EDCs are well known to have adverse effects on female reproductive organs, including the uterus. The main function of the uterus is to accept a fertilized ovum that then implants into the endometrium of the uterus and develops into a fetus until childbirth. During gestation, the uterus is relaxed so it can hold the fetus during development, which is controlled by P4. At the end of gestation, this muscular organ contracts to induce labor under the control of endogenous hormones such as E2, P4, OT, and PGF2α [24]. In the present study, we examined the effects of EE and OP on reproductive organs related to pregnancy in rats. First, we examined the expression levels of OT and hCG in the pituitary gland, which is a small organ located in the brain that secretes many hormones that function in the regulation of endocrine systems. The posterior pituitary gland consists mainly of neuronal projections (axons) of magnocellular neurosecretory cells extending from the supraoptic and paraventricular nuclei of the hypothalamus. These axons store and release neurohypophysial hormones such as OT and vasopressin into neurohypohyseal capillaries. In addition, gonadotrope cells of the anterior pituitary gland produce pituitary hCG [12]. There are four independent variants of hCG, each of which is produced by different cells. During pregnancy, hCG is mainly produced by placental villous syncytiotrophoblast cells. The hCG can also be produced by cytotrophoblast cells, in which its free β-subunit is made by multiple primary nontrophoblastic malignancies. Lastly, hCG can be produced by gonadotrope cells of the anterior pituitary gland [25]. Although they have different biological functions, all hCG molecules share the same β-subunit amino acid sequence. hCG is a heterodimer composed of a common α-subunit and hCG-specific β-subunit. The α-subunit of hCG is common to hyperglycosylated hCG, pituitary hCG, LH, FSH, and TSH. Reports have shown that pituitary hCG has approximately half biological activity than that of placenta in promoting progesterone production in the placenta [26]. E2 is known to regulate established anterior pituitary gland hormones as well as stimulate the release of OT from the hypothalamus [27]. The present results show that administration of EE and OP upregulated OT and hCG in the pituitary gland of pregnant rats. These results suggest that EE as well as OP have a stimulatory effect on the production of OT and hCG in the pituitary gland as a type of positive feedback mechanism. However, in the placenta, OT and hCG production was not stimulated by OP or EE, suggesting production of OT and hCG is regulated in a tissue-specific manner.
Regulation of myometrial contractility is modulated the expression of genes encoding CAPs. OT is a main mediator of uterine contraction and labor, and OTR expression dramatically increases in the myometrium at term [22,28]. Bossmar et al. demonstrated that OT expression is detectable in the uterus during pregnancy in rats, and this does not differ according to gestational age [29]. However, regulation of CAPs by E2 remains controversial. Recent studies have revealed that E2 upregulates OTR expression in the uterus of pregnant and non-pregnant animals [30,31]. In a previous study in ovariectomized rats, OT expression significantly stimulated by E2 was reduced by P4, even though P4 alone did not alter OT expression [29]. However, in another study, EE treatment alone did not show any effect on parturition [32]. Further, E2 alone or in combination with OT was shown to have no significant effect, whereas it enhances trypsin-induced contraction [33]. Our results suggest that administration of EE and OP does not modulate expression of OT and OTR in the uterus of pregnant rats. It is possible that the uterus during pregnancy may be already saturated with endogenous E2 and is thus unresponsive to exogenous administration.
Another important pathway for uterine contraction is PGF2α signaling, and elevation of FP is associated with induction of uterine contraction [34]. In the present study, expression of PGF2α signaling-related factors, including PGDH and FP, was evaluated. PGDH oxidizes and inactivates PG at the 15-hydroxyl group [35]. During pre-term and term, expression of PGDH is reduced in the uterus and placenta, which indicates that PG produced in these tissues are biologically active, thereby inducing uterine contraction and labor [23].
The effects of E2 on PGDH and FP expression are not well understood. E2 has been shown to increase the activity of PGDH in the rat uterus [36]. Furthermore, antagonist of E2 during the early menstrual cycle reduces PGDH activity and increases PG production, suggesting that E2 inactivates PGDH [37]. However, in another study, E2 was shown to have no effect on PGDH activity in cultured placenta cells [38]. Dong et al. demonstrated that E2 significantly increases FP mRNA expression in the uterus of ovariectomized rats [39].
In the present study, the effects of EE and OP on PGF2α-related gene expression were different. PGDH expression was up-regulated by OP but not EE, whereas FP was not regulated by either EE or OP. These results suggest that OP and EE may have different effects on the regulation of PGDH expression in the uterus. To clarify the effects of OP on uterine contraction during pregnancy, we performed uterine contraction assay. Uterine contractile activity is traditionally estimated by measuring the isometric force of longitudinal uterine sections using tension transducers in organ baths. Since this method can be affected by many factors such as buffer composition and other experimental conditions, we employed 3D collagen gel contraction assay originally developed by Dallot et al. [29]. We demonstrated for the first time that E2 did not affect contraction of primary uterine cells harvested from pregnant rats using a 3D collagen gel model. In contrast, OP significantly reduced uterine contraction. These results were consistent with our previous study showing that OP and BPA reduce uterine cell contractility in immature rats [4].
In summary, we examined the effects of EE and OP on uterine contraction and its associated factors in pregnant rats. Our results demonstrate that OP interferes with regulation of OT and hCG in the pituitary glandas well as PGDH in the uterus, thereby reducing uterine contraction activity. This result is in contrast to the action of endogenous E2. These results further suggest that exposure to EDCs such as OP can reduce uterine contractile ability, which may cause contraction-associated adverse effects such as metratonia, bradytocia, and uterine leiomyomata.