Activation of ΔF508 CFTR in an epithelial monolayer

Abstract

The ΔF508 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, ΔF508 CFTR conforms to this paradigm and is not present at the plasma membrane. When LLC-PK₁cells or human nasal polyp cells derived from a ΔF508 homozygous patient are grown on plastic dishes and treated with an epithelial differentiating agent (DMSO, 2% for 4 days) or when LLC-PK₁cells are grown as polarized monolayers on permeable supports, plasma membrane ΔF508 CFTR is significantly increased. Moreover, when confluent LLC-PK₁cells expressing ΔF508 CFTR were treated with DMSO and mounted in an Ussing chamber, a further increase in cAMP-activated short-circuit current (i.e., ∼7 μA/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 ΔF508 CFTR. The experiments describe a way to augment ΔF508 CFTR maturation in epithelial cells that appears to act through a novel mechanism and allows insertion of functional ΔF508 CFTR in the plasma membranes of transporting cell monolayers. The results raise the possibility that increased epithelial differentiation might increase the delivery of ΔF508 CFTR from the endoplasmic reticulum to the Golgi, where the ΔF508 protein is shielded from degradative pathways such as the proteasome and allowed to mature.