2-Tetradecylglycidic acid, an inhibitor of carnitine palmitoyltransferase-1, induces myocardial hypertrophy via the AT1 receptor

Sirt5 improves cardiomyocytes fatty acid metabolism and ameliorates cardiac lipotoxicity in diabetic cardiomyopathy via CPT2 de-succinylation

RATIONALE

The disruption of the balance between fatty acid (FA) uptake and oxidation (FAO) leads to cardiac lipotoxicity, serving as the driving force behind diabetic cardiomyopathy (DbCM). Sirtuin 5 (Sirt5), a lysine de-succinylase, could impact diverse metabolic pathways, including FA metabolism. Nevertheless, the precise roles of Sirt5 in cardiac lipotoxicity and DbCM remain unknown.

OBJECTIVE

This study aims to elucidate the role and underlying mechanism of Sirt5 in the context of cardiac lipotoxicity and DbCM.

METHODS AND RESULTS

The expression of myocardial Sirt5 was found to be modestly elevated in diabetic heart failure patients and mice. Cardiac dysfunction, hypertrophy and lipotoxicity were exacerbated by ablation of Sirt5 but improved by forced expression of Sirt5 in diabetic mice. Notably, Sirt5 deficiency impaired FAO without affecting the capacity of FA uptake in the diabetic heart, leading to accumulation of FA intermediate metabolites, which mainly included medium- and long-chain fatty acyl-carnitines. Mechanistically, succinylomics analyses identified carnitine palmitoyltransferase 2 (CPT2), a crucial enzyme involved in the reconversion of fatty acyl-carnitines to fatty acyl-CoA and facilitating FAO, as the functional succinylated substrate mediator of Sirt5. Succinylation of Lys424 in CPT2 was significantly increased by Sirt5 deficiency, leading to the inactivation of its enzymatic activity and the subsequent accumulation of fatty acyl-carnitines. CPT2 K424R mutation, which mitigated succinylation modification, counteracted the reduction of enzymatic activity in CPT2 mediated by Sirt5 deficiency, thereby attenuating Sirt5 knockout-induced FAO impairment and lipid deposition.

CONCLUSIONS

Sirt5 deficiency impairs FAO, leading to cardiac lipotoxicity in the diabetic heart through the succinylation of Lys424 in CPT2. This underscores the potential roles of Sirt5 and CPT2 as therapeutic targets for addressing DbCM.

Progress of potential drugs targeted in lipid metabolism research

Lipids are a class of complex hydrophobic molecules derived from fatty acids that not only form the structural basis of biological membranes but also regulate metabolism and maintain energy balance. The role of lipids in obesity and other metabolic diseases has recently received much attention, making lipid metabolism one of the attractive research areas. Several metabolic diseases are linked to lipid metabolism, including diabetes, obesity, and atherosclerosis. Additionally, lipid metabolism contributes to the rapid growth of cancer cells as abnormal lipid synthesis or uptake enhances the growth of cancer cells. This review introduces the potential drug targets in lipid metabolism and summarizes the important potential drug targets with recent research progress on the corresponding small molecule inhibitor drugs. The significance of this review is to provide a reference for the clinical treatment of metabolic diseases related to lipid metabolism and the treatment of tumors, hoping to deepen the understanding of lipid metabolism and health.

Carnitine Supplementation Attenuates Sunitinib-Induced Inhibition of AMP-Activated Protein Kinase Downstream Signals in Cardiac Tissues

This study has been initiated to investigate whether sunitinib (SUN) alters the expression of key genes engaged in mitochondrial transport and oxidation of long chain fatty acids (LCFA), and if so, whether these alterations should be viewed as a mechanism of SUN-induced cardiotoxicity, and to explore the molecular mechanisms whereby carnitine supplementation could attenuate SUN-induced cardiotoxicity. Adult male Wister albino rats were assigned to one of the four treatment groups: Rats in group 1 received no treatment but free access to tap water for 28 days. Rats in group 2 received L-carnitine (200 mg/kg/day) in drinking water for 28 days. Rats in group 3 received SUN (25 mg/kg/day) in drinking water for 28 days. Rats in group 4 received the same doses of L-carnitine and SUN in drinking water for 28 days. Treatment with SUN significantly increased heart weight, cardiac index, and cardiotoxicity enzymatic indices, as well as severe histopathological changes. Moreover, SUN significantly decreased level of adenosine monophosphate-activated protein kinase (AMPKα2), total carnitine, adenosine triphosphate (ATP) and carnitine palmitoyltransferase I (CPT I) expression and significantly increased acetyl-CoA carboxylase-2 (ACC2) expression and malonyl-CoA level in cardiac tissues. Interestingly, carnitine supplementation resulted in a complete reversal of all the biochemical, gene expression and histopathological changes-induced by SUN to the control values. In conclusion, data from this study suggest that SUN inhibits AMPK downstream signaling with the consequent inhibition of mitochondrial transport of LCFA and energy production in cardiac tissues. Carnitine supplementation attenuates SUN-induced cardiotoxicity.

