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

Serum Cystatin C Levels as a Predictor of Severity and Mortality Among Patients With COVID-19 Infection

INTRODUCTION

The pandemic caused by SARS Corona Virus-2 (COVID-19) has caused widespread mortality globally. The hallmark of the disease is the "cytokine storm," which is caused due to dysregulated immune system activation. Numerous inflammatory markers are used to predict the severity and mortality of the infection. Serum Cystatin C levels are associated with immune responses to exogenous and endogenous antigens. Our study was done to assess serum cystatin C as a marker of severity and mortality among patients admitted with COVID-19 infection.

METHODOLOGY

This cross-sectional study was conducted in a tertiary care center in South India. Sixty-nine patients with mild and severe COVID-19 infection admitted to the hospital were included in the study. Serum Cystatin C levels were estimated at admission. The levels were correlated with disease severity and mortality. Receiver operating characteristic curves (ROCs) was constructed for Cystatin C to predict severity and mortality. The computation of sensitivity, specificity, and positive and negative predictive values was done using optimal cut-off points. SPSS 18 was used for the statistical analysis. Version 18.0 of PASW Statistics for Windows. SPSS Inc., Chicago.

RESULTS

Out of 69 patients, 28 (40.5%) had a mild illness, and 41 patients (59.4%) had severe COVID-19 illness. Mean serum Cystatin C levels measured at the time of admission among patients with mild illness was 1.83 (SD-1.53), and among patients with severe illness was 3.84 (SD- 2.59) (p<0.001). The area under receiver operating characteristic curves (ROC) for serum cystatin C for predicting COVID-19 severity and mortality was 0.904 and 0.768, respectively (p<0.001).

CONCLUSION

Patients with severe COVID-19 disease had considerably higher serum levels of Cystatin C than those with mild COVID-19 illness. Cystatin C levels can be useful for predicting mortality and severity among patients admitted with COVID-19 infection.

Identification, expression profiling, and functional characterization of cystatin C from big-belly seahorse (Hippocampus abdominalis)

Cystatins are natural inhibitors of lysosomal cysteine proteases, including cathepsins B, L, H, and S. Cystatin C (CSTC) is a member of the type 2 cystatin family and is an essential biomarker in the prognosis of several diseases. Emerging evidence suggests the immune regulatory roles of CSTC in antigen presentation, the release of different inflammatory mediators, and apoptosis in various pathophysiologies. In this study, the 390-bp cystatin C (HaCSTC) cDNA from big-belly seahorse (Hippocampus abdominalis) was cloned and characterized by screening the pre-established cDNA library. Based on similarities in sequence, HaCSTC is a homolog of the teleost type 2 cystatin family with putative catalytic cystatin domains, signal peptides, and disulfide bonds. HaCSTC transcripts were ubiquitously expressed in all tested big-belly seahorse tissues, with the highest expression in ovaries. Immune challenge with lipopolysaccharides, polyinosinic:polycytidylic acid, Edwardsiella tarda, and Streptococcus iniae caused significant upregulation in HaCSTC transcript levels. Using a pMAL-c5X expression vector, the 14.29-kDa protein of recombinant HaCSTC (rHaCSTC) was expressed in Escherichia coli BL21 (DE3), and its protease inhibitory activity against papain cysteine protease was determined with the aid of a protease substrate. Papain was competitively blocked by rHaCSTC in a dose-dependent manner. In response to viral hemorrhagic septicemia virus (VHSV) infection, HaCSTC overexpression strongly decreased the expression of VHSV transcripts, pro-inflammatory cytokines, and pro-apoptotic genes; while increasing the expression of anti-apoptotic genes in fathead minnow (FHM) cells. Furthermore, HaCSTC overexpression protected VHSV-infected FHM cells against VHSV-induced apoptosis and increased cell viability. Our findings imply the profound role of HaCSTC against pathogen infections by modulating fish immune responses.

Cystatin C, COVID-19 severity and mortality: a systematic review and meta-analysis

Background

Combined markers of renal dysfunction and inflammation, e.g., cystatin C, might assist with risk stratification and clinical decisions in patients with coronavirus disease 19 (COVID-19). We conducted a systematic review and meta-analysis with meta-regression of serum cystatin C in COVID-19.

Methods

We searched PubMed, Web of Science and Scopus, between January 2020 and February 2021, for studies reporting serum cystatin C concentrations, measures of clinical severity and survival outcomes in hospitalized COVID-19 patients (PROSPERO registration number: CRD42021245295).

