Genetically different strains of laboratory mice vary dramatically in their sensitivity to estrogen, report researchers at the University of California, Davis, in the Aug. 20 issue of the journal Science.
The findings by Jimmy Spearow, a reproductive geneticist, and Marylynn Barkley, a reproductive endocrinologist, call into question the validity of current laboratory-animal-based safety tests of estrogen-like chemicals and suggest that an individual's genetic makeup should be considered when prescribing estrogen and related hormones for medical purposes.
"The use of laboratory animals that genetically are quite resistant to estrogen for the evaluation of possible reproductive effects of various chemicals might be misleading and may mask our appreciation of how global exposure to estrogen-like chemicals threatens wildlife, domestic animals and humans," said Spearow, a research geneticist in UC Davis' Neurobiology, Physiology and Behavior Section.
Estrogen is a naturally occurring hormone that is mimicked by other chemicals dubbed "endocrine disruptors" because they appear to hinder reproduction in fish, wildlife and other mammals by interfering with the normal function of the endocrine system. Such chemicals are found in certain pesticides, plastics, detergents and estrogens derived from plants.
The U.S. Environmental Protection Agency is preparing to screen thousands of pesticides and industrial chemicals for several endocrine-disrupting effects. Previous studies have indicated that estrogen-like endocrine disruptors found in the environment can cause decreased sperm counts, deformed genitals, aberrant mating behavior and sterility in wildlife.
Spearow and Barkley, who study reproductive hormones using mice as a research model, became interested in the possible genetic control over susceptibility to endocrine disruption by estrogen.
"Many commercial outbred lines of laboratory animals have been bred for large litter size and vigor," Spearow explained. "As a result, the males from these strains tend to have larger testes and a decreased sensitivity to the estrogen-triggered mechanism that temporarily 'turns off' the reproductive system."
He theorized that the process of breeding mice and rats that are genetically predisposed to producing large litters of offspring would also result in animals that are less sensitive to estrogen.
"Our concern was that the use of laboratory animals selected for large litter size in product-safety testing might underestimate the role of those estrogen-like chemicals in disrupting reproductive development and function," he said.
To test that notion, Spearow decided to study the effects of estradiol -- a common form of estrogen found in fish, amphibians, reptiles, birds and mammals -- on young male mice of different strains. He examined several strains of mice including: C57BL/6J (B6) mice that are widely used in producing genetically customized mice for biomedical research; C17 mice that were developed by random selection followed by inbreeding; S15 mice that were developed by selection for large litters followed by inbreeding; and CD-1 mice that produce large litters and are frequently used in toxicological and pharmacology studies.
When the mice were all 22 or 23 days old, the researchers surgically placed tiny tubules filled with increasing doses of estradiol under their skin. The implants were prepared in such a way as to gradually release estradiol.
When the mice were 43 days old, the researchers checked for possible endocrine-disrupting effects resulting from the estradiol by measuring the weight of the mice's testes. They discovered that testis weight of mice receiving empty, control implants differed between different strains of mice.
More importantly, while estradiol treatments suppressed testis weight in all strains of mice, strains differed dramatically in their sensitivity to estradiol. Of the treated mice, the B6 mice appeared to be most sensitive, experiencing a 60 percent suppression of testis weight even at the lowest dose of estradiol. C17 and S15 mice were almost as sensitive as B6 mice to the suppression of testis weight in response to estradiol. The CD-1 strain of mice, known for large litters, showed a high resistance to estrogen, exhibiting only a 30 percent suppression of testis weight even with the highest estradiol doses.
Testes of several mouse strains also were examined to see if sperm development and production was affected by the estradiol treatments. Spearow found that low doses of estradiol eliminated sperm development in both the B6 and C17 strains. Sperm maturation in CD-1 mice, however, was not inhibited by low doses of estradiol and showed little or no inhibition in response to the highest doses of estradiol. This provided further evidence that the highly prolific CD-1 strain of mice is much more resistant to the endocrine-disrupting effects of estrogen.
"It is clear that CD-1 is over 16 times more resistant to endocrine disruption by estrogen than B6 and C17 strain mice," Spearow said. "Furthermore, extrapolation of the CD-1 data suggests that this line of mice is about 100 times more resistant than those other strains.
"This study and a related study in rats potentially explain why doses of estrogenic chemicals resulting in endocrine disruption in fish and wildlife failed to disrupt reproductive development in previous laboratory animal studies," Spearow added. "The laboratory-animal endocrine-disruption studies to date seem to have used estrogen-resistant lines of mice and rats for product-safety testing."
He suggested that this demonstration of major genetic differences in sensitivity to the disruption of reproductive development and sperm formation in young male mice has widespread implications.
"Because genes controlling prolificacy are also associated with differences in estrogen sensitivity, there is likely to be a broad variation in estrogen sensitivity in various animal populations and species, including humans," he said. "Accurate monitoring of endocrine disruption will require that we consider an animal's genetic sensitivity to estrogen as well as its environmental exposure to estrogen-like chemicals."
Spearow contends that the issue of genetic variation in susceptibility to endocrine disruption should not be ignored by the Environmental Protection Agency in its testing of thousands of chemicals for this activity.
"Considering these genetic variations in the estrogen sensitivity of an individual or species will be important not only when testing for endocrine-disrupting properties in industrial chemicals and pesticides, but also when determining therapeutic doses of estrogen and related steroid compounds in human medicine," Spearow and Barkley emphasized.
For example, an individual's genetically controlled response to estrogen should be considered when determining the appropriate dose of hormones used in contraceptives, hormone replacement therapy, and prevention and treatment of breast and prostate cancer, they explained.
In other studies, Spearow has discovered major differences between strains of mice in how females respond to fertility drugs to produce estrogen and ovulate. Furthermore, he has mapped genes controlling hormone-induced ovulation rate and ovarian estrogen production to specific chromosomal regions. Information on these genes would optimize fertility drug treatments and improve the hormonal induction of reproduction in humans, farm animals and an increasing number of captive-bred endangered species.
Collaborating on this study were UC Davis undergraduate students Paul Doemeny, Robyn Sera and Rachael Leffler.
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