Lysosomal involvement in cell death and cancer

Determining signalling nodes for apoptosis by a genetic high-throughput screen

BACKGROUND

With the ever-increasing information emerging from the various sequencing and gene annotation projects, there is an urgent need to elucidate the cellular functions of the newly discovered genes. The genetically regulated cell suicide of apoptosis is especially suitable for such endeavours as it is governed by a vast number of factors.

METHODOLOGY/PRINCIPAL FINDINGS

We have set up a high-throughput screen in 96-well microtiter plates for genes that induce apoptosis upon their individual transfection into human cells. Upon screening approximately 100,000 cDNA clones we determined 74 genes that initiate this cellular suicide programme. A thorough bioinformatics analysis of these genes revealed that 91% are novel apoptosis regulators. Careful sequence analysis and functional annotation showed that the apoptosis factors exhibit a distinct functional distribution that distinguishes the cell death process from other signalling pathways. While only a minority of classic signal transducers were determined, a substantial number of the genes fall into the transporter- and enzyme-category. The apoptosis factors are distributed throughout all cellular organelles and many signalling circuits, but one distinct signalling pathway connects at least some of the isolated genes. Comparisons with microarray data suggest that several genes are dysregulated in specific types of cancers and degenerative diseases.

CONCLUSIONS/SIGNIFICANCE

Many unknown genes for cell death were revealed through our screen, supporting the enormous complexity of cell death regulation. Our results will serve as a repository for other researchers working with genomics data related to apoptosis or for those seeking to reveal novel signalling pathways for cell suicide.

Lysosomes and lysosomal cathepsins in cell death

Lysosomes are the key degradative compartments of the cell. Lysosomal cathepsins, which are enclosed in the lysosomes, help to maintain the homeostasis of the cell's metabolism by participating in the degradation of heterophagic and autophagic material. Following the targeted lysosomal membrane's destabilization, the cathepsins can be released into the cytosol and initiate the lysosomal pathway of apoptosis through the cleavage of Bid and the degradation of the anti-apoptotic Bcl-2 homologues. Cathepsins can also amplify the apoptotic signaling, when the lysosomal membranes are destabilized at a later stage of apoptosis, initiated by other stimuli. However, the functional integrity of the lysosomal compartment during apoptosis enables efficient autophagy, which can counteract apoptosis by providing the energy source and by disposing the damaged mitochondria, which generate the ROS. Impairing autophagy by disabling the lysosome function is being investigated as an adjuvant therapeutic approach to sensitize cells to apoptosis-inducing agents. Destabilization of the lysosomal membranes by the lysosomotropic detergents seems to be a promising strategy in this context as it would not only disable autophagy, but also promote apoptosis through the initiation of the lysosomal pathway. In contrast, the impaired autophagy and lysosomal degradation linked with the increased oxidative stress underlie degenerative changes in the aging neurons. This further suggests that lysosomes and lysosomal cathepsins have a dual role in cell death. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Lysosomal membrane permeabilization contributes to elemene emulsion-induced apoptosis in A549 cells

Elemene is a broad-spectrum antitumor agent. In the present study, lysosomal membrane permeabilization (LMP) was detected after short elemene emulsion--exposure (12 h) that preceded a decrease of the mitochondrial membrane potential and DNA damage (24 h) in A549 cells. At later time points (36 h) elemene emulsion caused the appearance of A549 cells with apoptotic features, including apoptotic morphology, phosphatidylserine exposure, and caspase-3 activation. A significant increase in protein expression for cathepsin D was also observed utilizing Western blot analysis after exposure to elemene emulsion for 12 h. The present study showed that elemene emulsion induced the increased levels of reactive oxygen species (ROS) and depletion of glutathione (GSH) in A549 cells. Cells treated with pepstatin A, an inhibitor for cathepsin D, showed a significant inhibition in DNA damage, mitochondrial membrane permeabilization, caspase-3 activation, and phosphatidylserine exposure. These results demonstrated that apoptosis induced by elemene emulsion in A549 cells is mediated in part through LMP and lysosomal protease cathepsin D.

