Analyses of cis-acting and trans-acting elements that are crucial to sustain pregnancy-specific glycoprotein gene expression in different cell types

PSG gene expression is up-regulated by lysine acetylation involving histone and nonhistone proteins

Background

Lysine acetylation is an important post-translational modification that plays a central role in eukaryotic transcriptional activation by modifying chromatin and transcription-related factors. Human pregnancy-specific glycoproteins (PSG) are the major secreted placental proteins expressed by the syncytiotrophoblast at the end of pregnancy and represent early markers of cytotrophoblast differentiation. Low PSG levels are associated with complicated pregnancies, thus highlighting the importance of studying the mechanisms that control their expression. Despite several transcription factors having been implicated as key regulators of PSG gene family expression; the role of protein acetylation has not been explored.

Methodology/Principal Findings

Here, we explored the role of acetylation on PSG gene expression in the human placental-derived JEG-3 cell line. Pharmacological inhibition of histone deacetylases (HDACs) up-regulated PSG protein and mRNA expression levels, and augmented the amount of acetylated histone H3 associated with PSG 5′regulatory regions. Moreover, PSG5 promoter activation mediated by Sp1 and KLF6, via the core promoter element motif (CPE, −147/−140), was markedly enhanced in the presence of the HDAC inhibitor trichostatin A (TSA). This effect correlated with an increase in Sp1 acetylation and KLF6 nuclear localization as revealed by immunoprecipitation and subcellular fractionation assays. The co-activators PCAF, p300, and CBP enhanced Sp1-dependent PSG5 promoter activation through their histone acetylase (HAT) function. Instead, p300 and CBP acetyltransferase domain was dispensable for sustaining co-activation of PSG5 promoter by KLF6.

Conclusions/Significance

Results are consistent with a regulatory role of lysine acetylation on PSG expression through a relaxed chromatin state and an increase in the transcriptional activity of Sp1 and KLF6 following an augmented Sp1 acetylation and KLF6 nuclear localization.

Krüppel-like factor 6 expression changes during trophoblast syncytialization and transactivates ßhCG and PSG placental genes

Background

Krüppel-like factor-6 (KLF6) is a widely expressed member of the Sp1/KLF family of transcriptional regulators involved in differentiation, cell cycle control and proliferation in several cell systems. Even though the highest expression level of KLF6 has been detected in human and mice placenta, its function in trophoblast physiology is still unknown.

Methodology/Principal Findings

Herein, we explored KLF6 expression and sub-cellular distribution in human trophoblast cells differentiating into the syncytial pathway, and its role in the regulation of genes associated with placental development and pregnancy maintenance. Confocal immunofluorescence microscopy demonstrated that KLF6 is expressed throughout human cytotrophoblast differentiation showing no evident modifications in its nuclear and cytoplasmic localization pattern. KLF6 transcript and protein peaked early during the syncytialization process as determined by qRT-PCR and western blot assays. Overexpression of KLF6 in trophoblast-derived JEG-3 cells showed a preferential nuclear signal correlating with enhanced expression of human β-chorionic gonadotropin (βhCG) and pregnancy-specific glycoprotein (PSG) genes. Moreover, KLF6 transactivated βhCG5, PSG5 and PSG3 gene promoters. Deletion of KLF6 Zn-finger DNA binding domain or mutation of the consensus KLF6 binding site abolished transactivation of the PSG5 promoter.

Conclusions/Significance

Results are consistent with KLF6 playing a role as transcriptional regulator of relevant genes for placental differentiation and physiology such as βhCG and PSG, in agreement with an early and transient increase of KLF6 expression during trophoblast syncytialization.

Expression and transcriptional regulation of individual pregnancy-specific glycoprotein genes in differentiating trophoblast cells

Human pregnancy-specific glycoproteins (PSGs), encoded by eleven highly conserved genes, are the major placental polypeptides. Low PSG levels in maternal circulation have been associated with complicated pregnancies. However, expression of each PSG gene and their regulation during cytotrophoblast cell differentiation remain poorly explored. Herein, we analyze the expression of five PSG genes and demonstrate that they are almost undetectable in undifferentiated trophoblast, but are all transcribed in differentiated cells. Among them, PSG1, PSG3 and PSG5 genes achieve high mRNA levels while PSG7 and PSG9 are poorly expressed. In addition, total PSG proteins and transcripts markedly increase during trophoblast differentiation, preceding morphological syncytialization and betahCG expression. The 5' regulatory region contributes to the transcriptional control of PSG gene induction in trophoblast cells undergoing differentiation. This responsive region in PSG3 maps within a 130 bp promoter sequence, which overlaps the transcription start site and requires a functional Retinoic Acid Responsive Element (RARE) and a GA-binding protein (GABP) consensus site for basal and differentiation-dependent promoter activity, respectively. Present findings provide novel data for understanding the control of PSG gene expression and demonstrate that their proteins and transcripts represent early markers of trophoblast differentiation.

