The evolving impact of g protein-coupled receptor kinases in cardiac health and disease

Abstract

G protein-coupled receptors (GPCRs) are important regulators of various cellular functions via activation of intracellular signaling events. Active GPCR signaling is shut down by GPCR kinases (GRKs) and subsequent β-arrestin-mediated mechanisms including phosphorylation, internalization, and either receptor degradation or resensitization. The seven-member GRK family varies in their structural composition, cellular localization, function, and mechanism of action (see sect. II). Here, we focus our attention on GRKs in particular canonical and novel roles of the GRKs found in the cardiovascular system (see sects. III and IV). Paramount to overall cardiac function is GPCR-mediated signaling provided by the adrenergic system. Overstimulation of the adrenergic system has been highly implicated in various etiologies of cardiovascular disease including hypertension and heart failure. GRKs acting downstream of heightened adrenergic signaling appear to be key players in cardiac homeostasis and disease progression, and herein we review the current data on GRKs related to cardiac disease and discuss their potential in the development of novel therapeutic strategies in cardiac diseases including heart failure.

Figure 1.

G protein-coupled receptor activation and reactivation. At rest, Gαβγ is bound to GDP and both receptor and Gαβγ are not associated. Upon agonist binding, Gαβγ associates with the receptor followed by GDP-GTP nucleotide exchange. Gα and Gβγ dissociate from the receptor with its respective effector proteins leading to downstream signaling. At the active receptor, GRKs bind and phosphorylate the receptor leading to receptor conformational changes and subsequent β-arrestin binding. β-Arrestin-bound receptors are then primed for clathrin-mediated endocytosis and sorted for proteosomal degradation or receptor recycling. Concurrently, RGS binds to Gα leading to GTP hydrolysis and promoting the receptor to its initial resting state.

Figure 2.

GRK topology and subfamily distribution. All GRKs are comprised of a NH₂-terminal region (green), followed by an RH domain (pink) interrupted by a kinase domain (yellow) and a COOH terminus (orange). Visual or rhodopsin-kinases subfamily comprises of GRK1 (also known as rhodopsin kinase) and GRK7, with the unique capability of prenylation in the COOH terminus. The βARK subfamily comprises of GRK2 and GRK3, having a characteristic PH domain. The GRK4 subfamily is comprised of GRK4, GRK5, and GRK6. GRK5 has a unique nuclear localization signal (in blue) and nuclear export signal (in dark green). Numbers in black correspond to amino acids at start or end of each region. GRK4 and GRK6 residue numbers correspond to GRK4α and GRK6A variants, respectively. Numbers in red correspond to residues responsible for catalytic activity.

Figure 3.

Mitochondrial GRK2 translocation in oxidative stress. A: cellular stressful events (such as ischemia/reperfusion) lead to the generation of reactive oxygen species (ROS) and subsequent activation of extracellular signal-regulated kinases (ERK), which in turn phosphorylate G protein-coupled receptor kinase 2 (GRK2) at residue 670. Phosphorylated GRK2 then interacts with heat shock protein 90 (Hsp90) mediating translocation to the mitochondria. βARKct is a peptide inhibitor of GRK2. Treatment with the antioxidant N-acetylcysteine inhibits ROS generation. Geldamycin (GA) is an inhibitor of Hsp90. B: immunogold electron microscopy of cardiac tissue from nontransgenic littermate control (NLC) on top and GRK2-overexpressing (GRK2Tg) animals on bottom. Gold particle labeling shows GRK2 present at baseline in mitochondria of both genotypes though overexpression of GRK2 seems to also translate to increase amount of GRK2 in the mitochondria. Original experiment first described in Chen et al. (43). Scale bars are 1 μm. It is clear that basally GRK2 is already residing within the mitochondria (B), independent of mechanisms shown in A. Therefore, GRK2 may play a role in normal mitochondrial homeostasis. It is not yet clear what this can be, but data suggest mitochondrial GRK2 may regulate ATP levels (83).

Figure 4.

Agonist-mediated GRK5 translocation to the nucleus. A: catecholamine or angiotensin II-mediated receptor activation leads to calmodulin activation through G_q. Calmodulin binds to the NH₂ terminus of GRK5 dislodging from the plasma membrane and promoting nuclear localization. Nuclear GRK5 interaction to calmodulin is then stabilized by IP₃-regulated calcium release. Nuclear GRK5 can act as an HDAC5 kinase, de-repressing MEF2 and inducing hypertrophic gene transcription. On the other hand, endothelin-1 receptor activation leads to its desensitization via GRK5 which cannot interact with calmodulin thus preventing nuclear translocation. B: cardiomyocytes from control rats (left column) and spontaneously hypertensive heart failure rats (SHHF) (right column). Myocytes are shown at different ages which correspond to: 2 (first row), 6 (second row), 12 (third row), and 24 mo old (fourth row). Confocal images labeling for GRK5 reveal a redistribution of GRK5 to the nucleus in SHHF rats. Scale bar equals 50 μm. [Image from studies of Yi et al. (281), with permission from John Wiley and Sons, Inc.]

Figure 5.

Potential mechanisms of interaction involving GRK5 and NFκB pathway. A: NFκB-IκBα complex can shuttle between cytoplasm and nucleus. Alternatively, IκBα can be phosphorylated by IκBα-kinase leading to its degradation. Free NFκB can then enter the nucleus and activate transcription. GRK5 RH domain can bind to IκBα COOH terminus. Upregulation of GRK5 leads to the increase of nuclear IκBα, which can in turn inhibit NFκB and NFκB-driven transcription. B: alternatively, upregulation of GRK5 has been proposed to upregulate NFκB proteins (p50 and p65). This upregulation in NFκB would lead to its translocation to the nucleus and activation of gene transcription. In addition, NFκB can also activate the transcription of GRK5.