The Role of Cysteine Protease Cathepsins B, H, C, and X/Z in Neurodegenerative Diseases and Cancer.

Papain-like cysteine proteases are composed of 11 human cysteine cathepsins, originally located in the lysosomes. They exhibit broad specificity and act as endopeptidases and/or exopeptidases. Among them, only cathepsins B, H, C, and X/Z exhibit exopeptidase activity. Recently, cysteine cathepsins have been found to be present outside the lysosomes and often participate in various pathological processes. Hence, they have been considered key signalling molecules. Their potentially hazardous proteolytic activities are tightly regulated. This review aims to discuss recent advances in understanding the structural aspects of these four cathepsins, mechanisms of their zymogen activation, regulation of their activities, and functional aspects of these enzymes in neurodegeneration and cancer. Neurodegenerative effects have been evaluated, particularly in Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, and neuropsychiatric disorders. Cysteine cathepsins also participate in tumour progression and metastasis through the overexpression and secretion of proteases, which trigger extracellular matrix degradation. To our knowledge, this is the first review to provide an in-depth analysis regarding the roles of cysteine cathepsins B, H, C, and X in neurodegenerative diseases and cancer. Further advances in understanding the functions of cysteine cathepsins in these conditions will result in the development of novel, targeted therapeutic strategies.

Menadione-induced apoptosis in U937 cells involves Bid cleavage and stefin B degradation.

Earlier studies showed that the oxidant menadione (MD) induces apoptosis in certain cells and also has anticancer effects. Most of these studies emphasized the role of the mitochondria in this process. However, the engagement of other organelles is less known. Particularly, the role of lysosomes and their proteolytic system, which participates in apoptotic cell death, is still unclear. The aim of this study was to investigate the role of lysosomal cathepsins on molecular signaling in MD-induced apoptosis in U937 cells. MD treatment induced translocation of cysteine cathepsins B, C, and S, and aspartic cathepsin D. Once in the cytosol, some cathepsins cleaved the proapoptotic molecule, Bid, in a process that was completely prevented by E64d, a general inhibitor of cysteine cathepsins, and partially prevented by the pancaspase inhibitor, z-VAD-fmk. Upon loss of the mitochondrial membrane potential, apoptosome activation led to caspase-9 processing, activation of caspase-3-like caspases, and poly (ADP-ribose) polymerase cleavage. Notably, the endogenous protein inhibitor, stefin B, was degraded by cathepsin D and caspases. This process was prevented by z-VAD-fmk, and partially by pepstatin A-penetratin. These findings suggest that the cleaved Bid protein acts as an amplifier of apoptotic signaling through mitochondria, thus enhancing the activity of cysteine cathepsins following stefin B degradation.

Tumor Necrosis Factor-α Induced Apoptosis in U937 Cells Promotes Cathepsin D-Independent Stefin B Degradation.

Lysosomal cathepsins were previously found to be involved in tumor necrosis factor-α (TNFα)-induced apoptosis. However, there are opposing views regarding their role as either initiators or amplifiers of the signaling cascade as well as the order of molecular events during this process. In this study, we investigated the role of cathepsin D (catD) in TNFα/cycloheximide-induced apoptosis in U937 human monocytic cells. TNFα-induced apoptosis proceeds through caspase-8 activation, processing of the pro-apoptotic molecule Bid, mitochondrial membrane permeabilization, and caspase-3 activation. The translocation of lysosomal catD into the cytosol was a late event, suggesting that lysosomal membrane permeabilization and the release of cathepsins are not required for the induction of apoptosis, but rather amplifies the process through the generation of reactive oxygen species. For the first time, we show that apoptosis is accompanied by degradation of the cysteine cathepsin inhibitor stefin B (StfB). CatD did not exhibit a crucial role in this step. However, this degradation was partially prevented through pre-incubation with the antioxidant N-acetyl cysteine, although it did not prevent apoptosis and its progression. These results suggest that the degradation of StfB, as a response to TNFα, could induce a cell death amplification effect as a result of progressive damage to lysosomes during TNFα treatment. J. Cell. Biochem. 118: 4813-4820, 2017. © 2017 Wiley Periodicals, Inc.

Proline Residues as Switches in Conformational Changes Leading to Amyloid Fibril Formation.

