1.8 Endocrine disruptors

During the past 50 years, there has been a huge increase in the number of chemical substances used worldwide as plasticizers, pesticides, detergents, paints, metal food cans, flame retardants, cosmetics, and chemical wastes, which exhibit the potential to interfere with the endocrine system of humans and animals. In addition, it has been found that many natural plant products have the same features (i.e. phyto-oestrogens). The public health risks related to these substances have raised reasonable concerns. Thus, the so-called endocrine disruptors have become the target of major scientific research.

According to the US Environmental Protection Agency ‘“an endocrine disruptor” is an exogenous agent that interferes 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’ (1). Many endocrine disrupters are biologically active at extremely low doses. Their effects on humans, wildlife, and the environment have been the focus of attention of the international scientific community, since they mimic endogenous hormones and are supposed to cause adverse health effects such as infertility, abnormal prenatal development, precocious or delayed puberty, thyroid dysfunction, obesity, behavioural disorders, and cancer. The scientific research is focused on three general principles that characterize the endocrine disruptors. First, the timing of exposure (as well as the ‘time window’ of exposure) seems to be critical for the outcome, since prenatal or early postnatal exposure could cause permanent malfunction of certain systems and could affect the individual throughout life. Second, endocrine disruptor have different dose–responses and act through different cellular mechanisms. Third, endocrine disruptors may affect the offspring of the exposed individual, via genomic or epigenetic modifications (2).

Endocrine disruptors have been known to exist since the 1930s, when the oestrogenic action of some chemicals, including bisphenol-A (BPA), was shown in laboratory animals. Later, in the 1950s, another chemical pesticide, dichloro-diphenyl-trichloroethane (DDT), was reported to have feminizing effects in roosters. During the 1970s, the use of diethylstilbestrol (DES), a synthetic oestrogen, for the prevention of abortions was common. Later, it was found that the children of those women treated with DES developed serious disorders such as vaginal carcinomas and infertility. The use of DES is now prohibited (3).

Endocrine disruptors interfere with the endocrine system, affecting the hormonal action, the hormonal concentration, or the hormonal receptor concentration. Exogenous compounds might have agonistic or antagonistic action when binding at a hormone receptor. If the endocrine disruptor binds at the binding site of a specific receptor with high affinity and activates it (agonistic action), the result is the same as that caused by the endogenous hormone. This is the most common mechanism of action of endocrine disruptors. They usually interact with the oestrogen receptor (i.e. BPA, DES), the androgen receptor (i.e. vinclozolin) and the aryl hydrocarbon receptor (i.e. dioxins). Other substances bind on the hormone receptor, resulting in a competitive or noncompetitive antagonistic action. In a competitive antagonistic action an endogenous agonist and an exogenous antagonist compete for the same active binding site. On the other hand, in a noncompetitive antagonistic action the antagonistic exogenous compound binds on an area of the receptor, other than the active binding site. The competitive antagonistic action usually leads to total deactivation of the receptor, while the noncompetitive antagonistic action causes the receptor to react slower or less efficiently. Typical antagonists for the binding site are the herbicides linuron and vinclozolin and their metabolites.

In addition, chemicals can affect the endocrine system by inhibiting enzyme-dependent chemical reactions (i.e. the aromatization of testosterone to oestrogen) by inducing hormone metabolizing enzymes (i.e. cytochrome P450 group) or by antagonizing the binding sites of the transport proteins (a reduction in the transfer proteins causes the concentration of the free/active hormone to increase). Finally, an endocrine disruptor could affect the hormone receptor concentration by down-regulation or by increasing the degradation rate of the receptor (4).

The most common categories of endocrine disruptors (Table 1.8.1) as well as their exposure routes are described below.

