Sorting of a secretory protein (gp80) to the apical surface of Caco-2 cells

We have investigated the synthesis and polarized secretion of the exogenous gp80 glycoprotein complex in the human epithelial adenocarcinoma cell line, Caco-2. gp80 is secreted at the apical surface of Madin-Darby canine kidney (MDCK) cells and should, therefore, display the signal(s) required for sorting into the apical exocytic pathway. In Caco-2 cells, no bona fide secretory protein released preferentially at the apical surface has been described so far. To address the question of whether Caco-2 cells possess a machinery capable of delivery of secretory proteins at the apical surface, we stably transfected the cells with a recombinant gene coding for the gp80 glycoprotein complex. Pulse-chase analysis showed that stably transfected Caco-2 cells secrete gp80 quantitatively into the medium. In polarized layers of filter-grown Caco-2 cells, the protein was secreted predominantly at the apical surface, demonstrating the ability of the cells to efficiently sort secretory proteins directly into the apical exocytic pathway. Our results further demonstrate that the apical targeting information of gp80 recognized by MDCK cells is also recognized by Caco-2 cells.