Individuals are exposed to a range of synthetic chemicals daily through consumer products. For many of these chemicals, the health effects are unknown. Of particular concern is the number of chemicals that may increase breast cancer risk and progression. Most breast cancers are hormonally responsive, classified as estrogen (ER)- and progesterone(P4)-receptor (PR)-positive.
Established breast cancer risk factors, such as hormone replacement therapy and reproductive history, are thought to act by increasing estradiol (E2) and P4 activity. Despite the potential effects of E2 and P4 on breast cancer risk and progression, little attention has been paid to chemicals that may increase synthesis of these hormones.
To identify these chemical risk factors, a new study conducted by researchers at the Silent Spring Institute searched through data on more than 2,000 chemicals generated by the U.S. Environmental Protection Agency (EPA) ToxCast program. The findings of the study were reported in the journal Environmental Health Perspectives (2021: https://doi.org/10.1289/EHP8608).
The team used data from a high-throughput in vitro steroidogenesis assay developed for the EPA ToxCast program to identify chemicals that increase the synthesis of estradiol (E2) or progesterone (P4) in human H295R adrenocortical carcinoma cells. The ToxCast program aims to improve scientists' ability to evaluate potential risks. It employs automated chemical screening methods to expose living cells to chemicals and investigate the various biological changes they cause.
The EPA's in vitro screening data identified several hundred chemicals that should be considered as potential risk factors for breast cancer because they increased E2 or P4 synthesis. In total, the team discovered 296 unique chemicals increased E2 (182) or P4 (185). Seventy-one chemicals were found to increase levels of both E2 and P4. These chemicals were more likely to be carcinogens or reproductive/developmental toxicants than to not cause those types of effects. Many of these have not previously been identified as potentially related to breast cancer risk. E2- and P4-up chemicals included pesticides; ingredients in personal care products, such as hair dyes; food additives; drinking water contaminants; and chemical flame retardants in building materials and furniture.
In vivo data often demonstrated similar results observed for chemicals that were active in vitro. A significant amount of the chemicals were identified as likely reproductive and developmental toxicants (33% E2 and 33% P4) or potentially carcinogenic (30% E2 and 28% P4) whereas only 5-13 percent were classified as unlikely. However, most of the chemicals had inadequate in vivo data to assess their effects.
Furthermore, of 45 chemicals associated with mammary gland effects and also tested in the H294R assay, 29 increased E2 or P4. In vivo evidence of effects of the 18 most effective E2 and P4 chemicals included increased hormone concentrations, mammary gland effects, and other reproductive and developmental toxicity than to not cause those types of effects. The researchers noted it is still unclear how these chemicals are causing cells to produce more hormones.
The team also outlined several recommendations in their study for improving chemical safety testing to help identify potential breast cancer risk factors before they end up in consumer products, and suggest strategies to decrease people's exposures, particularly during critical periods of development, such as during puberty or pregnancy. The project is part of Silent Spring Institute's Safer Chemicals Program, which is developing new cost-effective ways of screening chemicals for their effects on the breast.
Oncology Times caught up with co-author Ruthann Rudel, MS, for additional insights on the study. She is a toxicologist and Director of Research at Silent Spring Institute.
Oncology Times: Despite the known effects of E2 and P4 on breast cancer, why do you think so little attention has been paid to chemicals that can increase their activity?
Rudel: "When toxicologists test chemicals for safety, common approaches focus on identifying classical carcinogens that damage DNA, systemic toxicants that damage organs (e.g., liver), or chemicals that interfere with reproduction and some aspects of development. Breast carcinogens do not neatly fit that profile. They can act on hormonal pathways, but this possibility simply has not been routinely considered-possibly it is not widely known among toxicologists-and so has not been a priority in design of chemical screening and testing.
"A novel aspect of our study is that we identified many chemicals that increase synthesis of E2 and P4, a pathway that has not been widely considered in chemical testing despite the importance of these hormones in breast cancer progression and recurrence. The hypothesis that chemicals that bind to and activate the ER could increase breast cancer risk has been investigated in some studies, with some compelling findings, but many of the identified chemicals are weaker ER agonists compared with estradiol (Environ Health Perspect 1993; https://doi.org/10.1289/ehp.93101372). Chemicals that increase E2 and P4 synthesis may end up being more important health risks, because they are more biologically effective than the weak estrogens."
Oncology Times: What is one of the most compelling aspects of this study?
