Phospholipase A2 triggers the first phase of the thermal stress response and exhibits cell-type specificity

To understand the relationship of inflammatory and cellular stress responses, phospholipase A2 (PLA2) was examined for its role in the first phase of the transcriptional response to cellular stress. Electromobility shift analysis revealed heat shock transcription factor (HSF1)-DNA binding when HeLa S3 and Jurkat cells were exposed to exogenous PLA2. Although PLA2-inducible HSF1-DNA binding was comparable to thermal stress, it did not induce maximal heat shock gene expression. PLA2-induced HSF1 was not hyperphosphorylated relative to the heat-inducible form, thus suggesting that exogenous PLA2 affects the signal for HSF1 multimerization but not its phosphorylation. Because inflammation often involves elevated temperatures, the effect of PLA2 on thermal regulation of HSF1-DNA binding activity was examined. PLA2 exposure altered the thermal threshold for HSF1 activation, and pore-gradient gel analysis indicated that either conformational changes or other modifications of HSF1 are being induced when cells are treated by PLA2, thus creating a synergistic environment for HSF1 activation into its DNA-bound state. Surprisingly, the monocyte-like cell line, U-937, was insensitive to the action of exogenous PLA2. Neither HSF1-DNA binding or lowering of the temperature threshold for HSF1 activation was observed in PLA2-treated U-937 cells. These data suggest that inflammatory mediators such as PLA2 partially affect transcriptional switches mediating thermal stress in some cell types but not others. The purpose of HSF1 activation during inflammation and its differential induction are discussed relative to these observations.