Corpus luteum (CL) function: local control mechanisms

Local Expression Dynamics of Various Adipokines during Induced Luteal Regression (Luteolysis) in the Bovine Corpus Luteum

The study aimed to evaluate the mRNA expression levels of various local novel adipokines, including vaspin, adiponectin, visfatin, and resistin, along with their associated receptors, heat shock 70 protein 5, adiponectin receptor 1, and adiponectin receptor 2, in the corpus luteum (CL) during luteal regression, also known as luteolysis, in dairy cows. We selected Fleckvieh cows in the mid-luteal phase (days 8-12, control group) and administered cloprostenol (PGF analog) to experimentally induce luteolysis. We collected CL samples at different time points following PGF application: before treatment (days 8-12, control group) and at 0.5, 2, 4, 12, 24, 48, and 64 h post-treatment (n = 5) per group. The mRNA expression was measured via real-time reverse transcription polymerase chain reaction (RT-qPCR). Vaspin was characterized by high mRNA levels at the beginning of the regression stage, followed by a significant decrease 48 h and 64 h after PGF treatment. Adiponectin mRNA levels were elevated 48 h after PGF. Resistin showed upregulation 4 h post PGF application. In summary, the alterations observed in the adipokine family within experimentally induced regressing CL tissue potentially play an integral role in the local regulatory processes governing the sequence of events culminating in functional luteolysis and subsequent structural changes in the bovine ovary.

Current Knowledge on the Multifactorial Regulation of Corpora Lutea Lifespan: The Rabbit Model

Simple Summary

Corpora lutea (CL) are temporary endocrine structures that secrete progesterone, which is essential for maintaining a healthy pregnancy. A variety of regulatory factors come into play in modulating the functional lifespan of CL, with luteotropic and luteolytic effects. Many aspects of luteal phase physiology have been clarified, yet many others have not yet been determined, including the molecular and/or cellular mechanisms that maintain the CL from the beginning of luteolysis during early CL development. This paper summarizes our current knowledge of the endocrine and cellular mechanisms involved in multifactorial CL lifespan regulation, using the pseudopregnant rabbit model.

Abstract

Our research group studied the biological regulatory mechanisms of the corpora lutea (CL), paying particular attention to the pseudopregnant rabbit model, which has the advantage that the relative luteal age following ovulation is induced by the gonadotrophin-releasing hormone (GnRH). CL are temporary endocrine structures that secrete progesterone, which is essential for maintaining a healthy pregnancy. It is now clear that, besides the classical regulatory mechanism exerted by prostaglandin E2 (luteotropic) and prostaglandin F2α (luteolytic), a considerable number of other effectors assist in the regulation of CL. The aim of this paper is to summarize our current knowledge of the multifactorial mechanisms regulating CL lifespan in rabbits. Given the essential role of CL in reproductive success, a deeper understanding of the regulatory mechanisms will provide us with valuable insights on various reproductive issues that hinder fertility in this and other mammalian species, allowing to overcome the challenges for new and more efficient breeding strategies.

Preclinical Models of Altered Early Life Nutrition and Development of Reproductive Disorders in Female Offspring

Early epidemiology studies in humans have and continue to offer valuable insight into the Developmental Origins of Health and Disease (DOHaD) hypothesis, which emphasises the importance of early-life nutritional and environmental changes on the increased risk of metabolic and reproductive disease in later life. Human studies are limited and constrained by a range of factors which do not apply to preclinical research. Animal models therefore offer a unique opportunity to fully investigate the mechanisms associated with developmental programming, helping to elucidate the developmental processes which influence reproductive diseases, and highlight potential biomarkers which can be translated back to the human condition. This review covers the use and limitations of a number of animal models frequently utilised in developmental programming investigations, with an emphasis on dietary manipulations which can lead to reproductive dysfunction in offspring.

Angiopoietin expression in ovine corpora lutea during the luteal phase: Effects of nutrition, arginine and follicle stimulating hormone

The aim of this study was to evaluate angiopoietin (ANGPT) 1 and 2, and tyrosine-protein kinase receptor 2 (TIE2) expression in the corpora lutea (CL) of FSH-treated, or non-treated sheep administered arginine (Arg) or vehicle (saline, Sal), and fed a control (C), excess (O) or restricted (U) diet. Ewes from each dietary group were treated with Arg or Sal (experiment 1), and with FSH (experiment 2). Luteal tissues were collected at the early-, mid- and/or late-luteal phases of the estrous cycle. Protein and mRNA expression was determined using immunohistochemistry followed by image analysis, and quantitative RT-PCR, respectively. The results demonstrated that ANGPT1 and TIE2 proteins were localized to luteal capillaries and endothelial cells of larger blood vessels, and ANGPT2 was localized to tunica media of larger blood vessels. TIE2 protein was also present in luteal cells. In experiment 1, ANGPT1 protein expression was greater in O than C during early- and mid-luteal phases, and was greatest during late-luteal phase, less at the mid- and least at the early-luteal phase; 2) TIE2 protein expression was greatest at the mid-, less at the early- and least at the late-luteal phase; 3) ANGPT1 and 2 mRNA expression was greater at the mid- and late- than the early-luteal phase, and TIE2 mRNA expression was greatest at the late-, less at the mid- and least at the early-luteal phase. The ANGPT1/2 ratio was less at the early- than mid- or late-luteal phases. In experiment 2, ANGPT1 protein expression was greater in O during the mid-luteal phase than in other groups, and was greater at the mid- than early-luteal phase. TIE2 protein expression was highest at the mid-, less at the early- and least during the late-luteal phase. ANGPT1 and 2, and TIE2 mRNA expression was higher at the mid- than the early-luteal phase. During mid-luteal phase, ANGPT1 mRNA expression was greater in C than O and U, ANGPT2 was greatest in C, less in O and least in U, and TIE2 mRNA expression was greater in C than O and U. The ANGPT1/2 ratio was higher in U than in any other group. Comparison of FSH vs. Sal treatment effects (experiment 2 vs. experiment 1) demonstrated that FSH affected ANGPT1 and/or -2, and TIE2 protein and mRNA expression depending on luteal phase and/or diet. Thus, expression of ANGPTs and TIE2 in the CL changes during the luteal lifespan, indicating their involvement in luteal vascular formation, stabilization and degradation. Moreover, this study has demonstrated that plane of nutrition and/or FSH treatment affect the ANGPT system, and may alter luteal vascularity and luteal function in sheep.

