Expression and tissue concentration of vascular endothelial growth factor, its receptors, and localization in the bovine corpus luteum during estrous cycle and pregnancy

Maternal nutrient restriction in the ewe from early to midgestation programs reduced steroidogenic enzyme expression and tended to reduce progesterone content of corpora lutea, as well as circulating progesterone in nonpregnant aged female offspring

BACKGROUND

Previously we reported decreased circulating progesterone and fertility in one and two year old ewes born to undernourished mothers. This study was designed to investigate if this reduction in progesterone persisted into old age, and if it did, what mechanisms are involved.

METHODS

Ewes were fed a nutrient restricted (NR, 50% of NRC recommendations) or control (C, 100% of NRC) diets from day 28 to 78 of gestation, then all were fed to requirements through parturition and weaning. Female offspring (4 per treatment group) were maintained as a group and fed to requirements from weaning until assigned to this study at 6 years of age. Ewes were synchronized for estrus (day 0) and blood samples were collected daily from day 0 to day 11 before necropsy on day 12. Blood serum and luteal tissue were assayed for progesterone concentrations by validated radioimmunoassay.

RESULTS

Circulation progesterone concentrations tended to be lower (P = 0.06) in NR than C offspring from day 0 to 11 of the estrous cycle. While total luteal weight was similar across groups, total progesterone content also tended to be reduced (P = 0.07) in luteal tissue of NR than C offspring. Activity of hepatic progesterone catabolizing enzymes and selected angiogenic factors in luteal tissue were similar between groups. Messenger RNA expression of steroidogenic enzymes StAR and P450scc were reduced (P < 0.05), while protein expression of StAR tended to be reduced (P < 0.07) and P450scc was reduced (P < 0.05) in luteal tissue of NR versus C offspring.

CONCLUSIONS

There appears to be no difference in hepatic steroid catabolism that could have led to the decreased serum progesterone. However, these data are consistent with the programming of decreased steroidogenic enzyme expression in CL of NR offspring, leading to reduced synthesis and secretion of progesterone.

Fibroblast growth factor 2 induces the precocious development of endothelial cell networks in bovine luteinising follicular cells

The transition from follicle to corpus luteum represents a period of intense angiogenesis; however, the exact roles of angiogenic factors during this time remain to be elucidated. Thus, the roles of vascular endothelial growth factor (VEGF) A, fibroblast growth factor (FGF) 2 and LH in controlling angiogenesis were examined in the present study. A novel serum-free luteinising follicular angiogenesis culture system was developed in which progesterone production increased during the first 5 days and was increased by LH (P < 0.01). Blockade of signalling from FGF receptors (SU5402; P < 0.001) and, to a lesser extent, VEGF receptors (SU1498; P < 0.001) decreased the development of endothelial cell (EC) networks. Conversely, FGF2 dose-dependently (P < 0.001) induced the precocious transition of undeveloped EC islands into branched networks associated with a twofold increase in the number of branch points (P < 0.001). In contrast, VEGFA had no effect on the area of EC networks or the number of branch points. LH had no effect on the area of EC networks, but it marginally increased the number of branch points (P < 0.05) and FGF2 production (P < 0.001). Surprisingly, progesterone production was decreased by FGF2 (P < 0.01) but only on Day 5 of culture. Progesterone production was increased by SU5402 (P < 0.001) and decreased by SU1498 (P < 0.001). These results demonstrate that FGF and VEGF receptors play a fundamental role in the formation of luteal EC networks in vitro, which includes a novel role for FGF2 in induction of EC sprouting.

Expression of lymphangiogenic vascular endothelial growth factor family members in bovine corpus luteum

The aim of this study was to evaluate mRNA expression, protein concentration and localization of the assumedly important lymphangiogenic factors VEGFC and VEGFD and the receptor FLT4 in bovine corpora lutea (CL) during different physiological stages. In experiment 1, CL were collected in a slaughterhouse and stages (days 1-2, 3-4, 5-7, 8-12, 13-16, >18) of oestrous cycle and month <3, 3-5, 6-7 and >8 of pregnancy. In experiment 2, prostaglandin F2α (PGF)-induced luteolysis was performed in 30 cows, which were injected with PGF analogue on day 8-12 (mid-luteal phase), and CL were collected before and 0.5, 2, 4, 12, 24, 48 and 64 h after PGF injection. The mRNA expression was characterized by RT-qPCR. All three factors were clearly expressed and showed significant changes during different groups and periods examined in both experiments. Protein concentrations of VEGFD and FLT4 measured by ELISA were not detectable in early cyclic CL but increased to higher plateau levels during pregnancy. After PGF-induced luteolysis FLT4 protein showed an increase within 2-24 h after the injection. FLT4 localization by immunohistochemistry in the cytoplasm of luteal cells was relatively weak in early CL. It increased in late CL and especially in CL during pregnancy. During pregnancy, a positive FLT4 staining in both the nucleus and cytoplasm of lymphatic endothelial cells in peripheral tissue was observed. In conclusion, our results lead to the assumption that lymphangiogenic factors are produced and regulated in CL and may be involved in mechanisms regulating CL function, especially during pregnancy.

Restoration of corpus luteum angiogenesis in immature hypothyroid rdw rats after thyroxine treatment: morphologic and molecular evidence

Thyroxine (T4) plus gonadotropins might stimulate ovarian follicular angiogenesis in immature infertile hypothyroid rdw rats by upregulating mRNA expression of major angiogenic factors. Development of growing corpus luteum (CL) is strongly related to angiogenesis and to morphofunctional development of microcirculation. Our aim was to investigate if T4 is involved in CL angiogenesis and in the activation of capillary cells and angiogenic factors after ovulation in a spontaneous model of hypothyroidism, the rdw rat. Rdw rats were treated with T4 plus gonadotropins (equine chorionic gonadotropin plus human chorionic gonadotropin; eCG+hCG) or gonadotropins alone in order to evaluate the effects of T4 on early luteal angiogenesis, on microvascular cells and on expression of major growth factors which are involved in the regulation of angiogenesis. Wistar-Imamichi rats treated with gonadotropins were used as controls. The ovaries were collected 4 days after hCG administration and analyzed using morphologic and molecular approaches. Thyroxine plus gonadotropins stimulated the growth of CLs and follicles as in controls, differently from rdw rats treated only with gonadotropins, in which CLs were not found and only small follicles, often atretic, could be recognized. In T4 plus gonadotropin-treated rdw rats CLs showed increased microvasculature, numerous activated capillaries characterized by sprouting and other angiogenic figures, and associated pericytes. Quantitative analysis revealed that the number of pericytes in T4 plus gonadotropin-treated rdw rats was comparable with that found in control rats and was significantly higher than that found in gonadotropin-treated rdw rats. The mRNA expression of vascular endothelial growth factor and basic fibroblast growth factor was significantly higher in control rats and in T4 plus gonadotropin-treated rdw rats than in gonadotropin-treated rdw rats. mRNA expression of tumor necrosis factor α, transforming growth factor β, and epidermal growth factor did not show significant changes. Our data originally demonstrated that T4 promoted the growth of an active microcirculation in developing CLs of gonadotropin-primed hypothyroid rdw rats, mainly by inducing sprouting angiogenesis, pericyte recruitment, and upregulation of mRNA expression of vascular endothelial growth factor and basic fibroblast growth factor. In conclusion, we suggest that T4 plays a key role in restoring luteal angiogenesis in ovaries of immature hypothyroid rdw rats.

