Search

Aims: In this study, we investigated the involvement of apoptosis signal-regulating kinase 1 (ASK1) in oxidative stress and osmotic stress-induced hepatocyte death., Main Methods: Activation of ASK1-JNK/p38 cascade and resulting cell death induced by oxidative and osmotic stress was investigated by Western immunoblot analysis and cell toxicity assay using human hepatoma cell lines, Huh7 expressing high level of ASK1 and HepG2 cells expressing low level of ASK1. Gene knock-down of ASK1 using shRNA against ASK1 was conducted using mouse hepatocyte cell line, AML12., Key Findings: Activation of ASK1-JNK/p38 cascade and cell death in Huh7 expressing high level of ASK1 was markedly induced by the oxidative stress. HepG2 expressing low level of ASK1 was resistant to oxidative stress while cell death induced by osmotic stress was comparable between Huh7 and HepG2 cells. Although the phosphorylation of ASK1 was not observed by osmotic stress, the phosphorylation of p38 and JNK and resulting cell death was induced in both cell lines. The phosphorylation of ASK1 and p38/JNK in the mouse primary hepatocyte were also increased by oxidative stress. Knock-down of ASK1 mRNA in AML12 in vitro significantly reduced oxidative stress-induced cell death, however, knock-down of ASK1 in cells did not affect the osmotic stress-induced cell death., Significance: This study revealed that ASK1 regulates oxidative stress- but not osmotic stress-induced hepatocyte death, suggesting ASK1 plays a critical role in oxidative-stress induced hepatocyte death. These results raise the possibility that an ASK1 may be a promising therapeutic target for liver diseases caused by oxidative stress.

Apoptosis signal-regulating kinase (ASK) 1 is a mitogen-activated protein kinase kinase kinase (MAP3K) in the c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase pathways that play multiple important roles in cytokine and stress responses. Here we show that ASK2, a highly related serine/threonine kinase to ASK1, also functions as a MAP3K only in a heteromeric complex with ASK1. We found that endogenous ASK2 was constitutively degraded in ASK1-deficient cells, suggesting that ASK1 is required for the stability of ASK2. ASK2 in a heteromeric complex with a kinase-negative mutant of ASK1 (ASK1-KN) effectively activated MAP2K and was more competent to respond to oxidative stress than ASK2 alone. Knockdown of ASK2 revealed that ASK2 was required for oxidative stress-induced JNK activation. These results suggest that ASK2 forms a functional MAP3K complex with ASK1, in which ASK1 supports the stability and the active configuration of ASK2. Moreover, ASK2 was found to activate ASK1 by direct phosphorylation, suggesting that ASK1 and ASK2 in a heteromeric complex facilitate their activities to each other by distinct mechanisms. Such a formation of functional heteromeric complex between different MAP3Ks may be advantageous for cells to cope with a wide variety of stimuli by fine regulation of cellular responses.