Expression of three carnitine palmitoyltransferase-I isoforms in 10 regions of the rat brain during feeding, fasting, and diabetes

The Astrocyte: Metabolic Hub of the Brain

Astrocytic metabolism has taken center stage. Interposed between the neuron and the vasculature, astrocytes exert control over the fluxes of energy and building blocks required for neuronal activity and plasticity. They are also key to local detoxification and waste recycling. Whereas neurons are metabolically rigid, astrocytes can switch between different metabolic profiles according to local demand and the nutritional state of the organism. Their metabolic state even seems to be instructive for peripheral nutrient mobilization and has been implicated in information processing and behavior. Here, we summarize recent progress in our understanding of astrocytic metabolism and its effects on metabolic homeostasis and cognition.

Glycolytically impaired Drosophila glial cells fuel neural metabolism via β-oxidation

Neuronal function is highly energy demanding and thus requires efficient and constant metabolite delivery by glia. Drosophila glia are highly glycolytic and provide lactate to fuel neuronal metabolism. Flies are able to survive for several weeks in the absence of glial glycolysis. Here, we study how Drosophila glial cells maintain sufficient nutrient supply to neurons under conditions of impaired glycolysis. We show that glycolytically impaired glia rely on mitochondrial fatty acid breakdown and ketone body production to nourish neurons, suggesting that ketone bodies serve as an alternate neuronal fuel to prevent neurodegeneration. We show that in times of long-term starvation, glial degradation of absorbed fatty acids is essential to ensure survival of the fly. Further, we show that Drosophila glial cells act as a metabolic sensor and can induce mobilization of peripheral lipid stores to preserve brain metabolic homeostasis. Our study gives evidence of the importance of glial fatty acid degradation for brain function, and survival, under adverse conditions in Drosophila.

Mechanisms underlying neonate-specific metabolic effects of volatile anesthetics

Volatile anesthetics (VAs) are widely used in medicine, but the mechanisms underlying their effects remain ill-defined. Though routine anesthesia is safe in healthy individuals, instances of sensitivity are well documented, and there has been significant concern regarding the impact of VAs on neonatal brain development. Evidence indicates that VAs have multiple targets, with anesthetic and non-anesthetic effects mediated by neuroreceptors, ion channels, and the mitochondrial electron transport chain. Here, we characterize an unexpected metabolic effect of VAs in neonatal mice. Neonatal blood β-hydroxybutarate (β-HB) is rapidly depleted by VAs at concentrations well below those necessary for anesthesia. β-HB in adults, including animals in dietary ketosis, is unaffected. Depletion of β-HB is mediated by citrate accumulation, malonyl-CoA production by acetyl-CoA carboxylase, and inhibition of fatty acid oxidation. Adults show similar significant changes to citrate and malonyl-CoA, but are insensitive to malonyl-CoA, displaying reduced metabolic flexibility compared to younger animals.

Mitochondrial Metabolism in Astrocytes Regulates Brain Bioenergetics, Neurotransmission and Redox Balance

In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.

The Effect of S-15176 Difumarate Salt on Ultrastructure and Functions of Liver Mitochondria of C57BL/6 Mice with Streptozotocin/High-Fat Diet-Induced Type 2 Diabetes

Simple Summary

Type II diabetes mellitus (T2DM) is one of the most common diseases, which currently represents a major medical and social problem due to the chronic course, high rates of disability and mortality among patients. Mitochondria of the liver and other vital organs are one of the main targets of T2DM at the intracellular level. The pathological changes in the structure of mitochondria, hyperproduction of reactive oxygen species by the organelles, disorders in mitochondrial transport systems and ATP synthesis are now widely recognized as important factors in the development of diabetes. Therefore, treatment strategies to attenuate mitochondrial injury may result in cellular reprogramming and alleviation of the diabetes-related pathological complications. The aim of present work was to investigate the possible protective effect of S-15176, a potent derivative of the anti-ischemic agent trimetazidine, against mitochondrial damage in the liver of mice with experimental T2DM. The data indicate that S-15176 attenuates mitochondrial dysfunction and ultrastructural abnormalities in the liver of T2DM mice. The mechanisms underlying the protective effect of S-15176 are related to the stimulation of mitochondrial biogenesis and the inhibition of lipid peroxidation in the organelles. One may assume that the compound acts as a mitochondria-targeted metabolic reprogramming agent in T2DM.