Rudel: "Since these chemicals can increase synthesis of E2 and P4, their potential effects are tied very directly to breast cancer etiology and treatment. The Women's Health Study, a large, randomized trial, provided strong evidence that the mix of estrogen plus progesterone in hormone replacement therapy increased breast cancer. Interventions to prevent breast cancer in high-risk women will often reduce the levels of endogenous hormones, as in the removal of the ovaries in women with high-risk inherited genes (Best Pract Res Clin Obstet Gynaecol 2020; https://doi.org/10.1016/j.bpobgyn.2019.11.006). In experimental animals, both E2 and P4 are reported to increase mammary gland tumors and to increase the risk of breast cancer following ionizing radiation (Arch Toxicol 2020; https://doi.org/10.1007/s00204-020-02752-z; Cancer 2007; https://doi.org/10.1002/cncr.22653). Thus, chemicals that increase E2 or P4 might reasonably be assumed to increase breast cancer risk.
"Also, treatment for HR+ breast cancers relies on drugs that block estrogen action, either by antagonizing the estrogen receptor (ER) or by inhibiting the synthesis of E2 by the aromatase enzyme (Best Pract Res Clin Obstet Gynaecol 2020; https://doi.org/10.1016/j.bpobgyn.2019.11.006; Curr Opin Obstet 2014; doi: 10.1097/GCO.0000000000000039). Aromatase-the enzyme that converts androgens to estrogens-is a focus in HR+ breast cancer etiology and treatment. In fact, first-line treatments block aromatase action (aromatase inhibitors).
"At the same time, we know that women with a genetic variant that causes elevated aromatase expression have poor survival following ER-positive breast cancer (Horm Canc 2018; https://doi.org/10.1007/s12672-017-0317-2). Also, aromatase inhibitors have been shown to reduce breast cancer incidence in postmenopausal women by 50 percent (Lancet 2020; https://doi.org/10.1016/S0140-6736(19)32955-1). Thus, it is a compelling hypothesis that exposure to these steroidogenic chemicals-which are also expected to act additively-could increase production of E2 or P4 and thereby increase breast cancer risk."
Oncology Times: The results revealed that most of the chemicals had insufficient in vivo data. What are some of the challenges of investigating potentially relevant in vivo effects for the most compelling and effective E2- and P4-increasing chemicals?
Rudel: "One of the biggest challenges of investigating whether these chemicals can alter the hormonal environment in the breast is the difficulty in obtaining breast tissue from people with and without exposures to these chemicals in order to measure hormone concentrations. Studies suggest the local control of the hormone environment in the breast is important, so concentrations of hormones like estradiol in blood do not necessarily reflect levels in breast.
"Because human breast tissue for high-quality hormone measurements is difficult to obtain, another approach is to study mammary gland changes in animal models, such as rats, exposed to these chemicals.
"Reviewing existing in vivo rodent studies of these chemicals can provide some information, but in many of these studies the most relevant measurements were not included. For example, changes in mammary gland hormone levels and careful examination of mammary gland changes have usually not been measured. So new studies will need to be conducted to learn what the effects of steroidogenic chemicals are in vivo."
Oncology Times: What additional research do you propose to better characterize the significance of these effects?
Rudel: "As next steps, we are developing approaches to estimate risks from exposure to these steroidogenic chemicals, including their effects in combination. Specifically, we plan to use a rat model to measure changes in breast concentrations of estradiol following treatment with a mixture of these chemicals. We are also using computational methods to identify structural features of the chemicals that seem to be associated with this steroidogenic activity in order to develop predictive models to identify chemicals that are likely to have these effects based on chemical structure alone.
"The study highlights many opportunities to strengthen how we use scientific knowledge to evaluate chemicals for safety so that we will be better at finding chemicals that might increase risk of breast cancer. We found many surprising examples showing that, when chemicals are tested for safety, the mammary gland is not properly studied. We also found that many mammary effects-including tumors-are often dismissed in regulatory studies (Mol Cell Endocrinol 2020; https://doi.org/10.1016/j.mce.2020.110927) when they are noticed, for a variety of reasons that reflect outdated concepts of carcinogenesis and an eagerness to approve pesticides. Thus, we think that more rigor should be applied in interpreting observations of chemically induced changes in mammary tissue. Our ongoing work includes identifying ways to encourage more consistent and sensitive mammary gland effects."
Dibash Kumar Das is a contributing writer.