Abstract
Progesterone (P4) is synthesized in the ovary and acts directly on granulosa cells of developing ovarian follicles to suppress their rate of mitosis and apoptosis. Granulosa cells do not express nuclear progesterone receptor (PGR) but rather progesterone receptor membrane component-1 (PGRMC1). PGRMC1 binds P4 and mediates P4's actions, as evidenced by PGRMC1 siRNA studies. PGRMC1 acts by binding plasminogen activator inhibitor 1 RNA-binding protein and regulating gene expression. Specifically, PGRMC1 suppresses some genes that promote cell death (i.e., Bad, Caspase-3, Caspase-4). P4 regulates gene expression in part by inhibiting PGRMC1 binding to Tcf/Lef transcription sites, thereby reducing Tcf/Lef transcriptional activity. Since Tcf/Lef transcription sites are located within the promoters of genes that initiate mitosis and/or apoptosis (i.e., c-jun and c-myc), P4-PGRMC1 mediated suppression of these Tcf/Lef regulated genes could account for P4's actions. PGRMC1 expression is also altered in women with polycystic ovarian syndrome, premature ovarian failure and infertility. Collectively, these observations support a role for PGRMC1 in regulating human ovarian follicle development.
Introduction
Progesterone receptor membrane component 1 (PGRMC1) plays a clinically important role in regulating ovarian function as demonstrated by the fact that PGRMC1 levels are reduced in some women with polycystic ovarian syndrome (Schuster et al., ) or premature ovarian failure (Mansouri et al., ; Schuster et al., ). In contrast PGRMC1 over expression is associated with impaired follicular development in women undergoing gonadotropin-induced ovulation and in vitro fertilization as part of their infertility treatment (Elassar et al., ). In infertile patients, the elevated levels of PGRMC1 were detected within ovarian (granulosa/luteal) cells harvested at the time of oocyte (egg) retrieval. Thus, the change in PGRMC1 expression directly reflects altered ovarian function. Moreover, PGRMC1 is highly expressed in granulosa cells of ovarian follicles of women (Engmann et al., ) as well as in all mammalian ovaries thus far examined including mice (Cai and Stocco, ), rats (Peluso et al., ), monkeys (Bishop et al., ), and cows (Kowalik and Kotwica, ; Luciano et al., ).
Interestingly, PGRMC1 is detected at the plasma membrane and cytoplasm and occasionally in the nuclei of granulosa cells of growing preantral and antral follicles (Peluso et al., ). This expression pattern is consistent with PGRMC1 being a mediator of progesterone's actions in granulosa cells. Specially, the ability of progesterone (P4) to slow ovarian follicular growth has been demonstrated in hypophysectomized hamsters (Moore and Greenwald, ), gonadotropin-primed hamsters (Kim and Greenwald, ) rats (Buffler and Roser, ), mice (Peluso et al., ), and monkeys (Dizerega and Hodgen, ). Given these findings, this mini review will focus on the experimental evidence that supports a role for P4-PGRMC1 signaling in regulating the granulosa cell functions of mitosis and apoptosis.
PGRMC1 as a mediator of P4's actions
The first characteristic of a mediator of P4's action is the ability to bind P4 with high affinity. Although there are reports that PGRMC1 does not bind P4, these studies assessed P4-binding to bacterially-expressed PGRMC1 proteins (For review see Cahill, ). However, the bacterially-expressed PGRMC1 may not be properly folded and therefore unable to bind P4. In contrast, partially purified PGRMC1-fusion protein isolated from either spontaneously immortalized granulosa cells (SIGCs) or human granulosa/luteal cells (hGL5 cells) specifically binds P4 with high affinity (Peluso et al., , ; Figure 1A). In addition, PGRMC1 siRNA treatment of SIGCs for 48 h depletes PGRMC1 mRNA levels to 5% of scramble control (Peluso et al., ). Furthermore, depleting PGRMC1 levels results in a corresponding decrease in the capacity of these cells to bind P4 (Peluso et al., ). Thus, these two observations provide conclusive evidence that PGRMC1 binds P4.
