Differential effects of extracellular matrix proteins on human airway smooth muscle cell proliferation and phenotype

Diminished sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) expression contributes to airway remodelling in bronchial asthma

Phenotypic modulation of airway smooth muscle (ASM) is an important feature of airway remodeling in asthma that is characterized by enhanced proliferation and secretion of pro-inflammatory chemokines. These activities are regulated by the concentration of free Ca(2+) in the cytosol ([Ca(2+)](i)). A rise in [Ca(2+)](i) is normalized by rapid reuptake of Ca(2+) into sarcoplasmic reticulum (SR) stores by the sarco/endoplasmic reticulum Ca(2+) (SERCA) pump. We examined whether increased proliferative and secretory responses of ASM from asthmatics result from reduced SERCA expression. ASM cells were cultured from subjects with and without asthma. SERCA expression was evaluated by western blot, immunohistochemistry and real-time PCR. Changes in [Ca(2+)](i), cell spreading, cellular proliferation, and eotaxin-1 release were measured. Compared with control cells from healthy subjects, SERCA2 mRNA and protein expression was reduced in ASM cells from subjects with moderately severe asthma. SERCA2 expression was similarly reduced in ASM in vivo in subjects with moderate/severe asthma. Rises in [Ca(2+)](i) following cell surface receptor-induced SR activation, or inhibition of SERCA-mediated Ca(2+) re-uptake, were attenuated in ASM cells from asthmatics. Likewise, the return to baseline of [Ca](i) after stimulation by bradykinin was delayed by approximately 50% in ASM cells from asthmatics. siRNA-mediated knockdown of SERCA2 in ASM from healthy subjects increased cell spreading, eotaxin-1 release and proliferation. Our findings implicate a deficiency in SERCA2 in ASM in asthma that contributes to its secretory and hyperproliferative phenotype in asthma, and which may play a key role in mechanisms of airway remodeling.

Effect of extracellular matrix composition on airway epithelial cell and fibroblast structure: implications for airway remodeling in asthma

BACKGROUND

Airway remodeling in asthma is characterized by structural changes to the airways including extracellular matrix (ECM) deposition and epithelial metaplasia. Extracellular matrix deposition in the subepithelial region may play an important role in modulation of epithelial cell and fibroblast structure and function because it lies in immediate contact with these cell types and exists within the functional epithelial mesenchymal trophic unit.

OBJECTIVE

To investigate the effect of aberrant ECM components on airway epithelial cells and fibroblasts and the relationship among subepithelial ECM deposition, other remodeling changes, and airway hyperresponsiveness.

METHODS

BEAS-2B human airway epithelial cells and WI-38 human airway fibroblast cells were cultured on various ECM protein substrates (Matrigel, representing normal basement membrane matrix, or aberrant matrix proteins collagen I, collagen III, and fibronectin). Airway remodeling changes were determined using morphometry in sections from a murine model of chronic allergic airway disease. Airway reactivity to methacholine was determined, and these parameters correlated.

RESULTS

Abnormal ECM substrates induced epithelial and fibroblast proliferation and altered the cell morphology of both human airway epithelial cells and fibroblasts when compared with normal basement membrane ECM. Subepithelial matrix deposition in the mouse correlated with epithelial thickness, but only weak correlations were noted among the other parameters.

CONCLUSIONS

We have demonstrated that ECM may affect the growth of airway epithelial cells and fibroblasts in vitro and may influence epithelial thickness in the mouse. These findings may have implications for understanding the pathogenesis of asthma and future therapeutic targeting of airway remodeling.

Concurrent dual allergen exposure and its effects on airway hyperresponsiveness, inflammation and remodeling in mice

Experimental mouse models of asthma have broadened our understanding of the mechanisms behind allergen-induced asthma. Typically, mouse models of allergic asthma explore responses to a single allergen; however, patients with asthma are frequently exposed to, and tend to be allergic to, more than one allergen. The aim of the current study was to develop a new and more relevant mouse model of asthma by measuring the functional, inflammatory and structural consequences of chronic exposure to a combination of two different allergens, ovalbumin (OVA) and house dust mite (HDM), in comparison with either allergen alone. BALB/c mice were sensitized and exposed to OVA, HDM or the combination of HDM and OVA for a period of 10 weeks. Following allergen exposure, airway responsiveness was measured using the flexiVent small animal ventilator, and mice were assessed for indices of airway inflammation and remodeling at both 24 hours and 4 weeks after the final allergen exposure. Mice exposed to the HDM-OVA combination exhibited increased numbers of inflammatory cells in the bronchoalveolar lavage (BAL) when compared with mice exposed to a single allergen. Mice exposed to HDM-OVA also exhibited an elevated level of lung tissue mast cells compared with mice exposed to a single allergen. Following the resolution of inflammatory events, mice exposed to the allergen combination displayed an elevation in the maximal degree of total respiratory resistance (Max R(RS)) compared with mice exposed to a single allergen. Furthermore, trends for increases in indices of airway remodeling were observed in mice exposed to the allergen combination compared with a single allergen. Although concurrent exposure to HDM and OVA resulted in increased aspects of airway hyperresponsiveness, airway inflammation and airway remodeling when compared with exposure to each allergen alone, concurrent exposure did not result in a substantially more robust mouse model of allergic asthma than exposure to either allergen alone.

Use of a three-dimensional cell culture model to study airway smooth muscle-mast cell interactions in airway remodeling

Increased airway smooth muscle (ASM) mass and infiltration by mast cells are key features of airway remodeling in asthma. We describe a model to investigate the relationship between ASM, the extracellular matrix, mast cells, and airway remodeling. ASM cells were cultured in a three-dimensional (3-D) collagen I gel (3-D culture) alone or with mast cells. Immunocytochemistry and Western blotting of ASM in 3-D cultures revealed a spindle-shaped morphology and significantly lower alpha-smooth muscle actin and vimentin expression than in ASM cultured in monolayers on collagen type I or plastic (2-D culture). In 3-D cultures, basal ASM proliferation, examined by Ki67 immunocytochemistry, was reduced to 33 +/- 7% (P < 0.05) of that in 2-D cultures. The presence of mast cells in cocultures increased ASM proliferation by 1.8-fold (P < 0.05). Gelatin zymography revealed more active matrix metalloproteinase (MMP)-2 in 3-D than in 2-D culture supernatants over 7 days. Functional MMP activity was examined by gel contraction. The spontaneous gel contraction over 7 days was significantly inhibited by the MMP inhibitor ilomastat. Mast cell coculture enhanced ASM gel contraction by 22 +/- 16% (not significant). Our model shows that ASM has different morphology, with lower contractile protein expression and basal proliferation in 3-D culture. Compared with standard techniques, ASM synthetic function, as shown by MMP production and activity, is sustained over longer periods. The presence of mast cells in the 3-D model enhanced ASM proliferation and MMP production. Airway remodeling in asthma may be more accurately modeled by our system than by standard culture systems.

Fibrillar collagen clamps lung mesenchymal cells in a nonproliferative and noncontractile phenotype

Pulmonary fibrosis is characterized by phenotypic changes to mesenchymal cells and an increase in the deposition of fibrillar collagen (fCollagen). This study investigated the effect of type I fCollagen on the phenotypic plasticity of human parenchymal fibroblasts (PFbs) in vitro. Cell numbers were 45% lower when cultured on fCollagen as compared with culture on its degradation product, monomeric collagen (mCollagen). DNA profiles indicated that fCollagen is antiproliferative, rather than proapoptotic. fCollagen suppressed basic fibroblast growth factor-stimulated increases in the levels of cyclin E and CDK2 mRNA. fCollagen also suppressed transforming growth factor-beta (100 pM)-stimulated increases in the mRNA and protein levels of alpha-smooth muscle actin (alpha-SMA), a marker of the myofibroblast phenotype. However, in cells exposed to fCollagen, the levels of matrix metalloproteinase (MMP)-1 and -14 mRNA, as well as active MMP-2 protein, were increased by between two- and fivefold. The MMP inhibitors, ilomastat (10 microM) and doxycycline (30 microM), attenuated the dissolution of collagen fibrils by fibroblasts maintained on fCollagen, with a corresponding decrease in cell number. Ilomastat also reduced alpha-SMA expression and the capacity of PFb to contract three-dimensional fCollagen gels. Thus, exposure of fibroblasts to the fibrillar form of type I collagen in vitro reduces cell proliferation, increases MMP production and activation, and attenuates differentiation of PFb into myofibroblasts. fCollagen appears to apply a phenotypic clamp on lung fibroblasts that may be partially released by autocrine MMP activity.

