Del Piano, Filomena (2024) Study on the toxicity induced by polystyrene microplastic in gilthead seabream: focus on intestinal and hepatic homeostasis. [Tesi di dottorato]

Anteprima

Testo Del Piano_Filomena_36.pdf Download (19MB) | Anteprima

Abstract

Plastic pollution in the natural environment is a threat of growing concern that has drawn the attention of many scientists worldwide. Once discharged into the ecosystems, plastic debris fragment into smaller particles named microplastics (MPs). In aquatic environments, MPs bind to water and sediment phases and undergo bioaccumulation and biomagnification processes along the trophic chains, with health consequences not yet well understood. Polystyrene (PS) is one of the most abundant polymer types contaminating aquatic environments and is frequently encountered in biotic matrices. Ingestion is the main exposure route to MPs in aquatic species, and ingested MPs accumulate in the gastrointestinal tract, causing adverse local effects on the gut. However, MPs may also translocate from the gut to the circulatory system, reaching several tissues and organs where they can cause detrimental effects. The gut-liver axis is a bidirectional crosstalk between the gut and the liver that may be influenced by several factors, including environmental ones. Disorders of the intestinal barrier may result in an increased portal influx of bacteria or their products, as well as toxic substances to the liver, where they cause or worsen several hepatic diseases. One of these is the non-alcoholic fatty liver disease (NAFLD), which is characterized by an excessive accumulation of lipids in hepatocytes, inflammation, and oxidative stress (OS). The aim of this study was to assess the effects of MP ingestion on gut-liver axis balance in gilthead seabream (Sparus aurata Linnaeus, 1758) fed with a diet enriched with PS-MPs (1–20 μm; 0, 25, or 250 mg /kg b.w./day) for 21 days. Experimental analyses were carried out using histological, molecular, and biochemical methodologies. PS-MPs differently affected homeostasis of the anterior (AI) and posterior (PI) intestine. Inflammation and immune alterations were revealed in both tracts, but PI showed a greater susceptibility to MP-induced modifications. Specifically, PS-MPs triggered the TLRs-Myd88 signaling pathway with the following augmented release of pro-inflammatory cytokines. Also, PS-MPs raised the expression of other immune-associated genes, such as Lys, CSF1R, and ALP. PS-MP exposure also increased OS and nitrosative stress and impaired the antioxidant defense system, in PI more than in AI. Moreover, the MAPKs (i.e., p38 and ERK) were activated by PS-MPs, and the intestinal barrier integrity was disrupted, as evidenced by the reduced gene expression of tight junctions (i.e., ZO-1, Cldn15, Occludin, and Tricellulin), integrins (i.e., Itgb6) and mucins (i.e., Muc2-like and Muc13-like). On the other hand, an increased synthesis and accumulation of lipids were revealed in the liver of exposed fish. Specifically, PS-MPs induced the up-regulation of genes related to lipid synthesis (i.e., PPARy, Srebp1, Fasn) without modifications of those involved in lipid catabolism or transport (i.e., PPARa, HL, Pla2, Fabp1). Moreover, a dose-dependent increase of immune and pro-inflammatory cytokines gene expression was also observed in exposed fish. These findings were confirmed by hepatic histological evaluations reporting evidence of lipid accumulation, inflammation, and necrosis. Additionally, PS-MPs affected the functionality of hepatic antioxidant and detoxifying systems, resulting in OS, as shown by the augmented production of ROS and MDA. Specifically, the alteration of enzymatic (catalase, superoxide dismutase, and glutathione reductase) and non-enzymatic (glutathione) components of the antioxidant defense system was highlighted in the liver of exposed fish. Similarly, the activity of detoxifying enzymes (cytochrome P4501A and glutathione-S-transferase) was impaired by PS-MP exposure. Based on the results, PS-MPs affect intestinal and hepatic homeostasis, altering the gut-liver axis balance in gilthead seabreams subchronically exposed via diet. PS-MPs directly damage the intestinal barrier integrity and function, with indirect consequences on liver health. Indeed, the impairment of hepatic lipid metabolism and the following steatosis, the increase of inflammatory response, and the OS induced by PS-MPs cooperate in liver dysfunction. All these alterations may synergistically promote and influence each other in the onset, development, and progression of NAFLD.

Downloads

Actions (login required)

Modifica documento