Gene-dependent cell death in yeast

Caspase-dependent apoptotic cell death has been extensively studied in cultured cells and during embryonic development, but the existence of analogous molecular pathways in single-cell species is uncertain. This has reduced enthusiasm for applying the advanced genetic tools available for yeast to study cell death regulation. However, partial characterization in mammals of additional genetically encoded cell death mechanisms, which lead to a range of dying cell morphologies and necrosis, suggests potential applications for yeast genetics. In this light, we revisited the topic of gene-dependent cell death in yeast to determine the prevalence of yeast genes with the capacity to contribute to cell-autonomous death. We developed a rigorous strategy by allowing sufficient time for gene-dependent events to occur, but insufficient time to evolve new populations, and applied this strategy to the Saccharomyces cerevisiae gene knockout collection. Unlike sudden heat shock, a ramped heat stimulus delivered over several minutes with a thermocycler, coupled with assessment of viability by automated counting of microscopic colonies revealed highly reproducible gene-specific survival phenotypes, which typically persist under alternative conditions. Unexpectedly, we identified over 800 yeast knockout strains that exhibit significantly increased survival following insult, implying that these genes can contribute to cell death. Although these death mechanisms are yet uncharacterized, this study facilitates further exploration.

Proteases in autophagy

Autophagy is a process involved in the proteolytic degradation of cellular macromolecules in lysosomes, which requires the activity of proteases, enzymes that hydrolyse peptide bonds and play a critical role in the initiation and execution of autophagy. Importantly, proteases also inhibit autophagy in certain cases. The initial steps of macroautophagy depend on the proteolytic processing of a particular protein, Atg8, by a cysteine protease, Atg4. This processing step is essential for conjugation of Atg8 with phosphatidylethanolamine and, subsequently, autophagosome formation. Lysosomal hydrolases, known as cathepsins, can be divided into several groups based on the catalitic residue in the active site, namely, cysteine, serine and aspartic cathepsins, which catalyse the cleavage of peptide bonds of autophagy substrates and, together with other factors, dispose of the autophagic flux. Whilst most cathepsins degrade autophagosomal content, some, such as cathepsin L, also degrade lysosomal membrane components, GABARAP-II and LC3-II. In contrast, cathepsin A, a serine protease, is involved in inhibition of chaperon-mediated autophagy through proteolytic processing of LAMP-2A. In addition, other families of calcium-dependent non-lysosomal cysteine proteases, such as calpains, and cysteine aspartate-specific proteases, such as caspases, may cleave autophagy-related proteins, negatively influencing the execution of autophagic processes. Here we discuss the current state of knowledge concerning protein degradation by autophagy and outline the role of proteases in autophagic processes. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Attenuation of the lysosomal death pathway by lysosomal cholesterol accumulation

In the past decade, lysosomal membrane permeabilization (LMP) has emerged as a significant component of cell death signaling. The mechanisms by which lysosomal stability is regulated are not yet fully understood, but changes in the lysosomal membrane lipid composition have been suggested to be involved. Our aim was to investigate the importance of cholesterol in the regulation of lysosomal membrane permeability and its potential impact on apoptosis. Treatment of normal human fibroblasts with U18666A, an amphiphilic drug that inhibits cholesterol transport and causes accumulation of cholesterol in lysosomes, rescued cells from lysosome-dependent cell death induced by the lysosomotropic detergent O-methyl-serine dodecylamide hydrochloride (MSDH), staurosporine (STS), or cisplatin. LMP was decreased by pretreating cells with U18666A, and there was a linear relationship between the cholesterol content of lysosomes and their resistance to permeabilization induced by MSDH. U18666A did not induce changes in expression or localization of 70-kDa heat shock proteins (Hsp70) or antiapoptotic Bcl-2 proteins known to protect the lysosomal membrane. Induction of autophagy also was excluded as a contributor to the protective mechanism. By using Chinese hamster ovary (CHO) cells with lysosomal cholesterol overload due to a mutation in the cholesterol transporting protein Niemann-Pick type C1 (NPC1), the relationship between lysosomal cholesterol accumulation and protection from lysosome-dependent cell death was confirmed. Cholesterol accumulation in lysosomes attenuates apoptosis by increasing lysosomal membrane stability.

