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

Canonical or non-canonical, all aspects of G protein-coupled receptor kinase 2 in heart failure

G protein-coupled receptor kinase 2 (GRK2) with its multidomain structure performs various crucial cellular functions under both normal and pathological conditions. Overexpression of GRK2 is linked to cardiovascular diseases, and its inhibition or deletion has been shown to be protective. The functions of GRK2 extend beyond G protein-coupled receptor (GPCR) signaling, influencing non-GPCR substrates as well. Increased GRK2 in heart failure (HF) initially may be protective but ultimately leads to maladaptive effects such as GPCR desensitization, insulin resistance, and apoptosis. The multifunctional nature of GRK2, including its action in hypertrophic gene expression, insulin signaling, and cardiac fibrosis, highlights its complex role in HF pathogenesis. Additionally, GRK2 is involved in mitochondrial biogenesis and lipid metabolism. GRK2 also regulates epinephrine secretion from the adrenal gland and its increase in circulating lymphocytes can be used to monitor HF status. Overall, GRK2 is a multifaceted protein with significant implications for HF and the regulation of GRK2 is crucial for understanding and treating cardiovascular diseases.

RGS Proteins in Sympathetic Nervous System Regulation: Focus on Adrenal RGS4

The sympathetic nervous system (SNS) consists largely of two different types of components: neurons that release the neurotransmitter norepinephrine (NE, noradrenaline) to modulate homeostasis of the innevrvated effector organ or tissue and adrenal chromaffin cells, which synthesize and secrete the hormone epinephrine (Epi, adrenaline) and some NE into the blood circulation to act at distant organs and tissues that are not directly innervated by the SNS. Like almost every physiological process in the human body, G protein-coupled receptors (GPCRs) tightly modulate both NE release from sympathetic neuronal terminals and catecholamine (CA) secretion from the adrenal medulla. Regulator of G protein Signaling (RGS) proteins, acting as guanosine triphosphatase (GTPase)-activating proteins (GAPs) for the Gα subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins), play a central role in silencing G protein signaling from a plethora of GPCRs. Certain RGS proteins and, in particular, RGS4, have been implicated in regulation of SNS activity and of adrenal chromaffin cell CA secretion. More specifically, recent studies have implicated RGS4 in regulation of NE release from cardiac sympathetic neurons by means of terminating free fatty acid receptor (FFAR)-3 calcium signaling and in regulation of NE and Epi secretion from the adrenal medulla by means of terminating cholinergic calcium signaling in adrenal chromaffin cells. Thus, in this review, we provide an overview of the current literature on the involvement of RGS proteins, with a particular focus on RGS4, in these two processes, i.e., NE release from sympathetic nerve terminals & CA secretion from adrenal chromaffin cells. We also highlight the therapeutic potential of RGS4 pharmacological manipulation for diseases characterized by sympathetic dysfunction or SNS hyperactivity, such as heart failure and hypertension.

Developing a prediction model for preoperative acute heart failure in elderly hip fracture patients: a retrospective analysis

BACKGROUND

Hip fractures in the elderly are a common traumatic injury. Due to factors such as age and underlying diseases, these patients exhibit a high incidence of acute heart failure prior to surgery, severely impacting surgical outcomes and prognosis.

OBJECTIVE

This study aims to explore the potential risk factors for acute heart failure before surgery in elderly patients with hip fractures and to establish an effective clinical prediction model.

METHODS

This study employed a retrospective cohort study design and collected baseline and preoperative variables of elderly patients with hip fractures. Strict inclusion and exclusion criteria were adopted to ensure sample consistency. Statistical analyses were carried out using SPSS 24.0 and R software. A prediction model was developed using least absolute shrinkage and selection operator (LASSO) regression and multivariate logistic regression. The accuracy of the model was evaluated by analyzing the area under the receiver operating characteristic (ROC) curve (AUC) and a calibration curve was plotted to assess the model's calibration.

