By Dean A. Haycock
Special to BioWorld Today
On the molecular level, the cry "vive la diffirence" can be traced to some very small differences in the structures of estradiol and testosterone, two key sex hormones. The differences add up to one carbon and four hydrogen atoms in two functional groups attached to a much larger molecular carbon skeleton.
These small differences account for many of the secondary sex characteristics, the very traits that inspire the familiar French cry from each generation as it discovers them.
Estradiol, the hormone responsible for many female traits, is a naturally occurring estrogen. Few have trouble with that. The problems with estrogens emerge when they enter the environment and act as "endocrine disrupters." They are suspected of harming wildlife by interfering with reproduction and of harming humans by disrupting normal control of their endocrine systems, which regulate the secretion of hormones. Estrogens are present in some pesticides and in industrial products such as detergents and plastics.
Thus, they are present in the environment.
The Environmental Protection Agency (EPA) is planning to test thousand of compounds for their estrogen-like properties with the intention of determining the risks they pose for humans. Like most such studies, these will involve animal tests. A paper in the Aug. 20, 1999, issue of Science raises concerns about the choice of animals frequently used to test the effects of agents such as estrogens. The paper also may have implications for the use of hormone treatments in modern medical practice.
Popular Mouse Strain Shows Significant Resistance
Jimmy L. Spearow, assistant research geneticist at the University of California at Davis, is the first author of "Genetic Variation in Susceptibility to Endocrine Disruption by Estrogen in Mice" in today's Science. He and his colleagues found that different strains of lab mice differ greatly in their sensitivity to estradiol. They implanted capsules that slowly released the hormone in young male mice from four different strains. Then the researchers measured the resulting effects on testes weight and spermatogenesis. They found that a widely used mouse strain, CD-1, is more than 16 times more resistant to endocrine disruption than are other strains. CD-1 mice are popular with researchers because they produce large litters, a trait that might be linked to their relative resistance to estradiol.
"If that level of genetic variability in response to hormones exists in mice, it is likely to exist in other species as well," said Fred vom Saal, professor of reproductive biology at the University of Missouri in Columbia. "Now, that doesn't prove that point. It just is reasonable to assume that it is until proven otherwise."
He noted that the two major model animals being used by the EPA at this time are the CD-1 mouse and the Spague-Dawley rat. "There are findings already published [but] never really specifically focused in this way. One of the nice things about this article is that it really lays this out in a clear way," vom Saal told BioWorld Today. He added, "This [finding] is not out in left field. It fits into the literature. It suggests that the underlying variability to hormones in humans might be much greater than had been appreciated."
Gary Timm, a senior technical advisor at the EPA, wrote in a news item in Science, "Certainly we are interested in using the most sensitive species or strains." The agency is now designing its screen for estrogen disrupters.
Spearow's advice for organizations or companies conducting screens such as these is to test more than one genotype. He told BioWorld Today, "You need to do some, at least, initial studies to make sure that whatever effect you are looking at does not differ between strains before you just focus on using one animal strain."
Human Variability May Be Underestimated
Regulatory agencies such as the EPA and the FDA routinely estimate safe doses of chemicals for humans by estimating them to be 10 times lower than doses determined to be safe in animal tests, according to vom Saal. "The underlying assumption with regard to calculating amounts of environmental chemicals that are 'safe' for human consumption is that a factor of 10 accounts for variability in human response to environmental chemicals," he explained. "Spearow's data suggest that that correction factor is significantly underestimating the true level of variability within the given population. That is really important."
The results also may have important implications for other regulatory systems in the body. "Estrogen operates through a receptor system known as the nuclear receptor superfamily for steroid-like molecules. That would include the thyroid, glucocorticoid, androgen, estrogen, progestin and retinoid response systems," vom Saal said. "All of these operate through receptors that bind to DNA and activate genes. So, there are gene-regulating chemicals. If this is true for estrogen," vom Saal predicted, "it is going to be true for a very large class of chemicals that operate through the kinds of systems that estrogen operates through. This is a very large superfamily of regulatory molecules."
The fertility industry may also want to take note of the results by Spearow and his co-workers.
"It looks as though there is a tremendous amount of variation in the mechanisms that are controlling reproduction," Spearow said. "You can have two different individuals that would have approximately the same overall level of reproduction. And yet, when you give fertility drugs or contraceptives, they may respond totally differently because they are regulating reproduction a little bit differently. This is particularly pertinent now because it has only been in the last 50 years or so that hormones have really been used in order to alter reproduction.
"People have basically ignored the role of genetic differences in the control of reproduction," Spearow continued. "It is worse than ignoring it. In many cases they have used the most prolific strains for most of the tests of reproductive physiology, particularly back in the 1960s, 70s and 80s. Now people are shifting more to using inbreds with the transgenics and knockout mice. Those animals are not necessarily selected for large litter size."