Adrenal angiotensin II type 1 receptor biased signaling: The case for "biased" inverse agonism for effective aldosterone suppression

Angiotensin II (AngII) uses two distinct G protein-coupled receptor (GPCR) types, AT1R and AT2R, to exert a plethora of physiologic effects in the body and to significantly affect cardiovascular homeostasis. Although not much is known about the signaling of the AT2R, AT1R signaling is known to be quite pleiotropic, mobilizing a variety of signal transducers inside cells to produce a biological outcome. When the outcome in question is aldosterone production from the adrenal cortex, the main transducers activated specifically by the adrenocortical AT1R to signal toward that cellular effect are the Gq/11 protein alpha subunits and the β-arrestins (also known as Arrestin-2 and -3). The existence of various downstream pathways the AT1R signal can travel down on has led to the ever-expanding filed of GPCR pharmacology termed "biased" signaling, which refers to a ligand preferentially activating one signaling pathway over others downstream of the same receptor in the same cell. However, "biased" signaling or "biased" agonism is therapeutically desirable only when the downstream pathways lead to different or opposite cellular outcomes, so the pathway promoting the beneficial effect can be selectively activated over the pathway that leads to detrimental consequences. In the case of the adrenal AT1R, both Gq/11 proteins and β-arrestins mediate signaling to the same end-result: aldosterone synthesis and secretion. Therefore, both pathways need to remain inactive in the adrenal cortex to fully suppress the production of aldosterone, which is one of the culprit hormones elevated in chronic heart failure, hypertension, and various other cardiovascular diseases. Variations in the effectiveness of the AT1R antagonists, which constitute the angiotensin receptor blocker (ARB) class of drugs (also known as sartans), at the relative blockade of these two pathways downstream of the adrenal AT1R opens the door to the flip term "biased" inverse agonism at the AT1R. ARBs that are unbiased and equipotent inverse agonists for both G proteins and β-arrestins at this receptor, like candesartan and valsartan, are the most preferred agents with the best efficacy at reducing circulating aldosterone, thereby ameliorating heart failure. In the present review, the biased signaling of the adrenal AT1R, particularly in relation to aldosterone production, is examined and the term "biased" inverse agonism at the AT1R is introduced and explained, as a means of pharmacological categorization of the various agents within the ARB drug class.

The place of ARBs in heart failure therapy: is aldosterone suppression the key?

Since the launch of the first orally available angiotensin II (AngII) type 1 receptor (AT1R) blocker (ARB) losartan (Cozaar) in the late 1990s, the class of ARBs (or 'sartans', short for Angiotensin-RecepTor-ANtagonistS) quickly expanded to include candesartan, eprosartan, irbesartan, valsartan, telmisartan, and olmesartan. All ARBs have high affinity for the AT1 receptor, expressed in various tissues, including smooth muscle cells, heart, kidney, and brain. Since activation of AT1R, the target of these drugs, leads, among other effects, to vascular smooth muscle cell growth, proliferation and contraction, activation of fibroblasts, cardiac hypertrophy, aldosterone secretion from the adrenal cortex, thirst-fluid intake (hypervolemia), etc., the ARBs are nowadays one of the most useful cardiovascular drug classes used in clinical practice. However, significant differences in their pharmacological and clinical properties exist that may favor use of particular agents over others within the class, and, in fact, two of these drugs, candesartan and valsartan, continuously appear to distinguish themselves from the rest of the 'pack' in recent clinical trials. The reason(s) for the potential superiority of these two agents within the ARB class are currently unclear but under intense investigation. The present short review gives an overview of the clinical properties of the ARBs currently approved by the United States Food and Drug Administration, with a particular focus on candesartan and valsartan and the areas where these two drugs seem to have a therapeutic edge. In the second part of our review, we outline recent data from our laboratory (mainly) on the molecular effects of the ARB drugs on aldosterone production and on circulating aldosterone levels, which may underlie (at least in part) the apparent clinical superiority of candesartan (and valsartan) over most other ARBs currently in clinical use.

