Role of Cation Channel TRPV4 in Mechanosensing, Myofibroblast Differentiation, and Pulmonary Fibrosis

Abstract

Idiopathic pulmonary fibrosis (IPF) is a fatal fibrotic lung disorder with marginally effective medical treatment. Myofibroblasts are critical to the fibrogenic lung repair process; however, the mechanism whereby the mechanical signal that leads to myofibroblast generation is sensed and transmitted remains to be determined. As transient receptor potential vanilloid 4 (TRPV4) ion channels are activated by plasma membrane stretch/matrix stiffness, we investigated whether TRPV4 plays a role in myofibroblast differentiation and/or in vivo lung fibrosis. TRPV4 inhibition by small-molecule inhibitors almost completely abrogated both the calcium influx response to TPRV4 agonist in a dose-dependent manner and the myofibroblast differentiation response to transforming growth factor-β. Similar findings were noted on TRPV4 deletion with small interfering RNA or in TRPV4 KO murine lung fibroblasts. Further, TRPV4 exhibited its mechanosensing and myofibroblast-differentiating effect under conditions of matrix stiffness in the pathophysiological range (1–25 kPa). Mechanistically, TRPV4 activity modulates transforming growth factor β actions in a Smad-independent manner, in part through enhanced actomyosin remodeling with resultant nuclear translocation of the α–smooth muscle actin transcription coactivator (myocardin-related transcription factor-A). The myofibroblast differentiation response to actual fibrotic lung tissue was also completely inhibited on TRPV4 blockade. Furthermore, TRPV4 deficiency protects mice from fibrosis-inducing effects of bleomycin (1 or 4 U/kg dose) at the biochemical (75% less hydroxyproline; P < 0.05), histologic, and physiologic levels (57% less impairment of compliance; P < 0.05). These data identify TRPV4 as a long-elusive mechanosensor/transducer that mediates myofibroblast differentiation and in vivo pulmonary fibrogenesis. Successful manipulation of TRPV4 channel activity may be a novel therapeutic approach for fibrotic diseases of the lung and other organs.