Silencing of the gene for the alpha-subunit of human chorionic gonadotropin by the embryonic transcription factor Oct-3/4

Transcriptional control of IFNT expression

Once interferon-tau (IFNT) had been identified as a type I IFN in sheep and cattle and its functions were characterized, numerous studies were conducted to elucidate the transcriptional regulation of this gene family. Transfection studies performed largely with human choriocarcinoma cell lines identified regulatory regions of the IFNT gene that appeared responsible for trophoblast-specific expression. The key finding was the recognition that the transcription factor ETS2 bound to a proximal region within the 5'UTR of a bovine IFNT and acted as a strong transactivator. Soon after other transcription factors were identified as cooperative partners. The ETS2-binding site and the nearby AP1 site enable response to intracellular signaling from maternal uterine factors. The AP1 site also serves as a GATA-binding site in one of the bovine IFNT genes. The homeobox-containing transcription factor, DLX3, augments IFNT expression combinatorially with ETS2. CDX2 has also been identified as transactivator that binds to a separate site upstream of the main ETS2 enhancer site. CDX2 participates in IFNT epigenetic regulation by modifying histone acetylation status of the gene. The IFNT downregulation at the time of the conceptus attachment to the uterine endometrium appears correlated with the increased EOMES expression and the loss of other transcription coactivators. Altogether, the studies of transcriptional control of IFNT have provided mechanistic evidence of the regulatory framework of trophoblast-specific expression and critical expression pattern for maternal recognition of pregnancy.

Squelching of ETS2 transactivation by POU5F1 silences the human chorionic gonadotropin CGA subunit gene in human choriocarcinoma and embryonic stem cells

The subunit genes encoding human chorionic gonadotropin, CGA, and CGB, are up-regulated in human trophoblast. However, they are effectively silenced in choriocarcinoma cells by ectopically expressed POU domain class 5 transcription factor 1 (POU5F1). Here we show that POU5F1 represses activity of the CGA promoter through its interactions with ETS2, a transcription factor required for both placental development and human chorionic gonadotropin subunit gene expression, by forming a complex that precludes ETS2 from interacting with the CGA promoter. Mutation of a POU5F1 binding site proximal to the ETS2 binding site does not alter the ability of POU5F1 to act as a repressor but causes a drop in basal promoter activity due to overlap with the binding site for DLX3. DLX3 has only a modest ability to raise basal CGA promoter activity, but its coexpression with ETS2 can up-regulate it 100-fold or more. The two factors form a complex, and both must bind to the promoter for the combination to be transcriptionally effective, a synergy compromised by POU5F1. Similarly, in human embryonic stem cells, which express ETS2 but not CGA, ETS2 does not occupy its binding site on the CGA promoter but is found instead as a soluble complex with POU5F1. When human embryonic stem cells differentiate in response to bone morphogenetic protein-4 and concentrations of POU5F1 fall and hCG and DLX3 rise, ETS2 then occupies its binding site on the CGA promoter. Hence, a squelching mechanism underpins the transcriptional silencing of CGA by POU5F1 and could have general relevance to how pluripotency is maintained and how the trophoblast lineage emerges from pluripotent precursor cells.

Trophoblast stem cells

Trophoblast stem cells (TSC) are the precursors of the differentiated cells of the placenta. In the mouse, TSC can be derived from outgrowths of either blastocyst polar trophectoderm (TE) or extraembryonic ectoderm (ExE), which originates from polar TE after implantation. The mouse TSC niche appears to be located within the ExE adjacent to the epiblast, on which it depends for essential growth factors, but whether this cellular architecture is the same in other species remains to be determined. Mouse TSC self-renewal can be sustained by culture on mitotically inactivated feeder cells, which provide one or more factors related to the NODAL pathway, and a medium supplemented with FGF4, heparin, and fetal bovine serum. Repression of the gene network that maintains pluripotency and emergence of the transcription factor pathways that specify a trophoblast (TR) fate enables TSC derivation in vitro and placental formation in vivo. Disrupting the pluripotent network of embryonic stem cells (ESC) causes them to default to a TR ground state. Pluripotent cells that have acquired sublethal chromosomal alterations may be sequestered into TR for similar reasons. The transition from ESC to TSC, which appears to be unidirectional, reveals important aspects of initial fate decisions in mice. TSC have yet to be derived from domestic species in which remarkable TR growth precedes embryogenesis. Recent derivation of TSC from blastocysts of the rhesus monkey suggests that isolation of the human equivalents may be possible and will reveal the extent to which mechanisms uncovered by using animal models are true in our own species.

The EWS-Oct-4 fusion gene encodes a transforming gene

The t(6;22)(p21;q12) translocation associated with human bone and soft-tissue tumours results in a chimaeric molecule fusing the NTD (N-terminal domain) of the EWS (Ewing's sarcoma) gene to the CTD (C-terminal domain) of the Oct-4 (octamer-4) embryonic gene. Since the N-terminal domains of EWS and Oct-4 are structurally different, in the present study we have assessed the functional consequences of the EWS-Oct-4 fusion. We find that this chimaeric gene encodes a nuclear protein which binds DNA with the same sequence specificity as the parental Oct-4 protein. Comparison of the transactivation properties of EWS-Oct-4 and Oct-4 indicates that the former has higher transactivation activity for a known target reporter gene containing Oct-4 binding. Deletion analysis of the functional domains of EWS-Oct-4 indicates that the EWS (NTD), the POU domain and the CTD of EWS-Oct-4 are necessary for full transactivation potential. EWS-Oct-4 induced the expression of fgf-4 (fibroblast growth factor 4) and nanog, which are potent mitogens as well as Oct-4 downstream target genes whose promoters contain potential Oct-4-binding sites. Finally, ectopic expression of EWS-Oct-4 in Oct-4-null ZHBTc4 ES (embryonic stem) cells resulted in increased tumorigenic growth potential in nude mice. These results suggest that the oncogenic effect of the t(6;22) translocation is due to the EWS-Oct-4 chimaeric protein and that fusion of the EWS NTD to the Oct-4 DNA-binding domain produces a transforming chimaeric product.

