In vivo intra-luteal implants of prostaglandin (PG) E1 or E2 (PGE1, PGE2) prevent luteolysis in cows. I. Luteal weight, circulating progesterone, mRNA for luteal luteinizing hormone (LH) receptor, and occupied and unoccupied luteal receptors for LH

Lipid metabolism and endometrial receptivity

BACKGROUND

Obesity has now been recognized as a high-risk factor for reproductive health. Although remarkable advancements have been made in ART, a considerable number of infertile obese women still suffer from serial implantation failure, despite the high quality of embryos transferred. Although obesity has long been known to exert various deleterious effects on female fertility, the underlying mechanisms, especially the roles of lipid metabolism in endometrial receptivity, remain largely elusive.

OBJECTIVE AND RATIONALE

This review summarizes current evidence on the impacts of several major lipids and lipid-derived mediators on the embryonic implantation process. Emerging methods for evaluating endometrial receptivity, for example transcriptomic and lipidomic analysis, are also discussed.

SEARCH METHODS

The PubMed and Embase databases were searched using the following keywords: (lipid or fatty acid or prostaglandin or phospholipid or sphingolipid or endocannabinoid or lysophosphatidic acid or cholesterol or progesterone or estrogen or transcriptomic or lipidomic or obesity or dyslipidemia or polycystic ovary syndrome) AND (endometrial receptivity or uterine receptivity or embryo implantation or assisted reproductive technology or in vitro fertilization or embryo transfer). A comprehensive literature search was performed on the roles of lipid-related metabolic pathways in embryo implantation published between January 1970 and March 2022. Only studies with original data and reviews published in English were included in this review. Additional information was obtained from references cited in the articles resulting from the literature search.

OUTCOMES

Recent studies have shown that a fatty acids-related pro-inflammatory response in the embryo-endometrium boundary facilitates pregnancy via mediation of prostaglandin signaling. Phospholipid-derived mediators, for example endocannabinoids, lysophosphatidic acid and sphingosine-1-phosphate, are associated with endometrial receptivity, embryo spacing and decidualization based on evidence from both animal and human studies. Progesterone and estrogen are two cholesterol-derived steroid hormones that synergistically mediate the structural and functional alterations in the uterus ready for blastocyst implantation. Variations in serum cholesterol profiles throughout the menstrual cycle imply a demand for steroidogenesis at the time of window of implantation (WOI). Since 2002, endometrial transcriptomic analysis has been serving as a diagnostic tool for WOI dating. Numerous genes that govern lipid homeostasis have been identified and, based on specific alterations of lipidomic signatures differentially expressed in WOI, lipidomic analysis of endometrial fluid provides a possibility for non-invasive diagnosis of lipids alterations during the WOI.

WIDER IMPLICATIONS

Given that lipid metabolic dysregulation potentially plays a role in infertility, a better understanding of lipid metabolism could have significant clinical implications for the diagnosis and treatment of female reproductive disorders.

Loss of luteal sensitivity to luteinizing hormone underlies luteolysis in cattle: A hypothesis

