Activation of DeltaF508 CFTR in an epithelial monolayer.
The DeltaF508 mutation leads to retention of cystic fibrosis transmembrane conductance regulator (CFTR) in the endoplasmic reticulum and rapid degradation by the proteasome and other proteolytic systems. In stably transfected LLC-PK1 (porcine kidney) epithelial cells, DeltaF508 CFTR conforms to this paradigm and is not present at the plasma membrane. When LLC-PK1 cells or human nasal polyp cells derived from a DeltaF508 homozygous patient are grown on plastic dishes and treated with an epithelial differentiating agent (DMSO, 2% for 4 days) or when LLC-PK1 cells are grown as polarized monolayers on permeable supports, plasma membrane DeltaF508 CFTR is significantly increased. Moreover, when confluent LLC-PK1 cells expressing DeltaF508 CFTR were treated with DMSO and mounted in an Ussing chamber, a further increase in cAMP-activated short-circuit current (i.e., approximately 7 microA/cm2; P < 0.00025 compared with untreated controls) was observed. No plasma membrane CFTR was detected after DMSO treatment in nonepithelial cells (mouse L cells) expressing DeltaF508 CFTR. The experiments describe a way to augment DeltaF508 CFTR maturation in epithelial cells that appears to act through a novel mechanism and allows insertion of functional DeltaF508 CFTR in the plasma membranes of transporting cell monolayers. The results raise the possibility that increased epithelial differentiation might increase the delivery of DeltaF508 CFTR from the endoplasmic reticulum to the Golgi, where the DeltaF508 protein is shielded from degradative pathways such as the proteasome and allowed to mature.