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(Circulation. 2003;108:1611.)
© 2003 American Heart Association, Inc.

Basic Science Reports

Dual Inhibition of ß-Adrenergic and Angiotensin II Receptors by a Single Antagonist

A Functional Role for Receptor–Receptor Interaction In Vivo

From the Departments of Medicine, Cell Biology, and Genetics, Duke University Medical Center, Durham, NC.

Correspondence to Howard A. Rockman, MD, Department of Medicine, Duke University Medical Center, DUMC 3104, 226 CARL Building, Research Drive, Durham, NC, 27710. E-mail h.rockman{at}duke.edu

Received July 16, 2003; revision received August 8, 2003; accepted August 8, 2003.

	Abstract

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Background— Although the renin–angiotensin and the ß-adrenergic systems are interrelated, a direct interaction between ß-adrenergic receptors (ßARs) and angiotensin II type 1 receptors (AT₁Rs) has not been identified.

Methods and Results— Here, we provide evidence for a functional and physiological interaction between 2 G protein–coupled receptors: the ßAR and the AT₁R. Selective blockade of ßARs in mouse cardiomyocytes inhibits angiotensin-induced contractility with an IC₅₀ that is similar to its inhibition of isoproterenol-mediated contractility. Furthermore, administration of the angiotensin receptor blocker valsartan to intact mice results in a significant reduction in the maximal response to catecholamine-induced elevation of heart rate. The mechanism for this transinhibitory effect of ß-blockers and angiotensin receptor blockers is through receptor–G protein uncoupling; ie, ß-blockers interfere with AT₁R-G_q coupling, and valsartan interferes with ßAR-G_s coupling. Finally, we demonstrate that AT₁Rs and ßARs form constitutive complexes that are not affected by ligand stimulation. As a result of these interactions, a single receptor antagonist effectively blocks downstream signaling and trafficking of both receptors simultaneously.

Conclusions— We show that direct interactions between ßARs and AT₁Rs may have profound consequences on the overall response to drugs that antagonize these receptors.

Key Words: heart failure • signal transduction • receptors, adrenergic, beta • angiotensin • pharmacology

	Introduction

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Heart failure is a progressive disorder that involves dysfunction of the renin–angiotensin system and the adrenergic nervous system.^1,2 Sustained adrenergic drive after injury to the heart results in abnormalities of ß-adrenergic receptor (ßAR) signaling (downregulation of ß₁ARs and uncoupling of both ß₁ and ß₂ARs from G proteins^3,4), as well as a diminished number of angiotensin II (Ang II) type 1 receptors (AT₁Rs).¹ Despite evidence for positive feedback regulation between the renin–angiotensin system and the ßAR systems,⁵ to date there is no proof of direct cross talk at the receptor level between ß-adrenergic and angiotensin receptors.

Both ßARs and AT₁Rs belong to the superfamily of G protein–coupled receptors (GPCRs), which, once activated, interact with effector molecules through coupling to G proteins. AT₁Rs translate the actions of the agonist Ang II by coupling to the heterotrimeric G_q/11 family of G proteins and activation of phospholipase Cß (PLCß), whereas ßARs mediate the actions of norepinephrine through G_s-dependent activation of adenylyl cyclase. Although receptors in the GPCR superfamily were initially believed to function as monomeric entities, a growing body of evidence suggests that they exist as homodimers or heterodimers.⁶

In the present study, we demonstrate the existence of a direct interaction between AT₁Rs and ßARs and show that interfering specifically with the signaling of one receptor (ie, either the ßAR or the AT₁R) results in the uncoupling and inhibition of signaling by the reciprocal, interacting receptor.

