Regulation of progesterone production in the rabbit corpus luteum

Endocrine changes during the peripartal period related to colostrogenesis in mammalian species

This review discusses endocrine and functional changes during the transition from late gestation to lactation that are related to the production of colostrum in different mammalian species. Species covered in this article include ungulate species (cattle, sheep, goats, pigs, horses), rodents (rat, mouse), rabbits, and carnivores (cats, dogs), as well as humans. An immediate availability of high quality colostrum for the newborn after birth is crucial in species where a transfer of immunoglobulins (Ig) does not or only partially occur via the placenta during pregnancy. Declining activity of gestagens, in most species progesterone (P4), is crucial at the end of pregnancy to allow for the characteristic endocrine changes to initiate parturition and lactation, but the endocrine regulation of colostrogenesis is negligible. Both, the functional pathways and the timing of gestagen withdrawal differ considerably among mammalian species. In species with a sustaining corpus luteum throughout the entire pregnancy (cattle, goat, pig, cat, dog, rabbit, mouse, and rat), a prostaglandin F2α (PGF2α)-induced luteolysis shortly before parturition is assumed to be the key event to initiate parturition as well as lactogenesis. In species where the gestagen production is taken over by the placenta during the course of gestation (e.g., sheep, horse, and human), the reduction of gestagen activity is more complex, as PGF2α does not affect placental gestagen production. In sheep the steroid hormone synthesis is directed away from P4 towards estradiol-17β (E2) to achieve a low gestagen activity at high E2 concentrations. In humans the uterus becomes insensitive to P4, as parturition occurs despite still high P4 concentrations. However, lactogenesis is not completed as long as P4 concentration is high. Early colostrum and thus Ig intake for immune protection is not needed for the human newborn which allows a delayed onset of copious milk secretion for days until the placenta expulsion causes the P4 drop. Like humans, horses do not need low gestagen concentrations for successful parturition. However, newborn foals need immediate immune protection through Ig intake with colostrum. This requires the start of lactogenesis before parturition which is not fully clarified. The knowledge of the endocrine changes and related pathways to control the key events integrating the processes of colostrogenesis, parturition, and start of lactation are incomplete in many species.

Expression and Transcript Localization of star, sf-1, and dax-1 in the Early Brain of the Orange-Spotted Grouper Epinephelus coioides

We investigated the developmental expression and localization of sf-1 and dax-1 transcripts in the brain of the juvenile orange-spotted grouper in response to steroidogenic enzyme gene at various developmental ages in relation to gonadal sex differentiation. The sf-1 transcripts were significantly higher from 110-dah (day after hatching) and gradually increased up to 150-dah. The dax-1 mRNA, on the other hand, showed a decreased expression during this period, in contrast to sf-1 expression. At the same time, the early brain had increased levels of steroidogenic gene (star). sf-1 and star hybridization signals were found to be increased in the ventromedial hypothalamus at 110-dah; however, dax-1 mRNA signals decreased in the early brain toward 150-dah. Furthermore, the exogenous estradiol upregulated star and sf-1 transcripts in the early brain of the grouper. These findings suggest that sf-1 and dax-1 may have an antagonistic expression pattern in the early brain during gonadal sex differentiation. Increased expression of steroidogenic gene together with sf-1 during gonadal differentiation strongly suggests that sf-1 may play an important role in the juvenile grouper brain steroidogenesis and brain development.