RXRalpha regulates the pregnancy-specific glycoprotein 5 gene transcription through a functional retinoic acid responsive element

Human pregnancy-specific glycoproteins (PSG) are major placental polypeptides encoded by eleven highly conserved genes expressed by the syncytiotrophoblast. The minimal promoter region of all PSG genes contains a putative Retinoic Acid Responsive Element (RARE) though the ability of retinoids to regulate PSG gene expression has not been established. Retinoid signaling pathway plays a key role for overall placenta biology and is essential for trophoblast differentiation. In this work, we investigated the participation of the RARE motif in the regulation of PSG5 gene transcription by retinoic acid and its receptors. The minimal promoter region of PSG5 gene was activated by RXRalpha but not by RARalpha, in a ligand-dependent manner. The RARE sequence of PSG5 gene promoter was recognized by endogenous RXRalpha present in placental nuclear extracts as well as by RXRalpha either over expressed in cultured non-placental cells or in vitro translated. Mutations at specific nucleotides within the RARE motif abrogated both RXRalpha DNA binding and transcriptional activation of PSG5 promoter mediated by RXRalpha. Moreover, endogenous PSG expression was significantly induced in trophoblast-derived Jeg-3 cells upon 9-cis retinoic acid treatment. Interestingly, the induction level was higher following methotrexate-induced differentiation of Jeg-3 cells to syncytiotrophoblast-like structures. Altogether, these data provide the first evidences demonstrating that transcriptional activity of PSG5 gene is responsive to an external signal involving the retinoids-RXRalpha axis through a conserved RARE motif shared by all PSG gene family members.

Activation of the human pregnancy-specific glycoprotein PSG-5 promoter by KLF4 and Sp1

Pregnancy-specific glycoproteins (PSGs) are major placental proteins thought to be essential for the maintenance of gestation. Little is known about the regulation of expression of the 11 genes encoding these proteins. It was previously demonstrated that Krüppel-like factor 6 (KLF6) and specific-protein 1 (Sp1) bind to conserved sequence within the PSG-5 gene promoter. Informatics analysis revealed the presence of one potential binding site for Krüppel-like factor 4 (KLF4), in the PSG-5 promoter, suggesting a potential transcriptional regulator role for KLF4. Using gene promoter-reporter transfections and X-ChIP assays, we demonstrated that KLF4 is an activator of the PSG-5 promoter by binding to a KLF consensus like binding which includes the Core Promoter Element region (-147/-140). Furthermore, we used previous data showing the binding of Sp1 transcription factor to a GT-box (-443/-437) and co-transfection assays with KLF4 and Sp1 to demonstrate the strong synergic activity of these two factors on the PSG-5 promoter.

Transcriptional control of the human pregnancy-specific glycoprotein 5 gene is dependent on two GT-boxes recognized by the ubiquitous specificity protein 1 (Sp1) transcription factor

Pregnancy-specific glycoprotein 5 gene (PSG-5) belongs to the human pregnancy-specific glycoprotein family, encoded by eleven highly similar and transcriptionally active genes. High levels of PSG biosynthesis are restricted to the placenta syncytiotrophoblast and are essential for the maintenance of normal gestation in mammalian species. We have investigated here the nature of the transcription factors that recognize the FP1 (-455/-433) and the CPE (-147/-140) regulatory sequences that significantly contribute to basal PSG-5 promoter activity. Both elements bear a similar GT-box motif; and DNA-protein complex formation, as well as promoter activity, is largely dependent on the integrity of these GT-box sequences. Gel shift, super gel shift and UV-crosslinking experiments clearly demonstrate that the ubiquitous specificity protein 1 (Sp1) is the major transcription factor involved in complex formation with both cis-acting elements in normal term placenta tissue and in PSG-non-expressing COS-7 cells. Furthermore, transfection experiments indicate that Sp1 activates PSG-5 promoter constructs. In addition, we show that Sp1 is indeed co-expressed with PSG genes in the syncytiotrophoblast cells, stressing its potential role in the in vivo regulation of PSG expression.