Targeting of lysosomes by liposomes modified with octadecyl-rhodamine B

The use of lysosome-targeted liposomes may significantly improve a delivery of therapeutic enzymes into lysosomes for the treatment of lysosome-associated diseases. The aim of this research was to achieve a specific intracellular targeting of lysosomes, by using liposomes modified with the lysosomotropic octadecyl-rhodamine B (RhB) and loaded with a model compound, fluorescein isothiocyanate (FITC)-dextran (FD). Plain and RhB-modified liposomes were prepared by hydration of lipid films and loaded with FD or with 5-dodecanoylaminofluorescein di-β-d-galactopyranoside (C(12)FDG), a specific substrate for the intralysosomal β-galactosidase. The delivery of these liposomes and their content to lysosomes in HeLa cells was investigated by confocal microscopy, flow cytometry, and subcellular fractionation. Confocal microscopy demonstrated that RhB-liposomes co-localize well with the specific lysosomal markers, unlike plain liposomes. The comparison of the FITC fluorescence of the lysosomes isolated by subcellular fractionation also showed that the efficiency of FD delivery into lysosomes by RhB-modified liposomes was significantly higher compared with plain liposomes. These results were additionally confirmed by the flow cytometry of the intact cells treated with C(12)FDG-loaded liposomes that also demonstrated increased lysosomal targeting by RhB-modified liposomes. The modification of the liposomal surface with a lysosomotropic ligand, such as octadecyl-RhB, can significantly increase the delivery of liposomal loads to lysosomes.

Nonesterified cholesterol content of lysosomes modulates susceptibility to oxidant-induced permeabilization

Reactive oxygen species (ROS) can induce lysosomal membrane permeabilization (LMP). Photoirradiation of murine hepatoma 1c1c7 cultures preloaded with the photosensitizer NPe6 generates singlet oxygen within acidic organelles and causes LMP and the activation of procaspases. Treatment with the cationic amphiphilic drugs (CADs) U18666A, imipramine, and clozapine stimulated the accumulation of filipin-stainable nonesterified cholesterol/sterols in late endosomes/lysosomes, but not in mitochondria. Concentration-response studies demonstrated an inverse relationship between lysosomal nonesterified cholesterol/sterol contents and susceptibility to NPe6 photoirradiation-induced intracellular membrane oxidation, LMP, and activation of procaspase-9 and -3. Similarly, the kinetics of restoration of NPe6 photoirradiation-induced LMP paralleled the losses of lysosomal cholesterol that occurred upon replating U18666A-treated cultures in CAD-free medium. Consistent with the oxidation of lysosomal cholesterol, filipin staining in U18666A-treated cultures progressively decreased with increasing photoirradiating light dose. U18666A also suppressed the induction of LMP and procaspase activation by exogenously added hydrogen peroxide. However, neither U18666A nor imipramine suppressed the induction of apoptosis by agents that did not directly induce LMP. These studies indicate that lysosomal nonesterified cholesterol/sterol content modulates susceptibility to ROS-induced LMP and possibly does so by being an alternative target for oxidants and lowering the probability of damage to other lysosomal membrane lipids and/or proteins.

A high content screening assay for identifying lysosomotropic compounds

Lysosomes are acidic organelles that are essential for the degradation of old organelles and engulfed microbes. Furthermore, lysosomes play a key role in cell death. Lipophilic or amphiphilic compounds with a basic moiety can become protonated and trapped within lysosomes, causing lysosomal dysfunction. Therefore, high-throughput screens to detect lysosomotropism, the accumulation of compounds in lysosomes, are desirable. Hence, we developed a 96-well format, high content screening assay that measures lysosomotropism and cytotoxicity by quantitative image analysis. Forty drugs, including antidepressants, antipsychotics, antiarrhythmics and anticancer agents, were tested for their effects on lysosomotropism and cytotoxicity in H9c2 cells. The assay correctly identified drugs known to cause lysosomotropism and revealed novel information showing that the anticancer drugs, gefitinib, lapatinib, and dasatinib, caused lysosomotropism. Although structurally and pharmacologically diverse, drugs that were lysosomotropic shared certain physicochemical properties, possessing a ClogP>2 and a basic pKa between 6.5 and 11. In contrast, drugs which did not lie in this physicochemical property space were not lysosomotropic. The assay is a robust, rapid screen that can be used to identify lysosomotropic, as well as, cytotoxic compounds, and can be positioned within a screening paradigm to understand the role of lysosomotropism as a contributor to drug-induced toxicity.

