Regulation of the human prostanoid TPα and TPβ receptor isoforms mediated through activation of the EP1 and IP receptors

1 The intermolecular cross-regulation mediated by the prostanoid IF-receptor (IP)/EF1 receptor (EP1) agonists PGI(2) and 17 phenyl trinor PGE(2) on TP receptor (TP) signalling within platelets was compared to that which occurs to the individual TP alpha and TP beta receptors over-expressed in human embryonic kidney (HEK) 293 cells. Ligand mediated TP receptor activation was monitored by analysing mobilization of intracellular calcium ([Ca2+](i)) following stimulation with the selective thromboxane (TX) A(2) mimetic U46619. 2 Consistent with previous studies, in platelets, PGI(2) acting through endogenous IP receptors completely inhibited U46619-mediated TP receptor signalling in a protein kinase (PK) A-dependent, PKC-independent manner. 3 In HEK 293 cells, PGI(2), acting through endogenous AH6809 sensitive EP1 rather than IP receptors, and the selective EP1 receptor agonist 17 phenyl trinor PGE, antagonized U46619mediated signalling by both TP alpha and TP beta receptors in a PKC-dependent, PKA-independent manner. 4 The maximum response induced by either ligand was significantly (P<0.005) greater for the TP alpha receptor than the TP beta receptor, pointing to possible physiologic differences between the TP isoforms, although the potency of each ligand was similar for both TP receptors. 5 TPDelta 328 a truncated variant of TP receptor lacking the C-tail sequences unique to TP alpha or TP beta receptors, was not sensitive to EP1 receptor-mediated regulation by PGI(2) or 17 phenyl trinor PGE2. 6 In conclusion, these data confirm that TP alpha and TP beta receptors are subject to cross regulation by EP1 receptor signalling in HEK 293 cells mediated by PKC at sites unique to the individual TP receptors and that TP alpha receptor responses are significantly more reduced by EP1 receptor regulation than those of the TP beta receptor.