1. Environmental Endocrine
Disruptors
Part I: Toxicity
Pineal gland
Hypothalamus
Michael H. Dong
MPH, DrPA, PhD
Pituitary gland
Thyroid
Parathyroid
gland
gland
Adrenal
glands
Pancreas
This is the first of a three-part lecture characterizing the toxicity, bioaccumulation, and persistence properties of environmental endocrine disruptors (EEDs). The characterization is primarily from a health risk assessment perspective. Therefore, the effects and properties so characterized are mainly those pertinent to exposure and risk significance, rather than those dealing with adverse health effects, of which endocrine disruption is an important type. The lecture’s overall objective is to convey the concept that even for a short duration, exposure to a persistent bioaccumulative EED of low disruption potency could lead to a severe health outcome.
To facilitate the characterization so promised, this Part I presents an overview of the human endocrine system and the basic mechanisms of endocrine disruption. It then concludes with a literature review to ascertain the effects of endocrine disruption as a major health problem.
Part II reviews the factors fundamental to a substance’s fate and bioaccumulation in the environment. Another effort of Part II is to provide some evidence from the literature on bioaccumulation and long-range transport. These literature data are intended to support the asseveration that many endocrine disruptors are highly bioaccumulative.
Part III brings forth the main themes of this lecture. It illustrates how and why endocrine disruption is inducible from exposure to a persistent bioaccumulant, even when its disruption potency is low and the exposure duration is short. Part III further elucidates the notion that at times, even induction of this kind could end with a severe health outcome.
Ovaries
Testicles
(women)
(men)
Readings

2. 05/30/2004, Elk Grove, California, USA
Dr. Michael H. Dong, born in Hong Kong, holds a DrPA (Doctor of Public Administration) in health policy from the University of Southern California, and a PhD in Environmental Epidemiology from the University of Pittsburgh. He has also earned a BSc in biochemistry from the University of California at Davis/Riverside, a second BSc in forensic science from the California State University at Sacramento, and a MPH in environmental and nutritional sciences from UCLA. Dr. Dong currently is a Certified Nutrition Specialist (CNS) and a Diplomate of the American Board of Toxicology (DABT).
Dr. Dong has been working for 15 years as a regulatory toxicologist at the State of California Department of Pesticide Regulation. Previously, he was on active duty for several years as a U.S. Public Health Service Commissioned Corps officer, working at the Center for Drug Research and Evaluation, U.S. Food and Drug Administration. He also served for several years as a U.S. Army Medical Service Corp officer with various assignments working as a nutritional, clinical, or research biochemist. Between the two military services, Michael worked for a year as an occupational toxicologist as well as an environmental epidemiologist at the Occupational Safety and Health Administration, U.S. Department of Labor.
Michael's academic and professional trainings, his published work in such innovative concepts as physiologically-based pharmacokinetic (PB-PK) modeling and aggregate exposure assessment through Monte Carlo simulation, and the series of online lectures on such subjects as toxicologic epidemiology, herbal medicine, and environmental endocrine disruption, all point to his continuing interest in and commitment to promoting global health.
05/30/2004, Elk Grove, California, USA

3. Course Objectives
Be familiar with the terms used, especially those pertaining to environmental endocrine disruption.
Undertake a brief review of the human endocrine system, which regulates our body’s day-to-day function and development.
Understand the basic modes of endocrine disruption.
Appreciate the potential, yet at times even dreadful, effects of endocrine disruption.
In the simplest term, environmental endocrine disruptors (EEDs) are chemical pollutants in the environment that have the potential to affect the endocrine system in animals, especially in humans and wildlife. The endocrine system is a complex network of glands that synthesize and secrete hormones into the circulatory system to coordinate, regulate, and control many of the body's physiologic functions, including reproduction, growth, sexual characteristics, and homeostasis. A hormone, the word meaning to stimulate or to activate, is a chemical signal sent from cells to cells within the same organism; it is a chemical substance with the ability to transmit cellular instructions, and hence often referred to as a chemical messenger.
The objectives of this part (chapter), Part I of the lecture, are for students: (1) to be familiar with the terms and definitions used, especially those pertaining to EEDs; (2) to undertake a brief review of the human endocrine system, which is one of the two major networks regulating our body’s day-to-day function and development; (3) to understand the basic mechanisms of endocrine disruption, which involves one or more biochemical actions or reactions in the endocrine system; and (4) to appreciate the potential effects of endocrine disruption, which at times can even be life-threatening.
The health problems with EEDs are of global concern, mainly due to their vast availability for exposure, which is the topic discussed extensively in Part II continuing into Part III. Meantime, some incidences caused by endocrine disruption have been found to be severe to humans and wildlife. Selected cases are thus provided toward the end of this presentation to strengthen the student’s appreciation of the actual health impacts from these pollutants.

