Caspase cleavage of vimentin disrupts intermediate filaments and promotes apoptosis

Dual roles of intermediate filaments in apoptosis

New roles have emerged recently for intermediate filaments (IFs), namely in modulating cell adhesion and growth, and providing resistance to various forms of stress and to apoptosis. In this context, we first summarize findings on the IF association with the cell response to mechanical stress and growth stimulation, in light of growth-related signaling events that are relevant to death-receptor engagement. We then address the molecular mechanisms by which IFs can provide cell resistance to apoptosis initiated by death-receptor stimulation and to necrosis triggered by excessive oxidative stress. In the same way, we examine IF involvement, along with cytolinker participation, in sequential caspase-mediated protein cleavages that are part of the overall cell death execution, particularly those that generate new functional IF protein fragments and uncover neoantigen markers. Finally, we report on the usefulness of these markers as diagnostic tools for disease-related aspects of apoptosis in humans. Clearly, the data accumulated in recent years provide new and significant insights into the multiple functions of IFs, particularly their dual roles in cell response to apoptotic insults.

Acidic Triterpenes Compromise Growth and Survival of Astrocytoma Cell Lines by Regulating Reactive Oxygen Species Accumulation

Several studies have shown how pentacyclic triterpenes can inhibit proliferation and induce apoptosis of some tumor cell lines; however, its effect on astrocytic tumors, one of the most malignant forms of cancer, has rarely been reported. The aim of this study was to examine how the pentacyclic triterpenes, oleanolic acid and maslinic acid, isolated from olive juice, affected astrocytoma cell morphology and survival. Cell proliferation was inhibited in 1321N1 astrocytoma cells by using 1 to 50 micromol/L of either oleanolic acid or maslinic acid, with an average IC(50) of 25 micromol/L. Growth inhibition led to morphologic and cytoskeletal alterations associated with the loss of stellate morphology and characterized by a retraction of the cytoplasm and collapse of actin stress fibers. Using 4',6-diamidino-2-phenylindole and Annexin V, we showed that astrocytoma cell death induced by oleanolic acid or maslinic acid were mainly due to apoptotic events. Furthermore, we showed that caspase-3 is activated as a consequence of triterpene treatment. Finally, we found that exposure of the cells to oleanolic acid or maslinic acid resulted in a significant increase of intracellular reactive oxygen species, followed by loss of mitochondrial membrane integrity. Importantly, enzymatic scavengers, such as catalase, or phenolic antioxidants, such as butylated hydroxytoluene, rescued cells from the triterpene-mediated apoptosis, suggesting that the potential therapeutic effect of these acidic triterpenes is dependent on oxidative stress. Our data show that acidic triterpenes play a major role in 1321N1 astrocytoma morphology and viability and support the conclusion that oleanolic acid and maslinic acid may thus be promising new agents in the management of astrocytomas.

Novel functions of vimentin in cell adhesion, migration, and signaling

Vimentin is the major intermediate filament (IF) protein of mesenchymal cells. It shows dynamically altered expression patterns during different developmental stages and high sequence homology throughout all vertebrates, suggesting that the protein is physiologically important. Still, until recently, the real tasks of vimentin have been elusive, primarily because the vimentin-deficient mice were originally characterized as having a very mild phenotype. Recent studies have revealed several key functions for vimentin that were not obvious at first sight. Vimentin emerges as an organizer of a number of critical proteins involved in attachment, migration, and cell signaling. The highly dynamic and complex phosphorylation of vimentin seems to be a likely regulator mechanism for these functions. The implicated novel vimentin functions have broad ramifications into many different aspects of cell physiology, cellular interactions, and organ homeostasis.

Genes and proteins differentially expressed during in vitro malignant transformation of bovine pancreatic duct cells

Pancreatic carcinoma has an extremely bad prognosis due to lack of early diagnostic markers and lack of effective therapeutic strategies. Recently, we have established an in vitro model recapitulating the first steps in the carcinogenesis of the pancreas. SV40 large T antigen-immortalized bovine pancreatic duct cells formed intrapancreatic adenocarcinoma tumors on k-ras(mut) transfection after orthotopic injection in the nude mouse pancreas. Here we identified genes and proteins differentially expressed in the course of malignant transformation using reciprocal suppression subtractive hybridization and 2D gel electrophoresis and mass spectrometry, respectively. We identified 34 differentially expressed genes, expressed sequence tags, and 15 unique proteins. Differential expression was verified for some of the genes or proteins in samples from pancreatic carcinoma. Among these genes and proteins, the majority had already been described either to be influenced by a mutated ras or to be differentially expressed in pancreatic adenocarcinoma, thus proving the feasibility of our model. Other genes and proteins (e.g., BBC1, GLTSCR2, and rhoGDIalpha), up to now, have not been implicated in pancreatic tumor development. Thus, we were able to establish an in vitro model of pancreatic carcinogenesis, which enabled us to identify genes and proteins differentially expressed during the early steps of malignant transformation.

Intermediate filaments: a historical perspective

Intracellular protein filaments intermediate in size between actin microfilaments and microtubules are composed of a surprising variety of tissue specific proteins commonly interconnected with other filamentous systems for mechanical stability and decorated by a variety of proteins that provide specialized functions. The sequence conservation of the coiled-coil, alpha-helical structure responsible for polymerization into individual 10 nm filaments defines the classification of intermediate filament proteins into a large gene family. Individual filaments further assemble into bundles and branched cytoskeletons visible in the light microscope. However, it is the diversity of the variable terminal domains that likely contributes most to different functions. The search for the functions of intermediate filament proteins has led to discoveries of roles in diseases of the skin, heart, muscle, liver, brain, adipose tissues and even premature aging. The diversity of uses of intermediate filaments as structural elements and scaffolds for organizing the distribution of decorating molecules contrasts with other cytoskeletal elements. This review is an attempt to provide some recollection of how such a diverse field emerged and changed over about 30 years.

Apoptotic cleavage of NuMA at the C-terminal end is related to nuclear disruption and death amplification

NuMA is a nuclear matrix protein in interphase and distributes to the spindle poles during mitosis. While the essential function of NuMA for mitotic spindle assembly is well established, a structural role of NuMA in interphase nucleus has also been proposed. Several observations suggest that the apoptotic degradation of NuMA may relate to chromatin condensation and micronucleation. Here we demonstrate that four apoptotic cleavage sites are clustered at a junction between the globular tail and the central coiled-coil domains of NuMA. Cleavage of a caspase-6-sensitive site at D(1705) produced the R-form, a major tail-less product of NuMA during apoptosis. The other two cleavage sites were defined at D(1726) and D(1747) that were catalyzed, respectively, by caspase-3 and an unknown aspartase. A NuMA deletion mutant missing the entire cleavage region of residues 1701-1828 resisted degradation and protected cells from nuclear disruption upon apoptotic attack. Under such conditions, cytochrome c was released from mitochondria, but the subsequent apoptotic events such as caspase-3 activation, poly(ADP-ribose) polymerase degradation, and DNA fragmentation were attenuated. Conversely, the tail-less NuMA alone, a mutant mimicking the R-form, induced chromatin condensation and activated the death machinery. It supports that intact NuMA is a structural element in maintaining nuclear integrity.