Abstract

S-15176, a potent derivative of the anti-ischemic agent trimetazidine, was reported to have multiple effects on the metabolism of mitochondria. In the present work, the effect of S-15176 (1.5 mg/kg/day i.p.) on the ultrastructure and functions of liver mitochondria of C57BL/6 mice with type 2 diabetes mellitus (T2DM) induced by a high-fat diet combined with a low-dose streptozotocin injection was examined. An electron microscopy study showed that T2DM induced mitochondrial swelling and a reduction in the number of liver mitochondria. The number of mtDNA copies in the liver in T2DM decreased. The expression of Drp1 slightly increased, and that of Mfn2 and Opa1 somewhat decreased. The treatment of diabetic animals with S-15176 prevented the mitochondrial swelling, normalized the average mitochondrial size, and significantly decreased the content of the key marker of lipid peroxidation malondialdehyde in liver mitochondria. In S-15176-treated T2DM mice, a two-fold increase in the expression of the PGC-1α and a slight decrease in Drp 1 expression in the liver were observed. The respiratory control ratio, the level of mtDNA, and the number of liver mitochondria of S-15176-treated diabetic mice tended to restore. S-15176 did not affect the decrease in expression of Parkin and Opa1 in the liver of diabetic animals, but slightly suppressed the expression of these proteins in the control. The modulatory effect of S-15176 on dysfunction of liver mitochondria in T2DM can be related to the stimulation of mitochondrial biogenesis and the inhibition of lipid peroxidation in the organelles.

The metabolic regulation of fenofibrate is dependent on dietary protein content in male juveniles of Nile tilapia (Oreochromis niloticus)

The fenofibrate functions in mammals could be affected by many factors such as dietary nutrient levels and physiological status. However, this phenomenon has not been well studied in fish. The goal of our study was to investigate the effect of dietary protein contents on metabolic regulation of fenofibrate in Nile tilapia. An 8-week experiment was conducted to feed fish with four diets at two protein levels (28 and 38 %) with or without the supplementation of fenofibrate (200 mg/kg body weight per d). After the trial, the body morphometric parameters, plasma biochemical parameters and quantitative PCR data were examined. These results showed that fenofibrate significantly reduced the feeding intake and weight gain rate, increased the oxidative stress (increased plasma methane dicarboxylic aldehyde) and liver : body ratio (increased hepatosomatic index) in the low protein (LP)-fed fish. In contrast, fenofibrate exhibited a lipid-lowering (reduced hepatic lipid) effect and up-regulated the expressions of the genes related to lipid catabolism, transport and anabolic metabolism in the high protein (HP)-fed fish. The present study suggested that lipid-lowering effect of fenofibrate would be strengthened in the fish fed with the HP diet containing high energy, but in the fish fed with the LP diet containing low energy, the fenofibrate treatment would cause adverse effects for metabolism. Taking together, our study showed that the metabolic regulation of fenofibrate in Nile tilapia was dependent not only on feed energy content but also on dietary nutrient composition, such as dietary protein and/or lipid levels.

Metabolic profiling analysis upon acylcarnitines in tissues of hepatocellular carcinoma revealed the inhibited carnitine shuttle system caused by the downregulated carnitine palmitoyltransferase 2

The carnitine shuttle system (CSS) plays a crucial role in the transportation of fatty acyls during fatty acid β-oxidation for energy supplementation, especially in cases of high energy demand, such as in cancer. In this study, to systematically characterize alterations of the CSS in hepatocellular carcinoma (HCC), acylcarnitine metabolic profiling was carried out on 80 pairs of HCC tissues and adjacent noncancerous tissues (ANTs) by using ultra-performance liquid chromatography coupled to mass spectrometry. Twenty-four acylcarnitines classified into five categories were identified and characterized between HCCs and ANTs. Notably, increased saturated long-chain acylcarnitines (LCACs) and decreased short- and medium-chain acylcarnitines (S/MCACs) were simultaneously observed in HCC samples. Subsequent correlation network and heatmap analysis indicated low correlations between LCACs and S/MCACs. The mRNA and protein expressions of carnitine palmitoyltransferase 2 (CPT2) was significantly downregulated in HCC samples, whereas CPT1A expression was not significantly changed. Correspondingly, the relative levels of S/MCACs were reduced and those of LCACs were increased in BEL-7402/CPT2-knockdown cells compared to negative controls. Both results suggested that decreased shuttling efficiency in HCC might be associated with downregulation of CPT2. In addition, decreases in the mRNA expression of acetyl-CoA acyltransferase 2 were also observed in HCC tissues and BEL-7402/CPT2-knockdown cells, suggesting potential low β-oxidation efficiency, which was consistent with the increased expression of stearoyl-CoA desaturase 1 in both samples. The systematic strategy applied in our study illustrated decreased shuttling efficiency of the carnitine shuttle system in HCC and can provide biologists with an in-depth understanding of β-oxidation in HCC.

