# Cellular Effects of Silver Nanoparticle Suspensions on Lung Epithelial Cells and Macrophages
## 1 Introduction  
Nanoparticles are characterized by a diameter of 1-100 nm [1]. Many types of metal and metal oxide nanoparticles are currently being produced. Among these, silver nanoparticles (AgNPs) are particularly important because they are utilized not only for industrial applications such as catalysis, but also for familiar materials used in everyday life. AgNPs exhibit antimicrobial activity [2], and therefore they are widely used in various sanitary materials, such as antimicrobial textiles and deodorants [3; 4; 5]. In addition, AgNPs are utilized for wound-healing purposes as they inhibit microbial growth [6]. Compared with other metal or metal oxide nanoparticles, AgNPs are characterized by their antimicrobial activity. The mechanisms underlying the antimicrobial activity of AgNPs have been suggested: (1) the nanoparticles adhere to and accumulate on the cell wall, changing the cell structure or causing the formation of pits, thereby leading to cell death; (2) the nanoparticles generate reactive oxygen species (ROS); (3) the nanoparticles release silver ions, which exert bactericidal activity [2; 7; 8; 9]. The released silver ions bind to thiol protein groups and disrupt bacterial metabolism by inhibiting enzyme activity [10]. Furthermore, AgNPs damage nucleic acids directly or through ROS generation [9]. These antimicrobial properties of AgNPs also indicate their potential cytotoxic activity against mammalian cells. The cytotoxicity of AgNPs to mammalian cells has been reported [11]. For example, exposure to AgNPs leads to cell membrane damage and apoptosis in A549 human lung carcinoma-derived cells [12]. Moreover, AgNPs induce oxidative stress and DNA injury in normal human bronchial epithelial (BEAS-2B) cells [13]. Bioengineeringered AgNPs showed not only antimicrobial activity, but also cellular effects, such as a decrease of cell viabilityand cellular oxidative stress induction in A549 cells [14]. Another biologically synthesized AgNP caused a decrease of cell viability [15]. In many cases, these cytotoxic activities against mammalian cells share similar mechanisms and common molecular events with antimicrobial activities. In addition to these antimicrobial mechanisms, cellular particle uptake is an important factor in the cytotoxicity of AgNPs to mammalian cells. Several metal oxide nanoparticles exhibit stronger cytotoxicity than their water-soluble compounds owing to the cellular uptake and subsequent intracellular release of metal ions [16; 17; 18]. For example, the cytotoxicity of nickel oxide nanoparticles is more potent than that of nickel chloride [16]. Moreover, ZnO and CuO nanoparticles release Zn\({}^{2+}\) and Cu\({}^{2+}\) extracellularly and intracellularly, and these metal ions subsequently cause cytotoxicity [19; 20]. Phagocytes, such as macrophages and neutrophils, have a higher phagocytic capacity than epithelial cells. Therefore, a higher amount of AgNPs may be taken up by phagocytes. When AgNPs of different sizes were applied to macrophage-differentiated human monocyte U937 cells, smaller particles induced a more pronounced decrease in cell viability. In addition, AgNPs enhanced interleukin (IL)-8 secretion. Exposure to AgNPs 4 nm in diameter upregulated intracellular ROS levels. As the induction of IL-8 and intracellular ROS levels was suppressed by N-acetylcysteine treatment, oxidative stress may be involved in the upregulation of IL-8 secretion by AgNPs [21]. Moreover, AgNPs have been reported to increase IL-8 secretion in macrophage-like differentiated THP-1 (dTHP-1) cells [22]. In many cases, exposure to these metal-based nanoparticles increases intracellular ROS levels, activates cellular antioxidant systems such as HO-1, and induces chemokine expression, such as IL-8 [23]. Furthermore, metal-based nanoparticle exposure results in mitochondrial dysfunction, cell membrane damage, and subsequent cell death. In some cases, apoptotic cell death was observed following exposure to metal-base nanoparticles, including AgNPs [24; 25; 26; 27; 28; 29].