	Methods

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Preparation of Adult Cardiac Myocytes and Contractility Studies
Mice used in these studies were female adult wild-type C57BL/6 mice, 3 to 6 months of age and weighing 30 to 40 g (Jackson Laboratories, Bar Harbor, Maine). Animals were handled according to approved protocols and animal welfare regulations of the Institutional Review Board at Duke University Medical Center. Myocytes were isolated as described previously⁷ and visualized with an inverted microscope (Nikon Eclipse TE 300). Single-cell contraction was measured by video edge detection, and recordings were made under basal conditions and after administration of the different agents, as described in the figure legends.

Cell Culture and Transfection
COS-7 and human embryonic kidney (HEK) 293 cells were grown in DMEM or modified Eagle’s medium, respectively. Human umbilical vein endothelial cells (HUVECs) were cultured in EGM-2 complete medium. The rat Ang II type 1A–green fluorescent protein (AT₁R-GFP) and HA-tagged AT₁R cDNAs were a generous gift from Dr Larry Barak (Duke University, Durham, NC), and the human FLAG–ß₂AR was a generous gift from Dr Robert Lefkowitz (Duke University, Durham, NC). Transient transfections were performed on 60% to 80% confluent monolayers in 100-mm dishes by use of FUGENE6 (Roche Molecular Biochemicals). Agonist stimulations were performed at 37°C in serum-free media after preincubation with the indicated inhibitors.

Immunoblotting
HUVECs or COS-7 cells were serum-starved for 2 hours before stimulation. Reactions were rapidly terminated, and samples were separated by SDS-PAGE, transferred to nitrocellulose membranes, and probed for phosphorylated extracellular signal–regulated kinase (ERK) using a polyclonal rabbit phospho-ERK1/2–specific antibody (Cell Signaling Technology). Blots were visualized with electrochemiluminescence (Amersham Pharmacia), and the autoradiographs were quantified by use of a Fluor-S MultiImager (Bio-Rad). Blots were then stripped and reprobed with an anti-ERK1/2 antibody (Upstate Biotechnology, Inc) to normalize the level of phosphorylated ERK to total ERK.

Ligand Binding Studies
COS-7 cells were plated into 24-well dishes and allowed to reach confluence. Binding reactions were performed in 0.25 mL binding buffer as described previously.^{8 125}I-labeled angiotensin 0.5 nmol/L or ¹²⁵I-labeled cyanopindolol (CYP) 125 pmol/L and various concentrations of unlabeled competing ligands were added, then plates were incubated on ice for 4 hours. Nonspecific binding was determined by the addition of 100 µmol/L unlabeled Ang II or propranolol, respectively. For internalization studies, COS-7 cells were serum-starved for 2 hours and stimulated with the different agents, as indicated in the figure legend. Protein concentrations were determined with the Bradford assay (Bio-Rad). Data were analyzed by use of Prism 3.0 (GraphPad).

Inositol Phosphate Measurements
COS-7 cells were grown in 12-well dishes and incubated with 2 µCi/well myo-[³H]-inositol (76 Ci/mmol, Amersham) in serum-free medium for 24 hours. Samples were preincubated with propranolol for 30 minutes before stimulation with Ang II for 60 minutes more. Inositol phosphate levels were measured exactly as described previously.⁸

Preparation of Cardiac Membranes
Crude cardiac membranes were prepared from excised hearts as described previously.⁹

GTPS-Binding Studies
[³⁵S]-labeled GTPS binding to isolated cardiac membranes was assayed in a total reaction volume of 50 µL as described previously.¹⁰

Confocal Microscopy
HEK 293 cells expressing AT₁R-GFP or HA-AT₁R were plated on 35-mm glass-bottomed culture dishes and serum-starved for 3 hours before stimulation. Live cells were treated with the different agents as described in the figure legends. Fixing and dual staining of cells were performed as described previously.¹¹ Images were collected by use of an Olympus 1x70 laser scanning confocal microscope.