Vascular and immune regulation of corpus luteum development, maintenance, and regression in the cow

The bovine corpus luteum (CL) is a unique, transient organ with well-coordinated mechanisms by which its development, maintenance, and regression are effectively controlled. Angiogenic factors, such as vascular endothelial growth factor A and basic fibroblast growth factor, play an essential role in promoting progesterone secretion, cell proliferation, and angiogenesis. These processes are critically regulated, through both angiogenic and immune systems, by the specific immune cells, including macrophages, eosinophils, and neutrophils, that are recruited into the developing CL. The bovine luteolytic cascade appears to be similar to that of general acute inflammation in terms of time-dependent infiltration by immune cells (neutrophils, macrophages, and T lymphocytes) and drastic changes in vascular tonus and blood flow, which are regulated by luteal nitric oxide and the vasoconstrictive factors endothelin-1 and angiotensin II. Over the period of maternal recognition of pregnancy, the maternal immune system should be well controlled to accept the semiallograft fetus. The information on the presence of the developing embryo in the genital tract is suggested to be transmitted to the ovary by both the endocrine system and the circulating immune cells. In the bovine CL, the lymphatic system, but not the blood vascular system, is reconstituted during early pregnancy, and interferon tau from the embryo could trigger this novel phenomenon. Collectively, the angiogenic and vasoactive factors produced by luteal cells and the time-dependently recruited immune cells within the CL and their interactions appear to play critical roles in regulating luteal functions throughout the life span of the CL.

Dexamethasone-induced eosinopenia is associated with lower progesterone production in cattle

Eosinophilic cells accumulate in the capillaries of the bovine Graafian follicle shortly before ovulation and in the early developing corpus luteum (CL). Suppressing the migration of these eosinophilic cells by dexamethasone allowed us to evaluate their possible function in the CL development. Brown Swiss cows (n = 10) were randomly subdivided into two groups (n = 5). Every group was used once as control group and once as experimental group with two oestrous cycles between each treatment. Eighteen hours (h) after oestrus synchronization, dexamethasone or saline was given. Ovulation was induced 24 h later with gonadotropin-releasing hormone. Another injection of dexamethasone or saline was given 12 h later. Eosinophilic cells in the blood were counted daily until day 7 after the first dexamethasone injection. The collection of ovaries took place at days 1, 2 and 5. Gene expression, protein concentration and location of angiogenic factors, chemokines, insulin-like growth factor 1 (IGF1) and eosinophilic cells were studied. No eosinophilic cells were found in the CL of the treatment group. Blood progesterone decreased significantly in the dexamethasone group from day 8 to 17. The protein concentration of FGF2 increased significantly in CL tissue at day 2 and VEGFA decreased. Local IGF1 gene expression in the CL was not regulated. We assume from our data that the migration of eosinophilic cells into the early CL is not an essential, but an important stimulus for angiogenesis during early CL development in cattle.

Possible action of vasohibin-1 as an inhibitor in the regulation of vascularization of the bovine corpus luteum

The development of the corpus luteum (CL), which secretes large amounts of progesterone to establish pregnancy, is accompanied by active angiogenesis, vascularization, and lymphangiogenesis. Negative feedback regulation is a critical physiological mechanism. Vasohibin-1 (VASH1) was recently discovered as a novel endothelium-derived negative feedback regulator of vascularization. We therefore investigated the expression of VASH1 in the bovine CL. Expression of VASH1 mRNA and protein was predominantly localized to luteal endothelial cells (LECs). VASH1 expression in the CL was constant through the early to late luteal phases and decreased during CL regression relating with the action of luteolytic prostaglandin F2α in vivo. To investigate the role of VASH1, we determined whether VASH1 treatment affects angiogenesis and/or lymphangiogenesis using LECs and lymphatic endothelial cells (LyECs) in vitro. Vascular endothelial growth factor A (VEGFA) stimulated the expression of VASH1 in LECs but not in LyECs, and VASH1 completely blocked VEGFA-induced formation of capillary-like tube structures of LECs and LyECs in vitro. In summary, VASH1 is predominantly located on LECs in the bovine CL and inhibits the angiogenic and lymphangiogenic actions of VEGFA. Bovine CL therefore has a VEGFA–VASH1 system that may be involved in regulation of luteal function, especially in the development of the CL. The results indicate that VASH1 has the potential to act as a negative feedback regulator of angiogenesis and lymphangiogenesis in the CL in cows.

Immunocytochemical localization of cytoplasmic and nuclear intermediate filaments in the bovine ovary during folliculogenesis

The cellular cytoskeleton is composed of three fibrillar systems, namely actin microfilaments, microtubules and intermediate filaments (IFs). It not only is a structural system, which mediates functional compartmentalization, but also contributes to many cellular processes such as transport, mitosis, secretion, formation of cell extensions, intercellular communication and apoptosis. In this study, we have examined the distribution of four groups of IFs [cytokeratins (CKs), vimentin, desmin and lamins] in the somatic and germinal cells of the bovine ovary using RT-PCR and immunohistochemical techniques. Using RT-PCR, specific transcripts for all intermediate proteins studied (CK8, CK18, desmin, vimentin, lamin A/C and lamin B1) were detected. A characteristic immunohistochemical staining pattern was observed for the different IFs within the ovary. In this study, we used antibodies against type I CK (acidic CKs: CK14, CK18 and CK19) and type II CK (basic CKs: CK5 and CK8). Among these, only antibodies against CK18 gave a characteristic pattern of immunostaining in the ovary, which included the surface epithelium, the follicle cells, the endothelium of blood vessels and rete ovarii. Antibodies against all other CKs resulted in a weak staining of a limited number of cellular structures (CK5 and CK19) or were completely negative (CK8 and CK14, apart from the surface epithelium). Vimentin antibodies resulted occasionally in a weak staining of the granulosa cells of primary and secondary follicles. In late secondary follicles, the basal and the most apical follicle cells contacting the zona pellucida usually showed a marked immunostaining for vimentin. In antral follicles, three different immunostaining patterns for vimentin were observed. Desmin immunostaining was confined to the smooth muscle cells of blood vessels. Although mRNA for lamin A/C and lamin B1 could be demonstrated using RT-PCR, no immunostaining was found for lamins, neither in the follicle cells nor in the oocytes.

Fibroblast growth factor 2 is a key determinant of vascular sprouting during bovine luteal angiogenesis

Fibroblast growth factor (FGF) 2 and vascular endothelial growth factor (VEGF) A are thought to be key controllers of luteal angiogenesis; however, their precise roles in the regulation and coordination of this complex process remain unknown. Thus, the temporal and spatial patterns of endothelial network formation were determined by culturing mixed cell types from early bovine corpora lutea on fibronectin in the presence of FGF2 and VEGFA (6 h to 9 days). Endothelial cells, as determined by von Willebrand factor immunohistochemistry, initially grew in cell islands (days 0–3), before undergoing a period of vascular sprouting to display a more tubule-like appearance (days 3–6), and after 9 days in culture had formed extensive intricate networks. Mixed populations of luteal cells were treated with SU1498 (VEGF receptor 2 inhibitor) or SU5402 (FGF receptor 1 inhibitor) or control on days 0–3, 3–6 or 6–9 to determine the role of FGF2 and VEGFA during these specific windows. The total area of endothelial cells was unaffected by SU1498 treatment during any window. In contrast, SU5402 treatment caused maximal reduction in the total area of endothelial cell networks on days 3–6 vs controls (mean reduction 81%;P<0.001) during the period of tubule initiation. Moreover, SU5402 treatment on days 3–6 dramatically reduced the total number of branch points (P<0.001) and degree of branching per endothelial cell island (P<0.05) in the absence of changes in mean island area. This suggests that FGF2 is a key determinant of vascular sprouting and hence critical to luteal development.

