Overexpression of AMP-activated protein kinase or protein kinase D prevents lipid-induced insulin resistance in cardiomyocytes

Insulin signaling in the heart

Insulin receptors are highly expressed in the heart and vasculature. Insulin signaling regulates cardiac growth, survival, substrate uptake, utilization, and mitochondrial metabolism. Insulin signaling modulates the cardiac responses to physiological and pathological stressors. Altered insulin signaling in the heart may contribute to the pathophysiology of ventricular remodeling and heart failure progression. Myocardial insulin signaling adapts rapidly to changes in the systemic metabolic milieu. What may initially represent an adaptation to protect the heart from carbotoxicity may contribute to amplifying the risk of heart failure in obesity and diabetes. This review article presents the multiple roles of insulin signaling in cardiac physiology and pathology and discusses the potential therapeutic consequences of modulating myocardial insulin signaling.

Phosphatidylinositol 4-kinase IIIβ mediates contraction-induced GLUT4 translocation and shows its anti-diabetic action in cardiomyocytes

In the diabetic heart, long-chain fatty acid (LCFA) uptake is increased at the expense of glucose uptake. This metabolic shift ultimately leads to insulin resistance and a reduced cardiac function. Therefore, signaling kinases that mediate glucose uptake without simultaneously stimulating LCFA uptake could be considered attractive anti-diabetic targets. Phosphatidylinositol-4-kinase-IIIβ (PI4KIIIβ) is a lipid kinase downstream of protein kinase D1 (PKD1) that mediates Golgi-to-plasma membrane vesicular trafficking in HeLa-cells. In this study, we evaluated whether PI4KIIIβ is involved in myocellular GLUT4 translocation induced by contraction or oligomycin (an F₁F₀-ATP synthase inhibitor that activates contraction-like signaling). Pharmacological targeting, with compound MI14, or genetic silencing of PI4KIIIβ inhibited contraction/oligomycin-stimulated GLUT4 translocation and glucose uptake in cardiomyocytes but did not affect CD36 translocation nor LCFA uptake. Addition of the PI4KIIIβ enzymatic reaction product phosphatidylinositol-4-phosphate restored oligomycin-stimulated glucose uptake in the presence of MI14. PI4KIIIβ activation by PKD1 involves Ser294 phosphorylation and altered its localization with unchanged enzymatic activity. Adenoviral PI4KIIIβ overexpression stimulated glucose uptake, but did not activate hypertrophic signaling, indicating that unlike PKD1, PI4KIIIβ is selectively involved in GLUT4 translocation. Finally, PI4KIIIβ overexpression prevented insulin resistance and contractile dysfunction in lipid-overexposed cardiomyocytes. Together, our studies identify PI4KIIIβ as positive and selective regulator of GLUT4 translocation in response to contraction-like signaling, suggesting PI4KIIIβ as a promising target to rescue defective glucose uptake in diabetics.

Understanding the distinct subcellular trafficking of CD36 and GLUT4 during the development of myocardial insulin resistance

CD36 and GLUT4 are the main cardiac trans-sarcolemmal transporters for long-chain fatty acids and glucose, respectively. Together they secure the majority of cardiac energy demands. Moreover, these transporters each represent key governing kinetic steps in cardiac fatty acid and glucose fluxes, thereby offering major sites of regulation. The underlying mechanism of this regulation involves a perpetual vesicle-mediated trafficking (recycling) of both transporters between intracellular stores (endosomes) and the cell surface. In the healthy heart, CD36 and GLUT4 translocation to the cell surface is under short-term control of the same physiological stimuli, most notably increased contraction and insulin secretion. However, under chronic lipid overload, a condition that accompanies a Western lifestyle, CD36 and GLUT4 recycling are affected distinctly, with CD36 being expelled to the sarcolemma while GLUT4 is imprisoned within the endosomes. Moreover, the increased CD36 translocation towards the cell surface is a key early step, setting the heart on a route towards insulin resistance and subsequent contractile dysfunction. Therefore, the proteins making up the trafficking machinery of CD36 need to be identified with special focus to the differences with the protein composition of the GLUT4 trafficking machinery. These proteins that are uniquely dedicated to either CD36 or GLUT4 traffic may offer targets to rectify aberrant substrate uptake seen in the lipid-overloaded heart. Specifically, CD36-dedicated trafficking regulators should be inhibited, whereas such GLUT4-dedicated proteins would need to be activated. Recent advances in the identification of CD36-dedicated trafficking proteins have disclosed the involvement of vacuolar-type H⁺-ATPase and of specific vesicle-associated membrane proteins (VAMPs). In this review, we summarize these recent findings and sketch a roadmap of CD36 and GLUT4 trafficking compatible with experimental findings.

