A novel pathway of prostacyclin signaling-hanging out with nuclear receptors

Prostacylin (PGI(2)), one of the major prostaglandins, is derived from arachidonic acid by the action of the cyclooxygenase (COX) system coupled to PGI(2) synthase (PGIS). The presence of the COX-2/PGIS at the nuclear and endoplasmic reticular membrane suggests differential signaling pathways of PGI(2) actions involving both cell surface and nuclear receptors. Although the signaling of PGI(2) via its cell surface receptor, prostacyclin receptor (IP), is well documented in vascular biology, its action via nuclear receptors in other physiological responses is gradually being more appreciated. Peroxisomal proliferator-activated receptors (PPARs), PPARalpha, PPARgamma, and PPARdelta, though initially cloned as a family of orphan receptors, are now known for their ligand promiscuity. The ligands range from free fatty acids and their derivatives produced by the cyclooxygenase or lipoxygenase pathway to certain hypolipidemic drugs. The predisposition of PPARs to use a wide spectrum of ligands is well explained by their unusually large ligand-binding pocket. The promiscuous ligand usage by PPARs is also reflected by their involvement in various pathophysiological events. Several recent independent reports show that endogenously produced PGI(2) indeed activates PPARdelta in vivo, indicating that a novel signaling mechanism for this abundant eicosanoid is operative in certain systems. This review attempts to cover recent developments in nuclear actions of PGI(2) in diverse biological functions.