Figure 1
In addition to binding P4, PGRMC1 is an essential component in the mechanism through which P4 regulates granulosa cell function. This has been demonstrated by genetically depleting the expression of PGRMC1. Once PGRMC1 is depleted, P4 no longer inhibits apoptosis induced by serum withdrawal (Peluso et al.,
To resolve this issue SIGCs were exposed to PGRMC1 siRNA in media supplemented with steroid-free serum. After 48 h, PGRMC1 levels are reduced to ≈5% of that observed after scramble siRNA treatment. In the presence of steroid-free serum, P4 suppresses the rate of mitosis and apoptosis of SIGCs exposed to scramble siRNA. In contrast depleting PGRMC1 reduces P4's ability to suppress both of these cellular functions (Figures 1B,C) (Peluso and Griffin, unpublished). Also, depleting PGRMC1 accelerates the rate at which SIGCs enter metaphase, but this increase in metaphase cells does not result in an increase in the number of cells. Rather, the metaphase cells undergo apoptosis. These studies not only confirm the functionality of the P4-PGRMC1 signaling pathway but also are consistent with the concept that P4 activation of PGRMC1 slows that rate of cell division to insure that mitosis occurs properly with fewer granulosa cells undergoing apoptosis as a result of a “mitotic catastrophe” (Peluso et al.,
To gain insight into how P4 and PGRMC1 control mitosis, the rate at which SIGCs enter the metaphase stage of the cell cycle was determined by culturing these cells with colchicine, which arrests cells in metaphase. This study demonstrates that P4 suppresses and depletion of PGRMC1 accelerates entry into metaphase even in the presence of P4 (Figure 1D). Thus, P4 activation of PGRMC1 slows the rate of entry into the metaphase stage of mitosis. Interestingly, PGRMC1 localizes to the mitotic spindle and directly interacts with the major mitotic spindle protein, ß-tubulin, as revealed by in situ proximity ligation assay (Lodde and Peluso,
Figure 2

In (A) the co-localization of β-tubulin (green) and PGRMC1 (red) in relationship to metaphase chromosomes (blue) is shown in SKOV3 cell. Also shown is interaction between PGRMC1 and β-tubulin as revealed by in situ Proximity Ligation Assay (PLA). The presence of red fluorescent dots in the PLA assay indicates that two proteins are in close proximity (i.e., interacting). DNA is counterstained with DAPI (blue). Negative controls were conducted and did not show any staining. (Images are from Figures 5 to 7 from Lodde and Peluso,
Based on these in vitro experiments, it is hypothesized that reducing the level of PGRMC1 would disrupt granulosa cell proliferation and increase apoptosis, resulting in more ovarian follicles undergoing atresia. This hypothesis would be supported if an increase in atretic (dying) follicles and/or fewer antral follicles were observed in PGRMC1 conditional knockout mice. We have recently generated mice in which PGRMC1 was depleted from granulosa cells by mating floxed PGRMC1 mice with transgenic mice in which cre recombinase enzyme is expressed under the control of the Amhr2 promoter. Using these mice a role for PGRMC1 in regulating follicle growth is supported by the observation that in immature mice with reduced PGRMC1 levels (i.e., heterozygous mice; +/−) the number of antral follicles is the same as controls but the percentage of atretic follicles is higher (Figure 2B). Depleting PGRMC1 levels as in homozygous PGRMC1 knockout mice (−/−) results in fewer antral follicles (Figure 2B) (Pru and Peluso, unpublished observations). Since the numbers of primordial, primary and preantral follicles are not reduced, this implies that PGRMC1 plays a key role in promoting the growth of ovarian follicles between the preantral and antral stages of development. The precise mechanism through which PGRMC1 promotes the development of preantral into antral follicles is not defined by likely is due in part to P4 activation of a PGRMC1 mediated cell survival pathway (Peluso et al.,
These in vitro and in vivo studies together with the clinical observations imply that P4-PGRMC1 signaling plays an important role in ovarian follicle development by regulating granulosa cell proliferation and apoptosis. As indicated one site of action is at the mitotic spindle and more must be done to elucidate the details related to this site of action. Equally important is the mechanism through which P4 activation of PGRMC1 influences the rate at which granulosa cell enter into the cell cycle.
