Hypoxia promotes luteal cell death in bovine corpus luteum

Exposure to a Low-Oxygen Environment Causes Implantation Failure and Transcriptomic Shifts in Mouse Uteruses and Ovaries

Hypoxia is a condition in which tissues of the body do not receive sufficient amounts of oxygen supply. Numerous studies have elucidated the intricate roles of hypoxia and its involvement in both physiological and pathological conditions. This study aimed to clarify the impact of a forced low-oxygen environment in early pregnancy by exposing mice to low-oxygen conditions for 24-72 h after fertilization. The treatment resulted in the complete failure of blastocyst implantation, accompanied by vascular hyperpermeability in the uterus. A transcriptome analysis of the uterus revealed remarkable alterations in gene expression between control normoxic- and hypoxic-treatment groups. These alterations were characterized by the differentially expressed genes categorized into the immune responses and iron coordination. Furthermore, exposure to a low-oxygen environment caused apoptosis in the corpus luteum within the ovary and a reduction in progesterone secretion. Consequently, diminished plasma progesterone levels were considered to contribute to implantation failure in combination with the activation of the hypoxic pathway in the uterus. Additionally, previous studies have demonstrated the impact of hypoxic reactions on blastocyst development and the pre-implantation process in the endometrium. Our findings suggest that the corpus luteum exhibits elevated susceptibility to hypoxia, thereby elucidating a critical aspect of its physiological response.

Apoptosis, autophagic cell death, and necroptosis: different types of programmed cell death in bovine corpus luteum regression

In mammals, the corpus luteum (CL) is a transient organ that secretes progesterone (P4). In the absence of pregnancy, the CL undergoes regression (luteolysis), which is a crucial preparation step for the next estrous cycle. Luteolysis, initiated by uterine prostaglandin F_2α (PGF) in cattle, is usually divided into two phases, namely functional luteolysis characterized by a decline in P4 concentration and structural luteolysis characterized by the elimination of luteal tissues from the ovary. Programmed cell death (PCD) of luteal cells, including luteal steroidogenic cells (LSCs) and luteal endothelial cells (LECs), plays a crucial role in structural luteolysis. The main types of PCD are caspase-dependent apoptosis (type 1), autophagic cell death (ACD) via the autophagy-related gene (ATG) family (type 2), and receptor-interacting protein kinase (RIPK)-dependent programmed necrosis (necroptosis, type 3). However, these PCD signaling pathways are not completely independent and interact with each other. Over the past several decades, most studies on luteolysis have focused on apoptosis as the principal mode of bovine luteal cell death. Recently, ATG family members were reported to be expressed in bovine CL, and their levels increased during luteolysis. Furthermore, the expression of RIPKs, which are crucial mediators of necroptosis, is reported to increase in bovine CL during luteolysis and is upregulated by pro-inflammatory cytokines in bovine LSCs and LECs. Therefore, apoptosis, ACD, and necroptosis may contribute to bovine CL regression. In this article, we present the recent findings regarding the mechanisms of the three main types of PCD and the contribution of these mechanisms to luteolysis.

The Role of Reduced Oxygen Supply and Transcription Factors cJUN and CREB in Progesterone Production during the Corpus Luteum Rescue in Gilts

The corpus luteum plays a fundamental role in regulating reproduction via progesterone production. Still, there is little data on factors regulating the maintenance of luteal function during early pregnancy in gilts. Previous studies emphasize the role of hypoxia and HIF-1 in the regulation of steroidogenic and angiogenic genes expression and progesterone production by ovarian cells. Using the corpus luteum of cyclic and early pregnant gilts we analyzed: (1) the in vitro effects of reduced oxygen tension on progesterone production and mRNA expression of HIF1A and luteal function regulators, STAR and VEGFA; (2) the ex vivo profiles of mRNA and protein expression of HIF-1α, STAR, VEGFA and transcription factors-cJUN and CREB, regulating STAR expression, in the corpus luteum of cyclic and pregnant gilts. The synthesis of progesterone was gradually inhibited in cyclic or pregnant gilt luteal tissue (on day 13 of cycle or pregnancy) incubated in a decreasing concentration−20%, 10%, and 3% of oxygen (O2). Luteal tissues of pregnant gilts produced trace amounts of progesterone in 10% O2, which was similar to cyclic gilts in 3% O2. HIF1A expression increased after 24 h of incubation in tissues of cyclic gilts in 3% vs. 20% O2 (p < 0.01), whereas levels of STAR and VEGFA increased significantly in cyclic and pregnant gilt tissues incubated in 10% and 3% vs. 20% O2. The ex vivo mRNA expression of HIF1A and VEGFA was elevated (p < 0.001) on day 14 vs. day 12 of pregnancy. The protein expression of HIF-1 and VEGFA increased (p < 0.001), whereas the level of STAR (mRNA and protein) and progesterone dropped (p < 0.001) on day 14 of the estrous cycle vs. a parallel day of pregnancy and/or day 12 of the estrous cycle. The content of phosphorylated cJUN and CREB was elevated (p < 0.01) in the luteal tissue on day 12 or 14 of pregnancy vs. parallel days of the estrous cycle. These increases of phosphorylated cJUN and CREB may be involved in STAR expression in the luteal tissue during early pregnancy in gilts.

