Regulation of airway wall remodeling: prospects for the development of novel antiasthma drugs

The antimalarial drug artesunate inhibits primary human cultured airway smooth muscle cell proliferation

Airway smooth muscle (ASM) cell hyperplasia contributes to airway wall remodeling (AWR) in asthma. Glucocorticoids, which are used as first-line therapy for the treatment of inflammation in asthma, have limited impact on AWR, and protracted usage of high doses of glucocorticoids is associated with an increased risk of side effects. Moreover, patients with severe asthma often show reduced sensitivity to glucocorticoids. Artesunate, a semisynthetic artemisinin derivative used to treat malaria with minimal toxicity, attenuates allergic airway inflammation in mice, but its impact on AWR is not known. We examined the effects of artesunate on ASM proliferation in vitro and in vivo. Primary human ASM cells derived from nonasthmatic donors were treated with artesunate before mitogen stimulation. Artesunate reduced mitogen-stimulated increases in cell number and cyclin D1 protein abundance but had no significant effect on ERK1/2 phosphorylation. Artesunate, but not dexamethasone, inhibited phospho-Akt and phospho-p70(S6K) protein abundance. Artesunate, but not dexamethasone, inhibited mitogen-stimulated increases in cell number, cyclin D1, and phospho-Akt protein abundance on ASM cells derived from asthmatic donors. In a murine model of allergic asthma, artesunate reduced the area of α-smooth muscle actin-positive cells and decreased cyclin D1 protein abundance. Our study provides a basis for the future development of artesunate as a novel anti-AWR agent that targets ASM hyperplasia via the PI3K/Akt/p70(S6K) pathway and suggests that artesunate may be used as combination therapy with glucocorticoids.

Can we differentiate between airway and vascular smooth muscle?

1. Airway smooth muscle (ASM) has recently been termed the 'frustrated' cell of the lung given that contraction of ASM has no proven useful physiological function in adults and yet is indelibly associated with pathological conditions by virtue of its unwanted airflow-limiting actions in asthma. In contrast, pulmonary vascular smooth muscle contraction plays an essential role in the control of blood flow through the lung. 2. Little is known of the differences in phenotype between human ASM and pulmonary vascular smooth muscle (VSM) tissues, but differences in contractile protein and transcription factor expression and regulation of contractile protein promoter activity have been documented. Similarly, the embryological signals in mice required for differentiation of ASM versus pulmonary VSM are distinct. 3. Bronchoconstriction in asthma is currently treated with beta2-adrenoceptor agonists, which relax contracted ASM cells. An additional approach may be to use gene therapy to render ASM unable to contract (via disruption of their contractile apparatus organization). 4. Application of ASM-specific gene therapies would rely on minimal actions on other lung smooth muscle tissues, including pulmonary and bronchial vascular smooth muscle. The combination of mRNA analysis of laser-captured microdissected tissue with in situ immunohistochemical staining for protein should be very useful in terms of being able to characterize definitively the differences in mRNA and protein expression between the smooth muscle species of the lung. Any discovery of an ASM-selective target could provide a novel lead for ASM-directed anti-asthma therapy.

Factors controlling airway smooth muscle proliferation in asthma

Airway smooth muscle proliferation has been the focus of considerable attention, as it is a quantitatively important component of the airway wall remodeling response in asthma and has been suggested as a suitable target for the development of novel anti-asthma agents. Such agents are considered likely to reduce airway hyperresponsiveness and, consequently, airway obstruction, resulting in fewer symptoms and exacerbations. Identifying suitable drug targets has proved an elusive goal, as no dominant molecular mechanism for remodeling has emerged. Moreover, recent findings raise some doubt as to whether smooth muscle proliferation per se is the explanation of the increase in smooth muscle cell number in asthma, with alternative explanations including the proposal that cells migrate either from the interstitial compartment or from a circulating precursor stem cell population. Therefore, drug targeting of migration responses should be considered as an alternative approach to regulating the smooth muscle component of airway wall remodeling.

