Heparin accelerates the inhibition of cathepsin G by mucus proteinase inhibitor: potent effect of O-butyrylated heparin

Regulation of Peptidase Activity beyond the Active Site in Human Health and Disease

This comprehensive review addresses the intricate and multifaceted regulation of peptidase activity in human health and disease, providing a comprehensive investigation that extends well beyond the boundaries of the active site. Our review focuses on multiple mechanisms and highlights the important role of exosites, allosteric sites, and processes involved in zymogen activation. These mechanisms play a central role in shaping the complex world of peptidase function and are promising potential targets for the development of innovative drugs and therapeutic interventions. The review also briefly discusses the influence of glycosaminoglycans and non-inhibitory binding proteins on enzyme activities. Understanding their role may be a crucial factor in the development of therapeutic strategies. By elucidating the intricate web of regulatory mechanisms that control peptidase activity, this review deepens our understanding in this field and provides a roadmap for various strategies to influence and modulate peptidase activity.

Sulphated penta-galloyl glucopyranoside (SPGG) is glycosaminoglycan mimetic allosteric inhibitor of cathepsin G

Objective

Cathepsin G (CatG) is a cationic serine protease with wide substrate specificity. CatG is reported to play a role in several inflammatory pathologies. Thus, we aimed at identifying a potent and allosteric inhibitor of CatG to be used as a platform in further drug development opportunities.

Methods

Chromogenic substrate hydrolysis assays were used to evaluate the inhibition potency and selectivity of SPGG towards CatG. Salt-dependent studies, Michaelis-Menten kinetics and SDS-PAGE were exploited to decipher the mechanism of CatG inhibition by SPGG. Molecular modelling was also used to identify a plausible binding site.

Key findings

SPGG displayed an inhibition potency of 57 nM against CatG, which was substantially selective over other proteases. SPGG protected fibronectin and laminin against CatG-mediated degradation. SPGG reduced VMAX of CatG hydrolysis of a chromogenic substrate without affecting KM, suggesting an allosteric mechanism. Resolution of energy contributions indicated that non-ionic interactions contribute ~91% of binding energy, suggesting a substantial possibility of specific recognition. Molecular modelling indicated that SPGG plausibly binds to an anion-binding sequence of 109SRRVRRNRN117.

Conclusion

We present the discovery of SPGG as the first small molecule, potent, allosteric glycosaminoglycan mimetic inhibitor of CatG. SPGG is expected to open a major route to clinically relevant allosteric CatG anti-inflammatory agents.

Non-anticoagulant derivatives of heparin for the management of asthma: distant dream or close reality?

INTRODUCTION

Approximately 300 million people worldwide are currently affected by asthma. Improvements in the understanding of the mechanisms involved in such inflammatory airway disorders has led to the recognition of new therapeutic approaches. Heparin, a widely used anticoagulant, has been shown to be beneficial in the management of asthma. It belongs to the family of highly sulphated polysaccharides referred to as glycosaminoglycans, containing a heterogeneous mixture of both anticoagulant and non-anticoagulant polysaccharides. Experimental findings have suggested that heparin has potential anti-asthmatic properties owing to the ability of its non-anticoagulant oligosaccharides to bind and modulate the activity of a wide range of biological molecules involved in the inflammatory process.

AREAS COVERED

This review focuses on the potential mechanisms of action and clinical application of heparin as an anti-inflammatory agent for the management of asthma.

EXPERT OPINION

Heparin may play a significant role in the management of asthma. However, these properties are often hindered by the presence of anticoagulant oligosaccharides, which possess a significant risk of bleeding. Therefore, its therapeutic potential must be explored using well-designed clinical studies that focus on identifying and isolating the anti-inflammatory oligosaccharides of heparin and further elucidating the structure and mechanisms of actions of these non-anticoagulant oligosaccharides.

Inhibitors of cathepsin G: a patent review (2005 to present)

INTRODUCTION

Cathepsin G (CatG) is a neutral proteinase originating from human neutrophils. It displays a unique dual specificity (trypsin- and chymotrypsin-like); thus, its enzymatic activity is difficult to control. CatG is involved in the pathophysiology of several serious human diseases, such as chronic obstructive pulmonary disease (COPD), Crohn's disease, rheumatoid arthritis, cystic fibrosis and other conditions clinically manifested by excessive inflammatory reactions. For mentioned reasons, CatG was considered as good molecular target for the development of novel drugs. However, none of them have yet entered the market as novel therapeutic agents.

