Abstract
In severe asthma, bronchodilator- and steroid-insensitive airflow obstruction develops through unknown mechanisms characterized by increased lung airway smooth muscle (ASM) mass and stiffness. We explored the role of a Regulator of G-protein Signaling protein (RGS4) in the ASM hyperplasia and reduced contractile capacity characteristic of advanced asthma. Using immunocytochemical staining, ASM expression of RGS4 was determined 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 determined in cultured human ASM cells and in human precision cut lung slices. Using gain- and loss-of-function approaches, the precise role of RGS proteins was determined 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 G protein-coupled receptor (GPCR)-mediated bronchoconstriction, unexpectedly RGS4 was 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.
Highlights
Asthma, a common respiratory disease, manifests by airway inflammation, hyperresponsiveness and reversible luminal obstruction
Because Regulators of G protein signaling (RGS) transcription often dynamically responds to environmental cues, we analyzed RGS expression in human ASM (HASM) treated with growth factors or cytokines associated with airways dysfunction in asthma by real-time PCR
PDGF-induced RGS4 expression was dose- and time-dependent, with maximum expression occurring at 6 hours (Figure 1C), and required active transcription as it was inhibited by actinomycin D (Figure S1A)
Summary
A common respiratory disease, manifests by airway inflammation, hyperresponsiveness and reversible luminal obstruction. In biopsies of the bronchial wall, increases in the mass of airway smooth muscle (ASM), the pivotal cell regulating bronchomotor tone, in part defines airway remodeling in severe asthma [2]. The contribution of increases in smooth muscle mass to irreversible airway obstruction remains controversial. Phenotypic plasticity defines the ability of smooth muscle to switch between a contractile and synthetic state [3,4]. Characteristic of smooth muscle in cell culture, phenotypic plasticity of smooth muscle in disease states remains controversial [3,4]. Whether all or select populations of smooth muscle in vivo retain a proliferative capacity and whether distinct signaling pathways serve as master switches to promote smooth muscle growth and inhibit agonist-induced contraction remain unknown
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call