Human cystatin D. cDNA cloning, characterization of the Escherichia coli expressed inhibitor, and identification of the native protein in saliva

Serum proteins associated with periodontitis relapse post-surgery: A pilot study

BACKGROUND

The knowledge of which genes and proteins that are connected to the susceptibility to gingivitis with subsequent local tissue degradation seen in periodontitis is insufficient. Changes of serum proteins associated with recurrence of bleeding on probing (BOP) and increased periodontal pocket depths (PPD) after surgical treatment of periodontitis could reveal molecules that could be early signals of tissue destruction and/or of importance for systemic effects in other tissues or organs.

METHODS

We performed a longitudinal pilot study and followed 96 inflammation-related proteins over time in serum from patients who underwent surgical treatment of periodontitis (n= 21). The samples were taken before (time 0), and then at 3, 6, and 12 months after surgery. Changes in protein levels were analysed in relation to the clinical outcome measures, that is, proportion of surfaces affected by BOP and PPD.

RESULTS

Changes in treatment outcomes with early signs of relapse in periodontitis after surgical treatment, for example, increased BOP and PPDs, were during 12-months follow up associated with increased serum levels of high-sensitivity C-reactive protein (hs-CRP) and programmed death-ligand 1 (PD-L1), and reduced serum levels of cystatin-D protein.

CONCLUSION

This study shows that clinical signs of recurrence of periodontitis after surgery are reflected in serum, but larger studies are needed for verification. Our novel findings of an association between increased PD-L1- and decreased cystatin D-levels and recurrence in periodontitis are interesting because PD-L1 has been shown to facilitate bacterial infections and chronic inflammation and cystatin D to inhibit tissue destruction. Our results justify mechanistic studies regarding the role of these molecules in periodontitis.

Secreted cystatins decrease proliferation and enhance apoptosis of human leukemic cells

Cysteine proteases are implicated in proteolysis events favoring cancer cell growth, spread, and death by apoptosis. Herein, we have studied whether the net growth and survival of the leukemic cell lines Jurkat, U937, and HL‐60 are affected by external addition of five proteins acting as natural cysteine protease inhibitors. None of the cystatins examined (A, C, D, and E/M) or chagasin showed consistent effects on Fas‐induced apoptosis when evaluated at 1 µm. In contrast, when the intrinsic apoptosis pathway was activated by hydrogen peroxide, addition of cystatin D augmented caspase‐3‐like activity within all three cell lines. Flow cytometric analysis of U937 cells also showed increased numbers of annexin V‐positive cells when hydrogen peroxide was used to initiate apoptosis and cells were cultured in the presence of cystatin D or C. Moreover, stimulation of hydrogen peroxide‐induced apoptotic U937 cells with either cystatin C or D resulted in a dose‐dependent decrease in the number of cells. Cell viability was also decreased when U937 cells were cultured in the presence of cystatin C or D (1–9 µm) only, demonstrating that these cystatins can reduce cell proliferation by themselves in addition to enhancing apoptosis induced by oxidative stress. These effects on U937 cells were paralleled by internalization of cystatins C and D, indicating these effects are caused by downregulation of intracellular proteolysis. External addition of cystatins C and D to HL‐60 and Jurkat cells demonstrated similar degrees of cystatin D uptake and decreased viability as for U937 cells, indicating that these effects are general for leukemic cells.

Cystatin F as a regulator of immune cell cytotoxicity

Cysteine cathepsins are lysosomal peptidases involved in the regulation of innate and adaptive immune responses. Among the diverse processes, regulation of granule-dependent cytotoxicity of cytotoxic T-lymphocytes (CTLs) and natural killer (NK) cells during cancer progression has recently gained significant attention. The function of cysteine cathepsins is regulated by endogenous cysteine protease inhibitors-cystatins. Whereas other cystatins are generally cytosolic or extracellular proteins, cystatin F is present in endosomes and lysosomes and is thus able to regulate the activity of its target directly. It is delivered to endosomal/lysosomal vesicles as an inactive, disulphide-linked dimer. Proteolytic cleavage of its N-terminal part leads to the monomer, the only form that is a potent inhibitor of cathepsins C, H and L, involved in the activation of granzymes and perforin. In NK cells and CTLs the levels of active cathepsin C and of granzyme B are dependent on the concentration of monomeric, active cystatin F. In tumour microenvironment, inactive dimeric cystatin F can be secreted from tumour cells or immune cells and further taken up by the cytotoxic cells. Subsequent monomerization and inhibition of cysteine cathepsins within the endosomal/lysosomal vesicles impairs granzyme and perforin activation, and provokes cell anergy. Further, the glycosylation pattern has been shown to be important in controlling secretion of cystatin F from target cells, as well as internalization by cytotoxic cells and trafficking to endosomal/lysosomal vesicles. Cystatin F is therefore an important mediator used by bystander cells to reduce NK and T-cell cytotoxicity.

Cysteine cathepsins as a prospective target for anticancer therapies-current progress and prospects

Cysteine cathepsins (CTS), being involved in both physiological and pathological processes, play an important role in the human body. During the last 30 years, it has been shown that CTS are highly upregulated in a wide variety of cancer types although they have received a little attention as a potential therapeutic target as compared to serine or metalloproteinases. Studies on the increasing problem of neoplastic progression have revealed that secretion of cell-surface- and intracellular cysteine proteases is aberrant in tumor cells and has an impact on their growth, invasion, and metastasis by taking part in tumor angiogenesis, in apoptosis, and in events of inflammatory and immune responses. Considering the role of CTS in carcinogenesis, inhibition of these enzymes becomes an attractive strategy for cancer therapy. The downregulation of natural CTS inhibitors (CTSsis), such as cystatins, observed in various types of cancer, supports this claim. The intention of this review is to highlight the relationship of CTS with cancer and to present illustrations that explain how some of their inhibitors affect processes related to neoplastic progression.

Low-level internalization of cystatin E/M affects legumain activity and migration of melanoma cells

The ratio between proteases and their inhibitors is unbalanced in cancer. The cysteine protease inhibitor cystatin C is internalized by some cancer cells, which affects cellular properties. Here we aimed to investigate if uptake of cystatin C and the related inhibitor cystatin E/M occur in melanoma cell lines and to evaluate to what extent the uptake affects the legumain activity that is typically increased in melanoma. First we studied the basic expression, secretion, and intracellular content of all type 2 cystatins as well as expression and activity of their possible target enzymes legumain and cathepsin B in MDA-MB-435S, A375, and C8161 melanoma cells. Legumain activity was measureable in all cell lines, and of the potential legumain inhibitors, cystatin C, E/M, and F, cystatin C was the one mainly produced. All cells internalized cystatin C added to culture media, leading to increased intracellular cystatin C levels by 120-200%. Cystatin E/M was internalized as well but at a modest rate. The effects on intracellular legumain activity were nevertheless pronounced, probably because the cells lacked this inhibitor, and its affinity for legumain is 100-fold higher than that of cystatin C. Likewise, the low-degree uptake resulted in reduced migration and invasion of A375 cells in Matrigel to an extent comparable with the W106F variant of cystatin C with optimal uptake properties and resulting in much higher intracellular levels. Thus, cystatin E/M appears to be a good candidate to efficiently down-regulate the increased legumain activity, possibly important for the malignant phenotype of melanoma cells.