Heart Rate and Blood Pressure Measurements in Intact Mice
Mice were anesthetized with ketamine (100 mg/kg) and xylazine (2.5 mg/kg) and underwent bilateral vagotomy before catheterization to prevent reflex inhibition in heart rate as described previously.⁹ The left axillary artery was cannulated for continual recording of blood pressure and heart rate. Drugs were administered through the right external jugular vein. Control animals were injected with vehicle, followed by graded dose of isoproterenol (ISO; 50 to 10 000 pg). Two other groups of animals were injected with a single dose of valsartan (250 µg) or propranolol (1 µg) before administration of ISO.

Statistical Analysis
Data are expressed as mean±SEM. Statistical significance was determined by a 1-way ANOVA or a 2-way repeated-measures ANOVA when appropriate. Post hoc analysis was performed with a Tukey-Kramer multicomparison test or a Newman-Keuls test when appropriate. A value of P<0.05 was considered significant.

	Results

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To test the existence of possible cross-regulation between AT₁Rs and ßARs in the heart, we measured the effect of ß-blockers on Ang II–mediated contractility of freshly isolated mouse cardiomyocytes. Under basal conditions, stimulation of the cells with Ang II resulted in a significant increase in cell shortening and rate of cell shortening (-dL/dt) (Figure 1, A and B), similar to the response observed by stimulation with the ßAR agonist ISO.⁷ Remarkably, coadministration of the nonselective ß-blocker propranolol together with Ang II abolished the Ang II effect, demonstrating a direct action of ß-blockers on the Ang II–mediated contractile response (Figure 1, A and B). Because ß₁ARs are the predominant subtype of ßARs in the heart, we tested the effect of the selective ß₁AR inhibitor metoprolol on the Ang II–mediated increase in contractility. As depicted in Figure 1B, metoprolol also abolished the Ang II–mediated response. Neither of the ß-blockers had any effect on the basal levels of contractility. To confirm that the actions of ß-blockers on the AT₁R were receptor-mediated, we tested the effect of propranolol on a non–receptor-mediated augmentation in myocyte contractility induced by the Na⁺/K⁺-ATPase inhibitor ouabain. As shown in Figure 1B, ouabain induced a substantial increase in contractility that was not significantly affected by the presence of the ß-blocker.

View larger version (29K): [in this window] [in a new window]   Figure 1. ß-Blockers inhibit Ang II–mediated myocyte contractility. A, Representative tracings of single cells showing changes in cell length (top) and its first derivative (-dL/dt) (bottom) under basal conditions in which cells were field-stimulated at 0.5 Hz and after administration of 1 µmol/L propranolol (Prop), 10 µmol/L Ang II (AngII) or both. B, Summary of % cellular shortening and rate of cell shortening (-dL/dt) from 5 individual hearts (10 to 15 myocytes from each heart) at baseline and after stimulation with different agents: 1 µmol/L Prop, 1 µmol/L metoprolol (Met), and 100 µmol/L ouabain (Ouab). *P<0.05 vs basal. C, Summary of dose-response effect of propranolol on ISO- () and Ang II– () mediated cell shortening from 5 individual hearts. IC50 ISO=3.0x10-7 mol/L, IC50 Ang II=5.3x10-7 mol/L. D, Dose-response effect of propranolol on ISO- () and Ang II– () mediated -dL/dt from 5 individual hearts. IC50 ISO=2.41x10-7 mol/L, IC50 Ang II=4.45x10-7 mol/L.

To discriminate between ßAR-mediated effects of propranolol versus ßAR-independent actions (eg, direct antagonism of angiotensin receptors or nonspecific effects on membrane fluidity), we measured the IC₅₀ for inhibition of the ISO- and Ang II–enhanced contractility by propranolol. As shown in Figure 1C, increasing concentrations of propranolol resulted in comparable inhibition of both ISO- and Ang II–mediated contractility. Furthermore, the IC₅₀ values for propranolol inhibition of the effects of ISO and Ang II were not significantly different (3.0x10^-7 and 5.3x10^-7 mol/L, respectively, P=NS). The same result was obtained for inhibition in the rate of contractility in the presence of the ß-blocker (Figure 1D). The ability of ß-blockers to inhibit both Ang II– and ISO-stimulated myocyte contractility within the same range of concentrations supports our hypothesis that propranolol exerts its effect on angiotensin signaling via the ßAR and not through binding to the angiotensin receptor.

