Overexpression of apolipoprotein B in the heart impedes cardiac triglyceride accumulation and development of cardiac dysfunction in diabetic mice

The heart secretes apolipoprotein B (apoB) containing lipoproteins. Herein, we examined whether the overexpression of a human apoB transgene in the heart affects triglyceride accumulation and development of cardiac dysfunction in streptozotocin-treated diabetic mice. Blood glucose, plasma free fatty acids, and plasma triglycerides were similarly affected in diabetic wild type mice and diabetic apoB transgenic mice as compared with non-diabetic mice of the same genotype. After 12 weeks, heart triglycerides were increased by 48% in diabetic wild type mice. These mice displayed an increased expression of brain natriuretic peptide and deterioration of heart function on echocardiography. In diabetic apoB transgenic mice, heart triglyceride levels were identical to those in non-diabetic wild type and apoB transgenic mice, and brain natriuretic peptide expression as well as echocardiographic indexes of heart function were only marginally affected or unaffected. The findings suggest that triglyceride accumulation in the heart is important for development of diabetic cardiomyopathy in mice, and that lipoprotein formation by cardiomyocytes plays an integrated role in cardiac lipid metabolism.     

Cardiac-specific knock-out of lipoprotein lipase alters plasma lipoprotein triglyceride metabolism and cardiac gene expression

Fatty acids are the primary energy source for the heart. The heart acquires fatty acids associated with albumin or derived from lipoprotein lipase (LpL)-mediated hydrolysis of lipoprotein triglyceride (TG). We generated heart-specific LpL knock-out mice (hLpL0) to determine whether cardiac LpL modulates the actions of peroxisome proliferator-activated receptors and affects whole body lipid metabolism. Male hLpL0 mice had significantly elevated plasma TG levels and decreased clearance of postprandial lipids despite normal postheparin plasma LpL activity. Very large density lipoprotein-TG uptake was decreased by 72% in hLpL0 hearts. However, heart uptake of albumin-bound free fatty acids was not altered. Northern blot analysis revealed a decrease in the expression of peroxisome proliferator-activated receptor alpha-response genes involved in fatty acid beta-oxidation. Surprisingly, the expression of glucose transporters 1 and 4 and insulin receptor substrate 2 was increased and that of pyruvate dehydrogenase kinase 4 and insulin receptor substrate 1 was reduced. Basal glucose uptake was increased markedly in hLpL0 hearts. Thus, the loss of LpL in the heart leads to defective plasma metabolism of TG. Moreover, fatty acids derived from lipoprotein TG and not just albumin-associated fatty acids are important for cardiac lipid metabolism and gene regulation.     

Ceramide is a cardiotoxin in lipotoxic cardiomyopathy

Ceramide is among a number of potential lipotoxic molecules that are thought to modulate cellular energy metabolism. The heart is one of the tissues thought to become dysfunctional due to excess lipid accumulation. Dilated lipotoxic cardiomyopathy, thought to be the result of diabetes and severe obesity, has been modeled in several genetically altered mice, including animals with cardiac-specific overexpression of glycosylphosphatidylinositol (GPI)-anchored human lipoprotein lipase (LpL(GPI)). To test whether excess ceramide was implicated in cardiac lipotoxicity, de novo ceramide biosynthesis was inhibited pharmacologically by myriocin and genetically by heterozygous deletion of LCB1, a subunit of serine palmitoyltransferase (SPT). Inhibition of SPT, a rate-limiting enzyme in ceramide biosynthesis, reduced fatty acid and increased glucose oxidation in isolated perfused LpL(GPI) hearts, improved systolic function, and prolonged survival rates. Our results suggest a critical role for ceramide accumulation in the pathogenesis of lipotoxic cardiomyopathy.     