Quantitative salivary proteomic differences in oral chronic graft-versus-host disease

Purpose

Chronic graft-versus-host disease (cGVHD) is a severe immunological complication that occurs after allogeneic hematopoietic stem cell transplantation (HSCT). Although oral cGVHD occurs in >25 % of cGVHD patients and leads to decreased quality of life, its etiology is poorly understood. The present retrospective cross-sectional analysis of oral cGVHD patients sought to (1) test the feasibility of liquid chromatography tandem mass spectrometry (LC-MS/MS) to identify protein biomarkers of oral cGVHD and (2) to gain a clearer understanding of salivary proteins impacted by oral cGVHD.

Methods

Using unstimulated whole saliva, we compared pooled saliva from five patients with a diagnosis of moderate or severe oral cGVHD, with a gender-and age- matched pool of five cGVHD patients with no oral mucosal findings. LC-MS/MS was used to identify salivary proteins, followed by Ingenuity Pathway Analysis (IPA). Selected mass spectrometric findings, including lactotransferrin, lactoperoxidase, and albumin, were confirmed by targeted label-free quantification.

Results

LC-MS/MS led to confident identification of 180 proteins. Of these proteins, 102 changed in abundance at least 2 fold, including 12 proteins identified only in the No oral cGVHD group. Downregulation of ~0.4 fold was confirmed for both lactotransferrin and lactoperoxidase in Oral cGVHD saliva using targeted label-free quantification. IPA analysis implicated pathways involved in cellular metabolism and immunoregulation.

Conclusions

Reduction of salivary lactoperoxidase, lactotransferrin, and several cysteine proteinase inhibitor family proteins suggests impaired oral antimicrobial host immunity in cGVHD patients. This shotgun proteomic analysis of oral cGVHD saliva using targeted label-free quantification of select proteins supports the use of mass spectrometry for future validation in a large patient population as noninvasive tests for screening, early detection, and monitoring of cGVHD.

Electronic supplementary material

The online version of this article (doi:10.1007/s10875-012-9738-4) contains supplementary material, which is available to authorized users.

Antimicrobial peptides and periodontal disease

AIMS

The goal of this review is to identify the antimicrobial proteins in the oral fluids, saliva and gingival crevicular fluid and identify functional families and candidates for antibacterial treatment.

RESULTS

Periodontal biofilms initiate a cascade of inflammatory and immune processes that lead to the destruction of gingival tissues and ultimately alveolar bone loss and tooth loss. Treatment of periodontal disease with conventional antibiotics does not appear to be effective in the absence of mechanical debridement. An alternative treatment may be found in antimicrobial peptides and proteins, which can be bactericidal and anti-inflammatory and block the inflammatory effects of bacterial toxins. The peptides have co-evolved with oral bacteria, which have not developed significant peptide resistance. Over 45 antibacterial proteins are found in human saliva and gingival crevicular fluid. The proteins and peptides belong to several different functional families and offer broad protection from invading microbes. Several antimicrobial peptides and proteins (AMPs) serve as templates for the development of therapeutic peptides and peptide mimetics, although to date none have demonstrated efficacy in human trials.

CONCLUSIONS

Existing and newly identified AMPs may be developed for therapeutic use in periodontal disease or can serve as templates for peptide and peptide mimetics with improved therapeutic indices.

The effects of 1,25-dihydroxyvitamin D3 on colon cancer cells depend on RhoA-ROCK-p38MAPK-MSK signaling

Many studies support a protective action of vitamin D against colon cancer. 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) exerts wide gene regulatory effects in human colon cancer cells. We previously reported that 1,25(OH)2D3 increases cytosolic Ca2+ concentration and transiently activates RhoA and its effector the Rho-associated coiled-kinase (ROCK), and later p38MAPK-MSK. We found that the inhibition of ROCK signaling by Y27632 or that of MSK by Ro318220 prevent the formation of epithelioid islands of SW480-ADH cells by 1,25(OH)2D3 and disrupts the adhesive phenotype of HT29 cells. ROCK and MSK inhibition also abrogates the induction of 1,25(OH)2D3 24-hydroxylase (CYP24), E-cadherin, and vinculin and the repression of cyclin D1 by 1,25(OH)2D3. Moreover, 1,25(OH)2D3 does not promote the localization of the tight junction protein occludin at the plasma membrane in cells expressing a dominant negative RhoA (N19-RhoA). In addition, 1,25(OH)2D3 specifically increases the level of the cysteine protease-inhibitor cystatin D, whereas that of cystatin SN is unaffected. The increase of cystatin D protein caused by 1,25(OH)2D3 is abrogated in N19-RhoA cells. Thus, activation of the RhoA-ROCK-p38MAPK-MSK signaling pathway is essential for the regulation of the phenotype and of the CST5/cystatin D candidate tumor suppressor and other target genes by 1,25(OH)2D3 in colon cancer cells.

Cystatin E/M suppresses legumain activity and invasion of human melanoma

Background

High activity of cysteine proteases such as legumain and the cathepsins have been shown to facilitate growth and invasion of a variety of tumor types. In breast cancer, several recent studies have indicated that loss of the cysteine protease inhibitor cystatin E/M leads to increased growth and metastasis. Although cystatin E/M is normally expressed in the skin, its role in cysteine protease regulation and progression of malignant melanoma has not been studied.

Methods

A panel of various non-melanoma and melanoma cell lines was used. Cystatin E/M and C were analyzed in cell media by immunoblotting and ELISA. Legumain, cathepsin B and L were analyzed in cell lysates by immunoblotting and their enzymatic activities were analyzed by peptide substrates. Two melanoma cell lines lacking detectable secretion of cystatin E/M were transfected with a cystatin E/M expression plasmid (pCST6), and migration and invasiveness were studied by a Matrigel invasion assay.

Results

Cystatin E/M was undetectable in media from all established melanoma cell lines examined, whereas strong immunobands were detected in two of five primary melanoma lines and in two of six lines derived from patients with metastatic disease. Among the four melanoma lines secreting cystatin E/M, the glycosylated form (17 kD) was predominant compared to the non-glycosylated form (14 kD). Legumain, cathepsin B and L were expressed and active in most of the cell lines, although at low levels in the melanomas expressing cystatin E/M. In the melanoma lines where cystatin E/M was secreted, cystatin C was generally absent or expressed at a very low level. When melanoma cells lacking secretion of cystatin E/M were transfected with pCST6, their intracellular legumain activity was significantly inhibited. In contrast, cathepsin B activity was not affected. Furthermore, invasion was suppressed in cystatin E/M over-expressing melanoma cell lines as measured by the transwell Matrigel assay.

Conclusions

These results suggest that the level of cystatin E/M regulates legumain activity and hence the invasive potential of human melanoma cells.