Importance of vascular endothelial growth factor (VEGF) in ovarian physiology of mammals

Ovarian folliculogenesis in mammals is a complex process. Several compounds have been tested during in vitro culture of follicular cells for a better understanding of the mechanisms and factors related to ovarian folliculogenesis in mammals. From these compounds, vascular endothelial growth factor (VEGF) can be highlighted, as it is strongly associated with angiogenesis and, in recent years, its presence in ovarian cells has been investigated extensively. Previous studies have shown that the presence of VEGF protein, as well as mRNA expression of its receptor 2 (VEGFR-2) increases during follicular development. Therefore, it is likely that the interaction between VEGF and VEGFR-2 is crucial to promote follicular development. However, few studies on the influence of this factor on follicular development have been reported. This review addresses aspects related to the structural characterization and mechanism of action of VEGF and its receptors, and their biological importance in the ovary of mammals.

Possible involvement of IFNT in lymphangiogenesis in the corpus luteum during the maternal recognition period in the cow

The corpus luteum (CL), which secretes large amounts of progesterone and is thus essential for establishing pregnancy, contains various types of immune cells that may play essential roles in CL function by generating immune responses. The lymphatic system is the second circulation system and is necessary for immune function, but the lymphatic system of the bovine CL has not been characterized in detail. We collected bovine CLs on days 12 and 16 of the estrous cycle (C12 and C16) and days 16 and 40 of early pregnancy (P16 and P40). Lymphatic endothelial hyaluronan receptor 1 (LYVE1) protein was detected in the CL by immunohistochemistry and western blotting and increased at P40 compared with C16. The mRNA expression levels of lymphangiogenic factors, such as vascular endothelial growth factor-C (VEGFC), VEGFD, and their common receptor VEGFR3, as well as the lymphatic endothelial cell (LyEC) marker podoplanin, increased in P16 and P40 CLs. Thus, it is suggested that the lymphatic system of the bovine CL reconstitutes during early pregnancy. Interferon tau (IFNT) from the conceptus in the uterus is a candidate for activating luteal lymphangiogenesis during the maternal recognition period (MRP). We found that treatment of LyECs isolated from internal iliac lymphatic vessels with IFNT stimulated LyEC proliferation and significantly increased mRNA expression of VEGFC and IFN-stimulated gene 15. Moreover, both IFNT and VEGFC induced LyECs to form capillary-like tubes in vitro. In conclusion, it is suggested that new lymphangiogenesis in the bovine CL begins during the MRP and that IFNT may mediate this novel phenomenon.

A role for cysteine-rich 61 in the angiogenic switch during the estrous cycle in cows: regulation by prostaglandin F2alpha

The development and demise of the corpus luteum (CL) are accompanied by angiogenic and angioregressive processes; however, the mediators of these processes have not been fully identified and characterized. Transcriptional profiling studies revealed the upregulation of cysteine-rich 61 (CYR61) in the CL, about which nothing was previously known. In the present study, we found that over a 12-h period following a single injection of prostaglandin F(2alpha) (PGF(2alpha)), RT-PCR revealed the upregulation of CYR61 at 0.5 and 1 h, after which it declined. We also determined that luteal-derived endothelial cells as well as luteal steroidogenic cells are sources of CYR61. Treatment with PGF(2alpha) in vitro had no effect on CYR61 expression in luteal-derived endothelial cells, but it increased CYR61 expression in luteal steroidogenic cells. During the estrous cycle, CYR61/CYR61 (transcript/protein) was increased in the Day 4 but not in the Day 10 and Day 16 CL, suggesting that it may be associated with the switch to the angiogenic phenotype. In addition, the specific but transient upregulation of CYR61 by PGF(2alpha) in vivo, and in luteal steroidogenic cells but not endothelial cells in vitro, may be part of the mechanism underlying the previously reported transient increase in blood flow during the early onset of luteolysis. This is supported by our preliminary finding that CYR61 transiently inhibited endothelial cell expression of endothelin-converting enzyme 1 mRNA but not endothelin 1. Collectively, the increased expression of CYR61 in the Day 4 CL and its transient increase by PGF(2alpha) in Day 6, Day 10, and Day 16 CL indicate that CYR61 may play a role in regulating angiogenesis over the life span of the CL.

Contribution of hypoxia-inducible factor-1α to transcriptional regulation of vascular endothelial growth factor in bovine developing luteal cells

Vascular endothelial growth factor (VEGF)-dependent angiogenesis is crucial for corpus leteum formation and their functional maintenance in mammalian ovaries. The present study was designed to test the hypothesis that hypoxia-inducible factor (HIF)-1α-mediated transcriptional activation contributes to the increased expression of VEGF gene in response to hypoxia in the bovine developing luteal cells (LCs). By real-time RT-PCR analysis, VEGF messenger RNA (mRNA) expression was found to significantly increase under hypoxia or treatment with desferrioxamine (DFX), cobalt chloride (CoCl(2)) or even N-carbobenzoxyl-L-leucinyl-L-leucinyl-L-norvalinal (MG-132), while these increased VEGF mRNA expressions could also be blocked by ferrous ammonium sulfate (FAS) or cis-element oligodeoxynucleotide (dsODN) transfection under hypoxia. Further analysis also found that these changes of VEGF mRNA were consistent with HIF-1α expression or HIF-1 activity. Taken together, our results indicate that VEGF is transcriptionally activated by hypoxia through HIF-1α-mediated mechanisms in LCs. This hypoxia-induced transcriptional activation may be one of the important mechanisms mediating the increase of VEGF expression in developing LCs during mammalian corpus leteum formation.

Theca: the forgotten cell of the ovarian follicle

Theca cells function in a diverse range of necessary roles during folliculogenesis; to synthesize androgens, provide crosstalk with granulosa cells and oocytes during development, and provide structural support of the growing follicle as it progresses through the developmental stages to produce a mature and fertilizable oocyte. Thecal cells are thought to be recruited from surrounding stromal tissue by factors secreted from an activated primary follicle. The precise origin and identity of these recruiting factors are currently not clear, but it appears that thecal recruitment and/or differentiation involves not just one signal, but a complex and tightly controlled combination of multiple factors. It is clear that thecal cells are fundamental for follicular growth, providing all the androgens required by the developing follicle(s) for conversion into estrogens by the granulosa cells. Their function is enabled through the establishment of a vascular system providing communication with the pituitary axis throughout the reproductive cycle, and delivering essential nutrients to these highly active cells. During development, the majority of follicles undergo atresia, and the theca cells are often the final follicular cell type to die. For those follicles that do ovulate, the theca cells then undergo hormone-dependent differentiation into luteinized thecal cells of the corpus luteum. While the theca is an essential component of follicle development and ovulation, we do not yet fully understand the control of recruitment and function of theca cells, an important consideration since their function appears to be altered in certain causes of infertility.

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.

