Exposure to Fine Particulate Air Pollution Causes Vascular Insulin Resistance by Inducing Pulmonary Oxidative Stress

Abstract

Background: Epidemiological evidence suggests that exposure to ambient air fine particulate matter (PM2.5) increases the risk of developing type 2 diabetes and cardiovascular disease. However, the mechanisms underlying these effects of PM2.5 remain unclear.

Objectives: We tested the hypothesis that PM2.5 exposure decreases vascular insulin sensitivity by inducing pulmonary oxidative stress.

Methods: Mice fed control (10-13% kcal fat) and high-fat (60% kcal fat, HFD) diets, treated with 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) or mice overexpressing lung-specific extracellular superoxide dismutase (ecSOD) were exposed to HEPA-filtered air or to concentrated PM2.5 (CAP) for 9 or 30 days, and changes in systemic and organ-specific insulin sensitivity and inflammation were measured.

Results: In control diet-fed mice, exposure to CAP for 30 days decreased insulin-stimulated Akt phosphorylation in lung, heart, and aorta but not in skeletal muscle, adipose tissue, and liver and did not affect adiposity or systemic glucose tolerance. In HFD-fed mice, 30-day CAP exposure suppressed insulin-stimulated endothelial nitric oxide synthase (eNOS) phosphorylation in skeletal muscle and increased adipose tissue inflammation and systemic glucose intolerance. In control diet-fed mice, a 9-day CAP exposure was sufficient to suppress insulin-stimulated Akt and eNOS phosphorylation and to decrease IκBα (inhibitor of the transcription factor NF-κB levels in the aorta. Treatment with the antioxidant TEMPOL or lung-specific overexpression of ecSOD prevented CAP-induced vascular insulin resistance and inflammation.

Conclusions: Short-term exposure to PM2.5 induces vascular insulin resistance and inflammation triggered by a mechanism involving pulmonary oxidative stress. Suppression of vascular insulin signaling by PM2.5 may accelerate the progression to systemic insulin resistance, particularly in the context of diet-induced obesity. Citation: Haberzettl P, O'Toole TE, Bhatnagar A, Conklin DJ. 2016. Exposure to fine particulate air pollution causes vascular insulin resistance by inducing pulmonary oxidative stress. Environ Health Perspect 124:1830-1839; http://dx.doi.org/10.1289/EHP212.

Figure 1

Effects of concentrated fine particulate matter (CAP) exposure on systemic glucose homeostasis. Mice maintained on control diet (13% kcal fat) or placed on a high-fat diet (HFD, 60% kcal fat) were exposed to air or CAP for 30 days (Study I). After 25 days of exposure, systemic glucose tolerance was tested in both control (A) and HFD-fed (B) mice. The total excursion of glucose in the blood was calculated by integrating the area under the curve (AUC, inset). Data are the mean ± SE. ^# p < 0.05 air versus CAP; n = 8. GTT, glucose tolerance test.

Figure 2

Effects of concentrated fine particulate matter (CAP) exposure on organ-specific insulin sensitivity. Western blot analysis of Akt phosphorylation in lung (A) and heart (B) and phosphorylation of Akt (i) and eNOS (ii) in aorta (C) and skeletal muscle (D) in mice injected with saline or insulin (1.5 U/kg). Mice fed control diet (10% kcal fat) or placed on a high-fat diet (HFD, 60% kcal fat) were exposed to air or CAP for 30 days (Study II). Data are the mean ± SE normalized to controls. NS, not significant. *p < 0.05 saline vs. insulin; ^# p < 0.05, ⁺ p < 0.1 air versus CAP; n = 4.

Figure 3

Concentrated fine particulate matter (CAP) exposure induces aortic insulin resistance and vascular inflammation. (A) Western blot analysis of the insulin-stimulated (100 nM, 15 min) phosphorylation of Akt (i), eNOS (ii) and ERK (iii) and the abundance of IκBα (iv) in aortas isolated from mice maintained on control diet (13% kcal fat) or placed on a high-fat diet (HFD, 60% kcal fat) that were exposed to air or CAP for 9 days (Study III). A continuous Western blot to detect the abundance of IκBα is shown in Figure S4A. Data are the mean ± SE normalized to controls (^# p < 0.05 vs. air-exposed control diet-fed mice; phospho-Akt, n = 8; phospho-eNOS, n = 5; phospho-ERK, n = 6; IκBα, n = 4). (B) Dose dependency of CAP-induced vascular insulin resistance was analyzed in insulin-stimulated aortas isolated from mice exposed to air or CAP for 9 or 30 days. For each exposure performed between 2010 and 2013 (see Table S4), the extent of insulin-induced Akt phosphorylation in the aorta was measured by Western analysis as described. Data are shown as discrete points of insulin-induced Akt phosphorylation (mean ± SE, in percent of air-exposed controls) and the cumulative CAP dose for each exposure, and the curve is a best fit of a first-order exponential equation [(y = 39.0+ 60.9exp (–x/49.2); y = percent insulin-induced phospho-Akt, x = CAP concentration in micrograms per cubic meter] to the data. (C) Western blot analysis of the plasmatic abundance of protein–acrolein adducts (loading controls are shown in Figure S4C) and plasma TBARS levels (C, inset) in mice exposed to air or CAP for 9 days. Data are mean ± SE (^# p < 0.05, ⁺ p < 0.1 air vs. CAP; n = 5). ED₅₀, median effective dose; TBARS, thiobarbituric acid reactive substances.

Figure 4

TEMPOL treatment prevents concentrated fine particulate matter (CAP)-induced vascular insulin resistance and inflammation. Western blot analysis of (A) the insulin-stimulated phosphorylation of Akt and (B) the abundance of IκBα in aortas of mice treated with water or TEMPOL (1 mM, in drinking water) exposed to air or CAP for 9 days (Study IV). The continuous IκBα Western blot is shown in Figure S4B. Data are the mean ± SE normalized to controls. *p < 0.05 control versus insulin; ^# p < 0.05 air versus CAP. Phospho-Akt, n = 5–10; IκBα, n = 4–5.

Figure 5

Concentrated fine particulate matter (CAP)-induced vascular insulin resistance and inflammation are prevented in lung-specific ecSOD transgenic (ecSOD-Tg) mice. (A) Western blot analysis of the pulmonary ecSOD protein abundance (n = 4) in wild-type (WT) mice exposed to air or CAP for 9 days. (B) Western blots of the transgene ecSOD (t-ecSOD) protein abundance in lung and aorta isolated from WT mice exposed to air or CAP for 9 days and from ecSOD-Tg mice (Study V). Western blot analysis of the (C) pulmonary abundance of protein–acrolein adducts (loading controls are shown in Figure S4D, n = 4–5), (D) aortic insulin-stimulated Akt phosphorylation (n = 5–8) and (E) aortic IκBα abundance (n = 8–12) in WT and ecSOD-Tg mice exposed to air or CAP for 9 days. Data are the mean ± SE normalized to controls. *p < 0.05 control versus insulin; ^# p < 0.05 and ⁺ p < 0.1 air versus CAP.