Superfund Research Program
The Alteration of Estrogen Metabolism By Polychlorinated Biphenyls
Once widely used in industry, polychlorinated biphenyls (PCBs) are now among the most notorious of environmental contaminants. This class of synthetic organochlorine compounds consists of 209 molecules, each of which consists of two benzene rings and one to ten chlorine atoms. PCBs were first manufactured commercially in 1929; however, it was not until the late 1960s that the compounds were linked to extensive global contamination, and recognized as a serious environmental problem. Large quantities of these compounds are now widely dispersed in sediments, soils, lakes, and river basins throughout the world. Ironically, the exceptional chemical and thermal stability that made PCBs such ideal industrial agents now contribute to their persistence in the environment. PCBs also accumulate in food chains. In fact, most humans have traces of these industrial chemicals in the fatty tissues of their bodies. Knowing that PCBs are such widespread and persistent environmental contaminants concerns scientists and the public alike because PCB exposures are also associated with a number of adverse health effects.
In recent years, it has been suggested that PCBs in the environment can mimic the body's natural hormones, and that this "endocrine disruption" can lead to reproductive failure, developmental disorders, and impairments to the nervous and immune systems. These concerns have catalyzed the need to understand the possible endocrine effects of PCBs in humans. To determine whether PCBs initiate toxic effects by disrupting endocrine activity, a clear understanding of how PCBs interact with the body's biochemical and molecular machinery is needed.
Scientists at the University at Albany-SUNY are investigating the effects of PCBs on the cellular processes used to regulate estrogen levels in human cells. A major goal of these mechanistic studies is to determine whether PCBs can alter cellular levels of the endogenous estrogen, 17ß-estradiol (E2), by disrupting the biological mechanisms involved in metabolizing E2 to its inactive form. Determining whether PCBs can modify estrogen levels in humans is important because this hormone regulates a number of biological activities during fetal development (e.g., sexual differentiation and neural development), as well as many aspects of fertility in females.
A number of biological molecules are involved in regulating E2 levels in the human body. The Ah receptor is a key molecule that is thought to be involved in reducing the amount of intracellular E2 when the level of this hormone rises above its normal range. When activated, the Ah receptor increases the gene transcription of two enzymes, known as cytochrome P450 1A1 (CYP1A1) and cytochrome P450 1B1 (CYP1B1), that catalyze the hydroxylation of E2 to non-active forms. The endogenous ligand(s) for the Ah receptor have not been identified; however, the Ah receptor is known to be activated by a number of environmental chemicals, including PCBs.
Although PCBs have been previously shown to activate the Ah receptor of a number of vertebrate species, data on the interactions of specific congeners with the human Ah receptor are lacking. It is increasingly recognized that the biological activity of PCB mixtures is dependent on the congeners present in the mixture. In addition, little is known about the effects of PCBs on the induction and activity of CYP1B1, an enzyme that was only recently discovered. Therefore, the SUNY-Albany scientists investigated the effects of a series of coplanar PCB congeners on the induction and E2 hydroxylase activities of CYP1A1 and CYP1B1 in three human-derived cell lines. Experiments have been completed on all coplanar PCBs with three or more chlorines. In these in vitro studies, the scientists monitored the effects of individual PCB congeners on the expression of CYP1A1 and CYP1B1.
Results of these recent studies indicate the effects of coplanar PCBs on estrogen metabolism are quite complex, as they not only act as inducers of CYP1A1 and CYP1B1, but they also act as inhibitors for these enzymes. Thus, some PCB congeners can stimulate estrogen metabolism and lead to anti-estrogenic (estrogen blocking) effects, while others can inhibit estrogen metabolism and lead to enhanced estrogenic activity in in vitro systems. In addition, one PCB congener was shown to be a substrate for CYP1A1 and was metabolized to a potentially estrogen-like compound.
The three most potent coplanar PCB congeners for inducing CYP1A1 and CYP1B1 were PCB 81 (3,4,4',5-tetrachlorobiphenyl), PCB 126 (3,3',4,4',5-pentachlorobiphenyl), and PCB 39 (3,4',5-trichlorobiphenyl). Based on rodent studies, PCBs 81 and 39 were not previously thought to have significant Ah receptor activity. These findings are significant in that the relative potencies of PCB congeners for the human Ah receptor may differ considerably from those determined in rodents.
The inhibition of CYP1A1 and CYP1B1 by some coplanar PCB congeners suggests that the chemicals may bind tightly in the active sites of the enzymes, and that some may be hydroxylated as well. This latter possibility is potentially very important, as some hydroxy-PCB metabolites are known to interact with the estrogen receptor and exhibit estrogenic activity.
To investigate the possibility that coplanar PCBs are substrates for human CYP1A1 and CYP1B1 enzymes, the researchers developed a highly sensitive assay for hydroxy-PCB metabolites. Initial studies with this technique showed that metabolism of PCB 77 (3,3',4,4'-tetrachlorobiphenyl) occurred in liver cells (the HepG2 cell line). The major product was 3,3',4',5-tetrachloro-4-biphenylol, a metabolite suspected to have estrogenic activity.
These studies provide much needed understanding on the biological effects of individual PCB congeners and underscore the complexity of PCB interactions in human systems. Members of one subgroup of PCBs - the coplanar congeners - can elevate the cellular levels of enzymes that the body normally uses to reduce estrogen levels. This activity could lead to reduced intracellular levels of E2 and reduced estrogenic response. However, some PCBs can also inhibit these same enzymes, an activity that could enhance estrogenic response. The results with PCB 77 indicate this congener is also a substrate for CYP1A1, resulting in the formation of metabolites with putative estrogenic activity. While these effects on estrogen metabolism are not toxic endpoints per se, they do indicate potential mechanisms that may be involved to varying degrees in the suspected endocrine-disrupting effects of PCBs.
For More Information Contact:
David C. Spink
University of Albany - SUNY
Empire State Plaza
Albany, New York 12201
To learn more about this research, please refer to the following sources:
- Spink BC, Fasco MJ, Gierthy JF, Spink DC. 1998. 12-O-Tetradecanolyl-phorbol-13-acetate differentially alters CYP1A1 and CYP1B1 expression in MCF-7 breast cancer cells. J Cell Biochem 70(3):289-296.
- Spink DC, Spink BC, Cao JQ, DePasquale JA, Pentecost BT, Fasco MJ, Li Y, Sutter TR. 1998. Differential expression of CYP1A1 and CYP1B1 in human breast epithelial cells and breast tumor cells. Carcinogenesis 19(2):291-298. PMID:9498279
- Pang S, Cao JQ, Hayes CL, Sutter TR, Spink DC. 1997. Coplanar polychlorinated biphenyls as inducers and inhibitors of human cytochromes P450 1A1 and 1B1. Environ Health Perspect 36:158.
- Spink DC, Cao JQ, Gierthy JF, Hayes CL, Li Y, Sutter TR. 1997. Induction of cytochrome P450 1B1 and catechol estrogen metabolism in ACHN renal adenocarcinoma cells. Psychiatr Pol 62:223-232. PMID:9393958
- Hayes CL, Spink DC, Spink BC, Cao JQ, Walker NJ, Sutter TR. 1996. 17β-estradiol hydroxylation catalyzed by human cytochrome P450 1B1. Proc Natl Acad Sci U S A 93:9776-9781. PMID:8790407
- Pang S, Cao JQ, Spink DC. 1996. Effects of polychlorinated biphenyls on estrogen metabolism in human breast cancer cells. Environ Health Perspect 30:290.
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