Caspases leave the beaten track: caspase-mediated activation of NF-kappaB

Potential Hepatoprotective Effects of Irbesartan, an Accessible Angiotensin II Receptor Blocker, Against Cisplatin-Induced Liver Injury in a Rat Model

Objectives

Drug-induced liver injury is a common adverse reaction that frequently occurs with chemotherapeutic agents, such as cisplatin (CIS). This study seeks to enhance our understanding of drug actions and their associated adverse effects by examining the toxicity of CIS on rat liver tissue. We aimed to investigate the potential hepatoprotective effects of irbesartan (IRB), an easily accessible angiotensin II receptor blocker, in mitigating CIS-induced hepatotoxicity.

Materials and Methods

Wistar albino rats were divided into four groups. These groups included a control group [saline, per oral (p.o.)] for seven days, and 1 mL saline intraperitoneal [(i.p.) on the fourth day]; a CIS group (1 mL saline for seven days and 7.5 mg/kg CIS i.p. on the fourth day); a CIS + IRB group (IRB: 50 mg/kg p.o. for seven days and 7.5 mg/kg CIS i.p. on the fourth day), and an IRB group (50 mg/kg IRB p.o. for seven days). The effect of IRB on interleukin-1 beta (IL-1β) and caspase 3 levels was evaluated by immunohistochemical analysis, and its effects on mRNA expression levels of CCAAT/enhancer-binding protein homologous protein (CHOP) and immunoglobulin-heavy-chain-binding protein (BiP) were tested by quantitative real-time polymerase chain reaction.

Results

IRB administration mitigated CIS-induced liver toxicity by inhibiting endoplasmic reticulum (ER) stress. Specifically, this drug reduced the mRNA expression of ER stress markers, including CHOP and BiP. In addition, IRB treatment decreased oxidative stress, inflammatory responses, and apoptotic markers.

Conclusion

These findings suggest that IRB is a promising therapeutic option for preventing CIS-induced liver injury, potentially by modulating ER stress-related pathways.

Controlling Cancer Cell Death Types to Optimize Anti-Tumor Immunity

Over several decades, cell biology research has characterized distinct forms of regulated cell death, identified master regulators such as nuclear factor kappa B (NFκB), and contributed to translating these findings in order to improve anti-cancer therapies. In the era of immunotherapy, however, the field warrants a new appraisal-the targeted induction of immunogenic cell death may offer personalized strategies to optimize anti-tumor immunity. Once again, the spotlight is on NFκB, which is not only a master regulator of cancer cell death, survival, and inflammation, but also of adaptive anti-tumor immune responses that are triggered by dying tumor cells.

The Role of Inflammasome Activation in Early HIV Infection

The inflammasome pathway is an important arm of the innate immune system that provides antiviral immunity against many viruses. The main pathways involved in virus infections include the NLRP3, IFI16, and AIM2 pathways. However, a succinct understanding of its role in HIV is not yet well elucidated. In this review, we showed that NLRP3 inflammasome activation plays a vital role in inhibiting HIV entry into target cells via the purinergic pathway; IFI16 detects intracellular HIV ssDNA, triggers interferon I and III production, and inhibits HIV transcription; and AIM2 binds to HIV dsDNA and triggers acute inflammation and pyroptosis. Remarkably, by understanding these mechanisms, new therapeutic strategies can be developed against the disease.

Human Caspase 12 Enhances NF-κB Activity through Activation of IKK in Nasopharyngeal Carcinoma Cells

Human nasopharyngeal carcinoma (NPC) is a highly invasive cancer associated with proinflammation. Caspase-12 (Casp12), an inflammatory caspase, is implicated in the regulation of NF-κB-mediated cellular invasion via the modulation of the IκBα protein in NPC cells. However, the effect mechanisms of Casp12 need to be elucidated. NPC cells were transfected with the full length of human Casp12 cDNA (pC12) and the effect of human Casp12 (hCasp12) on the NF-κB activity was investigated. We found ectopic expression of hCasp12 increased the NF-κB activity accompanied by an increased p-IκBα expression and a decreased IκBα expression. Treatment of BMS, a specific IKK inhibitor, and pC12-transfected cells markedly decreased the NF-κB activity and ameliorated the expression level of IκBα reduced by hCasp12. Co-immunoprecipitation assays validated the physical interaction of hCasp12 with IKKα/β, but not with NEMO. Furthermore, the NF-κB activity of ΔCasp12-Q (a mutated catalytic of hCasp12) transfected cells was concentration-dependently induced, but lower than that of hCasp12-transfected cells. Importantly, the hCasp12-mediated NF-kB activity was enhanced by TNFα stimulation. That indicated a role of the catalytic motif of hCasp12 in the regulation of the NF-κB activity. This study indicated hCasp12 activated the NF-κB pathway through the activation of IKK in human NPC cells.

The role of caspase-8 in the tumor microenvironment of ovarian cancer

Caspase-8 is an aspartate-specific cysteine protease, which is best known for its apoptotic functions. Caspase-8 is placed at central nodes of multiple signal pathways, regulating not only the cell cycle but also the invasive and metastatic cell behavior, the immune cell homeostasis and cytokine production, which are the two major components of the tumor microenvironment (TME). Ovarian cancer often has dysregulated caspase-8 expression, leading to imbalance between its apoptotic and non-apoptotic functions within the tumor and the surrounding milieu. The downregulation of caspase-8 in ovarian cancer seems to be linked to high aggressiveness with chronic inflammation, immunoediting, and immune resistance. Caspase-8 plays therefore an essential role not only in the primary tumor cells but also in the TME by regulating the immune response, B and T lymphocyte activation, and macrophage differentiation and polarization. The switch between M1 and M2 macrophages is possibly associated with changes in the caspase-8 expression. In this review, we are discussing the non-apoptotic functions of caspase-8, highlighting this protein as a modulator of the immune response and the cytokine composition in the TME. Considering the low survival rate among ovarian cancer patients, it is urgently necessary to develop new therapeutic strategies to optimize the response to the standard treatment. The TME is highly heterogenous and provides a variety of opportunities for new drug targets. Given the variety of roles of caspase-8 in the TME, we should focus on this protein in the development of new therapeutic strategies against the TME of ovarian cancer.