Disturbed bovine mitochondrial lipid metabolism: a review

In mammals, excess energy is stored primarily as triglycerides, which are mobilized when energy demands arise and cannot be covered by feed intake. This review mainly focuses on the role of long chain fatty acids in disturbed energy metabolism of the bovine species. Long chain fatty acids regulate energy metabolism as ligands of peroxisome proliferator-activated receptors. Carnitine acts as a carrier of fatty acyl groups as long-chain acyl-CoA derivatives do not penetrate the mitochondrial inner membrane. There are two different types of disorders in lipid metabolism which can occur in cattle, namely the hypoglycaemic-hypoinsulinaemic and the hyperglycaemic-hyperinsulinaemic type with the latter not always associated with ketosis. There is general agreement that fatty acid β-oxidation capability is limited in the liver of (ketotic) cows. In accord, supplemental L-carnitine decreased liver lipid accumulation in periparturient Holstein cows. Of note, around parturition concurrent oxidation of fatty acids in skeletal muscle is highly activated. Also peroxisomal β-oxidation in liver of dairy cows may be part of the hepatic adaptations to a negative energy balance (NEB) to break down fatty acids. An elevated blood concentration of nonesterified fatty acids is one of the indicators of NEB in cattle among others like increased β-hydroxy butyrate concentration, and decreased concentrations of glucose, insulin, and insulin-like growth factor-I. Assuming that liver carnitine concentrations might limit hepatic fatty acid oxidation capacity in dairy cows, further study of the role of acyl-CoA dehydrogenases and/or riboflavin in bovine ketosis is warranted.

Inhibition of β-oxidation is not a valid therapeutic tool for reducing oxidative stress in conditions of neurodegeneration

According to recent reports, systemic treatment of rats with methylpalmoxirate (carnitine palmitoyltransferase-1 inhibitor) decreased peroxidation of polyunsaturated fatty acids in brain tissue. This was taken as evidence of mitochondrial β-oxidation in brain, thereby contradicting long-standing paradigms of cerebral metabolism, which claim that β-oxidation of activated fatty acids has minor importance for brain energy homeostasis. We addressed this controversy. Our experiments are the first direct experimental analysis of this question. We fueled isolated brain mitochondria or rat brain astrocytes with octanoic acid, but octanoic acid does not enhance formation of reactive oxygen species, neither in isolated brain mitochondria nor in astrocytes, even at limited hydrogen delivery to mitochondria. Thus, octanoic acid or l-octanoylcarnitine does not stimulate H₂O₂ release from brain mitochondria fueled with malate, in contrast to liver mitochondria (2.25-fold rise). This does obviously not support the possible occurrence of β-oxidation of the fatty acid octanoate in the brain. We conclude that a proposed inhibition of β-oxidation does not seem to be a helpful strategy for therapies aiming at lowering oxidative stress in cerebral tissue. This question is important, since oxidative stress is the cause of neurodegeneration in numerous neurodegenerative or inflammatory disease situations.

Role of Hypothalamic Creb-Binding Protein in Obesity and Molecular Reprogramming of Metabolic Substrates

We have reported a correlation between hypothalamic expression of Creb-binding protein (Cbp) and lifespan, and that inhibition of Cbp prevents protective effects of dietary restriction during aging, suggesting that hypothalamic Cbp plays a role in responses to nutritional status and energy balance. Recent GWAS and network analyses have also implicated Cbp as the most connected gene in protein-protein interactions in human Type 2 diabetes. The present studies address mechanisms mediating the role of Cbp in diabetes by inhibiting hypothalamic Cbp using a Cre-lox strategy. Inhibition of hypothalamic Cbp results in profound obesity and impaired glucose homeostasis, increased food intake, and decreased body temperature. In addition, these changes are accompanied by molecular evidence in the hypothalamus for impaired leptin and insulin signaling, a shift from glucose to lipid metabolism, and decreased Pomc mRNA, with no effect on locomotion. Further assessment of the significance of the metabolic switch demonstrated that enhanced expression of hypothalamic Cpt1a, which promotes lipid metabolism, similarly resulted in increased body weight and reduced Pomc mRNA.

