Caspase-mediated cleavage of DNA topoisomerase I at unconventional sites during apoptosis

A pathogenic proteolysis-resistant huntingtin isoform induced by an antisense oligonucleotide maintains huntingtin function

Huntington’s disease (HD) is a late-onset neurological disorder for which therapeutics are not available. Its key pathological mechanism involves the proteolysis of polyglutamine-expanded (polyQ-expanded) mutant huntingtin (mHTT), which generates N-terminal fragments containing polyQ, a key contributor to HD pathogenesis. Interestingly, a naturally occurring spliced form of HTT mRNA with truncated exon 12 encodes an HTT (HTT^Δ12) with a deletion near the caspase-6 cleavage site. In this study, we used a multidisciplinary approach to characterize the therapeutic potential of targeting HTT exon 12. We show that HTT^Δ12 was resistant to caspase-6 cleavage in both cell-free and tissue lysate assays. However, HTT^Δ12 retained overall biochemical and structural properties similar to those of wt-HTT. We generated mice in which HTT exon 12 was truncated and found that the canonical exon 12 was dispensable for the main physiological functions of HTT, including embryonic development and intracellular trafficking. Finally, we pharmacologically induced HTT^Δ12 using the antisense oligonucleotide (ASO) QRX-704. QRX-704 showed predictable pharmacology and efficient biodistribution. In addition, it was stable for several months and inhibited pathogenic proteolysis. Furthermore, QRX-704 treatments resulted in a reduction of HTT aggregation and an increase in dendritic spine count. Thus, ASO-induced HTT exon 12 splice switching from HTT may provide an alternative therapeutic strategy for HD.

Multifunctional Thio-Stabilized Gold Nanoparticles for Near-Infrared Fluorescence Detection and Imaging of Activated Caspase-3

Background

Gold nanoparticles (AuNPs) are commonly used in nanomedicine because of their unique spectral properties, chemical and biological stability, and ability to quench the fluorescence of organic dyes attached to their surfaces. However, the utility of spherical AuNPs for activatable fluorescence sensing of molecular processes have been confined to resonance-matched fluorophores in the 500 nm to 600 nm spectral range to maximize dye fluorescence quenching efficiency. Expanding the repertoire of fluorophore systems into the NIR fluorescence regimen with emission >800 nm will facilitate the analysis of multiple biological events with high detection sensitivity.

Objective

The primary goal of this study is to determine if spherical AuNP-induced radiative rate suppression of non-resonant near-infrared (NIR) fluorescent probes can serve as a versatile nanoconstruct for highly sensitive detection and imaging of activated caspase-3 in aqueous media and cancer cells. This required the development of activatable NIR fluorescence sensors of caspase-3 designed to overcome the nonspecific degradation and release of the surface coatings in aqueous media.

Method

We harnessed the fluorescence-quenching properties and multivalency of spherical AuNPs to develop AuNP-templated activatable NIR fluorescent probes to detect activated caspase-3, an intracellular reporter of early cell death. Freshly AuNPs were coated with a multifunctional NIR fluorescent dye-labeled peptide (LS422) consisting of an RGD peptide sequence that targets α_vβ₃-integrin protein (α_vβ₃) on the surface of cancer cells to mediate the uptake and internalization of the sensors in tumor cells; a DEVD peptide sequence for reporting the induction of cell death through caspase-3 mediated NIR fluorescence enhancement; and a multidentate hexacysteine sequence for enhancing self-assembly and stabilizing the multifunctional construct on AuNPs. The integrin binding affinity of LS422 and caspase-3 kinetics were determined by a radioligand competitive binding and fluorogenic peptide assays, respectively. Detection of intracellular caspase-3, cell viability, and the internalization of LS422 in cancer cells were determined by confocal NIR fluorescence spectroscopy and microscopy.

Results

Narrow size AuNPs (13 nm) were prepared and characterized by transmission electron microscopy and dynamic light scattering. When assembled on the AuNPs, the binding constant of LS422 for α_vβ₃ improved 11-fold from 13.2 nM to 1.2 nM. Whereas the catalytic turnover of caspase-3 by LS422-AuNPs was similar to the reference fluorogenic peptide, the binding affinity for the enzyme increased by a factor of 2. Unlike the α_vβ₃ positive, but caspase-3 negative breast cancer MCF-7 cells, treatment of the α_vβ₃ and caspase-3 positive lung cancer A549 cells with Paclitaxel showed significant fluorescence enhancement within 30 minutes, which correlated with caspase-3 specific activation of LS422-AuNPs fluorescence. Incorporation of a 3.5 mW NIR laser source into our spectrofluorometer increased the detection sensitivity by an order of magnitude (limit of detection ~0.1 nM of cypate) and significantly decreased the signal noise relative to a xenon lamp. This gain in sensitivity enabled the detection of substrate hydrolysis at a broad range of inhibitor concentrations without photobleaching the cypate dye.

Conclusion

The multifunctional AuNPs demonstrate the use of a non-resonant quenching strategy to design activatable NIR fluorescence molecular probes. The nanoconstruct offers a selective reporting method for detecting activated caspase-3, imaging of cell viability, identifying dying cells, and visualizing the functional status of intracellular enzymes. Performing these tasks with NIR fluorescent probes creates an opportunity to translate the in vitro and cellular analysis of enzymes into in vivo interrogation of their functional status using deep tissue penetrating NIR fluorescence analytical methods.

Accumulation of Nucleolar Inorganic Polyphosphate Is a Cellular Response to Cisplatin-Induced Apoptosis

The chemotherapeutic drug cisplatin, which targets DNA, serves as one of the main staples in cancer treatment. Yet, the therapeutic application of cisplatin is limited by two major challenges: the occurrence of reversible and irreversible side effects due to non-specific toxicity, and the intrinsic or developing resistance of tumor cells toward cisplatin. Here we demonstrate that cancer cells respond to cisplatin treatment with the nucleolar accumulation of inorganic polyphosphate (polyP), a universally conserved high-energy compound. PolyP accumulation positively correlates with the levels of activated caspase-3, suggesting a novel role of polyP in cisplatin-mediated apoptosis. In support of this finding, we discovered that administration of exogenous polyP increases cisplatin-induced toxicity in select cancer cell lines, raising the exciting possibility that enhancing endogenous polyP levels might be a novel mechanism to sensitize cancer cells to cisplatin treatment.

The Small Heat Shock Protein α-Crystallin B Shows Neuroprotective Properties in a Glaucoma Animal Model

Glaucoma is a neurodegenerative disease that leads to irreversible retinal ganglion cell (RGC) loss and is one of the main causes of blindness worldwide. The pathogenesis of glaucoma remains unclear, and novel approaches for neuroprotective treatments are urgently needed. Previous studies have revealed significant down-regulation of α-crystallin B as an initial reaction to elevated intraocular pressure (IOP), followed by a clear but delayed up-regulation, suggesting that this small heat-shock protein plays a pathophysiological role in the disease. This study analyzed the neuroprotective effect of α-crystallin B in an experimental animal model of glaucoma. Significant IOP elevation induced by episcleral vein cauterization resulted in a considerable impairment of the RGCs and the retinal nerve fiber layer. An intravitreal injection of α-crystallin B at the time of the IOP increase was able to rescue the RGCs, as measured in a functional photopic electroretinogram, retinal nerve fiber layer thickness, and RGC counts. Mass-spectrometry-based proteomics and antibody-microarray measurements indicated that a α-crystallin injection distinctly up-regulated all of the subclasses (α, β, and γ) of the crystallin protein family. The creation of an interactive protein network revealed clear correlations between individual proteins, which showed a regulatory shift resulting from the crystallin injection. The neuroprotective properties of α-crystallin B further demonstrate the potential importance of crystallin proteins in developing therapeutic options for glaucoma.

