Polystyrene bead ingestion promotes adiposity and cardiometabolic disease in mice

Abstract

Vast amounts of plastic materials are produced in the modern world and despite recycling efforts, large amounts are disposed in water systems and landfills. Under these storage conditions, physical weathering and photochemical processes break down these materials into smaller particles of the micro- and nano-scale. In addition, ecosystems can be contaminated with plastic particles which are manufactured in these size ranges for commercial purposes. Independent of source, microplastics are abundant in the environment and have found their way into water supplies and the food cycle where human exposure is inevitable. Nevertheless, the health consequences of microplastic ingestion, inhalation, or absorption are largely unknown. In this study we sought to determine if ingestion of microplastics promoted pre-clinical cardiovascular disease (CVD). To do this, we supplied mice with normal drinking water or that supplemented with polystyrene beads of two different sizes (0.5 µm and 5 µm) and two different doses (0.1 μg/ml and 1 μg/ml) each for 12 weeks and measured several indices of metabolism and glucose homeostasis. As early as 3 weeks of consumption, we observed an accelerated weight gain with a corresponding increase in body fat for some exposure groups versus the control mice. Some exposure groups demonstrated increased levels of fasting plasma glucose. Those mice consuming the smaller sized beads (0.5 µm) at the higher dose (1 μg/ml), had increased levels of fasting plasma insulin and higher homeostatic model assessment of insulin resistance (HOMA-IR) scores as well. This was accompanied by changes in the gut microbiome consistent with an obese phenotype. Using samples of perivascular adipose tissue collected from the same group, we observed changes in gene expression consistent with increased adipogenesis. These results suggest that ingestion of polystyrene beads promotes a cardiometabolic disease phenotype and thus may be an unrecognized risk factor for CVD.

Keywords: Cardiometabolic disease; Gut microbiome; Microplastics; Obesity; Polystyrene.

Figure 1.. Experimental approach.

Mice were supplied with normal drinking water or that containing polystyrene (PS) beads of a given size and dose (table) for 12 weeks. Illustrated are the times of analytical measures with the groups used for those measures in parentheses.

Figure 2.. Body weight and composition.

The weights of mice drinking normal water (control) and that containing 5μm PS beads (A) or 0.5μm PS beads (B) at the indicated concentrations were obtained after 3, 6, 9, and 12 weeks of consumption. Depicted are representative DEXA scan images from the indicated groups after 9 weeks of consumption (C). Also illustrated is the percent body fat determined for all mice and all groups (D). *: p<0.05.

Figure 3. Plasma glucose and insulin.

Fasting plasma glucose levels were obtained from mice drinking normal water (control) and that containing 5μm PS beads (A) or 0.5μm PS beads (B) at the indicted concentration after 3, 6, 9, and 12 weeks of consumption. Fasting plasma insulin levels were measured in samples collected from the 5 treatment groups at termination (C). Also illustrated are calculated HOMA-IR scores obtained from control mice and those mice ingesting 0.5μm beads at a dose of 1μg/ml for 12 weeks (D). *: p<0.05

Figure 4.. Microbiome analysis.

A) Illustrated is the principal coordinate analysis of beta diversity of gut microbiota from unweighted UniFrac distance values. Samples from mice consuming water containing the PS beads (blue) are more tightly clustered than the control samples which show higher variation. B) Illustrated are boxplots of beta diversity of the gut microbiota from the unweighted UniFrac distance values of two groups with q-value 0.028 as calculated from a PERMANOVA test. C) Linear discriminant analysis (LDA) effect size (LEfSe) analysis identifies differentially abundant gut microbiota with LDA score ≥ 2 in Phylum level between two groups based on a non-parametric Kruskal-Wallis test.

Figure 5.. Phylum level composition.

Illustrated is the relative abundance of Bacteriodetes and Firmicutes in the collected fecal samples (left) and the relative abundance of more lowly expressed phyla (right). PS: water containing polystyrene beads.