Air pollution effects on the gut microbiota

Abstract

Global incidence rates for inflammatory bowel disease (IBD) have gradually risen over the past 20 years. Genome-wide association studies (GWAS) have identified over 160 genetic loci associated with IBD; however, inherited factors only account for a partial contribution to the disease risk. We have recently shown that urban airborne particulate matter (PM) ingested via contaminated food can alter gut microbiome and immune function under normal and inflammatory conditions. In this addendum, we will discuss how PM can modify the gut microbial form and function, provide evidence on changes seen in intestinal barrier, and suggest a working hypothesis of how pollutants affect the gastrointestinal tract. The significance of the work presented could lead to identifying airborne pollutants as potential risk factors and thus provide better patient care management.

Keywords: :

• air pollution
• environment
• cytokines
• gut microbiota
• mucosal inflammation
• intestinal permeability
• short chain fatty acids

This article refers to:

Introduction

The world is seeing a dramatic rise in population growth in urban areas. As urban populations grow, the quality of the environment, and especially urban air pollution, will play an increasingly important role in public health. It has become clear that the disease burden due to urban air pollution is on the rise. Though research on airborne pollutants has mostly focused on cardiovascular and respiratory effects, emerging evidence suggests air pollutants are also having adverse effects on the gastrointestinal tract. Epidemiological studies have revealed an association between exposure to air pollution and different gastrointestinal diseases including inflammatory bowel disease (IBD),^1,2 appendicitis,³ irritable bowel syndrome,⁴ and enteric infections in infants.⁵

Components of Air Pollution

Air pollution is a heterogeneous mixture of a number of substances, including gases (e.g., carbon dioxide, carbon monoxide, ozone, nitric oxide, and sulfur dioxide), volatile organic compounds (e.g., benzene), and particulate matter.⁶ Particulate matter (PM) is a key pollutant in airborne pollution that has been associated with many adverse health conditions. Most airborne PM pollutants are derived from combustion of fossil fuels (e.g., from car exhaust) and industrial effluents. The major components of PM are pollen, sulfates, nitrates, organic carbon, mineral dust, polycyclic aromatic hydrocarbons (PAH), metals, ions, and biological components (microbial particles, lipopolysaccharide, and spores).⁷ These particles are defined according to their aerodynamic diameter as either fine particles of a diameter smaller than 2.5 μm (PM_2.5) or coarse with a diameter smaller than 10 μm (PM₁₀).⁷ The overall composition of air pollution depends upon the local sources; these include fossil fuel combustion from cars, home furnaces, and factories, as well as livestock emissions. The total pollution exposure of an individual will be determined not only by where they live but also where they travel and where they work. Two components of air pollution that have been extensively investigated for biological effects include ozone and particulate matter (PM). Studies have shown that exposure to ozone results in an increase in cellular permeability and a breakdown of tight junctions, while increased PM exposure is linked with increased cardiovascular mortality.^8,9 It is also likely that other components of air pollution yet to be studied may also have detrimental effects on human physiology.

Exposure of the Gut to Particulate Matter

The gastrointestinal tract is exposed to high concentrations of pollutant PM. Human studies have shown mucociliary transport of inhaled PM are quickly cleared from the lungs and into the intestine.¹⁰ Furthermore, pollutant PM contaminates both our food and water supply in significant amounts and, hence, account for additional oral route exposure.^11,12 It has been estimated that 10¹²–10¹⁴ particles are ingested per day by an individual on a typical Western diet, with an estimated mucosal uptake of ~1% (10⁹–10¹² per day).^13,14 As such, understanding what role PM might be playing in the recent rise of gastrointestinal diseases, particularly the rise in incidences of IBD, is a relevant point of study.

Importantly, air pollution is not the only inhaled environmental exposure that has been associated with gastrointestinal disease. Smoking is an example of an inhaled exposure that has been shown to influence the susceptibility of developing IBD.^15-18 Animal model studies suggest that smoking may mediate its effects through alterations of intestinal microbiota.¹⁹

