Hormones and endocrine-disrupting chemicals: low-dose effects and nonmonotonic dose responses.

Environmental Chemicals: Evaluating Low-Dose Effects

Chapter 7 - Nonmonotonic Responses in Endocrine Disruption

Chapter 7 - Low Dose Effects and Nonmonotonic Dose Responses for Endocrine Disruptors

Non-monotonic dose response curves (NMDRCs) have been demonstrated for natural hormones and endocrine disrupting chemicals (EDCs) in a variety of biological systems including cultured cells, whole organ cultures, laboratory animals and human populations. The mechanisms responsible for these NMDRCs are well known, typically related to the interactions between the ligand (hormone or EDC) and a hormone receptor. Although there are hundreds of examples of NMDRCs in the EDC literature, there are claims that they are not 'common enough' to influence the use of high-to-low dose extrapolations in risk assessments. Here, we chose bisphenol A (BPA), a well-studied EDC, to assess the frequency of non-monotonic responses. Our results indicate that NMDRCs are common in the BPA literature, occurring in greater than 20% of all experiments and in at least one endpoint in more than 30% of all studies we examined. We also analyzed the types of endpoints that produce NMDRCs in vitro and factors related to study design that influence the ability to detect these kinds of responses. Taken together, these results provide strong evidence for NMDRCs in the EDC literature, specifically for BPA, and question the current risk assessment practice where 'safe' low doses are predicted from high dose exposures.

Endocrine-disrupting chemicals (EDCs) interfere with hormone action by altering hormone synthesis, secretion, transport in the blood, binding to receptors, metabolism, or excretion. This chapter reviews the history of EDCs and other environmental chemicals, methods used to identify EDCs, and common uses for these chemicals in consumer products. It also describes major principles of endocrinology and how these features influence the actions of EDCs. This chapter will also evaluate controversies in the study and regulation of EDCs, including the concept of “low dose effects,” the question of whether humans are exposed to EDCs at levels that can cause harm, and the determination of “safe” doses of exposure. Finally, this chapter reviews other environmental factors that can influence the health of laboratory animals and interfere with the study of EDCs.

Regulatory decisions on endocrine disrupting chemicals should be based on the principles of endocrinology

Possible harm from endocrine disrupting chemicals (EDCs) in humans is speculated based on two types of evidence; 1) increasing trends of suspected diseases in ecological studies of populations and 2) findings from traditional epidemiological studies of individuals. However, ecological findings are not regarded as direct human evidence of the relation between EDCs and disease, while the evidence among epidemiological studies of individuals is often inconsistent. Thus, a criticism is that linking EDCs and health in human is naively presumed without solid evidence. However, human studies of EDCs are methodologically complex and understanding methodological issues will help to interpret findings from existing human studies and to properly design optimal human studies. The key issues are low reliability of exposure assessment of EDCs with short half-lives, EDC mixtures, possibility of non-monotonic dose-response relationships, non-existence of an unexposed group, difficulties in measuring exposure during critical periods, and interactions with established risk factors. Furthermore, EDC mixtures may affect human health through other mechanisms than traditional endocrine disruption, for example glutathione depletion or mitochondrial dysfunction. Given this complexity, the most plausible scenario in humans is that exposure to EDC mixtures leads to increasing risk of related diseases at the ecological level, but inconsistent associations would be expected in traditional epidemiological studies. Although epidemiologists have long relied on Bradford Hill's criteria to objectively evaluate whether associations observed in epidemiology can be interpreted as causal, there are challenges to use these criteria for EDCs, particularly concerning consistency across studies and the findings of linear dose-response relationships. At the individual level, compared to EDCs with short half-lives, epidemiological studies of EDCs with long half-lives among populations with a relatively low exposure dose range of exposure can likely produce relatively more reliable results, because the measurement of EDCs with long half-lives likely represents typical long-term exposure and populations with exposure in the low range of doses are likely to have a reference group closer to non-exposure.