Cystatin D is a candidate tumor suppressor gene induced by vitamin D in human colon cancer cells

The active vitamin D metabolite 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] has wide but not fully understood antitumor activity. A previous transcriptomic analysis of 1alpha,25(OH)2D3 action on human colon cancer cells revealed cystatin D (CST5), which encodes an inhibitor of several cysteine proteases of the cathepsin family, as a candidate target gene. Here we report that 1alpha,25(OH)2D3 induced vitamin D receptor (VDR) binding to, and activation of, the CST5 promoter and increased CST5 RNA and protein levels in human colon cancer cells. In cells lacking endogenous cystatin D, ectopic cystatin D expression inhibited both proliferation in vitro and xenograft tumor growth in vivo. Furthermore, cystatin D inhibited migration and anchorage-independent growth, antagonized the Wnt/beta-catenin signaling pathway, and repressed c-MYC expression. Cystatin D repressed expression of the epithelial-mesenchymal transition inducers SNAI1, SNAI2, ZEB1, and ZEB2 and, conversely, induced E-cadherin and other adhesion proteins. CST5 knockdown using shRNA abrogated the antiproliferative effect of 1alpha,25(OH)2D3, attenuated E-cadherin expression, and increased c-MYC expression. In human colorectal tumors, expression of cystatin D correlated with expression of VDR and E-cadherin, and loss of cystatin D correlated with poor tumor differentiation. Based on these data, we propose that CST5 has tumor suppressor activity that may contribute to the antitumoral action of 1alpha,25(OH)2D3 in colon cancer.

Biochemistry and Clinical Role of Human Cystatin C

Low molecular-mass plasma proteins play a key role in health and disease. Cystatin C is an endogenous cysteine proteinase inhibitor belonging to the type 2 cystatin superfamily. The mature, active form of human cystatin C is a single non-glycosylated polypeptide chain consisting of 120 amino acid residues, with a molecular mass of 13,343-13,359 Da, and containing four characteristic disulfide-paired cysteine residues. Human cystatin C is encoded by the CST3 gene, ubiquitously expressed at moderate levels. Cystatin C monomer is present in all human body fluids; it is preferentially abundant in cerebrospinal fluid, seminal plasma, and milk. Cystatin C L68Q variant is an amyloid fibril-forming protein with a high tendency to dimerize. It forms self-aggregates with massive amyloid deposits in the brain arteries of young adults, leading to lethal cerebral hemorrhage. The main catabolic site of cystatin C is the kidney: more than 99% of the protein is cleared from the circulation by glomerular ultrafiltration and tubular reabsorption. The diagnostic value of cystatin C as a marker of kidney dysfunction has been extensively investigated in multiple clinical studies on adults, children, and in the elderly. In almost all the clinical studies, cystatin C demonstrated a better diagnostic accuracy than serum creatinine in discriminating normal from impaired kidney function, but controversial results have been obtained by comparing this protein with other indices of kidney disease, especially serum creatinine-based equations. In this review, we present and discuss most of the available data from the literature, critically reviewing conclusions and suggestions for the use of cystatin C in clinical practice. Despite the multitude of clinical data in the literature, cystatin C has not been widely used, perhaps because of a combination of factors, such as a general diffidence among clinicians, the absence of definitive cut-off values, conflicting results in clinical studies, no clear evidence on when and how to request the test, the poor commutability of results, and no accurate examination of costs and of its routine use in a stat laboratory.

The cysteine protease inhibitors cystatins inhibit herpes simplex virus type 1-induced apoptosis and virus yield in HEp-2 cells

The role of cystatins in herpes simplex virus (HSV)-induced apoptosis and viral replication has been studied. Human epithelial (HEp-2) cells infected with wild-type HSV-1 (F), with a deletion virus lacking the anti-apoptotic gene Us3 (R7041) or with a deletion virus lacking the anti-apoptotic genes Us3 and ICP4 (d120) were treated with cystatin A, C or D. Cells and culture media were studied at different time points for replicating HSV-1 and for apoptosis. Cystatins C and D inhibited the yield of replicative HSV-1 significantly in HEp-2 cells. In addition, cystatin D inhibited R7041 and d120 virus-induced apoptosis. Moreover, cystatin A inhibited R7041-induced apoptosis. These inhibitory effects of cystatins on virus replication and apoptosis are likely to be separate functions. Cystatin D treatment decreased cellular cathepsin B activity in HSV-1 infection, suggesting that cathepsin B is involved in virus-induced apoptosis.

Down-regulation of human extracellular cysteine protease inhibitors by the secreted staphylococcal cysteine proteases, staphopain A and B

Of seven human cystatins investigated, none inhibited the cysteine proteases staphopain A and B secreted by the human pathogen Staphylococcus aureus. Rather, the extracellular cystatins C, D and E/M were hydrolyzed by both staphopains. Based on MALDI-TOF time-course experiments, staphopain A cleavage of cystatin C and D should be physiologically relevant and occur upon S. aureus infection. Staphopain A hydrolyzed the Gly11 bond of cystatin C and the Ala10 bond of cystatin D with similar Km values of approximately 33 and 32 microM, respectively. Such N-terminal truncation of cystatin C caused >300-fold lower inhibition of papain, cathepsin B, L and K, whereas the cathepsin H activity was compromised by a factor of ca. 10. Similarly, truncation of cystatin D caused alleviated inhibition of all endogenous target enzymes investigated. The normal activity of the cystatins is thus down-regulated, indicating that the bacterial enzymes can cause disturbance of the host protease-inhibitor balance. To illustrate the in vivo consequences, a mixed cystatin C assay showed release of cathepsin B activity in the presence of staphopain A. Results presented for the specificity of staphopains when interacting with cystatins as natural protein substrates could aid in the development of therapeutic agents directed toward these proteolytic virulence factors.

Extracellular production of human cystatin S and cystatin SA by Bacillus subtilis

We herein describe the development of a Bacillus subtilis system that can be used to produce large quantities of recombinant (r-) human salivary cystatins, a cysteine protease inhibitor of family 2 in the cystatin superfamily. The B. subtilis that lacked the alkaline protease E gene (DeltaaprE type mutant strain) was prepared by homologous recombination. The cDNA fragments coding for mature cystatins (S and SA) were ligated in frame to the DNA segment for the signal peptide of endoglucanase in the pHSP-US plasmid vector that was then use to transform the DeltaaprE type mutant strain of B. subtilis. The transformants carrying the expression vectors were cultivated in 5-L jar fermenters for 3 days at 30 degrees C. Both r-cystatin S and r-cystatin SA were successfully expressed and secreted into the culture broth, and were purified using a fast performance liquid chromatography system. The first use of DeltaaprE type mutant strain of B. subtilis made it possible to obtain a high yield of secreted protein, which makes this system an improvement over expression in Escherichia coli. We conclude that this system has high utility for expression of commercial quantities of secreted proteins.

Characterization of cathepsin L secreted by Sf21 insect cells

Sf21 cells, derived from the Spodoptera frugiperda pupa, are commonly used for the heterologous expression of proteins. While purifying recombinant proteins from this system we encountered a protease, secreted at high levels by Sf21 cells, that readily degraded recombinant proteins and also tended to co-purify with histidine-tagged proteins from Ni(2+) affinity columns. Purification and characterization of the protease revealed that it has many properties consistent with cysteine proteases of the papain family, including autoactivation under reducing conditions and acidic pH, and inhibition by E-64. Amino acid sequence analysis showed that the Sf21 enzyme may be identical to a putative insect procathepsin L cloned from the cotton bollworm. The subsite specificity of the Sf21 cathepsin and its inhibition profile by cystatins are consistent with the protease being an insect homologue of cathepsin L. Monoclonal antibodies useful for the detection and purification of the insect cathepsin L were developed.