Contribution of the p38MAPK signalling pathway to proliferation in human cultured airway smooth muscle cells is mitogen-specific

We have investigated the role of p38MAPK in human airway smooth muscle (HASM) proliferation in response to thrombin and bFGF. The regulation of cyclin D1 mRNA, cyclin D1, cyclin E and p21Cip1 protein levels, and the extent of retinoblastoma protein (pRb) phosphorylation in response to activation of p38MAPK have also been examined. Two distinct inhibitors of p38MAPK, SB 203580 (10 microm) and SB 202190 (10 microm), prevented bFGF (0.3-3 nm)-stimulated cell proliferation, but had no effect on the response to thrombin (0.3-3 U ml(-1)). In cells incubated with thrombin or bFGF for 20 h, there was an increase in p38MAPK phosphorylation in response to bFGF, but not to thrombin. Thrombin and bFGF-stimulated increases in ERK phosphorylation and cyclin D1 mRNA and protein levels were not influenced by SB 203580 pre-treatment. Similarly, cyclin E and p21Cip1 protein levels, measured after 20 h incubation with mitogen, did not appear to be regulated by SB 203580 (10 microm). Although both thrombin and bFGF significantly increased levels of pRb phosphorylation, SB 203580 (10 microm) inhibited only bFGF-stimulated pRb phosphorylation. In addition, SB 203580 (10 microm) selectively inhibited bFGF-stimulated DNA synthesis, suggesting that the antimitogenic actions of SB 203580 on pRb phosphorylation cause cell cycle arrest at late G1 phase. In conclusion, these results indicate that p38MAPK is involved in bFGF-, but not in thrombin-stimulated HASM proliferation. The activation of the p38MAPK pathway by bFGF, but not by thrombin, regulates the phosphorylation of pRb without influencing cyclin D1 expression.

Do inflammatory mediators influence the contribution of airway smooth muscle contraction to airway hyperresponsiveness in asthma?

It is now accepted that a host of cytokines, chemokines, growth factors, and other inflammatory mediators contributes to the development of nonspecific airway hyperresponsiveness in asthma. Yet, relatively little is known about how inflammatory mediators might promote airway structural remodeling or about the molecular mechanisms by which they might exaggerate smooth muscle shortening as observed in asthmatic airways. Taking a deep inspiration, which provides relief of bronchodilation in normal subjects, is less effective in asthmatic subjects, and some have speculated that this deficiency stems directly from an abnormality of airway smooth muscle and results in airway hyperresponsiveness to constrictor agonists. Here, we consider some of the mechanisms by which inflammatory mediators might acutely or chronically induce changes in the contractile apparatus that in turn might contribute to hyperresponsive airways in asthma.

Impact of extracellular matrix and strain on proliferation of bovine airway smooth muscle

1. The proliferation of airway smooth muscle (ASM) and collagen deposition are major contributors to airway remodelling in asthma that may be an important component of airway hyperresponsiveness. The ratio of collagen to laminin in asthma is increased as a result of fibrosis. 2. We investigated the effects of the extracellular matrix proteins laminin and collagen type I, as well as airway cell elongation (mechanical strain), on the proliferative responses of cultured bovine ASM cells and the impact of these biomechanical influences on glucocorticoid antimitogenic actions. 3. Bovine ASM cells were cultured onto a flexible silastic membrane coated with laminin or collagen. Cells were pretreated with dexamethasone (100 nmol/L) prior to incubation with the mitogen basic fibroblast growth factor (bFGF) for 72 h, at which time cells were enumerated. Cells were either subjected to mechanical strain (cell elongation) by stretching the flexible silastic membrane or were grown under static conditions. 4. Culture on collagen without elongation enhanced ASM proliferation compared with cells grown on laminin. Cells grown on laminin and subjected to 4% elongation did not respond to the mitogenic actions of bFGF. 5. Dexamethasone-mediated inhibition of bFGF-induced proliferation was unaffected by the extracellular matrix on which cells were seeded, or the degree of cell elongation. 6. These data support the hypothesis that the asthmatic airway wall microenvironment (collagen, reduced airway strain) enhances the proliferation of ASM cells.

