1 Introduction to Endocrine-Disrupting Chemicals

(1)

1 Introduction to Endocrine-Disrupting Chemicals

Andrea C. Gore, P

^H

D

CONTENTS

1 What is an Endocrine-Disrupting Chemical?

2 EDC Effects on Individuals and Populations Across the Life Cycle

3 Mechanisms of EDCs

4 Endocrine Disruption: A Translational Approach

1. WHAT IS AN ENDOCRINE-DISRUPTING CHEMICAL?

The United States Environmental Protection Agency (USEPA) defined endocrine- disrupting chemicals (EDCs) as “exogenous agents that interfere with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis, reproduction, development, and/or behavior.” Although this definition may seem all-encompassing, it is becoming clear that it needs to be extended. The USEPA specifies “exogenous agents,” implying that in order for a substance to be deemed an EDC, it must come from an external source.

However, an organism’s environment consists of not only external factors but also an organism’s internal hormonal milieu. For example, if an organism’s own endogenous hormonal systems are activated or inactivated at inappropriate times, it may disrupt endocrine processes. Another example is that of the mammalian fetus, in which the intrauterine environment may alter endocrine and homeostatic processes. Thus, the environment needs to be redefined to include not only exogenous environmental factors, as specified by the USEPA above, but also internal secretions such as inappropriate endogenous and maternal hormones.

As you will learn from the chapters in this book, EDCs comprise natural substances (phytoestrogens such as soy, alfalfa, and clover), pesticides [dichlorodiphenyl- trichloroethane (DDT)], fungicides (vinclozolin), substances used in production of plastics or as plasticizers (bisphenol A and phthalates), industrial chemicals [polychlo- rinated biphenyls (PCBs)], and metals (cadmium, lead, mercury, and uranium and arsenic, a metalloid; Chapter 5) (1). The components of this list seem to have nothing

From: Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice Edited by: A. C. Gore © Humana Press Inc., Totowa, NJ

3

(2)

in common, yet all are documented as endocrine disruptors. For a more comprehensive list of EDCs, and more details on their structures, functions, and mechanisms, I refer you to several chapters in this book. The chemical structures of specific EDCs are shown in Chapter 4, Fig. 2 (2). Within Chapter 9, Table 1 presents a helpful list of EDC categorization and their effects on female reproductive health in humans (3), and readers should note that one of those EDCs, bisphenol A, is discussed for its low-dose effects on female reproductive health in animal models (Chapter 2) (4). The same EDCs in Table 1 of Chapter 9 are also discussed in the context of male reproductive health (Chapters 3 and 10) (5,6) and neuroendocrine systems (Chapter 4) (2). Chapter 5 discusses how metals may act as EDCs (1). Finally, the end of Chapter 13 (7) provides an important list of references and resources for additional information on EDCs that falls beyond the scope of this book.

2. EDC EFFECTS ON INDIVIDUALS AND POPULATIONS ACROSS THE LIFE CYCLE

Endocrine disruption needs to be considered in the context of both individuals and populations (8). The importance of populations is made clear from epidemio- logic studies demonstrating clear evidence for environmental endocrine disruption in humans. As an example, Chapter 10 of this book (6) provides a careful discussion of the epidemiologic evidence for a link between exposure to EDCs and male reproductive dysfunction. In addition, populations of wildlife are impacted by endocrine disruption (9). However, not every individual within a population may be similarly affected (10). Some may experience overt toxicity, others may experience more subtle dysfunctions, and still others will not have any evident phenotype. These differences in responsiveness among individuals are due to differences in genomes, in combi- nation with an organism’s entire life history of experiences. With the rare exception of monozygotic twins (humans) or identical littermates (animals), each organism has unique sequences of DNA, which may undergo mutations such as point mutations or deletions that compromise the gene product (protein) to result in disease and dysfunction. Moreover, even identical twins are uniquely and differentially affected by their environments, beginning in the uterus and throughout the rest of life (11).

The embryonic period is particularly important in this regard, as it is a developmental window when DNA becomes modified through methylation, demethylation, and/or remethylation, a process that is thought to play a key role in cellular differentiation.

The final methylation patterns are not fixed until late in embryonic development and possibly into early neonatal life (12). Differences among organisms in epigenetic modifications to the DNA, such as DNA methylation or acetylation, result in differential gene functions and potentially, differential vulnerability to EDCs. New evidence indicates that this is a biologically plausible mechanism of action for EDCs in the developing embryo (8,13,14).

Although the developing fetus may be the most vulnerable to endocrine disruption (see Chapters 2–5 for specific examples) (1,2,4,5), effects of EDCs may be exerted in other phases of the life cycle. Moreover, as EDCs themselves exert epigenetic effects that may be passed to subsequent generations, if expressed in the germ line, this may create a “vicious cycle” by which an organism’s epigenome may predispose it to vulnerability to endocrine disruption, which in turn creates additional epigenetic

(3)

modifications that are passed on to the offspring. This concept is discussed most extensively in Chapter 7 of this book (11). Such a situation is further magnified if the descendants continue to live in contaminated environments. Thus, the genome, the epigenome, and the environment interact throughout the entire life cycle to influence the impact of EDCs. These life experiences, from the embryo through aging, are not only unique to each of us but need to be taken into account when considering cumulative effects of EDCs across the life cycle.