To directly exclude the possibility that ß-blockers inhibit Ang-mediated signaling by interfering with Ang II binding to its receptor, we performed competitive radioligand binding analyses in intact COS-7 cells. COS cells express ß₂ARs and AT₁Rs at comparable levels, with little expression of ß₁ARs. As shown in Figure 2A, increasing concentrations of unlabeled Ang II effectively displaced ¹²⁵I-Ang II binding. Conversely, both propranolol and the ß₂AR antagonist ICI-118,551 (ICI) were unable to compete with the labeled ligand, suggesting that ß-blockers do not interfere with binding of Ang II to the AT₁R. In a similar manner, we found that the Ang II receptor blocker valsartan, which binds specifically to the AT₁R,¹² did not displace the ß₂AR antagonist ¹²⁵I-cyanopindolol from the endogenously expressed ß₂ARs (Figure 2B). Together, these data demonstrate that propranolol and valsartan bind specifically to the ßAR and AT₁R, respectively, and that the transinhibitory effect on their reciprocal pathways is exerted through specific blockade at a point that is distal to ligand binding.

View larger version (20K): [in this window] [in a new window]   Figure 2. trans-Inhibition of receptor activity is not mediated through competitive ligand binding (A). Competitive binding of 125I-Ang II with Ang II (), Prop (), and ICI 118,551 (). Values represent means from 5 individual experiments for each ligand. B, Competitive binding of 125I-CYP to COS-7 cells. () ICI and () valsartan. Values represent average data from 5 individual experiments for each ligand.

To test whether selective antagonists for one receptor are able to uncouple the reciprocal receptor from binding to its cognate G protein, we assessed receptor–G protein coupling in mouse cardiac membranes. As depicted in Figure 3A, stimulation with Ang II enhanced AT₁R–G_q coupling, as determined by [³⁵S]GTPS loading of G_q. However, preincubation of the membranes with propranolol completely prevented AT₁R–G_q coupling. Consistent with receptor uncoupling, inositol phosphate formation after Ang II stimulation in COS-7 cells was abrogated in the presence of either propranolol (Figure 3B) or ICI (not shown). To test whether blockade of the AT₁R induced a similar effect on coupling of ßARs to G_s, cardiac membranes were stimulated with ISO in the absence or presence of valsartan. ISO-induced coupling of ßAR to G_s, which resulted in a significant increase in [³⁵S]GTPS loading, was completely blocked in the presence of valsartan (Figure 3C). Consistent with ßAR–G_s uncoupling, ISO-mediated generation of the second messenger cAMP was impaired in the presence of valsartan in COS-7 cells endogenously expressing both receptors (Figure 3D).

View larger version (20K): [in this window] [in a new window]   Figure 3. Blockade of one receptor elicits functional uncoupling of trans-receptor from its cognate G protein. A, Propranolol interferes with AT1R–Gq coupling. Ang II–stimulated [35S]GTPS binding to Gq in membranes of cardiomyocytes in absence or presence of 10 µmol/L propranolol. Shown are data from 6 separate experiments performed in triplicate. *P<0.01 vs basal. B, Ang II–stimulated increase in inositol phosphate (IP) formation in COS-7 cells in absence or presence of 10 µmol/L Prop. Shown are mean±SEM from 6 separate experiments performed in triplicate. *P<0.05 vs basal. C, Valsartan interferes with ßAR–Gs coupling. Ang II–stimulated [35S]GTPS binding to Gs in membranes of cardiomyocytes in absence or presence of 10 µmol/L valsartan (Val). Shown are means from 5 separate experiments performed in triplicate. *P<0.05 vs basal. D, Valsartan inhibits cAMP production after ISO stimulation. ISO-stimulated increase in cAMP production in presence or absence of 10 µmol/L valsartan. Shown are mean±SEM from 5 separate experiments performed in triplicate. *P<0.01 vs basal.