Dronedarone-Induced Cardiac Mitochondrial Dysfunction and Its Mitigation by Epoxyeicosatrienoic Acids

Dronedarone and amiodarone are structurally similar antiarrhythmic drugs. Dronedarone worsens cardiac adverse effects with unknown causes while amiodarone has no cardiac adversity. Dronedarone induces preclinical mitochondrial toxicity in rat liver and exhibits clinical hepatotoxicity. Here, we further investigated the relative potential of the antiarrhythmic drugs in causing mitochondrial injury in cardiomyocytes. Differentiated rat H9c2 cardiomyocytes were treated with dronedarone, amiodarone, and their respective metabolites namely N-desbutyldronedarone (NDBD) and N-desethylamiodarone (NDEA). Intracellular ATP content, mitochondrial membrane potential (Δψm), and inhibition of carnitine palmitoyltransferase I (CPT1) activity and arachidonic acid (AA) metabolism were measured in H9c2 cells. Inhibition of electron transport chain (ETC) activities and uncoupling of ETC were further studied in isolated rat heart mitochondria. Dronedarone, amiodarone, NDBD and NDEA decreased intracellular ATP content significantly (IC50 = 0.49, 1.84, 1.07, and 0.63 µM, respectively) and dissipated Δψm potently (IC50 = 0.5, 2.94, 12.8, and 7.38 µM, respectively). Dronedarone, NDBD, and NDEA weakly inhibited CPT1 activity while amiodarone (IC50 > 100 µM) yielded negligible inhibition. Only dronedarone inhibited AA metabolism to its regioisomeric epoxyeicosatrienoic acids (EETs) consistently and potently. NADH-supplemented ETC activity was inhibited by dronedarone, amiodarone, NDBD and NDEA (IC50 = 3.07, 5.24, 11.94, and 16.16 µM, respectively). Cytotoxicity, ATP decrease and Δψm disruption were ameliorated via exogenous pre-treatment of H9c2 cells with 11, 12-EET and 14, 15-EET. Our study confirmed that dronedarone causes mitochondrial injury in cardiomyocytes by perturbing Δψm, inhibiting mitochondrial complex I, uncoupling ETC and dysregulating AA-EET metabolism. We postulate that cardiac mitochondrial injury is one potential contributing factor to dronedarone-induced cardiac failure exacerbation.

Cardiac-specific down-regulation of carnitine palmitoyltransferase-1b (CPT-1b) prevents cardiac remodeling in obese mice

OBJECTIVE

To determine whether inhibiting cardiac carnitine palmitoyltransferase-1b (CPT-1b) improves obesity-related cardiomyopathy.

METHODS

Four-week-old male C57BL/6J mice were fed with high-fat diet (HFD) for 12 weeks to induce obesity. At 6 weeks of age, mice were subjected to intramyocardial injection with lentivirus to down-regulate the expression of either cardiac CPT-1b or green fluorescent protein. Morphological, biochemical, functional, histological, and ultrastructural profiles were assessed at 16 weeks of age.

RESULTS

HFD administration elicited obesity, cardiac hypertrophy, and systolic dysfunction accompanied with altered biochemical parameters. In addition, HFD consumption promoted lipid accumulation and reactive oxygen species generation in cardiomyocytes and damaged myocardial ultrastructure. Cardiac CPT-1b silencing protected against HFD-induced cardiac remodeling by decreasing heart weight/tibial length ratio and increasing left ventricular ejection fraction and fractional shortening, as well as normalizing left ventricular diameter. Meanwhile, CPT-1b inhibition mitigated the changes in biochemical parameters, aggravated myocardial lipid accumulation, reduced intramyocardial reactive oxygen species production, and partly amended myocardial ultrastructural alterations in obese mice.

