The third intracellular loop of the rat and mouse cholecystokinin-A receptors is responsible for different patterns of gene activation

It has previously been reported that the cholecystokinin analog JMV-180 beh aves differently on the rat and the mouse cholecystokinin-A receptor (CCK-A R). In mice this analog acts as an agonist on low- and high-affinity sites of the CCK-AR, whereas in rats this compound acts as an agonist on high-aff inity sites and as an antagonist on low- affinity sites. In an attempt to u nderstand why the same compound behaves differently on these two CCK-A rece ptors, we cloned the cDNA encoding the mouse CCK-AR. We then investigated a cellular model able to mimic the effect that was observed in rats and mice . HeLa cells were transiently cotransfected with plasmids leading to expres sion of the rat or mouse CCK-AR in the presence of pFos-Luc as reporter pla smid; such a plasmid placed the regulatory part of the human c-Fos gene ups tream from the firefly luciferase structural gene (Luc). We then observed t hat the two CCK-A receptors behaved differently, not only in the presence o f compound JMV-180 but also in the presence of cholecystokinin or even in a bsence of ligand; the rat CCK-AR was 2 to 3 times more potent than the mous e CCK-AR in inducing the reporter protein, whatever the ligand studied. Thi s result was confirmed using the same kind of experiment with the reporter plasmid p(TRE)(3)-tk-Luc. Using various mutated receptors, we investigated the role of the putative third intracellular loop. We concluded that both t he primary structure of the receptor and the cellular context are in part r esponsible for the differential behavior of these CCK-A receptors.