Results

Thirteen studies in 2510 COVID-19 patients, 1972 with low severity or survivor status and 538 with high severity or non-survivor status during follow up, were included in the meta-analysis. The pooled results showed that serum cystatin C concentrations were higher in patients with high disease severity or non-survivor status (standard mean deviation, SMD, 1.71, 95% CI 0.95 to 2.46, p < 0.001). Extreme between-study heterogeneity was observed (I² = 97.5%, p < 0.001). Sensitivity analysis, performed by sequentially removing each study and re-assessing the pooled estimates, showed that the magnitude and direction of the effect size was not substantially modified. The Begg’s and Egger’s t tests did not show publication bias. In meta-regression, the SMD of serum cystatin C was not associated with age, proportion of males, C-reactive protein, neutrophils, lymphocytes, aspartate aminotransferase, alanine aminotransferase, albumin, creatinine, creatine kinase-MB, lactate dehydrogenase, and proportion of patients with diabetes or hypertension.

Conclusions

Higher concentrations of serum cystatin C were associated with higher COVID-19 severity and mortality.

Graphic abstract

Cystatin M/E (Cystatin 6): A Janus-Faced Cysteine Protease Inhibitor with Both Tumor-Suppressing and Tumor-Promoting Functions

Alongside its contribution in maintaining skin homeostasis and its probable involvement in fetal and placental development, cystatin M/E (also known as cystatin 6) was first described as a tumor suppressor of breast cancer. This review aims to provide an update on cystatin M/E with particular attention paid to its role during tumorigenesis. Cystatin M/E, which is related to type 2 cystatins, displays the unique property of being a dual tight-binding inhibitor of both legumain (also known as asparagine endopeptidase) and cysteine cathepsins L, V and B, while its expression level is epigenetically regulated via the methylation of the CST6 promoter region. The tumor-suppressing role of cystatin M/E was further reported in melanoma, cervical, brain, prostate, gastric and renal cancers, and cystatin M/E was proposed as a biomarker of prognostic significance. Contrariwise, cystatin M/E could have an antagonistic function, acting as a tumor promoter (e.g., oral, pancreatic cancer, thyroid and hepatocellular carcinoma). Taking into account these apparently divergent functions, there is an urgent need to decipher the molecular and cellular regulatory mechanisms of the expression and activity of cystatin M/E associated with the safeguarding homeostasis of the proteolytic balance as well as its imbalance in cancer.

Correlation of serum cystatin C with inflammatory cytokines in patients with traumatic brain injury

Traumatic brain injury (TBI) is a world-wide leading health problem with high morbidity and mortality rates. Emerging studies have demonstrated that TBI is the consequence of a series of inflammatory responses in the body. The alteration of Cystatin C (Cys C) was reported in a variety of chronic inflammatory diseases and was also recommended as a biomarker for predicting renal diseases. In this study, we aimed to investigate the relationship between serum Cys C and TBI, and to evaluate the prognostic role of Cys C in TBI prediction. One hundred and seventy-six patients with TBI were recruited and 102 patients were finally analyzed, with 30 healthy control subjects. The concentrations of Cys C were significantly reduced in the healthy control group compared to the TBI group, and correlated with high GCS scores. The levels of hsCRP, counts of white blood cells, and levels of IL-6 and TNF-α were remarkably elevated in the TBI patients compared with the control group in a severity-dependent manner. Moreover, the concentration of Cys C was negatively correlated with TBI severity and positively correlated with inflammatory factors. In conclusion, serum Cys is an inflammatory cytokine-related factor and might indicate the severity of TBI thus serving as a prognostic biomarker.

Egg White Cystatin – A Review

Hen eggs are widely used, not only for human consumption, but also as an important material in food production and in pharmaceutical and cosmetics industry. Cystatin is a biologically active component of egg white, mostly used as an inhibitor of papain-like cysteine proteases. It was isolated from chicken egg white and has later been used in the nomenclature of structurally and functionally related proteins. Cystatins from animals, including mouse, rat, dog, cow and chicken egg white have been isolated and recently used in foodstuffs and drug administration. Cystatin has found its place and use in medicine due to its antimicrobial, antiviral and insecticidal effects, for the prevention of cerebral hemorrhage and control of cancer cell metastasis.

The association between cystatin C and COPD: a meta-analysis and systematic review

BACKGROUND

In recent years, many studies have discovered that cystatin C (Cys C) may play an important role in respiratory diseases, especially in chronic obstructive pulmonary disease (COPD). However, the findings of these studies were inconsistent. This systematic review and meta-analysis aimed to assess the relationship between serum Cys C and COPD.

