Monitoring Human Neutrophil Activation by a Proteinase 3 Near-Infrared Fluorescence Substrate-Based Probe.

A near-infrared fluorescent (NIRF) substrate-based probe (SBP) was conceived to monitor secreted human proteinase 3 (hPR3) activity. This probe, called pro3-SBP, is shaped by a fused peptide hairpin loop structure, which associates a hPR3 recognition domain (Val-Ala-Asp-Nva-Ala-Asp-Tyr-Gln, where Nva is norvaline) and an electrostatic zipper (consisting of complementary polyanionic (d-Glu)5 and polycationic (d-Arg)5 sequences) in close vicinity of the N- and C-terminal FRET couple (fluorescent donor, sulfoCy5.5; dark quencher, QSY21). Besides its subsequent stability, no intermolecular fluorescence quenching was detected following its complete hydrolysis by hPR3, advocating that pro3-SBP could further afford unbiased imaging. Pro3-SBP was specifically hydrolyzed by hPR3 (kcat/Km= 440 000 ± 5500 M-1·s-1) and displayed a sensitive detection threshold for hPR3 (subnanomolar concentration range), while neutrophil elastase showed a weaker potency. Conversely, pro3-SBP was not cleaved by cathepsin G. Pro3-SBP was successfully hydrolyzed by conditioned media of activated human neutrophils but not by quiescent neutrophils. Moreover, unlike unstimulated neutrophils, a strong NIRF signal was specifically detected by confocal microscopy following neutrophil ionomycin-induced degranulation. Fluorescence release was abolished in the presence of a selective hPR3 inhibitor, indicating that pro3-SBP is selectively cleaved by extracellular hPR3. Taken together, the present data support that pro3-SBP could be a convenient tool, allowing straightforward monitoring of human neutrophil activation.

Regulation of the Proteolytic Activity of Cysteine Cathepsins by Oxidants.

Besides their primary involvement in the recycling and degradation of proteins in endo-lysosomal compartments and also in specialized biological functions, cysteine cathepsins are pivotal proteolytic contributors of various deleterious diseases. While the molecular mechanisms of regulation via their natural inhibitors have been exhaustively studied, less is currently known about how their enzymatic activity is modulated during the redox imbalance associated with oxidative stress and their exposure resistance to oxidants. More specifically, there is only patchy information on the regulation of lung cysteine cathepsins, while the respiratory system is directly exposed to countless exogenous oxidants contained in dust, tobacco, combustion fumes, and industrial or domestic particles. Papain-like enzymes (clan CA, family C1, subfamily C1A) encompass a conserved catalytic thiolate-imidazolium pair (Cys25-His159) in their active site. Although the sulfhydryl group (with a low acidic pKa) is a potent nucleophile highly susceptible to chemical modifications, some cysteine cathepsins reveal an unanticipated resistance to oxidative stress. Besides an introductory chapter and peculiar attention to lung cysteine cathepsins, the purpose of this review is to afford a concise update of the current knowledge on molecular mechanisms associated with the regulation of cysteine cathepsins by redox balance and by oxidants (e.g., Michael acceptors, reactive oxygen, and nitrogen species).

Cigarette smoke induces overexpression of active human cathepsin S in lungs from current smokers with or without COPD.

Cigarette smoking has marked effects on lung tissue, including induction of oxidative stress, inflammatory cell recruitment, and a protease/antiprotease imbalance. These effects contribute to tissue remodeling and destruction resulting in loss of lung function in chronic obstructive pulmonary disease (COPD) patients. Cathepsin S (CatS) is a cysteine protease that is involved in the remodeling/degradation of connective tissue and basement membrane. Aberrant expression or activity of CatS has been implicated in a variety of diseases, including arthritis, cancer, cardiovascular, and lung diseases. However, little is known about the effect of cigarette smoking on both CatS expression and activity, as well as its role in smoking-related lung diseases. Here, we evaluated the expression and activity of human CatS in lung tissues from never-smokers and smokers with or without COPD. Despite the presence of an oxidizing environment, CatS expression and activity were significantly higher in current smokers (both non-COPD and COPD) compared with never-smokers, and correlated positively with smoking history. Moreover, we found that the exposure of primary human bronchial epithelial cells to cigarette smoke extract triggered the activation of P2X7 receptors, which in turns drives CatS upregulation. The present data suggest that excessive CatS expression and activity contribute, beside other proteases, to the deleterious effects of cigarette smoke on pulmonary homeostasis.