AREAS COVERED

This article presents an in-depth and detailed analysis of the therapeutic potential of CatG inhibitors based on a review of patent applications and academic publishing disclosed in patents and patent applications (1991 - 2012), with several exceptions for inhibitors retrieved from academic articles.

EXPERT OPINION

Among the discussed inhibitors of CatG, examples corresponding to derivatives of β-ketophosphonic acids, aminoalkylphosphonic esters and boswellic acids (BAs) could be regarded as the most promising. The most promising one seems to be analogues of compounds of Nature's origin (peptidic and BA derivates). Nevertheless, nothing is currently known about the clinical disposition of any of the CatG inhibitors discovered so far. This latter point suggests that there is still a lot of work to do in the design of stable, pharmacologically active compounds able to specifically regulate the in vivo activity of cathepsin G.

Modulatory effects of proteoglycans on proteinase activities

Proteoglycans (PGs), composed of a core protein and one or more covalently attached sulfated glycosaminoglycan (GAG) chains, interact with a wide range of bioactive molecules, such as growth factors and chemokines, to regulate cell behaviors in normal and pathological processes. Additionally, PGs, through their compositional diversity, play a broad variety of roles as modulators of proteinase activities. Interactions of proteinases with other molecules on the plasma membrane anchor and activate them at a specific location on the cell surface. These interactions with macromolecules other than their own protein substrates or inhibitors result in changes in their activity and/or may have important biological effects. Thus, GAG chains induce conformational changes upon their binding to peptides or proteins. This behavior may be related to the ability of GAGs to act as modulators for some proteins (1) by acting as crucial structural elements by the control of proteinase activities, (2) by increasing the protein stability, (3) by permitting some binding to occur, exposing binding regions on the target protein, or (4) by acting as coreceptors for some inhibitors, playing important roles for the acceleration of proteinase inhibition. Understanding the modulatory effects exerted by PGs on proteinase activities is expected to lead to new insights in the understanding of some molecular systems present in pathological states, providing new targets for drug therapy.

Neutrophil elastase, proteinase 3, and cathepsin G as therapeutic targets in human diseases

Polymorphonuclear neutrophils are the first cells recruited to inflammatory sites and form the earliest line of defense against invading microorganisms. Neutrophil elastase, proteinase 3, and cathepsin G are three hematopoietic serine proteases stored in large quantities in neutrophil cytoplasmic azurophilic granules. They act in combination with reactive oxygen species to help degrade engulfed microorganisms inside phagolysosomes. These proteases are also externalized in an active form during neutrophil activation at inflammatory sites, thus contributing to the regulation of inflammatory and immune responses. As multifunctional proteases, they also play a regulatory role in noninfectious inflammatory diseases. Mutations in the ELA2/ELANE gene, encoding neutrophil elastase, are the cause of human congenital neutropenia. Neutrophil membrane-bound proteinase 3 serves as an autoantigen in Wegener granulomatosis, a systemic autoimmune vasculitis. All three proteases are affected by mutations of the gene (CTSC) encoding dipeptidyl peptidase I, a protease required for activation of their proform before storage in cytoplasmic granules. Mutations of CTSC cause Papillon-Lefèvre syndrome. Because of their roles in host defense and disease, elastase, proteinase 3, and cathepsin G are of interest as potential therapeutic targets. In this review, we describe the physicochemical functions of these proteases, toward a goal of better delineating their role in human diseases and identifying new therapeutic strategies based on the modulation of their bioavailability and activity. We also describe how nonhuman primate experimental models could assist with testing the efficacy of proposed therapeutic strategies.