Different cysteine proteinases involved in bone resorption and osteoclast formation

Cysteine proteinases, especially cathepsin K, play an important role in osteoclastic degradation of bone matrix proteins and the process can, consequently, be significantly inhibited by cysteine proteinase inhibitors. We have recently reported that cystatin C and other cysteine proteinase inhibitors also reduce osteoclast formation. However, it is not known which cysteine proteinase(s) are involved in osteoclast differentiation. In the present study, we compared the relative potencies of cystatins C and D as inhibitors of bone resorption in cultured mouse calvariae, osteoclastogenesis in mouse bone marrow cultures, and cathepsin K activity. Inhibition of cathepsin K activity was assessed by determining equilibrium constants for inhibitor complexes in fluorogenic substrate assays. The data demonstrate that whereas human cystatins C and D are equipotent as inhibitors of bone resorption, cystatin D is 10-fold less potent as an inhibitor of osteoclastogenesis and 200-fold less potent as an inhibitor of cathepsin K activity. A recombinant human cystatin C variant with Gly substitutions for residues Arg8, Leu9, Val10, and Trp106 did not inhibit bone resorption, had 1,000-fold decreased inhibitory effect on cathepsin K activity compared to wildtype cystatin C, but was equipotent with wildtype cystatin C as an inhibitor of osteoclastogenesis. It is concluded that (i) different cysteine proteinases are likely to be involved in bone resorption and osteoclast formation, (ii) cathepsin K may not be an exclusive target enzyme in any of the two systems, and (iii) the enzyme(s) involved in osteoclastogenesis might not be a typical papain-like cysteine proteinase.

Crystal structure of human cystatin D, a cysteine peptidase inhibitor with restricted inhibition profile

Cystatins are natural inhibitors of papain-like (family C1) and legumain-related (family C13) cysteine peptidases. Cystatin D is a type 2 cystatin, a secreted inhibitor found in human saliva and tear fluid. Compared with its homologues, cystatin D presents an unusual inhibition profile with a preferential inhibition cathepsin S > cathepsin H > cathepsin L and no inhibition of cathepsin B or pig legumain. To elucidate the structural reasons for this specificity, we have crystallized recombinant human Arg(26)-cystatin D and solved its structures at room temperature and at cryo conditions to 2.5- and 1.8-A resolution, respectively. Human cystatin D presents the typical cystatin fold, with a five-stranded anti-parallel beta-sheet wrapped around a five-turn alpha-helix. The structures reveal differences in the peptidase-interacting regions when compared with other cystatins, providing plausible explanations for the restricted inhibitory specificity of cystatin D for some papain-like peptidases and its lack of reactivity toward legumain-related enzymes.

Murine monoclonal antibody which can distinguish cystatins SA1 and SA2

To develop a diagnostic trial enabling the selective examination for a target cystatin in human body fluids, we attempted to prepare monoclonal antibodies against human cystatin SA1 (originally cystatin SA) and its variant form (cystatin SA2). BALB/c mice were immunized with recombinant (r-) cystatins SA1 and SA2. Two monoclonal antibodies designated Cys3F11 and Cys2E5 were selected. By ELISA analyses, the Cys2E5 was shown to react with r-cystatin SA2 but also somewhat with r-cystatin SA1 (22% cross-reactivity) and with plasma cystatin C (18% cross-reactivity), indicating a high specificity for cystatin SA2. The Cys3F11 reacted not only with r-cystatin SA1 but also with r-cystatin SA2 (89% cross-reactivity) and plasma cystatin C (47% cross-reactivity). This finding was further emphasized by immunoblotting of human submandibular-sublingual saliva samples. ELISA additivity test suggests that the two monoclonal antibodies bind to distinct epitopes. In conclusion, we have succeeded in producing two antibodies that discriminate the structural differences between salivary cystatins S and SN, which share more than 90% identity in amino acid sequence with cystatin SA.

Specific cysteine protease inhibitors act as deterrents of western flower thrips, Frankliniella occidentalis (Pergande), in transgenic potato

In this study, the effects of the accumulation of cysteine protease inhibitors on the food preferences of adult female western flower thrips, Frankliniella occidentalis (Pergande), were investigated. Representative members of the cystatin and thyropin gene families (stefin A, cystatin C, kininogen domain 3 and equistatin) were expressed in potato (Solanum tuberosum) cv. Impala, Kondor and Line V plants. In choice assays, a strong time- and concentration-dependent deterrence from plants expressing stefin A and equistatin was observed. Cystatin C and kininogen domain 3 were not found to be active. All tested inhibitors were equally or more active than stefin A at inhibiting the proteolytic activity of thrips, but, in contrast with stefin A, they were all expressed in potato as partially degraded proteins. The resistance of cysteine protease inhibitors against degradation in planta by endogenous plant proteases may therefore be relevant in explaining the observed differences in the deterrence of thrips. The results demonstrate that, when given a choice, western flower thrips will select plants with low levels of certain cysteine protease inhibitors. The novel implications of the defensive role of plant cysteine protease inhibitors as both deterrents and antimetabolic proteins are discussed.

Interactive effects of microbial transglutaminase and recombinant cystatin on the mackerel and hairtail muscle protein

Interactive effects of microbial transglutaminase (MTGase) and recombinant cystatin on the mackerel and hairtail water soluble protein (WSP), salt soluble protein (SSP), and muscle protein (MP) were investigated. According to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and enzymic activity analyses, cross-linking of mackerel and hairtail myosin heavy chain and low molecular mass compounds and formation of epsilon-(gamma-glutamyl)lysine cross-links were observed on samples with MTGase, while the recombinant cystatin could effectively inhibit the cathepsins and subsequently prevent degradation of proteins during setting. The cathepsins and MTGase activities in WSP, SSP, and MP solutions decreased, but the recombinant cystatin activity increased during setting at 45 degrees C.

Modulation of host immune responses by nematode cystatins

Parasitic nematodes, living in the intestinal tract or within tissues of theirs hosts, are constantly exposed to an array of immune effector mechanisms. One strategy to cope with the immune response is the release of immunomodulatory components that block effector mechanisms or interact with the cytokine network. Among the secreted nematode immunomodulators, cysteine protease inhibitors (cystatins) are shown to be of major importance. Nematode cystatins inhibit, among others, proteases involved in antigen processing and presentation, which leads to a reduction of T cell responses. At the same time nematode cystatins modulate cytokine responses, the most prominent trait being the upregulation of IL-10, a Th2 cytokine, by macrophages. In this situation, IL-10 leads among others to downregulation of costimulatory surface molecules of macrophages. These properties contribute to induction of an anti-inflammatory environment, concomitant with a strong inhibition of cellular proliferation. This setting is believed to favour the survival of worms. An opposite activity of nematode cystatins is the upregulation of production of inducible nitric oxide by IFN-gamma activated macrophages, an intrinsic property of natural cysteine protease inhibitors. This shows that these proteins can act as proinflammatory molecules under certain circumstances. A comparison of the immunomodulatory effects of cystatins of filarial nematodes with homologous proteins of the free-living nematode Caenorhabditis elegans revealed distinct differences. Caenorhabditis elegans cystatins induce the production of the Th1 cytokine IL-12, in contrast to filarial cystatins that upregulate IL-10. Caenorhabditis elegans cystatins hardly inhibit cellular proliferation. These data suggest that cystatins of parasitic nematodes have multiple, specific capacities for immunomodulation, acting in parallel on different immune effector mechanisms. Elucidation of the mechanisms involved might be useful in the development of immunotherapeutic reagents in the future.