Equine Chorionic Gonadotropin Modulates the Expression of Genes Related to the Structure and Function of the Bovine Corpus Luteum

We hypothesized that stimulatory and superovulatory treatments, using equine chorionic gonadotropin (eCG), modulate the expression of genes related to insulin, cellular modelling and angiogenesis signaling pathways in the bovine corpus luteum (CL). Therefore, we investigated: 1—the effect of these treatments on circulating insulin and somatomedin C concentrations and on gene and protein expression of INSR, IGF1 and IGFR1, as well as other insulin signaling molecules; 2—the effects of eCG on gene and protein expression of INSR, IGF1, GLUT4 and NFKB1A in bovine luteal cells; and 3—the effect of stimulatory and superovulatory treatments on gene and protein expression of ANG, ANGPT1, NOS2, ADM, PRSS2, MMP9 and PLAU. Serum insulin did not differ among groups (P = 0.96). However, serum somatomedin C levels were higher in both stimulated and superovulated groups compared to the control (P = 0.01). In stimulated cows, lower expression of INSR mRNA and higher expression of NFKB1A mRNA and IGF1 protein were observed. In superovulated cows, lower INSR mRNA expression, but higher INSR protein expression and higher IGF1, IGFR1 and NFKB1A gene and protein expression were observed. Expression of angiogenesis and cellular modelling pathway-related factors were as follows: ANGPT1 and PLAU protein expression were higher and MMP9 gene and protein expression were lower in stimulated animals. In superovulated cows, ANGPT1 mRNA expression was higher and ANG mRNA expression was lower. PRSS2 gene and protein expression were lower in both stimulated and superovulated animals related to the control. In vitro, eCG stimulated luteal cells P4 production as well as INSR and GLUT4 protein expression. In summary, our results suggest that superovulatory treatment induced ovarian proliferative changes accompanied by increased expression of genes providing the CL more energy substrate, whereas stimulatory treatment increased lipogenic activity, angiogenesis and plasticity of the extracellular matrix (ECM).

Profiling of luteal transcriptome during prostaglandin F2-alpha treatment in buffalo cows: analysis of signaling pathways associated with luteolysis

In several species including the buffalo cow, prostaglandin (PG) F2α is the key molecule responsible for regression of corpus luteum (CL). Experiments were carried out to characterize gene expression changes in the CL tissue at various time points after administration of luteolytic dose of PGF2α in buffalo cows. Circulating progesterone levels decreased within 1 h of PGF2α treatment and evidence of apoptosis was demonstrable at 18 h post treatment. Microarray analysis indicated expression changes in several of immediate early genes and transcription factors within 3 h of treatment. Also, changes in expression of genes associated with cell to cell signaling, cytokine signaling, steroidogenesis, PG synthesis and apoptosis were observed. Analysis of various components of LH/CGR signaling in CL tissues indicated decreased LH/CGR protein expression, pCREB levels and PKA activity post PGF2α treatment. The novel finding of this study is the down regulation of CYP19A1 gene expression accompanied by decrease in expression of E2 receptors and circulating and intra luteal E2 post PGF2α treatment. Mining of microarray data revealed several differentially expressed E2 responsive genes. Since CYP19A1 gene expression is low in the bovine CL, mining of microarray data of PGF2α-treated macaques, the species with high luteal CYP19A1 expression, showed good correlation between differentially expressed E2 responsive genes between both the species. Taken together, the results of this study suggest that PGF2α interferes with luteotrophic signaling, impairs intra-luteal E2 levels and regulates various signaling pathways before the effects on structural luteolysis are manifest.

Expression and localization of ghrelin and its functional receptor in corpus luteum during different stages of estrous cycle and the modulatory role of ghrelin on progesterone production in cultured luteal cells in buffalo

Evidence obtained during recent years provided has insight into the regulation of corpus luteum (CL) development, function, and regression by locally produced ghrelin. The present study was carried out to evaluate the expression and localization of ghrelin and its receptor (GHS-R1a) in bubaline CL during different stages of the estrous cycle and investigate the role of ghrelin on progesterone (P4) production along with messenger RNA (mRNA) expression of P4 synthesis intermediates. The mRNA and protein expression of ghrelin and GHS-R1a was significantly greater in mid- and late luteal phases. Both factors were localized in luteal cells, exclusively in the cytoplasm. Immunoreactivity of ghrelin and GHS-R1a was greater during mid- and late luteal phases. Luteal cells were cultured in vitro and treated with ghrelin each at 1, 10, and 100 ng/mL concentrations for 48 h after obtaining 75% to 80% confluence. At a dose of 1 ng/mL, there was no significant difference in P4 secretion between control and treatment group. At 10 and 100 ng/mL, there was a decrease (P < 0.05) in P4 concentration, cytochrome P45011A1 (CYP11A1), and 3-beta-hydroxysteroid dehydrogenase mRNA expression and localization. There was no difference in mRNA expression of steroidogenic acute regulatory protein between control and treatment group. In summary, the present study provided evidence that ghrelin and its receptor are expressed in bubaline CL and are localized exclusively in the cell cytoplasm and ghrelin has an inhibitory effect on P4 production in buffalo.

