Xenoestrogen - Effects

Effects

Xenoestrogens have been implicated in a variety of medical problems, and during the last 10 years many scientific studies have found hard evidence of adverse effects on human and animal health.

There is a concern that xenoestrogens may act as false messengers and disrupt the process of reproduction. Xenoestrogens, like all estrogens, can increase growth of the endometrium, so treatments for endometriosis include avoidance of products which contain them. Likewise, they are avoided in order to prevent the onset or aggravation of adenomyosis. Studies have implicated observations of disturbances in wildlife with estrogenic exposure. For example, discharge from human settlement including runoff and water flowing out of wastewater treatment plants release a large amount of xenoestrogens into streams, which lead to immense alterations in aquatic life. With a bioaccumulation factor of 10^5 –10^6, fish are extremely susceptible to pollutants. Streams in more arid conditions are thought to have more effects due to higher concentrations of the chemicals arising from lack of dilution.

When comparing fish from above a wastewater treatment plant and below a wastewater treatment plant, studies found disrupted ovarian and testicular histopathology, gonadal intersex, reduced gonad size, vitellogenin induction, and altered sex ratios.

The sex ratios are female biased because xenoestrogens interrupt gonadal configuration causing complete or partial sex reversal. When comparing adjacent populations of white sucker fish, the exposed female fish can have up to five oocyte stages and asynchronously developing ovaries versus the unexposed female fish who usually have two oocyte stages and group-synchronously developing ovaries. Previously, this type of difference has only been found between tropical and temperate species.

Sperm concentrations and motility perimeters are reduced in male fish exposed to xenoestrogens in addition to disrupt stages of spermatogenesis. Moreover, xenoestrogens have been leading to vast amounts of intersex in fish. For example, one study indicates the numbers of intersex in white sucker fish to be equal to the number of males in the population downstream of a waste water treatment plant. No intersex members were found upstream from the plant. Also, they found differences in the proportion of testicular and ovarian tissue and it’s degree of organization between the intersex fish. Furthermore, xenoestrogens expose fish to CYP1A inducers through inhibiting a putative labile protein and enhancing the Ah receptor, which has been linked to epizootics of cancer and the initiation of tumors.

The induction of CYP1A has been established to be a good bioindicator for xenoestrogen exposure. In addition, xenoestrogens stimulate vitellogenin (Vtg), which acts as a nutrient reserve, and Zona readiata proteins (Zrp), which forms eggshells. Therefore, Vtg and Zrp are biomarkers to exposure for fish.

Another potential effect of xenoestrogens is on oncogenes, specifically in relation to breast cancer. Some scientists doubt that xenoestrogens have any significant biological effect, in the concentrations found in the environment. However, there is substantial evidence in a variety of recent studies to indicate that xenoestrogens can increase breast cancer growth in tissue culture.

It has been suggested that very low levels of a xenoestrogen, Bisphenol A, could affect fetal neural signalling more than higher levels, indicating that classical models where dose equals response may not be applicable in susceptible tissue. As this study involved intra-cerebellar injections, its relevance to environmental exposures is unclear, as is the role of an estrogenic effect compared to some other toxic effect of bisphenol A.

Other scientists argue that the observed effects are spurious and inconsistent, or that the quantities of the agents are too low to have any effect. A 1997 survey of scientists in fields pertinent to evaluating estrogens found that 13 percent regarded the health threats from xenoestrogens as "major," 62 percent as "minor" or "none," and 25 percent were unsure.

There has been speculation that falling sperm counts in males may be due to increased estrogen exposure in utero. Sharpe in a 2005 review indicated that external estrogenic substances are too weak in their cumulative effects to alter male reproductive functioning, but indicates that the situation appears to be more complex as external chemicals may affect the internal testosterone-estrogen balance.