4. Endocrine Disruption
Endocrine disruption can be defined as one or more biochemical actions disrupting the endocrine system, or as the resultant effects.
Its impact on health is not a new concept, as evident from the epidemic linking a rare form of vaginal cancer to the maternal use of the estrogen DES (diethylstilbestrol).
A good number of laboratory assays also have shown successfully that some pesticides and industrial chemicals can induce similar hormonal disruption.
Endocrine disruption can be broadly defined as one or more biochemical actions disrupting the endocrine system in our body, or as the resultant effects. In most cases where the biochemical action is not therapeutic, the resultant effects can be detrimental and even fatal. Therefore, in most instances, such a biochemical interference or disruption is toxicological in nature, rather than pharmaceutical.
To many health professionals, the concept that endocrine disruption can cause adverse health effects is not new. In 1970, for example, clinicians were able to link many women’s use of diethylstilbestrol (DES) to a rare form of vaginal cancer (clear cell carcinoma) found in their daughters from preteen to as old as in the 30s. DES is an estrogen first synthesized in 1938, and was commonly prescribed for prevention of miscarriages from 1940 until banned in the US in Some daughters of DES-treated women were further found to have an increased risk of reproductive and immunologic abnormalities; and some sons of the treated women were at an increased risk for non-cancerous epididymal cysts and abnormal spermatogenesis.
In recent years, a good number of laboratory assays have also shown successfully that some pesticides and other industrial chemicals can induce the DES type of hormonal disruption directly or indirectly. Because these chemicals are often present as environmental pollutants, their persistence in the environment is thus of great concern from a public health perspective. The health concern over their persistence effects, which is the heart of this three-part lecture, can be further appreciated with some understanding of how the endocrine system works. It is for this reason that an overview of the endocrine system is presented in the next few slides.

5. Endocrine System (I)
Cells, organs, and functions in the human or animal body are regulated practically every day by the endocrine system.
Structurally, the endocrine system is a collection of ductless glands that secrete chemical messages known as hormones.
Main function of the endocrine network is to maintain homeostasis of and long-term control in the body by means of chemical signals. It works in parallel with the nervous system to control many body functions.
The cells, the tissue organs, and the biological functions in the human or animal body are regulated or influenced practically every day by two anatomical and physiological systems. These are the nervous system and the endocrine system. The nervous system coordinates rapid and precise responses to stimuli through a momentary change in electrical potential in a cell or tissue. This momentary change occurs when a cell or tissue has been activated by a stimulus, and is commonly referred to as action potential. The functions performed by the nervous system are much more immediate and rapid, such as the control of breathing and body movement. Further discussion of this energy concept and the nervous system is beyond the scope of this lecture. Students with an interest in this subject area are referred to the basic textbooks on general physiology or neurophysiology.
The endocrine network, on the other hand, maintains homeostasis of and long-term control in our body by means of chemical signals. It works in parallel with the nervous system to control many body functions along with homeostasis. Homeostasis is a physiological term used to define a biological entity’s ability or tendency to maintain a steady internal state.
The endocrine system is a collection of ductless glands that secrete chemical messages which are commonly referred to as hormones. These signals are passed through the blood to arrive at target cells which possess the appropriate receptor, much like a lock of definite shape to which only one or certain keys can fit. Exocrine glands that secrete substances that are passed outside the body are ducted structures, such as digestive glands, sweat glands, and salivary glands. These ducted glands are thus by definition not part of the endocrine network.

