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.

Apoptosis signal-regulating kinase 1 (ASK1) is an evolutionarily conserved mitogen-activated protein 3-kinase that activates both Jnk and p38 mitogen-activated protein kinases. Here we used ASK1-deficient mice to show that ASK1 was selectively required for lipopolysaccharide-induced activation of p38 but not of Jnk or the transcription factor NF-kappaB. ASK1 was required for the induction of proinflammatory cytokines dependent on Toll-like receptor 4 (TLR4) but not TLR2 or other TLRs. Consistent with this, ASK1-deficient mice were resistant to lipopolysaccharide-induced septic shock. Lipopolysaccharide induced the production of intracellular reactive oxygen species, which was required for the formation of a complex of the adaptor molecule TRAF6 and ASK1 and subsequent activation of the ASK1-p38 pathway. Our data demonstrate that the reactive oxygen species-dependent TRAF6-ASK1-p38 axis is crucial for TLR4-mediated mammalian innate immunity.

Apoptosis signal-regulating kinase 1 (ASK1) plays a pivotal role in oxidative stress-induced cell death. Reactive oxygen species disrupt the interaction of ASK1 with its cellular inhibitor thioredoxin and thereby activates ASK1. However, the precise mechanism by which ASK1 freed from thioredoxin undergoes oligomerization-dependent activation has not been fully elucidated. Here we show that endogenous ASK1 constitutively forms a high molecular mass complex including Trx (ϳ1,500 -2,000 kDa), which we designate ASK1 signalosome. Upon H 2 O 2 treatment, the ASK1 signalosome forms a higher molecular mass complex at least in part because of the recruitment of tumor necrosis factor receptor-associated factor 2 (TRAF2) and TRAF6. Consistent with our previous findings that TRAF2 and TRAF6 activate ASK1, H 2 O 2 -induced ASK1 activation and cell death were strongly reduced in the cells derived from Traf2؊/؊ and Traf6؊/؊ mice. A novel signaling complex including TRAF2, TRAF6, and ASK1 may thus be the key component in oxidative stress-induced cell death.Apoptosis signal-regulating kinase 1 (ASK1) 2 is a mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK) family member that activates the JNK and p38 MAP kinase pathways and is activated by various stresses including oxidative stress, TNF␣, calcium overload, and endoplasmic reticulum stress (1-7). Recent analyses of ASK1-deficient mice have revealed that ASK1 is required for cell death induced by oxidative stress, TNF␣, and endoplasmic reticulum stress (4, 8).Among various stimuli tested, oxidative stress is one of the most potent activators of ASK1 (3). Thioredoxin (Trx), a reduction/oxidation (redox) regulatory protein, inhibits the kinase activity of ASK1 by its direct binding to the N-terminal noncatalytic region of ASK1 (6, 9). Reactive oxygen species (ROS) such as H 2 O 2 dissociate Trx from ASK1 and thereby activate ASK1 (6). Following the dissociation of Trx, autophosphorylation-dependent activation of ASK1 occurs most likely through mechanisms involving homo-oligomerization (10, 11); however, precise mechanisms of the oligomerization-dependent activation of ASK1 remained unknown. TNF receptor-associated factor 2 (TRAF2) has been shown to bind to and thereby activate ASK1 (5). TRAF2 is not only a pivotal intermediate in TNF␣-induced NF-B activation (12-16), but it also activates JNK in TNF␣ signaling through the association with MAPKKKs such as ASK1 and MEKK1 (5,17,18). A recent report has shown that AIP1/DAB2IP, a novel member of Ras-GAP family, regulates TRAF2-dependent activation of ASK1 in TNF␣-treated endothelial cells (19). In accordance with the findings that ROS function as second messengers in TNF␣ signaling (20, 21), TNF␣-induced association of ASK1 with TRAF2 and subsequent activation of ASK1 have been shown to depend largely on intracellular ROS production by TNF␣ (6,17).In addition to TRAF2, we have previously shown that among extensively characterized six members of the TRAF family (TRAF1-TRAF6) (22-24), TRAF5 and TRAF6 also interact with and ac...

Apoptosis and inflammation generally exert opposite effects on tumorigenesis: apoptosis serves as a barrier to tumour initiation, whereas inflammation promotes tumorigenesis. Although both events are induced by various common stressors, relatively little is known about the stress-induced signalling pathways regulating these events in tumorigenesis. Here, we show that stress-activated MAP3Ks, ASK1 and ASK2, which are involved in cellular responses to various stressors such as reactive oxygen species, differentially regulate the initiation and promotion of tumorigenesis. ASK2 in cooperation with ASK1 functioned as a tumour suppressor by exerting proapoptotic activity in epithelial cells, which was consistent with the reduction in ASK2 expression in human cancer cells and tissues. In contrast, ASK1-dependent cytokine production in inflammatory cells promoted tumorigenesis. Our findings suggest that ASK1 and ASK2 are critically involved in tumorigenesis by differentially regulating apoptosis and inflammation.

Estrogenic action has been suggested to be responsible for the strong female preponderance of autoimmune diseases, but the role of estrogens in the female has not been well characterized. We evaluated the effects of estrogen deficiency in a murine model for autoimmune exocrinopathy of Sjögren's syndrome (SS). Severe destructive autoimmune lesions developed in the salivary and lacrimal glands in estrogen-deficient mice, and these lesions were recovered by estrogen administration. We detected an intense estrogen receptor in splenic CD8(+) T cells compared with that in CD4(+) T cells, and concanavalin-A-stimulated blastogenesis of splenic CD8(+) T cells with estrogens was much higher than that of CD4(+) T cells. We found a significant increase in serum autoantibody production against the organ-specific autoantigen alpha-fodrin. Moreover, an increased proportion of TUNEL+ apoptotic epithelial duct cells was observed in estrogen-deficient mice. It was demonstrated that Fas-mediated apoptosis in cultured salivary gland cells was clearly inhibited by estrogens in vitro. These results indicate that dysfunction of regulatory T cells by estrogen deficiency may play a crucial role on acceleration of organ-specific autoimmune lesions, and estrogenic action further influences target epithelial cells through Fas-mediated apoptosis in a murine model for SS.