Prostaglandin F2alpha differentially affects mRNA expression relating to angiogenesis, vasoactivation and prostaglandins in the early and mid corpus luteum in the cow

Administration of prostaglandin (PG) F(2alpha) in cattle during the mid-luteal phase (Days 8-12 of the estrous cycle) drastically reduces the plasma progesterone concentrations and the volume of the corpus luteum (CL). However, PGF(2alpha) does not induce luteolysis during the early luteal phase (up to Day 5 of the estrous cycle). To characterize the possible distinct difference in acute response to a luteolytic dose of PGF(2alpha) administration, we determined various mRNA expressions in the early and mid CL relating to angiogenesis, vasoactivation and PG-related factors at 30 min after PGF(2alpha) injection in cyclic cows. The experiments were conducted on Day 4 (early CL) and Days 10-12 (mid CL). Cows were either injected with 500 microg PGF(2alpha) analogue or saline as the control (early CL control, n=5; early CL PGF(2alpha) treated, n=5; mid CL control, n=5; mid CL PGF(2alpha) treated, n=7). Thirty min after injection of PGF(2alpha) or saline, the cows were ovariectomized transvaginally, and the CL tissues were collected from regions designated as the periphery and center of the CL. Administration of PGF(2alpha) up-regulated the mRNA expressions of angiogenic-related factors such as vascular endothelial growth factors, vasohibin, fibroblast growth factor 2 and insulin-like growth factor-II in the early CL, whereas PGF(2alpha) down-regulated these mRNA expressions in the mid CL. In the vasoactive factors, PGF(2alpha) stimulated the mRNA expressions of endothelin-1, angiotensin converting enzyme, endothelial nitric oxide synthase (NOS) and inducible NOS in the periphery area of the mid CL, but not in the early CL. However, PGF(2alpha) drastically down-regulated PGF(2alpha) receptor mRNA expression in both regions of the early and mid CL. The results indicated a clear difference in the acute action of PGF(2alpha) depending not only on the luteal phase (immature vs. mature) but also the region (periphery vs. center) within the CL at 30 min after PGF(2alpha) injection in the cow.

Induction of endothelin-2 expression by luteinizing hormone and hypoxia: possible role in bovine corpus luteum formation

The pattern and regulation of endothlin-2 (EDN2) expression and its putative roles in bovine ovaries were investigated. EDN2 mRNA was determined in corpus luteum (CL) and during folliculoluteal transition induced by GnRH in vivo. EDN2 was elevated only in the early CL and was not present in older CL. In the young CL, EDN2 mRNA was identified mainly in luteal cells but not endothelial cells that expressed the EDN1 gene. Similarly, in preovulatory follicles, EDN2 was expressed in the granulosa cells (GCs) and not in the vascular theca interna. LH and hypoxia are two major stimulants of CL formation. Therefore, GCs were cultured with bovine LH, under hypoxic conditions. GCs incubated with bovine LH resulted in increased EDN2 mRNA 42 h later. CoCl2, a hypoxia-mimicking agent, elevated EDN2 in GCs in a dose-dependent manner. Incubation of the human GC line (Simian virus 40 large T antigen) under low oxygen tension (1%) augmented EDN2 6 and 24 h later. In these two cell types, along with EDN2, hypoxia augmented VEGF. EDN2 induced in GCs changes that characterize the developing CL: cell proliferation as well as up-regulation of vascular endothelial growth factor and cyclooxygenase-2 (mRNA and protein levels). Human chorionic gonadotropin also up-regulated these two genes. Small interfering RNA targeting EDN-converting enzyme-1 effectively reduced its mRNA levels. This treatment, expected to lower the mature EDN2 peptide production, inhibited VEGF mRNA levels and GC numbers. Together these data suggest that elevated EDN2 in the early bovine CL, triggered by LH surge and hypoxia, may facilitate CL formation by promoting angiogenesis, cell proliferation, and differentiation.

Effect of prostaglandin F2 alpha on local luteotropic and angiogenic factors during induced functional luteolysis in the bovine corpus luteum

The essential role of endometrial prostaglandin F2 alpha (PTGF) for induction of the corpus luteum (CL) regression is well documented in the cow. However, the acute effects of PTGF on known local luteotropic factors (oxytocin [OXT] and its receptor, insulin-like growth factor [IGF] 1, and progesterone and its receptor), the principal angiogenic factor vascular endothelial growth factor (VEGF) A and the capillary destabilization factor angiopoietin (ANGPT) 2 were not thoroughly studied in detail. The aim of this study was therefore to evaluate the tissue concentration of these factors during PTGF induced luteolysis. In addition the mRNA expression of progesterone receptor (PGR), OXT receptor (OXTR), IGF1, IGFBP1, ANGPT1, and ANGPT2 was determined at different times after PTGF treatment. Cows (n = 5 per group) in the mid-luteal phase (Days 8-12, control group) were injected with the PTGF analog (cloprostenol), and CL were collected by transvaginal ovariectomy at 0.5, 2, 4, 12, 24, 48, and 64 h after injection. The mRNA expression was analyzed by quantitative real-time PCR, and the protein concentration was evaluated by enzyme immunoassay or radioimmunoassay. Progesterone concentrations, as well as mRNA expression of PGR, in CL tissue were significantly down-regulated by 12 h after PTGF. Tissue OXT peptide and OXTR mRNA decreased significantly after 2 h, followed by a continuous decrease of OXT mRNA. IGF1 and VEGFA protein already decreased after 0.5 h. By contrast, the IGFBP1 mRNA was up-regulated significantly after 2 h to a high plateau. ANGPT2 protein and mRNA significantly increased during the first 2 h, followed by a steep decrease after 4 h. The acute decrease of local luteotropic activity and acute changes of ANGPT2 and VEGFA suggest that modulation of vascular stability may be a key component in the cascade of events leading to functional luteolysis.

Angiogenesis and vascular function in the ovary

Ovarian function is dependent on the establishment and continual remodelling of a complex vascular system. This enables the follicle and/or corpus luteum (CL) to receive the required supply of nutrients, oxygen and hormonal support as well as facilitating the release of steroids. Moreover, the inhibition of angiogenesis results in the attenuation of follicular growth, disruption of ovulation and drastic effects on the development and function of the CL. It appears that the production and action of vascular endothelial growth factor A (VEGFA) is necessary at all these stages of development. However, the expression of fibroblast growth factor 2 (FGF2) in the cow is more dynamic than that of VEGFA with a dramatic upregulation during the follicular–luteal transition. This upregulation is then likely to initiate intense angiogenesis in the presence of high VEGFA levels. Recently, we have developed a novel ovarian physiological angiogenesis culture system in which highly organised and intricate endothelial cell networks are formed. This system will enable us to elucidate the complex inter-play between FGF2 and VEGFA as well as other angiogenic factors in the regulation of luteal angiogenesis. Furthermore, recent evidence indicates that pericytes might play an active role in driving angiogenesis and highlights the importance of pericyte–endothelial interactions in this process. Finally, the targeted promotion of angiogenesis may lead to the development of novel strategies to alleviate luteal inadequacy and infertility.

Local regulation of corpus luteum development and regression in the cow: Impact of angiogenic and vasoactive factors

The corpus luteum (CL) of the estrous cycle in the cow is a dynamic organ which has a life time of approximately 17-18 days. The main function of the CL is to secrete a large amount of progesterone (P) thereby supporting the achievement of pregnancy. As the CL matures, the steroidogenic cells establish contact with many capillaries and the matured CL is composed of many vascular endothelial cells that account for up to 50% of all CL cells. The bovine CL produces several major angiogenic and vasoactive foctors such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), angiopoietin-1 and -2 (ANPT-1 and -2), prostaglandin F(2alpha) (PGF(2alpha)), endothelin-1 (EDN1), angiotensin II (Ang II) and nitric oxide (NO). These factors regulate P secretion directly and/or indirectly within the CL. Moreover, different actions of PGF(2alpha) in the early cycle CL (non-luteolytic) and the mid cycle CL (luteolytic) may provide insight into the luteolysis cascade in the cow. The aim of the present review is to describe the current concepts of the local mechanisms for the cascade of development and regression of the bovine CL as regulated by luteal angiogenic and vasoactive factors.

