Defective uptake and utilization of long chain fatty acids in muscle and adipose tissues of CD36 knockout mice

Plasmalemmal fatty acid transport is regulated in heart and skeletal muscle by contraction, insulin and leptin, and in obesity and diabetes

It has been assumed that the uptake of long chain fatty acids (LCFAs) into skeletal muscle and the heart muscle, as well as other tissues, occurred via passive diffusion. In recent years our work has shown that the LCFA uptake into skeletal muscle is a highly regulated process. The use of giant sarcolemmal vesicles obtained from skeletal muscle and heart has been used to demonstrate that LCFA uptake into these tissues occurs via a protein-mediated mechanism involving the 40 kDa plasma membrane associated fatty acid binding protein (FABPpm) and the 88 kDa fatty acid translocase, the homologue of human CD36 (FAT/CD36). Both are ubiquitously expressed proteins and correlate with LCFA uptake into heart and muscle, consistent with the known differences in LCFA metabolism in these tissues. It has recently been found that FAT/CD36 is present in an intracellular (endosomal) compartment from which it can be translocated to the plasma membrane within minutes by muscle contraction and by insulin, to stimulate LCFA uptake. In rodent models of obesity and type 1 diabetes LCFA uptake into heart and muscle is also increased, either by permanently relocating FAT/CD36 to the plasma membrane without altering its expression (obesity) or by increasing the expression of both FAT/CD36 and FABPpm (type 1 diabetes). Chronic leptin treatment decreases LCFA transporters and transport in muscle. Clearly, recent evidence has established that LCFA uptake into heart and muscle is regulated acutely and chronically.

Cardiac lipid accumulation associated with diastolic dysfunction in obese mice

Obesity may confer cardiac dysfunction due to lipid accumulation in cardiomyocytes. To test this idea, we examined whether obese ob/ob mice display heart lipid accumulation and cardiac dysfunction. Ob/ob mouse hearts had increased expression of genes mediating extracellular generation, transport across the myocyte cell membrane, intracellular transport, mitochondrial uptake, and beta-oxidation of fatty acids compared with ob/+ mice. Accordingly, ob/ob mouse hearts contained more triglyceride (6.8 +/- 0.4 vs. 2.3 +/- 0.4 microg/mg; P < 0.0005) than ob/+ mouse hearts. Histological examinations showed marked accumulation of neutral lipid droplets within cardiac myocytes but not increased deposition of collagen between myocytes in ob/ob compared with ob/+ mouse hearts. On echocardiography, the ratio of E to A transmitral flow velocities (an indicator of diastolic function) was 1.8 +/- 0.1 in ob/ob mice and 2.5 +/- 0.1 in ob/+ mice (P = 0.0001). In contrast, the indexes of systolic function and heart brain natriuretic peptide mRNA expression were only marginally affected and unaffected, respectively, in ob/ob compared with ob/+ mice. The results suggest that ob/ob mouse hearts have increased expression of cardiac gene products that stimulate myocyte fatty acid uptake and triglyceride storage and accumulate neutral lipids within the cardiac myocytes. The results also suggest that the cardiac lipid accumulation is paralleled by cardiac diastolic dysfunction in ob/ob mice.

Decreased liver fatty acid binding capacity and altered liver lipid distribution in mice lacking the liver fatty acid-binding protein gene

Although liver fatty acid-binding protein (L-FABP) is an important binding site for various hydrophobic ligands in hepatocytes, its in vivo significance is not understood. We have therefore created L-FABP null mice and report here their initial analysis, focusing on the impact of this mutation on hepatic fatty acid binding capacity, lipid composition, and expression of other lipid-binding proteins. Gel-filtered cytosol from L-FABP null liver lacked the main fatty acid binding peak in the fraction that normally comprises both L-FABP and sterol carrier protein-2 (SCP-2). The binding capacity for cis-parinaric acid was decreased >80% in this region. Molar ratios of cholesterol/cholesterol ester, cholesteryl ester/triglyceride, and cholesterol/phospholipid were 2- to 3-fold greater, reflecting up to 3-fold absolute increases in specific lipid classes in the order cholesterol > cholesterol esters > phospholipids. In contrast, the liver pool sizes of nonesterified fatty acids and triglycerides were not altered. However, hepatic deposition of a bolus of intravenously injected [14C]oleate was markedly reduced, showing altered lipid pool turnover. An increase of approximately 75% of soluble SCP-2 but little or no change of other soluble (glutathione S-transferase, albumin) and membrane (fatty acid transport protein, CD36, aspartate aminotransferase, caveolin) fatty acid transporters was measured. These results (i) provide for the first time a quantitative assessment of the contribution of L-FABP to cytosolic fatty acid binding capacity, (ii) establish L-FABP as an important determinant of hepatic lipid composition and turnover, and (iii) suggest that SCP-2 contributes to the accumulation of cholesterol in L-FABP null liver.

