Smoking-induced Wnt pathway downregulation

The Wnt pathway plays a central role in controlling differentiation of epithelial tissues; when Wnt is on, differentiation is suppressed, but when Wnt is off, differentiation is allowed to proceed. Based on this concept, we hypothesized that expression of key genes in the Wnt pathway are suppressed in the human airway epithelium under the stress of cigarette smoking, a stress associated with dysregulation of the differentiated state of the airway epithelium. For this purpose, HG-U133 Plus 2.0 microarrays were used to assess the expression of Wnt-related genes in the small airway (10th-12th generation) epithelium (SAE) obtained via bronchoscopy and brushing of healthy nonsmokers (n=47), healthy smokers (n=58), and smokers with established COPD (n=22). With expression defined as present in >20% of samples, microarray analysis demonstrated that 35 of 57 known Wnt-related genes are expressed in the adult SAE. Wnt pathway downstream targets β-catenin (p<0.05) and the transcription factor 7-like 1 were down-regulated in healthy smokers, and smokers with COPD, as were a number of Wnt target genes, including VEGFA, CCND1, MMP7, CLDN1, SOX9, RHOU (all p<0.05 compared to healthy nonsmokers). As a mechanism to explain this broad, smoking-induced suppression of the Wnt pathway, we assessed expression of the DKK and SFRP families, extracellular regulators that suppress the Wnt pathway. Among these, secreted frizzled-related protein 2 (SFRP2), was up-regulated 4.3-fold (p<0.0001) in healthy smokers and 4.9-fold (p<0.0001) in COPD smokers, an observation confirmed by TaqMan Real-time PCR. AT the protein levels, Western analysis demonstrated SFRP2 up-regulation, and immunohistochemistry demonstrated that the smoking-induced SFRP2 upregulation occurred in differentiated ciliated cells. Finally, cigarette smoke extract mediated up-regulation of SFRP2 and downregulation of Wnt target genes in airway epithelial cells in vitro. These observations are consistent with the hypothesis that the Wnt pathway plays a role in airway epithelial cell differentiation in the adult human airway epithelium, with smoking associated with down-regulation of Wnt pathway, contributing to the dysregulation of airway epithelial differentiation observed in the smoking-related airway disorders. Affymetrix arrays were used to assess gene expression data of genes in the Wnt pathway in small airway epithelium obtained by fiberoptic bronchoscopy of 47 healthy non-smokers and 58 healthy smokers and 22 smokers with COPD.