Mast cell tryptase activates extracellular-regulated kinases (p44/p42) in airway smooth-muscle cells: importance of proteolytic events, time course, and role in mediating mitogenesis

The emerging role of mast cell proteases in asthma

It is now well established that mast cells (MCs) play a crucial role in asthma. This is supported by multiple lines of evidence, including both clinical studies and studies on MC-deficient mice. However, there is still only limited knowledge of the exact effector mechanism(s) by which MCs influence asthma pathology. MCs contain large amounts of secretory granules, which are filled with a variety of bioactive compounds including histamine, cytokines, lysosomal hydrolases, serglycin proteoglycans and a number of MC-restricted proteases. When MCs are activated, e.g. in response to IgE receptor cross-linking, the contents of their granules are released to the exterior and can cause a massive inflammatory reaction. The MC-restricted proteases include tryptases, chymases and carboxypeptidase A3, and these are expressed and stored at remarkably high levels. There is now emerging evidence supporting a prominent role of these enzymes in the pathology of asthma. Interestingly, however, the role of the MC-restricted proteases is multifaceted, encompassing both protective and detrimental activities. Here, the current knowledge of how the MC-restricted proteases impact on asthma is reviewed.

Airway smooth muscle enlargement is associated with protease-activated receptor 2/ligand overexpression in patients with difficult-to-control severe asthma

BACKGROUND

Asthma is a complex disease with heterogeneous features of airway inflammation and remodeling. The increase in airway smooth muscle (ASM) mass is an essential component of airway remodeling in patients with severe asthma, yet the pathobiological mechanisms and clinical outcomes associated with ASM enlargement remain elusive.

OBJECTIVE

We sought to compare ASM area in control subjects and patients with mild-to-moderate or severe asthma and to identify specific clinical and pathobiological characteristics associated with ASM enlargement.

METHODS

Bronchial biopsy specimens from 12 control subjects, 24 patients with mild-to-moderate asthma, and 105 patients with severe asthma were analyzed for ASM area, basement membrane thickness, vessels, eosinophils, neutrophils, T lymphocytes, mast cells, and protease-activated receptor 2 (PAR-2). In parallel, the levels of several ASM mitogenic factors, including the PAR-2 ligands, mast cell tryptase, trypsin, tissue factor, and kallikrein (KLK) 5 and KLK14, were assessed in bronchoalveolar lavage fluid. Data were correlated with asthma severity and control both at inclusion and after 12 to 18 months of optimal management and therapy.

RESULTS

Analyses across ASM quartiles in patients with severe asthma demonstrated that patients with the highest ASM quartile (median value of ASM area, 26.3%) were younger (42.5 vs ≥50 years old in the other groups, P ≤ .04) and had lower asthma control after 1 year of optimal management (P ≤ .006). ASM enlargement occurred independently of features of airway inflammation and remodeling, whereas it was associated with PAR-2 overexpression and higher alveolar tryptase (P ≤ .02) and KLK14 (P ≤ .03) levels.

CONCLUSION

Increase in ASM mass, possibly involving aberrant expression and activation of PAR-2-mediated pathways, characterizes younger patients with severe asthma with poor asthma control.

Tryptase as an inflammatory marker in allergic disease and asthma

Asthma is a chronic inflammatory disease of the airways, varying from occasional episodes of wheezing and shortness of breath, to an irreversible, life-threatening obstructive disease. While many cases are managed with relative ease, others do not respond to the traditional inhaled therapy or even to oral glucocorticosteroids. Although it cannot be cured as yet, asthma can be controlled if properly diagnosed. Usually, functional clinical parameters form the basis for estimation of the disease severity. In addition, the growing database of cytokine and mediator profiles have allowed their exploitation as molecular markers for processes underlying airway inflammation in asthma. Tryptase is a potent and versatile mediator in allergic inflammation, orchestrating both acute and chronic events by acting on a vast array of cells and tissue components. For more than a decade, tryptase has been used as a marker for allergic inflammation in asthma as well as in a variety of other airway diseases. In this review, the current advantages and disadvantages of the use of tryptase as an inflammatory marker in asthma will be discussed.

Human subjects are protected from mast cell tryptase deficiency despite frequent inheritance of loss-of-function mutations

BACKGROUND

Mast cell tryptases have proposed roles in allergic inflammation and host defense against infection. Tryptase gene loci TPSAB1 and TPSB2 are known to be polymorphic, but the nature and extent of diversity at these loci have not been fully explored.