The apoptotic microtubule network preserves plasma membrane integrity during the execution phase of apoptosis

It has recently been shown that the microtubule cytoskeleton is reformed during the execution phase of apoptosis. We demonstrate that this microtubule reformation occurs in many cell types and under different apoptotic stimuli. We confirm that the apoptotic microtubule network possesses a novel organization, whose nucleation appears independent of conventional gamma-tubulin ring complex containing structures. Our analysis suggests that microtubules are closely associated with the plasma membrane, forming a cortical ring or cellular "cocoon". Concomitantly other components of the cytoskeleton, such as actin and cytokeratins disassemble. We found that colchicine-mediated disruption of apoptotic microtubule network results in enhanced plasma membrane permeability and secondary necrosis, suggesting that the reformation of a microtubule cytoskeleton plays an important role in preserving plasma membrane integrity during apoptosis. Significantly, cells induced to enter apoptosis in the presence of the pan-caspase inhibitor z-VAD, nevertheless form microtubule-like structures suggesting that microtubule formation is not dependent on caspase activation. In contrast we found that treatment with EGTA-AM, an intracellular calcium chelator, prevents apoptotic microtubule network formation, suggesting that intracellular calcium may play an essential role in the microtubule reformation. We propose that apoptotic microtubule network is required to maintain plasma membrane integrity during the execution phase of apoptosis.

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.

Caspase proteolysis of the integrin beta4 subunit disrupts hemidesmosome assembly, promotes apoptosis, and inhibits cell migration

Caspases are a conserved family of cell death proteases that cleave intracellular substrates at Asp residues to modify their function and promote apoptosis. In this report we identify the integrin beta4 subunit as a novel caspase substrate using an expression cloning strategy. Together with its alpha6 partner, alpha6beta4 integrin anchors epithelial cells to the basement membrane at specialized adhesive structures known as hemidesmosomes and plays a critical role in diverse epithelial cell functions including cell survival and migration. We show that integrin beta4 is cleaved by caspase-3 and -7 at a conserved Asp residue (Asp(1109)) in vitro and in epithelial cells undergoing apoptosis, resulting in the removal of most of its cytoplasmic tail. Caspase cleavage of integrin beta4 produces two products, 1) a carboxyl-terminal product that is unstable and rapidly degraded by the proteasome and 2) an amino-terminal cleavage product (amino acids 1-1109) that is unable to assemble into mature hemidesmosomes. We also demonstrate that caspase cleavage of integrin beta4 sensitizes epithelial cells to apoptosis and inhibits cell migration. Taken together, we have identified a previously unrecognized proteolytic truncation of integrin beta4 generated by caspases that disrupts key structural and functional properties of epithelial cells and promotes apoptosis.

A proteomic analysis of the effect of MAPK pathway activation on L-glutamate-induced neuronal cell death

Oxidative stress has been implicated in the pathogenesis of neuronal degenerative diseases. It is also widely known that oxidative stress induces mitogen-activated protein kinase (MAPK) signaling cascades. In this study, we used proteomic analysis to investigate the role of the MAPK pathway in oxidative stress-induced neuronal cell death. The results demonstrated that several proteins, including eukaryotic translation elongation factor 2 (eEF2) and enolase I, showed a differential expression pattern during the neuronal cell death process, and this was MAPK pathway dependent. Several chaperone and cytoskeletal proteins including heat shock protein 70, calreticulin, vimentin, prolyl 4-hydroxylase β polypeptide, and transgelin 2 were up-or down-regulated, despite their expressions not depending on the MAPK pathway. These findings strongly suggest that the expressions of proteins which play protective roles are independent of the MAPK pathway. On the other hand, eEF2 and enolase I may be the downstream targets of the MAPK pathway.

Calpain is required for the rapid, calcium-dependent repair of wounded plasma membrane

Mammalian cells require extracellular calcium ion to undergo rapid plasma membrane repair seconds after mechanical damage. Utilizing transformed fibroblasts from calpain small subunit knock-out (Capns1-/-) mouse embryos, we now show that the heterodimeric, typical subclass of calpains is required for calcium-mediated survival after plasma membrane damage caused by scraping a cell monolayer. Survival of scrape-damaged Capns1-/- cells was unaffected by calcium in the scraping medium, whereas more Capns1+/+ cells survived when calcium was present. Calcium-mediated survival was increased when Capns1-/- cells were scraped in the presence of purified m- or mu-calpain. Survival rates of scraped Capns1+/+, HFL-1, or Chinese hamster ovary cells were decreased by the calpain inhibitor, calpeptin, or the highly specific calpain inhibitor protein, calpastatin. Capns1-/- cells failed to reseal following laser-induced membrane disruption, demonstrating that their decreased survival after scraping resulted, at least in part, from failed membrane repair. Proteomic and immunologic analyses demonstrated that the known calpain substrates talin and vimentin were exposed at the cell surface and processed by calpain following cell scraping. Autoproteolytic activation of calpain at the scrape site was evident at the earliest time point analyzed and appeared to precede proteolysis of talin and vimentin. The results indicate that conventional calpains are required for calcium-facilitated survival after plasma membrane damage and may act by localized remodeling of the cortical cytoskeleton at the injury site.

A nestin scaffold links Cdk5/p35 signaling to oxidant-induced cell death

The intermediate filament protein, nestin, has been implicated as an organizer of survival-determining signaling molecules. When nestin expression was related to the sensitivity of neural progenitor cells to oxidant-induced apoptosis, nestin displayed a distinct cytoprotective effect. Oxidative stress in neuronal precursor cells led to downregulation of nestin with subsequent activation of cyclin-dependent kinase 5 (Cdk5), a crucial kinase in the nervous system. Nestin downregulation was a prerequisite for the Cdk5-dependent apoptosis, as overexpression of nestin efficiently inhibited induction of apoptosis, whereas depletion of nestin by RNA interference had a sensitizing effect. When the underlying link between nestin and Cdk5 was analyzed, we observed that nestin serves as a scaffold for Cdk5, with binding restricted to a specific region following the alpha-helical domain of nestin, and that the presence and organization of nestin regulated the sequestration and activity of Cdk5, as well as the ubiquitylation and turnover of its regulator, p35. Our data imply that nestin is a survival determinant whose action is based upon a novel mode of Cdk5 regulation, affecting the targeting, activity, and turnover of the Cdk5/p35 signaling complex.

Disassembly of the vimentin cytoskeleton disrupts articular cartilage chondrocyte homeostasis

Articular cartilage functions in dissipating forces applied across joints. It comprises an extracellular matrix containing primarily collagens, proteoglycans and water to maintain its functional properties, and is interspersed with chondrocytes. The chondrocyte cytoskeleton comprises actin microfilaments, tubulin microtubules and vimentin intermediate filaments. Previous studies have determined the contribution of actin and tubulin in regulating the synthesis of the extracellular matrix components aggrecan and type II collagen. The contribution of vimentin to extracellular matrix biosynthesis in any cell type has not previously been addressed. Therefore the aim of this study was to assess the role of vimentin in cartilage chondrocyte metabolism. Vimentin intermediate filaments were disrupted in high-density monolayer articular chondrocyte cultures using acrylamide for 7 days. De novo protein and collagen synthesis were measured by adding [3H]-proline, and sulphated glycosaminoglycan (sGAG) synthesis measured by adding [35S]-sulphate to cultures. Vimentin disruption resulted in decreased collagen synthesis, whilst sGAG synthesis was unaffected. In addition, there was a significant reduction in type II collagen and aggrecan gene transcription suggesting that the effects observed occur at both the transcriptional and translational levels. A 3-day cold chase demonstrated a significant inhibition of collagen and sGAG degradation; the reduction in collagen degradation was corroborated by the observed reduction in both pro-MMP 2 expression and activation. We have demonstrated that an intact vimentin intermediate filament network contributes to the maintenance of the chondrocyte phenotype and thus an imbalance favouring filament disassembly can disturb the integrity of the articular cartilage, and may ultimately lead to the development of pathologies such as osteoarthritis.