To test whether receptor transinhibition, either by Ang II receptor blockers or ß-blockers, affects downstream signaling by ß₂ARs and AT₁Rs, we measured ERK mitogen-activated protein kinase activation in intact COS-7 cells endogenously expressing both receptors. Stimulation of cells with either ISO or Ang II induced a significant increase in ERK phosphorylation (Figure 4, A and B) in a dose- and time-dependent manner (not shown), confirming the existence of both receptors in these cells. Similar to the observations made in the contractility studies, pretreatment of cells with propranolol abolished the Ang II–mediated activation of ERK (Figure 4B, left). In a reciprocal manner, stimulation of the cells with ISO in the presence of valsartan abrogated the ISO-mediated activation of the ERK pathway (Figure 4A). As expected, the ß₁AR-selective antagonist metoprolol had no effect on ERK activation by either ISO or Ang II (Figure 4A, right), consistent with the fact that COS-7 cells do not endogenously express detectable levels of ß₁ARs. The presence of primarily ß₂ARs in COS-7 cells was further confirmed by the observation that the ß₂AR-selective inverse agonist ICI blocked the Ang II response (Figure 4B). These data confirm that the effect of ß-blockers on Ang II–mediated signaling is specific to the subtype of receptors expressed in the tissue studied.

View larger version (33K): [in this window] [in a new window]   Figure 4. trans-Inhibition of downstream receptor signaling between ßARs and AT1Rs. A, Representative immunoblots showing accumulation of phospho-ERK after treatment with 10 µmol/L ISO or 1 µmol/L Ang II for 5 minutes compared with control (CN): 10 µmol/L of Prop, ICI, Val, or Met was added 30 minutes before agonist stimulation. B, Summary graph represents quantitative data from 4 to 5 independent experiments. *P<0.01 vs control. C, Representative immunoblot showing accumulation of p-ERK after treatment of HUVECs with 10 µmol/L ISO or 1 µmol/L Ang II for 5 minutes in absence or presence of 10 µmol/L Prop or Val.

The ability of the antagonists used in the present study to inhibit reciprocal receptor signaling was found to depend primarily on the expression levels of both receptors. For example, stimulation of HUVECs with ISO resulted in a marked elevation of ERK phosphorylation (Figure 4C). However, these cells did not respond to stimulation by Ang II, and the ISO-induced ERK activation was not sensitive to treatment with valsartan. Complementary binding studies showed that as opposed to the heart, HUVECs express ßARs and AT₁Rs at a ratio of 46:1 (79 versus 1.7 fmol/mg protein, respectively).

The finding that pharmacological blockade of either the ßAR or the AT₁R results in functional uncoupling of the reciprocal receptor to its cognate G protein led us to postulate that this effect is mediated through a direct interaction between AT₁R and ßARs. To determine whether AT₁Rs and ßARs form stable complexes, we used the strategy of differential epitope tagging and selective coimmunoprecipitation.⁶ FLAG-epitope–tagged ß₂ARs and HA-tagged AT₁Rs were expressed alone or together, at similar low expression levels, and the cells were exposed to Ang II, ISO, or propranolol. As shown in Figure 5A, immunoprecipitation of FLAG-ß₂ARs resulted in coimmunoprecipitation of HA-AT₁Rs in the absence of ligand (lane 3). Furthermore, the amount of complex precipitated was not affected by the presence of either receptor agonist or antagonist (lanes 4 to 6), suggesting that these receptors are assembled as oligomers before their localization on the plasma membrane.¹³

