Excessive progesterone impairs mouse decidualization via the Kyn-AhR pathway

Progesterone (P 4 ) is essential for pregnancy establishment and maintenance. Clinically, P 4 is widely used to regulate the menstrual cycle, maintain pregnancy, and treat luteal phase deficiency. However, P 4 administration protocols, particularly regarding routes, dosage, and timing remain poorly defined. Although excessive P 4 impairs embryo implantation and decidualization in mice, the underlying mechanism remains unclear. Our data show that decidualization in day 8 pregnant mice and artificial decidualization in day 8 pseudopregnant mice are impaired by 4 mg or 8 mg/mouse P 4 . The mRNA levels of Prl8a2 and Prl3c1, markers of in vitro decidualization are significantly downregulated by 10 or 20 μM P 4 . The uterine fluorescent signal of indoleamine 2,3-dioxygenase 1 (IDO1) and protein levels of tryptophan 2,3-dioxygenase (TDO) are increased after ovariectomized mice are treated with excessive P 4 . Treatment of uterine stromal cells with excessive P 4 also significantly upregulates the protein levels of IDO1 and TDO, and kynurenine (Kyn) secretion. Epacadostat (IDO1 antagonist) or RU486 (progesterone receptor antagonist) effectively block P 4 -induced Kyn elevation. The mRNA levels of Prl8a2 and Prl3c1 and the protein levels of BMP2 are significantly inhibited by Kyn. The high-dose of P 4 activates the aryl hydrocarbon receptor (AhR) and its downstream targets CYP1A1 and CYP1B1. Under in vitro decidualization, the mRNA levels of Prl8a2 and Prl3c1 are inhibited by 2-OH-E 2 and 4-OH-E 2 , the catalytic products of CYP1A1 and CYP1B1, respectively. CH-223191, a specific AhR antagonist, effectively counteracts the effects of Kyn on Cyp1a1 , Cyp1b1 , and Prl8a2 expression. Additionally, nucleolar size in stromal cells is increased both in vivo and in vitro following excessive P 4 treatment. Our findings suggest that excessive P 4 impairs mouse decidualization via the Kyn-AhR pathway.