Chloride Ion Transport in Transformed Normal and Cystic Fibrosis Epithelial Cells

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Abstract
The inability of β-adrenergic agonists (eg. isoproterenol) to activate a specific apical membrane chloride channel in epithelial cells is characteristic of cystic fibrosis (CF). The study of these channels has been facilitated by the transformation of human airway epithelial cells. Clonal populations of SV40 large T-antigen transformed airway epithelial cells from both normal and cystic fibrosis individuals have been established. A limitation in the use of these cell lines has been the loss of the ability to form tight junctions after multiple subcultures. In particular, this loss appears to be associated with cell “crisis”. A selection protocol that involves growing cells in medium that is high in Ca2+ and supplemented with fetal bovine serum (FBS) has been employed to facilitate progression through crisis. One cell line (lHAEo-) that has a normal phenotype and retains tight junctions post-crisis has been produced using this protocol. Immunofluorescent staining with a monoclonal antibody to the E-cadherin adhesion molecule shows a characteristic pericellular localization, indicating the presence of the junctional complex. The presence of tight junctions has been confirmed by electron microscopy. These cells produce elevated (>30 fold) levels of cAMP in response to exposure to isoproterenol or forskolin. Chloride ion transport, as measured by 36C1- efflux, is stimulated >2 fold by these agents. Three post-crisis CF cell lines which grow in serum-contai-ning medium have been established. These do not exhibit tight junctions. Elevated (>25 fold) levels of cAMP are detected in these cells after addition of isoproterenol or forskolin, but this increase in cAMP is not accompanied by an increase in 36C1- efflux. Both normal (lHAEo-) and CF cells show increased 36C1- efflux following addition of the calcium ionophore A23187. Acti-vation of the apical membrane chloride channel by elevated in-tracellular calcium is not thought to be affected in CF. When these transformed cells are grown in serum-free medium containing epinephrine the response to isoproterenol or forskolin is attenuated. This is consistent with down regulation of the β-adrenergic receptor upon chronic stimulation. One of the post-crisis CF lines (2CFSMEO-) is homozygous for the deltaF508 deletion; the remainder do not have this mutation. This may make it possible to correlate specific deletions with the degree of severity of the CF phenotype.