The sulfate groups of chondroitin sulfate- and heparan sulfate-containing proteoglycans in neutrophil plasma membranes are novel binding sites for human leukocyte elastase and cathepsin G

Human leukocyte elastase (HLE) and cathepsin G (CG) are expressed at high levels on the surface of activated human neutrophils (PMN) in catalytically active but inhibitor-resistant forms having the potential to contribute to tissue injury. Herein we have investigated the mechanisms by which HLE and CG bind to PMN plasma membranes. (125)I-Labeled HLE and CG bind to PMN at 0 degrees C in a saturable and reversible manner (K(D) = 5.38 and 4.36 x 10(-7) m and 11.5 and 8.1 x 10(6) binding sites/cell, respectively). Incubation of PMN with radiolabeled HLE and CG in the presence of a 200-fold molar excess of unlabeled HLE, CG, myeloperoxidase, lactoferrin, proteinase 3, phenylmethylsulfonyl fluoride (PMSF)-inactivated HLE, or PMSF-inactivated CG inhibited binding of radiolabeled ligands. This indicates that these PMN granule proteins share binding sites on PMN and that functional active sites of HLE and CG are not required for their binding to PMN. The sulfate groups of heparan sulfate- and chondroitin sulfate-containing proteoglycans are the PMN binding sites for HLE and CG since binding of HLE and CG to PMN was inhibited by incubating PMN with 1) trypsin, chondroitinase ABC, and heparitinases, but not other glycanases, and 2) purified chondroitin sulfates, heparan sulfate, and other sulfated molecules, but not with non-sulfated glycans. Thus, heparan sulfate- and chondroitin sulfate-containing proteoglycans are low affinity, high volume PMN surface binding sites for HLE and CG, which are well suited to bind high concentrations of active serine proteinases released from degranulating PMN.

Semi-synthetic heparin derivatives: chemical modifications of heparin beyond chain length, sulfate substitution pattern and N-sulfo/N-acetyl groups

The glycosaminoglycan heparin is a polyanionic polysaccharide most recognized for its anticoagulant activity. Heparin binds to cationic regions in hundreds of prokaryotic and eukaryotic proteins, termed heparin-binding proteins. The endogenous ligand for many of these heparin-binding proteins is a structurally similar glycosaminoglycan, heparan sulfate (HS). Chemical and biosynthetic modifications of heparin and HS have been employed to discern specific sequences and charge-substitution patterns required for these polysaccharides to bind specific proteins, with the goal of understanding structural requirements for protein binding well enough to elucidate the function of the saccharide-protein interactions and/or to develop new or improved heparin-based pharmaceuticals. The most common modifications to heparin structure have been alteration of sulfate substitution patterns, carboxyl reduction, replacement N-sulfo groups with N-acetyl groups, and chain fragmentation. However, an accumulation of reports over the past 50 years describe semi-synthetic heparin derivatives obtained by incorporating aliphatic, aryl, and heteroaryl moieties into the heparin structure. A primary goal in many of these reports has been to identify heparin-derived structures as new or improved heparin-based therapeutics. Presented here is a perspective on the introduction of non-anionic structural motifs into heparin structure, with a focus on such modifications as a strategy to generate novel reduced-charge heparin-based bind-and-block antagonists of HS-protein interactions. The chemical methods employed to synthesize such derivatives, as well as other unique heparin conjugates, are reviewed.

Interactions of low-molecular-weight semi-synthetic sulfated heparins with human leukocyte elastase and human Cathepsin G

Semi-synthetic low-molecular-weight heparin samples (LMWHs), having homogeneous degree of polymerization and saccharide backbone, but differing in the number and location of sulfate groups, were investigated in their ability to interfere with the pharmacologically relevant targets human leukocyte elastase (EL) and human Cathepsin G (CatG). Spectroscopic studies were performed for a quantitative evaluation of the enzyme-inhibitor dissociation constant, K(i), and of the IC(50) values for the inhibition of cleavage of target peptide sequences. Both proteases are inhibited by the tested polysaccharides through a mixed hyperbolic binding process. A non-linear relationship was found between degree of sulfation and binding affinity or enzyme inhibition properties, showing a composite correlation between heparin charge density and interference with EL/CatG activity.

