Environmental Factor, April 2007, National Institute of Environmental Health Sciences
Amphibian Specialist Challenges EPA and Pesticide Manufacturers
By Eddy Ball
On the afternoon of March 7, NIEHS scientists packed Rodbell C to hear a talk by biologist Tyrone Hayes, Ph.D. Hayes is a professor in the Department of Integrative Biology at the University of California - Berkley. He is also a speaker who does not mince words when it comes to manufacturers of the herbicide atrazine and the federal agency responsible for regulating the compound.
Hayes' talk, "From Silent Spring to Silent Night: The Connection between Pesticides, Amphibian Declines, and Cancer," was an indictment of practices by companies that make the widely used herbicide. The biologist also had harsh words for the ruling by the Environmental Protection Agency re-approving atrazine, as well as the agency's conclusion that the herbicide is "not likely to be carcinogenic to humans."
Hayes' research has uncovered in frogs reproductive anomalies linked to endocrine disruption associated with exposure to small amounts of atrazine. Hayes contends that the chemical has similar effects in humans. By up-regulating the enzyme aromatase, which converts the male hormone testosterone into estradiol and alters gene expression, atrazine exposure could trigger the development of hormone-stimulated cancers, such as breast cancer.
Ironically, it was Hayes' research during the 1990s for the industry-funded Atrazine Endocrine Disruption Panel that set the stage for his research. Working with African clawed frogs raised in his lab, Hayes began to see abnormalities in larynx growth in frogs exposed to doses of atrazine in concentrations as low as one part per billion (ppb).
Further research convinced Hayes that more than abnormalities of the larynx were involved when male frogs were exposed to the pesticide during their development. Hayes discovered that the up-regulation of aromatase and the subsequent conversion of male hormones to female hormones produced what he described as "chemical castration." Exposed frogs developed the reproductive organs of both sexes.
The results were disturbing, he explained, because under EPA guidelines, atrazine is considered safe in drinking water as long as it is found in levels no greater than three ppb, three times the level that causes endocrine disruption in amphibians. "I could take tap water that is regulated by the U.S. EPA," Hayes noted, "and I could chemically castrate frogs."
Although Hayes has never performed studies with mammals, the connection between abnormalities in frogs and carcinogenesis in humans was a logical one for him. "Frog hormones are very similar, and in some cases identical, to human hormones," he argued.
"So what affects a frog, may also affect humans."
If atrazine could cause endocrine disruption in frogs, he reasoned, it was likely to have similar effects in people, especially if they were exposed during development. His follow-up research at the bench with frogs and his extensive observations in the field soon convinced him that he was on to something important.
When Hayes' began to talk and write about his results, industry-supported researchers attacked the accuracy of his data and tried to explain away the effects as "normal variation." He continues to be a target of criticism by industry spokespersons, who, he believes, hope to discredit his research by claiming they cannot replicate his results.
To counter these attacks and EPA skepticism, Hayes pointed to results of over 40 studies that are in line with his hypothesis about atrazine's effects on mammals. He also cited Sygenta-funded research that reported results similar to his own.
Research with frogs can cover several generations in the course of a year. However, it may take decades before investigators can reach definitive conclusions in studies of humans and persuade regulators and manufacturers that atrazine poses a significant health threat to humans.
Hayes' talk was sponsored by the Laboratory of Experimental Pathology (LEP). LEP Veterinary Medical Officer Greg Travlos, D.V.M., was the host for the lecture.