PDX1, a cellular homeoprotein, binds to and regulates the activity of human cytomegalovirus immediate early promoter.

Cellular homeoproteins have been shown to regulate the transcription of several viruses, including herpes simplex viruses, human papillomaviruses, and mouse mammary tumor viruses. Previous studies investigating the anti-viral mechanisms of several cyclin-dependent kinase inhibitors showed that the homeoproteins, pre B-cell leukemia transcription factor 1 (PBX1) and PBX-regulating protein-1 (PREP1), function as transcriptional activators of Moloney murine leukemia virus. Here, we examined the involvement of cellular homeoproteins in regulating the activity of the human cytomegalovirus immediate early (CMV IE) promoter. We identified a 45-bp element located at position -593 to -549 upstream of the transcription start site of the CMV IE gene, which contains multiple putative homeoprotein binding motifs. Gel shift assays demonstrated the physical association between a homeodomain protein, pancreatic-duodenal homeobox factor-1 (PDX1) and the 45-bp cytomegalovirus (CMV) region. We further determined that PDX1 represses the CMV IE promoter activity in 293 cells. Overexpression of PDX1 resulted in a decrease in transcription of the CMV IE gene. Conversely, blocking PDX1 protein synthesis and mutating the PDX1 binding sites enhanced CMV IE-dependent transcription. Collectively, our results represent the first work demonstrating that a cellular homeoprotein, PDX1, may be a repressor involved in regulation of human CMV gene expression.

It has been hypothesized that the CDKIs block viral replication by inhibiting the transcription of specific cellular genes that are required for viral infection. Our previous work tested this hypothesis using microarray technology and identified a cellular homeoprotein, pre B-cell leukemia transcription factor 1 (PBX1), as a target of the CDKIs and a required cellular co-factor for Moloney MLV replication (3). PBX1 was shown to form a heterodimer with another homeodomain protein, PBXregulating protein-1 (PREP1), and function as a transcriptional activator of Moloney MLV (3). The PBX1-PREP1 DNA binding motif, TGATTGAC, was further shown to be conserved in the long terminal repeats of 14 other murine retroviruses (3), suggesting the importance of homeoproteins in regulating retroviral transcription.
A number of cellular homeoproteins, including OCT-1, Brn-3a, and Brn-3b, as well as CCAAT displacement protein (CDP), have previously been shown to play a role in viral transcription ranging across several virus families. The HSV transcriptional activator VP16, for example, directs the formation of a multiprotein-DNA complex on a specific element found in the promoters of HSV immediate-early genes (10). This VP16-induced complex is composed of two cellular proteins, HCF-1 and a homeoprotein OCT-1 (11). OCT-1 contains a POU (Pit-Oct-Unc) DNA-binding domain, which is composed of an amino-terminal POU-specific domain and a carboxyl-terminal POU homeodomain (12,13). The OCT-1 POU homeodomain plays an important role in directing the binding and formation of VP16-induced complexes (14,15). Other POU family transcription factors, Brn-3a (also known as Brn-3.0) and Brn-3b (also known as Brn-3.2), originally identified in neuronal cells (16 -18) and subsequently observed in cervical cells, have been shown to regulate viral transcription of human papillomavirus-16 (HPV-16) and HPV-18 (19,20). Both factors were shown to directly bind to the upstream regulatory region of the virus genome, however the two factors exert opposing effects, with Brn-3a activating transcription directed by the HPV upstream regulatory region, and Brn-3b repressing E6 and E7 expression (20). Furthermore, a possible involvement of Brn-3a in the pathogenesis of HSV has also been suggested (21). The silencer CDP, the mammalian homolog of the Drosophila CUT protein and a homeoprotein, negatively regulates HPV-dependent transcription by binding to specific DNA elements located in the viral promoters and long control regions, thus repressing HPV replication (22)(23)(24). In addition, CDP has been shown to block the transcription of mouse mammary tumor viruses through binding to the negative regulatory regions located in the long terminal repeats of mouse mammary tumor virus (25)(26)(27).