Abbreviated assessment of bisphenol A toxicology literature

The mission of National Institute of Environmental Health Sciences (NIEHS) is to improve the health of the American people by understanding the role of environmental exposures in disease and dysfunction. We accomplish this mission by conducting and funding research—including in vitro, animal, and human studies—on the health effects of environmental agents. Our goal is to prevent disease by identifying and reducing exposures to environmental agents that compromise health. It is clear that every complex disease has both an environmental and a genetic component. Thus, NIEHS-sponsored research must play an important role in understanding disease etiology. In the last few years there have been workshops (Melnick et al. 2002;vom Saal et al. 2007), manuscripts (Myers et al. 2009a, 2009b), and even society-position papers (The Endocrine Society 2009) indicating that increased use of environmental health science data by policy makers should lead to reductions in the human burden of disease. There are several recent examples of how research supported by the NIEHS is leading to paradigm shifts in understanding how environmental toxicants—even at very low-level exposures—can have significant consequences, including dysfunction and disease. These paradigm shifts are being informed by new approaches for dose measurement. NIEHS researchers are turning their attention to the “environmentally relevant dose,” which is the dose in the range of typical human exposure as measured in tissue, blood, and urine of study subjects. Simply put, the environmentally relevant dose is based on the internal concentration of the toxicant rather than the administered dose. In 2007, the NIEHS invited a panel of experts to Chapel Hill, North Carolina, for a scientific review of all literature published on bisphenol A (BPA). The expert panel then issued a consensus statement (vom Saal et al. 2007), which concluded that low environmentally relevant doses of BPA could cause numerous diseases in animal models, and that there was evidence for both low-dose effects and for nonmonotonic dose–response relationships. Overall, similar conclusions were reached by the National Toxicology Program’s Center for the Evaluation of Risks to Human Reproduction (NTP 2008), which focused on the developmental and reproductive effects of BPA. An article in this issue of Environmental Health Perspectives (Myers et al. 2009b) highlights this discussion of low-dose effects and notes that nonmonotonic, or biphasic, dose–response curves are commonly observed in endocrinology. This suggests that high doses may not be appropriate to predict the safety of low doses when hormonally active or modulating compounds are studied. Their conclusions are supported by the position statement published by the Endocrine Society (2009). This debate—whether chemicals with endocrine-disrupting activity can cause toxicity at environmentally relevant doses—has been under way for more than a decade (Melnick et al. 2002). There are now low-dose data not only on BPA but also on phthalates, polychlorinated biphenyls (PCBs), dioxins, heavy metals such as lead and mercury, perchlorate, and some diverse pesticides such as hexachlorobenzene and atrazine. Indeed, the doses used in many animal toxicology studies result in internal concentrations that are in the range of human exposures. Many of these low-dose studies demonstrate that the timing of exposure is critical to the outcome and that exposures during early life stages (fetal, infant, and pubertal) are particularly important. This recognition of critical windows of vulnerability not only demonstrates the developmental basis of disease but also that the timing, as well as the dose, makes the poison. Understanding the connection between our health and our environment, with its mixture of chemicals, diet, and lifestyle stres-human genome; just as we have moved beyond “one gene, one disease,” we must move beyond “one chemical, one dose (range), one health outcome.” Reliability and validity are established in science by replication of findings in multiple independent studies. A weight-of-evidence approach is essential in understanding the public health impacts of environmental exposures.

Low-dose effects and nonmonotonic dose–responses of endocrine disrupting chemicals: Has the case been made?

