Alteration of myocardial GRK2 produces a global metabolic phenotype

The Role of β3-Adrenergic Receptors in Cold-Induced Beige Adipocyte Production in Pigs

After exposure to cold stress, animals enhance the production of beige adipocytes and expedite thermogenesis, leading to improved metabolic health. Although brown adipose tissue in rodents is primarily induced by β3-adrenergic receptor (ADRB3) stimulation, the activation of major β-adrenergic receptors (ADRBs) in pigs has been a topic of debate. To address this, we developed overexpression vectors for ADRB1, ADRB2, and ADRB3 and silenced the expression of these receptors to observe their effects on the adipogenic differentiation stages of porcine preadipocytes. Our investigation revealed that cold stress triggers the transformation of subcutaneous white adipose tissue to beige adipose tissue in pigs by modulating adrenergic receptor levels. Meanwhile, we found that ADRB3 promotes the transformation of white adipocytes into beige adipocytes. Notably, ADRB3 enhances the expression of beige adipose tissue marker genes, consequently influencing cellular respiration and metabolism by regulating lipolysis and mitochondrial expression. Therefore, ADRB3 may serve as a pivotal gene in animal husbandry and contribute to the improvement of cold intolerance in piglets.

Increased cardiac PFK-2 protects against high-fat diet-induced cardiomyopathy and mediates beneficial systemic metabolic effects

A healthy heart adapts to changes in nutrient availability and energy demands. In metabolic diseases like type 2 diabetes (T2D), increased reliance on fatty acids for energy production contributes to mitochondrial dysfunction and cardiomyopathy. A principal regulator of cardiac metabolism is 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2), which is a central driver of glycolysis. We hypothesized that increasing PFK-2 activity could mitigate cardiac dysfunction induced by high-fat diet (HFD). Wild type (WT) and cardiac-specific transgenic mice expressing PFK-2 (GlycoHi) were fed a low fat or HFD for 16 weeks to induce metabolic dysfunction. Metabolic phenotypes were determined by measuring mitochondrial bioenergetics and performing targeted quantitative proteomic and metabolomic analysis. Increasing cardiac PFK-2 had beneficial effects on cardiac and mitochondrial function. Unexpectedly, GlycoHi mice also exhibited sex-dependent systemic protection from HFD, including increased glucose homeostasis. These findings support improving glycolysis via PFK-2 activity can mitigate mitochondrial and functional changes that occur with metabolic syndrome.

Animal Models of Dysregulated Cardiac Metabolism

As a muscular pump that contracts incessantly throughout life, the heart must constantly generate cellular energy to support contractile function and fuel ionic pumps to maintain electrical homeostasis. Thus, mitochondrial metabolism of multiple metabolic substrates such as fatty acids, glucose, ketones, and lactate is essential to ensuring an uninterrupted supply of ATP. Multiple metabolic pathways converge to maintain myocardial energy homeostasis. The regulation of these cardiac metabolic pathways has been intensely studied for many decades. Rapid adaptation of these pathways is essential for mediating the myocardial adaptation to stress, and dysregulation of these pathways contributes to myocardial pathophysiology as occurs in heart failure and in metabolic disorders such as diabetes. The regulation of these pathways reflects the complex interactions of cell-specific regulatory pathways, neurohumoral signals, and changes in substrate availability in the circulation. Significant advances have been made in the ability to study metabolic regulation in the heart, and animal models have played a central role in contributing to this knowledge. This review will summarize metabolic pathways in the heart and describe their contribution to maintaining myocardial contractile function in health and disease. The review will summarize lessons learned from animal models with altered systemic metabolism and those in which specific metabolic regulatory pathways have been genetically altered within the heart. The relationship between intrinsic and extrinsic regulators of cardiac metabolism and the pathophysiology of heart failure and how these have been informed by animal models will be discussed.

A Cardiac Amino-Terminal GRK2 Peptide Inhibits Maladaptive Adipocyte Hypertrophy and Insulin Resistance During Diet-Induced Obesity

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Heart disease remains the leading cause of death, in part due to increasing diabetes and obesity, though the exact mechanisms linking these disorders are not fully understood.

