-detected estrogen response element (ERE) sequences, one of which is evolutionarily conserved. Both of these sequences are of undocumented cis-regulatory function. We recently demonstrated that estradiol (E2) enhances PCNA mRNA expression in MCF7 breast cancer cells. MCF7 cells proliferate in response to E2.

Methodology/Principal Findings

Here, we demonstrate that E2 rapidly enhanced PCNA mRNA and protein expression in a process that requires ERα as well as de novo protein synthesis. One of the two upstream ERE sequences was specifically bound by ERα-containing protein complexes, in vitro, in gel shift analysis. Yet, each ERE sequence, when cloned as a single copy, or when engineered as two tandem copies of the ERE-containing sequence, was not capable of activating a luciferase reporter construct in response to E2. In MCF7 cells, neither ERE-containing genomic region demonstrated E2-dependent recruitment of ERα by sensitive ChIP-PCR assays.

Conclusion/Significance

We conclude that E2 enhances PCNA gene expression by an indirect process and that computational detection of EREs, even when evolutionarily conserved and when near E2-responsive genes, requires biochemical validation.

Human T-cell leukemia virus type 1 Tax relieves repression of proliferating cell nuclear antigen gene expression

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia. The transforming ability of Tax, the viral oncoprotein, is believed to depend on interactions with cell cycle regulators and on transactivation of genes that control cellular proliferation, including proliferating cell nuclear antigen (PCNA), a cofactor associated with DNA replication and repair. Tax associates with cellular transcription factors to alter their affinity for cognate DNA elements, leading to increased or decreased transcription from that promoter. Although it has been demonstrated that Tax transactivates the PCNA promoter, the mechanism of transcriptional activation is unknown. Here we report a cellular complex that binds specifically to a novel site within the minimal Tax-responsive element of the TATAA-less PCNA promoter. Mutation at this binding site or Tax expression inhibited complex formation and increased promoter activity, suggesting that the complex is a transcriptional repressor. The activation of PCNA gene expression by Tax and consequential decrease in nucleotide excision repair mediated by PCNA overexpression could contribute to the reduced DNA repair capacity and genomic instability observed in HTLV-1-infected cells.

Regulation of PCNA and cyclin D1 expression and epithelial morphogenesis by the ZO-1-regulated transcription factor ZONAB/DbpA

The tight junction protein ZO-1 inhibits G1/S-phase transition by cytoplasmic sequestration of a complex formed by CDK4 and the transcription factor ZONAB. Canine ZONAB is the homologue of human DbpA, an E2F target gene that is overexpressed in different carcinomas. Since the ZONAB target genes that are involved in G1/S-phase transition are unknown, we employed the mammary epithelial cell line MCF-10A and cDNA arrays to screen for such genes. We identified genes encoding cell cycle and replication proteins whose expression was altered due to increased ZONAB expression. For proliferative cell nuclear antigen and cyclin D1 genes, we show that increased mRNA levels resulted in increased protein levels and we identified ZONAB-responsive elements in their promoters by using different approaches, including chromatin immunoprecipitation assays. RNA interference and overexpression of ZONAB affected the proliferation of both MCF-10A and MDCK cells as well as the differentiation of MDCK cells into polarized cysts in three-dimensional cultures. These results indicate that ZONAB regulates the transcription of genes that are important for G1/S-phase progression and links tight junctions to the transcriptional control of key cell cycle regulators and epithelial cell differentiation.

Antagonistic regulation of the Drosophila PCNA gene promoter by DREF and Cut

The gene promoter of Drosophila proliferating cell nuclear antigen (dPCNA) contains several transcriptional regulatory elements, such as upstream regulatory element (URE), DNA replication-related element (DRE, 5'-TATCGATA), and E2F recognition sites. In the present study, a yeast one-hybrid screen using three tandem repeats of DRE in dPCNA promoter as the bait allowed isolation of a cDNA encoding Cut, a Drosophila homolog of mammalian CCAAT-displacement protein (CDP)/Cux. Electrophoretic mobility shift assays showed that Cut bound to both DRE and the sequence 5'-AATCAAAC in URE, with much higher affinity to the former. Measurement of dPCNA promoter activity by transient luciferase expression assays in Drosophila S2 cells after an RNA interference for Cut or DREF showed DREF activates the dPCNA promoter while Cut functions as a repressor. Chromatin immunoprecipitation assays in the presence or absence of 20-hydroxyecdysone further showed both DREF and Cut proteins to be localized in the genomic region containing the dPCNA promoter in S2 cells, especially in the Cut case upon induction of differentiation. These results indicate that Cut functions as a transcriptional repressor of dPCNA gene by binding to the promoter region in the differentiated state, while DREF binds to DRE to promote expression of dPCNA during cell proliferation.

En masse nascent transcription analysis to elucidate regulatory transcription factors

Despite exhaustively informing about steady-state mRNA abundance, DNA microarrays have been used with limited success to identify regulatory transcription factors (TFs). The main limitation of this approach is that altered mRNA stability also strongly governs the patterns of expressed genes. Here, we used nuclear run-on assays and microarrays to systematically interrogate changes in nascent transcription in cells treated with the topoisomerase inhibitor camptothecin (CPT). Analysis of the promoters of coordinately transcribed genes after CPT treatment suggested the involvement of TFs c-Myb and Rfx1. The predicted CPT-dependent associations were subsequently confirmed by chromatin immunoprecipitation assays. Importantly, after RNAi-mediated knockdown of each TF, the CPT-elicited induction of c-Myb- and/or Rfx1-regulated mRNAs was diminished and the overall cellular response was impaired. The strategies described here permit the successful identification of the TFs responsible for implementing adaptive gene expression programs in response to cellular stimulation.

Nucleophosmin/B23 regulates PCNA promoter through YY1

Ectopic over-expression of nucleophosmin/B23 caused a marked up-regulation in the amounts of Yin Yang 1 (YY1) and proliferating cellular nuclear antigen (PCNA) proteins. Transfection with nucleophosmin/B23-targeting siRNA induced a decrease in the intracellular amounts of nucleophosmin/B23, YY1, and PCNA. PCNA expression and its promoter activity were attenuated either by nucleophosmin/B23-siRNA or YY1-siRNA. The ChIP assay showed that positive regulation of PCNA is achieved by binding of YY1 to the initiation site of PCNA promoter. The binding of YY1 to the PCNA promoter and histone H4 acetylation was significantly decreased in nucleophosmin/B23-siRNA-treated cells as compared to control vector-transfected cells. Mutation in YY1 binding site resulted in the loss of PCNA promoter activity and the binding of YY1 to the promoter. The results have indicated that YY1 binds to the initiation site of the PCNA promoter along with histone H4 acetylation, leading to transcriptional activity. We have demonstrated that nucleophosmin/B23 plays an important role in the regulation of PCNA through YY1.

Characterization of dRFX2, a novel RFX family protein in Drosophila

A transcriptional regulatory element was identified in the region between URE (upstream regulatory element) and DRE (DNA replication-related element) in the Drosophila PCNA gene promoter. This element plays an important role in promoter activity in living flies. A yeast one-hybrid screening using this element as a bait allowed isolation of a cDNA encoding a protein which binds to the element in vitro. Nucleotide sequence analyses revealed that the cDNA encodes a novel protein containing a characteristic DNA-binding domain conserved among the regulatory factor X (RFX) family proteins. We termed this protein Drosophila RFX2 (dRFX2) and this element dRFX2 site. To investigate the function of dRFX2 in vivo, we took the strategy of analyzing the dominant negative effects against the endogenous dRFX2. Transgenic flies were established in which expression of HA-dRFX(202-480) carrying the amino acid sequences from 202 to 480 containing the RFX domain (DNA-binding domain) of dRFX2 was targeted to the cells in the eye imaginal discs. In the eye imaginal disc expressing the HA-dRFX(202-480), the G1-S transition and/or the progression of S phase were/was interrupted, and the ectopic apoptosis was induced, though photoreceptor cells differentiated normally. These results indicate that dRFX2 plays a role in G1-S transition and/or in progression of S phase.