Carnitine palmitoyltransferase 1C: From cognition to cancer

Carnitine palmitoyltransferase 1 (CPT1) C was the last member of the CPT1 family of genes to be discovered. CPT1A and CPT1B were identified as the gate-keeper enzymes for the entry of long-chain fatty acids (as carnitine esters) into mitochondria and their further oxidation, and they show differences in their kinetics and tissue expression. Although CPT1C exhibits high sequence similarity to CPT1A and CPT1B, it is specifically expressed in neurons (a cell-type that does not use fatty acids as fuel to any major extent), it is localized in the endoplasmic reticulum of cells, and it has minimal CPT1 catalytic activity with l-carnitine and acyl-CoA esters. The lack of an easily measurable biological activity has hampered attempts to elucidate the cellular and physiological role of CPT1C but has not diminished the interest of the biomedical research community in this CPT1 isoform. The observations that CPT1C binds malonyl-CoA and long-chain acyl-CoA suggest that it is a sensor of lipid metabolism in neurons, where it appears to impact ceramide and triacylglycerol (TAG) metabolism. CPT1C global knock-out mice show a wide range of brain disorders, including impaired cognition and spatial learning, motor deficits, and a deregulation in food intake and energy homeostasis. The first disease-causing CPT1C mutation was recently described in humans, with Cpt1c being identified as the gene causing hereditary spastic paraplegia. The putative role of CPT1C in the regulation of complex-lipid metabolism is supported by the observation that it is highly expressed in certain virulent tumor cells, conferring them resistance to glucose- and oxygen-deprivation. Therefore, CPT1C may be a promising target in the treatment of cancer. Here we review the molecular, biochemical, and structural properties of CPT1C and discuss its potential roles in brain function, and cancer.

Malony-CoA inhibits the S113L variant of carnitine-palmitoyltransferase II

Carnitine palmitoyltransferases (CPT), located both in the outer (CPT I) and inner membrane (CPT II) of mitochondria, are the key players for an efficient transport of long chain fatty acids into this cell compartment. The metabolite malonyl-CoA is known to inhibit CPT I, but not CPT II. His6-N-hCPT2 (wild type) and His6-N-hCPT2/ S113L (variant) were produced recombinantly in prokaryotic host, purified and characterized according to their functional and regulatory properties. The wild type and the variant showed the same enzymatic activity and were both inhibited by malonyl-CoA and malonate in a time-dependent manner. The inhibition was, however, significantly more pronounced in the mutated enzyme. The residual activities were 40% and 5% at temperatures of 4 °C and 30 °C, respectively. The inhibitory effect proceeded irreversibly with no recovery after postincubation of palmitoyl-CoA (Pal-CoA) as native substrate. A model of malonyl-CoA and malonate binding to human CPT II was suggested by docking studies to explain the action of the inhibitors regarding to the effect of the mutation on the protein conformation. Results indicated that not only CPT I, but also CPT II can be inhibited by malonyl-CoA. Thus, the complete inhibition of total CPT (i.e. CPT I and CPT II) in muscle homogenates by an established assay is not due to a lack of enzymatically active CPT II, but rather due to an abnormal regulation of the enzyme.

Glial β-oxidation regulates Drosophila energy metabolism

The brain's impotence to utilize long-chain fatty acids as fuel, one of the dogmas in neuroscience, is surprising, since the nervous system is the tissue most energy consuming and most vulnerable to a lack of energy. Challenging this view, we here show in vivo that loss of the Drosophila carnitine palmitoyltransferase 2 (CPT2), an enzyme required for mitochondrial β-oxidation of long-chain fatty acids as substrates for energy production, results in the accumulation of triacylglyceride-filled lipid droplets in adult Drosophila brain but not in obesity. CPT2 rescue in glial cells alone is sufficient to restore triacylglyceride homeostasis, and we suggest that this is mediated by the release of ketone bodies from the rescued glial cells. These results demonstrate that the adult brain is able to catabolize fatty acids for cellular energy production.

Effects of pomegranate seed oil on glucose and lipid metabolism-related organs in rats fed an obesogenic diet

Studies conducted in mice have revealed positive effects of punicic acid (PUA). The aim of this study was to analyze the effects of PUA on fat accumulation and glycemic control in rats fed an obesogenic diet. Rats were randomly divided into two groups: control group and PUA group (diet supplemented with 0.5% PUA). No changes were observed in adipose tissue weights. The glucose tolerance test showed that the glycemic value in the PUA group had decreased significantly at the final time (120 min) (-19.3%), as had fructosamine levels (-11.1%). However, homeostasis model assessment (HOMA-IR) showed that insulin resistance did not improve. No changes were observed in the liver, skeletal muscle composition, or peroxisome proliferator-activated receptors (PPARs) activation. Low levels (mg/g tissue) of PUA (0.04 ± 0.01 in both tissues) and higher levels of cis-9,trans-11 conjugated linoleic acid (0.31 ± 0.08 in liver, 0.52 ± 0.11 in muscle) were found. PUA supplementation induced hypoplasia (-16.1%) due to the antiproliferative effect on hepatocytes. In conclusion, dietary supplementation of 0.5% PUA did not lead to decreased fat accumulation in adipose tissue, liver, or skeletal muscle, or to improved glycemic control. The hypoplasia induced in liver is a negative effect that should be considered before proposing PUA as a functional ingredient.

