Insulin resistance is a characteristic feature of obesity and Type 2 diabetes and impacts the heart in various ways. Impaired insulin-mediated glucose uptake is a uniformly observed characteristic of the heart in these states, although changes in upstream kinase signaling are variable and dependent on the severity and duration of the associated obesity or diabetes. The understanding of the physiological and pathophysiological role of insulin resistance in the heart is evolving. To maintain its high energy demands, the heart is capable of utilizing many metabolic substrates. Although, insulin signaling may directly regulate cardiac metabolism, its main role is likely the regulation of substrate delivery from the periphery to the heart. In addition to promoting glucose uptake, insulin regulates long chain fatty acid uptake, protein synthesis, and vascular function in the normal cardiovascular system. Recent advances in understanding the role of metabolic, signaling, and inflammatory pathways in obesity have provided opportunities to better understand the pathophysiology of insulin resistance in the heart. This review will summarize our current understanding of metabolic mechanisms for and consequences of insulin resistance in the heart and discuss potential new areas for investigating novel mechanisms that contribute to insulin resistance in the heart.
High sugar intake increases heart disease risk in humans. In animals, sugar intake accelerates heart failure development via increased reactive oxygen species (ROS). Glucose 6-phosphate dehydrogenase (G6PD) can fuel ROS production by providing NADPH for superoxide generation by NADPH oxidase. On the other hand, G6PD also facilitates ROS scavenging via the glutathione pathway. We hypothesized that high sugar intake would increase flux through G6PD to increase myocardial [NADPH] and ROS, and accelerate cardiac dysfunction and death.
Research Methods & Procedures
Six-week old TO-2 hamsters, a nonhypertensive model of genetic cardiomyopathy caused by a δ-sarcoglycan mutation, were fed a long-term diet of either high starch or high sugar (57% of energy from sucrose+fructose).
After 24 weeks, δ-sarcoglycan deficient animals displayed expected decreases in survival and cardiac function associated with cardiomyopathy (ejection fraction: control=68.7±4.5%; TO-2 starch=46.1±3.7, p<0.05 TO-2 starch vs control; TO-2 sugar=58.0±4.2%, N.S. vs TO-2 starch or control; median survival: TO-2 starch=278 days, TO-2 sugar=318 days, P=0.133). Although we expected high sugar intake to exacerbate cardiomyopathy, surprisingly there was no further decrease in ejection fraction or survival with high sugar compared to starch in cardiomyopathic animals. Cardiomyopathic animals had systemic and cardiac metabolic abnormalities (elevated serum lipids and glucose, and decreased myocardial oxidative enzymes) which were unaffected by diet. High sugar intake increased myocardial superoxide, but [NADPH] and lipid peroxidation were unaffected.
A sugar enriched diet did not exacerbate ventricular function, metabolic abnormalities, or survival in heart failure despite an increase in NADPH and superoxide production.
heart failure; reactive oxygen species; δ-sarcoglycan; diet; sugar; fructose; sucrose
The impact of a high-fat diet on the failing heart is unclear, and the differences between polyunsaturated fatty acids (PUFA) and saturated fat have not been assessed. Here, we compared a standard low-fat diet to high-fat diets enriched with either saturated fat (palmitate and stearate) or PUFA (linoleic and α-linolenic acids) in hamsters with genetic cardiomyopathy.
Methods and results
Male δ-sarcoglycan null Bio TO2 hamsters were fed a standard low-fat diet (12% energy from fat), or high-fat diets (45% fat) comprised of either saturated fat or PUFA. The median survival was increased by the high saturated fat diet (P< 0.01; 278 days with standard diet and 361 days with high saturated fat)), but not with high PUFA (260 days) (n = 30–35/group). Body mass was modestly elevated (∼10%) in both high fat groups. Subgroups evaluated after 24 weeks had similar left ventricular chamber size, function, and mass. Mitochondrial oxidative enzyme activity and the yield of interfibrillar mitochondria (IFM) were decreased to a similar extent in all TO2 groups compared with normal F1B hamsters. Ca2+-induced mitochondrial permeability transition pore opening was enhanced in IFM in all TO2 groups compared with F1B hamsters, but to a significantly greater extent in those fed the high PUFA diet compared with the standard or high saturated fat diet.
These results show that a high intake of saturated fat improves survival in heart failure compared with a high PUFA diet or low-fat diet, despite persistent mitochondrial defects.
