Although SB202190 and SB203580 are described as specific p38 MAP kinase inhibitors, several reports have indicated that other enzymes are also sensitive to SB203580. Using a pharmacological approach, we report for the first time that compounds SB202190 and SB203580 were able to directly and selectively interact with a G-protein-coupled receptor, namely the cholecystokinin receptor subtype CCK1, but not with the CCK2 receptor. We demonstrated that these compounds were non-competitive antagonists of the CCK1 receptor at concentrations typically used to inhibit protein kinases. By chimeric construction of the CCK2 receptor, we determined the involvement of two CCK1 receptor intracellular loops in the binding of SB202190 and SB203580. We also showed that two CCK antagonists, L364,718 and L365,260, were able to regulate p38 mitogen-activated protein (MAP) kinase activity. Using a reporter gene strategy and immunoblotting experiments, we demonstrated that both CCK antagonists inhibited selectively the enzymatic activity of p38 MAP kinase. Kinase assays suggested that this inhibition resulted from a direct interaction with both CCK antagonists. Molecular modeling simulations suggested that this interaction occurs in the ATP binding pocket of p38 MAP kinase. These results suggest that SB202190 and SB203580 bind to the CCK1 receptor and, as such, these compounds should be used with caution in models that express this receptor. We also found that L364,718 and L365,260, two CCK receptor antagonists, directly interacted with p38 MAP kinase and inhibited its activity. These findings suggest that the CCK1 receptor shares structural analogies with the p38 MAP kinase ATP binding site. They open the way to potential design of either a new family of MAP kinase inhibitors from CCK1 receptor ligand structures or new CCK1 receptor ligands based on p38 MAP kinase inhibitor structures.

In cells overexpressing active MEKK1 to enhance c‐Jun trans‐activation, expression of rat cholecystokinin 1 receptor increased the activity of c‐Jun while in the same experimental conditions overexpression of mouse cholecystokinin 1 receptor repressed it. This differential trans‐activation is specific, since it was not observed for either the other overexpressed kinases (MEK, PKA) or for other transcription factors (ATF2, ELK‐1, CREB). This differential behaviour was also detected in a human colon adenocarcinoma cell‐line naturally producing high levels of endogenous MEKK1. This differential behaviour between the two receptors on the MEKK1‐induced c‐Jun trans‐activation was independent of the activation state of JNK, of the phosphorylation level of c‐Jun and of its ability to bind its specific DNA responsive elements. Two amino acids (Val43 and Phe50 in the mouse cholecystokinin 1 receptor, replaced by Leu43 and Ileu50 in the rat cholecystokinin 1 receptor) localized in the first transmembrane domain were found to play a crucial role in this differential behaviour. MEKK1 probably activates a transcriptional partner of c‐Jun whose activity is maintained or increased in the presence of the rat cholecystokinin 1 receptor but repressed in the presence of the mouse cholecystokinin 1 receptor. British Journal of Pharmacology (2006) 147, 951–958. doi: