A new UC San Diego study suggests that the byproducts of a chemical used in plastic found in the lining of cans may disrupt human hormone function more than the chemical itself.
The study, published today in PLOS ONE, may help explain why the chemical bisphenol A, or BPA, has been tied to many health problems even though the chemical doesn't have a strong effect on cells in a Petri dish.
"We have a candidate chemical that is doing the nasty stuff, or the endocrine disruption," said study co-author and UCSD structural biologist Michael E. Baker. "We know that BPA exposure causes a lot of endocrine problems, but if you're analyzing BPA in urine, you may not be analyzing the chemical that's really doing the endocrine disruption."
Several studies have linked BPA exposure to health problems such as breast cancer and birth defects in animals and humans. Researchers say the chemical may be harmful because it mimics hormones such as estrogen. A 2003-04 National Health and Nutrition Examination Survey found more than 90 percent of participants tested had measurable levels of the chemical in their urine.
But when BPA is put into a dish with cells that bind to estrogen, the cells don't proliferate, Baker said. For instance, breast cancer cells may grow prolifically in the presence of estrogen, but would need much more BPA to see the same growth, he said.
But in 2003, a Japanese research team found that the body breaks down BPA into another chemical: 4-methyl-2,4-bis(p-hydroxyphenyl)pent-1-ene, called MBP for short. MBP was 500 times more powerful at mimicking estrogen than BPA.
The researchers used computer simulations to model how MBP fit together with the part of cells called receptors, which bind estrogen. The simulations revealed that because the MBP molecule is longer, it fits better with the estrogen receptor and forms a stronger bond.
"It's like the key fits into all parts of the lock with MBP, and with BPA, only some of the key fits into the lock and other parts just don't fit," Baker said.
The findings suggest that researchers should be studying how MBP works in the body, Baker said. BPA leaves the body within a few days, but "how well MBP is cleared is not known, and it's not known if it accumulates in fatty tissue," he said.
But Steven Hentges, a spokesman for the American Chemistry Council, a plastics lobbying group, said in a prepared statement that MBP is not relevant for understanding BPA exposure.
"Multiple recent studies by the U.S. Food and Drug Administration and others demonstrate that BPA is rapidly metabolized and eliminated from the body, but BPA is not converted into MBP," he said in a prepared statement.
A second study by UC Berkeley researchers, published today in Environmental Health Perspectives, also ties BPA to disrupted hormone function. The results were drawn from a larger study of chemical exposures in Latino farm workers from the Salinas Valley.
Of the pregnant women studied, those with the highest concentrations of BPA in their urine had lower levels of a thyroid hormone called thyroxine, a trend toward a less active thyroid gland. But the male babies of those women had thyroid hormone levels consistent with a more active thyroid gland. It's not clear why babies showed the reverse trend of that found in the mothers.
"Thyroid hormone in pregnant women plays an essential role in fetal growth and brain development," said Jonathan Chevrier, UC Berkeley epidemiologist and study co-author.
By Tia Ghose, California Watch