THE CATION-INDEPENDENT MANNOSE 6-PHOSPHATE RECEPTOR IS NOT INVOLVED IN THE POLARIZED SECRETION OF LYSOSOMAL ALPHA-GLUCOSIDASE FROM CACO-2 CELLS

In the human adenocarcinoma cell line Caco-2 a substantial amount of a precursor form of the lysosomal enzyme alpha-glucosidase is not segregated into lysosomes, but instead secreted from the apical membrane. In this study we addressed the question whether this process is mediated by mannose 6-phosphate receptors. The subcellular distribution of the cation-independent mannose 6-phosphate receptor was studied by means of electron microscopic immunocytochemistry. The bulk of label was found in the perinuclear region in electron-lucent and dense vesicles, some of the latter bearing a coat. Receptor-containing dense vesicles were also found throughout the cytoplasm. In the apical part of the cells, label for the receptor was present over the surrounding membrane and the interior vesicles of multivesicular bodies, but not over lysosomes. Label on the plasma membrane was mainly restricted to the apical domain. In contrast to alpha-glucosidase, the secreted forms of the lysosomal enzymes cathepsin D, beta-hexosaminidase and beta-glucuronidase are mainly found in the basolateral medium. Enzyme activity measurements and immunoprecipitation of metabolically labeled cells showed that incubation with NH4Cl leads to an enhanced secretion of these enzymes into the basolateral medium, but has no effect on the basolateral secretion of alpha-glucosidase. In addition, NH4Cl caused a minor decrease in the secretion of these enzymes from the apical side and had little or no effect on the secretion of alpha-glucosidase. When buffer of low pH or medium supplemented with 20 mm mannose 6-phosphate to the apical side of the cells was added, no effect was found on the amount of immunocytochemically detectable alpha-glucosidase in the microvilli. We conclude that although mannose 6-phosphate receptor-mediated transport is functionally active in Caco-2 cells, an additional or alternative mechanism contributes to the transport of alpha-glucosidase to the apical membrane.