Inhibition of vascular endothelial growth factor in the primate ovary up-regulates hypoxia-inducible factor-1alpha in the follicle and corpus luteum

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.

Ovarian VEGF(165)b expression regulates follicular development, corpus luteum function and fertility

Angiogenesis and vascular regression are critical for the female ovulatory cycle. They enable progression and regression of follicular development, and corpora lutea formation and regression. Angiogenesis in the ovary occurs under the control of the vascular endothelial growth factor-A (VEGFA) family of proteins, which are generated as both pro-(VEGF(165)) and anti(VEGF(165)b)-angiogenic isoforms by alternative splicing. To determine the role of the VEGF(165)b isoforms in the ovulatory cycle, we measured VEGF(165)b expression in marmoset ovaries by immunohistochemistry and ELISA, and used transgenic mice over-expressing VEGF(165)b in the ovary. VEGF(165)b was expressed in the marmoset ovaries in granulosa cells and theca, and the balance of VEGF(165)b:VEGF(165) was regulated during luteogenesis. Mice over-expressing VEGF(165)b in the ovary were less fertile than wild-type littermates, had reduced secondary and tertiary follicles after mating, increased atretic follicles, fewer corpora lutea and generated fewer embryos in the oviduct after mating, and these were more likely not to retain the corona radiata. These results indicate that the balance of VEGFA isoforms controls follicle progression and luteogenesis, and that control of isoform expression may regulate fertility in mammals, including in primates.

Activation of PI3K/mTOR signaling pathway contributes to induction of vascular endothelial growth factor by hCG in bovine developing luteal cells

We recently reported that HIF-1α plays a critical role in the regulation of vascular endothelial growth factor (VEGF) expression in the developing letual cells (LCs) and VEGF-dependent angiogenesis is essential for normal luteal development. Although it is believed that hypoxia is the primary inducer of VEGF, recent reports have also shown that human chorionic gonadotrophin (hCG) up-regulates VEGF expression in developing corpus luteum (CL). Therefore the present study was designed to test the induced effects of hCG on the expression of VEGF and HIF-1α in LCs under normoxic and hypoxic conditions. In addition, we also investigated whether the signaling pathways such as phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) and mitogen-activated protein kinase (MAPK) are involved in hCG-induced VEGF in LCs. A significant increase of VEGF mRNA was found in LCs treated with hCG, which was consistent with the changes of HIF-1α protein, even under hypoxic conditions. However, there was no obvious changes of HIF-1α mRNA in hCG-treated LCs between normoxic and hypoxic conditions, indicating hCG induces VEGF expression by increasing transcription of HIF-1α, while hypoxia mainly increases HIF-1α protein stability. When LCs were pretreated with inhibitors, we found that the PI3K/mTOR signaling pathway is required for HIF-1α and VEGF expression induced by hCG, while the MAPK pathway is not required. Together, these results suggest that activation of IP3K/mTOR signaling pathway contributes to the induction of VEGF and HIF-1α in hCG-treated LCs. To our knowledge this will provide a new insight into the important mechanism of hCG/LH-induced VEGF-dependent angiogenesis in the bovine ovary.

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.

Advanced follicle development in xenografted prepubertal ovarian tissue: the common marmoset as a nonhuman primate model for ovarian tissue transplantation

OBJECTIVE

To establish a nonhuman primate model addressing follicular development in cryopreserved prepubertal ovarian tissue after xenografting.

DESIGN

Experimental study.

SETTING

Academic research center.

ANIMAL(S)

Ovarian tissue from female prepubertal common marmoset (Callytrix jacchus jacchus) grafted into immunodeficient nude mice (Crl:NU-FoxnI(nu)).

INTERVENTION(S)

Removal and subsequent cryopreservation of ovarian tissues with dimethyl sulfoxide, followed by grafting to subcutaneous sites of ovariectomized and intact nude mice.

MAIN OUTCOME MEASURE(S)

Histologic evaluation for the mean number of total and morphologically normal follicles in each class.

RESULT(S)

The mean number of unadvanced follicles in frozen-thawed grafted ovarian tissues was reduced compared with pregraft controls, but the prevalence of normal follicular morphology was either slightly increased (primordial follicles) or unchanged (primary follicles). Previous ovariectomy in graft recipients increased total follicle numbers without effect on normal follicular morphology and shifted the ratio of primordial to primary follicles toward an increase in primary follicles, indicating activation of follicular maturation.