Cardiomyopathy; Low-carbohydrate diet; Metabolism; Obesity
AMP-activated protein kinase (AMPK), is an important regulator of cardiac metabolism, but it’s role is not clearly understood in pressure overload induced hypertrophy. In addition, the relationship between AMPK and other important protein kinases such as p38 MAP kinase, Akt and Pim-1 are unclear. Thus we studied the time course of AMPK activity and phosphorylation of its α-subunit during the development of cardiac hypertrophy. In parallel, we examined the expression and activation of key kinases known to be involved in cardiac hypertrophy that could interact with AMPK (i.e. p38 MAP kinase, Akt and Pim-1). Male C57BL/6J mice underwent sham or transverse aortic constriction (TAC) surgery and the hearts were harvested 2, 4, 6 and 8 weeks later. Despite significant left ventricular (LV) hypertrophy, LV dilation and impaired LV contractile function at all time points in TAC compared to sham mice, the activity and phosphorylation of AMPK were similar to sham. In contrast, p38 and Pim-1 protein expression were transiently increased in TAC mice at 2 and 4 weeks and at 2, 4 and 6 weeks, respectively. In addition, p38 activation by phosphorylation was also transiently increased at 2 to 6 weeks. There were no differences between sham and TAC mice in p38, Akt or Pim-1 at 8 weeks. In conclusion, TAC resulted in a transient upregulation in the expression of p38 and Pim-1 despite no activation of AMPK or Akt.
heart failure; hypertrophy; AMP activated protein kinase; p38 MAPK; Pim-1; Akt
Mitochondria can depolarize and trigger cell death through the opening of the mitochondrial permeability transition pore (MPTP). We recently showed that an increase in the long chain n3 polyunsaturated fatty acids (PUFA) docosahexaenoic acid (DHA; 22:6n3) and depletion of the n6 PUFA arachidonic acid (ARA; 20:4n6) in mitochondrial membranes is associated with a greater Ca2+ load required to induce MPTP opening. Here we manipulated mitochondrial phospholipid composition by supplementing the diet with DHA, ARA or combined DHA+ARA in rats for 10 weeks. There were no effects on cardiac function, or respiration of isolated mitochondria. Analysis of mitochondrial phospholipids showed DHA supplementation increased DHA and displaced ARA in mitochondrial membranes, while supplementation with ARA or DHA+ARA increased ARA and depleted linoleic acid (18:2n6). Phospholipid analysis revealed a similar pattern, particularly in cardiolipin. Tetralinoleoyl cardiolipin was depleted by 80% with ARA or DHA+ARA supplementation, with linoleic acid side chains replaced by ARA. Both the DHA and ARA groups had delayed Ca2+-induced MPTP opening, but the DHA+ARA group was similar to the control diet. In conclusion, alterations in mitochondria membrane phospholipid fatty acid composition caused by dietary DHA or ARA was associated with a greater cumulative Ca2+ load required to induced MPTP opening. Further, high levels of tetralinoleoyl cardiolipin were not essential for normal mitochondrial function if replaced with very-long chain n3 or n6 PUFAs.
Mitofusin-2 (Mfn-2) is a dynamin-like protein that is involved in the rearrangement of the outer mitochondrial membrane. Research using various experimental systems has shown that Mfn-2 is a mediator of mitochondrial fusion, an evolutionarily conserved process responsible for the surveillance of mitochondrial homeostasis. Here, we find that cardiac myocyte mitochondria lacking Mfn-2 are pleiomorphic and have the propensity to become enlarged. Consistent with an underlying mild mitochondrial dysfunction, Mfn-2-deficient mice display modest cardiac hypertrophy accompanied by slight functional deterioration. The absence of Mfn-2 is associated with a marked delay in mitochondrial permeability transition downstream of Ca2+ stimulation or due to local generation of reactive oxygen species (ROS). Consequently, Mfn-2-deficient adult cardiomyocytes are protected from a number of cell death-inducing stimuli and Mfn-2 knockout hearts display better recovery following reperfusion injury. We conclude that in cardiac myocytes, Mfn-2 controls mitochondrial morphogenesis and serves to predispose cells to mitochondrial permeability transition and to trigger cell death.
