Muscarinic M3-receptors mediate cholinergic synergism of mitogenesis in airway smooth muscle

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.

Bradykinin augments EGF-induced airway smooth muscle proliferation by activation of conventional protein kinase C isoenzymes

This study aims to investigate the effects of bradykinin, alone and in combination with growth factors on proliferation of cultured bovine tracheal smooth muscle cells. Bradykinin did not induce mitogenic responses by itself, but concentration-dependently augmented growth factor-induced [3H]thymidine incorporation and cell proliferation. The bradykinin effect was mediated by bradykinin B2 receptors, and not dependent on cyclo-oxygenase. Bradykinin-induced synergism with epidermal growth factor (EGF) could be suppressed by the protein kinase C (PKC) inhibitors GF 109203X (Bisindolylmaleimide I; specific for conventional and novel PKCs) and Gö 6976 (12-(2-Cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole; specific for conventional PKCs). In addition, sole activation of PKC using Phorbol 12-myristate 13-acetate (PMA) was sufficient for a synergistic interaction with EGF. In contrast to bradykinin however, PMA was mitogenic by itself which was not at all affected by Gö 6976, but abolished by GF 109203X. Bradykinin transiently activated the p42/p44 MAP kinase pathway, whereas PMA-induced activation of p42/p44 mitogen activated protein (MAP) kinase was sustained. Neither the combination of bradykinin and EGF nor that of PMA and EGF induced synergistic activation of p42/p44 MAP kinase, however. These results show that bradykinin B2 receptor-stimulation augments growth factor-induced mitogenic responses of airway smooth muscle cells through activation of conventional PKC isozymes. In addition, the results show that PKC isozyme-specificity underlies stimulus-specific differences in mitogenic capacity for bradykinin and PMA.

Cooperative regulation of p70S6 kinase by receptor tyrosine kinases and G protein-coupled receptors augments airway smooth muscle growth

We have previously demonstrated that concomitant activation of receptor tyrosine kinases and certain G protein-coupled receptors (GPCRs) can promote a synergistic increase in the rate of airway smooth muscle cell (ASM) proliferation. Here we clarify the role of p70S6 kinase (p70S6K) as an integrator of receptor tyrosine kinase and GPCR signaling that augments ASM DNA synthesis by demonstrating that specific p70S6K phosphorylation sites receive distinct regulatory input from GPCRs that promotes sustained kinase activity critical to mitogenesis. Prolonged stimulation of ASM cells with EGF and thrombin induced a greater than additive effect in levels of p70S6K phosphorylated at residue T389, whereas a significant but more modest increase in the level of T229 and T421/S424 phosphorylation was also observed. The augmenting effects of thrombin could be dissociated from p42/p44 MAPK activation, as selective inhibition of thrombin-stimulated p42/p44 failed to alter the profile of cooperative p70S6K T389 phosphorylation, p70S6K kinase activity, or ASM [(3)H]thymidine incorporation. Thrombin stimulated a sustained increase in the level of Akt phosphorylation and also augmented EGF-stimulated Akt phosphorylation. The cooperative effects of thrombin on Akt/p70S6K phosphorylation and [(3)H]thymidine incorporation were all attenuated by heterologous expression of Gbetagamma sequestrants. These data suggest that PI3K-dependent T389/T229 phosphorylation is limiting in late-phase p70S6K activation by EGF and contributes to the cooperative effect of GPCRs on p70S6K activity and cell growth.

Endothelin-1 regulates proliferative responses, both alone and synergistically with PDGF, in rat tracheal smooth muscle cells

The peptide, endothelin-1 (ET-1) regulates proliferative responses in numerous cell types. Recently, a dual ET receptor antagonist was shown to prevent the increase in airway smooth muscle cell (SMC) proliferation that accompanies airway smooth muscle remodeling in a rat model of experimental asthma. Thus, we used [(3)H]-thymidine incorporation assays and western immunoblotting to identify signaling pathways that regulate proliferative responses in cultured rat tracheal SMC. Our data indicate that ET-1 activation of the ET A receptor subtype induced [(3)H]-thymidine incorporation and activation of ERK 1/2 in primary rat tracheal SMC. ET-1-induced [(3)H]-thymidine incorporation and activation of ERK 1/2 were inhibited by pretreatment of SMC with pertussis toxin or down regulation of phorbol ester responsive isoforms of PKC. While ET- 1-induced ERK 1/2 activation was unaffected following inhibition of Rho kinase, ET-1-induced [(3)H]-thymidine incorporation was abrogated. ET-1 also potentiated [(3)H]-thymidine incorporation as well as cell proliferation of SMC stimulated with PDGF-BB and this response did not appear to be regulated by ERK1/ 2. These data demonstrate that ET-1 induces activation of multiple G proteins that regulate rat tracheal SMC proliferative responses, likely through signaling pathways downstream of ERK1/2 and Rho kinase.

Control by cholinergic mechanisms

In the respiratory tract acetylcholine is neurotransmitter in ganglia and postganglionic parasympathetic nerves, but in addition is paracrine mediator released from various non-neuronal cells. Almost every cell type present in the respiratory tract expresses nicotinic and muscarinic receptors and therefore appears to be a target for acetylcholine. The present review describes the mechanisms of synthesis and release of acetylcholine from neuronal and non-neuronal cells and the differential control mechanisms. The different cholinoceptors, multiple nicotinic and muscarinic receptors and their signalling are outlined and their involvement in the modulation of the function of various target cells, smooth muscles, nerves, surface epithelial, secretory cells, fibroblasts and inflammatory cells is discussed in detail.