Lipoprotein lipase activity is required for cardiac lipid droplet production

The rodent heart accumulates TGs and lipid droplets during fasting. The sources of heart lipids could be either FFAs liberated from adipose tissue or FAs from lipoprotein-associated TGs via the action of lipoprotein lipase (LpL). Because circulating levels of FFAs increase during fasting, it has been assumed that albumin transported FFAs are the source of lipids within heart lipid droplets. We studied mice with three genetic mutations: peroxisomal proliferator-activated receptor α deficiency, cluster of differentiation 36 (CD36) deficiency, and heart-specific LpL deletion. All three genetically altered groups of mice had defective accumulation of lipid droplet TGs. Moreover, hearts from mice treated with poloxamer 407, an inhibitor of lipoprotein TG lipolysis, also failed to accumulate TGs, despite increased uptake of FFAs. TG storage did not impair maximal cardiac function as measured by stress echocardiography. Thus, LpL hydrolysis of circulating lipoproteins is required for the accumulation of lipids in the heart of fasting mice.     

DGAT1 Expression Increases Heart Triglyceride Content but Ameliorates Lipotoxicity

Intracellular lipid accumulation in the heart is associated with cardiomyopathy, yet the precise role of triglyceride (TG) remains unclear. With exercise, wild type hearts develop physiologic hypertrophy. This was associated with greater TG stores and a marked induction of the TG-synthesizing enzyme diacylglycerol (DAG) acyltransferase 1 (DGAT1). Transgenic overexpression of DGAT1 in the heart using the cardiomyocyte- specific α-myosin heavy chain (MHC) promoter led to approximately a doubling of DGAT activity and TG content and reductions of ∼35% in cardiac ceramide, 26% in DAG, and 20% in free fatty acid levels. Cardiac function assessed by echocardiography and cardiac catheterization was unaffected. These mice were then crossed with animals expressing long-chain acyl-CoA synthetase via the MHC promoter (MHC-ACS), which develop lipotoxic cardiomyopathy. MHC-DGAT1XMHC-ACS double transgenic male mice had improved heart function; fractional shortening increased by 74%, and diastolic function improved compared with MHC-ACS mice. The improvement of heart function correlated with a reduction in cardiac DAG and ceramide and reduced cardiomyocyte apoptosis but increased fatty acid oxidation. In addition, the survival of the mice was improved. Our study indicates that TG is not likely to be a toxic lipid species directly, but rather it is a feature of physiologic hypertrophy and may serve a cytoprotective role in lipid overload states. Moreover, induction of DGAT1 could be beneficial in the setting of excess heart accumulation of toxic lipids.     

Very low density lipoprotein (VLDL) receptor-deficient mice have reduced lipoprotein lipase activity. Possible causes of hypertriglyceridemia and reduced body mass with VLDL receptor deficiency.

Although very low density lipoprotein (VLDL) receptor (VLDLr) knockout mice have been reported to have no lipoprotein abnormalities, they develop less adipose tissue than control mice when fed a high calorie diet. Mice that are deficient in adipose tissue expression of lipoprotein lipase (LpL) also have less fat, but only when crossed with ob/ob mice. We hypothesized that the VLDLr, a protein that will bind and transport LpL, is required for optimal LpL actions in vivo and that hypertriglyceridemia due to VLDLr deficiency is exacerbated by either LpL deficiency or VLDL overproduction. Fasted VLDLr knockout (VLDLr0) mice were more hypertriglyceridemic than controls (2-fold greater triglyceride levels). The hypertriglyceridemia due to VLDLr0 was even more evident when VLDLr0 mice were crossed with heterozygous LpL-deficient (LpL1) and human apolipoprotein B (apoB) transgenic mice. This was due to an increase in apoB48-containing VLDL. [(3)H]VLDL turnover studies showed that VLDL-triglyceride clearance in VLDLr0/LpL1 mice was impaired by 50% compared with LpL1 mice. VLDLr0/LpL1 mice had less LpL activity in postheparin plasma, heart, and skeletal muscle. Infection of mice with an adenovirus-expressing receptor-associated protein, an inhibitor of the VLDLr, reduced LpL activity in wild type but not VLDLr0 mice. Therefore, the VLDLr is required for normal LpL regulation in vivo, and the disruption of VLDLr results in hypertriglyceridemia associated with decreased LpL activity.     

