Rescue of ΔF508-CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) by Curcumin: Involvement of the Keratin 18 Network

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Abstract
The most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, ΔF508, causes retention of ΔF508-CFTR in the endoplasmic reticulum and leads to the absence of CFTR Cl– channels in the plasma membrane. ΔF508-CFTR retains some Cl– channel activity so increased expression of ΔF508-CFTR in the plasma membrane can restore Cl– secretion deficiency. Recently, curcumin was shown to rescue ΔF508-CFTR localization and function. In our previous work, the keratin 18 (K18) network was implicated in ΔF508-CFTR trafficking. Here, we hypothesized that curcumin could restore a functional ΔF508-CFTR to the plasma membrane acting via the K18 network. First, we analyzed the effects of curcumin on the localization of ΔF508-CFTR in different cell lines (HeLa cells stably transfected with wild-type CFTR or ΔF508-CFTR, CALU-3 cells, or cystic fibrosis pancreatic epithelial cells CFPAC-1) and found that it was significantly delocalized toward the plasma membrane in ΔF508-CFTR-expressing cells. We also performed a functional assay for the CFTR chloride channel in CFPAC-1 cells treated or not with curcumin and detected an increase in a cAMP-dependent chloride efflux in treated ΔF508-CFTR-expressing cells. The K18 network then was analyzed by immunocytochemistry and immunoblot exclusively in curcumin-treated or untreated CFPAC-1 cells because of their endogenic ΔF508-CFTR expression. After curcumin treatment, we observed a remodeling of the K18 network and a significant increase in K18 Ser52 phosphorylation, a site directly implicated in the reorganization of intermediate filaments. With these results, we propose that K18 as a new therapeutic target and curcumin, and/or its analogs, might be considered as potential therapeutic agents for cystic fibrosis.