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RESEARCH |
Obstetrics and Gynaecology, Department of Reproductive and Developmental Sciences, University of Edinburgh, Royal Infirmary of Edinburgh Little France, 49 Little France Crescent, Old Dalkeith Road, Edinburgh EH16 4SB, UK
Correspondence should be addressed to W C Duncan; Email: W.C.Duncan{at}ed.ac.uk
| Abstract |
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-hydroxylase positive) and granulosalutein (aromatase positive) express progesterone receptors, as do stromal fibroblasts (vimentin positive, fibroblast antigen positive). Vascular endothelial cells (CD31 positive), pericytes (
-smooth muscle actin positive), macrophages (CD68 positive) and fibroblasts within the central clot do not express nuclear progesterone receptors. Progesterone is a candidate messenger molecule for the effects of hCG on the matrix metalloproteinase-producing stromal fibroblasts. Some of the effects of hCG on steroidogenic cells may be mediated by progesterone, but its effects on blood vessels and macrophages require alternate paracrine signalling mechanisms. In addition, there appears to be at least two fibroblast populations in the corpus luteum. | Introduction |
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These studies have revealed that the influx of macrophages into the corpus luteum in the late luteal phase is not seen in the presence of hCG (Duncan et al. 1998a). They have also demonstrated that matrix metalloproteinases (MMPs), notably MMP-2, increase in the late luteal phase and that this increase is not seen in the presence of hCG (Duncan et al. 1998b). In addition, mRNA in situ hybridisation identified the main cellular source of MMP-2 in the corpus luteum to be stromal fibroblasts (Duncan et al. 1998b). A further study showed an increase in endothelial cell proliferation, and endothelial cell and pericyte area in the rescued corpus luteum of simulated early pregnancy (Wulff et al. 2001a). It was clear that hCG had marked effects on the corpus luteum during luteal rescue.
hCG causes its effects by binding to, and activating, the luteinising hormone (LH) receptor, a seven transmembrane-domain G protein-coupled glycoprotein receptor. The LH/hCG receptor is localised to the steroidogenic cells of the corpus luteum (Nishimori et al. 1995, Duncan et al. 1996a). However, it was also clear that neither macrophages, the stromal fibroblasts that are the main source of MMP-2 expression, nor blood vessels express LH/hCG receptors that are detectable by mRNA in situ hybridisation (Duncan et al. 1996a, 1998a,Duncan et al. b). These receptors are present on the steroidogenic cells of the corpus luteum. This led us to hypothesise that macrophage influx and MMP-2 expression are influenced by steroidogenic cell products (Duncan 2000).
One strategy to identify putative paracrine molecules is to investigate factors synthesised in steroidogenic cells that have receptors on other cell types in the corpus luteum. It has been known for some years now that the corpus luteum of a variety of species including humans express genomic progesterone receptors (Hild-Petito et al. 1988, Suzuki et al. 1994). The possible paracrine effects of progesterone in the corpus luteum are still not clear however (Rothchild 1996, Vega & Devoto 1997, Stouffer 2003). As progesterone is hCG dependent, increases during luteal rescue and falls in the late luteal phase, it is an excellent candidate molecule for some of the effects of hCG during luteal rescue (Duncan 2000). This study aimed to identify the cell types capable of responding to progesterone in the corpus luteum by co-localising nuclear genomic progesterone receptors with specific cellular markers using dual-staining immunohistochemistry.
| Materials and Methods |
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-hydroxylase was kindly provided by Professor M R Waterman (Vanderbilt University, Nashville, TN, USA). The polyclonal rabbit antibody to human placental type I 3ß-hydroxysteroid dehydrogenase (3ß-HSD) was supplied by Professor V Luu-The (CHUL Research Centre, Quebec, Canada). The mouse monoclonal antibody to aromatase was provided by Professor E Simpson (PHIMR, Clayton, Victoria, Australia). The mouse monoclonal antibodies to CD68, CD31, vimentin and
-smooth muscle actin (
-SMA) were all obtained from Dako Ltd (Ely, Cambs, UK). The mouse monoclonal antibody to human fibroblast antigen was obtained from Oncogene Research (Boston, MA, USA). Secondary antibodies and the detection systems were either obtained from Vector Laboratories Ltd (Southgate, Peterborough, Cambs, UK) or from Dako Ltd.
Collection of tissue
Human corpora lutea were collected at the time of surgery from women undergoing hysterectomy for benign conditions (n = 12). All women had regular cycles and had not received any form of hormonal treatment in the 3 months prior to taking part in the study. Prior to surgery the women collected a daily early morning urine sample and the corpora lutea were dated on the basis of the urinary LH surge as described previously (Duncan et al. 1996a, Duncan 2000). In this study, five corpora lutea were classified as early luteal (LH + 1 to LH + 5), four as mid-luteal (LH + 6 to LH + 10) and three as late luteal (LH + 11 to LH + 14). In all cases an endometrial biopsy corroborated our urinary-based tissue-dating system. The collection of human corpora lutea, after informed consent, was approved by the reproductive medicine subcommittee of the Lothian research ethics committee.
