U73122 and U73343 inhibit receptor-mediated phospholipase D activation downstream of phospholipase C in CHO cells

The aminosteroid 1-(6-¿[17beta-3-methoxyestra- 1,3,5(10)-trien- 17-yl]-amino¿hexyl)- 1H-pyrrole-2,5-dione (U73122) and its inactive analogue 1-(6-¿[17beta-3-methoxyestra-1,3,5(10)-trien- 17-yl]-amino¿hexyl-2,5-pyrrolidine-dione (U73343) are widely used to study the involvement of G protein-coupled 1-phosphatidylinositol-phosphodiesterase, or phospholipase C, in receptor-mediated cell activation. The present work shows that both aminosteroids inhibit cholecystokinin-(26-33)-peptide amide (CCK-8)-induced phospholipase D activation equipotently in Chinese hamster ovary cells expressing the cholecystokinin-A receptor (CHO-CCK(A) cells). In addition, the two aminosteroids virtually completely inhibited thapsigargin- and 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced phospholipase D activation. Since the latter two drugs mimic inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation and 1,2-diacylglycerol-mediated protein kinase C activation. respectively, this suggests that both U73122 and U73343 act downstream of phospholipase C to inhibit receptor-mediated phospholipase D activation. U73122, but not U73343. effectively inhibited both TPA/Ca2+-stimulated phospholipase D activation and TPA/phosphatidylserine-stimulated protein kinase C activation in a homogenate of CHO-CCK(A) cells. The data presented suggest that U73122 may act at the level of protein kinase C to inhibit activation of phospholipase D. The exact site of action of U73343 is presently unknown.

Mutational analysis of the putative devazepide binding site of the CCK(A) receptor

Recently a molecular model was proposed for the binding site of the antagonist 3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl) -1H-indole-2-carboxamide (devazepide) on the cholecystokinin-A (CCK(A)) receptor (Van der Bent et al., 1994. Drug Design Discov. 12, 129-148). Fifteen amino acids were identified, including hydrophilic ones such as Ser139, Asn349 and Ser379, that might interact with the carboxamide moiety in devazepide. To provide mutational evidence for this model, wild-type and mutant receptors (S139A, N349A and S379A) were transiently expressed and compared with respect to the ability of devazepide to inhibit binding of radiolabelled cholecystokinin-(26-33)-peptide amide (CCK-8) and CCK-8-evoked Ca2+ mobilization. The data presented suggest the involvement of the three residues in antagonist binding, although to a different extent. However, it does not seem likely that hydrogen bonds are the driving force in view of the relatively minor changes in receptor affinity and activity.

Cholecystokinin-stimulated enzyme secretion from dispersed rabbit pancreatic acinar cells: phosphorylation-dependent changes in potency and efficacy

In order to establish a regulatory role for phosphoproteins in receptor-stimulated enzyme secretion, dispersed rabbit pancreatic acinar cells were stimulated with the COOH-terminal octapeptide of cholecystokinin (CCK8) in the absence and presence of staurosporine and/or 12-O-tetradecanoylphorbol 13-acetate (TPA) or forskolin. The dose/response curve for the stimulatory effect of CCK8 on amylase secretion was biphasic, with a mean half-maximal concentration (EC50) of 21 pM. Staurosporine (1 microM) did not affect secretion elicited by CCK8 concentrations below 0.1 nM, but reduced the response to CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 decreased to 8 pM and its efficacy to 70%. The phorbol ester TPA (0.1 microM) attenuated secretion evoked by CCK8 concentrations below 0.1 nM and potentiated the response to CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 increased to 0.14 nM and its efficacy to 300%. Staurosporine abolished both the inhibitory and the potentiating effect of TPA, thereby turning the inhibitory effect into a strong potentiating effect. As a result, the mean EC50 for CCK8 decreased to 3 pM, whereas its efficacy increased to 190%. Forskolin (30 microM) potentiated the response to both the lower and the higher CCK8 concentrations. As a result, the mean EC50 for CCK8 increased to 28 pM and its efficacy to 300%. Staurosporine enhanced the potentiating effect of forskolin at CCK8 concentrations below 0.1 nM, but abolished potentiation at CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 decreased to 1.4 pM, whereas its efficacy increased to 260%. The data presented demonstrate that the apparent sensitivity of dispersed pancreatic acinar cells to stimulation of the process of enzyme secretion by CCK8 decreases when kinases are activated and increases when kinases are inactivated. Moreover, they show that the efficacy of CCK8 increases by the action of kinases, both sensitive and insensitive to staurosporine.