FGF2 is crucial for the development of bovine luteal endothelial networks in vitro

The development of the corpus luteum requires angiogenesis, and involves the complex interplay between factors such as vascular endothelial growth factor A (VEGFA), fibroblast growth factor 2 (FGF2) and platelet-derived growth factor (PDGF). However, the relative role of these factors remains to be elucidated. This study used a new physiologically relevant mixed luteal cell culture system to test the hypotheses that: a) FGF2 and VEGFA are critical for bovine luteal angiogenesis; and b) local luteal PDGF signalling stimulates the formation of endothelial networks. Cells were treated with receptor tyrosine kinase inhibitors against VEGFA (SU1498), FGF2 (SU5402) or PDGF (AG1295) activity. After 9 days in culture, endothelial cells were immunostained for von Willebrand factor (VWF) and quantified by image analysis. Highly organised intricate endothelial networks were formed in the presence of exogenous VEGFA and FGF2. The inhibition of FGF2 activity reduced the total area of VWF staining versus controls (>95%; P<0.001). Inhibition of VEGF and PDGF activity reduced the endothelial network formation by more than 60 and 75% respectively (P<0.05). Progesterone production increased in all treatments from day 1 to 7 (P<0.001), and was unaffected by FGF2 or PDGF receptor kinase inhibition (P>0.05), but was reduced by the VEGF receptor inhibitor on days 5 and 7 (P<0.001). In conclusion, this study confirmed that VEGF signalling regulates both bovine luteal angiogenesis and progesterone production. However, FGF2 was crucial for luteal endothelial network formation. Also, for the first time, this study showed that local luteal PDGF activity regulates bovine luteal endothelial network formation in vitro.

Expression and possible role of fibroblast growth factor family members in porcine antral follicles during final maturation

The aim of this study was to investigate the possible participation of fibroblast growth factor (FGF) family members (FGF1, FGF2 and FGF7 and their receptors) in porcine follicles (polyovulatory species) under special consideration for FGF2 during final growth. A classification of follicles was done by size and follicular fluid content of oestradiol-17β, progesterone and prostaglandin F2α. The mRNA expression of examined factors was analysed by real-time PCR. The hormone concentration was estimated by enzyme immunoassay, protein characterisation by western blotting and localisation by immunohistochemistry. Follicle tissue separated in theca interna and granulosa cells was extracted and tested for mRNA of FGF1, FGF2, FGF7 and receptors (FGFR1IIIc, FGFRIIIb and FGFR2IIIc). Additionally, the mRNA expression of FSHR, LHR and aromatase cytochrome P450 for further characterisation of follicles was analysed. Significantly, higher FGF2 protein levels were measured in stroma when compared with total follicle or corpus luteum tissue. This result was confirmed by western blot with two strong bands. Immunological localisation of FGF2 only in stroma (fibroblasts) confirms the protein measurements. The results show a clear difference for FGF2 protein expression during final growth of follicles if monovulatory (bovine) and polyovulatory (porcine) species are compared. FGF2 protein in porcine ovary may be (due to localisation and concentration in stroma) important for support of angiogenesis of more follicles (polyovulatory species) and not of a single follicle like in cows.

The expression of apelin and its receptor APJ during different physiological stages in the bovine ovary

Recent studies implicate that apelin and its receptor APJ may have important role for the modulation of angiogenesis. The aim of this study was to further characterise the regulation of apelin/APJ system in bovine ovary. Experiment 1: corpora lutea (CL) were assigned to the following stages: days 1-2, 3-4, 5-7, 8-12, 13-16, >18 (after regression) of oestrous cycle and of gravidity (month <3, 3-5, 6-7 and >8). Experiment 2: Follicles during maturation were divided into granulosa cells (GC) and theca interna (TI) and were examined separately. Classification of follicles occurred by follicle size and oestradiol-17β (E2) concentration in the follicular fluid (FF) (<0.5 ng/ml, 0.5-5 ng/ml; 5-40 ng/ml; 40-180 ng/ml; >180 ng/ml). Real-time RT-PCR (qPCR) was applied to investigate mRNA expression of examined factors. In general, the expression level of apelin during the oestrous cycle was significantly higher compared to the one during pregnancy. Apelin mRNA levels were always high during the cycle with a tendency of decrease after CL regression. The APJ mRNA in the CL was significantly up regulated on days 5-7 and 8-12 followed by a decrease on days 13-16, and further on days >18. The expression of APJ does not show any significant regulation in the CL throughout pregnancy. The expression of apelin and APJ was not statistically regulated in GC, but was significantly up regulated in follicles with an E2 concentration of more than 5 ng/ml and showed an increase according to growth and maturation of follicles. In conclusion, our data suggest that apelin/APJ system is involved in the mechanism regulating angiogenesis during follicle maturation as well as during CL formation and function in the bovine ovary.

Induction of heparanase in bovine granulosa cells by luteinizing hormone: possible role during the ovulatory process

Follicular development, follicular rupture, and corpus luteum (CL) formation are accompanied by extensive tissue remodeling. We examined whether heparanase (HPSE), which cleaves heparan sulfate glycosaminoglycans, is induced during these processes. Prostaglandin F2alpha injection, which initiated luteolysis and the development of a preovulatory follicle, moderately increased HPSE mRNA in bovine granulosa cells (GCs). GnRH, used to induce gonadotropin surge, markedly augmented HPSE mRNA levels 12 h after its injection. The temporal pattern of HPSE gene expression in follicular-luteal transition was further examined in follicles collected before, and 4, 10, 20, 25, and 60 h after GnRH injection. HPSE mRNA increased transiently 10-20 h after GnRH injection to levels 10-fold higher than in untreated heifers. HPSE protein levels were similarly elevated 20 h after GnRH injection in GCs, but not in the theca layer. Cyclooxygenase-2 (PTGS2) mRNA peaked before ovulation when HPSE levels returned to baseline levels. HPSE mRNA abundance also remained low in the CLs. The antiprogesterone, RU-486, elevated HPSE levels in GC culture, suggesting that progesterone secreted by CLs may inhibit HPSE. HPSE immunostaining was more abundant in GCs than thecae. In cultured GCs, LH induced a transient increase in HPSE mRNA 3-6 h after its addition, but not at 24 h. However, PTGS2 mRNA was clearly induced at this time. These findings suggest that: 1) HPSE may play a role in ovulation but much less so during CL development, and 2) GC-derived HSPE may be a novel member of the LH-induced extracellular matrix-degrading enzyme family and may contribute to follicular rupture.

Expression and localization of gap junctional connexins 26 and 43 in bovine periovulatory follicles and in corpus luteum during different functional stages of oestrous cycle and pregnancy

The aim of this study was to characterize the regulation of connexins (Cx26 and Cx43) in the bovine ovary (experiment 1-3). Experiment 1: ovaries containing preovulatory follicles or corpora lutea (CL) were collected at 0, 4, 10, 20, 25 (follicles) and 60 h (CL) relative to injection of GnRH. Experiment 2: CL were assigned to the following stages: days 1-2, 3-4, 5-7, 8-12, 13-16, >18 (after regression) of oestrous cycle and of early and late pregnancy (<4 and >4 months). Experiment 3: induced luteolysis, cows on days 8-12 were injected with PGF2alpha analogue (Cloprostenol), and CL were collected by transvaginal ovariectomy before and 0.5, 2, 4, 12, 24, 48 and 64 h after PGF2alpha injection. Real-time RT-PCR was applied to investigate mRNA expression and immunofluorescence was utilized for protein localization. Cx26 mRNA increased rapidly 4 h after GnRH injection (during LH surge) and decreased afterwards during the whole experimental period. Cx43 mRNA expression decreased continuously after GnRH application. Cx26 mRNA in CL increased significantly in the second part of oestrous cycle and after regression. In contrast, the highest mRNA expression for Cx43 in CL was detected during the early luteal phase. After induced luteolysis the mRNA expression of Cx26 increased significantly at 24 h. As shown by immunofluorescence, Cx26 was predominantly localized in the connective tissue and blood vessels of bovine CL, whereas Cx43 was present in the luteal cells and blood vessels. This resulted in a strong increase of Cx26 expression during the late luteal phase and after luteal regression. Subsequently, Cx43 expression was distinctly decreased after luteal regression. These data suggest that Cx26 and Cx43 are involved in the local cellular mechanisms participating in tissue remodelling during the critical time around periovulation as well as during CL formation (angiogenesis), function and regression in the bovine ovary.

