Progesterone maintains basal intracellular adenosine triphosphate levels and viability of spontaneously immortalized granulosa cells by promoting an interaction between 14-3-3sigma and ATP synthase beta/precursor through a protein kinase G-dependent mechanism

Progesterone signaling in the regulation of luteal steroidogenesis

The corpus luteum is the major source of progesterone, the essential hormone for female reproductive function. While progesterone activity has been the subject of extensive research for decades, characterization of non-canonical progesterone receptor/signaling pathways provided a new perspective for understanding the complex signal transduction mechanisms exploited by the progesterone hormone. Deciphering these mechanisms has significant implications in the management of luteal phase disorders and early pregnancy complications. The purpose of this review is to highlight the complex mechanisms through which progesterone-induced signaling mediates luteal granulosa cell activity in the corpus luteum. Here, we review the literature and discuss the up-to-date evidence on how paracrine and autocrine effects of progesterone regulate luteal steroidogenic activity. We also review the limitations of the published data and highlight future research priorities.

Progesterone Signaling and Mammalian Ovarian Follicle Growth Mediated by Progesterone Receptor Membrane Component Family Members

How progesterone influences ovarian follicle growth is a difficult question to answer because ovarian cells synthesize progesterone and express not only the classic nuclear progesterone receptor but also members of the progestin and adipoQ receptor family and the progesterone receptor membrane component (PGRMC) family. Which type of progestin receptor is expressed depends on the ovarian cell type as well as the stage of the estrous/menstrual cycle. Given the complex nature of the mammalian ovary, this review will focus on progesterone signaling that is transduced by PGRMC1 and PGRMC2 specifically as it relates to ovarian follicle growth. PGRMC1 was identified as a progesterone binding protein cloned from porcine liver in 1996 and detected in the mammalian ovary in 2005. Subsequent studies focused on PGRMC family members as regulators of granulosa cell proliferation and survival, two physiological processes required for follicle development. This review will present evidence that demonstrates a causal relationship between PGRMC family members and the promotion of ovarian follicle growth. The mechanisms through which PGRMC-dependent signaling regulates granulosa cell proliferation and viability will also be discussed in order to provide a more complete understanding of our current concept of how progesterone regulates ovarian follicle growth.

Progesterone-Mediated Non-Classical Signaling

Progesterone is essential for pregnancy maintenance and menstrual cycle regulation. Hormone action has been primarily ascribed to the well-characterized classical signaling pathway involving ligand binding, activation of nuclear progesterone receptors (PRs), and subsequent activation of genes containing progesterone response elements (PREs). Recent studies have revealed progesterone actions via non-classical signaling pathways, often mediated by non-genomic signaling. Progesterone signaling, in conjunction with growth factor signaling, impacts on the function of growth factors and regulates important physiological actions such as cell growth and remodeling, as well as apoptosis. This review focuses on non-classical progesterone signaling pathways, both including and excluding PR, and highlights how research in this area will provide a better understanding of progesterone actions and may inform novel therapeutic strategies.

Effects of seasonal adiposity on ovarian activity of Vespertilionid bat, Scotophilus heathi: proteomics analysis

In present study comparative proteomics was utilized to identify ovarian protein profiles and correlate the expression of these proteins with adiposity induced changes in ovarian activity leading to suppression of ovulation (delayed ovulation) in the bat, S. heathi. To achieve this, two-dimension gel electrophoresis combined with protein identification by tandem mass spectrometry (LC-MS/MS) was applied. Protein profiles were obtained from intact ovaries of bats collected during recrudescence (basal body weight) and delayed ovulation (increased body weight) phases of reproductive cycle. Out of 42 differentially expressed protein spots, 15 protein spots were identified by LC-MS/MS. A majority of the 15 protein spots identified belonged to a group of enzymes within the glycolytic and citrate cycles. Greater concentrations of these enzymes were found during the period of delayed ovulation, which may be responsible for an increase in the production of ATP within the ovary. The increased metabolic activity and energy production observed within the ovary during winter dormancy may be required for increased steroidogenic activity during this period. The protein 14-3-3 identified by LC-MS/MS was verified by immunoblotting, which confirmed its increased expression during the period of delayed ovulation and may be associated with development of insulin resistance. Treatment with adipokines (adiponectin, resistin) is responsible for increased expression of 14-3-3 protein in the ovary of S. heathi. Adiposity-associated rise in adipokines are thus responsible for increased expression of 14-3-3 protein in the ovary of S. heathi, which may be responsible for prolonged survival of antral follicles and suppression of ovulation. The 14-3-3 protein may represent a new marker for adiposity associated ovarian anovulation (disorders).

