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RESEARCH |
Colleges of 1 Life Sciences and 2 Animal Medicine, Northeast Agricultural University, Harbin 150030, China
Correspondence should be addressed to Zeng-Ming Yang; Email: zmyang{at}mail.neau.edu.cn
| Abstract |
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| Introduction |
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PGE2 synthase (PGES) is a terminal prostanoid synthase that can enzymatically convert the cyclooxygenase product PGH2 to PGE2. There are two isoforms of PGES, microsomal PGES (mPGES) and cytosolic PGES (cPGES). cPGES is constitutively expressed in a wide variety of cells and tissues, and predominantly linked with COX-1 to promote the immediate response, during which relatively high concentrations of arachidonic acid are released in a short period (Tanioka et al. 2000). mPGES, a membrane-associated and inducible perinuclear enzyme with glutathione-dependent activity, is expressed in a variety of tissues, including prostate, testes, and small intestine (Jakobsson et al. 1999). mPGES is preferentially coupled with the inducible COX-2 to promote delayed PGE2 generation and, if COX-2 already exists in cells, it also regulates immediate PGE2 generation (Murakami et al. 2000).
The expression and regulation of the mPGES gene in mouse uterus was recently characterized (Ni et al. 2002). The strong mPGES expression in the implantation site and decidual cells suggests that mPGES might play an important role during implantation and, more importantly, decidualization (Ni et al. 2002). Recently, it has been found that COX-2 mRNA and protein and mPGES mRNA are differentially expressed and regulated in bovine endometrium during the estrous cycle. The COX-2 and mPGES pathway is responsible for the endometrial production of PGE2 in the bovine endometrium during the estrous cycle (Arosh et al. 2002). Expression of mPGE synthase was localized in glandular epithelial and endothelial cells in both basalis and functionalis regions of the human endometrium. By contrast, stromal staining was predominantly localized in the functionalis layer (Milne et al. 2001).
The expression of cPGES in the uterus was reported only in mice. There was a strong level of cPGES mRNA signal in the stromal cells at the implantation site on day 5 of pregnancy, while cPGES immunostaining was strongly detected in the luminal epithelium. The signals for both cPGES mRNA and immunostaining were strongly detected in the decidualized cells from days 68 of pregnancy. These data suggest that cPGES may play an important role during implantation and decidualization (Ni et al. 2003).
In spite of the extensive studies reported on COX expression, regulation and potential function in the reproductive processes of other species, relatively little is known on COX expression and regulation in the primate uterus. Kim et al.(1999) have reported the expression of COX-1 and COX-2 during the menstrual cycle and pregnancy in the baboon. In man, COX-2 was localized in the uterine glandular epithelium by immunohistochemistry throughout the menstrual cycle and in early pregnancy (Jones et al. 1997). However, COX expression during the menstrual cycle is still not defined in monkey. The aim of this study was to examine immunohistochemically the expression of COX-1, COX-2, mPGES and cPGES proteins in monkey endometrium during the menstrual cycle.
| Materials and Methods |
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Immunohistochemistry
The dorsal portion of monkey uterus was immediately cut into small pieces, fixed in Bouins solution for 24 h, dehydrated with graded ethanol and embedded in paraffin. Paraffin sections were cut, deparaffinized and hydrated gradually in PBS. For the sections used for COX-2 immunohistostaining, antigen retrieval was performed in 0.1 mM EDTA (pH 9.2) at 88 °C water bath for 10 min. Nonspecific binding was blocked with 10% horse serum in PBS at 37 °C for 1 h. The sections were incubated with goat anti-COX-1 immunoglobulin G (IgG) (1:100, Santa Cruz Biotechnology, Santa Cruz, CA, USA) or goat anti-COX-2 IgG (1:300, Santa Cruz) at 24 °C for 90 min. Normal goat IgG was used to replace each primary antibody for a negative control. The sections were then incubated with biotinylated rabbit anti-goat antibody followed by an avidinalkaline phosphatase complex and Vector Red according to the manufacturers protocol (Vectastain ABC-AP kit, Vector Laboratories, Burlingame, CA, USA). For the mPGES and cPGES immunostaining, the sections were incubated with rabbit anti-mPGES (1:200, Cayman Chemical, Ann Arbor, MI, USA) or rabbit anti-cPGES (1:400, Cayman Chemical) overnight at 4 °C. The sections were then incubated with biotinylated goat anti-rabbit antibody followed by an avidinalkaline phosphatase complex and Vector Red. Vector Red was visualized as a red color. Endogenous alkaline phosphatase activity was inhibited with levamisole (Sigma). The sections were counterstained with hematoxylin and mounted.
Additionally, normal rabbit IgG was used to replace the primary antibodies for a negative control. Moreover, rabbit anti-cPGES was preincubated with the blocking peptide for cPGES (Cat no. 360150, Cayman Chemical) for 30 min prior to incubating with the sections on day 25 of the menstrual cycle. Similarly, the rabbit anti-mPGES antibody was preincubated with mPGES blocking peptide (Cat No. 360140, Cayman Chemical) for 30 min prior to incubating with the sections on day 25 of the menstrual cycle.
