Cysteine proteinases and metastasis

Lysosome dynamics during human endometrial stromal cells decidualization: effect of para-nonylphenol

During the process of decidualization, the stromal cells of the endometrium change dynamically to create a favorable environment for embryo implantation. Lysosome activity has often been associated with physiological changes in the endometrium during the preimplantation period and early pregnancy. In this study, the effect of para-nonylphenol (p-NP), an endocrine disruptor, on human immortalized endometrial stromal cells (tHESCs) was investigated. After exposure to p-NP (1 nM and 1 pM), the cells were examined for the decidualization markers connexin-43, insulin like growth factor binding protein 1 (IGFBP1), and prolactin. In addition, the effect of p-NP on lysosome biogenesis and exocytosis was investigated by examining the expression and localization of the transcription factor EB (TFEB) and that of the lysosomal-associated membrane protein 1 (LAMP-1). Finally, we evaluated the effect of p-NP on extracellular matrix (ECM) remodeling using a fibronectin assay. Our results showed that p-NP reduced the expression of prolactin protein, increased the nuclear localization of TFEB, and induced the increase and translocation of the lysosomal protein LAMP-1 to the membrane of tHESCs. The data indicate an impairment of decidualization and suggest an increase in lysosomal biogenesis and exocytosis, which is supported by the higher release of active cathepsin D by tHESCs. Given the importance of cathepsins in the processing and degradation of the ECM during trophoblast invasiveness and migration into the decidua, our results appear to be clear evidence of the negative effects of p-NP on endometrial processes that are fundamental to reproductive success and the establishment of pregnancy.NEW & NOTEWORTHY Endocrine disruptors, such as para-nonylphenol, affect the decidualization of human endometrial stromal cells with an impact on decidualization itself, lysosome biogenesis and exocytosis, and extracellular matrix remodeling. All these alterations may negatively impact embryo implantation with the success of reproduction and the establishment of pregnancy.

Circ_0020014 mediates CTSB expression and participates in IL-1β-prompted chondrocyte injury via interacting with miR-24-3p

BACKGROUND

Recent studies have shown that circRNAs are involved in the pathogenesis of osteoarthritis (OA) by affecting various fundamental cellular characteristics of chondrocytes. The purpose of this paper is to investigate the role and regulatory mechanism of hsa_circ_0020014 (circ_0020014) in chondrocytes of OA.

METHODS

The inflammatory cytokine interleukin 1 beta (IL-1β) was used to stimulate human chondrocytes. Cell viability, proliferation, and apoptosis were evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT), 5-Ethynyl-2'-deoxyuridine (EdU), and flow cytometry assays. Several protein levels were determined by western blotting (WB). Levels of inflammatory cytokines and malondialdehyde (MDA) were determined by enzyme-linked immunosorbent assay (ELISA). Relative expression of circ_0020014 was estimated by real-time polymerase quantitative chain reaction (RT-qPCR). Bioinformatics prediction combined with dual-luciferase reporter, RIP and RNA pull-down assays were done to probe into the regulatory mechanism of circ_0020014.

RESULTS

Circ_0020014 was overexpressed in OA patient-derived articular cartilages and IL-1β-stimulated chondrocytes. Silencing of circ_0020014 lighted IL-1β-prompted chondrocyte proliferation repression, apoptosis, inflammation, and oxidative stress. Mechanically, circ_0020014 functioned as a miR-24-3p molecular sponge to regulate cathepsin B (CTSB) expression. Furthermore, miR-24-3p inhibition alleviated circ_0020014 knockdown-mediation repression of IL-1β-urged chondrocyte injury. In addition, CTSB overexpression whittled miR-24-3p upregulation-mediated suppression of IL-1β-urged chondrocyte injury.

CONCLUSION

Our findings demonstrated that the circ_0020014/miR-24-3p/CTSB axis was associated with IL-1β-prompted chondrocyte injury, supporting the involvement of circ_0020014 in the OA pathogenesis.

Disintegrin-like Protein Strategy to Inhibit Aggressive Triple-Negative Breast Cancer

Venoms are a rich source of bioactive compounds, and among them is leberagin-C (Leb-C), a disintegrin-like protein derived from the venom of Macrovipera lebetina transmediterrannea snakes. Leb-C has shown promising inhibitory effects on platelet aggregation. Previous studies have demonstrated that this SECD protein specifically targets α5β1, αvβ3, and αvβ6 integrins through a mimic mechanism of RGD disintegrins. In our current study, we focused on exploring the potential effects of Leb-C on metastatic breast cancer. Our findings revealed that Leb-C disrupted the adhesion, migration, and invasion capabilities of MDA-MB-231 breast cancer cells and its highly metastatic D3H2LN sub-population. Additionally, we observed significant suppression of adhesion, migration, and invasion of human umbilical vein endothelial cells (HUVECs). Furthermore, Leb-C demonstrated a strong inhibitory effect on fibroblast-growth-factor-2-induced proliferation of HUVEC. We conducted in vivo experiments using nude mice and found that treatment with 2 µM of Leb-C resulted in a remarkable 73% reduction in D3H2LN xenograft tumor size. Additionally, quantification of intratumor microvessels revealed a 50% reduction in tumor angiogenesis in xenograft after 21 days of twice-weekly treatment with 2 µM of Leb-C. Collectively, these findings suggest the potential utility of this disintegrin-like protein for inhibiting aggressive and resistant metastatic breast cancer.

FABP7 inhibits proliferation and invasion abilities of cutaneous squamous cell carcinoma cells via the Notch signaling pathway

Cutaneous squamous cell carcinoma (CSCC) is one of the most common non-melanoma skin cancers worldwide. Fatty acid-binding protein 7 (FABP7) has been reported to be involved in the occurrence, development, metastasis and prognosis of various tumors. In addition, downregulated FABP7 expression was demonstrated in cutaneous malignant melanoma in a previous study. Therefore, we speculated that FABP7 may be a biomarker for CSCC diagnosis. The aim of the present study was to determine the molecular mechanism underlying the effects of FABP7 in CSCC, which may provide a new diagnostic biomarker or treatment target for CSCC. Reverse transcription-PCR, western blotting and immunohistochemistry assays were performed to detect the expression levels of FABP7 in CSCC tissues and cells. Overexpression of FABP7 was achieved in A431 and colo-16 cell lines by transfection with an overexpression vector (oeFABP7). Cell proliferation, colony formation, migration and invasion were detected by Cell Counting Kit-8, crystal violet, scratch and Transwell assays, respectively. Following FABP7 overexpression, western blotting was used to determine the expression levels of proliferation-, invasion- and Notch pathway-associated proteins, including Snail, N-cadherin, Twist, matrix metalloproteinase (MMP)-2, MMP-7, Notch 1 and Notch 3. In addition a CSCC model in nude mice was constructed. Immunohistochemistry was used to determine the expression levels of FABP7, Ki67, Notch 1 and Notch 3. It was demonstrated that FABP7 expression levels were significantly reduced in human CSCC tissues and cells compared with normal samples. Overexpression of FABP7 inhibited the proliferation, invasion and migration abilities of A431 and colo-16 cells compared with those in the negative control group. In addition, transfection with oeFABP7 reduced the expression levels of proliferation-, invasion- and Notch pathway-associated proteins compared with those in the negative control group. Overexpression of FABP7 also reduced the growth of CSCC tumors in vivo and inhibited the expression of Ki67, Notch 1 and Notch 3. Therefore, the results of the present study suggested that FABP7 may inhibit the proliferation and invasion of CSCC cells via the Notch signaling pathway.