OBJECTIVE

We sought to compare functional and nonfunctional tryptase allele frequencies and establish haplotypes in human populations.

METHODS

Tryptase allele frequencies were determined by means of direct sequencing in 270 individuals from HapMap populations of European, African, Chinese, and Japanese ancestry. Haplotypes were predicted, validated in parent-child trios, and compared between populations.

RESULTS

We identify a new frame-shifted tryptase allele (betaIII(FS)) carried by 23% and 19% of individuals of European and African ancestry but 0% of Asian subjects. Homology models predict that betaIII(FS) is functionless. Our genotyping assay shows that allele and haplotype distributions in each population are unique. Strong linkage disequilibrium between TPSAB1 and TPSB2 (r(2)=0.83, D'=0.85) yields 2 major and 5 minor tryptase haplotypes.

CONCLUSIONS

Tryptase deficiency alleles (alpha and the newly discovered betaIII(FS)) are common, causing the number of inherited active genes to range from a minimum of 2 to a maximum of 4, with major differences between populations in the proportion of individuals inheriting 2 versus 4 active alleles. African and Asian populations are especially enriched in genes encoding functional and nonfunctional tryptases, respectively. Strong linkage of TPSAB1 and TPSB2 and pairing of deficiency alleles with functional alleles in observed haplotypes protect human subjects from "knockout" genomes and indeed from inheritance of fewer than 2 active alleles.

Mast cell tryptases and chymases in inflammation and host defense

Tryptases and chymases are the major proteins stored and secreted by mast cells. The types, amounts, and properties of these serine peptidases vary by mast cell subtype, tissue, and mammal of origin. Membrane-anchored gamma-tryptases are tryptic, prostasin-like, type I peptidases that remain membrane attached on release and act locally. Soluble tryptases, including their close relatives, mastins, form inhibitor-resistant oligomers that act more remotely. Befitting their greater destructive potential, chymases are quickly inhibited after release, although some gain protection by associating with proteoglycans. Most chymase-like enzymes, including mast cell cathepsin G, hydrolyze chymotryptic substrates, an uncommon capability in the proteome. Some rodent chymases, however, have mutations resulting in elastolytic activity. Secreted tryptases and chymases promote inflammation, matrix destruction, and tissue remodeling by several mechanisms, including destroying procoagulant, matrix, growth, and differentiation factors and activating proteinase-activated receptors, urokinase, metalloproteinases, and angiotensin. They also modulate immune responses by hydrolyzing chemokines and cytokines. At least one chymase protects mice from intestinal worms. Tryptases and chymases can also oppose inflammation by inactivating allergens and neuropeptides causing inflammation and bronchoconstriction. Thus, like mast cells themselves, mast cell serine peptidases play multiple roles in host defense, and any accounting of benefit versus harm is necessarily context specific.

Trypsin increases pseudorabies virus production through activation of the ERK signalling pathway

Extracellular proteases that are expressed in primary and secondary foci of viral infection are potentially important mediators of infectious inflammatory processes. For some viruses, such as influenza virus and rotaviruses, proteases such as trypsin enhance infectivity by a direct proteolytic effect on some virion proteins. By using an in vitro model of herpesvirus infection, the possibility that proteases modulate the viral cycle through signalling delivered to the infected cell was investigated. It is reported that exposure of pseudorabies virus-infected cells to trypsin increased virus production. Moreover, this treatment induced synergistic and sustained activation of the extracellular signal-regulated kinase (ERK) 1/2 signalling pathway, which appeared to be necessary for this increased viral production. These results suggest that herpesviruses could take advantage of the inflammatory context and particularly of the presence of proteases to increase their replication. Thus, these data point to a potentially important role of extracellular proteases in herpesvirus infection.

Tryptase activates phosphatidylinositol 3-kinases proteolytically independently from proteinase-activated receptor-2 in cultured dog airway smooth muscle cells