Characterization of the human p11 promoter sequence

p11 is expressed in many different cell types, and serves a variety of regulatory functions. In order to better understand the transcriptional control of this protein, the 5' promoter region of the human p11 gene was cloned and sequenced. After confirming the transcription start point (TSP) using 5' rapid amplification of cDNA ends analysis, the 5' promoter was analysed. The sequence lacks a TATA box, but contains a variety of putative regulatory elements. There are two GAS sites, two AP-1 sites, two overlapping Sp-1 sites, and a gamma-IRE site clustered between -1080 and -1450. There is another cluster of putative regulatory sites between the TSP and -550 which contains two Sp-1 sites, two AP-2 sites, one GAS site, one NF-kappaB site, an incomplete CAAT box (8/9) and an overlapping Sp-1/AP-2 site at -17 to -26. Reporter gene constructs containing 4225 and 1498 bases 5' of the TSP demonstrated excellent unidirectional transcriptional activity in both constructs. Reporter genes containing serial 5' deletions were compared to the -1498 construct. The reporter gene which contained base pairs (bp) -36 to +89 had almost no activity. The reporter gene containing -188 to +89 had 50% of the -1498 construct, indicating that this sequence contains at least the minimal promoter. The Sp-1/AP-2 site near the transcription start site was studied by electrophoretic mobility shift and reporter gene assays. Addition of HeLa cell nuclear extract to labeled double-stranded (ds) oligonucleotide containing this sequence resulted in a gel shift which was inhibited by excess unlabeled ds oligonucleotide and by a consensus cold Sp-1 ds oligonucleotide, indicating specific Sp-1 binding. Excess AP-2 or NF-kappaB ds oligonucleotide had no effect on nuclear protein binding to the sequence. Mutation of the p11 wild-type Sp-1/AP-2 sequence eliminated both nuclear protein binding and the sequences ability to compete with native sequence for nuclear binding protein. A -1048 to +89 reporter construct containing a mutated Sp-1/AP-2 site resulted in a 40% decrease in transcriptional activity. Therefore, the 5' flanking sequence of the p11 gene exhibits promoter activity which may be localized to a variety of controlling regions, of which the proximal Sp-1/AP-2 site appears to be important for basal activity via its Sp-1 binding ability.

Co-localization of KLF6 and KLF4 with pregnancy-specific glycoproteins during human placenta development

Pregnancy-specific glycoproteins (PSGs) are major placental proteins essential for the maintenance of normal gestation. However, little is known about their gene expression regulation during placentation. It was previously demonstrated that the human core promoter binding protein recently renamed Krüppel-like factor (KLF) 6 binds to a highly conserved sequence within the PSG promoters and is mainly expressed in human term placenta. Here, we determined the expression pattern of the 13 other KLFs during human placental development. We demonstrate that eight KLFs exhibit specific expression patterns in human placental tissues and membranes, in favor of a functional cooperation of specific KLFs during placentation. In addition, we demonstrate that KLF6, KLF4 and PSG proteins are co-expressed in same cell types of placental villi and membranes. This experimental evidence further strengthens the potential cross talk of both transcription factors for PSG gene regulation in vivo.

The Krüppel-like core promoter binding protein gene is primarily expressed in placenta during mouse development

The human core promoter binding protein (hCPBP) has been identified as a DNA-binding protein involved in the regulation of TATA box-less genes like those encoding the pregnancy-specific glycoproteins. Structurally, hCPBP contains three zinc fingers in the C-terminal domain, which is highly conserved in a number of proteins that constitute the Krüppel-like family of transcription factors. In the present work, we report the molecular cloning of the mouse CPBP (mCPBP) and its expression pattern during development as well as in adult tissues. The mouse cDNA encodes a protein of 283 amino acids that share 94.4% of identity with the hCPBP. The highest level of mCPBP transcript was detected in placenta, and its expression was lower in total embryos and in adult tissues. We also show by in situ hybridization that during embryonic development the mCPBP gene is mainly expressed in extra-embryonic structures throughout gestation; essentially no specific expression was detected in embryonic tissues. Our data demonstrate that CPBP transcript is enriched in the trophoblastic tissue and strongly suggest that its encoded polypeptide regulates target genes involved in placental development and pregnancy maintenance.

Cloning the cDNA for a new human zinc finger protein defines a group of closely related Krüppel-like transcription factors

We have identified a novel zinc finger protein that has been named ubiquitous Krüppel-like factor (UKLF) based on structural considerations and the pattern of gene expression. UKLF was isolated by the polymerase chain reaction approach using degenerate oligonucleotides corresponding to the DNA-binding domain of erythroid Krüppel-like factor (EKLF) and cDNA prepared from human vascular endothelial cells. The carboxyl-terminal portion of UKLF contains three zinc fingers of the Cys2-His2 type and binds in vitro to the CACCC motif of the beta-globin promoter and to the Sp1 recognition sequence. The amino-terminal portion of UKLF consists of a hydrophobic region rich in serines and a negatively charged segment with several glutamic acid residues. The first 47 amino acids of the acidic region are nearly identical to the amino-terminal portion of another Krüppel-like factor, the so-called core promoter-binding protein (CPBP) or Zf9. Like CPBP/Zf9, UKLF can function as a transcription activator in co-transfection assays. However, this activity is lost when the highly conserved amino-terminal segment is deleted. These findings indicate that UKLF and CPBP/Zf9 represent a distinct subgroup of closely related Krüppel-like activators of transcription. Mapping of the UKLF gene to chromosome 2 suggested that UKLF and CPBP/Zf9 translocated to different chromosomes following duplication from an ancestral gene.

