Recombinant cathepsin S propeptide attenuates cell invasion by inhibition of cathepsin L-like proteases in tumor microenvironment

Rational design of humanized antibody inhibitors for cathepsin S

Cathepsin S (CTSS) is involved in pathogenesis of many human diseases. Inhibitors blocking its protease activity hold therapeutic potential. In comparison to small-molecule inhibitors, monoclonal antibodies capable of inhibiting CTSS enzymatic activity may possess advantageous pharmacological properties. Here we designed and produced inhibitory antibodies targeting human CTSS by genetically fusing the propeptide of procathepsin S (proCTSS) with antibodies in clinic. The resulting antibody fusions in full-length or fragment antigen-binding format could be stably expressed and potently inhibit CTSS proteolytic activity in high specificity. These fusion antibodies not only demonstrate a new approach for facile synthesis of antibody inhibitors against CTSS, but also represent novel anti-CTSS therapeutic candidates.

Evaluation of variable new antigen receptors (vNARs) as a novel cathepsin S (CTSS) targeting strategy

Aberrant activity of the cysteine protease Cathepsin S (CTSS) has been implicated across a wide range of pathologies. Notably in cancer, CTSS has been shown to promote tumour progression, primarily through facilitating invasion and migration of tumour cells and augmenting angiogenesis. Whilst an attractive therapeutic target, more efficacious CTSS inhibitors are required. Here, we investigated the potential application of Variable New Antigen Receptors (vNARs) as a novel inhibitory strategy. A panel of potential vNAR binders were identified following a phage display panning process against human recombinant proCTSS. These were subsequently expressed, purified and binding affinity confirmed by ELISA and SPR based approaches. Selected lead clones were taken forward and were shown to inhibit CTSS activity in recombinant enzyme activity assays. Further assessment demonstrated that our lead clones functioned by a novel inhibitory mechanism, by preventing the activation of proCTSS to the mature enzyme. Moreover, using an intrabody approach, we exhibited the ability to express these clones intracellularly and inhibit CTSS activity whilst lead clones were also noted to impede cell invasion in a tumour cell invasion assay. Collectively, these findings illustrate a novel mechanistic approach for inhibiting CTSS activity, with anti-CTSS vNAR clones possessing therapeutic potential in combating deleterious CTSS activity. Furthermore, this study exemplifies the potential of vNARs in targeting intracellular proteins, opening a range of previously "undruggable" targets for biologic-based therapy.

Cathepsin S (CTSS) activity in health and disease - A treasure trove of untapped clinical potential

Amongst the lysosomal cysteine cathepsin family of proteases, cathepsin S (CTSS) holds particular interest due to distinctive properties including a normal restricted expression profile, inducible upregulation and activity at a broad pH range. Consequently, while CTSS is well-established as a member of the proteolytic cocktail within the lysosome, degrading unwanted and damaged proteins, it has increasingly been shown to mediate a number of distinct, more selective roles including antigen processing and antigen presentation, and cleavage of substrates both intra and extracellularly. Increasingly, aberrant CTSS expression has been demonstrated in a variety of conditions and disease states, marking it out as both a biomarker and potential therapeutic target. This review seeks to contextualise CTSS within the cysteine cathepsin family before providing an overview of the broad range of pathologies in which roles for CTSS have been identified. Additionally, current clinical progress towards specific inhibitors is detailed, updating the position of the field in exploiting this most unique of proteases.

An efficient and concise synthesis of a selective small molecule non-peptide inhibitor of cathepsin L: KGP94

KGP94 is a potent, selective, and competitive inhibitor of the lysosomal endopeptidase enzyme (Cathepsin L) currently in preclinical trials for the treatment of metastatic cancer, which is a leading cause of cancer-associated death. Herein, we report two new synthetic routes for synthesizing the target compound through four consecutive steps, using a Weinreb amide approach starting from a common 3-bromobenzoyl chloride. A key step in the approach is a coupling reaction of a readily available Grignard reagent with amide 4 to produce 6, a previously unreported coupling pattern. These new strategies offer an efficient and alternative approach to synthesis of target compound with an excellent overall yield.

A humanized antibody inhibitor for cathepsin L

Cathepsin L (CTSL) is a cysteine protease involved in a variety of physiological and pathological processes. Potent inhibitors against CTSL have long been sought for drug development. Due to insufficient specificity and suboptimal pharmacological properties for current CTSL inhibitors, novel agents are still required for selectively blocking CTSL activity. Here we generated a humanized antibody inhibitor of CTSL by genetically fusing the inhibitory propeptide of procathepsin L to the N-terminus of the light chain of a humanized antibody. The resulting antibody fusion could be stably expressed and displays highly potent inhibition activity and specificity toward CTSL. This work demonstrates a new approach for the rapid generation of antibody inhibitors of CTSL. It can possibly be extended to create inhibitory antibodies targeting other cathepsin proteases, providing novel research and therapeutic tools.

