Expansion of CAG trinucleotide repeats that encode polyglutamine is the underlying cause of at least nine inherited human neurodegenerative disorders, including Huntington's disease and spinocerebellar ataxias. PolyQ fragments accumulate as aggregates in the cytoplasm and/or in the nucleus, and induce neuronal cell death. However, the molecular mechanism of polyQ-induced cell death is controversial. Here, we show the following: (1) polyQ with pathogenic repeat length triggers ER stress through proteasomal dysfunction; (2) ER stress activates ASK 1 through formation of an IRE1-TRAF2-ASK1 complex; and (3) ASK1 −/− primary neurons are defective in polyQ-, proteasome inhibitor-, and ER stress-induced JNK activation and cell death. These findings suggest that ASK1 is a key element in ER stress-induced cell death that plays an important role in the neuropathological alterations in polyQ diseases.

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Cancer cells use aerobic glycolysis preferentially for energy provision and this metabolic change is important for tumour growth. Here, we have found a link between the tumour suppressor p53, the transcription factor NF-kappaB and glycolysis. In p53-deficient primary cultured cells, kinase activities of IKKalpha and IKKbeta and subsequent NF-kappaB activity were enhanced. Activation of NF-kappaB, by loss of p53, caused an increase in the rate of aerobic glycolysis and upregulation of Glut3. Oncogenic Ras-induced cell transformation and acceleration of aerobic glycolysis in p53-deficient cells were suppressed in the absence of p65/NF-kappaB expression, and were restored by GLUT3 expression. It was also shown that a glycolytic inhibitor diminished the enhanced IKK activity in p53-deficient cells. Moreover, in Ras-expressing p53-deficient cells, IKK activity was suppressed by p65 deficiency and restored by GLUT3 expression. Taken together, these data indicate that p53 restricts activation of the IKK-NF-kappaB pathway through suppression of glycolysis. These results suggest that a positive-feedback loop exists, whereby glycolysis drives IKK-NF-kappaB activation, and that hyperactivation of this loop by loss of p53 is important in oncogene-induced cell transformation.

Apoptosis signal-regulating kinase 1 (ASK1) is a MAPKKK family member which activates c-Jun N-terminal kinase (JNK) and p38. In non-stressed cells, ASK1 exists as an inactive complex with the reduced form of thioredoxin. Oxidative stress such as hydrogen peroxide (H2O2) disrupts the ASK1-thioredoxin complex by oxidization of thioredoxin and thereby activates ASK1. The precise mechanism by which ASK1 is activated after its release from thioredoxin is unknown. Here we show that phosphorylation of Thr845 at the activation loop is essential for ASK1 to be activated by H2O2. ASK1 appears to form a silent homo-oligomer through its C-terminal coiled-coil region in non-stressed cells. Following H2O2 treatment, pre-existing ASK1 oligomer undergoes conformational change and creates a new interface within an oligomer, which ultimately leads to trans-autophosphorylation of Thr845. Thus, direct interaction via the coiled-coil region is required for self-scaffolding but not sufficient for activation of ASK1. Importantly, Thr845 of ASK1 can also be trans-phosphorylated by an unidentified Thr845 kinase in response to H2O2 treatment. We propose that this potential Thr845 kinase may be an ignition kinase that triggers Thr845 phosphorylation in oligomerized and activation-competent forms of ASK1.

Apoptosis signal-regulating kinase 1 (ASK1) is a MAP kinase kinase kinase (MAPKKK) that activates the JNK and p38 MAP kinase cascades and is activated in response to oxidative stress such as hydrogen peroxide (H 2 O 2 ). A yeast two-hybrid screening identi®ed a serine/threonine protein phosphatase 5 (PP5) as a binding partner of ASK1. PP5 directly dephosphorylated an essential phospho-threonine residue within the kinase domain of ASK1 and thereby inactivated ASK1 activity in vitro and in vivo. The interaction between PP5 and ASK1 was induced by H 2 O 2 treatment and was followed by the decrease in ASK1 activity. PP5 inhibited not only H 2 O 2 -induced sustained activation of ASK1 but also ASK1-dependent apoptosis. Thus, PP5 appears to act as a physiological inhibitor of ASK1±JNK/p38 pathways by negative feedback.

Apoptosis, a molecularly regulated form of cell death, is essential for the normal functioning and homeostasis of most multicellular organisms, and can be induced by a range of environmental, physical, and chemical stresses. As the cellular decision to live or to die is made by the coordinated action and balancing of many different pro- and antiapoptotic factors, defects in control of this coordination and balance may contribute to a variety of human diseases, including cancer and autoimmune and neurodegenerative conditions. In recent years, multiple factors associated with the execution of apoptosis, such as caspases and Bcl-2 family members, have been discovered and their complicated signaling and molecular interactions have been demonstrated; however, the precise mechanistic basis for intracellular and/or extracellular stress-induced apoptosis remains to be fully characterized. Protein kinases contribute to regulation of life and death decisions made in response to various stress signals, and the actions of pro- and antiapoptotic factors are often affected by modulation of the phosphorylation status of key elements in the execution of apoptosis. Apoptosis signal-regulating kinase 1 (ASK1) is a member of the mitogen-activated protein (MAP) kinase kinase kinase family, which activates both the MKK4/MKK7-JNK and MKK3/MKK6-p38 MAP kinase pathways and constitutes a pivotal signaling pathway in various types of stress-induced apoptosis. We have recently shown through ASK1 gene ablation in mice that ASK1 plays essential roles in oxidative stress- and endoplasmic reticulum (ER) stress-induced apoptosis. These stresses are closely linked to physiological phenomena in the control of cell fate, and the resultant apoptosis is implicated in the pathophysiology of a broad range of human diseases. This article reviews our new findings on the physiological roles of ASK1-mediated signal transduction in stress responses and the molecular mechanisms by which ASK1 determines cell fate such as survival, differentiation, or apoptosis, with special focus on the regulatory mechanisms of ASK1-mediated apoptosis induced by oxidative stress and ER stress.

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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.