Epidemiological and experimental links between air pollution and type 2 diabetes

Ozone exposure triggers insulin resistance through muscle c-Jun N-terminal kinase activation

A growing body of evidence suggests that exposure to traffic-related air pollution is a risk factor for type 2 diabetes. Ozone, a major photochemical pollutant in urban areas, is negatively associated with fasting glucose and insulin levels, but most aspects of this association remain to be elucidated. Using an environmentally realistic concentration (0.8 parts per million), we demonstrated that exposure of rats to ozone induced whole-body insulin resistance and oxidative stress, with associated endoplasmic reticulum (ER) stress, c-Jun N-terminal kinase (JNK) activation, and disruption of insulin signaling in skeletal muscle. Bronchoalveolar lavage fluids from ozone-treated rats reproduced this effect in C2C12 myotubes, suggesting that toxic lung mediators were responsible for the phenotype. Pretreatment with the chemical chaperone 4-phenylbutyric acid, the JNK inhibitor SP600125, or the antioxidant N-acetylcysteine alleviated insulin resistance, demonstrating that ozone sequentially triggered oxidative stress, ER stress, and JNK activation to impair insulin signaling in muscle. This study is the first to report that ozone plays a causative role in the development of insulin resistance, suggesting that it could boost the development of diabetes. We therefore provide a potential mechanism linking pollutant exposure and the increased incidence of metabolic diseases.

Long-term exposure to air pollution and type 2 diabetes mellitus in a multiethnic cohort

Although air pollution has been suggested as a possible risk factor for type 2 diabetes mellitus (DM), results from existing epidemiologic studies have been inconsistent. We investigated the associations of prevalence and incidence of DM with long-term exposure to air pollution as estimated using annual average concentrations of particulate matter with an aerodynamic diameter of 2.5 μm or less (PM₂.₅) and nitrogen oxides at baseline (2000) in the Multi-Ethnic Study of Atherosclerosis. All participants were aged 45-84 years at baseline and were recruited from 6 US sites. There were 5,839 participants included in the study of prevalent DM and 5,135 participants without DM at baseline in whom we studied incident DM. After adjustment for potential confounders, we found significant associations of prevalent DM with PM₂.₅ (odds ratio (OR) = 1.09, 95% confidence interval (CI): 1.00, 1.17) and nitrogen oxides (OR = 1.18, 95% CI: 1.01, 1.38) per each interquartile-range increase (2.43 µg/m(3) and 47.1 ppb, respectively). Larger but nonsignificant associations were observed after further adjustment for study site (for PM₂.₅, OR = 1.16, 95% CI: 0.94, 1.42; for nitrogen oxides, OR = 1.29, 95% CI: 0.94, 1.76). No air pollution measures were significantly associated with incident DM over the course of the 9-year follow-up period. Results were partly consistent with a link between long-term exposure to air pollution and the risk of type 2 DM. Additional studies with a longer follow-up time and a greater range of air pollution exposures, including high levels, are warranted to evaluate the hypothesized association.

The association between air pollutants and morbidity for diabetes and liver diseases modified by sexes, ages, and seasons in Tianjin, China

With the generalized linear model and natural splines (ns), we examined the association between outdoor air pollutants and daily morbidity for diabetes and liver disease stratified by sexes and ages based on 4 years of daily data (2008-2011) in Tianjin, China. Season effects of air pollutants including particulate matter (PM), sulfur dioxide (SO2), and nitrogen dioxide (NO2) were also investigated. An increase of 10 μg/m(3) in a 2-day average concentrations of particulate matter with diameters of 10 μm or less (PM10), SO2, and NO2 corresponds to increases in diabetes morbidity of 0.39 % (95 % confidence interval (CI), -0.42-1.12), 0.15 % (95 % CI, -0.25-0.54), and 1.22 % (95 % CI, 0.51-2.96), respectively. As for liver morbidity, the increases were -0.84 % (95 % CI, -2.33-0.62), 0.90 % (95 % CI, 0.50-1.74), and 1.10 % (95 % CI, -2.58-4.78), respectively. The effects were stronger in the cool season than those in the warm season; females and the elderly were generally more vulnerable to outdoor air pollution. This study possesses scientific implications and instructional significance for local environmental standards and medical policymaking.