Role of Inflammasomes in HIV-1 and Drug Abuse Mediated Neuroinflammaging

Despite the effectiveness of combined antiretroviral therapy (cART) in suppressing virus replication, chronic inflammation remains one of the cardinal features intersecting HIV-1, cART, drug abuse, and likely contributes to the accelerated neurocognitive decline and aging in people living with HIV-1 (PLWH) that abuse drugs. It is also estimated that ~30–60% of PLWH on cART develop cognitive deficits associated with HIV-1-associated neurocognitive disorders (HAND), with symptomatology ranging from asymptomatic to mild, neurocognitive impairments. Adding further complexity to HAND is the comorbidity of drug abuse in PLWH involving activated immune responses and the release of neurotoxins, which, in turn, mediate neuroinflammation. Premature or accelerated aging is another feature of drug abusing PLWH on cART regimes. Emerging studies implicate the role of HIV-1/HIV-1 proteins, cART, and abused drugs in altering the inflammasome signaling in the central nervous system (CNS) cells. It is thus likely that exposure of these cells to HIV-1/HIV-1 proteins, cART, and/or abused drugs could have synergistic/additive effects on the activation of inflammasomes, in turn, leading to exacerbated neuroinflammation, ultimately resulting in premature aging referred to as “inflammaging” In this review, we summarize the current knowledge of inflammasome activation, neuroinflammation, and aging in central nervous system (CNS) cells such as microglia, astrocytes, and neurons in the context of HIV-1 and drug abuse.

The Role of Caspase-2 in Regulating Cell Fate

Caspase-2 is the most evolutionarily conserved member of the mammalian caspase family and has been implicated in both apoptotic and non-apoptotic signaling pathways, including tumor suppression, cell cycle regulation, and DNA repair. A myriad of signaling molecules is associated with the tight regulation of caspase-2 to mediate multiple cellular processes far beyond apoptotic cell death. This review provides a comprehensive overview of the literature pertaining to possible sophisticated molecular mechanisms underlying the multifaceted process of caspase-2 activation and to highlight its interplay between factors that promote or suppress apoptosis in a complicated regulatory network that determines the fate of a cell from its birth and throughout its life.

Cell death-independent activities of the death receptors CD95, TRAILR1, and TRAILR2

Since their identification more than 20 years ago, the death receptors CD95, TRAILR1, and TRAILR2 have been intensively studied with respect to their cell death-inducing activities. These receptors, however, can also trigger a variety of cell death-independent cellular responses reaching from the activation of proinflammatory gene transcription programs over the stimulation of proliferation and differentiation to induction of cell migration. The cell death-inducing signaling mechanisms of CD95 and the TRAIL death receptors are well understood. In contrast, despite the increasing recognition of the biological and pathophysiological relevance of the cell death-independent activities of CD95, TRAILR1, and TRAILR2, the corresponding signaling mechanisms are less understood and give no fully coherent picture. This review is focused on the cell death-independent activities of CD95 and the TRAIL death receptors and addresses mainly three questions: (a) how are these receptors linked to noncell death pathways at the molecular level, (b) which factors determine the balance of cell death and cell death-independent activities of CD95 and the TRAIL death receptors at the cellular level, and (c) what are the consequences of the cell death-independent functions of these receptors for their role in cancer and inflammatory diseases.

Nuclear translocation of nuclear factor kappa B is regulated by G protein signaling pathway in arsenite-induced apoptosis in HBE cell line

Arsenite is a certainly apoptosis inducer in various cell types. However, the detailed mechanism underlying how arsenite trigger apoptosis remains elusive. In this study, using human bronchial epithelial cell as a culture system, we demonstrated that arsenite-induced nuclear translocation of nuclear factor kappa B (NF-κB) resulted in the release of cytochrome c, the modulation of Fas and FasL, caspase activation, and ultimately leading to cell apoptosis. Importantly, we showed for the first time that the NF-κB-mediated apoptosis induced by arsenite was regulated by G protein-adenylate cyclase (AC)-cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) pathway. Inhibition of this classical G protein signaling pathway by a typical PKA inhibitor, H-89, caused the inactivation of NF-κB, the depletion of caspase-3, 8 and 9 activities, and thus reducing the level of cell apoptosis. Taken together, our results indicate that arsenite is able to trigger cell apoptosis in human bronchial epithelial cells through the nuclear translocation of NF-κB, which can be modulated by G protein signaling pathway. These findings further suggest that inhibition of G protein-mediated pathway by specific inhibitors may be a potential strategy for the prevention of arsenite toxicity. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1819-1833, 2016.

Tumstatin induces apoptosis and stimulates phosphorylation of p65NF-κB in human osteoblastic osteosarcoma Saos-2 cells

The present study was aimed to investigate the effect of tumstatin on inhibition of proliferation and induction of apoptosis in Saos-2 human osteosarcoma cells and to understand the mechanism involved. Inhibition of cell proliferation was analyzed by MTT assay and induction of apoptosis through nuclear fragmentation assay. Viability of Saos-2 cells was reduced to 19% on treatment with 25 µM concentration of tumstatin after 48 h. Presence of characteristic apoptotic nuclei, rounded cell shape and shrunken size were caused by tumstatin treatment at 25 µM concentration. The level of mRNA corresponding to PTEN, FasR and FasL was increased significantly in tumstatin treated Saos-2 cells compared to untreated control. Investigation of the mechanism revealed NF-κB activation by phosphorylation on serine 536. The activated NF-κB was translocated into the nucleus from the cytoplasm on treatment with tumstatin. Degradation of the IκBα by tumstatin was found to be much slower compared to that induced by treatment with TNF-α. Thus, tumstatin inhibits proliferation and induces apoptosis in Saos-2 cells through activation of NF-κB and its translocation to the nucleus. Therefore, tumstatin can play an important role in the treatment of osteosarcoma.

