Endocrine disruption of oestrogen action and female reproductive tract cancers

The oestrogenic activities of DES has been comprehensively studied and is described

Abstract

Endocrine disrupting chemicals (EDC) are ubiquitous and persistent compounds that have the capacity to interfere with normal endocrine homoeostasis. The female reproductive tract is exquisitely sensitive to the action of sex steroids, and oestrogens play a key role in normal reproductive function. Malignancies of the female reproductive tract are the fourth most common cancer in women, with endometrial cancer accounting for most cases. Established risk factors for development of endometrial cancer include high BMI and exposure to oestrogens or synthetic compounds such as tamoxifen. Studies on cell and animal models have provided evidence that many EDC can bind oestrogen receptors and highlighted early life exposure as a window of risk for adverse lifelong effects on the reproductive system. The most robust evidence for a link between early life exposure to EDC and adverse reproductive health has come from studies on women who were exposed in utero to diethylstilbestrol. Demonstration that EDC can alter expression of members of the HOX gene cluster highlights one pathway that might be vulnerable to their actions. In summary, evidence for a direct link between EDC exposure and cancers of the reproductive system is currently incomplete. It will be challenging to attribute causality to any single EDC when exposure and development of malignancy may be separated by many years and influenced by lifestyle factors such as diet (a source of phytoestrogens) and adiposity. This review considers some of the evidence collected to date.

Diethylstilbestrol

DES, a synthetic non-steroidal oestrogen, is often regarded as the archetypal endocrine disruptor. From about 1940 to 1970, DES was given to pregnant women in the mistaken belief that it would reduce the risk of pregnancy complications. Herbst et al. reported a probable link between DES and vaginal clear cell adenocarcinoma in girls and young women who had been exposed to this drug. It is estimated that five to ten million people were exposed to DES, including the pregnant mothers who received treatment and their offspring.

Of the several million women exposed to DES in utero, a cohort of 4653 DES-exposed women have been followed up to investigate the long-term consequences of exposure. Patient data stratified to account for the extent of exposure or dose effects of DES identified an association between treatment of mothers earlier during their pregnancy and adverse vaginal epithelial changes at a younger age in their offspring. While DES exposure is associated with increased risk of breast and cervical/vaginal clear cell adenocarcinoma, several studies have indicated that there is no associated risk of endometrial or ovarian cancer. As endometrial cancer is most likely to present after menopause, many of the DES-exposed women may not yet be old enough to determine whether they are at excess risk, as in the 2011 report only 27% were older than 50 years. Although to date epidemiological data indicate that DES-exposed women may not be at increased risk of developing endometrial cancer, studies in animal models provide evidence to the contrary.

In the early 1990s, Newbold et al. developed a mouse model for investigating hormonal carcinogenesis in mice by investigating the effects of neonatal exposure to oestrogens on cancer development. Treatment of CD1 neonatal mice with DES on postnatal days 1–5, which correspond to late prenatal human development, resulted in 90% of DES-exposed mice developing uterine adenocarcinomas after 18 months while none of the control animals had neoplastic lesions. Crucially, while administration of DES increased the risk of uterine adenocarcinoma, endogenous oestrogen was required for tumour development with prepubertal ovariectomy preventing tumour development. In DES-exposed women, vaginal and cervical carcinomas were only detected post-menarche consistent with a requirement for endogenous oestrogen in tumour development. ERα knockout mice (ERKO) did not develop tumours following neonatal DES exposure; transgenic mice overexpressing ERα displayed accelerated tumour development but mice with a dominant negative isoform of ERα (ERΔ3) were not protected, highlighting the complexity of the molecular signalling mechanisms involved.

Interestingly, gene expression analysis indicates that developmental DES exposure results in persistent altered gene expression of oestrogen-responsive genes in the uterus that may explain the increased susceptibility to tumour development. Gene ontology analysis of microarray data revealed altered expression of genes involved in cell growth, differentiation and adhesion. Kabbarah et al.  collected uterine cancer tissue RNA from DES-exposed mice by laser capture microdissection to minimise contamination with other cell types and performed targeted transcriptional profiling. Interestingly, the tumour suppressor PTEN was down-regulated in the majority of tumours, analogous to loss of PTEN expression in human tumours. In addition, genes associated with cell adhesion, such as Decorin, were down-regulated in DES-induced tumours while suppressor of cytokine signalling 3 (Socs3) was over-expressed. Other studies have also identified molecular similarities between DES-induced tumours in mice and endometrial cancer in humans, such as microsatellite instability brought about by defects in expression of DNA mismatch repair genes such as MSH2 and MSH6.

It could be argued that the apparent trans-generational effect of endocrine disruption is of greater significance. Following neonatal DES exposure in mice, the F1 generation of DES daughters have an increased incidence of uterine adenocarcinoma. Newbold et al. found that 31% of F1 females from the maternal germ cell lineage developed tumours after 18 months despite there being no exogenous endocrine exposure in these animals, highlighting the potential for future risk to the daughters of DES-exposed women. DES is reported to induce epigenetic changes. Altered methylation patterns have been reported for several uterine genes that are permanently dysregulated after developmental DES exposure; lactoferrin and c-Fos are permanently up-regulated following neonatal DES exposure to due to hypomethylation of the promoter region.

DES has been reported to promote hypermethylation of the homeobox gene Hoxa10 in mice exposed in utero to DES. DES exposure was also associated with increased expression of DNA methyltransferases 1 and 3b leading to long-term altered expression of Hoxa10. Contrary to the reported action of DES on Hoxa10, exposure to BPA in mice in utero results in hypomethylation of the Hoxa10 promoter, which leads to enhanced binding of ERα to EREs in the promoter region and an increase in an ERE-driven reporter gene in vitro.

Thus, epigenetic changes in uterine genes may indicate a possible mechanism for trans-generational effects of DES because altered expression of genes is reported to persist in DES-lineage females.

References

  • Full study (free access) : Endocrine disruption of oestrogen action and female reproductive tract cancers, Endocrine-related cancer, doi.org/10.1530/ERC-13-0342, 2014 Mar.
  • Featured image credit alpco.
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