View larger version (38K): [in this window] [in a new window]   Figure 5. ßARs and AT1Rs form stable complexes. A, COS-7 cells transiently transfected with either FLAG- ß2AR (lanes 1 and 7), HA-AT1R (lanes 2 and 8), or both (lanes 3 to 6) were treated for 30 minutes with 1 µmol/L Ang II (lane 4), 10 µmol/L ISO (lane 5), or 10 µmol/L propranolol (Prop) (lane 6). Immunoprecipitated (IP) proteins were resolved by SDS-PAGE and immunoblotted (IB) for FLAG-ß2AR and HA-AT1R using monoclonal M2 anti–FLAG-affinity agarose or monoclonal Affinity matrix HA-11, respectively. Appearance of heterogeneous populations of receptors is because of variable glycosylation and/or formation of higher-molecular-weight complexes. B, HEK 293 cells with endogenous expression of ßARs were transiently transfected with AT1R-GFP under conditions that yielded similar levels of both receptors (10 to 40 fmol/mg protein). Cells were treated with 10 µmol/L Ang II (left) or pretreated with 10 µmol/L propranolol for 15 to 20 minutes before stimulation with Ang II (right). Arrows indicate internalized vesicles formed after stimulation. Representative confocal image of 4 independent experiments. C, Representative images of HEK 293 cells with endogenous expression of ßAR and overexpression of HA-AT1R. Cells were preincubated in absence or presence of 10 µmol/L H-89 for 20 minutes before stimulation with 10 µmol/L ISO for 12 minutes. D, Internalization of endogenous AT1R after agonist stimulation. Serum-starved COS-7 cells were treated with 1 µmol/L Ang II or 10 µmol/L ISO for 30 minutes. Preincubation with 10 µmol/L H-89 was performed 20 minutes before stimulation. Values represent mean±SEM from 5 separate experiments performed in triplicate. *P<0.01 vs basal. E, Representative images of HEK 293 cells expressing AT1R-GFP (green) and FLAG-ß2AR (red) pretreated with 10 µmol/L Val for 20 minutes before stimulation with 10 µmol/L ISO. Arrows indicate appearance of ß2AR in endocytic vesicles, which is observed only in cells lacking AT1R.

To further determine whether there is a direct interaction between ßARs and AT₁Rs, we examined whether stimulation or blockade of 1 receptor affected the ability of the other receptor to undergo a process of internalization after agonist stimulation.^14,15 HEK 293 cells expressing similar levels of endogenous ßARs and transfected GFP-tagged AT₁Rs were stimulated with Ang II in the absence or presence of propranolol. Internalization of the GFP-AT₁R was visualized in the form of aggregates of receptors under the laser scanning confocal microscope. As expected, GFP-AT₁Rs underwent marked internalization after Ang II stimulation (Figure 5B, left). However, pretreatment of endogenous ßARs with propranolol prevented Ang II–induced GFP-AT₁R internalization, and the receptors remained on the surface of the plasma membrane (Figure 5B, right).

Analysis of AT₁R internalization showed that AT₁Rs internalize by either Ang II or ISO stimulation (Fig 5, C and D). Importantly, only 30% to 40% of AT₁Rs internalized in response to ISO, compared with 80% in response to Ang II. Because ßARs and AT₁Rs display different internalization patterns on the basis of their ability to bind ß-arrestin,¹⁶ the presence of AT₁R–ßAR dimers may alter the recycling properties of the AT₁R to that of a ßAR.

Because endocytosis of GPCRs is preceded by phosphorylation of the receptor, it is possible that ßAR-mediated activation of PKA may promote phosphorylation, desensitization, and internalization by means of heterologous desensitization.¹⁷ To test this possibility, we examined the effect of the PKA-selective inhibitor H-89 on ISO-induced internalization of the AT₁R. As seen by both confocal microscopy (Figure 5C) and ligand binding studies (Figure 5D), inhibition of PKA did not prevent internalization of the AT₁R in response to ISO, supporting the idea that a direct interaction between the 2 receptors, rather than the stimulation of protein kinase activity, is responsible for the effect on receptor trafficking.

