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 cla thrin-coated pits are also involved, at least in part, in VIP uptake. The distribution of I-125-Tf- or I-125-VIP-containing vesicles in sucr ose gradients revealed low and high density vesicle subpopulations. Th e low density vesicle subpopulation represented a transient compartmen t from which incoming vesicles containing N-leucyl-beta naphthylamidas e were recycled back to the membrane while those containing beta-hexos aminidase (HA) and ligand were mostly transferred into the high densit y compartment. Subsequent fusion of the latter with heavy vesicles was demonstrated by the shift of HA and ligand with vesicles that had bee n prelabeled with horseradish peroxidase (HRP). Simultaneous internali zation of Tf-HRP and I-125-VIP showed that both the low and high densi ty vesicle subpopulations comprised of two types of VIP-containing ves icle, as confirmed by the density shift reaction: two-thirds of VIP sh ifted with the Tf-HRP-containing vesicles to denser fractions and the remaining was found with unshifted vesicles. These findings indicate t hat the VIP-receptor complex processing in HT-29 cells follows two rou tes, the major route being common with Tf endocytosis.