Due to the emergence of a role for homeodomain proteins in regulation of viral transcriptional processes, we explored the possible involvement of these factors in regulating transcription of the ␤-herpes virus group member, human cytomegalovirus (CMV). Expression of the human CMV immediate early (IE) gene is critical for productive viral replication (28,29). It has been shown that the replication and transcription of human CMV depends on the cellular differentiation status of the host cell. For example, in undifferentiated monocytes, little or no human CMV IE gene products are expressed, and CMV remains latent. Abundant IE gene products are produced and infectious CMV is generated when the monocytes differentiate to macrophages (30). The molecular basis for the repression of CMV IE gene remains largely unknown. In this report, we test a panel of homeoproteins, including HOXA9, PBX1, PREP1, MEIS1, and pancreatic-duodenal homeobox factor-1 (PDX1), a protein previously shown to complex with PBX1 in the regulation of elastase and somatostatin gene expression (31)(32)(33), for their ability to modulate the activity of human CMV IE promoter. We identified a 45-bp element located within the promoter of the CMV IE gene, which contains 12 putative binding sites for these homeoproteins. Our results indicate that multiple cellular proteins bind to the 45-bp CMV element and that PDX1 is present in this specific protein-DNA complex. We further demonstrate the functional significance of this interaction in cell-based assays that measure CMV IE-dependent transcription. PDX1 was found to negatively regulate the activity of the CMV IE promoter in 293 cells, and mutations in the PDX1 binding motifs alleviated these repressive effects. From these experiments we conclude that PDX1 binds to a specific region of the human CMV IE promoter and represses the promoter activity.

EXPERIMENTAL PROCEDURES
Cells and Reagents-293, 293T, HeLa, PANC-1, and THP-1 cells were obtained from American Type Culture Collection. Differentiation of THP-1 cells was achieved with supplementation of growth media with 10 nM phorbol 12-myristate 13-acetate (PMA, Sigma) for 2 days. 293 cells were transfected with a CMV-firefly luciferase plasmid, and stable integrants were selected using 5 mg/ml blasticidin (Invitrogen) to generate the CMV-Luc stable cell line, 293-CMV-Luc.
Plasmids and Plasmid Construction-The Pdx1 (accession number: X99894) complementary DNA (cDNA) was reverse-transcribed with gene-specific primers using the Qiagen OneStep RT-PCR (reverse transcription-PCR) kit as prescribed by the manufacturer. In brief, the cDNA product was synthesized and amplified from 1 g of purified human pancreatic poly(A) mRNA (Clontech) using two primers (5Ј-AATAGGATCCGCCGCAGCCATGAACGGCGA and 5Ј-CTCCTCTA-GACTCTCATCGTGGTTCCTGCG). The resultant product was inserted into the multiple cloning site of pUB6/V5-His B (Invitrogen) using the appended BamHI and XbaI (underlined) restriction sites to generate pUB-PDX1, in which the expression of Pdx1 was driven by the human ubiquitin C promoter. Two PDX1 mutants, pUB-PDX1H189F and pUB-PDX1I192Q, were created using the QuikChange site-directed mutagenesis kit (Stratagene) as described previously (34). A pUB-PREP1 plasmid was generated by cloning the coding sequences of Prep1 into the pUB6 vector using CMV-PREP1 as the DNA template (3). The pCITE-PBX1a, pCITE-MEIS1, and pCITE-PREP1 plasmids were constructed as described previously (3). The pCITE-PBX1b and pCITE-PDX1 plasmids (including wild-type and mutant Pdx1) were created by cloning the coding regions of Pbx1b and Pdx1 into pCITE vectors using CMV-PBX1b and pUB-PDX1 (wild-type or mutant Pdx1) as templates (3). The pCITE constructs were used to synthesize translated proteins in vitro using TNT Quick Coupled Transcription/Translation Systems (Promega). Three luciferase plasmids were used in this study: ubiquitin-firefly luciferase (pUB-F-Luc), CMV-firefly luciferase (CMV-F-Luc; containing the human CMV IE promoter-enhancer), and CMV-Renilla luciferase (CMV-R-Luc; containing the human CMV IE promoter-enhancer). The firefly luciferase sequences of pGL2 control (Promega) were subcloned into pUB6 or pcDNA6 (Invitrogen) to generate pUB-F-Luc or CMV-F-Luc. CMV-R-Luc was purchased from Promega (i.e. pRL-CMV). Mutations and deletions of the human CMV IE promoter were constructed using the QuikChange site-Directed mutagenesis kit (Stratagene). All CMV-Luc mutants were generated using CMV-R-Luc as template. To mutate the potential homeoprotein binding tetramers, the middle two nucleotides were changed to two cytosines (for example, TAAT to TCCT).
EMSAs-Three different DNA fragments of the 45-nucleotide CMV sequences were used as the DNA probes for EMSAs: CMV1 (5Ј-GGCAT-TGATTATTGACTAGTTATTAATAGTAA), CMV2 (5Ј-AATAGTAAT-CAATTACGGGGTCATTAGTTCA), and CMV12 (5Ј-GGCATTGATTAT-TGACTAGTTATTAATAGTAATCAATTACGGGGTCATTAGTTCA), which contain the first half, second half, and entire regions of the 45-bp region, respectively. Electrophoretic mobility shift assays (EMSAs) were performed as described previously (3) using CMV1, CMV2, or CMV12 as the DNA probes. Briefly, anti-PBX1, anti-MEIS1, anti-PREP1, anti-PDX1, anti-HOXA9, or anti-SP1 antibodies (Santa Cruz Biotechnology, Inc.) were incubated with nuclear extracts prepared from 293 or HeLa cells for 10 min at room temperature before the 32 P-labeled probe was included. When in vitro translated proteins were used in EMSAs, DNA binding reactions were performed at 4°C for 30 min. DNA and DNA-protein complexes were resolved on 5% non-denaturing polyacrylamide gels at room temperature in 0.3ϫ TBE (27 mM Tris-borate, pH 8.3, 0.6 mM EDTA). Following electrophoresis, the gels were dried and exposed to x-ray film.
