Apoptosis Signal-regulating Kinase 1 Knockout Mice Research Articles

ASK1 signaling regulates phase-specific glial interactions during neuroinflammation

Neuroinflammation is well known to be associated with neurodegenerative diseases. Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase that has been implicated in neuroinflammation, but its precise cellular and molecular mechanisms remain unknown. In this study, we generated conditional knockout (CKO) mice that lack ASK1 in T cells, dendritic cells, microglia/macrophages, microglia, or astrocytes, to assess the roles of ASK1 during experimental autoimmune encephalomyelitis (EAE). We found that neuroinflammation was reduced in both the early and later stages of EAE in microglia/macrophage-specific ASK1 knockout mice, whereas only the later-stage neuroinflammation was ameliorated in astrocyte-specific ASK1 knockout mice. ASK1 deficiency in T cells and dendritic cells had no significant effects on EAE severity. Further, we found that ASK1 in microglia/macrophages induces a proinflammatory environment, which subsequently activates astrocytes to exacerbate neuroinflammation. Microglia-specific ASK1 deletion was achieved using a CX3CR1CreER system, and we found that ASK1 signaling in microglia played a major role in generating and maintaining disease. Activated astrocytes produce key inflammatory mediators, including CCL2, that further activated and recruited microglia/macrophages, in an astrocytic ASK1-dependent manner. Astrocyte-specific analysis revealed CCL2 expression was higher in the later stage compared with the early stage, suggesting a greater proinflammatory role of astrocytes in the later stage. Our findings demonstrate cell-type-specific roles of ASK1 and suggest phase-specific ASK1-dependent glial cell interactions in EAE pathophysiology. We propose glial ASK1 as a promising therapeutic target for reducing neuroinflammation.

Functional cooperation between ASK1 and p21Waf1/Cip1 in the balance of cell-cycle arrest, cell death and tumorigenesis of stressed keratinocytes

Both CDKN1A (p21 Waf1/Cip1) and Apoptosis signal-regulating kinase 1 (ASK1) play important roles in tumorigenesis. The role of p21 Waf1/Cip1 in attenuating ASK1-induced apoptosis by various stress conditions is well established. However, how ASK1 and p21 Waf1/Cip1 functionally interact during tumorigenesis is still unclear. To address this aspect, we crossed ASK1 knockout (ASK1KO) mice with p21Waf1/Cip1 knockout (p21KO) mice to compare single and double-mutant mice. We observed that deletion of p21Waf1/Cip1 leads to increased keratinocyte proliferation but also increased cell death. This is mechanistically linked to the ASK1 axis-induced apoptosis, including p38 and PARP. Indeed, deletion of ASK1 does not alter the proliferation but decreases the apoptosis of p21KO keratinocytes. To analyze as this interaction might affect skin carcinogenesis, we investigated the response of ASK1KO and p21KO mice to DMBA/TPA-induced tumorigenesis. Here we show that while endogenous ASK1 is dispensable for skin homeostasis, ASK1KO mice are resistant to DMBA/TPA-induced tumorigenesis. However, we found that epidermis lacking both p21 and ASK1 reacquires increased sensitivity to DMBA/TPA-induced tumorigenesis. We demonstrate that apoptosis and cell-cycle progression in p21KO keratinocytes are uncoupled in the absence of ASK1. These data support the model that a critical event ensuring the balance between cell death, cell-cycle arrest, and successful divisions in keratinocytes during stress conditions is the p21-dependent ASK1 inactivation.

Dysregulated Immune Responses by ASK1 Deficiency Alter Epithelial Progenitor Cell Fate and Accelerate Metaplasia Development during H. pylori Infection.

