Michel A. Fortier

The corpus luteum (CL) is a transient ovarian endocrine gland formed from the ovulated follicle. Progesterone is the primary secretory product of CL and is essential for establishment of pregnancy in mammals. In the cyclic female, the life span of CL is characterized by luteal development, maintenance, and regression regulated by complex interactions between luteotrophic and luteolytic mediators. It is universally accepted that prostaglandin (PG) F(2a) is the luteolysin whereas PGE(2) is considered as a luteotropin in most mammals. New emerging concepts emphasize the autocrine and paracrine actions of luteal PGs in CL function. However, there is no report on selective biosynthesis and cellular transport of luteal PGE(2) and PGF(2alpha) in the CL of any species. We have studied the expression of enzymes involved in the metabolism of PGE(2) and PGF(2alpha), cyclooxygenase (COX)-1 and -2, PGE and F synthases, PG 15-dehydrogenase, and PG transporter as well as receptors (EP2, EP3, and FP) throughout the CL life span using a bovine model. COX-1, PGF synthase, and PG 15-dehydrogenase are expressed at constant levels whereas COX-2, PGE synthase, PG transporter, EP2, EP3, and FP are highly modulated during different phases of the CL life span. The PG components are preferentially expressed in large luteal cells. The results indicate that PGE(2) biosynthesis, transport, and signaling cascades are selectively activated during luteal maintenance. By contrast the PGF(2alpha) system is activated during luteal regression. Collectively, our results suggest an integrated role for luteal PGE(2) and PGF(2alpha) in autoregulation of CL function.

In ruminants, endometrial prostaglandin F(2alpha) (PGF(2alpha)) is responsible for luteolysis and prostaglandin E(2) (PGE(2)) is thought to be involved in maternal recognition of pregnancy. In the present study, healthy uteri were collected from cows at the abattoir, and days of the estrous cycle were determined macroscopically. The uteri were classified into seven groups as Days 1-3, 4-6, 7-9, 10-12, 13-15, 16-18, and 19-21 of the estrous cycle. Endometrial scrapings were collected. The expression of cyclooxygenase (COX)-1 and COX-2 mRNAs and proteins and PGE synthase (PGES) mRNA was analyzed by Northern and Western blot. There was no expression of COX-1, either mRNA or protein, on any day of the estrous cycle. In contrast, COX-2 mRNA and protein were expressed at low and high levels on Days 1-12 and 13-21 of the estrous cycle, respectively. The level of expression of PGES was moderate, low, and high on Days 1-3, 4-12, and 13-21 of the estrous cycle, respectively. There were significant correlations between COX-2 mRNA and protein levels and between COX-2 and PGES mRNA levels. COX-1 mRNA and protein are not expressed on any day of the estrous cycle, whereas COX-2 mRNA and protein and PGES mRNA are differentially expressed and regulated in bovine endometrium during the estrous cycle. COX-2, rather than COX-1, is the primary isoenzyme involved in the endometrial production of prostaglandins, and the COX-2 and PGES pathway is responsible for the endometrial production of PGE(2) in the bovine endometrium during the estrous cycle.

Epithelial and stromal cells were isolated from endometrium of cyclic heifers by enzymic dispersion. These cells exhibited specific morphological and functional properties. Epithelial cells appeared cuboidal or columnal and showed contact inhibition as they reached confluence. Stromal cells were fibroblast-like and enlarged at the time of confluence after which they overgrew in multiple layers. The presence of specific receptors for PGE-2 and beta-adrenergic catecholamines (isoproterenol) was estimated by activation of adenylate cyclase. Stromal cells had more adenylate cyclase activity (P less than 0.01) than did epithelial cells before (basal) and after stimulation with guanosine triphosphate (GTP) and PGE-2. However, epithelial cells were much more responsive to isoproterenol (P less than 0.01). Treatment of cultured cells with indomethacin to block PG synthesis increased the sensitivity and maximal response to PGE-2 in stromal (P less than 0.01) but not in epithelial (P greater than 0.1) cells. The latter result suggested autologous desensitization of the PGE-2 response resulting from synthesis of PGs in cultured cells. Both cell types synthesized PGs in culture: PGF-2 alpha was synthesized in greater quantity in epithelial than in stromal cells (P less than 0.05) while stromal cells synthesized more PGE-2 than did epithelial cells (P less than 0.001). Endometrial cells separated in this way should prove useful for study of their specific role in the processes of implantation and maternal recognition of pregnancy.