PGRMC1 and its capacity to bind P4
It is likely that the first event in a PGRMC1-dependent signal transduction pathway is P4-binding to PGRMC1. Unfortunately, the P4-binding site within PGRMC1 has not been clearly defined. PGRMC1 is composed of a single amino acid chain of 194 amino acids. The first 20 amino acids encode the extracellular domain, amino acids 21–40 encode a transmembrane domain and amino acids 70–170 represent a cytochrome P450 b5-binding domain (Cahill,
PGRMC1 and its interaction with PAIRBP1
The sequence of molecular events that occurs once P4 binds PGRMC1 is not known. What is known is that the relationship between P4 and PGRMC1 is not a simple ligand-receptor interaction, because PGRMC1 also binds plasminogen activator inhibitor 1 mRNA-binding protein (PAIRBP1) (Peluso et al.,
To gain insight into this issue, the amino acid sequence in PGRMC1 that promotes its interaction with PAIRBP1 was recently identified. This was accomplished by making a series of PGRMC1-GFP deletion mutations and using them in pulldown assays (Peluso et al.,
These pulldown studies also revealed that the PGRMC1-GFP fusion protein without amino acids 131–194 binds more PAIRBP1 than the full-length PGRMC1-GFP (Peluso et al.,
Sumoylation of PGRMC1
To begin to validate this hypothesis, it is essential to demonstrate that PGRMC1 is sumoylated. Western blots detect PGRMC1 as a ≈22 kDa band but longer exposures often reveal several bands that are >50 kDa (Peluso et al.,
While these co-immunoprecipitation studies are important, they are rather incomplete. Since in silico analysis predictes that PGRMC1 can be sumoylated at lysine residues 136, 187, and/or 193 (see http://sumosp.biocuckoo.org/online.php), an expression construct was made that encodes a PGRMC1-Flag fusion protein in which the lysine within each of the three sumoylation sites was mutated to arginine (ΔSumo-PGRMC1-Flag), thus eliminating the ability of the ΔSumo-PGRMC1-Flag fusion protein to be sumoylated. When SIGCs are transfected with either ΔSUMO-PGRMC1-Flag or wild-type PGRMC1-Flag and placed under serum-free conditions for 5 h, fewer ΔSUMO-PGRMC1-Flag transfected SIGCs undergo apoptosis (i.e., 29 ± 3%) compared to those cells that express wild type-PGRMC1-Flag (50 ± 5% apoptotic cells, n = 4, p < 0.01). Importantly, the percentage of ΔSUMO-PGRMC1-Flag transfected SIGCs that undergo apoptosis is similar to the percentage of wild type-PGRMC1-Flag transfected cells observed after treatment with P4 (i.e., 25 ± 4%; Peluso, unpublished observations).
Although it is not known whether all three sumoylation sites are functional, the fact that mutating all of the putative sumoylation sites enhances the ability of PGRMC1 to maintain SIGC viability is consistent with the concept that sumoylation plays an important role in modulating PGRMC1's actions. The precise role that sumoylation of PGRMC1 plays is under investigation. Interestingly, ΔSUMO-PGRMC1-Flag tends to be localized in the cytoplasm compared to wild-type PGRMC1-Flag, which is more equally distributed between the membrane/cytoplasmic and nuclear fractions (Peluso, unpublished observation). Given that PGRMC1-GFP without sumoylation sites binds more PAIRBP1 than wild-type PGRMC1-GFP, it is possible that the ΔSUMO-PGRMC1 remains tethered to PAIRBP1 in the cytoplasm. This putative enhanced interaction between PAIRBP1 and ΔSUMO-PGRMC1 may account for the anti-apoptotic effects of ΔSUMO-PGRMC1, since PAIRBP1-PGRMC1 interaction is essential for cell survival (Peluso et al.,
In addition, sumoylation is an important posttranslational modification that often promotes the transport of a protein from cytoplasmic to the nucleus (Geiss-Friedlander and Melchior,
PGRMC1's nuclear localization and genomic action
Clearly, PGRMC1 sumoylation is an important event in the P4-PGRMC1 signal cascade but this signal cascade has additional components. This is evident because P4's ability to prevent apoptosis requires RNA synthesis (Peluso et al.,
To validate this transcription factor screen, filter and Tcf/Lef luciferase reporter assays were used and these assays confirmed that P4 decreases Tcf/Lef activity. Further, P4's ability to suppress Tcf/Lef luciferase reporter activity is PGRMC1-dependent because PGRMC1 siRNA attenuates P4's ability to suppress Tcf/Lef activity.