Cell stiffness, contractile stress and the role of extracellular matrix

Here we have assessed the effects of extracellular matrix (ECM) composition and rigidity on mechanical properties of the human airway smooth muscle (ASM) cell. Cell stiffness and contractile stress showed appreciable changes from the most relaxed state to the most contracted state: we refer to the maximal range of these changes as the cell contractile scope. The contractile scope was least when the cell was adherent upon collagen V, followed by collagen IV, laminin, and collagen I, and greatest for fibronectin. Regardless of ECM composition, upon adherence to increasingly rigid substrates, the ASM cell positively regulated expression of antioxidant genes in the glutathione pathway and heme oxygenase, and disruption of a redox-sensitive transcription factor, nuclear erythroid 2 p45-related factor (Nrf2), culminated in greater contractile scope. These findings provide biophysical evidence that ECM differentially modulates muscle contractility and, for the first time, demonstrate a link between muscle contractility and Nrf2-directed responses.

Characterization of primary cilia in human airway smooth muscle cells

BACKGROUND

Considerable evidence indicates a key role for primary cilia of mammalian cells in mechanochemical sensing. Dysfunctions of primary cilia have been linked to the pathogenesis of several human diseases. However, cilia-related research has been limited to a few cell and tissue types; to our knowledge, no literature exists on primary cilia in airway smooth muscle (ASM). The aim of this study was to characterize primary cilia in human ASM.

METHODS

Primary cilia of human bronchial smooth muscle cells (HBSMCs) were examined using immunofluorescence confocal microscopy, and scanning and transmission electron microscopy. HBSMC migration and injury repair were examined by scratch-wound and epidermal growth factor (EGF)-induced migration assays.

RESULTS

Cross-sectional images of normal human bronchi revealed that primary cilia of HBSMCs within each ASM bundle aggregated at the same horizontal level, forming a "cilium layer." Individual cilia of HBSMCs projected into extracellular matrix and exhibited varying degrees of deflection. Mechanochemical sensing molecules, polycystins, and alpha2-, alpha5-, and beta1-integrins were enriched in cilia, as was EGF receptor, known to activate jointly with integrins during cell migration. Migration assays demonstrated a ciliary contribution to HBSMC migration and wound repair.

CONCLUSIONS

The primary cilia of ASM cells exert a role in sensing and transducing extracellular mechanochemical signals and in ASM injury repair. Defects in ASM ciliary function could potentially affect airway wall maintenance and/or remodeling, possibly relating to the genesis of bronchiectasis in autosomal dominant polycystic kidney disease, a disease of ciliopathy.

Reconstruction is not renovation - the role of remodeling in asthma

The chronicity of asthma results not only in persistent lung inflammation but also in changes in structure and composition of this vital organ. These changes are most commonly referred to as remodeling, and include epithelial dysplasia, angiogenesis, changes in the extracellular matrix and increased smooth muscle mass. In this review we summarize recent findings on the contribution of remodeling to the pathological phenotype of asthma. We discuss how and why current treatment (such as corticosteroids) options fail to adequately treat remodeling.

Primary murine airway smooth muscle cells exposed to poly(I,C) or tunicamycin synthesize a leukocyte-adhesive hyaluronan matrix

Asthmatic attacks often follow viral infections with subsequent airway smooth muscle cell proliferation and the formation of an abnormal hyaluronan extracellular matrix with infiltrated leukocytes. In this study, we show that murine airway smooth muscle cells (MASM) treated with polyinosinic acid-polycytidylic acid (poly(I,C)), a double-stranded RNA that simulates a viral infection, synthesize an abnormal hyaluronan matrix that binds leukocytes (U937 cells). Synthesis of this matrix is initiated rapidly and accumulates linearly for approximately 10 h, reaching a plateau level approximately 7-fold higher than control cultures. MASM cells treated with tunicamycin, to induce endoplasmic reticulum stress, also rapidly initiate synthesis of the abnormal hyaluronan matrix with linear accumulation for approximately 10 h, but only reach a plateau level approximately 2-fold higher than control cultures. In contrast to poly(I,C), the response to tunicamycin depends on cell density, with pre-confluent cells producing more abnormal matrix per cell. Furthermore, U937 cell adhesion per hyaluronan content is higher in the sparse matrix produced in response to tunicamycin, suggesting that the structure in the poly(I,C)-induced matrix masks potential binding sites. When MASM cells were exposed to tunicamycin and poly(I,C) at the same time, U937 cell adhesion was partially additive, implying that these two toxins stimulate hyaluronan synthesis through two different pathways. We also characterized the size of hyaluronan produced by MASM cells, in response to poly(I,C) and tunicamycin, and we found that it ranges from 1500 to 4000 kDa, the majority of which was approximately 4000 kDa and not different in size than hyaluronan made by untreated cells.

High expression of alpha 2, 3-linked sialic acid residues is associated with the metastatic potential of human gastric cancer

BACKGROUND

Sialic acid, as a terminal saccharide residue on cell surface glycoconjugates, plays an important role in a variety of biological processes. However, the precise nature of the molecules in gastric cancers has not been unveiled nor documented to be of clinical relevance. Herein, we measured the expression of alpha 2, 3-linked sialic acid residues by using a specific lectin as well as the potential of invasion and metastasis of gastric cancer was analyzed.

METHODS

The expression of alpha 2, 3-linked sialic acid residues in 100 cases of gastric cancer samples was evaluated using Maackia amurensis leukoagglutinin (MAL) histochemical staining analysis. The assays of cytochemical staining and flow cytometry were employed to determine the MAL positive cells in the gastric cancer cell lines. The activities of invasion and migration were evaluated using the assays of cell adhesion and transwell chamber.

RESULTS

The staining of MAL in gastric cancer tissues showed that high levels of alpha 2, 3-linked sialic acid residues were closely associated with the invasive depth (P=0.0003) and lymph node metastasis (P=0.0441). In gastric adenocarcinoma cell lines, SGC-7901, the highly metastatic cell line, displayed the most positive reaction with MAL among the selected cell lines. The potential of invasion and migration was confirmed using the assays of adhesion and transwell chamber that SGC-7901 exhibited the high activity of adhesion to extracellular matrix (ECM) and penetration to Matrigel.

CONCLUSION

These results suggested that high level of alpha 2, 3-linked sialic acid residues was associated with metastatic potential of gastric cancer cells.

Insulin-induced laminin expression promotes a hypercontractile airway smooth muscle phenotype

Airway smooth muscle (ASM) plays a key role in the development of airway hyperresponsiveness and remodeling in asthma, which may involve maturation of ASM cells to a hypercontractile phenotype. In vitro studies have indicated that long-term exposure of bovine tracheal smooth muscle (BTSM) to insulin induces a functional hypercontractile, hypoproliferative phenotype. Similarly, the extracellular matrix protein laminin has been found to be involved in both the induction and maintenance of a contractile ASM phenotype. Using BTSM, we now investigated the role of laminins in the insulin-induced hypercontractile, hypoproliferative ASM phenotype. The results demonstrate that insulin-induced hypercontractility after 8 days of tissue culture was fully prevented by combined treatment of BTSM-strips with the laminin competing peptides Tyr-Ile-Gly-Ser-Arg (YIGSR) and Arg-Gly-Asp-Ser (RGDS). YIGSR also prevented insulin-induced increases in sm-myosin expression and abrogated the suppressive effects of prolonged insulin treatment on platelet-derived growth factor-induced DNA synthesis in cultured cells. In addition, insulin time-dependently increased laminin alpha2, beta1, and gamma1 chain protein, but not mRNA abundance in BTSM strips. Moreover, as previously found for contractile protein accumulation, signaling through PI3-kinase- and Rho kinase-dependent pathways was required for the insulin-induced increase in laminin abundance and contractility. Collectively, our results indicate a critical role for beta1-containing laminins, likely laminin-211, in the induction of a hypercontractile, hypoproliferative ASM phenotype by prolonged insulin exposure. Increased laminin production by ASM could be involved in the increased ASM contractility and contractile protein expression in asthma. Moreover, the results may be of interest for the use of inhaled insulin administrations by diabetics.

The role of the extracellular matrix and specific growth factors in the regulation of inflammation and remodelling in asthma

Asthma is a disease characterised by persistent inflammation and structural changes in the airways, referred to as airway remodelling. The mechanisms underlying these processes may be interdependent or they may be separate processes that are driven by common factors. The levels of a variety of growth factors (including transforming growth factor beta, granulocyte macrophage colony stimulating factor, and vascular endothelial growth factor) are known to be changed in the asthmatic airway. These and other growth factors can contribute to the development and persistence of inflammation and remodelling. One of the prominent features of the structural changes of the airways is the increased deposition and alterations in the composition of the extracellular matrix proteins. These proteins include fibronectin, many different collagen types and hyaluronan. There is a dynamic relationship between the extracellular matrix proteins and the airway mesenchymal cells such that the changes in the extracellular matrix proteins can also contribute to the persistence of inflammation and the airway remodelling. This review aims to summarise the role growth factors and extracellular matrix proteins play in the regulation of inflammation and airway remodelling in the asthmatic airway.