CONCLUSIONS

Cardiac CPT-1b suppression protects against the aggravation of cardiac morphology and function associated with HFD feeding. CPT-1b represents a potential therapeutic target for the treatment of cardiac dysfunction related to metabolic diseases such as obesity and diabetes.

Inhibition of gene expression of carnitine palmitoyltransferase I and heart fatty acid binding protein in cyclophosphamide and ifosfamide-induced acute cardiotoxic rat models

This study investigated whether cyclophosphamide (CP) and ifosfamide (IFO) therapy alters the expression of the key genes engaged in long-chain fatty acid (LCFA) oxidation outside rat heart mitochondria, and if so, whether these alterations should be viewed as a mechanism during CP- and IFO-induced cardiotoxicity. Adult male Wistar albino rats were assigned to one of the six treatment groups: Rats in group 1 (control) and group 2 (L-carnitine) were injected intraperitoneal (i.p.) with normal saline and L-carnitine (200 mg/kg/day), respectively, for 10 successive days. Animals in group 3 (CP group) were injected i.p. with normal saline for 5 days before and 5 days after a single dose of CP (200 mg/kg, i.p.). Rats in group 4 (IFO group) received normal saline for 5 successive days followed by IFO (50 mg/kg/day, i.p.) for 5 successive days. Rats in group 5 (CP-carnitine supplemented) were given the same doses of L-carnitine as group 2 for 5 days before and 5 days after a single dose of CP as group 3. Rats in group 6 (IFO-carnitine supplemented) were given the same doses of L-carnitine as group 2 for 5 days before and 5 days concomitant with IFO as group 4. Immediately, after the last dose of the treatment protocol, blood samples were withdrawn and animals were killed for biochemical, histopathological and gene expression studies. Treatment with CP and IFO significantly decreased expression of heart fatty acid binding protein (H-FABP) and carnitine palmitoyltransferase I (CPT I) genes in cardiac tissues. Moreover, CP but not IFO significantly increased acetyl-CoA carboxylase2 mRNA expression. Conversely, IFO but not CP significantly decreased mRNA expression of malonyl-CoA decarboxylase. Both CP and IFO significantly increased serum lactate dehydrogenase, creatine kinase isoenzyme MB and malonyl-CoA content and histopathological lesions in cardiac tissues. Interestingly, carnitine supplementation completely reversed all the biochemical, histopathological and gene expression changes induced by CP and IFO to the control values, except CPT I mRNA, and protein expression remained inhibited by IFO. Data from the current study suggest, for the first time, that (1) CP and IFO therapy is associated with the inhibition of the expression of H-FABP and CPT I genes in cardiac tissues with the consequent inhibition of mitochondrial transport and oxidation of LCFA. (2) The progressive increase in cardiotoxicity enzymatic indices and the decrease in H-FABP and CPT I expression may point to the possible contribution of these genes to CP- and IFO-induced cardiotoxicity.

Carnitine palmitoyltransferase-1b deficiency aggravates pressure overload-induced cardiac hypertrophy caused by lipotoxicity

BACKGROUND

Carnitine palmitoyltransferase-1 (CPT1) is a rate-limiting step of mitochondrial β-oxidation by controlling the mitochondrial uptake of long-chain acyl-CoAs. The muscle isoform, CPT1b, is the predominant isoform expressed in the heart. It has been suggested that inhibiting CPT1 activity by specific CPT1 inhibitors exerts protective effects against cardiac hypertrophy and heart failure. However, clinical and animal studies have shown mixed results, thereby creating concerns about the safety of this class of drugs. Preclinical studies using genetically modified animal models should provide a better understanding of targeting CPT1 to evaluate it as a safe and effective therapeutic approach.

METHODS AND RESULTS

Heterozygous CPT1b knockout (CPT1b(+/-)) mice were subjected to transverse aorta constriction-induced pressure overload. These mice showed overtly normal cardiac structure/function under the basal condition. Under a severe pressure-overload condition induced by 2 weeks of transverse aorta constriction, CPT1b(+/-) mice were susceptible to premature death with congestive heart failure. Under a milder pressure-overload condition, CPT1b(+/-) mice exhibited exacerbated cardiac hypertrophy and remodeling compared with wild-type littermates. There were more pronounced impairments of cardiac contraction with greater eccentric cardiac hypertrophy in CPT1b(+/-) mice than in control mice. Moreover, the CPT1b(+/-) heart exhibited exacerbated mitochondrial abnormalities and myocardial lipid accumulation with elevated triglycerides and ceramide content, leading to greater cardiomyocyte apoptosis.