Regulation of TGF-ß1-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-ß1-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-ß1. Moreover, the addition of Cat B generated a 25-kDa mature form of TGF-ß1 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-ß1-driven canonical Smad signaling pathway. Although mRNA expression of cystatin C was stable, its secretion, which was inhibited by brefeldin A, increased during TGF-ß1-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-ß1 promotes fibrosis by driving the effective cystatin C-dependent inhibition of extracellular matrix-degrading Cats.

Active site labeling of cysteine cathepsins by a straightforward diazomethylketone probe derived from the N-terminus of human cystatin C.

We designed a straightforward biotinylated probe using the N-terminal substrate-like region of the inhibitory site of human cystatin C as a scaffold, linked to the thiol-specific reagent diazomethylketone group as a covalent warhead (i.e. Biot-(PEG)2-Ahx-LeuValGly-DMK). The irreversible activity-based probe bound readily to cysteine cathepsins B, L, S and K. Moreover affinity labeling is sensitive since active cathepsins were detected in the nM range using an ExtrAvidin-peroxidase conjugate for disclosure. Biot-(PEG)2-Ahx-LeuValGly-DMK allowed a slightly more pronounced labeling for cathepsin S with a compelling second-order rate constant for association (kass = 2,320,000 M(-1) s(-1)). Labeling of the active site is dose-dependent as observed using 6-cyclohexylamine-4-piperazinyl-1,3,5-triazine-2-carbonitrile, as competitive inhibitor of cathepsins. Finally we showed that Biot-(PEG)2-Ahx-LeuValGly-DMK may be a simple and convenient tool to label secreted and intracellular active cathepsins using a myelomonocytic cell line (THP-1 cells) as model.

Cysteine cathepsins and cystatins: from ancillary tasks to prominent status in lung diseases.

Human cysteine cathepsins (family C1, clan CA) have long been regarded as ubiquitous household enzymes, primarily involved in the recycling and degradation of proteins in lysosomes. This opinion has changed considerably during recent decades, however, with the demonstration of their involvement in various physiological processes. A growing body of evidence supports the theory that cathepsins play specific functions in lung homeostasis and pathophysiological events such as asthma, lung fibrosis (including idiopathic pulmonary fibrosis), chronic obstructive pulmonary disease (embracing emphysema and chronic bronchitis), silicosis, bronchopulmonary dysplasia or tumor invasion. The objective of this review is to provide an update on the current knowledge of the role of these enzymes in the lung. Particular attention has been paid to the understanding of the role of these proteases and their natural inhibitors, cystatins (family I25, clan IH), in TGF-ß1-driven fibrotic processes with an emphasis on lung fibrosis.

Short-term exposure to cigarette smoke upregulates cathepsin S and alters expression of tight junction ZO-1.

A long-term exposure to cigarette smoke (CS) alters the integrity of airway epithelial barrier, contributes to lung dysfunction, and elicits the expression and activity of lung cathepsin S (CatS), a cysteine protease that participates in the remodeling of connective tissue and cell junctions. Here, we observed that a short-term (4 days) exposure of mice to CS increased the expression and activity of CatS, while the expression level of zonula occludens 1 (ZO-1), an epithelial tight junction protein that stabilizes barrier assembly, was reduced in lung tissue lysates. Present data support that proteolytically active CatS may contribute to the defect of ZO-1 in CS-exposed mice.

The Interplay of Glycosaminoglycans and Cysteine Cathepsins in Mucopolysaccharidosis.

Mucopolysaccharidosis (MPS) consists of a group of inherited lysosomal storage disorders that are caused by a defect of certain enzymes that participate in the metabolism of glycosaminoglycans (GAGs). The abnormal accumulation of GAGs leads to progressive dysfunctions in various tissues and organs during childhood, contributing to premature death. As the current therapies are limited and inefficient, exploring the molecular mechanisms of the pathology is thus required to address the unmet needs of MPS patients to improve their quality of life. Lysosomal cysteine cathepsins are a family of proteases that play key roles in numerous physiological processes. Dysregulation of cysteine cathepsins expression and activity can be frequently observed in many human diseases, including MPS. This review summarizes the basic knowledge on MPS disorders and their current management and focuses on GAGs and cysteine cathepsins expression in MPS, as well their interplay, which may lead to the development of MPS-associated disorders.

Cysteine cathepsins S and L modulate anti-angiogenic activities of human endostatin.

