In the present study, we determined the peak expression of S100P during the implantation window in terms of both the mRNA and protein levels, which was approximately 100 times high as those in the other phases of endometrium during the menstrual cycle. On the one hand, it suggests that S100P could be chosen as a unique biomarker of receptive endometrium. Moreover, the most prominent S100P signal in the MSE were located in the glands, which was not in accordance with the expression pattern found by Tong et al. , who showed that S100P is distributed in the nuclei and observed more intense S100P immune staining in the stroma compared to that in the glands. However, our results were similar to those for the gastric mucosa reported by Parkkila et al., who reported that the highest immunoreactivity was located in the surface epithelial cells and glands , suggesting that S100P has the potential to be secreted into the uterine cavity and detected in the uterine fluid as a non-invasive biomarker of endometrial receptivity. On the other hand, it suggests that S100P could be functionally related to the establishment and withdrawal of endometrial receptivity, the secretory activity of the endometrium and the maintenance of the inter-uterine environment. It has been reported that the expression of S100P increased 37-fold in stromal cells after their co-culture with trophoblast cells , which proved that S100P might be involved in interactions at the maternal-fetal interface.
Previous studies have shown that S100P can be localized in the cytoplasm and/or the nucleus of a wide range of cells, and as a secretory protein, it can also be secreted to extracellular regions [13, 31, 33]. In the present study, we observed that S100P is expressed in the nucleus as well as in the cytoplasm of endometrial cells and that it largely accumulated in the cytoplasm of EECs and ESCs. The cellular location of S100P has been reported to correspond to its molecular functions as follows: the intracellular component of S100P could interact with Ezrin, a multi-domain link protein of the membrane-cytoskeleton, thus playing a role in cell differentiation, adhesion, and migration ; S100P in the cytoplasm could bind to CacyBP/SIP, a component of the ubiquitin pathway, which is involved in the degradation of the cell signaling molecule ß-catenin ; the extracellular-soluble form of S100P could function as a ligand of the RAGE receptor to modulate cell proliferation and survival via activation of MAP kinase and NF-kappa B pathway ; and the component of S100P in the nucleus might bind to S100PBP (S100P binding protein), the exact role of which remains unclear.
It has been shown that cell proliferation, adhesion and motility are all vital to endometrial receptivity and embryo implantation, in which ß-catenin , MAP kinase  and the NF-kappa B pathway  play an important role. Ezrin, which regulates the reorganization of cytoskeleton-membrane connections , has also been shown to be involved. These findings suggest that S100P might act as a key linker participating in the formation of endometrial receptivity and embryo implantation.
Elucidating the regulation of S100P is an important step towards understanding the biological significance of and proposing strategies for targeted S100P modulation . Recent studies have shown that DNA methylation , IL-6 , bone morphogenic protein , prostaglandin E (PGE)/EP4 , glucocorticoid , and non-steroidal anti-inflammatory drugs  could regulate S100P expression during tumor progression. Steroid hormones were also reported to play a role in the expression of S100P, including the synthetic androgen R1881, which regulates the expression of S100P in prostate cancer , as well as P4 and six other types of clinically relevant synthetic progestins, which up-regulate S100P in progesterone receptor (PR)-positive cell lines of breast cancer .
In the present study, we verified that P4 can up-regulate the expression of S100P in primary endometrial cells. The results of the time course experiments showed a parabolic shape, with a peak at 48 h, after which a sharp drop was observed. The dose-dependent experiments showed that S100P expression increased with increasing P4 concentrations within a certain range. This response explained the special temporal expression of S100P during the window of implantation resulting from the influence of P4. The level of progesterone secretion may have peaked at the mid-secretory phase in concert with the peak value of S100P expression, and the expression of S100P may have declined significantly with the time extension of progesterone at the late-secretory phase, although the concentration of progesterone decreased little.