Uncovering Mediators of Collagen Degradation in the Tumor Microenvironment

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Collagen cleavage in tumors is primarily mediated by FAP⁺ cancer-associated fibroblasts.

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Collagen fibers are cleaved in an MMP-dependent manner.

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Released collagen fragments are internalized by M2-like tumor-associated macrophages and cancer-associated fibroblasts.

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The mannose receptor is central in collagen internalization by tumor-associated macrophages.

Increased remodeling of the extracellular matrix in malignant tumors has been shown to correlate with tumor aggressiveness and a poor prognosis. This remodeling involves degradation of the original extracellular matrix (ECM) and deposition of a new tumor-supporting ECM. The main constituent of the ECM is collagen and collagen turnover mainly occurs in a sequential manner, where initial proteolytic cleavage of the insoluble fibers is followed by cellular internalization of large well-defined collagen fragments for lysosomal degradation. However, despite extensive research in the field, a lack of consensus on which cell types within the tumor microenvironment express the involved proteases still exists. Furthermore, the relative contribution of different cell types to collagen internalization is not well-established. Here, we developed quantitative ex vivo collagen degradation assays and show that the proteases responsible for the initial collagen cleavage in two murine syngeneic tumor models are matrix metalloproteinases produced by cancer-associated fibroblasts and that collagen degradation fragments are endocytosed primarily by tumor-associated macrophages and cancer-associated fibroblasts from the tumor stroma. Using tumors from mannose receptor-deficient mice, we show that this receptor is essential for collagen-internalization by tumor-associated macrophages. Together, these findings identify the cell types responsible for the entire collagen degradation pathway, from initial cleavage to endocytosis of fragments for intracellular degradation.

The E3 ubiquitin ligase NEDD4 mediates cell migration signaling of EGFR in lung cancer cells

BACKGROUND

EGFR-dependent cell migration plays an important role in lung cancer progression. Our previous study observed that the HECT E3 ubiquitin ligase NEDD4 is significantly correlated with tumor metastasis and required for migration and invasion signaling of EGFR in gastric cancer cells. However, how NEDD4 promotes the EGFR-dependent lung cancer cell migration is unknown. This study is to elucidate the mechanism by which NEDD4 mediates the EGFR lung cancer migration signaling.

METHODS

Lentiviral vector-loaded NEDD4 shRNA was used to deplete endogenous NEDD4 in lung cancer cell lines. Effects of the NEDD4 knockdown on the EGFR-dependent or independent lung cancer cell migration were determined using the wound-healing and transwell assays. Association of NEDD4 with activated EGFR was assayed by co-immunoprecipitation. Co-expression of NEDD4 with EGFR or PTEN was determined by immunohistochemical (IHC) staining in 63 lung adenocarcinoma tissue samples. Effects of NEDD4 ectopic expression or knockdown on PTEN ubiquitination and down-regulation, AKT activation and lysosomal secretion were examined using the GST-Uba pulldown assay, immunoblotting, immunofluorescent staining and a human cathepsin B ELISA assay respectively. The specific cathepsin B inhibitor CA-074Me was used for assessing the role of cathepsin B in lung cancer cell migration.

RESULTS

Knockdown of NEDD4 significantly reduced EGF-stimulated cell migration in non-small cell lung carcinoma (NSCLC) cells. Co-immunoprecipitation assay found that NEDD4 is associated with EGFR complex upon EGF stimulation, and IHC staining indicates that NEDD4 is co-expressed with EGFR in lung adenocarcinoma tumor tissues, suggesting that NEDD4 might mediate lung cancer cell migration by interaction with the EGFR signaling complex. Interestingly, NEDD4 promotes the EGF-induced cathepsin B secretion, possibly through lysosomal exocytosis, as overexpression of the ligase-dead mutant of NEDD4 impedes lysosomal secretion, and knockdown of NEDD4 significantly reduced extracellular amount of cathepsin B induced by EGF. Consistent with the role of NEDD4, cathepsin B is pivotal for both basal and the EGF-stimulated lung cancer cell migration. Our studies propose a novel mechanism underlying the EGFR-promoted lung cancer cell migration that is mediated by NEDD4 through regulation of cathepsin B secretion.

CONCLUSION

NEDD4 mediates the EGFR lung cancer cell migration signaling through promoting lysosomal secretion of cathepsin B.

Tumor-Associated Macrophages Derived from Circulating Inflammatory Monocytes Degrade Collagen through Cellular Uptake

Physiologic turnover of interstitial collagen is mediated by a sequential pathway in which collagen is fragmented by pericellular collagenases, endocytosed by collagen receptors, and routed to lysosomes for degradation by cathepsins. Here, we use intravital microscopy to investigate if malignant tumors, which are characterized by high rates of extracellular matrix turnover, utilize a similar collagen degradation pathway. Tumors of epithelial, mesenchymal, or neural crest origin all display vigorous endocytic collagen degradation. The cells engaged in this process are identified as tumor-associated macrophage (TAM)-like cells that degrade collagen in a mannose receptor-dependent manner. Accordingly, mannose-receptor-deficient mice display increased intratumoral collagen. Whole-transcriptome profiling uncovers a distinct extracellular matrix-catabolic signature of these collagen-degrading TAMs. Lineage-ablation studies reveal that collagen-degrading TAMs originate from circulating CCR2+ monocytes. This study identifies a function of TAMs in altering the tumor microenvironment through endocytic collagen turnover and establishes macrophages as centrally engaged in tumor-associated collagen degradation.

Galactosidase Enzyme Fragment Complementation as a High-Throughput Screening Protease Technology

The authors describe a homogeneous, high-throughput screening (HTS) assay for measuring protease activity and detection of inhibitors. The assay comprises a cyclic β-galactosidase (β-gal) enzyme donor peptide (ED) containing a protease-selective cleavage sequence. Alone, the cyclic peptide is inactive, but when linearized following protease cleavage, ED complements with β-gal enzyme acceptor forming active β-gal enzyme. This then catalyzes the formation of either fluorescent or chemiluminescent products, with β-gal turnover providing a highly amplified signal, and thus an assay technology of high sensitivity. To demonstrate the utility of the technology, an EFC assay was developed to measure the activity of 2, caspase 3 and β-secretase. Using a cyclic ED containing the caspase 3 substrate sequence, DEVD, the EFC assay signal was linear with respect to caspase 3 concentration. The assay was very sensitive, being able to detect activity at low picogram amounts of caspase 3. For the β-secretase (BACE) EFC assay, a cyclic ED containing the Swedish mutant cleavage site of amyloid precursor protein (APP), SEVNLDAEFK, was used. In a similar fashion to the caspase 3 assay, the signal induced by BACE activity was linear with respect to enzyme concentration and was highly sensitive, being able to detect nanogram quantities of BACE. The assay was also more sensitive than a commercially available FRET-based assay of BACE activity. It is concluded that the EFC protease assay is a simple, flexible, and sensitive technology for HTS of proteases.

