Sphingolipids in asthma pathogenesis

Despite effective clinical management of a large number of asthmatics using inhaled bronchodilators and corticosteroids, significant heterogeneity amongst patients means that there remains a large unmet need in treatment, especially in severe, steroid-resistant asthma. In addition, as yet unidentified contractile agents may play a role in the pathogenesis of the disease. Genome-wide association studies have identified ORMDL3, a regulator of the sphingolipid pathway, as potentially associated with asthma. The end-product of this pathway, sphingosine-1-phosphate (S1P), has been shown at higher levels in the bronchoalveolar lavage of asthmatic patients than healthy subjects after allergen challenge, and has been shown to induce a pro-remodelling phenotype in airway smooth muscle cells. S1P has also been postulated to cause contraction in airway smooth muscle cells (ASM) in vitro. Using proliferation assays of primary ASM cells, contractile experiments of ex vivo animal and human airways, this study aims to identify the role of S1P in airway remodelling, bronchoconstriction, and airway hyperresponsiveness. Results show that S1P significantly increases proliferation in primary ASM cells with a 12-fold increase compared to controls, with S1PR2 as the major receptor isotype involved in this response, S1PR2 knockdown ablating this respose. In healthy ex vivo mouse airways, S1P does not cause contraction, but significantly increased airway hyperresponsiveness to a level comparable to that seen in HDM allergic airways, with responsiveness as a percentage of K+PSS response increasing from around 75% in control airways to around 200% in airways treated with S1P in 6 separate airways. In ex vivo human airways, S1P increased sensitivity causing around a 10-fold decrease in EC50 value, as well as increasing responsiveness. Significantly, a first-in-man clinical trial of S1P was performed in which healthy and asthmatic subjects were recruited, with their airway hyperresponsiveness assessed first by methacholine challenge and lung function assessed by spirometry. In vivo, human airways do not constrict in response to S1P in 6 healthy and 6 asthmatic patients, although S1P saline preparations were shown to be safe, with no adverse events reported. Hence, while S1P does not directly cause airway constriction, it significantly increases two of the other major phenotypic features of asthma, hyperresponsiveness and remodelling.