Artesunate activates mitochondrial apoptosis in breast cancer cells via iron-catalyzed lysosomal reactive oxygen species production

The antimalarial agent artesunate (ART) activates programmed cell death (PCD) in cancer cells in a manner dependent on the presence of iron and the generation of reactive oxygen species. In malaria parasites, ART cytotoxicity originates from interactions with heme-derived iron within the food vacuole. The analogous digestive compartment of mammalian cells, the lysosome, similarly contains high levels of redox-active iron and in response to specific stimuli can initiate mitochondrial apoptosis. We thus investigated the role of lysosomes in ART-induced PCD and determined that in MCF-7 breast cancer cells ART activates lysosome-dependent mitochondrial outer membrane permeabilization. ART impacted endolysosomal and autophagosomal compartments, inhibiting autophagosome turnover and causing perinuclear clustering of autophagosomes, early and late endosomes, and lysosomes. Lysosomal iron chelation blocked all measured parameters of ART-induced PCD, whereas lysosomal iron loading enhanced death, thus identifying lysosomal iron as the lethal source of reactive oxygen species upstream of mitochondrial outer membrane permeabilization. Moreover, lysosomal inhibitors chloroquine and bafilomycin A1 reduced ART-activated PCD, evidencing a requirement for lysosomal function during PCD signaling. ART killing did not involve activation of the BH3-only protein, Bid, yet ART enhanced TNF-mediated Bid cleavage. We additionally demonstrated the lysosomal PCD pathway in T47D and MDA-MB-231 breast cancer cells. Importantly, non-tumorigenic MCF-10A cells resisted ART-induced PCD. Together, our data suggest that ART triggers PCD via engagement of distinct, interconnected PCD pathways, with hierarchical signaling from lysosomes to mitochondria, suggesting a potential clinical use of ART for targeting lysosomes in cancer treatment.

A role for serglycin proteoglycan in mast cell apoptosis induced by a secretory granule-mediated pathway

Mast cell secretory granules (secretory lysosomes) contain large amounts of fully active proteases bound to serglycin proteoglycan. Damage to the granule membrane will thus lead to the release of serglycin and serglycin-bound proteases into the cytosol, which potentially could lead to proteolytic activation of cytosolic pro-apoptotic compounds. We therefore hypothesized that mast cells are susceptible to apoptosis induced by permeabilization of the granule membrane and that this process is serglycin-dependent. Indeed, we show that wild-type mast cells are highly sensitive to apoptosis induced by granule permeabilization, whereas serglycin-deficient cells are largely resistant. The reduced sensitivity of serglycin(-/-) cells to apoptosis was accompanied by reduced granule damage, reduced release of proteases into the cytosol, and defective caspase-3 activation. Mechanistically, the apoptosis-promoting effect of serglycin involved serglycin-dependent proteases, as indicated by reduced sensitivity to apoptosis and reduced caspase-3 activation in cells lacking individual mast cell-specific proteases. Together, these findings implicate serglycin proteoglycan as a novel player in mast cell apoptosis.

HSP70 and lysosomal storage disorders: novel therapeutic opportunities

Lysosomes, with their arsenal of catabolic enzymes and crucial metabolic housekeeping functions are experiencing a revived research interest after having lived a rather quiet life for the last few decades. With the discovery of the interaction of the lysosomes with another ancient component of cellular homoeostasis, the molecular chaperone HSP70 (heat-shock protein 70), the stage seems set for further discoveries of the mechanisms regulating cellular and physiological stress responses to otherwise detrimental challenges.

