Air Pollution and Insulin Resistance: Do All Roads Lead to Rome?

Air Pollution and Chronic Disease Susceptibility

The World Health Organization estimates that worldwide in 2012, nearly 7 million deaths occurred prematurely due to air pollution (1). In addition to respiratory and cardiovascular diseases, air pollution exposure is also linked to increased incidence of diabetes (2). Notably, the prevalence of diabetes and dyslipidemia escalated exponentially in the latter half of the 20th century coincident with the manufacture, use, and release of massive amounts of chemicals and pollution. The increase in the prevalence of these chronic conditions also coincides with an increase in sedentary lifestyles, calorie-rich diets, and human stress. Together, these factors contribute to metabolic disease. This complex tapestry suggests that many elements, including air pollution, are likely involved in an interactive manner to increase the risk of certain health conditions. Determining how air pollution might be linked to diabetes is useful not only in understanding how environmental factors contribute to the pathogenesis of this disease but also for identifying molecular targets for potential therapeutic strategies. Improved understanding of this dynamic also provides further rationale for improving air quality standards and public health.

Ozone is produced in the air by photochemical reaction of components of anthropogenic emissions, and it contributes substantially to the societal burden of respiratory and cardiovascular disease. The pulmonary effects of ozone have been studied for decades (3), with recent attention turning to the metabolic and cardiovascular effects of this exposure.

Many Mechanisms Have Been Proposed for Systemic Effects of Air Pollution

A number of mechanisms have been proposed for extrapulmonary effects of inhaled pollutants. On the basis of experimental studies examining the metabolic effects of ambient particulate matter (chemically complex mixtures consisting of many components) in mouse models, Rajagopalan and Brook (4) proposed that systemic inflammation, oxidative stress, and neuronal mechanisms might be involved in adipose inflammation and tissue insulin resistance. In this issue of Diabetes, Vella et al. (5) report that although rats exposed to ozone did not show systemic inflammation associated with acute metabolic effects, they did exhibit insulin resistance in muscle tissue resulting from lipid and protein oxidation by-products.

Neuronal Stress Response as a Potential Contributor to Insulin Resistance

Rats have a reversible decrease in body temperature when exposed to ozone (6,7). This is associated with impaired glucose homeostasis (8) and mobilization of energy sources during stress responses, an observation that might coincide with the adaptive insulin resistance in peripheral tissues (9) that was observed in the new work by Vella et al. (5). A growing body of evidence supports the hypothesis that ozone exposure induces a neuronal response that activates stress-sensitive centers in the nucleus tractus solitarius (10,11). Activation of catecholaminergic neurons in the nucleus tractus solitarius—central to systemic sympathetic stimulation—can mediate an immediate action (fight-or-flight response), activate a hypothalamus-pituitary-adrenal–mediated release of hormones from the adrenal cortex (12), and result in a release of glucose, free fatty acids, and branched-chain amino acids into the circulation (Fig. 1) (13). All of these responses have been implicated in insulin resistance (14).

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Figure 1

Many potential mechanisms might be involved in air pollution–induced insulin resistance and diabetes. While exposure to ozone or other gas and particulate matter components of air pollution upon inhalation can oxidatively modify surfactant lining and cellular components, it can also activate a classic stress response to injury through direct sympathetic stimulation and activation of hypothalamus-pituitary-adrenal axis. A variety of systemic homeostatic metabolic and inflammatory mechanisms might be involved through neurohormone regulation, which rapidly changes the metabolic processes in multiple tissues and the circulating factors. Many of these factors are involved in tissue-selective insulin resistance, dyslipidemia, and the neuronal modulation of homeostatic mechanisms. The question that remains is how these acute changes in processes might upon chronic air pollution exposure contribute to sustained peripheral insulin resistance, dyslipidemia, diabetes, and cardiovascular disease. PM, ambient particulate matter; O₃, ozone.

What Is Changed in the Circulation in Response to Ozone?