The multifunctional role of E1A in the transcriptional regulation of CREB/CBP-dependent target genes

Oncoproteins encoded by the early region 1A (E1A) of adenoviruses (Ads) have been shown to be powerful tools to study gene regulatory mechanisms. As E1A proteins lack a sequence-specific DNA-binding activity, they modulate viral and cellular gene expression by interacting directly with a diverse array of cellular factors, among them sequence-specific transcription factors, proteins of the general transcription machinery, co-activators and chromatin-modifying enzymes. By making use of these factors, E1A affects major cellular events such as cell cycle control, differentiation, apoptosis, and oncogenic transformation. In this review we will focus on the interaction of E1A with cellular components involved in the cAMP/PKA signal transduction pathway and we will discuss the consequences of these interactions in respect to the activation of CREB/CBP-dependent target genes.

The promoter of the human proliferating cell nuclear antigen gene is not sufficient for cell cycle-dependent regulation in organotypic cultures of keratinocytes

The proliferating cell nuclear antigen (PCNA) is essential for DNA replication of mammalian cells and their small DNA tumor viruses. The mechanism of the cell cycle-dependent regulation of the human PCNA promoter is not clear despite extensive investigations. In this report, we employed organotypic cultures of primary human keratinocytes, which closely resemble native skin comprising both proliferating and postmitotic, differentiated cells, to examine the cell cycle-dependent regulation of the human PCNA gene (hPCNA) in the absence or presence of the human papillomavirus type 18 (HPV-18) E7 protein. HPV-18 E7 promotes S phase re-entry in post-mitotic differentiated keratinocytes by abrogating the transcription repression of E2F transcription factors by the retinoblastoma susceptibility protein, pRb. We demonstrated that E7 reactivated the transcription of the endogenous hPCNA in differentiated keratinocytes. In contrast, with or without E7, the expression of a transduced hPCNA promoter-driven reporter did not correlate with that of the endogenous hPCNA gene in either proliferating or differentiated cells. Moreover, in Chinese hamster ovary and L-cells, HPV E7 and the adenovirus E1A protein repressed the transduced hPCNA promoter, but both activated an extended promoter construct spanning the first intron. Mutations of two E2F sites in the intron reduced the basal activity and abolished the response to E7 or E1A. Promoter repression or activation required the CR2 domain of E7 and, to a lesser extent, CR1 as well. However, in organotypic cultures, this extended promoter construct failed to recapitulate the cell cycle-dependent regulation of the endogenous hPCNA gene. Only when a full-length Myc-tagged hPCNA spanning the 5' promoter and all exons and introns was used was the native pattern of expression largely restored, indicative of the complexity of its regulation.

Cloning and characterization of dRFX, the Drosophila member of the RFX family of transcription factors

The RFX family of transcription factors is characterized by a unique DNA binding domain. Five genes have been isolated in mammals, one gene in Caenorhabditis elegans and in the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae. Whereas the roles of the RFX genes are beginning to be understood in yeasts, no clear function has been reported in multicellular organisms, except for RFX5, the most divergent member of the family. To study the physiological role of RFX transcription factors using an alternative multicellular model, we report the isolation and characterization of the Drosophila RFX gene (dRFX). The fruit fly protein shares highly conserved domains with the mammalian factors RFX1 to 3 and is more closely related to this subgroup. It binds DNA with the same target specificity as mammalian factors RFX1 to 3. dRFX is located on chromosome III and we characterized the entire locus. dRFX expression was analyzed during embryogenesis. dRFX mRNAs are detected only in the peripheral nervous system and in the brain of the embryo.

A methylation-responsive MDBP/RFX site is in the first exon of the collagen alpha2(I) promoter

DNA methylation inhibits transcription driven by the collagen alpha2(I) promoter and the 5' end of the gene in transient transfection and in vitro transcription assays. DNA-binding proteins in a unique family of ubiquitously expressed proteins, methylated DNA-binding protein (MDBP)/regulatory factor for X box (RFX), form specific complexes with a sequence overlapping the transcription start site of the collagen alpha2(I) gene. Complex formation increased when the CpG site at +7 base pairs from the transcription start site was methylated. The identity of the protein was demonstrated by co-migration and cross-competition for a characteristic slowly migrating doublet complex formed on MDBP/RFX recognition sequences and the collagen sequences by band shift assays. A RFX1-specific antibody supershifted the collagen DNA-protein complexes. Furthermore, in vitro translated RFX1 protein formed a specific complex with the collagen sequence that was also supershifted with the RFX1 antibody. MDBP/RFX displayed a higher affinity binding to the collagen sequence if the CpG at +7 was mutated in a manner similar to TpG. This same mutation within reporter constructs inhibited transcription in transfection and in vitro transcription assay. These results support the hypothesis that DNA methylation-induced inactivation of collagen alpha2(I) gene transcription is mediated, in part, by increased binding of MDBP/RFX to the first exon in response to methylation in this region.

The dimerization/repression domain of RFX1 is related to a conserved region of its yeast homologues Crt1 and Sak1: a new function for an ancient motif

The RFX protein family includes members from yeast to humans, which function in various biological systems, and share a DNA-binding domain and a conserved C-terminal region. In the human transcription regulator RFX1, the conserved C terminus is an independent functional domain, which mediates dimerization and transcriptional repression. This dimerization domain has a unique ability to mediate the formation of two alternative homodimeric DNA-protein complexes, the upper of which has been linked to repression. Here, we localize the complex formation capacity to several different RFX1 C-terminal subregions, each of which can function independently to generate the upper complex and repress transcription, thus correlating complex formation with repression. To gain an evolutionary perspective, we have examined whether the different properties of the RFX1 C terminus exist in the two yeast RFX proteins, which are involved in signaling pathways. Replacement of the RFX1 C terminus with those of Sak1 and Crt1, its orthologues from Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively, and analysis of fusions with the Gal4 DNA-binding domain, revealed that the ability to generate the two alternative complexes is conserved in the RFX family, from S. cerevisiae to man. While sharing this unique biochemical property, the three C termini differed from each other in their ability to mediate dimerization and transcriptional repression. In both functions, RFX1, Sak1, and Crt1 showed high capacity, moderate capacity, and no capacity, respectively. This comparative analysis of the RFX proteins, representing different evolutionary stages, suggests a gradual development of the conserved C terminus, from the appearance of the ancestral motif (Crt1), to the later acquisition of the dimerization/repression functions (Sak1), and finally to the enhancement of these functions to generate a domain mediating highly stable protein-protein interactions and potent transcriptional repression (RFX1).

In vivo structure of two divergent promoters at the human PCNA locus. Synthesis of antisense RNA and S phase-dependent binding of E2F complexes in intron 1

Proliferating cell nuclear antigen (PCNA) synthesis is strictly regulated during the cell cycle. To investigate PCNA transcriptional regulation, we have analyzed protein-DNA interactions at the promoter region and in the first intron in quiescent fibroblasts and following serum stimulation. Twenty putative protein-binding sites, distributed in two divergent promoters at the PCNA locus, were identified in vivo by genomic footprinting. These elements bind transcription factors continuously throughout the cell cycle with the exception of one E2F consensus site, located in the first intron at position +583. This E2F site becomes strongly occupied 18 h after serum stimulation, implying that an E2F activator complex plays a role in activation of the PCNA gene at the onset of S phase. We detected a 500-600-base pair-long antisense transcript by Northern blot analysis. This RNA has no apparent coding capacity and is constitutively transcribed from a promoter located within the first intron. We suggest that silencing of the PCNA gene is accomplished through base pairing between sense pre-mRNA and antisense RNA. The binding of S phase-specific E2F complexes at the +583 element may help to overcome the negative effect of the antisense transcript, which results in up-regulation of PCNA expression in proliferating cells.

