Sphingosine-1-phosphate induces airway smooth muscle hyperresponsiveness and proliferation

Abstract

The emerging role of sphingosine-1-phosphate (S1P) in regulating smooth muscle functions has led to the exploration of the possibility that this sphingolipid could represent a potential therapeutic target in asthma and other lung diseases. Several studies in animal surrogates have suggested a role for S1P-mediated signalling in the regulation of airway smooth muscle contraction, airway hyperresponsiveness and airway remodelling but evidence from human studies is lacking. We have compared the responsiveness of the airways to S1P in healthy and asthmatic individuals in vivo, in isolated human airways ex vivo and in murine airways dissected from healthy and house dust mite (HDM) sensitised animals. Airway responsiveness was measured by spirometry during inhalation challenges and by wire myography in airways isolated from human and mouse lungs. Thymidine incorporation and calcium mobilisation assays were used to study human airway smooth muscle cell responses. S1P did not induce contraction of airways isolated from healthy and HDM exposed mice, nor in human airways. Similarly, there was no airway constriction observed in healthy and asthmatic subjects in response to increasing concentrations of inhaled S1P. However, 30 minutes exposure to S1P induced a significant, concentration-dependent enhancement of airway reactivity to methacholine and to histamine in murine and human airways respectively. HDM-sensitised mice demonstrated a significant increase in methacholine responsiveness which was not further enhanced by S1P treatment. S1P also concentration-dependently enhanced proliferation of human airway smooth muscle cells, an effect mediated through the S1P type 2 receptor, as shown by selective antagonism and S1PR2 siRNA knockdown. Our data suggest that S1P released locally into the airways may be involved in the regulation of airway smooth muscle hyperresponsiveness and hyperplasia, defining a novel target for future therapies.