6. Endocrine System (II)
The glands that make up the (human’s) endocrine system are hypothalamus, pituitary, thyroid, parathyroid, adrenals, pineal body, pancreas, ovaries, and testicles.
The primary function of these glands is to synthesize and secrete hormones.
Acting as body’s messengers, hormones transfer information and instructions from one set of cells to another; the shape of each hormone molecule is specific and can bind to certain cellular receptors only.
Unlike the nervous system, the endocrine system operates in a less rapid and more long-lasting manner. It is important to note that when the endocrine glands and the hormones that they secrete do not work properly, a variety of disorders can arise. Acting as the body’s messengers by transferring important information and instructions from one set of cells to another, hormones regulate the mood, body development, sexual function, pregnancy, other reproductive processes, and much more.
The glands that make up the (human’s) endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenals, pineal body, pancreas, ovaries, and testicles. Of note is that by anatomical design, part of the pancreas is exocrine, as its bulk is connected to the digestive system and secretes digestive enzymes into the intestine. Endocrine glands are not the only ones secreting hormones in our body. Some non-endocrine organs, such as the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta, also produce (a small amount of) hormones.
The shape of each hormone molecule is specific and can bind to certain cells only. These unique binding sites on the target cells are called hormone receptors. Many hormones come in antagonistic pairs with opposite effects on the target organs. For example, glucagons and insulin have opposite effects on the liver’s control of sugar level in the blood. Insulin lowers the blood sugar levels by instructing the liver to take glucose out of circulation and store it, whereas glucagons instruct the liver to raise the blood sugar level by releasing some of the stored glucose. Many hormonal regulations depend on feedback loops to maintain balance and homeostasis.

7. Types of Hormones
Hormones are typically grouped into three classes: steroids, amines, and peptides.
Nearly all the steroid hormones are lipids synthesized from cholesterol; they are responsible for the development of many male and female sex characteristics.
Amine hormones are (all) derived from the amino acid tyrosine secreted by the thyroid.
Most hormones are peptides, thus each with only a short chain of amino acids; they are synthesized largely as proteins first.
Hormones are typically classified into three groups, based on their chemical structure rather than their function. These three groups are steroids, amines, and peptides.
All the steroid hormones, except retinoic acid, are lipids synthesized from cholesterol through a series of biochemical reactions. 17-Estradiol is perhaps one of the few most discussed hormones in this group, as it is the active form of estrogen responsible for the development of many female sex characteristics. Testosterone, having a structure similar to estradiol, is an androgen responsible for the development of many male characteristics. These steroids are referred to as sex hormones and are secreted by the adrenal cortex, the placenta, and mostly the gonads. Some other steroid hormones are aldosterone, corticosterone, and progesterone.
Amine hormones are (almost) all derived from the amino acid tyrosine. They are secreted by the adrenal medulla but largely by the thyroid, and are typically stored as granules in the cytoplasm until needed. Epinephrine and thyroxine are the notable ones in this group.
Most hormones are peptides, thus each having only a short chain of amino acids. They are secreted by several tissue organs including the pituitary, parathyroid, heart, stomach, liver, and kidneys. Many hypothalamic factors (e.g., gonadotropin- and growth hormone-releasing hormones) and pituitary hormones (e.g., growth hormone, thyroid-stimulating hormone) belong to this peptide group. Also included in this group are insulin and glucagons. These peptide hormones are synthesized largely as proteins on the ribosomes first, and then cleaved into the size of amino acids. Some of them also serve as neurotransmitters.

8. Hypothalamus, Pituitary
The hypothalamus is located below the thalamus, in the lower center part of the brain; beneath this gland is the pituitary, which has the size of a pea. Together, these two glands control many other endocrine functions.
Hormones from the two glands are crucial to pregnancy, birth, lactation, and a woman’s menstrual cycle, including ovulation.
Growth hormone and antidiuretic hormone are also crucial hormones secreted by the anterior and posterior pituitary, respectively.
The hypothalamus is located in the lower center part of the brain, below the thalamus. This gland links the nervous system to the endocrine network by producing hormones capable of directing the anterior pituitary to release other hormones into the circulatory system. The pituitary gland, located at the base of the brain just beneath the hypothalamus, is no bigger than a pea. Together, these two glands control many other endocrine functions. The pituitary gland is often called the “master gland,” and is structurally divided into the anterior and the posterior lobe. The stimulation and the suppression of hormone secretions from this master gland are regulated by factors released by nerve cells (neurons) of the hypothalamus.
The several hormone products secreted by the hypothalamus and the pituitary are very crucial to pregnancy, birth, lactation, and the menstrual cycle. These include the follicle-stimulating hormone (FSH) and the leutinizing hormone (LH). FSH stimulates the maturation of ovarian follicles, of which only one contains a mature egg. The release of this mature egg is induced by a large burst of LH secretion by the pituitary gland. The amounts of FSH and LH in the body vary throughout the menstrual cycle, and are highest just before ovulation.
One critical hormone secreted by the anterior pituitary is growth hormone. Its major role is in stimulating the liver and other tissues to secrete the insulin-like growth factor IGF-1. This factor in turn is a major participant in the control of several complex physiologic processes, including growth and metabolism. Another important hormone, secreted by the posterior pituitary, is antidiuretic hormone (also known as vasopressin). The single most important role of this peptide hormone is in reducing the formation of urine to conserve body water.

