Co-expression in rat heart and skeletal muscle of four genes coding for proteins implicated in long-chain fatty acid uptake

Administration of Fenofibrate Markedly Elevates Fabp3 in Rat Liver and Plasma and Confounds Its Use as a Preclinical Biomarker of Cardiac and Muscle Toxicity

Proteins involved in lipid homeostasis are often regulated through the nuclear peroxisome proliferator-activated receptors (PPAR). PPARα is the target for the fibrate-class of drugs. Fenofibrate has been approved for its lipid-lowering effects in patients with hypercholesterolemia and hypertriglyceridemia. We were interested in understanding the expression of the energy transporters in energy-utilizing tissues like liver, heart, muscle, and adipose tissues in rat with the hypothesis that the change in transporter expression would align with the known lipid-lowering effects of PPARα agonists like fenofibrate. We found that several fatty-acid transporter proteins had significantly altered levels following 8 days of fenofibrate dosing. The mRNA levels of the highly abundant Fatp2 and Fatp5 in rat liver increased approximately twofold and decreased fourfold, respectively. Several fatty-acid-binding proteins and acyl-CoA-binding proteins had a significant increase in mRNA abundance but not the major liver fatty-acid-binding protein, Fabp1. Of particular interest was the increased liver expression of Fabp3 also known as heart-fatty acid binding protein (H-FABP or FABP3). FABP3 has been proposed as a circulating clinical biomarker for cardiomyopathy and muscle toxicity, as well as a preclinical marker for PPARα-induced muscle toxicity. Here, we show that fenofibrate induces liver mRNA levels of Fabp3 ~5000-fold resulting in an approximately 50-fold increase in FABP3 protein levels in the whole liver. This increased liver expression complicates the interpretation and potential use of FABP3 as a specific biomarker for PPARα-induced muscle toxicities.

Differential Long-Chain Polyunsaturated Fatty Acids Status and Placental Transport in Adolescent Pregnancies

Adolescent pregnancy increases risk of adverse perinatal outcomes. Placental delivery of long-chain polyunsaturated fatty acids (LCPUFA) is essential for fetal growth and development. In this pilot study, we aimed to assess maternal and fetal status of fatty acids (FA) measured at birth and the expression of key genes involved in FA uptake, transport and metabolism in the placenta of fifteen adolescents and fifteen adults. FA were quantified by gas-liquid chromatography. Placental expression of FA transporters was assessed by quantitative real-time polymerase chain reaction (qRT-PCR) and peroxisome proliferator-activated receptor gamma (PPARγ) was quantified by Western Blot. Adolescents had lower docosahexaenoic acid (DHA, 22:6 n-3) and total n-3 FA levels in maternal erythrocytes and placenta, but these were not different in fetal erythrocytes. Arachidonic acid (AA, 20:4 n-6) concentration was increased in placenta but lower in fetal circulation. Plasma membrane fatty acid binding protein (FABPpm) and fatty acid transport protein (FATP) 4 mRNA expressions were not different, however FATP1, fatty acid translocase (FAT/CD36) and fatty acid binding protein 3 (FABP3) mRNA and PPARγ protein levels were decreased in placenta of adolescents. Despite significant downregulation of FATP1, CD36 and FABP3, there was only a modest decrease in LCPUFA (10%) and AA (12%) and no difference in DHA content in cord blood, suggesting that FA transfer to the fetus was partially protected by other factors in adolescents from this cohort.

β-Carotene Increases Muscle Mass and Hypertrophy in the Soleus Muscle in Mice

Supplements and naturally occurring nutraceuticals effective for maintenance or enhancement of skeletal muscle mass are expected to contribute to prevention of decreased mobility and increased risk of developing metabolic diseases. However, information about available food components remains widely unavailable. In the present study, we investigated the effects of dietary β-carotene on the quantity and quality of skeletal muscle under physiological conditions. Male ddY mice (8 wk old) were orally administered β-carotene (0.5 mg once daily) for 14 d. Dietary β-carotene had no influence on body weight, but increased the soleus muscle/body weight ratio. The cross-sectional area (CSA) in muscle fibers of the soleus muscle was increased, indicating that administration of β-carotene induces muscle hypertrophy. In the soleus muscle of the β-carotene-administered mice, twitch force tended to be increased (p=0.06) and tetanic force was significantly increased, whereas specific force (force per CSA) remained unchanged. Dietary β-carotene increased the mRNA level of insulin-like growth factor 1 (Igf-1) as its splicing variant Igf-1ea, but had no influence on the liver Igf-1 mRNA level or serum IGF-1 level. β-Carotene promoted protein synthesis in the soleus muscle and reduced levels of ubiquitin conjugates, but had no influence on the mRNA levels of two atrogenes, Atrogin-1 and Murf1. On the other hand, β-carotene had no influence on the processing of the autophagy marker protein light chain 3. These results indicate that in mice, administration of β-carotene increases mass and induces functional hypertrophy in the soleus muscle, perhaps by promoting IGF-1-mediated protein synthesis and by reducing ubiquitin-mediated protein degradation.

