Activation of ASK-1 and downstream MAP kinases in cytochrome P4502E1 potentiated tumor necrosis factor α liver injury

Abstract

Cytochrome P4502E1 (CYP2E1) potentiates TNFα toxicity by a mechanism involving increased oxidative stress and activation of JNK and p38 MAPKs. This study evaluated the upstream mediators of this MAPK activation with a special focus on studying whether apoptosis signal regulating kinase-1 (ASK-1) is activated in the CYP2E1–TNFα hepatotoxic model. Wild-type and CYP2E1 −/− mice were treated with pyrazole (PY) for 3 days to induce CYP2E1 and challenged with TNFα on day 3. Liver injury occurred between 8 and 12 h after TNFα administration only to the wild-type PY-treated mice. Oxidative stress was elevated in the PY mice at 4 h, a time before the liver injury. ASK-1 was dissociated from the thioredoxin–ASK-1 complex and was activated at 4 h after administration of TNFα to PY mice. This was followed by activation of MKK3/MKK6 and MKK4/MKK7 at 4–8 or 12 h and then JNK/p38 MAPK at 8 to 12 h. MAPK phosphatase-1 was decreased 12 to 24 h after TNFα administration. This may promote a sustained activation of JNK. Bax was elevated, whereas Bcl-2 and cFLIP S/L were lowered at 4 h after administration of TNFα. These changes were followed by increases in caspase 8 and 3 activities and apoptosis. None of the above changes were observed when TNFα was administered to PY-treated CYP2E1 −/− mice. These studies show that TNFα increases oxidative stress in mice with elevated CYP2E1, with subsequent activation of ASK-1 via a mechanism involving thioredoxin–ASK-1 dissociation, followed by activation of downstream MKK and MAPK. We speculate that similar interactions between CYP2E1 and TNFα may be important for alcohol-induced liver injury.