C-terminus of heat shock protein 70-interacting protein facilitates degradation of apoptosis signal-regulating kinase 1 and inhibits apoptosis signal-regulating kinase 1-dependent apoptosis

Ubiquitin-like sequence in ASK1 plays critical roles in the recognition and stabilization by USP9X and oxidative stress-induced cell death

Ubiquitination is an important posttranslational modification that regulates various cellular processes, including signal transduction. However, physiological roles of ubiquitination in the regulation of MAPK pathways are poorly understood. Here, we identified the deubiquitinating enzyme USP9X as a binding partner of ASK1 that mediates oxidative stress-induced cell death through activation of the JNK and p38 MAPK pathways. In the recognition of ubiquitin by deubiquitinating enzymes, the importance of a tandem glycine-glycine sequence in the ubiquitin C terminus has been suggested. Interestingly, ASK1 contains six amino acids identical to the ubiquitin C terminus (LRLRGG), and the GG sequence of ASK1 was required for the USP9X-ASK1 interaction. We also found that USP9X interacted with oxidative stress-activated ASK1 and prevented it from undergoing ubiquitin-dependent degradation. In USP9X-deficient cells, oxidative stress-induced JNK activation and subsequent cell death were reduced. These results demonstrate that USP9X-dependent stabilization of activated ASK1 plays a crucial role in oxidative stress-induced cell death.

C terminus of Hsc70-interacting protein promotes smooth muscle cell proliferation and survival through ubiquitin-mediated degradation of FoxO1

Forkhead transcription factors (FoxOs) play a pivotal role in controlling cellular proliferation and survival. The cellular level of these factors is tightly regulated through the phosphoinositide 3-kinase/Akt and ubiquitin-mediated degradation. However, the ubiquitin ligases responsible for the degradation of FoxO1 and the relevance of this regulation to smooth muscle cell (SMC) proliferation and survival have not been fully identified. Here we showed that overexpression of C terminus of Hsc70-interacting protein (CHIP) promoted ubiquitination and degradation of FoxO1 in SMCs in response to tumor necrosis factor-alpha. Both the U-box (containing ubiquitin ligase activity) and the charged (essential for FoxO1 binding) domains within CHIP were required for CHIP-mediated FoxO1 down-regulation. Moreover, interaction and ubiquitination of FoxO1 by CHIP depended on phosphorylation of FoxO1 at Ser-256. Furthermore, overexpression of CHIP repressed FoxO1-mediated transactivation and its proapoptotic function following tumor necrosis factor-alpha treatment. In contrast, knockdown of CHIP by small interfering RNA enhanced FoxO1-mediated transactivation and its effect on SMC proliferation and survival. Taken together, our data indicate that CHIP is a negative regulator of FoxO1 activity through ubiquitin-mediated degradation, and inhibition of CHIP may serve as a potential therapeutic target for reducing proliferative arterial diseases.

Stress-dependent Daxx-CHIP interaction suppresses the p53 apoptotic program

Our previous studies have implicated CHIP (carboxyl terminus of Hsp70-interacting protein) as a co-chaperone/ubiquitin ligase whose activities yield protection against stress-induced apoptotic events. In this report, we demonstrate a stress-dependent interaction between CHIP and Daxx (death domain-associated protein). This interaction interferes with the stress-dependent association of HIPK2 with Daxx, blocking phosphorylation of serine 46 in p53 and inhibiting the p53-dependent apoptotic program. Microarray analysis confirmed suppression of the p53-dependent transcriptional portrait in CHIP(+/+) but not in CHIP(-/-) heat shocked mouse embryonic fibroblasts. The interaction between CHIP and Daxx results in ubiquitination of Daxx, which is then partitioned to an insoluble compartment of the cell. In vitro ubiquitination of Daxx by CHIP revealed that ubiquitin chain formation utilizes non-canonical lysine linkages associated with resistance to proteasomal degradation. The ubiquitination of Daxx by CHIP utilizes lysines 630 and 631 and competes with the sumoylation machinery of the cell at these residues. These studies implicate CHIP as a stress-dependent regulator of Daxx that counters the pro-apoptotic influence of Daxx in the cell. By abrogating p53-dependent apoptotic pathways and by ubiquitination competitive with Daxx sumoylation, CHIP integrates the proteotoxic stress response of the cell with cell cycle pathways that influence cell survival.

