Reduced frizzled receptor 4 expression prevents WNT/b-catenin-driven alveolar lung repair in chronic obstructive pulmonary disease

Rationale: Chronic obstructive pulmonary disease (COPD), in particular emphysema, is characterized by loss of parenchymal alveolar tissue and impaired tissue repair. Wingless and INT-1 (WNT)/b-catenin signaling is reduced in COPD; however, the mechanisms thereof, specifically the role of the frizzled (FZD) family of WNT receptors, remain unexplored. Objectives: To identify and functionally characterize specific FZD receptors that control downstream WNT signaling in impaired lung repair in COPD. Methods: FZD expression was analyzed in lung homogenates and alveolar epithelial type II (ATII) cells of never-smokers, smokers, patients with COPD, and two experimental COPD models by quantitative reverse transcriptase-polymerase chain reaction, immunoblotting, and immunofluorescence. The functional effects of cigarette smoke on FZD4, WNT/b-catenin signaling, and elastogenic components were investigated in primary ATII cells in vitro and in three-dimensional lung tissue cultures ex vivo. Gain- and loss-of-function approaches were applied to determinethe effects of FZD4 signaling on alveolar epithelial cell wound healing and repair, as well as on expression of elastogenic components. Measurements and Main Results: FZD4 expression was reduced in human and experimental COPD lung tissues as well as in primary human ATII cells from patients with COPD. Cigarette smoke exposure down-regulated FZD4 expression in vitro and in vivo, along with reduced WNT/b-catenin activity. Inhibition of FZD4 decreased WNT/b-catenin-driven epithelial cell proliferation and wound closure, and it interfered with ATII-to-ATI cell transdifferentiation and organoid formation, which were augmented by FZD4 overexpression. Moreover, FZD4 restoration by overexpression or pharmacological induction led to induction of WNT/b-catenin signaling and expression of elastogenic components in threedimensional lung tissue cultures ex vivo. Conclusions: Reduced FZD4 expression in COPD contributes to impaired alveolar repair capacity.