The mechanism of H. pylori-induced atrophy and metaplasia has not been fully understood. Here, we demonstrate the novel role of Apoptosis signal-regulating kinase 1 (ASK1) and downstream MAPKs as a regulator of host immune responses and epithelial maintenance against H. pylori infection. ASK1 gene deficiency resulted in enhanced inflammation with numerous inflammatory cells including Gr-1+CD11b+ myeloid-derived suppressor cells (MDSCs) recruited into the infected stomach. Increase of IL-1β release from apoptotic macrophages and enhancement of TH1-polarized immune responses caused STAT1 and NF-κB activation in epithelial cells in ASK1 knockout mice. Dysregulated immune and epithelial activation in ASK1 knockout mice led to dramatic expansion of gastric progenitor cells and massive metaplasia development. Bone marrow transplantation experiments revealed that ASK1 in inflammatory cells is critical for inducing immune disorder and metaplastic changes in epithelium, while ASK1 in epithelial cells regulates cell proliferation in stem/progenitor zone without changes in inflammation and differentiation. These results suggest that H. pylori-induced immune cells may regulate epithelial homeostasis and cell fate as an inflammatory niche via ASK1 signaling.

Apoptosis signal regulating kinase 1 deletion mitigates α-synuclein pre-formed fibril propagation in mice

α-Synuclein (α-Syn) is a key pathogenic protein in α-synucleinopathies including Parkinson disease and dementia with Lewy bodies. Accumulating evidence has shown that misfolded fibrillar α-Syn is transmitted from cell-to-cell, a phenomenon that correlates with clinical progression of the disease. We previously showed that deleting the MAP3 kinase apoptosis signal-regulating kinase 1 (ASK1), which is a central player linking oxidative stress with neuroinflammation, mitigates the phenotype of α-Syn transgenic mice. However, whether ASK1 impacts pathology and disease progression induced by recombinant α-Syn pre-formed fibrils (PFF) remains unknown. Here, we compared the neuropathological and behavioral phenotype of ASK1 knock-out mice with that of wild-type mice following intrastriatal injections of α-Syn PFF. At 6 months post-injections, ASK1 null mice exhibited reduced amount of phosphorylated α-Syn aggregates in the striatum and cortex, and less pronounced degeneration of the nigrostriatal pathway. Additionally, the neuroinflammatory reaction to α-Syn PFF injection and propagation seen in wild-type mice was attenuated in ASK1 knock-out animals. These neuropathological markers were associated with better behavioral performance. These data suggest that ASK1 plays an important role in pathological α-Syn fibril transmission and, consequently, may impact disease progression. These findings collectively support inhibiting ASK1 as a disease modifying therapeutic strategy for Parkinson disease and related α-synucleinopathies.

A heteromeric molecular complex regulates the migration of lung alveolar epithelial cells during wound healing

Alveolar type II epithelial cells (ATII) are instrumental in early wound healing in response to lung injury, restoring epithelial integrity through spreading and migration. We previously reported in separate studies that focal adhesion kinase-1 (FAK) and the chemokine receptor CXCR4 promote epithelial repair mechanisms. However, potential interactions between these two pathways were not previously considered. In the present study, we found that wounding of rat ATII cells promoted increased association between FAK and CXCR4. In addition, protein phosphatase-5 (PP5) increased its association with this heteromeric complex, while apoptosis signal regulating kinase-1 (ASK1) dissociated from the complex. Cell migration following wounding was decreased when PP5 expression was decreased using shRNA, but migration was increased in ATII cells isolated from ASK1 knockout mice. Interactions between FAK and CXCR4 were increased upon depletion of ASK1 using shRNA in MLE-12 cells, but unaffected when PP5 was depleted. Furthermore, we found that wounded rat ATII cells exhibited decreased ASK1 phosphorylation at Serine-966, decreased serine phosphorylation of FAK, and decreased association of phosphorylated ASK1 with FAK. These changes in phosphorylation were dependent upon expression of PP5. These results demonstrate a unique molecular complex comprising CXCR4, FAK, ASK1, and PP5 in ATII cells during wound healing.

