Lipopolysaccharide Inhibits TNF-Induced Apoptosis: Role of Nuclear Factor-κB Activation and Reactive Oxygen Intermediates

Abstract

AbstractLPS, a component of the cell wall in Gram-negative bacteria, induces inflammation and septic shock syndrome by stimulating various inflammatory cytokines including TNF. How LPS affects the TNF-mediated cellular responses, however, is not understood. In this study, the effect of LPS on TNF-mediated apoptosis in human histiocytic lymphoma U-937 cells was investigated. We found that treatment of cells with LPS completely abolished TNF-mediated cytotoxicity and activation of caspase-3. LPS-chelating antibiotic, polymyxin B, suppressed the antiapoptotic activity, indicating the specificity of the effect. Within minutes, LPS through CD14 induced the activation of NF-κB, degradation of IκBα (inhibitory subunit of NF-κB) and IκBβ, and nuclear translocation of p65. An antioxidant, pyrrolidine dithiocarbamate, which blocked LPS-induced NF-κB activation, also abolished the antiapoptotic effects of LPS at the same time. Besides TNF, the apoptosis induced by taxol and okadaic acid was also sensitive to LPS-induced NF-κB activation, whereas that induced by H2O2, doxorubicin, daunomycin, vincristine, and vinblastine was NF-κB insensitive. Tumor cells that constitutively expressed NF-κB also showed resistance to the apoptotic effects of TNF, taxol, and okadaic acid, but sensitivity to all other agents, indicating the critical role of NF-κB in blocking apoptosis induced by certain agents. Overall, these results indicate that LPS induces resistance to the apoptotic effects of TNF and other agents, and that NF-κB activation, whether induced or constitutive, inhibits this apoptosis.