Hypothermia-Induced Loss of Endothelial Barrier Function Is Restored after Dopamine Pretreatment: Role of p42/p44 Activation

BACKGROUND

Donor dopamine usage is associated with improved immediate graft function after renal transplantation. Although prolonged cold preservation results in an increased vascular permeability, the present study was conducted to examine in vitro and in vivo if dopamine pretreatment influences endothelial barrier function under such conditions.

METHODS

To assess cold preservation injury in vitro and in vivo, cultured human umbilical vein endothelial cells (HUVEC) and Lewis donor rats were pretreated with dopamine or isotonic saline prior to cold storage. Injury was determined by lactate dehydrogenase (LDH) release, histology, and functional analysis.

RESULTS

In vitro cold storage resulted in intercellular gap formation in both untreated and dopamine pretreated HUVEC. In the latter monolayer integrity was completely restored upon rewarming and paracellular transport of fluorescein isothiocyanate-dextran was significantly reduced. In dopamine treated HUVEC, intercellular gap formation was independent of cell death and was associated with redistribution of junctional proteins and condensation of cytoskeleton proteins. In untreated HUVEC proteolysis and cell death were clearly evident after hypothermia. Closing of intercellular gaps was dependent on p42/p44 activation. Regeneration of adenosine triphosphate was only observed in dopamine pretreated cells. Only in dopamine treated Lewis renal allografts subjected to cold storage, activation of p42/p44 occurred upon rewarming. These grafts had a better renal function and displayed less inflammatory cells five days after transplantation.

CONCLUSION

Our study demonstrates beneficial effects of dopamine treatment on cold storage induced endothelial barrier disturbances. This may contribute to the positive effects of catecholamines on immediate graft function of renal allografts in men.

Condensin I recruitment and uneven chromatin condensation precede mitotic cell death in response to DNA damage

Mitotic cell death (MCD) is a prominent but poorly defined form of death that stems from aberrant mitosis. One of the early steps in MCD is premature mitosis and uneven chromatin condensation (UCC). The mechanism underlying this phenomenon is currently unknown. In this study, we show that DNA damage in cells with a compromised p53-mediated G2/M checkpoint triggers the unscheduled activation of cyclin-dependent kinase 1 (Cdk1), activation and chromatin loading of the condensin I complex, and UCC followed by the appearance of multimicronucleated cells, which is evidence of MCD. We demonstrate that these processes engage some of the players of normal mitotic chromatin packaging but not those that drive the apoptotic chromatin condensation. Our findings establish a link between the induction of DNA damage and mitotic abnormalities (UCC) through the unscheduled activation of Cdk1 and recruitment of condensin I. These results demonstrate a clear distinction between the mechanisms that drive MCD-associated and apoptosis-related chromatin condensation and provide mechanistic insights and new readouts for a major cell death process in treated tumors.

Expression and characterization of constitutively active human caspase-14

Caspase-14 is a cysteine endoproteinase that is expressed in the epidermis and a limited number of other tissues. It is activated during keratinocyte differentiation by zymogen processing, but its precise function is unknown. To obtain caspase-14 for functional studies, we engineered and expressed a constitutively active form of human caspase-14 (Rev-hC14) in Escherichia coli and cultured mammalian cells. Rev-hC14 required no proteolytic processing for activity, showed strong activity against the caspase substrate WEHD, and was inhibited by the pan-caspase inhibitor zVAD-fmk. Mammalian cells that expressed active caspase-14 showed normal cell adherence and morphology. Using positional scanning of synthetic tetrapeptide libraries, we determined the substrate preference of human caspase-14 to be W (or Y)-X-X-D. These studies affirm that caspase-14 has a substrate specificity similar to the group I caspases, and demonstrate that it functions in a distinct manner from executioner caspases to carry out specific proteolytic events during keratinocyte differentiation.

Role of vimentin in regulation of monocyte/macrophage differentiation

Maturation of blood cells depends on dramatic changes of expression profiles of specific genes. Although these changes have been extensively studied, their functional outcomes often remain unclear. In this study, we explored the identity and function of an unknown protein that was greatly overexpressed in v-myb-transformed BM2 monoblasts undergoing differentiation to macrophage-like cells. We identified this protein as vimentin, the intermediate filament protein. We show that an increased level of vimentin protein results from activation of the vimentin gene promoter occurring in monoblastic cells induced to differentiate by multiple agents. Furthermore, our studies reveal that the vimentin gene promoter is stimulated by Myb and Jun proteins, the key transcriptional regulators of myeloid maturation. Silencing of vimentin gene expression using siRNA markedly suppressed the ability of BM2 cells to form macrophage polykaryons active in phagocytosis and producing reactive oxygen species. Taken together, these findings document that up-regulation of vimentin gene expression is important for formation of fully active macrophage-like cells and macrophage polykaryons.

Overexpression of peptidylarginine deiminase IV features in apoptosis of haematopoietic cells

Peptidylarginine deiminases (PADIs) convert peptidylarginine into citrulline via posttranslational modification. One member of the family, PADI4, plays an important role in immune cell differentiation and cell death. To elucidate the participation of PADI4 in haematopoietic cell death, we examine whether inducible overexpression of PADI4 enhances the apoptotic cell death. PADI4 reduced the viability in a dose- and time-dependent manner of human leukemia HL-60 cells and human acute T leukemia Jurkat cells. The apoptosis-inducing activities were determined by nuclear condensation, DNA fragmentation, sub-G1 appearance, loss of mitochondrial membrane potential (delta psi(m)), release of mitochondrial cytochrome c into cytoplasm and proteolytic activation of caspase 9 and 3. Following PADI4 overexpression, cells arrest in G1 phase significantly before their entrance into apoptotic cell death. PADI4 increases tumor suppressor p53 and its downstream p21 to control cell cycle. In the detections of protein expression and kinase activity, all protein levels of cyclin-dependent kinases (CDKs) and cyclins are not reduced except cyclin D, however, CDK2 (G1 entry S phase) and CDK1 (G2 entry M phase) enzyme activities are inhibited by conditionally inducible PADI4. p53 also expands its other downstream Bax to induce cytochrome c release from mitochondria. According to these data, we suggest that PADI4 induces apoptosis mainly through cell cycle arrest and mitochondria-mediated pathway. Furthermore, p53 features in PADI4-induced apoptosis by increasing intracellular p21 to control cell cycle and by Bax accumulation to decline Bcl-2 function, destroy delta psi(m), release cytochrome c to cytoplasm and activate the caspase cascade.

Microtubules: forgotten players in the apoptotic execution phase

A cell entering the execution phase of apoptosis (regulated cell death) undergoes characteristic rearrangements, in which the cytoskeleton has major roles. Historically, this reorganisation has been attributed entirely to actomyosin contractility, with microtubule and intermediate filament systems both reported to be lost at an early stage. However, recent results indicate that microtubule networks re-form during the later stages of apoptosis and assist in the dispersal of nuclear and cellular fragments--steps that are thought to be important for preventing inflammation. Here, we discuss the roles of the cytoskeleton during apoptosis and challenge current thinking that actin is the sole functional component driving all major execution phase events.