The source of matrix-degrading enzymes in human cancer: Problems of research reproducibility and possible solutions

Matrix degradation is central to tumor pathogenesis. Enzymes that degrade extracellular matrix are abundant in tumors. But which out of the complex mixture of cells that form a tumor produces them? Surprisingly, several hundred studies devoted to this question have provided confusion rather than clarity. Our analysis of these studies identifies likely reasons as to why this may be the case, which has implications for the broader issue of research reproducibility.

Mechanistic insights into mode of action of novel natural cathepsin L inhibitors

Background

Development of a cancerous cell takes place when it ceases to respond to growth-inhibiting signals and multiplies uncontrollably and can detach and move to other parts of the body; the process called as metastasis. A particular set of cysteine proteases are very active during cancer metastasis, Cathepsins being one of them. They are involved in tumor growth and malignancy and have also been reported to be overexpressed in tumor cell lines. In the present study, a combinatorial approach comprising three-dimensional quantitative structure-activity relationship (3D QSAR), ligand-based pharmacophore modelling and search followed by cathepsin L structure-based high throughput screening was carried out using an initial set of 28 congeneric thiosemicarbazone derivatives as cathepsin L inhibitors. A 3D QSAR was derived using the alignment of a common thiosemicarbazone substructure. Essential structural features responsible for biological activity were taken into account for development of a pharmacophore model based on 29 congeneric thiosemicarbazone derivatives. This model was used to carry out an exhaustive search on a large dataset of natural compounds. A further cathepsin L structure-based screen identified two top scoring compounds as potent anti-cancer leads.

Results

The generated 3D QSAR model showed statistically significant results with an r² value of 0.8267, cross-validated correlation coefficient q² of 0.7232, and a pred_r² (r² value for test set) of 0.7460. Apart from these, a high F test value of 30.2078 suggested low probability of the model's failure. The pharmacophoric hypothesis chosen for searching the natural compound libraries was identified as DDHRR, where two Ds denote 2 hydrogen donors, H represents a hydrophobic group and two Rs represent aromatic rings, all of which are essential for the biological activity. We report two potential drug leads ZINC08764437 (NFP) and ZINC03846634 (APQ) obtained after a combined approach of pharmacophore-based search and structure-based virtual screen. These two compounds displayed extra precision docking scores of -7.972908 and -7.575686 respectively suggesting considerable binding affinity for cathepsin L. High activity values of 5.72 and 5.75 predicted using the 3D QSAR model further substantiated the inhibitory potential of these identified leads.

Conclusion

The present study attempts to correlate the structural features of thiosemicarbazone group with their biological activity by development of a robust 3D QSAR model. Being statistically valid, this model provides near accurate values of the activities predicted for the congeneric set on which it is based. These predicted activities are good for the test set compounds making it indeed a statistically sound 3D QSAR model. The identified pharmacophore model DDHRR.8 comprised of all the essential features required to interact with the catalytic triad of cathepsin L. A search for natural compounds based on this pharmacophore followed by docking studies further screened out two top scoring candidates: NFP and AFQ. The high binding affinity and presence of essential structural features in these two compounds make them ideal for consideration as natural anti-tumoral agents. Activity prediction using 3D QSAR model further validated their potential as worthy drug candidates against cathepsin L for treatment of cancer.

Proteomic analysis of seminal fluid from men exhibiting oxidative stress

BACKGROUND

Seminal plasma serves as a natural reservoir of antioxidants. It helps to remove excessive formation of reactive oxygen species (ROS) and consequently, reduce oxidative stress. Proteomic profiling of seminal plasma proteins is important to understand the molecular mechanisms underlying oxidative stress and sperm dysfunction in infertile men.

METHODS

This prospective study consisted of 52 subjects: 32 infertile men and 20 healthy donors. Once semen and oxidative stress parameters were assessed (ROS, antioxidant concentration and DNA damage), the subjects were categorized into ROS positive (ROS+) or ROS negative (ROS-). Seminal plasma from each group was pooled and subjected to proteomics analysis. In-solution digestion and protein identification with liquid chromatography tandem mass spectrometry (LC-MS/MS), followed by bioinformatics analyses was used to identify and characterize potential biomarker proteins.

RESULTS

A total of 14 proteins were identified in this analysis with 7 of these common and unique proteins were identified in both the ROS+ and ROS- groups through MASCOT and SEQUEST analyses, respectively. Prolactin-induced protein was found to be more abundantly present in men with increased levels of ROS. Gene ontology annotations showed extracellular distribution of proteins with a major role in antioxidative activity and regulatory processes.

CONCLUSIONS

We have identified proteins that help protect against oxidative stress and are uniquely present in the seminal plasma of the ROS- men. Men exhibiting high levels of ROS in their seminal ejaculate are likely to exhibit proteins that are either downregulated or oxidatively modified, and these could potentially contribute to male infertility.

Proteomic analysis of human skin treated with larval schistosome peptidases reveals distinct invasion strategies among species of blood flukes

Background

Skin invasion is the initial step in infection of the human host by schistosome blood flukes. Schistosome larvae have the remarkable ability to overcome the physical and biochemical barriers present in skin in the absence of any mechanical trauma. While a serine peptidase with activity against insoluble elastin appears to be essential for this process in one species of schistosomes, Schistosoma mansoni, it is unknown whether other schistosome species use the same peptidase to facilitate entry into their hosts.

Methods

Recent genome sequencing projects, together with a number of biochemical studies, identified alternative peptidases that Schistosoma japonicum or Trichobilharzia regenti could use to facilitate migration through skin. In this study, we used comparative proteomic analysis of human skin treated with purified cercarial elastase, the known invasive peptidase of S. mansoni, or S. mansoni cathespin B2, a close homolog of the putative invasive peptidase of S. japonicum, to identify substrates of either peptidase. Select skin proteins were then confirmed as substrates by in vitro digestion assays.

Conclusions

This study demonstrates that an S. mansoni ortholog of the candidate invasive peptidase of S. japonicum and T. regenti, cathepsin B2, is capable of efficiently cleaving many of the same host skin substrates as the invasive serine peptidase of S. mansoni, cercarial elastase. At the same time, identification of unique substrates and the broader species specificity of cathepsin B2 suggest that the cercarial elastase gene family amplified as an adaptation of schistosomes to human hosts.

