Lysosomal protease pathways to apoptosis. Cleavage of bid, not pro-caspases, is the most likely route

Lysosomal membrane permeabilization in cell death: concepts and challenges

Late endocytic compartments include late endosomes, lysosomes and hybrid organelles. In the acidic lumen, cargo material derived from endocytosed and phagocytosed extracellular material and autophagy-derived intracellular material is degraded. In the event of lysosomal membrane permeabilization (LMP), the function of endo/lysosomal compartment is affected and the luminal contents are released into the cytosol to various extents. LMP can be a result of osmotic lysis or direct membranolytic activity of the compounds that accumulate in the lumen of endo/lysosomes. In addition to several synthetic compounds, such as dipeptide methyl esters and lysosomotropic detergents, endogenous agents that can cause LMP include ROS and lipid metabolites such as sphingosine and phosphatidic acid. Depending on the cell type and the dose, LMP can initiate the lysosomal apoptotic pathway, pyroptosis or necrosis. LMP can also amplify cell death signaling that was initiated outside the endocytic compartment, and hamper cell recovery via autophagy. However, mechanisms that connect LMP with cell death signaling are poorly understood, with the exception of the proteolytic activation of Bid by aspartic cathepsin D and cysteine cathepsins. Determination of LMP in a cell model system is methodologically challenging. Even more difficult is to prove that LMP is the primary event leading to cell death. Nevertheless, LMP may prove to be a valuable approach in therapy, either as a trigger of cell death or as a mechanism of therapeutic drug release in the case of delivery systems that target the endocytic pathway.

Enhanced efficacy of photodynamic therapy via a sequential targeting protocol

This study was designed to examine determinants of the discovery that low-dose lysosomal photodamage (lyso-PDT) could potentiate the efficacy of subsequent low-dose mitochondrial photodamage (mito-PDT). The chlorin NPe6 and the benzoporphyrin derivative (BPD) were used to separately target lysosomes and mitochondria, respectively, in murine hepatoma cells. Lyso-PDT (LD(5) conditions) followed by mito-PDT (LD(15) conditions) enhanced the loss of the mitochondrial membrane potential, activation of procaspases-3/7 and photokilling. Reversing the sequence was less effective. The optimal sequence did not enhance reactive oxygen species formation above that obtained with low-dose mito-PDT. In contrast, alkalinization of lysosomes with bafilomycin also enhanced low-dose mito-PDT photokilling, but via a different pathway. This involves redistribution of iron from lysosomes to mitochondria leading to enhanced hydroxyl radical formation, effects not observed after the sequential procedure. Moreover, Ru360, an inhibitor of mitochondrial calcium and iron uptake, partially suppressed the ability of bafilomycin to enhance mito-PDT photokilling without affecting the enhanced efficacy of the sequential protocol. We conclude that sequential PDT protocol promotes PDT efficacy by a process not involving iron translocation, but via promotion of the pro-apoptotic signal that derives from mitochondrial photodamage.

Cysteine cathepsins and their potential in clinical therapy and biomarker discovery

Since their discovery, cysteine cathepsins were generally considered to be involved mainly in the nonspecific bulk protein degradation that takes place within the lysosomes. However, it has become clear that their proteolytical activity can also influence various specific pathological processes such as cancer, arthritis, and atherosclerosis. Furthermore, their localization was found not to be limited strictly to the lysosomes. In the light of those findings, it is not surprising that cysteine cathepsins are currently considered as highly relevant clinical targets. Moreover, recent development of proteomic-based methods for identification of novel physiological substrates of proteases provides a major opportunity also in the field of cysteine cathepsins. In this review, we will therefore present cysteine cathepsin roles in disease progression and discuss their potential relevance as prognostic and diagnostic biomarkers.

Inhibition of cysteine cathepsin B and L activation in astrocytes contributes to neuroprotection against cerebral ischemia via blocking the tBid-mitochondrial apoptotic signaling pathway

The roles of cathepsins in the ischemic astrocytic injury remain unclear. Here, we test the hypothesis that activation of cathepsin B and L contributes to the ischemic astrocyte injury via the tBid-mitochondrial apoptotic signaling pathways. In the rat models of pMCAO, CA-074Me or Clik148, a selective inhibitor of cathepsin B or cathepsin L, reduced the infarct volume, improved the neurological deficits and increased the MAP2 and GFAP levels. In OGD-induced astrocyte injury, CA-074Me or Clik148 decreased the LDH leakage and increased the GFAP levels. In the ischemic cortex or OGD-induced astrocytes injury, Clik148 or CA-074Me reversed pMCAO or OGD-induced increase in active cathepsin L or cathepsin B at 3 h or 6 h, increase in tBid, reduction in mitochondrial cytochrome-c (Cyt-c) and increase in cytoplastic Cyt-c and active caspase-3 at 12-24 h of the late stage of pMCAO or OGD. CA-074Me or Clik148 also reduced cytosolic and mitochondrial tBid, increased mitochondrial Cyt-c and decreased cytoplastic Cyt-c and active caspase-3 at 6 h of the early stage of Bid activation. CA-074Me or Clik148 blocked the pMCAO-induced release of cathepsin B or L from the lysosomes into the cytoplasm and activation of caspase-3 in ischemic astrocytes at 12 h after ischemia. Concurrent inhibition of cathepsin B and cathepsin L provided better protection on the OGD-induced astrocytic apoptosis than obtained with separate use of each inhibitor. These results suggest that inhibition of the cysteine cathepsin B and cathepsin L activation in ischemic astrocytes contributes to neuroprotection via blocking the tBid-mitochondrial apoptotic signaling pathway.

Preeclampsia is associated with alterations in the p53-pathway in villous trophoblast

BACKGROUND

Preeclampsia (PE) is characterized by exaggerated apoptosis of the villous trophoblast of placental villi. Since p53 is a critical regulator of apoptosis we hypothesized that excessive apoptosis in PE is mediated by abnormal expression of proteins participating in the p53 pathway and that modulation of the p53 pathway alters trophoblast apoptosis in vitro.

METHODS

Fresh placental villous tissue was collected from normal pregnancies and pregnancies complicated by PE; Western blotting and real-time PCR were performed on tissue lysate for protein and mRNA expression of p53 and downstream effector proteins, p21, Bax and caspases 3 and 8. To further assess the ability of p53 to modulate apoptosis within trophoblast, BeWo cells and placental villous tissue were exposed to the p53-activator, Nutlin-3, alone or in combination with the p53-inhibitor, Pifithrin-α (PFT-α). Equally, Mdm2 was knocked-down with siRNA.

RESULTS

Protein expression of p53, p21 and Bax was significantly increased in pregnancies complicated by PE. Conversely, Mdm2 protein levels were significantly depleted in PE; immunohistochemistry showed these changes to be confined to trophoblast. Reduction in the negative feedback of p53 by Mdm2, using siRNA and Nutlin-3, caused an imbalance between p53 and Mdm2 that triggered apoptosis in term villous explants. In the case of Nutlin, this was attenuated by Pifithrin-α.

