OUP accepted manuscript

BACKGROUND: Endometrium is a vital multicellular tissue for progression of pregnancy. Forkhead box O1 (FoxO1) transcription factor plays an important role in the endometrium as it regulates various cellular processes with its unique expression in different cell types. OBJECTIVE AND RATIONALE: This review focuses on the role of FoxO1 in endometrium with a particular emphasis on FoxO1 signaling in individual endometrial cell types during the menstrual cycle and early pregnancy. SEARCH METHODS: A literature search was conducted in PubMed, Web of Science and Scopus to select studies reporting the role of FoxO1 in endometrium using the keywords: FoxO1, endometrium, menstrual cycle, early pregnancy, endometrial receptivity, implantation, decidualization, angiogenesis and neoplasia. Papers published before October 2020 were selected. Drawing on advances in laboratory science and preclinical studies, we performed a narrative review of the scientific literature to provide a timely update on the roles of FoxO1 during the menstrual cycle and early pregnancy. OUTCOMES: FoxO1 is considered to be a decidualization marker in endometrial stromal cells, mainly because it regulates the transcription of decidual prolactin and insulin-like growth factor-binding protein 1 genes. Importantly, 507 of 3405 genes that are specifically regulated during decidualization of human endometrial stromal cells are expressed abnormally as a result of FOXO1 reduction. Epithelial FoxO1 is currently accepted as a novel endometrial receptivity marker for humans and mice owing to its timely and specific expression at the window of implantation. On the other hand, FOXO1 is essential in endometrial epithelial cell proliferation and differentiation to achieve endometrial homeostasis since loss of function of FOXO1 causes endometrial neoplasia. Last but not least, FoxO1 seems to act like a navigator molecule for embryo homing owing to its notably decreased nuclear expression in endometrial luminal epithelial cells, specifically at the blastocyst attachment region, which results in differentiation, entosis and apoptosis of endometrial epithelial cells during the peri-implantation period. In endothelium, FoxO1 expression coincides with the timing of increased vascular permeabilization during early pregnancy. There are limited data regarding the importance of FoxO1 upregulation in endometrial endothelial cells, therefore, it is time to investigate the role of endothelial FoxO1, which is the missing piece in the puzzle of the enigmatic endometrium. Another missing piece in the puzzle for the role of FoxO1 is on embryo development. WIDER IMPLICATIONS: FoxO1 is a cell-specific core transcription factor involved in efficient endometrial remodeling during the menstrual cycle and early pregnancy. A better understanding of the role of FoxO1 as a decidualization marker, as an emerging marker of endometrial receptivity, and as a therapeutic target to prevent endometrial neoplasia could help us to make sense of endometrial biology and thus to improve the outcomes of ART in the clinic.