Targeting the Intracellular Environment in Cystic Fibrosis: Restoring Autophagy as a Novel Strategy to Circumvent the CFTR Defect

Abstract

Cystic fibrosis (CF) patients harboring the most common deletion mutation of the CF transmembrane conductance regulator (CFTR), F508del, are poor responders to potentiators of CFTR channel activity which can be used to treat a small subset of CF patients who genetically carry plasma membrane (PM)-resident CFTR mutants. The misfolded F508del-CFTR protein is unstable in the PM even if rescued by pharmacological agents that prevent its intracellular retention and degradation. CF is a conformational disease in which defective CFTR induces an impressive derangement of general proteostasis resulting from disabled autophagy. In this review, we discuss how rescuing Beclin 1 (BECN1), a major player of autophagosome formation, either by means of direct gene transfer or indirectly by administration of proteostasis regulators, could stabilize F508del-CFTR at the PM. We focus on the relationship between the improvement of peripheral proteostasis and CFTR PM stability in F508del-CFTR homozygous bronchial epithelia or mouse lungs. Moreover, this article reviews recent pre-clinical evidence indicating that targeting the intracellular environment surrounding the misfolded mutant CFTR instead of protein itself could constitute an attractive therapeutic option to sensitize patients carrying the F508del-CFTR mutation to the beneficial action of CFTR potentiators on lung inflammation.

Keywords: BECN1; CFTR; autophagy; cystic fibrosis; proteostasis regulators.

Figure 1

Defective CFTR-induced perturbation of the post-translational network in CF epithelial cells. Defective CFTR leads to increased levels of reactive oxygen species (ROS) that increase the levels of the SUMO E3-ligase PIASy, causing TG2 SUMOylation, that, in turn, inhibits TG2 ubiquitination, and avoids its proteosomal degradation, thus sustaining increased TG2 protein levels. Sustained TG2 activation mediates crosslinking of PPARγ and IκBα, which undergo ubiquitination and proteasome degradation. This inhibits nuclear translocation of PPARγ and favors nuclear translocation of NF-κB, stimulating inflammation.

Figure 2

TG2-mediated inhibition of autophagy in CF epithelial cells. Defective CFTR-mediated TG2 activation leads to BECN1 crosslinking and displaces BECN1 interactome away from the endoplasmic reticulum (ER). This mislocalization inhibits autophagosome formation, disables autophagy, and induces accumulation of SQSTM1 (p62). SQSTM1 accumulation leads to proteasome overload and favors sequestration of cross-linked TG2 substrates (PPARγ, IκBα, BECN1) within HDAC6⁺ aggresomes. The combined inhibition of protein and aggresome turnover may also favor the accumulation of F508del-CFTR (together with SQSTM1) within HDAC6⁺/ubiquitin⁺ intracellular aggregates. Defective autophagy inhibits the clearance of damaged mitochondria that contribute to the generation of pro-inflammatory ROS.