Alteration of fatty acid metabolism in the liver, adipose tissue, and testis of male mice conceived through assisted reproductive technologies: fatty acid metabolism in ART mice

BACKGROUND

Lipid metabolism plays important roles in the whole process of pregnancy. Previous studies have demonstrated abnormalities of lipid metabolism in the placentas of pregnancies obtained by assisted reproductive technology (ART). Therefore, we hypothesized that ART micromanipulation may affect lipid metabolism in offspring, and focused on the fatty acid metabolism in ART male offspring in this study.

METHODS

The fatty acid metabolism in the liver, adipose tissue and testis was detected. The comparison between naturally conceived (NC), controlled ovarian hyperstimulation (COH), in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) mice was made to analyze the effect of ART on offspring. The mice models in this study included two age groups: adult group and old group. The fatty acid composition and the expression of lipid metabolism-related genes were analyzed by GC-MS and qRT-PCR.

RESULTS

The fatty acid composition in the liver and adipose tissue were significantly altered in ART mice, but no significant difference was found in the testis. In adipose tissue, ART mice showed decreased monounsaturated fatty acids (MUFAs) and increased polyunsaturated fatty acids (PUFAs) in both adult and old mice, while the alteration of saturated fatty acids (SFAs) in the adult disappeared in the old. In liver, the changes were much complex in adult mice, while increased MUFAs and decreased PUFAs were found in ART old mice. The activities of fatty acid metabolism-related enzymes and the expression of lipogenic and lipolytic proteins changed in ART groups, with the adult mice and old mice showing inconsistent alterations. Further analysis indicated that SFAs was closely associated with the alterations of fatty acid metabolism-related enzyme activities and the expression of lipogenic and lipolytic proteins. Furthermore, we also found that the effect of separated ART treatments on fatty acid metabolism varied with different ages and tissues.

CONCLUSIONS

ART treatments had effect on the fatty acid composition in adipose tissue and liver of male mice. The alteration of SFAs content was crucial for the regulation of fatty acid composition. These changes might have potential effects on the health of ART male offspring which need further investigation.

The CPT1C 5'UTR contains a repressing upstream open reading frame that is regulated by cellular energy availability and AMPK

BACKGROUND

Translational control is utilized as a means of regulating gene expression in many species. In most cases, posttranscriptional regulatory mechanisms play an important role in stress response pathways and can lead to dysfunctional physiology if blocked by mutations. Carnitine Palmitoyltransferase 1 C (CPT1C), the brain-specific member of the CPT 1 family, has previously been shown to be involved in regulating metabolism in situations of energy surplus.

PRINCIPAL FINDINGS

Sequence analysis of the CPT1C mRNA revealed that it contains an upstream open reading frame (uORF) in the 5' UTR of its mRNA. Using CPT1C 5' UTR/luciferase constructs, we investigated the role of the uORF in translational regulation. The results presented here show that translation from the CPT1C main open reading frame (mORF) is repressed by the presence of the uORF, that this repression is relieved in response to specific stress stimuli, namely glucose deprivation and palmitate-BSA treatment, and that AMPK inhibition can relieve this uORF-dependent repression.

SIGNIFICANCE

The fact that the mORF regulation is relieved in response to a specific set of stress stimuli rather than general stress response, hints at an involvement of CPT1C in cellular energy-sensing pathways and provides further evidence for a role of CPT1C in hypothalamic regulation of energy homeostasis.

Gender differences in locomotor and stereotypic behavior associated with l-carnitine treatment in mice

BACKGROUND

The carnitines exert neuroprotective and neuromodulatory actions, and carnitine supplementation increases locomotor activity (LMA) in experimental animals.

METHODS

We measured 13 indexes of LMA and 3 indexes of stereotypic activity (STA) in adult male and female caged mice. In a randomized 4-week trial, 10 males and 10 females received 50 mg/kg body weight PO l-carnitine, and another 10 males and 10 females received placebo.

