GRK5 Protects the Heart against Excessive Aldosterone by Phosphorylating and Inhibiting the Cardiac Mineralocorticoid Receptor

Objective: To study the role of GRK5 in cardiac MR regulation. Background: Aldosterone (Aldo) is one of several increased cardio-toxic hormones in chronic heart failure (HF), contributing to its morbidity & mortality. Most of Aldo`s cardiotoxic effects are mediated by the mineralocorticoid receptor (MR). G protein-coupled receptor (GPCR)-kinases (GRKs) are a family of seven serine/threonine kinases that primarily phosphorylate and desensitize GPCRs. GRK2 and GRK5, the most abundant GRKs in the heart, are also known to phosphorylate non-GPCR substrates. The MR is known to get phosphorylated at various serines, which diminishes its capacity to either translocate to the nucleus or to activate transcription (inhibitory phosphorylation). Methods: We used the cardiomyocyte cell line H9c2 and adult rat cardiomyocytes. We performed co-immunoprecipitation experiments for GRK interactions with MR. We measured MR phosphorylation via western blotting and MR transcriptional activity via the luciferase reporter assay. We also measured cellular apoptosis via TUNEL. Results: GRK5, but not GRK2, phosphorylates the MR in H9c2 cardiomyocytes. This effect is constitutive and is enhanced by beta2-adrenoceptor (but not beta1-adrenoceptor) stimulation. The GRK5-phosphorylated MR is incapable of activating gene transcription in cardiomyocytes, since MR transcriptional activity is markedly suppressed upon GRK5 overexpression. Conversely, CRISPR-mediated GRK5 gene deletion augments cardiac MR transcriptional activity. Finally, GRK5 is absolutely necessary for the anti-apoptotic effects of the MR antagonist drug eplerenone in cardiomyocytes. Conclusions: GRK5 blocks the cardio-toxic MR-dependent effects of Aldo. Thus, cardiac GRK5 stimulation with a beta2-adrenoceptor agonist may boost MR inhibitor therapy for severe chronic HF treatment. Grants: NSU`s PFRDG FY17 #33540