Repression of Ets-2-Induced Transactivation of the Tau Interferon Promoter by Oct-4

Tau interferons (IFN-τ) are structurally related to IFN-β, IFN-α, and IFN-ω (with approximately 25, 50, and 75% primary sequence identities, respectively) (38, 39). Although these IFNs exhibit typical properties, including the ability to induce an antiviral state in cells expressing the appropriate IFN receptor, their function is in reproduction and not in disease prevention (2, 11, 37, 38). The preattachment conceptuses of ruminant species, such as cattle, sheep, and deer, produce IFN-τ in order to prevent the regression of the maternal corpus luteum, an event that would normally occur at the end of the estrous cycle if the animal were not pregnant (2, 11, 37). Members of the multigene IFN-τ family are expressed weakly in the trophectoderm of cattle beginning when this epithelium differentiates at blastocyst formation (11, 14, 16, 37). The level of production of IFN-τ per cell increases markedly as the blastocyst enlarges and begins to elongate. Production is down-regulated when the trophectoderm forms contacts with the uterine wall and as the process of placentation is initiated. The expression of IFN-τ is confined to a single cell layer, the trophectoderm, for 1 to 2 weeks during the critical period when the corpus luteum of pregnancy must be rescued and progesterone production must be maintained. Neither the inner cell mass nor the embryo that is derived from it expresses this IFN (2, 11, 37). An analysis of the nucleotide sequences of the IFN-τ genes (IFNT genes) has indicated that the progenitor gene arose by duplication of a closely related gene, one encoding IFN-ω, approximately 35 million years ago (38, 39). This event appeared to involve a disruption in the upstream promoter region of the new gene relative to its parent and presumably led to the observed loss of viral inducibility (20) and a gain in the ability to be expressed constitutively in trophoblasts. A major divergence in sequence identity between the promoter sequences of IFNT genes and IFN-ω genes (IFNW genes) begins just proximal to a presumptive Ets binding site (−79 to −70) in the IFNT genes which is lacking in the IFNW genes (38, 39). This region of the IFNT promoter has been implicated in controlling high-level IFN-τ expression in the trophectoderm (10, 20). In experiments that used the yeast one-hybrid screen, Ets-2 was confirmed as the transcription factor that most likely bound to the Ets binding site in the IFNT proximal promoter region (13). Ets-2 strongly transactivates IFNT-luciferase (luc) gene reporters, and IFNT genes that lack the Ets site are poorly expressed. The Ets-2 protein is also expressed in the trophectoderm of ovine conceptuses when this tissue expresses IFNT genes. A role for Ets-2 in controlling the differentiation of trophoblast derivatives has been proposed because deletion of the murine ets-2 gene leads to embryonic death by day 8.5 as the result of defective placental development (56). Further, a number of genes in addition to IFNT genes, which are expressed in trophoblasts but not in the inner cell mass (i.c.m.), are regulated by the Ets factor (18, 30, 36, 50, 52). It was recently demonstrated that two human genes expressed in trophoblasts, the α and β subunits of human chorionic gonadotropin (hCG), are silenced in human choriocarcinoma cells by the transcription factor Oct-4 (22, 23). Oct-4 (48), also called Oct-3 (29), belongs to the family of POU transcription factors that contain a bipartite DNA binding domain (POU specific and POU homeodomain) (43). In mouse embryos, it is expressed predominantly in blastomeres, pluripotent early embryo cells, and the germ cell lineage (34, 42, 47). Its mRNA and protein are expressed throughout the cleavage stages of embryo development but not in the trophectoderm of blastocysts (32). The situation is slightly different in bovine embryos (19, 54). There, Oct-4 is expressed strongly in the i.c.m. However, the expression of Oct-4 can also be detected in the trophectoderm until day 10, approximately 3 days after blastocyst formation but before the massive up-regulation of IFN-τ that accompanies trophoblast elongation (14, 37). Oct-4 has been predicted to play a defining role in maintaining cells in the pluripotent state and in preventing differentiation of the i.c.m. of mouse embryos into the trophectoderm (28). Day 4.5 mouse embryos with their Oct-4 gene deleted lack a true i.c.m. and consist entirely of trophectoderm-like cells (26). It is not yet clear how Oct-4 prevents differentiation, but it likely activates certain key genes (3, 31, 41, 58) while repressing others (22, 23). Down-regulated Oct-4 expression presumably lifts the constraints operating on the differentiation process. These data together suggest that Ets-2 and Oct-4 might have counteracting roles in the functional differentiation of trophoblasts. While the former may have a promoting action, the role of Oct-4 may be in restraint. Here we have examined whether the two transcription factors interact in controlling IFNT gene expression, which is a biochemical marker for the functional differentiation of trophoblasts in bovine embryos.