High levels of catalase and glutathione peroxidase activity dampen H2O2 signaling in human alveolar macrophages

Results are presented which support the hypothesis that adequate steady-state levels of hydrogen peroxide (H2O2) are required to overcome the effects of high catalase and glutathione peroxidase (GPx) expression for p38 mitogen-activated protein (MAP) kinase activation and tumor necrosis factor (TNF)- gene expression in human alveolar macrophages stimulated with asbestos. We found significant differences in the types and amounts of reactive oxygen species generated in human blood monocytes compared with human alveolar macrophages. This difference in reactive oxygen species production is related, in part, to the differences in antioxidant enzyme expression and activity. Most importantly, catalase and GPx activities were significantly increased in alveolar macrophages compared with blood monocytes. Asbestos activated the p38 MAP kinase and induced TNF- gene expression only in blood monocytes. Increasing the steady-state levels of H2O2 by using polyethylene glycol superoxide dismutase, an antioxidant that crosses the cell membrane, or aminotriazole, an irreversible inhibitor of catalase, allowed the p38 MAP kinase to be activated in alveolar macrophages. In addition, asbestos-stimulated macrophages cultured with polyethylene glycol superoxide dismutase had a significant increase in gene expression mediated by the TNF- promoter. These results demonstrate that high catalase and GPx activity in human alveolar macrophages limits the effectiveness of H2O2 to act as a mediator of inflammatory gene expression. Macrophages have an important role in initiating inflammatory responses by secreting proinflammatory cytokines. One important characteristic of the pathogenesis of asbestosis and other chronic lung diseases is an inflammatory response generated by a mixture of mature and immature macrophages. This is thought to result from the recruitment of monocytes to sites of disease. Some of these cells spontaneously release cytokines, which accentuate the inflammatory response. The significance of these observations has been illustrated in animal studies showing that inhibition of cytokines, such as tumor necrosis factor (TNF)-, prevented the development of pulmonary fibrosis (1‐3). These disorders are also associated with the generation of reactive oxygen species (ROS), which are involved in the inflammatory response by regulating signaling and can cause tissue injury when