Protease-activated receptor (PAR)-independent growth and pro-inflammatory actions of thrombin on human cultured airway smooth muscle

(1) Thrombin, a mitogen for human cultured airway smooth muscle (HASM), has many actions that have been attributed to activation of protease-activated receptor (PARs). However, the role of PARs in the proliferative action has not been clearly identified. Moreover, thrombin elicits cytokine production in a number of cell types, but these effects have not been characterized in human ASM. (2) Thrombin (0.03-3 U ml(-1))-stimulated increases in the levels of the pro-inflammatory and fibrogenic cytokine, granulocyte-macrophage colony-stimulating factor (GM-CSF) were observed over the same concentration range observed for thrombin-stimulated mitogenesis. (3) Inhibition of thrombin proteolytic activity, with either D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone (PPACK)- or hirudin-treated thrombin (0.3 U ml(-1)) or in the presence of the thrombin serine protease-selective inhibitor, SDZ 217-766 (0.15 micro M), reduced the thrombin-stimulated GM-CSF levels by 91+/-3, 65+/-12 and 83+/-9% (n=8, P<0.05), respectively. PPACK treatment, hirudin and SDZ 217-766 inhibited thrombin-stimulated increase in cell number by 70+/-8, 63+/-11 and 69+/-8%, respectively. (4) PAR-selective peptides SFLLRN (PAR1; 10 micro M), SLIGKV (PAR2; 10 micro M), GYPGQV (PAR4; 100 micro M) or the combination of SFLLRN and GYPGQV elicited mitogenic responses of only 15% of that to thrombin and surprisingly, had no effect on GM-CSF levels (n=8). Nevertheless, inhibition of thrombin responses by pertussis toxin (50 ng ml(-1)) suggests that the PAR-independent actions also involve a G-protein-coupled receptor. (5) PAR1 receptor expression was evident by immunohistochemistry and these receptors were coupled to increases in intracellular calcium, but not to the phosphorylation of ERK or the increases in cyclin D1 protein levels that are essential for cell proliferation. Cross-desensitization of intracellular calcium increases by thrombin and the PAR1-selective peptide provides evidence that the PAR1 receptor responds to both ligands. (6) The failure of PAR-selective peptides to mimic thrombin responses together with the inhibition of thrombin responses by serine protease inhibitors suggest the involvement of novel proteolytic receptor targets for thrombin-induced mitogenesis and cytokine production.

Assessing the evidence for remodelling of the airway in asthma

Asthma is now described as being characterized by reversible airflow obstruction, with bronchial inflammation and tissue remodelling of the airway wall. The description of remodelling has been usefully invoked to account for a component of airflow obstruction that is unresponsive to usual bronchodilator therapy. It is crucial to examine critically the evidence for this view, particularly the quantitation of specific changes in the epithelium, mucus glands, cell infiltrate, collagen, vessels and smooth muscle of the bronchial wall. The useful tools of immunohistochemistry and molecular biology combined with airway biopsy and well-designed clinical trials will be essential to determine the specific roles of cells and cytokines in airway remodelling in asthma.

The importance of ERK activity in the regulation of cyclin D1 levels and DNA synthesis in human cultured airway smooth muscle

The relationship between persistent ERK (extracellular signal-regulated kinase) activity, cyclin D1 protein and mRNA levels and cell cycle progression in human cultured airway smooth muscle was examined in response to stimulation by ET-1 (endothelin-1), thrombin and bFGF (basic fibroblast growth factor). Thrombin (0.3 and 3 u ml(-1)) and bFGF (0.3 and 3 nM) increased ERK activity for more than 2 h and increased cell number, whereas ET-1 (100 nM) transiently stimulated ERK activity and was non-mitogenic. The MEK1 (mitogen-activated ERK kinase) inhibitor, PD 98059 (30 microM), inhibited both ERK phosphorylation and activity, and either prevented (thrombin 0.3 and 3 u ml(-1), bFGF 300 pM) or attenuated (bFGF 3 nM) DNA synthesis. Thrombin and bFGF increased both cyclin D1 mRNA and protein levels. PD 98059 decreased cyclin D1 protein levels stimulated by the lower but not higher thrombin concentrations. Moreover, increases in cyclin D1 mRNA levels were unaffected by PD 98059 pretreatment, irrespective of the mitogen or its concentration, suggesting that inhibition of cyclin D1 protein levels occurred by a post-transcriptional mechanism. These findings indicate that the control of cyclin D1 protein levels may occur independently of the MEK1/ERK signalling pathways. The inhibition of S phase entry by PD 98059 at higher thrombin concentrations appears to result from effects on pathways downstream or parallel to those regulating cyclin D1 protein levels. These findings suggest heterogeneity in the signalling of DNA synthesis in human cultured airway smooth muscle.