beta-Arrestins facilitate ubiquitin-dependent degradation of apoptosis signal-regulating kinase 1 (ASK1) and attenuate H2O2-induced apoptosis

beta-Arrestins are ubiquitously expressed proteins that play important roles in receptor desensitization, endocytosis, proteosomal degradation, apoptosis and signaling. It has been reported that beta-Arrestin2 acts as a scaffold by directly interacting with the JNK3 isoform and recruiting MKK4 and the apoptosis-signaling kinase-1 (ASK1). Here, we report a novel function of beta-Arrestins in regulating H(2)O(2)-induced apoptosis. Our study demonstrates that beta-Arrestins physically associate with C-terminal domain of ASK1, and moreover, both over-expression and RNA interference (RNAi) experiments indicate that beta-Arrestins down-regulate ASK1 protein. In detail, beta-Arrestin-induced reduction of ASK1 protein is due to ubiquitination and proteasome-dependent degradation of ASK1 in response to association of beta-Arrestins and ASK1. Upon H(2)O(2) stimulation, the protein binding between beta-Arrestins and ASK1 increases and ASK1 degradation is expedited. In consequence, beta-Arrestins prevent ASK1-JNK signaling and as a result attenuate H(2)O(2)-induced apoptosis. Structurally, C-terminal domain of ASK1 is essential for beta-Arrestins and ASK1 association. We also found that CHIP is required for beta-Arrestins-induced ASK1 degradation, which suggested that beta-Arrestins function as a scaffold of ASK1 and CHIP, leading to CHIP-mediated ASK1 degradation. All these findings indicate that beta-Arrestins play a negative regulatory role in H(2)O(2)-induced apoptosis signaling through associating with ASK1 and CHIP and facilitating ASK1 degradation, which provides a new insight for analyzing the effects of beta-Arrestins on protecting cells from oxidative stress-induced apoptosis.

CHIP represses myocardin-induced smooth muscle cell differentiation via ubiquitin-mediated proteasomal degradation

Myocardin, a coactivator of serum response factor (SRF), plays a critical role in the differentiation of vascular smooth muscle cells (SMCs). However, the molecular mechanisms regulating myocardin stability and activity are not well defined. Here we show that the E3 ligase C terminus of Hsc70-interacting protein (CHIP) represses myocardin-dependent SMC gene expression and transcriptional activity. CHIP interacts with and promotes myocardin ubiquitin-mediated degradation by the proteasome in vivo and in vitro. Furthermore, myocardin ubiquitination by CHIP requires its phosphorylation. Importantly, CHIP overexpression reduces the level of myocardin-dependent SMC contractile gene expression and diminishes arterial contractility ex vivo. These findings for the first time, to our knowledge, demonstrate that CHIP-promoted proteolysis of myocardin plays a key role in the physiological control of SMC phenotype and vessel tone, which may have an important implication for pathophysiological conditions such as atherosclerosis, hypertension, and Alzheimer's disease.

Degradation of activated protein kinases by ubiquitination

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular functions. Once a protein kinase is activated, its activity is subsequently downregulated through a variety of mechanisms. Accumulating evidence indicates that the activation of protein kinases commonly initiates their downregulation via the ubiquitin/proteasome pathway. Failure to regulate protein kinase activity or expression levels can cause human diseases.

Mechanisms of minocycline-induced suppression of simian immunodeficiency virus encephalitis: inhibition of apoptosis signal-regulating kinase 1

Human immunodeficiency virus (HIV) infection of the central nervous system (CNS) can lead to cognitive dysfunction, even in individuals treated with highly active antiretroviral therapy. Using an established simian immunodeficiency virus (SIV)/macaque model of HIV CNS disease, we previously reported that infection shifts the balance of activation of mitogen-activated protein kinase (MAPK) signaling pathways in the brain, resulting in increased activation of the neurodegenerative MAPKs p38 and JNK. Minocycline treatment of SIV-infected macaques reduced the incidence and severity of SIV encephalitis in this model, and suppressed the activation of p38 in the brain. The purpose of this study was to further examine the effects of minocycline on neurodegenerative MAPK signaling. We first demonstrated that minocycline also decreases JNK activation in the brain and levels of the inflammatory mediator nitric oxide (NO). We next used NO to activate these MAPK pathways in vitro, and demonstrated that minocycline suppresses p38 and c-Jun N-terminal kinase (JNK) activation by reducing intracellular levels, and hence, activation of apoptosis signal-regulating kinase 1 (ASK1), a MAPK kinase capable of selectively activating both pathways. We then demonstrated that ASK1 activation in the brain during SIV infection is suppressed by minocycline. By suppressing p38 and JNK activation pathways, which are important for the production of and responses to inflammatory mediators, minocycline may interrupt the vicious cycle of inflammation that both results from, and promotes, virus replication in SIV and HIV CNS disease.

