Polychlorinated Biphenyls (PCBs) and its potential effects on GI health, reproduction and immune functions of living organisms.
Polychlorinated biphenyls (PCBs) are persistent organic pollutants (POPs) that are spread throughout the world. These Polychlorinated biphenyls are widely distributed in soil sediments, water, air, land, human blood, and milk and can accumulate in living organisms.
The main route of excretion of PCBs is urine, feces and breast milk. Polychlorinated biphenyls have adverse effects on fertility and reproductive organs. They can also affect the gut microbial diversity and tight protein junction in gastrointestinal (GI) tract.
After exposure to Polychlorinated biphenyls, the size of thymus gland and the number of immune cells have been decreased. In conclusion, Polychlorinated biphenyls may impair the gastrointestinal, reproductive, and immune functions of living organisms.
Polychlorinated Biphenyls (PCBs) are persistence organic pollutants having one or more chlorine atoms attached to the joined benzene rings. There are more than 209 types of PCBs having different number and location of chlorine atoms attached to the benzene ring.
PCBs are distributed into coplanar, mono-ortho-substituted PCBs, and non-dioxin-like PCBs. Coplanar and some mono-ortho-substituted PCBs have dioxin-like properties. The toxicity level of PCBs depends upon the arrangement of chlorine atoms in the molecule.
Many type of PCB have dioxin-like properties. These PCBs have one chlorine atom at the orthoposition in benzene ring. The benzene rings of such molecules can interchange and adopt a coplanar arrangement, which have the similar toxicity as the polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).
However, many other types of PCB have more than two chlorine atoms at the ortho-position in the benzene ring. In such molecules, the two benzene rings are not in the similar plane, and slow non-dioxin-like properties.
Industrial PCB are a combination of various types PCB and comprises small quantity of PCDFs or PCDDs. The contamination level of such molecules is significantly higher than compared to PCBs.
PCBs are insulated and non-flammable therefore they were commonly used in making different things for approximately 50 years (1929 and 1977). They were used in producing diffusion pump oils, pesticide extenders, heat conversation, and dielectric liquids in electric appliances like transformers and capacitors, hydraulic and greasing liquids, as a constituent in sealing compounds, paint industry, glues, flame-retardants, and plasticizers.
In 1977, the United States banned the production of PCBs because of their potential carcinogenicity.
Source of PCBs
PCBs can be emitted in the environment by the removal of PCB-containing products in public landfills, prohibited or inappropriate depositing of waste material that contained PCBs, for example, transformer liquids leakages from electrical transformers and capacitors having PCBs, contaminated waste places.
PCBs have been categorized in at least 500 of the 1,598 harmful waste places on the U.S. EPA’s National Priorities List, and low concentrations of PCBs can be observed all over the world.
After emitted into the environment, PCBs accumulates in animal and human tissues as they shift up the food chain, gradually metabolize, and driven and transferred within the environment.
After emitted into the environment, PCBs adsorb effectively to water, air, soil, and sediment. Consequently, these mixtures have a tendency to persist in the environment, with half-lives from months to years.
PCBs can be transferred to the plants via soil inconsequentially. Cycling of PCBs includes volatilization from soil and water surfaces into the atmosphere, with the following elimination from the atmosphere by wet or dry deposition
Release and Exposure of PCBs
The major sources of PCBs contamination are the atmosphere and the factories. As they degrade slowly, highly chlorinated PCB complexes can persevere in the environment for years.
The principal route of exposure to PCBs is the ingestion of infected foodstuffs, especially beef, mutton, chicken meat, and fish. Industrial contact with PCBs occurs generally via the inhalation and dermal routes. Industrial contact with PCBs may also result in waste site cleaning or dumping activities.
Potential Health Effects of PCBs
Effects on gastrointestinal tract
There is a homology in bacterial colonization and immunity, digestive and immune physiologies, and immuno-histology between laboratory animals, and pigs. Zebrafish gut is an important biomarker to ascertain the effect of PCBs on gut microbiome and GI health in mammals.