The proteome of lysosomes

Lysosomes are organelles of eukaryotic cells that are critically involved in the degradation of macromolecules mainly delivered by endocytosis and autophagocytosis. Degradation is achieved by more than 60 hydrolases sequestered by a single phospholipid bilayer. The lysosomal membrane facilitates interaction and fusion with other compartments and harbours transport proteins catalysing the export of catabolites, thereby allowing their recycling. Lysosomal proteins have been addressed in various proteomic studies that are compared in this review regarding the source of material, the organelle/protein purification scheme, the proteomic methodology applied and the proteins identified. Distinguishing true constituents of an organelle from co-purifying contaminants is a central issue in subcellular proteomics, with additional implications for lysosomes as being the site of degradation of many cellular and extracellular proteins. Although many of the lysosomal hydrolases were identified by classical biochemical approaches, the knowledge about the protein composition of the lysosomal membrane has remained fragmentary for a long time. Using proteomics many novel lysosomal candidate proteins have been discovered and it can be expected that their functional characterisation will help to understand functions of lysosomes at a molecular level that have been characterised only phenomenologically so far and to generally deepen our understanding of this indispensable organelle.

Lysosomal-mitochondrial cross-talk during cell death

Lysosomes are membrane-bound organelles, which contain an arsenal of different hydrolases, enabling them to act as the terminal degradative compartment of the endocytotic, phagocytic and autophagic pathways. During the last decade, it was convincingly shown that destabilization of lysosomal membrane and release of lysosomal content into the cytosol can initiate the lysosomal apoptotic pathway, which is dependent on mitochondria destabilization. The cleavage of BID to t-BID and degradation of anti-apoptotic BCL-2 proteins by lysosomal cysteine cathepsins were identified as links to the mitochondrial cytochrome c release, which eventually leads to caspase activation. There have also been reports about the involvement of lysosome destabilization and lysosomal proteases in the extrinsic apoptotic pathway, although the molecular mechanism is still under debate. In the present article, we discuss the cross-talk between lysosomes and mitochondria during apoptosis and its consequences for the fate of the cell.

Combating apoptosis and multidrug resistant cancers by targeting lysosomes

Acquired therapy resistance is one of the prime obstacles for successful cancer treatment. Partial resistance is often acquired already during an early face of tumor development when genetic changes causing defects in classical caspase-dependent apoptosis pathway provide transformed cells with a growth advantage by protecting them against various apoptosis inducing stimuli including transforming oncogenes themselves and host immune system. Apoptosis defective cells are further selected during tumor progression and finally by apoptosis inducing treatments. Another form of resistance, multidrug resistance, arises during cancer treatment when cancer cells with effective efflux of cytotoxic agents escape the therapy. Remarkably, induction of lysosomal membrane permeabilization has recently emerged as an effective way to kill apoptosis resistant cancer cells and some lysosome targeting drugs can also re-sensitize multidrug resistant cells to classical chemotherapy. In this review, we highlight recent data on lysosomal cell death pathways and their implications for the future treatment of apoptosis defective and multidrug resistant aggressive tumors.

The autoimmunity-related GIMAP5 GTPase is a lysosome-associated protein

A mutation in the rat GIMAP5 gene predisposes for autoimmunity, most famously in the BB rat model of autoimmune type 1 diabetes mellitus. This mutation is associated with severe peripheral T lymphopenia, as is mutation of the same gene in mice, but the mechanism by which GIMAP5 normally protects T cells from death is unknown. GIMAP5 is a putative small GTPase, a class of proteins which often fulfil their functions in the vicinity of cellular membranes. The objective of this study was to determine the normal intracellular location of GIMAP5 in lymphoid cells. Combining studies in rat, mouse and human systems, novel monoclonal antibodies (mAbs) were used to examine the localization of GIMAP5 and the closely-related protein, GIMAP1, in lymphoid cells by means of confocal microscopy and sub-cellular fractionation combined with immunoblotting. Additionally, human Jurkat T cells that inducibly express epitope-tagged GIMAP5 were established and used in electron microscopy (EM). Endogenous GIMAP5 was found to be located in a membraneous compartment/s which was also detected by established markers of lysosomes. GIMAP1, by contrast, was found to be located in the Golgi apparatus. EM studies of the inducible Jurkat T cells also found GIMAP5 in lysosomes and, in addition, in multivesicular bodies. This study establishes that the endogenous location of GIMAP5 is in lysosomes and related compartments and provides a clearer context for hypotheses about its mechanism of action.