Schistosome parasites are a major cause of disease in the developing world, but the mechanism by which these parasites first infect their host has been studied at the molecular level only for S. mansoni. In this paper, we have mined recent genome annotations of S. mansoni and S. japonicum, a zoonotic schistosome species, to identify differential expansion of peptidase gene families that may be involved in parasite invasion and subsequent migration through skin. Having identified a serine peptidase gene family in S. mansoni and a cysteine peptidase gene family in S. japonicum, we then used a comparative proteomic approach to identify potential substrates of representative members of both classes of enzymes from S. mansoni in human skin. The results of this study suggest that while these species evolved to use different classes of peptidases in host invasion, both are capable of cleaving components of the epidermis and dermal extracellular matrix, as well as proteins involved in the host immune response against the migrating parasite.

Cathepsin B trafficking in thyroid carcinoma cells

BACKGROUND

The cysteine peptidase cathepsin B is important in thyroid physiology by being involved in prohormone processing initiated in the follicle lumen and completed in endo-lysosomal compartments. However, cathepsin B has also been localized to the extrafollicular space in thyroid cancer tissue, and is therefore suggested to promote invasiveness and metastasis in thyroid carcinomas through e.g. extracellular matrix degradation.

METHODS

Transport of cathepsin B in normal thyroid epithelial and carcinoma cells was investigated through immunolocalization of endogenous cathepsin B in combination with probing protease activity. Transport analyses of cathepsin B-eGFP and its active-site mutant counterpart cathepsin B-C29A-eGFP were used to test whether intrinsic sequences of a protease influence its trafficking.

RESULTS

Our approach employing activity based probes, which distinguish between active and inactive cysteine proteases, demonstrated that both eGFP-tagged normal and active-site mutated cathepsin B chimeras reached the endo-lysosomal compartments of thyroid epithelial cells, thereby ruling out alterations of sorting signals by mutagenesis of the active-site cysteine. Analysis of chimeric protein trafficking further showed that GFP-tagged cathepsin B was transported to the expected compartments, i.e. endoplasmic reticulum, Golgi apparatus and endo-lysosomes of normal and thyroid carcinoma cell lines. However, the active-site mutated cathepsin B chimera was mostly retained in the endoplasmic reticulum and Golgi of KTC-1 and HTh7 cells. Hence the latter, as the least polarized of the three carcinoma cell lines analyzed, exhibited severe transport defects in that it retained chimeras in pre-endolysosomal compartments. Furthermore, secretion of endogenous cathepsin B and of other cysteine peptidases, which occurs at the apical pole of normal thyroid epithelial cells, was most prominent and occurred in a non-directed fashion in thyroid carcinoma cells.

CONCLUSIONS

Transport of endogenous and eGFP-tagged active and inactive cathepsin B in the cultured thyroid carcinoma cells reflected the distribution patterns of this protease in thyroid carcinoma tissue. Hence, our studies showed that sub-cellular localization of proteolysis is a crucial step in regulation of tissue homeostasis. We conclude that any interference with protease trafficking resulting in altered regulation of proteolytic events leads to, or is a consequence of the onset and progression of thyroid cancer.

Molecular cloning, characterization and expression analysis of cathepsin A gene in Chinese mitten crab, Eriocheir sinensis

Cathepsins, a superfamily of hydrolytic enzymes produced and enclosed within lysosomes, function in immune response in vertebrates; however, their function within the innate immune system of invertebrates remains largely unknown. Therefore, we investigated the immune functionality of cathepsin A (catA) in Chinese mitten crab (Eriocheir sinensis), a commercially important and disease vulnerable aquaculture species. The full length catA cDNA (2200 bp) was cloned via PCR based upon an initial expressed sequence tag (EST) isolated from a hepatopancreatic cDNA library. The catA cDNA contained a 1398 bp open reading frame (ORF) that encoded a putative 465 amino acid (aa) protein. Comparisons with other reported vertebrate cathepsins sequences revealed percent identity range from 48 to 51%. CatA mRNA expression in E. sinensis was (a) tissue-specific, with the highest expression observed in gill and (b) responsive in hemocytes to a Vibrio anguillarum challenge, with peak exposure observed 12 h post-injection. Collectively, data demonstrate the successful isolation of catA from the Chinese mitten crab, and its involvement in the innate immune system of an invertebrate.

Nuclear cysteine cathepsin variants in thyroid carcinoma cells

The cysteine peptidase cathepsin B is important in thyroid physiology by being involved in thyroid prohormone processing initiated in the follicular lumen and completed in endo-lysosomal compartments. However, cathepsin B has also been localized to the extrafollicular space and is therefore suggested to promote invasiveness and metastasis in thyroid carcinomas through, e.g., ECM degradation. In this study, immunofluorescence and biochemical data from subcellular fractionation revealed that cathepsin B, in its single- and two-chain forms, is localized to endo-lysosomes in the papillary thyroid carcinoma cell line KTC-1 and in the anaplastic thyroid carcinoma cell lines HTh7 and HTh74. This distribution is not affected by thyroid stimulating hormone (TSH) incubation of HTh74, the only cell line that expresses a functional TSH-receptor. Immunofluorescence data disclosed an additional nuclear localization of cathepsin B immunoreactivity. This was supported by biochemical data showing a proteolytically active variant slightly smaller than the cathepsin B proform in nuclear fractions. We also demonstrate that immunoreactions specific for cathepsin V, but not cathepsin L, are localized to the nucleus in HTh74 in peri-nucleolar patterns. As deduced from co-localization studies and in vitro degradation assays, we suggest that nuclear variants of cathepsins are involved in the development of thyroid malignancies through modification of DNA-associated proteins.

Matrix metalloproteinases in cytotoxic lymphocytes impact on tumour infiltration and immunomodulation

To efficiently combat solid tumours, endogenously or adoptively transferred cytotoxic T cells and natural killer (NK) cells, need to leave the vasculature, traverse the interstitium and ultimately infiltrate the tumour mass. During this locomotion and migration in the three dimensional environment many obstacles need to be overcome, one of which is the possible impediment of the extracellular matrix. The first and obvious one is the sub-endothelial basement membrane but the infiltrating cells will also meet other, both loose and tight, matrix structures that need to be overridden. Matrix metalloproteinases (MMPs) are believed to be one of the most important endoprotease families, with more than 25 members, which together have function on all known matrix components. This review summarizes what is known on synthesis, expression patterns and regulation of MMPs in cytotoxic lymphocytes and their possible role in the process of tumour infiltration. We also discuss different functions of MMPs as well as the possible use of other lymphocyte proteases for matrix degradation.

