CALCITONIN IS A PROGESTERONE-REGULATED MARKER THAT FORECASTS THE RECEPTIVE STATE OF ENDOMETRIUM DURING IMPLANTATION

Previous studies established that in the rat, the uterus can accept a developing blastocyst for implantation only during a limited period of time on day 5 of gestation, termed the receptive phase. Our previous studies showed that the expression of calcitonin, a peptide hormone th at regulates calcium homeostasis, is induced in rat uterus between day s 3-5 of gestation and is switched off once the implantation process h as progressed to day 6. In the present study, we analyze in detail how the expression of calcitonin messenger RNA (mRNA) in the uterus is re gulated by the steroid hormones progesterone and estrogen and explore the possibility that calcitonin may serve as a potential marker of ute rine receptivity. We demonstrate by in situ hybridization that calcito nin mRNA is synthesized specifically in the glandular epithelial cells between days 3-5 of pregnancy. Interestingly, calcitonin synthesis is also induced in these cells during pseudopregnancy, indicating that t his peptide hormone is produced in the endometrium in response to mate rnal, rather than embryonic, signals. We also demonstrate that calcito nin mRNA expression during pseudopregnancy, like that in normal pregna ncy, is under progesterone regulation. We further examined the steroid hormone regulation of uterine calcitonin expression in a delayed impl antation model. In pregnant rats in which implantation is blocked upon removal of both ovaries on day 4 of gestation, continued administrati on of progesterone sustains calcitonin expression in the uterus for se veral days in the absence of estrogen. Administration of estrogen, whi ch allows delayed implantation, also rapidly reduces calcitonin expres sion, indicating a role for this steroid hormone in turning off calcit onin gene expression. In gene transfection studies, expression of the progesterone receptor B isoform in cultured endometrial cells induces RNA synthesis from a reporter gene containing a 1.3-kb calcitonin prom oter fragment in a hormone-dependent manner. As expected, mifepristone -complexed progesterone receptor B isoform fails to activate the calci tonin promoter. Progesterone acting through its nuclear receptor there fore regulates the expression of the calcitonin gene at the level of t ranscription. Finally, using RIA we investigated whether calcitonin is secreted from its glandular site of synthesis at the time of implanta tion by analyzing uterine flushings obtained from pregnant rats. We re port the detection of a significant amount of calcitonin in the lumina l secretions collected on day 4 and a lower amount on day 5 of gestati on, whereas similar samples collected from animals on either day 3 or 6 of gestation did not contain detectable amounts of this peptide horm one. A transient burst of calcitonin secretion into the uterine lumen therefore occurs immediately preceding implantation. Based on these re sults, we propose that calcitonin is a measurable marker that forecast s the receptive state of rat endometrium during blastocyst implantatio n.