Abstract
Previous studies indicated that calcitonin (CT), a peptide hormone involved in calcium homeostasis, is transiently expressed in the receptive rat and human endometrial epithelia within the window of implantation. Attenuation of uterine CT expression using antisense methods severely impaired implantation in the rat. The molecular pathway of CT in the pregnant uterus, however, remains unknown. In the present study, we investigated the cellular events following the binding of CT to its membrane receptors in human endometrial epithelial cell line Ishikawa. We observed that CT treatment triggers a transient rise in intracellular calcium in these cells. Most interestingly, CT treatment also led to the disappearance of E-cadherin, a critical cell adhesion molecule, from cell-cell contact sites. Blockade of intracellular calcium release by BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl) prevented the CT-induced disappearance of E-cadherin. Our studies further revealed that CT treatment markedly down-regulates the level of E-cadherin mRNA in Ishikawa cells. We also examined whether CT influences the expression of E-cadherin mRNA in intact rat uterine tissue during implantation. In pregnant rats, high levels of E-cadherin mRNA were expressed during the first 3 days of gestation when the CT mRNA in uterine epithelial cells is undetectable. Concomitant with a transient burst of CT expression during days 4-5 of pregnancy, the level of E-cadherin mRNA declined sharply. Furthermore, administration of exogenous CT to animals on day 2 of pregnancy led to a premature suppression of E-cadherin mRNA level on day 3, indicating a direct link between elevated levels of uterine CT and the down-regulation of E-cadherin expression in the surface epithelium. Collectively, our results are consistent with the hypothesis that CT-induced reduction in E-cadherin expression may remodel the adherens junctions between epithelial cells, and this change in epithelial cell phenotype might be a critical event during the implantation of the blastocyst.
Highlights
Implantation involves complex and progressively intimate interactions between the blastocyst and the uterine epithelium ʈ Supported by National Institutes of Health Grants R01-DK-50257 and U54-HD-13541
Our studies showed that the expression of CT is induced in the human endometrial epithelium during the midsecretory phase of the menstrual cycle, which closely overlaps with the putative window of implantation [12]
This study addresses the mechanism of action of CT as a paracrine or autocrine effector in the uterus during implantation
Summary
Implantation involves complex and progressively intimate interactions between the blastocyst and the uterine epithelium ʈ Supported by National Institutes of Health Grants R01-DK-50257 and U54-HD-13541. Suppression of the steady-state level of the CT mRNAs in the preimplantation rat uterus by antisense ODNs resulted in a dramatic reduction in the number of implanted embryos [13]. We observed that binding of CT to its cell surface receptor in Ishikawa cells leads to a transient rise in intracellular calcium, which in turn suppresses the expression of calciumdependent cell adhesion glycoprotein, E-cadherin, at cell-cell contact sites. Consistent with this in vitro observation, we found that administration of exogenous CT down-regulates E-cadherin expression in the endometrial epithelium of pregnant rats without altering the expression of ZO-1, a marker of tight junctions. We postulate that the hormone-induced down-regulation of E-cadherin expression results in the reorganization of the adherens junctions between
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