SERCA Pump Inhibitors Do Not Correct Biosynthetic Arrest of  F508 CFTR in Cystic Fibrosis

Deletion of phenylalanine 508 (Delta F508) accounts for nearly 70% of all mutations that occur in the cystic fibrosis transmembrane conductance regulator (CFTR). The Delta F508 mutation is a class 11 processing mutation that results in very little or no mature CFTR protein reaching the apical membrane and thus no cAMP-mediated Cl- conductance. Therapeutic strategies have been developed to enhance processing of the defective Delta F508 CFTR molecule so that a functional cAMP-regulated Cl- channel targets to the apical membrane. Sarcoplasmic/endoplasmic reticulum calcium (SERCA) inhibitors, curcumin and thapsigargin, have been reported to effectively correct the CF ion transport defects observed in the Delta F508 CF mice. We investigated the effect of these compounds in human airway epithelial cells to determine if they could induce Delta F508 CFTR maturation, and Cl- secretion. We also used Baby Hamster Kidney cells, heterologously expressing Delta F508 CFTR, to determine if SERCA inhibitors could interfere with the interaction between calnexin and CFTR and thereby correct the Delta F508 CFTR misfolding defect. Finally, at the whole animal level, we tested the ability of curcumin and thapsigargin to (1) induce Cl- secretion and reduce hyperabsorption of Na+ in the nasal epithelia of the Delta F508 mouse in vivo, and (2) induce Cl- secretion in intestine (jejunum and distal colon) and the gallbladder of the Delta F508 CF mouse. We conclude that curcumin and thapsigargin failed to induce maturation of Delta F508 CFTR, or induce Cl- secretion, as measured by biochemical and electrophysiologic techniques in a variety of model systems ranging from cultured cells to in vivo studies.