Heat shock inhibits radiation-induced activation of NF-kappa B via inhibition of I-kappa B kinase

Radiation stimulates signaling cascades that result in the activation of se veral transcription factors that are believed to play a central role in pro tective response(s) to ionizing radiation (IR). It is also well established that heat shock alters the regulation of signaling cascades and transcript ion factors and is a potent radiosensitizing agent. To explore the hypothes is that heat disrupts or alters the regulation of signaling factors activat ed by IR, the effect of heat shock on IR-induced activation of NF-kappa B w as determined. Irradiated HeLa cells demonstrated transient increases in NF -kappa B DNA binding activity and NF-kappa B protein nuclear localization. In addition, irradiated cells demonstrated increased I-kappa B phosphorylat ion and decreased I-kappa B alpha cytoplasmic protein levels, corresponding temporally with the increase of NF-kappa B DNA binding. Heat shock prior t o IR inhibited the increase in NF-kappa B DNA binding activity, nuclear loc alization of NF-kappa B, and the phosphorylation and subsequent degradation of I-kappa B. I-kappa B kinase (IKK) immunoprecipitation assays demonstrat ed an increase in IKK catalytic activity in response to IR that was inhibit ed by pretreatment with heat. Kinetic experiments determined that heat-indu ced inhibition of NF-kappa B activation in response to LR decayed within 5 h after heating. Furthermore, pretreatment with cycloheximide, to block de novo protein synthesis, did not alter heat shock inhibition of IR induction of NF-kappa B, These experiments demonstrate that heat shock transiently i nhibits IR induction of NF-kappa B DNA binding activity by preventing IKK a ctivation and suggests a mechanism independent of protein synthesis.