Immune response and expression analysis of cathepsin K in goldfish during Aeromonas hydrophila infection

The innate immunity and expression profiles of cathepsins D were determined in the goldfish (Carassius auratus) tissues after challenge with a fish pathogen Aeromonas hydrophila. The innate immunity of reactive oxygen species (ROS) and reactive nitrogen species (RNS) were determined by peripheral blood leucocytes. Blood and tissue samples of the muscle, gills, liver, kidney, heart, spleen, and intestine were sampled at 1, 3, 6 and 12 h post-infection for cathepsin D expression by semi-quantitative RT-PCR. The ROS and RNS production did not significantly increase at 1 h post-challenged goldfish. However, the ROS and RNS production was significantly increased after 3 h post-challenged fish compared to the control. The cathepsin D expression was found very low in muscle and kidney of the control fish, other tissues was not found the expression. A similar pattern was found in goldfish at 1 h post-challenge with A. hydrophila. However, at 3 h post-challenge goldfish, the cathepsin D expression was high only in the heart. At 6 h post-challenge goldfish, the cathepsin D expression was seen high all the tissues, except in the spleen. However, the expression was decreased at 12 h post-infection samples. This result was suggested that the goldfish infected with A. hydrophila decreased the innate immunity level in peripheral blood and expressed the cathepsin D in tissues.

Drug development against metastasis-related genes and their pathways: a rationale for cancer therapy

It is well recognized that the majority of cancer related deaths is caused by metastatic diseases. Therefore, there is an urgent need for the development of therapeutic intervention specifically targeted to the metastatic process. In the last decade, significant progress has been made in this research field, and many new concepts have emerged that shed light on the molecular mechanism of metastasis cascade which is often portrayed as a succession of six distinct steps; localized invasion, intravasation, translocation, extravasation, micrometastasis and colonization. Successful metastasis is dependent on the balance and complex interplay of both the metastasis promoters and suppressors in each step. Therefore, the basic strategy of our interventions is aimed at either blocking the promoters or potentiating the suppressors in this disease process. Toward this goal, various kinds of antibodies and small molecules have been designed. These include agents that block the ligand-recepter interaction of metastasis promoters (HGF/c-Met), antagonize the metastasis-promoting enzymes (AMF, uPA and MMP) and inhibit the transcriptional activity of metastasis promoter (beta-Catenin). On the other hand, the intriguing roles of metastasis suppressors and their signal pathways have been extensively studied and various attempts have been made to potentiate these factors. Small molecules have been developed to restore the expression or mimic the function of metastasis-suppressor genes such as NM23, E-cadherin, Kiss-1, MKK4 and NDRG1, and some of them are under clinical trials. This review summarizes our current understanding of the molecular pathway of tumor metastasis and discusses strategies and recent development of anti-metastatic drugs.

Tumor cell-organ microenvironment interactions in the pathogenesis of cancer metastasis

The process of cancer metastasis is sequential and selective and contains stochastic elements. The growth of metastases represents the endpoint of many lethal events that few tumor cells can survive. Primary tumors consist of multiple subpopulations of cells with heterogeneous metastatic properties, and the outcome of metastasis depends on the interplay of tumor cells with various host factors. The findings that different metastases can originate from different progenitor cells account for the biological diversity that exists among various metastases. Even within a solitary metastasis of proven clonal origin, however, heterogeneity of biological characteristics can develop rapidly. The pathogenesis of metastasis depends on multiple interactions of metastatic cells with favorable host homeostatic mechanisms. Interruption of one or more of these interactions can lead to the inhibition or eradication of cancer metastasis. For many years, all of our efforts to treat cancer have concentrated on the inhibition or destruction of tumor cells. Strategies both to treat tumor cells (such as chemotherapy and immunotherapy) and to modulate the host microenvironment (including the tumor vasculature) should offer additional approaches for cancer treatment. The recent advances in our understanding of the biological basis of cancer metastasis present unprecedented possibilities for translating basic research to the clinical reality of cancer treatment.

Tumor-endothelial cell interactions: therapeutic potential

Metastasis is the primary cause of death in cancer patients. However, the molecular mechanism of the metastatic process is poorly understood because it involves multiple steps with a high degree of complexity. A critical step for successful establishment of secondary colonization is the hematogenous dissemination of malignant cells. During this process, the attachment of cancer cells to the endothelial cells on microvasculature is considered to be an essential step and many adhesion molecules as well as chemokines have been found to be involved in this process. This interaction of cancer-endothelial cell is considered not only to determine the physical site of metastasis, but also to provide the necessary anchorage to facilitate tumor cell extravasation. However, recent evidence indicates that this interaction also serves as a host defense mechanism and hinders the process of metastasis. The tumor metastases suppressor gene, KAI1, has been known to block metastatic process without affecting the primary tumor growth, and this protein has been found to be able to bind to the chemokine receptor, Duffy antigen receptor for chemokines (DARC), which is expressed on endothelial cells. Importantly, this interaction markedly induces senescence of tumor cells. This novel finding is not only significant in the context of molecular dissection of metastatic process but also in the therapeutic implication to develop drugs inhibiting metastasis.

Towards novel anti-cancer strategies based on cystatin function

Cystatins have recently emerged as important players in a multitude of physiological and patho-physiological settings that range from cell survival and proliferation, to differentiation, cell signaling and immunomodulation. This group of cysteine protease inhibitors forms a large super-family of proteins composed of one, two, three, and, in some species, more than three cystatin domains. Over the last 20 years or so, members of the cystatin super-family have been primarily explored with respect to their capacity to inhibit intracellular cysteine proteases. Yet, this classical mode of action does not fully explain their remarkably diverse biological functions. Due to the space limitations, the author will discuss here the most recent findings that suggest that some of the single-domain, cytoplasmic and cell-secreted cystatins may play important roles in the promotion or suppression of tumor growth, invasion and metastasis. Based on the present understanding of cystatin function, novel avenues for anti-cancer strategies are proposed.

Significance and prognostic value of lysosomal enzyme activities measured in surgically operated adenocarcinomas of the gastroesophageal junction and squamous cell carcinomas of the lower third of esophagus

AIM: To establish whether there are fundamental differences in the biochemistries of adenocarcinomas of the gastroesophageal junction (GEJ) and the squamous cell carcinomas of the lower third of the esophagus (LTE).

METHODS: Between February 1, 1997 and February 1, 2000, we obtained tissue samples at the moment of resection from 54 patients for biochemical analysis. The full set of data could be comprehensively analyzed in 47 of 54 patients samples (81%). Of these, 29 were adenocarcinomas of the GEJ Siewert type I (n = 8), type II (n = 12), type III (n = 9), and 18 presented as squamous cell carcinomas of the LTE. We evaluated the mean values of 11-lysosomal enzyme and 1-cytosol protease activities of the tumorous and surrounding mucosae as well as their relative activities, measured as the ratio of activity in tumor and normal tissues from the same patient. These data were further analyzed to establish the correlation with tumor localization, TNM stage (lymph-node involvement), histological type (papillary, signet-ring cell, tubular), state of differentiation (good, moderate, poor), and survival (≤ 24 or ≥ 24 mo).