Mast cell tryptase is a potent mitogen for many cells in the airways and lung, but the cellular mechanisms for its growth stimulatory effects are poorly understood. Our major goal was to determine whether tryptase activates phosphatidylinositol 3-kinases (PI 3-kinases) in cultured dog tracheal smooth muscle cells to induce its mitogenic effects. After exposure to tryptase, cells were lysed. Immunocomplexes prepared from the lysates using an antibody to the p85 subunit of PI 3-kinase, but not using anti-phosphotyrosine antibodies, possessed increased capacity to phosphorylate inositol on its D3 hydroxyl group. Tryptase also increased phosphorylation of Akt, a downstream target of PI 3-kinases. This effect was abolished by one PI 3-kinase inhibitor, wortmannin, and attenuated by another, LY-294004, which also blocked tryptase's mitogenic effects. Treatment of tryptase with p-amidino phenylmethanesulfonyl fluoride, to abolish its proteolytic activity irreversibly, inhibited its stimulatory effects on Akt phosphorylation. Proteinase-activated receptor-2 (PAR-2)-activating peptides failed to increase Akt phosphorylation in cultured dog tracheal smooth muscle cells, but the PAR-2-activating peptides did induce brisk increases in Akt phosphorylation in Madin-Darby canine kidney cells. We concluded that tryptase activates PI 3-kinases in cultured dog tracheal smooth muscle cells to induce its potent mitogenic effects. These effects of tryptase on PI 3-kinases appear to occur via novel proteolytic mechanisms independent from PAR-2. Also, tryptase, although comparable in mitogenic potency to platelet-derived growth factor (PDGF), induces considerably less tyrosine phosphorylation on proteins than occur in response to PDGF.

The action of the mast cell product tryptase on cyclooxygenase-2 (COX2) and subsequent fibroblast proliferation involves activation of the extracellular signal-regulated kinase isoforms 1 and 2 (erk1/2)

The mast cell product tryptase, via protease-activated receptor 2 (PAR2), induces cyclooxygenase-2 (COX2) and 15-deoxy-prostaglandin J2 (15d-PGJ2) synthesis. 15d-PGJ2, through the nuclear peroxisome proliferator activated receptor gamma (PPARgamma), subsequently causes fibroblast proliferation. In this study we attempted to determine initial events of the tryptase/PAR2 signaling pathway leading to COX2 induction and fibroblast proliferation. In human fibroblasts (HFFF2), cDNA array, RT-PCR and Western blotting studies demonstrated that tryptase, but not 15d-PGJ2, up-regulates c-jun, c-fos and COX2 expression, and phosphorylates the extracellular signal-regulated kinase isoforms 1 and 2 (erk1/2). Furthermore, tryptase effects on erk1/2, c-jun, c-fos, COX2 and cell proliferation were prevented by PD98059, an inhibitor of the mitogen-activated protein kinase kinase (MEK). Other kinases [P38, stress-activated protein kinase/c-jun N-terminal kinase (SAPK/JUNK), erk5], intracellular Ca(2+) or cAMP were not affected by tryptase/PAR2. Our study identifies crucial intracellular events leading to induction of COX2 and fibroblast proliferation, i.e. a cornerstone of fibrosis.

Airway remodeling: a comparison between fatal and nonfatal asthma

BACKGROUND

Airway remodeling has been recently one of the main goals in asthma research because it has been implicated to influence airway behavior and evolution of asthma; hence, important in long-term followup of asthmatic patients.

METHODS

Airways of fatal asthma (n=3), non-fatal asthma (n=3) and control cases (n=4) were studied using morphometry and immunohistochemical and H&E staining.

RESULTS

The basement membrane was thicker in the cartilaginous and membranous airways of fatal and non-fatal asthma groups compared to the control group (p<0.05). Smooth muscle shortening was greater in airways of fatal asthma cases while submucosal gland area and mucus plug occupying ratio were greater in fatal asthma large airways compared to the two other groups (p<0.01). Increased intact and degranulated mast cells were observed in smooth muscle and in submucosal gland of fatal asthma airways (p<0.01) and were associated with greater degree of smooth muscle shortening and larger submucosal gland area, respectively. Eosinophil and EG2+ cell infiltrations were greatest in lamina propria of airways of fatal asthma than in nonfatal and control cases (p<0.01), but were not associated with any airway structural change.

CONCLUSION

Increased infiltration of eosinophils in the lamina propria and mast cells in smooth muscle and submucosal glands may have a role in airway remodeling of fatal asthma airways but needs further investigation. Moreover, mast cells in cartilaginous airways may participate in the regulation of smooth muscle tone and mucous gland secretion and hyperplasia.