Proinflammatory M1 Macrophages Inhibit RANKL-Induced Osteoclastogenesis

In response to a defined panel of stimuli, immature macrophages can be classified into two major phenotypes: proinflammatory (M1) and anti-inflammatory (M2). Although both phenotypes have been implicated in several chronic inflammatory diseases, their direct role in bone resorption remains unclear. The present study investigated the possible effects of M1 and M2 macrophages on RANKL-induced osteoclastogenesis. In osteoclastogenesis assays using RAW264.7 cells or bone marrow cells as osteoclast precursors, addition of M1 macrophages significantly suppressed RANKL-induced osteoclastogenesis compared to nonstimulated conditions (M0), addition of M2 macrophages, or no macrophage addition (P < 0.05), suggesting that M1 macrophages can downregulate osteoclastogenesis. This effect was maintained when direct contact between M1 and osteoclast precursors was interrupted by cell culture insertion, indicating engagement of soluble factors released from M1. M1 macrophages developed from interferon gamma (IFN-γ) knockout (IFN-γ-KO) mice lost the ability to downregulate osteoclastogenesis. Antibody-based neutralization of interleukin-12 (IL-12), but not IL-10, produced by M1 macrophages also abrogated M1-mediated downregulation of osteoclastogenesis. Real-time PCR analyses showed that IFN-γ suppressed gene expression of NFATc1, a master regulator of osteoclastogenesis, whereas IL-12 increased the apoptosis of osteoclasts, suggesting molecular mechanisms underlying the possible roles of IFN-γ or IL-12 in M1-mediated inhibition of osteoclastogenesis. These findings were confirmed in an in vivo ligature-induced mouse periodontitis model in which adoptive transfer of M1 macrophages showed a significantly lower level of bone loss and less tartrate-resistant acid phosphatase (TRAP)-positive cell induction than M0 or M2 macrophage transfer. In conclusion, by its secretion of IFN-γ and IL-12, M1, but not M0 or M2, was demonstrated to inhibit osteoclastogenesis.

Docosahexanoic acid antagonizes TNF-α-induced necroptosis by attenuating oxidative stress, ceramide production, lysosomal dysfunction, and autophagic features

OBJECTIVE

It was previously reported that docosahexanoic acid (DHA) reduces TNF-α-induced necrosis in L929 cells. However, the mechanisms underlying this reduction have not been investigated. The present study was designed to investigate cellular and biochemical mechanisms underlying the attenuation of TNF-α-induced necroptosis by DHA in L929 cells.

METHODS

L929 cells were pre-treated with DHA prior to exposure to TNF-α, zVAD, or Necrostatin-1 (Nec-1). Cell death and survival were assessed by MTT and caspase activity assays, and microscopic visualization. Reactive oxygen species (ROS) were measured by flow cytometry. C16- and C18-ceramides were measured by mass spectrometry. Lysosomal membrane permeabilization (LMP) was evaluated by fluorescence microscopy and flow cytometry using Acridine Orange. Cathepsin L activation was evaluated by immunoblotting and fluorescence microscopy. Autophagy was assessed by immunoblotting of LC3-II and Beclin.

RESULTS

Exposure of L929 cells to TNF-α alone for 24 h induced necroptosis, as evidenced by the inhibition of cell death by Nec-1, absence of caspase-3 activity and Lamin B cleavage, and morphological analysis. DHA attenuated multiple biochemical events associated with TNF-α-induced necroptosis, including ROS generation, ceramide production, lysosomal dysfunction, cathepsin L activation, and autophagic features. DHA also attenuated zVAD-induced necroptosis but did not attenuate the enhanced apoptosis and necrosis induced by the combination of TNF-α with Actinomycin D or zVAD, respectively, suggesting that its protective effects might be limited by the strength of the cell death insult induced by TNF-α.

CONCLUSIONS

DHA effectively attenuates TNF-α-induced necroptosis and autophagy, most likely via its ability to inhibit TNF-α-induced sphingolipid metabolism and oxidative stress. These results highlight the role of this Omega-3 fatty acid in antagonizing inflammatory cell death.

Expression and activation of caspase-6 in human fetal and adult tissues

Caspase-6 is an effector caspase that has not been investigated thoroughly despite the fact that Caspase-6 is strongly activated in Alzheimer disease brains. To understand the full physiological impact of Caspase-6 in humans, we investigated Caspase-6 expression. We performed western blot analyses to detect the pro-Caspase-6 and its active p20 subunit in fetal and adult lung, kidney, brain, spleen, muscle, stomach, colon, heart, liver, skin, and adrenals tissues. The levels were semi-quantitated by densitometry. The results show a ubiquitous expression of Caspase-6 in most fetal tissues with the lowest levels in the brain and the highest levels in the gastrointestinal system. Caspase-6 active p20 subunits were only detected in fetal stomach. Immunohistochemical analysis of a human fetal embryo showed active Caspase-6 positive apoptotic cells in the dorsal root ganglion, liver, lung, kidney, ovary, skeletal muscle and the intestine. In the adult tissues, the levels of Caspase-6 were lower than in fetal tissues but remained high in the colon, stomach, lung, kidney and liver. Immunohistological analyses revealed that active Caspase-6 was abundant in goblet cells and epithelial cells sloughing off the intestinal lining of the adult colon. These results suggest that Caspase-6 is likely important in most tissues during early development but is less involved in adult tissues. The low levels of Caspase-6 in fetal and adult brain indicate that increased expression as observed in Alzheimer Disease is a pathological condition. Lastly, the high levels of Caspase-6 in the gastrointestinal system indicate a potential specific function of Caspase-6 in these tissues.

Caspase-3 mediated release of SAC domain containing fragment from Par-4 is necessary for the sphingosine-induced apoptosis in Jurkat cells

Background

Prostate apoptosis response-4 (Par-4) is a tumor-suppressor protein that selectively activates and induces apoptosis in cancer cells, but not in normal cells. The cancer specific pro-apoptotic function of Par-4 is encoded in its centrally located SAC (Selective for Apoptosis induction in Cancer cells) domain (amino acids 137–195). The SAC domain itself is capable of nuclear entry, caspase activation, inhibition of NF-κB activity, and induction of apoptosis in cancer cells. However, the precise mechanism(s) of how the SAC domain is released from Par-4, in response to apoptotic stimulation, is not well explored.

Results

In this study, we demonstrate for the first time that sphingosine (SPH), a member of the sphingolipid family, induces caspase-dependant cleavage of Par-4, leading to the release of SAC domain containing fragment from it. Par-4 is cleaved at the EEPD131G site on incubation with caspase-3 in vitro, and by treating cells with several anti-cancer agents. The caspase-3 mediated cleavage of Par-4 is blocked by addition of the pan-caspase inhibitor z-VAD-fmk, caspase-3 specific inhibitor Ac-DEVD-CHO, and by introduction of alanine substitution for D131 residue. Moreover, suppression of SPH-induced Akt dephosphorylation also abrogated the caspase dependant cleavage of Par-4.

Conclusion

Evidence provided here shows that Par-4 is cleaved by caspase-3 during SPH-induced apoptosis. Cleavage of Par-4 leads to the generation of SAC domain containing fragment which may possibly be essential and sufficient to induce or augment apoptosis in cancer cells.

