Untoward Effects of Micro- and Nanoplastics: An Expert Review of Their Biological Impact and Epigenetic Effects

Abstract

The production of plastic has dramatically increased in the last 50 y. Because of their stability and durability, plastics are ubiquitously incorporated in both marine and terrestrial ecosystems. Plastic is acted upon by biological, chemical, and physical agents, leading to fragmentation into small pieces [i.e., microplastics (MPs) or nanoplastics (NPs)], classified depending on their size. MPs range from 0.1 to 5000 μm and NPs are fragments between 0.001 to 0.1 μm. MPs and, especially NPs, are easily incorporated into living beings via ingestion. The penetration of MPs and NPs into the food system is an important issue, for both food security and health risk assessment. Ingestion of different MPs and NPs has been associated with different issues in the intestine, such as direct physical damage, increased intestinal permeability, diminished microbiota diversity, and increases in local inflammatory response. However, the potential harmful effects of low-dose dietary plastic are still unclear. Some evidence indicates that intestinal uptake of plastic particles is relatively low and is mostly dependent on the particle's size. However, other evidence highlights that NPs dysregulate key molecular signaling pathways, modify the gut microbiota composition, and may induce important epigenetic changes, including transgenerational effects that might be involved in the onset of many different metabolic disorders. Until now, experiments have been mostly performed on marine organisms, Caenorhabditis elegans, and mouse models, but some research indicates accidental plastic dietary consumption by humans, raising the issue of detrimental health effects of MPs and NPs. This review discusses the impact that MPs and NPs could have on the intestinal tract and the biodistribution and systemic, cellular, and molecular levels. Accumulated evidence of MPs' effects on the human gut suggests that large exposure to MPs and NPs may have phenotypical untoward effects in humans, calling for urgent research in this field.

Keywords: biological effect; epigenetic; food; inflammation; miRNA; microbiota; microplastics; nanoplastics.

FIGURE 1

Plastic food-chain pyramid. The exposure of plastic debris to both chemical, biological and biotic factors causes mechanical degradation of large pieces of plastics, converting them to microplastics, which , produce nanoplastics. In marine ecosystems, marine organisms may take up microplastics into their organisms, which may remain for a long time in the gastrointestinal tract. The microplastics and nanoplastics were also observed in drinking water, edible fruits and vegetables, as well as in birds and animal farms. Therefore, the exposure of humans to micro- and nanoplastics is imminent, which can be observed at different levels, present in edible foods and in the trophic chain.

FIGURE 2

The effect of MPs and NPs on intestinal health. The exposure of the gastrointestinal tract to MPs produce short-term physical damage at the lumen, but also alter the production of mucus secretion and decrease the digestive enzymatic activity. This issue, in turn, alters the composition of the microbiota, by affecting the diversity of bacteria. Finally, the combination of all these factors produces an inflammatory response, in which a long exposure to MPs may induce a chronic inflammatory state at the gut level. With regard to NPs, these particles enter into the cell through endocytosis, and within the cell they interact with a number of biological molecules, altering many signaling pathways. They also induce changes at epigenetic levels, and such effects are also observed in transgenerational offspring. Abbreviations: EGF-ERK, epidermal growth factor—extracellular signal-regulated kinase 1/2; JNK, c-Jun N-terminal kinase; MAPK, mitogen activated protein kinase; miRNA, microRNA; MP, microplastic; NP, nanoplastic; NRF, nuclear factor E2-related factor.