Regulation of Na⁺/K⁺-ATPase α1 and ß1 Expression in Epithelial Cells

Abstract

In nearly all epithelia the Na⁺/K⁺-ATPase is confined to the basolateral membrane where it establishes the transepithelial Na⁺ gradient that drives the vectoral transport of solutes and ions across the epithelium. In some epithelial diseases such as polycystic kidney disease and ischemia the cell surface polarity of the Na⁺/K⁺-ATPase can be disrupted resulting in a loss of epithelial function and integrity (4,7). Though other investigators have examined Na⁺/K⁺-ATPase expression (3) and a subunit localization (1,2) in polarized cells, there is seldom reference as to the expression and cellular localization of the specific subunit isoforms. We have found by RNA and immunoblot analysis of Madin Darby canine kidney (MDCK) and Caco-2 (human colon carcinoma) cells that both cell lines express only α1 and ß1 isoforms (data not shown). Likewise in agreement with what other investigators have seen in MDCK cells using non-isoform specific antibodies (2), the α1 and ß1 subunits are primarily localized to the lateral plasma membrane in both cell lines (data not shown). With this in mind, we wished to express the individual Na⁺/K⁺-ATPase subunits in polarized epithelial cells to elucidate the role of each isoform in directing cellular sorting of the enzyme and in turn to identify possible structural features of each subunit that may participate in determining the localization.