Metabolic syndrome and the environmental pollutants from mitochondrial perspectives

The worldwide epidemic of diabetes and metabolic syndrome in the last few decades cannot be fully accounted for only by changes in the lifestyle factors, such as sedentary lifestyle and overeating. Besides genetic factors, there must be other causes to explain this rapid change. They could not be infectious in nature and induce insulin resistance as key biochemical abnormality. Mitochondrial dysfunction could be underlying mechanism behind the insulin resistance, thus metabolic syndrome. Then there have been increasing number of reports suggesting that chronic exposure to and accumulation of endocrine disrupting chemicals (EDCs), especially so-called the persistent organic pollutants (POPs) within the body might be associated with metabolic syndrome. Combining two concepts, we developed new "EDCs-induced mitochondrial dysfunction hypothesis of metabolic syndrome". In this review we suggest that classifying those chemicals into 5 groups might be clinically useful considering their removal or avoidance; POPs, non-persistent organic pollutants, heavy metals, air pollutants and drugs. We will also discuss briefly how those insights could be applied to clinical medicine.

Air pollution and risk of type 2 diabetes mellitus: a systematic review and meta-analysis

AIM

Whether exposure to relatively high levels of air pollution is associated with diabetes occurrence remains unclear. We sought to assess and quantify the association between exposure to major air pollutants and risk of type 2 diabetes.

METHODS

PubMed and EMBASE databases (through September 2013) were searched using a combination of terms related to exposure to gaseous (NO2 and NOx) or particulate matter pollutants (PM2.5, PM10 and PM10-2.5) and type 2 diabetes. Descriptive and quantitative information were extracted from selected studies. We used random-effects models meta-analysis to derive overall risk estimates per type of pollutant.

RESULTS

We included ten studies (five cross-sectional and five prospective), assessing the effects of air pollutants on the occurrence of diabetes. In prospective investigations, the overall effect on diabetes occurrence was significant for both NO2 (adjusted hazard ratio [HR], 1.13; 95% confidence interval [95%CI], 1.01-1.22; p < 0.001; I(2) = 36.4%, pheterogeneity = 0.208) and PM2.5 (HR, 1.11; 95%CI, 1.03-1.20; p < 0.001; I(2) = 0.0%, pheterogeneity = 0.827). Odds ratios were reported by two cross-sectional studies which revealed similar associations between both NO2 and PM2.5 with type 2 diabetes. Across studies, risk estimates were generally adjusted for age, gender, body mass index and cigarette smoking.

CONCLUSIONS

Available evidence supports a prospective association of main air pollutants with an increased risk for type 2 diabetes. This finding may have implications for population-based strategies to reduce diabetes risk.

Associations between ambient air pollution and blood markers of inflammation and coagulation/fibrinolysis in susceptible populations

The pathophysiological pathways linking particulate air pollution to cardiovascular disease are still not fully understood. We examined the association between ambient air pollutants and blood markers of inflammation and coagulation/fibrinolysis in three potentially susceptible populations. Three panels of non-smoking individuals were examined between 3/2007 and 12/2008: 1) with type 2 diabetes mellitus (T2D, n=83), 2) with impaired glucose tolerance (IGT, n=104), and 3) with a potential genetic predisposition which could affect detoxifying and inflammatory pathways (n=87) defined by the null polymorphism for glutathione S-transferase M1 (GSTM1) in combination with a certain single nucleotide polymorphism on the C-reactive protein (CRP) or the fibrinogen gene. Study participants had blood drawn up to seven times every four to six weeks. In total, 1765 blood samples were analysed for CRP, interleukin (IL)-6, soluble CD40 ligand (sCD40L), fibrinogen, myeloperoxidase (MPO), and plasminogen activator inhibitor-1 (PAI-1). Hourly mean values of particulate air pollutants, particle number concentrations in different size ranges and gaseous pollutants were collected at fixed monitoring sites and individual 24hour averages calculated. Associations between air pollutants and blood markers were analysed for each panel separately and taking the T2D panel and the IGT panel together, using additive mixed models adjusted for long-term time trend and meteorology. For the panel with potential genetic susceptibility, CRP and MPO increased for most lags, especially with the 5-day average exposure (% change of geometric mean and 95% confidence interval: 22.9% [12.0;34.7] for CRP and 5.0% [0.3;9.9] for MPO per interquartile range of PM2.5). Small positive associations were seen for fibrinogen while sCD40L, PAI-1 and IL-6 mostly decreased in association with air pollution concentrations. Except for positive associations for fibrinogen we did not see significant results with the two other panels. Participants with potential genetic susceptibility showed a clear association between inflammatory blood biomarkers and ambient air pollutants. Our results support the hypothesis that air pollution increases systemic inflammation especially in susceptible populations which may aggravate atherosclerotic diseases and induce multi-organ damage.