Augmented generation of protein fragments during wakefulness as the molecular cause of sleep: a hypothesis

Despite extensive understanding of sleep regulation, the molecular-level cause and function of sleep are unknown. I suggest that they originate in individual neurons and stem from increased production of protein fragments during wakefulness. These fragments are transient parts of protein complexes in which the fragments were generated. Neuronal Ca²⁺ fluxes are higher during wakefulness than during sleep. Subunits of transmembrane channels and other proteins are cleaved by Ca²⁺-activated calpains and by other nonprocessive proteases, including caspases and secretases. In the proposed concept, termed the fragment generation (FG) hypothesis, sleep is a state during which the production of fragments is decreased (owing to lower Ca²⁺ transients) while fragment-destroying pathways are upregulated. These changes facilitate the elimination of fragments and the remodeling of protein complexes in which the fragments resided. The FG hypothesis posits that a proteolytic cleavage, which produces two fragments, can have both deleterious effects and fitness-increasing functions. This (previously not considered) dichotomy can explain both the conservation of cleavage sites in proteins and the evolutionary persistence of sleep, because sleep would counteract deleterious aspects of protein fragments. The FG hypothesis leads to new explanations of sleep phenomena, including a longer sleep after sleep deprivation. Studies in the 1970s showed that ethanol-induced sleep in mice can be strikingly prolonged by intracerebroventricular injections of either Ca²⁺ alone or Ca²⁺ and its ionophore (Erickson et al., Science 1978;199:1219-1221; Harris, Pharmacol Biochem Behav 1979;10:527-534; Erickson et al., Pharmacol Biochem Behav 1980;12:651-656). These results, which were never interpreted in connection to protein fragments or the function of sleep, may be accounted for by the FG hypothesis about molecular causation of sleep.

Caspase-3 activation as a bifurcation point between plasticity and cell death

Death-mediating proteases such as caspases and caspase-3 in particular, have been implicated in neurodegenerative processes, aging and Alzheimer's disease. However, emerging evidence suggests that in addition to their classical role in cell death, caspases play a key role in modulating synaptic function. It is remarkable that active caspases-3, which can trigger widespread damage and degeneration, aggregates in structures as delicate as synapses and persists in neurons without causing acute cell death. Here, we evaluate this dichotomy, and discuss the hypothesis that caspase-3 may be a bifurcation point in cellular signaling, able to orient the neuronal response to stress down either pathological/apoptotic pathways or towards physiological cellular remodeling. We propose that temporal, spatial and other regulators of caspase activity are key determinants of the ultimate effect of caspase-3 activation in neurons. This concept has implications for differential roles of caspase-3 activation across the lifespan. Specifically, we propose that limited caspase-3 activation is critical for synaptic function in the healthy adult brain while chronic activation is involved in degenerative processes in the aging brain.

A short caspase-3 isoform inhibits chemotherapy-induced apoptosis by blocking apoptosome assembly

Alternative splicing of caspase-3 produces a short isoform caspase-3s that antagonizes caspase-3 apoptotic activity. However, the mechanism of apoptosis inhibition by caspase-3s remains unknown. Here we show that exogenous caspase-3 sensitizes MCF-7 and HBL100 breast cancers cells to chemotherapeutic treatments such as etoposide and methotrexate whereas co-transfection with caspase-3s strongly inhibits etoposide and methotrexate-induced apoptosis underlying thus the anti-apoptotic role of caspase-3s. In caspase-3 transfected cells, lamin-A and α-fodrin were cleaved when caspase-3 was activated by etoposide or methotrexate. When caspase-3s was co-transfected, this cleavage was strongly reduced. Depletion of caspase-3 by RNA interference in HBL100 containing endogenous caspase-3s caused reduction in etoposide and methotrexate-induced apoptosis, whereas the depletion of caspase-3s sensitized cells to chemotherapy. In the presence of caspase-3s, a lack of interaction between caspase-3 and caspase-9 was observed. Immunoprecipitation assays showed that caspase-3s binds the pro-forms of caspase-3. This result suggested that the absence of interaction with caspase-9 when both variants of caspase-3 are present contribute to block the apoptosome assembly and inhibit apoptosis. These data support that caspases-3s negatively interferes with caspase-3 activation and apoptosis in breast cancer, and that it can play key roles in the modulation of response to chemotherapeutic treatments.

Prostate apoptosis response 4 (Par-4), a novel substrate of caspase-3 during apoptosis activation

Prostate apoptosis response 4 (Par-4) is a ubiquitously expressed proapoptotic tumor suppressor protein. Here, we show for the first time, that Par-4 is a novel substrate of caspase-3 during apoptosis. We found that Par-4 is cleaved during cisplatin-induced apoptosis in human normal and cancer cell lines. Par-4 cleavage generates a C-terminal fragment of ~25 kDa, and the cleavage of Par-4 is completely inhibited by a caspase-3 inhibitor, suggesting that caspase-3 is directly involved in the cleavage of Par-4. Caspase-3-deficient MCF-7 cells do not show Par-4 cleavage in response to cisplatin treatment, and restoration of caspase-3 in MCF-7 cells produces a decrease in Par-4 levels, with the appearance of a cleaved fragment. Additionally, knockdown of Par-4 reduces caspase-3 activation and apoptosis induction. Site-directed mutagenesis reveals that Par-4 cleavage by caspase-3 occurs at an unconventional site, EEPD(131)↓G. Interestingly, overexpression of wild-type Par-4 but not the Par-4 D131A mutant sensitizes cells to cisplatin-induced apoptosis. Upon caspase-3 cleavage, the cleaved fragment of Par-4 accumulates in the nucleus and displays increased apoptotic activity. Overexpression of the cleaved fragment of Par-4 inhibits IκBα phosphorylation and blocks NF-κB nuclear translocation. We have identified a novel specific caspase-3 cleavage site in Par-4, and the cleaved fragment of Par-4 retains proapoptotic activity.

A quantitative method for the specific assessment of caspase-6 activity in cell culture

Aberrant activation of caspase-6 has recently emerged as a major contributor to the pathogeneses of neurodegenerative disorders such as Alzheimer's and Huntington disease. Commercially available assays to measure caspase-6 activity commonly use the VEID peptide as a substrate. However these methods are not well suited to specifically assess caspase-6 activity in the presence of other, confounding protease activities, as often encountered in cell and tissue samples. Here we report the development of a method that overcomes this limitation by using a protein substrate, lamin A, which is highly specific for caspase-6 cleavage at amino acid 230. Using a neo-epitope antibody against cleaved lamin A, we developed an electrochemiluminescence-based ELISA assay that is suitable to specifically detect and quantify caspase-6 activity in highly apoptotic cell extracts. The method is more sensitive than VEID-based assays and can be adapted to a high-content imaging platform for high-throughput screening. This method should be useful to screen for and characterize caspase-6 inhibitor compounds and other interventions to decrease intracellular caspase-6 activity for applications in neurodegenerative disorders.

A multi-factor model for caspase degradome prediction

BACKGROUND

Caspases belong to a class of cysteine proteases which function as critical effectors in cellular processes such as apoptosis and inflammation by cleaving substrates immediately after unique tetrapeptide sites. With hundreds of reported substrates and many more expected to be discovered, the elucidation of the caspase degradome will be an important milestone in the study of these proteases in human health and disease. Several computational methods for predicting caspase cleavage sites have been developed recently for identifying potential substrates. However, as most of these methods are based primarily on the detection of the tetrapeptide cleavage sites - a factor necessary but not sufficient for predicting in vivo substrate cleavage - prediction outcomes will inevitably include many false positives.

RESULTS

In this paper, we show that structural factors such as the presence of disorder and solvent exposure in the vicinity of the cleavage site are important and can be used to enhance results from cleavage site prediction. We constructed a two-step model incorporating cleavage site prediction and these factors to predict caspase substrates. Sequences are first predicted for cleavage sites using CASVM or GraBCas. Predicted cleavage sites are then scored, ranked and filtered against a cut-off based on their propensities for locating in disordered and solvent exposed regions. Using an independent dataset of caspase substrates, the model was shown to achieve greater positive predictive values compared to CASVM or GraBCas alone, and was able to reduce the false positives pool by up to 13% and 53% respectively while retaining all true positives. We applied our prediction model on the family of receptor tyrosine kinases (RTKs) and highlighted several members as potential caspase targets. The results suggest that RTKs may be generally regulated by caspase cleavage and in some cases, promote the induction of apoptotic cell death - a function distinct from their role as transducers of survival and growth signals.

