Characterization of Nuclear and Membrane Progesterone Receptors in Bovine Endometrium.

Abstract

Progesterone (P4) is essential for maintenance of pregnancy and also plays an important role in the timing of luteolysis in ruminants. The time from estrus to luteolysis is regulated by P4 since a) treatment of cows with P4 early in the estrous cycle results in premature luteolysis and b) delaying the action of P4 on the endometrium by the use of an antagonist delays luteolysis. However, the exact mechanism(s) of action of P4 in the endometrium is still not known. Nuclear progesterone receptors (nPR) occur naturally in A and B forms, which result from alternative promoter usage of the same gene. The two PR forms are hormonally and developmentally regulated. The specific role for each of these two PR subtypes in the regulation of endometrial function is unclear. However, the existence of elaborate mechanisms regulating their production suggests that the relative levels of PRA and PRB in cells is critical for appropriate cellular response to P4; for example in human parturition when there is no change in plasma P4 concentrations. P4 can also have non-genomic effects and membrane receptors (mPR) in the epithelial cells of the ruminant endometrium have been shown in sheep and cows, but no detailed studies have been performed. The hypothesis for this study is that changes in mPR and/or the ratio of PRA/PRB during the estrous cycle in the cow are responsible for regulating endometrial function, including induction of luteolysis. The objectives of this present study are to determine if changes in PRA, PRB and mPR occur during the estrous cycle. Endometrial samples, from uterine horns ipsi- and contra-lateral to the corpus luteum, were collected during the early, mid and late stages of the estrous cycle from cows (4 cows per stage) at the slaughterhouse and snap-frozen until analysis. Measurement of mRNA for PRA and PRB by RT-PCR has been established and the results show that both isoforms are expressed at all stages of the cycle. Quantitative PCR has now to be performed to determine if changes occur during the cycle. Protein expression (measured by Western blotting) shows PRB and PRA at all stages of the cycle. There was a higher expression of PRA in the ipsi-lateral horn (P=0.05, n=4) but no difference was observed at the different stages of the cycle in either horn. Further studies are in progress to determine if changes occur in the different cell types of the endometrium. Preliminary data show that mPR? mRNA is present in the endometrium throughout the estrous cycle. These data show that mPR and the different isoforms of the nPR are present in the bovine endometrium and could play an important role in its regulation. This work was supported by NSERC.