Raf kinase inhibitor protein: lessons of a better way for β-adrenergic receptor activation in the heart

Abstract

Stimulation of β-adrenergic receptors (βARs) provides the most efficient physiological mechanism to enhance contraction and relaxation of the heart. Activation of βARs allows rapid enhancement of myocardial function in order to fuel the muscles for running and fighting in a fight-or-flight response. Likewise, βARs become activated during cardiovascular disease in an attempt to counteract the restrictions of cardiac output. However, long-term stimulation of βARs increases the likelihood of cardiac arrhythmias, adverse ventricular remodelling, decline of cardiac performance and premature death, thereby limiting the use of βAR agonists in the treatment of heart failure. Recently the endogenous Raf kinase inhibitor protein (RKIP) was found to activate βAR signalling of the heart without adverse effects. This review will summarize the current knowledge on RKIP-driven compared to receptor-mediated signalling in cardiomyocytes. Emphasis is given to the differential effects of RKIP on β₁ - and β₂ -ARs and their downstream targets, the regulation of myocyte calcium cycling and myofilament activity.

Keywords: RKIP; beta-adrenergic receptors; heart failure.

Figure 1. Acute dobutamine application induces positive inotropy; chronic dobutamine application deteriorates cardiac function

Dobutamine activates β₁‐ and β₂‐adrenergic receptors (β₁AR and β₂AR). Activated βARs increase contractility and relaxation of cardiomyocytes via the activation of stimulatory G‐proteins (G_s), which in turn activate protein kinase A and Ca²⁺/calmodulin‐dependent protein kinase II. These kinases increase Ca²⁺ cycling: upon phosphorylation, phospholamban (PLN) dissociates from sarco‐/endoplasmatic reticulum Ca²⁺‐ATPase (SERCA2a). This leads to increased SERCA2a‐mediated Ca²⁺ re‐uptake into the sarcoplasmatic reticulum and cardiomyocyte contractility. Phosphorylation of troponin I (TnI) decreases Ca²⁺ sensitivity and thereby increases cardiomyocyte relaxation. However, G‐protein‐coupled receptor kinase (GRK) phosphorylates activated G‐protein‐coupled receptors (GPCR) as for example β₁AR and β₂AR, which induces receptor desensitization and internalization. This blunts βAR signalling and the initial increase in cardiomyocyte contractility upon dobutamine application. Further, chronic βAR stimulation induces apoptosis, fibrosis and arrhythmia, in particular via hyperphosphorylation of the ryanodine receptor 2 (RyR2) and L‐type Ca²⁺ channels (LTCC), thereby leading to increased diastolic Ca²⁺ leak.

Figure 2. RKIP induces positive inotropy and protects from cell death and diastolic Ca²⁺ leak

The Raf kinase inhibitor protein (RKIP) binds GRK2 and inhibits G‐protein‐coupled receptor kinase (GRK)‐mediated receptor phosphorylation, which prevents receptor desensitization and internalization and, thus, increases β‐adrenergic receptor signalling. RKIP increases contractility and relaxation of cardiomyocytes via activated β₁‐adrenergic receptor (β₁AR) coupled to stimulatory G‐proteins (G_s): phosphorylated phospholamban (PLN) dissociates from sarco‐/endoplasmatic reticulum Ca²⁺‐ATPase (SERCA2a) and thereby increases SERCA2a activity, Ca²⁺ loading of the sarcoplasmatic reticulum and cardiomyocyte contractility. Phosphorylation of troponin I (TnI) decreases Ca²⁺ sensivity and thereby increases cardiomyocyte relaxation. RKIP mediates anti‐apoptotic, anti‐fibrotic and anti‐arrhythmic effects via increased β₂‐adrenergic receptor (β₂AR) signalling. Continuous signalling of β₂AR coupled to inhibitory G‐proteins (G_i) prevents β₁AR‐stimulated increases in ryanodine receptor 2 (RyR2) and L‐type Ca²⁺ channel (LTCC) phosphorylation and protects from diastolic Ca²⁺ leak.