CONCLUSION

As a step towards the prediction of in vivo caspase substrates, we have developed an accurate method incorporating cleavage site prediction and structural factors. The multi-factor model augments existing methods and complements experimental efforts to define the caspase degradome on the systems-wide basis.

Role of oxidative stress in pancreatic inflammation

Reactive oxygen and reactive nitrogen species (ROS/RNS) have been implicated in the pathogenesis of acute and chronic pancreatitis. Clinical and basic science studies have indicated that ROS/RNS formation processes are intimately linked to the development of the inflammatory disorders. The detrimental effects of highly reactive ROS/RNS are mediated by their direct actions on biomolecules (lipids, proteins, and nucleic acids) and activation of proinflammatory signal cascades, which subsequently lead to activation of immune responses. The present article summarizes the possible sources of ROS/RNS formation and the detailed signaling cascades implicated in the pathogenesis of pancreatic inflammation, as observed in acute and chronic pancreatitis. A therapeutic ROS/RNS-scavenging strategy has been advocated for decades; however, clinical studies examining such approaches have been inconsistent in their results. Emerging evidence indicates that pancreatitis-inducing ROS/RNS generation may be attenuated by targeting ROS/RNS-generating enzymes and upstream mediators.

hScrib, a human homologue of Drosophila neoplastic tumor suppressor, is a novel death substrate targeted by caspase during the process of apoptosis

hScrib, human homologue of Drosophila neoplastic tumor suppressor, was identified as a target of human papillomavirus E6 oncoprotein for the ubiquitin-mediated degradation. Here, we report that hScrib is a novel death substrate targeted by caspase. Full-length hScrib was cleaved by caspase during death ligands-induced apoptosis, which generates a p170 C-terminal fragments in Hela cells. In vitro cleavage assay using recombinant caspases showed that hScrib is cleaved by the executioner caspases. DNA damage-induced apoptosis caused loss of expression of full-length hScrib, which was recovered by addition of capase-3 inhibitor in HaCat cells. TUNEL positive apoptotic cells, which were identified 4 h after UV irradiation in HaCat cells, showed loss of hScrib expression at the adherens junction. Mutational analysis identified the caspase-dependent cleavage site of hScrib at the position of Asp-504. Although MDCK cells transfected with GFP-fused wild-type hScrib showed loss of E-cadherin expression and shrinkage of cytoplasm by UV irradiation, cells transfected with hScrib with Ala substitution of Asp-504 showed resistance to caspase-dependent cleavage of hScrib and intact expression of E-cadherin. These results indicate that caspase-dependent cleavage of hScrib is a critical step for detachment of cell contact during the process of apoptosis.

Topoisomerase I requirement for death receptor-induced apoptotic nuclear fission

Topoisomerase I (Top1) is known to relax DNA supercoiling generated by transcription, replication, and chromatin remodeling. However, it can be trapped on DNA as cleavage complexes (Top1cc) by oxidative and carcinogenic DNA lesions, base damage, and camptothecin treatment. We show here that Top1 is also functionally involved in death receptor-induced programmed cell death. In cells exposed to TRAIL or Fas ligand, Top1cc form at the onset of apoptosis. Those apoptotic Top1cc are prevented by caspase inhibition and Bax inactivation, indicating that both caspases and the mitochondrial death pathway are required for their formation. Accordingly, direct activation of the mitochondrial pathway by BH3 mimetic molecules induces apoptotic Top1cc. We also show that TRAIL-induced apoptotic Top1cc are preferentially formed by caspase-3-cleaved Top1 at sites of oxidative DNA lesions with an average of one apoptotic Top1cc/100 kbp. Examination of Top1 knock-down cells treated with TRAIL revealed similar DNA fragmentation but a marked decrease in apoptotic nuclear fission with reduced formation of nuclear bodies. Thus, we propose that Top1 contributes to the full apoptotic responses induced by TRAIL.

Targets of caspase-6 activity in human neurons and Alzheimer disease

Caspase-6 activation occurs early in Alzheimer disease and sometimes precedes the clinical manifestation of the disease in aged individuals. The active Caspase-6 is localized in neuritic plaques, in neuropil threads, and in neurofibrillary tangles containing neurons that are not morphologically apoptotic in nature. To investigate the potential consequences of the activation of Caspase-6 in neurons, we conducted a proteomics analysis of Caspase-6-mediated cleavage of human neuronal proteins. Proteins from the cytosolic and membrane subcellular compartments were treated with recombinant active Caspase-6 and compared with undigested proteins by two-dimensional gel electrophoresis. LC/MS/MS analyses of the proteins that were cleaved identified 24 different potential protein substrates. Of these, 40% were cytoskeleton or cytoskeleton-associated proteins. We focused on the cytoskeleton proteins because these are critical for neuronal structure and function. Caspase-6 cleavage of alpha-Tubulin, alpha-Actinin-4, Spinophilin, and Drebrin was confirmed. At least one Caspase-6 cleavage site was identified for Drebrin, Spinophilin, and alpha-Tubulin. A neoepitope antiserum to alpha-Tubulin cleaved by Caspase-6 immunostained neurons, neurofibrillary tangles, neuropil threads, and neuritic plaques in Alzheimer disease and co-localized with active Caspase-6. These results imply that the early and neuritic activation of Caspase-6 in Alzheimer disease could disrupt the cytoskeleton network of neurons, resulting in impaired neuronal structure and function in the absence of cell death. This study provides novel insights into the pathophysiology of Alzheimer disease.

Identification of caspase 3 motifs and critical aspartate residues in human phospholipase D1b and phospholipase D2a

Stimulation of mammalian cells frequently initiates phospholipase D-catalyzed hydrolysis of phosphatidylcholine in the plasma membrane to yield phosphatidic acid (PA) a novel lipid messenger. PA plays a regulatory role in important cellular processes such as secretion, cellular shape change, and movement. A number of studies have highlighted that PLD-based signaling also plays a pro-mitogenic and pro-survival role in cells and therefore anti-apoptotic. We show that human PLD1b and PLD2a contain functional caspase 3 cleavage sites and identify the critical aspartate residues within PLD1b that affect its activation by phorbol esters and attenuate phosphatidylcholine hydrolysis during apoptosis.

Apoptosis-associated caspase activation assays

Caspases are aspartate-directed cysteine proteases that cleave a diverse group of intracellular substrates to contribute to various manifestations of apoptosis. These proteases are synthesized as inactive precursors and are activated as a consequence of signaling induced by a wide range of physiological and pathological stimuli. Caspase activation can be detected by measurement of catalytic activity, immunoblotting for cleavage of their substrates, immunolabeling using conformation-sensitive antibodies or affinity labeling followed by flow cytometry or ligand blotting. Here we describe methods for each of these assays, identify recent improvements in these assays and outline the strengths and limitations of each approach.

Caspase-mediated cleavage of the exosome subunit PM/Scl-75 during apoptosis

Recent studies have implicated the dying cell as a potential reservoir of modified autoantigens that might initiate and drive systemic autoimmunity in susceptible hosts. A number of subunits of the exosome, a complex of 3'-->5' exoribonucleases that functions in a variety of cellular processes, are recognized by the so-called anti-PM/Scl autoantibodies, found predominantly in patients suffering from an overlap syndrome of myositis and scleroderma. Here we show that one of these subunits, PM/Scl-75, is cleaved during apoptosis. PM/Scl-75 cleavage is inhibited by several different caspase inhibitors. The analysis of PM/Scl-75 cleavage by recombinant caspase proteins shows that PM/Scl-75 is efficiently cleaved by caspase-1, to a smaller extent by caspase-8, and relatively inefficiently by caspase-3 and caspase-7. Cleavage of the PM/Scl-75 protein occurs in the C-terminal part of the protein at Asp369 (IILD369 [see text] G), and at least a fraction of the resulting N-terminal fragments of PM/Scl-75 remains associated with the exosome. Finally, the implications of PM/Scl-75 cleavage for exosome function and the generation of anti-PM/Scl-75 autoantibodies are discussed.

