Recombinant human G protein-coupled lysophosphatidic acid receptors mediate intracellular calcium mobilization.
Mobilization of intracellular Ca2+ is a critical cellular response to lysophosphatidic acid (LPA) in many cell types. Recent identification of endothelial differentiation gene (Edg) 2 and Edg4 as subtypes of G protein-coupled receptors for LPA allowed examination of the Ca2+ mobilization mediated specifically by each subtype. To reduce endogenous background levels while enhancing recombinant receptor-specific signals, the aequorin luminescence method was used to quantify cytoplasmic Ca2+ levels. In TAg-Jurkat T cells transiently co-transfected with apoaequorin and human Edg2 or Edg4 cDNA, LPA dose-dependently increased light emission triggered by increased Ca2+ bound to aequorin. N-Palmitoyl-L-serine-phosphoric acid and N-palmitoyl-L-tyrosine-phosphoric acid, which had been previously shown to be antagonists for Xenopus laevis LPA receptors, did not antagonize the Ca2+-mobilizing effects of Edg2 and Edg4. Surprisingly, they acted as agonists or partial agonists for Edg2 and Edg4. The Ca2+ mobilization by Edg2 and Edg4 was further characterized in stable transfectants of rat HTC4 hepatoma cells. By using the fura-2 fluorescence method, a difference in the kinetics of Ca2+ flux with Edg2 and Edg4 was observed. With Edg2, but not Edg4, the initial increase in the Ca2+ concentration was followed by a sustained influx of extracellular Ca2+. The coincident production of inositol phosphates and the inhibition of Ca2+ mobilization by the phospholipase C inhibitor U73122 strongly suggested that Edg2 and Edg4 mobilize Ca2+ through inositol trisphosphate generated by phospholipase C activation. Pertussis toxin almost completely blocked LPA-induced Ca2+ mobilization by Edg2 but only partially blocked that by Edg4, which suggests that Edg2 transduces Ca2+ mobilization largely through pertussis toxin-sensitive Gi proteins, whereas Edg4 requires both Gi and Gq.
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Departments of Medicine and Microbiology-Immunology, University of California, San Francisco, California 94143-0711, USA. songzhu@itsa.ucsf.edu
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D000818:Animals; D002118:Calcium / Q000378:metabolism*; D018076:DNA, Complementary / Q000235:genetics / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D019169:Jurkat Cells / Q000378:metabolism; D007700:Kinetics; D008114:Liver Neoplasms, Experimental / Q000235:genetics / Q000378:metabolism; D008246:Lysophospholipids / Q000494:pharmacology; D051381:Rats; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D043562:Receptors, G-Protein-Coupled; D049368:Receptors, Lysophosphatidic Acid; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D014162:Transfection
D018076:DNA, Complementary; D008246:Lysophospholipids; D011956:Receptors, Cell Surface; D043562:Receptors, G-Protein-Coupled; D049368:Receptors, Lysophosphatidic Acid; D011994:Recombinant Proteins; D019204:GTP-Binding Proteins; D002118:Calcium