Epidemiologic Associations between Air Pollution and Gut Disease

Epidemiologic data are beginning to show that air pollution has important health effects on the gastrointestinal tract. Long-term exposure to higher concentrations of nitrogen dioxide and PM were associated with an increased risk of early-onset Crohn disease.¹ Among Crohn disease patients, the risk associated with air pollution increased linearly across concentration levels suggesting a dose response effect.¹ Additionally, a second study demonstrated that a rise in total measured air pollutants was associated with an increased risk of hospitalizations for IBD.² Short-term exposure to air pollution has also been associated with gastrointestinal diseases. A single city study demonstrated an association between ozone exposure and appendicitis.²⁰ A replication study conducted in 12 cities across Canada confirmed the association between ozone and appendicitis. Moreover, this study showed that higher levels of ozone exposure shifted the course of appendicitis toward a perforating phenotype.³ A multi-city study showed that an acute rise in air pollution was associated with increased emergency department visits for non-specific acute abdominal pain in young women.⁴ Another multi-city study demonstrated that infants were more likely to visit emergency departments for gastroenteritis when carbon dioxide levels were acutely elevated.⁵

However, important limitations of epidemiologic air pollution studies should be considered, including that facts that air pollution exposures are generally based on regional estimates, not personal monitoring; gastrointestinal diseases are identified based on administrative databases that may contain misclassification errors; false positive associations can occur when multiple air pollutants are considered; and the possibility of bias due to confounding factors. As more biological studies are performed, a clearer picture of the effects of particular air pollutants on gastrointestinal function is bound to emerge.

Ingested Particulate Matter Alters the Gut Microbiome

There are over 10¹⁴ microbes in the human gastrointestinal tract and they encode for over 100-fold more unique genes than our own genome.²¹ Recent evidence has implicated a dysfunctional gut microbiome in the development of several disorders, including diabetes, obesity, metabolic disorders, and IBD.²² As such, understanding the factors that cause alterations in the gut microbes is important to understand disease pathogenesis. Despite the well-studied effects of environmental pollutants on various health conditions, little is known on how air pollution impacts the gut microbiome. Using a human gastrointestinal simulator, Van de Wiele et al. illustrated that human intestinal bacteria metabolised inorganic arsenic from contaminated soils into various toxic species.²³ In another experiment, gut microbes were also shown to transform PAHs into compounds that exhibit estrogenic properties in the body.²³ These results highlight that gut microbes are engaged in bioactivation of inorganic compounds, which in turn may contribute to disease risk development and perpetuation of chronic diseases.

In our recent work, we showed that ingested pollutant particles altered gut microbiota composition and function.²⁴ IL-10^−/− mice (whose colitis development depends on the gut microbiota) given particulate matter in their chow showed significant changes in the relative amounts of Bacteroidetes, Firmicutes, and Verrucomicrobia. These changes in microbial abundance correlated with changes in short chain fatty acid production, in that mice fed the particles had increased concentrations of the branched chain fatty acids isobutyrate and isovalerate in the cecum. Isobutyrate and isovalerate in human stool originate from the degradation of amino acids valine, leucine, or isoleucine,²⁵ and the finding of an increase in these compounds indicates a shift from a carbohydrate to a protein fermentation environment, indicating changes in microbial composition. In addition, exposure of the gut to particulate matter resulted in a decreased concentration of butyrate. This is significant in that butyrate is an essential fatty acid for colonocytes and mucosal immune cells, and a depletion in butyrate is commonly associated with a decrease in barrier function and increased susceptibility to mucosal inflammation.²⁶ In a similar study, mice exposed to polychlorinated biphenyls (PCBs) from contaminated foods showed a substantial shift in abundance and composition of the gut microbiome.²⁷ The mechanism underlying this PM-induced shift in microbial composition remains to be shown; however, taken together, these results suggest that ingested environmental pollutants can significantly alter both gut microbe composition and also metabolic processes. These alterations can have detrimental effects on not only the digestion process occurring in the lumen, but also on the structure and function of the intestinal mucosa.

Particulate Matter Increases Intestinal Permeability

At the interface between the external luminal environment and the internal body proper lies the highly regulated epithelial monolayer that not only serves to absorb nutrients and water, but also provides important barrier and immune surveillance mechanisms.²⁸ Changes in intestinal barrier have been associated with a number of diseases including IBD and celiac disease.²⁹ In our study we found that upon ingestion of particulate matter, there was an increased small intestinal permeability that was accompanied with an inflammatory response. Analysis of lactulose/mannitol excretion in urine revealed that pollutant PM increased permeability compared with the control group. Mutlu et al. showed that decreased epithelial barrier caused by particulate matter was associated with rearrangement of the epithelial tight junction proteins.³⁰ They further showed that particulate matter-induced barrier disruptions were linked to free radical oxygen species (ROS) that were generated by epithelial cells. These observations suggest that either directly or indirectly, particulate matter contributes to increased gut permeability. As such, gut exposure to airborne pollutant may play a pathogenic role in contributing to gastrointestinal disorders through disruptions in the epithelial barrier.