Mechanisms of cell death in oxidative stress

Reactive oxygen or nitrogen species (ROS/RNS) generated endogenously or in response to environmental stress have long been implicated in tissue injury in the context of a variety of disease states. ROS/RNS can cause cell death by nonphysiological (necrotic) or regulated pathways (apoptotic). The mechanisms by which ROS/RNS cause or regulate apoptosis typically include receptor activation, caspase activation, Bcl-2 family proteins, and mitochondrial dysfunction. Various protein kinase activities, including mitogen-activated protein kinases, protein kinases-B/C, inhibitor-of-I-kappaB kinases, and their corresponding phosphatases modulate the apoptotic program depending on cellular context. Recently, lipid-derived mediators have emerged as potential intermediates in the apoptosis pathway triggered by oxidants. Cell death mechanisms have been studied across a broad spectrum of models of oxidative stress, including H2O2, nitric oxide and derivatives, endotoxin-induced inflammation, photodynamic therapy, ultraviolet-A and ionizing radiations, and cigarette smoke. Additionally ROS generated in the lung and other organs as the result of high oxygen therapy or ischemia/reperfusion can stimulate cell death pathways associated with tissue damage. Cells have evolved numerous survival pathways to counter proapoptotic stimuli, which include activation of stress-related protein responses. Among these, the heme oxygenase-1/carbon monoxide system has emerged as a major intracellular antiapoptotic mechanism.

Cellular stress triggers the human topoisomerase I damage response independently of DNA damage in a p53 controlled manner

The 'human topoisomerase I (htopoI) damage response' was reported to be triggered by various kinds of DNA lesions. Also, a high and persistent level of htopoI cleavage complexes correlated with apoptosis. In the present study, we demonstrate that DNA damage-independent induction of cell death using colcemid and tumor necrosis factor alpha is also accompanied by a strong htopoI response that correlates with the onset of apoptotic hallmarks. Consequently, these results suggest that htopoI cleavage complex formation may be caused by signaling pathways independent of the kind of cellular stress. Thus, protein interactions or signaling cascades induced by DNA damage or cellular stress might lead to the formation of stabilized cleavage complexes rather than the DNA lesion itself. Finally, we show that p53 not only plays a key role in the regulation of the htopoI response to UV-C irradiation but also to treatment with colcemid.

Differential susceptibility to etoposide in clones derived from a human ovarian cancer cell line

OBJECTIVES

To identify parameters/factors that may contribute to the differential sensitivity to etoposide in two clones isolated from the human ovarian carcinoma SKOV-3 cell line, which does not express p53 and is resistant to platinum-based regimens.

METHODS

Differential sensitivity of the cells to etoposide was monitored by microscopy to observe morphological changes, by flow cytometry analyses to detect cell cycle perturbations, and by molecular/biochemical assays to identify events involved in induction of apoptosis.

RESULTS

Etoposide treatment (1) induced apoptosis in one clone, ES, but not in another clone, ER, (2) had no effect on the expression of the antiapoptotic proteins Bcl-2 and Bcl-X(L) in both cell clones, whereas the proapoptotic proteins Bak and Bax were dramatically upregulated in ES, but not ER cells, and (3) induced more extensive processing of procaspase-8, procaspase-9, and the caspase-3-targeted substrates, topoisomerase I and PARP, in ES cells. Ectopic overexpression of Bcl-2 in ES cells failed to inhibit etoposide-induced apoptosis.

CONCLUSIONS

The differential susceptibility of ES and ER cells to etoposide-induced apoptosis is associated with differences in several events rather than with a specific single genetic regulator of the apoptotic machinery. We propose that the differential response of ovarian cancer patients to etoposide treatment is associated with the number of etoposide-sensitive cells in the tumor.

DNA topoisomerase I binding to fibroblasts induces monocyte adhesion and activation in the presence of anti-topoisomerase I autoantibodies from systemic sclerosis patients

OBJECTIVE

Systemic sclerosis (SSc) is an autoimmune disease characterized by fibrosis due to excessive and dysregulated collagen production by fibroblasts. Previously, we reported that anti-DNA topoisomerase I (anti-topo I) antibodies bound specifically to fibroblast surfaces; however, we had not identified their antigenic target. We undertook this study to characterize the target of anti-topo I antibodies on fibroblasts and the effects of their binding.

METHODS

Purified topo I or topo I released from apoptotic cells was tested for surface binding to a number of human cell types by cell-based enzyme-linked immunosorbent assay, flow cytometry, and indirect immunofluorescence. Antibodies purified from SSc patient and normal control sera were used to detect topo I binding. The consequences of topo I and anti-topo I binding to fibroblasts were assessed by coculture with THP-1 monocytes.

RESULTS

The autoantigen topo I itself was found to bind specifically to fibroblasts in a dose-dependent and saturable manner, where it was recognized by anti-topo I from SSc patients. The binding of anti-topo I subsequently stimulated adhesion and activation of cocultured monocytes. Topo I released from apoptotic endothelial cells was also found to bind specifically to fibroblasts.

CONCLUSION

The findings of this study thus confirm and extend the findings of our previous study by showing that topo I binding to fibroblast surfaces is both necessary and sufficient for anti-topo I binding. Second, topo I-anti-topo I complex binding can then trigger the adhesion and activation of monocytes, thus providing a plausible model for the amplification of the fibrogenic cascade in anti-topo I-positive SSc patients.

Cleavage at the carboxyl-terminus of Ku80 during apoptosis in human Jurkat T cells

We have previously reported that the amount of Apg-2, an Hsp110 family protein, decreases during apoptosis in Jurkat T cells. Since we hypothesized that Apg-2 would be cleaved by caspase-3 during apoptosis, a cleavage-site-directed antibody was raised against the carboxyl-terminus of the Apg-2 fragment that appears after the cleavage by caspase-3. Although this antibody could not detect the Apg-2 fragment in apoptotic cells, three additional fragments were unexpectedly detected. Based on the results of microsequencing, one of these fragments was identified as Ku80. Ku80 is a nuclear protein and a component of DNA-dependent protein kinase (DNA-PK). In this study, we observed that Ku80 is cleaved at Asp-730 residue during apoptosis, and this cleavage occurs in the nucleus in the early apoptotic phase. Furthermore, Ku80 is distributed in the cytoplasm of nuclear fragmented apoptotic cells, although the cleaved fragment contains the nuclear-localization signal (NLS). Our study clearly shows that Ku80 is cleaved in the nucleus, and distributes in the cytoplasm during apoptosis.

Selective up-regulation of caspase-3 gene expression following TCR engagement

Activation-induced cell death (AICD) in T lymphocytes depends on the expression of Fas-ligand, which triggers the apoptotic process after binding to its receptor Fas. This leads to the activation of cysteine proteases of the caspase family and especially of caspase-3, a critical effector protein during AICD. We have previously observed the up-regulation of caspase-3 expression in effector but not memory T cells stimulated in vivo. In this study, we further characterized the regulation of caspase expression following T cell receptor (TCR) signaling and demonstrate that a three-fold increase in caspase-3 mRNA levels was observed by semi-quantitative and real-time RT-PCR analysis. Caspase-3 expression was selectively increased among five different caspases following TCR stimulation, as assessed by RNase protection assay. Real-time RT-PCR analysis demonstrated that a three-fold up-regulation in caspase-3 mRNA levels was observed following TCR triggering, whereas caspase-8 mRNA levels remained unchanged. The increase in caspase-3 mRNA levels occurred before cleavage and activation of caspase-3 and in the absence of apoptosis. TCR-mediated induction in caspase-3 expression was not dependent on STAT1 activation, since following stimulation of KOX-14 cells the transcription factor was not phosphorylated. Together, these results show that TCR activation triggers the selective increase in caspase-3 mRNA levels, independently of caspase activity and the induction of apoptosis.

