Restoration of glucose metabolism in leptin-resistant mouse hearts after acute myocardial infarction through the activation of survival kinase pathways

Association of attenuated leptin signaling pathways with impaired cardiac function under prolonged high-altitude hypoxia

Cardiovascular function and adipose metabolism were markedly influenced under high altitudes. However, the interplay between adipokines and heart under hypoxia remains to be elucidated. We aim to explore alterations of adipokines and underlying mechanisms in regulating cardiac function under high altitudes. We investigated the cardiopulmonary function and five adipokines in Antarctic expeditioners at Kunlun Station (4,087 m) for 20 days and established rats exposed to hypobaric hypoxia (5,000 m), simulating Kunlun Station. Antarctic expeditioners exhibited elevated heart rate, blood pressure, systemic vascular resistance, and decreased cardiac pumping function. Plasma creatine phosphokinase-MB (CK-MB) and platelet-endothelial cell adhesion molecule-1 (sPecam-1) increased, and leptin, resistin, and lipocalin-2 decreased. Plasma leptin significantly correlated with altered cardiac function indicators. Additionally, hypoxic rats manifested impaired left ventricular systolic and diastolic function, elevated plasma CK-MB and sPecam-1, and decreased plasma leptin. Chronic hypoxia for 14 days led to increased myocyte hypertrophy, fibrosis, apoptosis, and mitochondrial dysfunction, coupled with reduced protein levels of leptin signaling pathways in myocardial tissues. Cardiac transcriptome analysis revealed leptin was associated with downregulated genes involved in rhythm, Na+/K+ transport, and cell skeleton. In conclusion, chronic hypoxia significantly reduced leptin signaling pathways in cardiac tissues along with significant pathological changes, thus highlighting the pivotal role of leptin in regulation of cardiac function under high altitudes.

Anti-Ischemic Effect of Leptin in the Isolated Rat Heart Subjected to Global Ischemia-Reperfusion: Role of Cardiac-Specific miRNAs

Background: Leptin is an obesity-associated adipokine that has been implicated in cardiac protection against ischemia-reperfusion injury (IRI). In this study, concentration-dependent effects of leptin on myocardial IRI were investigated in the isolated rat heart. In addition, we analyzed myocardial miRNAs expression in order to investigate their potential involvement in leptin-mediated cardioprotection. Methods: The effect of leptin on IRI was examined in Langendorff-perfused rat hearts preconditioned with two leptin concentrations (1.0 nM and 3.1 nM) for 60 min. The hearts were subjected to 30 min global ischemia and 120 min reperfusion with buffer containing leptin in the respective concentration. Heart function and arrhythmia incidence were analyzed. Infarct size was assessed histochemically. Expression of miRNA-144, -208a, -378, and -499 was analyzed in the ventricular myocardium using RT-PCR. Results: The addition of 1.0 nM leptin to the buffer exerted an infarct-limiting effect, preserved post-ischemic ventricular function, and prevented reperfusion arrhythmia compared to 3.1 nM leptin. Myocardial expression of miRNA-208a was decreased after heart exposure to 1.0 nM leptin and significantly elevated in the hearts perfused with leptin at 3.1 nM. Conclusion: Acute administration of leptin at low dose (1.0 nM) results in cardiac protection against IRI. This effect is associated with reduced myocardial expression of miRNA-208a.

Crosstalk Between Adipose Tissue and the Heart: An Update

It is important to understand how different human organs coordinate and interact with each other. Since obesity and cardiac disease frequently coincide, the crosstalk between adipose tissues and heart has drawn attention. We appreciate that specific peptides/proteins, lipids, nucleic acids, and even organelles shuttle between the adipose tissues and heart. These bioactive components can profoundly affect the metabolism of cells in distal organs, including heart. Importantly, this process can be dysregulated under pathophysiological conditions. This also opens the door to efforts targeting these mediators as potential therapeutic strategies to treat patients who manifest diabetes and cardiovascular disease. Here, we summarize the recent progress toward a better understanding of how the adipose tissues and heart interact with each other.

