A cause without a disease

Endocrine-disrupting chemicals have become a topic of public concern because they could potentially cause cancer and male infertility. But evidence for a human health problem is hard to find


Endocrine disruptors—or 'gender benders' as they are often referred to by the public—have become the focus of environmentalists and public health advocates who decry a slow poisoning of humans and the environment by the chemical and consumer goods industries. The term is a rather broad label for substances that are able to interfere with hormone receptors or hormonal pathways in the cell. Endocrine disruptors have caused serious public concern, because their interaction with the hormone system could potentially wreak havoc with prenatal and early development and affect a wide variety of organs. Theo Colborn, a researcher for the World Wildlife Fund, painted a bleak picture of their effects at a 2001 meeting of the US Department of the Interior: "... these chemicals can undermine the development of the brain, and intelligence and behaviour, and the endocrine, immune and reproductive systems. ... there is now a growing collection of studies revealing that some of these chemicals can affect our children's ability to learn, to socially integrate, to fend off disease and to reproduce" (Colborn, 2001).


However, as public fear mounted, the evidence for a creeping epidemic caused by endocrine disruptors in the environment remained elusive


In fact, early observations on wild and laboratory animals showed that some compounds that are able to interact with receptor molecules, in particular with the oestrogen receptor, exert effects on the reproductive system of these animals. These observations were accompanied by reports on the increasing incidence of breast and prostate cancer and declining male fertility, and it was only a matter of time before the press took up the issue and parents became concerned about this slow poisoning of their children. However, as public fear mounted, the evidence for a creeping epidemic caused by endocrine disruptors in the environment remained elusive. Although most scientists now acknowledge that many substances can have an effect on the human endocrine system, more recent analysis has shown that many of the claims about health effects were either exaggerated or based on flawed analysis of observations. As Stephen H. Safe, Professor of Veterinary Physiology and Pharmacology and of Biochemistry and Biophysics at Texas A&M University (College Station, TX, USA) put it: "The hypothesis is okay, but we don't even have a problem."

The scientific chapter of the endocrine disruptor story began in the early 1990s with a 'hypothesis' article in The Lancet in which Richard M. Sharpe from the MRC Reproductive Unit at the University of Edinburgh, UK, and Niels E. Skakkebaek from the Department of Growth and Reproduction at the University of Copenhagen, Denmark, wrote, "exposure to exogenous oestrogens, ... during foetal and neonatal life can lead to an increase in reproductive disorders" (Sharpe & Skakkebaek, 1993). On the basis of a meta-analysis of more than 60 studies published between 1940 and 1990, they suggested that abnormalities in the development of male sex organs and a 50% decline in sperm count could be attributed to exposure to oestrogens in utero. The finding that the prescription of an artificial oestrogen, diethylstilboestrol, for pregnant women from the 1940s to the 1970s had caused an increased rate of cervical cancer among the daughters of these women further supported Sharpe and Skakkebaek's hypothesis, and the fear that men could also be affected did not seem so far-fetched.

Observation of wildlife also provided evidence for the effects of endocrine disruptors on reproductive health. Various publications described how chemicals suspected to have endocrine-disrupting effects, including DDT, dioxins, polychlorinated biphenyls (PCBs), which are all banned, and various pesticides and fungicides, caused a wide range of reproductive disorders and deformities of sexual organs among wild animals in polluted areas. Nonylphenol, a degradation product from many detergents, herbicides, spermicides and cosmetics, has been shown to cause imposex in oysters, which is a pseudo-hermaphroditic condition in which females acquire male sex characteristics (Nice et al, 2003). Scientists in the UK found that oestrogenic compounds in human and agricultural wastewater triggered the feminization of male fish in British lakes and rivers. Else-where, US scientists found that female mosquito fish in Florida exposed to pulp-mill effluent developed a gonopodium, an organ normally found only in males. Similarly, male alligators in various contaminated lakes in Florida suffered from phallus deformations and an impaired immune system. Half of male carp caught in the Tama River in Japan were found to produce unusually large amounts of the yolk precursor protein vitellogenin, specific to female fish.

In 1996, Colborn, together with science writers Dianne Dumanoski and John Peterson Myers, compiled these observations into the book Our Stolen Future and drew a straight line between the effects observed in wild animals and human health effects, including breast and prostate cancer and decreasing male fertility caused by decreasing sperm counts, cryptorchidism (where one or both testicles fail to descend from the body) and hypospadias (deformation of the phallus). Often compared to Rachel Carson's Silent Spring, Colborn's book had an enormous impact on public opinion and triggered intense media coverage about the suspected epidemic of cancers and male infertility. The media obtained further ammunition when Fred vom Saal and co-workers at the University of Missouri (Columbia, MO, USA) showed that bisphenol A (BPA), a commonly used compound found in many plastics, caused abnormal prostate growth and decreased sperm production in rats at doses far lower than those considered to be safe (Nagel et al, 1997; vom Saal et al, 1998). Patricia Hunt at Case Western Reserve University (Cleveland, OH, USA) observed that BPA caused severe aberrations of the meiotic cell division in mouse oocytes in up to 40% of all cases (Hunt et al, 2003). Although industrial and academic researchers have so far failed to reproduce vom Saal's findings, his work has become the main argument for public health advocates who seek to ban chemicals such as BPA because they can exert their toxic effects at extremely low doses.

