Characterization of cholecystokinin receptors on human gastric smooth muscle tumors

Gastric smooth muscle cells are a physiological target for the polypeptide hormone cholecystokinin (CCK). Human tumors arising from this type of cell, leiomyosarcomas, can retain their ability to express a receptor for CCK. To begin to characterize the human CCK receptor, we established a scheme for fractionation of these tumors to yield a membrane preparation enriched in enzyme markers of plasmalemma that saturably binds CCK. In competition-binding studies using ¹²⁵I-CCK-8, only peptides structurally related to CCK competed for binding, with 50% of binding inhibited by 0.075 nM CCK-8, 0.9 nM CCK-8-desulfate, 0.9 nM gastrin-17, and 2.5 nM CCK tetrapeptide. Specificity of binding was demonstrated by showing that structurally unrelated peptides did not compete for binding. Association and dissociation of binding were temperature dependent. We have also performed affinity labeling studies to define the molecular properties of the CCK binding site. In these, the membranes were incubated with ¹²⁵I-CCK-33, washed, cross-linked with disuccinimidyl suberate, solubilized, and electrophoretically separated on a polyacrylamide gel. Autoradiography of the dried gel revealed labeling of a major component with M(r) 75,000 and minor components with M(r) 53,000, M(r) 100,000, M(r) 120,000, and M(r) > 200,000. Labeling was inhibited by CCK-8 in a concentration-dependent manner. This was also specific for CCK and structurally related peptides. These results demonstrate that gastric leiomyosarcomas are a very good source of a human CCK receptor and suggest that they may provide an easily cultured tissue with which this receptor can be fully characterized.