Internalization-sequestration and degradation of cholecystokinin (CCK) in tumoral rat pancreatic AR 4-2 J cells

Abstract

Cholecystokinin (CCK) receptors were investigated in the tumoral acinar cell line AR 4-2 J derived from rat pancreas, after preincubation with 20 nM dexamethasone. At steady state binding at 37°C (i.e., after a 5 min incubation), less than 10% of the radioactivity of [ 125I]BH-CCK-9 (3-(4-hydroxy-[ 125I]iodophenyl)propionyl (Thr 34, Nle 37) CCK(31–39)) could be washed away from intact cells with an ice-cold acidic medium, suggesting high and rapid internalization-sequestration of tracer. By contrast, more than 85% of the tracer dissociated rapidly after a similar acid wash from cell membranes prelabelled at steady state. In intact AR 4-2 J cells, internalization required neither energy nor the cytoskeleton framework. Tracer internalization was reversed partly but rapidly at 37°C but slowly at 4°C. In addition, two degradation pathways of the tracer were demonstrated, one intracellular and one extracellular. Intracellular degradation occurred at 37°C but not at 20°C and resulted in progressive intracellular accumulation of [ 125I]BH-Arg that corresponded, after 1 h at 37°C, to 35% of the radioactivity specifically bound. This phenomenon was not inhibited by serine proteinase inhibitors and modestly only by monensin and chloroquine. Besides, tracer degradation at the external cell surface was still observable at 20°C and yielded a peptide (probably [ 125I]BH-Arg-Asp-Tyr(SO 3H)-Thr-Gly). This degradation pathway was partly inhibited by bacitracin and phosphoramidon while thiorphan, an inhibitor of endopeptidase EC 3.4.24.11, was without effect.