METHODS

We conducted a systematic literature search in PubMed, Embase, Web of Science, Wanfang databases, and the China National Knowledge Infrastructure. The standardized mean difference (SMD), Fisher's Z-value and 95% confidence interval (CI) were calculated to investigate the effect sizes. Subgroup analyses were performed on disease status, ethnicity, assay method, and study design. Sensitivity was performed, and publication bias was assessed.

RESULTS

A total of 15 studies, including 4079 COPD patients and 5949 controls, were included in this meta-analysis. The results showed that serum Cys C levels in patients with COPD were significantly higher than those in controls (SMD = 0.99, 95% CI =0.62-1.37, P < 0.001), especially in AECOPD (SMD = 1.59, 95% CI =1.05-2.13, P < 0.001), and there were statistically different among AECOPD and SCOPD (SMD = 0.35, 95% CI =0.10-0.59, P = 0.005). The serum Cys C levels were negatively correlated with FEV1%pre (Z = - 0.45, 95%CI = -0.58--0.32, P = 0.011) and FEV1/FVC (Z = - 0.32, 95%CI = -0.50--0.14, P = 0.006). The serum Cys C levels were independent of ethnicity, assay method, and study design.

CONCLUSION

Serum Cys C levels were associated with COPD and COPD exacerbation, and they were inversely correlated with FEV1%pre and FEV1/FVC.

Serum cystatin C levels are decreased in type 1 diabetes mellitus patients with diabetic ketoacidosis

BACKGROUND

Cystatin C is a marker of renal function and risk factor for cardiovascular disease. Patients with acute myocardial infarction showed a significant decrease in cystatin C levels. It is unknown whether reduced serum cystatin C levels are connected to acute events or represent a negative acute phase response. The current study aimed to assess the association between cystatin C and the existence of diabetic ketoacidosis (DKA), an acute event in individuals with type 1 diabetes mellitus (T1DM).

METHODS

Cystatin C was measured in the control group (N.=322) and in T1DM patients with (N.=161) and without DKA (N.=146). Data were compared according to diabetes and ketoacidosis status. Correlation analysis was used to identify factors associated with cystatin C levels. A multiple stepwise regression analysis was used to determine which of the parameters that were significantly correlated with cystatin C in univariate analysis were independently related to cystatin C. Then, we assessed the independent association between cystatin C and the occurrence of DKA in T1DM patients.

RESULTS

Serum cystatin C levels were lower in patients with DKA than in patients without DKA. After adjustment for age, sex, fasting plasma glucose and creatinine, cystatin C was positively correlated with the duration of diabetes, systolic blood pressure (SBP), total cholesterol, triglycerides, uric acid and low-density lipoprotein cholesterol (P=0.004, P=0.022, P=0.013, P=0.035, P=0.006, P=0.012, respectively) and negatively correlated with hemoglobin (P<0.001). The duration of diabetes (P<0.001), total cholesterol (P=0.002), hemoglobin (P<0.001), SBP (P=0.011) and serum creatinine (P<0.001) were independently associated with cystatin C. Furthermore, we found that cystatin C was independently associated with the occurrence of DKA in T1DM patients (OR=0.004, 95% CI: 0.000-0.079, P<0.001).

CONCLUSIONS

Cystatin C was decreased in T1DM patients with DKA and was found to be an independent predictor of the occurrence of DKA in T1DM patients. The reduction in cystatin C levels might be significantly connected with acute events.

Suppressed dendritic cell functions by cystatin C lead to compromised immunity in vivo

Pathogenic microorganisms utilize multiple approaches to break down host immunity in favor of their invasion, of which, cystatin C is one of the soluble factors secreted by parasites reported to affect host immunity in vivo. The cellular targets and mechanisms of action in vivo of cystatin C, however, are far from clear. As professional antigen-presenting cells, dendritic cells (DCs) are first immune cells that contact foreign pathogenic agents or their products to initiate immune responses. We previously reported that cystatin C can regulate the functions of DCs in terms of suppressed CD4⁺ T cell activation but enhanced Th1/Th17 differentiation via different mechanisms. Here, we further verified these regulatory effects of cystatin C on DCs in vivo. We found that the suppressive role of DC-mediated CD4⁺ T cell proliferation by cystatin C was partly cell-contact independent and extended to CD8⁺ T cells in vivo. Although cystatin C-overexpressing DCs trafficked equally as their mock-transduced counterparts, their adoptive transfer suppressed CD8⁺ T cell immunity and resulted in compromised tumor rejection in both vaccination and treatment regimes. Compared with their role in promoting Th17 differentiation in vivo, cystatin C-transduced DCs had far greater ability to induce T regulatory cells (Tregs), leading to collectively a higher Treg/Th17 ratio in an adoptively transferred disease model, and thus relieved Th17-dependent autoimmunity. Collectively, these data demonstrated strong in vivo evidences for immune regulatory roles of cystatin C in DCs and provided theoretical basis for the application of cystatin C-transduced cell therapy in the treatment or remission of certain autoimmune diseases. (246).