Progesterone receptor membrane component-1 (PGRMC1) and PGRMC-2 interact to suppress entry into the cell cycle in spontaneously immortalized rat granulosa cells

Progesterone receptor membrane component 1 (PGRMC1) and PGRMC2 are expressed in rat granulosa cells and spontaneously immortalized granulosa cells (SIGCs) but their biological roles are not well defined. The present studies demonstrate that depleting either Pgrmc1 or Pgrmc2 in SIGCs increases entry into the cell cycle but does not increase cell proliferation. Rather, PGRMC1 and/or PGRMC2-deplete cells accumulate in metaphase and undergo apoptosis. Because both PGRMC1 and PGRMC2 localize to the mitotic spindle, their absence likely accounts for cells arresting in metaphase. Moreover, pull-down assays, colocalization studies and in situ proximity ligation assays (PLA) indicate that PGRMC1 binds PGRMC2. Disrupting the PGRMC1:PGRMC2 complex through the use of siRNA or the cytoplasmic delivery of a PGRMC2 antibody increases entry into the cell cycle. Conversely, overexpressing either PGRMC1-GFP or GFP-PGRMC2 fusion protein inhibits entry into the cell cycle. Subsequent studies reveal that depleting PGRMC1 and/or PGRMC2 reduces the percentage of cells in G0 and increases the percentage of cells in G1. These observations indicate that in addition to their role at metaphase, PGRMC1 and PGRMC2 are involved in regulating entry into the G1 stage of the cell cycle. Interestingly, both PGRMC1 and PGRMC2 bind GTPase-activating protein-binding protein 2 (G3BP2) as demonstrated by pull-down assays, colocalization assays, and PLAs. G3bp2 siRNA treatment also promotes entry into the G1 stage. This implies that dynamic changes in the interaction among PGRMC1, PGRMC2, and G3BP2 play an important protein regulating the rate at which SIGCs enter into the cell cycle.

Non-canonical progesterone signaling in granulosa cell function

It has been known for over 3 decades that progesterone (P4) suppresses follicle growth. It has been assumed that P4 acts directly on granulosa cells of developing follicles to slow their development, as P4 inhibits both mitosis and apoptosis of cultured granulosa cells. However, granulosa cells of developing follicles of mice, rats, monkeys, and humans do not express the A or B isoform of the classic nuclear receptor for P4 (PGR). By contrast, these granulosa cells express other P4 binding proteins, one of which is referred to as PGR membrane component 1 (PGRMC1). PGRMC1 specifically binds P4 with high affinity and mediates P4's anti-mitotic and anti-apoptotic action as evidenced by the lack of these P4-dependent effects in PGRMC1-depleted cells. In addition, mice in which PGRMC1 is conditionally depleted in granulosa cells show diminished follicle development. While the mechanism through which P4 activation of PGRMC1 affects granulosa cell function is not well defined, it appears that PGRMC1 controls granulosa cell function in part by regulating gene expression in T-cell-specific transcription factor/lymphoid enhancer factor-dependent manner. Clinically, altered PGRMC1 expression has been correlated with premature ovarian failure/insufficiency, polycystic ovarian syndrome, and infertility. These collective studies provide strong evidence that PGRMC1 functions as a receptor for P4 in granulosa cells and that altered expression results in compromised reproductive capacity. Ongoing studies seek to define the components of the signal transduction cascade through which P4 activation of PGRMC1 results in the regulation of granulosa cell function.