Interestingly, when PGRMC1 levels are increased by forcing the expression of PGRMC1-Flag fusion protein, Tcf/Lef activity is increased by 2-fold and P4 suppresses the increased Tcf/Lef luciferase reporter activity induced by the PGRMC1-Flag (Peluso et al.,
A putative model of the P4-PGRMC1 signal transduction pathway and entry into the cell cycle
Based on these studies, a preliminary mechanism can be outlined that defines the P4-PGRMC1 signaling cascade in the context of ovarian follicle development (Figure 2C). This mechanism assumes that (1) the source of P4 is from either within individual granulosa cells, secreted from granulosa cells within an individual follicle or secreted from adjacent ovarian follicles or corpora lutea, (2) regardless of the source, P4 acts on PGRMC1 that is present at or near the plasma membrane and within the nucleus, and (3) P4's actions are dose-dependent. With these assumptions in mind, it is proposed that this signal transduction cascade is initiated by P4-binding to PGRMC1 at or near the plasma membrane. This binding occurs at relatively low P4 concentrations since the Kd for PGRMC1-binding to P4 is between 10 and 40 nM (Peluso et al.,
Once in the nucleus, it is proposed that PGRMC1 interacts with Tcf/Lef-binding sites that are within the promoters of early immediate genes. PGRMC1 in the presence of mitogenic stimuli enhances Tcf/Lef activity, which in turn stimulates the expression of early immediate genes (e.g., c-myc and c-jun) and subsequently induces the granulosa cell to enter the cell cycle. This mechanism is consistent with the observations that in growing follicles PGRMC1 is localized to the nucleus of about 30% of granulosa cells, which is the same as the percentage of granulosa cells that are in the S-phase of the cell cycle (Pedersen,
While this proposed mechanism is incomplete at present, it is proposed to provide a framework for future studies on P4's action in granulosa cells. Since PGRMC1 is expressed in numerous tissues as outlined in several of the reviews in this issue, it is also hoped that this putative mechanism will served as a guide to help elucidate the mechanism of PGRMC1's action in other tissues.
Conflict of interest statement
The author was awarded a patent on non-genomic regulators of progesterone's action.
Statements
Acknowledgments
The author would like to thank those individuals that were involved in conducting the unpublished studies discussed in this review. They include Xiufang Liu and Daniel Griffin, MD who conducted the culture studies and Dr. Jim Pru who did the studies involving the conditional PGRMC1 knockout mice. The author would also like to acknowledge the grant support from the National Institute of Child Health and Development (RO1 HD052740 and ARRA 3RO1 HD052740-03S1 awarded to author and R21 RR030264-01 awarded to the Dr. Jim Pru and the author).
Conflict of interest
The author was awarded a patent on non-genomic regulators of progesterone's action.
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Summary
Keywords
progesterone, ovary, mitosis, apoptosis, progesterone receptor membrane component 1
Citation
Peluso JJ (2013) Progesterone receptor membrane component 1 and its role in ovarian follicle growth. Front. Neurosci. 7:99. doi: 10.3389/fnins.2013.00099
Received
23 April 2013
Accepted
22 May 2013
Published
13 June 2013
Volume
7 - 2013
Edited by
Sandra L. Petersen, University of Massachusetts Amherst, USA
Reviewed by
T. Rajendra Kumar, University of Kansas Medical Center, USA; Alberto Maria Luciano, University of Milan, Italy
Copyright
© 2013 Peluso.
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.
*Correspondence: John J. Peluso, Department of Cell Biology, University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030, USA e-mail: peluso@nso2.uchc.edu
This article was submitted to Frontiers in Neuroendocrine Science, a specialty of Frontiers in Neuroscience.
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