CONCLUSIONS

These data illustrate the potential for an imbalance in p53 and Mdm2 expression to promote excessive apoptosis in villous trophoblast. The upstream regulation of p53 and Mdm2, with regard to exaggerated apoptosis and autophagy in PE, merits further investigation.

Time resolved study of cell death mechanisms induced by amine-modified polystyrene nanoparticles

Positively charged polymers and nanoparticles (NPs) can be toxic to cells in various systems. Using human astrocytoma cells, we have previously shown that 50 nm amine-modified polystyrene NPs damage mitochondria and induce cell death by apoptosis. Here we provide comprehensive details of the cellular events occurring after exposure to the NPs in a time-resolved manner. We demonstrate that the accumulation of NPs in lysosomes plays a central role in the observed cell death, leading to swelling of the lysosomes and release of cathepsins into the cytosol, which ultimately propagates the damage to the mitochondria with subsequent activation of apoptosis. This is accompanied and sustained by other events, such as increasing ROS levels and autophagy. Using various inhibitors, we also show the interplay between apoptosis and autophagy as a response to NP accumulation in lysosomes.

Apoptosis and necrosis in the liver

Because of its unique function and anatomical location, the liver is exposed to a multitude of toxins and xenobiotics, including medications and alcohol, as well as to infection by hepatotropic viruses, and therefore, is highly susceptible to tissue injury. Cell death in the liver occurs mainly by apoptosis or necrosis, with apoptosis also being the physiologic route to eliminate damaged or infected cells and to maintain tissue homeostasis. Liver cells, especially hepatocytes and cholangiocytes, are particularly susceptible to death receptor-mediated apoptosis, given the ubiquitous expression of the death receptors in the organ. In a quite unique way, death receptor-induced apoptosis in these cells is mediated by both mitochondrial and lysosomal permeabilization. Signaling between the endoplasmic reticulum and the mitochondria promotes hepatocyte apoptosis in response to excessive free fatty acid generation during the metabolic syndrome. These cell death pathways are partially regulated by microRNAs. Necrosis in the liver is generally associated with acute injury (i.e., ischemia/reperfusion injury) and has been long considered an unregulated process. Recently, a new form of "programmed" necrosis (named necroptosis) has been described: the role of necroptosis in the liver has yet to be explored. However, the minimal expression of a key player in this process in the liver suggests this form of cell death may be uncommon in liver diseases. Because apoptosis is a key feature of so many diseases of the liver, therapeutic modulation of liver cell death holds promise. An updated overview of these concepts is given in this article.

Cysteine cathepsins in neurological disorders

Increased proteolytic activity is a hallmark of several pathological processes, including neurodegeneration. Increased expression and activity of cathepsins, lysosomal cysteine proteases, during degeneration of the central nervous system is frequently reported. Recent studies reveal that a disturbed balance of their enzymatic activities is the first insult in brain aging and age-related diseases. Leakage of cathepsins from lysosomes, due to their membrane permeability, and activation of pro-apoptotic factors additionally contribute to neurodegeneration. Furthermore, in inflammation-induced neurodegeneration the cathepsins expressed in activated microglia play a pivotal role in neuronal death. The proteolytic activity of cysteine cathepsins is controlled by endogenous protein inhibitors-the cystatins-which evidently fail to perform their function in neurodegenerative processes. Exogenous synthetic inhibitors, which may augment their inhibitory potential, are considered as possible therapeutic tools for the treatment of neurological disorders.

N-terminally truncated forms of human cathepsin F accumulate in aggresome-like inclusions

The contribution of individual cysteine cathepsins as positive mediators of programmed cell death is dependent on several factors, such as the type of stimuli, intensity and duration of the stimulus, and cell type involved. Of the eleven human cysteine cathepsins, cathepsin F is the only cathepsin that exhibits an extended N-terminal proregion, which contains a cystatin-like domain. We predicted that the wild-type human cathepsin F contains three natively disordered regions within the enzyme's propeptide and various amino acid stretches with high fibrillation propensity. Wild-type human cathepsin F and its N-terminally truncated forms, Ala(20)-Asp(484) (Δ(19)CatF), Pro(126)-Asp(484) (Δ(125)CatF), and Met(147)-Asp(484) (Δ(146)CatF) were cloned into the pcDNA3 vector and overexpressed in HEK 293T cells. Wild-type human cathepsin F displayed a clear vesicular labeling and colocalized with the LAMP2 protein, a lysosomal marker. However, all three N-terminally truncated forms of human cathepsin F were recovered as insoluble proteins, suggesting that the deletion of at least the signal peptides (Δ(19)CatF), results in protein aggregation. Noteworthy, they concentrated large perinuclear-juxtanuclear aggregates that accumulated within aggresome-like inclusions. These inclusions showed p62-positive immunoreactivity and were colocalized with the autophagy marker LC3B, but not with the LAMP2 protein. In addition, an approximately 2-3 fold increase in DEVDase activity was not sufficient to induce apoptotic cell death. These results suggested the clearance of the N-terminally truncated forms of human cathepsin F via the autophagy pathway, underlying its protective and prosurvival mechanisms.

Multiple cell death pathways are independently activated by lethal hypertonicity in renal epithelial cells

When hypertonicity is imposed with sufficient intensity and acuteness, cells die. Here we investigated the cellular pathways involved in death using a cell line derived from renal epithelium. We found that hypertonicity rapidly induced activation of an intrinsic cell death pathway-release of cytochrome c and activation of caspase-3 and caspase-9-and an extrinsic pathway-activation of caspase-8. Likewise, a lysosomal pathway of cell death characterized by partial lysosomal rupture and release of cathepsin B from lysosomes to the cytosol was also activated. Relationships among the pathways were examined using specific inhibitors. Caspase inhibitors did not affect cathepsin B release into the cytosol by hypertonicity. In addition, cathepsin B inhibitors and caspase inhibitors did not affect hypertonicity-induced cytochrome c release, suggesting that the three pathways were independently activated. Combined inhibition of caspases and cathepsin B conferred significantly more protection from hypertonicity-induced cell death than inhibition of caspase or cathepsin B alone, indicating that all the three pathways contributed to the hypertonicity-induced cell death. Similar pattern of sensitivity to the inhibitors was observed in two other cell lines derived from renal epithelia. We conclude that multiple cell death pathways are independently activated early in response to lethal hypertonic stress in renal epithelial cells.