Polychlorinated biphenyls (PCBs) are synthetic organic chemicals that were used as coolants and lubricants in transformers, capacitors, and other electrical equipment. The use of these substances was stopped in 1977, when scientists recognized their negative health effects. Today, these compounds may be found in old microscope oil, old hydraulic oil, old fluorescent lighting fixtures, or electrical devices that contain old PCB capacitors. One route of human exposure includes the inhalation of PCBs released in the air when old electrical devices get hot during operation. Another exposure route is through the ingestion of contaminated food or through skin exposure. Infants could be exposed to PCBs through their mother’s breast milk during nursing.

Phthalate esters are chemicals that are commonly used in plastics, in products such as wall coverings, vacuum pumps, tablecloths, floor tiles, furniture upholstery, shower curtains, garden hoses, swimming pool liners, rainwear, baby pants, squeeze toys and dolls, shoes, automobile upholstery and tops, packaging film and sheets, sheathing for wire and cable, medical tubing, and blood storage bags. They are also used as an additive in cosmetics. There is potential risk for exposure due to inhalation, however, there is minimal risk of exposure associated with drinking water due to the fact that it does not dissolve readily in water. Phthalates can enter the body during certain medical procedures. The greatest risks are run during blood transfusions, kidney dialysis, intravenous fluid administration and when a respirator for breathing support is used.

BPA is a light plastic with unique toughness, optical clarity, and high heat and electrical resistance. It is used widely in eyeglass lenses, medical equipment, water bottles, CDs and DVDs, cell phones, computers, household appliances, reusable food and drink containers, safety shields, sports equipment, industrial floorings, industrial protective coatings, can coatings, and electrical equipment. Although BPA is considered to be biodegradable, there is some risk of BPA leaching out of the lining of cans, which could potentially contaminate the foods and liquids inside (6).

Dioxins form a group of hundreds of chemicals that are highly persistent in the environment. The most toxic compound is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dioxin is formed as an unintentional byproduct of many industrial processes involving chlorine, such as waste incineration, chemical and pesticide manufacturing, and pulp and paper bleaching. The major sources of environmental dioxin are the various kinds of waste-burning incinerator. Dioxin pollution is also associated with paper mills, where chlorine bleaching is used in various processes. Dioxin is also present in the human diet; as it is fat soluble, it bioaccumulates, climbing up the food chain (7).

Pesticides and herbicides such as atrazine, DDT and trifuralin were used widely in the past. Atrazine is a white powder that is used to protect grasses and broadleaf weeds from pests. It dissolves in water and is taken up by plants growing in the soil. Atrazine may be inhaled as a dust and ingested through contaminated drinking water, but it is not absorbed through the skin.

DDT is extremely hydrophobic and signficantly absorbed by soils. Depending on conditions, its soil half-life can range from 22 days to 30 years. Routes of loss and degradation include run-off, volatilization, photolysis as well as aerobic and anaerobic biodegradation. When applied to aquatic ecosystems it is quickly absorbed by organisms and by the soil, or it evaporates, leaving little DDT dissolved in the water itself. Its breakdown products and metabolites, DDE and DDD, also persist for long periods and have similar chemical and physical properties.

Trifluralin is used as a herbicide for controlling the growth of grasses and some broadleaf weeds in a wide variety of vegetables and some fruit. It is usually directly incorporated into soils, although some trifluralin mixtures may be sprayed. Trifluralin may enter the aquatic environment via diffuse sources, resulting from its recommended use, e.g. in agricultural run-off bound mainly to soil particles. Industrial discharges, accidental spillages during transport, storage, and use are potential point sources of trifluralin contamination.

Phyto-oestrogens are a diverse group of naturally occurring nonsteroidal plant compounds that, because of their structural similarity with oestradiol (17-β-oestradiol), have the ability to cause oestrogenic or/and antioestrogenic effects. These compounds in plants are an important part of their defence system, mainly against fungi. Foods with the highest relative phyto-oestrogen content are nuts and oilseeds, followed by soya products, cereals and breads, legumes, meat products, and other processed foods that may contain soya, vegetables, fruits, and alcoholic and nonalcoholic beverages.