9. (Para)Thyroid, Adrenals
Thyroid is located in the front and middle of the lower neck; thyroxine and T3 are two important hormones from this gland.
Located within each of the thyroid lobes are a pair of tiny oval-shaped glands called parathyroid; hormones from this gland are the most important regulator of serum calcium.
The two adrenals are each situated atop of each kidney; their corticosteroid and catechol-amine hormones play an important role in metabolism, the immune system, and stress.
The thyroid gland is located in the front and middle of the lower neck, below the larynx. It is shaped like a bow tie with two halves (lobes). Tetraiodothyronine (T4, or better known as thyroxine) and triiodothyronine (T3) are two major hormones of the thyroid gland. These two iodine-containing peptides control the rate at which cells burn body fuels from food to produce energy. Other thyroid hormones, such as calcitonin, also play a key role in bone growth and in the development of the brain and nervous system in children. In short, thyroid hormones are vital to many body functions, including heat production, heart rate, respiratory rate, and digestion. The release of thyroid hormones is controlled by the pituitary gland’s thyroid-stimulating hormone.
Located within each of the thyroid lobes are (usually) a pair of tiny oval-shaped glands that function together called the parathyroid glands. With the aid of calcitonin secreted by the thyroid, the parathyroid hormone secreted by these parathyroid glands is the most important endocrine regulator of calcium levels in the blood and in the extracellular fluid.
The two adrenal glands are triangular in shape, with each situated atop of each kidney. Each adrenal gland’s outer part is called adrenal cortex whereas its inner part, adrenal medulla. The adrenal cortex produces the corticosteroid hormones, which play an important role in metabolism, the immune system, certain aspects of behavior, and sexual function. These corticosteroids include aldosterone, hydrocortisone, and adrenal androgens. The adrenal medulla secretes norepinephrine and epinephrine to aid the body in dealing with stress. Both of these catecholamine hormones are associated with higher blood pressure and heart rate.

10. Pineal, Pancreas
The pineal body is located near the center of the brain, having the shape of a tiny clone; its hormone melatonin has significant effects on reproduction and daily physiologic cycles, most notably the circadian rhythms.
Pancreas has both exocrine and endocrine functions; its bulk is a ducted gland secreting digestive enzymes into the small intestine. Its endocrine function is by means of its many small clusters of endocrine cells, from which the hormones glucagons and insulin play an important role in regulating blood sugar level.
The pineal gland in humans is a clone-shaped structure of about 1 cm in length. It is located near the center of the brain. The pine clone-like organ consists of specialized secretory cells called pinealocytes, which synthesize the hormone melatonin and secrete it into the bloodstream by means of the surrounding cerebrospinal fluid. Melatonin has significant effects on reproduction and daily physiologic cycles, most notably the circadian rhythms.
Seasonal affective disorder (SAD) is a condition caused by the overproduction of melatonin, especially during the longer nights of winter. Its symptoms include tiredness, weight gain, sadness, profound depression, and oversleeping. Treatment of SAD thus should consist of exposure to bright lights for several hours each day to inhibit melatonin production. There is indication that melatonin plays a role in cancer inhibition (see Slide 19).
The pancreas has both exocrine and endocrine functions. The bulk of it is a ducted gland secreting digestive enzymes into the small intestine through the pancreatic duct. Its other function is achieved by means of its many small clusters of endocrine cells called islets of Langerhans. These islets consist of four types of cells called alpha, beta, delta, and gamma.
The alpha cells secrete glucagons, which mobilize glucose, fatty acids, and amino acids from stores into the blood in response to a low sugar level. The beta cells are responsible for the secretion of insulin which is needed to metabolize glucose. Patients are said to have diabetes mellitus if their pancreas does not make enough insulin to lower the blood sugar level, or if their muscle, fat, and liver cells do not respond to insulin’s normal function, or both.