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.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

A bioavailable cathepsin S nitrile inhibitor abrogates tumor development

Background

Cathepsin S has been implicated in a variety of malignancies with genetic ablation studies demonstrating a key role in tumor invasion and neo-angiogenesis. Thus, the application of cathepsin S inhibitors may have clinical utility in the treatment of cancer. In this investigation, we applied a cell-permeable dipeptidyl nitrile inhibitor of cathepsin S, originally developed to target cathepsin S in inflammatory diseases, in both in vitro and in vivo tumor models.

Methods

Validation of cathepsin S selectivity was carried out by assaying fluorogenic substrate turnover using recombinant cathepsin protease. Complete kinetic analysis was carried out and true K_i values calculated. Abrogation of tumour invasion using murine MC38 and human MCF7 cell lines were carried out in vitro using a transwell migration assay. Effect on endothelial tube formation was evaluated using primary HUVEC cells. The effect of inhibitor in vivo on MC38 and MCF7 tumor progression was evaluated using cells propagated in C57BL/6 and BALB/c mice respectively. Subsequent immunohistochemical staining of proliferation (Ki67) and apoptosis (TUNEL) was carried out on MCF7 tumors.

Results

We confirmed that this inhibitor was able to selectively target cathepsin S over family members K, V, L and B. The inhibitor also significantly reduced MC38 and MCF7 cell invasion and furthermore, significantly reduced HUVEC endothelial tubule formation in vitro. In vivo analysis revealed that the compound could significantly reduce tumor volume in murine MC38 syngeneic and MCF7 xenograft models. Immunohistochemical analysis of MCF7 tumors revealed cathepsin S inhibitor treatment significantly reduced proliferation and increased apoptosis.

Conclusions

In summary, these results highlight the characterisation of this nitrile cathepsin S inhibitor using in vitro and in vivo tumor models, presenting a compound which may be used to further dissect the role of cathepsin S in cancer progression and may hold therapeutic potential.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-016-0513-7) contains supplementary material, which is available to authorized users.

Is There Any Role for Serum Cathepsin S and CRP Levels on Prognostic Information in Breast Cancer? The Swedish Mammography Cohort

Breast cancer is the most common cancer among women, and both low-grade inflammation and cathepsins might have important roles in breast cancer. We questioned whether prediagnostic circulating levels of C-reactive protein (CRP), cathepsin B, and cathepsin S were associated with breast cancer risk. Sixty-nine incident breast cancer cases diagnosed after blood collection and 719 controls from the Swedish Mammography Cohort were analyzed for systemic CRP, cathepsin B, and cathepsin S. Cathepsin S and inflammation (high-sensitivity CRP [hsCRP])-adjusted cathepsin S were inversely associated with breast cancer risk (cathepsin S: odds ratio [OR] for top vs. bottom tertile=0.46; 95% confidence interval [CI]=0.23-0.92; p(trend)=0.02; hsCRP-adjusted cathepsin S: OR of 0.44; 95% CI=0.22-0.87; p(trend)=0.02). hsCRP was significantly associated with increased breast cancer risk (OR for top vs. bottom tertile=2.01; 95% CI=1.02-3.95; p(trend)=0.04). No significant association was observed between cathepsin B and breast cancer risk (OR for top vs. bottom tertile=0.67; 95% CI=0.32-1.40; ptrend=0.30). These observations lead to the hypothesis that levels of cathepsin S and hsCRP observed in women who later developed breast cancer may provide prognostic information regarding tumor development and need to be evaluated in prospective studies.

Proteomic Identification of Cysteine Cathepsin Substrates Shed from the Surface of Cancer Cells

Extracellular cysteine cathepsins are known to drive cancer progression, but besides degradation of extracellular matrix proteins little is known about their physiological substrates and thus the molecular mechanisms they deploy. One of the major mechanisms used by other extracellular proteases to facilitate cancer progression is proteolytic release of the extracellular domains of transmembrane proteins or ectodomain shedding. Here we show using a mass spectrometry-based approach that cathepsins L and S act as sheddases and cleave extracellular domains of CAM adhesion proteins and transmembrane receptors from the surface of cancer cells. In cathepsin S-deficient mouse pancreatic cancers, processing of these cathepsin substrates is highly reduced, pointing to an essential role of cathepsins in extracellular shedding. In addition to influencing cell migration and invasion, shedding of surface proteins by extracellular cathepsins impacts intracellular signaling as demonstrated for regulation of Ras GTPase activity, thereby providing a putative mechanistic link between extracellular cathepsin activity and cancer progression. The MS data is available via ProteomeXchange with identifier PXD002192.

