Dimerization of the human receptors for prostacyclin and thromboxane facilitates thromboxane receptor-mediated cAMP generation

Abstract

Prostacyclin (PGI2) and thromboxane (TxA2) are biological epposites; PGI2, a vasodilator and inhibitor of platelet aggregation, limits the deleterious actions of TxA2, a vasoconstrictor and platelet activator. The molecular mechanisms involved in the counterregulation of PGI2/TxA2 signaling are unclear. We examined the interaction of the receptors for PGI2 (IP) and TxA 2 (TPa). IP-induced cAMP and TP-induced inositol phosphate generation were unaltered when the receptors were co-expressed in HEK 293 cells (IP/TPα-HEK). TP-cAMP generation, in response to TP agonists or a TP-dependent isoprostane, iPE2III, was evident in IP/TPα-HEK and in aortic smooth muscle cells, but not in cells expressing either receptor alone, or in IP-deficient aortic smooth muscle cells. Augmentation of TP-induced cAMP generation, with the IP agonist cicaprost, was ablated in IP-deficient cells and was independent of direct IP signaling. IP/TPα heterodimers were formed constitutively when the receptors were co-espressed, with no overt changes in ligand binding to the individual receptor sites. However, despite inefficient binding of iPE2III to either the IP or TPα, expressed alone or in combination, robust cAMP generation was evident in IP/TPα-HEK, suggesting the formation of an alternative receptor site. Thus, IP/TP α dimerization was coincident with TP-cAMP generation, promoting a "PGI2-like" cellular response to TP activation. This represents a previously unknown mechanism by which IP may limit the cellular effects of TP.