SILICA-INDUCED NUCLEAR FACTOR- k B ACTIVATION: INVOLVEMENT OF REACTIVE OXYGEN SPECIES AND PROTEIN TYROSINE KINASE ACTIVATION

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Abstract
Nuclear factor-kB (NF-kB) is a multiprotein complex that may regulate a variety of inflammatory cytokines involved in the initiation and progression of silicosis. The present study documents the ability of in vitro silica exposure to induce DNA-binding activity of NF-kB in a mouse peritoneal macrophage cell line (RAW264.7 cells) and investigates the role of reactive oxygen species (ROS) and/or protein tyrosine kinase in this activation. In vitro exposure of mouse macrophages to silica (100 µg/ml) resulted in a twofold increase in ROS production, measured as the generation of chemiluminescence (CL), and caused activation of NF-kB. Silica-induced CL was inhibited 100% by superoxide dismutase (SOD) and 75% by catalase, while NF-kB activation was inhibited by a variety of antioxidants (catalase, superoxide dismutase, alpha-tocopherol, pyrrolidine dithiocarbamate, or N-acetylcysteine). Further evidence for the involvement of ROS in NF-kB activation is that 1 mM H2O2 enhanced NF-kB/DNA binding and that this activation was inhibited by catalase. Specific inhibitors of protein tyrosine kinase, such as herbimycin A, genistein, and AG-494, prevented NF-kB activation in silica-treated cells. Genistein and AG-494 also reduced NF-kB activation in H2O2-treated cells. Results con firm that tyrosine phosphorylation of several cellular proteins (approximate molecular mass of 39, 58?70, and 103 kD) was increased in silica-exposed macrophages and that genistein inhibited this silica-induced phosphorylation. In contrast, inhibitors of protein kinase A or C, such as H89, staurosporin, calphostin C, and H7, had no marked inhibitory effect on silica-induced NF-kB activation. The results suggest that ROS may play a role in silica-induced NF-kB activation in macrophages and that phosphorylation events mediated by tyrosine kinase may be involved in this activation.

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