Transfection and Luciferase Assays-293 or 293T cells were grown to 50 -80% confluence in 96-well plates. Transfections were performed using FuGENE 6 (Roche Applied Science) or LipofectAMINE 2000 reagent (Invitrogen) as described in the manufacturers' manuals. For the overexpression assays, 293 or 293T cells were co-transfected with CMV-R-Luc, the indicated expression vectors (i.e. pUB-PDX1, -PDX1H189F, -PDX1I192Q, or -PREP1) and a pUB-F-Luc internal control plasmid for 48 h. In the CMV mutant assays, 293 and 293T cells were co-transfected with pUB-F-Luc (as internal control) and a wildtype or mutant CMV-R-Luc. Firefly and Renilla luciferases were measured using the Dual-Glo assay system (Promega), and the activities were determined using an Acquest multimode reader (LJL Biosystems, Inc.).
Short interfering RNAs (siRNA) targeting positions 29 -47 (Pdx1-1 siRNA) or 554 -572 (Pdx1-2 siRNA) of the Pdx1 open reading frame and positions 20 -38 of the Prep1 open reading frame (accession number: XM_033008) were purchased from Qiagen. A fluorescein-labeled luciferase GL2 duplex was obtained from Dharmacon and used to determine transfection efficiency. An siRNA targeting Renilla luciferase was used as a control (35). 293 and 293T cells were co-transfected with CMV-F-Luc and the appropriate siRNA (i.e. Pdx1, Prep1, or R-Luc siRNAs). The effects of the siRNAs on firefly luciferase expression and activity were measured using the Bright-Glo assay system (Promega) 48 h after transfection. The inhibitory effects of the Pdx1 and Prep1 siRNAs on cellular PDX1 and PREP1 protein synthesis were further examined by Western blot analysis using anti-PDX1 and anti-PREP1 antibodies (Santa Cruz Biotechnology, Inc.).

PDX1 Associates with a Specific Region of the Human CMV
IE Promoter-Our previous work identified two cellular homeodomain proteins, PBX1 and PREP1, as transcriptional activators of Moloney MLV (3). To further investigate a possible role for homeoproteins in the transcriptional regulation of other virus families, we examined the promoter-enhancer regions of the human CMV IE gene for consensus homeoprotein binding elements. A 45-nucleotide fragment located at position Ϫ593 to Ϫ549 upstream of the transcription start site of the CMV IE gene was found to contain numerous putative homeoprotein binding sites (Fig. 1). Including the reverse complementary sequences, 12 potential tetramer binding sites for homeoproteins were identified, including two PBX1 (i.e. TGAT) (36,37), two PREP1 or MEIS1 (i.e. TGAC) (36 -38), six PDX1 (i.e. TAAT) (39,40), and eight HOX binding sites (i.e. TAAT or TTAT) (41-43) (Fig. 1). It has been shown that PDX1 associates with the B element of the transcriptional enhancer of the pancreatic elastase I gene by forming a trimeric complex with PBX1b and MEIS2 in pancreatic acinar cell lines, whereas PDX1 binds to the B element alone in ␤-cell lines (32). It has additionally been observed that cooperative interactions occur between PBX1, PREP1, and PDX1 on the somatostatin mini-enhancer (31).
Based on these observations, we performed EMSAs to examine the possible association between the 45-bp CMV element and homeodomain proteins, including PBX1, MEIS1, PREP1, and PDX1.