Heart type fatty acid binding protein: structure, function and biosensing applications for early detection of myocardial infarction

Heart type fatty acid binding protein (HFABP) as an early marker of cardiac injury holds a promising future with studies indicating surpassing performance as compared to myoglobin. As a plasma marker, this cytoplasmic protein owing to its small size (∼15kDa) and water solubility, appears readily in the blood-stream following cardiomyocyte damage, reaching peak levels within 6h of symptom onset. Low plasma levels of HFABP as compared to tissue levels indicate that minute amounts of the protein when released during myocardial infarction leads to a greater proportional rise. These parameters of kinetic release make it an ideal candidate for rapid assessment of acute myocardial infarction (AMI). The need for development of rapid immunoassays and immunotests so as to use HFABP as an early marker for AMI exclusion is tremendous. In the present review, we outline the various immunoassays and immunosensors developed so far for the detection of HFABP in buffer, plasma or whole blood. The principles behind the detection techniques along with their performance parameters compared to standard ELISA techniques are elucidated.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

The emerging functions and mechanisms of mammalian fatty acid-binding proteins

Fatty acid-binding proteins (FABPs) are abundant intracellular proteins that bind long-chain fatty acids with high affinity. Nine separate mammalian FABPs have been identified, and their tertiary structures are highly conserved. The FABPs have unique tissue-specific distributions that have long suggested functional differences among them. In the last decade, considerable progress has been made in understanding the specific functions of the FABPs and, in some cases, their mechanisms of action at the molecular level. The FABPs appear to be involved in the extranuclear compartments of the cell by trafficking their ligands within the cytosol via interactions with organelle membranes and specific proteins. Several members of the FABP family have been shown to function directly in the regulation of cognate nuclear transcription factor activity via ligand-dependent translocation to the nucleus. This review will focus on these emerging functions and mechanisms of the FABPs, highlighting the unique functional properties of each as well as the similarities among them.

Expression pattern of fatty acid transport protein-1 (FATP-1), FATP-4 and heart-fatty acid binding protein (H-FABP) genes in human term placenta

Placental tissue from five women undergoing elective caesarean did not present differences in fatty acids or mRNA expression of FATP-1, FATP-4 and H-FABP in different placental locations. mRNA expression of FATP-1 and FATP-4 was significantly higher than H-FABP. The expression of L-FABP was too low in placenta for accurate quantification.

Fiber type- and fatty acid composition-dependent effects of high-fat diets on rat muscle triacylglyceride and fatty acid transporter protein-1 content

Intramuscular triacylglyceride (TAG) is considered an independent marker of insulin resistance in humans. Here, we examined the effect of high-fat diets, based on distinct fatty acid compositions (saturated, monounsaturated or n-6 polyunsaturated), on TAG levels and fatty acid transporter protein (FATP-1) expression in 2 rat muscles that differ in their fiber type, soleus, and gastrocnemius; the relationship to whole body glucose intolerance was also studied. Compared with carbohydrate-fed rats, the groups subjected to any one of the high-fat diets consistently exhibited enhanced body weight gain and adiposity, elevated plasma free fatty acids and TAG in the fed condition, hyperinsulinemia, and glucose intolerance. TAG content was consistently higher in soleus than in gastrocnemius, but was only significantly elevated by the n-6 polyunsaturated-based diet. FATP-1 levels in soleus were double those in gastrocnemius muscle in carbohydrate-fed animals. High-fat diets caused an elevation in FATP-1 protein content in soleus, but a reduction in gastrocnemius. In conclusion, the hyperinsulinemic hyperlipidemic condition upregulates FATP-1 expression in soleus and downregulates that of gastrocnemius. Hypercaloric saturated, monounsaturated, or n-6 polyunsaturated lipid diets cause equivalent whole body insulin resistance in rats, but only an n-6 polyunsaturated acid-based diet triggers intramuscular TAG accumulation.

