Endocrine disruptors and their impact on human health and the environment

What is an endocrine disruptor?

The WHO/IPCS, together with Japanese, USA, Canadian, OECD and European Union experts, developed a consensus working definition for endocrine disruptors that was also adopted by the European Community Strategy for Endocrine Disruptors [1]:

“An endocrine disruptor is an exogenous substance or mixture that alters function(s) of the endocrine system and consequently causes adverse health effects in an intact organism, or its progeny, or (sub)populations.”

An adverse effect was further specified in the context of reproductive effects: “A change in morphology, physiology, growth, reproduction, development or lifespan of an organism which results in impairment of functional capacity or impairment of capacity to compensate for additional stress or increased susceptibility to the harmful effects of other environmental influences.

Why this growing concern about endocrine disruption and its relation to some chemicals?

There is good evidence that wildlife populations have been affected, with sometimes widespread effects. During the last two decades evidence of increasing trends of many endocrine-related disorders in humans has strengthened, and unfavourable disease trends have become apparent. There are negative impacts on reproduction and development. The issue is that the correct description of how endocrine diseases develop is often complicated by a lack of uniform diagnostic criteria.

Multiple causes underlie these trends, and the evidence is strengthening that chemical exposures are involved. The notion that exposure to chemicals contribute to endocrine disorders in humans and wildlife is supported by extensive laboratory studies. Exposure during some critical periods of development where organisms are especially sensitive to hormonal messages can cause irreversible and delayed effects that do not become evident until later in life. Nevertheless, there are significant difficulties in identifying specific chemicals as contributing to risks especially where chemicals do not stay for long periods in tissues after exposures have occurred.

These toxicological properties justified consideration of endocrine disrupting chemicals (EDCs) as substances of concern equivalent to “carcinogens, mutagens and reproductive” toxicants (“CMRs”), as well as persistent, bioaccumulative and toxic chemicals (“PTBs”).

What are the main effects of endocrine disruptors that are considered?

There are four main groups of endpoints considered:

Impact on human reproductive health
This addresses one of the key topics of human health effects of endocrine disruptors: the impacts on male and female reproductive health. The issue of male reproductive health is dealt within one unifying hypothesis, the testicular dysgenesis syndrome. Due to a lack of similar theory advances, female reproductive health had to be discussed by considering relevant health outcomes separately, such as precocious puberty, female fecundity, polycystic ovary syndrome, fertility, endometriosis and uterine fibroids.

Hormonal cancers in humans
Key advances in understanding the role of endocrine disrupting chemicals in cancers of the breast, prostate, testes and thyroid are summarized. Due to a lack of relevant research findings, other hormonal cancers such as ovarian neoplasms and endometrial cancers are not considered.

Impact on development and metabolism in humans
In this part of a discussion of human health endpoints, the state of the science in the field of developmental neurotoxicity with respect to endocrine disruptors is summarised. This covers a considerable variety of hormone systems, including the thyroid system and aryl hydrocarbon receptor signaling. In addition, scientific advances in our understanding of receptor signalling and molecular biology are continuously blurring the borders between the nervous, immune and endocrine systems. The topic of chemical exposures and their role in the ongoing obesity epidemic is then dealt with in the section on the metabolic syndrome, an issue that has only fairly recently entered the mainstream of endocrine disruptor research.

Impact on wildlife
The summary of pertinent research findings in the arena of wildlife endocrine disruptor research is structured according to major species and phyla and deals with impact on invertebrates, fish, amphibians, reptiles, birds and mammals. The main endpoints that are considered when looking at animals have to do with reproduction and development.

What makes the characterization of endocrine disruptive chemicals (EDCs) particular?

It has been argued that the current risk assessment methods needs modification or, for some, has become obsolete, because EDCs can elicit effects at doses much lower than normally used in classical toxicology testing. According to the authors of the report, the existence of dose thresholds cannot be proven or ruled out by current experimental approaches, because all methods for measuring effects have their limits of detection low enough to obscure thresholds, if they exist.

