CFTR-associated ligand (CAL)-dependent post-Golgi trafficking of cystic fibrosis transmembrane conductance regulator (CFTR) is modulated by insulin-like growth factor 1 (IGF-1)

Cystic fibrosis is a genetic disease caused by the mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. Since CFTR is localized at the cell surface to secrete chloride ions, improper localization, insufficient expression levels, and/or defective channel function caused by mutations in CFTR result in abnormal physiological conditions. Thus, adequate cell-surface expression of functional CFTR is required for human health.;While ER-associated degradation and endocystic lysosomal degradation of CFTR have been widely studied, the post-Golgi trafficking of CFTR needs thorough study. CFTR-associated ligand (CAL) is involved in the post-Golgi trafficking of CFTR, since CAL resides at the Golgi and enhances lysosomal degradation of CFTR. The aims of the thesis are unveiling more about how CAL regulates CFTR expression and discovering upstream signals regulating CAL.;First, the CAL-CFTR binding at the Golgi was confirmed by using acceptor photobleaching fluorescent resonance energy transfer. The CAL-CFTR binding at the Golgi implies that CAL functions at the Golgi to enhance the lysosomal degradation. The cell surface biotinylation assay showed that overexpression of CAL inhibits forskolininduced cell surface expression of CFTR. IGF-1 and active TC10 reversed the CAL-mediated inhibition of the cell surface expression of CFTR. Our data support the model that the binding of CAL and CFTR at the Golgi inhibits the CFTR trafficking to the cell surface, probably leading to the lysosomal degradation pathway instead.;Insulin-like growth factor 1 (IGF-1) increases CFTR expression level via TC10 and CAL. IGF-I activates TC10, a small Rho GTPase in human bronchial epithelial cells and HeLa cells. And activated TC10 inhibits CAL-mediating degradation of CFTR. IGF1 does not increase the expression of ΔF508 CFTR, whose processing is arrested in the ER, consistent with our results that IGF-1 alters the CAL-CFTR binding at the Golgi. However, when ΔF508 CFTR is rescued with low temperature or the corrector, VRT-325, and proceeds to the Golgi, IGF-1 increases the expression of rescued ΔF508 CFTR. These findings offer a potential strategy of a combinational treatment of IGF-1 and correctors for cystic fibrosis patients bearing the ΔF508 mutation by increasing the post-Golgi expression of CFTR.