Here we discuss studies of the structure, folding, oligomerization and amyloid fibril formation of several proline mutants of human stefin B, which is a protein inhibitor of lysosomal cysteine cathepsins and a member of the cystatin family. The structurally important prolines in stefin B are responsible for the slow folding phases and facilitate domain swapping (Pro 74) and loop swapping (Pro 79). Moreover, our findings are compared to ß2-microglobulin, a protein involved in dialysis-related amyloidosis. The assessment of the contribution of proline residues to the process of amyloid fibril formation may shed new light on the critical molecular events involved in conformational disorders.

Fast profiling of protease specificity reveals similar substrate specificities for cathepsins K, L and S.

Proteases are important effectors of numerous physiological and pathological processes. Reliable determination of a protease's specificity is crucial to understand protease function and to develop activity-based probes and inhibitors. During the last decade, various proteomic approaches for profiling protease substrate specificities were reported. Although most of these approaches can identify up to thousands of substrate cleavage events in a single experiment, they are often time consuming and methodologically challenging as some of these approaches require rather complex sample preparation procedures. For such reasons their application is often limited to those labs that initially introduced them. Here, we report on a fast and simple approach for proteomic profiling of protease specificities (fast profiling of protease specificity (FPPS)), which can be applied to complex protein mixtures. FPPS is based on trideutero-acetylation of novel N-termini generated by the action of proteases and subsequent peptide fractionation on Stage Tips containing ion-exchange and reverse phase chromatographic resins. FPPS can be performed in 2 days and does not require extensive fractionation steps. Using this approach, we have determined the specificity profiles of the cysteine cathepsins K, L and S. We further validated our method by comparing the results with the specificity profiles obtained by the N-terminal combined fractional diagonal chromatography method. This comparison pointed to almost identical substrate specificities for all three cathepsins and confirmed the reliability of the FPPS approach. All MS data have been deposited in the ProteomeXchange with identifiers PXD001536 and PXD001553 (http://proteomecentral.proteomexchange.org/dataset/PXD001536; http://proteomecentral.proteomexchange.org/dataset/PXD001553).

A role for stefin B (cystatin B) in inflammation and endotoxemia.

Stefin B (cystatin B) is an endogenous cysteine cathepsin inhibitor, and the loss-of-function mutations in the stefin B gene were reported in patients with Unverricht-Lundborg disease (EPM1). In this study we demonstrated that stefin B-deficient (StB KO) mice were significantly more sensitive to the lethal LPS-induced sepsis and secreted higher amounts of pro-inflammatory cytokines IL-1ß and IL-18 in the serum. We further showed that increased caspase-11 gene expression and better pro-inflammatory caspase-1 and -11 activation determined in StB KO bone marrow-derived macrophages resulted in enhanced IL-1ß processing. Pretreatment of macrophages with the cathepsin inhibitor E-64d did not affect secretion of IL-1ß, suggesting that the increased cathepsin activity determined in StB KO bone marrow-derived macrophages is not essential for inflammasome activation. Upon LPS stimulation, stefin B was targeted into the mitochondria, and the lack of stefin B resulted in the increased destabilization of mitochondrial membrane potential and mitochondrial superoxide generation. Collectively, our study demonstrates that the LPS-induced sepsis in StB KO mice is dependent on caspase-11 and mitochondrial reactive oxygen species but is not associated with the lysosomal destabilization and increased cathepsin activity in the cytosol.

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.

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.

Cysteine cathepsins: from structure, function and regulation to new frontiers.

It is more than 50 years since the lysosome was discovered. Since then its hydrolytic machinery, including proteases and other hydrolases, has been fairly well identified and characterized. Among these are the cysteine cathepsins, members of the family of papain-like cysteine proteases. They have unique reactive-site properties and an uneven tissue-specific expression pattern. In living organisms their activity is a delicate balance of expression, targeting, zymogen activation, inhibition by protein inhibitors and degradation. The specificity of their substrate binding sites, small-molecule inhibitor repertoire and crystal structures are providing new tools for research and development. Their unique reactive-site properties have made it possible to confine the targets simply by the use of appropriate reactive groups. The epoxysuccinyls still dominate the field, but now nitriles seem to be the most appropriate "warhead". The view of cysteine cathepsins as lysosomal proteases is changing as there is now clear evidence of their localization in other cellular compartments. Besides being involved in protein turnover, they build an important part of the endosomal antigen presentation. Together with the growing number of non-endosomal roles of cysteine cathepsins is growing also the knowledge of their involvement in diseases such as cancer and rheumatoid arthritis, among others. Finally, cysteine cathepsins are important regulators and signaling molecules of an unimaginable number of biological processes. The current challenge is to identify their endogenous substrates, in order to gain an insight into the mechanisms of substrate degradation and processing. In this review, some of the remarkable advances that have taken place in the past decade are presented. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Prediction of Transmembrane Regions, Cholesterol, and Ganglioside Binding Sites in Amyloid-Forming Proteins Indicate Potential for Amyloid Pore Formation.

Besides amyloid fibrils, amyloid pores (APs) represent another mechanism of amyloid induced toxicity. Since hypothesis put forward by Arispe and collegues in 1993 that amyloid-beta makes ion-conducting channels and that Alzheimer's disease may be due to the toxic effect of these channels, many studies have confirmed that APs are formed by prefibrillar oligomers of amyloidogenic proteins and are a common source of cytotoxicity. The mechanism of pore formation is still not well-understood and the structure and imaging of APs in living cells remains an open issue. To get closer to understand AP formation we used predictive methods to assess the propensity of a set of 30 amyloid-forming proteins (AFPs) to form transmembrane channels. A range of amino-acid sequence tools were applied to predict AP domains of AFPs, and provided context on future experiments that are needed in order to contribute toward a deeper understanding of amyloid toxicity. In a set of 30 AFPs we predicted their amyloidogenic propensity, presence of transmembrane (TM) regions, and cholesterol (CBM) and ganglioside binding motifs (GBM), to which the oligomers likely bind. Noteworthy, all pathological AFPs share the presence of TM, CBM, and GBM regions, whereas the functional amyloids seem to show just one of these regions. For comparative purposes, we also analyzed a few examples of amyloid proteins that behave as biologically non-relevant AFPs. Based on the known experimental data on the ß-amyloid and α-synuclein pore formation, we suggest that many AFPs have the potential for pore formation. Oligomerization and α-TM helix to ß-TM strands transition on lipid rafts seem to be the common key events.

Human cathepsin X/Z is a biologically active homodimer.

Human cathepsin X belongs to the cathepsin family of 11 lysosomal cysteine proteases. We expressed recombinant procathepsin X in Pichia pastoris in vitro and cleaved it into its active mature form using aspartic cathepsin E. We found, using size exclusion chromatography, X-ray crystallography, and small-angle X-ray scattering, that cathepsin X is a biologically active homodimer with a molecular weight of ~53 kDa. The novel finding that cathepsin X is a dimeric protein opens new horizons in the understanding of its function and the underlying pathophysiological mechanisms of various diseases including neurodegenerative disorders in humans.

Cysteine cathepsins are not critical for TNF-alpha-induced cell death in T98G and U937 cells.

The tumor necrosis factor (TNF) is a cytokine known to be an important mediator of apoptosis and inflammation. It has been implicated in the pathogenesis of a number of diseases, including cancer and rheumatoid arthritis. TNF apoptosis has been known for a number of years to be critically dependent on caspases; however, recently it has been suggested that cysteine cathepsins might also be involved in the pathway. In the present work the hypothesis that cathepsins can act as an essential downstream mediator of TNF-alpha-triggered apoptosis was tested. The TNF-alpha apoptosis was investigated in two tumor-cell lines: U937 and T98G. Based on the use of pharmacological caspase inhibitors, the TNF-alpha induced caspase-dependent apoptotic cell death in both cell lines, which was accompanied by lysosomal destabilization and the release of cathepsins in the cytosol. However, blocking cysteine cathepsins with a broad-spectrum inhibitor, E64d, or a more specific cathepsin B inhibitor, CA-074Me, had no effect on the progression of the apoptosis in both cell lines, suggesting that the TNF-alpha apoptosis is not critically dependent on the cathepsins in these two cellular models.

A structural network associated with the kallikrein-kinin and renin-angiotensin systems.

The kallikrein-kinin and renin-angiotensin (KKS-RAS) systems represent two highly regulated proteolytic systems that are involved in several physiological and pathological processes. Although their protein-protein interactions can be studied using experimental approaches, it is difficult to differentiate between direct physical interactions and functional associations, which do not involve direct atomic contacts between macromolecules. This information can be obtained from an atomic-resolution characterization of the protein interfaces. As a result of this, various three-dimensional-based protein-protein interaction databases have become available. To gain insight into the multilayered interaction of the KKS-RAS systems, we present a protein network that is built up on three-dimensional domain-domain interactions. The essential domains that link these systems are as follows: Cystatin, Peptidase_C1, Thyroglobulin_1, Insulin, CIMR (Cation-independent mannose-6-phosphate receptor repeat), fn2 (Fibronectin type II domain), fn1 (Fibronectin type I domain), EGF, Trypsin, and Serpin. We found that the CIMR domain is located at the core of the network, thus connecting both systems. From the latter, all domain interactors up to level 4 were retrieved, thus displaying a more comprehensive representation of the KKS-RAS structural network.

