Secretory Phospholipase A2s in Airway Pathophysiology. Dysregulated eicosanoid synthesis plays a central role in the immunobiology of asthma. Emerging evidence indicates that the epithelium can regulate eicosanoid production by controlling the release of arachidonic acid (AA) used by leukocytes to generate pro-inflammatory eicosanoids. The phospholipase A2s (PLA2)s are a group of enzymes that catalyze the rate-limiting step of AA release from membrane phospholipids initiating the production of leukotrienes (LT)s and prostaglandins (PG)s. Recently, 9 secretory PLA2s (sPLA2)s have been identified that act in concert with the well-described group IVa cytosolic PLA2 (cPLA2a) to release AA and preferentially initiate synthesis of pro-inflammatory eicosanoids such as cysteinyl leukotrienes (CysLT)s. Our overall hypothesis is that upregulation of secretory PLA2s in the epithelium mediates increased production of pro-inflammatory eicosanoids in asthma. A phenotype of asthma where dysregulated eicosanoid synthesis plays a critical role is in exercise-induced bronchoconstriction (EIB). Asthmatics with this phenotype have increased basal levels of CysLTs in their airways, and the release of CysLTs and other eicosanoids such as PGD2 sustain bronchoconstriction during EIB. In preliminary studies, we found that sPLA2 group X (sPLA2-X) is elevated in induced sputum of asthmatics with EIB relative to normal controls, increases in the airways following exercise challenge, and is specifically expressed in airway epithelium. In the murine model of asthma, the epithelium is a major source of sPLA2-X, and genetic deficiency of sPLA2-X markedly inhibits the development airway inflammation, bronchial hyperresponsiveness and remodeling.
In Specific Aim 1, we will conduct a baseline endobronchial biopsy study to determine if asthmatics with EIB have upregulation of sPLA2s in the airway epithelium. Differences in the key regulatory points for AA release, and the synthetic pathways for CysLTs and other eicosanoids will be compared between asthmatics with EIB, asthmatics without EIB, and normal controls.
In Specific Aim 2 we will conduct an exercise challenge study in these three groups to determine if activation of sPLA2s in the epithelium initiates the sustained release of CysLTs and other eicosanoids during EIB. The major control points for AA release within the epithelium, from sPLA2s released into the airways, and in leukocytes residing within the airways will be examined.
In Specific Aim 3, we will conduct in vitro studies with primary bronchial epithelial cells isolated from each of the groups alone and co-cultured with peripheral blood eosinophils to examine the contribution of epithelial sPLA2s to the release of AA from the asthmatic epithelium and the epithelial activation of CysLT synthesis in leukocytes. These studies focusing on the immunobiology of eicosanoid production in a specific disease phenotype are designed to provide important information leading to new therapies for asthma.
. Asthma is the most prevalent chronic disease of young adults in the developed world with a prevalence of 7.2% in adults, and 10.8% in children. Asthma led to 484,000 hospitalizations, 1.9 million emergency department visits, and 13.9 million outpatient visits in 2002. The national annual economic burden of asthma is over 12.7 billion dollars. There is a pressing need to develop new therapies for asthma because long-term preventative treatments for asthma are ineffective in about a third of patients with asthma, and none of the current therapies alter the natural history of the disease. These data highlight the need to develop new therapies that will improve the effectiveness of treatments for asthma. In this application we focus on the mechanism leading to increased production of pro-inflammatory eicosanoid in asthma. Preliminary data strongly implicate secretory phospholipase A2 group X as a key regulator of pro-inflammatory eicosanoid production. Our goal is to determine the function of the sPLA2s in asthma so that novel therapies can be developed and tested in clinical trials that will improve the effectiveness of asthma treatments and alter the natural history of asthma.
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