Transcriptional regulation of HIF1α-mediated STAR expression in murine KK1 granulosa cell line involves cJUN, CREB and CBP-dependent pathways

Gonadal function is connected to hypoxia, with hypoxia-inducible factor (HIF) 1α, as a component of HIF1-complexes, regulating cellular adaptation to hypoxic conditions. In the ovary, it regulates follicular maturation, ovulation and luteal development. At the cellular level, HIF1-complexes coordinate the expression of steroidogenic acute regulatory protein (STAR), and thereby ovarian steroidogenesis. The functionality of STAR is associated with the cAMP/PKA-dependent pathways. In vitro, HIF1α is required for basal and cAMP-induced STAR expression, under ambient and reduced oxygen (O₂) tension. Lowering O₂ increases the responsiveness of the Star promoter towards cAMP and PKA mediates activation/phosphorylation (P) of several transcriptional factors, including cJUN and cAMP response element-binding protein (CREB), whose functionality is linked to HIF1 through utilization of CREB-binding protein (CBP). Since the mechanisms underlying HIF1α-dependent expression of STAR remain unknown, we investigated the involvement of HIF1α in CREB-, cJUN- and CBP-mediated expression of STAR using a well-characterized steroidogenic model, murine KK1 granulosa cells; ambient and lowered (10%) O₂ were applied. Our main findings were that while functional suppression of the α-subunit of HIF1 lowered STAR/P-STAR and steroidogenic output from granulosa cells, surprisingly the levels of P-CREB and its transcriptional activity were strongly induced. However, its association with the Star promoter was decreased, indicating dissociation of P-CREB from the promoter. Further, suppression of HIF1 activity ultimately diminished the expression of cJUN/P-cJUN and CBP. Finally, the study suggests that HIF1-complex: (1) regulates cJUN expression in granulosa cells, (2) is involved in regulating the recruitment of P-CREB to the Star promoter in (3) a mechanism which possibly involves the HIF1-dependent regulation of CBP expression.

HIF-1α Activation Promotes Luteolysis by Enhancing ROS Levels in the Corpus Luteum of Pseudopregnant Rats

The increase of oxidative stress is one of the important characteristics of mammalian luteal regression. Previous investigations have revealed the essential role of reactive oxygen species (ROS) in luteal cell death during luteolysis, while it is unknown how ROS is regulated in this process. Considering the decrease of blood flow and increase of PGF_2α during luteolysis, we hypothesized that the HIF-1α pathway may be involved in the regulation of ROS in the luteal cell of the late corpus luteum (CL). Here, by using a pseudopregnant rat model, we showed that the level of both HIF-1α and its downstream BNIP3 was increased during luteal regression. Consistently, we observed the increase of autophagy level during luteolysis, which is regulated in a Beclin1-independent manner. Comparing with early (Day 7 of pseudopregnancy) and middle CL (Day 14), the level of ROS was significantly increased in late CL, indicating the contribution of oxidative stress in luteolysis. Inhibition of HIF-1α by echinomycin (Ech), a potent HIF-1α inhibitor, ameliorated the upregulation of BNIP3 and NIX, as well as the induction of autophagy and the accumulation of ROS in luteal cells on Day 21 of pseudopregnancy. Morphologically, Ech treatment delayed the atrophy of the luteal structure at the late-luteal stage. An in vitro study indicated that inhibition of HIF-1α can also attenuate PGF_2α-induced ROS and luteal cell apoptosis. Furthermore, the decrease of cell apoptosis can also be observed by ROS inhibition under PGF_2α treatment. Taken together, our results indicated that HIF-1α signaling is involved in the regression of CL by modulating ROS production via orchestrating autophagy. Inhibition of HIF-1α could obviously hamper the apoptosis of luteal cells and the process of luteal regression.

HIF-1α/BNIP3-Mediated Autophagy Contributes to the Luteinization of Granulosa Cells During the Formation of Corpus Luteum

During the luteinization after ovulation in mammalian ovary, the containing cells undergo an energy consuming function re-determination process to differentiate into luteal cells under avascular environment. Previous evidences have delineated the contribution of autophagy to the cell differentiation and the catabolic homeostasis in various types of mammalian cells, whereas few interest had been focused on the involvement of autophagy in the luteinization of granulosa cells during the formation of early corpus luteum. Herein, the present study investigated that expression and contribution of autophagy during granulosa cell luteinization and early luteal development through in vivo and in vitro experiments. The results clearly demonstrated that HIF-1α/BNIP3-mediated autophagy plays a vital role in the luteinization of granulosa cells during the early luteal formation in vivo and in vitro. In the neonatal corpus luteum, HIF-1α up-regulated BNIP3 expressions, which contributed to the autophagic initiation by disrupting beclin1 from Bcl-2/beclin1 complex and protected cells from apoptosis by curbing the skew of mitochondria balance under avascular niche. Notably, Inhibition of HIF-1α activity by echinomycin enhanced the levels of cytoplasmic cytochrome c and cell apoptosis in the nascent corpus luteum. These findings revealed that HIF-1α/BNIP3-mediated autophagy enabled the process of granulosa cell luteinization and protected the granulosa-lutein cells from further apoptosis under hypoxia niche. To our knowledge, the present study firstly clarified that HIF-1α/BNIP3-mediated autophagy contributes to the luteinization of granulosa cells during the formation of pregnant corpus luteum, which will help us further understanding the luteal biology and provide us new clues for the treatment of luteal insufficiency.

