An antiapoptotic protein, c-FLIPL, directly binds to MKK7 and inhibits the JNK pathway

Inhibition of NF-kappa B activation increases susceptibility to tumor necrosis factor (TNF)alpha-induced cell death, concurrent with caspases and prolonged c-Jun N-terminal kinase (JNK) activation, and reactive oxygen species (ROS) accumulation. However, the detailed mechanisms are unclear. Here we show that cellular FLICE-inhibitory protein (c-FLIP) is rapidly lost in NF-kappa B activation-deficient, but not wild-type fibroblasts upon TNF alpha stimulation, indicating that NF-kappa B normally maintains the cellular levels of c-FLIP. The ectopic expression of the long form of c-FLIP (c-FLIPL) inhibits TNF alpha-induced prolonged JNK activation and ROS accumulation in NF-kappa B activation-deficient fibroblasts. Conversely, TNFa induces prolonged JNK activation and ROS accumulation in c-Flip(-/-) fibroblasts. Moreover, c-FLIPL directly interacts with a JNK activator, MAP kinase kinase (MKK) 7, in a TNF alpha-dependent manner and inhibits the interactions of MKK7 with MAP/ERK kinase kinase 1, apoptosis-signal-regulating kinase 1, and TGFb-activated kinase 1. This stimuli-dependent interaction of c-FLIPL with MKK7 might selectively suppress the prolonged phase of JNK activation. Taken that ROS promote JNK activation and activation of the JNK pathway may promote ROS accumulation, c-FLIPL might block this positive feedback loop, thereby suppressing ROS accumulation.