For example, because of pre-existing exposures to steroidal estrogens, either internal or from the regular diet (phyto-estrogens), it can be inferred that any amount of externally added estrogenic agent could have an effect, without a threshold. This is an important consideration for the role of estrogens during programming of the neuro-endocrine system and timing of puberty or in breast cancer.

Also, many chemicals are capable of interacting with steroid receptors (“endocrine activity”), but whether this always leads to adverse effects is often unclear.

This introduces considerable uncertainties, with the likelihood of overlooking harmful effects in humans and wildlife. Until better tests become available, hazard and risk identification has to rely also on epidemiological approaches.

What are the methods used to evaluate endocrine disruptive effects?

For the identification of the intrinsic endocrine disruptive properties (“hazard assessment”), the approach adopted in this report is the ‘‘weight-of-evidence’ (WoE) approach, which refers very generally to the synthesis or pooling of different lines of evidence and, more specifically to the evidence of harm following exposure to a specific chemical substance. In the ECHA guidance document on how to report weight of evidence, WoE is defined as “the process of considering the strengths and weaknesses of various pieces of information in reaching and supporting a conclusion concerning a property of the substance”. This approach is a summary of the intrinsic endocrine disruptive properties where the methodological approach includes various elements : systematic narrative reviews, criteria-based methods of causal inference, quantitative statistical techniques such as meta-analysis. This WoE approach is distinct from an alternative approach referred to as ‘strength of evidence’, which analyses the degree of positive evidence from a subset of key studies that demonstrate a statistically significant result.

For the evaluation of risks related to the exposure to endocrine disruptive chemicals, epidemiological criteria are used to establish a cause/effect relationship (“causal inference”) and are almost invariably based on criteria featuring a list of nine ‘considerations’ for causation combining statistically significant epidemiological evidence and experimental toxicological evidence.

The existence of epidemiological evidence of effects linked to endocrine disruption in humans and wildlife is indicative of a failure of accurately predicting the toxicity of a given substance. The existing controversy about associations between specific chemicals and their supposed endocrine disrupting properties can then be directly related to divergent interpretations and quantitative or qualitative weights that were attached in such analysis to different types of available evidence. This led to the recognition that elements of the epidemiological evidence need to be better integrated in the hazard characterisation of the endocrine disruptive potential of chemical substances.

For the identification of the mode of action of endocrine disruptors, which can also help determine the shape of the dose-response curve, particularly at low doses, the relevance of effects observed in experimental animals to humans and the variability of the response within the human population including susceptible subgroups are taken into account. To eliminate a postulated mode of action for humans based on experimental data, three elements have to be considered:

1. The mode of action in experimental animals;
2. whether human relevance of this mode of action could be reasonably excluded on the basis of fundamental qualitative differences between experimental animals and humans;
3. whether the relevance of the mode of action in experimental animals could be reasonably excluded on the basis of quantitative differences either in toxicokinetic or toxicodynamic factors between experimental animals and humans.

How are chemicals tested for their endocrine disrupting potential?

For a wide range of endocrine disrupting effects, there are not yet standard and validated test methods. In many cases, even scientific research models that could be developed into tests are missing, says the report.

Internationally agreed and validated test methods (OECD) for the identification of endocrine disruptors are generally regarded as useful, but it is acknowledged that they capture only a limited range of the known spectrum of endocrine disrupting effects. Considerable gaps exist for the identification of chemicals that can affect wildlife taxa. It is thus far not possible to infer the possibility of adverse effects from cost-effective screening-level assays.

The authors consider that information and testing requirements laid down in the EU chemicals regulation do not capture the range of endocrine disrupting effects that can be measured with internationally agreed and validated test methods. In particular, testing with the most sensitive and appropriate methods currently available and with exposure regimens that cover periods of heightened susceptibility during critical life stages, is not conducted.