Treatment with the ω-3 polyunsaturated fatty acids (PUFAs) docosahexanoic acid (DHA) and eicosapentanoic acid (EPA) exerts cardioprotective effects, and suppresses Ca2+-induced opening of the mitochondrial permeability transition pore (MPTP). These effects are associated with increased DHA and EPA, and lower arachidonic acid (ARA) in cardiac phospholipids. While clinical studies suggest the triglyceride lowering effects of DHA and EPA are equivalent, little is known about the independent effects of DHA and EPA on mitochondria function. We compared the effects of dietary supplementation with the ω-3 PUFAs DHA and EPA on cardiac mitochondrial phospholipid fatty acid composition and Ca2+-induced MPTP opening. Rats were fed a standard lab diet with either normal low levels of ω-3 PUFA, or DHA or EPA at 2.5% of energy intake for 8 weeks, and cardiac mitochondria were isolated and analyzed for Ca2+-induced MPTP opening and phospholipid fatty acyl composition. DHA supplementation increased both DHA and EPA and decreased ARA in mitochondrial phospholipid, and significantly delayed MPTP opening as assessed by increased Ca2+ retention capacity and decreased Ca2+-induced mitochondria swelling. EPA supplementation increased EPA in mitochondrial phospholipids, but did not affect DHA, only modestly lowered ARA, and did not affect MPTP opening. In summary, dietary supplementation with DHA but not EPA, profoundly altered mitochondrial phospholipid fatty acid composition and delayed Ca2+-induced MPTP opening.
cardiac; eicosapentaenoic acid; docosahexaenoic acid; fish oil; heart; mitochondrial permeability transition pore
Supplementation with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from fish oil may prevent development of heart failure through alterations in cardiac phospholipids that favorably impact inflammation and energy metabolism. A high-fat diet may block these effects in chronically stressed myocardium. Pathological left ventricle (LV) hypertrophy was generated by subjecting rats to pressure overload by constriction of the abdominal aorta. Animals were fed: (1) standard diet (10% of energy from fat), (2) standard diet with EPA+DHA (2.3% of energy intake as EPA+DHA), (3) high fat (60% fat); or (4) high fat with EPA+DHA. Pressure overload increased LV mass by ≈40% in both standard and high-fat diets without fish oil. Supplementation with fish oil increased their incorporation into cardiac phospholipids, and decreased the proinflammatory fatty acid arachidonic acid and urine thromboxane B2 with both the standard and high-fat diet. Linoleic acid and tetralinoloyl cardiolipin (an essential mitochondrial phospholipid) were decreased with pressure overload on standard diet, which was prevented by fish oil. Animals fed high-fat diet had decreased linoleic acid and tetralinoloyl cardiolipin regardless of fish oil supplemention. Fish oil limited LV hypertrophy on the standard diet, and prevented upregulation of fetal genes associated with heart failure (myosin heavy chain-β and atrial natriuetic factor). These beneficial effects of fish oil were absent in animals on the high-fat diet. In conclusion, whereas treatment with EPA+DHA prevented tetralinoloyl cardiolipin depletion, LV hypertrophy, and abnormal genes expression with pressure overload, these effects were absent with a high-fat diet.
Omega-3 fatty acids; cardiac hypertrophy; heart failure; cardiolipin; phospolipids
Consumption of ω-3 fatty acids from fish oil, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), decreases risk for heart failure and attenuates pathologic cardiac remodeling in response to pressure overload. Dietary supplementation with EPA+DHA may also impact cardiac mitochondrial function and energetics through alteration of membrane phospholipids. We assessed the role of EPA+DHA supplementation on left ventricular (LV) function, cardiac mitochondrial membrane phospholipid composition, respiration, and sensitivity to mitochondrial permeability transition pore (MPTP) opening in normal and infarcted myocardium. Rats were subjected to sham surgery or myocardial infarction by coronary artery ligation (n=10–14), and fed a standard diet, or supplemented with EPA+DHA (2.3% of energy intake) for 12 weeks. EPA+DHA altered fatty acid composition of total mitochondrial phospholipids and cardiolipin by reducing arachidonic acid content and increasing DHA incorporation. EPA+DHA significantly increased calcium uptake capacity in both subsarcolemmal and intrafibrillar mitochondria from sham rats. This treatment effect persisted with the addition of cyclosporin A, and was not accompanied by changes in mitochondrial respiration or coupling, or cyclophilin D protein expression. Myocardial infarction resulted in heart failure as evidenced by LV dilation and contractile dysfunction. Infarcted LV myocardium had decreased mitochondrial protein yield and activity of mitochondrial marker enzymes, however respiratory function of isolated mitochondria was normal. EPA+DHA had no effect on LV function, mitochondrial respiration, or MPTP opening in rats with heart failure. In conclusion, dietary supplementation with EPA+DHA altered mitochondrial membrane phospholipid fatty acid composition in normal and infarcted hearts, but delayed MPTP opening only in normal hearts.