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In a diet-induced obesity model, we found that cardiac expression of an amino-terminal peptide of GRK2, βARKnt, preserves systemic glucose tolerance and insulin sensitivity despite normal weight gain.

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βARKnt enhanced metabolic flexibility, increased energy expenditure, protected against maladaptive visceral adipocyte hypertrophy, and induced visceral fat browning.

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βARKnt further elicited cardioprotection and increased insulin-mediated AS160 signaling during metabolic stress.

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These data point to a noncanonical cardiac regulation of systemic metabolic homeostasis that may lead to new treatment modalities for metabolic syndrome.

Heart disease remains the leading cause of death, and mortality rates positively correlate with the presence of obesity and diabetes. Despite the correlation between cardiac and metabolic dysregulation, the mechanistic pathway(s) of interorgan crosstalk still remain undefined. This study reveals that cardiac-restricted expression of an amino-terminal peptide of GRK2 (βARKnt) preserves systemic and cardiac insulin responsiveness, and protects against adipocyte maladaptive hypertrophy in a diet-induced obesity model. These data suggest a cardiac-driven mechanism to ameliorate maladaptive cardiac remodeling and improve systemic metabolic homeostasis that may lead to new treatment modalities for cardioprotection in obesity and obesity-related metabolic syndromes.

The RAF Kinase Inhibitor Protein (RKIP): Good as Tumour Suppressor, Bad for the Heart

The RAF kinase inhibitor protein, RKIP, is a dual inhibitor of the RAF1 kinase and the G protein-coupled receptor kinase 2, GRK2. By inhibition of the RAF1-MAPK (mitogen-activated protein kinase) pathway, RKIP acts as a beneficial tumour suppressor. By inhibition of GRK2, RKIP counteracts GRK2-mediated desensitisation of G protein-coupled receptor (GPCR) signalling. GRK2 inhibition is considered to be cardioprotective under conditions of exaggerated GRK2 activity such as heart failure. However, cardioprotective GRK2 inhibition and pro-survival RAF1-MAPK pathway inhibition counteract each other, because inhibition of the pro-survival RAF1-MAPK cascade is detrimental for the heart. Therefore, the question arises, what is the net effect of these apparently divergent functions of RKIP in vivo? The available data show that, on one hand, GRK2 inhibition promotes cardioprotective signalling in isolated cardiomyocytes. On the other hand, inhibition of the pro-survival RAF1-MAPK pathway by RKIP deteriorates cardiomyocyte viability. In agreement with cardiotoxic effects, endogenous RKIP promotes cardiac fibrosis under conditions of cardiac stress, and transgenic RKIP induces heart dysfunction. Supported by next-generation sequencing (NGS) data of the RKIP-induced cardiac transcriptome, this review provides an overview of different RKIP functions and explains how beneficial GRK2 inhibition can go awry by RAF1-MAPK pathway inhibition. Based on RKIP studies, requirements for the development of a cardioprotective GRK2 inhibitor are deduced.

Sympatho-adrenergic mechanisms in heart failure: new insights into pathophysiology

The sympathetic nervous system is activated in the setting of heart failure (HF) to compensate for hemodynamic instability. However, acute sympathetic surge or sustained high neuronal firing rates activates β-adrenergic receptor (βAR) signaling contributing to myocardial remodeling, dysfunction and electrical instability. Thus, sympatho-βAR activation is regarded as a hallmark of HF and forms pathophysiological basis for β-blocking therapy. Building upon earlier research findings, studies conducted in the recent decades have significantly advanced our understanding on the sympatho-adrenergic mechanism in HF, which forms the focus of this article. This review notes recent research progress regarding the roles of cardiac β2AR or α1AR in the failing heart, significance of β1AR-autoantibodies, and βAR signaling through G-protein independent signaling pathways. Sympatho-βAR regulation of immune cells or fibroblasts is specifically discussed. On the neuronal aspects, knowledge is assembled on the remodeling of sympathetic nerves of the failing heart, regulation by presynaptic α2AR of NE release, and findings on device-based neuromodulation of the sympathetic nervous system. The review ends with highlighting areas where significant knowledge gaps exist but hold promise for new breakthroughs.