The 45-bp region of interest was divided into two segments extending from position Ϫ593 to Ϫ571 (CMV1) and from position Ϫ570 to Ϫ549 (CMV2). Incubation of 32 P-labeled CMV1 DNA with nuclear extracts prepared from 293 cells resulted in the formation of a major DNA-protein complex, complex C ( Fig.  2A; indicated as "C"). The specificity of the complex for CMV1 binding was confirmed using unlabeled specific competitor oligonucleotides (i.e. unlabeled CMV1; Fig The interaction between the CMV1-region and the associated protein complex was then examined using antibodies specific to the individual homeoproteins. Incubation of nuclear extracts with antibodies raised against PDX1 proteins resulted in the appearance of a supershifted complex ( Fig. 2A, arrow). However, the incubation of nuclear extracts with antibodies against PBX1, MEIS1, or PREP1 proteins did not produce any significant new species ( Fig. 2A). A parallel set of EMSAs performed using CMV2 as the DNA probe yielded the same complex C as indicated in Fig. 2A. The further addition of anti-PDX1 antibodies to the reaction mixture resulted in a supershift of complex C (Fig. 2B, arrow). No effects on the DNA-protein complex were seen using anti-PBX1, -MEIS1, or -PREP1 antibodies (data not shown). When the same EMSAs (CMV1 and CMV2 as DNA probes) were performed using nuclear extracts prepared from HeLa cells, instead of 293 cells, identical results were obtained. In agreement with our findings using 293 nuclear extracts, the supershifted C complex was observed only when anti-PDX1 antibodies were included in the reaction (data not shown).
It has been shown that HOXA9 can form a trimeric protein complex with PBX1-MEIS1 or PBX1-PREP1 (43,44). Because there are eight potential HOX binding sites present in the 45-bp CMV region, we investigated the possible association between HOXA9 proteins and the CMV DNA element. EMSAs were thus performed, incubating anti-HOXA9 antibodies with nuclear extracts. The presence of HOXA9 antibodies did not, however, affect the formation of the CMV DNA-protein complex (data not shown).
To further validate the EMSA data using nuclear extracts and antibodies, we also performed EMSAs using in vitro translated PBX1a, PBX1b, MEIS1, PREP1, HOXA9, and PDX1 proteins. The results showed that only the PDX1 protein was able to bind to the CMV1 DNA probe (Fig. 2C). PDX1 further did not appear to cooperate with the other homeodomain proteins in associating with CMV1 (Fig. 2C). Furthermore, the complex of CMV1-PDX1 (Fig. 2C) was much smaller than that observed in the EMSAs using nuclear extracts ( Fig. 2A), suggesting that other cellular proteins are also present in the CMV1-protein complexes. An identical set of results was obtained when CMV2 was used as the DNA probe (data not shown). Lastly, the in vitro synthesized HOXA9 protein did not appear to interact with the CMV DNA either as a monomer, or in combination with the other homeoproteins as a dimer (i.e. PBX1-HOXA9, MEIS1-HOXA9, or PREP1-HOXA9), or trimer (i.e. PBX1-MEIS1-HOXA9 or PBX1-PREP1-HOXA9) (data not shown). Collectively, these gel shift experiments indicate that PDX1 protein is a component of a multiprotein complex that binds a region of the CMV IE promoter.
The expression of Pdx1 was initially described to be restricted to the pancreas and duodenum (45,46). It was later reported that Pdx1 is also expressed in the developing brain (47). However, the expression of PDX1 in 293 and HeLa cells is unknown. Western blot analysis was thus performed using nuclear extracts prepared from 293 and HeLa cells (Fig. 3). In vitro synthesized recombinant PDX1 protein, which contains additional amino acids at the N terminus of PDX1, was used as a positive control (Fig. 3). Nuclear extracts of the pancreatic cell line PANC-1 were also analyzed by Western blot. As shown in Fig. 3, the expression of PDX1 in 293 and HeLa cells was confirmed.
We next wished to examine the expression of PDX1 in the CMV host cells. It has been shown that the expression of human CMV IE gene does not occur in monocytes but is expressed in terminally differentiated macrophages (30). THP-1, a human monocytic cell line, can be induced to differentiate into macrophage-like cells through treatment with PMA (48). Nuclear extracts of untreated and PMA-treated THP-1 cells were analyzed by Western blot using anti-PDX1 antibodies.
The results indicate that PDX1 is expressed in both undifferentiated and differentiated THP-1 cells (Fig. 3).