11. Ovaries, Testicles
The female ovaries and the male testicles, responsible for many sex characteristics, are referred to as the gonad glands or sex organs.
Female ovaries synthesize the hormones estrogen and progesterone in varying amounts depending on where in her cycle a woman is.
Testicular production of the sex hormone testosterone (a principle androgen) begins during fetal development, continues for a short time after birth, nearly ceases during childhood, and then resumes at puberty.
The gonads are the female’s ovaries and the male’s testicles, or otherwise more commonly referred to as the sex (or sometimes also the reproductive) organs. In addition to producing gametes (ova and sperms), these sex organs secrete hormones. As mentioned earlier (Slide 8), the secretion of these sex hormones by the gonads is controlled by pituitary gland hormones such as the FSH (follicle-stimulating hormones) and LH (leutinizing hormones). While both sexes make some of each of these hormones, male testicles secrete primarily androgens of which testosterone is the principle one. Female ovaries make estrogens and progesterone in varying amounts depending on where in her cycle a woman is. In a pregnant woman, the baby’s placenta also secretes hormones to help maintain the pregnancy.
The estrogens and progesterone contribute substantially to the development and the function of female reproductive organs and sex characteristics. At the onset of puberty, estrogens promote several physiologic processes: distribution of fat evidenced in the hips, legs, and breast; development of the breasts; and maturation of reproductive organs such as the uterus and vagina. Progesterone causes the uterine lining to thicken in preparation for pregnancy.
Production of testosterone in males begins during fetal development, continues for a short time after birth, nearly ceases during childhood, and then resumes at puberty. This steroid hormone is responsible for several growth and sexually-related functions. These functions include but are not limited to: growth and development of the male reproductive structures; increased skeletal and muscular growth; enlargement of the larynx accompanied by voice changes; growth and distribution of body hair; and increased male sexual drive.

12. Modes of Disruption (I)
There are two basic avenues of endocrine disruption, each of which involves primarily two modes of biochemical reactions. One avenue is on the function or the structure of the glands or the target cells, directly or not.
The other avenue is on the metabolism and the function of hormones that the endocrine glands secrete or that the target cells bind to.
In all cases, the disruption can lead to either an excessive activation or an excessive inhibition of a hormone’s normal function.
From a brief review of the endocrine system provided in the previous slides, it becomes clear that adverse health effects can result from unwanted interference with the production or the function of hormones. It also should be transparent that not all endocrine disruption per se is detrimental, as readily reflected in the intended use of the now banned synthetic estrogen DES (diethylstilbestrol). Within the context of this lecture, however, the focus hereupon is on the side of unintended disruption.
Endocrine disruption is actually a form of biochemical (re)actions which, when sufficient enough, can lead to one or more (adverse) health outcomes; that is, it is not necessarily an immediate toxicological endpoint per se. Despite the fact that often times the public health interest is in the epidemiologic link between a cause and the effect(s), an understanding of the causal mechanism is often advantageous and some times even necessary. In the case with endocrine disruption, students certainly would become more appreciative of the related health concerns if they could gain a basic understanding of the underlying mechanisms first.
There are mainly two avenues of endocrine disruption, each of which involves primarily two modes of biochemical actions. One avenue is directly or indirectly on the structure or the function of the endocrine glands or of the target cells, whereas the other is on the metabolism or the function of hormones that the glands secrete or that the target cells bind to. In either case, the disruption can lead to an excessive activation or an excessive inhibition of a hormone’s normal function. In short, the major modes of disruption involve modification of or damage to the endocrine system or to target cells, and binding of or damage to hormones.

13. Modes of Disruption (II)
The endocrine glands as a target organ can be impaired or affected directly or indirectly through certain toxicologic disruptions.
For example, their hormone secretion can be impaired by an intruder’s ability to inhibit the biosynthesis or the secretion process.
Other examples include the secondary endocrine toxicity of DES on the ovary and of testosterone secretion, and the primary toxicity of nicotine on the adrenal, nitrogen on the ovary, and estrogens on the pituitary.
The endocrine gland as a target organ can be impaired or affected directly through certain toxicologic disruptions. Or such adverse effects can be a secondary response as a result of certain actions or disruptions undertaken elsewhere in or outside the endocrine axis. Herein endocrine toxicity and endocrine disruption are two loosely interchangeable terms. Note that even with an endocrine gland being a toxicological target, the primary or ultimate concern here is with the potential production or secretion of inappropriate amounts of hormones.
A well-known case of secondary endocrine toxicity is the notorious synthetic estrogen DES that was banned worldwide mainly due to its carcinogenicity to the vagina of the user’s daughter. Harvey et al. (1999) also highlight this type of toxicity with a rat study, in which increased incidences of castration cells in the pituitary were shown as a secondary response to the direct effect that had caused a reduced testosterone secretion by the rat’s testes. In addition, these investigators provide a list of examples of drugs and chemicals that were seen to have caused primary endocrine toxicity. Some of their examples for primary toxicity are nicotine on the adrenal, nitrogen mustard on the ovary, and estrogens on the pituitary.
The production of hormones can be impaired not only by a toxicologic effect on the secreting gland, but also by an intruding agent’s ability to inhibit a specific enzymatic step required in the biosynthesis process. In addition, the production can be affected by the agent’s ability to regulate at the transcriptional or translational processes. Aminoglutethimide, cyanoketone, and ketoconazole are some of the chemicals referenced (Sikka and Naz, 2002; Sikka, 1997) as having an inhibitory activity on the biosynthesis of steroid hormones in the testes.