Long-chain fatty acid uptake by skeletal muscle is impaired in homozygous, but not heterozygous, heart-type-FABP null mice

Previous studies with cardiac myocytes from homozygous heart-type fatty acid (FA)-binding protein (H-FABP) -/- mice have indicated that this intracellular receptor protein for long-chain FA is involved in the cellular uptake of these substrates. Based on the knowledge that muscle FA uptake is a process highly sensitive to regulation by hormonal and mechanical stimuli, we studied whether H-FABP would play a role in this regulation. A suitable model system to answer this question is provided by H-FABP +/- mice, because in hindlimb muscles the content of H-FABP was measured to be 34% compared to wild-type mice. In these H-FABP +/- skeletal muscles, just as in H-FABP -/- muscles, contents of FA transporters, i.e., 43-kDa FABPpm and 88-kDa FAT/CD36, were similar compared to wild-type muscles, excluding possible compensatory mechanisms at the sarcolemmal level. Palmitate uptake rates were measured in giant vesicles prepared from hindlimb muscles of H-FABP -/-, H-FABP +/-, and H-FABP +/+ mice. For comparison, giant vesicles were isolated from liver, the tissue of which expresses a distinct type of FABP (i.e., L-FABP). Whereas in H-FABP -/- skeletal muscle FA uptake was reduced by 42-45%, FA uptake by H-FABP +/- skeletal muscle was not different from that in wild-type mice. In contrast, in liver from H-FABP -/- and from H-FABP +/- mice, FA uptake was not altered compared to wild-type animals, indicating that changes in FA uptake are restricted to H-FABP expressing tissues. It is concluded that H-FABP plays an important, yet merely permissive, role in FA uptake into muscle tissues.

Thyroid hormone transport by the rat fatty acid translocase

We examined the hypothesis that rat fatty acid translocase (rFAT) mediates the cellular uptake of T(3) and other iodothyronines. Uninjected Xenopus laevis oocytes and oocytes injected 4 d previously with rFAT cRNA were incubated for 60 min at 25 C in medium containing 0.01-10 micro M [(125)I]T(3) and 0.1% BSA, or 1-100 micro M [(3)H]oleic acid and 0.5% BSA. Injection of rFAT cRNA resulted in a 1.9-fold increase in uptake of T(3) (10 nM) and a 1.4-fold increase in uptake of oleic acid (100 micro M). Total T(3) uptake was lower in the presence than in the absence of BSA, but relative to the free T(3) concentration, uptake was increased by BSA. The fold induction of T(3) uptake by rFAT was not influenced by BSA. By analyzing uptake as a function of the ligand concentration, we estimated a K(m) value of 3.6 micro M for (total) T(3) and 56 micro M for (total) oleic acid. In addition to T(3), rFAT mediates the uptake of T(4), rT(3), 3,3'-diiodothyronine, and T(3) sulfate. The injection of human type III deiodinase cRNA with or without rFAT cRNA resulted in the complete deiodination of T(3) taken up by the oocytes, indicating that T(3) is indeed transported to the cytoplasm. In conclusion, our results demonstrate transport of T(3) and other iodothyronines by rFAT.

Rosiglitazone fails to improve hypertriglyceridemia and glucose tolerance in CD36-deficient BN.SHR4 congenic rat strain

The favorable metabolic effects of thiazolidinediones are supposedly related to the peroxisome proliferator-activated receptor-gamma (PPARgamma)-driven changes in lipid metabolism, particularly in free fatty acid (FFA) trafficking. The fatty acid translocase CD36 is one of the proposed PPARgamma targets to mediate this action. We assessed the effect of rosiglitazone (RSG, Avandia) administration in two inbred rat strains, BN/Cub and BN.SHR4 congenic strain, differing in 10 cM proximal segment of chromosome 4. Rats were fed high-sucrose diet with or without RSG for 1 wk. In BN.SHR4, which carries defective Cd36 allele of SHR origin, RSG failed to improve glucose tolerance (assessed by the oral glucose tolerance test), did not lower triglyceridemia, nor induced increases in epididymal and retroperitoneal adipose tissue weights and adipose tissue glucose utilization, effects observed in BN/Cub. On the other hand, the RSG-treated BN.SHR4 showed lower concentrations of FFA and substantial increase in glycogen synthesis and glucose oxidation in skeletal muscle. Altogether, these results support involvement of CD36 in RSG action, suggesting this pharmacogenetic interaction may be of particular importance in CD36-deficient humans.

