Cooperation of Gq, Gi, and G12/13 in Protein Kinase D Activation and Phosphorylation Induced by Lysophosphatidic Acid

To examine the contribution of different G-protein pathways to lysophosphatidic acid (LPA)-induced protein kinase D (PKD) activation, we tested the effect of LPA on PKD activity in murine embryonic cell lines deficient in Galpha q/11 (Galpha q/11 KO cells) or Galpha 12/13 (Galpha 12/13 KO cells) and used cells lacking rhodopsin kinase (RK cells) as a control. In RK and Galpha 12/13 KO cells, LPA induced PKD activation through a phospholipase C/protein kinase C pathway in a concentration-dependent fashion with maximal stimulation (6-fold for RK cells and 4-fold for Galpha 12/13 KO cells in autophosphorylation activity) achieved at 3 µM. In contrast, LPA did not induce any significant increase in PKD activity in Galpha q/11 KO cells. However, LPA induced a significantly increased PKD activity when Galpha q/11 KO cells were transfected with Galpha q. LPA-induced PKD activation was modestly attenuated by prior exposure of RK cells to pertussis toxin (PTx) but abolished by the combination treatments of PTx and Clostridium difficile toxin B. Surprisingly, PTx alone strikingly inhibited LPA-induced PKD activation in a concentration-dependent fashion in Galpha 12/13 KO cells. Similar results were obtained when activation loop phosphorylation at Ser-744 was determined using an antibody that detects the phosphorylated state of this residue. Our results indicate that Gq is necessary but not sufficient to mediate LPA-induced PKD activation. In addition to Gq, LPA requires additional G-protein pathways to elicit a maximal response with Gi playing a critical role in Galpha 12/13 KO cells. We conclude that LPA induces PKD activation through Gq, Gi, and G12 and propose that PKD activation is a point of convergence in the action of multiple G-protein pathways.