Classical and alternative macrophage activation in the lung following ozone-induced oxidative stress
Highlights
► Lung macrophages are highly sensitive to ozone induced oxidative stress. ► Ozone induces autophagy and apoptosis in lung macrophages. ► Proinflammatory and wound repair macrophages are activated early after ozone. ► Oxidative stress may contribute to regulating macrophage phenotype and function.
Introduction
Ozone is a highly reactive oxidant that induces lung injury and impairs pulmonary mechanics (Uysal and Schapira, 2003). Toxicity is initiated by ozone-induced peroxidation of polyunsaturated fatty acids in membrane lipids and in lung lining fluid, resulting in the generation of reactive oxygen species, and a mixture of lipid ozonation products including lipoperoxyl radicals, hydroperoxides, malonydialdehyde, isoprostanes and alkenals such as 4-hydroxy-nonenal (Kafoury et al., 1999, Mustafa, 1990, Pryor et al., 1996, Rahman et al., 2002). These reactive products cause oxidative stress in the lung. This leads to damage to the respiratory epithelium, disruption of alveolar epithelial barrier function, edema and inflammation (Al-Hegelan et al., 2011, Hollingsworth et al., 2007).
Evidence suggests that inflammatory macrophages accumulating in the lung in response to ozone-induced injury contribute to oxidative stress and pulmonary toxicity (reviewed in Hollingsworth et al., 2007 and Laskin et al., 2011). Thus, following exposure to products released from ozone-injured epithelial cells, lung macrophages are classically activated to release cytotoxic/proinflammatory mediators including reactive oxygen and nitrogen species, and tumor necrosis factor-alpha (TNFα) which promote tissue injury. This is supported by findings that blocking macrophages or the cytotoxic mediators which they release protects against ozone-induced lung injury (Cho et al., 2001, Fakhrzadeh et al., 2002, Fakhrzadeh et al., 2004a, Fakhrzadeh et al., 2004b, Pendino et al., 1995, Toward and Broadley, 2002). Accumulating data indicate that macrophages also play a protective role following ozone-induced lung injury, clearing oxidized products and cellular debris (Dahl et al., 2007, Ishii et al., 1998). They also augment lung antioxidant activity and release mediators that suppress inflammation and initiate wound repair (Backus et al., 2010, Dahl et al., 2007, Reinhart et al., 1999). It appears that these activities are mediated by a distinct subpopulation of macrophages that is alternatively activated (Byers and Holtzman, 2011, Laskin et al., 2011). The present studies demonstrate that oxidative stress caused by inhalation of ozone leads to activation of both cytotoxic/proinflammatory and anti-inflammatory/wound repair macrophages in the lung. Moreover, their appearance overlaps suggesting that processes of tissue injury and repair are initiated early in the pathogenic response to ozone.
Section snippets
Animals and exposure
Female specific pathogen-free Wistar rats (200–225 g) were obtained from Harlan Laboratories (IN). Animals were housed in filter top microisolation cages and maintained on food and water ad libitum. All animals received humane care in compliance with the institution's guidelines, as outlined in the Guide for the Care and Use of Laboratory Animals, published by the National Institutes of Health. Animals were exposed in groups of 4 to ultra-pure air (Messer Gas, Allentown, PA) or 2 ppm ozone for 3 h
Results
Ozone is a potent pulmonary irritant known to cause oxidative stress (Bocci, 2006, Ciencewicki et al., 2008, Yang and Omaye, 2009). Consistent with this activity, we observed a time-related increase in the DNA oxidation product, 8-OHdG in alveolar macrophages, as well as epithelial cells, which was most notable 24 h after ozone exposure (Fig. 1). Subsequently 8-OHdG levels declined. The antioxidant HO‐1 also increased in alveolar macrophages following ozone exposure, however this response was
Discussion
Ozone is a highly reactive molecule that directly or indirectly through the generation of free radicals, oxidizes proteins, lipids and DNA. The resulting oxidation products, together with reactive species produced by inflammatory cells, play a key role in ozone-induced tissue injury (Kafoury et al., 1999, Mustafa, 1990, Pryor et al., 1996, Rahman et al., 2002). The present studies demonstrate that lung macrophages are highly sensitive to ozone-induced oxidative stress, developing into
Conflict of interest statement
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was supported by NIH grants R01ES004738, R01GM034310, R01CA132624, U54AR055073 and P30ES05022.
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