Fertility in a high-altitude environment is compromised by luteal dysfunction: the relative roles of hypoxia and oxidative stress

BACKGROUND

At high altitudes, hypoxia, oxidative stress or both compromise sheep fertility. In the present work, we tested the relative effect of short- or long-term exposure to high altitude hypobaric hypoxia and oxidative stress on corpora luteal structure and function.

METHODS

The growth dynamics of the corpora lutea during the estrous cycle were studied daily by ultrasonography in cycling sheep that were either native or naïve to high-altitude conditions and that were supplemented or not supplemented with antioxidant vitamins. Arterial and venous blood samples were simultaneously drawn for determination of gases and oxidative stress biomarkers and progesterone measurement. On day five after ovulation in the next cycle, the ovaries were removed for immunodetection of luteal HIF-1alpha and VEGF and IGF-I and to detect IGF-II gene expression.

RESULTS

The results showed that both short- and long-term exposure to high-altitude conditions decreased luteal growth and IGF-I and IGF-II gene expression but increased HIF-1 alpha and VEGF immunoexpression. The level of plasma progesterone was also increased at a high altitude, although an association with increased corpus luteum vascularization was only found in sheep native to a high-altitude location. Administration of antioxidant vitamins resulted in a limited effect, which was restricted to decreased expression of oxidative stress biomarkers and luteal HIF-1alpha and VEGF immunoexpression.

CONCLUSIONS

Exposure of the sheep to high-altitude hypobaric hypoxia for short or long time periods affects the development and function of the corpus luteum. Moreover, the observed association of oxidative stress with hypoxia and the absence of any significant effect of antioxidant vitamins on most anatomical and functional corpus luteum traits suggests that the effects of high altitude on this ovarian structure are mainly mediated by hypoxia. Thus, these findings may help explain the decrease in sheep fertility at a high altitude.

Global gene expression in the bovine corpus luteum is altered after stimulatory and superovulatory treatments

Equine chorionic gonadotrophin (eCG) has been widely used in superovulation and artificial insemination programmes and usually promotes an increase in corpus luteum (CL) volume and stimulates progesterone production. Therefore, to identify eCG-regulated genes in the bovine CL, the transcriptome was evaluated by microarray analysis and the expression of selected genes was validated by qPCR and western blot. Eighteen Nelore crossbred cows were divided into control (n = 5), stimulated (n = 6) and superovulated groups (n = 7). Ovulation was synchronised using a progesterone device-based protocol. Stimulated animals received 400 IU of eCG at device removal and superovulated animals received 2000 IU of eCG 4 days prior. Corpora lutea were collected 7 days after gonadotrophin-releasing hormone administration. Overall, 242 transcripts were upregulated and 111 transcripts were downregulated in stimulated cows (P ≤ 0.05) and 111 were upregulated and 113 downregulated in superovulated cows compared to the control animals (1.5-fold, P ≤ 0.05). Among the differentially expressed genes, many were involved in lipid biosynthesis and progesterone production, such as PPARG, STAR, prolactin receptors and follistatin. In conclusion, eCG modulates gene expression differently depending on the treatment, i.e. stimulatory or superovulatory. Our data contribute to the understanding of the pathways involved in increased progesterone levels observed after eCG treatment.

Effect of oxytocin infusion on luteal blood flow and progesterone secretion in dairy cattle

The objective of this study was to investigate the effects of oxytocin infusion on corpus luteum (CL) function during early to mid-diestrus by measuring luteal size (LS) and luteal blood flow (LBF) along with plasma levels of progesterone (P4) and prostaglandin metabolites (13,14-dihydro-15-keto-prostaglandin F_2α, PGFM). On day (D) 7 of the estrus cycle (D1 = ovulation), seven cows received 100 IU of oxytocin (OXY) or placebo (PL) following a Latin square design. LS and LBF increased in both groups over time and no differences were observed between the groups. PGFM did not differ either within the groups over time or between the groups at any time point. P4 of the OXY group was higher compared to that of the the PL group 360 min after the infusion (p = 0.01) and tended to be higher at the time points 450 min, 48 h, and 72 h (all p = 0.08). Results from this study support the hypothesis that OXY is not directly involved in the mechanism(s) governing blood flow of the CL and has no remarkable effects either on luteal size or P4 and PGFM plasma levels. Further investigation is needed to elucidate the role of OXY in CL blood flow during early and late luteal phases.

Deleterious effects of progestagen treatment in VEGF expression in corpora lutea of pregnant ewes

The aim of the current study was to determine the possible effects of progestagen oestrous synchronization on vascular endothelial growth factor (VEGF) expression during sheep luteogenesis and the peri-implantation period and the relationship with luteal function. At days 9, 11, 13, 15, 17 and 21 of pregnancy, the ovaries from 30 progestagen treated and 30 ewes cycling after cloprostenol injection were evaluated by ultrasonography and, thereafter, collected and processed for immunohistochemical evaluation of VEGF; blood samples were drawn for evaluating plasma progesterone. The progestagen-treated group showed smaller corpora lutea than cloprostenol-treated and lower progesterone secretion. The expression of VEGF in the luteal cells increased with time in the cloprostenol group, but not in the progestagen-treated group, which even showed a decrease between days 11 and 13. In progestagen-treated sheep, VEGF expression in granulosa-derived parenchymal lobule capillaries was correlated with the size of the luteal tissue, larger corpora lutea had higher expression, and tended to have a higher progesterone secretion. In conclusion, the current study indicates the existence of deleterious effects from exogenous progestagen treatments on progesterone secretion from induced corpora lutea, which correlate with alterations in the expression of VEGF in the luteal tissue and, this, presumably in the processes of neoangiogenesis and luteogenesis.