Immunohistochemistry
Tissues were fixed in 4% paraformaldehyde for 24 h and embedded in paraffin wax for subsequent immunohistochemical examination. Sections (5 µm) were cut onto poly-L-lysine-coated slides, dewaxed and rehydrated into distilled water. Antigen retrieval was used as described in Table 1
. When proteolytic digestion was required, sections were incubated for 30 min at 37 °C in 0.1% trypsin with 0.1% calcium chloride buffered with 0.25 M TrisHCl. When microwave retrieval was required, slides were placed in a 0.01 M sodium citrate buffer, pH6, microwaved at 450 W for two rounds of 5 min and left to stand for 20 min.
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For peroxidase detection systems, ABC linked to HRP was added for 1 h and antibody binding was visualised using DAB to give a brown colour or DAB with nickel to give a black end product. In some cases, haematoxylin counterstaining was used to aid visualisation of the section. For the AP detection system, ABC-AP was added for 1 h and antibody binding was visualised with NBT to give a blue colour. When the APAAP detection method was used the non-biotinylated secondary was added for 30 min followed by the tertiary APAAP complex for a further 30 min. NBT was used to colour regions of antibody binding blue.
Dual staining immunohistochemistry involved detecting the anti-progesterone receptor antibody first followed by detection of the secondary antibody. Antigen retrieval was used only for the first primary antibody and the protocol adapted accordingly depending on which antibodies were being studied (Table 1
). The slides were dehydrated and mounted with pertex mounting medium before analysis. The optimal conditions for each antibody and antibody combination were detected in many trial runs. All sections were studied by two independent observers by high power microscopy. The presence of nuclear progesterone receptor staining in the same cells as the second primary antibody to various cytoplasmic components was carefully noted.
| Results |
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-hydroxylase (Fig. 2b
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-SMA. Dual-staining immunohistochemistry revealed that pericytes did not express genomic progesterone receptors (Fig. 3a
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Macrophages in the corpus luteum
Macrophages were identified by immunostaining for CD68 (Fig. 3h
). Macrophages were clearly visible in the stroma and within the steroidogenic cell layers. It was clear on dual staining of the macrophages with progesterone receptors that macrophages did not express immuno-detectable genomic progesterone receptors (Fig. 3i
). This was true in all areas of each section at all stages of the luteal phase (Table 2
).
| Discussion |
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We have found that some luteal fibroblasts express progesterone receptors. As the staining intensity appeared to be consistent throughout, these fibroblasts are able to respond to progesterone at all stages of the luteal phase. Fibroblasts have roles during luteolysis and luteal rescue. The expression of MMPs, notably MMP-2, is increased during luteolysis and inhibited by hCG during luteal rescue (Duncan et al. 1998b). Although several cell types may express MMPs, it is clear that the major source of both MMP-2 and MMP-1 in the human corpus luteum is fibroblasts (Duncan et al. 1998b). It is now clear that both stromal and perivascular fibroblasts express progesterone receptors. Progesterone has a regulatory role in the expression of MMPs in the endometrium (Salamonsen et al. 1997, Curry & Osteen 2003) and progesterone withdrawal up-regulates MMP-1 in the corpus luteum (Stouffer 2003). Studies on the effect of progesterone on ovarian fibroblast gene expression in vitro will help determine whether progesterone is involved in the local control of fibroblast function in the corpus luteum.
We have also found that steroidogenic cells, both theca-lutein and granulosa-lutein cells, express progesterone receptors. In our study, immunostaining was detected in steroidogenic cells at all stages of the luteal phase. However, of all the cell types examined, immunostaining of the steroidogenic cells was the most variable across the luteal phase. These dual-staining studies were designed to investigate localisation rather than variation in progesterone receptor immunostaining. However, previous reports have suggested a reduction in progesterone receptor immunostaining in the late luteal phase (Suzuki et al. 1994, Hild-Petito & Fazleabas 1997). As steroidogenic cell immunostaining was most variable, it is likely that these reports reflect changes in the immunostaining of the steroidogenic cell compartment. Our detection of progesterone receptor immunostaining in granulosa-lutein cells confirms the observations of Hild-Petito & Fazleabas (1997) in the primate corpus luteum. Therefore, although progesterone may function as a paracrine molecule in the corpus luteum, it may also have intracrine effects, as these are the cells responsible for progesterone synthesis. Therefore the steroidogenic cells have receptors to two potentially trophic hormones (hCG and progesterone) and dissecting the independent effect of each is difficult. It has been suggested that progesterone itself is involved in its own synthesis (Rothchild 1996).
There are certainly some effects of progesterone on luteal steroidogenic cells. Tissue inhibitor of matrix metalloproteinases-1 (TIMP-1) is a major product of granulosa-lutein cells (Duncan et al. 1996b) and withdrawal of gonadotrophins results in its reduction (Duncan et al. 1996c). Interestingly, withdrawal of progesterone using RU486 in luteinised granulosa cell cultures also inhibited TIMP-1 expression (Morgan et al. 1994) in the presence of constant gonadotrophin concentrations. Progesterone can directly promote luteinised granulosa cell survival (Makrigiannakis et al. 2000) and influence the expression of LH receptors (Jones et al. 1992) and steroidogenic enzymes (Chaffin et al. 2000). It is therefore possible that progesterone withdrawal has a direct effect on steroidogenic cell function and further studies are required.