Perfusion of hearts with triglyceride-rich particles reproduces the metabolic abnormalities in lipotoxic cardiomyopathy

Hearts with overexpression of anchored lipoprotein lipase (LpL) by cardiomyocytes (hLpL(GPI) mice) develop a lipotoxic cardiomyopathy. To characterize cardiac fatty acid (FA) and triglyceride (TG) metabolism in these mice and to determine whether changes in lipid metabolism precede cardiac dysfunction, hearts from young mice were perfused in Langendorff mode with [14C]palmitate. In hLpL(GPI) hearts, FA uptake and oxidation were decreased by 59 and 82%, respectively. This suggests reliance on an alternative energy source, such as TG. Indeed, these hearts oxidized 88% more TG. Hearts from young hLpL(GPI) mice also had greater uptake of intravenously injected cholesteryl ester-labeled Intralipid and VLDL. To determine whether perfusion of normal hearts would mimic the metabolic alterations found in hLpL(GPI) mouse hearts, wild-type hearts were perfused with [14C]palmitate and either human VLDL or Intralipid (0.4 mM TG). Both sources of TG reduced [14C]palmitate uptake (48% with VLDL and 45% with Intralipid) and FA oxidation (71% with VLDL and 65% with Intralipid). Addition of either heparin or LpL inhibitor P407 to Intralipid-containing perfusate restored [14C]palmitate uptake and confirmed that Intralipid inhibition requires local LpL. Our data demonstrate that reduced FA uptake and oxidation occur before mechanical dysfunction in hLpL(GPI) lipotoxicity. This physiology is reproduced with perfusion of hearts with TG-containing particles. Together, the results demonstrate that cardiac uptake of TG-derived FA reduces utilization of albumin-FA.     

Lipoprotein lipase deficiency and CETP in streptozotocin-treated apoB-expressing mice

Both hyperglycemia and hyperlipidemia have been postulated to increase atherosclerosis in patients with diabetes mellitus. To study the effects of diabetes on lipoprotein profiles and atherosclerosis in a rodent model, we crossed mice that express human apolipoprotein B (HuB), mice that have a heterozygous deletion of lipoprotein lipase (LPL1), and transgenic mice expressing human cholesteryl ester transfer protein (CETP). Lipoprotein profiles due to each genetic modification were assessed while mice were consuming a Western type diet. Fast-protein liquid chromatography analysis of plasma samples showed that HuB/LPL1 mice had increased VLDL triglyceride, and HuB/LPL1/CETP mice had decreased HDL and increased VLDL and IDL/LDL. All strains of mice were made diabetic using streptozotocin (STZ); diabetes did not alter lipid profiles or atherosclerosis in HuB or HuB/LPL1/CETP mice. In contrast, STZ-treated HuB/LPL1 mice were more diabetic, severely hyperlipidemic due to increased cholesterol and triglyceride in VLDL and IDL/LDL, and had more atherosclerosis.     

Metabolism of very-low-density lipoprotein and chylomicrons by streptozotocin-induced diabetic rat heart: effects of diabetes and lipoprotein preference

Very-low-density lipoprotein (VLDL) and chylomicrons (CM) are major sources of fatty acid supply to the heart, but little is known about their metabolism in diabetic myocardium. To investigate this, working hearts isolated from control rats and diabetic rats 2 wk following streptozotocin (STZ) injection were perfused with control and diabetic lipoproteins. Analysis of the diabetic lipoproteins showed that both VLDL and CM were altered compared with control lipoproteins; both were smaller and had different apolipoprotein composition. Heparin-releasable lipoprotein lipase (HR-LPL) activity was increased in STZ-induced diabetic hearts, but tissue residual LPL activity was decreased; moreover, diabetic lipoproteins stimulated HR-LPL activity in both diabetic and control hearts. Diabetic hearts oxidized lipoprotein-triacylglycerol (TAG) to a significantly greater extent than controls (>80% compared with deposition as tissue lipid), and the oxidation rate of exogenous lipoprotein-TAG was increased significantly in diabetic hearts regardless of TAG source. Significantly increased intracardiomyocyte TAG accumulation was found in diabetic hearts, although cardiac mechanical function was not inhibited, suggesting that lipotoxicity precedes impaired cardiac performance. Glucose oxidation was significantly decreased in diabetic hearts; additionally, however, diabetic lipoproteins decreased glucose oxidation in diabetic and control hearts. These results demonstrate increased TAG-rich lipoprotein metabolism concomitant with decreased glucose oxidation in type 1 diabetic hearts, and the alterations in cardiac lipoprotein metabolism may be due to the properties of diabetic TAG-rich lipoproteins as well as the diabetic state of the myocardium. These changes were not related to cardiomyopathy at this early stage of diabetes.     