Vascular dynamics in relation to immunolocalisation of VEGF-A, VEGFR-2 and Ang-2 in the bovine corpus luteum

Vascular dynamics during development and regression of the bovine corpus luteum were investigated morphometrically in relation to immunolocalisation of vascular endothelial growth factor A (VEGF-A), vascular endothelial growth factor receptor 2 (VEGFR-2) and angiopoietin 2 (Ang-2) in the different cell types. Angiogenesis and remodelling of the capillary network between day 8 and 17 of the oestrous cycle was driven by luteal cells, which were highly immunopositive for VEGF-A. Thereafter, immunoreactivity for VEGF-A and VEGFR-2 was mainly found in vascular smooth muscle cells, the predominant cell type in late luteolysis. During early regression, apoptosis of luteal and endothelial cells was closely correlated, resulting in a significant decrease of capillarity. In late regression, an increase in capillary density was found, suggesting that regression and transformation into the corpus albicans requires adequate perfusion. In the phases of vascular remodelling, i.e. mature and late regressing corpus luteum, high scores of Ang-2-immunopositive endothelial and smooth muscle cells were found. Therefore, it may be hypothesised that Ang-2 supports the angiogenic effects of VEGF-A in these luteal stages. Results emphasise that precise staging and a differentiated view on the cellular populations is important in evaluating the controlled regression and transformation of the corpus luteum.

Assessment of VEGF-receptor system expression in the porcine endometrial stromal cells in response to insulin-like growth factor-I, relaxin, oxytocin and prostaglandin E2

Several factors participate in regulation of growth and development as well as angiogenesis of the uterus during pregnancy, and hence little is known about the role of hormonal regulation of vascular endothelial growth factor (VEGF)-receptor system expression. This study has examined the effect of insulin-like growth factor-I (IGF-I), relaxin (RLX), oxytocin (OT) and prostaglandin (PG) E(2), on VEGF secretion and VEGF-receptor system mRNA expression in the porcine endometrial stromal cells. IGF-I and RLX were identified as the most effective inducers of VEGF secretion and mRNA expression. Although PGE(2) stimulated VEGF secretion and VEGF164 mRNA expression, OT inhibited both secretion and mRNA expression of VEGF. When tested for VEGF receptors (R), all factors failed to affect their mRNA expression. Media conditioned by stromal cells collected after IGF-I and RLX treatment significantly increased endothelial cell proliferation and this effect was blocked by soluble VEGFR-1. These data suggest that during early pregnancy IGF-I, RLX and PGE(2) can affect VEGF expression in the endometrium and therefore may support uterine and embryo development, implantation and pregnancy.

HCG up-regulates hypoxia inducible factor-1 alpha in luteinized granulosa cells: implications for the hormonal regulation of vascular endothelial growth factor A in the human corpus luteum

Vascular endothelial growth factor (VEGF)-dependent angiogenesis is essential for normal luteal development. Although it is believed that hypoxia is the primary inducer of VEGF, in the corpus luteum it is up-regulated by human chorionic gonadotrophin (hCG). As hypoxia-inducible factor (HIF)1A has been shown to regulate VEGFA under ligand-stimulated conditions, we hypothesized that the effect of hCG on luteal VEGFA was mediated through HIF1A. We studied the effect of hCG on VEGFA and HIF1A expression in human luteinized granulosa cells in vitro and in human corpora lutea in vivo. HCG up-regulated VEGFA (P < 0.05) and HIF1A (P < 0.001) in vitro and VEGFA (P < 0.05) and HIF1A (P < 0.05) in vivo. There was a correlation between HIF1A and VEGFA in vivo (P < 0.005) and in vitro (P < 0.05). Nuclear HIF1A in granulosa-lutein cells was highest during luteal formation and absent from the fully functional corpus luteum (P < 0.05). Both VEGFA (P < 0.001) and HIF1A (P < 0.01) were up-regulated by dibutyryl-cAMP, through a PKA pathway. Hypoxia increased VEGFA (P < 0.001) and HIF1A (P < 0.05) expression and hCG further augmented VEGFA (P < 0.001) and HIF1A (P < 0.01) under hypoxic conditions. However, progesterone increased hCG-stimulated VEGFA but had no effect on HIF1A expression. The expression of HIF1A is therefore hormonally regulated in luteal cells in vitro and in vivo and may regulate VEGFA expression under normoxic and hypoxic conditions. However, the differential effects of progesterone suggest that not all regulation of VEGFA is associated with an up-regulation of HIF1A.

A novel physiological culture system that mimics luteal angiogenesis

Luteal inadequacy is a major cause of poor embryo development and infertility. Angiogenesis, the formation of new blood vessels, is an essential process underpinning corpus luteum (CL) development and progesterone production. Thus, understanding the factors that regulate angiogenesis during this critical time is essential for the development of novel strategies to alleviate luteal inadequacy and infertility. This study demonstrates the development of a physiologically relevant primary culture system that mimics luteal angiogenesis. This system incorporates all luteal cell types (e.g. endothelial, steroidogenic cells, fibroblasts and pericytes). Using this approach, endothelial cells, identified by the specific marker von Willebrand factor (VWF), start to form clusters on day 2, which then proliferate and develop thread-like structures. After 9 days in culture, these tubule-like structures lengthen, thicken and form highly organized intricate networks resembling a capillary bed. Development of the vasculature was promoted by coating wells with fibronectin, as determined by image analysis (P<0.001). Progesterone production increased with time and was stimulated by LH re-enforcing the physiological relevance of the model in mimicking in vivo luteal function. LH also increased the area stained positively for VWF by twofold (P<0.05). Development of this endothelial cell network was stimulated by fibroblast growth factor 2 and vascular endothelial growth factor A, which increased total area of VWF positive staining on day 9, both independently (three- to fourfold; P<0.01) and in combination (tenfold; P<0.001). In conclusion, the successful development of endothelial cell networks in vitro provides a new opportunity to elucidate the physiological control of the angiogenic process in the developing CL.