The degree of staining was assessed subjectively by blinded examination of the slides by two investigators. A scoring system was designated as 0 = no stain, 1 = pale stain, 2 = moderate stain, 3 = strong stain and 4 = very intense stain. At least three monkeys at each stage were used under this analysis. The means ± S.E.M. of the staining intensity for each cellular compartment were shown graphically.
| Results |
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| Discussion |
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It has been shown that cPGES is predominantly linked with COX-1 to promote the immediate response, during which relatively high concentrations of arachidonic acid are released in a short period (Tanioka et al. 2000). In our study, cPGES was colocalized with COX-1 in the luminal epithelium and glandular epithelium, and both cPGES and COX-1 were highly expressed on days 16 and 20 of the menstrual cycle. In addition, culture studies have demonstrated that both human epithelial and stromal cells can synthesize PGE2 in vitro (Smith & Kelly 1988). These data suggest that the coupling of cPGES and COX-1 in the luminal epithelium may be responsible for the synthesis of PGE2 in monkey endometrium.
In this study, mPGES immunostaining was mainly detected in the glandular epithelium on days 20 and 25 of the menstrual cycle. In human endometrium, mPGES immunoreactivity in the functionalis layer was expressed in glandular epithelial, stromal, and endothelial cells (Milne et al. 2001). mPGES is preferentially coupled with the inducible COX-2 to promote delayed PGE2 generation and, if COX-2 already exists in cells, it also regulates immediate PGE2 generation (Jakobsson et al. 1999, Murakami et al. 2000). In the present study, COX-2 immunostaining was strongly detected in the luminal epithelium and glandular epithelium on day 16, and weakly on day 20. However, mPGES immunostaining was strongly seen only in the glandular epithelium on days 20 and 25. The expression peak of COX-2 was not matched with that of mPGES. COX-2 was colocated with mPGES in the glandular epithelium only on day 20 of the menstrual cycle, which was within the potential implantation window. Nevertheless, it is important to see whether COX-2 and mPGES are highly coexpressed in the endometrium of rhesus monkey during the periimplantation period. During early pregnancy in mice, both COX-2 and mPGES were highly expressed at implantation sites on day 5 of pregnancy (Chakraborty et al. 1996, Ni et al. 2002). Furthermore, COX-2-deficient mice had multiple failures in female reproductive processes, including ovulation, fertilization, implantation and decidualization (Lim et al. 1997). Conversely, Cheng and Stewart (2003) reported that COX-2-deficient mice did not show any abnormalities in embryo implantation except for the reduction in the numbers of ovulated and fertilized eggs, and a delay in decidual growth. This difference may be caused by the different genetic backgrounds of the COX-2-deficient mice used by the two groups. The role of COX-2 during embryo implantation also needs to be further investigated.
The exact role of PGE2 in the monkey endometrium is not fully elucidated. It seems that COX-1 and cPGES are mainly responsible for the synthesis of PGE2 in the monkey endometrium during the menstrual cycle. In bovine endometrium, COX-1 mRNA and protein are not expressed on any day of the estrous cycle, whereas COX-2 mRNA, and protein and mPGES mRNA are differentially expressed and regulated in bovine endometrium during the estrous cycle. The COX-2 and mPGES pathway is responsible for the endometrial production of PGE2 in the bovine endometrium during the estrous cycle (Arosh et al. 2002). In human colon epithelial cells, overexpression of COX-2 and enhanced synthesis of PGE2 have been shown to promote the proliferation and survival of cells through inhibition of apoptosis. PGE2 has been shown to upregulate expression of antiapoptotic genes such as bcl-2 (Sheng et al. 1998). It is plausible that, in the human endometrium, PGE2 may be activating similar mechanisms that promote glandular epithelial cell proliferation and/or survival (Milne et al. 2001). PGE2 function in glandular epithelial cells may also be associated with the regulation of uterine angiogenesis. In the human endometrium, vascular endothelial growth factor expression is localized to the glandular epithelial cells throughout the menstrual cycle (Torry et al. 1996). Moreover, PGE2 has been previously shown to upregulate the expression of vascular endothelial growth factor in a number of different cell types (Harada et al. 1994, Cheng et al. 1998). Nevertheless, the availability of arachidonic acid may be a rate-limiting step in synthesizing PGE2. Cytosolic phospholipase A2
(cPLA2
) is a major provider of arachidonic acid in the COX system for PG biosynthesis. cPLA2
deficiency results in aberrant uterine spacing of embryos and deferred implantation (Song et al. 2002). It remains to determine whether arachidonic acid is provided for PGE2 synthesis through the COX-1/cPGES and/or COX-2/mPGES pathway.
In conclusion, our data suggest that the coupling of cPGES and COX-1 in the luminal epithelium may be responsible for the synthesis of PGE2 in monkey endometrium, and the coupling of mPGES and COX-2 in the glandular epithelium may be important in preparing the receptive endometrium.
| Acknowledgements |
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| Footnotes |
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| References |
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