Alpha2-adrenergic receptor subtype-specific activation of NF-kappaB in PC12 cells

In the present study we sought to investigate the signal transduction mechanisms that underlie the alpha2-adrenergic receptor (AR)-induced, subtype-specific neuronal differentiation of PC12 cells. Alpha2-ARs induced NF-kappaB transcriptional activity and p21(waf-1) gene transcription in the same subtype-specific manner (alpha2A Collapse

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MESH Headings

• Animals
• Cell Differentiation
• Cyclin-Dependent Kinase Inhibitor p21/genetics
• Humans
• Mitogen-Activated Protein Kinase 1/physiology
• Mitogen-Activated Protein Kinase 3/physiology
• NF-kappa B/genetics
• NF-kappa B/metabolism
• PC12 Cells
• Phosphatidylinositol 3-Kinases/physiology
• Rats
• Receptors, Adrenergic, alpha-2/physiology
• Signal Transduction
• Transcription, Genetic

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Not all arrestins are created equal: Therapeutic implications of the functional diversity of the β-arrestins in the heart

The two ubiquitous, outside the retina, G protein-coupled receptor (GPCR) adapter proteins, β-arrestin-1 and -2 (also known as arrestin-2 and -3, respectively), have three major functions in cells: GPCR desensitization, i.e., receptor decoupling from G-proteins; GPCR internalization via clathrin-coated pits; and signal transduction independently of or in parallel to G-proteins. Both β-arrestins are expressed in the heart and regulate a large number of cardiac GPCRs. The latter constitute the single most commonly targeted receptor class by Food and Drug Administration-approved cardiovascular drugs, with about one-third of all currently used in the clinic medications affecting GPCR function. Since β-arrestin-1 and -2 play important roles in signaling and function of several GPCRs, in particular of adrenergic receptors and angiotensin II type 1 receptors, in cardiac myocytes, they have been a major focus of cardiac biology research in recent years. Perhaps the most significant realization coming out of their studies is that these two GPCR adapter proteins, initially thought of as functionally interchangeable, actually exert diametrically opposite effects in the mammalian myocardium. Specifically, the most abundant of the two β-arrestin-1 exerts overall detrimental effects on the heart, such as negative inotropy and promotion of adverse remodeling post-myocardial infarction (MI). In contrast, β-arrestin-2 is overall beneficial for the myocardium, as it has anti-apoptotic and anti-inflammatory effects that result in attenuation of post-MI adverse remodeling, while promoting cardiac contractile function. Thus, design of novel cardiac GPCR ligands that preferentially activate β-arrestin-2 over β-arrestin-1 has the potential of generating novel cardiovascular therapeutics for heart failure and other heart diseases.

β Adrenergic Receptor Kinase C-Terminal Peptide Gene-Therapy Improves β2-Adrenergic Receptor-Dependent Neoangiogenesis after Hindlimb Ischemia

After hindlimb ischemia (HI), increased catecholamine levels within the ischemic muscle can cause dysregulation of β2-adrenergic receptor (β2AR) signaling, leading to reduced revascularization. Indeed, in vivo β2AR overexpression via gene therapy enhances angiogenesis in a rat model of HI. G protein-coupled receptor kinase 2 (GRK2) is a key regulator of βAR signaling, and β adrenergic receptor kinase C-terminal peptide (βARKct), a peptide inhibitor of GRK2, has been shown to prevent βAR down-regulation and to protect cardiac myocytes and stem cells from ischemic injury through restoration of β2AR protective signaling (i.e., protein kinase B/endothelial nitric oxide synthase). Herein, we tested the potential therapeutic effects of adenoviral-mediated βARKct gene transfer in an experimental model of HI and its effects on βAR signaling and on endothelial cell (EC) function in vitro. Accordingly, in this study, we surgically induced HI in rats by femoral artery resection (FAR). Fifteen days of ischemia resulted in significant βAR down-regulation that was paralleled by an approximately 2-fold increase in GRK2 levels in the ischemic muscle. Importantly, in vivo gene transfer of the βARKct in the hindlimb of rats at the time of FAR resulted in a marked improvement of hindlimb perfusion, with increased capillary and βAR density in the ischemic muscle, compared with control groups. The effect of βARKct expression was also assessed in vitro in cultured ECs. Interestingly, ECs expressing the βARKct fenoterol, a β2AR-agonist, induced enhanced β2AR proangiogenic signaling and increased EC function. Our results suggest that βARKct gene therapy and subsequent GRK2 inhibition promotes angiogenesis in a model of HI by preventing ischemia-induced β2AR down-regulation.

Transcription factor Sp1 expression is upregulated in human glomerulonephritis: correlation with pSmad2/3 and p300 expression and renal injury

BACKGROUND

Sp1 is a ubiquitous transcription factor that mediates the fibrogenic factor transforming growth factor beta (TGF-beta) signals through cooperation with Smad proteins. The transcriptional coactivator p300 is also suggested to play a role in Smad signal transduction.

METHODS

We investigated the immunohistochemical expression of Sp1 as well as the expression of pSmad2/3 and the coactivator p300 in 157 renal biopsy specimens from patients with various types of glomerulonephritis (GN). Correlations between immunohistochemical, clinical, and histologic parameters were performed.