As disruption of endocrine processes may result in a complex pattern of effects, it is highly unlikely that the information required to determine how adverse effects occur (something refered to as “adverse outcome pathway”) or that the key data that would establish causality, will be available for most suspected endocrine disruptors.

There are thus still enormous knowledge gaps that need to be addressed through research and development projects. Urgently needed are further methods for the identification of endocrine disruptors. Concerted efforts should be undertaken to identify the full spectrum of endocrine disruptors present in the environment and in human tissues.

What is the “State of the Science” for the various families of chemical substances most often considered?

An updated summary of the State of the Science on Endocrine Disruptors was published in January 2012 as the Annex 1 of the report to the European Commission State of the Art Assessment of Endocrine Disruptors”.

 http://ec.europa.eu/environment/chemicals/endocrine/pdf/annex1_summary_state_of_science.pdf

Here are some highlights of the report for some of the main families of chemicals considered for their endocrine disrupting potential.

• Bisphenol A has been researched very thoroughly over the last few years. Its effects are multifaceted, mediated by its ability to bind to both the estrogen and progesterone receptors, and its properties as a thyroid hormone antagonist. Exposure during organogenesis has been demonstrated to have irreversible adverse effects on reproductive development.
• Phtahlates. Two key studies conducted in the USA among young boys are considered in the report to provide good evidence of associations of irreversible effects in the form of altered hallmarks of sexual differentiation with elevated phthalate exposures during fetal life. The effects on male development are attributed to the ability of phthalates to directly interfere with testosterone synthesis. Some phthalates, including benzyl butyl phthalate (BBP) and DEHP are also agonists of the estrogen receptor. A mixture of phthalates tested for antiandrogenic properties in vivo acted together in line with the dose addition model. Effects on wildlife remain largely uninvestigated.
• Parabens. Epidemiological evidence of endocrine effects in humans is very limited. An association emerged recently between blood serum paraben levels and mammographic breast density in postmenopausal women, but there is no evidence that exposure may increase breast cancer risk.
• PCBs (polychloro biphenyls) According to the report, a convincing link has been made between PCB exposure and an increased risk of breast cancer in women and there is some evidence that the incidences of fibroids, thyroid cancer and prostate cancer are also increased. Neuro-developmental endpoints are also known targets of PCB exposure, which has been linked to serious and irreversible effects on cognition, motor and sensory function.In a variety of exposed vertebrate wildlife species, estrogen-related developmental perturbations, thyroid irregularities and suppression of thyroid hormone synthesis, have also been found.
• Dioxins (PCDDs and PCDFs) These chemicals share some of the same mechanisms of action as “co-planar” PCBs, binding to the same cellular receptor, perturbing thyroid function, but they do not activate the cellular estrogen receptor. Exposure during infancy to TCDD (the dioxin that was released a as consequence of the Seveso accident in 1976), has been shown to lead to irreversible reductions in sperm motility and sperm concentration. Strikingly, the opposite effect was observed among men who were exposed during puberty.
  Reduced age at menopause has been associated with exposure in adult women. There is an association with high PCDD/PCDF exposures and breast cancer, and a suggestive, almost significant association with thyroid cancer.
• Polybrominated biphenylethers (used a.o. as flame retardants) Exposure in utero or during early development can have profound and irreversible effects on neurodevelopment. Various possible effects on the reproductive function are described in the report but in most cases the evidence still remains hypothetical.
• Perfluorinated compounds (PFCs). In a human study, association was found between exposure to one PFC, perfluorooctanoic acid (PFOA), and thyroid hormones, suggesting that these chemicals interfere with thyroid hormone conversion. Although a causal link has yet to be established, mechanistic and epidemiological studies show a consistent positive association between PFC body burdens and increased cholesterol levels, indicating that they could create metabolic disorders. The endocrine effects of PFCs on wildlife have yet to be determined.
• Pesticides. Various families of pesticides are considered:
  