Lipid metabolites as regulators of airway smooth muscle function

Compelling evidence identifies airway smooth muscle (ASM) not only as a target but also a cellular source for a diverse range of mediators underlying the processes of airway narrowing and airway hyperresponsiveness in diseases such as asthma. These include the growing family of plasma membrane phospholipid-derived polyunsaturated fatty acids broadly characterised by the prostaglandins, leukotrienes, lipoxins, isoprostanes and lysophospholipids. In this review, we describe the enzymatic and non-enzymatic biosynthetic pathways of these lipid mediators and how these are influenced by drug treatment, oxidative stress and airways disease. Additionally, we outline their cognate receptors, many of which are expressed by ASM. We describe potential deleterious and protective roles for these lipid mediators in airway inflammatory and remodelling processes by describing their effects on diverse functions of ASM in asthma that have the potential to contribute to asthma pathogenesis and symptoms. These functions include contractile tone development, cytokine and extracellular matrix production, and cellular proliferation and migration.

Airway smooth muscle in asthma: phenotype plasticity and function

Clinical asthma is characterized by reversible airway obstruction which is commonly due to an exaggerated airway narrowing referred to as airway hyperresponsiveness (AHR). Although debate exists on the complex etiology of AHR, it is clear that airway smooth muscle (ASM) mediated airway narrowing is a major contributor to airway dysfunction. More importantly, it is now appreciated that smooth muscle is far from being a simple cell with only contractile ability properties. Rather, it is more versatile with the capacity to exhibit numerous cellular functions as it adapts to the microenvironment to which it is exposed. The emerging ability of individual smooth muscle cells to undergo changes in their phenotype (phenotype plasticity) and function (functional plasticity) in response to physiological and pathological cues is an important and active area of research. This article provides a brief review of the current knowledge and emerging concepts in the field of ASM phenotype and function both under healthy and asthmatic conditions.

Tissue and matrix influences on airway smooth muscle function

Asthma is characterized by structural changes in the airways - airway remodelling. These changes include an increase in the bulk of the airway smooth muscle (ASM) and alterations in the profile of extracellular matrix (ECM) proteins in the airway wall. The mechanisms leading to airway remodelling are not well understood. ASM cells have the potential to play a key role in these processes through the production and release of ECM proteins. The ASM cells and ECM proteins are each able to influence the behaviour and characteristics of the other. The modified ECM profile in the asthmatic airway may contribute to the altered behaviour of the ASM cells, such responses to ECM proteins are modulated through the cell surface expression of integrin receptors. ASM cells from asthmatic individuals express different levels of some integrin subunits compared to nonasthmatic ASM cells, which have the potential to further influence their responses to the ECM proteins in the airways. ECM homeostasis requires the presence and activation of matrix metalloproteinases and their tissue inhibitors, which in turn modulate the interaction of the ASM cells and the ECM proteins. Furthermore, the complex interactions of the ASM cells and the ECM in the asthmatic airways and the role played by external stimuli, such as viral infections, to modulate airway remodelling are currently unknown. This review summarises our current understanding of the influence of the ECM on ASM function.

Structural aspects of airway remodeling in asthma

Airway remodeling in asthma is a complex process that involves structural changes in virtually all tissues of the airway wall. The histologic changes to the airways consist of epithelial proliferation and goblet cell differentiation, subepithelial fibrosis, airway smooth muscle (ASM) growth, angiogenesis, matrix protein deposition, gland hyperplasia and hypertrophy, and nerve proliferation. Cytokines, chemokines, and growth factors from inflammatory cells and structural cells contribute to remodeling. There are complex interactions among the various signaling pathways involving matrix metalloproteinases that are required for growth factor release. The physiologic consequences of remodeling are airway hyperresponsiveness from ASM growth and mucus hypersecretion from gland and goblet cell hyperplasia. Airway stiffening is a probable contributor to airway hyperresponsiveness through attenuation of the transmission of potently bronchodilating cyclical stress to the ASM during breathing. The epidermal growth factor receptor's role in remodeling and its interaction with other potential causes of remodeling are discussed.

Glucocorticoid- and protein kinase A-dependent transcriptome regulation in airway smooth muscle

Glucocorticoids (GCs) and protein kinase A (PKA)-activating agents (beta-adrenergic receptor agonists) are mainstream asthma therapies based on their ability to prevent or reverse excessive airway smooth muscle (ASM) constriction. Their abilities to regulate another important feature of asthma--excessive ASM growth--are poorly understood. Recent studies have suggested that GCs render agents of inflammation such as IL-1 beta and TNF-alpha mitogenic to ASM, via suppression of (antimitogenic) induced cyclooxygenase-2-dependent PKA activity. To further explore the mechanistic basis of these observations, we assessed the effects of epidermal growth factor and IL-1 beta stimulation, and the modulatory effects of GC treatment and PKA inhibition, on the ASM transcriptome by microarray analysis. Results demonstrate that ASM stimulated with IL-1 beta, in a manner that is often cooperative with stimulation with epidermal growth factor, exhibit a profound capacity to function as immunomodulatory cells. Moreover, results implicate an important role for induced autocrine/paracrine factors (many whose regulation was minimally affected by GCs or PKA inhibition) as regulators of both airway inflammation and ASM growth. Induction of numerous chemokines, in conjunction with regulation of proteases and agents of extracellular matrix remodeling, is suggested as an important mechanism promoting upregulated G protein-coupled receptor signaling capable of stimulating ASM growth. Additional functional assays suggest that intracellular PKA plays a critical role in suppressing the promitogenic effects of induced autocrine factors in ASM. Finally, identification and comparison of GC- and PKA-sensitive genes in ASM provide insight into the complementary effects of beta-agonist/GC combination therapies, and suggest specific genes as important targets for guiding the development of new generations of GCs and adjunct asthma therapies.

Integrin-linked kinase regulates smooth muscle differentiation marker gene expression in airway tissue

Phenotypic changes in airway smooth muscle occur with airway inflammation and asthma. These changes may be induced by alterations in the extracellular matrix that initiate signaling pathways mediated by integrin receptors. We hypothesized that integrin-linked kinase (ILK), a multidomain protein kinase that binds to the cytoplasmic tail of beta-integrins, may be an important mediator of signaling pathways that regulate the growth and differentiation state of airway smooth muscle. We disrupted signaling pathways mediated by ILK in intact differentiated tracheal muscle tissues by depleting ILK protein using ILK antisense. The depletion of ILK protein increased the expression of the smooth muscle differentiation marker genes myosin heavy chain (SmMHC), SM22alpha, and calponin and increased the expression of SmMHC protein. Conversely, the overexpression of ILK protein reduced the mRNA levels of SmMHC, SM22alpha, and calponin and SmMHC protein. Analysis by chromatin immunoprecipitation showed that the binding of the transcriptional regulator serum response factor (SRF) to the promoters of SmMHC, SM22alpha, and calponin genes was increased in ILK-depleted tissues and decreased in tissues overexpressing ILK. ILK depletion also increased the amount of SRF that localized within the nucleus. ILK depletion and overexpression, respectively, decreased and increased the activation of its downstream substrate protein kinase B (PKB/Akt). The pharmacological inhibition of Akt activity also increased SRF binding to the promoters of smooth muscle-specific genes and increased expression of smooth muscle proteins, suggesting that ILK may exert its effects by regulating the activity of Akt. We conclude that ILK is a critical regulator of airway smooth muscle differentiation. ILK may mediate signals from integrin receptors that control airway smooth muscle differentiation in response to alterations in the extracellular matrix.