Loss of lipoprotein lipase-derived fatty acids leads to increased cardiac glucose metabolism and heart dysfunction

Long-chain fatty acids (FAs) are the predominant energy substrate utilized by the adult heart. The heart can utilize unesterified FA bound to albumin or FA obtained from lipolysis of lipoprotein-bound triglyceride (TG). We used heart-specific lipoprotein lipase knock-out mice (hLpL0) to test whether these two sources of FA are interchangeable and necessary for optimal heart function. Hearts unable to obtain FA from lipoprotein TG were able to compensate by increasing glucose uptake, glycolysis, and glucose oxidation. HLpL0 hearts had decreased expression of pyruvate dehydrogenase kinase 4 and increased cardiomyocyte expression of glucose transporter 4. Conversely, FA oxidation rates were reduced in isolated perfused hLpL0 hearts. Following abdominal aortic constriction expression levels of genes regulating FA and glucose metabolism were acutely up-regulated in control and hLpL0 mice, yet all hLpL0 mice died within 48 h of abdominal aortic constriction. Older hLpL0 mice developed cardiac dysfunction characterized by decreased fractional shortening and interstitial and perivascular fibrosis. HLpL0 hearts had increased expression of several genes associated with transforming growth factor-beta signaling. Thus, long term reduction of lipoprotein FA uptake is associated with impaired cardiac function despite a compensatory increase in glucose utilization.     

Streptozotocin-induced diabetes in human apolipoprotein B transgenic mice. Effects on lipoproteins and atherosclerosis

The effects of diabetes and lipoprotein lipase (LpL) on plasma lipids were studied in mice expressing human apolipoprotein B (HuBTg). Our overall objective was to produce a diabetic mouse model in which the sole effects of blood glucose elevation on atherosclerosis could be assessed. Mice were made diabetic by intraperitoneal injection of streptozotocin, which led to a 2- to 2. 5-fold increase in plasma glucose. Lipids were assessed in mice on chow and on an atherogenic Western type diet (WTD), consisting of 21% (wt/wt) fat and 0.15% (wt/wt) cholesterol. Plasma triglyceride and cholesterol were the same in diabetic and non-diabetic mice on the chow diet. On the WTD, male diabetic HuBTg mice had a >50% increase in plasma cholesterol and more very low density lipoprotein (VLDL) cholesterol and triglyceride as assessed by FPLC analysis. A Triton study showed no increase in triglyceride or apolipoprotein B production, suggesting that the accumulation of VLDL was due to a decrease in lipoprotein clearance. Surprisingly, the VLDL increase in these mice was not due to a decrease in LpL activity in postheparin plasma. To test whether LpL overexpression would alter these diabetes-induced lipoprotein changes, HuBTg mice were crossed with mice expressing human LpL in muscle. LpL overexpression reduced plasma triglyceride, but not cholesterol, in male mice on WTD. Aortic root atherosclerosis assessed in 32-week-old mice on the WTD was not greater in diabetic mice. In summary, diabetes primarily increased plasma VLDL in HuBTg mice. LpL activity was not decreased in these animals. However, additional LpL expression eliminated the diabetic lipoprotein changes. These mice did not have more atherosclerosis with diabetes.     

Cardiac overexpression of hormone-sensitive lipase inhibits myocardial steatosis and fibrosis in streptozotocin diabetic mice