[The mitochondrial organelle and the heart]

The heart is highly dependent for its function on oxidative energy generated in mitochondria, primarily by fatty acid beta-oxidation, respiratory electron chain and oxidative phosphorylation. Defects in mitochondrial structure and function have been found in association with cardiovascular diseases such as dilated and hypertrophy cardiomyopathy, cardiac conduction defects and sudden death, ischemic and alcoholic cardiomyopathy, as well as myocarditis. While a subset of these mitochondrial abnormalities have a defined genetic basis (e.g. mitochondrial DNA changes leading to oxidative phosphorylation dysfunction,fatty acid beta-oxidation defects due to specific nuclear DNA mutations), other abnormalities appear to be due to a more sporadic or environmental cardiotoxic insult or have not yet been characterized.This review focuses on abnormalities in mitochondrial bioenergetic function and mitochondrial DNA defects associated with cardiovascular diseases, their significance in cardiac pathogenesis as well as on the available diagnostic and therapeutic options. A concise background concerning mitochondrial biogenesis and bioenergetic pathways during cardiac growth,development and aging will also be provided.

New insights into long-chain fatty acid uptake by heart muscle: a crucial role for fatty acid translocase/CD36

Long-chain fatty acids are an important source of energy for several cell types, in particular for the heart muscle cell. Three different proteins, fatty acid translocase (FAT)/CD36, fatty acid transport protein and plasma membrane fatty acid binding protein, have been identified as possible membrane fatty acid transporters. Much information has been accumulated recently about the fatty acid transporting function of FAT/CD36. Several experimental models to study the influence of altered FAT/CD36 expression on fatty acid homoeostasis have been identified or developed, and underscore the importance of FAT/CD36 for adequate fatty acid transport. These models include the FAT/CD36 null mouse, the spontaneously hypertensive rat and FAT/CD36-deficient humans. The fatty acid transporting role of FAT/CD36 is further demonstrated in mice overexpressing muscle-specific FAT/CD36, and in transgenic mice generated using a genetic-rescue approach. In addition, a wealth of information has been gathered about the mechanisms that regulate FAT/CD36 gene expression and the presence of functional FAT/CD36 on the plasma membrane. Available data also indicate that FAT/CD36 may have an important role in the aetiology of cardiac disease, especially cardiac hypertrophy and diabetic cardiomyopathy. This review discusses our current knowledge of the three candidate fatty acid transporters, the metabolic consequences of alterations in FAT/CD36 levels in different models, and the mechanisms that have been identified for FAT/CD36 regulation.

Effects of cAMP modulators on long-chain fatty-acid uptake and utilization by electrically stimulated rat cardiac myocytes

Recently, we established that cellular contractions increase long-chain fatty-acid (FA) uptake by cardiac myocytes. This increase is dependent on the transport function of an 88 kDa membrane FA transporter, FA translocase (FAT/CD36), and, in analogy to skeletal muscle, is likely to involve its translocation from an intracellular pool to the sarcolemma. In the present study, we investigated whether cAMP-dependent signalling is involved in this translocation process. Isoproterenol, dibutyryl-cAMP and the phosphodiesterase (PDE) inhibitor, amrinone, which markedly raised the intracellular cAMP level, did not affect cellular FA uptake, but influenced the fate of intracellular FAs by directing these to mitochondrial oxidation in electrostimulated cardiac myocytes. The PDE inhibitors 3-isobutyl-1-methylxanthine, milrinone and dipyridamole each significantly stimulated FA uptake as well as intracellular cAMP levels, but these effects were quantitatively unrelated. The stimulatory effects of these PDE inhibitors were antagonized by sulpho- N -succinimidylpalmitate, indicating the involvement of FAT/CD36, albeit that the different PDE inhibitors use different molecular mechanisms to stimulate FAT/CD36-mediated FA uptake. Notably, 3-isobutyl-1-methylxanthine and milrinone increased the intrinsic activity of FAT/CD36, possibly through its covalent modification, and dipyridamole induces translocation of FAT/CD36 to the sarcolemma. Elevation of intracellular cGMP, but not of cAMP, by the PDE inhibitor zaprinast did not have any effect on FA uptake and metabolism by cardiac myocytes. The stimulatory effects of PDE inhibitors on cardiac FA uptake should be considered when applying these agents in clinical medicine.

