Abstract 074: GRK5 Increases Cardiac NFAT Transcriptional Activity
Introduction: G protein-coupled receptor (GPCR) kinase-2 and -5 (GRK2 and GRK5) have been shown to be up-regulated in failing human myocardium. While the canonical role of GRKs is to desensitize receptors via phosphorylation, it has been shown that GRK5, unlike GRK2, can reside in the nucleus of cardiomyocytes and exert GPCR-independent effects that promote maladaptive cardiac hypertrophy and heart failure (HF). Previously, our lab has shown that GRK5 increases hypertrophic gene transcription through the phosphorylation of histone deacetylase 5 (HDAC5) and subsequent disinhibition of the transcription factor, myocyte enhancer factor-2 (MEF2). Use of GRK5 knockout (KO) mice has recently proved that GRK5 is indeed a physiological HDAC kinase during hypertrophic stress. Through experiments described below we have now identified the nuclear factor of activated T cells (NFAT) pathway as another potential target of GRK5 during cardiac hypertrophy that may play a role in its facilitation of HF.
Methods and Results: Cardiac-specific NFAT-luciferase reporter mice were crossed with mice that overexpress wild-type GRK5 in myocytes to determine the role for GRK5 in NFAT signaling. These double-transgenic mice along with controls were stressed using the hypertrophic α1-adrenergic agonist phenylephrine (PE) as well as ventricular pressure-overload via transverse aortic constriction (TAC). NFAT activity was determined by both in vivo and ex vivo luciferase assays as well as RT-PCR for the NFAT target gene RCAN. Cardiac specific GRK5 overexpression leads to an increase in NFAT activity in the basal state as well as after both forms of hypertrophic stress. This was reproduced in cultured myocytes using a NFAT-reporter construct. Overexpression of a mutant GRK5 that cannot enter the nucleus appears to not activate NFAT demonstrating a nuclear-dependence. Consistent with this, GRK5 KO mice after TAC showed a decrease in the up-regulation of RCAN transcription as compared to wild-type mice. Studies are ongoing to determine the mechanism of GRK5’s regulation of cardiac NFAT activity.
Conclusions: This study provides another non-canonical role for GRK5 in activating the hypertrophic NFAT pathway in cardiomyocytes.
- © 2013 by American Heart Association, Inc.