A novel role for microtubules in apoptotic chromatin dynamics and cellular fragmentation

Dramatic changes in cellular dynamics characterise the apoptotic execution phase, culminating in fragmentation into membrane-bound apoptotic bodies. Previous evidence suggests that actin-myosin plays a dominant role in apoptotic cellular remodelling, whereas all other cytoskeletal elements dismantle. We have used fixed cells and live-cell imaging to confirm that interphase microtubules rapidly depolymerise at the start of the execution phase. Around this time, pericentriolar components (pericentrin, ninein and gamma-tubulin) are lost from the centrosomal region. Subsequently, however, extensive non-centrosomal bundles of densely packed, dynamic microtubules rapidly assemble throughout the cytoplasm in all cell lines tested. These microtubules have an important role in the peripheral relocation of chromatin in the dying cell, because nocodazole treatment restricts the dispersal of condensed apoptotic chromatin into surface blebs, and causes the withdrawal of chromatin fragments back towards the cell centre. Importantly, nocodazole and taxol are both potent inhibitors of apoptotic fragmentation in A431 cells, implicating dynamic microtubules in apoptotic body formation. Live-cell-imaging studies indicate that fragmentation is accompanied by the extension of rigid microtubule-rich spikes that project through the cortex of the dying cell. These structures enhance interactions between apoptotic cells and phagocytes in vitro, by providing additional sites for attachment to neighbouring cells.

Proteomic characterization of the effects of clofibrate on protein expression in rat liver

Clofibrate is a peroxisome proliferator known to induce liver tumours in rats. A proteomics study was conducted to provide new insights into the molecular mechanisms of clofibrate-induced non-genotoxic hepatocarcinogenesis. Rats were treated with 250 mg/kg day clofibrate orally and sacrificed after 7 days. Proteins extracted from the liver were analysed by 2-DE using DIGE technology. The protein identification performed by MS showed that clofibrate induced up-regulation of 77 proteins and down-regulation of 27 proteins. The highest expression ratios corresponded to proteins involved in a series of biochemical pathways such as lipid metabolism, fatty acid metabolism, amino acid metabolism, protein metabolism, citric acid cycle, xenobiotic detoxification and oxidative stress. Proteins implicated in cell proliferation and apoptosis, such as prohibitin, 10-formyl tetrahydrofolate dehydrogenase, senescence marker protein-30, pyridoxine 5'-phosphate oxidase and vimentin, were also identified as being regulated. These results provide leads for further investigations into the molecular mechanisms of liver tumours induced by clofibrate. In addition, MS results showed that a series of regulated proteins were detected as several spots corresponding to different pI and/or M(r). Differential effects on those variants could result from specific PTM and could be a specific molecular signature of the clofibrate-induced protein expression modulation in rat liver.

Proteomics study of anthrax lethal toxin-treated murine macrophages

The anthrax lethal toxin (LeTx) is composed of two proteins, protective antigen and lethal factor, which bind and enter the cell through a host receptor termed the anthrax toxin receptor (ATR). In the cell, LeTx targets p38, part of the MAP kinase signaling pathway. The toxin appears to initiate an apoptotic pathway in infected cells, indicating additional downstream targets of the toxin. We have applied a proteomics approach to investigate these downstream targets and the affected processes. In this study we have used an improved strategy for fractionation based on protein pI, off-gel electrophoresis, employed in conjunction with relative quantitation using the mass labeling approach. In our survey, 67 proteins were observed and quantified from the cytosol of RAW 264.7 cells with respect to control versus toxin-treated cells. Many of these proteins are involved in the oxidative stress response, as well as apoptosis, and thus likely to be relevant to the effects of anthrax in infected cells. Our results indicate that the tumor necrosis factor-alpha-mediated pathway is compromised in intoxicated cells. The knowledge of such changes and the pathways leading to the changes should be of great value in understanding and combating this disease.

Caspase redundancy and release of mitochondrial apoptotic factors characterize interdigital apoptosis

Here we show a detailed analysis of cellular and molecular events during in vivo apoptotic cell death in the INZs (interdigital necrotic zones) of the embryonic limb. As the apoptotic mechanisms proceed, the transcriptionally active chromatin and nuclear traffic of RNAs are disrupted, cytoskeletal components are disorganized and the adhesive properties of cells are compromised as Paxillin, a clue member of the focal adhesion complex, decreases in early apoptotic cells. Activation of effector caspases 3 and 7 follow nuclear degradation. In addition, active caspase2 is localized in the nuclei and cytoplasm of early apoptotic cells suggesting a major role in physiological conditions supported by its down-regulation in tissue survival experiments. However in caspase 2 siRNA assays we observed translocation of caspase 3 to the nuclei suggesting functional redundancy. We also observed release of cytochrome c and AIF from the mitochondria, and interestingly AIF becomes intranuclear in a caspase independent manner.

The Differentiation-dependent Desmosomal Cadherin Desmoglein 1 Is a Novel Caspase-3 Target That Regulates Apoptosis in Keratinocytes

Although a number of cell adhesion proteins have been identified as caspase substrates, the potential role of differentiation-specific desmosomal cadherins during apoptosis has not been examined. Here, we demonstrate that UV-induced caspase cleavage of the human desmoglein 1 cytoplasmic tail results in distinct 17- and 140- kDa products, whereas metalloproteinase-dependent shedding of the extracellular adhesion domain generates a 75-kDa product. In vitro studies identify caspase-3 as the preferred enzyme that cleaves desmoglein 1 within its unique repeating unit domain at aspartic acid 888, part of a consensus sequence not conserved among the other desmosomal cadherins. Apoptotic processing leads to decreased cell surface expression of desmoglein 1 and re-localization of its C terminus diffusely throughout the cytoplasm over a time course comparable with the processing of other desmosomal proteins and cytoplasmic keratins. Importantly, whereas classic cadherins have been reported to promote cell survival, short hairpin RNA-mediated suppression of desmoglein 1 in differentiated keratinocytes protected cells from UV-induced apoptosis. Collectively, our results identify desmoglein 1 as a novel caspase and metalloproteinase substrate whose cleavage likely contributes to the dismantling of desmosomes during keratinocyte apoptosis and also reveal desmoglein 1 as a previously unrecognized regulator of apoptosis in keratinocytes.

HSP90 protects apoptotic cleavage of vimentin in geldanamycin-induced apoptosis

Heat shock protein (HSP) 90 is of interest as an anticancer drug target because of its importance in maintaining the conformation, stability and function of the client proteins involved in signal transduction pathways leading to proliferation, cell cycle progression, and apoptosis. Geldanamycin, a specific antagonist of HSP90, binds directly to HSP90 and promotes proteolytic degradation of client proteins of HSP90. The aim of the present study was to identify novel client proteins of HSP90 and to elucidate HSP90 function through inhibition of HSP90 binding to its client proteins, by using of geldanamycin. We investigated changes in protein profile when apoptosis was induced by exposure to geldanamycin. Differentially expressed proteins were identified by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS), in human neuroblastoma SK-N-SH cells. The vimentin level was found to decrease dramatically by the treatment of geldanamycin. We observed subcellular co-localization of vimentin and HSP90. Physical association of vimentin with HSP90 was detected by an immunoprecipitation assay. The caspase inhibitors, Z-VAD-FMK and Ac-DEVD-CHO, completely abolished geldanamycin-induced cleavage of vimentin. Changes of HSP90 level by antisense treatment or transfection of HSP90-overexpressing vector affected geldanamycin-induced cleavage of vimentin. These results suggest that HSP90 protects vimentin by physical interaction in the geldanamycin-induced apoptotic pathway.

