Effect of in vivo administration of epidermal growth factor on prostaglandin production and NOS activity in term rat placentae. Possible participation of placental EGF receptors
Introduction
Epidermal growth factor (EGF) was first discovered by Cohen in submaxillary glands of mice [1]. From then on, most EGF studies have been focused on the stimulation of in vivo and in vitro cell proliferation. However, EGF seems to display other biological activities as well. It is present in maternal and fetal blood and in the amniotic fluid [2], [27]. It is also believed to be involved in feto-placental growth and development through its mitogenic action, increasing placental and fetal membrane hormone secretion [3]. It has been reported that, in 4–5 week human placentae, EGF and its receptor (EGF-R) are located in cytotrophoblasts, and that EGF increases cytotrophoblast cell proliferation without affecting their ability to secrete human chorionic gonadotrophin (hCG) and human placental lactogen (hPL) [4]. In contrast, in 6–12 week placentae, EGF and its receptor are located in syncytiotrophoblasts and EGF stimulates hCG and hPL secretion without affecting syncytiotrophoblast cell proliferation. Previous studies carried out in our laboratory [5] showed that intrauterine (i/u) administration of 500 ng EGF to 21-day pregnant rats delayed the onset of parturition 19.0 ± 0.6 h (EGF-treated rats delivered on day 23 of pregnancy instead of day 22 as sham animals). Our results also demonstrated that exogenous EGF elicited its effect modulating nitric oxide (NO) and prostaglandin (PG) production in the uterus.
It is well known that, in mammals, NO plays an important role during pregnancy regulating myometrial relaxation and placental circulation [6], [7]. Likewise, the ability of the villous vascular tree of human term placenta to both generate and respond to NO, and the significant role of NO in the maintenance of adequate fetal–placental circulation have also been demonstrated [8]. Moreover, three NOS isoforms have been identified in placental tissue: neuronal (nNOS), endothelial (eNOS) and inducible NOS (iNOS) [9].
On the other hand, PGs are active lipid mediators involved in the parturition process [10]. Their biosynthesis is catalyzed by the cyclooxygenase enzyme (COX), of which two isoforms are known: COX-I and COX-II [11]. Even though the uterus is the main target of PGs during parturition, it is not the primary source of these lipid mediators. It has been proposed that PGs derived from feto-placental tissues is the way by which they reach the uterus generating the contractions necessary for the expulsion of the fetus.
Since the placenta is a complex tissue located in close contact with the uterus [12], the first aim of our study was to investigate whether the placenta was also related to EGF-delayed labor onset mechanism. Thus, we analyzed if i/u administration of 500 ng EGF administered on day 21 of gestation regulated NOS and COX systems in this tissue.
Specific receptors for EGF were detected in fully developed placentae from pregnant mares [13], cats [14], mice [15] and women [16]. However, no descriptions can be found on the expression of EGF-R in placentae from pregnant rats. Therefore, our second aim was to characterize the expression of EGF-R in placentae from mid and term pregnant rats.
Section snippets
Drugs and reagents
Epidermal Growth Factor (murine submaxillary glands, culture grade, PM = 6100) was purchased from Calbiochem, Norabiochem Corporation (La Jolla, CA). [14C]-l-citrulline, [5,6,8,9,11,12,14,15(n)-3H]-prostaglandin F2α (160 Ci/mmol) and [5,6,8,9,11,12,14,15(n)-3H]-prostaglandin E (130 Ci/mmol) were from Amersham Corporation (Arlington Heights, IL, USA). Monoclonal nNOS, eNOS and iNOS first antibodies were from Transduction Co. (Devon, United Kingdom), whereas polyclonal COX-I, COX-II, EGF-R and EGF-pR
Effect of i/u EGF administration on PG synthesis
We decided to investigate if the delay in the onset of labor caused by the administration of EGF was due to a modulation on PG synthesis in the placenta. Therefore, we compared PG production in sham and EGF-treated rats.
We found that i/u administration of 500 ng EGF on day 21 of gestation significantly decreased both PGE (Figure 1A) and PGF2α (Figure 1B) production in placentae. Placentae from EGF-treated rats produced less PGs during day 22 than those from sham animals (50% PGE2 and 33% PGF2α).
Effect of i/u EGF administration on NO production
Discussion
The placenta is known to be an exceedingly rich source of receptors for several growth factors including EGF. This factor, which is primarily mitogenic for a variety of cells and also affects differentiated cellular functions in the absence of a mitogenic effect, is presumed to play a role not only in feto-placental development, but also as a signal during labor.
Previous results from our laboratory show that intrauterine administration of EGF to pregnant rats delays the onset of labor [5].
Acknowledgments
We would like to thanks Mrs. Ana Inés Casella and Ramona Morales for their technical assistance.
This work was supported by PICT 98, Project 05-04426 and FONDECYT 1040804.
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