RIP1 Cleavage in the Kinase Domain Regulates TRAIL-Induced NF-κB Activation and Lymphoma Survival

Although TRAIL is considered a potential anticancer agent, it enhances tumor progression by activating NF-κB in apoptosis-resistant cells. Cellular FLICE-like inhibitory protein (cFLIP) overexpression and caspase-8 activation have been implicated in TRAIL-induced NF-κB activation; however, the underlying mechanisms are unknown. Here, we report that caspase-8-dependent cleavage of RIP1 in the kinase domain (KD) and intermediate domain (ID) determines the activation state of the NF-κB pathway in response to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) treatment. In apoptosis-sensitive cells, caspase-8 cleaves RIP1 in the KD and ID immediately after the recruitment of RIP1 to the receptor complex, impairing IκB kinase (IKK) recruitment and NF-κB activation. In apoptosis-resistant cells, cFLIP restricts caspase-8 activity, resulting in limited RIP1 cleavage and generation of a KD-cleaved fragment capable of activating NF-κB but not apoptosis. Notably, depletion of the cytoplasmic pool of TRAF2 and cIAP1 in lymphomas by CD40 ligation inhibits basal RIP1 ubiquitination but does not prompt cell death, due to CD40L-induced cFLIP expression and limited RIP1 cleavage. Inhibition of RIP1 cleavage at the KD suppresses NF-κB activation and cell survival even in cFLIP-overexpressing lymphomas. Importantly, RIP1 is constitutively cleaved in human and mouse lymphomas, suggesting that cFLIP-mediated and caspase-8-dependent limited cleavage of RIP1 is a new layer of mechanism that promotes NF-κB activation and lymphoma survival.

Executioner Caspase-3 and 7 Deficiency Reduces Myocyte Number in the Developing Mouse Heart

Executioner caspase-3 and -7 are proteases promoting cell death but non-apoptotic roles are being discovered. The heart expresses caspases only during development, suggesting they contribute to the organ maturation process. Therefore, we aimed at identifying novel functions of caspases in heart development. We induced simultaneous deletion of executioner caspase-3 and -7 in the mouse myocardium and studied its effects. Caspase knockout hearts are hypoplastic at birth, reaching normal weight progressively through myocyte hypertrophy. To identify the molecular pathways involved in these effects, we used microarray-based transcriptomics and multiplexed quantitative proteomics to compare wild type and executioner caspase-deficient myocardium at different developmental stages. Transcriptomics showed reduced expression of genes promoting DNA replication and cell cycle progression in the neonatal caspase-deficient heart suggesting reduced myocyte proliferation, and expression of non-cardiac isoforms of structural proteins in the adult null myocardium. Proteomics showed reduced abundance of proteins involved in oxidative phosphorylation accompanied by increased abundance of glycolytic enzymes underscoring retarded metabolic maturation of the caspase-null myocardium. Correlation between mRNA expression and protein abundance of relevant genes was confirmed, but transcriptomics and proteomics indentified complementary molecular pathways influenced by caspases in the developing heart. Forced expression of wild type or proteolytically inactive caspases in cultured cardiomyocytes induced expression of genes promoting cell division. The results reveal that executioner caspases can modulate heart’s cellularity and maturation during development, contributing novel information about caspase biology and heart development.

Old, new and emerging functions of caspases

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

TRAIL activates JNK and NF-κB through RIP1-dependent and -independent pathways

The death receptor (DR) ligand TRAIL is being evaluated in clinical trials as an anti-cancer agent; however, many studies have found that TRAIL also enhances tumor progression by activating the NF-κB pathway in apoptosis-resistant cells. Although RIP1, cFLIP and caspase-8 have been implicated in TRAIL-induced JNK and NF-κB activation, underlying mechanisms are unclear. By examining the kinetics of pathway activation in TRAIL-sensitive lymphoma cells wild-type or deficient for RIP1, TRAF2, cIAP1/2 or HOIP, we report here that TRAIL induces two phases of JNK and NF-κB activation. The early phase is activated by TRAF2- and cIAP1-mediated ubiquitination of RIP1, whereas the delayed phase is induced by caspase-dependent activation of MEKK1 independent of RIP1 and TRAF2 expression. cFLIP overexpression promotes the early phase but completely suppresses the delayed phase of pathway activation in lymphoma cells, whereas Bcl-2 overexpression promotes both the early and delayed phases of the pathways. In addition, stable overexpression of cFLIP in RIP1- or TRAF2-deficient cells confers resistance to apoptosis, but fails to mediate NF-κB activation. HOIP is not essential for, but contributes to, TRAIL-induced NF-κB activation in cFLIP-overexpressing cells. These findings not only elucidate details of the mechanisms underlying TRAIL-induced JNK and NF-κB activation, but also clarify conflicting reports in the field.

Putting the pieces together: How is the mitochondrial pathway of apoptosis regulated in cancer and chemotherapy?

In order to solve a jigsaw puzzle, one must first have the complete picture to logically connect the pieces. However, in cancer biology, we are still gaining an understanding of all the signaling pathways that promote tumorigenesis and how these pathways can be pharmacologically manipulated by conventional and targeted therapies. Despite not having complete knowledge of the mechanisms that cause cancer, the signaling networks responsible for cancer are becoming clearer, and this information is serving as a solid foundation for the development of rationally designed therapies. One goal of chemotherapy is to induce cancer cell death through the mitochondrial pathway of apoptosis. Within this review, we present the pathways that govern the cellular decision to undergo apoptosis as three distinct, yet connected puzzle pieces: (1) How do oncogene and tumor suppressor pathways regulate apoptosis upstream of mitochondria? (2) How does the B-cell lymphoma 2 (BCL-2) family influence tumorigenesis and chemotherapeutic responses? (3) How is post-mitochondrial outer membrane permeabilization (MOMP) regulation of cell death relevant in cancer? When these pieces are united, it is possible to appreciate how cancer signaling directly impacts upon the fundamental cellular mechanisms of apoptosis and potentially reveals novel pharmacological targets within these pathways that may enhance chemotherapeutic success.

TCR-induced, PKC-θ-mediated NF-κB activation is regulated by a caspase-8-caspase-9-caspase-3 cascade

It has been documented that caspase-8, a central player in apoptosis, is also crucial for TCR-mediated NF-κB activation. However, whether other caspases are also involved this process is unknown. In this report, we showed that in addition to caspase-8, caspase-9 is required for TCR-mediated NF-κB activation. Caspase-9 induces activation of PKC-θ, phosphorylation of Bcl10 and NF-κB activation in a caspase-3-dependent manner, but it appears that Bcl10 phosphorylation is uncoupled from NF-κB activation. Furthermore, caspase-8 lies upstream of caspase-9 during T cell activation. Therefore, TCR ligation elicits a caspase cascade involving caspase-8, caspase-9 and caspase-3 which initiates PKC-θ-dependent pathway leading to NF-κB activation and PKC-θ-independent Bcl10 phosphorylation which limits NF-kB activity.