Interleukin-1alpha and tumour necrosis factor-alpha modulate airway smooth muscle DNA synthesis by induction of cyclo-oxygenase-2: inhibition by dexamethasone and fluticasone propionate

1. Previous studies have established that glucocorticoids inhibit airway smooth muscle DNA synthesis. The effects of a combination of the pro-inflammatory cytokines, interleukin-1alpha (IL-1alpha) and tumour necrosis factor-alpha (TNF-alpha) on the inhibition of DNA synthesis by glucocorticoids in human cultured airway smooth muscle have now been investigated, since these cytokines are chronically expressed in asthmatic airways. 2. Thrombin (0.3 u ml(-1)) and basic fibroblast growth factor (bFGF, 300 pM) stimulated increases in DNA synthesis which were concentration-dependently inhibited by dexamethasone (1-1000 nM). 3. The cytokine mixture, comprising IL-1alpha (0.01 and 0.1 pM) and TNF-alpha (3 and 30 pM), directly evoked increases in DNA synthesis which were attenuated by dexamethasone. However, the cytokine mixture prevented responses to bFGF or thrombin. 4. Paradoxically, in the presence of the cytokine mixture and bFGF, dexamethasone (1-1000 nM) concentration-dependently increased DNA synthesis. Furthermore, neither dexamethasone (100 nM) nor fluticasone propionate (1 nM) inhibited DNA synthesized in response to bFGF/cytokine mixture combination and dexamethasone was similarly inactive against the thrombin/cytokine mixture. 5. The levels of prostaglandin E2 (PGE2), an established inhibitor of airway smooth muscle DNA synthesis, remained below the limits of assay detection (0.05 nM) under basal conditions or following stimulation with either thrombin or bFGF. In contrast, the cytokine mixture alone, and in the presence of thrombin or bFGF, induced biologically active levels of PGE2. Dexamethasone (100 nM), the non-selective cyclo-oxygenase (COX) inhibitor indomethacin (3 microM) or the selective COX-2 inhibitor L-745,337 (0.3 microM) completely inhibited synthesis of PGE2. 6. Neither indomethacin (3 microM) nor L-745,337 (0.3 microM) influenced thrombin- or bFGF-induced DNA synthesis. However, each COX inhibitor enhanced DNA synthesis in cytokine-treated cells. 7. In unstimulated airway smooth muscle cells, COX-1, but not COX-2 protein was detectable by Western blotting. The induction of COX-2 protein by the cytokine mixture was attenuated by dexamethasone (100 nM), whereas the level of COX-1 protein was unaffected by either the cytokines or by dexamethasone. 8. Cytokine-induced, COX-2-dependent eicosanoid production inhibits DNA synthesis. The paradoxical increase in DNA synthesis observed in glucocorticoid treated airway smooth muscle stimulated by cytokine/bFGF combinations may be explained by the ability of glucocorticoids to repress COX-2 induction and prevent cytokine-induction of the DNA synthesis inhibitor, PGE2.

Synthetic functions of airway smooth muscle in asthma

Studies now show that the airway smooth muscle cell, in addition to its contractile function, can participate in and coordinate the inflammatory response. In response to inflammatory cytokines, the airway smooth muscle cell produces cytokines, chemokines, growth factors and cell adhesion molecules leading to inflammatory cell chemotaxis, hyperplasia and hypertrophy. Anti-inflammatory mediators are also produced suggesting a dual role in inflammation. Moreover, in response to growth stimuli the cells may adopt a phenotype more suited to 'synthetic' function. These issues are discussed in this article by Simon Johnson and Alan Knox, who also point to ways in which some of these pathways offer future therapeutic potential in asthma.