Salivary (SD-type) cystatins: over one billion years in the making--but to what purpose?

Human saliva contains relatively abundant proteins that are related ancestrally in sequence to the cystatin superfamily. Most, although not all, members of this superfamily are potent inhibitors of cysteine peptidases. Four related genes have been identified, CST1, 2, 4 and 5, encoding cystatins SN, SA, S, and D, respectively. CST1, 4, and probably CST5 are now known to be expressed in a limited number of other tissues in the body, primarily in exocrine epithelia, and the term SD-type cystatin is more appropriate than 'salivary cystatin'. These genes are co-ordinately regulated in the submandibular gland during post-natal development. The organization of these tissue-specifically-expressed genes in the genome, and their phylogeny, indicate that they evolved from an ancestral housekeeping gene encoding the ubiquitously expressed cystatin C, and are members of a larger protein family. Their relationship to rat cystatin S, a developmentally regulated rodent submandibular gland protein, remains to be established. In this review, the evolution of the SD-type cystatins in the cystatin superfamily, their genomics, expression, and structure-function relationships are examined and compared with known cystatin functions, with the goal of providing clues to their biological roles.

Cystatins C, E/M and F in human pleural fluids of patients with neoplastic and inflammatory lung disorders

Secretory type 2 cystatins, like cystatins C, E/M and F, are thought to be involved in many pathobiological processes, including vascular amyloidosis, rheumatoid arthritis, Alzheimer's disease, osteoporosis, viral and bacterial infections, inflammatory disorders and tumour invasion and metastasis. In order to define the levels of cystatins C, E/M, and F in pleural effusions and to investigate whether these cystatins correlate with diagnostic parameters of pleural and lung diseases, we determined their concentrations in 160 pleural effusions. The median concentration of cystatin C in pleural effusions was 1437 microg/l (95.8 nM), ranging between 18-3967 microg/l. Cystatin C did neither correlate with malignant nor with benign diseases. The concentration of cystatin E/M was significantly higher in effusions of primary pleural tumours (mesotheliomas) compared to secondary pleural tumours and benign diseases. Furthermore, there was a significant correlation between the concentration of cystatin E/M of mesotheliomas and the pleural fluid tumour cell count and of cystatin C. The median values of cystatin F were significantly increased in parapneumonic/empyema thoracis pleural effusions and tuberculous pleurisy compared to malignant pleural effusions, respectively. The concentration of cystatin F in benign effusions correlated significantly with diagnostic parameters and inflammation (total protein; lactate dehydrogenase; C-reactive protein). Finally, only in the group of parapneumonic/empyema thotatin F and the neutrophil count. In conclusion, pleural effusions of different origin contain high levels of cystatin C, perhaps constituting the major part of an inhibitor reservoir. The level of cystatin E/M appears to be significantly associated with primary pleural tumours and cystatin F correlates with inflammatory processes of lung disorders.

Molecular cloning and characterization of a novel cystatin-like molecule, CLM, from human bone marrow stromal cells

The cystatins are physiological cysteine proteinase inhibitors. Here we report the cloning of a novel human cystatin-like molecule (CLM) from human bone marrow stromal cell (BMSC) cDNA library. The putative CLM protein contained 159 residues with a 29-residue signal peptide. CLM protein was highly homologous to family 2 cystatins, especially mouse and human testatin. The CLM gene spanned two exons and was mapped on chromosome 20p11.2, among cystatin superfamily gene clusters. CLM mRNA was barely detected in most tumor cell lines except for breast adenocarcinoma MCF-7 cells and glioblastoma U251 cells, but after LPS or PMA stimulation, CLM expression was increased in myelogenous leukemia cell lines HL-60 and U-937. Northern blot analysis revealed CLM was ubiquitously expressed in normal tissues, which was clearly different from the testis-specific expression pattern of most family 2 cystatins. When overexpressed in 293 cells, GFP-fused CLM targeted extracellularly through secretory pathway by Golgi apparatus. The results indicated that the secreted CLM protein might play roles in hematopoietic differentiation or inflammation.

Expression of type 2 cystatin genes CST1-CST5 in adult human tissues and the developing submandibular gland

Type 2 cystatins comprise a class of cysteine peptidase inhibitor presumed to mediate protective functions at various locations, including the oral cavity. Seven cystatin genes are clustered within a 300-kb region of human 20p11.2. "Salivary" cystatins, encoded by CST1, 2, 4, and 5, are present in saliva at significant levels but have also been reported in other secretions, such as tears, suggesting that during their evolution, these genes have acquired mechanisms directing differential tissue-specific expression. However, their patterns of expression, which might also provide additional clues to their individual functions, have not been determined. Gene-specific RNase protection assays were used to examine the qualitative and quantitative distribution of expression of these seven genes within a collection of 23 adult human tissues. The CST3 gene, encoding cystatin C, was expressed at modest levels in all tissues examined. The presumptive pseudogenes CSTP1 and CSTP2 were not expressed at detectable levels in any tissue. The CST1, 2, 4, and 5 genes were expressed in differential, tissue-specific patterns. Expression of CST2 and CST5 was restricted to the submandibular and parotid glands, while CST1 and CST4 were expressed in these tissues and in the lacrimal gland. Immunohistochemistry studies localized expression to the serous-type secretory end pieces. Coexpression of CST1 and CST4 was also observed in the epithelial lining of the gallbladder and seminal vesicle. The CST1 product was detected in the tracheal glands and CST4 in the kidney and prostate. Despite their different adult patterns of expression, analysis of CST1, 2, 4, and 5 mRNA levels in infant submandibular glands demonstrated a coordinate upregulation of expression of between 3.5 and 9 months of age. The patterns of cystatin gene expression are consistent with several proposed oral functions of the salivary cystatins but also suggest they are important in other locations and that, despite their close sequence similarity, they are individually specialized.

