Vitamin C controls the cystic fibrosis transmembrane conductance regulator chloride channel

doi:10.1073/pnas.0308393100

Vitamin C controls the cystic fibrosis transmembrane conductance regulator chloride channel

Children's Hospital Oakland Research Institute, 5700 Martin Luther King Jr. Way, Oakland, CA 94609

Communicated by Bruce N. Ames, University of California, Berkeley, CA, December 16, 2003 (received for review October 29, 2003)

Abstract

Vitamin C (l-ascorbate) is present in the respiratory lining fluid of human lungs, and local deficits occur during oxidative stress. Here we report a unique function of vitamin C on the cystic fibrosis (CF) transmembrane conductance regulator (CFTR), a cAMP-dependent Cl channel that regulates epithelial surface fluid secretion. Vitamin C (100 μM) induced the openings of CFTR Cl channels by increasing its average open probability from 0 to 0.21 ± 0.08, without a detectable increase in intracellular cAMP levels. Exposure of the apical airway surface to vitamin C stimulated the transepithelial Cl secretion to 68% of forskolin-stimulated currents. The average half-maximal stimulatory constant was 36.5 ± 2.9 μM, which corresponds to physiological concentrations. When vitamin C was instilled into the nasal epithelium of human subjects, it effectively activated Cl transport in vivo. In CF epithelia, previous treatment of the underlying trafficking defect with trimethylamine oxide or expression of WT CFTR restored the activation of Cl transport by vitamin C. Sodium dependency and phloretin sensitivity, as well as the expression of transcripts for sodium-dependent vitamin C transporter (SVCT)-1 and SVCT2, support a model in which an apical vitamin C transporter is central for relaying the effect of vitamin C to CFTR. We conclude that cellular vitamin C is a biological regulator of CFTR-mediated Cl secretion in epithelia. The pool of vitamin C in the respiratory tract represents a potential nutraceutical and pharmaceutical target for the complementary treatment of sticky airway secretions by enhancing epithelial fluid secretion.

Footnotes

↵ * To whom correspondence should be addressed. E-mail: billek{at}chori.org.
Abbreviations: ASL, airway surface liquid; CF, cystic fibrosis; CFTR, CF transmembrane conductance regulator; I _sc, short-circuit current; NPD, nasal potential difference; RPD, rectal potential difference, SVCT, sodium-dependent vitamin C transporter; TMAO, trimethylamine oxide; hTE, normal human tracheal primary cultures; EGFP, enhanced GFP.
Data deposition: SVCT2 was cloned from human tracheal epithelial cultures, and the nucleotide sequence reported in this article has been deposited in the GenBank database (accession no. AY380556).