The gut microbiota consists of several bacteria diversities in homeostatic equilibrium. Ingestion of PCBs can disrupt the gut microbiota homeostasis to cause dysbiosis.
Dysbiosis is a decrease in gut microbial diversity, with surge in relative abundance of pathobionts. Recent studies employing 16sRNA sequencing technique suggest that PCBs reduced the α diversity in cecum, and increase Firmicutes: Bacteroidetes ratio in mice.
It caused a shift in gut microbiota populations at phyla and genera levels. And it caused a surge in the population of bifidobacterium in the gut. The observed dysbiosis is likely due to alteration in metabolism of the short-chain fatty acids (SCFA) and bile acids (BA) in the guts. This disrupts tight junction proteins that regulate intestinal barrier functions.
Effects on reproduction
Adverse impacts of PCB on fertility and reproductive organs have been tested in laboratory animals like rodents, mice, and monkeys. Rodents that were nourished comparatively high concentration of PCBs showed an increased testes size (3.5 mg/kg BW/day), decreased sperm quantity (25 mg/kg BW/day), reduced fertility (12.5 mg/kg BW/day), and reduced litter size (35.4 mg/kg BW/day).
Harmful reproductive impacts have been detected in the progeny of female rodents presented to a blend of comparatively high chlorinated PCB molecule during lactation. This indicates that PCBs transmitted during breastfeeding can have a long-term effect.
Monkeys that were fed PCBs indicated a delayed menstruation (0.1 mg/kg BW/day), and a decreased fertility (0.2 mg/kg BW/day). No harmful impacts on fertility were detected under 0.005 mg/kg BW/day.
Female hormonal movement, was broadly examined in-vitro and in-vivo with rodents and rats. The study shows that PCBs may simulate the estrogen movement or, on the other hand, it prevents estrogens from acting like in case of dioxin-like PCBs.
Growing effects of swallowed PCBs had been confirmed on many species of animal. Contact of females with PCBs during gestation and lactation can restrict the growth of fetus.
In offspring of exposed females of monkey, several behavior tests were performed. Monkeys offspring presented to a mixture similar to PCB in human milk displayed behavioral change at 0.0075 mg/kg BW/day.
In another study, changes in growth of nervous tissues were detected after contact with higher concentration of nonplanar PCBs, but not with coplanar (dioxin-like) PCBs.
Effects on immune system
The influences of PCBs on the immune tissues of young rodents were investigated. Fifteen-weeks old rodents were exposed to higher concentration of PCBs lead to a reduced size of thymus gland and a decreased immune cells number indicating that PCBs have damaged the immune functions. 1 mg/kg BW/day was the minimum dose on which no harmful impacts on immune system of rodents were detected.
Mice exposed to subclinical doses of PCB showed no mortality but upon an introduction of Salmonella typhimurium, higher mortality, and higher numbers of the bacteria in immune organs and blood were observed, along with increased sensitivity to endotoxin.
The effect of PCBs of host lipid metabolism and intestinal microbe population dynamics further suppresses humoral and adaptive immune responses. Toll-like receptor 4 (TLR4) may decrease in the jejunum and colon as observed in mice.
PCBs-induced GI health abnormalities also affect immune functions by reducing in antibodies production. It is therefore apparent that PCBs contamination can cause immunosuppression and increase susceptibility to inflammatory bowel diseases such as porcine epidemic diarrhea.
Highly chlorinated PCBs have impacts that are more adverse on immune system of mice than those with minimal chlorine atoms.
PCBs can induce gastrointestinal, reproductive and immune system malfunctions. Through the “one health’ concept, herbal plants can be harnessed to provide phytoremediation, and immunostimulation for sustainable improvement in environmental, animal and public health.
This article is jointly written by Aftab Qamar1 and JinJun Chen1* from 1Department of Veterinary Medicine, College of Coastal Agricultural Sciences, Guangdong Ocean University, Zhanjiang, Guangdong 524088 China. *Corresponding author can be reached at: firstname.lastname@example.org