The uterus is a primary target for sex steroid action in vivo during the estrous cycle and pregnancy. Cell cultures have been used to determine the specific function of the different cell types forming the uterus. We used endometrial cell cultures previously characterized in our laboratory to study the effect of estradiol (E) and progesterone (P4) on prostaglandin (PG) production and on regulation of the response of the cells to oxytocin (OT). The studies were performed on confluent cultures of epithelial cells grown as a monolayer either on plastic or on filter inserts to allow basal-apical polarization. As described previously, prostaglandin F2 alpha (PGF 2 alpha) production was greater (3.7-fold, p < 0.0001) than prostaglandin E2 (PGE2) production in epithelial cells, and the opposite was true in stromal cells (PGE2 9.9-fold > PGF2 alpha, p < 0.0001). In epithelial cells, the basal production of PGE2 (-61.6%, p < 0.0001) and PGF2 alpha (-51.7%, p < 0.0001) was reduced significantly by E and increased significantly by P4 (PGE2, + 30.0% [p < 0.002]; PGF2 alpha, + 22.2% [p < 0.006]). No significant effect of sex steroids on the basal production of PGs was detected in stromal cells. OT stimulated the production of PGF2 alpha (6.7-fold, p < 0.0001) and PGE2 (9.1-fold, p < 0.0001) in epithelial but not stromal cells. Treatment of the cells with E significantly (p < 0.001) increased OT-stimulated PGF2 alpha production in both the epithelial and stromal cells and that of PGE2 in epithelial cells only. The effect of steroids and OT was similar in polarized (filter) and nonpolarized (plastic) epithelial cells. Analysis of the vectorial secretion of PGs in epithelial cells grown on filter inserts revealed that PGF2 alpha is preferentially secreted in the basal (p < 0.001) compared to the apical compartment. The direction of secretion was not influenced by steroid or OT treatments. The results suggest that epithelial cells of the endometrium are a preferred target for the regulation of PG synthesis by sex steroids and OT.

Recognition and establishment of pregnancy involve several molecular and cellular interactions among the conceptus, uterus, and corpus luteum (CL). In ruminants, interferon-tau (IFNtau) of embryonic origin is recognized as the pregnancy recognition signal. Endometrial prostaglandin F(2alpha) (PGF(2alpha)) is the luteolysin, whereas PGE(2) is considered a luteoprotective or luteotrophic mediator at the time of establishment of pregnancy. The interplay between IFNtau and endometrial PGs production, transport, and signaling at the time of maternal recognition of pregnancy (MRP) is not well understood. We have studied the expression of enzymes involved in metabolism of PGE(2) and PGF(2alpha), cyclooxygenase-1 (COX-1) and COX-2, PG synthases (PGES and PGFS), PG 15-dehydrogenase, and PG transporter as well as PGE(2) (EP2 and EP3) and PGF(2alpha) receptors. IFNtau influences cell-specific expression of COX-2, PGFS, EP2, and EP3 in endometrium, myometrium, and CL in a spatio-temporal and tissue-specific manner, whereas it does not alter COX-1, PGES, PG 15-dehydrogenase, PG transporter, or PGF(2alpha) receptor expression in any of these tissues. In endometrium, IFNtau decreases PGFS in epithelial cells and increases EP2 in stroma. In myometrium, IFNtau decreases PGFS and increases EP2 in smooth muscle cells. In CL, IFNtau increases PGES and decreases EP3. Together, our results show that IFNtau directly or indirectly increases PGE(2) biosynthesis and EP2-associated signaling in endometrium, myometrium, and CL during MRP. Thus, PGE(2) may play pivotal roles in endometrial receptivity, myometrial quiescence, and luteal maintenance, indicating polycrine (endocrine, exocrine, paracrine, and autocrine) actions of PGE(2) at the time of MRP. Therefore, the establishment of pregnancy may depend not only on inhibition of endometrial PGF(2alpha), but also on increased PGE(2) production in cattle.