Intracellular lipid accumulation (steatosis) and resultant lipotoxicity are key features of diabetic cardiomyopathy. Since cardiac hormone-sensitive lipase (HSL) is activated in diabetic mice, we sought to explore a pathophysiological function of cardiac HSL in the development of diabetic cardiomyopathy. Transgenic (Tg) mice with heart-specific HSL overexpression were generated, and cardiac histology, function, lipid profile, and gene expressions were analyzed after induction of diabetes by streptozotocin. Electron microscopy showed numerous lipid droplets in wild-type (Wt) hearts after 3 wk of diabetes, whereas Tg mice showed no lipid droplet accumulation. Cardiac content of acylglycerides was increased approximately 50% with diabetes in Wt mice, whereas this was blunted in Tg hearts. Cardiac lipid peroxide content was twofold lower in Tg hearts than in Wt hearts. The mRNA expressions for peroxisome proliferator-activated receptor-alpha, genes for triacylglycerol synthesis, and lipoprotein lipase were increased with diabetes in Wt hearts, whereas this induction was absent in Tg hearts. Expression of genes associated with lipoapoptosis was decreased, whereas antioxidant protein metallothioneins were increased in diabetic Tg hearts. Diabetic Wt hearts showed interstitial fibrosis and increased collagen content. However, Tg hearts displayed no overt fibrosis, concomitant with decreased expression of collagens, transforming growth factor-beta, and matrix metalloproteinase 2. Notably, mortality during the experimental period was approximately twofold lower in diabetic Tg mice compared with Wt mice. In conclusion, since HSL overexpression inhibits cardiac steatosis and fibrosis by apparently hydrolyzing toxic lipid metabolites, cardiac HSL could be a therapeutic target for regulating diabetic cardiomyopathy.     

Cardiac metabolic compensation to hypertension requires lipoprotein lipase

Fatty acids (FAs) are acquired from free FA associated with albumin and lipoprotein triglyceride that is hydrolyzed by lipoprotein lipase (LpL). Hypertrophied hearts shift their substrate usage pattern to more glucose and less FA. However, FAs may still be an important source of energy in hypertrophied hearts. The aim of this study was to examine the importance of LpL-derived FAs in hypertensive hypertrophied hearts. We followed cardiac function and metabolic changes during 2 wk of angiotensin II (ANG II)-induced hypertension in control and heart-specific lipoprotein lipase knockout (hLpL0) mice. Glucose metabolism was increased in ANG II-treated control (control/ANG II) hearts, raising it to the same level as hLpL0 hearts. FA uptake-related genes, CD36 and FATP1, were reduced in control/ANG II hearts to levels found in hLpL0 hearts. ANG II did not alter these metabolic genes in hLpL0 mice. LpL activity was preserved, and mitochondrial FA oxidation-related genes were not altered in control/ANG II hearts. In control/ANG II hearts, triglyceride stores were consumed and reached the same levels as in hLpL0/ANG II hearts. Intracellular ATP content was reduced only in hLpL0/ANG II hearts. Both ANG II and deoxycorticosterone acetate-salt induced hypertension caused heart failure only in hLpL0 mice. Our data suggest that LpL activity is required for normal cardiac metabolic compensation to hypertensive stress.     

Cardiomyocyte Aldose Reductase Causes Heart Failure and Impairs Recovery from Ischemia

Aldose reductase (AR), an enzyme mediating the first step in the polyol pathway of glucose metabolism, is associated with complications of diabetes mellitus and increased cardiac ischemic injury. We investigated whether deleterious effects of AR are due to its actions specifically in cardiomyocytes. We created mice with cardiac specific expression of human AR (hAR) using the α–myosin heavy chain (MHC) promoter and studied these animals during aging and with reduced fatty acid (FA) oxidation. hAR transgenic expression did not alter cardiac function or glucose and FA oxidation gene expression in young mice. However, cardiac overexpression of hAR caused cardiac dysfunction in older mice. We then assessed whether hAR altered heart function during ischemia reperfusion. hAR transgenic mice had greater infarct area and reduced functional recovery than non-transgenic littermates. When the hAR transgene was crossed onto the PPAR alpha knockout background, another example of greater heart glucose oxidation, hAR expressing mice had increased heart fructose content, cardiac fibrosis, ROS, and apoptosis. In conclusion, overexpression of hAR in cardiomyocytes leads to cardiac dysfunction with aging and in the setting of reduced FA and increased glucose metabolism. These results suggest that pharmacological inhibition of AR will be beneficial during ischemia and in some forms of heart failure.     