Poly(ADP-ribose) polymerase-1 and its cleavage products differentially modulate cellular protection through NF-kappaB-dependent signaling

Poly(ADP-ribose) polymerase-1 (PARP-1) and its cleavage products regulate cell viability and NF-kappaB activity when expressed in neurons. PARP-1 cleavage generates a 24 kDa (PARP-1(24)) and an 89 kDa fragment (PARP-1(89)). Compared to WT (PARP-1WT), the expression of an uncleavable PARP-1 (PARP-1(UNCL)) or of PARP-1(24) conferred protection from oxygen/glucose deprivation (OGD) or OGD/restoration of oxygen and glucose (ROG) damage in vitro, whereas expression of PARP-1(89) was cytotoxic. Viability experiments were performed in SH-SY5Y, a human neuroblastoma cell line, as well as in rat primary cortical neurons. Following OGD, the higher viability in the presence of PARP-1UNCL or PARP-1(24) was not accompanied with decreased formation of poly(ADP-riboses) or higher NAD levels. PARP-1 is a known cofactor for NF-kappaB, hence we investigated whether PARP-1 cleavage influences the inflammatory response. All PARP-1 constructs mimicked PARP-1WT in regard to induction of NF-kappaB translocation into the nucleus and its increased activation during ischemic challenge. However, expression of PARP-1(89) construct induced significantly higher NF-kB activity than PARP-1WT; and the same was true for NF-kappaB-dependent iNOS promoter binding activity. At a protein level, PARP-1UNCL and PARP-1(24) decreased iNOS (and lower levels of iNOS transcript) and COX-2, and increased Bcl-xL The increased levels of NF-kB and iNOS transcriptional activities, seen with cytotoxic PARP-189, were accompanied by higher protein expression of COX-2 and iNOS (and higher levels of INOS transcript) and lower protein expression of Bcl-xL Taken together, these findings suggest that PARP-1 cleavage products may regulate cellular viability and inflammatory responses in opposing ways during in vitro models of "ischemia".

Intrinsic-mediated caspase activation is essential for cardiomyocyte hypertrophy

Cardiomyocyte hypertrophy is the cellular response that mediates pathologic enlargement of the heart. This maladaptation is also characterized by cell behaviors that are typically associated with apoptosis, including cytoskeletal reorganization and disassembly, altered nuclear morphology, and enhanced protein synthesis/translation. Here, we investigated the requirement of apoptotic caspase pathways in mediating cardiomyocyte hypertrophy. Cardiomyocytes treated with hypertrophy agonists displayed rapid and transient activation of the intrinsic-mediated cell death pathway, characterized by elevated levels of caspase 9, followed by caspase 3 protease activity. Disruption of the intrinsic cell death pathway at multiple junctures led to a significant inhibition of cardiomyocyte hypertrophy during agonist stimulation, with a corresponding reduction in the expression of known hypertrophic markers (atrial natriuretic peptide) and transcription factor activity [myocyte enhancer factor-2, nuclear factor kappa B (NF-κB)]. Similarly, in vivo attenuation of caspase activity via adenoviral expression of the biologic effector caspase inhibitor p35 blunted cardiomyocyte hypertrophy in response to agonist stimulation. Treatment of cardiomyocytes with procaspase 3 activating compound 1, a small-molecule activator of caspase 3, resulted in a robust induction of the hypertrophy response in the absence of any agonist stimulation. These results suggest that caspase-dependent signaling is necessary and sufficient to promote cardiomyocyte hypertrophy. These results also confirm that cell death signal pathways behave as active remodeling agents in cardiomyocytes, independent of inducing an apoptosis response.

Non-apoptotic function of caspases in a cellular model of hydrogen peroxide-associated colitis

Oxidative stress, caused by reactive oxygen species (ROS), is a major contributor to inflammatory bowel disease (IBD)-associated neoplasia. We mimicked ROS exposure of the epithelium in IBD using non-tumour human colonic epithelial cells (HCEC) and hydrogen peroxide (H2 O2 ). A population of HCEC survived H2 O2 -induced oxidative stress via JNK-dependent cell cycle arrests. Caspases, p21(WAF1) and γ-H2AX were identified as JNK-regulated proteins. Up-regulation of caspases was linked to cell survival and not, as expected, to apoptosis. Inhibition using the pan-caspase inhibitor Z-VAD-FMK caused up-regulation of γ-H2AX, a DNA-damage sensor, indicating its negative regulation via caspases. Cell cycle analysis revealed an accumulation of HCEC in the G1 -phase as first response to oxidative stress and increased S-phase population and then apoptosis as second response following caspase inhibition. Thus, caspases execute a non-apoptotic function by promoting cells through G1 - and S-phase by overriding the G1 /S- and intra-S checkpoints despite DNA-damage. This led to the accumulation of cells in the G2 /M-phase and decreased apoptosis. Caspases mediate survival of oxidatively damaged HCEC via γ-H2AX suppression, although its direct proteolytic inactivation was excluded. Conversely, we found that oxidative stress led to caspase-dependent proteolytic degradation of the DNA-damage checkpoint protein ATM that is upstream of γ-H2AX. As a consequence, undetected DNA-damage and increased proliferation were found in repeatedly H2 O2 -exposed HCEC. Such features have been associated with neoplastic transformation and appear here to be mediated by a non-apoptotic function of caspases. Overexpression of upstream p-JNK in active ulcerative colitis also suggests a potential importance of this pathway in vivo.