Critical steps in cellular fatty acid uptake and utilization

Despite decades of extensive research, the transport routes, mechanisms of uptake and points of flux control of long-chain fatty acids (FA) in mammalian organs are still incompletely understood. In non-fenestratred organs such as heart and skeletal muscle, membrane barriers for blood-borne FA are the luminal and abluminal membranes of endothelial cells, the sarcolemma and the mitochondrial membranes. Transport of FA through the phospholipid bilayer of the cellular membrane is most likely accomplished by diffusion of protonated FA. Evidence is accumulating that membrane-associated proteins, such as plasmalemmal fatty acid-binding protein (FABPpm) and fatty acid translocase (FAT/CD36), either alone or in conjunction with albumin binding protein (ABP), are instrumental in enhancing the delivery of FA to the cellular membrane. Inside the cell, cytoplasmic fatty acid-binding proteins (FABPc) are involved in diffusion of FA from the plasmalemma to the intracellular sites of conversion, such as the mitochondrial outer membrane. After conversion of FA to FACoA, the fatty acyl chain is transported across the mitochondrial inner membrane in a carnitine-mediated fashion. Uptake and utilization of FA by muscle cells are finely tuned, most likely to avoid the intracellular accumulation of FA, as these are cytotoxic at high concentrations. On a short-term basis, net uptake is, among others, regulated by intracellular translocation of FAT from intracellular stores to the sarcolemma and by the concentration gradient of FA across the sarcolemma. The latter implies that, among others, the rate of FA utilization determines the rate of uptake. The rate of utilization is governed by a variety of factors, including malonylCoA, the ratio acetylCoA/CoA and the availability of competing substrates such as glucose, lactate, and ketone bodies. Long-term regulation of uptake and utilization is accomplished by alterations in the rate of expression of genes, encoding for FA-handling proteins. Circumstantial evidence indicates that FA themselves are able to modulate the expression of FA-handling genes via nuclear transcription factors such as peroxisome proliferator-activated receptors (PPARs).

Understanding the impact of mitochondrial defects in cardiovascular disease: a review

OBJECTIVE

Defects in mitochondrial structure and function have been found in association with cardiovascular diseases such as dilated and hypertrophic cardiomyopathy, cardiac conduction defects and sudden death, ischemic and alcoholic cardiomyopathy, and myocarditis. A genetic basis has been established for some mitochondrial abnormalities (eg, mitochondrial DNA changes leading to oxidative phosphorylation dysfunction, fatty acid beta-oxidation (FAO) defects resulting from specific nuclear mutations) whereas other abnormalities appear to be due to a more sporadic or environmental cardiotoxic insult or have not yet been characterized.

METHODS

This article reviews mitochondrial abnormalities in structure or function reported in cardiac diseases highlighting information about their potential etiology, significance in cardiac pathogenesis, and diagnostic and therapeutic options available to the clinician. We also provide a brief background concerning mitochondrial biogenesis and bioenergetic pathways in cardiac growth, development, and aging.

CONCLUSIONS

Although aberrations in bioenergetic functioning of mitochondria appear to be most often related to cardiac dysfunction, the primary defect(s) causing bioenergetic dysfunction may reside in a nonbioenergetic pathway (eg, signaling between mitochondria and nucleus) or in overall mitochondrial biogenesis or degradation pathways.

Changes in fatty acid transport and transporters are related to the severity of insulin deficiency

We have examined the effects of streptozotocin (STZ)-induced diabetes (moderate and severe) on fatty acid transport and fatty acid transporter (FAT/CD36) and plasma membrane-bound fatty acid binding protein (FABPpm) expression, at the mRNA and protein level, as well as their plasmalemmal localization. These studies have shown that, with STZ-induced diabetes, 1) fatty acid transport across the plasma membrane is increased in heart, skeletal muscle, and adipose tissue and is reduced in liver; 2) changes in fatty acid transport are generally not associated with changes in fatty acid transporter mRNAs, except in the heart; 3) increases in fatty acid transport in heart and skeletal muscle occurred with concomitant increases in plasma membrane FAT/CD36, whereas in contrast, the increase and decrease in fatty acid transport in adipose tissue and liver, respectively, were accompanied by concomitant increments and reductions in plasma membrane FABPpm; and finally, 4) the increases in plasma membrane transporters (FAT/CD36 in heart and skeletal muscle; FABPpm in adipose tissue) were attributable to their increased expression, whereas in liver, the reduced plasma membrane FABPpm appeared to be due to its relocation within the cell in the face of slightly increased expression. Taken together, STZ-induced changes in fatty acid uptake demonstrate a complex and tissue-specific pattern, involving different fatty acid transporters in different tissues, in combination with different underlying mechanisms to alter their surface abundance.