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

Simple Summary

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

Abstract

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

Immunohistochemical localization of 3β-Hydroxysteroid dehydrogenase and progesterone receptors in the ovary and placenta during gestation of the placentotrophic lizard Mabuya sp (Squamata: Scincidae)

In squamates, progesterone (P) plays a key role in the inhibition of uterine mobility during egg retention in oviparous species, and during gestation in viviparous species. The corpus luteum (CL) is the main organ responsible for the production of P; however, in some species, the CL degenerates early and the P needed for gestation maintenance should be produced in other tissues. Mabuya sp (Scincidae) is a viviparous lizard with a prolonged gestation, it produces microlecithal eggs and, consequently, has an obligate placentotrophy related with a highly complex placenta. Its CL degenerates at early stages of gestation and therefore, other sources of P should exist. The aim of this study was to determine and localize by immunohistochemistry the production of P by detection of the enzyme 3β-Hydroxysteroid dehydrogenase (3β-HSD) and P receptors (PR) during gestation in the ovary and placenta of Mabuya sp. Positive and negative control sections were used. The ovary of this species localizes 3β-HSD and PR in the same tissues. The CL of the ovaries of females at early stages of gestation were positive for both molecules, whereas they did not localize from mid gestation to the end of pregnancy. Previtellogenic and vitellogenic follicles labelled for both molecules in the follicular epithelium and thecae. The placenta of Mabuya sp. demonstrated the potential for P production from mid gestation to the end of gestation in the uterine and chorionic tissues. PR were located in the uterine tissues throughout gestation, with a decrease towards its completion. Western blot analysis confirmed the presence of 3β-HSD mainly in the ovary of early pregnant females and in the placental tissues at mid gestation stages. Therefore, the chorioallantoic placenta of Mabuya sp. has an endocrine function producing the P needed for gestation and replacing the CL from mid gestation to the end of pregnancy.

Mitogen-Activated Protein Kinase 8 (MAP3K8) Mediates the Signaling Pathway of Estradiol Stimulating Progesterone Production Through G Protein-Coupled Receptor 30 (GPR30) in Mouse Corpus Luteum

The corpus luteum (CL) is a transient endocrine gland developed from the ovulated follicles, and the most important function is to synthesize and secrete progesterone (P(4)), a key hormone to maintain normal pregnancy and estrous cycle in most mammals. It is known that estrogen has a vital role in stimulating P(4) synthesis in CL, but it still remains unclear about the mechanism of estradiol (E(2)) regulating P(4) production in CL. Our results here first show that all of the CL cells express MAPK 8 (MAP3K8), and the MAP3K8 level is much higher at the midstage than at the early and late stages during CL development. The further functional studies show that the forced inhibition of endogenous MAP3K8 by using MAP3K8 small interfering RNA and MAP3K8 signaling inhibitor (MAP3K8i) in the luteal cells significantly block the P(4) synthesis and neutralize the enhancing effect of E(2) on P(4) production in the CL. In addition, our results here demonstrate that the stimulating effect of E(2) on P(4) synthesis relies on the estrogen no-classical protein-coupled receptor 30, and MAP3K8 is involved in mediating the protein-coupled receptor 30signaling of E(2) affecting P(4) synthesis via stimulating ERK phosphorylation. These novel findings are critical for our understanding the ovary physiology and pathological mechanism.

Bisphenol A modulates expression of sex differentiation genes in the self-fertilizing fish, Kryptolebias marmoratus

Endocrine disrupting chemicals (EDCs) have been a major concern in the normal reproduction and development of aquatic organisms. In the teleost, steroid hormones are synthesized via the steroidogenesis pathway, and play a key physiological role in the regulation of gonadal sex differentiation. The protogynous hermaphroditic fish, Kryptolebias marmoratus is the only vertebrate capable of reproducing through internal self-fertilization. To uncover the effect of bisphenol A (BPA) on sex differentiation genes on transcription, we investigated the expression patterns of several sex differentiation-related genes such as dax1, dmrt1, mis, sf1, figlα, StAR and wt1 after BPA exposure with controls (E2 and TMX). In response to 17β-estradiol (E2) exposure, a testis-specific gene, dmrt1 mRNA was down-regulated in the gonad of the secondary male but the expression of the female-specific gene, dax1 mRNA was significantly elevated in the brain and gonad. A high level of StAR mRNA was detected in the brain and gonad of both hermaphrodite and secondary males, suggesting that the elevated expression of dax1 and StAR genes would be involved in E2 exposure. As expected, upon BPA exposure, the dmrt1 and MIS mRNA level decreased in both hermaphrodite and secondary males, while the female-specific gene, figlα mRNA level increased in the gonad of both genders. BPA showed an opposite mode of action on the expression of dax1 (induction, P>0.05) and sf1 mRNA (inhibition, P>0.05) in the brain and gonad against both genders. The sensitivity of dax1 to BPA on expression was relatively high in the secondary male. The wt1 mRNA was up-regulated in most tissues except in the liver of BPA-exposed secondary males. Regarding the time course study, the figlα mRNA level increased at 6 h after BPA exposure. In addition, BPA elevated the expression of StAR, dax1, and wt1 mRNA but repressed sf1 mRNA. In this paper, we demonstrated that BPA may modulate the expression of sex differentiation and steroidogenesis pathway genes, and this finding would provide a better understanding on the modulation of transcription upon BPA exposure in steroidogenesis and sex differentiation in the hermaphroditic fish, K. marmoratus.

