Cysteinyl Leukotrienes: An Innate System for Epithelial Control of Airway Smooth Muscle Proliferation?
Authors: Joshua A. Boyce, Nora A. Barrett
Journal: American Journal of Respiratory and Critical Care Medicine (2015)

Cysteinyl leukotrienes (cysLTs) are products of arachidonic acid metabolism by the 5-lipoxygenase/leukotriene C4 synthase (5-LO/LTC4S) synthetic pathway. LTC4, the parent cysLT, is rapidly synthesized in response to cellular activation, released by a specific transporter, and converted extracellularly to LTD4 and LTE4 through the sequential actions of extracellular enzymes (1). CysLTs are generated by activated mast cells, eosinophils, basophils, macrophages, and myeloid dendritic cells, all of which constitutively express the requisite enzymes necessary for the formation of LTC4. All are hematopoietic cellular constituents of allergic inflammation. CysLTs signal through at least three cognate receptors, termed the type 1 and type 2 cysLT receptors (CysLT1R and CysLT2R) (2, 3), and GPR99 (4). Because of their potent effects as smooth muscle spasmogens (5) (an effect attributed primarily to CysLT1R), and because of evidence that the production of cysLTs increased during spontaneous asthma exacerbations (6), cysLTs drew early interest as potential mediators of asthma. This led to the development of some of the first molecularly targeted drugs for asthma therapy, the 5-lipoxygenase inhibitor, zileuton, and selective CysLT1R antagonists. Both classes of drugs show efficacy in improving baseline airflow measurements and reducing the frequency of asthma exacerbations (7, 8), supporting the pathobiologic role of the cysLTs in regulating bronchomotor tone in asthma.

Advances in molecular biology have significantly expanded our understanding of the potential pathobiologic functions of cysLTs. In mouse models of allergic pulmonary inflammation, cysLTs play a critical role in the induction and amplification of Th2-type immune responses. Mice lacking LTC4S or CysLT1R, with impaired cysLT generation or signaling, respectively, showed a marked reduction in the development of eosinophilic pulmonary inflammation and Th2 immunity in response to intranasal extracts from the house dust mites Dermatophagoides farinae or Dermatophagoides pteronyssinus (Dp) (9–11). CysLT generation in these studies conditioned dendritic cells for Th2 priming via an autocrine/paracrine action at CysLT1R. In another model, intranasal delivery of LTC4 to sensitized mice receiving ovalbumin challenges markedly amplified pulmonary eosinophil recruitment by a pathway involving the activation of CysLT2R on platelets (12). In a model of intranasal sensitization and challenge to the mold Alternaria alternata, the recruitment and activation of type 2 innate lymphoid cells depended on cysLTs (13). Last, in a model of chronic airway inflammation induced by repetitive ovalbumin challenges of sensitized mice, the administration of the CysLT1R antagonist montelukast inhibited peribronchial collagen deposition and airway smooth muscle hyperplasia (14). Although the latter finding supports a potential direct role for cysLTs (and CysLT1R) in driving airway remodeling, the pleiotropic actions of cysLTs at multiple steps in the proximal innate immune response require that downstream effects on structural cells be interpreted with caution.

In this issue of the Journal, Trian and colleagues (pp. 538–546) demonstrate a potential pathway by which cysLTs may elicit airway smooth muscle proliferation independent of the proximal immune response (15). They demonstrate that airway smooth muscle cells (ASM) cultured from bronchial biopsies of patients with severe asthma proliferated in response to cysLTs generated by Dp-stimulated airway epithelial cells. ASM from nonasthmatic controls did not respond to the epithelial supernatants and expressed substantially less CysLT1R mRNA and protein than did cells from severe asthmatics. The effect of Dp on epithelial cells was inhibited by antibodies against protease-activated receptor 2, suggesting Dp proteases were responsible for the activation of the 5-LO/LTC4S pathway in this system. Treatment of the epithelial cells with either zileuton or dexamethasone eliminated Dp induction of the growth factor activity and LTC4 synthesis. The epithelial cells showed substantial up-regulation of 5-LO, 5-LO-activating protein, and LTC4S mRNA transcript expression in response to Dp, suggesting epithelial cells (in contrast to hematopoietic cells) may require transcriptional induction to become competent to generate cysLTs at a level sufficient to cause physiologic effects on smooth muscle.

The study by Trian and colleagues carries several potential implications and caveats. First, although hematopoietic cells almost certainly provide the majority of cysLTs under virtually all circumstances, the study suggests epithelial cells can inducibly express the 5-LO/LTC4S system and could contribute cysLT synthesis when perturbed by environmental danger signals such as proteases. This would be consistent with previous observations that bronchial epithelial cells up-regulate 5-LO, 5-LO–activating protein, and LTC4S expression in response to stimulation with bradykinin or lipopolysaccharide (16). Second, epithelially derived cysLTs elicited proliferation only in asthmatic ASM, but not control ASM, which was attributed to the markedly higher levels of CysLT1R expression by asthmatic ASM. This phenomenon has not been reported previously and seems at odds with the previous reports that patients with asthma and nonasthmatic controls exhibit equivalent sensitivity to bronchoconstriction elicited by inhalation challenges with LTD4, the preferred ligand for CysLT1R (17, 18). Because the ASM cells used in the study by Trian and colleagues all came from donors with severe disease, it is not possible to determine whether CysLT1R expression by ASM varies with disease severity. The molecular basis for the up-regulated expression of CysLT1R in ASM from patients with severe asthma identified in this study could reflect an epigenetic change, as it is not only evident in cells obtained directly by microdissection but is also retained through multiple passages of culture. Finally, the proliferative action of cysLTs for ASM, which was identified in prior studies (19), suggests a potential application for CysLT1R antagonists in the prevention of airway remodeling. This possibility awaits a carefully done longitudinal study using CysLT1R antagonism with careful monitoring of physiologic and histologic endpoints.