Modulation of the exocellular serine-thiol proteinase activity of Paracoccidioides brasiliensis by neutral polysaccharides

Our group characterized an exocellular serine-thiol proteinase activity in the yeast phase of Paracoccidioides brasiliensis (PbST), a dimorphic human pathogen. The fungal proteinase is able to cleave in vitro, at pH 7.4, proteins associated with the basal membrane, such as human laminin and fibronectin, type IV collagen and proteoglycans. In the present study, we investigated the influence of glycosaminoglycans (GAGs) and neutral polysaccharides upon the serine-thiol proteinase activity by means of kinetic analysis monitored with fluorescence resonance energy transfer (FRET) peptides using the substrate Abz-MKALTLQ-EDDnp (Abz=ortho-aminobenzoic acid; EDDnp=ethylenediaminedinitrophenyl). Only neutral polysaccharides exhibited patterns of interaction with the proteinase, while sulfated GAGs had no effect. Incubation with neutral polysaccharides resulted in a powerful modulation of the enzyme activity, intensely changing the enzyme kinetic parameters of catalysis and affinity for the substrate. Commercial dextran at the highest concentration of 20 microM increased 6.8-fold the enzyme affinity for the substrate. In the presence of 8 microM of purified baker's yeast mannan, the apparent KM of the enzyme increased about 5.5-fold, reflecting a significant inhibition in binding to the peptide substrate. When an exocellular galactomannan (GalMan) complex isolated from P. brasiliensis was added to the reaction mixture at 400 nM, the apparent KM and VMAX decreased about threefold. Moreover, GalMan was able to protect the enzymatic activity at high temperatures, but it caused no effect on the optimum cleavage pH. Our results show a novel modulation mechanism in P. brasiliensis, where a fungal polysaccharide-rich component can stabilize a serine-thiol proteolytic activity, which is possibly involved in fungal dissemination.

Competition between elastase and related proteases from human neutrophil for binding to alpha1-protease inhibitor

The protease-antiprotease imbalance that is characteristic of most inflammatory lung disorders depends on the spatial-temporal regulation of active inhibitor and protease concentrations in lung secretions. We have studied the competition between the three main serine proteases from human neutrophil primary granules in their binding to alpha1-Pi, the main serine proteases inhibitor in lung secretions. Elastase was the only target of alpha1-Pi when identical molar amounts of purified inhibitor and the three proteases were tested together. The other two proteases were only inhibited once elastase was saturated. Elastase remained the preferred target of inhibitors when bronchoalveolar lavage fluids from patients with lung pneumonia and acute respiratory distress syndrome were used as the source of inhibitors, in spite of the presence of additional inhibitors in lung secretions. Since neutrophil proteases are expressed at the neutrophil surface, we also measured residual activities of membrane-bound proteases after purified neutrophils were incubated with bronchoalveolar fluids. Again, elastase was the preferred target of the inhibitors. We conclude that protease 3 and cathepsin G are not controlled as efficiently as elastase in lung secretions, a feature that must be taken into account when developing inhibitor-based anti-inflammatory therapies.

Involvement of stromal proteoglycans in tumour progression

Glycosaminoglycans (GAGs) and proteoglycans (PGs) belong to a class of extracellular macromolecules necessary for the growth of any multicellular structures, including tumours. Transformed cells induce stromal reaction either per se or by activation of the mesenchymal cells. Tumour stroma contains several chondroitin sulphate and heparan sulphate proteoglycans. These proteoglycans and their glycosaminoglycan chains modify cell behaviour by interacting with different molecules such as growth factors, cytokines, chemokines, proteinases and their inhibitors. This review describes the main proteoglycans of tumour stoma and discusses their implication in the regulation of the activity of extracellular proteins and peptides.

Intracellular proteoglycans

Proteoglycans (PGs) are proteins with glycosaminoglycan chains, are ubiquitously expressed and have a wide range of functions. PGs in the extracellular matrix and on the cell surface have been the subject of extensive structural and functional studies. Less attention has so far been given to PGs located in intracellular compartments, although several reports suggest that these have biological functions in storage granules, the nucleus and other intracellular organelles. The purpose of this review is, therefore, to present some of these studies and to discuss possible functions linked to PGs located in different intracellular compartments. Reference will be made to publications relevant for the topics we present. It is beyond the scope of this review to cover all publications on PGs in intracellular locations.