Modulation of salmon ovarian steroidogenesis and growth factor responses by the xenoestrogen, 4-nonylphenol

Endocrine-disrupting chemicals are known to influence organismal reproductive processes, including the production and regulation of gonadal steroids. This study evaluated the effects of a xenoestrogen (nonylphenol: NP) on salmon ovarian steroidogenesis and growth factors using an in vitro organ culture system. Ovarian tissues were cultivated for 3 and 7d with different concentrations of NP (0 (control), 1, 10 and 50 microM) dissolved in ethanol (0.1%). The mRNA expressions of steroidogenic acute regulatory (StAR) protein, P450-mediated cholesterol side-chain cleavage (P450scc), aromatase isoforms, 3beta-hydroxysteroid dehydrogenase (3beta-HSD), Cyp11beta-, Cyp17 and 21-hydroxylase, and insulin-like growth factors (IGF-1 and IGF-2) and IGF1-receptor (IGF1-R) were quantified by real-time PCR. Tissue levels of estradiol-17beta (E2), testosterone (T) and 11-ketotestosterone (11-KT) were quantified using enzyme immunoassays. Our data show that nominal NP levels produced time- and concentration-specific effects on the expression of steroidogenesis- and IGF-related transcripts in salmon ovarian tissues. Tissue levels of ovarian E2, T and 11-KT were significantly modulated after NP exposure. Interestingly, elevated ovarian E2 levels after 10 microM NP exposure at day 3 paralleled P450Arom isoforms mRNA expression at the same time interval. The expression patterns of other steroidogenic protein and enzyme genes, such as StAR, P450scc, 3beta-HSD and Cyp17 inversely paralleled this pattern, displaying consistent decreased transcript levels. These findings show that NP (an ubiquitous environmental pollutant) can produce variations in gonadal steroidogenesis and growth regulating responses with potential consequences for overt fecundity in teleosts.

Establishment of the need for oestrogen during implantation in non-human primates

Deprivation of oestrogen during post-ovulatory mated cycles in proven fertile female bonnet monkeys by tamoxifen, aromatase inhibitor or oestrogen antiserum resulted in inhibition of pregnancy establishment in all three groups of animals. However, more than 85% of the animals became pregnant within three exposures to proven fertile males in the control group. These results suggest the requirement for oestrogen in pregnancy establishment in primates. Based on this conclusion, it is suggested that use of a suitable and potent anti-oestrogenic compound can be exploited as an alternative approach to contraception.