Cystatin C regulates major histocompatibility complex-II-peptide presentation and extracellular signal-regulated kinase-dependent polarizing cytokine production by bone marrow-derived dendritic cells

Cystatin C is a ubiquitously expressed cysteine protease inhibitor that protects cells from either improper hydrolysis by endogenous proteases or pathogen growth/virulence by exogenous proteases. Although commonly used as a serum biomarker for evaluating renal function, cystatin C is associated with many immunological disorders under various pathophysiological conditions. How cystatin C affects immune cells, especially dendritic cells (DCs), however, is far from clear. In this study, we found that pharmacological treatment with or genetic overexpression of cystatin C in bone marrow-derived DCs (BMDCs) reduced their capacity to stimulate CD4⁺ T-cell proliferation, despite increased antigen uptake. This reduced capacity corresponded with reduced major histocompatibility complex-II presentation owing to diminished levels of the chaperon H2-DM in BMDCs. Instead of promoting proliferation, cystatin C promoted skewing of T cells toward proinflammatory T-helper (Th)1/Th17 differentiation. This was mediated by augmented extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase phosphorylation in BMDCs, leading to secretion of polarizing cytokines, which in turn led to the Th deviation. Collectively, our study explained the cellular and molecular basis of how this protease inhibitor can regulate immune responses, namely by affecting BMDCs and their cytokine pathway. Our results might open up an avenue for the development of therapeutic agents for the treatment of cystatin C-related immunological diseases.

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.

Involvement of cystatin C in immunity and apoptosis

As an abundantly expressed cysteine protease inhibitor widely distributed in the organisms, cystatin C is involved in various physiological processes. Due to its relatively small molecular weight and easy detection, cystatin C is commonly used as a measure for glomerular filtration rate. In pathological conditions, however, growing evidences suggest that cystatin C is associated with various immune responses against either exogenous or endogenous antigens, which ultimately result in inflammatory autoimmune diseases or tumor development if not properly controlled. Thus the fluctuation of cystatin C levels might have more clinical implications than a reflection of kidney functions. Here, we summarize the latest development of studies on the pathophysiological functions of cystatin C, with focus on its immune regulatory roles at both cellular and molecular levels including antigen presentation, secretion of cytokines, synthesis of nitric oxide, as well as apoptosis. Finally, we discuss the clinical implications and therapeutic potentials of what this predominantly expressed protease inhibitor can bring to us.

Cystatin F Affects Natural Killer Cell Cytotoxicity

Cystatin F is a cysteine peptidase inhibitor which, unlike other cystatin family members, is targeted to endosomal/lysosomal compartments. It is synthesized as an inactive disulfide-linked dimer which is then converted to an active monomer by proteolytic cleavage of 15 N-terminal residues. Cystatin F has been suggested to regulate the cytotoxicity of natural killer (NK) cells by inhibiting the major granzyme convertases, cathepsins C and H. To test this hypothesis, we prepared variants of cystatin F and analyzed their uptake, subcellular trafficking, and peptidase inhibition, as well as their impact on the cytotoxicity of NK-92 cells and primary NK cells. The N-glycosylation pattern is responsible for the secretion, uptake, and subcellular sorting of cystatin F in HeLa and Hek293 cells, whereas the legumain binding site had no effect on these processes. Active, N-terminally truncated, monomeric cystatin F can also be internalized by recipient cells and targeted to endo/lysosomes, affecting also cells lacking the activating peptidase. Cystatin F mutants capable of cell internalization and trafficking through the endo/lysosomal pathway significantly decreased cathepsin C and H activities, both in situ, following transfection and in trans, using conditioned media. Further, incubation of IL-2 stimulated NK-92 and primary NK cells with full-length and N-terminally truncated cystatin F mutants led to suppression of their granule-mediated cytotoxicity. This effect was most significant with the N-terminally truncated mutants. These results suggest that cystatin F can be an important mediator within tumor microenvironment affecting the cytotoxicity of NK cells and consequently antitumor immune response.