PDX1 Binds to Multiple Sites within the Human CMV IE Promoter-We next wished to determine the exact PDX1 binding site(s) within the 45-bp CMV region. There are six putative PDX1 binding tetramers, TAAT, present in this region (Fig. 1). Of note, there are also two TTAT and two TGAT tetramers that contain one mismatch at a single position and could therefore comprise PDX1 binding sites. To determine which tetramer conferred association with PDX1, point mutations at each of the ten potential PDX1 binding sites (i.e. TAAT, TTAT, and TGAT) were generated, by changing the central two nucleotides to two cytosines. Six possible PDX1 sites are present within CMV1 (tetramer 1-6; Fig. 4A) and four in CMV2 (tet- ramer 7-10; Fig. 4C). EMSAs were thus performed using in vitro synthesized PDX1 protein and a wild-type or site-mutated CMV1 DNA probe. As shown in Fig. 4B, although mild loss of binding effects were observed for mutations at each site, mutations at tetramers 4 and 6 (both are TAAT) and to a lesser extent, tetramer 3 (i.e. TTAT), prevented PDX1 binding, suggesting that PDX1 binds to these regions preferentially over sites 1, 2, and 5. A similar approach was utilized to examine the potential PDX1 binding motifs in the CMV2 nucleotides (Fig.  4C). Only slight effects on PDX1-CMV2 complexes were observed when sites 7-9 were changed (Fig. 4D). On the other hand, a significant reduction in PDX1 binding was detected when a mutation was introduced into site 10 (which was TAAT), suggesting that PDX1 binds to site 10 versus tetramers 7-9 (Fig. 4D). To further validate the interaction between PDX1 and the 45-bp region of the CMV IE promoter, two PDX1 mutant proteins (i.e. PDX1H189F and PDX1I192Q), which contain mutations within the homeodomain and fail to bind to the PDX1 DNA motif, were generated and analyzed by EMSAs (34). As shown in Fig. 4E, both PDX1 mutants did not associate with the 45-bp region, which reconfirmed the specific interaction between PDX1 protein and the 45-bp element. Collectively,  7-10). Anti-SP1 antibodies were used as the negative control (lane 10). B, an identical set of EMSA was performed using radiolabeled CMV2 (containing the second half of the 45-bp CMV element, 5Ј-GTAATCAATTACGGGGTCATTA) as the DNA probe. The position of the supershifted complex C is indicated by an arrow. C, EMSA was carried out using the CMV1 probe and in vitro translated PBX1a, PBX1b, MEIS1, PREP1, and PDX1 proteins produced by a coupled reticulocyte lysate system. The first lane (indicated by an asterisk) contained lysates alone that did not include an expression construct and demonstrates the binding of endogenous complexes in lysates. The arrow indicates the binding of PDX1 proteins. Similar results were obtained when the CMV2 probe was used (data not shown). these experiments identify three TAAT sites (sites 4, 6, and 10) as major PDX1 binding sites in the CMV IE promoter.
The correlation between major PDX1 binding elements (i.e. sites 4, 6, and 10) and the formation of the CMV-protein complexes was also examined using 293 nuclear extracts instead of in vitro translated PDX1 protein in the assays. Mutations at sites 4 and 6 caused significant, but not complete, disruption of CMV1-PDX1 association, whereas mutation of site 10 completely abrogated CMV2-PDX1 complex formation (data not shown). These data using nuclear extracts suggested that

FIG. 4. Identification of PDX1 binding sites in the 45-bp CMV region.
A, potential PDX1 binding motifs present within CMV1 DNA are indicated, including three TAATs, two TTATs, and one TGAT. Six individual mutant CMV1 oligonucleotides were generated, each destroying a specific possible PDX1-binding tetramer (positions 1-6). For each mutant, the middle two nucleotides of the tetramers were changed to two cytosines (underlined). B, EMSA was carried out utilizing a wild-type (WT) or mutated oligonucleotide (mutations 1-6) combined with in vitro translated PDX1 proteins. The first lane contained the WT CMV1 DNA probe and lysates without the Pdx1 expression plasmid, whereas the PDX1 proteins were included in the remaining lanes. The complexes containing PDX1 proteins are indicated by an arrow. The signal intensities of the PDX1-CMV1 DNA complexes were quantitated using the NIH Java-based image-processing program (National Institutes of Health Image/J) as shown at the bottom of the gel. The quantitative data are presented as the percentage relative to the control, i.e. the PDX1-WT CMV1 complex. C, four potential PDX1 binding motifs are present within CMV2 DNA, including three TAAT and one TGAT tetramers. Four individual mutant CMV2 oligonucleotides were generated, each of which destroyed a specific possible PDX1 tetramer (positions 7-10). The middle two nucleotides of the tetramers were changed to two cytosines (underlined). D, EMSA was performed using a wild-type (WT) or mutated oligonucleotide (mutations 7-10) and the in vitro translated PDX1 proteins. The first lane contained the WT CMV2 DNA and lysates without the Pdx1 expression plasmid, whereas the PDX1 proteins were included in the remaining reactions. The binding of PDX1 protein is indicated by an arrow, and the quantitation of the PDX1-CMV2 DNA complexes are shown at the bottom of the gel. E, the CMV12 oligonucleotide and the wild-type or mutant in vitro synthesized PDX1 proteins were utilized in the gel shift assay. The first lane contained the CMV12 DNA and lysates without the Pdx1 expression plasmid. The binding of wild-type PDX1 proteins is indicated by an arrow. PDX1 is likely not the only determinant in the formation of these CMV DNA-protein complexes. It is possible that other cellular factors present in the CMV-protein complexes may also participate in CMV DNA binding and the formation of these complexes.