Active relocation of chromatin and endoplasmic reticulum into blebs in late apoptotic cells

Plasma membrane blebbing is a defining characteristic of apoptosis, but its significance is not understood. Using live-cell imaging we have identified two phases of apoptotic blebbing. The early phase is restricted to adherent cells, and is prevented by the Rho-activated kinase inhibitor Y27632. The late phase is partially resistant to Y27632, and generates morphologically distinct membrane protrusions that are likely precursors to apoptotic bodies. Late blebbing is observed in all apoptotic cells tested. It occurs at a fixed period before phosphatidyl serine exposure, indicating that it is a universal and important feature of apoptosis. Late blebs contain a cortical layer of endoplasmic reticulum that often surrounds condensed chromatin, while other organelles are excluded. The appearance in some apoptotic cells of partially formed sheets of endoplasmic reticulum suggest that these cortical layers are newly formed by the remodelling of the endoplasmic reticulum of interphase cells. Formation of endoplasmic reticulum and chromatin-containing blebs requires both actin and microtubules, and is prevented by the caspase-6 inhibitor zVEID.fmk.

Mutations in vimentin disrupt the cytoskeleton in fibroblasts and delay execution of apoptosis

To get new insights into the function of the intermediate filament (IF) protein vimentin in cell physiology, we generated two mutant cDNAs, one with a point mutation in the consensus motif in coil1A (R113C) and one with the complete deletion of coil 2B of the rod domain. In keratins and glia filament protein (GFAP), analogous mutations cause keratinopathies and Alexander disease, respectively. Both mutants prevented filament assembly in vitro and inhibited assembly of wild-type vimentin when present in equal amounts. In stably transfected preadipocytes, these mutants caused the complete disruption of the endogenous vimentin network, demonstrating their dominant-negative behaviour. Cytoplasmic vimentin aggregates colocalised with the chaperones alphaB-crystallin and HSP40. Moreover, vimR113C mutant cells were more resistant against staurosporine-induced apoptosis compared to controls. We hypothesise that mutations in the vimentin gene, like in most classes of IF genes, may contribute to distinct human diseases.

Cleavage of p53-vimentin complex enhances tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis of rheumatoid arthritis synovial fibroblasts

Rheumatoid arthritis synovial fibroblasts (RASFs) contribute to arthritic cartilage degradation. Although RASFs are normally resistant to apoptosis, Apo2L/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-based gene therapy has been successfully used in a mouse model of arthritis. We investigated this further by treating human RASFs with nontoxic doses of the proteasome inhibitor lactacystin. Treatment induced cytosolic accumulation of p53 and enhanced the susceptibility of RASFs to apoptosis mediated by TRAIL-R2 (DR5) but not Fas. A specific role for p53 in TRAIL-R2-mediated apoptosis was indicated by the ability of p53 siRNA to significantly reduce RASF apoptosis and by the reduced apoptosis of RASFs bearing p53 mutations on treatment with anti-DR5 antibody or anti-DR5 antibody plus lactacystin. p53 immunoprecipitation followed by mass spectrometry identified a vimentin-p53 complex, an interaction that was confirmed by reciprocal vimentin-p53 immunoprecipitation and by co-immunofluorescence. Interestingly, human caspase-4 cleaved human vimentin, and blockade of caspase-4 with a chemical inhibitor or with specific siRNA significantly inhibited TRAIL-R2-mediated apoptosis of RASFs. Furthermore, blockade of caspase-4 was paralleled by persistence of a cytosolic pattern of p53 and absence of p53 translocation to the nucleus. Taken together, our findings suggest a unique role for caspase-4 in cleaving vimentin and releasing cytosolic p53 for nuclear translocation, events that may regulate the sensitivity of RASFs to receptor-mediated apoptosis.

Changes in the proteomic profile during differentiation and maturation of human monocyte-derived dendritic cells stimulated with granulocyte macrophage colony stimulating factor/interleukin-4 and lipopolysaccharide

Dendritic cells (DCs) are highly specialized antigen-presenting cells that play an essential role in the immune response. We used the proteomic approach based on two-dimensional gel electrophoresis and mass spectrometry to identify the protein changes that occur during differentiation of DCs from monocytes (Mo) stimulated with granulocyte macrophage colony stimulating factor/interleukin-4 (GM-CSF/IL-4) and during the maturation of immature DCs stimulated with lipopolysaccharide. Sixty-three differentially expressed proteins (+/- two-fold) were unambiguously identified with sequence coverage greater than 20%. They corresponded to only 36 different proteins, because 11 were present as 38 electrophoretic forms. Some proteins such as tropomyosin 4 and heat shock protein 71 presented differentially expressed electrophoretic forms, suggesting that many of the changes in protein expression that accompany differentiation and maturation of DCs occur in post-translationally modified proteins. The largest differences in expression were observed for actin (21-fold in Mo), Rho GDP-dissociation inhibitor 2 (20-fold in Mo), vimentin (eight-fold in immature DCs), lymphocyte-specific protein 1 (12-fold in mature DCs) and thioredoxin (14-fold in mature DCs). Several proteins are directly related to functional and morphological characteristics of DCs, such as cytoskeletal proteins (cytoskeleton rearrangement) and chaperones (antigen processing and presentation), but other proteins have not been assigned specific functions in DCs. Only a few proteins identified here were the same as those reported in proteomic studies of DCs, which used different stimuli to produce the cells (GM-CSF/IL-4 and tumor necrosis factor-alpha). These data suggest that the DC protein profile depends on the stimuli used for differentiation and especially for maturation.

A proteomic investigation into etoposide chemo-resistance of neuroblastoma cell lines

Neuroblastoma, one of the most common pediatric solid tumors, originates from the peripheral sympathetic nervous system and is responsible for approximately 15% of all childhood cancer deaths. Among the several antineoplastic drugs used in neuroblastoma chemotherapeutic protocols, topoisomerase inhibitors (i.e., etoposide) represent the most commonly used. Several resistance mechanisms limit the clinical success of topoisomerase-targeting drugs, mainly reducing the ability of neoplastic cells to start programmed cell death when exposed to antineoplastic drugs. The aim of this study was to determine, by means of proteomics, potential markers of etoposide resistance in human neuroblastoma cell lines as well as to investigate protein levels and modifications possibly involved in the onset of resistance. The etoposide resistant clone showed overexpression of the following proteins: peroxiredoxin 1, beta-galactoside soluble lectin binding protein, vimentin (three protein spots), heat shock 27 kDa protein (two protein spots) and heterogeneous nuclear ribonucleoprotein K. In addition, we also found down-regulation of dUTP pyrophosphatase. This investigation might represent a first step towards the development of novel prognostic markers of neuroblastoma chemotherapy.

Notch1 antiapoptotic activity is abrogated by caspase cleavage in dying T lymphocytes

Excessive signaling via the Notch1 receptor inhibits apoptosis in T lymphocytes. Since several antiapoptotic proteins are cleaved by caspases during cell death, we investigated whether Notch1 was a caspase substrate. Results demonstrate that the intracellular domain of Notch1 (NICD) is cleaved into six fragments during apoptosis in Jurkat cells or peripheral T lymphocytes. Notch1 cleavage is prevented by the caspase inhibitors DEVD-fmk and VEID-fmk or by Bcl-2 expression. Caspase-3 and caspase-6 cleave the NICD into six fragments using sites located within the NF-kappaB binding domain, the ankyrin repeats and the transactivation domain. Notch1 cleavage correlates with the loss of HES-1 expression in apoptotic T cells. Notch1 fragments cannot inhibit activation-induced cell death in a T-cell hybridoma, confirming the abrogation of Notch1 antiapoptotic activity by caspases. The ability of the NICD but not the fragments to antagonize Nur77 activity supports a role for this factor in Notch1 antiapoptotic function.

