FK506 binding protein 8 peptidylprolyl isomerase activity manages a late stage of cystic fibrosis transmembrane conductance regulator (CFTR) folding and stability.

Cystic fibrosis (CF) is caused by mutations in the apical chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) with 90% of patients carrying at least one deletion of the F508 (ΔF508) allele. This mutant form of CFTR is characterized by a folding and trafficking defect that prevents exit from the endoplasmic reticulum. We previously reported that ΔF508 CFTR can be recovered in a complex with Hsp90 and its co-chaperones as an on-pathway folding intermediate, suggesting that Δ508 CF disease arises due to a failure of the proteostasis network (PN), which manages protein folding and degradation in the cell. We have now examined the role of FK506-binding protein 8 (FKBP8), a component of the CFTR interactome, during the biogenesis of wild-type and ΔF508 CFTR. FKBP8 is a member of the peptidylprolyl isomerase family that mediates the cis/trans interconversion of peptidyl prolyl bonds. Our results suggest that FKBP8 is a key PN factor required at a post-Hsp90 step in CFTR biogenesis. In addition, changes in its expression level or alteration of its activity by a peptidylprolyl isomerase inhibitor alter CFTR stability and transport. We propose that CF is caused by the sequential failure of the prevailing PN pathway to stabilize ΔF508-CFTR for endoplasmic reticulum export, a pathway that can be therapeutically managed.

Small-molecule proteostasis regulators for protein conformational diseases.

Protein homeostasis (proteostasis) is essential for cellular and organismal health. Stress, aging and the chronic expression of misfolded proteins, however, challenge the proteostasis machinery and the vitality of the cell. Enhanced expression of molecular chaperones, regulated by heat shock transcription factor-1 (HSF-1), has been shown to restore proteostasis in a variety of conformational disease models, suggesting this mechanism as a promising therapeutic approach. We describe the results of a screen comprised of ∼900,000 small molecules that identified new classes of small-molecule proteostasis regulators that induce HSF-1-dependent chaperone expression and restore protein folding in multiple conformational disease models. These beneficial effects to proteome stability are mediated by HSF-1, FOXO, Nrf-2 and the chaperone machinery through mechanisms that are distinct from current known small-molecule activators of the heat shock response. We suggest that modulation of the proteostasis network by proteostasis regulators may be a promising therapeutic approach for the treatment of a variety of protein conformational diseases.

Potential Agents for Treating Cystic Fibrosis: Cyclic Tetrapeptides that Restore Trafficking and Activity of ΔF508-CFTR.

Cystic fibrosis (CF) is a loss-of-function disease caused by mutations in the CF transmembrane conductance regulator (CFTR) protein, a chloride ion channel that localizes to the apical plasma membrane of epithelial cells. The most common form of the disease results from the deletion of phenylalanine-508 (ΔF508), leading to the accumulation of CFTR in the endoplasmic reticulum with a concomitant loss of chloride flux. We discovered that cyclic tetrapeptides, such as 11, 14, and 15, are able to correct the trafficking defect and restore cell surface activity of ΔF508-CFTR. Although this class of cyclic tetrapeptides is known to contain inhibitors of certain histone deacetylase (HDAC) isoforms, their HDAC inhibitory potencies did not directly correlate with their ability to rescue ΔF508-CFTR. In full HDAC profiling, 15 strongly inhibited HDACs 1, 2, 3, 10 and 11, but not HDACs 4-9. Although 15 had less potent IC(50) values than reference agent vorinostat (2) in HDAC profiling, it was markedly more potent than 2 in rescuing ΔF508-CFTR. We suggest that specific HDACs can have a differential influence on correcting ΔF508-CFTR, which may reflect both deacetylase and protein scaffolding actions.