The effects of in utero exposure to endocrine disruptors are a subject of scientific research. There is evidence suggesting that exposure at critical time points (‘time window’) during fetal development could cause a number of disorders, most of which are not reversible. Several chemicals or classes of chemicals can cause neurodevelopmental alterations by interfering with neuroendocrine function, including PCBs, dioxins, metals, pesticides, phyto-oestrogens, synthetic steroids, and triazine herbicides. It seems plausible that any compound which mimics or antagonizes the action of neurotransmitters, hormones, and growth factors in the developing brain, could cause adverse effects in the fetal neurodevelopment. The nature of the nervous system deficit, which could include cognitive dysfunction, altered neurological development, or sensory deficits, depends on the severity of the thyroid disturbance and the specific developmental period when exposure to the chemical occurred (Fig. 1.8.1) (8).

The almost classical case for endocrine disruption in utero which leads to adult disease in the offspring is that of previously described prenatal exposure to DES. Studies have confirmed the association between maternal treatment with the hormone and cervicovaginal cancer in daughters. This was the first demonstration of transplacental carcinogenesis in humans. In addition to a small number of genital tract cancers, the daughters of DES-exposed mothers also had functional and anatomical abnormalities of the uterus and fallopian tubes, and fertility was also compromised (9).

In addition, studies have found neuromuscular disorders and lower IQ in newborns, associated with in utero exposure to PCBs. Recent studies have demonstrated that exposure to DDE and its metabolites during fetal development is negatively associated with cord serum T₄ levels in infants, emphasizing the need to further investigate the adverse effects on thyroid hormones, growth, and neural development in children exposed in early life to high doses of DDT (5). An additional study revealed neuromuscular disorders in infants associated with in utero and lactational exposure to PCBs. Moreover, a lower IQ was reported in children 4–11 years of age, prenatally exposed to PCBs (5). Another chemical that belongs to the phenols group and is related to BPA, pentachlorophenol (PCP), has been found to alter thyroid hormones levels in newborns and consequently may lead to adverse neurodevelopmental defects. One study has demonstrated that the pesticide Nitrofen induces lung hypoplasia in rat fetuses when administered to the mother during gestation (5).

A clinical study in Japan revealed that following contamination of rice oil with dioxins in 1968 in the city Yusho, significant adverse effects were observed in babies born to exposed women, including low birthweight, skin discoloration, bronchitis, and developmental retardation. Behavioural effects in the Yusho infants included hypoactivity and hypotony. Intrauterine exposure to dioxins causes a significant degree of thyroid dysfunction and affects development of newborns (5).

In addition, the maternal exposure to environmental pollutants during pregnancy and the high oestrogenic bioactivity in the serum of newborns strongly suggests that ambiguous genitalia are related to fetal exposure to endocrine disruptors (10). The literature states that perinatal exposure to BPA causes irregular cycles in mice, although there is not enough evidence of this in humans (11). Moreover, perinatal exposure to endocrine disruptors with oestrogenic activity is proposed to induce development of obesity later in life (12).

Phyto-oestrogens and xeno-oestrogens generally inhibit key steroidogenic enzymes, including 3β- and 17β- hydroxyl-steroid-dehydrogenase, aromatase, sulfatases, and sulfotransferases. There is also evidence that both phyto-oestrogens and xeno-oestrogens can modulate intracellular signalling pathways, thus inhibiting the synthesis and activity of steroidogenic enzymes. The ability of these compounds to modulate enzyme activity could be important in sexual differentiation and development as well as in the protection (or promotion) of hormone-dependent cancers (13).