RESULTS

Between 2018 and 2019, 1962 elderly fracture patients were included in the study. After filtering, 1273 were analyzed. Approximately 25.7% of the patients experienced acute heart failure preoperatively. Through LASSO and logistic regression analyses, predictors for preoperative acute heart failure in elderly patients with hip fractures were identified as Gender was male (OR = 0.529, 95% CI: 0.381-0.734, P < 0.001), Age (OR = 1.760, 95% CI: 1.251-2.479, P = 0.001), Coronary Heart Disease (OR = 1.977, 95% CI: 1.454-2.687, P < 0.001), Chronic Obstructive Pulmonary Disease (COPD) (OR = 2.484, 95% CI: 1.154-5.346, P = 0.020), Complications (OR = 1.516, 95% CI: 1.033-2.226, P = 0.033), Anemia (OR = 2.668, 95% CI: 1.850-3.847, P < 0.001), and Hypoalbuminemia (OR 2.442, 95% CI: 1.682-3.544, P < 0.001). The linear prediction model of acute heart failure was Logit(P) = -2.167-0.637×partial regression coefficient for Gender was male + 0.566×partial regression coefficient for Age + 0.682×partial regression coefficient for Coronary heart disease + 0.910×partial regression coefficient for COPD + 0.416×partial regression coefficient for Complications + 0.981×partial regression coefficient for Anemia + 0.893×partial regression coefficient for Hypoalbuminemia, and the nomogram prediction model was established. The AUC of the predictive model was 0.763, indicating good predictive performance. Decision curve analysis revealed that the prediction model offers the greatest net benefit when the threshold probability ranges from 4 to 62%.

CONCLUSION

The prediction model we developed exhibits excellent accuracy in predicting the onset of acute heart failure preoperatively in elderly patients with hip fractures. It could potentially serve as an effective and useful clinical tool for physicians in conducting clinical assessments and individualized treatments.

Using the sympathetic system, beta blockers and alpha-2 agonists, to address acute respiratory distress syndrome

Acute Respiratory Distress Syndrome (ARDS) manifests as an acute inflammatory lung injury characterized by persistent hypoxemia, featuring a swift onset, high mortality, and predominantly supportive care as the current therapeutic approach, while effective treatments remain an area of active investigation. Adrenergic receptors (AR) play a pivotal role as stress hormone receptors, extensively participating in various inflammatory processes by initiating downstream signaling pathways. Advancements in molecular biology and pharmacology continually unveil the physiological significance of distinct AR subtypes. Interventions targeting these subtypes have the potential to induce specific alterations in cellular and organismal functions, presenting a promising avenue as a therapeutic target for managing ARDS. This article elucidates the pathogenesis of ARDS and the basic structure and function of AR. It also explores the relationship between AR and ARDS from the perspective of different AR subtypes, aiming to provide new insights for the improvement of ARDS.

Adrenoceptor Desensitization: Current Understanding of Mechanisms

G-protein coupled receptors (GPCRs) transduce a wide range of extracellular signals. They are key players in the majority of biologic functions including vision, olfaction, chemotaxis, and immunity. However, as essential as most of them are to body function and homeostasis, overactivation of GPCRs has been implicated in many pathologic diseases such as cancer, asthma, and heart failure (HF). Therefore, an important feature of G protein signaling systems is the ability to control GPCR responsiveness, and one key process to control overstimulation involves initiating receptor desensitization. A number of steps are appreciated in the desensitization process, including cell surface receptor phosphorylation, internalization, and downregulation. Rapid or short-term desensitization occurs within minutes and involves receptor phosphorylation via the action of intracellular protein kinases, the binding of β-arrestins, and the consequent uncoupling of GPCRs from their cognate heterotrimeric G proteins. On the other hand, long-term desensitization occurs over hours to days and involves receptor downregulation or a decrease in cell surface receptor protein level. Of the proteins involved in this biologic phenomenon, β-arrestins play a particularly significant role in both short- and long-term desensitization mechanisms. In addition, β-arrestins are involved in the phenomenon of biased agonism, where the biased ligand preferentially activates one of several downstream signaling pathways, leading to altered cellular responses. In this context, this review discusses the different patterns of desensitization of the α 1-, α 2- and the β adrenoceptors and highlights the role of β-arrestins in regulating physiologic responsiveness through desensitization and biased agonism. SIGNIFICANCE STATEMENT: A sophisticated network of proteins orchestrates the molecular regulation of GPCR activity. Adrenoceptors are GPCRs that play vast roles in many physiological processes. Without tightly controlled desensitization of these receptors, homeostatic imbalance may ensue, thus precipitating various diseases. Here, we critically appraise the mechanisms implicated in adrenoceptor desensitization. A better understanding of these mechanisms helps identify new druggable targets within the GPCR desensitization machinery and opens exciting therapeutic fronts in the treatment of several pathologies.

Heart failure management with β-blockers: can we do better?