Silencing of endogenous IGFBP-5 by micro RNA interference affects proliferation, apoptosis and differentiation of neuroblastoma cells

Signal transduction through the IGF axis is implicated in proliferation, differentiation and survival during development and adult life. The IGF axis includes the IGF binding proteins (IGFBPs) that bind IGFs with high affinity and modulate their activity. In neuroblastoma (NB), a malignant childhood tumor, we found that IGFBP-5 is frequently expressed. Since NB is an IGF2-sensitive tumor, we investigated the relevance and the function of endogenous IGFBP-5 in LAN-5 and in SY5Y(N) cell lines transfected with micro and small interfering RNAs directed to IGFBP-5 mRNA. Cells in which IGFBP-5 expression was suppressed were growth-inhibited and more prone to apoptosis than the parental cell line and controls. Apoptosis was further enhanced by X-ray irradiation. The ability of these cells to undergo neuronal differentiation was impaired after IGFBP-5 inhibition but the effect was reversed by exposure to recombinant IGFBP-5. Together, these data demonstrate the importance of IGFBP-5 for NB cell functions and suggest that IGFBP-5 might serve as a novel therapeutic target in NB.

Modulation of IMPDH2, survivin, topoisomerase I and vimentin increases sensitivity to methotrexate in HT29 human colon cancer cells

We determined differentially expressed genes in HT29 human colon cancer cells, both after short treatment with methotrexate (MTX) and after the resistance to MTX had been established. Screening was performed using Atlas Human Cancer 1.2K cDNA arrays. The analysis was carried out using Atlas image 2.01 and genespring 6.1 software. Among the differentially expressed genes we chose for further validation were inosine monophosphate dehydrogenase type II (IMPDH2), inosine monophosphate cyclohydrolase and survivin as up-regulated genes, and topoisomerase I (TOP1) and vimentin as down-regulated genes. Changes in mRNA levels were validated by quantitative RT-PCR. Additionally, functional analyses were performed inhibiting the products of the selected genes or altering their expression to test if these genes could serve as targets to modify MTX cytotoxicity. Inhibition of IMPDH or TOP1 activity, antisense treatment against survivin, or overexpression of vimentin, sensitized resistant HT29 cells to MTX. Therefore, these proteins could constitute targets to develop modulators in MTX chemotherapy.

The small heat shock protein alpha B-crystallin is a novel inhibitor of TRAIL-induced apoptosis that suppresses the activation of caspase-3

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor alpha family of cytokines that preferentially induces apoptosis in transformed cells, making it a promising cancer therapy. However, many neoplasms are resistant to TRAIL-induced apoptosis by mechanisms that are poorly understood. We demonstrate that the expression of the small heat shock protein alpha B-crystallin (but not other heat shock proteins or apoptosis-regulating proteins) correlates with TRAIL resistance in a panel of human cancer cell lines. Stable expression of wild-type alpha B-crystallin, but not a pseudophosphorylation mutant impaired in its assembly and chaperone function, protects cancer cells from TRAIL-induced caspase-3 activation and apoptosis in vitro. Furthermore, selective inhibition of alpha B-crystallin expression by RNA interference sensitizes cancer cells to TRAIL. In addition, wild-type alpha B-crystallin promotes xenograft tumor growth and inhibits TRAIL-induced apoptosis in vivo in nude mice, whereas a pseudophosphorylation alpha B-crystallin mutant impaired in its anti-apoptotic function inhibits xenograft tumor growth. Collectively, these findings indicate that alpha B-crystallin is a novel regulator of TRAIL-induced apoptosis and tumor growth. Moreover, these results demonstrate that targeted inhibition of alpha B-crystallin promotes TRAIL-induced apoptosis, thereby suggesting a novel strategy to overcome TRAIL resistance in cancer.

Intermediate filament associated proteins

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Breaking the Connection: Caspase 6 Disconnects Intermediate Filament-Binding Domain of Periplakin from its Actin-Binding N-Terminal Region

Periplakin is a member of the plakin family of cytolinkers that connect cytoskeletal networks to each other as well as to the cell junctional complexes. Here, we demonstrate a direct molecular interaction between actin and periplakin. Furthermore, the oligomerization state of periplakin was shown to determine specificity of its binding to intermediate filaments (IF) in vitro. Both the filament association and the cell membrane localization of periplakin were confirmed in the cells overexpressing human periplakin. Double labeling of the N- and C-terminally tagged periplakin revealed unexpected lack of co-localization of periplakin ends in a confluent culture, and separation of the periplakin ends was even more pronounced in apoptotic cells. Western analysis revealed that after induction of apoptosis, periplakin becomes cleaved close to its C-terminal tail. Only the distinct cleavage products, but not the full-length periplakin, were present in the cells detached from the solid support during the apoptotic process. We show that caspase 6 cleaves periplakin at an unconventional recognition site, amino acid sequence TVAD. Thus, the separation of periplakin ends disconnects the actin-binding head-rod domain from the IF-binding C-terminal domain. We show that specific cleavage products co-exist with the full-length periplakin in cells, suggesting physiological consequences due to their altered binding specificities.

Peroxisome proliferator-activated receptor gamma agonists promote TRAIL-induced apoptosis by reducing survivin levels via cyclin D3 repression and cell cycle arrest

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising cancer therapy that preferentially induces apoptosis in cancer cells. However, many neoplasms are resistant to TRAIL by mechanisms that are poorly understood. Here we demonstrate that human breast cancer cells, but not normal mammary epithelial cells, are dramatically sensitized to TRAIL-induced apoptosis and caspase activation by peroxisome proliferator-activated receptor gamma (PPARgamma) agonists of the thiazolidinedione (TZD) class. Although TZDs do not significantly alter the expression of components of the TRAIL signaling pathway, they profoundly reduce protein levels of cyclin D3, but not other D-type cyclins, by decreasing cyclin D3 mRNA levels and by inducing its proteasomal degradation. Importantly, both TRAIL sensitization and reduction in cyclin D3 protein levels induced by TZDs are likely PPARgamma-independent because a dominant negative mutant of PPARgamma did not antagonize these effects of TZDs, nor were they affected by the expression levels of PPARgamma. TZDs also inhibit G(1) to S cell cycle progression. Furthermore, silencing cyclin D3 by RNA interference inhibits S phase entry and sensitizes breast cancer cells to TRAIL, indicating a key role for cyclin D3 repression in these events. G(1) cell cycle arrest sensitizes breast cancer cells to TRAIL at least in part by reducing levels of the anti-apoptotic protein survivin: ectopic expression of survivin partially suppresses apoptosis induced by TRAIL and TZDs. We also demonstrate for the first time that TZDs promote TRAIL-induced apoptosis of breast cancer in vivo, suggesting that this combination may be an effective therapy for cancer.

Plakin proteins are coordinately cleaved during apoptosis but preferentially through the action of different caspases

In epithelial cells, cell-cell and cell-matrix junctions, desmosomes and hemidesmosomes, provide anchorage sites for the keratin-intermediate filaments. The plakin proteins desmoplakin (DP), plectin, and periplakin represent intracellular constituents of these adhesion junctions. In staurosporine-treated apoptotic HaCaT cells, DP, plectin, and periplakin became cleaved coordinately with the elimination of keratins 10 and 14, while involucrin, actin, and keratin 18 displayed considerable stability. The caspase inhibitor zVAD-fmk prevented both the cell detachment and protein cleavage, indicating the function of caspases in these events. Closer examination in vitro revealed that while caspases 2 and 4 most efficiently cleaved DP, and plectin served as a target for caspases 3 and 7, periplakin as well as keratins were cleaved by caspase 6. The involvement of multiple caspases in the destruction of epithelial cell integrity ensures the efficient elimination of cytoskeleton, but also provides specificity for selectively targeting individual adhesion molecules.