In utero exposure to phthalate esters is associated with morphological abnormalities of the male reproductive tract, including decreased anogenital distance, cryptorchidism, hypospadias, diminished Leydig cell population, and decreased secretion of testicular testosterone. Testicular androgen signalling may also be impaired through suppression of the normal hypothalamus–pituitary–testis (HPTe) regulation of Leydig cell steroidogenesis. Disruption of the HPTe axis, resulting in low testicular testosterone levels, was demonstrated in the rat following exposure to a range of endocrine disruptors. Exposure to oestrogen-like DES impaired HPTe signalling in the rat, reducing plasma testosterone and increasing plasma follicle-stimulating hormone (FSH) levels. The HPTe axis was disrupted by PCB-169 exposure in utero, resulting in decreased spermatogenesis, Leydig cell number, and plasma testosterone levels in the rat. Atrazine, another herbicide that with antiandrogenic and oestrogenic properties, has been found to produce a number of adverse reproductive effects in the male rat. Atrazine was implicated in reduced secretion of testicular testosterone in males. Atrazine has a low affinity for androgen and oestrogen receptors, reduces androgen synthesis and enhances oestrogen production via the induction of aromatase. Both Leydig and Sertoli cells represent an intratesticular source of oestrogens via androgen aromatization (14).

Recent studies have revealed the role of endocrine disruptors on the onset of puberty. Animal studies have demonstrated that both male and female pubertal timing is vulnerable to endocrine disruptors, particularly compounds that have oestrogenic or antiandrogenic effects (15). Endocrine disruptors can disturb the hypothalamic–pituitary–gonadal axis through negative feedback mechanisms as well as direct effects both centrally (hypothalamus and pituitary) and peripherally (ovary and testis). The effects may be seen after gestational, lactational, or juvenile exposure. Lead exposure has been shown to be associated with delayed pubertal onset, while phyto-oestrogen, PCBs, pesticides, and BPA exposure was related to precocious female development. On the other hand, transient neonatal androgen exposure resulted in reduced testis weight and testosterone production in rodents (16).

Another study revealed that exposure of adolescent girls in Canada to certain chemicals such as PCBs with potential oestrogenic features and lead may affect attainment of menarche. Lead was associated with a later median predicted age at menarche, when controlling for other toxicants, age, and socioeconomic status. However, at much higher or lower levels of lead and/or PCBs, different effects may occur (17).

Age at menarche is reduced in girls exposed to oestrogenic organochlorines, but the exact contribution of these substances to precocious menarche is unknown because of the numerous environmental variables influencing menarche. A study of women exposed to DDE through consumption of Great Lakes fish found a 1-year reduction in age at menarche for each increase of 15 mg/l serum DDE. Another study in Chinese textile workers showed that a 10 mg/l serum DDT increase was associated with 0.2-year reduction in menarcheal age (11).

The impact of exposure to environmental contaminants on human fertility remains controversial. However, many studies have illuminated some aspects of the impact of endocrine disruptors on human reproduction. First, the ovarian effects of the exposure to endocrine disruptors will be discussed. Ten years ago, an observation that mice housed in damaged polycarbonate plastic cages had a high incidence of oocytes with meiotic disturbances led to investigations into the oocyte-damaging effect of the oestrogenic plasticizer BPA. It was determined that BPA was leaching into the water of animals in damaged cages, and when BPA purposely was added to the water in nondamaged cages similar oocyte meiotic disturbances were induced. Some of these meiotic disturbances resulted in aneuploidy. Experimental data from three different laboratories supported the conclusion that BPA exposure has a detrimental impact on the maturing oocyte. Besides BPA, other endocrine disruptors, such as DES, have been shown to cause meiotic disturbances.

Another common disturbance of ovarian function is the polycystic ovary syndrome (PCOS). An endocrine disruptor that has been associated with PCOS is BPA. BPA has been measured in serum and follicular fluid (1–2 ng/ml), as well as in fetal serum and term amniotic fluid, confirming passage through the placenta. There is a significant increase in serum BPA levels in women with PCOS. These results may partly prove the association of BPA with PCOS.

In addition, human cyclicity seems to be affected by endocrine disruptors. Experimental studies of exposure of neonatal mice to physiologically relevant concentrations of the phyto-oestrogen genistein causes prolonged and abnormal cycles in adult animals. In humans, altered cyclicity has been linked to adult exposures to persistent organic pollutants and contemporary pesticides. Studies examining the influence of organochlorine pesticide exposure on cyclicity and fecundity suggest that organochlorine exposure shortens the menstrual cycle, whereas women who are exposed to hormonally active pesticides (nonorganochlorine) have a 60–100% increased risk of long cycles, intermenstrual bleeding, and missed periods (18).