eicosapentaenoic acid; docosahexaenoic acid; myocardial infarction; mitochondrial permeability transition pore
Heart failure (HF) is a complex clinical syndrome with multiple aetiologies. Current treatment options can slow the progression to HF, but overall the prognosis remains poor. Clinical studies suggest that high dietary intake of the ω-3 polyunsaturated fatty acids (ω-3PUFA) found in fish oils (eicosapentaenoic and docosahexaenoic acids) may lower the incidence of HF, and that supplementation with pharmacological doses prolongs event-free survival in patients with established HF. The mechanisms for these potential benefits are complex and not well defined. It is well established that fish oil supplementation lowers plasma triglyceride levels, and more recent work demonstrates anti-inflammatory effects, including reduced circulating levels of inflammatory cytokines and arachidonic acid-derived eicosanoids, and elevated plasma adiponectin. In animal studies, fish oil favourably alters cardiac mitochondrial function. All of these effects may work to prevent the development and progression of HF. The ω-3PUFA found in plant sources, α-linolenic acid, may also be protective in HF; however, the evidence is not as compelling as for fish oil. This review summarizes the evidence related to use of ω-3PUFA supplementation as a potential treatment for HF and discusses possible mechanisms of action. In general, there is growing evidence that supplementation with ω-3PUFA positively impacts established pathophysiological targets in HF and has potential therapeutic utility for HF patients.
α-Linolenic acid; Cardiac; Docosahexaenoic acid; Eicosapentaenoic acid; Inflammation; Metabolism
Pathological left ventricular (LV) hypertrophy frequently progresses to dilated heart failure with suppressed mitochondrial oxidative capacity. Dietary marine ω-3 polyunsaturated fatty acids (ω-3 PUFA) up-regulate adiponectin and prevent LV dilation in rats subjected to pressure overload. This study 1) assessed the effects of ω-3 PUFA on LV dilation and down-regulation of mitochondrial enzymes in response to pressure overload; and 2) evaluated the role of adiponectin in mediating the effects of ω-3 PUFA in heart.
Wild type (WT) and adiponectin-/- mice underwent transverse aortic constriction (TAC) and were fed standard chow ± ω-3 PUFA for 6 weeks. At 6 weeks, echocardiography was performed to assess LV function, mice were terminated, and mitochondrial enzyme activities were evaluated.
TAC induced similar pathological LV hypertrophy compared to sham mice in both strains on both diets. In WT mice TAC increased LV systolic and diastolic volumes and reduced mitochondrial enzyme activities, which were attenuated by ω-3 PUFA without increasing adiponectin. In contrast, adiponectin-/- mice displayed no increase in LV end diastolic and systolic volumes or decrease in mitochondrial enzymes with TAC, and did not respond to ω-3 PUFA.
These findings suggest ω-3 PUFA attenuates cardiac pathology in response to pressure overload independent of an elevation in adiponectin.
Clinical studies suggest that intake of ω-3 polyunsaturated fatty acids (ω-3 PUFA) may lower the incidence of heart failure. Dietary supplementation with ω-3 PUFA exerts metabolic and anti-inflammatory effects that could prevent left ventricle (LV) pathology; however, it is unclear whether these effects occur at clinically relevant doses and whether there are differences between ω-3 PUFA from fish [eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] and vegetable sources [α-linolenic acid (ALA)].
Methods and results
We assessed the development of LV remodelling and pathology in rats subjected to aortic banding treated with ω-3 PUFA over a dose range that spanned the intake of humans taking ω-3 PUFA supplements. Rats were fed a standard food or diets supplemented with EPA+DHA or ALA at 0.7, 2.3, or 7% of energy intake. Without supplementation, aortic banding increased LV mass and end-systolic and -diastolic volumes. ALA supplementation had little effect on LV remodelling and dysfunction. In contrast, EPA+DHA dose-dependently increased EPA and DHA, decreased arachidonic acid in cardiac membrane phospholipids, and prevented the increase in LV end-diastolic and -systolic volumes. EPA+DHA resulted in a dose-dependent increase in the anti-inflammatory adipokine adiponectin, and there was a strong correlation between the prevention of LV chamber enlargement and plasma levels of adiponectin (r = −0.78). Supplementation with EPA+DHA had anti-aggregatory and anti-inflammatory effects as evidenced by decreases in urinary thromboxane B2 and serum tumour necrosis factor-α.
