Heat Shock Response Regulates IL-18 Gene Expression In Murine Macrophages

Heat shock response is a conserved stress response. Exposure of cells to physicochemical stimuli leads to the heat shock response, through activation of heat shock factors and induction of heat shock proteins, it has been implicated in cytoprotective effects from cellular damage. Recent studies have revealed that it had anti-inflammatory effects and involved in regulating the expression of cytokine. Macrophages play a critical role in spanning innate immunity and adaptive immunity. After the stimulation of LPS, it could produce a series cytokine. Among these cytokines secreted by macrophages, IL-18 is a proinflammatory cytokine that has diverse immune regulatory effects on T cells, B cells, NK cells and nonimmune cells. In here, we studied the effect of heat shock on LPS-induced IL-18 gene expression. Our results show that the augmentation of LPS-induced IL-18 mRNA and protein was significantly suppressed in murine macrophages after 43℃ heat shock treatment. The novel observation on down-regulation of IL-18 by heat shock response adds to the mechanism by which heat shock response exerts its anti-inflammatory effects.We further investigated the signaling mechanism that heat shock response inhibited LPS-induced IL-18 expression. We detected IL-18 mRNA expression after cells were pre-incubated with different MAPK inhibitors, and we found that the JNK MAPK inhibitor SP600125 inhibited IL-18 mRNA transcription in a dose-dependent manner. To examine the possibility that the inhibition of IL-18 may be mediated through the inactivation of JNK, the activity of JNK was measured by using Western blot and kinase assays. Our data show that heat shock response decreased LPS-induced phosphorylation of JNK and its downstream substrate c-Jun. The expression of IL-18 is regulated by several transcription factors. It has been shown that AP-1 could bind the sequence TGA(C/G) TCA located -1120 to -1083 of the IL-18 promoter and up-regulate the expression of IL-18. AP-1 is the down stream substrates of JNK which consisting of either Jun homodimer or a Fos/Jun heterodimeric complex. It is naturally to think that the activity of JNK suppressed by heat shock may inhibit AP-1 binding with the promoter of IL-18. Then we examined AP-1 DNA-binding-activity by EMSA, the results proved that the heat shock response and the inhibitor of JNK MAPK signal pathway SP600125 decreased the DNA-binding-activity of AP-1. These findings suggest that treatment of heat stress results in inhibition of IL-18 production in macrophages is regulated mainly through the JNK/AP-1 signaling pathway.