Long-term air pollution exposure and diabetes in a population-based Swiss cohort

Air pollution is an important risk factor for global burden of disease. There has been recent interest in its possible role in the etiology of diabetes mellitus. Experimental evidence is suggestive, but epidemiological evidence is limited and mixed. We therefore explored the association between air pollution and prevalent diabetes, in a population-based Swiss cohort. We did cross-sectional analyses of 6392 participants of the Swiss Cohort Study on Air Pollution and Lung and Heart Diseases in Adults [SAPALDIA], aged between 29 and 73 years. We used estimates of average individual home outdoor PM10 [particulate matter <10μm in diameter] and NO2 [nitrogen dioxide] exposure over the 10 years preceding the survey. Their association with diabetes was modeled using mixed logistic regression models, including participants' study area as random effect, with incremental adjustment for confounders. There were 315 cases of diabetes (prevalence: 5.5% [95% confidence interval (CI): 2.8, 7.2%]). Both PM10 and NO2 were associated with prevalent diabetes with respective odds ratios of 1.40 [95% CI: 1.17, 1.67] and 1.19 [95% CI: 1.03, 1.38] per 10μg/m(3) increase in the average home outdoor level. Associations with PM10 were generally stronger than with NO2, even in the two-pollutant model. There was some indication that beta blockers mitigated the effect of PM10. The associations remained stable across different sensitivity analyses. Our study adds to the evidence that long term air pollution exposure is associated with diabetes mellitus. PM10 appears to be a useful marker of aspects of air pollution relevant for diabetes. This association can be observed at concentrations below air quality guidelines.

Ambient Air Pollution and Type 2 Diabetes: A Systematic Review of Epidemiologic Research

Recent experimental and epidemiologic studies have suggested air pollution as a new risk factor for type 2 diabetes mellitus (T2DM). We conducted a systematic review of the epidemiologic studies on the association of air pollution with T2DM and related outcomes published by December 2013. We identified 22 studies: 6 prospective studies on incident T2DM; 2 prospective study on diabetes mortality; 4 cross-sectional studies on prevalent T2DM; 7 ecological studies on mortality or morbidity from diabetes; and 3 studies on glucose or insulin levels. The evidence of the association between long-term exposure to fine particles (PM_2.5) and the risk of T2DM is suggestive. The summary hazard ratio of the association between long-term PM_2.5 exposure and incident T2DM was 1.11 (95% CI, 1.03-1.19) for a 10 μg/m³ increase. The evidence on the association between long-term traffic-related exposure (measured by nitrogen dioxide or nitrogen oxides) and the risk of T2DM was also suggestive although most studies were conducted in women. For short-term effects of air pollution on diabetes mortality or hospital/emergency admissions, we conclude that the evidence is not sufficient to infer a causal relationship. Because most studies were conducted in North America or in Europe where exposure levels are relatively low, more studies are needed in recently urbanized areas in Asia and Latin America where air pollution levels are much higher and T2DM is an emerging public health concern.

