A short lived protein involved in the heat shock sensing mechanism responsible for stress-activated protein kinase 2 (SAPK2/p38) activation

The stress-activated protein kinase 2 (SAPK2/p38) is activated by various e nvironmental stresses and also by a vast array of agonists including growth factors and cytokines, This implies the existence of multiple proximal sig naling pathways converging to the SAPK2/p38 activation cascade. Here, we sh ow that there is a sensing mechanism highly specific to heat shock for acti vation of SAPK2/p38, After mild heat shock, cells became refractory to rein duction of the SAPK2/p38 pathway by a second heat shock. This was not the r esult of a toxic effect because the cells remained fully responsive to rein duction by other stresses, cytokines, or growth factors. Neither the activi ty of SAPK2/p38 itself nor the accumulation of the heat shock proteins was essential in the desensitization process. The cells were not desensitized t o heat shock by other treatments that activated SAPK2/p38, Moreover, inhibi ting SAPK2/p38 activity during heat shock did not block desensitization. Al so, overexpression of HSP70, HSP27, or HSP90 by gene transfection did not c ause desensitization, and inhibiting their synthesis after heat shock did n ot prevent desensitization. Desensitization rather appeared to be Linked cl osely to the turnover of a putative upstream activator of SAPK2/p38. Cycloh eximide induced a progressive and eventually complete desensitization. The effect was specific to heat shock and minimally affected activation by othe r stress inducers. Inhibiting protein degradation with MG132 caused the con stitutive activation of SAPK2/p38, which was blocked by a pretreatment with either cycloheximide or heat shock. The results thus indicate that there i s a sensing pathway highly specific to heat shock upstream of SAPK2/p38 act ivation. The pathway appears to involve a short Lived protein that is the t arget of rapid successive up- and downregulation by heat shock.