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Emerging Role of Adipocyte Dysfunction in Inducing Heart Failure Among Obese Patients With Prediabetes and Known Diabetes Mellitus

Adipose tissue dysfunction is a predictor for cardiovascular (CV) events and heart failure (HF) in patient population with obesity, metabolic syndrome, and known type 2 diabetes mellitus. Previous preclinical and clinical studies have yielded controversial findings regarding the role of accumulation of adipose tissue various types in CV risk and HF-related clinical outcomes in obese patients. There is evidence for direct impact of infiltration of epicardial adipocytes into the underlying myocardium to induce adverse cardiac remodeling and mediate HF development and atrial fibrillation. Additionally, perivascular adipocytes accumulation is responsible for release of proinflammatory adipocytokines (adiponectin, leptin, resistin), stimulation of oxidative stress, macrophage phenotype switching, and worsening vascular reparation, which all lead to microvascular inflammation, endothelial dysfunction, atherosclerosis acceleration, and finally to increase in CV mortality. However, systemic effects of white and brown adipose tissue can be different, and adipogenesis including browning of adipose tissue and deficiency of anti-inflammatory adipocytokines (visfatin, omentin, zinc-α2-glycoprotein, glypican-4) was frequently associated with adipose triglyceride lipase augmentation, altered glucose homeostasis, resistance to insulin of skeletal muscles, increased cardiomyocyte apoptosis, lowered survival, and weak function of progenitor endothelial cells, which could significantly influence on HF development, as well as end-organ fibrosis and multiple comorbidities. The exact underlying mechanisms for these effects are not fully understood, while they are essential to help develop improved treatment strategies. The aim of the review is to summarize the evidence showing that adipocyte dysfunction may induce the onset of HF and support advance of HF through different biological mechanisms involving inflammation, pericardial, and perivascular adipose tissue accumulation, adverse and electrical cardiac remodeling, and skeletal muscle dysfunction. The unbalancing effects of natriuretic peptides, neprilysin, and components of renin–angiotensin system, as exacerbating cause of altered adipocytokine signaling on myocardium and vasculature, in obesity patients at high risk of HF are disputed. The profile of proinflammatory and anti-inflammatory adipocytokines as promising biomarker for HF risk stratification is discussed in the review.

Role of Leptin in Cardiovascular Diseases

The adipocyte-derived adipokine leptin exerts pleiotropic effects, which are essential for the regulation of energy balance and cell metabolism, for controlling inflammatory and immune responses, and for the maintenance of homeostasis of the cardiovascular system. Leptin resistance in obese or type 2 diabetes mellitus (T2DM) patients is defined as a decrease in tissue response to leptin. In the cardiovascular system, leptin resistance exhibits the adverse effect on the heart's response to stress conditions and promoting cardiac remodeling due to impaired cardiac metabolism, increased fibrosis, vascular dysfunction, and enhanced inflammation. Leptin resistance or leptin signaling deficiency results in the risk increase of cardiac dysfunction and heart failure, which is a leading cause of obesity- and T2DM-related morbidity and mortality. Animal studies using leptin- and leptin receptor- (Lepr) deficient rodents have provided many useful insights into the underlying molecular and pathophysiological mechanisms of obese- and T2DM-associated metabolic and cardiovascular diseases. However, none of the animal models used so far can fully recapitulate the phenotypes of patients with obese or T2DM. Therefore, the role of leptin in the human cardiovascular system, and whether leptin affects cardiac function directly or acts through a leptin-regulated neurohumoral pathway, remain elusive. As the prevalence of obesity and diabetes is continuously increasing, strategies are needed to develop and apply human cell-based models to better understand the precise role of leptin directly in different cardiac cell types and to overcome the existing translational barriers. The purpose of this review is to discuss the mechanisms associated with leptin signaling deficiency or leptin resistance in the development of metabolic and cardiovascular diseases. We analyzed and comprehensively addressed substantial findings in pathophysiological mechanisms in commonly used leptin- or Lepr-deficient rodent models and highlighted the differences between rodents and humans. This may open up new strategies to develop directly and reliably applicable models, which resemble the human pathophysiology in order to advance health care management of obesity- and T2DM-related cardiovascular complications.