In fact, a series of studies that closely investigated the original publications claiming an increase in breast and prostate cancer and a decline in male fertility found that this is not so

The political reaction to these reports was swift, particularly in the USA. The US Environmental Protection Agency (EPA) convened two workshops in 1995 to make recommendations for research into the health threat of endocrine disruptors, including their effects on reproductive, neurological and immunological function and carcinogenic activity. In 1996, the US Congress amended the Food Quality Protection Act and the Safe Drinking Water Act to require the testing of food-use pesticides and drinking water contaminants for endocrine activity, which mandated the EPA to screen up to 70,000 chemicals regulated under the Toxic Substances Control Act for endocrine-disruptive effects. In 1999, the EPA launched the Endocrine Disruptor Screening Program (EDSP) and is now developing animal tests and other assays to screen for hormone activity. In Japan, the Ministry of the Environment decided to start risk assessment studies on more than 40 substances suspected to have endocrine-disrupting effects (Iguchi et al, 2002). On 29 October 2003, the European Commission proposed a new regulatory framework for all chemicals manufactured or imported in quantities of more than a tonne per year. Among the chemicals labelled as being of 'very high concern' that require authorization for particular use are substances that could cause reproductive damage or affect fetal development—in other words, endocrine disruptors.

The only problem is that nobody actually knows whether the levels of endocrine disruptors in the environment are a threat to public health. "The so-called epidemic of endocrine diseases remains to be established," said Raphael J. Witorsch, Professor of Physiology at Virginia Commonwealth University in Richmond, VA, USA. A working group, convened by the Royal Society of London, UK, that investigated the health threat of endocrine- disrupting chemicals (EDCs) came to the same conclusion: "whilst high levels of exposure to some EDCs could theoretically increase the risk of such disorders, no direct evidence is available at present" (The Royal Society, 2000). Richard Sharpe, one of the original authors of the endocrine disruptor hypothesis, also acknowledged that "the threat [to human health] is minimal." In fact, a series of studies that closely investigated the original publications claiming an increase in breast and prostate cancer and a decline in male fertility found that this is not so. "We now know that this is absolutely not true," Safe said about health advocates who warn that endocrine disruptors could cause a worldwide epidemic of disorders and diseases. According to Witorsch, many of the original epidemiological analyses were flawed and lacked confounding factors.

In addition, large-scale studies among elderly women in the USA and the UK showed that the increase in breast and cervical cancer was caused mainly by hormone replacement therapy for post-menopausal women (Brower, 2003) rather than hormonally active compounds in the environment. In fact, many of the chemicals under suspicion bind only weakly to the oestrogen receptor and it is not clear whether they have an estrogenic, anti-estrogenic or anti-androgenic effect. Furthermore, critics maintain that EDCs have to compete with more effectively binding natural oestrogens that are abundant in the diet, in medicines and in contraceptives at much higher concentrations. "In terms of magnitude and extent, all such exposures to so-called endocrine disruptors are dwarfed by the extensive use of oral contraceptives and estrogens for the treatment of menopausal and post-menopausal disorders. Also, the exposure to hormonally active xenobiotics is virtually insignificant when compared with the intake of the phytoestrogens that are present in food and beverages," commented Robert Nilsson, Professor of Toxicology at Stockholm University, Sweden (Nilsson, 2000). "So we've got all these [phytohormones] out there in the diet," Safe concluded, but "my scepticism is how could small concentrations [of other chemicals] in the environment be a problem?"

Equally, the low-dose effects that vom Saal observed in mice have come under criticism. "I remain completely unconvinced by the low-dose studies. ... How does BPA in Fred vom Saal's study induce an effect at concentrations at which we know that it doesn't bind to the oestrogen receptor?" Sharpe said. "I want to see that these effects can lead to disorders that are directly related to human health issues." But Sharpe still sees a potential problem with compounds that act elsewhere in the signalling pathway and modify the internal balance of hormones. "If you alter the endogenous hormonal milieu, then you get a disease," he said. "I have no problem with understanding that chemicals that have that ability can cause health problems."

... critics maintain that endocrine-disrupting chemicals have to compete with more effectively binding natural oestrogens that are abundant in the diet, in medicines and in contraceptives at much higher concentrations

Indeed, the evidence of detrimental effects of endocrine disruptors in animals, particularly for aquatic organisms, is quite convincing, Sharpe maintained. He cited the example of TBT (tributyltin), widely used in ship paint. The chemical does not bind to a hormone receptor but modulates the endogenous hormonal milieu in mussels elsewhere to cause imposex. Consequently, as the human fetus is "kind of an aquatic organism," as Sharpe put it, such chemicals could potentially impair prenatal development and cause effects later in life. But "humans are different from fish," Safe countered, and he questioned the sense of extrapolating observations made in wild or laboratory animals to humans, who often have very different hormonal metabolisms. "There isn't a single chemical that doesn't have an effect," he said, but "what are these animal studies telling us?" Witorsch agreed that making such assumptions is like "shooting from the hip". "To judge in utero effects in rodents and try to extrapolate them to humans has to be done with caution," he said. "An observation doesn't mean that you should ban a substance and a lack of observation doesn't necessarily mean that it is safe."