ASK1, a Novel Regulator of Thrombosis, Is Activated Downstream of G-Protein Coupledreceptors in Human Platelets

In the event of vascular injury, platelets rapidly adhere to sub-endothelial matrix proteins such as collagen and Von Willebrand factor and activate to form a stable hemostatic platelet plug. Defects in the molecular mechanisms dictating platelet plug formation are responsible for numerous thrombotic disorders. Elucidating the signaling pathways and molecular mechanisms of platelet activation is paramount to the development of safer and more effective anti-thrombotic drugs. While it is known that MAP-Kinase signaling participates in platelet activation, it is unknown how MAP-Kinase signaling specifically mediates platelet activation. Our laboratory has identified the presence and activation of a MAP-Kinase Kinase Kinase known as Apoptosis Signal Regulating Kinase 1 (ASK1). We have demonstrated using an ASK1 knockout mouse model that ablation of ASK1 leads to a significantly increased (p = .0003) time of vessel occlusion associated with unstable thrombus formation following a carotid artery injury induced by 10% FeCl3. Furthermore, ASK1 knockout mice display protection from pulmonary thromboembolism induced by an intravenous injection of collagen and epinephrine. In order to determine the kinetics of ASK1 activation by physiological agonists, washed human platelets (4 x 108 platelets/mL) were treated with 0.1 U/mL of thrombin for 30”, 1’, 3’, 5’, and 8’. Robust activation of ASK1 by thrombin occurred as early as 30 seconds up until 5 min, after which ASK1 activation decreased sharply. Platelets treated with 100 µM of PAR1 (SFLLRN) or PAR4 (AYPGKF) peptides resulted in strong ASK1 activation, suggesting that both the PAR1 and PAR4 receptors lead to ASK1 activation. Inhibition of Src family kinases by PP2 or PI3K by wortmannin or Rho kinase by Y-27632 had no effect on thrombin-induced ASK1 activation. However, inhibition of PLC-β2, a mediator of platelet activation downstream of the PAR1/4 receptors, strongly inhibited ASK1 activation by thrombin. We next determined whether TxA2 generation was responsible for ASK1 activation by thrombin. Washed platelets were pre-treated with 1 mM aspirin to block TxA2 generation, followed by treatment with 0.1 U/mL of thrombin. It was found that blocking TxA2 generation eliminated ASK1 activation by thrombin at 30” and 1’, but not at a later time point, suggesting there may be an additional pathway contributing to ASK1 activation. The observation that TxA2 generation contributes to ASK1 activation by thrombin seemed to correlate with the finding that treatment of platelets with 1 µM of the TxA2 mimetic U46619, which activates the TP-α receptor, could also activate ASK1. We also determined whether ADP released from dense granules, which would activate the P2Y1 and P2Y12 receptors, leads to ASK1 activation. To test this, washed platelets were pre-treated with 1 U/mL of apyrase to hydrolyze secreted ADP. It was found that apyrase treatment completely eliminates ASK1 activation by thrombin, suggesting a strong dependency of thrombin-induced ASK1 activation on ADP release from dense granules. To further investigate this possibility, washed platelets were pre-treated with 50 µM of the P2Y1 antagonist MRS2179 or P2Y12 antagonist 2-MeSAMP, followed by treatment with 0.1 U/mL of thrombin. Antagonism of the P2Y12 receptor and not P2Y1 receptor severely diminished ASK1 activation by thrombin. This indicates that ASK1 activation by thrombin is also dependent on ADP released from dense granules and subsequent activation of the P2Y12 receptor. Surprisingly, collagen, a strong activator of platelets, was unable to activate ASK1 in washed platelets at a concentration of 2 µg/mL. Similarly, 2 µM epinephrine treatment also had no effect. However, when washed platelets were treated with 2 µg/mL collagen and 2 µM epinephrine together, a strong ASK1 activation was observed (p=.0012). This suggests the existence of a novel mechanism for ASK1 activation by simultaneous stimulation of the collagen receptors GPVI/α2β1 and epinephrine receptor α2A. The finding that ASK1 activation occurs downstream of TP-α, P2Y12, and possibly α2A receptors highlights the importance of ASK1 in regulation of these G-Protein Coupled Receptors in platelet activation. In conclusion, our data indicates ASK1 to be a key mediator in platelet activation and represents a novel target for anti-thrombotic drug therapy. DisclosuresNo relevant conflicts of interest to declare.

Inhibition of apoptosis signal-regulating kinase 1 enhances endochondral bone formation by increasing chondrocyte survival.