Regulation of apoptosis-associated lysosomal membrane permeabilization

Lysosomal membrane permeabilization (LMP) occurs in response to a large variety of cell death stimuli causing release of cathepsins from the lysosomal lumen into the cytosol where they participate in apoptosis signaling. In some settings, apoptosis induction is dependent on an early release of cathepsins, while under other circumstances LMP occurs late in the cell death process and contributes to amplification of the death signal. The mechanism underlying LMP is still incompletely understood; however, a growing body of evidence suggests that LMP may be governed by several distinct mechanisms that are likely engaged in a death stimulus- and cell-type-dependent fashion. In this review, factors contributing to permeabilization of the lysosomal membrane including reactive oxygen species, lysosomal membrane lipid composition, proteases, p53, and Bcl-2 family proteins, are described. Potential mechanisms to safeguard lysosomal integrity and confer resistance to lysosome-dependent cell death are also discussed.

Sculpturing digit shape by cell death

Physiological cell death is a key mechanism that ensures appropriate development and maintenance of tissues and organs in multicellular organisms. Most structures in the vertebrate embryo exhibit defined areas of cell death at precise stages of development. In this regard the areas of interdigital cell death during limb development provide a paradigmatic model of massive cell death with an evident morphogenetic role in digit morphogenesis. Physiological cell death has been proposed to occur by apoptosis, cellular phenomena genetically controlled to orchestrate cell suicide following two main pathways, cytochrome C liberation from the mitochondria or activation of death receptors. Such pathways converge in the activation of cysteine proteases known as caspases, which execute the cell death program, leading to typical morphologic changes within the cell, termed apoptosis. According to these findings it would be expected that caspases loss of function experiments could cause inhibition of interdigital cell death promoting syndactyly phenotypes. A syndactyly phenotype is characterized by absence of digit freeing during development that, when caused by absence of interdigital cell death, is accompanied by the persistence of an interdigital membrane. However this situation has not been reported in any of the KO mice or chicken loss of function experiments ever performed. Moreover histological analysis of dying cells within the interdigit reveals the synchronic occurrence of different types of cell death. All these findings are indicative of caspase alternative and/or complementary mechanisms responsible for physiological interdigital cell death. Characterization of alternative cell death pathways is required to explain vertebrate morphogenesis. Today there is great interest in cell death via autophagy, which could substitute or act synergistically to the apoptotic pathway. Here we discuss what is known about physiological cell death in the developing interdigital tissue of vertebrate embryos, paying special attention to the avian species.

Prevention of 7β-hydroxycholesterol-induced cell death by mangafodipir is mediated through lysosomal and mitochondrial pathways

Mangafodipir, a MRI contrast agent, has been used as a viability marker in patients with myocardial infarction and showed vascular relaxation effect. It confers myocardial protection against oxidative stress. However mechanisms underlying such protection have not yet been investigated. In this investigation we first studied whether mangafodipir inhibits apoptosis induced by 7beta-hydroxycholesterol (7betaOH), a cytotoxic cholesterol oxidation product found in atherosclerotic lesions in humans and in heart of ethanol-fed rats. We then focused on whether mangafodipir influences the production of reactive oxygen species, lysosomal and mitochondrial membrane permeabilities in the cell model. Our results revealed that pre-treatment with mangafodipir (400 microM) protected against cellular reactive oxygen species production, apoptosis, and permeabilization of lysosomal and mitochondrial membranes induced by 7betaOH. In conclusion, a novel effect of mangafodipir on 7betaOH-induced apoptosis is via reduction of cellular reactive oxygen species and stabilization of lysosomal and mitochondrial membranes. This is the first report to show the additional cytoprotective effect of mangafodipir, which may suggest possible use of the drug.

Diverse regulatory roles for lysosomal proteases in the immune response

The innate and adaptive immune system utilise endocytic protease activity to promote functional immune responses. Cysteine and aspartic proteases (cathepsins) constitute a subset of endocytic proteases, the immune function of which has been described extensively. Although historically these studies have focused on their role in processes such as antigen presentation and zymogen processing within the endocytic compartment, recent discoveries have demonstrated a critical role for these proteases in other intracellular compartments, and within the extracellular milieu. It has also become clear that their pattern of expression and substrate specificities are more diverse than was first envisaged. Here, we discuss recent advances addressing the role of lysosomal proteases in various aspects of the immune response. We pay attention to reports demonstrating cathepsin activity outside of its canonical endosome/lysosome microenvironment.