The interface of the immune and reproductive systems in the ovary: lessons learned from the corpus luteum of domestic animal models

The dynamic changes that characterize the female reproductive system are regulated by hormones. However, local cell-to-cell interactions may mediate responsiveness of tissues to hormonal signals. The corpus luteum (CL) is an excellent model for understanding how immune cells are recruited into tissues and the role played by those cells in regulating tissue homeostasis or demise. Leukocytes are recruited into the CL throughout its lifespan, and leukocyte-derived cytokines have been found in corpora lutea of all species examined. The proinflammatory cytokines inhibit gonadotropin-stimulated steroidogenesis, profoundly stimulate prostaglandin synthesis by luteal cells, and promote apoptosis. However, there is mounting evidence that leukocytes and luteal cells communicate in different ways to maintain homeostasis within the functional CL. Domestic animals have provided important information regarding the presence and role of immune cells in the CL.

Expression of luteal estrogen receptor, interleukin-1, and apoptosis-associated genes after PGF2alpha administration in rabbits at different stages of pseudopregnancy

The dynamic expression for estrogen receptor subtype-1 (ESR1), interleukin-1beta (IL1B), and apoptosis-associated genes, as well as nitric oxide synthase activity, were examined in corpora lutea (CL) of rabbits after prostaglandin F(2alpha) (PGF(2alpha)) administration on either day 4 or day 9 of pseudopregnancy. By reverse transcriptase polymerase chain reaction, the steady-state level of ESR1 transcript was lower (P < 0.01) and that of anti-apoptotic B-cell CLL/lymphoma 2 (BCL2) -like 1 (BCL2L1) was greater in day 4 (P < 0.01) than in day 9 CL. Western blot analysis revealed that BCL2-associated X protein (BAX) abundance was greater in day 4 (P < 0.01) than in day 9 CL, whereas BCL2L1 protein was undetectable at both luteal stages. After PGF(2alpha), ESR1 transcript decreased (P < 0.01) in day 9 CL, whereas IL1B mRNA showed a transitory increase (P < 0.01) at both stages. The pro-apoptotic tumor protein p53 (TP53) gene had diminished (P < 0.01) on day 4 and on day 9 after a transitory increase (P < 0.01), whereas the BAX/BCL2L1 expression ratio increased (P < 0.01) in day 9 CL 24 h after treatment. Following PGF(2alpha), TP53 protein increased (P < 0.01) at both luteal stages, and BAX decreased (P < 0.01) in day 4 CL but increased (P < 0.01) 24 h later in day 9 CL; BCL2L1 became detectable 6 h later in day 4 CL. Nitric oxide synthase activity temporarily increased (P < 0.01) following PGF(2alpha). These findings suggest that PGF(2alpha) regulates luteolysis by ESR1 mRNA down-regulation and modulation of pro- and anti-apoptotic pathways in CL that have acquired a luteolytic capacity.

Survival role of locally produced acetylcholine in the bovine corpus luteum

The present study was conducted to explore the source of acetylcholine (ACH) in the corpus luteum (CL) and to test our hypothesis of an antiapoptotic role of ACH in the bovine CL and, further, to investigate whether nerve growth factor (NGF), insulin-like growth factor 1 (IGF1), and transforming growth factor beta1 (TGFB1) influence the expression of choline acetyltransferase (CHAT), the biosynthetic enzyme of ACH, in cultured bovine luteal cells. Protein expression and immunolocalization of CHAT were carried out at different stages throughout the luteal phase and in cultured luteal and endothelial cells. ACH was measured in luteal tissue at the different luteal stages and in luteal cells cultured for 8 and 24 h. Cell viability and TUNEL assays were performed on cultured midluteal cells treated with or without tumor necrosis factor alpha (TNF)/interferon gamma (IFNG) in the presence of ACH and its muscarinic (atropine) and nicotinic (mecamylamine) receptor antagonists. The CL was devoid of cholinergic nerve fibers. CHAT immunostaining was evident in luteal, endothelial, and stromal cells in luteal tissue sections and in cultured luteal and endothelial cells. CHAT protein was expressed throughout the cycle without any significant changes. ACH concentration in luteal tissue was not changed during the luteal stages but increased over time and with increased cell numbers in luteal cell cultures. ACH increased cell viability and prevented cell death induced by TNF/IFNG. Atropine significantly attenuated ACH action, whereas mecamylamine had no effect. TNF/IFNG treatment downregulated CHAT expression, whereas NGF, IGF1, and TGFB1 upregulated CHAT expression, in cultured luteal cells. The overall findings strongly suggest a nonneural source and antiapoptotic role of ACH in the bovine CL. Locally produced ACH appears to be regulated by NGF, IGF1, and TGFB1.

The role of endocrine insulin-like growth factor-I (IGF-I) in female bovine reproduction

Insulin-like growth factor-I (IGF-I) plays a pivotal role in cattle fertility, acting as a monitoring signal that allows reproductive events to occur when nutritional conditions for successful reproduction are reached. However, endocrine IGF-I is not a predictor of reproductive events, but rather an indirect estimator of the suitability of the animal to achieve the reproductive event in question. Although measuring circulating IGF-I concentrations might not have any clinical application in the cattle industry, endocrine IGF-I screening will continue to be important for the study of interactions between nutrition and reproduction. In addition, endocrine IGF-I screening could be used as an ancillary test for the selection of cattle for high reproductive potential, especially in herds of high genetic merit for milk production, in which a decline in fertility has been identified.