Interaction between gemcitabine and topotecan in human non-small-cell lung cancer cells: effects on cell survival, cell cycle and pharmacogenetic profile

The pyrimidine analogue gemcitabine is an established effective agent in the treatment of non-small-cell lung cancer (NSCLC). The present study investigates whether gemcitabine would be synergistic with the topoisomerase I inhibitor topotecan against the NSCLC A549 and Calu-6 cells. Cells were treated with gemcitabine and topotecan for 1 h and the type of drug interaction was assessed using the combination index (CI). Cell cycle alterations were analysed by flow cytometry, while apoptosis was examined by the occurrence of DNA internucleosomal fragmentation, nuclear condensation and caspase-3 activation. Moreover, the possible involvement of the PI3K-Akt signalling pathway was investigated by the measurement of Akt phosphorylation. Finally, quantitative, real-time PCR (QRT-PCR) was used to study modulation of the gemcitabine-activating enzyme deoxycytidine kinase (dCK) and the cellular target enzyme ribonucleotide reductase (RR). In results, it was found that simultaneous and sequential topotecan → gemcitabine treatments were synergistic, while the reverse sequence was antagonistic in both cell lines. DNA fragmentation, nuclear condensation and enhanced caspase-3 activity demonstrated that the drug combination markedly increased apoptosis in comparison with either single agent, while cell cycle analysis showed that topotecan increased cells in S phase. Furthermore, topotecan treatment significantly decreased the amount of the activated form of Akt, and enhanced the expression of dCK (+155.0 and +115.3% in A549 and Calu-6 cells, respectively), potentially facilitating gemcitabine activity. In conclusion, these results indicate that the combination of gemcitabine and topotecan displays schedule-dependent activity in vitro against NSCLC cells. The gemcitabine → topotecan sequence is antagonistic while drug synergism is obtained with the simultaneous and the sequential topotecan → gemcitabine combinations, which are associated with induction of decreased Akt phosphorylation and increased dCK expression.

A substrate-phage approach for investigating caspase specificity

We have developed a substrate-phage approach for examining the substrate specificities of an important group of proteases involved in apoptosis--the caspases. After establishing selection conditions with caspases-3 and caspase-8 vs control substrate-phage, we sorted X4 and X6 diversity libraries, identified consensus motifs that agree with previously defined caspase substrate motifs, confirmed the selection of active substrates using synthetic peptide rate assays under a range of buffer conditions, and compared kinetic parameters for selected substrates. The libraries produced some variations on the canonical motifs. From caspase-3 selections, a phage-derived synthetic peptide, DLVD, was hydrolyzed up to 170% faster than the canonical substrate DEVD. The P4 Asp residue was essential for good protease-sensitivity, but even substrates with substitutions at P4 were selected by phage and shown to be hydrolyzed. Caspase-8 selections, as expected, yielded predominantly clones containing a Glu at P3. In this case, the most frequent phage-derived peptide, LEVD, was cleaved at a rate of only 20% of the canonical caspase-8 substrate LETD. However, based on substitutions observed in the phage selectants at P4, a substrate peptide, AETD, was designed and shown to be hydrolyzed up to 160% faster than LETD. We consider factors that may contribute to differences in caspase substrate-phage selections vs synthetic peptide studies on the caspases, and suggest that the two approaches may offer complementary information.

Involvement of lysosomal cathepsins in the cleavage of DNA topoisomerase I during necrotic cell death

OBJECTIVE

Autoantibodies to DNA topoisomerase I (topo I) are associated with diffuse systemic sclerosis (SSc), appear to be antigen driven, and may be triggered by cryptic epitopes exposed during in vivo topo I fragmentation. These autoantibodies recognize topo I and fragments of this autoantigen generated during apoptosis and necrosis. We undertook this study to determine whether lysosomal cathepsins are involved in topo I fragmentation during necrosis.

METHODS

Topo I cleavage during necrosis was assessed by immunoblotting of lysates from L929 fibroblasts exposed to tumor necrosis factor alpha (TNFalpha) and the broad caspase inhibitor Z-VAD-FMK, and by immunoblotting of lysates from endothelial cells treated with HgCl2. Purified topo I and L929 nuclei were incubated with cathepsins B, D, G, H, and L, and topo I cleavage was detected by immunoblotting. The intracellular localization of cathepsin L activity and topo I in necrotic cells was examined using fluorescence microscopy.

RESULTS

Treatment of L929 cells with TNFalpha and Z-VAD-FMK induced caspase-independent cell death with necrotic morphology. This cell death involved topo I cleavage into fragments of approximately 70 kd and 45 kd. This cleavage profile was reproduced in vitro by cathepsins L and H and was inhibited by the cathepsin L inhibitor Z-FY-CHO. During necrosis, cathepsin L activity diffused from lysosomes into the cytoplasm and nucleus, whereas topo I partially relocalized to the cytoplasm. Z-FY-CHO delayed necrosis and partially blocked topo I cleavage. The topo I cleavage fragments were also detected in necrotic endothelial cells and recognized by SSc sera containing anti-topo I antibodies.

CONCLUSION

These results implicate cathepsins, particularly cathepsin L, in the cleavage of topo I during necrosis. This cleavage may generate potentially immunogenic fragments that could trigger anti-topo I immune responses in SSc.

Apoptosis: The Sculptor of Development

Apoptosis is a programmed mechanism of cell death recognized by its characteristic morphological and biochemical changes. Over the last decade, our understanding of the biochemistry of apoptosis has flourished. However, the physiological relevance of apoptosis remains elusive. Here, I propose that the process of programmed cell death plays an essential role in structural development. From pioneering studies almost a century ago to recent findings using modern technology, similar conclusions have emerged that highlight the fundamental role of apoptosis in vascular development. This review will recount these classic and modern studies as I survey evidence that implicates apoptosis in other aspects of development and ask how cell death can possibly contribute to homeostasis and development of the immune system. I briefly consider the mechanisms that may determine the fate of cells within the vasculature and propose new roles for the contribution of apoptosis to development and differentiation. More provocatively, I explore the possibilities that arise from this growing field of study, including prevention of developmental defects and even abnormal development after birth, such as neoplastic development. To realize these end points, the biochemical bases of apoptosis must be thoroughly understood.

Role of caspases in the regulation of apoptotic pancreatic islet beta-cells death

The homeostatic control of beta-cell mass in normal and pathological conditions is based on the balance of proliferation, differentiation, and death of the insulin-secreting cells. A considerable body of evidence, accumulated during the last decade, has emphasized the significance of the disregulation of the mechanisms regulating the apoptosis of beta-cells in the sequence of events that lead to the development of diabetes. The identification of agents capable of interfering with this process needs to be based on a better understanding of the beta-cell specific pathways that are activated during apoptosis. The aim of this article is fivefold: (1) a review of the evidence for beta-cell apoptosis in Type I diabetes, Type II diabetes, and islet transplantation, (2) to review the common stimuli and their mechanisms in pancreatic beta-cell apoptosis, (3) to review the role of caspases and their activation pathway in beta-cell apoptosis, (4) to review the caspase cascade and morphological cellular changes in apoptotic beta-cells, and (5) to highlight the putative strategies for preventing pancreatic beta-cells from apoptosis.

