Abstract
Human endometrium resists embryo implantation except during the 'window of receptivity'. A change in endometrial gene expression is required for the development of receptivity. Uterine calbindin-D28k (CaBP-28k) is involved in the regulation of endometrial receptivity by intracellular Ca2+. Currently, this protein is known to be mainly expressed in brain, kidneys, and pancreas, but potential role(s) of CaBP-28k in the human uterus during the menstrual cycle remain to be clarified. Thus, in this study we demonstrated the expression of CaBP-28k in the human endometrium in distinct menstrual phases. During the human menstrual cycle, uterine expression levels of CaBP-28k mRNA and protein increased in the proliferative phase and fluctuated in these tissues, compared with that observed in other phases. We assessed the effects of two sex-steroid hormones, 17beta-estradiol (E2) and progesterone (P4), on the expression of CaBP-28k in Ishikawa cells. A significant increase in the expression of CaBP-28k mRNA was observed at the concentrations of E2 (10(-9 to -7) M). In addition, spatial expression of CaBP-28k protein was detected by immunohistochemistry. CaBP-28k was abundantly localized in the cytoplasm of the luminal and glandular epithelial cells during the proliferative phases (early-, mid-, late-) and early-secretory phase of menstrual cycle. Taken together, these results indicate that CaBP-28k, a uterine calcium binding protein, is abundantly expressed in the human endometrium, suggesting that uterine expression of CaBP-28k may be involved in reproductive function during the human menstrual cycle.
Highlights
Intracellular calcium binding proteins are critical for regulating the availability of calcium ions (Ca2+) within cells
Pattern of endometrial calbindin-D28k expression To investigate the expression of endometrial CaBP-28k during the menstrual cycle, we divided the endometrial tissues into seven groups [menstrual phase, proliferative phase, and secretory phase according to the most recent menstrual period, and histology according to the criteria of Noyes et al [32]
Effect of sex-steroid hormone E2 and P4 on regulation of calbindin-D28k expression in Ishikawa cells We assessed the effects of sex-steroid hormones E2 and P4 on the regulation of CaBP-28k in Ishikawa cells
Summary
Intracellular calcium binding proteins (calbindins) are critical for regulating the availability of calcium ions (Ca2+) within cells. There are two types of cytosolic calbindins, calbindin-D9k (CaBP-9k) and calbindin-D28k (CaBP-28k), which are cytosolic proteins differentially regulated by steroid hormones in the uterus [1,2]. In addition to its traditional role in extracellular calcium homeostasis [3], vitamin D influences a broad range of cellular events, ranging from oncogene expression [4] and immunoregulation [5] to cellular differentiation [6,7] and intracellular calcium metabolism [8]. The expression of CaBP-9k mRNA in the mouse uterus was significantly increased by treatment with P4 and E2 plus P4, but not by E2 alone [18,19]. Uterine expression of the larger calbindin-D28k increased by the addition of testosterone [15]
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