RESULTS

Compared with placebo-treated females, placebo-treated males had a greater number of stereotypies (NSTs), stereotypy counts (STCs), stereotypy time (STT), and right front time (RFT), but smaller total distance traveled (TDT), margin distance (MD), number of vertical movements (NVMs), and left rear time (LRT). Compared with placebo-treated males, carnitine-treated males had greater horizontal activity (HA), movement time (MT), NVM, STT, TDT, STC, MD, LRT, and clockwise revolutions (CRs), but smaller left front time (LFT) and RFT. Compared with placebo-treated females, carnitine-treated females had greater NST, STC, STT, LFT, and RFT, but smaller NM, HA, NVM, VA, MT, anticlockwise revolutions (ACRs), CR, TDT, and MD; right rear time (RRT) remained statistically insignificant across all comparisons.

CONCLUSIONS

In summary, l-carnitine caused gender differences to persist for STC, diminish for NST and STT, disappear for LRT and NVM, change in the opposite direction for TDT and MD, appear de novo for HA, VA, NM, MT, and LFT, and remain absent for RRT and ACR. Some indexes of LMA and STA are sexually dimorphic in adult mice, and l-carnitine differentially maintains, diminishes/cancels, inverts, or creates the sexual dimorphism of particular indexes.

Enhanced susceptibility of Cpt1c knockout mice to glucose intolerance induced by a high-fat diet involves elevated hepatic gluconeogenesis and decreased skeletal muscle glucose uptake

AIMS/HYPOTHESIS

Carnitine palmitoyltransferase-1 (CPT1)c is a novel isoform in the CPT1 family and is found specifically in the brain. Cpt1c knockout (KO) mice are more susceptible to high-fat diet (HFD)-induced obesity. However, the underlying mechanism of this phenotype and the question of whether CPT1c is involved in the pathogenesis of diet-induced insulin resistance are unclear.

METHODS

To assess the potential role of CPT1c in the regulation of whole-body glucose homeostasis, we generated Cpt1c KO mice and challenged them with HFD or standard chow. Glucose homeostasis of each group was assessed weekly.

RESULTS

After 8 weeks of HFD feeding, Cpt1c KO mice developed a phenotype of more severe insulin resistance than that in wild-type controls. The increased susceptibility of Cpt1c KO mice to HFD-induced insulin resistance was independent of obesity. Impaired glucose tolerance in Cpt1c KO mice was attributable to elevated hepatic gluconeogenesis and decreased glucose uptake in skeletal muscle. These effects correlated with decreased hepatic and intramuscular fatty acid oxidation and expression of oxidative genes as well as with elevated triacylglycerol content in these tissues. Interestingly, Cpt1c deletion caused a specific elevation of hypothalamic CPT1a and CPT1b isoform expression and activity. We demonstrated that elevated plasma NEFA concentration is one mechanism via which this compensatory effect is induced.

CONCLUSIONS/INTERPRETATION

These results further establish the role of CPT1c in controlling whole-body glucose homeostasis and in the regulation of hypothalamic Cpt1 isoform expression. We identify changes in hepatic and skeletal muscle glucose metabolism as important mechanisms determining the phenotype of Cpt1c KO mice.

Malonyl-CoA, a key signaling molecule in mammalian cells

Malonyl-CoA can be formed within the mitochondria, peroxisomes, and cytosol of mammalian cells. Besides being an intermediate in the pathways of de novo fatty acid biosynthesis and fatty acid elongation, malonyl-CoA has an important signaling function through its allosteric inhibition of carnitine palmitoyltransferase 1, the enzyme that normally exerts flux control over mitochondrial beta-oxidation. Malonyl-CoA is rapidly turned over in mammalian cells, and the activities of acetyl-CoA carboxylase and malonyl-CoA decarboxylase are important determinants of its cytosolic concentration. It is now recognized that malonyl-CoA participates in a diverse range of physiological or pathological responses and systems. These include the ketogenic response of the liver to fasting and diabetes, carbohydrate versus fat fuel selection in muscle tissues, metabolic changes in muscle during contracture, alterations in fatty acid metabolism during cardiac ischemia and postischemic reperfusion, stimulation of B cell insulin secretion by glucose, and the hypothalamic control of appetite.

