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In humans, progesterone stimulation of the estradiol-primed endometrium initiates decidualization of the stromal cells around the spiral arterioles to establish the decidual cell as a major cell type of the secretory phase and pregnant endometrium. Prior studies from our laboratory have established that progestins stimulate the endometrial decidual/ stromal expression of two crucial modulators of hemostasis: tissue factor (TF) and plasminogen activator inhibitor type 1 (PAI-1). Neither one of these genes contains consensus estrogen or progesterone response elements My studies focused on the mechanisms underlying the enhanced expression by progestin of TF and PAI-1 in vivo as well as in primary human endometrial stromal cell cultures. It was determined that the prolonged progestational upregulation occurred at the transcriptional level. Transient transfections of human endometrial stromal cells were carried out with truncated TF or PAI-1 promoters as well as promoters undergoing site directed mutagenesis.
These studies identified specific Sp1 sites as the key regulators of progestin-induced activity of the TF and PAI-1 genes. Co-expression of these promoters with overexpression vectors for Sp1 and/or Sp3 demonstrated the crucial role for Sp1 but not Sp3 in the transcriptional regulation of both TF and PAI-1 (see figure below). Most interestingly, immunohistochemical studies demonstrated increased Sp1 levels in perivascular stromal cells in secretory phase compared to proliferative phase endometria. Conversely, Sp3 expression was greatly decreased in stromal cells of secretory compared to proliferative phase tissues. We proposed that the changes observed in the cycling endometrium altered the ratio of Sp1 to Sp3 transcription factors and that these changes in turn were involved in TF and PAI-1 gene expression in vivo and in vitro.