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Am J Physiol Gastrointest Liver Physiol 277: G506-G514, 1999;
0193-1857/99 $5.00
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Vol. 277, Issue 3, G506-G514, September 1999

Biosynthesis and secretion of the mannose 6-phosphate receptor and its ligands in polarized Caco-2 cells

Debra A. Wick1, Bellur Seetharam1,2, and Nancy M. Dahms1

1 Department of Biochemistry, and 2 Department of Medicine, Division of Gastroenterology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226

We have analyzed the transport of newly synthesized mannose 6-phosphate (Man-6-P)-bearing proteins (i.e., lysosomal enzymes) in the polarized human colon adenocarcinoma cell line, Caco-2, by subjecting filter-grown cells to a pulse-chase labeling protocol using [35S]methionine, and the resulting cell lysate, apical medium, and basolateral medium were immunoprecipitated with insulin-like growth factor II/Man-6-P receptor (IGF-II/MPR)-specific antisera. The results showed that the majority of secreted lysosomal enzymes accumulated in the apical medium at >2 h of chase and that this polarized distribution was facilitated by the IGF-II/MPR selectively endocytosing lysosomal enzymes from the basolateral surface. Treatment with various agents known to affect vesicular transport events demonstrated that incubations at 16°C or incubations with brefeldin A inhibited the secretion of lysosomal enzymes from both the apical and basolateral surface, whereas treatment with nocodazole selectively blocked apical secretion. In contrast, incubation with NH4Cl or nocodazole had a stimulatory effect on basolateral secretion. Taken together, these results demonstrate that the sorting of Man-6-P-containing proteins into the apical and basolateral secretory pathways is regulated by distinct components of the intracellular trafficking machinery.

lysosomal enzymes; intracellular trafficking; insulin-like growth factor II receptor


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