Molecular Recognition Patterns of Serum Anti-DNA Topoisomerase I Antibody in Systemic Sclerosis

Autoreactive anti-DNA topoisomerase I (anti-Topo I) Abs are commonly detected in sera of systemic sclerosis (SSc) patients. Our studies have established a positive correlation between the levels of serum anti-Topo I Abs and both disease severity and activity of SSc. The molecular targets of anti-Topo I Ab on Topo I domains remain to be further defined. In this report, we studied the molecular recognition pattern of serum anti-Topo I Ab in 52 SSc patients. The highest reactivity of serum anti-Topo I Abs was against the core subdomains I and II (aa 207-441) and, to a lesser extent, against the core subdomain III (aa 433-636) of Topo I. The linker domain (aa 636-712) and the C-terminal domain (aa 713-765) had much less reactivity than the core domain (aa 207-636). Strikingly, very little reactivity was directed against the N-terminal domain (aa 1-213) by serum anti-Topo I Ab. This molecular recognition pattern was consistent among all SSc serum samples studied. Results from patients with serial serum samples indicated that this pattern remained unchanged over time. Interestingly, some naive B cells from healthy controls, upon transformation by EBV, produced IgM Abs against Topo I. These Abs had low affinity for Topo I and reacted equally to all domains of Topo I. The molecular recognition pattern of serum anti-Topo I Ab in SSc suggests the presence of a unique antigenic stimulation in vivo in this disease.

Nucleosomes in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterized by the development of a large array of autoantibodies that primarily are directed against the whole chromatin (antinucleosome) and its individual components, dsDNA and histones. Apoptotic defects and impaired removal of apoptotic cells could contribute to an overload of autoantigens (and in particular of nucleosomes) in circulation or in target tissues that could become available to initiate an autoimmune response. In susceptible individuals, this can lead to autoantibody-mediated tissue damage. In addition to intrinsic or secondary apoptosis/apoptotic cell removal defects, certain apoptotic stimuli (eg, UV, viruses) could lead to posttranscriptional modifications that generate autoantigen cryptic fragments for which cells of the immune system have not been tolerized. Besides their role as a major immunogen in lupus, nucleosomes participate in antibody-mediated renal pathogenicity and act as a bridging molecule that recognizes heparin sulfate/collagen V components of the glomerular basement membrane. New tools that were developed to detect antinucleosome antibodies in the serum of patients (by ELISA) have shown the specificity and the high sensitivity of antinucleosome antibody reactivity in SLE. In particular, antinucleosome could be a useful marker of patients who have SLE and lack anti-dsDNA antibodies, a prognosis marker for imminent relapse, and a diagnosis marker of lupus nephritis.

Mcl-1 is required for Akata6 B-lymphoma cell survival and is converted to a cell death molecule by efficient caspase-mediated cleavage

Enforced expression of the antiapoptotic Bcl-2 family protein Mcl-1 promotes lymphomagenesis in the mouse; however, the functional role of Mcl-1 in human B-cell lymphoma remains unclear. We demonstrate that Mcl-1 is widely expressed in malignant B-cells, and high-level expression of Mcl-1 is required for B-lymphoma cell survival, since transfection of Mcl-1-specific antisense oligodeoxynucleotides was sufficient to promote apoptosis in Akata6 lymphoma cells. Mcl-1 was efficiently cleaved by caspases at evolutionarily conserved aspartic acid residues in vitro, and during cisplatin-induced apoptosis in B-lymphoma cell lines and spontaneous apoptosis of primary malignant B-cells. Overexpression of the Mcl-1 cleavage product that accumulated during apoptosis was sufficient to kill cells. Therefore, Mcl-1 is an essential survival molecule for B-lymphoma cells and is cleaved by caspases to a death-promoting molecule during apoptosis. In contrast to Mcl-1, Bcl-2 and Bcl-XL were relatively resistant to caspase cleavage in vitro and in intact cells. Interfering with Mcl-1 function appears to be an effective means of inducing apoptosis in Mcl-1-positive B-cell lymphoma, and the unique sensitivity of Mcl-1 to caspase-mediated cleavage suggests an attractive strategy for converting it to a proapoptotic molecule.

Topoisomerase I-DNA complexes contribute to arsenic trioxide-induced apoptosis

Topoisomerase I is an essential enzyme that relaxes DNA supercoiling by forming covalent DNA cleavage complexes, which are normally transient. Topoisomerase I-DNA complexes can be trapped by anticancer drugs (camptothecins) as well as by endogenous and exogenous DNA lesions. We show here that arsenic trioxide (a potent inducer of apoptosis that induces the intracellular accumulation of reactive oxygen species and targets mitochondria) induces cellular topoisomerase I cleavage complexes. Bcl-2 overexpression and quenching of reactive oxygen species, which prevent arsenic trioxide-induced apoptosis, also prevent the formation of topoisomerase I-DNA complexes, whereas enhancement of reactive oxygen species accumulation promotes these complexes. The caspase inhibitor, benzyloxycarbonyl-VAD partially prevents arsenic trioxide-induced topoisomerase I-DNA complexes and apoptosis, suggesting that activated caspases further maintain intracellular levels of reactive oxygen species that induce the formation of topoisomerase I-DNA complexes. Down-regulation of topoisomerase I expression decreases arsenic trioxide-induced apoptotic DNA fragmentation. Thus, we propose that arsenic trioxide induces topoisomerase I-DNA complexes that participate in chromatin fragmentation and programmed cell death during apoptosis.

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.

The human topoisomerase I damage response plays a role in apoptosis

Previous studies have shown that human topoisomerase I cleavage complexes form as a response to various DNA damages in vivo, the so called human topoisomerase I "damage response". It was suggested that this damage response may play a role in DNA repair as well as in apoptosis, but only very few investigations have been done and the significance of the damage response still remains unclear. Here we demonstrate that human topoisomerase I cleavage complexes induced by high doses of UV irradiation are highly stable for up to 48 h. Furthermore, we show that human topoisomerase I cleavage complexes correlate with apoptosis. However, at low UV doses the cleavage complex level was very low and the complexes were repaired. Surprisingly, we found that high levels of stable cleavage complexes were not only found in UV-irradiated cells but also in untreated cells that underwent apoptosis. A possible role of human topoisomerase I in apoptosis is discussed.

Downregulation of genes involved in DNA repair and differential expression of transcription regulators and phosphatases precede IgM-induced apoptosis in the Burkitt's lymphoma cell line BL60-2

Apoptosis of lymphocytes recognizing self-antigens is an essential mechanism to protect the organism against autoimmune diseases. Programmed cell death of susceptible B cells occurs in response to surface IgM cross-linking mediated by self-antigens. This effect can be mimicked in the Burkitt's lymphoma line BL60-2 by addition of anti-IgM antibodies. In order to identify genes with differential expression in response to the apoptotic stimulus, total RNA prepared from BL60-2 cells before and at different points in time after IgM cross-linking was used for Atlas arrays, high-density oligonucleotide microarrays (GeneChip arrays, Affymetrix) and in RNase protection assays (RPA). One of our major observations was the downregulation of six genes involved in the ligation of DNA strand breaks, like DNA ligases and DNA-PK, indicating a shutdown of DNA repair mechanisms in apoptotic cells. In addition, we found changes on mRNA level for several transcription regulators, including early growth response genes 1 and 2, TAFII30 and topoisomerase I. Furthermore, we show accumulation of mRNA for the phosphatases CD45 and DUSP5 in anti-IgM stimulated BL60-2 cells. Our data provide a basis for further analysis of the differentially expressed genes and their roles in IgM-induced B cell death as well as in apoptosis in other cellular systems.

Mechanisms of resistance to topoisomerase I-targeting drugs

DNA topoisomerases are a class of enzymes that alter the topology of DNA and are targets of several anticancer drugs. Camptothecins (CPTs) are a relatively new family of compounds that specifically target topoisomerase I (Top1). These compounds "poison" Top1 by binding to the Top1-DNA complex in a manner that prevents the religation of DNA. Topotecan and irinotecan are two CPTs that are approved for the treatment of a variety of malignancies, including colorectal, ovarian, and small cell lung cancers, as well as myeloid malignancies. Although CPTs have proven to be effective anticancer drugs, resistance is still a critical clinical problem. The mechanisms underlying de novo and acquired clinical resistance to CPTs and the newer classes of Top1 poisons are unclear. However, based on preclinical studies, it is likely that clinical resistance to these drugs is the result of: (1) inadequate accumulation of drug in the tumor, (2) resistance-conferring alterations in Top1, or (3) alterations in the cellular response to the Top1-CPT interaction. This review will focus on the current knowledge regarding mechanisms of resistance to CPTs and other Top1-targeting drugs.