Involvement of RFX1 protein in the regulation of the human proliferating cell nuclear antigen promoter

The proliferating cell nuclear antigen (PCNA) is an essential eukaryotic DNA replication factor that is transcriptionally regulated by the adenovirus oncoprotein E1A 243R. Inducibility of the human PCNA promoter by E1A 243R is conferred by the cis-acting PCNA E1A-responsive element (PERE), which associates with the ATF-1, cAMP response element-binding protein (CREB), and RFX1 transcription factors and is modulated by cellular proteins such as the coactivator CREB-binding protein (CBP) and tumor suppressor p107 (Labrie, C., Lee, B. H., and Mathews, M. B. (1995) Nucleic Acids Res. 23, 3732-3741; Lee, B. H., and Mathews, M. B. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 4481-4486; Lee, B. H., Liu, M., and Mathews, M. B. (1998) J. Virol. 72, 1138-1145). RFX1 also forms a complex with sequences in the PCNA promoter of mouse and rat that share homology with the RFX1 consensus site. To explore the role of RFX1 in regulating the PCNA promoter, we examined the effects of mutations in the human PERE on RFX1 binding and gene expression. Mutations within the RFX1 consensus binding site reduced RFX1 binding, whereas mutations upstream of the site, or on its border, increased RFX1 binding. These mutations also affected the transcriptional activity of PCNA-chloramphenicol acetyltransferase reporter constructs in transient expression assays. The relative transcriptional activity of mutant PCNA promoters, both in the presence and absence of E1A 243R, was inversely related to their ability to complex with RFX1. These findings suggest that the binding of RFX1 is influenced by sequences outside its consensus binding site and that this transcription factor plays an inhibitory role in the regulation of PCNA gene expression.

Serum responsiveness of the rat PCNA promoter involves the proximal ATF and AP-1 sites

We have previously shown that the rat PCNA (proliferating cell nuclear antigen) gene promoter is responsive to serum stimulation. In this study, the sequence of the promoter responsive to serum stimulation has been localized in the region between nucleotides -70 and +125 relative to the transcription initiation site. This region contains an ATF site (nucleotides -51 to -44) and an AP-1 site (nucleotides -64 to -58). Mutation at either the ATF or the AP-1 site reduced the serum responsiveness of the promoter. In gel mobility shift assays, nuclear extracts from serum stimulated cells, compared to those from quiescent cells, exhibit an increasing binding activity toward a promoter related oligonucleotide (-70 to -42) which includes the ATF site and the AP-1 site. Formation of the DNA:protein complexes requires the simultaneous involvement of ATF and AP-1 sites as either element can abrogate the complexes in the competition experiment. Both the distance and sequence are essential to complex formation. Moreover, ATF-1 but not ATF-2 (or CREB) has been identified as a major component of the complexes in the antibody supershift or interference experiment. The results of this study suggest that ATF-1 in association with other factors is involved in regulating the serum stimulation of the rat PCNA promoter activity via the proximal ATF and AP-1 sites.

RFX1, a single DNA-binding protein with a split dimerization domain, generates alternative complexes

The transcription of various viral and cellular genes is regulated by palindromic and nonpalindromic DNA sites resembling the EP element of the hepatitis B virus enhancer, which generate similar DNA-protein complexes. The upper EP complex contains homodimers of the transcription regulator RFX1. We show that RFX1 possesses a split, extended dimerization domain composed of several evolutionarily conserved boxes, one of which was previously shown to mediate dimerization. Such an unusually long and complex dimerization domain could potentially serve for generating multiple complexes. In addition to the previously characterized complex, RFX1 generated a novel DNA-protein complex of extremely low mobility, formed only with palindromic DNA sites. Different deletions within the dimerization domain altered the relative abundance of the two complexes, suggesting an interplay between them. Formation of the low mobility complex correlated with transcriptional repression, in that both activities were mediated by several portions of the conserved region. Our results propose a mechanism by which the extended dimerization domain mediates the formation of alternative homodimeric complexes, which differ in the nature of the intersubunit interaction. By participating in different types of interactions, this domain may regulate the relative abundance of the different complexes, thus affecting transcriptional activity.