PDX1 Negatively Regulates the Human CMV IE-dependent Transcription in 293 and 293T Cells-We further investigated the importance of PDX1 in the regulation of the human CMV IE promoter. 293T cells were thus co-transfected with a reporter vector (CMV-Renilla luciferase, or CMV-R-Luc) and an expression plasmid encoding PREP1, wild-type or mutant PDX1 (i.e. pUB-PREP1, pUB-PDX1WT, pUB-PDX1H189F, or pUB-PDX1I192Q). The ubiquitin-firefly luciferase plasmid (i.e. pUB-F-Luc) was utilized as an internal control for normalization of transfection efficiency. Overexpression of PDX1 resulted in a 33% decrease in CMV-dependent transcription, whereas no effects were detected when PDX1H189F or PDX1I192F were overexpressed in 293T cells (Fig. 5A). Furthermore, in agreement with the gel shift data (Fig. 2), no significant effects were observed when PREP1 was overexpressed (Fig. 5A). The cellbased assays were performed using 293 cells and a 52% decrease in CMV IE-dependent expression was detected when PDX1 was overexpressed (data not shown).
We also used the Pdx1-targeted siRNA to evaluate the effect of removing the PDX1 protein on CMV-mediated transcription. Two Pdx1 siRNAs (i.e. Pdx1-1 and Pdx1-2 siRNAs) were used in the assays. 293 cells were thus co-transfected with CMV-Luc and siRNAs directed against Pdx1 or Prep1. A 6-fold increase in CMV transcription was observed in the Pdx1-2 siRNA-transfected cells, whereas no effects were detected when treating with Prep1 or Pdx1-1 siRNA (Fig. 5B; data not shown). The identical experiments were carried out using 293T cells, and similar results were obtained (data not shown). The Pdx1-2 and Prep1 siRNAs did not cause any significant effects on cellular toxicity or proliferation as determined by Alamar Blue cell viability assays (data not shown). We also generated a stable 293 cell line expressing luciferase from the human CMV IE promoter (293-CMV-Luc cells) and performed a set of experiments identical to the ones described above. Overexpression of PDX1 caused a 30% decrease in CMV-dependent transcription (data not shown). In addition, a greater than 3-fold increase in luciferase activity was observed when 293-CMV-Luc cells were transfected with Pdx1-2 siRNA (data not shown).
The effects of Pdx1 siRNAs on endogenous PDX1 protein levels were determined by Western blot analysis (Fig. 5C). As shown in Fig. 5C, Pdx1-2 but not Pdx1-1 siRNA induced a significant reduction in steady-state PDX1 levels within the cell. A 40 -50% reduction of cellular PREP1 protein was also observed when cells were treated with Prep1 siRNA (data not shown). These findings likely understate the effect of the siRNAs under these conditions, because the transfection efficiency of these siRNAs on 293 cells was determined to be around 50% using fluorescein-labeled luciferase GL2 siRNA (data not shown).
To further investigate the importance of PDX1 in regulating the activity of the CMV IE promoter, mutations in each identified or putative PDX1 binding site were generated using site-directed mutagenesis and tested for differences in expression (Fig. 6A). 293T cells were transiently transfected with a CMV-Luc plasmid, which contained either the wild-type CMV IE promoter or a mutation in the indicated PDX1 site. As shown in Fig. 6B, five mutations resulted in a greater than 2-fold induction in luciferase activity, including four TAAT tetramers (sites 5, 6, 9, and 10). A subset of these mutants (sites 6 and 10) was also identified as comprising major PDX1 binding sites in the EMSA experiments (Fig. 4, B and D). A CMV-Luc construct with double mutations at sites 6 and 10 was also generated and tested, however, no additive effects on luciferase activity were detected (Fig. 6B). Similar results were obtained when 293 cells were utilized in transfection assays (data not shown).
We next wished to determine the effects of deleting regions of the 45-bp element on repression of the CMV IE promoter. Three CMV-Luc deletion constructs, CMV-Luc-del-1, CMV-Luc-del-2, and CMV-Luc-del-3, which lack the first 11, first 33, or the entire 45 bp of the 45-bp region, were generated (Fig.  7A). CMV-Luc-del-1 retains all three major PDX1 binding sites, whereas CMV-Luc-del-2 and CMV-Luc-del-3 lack two or three of the identified PDX1 sites, respectively (Fig. 7A). 293 and 293T cells were transiently transfected with the indicated CMV-Luc deletion mutant or a wild-type CMV-Luc, which served as a control. No significant effects on the luciferase activity were detected on the CMV-Luc-del-1 construct, however, CMV-Luc-del-2 and CMV-Luc-del-3 resulted in up to 2.5fold increase in CMV IE-dependent expression (Fig. 7B).
