CardioPulmonary Research Center, University of Cincinnati College of Medicine, 231 Albert Sabin Way, Room G-062, Cincinnati, OH 45267-0564, USA
* Corresponding author. Tel: +1 513 558 0484; fax: +1 513 558 0835. E-mail address: stephen.liggett{at}uc.edu
This editorial refers to Elevated myocardial and lymphocyte GRK2 expression and activity in human heart failure
by G. Iaccarino et al., on page 1752
Seven transmembrane (7-TM) receptors, also termed G-protein coupled receptors, represent one of the largest signalling families in the human genome. The ß1- and ß2-adrenergic receptors (ß1- and ß2ARs) are prototypic of many members of this family. In the classic paradigm, when occupied by agonist, both ßAR subtypes couple to the stimulatory G-protein Gs, whose subunit activates adenylyl cyclase, increasing intracellular cAMP and activating protein kinase A (PKA), which phosphorylates multiple proteins in the heart leading to enhanced inotropy and chronotropy. This scenario, however, is not a simple, series-type switch. Instead, it can be viewed as a network consisting of switches, potentiometers, amplifiers, and attenuators in series and in parallel. Indeed, we now know that adrenergic receptors can couple to multiple G-proteins and can even signal by direct interaction of their cytoplasmic tails with effectors without the need for G-proteins as transducers.1 Such complexity (and plasticity) in signalling is necessary for the cell to integrate the large number of signals presented to it, so as to respond appropriately to both physiologic and pathologic conditions. Of particular relevance to this network and the subject of a report by Iaccarino et al.2 are G-protein coupled receptor kinases (GRKs). These kinases are known by several names: GRK2 (ßARK or ßARK1), GRK3 (ßARK2), GRK4 (IT11), GRK5, GRK6, and the two phototransduction kinases GRK1 (rhodopsin kinase) and GRK7 (cone opsin kinase). The initial function of GRKs, typified by studies with the ß2AR and
2AR, and GRK2, was considered to be the mediator of agonist-promoted (homologous) desensitization.1 The developing hypothesis was that the agonist-occupied conformation of 7-TM receptors promoted their phosphorylation by GRKs on specific serines or threonines, which served to uncouple the receptor from its G-protein and thus decrease function. Subsequently, it was found that this required the binding of another class of proteins, the arrestins, which acted to decrease receptorG-protein interaction.1 Arrestins also appear to act as scaffolding and adaptor proteins, bringing together other proteins into a microdomain with the receptor and the other components of the network.1 As an example, ß-arrestin is recruited from the cytoplasm to the ß2ARs at the cell surface upon GRK phosphorylation, and ß-arrestin shaparones phosphodiesterase type 4 to the complex, thereby accelerating cAMP degradation and enhancing desensitization. ß-arrestin also plays a role in ßAR activation of mitogen-activated protein kinase. Critical to events such as those described earlier are the quantity and availability of GRKs within the cell to phosphorylate receptors.
In heart failure, ßAR signalling and GRK expression and activity are dynamically regulated.3 Early in any condition in which cardiac output is depressed, the sympathetic nervous system responds by increasing epinephrine and norepinephrine. These act at ßAR to acutely increase cardiac inotropy and chronotropy and at 1AR to increase vascular smooth muscle contraction, limiting perfusion to non-critical areas. These responses serve acute haemodynamic insufficiency fairly well; but over the long term may act to worsen failure by promoting specific hypertrophic and apoptotic events (particularly via the ß1AR subtype) and by increasing the metabolic demand on a heart which has limited physiologic, anatomic, and biochemical reserves. One response to chronic heart failure is the desensitization of ßAR signalling, which is considered a consequence of elevated catecholamines.3 Such desensitization, though, limits the heart in its capacity to acutely increase output (such as with exercise) and is typically considered indicative of worsening failure. Paradoxically, ßAR blockade serves to break this vicious cycle, and indeed ßAR expression and function, cardiac contractility, and potentially the ability to acutely respond to stress, begin to normalize with properly titrated ß-blocker therapy. So it appears that a delicate balance between ßAR signalling vs. its desensitization is necessary for a favourable outcome in heart failure.4 Interestingly, cardiac GRK expression, or function as determined by an in vitro assay using rhodopsin as substrate, is increased in animal models of hypertrophy or failure5 as well as in the human syndrome.6
One successful genetic approach for normalizing ßAR signalling, and in many cases ventricular function, in animal models of heart failure has been the inhibition of GRK2 activity.7 Overexpression of ßARs themselves, particularly the ß1AR, can evoke a cardiomyopathy8,9 and thus such forced amplification is not likely to be a viable approach. One consideration of the findings from such rescue matings is that enhanced GRK activity in heart failure is not only desensitizing ßAR function, but through its promotion of ß-arrestin signalling is contributing to its pathology. Under this hypothesis, increasing ßAR expression would not have the same downstream effect as inhibiting GRK:ß-arrestin function. Taken together, knowing the levels of GRK expression in the heart may be predictive of the status of ßAR desensitization, the remodelling imposed by GRK-mediated ß-arrestin signalling, and other biochemical abnormalities in heart failure.
The report by Iaccarino et al.2 indicates that GRK expression (or function) of circulating lymphocytes may be a useful biomarker for GRK levels in the heart. Such a hypothesis is based on the idea that both myocytes and lymphocytes are potentially exposed to a comparable milieu. This environment could be that of catecholamines (lymphocytes express the ß2AR subtype) or the result of other neurohumoral signalling abnormalities in the syndrome. The critical figure in this report is Figure 2, in which the correlation is shown between GRK2 expression in lymphocytes and right atrial appendages from 10 subjects admitted to the intensive care unit (presumably with various degrees of heart failure) undergoing elective surgery. There are two important observations. First, the coefficient of determination, r2, is 0.568. Thus, knowing the GRK expression in lymphocytes explains 57% of the variance in GRK expression in the heart. Given the complexity of the syndrome, this is a reasonable correlation, particularly with the small sample size. It is intriguing to note that the range of expression from this study is approximately six-fold. It would be interesting to know about the clinical status of those with the highest expression vs. those with the lowest, as well as the range of lymphocyte GRK2 expression or function found in normal individuals. To begin to assess this in heart failure, lymphocyte GRK activity was assessed in a larger group of patients admitted to the intensive care unit. A moderate association was noted between lymphocyte GRK activity and ejection fraction and NYHA class. Some important issues remain in terms of both the scientific basis for measuring GRK expression/activity and its potential value as a biomarker in heart failure. First, there was a relatively weak negative correlation (r2=0.215) for cardiac adenylyl cyclase activation by the agonist isoproterenol and cardiac GRK activity. As discussed earlier, GRK activity not only promotes desensitization, but via receptor phosphorylation provides a substrate for ß-arrestin binding and its subsequent initiation of other signalling pathways. So, the current data confirms the notion that some of the deleterious effects of elevated GRK levels may not be related to its direct effect on ßAR function. It is also of interest to note the relatively low correlation (r=0.609, r2=0.361) of cardiac GRK activity and GRK2 expression. Which of these is the relevant measurement? This is particularly interesting in light of the fact that the data in Figure 2 use GRK2 expression by western blots to show a correlation between lymphocyte and cardiac GRK levels, whereas in the clinical study (Figure 3), GRK activity of lymphocytes was measured. Thus, it is important to define the most biologically valid, reproducible, and quantitative measurement. Despite these caveats, this commendable work now sets the stage for larger studies potentially providing for a new and desperately needed biomarker for heart failure status.
Footnotes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
References
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