RESULTS

Sp1 exhibited an increased glomerular and proximal tubular expression in all forms of GN compared to controls. The proximal tubular expression of Sp1 was significantly increased in proliferative GNs (p = 0.025), whereas in secondary GNs, there was a significant increase in the molecule's glomerular expression (p = 0.008). Sp1 correlated positively with pSmad2/3 and p300 expression in proximal tubules (r = 0.241, p = 0.018 and r = 0.244, p = 0.014, respectively), while in proliferative GNs, its expression correlated positively with pSmad2/3 expression in glomeruli (r = 0.32, p = 0.028). Sp1 glomerular and proximal tubular immunostaining correlated positively with serum creatinine levels (r = 0.265, p = 0.02 and r = 0.306, p = 0.006, respectively), while its proximal tubular expression showed a similar correlation with interstitial fibrosis (r = 0.213, p = 0.025). Sp1 was constantly detected in hyperplastic lesions and cellular crescents (each 100%), and very often in micro adhesions (94%) and segmentally or globally sclerotic areas (each 83%).

CONCLUSIONS

This study documents the upregulation of Sp1 expression in glomeruli and proximal tubules of GN specimens. Our findings suggest a possible cooperation of Sp1 with pSmad2/3 and p300 in mediating renal injury as well as a possible role for this molecule in the pathogenesis and the progression of human GN.

β1-adrenoceptor Arg389Gly polymorphism confers differential β-arrestin-binding tropism in cardiac myocytes

AIM

The β1-adrenergic receptor (AR) Arg389Gly polymorphism affects efficacy of its procontractile signaling in cardiomyocytes and carriers' responses to β-blockers. To identify molecular mechanisms underlying functional differences between Arg389 and Gly389 β1ARs, we examined their binding to β-arrestins (βarr-1 and -2), which mediate β1AR signaling, in neonatal rat ventricular myocytes.

METHODS

We tested the β1AR-βarr interaction via β1AR immunoprecipitation followed by βarr immunoblotting.

RESULTS

βarr1 binds both variants upon isoproterenol, carvedilol or metoprolol treatment in neonatal rat ventricular myocytes. Conversely, the potentially beneficial in the heart βarr2 only interacts with the Arg389 receptor in response to isoproterenol or carvedilol.

CONCLUSION

Arg389 confers unique βarr2-interacting tropism to the β1AR in cardiac myocytes, potentially underlying this variant's gain-of-function phenotype and better clinical responses to β-blockers.

Adrenal G Protein-Coupled Receptors and the Failing Heart: A Long-distance, Yet Intimate Affair

Systolic heart failure (HF) is a chronic clinical syndrome characterized by the reduction in cardiac function and still remains the disease with the highest mortality worldwide. Despite considerable advances in pharmacological treatment, HF represents a severe clinical and social burden. Chronic human HF is characterized by several important neurohormonal perturbations, emanating from both the autonomic nervous system and the adrenal glands. Circulating catecholamines (norepinephrine and epinephrine) and aldosterone elevations are among the salient alterations that confer significant hormonal burden on the already compromised function of the failing heart. This is why sympatholytic treatments (such as β-blockers) and renin-angiotensin system inhibitors or mineralocorticoid receptor antagonists, which block the effects of angiotensin II (AngII) and aldosterone on the failing heart, are part of the mainstay HF pharmacotherapy presently. The adrenal gland plays an important role in the modulation of cardiac neurohormonal stress because it is the source of almost all aldosterone, of all epinephrine, and of a significant amount of norepinephrine reaching the failing myocardium from the blood circulation. Synthesis and release of these hormones in the adrenals is tightly regulated by adrenal G protein-coupled receptors (GPCRs), such as adrenergic receptors and AngII receptors. In this review, we discuss important aspects of adrenal GPCR signaling and regulation, as they pertain to modulation of cardiac function in the context of chronic HF, by focusing on the 2 best studied adrenal GPCR types in that context, adrenergic receptors and AngII receptors (AT 1 Rs). Particular emphasis is given to findings from the past decade and a half that highlight the emerging roles of the GPCR-kinases and the β-arrestins in the adrenals, 2 protein families that regulate the signaling and functioning of GPCRs in all tissues, including the myocardium and the adrenal gland.

Pharmacogenomics of adrenoceptors

Adrenoceptors (ARs) consist of nine subtypes (alpha(1A)-, alpha(1B)-, alpha(1D)-, beta(1)-, beta(2)-, beta(3)-, alpha(2A)-, alpha(2B)- and alpha(2C)-AR), which are involved in a wide spectrum of physiological functions and are the site of action for a considerable percentage of currently prescribed therapeutics. With the exception of alpha(1D), all AR subtypes are polymorphic with genetic variations in the coding and non-coding regions. This review discusses the biochemical consequences of these genetic variations and their impact in receptor function, disease pathophysiology, and drug response. Pharmacogenomic principles that have been discovered are also discussed.

