Tryptophan and kynurenine stimulate human decidualization via activating Aryl hydrocarbon receptor: Short title: Kynurenine action on human decidualization.

Decidualization is essential for successful pregnancy in rodents and primates. Although L-Tryptophan and its metabolites are essential for mammalian pregnancy, the underlying mechanism is poorly defined. We explored effects of tryptophan and kynurenine on human in vitro decidualization in human endometrial stromal cell line and primary endometrial stromal cells. Tryptophan significantly stimulates the expression of prolactin and insulin growth factor binding protein 1, reliable markers for human decidualization. When stromal cells are treated with tryptophan, tryptophan hydroxylase-1 remains unchanged, but indoleamine 2,3-dioxygenase 1 is significantly increased, suggesting tryptophan is mainly metabolized through kynurenine pathway. Kynurenine significantly stimulates insulin growth factor binding protein 1 expression. Aryl hydrocarbon receptor and its target genes (P450 1A1 and P450 1B1) are significantly increased by tryptophan and kynurenine. The induction of tryptophan and kynurenine on insulin growth factor binding protein 1 is abrogated by CH223191, an aryl hydrocarbon receptor inhibitor. Cytochrome P450 1A1 and P450 1B1 catalyze the oxidative metabolism of estradiol to catechol estrogens (2-hydroxy estradiol and 4-hydroxy estradiol), respectively. Insulin growth factor binding protein 1 is up-regulated by 2-hydroxy estradiol and 4-hydroxy estradiol. Interferon-γ significantly induces the expression of indoleamine 2,3-dioxygenase 1, aryl hydrocarbon receptor and insulin growth factor binding protein 1. All the data are also verified in primary human stromal cells. Our data indicate that Interferon-γ-induced kynurenine pathway promotes human decidualization via aryl hydrocarbon receptor signaling.