CONCLUSION(S)

The marmoset is a suitable primate model for studies on the subsequent use of cryopreserved ovarian tissue, demonstrating graft sustainment and the development of follicles from prepubertal ovarian tissue in immunodeficient hosts up to secondary and preantral stages.

Transcription factors and ovarian functions

This study represents a first review of contemporarily knowledge concerning involvement of transcription factors in control of different ovarian functions. After introduction of basic functions and classification of transcription factors, the available data concerning involvement of transcription factors in control of the following ovarian events are present: follicular development and selection, ovarian cell proliferation and cancerogenesis, ovarian cell apoptosis, ovarian secretory activity, oocyte/cumulus maturation, ovulation and luteogenesis, mediation effect of hormones, growth factors, and cytokines. The importance of transcription factors of Smad family, of forkhead transcription factor (Fox) family, of breast cancer-associated genes/transcription factor, hypoxia-induced transcription factors and of other transcription factors in control of these processes has been demonstrated.

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.

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.

SRB Reproduction, Fertility and Development Award Lecture 2008. Regulation and manipulation of angiogenesis in the ovary and endometrium

The marked cyclical physiological angiogenesis in the developing follicle, corpus luteum and endometrium implies a critical role in health and disease. Our approach to understanding its regulation has been to localise and quantify the temporal changes in putative angiogenic factors, and their receptors, in human and non-human primate tissue and to use antagonists to dissect their role by specific inhibition at defined periods during the ovulatory cycle in non-human primates in vivo. The course of angiogenesis throughout the cycle and the cellular and molecular effects of inhibitory treatments have been investigated in the marmoset ovary and uterus, whereas consequences on pituitary-ovarian function have been monitored in macaques. Inhibition of vascular endothelial growth factor (VEGF) at the time of follicle recruitment or selection prevents endothelial cell proliferation, leading to inhibition of follicular development. VEGF inhibition during the early luteal phase prevents angiogenesis and restricts development of the luteal microvasculature. Inhibition of angiogenesis at all stages of the cycle leads to profound suppression of ovarian function. Even during the 'post-angiogenic' period of the luteal phase, inhibition of VEGF precipitates a suppression of progesterone secretion, pointing to additional roles for VEGF in the ovary. In the endometrium, oestrogen drives endometrial angiogenesis through VEGF. Thus, oestrogen can restore angiogenesis after ovariectomy, but not in the presence of VEGF inhibitors. These investigations enhance our understanding of the regulation of angiogenesis in the ovary and uterus and inform studies on conditions with abnormal vascularisation, such as polycystic ovary syndrome, endometriosis, uterine fibroids and menstrual dysfunction.

The role of vascular endothelial growth factor and estradiol in the regulation of endometrial angiogenesis and cell proliferation in the marmoset

The present studies explore the roles of vascular endothelial growth factor (VEGF) and estradiol on angiogenesis and stromal and epithelial cell proliferation in the marmoset endometrium during the proliferative phase of the ovulatory cycle. At the start of the proliferative phase, marmosets were 1) treated with vehicle, 2) treated with a VEGF inhibitor (VEGF Trap, aflibercept), 3) ovariectomized, 4) ovariectomized and given replacement estradiol, or 5) treated with VEGF Trap and given replacement estradiol. The uterus was examined 10 d later in the late proliferative phase. Changes in endothelial and epithelial cell proliferation were quantified using a volumetric density method after immunohistochemistry for bromodeoxyuridine to localize proliferating cells, CD31 to visualize endothelial cells, and dual staining to distinguish endothelial cell proliferation. Endothelial proliferation was elevated in late proliferative controls but virtually absent after VEGF Trap. Ovariectomy had a similar inhibitory effect, whereas angiogenesis was restored by estrogen replacement. Estradiol replacement in VEGF Trap-treated marmosets resulted in only a small increase in endothelial cell proliferation that remained significantly below control values. VEGF Trap treatment and ovariectomy also markedly reduced stromal cell proliferation but resulted in increased stromal cell density associated with a reduction in overall endometrial volume. Estrogen replacement in both ovariectomized and VEGF Trap-treated animals restored stromal proliferation rates and cell density. These results show that endometrial angiogenesis and stromal proliferation during the proliferative phase are driven by estradiol and that the effect of estrogen on angiogenesis is mediated largely by VEGF.

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.