RGS proteins and cardiovascular Angiotensin II Signaling: Novel opportunities for therapeutic targeting

Angiotensin II (AngII), as an octapeptide hormone normally ionized at physiological pH, cannot cross cell membranes and thus, relies on, two (mainly) G protein-coupled receptor (GPCR) types, AT1R and AT2R, to exert its intracellular effects in various organ systems including the cardiovascular one. Although a lot remains to be elucidated about the signaling of the AT2R, AT1R signaling is known to be remarkably versatile, mobilizing a variety of G protein-dependent and independent signal transduction pathways inside cells to produce a biological outcome. Cardiac AT1R signaling leads to hypertrophy, adverse remodeling, fibrosis, while vascular AT1R signaling raises blood pressure via vasoconstriction, but also elicits hypertrophic, vascular growth/proliferation, and pathological remodeling sets of events. In addition, adrenal AT1R is the major physiological stimulus (alongside hyperkalemia) for secretion of aldosterone, a mineralocorticoid hormone that contributes to hypertension, electrolyte abnormalities, and to pathological remodeling of the failing heart. Regulator of G protein Signaling (RGS) proteins, discovered about 25 years ago as GTPase-activating proteins (GAPs) for the Gα subunits of heterotrimeric G proteins, play a central role in silencing G protein signaling from a plethora of GPCRs, including the AngII receptors. Given the importance of AngII and its receptors, but also of several RGS proteins, in cardiovascular homeostasis, the physiological and pathological significance of RGS protein-mediated modulation of cardiovascular AngII signaling comes as no surprise. In the present review, we provide an overview of the current literature on the involvement of RGS proteins in cardiovascular AngII signaling, by discussing their roles in cardiac (cardiomyocyte and cardiofibroblast), vascular (smooth muscle and endothelial cell), and adrenal (medulla and cortex) AngII signaling, separately. Along the way, we also highlight the therapeutic potential of enhancement of, or, in some cases, inhibition of each RGS protein involved in AngII signaling in each one of these cell types.

A case report: Pharmacology and resistance patterns of three generations of ALK inhibitors in metastatic inflammatory myofibroblastic sarcoma

BACKGROUND

Little exists currently in research about the mechanisms of resistance of ALK inhibitors in inflammatory myofibroblastic sarcoma. It is known, however, that ALK gene rearrangements are common in inflammatory myofibroblastic tumors, similar to non-small cell lung cancer. In roughly 50% of inflammatory myofibroblastic tumors, gene rearrangement has been found to occur on chromosome 2 at band 2p23. In non-small cell lung cancer, it has been shown that about a third of patients who progress on the first generation ALK inhibitor, crizotinib develops mutations in the ALK kinase domain. The remaining two-thirds of patients tend to develop amplification of ALK or activation of alternative signaling pathways. Chromoplexy has also been described as a mechanism of resistance, where multiple closed chain rearrangements cause loss-of-function of tumor suppressor genes and gain-in-function of oncogenic fusions. Partner genes that have been identified in IMTs are tropomyosin 3 (TPM3), tropomyosin 4 (TPM4), clathrin heavy chain (CLTC), Ran-binding protein 2 (RANBP2), cysteinyl-tRNA synthetase (CARS), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC), and SEC31L1. All are active promoters for the fusion gene, in response to NPM binding. Several inflammatory myofibroblastic tumor case reports indicated that fusion of ALK and RANBP2 led to a more aggressive clinical course. Although the majority of inflammatory myofibroblastic tumor case reports have utilized first and second generation ALK inhibitors, all generations of ALK inhibitors have demonstrated some ability to impair disease progression and extend life expectancy. However, at some point in the course of therapy with each generation of ALK inhibitor, resistance ultimately developed. In order to better understand the pharmacology and resistance patterns behind three generations of ALK inhibitors, we sought to examine a patient with metastatic anaplastic lymphoma kinase-1-rearranged inflammatory myofibroblastic sarcoma to the brain. We also explored the similarities and differences of this clinical case to other inflammatory myofibroblastic sarcoma case reports involving the use of ALK inhibitors.

CASE REPORT

A rare case of pulmonary IMS with ALK-1 gene rearrangement and multiple brain metastases responded to three generations of ALK inhibitors. However, similar to other case reports, due to the development of resistance and recurrence, the patient eventually succumbed to the disease.