Characterization of anti-inflammatory compounds using transcriptomics, proteomics, and metabolomics in combination with multivariate data analysis

The discovery of new anti-inflammatory drugs is often based on an interaction with a specific target, although other pathways often play a primary or secondary role. Anti-inflammatory drugs can be categorized into classes, based on their mechanism of action. In this article we investigate the possibility to characterize novel anti-inflammatory compounds by three holistic methods. For this purpose, we make use of macrophage-like U937 cells which are stimulated with LPS in the absence or presence of an anti-inflammatory compound. Using micro-arrays, 2-D gel electrophoresis and a LC-MS method for lipids the effects on the transcriptome, proteome and metabolome of the exposed cells is investigated. The expression patterns are subsequently analyzed using in-house developed pattern recognition tools. Using the methods described above, we have examined the effects of six anti-inflammatory compounds. Our results demonstrate that different classes of anti-inflammatory compounds show distinct and characteristic mRNA, protein, and lipid expression patterns, which can be used to categorise known molecules and to discover and classify new leads. The potential of our approach is illustrated by the analysis of several beta (2)-adrenergic agonists (beta2-agonists). In addition to their primary pharmacological target, beta2-agonists posses certain anti-inflammatory properties. We were able to show that zilpaterol, a poorly characterized beta2-agonist, gives rise to an almost identical expression pattern as the beta2-agonists clenbuterol and salbutamol. Furthermore we have identified specific mRNA, protein and lipid markers for the anti-inflammatory compounds investigated in this study.

Active caspase-6 and caspase-6-cleaved tau in neuropil threads, neuritic plaques, and neurofibrillary tangles of Alzheimer's disease

Previously, we have shown that caspase-6 but not caspase-3 is activated by serum deprivation and induces a protracted cell death in primary cultures of human neurons (LeBlanc AC, Liu H, Goodyer C, Bergeron C, Hammond J: Caspase-6 role in apoptosis of human neurons, amyloidogenesis and Alzheimer's disease. J Biol Chem 1999, 274:23426-23436 and Zhang Y, Goodyer C, LeBlanc A: Selective and protracted apoptosis in human primary neurons microinjected with active caspase-3, -6, -7, and -8. J Neurosci 2000, 20:8384-8389). Here, we show with neoepitope antibodies that the p20 subunit of active caspase-6 increases twofold to threefold in the affected temporal and frontal cortex but not in the unaffected cerebellum of Alzheimer's disease brains and is present in neurofibrillary tangles, neuropil threads, and the neuritic plaques. Furthermore, a neoepitope antibody to caspase-6-cleaved Tau strongly detects intracellular tangles, extracellular tangles, pretangles, neuropil threads, and neuritic plaques. Immunoreactivity with both antibodies in pretangles indicates that the caspase-6 is active early in the pathogenesis of Alzheimer's disease. In contrast to the nuclear and cytosolic localization of active caspase-6 in apoptotic neurons of fetal and adult ischemic brains, the active caspase-6 in Alzheimer's disease brains is sequestered into the tangles or neurites. The localization of active caspase-6 may strongly jeopardize the structural integrity of the neuronal cytoskeletal system leading to inescapable neuronal dysfunction and eventual cell death in Alzheimer's disease neurons. Our results suggest that active caspase-6 is strongly implicated in human neuronal degeneration and apoptosis.

Cytoplasmic intermediate filaments revealed as dynamic and multipurpose scaffolds

Intermediate filaments are cytoskeletal polymers encoded by a large family of differentially expressed genes that provide crucial structural support in the cytoplasm and nucleus of higher eukaryotes. Perturbation of their function accounts for several genetically determined diseases in which fragile cells cannot sustain mechanical and non-mechanical stresses. Recent studies shed light on how this structural support is modulated to meet the changing needs of cells, and reveal a novel role whereby intermediate filaments influence cell growth and death through dynamic interactions with non-structural proteins.

Proteasomal degradation of caspase-6 in 17beta-estradiol-treated neurons

In primary cultures of human neurons, 17beta-estradiol (17beta-E2) prevents caspase-6-mediated cell death and induces a caspase inhibitory factor (CIF) inhibiting active caspase-6 (Csp-6) in vitro. Here, we show that treatment of neurons with 17beta-E2 results in a proteasomal-dependent but ubiquitin-independent degradation of endogenous and exogenous active Csp-6 in live neurons and in cell free assays, respectively. We further show that the proteasomal-dependent degradation of Csp-6 is not required for its inhibition. Using several protease inhibitors, we find that leupeptin, E-64, and ALLN prevent inhibition of recombinant active Csp-6 (R-Csp-6) in 17beta-E2-treated neuronal protein extracts. Because all three protease inhibitors have the ability to inhibit cysteine proteases, we believe that a cysteinyl protease activity may be required for 17beta-E2-mediated inhibition of active Csp-6. However, we exclude caspases, calpains, and cathepsins as potential cysteinyl proteases involved in the 17beta-E2-mediated Csp-6 inhibition. The results suggest that a proteolytic activity inhibited by leupeptin, E-64, and ALLN is needed to inhibit Csp-6 and that the inhibited Csp-6 is subsequently degraded by the proteasome. The mechanism of 17beta-E2-mediated inhibition of Csp-6 is different from the ubiquitin-dependent proteasomal degradation of Csp-3 and Csp-7 by XIAP and cIAP2 but consistent with the mechanism of Baculovirus p35 inhibition of caspases.

Proteolysis of the mismatch repair protein MLH1 by caspase-3 promotes DNA damage-induced apoptosis

Caspases are critical proapoptotic proteases that execute cell death signals by selectively cleaving proteins at Asp residues to alter their function. Caspases trigger apoptotic chromatin degradation by activating caspase-activated DNase and by inactivating a number of enzymes that sense or repair DNA damage. We have identified the mismatch repair protein MLH1 as a novel caspase-3 substrate by screening small pools of a human prostate adenocarcinoma cDNA library for cDNAs encoding caspase substrates. In this report, we demonstrate that human MLH1 is specifically cleaved by caspase-3 at Asp(418) in vitro. Furthermore, MLH1 is rapidly proteolyzed by caspase-3 in cancer cells induced to undergo apoptosis by treatment with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) or the topoisomerase II inhibitor etoposide, which damages DNA. Importantly, proteolysis of MLH1 by caspase-3 triggers its partial redistribution from the nucleus to the cytoplasm and generates a proapoptotic carboxyl-terminal product. In addition, we demonstrate that a caspase-3 cleavage-resistant D418E MLH1 mutant inhibits etoposide-induced apoptosis but has little effect on TRAIL-induced apoptosis. These results indicate that the proteolysis of MLH1 by caspase-3 plays a functionally important and previously unrecognized role in the execution of DNA damage-induced apoptosis.

Expression and activity of citrullinating peptidylarginine deiminase enzymes in monocytes and macrophages

BACKGROUND

Antibodies directed to proteins containing the non-standard amino acid citrulline, are extremely specific for rheumatoid arthritis (RA). Peptidylcitrulline can be generated by post-translational conversion of arginine residues. This process, citrullination, is catalysed by a group of calcium dependent peptidylarginine deiminase (PAD) enzymes.

OBJECTIVE

To investigate the expression and activity of four isotypes of PAD in peripheral blood and synovial fluid cells of patients with RA.