Beta 2-adrenoceptor agonist-mediated inhibition of human airway smooth muscle cell proliferation: importance of the duration of beta 2-adrenoceptor stimulation

1. Airway hyperresponsiveness in asthma has been ascribed to airway wall thickening as a result of smooth muscle proliferation and hypertrophy. We have previously shown that continuous exposure to the beta 2-adrenoceptor agonist, salbutamol inhibits mitogen-induced proliferation of airway smooth muscle cells. In the present study, the effects of variable durations and repeated periods of exposure to beta 2-adrenoceptor agonists on DNA synthesis in human cultured airway smooth muscle have been investigated to model some of the possible pharmacokinetic profiles of these agents following inhalation. DNA synthesis was measured by [3H]-thymidine incorporation. 2. Shorter periods of exposure (up to 2.5 h) of airway smooth muscle cells to salbutamol (100 nM) commencing 30 min before thrombin (0.3 u ml-1) stimulation had no effect on the subsequent increase in [3H]-thymidine incorporation. However, inhibition by salbutamol was evident with a 4.5 h exposure and was maximal after an 8.5 h exposure. Similar patterns of results were observed when fenoterol (100 nM) was used in place of salbutamol as the beta 2-adrenoceptor agonist or when epidermal growth factor (300 pM) was used in place of thrombin as the mitogen. Salbutamol had no effect on thrombin-stimulated [3H]-leucine incorporation after 8.5 h of exposure, but a statistically significant effect was observed after 48 h of exposure. 3. Experiments in which DNA synthesis was measured up to 52 h after the addition of thrombin indicated that exposure to salbutamol during the first 8 h of mitogen stimulation delayed rather than inhibited the DNA synthesis. 4. Addition of salbutamol (100 nM) at different times either before or up to 24 h after the addition of thrombin indicated that [3H]-thymidine incorporation (measured between 24 and 28 h after thrombin) could be significantly attenuated when salbutamol was added as late as 18 h after the addition of thrombin. 5. The effects of more prolonged exposure to salbutamol were investigated by the addition of salbutamol for either 15 or 24 h per day for a total of 3 days. There were no significant differences in the level of inhibition of thrombin-stimulated [3H]-thymidine incorporation between continuous and intermittent salbutamol over the 3 day period and the inhibition was also not different to that achieved with a single continuous exposure to salbutamol over 28 h. 6. These results indicate that although exposure to beta 2-adrenoceptor agonists during the first 8 h of mitogen stimulation does not have a sustained inhibitory effect on DNA synthesis, repeated intermittent or prolonged continuous exposures to salbutamol do inhibit DNA synthesis, without evidence of marked desensitization.

The effect of glucocorticoids on proliferation of human cultured airway smooth muscle

1. Airway smooth muscle proliferation is a significant component of the airway wall remodelling that occurs in asthma. In this study, the effects of glucocorticoids on mitogenic responses of human airway smooth muscle have been examined. 2. Pretreatment of smooth muscle cells with dexamethasone (100 nM, 60 min) inhibited thrombin-induced increases in [3H]-thymidine incorporation (DNA synthesis) and cell number. 3. Inhibition of thrombin-induced [3H]-thymidine incorporation was also observed with hydrocortisone (0.01-1 microM) and methylprednisolone (0.001-0.1 microM) pretreatment. In contrast, pretreatment with either testosterone (0.001-1 microM) progesterone (0.001-1 microM), 17 beta-oestradiol (0.001-1 microM), or aldosterone (0.001-1 microM) had no effect on the response to thrombin. 4. Responses to a range of mitogens including thrombin (0.01-. 10 u ml-1), epidermal growth factor (EGF, 3-3000 pM), basic fibroblast growth factor (bFGF, 0.3-300 pM) and foetal calf serum (FCS, 0.1-10% v/v) were inhibited by dexamethasone (100 nM) pretreatment. However, the magnitude of the inhibitory effect was dependent on the mitogen, with EGF being the least, and thrombin being the most sensitive to the inhibitory effect. 5. The potency of hydrocortisone as an inhibitor of [3H]-thymidine incorporation was reduced when FCS (10% v/v, which caused a 40 fold increase in [3H]-thymidine incorporation) was used as the mitogen in place of thrombin (0.3 u ml-1, which caused a 10 fold increase in [3H]-thymidine incorporation). 6. The effect of post-treatment with dexamethasone (100 nM) indicated that addition of the glucocorticoid up to 17-19 h after thrombin (0.3 u ml-1) produced similar degrees of inhibition to those obtained when it was added as a pretreatment. Dexamethasone no longer produced an inhibitory effect if added 21 h or more after the addition of thrombin. 7. These results suggest that glucocorticoids regulate airway smooth muscle proliferation initiated by a range of stimuli. This effect may be of importance in the therapeutic actions of these compounds in asthma, particularly when they are used for prolonged periods of time.