Influence of PMA and a low extracellular Ca2+ concentration on the development of the Na(+)-dependent hexose carrier in LLC-PK1 cells

We have analyzed the development of Na(+)-dependent hexose transport during differentiation and during polarization of LLC-PK1, an established cell line with characteristics of the proximal tubule. When cell-cell contact was disturbed by a low extracellular Ca2+ concentration or by a phorbol myristate acetate (PMA) treatment, the development of Na(+)-dependent hexose transport was completely inhibited. The effect of PMA on the development of hexose transport could be uncoupled from its effect on the tight junctions. The PMA concentration needed for the latter effect was approx. 10-fold higher than for the former. As the primary cause of the PMA effect, an influence on the cytoskeleton is suggested. In contrast to PMA, the concentration dependence of both phenomena on the extracellular Ca2+ concentration was almost the same. Moreover, the incorporation of hexose carriers in the plasma membrane could be induced by changing the extracellular CA2+ concentration from low to normal. We conclude that there is a relation between the formation of tight junctions and the development of the Na(+)-dependent hexose carrier, possibly because Ca(2+)-dependent cell adhesion molecules play a role in both phenomena. However, a direct relation between Ca(2+)-dependent elements of the tight junctions and the insertion of the hexose carrier can not be excluded. The Ca(2+)-dependent development seems to be a common characteristic of apical membrane proteins in contrast to the development of the basolateral membrane protein, (Na(+)+K+)-ATPase.