Augmenting Vacuolar H+-ATPase Function Prevents Cardiomyocytes from Lipid-Overload Induced Dysfunction

The diabetic heart is characterized by a shift in substrate utilization from glucose to lipids, which may ultimately lead to contractile dysfunction. This substrate shift is facilitated by increased translocation of lipid transporter CD36 (SR-B2) from endosomes to the sarcolemma resulting in increased lipid uptake. We previously showed that endosomal retention of CD36 is dependent on the proper functioning of vacuolar H⁺-ATPase (v-ATPase). Excess lipids trigger CD36 translocation through inhibition of v-ATPase function. Conversely, in yeast, glucose availability is known to enhance v-ATPase function, allowing us to hypothesize that glucose availability, via v-ATPase, may internalize CD36 and restore contractile function in lipid-overloaded cardiomyocytes. Increased glucose availability was achieved through (a) high glucose (25 mM) addition to the culture medium or (b) adenoviral overexpression of protein kinase-D1 (a kinase mediating GLUT4 translocation). In HL-1 cardiomyocytes, adult rat and human cardiomyocytes cultured under high-lipid conditions, each treatment stimulated v-ATPase re-assembly, endosomal acidification, endosomal CD36 retention and prevented myocellular lipid accumulation. Additionally, these treatments preserved insulin-stimulated GLUT4 translocation and glucose uptake as well as contractile force. The present findings reveal v-ATPase functions as a key regulator of cardiomyocyte substrate preference and as a novel potential treatment approach for the diabetic heart.

Chronic AICAR treatment prevents metabolic changes in cardiomyocytes exposed to free fatty acids

The stimulation of glucose transport by metabolic stress is an important determinant of myocardial susceptibility to ischemia and reperfusion injury. Stimulation of glucose transport is markedly impaired in cardiomyocytes chronically exposed to excess free fatty acids (FFA), as occurs in vivo in type 2 diabetes. To determine whether chronic low-grade activation of AMP-activated kinase (AMPK) improves substrate metabolism in cardiomyocytes exposed to FFA, isolated cultured cardiomyocytes were exposed for 7 days to FFA ± the AMPK agonist 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR). Glucose transport and glycolysis were then measured during acute metabolic stress provoked by oligomycin. Chronic treatment with AICAR improved basal and oligomycin-stimulated glucose transport in FFA-exposed but not in control cardiomyocytes. Similarly, basal and oligomycin-stimulated glycolysis was reduced in FFA-exposed cardiomyocytes but restored by chronic AICAR treatment. Conversely, fatty acid oxidation was increased in FFA-exposed cardiomyocytes and reduced by chronic AICAR treatment. Chronic AICAR treatment induced in FFA-exposed cardiomyocytes the biogenesis of numerous lipid droplets. Curiously, whereas acute treatment of cardiomyocytes with AICAR increased phosphorylation of the AMPKα subunit on T¹⁷², a classical marker of AMPK activation, chronic AICAR treatment almost completely obliterated T¹⁷² phosphorylation. However, phosphorylation of the AMPK target protein raptor on S⁷⁹² was reduced in FFA-exposed cardiomyocytes but restored by AICAR treatment. In conclusion, chronic AICAR treatment induces a metabolic shift in FFA-exposed cardiomyocytes, characterized by improved glucose transport and glycolysis and redirection of fatty acids towards neutral storage. Such metabolic changes in vivo could protect the hearts of patients with type 2 diabetes against ischemia-reperfusion injury.