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

We investigated the mechanism of lysosome-mediated cell death using purified recombinant pro-apoptotic proteins, and cell-free extracts from the human neuronal progenitor cell line NT2. Potential effectors were either isolated lysosomes or purified lysosomal proteases. Purified lysosomal cathepsins B, H, K, L, S, and X or an extract of mouse lysosomes did not directly activate either recombinant caspase zymogens or caspase zymogens present in an NT2 cytosolic extract to any significant extent. In contrast, a cathepsin L-related protease from the protozoan parasite Trypanosoma cruzi, cruzipain, showed a measurable caspase activation rate. This demonstrated that members of the papain family can directly activate caspases but that mammalian lysosomal members of this family may have been negatively selected for caspase activation to prevent inappropriate induction of apoptosis. Given the lack of evidence for a direct role in caspase activation by lysosomal proteases, we hypothesized that an indirect mode of caspase activation may involve the Bcl-2 family member Bid. In support of this, Bid was cleaved in the presence of lysosomal extracts, at a site six residues downstream from that seen for pathways involving capase 8. Incubation of mitochondria with Bid that had been cleaved by lysosomal extracts resulted in cytochrome c release. Thus, cleavage of Bid may represent a mechanism by which proteases that have leaked from the lysosomes can precipitate cytochrome c release and subsequent caspase activation. This is supported by the finding that cytosolic extracts from mice ablated in the bid gene are impaired in the ability to release cytochrome c in response to lysosome extracts. Together these data suggest that Bid represents a sensor that allows cells to initiate apoptosis in response to widespread adventitious proteolysis.

RINX(VSX1), a novel homeobox gene expressed in the inner nuclear layer of the adult retina

The locus control region (LCR) of the human red and green visual pigment genes is critical for the formation of functional red and green cones in the retina. A 37-bp core of the LCR is perfectly conserved among mammals and binds specific retinal nuclear proteins. Here, we employed a yeast one-hybrid screen of an adult retinal cDNA library to clone and characterize these proteins. We identified clones encoding homeodomain (HD) transcription factors Pax6, Rx, and Chx10 and a novel paired-like HD protein, RINX. In the adult retina, RINX is exclusively expressed in a subset of cells (likely to be bipolar cells) of the retinal inner nuclear layer (INL). RINX is closely related to Chx10, which is also exclusively expressed in the INL of the adult retina and is critical for retinal development. The RINX gene is expressed in two classes of mRNA. One class encodes proteins that lack either part of or all of the HD, but retain the transcriptional activation domain. The RINX gene maps to chromosome 20p11.2 to which no retinal disease has been assigned. In conclusion, the LCR contains two adjacent motifs that are targets for binding of HD proteins that may specify the development and differentiation of cone photoreceptors and a subset of INL bipolar cells. Mutations in the related human CHX10 gene cause microphthalmia in a subset of families, and, therefore, the RINX gene is a candidate for this phenotype in another subset of patients. Since the RINX gene is likely an ortholog of the goldfish Vsx1 gene, it has been named VSX1 by the Human Gene Nomenclature Committee.

Rainbow trout (Oncorhynchus mykiss) cystatin C: expression in Escherichia coli and properties of the recombinant protease inhibitor

Cystatins are a superfamily of low Ki cysteine proteinase inhibitors found in both plants and animals. Cystatin C, a secreted molecule of this family, is of interest from biochemical and evolutionary points of view, and also has biotechnological applications. Recently we cloned and sequenced the cDNA for rainbow trout (Oncorhynchus mykiss) cystatin C [Li et al., 1998. Molecular cloning, sequence analysis and expression distribution of rainbow trout (Oncorhynchus mykiss) cystatin C. Comp. Biochem. Phys. B 121, 135-143]. To explore the relationship between protein structure and function of trout cystatin C, we established a bacterial system for expression of the protein. Trout cystatin C expressed in the cytoplasm of bacterial cells did not have detectable protease inhibitory activity. Activity was regained by Ni-NTA chromatography under denaturing conditions followed by dialysis-based refolding. Titration of purified cystatin C preparations with papain indicated that approximately 20% of the total protein had been converted to the active form after one refolding cycle. Expression levels were 3-5 mg/l. The protease-inhibitory properties of recombinant trout cystatin C were similar to those of human and chicken cystatin C derived from biological sources and recombinant cystatin C derived from rat and mouse genes. The Ki for papain was 1.2 x 10(-15) M, exhibiting the high affinity binding unique to this family of protease inhibitors.

Cystatin C--properties and use as diagnostic marker

This chapter focuses on the most well characterized inhibitors—cystatin C—and provide some information on its structure, biochemical properties, its role in normal and abnormal physiological processes, as well as on its use as a diagnostic marker. A major part of the cysteine proteases are evolutionary related to the structurally well–defined cysteine protease papain and are called papain–like cysteine proteases. The biological roles and the cystatin superfamily inhibitors of papain–like cystein proteases are also discussed. The aminoacid sequence and schematic structure of human cystatin C is also presented. The evolutionary relationships among all known inhibitory active human cystatins and kininogen cystatin domains are diagrammatically represented. The distribution of cystatins in body fluids and additional functions attributed to cystatin C are described. The serum or plasma cystatin C is used as a marker for glomerular filtration rate (GFR). The urine cystatin C is used as a marker for proximal tubular damage. The two types of brain hemorrhage associated with Cystatin C amyloid deposits are also demonstrated. The conditions connected with deposition of amyloid β–protein in cystatin C and cerebral hemorrhage is also provided.

Inhibition of mammalian legumain by some cystatins is due to a novel second reactive site

We have investigated the inhibition of the recently identified family C13 cysteine peptidase, pig legumain, by human cystatin C. The cystatin was seen to inhibit enzyme activity by stoichiometric 1:1 binding in competition with substrate. The Ki value for the interaction was 0.20 nM, i.e. cystatin C had an affinity for legumain similar to that for the papain-like family C1 cysteine peptidase, cathepsin B. However, cystatin C variants with alterations in the N-terminal region and the "second hairpin loop" that rendered the cystatin inactive against cathepsin B, still inhibited legumain with Ki values 0.2-0.3 nM. Complexes between cystatin C and papain inhibited legumain activity against benzoyl-Asn-NHPhNO2 as efficiently as did cystatin C alone. Conversely, cystatin C inhibited papain activity against benzoyl-Arg-NHPhNO2 whether or not the cystatin had been incubated with legumain, strongly indicating that the cystatin inhibited the two enzymes with non-overlapping sites. A ternary complex between legumain, cystatin C, and papain was demonstrated by gel filtration supported by immunoblotting. Screening of a panel of cystatin superfamily members showed that type 1 inhibitors (cystatins A and B) and low Mr kininogen (type 3) did not inhibit pig legumain. Of human type 2 cystatins, cystatin D was non-inhibitory, whereas cystatin E/M and cystatin F displayed strong (Ki 0.0016 nM) and relatively weak (Ki 10 nM) affinity for legumain, respectively. Sequence alignments and molecular modeling led to the suggestion that a loop located on the opposite side to the papain-binding surface, between the alpha-helix and the first strand of the main beta-pleated sheet of the cystatin structure, could be involved in legumain binding. This was corroborated by analysis of a cystatin C variant with substitution of the Asn39 residue in this loop (N39K-cystatin C); this variant showed a slight reduction in affinity for cathepsin B (Ki 1.5 nM) but >>5,000-fold lower affinity for legumain (Ki >>1,000 nM) than wild-type cystatin C.

