REDOX reaction at ASK1-Cys250 is essential for activation of JNK and induction of apoptosis.

ASK1 cysteine oxidation allows JNK activation upon oxidative stress. Trx1 negatively regulates this pathway by reducing the oxidized cysteines of ASK1. However, precisely how oxidized ASK1 is involved in JNK activation and how Trx1 regulates ASK1 oxidoreduction remains elusive. Here, we describe two different thiol reductase activities of Trx1 on ASK1. First, in H(2)O(2)-treated cells, Trx1 reduces the various disulfide bonds generated between cysteines of ASK1 by a rapid and transient action. Second, in untreated cells, Trx1 shows a more stable thiol reductase activity on cysteine 250 (Cys250) of ASK1. After H(2)O(2) treatment, Trx1 dissociates from Cys250, which is not sufficient to activate the ASK1-JNK pathway. Indeed, in untreated cells, a Cys250 to alanine mutant of ASK1 (C250A), which cannot bind Trx1, does not constitutively activate JNK. On the other hand, in H(2)O(2)-treated cells, this mutant (C250A) fails to activate JNK and does not induce apoptosis, although it remains fully phosphorylated on Threonine 838 (Thr838) in its activation loop. Overall, our data show that Cys250 is essential for H(2)O(2)-dependent signaling downstream from ASK1 but at a step subsequent to the phosphorylation of ASK1 Thr838. They also clarify the thiol reductase function of Trx1 on ASK1 activity.