Multiple roles of hypoxia in bovine corpus luteum

There has been increasing interest in the role of hypoxia in the microenvironment of organs, because of the discovery of hypoxia-inducible factor-1 (HIF1), which acts as a transcription factor for many genes activated specifically under hypoxic conditions. The ovary changes day by day during the estrous cycle as it goes through phases of follicular growth, ovulation, and formation and regression of the corpus luteum (CL). These phenomena are regulated by hypothalamic and pituitary hormones, sex steroids, peptides and cytokines, as well as oxygen conditions. Hypoxia strongly induces angiogenesis via transcription of a potent angiogenic factor, vascular endothelial growth factor (VEGF), that is regulated by HIF1. A CL forms with a rapid increase of angiogenesis that is mainly induced by HIF1-VEGF signaling. Hypoxia also contributes to luteolysis by down-regulating progesterone synthesis and by up-regulating apoptosis of luteal cells. This review focuses on recent studies on the roles of hypoxia- and HIF1-regulated genes in the regulation of bovine CL function.

Parenteral Zinc Supplementation Increases Pregnancy Rates in Beef Cows

Zinc (Zn) is required for normal reproductive performance in cattle. The aim of this study was to evaluate the effect of subcutaneous injection of 400 mg Zn at the beginning of fixed-time artificial insemination (FTAI) on preovulatory follicle and corpus luteum (CL) size, plasma estradiol (E2) and progesterone (P4) concentrations, and pregnancy rates in beef cows. Copper (Cu) concentration and alkaline phosphatase (ALP) activity in plasma were also evaluated. Zinc supplementation at the beginning of the FTAI protocol (day 0) increased the area of preovulatory follicle (APF, day 9; P = 0.042) and plasma P4 concentration (day 16; P = 0.01), whereas plasma E2 concentration (day 9) and area of CL (ACL; day 16) were not modified by Zn supplementation in cows with adequate plasma Zn concentration. Zinc supplementation in Zn-deficient cows increased ACL with respect to controls (P = 0.048) but did not modify plasma E2 concentration. Pregnancy rate on day 41 after FTAI was higher in cows supplemented with Zn compared with controls (80.95% and 51.61%, respectively; P = 0.042). Plasma Zn and Cu concentrations on days 7, 9, and 16 were not affected by Zn supplementation. In conclusion, the results obtained in the present study determined that parenteral Zn supplementation at the beginning of the FTAI protocol increased preovulatory follicle size, plasma P4 concentration, and pregnancy rates in beef cows.

The Interaction Between Nodal, Hypoxia-Inducible Factor 1 Alpha, and Thrombospondin 1 Promotes Luteolysis in Equine Corpus Luteum

The regulation of corpus luteus (CL) luteolysis is a complex process involving a myriad of factors. Previously, we have shown the involvement of Nodal in functional luteolysis in mares. Presently, we ask the extent of which Nodal mediation of luteolysis is done through regulation of angioregression. We demonstrated the interaction between Nodal and hypoxia-inducible factor 1 α (HIF1α) and thrombospondin 1/thrombospondin receptor (TSP1/CD36) systems, could mediate angioregression during luteolysis. First, we demonstrated the inhibitory effect of Nodal on the vascular marker platelet/endothelial cell adhesion molecule 1 (CD31). Also, treatment of mid CL explants with vascular endothelial growth factor A (VEGFA) showed a trend on activin-like kinase 7 (Alk7) protein inhibition. Next, Nodal was also shown to activate HIF1α and in vitro culture of mid CL explants under decreased oxygen level promoted Nodal expression and SMAD family member 3 (Smad3) phosphorylation. In another experiment, the crosstalk between Nodal and TSP1/CD36 was investigated. Indeed, Nodal increased the expression of the anti-angiogenic TSP1 and its receptor CD36 in mid CL explants. Finally, the supportive effect of prostaglandin F2α (PGF2α) on TSP1/CD36 was blocked by SB431542 (SB), a pharmacological inhibitor of Nodal signaling. Thus, we evidenced for the first time the in vitro interaction between Nodal and both HIF1α and TSP1 systems, two conserved pathways previously shown to be involved in vascular regression during luteolysis. Considering the given increased expression of Nodal in mid CL and its role on functional luteolysis, the current results suggest the additional involvement of Nodal in angioregression during luteolysis in the mare, particularly in the activation of HIF1α and TSP1/CD36.