Targeting viperin improves diet-induced glucose intolerance but not adipose tissue inflammation

Viperin is an interferon-inducible antiviral protein, responsible for antiviral response to a variety of viral infections. Here, we show that silencing viperin by antisense oligonucleotides (ASO) protects against diet-induced glucose intolerance, and yet exacerbates adipose tissue inflammation. In high-fat diet-fed mice, viperin ASO improves glucose homeostasis, reduces plasma triglyceride concentrations and ameliorates diet-induced hepatic steatosis. Peripheral delivery of viperin by adeno-associated virus elevates fasting plasma glucose and insulin concentrations and reduces insulin-stimulated glucose uptake in skeletal muscle. Viperin overexpression reduces epinephrine- stimulated lipolysis in white adipose tissue, whereas viperin ASO increases expression of lipolytic genes. Targeting viperin by antisense oligonucleotides promotes reciprocal regulation of hepatic and adipose lipogenesis by reducing hepatic lipid content and increasing triacylglycerol content in adipose tissue. These findings reveal viperin as an important target to improve glucose metabolism, and suggest that suppressing antiviral potential may improve the metabolic adaptability to high-fat diet.

Emergency Spatiotemporal Shift: The Response of Protein Kinase D to Stress Signals in the Cardiovascular System

Protein Kinase D isoforms (PKD 1-3) are key mediators of neurohormonal, oxidative, and metabolic stress signals. PKDs impact a wide variety of signaling pathways and cellular functions including actin dynamics, vesicle trafficking, cell motility, survival, contractility, energy substrate utilization, and gene transcription. PKD activity is also increasingly linked to cancer, immune regulation, pain modulation, memory, angiogenesis, and cardiovascular disease. This increasing complexity and diversity of PKD function, highlights the importance of tight spatiotemporal control of the kinase via protein–protein interactions, post-translational modifications or targeting via scaffolding proteins. In this review, we focus on the spatiotemporal regulation and effects of PKD signaling in response to neurohormonal, oxidant and metabolic signals that have implications for myocardial disease. Precise targeting of these mechanisms will be crucial in the design of PKD-based therapeutic strategies.

Signaling pathways involved in cardiac energy metabolism

Various signaling pathways coordinate energy metabolism and contractile function in the heart. Myocardial uptake of long-chain fatty acids largely occurs by facilitated diffusion, involving the membrane-associated protein, CD36. Glucose uptake, the rate-limiting step in glucose utilization, is mediated predominantly by the glucose transporter protein, GLUT4. Insulin and contraction-mediated AMPK signaling each are implicated in tightly regulating these myocardial 'gate-keepers' of energy balance, that is, CD36 and GLUT4. The insulin and AMPK signaling cascades are complex and their cross-talk is only beginning to be understood. Moreover, transcriptional regulation of the CD36 and GLUT4 is significantly understudied. This review focuses on recent advances on the role of these signaling pathways and transcription factors involved in the regulation of CD36 and GLUT4.

Fatty acid transport proteins in disease: New insights from invertebrate models

The dysregulation of lipid metabolism has been implicated in various diseases, including diabetes, cardiopathies, dermopathies, retinal and neurodegenerative diseases. Mouse models have provided insights into lipid metabolism. However, progress in the understanding of these pathologies is hampered by the multiplicity of essential cellular processes and genes that modulate lipid metabolism. Drosophila and Caenorhabditis elegans have emerged as simple genetic models to improve our understanding of these metabolic diseases. Recent studies have characterized fatty acid transport protein (fatp) mutants in Drosophila and C. elegans, establishing new models of cardiomyopathy, retinal degeneration, fat storage disease and dermopathies. These models have generated novel insights into the physiological role of the Fatp protein family in vivo in multicellular organisms, and are likely to contribute substantially to progress in understanding the etiology of various metabolic disorders. Here, we describe and discuss the mechanisms underlying invertebrate fatp mutant models in the light of the current knowledge relating to FATPs and lipid disorders in vertebrates.