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.

Serum Cystatin C as an Inflammatory Marker in Exacerbated and Convalescent COPD Patients

Chronic obstructive pulmonary disease (COPD) is a common chronic inflammatory disease with high morbidity and mortality rates. Cystatin C (Cys C) is a sensitive indicator for various chronic inflammatory diseases. In this study, we aimed to evaluate the role of Cys C in COPD patients comparing with the other well-known inflammatory markers. Ninety patients with acute exacerbated COPD were studied and were reassessed when convalescent. Ninety controls were matched for age, gender, body mass index, smoking index, and comorbidity. Serum Cys C was significantly increased in convalescent COPD patients compared with healthy controls and further increased in COPD patients with an acute exacerbation. Serum Cys C was positively correlated with hsCRP both in the exacerbation and convalescence periods of COPD and negatively correlated with FEV1% predicted and FEV1/FVC in the convalescent COPD patients. In conclusion, serum Cys C is a positive acute-phase reactant in COPD patients and might indicate systemic inflammation during the progression of COPD.

Serum cystatin C and indices of lung function in elderly Chinese men with chronic obstructive pulmonary disease

OBJECTIVE

The aim of this cross-sectional case-control study was to determine the relationship between serum cystatin C (sCysC) levels and lung function in elderly male patients with chronic obstructive pulmonary disease (COPD).

METHODS

This study included 251 Chinese men (age ≥ 65 years) who were divided into COPD (n = 129) and non-COPD (n = 122) groups. Participants underwent lung function and laboratory testing, including measurement of sCysC levels. Relationships between sCysC concentration and indices of lung function were assessed by multiple regression analysis.

RESULTS

Participants in the COPD group displayed higher sCysC concentrations (P = 0.041) and lower lung function (P < 0.001) compared to participants in the non-COPD group. Multiple linear regression analyses revealed that the reciprocal of the sCysC concentration (1/sCysC) was positively associated with the predicted forced expiratory volume in 1 s in all subjects (β = 0.156, P = 0.009). The findings indicate that high sCysC levels were directly associated with decreased lung function in elderly Chinese men with COPD.

CONCLUSIONS

High sCysC concentration may be a potential indicator of impaired lung function, and its application may improve the diagnosis and assessment of COPD severity in elderly male patients.

Cystatins in immune system

Cystatins comprise a large superfamily of related proteins with diverse biological activities. They were initially characterised as inhibitors of lysosomal cysteine proteases, however, in recent years some alternative functions for cystatins have been proposed. Cystatins possessing inhibitory function are members of three families, family I (stefins), family II (cystatins) and family III (kininogens). Stefin A is often linked to neoplastic changes in epithelium while another family I cystatin, stefin B is supposed to have a specific role in neuredegenerative diseases. Cystatin C, a typical type II cystatin, is expressed in a variety of human tissues and cells. On the other hand, expression of other type II cystatins is more specific. Cystatin F is an endo/lysosome targeted protease inhibitor, selectively expressed in immune cells, suggesting its role in processes related to immune response. Our recent work points on its role in regulation of dendritic cell maturation and in natural killer cells functional inactivation that may enhance tumor survival. Cystatin E/M expression is mainly restricted to the epithelia of the skin which emphasizes its prominent role in cutaneous biology. Here, we review the current knowledge on type I (stefins A and B) and type II cystatins (cystatins C, F and E/M) in pathologies, with particular emphasis on their suppressive vs. promotional function in the tumorigenesis and metastasis. We proposed that an imbalance between cathepsins and cystatins may attenuate immune cell functions and facilitate tumor cell invasion.

Potentials of cystatin C and uric acid for predicting prognosis of heart failure

Few studies have explored the clinical potentials of combined Cystatin C (Cys) and uric acid (UA) in heart failure (HF). The authors evaluated Cys and UA as predictors of clinical outcomes compared with conventional renal biomarkers. This prospective cohort study included 587 HF patients presenting with dyspnea. At admission, Cys, UA, and other renal measures including serum urea nitrogen (BUN), creatinine (Cr), and glomerular filtration rate (GFR) were obtained. The primary endpoint was the composite of cardiac death and rehospitalization for worsening HF. During a 25-month median follow-up period, 68 patients experienced clinical outcomes: 9 cardiac deaths and 59 HFs. They showed higher BUN and Cr values and lower GFR. Within these parameters, Cys and UA had the most favorable area under the curves, and patients with Cys ≥0.8 mg/L and UA ≥6.6 mg/dL showed more frequent events. The net reclassification improvement analysis showed the combination of Cys and UA had a greater incremental effect for cardiac prognosis. On multivariate Cox hazard analysis, Cys and UA were independent predictive markers for clinical outcomes. In HF patients presenting with dyspnea, Cys and UA appear to be more useful predictors of clinical events than other renal measures.

