Identification of a putative membrane receptor for the bioactive phospholipid, lysophosphatidic acid.

Abstract

Lysophosphatidic acid (LPA) is a naturally occurring phospholipid with hormoneand growth factor-like activities. Exogenous LPA stimulates GTP-dependent phosphoinositide hydrolysis and inhibits adenylate cyclase in its target cells, but the site of action of LPA is unknown. We now report the identification by photoafTinity labeling of a putative LPA membrane receptor in various LPA-responsive cell types. A 32p_ labeled LPA analogue containing a photoreactive fatty acid, [32P]diazirine-LPA, labels a membrane protein of apparent molecular mass of 38-40 kDa in various cell types, including neuronal cells, brain homogenates, carcinoma cells, leukemic cells and normal fibroblasts. Labeling of the 38-40 kDa protein is competitively inhibited by unlabeled 1-oleoyl-LPA (IC50 -10 nM), but not by other phospholipids. Specific labeling is not detected in rat liver membranes or in human neutrophils, which are physiologically unresponsive to LPA. Suraniin, an inhibitor of both early and late events in the action of LPA, completely inhibits the binding of photoreactive LPA. We suggest that the 38-40 kDa protein represents a specific LPA cell surface receptor mediating at least part of the multiple cellular responses to LPA.

Human Lysophosphatidic Acid Acyltransferase

“…4) demonstrated that the LPAAT message is expressed at very high levels in the liver and pancreas, at low levels in the placenta and certain regions of the brain, and at intermediate levels in numerous other tissues. This pattern of expression is in marked contrast to the pattern seen with the LPA receptor, which is expressed most abundantly in the brain, not in the liver, and at intermediate levels in other tissues (7,8). This suggests the possibility that, at least in the brain and liver, LPA signaling and metabolism are mediated by additional receptors and enzymes.…”

“…In addition, LPA is a potent promoter of tumor cell growth and invasion (5,6). LPA exerts its biological effects via at least one, but perhaps multiple, specific G protein-coupled receptors (7)(8)(9). Engagement of the receptor results in activation of Ras and the Raf/mitogen-activated protein kinase pathway, stimulation of phospholipases C and D, inhibition of adenylyl cyclase, and tyrosine phosphorylation of focal adhesion proteins along with actin cytoskeleton remodeling (1).…”

“…Consistent with this, there appear to be at least three mechanisms whereby LPA bioactivity might be attenuated. (i) LPA can be converted to PA in cells by the action of LPA acyltransferase (LPAAT) (7,10). This enzyme is found in microsomes and the plasma membrane (29) and may be regulated by phosphorylation in response to interleukin-1 (IL-1) (29,34).…”

Lysophosphatidic acid inhibits gap-junctional communication and stimulates phosphorylation of connexin-43 in WB cells: possible involvement of the mitogen-activated protein kinase cascade

“…Overall, the evidence supports the view that this simple phospholipid interacts directly with receptors on the surface of cells, rather than by acting as a general membrane perturbant, or as a precursor of intracellular metabolites. Thus photoaffinity-labelling studies with a LPA analogue have identified a binding protein of apparent molecular mass 38-40 kDa in a range of cell types, including fibroblasts [6]. Binding of LPA to this protein was inhibited by suramin, a polysulphonated naphthylurea derivative that also blocks the biological response of intact cells to LPA [7].…”

Synthesis of Photoactivatable Analogues of Lysophosphatidic Acid and Covalent Labeling of Plasma Proteins

“…One previous study reported the synthesis and characterization of a photoreactive LPA analogue containing a trifluoromethylphenyldiazirine group for labeling of fetal bovine serum and LPA responsive cells. 19 In this paper, we report the syntheses of additional photoactivable analogues of acyland O-alkyl-linked LPAs in which the long chain resembles an oleoyl (ester) or oleyl (ether) chain, respectively. The rationale for this approach is that the oleoyl chain has been shown to represent the maximally effective fatty acyl chain of natural LPAs in previous structure-activity studies.…”