Perturbations in Lineage Specification of Granulosa and Theca Cells May Alter Corpus Luteum Formation and Function

Anovulation is a major cause of infertility, and it is the major leading reproductive disorder in mammalian females. Without ovulation, an oocyte is not released from the ovarian follicle to be fertilized and a corpus luteum is not formed. The corpus luteum formed from the luteinized somatic follicular cells following ovulation, vasculature cells, and immune cells is critical for progesterone production and maintenance of pregnancy. Follicular theca cells differentiate into small luteal cells (SLCs) that produce progesterone in response to luteinizing hormone (LH), and granulosa cells luteinize to become large luteal cells (LLCs) that have a high rate of basal production of progesterone. The formation and function of the corpus luteum rely on the appropriate proliferation and differentiation of both granulosa and theca cells. If any aspect of granulosa or theca cell luteinization is perturbed, then the resulting luteal cell populations (SLC, LLC, vascular, and immune cells) may be reduced and compromise progesterone production. Thus, many factors that affect the differentiation/lineage of the somatic cells and their gene expression profiles can alter the ability of a corpus luteum to produce the progesterone critical for pregnancy. Our laboratory has identified genes that are enriched in somatic follicular cells and luteal cells through gene expression microarray. This work was the first to compare the gene expression profiles of the four somatic cell types involved in the follicle-to-luteal transition and to support previous immunofluorescence data indicating theca cells differentiate into SLCs while granulosa cells become LLCs. Using these data and incorporating knowledge about the ways in which luteinization can go awry, we can extrapolate the impact that alterations in the theca and granulosa cell gene expression profiles and lineages could have on the formation and function of the corpus luteum. While interactions with other cell types such as vascular and immune cells are critical for appropriate corpus luteum function, we are restricting this review to focus on granulosa, theca, and luteal cells and how perturbations such as androgen excess and inflammation may affect their function and fertility.

BNIP3 expression in bovine follicle and corpus luteum

BNIP3 (BCL2/adenovirus E1B nineteen kilodalton interacting protein-3), a member of the BCL2 family, is activated under hypoxic conditions and induces apoptosis or mitochondrial autophagy for adapting cells to hypoxia. The physiological roles of BNIP3 in the mammalian ovary are still unclear. In order to understand the role of BNIP3 in the bovine ovary, we examined its mRNA and protein expressions of BNIP3 in follicular granulosa cells and corpus luteum (CL). BNIP3 mRNA and protein expressions in granulosa cells from large follicles (>10 mm) at the follicular stage were much higher than those in small follicles (2–8 mm). BNIP3 mRNA and protein expressions in the CL peaked at the early luteal stage. In bovine granulosa cells cultured for 6 hr under hypoxia (3% O₂) and normoxia (20% O₂), BNIP3 mRNA expression was higher under hypoxia. These results of the present study suggest that BNIP3 has some roles in luteal formation in the bovine ovary, and that the highly expressed BNIP3 in the granulosa cells from large follicles at the follicular stage is related to the roles of BNIP3 in the luteal formation.

Endocrine targets of hypoxia-inducible factors

Endocrine is an important and tightly regulated system for maintaining body homeostasis. Endocrine glands produce hormones, which are released into blood stream to guide the target cells responding to all sorts of stimulations. For maintaining body homeostasis, the secretion and activity of a particular hormone needs to be adjusted in responding to environmental challenges such as changes in nutritional status or chronic stress. Hypoxia, a status caused by reduced oxygen availability or imbalance of oxygen consumption/supply in an organ or within a cell, is a stress that affects many physiological and pathological processes. Hypoxic stress in endocrine organs is especially critical because endocrine glands control body homeostasis. Local hypoxia affects not only the particular gland but also the downstream cells/organs regulated by hormones secreted from this gland. Hypoxia-inducible factors (HIFs) are transcription factors that function as master regulators of oxygen homeostasis. Recent studies report that aberrant expression of HIFs in endocrine organs may result in the development and/or progression of diseases including diabetes, endometriosis, infertility and cancers. In this article, we will review recent findings in HIF-mediated endocrine organ dysfunction and the systemic syndromes caused by these disorders.

Hypoxia-inducible factor 1 mediates hypoxia-enhanced synthesis of progesterone during luteinization of granulosa cells

Hypoxia has been suggested to enhance progesterone (P4) synthesis in luteinizing granulosa cells (GCs), but the mechanism is unclear. The present study was designed to test the hypothesis that the hypoxia-induced increase in P4 synthesis during luteinization in bovine GCs is mediated by hypoxia-inducible factor 1 (HIF-1). GCs obtained from small antral follicles were cultured with 2 µg/ml insulin in combination with 10 µM forskolin for 24 h as a model of luteinizing GCs. To examine the influence of HIF-1 on P4 synthesis, we determined the effect of changes in protein expression of the α-subunit of HIF-1 (HIF1A) on P4 production and on the expression levels of StAR, P450scc, and 3β-HSD. CoCl₂ (100 µM), a hypoxia-mimicking chemical, increased HIF-1α protein expression in luteinizing GCs. After the upregulation of HIF-1α, we observed an increase in P4 production and in the gene and protein expression levels of StAR in CoCl₂-treated luteinizing GCs. In contrast, CoCl₂ did not affect the expression of either P450scc or 3β-HSD. Echinomycin, a small-molecule inhibitor of HIF-1's DNA-binding activity, attenuated the effects of CoCl₂ and of low oxygen tension (10% O₂) on P4 production and StAR expression in luteinizing GCs. Overall, these findings suggest that HIF-1 is one of the factors that upregulate P4 in GCs during luteinization.

