Implantation of the embryo is an initial step in the establishment of a successful pregnancy, and involves a complex sequence of signaling events. A large number of molecular mediators have been identified and postulated to be involved in this early fetal–maternal interaction, including a variety of inter-related molecules such as adhesion molecules, cytokines, growth factors and lipids . Several benign gynecological disorders, including hydrosalpinx, may hinder implantation by influencing endometrial receptivity, and salpingectomy or ligation of the salpinx before IVF can increase the success rate of IVF . The presence of hydrosalpinx may affect the expression of molecules which mediate endometrial receptivity in the endometrium, such as αvβ3 integrin and leukemia inhibitory factor (LIF) ; however, the exact mechanisms by which a hydrosalpinx affects implantation are unclear and require further research.
CFTR plays an important role in the implantation process . As an apical Cl− channel, CFTR regulates Cl− secretion and thus fluid volume. Additionally, CFTR is co-expressed with the epithelial NaC channel (ENaC), which has been proposed to be the major mechanism regulating uterine fluid absorption. Upregulation of ENaC and downregulation of CFTR have been observed in the endometrial epithelia of mice during preimplantation, providing a molecular basis for the ‘closure’ of the uterine lumen observed on the day of implantation. Interestingly, in this study we observed that the expression level of CFTR was significantly lower in the endometrium of infertile patients with hydrosalpinx. However, a previous study conducted in mice suggested that LPS produced by uterine infection with C. trachomatis induced upregulation of CFTR and abnormal fluid accumulation in the mouse uterus at diestrus . There are several reasons which may lead to the inconsistencies between these studies. Firstly, different species were examined in each study. Secondly, in this study , the endometrial epithelia were freshly isolated from mice, and CFTR expression was analyzed only 24 hours after C. trachomatis LPS inoculation; therefore, it is probable that the uteri were still in the acute inflammatory phase. In contrast, the patients in our study had a history of hydrosalpinx ranging from 1 to 13 years; therefore, the disease had progressed to the chronic phase. It is possible that the decreased CFTR expression observed in infertile patients with hydrosalpinx in our study may be the result of a compensatory effect. Further studies are required to determine the expression and regulation of CFTR in the endometrium of infertile patients with hydrosalpinx.
NF KappaB is a family of transcription factors involved inflammatory and immune responses, which control cytokine and adhesion molecule gene expression in a variety of cell types, including the human endometrium. Similarly to CFTR, a non-significant trend toward increased NF KappaB mRNA and protein were observed throughout the menstrual cycle in this study. However, NF KappaB expression was mainly distributed in the cytoplasm. Nuclear expression of NF KappaB is associated with the activation of NF KappaB in pathological processes . However, nuclear translocation of NF KappaB is a transient, rapid event. It is possible that the tissue sampling method or immunohistochemical techniques used in this study were not suitable for detecting subtle changes in NF KappaB activation; therefore, the lack of detectable nuclear NF KappaB expression in this study may not reflect a lack of NF KappaB activation .
More importantly, our study demonstrates that NF KappaB mRNA and protein expression are increased in the endometrium of infertile patients with hydrosalpinx, which has not been previously described. Hydrosalpinx is the result of chronic pelvic inflammatory disease and is characterized by fluid accumulation in the fallopian tube. NF KappaB is one of the most important regulators of pro-inflammatory gene expression, and the synthesis of a variety of cytokines, such as interleukia ( IL)-6 and LIF, are mediated by NF KappaB [17–19]. The presence of high concentrations of cytokines, including (IL)-8, tumor necrosis factor (TNF)-α and LIF, have been detected in hydrosalpingeal fluid . Hydrosalpingeal fluid can flow into the uterine cavity, where the abundant cytokines may stimulate production of NF KappaB in the endometrium. In turn, increased NF KappaB expression in the endometrium may negatively influence implantation. For example, overexpression of NF KappaB in the endometrium of infertile patients with hydrosalpinx may lead to the recruitment of excessive numbers of inflammatory cells and stimulate the production of proinflammatory mediators such as IL-1, IL-6, IL-8 and TNF-α, thus leading to excessive inflammatory and immune responses in the endometrium during the peri-implantation window, which may inhibit or reduce embryo implantation. However, the exact mechanisms by which NF KappaB affects implantation in women with hydrosalpinx require further investigation.
MUC1, an important member of the mucin family, is a highly glycosylated macromolecule. When at high levels expressed on the cell surface, MUC1 can interfere with cellular adhesion via a steric hindrance phenomenon. Therefore, MUC1 may prevent adherence of the blastocyst to the endometrium by acting as an anti-adhesion molecule. The expression of MUC1 normally reduces during implantation in other species ; however, MUC1 is upregulated in the human endometrium during the peri-implantation period . Furthermore, both MUC1 mRNA and protein expression increase several-fold from the proliferative phase to the mid-secretory phase . It has been suggested that the repellent effect of MUC-1 could be of importance in guiding the blastocyst to the precise area fittest for implantation. Significantly reduced expression of MUC1 can also affect the embryo selection function of endometrium, subsequently increasing the miscarriage rate or reducing the implantation rate . MUC1, an important implantation-related immune factor in the endometrium, is also expressed on the surface of T cells where it acts as an immunomodulator. In this study, we observed that MUC1 was significantly downregulated in the endometrium of infertile patients with hydrosalpinx. This result is partially in agreement with research by Li et al. , who reported that MUC1 expression was significantly reduced in the endometrium during the peri-implantation in patients with hydrosalpinx. Low levels of MUC1 could damage the embryo selection function of the endometrium in infertile patients with hydrosalpinx, thus increasing the miscarriage rate or reducing the implantation rate . Furthermore, our study also indicated a trend towards increased MUC1 mRNA and protein expression throughout the menstrual cycle, suggesting that MUC1 is hormonally regulated during the menstrual cycle.
Interestingly, we observed a positive correlation between CFTR and MUC1 mRNA and a negative correlation between CFTR and NF KappaB mRNA in the endometrium of infertile women. This indicates that CFTR can affect implantation in the endometrium not only by functioning as an anion channel, but also by acting as a molecular regulator. Previous studies have suggested a relationship between the expression of CFTR and MUC1 or NF KappaB [7, 8, 24]. Kuver et al. described lower MUC1 expression in cftr (−/−) cells compared to wild-type cells . Hunter et al. indicated that repression of NF KappaB signaling is normally mediated by CFTR (7). Moreover, Chen et al. observed a similar negative correlation between CFTR and NF KappaB expression in lung cells, and proposed that a functional negative regulation loop exists between these molecules in the lung . Although the data is limited, we tentatively suggest that CFTR may positively regulate MUC1 in the endometrium, and negatively regulate NF KappaB (and possibly other factors) in the endometrium, via unknown mechanisms. It is possible that CFTR, as a chloride channel, may affect the chloride concentration in the cell, which may activate signaling pathways such as the WNK-OSR1/SPAK pathway. CFTR is also known to be either directly or indirectly involved in bicarbonate transport, which is important for pH regulation . The ability of CFTR to alter pH and ion concentrations suggests that it plays a key role in the maintenance of cellular homoeostasis. Decreased CFTR expression may disrupt the cellular microenvironment and thus affect a number other factors; however, the possible role of CFTR in these processes and its ability to regulate MUC1 and NF KappaB in the endometrium require further investigation.
This study has some limitations, as we did not limit patient selection to the implantation window. The precise expression levels and role of CFTR, MUC1 and NF KappaB during the implantation window in infertile patients with hydrosalpinx remain to be explored.