Dietary supplementation with ω-3 PUFA derived from fish, but not from vegetable sources, increased plasma adiponectin, suppressed inflammation, and prevented cardiac remodelling and dysfunction under pressure overload conditions.
α-linolenic acid; Diet; Docosahexaenoic acid; Eicosapentaenoic acid; Heart failure
Epidemiological studies suggest that consumption of ω-3 polyunsaturated fatty acids (ω-3 PUFA) decreases the risk of heart failure. We assessed the effects of dietary supplementation with ω-3 PUFA from fish oil on the response of the left ventricle (LV) to arterial pressure overload.
Male Wistar rats were fed a standard chow or a ω-3 PUFA-supplemented diet. After 1 week rats underwent abdominal aortic banding or sham surgery (n=9-12/group). LV function was assessed by echocardiography after 8 weeks. In addition, we studied the effect of ω-3 PUFA on the cardioprotective adipocyte-derived hormone adiponectin, which may alter the pro-growth serine-threonine kinase Akt.
Banding increased LV mass to a greater extent with the standard chow (31%) than with ω-3 PUFA (18%). LV end diastolic and systolic volumes were increased by 19% and 105% with standard chow, respectively, but were unchanged with ω-3 PUFA. The expression of adiponectin was up-regulated in adipose tissue, and the plasma adiponectin concentration was significantly elevated. Treatment with ω-3 PUFA increased total Akt protein expression in the heart, but decreased the fraction of Akt in the active phosphorylated form, and thus did not alter the amount of active phospho-Akt.
Dietary supplementation with ω-3 PUFA attenuated pressure overload-induced LV dysfunction, which was associated with elevated plasma adiponectin.
It is not known how carbohydrate and fat intake impact the development of left ventricular (LV) hypertrophy and contractile dysfunction in response to pressure overload. We hypothesized that a low carbohydrate/high fat diet prevents LV hypertrophy and dysfunction compared to high carbohydrate diets.
Methods and Results
Rats were fed high carbohydrate diets comprised of either starch or sucrose, or a low carbohydrate/high fat diet, and underwent abdominal aortic banding (AAB) for two months. AAB increased LV mass with all diets. LV end diastolic and systolic volumes, and the ratio of the mRNA for myosin heavy chainβ/α were increased with both high carbohydrate diets, but not with the low carbohydrate/high fat diet. Circulating levels of insulin and leptin, both stimulants for cardiac growth, were lower, and free fatty acids higher, with the low carbohydrate/high fat diet compared to high carbohydrate diets. Among AAB animals LV volumes were positively correlated with insulin, and LV mass correlated with leptin.
A low carbohydrate/high fat diet attenuated pressure overload-induced LV remodeling compared to high carbohydrate diets. This effect corresponded to lower insulin and leptin concentrations, suggesting they may contribute to the development of LV hypertrophy and dysfunction under conditions of pressure overload.
Currently, a high carbohydrate/low fat diet is recommended for patients with hypertension; however, the potentially important role that the composition of dietary fat and carbohydrate plays in hypertension and the development of pathological left ventricular hypertrophy (LVH) has not been well characterized. Recent studies demonstrate that LVH can also be triggered by activation of insulin signaling pathways, altered adipokine levels, or the activity of peroxisome proliferator-activated receptors (PPARs), suggesting that metabolic alterations play a role in the pathophysiology of LVH. Hypertensive patients with high plasma insulin or metabolic syndrome have a greater occurrence of LVH, which could be due to insulin activation of the serine-threonine kinase Akt and its downstream targets in the heart, resulting in cellular hypertrophy. PPARs also activate cardiac gene expression and growth and are stimulated by fatty acids and consumption of a high fat diet. Dietary intake of fats and carbohydrate and the resultant effects of plasma insulin, adipokine, and lipid concentrations may affect cardiomyocyte size and function, particularly in the setting of chronic hypertension. This review discusses potential mechanisms by which dietary carbohydrates and fats ca affect cardiac growth, metabolism, and function, mainly in the context of pressure overload-induced LVH.
diet; fat; glucose; hypertension; hypertrophy; metabolism