Phytoestrogens and their metabolites inhibit the sensitivity of the bovine corpus luteum to luteotropic factors

The aim of this study was to examine whether active metabolites of phytoestrogens (equol and para-ethyl-phenol) inhibit sensitivity of bovine corpus luteum (CL) to luteinizing hormone (LH) and to auto/paracrine luteotropic factors (prostaglandin E2-PGE2 and prostaglandin F(2alpha)-PGF(2alpha)), and whether they influence pulsatile progesterone (P4) secretion by the bovine CL. In in vivo experiments, high levels of equol and para-ethyl-phenol were found in plasma and in the CL tissue of heifers and cows fed a soy bean diet (2.5 kg/animal/day), along with lower concentrations of P4 (P < 0.05). Both Prostaglandins (PG) and LH strongly stimulated P4 secretion in cultured pieces of CL that were collected from cows fed a standard diet (P < 0.01). There was no effect of PGs and LH on P4 stimulation in CLs obtained from cows fed a diet rich in soy bean. Finally, we examined whether active metabolites of phytoestrogens participated in regulation of pulsatile P4 secretion and LH-stimulated P4 secretion in vitro using a microdialysis system. Equol and para-ethyl-phenol had no effect on basic and pulsatile P4 secretion in CLs during 240 min of perfusion when compared to the control (P < 0.05). However, they inhibited LH-stimulated P4 secretion (P < 0.05). Phytoestrogens and their metabolites may disrupt CL function by inhibiting PG- and LH-stimulated P4 secretion.

Ovarian function in ruminants

The purpose of this overview is to highlight important steps of ovarian regulation during follicle development, ovulation and the life span of corpus luteum (CL) in ruminants. The ovarian cycle is central to reproductive function. It is characterized by repeating patterns of cellular proliferation, differentiation and transformation that encompass follicular development and ovulation as well as the formation, function and regression of the CL. In the first part, the importance and regulation of final follicle growth and especially of angiogenesis and blood flow during folliculogenesis, dominant follicle development and CL formation are described. Our results underline the importance of growth factors especially of insulin-like growth factor (IGF), vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) for development and completion of a dense network of capillaries (angiogenesis) during follicle growth and CL formation. In the second part, the regulation of CL function by endocrine/paracrine and autocrine acting regulators is discussed. There is evidence that besides the main endocrine hormones luteinizing hormone (LH) and growth hormone (GH) local regulators as growth factors, peptides, steroids and prostaglandins are important modulators of luteal function. During early CL development until midluteal stage oxytocin (OT), prostaglandins and progesterone (P) itself stimulate luteal cell proliferation and function supported by the luteotropic action of a number of growth factors. The still high mRNA expression, protein concentration and localization of VEGF, FGF and IGF family members in the cytoplasm of luteal cells during midluteal stage suggest that they play pivotal role in the maintenance (survival) of this endocrine tissue. The major function of the CL is to secrete P. Progesterone itself regulates the length of the estrous cycle via influencing the timing of the luteolytic PGF2alpha signal from the endometrium. At the end of a nonfertile cycle, the regression of CL commences, steroidogenic capacity is lost (functional luteolysis), cell death is initiated, and tissue involution as well as resorption occurs within a few days (structural luteolysis). The cascade of mediators during luteolysis is very complex and still awaits elucidation. Evidence is given for participation of blood flow, inflammatory cytokines, vasoactive peptides (angiotensin II and endothelin-1), and decrease of the classical luteotropic mediators.