CONCLUSIONS

ALK inhibitors are beneficial in the temporary prevention of progression of disease in patients with inflammatory myofibroblastic tumors. In this case, due to the inability to reveal the fusion partner in this patient via DNA sequencing, it is unknown exactly if that partner was RANBP2 or another ALK partner gene. Brain biopsy tissue was also unobtainable during sequence of ALK due to risk versus benefit, which would have provided insight as which type of ALK resistance mutations the patient was developing. It is likely that this patient had some form of chromoplexy occurring.

Adrenal G protein-coupled receptor kinase-2 in regulation of sympathetic nervous system activity in heart failure

Heart failure (HF), the number one cause of death in the western world, is caused by the insufficient performance of the heart leading to tissue underperfusion in response to an injury or insult. It comprises complex interactions between important neurohormonal mechanisms that try but ultimately fail to sustain cardiac output. The most prominent such mechanism is the sympathetic (adrenergic) nervous system (SNS), whose activity and outflow are greatly elevated in HF. SNS hyperactivity confers significant toxicity to the failing heart and markedly increases HF morbidity and mortality via excessive activation of adrenergic receptors, which are G protein-coupled receptors. Thus, ligand binding induces their coupling to heterotrimeric G proteins that transduce intracellular signals. G protein signaling is turned-off by the agonist-bound receptor phosphorylation courtesy of G protein-coupled receptor kinases (GRKs), followed by βarrestin binding, which prevents the GRK-phosphorylated receptor from further interaction with the G proteins and simultaneously leads it inside the cell (receptor sequestration). Recent evidence indicates that adrenal GRK2 and βarrestins can regulate adrenal catecholamine secretion, thereby modulating SNS activity in HF. The present review gives an account of all these studies on adrenal GRKs and βarrestins in HF and discusses the exciting new therapeutic possibilities for chronic HF offered by targeting these proteins pharmacologically.

Sustained GRK2-dependent CREB activation is essential for α2-adrenergic receptor-induced PC12 neuronal differentiation

Many aspects of neuronal development, such as neuronal survival and differentiation, are regulated by the transcription factor cAMP-response element-binding protein (CREB). We have previously reported that α2-adrenergic receptors (ARs), members of the G protein-coupled receptor (GPCR) superfamily, induce neuronal differentiation of rat pheochromocytoma (PC)-12 cells in a subtype-specific manner, i.e. α2A<α2B<α2C. Herein, we sought to investigate CREB`s involvement in this α2AR-dependent neurogenic process. We used a combination of gene reporter assays and immunoblotting analysis, coupled with co-immunoprecipitation and morphological analysis, in transfected PC12 cell lines. Chronic α2B- or α2C-AR activation results in sustained CREB activation, which is both necessary and sufficient for neuronal differentiation of PC12 cells expressing these two α2ARs. In contrast, chronic α2A activation only leads to transient CREB activation, insufficient for PC12 neuronal differentiation. However, upon CREB overexpression, α2A-AR triggers neuronal differentiation similarly to α2B- or α2C-ARs. Importantly, NGF (Nerve Growth Factor)`s TrkA receptor transactivation is essential for the sustained activation of CREB by all three α2 subtypes in PC12 cells, whereas protein kinase A (PKA), the prototypic kinase that phosphorylates CREB, is not. Instead, TrkA-activated GPCR-kinase (GRK)-2 mediates the sustained CREB activation during α2AR-induced neuronal differentiation of PC12 cells. In conclusion, catecholaminergic-induced neuronal differentiation of PC12 cells through α2ARs uses a non-canonical pathway involving TrkA transactivation and subsequent GRK2-dependent, sustained phosphorylation/activation of CREB. These findings provide novel insights into catecholaminergic neurogenesis and suggest that α2AR agonists, combined with NGF analogs or GRK2 stimulators, may exert neurogenic/neuroprotective effects.

GRK2-Mediated Crosstalk Between β-Adrenergic and Angiotensin II Receptors Enhances Adrenocortical Aldosterone Production In Vitro and In Vivo