Acetylcholine: a novel regulator of airway smooth muscle remodelling?

Increased airway smooth muscle mass is a pathological feature that asthma and chronic obstructive pulmonary disease (COPD) have in common. This increase has gained renewed interest in view of recent developments showing that airway smooth muscle, instead of solely being a contractile partner, is capable of interacting dynamically with its environment, especially under inflammatory conditions. Airway smooth muscle cells are able to proliferate, to migrate, and to secrete chemokines, cytokines, extracellular matrix proteins and growth factors, and most importantly, to adapt to these functions by changing its phenotype from contractile to proliferative/synthetic. Conversely, switching to a (hyper)contractile phenotype may also occur. A vast number of inflammatory stimuli regulate these functions and exert their effects via excitatory G(q) or G(i)-coupled receptors. Since acetylcholine activates muscarinic M(2) and M(3) receptors in the airway smooth muscle cell membrane, which are coupled to G(i) and G(q) proteins, respectively, and since acetylcholine release may be enhanced in airway inflammation, a pathophysiological role of acetylcholine related to the above processes and exceeding contraction could be envisaged. In this review, evidence in favour of this hypothesis, based on recent data that show a role for muscarinic receptors in modulating airway smooth muscle proliferation, contractility and contractile protein expression is discussed. Based on these findings, we postulate that endogenous acetylcholine contributes to airway remodeling in asthma and COPD.

Protective effects of tiotropium bromide in the progression of airway smooth muscle remodeling

RATIONALE

Recent findings have demonstrated that muscarinic M(3) receptor stimulation enhances airway smooth muscle proliferation to peptide growth factors in vitro. Because both peptide growth factor expression and acetylcholine release are known to be augmented in allergic airway inflammation, it is possible that anticholinergics protect against allergen-induced airway smooth muscle remodeling in vivo.

OBJECTIVE

We investigated the effects of treatment with the long-acting muscarinic receptor antagonist tiotropium on airway smooth muscle changes in a guinea pig model of ongoing allergic asthma.

RESULTS

Twelve weekly repeated allergen challenges induced an increase in airway smooth muscle mass in the noncartilaginous airways. This increase was not accompanied by alterations in cell size, indicating that the allergen-induced changes were entirely from increased airway smooth muscle cell number. Morphometric analysis showed no allergen-induced changes in airway smooth muscle area in the cartilaginous airways. However, repeated ovalbumin challenge enhanced maximal contraction of open tracheal ring preparations ex vivo. This was associated with an increase in smooth muscle-specific myosin expression in the lung. Treatment with inhaled tiotropium considerably inhibited the increase in airway smooth muscle mass, myosin expression, and contractility.

CONCLUSIONS

These results indicate a prominent role for acetylcholine in allergen-induced airway smooth muscle remodeling in vivo, a process that has been thus far considered to be primarily caused by growth factors and other mediators of inflammation. Therefore, muscarinic receptor antagonists, like the long-acting anticholinergic tiotropium bromide, could be beneficial in preventing chronic airway hyperresponsiveness and decline in lung function in allergic asthma.

Muscarinic M(3) receptor-dependent regulation of airway smooth muscle contractile phenotype

1. Airway smooth muscle (ASM) cells are known to switch from a contractile to a proliferative and synthetic phenotype in culture in response to serum and growth factors. Phenotype switching in response to contractile agonists, however, is poorly characterised, despite the possible relationship between ASM phenotype and airway remodelling in asthma. 2. To investigate the effects of muscarinic receptor stimulation on ASM phenotype, we used organ-cultured bovine tracheal smooth muscle (BTSM) strips, in which contractile responsiveness, contractile protein expression and proliferation were measured after pretreatment with methacholine. 3. Long-term methacholine pretreatment (8 days) decreased maximal contraction and sensitivity to methacholine as well as to histamine and KCl. This decrease was dose-dependent (pEC(50)=5.2+/-0.1). Pretreatment with the highest concentration of methacholine applied (100 microm) could suppress maximal histamine-induced contraction to 8+/-1% of control. In addition, contractile protein expression (myosin, actin) was downregulated two-fold. No concomitant increase in proliferative capacity was observed. 4. The M(3)/M(2) muscarinic receptor antagonist DAU 5884 (0.1 microm) completely inhibited the observed decrease in contractility. In contrast, the M(2)/M(3) muscarinic receptor antagonist gallamine (10 microm) was ineffective, demonstrating that M(2) receptors were not involved. 5. Pretreatment (8 days) with 60 mm KCl could mimick the strong decreases in contractility. This was completely prevented by pretreatment with verapamil (1 microm). 6. Regulation of contractility was not affected by protein kinase C inhibition, whereas inhibitors of phosphatidyl inositol 3-kinase and p42/p44 mitogen activated protein kinase were partially effective. 7. These results show that long-term methacholine pretreatment (8 days) induces an M(3) receptor-dependent decrease in BTSM contractility without increased proliferative capacity.