Therapy with Astragalus polysaccharides rescues lipotoxic cardiomyopathy in MHC-PPARα mice

Obesity-related diabetes mellitus leads to lipotoxic cardiomyopathy resulting in a form of cardiac dysfunction. Mice with heart-specific overexpression of peroxisome proliferator-activated receptor (PPAR) α showed a metabolic and cardiomyopathic phenotype similar to the diabetic heart. To define the role of Astragalus Polysaccharides (APS) treatment for PPARα-mediated lipotoxicity in the pathogenesis of diabetic cardiomyopathy, myosin heavy chain [MHC]-PPARα mice with high-fat diet were administrated with APS or vehicle for 16 weeks. The APS treatment prevented myocardial long-chain triglyceride accumulation, ultrastructure abnormality in cardiac myocytes and cardiac dysfunction in the MHC-PPARα mice, which was associated with reduced free fatty acids (FFA) utilization and increased glucose uptake and oxidation in hearts by APS treatment. Consistent with the metabolic changes, activation of PPARα gene regulatory pathway involved in FFA-oxidation in the MHC-PPARα heart was down-regulated by APS treatment, while suppression of PPARα target genes involved in glucose uptake and oxidation in the MHC-PPARα hearts was normalized by APS administration. We conclude that therapy with APS might lead to the inhibition of PPARα-mediate lipotoxicity in the pathogenesis of diabetic cardiomyopathy.     

Differential effects of high-fat diet on myocardial lipid metabolism in failing and nonfailing hearts with angiotensin II-mediated cardiac remodeling in mice

Normal myocardium adapts to increase of nutritional fatty acid supply by upregulation of regulatory proteins of the fatty acid oxidation pathway. Because advanced heart failure is associated with reduction of regulatory proteins of fatty acid oxidation, we hypothesized that failing myocardium may not be able to adapt to increased fatty acid intake and therefore undergo lipid accumulation, potentially aggravating myocardial dysfunction. We determined the effect of high-fat diet in transgenic mice with overexpression of angiotensinogen in the myocardium (TG1306/R1). TG1306/R1 mice develop ANG II-mediated left ventricular hypertrophy, and at one year of age approximately half of the mice present heart failure associated with reduced expression of regulatory proteins of fatty acid oxidation and reduced palmitate oxidation during ex vivo working heart perfusion. Hypertrophied hearts from TG1306/R1 mice without heart failure adapted to high-fat feeding, similarly to hearts from wild-type mice, with upregulation of regulatory proteins of fatty acid oxidation and enhancement of palmitate oxidation. There was no myocardial lipid accumulation or contractile dysfunction. In contrast, hearts from TG1306/R1 mice presenting heart failure were unable to respond to high-fat feeding by upregulation of fatty acid oxidation proteins and enhancement of palmitate oxidation. This resulted in accumulation of triglycerides and ceramide in the myocardium, and aggravation of contractile dysfunction. In conclusion, hearts with ANG II-induced contractile failure have lost the ability to enhance fatty acid oxidation in response to increased fatty acid supply. The ensuing accumulation of lipid compounds may play a role in the observed aggravation of contractile dysfunction.     

Adverse effects of constitutively active alpha(1B)-adrenergic receptors after pressure overload in mouse hearts

Cardiac hypertrophy and function were studied 6 wk after constriction of the thoracic aorta (TAC) in transgenic (TG) mice expressing constitutively active mutant alpha(1B)-adrenergic receptors (ARs) in the heart. Hearts from sham-operated TG animals and nontransgenic littermates (WT) were similar in size, but hearts from TAC/TG mice were larger than those from TAC/WT mice, and atrial natriuretic peptide mRNA expression was also higher. Lung weight was markedly increased in TAC/TG animals, and the incidence of left atrial thrombus formation was significantly higher. Ventricular contractility in anesthetized animals, although it was increased in TAC/WT hearts, was unchanged in TAC/TG hearts, implying cardiac decompensation and progression to failure in TG mice. There was no increase in alpha(1A)-AR mRNA expression in TAC/WT hearts, and expression was significantly reduced in TAC/TG hearts. These findings show that cardiac expression of constitutively actively mutant alpha(1B)-ARs is detrimental in terms of hypertrophy and cardiac function after pressure overload and that increased alpha(1A)-AR mRNA expression is not a feature of the hypertrophic response in this murine model.     