Nonclassical progesterone signalling molecules in the nervous system

Progesterone (P4) regulates a wide range of cognitive, neuroendocrine, neuroimmune and neuroprotective functions. Therefore, it is not surprising that this ovarian hormone acts through multiple receptors. Ever since the 1980s, studies investigating the neural effects of P4 have focused mainly on genomic and nongenomic actions of the classical progestin receptor (PGR). More recently, two groups of nonclassical P4 signalling molecules have been identified: (i) the class II progestin and adipoQ receptor (PAQR) family, which includes PAQR 5, 6, 7, 8 and 9, also called membrane progestin receptor α (mPRα; PAQR7), mPRβ (PAQR8), mPRγ (PAQR5), mPRδ (PAQR6) and mPRε (PAQR9), and (ii) the b5-like haeme/steroid-binding protein family, which includes progesterone receptor membrane component 1 (Pgrmc1), Pgrmc2, neudesin and neuferricin. In this review, we describe the structures, neuroanatomical localisation and signalling mechanisms of these molecules. We also discuss gonadotrophin-releasing hormone regulation as an example of a physiological function regulated by multiple progesterone receptors but through different mechanisms.

Novel progesterone receptors: neural localization and possible functions

Progesterone (P₄) regulates a wide range of neural functions and likely acts through multiple receptors. Over the past 30 years, most studies investigating neural effects of P₄ focused on genomic and non-genomic actions of the classical progestin receptor (PGR). More recently the focus has widened to include two groups of non-classical P₄ signaling molecules. Members of the Class II progestin and adipoQ receptor (PAQR) family are called membrane progestin receptors (mPRs) and include: mPRα (PAQR7), mPRβ (PAQR8), mPRγ (PAQR5), mPRδ (PAQR6), and mPRε (PAQR9). Members of the b5-like heme/steroid-binding protein family include progesterone receptor membrane component 1 (PGRMC1), PGRMC2, neudesin, and neuferricin. Results of our recent mapping studies show that members of the PGRMC1/S2R family, but not mPRs, are quite abundant in forebrain structures important for neuroendocrine regulation and other non-genomic effects of P₄. Herein we describe the structures, neuroanatomical localization, and signaling mechanisms of these molecules. We also discuss possible roles for Pgrmc1/S2R in gonadotropin release, feminine sexual behaviors, fluid balance and neuroprotection, as well as catamenial epilepsy.

Progesterone directly and rapidly inhibits GnRH neuronal activity via progesterone receptor membrane component 1

GnRH neurons are essential for reproduction, being an integral component of the hypothalamic-pituitary-gonadal axis. Progesterone (P4), a steroid hormone, modulates reproductive behavior and is associated with rapid changes in GnRH secretion. However, a direct action of P4 on GnRH neurons has not been previously described. Receptors in the progestin/adipoQ receptor family (PAQR), as well as progesterone receptor membrane component 1 (PgRMC1) and its partner serpin peptidase inhibitor, clade E (nexin, plasminogen activator inhibitor type 1) mRNA binding protein 1 (SERBP1), have been shown to mediate rapid progestin actions in various tissues, including the brain. This study shows that PgRMC1 and SERBP1, but not PAQR, are expressed in prenatal GnRH neurons. Expression of PgRMC1 and SERBP1 was verified in adult mouse GnRH neurons. To investigate the effect of P4 on GnRH neuronal activity, calcium imaging was used on primary GnRH neurons maintained in explants. Application of P4 significantly decreased the activity of GnRH neurons, independent of secretion of gamma-aminobutyric acidergic and glutamatergic input, suggesting a direct action of P4 on GnRH neurons. Inhibition was not blocked by RU486, an antagonist of the classic nuclear P4 receptor. Inhibition was also maintained after uncoupling of the inhibitory regulative G protein (G(i/o)), the signal transduction pathway used by PAQR. However, AG-205, a PgRMC1 ligand and inhibitor, blocked the rapid P4-mediated inhibition, and inhibition of protein kinase G, thought to be activated downstream of PgRMC1, also blocked the inhibitory activity of P4. These data show for the first time that P4 can act directly on GnRH neurons through PgRMC1 to inhibit neuronal activity.