We further wished to determine if the effects of PDX1 on the CMV IE promoter was restricted to the 45-bp region under study. 293T cells were transiently transfected with a PDX1 expression vector and the wild-type or del-3 CMV-Luc reporter constructs. As seen previously (Fig. 5A), the overexpression of PDX1 caused a 60% decrease in luciferase expression driven by the wild-type CMV IE promoter. When the CMV-Luc-del-3 plasmid was alternatively used in the assay, the repressive effect of PDX1 overexpression was found to be less modest, resulting in a 40% inhibition (Fig. 7C). Although these results confirm that the 45-bp region contains PDX1 binding sites that may partially account for the PDX1-mediated repression, they also suggest that there might be additional PDX1 binding motifs outside the 45-bp region. Indeed, in our more recent analyses of the CMV IE promoter, we have identified additional PDX1 binding sites upstream of the 45-bp element (data not shown). Taken together, our findings suggest that PDX1 represses the promoter activity of the human CMV IE gene in 293 and 293T cells. DISCUSSION The human CMV IE promoter, one of the most potent RNA polymerase II promoters identified thus far, is commonly used for overproduction of recombinant proteins in a wide range of mammalian cells. Here we demonstrate that a cellular homeodomain protein, PDX, binds to the CMV IE promoter and down-regulates its activity in 293 and 293T cells. The identification of a specific 45-bp element, which spans nucleotides Ϫ593 to Ϫ549 of the CMV IE promoter and contains multiple putative homeoprotein binding sites, led to the discovery of PDX1 involvement in CMV-dependent transcription. Sites within this 45-nucleotide region demonstrated varying degrees of PDX1 binding by EMSAs (Fig. 4). In cell-based reporter gene assays, ectopic expression of PDX1 resulted in significant reduction in CMV IE-dependent luciferase activity, whereas PDX1 knockdown by the siRNA caused a 6-fold increase in transcription (Fig. 5, A and B). Furthermore, an increase in CMV IE promoter activity was observed when the PDX1 DNAbinding motifs were mutated or the 45-bp region was deleted ( Fig. 6 and 7). Collectively, these data imply a novel role for PDX1 as a cellular regulator of the human CMV IE promoter.
The region upstream of the CMV IE enhancer between Ϫ750 and Ϫ550 has previously been described as a unique region containing multiple NF-1 binding sites (30,49,50). All of the identified NF-1 binding sites are located upstream of position Ϫ602 (49,50). Interactions between endogenous nuclear proteins and the nucleotide sequence between Ϫ660 and Ϫ540 revealed five specific nuclear protein binding regions, including sequences between Ϫ602 and Ϫ557, Ϫ563 and Ϫ540, and Ϫ602 and Ϫ582, by DNase I protection experiments (49). However, the identities of these cellular proteins have not been reported. The association between the cellular homeoprotein PDX1 and the CMV 45-bp region (between Ϫ593 and Ϫ549) demonstrated here presents a novel finding in understanding CMV biology.
The importance of this unique region in human CMV IE transcription and viral replication has been investigated previously. Recombinant CMV containing deletions from positions Ϫ640 to Ϫ583, which include the putative homeoprotein binding sites 1, 2, and 5 ( Figs. 1 and 4A), did not result in any significant effects on IE transcription (51). However, a deletion of the sequence between Ϫ521 and Ϫ579, which spans tetramers 3, 4, 6, 7, 8, 9, and 10, including the preferred PDX1 binding  (28). In these experiments, viral replication was shown to be comparable to that of wild-type CMV at high m.o.i., however, the replication rates of the mutant viruses were significantly increased at low m.o.i. after 6 days post-infection (52). The IE gene products, IE1 and IE2, have been shown to be required for initiating viral replication (28,29,53,54). The results from our study and the previous recombinant CMV assays (51,52) suggest the possible involvement of PDX1 in human CMV replication through the transcriptional regulation of IE gene expression. Perhaps the inhibitory effects exerted by PDX1 on CMV IE transcription, and thus replication, are masked at high m.o.i. levels of wild-type CMV relative to PDX1, whereas at low m.o.i., PDX1 is not limiting, and the repression is manifest.
In the human CMV genome, the unique region is located between the IE enhancer and the UL127 open reading frame. It has been demonstrated that the unique region negatively regulates the expression from the UL127 promoter and may function as a boundary that efficiently blocks enhancer-promoter interactions (51,55). Ghazal and co-workers showed that a boundary domain, which contains the nucleotides from Ϫ532 to Ϫ598 (including the entire 45-bp element investigated in our work), confers repression on a heterologous promoter when placed between the enhancer and promoter (55). These previous findings help to explain how the CMV IE enhancer could selectively activate the expression of CMV IE gene, instead of UL127 promoter. However, the repressive mechanism of the boundary domain on viral transcription still remains unknown. Our finding of association between the cellular protein PDX1 and the 45-bp region provides a novel insight to investigate the regulation of the boundary domain on CMV IE promoter activity.