Successful metabolic adaptations leading to the prevention of high fat diet-induced murine cardiac remodeling

Background

Cardiomyopathy is a devastating complication of obesity and type 2 diabetes mellitus (T2DM). It arises even in patients with normoglycemia (glycosylated hemoglobin, A1C ≤7 %). As obesity and T2DM are approaching epidemic levels worldwide, the cardiomyopathy associated with these diseases must be therapeutically addressed. We have recently analyzed the systemic effects of a 12-week high fat diet (HFD) on wild type mice from the C57Bl/6 (B6) strain and the wild type super-healing Murphy Roths Large (MRL) mouse strain. The MRL HFD mice gained significantly more weight than their control diet counterparts, but did not present any of the other usual systemic T2DM phenotypes.

Methods

Cardiac pathology and adaptation to HFD-induced obesity in the MRL mouse strain compared to the HFD C57Bl/6 mice were thoroughly analyzed with echocardiography, histology, qPCR, electron microscopy and immunoblots.

Results

The obese HFD C57Bl/6 mice develop cardiac hypertrophy, cardiomyocyte lipid droplets, and initiate an ineffective metabolic adaptation of an overall increase in electron transport chain complexes. In contrast, the obese HFD MRL hearts do not display hypertrophy nor lipid droplets and their metabolism adapts quite robustly by decreasing pAMPK levels, decreasing proteins in the carbohydrate metabolism pathway and increasing proteins utilized in the β-oxidation pathway. The result of these metabolic shifts is the reduction of toxic lipid deposits and reactive oxygen species in the hearts of the obese HFD fed MRL hearts.

Conclusions

We have identified changes in metabolic signaling in obese HFD fed MRL mice that confer resistance to diabetic cardiomyopathy. The changes include a reduction of cardiac pAMPK, Glut4 and hexokinase2 in the MRL HFD hearts. Overall the MRL hearts down regulate glucose metabolism and favor lipid metabolism. These adaptations are essential to pursue for the identification of novel therapeutic targets to combat obesity related cardiomyopathy.

Cardiac contraction-induced GLUT4 translocation requires dual signaling input

Contraction-induced translocation of glucose transporter type-4 (GLUT4) to the sarcolemma is essential to stimulate cardiac glucose uptake during increased energy demand. As such, this process is a target for therapeutic strategies aiming at increasing glucose uptake in insulin-resistant and/or diabetic hearts. AMP-activated protein kinase (AMPK) and its upstream kinases form part of a signaling axis essential for contraction-induced GLUT4 translocation. Recently, activation of protein kinase-D1 (PKD1) was also shown to be as obligatory for contraction-induced GLUT4 translocation in cardiac muscle. However, contraction-induced PKD1 activation in this context occurs independently from AMPK signaling, suggesting that contraction-induced GLUT4 translocation requires the input of two separate signaling pathways. Necessity for dual input would more tightly couple GLUT4 translocation to stimuli that are inherent to cardiac contraction.

The PKD inhibitor CID755673 enhances cardiac function in diabetic db/db mice

The development of diabetic cardiomyopathy is a key contributor to heart failure and mortality in obesity and type 2 diabetes (T2D). Current therapeutic interventions for T2D have limited impact on the development of diabetic cardiomyopathy. Clearly, new therapies are urgently needed. A potential therapeutic target is protein kinase D (PKD), which is activated by metabolic insults and implicated in the regulation of cardiac metabolism, contractility and hypertrophy. We therefore hypothesised that PKD inhibition would enhance cardiac function in T2D mice. We first validated the obese and T2D db/db mouse as a model of early stage diabetic cardiomyopathy, which was characterised by both diastolic and systolic dysfunction, without overt alterations in left ventricular morphology. These functional characteristics were also associated with increased PKD2 phosphorylation in the fed state and a gene expression signature characteristic of PKD activation. Acute administration of the PKD inhibitor CID755673 to normal mice reduced both PKD1 and 2 phosphorylation in a time and dose-dependent manner. Chronic CID755673 administration to T2D db/db mice for two weeks reduced expression of the gene expression signature of PKD activation, enhanced indices of both diastolic and systolic left ventricular function and was associated with reduced heart weight. These alterations in cardiac function were independent of changes in glucose homeostasis, insulin action and body composition. These findings suggest that PKD inhibition could be an effective strategy to enhance heart function in obese and diabetic patients and provide an impetus for further mechanistic investigations into the role of PKD in diabetic cardiomyopathy.