Human endostatin, a potent anti-angiogenic protein, is generated by release of the C terminus of collagen XVIII. Here, we propose that cysteine cathepsins are involved in both the liberation and activation of bioactive endostatin fragments, thus regulating their anti-angiogenic properties. Cathepsins B, S, and L efficiently cleaved in vitro FRET peptides that encompass the hinge region corresponding to the N terminus of endostatin. However, in human umbilical vein endothelial cell-based assays, silencing of cathepsins S and L, but not cathepsin B, impaired the generation of the ∼22-kDa endostatin species. Moreover, cathepsins L and S released two peptides from endostatin with increased angiostatic properties and both encompassing the NGR sequence, a vasculature homing motif. The G10T peptide (residues 1455-1464: collagen XVIII numbering) displayed compelling anti-proliferative (EC(50) = 0.23 nm) and proapoptotic properties. G10T inhibited aminopeptidase N (APN/CD13) and reduced tube formation of endothelial cells in a manner similar to bestatin. Combination of G10T with bestatin resulted in no further increase in anti-angiogenic activity. Taken together, these data suggest that endostatin-derived peptides may represent novel molecular links between cathepsins and APN/CD13 in the regulation of angiogenesis.

Cathepsin V: Molecular characteristics and significance in health and disease.

Human cysteine cathepsins form a family of eleven proteases (B, C, F, H, K, L, O, S, V, W, X/Z) that play important roles in a considerable number of biological and pathophysiological processes. Among them, cathepsin V, also known as cathepsin L2, is a lysosomal enzyme, which is mainly expressed in cornea, thymus, heart, brain, and skin. Cathepsin V is a multifunctional endopeptidase that is involved in both the release of antigenic peptides and the maturation of MHC class II molecules and participates in the turnover of elastin fibrils as well in the cleavage of intra- and extra-cellular substrates. Moreover, there is increasing evidence that cathepsin V may contribute to the progression of diverse diseases, due to the dysregulation of its expression and/or its activity. For instance, increased expression of cathepsin V is closely correlated with malignancies (breast cancer, squamous cell carcinoma, or colorectal cancer) as well vascular disorders (atherosclerosis, aortic aneurysm, hypertension) being the most prominent examples. This review aims to shed light on current knowledge on molecular aspects of cathepsin V (genomic organization, protein structure, substrate specificity), its regulation by protein and non-protein inhibitors as well to summarize its expression (tissue and cellular distribution). Then the core biological and pathophysiological roles of cathepsin V will be depicted, raising the question of its interest as a valuable target that can open up pioneering therapeutic avenues.

Cleavage of Occludin by Cigarette Smoke-Elicited Cathepsin S Increases Permeability of Lung Epithelial Cells.

BACKGROUND: Chronic obstructive pulmonary disease (COPD) is an irreversible disease mainly caused by smoking. COPD is characterized by emphysema and chronic bronchitis associated with enhanced epithelial permeability. HYPOTHESIS: Lung biopsies from smokers revealed a decreased expression level of occludin, which is a protein involved in the cohesion of epithelial tight junctions. Moreover, the occludin level correlated negatively with smoking history (pack-years), COPD grades, and cathepsin S (CatS) activity. Thus, we examined whether CatS could participate in the modulation of the integrity of human lung epithelial barriers. METHODS AND RESULTS: Cigarette smoke extract (CSE) triggered the upregulation of CatS by THP-1 macrophages through the mTOR/TFEB signaling pathway. In a co-culture model, following the exposure of macrophages to CSE, an enhanced level of permeability of lung epithelial (16HBE and NHBE) cells towards FITC-Dextran was observed, which was associated with a decrease in occludin level. Similar results were obtained using 16HBE and NHBE cells cultured at the air-liquid interface. The treatment of THP-1 macrophages by CatS siRNAs or by a pharmacological inhibitor restored the barrier function of epithelial cells, suggesting that cigarette smoke-elicited CatS induced an alteration of epithelial integrity via the proteolytic injury of occludin. CONCLUSIONS: Alongside its noteworthy resistance to oxidative stress induced by cigarette smoke oxidants and its deleterious elastin-degrading potency, CatS may also have a detrimental effect on the barrier function of epithelial cells through the cleavage of occludin. The obtained data emphasize the emerging role of CatS in smoking-related lung diseases and strengthen the relevance of targeting CatS in the treatment of emphysema and COPD.

Binding of heparan sulfate to human cystatin C modulates inhibition of cathepsin L: Putative consequences in mucopolysaccharidosis.