RESULTS: In adenocarcinomas, the activity of α-mannosidase (AMAN), cathepsin B (CB) and dipeptidyl-peptidase I (DPP I) increased significantly as compared to the normal gastric mucosa. In squamous cell carcinomas of the esophagus, we also found a significant difference in the activity of cathepsin L and tripeptidyl-peptidase I in addition to these three. There was a statistical correlation of AMAN, CB, and DPP I activity between the level of differentiation of adenocarcinomas of the GEJ and lymph node involvement, because tumors with no lymph node metastases histologically confirmed as well-differentiated, showed a significantly lower activity. The differences in CB and DPP I activity correlated well with the differences in survival rates, since the CB and DPP I values of those who died within 24 mo following surgical intervention were significantly higher than of those who survived for 2 years or more.

CONCLUSION: Adenocarcinomas of the GEJ form a homogenous group from a tumor-biochemical aspect, and differ from the biochemical characteristics of squamous cell carcinomas of the LTE on many points. When adenocarcinomas of the GEJs are examined at the preoperative phase, the ratio of the performed AMAN, CB, and DPP I enzymatic activity of the tissue sample from the tumor and adjacent intact mucosa within2 cm of the tumor may have a prognostic value even in the preoperative examination period, and may indicate that ranking of these patients into the neo-adjuvant treatment group should be considered.

Brain metastases in melanoma: roles of neurotrophins

Brain metastasis, which occurs in 20% to 40% of all cancer patients, is an important cause of neoplastic morbidity and mortality. Successful invasion into the brain by tumor cells must include attachment to microvessel endothelial cells, penetration through the blood-brain barrier, and, of relevance, a response to brain survival and growth factors. Neurotrophins (NTs) are important in brain-invasive steps. Human melanoma cell lines express low-affinity NT receptor p75NTR in relation to their brain-metastatic propensity with their invasive properties being regulated by NGF, or nerve growth factor, the prototypic NT. They also express functional TrkC, the putative receptor for the invasion-promoting NT-3. In brain-metastatic melanoma cells, NTs promote invasion by enhancing the production of extracellular matrix (ECM)-degradative enzymes such as heparanase, an enzyme capable of locally destroying both ECM and the basement membrane of the blood-brain barrier. Heparanase is an endo-beta-d-glucuronidase that cleaves heparan sulfate (HS) chains of ECM HS proteoglycans, and it is a unique metastatic determinant because it is the dominant mammalian HS degradative enzyme. Brain-metastatic melanoma cells also produce autocrine/paracrine factors that influence their growth, invasion, and survival in the brain. Synthesis of these factors may serve to regulate NT production by brain cells adjacent to the neoplastic invasion front, such as astrocytes. Increased NT levels have been observed in tumor-adjacent tissues at the invasion front of human brain melanoma. Additionally, astrocytes may contribute to the brain-metastatic specificity of melanoma cells by producing NT-regulated heparanase. Trophic, autocrine, and paracrine growth factors may therefore determine whether metastatic cells can successfully invade, colonize, and grow in the CNS.

MMP-2 activation and stepwise progression of pulmonary adenocarcinoma: Analysis of MMP-2 and MMP-9 with gelatin zymography

Small pulmonary adenocarcinomas can be classified on the basis of their histological characteristics and prognosis, and when classified as such, the prognosis of replacing-type adenocarcinoma with active fibroblast proliferation is significantly worse than adenocarcinoma without fibroblast proliferation. In order to clarify the biological mechanisms of the key to the morphological changes associated with active fibroblast proliferation, we examined the activities of matrix metalloproteinase (MMP)-2 and MMP-9, which are important enzymes in the stromal invasion by cancers. The active MMP-2 and MMP-9 content of 40 pulmonary adenocarcinomas that were less than 20 mm in diameter was measured by the gelatin zymography method. The quantity of active MMP-2 in the pulmonary adenocarcinomas with active fibroblast proliferation was higher than in the pulmonary adenocarcinomas without proliferation (P < 0.001), but there were no correlations between the histological features and the activation of MMP-9. The presence of active fibroblast proliferation in small pulmonary adenocarcinomas suggests that the cancer cells have acquired the ability to invade through the action of active MMP-2, and this is thought to be one of the reasons for the worse prognosis of pulmonary adenocarcinoma with active fibroblast proliferation.

Cathepsin B in the rat eye

BACKGROUND

Cathepsin B is a mammalian cysteine protease. The enzyme has been suggested to participate in the patophysiological processes of keratoconus as well as in the corneal response to infectious agents. This study describes the localization of cathepsin B in the rat eye.

METHODS

Cathepsin B was identified in rat ocular tissues by Western blotting and immunohistochemistry. Cathepsin B mRNA levels were analyzed in the tissues by quantitative real-time cDNA amplification (QRT-PCR).

RESULTS

Cathepsin B is present in the epithelium, in stromal cells and in the endothelium of the cornea. It is also present in the epithelium lining the ciliary processes, in occasional stromal cells in the iris, in the anterior subcapsular lens epithelium and in various cell types in the retina. At all locations cathepsin B is present in cytoplasmic granules, presumably lysosomes. QRT-PCR analysis detected cathepsin B mRNA in all these tissues in amounts correlating to the immunodetection results, suggesting that the enzyme detected is locally produced.

CONCLUSIONS

Cathepsin B is present in several tissues and cell types throughout the rat eye. It is localized to cytoplasmic granules, presumably lysosomes. Our results suggest that it is probably also produced in the same cell types.

Exploring the role of 5' alternative splicing and of the 3'-untranslated region of cathepsin B mRNA

The cysteine peptidase cathepsin B is responsible for connective tissue breakdown in several diseases. The pathological expression of cathepsin B may depend on the structure of its mRNA. We investigated the translational efficiency of the cathepsin B mRNA untranslated regions (UTRs) using fusion constructs to green fluorescent protein (GFP) and luciferase. Transfection of fusion constructs with GFP and luciferase containing the full-length 5'-UTR, the variant lacking exon 2, and that lacking exons 2 and 3 into mammalian cells, resulted in modulation of the biosynthetic rate of cathepsin B in a cell-specific manner. Constructs missing these exons were biosynthetically more efficient than the full-length counterpart. Luciferase was cloned upstream of the 3'-UTR, downstream of the 5'-UTR, or sandwiched between the 5'- and the 3'-UTR. The UTRs of cathepsin B downregulated luciferase biosynthesis moderately when present individually, with the 3'-UTR being more efficient than the 5'-UTR, and downregulated it even more when present simultaneously. A truncated cathepsin B-GFP chimeric product derived from the 5'-UTR missing exons 2 and 3 induced cell death. The increased biosynthetic rate and abnormal trafficking of cathepsin B observed in pathologies such as cancer and osteoarthritis may depend on alternative splicing of pre-mRNA.