Histological analysis of arteriovenous anastomosis-like vessels established in the corpus luteum of cows during luteolysis

Background

The mechanisms regulating the function and regression of the corpus luteum (CL) have not yet been elucidated in detail. The regressed CL of cows was previously reported to be filled with unusual vessels like arteriovenous anastomosis (AVA); however how these vessels are being established during luteolysis remains unknown.

Methods

The bovine CL at different luteal stages and regressing bovine CL induced by prostaglandin F_2α (PGF) were histologically analyzed using light and electron microscopic levels. The changes in mRNA expression of genes encoding α-smooth muscle actin (SMA; Acta2) and transforming growth factor β1 (Tgfb1) in luteal tissues were analyzed by quantitative RT-PCR.

Results

AVA-like vessels appeared in the regressed CL with a diameter less than 1.5 cm in which no functional luteal cells and macrophages were observed. Epithelioid cells in the AVA-like vessel wall were immunoreactive for SMA, and the lumen of the vessels were narrow. Immunoreaction for SMA was found in the tunica media of typical arteries and arterioles, and pericytes around capillary vessel. Cells with elongated cytoplasmic processes ―resident fibroblasts expressing vimentin― distributed in the CL parenchyma without any association with blood vessels are also immunoreactive for SMA, and accumulated around arteries and arterioles during the late-luteal stage. In the regressed CL, walls of arteries and arterioles consisted of more than two layers of epithelioid cells positive for both SMA and desmin, suggesting that they are myofibroblasts transformed from fibroblasts. The percentage of the area positive for SMA and the mRNA expression of Acta2 were significantly increased in the regressed CL; however, they did not alter when a luteolytic dose of PGF was injected in vivo and collected within 24 h after the injection. On the other hand, Tgfb1, a known regulator for myofibroblast transformation, was significantly increased in PGF-induced regressing CL as well as in the CL during the late-luteal stage.

Conclusions

SMA-positive myofibroblasts accumulates around the arteries and arterioles to form AVA-like vessels during luteolysis in cows. PGF indirectly regulates myofibroblast transformation through enhancing the expression of TGFβ1. These peculiar AVA-like vessels may be involved in the regulation of blood flow in the bovine CL during luteolysis.

Roles of EDNs in regulating oviductal NO synthesis and smooth muscle motility in cows

Endothelins (EDNs) participate in various physiological events including smooth muscle contraction, nitric oxide (NO) synthesis, and embryonic development. In this study, we investigated the regional roles of EDNs produced by bovine oviductal epithelial cells in NO synthesis and smooth muscle motility. Quantification of mRNA expressions indicated that expression of EDN receptor B (EDNRB) in the ampullary region was higher after ovulation than before ovulation, whereas expression of EDNRA in the isthmic region was higher after ovulation than before ovulation. Immunohistochemistry revealed that the EDN receptors (EDNRA and EDNRB) were expressed in the epithelium, whereas smooth muscle showed positive staining only for EDNRA. The expressionsPlease suggest whether 'NOS2' can be treated as the updated symbol for 'iNOS' as per gene nomenclature. of inducible NO synthase (iNOS) protein and its mRNA (NOS2) in cultured epithelial cells isolated from the ampulla were stimulated by EDN1, but not by EDN2 or EDN3, after 1h of incubation. In isthmic epithelial cells, none of the EDNs affected the expression of NOS2 Isometric contraction tests indicated that spontaneous waves were strong in the isthmic region but weak in the ampullary region. EDN1 modulated smooth muscle motility in both the regions. The overall findings suggest that EDN1 plays region-specific roles in smooth muscle motility and epithelial NO synthesis, providing an optimal oviductal microenvironment for transport of gametes, fertilization, and development/transport of early embryo.

Steroid hormones, prostanoids, and angiogenic systems during rescue of the corpus luteum in pigs

In order to characterize the transition of the corpora lutea (CL) from acquisition of luteolytic sensitivity to rescue of luteal function: i) the expression of 38 factors associated with steroids, prostanoids, and angiogenic systems and ii) concentrations of the main hormones responsible for maintenance of CL function in cyclic and pregnant pigs were examined. Additionally, the effect of prostaglandin (PG) E2 and F2 α on luteal function during the estrous cycle and pregnancy was evaluated in vitro. Significantly up-regulated gene expression was revealed in CL collected on day 14 of the estrous cycle (CYP19A1, ESR2, PTGS2, HIF1A, and EDN1) and on days 12-14 of pregnancy (SCARB1, PGRMC1, STAR, HSD3B1, NR5A1, PTGFR, PTGER4, and VEGFA). Elevated concentrations of estradiol-17β and PGE2 occurred in CL on days 12 and 14 of pregnancy respectively, while an increased intraluteal PGF2 α content was noted on day 14 of the estrous cycle. Both PGs increased the synthesis of progesterone by cultured luteal slices obtained on day 14 of pregnancy, in contrast to the action of PGF2 α on the corresponding day of the estrous cycle. PGE2 stimulated cAMP production via PTGER2 and PTGER4, while PGF2 α elevated the content of CREB in cultured luteal slices from CL of pregnant pigs. In silico analysis showed that infiltration of lymphocytes and apoptosis of microvascular endothelium were activated in CL on day 12 of the estrous cycle vs pregnancy. Summarizing, an abundance of E2 and PGE2 during pregnancy regulates specific pathways responsible for steroidogenesis, the prostanoid signaling system and angiogenesis during rescue from luteolysis in porcine CL.

