Temporal and Mechanistic Effects of Heat Shock on LPS-Mediated Degradation of IκBα in Macrophages

Previous studies demonstrated important interactions between the heat shock response and the IκBα/NF-κB pathway when these two pathways are induced sequentially. One such interaction involves the ability of heat shock to inhibit subsequent degradation of IκBα in response to a proinflammatory signal. Herein we investigated the temporal relationship between recovery from heat shock and inhibition of IκBα degradation, and the proximal mechanisms by which heat shock inhibits degradation of IκBα in macrophages. In RAW 264.7 murine macrophages, prior heat shock inhibited LPS-mediated IκBα degradation up to 4 h after recovery from heat shock, and this effect correlated with inhibition of LPS-mediated activation of NF-κB. Beyond these recovery periods, heat shock did not inhibit IκBα degradation. IκB kinase (IKK) assays demonstrated that heat shock inhibited LPS-mediated activation of IKK up to 1 h after recovery from heat shock. Heat shock also increased intracellular phosphatase activity, and inhibition of intracellular phosphatase activity partially reversed the ability of heat shock to inhibit both LPS-mediated degradation of IκBα and LPS-mediated activation of IKK. These data demonstrate that the ability of heat shock to inhibit degradation of IκBα is dependent on the recovery period between the heat shock stimulus and the proinflammatory stimulus. The mechanism by which heat shock inhibits degradation of IκBα involves dual modulation of IKK and intracellular phosphatase activity.