High-fat diet feeding causes rapid, non-apoptotic cleavage of caspase-3 in astrocytes

Astrocytes respond to multiple forms of central nervous system (CNS) injury by entering a reactive state characterized by morphological changes and a specific pattern of altered protein expression. Termed astrogliosis, this response has been shown to strongly influence the injury response and functional recovery of CNS tissues. This pattern of CNS inflammation and injury associated with astrogliosis has recently been found to occur in the energy homeostasis centers of the hypothalamus during diet-induced obesity (DIO) in rodent models, but the characterization of the astrocyte response remains incomplete. Here, we report that astrocytes in the mediobasal hypothalamus respond robustly and rapidly to purified high-fat diet (HFD) feeding by cleaving caspase-3, a protease whose cleavage is often associated with apoptosis. Although obesity develops in HFD-fed rats by day 14, caspase-3 cleavage occurs by day 3, prior to the development of obesity, suggesting the possibility that it could play a causal role in the hypothalamic neuropathology and fat gain observed in DIO. Caspase-3 cleavage is not associated with an increase in the rate of apoptosis, as determined by TUNEL staining, suggesting it plays a non-apoptotic role analogous to the response to excitotoxic neuron injury. Our results indicate that astrocytes in the mediobasal hypothalamus respond rapidly and robustly to HFD feeding, activating caspase-3 in the absence of apoptosis, a process that has the potential to influence the course of DIO.

Caspase activation contributes to astrogliosis

Caspases, a family of cysteine proteases, are widely activated in neurons and glia in the injured brain, a response thought to induce apoptosis. However, caspase activation in astrocytes following injury is not strongly associated with apoptosis. The present study investigates the potential role of caspase activation in astrocytes with another characteristic response to neural injury, astrogliosis. Caspase activity and morphological and biochemical indices of astrogliosis and apoptosis were assessed in (i) cultured neonatal rat astrocytes treated with astrogliosis-inducing stimuli (dibutryl cAMP, β-amyloid peptide), and (ii) cultures of adult rat hippocampal astrocytes generated from control and kainate-lesioned rats. The effects of broad spectrum and specific pharmacological caspase inhibitors were assessed on indicators of astrogliosis, including stellate morphology and expression of glutamine synthetase and fibroblast growth factor-2. Reactive neonatal and adult astrocytes demonstrated an increase in total caspase activity with a corresponding increase in the expression of active caspase-3 in the absence of cell death. Broad spectrum caspase inhibition with zVAD significantly attenuated increases in glutamine synthetase and fibroblast growth factor-2 in the reactive astrocytes. In the reactive neonatal astrocyte cultures, specific inhibition of caspases-3 and -11 also attenuated glutamine synthetase and fibroblast growth factor-2 expression, but did not reverse the morphological reactive phenotype. Astrogliosis is observed in all forms of brain injury and despite extensive study, its molecular triggers remain largely unknown. While previous studies have demonstrated active caspases in astrocytes following acute brain injury, here we present evidence functionally implicating the caspases in astrogliosis.

Deregulated inflammasome signaling in disease

Inflammasomes are multi-protein complexes that sense microbial molecules and endogenous danger signals in intracellular compartments. Inflammasome assembly results in caspase-1 activation, which in turn drives maturation and secretion of the pro-inflammatory cytokines interleukin 1β (IL-1β) and IL-18, and induces pyroptosis to eliminate the infectious agent. The importance of inflammasomes in regulating immune responses was recognized with the discovery of polymorphisms in genes encoding inflammasome components and their linkage to aberrant production of IL-1β and IL-18 in autoimmune and hereditary periodic fevers syndromes. We review the current knowledge on the role of inflammasomes in regulating innate and adaptive immune responses with an emphasis on the role of these immune complexes in autoinflammatory disorders and autoimmune diseases such as colitis, type I diabetes, multiple sclerosis and vitiligo.

Inflammasomes: caspase-1-activating platforms with critical roles in host defense

Activation of the inflammatory cysteine protease caspase-1 in inflammasome complexes plays a critical role in the host response to microbial infections. Inflammasome activation induces inflammation through secretion of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and through extracellular release of the alarmin high mobility group box 1. Moreover, caspase-1 activation by inflammasomes counters bacterial replication and induces pyroptosis, a specialized cell death program that removes infected immune cells as part of the host defense system. It is thus not surprising that bacterial and viral pathogens evolved virulence factors targeting inflammasome activation and activity. Here, we provide an overview of the distinct inflammasome complexes that are activated in a pathogen-specific manner and discuss the diverse strategies employed by viruses and bacteria to modulate inflammasome function.

Neutrophils activate alveolar macrophages by producing caspase-6-mediated cleavage of IL-1 receptor-associated kinase-M

Alveolar macrophages (AMs) are exposed to respirable microbial particles. Similar to phagocytes in the gastrointestinal tract, AMs can suppress inflammation after exposure to nonpathogenic organisms. IL-1R-associated kinase-M (IRAK-M) is one inhibitor of innate immunity, normally suppressing pulmonary inflammation. During pneumonia, polymorphonuclear neutrophils (PMNs) are recruited by chemotactic factors released by AMs to produce an intense inflammation. We report that intact IRAK-M is strongly expressed in resting human AMs but is cleaved in patients with pneumonia via PMN-mediated induction of caspase-6 (CASP-6) activity. PMN contact is necessary and PMN membranes are sufficient for CASP-6 induction in macrophages. PMNs fail to induce TNF-α fully in macrophages expressing CASP-6 cleavage-resistant IRAK-M. Without CASP-6 expression, PMN stimulation fails to cleave IRAK-M, degrade IκBα, or induce TNF-α. CASP-6(-/-) mice subjected to cecal ligation and puncture have impaired TNF-α production in the lung and decreased mortality. LPS did not induce or require CASP-6 activity demonstrating that TLR2/4 signaling is independent from the CASP-6 regulated pathway. These data define a central role for CASP-6 in PMN-driven macrophage activation and identify IRAK-M as an important target for CASP-6. PMNs de-repress AMs via CASP-6-mediated IRAK-M cleavage. This regulatory system will blunt lung inflammation unless PMNs infiltrate the alveolar spaces.