Multiple roles of hypoxia in ovarian function: roles of hypoxia-inducible factor-related and -unrelated signals during the luteal phase

There is increasing interest in the role of oxygen conditions in the microenvironment of organs because of the discovery of a hypoxia-specific transcription factor, namely hypoxia-inducible factor (HIF) 1. Ovarian function has several phases that change day by day, including ovulation, follicular growth and corpus luteum formation and regression. These phases are regulated by many factors, including pituitary hormones and local hormones, such as steroids, peptides and cytokines, as well as oxygen conditions. Hypoxia strongly induces angiogenesis because transcription of the potent angiogenic factor vascular endothelial growth factor (VEGF) is regulated by HIF1. Follicular development and luteal formation are accompanied by a marked increase in angiogenesis assisted by HIF1-VEGF signalling. Hypoxia is also one of the factors that induces luteolysis by suppressing progesterone synthesis and by promoting apoptosis of luteal cells. The present review focuses on recent studies of hypoxic conditions, as well as HIF1-regulated genes and proteins, in the regulation of ovarian function.

Hypoxia promotes progesterone synthesis during luteinization in bovine granulosa cells

To determine whether hypoxia has an effect on luteinization, we examined the influence of hypoxia on a model of bovine luteinizing and non-luteinizing granulosa cell culture. The granulosa cells were obtained from small antral follicles (≤ 6 mm in diameter). To induce luteinization, the cells were treated for 24 h with insulin (2 µg/ml), forskolin (10 µM) or insulin in combination with forskolin at 20% O2. After 24 h, progesterone (P4) production was higher in the treated cells, which we defined as luteinizing granulosa cells, than in non-treated cells, which we defined as non-luteinizing granulosa cells. P4 production by non-luteinizing granulosa cells was not affected by hypoxia (24 h at 10% and 5% O2), while P4 production by granulosa cells treated with insulin in combination with forskolin was significantly increased under hypoxia (24 h at 10% and 5% O2). Because hypoxia affected P4 production by the luteinizing granulosa cells but not by the non-luteinizing granulosa cells, hypoxia seems to promote P4 production during, rather than before, luteinization. In the cells treated with insulin in combination with forskolin, mRNA and protein expression of steroidogenic acute regulatory protein (StAR) and protein expression of 3β-hydroxysteroid dehydrogenase (3β-HSD) increased under 10% O2, while mRNA and protein expressions of key protein and enzymes in P4 biosynthesis did not increase under 5% O2. The overall results suggest that hypoxia plays a role in progressing and completing the luteinization by enhancing P4 production through StAR as well as 3β-HSD expressions in the early time of establishing the corpus luteum.

Glucose transporter 1 expression accompanies hypoxia sensing in the cyclic canine corpus luteum

The canine corpus luteum (CL) functions as a source of progesterone (P4) and 17β-oestradiol (E2); however, the transport of energy substrates to maintain its high hormonal output has not yet been characterised. This study involved the localisation and temporal distribution of the facilitative glucose transporter 1 and the quantification of the corresponding protein (GLUT1) and gene (SLC2A1) expression. Some GLUT1/SLC2A1 regulatory proteins, such as hypoxia-inducible factor 1α (HIF1A) and fibroblast growth factor 2 (FGF2); mRNAs, such as HIF1A, FGF2 and vascular endothelial growth factor A (VEGFA); and VEGFA receptors 1 and 2 (FLT1 and KDR) were also analysed from days 10 to 70 after ovulation. Additionally, plasma P4 and E2 levels were assessed via chemiluminescence. Moreover, the canine KDR sequence has been cloned, thereby enabling subsequent semi-quantitative PCR analysis. Our results demonstrate time-dependent variations in the expression profile of SLC2A1 during dioestrus, which were accompanied by highly correlated changes (0.84<r<0.98; P<0.03) in the gene expression of HIF1A, VEGF and FLT1 as well as in P4 plasma concentrations. FGF2 mRNA correlated with E2 plasma concentrations (r=0.61; P=0.01). Our data reveal that the glucose transporter is regulated throughout the CL lifespan and suggest that CL depends on the sensing of hypoxia and the status of luteal vascularisation. Moreover, time-dependent expression of GLUT1/SLC2A1 may lie underneath increased metabolic and energetic requirements for sustaining P4 production.

