WNT5A and chronic obstructive pulmonary disease: Cigarette smoke can reduce the expression of Frizzled receptor 4 (FZD4) in alveolar epithelial cells and enhance the phosphorylation of β-catenin, inhibiting epithelial proliferation and alveolar repair mediated by the classic Wnt signalling pathway.5 (2) The expression of fibroblast-derived Wnt5a is increased in COPD.