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 environmental stresses and also by a vast array of agonists including growth factors and cytokines. This implies the existence of multiple proximal signaling pathways converging to the SAPK2/p38 activation cascade. Here, we show that there is a sensing mechanism highly specific to heat shock for activation of SAPK2/p38. After mild heat shock, cells became refractory to reinduction of the SAPK2/p38 pathway by a second heat shock. This was not the result of a toxic effect because the cells remained fully responsive to reinduction by other stresses, cytokines, or growth factors. Neither the activity of SAPK2/p38 itself nor the accumulation of the heat shock proteins was essential in the desensitization process. The cells were not desensitized to heat shock by other treatments that activated SAPK2/p38. Moreover, inhibiting SAPK2/p38 activity during heat shock did not block desensitization. Also, overexpression of HSP70, HSP27, or HSP90 by gene transfection did not cause desensitization, and inhibiting their synthesis after heat shock did not prevent desensitization. Desensitization rather appeared to be linked closely to the turnover of a putative upstream activator of SAPK2/p38. Cycloheximide induced a progressive and eventually complete desensitization. The effect was specific to heat shock and minimally affected activation by other stress inducers. Inhibiting protein degradation with MG132 caused the constitutive activation of SAPK2/p38, which was blocked by a pretreatment with either cycloheximide or heat shock. The results thus indicate that there is a sensing pathway highly specific to heat shock upstream of SAPK2/p38 activation. The pathway appears to involve a short lived protein that is the target of rapid successive up- and down-regulation by heat shock.