Background: Endocrine Disruption

What is the Endocrine System?

The entirety of an organism’s hormone secreting cells is called the endocrine system and organs that secrete hormones are called endocrine glands. There are over 50 known hormones in the human body that regulate a range of functions such as sexual maturation to the control of one’s blood sugar levels. The endocrine glands include the hypothalamus, pituitary, thyroid, parathyroid, pineal, adrenal, thymus, pancreas, ovaries, and testes. These glands release hormones into the bloodstream, or in some cases into the cell, which then bind to cellular receptors. The biological action of hormones is due to changes in various cellular characteristics and processes that are triggered by hormone binding to specific receptors. These biological responses can be varied, such as changes in cell membrane ion permeability or gene transcription.

In fish and other marine animals, hormones play a similar role regulating various biological processes such as molting of the exoskeleton, growth, reproduction, and development. While the endocrine organs of fish may differ from those in humans (Figure 1), their hormones that are similar in structure, similarly synthesized and have many of the same mechanisms of action. Many of the same building blocks in the biosynthesis of these hormones are shared between species and conserved in human hormones, thus contributing to non target organism exposures and biological effects.

Figure 1. The fish endocrine system is composed of many different tissues (Bond 1979 Biology of Fishes).

What is an Endocrine Disrupting Chemical (EDC)?

An endocrine disrupting chemical is a chemical that, when absorbed into the body, has the capability of disrupting the organism’s normal functions, through effects on hormone production, transport, or response. Many different tissues and receptors can be affected by EDCs, especially those that participate in hormone biosynthesis, transport, activity, and metabolism. Because of the diversity of ways that EDCs produce effects, there are significant challenges in detecting endocrine disruption and predicting which chemicals are EDCs.

One of the challenges in predicting an EDC is that different types of chemicals can produce similar endocrine disruption effects. Figure 2 shows examples of just a few distinct environmental chemicals found capable of altering estrogen levels in aquatic organisms. For some EDCs, the parent compound may have no endocrine disrupting activity whereas the metabolites of the same chemical may be biologically active. In addition, different metabolites of the same compound may produce different types of effects. Another challenge is that a single EDC may impact several biological systems within the organism, complicating the detection and interpretation of biological effects.

Figure 2. Example estrogenic compounds.

Mechanisms of Endocrine Disruption

Much of our knowledge about endocrine disruption is based on studies of growth, development, and reproduction. These processes are regulated by steroid and thyroid hormones that can be affected by many environmental chemicals. One of the more common mechanisms of endocrine disruption occurs when the chemical mimicks a steroid hormone’s effect by binding to the receptor for that hormone. A chemical that has this mechanism of action is termed an agonist. Other EDCs may be hormone antagonists, where their binding to hormone receptors reduces hormone activity by preventing the endogenous hormone from binding to that receptor and producing the intended cellular response.

Endocrine disrupting chemicals may also act by indirect mechanisms, by interfering with hormone synthesis, degradation, or transport. For example there are many enzymes that are responsible for hormone catabolism. Some EDCs may induce an up regulation of these enzymes leading to increased catabolism and subsequently reduced hormone levels. Many hormones that circulate in the blood are actually bound to plasma proteins; some EDCs bind to these proteins and displace the hormone. The end result is an increase in blood hormone clearance and a decrease in circulating hormone levels. Table 1 provides examples of endocrine disrupting chemicals, hormone targets, and types of organisms impacted.

Table 1. Example EDCs, hormone targets, and aquatic receptors.

Ecological Effects

The ecological effects of EDCs have been documented for over 50 years. Reproductive abnormalities in fish eating birds and marine mammals were some of the earliest recorded effects of these chemicals. Exposure to DDT has been implicated in eggshell thinning and impaired reproduction of pelicans and sea gulls. Recent research has also documented endocrine disruption in other aquatic life, such as alligators, frogs, fish and invertebrates. Several species of California fish have shown evidence of endocrine disruption from estrogenic chemicals, such as the presence of female reproductive proteins and developing eggs in males, known as ovo-testis (Figure 3). The SCCWRP Toxicology Department is conducting several research projects that are directed towards understanding the extent, significance, and causes of endocrine disruption in fish.

Figure 3. Section of male hornyhead turbot testis showing the presence of a large developing egg.

For more information on Background: Endocrine Disruption, contact Doris Vidal-Dorsch at dorisv@sccwrp.org (714) 755-3216.

This page was last updated on: 11/14/2011