Regulation of Corpus Luteum Function in Cattle - an Overview

The corpus luteum (CL) is a transient reproductive gland that produces progesterone (P), required for the establishment and maintenance of pregnancy. Although the regulation of bovine luteal function has been studied for several decades, many of the regulatory mechanisms involved are incompletely understood. We are far from understanding how these complex mechanisms function in unison. The purpose of this overview is to stress important steps of regulation during the lifetime of CL. In the first part, the importance and regulation of angiogenesis and blood flow during CL formation is described. The results underline the importance of growth factors especially of vascular endothelial growth factor A (VEGF A) and basic fibroblast growth factor (FGF-2) for development and completion of a dense network of capillaries. In the second part, the regulation of function by endocrine/paracrine- and autocrine-acting regulators is discussed. There is now more evidence that besides the main endocrine hormones LH and GH local regulators as growth factors, peptides, steroids and prostaglandins are important modulators of luteal function. During early CL development until mid-luteal stage oxytocin, prostaglandins and P itself stimulate luteal cell proliferation and function supported by the luteotropic action of a number of growth factors. The still high mRNA expression, protein concentration and localization of growth factors [VEGF, FGF-1, FGF-2, insulin-like growth factors (IGFs)] in the cytoplasm of luteal cells during mid-luteal stage suggest maintenance (survival) functions for growth factors. In the absence of pregnancy regression (luteolysis) of CL occurs. Progesterone itself regulates the length of the oestrous cycle by influencing the timing of the luteolytic signal prostaglandin F2alpha (PGF2alpha) from the endometrium. The cascade of mediators afterwards is very complex and still not well-elucidated. Evidence is given for participation of blood flow, inflammatory cytokines, vasoactive peptides (angiotensin II and endothelin-1), reactive oxygen species, angiogenic growth factors (VEGFs, FGFs, IGFs) and decrease of the classical luteotropic components as LH-R, GH-R, P450(scc) and 3beta-HSD. Despite of differences in methodology and interpretations, progress has been made and will continue to be made.

Steroids as local regulators of ovarian activity in domestic animals

The presented overview gives clear evidence for steroids as local regulators of follicular and luteal activity. In the follicle, estrogen receptor-alpha (ERalpha) and ERbeta expression are demonstrated in cow, ewe and pig. Besides species specific effects in general, there is evidence that estradiol-17beta (E(2)) exerts a dose-dependent inhibition on the secretion of progesterone (P(4)) by both theca interna cells (TI) and granulosa cells (GC). GC enhance the ability of the TI to produce androstendione by supplying them with progestin precursor. Androgen produced by TI enhances the ability of the GC to make E(2), and high concentrations of E(2) in the preovulatory follicle inhibit 3beta-HSD in both TI and GC and thus, may promote the use of the pathway Delta(5) for TI androgen production. The authors suggest that E(2) acts within the follicle to exert positive feedback on androgen and E(2) production, and exerts mitotic and anti-atretic or anti-apoptotic effects on follicular cells. Parts of the E(2)-mediated local action are regulated by stimulating effects on hormone receptors (LH, FSH, oxytocin). Gap junctions permit transfer of nutrients and cytokines to and from the avascular GC and oocyte, and formation is stimulated by estrogens. In bovine corpus luteum (CL) there is evidence that P(4) may directly regulate the production of P(4), oxytocin and prostaglandins (PGs) in a cycle dependent fashion. In most of domestic animal species, there is clear evidence for CL production of E(2) with clear stimulatory and luteotropic effects on P(4), and an intraluteal circuit that involves paracrine effects of E(2), oxytocin and PGF(2alpha) (especially in pigs). In contrast, there are species (ruminants, mares) in which the evidence for important local effects of E(2) is less clear, although expression of ERalpha, ERbeta and progesterone receptor (PR) is documented. Progesterone is very important for the regulation of CL lifetime by effects on the endometrium and release of the luteolytic signal PGF(2alpha). In conclusion, steroids as local regulators of ovarian activity are now documented and may stimulate further research in this field.

Estradiol-17beta is produced in bovine corpus luteum

The aim of this study was to investigate the expression of cytochrome P450 aromatase (aromatase) mRNA, its activity, and estradiol-17beta (estradiol) secretion in bovine corpus luteum (CL) during the estrous cycle. Expression of aromatase mRNA was examined in CL at the early, mid, late, and regressed luteal stages by using a reverse transcription-polymerase chain reaction. Aromatase mRNA was detected in all luteal stages examined, although aromatase expression was significantly lower during the early and regressed luteal phases compared to the mid and late luteal phases. Moreover, cultured midluteal cells clearly converted exogenous [(3)H]androstenedione into estradiol, and an aromatase inhibitor significantly inhibited this conversion. To characterize the local release of estradiol within the CL during the estrous cycle, an in vitro microdialysis system (MDS) of CL was conducted. Estradiol in MDS perfusate was confirmed by a reverse-phase high-performance liquid chromatography in combination with enzyme immunoassays. Basal release of estradiol from microdialyzed CL did not change during the estrous cycle. Additionally, when freshly prepared midluteal cells were exposed to estradiol (10(-14) to 10(-9) M), estradiol stimulated prostaglandin (PG) F(2alpha) secretion (P < 0.05), although it did not affect progesterone and oxytocin secretion. The overall results indicate that estradiol is produced locally in bovine CL throughout the estrous cycle, and they suggest that estradiol plays a role in regulating PGF(2alpha) production in CL as an autocrine/paracrine factor.