Characteristics, Structure, and Biological Role of Stefins (Type-1 Cystatins) of Human.

The majority of lysosomal cysteine cathepsins are ubiquitously expressed enzymes. However, some of them differ in their specific cell or tissue distribution and substrate specificity, suggesting their involvement in determining normal cellular processes, as well as pathologies. Their proteolytic activities are potentially harmful if uncontrolled. Therefore, living organisms have developed several regulatory mechanisms such as endogenous protein inhibitors of the cystatin family, including the group of small cytosolic proteins, the stefins. The main focus of this review is stefins of various origins and their properties, structure, and mechanism of interaction with their target enzymes. Furthermore, oligomerization and fibrillogenesis in stefins and/or cystatins provide insights into conformational diseases. The present status of the knowledge in this field and current trends might contribute to identifying novel therapeutic targets and approaches to treat various diseases.

Prolines Affect the Nucleation Phase of Amyloid Fibrillation Reaction; Mutational Analysis of Human Stefin B.

Proline residues play a prominent role in protein folding and aggregation. We investigated the influence of single prolines and their combination on oligomerization and the amyloid fibrillation reaction of human stefin B (stB). The proline mutants influenced the distribution of oligomers between monomers, dimers, and tetramers as shown by the size-exclusion chromatography. Only P74S showed higher oligomers, reminiscent of the molten globule reported previously for the P74S of stB-Y31 variant. The proline mutants also inhibited to various degree the amyloid fibrillation reaction. At 30 and 37 °C, inhibition was complete for the P74S single mutant, two double mutants (P6L P74S and P74S P79S), and for the triple mutant P6L P11S P74S. At 30 °C the single mutant P6L completely inhibited the reaction, while P11S and P79S formed amyloid fibrils with a prolonged lag phase. P36D did not show a lag phase, reminiscent of a downhill polymerization model. At 37 °C in addition to P36D, P11S, and P79S, P6L and P11S P74S also started to fibrillate; however, the yield of the fibrils was much lower than that of the wild-type protein as judged by transmission electron microscopy. Thus, Pro 74 cis/trans isomerization proves to be the key event, acting as a switch toward an amyloid transition. Using our previous model of nucleation and growth, we simulated the kinetics of all the mutants that exhibited sigmoidal fibrillation curves. To our surprise, the nucleation phase was most affected by Pro cis/trans isomerism, rather than the fibril elongation phase.

Increased expression and altered subcellular distribution of cathepsin B in microglia induce cognitive impairment through oxidative stress and inflammatory response in mice.

During normal aging, innate immunity progresses to a chronic state. However, how oxidative stress and chronic neuroinflammation arise during aging remains unclear. In this study, we found that genetic ablation of cathepsin B (CatB) in mice significantly reduced the generation of reactive oxygen species (ROS) and neuroinflammation and improved cognitive impairment during aging. In cultured microglia, pharmacological inhibition of CatB significantly reduced the generation of mitochondria-derived ROS and proinflammatory mediators induced by L-leucyl-L-leucine methyl ester (LLOMe), a lysosome-destabilizing agent. In the CatB-overexpressing microglia after treatment with LLOMe, which mimicked the aged microglia, CatB leaked in the cytosol is responsible for the degradation of the mitochondrial transcription factor A (TFAM), resulting in the increased generation of mitochondria-derived ROS and proinflammatory mediators through impaired mtDNA biosynthesis. Furthermore, intralateral ventricle injection of LLOMe-treated CatB-overexpressing microglia induced cognitive impairment in middle-aged mice. These results suggest that the increase and leakage of CatB in microglia during aging are responsible for the increased generation of mitochondria-derived ROS and proinflammatory mediators, culminating in memory impairment.

Cystatins: biochemical and structural properties, and medical relevance.

