Novel clusters of receptors for sphingosine‐1‐phosphate, sphingosylphosphorylcholine, and (lyso)‐phosphatidic acid: New receptors for “Old” ligands

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Abstract
The (lyso)phospholipid mediators sphingosine‐1‐phosphate (S1P), lysophosphatidic acid (LPA), sphingosylphosphorylcholine (SPC), and phosphatidic acid (PA) regulate diverse cellular responses such as proliferation, survival and death, cytoskeletal rearrangements, cell motility, and differentiation among many others. Signaling is complex and many signaling events are mediated through the activation of cell surface seven transmembrane (7TM) G protein coupled receptors. Five high affinity receptors for S1P have been identified so far and named S1P1, 2 ,3 ,4 ,5 (formerly referred to as endothelial differentiation gene (edg)1, 5, 3, 6, 8). Recently, the orphan receptor GPR63 was identified a low affinity S1P receptor structurally distant from the S1P1–5 family. The orphan GPR3, 6, 12 cluster, phylogenetically related to the edg and melanocortin receptors appears to be subject to modulation by S1P and SPC although all three receptors are strong constitutive stimulators of the Gαs‐adenylyl cyclase (AC) pathway and would not require additional ligand stimulation but rather inverse agonism to control activity. Ovarian cancer G protein coupled receptor 1 (OGR1) and GPR4, two structurally closely related receptors were assigned in functional and binding studies as high affinity molecular targets for SPC. Very recently, however, both OGR1 and GPR4 were described as receptors endowed with the ability to signal cells in response to protons. LPA exerts its biological effects through the activation of G protein coupled LPA1–3 receptors (formerly referred to as edg2, 4, 7). A fourth high affinity LPA receptor has been identified: P2Y9 (GPR23) structurally related to nucleotide receptors and phylogenetically quite distant from the high affinity LPA1–3 cluster. This review attempts to give an overview about the existing families of lysophosholipid receptors and the spectrum of lipid agonists they use as high or low affinity ligands to relay extracellular signals into intracellular responses. Recently deorphaned lipid receptors, within and outside the known lipid receptor clusters will receive particular attention.