Hormesis (defined operationally as low-dose stimulation, high-dose inhibition) is often used to promote the notion that while high-level exposures to toxic chemicals could be detrimental to human health, low-level exposures would be beneficial. Some proponents claim hormesis is an adaptive, generalizable phenomenon and argue that the default assumption for risk assessments should be that toxic chemicals induce stimulatory (i.e., “beneficial”) effects at low exposures. In many cases, nonmonotonic dose–response curves are called hormetic responses even in the absence of any mechanistic characterization of that response. Use of the term “hormesis,” with its associated descriptors, distracts from the broader and more important questions regarding the frequency and interpretation of nonmonotonic dose responses in biological systems. A better understanding of the biological basis and consequences of nonmonotonic dose–response curves is warranted for evaluating human health risks. The assumption that hormesis is generally adaptive is an oversimplification of complex biological processes. Even if certain low-dose effects were sometimes considered beneficial, this should not influence regulatory decisions to allow increased environmental exposures to toxic and carcinogenic agents, given factors such as interindividual differences in susceptibility and multiplicity in exposures. In this commentary we evaluate the hormesis hypothesis and potential adverse consequences of incorporating low-dose beneficial effects into public health decisions.

Critical comments on the WHO-UNEP State of the Science of Endocrine Disrupting Chemicals – 2012

In the fields of endocrinology and toxicology, there are ongoing debates about whether endocrine disrupting chemicals (EDCs) produce non-monotonic dose responses. This type of response is typically characterized by a U- or inverted U-shaped relationship between dose and effect. In a recent report, a US EPA panel concluded that non-monotonicity is observed for EDCs, but that these responses are not expected in vivo, and are not typically observed in apical endpoints, i.e. endpoints that are indicators of adverse effects. Here, we have analyzed the shapes of the dose response curves in an EPA report analyzing the effectiveness of the Endocrine Disruptor Screening Program (EDSP) Tier 1 assays. This report included the results of 11 guideline assays for each of 4 chemicals. We found indications of non-monotonic dose response curves (iNMDRCs) for 3 of the 4 coded chemicals. In total, 27% of assays with dose response data included at least one iNMDRC. When the endpoints from the 4 test chemicals with dose response data were considered together, 9% were consistent with non-monotonicity. Collectively, these results indicate that non-monotonic responses occur in guideline endpoints including the kinds of adverse outcomes that regulatory agencies use in chemical safety assessments such as circulating hormone concentrations and organ weights. These results should inform discussions about whether NMDRCs are 'real' and occur frequently enough to be important. Because risk assessments typically involve high dose testing, limited dose groups (often only 3) and extrapolation to lower doses that are not expected to have adverse effects, the presence of NMDRCs challenges this status quo.

Non-monotonic dose response curves (NMDRCs) occur in cells, tissues, animals and human populations in response to nutrients, vitamins, pharmacological compounds, hormones and endocrine disrupting chemicals (EDCs). Yet, regulatory agencies have argued that NMDRCs are not common, are not found for adverse outcomes, and are not relevant for regulation of EDCs. Under the linear dose response model, high dose testing is used to extrapolate to lower doses that are anticipated to be ‘safe’ for human exposures. NMDRCs that occur below the toxicological no-observed-adverse-effect level (NOAEL) would falsify a fundamental assumption, that high dose hazards can be used to predict low dose safety. In this commentary, we provide examples of NMDRCs and discuss how their presence in different portions of the dose response curve might affect regulatory decisions. We provide evidence that NMDRCs do occur below the NOAEL dose, and even below the ‘safe’ reference dose, for chemicals such as resveratrol, permethrin, chlorothalonil, and phthalates such as DEHP. We also briefly discuss the recent CLARITY-BPA study, which reported mammary adenocarcinomas only in rats exposed to the lowest BPA dose. We conclude our commentary with suggestions for how NMDRCs should be acknowledged and utilized to improve regulatory toxicity testing and in the calculation of reference doses that are public health protective.

