OR09-04 Ovarian Stimulation Disrupts Expression Of The Retinoic Acid (RA) Signaling Pathway In The Human Endometrium

Disclosure: Q. Zhao: None. P. Timmins: None. M. Saad-Naguib: None. C. Samuels: None. R. Loia: None. T. Wu: None. A. Chemerinski: None. S.S. Morelli: None. A.V. Babwah: None. N. Douglas: None. Ovarian stimulation (OS) is employed to recruit multiple dominant follicles and mature oocytes for in vitro fertilization (IVF). OS followed by a fresh embryo transfer (ET) is associated with decreased pregnancy rates. By better understanding the processes which prepare the endometrium for implantation, we hope to improve outcomes following OS. To understand the impact of OS on the endometrium, we performed an in-depth comparative analysis of the human endometrium under natural vs OS conditions during the periovulatory (PO) and mid-secretory (MSE) phase of the menstrual cycle. Our data showed that OS, which significantly elevated serum E2 and P4 levels, was associated with dyssynchronous glandular-stromal development, increased glandular epithelial volume, and increased macrophage and B cell abundance. We then performed bulk and single-cell RNA sequencing (RNA-seq) to identify the molecular pathways and cell-type specific transcriptomes associated with these changes. We found that OS significantly changed the expression of numerous genes in the retinoic acid (RA) signaling pathway, a pathway known to be crucial for a successful pregnancy. Our data showed that OS affected the fibroblast and epithelial cell-specific expression of genes that regulate RA signaling, with increased expression of RA synthesis genes and RA target genes, suggesting that OS increases endometrial RA signaling. RA is the metabolically active form of vitamin A which regulates the transcription of over 500 genes and adversely affects pregnancy when deficient or in excess. Thus, these findings led us to hypothesize that OS disrupts normal endometrial RA signaling and thereby endometrial differentiation that is required for implantation. We investigated whether RA is required for endometrial differentiation in humans and the establishment of pregnancy in mice. Using siRNAs against the RA receptor RARA, we observed that reduced expression of RARA mRNA in human endometrial stromal cells reduced their ability to undergo decidualization, an essential requirement for successful embryo implantation. We found that administration of BMS93, a pan RAR receptor antagonist, to pregnant dams prior to implantation from E0.5 or E2.5 until E7.5 resulted in pregnancy failure. Whereas BMS493 administration to pregnant dams from E4.5 to E7.5, resulted in pregnancies with reduced decidual vascular development in the anti-mesometrial region. Together, these data suggest that RA is required for the proper preparation of the endometrium for embryo implantation and loss of RA signaling in the endometrium, not the embryo, is a cause of pregnancy failure. Importantly, OS induced disruptions of RA signaling might contribute to reduced implantation rates in fresh ET, uncovering a novel pharmacologic target for improving pregnancy outcomes in fresh ET following OS. Presentation: Friday, June 16, 2023