Regulation of Normal and Asthmatic Lung Function by G-Protein-Coupled Receptors

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Asthma, a pathological condition of reversible airway obstruction, is comprised of both inflammation of the lung and hyper-contractility of the bronchiolar smooth muscle. Such airway hyperresponsiveness (AHR) can exist in the absence of frank inflammatory infiltrates, however, suggesting that primary abnormalities in airway smooth muscle (ASM) number or contraction may exist in this disease. The major substances that induce bronchial smooth muscle contraction are natural ligands of GPCRs, such as allergen proteases and those contained in allergen-IgE activated mast cell granules (e.g. histamine, cysteinyl leukotrienes (LTD4), endothelin 1, adenosine, and bradykinin). In general, these agonists induce activation of the heterotrimeric G protein G-alpha q, which increases the concentration of intracellular calcium in smooth muscle cells, promoting actin-myosin interactions and muscle fiber shortening. In contrast, ligands acting on G-alpha-s-coupled receptors, such as albuterol, increase intracellular levels of cyclic AMP (cAMP), facilitating ASM relaxation. A large family of Regulators of G protein signaling (RGS) proteins binds to the G protein alpha subunits Gi and Gq (but not Gs) through a conserved RGS domain and inactivates them by accentuating their intrinsic GTPase activity and by blocking downstream effector interactions. Although they are generally considered to act as negative regulators of GPCR signaling pathways, the physiological function of RGS proteins in the lung is unknown. Using immunohistochemistry and immunoblotting with specific antibodies, we identified expression of RGS4 and RGS5 in bronchial smooth muscle. In severe asthma, bronchodilator- and steroid-insensitive airflow obstruction develops through unknown mechanisms characterized by increased lung ASM mass and stiffness. We explored the role of RGS4 and RGS5 in the ASM hyperplasia and reduced contractile capacity characteristic of advanced asthmatic ASM. Using immunocytochemical staining, we investigated RGS4 expression in endobronchial biopsies from healthy subjects and those from subjects with mild, moderate and severe asthma. Cell proliferation assays, agonist-induced calcium mobilization and bronchoconstriction were evaluated in cultured human ASM cells and in human precision cut lung slices. Using gain- and loss-of-function approaches, we studied the precise role of RGS proteins in stimulating human ASM proliferation and inhibiting bronchoconstriction. RGS4 expression was restricted to a subpopulation of ASM and was specifically upregulated by mitogens, which induced a hyperproliferative and hypocontractile ASM phenotype similar to that observed in recalcitrant asthma. RGS4 expression was markedly increased in bronchial smooth muscle of patients with severe asthma, and expression correlated significantly with reduced pulmonary function. Whereas RGS4 inhibited GPCR-mediated bronchoconstriction, RGS4 was unexpectedly required for PDGF-induced proliferation and sustained activation of PI3K, a mitogenic signaling molecule that regulates ASM proliferation. These studies indicate that increased RGS4 expression promotes a phenotypic switch of ASM, evoking irreversible airway obstruction in subjects with severe asthma. We also investigated the role of RGS5 in the development of fixed airway obstruction. We compared the GPCR-induced calcium mobilization and expression of GPCRs and signaling proteins related to procontractile signaling in ASM derived postmortem from subjects who died of nonrespiratory causes, with cells from subjects who died of asthma. Despite the increased or comparable expression of contraction-promoting GPCRs (bradykinin B2, histamine H1, and PAR1) in asthmatic ASM cells relative to cells from healthy donors, asthmatic ASM cells exhibited reduced histamine-induced calcium mobilization and comparable responses to bradykinin and thrombin, suggesting a postreceptor signaling defect. Accordingly, the expression of RGS5, an inhibitor of ASM contraction, was increased in cultured, asthmatic ASM cells and in bronchial smooth muscle bundles of both asthmatic human subjects and allergen-challenged mice, relative to those of healthy human donors or naive mice. The overexpression of RGS5 impaired the release of calcium to thrombin, histamine, and carbachol, and reduced the contraction of precision-cut lung slices to carbachol. These results suggest that increased RGS5 expression also contributes to decreased myocyte shortening in severe and fatal asthma. In collaboration with Dr. Neubig, we will examine the effect of an RGS4-specific inhibitor on the development of the asthma phenotype and ASM hyperplasia and contraction in animal models and cell culture. This is the first RGS inhibitory compound that has emerged.