Serum cystatin C and emphysema: results from the National Health and Nutrition Examination Survey (NHANES)

BACKGROUND

Cystatin C (CysC) is a potent nonorgan-specific cysteine protease inhibitor and may contribute to elastolysis and tissue destruction by a mechanism of protease–antiprotease imbalance. Given the prevalence of CysC in the serum of smokers and its role in tissue destruction, we aimed to evaluate the association between CysC and emphysema.

METHODS

Pooled cross-sectional data from the National Health and Nutrition Examination Survey 1999–2002 were used. Emphysema and chronic bronchitis were defined by a self-reported history ascertained using standardized questionnaires. Active smokers were defined as self-reported current smokers or measured serum cotinine ≥10 ng/mL. Nonactive smokers with a serum cotinine level >0.05 ng/mL were defined as environmental tobacco smoke (ETS)-exposed.

RESULTS

The prevalence (95% CI) of emphysema was 1.3% (range = 0.9–1.8%). The mean (SE) CysC level in the emphysema group was significantly higher than in normal controls [1,139 (22) vs. 883 (8) μg/L; p = 0.001]. Upon stratification of the study population by C-reactive protein (CRP) concentrations, we demonstrated a progressive increase in the mean serum CysC level with serially increasing CRP concentrations. Active smokers with emphysema had 115.4 (46.5) μg/L higher mean (SE) CysC levels than the normal controls (p < 0.001). Upon adjusted analysis, we observed that nonactive smokers with significant ETS exposure had 31.2 (15.2) μg/L higher mean (SE) serum CysC levels as compared to ETS unexposed nonactive smokers (p = 0.04).

CONCLUSION

In a large representative noninstitutionalized US population, we demonstrated an association between emphysema and serum CysC. Active smokers with emphysema had significantly higher CysC levels. These findings suggest that CysC may play a role in the pathogenesis of smoking-related emphysema.

Intra- and extracellular regulation of activity and processing of legumain by cystatin E/M

Legumain, an asparaginyl endopeptidase, is up-regulated in tumour and tumour-associated cells, and is linked to the processing of cathepsin B, L, and proMMP-2. Although legumain is mainly localized to the endosomal/lysosomal compartments, legumain has been reported to be localized extracellularly in the tumour microenvironment and associated with extracellular matrix and cell surfaces. The most potent endogenous inhibitor of legumain is cystatin E/M, which is a secreted protein synthesised with an export signal. Therefore, we investigated the cellular interplay between legumain and cystatin E/M. As a cell model, HEK293 cells were transfected with legumain cDNA, cystatin E/M cDNA, or both, and over-expressing monoclonal cell lines were selected (termed M38L, M4C, and M3CL, respectively). Secretion of prolegumain from M38L cells was inhibited by treatment with brefeldin A, whereas bafilomycin A1 enhanced the secretion. Cellular processing of prolegumain to the 46 and 36 kDa enzymatically active forms was reduced by treatment with either substance alone. M38L cells showed increased, but M4C cells decreased, cathepsin L processing suggesting a crucial involvement of legumain activity. Furthermore, we observed internalization of cystatin E/M and subsequently decreased intracellular legumain activity. Also, prolegumain was shown to internalize followed by increased intracellular legumain processing and activation. In addition, in M4C cells incomplete processing of the internalized prolegumain was observed, as well as nuclear localized cystatin E/M. Furthermore, auto-activation of secreted prolegumain was inhibited by cystatin E/M, which for the first time shows a regulatory role of cystatin E/M in controlling both intra- and extracellular legumain activity.