Mice Transgenic for the Human CGM6 Gene Express Its Product, the Granulocyte Marker CD66b, Exclusively in Granulocytes

The nonspecific cross-reacting antigen-95 (NCA-95/CD66b), is a member of the human carcinoembryonic antigen (CEA) family encoded by the CGM6 gene that is exclusively expressed in neutrophils and eosinophils. No murine counterpart is known to exist. We have analyzed a cosmid containing the complete CGM6 gene. The coding sequence is contained within six exons spanning a 16.5 kb region. The main transcriptional start site was mapped to a tight cluster between nucleotides -95 and -101 relative to the translational start site. As with other members of the CEA gene family, no typical TATA or CAAT-box sequences were found in the CGM6 gene. Transgenic mice were established with the cosmid insert. CD66b expression is first seen in the fetal liver on day 12.5 of mouse embryonic development, and it first appears in the bone marrow at day 17.5. Northern blot analysis showed that CD66b transcripts are confined to the bone marrow of adult mice, whereas immunohistochemistry also showed CD66b-positive granulocytes in the spleen, thymus, and lungs. FACScan analyses of bone marrow and spleen cells showed CD66b expression to be exclusive to granulocytes. Thus, all the elements necessary for regulating granulocyte-specific expression are present within this cosmid clone. These mice could provide a model for transplantation and for inflammation studies using CD66b as a granulocyte-specific marker.

Mice Transgenic for the Human CGM6 Gene Express Its Product, the Granulocyte Marker CD66b, Exclusively in Granulocytes

The nonspecific cross-reacting antigen-95 (NCA-95/CD66b), is a member of the human carcinoembryonic antigen (CEA) family encoded by the CGM6 gene that is exclusively expressed in neutrophils and eosinophils. No murine counterpart is known to exist. We have analyzed a cosmid containing the complete CGM6 gene. The coding sequence is contained within six exons spanning a 16.5 kb region. The main transcriptional start site was mapped to a tight cluster between nucleotides -95 and -101 relative to the translational start site. As with other members of the CEA gene family, no typical TATA or CAAT-box sequences were found in the CGM6 gene. Transgenic mice were established with the cosmid insert. CD66b expression is first seen in the fetal liver on day 12.5 of mouse embryonic development, and it first appears in the bone marrow at day 17.5. Northern blot analysis showed that CD66b transcripts are confined to the bone marrow of adult mice, whereas immunohistochemistry also showed CD66b-positive granulocytes in the spleen, thymus, and lungs. FACScan analyses of bone marrow and spleen cells showed CD66b expression to be exclusive to granulocytes. Thus, all the elements necessary for regulating granulocyte-specific expression are present within this cosmid clone. These mice could provide a model for transplantation and for inflammation studies using CD66b as a granulocyte-specific marker.

A novel human zinc finger protein that interacts with the core promoter element of a TATA box-less gene

We describe a novel human cDNA isolated by target site screening of a placental expression library, using as a probe, an essential element of a TATA box-less promoter corresponding to a pregnancy-specific glycoprotein gene. The cDNA encoded a predicted protein of 290 amino acids, designated core promoter-binding protein (CPBP), which has three zinc fingers (type Cys2-His2) at the end of its C-terminal domain, a serine/threonine-rich central region and an acidic domain lying within the N-terminal region. Additional sequence analysis and data base searches revealed that only the zinc finger domains are conserved (60-80% identity) in other transcription factors. In cotransfection assays, CPBP increased the transcription from a minimal promoter containing its natural DNA-binding site. Moreover, a chimeric protein between CPBP and Gal4 DNA binding domain also increased the activity of an heterologous reporter gene containing Gal4 DNA binding sites. The tissue distribution analysis of CPBP mRNA revealed that it is differentially expressed with an apparent enrichment in placental cells. The DNA binding and transcriptional activity of CPBP, in conjunction with its expression pattern, strongly suggests that this protein may participate in the regulation and/or maintenance of the basal expression of PSG and possibly other TATA box-less genes.