Aldosterone is produced by adrenocortical zona glomerulosa (AZG) cells in response to angiotensin II (AngII) acting through its type I receptors (AT1Rs). AT1R is a G protein-coupled receptor (GPCR) that induces aldosterone via both G proteins and the adapter protein βarrestin1, which binds the receptor following its phosphorylation by GPCR-kinases (GRKs) to initiate G protein-independent signaling. β-adrenergic receptors (ARs) also induce aldosterone production in AZG cells. Herein, we investigated whether GRK2 or GRK5, the two major adrenal GRKs, is involved in the catecholaminergic regulation of AngII-dependent aldosterone production. In human AZG (H295R) cells in vitro, the βAR agonist isoproterenol significantly augmented both AngII-dependent aldosterone secretion and synthesis, as measured by the steroidogenic acute regulatory (StAR) protein and CYP11B2 (aldosterone synthase) mRNA inductions. Importantly, GRK2, but not GRK5, was indispensable for the βAR-mediated enhancement of aldosterone in response to AngII. Specifically, GRK2 inhibition with Cmpd101 abolished isoproterenol's effects on AngII-induced aldosterone synthesis/secretion, whereas the GRK5 knockout via CRISPR/Cas9 had no effect. It is worth noting that these findings were confirmed in vivo, since rats overexpressing GRK2, but not GRK5, in their adrenals had elevated circulating aldosterone levels compared to the control animals. However, treatment with the β-blocker propranolol prevented hyperaldosteronism in the adrenal GRK2-overexpressing rats. In conclusion, GRK2 mediates a βAR-AT1R signaling crosstalk in the adrenal cortex leading to elevated aldosterone production. This suggests that adrenal GRK2 may be a molecular link connecting the sympathetic nervous and renin-angiotensin systems at the level of the adrenal cortex and that its inhibition might be therapeutically advantageous in hyperaldosteronism-related conditions.

Impact of Aldosterone on the Failing Myocardium: Insights from Mitochondria and Adrenergic Receptors Signaling and Function

The mineralocorticoid aldosterone regulates electrolyte and blood volume homeostasis, but it also adversely modulates the structure and function of the chronically failing heart, through its elevated production in chronic human post-myocardial infarction (MI) heart failure (HF). By activating the mineralocorticoid receptor (MR), a ligand-regulated transcription factor, aldosterone promotes inflammation and fibrosis of the heart, while increasing oxidative stress, ultimately induding mitochondrial dysfunction in the failing myocardium. To reduce morbidity and mortality in advanced stage HF, MR antagonist drugs, such as spironolactone and eplerenone, are used. In addition to the MR, aldosterone can bind and stimulate other receptors, such as the plasma membrane-residing G protein-coupled estrogen receptor (GPER), further complicating it signaling properties in the myocardium. Given the salient role that adrenergic receptor (ARs)-particularly βARs-play in cardiac physiology and pathology, unsurprisingly, that part of the impact of aldosterone on the failing heart is mediated by its effects on the signaling and function of these receptors. Aldosterone can significantly precipitate the well-documented derangement of cardiac AR signaling and impairment of AR function, critically underlying chronic human HF. One of the main consequences of HF in mammalian models at the cellular level is the presence of mitochondrial dysfunction. As such, preventing mitochondrial dysfunction could be a valid pharmacological target in this condition. This review summarizes the current experimental evidence for this aldosterone/AR crosstalk in both the healthy and failing heart, and the impact of mitochondrial dysfunction in HF. Recent findings from signaling studies focusing on MR and AR crosstalk via non-conventional signaling of molecules that normally terminate the signaling of ARs in the heart, i.e., the G protein-coupled receptor-kinases (GRKs), are also highlighted.

Adrenal βarrestin1 targeting for tobacco-associated cardiac dysfunction treatment: Aldosterone production as the mechanistic link

Tobacco kills 6 million people annually and its global health costs are continuously rising. The main addictive component of every tobacco product is nicotine. Among the mechanisms by which nicotine, and its major metabolite, cotinine, contribute to heart disease is the renin-angiotensin-aldosterone system (RAAS) activation. This increases aldosterone production from the adrenals and circulating aldosterone levels. Aldosterone is a mineralocorticoid hormone with various direct harmful effects on the myocardium, including increased reactive oxygen species (ROS) generation, which contributes significantly to cardiac mitochondrial dysfunction and cardiac aging. Aldosterone is produced in the adrenocortical zona glomerulosa (AZG) cells in response to angiotensin II (AngII), activating its type 1 receptor (AT1R). The AT1R is a G protein-coupled receptor (GPCR) that leads to aldosterone biosynthesis and secretion, via signaling from both Gq/11 proteins and the GPCR adapter protein βarrestin1, in AZG cells. Adrenal βarrestin1 is essential for AngII-dependent adrenal aldosterone production, which aggravates heart disease. Since adrenal βarrestin1 is essential for raising circulating aldosterone in the body and tobacco compounds are also known to elevate aldosterone levels in smokers, accelerating heart disease progression, our central hypothesis is that nicotine and cotinine increase aldosterone levels to induce cardiac injury by stimulating adrenal βarrestin1. In the present review, we provide an overview of the current literature of the physiology and pharmacology of adrenal aldosterone production regulation, of the effects of tobacco on this process and, finally, of the effects of tobacco and aldosterone on cardiac structure and function, with a particular focus on cardiac mitochondrial function. We conclude our literature account with a brief experimental outline, as well as with some therapeutic perspectives of our pharmacological hypothesis, that is that adrenal βarrestin1 is a novel molecular target for preventing tobacco-induced hyperaldosteronism, thereby also ameliorating tobacco-related heart disease development.