Proteasome inhibitor PS-341 (bortezomib) induces calpain-dependent IkappaB(alpha) degradation

The proteasome, a key component of the ubiquitin-proteasome pathway, has emerged as an important cancer therapeutic target. PS-341 (also called Bortezomib or Velcade) is the first proteasome inhibitor approved for newly diagnosed and relapsed multiple myeloma and is currently being tested in many clinical trials against other types of cancers. One proposed mechanism by which PS-341 exerts its anticancer effect is inactivation of nuclear factor-kappaB (NF-kappaB) through prevention of IkappaB(alpha) degradation. In this study, we show that PS-341 at concentrations that effectively inhibited the growth of human cancer cells, instead of increasing IkappaB(alpha) stability, paradoxically induced IkappaB(alpha) degradation. As a result, PS-341 facilitated p65 nuclear translocation and increased NF-kappaB activity. Moreover, IkappaB(alpha) degradation by PS-341 occurred early before induction of apoptosis and could not be inhibited by a pan-caspase inhibitor or caspase-8 silencing; however, it could be prevented with calpain inhibitors, calcium-chelating agents, calpain knockdown, or calpastatin overexpression. In agreement, PS-341 increased calpain activity. These data together indicate that PS-341 induces a calpain-mediated IkappaB(alpha) degradation independent of caspases. In the presence of a calpain inhibitor, the apoptosis-inducing activity of PS-341 was dramatically enhanced. Collectively, these unexpected findings suggest not only a novel paradigm regarding the relationship between proteasome inhibition and NF-kappaB activity but also a strategy to enhance the anticancer efficacy of PS-341.

TNF induces caspase-dependent inflammation in renal endothelial cells through a Rho- and myosin light chain kinase-dependent mechanism

The pathogenesis of LPS-induced acute kidney injury (AKI) requires signaling through tumor necrosis factor-alpha (TNF) receptor 1 (TNFR1), which within the kidney is primarily located in the endothelium. We showed previously that caspase inhibition protected mice against LPS-induced AKI and in parallel significantly inhibited LPS-induced renal inflammation. Therefore we hypothesized that caspase activation amplifies TNF-induced inflammation in renal endothelial cells (ECs). In cultured renal ECs, TNF induced apoptosis through a caspase-8-dependent pathway. TNF caused translocation of the p65 subunit of NF-kappaB to the nucleus, resulting in upregulation of inflammatory markers such as adhesion molecules ICAM-1 and VCAM-1. However, the broad-spectrum caspase inhibitor Boc-d-fmk reduced NF-kB activation as assessed by gel shift assay, reduced phosphorylation of subunit IkappaBalpha, and significantly inhibited TNF-induced expression of ICAM-1 and VCAM-1 as assessed by both real-time PCR and flow cytometry. Broad-spectrum caspase inhibition markedly inhibited neutrophil adherence to the TNF-activated endothelial monolayer, supporting the functional significance of this effect. Specific inhibitors of caspases-8 and -3, but not of caspase-1, reduced TNF-induced NF-kappaB activation. Caspase inhibition also reduced TNF-induced myosin light chain (MLC)-2 phosphorylation, and activation of upstream regulator RhoA. Consistent with this, MLC kinase (MLCK) inhibitor ML-7 reduced TNF-induced NF-kappaB activation. Thus caspase activation influences NF-kappaB signaling via its affect on cytoskeletal changes occurring through RhoA and MLCK pathways. These cell culture experiments support a role for caspase activation in TNF-induced inflammation in the renal endothelium, a key event in LPS-induced AKI.

Viral subversion of apoptotic enzymes: escape from death row

To prolong cell viability and facilitate replication, viruses have evolved multiple mechanisms to inhibit the host apoptotic response. Cellular proteases such as caspases and serine proteases are instrumental in promoting apoptosis. Thus, these enzymes are logical targets for virus-mediated modulation to suppress cell death. Four major classes of viral inhibitors antagonize caspase function: serpins, p35 family members, inhibitor of apoptosis proteins, and viral FLICE-inhibitory proteins. Viruses also subvert activity of the serine proteases, granzyme B and HtrA2/Omi, to avoid cell death. The combined efforts of viruses to suppress apoptosis suggest that this response should be avoided at all costs. However, some viruses utilize caspases during replication to aid virus protein maturation, progeny release, or both. Hence, a multifaceted relationship exists between viruses and the apoptotic response they induce. Examination of these interactions contributes to our understanding of both virus pathogenesis and the regulation of apoptotic enzymes in normal cellular functions.

Caspases - an update

Caspases belong to a family of highly conserved aspartate-specific cysteine proteases and are members of the interleukin-1beta-converting enzyme family, present in multicellular organisms. The caspase gene family consists of 15 mammalian members that are grouped into two major sub-families, namely inflammatory caspases and apoptotic caspases. The apoptotic caspases are further subdivided into two sub-groups, initiator caspases and executioner caspases. The caspases form a caspase-cascade system that plays the central role in the induction, transduction and amplification of intracellular apoptotic signals for cell fate determination, regulation of immunity, and cellular proliferation and differentiation. The substrates of apoptotic caspases have been associated with cellular dismantling, while inflammatory caspases mediate the proteolytic activation of inflammatory cytokines. The activation of this delicate caspase-cascade system and its functions are regulated by a variety of regulatory molecules, such as the inhibitor of apoptosis protein (IAP), FLICE, calpain, and Ca(2+). Based on the available literature we have reviewed and discussed the members of the caspase family, caspase-cascade system, caspase-regulating molecules and their apoptotic and non-apoptotic functions in cellular life and death. Also recent progress in the molecular structure and physiological role of non-mammalian caspases such as paracaspases, metacaspases and caspase-like-protease family members are included in relation to that of mammalian species.

