Shuiying Hu

Date of Graduation


Document Type



Exposure to silica dust induces lung inflammation and a progressive development of pulmonary fibrosis. The objective of the current study is to provide in vitro and in vivo correlations of a mechanism through which silica induces apoptosis in Alveolar Macrophage (AM), and to examine the potential relationship between silica-induced apoptosis and cellular production of proinflammatory cytokines. For in vivo studies, AM from male Sprague-Dawley rats exposed to saline or 20 mg/rat silica by intratracheal instillation were isolated and characterized for cytokine production and apoptosis. For in vitro studies, AM from normal rats were isolated and characterized for cytokine production and apoptosis in response to silica exposure (100 μg/106 cells). Silica exposure was found to induce macrophage apoptosis in vivo through a mechanism that includes decreased mitochondrial membrane potential, mitochondrial release of cytochrome c, activation of caspase 9 and PARP degradation. There was a sustained presence of apoptotic AM in silica exposed rats even at two weeks post-exposure. Silica exposure also led to cellular production of reactive oxygen species (ROS), most notably superoxide anion, and a depletion of intracellular GSH. The observed in vivo mechanism of silica-induced apoptosis was fully confirmed through in vitro studies using N-acetyl-L-cysteine (NAC) and rhodamine (R6G) as probes for ROS generation by the mitochondria. The results indicate that silica-induced apoptosis is through direct action of silica to induce mitochondrial generation of ROS, which may be inhibited by the presence of high levels of mitochondrial GSH. The secretion of IL-1β and TNF-α in response to silica exposure was markedly inhibited by NAC and R6G, suggesting that the production of these cytokines is also ROS-dependent. Inhibition of cytokine production by taxol or tetrandrine did not alter silica-induced apoptosis, thus suggesting an absence of a cytokine-mediated apoptotic process. Acute silica exposure, either in vivo or in vitro, did not alter AM production of TGF-β1. In summary, the present study shows that silica induces apoptosis to AM as an acute toxicity that adds to the inflammatory responses, and that the mechanism of apoptosis is through the mitochondrial pathway initiated by ROS generation in the mitochondria.