Expression of 14-3-3 protein isoforms in mouse oocytes, eggs and ovarian follicular development

BACKGROUND

The 14-3-3 (YWHA) proteins are a highly conserved, ubiquitously expressed family of proteins. Seven mammalian isoforms of 14-3-3 are known (β, γ, ε, ζ, η, τ and, σ). These proteins associate with many intracellular proteins involved in a variety of cellular processes including regulation of the cell cycle, metabolism and protein trafficking. We are particularly interested in the role of 14-3-3 in meiosis in mammalian eggs and the role 14-3-3 proteins may play in ovarian function. Therefore, we examined the expression of 14-3-3 proteins in mouse oocyte and egg extracts by Western blotting after polyacrylamide gel electrophoresis, viewed fixed cells by indirect immunofluorescence, and examined mouse ovarian cells by immunohistochemical staining to study the expression of the different 14-3-3 isoforms.

RESULTS

We have determined that all of the mammalian 14-3-3 isoforms are expressed in mouse eggs and ovarian follicular cells including oocytes. Immunofluorescence confocal microscopy of isolated oocytes and eggs confirmed the presence of all of the isoforms with characteristic differences in some of their intracellular localizations. For example, some isoforms (β, ε, γ, and ζ) are expressed more prominently in peripheral cytoplasm compared to the germinal vesicles in oocytes, but are uniformly dispersed within eggs. On the other hand, 14-3-3η is diffusely dispersed in the oocyte, but attains a uniform punctate distribution in the egg with marked accumulation in the region of the meiotic spindle apparatus. Immunohistochemical staining detected all isoforms within ovarian follicles, with some similarities as well as notable differences in relative amounts, localizations and patterns of expression in multiple cell types at various stages of follicular development.

CONCLUSIONS

We found that mouse oocytes, eggs and follicular cells within the ovary express all seven isoforms of the 14-3-3 protein. Examination of the differential expression of these 14-3-3 isoforms in female germ cells and ovarian follicles provides the foundation for further investigating 14-3-3 isoform-specific interactions with key proteins involved in ovarian development, meiosis and oocyte maturation. This will lead to a better understanding of the individual functional roles of the 14-3-3 protein isoforms in mammalian oogenesis and female reproductive development.

Distribution of mRNAs encoding classical progestin receptor, progesterone membrane components 1 and 2, serpine mRNA binding protein 1, and progestin and ADIPOQ receptor family members 7 and 8 in rat forebrain

Several lines of evidence suggest the existence of multiple progestin receptors that may account for rapid and delayed effects of progesterone in the CNS. The delayed effects have been long attributed to activation of the classical progestin receptor (Pgr). Recent studies have discovered novel progestin signaling molecules that may be responsible for rapid effects. These include progesterone receptor membrane component 1 (Pgrmc1), Pgrmc2, progestin and adipoQ receptor 7 (Paqr7) and Paqr8. The functions of these molecules have been investigated extensively in non-neural, but not in neural tissues, partly because it is unclear which are expressed in the brain and where they are expressed. To address these issues, we compared the distributions of mRNAs encoding Pgr, Pgrmc1, Pgrmc2, Paqr7 and Paqr8 using in situ hybridization with radiolabeled oligodeoxynucleotidyl probes in forebrain tissues of estradiol-treated female rats. We also examined the distribution of serpine mRNA binding protein 1 (Serbp1), a putative binding partner of Pgrmc1. Analyses of adjacent brain sections showed that the highest expression of mRNAs encoding Pgr, Pgrmc1, Pgrmc2 and Serbp1 was detected in several hypothalamic nuclei important for female reproduction. In contrast, expression patterns of Paqr7 and Paqr8 were low and homogeneous in the hypothalamus, and more abundant in thalamic nuclei. The neuroanatomical distributions of these putative progestin signaling molecules suggest that Pgrmc1 and Pgrmc2 may play roles in neuroendocrine functions while Paqr7 and Paqr8 are more likely to regulate sensory and cognitive functions.