The effect of valsartan on internalization of ßARs was assessed in HEK 293 cells, which did not show detectable levels of AT₁Rs by ligand binding. Cells transfected with either FLAG-ß₂AR alone or with both FLAG-ß₂AR and GFP-AT₁R were stimulated with ISO in the presence of valsartan. Cells that expressed only ß₂ARs (red) showed marked internalization after ISO stimulation (Figure 5E, white arrows). However, internalization in cells expressing both ßARs (red) and GFP-AT₁Rs (green) in response to ISO was significantly impaired in the presence of valsartan.

To determine whether our in vitro findings could be translated into the in vivo context, we tested the effect of a single dose of valsartan on the ISO-stimulated elevation of heart rate in intact, vagotomized, wild-type mice. As shown in Figure 6A, increasing doses of ISO yielded a marked elevation in heart rate, with half of the maximal response occurring at a dose of 1 ng per mouse. Pretreatment with a single dose of propranolol resulted in a marked shift of the ISO response curve to the right, as expected from a classic competitive ß-antagonist. In contrast, a single dose of valsartan resulted in a significant 25% reduction in heart rate, with half the maximal response occurring at a dose of 1.4 ng per mouse. The ability of valsartan to attenuate ISO-stimulated heart rate without altering the half-maximal response indicates a decrease in agonist efficacy without an effect on agonist potency. Administration of valsartan or propranolol had no effect on peripheral blood pressure compared with the control group, suggesting a direct effect of valsartan on the heart (Figure 6B).

View larger version (21K): [in this window] [in a new window]   Figure 6. Valsartan decreases ISO-stimulated elevation in heart rate. A, In vivo assessment of change in heart rate in response to increasing doses of ISO (, n=12) after acute administration of 250 µg valsartan (, n=15) or 1 µg propranolol (, n=6). Statistical analysis was performed with repeated-measures ANOVA. Post hoc testing was done with Newman-Keuls test (*P<0.005 valsartan vs control, P<0.0005 prop vs control). A significant between-group main effect in response to both drugs was found (P<0.001). Pattern of change between groups was also statistically different (P<0.00001). B, Change in blood pressure in response to increasing doses of ISO (, n=12) in presence of 250 µg valsartan (, n=15) or 1 µg propranolol (, n=6).

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
The major finding of this study is that AT₁Rs can directly interact with both ßAR subtypes and that this interaction elicits a phenomenon by which selective ßAR blockade inhibits signaling of AT₁Rs, whereas selective AT₁R blockade inhibits downstream signaling of ßARs. Moreover, the mechanism for dual transinhibition of 2 independent receptors by a single antagonist is via functional uncoupling of the signaling receptor from its cognate G protein.

The accepted paradigm for signaling of GPCRs is that these receptors function as single units (monomers), independently capable of coupling to a G protein and activating/inhibiting effector molecules. On the basis of our results and the results obtained for other GPCRs,⁸ we propose a shift in this paradigm by suggesting that more than 1 receptor may be involved in efficient receptor–G protein coupling, thus providing a new role for in vivo oligomerization of GPCRs. Because one of the receptors in the complex (the "nonsignaling" receptor) has to be free of antagonist to permit coupling and signaling of the ligand-activated receptor, it may have a role in stabilizing the interaction of the activated receptor with its cognate G protein.¹⁸ It is therefore possible that blockade of either ßARs or AT₁Rs induces a conformational change that is no longer favorable to support the interaction of the other receptor with its G protein (ie, AT₁R–G_q or ßARs–G_s). The nature of conformational changes in the presence of either agonists or antagonists and their effect on the reciprocal receptor is currently under investigation.

