Hyperexpression of recombinant CFTR in heterologous cells alters its physiological properties

We investigated whether high levels of expression of the cysticfibrosis transmembrane conductance regulator (CFTR) would alter thefunctional properties of newly synthesized recombinant proteins. COS-7,CFPAC-1, and A549 cells were intranuclearly injected with a Simianvirus 40-driven pECE-CFTR plasmid and assayed for halide permeabilityusing the6-methoxy-N-(3-sulfopropyl)quinolinium fluorescent probe. With increasing numbers of microinjected pECE-CFTR copies, the baseline permeability to halide dose dependently increased, and the response to adenosine 3',5'-cyclic monophosphate(cAMP) stimulation decreased. In cells hyperexpressing CFTR, the high level of halide permeability was reduced when a cell metabolism poisoning cocktail was applied to decrease intracellular ATP and, inversely, was increased by orthovanadate. In CFPAC-1 cellsinvestigated with the patch-clamp technique, CFTR hyperexpression ledto a time-independent nonrectifying chloride current that was notsensitive to cAMP stimulation. CFPAC-1 cells hyperexpressing CFTRexhibited no outward rectifying chloride current nor inward rectifyingpotassium current either spontaneously or under cAMP stimulation. Weconclude that hyperexpression of recombinant CFTR proteins modifiestheir properties inasmuch as 1) CFTRchannels are permanently activated and not susceptible to cAMPregulation and 2) they lose their capacity to regulate heterologous ionic channels.