Caspase-4 interacts with TNF receptor-associated factor 6 and mediates lipopolysaccharide-induced NF-kappaB-dependent production of IL-8 and CC chemokine ligand 4 (macrophage-inflammatory protein-1 )

Human caspase-4 does not have a corresponding mouse ortholog. Caspase-4 falls within the class of "inflammatory caspases," being homologous with human caspases 1 and 5 and mouse caspases 1, 11, and 12. To address the function of caspase-4, we generated caspase-4-deficient human THP1 monocytic cell lines which exhibited substantially reduced LPS-induced secretion of several chemokines and cytokines, including IL-8 (CXCL8), CCL4 (macrophage-inflammatory protein-1beta), CCL20 (macrophage-inflammatory protein-3alpha), and IL-1beta. The LPS-induced expression of the mRNAs encoding these cytokines was correspondingly reduced in the caspase-4-deficient clones. Because a specific NF-kappaB inhibitor blocked LPS-induced IL-8 and CCL4 mRNA expression as well as IL-8 and CCL4 secretion in THP1 cells, we investigated the role of caspase-4 in NF-kappaB signaling. LPS-induced NF-kappaB nuclear translocation and activation were inhibited in all caspase-4-deficient clones. LPS stimulation led to the interaction of endogenous caspase-4 and TNFR-associated factor 6 (TRAF6) via a TRAF6-binding motif (PPESGE), which we identified in caspase-4. Mutation of this site in caspase-4 resulted in the loss of the TRAF6-caspase-4 interaction. Similar TRAF6-binding motifs are known to be functionally important for TRAF6 interactions with other molecules including caspase-8, and for mediating NF-kappaB activation in various immune and nonimmune cell types. Our data suggest that the TRAF6-caspase-4 interaction, triggered by LPS, leads to NF-kappaB-dependent transcriptional up-regulation and secretion of important cytokines and chemokines in innate immune signaling in human monocytic cells.

Innate immunity meets with cellular stress at the IKK complex: regulation of the IKK complex by HSP70 and HSP90

Several research models have shown that if cellular stress induces the heat shock response then this will suppress the NF-kappaB-mediated inflammatory response. The NF-kappaB signaling pathway mediates both stress signals and innate immunity signals. Heat shock proteins HSP70 and HSP90 regulate several signaling cascades to maintain cellular homeostasis. Recent studies have revealed that HSP70 and HSP90 proteins regulate the function of the IKK complex which is the major activator of the NF-kappaB complex. The heat shock response can cause the dissociation of the IKK complex, composed of protein kinase subunits IKKalpha and IKKbeta and the regulatory unit NEMO, and inhibit the activation of NF-kappaB signaling. Suppression of immune signaling during cellular stress may be a useful feedback response for helping cells to survive tissue injury. Furthermore, IKKalpha and IKKbeta kinases are important activators of tumorigenesis and hence the inhibition of long-term activation of the IKK complex by HSP70 and HSP90 proteins may prevent cancer development during chronic inflammation.

Signaling perturbations induced by invading H. pylori proteins in the host epithelial cells: a mathematical modeling approach

Helicobacter pylori (H. pylori), a gram-negative bacterium, infects the stomach of approximately 50% of the world population. H. pylori infection is a risk factor for developing chronic gastric ulcers and gastric cancer. The bacteria produce two main cytotoxic proteins: Vacuolating cytotoxin A (VacA) and Cytotoxin-Associated gene A (CagA). When these proteins enter the host cell they interfere with the host MAP Kinase and Apoptosis signaling pathways leading to aberrant cell growth and premature apoptosis. The present study expanded existing quantitative models of the MAP Kinase and Apoptosis signaling pathways to take into account the protein interactions across species using the CellDesigner tool. The resulting network contained hundreds of differential equations in which the coefficients for the biochemical rate constants were estimated from previously published studies. The effect of VacA and CagA on the function of this network were simulated by increasing levels of bacterial load. Simulations showed that increasing bacterial load affected the MAP Kinase signaling in a dose dependant manner. The introduction of CagA decreased the activation time of mapK signaling and extended activation indefinitely despite normal cellular activity to deactivate the protein. Introduction of VacA produced a similar response in the apoptosis pathway. Bacterial load activated both pathways even in the absence of external stimulation. Time course of emergence of transcription factors associated with cell division and cell death predicted by our simulation showed close agreement with that determined from a publicly accessible microarray data set of H. pylori infected stomach epithelium. The quantitative model presented in this study lays the foundation for investigating the affects of single nucleotide polymorphisms (SNPs) on the efficiency of drug treatment.

RIP1, a kinase on the crossroads of a cell's decision to live or die

Binding of inflammatory cytokines to their receptors, stimulation of pathogen recognition receptors by pathogen-associated molecular patterns, and DNA damage induce specific signalling events. A cell that is exposed to these signals can respond by activation of NF-kappaB, mitogen-activated protein kinases and interferon regulatory factors, resulting in the upregulation of antiapoptotic proteins and of several cytokines. The consequent survival may or may not be accompanied by an inflammatory response. Alternatively, a cell can also activate death-signalling pathways, resulting in apoptosis or alternative cell death such as necrosis or autophagic cell death. Interplay between survival and death-promoting complexes continues as they compete with each other until one eventually dominates and determines the cell's fate. RIP1 is a crucial adaptor kinase on the crossroad of these stress-induced signalling pathways and a cell's decision to live or die. Following different upstream signals, particular RIP1-containing complexes are formed; these initiate only a limited number of cellular responses. In this review, we describe how RIP1 acts as a key integrator of signalling pathways initiated by stimulation of death receptors, bacterial or viral infection, genotoxic stress and T-cell homeostasis.

Dual Role of Intrauterine Immune Challenge on Neonatal and Adult Brain Vulnerability to Hypoxia-Ischemia

Epidemiologic evidence has underlined the impact of prenatal inflammation on the development of postnatal hypoxia-ischemia (HI) brain injury. To study to what extent prenatal inflammation affects CNS vulnerability later during development, C57BL/6 mice were subjected to intrauterine injection of lipopolysaccharide (LPS) at gestational day 15. At postnatal day (PND) 5, 9, and 70, the offspring were subjected to HI. It was found that, in neonatal mice, LPS-exposed brains showed markedly enhanced brain injury after HI, whereas in adult mice, LPS exposure resulted in a significant reduction in tissue loss after HI. Reduced myelin in subcortical white matter was noticed after HI in the LPS-exposed brains at PND14 and PND75. Increased activities of nuclear factor-kappaB and caspase-3 were obtained in fetal/neonatal brain after LPS administration. Conclusions were that 1) a prenatal low dose of LPS sensitized to HI-induced brain injury in neonates but confers protection in adulthood, 2) reduced myelination is seen after prenatal LPS exposure and HI in both neonatal and adult mice despite the fact that LPS reduced total tissue loss in adult mice; and 3) nuclear factor-kappaB and caspase-3 activation early after LPS exposure may play a role in the sensitization/protection (preconditioning) effects.

