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

The production of collagen and the activity of mast-cell chymase increase in human skin after irradiation therapy

Fibrosis is a common complication of radiotherapy. The pathogenesis of radiation-induced fibrosis is not known in detail. There is increasing evidence to suggest that mast cells contribute to various fibrotic conditions. Several mast-cell mediators have been proposed to have a role in fibrogenesis. Tryptase and chymase, the predominant proteins in mast cells, have been shown to induce fibroblast proliferation and collagen synthesis in vitro. In order to explore the role of mast cells in irradiation-induced fibrosis, we analyzed skin biopsies and suction blister fluid (SBF) samples from the lesional and healthy-looking skin of 10 patients who had been treated for breast cancer with surgery and radiotherapy. The biopsies were analyzed histochemically for mast-cell tryptase, chymase, kit receptor, and tumor necrosis factor-alpha. Skin collagen synthesis was assessed by determining the levels of type I and III procollagen amino-terminal propeptides (PINP and PIIINP) in SBF and using immunohistochemical staining for PINP. Immunohistochemical stainings for prolyl-4-hydroxylase reflecting collagen synthesis and chymase immunoreactivity in irradiated and control skin were also performed. The mean level of procollagen propeptides in SBF, which reflects actual skin collagen synthesis in vivo, was markedly increased in irradiated skin compared to corresponding healthy control skin areas. The mean number of PINP-positive fibroblasts was also significantly increased in the upper dermis of radiotherapy-treated skin. The number of cells positive for tryptase, chymase and kit receptor was markedly increased in irradiated skin. In addition, using double-staining techniques, it was possible to demonstrate that in some areas of the dermis, tryptase-positive mast cells and fibroblasts are closely associated. These findings suggest a possible role of mast cells in enhanced skin collagen synthesis and fibrosis induced by radiotherapy.

Vascular endothelial growth factor-induced secretion of fibronectin is ERK dependent

In severe asthma, cytokines and growth factors contribute to the proliferation of smooth muscle cells and blood vessels, and to the increased extracellular matrix deposition that constitutes the process of airway remodeling. Vascular endothelial growth factor (VEGF), which regulates vascular permeability and angiogenesis, also modulates the function of nonendothelial cell types. In this study, we demonstrate that VEGF induces fibronectin secretion by human airway smooth muscle (ASM) cells. In addition, stimulation of ASM with VEGF activates ERK, but not p38MAPK, and fibronectin secretion is ERK dependent. Both ERK activation and fibronectin secretion appear to be mediated through the VEGF receptor flt-1, as evidenced by the effects of the flt-1-specific ligand placenta growth factor. Finally, we demonstrate that ASM cells constitutively secrete VEGF, which is increased in response to PDGF, transforming growth factor-beta, IL-1beta, and PGE(2). We conclude that ASM-derived VEGF, through modulation of the extracellular matrix, may play an important role in airway remodeling seen in asthma.

The chymase, mouse mast cell protease 4, constitutes the major chymotrypsin-like activity in peritoneum and ear tissue. A role for mouse mast cell protease 4 in thrombin regulation and fibronectin turnover

To gain insight into the biological role of mast cell chymase we have generated a mouse strain with a targeted deletion in the gene for mast cell protease 4 (mMCP-4), the mouse chymase that has the closest relationship to the human chymase in terms of tissue localization and functional properties. The inactivation of mMCP-4 did not affect the storage of other mast cell proteases and did not affect the number of mast cells or the mast cell morphology. However, mMCP-4 inactivation resulted in complete loss of chymotryptic activity in the peritoneum and in ear tissue, indicating that mMCP-4 is the main source of stored chymotrypsin-like protease activity at these sites. The mMCP-4 null cells showed markedly impaired ability to perform inactivating cleavages of thrombin, indicating a role for mMCP-4 in regulating the extravascular coagulation system. Further, a role for mMCP-4 in connective tissue remodeling was suggested by the inability of mMCP-4 null peritoneal cells to process endogenous fibronectin.

The liberation of CD44

CD44 was once thought to simply be a transmembrane adhesion molecule that also played a role in the metabolism of its principal ligand hyaluronan. Investigations of CD44 over the past approximately 20 yr have established additional functions for CD44, including its capacity to mediate inflammatory cell function and tumor growth and metastasis. It has also become evident that intricate posttranslational modifications of CD44 regulate the affinity of the receptor for its ligands. In this review, we focus on emerging evidence that functional fragments of the cytoplasmic and ectodomain of CD44 can be liberated by enzymatic modification of cell surfaces as well as of cell-associated matrix. Based on the evidence discussed, we propose that CD44 exists in three phases, as a transmembrane receptor, as an integral component of the matrix, and as a soluble protein found in body fluids, each with biologically significant functions of which some are shared and some distinct. Thus, CD44 represents a model for understanding posttranslational processing and its emerging role as a general mechanism for regulating cell behavior.

The role of the mast cell in asthma: a reassessment

PURPOSE OF REVIEW

Interest in the role of the mast cell in bronchial asthma has waxed and waned over several decades, but there is now compelling evidence that mast cells make an important contribution to the pathophysiology of this disease. This review will discuss current advances in this field.

RECENT FINDINGS

Mast cells, but not T cells or eosinophils, localize within the bronchial smooth muscle bundles in patients with asthma but not in normal individuals or those with eosinophilic bronchitis. Smooth muscle mast cell density correlates significantly with indices of bronchial hyperresponsiveness, and is likely to be an important factor determining the asthmatic phenotype. Tryptase induces proliferation in human airway smooth muscle, and the recently identified transmembrane form induces the development of bronchial hyperresponsiveness in mice. IL-4 and IL-13, known mast cell products, also induce bronchial hyperresponsiveness in the mouse, in the absence of an inflammatory response. There are therefore several pathways by which the close approximation of mast cells with airway smooth muscle cells might lead to disordered smooth muscle function. Mast cells also infiltrate the airway mucous glands in patients with asthma, showing features of degranulation, and a positive correlate with the amount of mucus obstructing the airway lumen. Taken together these observations suggest that mast cells also play an important role in regulating mucous gland secretion.

SUMMARY

The development of potent and specific inhibitors of mast cell secretion, which remain active when administered long term to asthmatic airways, should offer a novel approach to the treatment of asthma.

Airway remodeling and long-term decline in lung function in asthma

Asthma is a heterogeneous disease that frequently shows progression of airway obstruction. There are indications that ongoing airway inflammation is responsible for the more severe hyperresponsiveness, lower lung function, and accelerated loss of forced expiratory volume in 1 second. At this moment, we are not well informed about the contributing factors to and the exact relation between airway inflammation and airway remodeling. Exciting new findings during the past years are the potential role of interleukin-13, the further exploration of the epithelial-mesenchymal trophic unit, and the presence of degranulated mast cells in smooth muscle bundles from the bronchi.