Lysophosphatidic acid inhibits Ca2+ signaling in response to epidermal growth factor receptor stimulation in human astrocytoma cells by a mechanism involving phospholipase C gamma and a G(alpha i) protein

The effect of the lysophospholipid mediators lysophosphatidic acid (LPA) an d sphingosine 1-phosphate and the polypeptide growth factor epidermal growt h factor (EGF) on the human astrocytoma cell line 1321N1 was assessed. Thes e agonists produced a rapid and transient increase of the intracellular Ca2 + concentration, When LPA was perfused before addition of EGF, the EGF-depe ndent Ca2+ transient was abrogated, whereas this was not observed when EGF preceded LPA addition. This inhibitory effect was not found for other EGF-m ediated responses, e.g., activation of the mitogen-activated protein kinase cascade and cell proliferation, thus pointing to the existence of cross-ta lk between LPA and EGF for only a branch of EGF-induced responses. As 1321N 1 cells expressed mRNA encoding the LPA receptors endothelial differentiati on gene (Edg)-2, Edg-4, and Edg-7 and as sphingosine 1-phosphate did not in terfere 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 a ddress the biochemical mechanism involved in the cross-talk between the rec eptors were conducted by the immunoprecipitation approach using antibodies reacting with the EGF receptor (EGFR), phosphotyrosine, phospholipase C gam ma (PLC gamma)-1, and G(alpha i) protein. LPA was found to induce coupling of PLC gamma-1 to the EGFR by a mechanism involving a G(alpha i) protein, i n the absence of tyrosine phosphorylation of both PLC gamma and the EGFR. T hese 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, G(alpha i)-pr otein-mediated translocation of PLC gamma-1 to EGFR in the absence of detec table tyrosine phosphorylation of both proteins.