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G_12/13- and Rho-Dependent Activation of Phospholipase C- by Lysophosphatidic Acid and Thrombin Receptors

Department of Pharmacology, University of North Carolina School of Medicine, Chapel Hill, North Carolina

Because phospholipase C (PLC-) is activated by G_12/13 and Rho family GTPases, we investigated whether these G proteins contribute to the increased inositol lipid hydrolysis observed in COS-7 cells after activation of certain G protein-coupled receptors. Stimulation of inositol lipid hydrolysis by endogenous lysophosphatidic acid (LPA) or thrombin receptors was markedly enhanced by the expression of PLC-. Expression of the LPA₁ or PAR1 receptor increased inositol phosphate production in response to LPA or SFLLRN, respectively, and these agonist-stimulated responses were markedly enhanced by coexpression of PLC-. Both LPA₁ and PAR1 receptor-mediated activation of PLC- was inhibited by coexpression of the regulator of G protein signaling (RGS) domain of p115RhoGEF, a GTPase-activating protein for G_12/13 but not by expression of the RGS domain of GRK2, which inhibits G_q signaling. In contrast, activation of the G_q-coupled M1 muscarinic or P2Y₂ purinergic receptor was neither enhanced by coexpression with PLC- nor inhibited by the RGS domain of p115RhoGEF but was blocked by expression of the RGS domain of GRK2. Expression of the Rho inhibitor C3 botulinum toxin did not affect LPA- or SFLLRN-stimulated inositol lipid hydrolysis in the absence of PLC- but completely prevented the PLC--dependent increase in inositol phosphate accumulation. Likewise, C3 toxin blocked the PLC--dependent stimulatory effects of the LPA₁, LPA₂, LPA₃, or PAR1 receptor but had no effect on the agonist-promoted inositol phosphate response of the M1 or P2Y₂ receptor. Moreover, PLC--dependent stimulation of inositol phosphate accumulation by activation of the epidermal growth factor receptor, which involves Ras- but not Rho-mediated activation of the phospholipase, was unaffected by C3 toxin. These studies illustrate that specific LPA and thrombin receptors promote inositol lipid signaling via activation of G_12/13 and Rho.

Address correspondence to: Dr. T. K. Harden, Department of Pharmacology, University of North Carolina School of Medicine, Chapel Hill, NC 27599-7365. E-mail: tkh{at}med.unc.edu

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