Role of the cystic fibrosis transmembrane conductance channel in human airway smooth muscle

Patients with cystic fibrosis (CF) suffer from asthma-like symptoms and gastrointestinal cramps, attributed to a mutation in the CF transmembrane conductance regulator (CFTR) gene present in a variety of cells. Pulmonary manifestations of the disease include the production of thickened mucus and symptoms of asthma, such as cough and wheezing. A possible alteration in airway smooth muscle (ASM) cell function of patients with CF has not been investigated. The aim of this study was to determine whether the (CFTR) channel is present and affects function of human ASM cells. Cell cultures were obtained from the main or lobar bronchi of patients with and without CF, and the presence of the CFTR channel detected by immunofluorescence. Cytosolic Ca(2+) was measured using Fura-2 and dual-wavelength microfluorimetry. The results show that CFTR is expressed in airway bronchial tissue and in cultured ASM cells. Peak Ca(2+) release in response to histamine was significantly decreased in CF cells compared with non-CF ASM cells (357 +/- 53 nM versus 558 +/- 20 nM; P < 0.001). The CFTR pharmacological blockers, glibenclamide and N-phenyl anthranilic acid, significantly reduced histamine-induced Ca(2+) release in non-CF cells, and similar results were obtained when CFTR expression was varied using antisense oligonucleotides. In conclusion, these data show that the CFTR channel is present in ASM cells, and that it modulates the release of Ca(2+) in response to contractile agents. In patients with CF, a dysfunctional CFTR channel could contribute to the asthma diathesis and gastrointestinal problems experienced by these patients.

Airway smooth muscle cell tone amplifies contractile function in the presence of chronic cyclic strain

Chronic contractile activation, or tone, in asthma coupled with continuous stretching due to breathing may be involved in altering the contractile function of airway smooth muscle (ASM). Previously, we (11) showed that cytoskeletal remodeling and stiffening responses to acute (2 h) localized stresses were modulated by the level of contractile activation of ASM. Here, we investigated if altered contractility in response to chronic mechanical strain was dependent on repeated modulation of contractile tone. Cultured human ASM cells received 5% cyclic (0.3 Hz), predominantly uniaxial strain for 5 days, with once-daily dosing of either sham, forskolin, carbachol, or histamine to alter tone. Stiffness, contractility (KCl), and "relaxability" (forskolin) were then measured as was cell alignment, myosin light-chain phosphorylation (pMLC), and myosin light-chain kinase (MLCK) content. Cells became aligned and baseline stiffness increased with strain, but repeated lowering of tone inhibited both effects (P < 0.05). Strain also reversed a negative tone-modulation dependence of MLCK, observed in static conditions in agreement with previous reports, with strain and tone together increasing both MLCK and pMLC. Furthermore, contractility increased 176% (SE 59) with repeated tone elevation. These findings indicate that with strain, and not without, repeated tone elevation promoted contractile function through changes in cytoskeletal organization and increased contractile protein. The ability of repeated contractile activation to increase contractility, but only with mechanical stretching, suggests a novel mechanism for increased ASM contractility in asthma and for the role of continuous bronchodilator and corticosteroid therapy in reversing airway hyperresponsiveness.

Heparin and structurally related polymers attenuate eotaxin-1 (CCL11) release from human airway smooth muscle

BACKGROUND AND PURPOSE

The glycosaminoglycan heparin has anti-inflammatory activity and is exclusively found in mast cells, which are localized within airway smooth muscle (ASM) bundles of asthmatic airways. Interleukin (IL)-13 induces the production of multiple inflammatory mediators from ASM including the eosinophil chemoattractant chemokine, eotaxin-1. Heparin and related glycosaminoglycan polymers having structurally heterogeneous polysaccharide side chains that varied in molecular weight, sulphation and anionic charge were used to identify features of the heparin molecule linked to anti-inflammatory activity.

EXPERIMENTAL APPROACH

Cultured human ASM cells were stimulated with interleukin (IL)-13 in the absence or presence of heparin and related polymers. Eotaxin-1 was quantified using chemokine antibody arrays and ELISA.

KEY RESULTS

Unfractionated heparin attenuated IL-13-dependent eotaxin-1 production and this effect was reproduced with low molecular weight heparins (3 and 6 kDa), demonstrating a minimum activity fragment of at least 3 kDa. N-desulphated, 20% re-N-acetylated heparin (anticoagulant) was ineffective against IL-13-dependent eotaxin-1 production compared with 90% re-N-acetylated (anticoagulant) or O-desulphated (non-anticoagulant) heparin, suggesting a requirement for N-sulphation independent of anticoagulant activity. Other sulphated molecules with variable anionic charge and molecular weight exceeding 3 kDa (dextran sulphate, fucoidan, chondroitin sulphate B) inhibited IL-13-stimulated eotaxin-1 release to varying degrees. However, non-sulphated dextran had no effect.

CONCLUSIONS

Inhibition of IL-13-dependent eotaxin-1 release by heparin involved but did not depend upon sulphation, though loss of N-sulphation reduced the attenuating activity, which could be restored by N-acetylation. This anti-inflammatory effect was also partially dependent on anionic charge, but independent of molecular size above 3 kDa and the anticoagulant action of heparin.

Expression of smooth muscle and extracellular matrix proteins in relation to airway function in asthma

BACKGROUND

Smooth muscle content is increased within the airway wall in patients with asthma and is likely to play a role in airway hyperresponsiveness. However, smooth muscle cells express several contractile and structural proteins, and each of these proteins may influence airway function distinctly.

OBJECTIVE

We examined the expression of contractile and structural proteins of smooth muscle cells, as well as extracellular matrix proteins, in bronchial biopsies of patients with asthma, and related these to lung function, airway hyperresponsiveness, and responses to deep inspiration.

METHODS

Thirteen patients with asthma (mild persistent, atopic, nonsmoking) participated in this cross-sectional study. FEV(1)% predicted, PC(20) methacholine, and resistance of the respiratory system by the forced oscillation technique during tidal breathing and deep breath were measured. Within 1 week, a bronchoscopy was performed to obtain 6 bronchial biopsies that were immunohistochemically stained for alpha-SM-actin, desmin, myosin light chain kinase (MLCK), myosin, calponin, vimentin, elastin, type III collagen, and fibronectin. The level of expression was determined by automated densitometry.

RESULTS

PC(20) methacholine was inversely related to the expression of alpha-smooth muscle actin (r = -0.62), desmin (r = -0.56), and elastin (r = -0.78). In addition, FEV(1)% predicted was positively related and deep inspiration-induced bronchodilation inversely related to desmin (r = -0.60), MLCK (r = -0.60), and calponin (r = -0.54) expression.

CONCLUSION

Airway hyperresponsiveness, FEV(1)% predicted, and airway responses to deep inspiration are associated with selective expression of airway smooth muscle proteins and components of the extracellular matrix.

Pharmacology of airway smooth muscle proliferation

Airway smooth muscle thickening is a pathological feature that contributes significantly to airflow limitation and airway hyperresponsiveness in asthma. Ongoing research efforts aimed at identifying the mechanisms responsible for the increased airway smooth muscle mass have indicated that hyperplasia of airway smooth muscle, due in part to airway myocyte proliferation, is likely a major factor. Airway smooth muscle proliferation has been studied extensively in culture and in animal models of asthma, and these studies have revealed that a variety of receptors and mediators contributes to this response. This review aims to provide an overview of the receptors and mediators that control airway smooth muscle cell proliferation, with emphasis on the intracellular signalling mechanisms involved.

Transforming growth factor beta 1 increases fibronectin deposition through integrin receptor alpha 5 beta 1 on human airway smooth muscle

BACKGROUND

Integrin receptors signal to and from the extracellular matrix. Altered expression of the integrin receptors, such as the fibronectin receptor alpha(5)beta(1), might be implicated in extracellular matrix accumulation in airway remodeling in asthma.

OBJECTIVE

We examined the effect of TGF-beta stimulation on integrin alpha(5)beta(1) expression and the role of alpha(5)beta(1) in fibronectin deposition and proliferation.

METHODS

Integrin subunit alpha(5) and beta(1) expression in airway smooth muscle (ASM) from subjects with and without asthma was examined by means of PCR and flow cytometry. The effect of blocking alpha(5)beta(1) receptor on ASM proliferation to FBS was assessed by using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium assay. Cells were stimulated with TGF-beta in the presence or absence of extracellular signal-regulated kinase, phosphoinositide-3 kinase, or p38 inhibitors and antibodies to the alpha(5) and beta(1) subunits. The effect of blocking alpha(5)beta(1) receptor on fibronectin deposition was assessed by means of immunocytochemistry.

RESULTS

Proliferation of ASM cells from asthmatic and nonasthmatic subjects was inhibited by blocking the fibronectin receptor subunit alpha(5)beta(1). TGF-beta-induced alpha(5)beta(1) was extracellular signal-regulated kinase dependent but not phosphoinositide-3 kinase or p38 dependent. Blockade of the alpha(5)beta(1) receptor inhibited TGF-beta-induced fibronectin matrix deposition.

CONCLUSION

Through its increased expression by the profibrotic stimulus TGF-beta, integrin alpha(5)beta(1) might be important in regulating fibronectin deposition.

