Sodium 4-phenylbutyrate downregulates Hsc70: implications for intracellular trafficking of Delta F508-CFTR

1 Division of Pulmonary Medicine, Children's Hospital of Philadelphia and Department of Pediatrics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104; and 2 Eudowood Division of Pediatric Respiratory Sciences, Johns Hopkins Medical Institutes, Baltimore, Maryland 21287 The most common mutation of the cystic fibrosis transmembrane conductance regulator (CFTR), F508, is a trafficking mutant that has prolonged associations with molecular chaperones and is rapidly degraded, at least in part by the ubiquitin-proteasome system. Sodium 4-phenylbutyrate (4PBA) improves F508-CFTR trafficking and function in vitro in cystic fibrosis epithelial cells and in vivo. To further understand the mechanism of action of 4PBA, we tested the hypothesis that 4PBA modulates the targeting of F508-CFTR for ubiquitination and degradation by reducing the expression of Hsc70 in cystic fibrosis epithelial cells. IB3-1 cells (genotype F508/W1282X) that were treated with 0.05-5 mM 4PBA for 2 days in culture demonstrated a dose-dependent reduction in Hsc70 protein immunoreactivity and mRNA levels. Immunoprecipitation with Hsc70-specific antiserum demonstrated that Hsc70 and CFTR associated under control conditions and that treatment with 4PBA reduced these complexes. Levels of immunoreactive Hsp40, Hdj2, Hsp70, Hsp90, and calnexin were unaffected by 4PBA treatment. These data suggest that 4PBA may improve F508-CFTR trafficking by allowing a greater proportion of mutant CFTR to escape association with Hsc70. cystic fibrosis; cystic fibrosis transmembrane conductance regulator; chaperones; Hsc70; phenylbutyrate