Hsp70 stabilizes lysosomes and reverts Niemann-Pick disease-associated lysosomal pathology

Heat shock protein 70 (Hsp70) is an evolutionarily highly conserved molecular chaperone that promotes the survival of stressed cells by inhibiting lysosomal membrane permeabilization, a hallmark of stress-induced cell death. Clues to its molecular mechanism of action may lay in the recently reported stress- and cancer-associated translocation of a small portion of Hsp70 to the lysosomal compartment. Here we show that Hsp70 stabilizes lysosomes by binding to an endolysosomal anionic phospholipid bis(monoacylglycero)phosphate (BMP), an essential co-factor for lysosomal sphingomyelin metabolism. In acidic environments Hsp70 binds with high affinity and specificity to BMP, thereby facilitating the BMP binding and activity of acid sphingomyelinase (ASM). The inhibition of the Hsp70-BMP interaction by BMP antibodies or a point mutation in Hsp70 (Trp90Phe), as well as the pharmacological and genetic inhibition of ASM, effectively revert the Hsp70-mediated stabilization of lysosomes. Notably, the reduced ASM activity in cells from patients with Niemann-Pick disease (NPD) A and B-severe lysosomal storage disorders caused by mutations in the sphingomyelin phosphodiesterase 1 gene (SMPD1) encoding for ASM-is also associated with a marked decrease in lysosomal stability, and this phenotype can be effectively corrected by treatment with recombinant Hsp70. Taken together, these data open exciting possibilities for the development of new treatments for lysosomal storage disorders and cancer with compounds that enter the lysosomal lumen by the endocytic delivery pathway.

Apoptosis and lysosome membrane permeabilization induction on breast cancer cells by an anticarcinogenic Bowman-Birk protease inhibitor from Vigna unguiculata seeds

In this work, we report the effects of a Bowman-Birk protease inhibitor, the Black-Eyed Pea Trypsin/Chymotrypsin Inhibitor - BTCI, purified from Vigna unguiculata seeds, on the MCF-7 breast cancer cells. The treatment of MCF-7 with 200microM BTCI for 72h induced significant reduction of the cell viability and proliferation (arrest in S and G2/M phase). These cytostatic effects were accompanied by acute morphological modifications including the alteration of the nuclear morphology, plasma membrane fragmentation, cytoplasm disorganization, presence of double-membrane vesicles, mitochondrial swelling, and an increase in the size of lysosomes. Significative DNA fragmentation, annexin-V(+) cell number increase, mitochondrial membrane potential reduction, and cytoplasm acidification were also detected. All together, these cytostatic and cytotoxic results point out to BTCI-induced apoptosis cell death associated with severe cell morphological alterations and lysosome membrane permeabilization. Our study confirms the anticarcinogenic potential of Bowman-Birk protease inhibitors and identifies BTCI as a promising tool for drug developments aimed at the treatment of breast cancer.

Direct measurement of cathepsin B activity in the cytosol of apoptotic cells by an activity-based probe

Cells control their own death through a program termed apoptosis, which is indispensable for development and homeostasis in all metazoans. Lysosomal cysteine proteases are not normally thought of as participating in apoptosis; however, recent reports have shown that the cathepsin proteases can be released from the lysosome during apoptosis, where they can participate in cell death. We report here the development of an activity-based probe that, under optimized conditions, reports on cathepsin B activity only in apoptotic cells by reading out the release of cathepsin B from the lysosomes. Biochemical characterization of apoptosis in cells from cathepsin B null mice shows delayed and suboptimal activation of caspases. Our data further supports a role for cathepsin B in the cytosol as a positive regulator of a cell death feed-forward loop and provides a chemical tool for future investigations.

