Stromal PRs mediate induction of 17 beta-hydroxysteroid dehydrogenase type2 expression in human endometrial epithelium: A paracrine mechanism for inactivation of E2

Progesterone stimulates the expression of 17 beta -hydroxysteroid dehydroge nase (HSD) type 2, which catalyzes the conversion of the potent estrogen, E 2, to an inactive form, estrone, in epithelial cells of human endometrial t issue. Various effects of progesterone on uterine epithelium have recently been shown to be mediated by stromal PRs in mice. We describe herein a crit ical paracrine mechanism whereby progesterone induction of 17 beta -HSD typ e 2 enzyme activity, transcript levels, and promoter activity in human endo metrial epithelial cells are mediated primarily by PR in endometrial stroma l cells. Medium conditioned with progestin-pretreated human endometrial str omal cells robustly increased 17 beta -HSD) type 2 enzyme activity (2-fold) and mRNA levels (13.2-fold) in Ishikawa malignant endometrial epithelial c ells. In contrast, direct progestin treatment of Ishikawa epithelial cells gave rise to much smaller increases in enzyme activity (1.2-fold) and mRNA levels (4-fold). These results suggest that progesterone-dependent paracrin e factors arising from stromal cells are primarily responsible for the indu ction of epithelial 17 beta -HSD type 2 expression in the endometrium. We t ransfected serial deletion mutants of the -1,244 bp 5'-flanking region of t he 17 beta -HSD type 2 gene into Ishikawa cells. No progesterone response e lements could be identified upstream of the 17 beta -HSD type 2 promoter. S tromal PR-dependent induction of the 17 beta -HSD type 2 promoter was media ted by a critical regulatory region mapped to the -200/-100 bp sequence. Di rect treatment of Ishikawa cells with progestin gave rise to a maximal incr ease in the activity of -200 bp/Luciferase construct only by 1.2-fold, wher eas medium conditioned by progestin-pretreated endometrial stromal cells in creased promoter activity up to 2.4-fold in a time- and concentration-depen dent manner. The stimulatory effect of medium conditioned by progestin-pret reated stromal cells was enhanced strikingly by increasing stromal cell PR levels with the addition of estrogen. This epithelial-stromal interaction w as specific for endometrial epithelial cells, since 17 beta -HSD type 2 cou ld not be induced in malignant breast epithelial cells by media conditioned with progestin-treated breast or endometrial stromal cells. In conclusion, progesterone regulates the conversion of biologically active E2 to estrone by inducing the 17 beta -HSD type 2 enzyme in human endometrial epithelium primarily via PR in stromal cells, which secrete factors that induce trans cription mediated primarily by the -200/-100 bp 5'-regulatory region of the 17 beta -HSD type 2 promoter.