Bisphenol A, the widely used compound in polycarbonate plastic, has the ability to alter the activity of genes in normal breast cells in ways that resemble what is found in extremely dangerous breast cancers, according to a new study.
The study, conducted by researchers in California and published this month in the journal Cancer Research, found that many genes in non-cancerous breast cells exposed to trace amounts of bisphenol A began acting in a way that closely resembled the gene activity in highly aggressive breast tumours that led to an increased likelihood that women would die of the disease.
The link "is highly supportive of the concept that overexposure to BPA and/or similar compounds could be an underlying factor in the aggressiveness, if not in the causality" of breast cancers, said Shanaz Dairkee, lead author of the study and senior scientist at the California Pacific Medical Center Research Institute in San Francisco, in an e-mail.
The study results were designated a "priority report" by the journal, published by the American Association for Cancer Research, one of the world's largest scientific organizations devoted to cancer studies.
Earlier this week, it was disclosed that Health Canada has concluded bisphenol A, or BPA, is a dangerous substance, making it the first regulatory body in the world to reach such a determination. An announcement on possible measures to curb its use is expected soon from federal Health Minister Tony Clement.
BPA, is one of the most widely used synthetic chemicals in modern industry. It is the basic building block for polycarbonate, the see-through, shatterproof plastic that resembles glass.
Bisphenol A is also used to make the epoxy resins lining most tin cans, along with some dental sealants, sports helmets and compact discs.
Many scientists are looking at the possible links between breast cancer and BPA because the synthetic chemical is able to mimic estrogen and was tested in the search for estrogen drugs before industry began using it to make polycarbonate plastic.
There is strong evidence linking breast cancer to long-term estrogen exposure, and many researchers have been worried that BPA leaching from consumer products is giving women what amounts to an extra dose of hormones, raising lifetime risk of malignancy.
Animal experiments have found that fetal or early life exposures to BPA cause lesions that may lead to increased susceptibility to mammary gland tumours later in life, but the scientific evidence hasn't been deemed strong enough to conclude that the chemical is also a human carcinogen. Other researchers have exposed already cancerous breast cells to BPA and found the chemical causes them to proliferate.
But the new study strengthens the case that BPA and cancer could be connected by showing the chemical has profound effects on the genes in breast cells that do not yet show cancerous tendencies.
The researchers, who also included scientists at Stanford Genome Technology Center at Stanford University's medical school, conducted the study by taking a small sample of normal, non-cancerous cells from the unaffected breasts of eight women who already had breast cancer.
Test tube experiments are often criticized because they only expose tissues to a few influences, while in living things there are thousands of chemicals acting on cells.
But to try to recreate realistic conditions that might mirror what is actually happening in women, the researchers exposed these cells to estrogen and progesterone in concentrations similar to what women experience in the last half of their menstrual cycles, and minute concentrations of BPA, equal to 22 parts per billion, an amount similar to what has been found leaching into some food from cans. A part per billion is a tiny amount, equivalent to approximately one second of elapsed time over a span of 32 years.
Tests were also run giving the cells only estrogen and mixtures of estrogen and progesterone.
The researchers found that the hormones and BPA left unique fingerprints on genes, inducing some of them to turn on or off, what in science is termed gene expression. They also observed that progesterone would turn off dangerous genes that had been activated by estrogen, only to have BPA turn the genes on again.
"As soon as BPA was presented to the cells, they reversed back as if they had gotten another whopping dose of estrogen and completely reversed the progesterone calming effect on the cells," Dr. Dairkee said.
Estrogen and BPA influenced 123 genes in common. But estrogen had the unique ability to affect 28 genes, while BPA was even more influential, being able to activate or deactivate another 52 genes.
The researchers then compared the response they found in normal tissue with publicly available gene expression profiles from about 525 breast tumours taken from cancer patients in the United States, Britain, Norway, the Netherlands and Sweden. They noted that the tissue they had exposed to BPA had a "striking association with tumour aggressiveness," according to the study.