CONCLUSIONS

CPT1b deficiency can cause lipotoxicity in the heart under pathological stress, leading to exacerbation of cardiac pathology. Therefore, caution should be exercised in the clinical use of CPT1 inhibitors.

Sustained activation of the mammalian target of rapamycin nutrient sensing pathway is associated with hepatic insulin resistance, but not with steatosis, in mice

AIMS/HYPOTHESIS

Activation of nutrient sensing through mammalian target of rapamycin (mTOR) has been linked to the pathogenesis of insulin resistance. We examined activation of mTOR-signalling in relation to insulin resistance and hepatic steatosis in mice.

MATERIALS AND METHODS

Chronic hepatic steatosis and hepatic insulin resistance were induced by high-fat feeding of male C57BL/6Jico mice for 6 weeks. In addition, acute hepatic steatosis in the absence of insulin resistance was induced by pharmacological blockade of beta-oxidation using tetradecylglycidic acid (TDGA). mTOR signalling was examined in liver homogenates.

RESULTS

High-fat feeding caused obesity (p<0.001), hepatic steatosis (p<0.05) and hepatic insulin resistance (p<0.05). The phosphorylation of mTOR and its downstream targets p70S6 kinase and S6 ribosomal protein was two-fold higher in mice on a high-fat diet than in mice fed standard chow (all p<0.05) and associated with enhanced rates of protein synthesis. Acute induction of hepatic steatosis with TDGA had no effect on mTOR activity. The increased activity of the mTOR pathway in livers from mice on a high-fat diet could not be ascribed to diet-induced alterations in known modulators of mTOR activity such as circulating plasma leucine levels, phosphorylation of protein kinase B and AMP-activated protein kinase, and changes in mitochondrial function.

CONCLUSIONS/INTERPRETATION

High-fat diet induces increase of the mTOR nutrient sensing pathway in association with hepatic insulin resistance, but not with hepatic lipid accumulation as such.

Acute hepatic steatosis in mice by blocking beta-oxidation does not reduce insulin sensitivity of very-low-density lipoprotein production

Accumulation of triglycerides (TG) in the liver is generally associated with hepatic insulin resistance. We questioned whether acute hepatic steatosis induced by pharmacological blockade of beta-oxidation affects hepatic insulin sensitivity, i.e., insulin-mediated suppression of VLDL production and insulin-induced activation of phosphatidylinositol 3-kinase (PI3-kinase) and PKB. Tetradecylglycidic acid (TDGA), an inhibitor of carnitine palmitoyl transferase-1 (CPT1), was used for this purpose. Male C57BL/6J mice received 30 mg/kg TDGA or its solvent intraperitoneally and were subsequently fasted for 12 h. CPT1 inhibition resulted in severe microvesicular hepatic steatosis (19.9 +/- 8.3 vs. 112.4 +/- 25.2 nmol TG/mg liver, control vs. treated, P < 0.05) with elevated plasma nonesterified fatty acid (0.68 +/- 0.25 vs. 1.21 +/- 0.41 mM, P < 0.05) and plasma TG (0.39 +/- 0.16 vs. 0.60 +/- 0.10 mM, P < 0.05) concentrations. VLDL-TG production rate was not affected on CPT1 inhibition (74.9 +/- 15.2 vs. 79.1 +/- 12.8 mumol TG.kg(-1).min(-1), control vs. treated) although treated mice secreted larger VLDL particles (59.3 +/- 3.6 vs. 66.6 +/- 4.5 nm diameter, P < 0.05). Infusion of insulin under euglycemic conditions suppressed VLDL production rate in control and treated mice by 43 and 54%, respectively, with formation of smaller VLDL particles (51.2 +/- 2.5 and 53.2 +/- 2.8 nm diameter). Insulin-induced insulin receptor substrate (IRS)1- and IRS2-associated PI3-kinase activity and PKB-phosphorylation were not affected on TDGA treatment. In conclusion, acute hepatic steatosis caused by pharmacological inhibition of beta-oxidation is not associated with reduced hepatic insulin sensitivity, indicating that hepatocellular fat content per se is not causally related to insulin resistance.