Summer heat stress affects prostaglandin synthesis in the bovine oviduct

Summer heat stress (HS) negatively affects reproductive functions, including prostaglandin (PG) F2α secretion in the endometrium, and decreases fertility in cattle. In the present study, we examined the effects of elevated temperatures on PG synthesis in oviductal epithelial cells. The epithelial cells obtained from the ampulla and isthmus of the oviduct were incubated at various temperatures (38.5, 39.5, 40.0, and 40.5 °C) for 24 h. In the ampulla, PGE2 concentration was higher at 40.5 °C than at 38.5 °C, while PGF2α production was not affected by the temperatures in this range. The expressions of microsomal PGE synthase 1 (PTGES (mPGES1)), cytosolic PGES (PTGES3 (cPGES)), and heat shock protein 90 (HSP90AA1 (HSP90)) mRNAs and proteins were higher at 40.5 °C than at 38.5 °C in the ampullary epithelial cells. Seasonal changes in the expressions of PGES and HSP90AA1 mRNAs in oviductal tissues were also investigated. The expressions of PTGES3 and HSP90AA1 mRNAs were higher in the ampullary tissues in summer than in winter. In summary, elevated temperatures stimulated PGE2 production in the ampullary oviduct by increasing the expressions of PGESs and HSP90AA1, which can activate cPGES. The overall results suggest that HS upsets PG secretions and reduces oviductal smooth muscle motility, which in turn could decrease gamete/embryo transport through the oviduct.

Role of the cell cycle in regression of the corpus luteum

The corpus luteum contains differentiated steroidogenic cells that have exited the cell cycle of proliferation. In some tissues, deletion of quiescent, differentiated cells by apoptosis in response to injury or pathology is preceded by reentry into the cell cycle. We tested whether luteal cells reenter the cell cycle during the physiological process of luteolysis. Ovaries were obtained after injection of cows with a luteolytic dose of prostaglandin F(2)(α) (PGF). In luteal sections, cells co-staining for markers of cell proliferation (MKI67) and apoptosis (cPARP1) increased 24  h after PGF, indicating that cells that reenter the cell cycle undergo apoptosis. The percent of steroidogenic cells (CYP11A1-positive) co-staining for MKI67 increased after PGF, while co-staining of non-steroidogenic cells did not change. Dispersed luteal cells were stained with Nile Red to distinguish lipid-rich steroidogenic cells from nonsteroidogenic cells and co-stained for DNA. Flow cytometry showed that the percent of steroidogenic cells progressing through the cell cycle and undergoing apoptosis increased after PGF. Culturing luteal cells induced reentry of steroidogenic cells into the cell cycle, providing a model to test the influence of the cell cycle on susceptibility to apoptosis. Blocking cells early in the cell cycle using inhibitors reduced cell death in response to treatment with the apoptosis-inducing protein, Fas ligand (FASL). Progesterone treatment reduced progression through the cell cycle and decreased FASL-induced apoptosis. In summary, steroidogenic cells reenter the cell cycle upon induction of luteal regression. While quiescent cells are resistant to apoptosis, entry into the cell cycle promotes susceptibility to apoptosis.

Roles of antioxidant enzymes in corpus luteum rescue from reactive oxygen species-induced oxidative stress

Progesterone produced by the corpus luteum (CL) regulates the synthesis of various endometrial proteins required for embryonic implantation and development. Compromised CL progesterone production is a potential risk factor for prenatal development. Reactive oxygen species (ROS) play diverse roles in mammalian reproductive biology. ROS-induced oxidative damage and subsequent adverse developmental outcomes constitute important issues in reproductive medicine. The CL is considered to be highly exposed to locally produced ROS due to its high blood vasculature and steroidogenic activity. ROS-induced apoptotic cell death is involved in the mechanisms of CL regression that occurs at the end of the non-fertile cycle. Luteal ROS production and propagation depend upon several regulating factors, including luteal antioxidants, steroid hormones and cytokines, and their crosstalk. However, it is unknown which of these factors have the greatest contribution to the maintenance of CL integrity and function during the oestrous/menstrual cycle. There is evidence to suggest that antioxidants play important roles in CL rescue from luteolysis when pregnancy ensues. As luteal phase defect impacts fertility by preventing implantation and early conceptus development in livestock and humans, this review attempts to address the importance of ROS-scavenging antioxidant enzymes in the control of mammalian CL function and integrity. The corpus luteum (CL) is a transient endocrine organ that develops after ovulation from the ovulated follicle during each reproductive cycle. The main function of the CL is the production and secretion of progesterone which is necessary for embryonic implantation and development. Compromised CL progesterone production is a potential risk factor for prenatal development and pregnancy outcomes. Reactive oxygen species (ROS), which are natural by-products of cellular respiration and metabolism, play diverse roles in mammalian reproductive biology. ROS-induced oxidative damage and subsequent development of adverse pregnancy outcomes constitute important issues in reproductive medicine. Before the end of the first trimester, a high rate of human and animal conceptions end in spontaneous abortion and most of these losses occur at the time of implantation in association with ROS-induced oxidative damage. Every cell in the body is normally able to defend itself against the oxidative damage caused by the ROS. The cellular antioxidant enzymes constitute the first line of defence against the toxic effects of ROS. The CL is considered to be highly exposed to locally produced ROS due to its high blood vasculature and metabolic activity. There is now evidence to suggest that cellular antioxidants play important roles in CL rescue from regression when pregnancy ensues. As defective CL function impacts fertility by preventing implantation and early conceptus development in livestock and humans, this review attempts to address the importance of antioxidant enzymes in the control of mammalian CL function and integrity.

