(This post is part of the Plastics theme for 2020)

The Endocrine System

The Endocrine System controls the functioning of the body ^[1].

It is composed of glands that produce messenger molecules called hormones. Hormones are released into the bloodstreams as a ‘message’ to be transported to a certain target organ. Indeed, in the cells of this organ or on their membranes, we can find another molecule called “hormone receptor”, that specifically recognizes that hormone, takes the message and activates a certain response ^[1] (Fig. 1).

The complex hormone-receptor can trigger or inhibit specific biological processes in the cells or in the tissues or in the organs. As consequence-specific functions can be controlled, the time and dose of occurrence of these hormones are two parameters really important for the wellness of the organism. If the hormone is released in a lower amount, endocrine disorders could occur, jeopardizing the general health of an individual ^{[1, 2]}.

Common examples of hormones include the following:

1. estrogens, the female sexual hormones, and androgens, the male sexual hormones, that control the reproductive sphere;
2. the thyroid hormones that control metabolism, growth, and reproduction;
3. cortisol, that has a fundamental role in the stress response ^{[1, 2]}.

Figure 1. Mechanism of action of the endocrine system (a simplified version). Secretory cells, that generally compose the glands, secrete the hormones (green spheres) into the bloodstream. Target cells host specific receptors (blue molecule) able to recognize that hormone binding to it. Non target cells have a different receptor (the yellow molecule) that is not able to recognize that hormone (green dot). Image credit: Martina Capriotti

You are probably wondering why you are reading about the functioning of the hormonal system when you are supposed to learn about plastic pollution. Read on and you will see my point about why this is important.

Chemical Pollutants in marine environment

Chemical pollution is one of the bigger threats our planet is facing. Essentially, it means that toxic molecules generally produced by human activities, can reach the environment, contaminating air, soil, lakes, rivers, and seas, and eventually, affecting life on Earth itself ^[3]. Pesticides used in agriculture are absorbed by the soil, then washed out reaching the rivers or aquifers, eventually ending in the sea. Polycyclic Aromatic Hydrocarbons, are released in the air during burning (ex. car exhaust or fire) and from the air transferred into the sea, or are released by oil leakages into the water. Plasticizers are plastic additives, used during plastic manufacturing for providing specific characteristics to plastic objects. They can be released if plastic is exposed to the sun’s ultraviolet rays, or to heat. Dioxins and furans are 2 other categories of chemicals released into the air from combustion processes. Polychlorinated Biphenyls widely used in the past as plasticizers, insulating materials, dyes or for electrical equipment, can be found in many natural compartments due to their ability to persist in the environment ^{[3, 4, 5, 6]}. It is important to also mention the PFAS, Per- and Polyfluoroalkyl Substances, found in a variety of products like water-resistant fabrics, cleaning products, food packaging or cookware ^[7].

Figure 2: A "vessel of opportunity" skims oil spilled after the Deepwater Horizon blowout in Gulf of Mexico. Oil Spill is another important source of hydrocarbons in marine environment. Photo credit: NOAA's Office of Response and Restoration

Chemical pollutants in marine environment can vary in amount or in diversity based on the vicinity to certain polluting sources like rivers, industrial runoff and oil extraction platforms for example. As a consequence, marine life can be exposed to these toxicants via respiration, feeding or contact. Different chemicals can provoke a different kind of toxicity, from DNA mutation to immune system impairment, disorders ranging from reproductive to development-related, or diseases ranging from cancer to chronic ones. But it is almost impossible to be exposed to only 1 compound in the environment. Indeed, what generally scares scientists studying pollution is the action of the mixture of contaminants, which in most of the cases is really much more severe than the action of a single compound ^{[3, 6, 8]}

Endocrine disruptors

The “impostors of the hormones”. Yes, it’s exactly like that. Endocrine disruptors are chemical compounds able to mimic the endogenous hormones, as they have a really similar chemical structure. Endogenous hormones are those hormones that we physiologically produce ^{[1, 8, 9, 10]}. So a hormone receptor can wrongly interpret these dangerous molecules activating or blocking as a consequence of the normal functions of the endocrine system ^{[1, 8, 9, 10, 11]} (Fig. 3).

Figure 3. Hormone receptors misleading. Endogenous hormones (red spheres) and Endocrine Disruptors (green spheres) are really similar. That’s why hormone receptors can be mislead in recognizing them as natural hormones. Image credit: Martina Capriotti

Common examples can be the phthalates and bisphenol A (BPA), used in plastic production, or DDT, widely used in the past as pesticide in agriculture or in public places. Even if many of these substances are now banned, their residue still remains in the environment and continues to pollute ^[6].

Some of the effects of these hormone-mimicking contaminants include interference with estrogen or androgen system affecting the reproductive functions, or interference with the thyroid system affecting the metabolism. That is why many forms of prostate/testis cancer or mammal/vaginal cancer or diabetes/obesity diseases are connected to endocrine disruptors ^{[8, 9, 10, 11]}.