Propeptides as modulators of functional activity of proteases

Most proteases are synthesized in the cell as precursor-containing propeptides. These structural elements can determine the folding of the cognate protein, function as an inhibitor/activator peptide, mediate enzyme sorting, and mediate the protease interaction with other molecules and supramolecular structures. The data presented in this review demonstrate modulatory activity of propeptides irrespective of the specific mechanism of action. Changes in propeptide structure, sometimes minor, can crucially alter protein function in the living organism. Modulatory activity coupled with high variation allows us to consider propeptides as specific evolutionary modules that can transform biological properties of proteases without significant changes in the highly conserved catalytic domains. As the considered properties of propeptides are not unique to proteases, propeptide-mediated evolution seems to be a universal biological mechanism.

Repurposing de novo designed entities reveals phosphodiesterase 3B and cathepsin L modulators

Scaffold hopping: a computational algorithm correctly predicted the macromolecular target ofde novogenerated small molecular entities.

miR-31 dysregulation in cystic fibrosis airways contributes to increased pulmonary cathepsin S production

RATIONALE

Cathepsin S (CTSS) activity is increased in bronchoalveolar lavage (BAL) fluid from patients with cystic fibrosis (CF). This activity contributes to lung inflammation via degradation of antimicrobial proteins, such as lactoferrin and members of the β-defensin family.

OBJECTIVES

In this study, we investigated the hypothesis that airway epithelial cells are a source of CTSS, and mechanisms underlying CTSS expression in the CF lung.

METHODS

Protease activity was determined using fluorogenic activity assays. Protein and mRNA expression were analyzed by ELISA, Western blotting, and reverse-transcriptase polymerase chain reaction.

MEASUREMENTS AND MAIN RESULTS

In contrast to neutrophil elastase, CTSS activity was detectable in 100% of CF BAL fluid samples from patients without Pseudomonas aeruginosa infection. In this study, we identified epithelial cells as a source of pulmonary CTSS activity with the demonstration that CF airway epithelial cells express and secrete significantly more CTSS than non-CF control cells in the absence of proinflammatory stimulation. Furthermore, levels of the transcription factor IRF-1 correlated with increased levels of its target gene CTSS. We discovered that miR-31, which is decreased in the CF airways, regulates IRF-1 in CF epithelial cells. Treating CF bronchial epithelial cells with a miR-31 mimic decreased IRF-1 protein levels with concomitant knockdown of CTSS expression and secretion.

CONCLUSIONS

The miR-31/IRF-1/CTSS pathway may play a functional role in the pathogenesis of CF lung disease and may open up new avenues for exploration in the search for an effective therapeutic target.

Antibody targeting of cathepsin S inhibits angiogenesis and synergistically enhances anti-VEGF

BACKGROUND

Angiogenesis is a key hallmark of tumourigenesis and its inhibition is a proven strategy for the development of novel anti-cancer therapeutics. An important aspect of early angiogenesis is the co-ordinated migration and invasion of endothelial cells through the hypoxic tumour tissue. Cathepsin S has been shown to play an important role in angiogenesis as has vascular endothelial growth factor (VEGF). We sought to assess the anti-angiogenic effect of Fsn0503, a novel cathepsin S inhibitory antibody, when combined with anti-VEGF on vascular development.

METHODOLOGY/PRINCIPAL FINDINGS

Cathepsin S expression and secretion from endothelial cells was characterised using RT-PCR and western blotting. We further show that cathepsin S promotes pericellular hydrolysis of extracellular matrix components in the tumour microenvironment and facilitates endothelial invasion. The cathepsin S inhibitory antibody, Fsn0503, blocks extracellular proteolysis, inhibiting endothelial invasion and tube formation in cell-based assays. The anti-angiogenic effects of Fsn0503 were also shown in vivo where it significantly retarded the development of vasculature in human xenograft models. Furthermore, when Fsn0503 was combined with an anti-VEGF antibody, a synergistic inhibition of microvascular development was observed.

CONCLUSIONS/SIGNIFICANCE

Taken together, this data demonstrates that the antibody-mediated targeting of cathepsin S represents a novel method of inhibiting angiogenesis. Furthermore, when used in combination with anti-VEGF therapies, Fsn0503 has the potential to significantly enhance current treatments of tumour neovascularisation and may also be of use in the treatment of other conditions associated with inappropriate angiogenesis.