β-adrenergic signaling inhibits Gq-dependent protein kinase D activation by preventing protein kinase D translocation

RATIONALE

Both β-adrenergic receptor (β-AR) and Gq-coupled receptor (GqR) agonist-driven signaling play key roles in the events, leading up to and during cardiac dysfunction. How these stimuli interact at the level of protein kinase D (PKD), a nodal point in cardiac hypertrophic signaling, remains unclear.

OBJECTIVE

To assess the spatiotemporal dynamics of PKD activation in response to β-AR signaling alone and on coactivation with GqR-agonists. This will test our hypothesis that compartmentalized PKD signaling reconciles disparate findings of PKA facilitation and inhibition of PKD activation.

METHODS AND RESULTS

We report on the spatial and temporal profiles of PKD activation using green fluorescent protein-tagged PKD (wildtype or mutant S427E) and targeted fluorescence resonance energy transfer-based biosensors (D-kinase activity reporters) in adult cardiomyocytes. We find that β-AR/PKA signaling drives local nuclear activation of PKD, without preceding sarcolemmal translocation. We also discover pronounced interference of β-AR/cAMP/PKA signaling on GqR-induced translocation and activation of PKD throughout the cardiomyocyte. We attribute these effects to direct, PKA-dependent phosphorylation of PKD-S427. We also show that phosphomimetic substitution of S427 likewise impedes GqR-induced PKD translocation and activation. In neonatal myocytes, S427E inhibits GqR-evoked cell growth and expression of hypertrophic markers. Finally, we show altered S427 phosphorylation in transverse aortic constriction-induced hypertrophy.

CONCLUSIONS

β-AR signaling triggers local nuclear signaling and inhibits GqR-mediated PKD activation by preventing its intracellular translocation. PKA-dependent phosphorylation of PKD-S427 fine-tunes the PKD responsiveness to GqR-agonists, serving as a key integration point for β-adrenergic and Gq-coupled stimuli.

Four-and-a-half LIM domains proteins are novel regulators of the protein kinase D pathway in cardiac myocytes

PKD (protein kinase D) is a serine/threonine kinase implicated in multiple cardiac roles, including the phosphorylation of the class II HDAC5 (histone deacetylase isoform 5) and thereby de-repression of MEF2 (myocyte enhancer factor 2) transcription factor activity. In the present study we identify FHL1 (four-and-a-half LIM domains protein 1) and FHL2 as novel binding partners for PKD in cardiac myocytes. This was confirmed by pull-down assays using recombinant GST-fused proteins and heterologously or endogenously expressed PKD in adult rat ventricular myocytes or NRVMs (neonatal rat ventricular myocytes) respectively, and by co-immunoprecipitation of FHL1 and FHL2 with GFP–PKD1 fusion protein expressed in NRVMs. In vitro kinase assays showed that neither FHL1 nor FHL2 is a PKD1 substrate. Selective knockdown of FHL1 expression in NRVMs significantly inhibited PKD activation and HDAC5 phosphorylation in response to endothelin 1, but not to the α₁-adrenoceptor agonist phenylephrine. In contrast, selective knockdown of FHL2 expression caused a significant reduction in PKD activation and HDAC5 phosphorylation in response to both stimuli. Interestingly, neither intervention affected MEF2 activation by endothelin 1 or phenylephrine. We conclude that FHL1 and FHL2 are novel cardiac PKD partners, which differentially facilitate PKD activation and HDAC5 phosphorylation by distinct neurohormonal stimuli, but are unlikely to regulate MEF2-driven transcriptional reprogramming.

Protein kinase D has multiple roles in cardiac myocytes, where its regulatory mechanisms remain incompletely defined. In the present study we identify four-and-a-half LIM domains proteins 1 and 2 as novel binding partners and regulators of protein kinase D in this cell type.