Second, endocrine disruptors cause structural changes in the human uterus. There are data implicating a role of endocrine disruptors in the development of uterine fibroids (leiomyomas). The consumption of phyto-oestrogens in a study conducted in Japanese women found that individuals consuming soya had a decreased incidence of hysterectomy. Considering that the principal diagnosis in women undergoing a hysterectomy is uterine fibroids, this study suggests a protective effect of modest phyto-oestrogen consumption (9).

The association between endometriosis and endocrine disruptors is still not clear. It has been shown that nonhuman primates exposed to the widespread environmental contaminant TCDD have a high rate of endometriosis. A recent evaluation of the cohort of women exposed to massive doses of dioxin after a chemical accident in Seveso, Italy, does not support these earlier findings in nonhuman primates (9). However, data linking organochlorine exposure and endometriosis in humans are equivocal, with some studies reporting significant correlations and others failing to find any significant relationship (11).

Finally, it seems that there is a correlation between exposure to endocrine disruptors and lactation. It is well known that exogenous oestrogens such as DES will effectively suppress lactation (9). Duration of lactation is reduced in women with elevated serum concentrations of PCBs and DDE. The effect of DDE and PCBs on duration of lactation is dose-dependent, with each additional part per million increase in serum concentration being associated with a 1-week reduction in lactation duration (11).

As far as the male reproductive system is concerned, there is a significant body of toxicological data based on laboratory and wildlife studies suggesting that exposure to certain endocrine disruptors is associated with reproductive toxicity, including abnormalities of the male reproductive tract (cryptorchidism, hypospadias), reduced semen quality, and impaired fertility in the adult. Endocrine disruption of spermatogenesis may occur by four mechanisms, including: (1) epigenetic changes to the genome; (2) apoptosis of germ cells; (3) dysregulation of androgenic signalling; and (4) disruption of Sertoli and other spermatogenesis supporting cells (14). The effects of some endocrine disruptors and their relation to male reproductive anomalies are presented in Table 1.8.2.

There is increasing concern about development of cancer after exposure to endocrine disruptors. Gestational and perinatal exposures to endocrine disruptors may have long-term effects on the endocrine system that can influence tumour development later in life. That a synthetic oestrogen such as DES could cause cancer in offspring should not be surprising, given that even elevated levels of natural oestrogens during gestation have been associated with an increase in breast cancer in the children later in life (18).

Developmental toxicants of the mammary gland may lead to an increase in the incidence of mammary tumours if they alter circulating or tissue-localized hormone levels, gland receptor expression patterns, hormone transport, or metabolism that results in altered response to endogenous hormones or growth factors. Many environmental chemicals with oestrogenic activity have been measured in the human breast and this could be associated to increased incidence of breast cancer. However, although animal models seem to support this point of view, studies are required to clarify the effects of endocrine disruptors on human mammary gland. The impact of multiple estrogen-mimicking compounds, as well as the dose to which the gland is more susceptible needs to be investigated. In addition, research is needed to determine whether the type of endocrine disruptor is an independent carcinogenic factor, or the exposure time and route are the most important factors in the development of neoplasia.

Studies conducted in the 1990s did not provide any significant evidence of association of PCBs exposure and breast cancer. In addition to these data, no association was found between DDT, its metabolite DDE and breast cancer, but further studies are needed. The role of phyto-oestrogens in the risk of developing breast cancer is controversial. Some studies have demonstrated that there may be an association with breast cancer, whereas others have found these compounds to have protective effects (19).

Although the initial thought that testicular cancer may be related to early life-stage exposure to environmental oestrogens and/or antiandrogens seemed logical, there is little evidence to support this notion. There is currently no compelling evidence that exposure to environmental oestrogenic or other hormonally active substances is contributing to the rise in testicular cancer incidence observed in Western countries over the past several decades; however, this question has not been extensively studied. Several factors have greatly hindered the understanding of environmental influences on the risk of testicular cancer: the rarity of this condition, the long lag time between the presumed sensitive period during fetal development and clinical appearance of the condition, and the lack of a good animal model to study the progression of the disease (20).

