Selective basolateral localization of overexpressed Na-K-ATPase β1- and β2-subunits is disrupted by butryate treatment of MDCK cells

The exclusive basolateral localization of the Na-K-ATPase in kidney epithelium is a critical aspect of nephron function. It has been suggested that mislocalized delivery of the Na-K-ATPase to the apical surface in autosomal dominant polycystic kidney disease (ADPKD) is due to the inappropriate expression of an alternative isoform of the beta-subunit, the beta(2)-isoform. It has been reported that heterologous expression of this beta(2)-isoform in Madin-Darby canine kidney (MDCK) cells results in apical delivery of the Na-K-ATPase. We created a MDCK cell line containing a tetracycline-inducible promoter and expressed either myc-tagged beta(2)- or flag-tagged beta(1)-subunits to study the surface localization of these beta-subunit isoforms in polarized monolayers. We find that the beta(2)-isoform is targeted to the basolateral surface of the plasma membrane in a polarization pattern indistinguishable from the beta(1)-isoform. However, inclusion of butyrate in the growth medium leads to upregulation of overexpressed beta(1)- or beta(2)-subunits and to their appearance at the apical surface. The beta(2)-isoform expressed in MDCK cells does not assemble into alpha(1)beta(2) heterodimers with the endogenous alpha(1). Our findings demonstrate that expression of the beta(2)-isoform does not lead to apical localization of the Na-K-ATPase in MDCK cells and provides evidence for an unexpected effect of butyrate on the trafficking of Na pump subunits.