In aquatic environment the feminization of some fish populations exposed to pollutants that mimic estrogens, was reported, where all the individuals became females. Also imposex (sexual impotence) was recorded in several marine gastropods populations exposed to TBT (Tributyltin) used in anti-fouling paints for the boats ^[12].

Microplastics as carrier of endocrine disruptors

Microplastics are by definition, plastic particles smaller than 5mm in their longest dimension (Fig. 4). They are invading our oceans due to the breakdown of larger plastic objects exposed to the UV sunlight (photochemical degradation) together with mechanical forces like waves. This mechanism drives them to break down into smaller and smaller pieces, which are called “secondary microplastics”.

Figure 4. Microplastic collected from San Benedetto del Tronto maritime district in Adriatic Sea (Italy). Photo credit: Martina Capriotti

“Primary microplastics” on the contrary are those particles manufactured intentionally of these dimensions to be small, like resin pellets or cosmetic abrasive microspheres ^[6]. Microfibers constitute a huge component of microplastics in aquatic environment: they are released through the washing of synthetic fibers clothes like acrylic, polyester, polyamide or nylon ^[6].

One of the main threats associated with the presence of microplastics in the water, is their ingestion by animals with implications for the health. Despite evidence showing the transfer of plastic particles from one level to another in the food web, their real effect in natural populations still remain unclear ^[6]. One of the risks associated to microplastics is their ability to attract chemical compounds on their surface, the same toxic compounds described above. As a consequence of this property, microplastics can become carriers of these pollutants once ingested, thus representing an additional source of toxicants to the animals ^{[6, 11]}.

I conducted studies on organic pollutants on microplastics in the Adriatic Sea (Italy; Fig. 5), with a project supported by a grant from the National Geographic Society and the SKY Ocean Rescue campaign. I extracted persistent organic pollutants from the surface of microplastics collected inside the San Benedetto del Tronto maritime district, in front of Marche Region coasts, and detected several categories of compounds like pesticides, polycyclic aromatic hydrocarbons or polychlorinated biphenyls, testing the endocrine activity of this cocktail of substances ^[11].

Figure 5 Manta trawl during microplastic sampling from surface sea water in Adriatic Sea, Italy. Photo credit: Martina Capriotti

Even if it’s not part of my research, it’s interesting to mention that the transfer of endocrine disruptors from the microplastics to the gut, after ingestion, seems to be related not only to those compounds attached to the particles’ surface, but also to the plastic itself that release its additives ^[6].

Unfortunately, more data about the real health consequences of microplastic and endocrine disruptors at the concentrations found in the environment (not in laboratory experiments), are still needed. This part of the microplastic research is at an early stage. Moving forward, scientists will need to collect more proof to have a clearer idea of the dangerous mechanism of endocrine disruptors action and how it is related to microplastics.

References

[1] https://www.epa.gov/endocrine-disruption/what-endocrine-system
[2] https://www.hormone.org/what-is-endocrinology/the-endocrine-system
[3] https://www.who.int/ceh/risks/cehchemicals/en/
[4]https://www.fda.gov/food/chemicals-metals-pesticides-food/chemicals
[5] https://www.who.int/news-room/fact-sheets/detail/dioxins-and-their-effects-on-human-health
[6] M. Bergmann, L. Gutow, & M. Klages (Eds.). “Marine anthropogenic litter.” Springer, 2015.
[7] https://www.epa.gov/pfas/basic-information-pfas
[8] P. Cocci, M. Capriotti, G. Mosconi, A. Campanelli, E. Frapiccini, M. Marini, ... & F.A. Palermo. “Alterations of gene expression indicating effects on estrogen signaling and lipid homeostasis in seabream hepatocytes exposed to extracts of seawater sampled from a coastal area of the central Adriatic Sea (Italy).” Marine environmental research, 2017, 123: 25-37.
[9] E. R. Kabir, M. S.  Rahman  and I. Rahman. "A review on endocrine disruptors and their possible impacts on human health." Environmental toxicology and pharmacology. 2015, 40.1: 241-258.
[10] A. C. Godswill and A. C. Godspel. "Physiological Effects of Plastic Wastes on the Endocrine System (Bisphenol A, Phthalates, Bisphenol S, PBDEs, TBBPA)." International Journal of Bioinformatics and Computational Biology. 2019, 4.2: 11-29.
[11] M. Capriotti et al. "The Estrogenic Potentiality of Hydrophobic Organic Pollutants Contaminating Microplastics." Ocean Sciences Meeting 2020. San Diego, United States of America, 2020.
[12] A. Pickering and J. P. Sumpter. “Comprehending endocrine disrupters in aquatic environments.” Environmental science & technology. 2003, 37 17: 331A-336A .

Martina Capriotti is a marine biologist researching on plastic and chemical pollution. She is a 2018 National Geographic Explorer and SKY Ocean Rescue Scholar. She is currently a postdoctoral researcher at University of Connecticut studying the feeding physiology of suspension feeders.

Linkedin Handle: Martina Capriotti
Twitter handle: @12Martina_C
Instagram handle: @martinacapriotti12