BACKGROUND: A decrease in maternal serum progesterone (P4) concentrations precedes the onset of labor in most placental mammals. Humans differ by maintaining high levels of P4 throughout birth. Parturition in humans probably includes mechanisms that undercut the pregnancy sustaining function of P4. One attractive hypothesis is the isoform switching hypothesis (ISH). ISH is supported by in vitro evidence that progesterone receptor isoform A (PR-A) inhibits PR-B and that the PR-A/PR-B ratio increases towards term. MATERIALS AND METHODS: Here, we test the hypothesis that isoform switching is an adaptation to high levels of P4 at term, predicting that, in humans, PR-A mediated repression of PR-B is stronger than in mouse. We use reporter assays with human and mouse PRs to detect species differences in the repressive effects of PR-A. RESULTS: We found that human PR-B is less sensitive to repression by human PR-A than mouse PR-B, contrary to our prediction. The difference between human and mouse PR-B sensitivity is most pronounced at PR-A/PR-B ratios typical for the preterm myometrium. CONCLUSIONS: Our results are inconsistent with the ISH. We speculate that, instead, the lower sensitivity of human PR-B to PR-A may be relevant for the maintenance of pregnancy at high progesterone levels and increasing PR-A concentrations towards term.
BACKGROUND: A decrease in maternal serum progesterone (P4) concentrations precedes the onset of labor in most placental mammals. Humans differ by maintaining high levels of P4 throughout birth. Parturition in humans probably includes mechanisms that undercut the pregnancy sustaining function of P4. One attractive hypothesis is the isoform switching hypothesis (ISH). ISH is supported by in vitro evidence that progesterone receptor isoform A (PR-A) inhibits PR-B and that the PR-A/PR-B ratio increases towards term. MATERIALS AND METHODS: Here, we test the hypothesis that isoform switching is an adaptation to high levels of P4 at term, predicting that, in humans, PR-A mediated repression of PR-B is stronger than in mouse. We use reporter assays with human and mouse PRs to detect species differences in the repressive effects of PR-A. RESULTS: We found that humanPR-B is less sensitive to repression by humanPR-A than mousePR-B, contrary to our prediction. The difference between human and mousePR-B sensitivity is most pronounced at PR-A/PR-B ratios typical for the preterm myometrium. CONCLUSIONS: Our results are inconsistent with the ISH. We speculate that, instead, the lower sensitivity of humanPR-B to PR-A may be relevant for the maintenance of pregnancy at high progesterone levels and increasing PR-A concentrations towards term.
Authors: Carolina Hofmeister; Maria de Lourdes Brizot; Adolfo Liao; Rossana P V Francisco; Marcelo Zugaib Journal: J Perinat Med Date: 2010-09 Impact factor: 1.901
Authors: Pooja Mittal; Roberto Romero; Adi L Tarca; Juan Gonzalez; Sorin Draghici; Yi Xu; Zhong Dong; Chia-Ling Nhan-Chang; Tinnakorn Chaiworapongsa; Stephen Lye; Juan Pedro Kusanovic; Leonard Lipovich; Shali Mazaki-Tovi; Sonia S Hassan; Sam Mesiano; Chong Jai Kim Journal: J Perinat Med Date: 2010-07-14 Impact factor: 1.901
Authors: M Mazor; R Hershkovitz; W Chaim; J Levy; Y Sharony; J R Leiberman; M Glezerman Journal: Am J Obstet Gynecol Date: 1994-07 Impact factor: 8.661