Epigenetic mechanisms in the actions of endocrine-disrupting chemicals

Gonadal effects and role in female reproduction

2014 Study Abstract

There is a heightened interest and concern among scientists, clinicians, and regulatory agencies as well as the general public, regarding the effects of environmental endocrine-disrupting chemicals (EDCs). In this review, we identify the main epigenetic mechanisms and describe key ovarian processes that are vulnerable to the epigenetic actions of EDCs. We also provide an overview of the human epidemiological evidence documenting the detrimental effects of several common environmental EDCs on female reproduction. We then focus on experimental evidence demonstrating the epigenetic effects of these EDCs in the ovary and female reproductive system, with an emphasis on methoxychlor, an organochlorine pesticide. We conclude the review by describing several critical issues in studying epigenetic effects of EDCs in the ovary, including transgenerational epigenetic effects.

Diethylstilbestrol DES

The most convincing human evidence that estrogenic EDC exposure during development can permanently affect female reproduction comes from reports of the effects of DES, a nonsteroidal synthetic estrogen. From the 1940s to the 1970s, DES was prescribed at doses of 5–150 mg⁄ day to prevent miscarriages. Numerous abnormalities in the reproductive, cardiovascular, and immune systems have since been reported in both male and female offspring of women treated with DES, and similar effects have been demonstrated in animal models. Some of these effects, such as irregular cylicity or ovarian cancer, are being observed in the granddaughters of DES-treated women as well. In light of this multigenerational aspect, whether epigenetic mechanisms are involved is a significant question.

Experimental evidence from in vivo studies regarding the effects of EDCs in the ovary

Characteristic sets of defects in the ovary are the hallmark of EDC exposure in rodent models. For example, mice injected as little as a single dose of 10 μg/kg DES on E15 and examined at 7 months of age had numerous atretic follicles, and no corpora lutea (an indicator of ovulation). Other studies in rodents of varying doses of DES administered either in utero (E9-16) or neonatally (PND1-5) demonstrated similar defects by adulthood. Estrous cyclicity was completely disrupted and high levels of testosterone were found. A vacuolated interstitial tissue with lipid droplet inclusions and hemorrhagic cysts were also observed. A recent study proposes that while increased lipid droplet are caused by impaired steroidogenesis due to suppressed LH levels, the hemorrhagic follicles are results of direct effects of DES in the ovary. Furthermore, there was a dose-dependent reduction in the number of litters as well as the number of oocytes ovulated after stimulation with exogenous gonadotropins with such oocytes when used in IVF showing lower levels of fertilizability, suggesting reduced oocyte quality. Most striking of all, an excessive number of MOFs is found in adult ovaries; such a finding is considered to be an indicator of reduced reproductive lifespan.

Such estrogenic actions of these EDCs are mediated via the ER signaling pathway. Recent studies have shown that MOFs induced by neonatal exposure to 3 μg/kg DES is mediated by ERβ and not ERα. DES exposure was shown to reduce oocyte apoptosis (potentially suppressing oocyte nest breakdown) via ERβ signaling mechanisms. Furthermore, it was hypothesized that alterations in the germ cell–to–somatic cell ratio may affect the invasion of pregranulosa cells and basement membrane remodeling during primordial follicle formation. In contrast to ERβ signaling mechanisms involved in mediating ovarian effects, Couse and colleagues reported that ERα is essential for the mediation of DES effects in the uterus: αERKO female mice exhibited a complete resistance to the effects of DES while βERKO mice did not.

Epigenetic mechanisms associated with the female reproductive system

Although there are now well-documented studies of the physiological and morphological effects of EDCs on the ovary, the early research providing evidence of an epigenetic component of EDC actions was predominantly on the uterus. For example, it is well known that DES caused T-shaped uteri and clear cell adenocarcinoma of the uterus, cervix, and vagina in women whose mothers were exposed to DES during pregnancy. In the numerous animal studies validating these human reports, developmentally DES-treated mice manifest malformations of the uterus, squamous metaplasia of the luminar and glandular epithelium, endometrial hyperplasia and leiomyomas, and oviductal proliferative lesions. Ovariectomized animals when supplemented with E2 are able to respond by a transient increase in gene expression and concomitant uterine proliferation and growth. When such a stimulus was removed, the uterus returned to its unstimulated state. However, when DES or E2 is administered during neonatal development, expression of immediate early genes such as c-fos, c-jun, and c-myc as well as lactoferrin and EGF are upregulated even into adulthood. This was associated with hypomethylation of the promoter region of the lactoferrin gene in the adult uterus. However, if animals were exposed for the same interval during adulthood, no such methylation or expression defects were observed, indicating the importance of developmental exposure. Subsequently, it was also found that exon 4 of the c-fos gene was extensively hypomethylated while the promoter region and intron 1 were unaffected, thereby potentially allowing for the upregulation of c-fos expression. Furthermore, recent work by Block and coworkers has shown that DES disrupts the regionalization of expression of Hoxa-9 and Hoxa-10 and the homeotic anterior transformations associated with hypermethylation in the promoter and intron 1 regions of Hoxa-10 gene. Additionally, ERα induction is necessary for activation of estrogen-responsive gene expression in the uterus, including that of the lactoferrin and c-fos genes. Since many of the above genes are downstream of ER signaling, which is involved in direct actions of EDCs, it is imperative to thoroughly examine the potential role of epigenetic mechanisms in the regulation of ER expression after EDC exposure. In addition, PI3K/Akt signaling downstream of membrane-associated ER signaling caused reduction in trimethylation of the histone H3K27 in response to E2 and DES exposures. More interestingly, activation of this non-genomic signaling caused reprogramming of the uterine gene expression profile . Another example is that of the investigation by Tang and colleagues, wherein it was demonstrated that neonatal DES/genistein exposure reduced DNA methylation and increased gene expression of nucleosomal binding protein 1 in adult uteri. These studies highlighted the age-dependent aspect of epigenetic reprogramming and also its interaction with steroid hormones.

Sources

  • Full study (free access) : Epigenetic mechanisms in the actions of endocrine-disrupting chemicals: Gonadal effects and role in female reproduction, HHS Author Manuscripts, NCBI PubMed PMC4151320, 2014 Sep 2.
  • Featured image by Christian Perner.
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