Human cystatin C: a new biomarker of idiopathic pulmonary fibrosis?

Human cystatin C in fibrotic diseases

Human cystatin C (hCC), which has a pervasive distribution within body fluids and is ubiquitously expressed by numerous cells and tissues, is a highly potent extracellular inhibitor of cysteine proteases. Besides measurement of serum creatinine, which is the most widely used technique for appraising glomerular filtration rate (GFR), hCC has emerged as a relevant GFR biomarker, because its quantification in serum is less sensitive to interferences with factors such as age, muscle mass or diet. Moreover, there are growing body of evidence that hCC overexpression and/or oversecretion, which is primarily driven by TGF-β1, occur during fibrogenesis (cardiac, liver, oral, and lung fibrosis). Even though molecular mechanisms and signaling pathways governing the regulation of hCC remain to be deciphered more acutely, current data sustain that hCC expression relates to myofibrogenesis and that hCC could be a specific and valuable biomarker of fibrotic disease.

Potential biomarkers for diagnosis and disease evaluation of idiopathic pulmonary fibrosis

ABSTRACT

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive lung disease characterized by progressive lung fibrogenesis and histological features of usual interstitial pneumonia. IPF has a poor prognosis and presents a spectrum of disease courses ranging from slow evolving disease to rapid deterioration; thus, a differential diagnosis remains challenging. Several biomarkers have been identified to achieve a differential diagnosis; however, comprehensive reviews are lacking. This review summarizes over 100 biomarkers which can be divided into six categories according to their functions: differentially expressed biomarkers in the IPF compared to healthy controls; biomarkers distinguishing IPF from other types of interstitial lung disease; biomarkers differentiating acute exacerbation of IPF from stable disease; biomarkers predicting disease progression; biomarkers related to disease severity; and biomarkers related to treatment. Specimen used for the diagnosis of IPF included serum, bronchoalveolar lavage fluid, lung tissue, and sputum. IPF-specific biomarkers are of great clinical value for the differential diagnosis of IPF. Currently, the physiological measurements used to evaluate the occurrence of acute exacerbation, disease progression, and disease severity have limitations. Combining physiological measurements with biomarkers may increase the accuracy and sensitivity of diagnosis and disease evaluation of IPF. Most biomarkers described in this review are not routinely used in clinical practice. Future large-scale multicenter studies are required to design and validate suitable biomarker panels that have diagnostic utility for IPF.

Cystatin SN in type 2 inflammatory airway diseases

Cystatin SN, encoded by CST1, belongs to the type 2 (T2) cystatin protein superfamily. In the past decade, several publications have highlighted the association between cystatin SN and inflammatory airway diseases including chronic rhinosinusitis, rhinitis, asthma, chronic obstructive pulmonary disease, and chronic hypersensitivity pneumonitis. It is, therefore, crucial to understand the role of cystatin SN in the wider context of T2 inflammatory diseases. Here, we review the expression of cystatin SN in airway-related diseases with different endotypes. We also emphasize the physiological and pathological roles of cystatin SN. Physiologically, cystatin SN protects host tissues from destructive proteolysis by cysteine proteases present in the external environment or produced via internal dysregulated expression. Pathologically, the secretion of cystatin SN from airway epithelial cells initiates and amplifies T2 immunity and subsequently leads to disease. We further discuss the development of cystatin SN as a T2 immunity marker that can be monitored noninvasively and assist in airway disease management. The discovery, biology, and inhibition capability are also introduced to better understand the role of cystatin SN in airway diseases.

Therapeutic potential of targeting cathepsin S in pulmonary fibrosis

Cathepsin S (CTSS), a lysosomal protease, belongs to a family of cysteine cathepsin proteases that promote degradation of damaged proteins in the endolysosomal pathway. Aberrant CTSS expression and regulation are associated with the pathogenesis of several diseases, including lung diseases. CTSS overexpression causes a variety of pathological processes, including pulmonary fibrosis, with increased CTSS secretion and accelerated extracellular matrix remodeling. Compared to many other cysteine cathepsin family members, CTSS has unique features that it presents limited tissue expression and retains its enzymatic activity at a neutral pH, suggesting its decisive involvement in disease microenvironments. In this review, we investigated the role of CTSS in lung disease, exploring recent studies that have indicated that CTSS mediates fibrosis in unique ways, along with its structure, substrates, and distinct regulation. We also outlined examples of CTSS inhibitors in clinical and preclinical development and proposed CTSS as a potential therapeutic target for pulmonary fibrosis.