DDTD, an isoflavone derivative, induces cell apoptosis through the reactive oxygen species/apoptosis signal-regulating kinase 1 pathway in human osteosarcoma cells

Osteosarcoma is the most common primary bone tumor associated with childhood and adolescence. In the present study, we investigated the anticancer effect of a new isoflavone derivative, 3',4'-dichloro-3-(3,4-dichlorophenylacetyl)-2,4,6-trihydroxydeoxybenzoin (DDTD) in human osteosarcoma cells. DDTD induced cell apoptosis in human osteosarcoma cell lines (including: U2OS, MG-63, Saos2 and ROS 17/2.8). We found that the accumulation of reactive oxygen species is a critical mediator in DDTD-induced cell death. DDTD induced apoptosis signal-regulating kinase 1 (ASK1) dephosphorylation and its dissociation from 14-3-3. Treatment of osteosarcoma cells with DDTD induced p38 and p53 phosphorylation. Transfection with ASK1, mitogen activated protein kinase (MAPK) kinase (MKK)3/6, and p38 small interfering RNA (siRNA) antagonized the DDTD-induced cell apoptosis. DDTD also triggered the mitochondrial apoptotic pathway, as indicated by a change in Bax/Bcl2 ratio and Caspase-9 activation. Bax knockdown using a Bax siRNA strategy reduced Bax expression and subsequent cell death. In addition, transfection of cells with ASK1, MKK3/6, and p38 siRNA reduced DDTD-induced p38 activation, p53 phosphorylation and Bax expression. These results suggest that DDTD generates reactive oxygen species and activates the ASK1-MKK3/6-p38-p53-Bax pathway to cause osteosarcoma cell death.

HBP21: a novel member of TPR motif family, as a potential chaperone of heat shock protein 70 in proliferative vitreoretinopathy (PVR) and breast cancer

A large number of tetratricopeptide repeat (TPR)-containing proteins have been shown to interact with the C-terminal domain of the 70 kDa heat-shock protein (Hsp70), especially those with three consecutive TPR motifs. The TPR motifs in these proteins are necessary and sufficient for mediating the interaction with Hsp70. Here, we investigate HBP21, a novel human protein of unknown function having three tandem TPR motifs predicted by computational sequence analysis. We confirmed the high expression of HBP21 in breast cancer and proliferative vitreoretinopathy (PVR) proliferative membrane and examined whether HBP21 could interact with Hsp70 using a yeast two-hybrid system and glutathione S-transferase pull-down assay. Previous studies have demonstrated the importance of Hsp70 C-terminal residues EEVD and PTIEEVD for interaction with TPR-containing proteins. Here, we tested an assortment of truncation and amino acid substitution mutants of Hsp70 to determine their ability to bind to HBP21 using a yeast two-hybrid system. The newly discovered interaction between HBP21 and Hsp70 along with observations from other studies leads to our hypothesis that HBP21 may be involved in the inhibition of progression and metastasis of tumor cells.

Integrated activation of MAP3Ks balances cell fate in response to stress

In vivo, tissues and organs are exposed to numerous stressors that require cells to respond appropriately for viability and homeostasis. Cells respond to these stressors, which range from UV irradiation, heat shock, chemicals, and changes in osmolality, to oxidative stress and inflammatory cytokines, by activating pathways that protect cells from damage. If the stress is too great, cells commit to undergo apoptosis. Such cell fate decisions involve the stress-mediated activation of mitogen-activated protein kinase (MAPK) networks, ultimately under the control of MAPK kinase kinases, or MAP3Ks. It is the MAP3Ks that coordinate the localization, duration and magnitude of MAPK activation in response to cell stress. A single stressor may activate several MAP3Ks, each of which impacts the balance between survival and apoptotic signaling. In this prospect article, we review the specific MAP3Ks that integrate the physiological response to cell stressors. The interrelationships among different stressors are discussed, with an emphasis on how the balance of signaling through MAP3Ks controls the MAPK response to determine cell fate.

