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Kimberly L. Dodge, Barbara M. Sanborn, Evidence for Inhibition by Protein Kinase A of Receptor/Gαq/Phospholipase C (PLC) Coupling by a Mechanism Not Involving PLCβ2, Endocrinology, Volume 139, Issue 5, 1 May 1998, Pages 2265–2271, https://doi.org/10.1210/endo.139.5.5963
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Abstract
The effects of cAMP on the oxytocin-stimulated increase in phosphatidylinositide turnover and the possible pathways involved were investigated in a human myometrial cell line (PHM1–41) and in COS-M6 cells overexpressing the oxytocin receptor. Preincubation with chlorophenylthio-cAMP (CPT-cAMP), forskolin, or relaxin inhibited oxytocin-stimulated phosphatidylinositide turnover in PHM1–41 cells, and the inhibition was reversed by H-89, a relatively specific protein kinase A inhibitor. Both CPT-cAMP and transiently expressed protein kinase A catalytic subunit inhibited stimulation by oxytocin and carbachol of [3H]inositol 1,3,4-trisphosphate formation in COS-M6 cells expressing oxytocin or muscarinic M1 receptors, respectively. CPT-cAMP also inhibited phosphatidylinositide turnover stimulation by endothelin-1 in PHM1–41 cells, further demonstrating the generality of the cAMP-inhibitory mechanism. Since Gβγ activation of phospholipase Cβ2 (PLCβ2) is a suggested target of protein kinase A, the possibility that the oxytocin receptor couples to PLCβ2 via GαiGβγ activation was explored. Western blot analysis of PHM1–41cells and COS-M6 cells detected PLCβ1 and PLCβ3, but not PLCβ2. In PHM1–41 cells, pertussis toxin reduced the oxytocin-stimulated increase in[ 3H]inositol 1,3,4-trisphosphate by 53%, and this was reversed completely by H-89. Thus, the inhibitory effect of pertussis toxin may result from an indirect effect of cAMP elevation. These data suggest that receptor/Gαq-coupled stimulation of PLCβ1 or PLCβ3 can be inhibited by cAMP through a phosphorylation mechanism involving protein kinase A that does not involve PLCβ2. In smooth muscle, this mechanism could constitute potentially important cross-talk between pathways regulating contraction and relaxation.
