IκBα is essential for maintaining basal c-Jun N-terminal kinase (JNK) activation and regulating JNK-mediated resistance to tumor necrosis factor cytotoxicity in L929 cells

Early activation of c-Jun N-terminal kinase (JNK) is believed to block apoptosis in response to death signals such as tumor necrosis factor (TNF). Brief exposure of murine L929 fibroblasts to anisomycin for 1 hr to activate JNK resulted in resistance to TNF killing. TNF rapidly induced cytoplasmic shrinkage in control cells, but not in the anisomycin-pretreated L929 cells. However, the induced TNF resistance was suppressed in the L929 cells which were engineered to stably inhibit IκBα protein expression by antisense mRNA (~ 80% reduction in protein expression). No constitutive NF-κB nuclear translocation and increased TNF resistance were found in these IκBα antisense cells. Notably, these cells had a significantly reduced basal level of JNK activation (50-70%), compared to vector control cells. Furthermore, brief exposure of L929 cells to wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), resulted in resistance to TNF killing, probably due to preconsumption of caspases by wortmannin. Nonetheless, wortmannin-induced TNF resistance was suppressed in the IκBα antisense cells. Thus, these observations indicate that IκBα is essential for maintaining the basal level of JNK activation and regulating the JNK-induced TNF resistance.