Cook and Calabrese (2006) make inaccurate claims about our perspective on hormesis (Thayer et al. 2005). They define hormesis as “low-dose stimulation and high-dose inhibition,” declaring “beneficial/ harmful effects should not be part of the definition, but reserved to subsequent evaluation. . . .” Yet, they advocate higher permissible environmental levels of hazardous agents based on purported health benefits. Cook and Calabrese promote changing the way carcinogens are regulated to accommodate hormesis, recognizing that this “would result in cancer risk assessment values about 100- to 200-fold higher than currently employed” (Calabrese and Cook 2005). Previously, Calabrese and Baldwin (2003a) stated, “agencies will need to accept the possibility (actually, the likelihood) that toxic substances, even the most highly toxic (e.g., cadmium, lead, mercury, dioxin, PCBs, etc.) can cause beneficial effects at low doses.” We are concerned that changing health policies to permit higher exposures based on alleged benefits would be harmful, particularly to susceptible subgroups and individuals exposed to mixtures (Thayer et al. 2005). Instead Cook and Calabrese (2006) suggest that policy decision making “may tend to bring various subgroups in the population together to debate one group’s health benefit against another group’s health risk.” To pit one group against another is absurd. Health-protective default assumptions that are used to compensate for uncertainties should not be dismissed based on untested propositions that likely incur greater risks. Contrary to statements made by Cook and Calabrese (2006), in our article (Thayer et al. 2005) we never claimed that hormetic responses are rare. Rather, we argued that hormesis should not be assumed as universal. In fact, we have published on nonmonotonic dose responses in biological systems (Kohn and Melnick 2002; Welshons et al. 2003). We argue against the assumption that “an exposure limit in the range of the maximum stimulation could promote appreciable benefits in public health” for the general population (Cook and Calabrese 2006). Yet, we fully support addressing non-monotonic dose–response relationships in risk assessments. Further, we never claimed that “comprehensive mechanistic knowledge is necessary” before making a public health decision. In fact, we have a history of arguing the contrary. Indeed, if this standard were operating today, we might still be debating the dangers of tobacco smoke and benzene, among many others. Calabrese appears to overstate the frequency of hormetic dose–response curves. Some responses considered “stimulatory” are not, such as decreased interleukin-2 release, blood pressure, memory, and prolactin level (Calabrese and Baldwin 2003b). His hormesis database contains U- or J-shaped curves where the low dose “stimulation” is actually decreased compared to control values (Calabrese and Baldwin 2001). There should be some mechanistic indication of what specifically is being stimulated (and inhibited at higher doses) before considering a curve hormetic. Otherwise, the empirical observations of Calabrese and colleagues simply reflect nonmonotonic dose responses. The quote we used from the BEIR VII (National Research Council 2005) that draws attention to the lack of evidence of a health benefit from low doses of ionizing radiation was not misleading; Kaiser (2005) also reported that the National Research Council dismissed “the hypothesis that tiny amounts of [ionizing] radiation are harmless or even beneficial,” noting that cancer risk increases proportionally with exposure. In contrast, Calabrese and Cook (2005) claimed that all or most carcinogens have a hormetic dose(s) at which tumors will be decreased. This is contrary to what we know about the carcinogenicity of chemicals and radiation. Labeling a dose response as hormetic to justify higher exposures and claimed benefits for the general population without providing scientific evidence is counter to public-health protective assumptions. For example, cadmium has been touted as a hormetic agent with benefits (Calabrese and Baldwin 2003) because low doses are associated with decreases in testicular tumors in rats. However, Waalkes et al. (1997, 1988) reported increases in prostate tumors within the hormetic dose range for testicular tumors. In our article (Thayer et al. 2005), we emphasized the latter, whereas it was seemingly ignored by Calabrese and Baldwin (2003), because cadmium is a human carcinogen and includes associations with cancer of the prostate and other organs [National Toxicology Program (NTP) 2004]. In addition, differential susceptibility must be addressed because it is well established that cancer and other health risks from ionizing radiation, some chemotherapeutics, and passive tobacco smoke are much greater for those exposed in utero or as children. We should not allow another tragedy such as the one caused by diethylstilbestrol. Disease prevention strategies should not rely on higher environmental exposures to known toxicants (e.g., cadmium, lead, mercury, dioxin, polychlorinated biphenyls). Setting environmental exposure limits based on ranges of maximum stimulation (i.e., equated with postulated hormetic benefits) is a totally unjustified public health policy that would impose greater involuntary risks on sizable segments of the population.