Cathepsin L silencing enhances arsenic trioxide mediated in vitro cytotoxicity and apoptosis in glioblastoma U87MG spheroids

Despite improved treatment options, glioblastoma multiforme (GBM) remains the most aggressive brain tumour with the shortest post-diagnostic survival. Arsenite (As2O3) is already being used in the treatment of acute promyelocytic leukaemia (APL), yet its effects on GBM have not been evaluated in detail. In U87MG cell monolayers, we have previously shown that arsenite cytotoxicity significantly increases upon transient inhibition of lysosomal protease Cathepsin L (CatL). As multicellular spheroids more closely represent in vivo tumours, we aimed to evaluate the impact of permanent CatL silencing on arsenite treatment in U87MG spheroids. CatL was stably silenced using shRNA expression plasmid packed lentiviruses. By using metabolic- and cell viability assays, we demonstrated that long-term CatL silencing significantly increased arsenite cytotoxicity in U87MG spheroids. Silenced CatL also increased arsenite-mediated apoptosis in spheroids via elevated p53 expression, Bax/Bcl2 ratio and caspase 3/7 activity, though with lower efficacy than in monolayers. Arsenite cytotoxicity was enhanced by lower CatL activity, since similar cytotoxicity increase was also observed using the novel CatL inhibitor AT094. The results have significant translational impact, since stable CatL silencing would enable the application of lower systemic doses of arsenite to achieve the desired cytotoxic effects on GBMs in vivo.

Induction of apoptosis by the Amsacta moorei entomopoxvirus

CF-70-B2 cells derived from the spruce budworm (Choristoneura fumiferana) undergo apoptosis when infected with Amsacta moorei entomopoxvirus (AMEV), as characterized by membrane blebbing, formation of apoptotic bodies, TdT-mediated dUTP nick-end labelling (TUNEL) staining, condensed chromatin and induction of caspase-3/7 activity. The apoptotic response was reduced when cells were infected with UV-inactivated AMEV, but not when infected in the presence of the DNA synthesis inhibitor, cytosine β-d-arabinofuranoside. Hence, only pre-DNA replication events were involved in inducing the antiviral response in CF-70-B2 cells. The virus eventually overcame the host’s antiviral response and replicated to high progeny virus titres accompanied by high levels of caspase-3/7 activity. The CF-70-B2 cells were less productive of progeny virus in comparison to LD-652, a Lymantria dispar cell line routinely used for propagation of AMEV. At late stages of infection, LD-652 cells also showed characteristics of apoptosis such as oligosomal DNA fragmentation, TUNEL staining, condensed chromatin and increased caspase-3/7 activity. Induction of apoptosis in LD-652 cells was dependent on viral DNA replication and/or late gene expression. A significantly reduced rate of infection was observed in the presence of general caspase inhibitors Q-VD-OPH and Z-VAD-FMK, indicating caspases may be involved in productive virus infection.

Gene expression profiling of murine T-cell lymphoblastic lymphoma identifies deregulation of S-phase initiating genes

In a search for genes and pathways implicated in T-cell lymphoblastic lymphoma (T-LBL) development, we used a murine lymphoma model, where mice of the NMRI-inbred strain were inoculated with murine leukemia virus mutants. The resulting tumors were analyzed by integration analysis and global gene expression profiling to determine the effect of the retroviral integrations on the nearby genes, and the deregulated pathways in the tumors. Gene expression profiling identified increased expression of genes involved in the minichromosome maintenance and origin of recognition pathway as well as downregulation in negative regulators of G1/S transition, indicating increased S-phase initiation in murine T-LBLs.

The radioprotective agent, amifostine, suppresses the reactivity of intralysosomal iron

Amifostine (2-[(3-aminopropyl)amino]ethane-thiol dihydrogen phosphate ester; WR-2721) is a radioprotective agent used clinically to minimize damage from radiation therapy to adjacent normal tissues. This inorganic thiophosphate requires dephosphorylation to produce the active, cell-permeant thiol metabolite, WR-1065. The activation step is presumably catalyzed by membrane-bound alkaline phosphatase, activity of which is substantially higher in the endothelium of normal tissues. This site-specific delivery may explain the preferential protection of normal versus neoplastic tissues. Although it was developed several decades ago, the mechanisms through which this agent exerts its protective effects remain unknown. Because WR-1065 is a weak base (pKa = 9.2), we hypothesized that the drug should preferentially accumulate (via proton trapping) within the acidic environment of intracellular lysosomes. These organelles contain abundant 'loose' iron and represent a likely initial target for oxidant- and radiation-mediated damage. We further hypothesized that, within the lysosomal compartment, the thiol groups of WR-1065 would interact with this iron, thereby minimizing iron-catalyzed lysosomal damage and ensuing cell death. A similar mechanism of protection via intralysosomal iron chelation has been invoked for the hexadentate iron chelator, desferrioxamine (DFO; although DFO enters the lysosomal compartment by endocytosis, not proton trapping). Using cultured J774 cells as a model system, we found substantial accumulation of WR-1065 within intracellular granules as revealed by reaction with the thiol-binding fluorochrome, BODIPY FL L-cystine. These granules are lysosomes as indicated by co-localization of BODIPY staining with LysoTracker Red. Compared to 1 mM DFO, cells pre-treated with 0.4 microM WR-1065 are protected from hydrogen peroxide-mediated lysosomal rupture and ensuing cell death. On a molar basis in this experimental system, WR-1065 is approximately 2500 times more effective than DFO in preventing oxidant-induced lysosomal rupture and cell death. This increased effectiveness is most likely due to the preferential concentration of this weak base within the acidic lysosomal apparatus. By electron spin resonance, we found that the generation of hydroxyl radical, which normally occurs following addition of hydrogen peroxide to J774 cells, is totally blocked by pretreatment with either WR-1065 or DFO. These findings suggest a single and plausible explanation for the radioprotective effects of amifostine and may provide a basis for the design of even more effective radio- and chemoprotective drugs.

Role of cathepsin B in dengue virus-mediated apoptosis

Dengue virus (DENV) infection is one of the most important mosquito-borne viral diseases, which is endemic in the tropical and sub-tropical regions. Patients with dengue hemorrhagic fever (DHF) generally present hemorrhagic tendencies, plasma leakage, thrombocytopenia, and hemoconcentration. Hepatic dysfunction is also a crucial feature of DENV infection. Hepatic biopsy specimens obtained from fatal cases of DENV infection show cellular apoptosis, which apparently relate to the pathogenesis. Cathepsins, which are cysteine proteases inside the lysosome, were previously reported to be up-regulated in patients with DHF. However, their functions during DENV infection have not been thoroughly investigated. We show for the first time that DENV induces lysosomal membrane permeabilization. The resulting cytosolic cathepsin B and S contributed to apoptosis via caspase activation. The activity of caspase 3 was significantly reduced in DENV-infected HepG2 cells treatedwith cathepsin B or S inhibitors. Treatment with cathepsin B inhibitor also reduced the activity of caspase 9, suggesting that cathepsin B activates both caspase-9 and caspase-3. Reduced cathepsin B expression, effected by RNA interference, mimicked pharmacological inhibition of the enzyme and confirmed the contribution of cathepsin B to apoptotic events induced by DENV in HepG2 cells.

