Ozone Exposure Increases Circulating Stress Hormones and Lipid Metabolites in Humans

Abstract

Rationale: Air pollution has been associated with increased prevalence of type 2 diabetes; however, the mechanisms remain unknown. We have shown that acute ozone exposure in rats induces release of stress hormones, hyperglycemia, leptinemia, and glucose intolerance that are associated with global changes in peripheral glucose, lipid, and amino acid metabolism.

Objectives: To examine ozone-induced metabolic derangement in humans using serum metabolomic assessment, establish human-to-rodent coherence, and identify novel nonprotein biomarkers.

Methods: Serum samples were obtained from a crossover clinical study that included two clinic visits (n = 24 each) where each subject was blindly exposed in the morning to either filtered air or 0.3 parts per million ozone for 2 hours during 15-minute on-off exercise. Serum samples collected within 1 hour after exposure were assessed for changes in metabolites using a metabolomic approach.

Measurements and main results: Metabolomic analysis revealed that ozone exposure markedly increased serum cortisol and corticosterone together with increases in monoacylglycerol, glycerol, and medium- and long-chain free fatty acids, reflective of lipid mobilization and catabolism. Additionally, ozone exposure increased serum lysolipids, potentially originating from membrane lipid breakdown. Ozone exposure also increased circulating mitochondrial β-oxidation-derived metabolites, such as acylcarnitines, together with increases in the ketone body 3-hydroxybutyrate. These changes suggested saturation of β-oxidation by ozone in exercising humans.

Conclusions: As in rodents, acute ozone exposure increased stress hormones and globally altered peripheral lipid metabolism in humans, likely through activation of a neurohormonally mediated stress response pathway. The metabolomic assessment revealed new biomarkers and allowed for establishment of rodent-to-human coherence. Clinical trial registered with www.clinicaltrials.gov (NCT 01492517).

Trial registration: ClinicalTrials.gov NCT01492517.

Keywords: air pollution; fatty acids; lipid mediators; stress response.

Figures

Figure 1.

Acute 2-hour ozone exposure increases serum levels of the stress hormones cortisol (A) and corticosterone (B) in humans. Box-and-whisker plots convey the spread of the data with the interquartile range represented by the box and the range of the data shown by the whiskers(n = 24 per group). Outlier values are defined as those that exceed, in either direction, 1.5 times the interquartile range (shown asopen circles in each plot). The solid baracross the box represents the median value, and +symbol represents the mean. *Significantly different from air group (P < 0.05). For each biochemical, data are median scaled with the median value across all samples set to 1.0. The y-axis thus reflects scaled intensity for each metabolite.

Figure 2.

Ozone altered lipid metabolism pathways in humans. Pathway enrichment analysis (using MetaboSync) identified seven specific biochemical pathways (sphingolipid metabolism, endocannabinoid synthesis, fatty acid metabolism, β-oxidation, dicarboxylic acid metabolism, steroid hormone biosynthesis, and phospholipid metabolism) as having a significant (P < 0.05) fold enrichment value for ozone exposure compared with air. This analysis was based on several parameters including the total number of detected metabolites in the study and selected pathway, and the abundance of total metabolites associated with each pathway.

Figure 3.

Acute ozone exposure increases circulating polyunsaturated lipids in humans. Polyunsaturated lipids (A) linolenate (α or γ; 18:3n3 or n6), (B) eicosapentaenoate (EPA; 20:5n3), (C) docosapentaenoate (n3 DPA; 22:5n3), (D) decosahexaenoate (DHA; 22:6n3), (E) linoleate (18:2n6), (F) dihomo-linolenate (DHGLA; 20:3n3 or n6), and (G) arachidonate (AA; 20:4n6) were increased significantly in the serum after ozone exposure in subjects. The metabolic pathways in which these metabolites were generated are summarized in themiddle panel. Box-and-whisker plots convey the spread of the data with the interquartile range represented by thebox and the range of the data shown by thewhiskers (n = 24 per group). Outlier values are defined as those that exceed, in either direction, 1.5 times the interquartile range (shown as open circle in each plot). Thesolid bar across the box represents the median value, and the + symbol represents the mean. For each biochemical, data are median scaled with the median value across all samples set to 1.0. They-axis thus reflects scaled intensity for each metabolite. Significant difference between air and ozone (P ≤ 0.05 forA–F;P ≤ 0.1 for G). 13-HODE = 13-hydroxy octadecadienoic acid; HETE = hydroxytetraenoic acid; PUFA = polyunsaturated fatty acid.