Inhibition of cathepsin L-like proteases by cathepsin V propeptide

In silico approaches for better understanding cysteine cathepsin-glycosaminoglycan interactions

Cysteine cathepsins constitute the largest cathepsin family, with 11 proteases in human that are present primarily within acidic endosomal and lysosomal compartments. They are involved in the turnover of intracellular and extracellular proteins. They are synthesized as inactive procathepsins that are converted to mature active forms. Cathepsins play important roles in physiological and pathological processes and, therefore, receive increasing attention as potential therapeutic targets. Their maturation and activity can be regulated by glycosaminoglycans (GAGs), long linear negatively charged polysaccharides composed of recurring dimeric units. In this review, we summarize recent computational progress in the field of (pro)cathepsin-GAG complexes analyses.

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.

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.

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.

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.

Proteolysis mediated by cysteine cathepsins and legumain-recent advances and cell biological challenges

Proteases play essential roles in protein degradation, protein processing, and extracellular matrix remodeling in all cell types and tissues. They are also involved in protein turnover for maintenance of homeostasis and protein activation or inactivation for cell signaling. Proteases range in function and specificity, with some performing distinct substrate cleavages, while others accomplish proteolysis of a wide range of substrates. As such, different cell types use specialized molecular mechanisms to regulate the localization of proteases and their function within the compartments to which they are destined. Here, we focus on the cysteine family of cathepsin proteases and legumain, which act predominately within the endo-lysosomal pathway. In particular, recent knowledge on cysteine cathepsins and their primary regulator legumain is scrutinized in terms of their trafficking to endo-lysosomal compartments and other less recognized cellular locations. We further explore the mechanisms that regulate these processes and point to pathological cases which arise from detours taken by these proteases. Moreover, the emerging biological roles of specific forms and variants of cysteine cathepsins and legumain are discussed. These may be decisive, pathogenic, or even deadly when localizing to unusual cellular compartments in their enzymatically active form, because they may exert unexpected effects by alternative substrate cleavage. Hence, we propose future perspectives for addressing the actions of cysteine cathepsins and legumain as well as their specific forms and variants. The increasing knowledge in non-canonical aspects of cysteine cathepsin- and legumain-mediated proteolysis may prove valuable for developing new strategies to utilize these versatile proteases in therapeutic approaches.

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.

Plasminogen hydrolysis by cathepsin S and identification of derived peptides as selective substrate for cathepsin V and cathepsin L inhibitor

Plasminogen is a glycoprotein implicated in angiogenesis and fibrin clot degradation associated with the release of angiostatin and plasmin activation, respectively. We have recently reported that cathepsin V, but not cathepsins L, B, and K, can release angiostatin-like fragments from plasminogen. Here, we extended the investigation to cathepsin S which has been implicated in angiogenesis and tumor cell proliferation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of plasminogen hydrolysis by cathepsin S revealed generation of two fragments (60 and 38 kDa). Amino-terminal sequencing indicated that cleavage occurs at the Leu469-Leu470 peptide bond. In contrast to cathepsin V, which possesses antiangiogenic activity, cathepsin S plasminogen cleavage products were not capable of inhibiting angiogenesis on endothelial cells. Moreover, we explored the different selectivities presented by cathepsins V and S towards plasminogen and synthesized fluorescence resonance energy transfer peptides encompassing the hydrolyzed peptide bonds by both enzymes. The peptide Abz-VLFEKKQ-EDDnp (Abz=ortho-aminobenzoic acid; EDDnp= N-[2,4-dinitrophenyl]ethylenediamine), hydrolyzed by cath-epsin V at the Phe-Glu bond, is a selective substrate for the enzyme when compared with cathepsins B, L, and S, whereas Abz-VLFEKKVYLQ-EDDnp is an efficient cathepsin L inhibitor. The demonstrated importance of the S3′-P3′ interaction indicates the significance of the extended subsites for enzyme specificity and affinity.

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.

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

Human cathepsin L along with cathepsin S, K, and V are collectively known as cathepsin L-like proteases due to their high homology. The overexpression and aberrant activity of each of these proteases has been implicated in tumorigenesis. These proteases contain propeptide domains that can potently inhibit both their cognate protease and other proteases within the cathepsin L-like subfamily. In this investigation, we have produced the cathepsin S propeptide recombinantly and have shown that it is a potent inhibitor of the peptidolytic, elastinolytic, and gelatinolytic activities of the cathepsin L-like proteases. In addition, we show that this peptide is capable of significantly attenuating tumor cell invasion in a panel of human cancer cell lines. Furthermore, fusion of an IgG Fc-domain to the COOH terminus of the propeptide resulted in a chimeric protein with significantly enhanced ability to block tumor cell invasion. This Fc fusion protein exhibited enhanced stability in cell-based assays in comparison with the unmodified propeptide species. This approach for the combined inhibition of the cathepsin L-like proteases may prove useful for the further study in cancer and other conditions where their aberrant activity has been implicated. Furthermore, this strategy for simultaneous inhibition of multiple cysteine cathepsins may represent the basis for novel therapeutics to attenuate tumorigenesis.