Discoidin domain receptor (DDR)1 expression is downregulated in human bronchial epithelial cells (HBECs) following epithelial mesenchymal transition (EMT) whereas DDR2 expression is unaltered. (a) Quantitative polymerase chain reaction analysis shows a 35% and 50% reduction of DDR1 expression after transforming growth factor (TGF)-β1 stimulation for 5 days in HBECs on plastic and PDL, respectively (top panel). However, DDR2 expression does not appear to change significantly in response to TGF-β1 treatment (bottom panel). The relative expression level of DDRs was normalized to glyceraldehyde 3-phosphate dehydrogenase mRNA in the same sample. Statistical significance was determined by one-way ANOVA followed by Tukey test; **P < 0.01, ***P < 0.001. (b) Western-blot analysis confirms a reduction of DDR1 protein expression level upon TGF-β1 treatment in HBECs in two different media.