There is increasing evidence both from epidemiology studies and animal models that specific endocrine disruptors may influence the development or progression of prostate cancer. In large part, these effects appear to be linked to oestrogen signalling, either through interactions with endocrine disruptors or by influencing steroid metabolism and altering oestrogen levels in the body. In human studies, PCBs and inorganic arsenic exposure have been associated with an elevated risk of prostate cancer. However, this risk seems to exist only if the exposure took place during critical developmental ‘time windows’ (in utero, neonatal, puberty). Thus, infants and children may be considered as a highly susceptible population with regard to exposure to endocrine disruptors and the increased risk of prostate cancer on ageing (21).

Another cancer whose development is directly related to the action of some hormones is ovarian cancer. Pesticides with endocrine-disrupting activity remain in use in different countries. Scientific research to date suggests a link between atrazine and risk of ovarian cancer, and other environmental and occupational exposures may also be associated with ovarian cancer. It remains to be determined whether these risks can be modified by hormone use or genetic susceptibility (22).

The existence of thyroid-disrupting chemicals has been confirmed through many animal and human studies. The disruption occurs at many different levels of thyroid hormone synthesis, binding, action, and metabolism. It has been demonstrated that the most common endocrine disruptors that affect thyroid function are BPA, pentachlorophenol, PCBs, and phyto-oestrogens. These compounds usually influence the hypothalamus–pituitary–thyroid axis, sodium-iodide symporter, thyroid-binding protein, the enzyme thyroperoxidase and many other sites. It seems possible that the endocrine disruptors that affect the thyroid could cause hypo- or hyperthyroidism, thyroid nodules and thyroid tumorigenesis. However, the limited data does not allow making reasonable conclusions about the effects of endocrine disruptors on thyroid function, and more scientific research is of crucial importance (5, 23).

The role of environmental chemicals in the development of obesity is an emerging area of research that is focusing on the identification of obesogens. Although until now data have been scant, some epidemiological and in vitro studies have suggested a link between environmental chemical exposure and obesity. Endocrine disruptors mimic natural lipophilic hormones that mediate their effects through members of the nuclear receptor transcription factors superfamily. Environmental estrogenic chemicals, such as BPA and nonylphenol, can promote adipocyte differentiation or proliferation of murine preadipocyte cell lines. Recently, studies have shown that chemicals including pesticides, organophosphates, polychlorinated biphenyls, polybrominated biphenyls, phthalates, BPA, heavy metals and solvents might cause weight gain possibly by interfering with weight homoeostasis via alterations in weight-controlling hormones, altered sensitivity to neurotransmitters, or altered activity of the sympathetic nervous system. However, more research is needed in this area (24).

It has been shown that endocrine disruptors might affect other systems such as the immune system and the endocrine pancreas. For example, some data suggest that EDs are involved in autoantibody production by B1 cells and could be an aetiologic factor in the development of autoimmune diseases. Other studies suggest that levels of BPA, phthalates, dioxins, and persistent organic pollutants are correlated with alterations of blood glucose homoeostasis in humans. However, these initial data about endocrine disruptor activity must be interpreted with caution (25, 26).

The impact of endocrine disruptors has been a matter of concern since the past 50 years. However, the results of the research remain controversial. This is because of the multitude of environmental effects on humans and because the genetic make-up of every individual is different, and the endocrine disruptor exposure duration and route may determine the outcome. In addition, the exact time point of the exposure is crucial. In utero exposure seems to cause irreversible outcomes. Moreover, experimental studies may not agree with studies in humans because exposure to endocrine disruptors varies and laboratory animals (rats, rodents, etc.) may also react differently. However, more experimental research is needed to clarify the possible mechanisms of action of endocrine disruptors.