Developmentally regulated 75‐kilodalton protein expressed in LLC‐PK1 cultures is a component of the renal Na+/glucose cotransport system

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Abstract
Na+/D‐glucose symport is a secondary active glucose transport mechanism expressed only in kidney proximal tubule and in small intestine. A monoclonal antibody that recognized the Na+/glucose symporter of pig renal brush border membranes also recognized a 75‐kD protein in apical membranes isolated from highly differentiated LLC‐PK1 cultures, an epithelial cell line of pig renal proximal tubule origin. The 75‐kD antigen was enriched from solubilized LLC‐PK1 apical membranes by means of high‐pressure liquid chromatography. The symporter antigen became apparent on the apical membrane surface after the development of a confluent monolayer in correlation with the expression of transport activity. Long‐term treatment of cultures with the differentiation inducer hexamethylene bisacetamide was accompanied by a dramatically increased expression of the symporter antigen as detected quantitatively by Western blot analysis and qualitatively by immunofluorescence staining. The number of symporter‐positive cells was dramatically increased after inducer treatment as predicted for differentiation‐regulated expression. These results identify a 75‐kD protein as a component of a developmentally regulated renal Na+/glucose symporter expressed in cell culture.

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