Excessive progesterone impairs mouse decidualization via the Kyn-AhR pathway

Progesterone (P₄) is essential for pregnancy establishment and maintenance. Clinically, P₄ is widely used to regulate the menstrual cycle, maintain pregnancy, and treat luteal phase deficiency. However, P₄ administration protocols, particularly regarding routes, dosage, and timing remain poorly defined. Although excessive P₄ 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 P₄. The mRNA levels of <i>Prl8a2 and Prl3c1,</i> markers of <i>in vitro</i> decidualization are significantly downregulated by 10 or 20 μM P₄. 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 P₄. Treatment of uterine stromal cells with excessive P₄ also significantly upregulates the protein levels of IDO1 and TDO, and kynurenine (Kyn) secretion. Epacadostat (IDO1 antagonist) or RU486 (progesterone receptor antagonist) effectively block P₄-induced Kyn elevation. The mRNA levels of <i>Prl8a2</i> and <i>Prl3c1 and</i> the protein levels of BMP2 are significantly inhibited by Kyn. The high-dose of P₄ activates the aryl hydrocarbon receptor (AhR) and its downstream targets CYP1A1 and CYP1B1. Under <i>in vitro</i> decidualization, the mRNA levels of <i>Prl8a2</i> and <i>Prl3c1</i> are inhibited by 2-OH-E₂ and 4-OH-E₂, the catalytic products of CYP1A1 and CYP1B1, respectively. CH-223191, a specific AhR antagonist, effectively counteracts the effects of Kyn on <i>Cyp1a1</i>, <i>Cyp1b1</i>, and <i>Prl8a2</i> expression. Additionally, nucleolar size in stromal cells is increased both <i>in vivo</i> and <i>in vitro</i> following excessive P₄ treatment. Our findings suggest that excessive P₄ impairs mouse decidualization via the Kyn-AhR pathway.