The carboxy-terminal region of adenovirus E1A activates transcription through targeting of a C-terminal binding protein-histone deacetylase complex

Binding of the C-terminal binding protein, CtBP, to the adenovirus E1A moiety of a Gal4-E1A fusion protein abolishes conserved region (CR) 1-dependent transcription activation. In contrast, a non-promoter targeted E1A peptide, capable of binding CtBP, can induce transcription from the proliferating cell nuclear antigen (PCNA) promoter. CtBP is shown here to bind the histone deacetylase HDAC1, suggesting that a promoter targeted CtBP-HDAC1 complex can silence transcription from the PCNA promoter through a deacetylation mechanism. Expression of the CtBP binding domain of E1A is sufficient to alleviate repression, possibly due to the displacement of the CtBP-HDAC1 complex from the promoter.

Regulation of the human proliferating cell nuclear antigen promoter by the adenovirus E1A-associated protein p107

The adenovirus E1A 243R oncoprotein is capable of transactivating the expression of the human proliferating cell nuclear antigen (PCNA) promoter. Mutational analysis of the E1A 243R protein suggested that both its p300/CBP- and p107-binding regions are required for optimal induction of the PCNA promoter (C. Kannabiran, G. F. Morris, C. Labrie, and M. B. Mathews, J. Virol. 67:425-437, 1993). We show that overexpression of p107 antagonizes the induction of PCNA by E1A 243R in transient expression assays. This inhibition is largely independent of p107's ability to interact with E1A 243R, because p107 mutants unable to bind to E1A 243R retain the ability to repress the E1A-activated PCNA promoter. Electrophoretic mobility shift assays with the PCNA promoter detected the presence of p107 in one of the major DNA-protein complexes, EH1, formed with HeLa cell nuclear extracts. Promoter mutations that disrupt the formation of complex EH1 abrogated p107's ability to reverse E1A 243R-induced PCNA expression. The same mutations characterize a sequence important for the binding of transcription factor RFX1 (C. Labrie, G. F. Morris, and M. B. Mathews, Nucleic Acids Res. 23:3732-3741, 1995), implying that p107 antagonizes E1A 243R-induced PCNA expression through this RFX1-binding site. Our data are suggestive of a novel cooperative mechanism for transactivation of PCNA expression, in which E1A 243R relieves transcriptional repression exerted by p107 on the promoter.

The transcriptional activation and repression domains of RFX1, a context-dependent regulator, can mutually neutralize their activities

EP is a DNA element found in regulatory regions of viral and cellular genes. While being a key functional element in viral enhancers, EP has no intrinsic enhancer activity but can stimulate or silence transcription in a context-dependent manner. The EP element is bound by RFX1, which belongs to a novel, evolutionarily conserved protein family. In an attempt to decipher the mechanism by which EP regulates transcription, the intrinsic transcriptional activity of RFX1 was investigated. A functional dissection of RFX1, by analysis of deletion mutants and chimeric proteins, identified several regions with independent transcriptional activity. An activation domain containing a glutamine-rich region is found in the N-terminal half of RFX1, while a region with repressor activity overlaps the C-terminal dimerization domain. In RFX1 these activities were mutually neutralized, producing a nearly inactive transcription factor. This neutralization effect was reproduced by fusing RFX1 sequences to a heterologous DNA-binding domain. We propose that relief of self-neutralization may allow RFX1 to act as a dual-function regulator via its activation and repression domains, accounting for the context-dependent activity of EP.