  • Dicarboxamides, despite experimental evidence of endocrine disrupting mechanisms for some substances (vinclozolin), no direct evidence of exposure-disease associations exists yet for either humans or wildlife.
  • Azole fungicides (including triazoles and imidazoles), studies of agricultural workers have detected statistically significant increases in reproductive abnormalities in the children of mothers exposed to these pesticides during pregnancy. Unfortunately, these studies do not identify the individual compounds or compound classes that the women were exposed to.
  • Triazines, with atrazine and simazine as the two most widely used herbicides), feminised secondary sexual characteristics, intersex and gonadal dysgenesis, have been observed in wild frogs collected from contaminated sites. Effects in mammals remain largely undetermined and the report does not mention specific effects on humans.
• Heavy metals.
  
  • Methylmercury has multiple modes of action relating to the endocrine system.
  • Lead exerts an endocrine mediated action via the enhanced pituitary release of the thyroid hormone TSH.
  • Cadmium, a weak link is made by some epidemiological studies of occupational exposure and breast cancer. There is also some evidence that cadmium may contribute to menstrual abnormalities and increased time to pregnancy but no association has been found with endometriosis. Associations between cadmium exposure and prostate cancer are weak, although these associations tend to be stronger for more aggressive forms of the disease. Investigations showed also a dose-response relationship between cadmium exposure and uterine weight. Its ED effects in wildlife are understudied.
• Other chemicals.. Multiple new chemicals and groups of chemicals have come to the fore over the last decade as being of potential concern. For sunscreen UV filters and for artificial musks, which are present in many cosmetic and personal care products, there is not yet direct evidence of either group of compounds having ED effects on humans, but some evidence exists for some of them from studies conducted in animals and in vitro.

How to regulate endocrine disruptors in the E.U?

An overview of proposals for regulating endocrine disruptors by EU Member States and other organisations revealed some commonalities and areas of agreement. More controversial are the proposals to classify endocrine disruptors on the basis of values reflecting their potency according to the Regulation on classification, labeling and packaging of substances and mixtures (CLP). The authors consider such rigid cut-off values largely arbitrary, and not scientifically justifiable.

According to the authors, defining endocrine disruptors for regulatory purposes, will have to rely on criteria for adversity and endocrine-related modes of action. Based on earlier proposals by various EU Member States and organisations, including ECETOC, a decision tree approach is proposed that proceeds in a step-wise manner by excluding substances that neither produce adverse effects, nor show endocrine-related modes of action. Substances producing effects shown to be of no relevance for humans or wildlife can also leave the decision tree, but in the absence of appropriate evidence, relevance should be assumed by default.

The final regulatory decision should be based on a consideration of the toxicological profile of the substances in a Weight-of-Evidence approach, which has yet to be developed, (see 5.1 above) and that would have to consider other factors such as severity and specificity of effect and irreversibility. Procedures that incentivise the provision of data in the case of data gaps are suggested.

What are the main recommendations of the report?

The six recommendations made in this report are:

1. Implement recently updated or enhanced validated and internationally recognised test methods in the testing and information requirements for PPPR (Plant Protection Products Regulations) and REACH,
2. Develop further guidance documents for the interpretation of test data;
3. Consider the creation of a separate regulatory class “Endocrine Disruptor” (ED);
4. Develop weight-of-evidence procedures that deal with the available evidence by weighing the criteria “adversity” and “mode of action” in parallel, but not by applying these criteria sequentially to exclude substances from the assessment;
5. Abandon “potency” as a rigid and decisive cut-off criterion for endocrine disruptors of regulatory concern, and consider potency, together with other criteria such as toxicity, specificity, severity and irreversibility in a weight-of-evidence approach.
6. Create regulatory categories that stimulate the generation of the necessary data, including test methods that are not validated yet, beyond the OECD Conceptual Framework.