A key molecule in the pathology of ongoing inflammation in COPD is the collagen fragment proline-glycine- proline (PGP). From the CCRN Macrolide trial, we found that PGP levels markedly declined in concert with azithromycin's ability to reduce COPD exacerbation frequency. To our knowledge, PGP was the only biomarker studied that showed such a profound response. More recently, our laboratory has demonstrated that PGP is negatively regulated by leukotriene A4 hydrolase (LTA4H). LTA4H is a unique enzyme which possesses both a hydrolase and aminopeptidase catalytic site. Although the hydrolase site of LTA4H is well- known to catalyze the conversion of leukotriene A4 into leukotriene B4 (a PMN chemoattractant), the substrate of the aminopeptidase activity was unknown. In a pivotal Science paper in 2010, our group demonstrated that the aminopeptidase activity degrades PGP, thereby leading to less neutrophilic (PMN) inflammation. As such, LTA4H is both pro-inflammatory (hydrolase) and anti-inflammatory (aminopeptidase) at baseline serving as a critical pivot in inflammatory responses. Cigarette smoke can upregulate the total amount of LTA4H as well as chemically modify and inactivate LTA4H's aminopeptidase but not hydrolase activity thus leaving LTB4 generation intact while allowing for chronic accumulation of inflammatory levels of PGP. Therefore, the regulation of this enzyme's amount and activities may have a crucial role in airway inflammation associated with COPD. In fact, we found marked elevation in amount but reduction in LTA4H aminopeptidase activity in smoking and COPD cohorts compared to non-lung disease cohorts although there was notable variability in each of these groups suggesting that the interaction between environment and genetics may play a key feature in COPD disease progression. In fact, previous genetic analyses have revealed a number of polymorphisms in the promoter and coding regions of LTA4H that showed associations with a variety of inflammatory disorders including COPD. In this proposal to assign alterations in LTA4H protein levels and enzyme activity to associated SNPs, we will examine genetic variation in the putative promoter region of LTA4H to determine impact on LTA4H protein levels both in vitro and in vivo. We will also explore the role of SNPs in the coding regio of LTA4H on its activity. The overall hypothesis of this proposal is that variability in expression and enzymatic activity of LTA4H impacts COPD susceptibility and related phenotypes, thus genetic variation affecting LTA4H expression and aminopeptidase function impacts COPD-related phenotypes. Such human studies are only now possible as a result of the COPDGene program, the renewal of which provides a time-sensitive window to conduct these paradigm establishing studies.
COPD is a critical public health problem with a high associated morbidity and mortality. It is a complicated condition which highlights the interplay between genetics and smoking. This proposal will examine an enzyme critical in regulating inflammation in COPD, leukotriene A4 hydrolase (LTA4H), in the context of known genetic variations and the interaction of these variants with smoke in both cell-based models and in a well-genotyped, well-phenotyped population of 10,000 COPD subjects. The successful completion of the aim will provide important insights into the importance of genetic variation of the enzyme in the progression of COPD. (End of Abstract)
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