Accumulating evidence indicates that oligomer formation by GPCRs adds a level of complexity to their signaling. Here, we demonstrate an interaction between AT₁Rs and ßARs and present evidence that as a result of this interaction, a single receptor antagonist simultaneously blocks signaling and trafficking of both receptors. These findings are supported by recent observations made in prostate cancer cells, in which blockade of either bradykinin type 1 receptor or bradykinin type 2 receptor resulted in impaired coupling and signaling by the reciprocal receptor without interfering with ligand binding.⁸ Because our data, as well as other studies,¹⁹ indicate that dimerization is a constitutive process, the overall response of an organ to an agonist or an antagonist will most likely depend on the levels and combinations of GPCR complexes it expresses. In this respect, it was recently demonstrated that AT₁R–bradykinin receptor dimers increase the efficacy and potency of Ang II²⁰ and that an increase in the number of these dimers mediates Ang II–induced hypersensitivity in preeclampsia.²¹ Here, we also demonstrate that whereas ßARs are sensitive to valsartan in the heart, they are unaffected by this drug in endothelial cells, which express a far greater ratio of ßARs to AT₁Rs. Together, these findings indicate that expression levels of different receptors have a critical effect on the overall response of a specific tissue to selective receptor antagonists.

Selective blockade of angiotensin and adrenergic signaling pathways in patients with moderate and severe heart failure has been shown to improve survival and quality of life.^22–25 Because some of the hallmarks of the failing heart are distinct abnormalities in the ßAR system, it is currently believed that many of the beneficial effects of ß-blockers are a result of antagonism of the cytotoxic effects of elevated circulating catecholamines and normalization of ßAR signaling.² In view of our in vitro and in vivo data, we propose that ß-blockers may have an additional novel role in directly blocking Ang II–mediated pathways, thus gaining control over 2 signaling pathways that play a pivotal role in cardiac function and are strongly implicated in the pathophysiology of the failing heart.

Although there is little direct physical evidence for AT₁R–ßAR dimerization in the heart, we believe that our data strongly support the existence of direct receptor cross talk in vivo. Therefore, it is possible that transinhibition of receptor signaling has potentially broad implications when the use of AT₁R and ßAR antagonists is considered in treatment of disease states such as heart failure, particularly because the use of 1 antagonist may block more than 1 signaling pathway. Although caution is warranted with regard to the translation of in vitro data to the outcome of clinical trials, our data may provide a biological explanation for unexpected findings from the study of valsartan in heart failure patients (the Val-Heft clinical trial 6), in which treatment of patients with a combination of ß-blockers, valsartan, and ACE inhibitors resulted in an increase in adverse events.²⁶ If indeed AT₁R–ßAR interactions occur in vivo, as our data suggest, ß-blockers in conjunction with valsartan would produce near-complete inhibition of both receptor signaling pathways, because each antagonist blocks not only its own receptor but also the signaling of the reciprocal receptor by a mechanism of transinhibition. The addition of ACE inhibitors would lower the levels of circulating Ang II and norepinephrine through inhibition of the renin–angiotensin system and the sympathetic nervous system.²⁷ Therefore, a combination of all 3 drugs may act together to severely suppress the 2 signaling systems beyond a critical point necessary to maintain homeostasis.

In conclusion, the present study shows that direct interactions between ßARs and AT₁Rs in vivo have a profound role in determining the overall response to drugs designed to block these receptors. Better understanding of the complex interactions between different GPCRs in vivo may therefore have important implications for the development of highly specified pharmacological treatment for a wide range of cardiovascular disorders.

	Acknowledgments

 
This work was supported by in part by National Institutes of Health grant HL-56687 and the Burroughs Wellcome Fund (both to Dr Rockman). Dr Rockman is a recipient of a Burroughs Wellcome Fund Clinical Scientist Award in Translational Research. The authors thank Kristine Hesser Porter, Dr Lan Mao, and Weili Zou for excellent technical assistance.

	Footnotes

 
This article originally appeared Online on September 8, 2003 (Circulation. 2003;108:r79–r86).

	References

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