Secreted proteins as a fundamental source for biomarker discovery

The proteins secreted by various cells (the secretomes) are a potential rich source of biomarkers as they reflect various states of the cells at real time and at given conditions. To have accessible, sufficient and reliable protein markers is desirable as they mark various stages of disease development and their presence/absence can be used for diagnosis, prognosis, risk stratification and therapeutic monitoring. As direct analysis of blood/plasma, a common and noninvasive patient screening method, can be difficult for candidate protein biomarker identification, the alternative/complementary approaches are required, one of them is the analysis of secretomes in cell conditioned media in vitro. As the proteins secreted by cells as a response to various stimuli are most likely secreted into blood/plasma, the identification and pre-selection of candidate protein biomarkers from cell secretomes with subsequent validation of their presence at higher levels in serum/plasma is a promising approach. In this review, we discuss the proteins secreted by three progenitor cell types (smooth muscle, endothelial and cardiac progenitor cells) and two adult cell types (neonatal rat ventrical myocytes and smooth muscle cells) which can be relevant to cardiovascular research and which have been recently published in the literature. We found, at least for secretome studies included in this review, that secretomes of progenitor and adult cells overlap by 48% but the secretomes are very distinct among progenitor cell themselves as well as between adult cells. In addition, we compared secreted proteins to protein identifications listed in the Human Plasma PeptideAtlas and in two reports with cardiovascular-related proteins and we performed the extensive literature search to find if any of these secreted proteins were identified in a biomarker study. As expected, many proteins have been identified as biomarkers in cancer but 18 proteins (out of 62) have been tested as biomarkers in cardiovascular diseases as well.

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.

Role of N-terminal pro-brain natriuretic peptide and cystatin C to estimate renal function in patients with and without heart failure

Blood cystatin C has increasingly been used as an endogenous marker for estimating glomerular filtration rate (GFR) and evaluating prognosis in patients with acute or chronic heart failure. The goal of the study was to investigate the impact of heart failure on the determination of renal function based on cystatin C or creatinine in nonacute cardiac patients. A total of 880 consecutive and clinically stable patients with heart disease were prospectively evaluated. Serum N-terminal pro-brain natriuretic peptide (NT-pro-BNP) showed a stronger correlation with cystatin C (r = 0.60, p <0.001) compared with creatinine (r = 0.46, p <0.001). Multivariate analysis identified estimated GFR according to the MDRD Study formula (p <0.001), serum NT-pro-BNP (p <0.001), use of immunosuppressive agents (p <0.001), and allopurinol treatment (p <0.001) as the strongest independent predictors of serum cystatin C. Parallel measurement of creatinine clearance using timed urine collection in a subgroup of 160 patients showed that estimated GFR according to cystatin C was almost identical to measured creatinine clearance independent of NT-proBNP. Conversely, creatinine-based calculation using the MDRD Study formula underestimated GFR in patients from the low (12 to 238 pg/ml) and medium (241 to 990 pg/ml) NT-pro-BNP tertiles. In conclusion, in patients without severe heart failure, indicated by low serum NT-pro-BNP, estimation of GFR using creatinine-based formulas underestimated renal function. The known prognostic impact of cystatin C in cardiac patients might result from a strong correlation with NT-pro-BNP, as well as its superior ability to predict renal function in patients with and without heart failure.

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.

Silencing of cystatin M in metastatic oral cancer cell line MDA-686Ln by siRNA increases cysteine proteinases and legumain activities, cell proliferation and in vitro invasion

Cystatins are inhibitors of lysosomal cysteine proteinases. Cystatin M demonstrates more diverse tissue distribution, target specificity and biological function than other cystatins from the same family. We utilized small interference RNAs (siRNA) to silence cystatin M gene expression in a metastatic oral cancer cell line (MDA-686Ln) that expresses a high level of cystatin M. We tested four different siRNAs targeted to different sites of the cystatin M mRNA, and found three out of the four siRNAs were effective in suppressing cystatin M expression by >50% at both mRNA and protein levels, as measured by quantitative real-time RT-PCR and Western blotting. We used siRNA-#1, which demonstrated highest efficiency of silencing cystatin M, to evaluate the phenotypic outcome of silencing cystatin M in MDA-686Ln cells. Cystatin M inhibition significantly increased the enzymatic activities of cathepsins B and L and legumain while reducing cysteine protease inhibitor activity both in the media and intracellularly. MDA-686Ln cells treated with siRNA#1 demonstrated markedly increased proliferation rate, in vitro motility and Matrigel invasiveness. Collectively, our data show that silencing of cystatin M in tumor cells not only increases their invasion and motility via cysteine-proteinase-dependent pathways, but also renders them hyperproliferative through a currently unknown mechanism.