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

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

Impact of an enriched-cholesterol diet on enzymatic cholesterol metabolism during rabbit gestation

An appropriate cholesterol homeostasis is vital for the maintenance and the optimal fetal development. The cholesterol is essential for the synthesis of progesterone and 17beta-estradiol, hormones that actively participate to sustain gestation. However, the administration of 0.2% enriched cholesterol diet (ECD) during rabbit gestation significantly increased the cholesterol blood profile (total-cholesterol, LDL, HDL, esterified-cholesterol and free-cholesterol) of dams and offspring, and induced a reduction of the offspring weight of 15% as compared to the control group. Enzymes involved in cholesterol metabolism (ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase) are greatly influenced by cholesterol profile. We hypothesized that the administration of an ECD during rabbit gestation modifies the activity of those enzymes. Female rabbits (pregnant or not) were fed with a standard diet or an ECD. At term, livers (dams and offspring) and placentas were collected and ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase activities were assayed. Our results demonstrate that gestation induced a reduction of ACAT activity (48.9%) in dam's liver and, an augmentation of HMG-CoA-reductase activity (142.4%) whereas it has no effect on cholesterol-7alpha-hydroxylase activity. The administration of the ECD has no additive effect on ACAT, but significantly reduced the HMG-CoA-reductase activity and cholesterol-7alpha-hydroxylase activity as compared with the pregnant control group. In placentas the ECD supplementation has an influence for HMG-CoA-reductase activity, where a 43% increased in observed. Any ACAT activity was detected in placenta and the ECD has no influence on the cholesterol-7alpha-hydroxylase activity. Whereas their offspring's liver present a reduction of ACAT and HMG-CoA-reductase activity. Gestation associated with ECD reduces significantly the HMG-CoA-reductase activity, decreasing the cholesterol synthesis, but placenta seems to compensate this effect by increasing its HMG-CoA-reductase activity.

Mechanisms controlling the function and life span of the corpus luteum

The primary function of the corpus luteum is secretion of the hormone progesterone, which is required for maintenance of normal pregnancy in mammals. The corpus luteum develops from residual follicular granulosal and thecal cells after ovulation. Luteinizing hormone (LH) from the anterior pituitary is important for normal development and function of the corpus luteum in most mammals, although growth hormone, prolactin, and estradiol also play a role in several species. The mature corpus luteum is composed of at least two steroidogenic cell types based on morphological and biochemical criteria and on the follicular source of origin. Small luteal cells appear to be of thecal cell origin and respond to LH with increased secretion of progesterone. LH directly stimulates the secretion of progesterone from small luteal cells via activation of the protein kinase A second messenger pathway. Large luteal cells are of granulosal cell origin and contain receptors for PGF(2alpha) and appear to mediate the luteolytic actions of this hormone. If pregnancy does not occur, the corpus luteum must regress to allow follicular growth and ovulation and the reproductive cycle begins again. Luteal regression is initiated by PGF(2alpha) of uterine origin in most subprimate species. The role played by PGF(2alpha) in primates remains controversial. In primates, if PGF(2alpha) plays a role in luteolysis, it appears to be of ovarian origin. The antisteroidogenic effects of PGF(2alpha) appear to be mediated by the protein kinase C second messenger pathway, whereas loss of luteal cells appears to follow an influx of calcium, activation of endonucleases, and an apoptotic form of cell death. If the female becomes pregnant, continued secretion of progesterone from the corpus luteum is required to provide an appropriate uterine environment for maintenance of pregnancy. The mechanisms whereby the pregnant uterus signals the corpus luteum that a conceptus is present varies from secretion of a chorionic gonadotropin (primates and equids), to secretion of an antiluteolytic factor (domestic ruminants), and to a neuroendocrine reflex arc that modifies the secretory patterns of hormones from the anterior pituitary (most rodents).

Effects of induced endometritis on the life-span of corpora lutea in pseudopregnant rabbits and incidence of spontaneous uterine infections related to fertility of breeding does