Heart failure (HF) is associated with disabling symptoms, poor quality of life, and a poor prognosis with substantial excess mortality in the years following diagnosis. Overactivation of the sympathetic nervous system is a key feature of the pathophysiology of HF and is an important driver of the process of adverse remodelling of the left ventricular wall that contributes to cardiac failure. Drugs which suppress the activity of the renin-angiotensin-aldosterone system, including β-blockers, are foundation therapies for the management of heart failure with reduced ejection fraction (HFrEF) and despite a lack of specific outcomes trials, are also widely used by cardiologist in patients with HF with preserved ejection fraction (HFpEF). Today, expert opinion has moved away from recommending that treatment for HF should be guided solely by the LVEF and interventions should rather address signs and symptoms of HF (e.g. oedema and tachycardia), the severity of HF, and concomitant conditions. β-blockers improve HF symptoms and functional status in HF and these agents have demonstrated improved survival, as well as a reduced risk of other important clinical outcomes such as hospitalisation for heart failure, in randomised, placebo-controlled outcomes trials. In HFpEF, β-blockers are anti-ischemic and lower blood pressure and heart rate. Moreover, β-blockers also reduce mortality in the setting of HF occurring alongside common comorbid conditions, such as diabetes, CKD (of any severity), and COPD. Higher doses of β-blockers are associated with better clinical outcomes in populations with HF, so that ensuring adequate titration of therapy to their maximal (or maximally tolerated) doses is important for ensuring optimal outcomes for people with HF. In principle, a patient with HF could have combined treatment with a β-blocker, renin-angiotensin-aldosterone system inhibitor/neprilysin inhibitor, mineralocorticoid receptor antagonist, and a SGLT2 inhibitor, according to tolerability.

β-adrenergic receptor signaling mediated by β-arrestins and its potential role in heart failure

The lethality of heart failure (HF), particularly in the context of post-acute sequelae SARS-CoV-2 infection (PASC)-related myocarditis, necessitates the discovery of the cellular pathways implicated in cardiovascular disease (CVD). We summarize the signaling mechanisms of the catecholamine-binding β-adrenergic receptors (β-ARs), with an emphasis on the role of β-arrestins. β-ARs, a subset of G protein-coupled receptors (GPCRs), canonically propagate signals through heterotrimeric G proteins. However, since their discovery in the late 1980s, β-arrestins have been shown to, both (i) quench G protein signaling and (ii) initiate their own independent signaling cascades, which is influenced by post-translational modifications. β-arrestin-biased agonism by the beta-blocker carvedilol and its allosteric modulation can serve a cardioprotective role. The increasingly labyrinthine nature of GPCR signaling suggests that ligand-dependent β-AR signaling, either stimulated by an agonist or blocked by an antagonist, is selectively enhanced or suppressed by allosteric modulations, which are orchestrated by novel drugs or endogenous post-translational modifications.

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.

Clinical pharmacology of cardiac cyclic AMP in human heart failure: too much or too little?

INTRODUCTION

Cyclic 3', 5'-adenosine monophosphate (cAMP) is a major signaling hub in cardiac physiology. Although cAMP signaling has been extensively studied in cardiac cells and animal models of heart failure (HF), not much is known about its actual amount present inside human failing or non-failing cardiomyocytes. Since many drugs used in HF work via cAMP, it is crucial to determine the status of its intracellular levels in failing vs. normal human hearts.

AREAS COVERED

Only studies performed on explanted/excised cardiac tissues from patients were examined. Studies that contained no data from human hearts or no data on cAMP levels per se were excluded from this perspective's analysis.

EXPERT OPINION

Currently, there is no consensus on the status of cAMP levels in human failing vs. non-failing hearts. Several studies on animal models may suggest maladaptive (e.g. pro-apoptotic) effects of cAMP on HF, advocating for cAMP lowering for therapy, but human studies almost universally indicate that myocardial cAMP levels are deficient in human failing hearts. It is the expert opinion of this perspective that intracellular cAMP levels are too low in human failing hearts, contributing to the disease. Strategies to increase (restore), not decrease, these levels should be pursued in human HF.