By virtue of the secretion of progesterone (P4), corpus luteum (CL) is important not only for normal cyclicity but also for conception and continuation of pregnancy in female mammals. Luteolysis (also called luteal regression) is defined as loss of the capacity to synthesize and secrete P4 followed by the demise of the CL. There is strong evidence that sequential pulses of prostaglandin F2α (PGF) secreted from the uterus near the end of luteal phase induces luteolysis in farm animals. Loss of luteal sensitivity to luteinizing hormone (LH) at the end of menstrual cycle has been reported to be critical for initiation of luteolysis in primates, however this has not been investigated in farm animals. A closer observation of the published real-time profiles of circulating hormones (P4, LH, and PGF) and their inter-relationships around the time of the beginning of spontaneous luteolysis in cattle revealed- 1) A natural pulse of PGF causes a transient P4 suppression lasting a couple of hours followed by a rebound in P4 concentration, 2) The P4 secretions that occur in response to LH pulses before the beginning of luteolysis (i.e., preluteolysis) either fail or do so to a lesser extent during luteolysis indicating a loss of sensitivity to LH, and 3) The loss of sensitivity coincides with the beginning of luteolysis (i.e., transition), and apparently luteolysis does not initiate until there is loss of sensitivity to LH. The CL is sensitive to LH during preluteolysis, and the LH-stimulated P4-dependent and/or independent local survival mechanisms maintain the steroidogenic capability and viability of the CL until the very end of preluteolysis. Luteolysis does not appear to initiate with the PGF pulse(s) that occur during this period. With the loss of sensitivity to LH at the transition, however, a progressive decline in P4 begins initiating luteolysis. Also, the survival mechanisms become compromised making the CL less viable. The uterine PGF pulses that occur after the beginning of luteolysis induces increase in the local luteolytic factors, which contribute to further luteolysis, more importantly, structural luteolysis with ultimate demise of the CL. Therefore, I hypothesize that the loss of luteal sensitivity to LH underlies luteolysis in cattle. The hypothesis not only unifies the basic mechanism of luteolysis in a farm animal and primates but also provides a perspective to view luteolysis as a process rather than a factor-mediated event. A novel unified working model for luteolysis in a farm animal and primates is described. A better understanding of the luteal physiology including how responsiveness to LH diminishes in aging CL would help in the development of novel strategies in modulating CL structure-function to improve and/or control fertility in humans as well as in animals.

Oxylipin concentrations in bovine corpora lutea during maternal recognition of pregnancy

The objective was to determine the effects of pregnancy status on oxylipin profiles and eicosanoid metabolizing enzymes and in corpora lutea (CL) or endometrial (caruncle; CAR and intercaruncle; IC) tissues. Angus crossed cattle were synchronized with the CO-Synch protocol and artificially inseminated (AI). Sixteen days after AI, cattle were euthanized, and reproductive tracts collected from 6 non-pregnant and 6 pregnant cows. Oxylipin profiles and concentrations of progesterone (P₄) were obtained from CL tissues. The activity of cytochrome P450 1A (CYP1A) and UDP-glucuronosyltransferase (UGT) enzymes were determined using specific luminogenic substrates. Data were analyzed using the MIXED procedure of SAS, and the model included pregnancy status. Corpora lutea of pregnant cattle contained greater (P < 0.05) concentrations of 9,10-DiHODE, 15,16-DiHODE, and 9,10-DiHOME. These oxylipins have been observed to increase cellular proliferation and vasodilation. Activity of CYP1A in the CL and UGT in CAR and IC was not different (P > 0.05) between pregnant and non-pregnant cattle. In the CL, activity of UGT was decreased (P < 0.05) in pregnant vs. non-pregnant cattle. The decrease in CL UGT activity during pregnancy indicates alterations in local hormone metabolism, while no differences in CL weight nor amount of P₄ in CL were different between pregnant and non-pregnant cattle. Moreover, the increase in specific concentrations of oxylipins in the CL may indicate a novel pathway of steroid and eicosanoid metabolism during maternal recognition of pregnancy.

Nodal Promotes Functional Luteolysis via Down-Regulation of Progesterone and Prostaglandins E2 and Promotion of PGF2α Synthetic Pathways in Mare Corpus Luteum