Which specific causes of death are associated with short term exposure to fine and coarse particles in Southern Europe? Results from the MED-PARTICLES project

We investigated the short-term effects of particles with aerodynamic diameter less than 2.5μm (PM2.5), between 2.5 and 10μm (PM2.5-10) and less than 10μm (PM10) on deaths from diabetes, cardiac and cerebrovascular causes, lower respiratory tract infections (LRTI) and chronic obstructive pulmonary disease (COPD) in 10 European Mediterranean metropolitan areas participating in the MED-PARTICLES project during 2001-2010. In the first stage of the analysis, data from each city were analyzed separately using Poisson regression models, whereas in the second stage, the city-specific air pollution estimates were combined to obtain overall estimates. We investigated the effects following immediate (lags 0-1), delayed (lags 2-5) and prolonged exposure (lags 0-5) and effect modification patterns by season. We evaluated the sensitivity of our results to co-pollutant exposures or city-specific model choice. We applied threshold models to investigate the pattern of selected associations. For a 10μg/m(3) increase in two days' PM2.5 exposure there was a 1.23% (95% confidence interval (95% CI): -1.63%, 4.17%) increase in diabetes deaths, while six days' exposure statistically significantly increased cardiac deaths by 1.33% (95% CI: 0.27, 2.40%), COPD deaths by 2.53% (95% CI: -0.01%, 5.14%) and LRTI deaths by 1.37% (95% CI: -1.94%, 4.78%). PM2.5 results were robust to co-pollutant adjustments and alternative modeling approaches. Stronger effects were observed in the warm season. Coarse particles displayed positive, even if not statistically significant, associations with mortality due to diabetes and cardiac causes that were more variable depending on exposure period, co-pollutant and seasonality adjustment. Our findings provide support for positive associations between PM2.5 and mortality due to diabetes, cardiac causes, COPD, and to a lesser degree to cerebrovascular causes, in the European Mediterranean region, which seem to drive the particles short-term health effects.

Ozone induces glucose intolerance and systemic metabolic effects in young and aged Brown Norway rats

Air pollutants have been associated with increased diabetes in humans. We hypothesized that ozone would impair glucose homeostasis by altering insulin signaling and/or endoplasmic reticular (ER) stress in young and aged rats. One, 4, 12, and 24 month old Brown Norway (BN) rats were exposed to air or ozone, 0.25 or 1.0 ppm, 6 h/day for 2 days (acute) or 2 d/week for 13 weeks (subchronic). Additionally, 4 month old rats were exposed to air or 1.0 ppm ozone, 6 h/day for 1 or 2 days (time-course). Glucose tolerance tests (GTT) were performed immediately after exposure. Serum and tissue biomarkers were analyzed 18 h after final ozone for acute and subchronic studies, and immediately after each day of exposure in the time-course study. Age-related glucose intolerance and increases in metabolic biomarkers were apparent at baseline. Acute ozone caused hyperglycemia and glucose intolerance in rats of all ages. Ozone-induced glucose intolerance was reduced in rats exposed for 13 weeks. Acute, but not subchronic ozone increased α2-macroglobulin, adiponectin and osteopontin. Time-course analysis indicated glucose intolerance at days 1 and 2 (2>1), and a recovery 18 h post ozone. Leptin increased day 1 and epinephrine at all times after ozone. Ozone tended to decrease phosphorylated insulin receptor substrate-1 in liver and adipose tissues. ER stress appeared to be the consequence of ozone induced acute metabolic impairment since transcriptional markers of ER stress increased only after 2 days of ozone. In conclusion, acute ozone exposure induces marked systemic metabolic impairments in BN rats of all ages, likely through sympathetic stimulation.

Diabetes mellitus and inflammation

Type 2 diabetes mellitus (T2DM) is increasingly common worldwide. Related complications account for increased morbidity and mortality, and enormous healthcare spending. Knowledge of the pathophysiological derangements involved in the occurrence of diabetes and related complications is critical for successful prevention and control solutions. Epidemiologic studies have established an association between inflammatory biomarkers and the occurrence of T2DM and complications. Adipose tissue appears to be a major site of production of those inflammatory biomarkers, as a result of the cross-talk between adipose cells, macrophages, and other immune cells that infiltrate the expanding adipose tissue. The triggering mechanisms of the inflammation in T2DM are still ill-understood. Inflammatory response likely contributes to T2DM occurrence by causing insulin resistance, and is in turn intensified in the presence of hyperglycemia to promote long-term complications of diabetes. Targeting inflammatory pathways could possibly be a component of the strategies to prevent and control diabetes and related complications.