Rat liver mitochondrial carnitine palmitoyltransferase-I, hepatic carnitine, and malonyl-CoA: effect of starvation

Hepatic mitochondrial fatty acid oxidation and ketogenesis increase during starvation. Carnitine palmitoyltransferase I (CPT-I) catalyses the rate-controlling step in the overall pathway and retains its control over beta-oxidation under fed, starved and diabetic conditions. To determine the factors contributing to the reported several-fold increase in fatty acid oxidation in perfused livers, we measured the V(max) and K(m) values for palmitoyl-CoA and carnitine, the K(i) (and IC(50)) values for malonyl-CoA in isolated liver mitochondria as well as the hepatic malonyl-CoA and carnitine contents in control and 48 h starved rats. Since CPT-I is localized in the mitochondrial outer membrane and in contact sites, the kinetic properties of CPT-I also was determined in these submitochondrial structures. After 48 h starvation, there is: (a) a significant increase in K(i) and decrease in hepatic malonyl-CoA content; (b) a decreased K(m) for palmitoyl-CoA; and (c) increased catalytic activity (V(max)) and CPT-I protein abundance that is significantly greater in contact sites compared with outer membranes. Based on these changes the estimated increase in mitochondrial fatty acid oxidation is significantly less than that observed in perfused liver. This suggests that CPT-I is regulated in vivo by additional mechanism(s) lost during mitochondrial isolation or/and that mitochondrial oxidation of peroxisomal beta-oxidation products contribute to the increased ketogenesis by bypassing CPT-I. Furthermore, the greater increase in CPT-I protein in contact sites as compared to outer membranes emphasizes the significance of contact sites in hepatic fatty acid oxidation.

Localization and effect of ectopic expression of CPT1c in CNS feeding centers

Hypothalamic neurons monitor peripheral energy status and produce signals to adjust food intake and energy expenditure to maintain homeostasis. However, the molecular mechanisms by which these signals are generated remain unclear. Fluctuations in the level of hypothalamic malonyl-CoA are known to serve as an intermediary in regulating energy homeostasis and it has been proposed that the brain-specific carnitine palmitoyltransferase-1c (CPT1c) serves as a target of malonyl-CoA in the central nervous system (CNS). Here, we report that CPT1c is widely expressed in neurons throughout the CNS including the hypothalamus, hippocampus, cortex, and amygdala. CPT1c is enriched in neural feeding centers of the hypothalamus with mitochondrial localization as an outer integral membrane protein. Ectopic over-expression of CPT1c by stereotactic hypothalamic injection of a CPT1c adenoviral vector is sufficient to protect mice from body weight gain when fed a high-fat diet. These findings show that CPT1c is appropriately localized in regions and cell types to regulate energy homeostasis and that its over-expression in the hypothalamus is sufficient to protect mice from adverse weight gain caused by high-fat intake.

Regulation of acetyl CoA carboxylase and carnitine palmitoyl transferase-1 in rat adipocytes

OBJECTIVE

Acetyl CoA carboxylase (ACC) is a key enzyme in energy balance. It controls the synthesis of malonyl-CoA, an allosteric inhibitor of carnitine palmitoyltransferase-1 (CPT-I). CPT-I is the gatekeeper of free fatty acid (FFA) oxidation. To test the hypothesis that both enzymes play critical roles in regulation of FFA partitioning in adipocytes, we compared enzyme mRNA expression and specific activity from fed, fasted, and diabetic rats.

RESEARCH METHODS AND PROCEDURES

Direct effects of nutritional state, insulin, and FFAs on CPT-I and ACC mRNA expression were assessed in adipocytes, liver, and cultured adipose tissue explants. We also determined FFA partitioning in adipocytes from donors exposed to different nutritional conditions.

RESULTS

CPT-I mRNA and activity decreased in adipocytes but increased in liver in response to fasting. ACC mRNA and activity decreased in both adipocytes and liver during fasting. These changes were not caused directly by fasting-associated changes in plasma insulin and FFA concentrations because insulin suppressed CPT-I mRNA and did not affect ACC mRNA in vitro, whereas exogenous oleate had no effect on either. Despite the decrease in adipocyte CPT-I mRNA and specific activity, CO(2) production from endogenous FFAs increased, suggesting increased FFA transport through CPT-I for beta-oxidation.

DISCUSSION

Stimulation of FFA transport through CPT-I occurs in both tissues, but CPT-I mRNA and specific activity correlate with FFA transport in liver and not in adipocytes. We conclude that the mechanism responsible for increasing FFA oxidation in adipose tissue during fasting involves mainly allosteric regulation, whereas altered gene expression may play a central role in the liver.