NLRP3 inflammasome activation induced by engineered nanomaterials

Engineered nanomaterials (ENMs) continue to attract significant attention because they have novel physicochemical properties that can improve the functions of products that will benefit human lives. However, the physicochemical properties that make ENMs attractive could interact with biological systems and induce cascades of events that cause toxicological effects. Recently, there have been more studies suggesting inflammasome activation may play an important role in ENM-induced biological responses. Inflammasomes are a family of multiprotein complexes that are increasingly recognized as major mediators of the host immune system. Among these, NLRP3 inflammasome is the most studied that could directly interact with ENMs to generate inflammatory responses. In this review, the ENM physicochemical properties are linked to NLRP3 inflammasome activation. An understanding of the mechanisms of ENM-NLRP3 inflammasome interactions will provide us with strategies for safer nanomaterial design and therapy.

Role of lysosome rupture in controlling Nlrp3 signaling and necrotic cell death

The Nod-like receptor, Nlrp3, has been linked to inflammatory diseases and adjuvant-mediated immune responses. A wide array of structurally diverse agents does not interact directly with Nlrp3, but is thought to activate the Nlrp3 inflammasome by inducing a common upstream signal, such as lysosome rupture. To test the connection between lysosome integrity and Nlrp3 signaling, we analyzed inflammasome activation following stimulation of murine macrophages with lysosome-destabilizing agents and pyroptosis inducers. Here we provide evidence that lysosomal rupture and the corresponding release of lysosomal hydrolases is an early event in macrophages exposed to the lysosome-destabilizing adjuvants LLOMe and alum. Lysosome rupture preceded cell death induction mediated by these agents and was associated with the degradation of low-molecular weight proteins, including the inflammasome component caspase-1. Proteolysis of caspase-1 was controlled by specific cathepsins, but was independent of autocatalytic processes and Nlrp3 signaling. Consistent with these findings, lysosome-disrupting agents triggered only minimal caspase-1 activation and failed to cause caspase-1-dependent cell death (pyroptosis), generally associated with Nlrp3 signaling. In contrast, lysosome rupture was a late event in macrophages exposed to prototypical pyroptosis inducers. These agents triggered extensive Nlrp3 signaling prior to lysosome rupture with only minimal impact on the cellular proteome. Taken together, our findings suggest that lysosome impairment triggers a cascade of events culminating in cell death but is not crucial for Nlrp3 signaling. The significant differences observed between lysosome-disrupting agents and pyroptosis inducers might explain the distinct immunologic responses associated with these compounds.

DRAM1 regulates autophagy flux through lysosomes

We have previously reported that the mitochondria inhibitor 3-nitropropionic acid (3-NP), induces the expression of DNA damage-regulated autophagy modulator1 (DRAM1) and activation of autophagy in rat striatum. Although the role of DRAM1 in autophagy has been previously characterized, the detailed mechanism by which DRAM1 regulates autophagy activity has not been fully understood. The present study investigated the role of DRAM1 in regulating autophagy flux. In A549 cells expressing wilt-type TP53, 3-NP increased the protein levels of DRAM1 and LC3-II, whereas decreased the levels of SQSTM1 (sequestosome 1). The increase in LC3-II and decrease in SQSTM1 were blocked by the autophagy inhibitor 3-methyl-adenine. Lack of TP53 or knock-down of TP53 in cells impaired the induction of DRAM1. Knock-down of DRAM1 with siRNA significantly reduced 3-NP-induced upregulation of LC3-II and downregulation of SQSTM1, indicating DRAM1 contributes to autophagy activation. Knock-down of DRAM1 robustly decreased rate of disappearance of induced autophagosomes, increased RFP-LC3 fluorescence dots and decreased the decline of LC3-II after withdraw of rapamycin, indicating DRAM1 promotes autophagy flux. DRAM1 siRNA inhibited lysosomal V-ATPase and acidification of lysosomes. As a result, DRAM1 siRNA reduced activation of lysosomal cathepsin D. Similar to DRAM1 siRNA, lysosomal inhibitors E64d and chloroquine also inhibited clearance of autophagosomes and activation of lysosomal cathapsin D after 3-NP treatment. These data suggest that DRAM1 plays important roles in autophagy activation induced by mitochondria dysfunction. DRAM1 affects autophagy through argument of lysosomal acidification, fusion of lysosomes with autophagosomes and clearance of autophagosomes.

DRAM triggers lysosomal membrane permeabilization and cell death in CD4(+) T cells infected with HIV

Productive HIV infection of CD4⁺ T cells leads to a caspase-independent cell death pathway associated with lysosomal membrane permeabilization (LMP) and cathepsin release, resulting in mitochondrial outer membrane permeabilization (MOMP). Herein, we demonstrate that HIV infection induces damage-regulated autophagy modulator (DRAM) expression in a p53-dependent manner. Knocking down the expression of DRAM and p53 genes with specific siRNAs inhibited autophagy and LMP. However, inhibition of Atg5 and Beclin genes that prevents autophagy had a minor effect on LMP and cell death. The knock down of DRAM gene inhibited cytochrome C release, MOMP and cell death. However, knocking down DRAM, we increased viral infection and production. Our study shows for the first time the involvement of DRAM in host-pathogen interactions, which may represent a mechanism of defense via the elimination of infected cells.

Lysosomes are acidic organelles capable of digesting macromolecules and regulating autophagy. In the context of host-pathogen interactions, productive viral infections are associated with lysosome membrane permeabilization (LMP) and programmed cell death (PCD). At a molecular level, the tumor suppressor protein 53 (p53), which is a key player in the detection of DNA damage, acts also as a sensor of pathogen replication. Activation of p53 has been considered to be an altruistic cell suicide mechanism that limits viral infection. Here, we provide new evidence that damage-regulated autophagy modulator (DRAM), a p53 target gene, regulates both LMP and PCD of HIV-infected CD4 T cells. Whereas the inhibition of DRAM or p53 prevents autophagy in infected cells, the inhibition of the autophagy machinery has a minor role in this context. As a consequence, the silencing of DRAM leads to increased HIV viral infection. This is the first report describing the role of DRAM in the context of host-pathogen interaction. Whereas it is to the advantage of the pathogens to preserve their hosts and thus facilitate their multiplication and dissemination, hosts have developed altruistic cellular processes to defend themself and limit the spread of the infectious agent in multicellular organisms. We propose that the ancestral DRAM protein represents a mechanism of self-defense, inducing elimination of infected cells through LMP.

The influence of uremic high cystatin C concentration on neutrophil apoptosis and selected neutrophil functions isolated from healthy subjects

Background

Cystatin C (cC) is a cysteine protease inhibitor that may influence immune response. Our aim was to test the effect of a high concentration of cC, characteristic for uremic patients, on neutrophil (PMN) apoptosis and respiratory burst, as well as the cC secretion from PMNs stimulated with proinflammatory cytokines.