A significant percentage of rabbit does fail to become pregnant after AI. We hypothesized that uterine infections induced by the insemination procedure are related to delayed luteolysis and high progesterone concentrations noted to present at the time of AI. The rabbits, randomly assigned to 4 groups (3 animals/group), were given 0.8 microgram GnRH analogue (Day 0) just prior to infusing the uterus with sterile extender (control group) or with extender inoculated with 0.5, 1, and 2 x 10(6) Pasteurella multocida (treated groups). The effects of treatments on functional life-span of CL were assessed by evaluating plasma progesterone from Day 0 to Day 23 of pseudopregnancy. In treated rabbits, the progesterone profiles closely overlapped those found in controls until approximately Day 14. Thereafter, they varied greatly between animals, but luteolysis was delayed by at least 5-6 d and developed less rapidly than in controls. On Day 21, progesterone concentrations were higher than normal in 4 treated does. In a field survey, vaginal swabs were collected at the time of the second AI from 114 non-pregnant rabbits and those positive to bacteriological culture, were killed humanely 16 d later to collect uterine swabs. Positive uterine swabs were found only in 19 of the 34 does having a positive vaginal swabs and all of them were not pregnant. The most frequent pathogen isolated was S. aureus (50%), followed by E. coli (37.5%) and P. multocida (12.5%). We demonstrated that uterine infection increases the life-span of CL in non-pregnant does and that infections of the genital tract system are quite common among does on breeding farms, probably related to using AI.

Estradiol-mediated suppression of apoptosis in the rabbit corpus luteum is associated with a shift in expression of bcl-2 family members favoring cellular survival

In the rabbit, estradiol is the primary luteotropic hormone. Estradiol withdrawal results in a rapid decline in serum progesterone and eventually in corpus luteum (CL) regression. The objective of this study was to determine whether estradiol modulates luteal cell apoptosis. In the first experiment, rabbits were randomly assigned to one of five experimental groups. An empty capsule (control) or estradiol-filled Silastic capsule was inserted s.c. on Day 0 of pseudopregnancy (day of hCG administration). On Day 11 of pseudopregnancy, some of the group I (control) and group II (estradiol capsule) rabbits were subjected to laparotomy, and one ovary from each rabbit was perfused in vitro to determine progesterone secretion rates. The CL from the contralateral ovary were dissected, snap-frozen, and stored at -70 degrees C until analyzed for internucleosomal DNA cleavage (apoptosis). Estradiol-containing capsules were removed from some of the remaining rabbits on Days 8, 9, and 10 to initiate estradiol deprivation. Rabbits were then subjected to laparotomy 24, 48, or 72 h after capsule removal (groups III, IV, and V, respectively), and ovaries or CL were processed as described above. Deprivation of estradiol for 24 (group III), 48 (group IV), or 72 (group V) h in vivo reduced in vitro progesterone secretion rates by more than 90% as compared to that in ovaries collected from estradiol capsule-intact animals. After in vivo endogenous estradiol suppression, withdrawal of exogenous estradiol resulted in luteal cell apoptosis, which increased in a time-dependent manner. Northern blot analysis revealed an increase in bax mRNA levels and a decrease in bcl-x mRNA levels coincident with luteal cell apoptosis induced by estradiol withdrawal. These data demonstrate that changes in progesterone production caused by estradiol exposure and deprivation are in part related to luteal cell apoptosis, and alterations in the expression of bcl-2 gene family members may be one of the mechanisms by which estradiol exerts its luteotropic effect in the rabbit CL.

Changes in refractoriness of rabbit corpora lutea to a prostaglandin F2 alpha analogue, alfaprostol, during pseudopregnancy

The responsiveness of rabbit corpus luteum to 200 micrograms of the prostaglandin F2 alpha (PGF2 alpha) analogue, alfaprostol, between Days 3 and 9 of pseudopregnancy was assessed by evaluating the decline in plasma progesterone after treatment with PGF2 alpha in 81 New Zealand White (NZW) rabbits. On Days 3-5, functional luteolysis was not observed. On Days 6, 7, and 8 of pseudopregnancy, the number of rabbits responsive to PGF2 alpha, rose from 38% to 71% and 83%, respectively. In the other cases, the effect of the PGF2 alpha analogue was transient as CL recovered in the following 2 or 3 days. By contrast, on Day 9 luteolysis was effective and persistent in all the animals. In rabbits treated on Day 9, progesterone decreased gradually from 10.6 +/- 0.7 within the first 6 h, but fell to 3.6 +/- 1.5 ng/mL (p < 0.01) 12 h after PGF2 alpha and to 0.2 +/- 0.1 ng/mL (p < 0.01) 24 h later.

