Expression of insulin-like growth factor (IGF)-I receptors, IGF-II/cation-independent mannose 6-phosphate receptors (CI-MPRs), and cation-dependent MPRs in polarized human intestinal Caco-2 cells

Abstract

We have analyzed the surface distribution and functional expression of the insulin-like growth factor I (IGF-I) receptor and the IGF-II/cation-independent mannose 6-phosphate receptor (IGF-II/CI-MPR) in the polarized human colon adenocarcinoma cell line, Caco-2. Domain-selective biotinylation of the apical and basolateral surfaces of Caco-2 cells grown on filter supports revealed a 3–4-fold enrichment of these receptors on basolateral membranes. In addition, the biotinylation studies revealed the presence of the cation-dependent MPR on both membrane surfaces, with a 3.4-fold enrichment on basolateral membranes. Binding of 125I-IGF-I at 4°C confirmed similar higher levels of expression of the IGF-I receptor at the basolateral surface than at the apical surface. Cell surface-specific binding of the iodinated lysosomal enzyme β-glucuronidase was detected at 4°C on both plasma membrane domains. However, significant uptake of β-glucuronidase at 37°C was observed only from the basolateral surface. These results indicate that the MPRs and the IGF-I receptor are expressed in a polarized fashion in Caco-2 cells and that the IGF-II/CI-MPR present on apical membranes, unlike the IGF-II/CI-MPR expressed on the basolateral surface, is not functional in endocytosing lysosomal enzymes.