Expression of fibroblast growth factor receptors during development and regression of the bovine corpus luteum

There is evidence that fibroblast growth factors (FGFs) are involved in the regulation of growth and regression of the corpus luteum (CL). However, the expression pattern of most FGF receptors (FGFRs) during CL lifespan is still unknown. The objective of the present study was to determine the pattern of expression of ‘B’ and ‘C’ splice variants of FGFRs in the bovine CL. Bovine CL were collected from an abattoir and classed as corpora hemorrhagica (Stage I), developing (Stage II), developed (Stage III) or regressed (Stage IV) CL. Expression of FGFR mRNA was measured by semiquantitative reverse transcription-polymerase chain reaction and FGFR protein was localised by immunohistochemistry. Expression of mRNA encoding the ‘B’ and ‘C’ spliced forms of FGFR1 and FGFR2 was readily detectable in the bovine CL and was accompanied by protein localisation. FGFR1C and FGFR2C mRNA expression did not vary throughout CL lifespan, whereas FGFR1B was upregulated in the developed (Stage III) CL. FGFR3B, FGFR3C and FGFR4 expression was inconsistent in the bovine CL. The present data indicate that FGFR1 and FGFR2 splice variants are the main receptors for FGF action in the bovine CL.

Feeding level does not affect progesterone levels in intermittently suckled sows with lactational ovulation

The aim of this study was to examine whether the low post-ovulatory plasma P(4) levels found in intermittently suckled (IS) sows are related to the high feeding level during intermittent lactation. Multiparous sows (n=21) were separated from their piglets for 12h per day starting at day 14 of lactation until 6 days after ovulation. At day 28 of lactation, 9 sows had ovulated (spontaneous) and the remaining 12 sows were treated with PG600 (PG600-induced). At ovulation sows were allocated to either a high (H) feeding level (on average 6.5 kg) or a low (L) feeding level (high feeding level minus 2.5 kg) until 6 days after ovulation. Plasma P(4) levels were not affected by feeding level or type of ovulation (P>0.10), and neither were CL parameters, embryo survival rate and embryo development at day 30 of gestation. In conclusion, low levels of plasma P(4) were found in IS sows with lactational ovulation but these were not affected by feeding level during the first week after ovulation. Further studies are needed to investigate which factors cause the lower plasma P(4) levels in these sows.

Reproductive role of prolactin

The biological actions of prolactin (PRL), a polypeptide hormone, are mostly related to lactation and reproduction. These actions have been clarified by studies of PRL and PRL-deficient receptor mice, which have a clear phenotype of reproductive failure at multiple sites. This review aims to summarize current knowledge about PRL and its receptor, role in reproductive axis and presents information of hyperprolactinemia in reproductive medicine. Our understanding of the physiology and transduction pathway of PRL has largely increased in the past 20 years with the cloning of PRL and its receptor gene.

Vasoactive Peptides in the Luteolytic Process Activated by PGF2alpha in Pseudopregnant Rabbits at Different Luteal Stages1

To study the role of endothelial factors in luteal function, the dynamic profiles of genes for endothelin 1 (EDN1), its receptor subtypes, EDNRA and EDNRB, and angiotensin converting enzyme (ACE) were examined in corpora lutea (CL) obtained from rabbits on Days 4 and 9 of pseudopregnancy after prostaglandin (PG) F2alpha analogue (alfaprostol) treatment. The cell type distribution of EDN1 in the ovaries and its mechanisms of actions in vitro and in vivo were also studied. Positive immunostaining for EDN1 was localized in the luteal and endothelial cells, in granulosa cells of the follicles, and in the ovarian epithelium. The basal mRNA levels for EDNRA, EDNRB, and ACE were lower (P </= 0.01) in Day-4 CL than in Day-9 CL, whereas those for EDN1 did not differ between these two time-points. On Day 4, the luteal EDN1, EDNRA, EDNRB, and ACE mRNA levels were similarly increased two-fold (P </= 0.01) 1.5 h after alfaprostol injection, and did not show further changes in the subsequent 24 h. On Day 9, alfaprostol challenge transiently up-regulated (P </= 0.01) the luteal ACE transcripts at 1.5 h, and those of EDN1 at 1.5 h and 3 h, whereas the EDNRA and EDNRB transcript levels remained unchanged during the course of luteal regression. EDN1 decreased (P </= 0.01) progesterone release and increased (P </= 0.01) PGF2alpha secretion and NOS activity via the PLC/PKC pathway in Day-9 CL, but not in Day-4 CL, cultured in vitro. EDN1-induced, but not alfaprostol-induced luteolysis, was blocked by cotreatment in vivo with the ACE antagonist captopril. These findings support the hypothesis that PGF2alpha regulates luteolysis through intraluteal activation of the renin-angiotensin/EDN1 systems in CL that have acquired luteolytic competence.

The molecular control of corpus luteum formation, function, and regression

The corpus luteum (CL) is one of the few endocrine glands that forms from the remains of another organ and whose function and survival are limited in scope and time. The CL is the site of rapid remodeling, growth, differentiation, and death of cells originating from granulosa, theca, capillaries, and fibroblasts. The apparent raison d'etre of the CL is the production of progesterone, and all the structural and functional features of this gland are geared toward this end. Because of its unique importance for successful pregnancies, the mammals have evolved a complex series of checks and balances that maintains progesterone at appropriate levels throughout gestation. The formation, maintenance, regression, and steroidogenesis of the CL are among the most significant and closely regulated events in mammalian reproduction. During pregnancy, the fate of the CL depends on the interplay of ovarian, pituitary, and placental regulators. At the end of its life span, the CL undergoes a process of regression leading to its disappearance from the ovary and allowing the initiation of a new cycle. The generation of transgenic, knockout and knockin mice and the development of innovative technologies have revealed a novel role of several molecules in the reprogramming of granulosa cells into luteal cells and in the hormonal and molecular control of the function and demise of the CL. The current review highlights our knowledge on these key molecular events in rodents.

Can luteal regression be reversed?

The corpus luteum is an endocrine gland whose limited lifespan is hormonally programmed. This debate article summarizes findings of our research group that challenge the principle that the end of function of the corpus luteum or luteal regression, once triggered, cannot be reversed. Overturning luteal regression by pharmacological manipulations may be of critical significance in designing strategies to improve fertility efficacy.