The lipid-rich alveolar lining fluid is known to react with ozone and produce oxidation by-products. Vella et al. (5) showed that oxidation of lipids and proteins can occur in the lung, leading to increases in oxidation by-products not only in the lung but also systemically and in muscle. They confirmed this observation by demonstrating the effectiveness of ingested N-acetyl cysteine (NAC) in reducing ozone-induced oxidation of lipids and proteins. We have recently observed that exposure to ozone is associated with marked increases in a variety of free fatty acids, branched-chain amino acids, glucose, cholesterols, and stress and metabolic hormones in the circulation (13). Many of these constituents can remain unaltered in the circulation, be taken up by liver and other peripheral tissues, or have the potential to be oxidized within the circulation and in cells. Thus, in addition to local pulmonary-derived oxidation by-products, the flux of these lipid and protein catabolism by-products in the circulation likely plays a role in acute peripheral insulin resistance (Fig. 1).

Mechanisms of Peripheral Insulin Resistance

Most research on insulin resistance has shown a role for circulating factors and mediators in impairing insulin signaling in different tissues. Many published studies focus on mediators that induce activation of stress kinases. These include c-Jun N-terminal kinases and tissue-specific isoforms of protein kinase C, which affect tyrosine versus serine phosphorylation of insulin receptor substrate-1. Activation of these stress kinases leads to impaired Akt-mediated glucose transporter-4 translocation to the cell membrane and glucose intracellular transport (15). A variety of extracellular and intracellular biological components has been implicated in impairment of insulin signaling. These include free fatty acids (15), branched-chain amino acids (16), steroid and stress hormones (17), cytokines (18), diacylglycerol, ceramides (15), and intracellular free radicals derived from mitochondria (19). Although many studies have supported the contribution of oxidatively modified biological molecules, nonoxidized metabolites have also been implicated in insulin resistance (15,20). The intracellularly generated ceramides from fatty acyl-CoA and sphingosine have contributed to insulin resistance in muscle and liver cells (15). With regard to impairment of insulin signaling by air pollution, a key goal is to identify systemically released metabolic by-products/biological components and to determine their oxidation status.

The most intriguing finding of Vella et al. (5) is that ozone increases lipid and protein oxidation by-products in the bronchoalveolar lavage fluid (BALF) and serum in parallel with increased muscle insulin resistance. The authors show that although the ozone-induced lung injury (protein leak) and inflammatory cell influx were not reduced in rats pretreated with NAC, the levels of thiobarbituric acid–reactive substances and protein carbonyls were decreased in BALF and in the circulation. This suggests that although NAC pretreatment was ineffective in protecting the lung from vascular leakage and inflammation, it reduced oxidation systemically and improved insulin signaling in muscle. Because incubation of myoblast cultures with BALF from ozone-exposed rats stimulated c-Jun N-terminal kinase–mediated signaling and replicated in vivo ozone effect on insulin signaling, Vella et al. (5) believe that these oxidants are involved in muscle insulin resistance. This observation is supported by detection of labeled oxygen in the blood after inhalation of labeled ozone in rats (21). However, more studies are warranted to precisely identify individual lipid oxidation by-products and their sites of generation and clearance and to determine the contribution (and possible chemical modifications) of systemically released fatty acids, amino acids, and hormones in peripheral insulin resistance.

Factors to Consider in Future Studies

The outcome of NAC intervention in the study by Vella et al. (5) is interesting and it raises questions about the role of oxidation by-products and where they may be generated after ozone inhalation. However, high-concentration ozone exposure, especially during the night and in experiments of exceedingly long duration (16 h), which has been shown to cause remarkable lung injury in rats, is not likely to occur in a real-world scenario for humans. Moreover, we are still far from understanding if transient insulin resistance can be exacerbated or if adaptation is likely over extended periods, as noted with other biological end points on repeated chronic episodic ozone exposure (7,8). Considering the magnitude of the health burden of air pollution on chronic neurological and cardiovascular disease, lipidemia, ectopic lipid accumulation, nonalcoholic steatohepatitis, and diabetes in healthy and genetically compromised individuals, more studies like that by Vella et al. (5) are needed to fully understand the impact of this exposure on chronic disease. However, based on observed changes in a myriad of circulating factors (hormones, fatty acids, cholesterol, amino acids, and oxidation by-products of lipids and proteins) and alterations of peripheral metabolic homeostasis, it is quite possible that multiple mechanisms are involved in in vivo ozone-induced insulin resistance.