Mechanisms of heterologous agonist-stimulated phosphorylation of cholecystokinin receptor

The phosphorylation of one receptor that occurs as a result of the stimulation of a different receptor on a cell is a common mechanism for heterologous regulation or 'cross-talk,' which has been implicated in desensitization. In this work, we focus on the mechanisms of phosphorylation of the rat pancreatic acinar cell cholecystokinin (CCK) receptor that occur upon stimulation of this cell by various agonists. Phosphorylation was allowed to occur in dispersed intact acinar cells in response to the experimental manipulation, and the phosphoreceptor was, subsequently purified and quantified as an indication of response. Agonists such as vasoactive intestinal polypeptide and secretin, which act via activation of adenylate cyclase, had no effect on CCK receptor phosphorylation, whereas carbamylcholine and bombesin stimulated increased phosphorylation of the CCK receptor. Because these agents would be expected to activate protein kinase C (PKC) as well as a number of calcium-sensitive kinases and phosphatases, these activities were further dissociated by using more direct activators and inhibitors acting intracellularly. Manipulation of calcium independent of PKC by using a calcium ionophore, inhibition of calcium/calmodulin-dependent kinase II, and inhibition of calcium-dependent protein phosphatase type 2B had no effect on the state of CCK receptor phosphorylation. In contrast, direct activation of PKC with phorbol 12-myristate 13-acetate or 1-oleoyl-2- acetyl-sn-glycerol (OAG) stimulated receptor phosphorylation, whereas inhibition of this enzyme blocked CCK receptor phosphorylation stimulated by phorbol ester, carbamylcholine, and bombesin. Differences were observed in the time course of CCK receptor phosphorylation in response to the hormonal agonists and phorbol ester, but OAG elicited a time course similar to that elicited by hormonal agonists. Thus PKC appears to represent the predominant mediator of heterologous agonist-stimulated phosphorylation of the CCK receptor, with differences likely in the patterns of PKC isozyme activation and action on this important substrate.