Brain-metastatic melanoma: a neurotrophic perspective

The brain is a unique microenvironment enclosed by the skull and maintaining a highly regulated vascular transport barrier. To metastasize to the brain, malignant tumor cells must attach to microvessel endothelial cells, invade the blood-brain barrier (BBB), and respond to brain survival and growth factors. Neurotrophins (NT) are important in brain invasion because they stimulate this process. In brain-metastatic melanoma cells, NT can promote invasion by enhancing the production of extracellular matrixdegradative enzymes such as heparanase, an enzyme capable of locally destroying both the extracellular matrix and the basement membrane of the BBB. We have examined human and murine melanoma cell lines exhibiting varying abilities to form brain metastases, and have found that they express low-affinity neurotrophin receptor p75NTR in relation to their brain-metastatic potentials. They do not, however, express trkA, the gene encoding the tyrosine kinase receptor TrkA, the high-affinity receptor for nerve growth factor (NGF), the prototypic NT. Presence of functional TrkC, the putative receptor for the invasion-promoting neurotrophin NT-3, was also expressed in these cells. Brain-metastatic melanoma cells can also produce autocrine factors and inhibitors that influence their growth, invasion, and survival in the brain. Synthesis of these factors may influence NT production by brain cells adjacent to the neoplastic invasion front, such as oligodendrocytes and astrocytes. In brain biopsies, we observed increased amounts of NGF and NT-3 in tumor-adjacent tissues at the invasion front of human melanoma tumors. Additionally, we found that astrocytes contribute to the brain-metastatic specificity of melanoma cells by producing NT-regulated heparanase. Trophic, autocrine, and paracrine growth factors may therefore determine whether metastatic cells can successfully invade, colonize, and grow in the central nervous system (CNS).

Isolation of cathepsin B inhibitory peptides, Cabin-A1 and -A2, from a tryptic and chymotryptic hydrolysate of human serum albumin

Two novel peptides that inhibit cathepsin B were isolated from a tryptic and chymotryptic hydrolysate of human serum albumin, and designated as Cabin-A1 and -A2. Cabin-A1 and -A2 were purified by reversed-phase HPLC and identified as Ser-Leu-His-Thr-Leu-Phe and Phe-Gln-Asn-Ala-Leu, respectively. These peptides correspond to f(65-70) and f(403-407) of human serum albumin. Human albutensin A (Ala-Phe-Lys-Ala-Trp-Ala-Val-Ala-Arg), which corresponds to f(210-218), was also isolated as a potent cathepsin B inhibitor. Synthetic Cabin-A1, -A2, and human albutensin A showed dose-dependent inhibition of cathepsin B, with K(i) values of 2.4, 290, and 3.8 microM, respectively.

Putative role of 67 kDa elastin-laminin receptor in tumor invasion

Cellular regulatory mechanisms normally maintain a delicate balance between cell proliferation, quiescence and death. The imbalance between these functions resulting from molecular intracellular changes is a key factor in tumorigenesis. Tumor cells detaching from the primary tumor possess a propension for invasion and metastasis formation. These tumor cells can attach, migrate, proliferate and grow in host tissue. The surrounding extracellular matrix (ECM) modulates these functions. It is now widely accepted that cell-matrix interactions play an important role in these processes. Most investigators concentrated their attention on the role of integrins in the above processes. There are, however, only scant data on the role of elastin and its receptors in tumor invasion. Nevertheless, experimental evidence indicates that the 67 kDa elastin-laminin receptor (ELR) subunit plays an important role in tumor invasion by mediating essential tumor cell functions leading to metastases. In this review we will concentrate on the putative role of the 67 kDa ELR subunit in tumor invasion.

Isolation of novel peptides, cabin-1, -2, -3, and -4, that inhibit cathepsin B from a thermolysin digest of human plasma

Four novel peptides that inhibit cathepsin B, designated as Cabin-1, -2, -3, and -4, were isolated from a thermolysin digest of human plasma. After gel filtration and cation-exchange chromatography, the peptide mixture was purified by reverse-phase HPLC to isolate Cabin-1, -2, -3, and 4, with the amino acid sequences LGPVTQE, VLQSSGLYS, VVSVLT, and LVYDAY, respectively. These peptides correspond to f(64-70) of human apolipoprotein A-I for Cabin-1, f(56-64) and f(185-190) of the human immunoglobulin G gamma chain for Cabin-2 and -3, and f(66-71) of human transferrin for Cabin-4. Synthetic Cabin-1, -2, -3, and -4 showed dose-dependent inhibition of cathepsin B. Their IC50 values were 450, 500, 20, and 5.0 micromol/l, respectively. Lineweaver-Burk plots suggested that Cabin-3 is a noncompetitive inhibitor, while Cabin-4 is a competitive inhibitor. Among the N- and C-terminal deletion peptides of Cabin-2 and -4, Cabin-2(1-8), VLQSSGLY, was found to have the most potent inhibitory activity, with an IC50 of 3.8 micromol/l.

Polymer-supported approach for solution-phase synthesis of cysteine trap protease inhibitors: procedure for straightforward optimization of the P1-P1' pocket

Peptide-based reversible and irreversible cysteine proteases inhibitors are well reported in the literature. Many of these compounds have an electrophilic carbonyl group as a cysteine trap in the place of a scissile amide moiety of the natural substrate. As a common mechanism strategy, we have designed a probe library of a cysteine trap for rapid optimization of P1-P1' pockets of different cysteine proteases. The synthesis of this library using a straightforward methodology based on polymer-supported reagents and scavengers to avoid tedious purification steps has been achieved. For the selective monobromination of diazo ketones, preparation of a new supported reagent, piperidinoaminomethylpolystyrene hydrobromide, is also described.

A mutated cathepsin-D devoid of its catalytic activity stimulates the growth of cancer cells

Cathepsin-D, a lysosomal aspartyl proteinase, is highly secreted by breast cancer cells and its over-expression by transfection stimulates cancer cell proliferation. The mechanism by which this protease affects proliferation remains, however, unknown. In order to determine whether proteolytic activity is necessary, we abolished its enzymatic activity using site-directed mutagenesis followed by stable transfection in 3Y1-Ad12 cancer cells. Substitution of the aspartic acid residue 231 by an asparagine residue in its catalytic site abrogated the cathepsin-D proteolytic activity but did not affect its expression level, processing or secretion. However, like wild-type cathepsin-D, this mutated catalytically-inactive cathepsin-D retained its capacity to stimulate proliferation of cells embedded in Matrigel or collagen I matrices, colony formation in soft agar and tumor growth in athymic nude mice. Addition on the mock-transfected cells, of either conditioned media containing the wild-type or the mutated pro-cathepsin-D, or of the purified mutated pro-cathepsin-D, partially mimicked the mitogenic activity of the transfected cathepsin-D, indicating a role of the secreted pro-enzyme. Moreover, addition of two anti-cathepsin-D antibodies on the cathepsin-D transfected cells inhibited their proliferation, suggesting an action of the secreted pro-cathepsin-D via an autocrine loop. A synthetic peptide containing the 27-44 residue moiety of the cathepsin-D pro-fragment was, however, not mitogenic suggesting that a receptor for the pro-fragment was not involved. Furthermore, the cathepsin-D mitogenicity was not blocked by inhibiting the interaction of pro-cathepsin-D with the mannose-6-phosphate receptors. Our results altogether demonstrate that a mutated cathepsin-D devoid of catalytic activity is still mitogenic and suggest that it is acting extra-cellularly by triggering directly or indirectly a yet unidentified cell surface receptor.