Metoprolol improves cardiac function and modulates cardiac metabolism in the streptozotocin-diabetic rat

The effects of diabetes on heart function may be initiated or compounded by the exaggerated reliance of the diabetic heart on fatty acids and ketones as metabolic fuels. beta-Blocking agents such as metoprolol have been proposed to inhibit fatty acid oxidation. We hypothesized that metoprolol would improve cardiac function by inhibiting fatty acid oxidation and promoting a compensatory increase in glucose utilization. We measured ex vivo cardiac function and substrate utilization after chronic metoprolol treatment and acute metoprolol perfusion. Chronic metoprolol treatment attenuated the development of cardiac dysfunction in streptozotocin (STZ)-diabetic rats. After chronic treatment with metoprolol, palmitate oxidation was increased in control hearts but decreased in diabetic hearts without affecting myocardial energetics. Acute treatment with metoprolol during heart perfusions led to reduced rates of palmitate oxidation, stimulation of glucose oxidation, and increased tissue ATP levels. Metoprolol lowered malonyl-CoA levels in control hearts only, but no changes in acetyl-CoA carboxylase phosphorylation or AMP-activated protein kinase activity were observed. Both acute metoprolol perfusion and chronic in vivo metoprolol treatment led to decreased maximum activity and decreased sensitivity of carnitine palmitoyltransferase I to malonyl-CoA. Metoprolol also increased sarco(endo)plasmic reticulum Ca(2+)-ATPase expression and prevented the reexpression of atrial natriuretic peptide in diabetic hearts. These data demonstrate that metoprolol ameliorates diabetic cardiomyopathy and inhibits fatty acid oxidation in streptozotocin-induced diabetes. Since malonyl-CoA levels are not increased, the reduction in total carnitine palmitoyltransferase I activity is the most likely factor to explain the decrease in fatty acid oxidation. The metabolism changes occur in parallel with changes in gene expression.     

Remnant lipoprotein particles are taken up into myocardium through VLDL receptor--a possible mechanism for cardiac fatty acid metabolism

The VLDL (very low-density lipoprotein) receptor is a peripheral lipoprotein receptor expressing in fatty acid active tissues abundantly. In the Balb/c fasting mice, VLDL receptor as well as LPL (lipoprotein lipase), FAT (fatty acid translocase)/CD36, H-FABP (heart-type fatty acid-binding protein), ACS (acyl-CoA synthetase) and LCAD (long-chain acyl-CoA dehydrogenase) expressions increased. An electron microscopic examination indicated the lipid droplets that accumulated in the hearts of fasting Balb/c mice. During the development of SD (Sprague-Dawley) rats, VLDL receptor, LPL, FAT/CD36, H-FABP, ACS, and LCAD mRNAs concomitantly increased with growth. However, PK (pyruvate kinase) mRNA expression was negligible. In cultured neonatal rat cardiomyocytes, VLDL receptor expression increased with days in culture. Oil red-O staining showed that cardiomyocytes after 7 days in culture (when the VLDL receptor protein is present) accumulated beta-migrating VLDL. Thereby, we showed that the cardiac VLDL receptor pathway for delivery of remnant lipoprotein particles might be part of a cardiac fatty acid metabolism.     

Glucose metabolism in perfused mouse hearts overexpressing human GLUT-4 glucose transporter

Glucose and fatty acid metabolism was assessed in isolated working hearts from control C57BL/KsJ-m+/+db mice and transgenic mice overexpressing the human GLUT-4 glucose transporter (db/+-hGLUT-4). Heart rate, coronary flow, cardiac output, and cardiac power did not differ between control hearts and hearts overexpressing GLUT-4. Hearts overexpressing GLUT-4 had significantly higher rates of glucose uptake and glycolysis and higher levels of glycogen after perfusion than control hearts, but rates of glucose and palmitate oxidation were not different. Insulin (1 mU/ml) significantly increased glycogen levels in both groups. Insulin increased glycolysis in control hearts but not in GLUT-4 hearts, whereas glucose oxidation was increased by insulin in both groups. Therefore, GLUT-4 overexpression increases glycolysis, but not glucose oxidation, in the heart. Although control hearts responded to insulin with increased rates of glycolysis, the enhanced entry of glucose in the GLUT-4 hearts was already sufficient to maximally activate glycolysis under basal conditions such that insulin could not further stimulate the glycolytic rate.