Myocardial recovery from ischemia is impaired in CD36-null mice and restored by myocyte CD36 expression or medium-chain fatty acids

Long-chain fatty acid uptake, which provides a large part of myocardial energy, is impaired in human and murine hearts deficient in the membrane fatty acid translocase, FAT/CD36. We examined myocardial function in CD36-null mice using the working heart. Fatty acid oxidation and stores of glycogen, triglycerides, and ATP were reduced in CD36-deficient hearts and were restored to WT levels by rescue of myocyte CD36. Under normal perfusion conditions, CD36-null hearts had similar cardiac outputs and end-diastolic pressures as WT or transgenic hearts. After 6 min of ischemia, cardiac output decreased by 41% and end diastolic pressure tripled for CD36-null hearts, with no significant changes in WT or transgenic hearts. Null hearts also failed more frequently after ischemia as compared with WT or transgenics. To dissect out contribution of fatty acid uptake, a perfusate-lacking fatty acids was used. This decreased cardiac output after ischemia by 30% in WT hearts as compared with 50% for CD36-deficient hearts. End diastolic pressure, a negative index of myocardial performance, increased after ischemia in all heart types. Addition to the perfusate of a medium-chain fatty acid (caprylic acid) that does not require CD36 for uptake alleviated poor ischemic tolerance of CD36-null hearts. In summary, recovery from ischemia is compromised in CD36-deficient hearts and can be restored by CD36 rescue or by supplying medium-chain fatty acids. It would be important to determine whether the findings apply to the human situation where polymorphisms of the CD36 gene are relatively common.

Characterization of a heart-specific fatty acid transport protein

Fatty acids are a major source of energy for cardiac myocytes. Changes in fatty acid metabolism have been implicated as causal in diabetes and cardiac disease. The mechanism by which long chain fatty acids (LCFAs) enter cardiac myocytes is not well understood but appears to occur predominantly by protein-mediated transport. Here we report the cloning, expression pattern, and subcellular localization of a novel member of the fatty acid transport protein (FATP) family termed FATP6. FATP6 is principally expressed in the heart where it is the predominant FATP family member. Similar to other FATPs, transient and stable transfection of FATP6 into 293 cells enhanced uptake of LCFAs. FATP6 mRNA was localized to cardiac myocytes by in situ hybridization. Immunofluorescence microscopy of FATP6 in monkey and murine hearts revealed that the protein is exclusively located on the sarcolemma. FATP6 was restricted in its distribution to areas of the plasma membrane juxtaposed with small blood vessels. In these membrane domains FATP6 also colocalizes with another molecule involved in LCFA uptake, CD36. These findings suggest that FATP6 is involved in heart LCFA uptake, in which it may play a role in the pathogenesis of lipid-related cardiac disorders.

CD36 deficiency and absent myocardial iodine-123-(R,S)-15-(p-iodophenyl)-3-methylpentadecanoic acid uptake in a girl with cardiomyopathy

A 10-year-old girl presented left ventricular failure 1 month after the onset of hemolytic uremic syndrome (HUS) caused by an Escherichia coli O157 infection and was diagnosed as having dilated cardiomyopathy. Thallium myocardial scintigraphy showed normal perfusion, but no myocardial uptake of iodine-123-( R, S)-15-( p-iodophenyl)-3-methylpentadecanoic acid ((123)I-BMIPP) was observed. We analyzed the CD36 expression in platelets and monocytes by using a flowcytometer, and she turned out to have CD36 deficiency type I.

CONCLUSION

Some patients may be predisposed to myocardial damage in the presence of CD36 deficiency. It is necessary to clarify the etiological significance of the relationship between cardiac impairment and CD36 deficiency in children.

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.

