Activation of phospholipase C by α_(1-)adrenergic receptors is mediated by the α subunits of Gq family

Abstract

High efficiency transient transfection of Cos-7 cells was previously used to establish the functional coupling between Gαq/Gα11 and phospholipase C β_1 (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 Gα subunits mediate the activation of phospholipase C by the α_1-adrenergic receptor subtypes, α_1A, α_1B, and α_1C. We found that Gα14 and Gα16 behaved like Gα11 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 Gα subunits and phospholipase C β_1 indicates that both Gα 16 and Gα14 can activate phospholipase C β_1. The activation of phospholipase C β_1 was restricted to members of the Gq subfamily of α subunits. They activated phospholipase C β_1 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 Gαq or Gα11 leads to an increase in norepinephrine-dependent inositol phosphate release. This indicates that Gαq or Gα11 can mediate the activation of phospholipase C by all three subtypes of α_1-adrenergic receptors. With the same assay system, Gα16 and Gα14 appear to be differentially involved in the activation of phospholipase C by the α_1-adrenergic receptors. The α_1B subtype receptor gave a ligand-mediated synergistic response in the cells cotransfected with either Gα14 or Gα16. However, the α_1C receptor responded in cells cotransfected with Gα14 but not Gα16, and the α_1A receptor showed little synergistic response in cells transfected with either Gα14 or Gα16. The ability of the α_1A and α_1C receptors to activate phospholipase C through Gαq and Gα11 was also demonstrated in a cell-free system. Clearly Gαq and Gα ll can couple all three of the α_1-adrenergic receptors to activate phospholipase C β_1, however, there are differences in the relative coupling of the Gαl4 and Gαl6 subunits to these receptors.

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