Biased β2‐adrenoceptor signalling in heart failure: pathophysiology and drug discovery

The body is constantly faced with a dynamic requirement for blood flow. The heart is able to respond to these changing needs by adjusting cardiac output based on cues emitted by circulating catecholamine levels. Cardiac β‐adrenoceptors transduce the signal produced by catecholamine stimulation via G_s proteins to their downstream effectors to increase heart contractility. During heart failure, cardiac output is insufficient to meet the needs of the body; catecholamine levels are high and β‐adrenoceptors become hyperstimulated. The hyperstimulated β₁‐adrenoceptors induce a cardiotoxic effect, which could be counteracted by the cardioprotective effect of β₂‐adrenoceptor‐mediated G_i signalling. However, β₂‐adrenoceptor‐G_i signalling negates the stimulatory effect of the G_s signalling on cardiomyocyte contraction and further exacerbates cardiodepression. Here, further to the localization of β₁‐ and β₂‐adrenoceptors and β₂‐adrenoceptor‐mediated β‐arrestin signalling in cardiomyocytes, we discuss features of the dysregulation of β‐adrenoceptor subtype signalling in the failing heart, and conclude that G_i‐biased β₂‐adrenoceptor signalling is a pathogenic pathway in heart failure that plays a crucial role in cardiac remodelling. In contrast, β₂‐adrenoceptor‐G_s signalling increases cardiomyocyte contractility without causing cardiotoxicity. Finally, we discuss a novel therapeutic approach for heart failure using a G_s‐biased β₂‐adrenoceptor agonist and a β₁‐adrenoceptor antagonist in combination. This combination treatment normalizes the β‐adrenoceptor subtype signalling in the failing heart and produces therapeutic effects that outperform traditional heart failure therapies in animal models. The present review illustrates how the concept of biased signalling can be applied to increase our understanding of the pathophysiology of diseases and in the development of novel therapies.

Linked Articles

This article is part of a themed section on Chinese Innovation in Cardiovascular Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-23

Abbreviations

ACEI

ACE inhibitors

CaMKII

Ca²⁺/calmodulin‐dependent kinase II

ct

carboxy terminus

EGFR

epidermal growth receptor

Epac

exchange protein directly activated by cAMP

G_i

inhibitory G protein

GRK

GPCR kinase

G_s

stimulatory G protein

HF

heart failure

PKA

cAMP‐dependent protein kinase

SNS

sympathetic nervous system

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Role for engagement of β‐arrestin2 by the transactivated EGFR in agonist‐specific regulation of δ receptor activation of ERK1/2

Background and Purpose

β‐Arrestins function as signal transducers linking GPCRs to ERK1/2 signalling either by scaffolding members of ERK1/2s cascades or by transactivating receptor tyrosine kinases through Src‐mediated release of transactivating factor. Recruitment of β‐arrestins to the activated GPCRs is required for ERK1/2 activation. Our previous studies showed that δ receptors activate ERK1/2 through a β‐arrestin‐dependent mechanism without inducing β‐arrestin binding to the δ receptors. However, the precise mechanisms involved remain to be established.

Experimental Approach

ERK1/2 activation by δ receptor ligands was assessed using HEK293 cells in vitro and male Sprague Dawley rats in vivo. Immunoprecipitation, immunoblotting, siRNA transfection, intracerebroventricular injection and immunohistochemistry were used to elucidate the underlying mechanism.

Key Results

We identified a new signalling pathway in which recruitment of β‐arrestin2 to the EGFR rather than δ receptor was required for its role in δ receptor‐mediated ERK1/2 activation in response to H‐Tyr–Tic–Phe–Phe–OH (TIPP) or morphine stimulation. Stimulation of the δ receptor with ligands leads to the phosphorylation of PKCδ, which acts upstream of EGFR transactivation and is needed for the release of the EGFR‐activating factor, whereas β‐arrestin2 was found to act downstream of the EGFR transactivation. Moreover, we demonstrated that coupling of the PKCδ/EGFR/β‐arrestin2 transactivation pathway to δ receptor‐mediated ERK1/2 activation was ligand‐specific and the Ser³⁶³ of δ receptors was crucial for ligand‐specific implementation of this ERK1/2 activation pathway.

Conclusions and Implications

The δ receptor‐mediated activation of ERK1/2 is via ligand‐specific transactivation of EGFR. This study adds new insights into the mechanism by which δ receptors activate ERK1/2.

Abbreviations

DPDPE

[D‐Pen2, D‐Pen5] enkephalin

HB‐EGF

heparin‐binding EGF‐like growth factor

IGFR

insulin‐like growth factor receptor

NG108‐15

cell mouse neuroblastoma x rat glioma hybrid cell

RTK

receptor tyrosine kinase

TIPP

H‐Tyr‐Tic‐Phe‐Phe‐OH

     

Cannabidiol is a negative allosteric modulator of the cannabinoid CB1 receptor

Background and Purpose

Cannabidiol has been reported to act as an antagonist at cannabinoid CB₁ receptors. We hypothesized that cannabidiol would inhibit cannabinoid agonist activity through negative allosteric modulation of CB₁ receptors.

Experimental Approach

Internalization of CB₁ receptors, arrestin2 recruitment, and PLCβ3 and ERK1/2 phosphorylation, were quantified in HEK 293A cells heterologously expressing CB₁ receptors and in the STHdh ^Q7/Q7 cell model of striatal neurons endogenously expressing CB₁ receptors. Cells were treated with 2‐arachidonylglycerol or Δ⁹‐tetrahydrocannabinol alone and in combination with different concentrations of cannabidiol.

Key Results

Cannabidiol reduced the efficacy and potency of 2‐arachidonylglycerol and Δ⁹‐tetrahydrocannabinol on PLCβ3‐ and ERK1/2‐dependent signalling in cells heterologously (HEK 293A) or endogenously (STHdh ^Q7/Q7) expressing CB₁ receptors. By reducing arrestin2 recruitment to CB₁ receptors, cannabidiol treatment prevented internalization of these receptors. The allosteric activity of cannabidiol depended upon polar residues being present at positions 98 and 107 in the extracellular amino terminus of the CB₁ receptor.

Conclusions and Implications

Cannabidiol behaved as a non‐competitive negative allosteric modulator of CB₁ receptors. Allosteric modulation, in conjunction with effects not mediated by CB₁ receptors, may explain the in vivo effects of cannabidiol. Allosteric modulators of CB₁ receptors have the potential to treat CNS and peripheral disorders while avoiding the adverse effects associated with orthosteric agonism or antagonism of these receptors.

Abbreviations

2‐AG

2‐arachidonyl glycerol

BRET_Eff

BRET efficiency

CBD

cannabidiol

FAAH

fatty acid amide hydrolase

NAM

negative allosteric modulator

THC

Δ⁹‐tetrahydrocannabinol