Lysophosphatidic Acid Inhibits Ca2+ Signaling in Response to Epidermal Growth Factor Receptor Stimulation in Human Astrocytoma Cells by a Mechanism Involving Phospholipase Cγ and a Gαi Protein

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Abstract
The effect of the lysophospholipid mediators lysophosphatidic acid (LPA) and sphingosine 1‐phosphate and the polypeptide growth factor epidermal growth factor (EGF) on the human astrocytoma cell line 1321N1 was assessed. These agonists produced a rapid and transient increase of the intracellular Ca2+ concentration. When LPA was perfused before addition of EGF, the EGF‐dependent Ca2+ transient was abrogated, whereas this was not observed when EGF preceded LPA addition. This inhibitory effect was not found for other EGF‐mediated responses, e.g., activation of the mitogen‐activated protein kinase cascade and cell proliferation, thus pointing to the existence of cross‐talk between LPA and EGF for only a branch of EGF‐induced responses. As 1321N1 cells expressed mRNA encoding the LPA receptors endothelial differentiation gene (Edg)‐2, Edg‐4, and Edg‐7 and as sphingosine 1‐phosphate did not interfere with LPA signaling, Edg‐2, Edg‐4, and/or Edg‐7 could be considered as the LPA receptors mediating the aforementioned cross‐talk. Attempts to address the biochemical mechanism involved in the cross‐talk between the receptors were conducted by the immunoprecipitation approach using antibodies reacting with the EGF receptor (EGFR), phosphotyrosine, phospholipase Cγ (PLCγ)‐1, and Gαi protein. LPA was found to induce coupling of PLCγ‐1 to the EGFR by a mechanism involving a Gαi protein, in the absence of tyrosine phosphorylation of both PLCγ and the EGFR. These data show a cross‐talk between LPA and EGF limited to a branch of EGFR‐mediated signaling, which may be explained by a LPA‐induced, Gαi‐protein‐mediated translocation of PLCγ‐1 to EGFR in the absence of detectable tyrosine phosphorylation of both proteins.