14. Modes of Disruption (III)
Many foreign substances can mimic certain hormones and hence can bind to those target cellular sites receptive of natural hormones.
Some others can modulate the metabolic pathway of certain (sex) hormones; still some others can speed up the metabolism.
In short, the modes of endocrine disruption include agonistic and antagonistic receptor binding, and those actions that affect the biosynthesis, storage, release, transport, and clearance of hormones.
In addition to interference with the endocrine system’s ability to synthesize and secrete hormones, exogenous chemicals can disrupt a hormone’s normal function in several ways. These foreigners can mimic some hormones and hence can bind to those target cellular sites receptive of the natural hormones. When these receptors mistreat an intruder as the natural hormone, they respond as they would to the hormone. In mimicking the natural hormone, some intruders can serve as inhibitors by reducing the number of receptors for binding. Some other intruders can cause the body into over-responding as more receptors are activated.
Still some exogenous chemicals, such as the insecticide lindane and the herbicide atrazine, can modulate the metabolic pathway of certain sex hormones (e.g., Crain et al., 1997; Hayes et al., 2002). There are also some intruders that can activate enzymes that speed up the metabolism of some hormones. For example, certain enzymes in the testes are known to be capable of metabolizing estrogens (Toppari, et al., 1996). These enzymes can break down an estrogen rapidly to a form where the target receptors can no longer recognize this hormone. Either way, some exogenous chemicals can directly or indirectly modify the metabolic pathways of certain hormones substantially. Such a disruption in turn can cause the natural hormone not to be able to bind to the target receptors at the right time or in the right manner.
In short, the modes of endocrine disruption include mainly, but are not limited to, agonistic and antagonistic receptor binding, and those actions that affect the biosynthesis, storage, release, transport, and clearance of hormones (Kavlock, et al., 1996). In either case, hormonal functions can be altered substantially, possibly leading to a severe health outcome.

15. Pesticides as EEDs (I)
Many pesticides have been identified as environmental endocrine disruptors (EEDs).
More pesticides appear to have an adverse effect on the thyroid hormones than on others.
Some pesticides that are prominent thyroid hormone disruptors are: acetochlor, alachlor, ethylene thiourea, fipronil, heptachlor, maneb, methomyl, and zineb.
Mercury, pentachlorophenol, and PCBs are some of the thyroid hormone disruptors that are not or no longer used as pesticides.
Our Stolen Future (Colborn et al., 1996) is perhaps the one book that should be read by those who have special interest in the science and stories of endocrine disruption. This book, forwarded by former U.S. Vice President Al Gore, was published in over a dozen of foreign languages. In essence, the book explores the emerging science of how some synthetic chemicals interfere with the ways in which hormones function in humans and wildlife. It begins as a scientific detective story, but ends with a conclusion about the dreadful effects of persistent toxic substances, of which many are potential endocrine disruptors.
A successful spin-off of the book is the website where the authors provide regular updates to present the cutting edge of science related to endocrine disruption. According to this website, there appear to be more pesticides having an adverse effect on the thyroid hormones than on other groups of hormones in our body. The runner-ups that are likewise susceptible to pesticide interference are those hormones present in or released by the sex and the reproductive organs.
Below are the more prominent pesticides listed, along with references, on the above website that have been suspected to affect the thyroid hormone levels: acetochlor, alachlor, amitrol, chlofentezine, ethylene thiourea, fenbuconazole, fipronil, heptachlor, heptachlor-epoxide, karate, malathion, mancozeb, maneb, methomyl, metribuzin, nitrofen, pendimethalin, prodiamine, pyrimethanil, tarstar, thiazopyr, zineb, and ziram. The website also listed mercury, pentachlorophenol, and PCBs (polychlorinated biphenyls) as some of the thyroid hormone disruptors that are not typically or no longer used as pesticides.