Design and Use of Highly Specific Substrates of Neutrophil Elastase and Proteinase 3

We have exploited differences in the structures of S2' subsites of proteinase 3 (Pr3) and human neutrophil elastase (HNE) to prepare new fluorogenic substrates specific for each of these proteases. The positively charged residue at position 143 in Pr3 prevents it from accommodating an arginyl residue at S2' and improves the binding of P2' aspartyl-containing substrates, as judged by the decreased K(m). As a result, the k(cat)/K(m) for Abz-VADCADQ-EDDnp is over 500 times greater for Pr3 than for HNE, and that for Abz-APEEIMRRQ-EDDnp is over 500 times greater for HNE than for Pr3. This allows each protease activity to be measured in the presence of a large excess of the other, as might occur in vivo. Placing a prolyl residue in position P2' greatly impaired substrate binding to both HNE and Pr3, which further emphasizes the importance of S' subsites in these proteases. HNE and Pr3 activities were measured with these substrates at the surface of fixed polymorphonuclear leukocytes (PMNs) before and after activation. This demonstrated that their active site remains accessible when they are exposed to the cell surface. Both membrane-bound proteases were strongly inhibited by low M(r) serine protease inhibitors, but only partially by inhibitors of larger M(r) such as alpha1-protease inhibitor, the main physiologic inhibitor in lung secretions. Most of membrane-bound HNE and Pr3 can be released from the membrane surface of fixed cells by a buffer containing detergent, suggesting that hydrophobic interactions are involved in membrane binding.

Heparin-like dextran derivatives as well as glycosaminoglycans inhibit the enzymatic activity of human cathepsin G

Some synthetic dextran derivatives that mimic the action of heparin/heparan sulfate were previously shown to inhibit neutrophil elastase and plasmin. Here we report that these derivatized dextrans also inhibit cathepsin G (CatG). Dextran containing carboxymethyl and benzylamide groups (RG1150) as well as those containing carboxymethyl, sulfate and benzylamide groups (RG1192), were the most efficient inhibitors of CatG activity. RG1192 and RG1150 bind CatG with a K(i) of 0.11 and 0.17 nM, respectively, while carboxymethylated sulfated dextran (RG1503) as well as heparin, heparan sulfate and dermatan sulfate bind CatG with a 7- to 30-fold lower affinity. Variation of K(i) with ionic strength indicates that ionic interactions account for 26% of the RG1503-CatG binding energy, while binding of RG1192 or RG1150 to CatG is mainly governed by non-electrostatic interactions. This, together with the fact that these compounds both protect fibronectin and laminin against CatG-mediated degradation, suggest that specific dextran derivatives can contribute to the regulation of CatG activity.

Measurement of free and membrane-bound cathepsin G in human neutrophils using new sensitive fluorogenic substrates

Activated human polymorphonuclear neutrophils at inflammatory sites release the chymotrypsin-like protease cathepsin G, together with elastase and proteinase 3 (myeloblastin), from their azurophil granules. The low activity of cathepsin G on synthetic substrates seriously impairs studies designed to clarify its role in tissue inflammation. We have solved this problem by producing new peptide substrates with intramolecularly quenched fluorescence. These substrates were deduced from the sequence of putative protein targets of cathepsin G, including the reactive loop sequence of serpin inhibitors and the N-terminal domain of the protease-activated receptor of thrombin, PAR-1. Two substrates were selected, Abz-TPFSGQ-EDDnp and Abz-EPFWEDQ-EDDnp, that are cleaved very efficiently by cathepsin G but not by neutrophil elastase or proteinase 3, with specificity constants (k(cat)/K(m)) in the 10(5) M(-1).s(-1) range. They can be used to measure subnanomolar concentrations of free enzyme in vitro and at the surface of neutrophils purified from fresh human blood. Purified neutrophils express 0.02-0.7 pg of cathepsin G/cell (n=15) at their surface. This means that about 10(4) purified cells may be enough to record cathepsin G activity within minutes. This may be most important for investigating the role of cathepsin G as an inflammatory agent, especially in bronchoalveolar lavage fluids from patients with pulmonary inflammatory disorders.