Endochondral ossification is the result of chondrocyte differentiation, hypertrophy, death and replacement by bone. The careful timing and progression of this process is important for normal skeletal bone growth and development, as well as fracture repair. Apoptosis Signal-Regulating Kinase 1 (ASK1) is a mitogen-activated protein kinase (MAPK), which is activated by reactive oxygen species and other cellular stress events. Activation of ASK1 initiates a signaling cascade known to regulate diverse cellular events including cytokine and growth factor signaling, cell cycle regulation, cellular differentiation, hypertrophy, survival and apoptosis. ASK1 is highly expressed in hypertrophic chondrocytes, but the role of ASK1 in skeletal tissues has not been investigated. Herein, we report that ASK1 knockout (KO) mice display alterations in normal growth plate morphology, which include a shorter proliferative zone and a lengthened hypertrophic zone. These changes in growth plate dynamics result in accelerated long bone mineralization and an increased formation of trabecular bone, which can be attributed to an increased resistance of terminally differentiated chondrocytes to undergo cell death. Interestingly, under normal cell culture conditions, mouse embryonic fibroblasts (MEFs) derived from ASK1 KO mice show no differences in either MAPK signaling or osteogenic or chondrogenic differentiation when compared with wild-type (WT) MEFs. However, when cultured with stress activators, H2O2 or staurosporine, the KO cells show enhanced survival, an associated decrease in the activation of proteins involved in death signaling pathways and a reduction in markers of terminal differentiation. Furthermore, in both WT mice treated with the ASK1 inhibitor, NQDI-1, and ASK1 KO mice endochondral bone formation was increased in an ectopic ossification model. These findings highlight a previously unrealized role for ASK1 in regulating endochondral bone formation. Inhibition of ASK1 has clinical potential to treat fractures or to slow osteoarthritic progression by enhancing chondrocyte survival and slowing hypertrophy.

Deletion of Apoptosis Signal–Regulating Kinase–1 Prevents Ventilator-Induced Lung Injury in Mice

Both hyperoxia and mechanical ventilation can independently cause lung injury. In combination, these insults produce accelerated and severe lung injury. We recently reported that pre-exposure to hyperoxia for 12 hours, followed by ventilation with large tidal volumes, induced significant lung injury and epithelial cell apoptosis compared with either stimulus alone. We also reported that such injury and apoptosis are inhibited by antioxidant treatment. In this study, we hypothesized that apoptosis signal-regulating kinase-1 (ASK-1), a redox-sensitive, mitogen-activated protein kinase kinase kinase, plays a role in lung injury and apoptosis in this model. To determine the role of ASK-1 in lung injury, the release of inflammatory mediators and apoptosis, attributable to 12 hours of hyperoxia, were followed by large tidal volume mechanical ventilation with hyperoxia. Wild-type and ASK-1 knockout mice were subjected to hyperoxia (Fi(O(2)) = 0.9) for 12 hours before 4 hours of large tidal mechanical ventilation (tidal volume = 25 μl/g) with hyperoxia, and were compared with nonventilated control mice. Lung injury, apoptosis, and cytokine release were measured. The deletion of ASK-1 significantly inhibited lung injury and apoptosis, but did not affect the release of inflammatory mediators, compared with the wild-type mice. ASK-1 is an important regulator of lung injury and apoptosis in this model. Further study is needed to determine the mechanism of lung injury and apoptosis by ASK-1 and its downstream mediators in the lung.

Abstract 3133: Apoptosis signal-regulating kinase 1 and cyclin D1 compose a positive feedback loop contributing to tumor growth in gastric cancer