Angiotensin II-dependent aldosterone production in the adrenal cortex

The adrenal cortex is responsible for production of adrenal steroid hormones and is anatomically divided into three distinct zones: zona glomerulosa secreting mineralocorticoids (mainly aldosterone), zona fasciculata secreting glucocorticoids (cortisol), and zona reticularis producing androgens. Importantly, due to their high lipophilicity, no adrenal steroid hormone (including aldosterone) is stored in vesicles but rather gets synthesized and secreted instantly upon cell stimulation with specific stimuli. Aldosterone is the most potent mineralocorticoid hormone produced from the adrenal cortex in response to either angiotensin II (AngII) or elevated K+ levels in the blood (hyperkalemia). AngII, being a peptide, cannot cross cell membranes and thus, uses two distinct G protein-coupled receptor (GPCR) types, AngII type 1 receptor (AT1R) and AT2R to exert its effects inside cells. In zona glomerulosa cells, AT1R activation by AngII results in aldosterone synthesis and secretion via two main pathways: (a) Gq/11 proteins that activate phospholipase C ultimately raising intracellular free calcium concentration; and (b) βarrestin1 and -2 (also known as Arrestin-2 and -3, respectively) that elicit sustained extracellular signal-regulated kinase (ERK) activation. Both pathways induce upregulation and acute activation of StAR (steroidogenic acute regulatory) protein, the enzyme that catalyzes the rate-limiting step in aldosterone biosynthesis. This chapter describes these two salient pathways underlying AT1R-induced aldosterone production in zona glomerulosa cells. We also highlight some pharmacologically important notions pertaining to the efficacy of the currently available AT1R antagonists, also known as angiotensin receptor blockers (ARBs) or sartans at suppressing both pathways, i.e., their inverse agonism efficacy at G proteins and βarrestins.

Regulator of G-Protein Signaling-4 Attenuates Cardiac Adverse Remodeling and Neuronal Norepinephrine Release-Promoting Free Fatty Acid Receptor FFAR3 Signaling

Propionic acid is a cell nutrient but also a stimulus for cellular signaling. Free fatty acid receptor (FFAR)-3, also known as GPR41, is a Gi/o protein-coupled receptor (GPCR) that mediates some of the propionate's actions in cells, such as inflammation, fibrosis, and increased firing/norepinephrine release from peripheral sympathetic neurons. The regulator of G-protein Signaling (RGS)-4 inactivates (terminates) both Gi/o- and Gq-protein signaling and, in the heart, protects against atrial fibrillation via calcium signaling attenuation. RGS4 activity is stimulated by β-adrenergic receptors (ARs) via protein kinase A (PKA)-dependent phosphorylation. Herein, we examined whether RGS4 modulates cardiac FFAR3 signaling/function. We report that RGS4 is essential for dampening of FFAR3 signaling in H9c2 cardiomyocytes, since siRNA-mediated RGS4 depletion significantly enhanced propionate-dependent cAMP lowering, Gi/o activation, p38 MAPK activation, pro-inflammatory interleukin (IL)-1β and IL-6 production, and pro-fibrotic transforming growth factor (TGF)-β synthesis. Additionally, catecholamine pretreatment blocked propionic acid/FFAR3 signaling via PKA-dependent activation of RGS4 in H9c2 cardiomyocytes. Finally, RGS4 opposes FFAR3-dependent norepinephrine release from sympathetic-like neurons (differentiated Neuro-2a cells) co-cultured with H9c2 cardiomyocytes, thereby preserving the functional βAR number of the cardiomyocytes. In conclusion, RGS4 appears essential for propionate/FFAR3 signaling attenuation in both cardiomyocytes and sympathetic neurons, leading to cardioprotection against inflammation/adverse remodeling and to sympatholysis, respectively.

Short-Chain Fatty Acid Receptors and Cardiovascular Function

Increasing experimental and clinical evidence points toward a very important role for the gut microbiome and its associated metabolism in human health and disease, including in cardiovascular disorders. Free fatty acids (FFAs) are metabolically produced and utilized as energy substrates during almost every biological process in the human body. Contrary to long- and medium-chain FFAs, which are mainly synthesized from dietary triglycerides, short-chain FFAs (SCFAs) derive from the gut microbiota-mediated fermentation of indigestible dietary fiber. Originally thought to serve only as energy sources, FFAs are now known to act as ligands for a specific group of cell surface receptors called FFA receptors (FFARs), thereby inducing intracellular signaling to exert a variety of cellular and tissue effects. All FFARs are G protein-coupled receptors (GPCRs) that play integral roles in the regulation of metabolism, immunity, inflammation, hormone/neurotransmitter secretion, etc. Four different FFAR types are known to date, with FFAR1 (formerly known as GPR40) and FFAR4 (formerly known as GPR120) mediating long- and medium-chain FFA actions, while FFAR3 (formerly GPR41) and FFAR2 (formerly GPR43) are essentially the SCFA receptors (SCFARs), responding to all SCFAs, including acetic acid, propionic acid, and butyric acid. As with various other organ systems/tissues, the important roles the SCFARs (FFAR2 and FFAR3) play in physiology and in various disorders of the cardiovascular system have been revealed over the last fifteen years. In this review, we discuss the cardiovascular implications of some key (patho)physiological functions of SCFAR signaling pathways, particularly those regulating the neurohormonal control of circulation and adipose tissue homeostasis. Wherever appropriate, we also highlight the potential of these receptors as therapeutic targets for cardiovascular disorders.