Effects of decorin and biglycan on human airway smooth muscle cell proliferation and apoptosis

Proteoglycans (PG) are altered in the asthmatic airway wall. Because PGs are known to affect cell proliferation and apoptosis, we hypothesized that alterations in PG might influence the airway smooth muscle (ASM) hyperplasia observed in the asthmatic airway. Human ASM cells were seeded on plastic or plates coated with decorin (Dcn), biglycan (Bgn), or collagen type I (Col I) (1, 3, and 10 microg/ml). Cells were stimulated with platelet-derived growth factor (PDGF), and cell number was assessed at 0, 48, and 96 h. Cell proliferation was measured by bromodeoxyuridine (BrdU) incorporation and apoptosis by annexin V and propidium iodide staining at 48 h post-PDGF stimulation. A significant decrease in cell number was observed with cells seeded on Dcn (10 microg/ml) at 0, 48, and 96 h (P < 0.01). Dcn induced both decreases in BrdU incorporation and increases in annexin V staining (P < 0.05). Bgn decreased cell number at time 0 only (P < 0.05) and affected neither proliferation nor apoptosis. Col I (10 mug/ml) caused a significant increase in cell number at 48 and 96 h (P < 0.01). Adding exogenous Dcn (1-30 microg/ml) to the medium had no effect on cell number. Exposing Dcn-coated matrices to chondroitinase ABC, an enzyme that degrades glycosaminoglycan side chains, reversed the Dcn-induced decrease in cell number. These studies demonstrate that different PGs have variable effects on ASM cell proliferation and apoptosis. Recently described decreases in Dcn in the asthmatic airway wall could potentially permit more exuberant ASM growth.

Small-diameter human vessel wall engineered from bone marrow-derived mesenchymal stem cells (hMSCs)

Using biodegradable scaffold and a biomimetic perfusion system, our lab has successfully engineered small-diameter vessel grafts using endothelial cells (ECs) and smooth muscle cells (SMCs) obtained from vessels in various species. However, translating this technique into humans has presented tremendous obstacles due to species and age differences. SMCs from elderly persons have limited proliferative capacity and a reduction in collagen production, which impair the mechanical strength of engineered vessels. As an alternative cell source, adult human bone marrow-derived mesenchymal stem cells (hMSCs) were studied for their ability to differentiate into SMCs in culture plates as well as in a bioreactor system. In the former setting, immunofluorescence staining showed that MSCs, after induction for 14 days, expressed smooth muscle alpha-actin (SMA) and calponin, early and mid-SMC phenotypic markers, respectively. In the latter setting, vessel walls were constructed with MSC-derived SMCs. Various factors (i.e., matrix proteins, soluble factors, and cyclic strain) in the engineering system were further investigated for their effects on hMSC cell proliferation and differentiation into SMCs. Based on a screening of multiple factors, the engineering system was optimized by dividing the vessel culture into proliferation and differentiation phases. The vessel walls engineered under the optimized conditions were examined histologically and molecularly, and found to be substantially similar to native vessels. In conclusion, bone marrow-derived hMSCs can serve as a new cell source of SMCs in vessel engineering. Optimization of the culture conditions to drive SMC differentiation and matrix production significantly improved the quality of the hMSC-derived engineered vessel wall.

Extracellular matrix and airway smooth muscle interactions: a target for modulating airway wall remodelling and hyperresponsiveness?

The airway smooth muscle from asthmatic airways produces increased amounts and an altered composition of extracellular matrix proteins. The extracellular matrix can in turn influence the phenotype and function of airway smooth muscle cells, affecting the biochemical, geometric, and mechanical properties of the airway wall. This review provides a brief overview of the current understanding of the biology associated with airway smooth muscle interactions with the extracellular matrix. We present future directions needed for the study of cellular and molecular mechanisms that determine the outcomes of extracellular matrix - airway smooth muscle interactions, and discuss their possible importance as determinants of airway function in asthma.

Laminin-binding integrin alpha7 is required for contractile phenotype expression by human airway myocytes

Contractile airway smooth muscle (ASM) cells retain the ability for phenotype plasticity in response to multiple stimuli, which equips them with capacity to direct modeling and remodeling during development, and in disease states such as asthma. We have shown that endogenously expressed laminin is required for maturation of human ASM cells to a contractile phenotype, as occurs during ASM thickening in asthma. In this study, we profiled the expression of laminin-binding integrins alpha3beta1, alpha6beta1, and alpha7beta1, and tested whether they are required for laminin-induced myocyte maturation. Immunoblotting revealed that myocyte maturation induced by prolonged serum withdrawal, which was marked by the accumulation of contractile phenotype marker protein desmin, was also associated with the accumulation of alpha3A, alpha6A, and alpha7B. Flow cytometry revealed that alpha7B expression was a distinct feature of individual myocytes that acquired a contractile phenotype. siRNA knockdown of alpha7, but not alpha3 or alpha6, suppressed myocyte maturation. Thus, alpha7B is a novel marker of the contractile phenotype, and alpha7 expression is essential for human ASM cell maturation, which is a laminin-dependent process. These observations provide new insight into mechanisms that likely underpin normal development and remodeling associated with airways disease.

Airway smooth muscle dynamics: a common pathway of airway obstruction in asthma

Excessive airway obstruction is the cause of symptoms and abnormal lung function in asthma. As airway smooth muscle (ASM) is the effecter controlling airway calibre, it is suspected that dysfunction of ASM contributes to the pathophysiology of asthma. However, the precise role of ASM in the series of events leading to asthmatic symptoms is not clear. It is not certain whether, in asthma, there is a change in the intrinsic properties of ASM, a change in the structure and mechanical properties of the noncontractile components of the airway wall, or a change in the interdependence of the airway wall with the surrounding lung parenchyma. All these potential changes could result from acute or chronic airway inflammation and associated tissue repair and remodelling. Anti-inflammatory therapy, however, does not "cure" asthma, and airway hyperresponsiveness can persist in asthmatics, even in the absence of airway inflammation. This is perhaps because the therapy does not directly address a fundamental abnormality of asthma, that of exaggerated airway narrowing due to excessive shortening of ASM. In the present study, a central role for airway smooth muscle in the pathogenesis of airway hyperresponsiveness in asthma is explored.

Components of airway hyperresponsiveness and their associations with inflammation and remodeling in mice

BACKGROUND

Pathologic changes, including inflammation and remodeling, occur in the asthmatic airway. However, their relative contribution to the components of airway hyperresponsiveness (AHR) remains unclear.

OBJECTIVE

Attempting to delineate AHR into discrete immune-mediated and structural remodeling components, we performed a detailed time course of the development, progression, and persistence of maximal respiratory system resistance, airway reactivity, and airway sensitivity.

METHODS

Mice exposed to increasing durations of persistent allergen were assessed for airway function, morphometry, and inflammation.

RESULTS

Allergen exposure resulted in increases for all indices of AHR that persisted for at least 4 weeks after chronic allergen exposure (P < .01 for all values). Early increases in AHR were associated with increases in immune-mediated events, including airway eosinophils (P < .01), whereas sustained AHR was associated with structural remodeling events. Increased maximal respiratory system resistance, evident by 6 weeks postallergen and persisting for at least 4 weeks after 8 weeks of chronic exposure, was associated with an increase in collagen deposition (P < .01). Increased airway reactivity and sensitivity, each evident by 1 week after allergen and persisting for at least 4 weeks after 8 weeks of chronic exposure, were associated with an increase in airway smooth muscle area (P < .01).

CONCLUSION

Our novel observation of distinct temporal relationships in the development, progression, and persistence of the individual indices of AHR supports our hypothesis that multiple underlying factors contribute to airway dysfunction.

CLINICAL IMPLICATIONS

These findings illustrate the importance of clearly addressing specific components of airway dysfunction to provide greater insight into specific pathophysiologic mechanisms in airway disease.

Extracellular matrix proteins differentially regulate airway smooth muscle phenotype and function

Changes in the ECM and increased airway smooth muscle (ASM) mass are major contributors to airway remodeling in asthma and chronic obstructive pulmonary disease. It has recently been demonstrated that ECM proteins may differentially affect proliferation and expression of phenotypic markers of cultured ASM cells. In the present study, we investigated the functional relevance of ECM proteins in the modulation of ASM contractility using bovine tracheal smooth muscle (BTSM) preparations. The results demonstrate that culturing of BSTM strips for 4 days in the presence of fibronectin or collagen I depressed maximal contraction (E(max)) both for methacholine and KCl, which was associated with decreased contractile protein expression. By contrast, both fibronectin and collagen I increased proliferation of cultured BTSM cells. Similar effects were observed for PDGF. Moreover, PDGF augmented fibronectin- and collagen I-induced proliferation in an additive fashion, without an additional effect on contractility or contractile protein expression. The fibronectin-induced depression of contractility was blocked by the integrin antagonist Arg-Gly-Asp-Ser (RGDS) but not by its negative control Gly-Arg-Ala-Asp-Ser-Pro (GRADSP). Laminin, by itself, did not affect contractility or proliferation but reduced the effects of PDGF on these parameters. Strong relationships were found between the ECM-induced changes in E(max) in BTSM strips and their proliferative responses in BSTM cells and for E(max) and contractile protein expression. Our results indicate that ECM proteins differentially regulate both phenotype and function of intact ASM.