Sequence variations of the CST2 gene related to the polymorphism of salivary cystatin SA

The CST2 locus has two polymorphic alleles, CST2*1 and CST2*2, which produce cystatin proteins SAI and SA2, respectively (Shintani et al., 1994). The purpose of this study was to define nucleotide sequence variations of the protein-coding region of the two alleles. The variations were investigated by direct sequencing of amplified DNA from individuals with different CST2 phenotypes. The sequence of three exons obtained from DNA of the CST2 1 phenotype was found to be identical to the published sequence of the CST2 gene (Saitoh et al., 1987), whereas two-point mutations were found in the sequence obtained from DNA of the CST2 2 phenotype. One of the mutations was a G --> A transition in exon 2, resulting in loss of a commonly occurring AciI restriction site. This mutation resulted in a Gly59 --> Asp59 substitution in the protein. The other mutation was an A --> T transversion in exon 3, resulting in the generation of a SfaNI restriction site. This mutation also produced a Glu120 --> Asp120 substitution in the protein. PCR-RFLP assay with AciI and SfaNI restriction enzymes revealed that the two-point mutations were always correlated with cystatin SA polymorphism. The difference in the electrophoretic positions of the two proteins, SA1 and SA2, in a basic gel and in an isoelectric focusing gel agreed with the expected mobilities of the proteins with the SA2 variant at a more anodal position. The CST2*2 allele is a unique allele, which shows amino acid substitution in one of the most conserved regions responsible for cysteine proteinase inhibitory activity.

Cystatin F is a glycosylated human low molecular weight cysteine proteinase inhibitor

A previously undescribed human member of the cystatin superfamily called cystatin F has been identified by expressed sequence tag sequencing in human cDNA libraries. A full-length cDNA clone was obtained from a library made from mRNA of CD34-depleted cord blood cells. The sequence of the cDNA contained an open reading frame encoding a putative 19-residue signal peptide and a mature protein of 126 amino acids with two disulfide bridges and enzyme-binding motifs homologous to those of Family 2 cystatins. Unlike other human cystatins, cystatin F has 2 additional Cys residues, indicating the presence of an extra disulfide bridge stabilizing the N-terminal region of the molecule. Recombinant cystatin F was produced in a baculovirus expression system and characterized. The mature recombinant protein processed by insect cells had an N-terminal segment 7 residues longer than that of cystatin C and displayed reversible inhibition of papain and cathepsin L (Ki = 1.1 and 0.31 nM, respectively), but not cathepsin B. Like cystatin E/M, cystatin F is a glycoprotein, carrying two N-linked carbohydrate chains at positions 36 and 88. An immunoassay for quantification of cystatin F showed that blood contains low levels of the inhibitor (0.9 ng/ml). Six B cell lines in culture secreted barely detectable amounts of cystatin F, but several T cell lines and especially one myeloid cell line secreted significant amounts of the inhibitor. Northern blot analysis revealed that the cystatin F gene is primarily expressed in peripheral blood cells and spleen. Tissue expression clearly different from that of the ubiquitous inhibitor, cystatin C, was also indicated by a high incidence of cystatin F clones in cDNA libraries from dendritic and T cells, but no clones identified by expressed sequence tag sequencing in several B cell libraries and in >600 libraries from other human tissues and cells.

Leukocystatin, a new Class II cystatin expressed selectively by hematopoietic cells

We describe a new cystatin in both mice and humans, which we termed leukocystatin. This protein has all the features of a Class II secreted inhibitory cystatin but contains lysine residues in the normally hydrophobic binding regions. As determined by cDNA library Southern blots, this cystatin is expressed selectively in hematopoietic cells, although fine details of the distribution among these cell types differ between the human and mouse mRNAs. In addition, we have determined the genomic organization of mouse leukocystatin, and we found that in contrast to most cystatins, the leukocystatin gene contains three introns. The recombinant proteins corresponding to these cystatins were expressed in Escherichia coli as N-terminal glutathione S-transferase or FLAGTM fusions, and studies showed that they inhibited papain and cathepsin L but with affinities lower than other cystatins. The unique features of leukocystatin suggests that this cystatin plays a role in immune regulation through inhibition of a unique target in the hematopoietic system.

Production and characterization of two variants of human cystatin SA encoded by two alleles at the CST2 locus of the type 2 cystatin gene family

Two variants of cystatin SA encoded by two alleles at the CST2 locus of the type 2 human cystatin gene family were expressed in Escherichia coli. One, termed cystatin SA1, is identical to cystatin SA [S. Isemura, E. Saitoh, and K. Sanada J. Biochem. 102, 693-704, 1987]. Another, termed cystatin SA2, carries two amino acid substitutions (59Gly-->Asp; 120Glu-->Asp), one of which is in the so-called QXVXG region (the first hairpin loop) and another in the C-terminal portion of the molecule. Four recombinant cystatins [full-sized cystatin SA1, two N-terminally truncated cystatin SA1 lacking four residues (WSPQ) and six residues (WSPQEE), and full-sized cystatin SA2] were purified from the periplasmic fractions of E. coli cells. Two N-terminally truncated recombinant cystatin SA1 inhibited bovine cathepsin C with 2- to 20-fold lower Ki values than that of the full-sized one. In the inhibition of papain and ficin, however, both of the N-terminally truncated cystatin SA1 displayed a 10-fold higher Ki value than that of full-sized one. In the inhibition of papain, ficin, and recombinant human cathepsin K, recombinant cystatin SA2 showed, respectively, 3826-, 1090-, and 30-fold higher Ki values compared with those of SA1. Recombinant cystatin SA2 inhibited bovine cathepsin C with a 50-fold lower Ki value compared with that of SA1. Recombinant cystatin SA1 did not inhibit human cathepsin H but SA2 inhibited it slightly (Ki = 528 nM). Neither of the recombinant variants inhibited bovine cathepsin B. Our data supply evidence indicating that the amino acid sequence of the first hairpin loop of the cystatin superfamily is important in the inhibition of papain, ficin, cathepsin C, cathepsin H, and cathepsin K.

Structural basis for different inhibitory specificities of human cystatins C and D

Human cystatins C and D share almost identical primary structures of two out of the three segments proposed to be of importance for enzyme interactions but have markedly different profiles for inhibition of the target cysteine peptidases, cathepsins B, H, L, and S. To investigate if the N-terminal binding regions of the inhibitors are responsible for the different inhibition profiles, and thereby confer biological selectivity, two hybrid cystatins were produced in Escherichia coli expression systems. In one hybrid, the N-terminal segment of cystatin C was placed on the framework of cystatin D, and the second was engineered with the N-terminal segment of cystatin D on the cystatin C scaffold. Truncated cystatin C and D variants, devoid of their N-terminal segments, were obtained by incubation with glycyl endopeptidase and isolated, in a second approach to assess the importance of the N-terminal binding regions for cystatin function and specificity. The affinities of the four cystatin variants for cathepsins B, H, L, and S were measured. By comparison with corresponding results for wild-type cystatins C and D, it was concluded (1) that both the N-terminal and framework part of the molecules significantly contribute to the observed differences in inhibitory activities of cystatins C and D and (2) that the N-terminal segment of cystatin C increases the inhibitory activity of cystatin D against cathepsin S and cathepsin L but results in decreased activity against cathepsin H. These differences in specificity were explained by the residues interacting with the S2 subsite of peptidases (Val- and Ala-10 in cystatin C and D, respectively). Also, removal of the N-terminal segment results in total loss of enzyme affinity for cystatin D but not for cystatin C. Therefore, structural differences in the framework parts, as well as in the N-terminal segments, are critical for both inhibitory specificity and potency. Homology modeling was used to identify residues likely responsible for the generally reduced inhibitory potency of cystatin D.

