Mechanism of autodegradation of cell-surface macromolecules shed by human melanoma cells

The mechanism of autodegradation of cell-surface macromolecules shed by human melanoma cells was studied by incubating radio-iodinated shed macromolecules with unlabeled sister cells and measuring the appearance of acid-soluble radioactivity. After a preliminary latent period of 1-3 h, degradation continually increased up to 24 h and was concentration-dependent. By contrast, binding to cells was very rapid reaching half-maximal value within 15 min. Autodegradation was markedly reduced (44-82%) by pharmacological agents which interfere with endocytosis or lysosomal enzyme activity, including drugs which inhibit receptor migration into coated pits (dansylcadaverine), endocytosis and intracellular transport (colchicine, cytochalasin B, and monensin), and the activity of lysosomal enzymes (chloroquine, ammonium chloride, leupeptin). Degradation was almost totally suppressed (95%) at 4 degrees C. These data suggest that surface macromolecules shed by melanoma cells are autodegraded in part by re-uptake into melanoma cells followed by degradation in lysosomes.