Anna Planavila

Fibroblast growth factor 21 protects against cardiac hypertrophy in mice

Fibroblast growth factor 21 is an endocrine factor, secreted mainly by the liver, that exerts metabolic actions that favour glucose metabolism. Its role in the heart is unknown. Here we show that Fgf21(-/-) mice exhibit an increased relative heart weight and develop enhanced signs of dilatation and cardiac dysfunction in response to isoproterenol infusion, indicating eccentric hypertrophy development. In addition, Fgf21(-/-) mice exhibit enhanced induction of cardiac hypertrophy markers and pro-inflammatory pathways and show greater repression of fatty acid oxidation. Most of these alterations are already present in Fgf21(-/-) neonates, and treatment with fibroblast growth factor 21 reverses them in vivo and in cultured cardiomyocytes. Moreover, fibroblast growth factor 21 is expressed in the heart and is released by cardiomyocytes. Fibroblast growth factor 21 released by cardiomyocytes protects cardiac cells against hypertrophic insults. Therefore, the heart appears to be a target of systemic, and possibly locally generated, fibroblast growth factor 21, which exerts a protective action against cardiac hypertrophy.

SIRT1 Controls the Transcription of the Peroxisome Proliferator-activated Receptor-γ Co-activator-1α (PGC-1α) Gene in Skeletal Muscle through the PGC-1α Autoregulatory Loop and Interaction with MyoD

Peroxisome proliferator activated receptor-γ co-activator-1α (PGC-1α) is a transcriptional co-activator that coordinately regulates the expression of distinct sets of metabolism-related genes in different tissues. Here we show that PGC-1α expression is reduced in skeletal muscles from mice lacking the sirtuin family deacetylase SIRT1. Conversely, SIRT1 activation or overexpression in differentiated C2C12 myotubes increased PGC-1α mRNA expression. The transcription-promoting effects of SIRT1 occurred through stimulation of PGC-1α promoter activity and were enhanced by co-transfection of myogenic factors, such as myocyte enhancer factor 2 (MEF2) and, especially, myogenic determining factor (MyoD). SIRT1 bound to the proximal promoter region of the PGC-1α gene, an interaction potentiated by MEF2C or MyoD, which also interact with this region. In the presence of MyoD, SIRT1 promoted a positive autoregulatory PGC-1α expression loop, such that overexpression of PGC-1α increased PGC-1α promoter activity in the presence of co-expressed MyoD and SIRT1. Chromatin immunoprecipitation showed that SIRT1 interacts with PGC-1α promoter and increases PGC-1α recruitment to its own promoter region. Immunoprecipitation assays further showed that SIRT1-PGC-1α interactions are enhanced by MyoD. Collectively, these data indicate that SIRT1 controls PGC-1α gene expression in skeletal muscle and that MyoD is a key mediator of this action. The involvement of MyoD in SIRT1-dependent PGC-1α expression may help to explain the ability of SIRT1 to drive muscle-specific gene expression and metabolism. Autoregulatory control of PGC-1α gene transcription seems to be a pivotal mechanism for conferring a transcription-activating response to SIRT1 in skeletal muscle.

Nuclear factor-κB activation leads to down-regulation of fatty acid oxidation during cardiac hypertrophy

Little is known about the mechanisms responsible for the fall in fatty acid oxidation during the development of cardiac hypertrophy. We focused on the effects of nuclear factor (NF)-κB activation during cardiac hypertrophy on the activity of peroxisome proliferator-activated receptor (PPAR) β/δ, which is the predominant PPAR subtype in cardiac cells and plays a prominent role in the regulation of cardiac lipid metabolism. Phenylephrine-induced cardiac hypertrophy in neonatal rat cardiomyocytes caused a reduction in the expression of pyruvate dehydrogenase kinase 4 (Pdk4), a target gene of PPARβ/δ involved in fatty acid utilization, and a fall in palmitate oxidation that was reversed by NF-κB inhibitors. Lipopolysaccharide stimulation of NF-κB in embryonic rat heart-derived H9c2 myotubes, which only express PPARβ/δ, caused both a reduction in Pdk4 expression and DNA binding activity of PPARβ/δ to its response element, effects that were reversed by NF-κB inhibitors. Coimmunoprecipitation studies demonstrated that lipopolysaccharide strongly stimulated the physical interaction between the p65 subunit of NF-κB and PPARβ/δ, providing an explanation for the reduced activity of PPARβ/δ. Finally, we assessed whether this mechanism was present in vivo in pressure overload-induced cardiac hypertrophy. In hypertrophied hearts of banded rats the reduction in the expression of Pdk4 was accompanied by activation of NF-κB and enhanced interaction between p65 and PPARβ/δ. These results indicate that NF-κB activation during cardiac hypertrophy down-regulates PPARβ/δ activity, leading to a fall in fatty acid oxidation, through a mechanism that involves enhanced protein-protein interaction between the p65 subunit of NF-κB and PPARβ/δ.