Collagen I and thrombin activate MMP-2 by MMP-14-dependent and -independent pathways: implications for airway smooth muscle migration

Increased proinflammatory mediators and ECM deposition are key features of the airways in asthma. Matrix metalloproteinases (MMPs) are produced by airway smooth muscle (ASM) cells and have multiple roles in inflammation and tissue remodeling. We hypothesized that components of the asthmatic airway would stimulate MMP production and activation by ASM and contribute to airway remodeling. We measured human ASM-derived MMP mRNA, protein, and activity by real-time RT-PCR, zymography, Western blotting, and MMP activity assay. Collagen I and thrombin caused a synergistic increase in MMP-2 protein and total MMP activity but paradoxically decreased MMP-2 mRNA. Additionally, collagen I activated MMP-2 in culture supernatants independent of the cell surface. Together, collagen I and thrombin strongly enhanced MMP-14 mRNA and protein but had no effect individually, suggesting increased MMP-14, the activating protease for MMP-2, may be partially responsible for MMP-2 activation. Furthermore, collagen I reduced tissue inhibitor of metalloproteinase-2 protein (TIMP-2). We examined the role of MMPs in functions of ASM related to airway remodeling and found migration and proliferation were MMP dependent, whereas adhesion and apoptosis were not. Ilomastat inhibited migration by 25%, which was also inhibited by TIMPs 1-4 and increased by the MMP-2 activator thrombin. These in vitro findings suggest that the environment within the airways of patients with asthma enhances MMP-2 and -14 protein and activity by a complex interaction of transcriptional and posttranscriptional mechanisms, which may contribute to ASM migration.

Connective tissue growth factor: a role in airway remodelling in asthma?

1. Severe persistent asthma is accompanied by structural changes in the airway, referred to as remodelling. The mechanisms driving airway remodelling are poorly understood. 2. Transforming growth factor (TGF)-beta is increased in the airways of patients with asthma. Many of the effects of TGF-beta are mediated by connective tissue growth factor (CTGF). 3. Overexpression of CTGF is linked to many fibrotic diseases, but its exact role in airway remodelling is unknown. 4. Connective tissue growth factor mediates cell adhesion, migration, proliferation, survival, extracellular matrix synthesis and has a role in angiogenesis. 5. Current asthma therapies do not inhibit CTGF induction. 6. Understanding the mechanisms underlying the role of CTGF in airway remodelling may lead to new therapeutic strategies for asthma.

Endogenous laminin is required for human airway smooth muscle cell maturation

Background

Airway smooth muscle (ASM) contraction underlies acute bronchospasm in asthma. ASM cells can switch between a synthetic-proliferative phenotype and a contractile phenotype. While the effects of extracellular matrix (ECM) components on modulation of ASM cells to a synthetic phenotype have been reported, the role of ECM components on maturation of ASM cells to a contractile phenotype in adult lung is unclear. As both changes in ECM components and accumulation of contractile ASM are features of airway wall remodelling in asthma, we examined the role of the ECM protein, laminin, in the maturation of contractile phenotype in human ASM cells.

Methods

Human ASM cells were made senescence-resistant by stable expression of human telomerase reverse transcriptase. Maturation to a contractile phenotype was induced by 7-day serum deprivation, as assessed by immunoblotting for desmin and calponin. The role of laminin on ASM maturation was investigated by comparing the effects of exogenous laminin coated on culture plates, and of soluble laminin peptide competitors. Endogenous expression of laminin chains during ASM maturation was also measured.

Results

Myocyte binding to endogenously expressed laminin was required for ASM phenotype maturation, as laminin competing peptides (YIGSR or GRGDSP) significantly reduced desmin and calponin protein accumulation that otherwise occurs with prolonged serum deprivation. Coating of plastic cell culture dishes with different purified laminin preparations was not sufficient to further promote accumulation of desmin or calponin during 7-day serum deprivation. Expression of α2, β1 and γ1 laminin chains by ASM cells was specifically up-regulated during myocyte maturation, suggesting a key role for laminin-2 in the development of the contractile phenotype.

Conclusion

While earlier reports suggest exogenously applied laminin slows the spontaneous modulation of ASM to a synthetic phenotype, we show for the first time that endogenously expressed laminin is required for ASM maturation to the contractile phenotype. As endogenously expressed laminin chains α2, β1 and γ1 are uniquely increased during myocyte maturation, these laminin chains may be key in this process. Thus, human ASM maturation appears to involve regulated endogenous expression of a select set of laminin chains that are essential for accumulation of contractile phenotype myocytes.

Collagen impairs glucocorticoid actions in airway smooth muscle through integrin signalling

BACKGROUND AND PURPOSE

Airway wall remodelling in asthma is characterised by a number of structural changes, including an increase in the volume of airway smooth muscle (ASM), and the abundance of the extracellular matrix (ECM) protein, collagen, is increased. We have investigated the mechanism of collagen-induced glucocorticoid resistance of proliferation, and migration of ASM.

EXPERIMENTAL APPROACH

ASM cultured from human airways has been seeded on to either type I monomeric collagen or a laminin pentapeptide, YIGSR. The role of alpha2beta1 integrin in the collagen-induced glucocorticoid resistance was investigated using a function blocking monoclonal antibody.

KEY RESULTS

Culture of ASM on collagen I, but not laminin, led to a greater proliferative response that was insensitive to regulation by dexamethasone (100 nM). The anti-migratory effects of the glucocorticoid, fluticasone propionate (1 nM) were also impaired by contact of ASM with collagen. The impaired anti-mitogenic action of dexamethasone was associated with a failure to reduce the levels of the rate-limiting cell cycle regulatory protein, cyclin D1. When signalling through the alpha2beta1 integrin was reduced, dexamethasone-mediated reductions in proliferation and cyclin D1 levels were restored.

CONCLUSIONS AND IMPLICATIONS

In the collagen-rich microenvironment of the inflamed and fibrotic asthmatic airway, integrin/ECM interactions may contribute to glucocorticoid resistance.

Airway smooth muscle and asthma

The airway smooth muscle is the key determinant of airway narrowing in asthma but its function in the absence of disease is unknown. Evidence for an intrinsic abnormality in the muscle in asthma is only just emerging. The airway smooth muscle is not merely a contractile cell, but also one which determines the composition of, and interacts with the extracellular matrix, and which may participate in inflammatory and allergic reactions and viral infections. The reason for the differences which have been observed in the in vitro properties of airway smooth muscle derived from asthmatic individuals may result from an inherent "supercontractility", an increased tendency to proliferate due to the absence of an inhibitory transcription factor C/EBP-alpha, the influence of an altered extracellular matrix and/or a decrease in release of factors such as PGE(2) which would under normal circumstances inhibit both proliferation and contraction. Although long acting beta agonists and corticosteroids are successful treatments for inflammation and bronchoconstriction, the structural changes which constitute airway remodelling may require additional therapeutic intervention, the nature of which will be determined by thorough investigation of the mechanisms underlying the asthmatic phenotype.

Effect of IL-6 trans-signaling on the pro-remodeling phenotype of airway smooth muscle

Increased levels of IL-6 are documented in asthma, but its contribution to the pathology is unknown. Asthma is characterized by airway wall thickening due to increased extracellular matrix deposition, inflammation, angiogenesis, and airway smooth muscle (ASM) mass. IL-6 binds to a specific membrane-bound receptor, IL-6 receptor-alpha (mIL-6Ralpha), and subsequently to the signaling protein gp130. Alternatively, IL-6 can bind to soluble IL-6 recpetor-alpha (sIL-6Ralpha) to stimulate membrane receptor-deficient cells, a process called trans-signaling. We discovered that primary human ASM cells do not express mIL-6Ralpha and, therefore, investigated the effect of IL-6 trans-signaling on the pro-remodeling phenotype of ASM. ASM required sIL-6Ralpha to activate signal transducer and activator 3, with no differences observed between cells from asthmatic subjects compared with controls. Further analysis revealed that IL-6 alone or with sIL-6Ralpha did not induce release of matrix-stimulating factors (including connective tissue growth factor, fibronectin, or integrins) and had no effect on mast cell adhesion to ASM or ASM proliferation. However, in the presence of sIL-6Ralpha, IL-6 increased eotaxin and VEGF release and may thereby contribute to local inflammation and vessel expansion in airway walls of asthmatic subjects. As levels of sIL-6Ralpha are increased in asthma, this demonstration of IL-6 trans-signaling in ASM has relevance to the development of airway remodeling.