Mucopolysaccharidoses (MPS) are a group of rare lysosomal storage diseases characterized by glycosaminoglycan (GAG) accumulation causing progressive multi-organs dysfunction and ultimately severe cardio-respiratory damages. Human cystatin C (hCC), a potent inhibitor of cysteine cathepsins, plays an important role in respiratory diseases. However, its regulation remained unknown in MPS. Herein, elevated hCC levels were measured in respiratory specimens from MPS-I, -II, and -III patients and were significantly correlated with severe respiratory symptoms (rs = 0.7173). Heparan sulfate (HS), a prominent GAG, dampened its inhibitory activity toward cathepsin L in a dose-dependent manner. HS and HS-oligosaccharides bound tightly hCC, in combination with a secondary structure rearrangement. Molecular modeling studies identified three HS binding regions in hCC, including the N-terminus, which is crucial in the inhibition of cathepsins. Impairment of inhibitory potential of hCC may reflect abnormal regulation of proteolytic activity of cathepsin L in lung, ultimately contributing to the severity of MPS.

SLPI and trappin-2 as therapeutic agents to target airway serine proteases in inflammatory lung diseases: current and future directions.

It is now clear that NSPs (neutrophil serine proteases), including elastase, Pr3 (proteinase 3) and CatG (cathepsin G) are major pathogenic determinants in chronic inflammatory disorders of the lungs. Two unglycosylated natural protease inhibitors, SLPI (secretory leucocyte protease inhibitor) and elafin, and its precursor trappin-2 that are found in the lungs, have therapeutic potential for reducing the protease-induced inflammatory response. This review examines the multifaceted roles of SLPI and elafin/trappin-2 in the context of their possible use as inhaled drugs for treating chronic lung diseases such as CF (cystic fibrosis) and COPD (chronic obstructive pulmonary disease).

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.

Upregulation of gut cathepsin L during Eimeria tenella infection.

Coccidiosis is a disease caused by Eimeria, which represents the first parasitic disease in poultry farming. Among them, E. tenella is a virulent species which specifically colonizes the caecum. The inflammatory response to infection is associated to numerous host proteases including cysteine cathepsins that can be deleterious for tissue and innate immunity integrity. Here, germ-free and conventional chickens were used as models to find out whether the microbiota could modify the intestinal expression of host cysteine cathepsins during coccidiosis. The basal caecal peptidase activity primarily relies on host proteases rather than proteases from the commensal flora. While mRNA levels of E. tenella cathepsins B and L remained unchanged in germ-free and conventional broilers, an overall increase in endopeptidase activity of cysteine cathepsins was found in E. tenella-infected caeca in both experimental models (P < 0.005). A significant decrease in avian cystatin C transcription was also observed in infected conventional, but not in infected germ-free broilers. Despite an unchanged mRNA level of avian cathepsin L (CatL), its protein expression raised following infection, in parallel with an increased transcription of antimicrobial ß-defensins (AvBD1, AvBD2, AvBD4, AvBD6, and AvBD7). Taken together, data support that host CatL is post-translationally upregulated during E. tenella infection, and thus may be involved in the alteration of the gut proteolytic balance. Furthermore, CatL may participate to inflammation occurring during coccidiosis through its known ability to proteolytically inactivates up-regulated avian ß-defensins that are key molecules of innate immunity.

The abnormal accumulation of heparan sulfate in patients with mucopolysaccharidosis prevents the elastolytic activity of cathepsin V.

Mucopolysaccharidosis (MPS) are rare inherited diseases characterized by accumulation of lysosomal glycosaminoglycans, including heparan sulfate (HS). Patients exhibit progressive multi-visceral dysfunction and shortened lifespan mainly due to a severe cardiac/respiratory decline. Cathepsin V (CatV) is a potent elastolytic protease implicated in extracellular matrix (ECM) remodeling. Whether CatV is inactivated by HS in lungs from MPS patients remained unknown. Herein, CatV colocalized with HS in MPS bronchial epithelial cells. HS level correlated positively with the severity of respiratory symptoms and negatively to the overall endopeptidase activity of cysteine cathepsins. HS bound tightly to CatV and impaired its activity. Withdrawal of HS by glycosidases preserved exogenous CatV activity, while addition of Surfen, a HS antagonist, restored elastolytic CatV-like activity in MPS samples. Our data suggest that the pathophysiological accumulation of HS may be deleterious for CatV-mediated ECM remodeling and for lung tissue homeostasis, thus contributing to respiratory disorders associated to MPS diseases.

Oxidation of cathepsin S by major chemicals of cigarette smoke.