Strategies for the diagnosis of mitochondrial fatty acid beta-oxidation disorders

Mitochondrial fatty acid beta-oxidation disorders (FAOD) are a group of clinically and biochemically heterogeneous inherited metabolic defects. The spectrum of phenotypes has expanded from hepatic encephalopathy to encompass myopathy, cardiomyopathy, peripheral neuropathy, sudden death and pregnancy complicated by fetal FAOD. Pre-symptomatic diagnosis is important to prevent morbidity and this is now achievable through newborn screening using tandem mass spectrometry (MS/MS). Moreover, most of the diagnosed defects are treatable and the prognosis is generally favourable. This article reviews the features of FAOD, critically evaluates methods of investigation including metabolite analyses in body fluids, in vitro oxidation rates and acylcarnitine profiling studies, enzymatic and mutational tests, and discusses genotype-phenotype correlation, treatment and monitoring options. Based on this knowledge, strategies for the biochemical investigation and differential diagnosis of patients presenting clinically, asymptomatic neonates detected by newborn screening, infants born after complications during late pregnancy, and cases of sudden death with suspected FAOD are presented. Laboratory investigation commonly begins with a search for diagnostic metabolites in physiological fluids, followed by in vitro functional studies if the initial findings are inconclusive, and confirmation by enzymology and molecular analyses. Occasionally a stress test in vivo may be required. At other times there may be no firm diagnosis achieved.

Exercise and insulin increase muscle fatty acid uptake by recruiting putative fatty acid transporters to the sarcolemma

PURPOSE OF REVIEW

Skeletal muscle metabolic energy, needed to maintain contractile activity, is mainly obtained from glucose and long-chain fatty acids. Recent studies have revealed a remarkable parallel between the regulation of uptake of glucose and fatty acids by muscle, in that each is mediated by sarcolemmal transporters that are recruited from an intracellular storage site. The focus of this review is to describe newly obtained insights on the recruitment of fatty acid transporters and their malfunctioning in diabetes.

RECENT FINDINGS

The major fatty acid transporter involved is fatty acid translocase (CD36). Translocation of this protein to the membrane is triggered by muscle contraction and by insulin, and presumably occurs from distinct intracellular pools. This resembles the well documented exercise and insulin-induced recruitment of glucose transporter-4. Whether another transporter, plasma membrane fatty acid-binding protein, is also subject to such recycling is not yet clear. In a rodent model of insulin-dependent (type 1) diabetes, the increased rate of muscle fatty acid uptake could be associated with an increased total amount of fatty acid translocase (CD36). In a model of non-insulin dependent (type 2) diabetes, this increased rate could be associated with a permanent relocalization of fatty acid translocase to the sarcolemma.

SUMMARY

These findings indicate a pivotal role for the membrane transporter fatty acid translocase in the exercise and insulin-induced increases of muscle fatty acid uptake and utilization, and suggest that malfunctioning of the cellular recycling of fatty acid translocase is involved in the etiology of insulin resistance and type 2 diabetes.

Prolonged unloading of rat soleus muscle causes distinct adaptations of the gene profile

Using commercially available microarray technology, we investigated a series of transcriptional adaptations caused by atrophy of rat m. soleus due to 35 days of hindlimb suspension. We detected 395 out of 1,200 tested transcripts, which reflected 1%-5% of totally expressed genes. From various cellular functional pathways, we detected multiple genes that spanned a 200-fold range of gene expression levels. Statistical analysis combining L1 regression with the sign test based on the conservative Bonferroni correction identified 105 genes that underwent transcriptional adaptations with atrophy. Generally, expressional changes were discrete (<50%) and pointed in the same direction for genes belonging to the same cellular functional units. In particular, a distinct expressional adaptation of genes involved in fiber transformation; that is, metabolism, protein turnover, and cell regulation were noted and matched to corresponding transcriptional changes in nutrient trafficking. Expressional changes of extracellular proteases, and of genes involved in nerve-muscle interaction and excitation-contraction coupling identify previously not recognized adaptations that occur in atrophic m. soleus. Considerations related to technical and statistical aspects of the array approach for profiling the skeletal muscle genome and the impact of observed novel adaptations of the m. soleus transcriptome are put into perspective of the physiological adaptations occurring with muscular atrophy.

