Sequestration and phosphorylation of the prostaglandin E2 EP4 receptor: Dependence on the C-terminal tail

The prostaglandin E2 (PGE2) EP4 subtype is one of four prostanoid receptors that use PGE2 as the preferred ligand. We have investigated the agonist-mediated regulation of EP4 using a multifaceted approach. Short-term (30 min) agonist challenge of recombinant EP4 expressed in human embryonic kidney 293 cells (EP4-HEK293 cells) with PGE2 (1 microM) resulted in the desensitization of intracellular cyclic AMP (cAMP) accumulation and a reduction in cell surface [3H]PGE2 specific binding sites. These events correlated with sequestration of EP4, as visualized by immunofluorescence confocal microscopy and phosphorylation, as shown by [32P]orthophosphate labeling of the receptor. Stimulation of protein kinase A activity in EP4-HEK293 cells (10 microM forskolin or 1 mM 8-bromo-cAMP) did not induce EP4 desensitization, sequestration, or phosphorylation. In contrast, stimulation of protein kinase C activity (100 nM phorbol 12-myristate 13-acetate) attenuated PGE2-induced adenylyl cyclase activity and increased EP4 phosphorylation, but did not induce sequestration or a reduction in [3H]PGE2 specific binding sites. EP4 receptors containing a third intracellular loop deletion [EP4 (del. 215-263)] or a carboxyl-terminal tail truncation [EP4 (del. 355)] of EP4 were used to demonstrate that the C-terminal tail governs sequestration as well as phosphorylation of the receptor.