Cathepsin S: investigating an old player in lung disease pathogenesis, comorbidities, and potential therapeutics

Dysregulated expression and activity of cathepsin S (CTSS), a lysosomal protease and a member of the cysteine cathepsin protease family, is linked to the pathogenesis of multiple diseases, including a number of conditions affecting the lungs. Extracellular CTSS has potent elastase activity and by processing cytokines and host defense proteins, it also plays a role in the regulation of inflammation. CTSS has also been linked to G-coupled protein receptor activation and possesses an important intracellular role in major histocompatibility complex class II antigen presentation. Modulated CTSS activity is also associated with pulmonary disease comorbidities, such as cancer, cardiovascular disease, and diabetes. CTSS is expressed in a wide variety of immune cells and is biologically active at neutral pH. Herein, we review the significance of CTSS signaling in pulmonary diseases and associated comorbidities. We also discuss CTSS as a plausible therapeutic target and describe recent and current clinical trials examining CTSS inhibition as a means for treatment.

Curcumin inhibits the TGF-β1-dependent differentiation of lung fibroblasts via PPARγ-driven upregulation of cathepsins B and L

Pulmonary fibrosis is a progressive disease characterized by a widespread accumulation of myofibroblasts and extracellular matrix components. Growing evidences support that cysteine cathepsins, embracing cathepsin B (CatB) that affects TGF-β1-driven Smad pathway, along with their extracellular inhibitor cystatin C, participate in myofibrogenesis. Here we established that curcumin, a potent antifibrotic drug used in traditional Asian medicine, impaired the expression of both α-smooth muscle actin and mature TGF-β1 and inhibited the differentiation of human lung fibroblasts (CCD-19Lu cells). Curcumin induced a compelling upregulation of CatB and CatL. Conversely cystatin C was downregulated, which allowed the recovery of the peptidase activity of secreted cathepsins and the restoration of the proteolytic balance. Consistently, the amount of both insoluble and soluble type I collagen decreased, reaching levels similar to those observed for undifferentiated fibroblasts. The signaling pathways activated by curcumin were further examined. Curcumin triggered the expression of nuclear peroxisome proliferator-activated receptor γ (PPARγ). Contrariwise PPARγ inhibition, either by an antagonist (2-chloro-5-nitro-N-4-pyridinyl-benzamide) or by RNA silencing, restored TGF-β1-driven differentiation of curcumin-treated CCD-19Lu cells. PPARγ response element (PPRE)-like sequences were identified in the promoter regions of both CatB and CatL. Finally, we established that the transcriptional induction of CatB and CatL depends on the binding of PPARγ to PPRE sequences as a PPARγ/Retinoid X Receptor-α heterodimer.

The impaired proteases and anti-proteases balance in Idiopathic Pulmonary Fibrosis

Idiopathic Pulmonary Fibrosis (IPF) is a devastating chronic, progressive and irreversible disease that remains refractory to current therapies. Matrix metalloproteinases (MMPs) and their inhibitors, tissue inhibitors of MMPs (TIMPs), have been implicated in the development of pulmonary fibrosis since decades. Coagulation signalling deregulation, which influences several key inflammatory and fibro-proliferative responses, is also essential in IPF pathogenesis, and a growing body of evidence indicates that Protease-Activated Receptors (PARs) inhibition in IPF may be promising for future evaluation. Therefore, proteases and anti-proteases aroused great biomedical interest over the past years, owing to the identification of their potential roles in lung fibrosis. During these last decades, numerous other proteases and anti-proteases have been studied in lung fibrosis, such as matriptase, Human airway trypsin-like protease (HAT), Hepatocyte growth factor activator (HGFA)/HGFA activator inhibitor (HAI) system, Plasminogen activator inhibitor (PAI)-1, Protease nexine (PN)-1, cathepsins, calpains, and cystatin C. Herein, we provide a general overview of the proteases and anti-proteases unbalance during lung fibrogenesis and explore potential therapeutics for IPF.

