In Vitro and In Vivo Toxicity Mechanisms of ZnO and TiO2 Nanoparticles

Abstract

With the growth of nanotechnology, there has been a tremendous development in the diverse applications of nanomaterials such as antibacterial materials, drug delivery systems, electronics, cosmetics, etc. Nanomaterials have dramatically different physicochemical properties compared to fine particles of the similar composition. The smaller size of nanomaterials ensures that a large portion of atoms will be on the particle surface. Since surface properties, such as electronic structure, energy level and reactivity are quite different from interior states, the bioactivity of nanoparticles may likely differ from that of the fine size analogue. The nanomaterials is categorized as polymer based, ceramics, oxide or metal materials, carbon-based and silicon. However, the toxicity studies of nanomaterials have not fully elucidated. In this research, we studied the toxicity mechanisms of ZnO and TiO2 nanoparticles. In Part I, zinc oxide (ZnO) nanoparticles, which have been used in an increasing number of industrial products such as paint, coating and cosmetics, and in other biological applications, were chosen for toxicity mechanism. There have been many suggestions of ZnO nanoparticles toxicity paradigm but the underlying molecular mechanisms of the toxicity of ZnO nanoparticles remain unclear. This study was done to determine the potential toxicity of ZnO nanoparticles and to assess the toxicity mechanism in normal skin cells. Synthesized ZnO nanoparticles generated reactive oxygen species (ROS), as determined by electron spin resonance. After uptake into cells, ZnO nanoparticles induced ROS in a concentration- and time-dependent manner. To demonstrate ZnO nanoparticles toxicity mechanism related to ROS, we detected abnormal autophagic vacuoles accumulation and mitochondria dysfunction after ZnO nanoparticles treatment. Furthermore mitochondria membrane potential and adenosine triphosphate (ATP) production were decreased by the treatment of ZnO nanoparticles. Our results suggested that ZnO nanoparticles led to cell death through autophagic vacuole accumulation and mitochondria damage in normal skin cells via ROS induction. Nanomaterials are also used in diverse fields including food, cosmetic, and medical industries. Titanium dioxide (TiO2) nanoparticles are widely used, but their effects on biological systems and mechanism of toxicity have not been elucidated fully. In Part II, we report the toxicological mechanism of TiO2 nanoparticles in cell organelles. Human bronchial epithelial cells (16HBE14o-) were exposed to 50 and 100 μg/mL TiO2 nanoparticles for 24 and 48 h. Our results showed that TiO2 nanoparticles induced endoplasmic reticulum (ER) stress in the cells and disrupted the mitochondria-associated endoplasmic reticulum membranes (MAMs) and calcium ion balance, thereby increasing autophagy. In contrast, an inhibitor of ER stress, tauroursodeoxycholic acid (TUDCA), mitigated the cellular toxic response, suggesting that TiO2 nanoparticles promoted toxicity via ER stress. This novel mechanism of TiO2 nanoparticles toxicity in human bronchial epithelial cells suggests that further exhaustive research on the harmful effects of these nanoparticles in relevant organisms is needed for their safe application. In the workplace, the primary exposure route for TiO2 nanoparticles is inhalation through the respiratory system. Because TiO2 nanoparticles have different physiological properties, in terms of size and bioactivity, their toxic effects in the respiratory system must be determined. In Part III, to determine the toxic effect of inhaled TiO2 nanoparticles in the lung and the underlying mechanism, we used a whole-body chamber inhalation system to expose A/J mice to TiO2 nanoparticles for 28 days. During the experiments, the inhaled TiO2 nanoparticles were characterized using a cascade impactor and transmission electron microscopy. After inhalation of the TiO2 nanoparticles, hyperplasia and inflammation were observed in a TiO2 nanoparticles treated murine lung. To determine the biological mechanism of the toxic response in the lung, we examined endoplasmic reticulum (ER) and mitochondria in lung tissues. The ER and mitochondria were disrupted and dysfunctional in the TiO2 nanoparticles exposed lung leading to abnormal autophagy. In summary, we assessed the potential risk of TiO2 nanoparticles in the respiratory system, which contributed to our understanding of the mechanism underlining TiO2 nanoparticles toxicity in the lung.