Leukotriene E4 receptors in asthma pathogenesis

The incidence of bronchial asthma has increased during last two decades in the UK, affecting more than 5 million people and putting the UK in the first place among European countries with the highest prevalence of the disease. In spite of great efforts we still do not fully understand the primary mechanisms leading to development of asthmatic symptoms. Cysteinyl leukotrienes (cysLTs) appear to be one of the major substances causing asthma and asthmatic lungs are very sensitive to the effects of these substances, especially to leukotriene E4 (LTE4). This disproportionate sensitivity to LTE4 is even more increased in a subpopulation of patients with aspirin-sensitive asthma. We propose to characterise new receptors for cysLTs and define the way that cysLTs exert their action on lung cells. Our research will allow an understanding of the role of the cysLTs in the mechanism of bronchial asthma and may lead to new treatments.

Basic and clinical studies have indicated that cysteinyl leukotrienes (cysLTs) are one of the major proinflammatory mediators involved in the pathophysiology of bronchial asthma. Although leukotrienes have been known for more than 20 years we still do not fully understand their signalling and functions in pathological processes such as bronchial asthma.
Asthmatic airways respond with enhanced bronchoconstriction to inhaled cysLTs and especially to leukotriene E4 (LTE4) in comparison with normal subjects. This disproportionate hyperresponsiveness to LTE4 is even more increased in a subpopulation of asthmatics (about 10% of asthmatics), with aspirin-sensitive asthma. LTE4 has also been shown to be the most potent cysLT in eliciting influx of inflammatory cells such as eosinophils and basophils into bronchial mucosa of asthmatic subjects. None of these phenotypes can be explained based on current knowledge of cysLT signalling through their specific receptors. The existence of separate LTE4 receptors has been recently postulated. The P2Y12 has been suggested to be a novel LTE4 receptor in human cells and another, as yet unidentified LTE4 receptor has been described in CysLT1/CysLT2 double knock-out murine model. Our goal is to characterise the P2Y12 as the LTE4 specific receptor in human primary cells, define the human P2Y12 gene structure, including promoter and alternatively spliced transcript expression and mechanisms responsible for regulation of the receptor expression. To address whether P2Y12 may have a role in asthma pathogenesis, the receptor expression at the mRNA level as well responsiveness to LTE4 will be studied in cells derived from asthmatic patients in comparison to healthy control subjects. The role of currently used P2Y12 inhibitors as a potential, new anti-asthmatic drugs will be evaluated in in vitro models. Clinical data showing increased LTE4 responsiveness in asthmatic patients strongly suggest that an additional LTE4 receptor is expressed on airway smooth muscle cells, apart from P2Y12, with enhanced expression in asthmatics and with the highest expression present in bronchi of aspirin sensitive subjects. In order to identify this novel LTE4 receptor we will characterise the pattern of G-protein coupled receptor (GPCR) expression and LTE4-induced signalling in cells from asthmatics with different asthma phenotypes and healthy controls using real-time PCR array and calcium mobilization assay. Identified genes will be cloned, expressed heterologously and analysed for LTE4 responsiveness.
This project presents a unique opportunity to provide important insights on the role of LTE4 in asthma and the possibility of identifying new molecular targets for patient benefit.