Transcriptional coactivator cAMP response element binding protein mediates induction of the human proliferating cell nuclear antigen promoter by the adenovirus E1A oncoprotein

The proliferating cell nuclear antigen (PCNA), a crucial component of eukaryotic cell cycle and DNA replication complexes, is induced by the adenovirus E1A 243R oncoprotein through a cis-acting element termed the PERE (PCNA-E1A responsive element). The PERE contains a sequence homologous to an activating transcription factor (ATF) motif, and ATF-1 is a major component of PERE-protein complexes. We have identified a second PERE-binding protein, the cAMP response element binding protein (CREB) transcription factor, which forms heterodimers with ATF-1 at this site. CREB, but not ATF-1, is able to mediate transactivation of a minimal PCNA-chloramphenicol acetyltransferase reporter by E1A 243R. Further analysis revealed that the transcriptional coactivator, the CREB-binding protein (CBP), associates with PERE-related complexes, and that CBP is able to mediate a strong transactivation response to E1A 243R at the PCNA promoter. Experiments conducted with mutants in the E1A or CREB components support a model whereby E1A 243R transactivates the PCNA promoter via a CBP-CREB-PERE pathway. These findings delineate a paradigm by which E1A 243R can target and transactivate specific DNA promoter sequences.

The role of ATF in regulating the human cytomegalovirus DNA polymerase (UL54) promoter during viral infection

Previous analysis of the human cytomegalovirus (HCMV) DNA polymerase (UL54) early gene promoter demonstrated that transcriptional activation of this gene is dependent upon the interaction of cellular transcription factors with viral transactivators (J. A. Kerry, M. A. Priddy, T. Y. Jervey, C. P. Kohler, T. L. Staley, C. D. Vanson, T. R. Jones, A. C. Iskenderian, D. G. Anders, and R. M. Stenberg, J. Virol. 70:373-382, 1996). A sequence element, IR1, was shown to be the primary regulatory element of this promoter in transient assays. However, assessment of this element in the context of the viral genome revealed IR1-independent activation at late times after infection. To extend these studies, we aim to identify additional sequence elements involved in the activation of the UL54 promoter. Our present studies demonstrate that the level of binding of proteins to the ATF site in the UL54 promoter is enhanced by viral infection. Furthermore this increase is sensitive to treatment with phosphonoacetic acid (PAA), a DNA synthesis inhibitor. These data suggest that the increase in the level of ATF binding activity is regulated, either directly or indirectly, by HCMV late gene expression. By using specific antibodies, we determined that ATF-1 was a major component of the proteins binding to the UL54 ATF site at late times. In addition, we have demonstrated direct binding of recombinant ATF-1 to the UL54 ATF site. To assess the biological significance of these events, a recombinant virus construct was generated that contained the UL54 promoter with a mutation in the ATF site regulating expression of the chloramphenicol acetyltransferase (CAT) reporter gene inserted between open reading frames US9 and US10. Analysis of this virus (RVATFmCAT) revealed that mutation of the ATF site does not alter the kinetics of UL54 promoter activation. However, levels of CAT mRNA and activity were reduced by 5- to 10-fold compared to those of the wild-type promoter at all stages of infection. These findings indicate that ATF-1 can regulate the levels of UL54 promoter activity at both early and late times. Furthermore, these results imply that HCMV can regulate the activity of cellular factors involved in early gene regulation.

Evidence for the Role of Cyclic AMP-Responsive Elements in Human Virus Replication and Disease

We review the involvement of the cyclic AMP responsive DNA element (CRE) and the ATF/CREB (activating transcription factor/CRE binding protein) family of transcription factors in the regulation and pathology of clinically important viruses that infect humans, including the herpesviridae, adenoviridae, parvoviridae, hepadnaviridae, and retroviridae families. CRE sequences found in specific regulatory elements of human viruses are listed, and the functional evidence for CRE activity, in the form of DNA binding assays, mutational studies, transfection and transcriptional activation experiments, or in vitro transcription assays, is summarized. Manipulation of cellular processes is required for virus replication in human cells following infection. A primary target of many viruses is the cellular transcription machinery, and several human viruses contain transcriptional activator and repressor proteins that affect cellular transcription. Through their effect on cellular transcription, viral genes alter the pattern of cellular gene expression, and thereby affect the differentiation state and cell cycle progression of the infected cell. We summarize evidence demonstrating that the CRE and its binding proteins are involved in the activity of the viruses, implicating their function in the pathogenesis of human diseases. The targeting of specific transcription factor pathways as a potential therapeutic approach is discussed. Copyright 1996 S. Karger AG, Basel