16. Pesticides as EEDs (II)
Pesticides identified as environmental endocrine disruptors (EEDs) with estrogenic effects include: DDT, dieldrin, endosulfan, fenvalerate, kepone, lindane, methoxychlor, permethrin, triadimefon, and triadimenol.
Pesticide EEDs considered as androgenic or antiandrogenic are: atrazine, p,p’-DDE, lindane, procymidone, vinclozolin, etc.
Those affecting the reproductive system are fewer, including: ketoconazole, oxy-chlordane, tributyltin, and trifluraline.
As mentioned in the preceding slide, the website is a spin-off of the book Our Stolen Future (Colborn et al., 1996). According to this website, pesticides that have been considered or suspected as estrogenic are: DDT, dicofol, dieldrin, endosulfan, fenvalerate, kepone, lindane, methoxychlor, permethrin, triadimefon, and triadimenol. Also listed as estrogenic are the food antioxidant BHA (butylated hydroxyanisole), the phthalate compounds bisphenol A and bisphenol F, and the persistent organohalogens dioxins, furans, and PCBs.
The fewer pesticides listed as androgenic or antiandrogenic on that website are: atrazine, p,p’-DDE, lindane, procymidone, and vinclozolin. Other chemicals not used as pesticides, such as PCBs and some phthalate compounds, are also listed as androgenic.
The reproductive system is the last of the more prominent places in the body where pesticides and other persistent organochlorines are listed on the website as having the ability to disrupt hormone levels. Endocrine disruptors in this group include the pesticides ketoconazole, oxychlordane, tributyltin, and trifluralin, and the non-pesticides BHC (benzenhexachloride), 1,2-dibromoethane, chloroform, PCBs, carbendazim, lead, and mercury.
For an understanding of the actual disruption mechanisms involved, students are referred to the supporting references listed on that website. They may start with the work by Soto et al. (1994) and by Hurley et al. (1998). In fact, many of the supporting references cited on that website were journal papers published in Environmental Health Perspectives.

17. Literature Evidence (I)
Many in vitro assays are currently used in U.S. EPA’s mandated Endocrine Disruptor Screening Program; these assays are used as the first tiered test to identify and confirm the potential of hundreds (or perhaps thousands) of chemicals as endocrine disruptors.
There are also sufficient in vivo studies showing that many chemicals are capable of inducing endocrine disruption, even at very low doses comparable to background levels in people living in many places.
As expected, there are three major sources of evidence supporting the existence of endocrine disruption by environmental pollutants. The first source of evidence is the typical laboratory studies that confirm a chemical’s disruption mechanism at the in vivo or in vitro level. The second source is the observations gathered on wildlife animals, primarily on the effects on their population growth and reproductive activities. The third source is the data from epidemiology studies or from well-designed exposure trials in humans (or even in wildlife).
Many in vitro assays are currently being used in U. S. EPA’s mandated Endocrine Disruptor Screening Program (EDSP). These assays are used as the first tiered test to confirm the potential of hundreds to thousands of chemicals as endocrine disruptors, while currently EDSP’s main focus is on those disruptors that are estrogenic or antiandrogenic.
Some of the more recent in vivo examples include the study by Markowski et al. (2001), in which rats were shown to reduce motivation in standard behavioral tests following perinatal exposure to extraordinarily low doses (parts per trillion) of TCDD (dioxin). The doses tested were comparable to the background levels in people living in many places. Another study by Rice (2000) showed that infant monkeys developed a syndrome analogous to attention deficit hyperactivity disorder when these primates were exposed to very low levels of PCBs from birth. Still another study (Gray et al., 1999) demonstrated that low doses (parts per million) of the fungicide vinclozolin given to pregnant rats were capable of altering the sexual development in the male offspring, including reduction of sperm counts, retention of nipples, shortening of anal-genital distance, and some other reproductive malformations.

18. Literature Evidence (II)
Field observations in wildlife over the years reveal a worrying trend of endocrine disruption affecting their population growth.
From 1970s to 1980s, a great number of herring gulls and other fish-eating birds were noted to have deformities caused by exposure to dioxin released into the Great Lakes.
More astounding is the observation in the 1980s that male alligators in Lake Apopka had testosterone levels as low as a female’s and penises 25% smaller than the normal males’.
Field observations in wildlife over the past 50 years reveal a worrying trend of adverse effects on their population growth and reproductive health in the ecosystem. It is believed that much of these adverse health effects are caused by endocrine disruption of pollutants persistently available in the environment. As a matter of fact, there is already evidence of reproductive abnormalities seen in birds, fish, and mammals. Cross-sex masculinization and feminization are also seen in fish, birds, turtles, and other wildlife species.
Between early 1970s and late 1980s, a great number of the herring gulls and other colonial fish-eating birds near Lake Ontario were found to have some type of edema disease and deformities. It was Gilbertson et al. (1991) who first associated the edema and deformities syndrome with dioxin, which is a well-recognized environmental endocrine disruptor. Dioxin has been released into the Great Lakes from manufacturing plants since the 1930s.
Even more astounding is the observation in the 1980s that alligators in Lake Apopka, Florida had testosterone levels as low as a typical female’s, and penises about 25% smaller than those in normal males. Subsequent studies by Guillette et al. (1994, 1996) indicated that the p,p’-DDE, a major contaminant in Lake Apopka, was likely the chemical culprit.
In the quasi-field settings, tadpoles of the gray treefrog species were seen to be less capable of avoiding predators following exposure to low level of the pesticide carbaryl, (Relyea and Mills, 2001). More green frog tadpoles were also seen to reach metamorphosis after exposure to the carbaryl (up to) 3 times at various developmental stages (Boone and Bridges, 2003).