Effect of the luteinising hormone surge on regulation of vascular endothelial growth factor and extracellular matrix-degrading proteinases and their inhibitors in bovine follicles

The aim of the present study was to evaluate the pattern of regulation of vascular endothelial growth factor (VEGF)-A (isoforms 121, 165, 189), VEGF receptor tyrosine kinases (VEGF-R1 and VEGF-R2), matrix metalloproteinase (MMP)-1, MMP-2, MMP-14, MMP-19, tissue-specific inhibitor of metalloproteinases (TIMP)-1, TIMP-2, tissue plasminogen activator (tPA), urokinase plasminogen activator (uPA), urokinase plasminogen activator receptor (uPAR) and plasminogen activator inhibitor-1 (PAI-1) in time-defined follicle classes before (0 h) and after the application of gonadotrophin-releasing hormone (GnRH). Bovine ovaries containing periovulatory follicles or new corpora lutea (CL; Days 1-2) were collected 0, 4, 10, 20 and 25 h (follicles) or 60 h (CL) after the injection of GnRH. Transcripts of VEGF isoforms (VEGF(121), VEGF(165), VEGF(189)) were upregulated 4 h after GnRH injection (during the luteinising hormone (LH) surge) and decreased thereafter to lowest levels around ovulation. All VEGF isoforms and their receptors were upregulated again after ovulation. The VEGF peptide concentration in follicular fluid decreased 20 h after GnRH injection, followed by an increase in follicles 25 h after GnRH. Expression of MMP-1 mRNA increased rapidly 4 h after GnRH injection and remained high during the entire experimental period. In contrast, MMP-19 mRNA increased significantly only after ovulation. Expression of TIMP-1 mRNA increased 4 h after GnRH and again after ovulation. Expression of tPA mRNA increased 4 h after GnRH and remained high during the entire experimental period, whereas expression of uPA transcripts increased significantly only after ovulation. Both uPAR and PAI-1 mRNA levels increased in follicles 4 h after GnRH and again after ovulation. The amount of MMP-1 protein (immunolocalisation) increased in follicles 10 h after GnRH: additional staining was observed in the granulosa cell layer. In conclusion, the temporal and spatial pattern of regulation of VEGF and extracellular matrix-degrading proteinases during periovulation suggests they are important mediators of the LH-dependent rupture of bovine follicles and for early CL formation (angiogenesis).

Prostaglandin F2alpha increases endothelial nitric oxide synthase in the periphery of the bovine corpus luteum: the possible regulation of blood flow at an early stage of luteolysis

Prostaglandin F(2)(alpha) (PGF(2)(alpha)) released from the uterus causes alterations in luteal blood flow, reduces progesterone secretion, and induces luteolysis in the bovine corpus luteum (CL). We have recently discovered that luteal blood flow in the periphery of the mature CL acutely increases coincidently with pulsatile increases in a metabolite of PGF(2)(alpha) (PGFM). In this study, we characterized changes in regional luteal blood flow together with regional alterations in endothelial nitric oxide synthase (eNOS) expression during spontaneous luteolysis and in response to PGF(2)(alpha). Smooth muscle actin-positive blood vessels larger than 20 microm were observed mainly in the periphery of mature CL. PGF(2)(alpha) receptor was localized to luteal cells and large blood vessels in the periphery of mid-CL. PGF(2)(alpha) acutely stimulated eNOS expression in the periphery but not in the center of mature CL. Injection of the NO donor S-nitroso-N-acetylpenicillamine into CL induced an acute increase in luteal blood flow and shortened the estrous cycle. In contrast, injection of the NOS inhibitor l-NAME into CL completely suppressed the acute increase in luteal blood flow induced by PGF(2)(alpha) and delayed the onset of luteolysis. In conclusion, PGF(2)(alpha) has a site-restricted action depending on not only luteal phase but also the region in the CL. PGF(2)(alpha) stimulates eNOS expression, vasodilation of blood vessels, and increased luteal blood flow in periphery of mature CL. Furthermore, the increased blood flow is mediated by NO, suggesting that the acute increase in peripheral blood flow to CL is one of the first physiological indicators of NO action in response to PGF(2)(alpha).

VEGF system expression in different stages of estrous cycle in the corpus luteum of non-treated and superovulated water buffalo

Water buffaloes are easily adaptable animals, whose raising and economical exploitation have been growing in the last three decades all over the world. Hyperstimulation of ovarian function in this species is a common technique aiming to improve reproductive performance. Superovulatory treatment affects corpus luteum (CL) function, which is highly correlated to angiogenic process. The aim of this study was therefore to assess the temporal protein and mRNA expression of VEGF and its receptors in the CL of non-treated and superovulated buffaloes. For that purpose blood samples and CL from 36 healthy (30 untreated, groups 1-5, and 6 superovulated, group 6) non-pregnant buffaloes were collected and the samples were divided into 6 groups according to the age of CL. Plasma samples were submitted to RIA to measure progesterone concentration and CL were subjected to immunohistochemistry and real time PCR for VEGF (vascular endothelial growth factor), Flt-1 (fms-like tyrosine kinase receptor 1) and KDR (kinase insert domain containing region). The VEGF system protein and mRNA expression during CL life span of untreated animals showed a specific time-dependent profile, although protein did not always reflect mRNA concentrations. VEGF expression in luteal cells was high correlated to plasma progesterone levels. Superovulated CL showed a significant increase of the VEGF-system protein and a significant decrease of mRNA expression compared to untreated animals in the same stage of the oestrous cycle. We conclude that VEGF, Flt-1 and KDR protein and mRNA expression in buffalo CL is dependent of estrous cycle stage and superovulatory treatment is able to increase the translation rate of this system.

Expression and localisation of extracellular matrix degrading proteases and their inhibitors during the oestrous cycle and after induced luteolysis in the bovine corpus luteum

The corpus luteum (CL) offers the opportunity to study high proliferative processes during its development and degradation processes during its regression. We examined the mRNA expression of matrix metalloproteases (MMP)-1, MMP-2, MMP-9, MMP-14, MMP-19, tissue inhibitor of MMP (TIMP)-1, TIMP-2, tissue plasminogen activator (tPA), urokinase plasminogen activator (uPA), uPA-receptor (uPAR), PA-inhibitors (PAI)-1, PAI-2 in follicles 20 h after GnRH application, CLs during days 1–2, 3–4, 5–7 and 8–12 of the oestrous cycle as well as after induced luteolysis. Cows in the mid-luteal phase were injected with Cloprostenol and the CLs were collected at 0.5, 2, 4, 12, 24, 48 and 64 h after PGF2α injection. Real-time RT-PCR determined mRNA expressions. Expression from 20 h after GnRH to day 12: MMP-1, MMP-2, MMP-14 and tPA showed a clear expression, but no regulation. TIMP-1 and uPAR mRNA increased when compared with the follicular phase. TIMP-2, MMP-9, MMP-19 and uPA increased from the follicular phase to days 8–12. PAI-1 and PAI-2 expression increased from days 1–7 and decreased to days 8–12. Induced luteolysis: MMP-1, MMP-2, MMP-9, MMP-14, MMP-19 and TIMP-1 all increased at different time points and intensities, whereas TIMP-2 was constantly decreased from 24 to 64 h. The plasminogen activator system and their inhibitors were up-regulated from 2 to 64 h, tPA was already increased after 0.5 h. Immunohistochemistry for MMP-1, MMP-2, MMP-14: an increased staining for MMP-1 and MMP-14 was seen in large luteal cells beginning 24 h after PGF2α application. MMP-2 showed a strong increase in staining in endothelial cells at 48 h.

