Our Stolen Futurea book by Theo Colborn, Dianne Dumanoski, and John Peterson Myers
 
 

 

 

Wetherill, YB, CE Petre, KR Monk, A Puga, and KE Knudsen. 2002. The Xenoestrogen Bisphenol A Induces Inappropriate Androgen Receptor Activation and Mitogenesis in Prostatic Adenocarcinoma Cells. Molecular Cancer Therapeutics 1: 515–524.


common sense recommendations for men undergoing prostate cancer treatment...

Most previous work on endocrine disruption by bisphenol A has focused on exposures in the womb. This study identifies a new target of vulnerability: adult men undergoing hormonal treatment for prostate cancer. Wetherill et al. report that extremely low doses of bisphenol A increase a process in prostate cancer cells that renders them less responsive to the standard hormone treatment used to force prostatic adenocarcinomas into remission. The effects, observed in cell culture, take place at levels that have been measured in circulating blood of adult men.

Some background on prostate cancer (adapted from Wetherill et al):

  • In early stages of prostate cancer, treatment focuses on the androgen sensitivity of prostate cancer cells. These cells require serum androgen to proliferate and survive. Targeting this vulnerability, doctors use hormone treatments that reduce androgen levels as one of the first lines of therapy, or that inhibit the biochemical pathways that are stimulated by androgen receptor activity. Deprived of androgen stimulation, the androgen-dependent tumors go into remission.
  • Unfortunately, remission is not permanent. The median time for a relapse is between 12 and 30 months. For reasons that are not completely understood, tumors begin to appear in the cancer that do not require androgen for proliferation. The cancer becomes androgen-independent (also called androgen-refractory) and no longer responds to this intervention.
  • While the tumors are not androgen-dependent, they still have active androgen receptors. It appears that mutations in these receptors are involved in the formation of androgen-independent tumors. These mutations apparently make the AR less discriminating in its response to potential hormonal stimulation, i.e., instead of being responsive only to androgens, the androgen receptor will also respond to 17ß estradiol, progesterone and some anti-androgens.

Thus Wetherill et al.'s finding that bisphenol A (BPA) initiates the proliferation of androgen-independent prostate cancer cells at extremely low exposure levels is of great interest. Their work was done in cell culture. If the same mechanisms take place in adult men, BPA could be interfering with one of the key weapons used against treatment for prostate cancer.

What did they do? Wetherill et al. used a series of cell culture experiments and biochemical techniques to tease apart the effects of exposing human prostatic adenocarcinoma cells (called LNCaP cells) to low levels of BPA, first to establish the effects and then to hone in on their biochemical mechanisms.

  • In their first round of experiments, they exposed LNCaP cells to BPA at levels ranging from 0.1 nanoMolar to 100 nM to determine a dose-response relationship for the impact of BPA on cell proliferation.
  • A second round was performed to confirm that BPA was directly responsible for stimulating proliferation, by comparing BPA's impact on gene expression with that of a natural androgen, DHT, and a control.
  • They performed additional experiments to determine whether the impact of BPA on cell proliferation was related to the activation of mutated androgen receptors.

What did they find? Wetherill et al. first observed a dramatic nonmonotonic dose-response relationship between bisphenol A exposure and proliferation of prostatic adenocarcinoma cells. Their experiments were all conducted at very low levels of BPA (the "nanomolar" range, parts per billion). As shown in the figure below, the largest response to BPA was observed mid-way along the dose-response curve, at 1 nanomolar. As required, for a successful experiment, the positive control, dihydrotestosterone, increased proliferation. Low levels of BPA also stimulated proliferation while the highest level of BPA used, 100 nM, actually suppressed proliferation slightly.

 

BPA stimulates LNCaP proliferation. Wetherill et al. used a standard assay of DNA synthesis (BrdUrd incorporation rate) to monitor the proliferation of prostatic adenocarcinoma cells. BPA effects were compared to a vehicle control (ETOH, labelled 'control') and a positive control, dihydrotestosterone (DHT).

As would be expected on the basis of these results, cell numbers of the positive control (DHT) and BPA-exposed cultures increased over time, demonstrating proliferation.

Cell proliferation following exposure to 1 nanomolar BPA (note the log scale).

 

Previous research in their laboratory at the University of Cincinnati had characterized the way that DHT stimulates a series of biochemical events in LNCaP cells that allows them to proliferate. In the next round of experiments, Wetherill et al. showed that BPA is capable of stimulating the same series, demonstrating that BPA can initiate proliferation in prostatic adenocarcinoma cells via the same biochemical pathways and thus allowing the cells "to bypass the requirement for androgen."

Pushing more deeply into the biochemical mechanisms involved, Wetherhill et al. demonstrated through a series of experiments that BPA activates a mutant form of the androgen receptor present in androgen independent prostate cells (AR-T877A) and that the BPA-AR-T877A complex then induces gene expression, exploiting the same biochemical pathway used by DHT in androgen dependent cells. Their data demonstrate that "AR-T877A activation results in induction of endogenous AR target genes."