3. MECHANISMS OF EDCS

The potential mechanisms underlying the effects of EDCs are incredibly diverse, making studies on their biological effects daunting (for a review of traditional and non- traditional mechanisms of EDCs, see Chapter 6) (15). Indeed, in the case of humans, the latency between early exposures to EDCs and adult dysfunction may be 60 years.

The first important challenge is the nature of hormonal systems. Endogenous hormones act through several mechanisms. The classical mechanism of action for hormones such as estrogens, androgens, thyroid, and progesterone involves the binding of the hormone to its receptor, the interaction of this hormone–receptor complex with other cofactors in a cell, and the activation or inactivation of transcription of a target gene. More recently, membrane steroid hormone receptors have been identified, and these appear to use different intracellular signaling pathways for activation of subcellular processes (16). An important consideration is that the same ligand, for example, estradiol, activates a diversity of target receptors, signaling mechanisms, and may interact with completely different complements of cofactors depending upon the phenotype of the target cell. In addition, hormone signaling also involves the synthesis, degradation, or inactivation of hormones by specific enzymes, any or all of which may be targeted by EDCs. Another consideration is that endogenous hormones, particularly estrogens, androgens, and thyroid, bind to proteins in blood that reduce their bioavailability (i.e., their ability to act upon their receptors). EDCs may not bind to the same binding proteins, thereby increasing their bioavailability relative to endogenous hormones. This concept also applies to the enzymes that synthesize or degrade endogenous hormones. If EDCs are not as rapidly metabolized as endogenous steroid hormones, they may remain bioavailable far longer and get incorporated into the body burden, generally fat stores, as most EDCs are lipophilic. There are numerous other mechanisms for the regulation of steroid hormone actions that are beyond the scope of this introduction, but it is clear that the complexity is enormous. A specific example of the activation of diverse signaling pathways by EDCs is presented for the thyroid system in Chapter 8 of this book (17). When put into the context of how an EDC may cause an effect on an endocrine system, all of these potential targets and pathways need to be considered.

The second and third challenges for endocrine disruption are, respectively, trying to reconcile and understand how extremely low doses of EDCs can exert potent effects on endocrine and homeostatic systems, and related to this, why EDCs exert non-traditional dose–response curves (14,18). Although it is unclear how EDCs can act at such low levels, the implications are extremely clear, that low-dose EDC exposure, particularly at vulnerable developmental windows, can have long-term consequences on later health.

Furthermore, the biological evidence for such low-dose effects is extremely strong

(4)

from basic science studies (Chapter 2) (4). In addition, the finding that EDCs, act in non-linear dose–response curves, often U or inverted U in shape, was initially puzzling to toxicologists. However, these findings were not a surprise to endocrinologists, as hormones often act in non-linear manners because of the diversity of target receptors, cofactors, and other signaling pathways that may not all be activated or inactivated at the same range of dosages. The overall shape of the dose–response curve thus reflects the cumulative action of EDCs upon a range of targets. As a whole, the non-monotonic dose–response curves emphasize the need for communication among basic scientists across disciplines, from toxicology to endocrinology (14). Related to the low-dose phenomenon is the question of whether thresholds for different EDCs actually exist. Environmental toxicological protocols continue to use single doses of a single chemical at different concentrations, seeking the lowest dose at which no adverse effects are observed in the animal subject (the no-observed-adverse-effect level or NOAEL). The NOAEL for the chemical in question is then used as a threshold dose in risk assessments for human exposure. However, a power-analysis study revealed that thresholds may not exist for estrogenic EDCs, as any amount of the exogenous steroidal agent automatically exceeds the organism’s threshold (19).

A fourth challenge is that exposure to EDCs rarely occurs for a single substance, with the rare exception of toxic spills. For the most part, an environment that is contaminated by one industrial waste product will be contaminated by a complex mixture (10). Again, this makes the design of experiments difficult, because there is no “typical” exposure.

Studying the body burdens of humans, as discussed in Chapter 11 (20), has proven to be very important in informing researchers about what we may be exposed to, but it is not possible to be absolutely comprehensive in analyzing these exposures. Thus, some animal studies are designed using single toxicants or phytoestrogens; others use more complex mixtures. There is rationale but also criticism for both of these approaches, making the perfect experiment elusive.

4. ENDOCRINE DISRUPTION: A TRANSLATIONAL APPROACH Although the complexity of endocrine disruption makes understanding and mitigating exposures seem impossible, the chapters in this book help to clarify many of the key issues about endocrine disruption. Part I of this book focuses on the basic biology of EDCs, with greatest emphasis on animal models. The science of endocrine disruption depends on basic scientists to provide the fundamental and strong science that first identifies the EDCs; second, characterizes their effects; third, elucidates the mechanisms; and fourth, understands their functional implications. Chapters 2 through 7 provide such a basis by reviewing the scientific literature and demonstrating that there is conclusive evidence for biological and physiological effects of EDCs upon endocrine systems. In Part II, the biology of EDCs in humans is discussed in Chapters 8 through 11. These chapters discuss the evidence that humans are indeed exposed to EDCs, the mechanisms and implications for these effects, and how exposures are ascertained and measured. Finally, Part III of the book discusses the human health implications and provides information for actually dealing with the problem of EDCs.

