The conditional kinase ΔMEKK1:ER* selectively activates the JNK pathway and protects against serum withdrawal-induced cell death

The conditional protein kinase ΔMEKK3:ER* allows activation of the mitogen-activated and stress-activated protein kinases (MAPKs and SAPKs) without imposing a primary cellular stress or damage. Such separation of stress from stress-induced signalling is particularly important in the analysis of apoptosis. Activation of ΔMEKK3:ER* in cycling CCl39 cells caused a rapid stimulation of the ERK1/2, JNK and p38 pathways but resulted in a slow, delayed apoptotic response. Paradoxically, activation of the same pathways inhibited the rapid expression of BimEL and apoptosis following withdrawal of serum. Inhibition of the ERK1/2 pathway prevented the down-regulation of BimEL but caused only a partial reversion of the cyto-protective effect of ΔMEKK3:ER*. In contrast, inhibition of p38 had no effect, raising the possibility that activation of JNK might also exert a protective effect. To test this we used CCl39 cells expressing ΔMEKK1:ER* which activates JNK but not ERK1/2, p38, PKB or IκB kinase. Activation of ΔMEKK1:ER* inhibited serum withdrawal-induced conformational changes in Bax and apoptosis. These results suggest that in the absence of any overt cellular damage or chemical stress activation of JNK can act independently of the ERK1/2 or PKB pathways to inhibit serum withdrawal-induced cell death.