Airway smooth muscle as a regulator of immune responses and bronchomotor tone

The traditional view of airway smooth muscle (ASM) in asthma, as a purely contractile tissue, seems to be inadequate. Compelling evidence now suggests that ASM plays an important role in regulating bronchomotor tone, in perpetuating airway inflammation, and in remodeling of the airways. This article reviews three distinct functions of ASM cells: the process of excitation-contraction coupling, with a particular focus on the role of cytokines in modulating calcium responses; the processes of smooth muscle cell proliferation and migration; and the synthetic and immunomodulatory function of ASM cells. This article also discusses how altered synthetic function contributes to airway remodeling.

Airway remodelling in asthma: current understanding and implications for future therapies

Airway remodelling refers to the structural changes that occur in the airway wall in asthma. These include epithelial hyperplasia and metaplasia, subepithelial fibrosis, muscle cell hyperplasia and angiogenesis. These structural changes result in thickening of the airway wall, airway hyperresponsiveness (AHR), and a progressive irreversible loss of lung function. The precise sequence of events that take place during the remodelling process and the mechanisms regulating these changes remain poorly understood. It is thought that airway remodelling is initiated and promoted by repeated episodes of allergic inflammation that damage the surface epithelium of the airway. However, other mechanisms are also likely to contribute to this process. Moreover, the interrelationship between airway remodelling, inflammation and AHR has not been clearly defined. Currently, there are no effective treatments that halt or reverse the changes of airway remodelling and its effects on lung function. Glucocorticoids have been unable to eliminate the progression of remodelling changes and there is limited evidence of a beneficial effect from other available therapies. The search for novel therapies that can directly target individual components of the remodelling process should be made a priority. In this review, we describe the current understanding of the airway remodelling process and the mechanisms regulating its development. The impact of currently available asthma therapies on airway remodelling is also discussed.

Extracellular matrix regulates enhanced eotaxin expression in asthmatic airway smooth muscle cells

RATIONALE

Altered airway smooth muscle (ASM) function and enrichment of the extracellular matrix (ECM) with fibronectin and collagen are key features of asthma. Previously, we have reported these ECM proteins enhance ASM synthetic function.

OBJECTIVE

We compared ASM cultured from endobronchial biopsies from subjects with and without asthma to assess if asthmatic cells were hypersecretory and determined whether the underlying mechanism involved autocrine ECM production.

METHODS AND MEASUREMENTS

Cells from subjects with and without asthma were cultured on plastic or in plates precoated with ECM proteins. Cytokine production was evaluated by enzyme-linked immunosorbent assay and by reverse transcriptase-polymerase chain reaction. Function-blocking integrin antibodies were used to identify integrin involvement.

RESULTS

Baseline eotaxin and its production after stimulation with interleukin (IL)-13, IL-1beta, or tumor necrosis factor-alpha was increased (2.5- to 6.0-fold) in ASM cells cultured from subjects with asthma compared with healthy subjects. When seeded on ECM from asthmatic ASM, IL-13-dependent eotaxin release from healthy or asthmatic ASM was enhanced compared with culture on healthy ECM. The ECM substrates fibronectin and type I collagen each enhanced IL-13-dependent eotaxin release, and Western immunoblot indicated that fibronectin expression was higher in asthmatic ASM cells. Integrin-blocking antibodies revealed that alpha5beta1 was required for more than 50% of the enhanced IL-13-dependent eotaxin release by ASM cells from subjects with asthma, whereas alpha2beta1 or alphavbeta3 neutralization lacked effect.

CONCLUSION

The data indicate that ASM cells cultured from subjects with asthma are hypersecretory compared with cells from healthy donors and that autocrine fibronectin secretion acting via alpha5beta1 in part underlies this effect.

Resting smooth muscle cells as a model for studying vascular cell activation

Vascular smooth muscle (VSM) cells constitute the main structural components of tunica media. Under physiological conditions, these cells display a contractile phenotype and a low proliferative activity. However, they may also acquire a synthetic phenotype and become predominantly proliferative if stimulated under certain stress conditions. This capacity plays a major role in the inception and progression of such cardiovascular diseases as atherosclerosis, hypertension and restenosis. Porcine coronary smooth muscle (PCSM) cells exhibit a synthetic phenotype (ON cells) under standard culturing conditions, but they can be reverted to a contractile phenotype (OFF cells) in a serum-free medium. However, OFF cells can also re-acquire a synthetic phenotype (OFF/ON cells) upon serum administration. In the present study, proliferative and contractile behaviors were characterized by expression of specific differentiation markers. Taken together, these results demonstrate that porcine vascular smooth muscle cells can retain their phenotypic plasticity in culture, and thus mimic in vitro their in vivo differentiation states. OFF cells may thus provide a suitable model system in studying the mechanism(s) by which either known or unknown serum factors may trigger vascular smooth muscle activation. In the present study, this possibility was actually tested by exposing OFF cells to fetal bovine serum (FBS), PDGF-BB and IGF-I. Data show that only FBS could induce a synthetic phenotype in OFF cells, while both PDGF-BB and IGF-I failed to induce any VSM activation.

The association of caveolae, actin, and the dystrophin-glycoprotein complex: a role in smooth muscle phenotype and function?

Smooth muscle cells exhibit phenotypic and mechanical plasticity. During maturation, signalling pathways controlling actin dynamics modulate contractile apparatus-associated gene transcription and contractile apparatus remodelling resulting from length change. Differentiated myocytes accumulate abundant caveolae that evolve from the structural association of lipid rafts with caveolin-1, a protein with domains that confer unique functional properties. Caveolae and caveolin-1 modulate and participate in receptor-mediated signalling, and thus contribute to functional diversity of phenotypically similar myocytes. In mature smooth muscle, caveolae are partitioned into discrete linear domains aligned with structural proteins that tether actin to the extracellular matrix. Caveolin-1 binds with beta-dystroglycan, a subunit of the dystrophin glycoprotein complex (DGC), and with filamin, an actin binding protein that organizes cortical actin, to which integrins and focal adhesion complexes are anchored. The DGC is linked to the actin cytoskeleton by a dystrophin subunit and is a receptor for extracellular laminin. Thus, caveolae and caveolin-associated signalling proteins and receptors are linked via structural proteins to a dynamic filamentous actin network. Despite development of transgenic models to investigate caveolins and membrane-associated actin-linking proteins in skeletal and cardiac muscle function, only superficial understanding of this association in smooth muscle phenotype and function has emerged.

Transforming growth factor beta2-induced myofibroblastic differentiation of human retinal pigment epithelial cells: regulation by extracellular matrix proteins and hepatocyte growth factor

Retinal pigment epithelial (RPE) cells possess the potential to transdifferentiate into myofibroblasts after stimulation with transforming growth factor beta (TGFbeta) and are implicated in the pathogenesis of proliferative vitreoretinopathy. In this study we evaluated how TGFbeta2 and various extracellular matrix (ECM) proteins modulate the transdifferentiation of human fetal retinal pigment epithelial cells (RPE) cells into myofibroblast-like cells. Furthermore, we investigated whether hepatocyte growth factor (HGF) can suppress this transdifferentiation. RPE cells were cultured on ECM coated or uncoated surfaces in the presence or absence of TGFbeta2. HGF was added to certain cultures only once or on a daily basis during the treatment. Transdifferentiation of RPE cells into myofibroblasts was assessed by the quantitation of alpha-smooth muscle actin (alpha-SMA) using immunocytochemistry, flow cytometry, real-time PCR and Western blotting. TGFbeta2 induced a significant increase of alpha-SMA expression in a dose-dependent manner. Compared with growth on uncoated surfaces, RPE cultured on fibronectin (FN)-coated surfaces and stimulated with TGFbeta2 showed a significantly higher alpha-SMA expression than untreated cells. This upregulation of alpha-SMA could be markedly reduced by daily treatment with HGF; however, a single HGF administration did not significantly reduce alpha-SMA. These findings are important for further understanding the interaction of cytokines, RPE cells and their environment in mesenchymal transformation as well as its possible modulation. Continuous or long-term treatment with HGF should be further investigated for its potential to prevent mesenchymal transdifferentiation of RPE cells, and ultimately, PVR in vivo.