GRK2 contributes to glucose mediated calcium responses and insulin secretion in pancreatic islet cells

Diabetes is a metabolic syndrome rooted in impaired insulin and/or glucagon secretory responses within the pancreatic islets of Langerhans (islets). Insulin secretion is primarily regulated by two key factors: glucose-mediated ATP production and G-protein coupled receptors (GPCRs) signaling. GPCR kinase 2 (GRK2), a key regulator of GPCRs, is reported to be downregulated in the pancreas of spontaneously obesogenic and diabetogenic mice (ob/ob). Moreover, recent studies have shown that GRK2 non-canonically localizes to the cardiac mitochondrion, where it can contribute to glucose metabolism. Thus, islet GRK2 may impact insulin secretion through either mechanism. Utilizing Min6 cells, a pancreatic ß-cell model, we knocked down GRK2 and measured glucose-mediated intracellular calcium responses and insulin secretion. Silencing of GRK2 attenuated calcium responses, which were rescued by pertussis toxin pre-treatment, suggesting a Gαi/o-dependent mechanism. Pancreatic deletion of GRK2 in mice resulted in glucose intolerance with diminished insulin secretion. These differences were due to diminished insulin release rather than decreased insulin content or gross differences in islet architecture. Furthermore, a high fat diet feeding regimen exacerbated the metabolic phenotype in this model. These results suggest a new role for pancreatic islet GRK2 in glucose-mediated insulin responses that is relevant to type 2 diabetes disease progression.

Why Do We Not Assess Sympathetic Nervous System Activity in Heart Failure Management: Might GRK2 Serve as a New Biomarker?

Heart failure (HF) represents the end-stage condition of several structural and functional cardiovascular diseases, characterized by reduced myocardial pump function and increased pressure load. The dysregulation of neurohormonal systems, especially the hyperactivity of the cardiac adrenergic nervous system (ANS), constitutes a hallmark of HF and exerts a pivotal role in its progression. Indeed, it negatively affects patients’ prognosis, being associated with high morbidity and mortality rates, with a tremendous burden on global healthcare systems. To date, all the techniques proposed to assess the cardiac sympathetic nervous system are burdened by intrinsic limits that hinder their implementation in clinical practice. Several biomarkers related to ANS activity, which may potentially support the clinical management of such a complex syndrome, are slow to be implemented in the routine practice for several limitations due to their assessment and clinical impact. Lymphocyte G-protein-coupled Receptor Kinase 2 (GRK2) levels reflect myocardial β-adrenergic receptor function in HF and have been shown to add independent prognostic information related to ANS overdrive. In the present manuscript, we provide an overview of the techniques currently available to evaluate cardiac ANS in HF and future perspectives in this field of relevant scientific and clinical interest.

A peptide of the amino-terminus of GRK2 induces hypertrophy and yet elicits cardioprotection after pressure overload