Material/Methods

PMNs from 35 healthy volunteers aged 27–61 years were cultured in presence of cC, IL-1β or TNF-α. The percentage of apoptotic cells based on DNA depletion, Fas, FasL and caspase -3 expression were assessed. CC concentrations were determined by ELISA test. The influence of cC on spontaneous, fMLP-, PMA- or OZ-induced burst response of PMNs was tested using chemiluminescence.

Results

PMN cultured in the presence of cC resulted in a significant drop in apoptotic cell percentage (38% [11%; 65%]) compared both to control (70% [29%; 92%], and to the cells cultured with TNF-α (58% [24%; 85%]). These differences were not accompanied by Fas, FasL and caspase-3 expression changes. Spontaneous, fMLP- and PMA-stimulated oxidative burst of PMNs preincubated with cC were significantly downregulated. IL-1β markedly diminished and TNF-α significantly increased cC concentration in culture supernatants.

Conclusions

The presented results suggest that antiapoptotic activity of cC results from its inhibitory effect on ROS production. Thus, the higher concentration of cC characteristic for uremic patients may modulate acute inflammation through maintaining PMN longevity and inhibiting their respiratory burst and proinflammatory cytokine-related changes in cC release from PMNs.

Getting away with murder: how does the BCL-2 family of proteins kill with immunity?

The adult human body produces approximately one million white blood cells every second. However, only a small fraction of the cells will survive because the majority is eliminated through a genetically controlled form of cell death known as apoptosis. This review places into perspective recent studies pertaining to the BCL-2 family of proteins as critical regulators of the development and function of the immune system, with particular attention on B cell and T cell biology. Here we discuss how elegant murine model systems have revealed the major contributions of the BCL-2 family in establishing an effective immune system. Moreover, we highlight some key regulatory pathways that influence the expression, function, and stability of individual BCL-2 family members, and discuss their role in immunity. From lethal mechanisms to more gentle ones, the final portion of the review discusses the nonapoptotic functions of the BCL-2 family and how they pertain to the control of immunity.

Reconsider Alzheimer's disease by the 'calpain-cathepsin hypothesis'--a perspective review

Alzheimer's disease (AD) is characterized by slowly progressive neuronal death, but its molecular cascade remains elusive for over 100 years. Since accumulation of autophagic vacuoles (also called granulo-vacuolar degenerations) represents one of the pathologic hallmarks of degenerating neurons in AD, a causative connection between autophagy failure and neuronal death should be present. The aim of this perspective review is at considering such underlying mechanism of AD that age-dependent oxidative stresses may affect the autophagic-lysosomal system via carbonylation and cleavage of heat-shock protein 70.1 (Hsp70.1). AD brains exhibit gradual but continual ischemic insults that cause perturbed Ca(2+) homeostasis, calpain activation, amyloid β deposition, and oxidative stresses. Membrane lipids such as linoleic and arachidonic acids are vulnerable to the cumulative oxidative stresses, generating a toxic peroxidation product 'hydroxynonenal' that can carbonylate Hsp70.1. Recent data advocate for dual roles of Hsp70.1 as a molecular chaperone for damaged proteins and a guardian of lysosomal integrity. Accordingly, impairments of lysosomal autophagy and stabilization may be driven by the calpain-mediated cleavage of carbonylated Hsp70.1, and this causes lysosomal permeabilization and/or rupture with the resultant release of the cell degradation enzyme, cathepsins (calpain-cathepsin hypothesis). Here, the author discusses three topics; (1) how age-related decrease in lysosomal and autophagic activities has a causal connection to programmed neuronal necrosis in sporadic AD, (2) how genetic factors such as apolipoprotein E and presenilin 1 can facilitate lysosomal destabilization in the sequential molecular events, and (3) whether a single cascade can simultaneously account for implications of all players previously reported. In conclusion, Alzheimer neuronal death conceivably occurs by the similar 'calpain-hydroxynonenal-Hsp70.1-cathepsin cascade' with ischemic neuronal death. Blockade of calpain and/or extra-lysosomal cathepsins as well as scavenging of hydroxynonenal would become effective AD therapeutic approaches.

Mechanisms regulating neutrophil survival and cell death

Neutrophils not only play a critical role as a first line of defense against bacteria and fungi infections but also contribute to tissue injury associated with autoimmune and inflammatory diseases. Neutrophils are rapidly and massively recruited from the circulation into injured tissues displaying an impressive arsenal of toxic weapons. Although effective in their ability to kill pathogens, these weapons were equally effective to induce tissue damage. Therefore, the inflammatory activity of neutrophils must be regulated with exquisite precision and timing, a task mainly achieved through a complex network of mechanisms, which regulate neutrophil survival. Neutrophils have the shortest lifespan among leukocytes and usually die via apoptosis although new forms of cell death have been characterized over the last few years. The lifespan of neutrophils can be dramatically modulated by a large variety of agents such as cytokines, pathogens, danger-associated molecular patterns as well as by pharmacological manipulation. Recent findings shed light about the complex mechanisms responsible for the regulation of neutrophil survival in different physiological, pathological, and pharmacological scenarios. Here, we provide an updated review on the current knowledge and new findings in this field and discuss novel strategies that could be used to drive the resolution of neutrophil-mediated inflammatory diseases.

A ROS-mediated lysosomal-mitochondrial pathway is induced by a novel Amonafide analogue, 7c, in human Hela cervix carcinoma cells

In this study, a novel naphthalimide derivative 7c was designed which is topo II inhibiting though owning weak DNA binders. It was shown that 7c could induce cancer cells apoptosis and have less cytotoxicity in normal human cell. Further investigations on Hela cells revealed that 7c could also induce ROS generation, lysosome rupture as well as cathepsin B release. Subsequent mitochondrial damages including mitochondrial membrane permeabilization and the release of cytochrome c were also found in 7c when treating with Hela cells. According to our data, 7c may act as a lead compound for potential anticancer drugs. The idea of naphthalimides modification may also provide a novel strategy for naphthalimides design.

Cathepsin B and cystatin B in HIV-seropositive women are associated with infection and HIV-1-associated neurocognitive disorders

OBJECTIVE

HIV-1-associated neurocognitive disorders (HAND) is triggered by immune activation of brain cells and remain prevalent during progressive viral infection despite antiretroviral therapy. Cathepsins and cystatins are lysosomal proteins secreted by macrophages and microglia, and may play important roles in neuroregulatory responses. Our laboratory has shown increased secretion and neurotoxicity of cathepsin B from in-vitro HIV-infected monocyte-derived macrophages, and increased expression in postmortem brain tissue with HIV encephalitis and HAND. We hypothesized that cystatin B and cathepsin B could represent potential biomarkers for HAND.

METHODS

Monocytes, plasma, and cerebrospinal fluid (CSF) from retrospective samples from 63 HIV-seropositive Hispanic women were selected for this study. These were stratified as 27 normal, 14 asymptomatic, and 22 HIV dementia, and as 14 progressors and 17 nonprogressors. Samples were evaluated for cystatins B and C and cathepsin B expression and activity.