In the present work, we investigated the role of Nodal, an embryonic morphogen from the TGFβ superfamily in corpus luteum (CL) secretory activity using cells isolated from equine CL as a model. Expression pattern of Nodal and its receptors activin receptor A type IIB (ACVR2B), activin receptor-like kinase (Alk)-7, and Alk4, as well as the Nodal physiological role, demonstrate the involvement of this pathway in functional luteolysis. Nodal and its receptors were immune localized in small and large luteal cells and endothelial cells, except ACVR2B, which was not detected in the endothelium. Nodal mRNA in situ hybridization confirmed its transcription in steroidogenic and endothelial cells. Expression analysis of the aforementioned factors evidenced that Nodal and Alk7 proteins peaked at the mid-CL (P < .01), the time of luteolysis initiation, whereas Alk4 and ACVR2B proteins increased from mid- to late CL (P < .05). The Nodal treatment of luteal cells decreased progesterone and prostaglandin (PG) E2 concentrations in culture media (P < .05) as well as mRNA and protein of secretory enzymes steroidogenic acute regulatory protein, cholesterol side-chain cleavage enzyme, cytosolic PGE2 synthase, and microsomal PGE2 synthase-1 (P < .05). Conversely, PGF2α secretion and gene expression of PG-endoperoxidase synthase 2 and PGF2α synthase were increased after Nodal treatment (P < .05). Mid-CL cells cultured with PGF2α had increased Nodal protein expression (P < .05) and phosphorylated mothers against decapentaplegic-3 phosphorylation (P < .05). Finally, the supportive interaction between Nodal and PGF2α on luteolysis was shown to its greatest extent because both factors together more significantly inhibited progesterone (P < .05) and promoted PGF2α (P < .05) synthesis than Nodal or PGF2α alone. Our results neatly pinpoint the sites of action of the Nodal signaling pathway toward functional luteolysis in the mare.

Effects of intraluteal implants of prostaglandin E1 or E2 on angiogenic growth factors in luteal tissue of Angus and Brahman cows

Previously, it was reported that intraluteal implants containing prostaglandin E1 or E2 (PGE1 and PGE2) in Angus or Brahman cows prevented luteolysis by preventing loss of mRNA expression for luteal LH receptors and luteal unoccupied and occupied LH receptors. In addition, intraluteal implants containing PGE1 or PGE2 upregulated mRNA expression for FP prostanoid receptors and downregulated mRNA expression for EP2 and EP4 prostanoid receptors. Luteal weight during the estrous cycle of Brahman cows was reported to be lesser than that of Angus cows but not during pregnancy. The objective of this experiment was to determine whether intraluteal implants containing PGE1 or PGE2 alter vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2), angiopoietin-1 (ANG-1), and angiopoietin-2 (ANG-2) protein in Brahman or Angus cows. On Day 13 of the estrous cycle, Angus cows received no intraluteal implant and corpora lutea were retrieved, or Angus and Brahman cows received intraluteal silastic implants containing vehicle, PGE1, or PGE2 on Day 13 and corpora lutea were retrieved on Day 19. Corpora lutea slices were analyzed for VEGF, FGF-2, ANG-1, and ANG-2 angiogenic proteins via Western blot. Day-13 Angus cow luteal tissue served as preluteolytic controls. Data for VEGF were not affected (P > 0.05) by day, breed, or treatment. PGE1 or PGE2 increased (P < 0.05) FGF-2 in luteal tissue of Angus cows compared with Day-13 and Day-19 Angus controls but decreased (P < 0.05) FGF-2 in luteal tissue of Brahman cows when compared w Day-13 or Day-19 Angus controls. There was no effect (P > 0.05) of PGE1 or PGE2 on ANG-1 in Angus luteal tissue when compared with Day-13 or Day-19 controls, but ANG-1 was decreased (P < 0.05) by PGE1 or PGE2 in Brahman cows when compared with Day-19 Brahman controls. ANG-2 was increased (P < 0.05) on Day 19 in Angus Vehicle controls when compared with Day-13 Angus controls, which was prevented (P < 0.05) by PGE1 but not by PGE2 in Angus cows. There was no effect (P > 0.05) of PGE1 or PGE2 on ANG-2 in Brahman cows. PGE1 or PGE2 may alter cow luteal FGF-2, ANG-1, or ANG-2 but not VEGF to prevent luteolysis; however, species or breed differences may exist.