Inflammatory Pathways Are Activated during Cardiomyocyte Hypertrophy and Attenuated by Peroxisome Proliferator-activated Receptors PPARα and PPARδ

Accumulating evidence indicates an important role for inflammation in cardiac hypertrophy and failure. Peroxisome proliferator-activated receptors (PPARs) have been reported to attenuate inflammatory signaling pathways and, as such, may interfere with cardiac remodeling. Accordingly, the objectives of the present study were to explore the relationship between cardiomyocyte hypertrophy and inflammation and to investigate whether PPARα and PPARδ are able to inhibit NF-κB activation and, consequently, the hypertrophic growth response of neonatal rat cardiomyocytes (NCM). mRNA levels of markers of both hypertrophy and inflammation were increased following treatment with the pro-hypertrophic factor phenylephrine (PE) or the chemokine TNF-α. Induction of inflammatory genes was found to be fast (within 2 h after stimulation) and transient, while induction of hypertrophic marker genes was more gradual (peaking at 24–48 h). Inflammatory and hypertrophic pathways appeared to converge on NF-κB as both PE and TNF-α increased NF-κB binding activity as measured by electrophoretic mobility shift assay. Following transient transfection, the p65-induced transcriptional activation of a NF-κB reporter construct was significantly blunted after co-transfection of PPARα or PPARδ in the presence of their respective ligands. Finally, adenoviral overexpression of PPARα and PPARδ markedly attenuated cell enlargement and the expression of hypertrophic marker genes in PE-stimulated NCM. The collective findings reveal a close relationship between hypertrophic and inflammatory signaling pathways in the cardiomyocyte. It was shown that both PPARα and PPARδ are able to mitigate cardiomyocyte hypertrophy in vitro by inhibiting NF-κB activation.

Fibroblast growth factor 21 protects the heart from oxidative stress

AIMS Oxidative stress mediated by reactive oxygen species (ROS) plays a striking role in the pathogenesis of heart failure, and antioxidants have been shown to attenuate cardiac remodelling in experimental models of cardiac damage. We recently showed that fibroblast growth factor 21 (Fgf21) is produced by the heart and exerts protective effects, preventing cardiac hypertrophy development. The aim of the study was to determine the effects of Fgf21 during oxidative stress signalling in the heart. METHODS AND RESULTS Fgf21 treatment in cardiomyocytes in culture induced the expression of genes encoding proteins involved in antioxidative pathways, including mitochondrial uncoupling proteins (Ucp2 and Ucp3) and superoxide dismutase-2 (Sod2) and reduced ROS production. In keeping with this, expression of antioxidant genes in response to lipopolysaccharide (LPS)-induced stimulation of pro-inflammatory pathways or isoproterenol-induced cardiac hypertrophy in the heart was reduced in Fgf21-null mice. Moreover, we found that Fgf21 is expressed in and released by cardiomyocytes in response to LPS, and its expression is under the control of the Sirt1 (sirtuin-1) pathway. This Fgf21 released by cardiomyocytes acts in an autocrine manner to protect cells against oxidative stress. Finally, failing human hearts showed up-regulation of Fgf21, Ucp3, and Sod2, confirming the association between Fgf21 induction and the control of cardiac oxidative stress pathways. CONCLUSION Our data indicate that Fgf21 regulates genes involved in antioxidant pathways in an autocrine manner, thus preventing ROS production in cardiac cells. Therefore, Fgf21 acts as an antioxidant factor in the heart, preventing induction of pro-oxidative pathways by inflammatory or hypertrophic conditions.

Impaired expression of NADH dehydrogenase subunit 1 and PPARγ coactivator-1 in skeletal muscle of ZDF rats restoration by troglitazone

Type 2 diabetes has been related to a decrease of mitochondrial DNA (mtDNA) content. In this study, we show increased expression of the peroxisome proliferator-activated receptor-α (PPARα) and its target genes involved in fatty acid metabolism in skeletal muscle of Zucker Diabetic Fatty (ZDF) (fa/fa) rats. In contrast, the mRNA levels of genes involved in glucose transport and utilization (GLUT4 and phosphofructokinase) were decreased, whereas the expression of pyruvatedehydrogenase kinase 4 (PDK-4), which suppresses glucose oxidation, was increased. The shift from glucose to fatty acids as the source of energy in skeletal muscle of ZDF rats was accompanied by a reduction of subunit 1 of complex I (NADH dehydrogenase subunit 1, ND1) and subunit II of complex IV (cytochrome c oxidase II, COII), two genes of the electronic transport chain encoded by mtDNA. The transcript levels of PPARγ Coactivator 1 (PGC-1) showed a significant reduction. Treatment with troglitazone (30 mg/kg/day) for 15 days reduced insulin values and reversed the increase in PDK-4 mRNA levels, suggesting improved insulin sensitivity. In addition, troglitazone treatment restored ND1 and PGC-1 expression in skeletal muscle. These results suggest that troglitazone may avoid mitochondrial metabolic derangement during the development of diabetes mellitus 2 in skeletal muscle.

