Activation of the unfolded protein response by ΔF508 CFTR

Environmental insults and misfolded proteins cause endoplasmic reticulum (ER) stress and activate the unfolded protein response (UPR). The UPR decreases endogenous cystic fibrosis transmembrane conductance regulator (CFTR) mRNA levels and protein maturation efficiency. Herein, we investigated the effects of the folding-deficient Delta F508 CFTR on ER stress induction and UPR activation. For these studies, we developed and characterized stable clones of Calu3 Delta F cells that express different levels of endogenous wild-type (WT) and recombinant Delta F508 CFTR. We also present a novel RT-PCR-based assay for differential quantification of wild-type CFTR mRNA in the presence of Delta F508 CFTR message. The assay is based on a TaqMan minor groove binding (MGB) probe that recognizes a specific TTT sequence (encoding phenylalanine at position 508 in human CFTR). The MGB probe is extremely specific and sensitive to changes in WT CFTRmessage levels. In RNA samples that contain both WT and Delta F508 CFTR mRNAs, measurement of WT CFTR mRNA levels (using the MGB probe) and total CFTR mRNA (using commercial primers) allowed us to calculate Delta F508 CFTR mRNA levels. The results indicate that overexpression of Delta F508 CFTR causes ER stress and activates the UPR. UPR activation precedes a marked decrease in endogenous WT CFTR mRNA expression. Furthermore, polarized airway epithelial cell lines are important tools in cystic fibrosis research, and herein we provide an airway epithelial model to study the biogenesis and function of WT and Delta F508 CFTR expressed within the same cell.