Increased rates of fatty acid uptake and plasmalemmal fatty acid transporters in obese Zucker rats

Giant vesicles were used to study the rates of uptake of long-chain fatty acids by heart, skeletal muscle, and adipose tissue of obese and lean Zucker rats. With obesity there was an increase in vesicular fatty acid uptake of 1.8-fold in heart, muscle and adipose tissue. In some tissues only fatty acid translocase (FAT) mRNA (heart, +37%; adipose, +80%) and fatty acid-binding protein (FABPpm) mRNA (heart, +148%; adipose, +196%) were increased. At the protein level FABPpm expression was not changed in any tissues except muscle (+14%), and FAT/CD36 protein content was altered slightly in adipose tissue (+26%). In marked contrast, the plasma membrane FAT/CD36 protein was increased in heart (+60%), muscle (+80%), and adipose tissue (+50%). The plasma membrane FABPpm was altered only in heart (+50%) and adipose tissues (+70%). Thus, in obesity, alterations in fatty acid transport in metabolically important tissues are not associated with changes in fatty acid transporter mRNAs or altered fatty acid transport protein expression but with their increased abundance at the plasma membrane. We speculate that in obesity fatty acid transporters are relocated from an intracellular pool to the plasma membrane in heart, muscle, and adipose tissues.

Electrostimulation enhances FAT/CD36-mediated long-chain fatty acid uptake by isolated rat cardiac myocytes

We investigated palmitate uptake and utilization by contracting cardiac myocytes in suspension to explore the link between long-chain fatty acid (FA) uptake and cellular metabolism, in particular the role of fatty acid translocase (FAT)/CD36-mediated transsarcolemmal FA transport. For this, an experimental setup was developed to electrically stimulate cardiomyocytes in multiple parallel incubations. Electrostimulation at voltages > or =170 V resulted in cellular contraction with no detrimental effect on cellular integrity. At 200 V and 4 Hz, palmitate uptake (measured after 3-min incubation) was enhanced 1.5-fold. In both quiescent and contracting myocytes, after their uptake, palmitate was largely and rapidly esterified, mainly into triacylglycerols. Palmitate oxidation (measured after 30 min) contributed to 22% of palmitate taken up by quiescent cardiomyocytes and, after stimulation at 4 Hz, was increased 2.8-fold to contribute to 39% of palmitate utilization. The electrostimulation-mediated increase in palmitate uptake was blocked in the presence of either verapamil, a contraction inhibitor, or sulfo-N-succinimidyl-FA esters, specific inhibitors of FAT/CD36. These data indicate that, in contracting cardiac myocytes, palmitate uptake is increased due to increased flux through FAT/CD36.

Demonstration of direct effects of growth hormone on neonatal cardiomyocytes

The cellular and molecular basis of growth hormone (GH) actions on the heart remain poorly defined, and it is unclear whether GH effects on the myocardium are direct or mediated at least in part via insulin-like growth factor (IGF-1). Here, we demonstrate that the cultured neonatal cardiomyocyte is not an appropriate model to study the effects of GH because of artifactual loss of GH receptors (GHRs). To circumvent this problem, rat neonatal cardiomyocytes were infected with a recombinant adenovirus expressing the murine GHR. Functional integrity of GHR was suggested by GH-induced activation of the cognate JAK2/STAT5, MAPK, and Akt intracellular pathways in the cells expressing GHR. Although exposure to GH resulted in a significant increase in the size of the cardiomyocyte and increased expression of c-fos, myosin light chain 2, and skeletal alpha-actin mRNAs, there were no significant changes in IGF-1 or atrial natriuretic factor mRNA levels in response to GH stimulation. In this model, GH increased incorporation of leucine, uptake of palmitic acid, and abundance of fatty acid transport protein mRNA. In contrast, GH decreased uptake of 2-deoxy-d-glucose and levels of Glut1 protein. Thus, in isolated rat neonatal cardiomyocytes expressing GHR, GH induces hypertrophy and causes alterations in cellular metabolic profile in the absence of demonstrable changes in IGF-1 mRNA, suggesting that these effects may be independent of IGF-1.