19. Literature Evidence (III)
Although epidemiologic evidence is harder to come by, intriguing observations from epidemiology studies continue to emerge.
For example, two epidemiology studies had linked atrazine to ovarian tumors in Italian women; this antiandrogenic herbicide is known to be capable of affecting steroid metabolism and ovarian functions.
A more recent study showed that perinatal exposure to PCBs is linked to children’s play behavior.
Epidemiologic evidence of environmental endocrine disruption is harder to come by in part due to the usual ethical constraints. Also, evidence of this type cannot demonstrate endocrine disruption directly as the mechanism inducing the effects. Yet in spite of such limitations, intriguing observations from epidemiology studies continue to emerge at a steady pace.
Two studies had linked atrazine to ovarian tumors in women (Donna et al., 1984, 1989). This antiandrogenic herbicide is known to be capable of affecting ovarian function (Cooper et al., 1996) and steroid metabolism (Babic-Gojmerac et al., 1989; Crain et al., 1997; Hayes et al., 2002) in animals. Such animal data offer further support for the above epidemiologic link.
A more recent study by Vreugdenhil et al. (2002) reported that perinatal exposure to PCBs is associated with changes in play behavior of children. This epidemiology study also showed more feminized behaviors in both boys and girls exposed prenatally to higher levels of dioxins. Although this is perhaps the first study ever reported linking disruption effects of this type to child behavior, its results are consistent with findings observed in animal studies.
The link between light at night and an increased risk of breast cancer was provided in two independent epidemiology studies in the same year (Davis et al., 2001; Schernhammer et al., 2001). It was explain that, through photic information from the retina, visible light would suppress the normal nocturnal production of melatonin by the pineal gland (Hansen, 2001). The two epidemiology studies thus support the earlier in vitro work by Blask et al. (1997), that the pineal hormone has the potential to restrain tumor growth, especially in breast cells.

20. Impacts of Toxicity Data
Evidence shown thus far represents only a small fraction of the vast amount of toxicity data on endocrine disruption; yet it appears to have carried the collective weight of evidence for biologic plausibility and consistency.
A disruptor’s toxicity is critical, but the lesser of the two components of a health risk.
The other component tends to offer more preventive measures and hence appears to play a greater role, in that it counts on the disruptor’s availability for exposure.
The evidence presented in the last three slides represents only a very small fraction of the enormous amount of toxicity data on endocrine disruption. Neither are the few studies all that representative of the most relevant collection of the work done on the subject today. Yet the literature data provided here appear to have carried the collective weight of evidence for biologic plausibility and consistency, which are the two most crucial criteria for establishing a causal relationship. These limited data thus collectively are still considered as providing sufficient evidence for the concern that certain environmental endocrine disruptors (EEDs) can be toxic and potent, and that their disruption effects in humans and wildlife can be severe.
In closing this first chapter (part), it is important to note that from a public health standpoint, a disruptor’s toxicity or potency is the lesser important of the two key components of a health risk. The other key component tends to offer more preventive measures and hence appears to play a greater role, in that it counts on the disruptor’s availability for exposure. A chemical’s availability, especially that of an EED, is measured by its usage, its persistence, and the potential for its bioaccumulation by organisms in an ecosystem. Though often not considered as the most desirable, an effective or complete prevention or mitigation measure is of course to ban the chemical in question.
Whether or not a chemical’s production and use should be banned or substantially reduced depends primarily on its toxicity, persistence, and bioaccumulation properties. Part I here has just presented the general toxicity of EEDs. Their bioaccumulation and persistence properties are the topics of extensive discussion given in Part II and Part III, respectively.