Increases in gut permeability have been linked with intestinal inflammation in several animal models and in human diseases.²⁹ In the IL-10−/− mouse, ingested particulate matter both increased gut permeability and exacerbated colonic inflammation.²⁴ Past studies have demonstrated that ingested particulate matter reduces colonic contractility due to neuronal impairment rather than muscular mechanisms.⁴ As such, colonic mucosal immune cells will have prolonged exposure to particulate matter. In addition, several studies have demonstrated in vitro exposure of immune cells to particulate matter to result in an enhanced pro-inflammatory cytokine secretion, due in part to increased levels of oxidative stress and activation of MyD88.^31-35

In addition to gastrointestinal disease, inflammation has been shown to have a strong association with many chronic diseases seen in western countries, including asthma, type 1 and 2 diabetes, and atherosclerosis.^36,37 Type 1 diabetes is associated with type 1 interferon production and alterations in T cell signaling, suggesting an autoimmune response, while type 2 diabetes is associated with enhanced inflammatory responses in adipose tissue and insulin resistance.³⁸ Obesity and atherosclerosis have also been associated with chronic inflammation as evidenced by increased levels of serologic markers such as C-reactive protein (CRP).^36,37 Thus, a particulate matter-induced increase in gut permeability may have a significant contribution to enhanced levels of systemic inflammation, due to an unrestrained influx of microbial products from the gut into the systemic circulation.

Model of Particulate Matter Induction of Intestinal and Systemic Inflammation

As epidemiological evidence mounts for ingested pollutant PM having a role in gastrointestinal disorders, dissecting the mechanisms behind such alterations will provide insight into how particulate matter can trigger disease onset or reoccurrence. Based on the research evidence provided, we would propose the following mechanism (Fig. 1).

Figure 1. Proposed model of particulate matter induction of intestinal and systemic inflammation. Particles will have both direct effects on the gut epithelial cells and be metabolized by resident gut microbes into toxic metabolites. PM-induced changes in epithelial tight junctions through ROS production increase gut permeability allowing for an influx of microbial products and particulate matter into the lamina propria and increased interactions with immune cells. This induces a pro-inflammatory response from resident immune cells which will further increase gut permeability and alter the luminal environment of the gut to allow for the growth of particular microbial strains more suited to survival in an inflammatory environment. Select microbial groups metabolize particulate matter into toxic metabolites which can directly influence microbial survival and epithelial function. The altered microbial community subsequently leads to changes in metabolic processes within the host whereby a decrease in short chain fatty acid production occurs along with enhanced production of non-beneficial metabolites.

Ingested particulate matter enters the gastrointestinal tract either through the mucociliary clearance of inhaled airborne pollutants from the lungs and/or dietary intake of contaminated food and drinking water. Once in the intestinal lumen, the particles will have both direct effects on the gut epithelial cells and be metabolized by resident gut microbes into toxic metabolites. PM-induced changes in epithelial tight junctions through ROS production will increase gut permeability allowing for an influx of microbial products and particulate matter into the lamina propria, and increased interactions with immune cells. This microbial influx induces a pro-inflammatory response from resident macrophage and dendritic cells, which can drive systemic inflammation, further worsen gut permeability, and alter the luminal environment of the gut to allow for the growth of particular microbial strains more suited to survival in an inflammatory environment. Further, select microbial groups may also metabolize some particulate matter into toxic metabolites, which can also directly influence microbial survival and epithelial function. The altered microbial community subsequently leads to changes in metabolic processes within the host whereby a decrease in short chain fatty acid production occurs, along with enhanced production of non-beneficial metabolites. Decreased butyrate production by gut microbes affects hosts homeostasis by increasing susceptibility to intestinal permeability and mucosal inflammation.

Conclusion

Together, our study in IL-10^−/− mice, in conjunction with previous experimental and epidemiological observations, strongly suggests that ingested particulate matter could trigger and accelerate the development of gastrointestinal inflammatory diseases, particularly in genetically susceptible individuals. This can occur through a combination of factors, including increased gut permeability, decreased colonic motility and clearance, and altered gut microbial composition and metabolic function.

Disclosure of Potential Conflicts of Interest

No potential conflict of interest was disclosed.

10.4161/gmic.27251