Leptin increases mitochondrial OPA1 via GSK3-mediated OMA1 ubiquitination to enhance therapeutic effects of mesenchymal stem cell transplantation

Accumulating evidence revealed that mesenchymal stem cells (MSCs) confer cardioprotection against myocardial infarction (MI). However, the poor survival and engraftment rate of the transplanted cells limited their therapeutic efficacy in the heart. The enhanced leptin production associated with hypoxia preconditioning contributed to the improved MSCs survival. Mitochondrial integrity determines the cellular fate. Thus, we aimed to investigate whether leptin can enhance mitochondrial integrity of human MSCs (hMSCs) to protect against various stress. In vivo, transplantation of leptin-overexpressing hMSCs into the infarcted heart resulted in improved cell viability, leading to enhanced angiogenesis and cardiac function. In vitro, pretreatment of hMSCs with recombinant leptin (hMSCs-Lep^pre) displayed improved cell survival against severe ischemic condition (glucose and serum deprivation under hypoxia), which was associated with increased mitochondrial fusion. Subsequently, Optic atrophy 1 (OPA1), a mitochondrial inner membrane protein that regulates fusion and cristae structure, was significantly elevated in the hMSCs-Lep^pre group, and the protection of leptin was abrogated by targeting OPA1 with a selective siRNA. Furthermore, OMA1, a mitochondrial protease that cleaves OPA1, decreased in a leptin-dependent manner. Pretreatment of cells with an inhibitor of the proteasome (MG132), prevented leptin-induced OMA1 degradation, implicating the ubiquitination/proteasome system as a part of the protective leptin pathway. In addition, GSK3 inhibitor (SB216763) was also involved in the degradation of OMA1. In conclusion, in the hostile microenvironment caused by MI, (a) leptin can maintain the mitochondrial integrity and prolong the survival of hMSCs; (b) leptin-mediated mitochondrial integrity requires phosphorylation of GSK3 as a prerequisite for ubiquitination-depended degradation of OMA1 and attenuation of long-OPA1 cleavage. Thus, leptin targeting the GSK3/OMA1/OPA1 signaling pathway can optimize hMSCs therapy for cardiovascular diseases such as MI.

RNA SEQ Analysis Indicates that the AE3 Cl-/HCO3- Exchanger Contributes to Active Transport-Mediated CO2 Disposal in Heart

Loss of the AE3 Cl⁻/HCO₃⁻ exchanger (Slc4a3) in mice causes an impaired cardiac force-frequency response and heart failure under some conditions but the mechanisms are not known. To better understand the functions of AE3, we performed RNA Seq analysis of AE3-null and wild-type mouse hearts and evaluated the data with respect to three hypotheses (CO₂ disposal, facilitation of Na⁺-loading, and recovery from an alkaline load) that have been proposed for its physiological functions. Gene Ontology and PubMatrix analyses of differentially expressed genes revealed a hypoxia response and changes in vasodilation and angiogenesis genes that strongly support the CO₂ disposal hypothesis. Differential expression of energy metabolism genes, which indicated increased glucose utilization and decreased fatty acid utilization, were consistent with adaptive responses to perturbations of O₂/CO₂ balance in AE3-null myocytes. Given that the myocardium is an obligate aerobic tissue and consumes large amounts of O₂, the data suggest that loss of AE3, which has the potential to extrude CO₂ in the form of HCO₃⁻, impairs O₂/CO₂ balance in cardiac myocytes. These results support a model in which the AE3 Cl⁻/HCO₃⁻ exchanger, coupled with parallel Cl⁻ and H⁺-extrusion mechanisms and extracellular carbonic anhydrase, is responsible for active transport-mediated disposal of CO₂.

Complex Energy Metabolic Changes in Heart Failure With Preserved Ejection Fraction and Heart Failure With Reduced Ejection Fraction

Alterations in cardiac energy metabolism contribute to the severity of heart failure. However, the energy metabolic changes that occur in heart failure are complex, and are dependent not only on the severity and type of heart failure present, but also on the coexistence of common comorbidities such as obesity and type 2 diabetes. In this article we review the cardiac energy metabolic changes that occur in heart failure. An emphasis is made on distinguishing the differences in cardiac energy metabolism between heart failure with preserved ejection fraction (HFpEF) and heart failure with reduced ejection fraction (HFrEF) and in clarifying the common misconceptions surrounding the fate of fatty acids and glucose in the failing heart. The major key points from this article are: (1) mitochondrial oxidative capacity is reduced in HFpEF and HFrEF; (2) fatty acid oxidation is increased in HFpEF and reduced in HFrEF (however, oxidative metabolism of fatty acids in HFrEF still exceeds that of glucose); (3) glucose oxidation is decreased in HFpEF and HFrEF; (4) there is an uncoupling between glucose uptake and oxidation in HFpEF and HFrEF, resulting in an increased rate of glycolysis; (5) ketone body oxidation is increased in HFrEF, which might further reduce fatty acid and glucose oxidation; and finally, (6) branched chain amino acid oxidation is impaired in HFrEF. The understanding of these changes in cardiac energy metabolism in heart failure are essential to allow the development of metabolic modulators in the treatment of heart failure.