Unique expression and regulatory mechanisms of EG-VEGF/prokineticin-1 and its receptors in the corpus luteum

Endocrine gland-derived vascular endothelial growth factor (EG-VEGF) or Prokineticin-1 (PK-1) is a novel cysteine-rich protein that belongs to the AVIT protein family. EG-VEGF/PK-1, described as selective angiogenic mitogen, is widely expressed in different tissues including steroidogenic endocrine glands. This review summarizes the expression and functions of EG-VEGF/PK-1 in corpus luteum (CL)-derived cells: endothelial and steroidogenic cell types. EG-VEGF/PK-1 mRNA is expressed by luteal steroidogenic cells of human, rat and bovine ovaries, but was absent from the luteal Endothelial cells CLEC. Luteal EC expressed high levels of both PK-receptors PK-R1 and PK-R2 - the two G protein-coupled PK-1 receptors. Interestingly, expression of EG-VEGF/PK-1 and VEGF were inversely regulated in human and bovine luteinized granulosa cells. EG-VEGF/PK-1 elevated [3H]-thymidine incorporation, MAPK activation and c-jun/fos mRNA expression and enhanced LEC proliferation. EG-VEGF/PK-1 also inhibited serum starvation-induced apoptosis in these cells. Stress conditions such as serum withdrawal, TNFalpha and chemical hypoxia markedly increase PK-R2 expression, whereas mRNA levels of PK-R1 remain unchanged, implying that the anti-apoptotic effect of PK-1 on LEC may be mediated via PK-R2. Besides its direct mitogenic and anti-apoptotic effects, EG-VEGF/PK-1 elevated VEGF mRNA expression in bovine luteal steroidogenic cells, which possesses only PK-R1. Together, these findings suggest an important role for PK-1 in luteal function by acting as a mitogen and survival factor in LEC. Nevertheless, the inverse regulation of EG-VEGF/PK1 and VEGF mRNA expression by ovarian cells and the distribution of its receptors may suggest that in addition to its angiogenic effects, EG-VEGF/PK-1 may also play other roles in ovary.

Gene expression and immunolocalization of heparin-binding epidermal growth factor-like growth factor and human epidermal growth factor receptors in human corpus luteum

BACKGROUND

The objective of this study was to elucidate gene expression and immunolocalization of heparin-binding epidermal growth factor-like growth factor (HB-EGF) and human epidermal growth factor receptor (HER) family in the human ovary during luteal growth and regression.

METHODS

Ovaries obtained from pre-menopausal women were used for immunohistochemistry and semiquantitative RT-PCR analysis.

RESULTS

Immunoreactive HB-EGF was not detected in follicles or oocyte, while HB-EGF became apparent in granulosa luteal cells in the early luteal phase, and most abundant in the mid-luteal phase, but less abundant in the late luteal phase. Immunostaining for HER1 was very weak in granulosa luteal cells in the early and mid-luteal phases, and was not detected in the late luteal phase. Immunoreactive HER4 was abundant in the early luteal phase and became less abundant in the mid-luteal phase, whereas it was negative in the late luteal phase. Semiquantitative RT-PCR analysis revealed that HB-EGF and HER1 mRNA levels were high in the mid-luteal phase, whereas HER4 mRNA expression was high in the early luteal phase.

CONCLUSIONS

HB-EGF may play a vital role in regulating luteal growth in a juxtacrine manner and through activating HER4 signalling.

Role of the endothelin-1 system in the luteolytic process of pseudopregnant rabbits

The aim of this study was to better understand the role of the endothelin-1 (ET-1) system in the process of controlling the corpora lutea (CL) life span in rabbits. ET-1 (10 microg iv) administration at d 9 and 12 of pseudopregnancy induced a functional luteolysis within 24 h of injection, but it was ineffective at both d 4 and 6. Pretreatments with Bosentan, a dual ET(A)/ET(B) receptor antagonist, or cyclooxygenase (COX) inhibitor blocked the luteolytic action of ET-1 but not that induced by prostaglandin F2alpha (PGF2alpha). In CL cultured in vitro, ET-1 increased (P </= 0.01) both PGF(2alpha) production and luteal nitric oxide synthase activity but decreased (P < or = 0.01) progesterone release. Addition of ET(A) receptor antagonist BQ123 or COX inhibitor blocked the ET-1 luteolytic effects. Positive staining for ET-1 receptors was localized in ovarian blood vessels, granulosa cells of large follicles, and luteal cells. Immunoblot analysis of ET-1 receptor protein revealed a strong band of approximately 48 kDa in d-9 CL. Up to d 6 of pseudopregnancy, ET-1 mRNA abundance in CL was poorly expressed but then increased (P < or = 0.01) at d 9 and 13. ET(A)-receptor transcript increased (P < or = 0.01) at d 6, remained at the same level up to d 13, and then declined to the lowest (P < or = 0.01) levels at d 22. ET(B)-receptor mRNA increased (P < or = 0.01) throughout the late-luteal stage from d 13 up to d 18. Our data suggest that the luteolytic action of ET-1 may be a result of PGF2alpha synthesis from both luteal and accessory cells, via the COX pathways.

