Phosphorylation of tumor necrosis factor receptor 1 (p55) protects macrophages from silica-induced apoptosis.

Abstract Macrophages play a fundamental role in silicosis in part by removing silica particles and producing inflammatory mediators in response to silica. Tumor necrosis factor α (TNFα) is a prominent mediator in silicosis. Silica induction of apoptosis in macrophages might be mediated by TNFα. However, TNFα also activates signal transduction pathways (NF-κB and AP-1) that rescue cells from apoptosis. Therefore, we studied the TNFα-mediated mechanisms that confer macrophage protection against the pro-apoptotic effects of silica. We will show that exposure to silica induced TNFα production by RAW 264.7 cells, but not by IC-21. Silica-induced activation of NF-κB and AP-1 was only observed in RAW 264.7 macrophages. ERK activation in response to silica exposure was only observed in RAW 264.7 macrophages, whereas activation of p38 phosphorylation was predominantly observed in IC-21 macrophages. No changes in JNK activity were observed in either cell line in response to silica exposure. Silica induced apoptosis in both macrophage cell lines, but the induction of apoptosis was significantly larger in IC-21 cells. Protection against apoptosis in RAW 264.7 cells in response to silica was mediated by enhanced NF-κB activation and ERK-mediated phosphorylation of the p55 TNFα receptor. Inhibition of these two protective mechanisms by specific pharmacological inhibitors or transfection of dominant negative mutants that inhibit IκBα or ERK phosphorylation significantly increased silica-induced apoptosis in RAW 264.7 macrophages. These data suggest that NF-κB activation and ERK-mediated phosphorylation of the p55 TNF receptor are important cell survival mechanisms in the macrophage response to silica exposure.