CHARACTERIZATION OF THE CCK-B GASTRIN-LIKE RECEPTOR IN HUMAN COLON-CANCER/

The gastrointestinal peptide, gastrin, tonically stimulates growth of human colon cancer cells in vivo and in vitro, and does so in a recept or-mediated fashion. This study defined the nature of gastrin binding in human colon cancer using [H-3]L-365,260, a specific cholecystokinin B (CCK-B)/gastrin antagonist found to block gastrin's effects on grow th. Following elucidation of optimal binding conditions (e.g., pH, tim e, and temperature) in log phase HT-29 human colon cancer cells, speci fic and saturable binding with a dissociation constant of 4.8 +/- 0.7 nM and a maximal binding capacity (B-max) of 320 +/- 120 fmol/mg prote in, consistent with a single binding site, was recorded. Binding was l ocalized to the membrane fraction. Exposure to gastrin or receptor ant agonist decreased and increased, respectively, the B-max. Competition experiments indicated that L-365,260 was 25- and 200-fold more effecti ve at displacing radiolabeled L-365,260 than gastrin and cholecystokin in, respectively. In contrast to log phase cells, the B-max was decrea sed by 67 to 76% in confluent and postconfluent cultures. Binding acti vity was observed in other cell lines examined, as well as in xenograf ts and colon cancers obtained at surgery. Binding in normal human colo nic mucosa was 10-fold less than in colon cancer. These results provid e the first comprehensive identification and characterization of a CCK -B/gastrin-like receptor in human colon cancer.