Progestin-regulated expression of tissue factor in decidual cells: implications in endometrial hemostasis, menstruation and angiogenesis

Expression of tissue factor (TF), the primary initiator of hemostasis via thrombin formation, is induced during progesterone (P4)-stimulated decidualization of human endometrial stromal cells (HESCs), and remains elevated in decidualized HESCs of luteal and gestational endometrium. In HESC monolayers, progestins elevate TF mRNA and protein levels and estradiol (E2) plus progestin further enhance TF levels for weeks despite no response to E2 alone. This in vitro model mimics the chronic differential ovarian steroid upregulation of TF levels associated with in vivo decidualization. After incubation of HESCs with E2 plus progestin to elevate TF expression, the antiprogestin RU486 completely reversed this upregulation. Thus, progesterone withdrawal transformed decidualization-associated hemostasis of the luteal phase endometrium to the hemorrhagic milieu of menstruation. Transient transfections with TF promoter constructs containing SP and EGR-1 binding sites before and after inactivation by site-directed mutagenesis revealed that Sp1 mediates basal and progestin-enhanced TF transcriptional activity. Progesterone receptor involvement in TF expression was further confirmed since RU486 was a pure antagonist of progestin-enhanced TF mRNA and protein expression, and progestin-enhanced, but not basal, Sp1-mediated transcriptional activity. Enhanced TF mRNA and protein levels in HESCs require co-incubation with progestin and epidermal growth factor receptor (EGFR) agonist indicating that the EGFR mediates progestin-enhanced TF expression. A peak in the primary angiogenic agent, vascular endothelial growth factor (VEGF) in luteal phase endometrium may be indirectly regulated by P4. Neither E2, nor progestin, nor E2 plus progestin affected VEGF expression in glandular epithelial and stromal cells, whereas thrombin enhanced VEGF mRNA and protein levels in decidualized HESCs, but not in the epithelial cells. Transudation of clotting factors to perivascular decidual cell TF in the luteal phase would generate thrombin, enabling it to act as an autocrine enhancer of VEGF in decidualized HESCs. Abnormal uterine bleeding complicates long-term progestin only contraceptive use. After Norplant administration, endometrial VEGF levels are elevated and TF levels are selectively enhanced in decidualized HESCs at bleeding sites. Over-expressed VEGF causes blood vessels to become leaky, increasing clotting factor access to decidualized HESC-expressed TF to promote feed-forward thrombin and VEGF formation. Since thrombin and VEGF induce angiogenesis via separate endothelial cell receptors, they may synergize to elicit aberrant angiogenesis, and ultimately lead to focal bleeding.