G protein-coupled receptor (GPCR) kinase 2 (GRK2) expression and activity are elevated early on in response to several forms of cardiovascular stress and are a hallmark of heart failure. Interestingly, though, in addition to its well-characterized role in regulating GPCRs, mounting evidence suggests a GRK2 "interactome" that underlies a great diversity in its functional roles. Several such GRK2 interacting partners are important for adaptive and maladaptive myocyte growth; therefore, an understanding of domain-specific interactions with signaling and regulatory molecules could lead to novel targets for heart failure therapy. Herein, we subjected transgenic mice with cardiac restricted expression of a short, amino terminal fragment of GRK2 (βARKnt) to pressure overload and found that unlike their littermate controls or previous GRK2 fragments, they exhibited an increased left ventricular wall thickness and mass prior to cardiac stress that underwent proportional hypertrophic growth to controls after acute pressure overload. Importantly, despite this enlarged heart, βARKnt mice did not undergo the expected transition to heart failure observed in controls. Further, βARKnt expression limited adverse left ventricular remodeling and increased cell survival signaling. Proteomic analysis to identify βARKnt binding partners that may underlie the improved cardiovascular phenotype uncovered a selective functional interaction of both endogenous GRK2 and βARKnt with AKT substrate of 160 kDa (AS160). AS160 has emerged as a key downstream regulator of insulin signaling, integrating physiological and metabolic cues to couple energy demand to membrane recruitment of Glut4. Our preliminary data indicate that in βARKnt mice, cardiomyocyte insulin signaling is improved during stress, with a coordinate increase in spare respiratory activity and ATP production without metabolite switching. Surprisingly, these studies also revealed a significant decrease in gonadal fat weight, equivalent to human abdominal fat, in male βARKnt mice at baseline and following cardiac stress. These data suggest that the enhanced AS160-mediated signaling in the βARKnt mice may ameliorate pathological cardiac remodeling through direct modulation of insulin signaling within cardiomyocytes, and translate these to beneficial effects on systemic metabolism.

Cardiac Endocrinology: Heart-Derived Hormones in Physiology and Disease

The heart plays a central role in the circulatory system and provides essential oxygen, nutrients, and growth factors to the whole organism. The heart can synthesize and secrete endocrine signals to communicate with distant target organs. Studies of long-known and recently discovered heart-derived hormones highlight a shared theme and reveal a unified mechanism of heart-derived hormones in coordinating cardiac function and target organ biology. This paper reviews the biochemistry, signaling, function, regulation, and clinical significance of representative heart-derived hormones, with a focus on the cardiovascular system. This review also discusses important and exciting questions that will advance the field of cardiac endocrinology.

Deletion of Cardiomyocyte Glycogen Synthase Kinase-3 Beta (GSK-3β) Improves Systemic Glucose Tolerance with Maintained Heart Function in Established Obesity

Obesity is an independent risk factor for cardiovascular diseases (CVD), including heart failure. Thus, there is an urgent need to understand the molecular mechanism of obesity-associated cardiac dysfunction. We recently reported the critical role of cardiomyocyte (CM) Glycogen Synthase Kinase-3 beta (GSK-3β) in cardiac dysfunction associated with a developing obesity model (deletion of CM-GSK-3β prior to obesity). In the present study, we investigated the role of CM-GSK-3β in a clinically more relevant model of established obesity (deletion of CM-GSK-3β after established obesity). CM-GSK-3β knockout (GSK-3β^fl/flCre+/-) and controls (GSK-3β^fl/flCre-/-) mice were subjected to a high-fat diet (HFD) in order to establish obesity. After 12 weeks of HFD treatment, all mice received tamoxifen injections for five consecutive days to delete GSK-3β specifically in CMs and continued on the HFD for a total period of 55 weeks. To our complete surprise, CM-GSK-3β knockout (KO) animals exhibited a globally improved glucose tolerance and maintained normal cardiac function. Mechanistically, in stark contrast to the developing obesity model, deleting CM-GSK-3β in obese animals did not adversely affect the GSK-3αS21 phosphorylation (activity) and maintained canonical β-catenin degradation pathway and cardiac function. As several GSK-3 inhibitors are in the trial to treat various chronic conditions, including metabolic diseases, these findings have important clinical implications. Specifically, our results provide critical pre-clinical data regarding the safety of GSK-3 inhibition in obese patients.

Metabolic Crosstalk between the Heart and Fat

It is now recognized that the heart can behave as a true endocrine organ, which can modulate the function of other tissues. Emerging evidence has shown that visceral fat is one such distant organ the heart communicates with. In fact, it appears that bi-directional crosstalk between adipose tissue and the myocardium is crucial to maintenance of normal function in both organs. In particular, factors secreted from the heart are now known to influence the metabolic activity of adipose tissue and other organs, as well as modulate the release of metabolic substrates and signaling molecules from the periphery. This review summarizes current knowledge regarding primary cardiokines and adipokines involved in heart-fat crosstalk, as well as implications of their dysregulation for cardiovascular health.