Relationship between apoptosis and E-cadherin expression in bronchial epithelium of smoking mouse

Tobacco smoke induces epithelial barrier dysfunction via receptor EphA2 signaling

Erythropoietin-producing human hepatocellular carcinoma (Eph) receptors are the largest family of receptor tyrosine kinases (RTKs) that mediate various cellular and developmental processes. The degrees of expression of these key molecules control the cell-cell interactions. Although the role of Eph receptors and their ligand Ephrins is well studied in developmental processes, their function in tobacco smoke (TS)-induced epithelial barrier dysfunction is unknown. We hypothesized that TS may induce permeability in bronchial airway epithelial cell (BAEpC) monolayer by modulating receptor EphA2 expression, actin cytoskeleton, adherens junction, and focal adhesion proteins. Here we report that in BAEpCs, acute TS exposure significantly upregulated EphA2 and EphrinA1 expression, disrupted the actin filaments, decreased E-cadherin expression, and increased protein permeability, whereas the focal adhesion protein paxillin was unaffected. Silencing the receptor EphA2 expression with silencing interference RNA (siRNA) significantly attenuated TS-induced hyperpermeability in BAEpCs. In addition, when BAEpC monolayer was transfected with EphA2-expressing plasmid and treated with recombinant EphrinA1, the transepithelial electrical resistance decreased significantly. Furthermore, TS downregulated E-cadherin expression and induced hyperpermeability across BAEpC monolayer in a Erk1/Erk2, p38, and JNK MAPK-dependent manner. TS induced hyperpermeability in BAEpC monolayer by targeting cell-cell adhesions, and interestingly cell-matrix adhesions were unaffected. The present data suggest that TS causes significant damage to the BAEpCs via induction of EphA2 and downregulation of E-cadherin. Induction of EphA2 in the BAEpCs exposed to TS may be an important signaling event in the pathogenesis of TS-induced epithelial injury.

Upregulation of pirin expression by chronic cigarette smoking is associated with bronchial epithelial cell apoptosis

Background

Cigarette smoke disrupts the protective barrier established by the airway epithelium through direct damage to the epithelial cells, leading to cell death. Since the morphology of the airway epithelium of smokers does not typically demonstrate necrosis, the most likely mechanism for epithelial cell death in response to cigarette smoke is apoptosis. We hypothesized that cigarette smoke directly up-regulates expression of apoptotic genes, which could play a role in airway epithelial apoptosis.

Methods

Microarray analysis of airway epithelium obtained by bronchoscopy on matched cohorts of 13 phenotypically normal smokers and 9 non-smokers was used to identify specific genes modulated by smoking that were associated with apoptosis. Among the up-regulated apoptotic genes was pirin (3.1-fold, p < 0.002), an iron-binding nuclear protein and transcription cofactor. In vitro studies using human bronchial cells exposed to cigarette smoke extract (CSE) and an adenovirus vector encoding the pirin cDNA (AdPirin) were performed to test the direct effect of cigarette smoke on pirin expression and the effect of pirin expression on apoptosis.

Results

Quantitative TaqMan RT-PCR confirmed a 2-fold increase in pirin expression in the airway epithelium of smokers compared to non-smokers (p < 0.02). CSE applied to primary human bronchial epithelial cell cultures demonstrated that pirin mRNA levels increase in a time-and concentration-dependent manner (p < 0.03, all conditions compared to controls).

Overexpression of pirin, using the vector AdPirin, in human bronchial epithelial cells was associated with an increase in the number of apoptotic cells assessed by both TUNEL assay (5-fold, p < 0.01) and ELISA for cytoplasmic nucleosomes (19.3-fold, p < 0.01) compared to control adenovirus vector.

Conclusion

These observations suggest that up-regulation of pirin may represent one mechanism by which cigarette smoke induces apoptosis in the airway epithelium, an observation that has implications for the pathogenesis of cigarette smoke-induced diseases.