Cross-talk between transforming growth factor-ß1 and muscarinic M2 receptors augments airway smooth muscle proliferation.

Transforming growth factor-ß1 (TGF-ß1) is a central mediator in tissue remodeling processes, including fibrosis and airway smooth muscle (ASM) hyperplasia, as observed in asthma. The mechanisms underlying this response, however, remain unclear because TGF-ß1 exerts only weak mitogenic effects on ASM cells. In this study, we hypothesized that the mitogenic effect of TGF-ß1 on ASM is indirect and requires prolonged exposure to allow for extracellular matrix (ECM) deposition. To address this hypothesis, we investigated the effects of acute and prolonged treatment with TGF-ß1, alone and in combination with the muscarinic receptor agonist methacholine, on human ASM cell proliferation. Acutely, TGF-ß1 exerted no mitogenic effect. However, prolonged treatment (for 7 d) with TGF-ß1 increased ASM cell proliferation and potentiated the platelet-derived growth factor-induced mitogenic response. Muscarinic receptor stimulation with methacholine synergistically enhanced the effect of TGF-ß1. Interestingly, the integrin-blocking peptide Arg-Gly-Asp-Ser, as well as integrin α5ß1 function-blocking antibodies, inhibited the effects of TGF-ß1 and its combination with methacholine on cell proliferation. Accordingly, prolonged treatment with TGF-ß1 increased fibronectin expression, which was also synergistically enhanced by methacholine. The synergistic effects of methacholine on TGF-ß1-induced proliferation were reduced by the long-acting muscarinic receptor antagonist tiotropium and the M2 receptor subtype-selective antagonist gallamine, but not the M3-selective antagonist DAU5884. In line with these findings, the irreversible Gi protein inhibitor pertussis toxin also prevented the potentiation of TGF-ß1-induced proliferation by methacholine. We conclude that prolonged exposure to TGF-ß1 enhances ASM cell proliferation, which is mediated by extracellular matrix-integrin interactions, and which can be enhanced by muscarinic M2 receptor stimulation.