Ets-2 and C/EBP-beta are important mediators of ovine trophoblast Kunitz domain protein-1 gene expression in trophoblast

Background

The trophoblast Kunitz domain proteins (TKDPs) constitute a highly expressed, placenta-specific, multigene family restricted to ruminant ungulates and characterized by a C-terminal "Kunitz" domain, preceded by one or more unique N-terminal domains. TKDP-1 shares an almost identical expression pattern with interferon-tau, the "maternal recognition of pregnancy protein" in ruminants. Our goal here has been to determine whether the ovine (ov) Tkdp-1 and IFNT genes possess a similar transcriptional code.

Results

The ovTkdp-1 promoter has been cloned and characterized. As with the IFNT promoter, the Tkdp-1 promoter is responsive to Ets-2, and promoter-driven reporter activity can be increased over 700-fold in response to over-expression of Ets-2 and a constitutively active form of protein Kinase A (PKA). Unexpectedly, the promoter element of Tkdp-1 responsible for this up-regulation, unlike that of the IFNT, does not bind Ets-2. However, mutation of a CCAAT/enhancer binding element within this control region not only reduced basal transcriptional activity, but prevented Ets-2 as well as cyclic adenosine 5'-monophosphate (cAMP)/PKA and Ras/mitogen-activated protein kinase (MAPK) responsiveness. In vitro binding experiments and in vivo protein-protein interaction assays implicated CCAAT/enhancer binding protein-beta (C/EBP-β) as involved in up-regulating the Tkdp-1 promoter activity. A combination of Ets-2 and C/EBP-β can up-regulate expression of the minimal Tkdp-1 promoter as much as 930-fold in presence of a cAMP analog. An AP-1-like element adjacent to the CCAAT enhancer, which binds Jun family members, is required for basal and cAMP/ C/EBP-β-dependent activation of the gene, but not for Ets-2-dependent activity.

Conclusion

This paper demonstrates how Ets-2, a key transcription factor for trophoblast differentiation and function, can control expression of two genes (Tkdp-1 and IFNT) having similar spatial and temporal expression patterns via very different mechanisms.

Trophoblast gene expression: transcription factors in the specification of early trophoblast

Azone of trophoblast specification is established when the embryo is a morula, presumably reflecting a unique combination of transcription factors in that zone of cells and the influence of various environmental cues and growth factors on them. A key first step in this process of specification is the down-regulation of Oct4, a transcription factor that acts as a negative regulator of trophoblast specification and of genes normally up-regulated as the trophectoderm first forms. The transcription factors believed to have a positive association with trophectoderm specification have been inferred primarily in two ways: by their expression patterns in embryos, ES cells and TS cells and by the consequences of gene disruption on embryonic development. Many of these transcription factors also control the expression of genes characteristically expressed in trophoblast but not in the epiblast, primitive endoderm and their derivatives. ES and TS cells from the mouse and other species are beginning to provide insights into the changes in gene expression that accompany lineage specification and the subsequent post-specification events that lead to functional trophoblast derivatives.

Repression of Ets-2-induced transactivation of the tau interferon promoter by Oct-4

Oct-4 is a POU family transcription factor associated with potentially totipotent cells. Genes expressed in the trophectoderm but not in embryos prior to blastocyst formation may be targets for silencing by Oct-4. Here, we have tested this hypothesis with the tau interferon genes (IFNT genes), which are expressed exclusively in the trophectoderm of bovine embryos. IFNT promoters contain an Ets-2 enhancer, located at -79 to -70, and are up-regulated about 20-fold by the overexpression of Ets-2 in human JAr choriocarcinoma cells, which are permissive for IFNT expression. This enhancement was reversed in a dose-dependent manner by coexpression of Oct-4 but not either Oct-1 or Oct-2. When cells were transfected with truncated bovine IFNT promoters designed to eliminate potential octamer sites sequentially, luciferase reporter expression from each construct was still silenced by Oct-4. Full repression required both the N-terminal and POU domains of Oct-4, but neither domain used alone was an effective silencer. Oct-4 and Ets-2 formed a complex in vitro in the absence of DNA through binding of the POU domain of Oct-4 to a site located between the "pointed" and DNA binding domains of Ets-2. The two transcription factors were also coimmunoprecipitated after being expressed together in JAr cells. Oct-4, therefore, silences IFNT promoters by quenching Ets-2 transactivation. The POU domain most probably binds to Ets-2 directly, while the N-terminal domain inhibits transcription. These findings provide further evidence that the developmental switch to the trophectoderm is accompanied by the loss of Oct-4 silencing of key genes.