Abstract Gastric cancer is a common cancer worldwide, associated with a high mortality despite its declining incidence in recent decades. Although the role of H. pylori in causing mucosal effects has been investigated, which molecular signal(s) initiate the program of irreversible transformation remain unclear, and thus molecular targeting therapies for GC have not been well established. Mitogen-activated protein kinase (MAPK) pathways regulate multiple cellular functions and have been demonstrated to be highly active in many types of human cancers, and we have previously reported that c-Jun-N-terminal kinase 1 (JNK1) is important for gastric tumorigenesis. Apoptosis signal-regulating kinase 1 (ASK1) is an upstream MAPK known to be involved in apoptosis, inflammation, and carcinogenesis. In the current study, we investigated the role of ASK1 in the development of gastric cancer. In human gastric cancer specimens, we observed increased ASK1 expression compared to non-tumor epithelium, while we found no difference between colon cancer and non-tumor colon epithelium. Using the chemically induced murine gastric tumorigenesis model, we observed increased tumor ASK1 expression, and ASK1 knockout mice displayed both a lower number and smaller tumor size compared to wild-type mice. In gastric cancer cell lines, immunoprecipitation analysis revealed constitutive ASK1 activation in several cell lines. ASK1 siRNA inhibited cell proliferation through the accumulation of cells in G1 phase of the cell cycle, and reduced cyclin D1 expression in gastric cancer cells, while these effects were uncommon in other cancer cells, suggesting that the ASK1-dependent regulation of the cyclin D1 level and cellular proliferation is specific to gastric cancer. Overexpression of ASK1 induced the transcription of cyclin D1 through AP-1 activation in a kinase-dependent manner. In addition, we found that the expression of ASK1 was elevated following epidermal growth factor (EGF) or Helicobacter pylori (H.pylori) treatment in control cells but not in cyclin D1 silencing cells. Overexpression of cyclin D1 enhanced phosphorylation of Rb, followed by the increase in ASK1 expression. Chromatin immunoprecipitation (ChIP) assay showed that cyclin D1 expression increase E2F1 binding to ASK1 promoter, suggesting that the levels of ASK1 were regulated by cyclin D1 through the Rb-E2F pathway. Exogenous ASK1 induced cyclin D1 expression, followed by an elevated expression of endogenous ASK1. These results indicate an autoregulatory mechanism of ASK1 in the development of gastric cancer. Through targeting this positive feedback loop, ASK1 may present a potential therapeutic target for the treatment of advanced gastric cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3133. doi:10.1158/1538-7445.AM2011-3133

Apoptosis signal-regulating kinase 1 and cyclin D1 compose a positive feedback loop contributing to tumor growth in gastric cancer

Mitogen-activated protein kinase (MAPK) pathways regulate multiple cellular functions and are highly active in many types of human cancers. Apoptosis signal-regulating kinase 1 (ASK1) is an upstream MAPK involved in apoptosis, inflammation, and carcinogenesis. This study investigated the role of ASK1 in the development of gastric cancer. In human gastric cancer specimens, we observed increased ASK1 expression, compared to nontumor epithelium. Using a chemically induced murine gastric tumorigenesis model, we observed increased tumor ASK1 expression, and ASK1 knockout mice had both fewer and smaller tumors than wild-type (WT) mice. ASK1 siRNA inhibited cell proliferation through the accumulation of cells in G1 phase of the cell cycle, and reduced cyclin D1 expression in gastric cancer cells, whereas these effects were uncommon in other cancer cells. ASK1 overexpression induced the transcription of cyclin D1, through AP-1 activation, and ASK1 levels were regulated by cyclin D1, via the Rb-E2F pathway. Exogenous ASK1 induced cyclin D1 expression, followed by elevated expression of endogenous ASK1. These results indicate an autoregulatory mechanism of ASK1 in the development of gastric cancer. Targeting this positive feedback loop, ASK1 may present a potential therapeutic target for the treatment of advanced gastric cancer.

Role of Apoptosis Signal-Regulating Kinase 1 in Stress-Induced Neural Cell Apoptosis in Vivo

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase that plays an important role in oxidative stress-induced apoptosis. In the present study, we used ASK1 knockout (KO) mice to examine the possibility that ASK1 is involved in the neural cell apoptosis that occurs during retinal development and ischemic injury. ASK1 was expressed in retinal neurons, including retinal ganglion cells (RGCs), but retinal structure and extent of cell death during development were normal in ASK1 KO mice. On the other hand, the strain was less susceptible to ischemic injury, and the number of surviving retinal neurons was significantly increased compared with that in wild-type mice. Interestingly, ischemia-induced phosphorylation of p38 mitogen-activated protein kinase (p38), which mediates RGC apoptosis, was almost completely suppressed in ASK1 KO mice. In such retinas, the numbers of cleaved caspase-3- and TUNEL-positive neurons were apparently decreased compared with those in wild-type mice. Furthermore, cultured RGCs from ASK1 KO mice were resistant to H(2)O(2)-induced apoptosis. Our findings suggest that ASK1 is involved in the neural cell apoptosis after various kinds of oxidative stress. Thus, inhibition of the ASK1-p38 pathway could be useful for the treatment of neurodegenerative diseases including glaucoma.

