Phosphodiesterase-4 influences the PKA phosphorylation status and membrane translocation of G-protein receptor kinase 2 (GRK2) in HEK-293β2 cells and cardiac myocytes

Membrane-recruitment of GRK2 (G-protein receptor kinase 2) provides a fundamental step in the desensitization process controlling GPCRs (G-protein-coupled receptors), such as the β₂AR (β₂-adrenergic receptor). In the present paper, we show that challenge of HEK-293β2 [human embryonic kidney cells stably overexpressing the FLAG-tagged β₂AR–GFP (green fluorescent protein)] cells with the β-adrenoceptor agonist, isoprenaline, causes GRK2 to become phosphorylated by PKA (cAMP-dependent protein kinase). This action is facilitated when cAMP-specific PDE4 (phosphodiesterase-4) activity is selectively inactivated, either chemically with rolipram or by siRNA (small interfering RNA)-mediated knockdown of PDE4B and PDE4D. PDE4-selective inhibition by rolipram facilitates the isoprenaline-induced membrane translocation of GRK2, phosphorylation of the β₂AR by GRK2, membrane translocation of β-arrestin and internalization of β₂ARs. PDE4-selective inhibition also enhances the ability of isoprenaline to trigger the PKA phosphorylation of GRK2 in cardiac myocytes. In the absence of isoprenaline, rolipram-induced inhibition of PDE4 activity in HEK-293β2 cells acts to stimulate PKA phosphorylation of GRK2, with consequential effects on GRK2 membrane recruitment and GRK2-mediated phosphorylation of the β₂AR. We propose that a key role for PDE4 enzymes is: (i) to gate the action of PKA on GRK2, influencing the rate of GRK2 phosphorylation of the β₂AR and consequential recruitment of β-arrestin subsequent to β-adrenoceptor agonist challenge, and (ii) to protect GRK2 from inappropriate membrane recruitment in unstimulated cells through its phosphorylation by PKA in response to fluctuations in basal levels of cAMP.