The lipid sensor GPR120 promotes brown fat activation and FGF21 release from adipocytes.

The thermogenic activity of brown adipose tissue (BAT) and browning of white adipose tissue are important components of energy expenditure. Here we show that GPR120, a receptor for polyunsaturated fatty acids, promotes brown fat activation. Using RNA-seq to analyse mouse BAT transcriptome, we find that the gene encoding GPR120 is induced by thermogenic activation. We further show that GPR120 activation induces BAT activity and promotes the browning of white fat in mice, whereas GRP120-null mice show impaired cold-induced browning. Omega-3 polyunsaturated fatty acids induce brown and beige adipocyte differentiation and thermogenic activation, and these effects require GPR120. GPR120 activation induces the release of fibroblast growth factor-21 (FGF21) by brown and beige adipocytes, and increases blood FGF21 levels. The effects of GPR120 activation on BAT activation and browning are impaired in FGF21-null mice and cells. Thus, the lipid sensor GPR120 activates brown fat via a mechanism that involves induction of FGF21.

Peroxisome proliferator-activated receptors and the control of fatty acid oxidation in cardiac hypertrophy.

Cardiac hypertrophy is a response of the heart to a wide range of extrinsic stimuli, such as arterial hypertension, valvular heart disease, myocardial infarction, and cardiomyopathy. Although this process is initially compensatory for an increase workload, its prolongation frequently results in congestive heart failure, arrhythmia, and sudden death. Cardiac hypertrophy is associated with an increase in glucose utilization and a decrease in fatty acid oxidation. It is unclear at present, however, which consequences might result from impaired oxidation of fatty acids in the heart, but several studies have demonstrated that substrate utilization is important in the pathogenesis of cardiac hypertrophy. Here we will focus on the effects of cardiac hypertrophy on the activity of Peroxisome proliferator-activated receptors (PPARs), ligand-activated transcription factors that regulate the expression of genes involved in fatty acid uptake and oxidation, lipid metabolism and inflammation. Interestingly, activation of the Nuclear Factor (NF)-kappaB signaling pathway, which is one of the most important signal transduction pathways involved in the hypertrophic growth of the myocardium, may suppress the activity of the PPARs, affording a link between cardiac hypertrophy and the fall in fatty acid oxidation in the hypertrophied heart. As a result, inhibition of NF-kappaB activation during cardiac hypertrophy may also ameliorate cardiac fatty acid oxidation, achieving a better improvement in the prevention or inhibition of this pathological process.

FGF21 and cardiac physiopathology

The heart is not traditionally considered either a target or a site of fibroblast growth factor-21 (FGF21) production. However, recent findings indicate that FGF21 can act as a cardiomyokine; that is, it is produced by cardiac cells at significant levels and acts in an autocrine manner on the heart itself. The heart is sensitive to the effects of FGF21, both systemic and locally generated, owing to the expression in cardiomyocytes of β-Klotho, the key co-receptor known to confer specific responsiveness to FGF21 action. FGF21 has been demonstrated to protect against cardiac hypertrophy, cardiac inflammation, and oxidative stress. FGF21 expression in the heart is induced in response to cardiac insults, such as experimental cardiac hypertrophy and myocardial infarction in rodents, as well as in failing human hearts. Intracellular mechanisms involving PPARα and Sirt1 mediate transcriptional regulation of the FGF21 gene in response to exogenous stimuli. In humans, circulating FGF21 levels are elevated in coronary heart disease and atherosclerosis, and are associated with a higher risk of cardiovascular events in patients with type 2 diabetes. These findings provide new insights into the role of FGF21 in the heart and may offer potential therapeutic strategies for cardiac disease.

Fatty acids and cardiac disease: fuel carrying a message

From the viewpoint of the prevention of cardiovascular disease (CVD) burden, there has been a continuous interest in the detrimental effects of the Western‐type high‐fat diet for more than half a century. More recently, this general view has been subject to change as epidemiological studies showed that replacing fat by carbohydrate may even be worse and that various polyunsaturated fatty acids (FA) have beneficial rather than detrimental effects on CVD outcome. At the same time, advances in lipid biology have provided insight into the mechanisms by which the different lipid components of the Western diet affect the cardiovascular system. In fact, this still is a rapidly growing field of research and in recent years novel FA derivatives and FA receptors have been discovered. This includes fish‐oil derived FA‐derivatives with anti‐inflammatory properties, the so‐called resolvins, and various G‐protein‐coupled receptors that recognize FA as ligands. In the present review, we will extensively discuss the role of FA and their metabolites on cardiac disease, with special emphasis on the role of the different saturated and polyunsaturated FA and their respective metabolites in cellular signal transduction and the possible implications for the development of cardiac hypertrophy and cardiac failure.