Abstract P195: Nuclear Translocation of GRK5 Following Hypertrophic Stimuli Is a Calmodulin-Dependent Process
Maladaptive ventricular hypertrophy often precedes the pathological endpoint of Heart Failure (HF). G protein Coupled Receptors (GPCRs), such as the β-adrenergic receptor, play a crucial role in the development of cardiac hypertrophy. GPCR Kinase 5 (GRK5) is a primary GRK expressed in cardiomyocytes. Traditionally, it phosphorylates and desensitizes agonist-bound GPCRs. Recent, exciting data from our lab shows that GRK5 translocates into the nucleus and phosphorylates Histone Deacetylase 5 (HDAC5) in response to hypertrophic stimuli. This novel kinase activity of GRK5 causes nuclear export of HDAC5, enhancing transcription of hypertrophic genes regulated by myocyte elongation factor 2 (MEF2). We hypothesize that that nuclear GRK5 activity plays a critical role in cardiomyocytes, facilitating pathological hypertrophy and HF, and that hypertrophic GPCR signaling induces GRK5 nuclear translocation. To test this hypothesis, we utilized neonatal rat ventricular cardiomyocytes (NRVMs) to determine GRK5 localization following the hypertrophic stimulus of phenylephrine (PE) treatment. To determine the mechanism causing nuclear translocation, we analyzed the effects targeted inhibition of key post-GPCR hypertrophic signaling pathways on GRK5 cellular localization. Treatment of NRVMs with PE for 1 hour increases nuclear GRK5 accumulation by 100%. Chronic infusion of PE also lead to a 50% increase in nuclear GRK5 in the hearts of mice overexpressing cardiac-specific GRK5. Calmodulin inhibition (CaM) in NRVMs via calmidizolium chloride decreases nuclear GRK5 accumulation, while inhibition of PKC and PKD shows no significant effect. Using a luciferase assay, we have shown that overexpression of CaM increases MEF2 activity 3-fold and causes increased nuclear accumulation of GRK5. Further, overexpression of a GRK5 mutant that lacks CaM binding activity, results in a lack of PE- and CaM-induced nuclear translocation. We have now found that hypertrophic stimuli increases myocyte GRK5 nuclear translocation via a CaM-dependent process. Defining the role of CaM signaling in pathological GRK5 nuclear localization may offer a new therapeutic target for maladaptive hypertrophy and HF.
- © 2011 by American Heart Association, Inc.