The expression of cytosolic and mitochondrial type II acyl-CoA thioesterases is upregulated in the porcine corpus luteum during pregnancy

Acyl-CoA thioesterases hydrolyze acyl-CoAs to free fatty acids and CoASH, thereby regulating fatty acid metabolism. This activity is catalyzed by numerous structurally related and unrelated enzymes, of which several acyl-CoA thioesterases have been shown to be regulated via the peroxisome proliferator-activated receptor alpha, strongly linking them to fatty acid metabolism. Two protein families have recently been characterized, the type I acyl-CoA thioesterase gene family and the type II protein family, which are expressed in cytosol, mitochondria and peroxisomes. Still, only little is known about regulation of their expression and precise functions in vivo. In the present study, we have investigated the activity and expression of acyl-CoA thioesterase in the porcine ovary during different phases of the estrus cycle. The activity was low in homogenates obtained during the immature and follicular phases, increasing nearly 4-fold during the luteal phase, with the highest activity being found in the pregnant corpus luteum (about 7-fold higher than in immature follicles). The increase in homogenate activity in corpus luteum from pregnant pigs was due to a moderate increase in the cytosolic activity, and an approximately 20-25-fold increase in the mitochondrial fraction. Western blot analysis showed no detectable expression of the type I acyl-CoA thioesterases (CTE-I and MTE-I) and revealed that the increased activity in cytosol and mitochondria is due to increased expression of the type II acyl-CoA thioesterases (CTE-II and MTE-II). This apparent hormonal regulation of expression of the type II acyl-CoA thioesterase may provide new insights into the functions of these enzymes in the mammalian ovary.

Analysis of gene expression in the bovine corpus luteum through generation and characterisation of 960 ESTs

To gain new insights into gene identity and gene expression in the bovine corpus luteum (CL) a directionally cloned CL cDNA library was constructed, screened with a total CL cDNA probe and clones representing abundant and rare mRNA transcripts isolated. The 5'-terminal DNA sequence of 960 cDNA clones, composed of 192 abundant and 768 rare mRNA transcripts was determined and clustered into 351 non-redundant expressed sequence tag (EST) groups. Bioinformatic analysis revealed that 309 (88%) of the ESTs showed significant homology to existing sequences in the protein and nucleotide public databases. Several previously unidentified bovine genes encoding proteins associated with key aspects of CL function including extracellular matrix remodelling, lipid metabolism/steroid biosynthesis and apoptosis, were identified. Forty-two (12%) of the ESTs showed homology with human or with other uncharacterised ESTs, some of these were abundantly expressed and may therefore play an important role in primary CL function. Tissue-specificity and temporal CL gene expression of selected clones previously unidentified in bovine CL tissue was also examined. The most interesting finds indicated that mRNA encoding squalene epoxidase was constitutively expressed in CL tissue throughout the oestrous cycle and 7-fold down-regulated (P < 0.05) in late luteal tissue, concomitant with the disappearance of systemic progesterone, suggesting that de novo cholesterol biosynthesis plays an important role in steroidogenesis. The mRNA encoding the growth factor, insulin-like growth factor-binding protein-related protein 1 (IGFBP-rP1), remained constant during the oestrous cycle and was 1.8-fold up-regulated (P < 0.05) in late luteal tissue implying a role in CL regression.

Cytokines and steroidogenesis

Cytokines interfere with steroidogenesis at the level of the adrenals, testes, and ovaries. Within the adrenal, macrophages, and lymphocytes, physiologically widely infiltrating the adrenal cortex, and adrenocortical, and chromaffin cells produce cytokines, as IL-1, IL-6, TNFalpha, leukemia inhibitory factor (LIF), and IL-18 which have a key role in the immune-adreno-cortical communication. In addition to cytokines interacting with adrenal function, cytokine independent mechanisms are responsible for a cell to cell-mediated immune regulation of the adrenal. The importance of this immune-endocrine cross-talk becomes evident in the case of autoimmune and inflammatory diseases being necessary for an adequate adrenal stress response. Secretory products of macrophages are involved in the regulation of steroidogenesis, Sertoli cell activity, and germ cell survival in the human testes. In rats, IL-1 is involved in the paracrine regulation of Leydig cell steroidogenesis. IL-6 has been suggested to exert adverse effects on the male reproductive function, inducing persistent testicular resistance to luteinizing hormone (LH) action and/or suppression of Leydig cell steroidogenesis. Cytokines such as IL-8 and MCP-1 (monocyte chemotactic protein-1) are involved in follicular development and atresia, ovulation, steroidogenesis, and corpus luteum function. In undifferentiated ovarian cells TNF and IL-1 inhibit steroidogenesis, whereas in differentiated ovaries these cytokines stimulate progesterone synthesis. Some ovarian cancer cells secrete TNF and IL-1 which stimulate growth of these cells. In conclusion, cytokines interact with steroidogenesis in a systemic and complex manner, influencing development, function, and hormone production of the adrenals, testes, and ovaries.

Expression patterns of cytokines, p53 and nitric oxide synthase isoenzymes in corpora lutea of pseudopregnant rabbits during spontaneous luteolysis

The gene expressions for macrophage chemoattractant protein-1 (MCP-1), interleukin (IL)-1β, IL-2 and p53 were examined by semi-quantitative RT-PCR in corpora lutea (CL) of rabbits during spontaneous luteolysis at days 13, 15, 18 and 22 of pseudopregnancy. In the same luteal tissue, total activity of nitric oxide (NO) synthase (NOS) and genes for both endothelial (eNOS) and inducible (iNOS) isoforms were also analysed. From day 13 to 15, MCP-1 and IL-1βmRNA levels rose (P≤ 0.01) almost 2-fold, and the transcript for p53 almost 8-fold, but then all dropped (P≤ 0.05) from day 18 onward. IL-2 mRNA abundance was higher (P≤ 0.01) on day 13 and then gradually declined. During luteolysis, eNOS mRNA decreased 40% (P≤ 0.05) by day 15, but thereafter remained unchanged, while iNOS mRNA was barely detectable and did not show any clear age-related pattern throughout the late luteal stages. Total NOS activity progressively increased (P≤ 0.01) from day 13 to 18 of pseudopregnancy and then dropped to the lowest (P≤ 0.01) levels on day 22. Luteal progesterone content also declined during CL regression from 411 to 17 pg/mg found on days 13 and 22 respectively, in parallel with the decrease in blood progesterone concentrations. These data further support a physiological role of NO as modulator of luteal demise in rabbits. Locally, luteal cytokines may be involved in the up-regulation of NOS activity, while downstream NO may inhibit steroroidogenesis and induce expression of p53 gene after removal of the protective action of progesterone.