Induction of early growth-response factor 1 by platelet-derived growth factor in human airway smooth muscle

Platelet-derived growth factors (PDGF) may contribute to the activation and growth of smooth muscle that is characteristic of airway remodeling in asthmatic patients. Early growth response 1 (EGR-1) is a transcription factor that is induced in several cell types by PDGF and may mediate some of the effects of PDGF. We show that human airway smooth muscle cells in cell culture express EGR-1 1 h after addition of PDGF. Analysis of the EGR-1 promoter indicates that a serum response element located between 663 and 654 bp 5' to the ATG start site is essential for this induction. Serum response factor, E26 transcription factor-like protein 1, and serum protein 1 bind to this region. PDGF causes phosphorylation of ERK1/2 and is temporally associated with E26 transcription factor-like protein 1 phosphorylation. Finally, the specific ERK1/2 inhibitor U-0126 abolishes PDGF-induced expression of EGR-1 in these cells. On the basis of these data, we speculated that EGR-1 would be increased in airway smooth muscle of asthmatic patients compared with nonasthmatic controls. Using immunohistochemistry, we found that EGR-1 protein was expressed in airway smooth muscle cells and epithelial cells of asthmatic patients and nonasthmatic controls; however, there was no significant difference in the intensity of staining between groups. EGR-1 was similarly expressed in the lungs of mice with and without ovalbumin-induced airway inflammation; however, there was no difference between groups by immunohistochemistry and quantitative PCR. Although EGR-1 is induced by PDGF in human airway smooth muscle cells in cell culture, the role of EGR-1 in airway remodeling and asthma remains to be established.

Mast cell chymase modifies cell-matrix interactions and inhibits mitogen-induced proliferation of human airway smooth muscle cells

The hallmarks of chronic, severe asthma include prominent airway inflammation and airway smooth muscle (ASM) hypertrophy and hyperplasia. One of the factors that contribute to the injury and repair process within the airway is activation of proteases and turnover of extracellular matrix components. Mast cells, which are present in increased numbers in the asthmatic airway, are a rich source of the neutral protease chymase, which can degrade several basement membrane components. Recent data suggest that proteases also play a critical role in regulating the expression of CD44, the primary receptor for the matrix glycosaminoglycan hyaluronan. In this study we investigated the effects of chymase treatment on human ASM cell function. We found that chymase degraded the smooth muscle cell pericellular matrix. This was accompanied by an increased release of fibronectin and soluble CD44, but not soluble ICAM-1 or soluble hyaluronan, into the conditioned medium. In addition, chymase inhibited T cell adhesion to ASM and dramatically reduced epidermal growth factor-induced smooth muscle cell proliferation. These data suggest that the local release of mast cell chymase may have profound effects on ASM cell function and airway remodeling.

Proteolytic action of duodenase is required to induce DNA synthesis in pulmonary artery fibroblasts

Duodenase is a 29-kDa serine endopeptidase that displays selective trypsin- and chymotrypsin-like substrate specificity. This enzyme has been localized to epitheliocytes of Brunner's glands, and as described here, to mast cells within the intestinal mucosa and lungworm-infected lung, implying an important additional role in inflammation and tissue remodelling. In primary cultures of pulmonary artery fibroblasts, duodenase induced a concentration-dependent increase in [3H]thymidine incorporation with a maximal effect observed at 30 nm. Pretreating duodenase with soybean trypsin inhibitor abolished DNA synthesis, confirming that proteolytic activity was an essential requirement for this response. PAR1, PAR2 and PAR4 activating peptides were unable to induce [3H]thymidine incorporation in pulmonary artery fibroblasts. Likewise, pretreatment of fibroblasts with TNFalpha, known to up-regulate PAR2 expression in other systems, and IL-1beta, did not enhance the potential of duodenase to induce DNA synthesis. Furthermore, duodenase increased GTPgammaS binding to fibroblast membranes indicating that a G-protein-coupled receptor may mediate the effects of duodenase. Duodenase-induced DNA synthesis and GTPgammaS binding were both found to be inhibited by pertussis toxin, implying a role for Gi/o. Selective inhibitors of MEK1 (PD98059) and protein kinase C (GF109203X) only partially inhibited duodenase-induced DNA synthesis, but both wortmannin (100 nm) and LY294002 (10 microm) inhibited this response completely, indicating a key role for PtdIns 3-kinase. Furthermore, duodenase induced a 2.3 plus minus 0.1-fold increase in PtdIns 3-kinase activity in p85 immunoprecipitates, which was sensitive to inhibition by wortmannin. These results suggest that duodenase can induce pulmonary artery fibroblast DNA synthesis in a PtdIns 3-kinase-dependent manner via a G-protein-coupled receptor which is activated by a proteolytic mechanism.