Female Reproductive Tract

Expression of Key Prostaglandin Synthases in Equine Endometrium During Late Diestrus and Early Pregnancy

Abstract
Luteolysis in domestic species is mediated by the release of luteolytic pulses of prostaglandin (PG) F₂ by the uterus at the end of diestrus, which must be suppressed by the conceptus to permit maternal recognition of pregnancy. In many species including the horse, both the conceptus and the endometrium also synthesize PGE₂, which may antagonize PGF₂ by playing a luteotropic and/or anti-luteolytic role. While the release of PGE₂ and PGF₂ by the equine endometrium in late diestrus and early pregnancy has been previously studied, the underlying prostaglandin synthase gene regulatory mechanisms remain poorly defined. To resolve this issue, cyclooxygenase-2 (COX-2), microsomal PGE₂ synthase (PGES) and PGF₂ synthase (PGFS) expression were examined in a series of endometrial biopsies obtained from cycling mares on days 10, 13 and 15 post-ovulation, as well as from pregnant mares on day 15. Quantification of COX-2 expression revealed significant (P < 0.01) increases in both mRNA and protein levels at day 15 in cycling endometrium, relative to other timepoints. Importantly, the level of COX-2 expression in day 15 pregnant endometrium was found to be comparable to that observed in day 10 and day 13 cycling animals, suggesting that the presence of the conceptus blocks the induction of COX-2. Immunohistochemistry demonstrated that the induction of COX-2 expression on day 15 occurs specifically in surface epithelial cells in cycling animals only. As equine PGFS had not been previously characterized, a 1380 bp cDNA transcript was cloned by a combination of RT-PCR techniques, and found to be highly homologous to bovine liver type PGFS. The pattern of expression observed for the terminal PG synthases was distinct from that of COX-2, as PGES and PGFS mRNA and protein levels were found to be invariant throughout the timecourse and unaffected by pregnancy. Similarly to COX-2 however, the PGES and PGFS proteins were found to localize mainly to the surface epithelium. Thus, this study describes for the first time the regulation and spatial distribution of COX-2, PGES and PGFS expression in equine endometrium in late diestrus, with a marked induction of COX-2, but not of PGES and PGFS, expression in uterine epithelial cells at day 15. Furthermore, the presence of the conceptus was shown to block the induction of COX-2 expression at day 15, suggesting an important mechanism by which it may suppress uterine PGF₂ release and prevent luteolysis during early pregnancy.

Key words: Female Reproductive Tract • Pregnancy • Gene regulation • Ovulatory cycle • Uterus

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