Lung cysteine cathepsin S (CatS) that is a potent elastase plays a deleterious role in alveolar remodeling during smoke-induced emphysema. Despite the presence of a reactive nucleophilic cysteine (Cys25) within its active site, most of its elastinolytic activity is preserved after exposure to cigarette smoke extract (CSE), a major source of sulfhydryl oxidants. This result led us to decipher CatS resistance to major and representative CSE oxidants: hydrogen peroxide, formaldehyde, acrolein and peroxynitrite. CatS was inactivated by hydrogen peroxide, peroxynitrite and acrolein in a time- and dose-dependent manner, while formaldehyde was a weaker oxidant. Hydrogen peroxide, but not CSE, formaldehyde, and peroxynitrite impaired the autocatalytic maturation of pro-CatS, whereas acrolein prevented the formation of mature CatS without hindering the initial step of the two-step autocatalytic process. Far-UV CD spectra analysis supported that oxidation by CSE and hydrogen peroxide did not led to a structural alteration of CatS, despite a notable increase of protein carbonylation, a major hallmark of oxidative damage. Evaluation of the oxidation status of Cys25 by specific biotinylated redox sensing probes suggested the formation of sulfenic acid followed by a slower conversion to sulfinic acid after incubation with hydrogen peroxide. Addition of reducing reagents (dithiothreitol, glutathione and N-acetyl cysteine) led to a partial recovery of CatS activity following incubation with CSE, hydrogen peroxide and peroxynitrite. Current results provide some mechanistic evidence of CatS stability and activity in the presence of CSE, supporting its harmful contribution to the pathophysiology of emphysema.

Rat cathepsin K: Enzymatic specificity and regulation of its collagenolytic activity.

Human cathepsin K (hCatK), which is highly expressed in osteoclasts, has the noteworthy ability to cleave type I and II collagens in their helical domain. Its collagenase potency depends strictly on the formation of an oligomeric complex with chondroitin 4-sulfate (C4-S). Accordingly, hCatK is a pivotal protease involved in bone resorption and is an attractive target for the treatment of osteoporosis. As rat is a common animal model for the evaluation of hCatK inhibitors, we conducted a comparative analysis of rat CatK (rCatK) and hCatK, which share a high degree of identity (88%) and similarity (93%). The pH activity profile of both enzymes displayed a similar bell-shaped curve (optimal pH: 6.4). Presence of Ser134 and Val160 in the S2 pocket of rCatK instead of Ala and Leu residues, respectively, in hCatK, led to a weaker peptidase activity, as observed for mouse CatK. Also, regardless of the presence of C4-S, rCatK cleaved in the nonhelical telopeptide regions of both type I (tail) and type II (articular joint) rat collagens. Structure-based computational analyses (electrostatic potential, molecular docking, molecular dynamics, free energy calculations) sustained that the C4-S mediated collagenolytic activity of rCatK obeys distinct molecular interactions from those of hCatK. Additionally, T-kininogen (a.k.a. thiostatin), a unique rat serum acute phase molecule, acted as a tight-binding inhibitor of hCatK (Ki = 0.11 ± 0.05 nM). Taken into account the increase of T-Kininogen level in inflamed rat sera, this may raise the question of the appropriateness to evaluate pharmacological hCatK inhibitors in this peculiar animal model.

Combined targeting of interleukin-6 and vascular endothelial growth factor potently inhibits glioma growth and invasiveness.

Interleukin-6 (IL6) and vascular endothelial growth factor (VEGFA) are abundantly produced by glioma cells and contribute to malignancy by promoting angiogenesis, cell proliferation and resistance to apoptosis. We compared the effect of inhibiting IL6 and VEGF on U87-derived experimental glioma grown on the chick chorio-allantoic membrane (CAM) or in the brain of xenografted mice. Tumor growth was monitored by biomicroscopy and immunohistology. In vitro, IL6 knockdown had no effect on proliferation but substantially enhanced invasion. In the CAM experimental glioma, IL6 or VEGF knockdown reduced growth and vascularization of the tumors with a comparable efficiency, but increased invasion of residual tumor cells. In contrast, combined IL6/VEGF knockdown not only showed enhanced reduction of tumor growth and angiogenesis but also significantly prevented invasion of residual tumor cells. In mice, combining IL6 knockdown and Avastin treatment completely abrogated tumor development and infiltration. Molecular response of tumor cells to single or combined treatment was studied by transcriptomic profiling. Many cell cycle promoting genes and chromatin components were silenced in the double knockdown. In addition, specific migratory signatures detected in tumors under single IL6 or VEGF knockdown were partially erased in combined IL6/VEGF knockdown tumors. Our results show that treatment with a combination of IL6 and VEGF inhibitors brings synergistic antitumoral benefit and reduces global activity of major pathways of cell survival, proliferation and invasiveness in remaining tumor cells that may be induced by using VEGF or IL6 inhibitors alone.

Curcumin inhibits the TGF-ß1-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-ß1-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-ß1 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-ß1-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.