Proteinase activity regulation by glycosaminoglycans

There are few reports concerning the biological role and the mechanisms of interaction between proteinases and carbohydrates other than those involved in clotting. It has been shown that the interplay of enzymes and glycosaminoglycans is able to modulate the activity of different proteases and also to affect their structures. From the large number of proteases belonging to the well-known protease families and also the variety of carbohydrates described as widely distributed, only few events have been analyzed more deeply. The term "family" is used to describe a group of proteases in which every member shows an evolutionary relationship to at least one other protease. This relationship may be evident throughout the entire sequence, or at least in that part of the sequence responsible for catalytic activity. The majority of proteases belong to the serine, cysteine, aspartic or metalloprotease families. By considering the existing limited proteolysis process, in addition to the initial idea that the proteinases participate only in digestive processes, it is possible to conclude that the function of the enzymes is strictly limited to the cleavage of intended substrates since the destruction of functional proteins would result in normal tissue damage. In addition, the location as well as the eventual regulation of protease activity promoted by glycosaminoglycans can play an essential role in the development of several physiopathological conditions.

Inhibition of neutrophil cathepsin G by oxidized mucus proteinase inhibitor. Effect of heparin

Oxidation of mucus proteinase inhibitor (MPI) transforms Met73, the P'1 residue of its active center into methionine sulfoxide and lowers its affinity for neutrophil elastase [Boudier, C., and Bieth, J. G. (1994) Biochem. J. 303, 61-68]. Here, we show that the oxidized inhibitor has also a decreased affinity for neutrophil cathepsin G and pancreatic chymotrypsin. The Ki of the oxidized MPI-cathepsin G complex (1.2 microM) is probably too high to be compatible with significant inhibition of cathepsin G in inflammatory lung secretions. Stopped-flow kinetics shows that, within the inhibitor concentration range used, the mechanism of inhibition of cathepsin G and chymotrypsin by oxidized MPI is consistent with a one-step reaction, [equation in text] whereas the inhibition of elastase takes place in two steps, [equation in text]. Heparin, which accelerates the inhibition of the three proteinases by native MPI, also favors their interaction with oxidized MPI. Flow calorimetry shows that heparin binds oxidized MPI with Kd, Delta H degrees, and Delta S degrees values close to those reported for native MPI. In the presence of heparin, oxidized MPI inhibits cathepsin G via a two-step reaction characterized by Ki = 0.22 microM, k2 = 0.1 s-1, k-2 = 0.023 s-1, and Ki = 42 nM. Under these conditions, in vivo inhibition of cathepsin G is again possible. Heparin also improves the inhibition of chymotrypsin and elastase by oxidized MPI by increasing their kass or k2/Ki and decreasing their Ki. Our data suggest that oxidation of MPI during chronic bronchitis may lead to cathepsin G-mediated lung tissue degradation and that heparin may be a useful adjuvant of MPI-based therapy of acute lung inflammation in cystic fibrosis.

Effect of polynucleotides on the inhibition of neutrophil elastase by mucus proteinase inhibitor and alpha 1-proteinase inhibitor

DNA released from neutrophils at sites of inflammation may modulate tissue proteolysis. We used tRNA and synthetic polynucleotides as models of DNA to study the influence of polynucleotides on the inhibition of neutrophil elastase by its endogenous inhibitors alpha1-proteinase inhibitor (alpha1-PI) and mucus proteinase inhibitor (MPI). Affinity chromatography showed that polynucleotides form electrostatic complexes with elastase and MPI but not with alpha1-PI, the highest affinity being for MPI. The tight-binding partial inhibition of elastase by polynucleotides was used to calculate the Kd of the elastase-polynucleotide complexes which ranged from 4 microM to 21 nM. One mole of tRNA was able to bind 9 mol of elastase. Polydeoxycytosine and tRNA significantly impaired the reversible inhibition of elastase by MPI: they moderately increased the rate of enzyme-inhibitor association, strongly enhanced the rate of complex dissociation, and lowered the enzyme-inhibitor affinity by factors of 34 and 134, respectively. The two polynucleotides also decreased the rate of the irreversible inhibition of elastase by alpha1-PI by factors of 30 and 3, respectively. Polynucleotides also changed the mechanism of inhibition of elastase by the two inhibitors from a one-step inhibition reaction to a two-step binding mechanism. Our data may help explain why proteolysis may occur at sites of inflammation despite the presence of active proteinase inhibitors.