Activation of leptin expression by an inhibitor of carnitine palmitoyltransferase-1

Leptin may regulate peripheral fatty acid oxidation and invoke a feedback mechanism that affects leptin expression in adipocytes. The objective of this study, therefore, was to determine whether inhibiting systemic fatty acid oxidation at the level of carnitine palmitoyltransferase-1 (CPT1) affects leptin expression. To accomplish this objective, fed or overnight fasted rats were treated with 2-tetradecylglycidic acid (TDGA), a specific, irreversible CPT1 inhibitor, and acute changes in rat epididymal leptin expression and serum leptin content were measured using Northern, RT-PCR, and radioimmunoassay analyses. Overnight fasting decreased both epididymal leptin mRNA content and serum leptin. Treating overnight fasted rats with TDGA increased both their epididymal leptin mRNA and their serum leptin significantly in a time- and concentration-dependent manner. TDGA affected neither epididymal leptin mRNA nor serum leptin in fed rats where systemic fatty acid oxidation is low. These results support the conclusion that CPT1-linked fatty acid oxidation is a key modulator of leptin expression in fasting rats.

Lipid metabolism in the heart--contribution of BMIPP to the diseased heart

Lipid contributes greatly in cardiac metabolism to produce high energy ATPs, and is suggested to be related to the progression and deterioration of heart disease. It is fortunate that the I-123-betamethyliodophenylpentadecanoic acid (BMIPP) imaging technique is now available in determining heart condition, but we must be cautious about the interpretation of images obtained with this new tracer. From the uptake of BMIPP into the cell to breakdown and catabolism of it, there exist so many critical enzymatical pathways relating to the modification of BMIPP imaging. In clinical evaluation, the image will be translated as the integral effects of these pathways. In other words, we must be aware of these critical pathways regulating lipid metabolism and modifying factors in order to correctly understand BMIPP imaging. Lipid transport is affected by the albumin/FFA ratio in the blood, and extraction with membrane transporter proteins. Fatty acid binding protein (FABP) in the cytosole will play an important role in regulating lipid flux and following metabolism. Lipid will be utilized either for oxidation, triglyceride or phospholipid formation. For oxidation, carnitine palmitoil transferase is the key enzyme for the entrance of lipid into mitochondria, and oxidative enzymes such as acyl CoA dehydrogenase (MCAD, LCAD, HAD) will determine lipid use for the TCA cycle. ATPs produced in the mitochondria again limit the TG store. It is well known that BMIPP imaging completely changes in the ischemic condition, and is also shown that lipid metabolical regulation completely differs from normal in the very early phase of cardiac hypertrophy. In the process of deteriorating heart failure, metabolical switching of lipid with glucose will take place. In such a different heart disease conditions, it is clear that lipid metabolical regulation, including many lipid enzymes, works differently from in the healthy condition. These lipid enzymes are regulated by nuclear factor peroxisome proliferator-activated receptors (PPAR) just like a conductor of an orchestra. Most of the regulating mechanisms of the PPAR are still unknown, but reduction of this nuclear factor is shown in the process of decompensated heart failure. This review is based by mostly on our fundamental and Japanese clinical data. BMIPP has been used clinically in abundant cases in Japan. In such situations, further correct information on lipid metabolism, including BMIPP, will contribute to the understanding of deteriorating heart disease and its prognosis.

Effects of carnitine palmitoyltransferase I inhibitors on hepatic hypertrophy

We investigated the effect of two types of carnitine palmitoyltransferase I inhibitors, ethyl 2-(6-(4-chlorophenoxy)hexyl)oxirane-2-carboxylate (etomoxir) and (R)-3-carboxy-N,N, N-trimethyl-2-¿[hydroxy(tetradecyloxy)phosphinyl]oxy¿-1-propana minium hydroxide (SDZ CPI 975), on cardiac and hepatic hypertrophy in ddY mice. One-week administration of etomoxir caused cardiac and hepatic hypertrophy, 19% and 22% as a ratio to body weight, respectively. Although 4-week administration of etomoxir caused hepatic hypertrophy, there was no significant change in liver triglyceride content in the first or second week. In cultured HepG(2) cells, etomoxir treatment (1 week) did not cause triglyceride to accumulate. One-week administration of SDZ CPI 975 caused neither cardiac nor hepatic hypertrophy. In vitro, neither drug had selectivity for carnitine palmitoyltransferase I isozymes. These findings suggest that the hepatic hypertrophy following 1- or 2-week treatment with etomoxir is caused by mechanisms different from those responsible for triglyceride accumulation, and that inhibition of carnitine palmitoyltransferase I may not necessarily induce hepatic hypertrophy.