Processing of vasoactive intestinal peptide and transferrin in human cancerous colonic cells

Endocytosis of vasoactive intestinal peptide (VIP) and of transferrin (Tf) was comparatively studied in human cancerous colonic HT-29 cells. Cellular depletion in potassium inhibits the internalization of VIP (23%) and to a greater extent (42%) that of Tf. This indicates that clathrin-coated pits are also involved, at least in part, in VIP uptake. The distribution of 125I-Tf- or 125I-VIP-containing vesicles in sucrose gradients revealed low and high density vesicle subpopulations. The low density vesicle subpopulation represented a transient compartment from which incoming vesicles containing N-leucyl-β naphthylamidase were recycled back to the membrane while those containing β-hexosaminidase (HA) and ligand were mostly transferred into the high density compartment. Subsequent fusion of the latter with heavy vesicles was demonstrated by the shift of HA and ligand with vesicles that had been prelabeled with horseradish peroxidase (HRP). Simultaneous internalization of Tf-HRP and 125I-VIP showed that both the low and high density vesicle subpopulations comprised of two types of VIP-containing vesicle, as confirmed by the density shift reaction: two-thirds of VIP shifted with the Tf-HRP-containing vesicles to denser fractions and the remaining was found with unshifted vesicles. These findings indicate that the VIP-receptor complex processing in HT-29 cells follows two routes, the major route being common with Tf endocytosis.