Cystatin D, a natural salivary cysteine protease inhibitor, inhibits coronavirus replication at its physiologic concentration

This study was conducted to examine the effect of cystatin D, a newly discovered salivary cysteine protease inhibitor, on human coronavirus replication. When MRC-5, human diploid lung cells, were incubated with dilutions of recombinant human cystatin D from 0.65-10 microM for 1 h prior to, during and after infection with coronavirus OC43 and 229e strains, a decrease in virus yield was observed resulting in an IC50 of 0.8 microM for both virus strains. This dose is within the normal concentration range of cystatin D, 0.12-1.9 microM found in saliva. When a single dose, 2.5 microM, was applied, cystatin inhibition of release of virus progeny was not overcome until three days post infection whereas inhibition by leupeptin, a serine and cysteine protease inhibitor, was completely abrogated by two days. When cellular toxicity was measured by 3H-thymidine uptake, cystatin D did not markedly affect cell proliferation below a 10 microM dose. The results demonstrate that cystatin D is a potent inhibitor of coronavirus replication.

Cystatin E is a novel human cysteine proteinase inhibitor with structural resemblance to family 2 cystatins

A new member of the human cystatin superfamily, called cystatin E, has been found by expressed sequence tag (EST) sequencing in amniotic cell and fetal skin epithelial cell cDNA libraries. The sequence of a full-length amniotic cell cDNA clone contained an open reading frame encoding a putative 28-residue signal peptide and a mature protein of 121 amino acids, including four cysteine residues and motifs of importance for the inhibitory activity of Family 2 cystatins like cystatin C. Recombinant cystatin E was produced in a baculovirus expression system and isolated. An antiserum against the recombinant protein could be used for affinity purification of cystatin E from human urine, as confirmed by N-terminal sequencing. The mature recombinant protein processed by insect cells started at amino acid 4 (cystatin C numbering), and displayed reversible inhibition of papain and cathepsin B (Ki values of 0.39 and 32 nM, respectively), in competition with substrate. Cystatin E is thus a functional cysteine proteinase inhibitor despite relatively low amino acid sequence similarities with human cystatins (26-34% identity with sequences for the Family 2 cystatins C, D, S, SN, and SA; <30% with the Family 1 cystatins, A and B, and domains 2 and 3 of the Family 3 cystatin, kininogen). Unlike other human low Mr cystatins, cystatin E is a glycoprotein, carrying an N-linked carbohydrate chain at position 108. Northern blot analysis revealed that the cystatin E gene is expressed in most human tissues, with the highest mRNA amounts found in uterus and liver. A strikingly high incidence of cystatin E clones in cDNA libraries from fetal skin epithelium and amniotic membrane cells (>0.5% of clones sequenced) indicates a protective role of cystatin E during fetal development.

Identification, cloning, and characterization of cystatin M, a novel cysteine proteinase inhibitor, down-regulated in breast cancer

A novel human cystatin gene was identified in a differential display comparison aimed at the isolation of transcriptionally regulated genes involved in invasion and metastasis of breast cancer. Messenger RNAs from primary and metastatic tumor cells isolated from the same patient were compared. A partial cDNA was isolated that was expressed in the primary tumor cell line but not in the metastatic line. The full-length cDNA was cloned and sequenced, and the inferred amino acid sequence was found to encode a novel protein, which we named cystatin M, with 40% homology to human family 2 cystatins and similar overall structure. Cystatin M is expressed by normal mammary cells and a variety of human tissues. The mature cystatin M protein was produced in Escherichia coli as a glutathione S-transferase fusion protein using the pGEX-2T expression system and purified by affinity chromatography. The cystatin M fusion protein displayed inhibitory activity against papain. Native cystatin M protein of approximately 14.5 kDa is secreted and was immunoprecipitated from supernatants of mammary cell cultures using affinity-purified antisera raised against recombinant cystatin M. An N-glycosylated form of cystatin M of 20-22 kDa was co-immunoprecipitated and accounted for about 30-40% of total cystatin M protein. Both forms of native cystatin M also occurred intracellularly. Consistent with the mRNA differential expression, no cystatin M protein was detected in metastatic mammary epithelial tumor cells. Loss of expression of cystatin M is likely associated with the progression of a primary tumor to a metastatic phenotype.

Mouse and rat cystatin C: Escherichia coli production, characterization and tissue distribution

Recombinant mouse (Mus musculus) and rat (Rattus norvegicus) cystatin C were produced by expression in Escherichia coli, isolated and functionally characterized. The mouse and rat inhibitors were both fully active in titrations of papain. Determination of equilibrium constants for dissociation (Ki) for their complexes with the target proteinase, cathepsin B, produced values not largely different from that for human cystatin C (Ki 0.07-0.13 nM). Rabbit antisera against mouse and rat cystatin C were produced and used for improved affinity purification of the recombinant inhibitors. Affinity purified immunoglobulins isolated from the antiserum against mouse cystatin C were used for construction of a sensitive enzyme-linked immunosorbent assay. The assay was used to demonstrate a high degree of immunological cross-reactivity between mouse and rat cystatin C and could be used for cystatin C quantification in mouse and rat tissue homogenates. All tissues analyzed contained cystatin C, with a relative content very similar to that of human tissues. For all species, brain tissue contained the highest cystatin C amounts and liver the lowest, whereas kidney, spleen and muscle tissues were intermediate in content. In the mouse, a notable high cystatin C content in parotid gland tissue was observed. The high degree of similarity in distribution pattern and functional properties for mouse, rat and human cystatin C indicates that a murine model should be relevant for studies of the human disease, hereditary cystatin C amyloid angiopathy.

Psoriasin: a novel chemotactic protein

Inflammatory skin disorders such as psoriasis show a preferential epidermal infiltration of neutrophils and T lymphocytes. This observation raises a question as to which factors determine the appearance and composition of leukocyte tissue infiltrations. Previously, we described a low molecular mass calcium-binding protein (psoriasin, molecular mass 11,457 Da, pI 6.77) belonging to the S1OO family that is highly upregulated in psoriatic keratinocytes and whose expression patterns implied a role in the inflammatory response. Here we report that human psoriasin is a potent and selective chemotactic inflammatory protein for CD4+ T lymphocytes and neutrophils at concentrations of about 10(-11) M. Psoriasin is not structurally related to the alpha or the beta chemokine subfamilies or to lymphotactin, a member of a newly described class of chemokines. Thus, we have observed a chemotactic protein outside the chemokine subfamilies that could be an important new inflammatory mediator.

Cystatins in health and disease

Proteolytic enzymes have many physiological functions in plants, bacteria, viruses, protozoa and mammals. They play a role in processes such as food digestion, complement activation or blood coagulation. The action of proteolytic enzymes is biologically controlled by proteinase inhibitors and increasing attention is being paid to the physiological significance of these natural inhibitors in pathological processes. The reason for this growing interest is that uncontrolled proteolysis can lead to irreversible damage e.g. in chronic inflammation or tumor metastasis. This review focusses on the possible role of the cystatins, natural and specific inhibitors of the cysteine proteinases, in pathological processes.