Activation of phospholipase C by α1-adrenergic receptors is mediated by the a subunits of Gq family

Abstract

High efficiency transient transfection of Cos-7 cells was previously used to establish the functional cou-pling between Gaq/Gall and phospholipase C β1 (Wu, D., Lee, C-H., Rhee, S. G., and Simon, M. I. (1992) J. Biol. Chem. 267, 1811-1817). Here the same system was used to study the functional coupling between other guanine nucleotide-binding regulatory protein (G-protein) α subunits and phospholipases and to study which Ga subunits mediate the activation of phospholipase C by the α1-adrenergic receptor subtypes, α1A., α1b, and α1C. We found that Gα14 and Gα16 behaved like Gall or Gαq, i.e. they could activate endogenous phospholipases in Cos-7 cells in the presence of AIFn. The synergistic increase in inositol phosphate release in Cos-7 cells after they were cotransfected with cDNAs encoding Ga subunits and phospholipase C β1 indicates that both Ga 16 and Gα14 can activate phospholipase C β1. The activation of phospholipase C β1 was restricted to members of the Gq subfamily of a subunits. They activated phospholipase C β1 but not phospholipase C γ1, γ2, or phospholipase C δ3. The cotransfection of Cos-7 cells with cDNAs encoding three different α1-adrenergic receptors and Gaq or Gαl 1 leads to an increase in norepinephrine-dependent inositol phosphate release. This indicates that Gaq or Gall can mediate the activation of phospholipase C by all three subtypes of α1-adrenergic receptors. With the same assay system, Gα16 and Gα14 appear to be differentially involved in the activation of phospholipase C by the α1-adrenergic receptors. The α1B subtype receptor gave a ligand-mediated synergistic response in the cells cotransfected with either Gα14 or Gα16. However, the α1C receptor responded in cells cotransfected with Gα14 but not Gα16, and the α1A receptor showed little synergistic response in cells transfected with either Gα14 or Gα16. The ability of the α1A and α1C receptors to activate phospholipase C through Gaq and Gα11 was also demonstrated in a cell-free system. Clearly GαQ and Gα11 can couple all three of the α1-adrenergic receptors to activate phospholipase C β1, however, there are differences in the relative coupling of the Gα14 and Gα16 subunits to these receptors.