Mechanism and function of heat shock-dependent IκBα expression

Objective: Heat shock is known to inhibit activation of the NF-kappa B pathway. One potential mechanism of this effect is de novo expression of the intracellular NF-kappa B inhibitor, I kappa B alpha. Herein we sought to elucidate the mechanisms by which heat shock induces I kappa B alpha gene expression and the functional consequences of heat shock-mediated I kappa B alpha gene expression in A549 cells. Methods: Nuclear run-on assays demonstrated that heat shock had a small effect on transcription of the I kappa B alpha gene relative to the level of steady state I kappa B alpha mRNA that is seen following heat shock. Accordingly, we determined the effect of heat shock on I kappa B alpha mRNA stability by treating cells with actinomycin D to induce transcriptional arrest. Results: The half-life of I kappa B alpha mRNA was 36 +/- 7.2min in control cells and 101 +/- 3.7min in cells subjected to heat shock. These data were consistent with heat shock-mediated increased stability of I kappa B alpha mRNA. Heat shock induced activation of p38 MAP kinase and inhibition of p38 MAP kinase substantially reduced heat shock-dependent expression of I kappa B alpha mRNA. After a 4h recovery period from heat shock, there was inhibition of tumor necrosis factor-a-mediated NF-kappa B activation. The introduction of an I kappa B alpha anti-sense oligonucleotide reversed this inhibitory effect of heat shock. Conclusions: We conclude that heat shock increases I kappa B alpha gene expression primarily by increasing I kappa B alpha mRNA stability and this effect is partially dependent on p38 MAP kinase. The functional consequence of heat shock-mediated.