Caspase 3-mediated proteolysis of the N-terminal cytoplasmic domain of the human erythroid anion exchanger 1 (band 3)

The N-terminal cytoplasmic domain of the anion exchanger 1 (AE1 or band 3) of the human erythrocyte associates with peripheral membrane proteins to regulate membrane-cytoskeleton interactions, with glycolytic enzymes such as glyceraldehyde-3-phosphate dehydrogenase and aldolase, with the protein-tyrosine kinase p72syk, with hemoglobin and with hemichromes. We have demonstrated that the N-terminal cytoplasmic domain of band 3 (CDB3) is a substrate of the apoptosis executioner caspase 3 (1). CDB3 has two non-conventional caspase 3 cleavage sites, TATD45 and EQGD205 (2). In vitro treatment of recombinant CDB3 with caspase 3 generated two fragments, which could be blocked by pretreatment with the caspase 3 inhibitor Z-DEVD-fmk (3). Recombinant CDB3 in which the caspase 3 cleavage sites Asp45 and Asp205 were mutated, was resistant to proteolysis (4). Proteolytically derived fragments crossreactive with polyclonal anti-band 3 antibody appeared with simultaneous cleavage of poly (ADP-ribose) polymerase and procaspase 3 in staurosporine (STS)-treated HEK293 cells transiently transfected with CDB3 (5). In vivo cleavage of CDB3 could be blocked by pretreatment of cells with Z-DEVD-fmk or in cells transfected with mutant CDB3 (D45A, D205A) (6). Co-transfection experiments showed that STS-mediated cleavage of CDB3 diminished its interaction with the N-terminal domain of protein 4.2, confirming that such cleavage interferes with the interaction of CDB3 with cytoskeletal proteins (7). Active caspase 3 was observed in aged red cells but not in young cells. This red cell caspase 3 could cleave band 3 present in inside-out vesicles prepared from young erythrocytes arguing in favor of a physiological role of caspase 3 in aged erythrocytes.

Caspase 3-mediated inactivation of rac GTPases promotes drug-induced apoptosis in human lymphoma cells

The Rac members of the Rho family GTPases control signaling pathways that regulate diverse cellular activities, including cytoskeletal organization, gene transcription, and cell transformation. Rac is implicated in apoptosis, but little is known about the mechanism by which it responds to apoptotic stimuli. Here we demonstrate that endogenous Rac GTPases are caspase 3 substrates that are cleaved in human lymphoma cells during drug-induced apoptosis. Cleavage of Rac1 occurs at two unconventional caspase 3 sites, VVGD11/G and VMVD47/G, and results in inactivation of the GTPase and effector functions of the protein (binding to the p21-activated protein kinase PAK1). Expression of caspase 3-resistant Rac1 mutants in the cells suppresses drug-induced apoptosis. Thus, proteolytic inactivation of Rac GTPases represents a novel, irreversible mechanism of Rac downregulation that allows maximal cell death following drug treatment.

Differential susceptibility to 9-nitrocamptothecin (9-NC)-induced apoptosis in clones derived from a human ovarian cancer cell line: possible implications in the treatment of ovarian cancer patients with 9-NC

We have investigated whether variability in the apoptotic pathway may account for the differential susceptibility to apoptosis-induction by 9-nitrocamptothecin (9-NC) in cell subpopulations derived from the human ovarian cancer cell line, SKOV-3. Quantitative differences in the apoptotic fractions of cells were assessed by flow cytometry, whereas major regulatory and executing components of the apoptotic machinery were investigated by Western blot analysis using specific antibodies. The results indicate that indeed the apoptotic pathway was activated by 9-NC in some, but not all, cells of the SKOV-3 cell line, suggesting that 9-NC alone may partially be effective for treatment of patients with ovarian cancer.

Antinuclear antibodies recognize cellular autoantigens driven by apoptosis

OBJECTIVE

Present study addresses the issue whether cellular antigens recognised by antinuclear autoantibodies are driven by apoptosis.

MATERIALS AND METHODS

HEp-2 cells were committed to apoptosis by camptothecin; DNA fragmentation and FasL and Bax expression monitored apoptosis. Autoantigens were probed by indirect immunofluorescence and Western blot with autoantibodies or monoclonals against: DNA, Ro60, La, U1-RNP, CENP-B, DNA Topoisomerase I, Jo-1 and NuMA. A comparison of antinuclear antibody reactivity between living and apoptotic cells was performed by ELISA.

RESULTS

Apoptotic changes such as chromatin fragmentation, blebs and apoptotic bodies were induced with 20 mM camptothecin. Autoantigens were better detected in apoptotic cells. U1-RNP, Jo1, DNA-Topoisomerase I, CENP-B and NuMA exhibited fragmentation and redistribution as a consequence of apoptosis; in contrast, Ro60 and La ribonucleoproteins did not show proteolysis. Additionally the ELISA titers of antinuclear antibodies were higher in apoptotic cells than in normal cells.

CONCLUSION

Apoptosis induces molecular changes in different autoantigens, this modification increases the antigen-driven response of autoantibodies such as anti-RNP, anti-DNA Topoisomerase I, anti-CENP-B and anti-Jo1. Apoptotic changes would contribute to break down the tolerance in autoimmune connective tissue disease.

Caspase-mediated cleavage of the feline calicivirus capsid protein

Feline calicivirus (FCV) is responsible for an acute upper respiratory tract disease in cats. The FCV capsid protein is synthesized as a precursor (76 kDa) that is post-translationally processed into the mature 62 kDa capsid protein by removal of the N-terminal 124 amino acids. Our previous studies have also detected a 40 kDa protein, related to the FCV capsid protein, produced during infection. Here we demonstrate that cleavage of the FCV capsid protein, during infection of cells in culture, was prevented by caspase inhibitors. In addition, caspase-2, -3 and -7 were activated during FCV infection, as shown by pro-form processing, an increase in N-acetyl-Asp-Glu-Val-Asp-7-amido-4-trifluoromethylcoumarin cleavage activity and in situ poly(ADP-ribose) polymerase cleavage. Caspase activation coincided with the induction of apoptosis and capsid cleavage to the 40 kDa fragment. An in vitro cleavage assay, using recombinant human caspases and in vitro-derived FCV capsid protein, revealed that caspase-2, and to a lesser extent caspase-6, cleaved the capsid protein to generate a 40 kDa fragment. Taken together, these results suggest that FCV triggers apoptosis within infected cells and that caspase-induced capsid cleavage occurs concomitantly with apoptosis. The possible role of capsid cleavage in the pathogenesis of FCV infection is discussed.

Caspase-mediated cleavage of the U snRNP-associated Sm-F protein during apoptosis

Recent studies have implicated the dying cell as a potential reservoir of modified autoantigens that might initiate and drive systemic autoimmunity in susceptible hosts. The spliceosomal Sm proteins are recognized by the so-called anti-Sm autoantibodies, an antibody population found exclusively in patients suffering from systemic lupus erythematosus. We have studied the effects of apoptosis on the Sm proteins and demonstrate that one of the Sm proteins, the Sm-F protein, is proteolytically cleaved in apoptotic cells. Cleavage of the Sm-F protein generates a 9-kDa apoptotic fragment, which remains associated with the U snRNP complexes in apoptotic cells. Sm-F cleavage is dependent on caspase activation and the cleavage site has been located near the C-terminus, EEED(81) downward arrow G. Use of different caspase inhibitors suggests that besides caspase-8 other caspases are implicated in Sm-F cleavage. A C-terminally truncated mutant of the Sm-F protein, representing the modified form of the protein, is capable of forming an Sm E-F-G complex in vitro that is recognized by many anti-Sm patient sera.