Canine retinal angioblasts are multipotent

The purpose of this study was to culture and characterize endothelial cells and angioblasts, vascular precursors, from adult and neonatal dog retina and determine if angioblasts are committed to endothelial cell lineage or have the potential to be multipotent, i.e. express phenotypic characteristics of other vascular cell types. Endothelial cells were established from adult dog retina (ADREC) by the technique of Gitlin and D'Amore. For angioblasts, pieces of neonatal day 2 (P2) avascular peripheral retina were placed under coverslips until sufficient cells had explanted. All cells were maintained initially on hyaluronic acid (HA)/fibronectin (FN) substratum. Neonatal canine retinal angioblasts (NCRA) were maintained initially on retinal-derived growth factor with alpha-amino adipic acid to inhibit growth of Muller cells. Cell lines were characterized by enzyme histochemistry [menadione-dependent alpha glycerophosphate dehydrogenase (alphaGPDH), marker for angioblasts] and immunocytochemistry. Once characterized, cells were grown on FN, or collagens I or IV substrata and fed platelet-derived growth factor-BB (PDGF-BB) or fibroblast growth factor-2 (FGF-2). The phenotypic expression of a marker for endothelial cells [acetylated LDL (acLDL) uptake] or a marker for pericytes and smooth muscle cells, production of alpha smooth muscle actin (alphaSMA), was evaluated under those conditions. The canine retinal cell lines that were established had the following characteristics when maintained on serum and a retinal extract. Angioblasts had low expression of vWf and VEGF-R2 (two markers for canine endothelial cells), and very low uptake of acLDL but high expression of alphaGPDH and adenosine A2a receptors (A2aR) (two markers for canine angioblasts in vivo). ADREC had high expression of endothelial cell markers (vWf, VEGF-R2, and acLDL uptake) but minimal expression of alphaGPDH and A2aR. Both angioblasts and endothelial cells expressed CXCR4, a marker for hemangioblasts. Angioblasts grown on any of the substrata in the presence of FGF-2 had high uptake of acLDL and low expression of alphaSMA, while those grown in the presence of PDGF-BB had high expression of alphaSMA and low uptake of acLDL. In conclusion, angioblasts cultured from peripheral vascular retina have low expression of endothelial cell markers and high alphaGPDH and A2aR, markers for canine angioblasts in vivo. Angioblasts will internalize acLDL when maintained on FGF-2 and express alphaSMA when maintained on PDGF-BB, suggesting that they have the potential to become endothelial cells or pericytes, i.e. are multipotent.

Mechanisms underlying the relaxation induced by isokaempferide from Amburana cearensis in the guinea-pig isolated trachea

The present study examines possible mechanisms by which the flavonoid isokaempferide (IKPF; 5,7,4'-trihydroxy-3-methoxyflavone) from Amburana cearensis, a Brazilian medicinal plant popularly used as bronchodilator, induces relaxation of guinea-pig isolated trachea. In the trachea (with intact epithelium) contracted by carbachol, IKPF (1-1000 microM) caused a graded relaxation, and the epithelium removal increased the sensitivity of the airway smooth muscle to IKPF (EC50, in intact tissue: 77.4 [54.8-109.2] microM; in denuded epithelium: 15.0 [11.3-20.1] microM). The IKPF-induced relaxation was inhibited in 41% by the nitric oxide (NO) synthase inhibitor L-NAME (100 microM); in 31% and 50% by the soluble guanylate cyclase (sGC) inhibitor ODQ (3 and 33 microM); by propranolol (31%) and also by capsaicin (37%). In the trachea pre-contracted by 40 mM KCl the pre-incubation with glibenclamide (33 microM) or iberiotoxin (IbTX, 0.1 microM), selective K(+) channel inhibitors, inhibited the IKPF-induced relaxation by 39% and 38%, respectively. On the other hand, 4-aminopyridine (100 microM), a nonselective K(+) channel antagonist, did not significantly influence the effect of IKPF, while IbTX induced a rightward displacement of the IKPF concentration-response curve. However, in muscle pre-contracted with 120 mM KCl the relaxant effect of IKPF was significantly reduced and not affected by glibenclamide. In conclusion, these results indicate a direct and epithelium-independent relaxant effect of IKPF on smooth muscle fibers. Although this IKPF relaxant action seems to be multi-mediated, it occurs via both Ca(2+) and ATP-sensitive K(+) channels, but some other possible mechanisms unrelated to K(+) channels cannot be excluded.

(+)Insert smooth muscle myosin heavy chain (SM-B): From single molecule to human

In smooth muscle, alternative mRNA splicing of a single gene produces four myosin heavy chain (SMMHC) isoforms. Two of these isoforms differ by the presence [(+)insert] or absence [(-)insert] of a seven amino acid insert in the motor domain. This insert enhances the kinetic properties of myosin at the molecular level but its exact role at the cell and tissue levels still has to be elucidated. This review focuses on the expression and biological functions of the (+)insert isoform. Current knowledge is summarized regarding its tissue distribution in animals and humans. Studies at the molecular, cellular and tissue levels that aimed at understanding the contribution of this isoform to smooth muscle mechanical function are presented with a particular focus on velocity of shortening. In addition, the altered expression of the (+)insert isoform in diseases and models of diseases and the compensatory mechanisms that occur when the (+)insert is knocked out are discussed. The need for additional studies on the relationship of this isoform to contractile performance and how expression of this isoform is regulated are also considered.

Activation of valvular interstitial cells is mediated by transforming growth factor-beta1 interactions with matrix molecules

Strategies for the tissue-engineering of living cardiac valve replacements are limited by a lack of appropriate scaffold materials that both permit cell viability and actively contribute to the growth of functional tissues. Components of the extracellular matrix can localize and modify growth factor signals, and by doing so impart instructional stimuli for direction of cell phenotype. Fibronectin, collagen I, and heparin were explored as affinity matrices for sequestering and presenting soluble signaling molecules to control differentiation of valvular interstitial cells (VICs) to myofibroblasts. VIC differentiation is commonly characterized by expression of stress fibers containing alpha smooth muscle actin (alpha-SMA), and transforming growth factor-beta1 (TGF-beta1) is a central mediator of this transition. Both fibronectin and heparin, which are known to possess TGF-beta1 binding interactions, were found to increase VIC alpha-SMA expression (120% and 258% of expression in controls), while VICs cultured on collagen I-modified substrates had diminished alpha-SMA expression (66% of control). Heparin treatment significantly stimulated VIC production of TGF-beta1 at all concentrations tested (50 to 400 mug/ml). Heparin-modified substrates were found to alter cell morphology through increased adsorption of serum proteins, specifically TGF-beta1. In sum, heparin produced alpha-SMA-positive myofibroblasts through both the de novo production of TGF-beta1, and its localization in the pericellular environment. The addition of heparin to fibronectin-modified substrates led to a synergistic increase in VIC alpha-SMA expression, produced by the reciprocal binding of fibronectin, heparin, cell-produced TGF-beta1. The characterization of molecules, both soluble and insoluble, that control VIC activation will be important for the development of tailored 3D culture environments for tissue-engineering applications.

Cyclin-Dependent Kinase Inhibitor p27 Kip1 , But Not p21 WAF1/Cip1 , Is Required for Inhibition of Hypoxia-Induced Pulmonary Hypertension and Remodeling by Heparin in Mice

Heparin has growth inhibitory effects on pulmonary artery smooth muscle cell (PASMC) in vitro and in vivo. However, the mechanism has not been fully defined. In this study, we investigated the role of cyclin-dependent kinase inhibitors, p21(WAF1/cip1) (p21) and p27Kip1 (p27), in the inhibitory effect of heparin on PASMC proliferation in vitro and on hypoxia-induced pulmonary hypertension in vivo using p21 and p27-null mice. In vitro, loss of the p27 gene negated the inhibitory effect of heparin on PASMC proliferation, but p21 was not critical for this inhibition. In vivo, heparin significantly inhibited the development of hypoxia-induced pulmonary hypertension and remodeling, as evidenced by decreased right ventricular systolic pressure, ratio of right ventricular weight to left ventricle plus septum weight, and percent wall thickness of pulmonary artery, in p21(+/+), p21(-/-), p27(+/+), and p27(+/-), but not in p27(-/-) mice. We also observed that hypoxia decreased p27 expression significantly in mouse lung, which was restored by heparin. Heparin inhibited Ki67 proliferative index in terminal bronchial vessel walls in p27(+/+) and p27(+/-), but not in p27(-/-) mice exposed to hypoxia. Therefore, we conclude that the cyclin-dependent kinase inhibitor p27, but not p21, is required for the inhibition of hypoxic pulmonary vascular remodeling by heparin.