  • First, they showed that BPA exposure caused accumulation of one of the mutant forms of the androgen receptor, AR-T877A, in the nucleus of proliferating cells after BPA exposure, as does DHT exposure. This observation links BPA-induced cell division to the activation of the mutant androgen receptor. Nuclear accumulation takes place when the receptor binds to its ligand (hormone), becomes activited and migrates into the cell nucleus; unless it has been activated it won't accumulate.
7 hours after treatment, most prostate cancer cells have accumulated AR-T877A in the presence of DHT and BPA.

 

  • Second, they showed that BPA can activate the binding of the mutated AR-ligand complex to the DNA sequences specific to where the DHT-activated complex binds. The BPA activation can take place even in the absence of other steroids.
  • Third, they studied PSA, a protein secreted by the prostate and under the control of a gene that is expressed following androgen activation of the unmutated AR receptor. [PSA, or prostrate specific antigen, is elevated in patients with prostate cancer and hence is used as a marker for prostate cancer.] First they established that PSA gene expression was not increased by BPA via the unmutated androgen receptor. This is what they expected because the unmutated form is responsive to a narrow suite of androgens but not to BPA. In contrast, PSA expression was increased by BPA activation of the mutated form of the androgen receptor, AR-T877A. In fact, BPA induced a 5-fold increase in PSA expression, compared to DHT's 5.9 fold increase.
  • And fourth, they demonstrated that AR-T877A activity was required for BPA to have a proliferative effect. In otherwords, unless the androgen receptor has mutated into a form that is less ligand-specific (called "promiscuous"), BPA cannot initiate AR-dependent processes. This makes sense because BPA does not bind to the normal androgen receptor.

 

What does it mean? As the authors observe, "these data implicate BPA exposure as a potential mechanism that could facilitate the transition of prostatic adenocarcinomas to androgen independence." Transition is what causes the failure of hormonal treatment for prostate cancer, leading to patient morbidity and death.

What makes this finding even more important is the nanomolar levels at which the effects were highly significant. These are levels that other scientists have measured in adult men leading normal lives (1.4 to 6.5 nM).

Several scientific questions arise. Is the AR-T877A mutant androgen receptor the only mutant form with which bisphenol A binds and then activates gene expression? Do other xenoestrogens exert similar effects, through AR-T877A or other forms? Is this mechanism at work in prostate cells in men? Are there detectable effects of BPA or other xenoestrogens on the progression of prostate cancer? Currently, none of these questions can be answered.

What do these results mean for men undergoing treatment for prostate cancer? Until further study establishes that BPA does not exert this influence in men with prostate cancer (as opposed to cells in cell culture), men undergoing hormone treatment for prostate cancer who are looking for ways to make their treatment more effective should consider taking steps to reduce their exposure to bisphenol A as a precautionary measure.

BPA exposure comes from multiple sources. For example, bisphenol A-based polycarbonate is used as a plastic coating for teeth to prevent cavities, as a coating in metal cans to prevent the metal from contact with food contents, and as the plastic in food containers, refrigerator shelving, baby bottles, returnable containers for juice, milk and water, micro-wave ovenware and eating utensils (more...).

The simplest and least controversial of these exposures to reduce is via food items that have been heated (e.g., microwaved) in polycarbonate containers. Don't do it. The leaching rate from polycarbonate is temperature-dependent: the hotter it gets, the faster the leaching. There are other, readily available containers in which to heat food. Food stored in refrigerators in polycarbonate containers is not likely to be a major source because of the cold temperatures. Also worth noting is that old polycarbonate leaches much more rapidly than new polycarbonate. While this recommendation is simple to implement at home, it is more of a challenge with fast food, where plastic of a number of varieties, including polycarbonate, is commonly used a food container for heating.

The use of bisphenol A to coat the interior of food cans is problematic because it is impossible to tell, given current labeling requirements, which food cans use it and which don't. For those cans with bisphenol A lining, the type of food (fatty?) and the way it was placed in the can (still hot?) will also affect how much leaching will occur. A cautionary step would be to reduce consumption of canned food, especially with fat (like meat- or milk-based soups), at least until labeling catches up with this issue.

The British "Food Standards Agency" surveyed canned foods for bisphenol A content and found wide variability among products and countries. They concluded that there was no health risk. This conclusion was reached (2001) when there was lingering controversy about low level effects of BPA because of claims by industry that results demonstrating adverse effects could not be replicated. Those debated results have now been confirmed, and many other studies have been added to the litany of concerns about BPA, including this one by Wetherill et al. The Food Standards Agency's decision needs review in Britain. The US Food and Drug Administration should review this also.

Some plastic dental sealants leach bisphenol A, others do not. According to some estimates, the use of bisphenol A in sealants is declining and no sealants currently recommended by the American Dental Association leach BPA. Ask your dentist about the details of the sealants he/she is using.

 

 

 

 
     
     

 

 

 

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