Challenge of the β2Ar (β2-adrenergic receptor) with isoprenaline in HEK-293β2 cells (human embryonic kidney cells stably overexpressing a FLAG- and green fluorescent protein-tagged β2Ar) results in the PKA (cAMP-dependent protein kinase) phosphorylation of GRK2 (G-protein receptor kinase-2). This response was enhanced when PDE4 (phosphodiesterase-4) activity was attenuated using either rolipram, a PDE4-selective inhibitor, or with siRNA (small interfering RNA) knockdown of both PDE4B and PDE4D. Rolipram also facilitated GRK2 recruitment to the membrane and phosphorylation of the β2Ar by GRK2 in response to isoprenaline challenge of cells. In resting cells, rolipram treatment alone is sufficient to promote PKA phosphorylation of GRK2, with consequential effects on GRK2 translocation and GRK2 phosphorylation of the β2Ar. Similar effects are observed in cardiac myocytes. We propose that PDE4 activity protects GRK2 from inappropriate phosphorylation by PKA in resting cells that might have occurred through fluctuations in basal cAMP levels. Thus PDE4 gates the action of PKA to phosphorylate GRK2.

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