Acute changes in circulating concentrations of progesterone and nitric oxide and partial pressure of oxygen during prostaglandin F2alpha-induced luteolysis in cattle

To examine whether oxygen (O(2)) and nitric oxide (NO) are temporally associated with the acute changes in luteal function during luteolysis, we determined the real-time changes in the circulating concentrations of progesterone (P4) and nitrite/nitrate (the stable metabolites of NO) and the partial pressure of oxygen (pO(2)) during prostaglandin F(2alpha) (PGF(2alpha))-induced luteolysis in cattle. Catheters for frequent blood sample collection were inserted into the ovarian vein (OV), jugular vein (JV) and aorta abdominalis (AA) in 12 cows on Day 9 of the oestrous cycle (oestrus=Day 0). On Day 10, the cows were randomly divided into two groups and treated with a luteolytic dose of a PGF(2alpha) analogue or saline solution (control). Blood samples were collected at -2, -1, 0, 0.25, 0.5, 0.75, 1 and 2 h and then at 2-h intervals until 12 h after treatment (0 h). Injection of a PGF(2alpha) induced a significant decrease in the concentrations of P4 in OV plasma within 2 h. The decrease in P4 concentrations was preceded by an increase in the NO concentrations in the blood collected from OV, JV and AA. Basal pO(2) was significantly higher in OV blood than in JV blood (P<0.05). PGF(2alpha) injection increased pO(2) in OV blood between 0.5 and 2 h. These results demonstrate that PGF(2alpha) induced an acute increase in pO(2) and NO in the ovarian circulation and suggest that O(2) and NO are involved in the early events of CL regression, including inhibition of P4 secretion and output, in cattle.

Regulation of luteal function and corpus luteum regression in cows: hormonal control, immune mechanisms and intercellular communication

The main function of the corpus luteum (CL) is production of progesterone (P4). Adequate luteal function to secrete P4 is crucial for determining the physiological duration of the oestrous cycle and for achieving a successful pregnancy. The bovine CL grows very fast and regresses within a few days at luteolysis. Mechanisms controlling development and secretory function of the bovine CL may involve many factors that are produced both within and outside the CL. Some of these regulators seem to be prostaglandins (PGs), oxytocin, growth and adrenergic factors. Moreover, there is evidence that P4 acts within the CL as an autocrine or paracrine regulator. Each of these factors may act on the CL independently or may modify the actions of others. Although uterine PGF(2 alpha) is known to be a principal luteolytic factor, its direct action on the CL is mediated by local factors: cytokines, endothelin-1, nitric oxide. The changes in ovarian blood flow have also been suggested to have some role in regulation of CL development, maintenance and regression.

Anti-apoptotic roles of prostaglandin E2 and F2alpha in bovine luteal steroidogenic cells

Production of prostaglandins (PGs) and expression of their receptors have been demonstrated in bovine corpus luteum (CL). The aim of the present study was to determine whether PGE2 and PGF2alpha have roles in bovine luteal steroidogenic cell (LSC) apoptosis. Cultured bovine LSCs obtained at the midluteal stage (Days 8-12 of the cycle) were treated for 24 h with PGE2 (0.001-1 microM) and PGF2alpha (0.001-1 microM). Prostaglandin E2 (1 microM) and PGF2alpha (1 microM) significantly stimulated progesterone (P4) production and reduced the levels of cell death in the cells cultured with or without tumor necrosis factor alpha (TNF)/interferon gamma (IFNG), in the presence and absence of FAS ligand (P < 0.05). Furthermore, DNA fragmentation induced by TNF/IFNG was observed to be suppressed by PGE2 and PGF2alpha. Prostaglandin E2 and PGF2alpha also attenuated mRNA expression of caspase 3 and caspase 8, as well as caspase 3 activity (P < 0.05) in TNF/IFNG-treated cells. FAS mRNA and protein expression were decreased only by PGF2alpha (P < 0.05). A specific P4 receptor antagonist (onapristone) attenuated the apoptosis-inhibitory effects of PGE2 and PGF2alpha in the absence of TNF/IFNG (P < 0.05). A PG synthesis inhibitor (indomethacin) reduced cell viability in PGE2- and PGF2alpha-treated cells (P < 0.05). A specific inhibitor of cyclooxygenase (PTGS), PTGS2 (NS-398), also reduced cell viability, whereas an inhibitor of PTGS1 (FR122047) did not affect it. The overall results suggest that PGE2 and PGF2alpha locally play luteoprotective roles in bovine CL by suppressing apoptosis of LSCs.