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Lung Fibrotic Responses to Particle ExposureRespiratory Biology Program, Division of Biological Sciences, CIIT Centers for Health Research, Research Triangle Park, NC 27709, USA Correspondence: Address correspondence to: James C. Bonner, CIIT Centers for Health Research, P.O. Box 12137, Research Triangle Park, NC 27709, USA; e-mail:jbonner{at}ciit.org
Particles generated from numerous anthropogenic sources have the potential to cause or exacerbate lung diseases, including asthma, bronchitis, and COPD. Fibrotic reactions are a component of all of these pulmonary diseases, and involve the progressive deposition of collagen by pulmonary fibroblasts. The reactivity, toxicity, and fibrogenic potential of particles in the lung depends on a variety of factors including particle size, surface area, and composition. Smaller particles, particularly in the nanosized range, have more toxic and fibrogenic capacity due to a higher surface-to-mass ratio and greater oxidant-generating potential. Composition is also an important determinant in the fibrotic response to particles. Transition metals, bacterial lipopolysaccaride, and polycyclic aromatic hydrocarbons are some of the toxic components of particles that activate intracellular signaling pathways that culminate in the production of profibrotic cytokines and growth factors.
Key Words: Metals endotoxin oxidants cytokines growth factors
Not only can the exposure of the lungs to inhaled particles have numerous pathologic effects on the respiratory system, it can also affect other organ systems. For example, air pollution particle exposure is associated with increased morbidity and mortality due to impact on the respiratory and cardiovascular systems (Schwartz, 1993). Metal particles such as manganese are deposited within the nasal mucosa and rapidly transported to the brain where they exert neurotoxicity (Erikson et al., 2005). In the lung, particles mediate their toxic effects by exacerbating preexisting respiratory diseases, including asthma, bronchitis, and COPD (Pope et al., 1995; Anderson et al., 1998). All of these diseases involve considerable lung remodeling, including fibrotic reactions that are defined by increased numbers of fibroblasts and deposition of collagen by these cells. In general, particle size and composition are critical determinants of reactivity, toxicity, and fibrogenic potential of inhaled particles in the lung.
Particle Size
Particle Composition
Particle-Induced Oxidative Stress Oxidative injury is thought to play a major role in lung fibrogenesis. A variety of organic and inorganic constituents of particles have the potential to cause ROS generation, either directly via redox chemistry or through stimulating pulmonary cells to increase ROS production. Inhaled particles that deposit within the distal lung have the potential to generate ROS, depending on the surface area of the particle and the reactive composition of the particle. Many transition metals present on particles serve as catalysts for ROS production. ROS are produced by Fenton-like reactions, or by stimulating an oxidative burst in cells that have taken up metal-containing particles through phagocytosis. For example, residual oil fly ash particles rich in vanadium induce oxidant generation in alveolar macrophages and epithelial cells (Kadiiska et al., 1997; Goldsmith et al., 1998; Hiura et al., 1999). Diesel exhaust particles (DEP) and lipopolysaccharide (LPS) from gram negative bacteria have also been shown to stimulate generation of ROS in alveolar macrophages. Phagocytosis of DEP by macrophages induces rapid oxidant generation leading to activation of apoptotic pathways (Hiura et al., 1999). Additionally, semi-quinone radicals derived from combustion of DEP mediate redox cycling and sustained production of ROS (Squadrito et al., 2001). Extraction of PAHs and quinones from DEP reduces ROS generation and apoptosis, indicating that organic constituents of DEP mediate these effects. ROS generated by particle exposure serve as signaling intermediates to activate intracellular signaling targets, including receptor tyrosine kinases, mitogen-activated protein (MAP) kinases, and transcription factors. These signaling intermediates drive transcriptional activation and the expression of genes involved in inflammation and fibrosis. Some of the signaling molecules targeted by particle-derived ROS are illustrated in Figure 1. Increased generation of reactive oxygen species, such as superoxide anion (O.2–) and hydrogen peroxide (H2O2), has been associated with inflammation following tissue injury (Rosen et al., 1995). In addition to ROS, reactive nitrogen species (RNS) are also generated by particle exposure in the lung. Nitric oxide (NO.) released by inflammatory cells reacts with O.2– to form peroxynitrite (ONOO–) (Pryor et al., 1995). ONOO– causes nitration of tyrosine residues on proteins and thereby modifies protein function. Nitrated proteins have been identified in lung tissue following exposure to particles and fibers (Rosen et al., 1995; Zhu et al., 1998). Therefore, ONOO– generation and subsequent tyrosine nitration leading to protein dysfunction appear to contribute to disease progression following particle-induced lung injury. Oxidants serve as signaling intermediates required for receptor tyrosine kinase function and downstream activation of mitogen-activated protein (MAP) kinases. Low levels of oxidants (<10 µM) are essential mediators of normal cell physiologic function, including proliferation, migration and differentiation (Sundaresan et al., 1995; Bae et al., 1997). Quiescent lung fibroblasts in cell culture systems generate micromolar levels of H2O2 that likely maintain their proliferative potential when stimulated with growth factors (Wang et al., 2003). This endogenous pool of H2O2 could also react with constituents of particles to affect cytotoxic responses of cells. Particle-associated metals such as vanadium react with cell-derived H2O2 to form the protein tyrosine phosphatase (PTP) inhibitor, peroxovanadate. PTP inhibition by peroxovanadate is irreversible and results in prolonged MAP kinase activation and cellular stress (Ingram et al., 2003). Therefore, vanadium causes prolonged phosphorylation of intracellular signaling molecules, resulting in cellular stress, altered gene expression patterns, and apoptosis.
Particle-Induced Intracellular Signaling
Transcription factors are activated by particles through ROS, metals, or LPS. Vanadium is a strong activator of NF- Elucidating the intracellular signaling pathways that mediate the various pathologic outcomes associated with particle exposure is complex due to the diversity of constituents associated with particles. Moreover, little is known about the interactive effects of these constituents in stimulating biologic responses. Further research is needed to better understand how mixtures of metals and/or organic consitituents (LPS, PAH) interact to stimulate cell signaling and the production of inflammatory mediators.
Particle-Induced Expression of Pro-Fibrotic Cytokines and Growth Factors
Particle-Induced Exacerbation of Asthma and Airway Fibrosis
Epithelial-Mesenchymal Cell Interactions in Particle-Induced Bronchitis The epithelial-mesenchymal cell trophic unit does appear to be important in V2O5-induced lung injury and the subsequent development of airway fibrosis. Exposure of rats to V2O5 causes airway remodeling similar to that seen in humans with chronic bronchitis arising from occupational exposure to V2O5-rich particles (Bonner et al., 2000; Woodin et al., 2000). The pathology of V2O5-induced lesions in rats includes airway fibrosis, smooth muscle thickening, and mucous cell metaplasia (Figure 2). Experiments with human bronchial epithelial cells exposed to V2O5-induced oxidative stress in vitro show that these cells release heparin-binding epidermal growth factor (HB-EGF), a potent mitogen for human lung fibroblasts (Zhang et al., 2001). HB-EGF binds to the EGFR on fibroblasts and epithelial cells, thereby promoting proliferation of fibroblasts and differentiation of epithelial cells, respectively. Moreover, pharmacologic inhibition of the EGFR reduces V2O5-induced fibrotic responses in rats (Rice et al., 1999). Therefore, HB-EGF appears to be important to airway fibrogenesis and remodeling after exposure to particles rich vanadium. In general, the combined use of in vivo rodent models of V2O5-induced airway remodeling and in vitro cell culture systems with epithelial cells and fibroblast have proven useful in dissecting the cellular and molecular mechanisms that mediate V2O5-induced occupational bronchitis.
The potential of particles to cause fibrotic reactions within the lung depend on the both particle size and composition. Particles containing certain transition metals, such as vanadium and zinc, have a greater capacity to generate ROS and cause tissue injury. Decreasing particle size is also a critical determinant of toxicity due to the greater surface area of an equal mass of smaller particles. ROS generated by particle exposure activates intracellular signaling proteins, including MAP kinases and redox-sensitive transcription factors. These activated molecules play key roles in cell survival and the production of growth factors that drive fibrotic reactions within the lung following exposure to toxic particles.
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B and STAT-1 (
. While these cytokines do not directly promote fibroblast growth or the deposition of extracellular matrix proteins, they increase the expression of pro-fibrotic growth factors and their receptors. For example, TNF-

