Interaction of CFTR Modulators with Mammalian Membrane Mimetics: The Role of Cholesterol.

Lumacaftor and Ivacaftor are two FDA-approved medications currently used to treat cystic fibrosis (CF), a genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride ion channel located in epithelial cell membranes; however, the detailed mechanism(s) of their action remains to be elucidated. Both drugs, termed modulators, bind CFTR at a protein-lipid interface, yet Lumacaftor acts at the endoplasmic reticulum (ER), while Ivacaftor acts at the plasma membrane (PM). A major difference among biological membranes is their level of cholesterol (viz., the ER, 5% cholesterol; the Golgi apparatus, 12.5%; and the PM, 30%). Therefore, we investigated the ability of each molecule to interact with membranes of the corresponding cholesterol content to determine if lipid cholesterol content provides a physical basis for their observed localized activity. Using differential scanning calorimetry and a terbium-based liposome disruption assay, we show that both Lumacaftor (a corrector) and Ivacaftor (a potentiator) penetrate/diffuse through membranes containing high cholesterol concentrations, such as in Golgi and the PM. The results further suggest that (1) Lumacaftor resides within membranes containing 5% cholesterol, supporting the proposition that Lumacaftor acts as a corrector of the CFTR channel at the ER level where the nascent protein is in its initial folding stage; and (2) Ivacaftor is well-suited to penetrate the PM and reach its binding pocket on CFTR. Our findings provide evidence that membrane cholesterol levels significantly modulate CFTR corrector/potentiator activity and consequently may affect sensitivity to clinical therapeutics in CF patients.