For that very reason, many experts think that broad screening programmes based on animal testing, such as the EDSP, do not make much sense. "They're going to find some [chemicals] that are active and some that are inactive—so what?" Safe said, adding, "Is that the best use of taxpayer's money?" Nevertheless, the EPA has devised new testing guidelines that should address the subtleties of endocrine action better. Their new multi-generation tests investigate several endpoints relevant to reproductive performance, such as the female oestrous cycle, various parameters on sperm count and quality in parental and F1 generations, and the age at puberty in the F1 generation; they also include pathological tests for the impaired development of various organs. However, Witorsch remains unconvinced. "I'm not enthralled by the EDSP. ... These tests have been designed without appreciation of the complexity of the endocrine system," he said, adding, "I'm as underwhelmed with the EU approach as I am with the EDSP." Furthermore, as most of the testing and research concentrates on oestrogen and thyroid receptors, Witorsch thinks that it misses other equally important players in the endocrine system. "One of the benchmarks of naïveté is to totally ignore glucocorticoids and the physiological role of stress response," he said.

Sharpe is equally unconvinced by broad screening programmes and would rather put the focus on good epidemiology. "We need better exposure data. And that's the area where we have the weakest information," he said. Starting with chemicals for which there is genuine concern about cancer and male reproductive disorders, investigating their exposure particularly during pregnancy and then looking at their activity "would be a hell of a lot cheaper. ... It is possible that there are compounds out there that [modulate the endogenous system] and that they are very potent. But you can't find those with a screening system." The Royal Society's working group came to the same conclusion: "The report emphasises the difficulties of making generalised assumptions based on isolated experiments and the problems of developing policy in areas in which scientific understanding is still being developed. ... In order to improve our understanding of the relationship of EDCs to health and disease, further investigation is needed."

Instead of "fishing expeditions", Witorsch thus advocates "good mechanistic research" to gain more knowledge about hormonal action and physiological pathways before making any further assumptions about potential health problems. "We can only accept at this particular stage that it is a mechanism that we have to explore more," he said. In the light of public fears—unfounded or not—it is certainly necessary to shift the debate about the human health dangers of endocrine disruptors to a more factual level. And that includes scientists. "The important thing is not to listen to what I say or what vom Saal says but to look at the facts," Safe said, "And then the question you should ask is 'is there anything?'"

By Holger Breithaupt

References

Brower V ( 2003) A second chance for hormone replacement therapy? EMBO rep 4: 1112–1115 | Article | PubMed | ChemPort |

Colborn T ( 2001) The path before us: environmental stewardship in the 21st century. www.worldwildlife.org/toxics/progareas/ed/doi_speech.pdf"

Hunt PA, Koehler KE, Susiarjo M, Hodges CA, Ilagan A, Voigt RC, Thomas S, Thomas BF, Hassold TJ ( 2003) Bisphenol A exposure causes meiotic aneuploidy in the female mouse. Curr Biol 13: 546–553 | Article | PubMed | ChemPort |

Iguchi T, Sumi M, Tanabe S ( 2002) Endocrine disruptor issues in Japan. Congen Anom 42: 106–119 | ChemPort |

Nagel SC, vom Saal FS, Thayer KA, Dhar MG, Boechler M, Welshons WV ( 1997) Relative binding affinity-serum modified access (RBA-SMA) assay predicts the relative in vivo bioactivity of the xenoestrogens bisphenol A and octylphenol. Environ Health Perspect 105: 70–76 | PubMed | ChemPort |

Nice HE, Morritt D, Crane M, Thorndyke M ( 2003) Long-term and transgenerational effects of nonylphenol exposure at a key stage in the development of Crassostrea gigas. Possible endocrine disruption? Mar Ecol Prog Ser 256: 293–300 | ChemPort |

Nilsson R ( 2000) Endocrine modulators in the food chain and environment. Toxicol Pathol 28: 420–431 | PubMed | ChemPort |

Sharpe RM, Skakkebaek NE ( 1993) Are oestrogens involved in falling sperm counts and disorders of the male reproductive tract? Lancet 341: 1392–1395 | Article | PubMed | ChemPort |

The Royal Society ( 2000) Endocrine disrupting chemicals www.royalsoc.ac.uk/files/statfiles/document-111.pdf .

Vom Saal FS, Cooke PS, Buchanan DL, Palanza P, Thayer KA, Nagel SC, Parmigiani S, Welshons WV ( 1998) A physiologically based approach to the study of bisphenol A and other estrogenic chemicals on the size of reproductive organs, daily sperm production, and behaviour. Toxicol Indust Health 14: 239–260 | ChemPort |