Characterization of Aedes Dredd: a novel initiator caspase from the yellow fever mosquito, Aedes aegypti

Caspases play an essential role during programmed cell death in all metazoans. These enzymes are cysteine proteases and comprise a multi-gene family with more than a dozen mammalian family members. Although caspases have been characterized in many animals, including Drosophila melanogaster, little is known about the caspases that exist in mosquitoes. Here we describe the identification and characterization of Aedes Dredd (AeDredd), a novel caspase in the yellow fever mosquito, Aedes aegypti. AeDredd contains two N-terminal death effector domains and the well conserved caspase catalytic domain. Multiple sequence alignments and functional substrate assays of recombinant protein suggest that AeDredd is an orthologue of Drosophila Dredd and human caspase-8, both central effectors of the death receptor-mediated apoptotic pathway. AeDredd exhibits substrate specificity most similar to human caspase-8. AeDredd transcripts were found in all developmental stages with highest expression in early pupae. Within adults, AeDredd was found in all the tissues examined, with the highest transcript levels detected in fat body tissues. This is the first functional characterization of a death domain-containing caspase in an insect vector of human disease, and will initiate studies on the role of apoptosis in the innate immune response of vectors towards intracellular parasites such as viruses.

Chronic inflammation: importance of NOD2 and NALP3 in interleukin-1beta generation

Inflammation is part of the non-specific immune response that occurs in reaction to any type of bodily injury. In some disorders, the inflammatory process - which under normal conditions is self-limiting - becomes continuous and chronic inflammatory diseases might develop subsequently. Pattern recognition molecules (PRMs) represent a diverse collection of molecules responsible for sensing danger signals, and together with other immune components they are involved in the first line of defence. NALP3 and NOD2, which belong to a cytosolic subgroup of PRMs, dubbed Nod-like-receptors (NLRs), have been associated recently with inflammatory diseases, specifically Crohn's disease and Blau syndrome (NOD2) and familial cold autoinflammatory syndrome, Muckle-Wells syndrome and chronic infantile neurological cutaneous and articular syndrome (NALP3). The exact effects of the defective proteins are not fully understood, but activation of nuclear factor (NF)-kappaB, transcription, production and secretion of interleukin (IL)-1beta and activation of the inflammasome are some of the processes that might hold clues, and the present review will provide a thorough update in this area.

The role of STAT1/IRF-1 on synergistic ROS production and loss of mitochondrial transmembrane potential during hepatic cell death induced by LPS/d-GalN

Previously, we demonstrated that signal transducer and activator of transcription factor 1 (STAT1) plays an essential role in liver injury induced by lipopolysaccharide (LPS)/D-galactosamine (D-GalN); however, the underlying mechanism involved remains unclear. Here, we showed that LPS/D-GalN administration induced secretion of tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma), which mediated apoptosis synergistically. Moreover, LPS/D-GalN-induced apoptosis was associated with increased inducible nitric oxide synthase (iNOS) and nitric oxide (NO) production, as well as elevated reactive oxygen species (ROS) production, which were all strongly inhibited by treatment with the antioxidant N-acetyl-L-cysteine (NAC) and an iNOS/NO inhibitor, L-NMMA. Although STAT1 activation and expression did not change significantly in TNF-alpha/IFN-gamma-cotreated cells compared with cells treated with IFN-gamma alone, the absence of STAT1 or interferon regulatory factor 1 (IRF-1) in genetic knockout mice strongly abrogated the observed effects of TNF-alpha/IFN-gamma on iNOS/NO induction, ROS production, loss of mitochondrial transmembrane potential (DeltaPsim), and apoptosis compared with STAT1(+/+) and IRF-1(+/+) mice. Additionally, the synergistic effects of TNF-alpha/IFN-gamma on iNOS/NO induction, ROS production, and apoptosis were significantly inhibited by overexpression of dominant negative STAT1 in contrast to overexpression of wild-type STAT1. In STAT1-deficient mice, nuclear factor kappaB (NF-kappaB) activation by TNF-alpha/IFN-gamma was attenuated and strongly inhibited by both NAC and L-NMMA. Moreover, the proteasome inhibitor, MG132, inhibited NF-kappaB activation and strongly inhibited iNOS/NO induction, ROS production, and loss of DeltaPsim induced by TNF-alpha/IFN-gamma, thereby inhibiting apoptosis. Interestingly, it appears peroxynitrite, which is produced by TNF-alpha/IFN-gamma, may interfere with STAT1 phosphorylation by inducing STAT1 nitration. Collectively, these findings demonstrate that TNF-alpha/IFN-gamma synergistically potentiates iNOS/NO induction, ROS production, and loss of DeltaPsim via STAT1 overexpression, playing an important role in promoting apoptosis and liver injury induced by LPS/D-GalN.

Caspase-3 activation in astrocytes following postnatal excitotoxic damage correlates with cytoskeletal remodeling but not with cell death or proliferation

Caspase-3 has classically been defined as the main executioner of programmed cell death. However, recent data supports the participation of this protease in non-apoptotic cellular events including cell proliferation, cell cycle regulation, and cellular differentiation. In this study, astroglial cleavage of caspase-3 was analyzed following excitotoxic damage in postnatal rats to determine if its presence is associated with apoptotic cell death, cell proliferation, or cytoskeletal remodeling. A well-characterized in vivo model of excitotoxicity was studied, where damage was induced by intracortical injection of N-methyl-D-asparate (NMDA) in postnatal day 9 rats. Our results demonstrate that cleaved caspase-3 was mainly observed in the nucleus of activated astrocytes in the lesioned hemisphere as early as 4 h postlesion and persisted until the glial scar was formed at 7-14 days, and it was not associated with TUNEL labeling. Caspase-3 enzymatic activity was detected at 10 h and 1 day postlesion in astrocytes, and co-localized with caspase-cleaved fragments of glial fibrillary acidic protein (CCP-GFAP). However, at longer survival times, when astroglial hypertrophy was observed, astroglial caspase-3 did not generally correlate with GFAP cleavage, but instead was associated with de novo expression of vimentin. Moreover, astroglial caspase-3 cleavage was not associated with BrdU incorporation. These results provide further evidence for a nontraditional role of caspases in cellular function that is independent of cell death and suggest that caspase activation is important for astroglial cytoskeleton remodeling following cellular injury.