Superfund Research Program
Studies Investigate the Effects of TCDD and Benzo(a)pyrene on Placental-Uterine Function
In recent years considerable attention has been focused on understanding the potential adverse reproductive effects of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) and other related environmental aromatic hydrocarbon compounds in humans. Exactly how these compounds may affect the reproductive capacity of humans is currently not well understood. Some experimental reproductive studies have demonstrated that TCDD has the ability to cause pre- and/or post-implantation losses of fertilized eggs in animals, implying that TCDD may somehow disrupt normal placental-uterine functioning. Other studies have implicated TCDD-like compounds to be environmental growth dysregulators by altering gene expression through the aromatic hydrocarbon receptor (AhR). Based on this combined knowledge, researchers at the University of Florida have been carrying out studies that are investigating whether TCDD and another AhR agonist, benzo(a)pyrene, disrupt placental-uterine function by mechanisms involving the Ah receptor and the growth regulatory networks which are essential to placental invasiveness, proliferation, and hormone secretion, as well as uterine cell growth.
Experiments to date have involved in vitro studies with human placental and uterine cell lines in culture. Two different human placental cell lines, namely human placental choriocarcinoma JEG-3 and BeWo cells, have been used to profile the effects of TCDD and benzo(a)pyrene on the expression of the major growth regulatory genes involved in placental growth networks. The placental cell lines were chosen as a model of early pregnancy trophoblast cells which are highly proliferative, invasive, and exhibit endocrine function. Trophoblasts are essential during early pregnancy to establish contact with the maternal circulation and to establish the basic structure of the placenta. TCDD serves as a prototype environmental halogenated aromatic compound in these studies, while benzo(a)pyrene is a major polycyclic aromatic hydrocarbon found both in cigarette smoke and at many hazardous waste sites. The major growth regulatory genes that have been examined are epidermal growth factor (EGF) receptors, c-myc, and transforming growth factor (TGF-B1). AhR activity was also monitored by assaying for cytochrome P450 1A1 (CYP1A1), which is a very sensitive biomarker of exposure to AhR agonists.
In the placental cell lines, cytochrome P450 1A1 (CYP1A1) was induced by both TCDD and benzo(a)pyrene in a concentration and time dependent manner. Benzo(a)pyrene significantly decreased cell proliferation, the levels of EGF receptors, and c-myc, whereas the mRNA levels for TGF-B1 were significantly increased. In contrast, TCDD showed no effect on EGF receptors, nor on the mRNA levels of TGF-B1 and c-myc. Thus, benzo(a)pyrene has a more disruptive effect than TCDD on the important growth regulatory genes in the placental cell lines. These findings suggest that the benzo(a)pyrene mediated increase in TGF-B1 gene expression may block the invasion of early pregnancy trophoblasts, whereas the decrease in c-myc expression may directly interfere with normal trophoblast proliferation.
A separate investigation with a human uterine endometrial cell line has examined the effects of TCDD on similar endpoints. These cells were chosen for study because they express high affinity EGF and estrogen receptors which are important markers of uterine endometrial tissue. CYP1A1 was markedly induced by TCDD. In contrast, TCDD had no effect on cell proliferation, the level of high affinity EGF receptor, nor the mRNA level for TGF-B1. However, mRNA levels for the serine protease, urokinase plasminogen activator, were significantly increased. This protease has a major role in uterine endometrial cell migration. TCDD also significantly increased mRNA levels for interleukin-1 beta and tumor necrosis factor-alpha, two proinflammatory cytokines. Therefore, these findings suggest that in uterine endometrial cells TCDD may alter gene expression in a direction that favors migration, invasion, and inflammatory processes, which are consistent with endometriosis and infertility.
These studies are significant for providing a possible explanation for some of the observed adverse reproductive effects of benzo(a)pyrene and TCDD-like compounds. The results suggest that (1) benzo(a)pyrene may interfere with normal placental and vascular development, which may predispose to intrauterine growth retardation, and (2) TCDD may be implicated in the ontogenesis of uterine diseases without direct disruption of endocrine-mediated pathways. It is very important at this time to gain a better understanding of the effects of environmental contaminants on human reproductive capacity, therefore, these studies are both timely and highly relevant to current environmental health concerns.
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To learn more about this research, please refer to the following sources:
- Zhang L, Shiverick KT. 1998. Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene on proliferation and growth factor gene expression in human choriocarcinoma BeWo cells. Trophoblast Research 19:177-191.
- Charles GD, Shiverick KT. 1997. 2,3,7,8-Tetrachlorodibenzo-p-dioxin increases mRNA levels for interleukin-1B, urokinase plasminogen activator and tumor necrosis factor-a in human uterine endometrial adenocarcinoma RL95-2 cells. Biochem Biophys Res Commun 238:338-342. doi:10.1006/bbrc.1997.7291 PMID:9299508
- Zhang L, Shiverick KT. 1997. Benzo(a)pyrene, but not 2,3,7,8-Tetrachlorodibenzo-p-dioxin, alters, cell proliferation, c-myc and growth factor expression in human placental choriocarcinoma JEG-3 cells. Biochem Biophys Res Commun 231:117-120. PMID:9070231
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