RESULTS

The data presented here show that citrullination of proteins by PAD enzymes is a process regulated at three levels: transcription-in peripheral blood PAD2 and PAD4 mRNAs are expressed predominantly in monocytes; PAD4 mRNA is not detectable in macrophages, translation-translation of PAD2 mRNA is subject to differentiation stage-specific regulation by its 3' UTR, and activation-the PAD proteins are only activated when sufficient Ca(2+) is available. Such high Ca(2+) concentrations are normally not present in living cells. In macrophages, which are abundant in the inflamed RA synovium, vimentin is specifically citrullinated after Ca(2+) influx.

CONCLUSION

PAD2 and PAD4 are the most likely candidate PAD isotypes for the citrullination of synovial proteins in RA. Our results indicate that citrullinated vimentin is a candidate autoantigen in RA.

Intermediate filaments control the intracellular distribution of caspases during apoptosis

Caspases are responsible for a cascade of events controlling the disassembly of apoptotic cells. We now demonstrate that caspase-9 is activated at an early stage of apoptosis in epithelial cells and all its detectable, catalytically active large subunits (both the p35 and p37) are concentrated on cytokeratin fibrils. Immunolabeling of distinctive neoepitopes, exposed by cleavage of procaspase-9 at either Asp315 or Asp330, was co-localized on these fibrils with active caspase-3, caspase-cleaved cytokeratin-18, death-effector-domain containing DNA-binding protein and ubiquitin. Cytokeratin filaments may thus provide a scaffold whereby active subunits of caspase-9 can activate caspase-3 which, in turn, can activate more caspase-9 so forming an amplification loop to facilitate cleavage of cytokeratin-18, disruption of the cytoskeleton and the ensuing formation of cytoplasmic inclusions. These inclusions, formed from the collapse of fibrils, together with their associated components, also contain ubiquitinated proteins, vimentin, heat-shock protein 72, and tumor necrosis factor receptor type-1-associated death domain protein. Many of their constituents, including active caspases, remain sequestered within these inclusions, even after detergent treatment and isolation. Thus, such inclusions do not merely accumulate disrupted cytokeratins but also sequestrate potentially noxious proteins that could injure healthy neighboring cells.

Reduced intratesticular testosterone concentration alters the polymerization state of the Sertoli cell intermediate filament cytoskeleton by degradation of vimentin

The Sertoli cell intermediate filament cytoskeleton is composed of the type III family member vimentin. The distribution of Sertoli cell vimentin varies with the stage of spermatogenesis, with shortening of the filaments at stages VII-VIII, the stages of spermiation. Experimental reduction in intratesticular testosterone (T) concentration also results in the sloughing of advanced spermatids from the Sertoli cells, as well as in the apoptotic death of spermatocytes. We hypothesized that alteration of the distribution of Sertoli cell vimentin might play a role in the loss of germ cells that occurs in response to reduced intratesticular T. To test this hypothesis, intratesticular T was reduced by implanting LH-suppressive SILASTIC brand capsules containing T and estradiol into adult rats for 8 wk. Immunohistochemical analyses revealed that, in response to the implants, the vimentin cytoskeleton collapsed around the Sertoli cell nuclei at all stages of the cycle, losing the extensive branching and structure normally seen at most stages of the cycle. Western blots of isolated Sertoli cells revealed that protein levels did not differ significantly between control and T- and estradiol-treated rats. However, Sertoli cell fractions containing the vimentin monomer revealed that vimentin was cleaved into four to five fragments in Sertoli cells in response to the implants, suggestive of proteolysis. These results indicate that, in response to reduced intratesticular T, the vimentin cytoskeleton of the Sertoli cell collapses to a perinuclear localization, and suggest that this collapse is associated with, and perhaps caused by, the degradation of the vimentin monomer rather than by loss of its expression.

High molecular weight vimentin complex is formed after proteolytic digestion of vimentin by caspase-3: detection by sera of patients with interstitial pneumonia

In a previous study, we demonstrated anti-vimentin antibodies in sera of patients with interstitial pneumonia. We hypothesized that antibodies in sera might detect vimentin fragments formed during the process of apoptosis. To prove this, recombinant human vimentin was digested by recombinant human caspase-3 or caspase-8. Then, Western blotting using several commercially available antibodies against human vimentin or patients' sera which had anti-vimentin autoantibodies, was performed. As a result, after recombinant human vimentin was digested by caspase-3 or caspase-8, several vimentin fragments were formed and detected by 2 kinds of monoclonal anti-vimentin antibodies (clone 3B4 and clone V9) as well as by polyclonal sheep anti-human vimentin antibody. It was demonstrated that high molecular weight vimentin was formed after the digestion of vimentin by caspase-3, which was only detected by patients' sera. The high molecular weight vimentin was not formed after digestion of vimentin by caspase-8. Our present results show that high molecular weight vimentin was formed after the digestion of vimentin by caspase-3. In addition, it is suggested that this high molecular weight vimentin acted as an autoantigen to form anti-vimentin autoantibody in vivo.

Macroarray analysis of the effects of JP-8 jet fuel on gene expression in Jurkat cells

The jet fuel JP-8 is widely used and a large number of military and civilian personnel is, therefore, exposed to it. Treatment of several cell lines, including human Jurkat cells, with JP-8 induces cell death that exhibits various biochemical and morphological characteristics of apoptosis. The molecular mechanism of JP-8 cytotoxicity, however, has remained unclear. The effects of exposure of Jurkat cells to JP-8 (1/10,000 dilution) for 4 h on gene expression have now been examined by cDNA macroarray analysis. We had previously shown in these cells that under the above conditions, JP-8 causes significant apoptosis, based upon the observation that caspase-3 activation occurs at approximately 4 h and consequently most of the other classical apoptotic biochemical and morphological alterations progress until apoptotic cell death at 24 h. Of the 439 apoptosis- or stress response-related genes examined, the expression of 16 genes was up-regulated and that of ten genes was down-regulated by a factor of > or =2. The changes in the expression of 11 of these 26 genes were confirmed by reverse transcription and polymerase chain reaction analysis. These results provide insight into the mechanism of JP-8 toxicity and the associated induction of apoptosis.

Binding of protein kinase B to the plakin family member periplakin

The serine/threonine kinase protein kinase B (PKB/c-Akt) acts downstream of the lipid kinase phosphoinositide 3-kinase (PI3K) and functions as an essential mediator in many growth-factor-induced cellular responses such as cell cycle regulation, cell survival and transcriptional regulation. PI3K activation generates 3'-phosphorylated phosphatidylinositol lipids (PtdIns3P) and PKB activation requires PtdIns3P-dependent membrane translocation and phosphorylation by upstream kinases. However PKB activation and function is also regulated by interaction with other proteins. Here we show binding of PKB to periplakin, a member of the plakin family of cytolinker proteins. Interaction between PKB and periplakin was mapped to part of the pleckstrin homology (PH) domain of PKB, which is probably not involved in lipid binding, and indeed binding to periplakin did not affect PKB activation. We therefore investigated the possibility that periplakin may act as a scaffold or localization signal for PKB. In cells endogenous periplakin localizes to different cellular compartments, including plasma membrane, intermediate filament structures, the nucleus and mitochondria. Overexpression of the C-terminal part of periplakin, encompassing the PKB binding region, results in predominant intermediate filament localization and little nuclear staining. This also resulted in inhibition of nuclear PKB signalling as indicated by inhibition of PKB-dependent Forkhead transcription factor regulation. These results suggest a possible role for periplakin as a localization signal in PKB-mediated signalling.