Apoptosis Signal-Regulating Kinase 1 in Stress and Immune Response

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase (MAPK) kinase kinase of the c-Jun N-terminal kinase (JNK) and p38 MAPK pathways. ASK1 is preferentially activated by various cytotoxic stressors and plays pivotal roles in a wide variety of cellular response to them. Recent analyses of ASK1 orthologs in Caenorhabditis elegans and Drosophila melanogaster have revealed that ASK1 is an evolutionarily conserved signaling intermediate in stress responses and appears to constitute a primitive but efficient defense system against stimuli harmful to organisms. Consistent with this notion, ASK1 has been shown to be required for the innate immune response, which is essential for host defense against a wide range of pathogens. In this review, we focus on the molecular mechanisms by which ASK1 functions in stress and immune responses and discuss the possible involvement of ASK1 in human diseases.

Isoobtusilactone A induces cell cycle arrest and apoptosis through reactive oxygen species/apoptosis signal-regulating kinase 1 signaling pathway in human breast cancer cells

This study is the first to investigate the anticancer effect of isoobtusilactone A (IOA) in two human breast cancer cell lines, MCF-7 and MDA-MB-231. IOA exhibited effective cell growth inhibition by inducing cancer cells to undergo G(2)-M phase arrest and apoptosis. Further investigation revealed that IOA's inhibition of cell growth was also evident in a nude mice model. Cell cycle blockade was associated with increased levels of p21 and reduced amounts of cyclin B1, cyclin A, cdc2, and cdc25C. IOA also enhanced the levels of inactivated phosphorylated cdc2 and cdc25C. IOA triggered the mitochondrial apoptotic pathway, as indicated by a change in Bax/Bcl-2 ratios, resulting in mitochondrial membrane potential loss, cytochrome c release, and caspase-9 activation. We also found that the generation of reactive oxygen species (ROS) is a critical mediator in IOA-induced cell growth inhibition. Enhancement of ROS by IOA activated apoptosis signal-regulating kinase 1 (ASK1) resulted in the increased activation of c-Jun NH(2)-terminal kinase and p38. Antioxidants EUK8 and N-acetyl cystenine significantly decreased apoptosis by inhibiting the ASK1 dephosphorylation at Ser(967) and subsequently increased the interaction of ASK1 with thioredoxin or 14-3-3 proteins. Moreover, blocking ASK1 by small interfering RNA inhibition completely suppressed IOA-induced apoptosis. Taken together, these results imply a critical role for ROS and ASK1 in IOA's anticancer activity.

Regulation of apoptosis signal‐regulating kinase 1 degradation by Gα13

Apoptosis signal-regulating kinase (ASK1) is a mitogen-activated protein kinase (MAPK) that transduces apoptotic signals from a variety of stresses. We have shown previously that alpha subunits of heterotrimeric G12 and G13 proteins stimulate ASK1 kinase activity and ASK1-dependent apoptosis. Here, we report a novel mechanism of G-protein-dependent regulation of ASK1. We demonstrated that G alpha13 forms a complex with ASK1 in an activation-independent manner. Both N- and C-terminal regulatory domains of ASK1 were essential for the efficient interaction, while its kinase domain was not required. Formation of the G alpha13-ASK1 complex was enhanced by JNK-interacting leucine zipper protein, JLP. Constitutively activated G alpha13Q226L increased ASK1 expression. Short-term activation of a serotonin 5-HT4 receptor that is coupled to G alpha13 also increased ASK1 expression. Importantly, prolonged activation of 5-HT4 receptor in COS-7 cells or prolonged treatment of human umbilical vein endothelial cells with thrombin concomitantly down-regulated both G alpha13 and ASK1. Data showed that G alpha13Q226L reduced the rate of ASK1 degradation, decreased ASK1 ubiquitination, and reduced association of ASK1 with an E3 ubiquitin ligase CHIP, previously shown to mediate ASK1 degradation. Our findings indicate that ASK1 expression levels can be regulated by G alpha13, at least in part via control of ASK1 ubiquitination and degradation.