To further investigate the importance of the PDX1 binding motifs, CMV-Luc constructs harboring mutations within individual PDX1 binding sites or deletions within the 45-bp element were generated and examined in a cellular context (Figs. 6 and 7). However, unlike the data obtained from Pdx1-2 siRNA-treated cells (Fig. 5B), no greater than a 3-fold increase in the activity of CMV IE promoter-enhancer was observed even with the double mutations at two major PDX1 binding sites (sites 6 and 10) (Fig. 6). It is likely that PDX1 could bind to other non-mutated PDX1 binding sites within the 45-bp region and still participate in complexes that partially repress CMV-dependent transcription. Alternatively, there might be other PDX1 binding sites located outside the 45-bp region. Indeed in our current study of the human CMV IE promoter, the additional PDX1 binding region was identified upstream of the 45-bp element (data not shown). A greater increase in CMV IE transcription may be expected if all PDX1 binding sites present in the promoter-enhancer region could be eliminated. Furthermore, our results also demonstrated that PDX1 might not be the only factor that determines the formation of the specific CMV-protein complexes (Fig. 2). It is possible that one or more other unknown proteins present in the CMV DNAprotein complex might contribute binding energy and partially compensate for losses in PDX1-DNA binding affinity.
PDX1 has been shown to function as a transcriptional activator of several genes, including insulin (56,57), Glut2 (39), glucokinase (58), islet amyloid polypeptide (59, 60), and somatostatin (31). Our data demonstrate a unique function for PDX1 whereby PDX1 negatively regulates the activity of the CMV IE promoter. It is possible that the additional proteins present in the PDX1-CMV DNA complexes could contribute to the inhibitory function of the multiprotein complex on CMV IE transcription. Interestingly, Lundquist et al. (51) have identified a putative human papillomavirus (HPV) silencing motif at position Ϫ591 to Ϫ584 of the CMV IE promoter. It has been shown that the transcriptional repressor CDP binds to this silencing element and blocks transcription and replication of HPV (23). Future experiments will be carried out to examine the potential involvement of CDP in modulating the activity of the human CMV IE promoter. The identification of other unknown cellular proteins present in the CMV IE DNA-protein complexes could provide new insights into our understanding of the transcriptional regulation of the human CMV IE gene.
Although PDX1 functions as a transcription activator in pancreatic cells, its effects on other genes or in other cells/ tissues remain largely unknown. Here we show that PDX1 is also expressed in kidney (293 cells), cervix (HeLa cells), and monocytes (THP-1 cells) (Fig. 3). It is possible that the effects of PDX1 on transcriptional regulation could be tissue-specific and PDX1 could negatively regulate certain specific genes, including viral genes. However, the regulatory mechanism of PDX1 FIG. 7. Deletion analysis of the 45-bp region of the human CMV IE promoter. A, diagram of three CMV-Luc deletion constructs, CMV-Luc-del-1, CMV-Luc-del-2, and CMV-Luc-del-3, in which the first 11, first 33, or entire 45 bp are deleted. The three major PDX1 binding sites are underlined. B, 293 or 293T cells were transiently transfected with the indicated deletion CMV-Luc plasmids or wild-type (WT) CMV-Luc, which served as a control. Luciferase activity was measured 24 -30 h after transfection. C, 293T cells were transiently transfected with pUB-PDX1 and a CMV-Luc reporter plasmid, WT, or del-3. Luciferase activity was measured 48 h after transfection. on the activity of CMV IE promoters remains to be determined. PDX1 might directly affect CMV IE-dependent transcription through association with the CMV IE promoter and interfere with the interaction between the promoter and other transcription factors. Alternatively, PDX1 could also indirectly inhibit the CMV IE promoter by influencing the expression of the other cellular genes that are involved in the regulation of the CMV IE-dependent transcription.
We utilized 293 and 293T cells, which are non-permissive cell lines for human CMV but allow CMV IE-dependent transcription, in our study. Our data demonstrate that PDX1 is involved in the transcriptional repression of CMV IE-dependent transcription in 293 and 293T cells. It will be interesting to determine the effects of PDX1 on human CMV IE transcription in the CMV host cells, such as monocytes. Cells of the monocyte lineage serve as reservoirs of latent human CMV and as vehicles for disseminating viral infection (30). Here we showed that PDX1 is expressed in human monocytes raising the possibility that PDX1 could be involved in the regulation of human CMV transcription (Fig. 3). Future work will be performed to determine the functional role of PDX1 in human CMV transcription and replication.