Activation of apoptosis signal-regulating kinase 1 by reactive oxygen species through dephosphorylation at serine 967 and 14-3-3 dissociation.

Oxidative stress has been indicated in a variety of pathological processes such as atherosclerosis, diabetes, and neurodegenerative diseases. Understanding how intracellular signaling pathways respond to oxidative insults such as hydrogen peroxide (H(2)O(2)) would have significant therapeutic implications. Recent genetic studies have placed apoptosis signal-regulating kinase 1 (ASK1) in a pivotal position in transmitting H(2)O(2)-initiated signals. How ASK1 is activated by H(2)O(2), though, remains a subject of intense investigation. Here we report a mechanism by which H(2)O(2) induces ASK1 activation through dynamic control of its phosphorylation at serine 967. We found that treatment of COS7 cells with H(2)O(2) triggers dephosphorylation of Ser-967 through an okadaic acid-sensitive phosphatase, resulting in dissociation of the ASK1.14-3-3 complex with concomitant increase of ASK1 catalytic activity and ASK1-mediated activation of JNK and p38 pathways.

Targeted deletion of apoptosis signal-regulating kinase 1 attenuates left ventricular remodeling.

Left ventricular remodeling that occurs after myocardial infarction (MI) and pressure overload is generally accepted as a determinant of the clinical course of heart failure. The molecular mechanism of this process, however, remains to be elucidated. Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase that plays an important role in stress-induced apoptosis. We used ASK1 knockout mice (ASK-/-) to test the hypothesis that ASK1 is involved in development of left ventricular remodeling. ASK-/- hearts showed no morphological or histological defects. Echocardiography and cardiac catheterization revealed normal global structure and function. Left ventricular structural and functional remodeling were determined 4 weeks after coronary artery ligation or thoracic transverse aortic constriction (TAC). ASK-/- had significantly smaller increases in left ventricular end-diastolic and end-systolic ventricular dimensions and smaller decreases in fractional shortening in both experimental models compared with WT mice. The number of terminal deoxynucleotidyl transferase biotin-dUDP nick end-labeling-positive myocytes after MI or TAC was decreased in ASK-/- compared with that in WT mice. Overexpression of a constitutively active mutant of ASK1 induced apoptosis in isolated rat neonatal cardiomyocytes, whereas neonatal ASK-/- cardiomyocytes were resistant to H2O2-induced apoptosis. An in vitro kinase assay showed increased ASK1 activity in heart after MI or TAC in WT mice. Thus, ASK1 plays an important role in regulating left ventricular remodeling by promoting apoptosis.

ASK1 regulates influenza virus infection-induced apoptotic cell death

Apoptosis occurs in influenza virus (IV)-infected cells. There are a number of mechanisms for the regulation of apoptosis. However, the molecular mechanism of IV infection-induced apoptosis is still controversial. Apoptosis signal-regulating kinase1 (ASK1) is a ubiquitously expressed mitogen-activated protein kinase kinase kinase (MAPKKK) that activates the SEK1-c-Jun N-terminal kinase (JNK) and MKK3/MKK6-p38 MAPK signaling cascades. ASK1 has been implicated in cytokine- and stress-induced apoptosis. Here, we show the following: (1) IV infection activated ASK1 and concomitantly phosphorylated JNK and p38 MAPK in human bronchial epithelial cells; (2) the activation of JNK and p38 MAPK but not extracellular-regulated kinase (ERK) in embryonic fibroblasts (MEFs) derived from ASK1 knockout mice (ASK1 −/− MEFs) was depressed compared to MEFs derived from wild type mice (ASK1 +/+ MEFs); and (3) ASK1 −/− MEFs were defective in IV infection-induced caspase-3 activation and cell death. These results indicate that apoptosis in IV-infected BEC is mediated through ASK1-dependent cascades.

Physiological roles of ASK1-mediated signal transduction in oxidative stress- and endoplasmic reticulum stress-induced apoptosis: advanced findings from ASK1 knockout mice.

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.