Kei Tobiume

Mammalian thioredoxin is a direct inhibitor of apoptosis signal‐regulating kinase (ASK) 1

Apoptosis signal‐regulating kinase (ASK) 1 was recently identified as a mitogen‐activated protein (MAP) kinase kinase kinase which activates the c‐Jun N‐terminal kinase (JNK) and p38 MAP kinase pathways and is required for tumor necrosis factor (TNF)‐α‐induced apoptosis; however, the mechanism regulating ASK1 activity is unknown. Through genetic screening for ASK1‐binding proteins, thioredoxin (Trx), a reduction/oxidation (redox)‐regulatory protein thought to have anti‐apoptotic effects, was identified as an interacting partner of ASK1. Trx associated with the N‐terminal portion of ASK1 in vitro and in vivo. Expression of Trx inhibited ASK1 kinase activity and the subsequent ASK1‐dependent apoptosis. Treatment of cells with N‐acetyl‐L‐cysteine also inhibited serum withdrawal‐, TNF‐α‐ and hydrogen peroxide‐induced activation of ASK1 as well as apoptosis. The interaction between Trx and ASK1 was found to be highly dependent on the redox status of Trx. Moreover, inhibition of Trx resulted in activation of endogenous ASK1 activity, suggesting that Trx is a physiological inhibitor of ASK1. The evidence that Trx is a negative regulator of ASK1 suggests possible mechanisms for redox regulation of the apoptosis signal transduction pathway as well as the effects of antioxidants against cytokine‐ and stress‐induced apoptosis.

ASK1 is required for sustained activations of JNK/p38 MAP kinases and apoptosis.

Apoptosis signal‐regulating kinase (ASK) 1 is activated in response to various cytotoxic stresses including TNF, Fas and reactive oxygen species (ROS) such as H2O2, and activates c‐Jun NH2‐terminal kinase (JNK) and p38. However, the roles of JNK and p38 signaling pathways during apoptosis have been controversial. Here we show that by deleting ASK1 in mice, TNF‐ and H2O2‐induced sustained activations of JNK and p38 are lost in ASK1−/− embryonic fibroblasts, and that ASK1−/− cells are resistant to TNF‐ and H2O2‐induced apoptosis. TNF‐ but not Fas‐induced apoptosis requires ROS‐dependent activation of ASK1–JNK/p38 pathways. Thus, ASK1 is selectively required for TNF‐ and oxidative stress‐induced sustained activations of JNK/p38 and apoptosis.

p53 regulates glucose metabolism through an IKK-NF-kappaB pathway and inhibits cell transformation.

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-κB and glycolysis. In p53-deficient primary cultured cells, kinase activities of IKKα and IKKβ and subsequent NF-κB activity were enhanced. Activation of NF-κB, 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-κB 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-κB pathway through suppression of glycolysis. These results suggest that a positive-feedback loop exists, whereby glycolysis drives IKK–NF-κB activation, and that hyperactivation of this loop by loss of p53 is important in oncogene-induced cell transformation.

Activation of apoptosis signal‐regulating kinase 1 by the stress‐induced activating phosphorylation of pre‐formed oligomer

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. J. Cell. Physiol. 191: 95–104, 2002.

Negative feedback regulation of ASK1 by protein phosphatase 5 (PP5) in response to oxidative stress

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 (H2O2). A yeast two‐hybrid screening identified 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 H2O2 treatment and was followed by the decrease in ASK1 activity. PP5 inhibited not only H2O2‐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.

Involvement of ASK1 in Ca2+-induced p38 MAP kinase activation

The mammalian mitogen‐activated protein (MAP) kinase kinase kinase apoptosis signal‐regulating kinase 1 (ASK1) is a pivotal component in cytokine‐ and stress‐induced apoptosis. It also regulates cell differentiation and survival through p38 MAP kinase activation. Here we show that Ca2+ signalling regulates the ASK1–p38 MAP kinase cascade. Ca2+ influx evoked by membrane depolarization in primary neurons and synaptosomes induced activation of p38, which was impaired in those derived from ASK1‐deficient mice. Ca2+/calmodulin‐dependent protein kinase type II (CaMKII) activated ASK1 by phosphorylation. Moreover, p38 activation induced by the expression of constitutively active CaMKII required endogenous ASK1. Thus, ASK1 is a critical intermediate of Ca2+ signalling between CaMKII and p38 MAP kinase.

Activated p53 induces NF-κB DNA binding but suppresses its transcriptional activation

NF-kappaB plays an important role in oncogenesis. Recently, we have demonstrated that loss of p53 function enhances DNA binding and transcriptional activities of NF-kappaB via IKKalpha and IKKbeta, and that glycolysis, activated by NF-kappaB, has an integral role in oncogene-induced cell transformation. Here, we show that ectopically expressed p53 induces acetylation and phosphorylation at Ser 536 of p65, an NF-kappaB component, and enhances DNA-binding activity of NF-kappaB. However, activated p53 suppresses transcriptional activity of NF-kappaB. Under non-stimulating conditions, p65 formed a complex with IKKalpha and IKKbeta. Activated p53 bound to p65 on DNA and disrupted binding of p65 to IKKbeta. Moreover, histone H3 kinase activity, which requires transcriptional activation of NF-kappaB, was diminished by p53. Thus, activated p53 may suppress transcriptional activity of NF-kappaB through inhibition of IKK and histone H3 kinase on DNA, suggesting a novel p53-mediated suppression system for tumorigenesis.