Numerous studies have shown that short-term exposure to particulate air pollution is associated with increased morbidity and mortality, primarily from cardiovascular disease (CVD) (Katsouyanni et al., 2001; Kinney and Ozkaynak, 1991; Schwartz and Dockery, 1992). The common and potentially toxic metal constituents of particulate matter (PM) air pollution have been associated with CVD in experimental and epidemiological studies (Gerhardsson et al., 1995; Lustberg and Silbergeld, 2002; Schafer et al., 2005; Schwartz, 1995). Foundry work has also been associated with CVD (IARC, 1987). In spite of the incorporation of state of-the-art methods to reduce noxious exposures in modern foundry facilities, workers are still exposed to high levels of airborne metal-rich PM (Tarantini et al., 2009).
The mechanisms by which PM exposure leads to CVD have not yet been fully elucidated. Inhaled fine PM may enter the circulatory system through the pulmonary capillary bed and promote atherothrombosis by breaching endothelial integrity, thereby inciting a local inflammatory reaction (Nemmar et al., 2001). However, whether fine particles physically enter and deposit in blood vessels is still contentious (Tamagawa et al., 2008). There is evidence that a small fraction of fine and ultrafine particles accumulate in extrapulmonary organs, such as the liver and spleen (Mills et al., 2006). Alternatively, it has been proposed that ambient particles trigger pulmonary oxidative stress and inflammatory responses that lead to the release of proinflammatory signals into the circulatory system. Previous research has mainly focused on general inflammatory mediators and acute phase reactants, such as C-reactive protein, interleukin-6 and fibrinogen, rather than on mediators specifically induced by PM (Al-Nedawi et al., 2009). Moreover, C-reactive protein and fibrinogen are not produced in the lungs and, therefore, do not represent a direct link between lung inflammatory responses and CVD.
Microvesicles (MVs) are circular fragments of the plasma membrane actively released from human cells. MV release may be induced by soluble agonists or in response to physical or chemical stress, such as oxidative stress (Ratajczak et al., 2006). Growing evidence indicate that MVs represent a novel means for intercellular and between-tissue communication (Al-Nedawi et al., 2009). MVs can travel from the tissue of origin to target cells, to which MVs transfer their contents by being internalized (Hunter et al., 2008). MiRNAs are small, endogenous, single-stranded noncoding RNAs of 20-22 nucleotides (Mordukhovich et al., 2009) that regulate gene expression post-transcriptionally by either triggering mRNA cleavage or by repressing translation (He and Hannon, 2004). Recent findings indicate that microRNAs (miRNAs) contained in MVs may determine reprogramming of gene expression in target cells and have therefore been indicated as potential determinants of intercellular and interorgan communication (Hunter et al., 2008).
Bioinformatics models estimate that approximately one-third of the several thousand human genes may be regulated by miRNAs (Griffiths-Jones et al., 2008). A single miRNA can regulate hundreds of target mRNAs in interrelated genetic pathways, and a single mRNA can be targeted by several different miRNAs (Lewis et al., 2005). MiRNA expression in circulating MVs has been detected in the plasma of healthy individuals, and growing evidence suggests that peripheral blood miRNA levels may play a role in predicting human diseases, such as cancer (Hunter et al., 2008). In addition, alterations in the expression of several miRNAs have been found during oxidative stress (Babar et al., 2008) and inflammation (Xiao and Rajewsky, 2009). We recently showed modified expression of miRNAs involved in oxidative stress and inflammation in circulating blood leukocytes from electric-steel plant facility workers who were exposed to air particles rich in lead and cadmium (Bollati et al., 2010). Other in vitro and in vivo studies have also shown effects on the expression of intracellular miRNAs from PM (Bleck et al., 2013; Fossati et al., 2014) or other exposures that include PM, such as diesel exhaust (Yamamoto et al., 2013) or tobacco smoking (De Flora et al., 2012; Maccani et al., 2010).
Although the mechanisms linking air pollution and pulmonary inflammatory responses to vascular diseases are currently unknown, MVs are a plausible link because they can be produced by the respiratory system (Kesimer et al., 2009), can be disseminated through the circulatory system (Orozco and Lewis, 2010) and can lead to cardiovascular dysfunction (Puddu et al., 2010). We investigated the hypothesis that PM and PM-associated metals could modify specific MV-associated miRNAs from plasma. To our knowledge, this is the first study to address this question. We measured the expression levels of 88 miRNAs contained within plasma MVs from 55 steel plant workers taken at the beginning of a working week (baseline) and after 3 days of work (postexposure). To evaluate a possible source of MV miRNAs, we also assessed the expression of miRNAs in MVs from A549 pulmonary cells treated with PM.