RESULTS

Increased cathepsin B and cystatins B and C were found in plasma of HIV-seropositive women. Higher intracellular expression of cathepsin B and cystatin B were found in monocytes from women with HIV-associated dementia (P < 0.05). Significant increase in cystatin B concentration in CSF was found in women with dementia compared with HIV-seropositive asymptomatic women.

CONCLUSION

These results demonstrate that dysregulation of cystatin B-cathepsin B system is operative in HIV-associated neurocognitive impairment and suggests that intracellular expression of cystatin B and cathepsin B in monocytes could be potential candidate biomarkers for HIV dementia, whereas increased cathepsin B and cystatins B and C in plasma are potential candidate markers of chronic HIV-1 activation.

Cathepsin-mediated necrosis controls the adaptive immune response by Th2 (T helper type 2)-associated adjuvants

Immunologic adjuvants are critical components of vaccines, but it remains unclear how prototypical adjuvants enhance the adaptive immune response. Recent studies have shown that necrotic cells could trigger an immune response. Although most adjuvants have been shown to be cytotoxic, this activity has traditionally been considered a side effect. We set out to test the role of adjuvant-mediated cell death in immunity and found that alum, the most commonly used adjuvant worldwide, triggers a novel form of cell death in myeloid leukocytes characterized by cathepsin-dependent lysosome-disruption. We demonstrated that direct lysosome-permeabilization with a soluble peptide, Leu-Leu-OMe, mimics the alum-like form of necrotic cell death in terms of cathepsin dependence and cell-type specificity. Using a combination of a haploid genetic screen and cathepsin-deficient cells, we identified specific cathepsins that control lysosome-mediated necrosis. We identified cathepsin C as critical for Leu-Leu-OMe-induced cell death, whereas cathepsins B and S were required for alum-mediated necrosis. Consistent with a role of necrotic cell death in adjuvant effects, Leu-Leu-OMe replicated an alum-like immune response in vivo, characterized by dendritic cell activation, granulocyte recruitment, and production of Th2-associated antibodies. Strikingly, cathepsin C deficiency not only blocked Leu-Leu-OMe-mediated necrosis but also impaired Leu-Leu-OMe-enhanced immunity. Together our findings suggest that necrotic cell death is a powerful mediator of a Th2-associated immune response.

The endolysosomal system in cell death and survival

The endocytic pathway is a system specialized for the uptake of compounds from the cell microenvironment for their degradation. It contains an arsenal of hydrolases, including proteases, which are normally enclosed in membrane-bound organelles, but if released to the cytosol can initiate apoptosis signaling pathways. Endogenous and exogenous compounds have been identified that can mediate destabilization of lysosomal membranes, and it was shown that lysosomal proteases are not only able to initiate apoptotic signaling but can also amplify the apoptotic pathways initiated in other cellular compartments. The endocytic pathway also receives cargo destined for degradation via the autophagic pathway. By recycling energy and biosynthetic substrates, and by degrading damaged organelles and molecules, the endocytic system assists the autophagic system in resisting apoptotic stimuli. Steps leading to lysosomal membrane permeabilization and subsequent triggering of cell death as well as the therapeutic potential of intervention in lysosomal membrane permeabilization will be discussed.

Reovirus receptors, cell entry, and proapoptotic signaling

Mammalian orthoreoviruses (reoviruses) are members of the Reoviridae. Reoviruses contain 10 double-stranded (ds) RNA gene segments enclosed in two concentric protein shells, called outer capsid and core. These viruses serve as a versatile experimental system for studies of viral replication events at the virus-cell interface, including engagement of cell-surface receptors, internalization and disassembly, and activation of the innate immune response, including NF-κB-dependent cellular signaling pathways. Reoviruses also provide a model system for studies of virus-induced apoptosis and organ-specific disease in vivo.Reoviruses attach to host cells via the filamentous attachment protein, σ1. The σ1 protein of all reovirus serotypes engages junctional adhesion molecule-A (JAM-A), an integral component of intercellular tight junctions. The σ1 protein also binds to cell-surface carbohydrate, with the type of carbohydrate bound varying by serotype. Following attachment to JAM-A and carbohydrate, reovirus internalization is mediated by β1 integrins, most likely via clathrin-dependent endocytosis. In the endocytic compartment, reovirus outer-capsid protein σ3 is removed by acid-dependent cysteine proteases in most cell types. Removal of σ3 results in the exposure of a hydrophobic conformer of the viral membrane-penetration protein, μ1, which pierces the endosomal membrane and delivers transcriptionally active reovirus core particles into the cytoplasm.Reoviruses induce apoptosis in both cultured cells and infected mice. Perturbation of reovirus disassembly using inhibitors of endosomal acidification or protease activity abrogates apoptosis. The μ1-encoding M2 gene is genetically linked to strain-specific differences in apoptosis-inducing capacity, suggesting a function for μ1 in induction of death signaling. Reovirus disassembly leads to activation of transcription factor NF-κB, which modulates apoptotic signaling in numerous types of cells. Inhibition of NF-κB nuclear translocation using either pharmacologic agents or expression of transdominant forms of IκB blocks reovirus-induced apoptosis, suggesting an essential role for NF-κB activation in the death response. Multiple effector pathway s downstream of NF-κB-directed gene expression execute reovirus-induced cell death. This chapter will focus on the mechanisms by which reovirus attachment and disassembly activate NF-κB and stimulate the cellular proapoptotic machinery.

TLR4 activation under lipotoxic conditions leads to synergistic macrophage cell death through a TRIF-dependent pathway

Macrophage dysfunction in obesity and diabetes may predispose to the development of diabetic complications, such as infection and impaired healing after tissue damage. Saturated fatty acids, such as palmitate, are present at elevated concentrations in the plasma of patients with metabolic disease and may contribute to the pathogenesis of diabetes and its sequelae. To examine the effect of lipid excess on macrophage inflammatory function, we determined the influence of palmitate on LPS-mediated responses in peritoneal macrophages. Palmitate and LPS led to a profound synergistic cell death response in both primary and RAW 264.7 macrophages. The cell death had features of apoptosis and necrosis and was not dependent on endoplasmic reticulum stress, ceramide generation, or reactive oxygen species production. Instead, we uncovered a macrophage death pathway that required TLR4 signaling via TRIF but was independent of NF-κB, MAPKs, and IRF3. A significant decrease in macrophage lysosomal content was observed early in the death pathway, with evidence of lysosomal membrane damage occurring later in the death response. Overexpression of the transcription factor TFEB, which induces a lysosomal biogenic program, rescued the lysosomal phenotype and improved viability in palmitate- and LPS-treated cells. Our findings provide new evidence for cross-talk between lipid metabolism and the innate immune response that converges on the lysosome.