Expression analysis of cystatin C and M in laser-capture microdissectioned human breast cancer cells--a preliminary study

Cathepsins B and L, implicated in the progression of malignant tumors, are regulated by a family of endogenous inhibitors referred to as the cystatins. Cystatin M was identified by differential display as down-regulated gene in metastatic breast cancer cells. However, this finding has yet to be confirmed in clinical breast cancer specimens. Our objective is to examine the expression levels of cystatins C, M, and cathepsins B and L mRNA in breast cancer cells isolated by laser capture microdissection. The mRNA and protein levels of cathepsin B, L, and cystatin C and M in breast cancer specimens were determined utilizing laser capture microdissection/RT-PCR, Western blotting, and immunohistochemical methods. Expression levels of either cystatin M or C were not significantly different between lymph node-positive and -negative breast carcinomas. Increased expression levels of both cystatin M and C correlated significantly with larger tumor size. Cystatin M mRNA was detected by in situ hybridization in both primary and metastatic breast cancer cells. Our findings are at variance with a previous report proposing a metastasis suppressive function for cystatin M. Therefore, additional studies in a larger series with adequate follow-up are necessary to elucidate the biologic significance of cystatin M expression in breast cancer metastasis.

Inhibitory properties of cystatin F and its localization in U937 promonocyte cells

Cystatin F is a recently discovered type II cystatin expressed almost exclusively in immune cells. It is present intracellularly in lysosome-like vesicles, which suggests a potential role in regulating papain-like cathepsins involved in antigen presentation. Therefore, interactions of cystatin F with several of its potential targets, cathepsins F, K, V, S, H, X and C, were studied in vitro. Cystatin F tightly inhibited cathepsins F, K and V with Ki values ranging from 0.17 nM to 0.35 nM, whereas cathepsins S and H were inhibited with 100-fold lower affinities (Ki approximately 30 nM). The exopeptidases, cathepsins C and X were not inhibited by cystatin F. In order to investigate the biological significance of the inhibition data, the intracellular localization of cystatin F and its potential targets, cathepsins B, H, L, S, C and K, were studied by confocal microscopy in U937 promonocyte cells. Although vesicular staining was observed for all the enzymes, only cathepsins H and X were found to be colocalized with the inhibitor. This suggests that cystatin F in U937 cells may function as a regulatory inhibitor of proteolytic activity of cathepsin H or, more likely, as a protection against cathepsins misdirected to specific cystatin F containing endosomal/lysosomal vesicles. The finding that cystatin F was not colocalized with cystatin C suggests distinct functions for these two cysteine protease inhibitors in U937 cells.

Factors influencing serum cystatin C levels other than renal function and the impact on renal function measurement

BACKGROUND

It is well known that serum creatinine may be used as a marker of renal function only if taking into account factors that influence creatinine production, such as age, gender, and weight. Serum cystatin C has been proposed as a potentially superior marker than serum creatinine, because serum cystatin C level is believed to be produced at a constant rate and not to be affected by such factors. However, there are limited data on factors that may influence serum cystatin C levels, and there are limited data comparing cystatin C-based estimates of renal function with creatinine-based estimates that adjust for such factors, especially in individuals with normal, or mildly reduced, renal function.

METHODS

This was a cross-sectional study of 8058 inhabitants of the city of Groningen, The Netherlands, 28 to 75 years of age. Serum cystatin C and serum creatinine levels were measured, and creatinine clearance was determined from the average of two separate 24-hour urine collections. We performed multivariate analyses to identify factors independently associated with serum cystatin C levels after adjusting for creatinine clearance. Then, partial Spearman correlations were obtained after adjusting for factors that may influence serum cystatin C and creatinine levels. We also compared the goodness-of-fit (R(2)) of different multivariate linear regression models including serum cystatin C level and serum creatinine level for the outcome of creatinine clearance.

RESULTS

Older age, male gender, greater weight, greater height, current cigarette smoking, and higher serum C-reactive protein (CRP) levels were independently associated with higher serum cystatin C levels after adjusting for creatinine clearance. After adjusting for age, weight, and gender, the partial Spearman correlations between creatinine and, respectively, serum cystatin C level and serum creatinine level were -0.29 (P < 0.001) and -0.42 (P < 0.001), respectively. The R(2) values for serum cystatin C level and serum creatinine level adjusted for age, weight, and gender were 0.38 and 0.42, respectively. The addition of cigarette smoking and serum CRP levels did not improve the R(2) value for the multivariate serum cystatin C-based model.

CONCLUSION

Serum cystatin C appears to be influenced by factors other than renal function alone. In addition, we found no evidence that multivariate serum cystatin C-based estimates of renal function are superior to multivariate serum creatinine-based estimates.