Auto-amplification system for prostaglandin F2α in bovine corpus luteum

The bovine corpus luteum (CL) is hypothesized to utilize a local auto-amplification system for prostaglandin (PG) F2α production. The objective of the present study was to determine if such a PGF2α auto-amplification system exists in the bovine CL, and if so, which factors regulate it. PGF2α significantly stimulated intra-luteal PGF2α production in all luteal phases, but did not affect PGE2 production. The stimulatory effect of exogenous PGF2α on CL PGF2α production was lower at the early luteal phase. Indomethacin, an inhibitor of prostaglandin-endoperoxide synthase (PTGS), significantly suppressed the PGF2α-stimulated PGF2α production by luteal tissue, indicating that the PGF2α in the medium was of luteal origin. Consistent with these secreted-PGF2α profiles, PGF2α receptor (PTGFR) protein expression was higher during the mid and late luteal phases than at early and developing luteal phases. Treatment of cultured bovine luteal cells obtained from the mid-luteal phase with PGF2α (1 µM) significantly increased the expressions of PTGS2, PGF synthase (PGFS), and carbonyl reductase1 (CBR1) at 24 hr post-treatment. Together, these results suggest the presence of a local auto-amplification system for PGF2α mediated by PTGS2, PGFS, and CBR1 in the bovine CL, which may play an important role in luteolysis.

Different expression of PGE synthase, PGF receptor, TNF, Fas and oxytocin in the bovine corpus luteum of the estrous cycle and pregnancy

Functional differences between the corpus luteum (CL) of pregnancy and CL of the cycle in cows were examined. Messenger RNA and protein levels of prostaglandin (PG) E synthase (PGES), PGF2α receptor (PGFR), tumor necrosis factor-α (TNF) and Fas were found to be higher in the CL of pregnancy than in CL of the cycle. Oxytocin (OT) mRNA and protein levels were lower in the CL of pregnancy. Messenger RNA levels of progesterone receptor (PR), luteinizing hormone receptor (LHR), PGE2 receptor (PGER), PGF synthase (PGFS), TNF receptor type I (TNFRI) and TNF receptor type II (TNFRII) did not differ between the cycle and pregnancy. PGE2 and PGF2α production by cultured bovine endometrial tissues was decreased by a supernatant derived from the homogenized CL of pregnancy but not by that of the CL of the cycle, suggesting that specific substances in the CL of pregnancy affect endometrial PG production in cows. Collectively, PGES, PGFR, TNF, Fas or OT may contribute to differences between the CL of pregnancy and CL of the estrous cycle in cows.

Prostaglandin E2 functions as a luteotrophic factor in the dog

The luteal phase in dogs is governed by many poorly understood regulatory mechanisms. Functioning of the corpus luteum (CL) is unaffected by hysterectomy. Recently, the role of prostaglandins in regulating canine CL function was addressed suggesting a luteotrophic effect of prostaglandin E2 (PGE2) during the early luteal phase. However, compelling functional evidence was lacking. The potential of PGE2 to stimulate steroidogenesis was tested in canine primary luteal cells isolated from developing CL of non-pregnant dogs. In addition, the luteal expression of prostaglandin transporter (PGT) and steroidogenic acute regulatory protein (STAR) was demonstrated and characterized in CL from non-pregnant bitches during the course of dioestrus as well as from pregnant animals during the pre-implantation, post-implantation and mid-gestation periods of pregnancy and during luteolysis; the luteal expression of PGE2 receptors (EP2 and EP4) has been investigated at the protein level throughout pregnancy. Our findings show that PGE2 is an activator of STAR expression in canine luteal cells from early luteal phase, significantly up-regulating STAR promoter activity and protein expression resulting in increased steroidogenesis. The 3βHSD (HSD3B2) and P450scc (CYP11A1) expression remained unaffected by PGE2 treatment. The expression of PGT was confirmed in CL during both pregnancy and dioestrus and generally localized to the luteal cells. After initial up-regulation during the earlier stages of the CL phase, its expression declined towards the luteal regression. Together with the demonstration of EP2 and EP4 throughout pregnancy, and the decline in EP2 at prepartum, our findings further support our hypothesis that intra-luteal PGE2 may play an important role in regulating progesterone secretion in the canine CL.