Autophagy inhibitor chloroquine enhanced the cell death inducing effect of the flavonoid luteolin in metastatic squamous cell carcinoma cells

BACKGROUND

Flavonoids are widely proposed as very interesting compounds with possible chemopreventive and therapeutic capacities.

METHODS & RESULTS

In this study, we showed that in vitro treatment with the flavonoid Luteolin induced caspase-dependent cell death in a model of human cutaneous squamous cell carcinoma (SCC) derived cells, representing a matched pair of primary tumor and its metastasis. Notably, no cytotoxic effects were observed in normal human keratinocytes when treated with similar doses of Luteolin. Luteolin-induced apoptosis was accompanied by inhibition of AKT signaling, and sensitivity decreased with tumor progression, as the primary MET1 SCC cells were considerably more sensitive to Luteolin than the isogenic metastatic MET4 cells. Extensive intracellular vacuolization was observed in Luteolin-treated MET4 cells, which were characterized as acidic lysosomal vacuoles, suggesting the involvement of autophagy. Transmission electron microscopy, mRFP-GFP-LC3 assay and p62 protein degradation, confirmed that Luteolin stimulated the autophagic process in the metastatic MET4 cells. Blocking autophagy using chloroquine magnified Luteolin-induced apoptosis in the metastatic SCC cells.

CONCLUSION

Together, these results suggest that Luteolin has the capacity to induce selectively apoptotic cell death both in primary cutaneous SCC cells and in metastatic SCC cells in combination with chloroquine, an inhibitor of autophagosomal degradation. Hence, Luteolin might be a promising agent for the treatment of cutaneous SCC.

Autophagic degradation of the BCR-ABL oncoprotein and generation of antileukemic responses by arsenic trioxide

We provide evidence that arsenic trioxide (As(2)O(3)) targets the BCR-ABL oncoprotein via a novel mechanism involving p62/SQSTM1-mediated localization of the oncoprotein to the autolysosomes and subsequent degradation mediated by the protease cathepsin B. Our studies demonstrate that inhibitors of autophagy or cathepsin B activity and/or molecular targeting of p62/SQSTM1, Atg7, or cathepsin B result in partial reversal of the suppressive effects of AS(2)O(3) on BCR-ABL expressing leukemic progenitors, including primitive leukemic precursors from chronic myelogenous leukemia (CML) patients. Altogether, these findings indicate that autophagic degradation of BCR-ABL is critical for the induction of the antileukemic effects of As(2)O(3) and raise the potential for future therapeutic approaches to target BCR-ABL expressing cells by modulating elements of the autophagic machinery to promote BCR-ABL degradation.

Type 1 inositol-1,4,5-trisphosphate receptor is a late substrate of caspases during apoptosis

Apoptosis is characterized by the proteolytic cleavage of hundreds of proteins. One of them, the type 1 inositol-1,4,5-trisphosphate receptor (IP(3) R-1), a multimeric receptor located on the endoplasmic reticulum (ER) membrane that is critical to calcium homeostasis, was reported to be cleaved during staurosporine (STS) induced-apoptosis in Jurkat cells. Because the reported cleavage site separates the IP(3) binding site from the channel moiety, its cleavage would shut down a critical signaling pathway that is common to several cellular processes. Here we show that IP(3) R-1 is not cleaved in 293 cells treated with STS, TNFα, Trail, or ultra-violet (UV) irradiation. Further, it is not cleaved in Hela or Jurkat cells induced to undergo apoptosis with Trail, TNFα, or UV. In accordance with previous reports, we demonstrate that it is cleaved in a Jurkat cell line treated with STS. However its cleavage occurs only after poly(ADP-ribose) polymerase (PARP), which cleavage is a hallmark of apoptosis, and p23, a poor caspase-7 substrate, are completely cleaved, suggesting that IP(3) R-1 is a relatively late substrate of caspases. Nevertheless, the receptor is fully accessible to proteolysis in cellulo by ectopically overexpressed caspase-7 or by the tobacco etch virus (TEV) protease. Finally, using recombinant caspase-3 and microsomal fractions enriched in IP(3) R-1, we show that the receptor is a poor caspase-3 substrate. Consequently, we conclude that IP(3) R-1 is not a key death substrate.

New insights into the mechanisms for photodynamic therapy-induced cancer cell death

Photodynamic therapy (PDT) is a promising therapeutic modality for cancer treatment; however, a more detailed understanding is needed to improve the clinical use of this therapy. PDT induces cancer cell death by apoptosis, necrosis, and autophagy, and these mechanisms can be concurrently occurred. PDT destroys cancer cells by inducing apoptosis through diverse signaling pathways coupled with Bcl-2 family members, caspases, and apopotosis-inducing factor. When the apoptotic pathway is unavailable, PDT can cause cancer cell death through induction of a necrotic or autophagic mechanism. Autophagy is occurred in a Bax-independent manner and can be stimulated in parallel with apoptosis. PDT directly destroys cancer cells by inducing either apoptotic or necrotic death. PDT also can induce autophagy as a death or a survival mechanism. These mechanisms are dependent on a variety of parameters including the nature of the photosensitizer, PDT dose, and cell genotype. Understanding the complex cross talk between these pathways may improve the effectiveness of PDT. Here, we discuss the interplay between these mechanisms based on recent evidence and suggest prospects with regard to advances in PDT.

Photodynamic therapy with talaporfin sodium induces dose-dependent apoptotic cell death in human glioma cell lines

OBJECTIVE

To investigate the kinetics of cell death in human glioma cell lines induced by photodynamic therapy (PDT) with the second-generation photosensitizer talaporfin sodium (TS) and a 664-nm diode laser.

MATERIALS AND METHODS

Three human glioma cell lines (T98G, A172, U251) were studied. After incubation of the cell lines with various concentrations of TS for 4 h, PDT using diode laser irradiation at 33 mW/cm² and 10 J/cm² was performed. Cell viability and changes in cell morphology were examined by the Cell Counting Kit-8 assay and phase-contrast microscopy, respectively. In addition, to evaluate the pathology of cell death, changes in cell viability after treatment with a caspase activation inhibitor and an autophagy inhibitor were also examined.

RESULTS

In all 3 human glioma cell lines, TS induced dose-dependent cell death. However, the 50% lethal dose of TS varied among these cell lines. The main morphological feature of cell death was shrinkage of the cell body, and the number of cells with this morphological change increased in a time-dependent manner, resulting in cell death. In addition, a dose-dependent improvement in cell viability by the caspase inhibitor Z-VAD-fmk was observed.

CONCLUSION

PDT with TS induces dose-dependent apoptosis in human glioma cell lines. However, the sensitivity to PDT varied among the cell lines, indicating a possible difference in the intracellular content of TS, or a difference in the susceptibility to the intracellular oxidative stress caused by PDT.