We did not detect progesterone receptors on macrophages at any stage of the luteal phase. Immune cells are important mediators of luteal and ovarian function (Brännström & Norman 1993, Bukulmez & Arici 2000). Macrophages increase during luteolysis in many different species (Naftalin et al. 1997) and macrophage accumulation is not present during simulated early pregnancy (Duncan et al. 1998a). The hCG-responsive luteal cell product affecting macrophage migration does not therefore appear to be progesterone. Some immune cells, however, do express steroid receptors. It has recently been shown that uterine natural killer cells have genomic receptors to some steroid hormones (Henderson et al. 2003), but interestingly not progesterone. Another molecule, such as monocyte chemotactic protein-1 (MCP-1), must be involved in the regulation of macrophage influx (Penny 2000). MCP-1 does not appear to be a granulosa luteal cell product as it has a perivascular localisation in the human corpus luteum (Senturk et al. 1998). It is possible that it is expressed by perivascular fibroblasts that express genomic progesterone receptors and progesterone again is implicated as a potential regulator of macrophage influx. Further functional studies are clearly required to dissect this pathway.
The endothelial cells and pericytes of blood vessels did not express progesterone receptors at any stages of the luteal phase. Luteal rescue with hCG results in vascular changes involving an increase in endothelial cell and pericyte cell area (Wulff et al. 2001a) but these cells do not express LH/hCG receptors (Duncan et al. 1996a). However, they do have receptors to vascular endothelial growth factor (VEGF), a granulosa-luteal cell product (Wulff et al. 2000), and inhibition of VEGF in vivo significantly inhibits the development of the luteal vasculature (Wulff et al. 2001b). Steroidogenic cell VEGF expression is increased by hCG during luteal rescue (Wulff et al. 2000). In vitro, luteinised granulosa cells respond to hCG by secreting both progesterone and VEGF (Christenson & Stouffer 1997). It seems likely that the major hCG-regulated paracrine regulator of the luteal vasculature is VEGF. However, the cellular sources of VEGF in the corpus luteum express progesterone receptors. Whether progesterone has a local role in the expression of VEGF remains to be determined.
We have demonstrated that there are at least two different type of fibroblast in the corpus luteum: those that express genomic progesterone receptors and those that do not. The contribution of fibroblasts in different tissues has been a neglected area of study. What is clear is that fibroblasts from different tissues have different characteristics (Fries et al. 1994). In addition, different types of fibroblasts within tissues may have different phenotypes (Grupp & Muller 1999). As perivascular and stromal fibroblasts have potentially important roles in the development and regression of the corpus luteum, further work on ovarian fibroblasts is required.
It has been appreciated for a long time that not all the effects of progesterone can be explained by genomic progesterone receptors (Bramley 2003). Recently, two potential related membrane progesterone receptors have been cloned and sequenced (Zhu et al. 2003). These have a wide expression profile. It is still not known whether these are expressed on the various cell types in the human corpus luteum and these studies are underway. However, it is known that the corpus luteum does have membrane-linked progesterone-binding activity (Bramley et al. 2002). The role of these receptors is not clear but their presence offers further potentials for progesterone as a paracrine signalling molecule. Clearly, if progesterone has effects on cells in vitro, careful experiments with specific inhibitors are required to dissect possible membrane and genomic actions.
What is clear from these studies is that progesterone receptors are found in the same cell types at all stages of the luteal phase. Although it might have been expected that progesterone receptor immunostaining would be reduced in the late luteal phase (Suzuki et al. 1994), these studies confirm that all cell types with progesterone receptors maintain the potential to respond to progesterone in the late luteal phase. Indeed, these studies were focussed on the localisation of progesterone receptors rather than subtle changes in dual-staining intensities. Although we have optimised our sensitive assay to give clear, remarkably consistent positive or negative staining, we are aware that there may be genomic progesterone receptors in cells at low levels that cannot be detected by immunohistochemistry, or changes in expression levels that cannot be detected using dual-staining protocols.
Although it has been known for many years that the corpus luteum expresses genomic receptors to the progesterone it produces, the role of these receptors is not clear (Stouffer 2003). Inhibition of progesterone in vivo has endocrine and paracrine effects (Duffy et al. 1994) and these are difficult to dissect and interpret (Duffy & Stouffer 1997, Stouffer 2003). There are parallels between luteal remodelling and endometrial remodelling (Curry & Osteen 2003) and changing concentrations of progesterone have a clearly defined role in the later. It is likely that progesterone has some parallel paracrine roles in the corpus luteum. Now that we know the cell types to focus on, further interventional and in vitro studies are required to dissect and characterise these roles further.
| Acknowledgements |
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| Footnotes |
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| References |
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