EDCs are already in our world, and we need to be able to talk about them with not only scientists and physicians but also industrialists, manufacturers, end users, and the community. Thus, this section of the book has chapters on public policy

(5)

(Chapter 12) (21), on practical advice about what to do to avoid EDCs and provides resources on where to get more scientific information (Chapter 13) (7). Importantly, Chapter 14 (22) talks about a community intervention to reduce exposure to some EDCs, and Chapter 15 (23) poses some potential solutions to the problem.

In the field of endocrine disruption, it is necessary to draw parallels between basic science in animal models and clinical implications in humans. Such a translational approach is the goal of this book, which synthesizes the field for readers such that basic scientists will learn about clinical relevance and clinicians may better understand the basic biology of EDCs for their practice or clinical research laboratories. As a whole, this book addresses the key themes in understanding the mechanisms of endocrine disruption, their relevance to humans, and how we may deal with a problem that is already widespread in our world.

REFERENCES

1. Dyer CA. (2007) Heavy metals as endocrine-disrupting chemicals. In: Gore AC, ed. Endocrine- Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

2. Walker DM, Gore AC. (2007) Endocrine-disrupting chemicals and the brain. In: Gore AC, ed.

Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

3. Janssen S, Fujimoto VY, Giudice LC. (2007) Endocrine disruption and reproductive outcomes in women. In: Gore AC, ed. Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

4. Soto AM, Rubin BS, Sonnenschein C. (2007) Endocrine disruption and the female. In: Gore AC, ed.

5. Cowin PA, Foster P, Risbridger GP. (2007) Endocrine disruption in the male. In: Gore AC, ed.

6. Hauser R, Barthold JS, Meeker JD. (2007) Epidemiologic evidence on the relationship between environmental endocrine disruptors and male reproductive and developmental health. In: Gore AC, ed.

7. Solomon G, Janssen S. (2007) Talking with patients and the public about endocrine-disrupting chemicals. In: Gore AC, ed. Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

8. Crews D, McLachlan JA. (2006) Epigenetics, evolution, endocrine disruption, health, and disease.

Endocrinology 147:S4–10.

9. Zala SM, Penn DJ. (2004) Abnormal behaviours induced by chemical pollution: a review of the evidence and new challenges. Anim Behav 68:649–64.

10. Crews D, Willingham E, Skipper JK. (2000) Endocrine disruptors: present issues, future directions.

Q Rev Biol 75:243–60.

11. Guerrero-Bosagna C, Valladares L. (2007) Endocrine disruptors, epigenetically induced changes, transgenerational transmission of characters, epigenetic states. In: Gore AC, ed. Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

12. Baylin SB, Herman JG, Graff JR, Vertino PM, Issa JP. (1998) Alterations in DNA methylation: a fundamental aspect of neoplasia. Adv Cancer Res 72:141–96.

13. Anway MD, Skinner MK. (2006) Epigenetic transgenerational actions of endocrine disruptors.

14. Gore AC, Heindel JJ, Zoeller RT. (2006) Endocrine disruption for endocrinologists (and others).

15. Adler SR. (2007) Cellular mechanisms of endocrine disruption: traditional and novel actions. In:

Gore AC, ed. Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

(6)

16. Thomas P, Pang Y, Filardo EJ, Dong J. (2005) Identity of an estrogen membrane receptor coupled to a G protein in human breast cancer cells. Endocrinology 146:624–32.

17. Armstrong DL. (2007) Implications of thyroid hormone signaling through the phosphoinositide-3 kinase for xenobiotic disruption of human health. In: Gore AC, ed. Endocrine-Disrupting Chemicals:

From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

18. Welshons WV, Nagel SC, vom Saal FS. (2006) Large effects from small exposures. III. Endocrine mechanisms mediating effects of bisphenol A at levels of human exposure. Endocrinology 147:S56–69.

19. Sheehan DM, Willingham EJ, Bergeron JM, Osborn CT, Crews D. (1999) No threshold dose for estradiol-induced sex reversal of turtle embryos: how little is too much. Environ Health Perspect 107:155–9.

20. Calafat AM, Needham LL. (2007) Human exposures and body burdens of endocrine-disrupting chemicals. In: Gore AC, ed. Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

21. Woodruff TJ. (2007) Policy implications of endocrine-disrupting chemicals in humans. In: Gore AC, ed. Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ:

Humana Press.

22. Brenner B, Galvez M. (2007) Community interventions to reduce exposure to chemicals with endocrine disrupting properties. In: Gore AC, ed. Endocrine-Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.

23. Thornton JW. (2007) Solutions to the problem of endocrine disruption. In: Gore AC, ed. Endocrine- Disrupting Chemicals: From Basic Research to Clinical Practice. Totowa, NJ: Humana Press.