Study of the unfolded protein response (UPR) in cystic fibrosis (CF)

M. Kerbiriou, P. Trouvé, C. Ferec.
INSERM Brest, France

Cystic fibrosis transmembrane conductance regulator (CFTR) functions as a chloride (Cl–) channel in the apical membrane of epithelial cells. The most common mutation in CF is a deletion of a phenylalanine residue at position 508 (?F508). The ?F508-CFTR protein is incorrectly folded and accumulates in the endoplasmic reticulum (ER). This accumulation could trigger the UPR pathway which leads to a global decrease of the protein synthesis in cells and to apoptosis.

Our aim was to determine whether UPR is triggered in ?F508-CFTR expressing cells. Because infection and inflammation observed in CF can trigger UPR we studied the cell susceptibility to exogenous UPR inducers tunicamycin (Tu) and thapsigargin (Tg). UPR was studied by the expression of two specific markers: the glucose-regulated protein 78 (GRP78) and the transcription factor 6 (ATF6) which were compared in wild type (Wt) and in ?F508-CFTR transfected cells. The model was validated by immunofluorescence and by Cl– channel function studies (SPQ) and we showed by western blotting that both GRP78 and ATF6
are over-expressed in ?F508CFTR cells. Furthermore ATF6 was activated in ?F508-CFTR cells since it was cleaved and relocated to the nuclei.

The SPQ results indicated that the inhibition of GRP78 expression by a specific siRNA had no significant effect upon the CFTR activity whereas the inhibition of ATF6 expression induced an increased Cl– channel function of CFTR and ?F508-CFTR.
The response of Wt and ?F508-CFTR cells to Tu and Tg were not different.

In conclusion we show that UPR is triggered in ?F508-CFTR cells and that a decreased ATF6 expression leads to an increased CFTR and ?F508-CFTR function. We propose that the decreased ATF6 expression is a therapeutic target.

Supported by: VLM.