Fibroblast Growth Factor 2 Is More Dynamic than Vascular Endothelial Growth Factor A During the Follicle-Luteal Transition in the Cow1

Luteal inadequacy is a major cause of infertility in a number of species. During the early luteal phase, progesterone production requires the rapid growth of the corpus luteum (CL), which is in turn dependent on angiogenesis. In the present study, we examined the temporal changes in vascular endothelial growth factor A (VEGFA), fibroblast growth factor 2 (FGF2) and secreted protein, acidic, cysteine-rich (osteonectin) (SPARC) during the follicular-luteal transition and CL development in the cow. Luteal VEGFA concentrations increased as the CL developed but were lower in the regressing CL. Conversely, luteal FGF2 concentrations were highest immediately postovulation in the collapsed follicle and declined as the CL developed. Furthermore, three FGF2 isoforms were present in the collapsed follicle, but only one isoform was detected in older CL. Interestingly, FGF2 concentrations increased in the regressing CL. Western blot analysis for SPARC showed the presence of two isoforms, which were constitutively expressed throughout CL development. Further studies investigated the regulation of FGF2 by LH, which showed that FGF2 concentrations in preovulatory follicular fluid were higher in those animals that had experienced an LH surge. Moreover, LH stimulated FGF2 production in dispersed luteal cells. Conversely, the LH surge had no effect on follicular fluid VEGFA concentrations. In conclusion, FGF2 was more dynamic than VEGFA and SPARC during the follicular-luteal transition, which suggests that FGF2 plays a key role in the initiation of angiogenesis at this time. Furthermore, it is likely that this is stimulated by the LH surge. The results also suggest that VEGFA and SPARC have a more constitutive, but essential, role in the development of the CL vasculature.

Characterization and differential expression of vascular endothelial growth factor isoforms and receptors in swine corpus luteum throughout estrous cycle

Corpus luteum (CL) undergoes growth and regression during each estrous cycle; these processes are accompanied by growth and regression of the luteal vascular bed. Vascular endothelial growth factor (VEGF) is the main regulator of angiogenesis, inducing endothelial cell proliferation, migration, vascular permeability, and vessel lumen formation. VEGF presents several isoforms that are produced by alternative splicing of the same mRNA transcript. We determined by real time RT-PCR the expression patterns of VEGF isoform and receptor mRNAs, as well as the VEGF protein levels in pig CL throughout a whole estrous cycle. Four novel VEGF isoforms (VEGF144, VEGF147, VEGF182, and VEGF164b) were found for the first time in swine and the seven identified isoforms can be grouped in four different patterns of expression. The most expressed splice variants were VEGF120 and VEGF164. All isoforms showed their highest mRNA levels in newly formed CLs (day 1), followed by a decrease during mid-late luteal phase (days 10-17), except for VEGF182, VEGF188 and VEGF144 that showed a differential regulation during late luteal phase (day 14) or at luteolysis (day 17). VEGF protein levels paralleled the most expressed and secreted VEGF120 and VEGF164 isoforms. The VEGF receptors mRNAs showed a different pattern of expression in relation to their ligands, increasing between day 1 and 3 and gradually decreasing during the mid-late luteal phase. The differential regulation of VEGF isoforms may suggest specific physiological roles for some of them, particularly in angioregression occurring during the apoptotic structural luteolysis.

Expression of vascular endothelial growth factor and its receptors in the porcine corpus luteum during the estrous cycle and early pregnancy

The present study was undertaken to determine the expression of vascular endothelial growth factor (VEGF) and its receptors, fms-like tyrosine kinase (Flt-1) and fetal liver kinase-1/kinase insert domain-containing receptor (Flk-1/KDR), in the porcine corpus luteum (CL) during the estrous cycle and early pregnancy. Immunohistochemical studies localized proteins of VEGF ligand-receptor system in the cytoplasm of luteal cells and in some blood vessels. Western blot analysis revealed significantly higher levels of VEGF protein during early and mid-luteal phase (vs. late luteal phase; P<0.001 and P<0.01, respectively). Quantification of VEGF mRNA in the CL showed increased mRNA levels during entire luteal phase (vs. Days 16-17; P<0.05). Expression of Flt-1 protein remained high during luteal phase (P<0.001), but the mRNA levels tended to increase from the early to the late luteal phase. Elevated protein expression of Flk-1/KDR was found in the mid-luteal phase (vs. Days 16-17; P<0.05). However, induction of Flk-1/KDR mRNA expression occurred earlier, in early luteal phase. The lowest VEGF, Flt-1 and Flk-1/KDR mRNA and protein levels were observed in regressed CL (P<0.001). During pregnancy, VEGF, Flt-1 and Flk-1/KDR mRNA and protein expression was comparable to the mid-luteal phase. In conclusion, the present study has demonstrated dynamic expression of VEGF and its receptors in the porcine CL during the estrous cycle and early pregnancy. These data suggest that the VEGF ligand-receptor system may play an important role in the development and maintenance of the CL in pigs.

Gelatinases, endonuclease and Vascular Endothelial Growth Factor during development and regression of swine luteal tissue

BACKGROUND

The development and regression of corpus luteum (CL) is characterized by an intense angiogenesis and angioregression accompanied by luteal tissue and extracellular matrix (ECM) remodelling. Vascular Endothelial Growth Factor (VEGF) is the main regulator of angiogenesis, promoting endothelial cell mitosis and differentiation. After the formation of neovascular tubes, the remodelling of ECM is essential for the correct development of CL, particularly by the action of specific class of proteolytic enzymes known as matrix metalloproteinases (MMPs). During luteal regression, characterized by an apoptotic process and successively by an intense ECM and luteal degradation, the activation of Ca++/Mg++-dependent endonucleases and MMPs activity are required. The levels of expression and activity of VEGF, MMP-2 and -9, and Ca++/Mg++-dependent endonucleases throughout the oestrous cycle and at pregnancy were analyzed.

RESULTS

Different patterns of VEGF, MMPs and Ca++/Mg++-dependent endonuclease were observed in swine CL during different luteal phases and at pregnancy. Immediately after ovulation, the highest levels of VEGF mRNA/protein and MMP-9 activity were detected. On days 5-14 after ovulation, VEGF expression and MMP-2 and -9 activities are at basal levels, while Ca++/Mg++-dependent endonuclease levels increased significantly in relation to day 1. Only at luteolysis (day 17), Ca++/Mg++-dependent endonuclease and MMP-2 spontaneous activity increased significantly. At pregnancy, high levels of MMP-9 and VEGF were observed.

CONCLUSION

Our findings, obtained from a precisely controlled in vivo model of CL development and regression, allow us to determine relationships among VEGF, MMPs and endonucleases during angiogenesis and angioregression. Thus, CL provides a very interesting model for studying factors involved in vascular remodelling.

Immunohistochemical localization of VEGF and its receptors in the corpus luteum of the bitch during diestrus and anestrus

The corpus luteum (CL) is a temporary endocrine gland, whose life span depends on the interaction of luteotrophic and luteolytic factors. Since development and maintenance of CL is based on angiogenesis, angiogenic growth factors may play a role in CL-function of the bitch, as described for other species. The aim of this study was to detect the presence of the vascular endothelial growth factor (VEGF) system in the bitch CL throughout diestrus and early anestrus. For that purpose, blood samples from 24 bitches were collected and analyzed for progesterone to determine ovulation time and the animals were subjected to ovariosalpingohysterectomy 10, 20, 30, 40, 50, 60 or 70 days after ovulation. The corpora lutea were fixed in formalin and embedded in Paraplast resin. Five micrometers sections were submitted to standard immunohistochemistry protocol using three primary antibodies (SC-315, SC-316 and VG76e) for detection of kinase domain region (KDR), fms-like tyrosine kinase 1 (Flt-1) and VEGF, respectively. The VEGF system expression could be detected in all diestrus stages in endothelial as well as luteal cells (responsible for blood vessel formation and progesterone production, respectively), indicating time dependent changes: immunostaining tended to increase from Day 10 to 50 and to decrease until Day 70 post-ovulation. In the CL of the bitch, structure related cells, like pericytes and stroma cells, expressed it in determined time points of diestrus with little intensity variation. We concluded that VEGF might have a modulatory effect in the CL of the dog acting as paracrine and autocrine factor through its receptors, Flt-1 and KDR.