Identification and pre-clinical testing of a reversible cathepsin protease inhibitor reveals anti-tumor efficacy in a pancreatic cancer model

Proteolytic activity is required for several key processes in cancer development and progression, including tumor growth, invasion and metastasis. Accordingly, high levels of protease expression and activity have been found to correlate with malignant progression and poor patient prognosis in a wide variety of human cancers. Members of the papain family of cysteine cathepsins are among the protease classes that have been functionally implicated in cancer. Therefore, the discovery of effective cathepsin inhibitors has considerable potential for anti-cancer therapy. In this study we describe the identification of a novel, reversible cathepsin inhibitor, VBY-825, which has high potency against cathepsins B, L, S and V. VBY-825 was tested in a pre-clinical model of pancreatic islet cancer and found to significantly decrease tumor burden and tumor number. Thus, the identification of VBY-825 as a new and effective anti-tumor drug encourages the therapeutic application of cathepsin inhibitors in cancer.

Biologic protease inhibitors as novel therapeutic agents

Deregulated proteolytic activities frequently have causative or exacerbative functions in pathological conditions such as cancer and inflammatory disease. Many proteases therefore represent therapeutic targets, but the generation of successful small molecule drugs is often limited by the ability to achieve sufficient specificity of action. Consequently, several proteases have been deemed as unsuitable drug targets due to the inability to target them successfully. In an effort to circumvent these issues, much interest has recently focused on the development and application of biologic inhibitors. In this review, the latest research in the development of biologic protease inhibitors is examined. This includes a review of engineered kunitz and other inhibitory domains as well as the application of antibodies as therapeutically viable inhibitors.

Heparin enhances serpin inhibition of the cysteine protease cathepsin L

The glycosaminoglycan heparin is known to possess antimetastatic activity in experimental models and preclinical studies, but there is still uncertainty over its mechanism of action in this respect. As an anticoagulant, heparin enhances inhibition of thrombin by the serpin antithrombin III, but a similar cofactor role has not been previously investigated for proteases linked to metastasis. The squamous cell carcinoma antigens (serpins B3 and B4) are tumor-associated proteins that can inhibit papain-like cysteine proteases, including cathepsins L, K, and S. In this study, we show that SCCA-1 (B3) and SCCA-2 (B4) can bind heparin as demonstrated by affinity chromatography, native PAGE gel shifts, and intrinsic fluorescence quenching. Binding was specific for heparin and heparan sulfate but not other glycosaminoglycans. The presence of heparin accelerated inhibition of cathepsin L by both serpins, and in the case of SCCA-1, heparin increased the second order inhibition rate constant from 5.4 x 10(5) to >10(8), indicating a rate enhancement of at least 180-fold. A templating mechanism was shown, consistent with ternary complex formation. Furthermore, SCCA-1 inhibition of cathepsin L-like proteolytic activity secreted from breast and melanoma cancer cell lines was significantly enhanced by heparin. This is the first example of glycosaminoglycan enhancement of B-clade serpin activity and the first report of heparin acting as a cofactor in serpin cross-class inhibition of cysteine proteases. Most importantly, this finding raises the possibility that the anticancer properties of heparin may be due, at least partly, to enhanced inhibition of prometastatic proteases.

Roles for cathepsins S, L, and B in insulitis and diabetes in the NOD mouse

We developed a panel of non-obese diabetic (NOD) mice deficient in major lysosomal cysteine proteases (cathepsins S, L and B) to identify protease enzymes essential for autoimmune diabetes. Null alleles for cathepsins (Cts) S, L or B were introgressed onto the NOD genetic background with 19 Idd markers at homozygosity. Diabetes onset was determined among females aged up to 6 months. We evaluated insulitis and sialadenitis in tissues using histology and computer assisted morphology. NOD mice deficient in Ctss or Ctsb were partially protected from diabetes with incidence at 33% and 28%, respectively, versus wild-type NOD (69%; p < 0.00001). NODs lacking cathepsin L (Ctsl-/-) are completely protected from IDDM, as originally shown by others. Ctsl, Ctss, or Ctsb heterozygous mice were able to develop IDDM, although incidence levels were significantly lower for Ctsb+/- (50%) and Ctsl+/- (55%) as compared to NODs (69%; p < 0.03). Ctsl-/- mice contain functional, diabetogenic T cells and an enriched Foxp3+ regulatory T cell population, and diabetes resistance was due to the presence of an expanded population of regulatory T cells. These data provide additional information about the potency of the diabetogenic T cell population in Ctsl-/- mice which were comparable in potency to wild-type NOD mice. These data illustrate the critical contribution of each of these proteases in determining IDDM in the NOD mouse and provide a useful set of models for further studies.