Local interaction of prostaglandin F2α with endothelin-1 and tumor necrosis factor-α on the release of progesterone and oxytocin in ovine corpora lutea in vivo: a possible implication for a luteolytic cascade

Endothelin-1 (ET-1) and tumor necrosis factor-α (TNFα) participate in the cascade of luteolysis. Thus, in the present study the interactions of ET-1 and TNFα with prostaglandin F2α (PGF2α) on the release of progesterone and oxytocin (OT) within the corpus luteum (CL) were investigated. A microdialysis system (MDS) was surgically implanted in ovine CL (one MDS line/CL; 5–10 lines/ewe) formed after super-ovulation. A 4-h perfusion with PGF2α (0.01–1 μmol l −1) induced no clear effect on progesterone release, but acutely stimulated OT release in a dose-dependent manner. A perfusion of PGF2α (1 μmol l −1) increased ET-1 release over a period of 12 h. Two perfusions of ET-1 (0.1 μmol l−1) or a perfusion of ET-1 followed by TNFα (200 ng ml−1) decreased progesterone release (56–64% at 36–48 h). When the CL were pre-perfused with PGF2α (1 μmol l−1), two consecutive perfusions of ET-1 decreased progesterone release more rapidly. Similarly, a pre-perfusion with PGF2α followed by consecutive perfusions of ET-1 and then TNFα rapidly decreased progesterone release, with the inhibition most pronounced (35%) at 36–48 h. The simultaneous infusion of ET-1 with PGF2α induced a rapid decrease in progesterone release (36% at 36–48 h). In a further study, the possible second messenger systems involved in PGF2α action on the release of progesterone, OT and ET-1 were investigated. A perfusion with 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 10 μmol l−1), A23187 (10 μmol l−1), or PGF2α + A23187 increased progesterone release during infusion, but decreased it after perfusion. All treatments induced a massive release of OT during infusion, and increased ET-1 release after infusion. These results show that ET-1 is capable of suppressing progesterone release in the PGF2α-primed ovine CL in vivo and thus ET-1 works as a local luteolysin together with PGF2α during the process of functional luteolysis. During structural luteolysis, TNFα may interact with PGF2α and ET-1 to cause a rapid drop in progesterone release and accelerate the process of luteolysis. This result supports the contention that ET-1 and TNFα interact with PGF2α as local luteolytic mediators in the ewe as previously suggested.

Cholesterol transport and steroidogenesis by the corpus luteum

The synthesis of progesterone by the corpus luteum is essential for the establishment and maintenance of early pregnancy. Regulation of luteal steroidogenesis can be broken down into three major events; luteinization (i.e., conversion of an ovulatory follicle), luteal regression, and pregnancy induced luteal maintenance/rescue. While the factors that control these events and dictate the final steroid end products are widely varied among different species, the composition of the corpus luteum (luteinized thecal and granulosa cells) and the enzymes and proteins involved in the steroidogenic pathway are relatively similar among all species. The key factors involved in luteal steroidogenesis and several new exciting observations regarding regulation of luteal steroidogenic function are discussed in this review.

Chemokines in the corpus luteum: implications of leukocyte chemotaxis

Chemokines are small molecular weight peptides responsible for adhesion, activation, and recruitment of leukocytes into tissues. Leukocytes are thought to influence follicular atresia, ovulation, and luteal function. Many studies in recent years have focused attention on the characterization of leukocyte populations within the ovary, the importance of leukocyte-ovarian cell interactions, and more recently, the mechanisms of ovarian leukocyte recruitment. Information about the role of chemokines and leukocyte trafficking (chemotaxis) during ovarian function is important to understanding paracrine-autocrine relationships shared between reproductive and immune systems. Recent advances regarding chemokine expression and leukocyte accumulation within the ovulatory follicle and the corpus luteum are the subject of this mini-review.

Role of tumor necrosis factor-alpha and the modulating effect of the caspases in rat corpus luteum apoptosis

Tumor necrosis factor-alpha (TNFalpha) is a pleiotropic cytokine that has been implicated in apoptosis of many cell systems. However, the signal transduction of TNFalpha during the structural and functional regression of the corpus luteum (CL) is largely unknown. In this study, we investigate the role of TNFalpha in rat CL apoptosis and the involvement of monocyte chemoattractant protein-1 (MCP-1) and the modulating effect of the caspases in this process. An in vivo study of CL during pregnancy and postpartum using immunohistochemistry and Western blot analysis indicated that increases in TNFalpha correspond with luteal apoptosis approaching term (Day 22) and at postpartum (Day 3). CL apoptosis was further investigated using a whole-CL culture model of tropic withdrawal. An increase was observed in both low molecular weight (MW) DNA fragmentation and TUNEL staining from 0 h to 8 h in culture. CL apoptosis in vitro was associated with increased protein expression of both TNFalpha and MCP-1 as measured by immunohistochemistry and Western blot analysis. Using a whole-CL culture model, apoptosis was induced in vitro by TNFalpha as demonstrated by a dose-dependent increase in DNA fragmentation. Treatment of luteal cells with TNFalpha and both specific caspase inhibitors (Z-DEVD-FMK, Z-VEID-FMK, Z-IETD-FMK) or a general caspase inhibitor (Boc-D-FMK) prevented the effect of TNFalpha. CL regression involves the apoptotic deletion of luteal cells; the results of this study suggest that TNFalpha is possibly involved in this process. The observed increases in MCP-1 expression suggest the coordination of TNFalpha expression with the infiltration and activation of macrophages. Furthermore, the results demonstrate the importance of the caspases in the TNFalpha signal transduction pathway and suggest a hierarchy within the caspase family.