Lipotoxicity-mediated cell dysfunction and death involve lysosomal membrane permeabilization and cathepsin L activity

Lipotoxicity, which is triggered when cells are exposed to elevated levels of free fatty acids, involves cell dysfunction and apoptosis and is emerging as an underlying factor contributing to various pathological conditions including disorders of the central nervous system and diabetes. We have shown that palmitic acid (PA)-induced lipotoxicity (PA-LTx) in nerve growth factor-differentiated PC12 (NGFDPC12) cells is linked to an augmented state of cellular oxidative stress (ASCOS) and apoptosis and that these events are inhibited by docosahexanoic acid (DHA). The mechanisms of PA-LTx in nerve cells are not well understood, but our previous findings indicate that it involves ROS generation, mitochondrial membrane permeabilization (MMP), and caspase activation. The present study used nerve growth factor differentiated PC12 cells (NGFDPC12 cells) and found that lysosomal membrane permeabilization (LMP) is an early event during PA-induced lipotoxicity that precedes MMP and apoptosis. Cathepsin L, but not cathepsin B, is an important contributor in this process since its pharmacological inhibition significantly attenuated LMP, MMP, and apoptosis. In addition, co-treatment of NGFDPC12 cells undergoing lipotoxicity with DHA significantly reduced LMP, suggesting that DHA acts by antagonizing upstream signals leading to lysosomal dysfunction. These results suggest that LMP is a key early mediator of lipotoxicity and underscore the value of interventions targeting upstream signals leading to LMP for the treatment of pathological conditions associated with lipotoxicity.

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.

Docetaxel-induced prostate cancer cell death involves concomitant activation of caspase and lysosomal pathways and is attenuated by LEDGF/p75

Background

Hormone-refractory prostate cancer (HRPC) is characterized by poor response to chemotherapy and high mortality, particularly among African American men when compared to other racial/ethnic groups. It is generally accepted that docetaxel, the standard of care for chemotherapy of HRPC, primarily exerts tumor cell death by inducing mitotic catastrophe and caspase-dependent apoptosis following inhibition of microtubule depolymerization. However, there is a gap in our knowledge of mechanistic events underlying docetaxel-induced caspase-independent cell death, and the genes that antagonize this process. This knowledge is important for circumventing HRPC chemoresistance and reducing disparities in prostate cancer mortality.

Results

We investigated mechanistic events associated with docetaxel-induced death in HRPC cell lines using various approaches that distinguish caspase-dependent from caspase-independent cell death. Docetaxel induced both mitotic catastrophe and caspase-dependent apoptosis at various concentrations. However, caspase activity was not essential for docetaxel-induced cytotoxicity since cell death associated with lysosomal membrane permeabilization still occurred in the presence of caspase inhibitors. Partial inhibition of docetaxel-induced cytotoxicity was observed after inhibition of cathepsin B, but not inhibition of cathepsins D and L, suggesting that docetaxel induces caspase-independent, lysosomal cell death. Simultaneous inhibition of caspases and cathepsin B dramatically reduced docetaxel-induced cell death. Ectopic expression of lens epithelium-derived growth factor p75 (LEDGF/p75), a stress survival autoantigen and transcription co-activator, attenuated docetaxel-induced lysosomal destabilization and cell death. Interestingly, LEDGF/p75 overexpression did not protect cells against DTX-induced mitotic catastrophe, and against apoptosis induced by tumor necrosis factor related apoptosis inducing ligand (TRAIL), suggesting selectivity in its pro-survival activity.

Conclusion

These results underscore the ability of docetaxel to induce concomitantly caspase-dependent and independent death pathways in prostate cancer cells. The results also point to LEDGF/p75 as a potential contributor to cellular resistance to docetaxel-induced lysosomal destabilization and cell death, and an attractive candidate for molecular targeting in HRPC.

Engaging the lysosomal compartment to combat B cell malignancies

The combination of rituximab, a type I anti-CD20 mAb, with conventional chemotherapy has significantly improved the outcome of patients with B cell malignancies. Regardless of this success, many patients still relapse with therapy-resistant disease, highlighting the need for the development of mAbs with higher capacity to induce programmed cell death. The so-called type II anti-CD20 mAbs (e.g., tositumomab) that trigger caspase-independent B cell lymphoma cell death in vitro and show superior efficacy as compared with rituximab in eradicating target cells in mouse models are emerging as the next generation of therapeutic anti-CD20 mAbs. In this issue of the JCI, Ivanov and colleagues identify the lysosomal compartment as a target for type II mAbs (see the related article beginning on page 2143). These data encourage the further clinical development of type II mAbs as well as other lysosome-targeting drugs in the treatment of B cell malignancies.