Alternative messenger RNA splicing and enzyme forms of cathepsin B in human osteoarthritic cartilage and cultured chondrocytes

OBJECTIVE

In previous studies, we suggested that cathepsin B, which is present at sites of cartilage remodeling in osteoarthritis (OA), may act as an antagonist of cartilage repair, an enhancer of the action of metalloproteinases, and a mediator of cartilage neovascularization and mineralization. Alternative splicing of cathepsin B pre-messenger RNA (pre-mRNA) and/or mRNA overexpression is a plausible regulatory mechanism. In the present study, we investigated the abundance of cathepsin B transcripts and the properties of cathepsin B protein in normal and OA cartilage, osteophytes, and cultured chondrocytes.

METHODS

Cathepsin B mRNA splice variants containing the full-length transcript (CB) and the variants lacking either exon 2 (CB[-2]) or lacking exons 2 and 3 (CB[-2,3]) were measured by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot assays and were localized by in situ RT-PCR. Cathepsin B protein was analyzed by electrophoretic, Western blot, and chromatographic methods.

RESULTS

The relative content of CB, CB(-2), and CB(-2,3) varied considerably in OA cartilage and osteophytes, with less variation in normal cartilage. The mean cathepsin B mRNA level was significantly higher in OA cartilage and osteophytes than in normal cartilage. Normal cultured chondrocytes attained cathepsin B mRNA levels similar to those in OA cartilage. Enzyme overexpression resulted in the secretion of procathepsin B, followed by activation to the proteolytically active form.

CONCLUSION

The high levels of CB and CB(-2) are consistent with an overproduction of secreted procathepsin B in OA. Up-regulation of chondrocyte cathepsin B, which takes place at both the transcriptional and the translational level, suggests a leading role of the enzyme in the progression of OA.

Noncaspase proteases in apoptosis

Biochemical and genetic analysis of apoptosis has determined that intracellular proteases are key effectors of cell death pathways. In particular, early studies have pointed to the primacy of caspase proteases as mediators of execution. More recently, however, evidence has accumulated that noncaspases, including cathepsins, calpains, granzymes, and the proteasome complex, also have roles in mediating and promoting cell death. An important goal is to understand the importance of distinct noncaspases in various forms of apoptosis, and to determine whether pathways mediated by noncaspase proteases intersect with those mediated by caspases. In this review the roles of noncaspase proteases in the biochemistry of apoptosis will be discussed.

Differential effect toward inhibition of papain and cathepsin C by recombinant human salivary cystatin SN and its variants produced by a baculovirus system

Human salivary cystatin SN (CsnSN) is a member of the cystatin superfamily of cysteine proteinase inhibitors. In this study we used a baculovirus expression system to produce a full-length unaltered CsnSN and its variants. The variants were constructed with the changes in the three predicted proteinase-binding regions: the N-terminus (variant N(12-13), G12A-G13A), beta-hairpin loop I (variant L(56-58), Q56G-T57G-V58G) and beta-hairpin loop II (variant L(106-107), P106G-W107G). The secreted CsnSNs were purified using sequential spiral cartridge ultrafiltration and DE-52 radial flow chromatography. The purified proteins were examined for papain- and cathepsin C-inhibition. The wild-type CsnSN, and variants N(12-13) and L(106-107) bound tightly to papain (K(i) < 10 pM), whereas mutation in the loop I reduced binding affinity 5700-fold (K(i) = 57 nM). On the other hand, the wild-type CsnSN bound to cathepsin C less tightly (K(i) = 100 nM). The mutation in the N-terminus or loop I reduced binding affinity by 16 (K(i) = 1.6 microM)- and 19-fold (K(i) = 1.9 microM), respectively, while mutation in loop II resulted in an ineffective cathepsin C inhibitor (K(i) = 14 microM). Collectively, these results suggest that the N-terminal G12-G13 residues of CsnSN are not essential for papain inhibition but play a role in cathepsin C inhibition; residues Q56-T57-V58 in the loop I are essential for both papain and cathepsin C inhibitions, and residues P106-W107 in the loop II are not important for papain inhibition but essential for cathepsin C inhibition. These results demonstrated that CsnSN variants have different effects toward different cysteine proteinases.

Crystal structure of human procathepsin X: a cysteine protease with the proregion covalently linked to the active site cysteine

Human cathepsin X is one of many proteins discovered in recent years through the mining of sequence databases. Its sequence shows clear homology to cysteine proteases from the papain family, containing the characteristic residue patterns, including the active site. However, the proregion of cathepsin X is only 38 residues long, the shortest among papain-like enzymes, and the cathepsin X sequence has an atypical insertion in the regions proximal to the active site. This protein was recently expressed and partially characterized biochemically. Unlike most other cysteine proteases from the papain family, procathepsin X is incapable of autoprocessing in vitro but can be processed under reducing conditions by exogenous cathepsin L. Atypically, the mature enzyme is primarily a carboxypeptidase and has extremely poor endopeptidase activity. We have determined the three-dimensional structure of the procathepsin X at 1.7 A resolution. The overall structure of the mature enzyme is characteristic for enzymes of the papain superfamily, but contains several novel features. Most interestingly, the short proregion binds to the enzyme with the aid of a covalent bond between the cysteine residue in the proregion (Cys10p) and the active site cysteine residue (Cys31). This is the first example of a zymogen in which the inhibition of enzyme's proteolytic activity by the proregion is achieved through a reversible covalent modification of the active site nucleophile. Such mode of binding requires less contact area between the proregion and the enzyme than observed in other procathepsins, and no auxiliary binding site on the enzyme surface is used. A three-residue insertion in a highly conserved region, just prior to the active site cysteine residue, confers a significantly different shape on the S' subsites, compared to other proteases from papain family. The 3D structure provides an explanation for the rather unusual carboxypeptidase activity of this enzyme and confirms the predictions based on homology modeling. Another long insertion in the cathepsin X amino acid sequence forms a beta-hairpin pointing away from the active site. This insertion, thought to be an equivalent of cathepsin B occluding loop, is located on the side of the protein, distant from the substrate binding site.

The lysosomal cysteine proteases

A significant number of exciting papain-like cysteine protease structures have been determined by crystallographic methods over the last several years. This trove of data allows for an analysis of the structural features that empower these molecules as they efficiently carry out their specialized tasks. Although the structure of the paradigm for the family, papain, has been known for twenty years, recent efforts have reaped several structures of specialized mammalian enzymes. This review first covers the commonalities of architecture and purpose of the papain-like cysteine proteases. From that broad platform, each of the lysosomal enzymes for which there is an X-ray structure (or structures) is then examined to gain an understanding of what structural features are used to customize specificity and activity. Structure-based design of inhibitors to control pathological cysteine protease activity will also be addressed.