Progesterone inhibits apoptosis in part by PGRMC1-regulated gene expression

Progesterone receptor membrane component-1 (PGRMC1) is present in both the cytoplasm and nucleus of spontaneously immortalized granulosa cells (SIGCs). PGRMC1 is detected as a monomer in the cytoplasm and a DTT-resistant PGRMC1 dimer in the nucleus. Transfected PGRMC1-GFP localizes mainly to the cytoplasm and does not form a DTT-resistant dimer. Moreover, forced expression of PGRMC1-GFP increases the sensitivity of the SIGCs to progesterone (P4)'s anti-apoptotic action, indicating that the PGRMC1 monomer is functional. However, when endogenous PGRMC1 is depleted by siRNA treatment and replaced with PGRMC1-GFP, P4 responsiveness is not enhanced, although overall levels of PGRMC1 are increased. P4's anti-apoptotic action is also attenuated by actinomycin D, an inhibitor of RNA synthesis, and P4 activation of PGRMC1 suppresses Bad and increases Bcl2a1d expression. Taken together, the present studies suggest a genomic component to PGRMC1's anti-apoptotic mechanism of action, which requires the presence of the PGRMC1 dimer.

Progesterone activates a progesterone receptor membrane component 1-dependent mechanism that promotes human granulosa/luteal cell survival but not progesterone secretion

CONTEXT

Progesterone (P4) promotes its own secretion and the survival of human granulosa/luteal (GL) cells.

OBJECTIVE

The objective of these studies was to determine whether progesterone receptor membrane component-1 (PGRMC1) mediates P4's actions.

DESIGN

In vitro studies were conducted on GL cells from women undergoing in vitro fertilization and GL5 cells, which are derived from GL cells.

SETTING AND PATIENTS

GL cells were obtained from women undergoing fertility treatment at a university-based clinic and used for in vitro studies.

MAIN OUTCOME MEASURES

PCR, Western blot, and immunocytochemistry were used to detect various progestin binding proteins. (3)H-P4 binding kinetics were assessed on partially purified PGRMC1. Apoptosis was determined after culture by either TUNEL or DAPI staining. P4 was measured by an ELISA assay. PGRMC1 was depleted using small interfering RNA.

RESULTS

GL and GL5 cells expressed several P4 binding proteins including the nuclear progesterone receptor (PGR), progestin/adipoQ receptors (PAQR 7, 8, and 5) and PGRMC1. Ligand binding studies revealed that both P4 and the progestin, R5020, bound PGRMC1 with an EC(50) of approximately 10 nm. Interestingly, P4 inhibited apoptosis at concentrations in the 10 nm range, whereas R5020 stimulated P4 secretion at concentrations of at lease 16 mum. Depleting PGRMC1 attenuated P4's antiapoptotic action but failed to influence R5020-induced P4 secretion.

CONCLUSIONS

These studies conclusively demonstrate that in human GL cells PGRMC1 functions as a receptor through which P4 activates a signal cascade that prevents apoptosis. In contrast, PGRMC1 does not mediate P4's ability to acutely promote its own secretion.

Non-genomic progesterone actions in female reproduction

BACKGROUND

The steroid hormone progesterone is indispensable for mammalian procreation by controlling key female reproductive events that range from ovulation to implantation, maintenance of pregnancy and breast development. In addition to activating the progesterone receptors (PRs)-B and -A, members of the superfamily of ligand-dependent transcription factors, progesterone also elicits a variety of rapid signalling events independently of transcriptional or genomic regulation. This review covers our current knowledge on the mechanisms and relevance of non-genomic progesterone signalling in female reproduction.

METHODS

PubMed was searched up to August 2008 for papers on progesterone actions in ovary/breast/endometrium/myometrium/brain, focusing primarily on non-genomic signalling mechanisms.

RESULTS

Convergence and intertwining of rapid non-genomic events and the slower transcriptional actions critically determine the functional response to progesterone in the female reproductive system in a cell-type- and environment-specific manner. Several putative progesterone-binding moieties have been implicated in rapid signalling events, including the 'classical' PR and its variants, progesterone receptor membrane component 1, and the novel family of membrane progestin receptors. Progesterone and its metabolites have also been implicated in the allosteric regulation of several unrelated receptors, such as gamma-aminobutyric acid type A, oxytocin and sigma(1) receptors.

CONCLUSIONS

Identification of the mechanisms and receptors that relay rapid progesterone signalling is an area of research fraught with difficulties and controversy. More in-depth characterization of the putative receptors is required before the non-genomic progesterone pathway in normal and pathological reproductive function can be targeted for pharmacological intervention.