A decline in the levels of progesterone receptor coactivators in the pregnant uterus at term may antagonize progesterone receptor function and contribute to the initiation of parturition

Abstract

The molecular events that lead to the onset of labor in humans and in other mammalian species remain unclear. We propose that a decline in coactivators containing histone acetylase activity in myometrium may contribute to the onset of labor by impairing the function of the progesterone-progesterone receptor (PR) complex. As assessed by semiquantitative and real-time RT-PCR, immunohistochemistry, and immunoblotting, expression of the PR coactivators cAMP-response element-binding protein (CREB)-binding protein and steroid receptor coactivators 2 and 3 was decreased in fundal uterine tissue of women in labor. Using the mouse as an animal model, we also found decreased coactivator levels in uterine tissues at term. In both human and mouse, the levels of acetylated histone H3 were also decreased in uterine tissues at term. Administration of trichostatin A, a specific and potent histone deacetylase inhibitor, to pregnant mice late in gestation increased histone acetylation and delayed the initiation of parturition by 24-48 h, suggesting the functional importance of the decline in histone acetylation in the initiation of labor. These findings suggest that the decline in PR coactivator expression and in histone acetylation in the uterus near term may impair PR function by causing a functional progesterone withdrawal. The resulting decrease in expression of PR-responsive genes should increase sensitivity of the uterus to contractile stimuli.

Fig. 1.

mRNA levels for CBP and SRC-2 and -3 are decreased at term in fundal myometrium from women in labor, as compared with tissues from women not in labor. (A) Semiquantitative RT-PCR was used to analyze mRNA expression levels for CBP and SRC-1, -2, and -3 in myometrial biopsies obtained from the uterine fundus of women in labor as compared with those not in labor. G3PDH mRNA levels were analyzed to ensure that equal amounts of cDNA were added to each reaction. The wedges signify increasing numbers of PCR cycles. (B) Semiquantitative RT-PCR analysis of CnX43, PGF_2αR, and OTR mRNA levels as an index of “labor status.” The wedges indicate increasing numbers of PCR cycles. (C) Real-time RT-PCR analysis was used to compare mRNA expression levels of CBP and SRC-1, -2, and -3 in fundal myometrium biopsies from 12 women in labor (solid bars) and 12 not in labor (open bars). Data are the mean ± SEM. (n = 12). *, P < 0.01; **, P < 0.002; ***, P < 0.001.

Fig. 2.

Nuclear protein levels of CBP and SRC-2 are markedly decreased in myometrial tissues from women in labor, as compared with tissues from women not in labor. (A) Levels of expression of CBP and SRC-1 and -3 proteins were analyzed in nuclear fractions isolated from fundal myometrial biopsies from six in-labor and six not-in-labor subjects by immunoblotting (three in-labor and three not-in-labor subjects shown). (B) Immunohistochemistry was used to analyze expression and subcellular localization of CBP and SRC-2 and -3 (red reaction product) in fundal myometrial samples obtained at term from six women in labor and six women not in labor. Shown are representative sections from one subject in labor and one not in labor.

Fig. 3.

CBP and SRC-1 and -2 protein levels decline markedly in smooth muscle cell nuclei of the pregnant mouse uterus at term. (A) Levels of immunoreactive CBP and SRC-1 and -3 proteins were analyzed by immunoblotting in nuclear fractions from pregnant mouse uteri obtained on days 16–19 (in labor) of gestation. (B) Immunohistochemical analysis of CBP and SRC-1 and -2 proteins (red reaction product) in pregnant mouse uteri on days 16–19 (in labor) of gestation.

Fig. 4.

Levels of acetyl H3 are greatly reduced in fundal myometrium from at-term pregnant women in labor, as compared with tissues from women at term who are not in labor and in pregnant mouse uterus at term. (A) Nuclear protein levels of acetyl H3 and total histone H3 were analyzed in nuclear extracts of fundal myometrium from four women in labor and four women not in labor by immunoblotting. (B) Densitometric analysis of acetyl H3 relative to total histone H3 levels (mean ± SEM) in four in-labor and four not-in-labor human myometrium samples. (C) Nuclear levels of acetylated and total histone H3 in uteri of pregnant mice at days 16–19 (in labor) of gestation were analyzed by immunoblotting. (D) Densitometric analysis of acetyl H3 relative to total histone H3 levels (mean ± SEM) in pregnant mouse uteri at days 16–19 (in labor) of gestation.

Fig. 5.

Histone acetylation is greatly increased in uteri of at-term pregnant mice injected with TSA. (A) Levels of acetylated and total histone H3 proteins were analyzed in uterine nuclear fractions prepared from noninjected (N.I.), DMSO-injected controls, and TSA-injected mice on gestation day 19 by immunoblotting. (B) Densitometric measurements of acetylated relative to total histone H3 levels (mean ± SEM) for N.I., DMSO-injected controls, and TSA injected animals (n = 3).