Carvedilol Selectively Stimulates βArrestin2-Dependent SERCA2a Activity in Cardiomyocytes to Augment Contractility

Heart failure (HF) carries the highest mortality in the western world and β-blockers [β-adrenergic receptor (AR) antagonists] are part of the cornerstone pharmacotherapy for post-myocardial infarction (MI) chronic HF. Cardiac β1AR-activated βarrestin2, a G protein-coupled receptor (GPCR) adapter protein, promotes Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2a SUMO (small ubiquitin-like modifier)-ylation and activity, thereby directly increasing cardiac contractility. Given that certain β-blockers, such as carvedilol and metoprolol, can activate βarrestins and/or SERCA2a in the heart, we investigated the effects of these two agents on cardiac βarrestin2-dependent SERCA2a SUMOylation and activity. We found that carvedilol, but not metoprolol, acutely induces βarrestin2 interaction with SERCA2a in H9c2 cardiomyocytes and in neonatal rat ventricular myocytes (NRVMs), resulting in enhanced SERCA2a SUMOylation. However, this translates into enhanced SERCA2a activity only in the presence of the β2AR-selective inverse agonist ICI 118,551 (ICI), indicating an opposing effect of carvedilol-occupied β2AR subtype on carvedilol-occupied β1AR-stimulated, βarrestin2-dependent SERCA2a activation. In addition, the amplitude of fractional shortening of NRVMs, transfected to overexpress βarrestin2, is acutely enhanced by carvedilol, again in the presence of ICI only. In contrast, metoprolol was without effect on NRVMs' shortening amplitude irrespective of ICI co-treatment. Importantly, the pro-contractile effect of carvedilol was also observed in human induced pluripotent stem cell (hIPSC)-derived cardiac myocytes (CMs) overexpressing βarrestin2, and, in fact, it was present even without concomitant ICI treatment of human CMs. Metoprolol with or without concomitant ICI did not affect contractility of human CMs, either. In conclusion, carvedilol, but not metoprolol, stimulates βarrestin2-mediated SERCA2a SUMOylation and activity through the β1AR in cardiac myocytes, translating into direct positive inotropy. However, this unique βarrestin2-dependent pro-contractile effect of carvedilol may be opposed or masked by carvedilol-bound β2AR subtype signaling.

Autonomic Nervous System Regulation of Epicardial Adipose Tissue: Potential Roles for Regulator of G Protein Signaling-4

The epicardial adipose tissue (EAT) or epicardial fat is a visceral fat depot in the heart that contains intrinsic adrenergic and cholinergic nerves, through which it interacts with the cardiac sympathetic (adrenergic) and parasympathetic (cholinergic) nervous systems. These EAT nerves represent a significant source of several adipokines and other bioactive molecules, including norepinephrine, epinephrine, and free fatty acids. The production of these molecules is biologically relevant for the heart, since abnormalities in EAT secretion are implicated in the development of pathological conditions, including coronary atherosclerosis, atrial fibrillation, and heart failure. Sympathetic hyperactivity and parasympathetic (cholinergic) derangement are associated with EAT dysfunction, leading to a variety of adverse cardiac conditions, such as heart failure, diastolic dysfunction, atrial fibrillation, etc.; therefore, several studies have focused on exploring the autonomic regulation of EAT as it pertains to heart disease pathogenesis and progression. In addition, Regulator of G protein Signaling (RGS)-4 is a protein with significant regulatory roles in both adrenergic and muscarinic receptor signaling in the heart. In this review, we provide an overview of the autonomic regulation of EAT, with a specific focus on cardiac RGS4 and the potential roles this protein plays in this regulation.

