NADPH Oxidase 4 Expression Is Increased through TRPV4 Channel in D. farinae‐Induced Airway Remodeling and TGF‐β1‐Mediated Fibroblast Differentiation into Myofibroblasts

Abstract

NADPH oxidase 4 (NOX4) is a member of the NOX family of NADPH oxidases, encoded by NOX4 gene and has been implicated in TGF‐β1‐induced human cardiac fibroblast to myofibroblast differentiation. Further, NOX4 has been shown to potentiate lung fibroblast differentiation in vitro and its transcript level is enhanced in fibroblasts isolated from idiopathic pulmonary fibrosis (IPF) patients compared to normal ones. Our lab has previously identified that Transient Receptor Potential Vanilloid 4 (TRPV4) TRPV4, a mechano‐sensitive ion channel is involved in D. farinae‐induced airway remodeling during asthma in vivo and myofibroblast differentiation in vitro. However, the role of NOX4 in asthma, its interaction with TRPV4 and the mechanism of action still remains elusive. In the present study, we show that NOX4 is up‐regulated during lung remodeling in asthma and fibroblast differentiation in vitro in a TRPV4‐dependent pathway.Using NOX4 antagonist and NOX4 siRNA, we demonstrate that NOX4 is involved in TGFβ‐1 induced α smooth muscle actin (α SMA) and fibronectin (FN) expression in human lung fibroblasts (HLF). Further, TGFβ‐1 treatment up‐regulated NOX4 gene expression as determined by real time PCR.. Further, to investigate the effect of TRPV4 on NOX4, we inhibited TRPV4 channel using a pharmacological antagonist RN 1734, and this led to a reduction in TGFβ‐1‐mediated NOX4 expression and differentiation. Interestingly, NOX4 transcripts were up‐regulated in lung tissues in response to D. farinae in an asthma model. More importantly, TRPV4 KO mice which were protected from D. farinae‐induced asthma also exhibited lower levels of NOX4 gene expression, suggesting that TRPV4 promotes airway remodeling through NOX4. Our results unravel a unique role for NOX4 in TRPV4 mediated fibroblast differentiation and lung remodeling and can be used as a new therapeutic target in asthma treatment.