Increased expression of stefin B in the nucleus of T98G astrocytoma cells delays caspase activation

Stefin B (cystatin B) is an endogenous inhibitor of cysteine proteinases localized in the nucleus and the cytosol. Loss-of-function mutations in the stefin B gene (CSTB) gene were reported in patients with Unverricht-Lundborg disease (EPM1). Our previous results showed that thymocytes isolated from stefin B-deficient mice are more sensitive to apoptosis induced by the protein kinase C (PKC) inhibitor staurosporin (STS) than the wild-type control cells. We have also shown that the increased expression of stefin B in the nucleus of T98G astrocytoma cells delayed cell cycle progression through the S phase. In the present study we examined if the nuclear or cytosolic functions of stefin B are responsible for the accelerated induction of apoptosis observed in the cells from stefin B-deficient mice. We have shown that the overexpression of stefin B in the nucleus, but not in the cytosol of astrocytoma T98G cells, delayed caspase-3 and -7 activation. Pretreatment of cells with the pan-caspase inhibitor z-Val-Ala-Asp(OMe)-fluoromethylketone completely inhibited caspase activation, while treatment with the inhibitor of calpains- and papain-like cathepsins (2S,3S)-trans-epoxysuccinyl-leucylamido-3-methyl-butane ethyl ester did not prevent caspase activation. We concluded that the delay of caspase activation in T98G cells overexpressing stefin B in the nucleus is independent of cathepsin inhibition.

Phosphatidic acid mediates the targeting of tBid to induce lysosomal membrane permeabilization and apoptosis

Upon apoptotic stimuli, lysosomal proteases, including cathepsins and chymotrypsin, are released into cytosol due to lysosomal membrane permeabilization (LMP), where they trigger apoptosis via the lysosomal-mitochondrial pathway of apoptosis. Herein, the mechanism of LMP was investigated. We found that caspase 8-cleaved Bid (tBid) could result in LMP directly. Although Bax or Bak might modestly enhance tBid-triggered LMP, they are not necessary for LMP. To study this further, large unilamellar vesicles (LUVs), model membranes mimicking the lipid constitution of lysosomes, were used to reconstitute the membrane permeabilization process in vitro. We found that phosphatidic acid (PA), one of the major acidic phospholipids found in lysosome membrane, is essential for tBid-induced LMP. PA facilitates the insertion of tBid deeply into lipid bilayers, where it undergoes homo-oligomerization and triggers the formation of highly curved nonbilayer lipid phases. These events induce LMP via pore formation mechanisms because encapsulated fluorescein-conjugated dextran (FD)-20 was released more significantly than FD-70 or FD-250 from LUVs due to its smaller molecular size. On the basis of these data, we proposed tBid-PA interactions in the lysosomal membranes form lipidic pores and result in LMP. We further noted that chymotrypsin-cleaved Bid is more potent than tBid at binding to PA, inserting into the lipid bilayer, and promoting LMP. This amplification mechanism likely contributes to the culmination of apoptotic signaling.

Particulate systems for targeting of macrophages: basic and therapeutic concepts

Particulate systems in the form of liposomes, polymeric micelles, polymeric nano- and microparticles, and many others offer a rational approach for selective delivery of therapeutic agents to the macrophage from different physiological portals of entry. Particulate targeting of macrophages and intracellular drug release processes can be optimized through modifications of the drug carrier physicochemical properties, which include hydrodynamic size, shape, composition and surface characteristics. Through such modifications together with understanding of macrophage cell biology, targeting may be aimed at a particular subset of macrophages. Advances in basic and therapeutic concepts of particulate targeting of macrophages and related nanotechnology approaches for immune cell modifications are discussed.

Cathepsin D primes caspase-8 activation by multiple intra-chain proteolysis

During the resolution of inflammatory responses, neutrophils rapidly undergo apoptosis. A direct and fast activation of caspase-8 by cathepsin D was shown to be crucial in the initial steps of neutrophil apoptosis. Nevertheless, the activation mechanism of caspase-8 remains unclear. Here, by using site-specific mutants of caspase-8, we show that both cathepsin D-mediated proteolysis and homodimerization of caspase-8 are necessary to generate an active caspase-8. At acidic pH, cathepsin D specifically cleaved caspase-8 but not the initiator caspase-9 or -10 and significantly increased caspase-8 activity in dimerizing conditions. These events were completely abolished by pepstatin A, a pharmacological inhibitor of cathepsin D. The cathepsin D intra-chain proteolysis greatly stabilized the active site of caspase-8. Moreover, the main caspase-8 fragment generated by cathepsin D cleavage could be affinity-labeled with the active site probe biotin-VAD-fluoromethyl ketone, suggesting that this fragment is enzymatically active. Importantly, in an in vitro cell-free assay, the addition of recombinant human caspase-8 protein, pre-cleaved by cathepsin D, was followed by caspase-3 activation. Our data therefore indicate that cathepsin D is able to initiate the caspase cascade by direct activation of caspase-8. As cathepsin D is ubiquitously expressed, this may represent a general mechanism to induce apoptosis in a variety of immune and nonimmune cells.

Dysregulation of macrophage-secreted cathepsin B contributes to HIV-1-linked neuronal apoptosis

Chronic HIV infection leads to the development of cognitive impairments, designated as HIV-associated neurocognitive disorders (HAND). The secretion of soluble neurotoxic factors by HIV-infected macrophages plays a central role in the neuronal dysfunction and cell death associated with HAND. One potentially neurotoxic protein secreted by HIV-1 infected macrophages is cathepsin B. To explore the potential role of cathepsin B in neuronal cell death after HIV infection, we cultured HIV-1_ADA infected human monocyte-derived macrophages (MDM) and assayed them for expression and activity of cathepsin B and its inhibitors, cystatins B and C. The neurotoxic activity of the secreted cathepsin B was determined by incubating cells from the neuronal cell line SK-N-SH with MDM conditioned media (MCM) from HIV-1 infected cultures. We found that HIV-1 infected MDM secreted significantly higher levels of cathepsin B than did uninfected cells. Moreover, the activity of secreted cathepsin B was significantly increased in HIV-infected MDM at the peak of viral production. Incubation of neuronal cells with supernatants from HIV-infected MDM resulted in a significant increase in the numbers of apoptotic neurons, and this increase was reversed by the addition of either the cathepsin B inhibitor CA-074 or a monoclonal antibody to cathepsin B. In situ proximity ligation assays indicated that the increased neurotoxic activity of the cathepsin B secreted by HIV-infected MDM resulted from decreased interactions between the enzyme and its inhibitors, cystatins B and C. Furthermore, preliminary in vivo studies of human post-mortem brain tissue suggested an upregulation of cathepsin B immunoreactivity in the hippocampus and basal ganglia in individuals with HAND. Our results demonstrate that HIV-1 infection upregulates cathepsin B in macrophages, increases cathepsin B activity, and reduces cystatin-cathepsin interactions, contributing to neuronal apoptosis. These findings provide new evidence for the role of cathepsin B in neuronal cell death induced by HIV-infected macrophages.