Secretion of inflammatory cytokines after cellular sensing of the stressor leading to plaque progression

Sarah Søs Poulsen, The National Research Centre for the Working Environment Ulla Vogel, The National Research Centre for the Working Environment Håkan Wallin, Statens Arbeidsmiljøinstitutt Sabina Halappanavar, Health Canada Carole Yauk, Health Canada

Cardiovascular disease (CVD) is the leading cause of death worldwide, being responsible for 31% of all deaths in 2012 (WHO: http://www.who.int). The term CVD covers all diseases of the cardiovascular system, including atherosclerosis, which is manifested as increased plaque deposition or build-up in the arteries. Atherosclerosis is normally asymptotic disease and is initiated by a biological, chemical or physical insult to the artery walls. This leads to the expression of cell adhesion molecules (selectins, VCAM-1 and ICAM-1) on the endothelial lining of the arteries, which facilitates the activation, recruitment, and migration of monocytes through the endothelial monolayer [1;2]. Inside the intima layer, the monocytes differentiate into macrophages and internalize fatty deposits (mainly oxidized low-density lipoprotein). This results in them transforming into foam cells, which is a major component of the atherosclerotic fatty streaks. The fatty streaks reduce the elasticity of the artery walls and the foam cells promote a pro-inflammatory environment by secretion of cytokines and ROS. In addition, foam cells also induce the recruitment of smooth muscle cells to the intima. Added together, these changes lead to the formation of plaques on the artery walls. A fibrous cap of collagen and vascular smooth muscle cells protects the necrotic core and stabilizes the plaque [3;4]. However, blood clots can be formed if the plaque ruptures. These may travel with the bloodstream and obstruct the blood flow of smaller vessels, eg. the coronary arteries, which ultimately can lead to myocardial infarction.

Inhalation of particulate matter, chemicals and pathogens have been related to increased pulmonary inflammation. Whereas a normal immune reaction is crucial for effective elimination of incoming threats, chronic and unresolved inflammation has been linked to both adverse pulmonary and adverse systemic effects in humans. In concordance with this, various retrospective and prospective epidemiological studies have linked pulmonary exposure to respirable air particulates with increased the risk of developing CVD [5-8]. Inhalation of particles has been proposed to affect the cardiovascular system in several different ways, including through disruption of vasomotor function and through acceleration of plaque progression in atherosclerosis [9;10]. We recently showed that a sustained pulmonary inflammatory response occurs concurrently with a persistent acute phase response (APR) in the lungs and in the plasma after exposure to particulate matter in mice [11-13]. Both responses were dose-dependent [14] and the most differentially expressed genes were the serum amyloid A (Saa) isoforms, with Saa3 showing the greatest fold changes [11;13-15]. The SAAs are characterized as APR proteins. Similar to the APR protein C-reactive protein (CRP), elevated plasma levels of SAA protein are a risk factor for CVD in human [16-19]. However, in contrast to CRP, increased plasma protein levels of SAA is still related to CVD after Mendelian randomization, suggesting a causal relationship [20;21]. Indeed, studies in rodents have shown that increased levels of SAA increase plaque progression in ApoE−/− mice [22;23].

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