The role of insulin-like growth factor-I (IGF-I) and estradiol in rabbit corpus luteum progesterone production

To determine whether insulin-like growth factor-I (IGF-I) plays a role in rabbit corpus luteum (CL) physiology the authors examined: IGF-I expression, the effect of IGF-I on progesterone (P) production in vitro, and the interaction of IGF-I with estradiol, the primary luteotropin in the rabbit. Northern blot analysis revealed that IGF-I mRNA is present in the rabbit CL throughout pseudopregnanacy. An intact ovarian in vitro perfusion model and dispersed luteal cell culture were used to determine effects of IGF-I on P production and interactions with estradiol. IGF-I significantly stimulated P production compared to control medium by the isolated, intact perfused rabbit ovary and by dispersed, cultured luteal cells. Estradiol alone did not stimulate P production in vitro. Estradiol did augment IGF-I stimulated P production in the intact perfused ovary and in luteal cell culture. These findings support a role for IGF-I in rabbit CL P production.

Hormonal regulation of PKC-delta protein and mRNA levels in the rabbit corpus luteum

We have previously reported that rabbit corpora lutea exhibit a prominent phosphorylated substrate protein at 76 kDa which corresponds to the autophosphorylated form of protein kinase C (PKC) delta and that the expression of PKC-delta protein is increased in rabbit corpora lutea of pseudopregnancy at least 2-fold when serum estrogen levels are raised by the presence of an estrogen implant inserted at the time of human chorionic gonadotropin (hCG)-induced ovulation. The purpose of the experiments described herein was to evaluate further the hormonal regulation of PKC-delta in the rabbit corpus luteum. Results demonstrate that luteal PKC-delta protein and mRNA are concomitantly induced some 5-fold within 48 h in response to an ovulatory surge of hCG; that, as in corpora lutea of pseudopregnancy, luteal PKC-delta expression is relatively constant during the life span of the corpus luteum following a fertile mating; that exogenous estrogen does not modulate the induction of luteal PKC-delta during luteinization but promotes an additional two-fold increase in steady state PKC-delta mRNA (and protein) levels in corpora lutea by day 10 of pseudopregnancy; and that luteal PKC-delta expression can be abruptly and reversibly modulated upon withdrawal and subsequent replacement of an estrogen implant to pseudopregnant rabbits. These results demonstrate that an ovulatory surge of luteinizing hormone induces the expression of PKC-delta mRNA and protein in rabbit corpora lutea, and that once the corpus luteum becomes estrogen responsive, estrogen then regulates expression of PKC-delta mRNA and protein.

Giant and binucleate trophoblast cells of mammals

The cellular origin, structure, and function of trophoblastic giant cells (GC) and binucleate cells (BNC) are reviewed. Mammals in which these cells have received the greatest attention include rodents, rabbits, and humans (GCs), and ruminants and equids (BNCs). In almost all cases these cells arise from the cytotrophoblast. All are large cells and contain either two diploid nuclei (BNCs), multiple nuclei (human placental bed GCs), or single nuclei with amplified DNA content (rodent and rabbit GCs). Giant and binucleate cells typically exhibit the capacity for migration or invasion, although the degree of migratory activity varies between species. While most end up within, or at the interface with, endometrial tissue, in some instances the GCs or BNCs contribute directly to the interhemal membrane of the placenta. Hormone production is a property which most GC-BNC populations have in common. Lactogen or gonadotropin has been documented in almost all cells of this type examined to date, and in some animals they are also steroidogenic (e.g., rats and sheep). In spite of some common features, both structural and functional differences remain and it is suggested that use of terms such as mononuclear giant cells, multinucleate giant cells, and binucleate cells be continued rather than assuming that these cells are all members of a single trophoblastic subtype.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.