Dioleoyl phosphatidic acid increases intracellular Ca²⁺ through endogenous LPA receptors in C6 glioma and L2071 fibroblasts

Phosphatidic acid (PA) increased intracellular Ca²⁺ concentration ([Ca²⁺]_i) in C6 rat glioma and L2071 mouse fibroblast cells. Dioleoyl PA (PA, 18:1) was the most efficacious, followed by dipalmitoyl PA (16:0 PA) and dimyristoyl PA (14:0 PA). Lysophosphatidic acid (LPA) also increased the [Ca²⁺]_i in the both cells. PA desensitized LPA-induced Ca²⁺ response completely in C6 cells, but partly in L2071 cells. Treatment of pertussis toxin (PTX), a specific inhibitor of G_i/o-type G proteins, completely ameliorated LPA- and PA-induced Ca²⁺ response in C6 cells. However, in L2071 cells, PTX inhibited PA-induced Ca²⁺ increase by 80% and LPA-induced one by 20%. Ki16425, a specific inhibitor of LPA₁/LPA₃ receptors, completely inhibited both LPA- and PA-induced Ca²⁺ responses in C6 cells. On the other hand, in L2071 cells, Ki16425 completely inhibited PA-induced Ca²⁺ response, but partly LPA-induced one. VPC32183, another specific inhibitor of LPA₁/LPA₃ receptors, completely inhibited LPA- and PA-induced Ca²⁺ responses in both C6 and L2071 cells. Therefore, PA and LPA appear to increase [Ca²⁺]_i through Ki16425/VPC32183-sensitive LPA receptor coupled to PTX-sensitive G proteins in C6 cells. In L2071 cells, however, LPA increases [Ca²⁺]_i through Ki16425-insensitive LPA receptor coupled to PTX-insensitive G proteins and Ki16425-sensitive LPA receptor coupled to PTX-sensitive G protein, whereas PA utilized only the latter pathway. Our results suggest that PA acts as a partial agonist on endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-sensitive G protein, but not on LPA receptors, which are not sensitive to Ki16425 and coupled to PTX-insensitive G protein.