Retinoic acid synthesis and expression of cellular retinol-binding proteinand cellular retinoic acid-binding protein type II are concurrent with decidualization of rat uterine stromal cells

Decidualization of stromal cells at the site of embryo implantation in the rat uterus is accompanied by expression of cellular retinol-binding protein and cellular retinoic acid-binding protein [CRABP(II)], whose presence has been shown to correlate with gain of ability to synthesize retinoic acid i n other cells. Here we examined whether decidual cells also acquired the ab ility to synthesize retinoic acid, which would have important implications for understanding the implantation process. Decidual cells were isolated fr om the uterus on day 8 of pregnancy and cultured. When provided with retino l, they indeed synthesized and released retinoic acid to the medium. To fol low acquisition of this ability more closely, artificial induction of decid ualization was exploited. Ovariectomized rats were placed on a hormonal reg imen that allows decidualization to occur in vivo, with oil stimulation, or in vitro, if cells are isolated on day 5 of the regimen and then cultured. Decidualization in vivo reproduced the expression of cellular retinol-bind ing protein and CRABP(II) seen during pregnancy. Stromal cells isolated on regimen day 2 synthesized little retinoic acid and expressed little alkalin e phosphatase, a marker of decidualization. Stromal cells isolated on regim en day 5 had elevated levels of alkaline phosphatase, increasing during the 3 days of culture examined. The ability of the stromal cells to synthesize retinoic acid showed the same pattern: a substantially elevated production from that previously observed, on day 2, with production increasing signif icantly over the next 2 culture days. Thus, expression of CRABP(II) was cor related with gain of ability to synthesize retinoic acid. Retinoid signalin g may be an important part of the process of embryo implantation.