The Polarized Expression of Na+,K+-ATPase in Epithelia Depends on the Association between β-Subunits Located in Neighboring Cells

Abstract

The polarized distribution of Na+,K+-ATPase plays a paramount physiological role, because either directly or through coupling with co- and countertransporters, it is responsible for the net movement of, for example, glucose, amino acids, Ca2+, K+, Cl-, and CO3H-across the whole epithelium. We report here that the β-subunit is a key factor in the polarized distribution of this enzyme. 1) Madin-Darby canine kidney (MDCK) cells (epithelial from dog kidney) express the Na+,K+-ATPase over the lateral side, but not on the basal and apical domains, as if the contact with a neighboring cell were crucial for the specific membrane location of this enzyme. 2) MDCK cells cocultured with other epithelial types (derived from human, cat, dog, pig, monkey, rabbit, mouse, hamster, and rat) express the enzyme in all (100%) homotypic MDCK/MDCK borders but rarely in heterotypic ones. 3) Although MDCK cells never express Na+,K+-ATPase at contacts with Chinese hamster ovary (CHO) cells, they do when CHO cells are transfected with β1-subunit from the dog kidney (CHO-β). 4) This may be attributed to the adhesive property of the β1-subunit, because an aggregation assay using CHO (mock-transfected) and CHO-β cells shows that the expression of dog β1-subunit in the plasma membrane does increase adhesiveness. 5) This adhesiveness does not involve adherens or tight junctions. 6) Transfection of β1-subunit forces CHO-β cells to coexpress endogenous α-subunit. Together, our results indicate that MDCK cells express Na+,K+-ATPase at a given border provided the contacting cell expresses the dog β1-subunit. The cell–cell interaction thus established would suffice to account for the polarized expression and positioning of Na+,K+-ATPase in epithelial cells.