Contrasting roles of leu(356) in the human CCK(1) receptor for antagonist SR 27897 and agonist SR 146131 binding

A new highly specific, potent non-peptide agonist for the cholecystokinin subtype 1 receptor (CCK(1)), SR 146131 (2-[4-(4-chloro-2, 5-dimethoxyphenyl)-5-(2-cyclohexyl-ethyl)-thiazol-2-ylcarbamoyl ]-5, 7-dimethyl-indol-1-yl-1-acetic acid) was recently described [Bignon, E., Bachy, A., Boigegrain, R., Brodin, R., Cottineau, M., Gully, D., Herbert, J.-M., Keane, P., Labie, C., Molimard, J.-C., Olliero, D., Oury-Donat, F., Petereau, C., Prabonneaud, V., Rockstroh, M.-P., Schaeffer, P., Servant, O.Thurneyssen, O., Soubrié, P., Pascal, M., Maffrand, J.-P., Le Fur, G., 1999. SR 146131: a new, potent, orally active and selective non-peptide cholecystokinin subtype I receptor agonist: I. In vitro studies. J. Pharmacol. Exp. Ther. 289, 742-751]. From binding and activity assays with chimeric constructs of human CCK(1) and the cholecystokinin subtype 2 receptor (CCK(2)) and receptors carrying point mutations, we show that Leu(356), situated in transmembrane domain seven in the CCK(1) receptor, is a putative contact point for SR 146131. In contrast, Leu(356) is probably not in contact with the CCK(1) receptor specific antagonist SR 27897 (1-[2-(4-(2-chlorophenyl)thiazol-2-yl)aminocarbonyl indoyl]acetic acid), a compound structurally related to SR 146131, since its replacement by alanine, histidine or asparagine gave receptors having wild-type CCK(1) receptor SR 27897 binding affinity. Previous mutational analysis of His(381), the cognate position in the rat CCK(2) receptor, had implicated it as being involved in subtype specificity for SR 27897, results which we confirm with corresponding mutations in the human CCK(2) receptor. Moreover, binding and activity assays with the natural CCK receptor agonist, CCK-8S, show that CCK-8S is more susceptible to the mutations in that position in the CCK(1) receptor than in the CCK(2) receptor. The results suggest different binding modes for SR 27897, SR 146131 and CCK-8S in each CCK receptor subtype.