Unravelling the significance of cellular fatty acid-binding proteins

Cellular long-chain fatty acid (FA) transport and metabolism are believed to be regulated by membrane-associated and soluble proteins that bind and transport FAs. Several different classes of membrane proteins have been proposed as FA acceptors or transmembrane FA transporters. New evidence from in-vitro and whole-animal studies supports the existence of protein-mediated transmembrane transport of FAs, which is likely to coexist with passive diffusional uptake. The trafficking of FAs by intracellular fatty acid-binding proteins may involve their interaction with specific membrane or protein targets. Evidence is also emerging for concerted actions between the membrane and cytoplasmic fatty acid-binding proteins that allow for efficient regulation of FA transport and metabolism.

Role of plasma membrane transporters in muscle metabolism

Muscle plays a major role in metabolism. Thus it is a major glucose-utilizing tissue in the absorptive state, and changes in muscle insulin-stimulated glucose uptake alter whole-body glucose disposal. In some conditions, muscle preferentially uses lipid substrates, such as fatty acids or ketone bodies. Furthermore, muscle is the main reservoir of amino acids and protein. The activity of many different plasma membrane transporters, such as glucose carriers and transporters of carnitine, creatine and amino acids, play a crucial role in muscle metabolism by catalysing the influx or the efflux of substrates across the cell surface. In some cases, the membrane transport process is subjected to intense regulatory control and may become a potential pharmacological target, as is the case with the glucose transporter GLUT4. The goal of this review is the molecular characterization of muscle membrane transporter proteins, as well as the analysis of their possible regulatory role.

Effects of fatty acids on uncoupling protein-2 expression in the rat heart

Fatty acids are thought to play a role in the activity of uncoupling proteins (UCP) and have been shown to regulate the expression of genes encoding proteins involved in fatty acid handling. Therefore, we investigated whether fatty acids, which are the main substrates for the heart, affect rat cardiac UCP-2 expression in vivo and in vitro. After birth, when the contribution of fatty acid oxidation to cardiac energy conversion increases, UCP-2 expression enhanced rapidly. In the adult heart, however, UCP-2 mRNA levels did not alter during conditions associated with either enhanced (fasting, diabetes) or decreased (hypertrophy) fatty acid utilization. Exposure of neonatal cardiomyocytes and embryonic rat heart-derived H9c2 cells to fatty acids (palmitic and oleic acid) for 48 h strongly induced UCP-2 expression. Stimulation of neonatal cardiomyocytes with triiodothyronine also increased UCP-2 mRNA levels, though only in the presence of fatty acids. Ligands specific to the fatty acid-activated transcription factor PPARalpha, but not to PPARgamma, acted as inducers of cardiomyocyte UCP-2 expression. It is concluded that fatty acids promote UCP-2 expression in neonatal cardiomyocytes, which might explain the rapid increase in UCP-2 mRNA in the postnatal heart. However, UCP-2 mRNA levels in the adult heart appear to be insensitive to changes in cardiac fatty acid handling under various pathological conditions.

Cellular fatty acid transport in heart and skeletal muscle as facilitated by proteins

Despite the importance of long-chain fatty acids (FA) as fuels for heart and skeletal muscles, the mechanism of their cellular uptake has not yet been clarified. There is dispute as to whether FA are taken up by the muscle cells via passive diffusion and/or carrier-mediated transport. Kinetic studies of FA uptake by cardiac myocytes and the use of membrane protein-modifying agents have suggested the bulk of FA uptake is due to a protein component. Three membrane-associated FA-binding proteins were proposed to play a role in FA uptake, a 40-kDa plasma membrane FA-binding protein (FABPpm), an 88-kDa FA translocase (FAT/CD36), and a 60-kDa FA transport protein (FATP). In cardiac and skeletal myocytes the intracellular carrier for FA is cytoplasmic heart-type FA-binding protein (H-FABP), which likely transports FA from the sarcolemma to their intracellular sites of metabolism. A scenario is discussed in which FABPpm, FAT/CD36, and H-FABP, probably assisted by an albumin-binding protein, cooperate in the translocation of FA across the sarcolemma.