The cystatin superfamily comprises a large group of the cystatin domain containing proteins, present in a wide variety of organisms, including humans. Cystatin inhibitory activity is vital for the delicate regulation of normal physiological processes by limiting the potentially highly destructive activity of their target proteases such as the papain (C1) family, including cysteine cathepsins. Some of the cystatins also inhibit the legumain (C13) family of enzymes. Failures in biological mechanisms controlling protease activities result in many diseases such as neurodegeneration, cardiovascular diseases, osteoporosis, arthritis, and cancer. Cystatins have been classified into three types: the stefins, the cystatins and the kininogens, although other cystatin-related proteins, such as CRES proteins, are emerging. The stefins are mainly intracellular proteins, whereas the cystatins and the kininogens are extracellular. The cystatins are tight binding and reversible inhibitors. The basic mechanism of interaction between cystatins and their target proteases has been established, based mainly on the crystal structures of various cathepsins, stefins and cystatins and their enzyme-inhibitor complexes. Cystatins, as rather non-selective inhibitors, discriminate only slightly between endo- and exopeptidases. They are also prone to form amyloids. The levels of some stefins and cystatins in tissue and body fluids can serve as relatively reliable markers for a variety of diseases. In this review we summarize present knowledge about cystatins and their role in some diseases.

Purification and characterization of two cysteine peptidases of the Mediterranean fruit fly Ceratitis capitata during metamorphosis.

In holometabolous insects, there is a complete body remodeling from larva to adult. We determined in Ceratitis capitata that the transition from pre-pupa to pupa, 40 to 48 h after puparium formation (h APF), is a key moment of metamorphosis; when salivary glands, intestine, fat body, and muscles are in different stages of cell death. At 44-46 h APF, muscles from segments 1-3 (thoracic region) appeared fully disintegrated, whereas posterior muscles just started death processes. To understand some of the biochemical events eventually involved in histolytic processes during early metamorphosis, two cysteine peptidases coined "Metamorphosis Associated Cysteine Peptidase" (MACP-I and MACP-II) were purified to homogeneity from 40-46-h APF insects. Both enzymes were inhibited by Ep-475, a specific inhibitor of papain-like cysteine-peptidases. MACP-I is a single chain protein with an apparent molecular mass of 80 kDa and includes several isoforms with pI values of pH 6.25-6.35, 6.7, and 7.2. The enzyme has an optimum pH of 5.0 and its pH stability ranges from pH 4.0 to 6.0. The molecular weight and N-terminal sequence suggest that MACP-I might be a novel enzyme. MACP-II is an acidic single chain protein with a pI of pH 5.85 and an apparent molecular mass of 30 kDa. The enzyme is labile with a maximum stability in the pH range of 4.0 to 6.0 and an optimum pH among 5.0 to 6.0. MAPCP-II characteristics suggest it is a cathepsin B-like enzyme.

Glucosamine prevents polarization of cytotoxic granules in NK-92 cells by disturbing FOXO1/ERK/paxillin phosphorylation.

Glucosamine (GlcN) is a naturally occurring derivative of glucose and an over-the-counter food additive. However, the mechanism underlying GlcN action on cells is unknown. In this study, we investigated the effect of GlcN on natural killer (NK) cells. We demonstrate that GlcN affects NK-92 cell cytotoxicity by altering the distribution of cathepsin C, a cysteine protease required for granzyme processing in cytotoxic granules. The relocation of cathepsin C due to GlcN was shown to be accompanied by a decrease in the intracellular enzyme activity and its extracellular secretion. Similarly, the relocation of endosomal aspartic cathepsin E was observed. Furthermore, we elucidated that repositioning of cathepsin C is a consequence of altered signaling pathways of cytotoxic granule movement. The inhibition of phosphorylation upstream and downstream of ERK by GlcN disturbed the polarized release of cytotoxic vesicles. Considerable changes in the ERK phosphorylation dynamics, but not in those of p38 kinase or JNK, were observed in the IL2-activated NK-92 cells. We found decreased phosphorylation of the transcription factor FOXO1 and simultaneous prolonged phosphorylation of ERK as well as its nuclear translocation. Additionally, a protein downstream of the ERK phosphorylation cascade, paxillin, was less phosphorylated, resulting in a diffuse distribution of cytotoxic granules. Taken together, our results suggest that dietary GlcN affects signaling pathway activation of NK-92 immune cells.

Protease signalling in cell death: caspases versus cysteine cathepsins.

Proteases were, for a long time, mainly considered as protein degrading enzymes. However, in the last decade this view has changed dramatically, and the focus is now on proteases as signalling molecules. One of the best examples is apoptosis, the major mechanism used by eukaryotes to remove superfluous, damaged and potentially dangerous cells, in which a number of proteases have been found to play a central role. Of these the caspases have been considered to be the major players. However, more recently, other proteases have been increasingly suggested as being important in apoptosis, in particular the cysteine cathepsins. In this review the roles of caspases and cysteine cathepsins in apoptosis signalling are compared and discussed.