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

Abstract

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 β-subunit, the β₂-isoform. It has been reported that heterologous expression of this β₂-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 β₂- or flag-tagged β₁-subunits to study the surface localization of these β-subunit isoforms in polarized monolayers. We find that the β₂-isoform is targeted to the basolateral surface of the plasma membrane in a polarization pattern indistinguishable from the β₁-isoform. However, inclusion of butyrate in the growth medium leads to upregulation of overexpressed β₁- or β₂-subunits and to their appearance at the apical surface. The β₂-isoform expressed in MDCK cells does not assemble into α₁β₂heterodimers with the endogenous α₁. Our findings demonstrate that expression of the β₂-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.