Transcriptional down-regulation of the human alpha2C-adrenergic receptor by cAMP

The heterologous regulation of the alpha2C-adrenergic receptor (alpha2C-AR) was investigated in the HepG2 cell line. Binding of [(3)H]MK912 (alpha2-antagonist) to membranes from cells submitted to various treatments showed that exposure to insulin, phorbol 12-myristate 13-acetate, or dexamethasone did not affect receptor density. On the other hand, treatment with forskolin resulted in a large reduction of alpha2C-AR number. The effect of forskolin was mimicked by 8-br-cAMP and was abolished by the protein kinase A inhibitor, H89. The action of cAMP was slow (t(1/2) = 23 h), dose-dependent, and additive to the receptor down-regulation elicited by the alpha2-agonist, UK14304. Furthermore, the diminution of receptor was not caused by an increased rate of its degradation but resulted from a decrease in the steady state amounts of alpha2C4-mRNA. As assessed by experiments in the presence of actinomycin D, the stability of alpha2C4-mRNA was not affected by 8-br-cAMP or forskolin. By contrast, the activity of a luciferase construct containing the entire promoter region of the alpha2C4 gene (1.9 kilobase pairs) was inhibited, indicating that the primary mechanism of action of the two compounds is at the transcriptional level. Deletions in the 5'-end of this construct showed that the elements responsible for cAMP responsiveness lie within a 242-base-pair fragment of the gene promoter (nucleotides -236/+6 relative to transcription start). Band-shift experiments indicated that nuclear factors bind to this region in a cAMP-dependent manner. The determination of the actual cis- and trans-acting elements involved will be the object of future investigation, but the present study provides evidence for transcriptional regulation of human alpha2C-AR by cAMP.

Pharmacogenomics of heart failure

The combination of angiotensin-converting enzyme (ACE) inhibitors and β-adrenergic receptor (βAR) blockers remains the essential component of heart failure (HF) pharmacotherapy. However, individual patient responses to these pharmacotherapies vary widely. The variability in response cannot be explained entirely by clinical characteristics, and genetic variation may play a role. The purpose of this chapter is to examine the current knowledge in the field of beta-blocker and ACE inhibitor pharmacogenetics in HF. β-blocker and ACE inhibitor pharmacogenetic studies performed in patients with HF were identified from the PubMed database from 1966 to July 2011. Thirty beta-blocker and 10 ACE inhibitor pharmacogenetic studies in patients with HF were identified.The ACE deletion variant was associated with greater survival benefit from ACE inhibitors and beta-blockers compared with the ACE insertion. Ser49 in the β1AR, the insertion in the α2CAR, and Gln41 in G protein-coupled receptor (GPCR) kinase (GRK)-5 are associated with greater survival benefit from β-blockers, compared with Gly49, the deletion, and Leu41, respectively. However, many of these associations have not been validated. The HF pharmacogenetic literature is still in its very early stages, but there are promising candidate genetic variants that may identify which HF patients are most likely to benefit from beta-blockers and ACE inhibitors and patients that may require additional therapies.

Nicotine-induced adrenal beta-arrestin1 upregulation mediates tobacco-related hyperaldosteronism leading to cardiac dysfunction

BACKGROUND

Tobacco-related products, containing the highly addictive nicotine together with numerous other harmful toxicants and carcinogens, have been clearly associated with coronary artery disease, heart failure, stroke, and other heart diseases. Among the mechanisms by which nicotine contributes to heart disease is elevation of the renin-angiotensin-aldosterone system (RAAS) activity. Nicotine, and its major metabolite in humans cotinine, have been reported to induce RAAS activation, resulting in aldosterone elevation in smokers. Aldosterone has various direct and indirect adverse cardiac effects. It is produced by the adrenal cortex in response to angiotensin II (AngII) activating AngII type 1 receptors. RAAS activity increases in chronic smokers, causing raised aldosterone levels (nicotine exposure causes the same in rats). AngII receptors exert their cellular effects via either G proteins or the two βarrestins (βarrestin1 and-2).

AIM

Since adrenal ßarrestin1 is essential for adrenal aldosterone production and nicotine/cotinine elevate circulating aldosterone levels in humans, we hypothesized that nicotine activates adrenal ßarrestin1, which contributes to RAAS activation and heart disease development.

METHODS

We studied human adrenocortical zona glomerulosa H295R cells and found that nicotine and cotinine upregulate βarrestin1 mRNA and protein levels, thereby enhancing AngII-dependent aldosterone synthesis and secretion.

RESULTS

In contrast, siRNA-mediated βarrestin1 knockdown reversed the effects of nicotine on AngII-induced aldosterone production in H295R cells. Importantly, nicotine promotes hyperaldosteronism via adrenal βarrestin1, thereby precipitating cardiac dysfunction, also in vivo, since nicotine-exposed experimental rats with adrenal-specific βarrestin1 knockdown display lower circulating aldosterone levels and better cardiac function than nicotine-exposed control animals with normal adrenal βarrestin1 expression.

CONCLUSION

Adrenal βarrestin1 upregulation is one of the mechanisms by which tobacco compounds, like nicotine, promote cardio-toxic hyperaldosteronism in vitro and in vivo. Thus, adrenal βarrestin1 represents a novel therapeutic target for tobacco-related heart disease prevention or mitigation.