CHIP overexpression reduces mutant androgen receptor protein and ameliorates phenotypes of the spinal and bulbar muscular atrophy transgenic mouse model

Spinal and bulbar muscular atrophy (SBMA) is an inherited motor neuron disease caused by the expansion of a polyglutamine tract within the androgen receptor (AR). The pathologic features of SBMA are motor neuron loss in the spinal cord and brainstem and diffuse nuclear accumulation and nuclear inclusions of the mutant AR in the residual motor neurons and certain visceral organs. Many components of the ubiquitin-proteasome and molecular chaperones are also sequestered in the inclusions, suggesting that they may be actively engaged in an attempt to degrade or refold the mutant AR. C terminus of Hsc70 (heat shock cognate protein 70)-interacting protein (CHIP), a U-box type E3 ubiquitin ligase, has been shown to interact with heat shock protein 90 (Hsp90) or Hsp70 and ubiquitylates unfolded proteins trapped by molecular chaperones and degrades them. Here, we demonstrate that transient overexpression of CHIP in a neuronal cell model reduces the monomeric mutant AR more effectively than it does the wild type, suggesting that the mutant AR is more sensitive to CHIP than is the wild type. High expression of CHIP in an SBMA transgenic mouse model also ameliorated motor symptoms and inhibited neuronal nuclear accumulation of the mutant AR. When CHIP was overexpressed in transgenic SBMA mice, mutant AR was also preferentially degraded over wild-type AR. These findings suggest that CHIP overexpression ameliorates SBMA phenotypes in mice by reducing nuclear-localized mutant AR via enhanced mutant AR degradation. Thus, CHIP overexpression would provide a potential therapeutic avenue for SBMA.

CHIP and HSPs interact with beta-APP in a proteasome-dependent manner and influence Abeta metabolism

The C-terminus Hsp70 interacting protein (CHIP) has dual function as both co-chaperone and ubiquitin ligase. CHIP is increasingly implicated in the biology of polyglutamine expansion disorders, Parkinson's disease and tau protein in Alzheimer's disease. We investigated the involvement of CHIP in the metabolism of the beta-amyloid precursor protein and its derivative beta-amyloid (Abeta). Using immunoprecipitation, fluorescence localization and crosslinking methods, endogenous CHIP and betaAPP interact in brain and cultured skeletal myotubes as well as when they are expressed in stable HEK cell lines. Their interaction is confined to Golgi and ER compartments. In the presence of the proteasome inhibitor with MG132, endogenous and expressed betaAPP levels are significantly increased and accordingly, the interaction with CHIP enhanced. Concurrently, levels of Hsp70 were most consistently induced by proteasome inhibition among the various heat shock proteins (HSPs) tested. Thus, complexes of CHIP, Hsp70 and holo-betaAPP (as well as C-terminal fragments) were stabilized by the action of MG132. Moreover, CHIP itself is shown to both increase cellular holo-betaAPP levels and protect it from oxidative stress and degradation. Interestingly, CHIP also promotes the association of ubiquitin with betaAPP, implying that a smaller pool of betaAPP is destined for proteasomal processing. In neuronal cultures, CHIP and Hsp70/90 expression reduce steady-state cellular Abeta levels and hasten its degradation in pulse-chase experiments. The functional significance of CHIP and HSP interactions, especially with Hsp70, was tested using siRNA and in neuronal cells where protection from Abeta-induced toxicity is shown. We conclude that CHIP, as a bimolecular switch, interacts with HSP to stabilize normal holo-betaAPP on the one hand while also assisting in the ubiquitination of a subpopulation of betaAPP molecules that are destined for proteasome degradation. CHIP also hastens the clearance of Abeta in a manner consistent with its known neuroprotective properties.

Ubiquitin-proteasome degradation of serum- and glucocorticoid-regulated kinase-1 (SGK-1) is mediated by the chaperone-dependent E3 ligase CHIP

SGK-1 (serum- and glucocorticoid-regulated kinase-1) is a stress-induced serine/threonine kinase that is phosphorylated and activated downstream of PI3K (phosphoinositide 3-kinase). SGK-1 plays a critical role in insulin signalling, cation transport and cell survival. SGK-1 mRNA expression is transiently induced following cellular stress, and SGK-1 protein levels are tightly regulated by rapid proteasomal degradation. In the present study we report that SGK-1 forms a complex with the stress-associated E3 ligase CHIP [C-terminus of Hsc (heat-shock cognate protein) 70-interacting protein]; CHIP is required for both the ubiquitin modification and rapid proteasomal degradation of SGK-1. We also show that CHIP co-localizes with SGK-1 at or near the endoplasmic reticulum. CHIP-mediated regulation of SGK-1 steady-state levels alters SGK-1 kinase activity. These data suggest a model that integrates CHIP function with regulation of the PI3K/SGK-1 pathway in the stress response.