Specifically Formed Corona on Silica Nanoparticles Enhances Transforming Growth Factor β1 Activity in Triggering Lung Fibrosis

A corona is a layer of macromolecules formed on a nanoparticle surface in vivo. It can substantially change the biological identity of nanomaterials and possibly trigger adverse responses from the body tissues. Dissecting the role of the corona in the development of a particular disease may provide profound insights for understanding toxicity of nanomaterials in general. In our present study, we explored the capability of different silica nanoparticles (SiNPs) to induce silicosis in the mouse lung and analyzed the composition of coronas formed on these particles. We found that SiNPs of certain size and surface chemistry could specifically recruit transforming growth factor β1 (TGF-β1) into their corona, which subsequently induces the development of lung fibrosis. Once embedded into the corona on SiNPs, TGF-β1 was remarkably more stable than in its free form, and its fibrosis-triggering activity was significantly prolonged. Our study meaningfully demonstrates that a specific corona component on a certain nanoparticle could initiate a particular pathogenic process in a clinically relevant disease model. Our findings may shed light on the understanding of molecular mechanisms of human health risks correlated with exposure to small-scale substances.

Discordance in cathepsin B and cystatin C expressions in bronchoalveolar fluids between murine bleomycin-induced fibrosis and human idiopathic fibrosis

The activity of cysteine cathepsin B increased markedly in lung homogenates and in bronchoalveolar lavage fluids (BALF) of the mouse model of bleomycin-induced lung fibrosis after 14 days of challenge. In contrast the level of the cysteine cathepsin inhibitor cystatin C was unaffected in BALF of wild-type and cathepsin B-deficient mice. Therefore, murine cystatin C is not a reliable marker of fibrosis during bleomycin-induced lung fibrosis. Current data are in sharp contrast to previous analysis carried on human BALF from patients with idiopathic pulmonary fibrosis, for which the level of cathepsin B remained unchanged while cystatin C was significantly increased.

Six-SOMAmer Index Relating to Immune, Protease and Angiogenic Functions Predicts Progression in IPF

RATIONALE

Biomarkers in easily accessible compartments like peripheral blood that can predict disease progression in idiopathic pulmonary fibrosis (IPF) would be clinically useful regarding clinical trial participation or treatment decisions for patients. In this study, we used unbiased proteomics to identify relevant disease progression biomarkers in IPF.

METHODS

Plasma from IPF patients was measured using an 1129 analyte slow off-rate modified aptamer (SOMAmer) array, and patient outcomes were followed over the next 80 weeks. Receiver operating characteristic (ROC) curves evaluated sensitivity and specificity for levels of each biomarker and estimated area under the curve (AUC) when prognostic biomarker thresholds were used to predict disease progression. Both logistic and Cox regression models advised biomarker selection for a composite disease progression index; index biomarkers were weighted via expected progression-free days lost during follow-up with a biomarker on the unfavorable side of the threshold.

RESULTS

A six-analyte index, scaled 0 to 11, composed of markers of immune function, proteolysis and angiogenesis [high levels of ficolin-2 (FCN2), cathepsin-S (Cath-S), legumain (LGMN) and soluble vascular endothelial growth factor receptor 2 (VEGFsR2), but low levels of inducible T cell costimulator (ICOS) or trypsin 3 (TRY3)] predicted better progression-free survival in IPF with a ROC AUC of 0.91. An index score ≥ 3 (group ≥ 2) was strongly associated with IPF progression after adjustment for age, gender, smoking status, immunomodulation, forced vital capacity % predicted and diffusing capacity for carbon monoxide % predicted (HR 16.8, 95% CI 2.2-126.7, P = 0.006).

CONCLUSION

This index, derived from the largest proteomic analysis of IPF plasma samples to date, could be useful for clinical decision making in IPF, and the identified analytes suggest biological processes that may promote disease progression.

Antimicrobial Peptide LL-37 Is Both a Substrate of Cathepsins S and K and a Selective Inhibitor of Cathepsin L

Lung cysteine cathepsins B, K, L, and S contribute to physiological and pathological processes including degradation of antimicrobial peptides/proteins (AMPs) such as surfactant protein SP-A, lactoferrin, secretory leukocyte peptidase inhibitor, and beta-defensins-2 and -3. Substantial amounts of uncleaved LL-37, a 37-mer cationic AMP, were observed in the sputum of patients with cystic fibrosis (CF). Nevertheless LL-37 was degraded after prolonged incubation in CF sputum, and the hydrolysis was blocked by E-64, a selective inhibitor of cysteine proteases. Cathepsins K and S, expressed in human alveolar macrophages, thoroughly hydrolyzed LL-37 in vitro, whereas it competitively inhibited cathepsin L (Ki = 150 nM). Cleavage of LL-37 by cathepsins S and K impaired its antimicrobial activity against Pseudomonas aeruginosa and Staphylococcus aureus, in a time- and concentration-dependent manner. The exchange of residues 67 and 205 in the S2 pockets of cathepsins L (Leu67Tyr/Ala205Leu) and K (Tyr67Leu/Leu205Ala) switched the specificity of these mutants toward LL-37. Molecular modeling suggested that LL-37 interacted with the active site of cathepsin L in both forward (i.e., substrate-like) and reverse orientations with similar binding energies. Our data support the hypothesis that cysteine cathepsins modulate the innate immunity response by degrading distinct and representative members of the AMP family.

Quantitative proteomics of bronchoalveolar lavage fluid in idiopathic pulmonary fibrosis

The proteomic analysis of bronchoalveolar lavage fluid (BALF) can give insight into pulmonary disease pathology and response to therapy. Here, we describe the first gel-free quantitative analysis of BALF in idiopathic pulmonary fibrosis (IPF), a chronic and fatal scarring lung disease. We utilized two-dimensional reversed-phase liquid chromatography and ion-mobility-assisted data-independent acquisition (HDMSE) for quantitation of >1000 proteins in immunodepleted BALF from the right middle and lower lobes of normal controls and patients with IPF. Among the analytes that were increased in IPF were well-described mediators of pulmonary fibrosis (osteopontin, MMP7, CXCL7, CCL18), eosinophil- and neutrophil-derived proteins, and proteins associated with fibroblast foci. For additional discovery and targeted validation, BALF was also screened by multiple reaction monitoring (MRM), using the JPT Cytokine SpikeMix library of >400 stable isotope-labeled peptides. A refined MRM assay confirmed the robust expression of osteopontin, and demonstrated, for the first time, upregulation of the pro-fibrotic cytokine, CCL24, in BALF in IPF. These results show the utility of BALF proteomics for the molecular profiling of fibrotic lung diseases and the targeted quantitation of soluble markers of IPF. More generally, this study addresses critical quality control measures that should be widely applicable to BALF profiling in pulmonary disease.

Regulation of TGF-β1-driven differentiation of human lung fibroblasts: emerging roles of cathepsin B and cystatin C

Lung matrix homeostasis partly depends on the fine regulation of proteolytic activities. We examined the expression of human cysteine cathepsins (Cats) and their relative contribution to TGF-β1-induced fibroblast differentiation into myofibroblasts. Assays were conducted using both primary fibroblasts obtained from patients with idiopathic pulmonary fibrosis and human lung CCD-19Lu fibroblasts. Pharmacological inhibition and genetic silencing of Cat B diminished α-smooth muscle actin expression, delayed fibroblast differentiation, and led to an accumulation of intracellular 50-kDa TGF-β1. Moreover, the addition of Cat B generated a 25-kDa mature form of TGF-β1 in Cat B siRNA-pretreated lysates. Inhibition of Cat B decreased Smad 2/3 phosphorylation but had no effect on p38 MAPK and JNK phosphorylation, indicating that Cat B mostly disturbs TGF-β1-driven canonical Smad signaling pathway. Although mRNA expression of cystatin C was stable, its secretion, which was inhibited by brefeldin A, increased during TGF-β1-induced differentiation of idiopathic pulmonary fibrosis and CCD-19Lu fibroblasts. In addition, cystatin C participated in the control of extracellular Cats, because its gene silencing restored their proteolytic activities. These data support the notion that Cat B participates in lung myofibrogenesis as suggested for stellate cells during liver fibrosis. Moreover, we propose that TGF-β1 promotes fibrosis by driving the effective cystatin C-dependent inhibition of extracellular matrix-degrading Cats.