Lean heart: Role of leptin in cardiac hypertrophy and metabolism

Leptin is an adipokine that has been linked with the cardiovascular complications resulting from obesity such as hypertension and heart disease. Obese patients have high levels of circulating leptin due to increased fat mass. Clinical and population studies have correlated high levels of circulating leptin with the development of cardiac hypertrophy in obesity. Leptin has also been demonstrated to increase the growth of cultured cardiomyocytes. However, several animal studies of obese leptin deficient mice have not supported a role for leptin in promoting cardiac hypertrophy so the role of leptin in this pathological process remains unclear. Leptin is also an important hormone in the regulation of cardiac metabolism where it supports oxidation of glucose and fatty acids. In addition, leptin plays a critical role in protecting the heart from excess lipid accumulation and the formation of toxic lipids in obesity a condition known as cardiac lipotoxicity. This paper focuses on the data supporting and refuting leptin’s role in promoting cardiac hypertrophy as well as its important role in the regulation of cardiac metabolism and protection against cardiac lipotoxicity.

Association of serum leptin with future left ventricular structure and function: The Multi-Ethnic Study of Atherosclerosis (MESA)

BACKGROUND/OBJECTIVES

Earlier studies differ on whether serum leptin is associated with adverse or beneficial cardiac structure. We determined the association between serum leptin with subsequent cardiac structure and function.

METHODS

MESA is a multicenter longitudinal study of Black, White, Hispanic and Asian-American men and women. Cardiac MRI (CMR) was completed 6 to 8 years after leptin was measured. Left ventricular (LV) mass and volumes were indexed to body surface area. Multivariable linear regression models were constructed to assess the associations between leptin and risk factor adjusted (age, race, gender, systolic blood pressure, anti-hypertensive usage, LDL, HDL, hyperlipidemia medication usage, diabetes, diabetic medication usage, chronic kidney disease, alcohol and tobacco use, adiponectin and BMI) CMR variables.

RESULTS

Relative to participants in the lowest quintile of leptin concentration, participants in the highest quintile had a lower risk factor adjusted LV mass (-14 g), LV mass index (-9 g/m(2)), LV end diastolic volume index (LVEDVi) (-7 ml/m(2)), LV end systolic volume index (LVESVi) (-3 ml/m(2)) and stroke volume (-5 ml) (all p≤0.05). On regression analysis, a doubling of leptin concentration was associated with lower LV mass (-2.5 g ± 0.7 g), LV mass index (-1.7 ± 0.3g/m(2)), LVEDVi (-1.5 ± 0.4 ml/m(2)), LVESVi (-0.7 ± 0.2 ml/m(2)) and stroke volume (-1.0 ± 0.5 ml) (all p ≤ 0.05).

CONCLUSIONS

Higher leptin was associated with more favorable subsequent cardiac structure. Further study is needed to assess the prognostic and therapeutic implications of these observations.

Cardiomyocyte-specific deletion of leptin receptors causes lethal heart failure in Cre-recombinase-mediated cardiotoxicity

Although disruption of leptin signaling is associated with obesity as well as cardiac lipid accumulation and dysfunction, it has been difficult to separate the direct effects of leptin on the heart from those associated with the effects of leptin on body weight and fat mass. Using Cre-loxP recombinase technology, we developed tamoxifen-inducible, cardiomyocyte-specific leptin receptor-deficient mice to assess the role of leptin in regulating cardiac function. Cre recombinase activation in the heart resulted in transient reduction in left ventricular systolic function which recovered to normal levels by day 10. However, when cardiomyocyte leptin receptors were deleted in the setting of Cre recombinase-induced left ventricular dysfunction, irreversible lethal heart failure was observed in less than 10 days in all mice. Heart failure after leptin receptor deletion was associated with marked decreases of cardiac mitochondrial ATP, phosphorylated mammalian target of rapamycin (mTOR), and AMP-activated kinase (pAMPK). Our results demonstrate that specific deletion of cardiomyocyte leptin receptors, in the presence of increased Cre recombinase expression, causes lethal heart failure associated with decreased cardiac energy production. These observations indicate that leptin plays an important role in regulating cardiac function in the setting of cardiac stress caused by Cre-recombinase expression, likely through actions on cardiomyocyte energy metabolism.