Hypothalamic sensing of fatty acids

Selective regions of the brain, including the hypothalamus, are capable of gathering information on the body's nutritional status in order to implement appropriate behavioral and metabolic responses to changes in fuel availability. This review focuses on direct metabolic signaling within the hypothalamus. There is growing evidence supporting the idea that fatty acid metabolism within discrete hypothalamic regions can function as a sensor for nutrient availability that can integrate multiple nutritional and hormonal signals.

Peroxisomal proliferator activated receptor gamma coactivator (PGC-1alpha) stimulates carnitine palmitoyltransferase I (CPT-Ialpha) through the first intron

Peroxisomal proliferator activated receptor gamma coactivator-1 (PGC-1alpha) is a transcriptional coactivator that promotes mitochondrial biogenesis and energy metabolism in brown fat, skeletal muscle and heart. Previous studies demonstrated that PGC-1alpha is present at low levels in the liver but that the hepatic abundance of PGC-1alpha is elevated in diabetic and fasted animals. Elevated PGC-1alpha expression is associated with increased fatty acid oxidation and hepatic glucose production. Carnitine palmitoyltransferase-I (CPT-I) is a rate controlling step in the mitochondrial oxidation of long chain fatty acids. CPT-I transfers the acyl moiety from fatty acyl-CoA to carnitine for the translocation of long chain fatty acids across the mitochondrial membrane. There are two isoforms of CPT-I including a liver isoform CPT-Ialpha and a muscle isoform CPT-Ibeta. Here, we characterized the regulation of CPT-Ialpha isoform by PGC-1alpha. PGC-1alpha stimulates CPT-Ialpha primarily through multiple sites in the first intron. We found that PGC-1alpha can induce CPT-Ialpha gene expression in cardiac myocytes and primary hepatocytes. Our results indicate that PGC-1alpha elevates the expression of CPT-Ialpha via a unique mechanism that utilizes elements within the intron.

Expression of genes participating in regulation of fatty acid and glucose utilization and energy metabolism in developing rat hearts

The heart is a unique organ that can use several fuels for energy production. During development, the heart undergoes changes in fuel supply, and it must be able to respond to these changes. We have examined changes in the expression of several genes that regulate fuel transport and metabolism in rat hearts during early development. At birth, there was increased expression of fatty acid transporters and enzymes of fatty acid metabolism that allow fatty acids to become the major source of energy for cardiac muscle during the first 2 wk of life. At the same time, expression of genes that control glucose transport and oxidation was downregulated. After 2 wk, expression of genes for glucose uptake and oxidation was increased, and expression of genes for fatty acid uptake and utilization was decreased. Expression of carnitine palmitoyltransferase I (CPT I) isoforms during development was different from published data obtained from rabbit hearts. CPT Iα and Iβ isoforms were both highly expressed in hearts before birth, and both increased further at birth. Only after the second week did CPT Iα expression decrease appreciably below the level of CPT Iβ expression. These results represent another example of different expression patterns of CPT I isoforms among various mammalian species. In rats, changes in gene expression followed nutrient availability during development and may render cardiac fatty acid oxidation less sensitive to factors that influence malonyl-CoA content (e.g., fluctuations in glucose concentration) and thereby favor fatty acid oxidation as an energy source for cardiomyocytes in early development.

Peroxisomal proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) enhances the thyroid hormone induction of carnitine palmitoyltransferase I (CPT-I alpha)

Carnitine palmitoyltransferase I (CPT-I) catalyzes the rate-controlling step in the pathway of mitochondrial fatty acid oxidation. Thyroid hormone will stimulate the expression of the liver isoform of CPT-I (CPT-I alpha). This induction of CPT-I alpha gene expression requires the thyroid hormone response element in the promoter and sequences within the first intron. The peroxisomal proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) is a coactivator that promotes mitochondrial biogenesis, mitochondrial fatty acid oxidation, and hepatic gluconeogenesis. In addition, PGC-1 alpha will stimulate the expression of CPT-I alpha in primary rat hepatocytes. Here we report that thyroid hormone will increase PGC-1 alpha mRNA and protein levels in rat hepatocytes. In addition, overexpression of PGC-1 alpha will enhance the thyroid hormone induction of CPT-I alpha indicating that PGC-1 alpha is a coactivator for thyroid hormone. By using chromatin immunoprecipitation assays, we show that PGC-1 alpha is associated with both the thyroid hormone response element in the CPT-I alpha gene promoter and the first intron of the CPT-I alpha gene. Our data demonstrate that PGC-1 alpha participates in the stimulation of CPT-I alpha gene expression by thyroid hormone and suggest that PGC-1 alpha is a coactivator for thyroid hormone.