Rapid flip-flop of oleic acid across the plasma membrane of adipocytes

Nonesterified long-chain fatty acids may enter cells by free diffusion or by membrane protein transporters. A requirement for proteins to transport fatty acids across the plasma membrane would imply low partitioning of fatty acids into the membrane lipids, and/or a slower rate of diffusion (flip-flop) through the lipid domains compared to the rates of intracellular metabolism of fatty acids. We used both vesicles of the plasma membrane of adipocytes and intact adipocytes to study transmembrane fluxes of externally added oleic acid at concentrations below its solubility limit at pH 7.4. Binding of oleic acid to the plasma membrane was determined by measuring the fluorescent fatty acid-binding protein ADIFAB added to the external medium. Changes in internal pH caused by flip-flop and metabolism were measured by trapping a fluorescent pH indicator in the cells. The metabolic end products of oleic acid were evaluated over the time interval required for the return of intracellular pH to its initial value. The primary findings were that (i) oleic acid rapidly binds with high avidity in the lipid domains of the plasma membrane with an apparent partition coefficient similar to that of protein-free phospholipid bilayers; (ii) oleic acid rapidly crosses the plasma membrane by the flip-flop mechanism (both events occur within 5 s); and (iii) the kinetics of esterification of oleic acid closely follow the time dependence of the recovery of intracellular pH. Any postulated transport mechanism for facilitating translocation of fatty acid across the plasma membrane of adipocytes, including a protein transporter, would have to compete with the highly effective flip-flop mechanism.

Contribution of fatty acids released from lipolysis of plasma triglycerides to total plasma fatty acid flux and tissue-specific fatty acid uptake

There is controversy over the extent to which fatty acids (FAs) derived from plasma free FAs (FFAs) or from hydrolysis of plasma triglycerides (TGFAs) form communal or separate pools and what the contribution of each FA source is to cellular FA metabolism. Chylomicrons and lipid emulsions were labeled with [(3)H]triolein, injected into mice, and appearance in plasma of [(3)H]oleic acid was estimated, either through a steady-state approach or by compartmental modeling. [(14)C]oleic acid was included to trace plasma FFA. Eighty to 90% of triglyceride (TG) label was recovered in plasma, irrespective of tracer method or TG source. The contribution of TG lipolysis to total plasma FA turnover was 10-20%. After infusion of [(3)H]TG and [(14)C]FA, the retention of these labels varied substantially among liver, adipose tissue, and skeletal and heart muscle. Retention of TG label changed during fasting in the same direction as lipoprotein lipase (LPL) activity is regulated. We propose a model that reconciles the paradoxical 80-90% loss of TG label into plasma with LPL-directed differential uptake of TGFA in tissues. In this model, TGFAs mix locally at the capillaries with plasma FFAs, where they would lead to an increase in the local FA concentration, and hence, FA uptake. Our data indicate that a distinction between TG-derived FA and plasma FFA cannot be made.

Routes of FA delivery to cardiac muscle: modulation of lipoprotein lipolysis alters uptake of TG-derived FA

Long-chain fatty acids (FA) supply 70-80% of the energy needs for normal cardiac muscle. To determine the sources of FA that supply the heart, [(14)C]palmitate complexed to bovine serum albumin and [(3)H]triolein [triglyceride (TG)] incorporated into Intralipid were simultaneously injected into fasted male C57BL/6 mice. The ratio of TG to FA uptake was much greater for hearts than livers. Using double-labeled Intralipid with [(3)H]cholesteryl oleoyl ether (CE) and [(14)C]TG, we observed that hearts also internalize intact core lipid. Inhibition of lipoprotein lipase (LPL) with tetrahydrolipstatin or dissociation of LPL from the heart with heparin reduced cardiac uptake of TG by 82 and 64%, respectively (P < 0.01). Palmitate uptake by the heart was not changed by either treatment. Uptake of TG was 88% less in hearts from LPL knockout mice that were rescued via LPL expression in the liver. Our data suggest that the heart is especially effective in removal of circulating TG and core lipids and that this is due to LPL hydrolysis and not its bridging function.

Insulin sensitivity and lipid metabolism in human CD36 deficiency

OBJECTIVE

CD36 has been proposed as a fatty acid translocase and a receptor for HDL and oxidized LDL. The association between CD36 deficiency and insulin resistance remains controversial. We investigated glucose and lipid metabolism in human CD36 deficiency.

RESEARCH DESIGN AND METHODS

A total of 61 type I CD36-deficient patients and 25 control subjects were examined. Diabetes was defined as fasting glucose level > or =7 mmol/l or use of hypoglycemic agents. A homeostasis model assessment (HOMA) index was evaluated in patients without diabetes. Insulin resistance was defined as a HOMA index > or =1.73 (sensitivity 64.3%, specificity 78.9%; J Japan Diab Soc, 2000).

RESULTS

Diabetes was identified in 12 (20%) of the 61 CD36-deficient patients. Fasting glucose, HbA(1c), and total cholesterol levels in the diabetic CD36-deficient patients were significantly higher than in the control subjects and the nondiabetic CD36-deficient patients. Regardless of diabetes, HDL cholesterol concentrations in the CD36-deficient patients were significantly higher than in the control subjects. The nondiabetic CD36-deficient patients had higher triglyceride concentrations than the control subjects, and triglyceride concentrations were higher in the diabetic CD36-deficient patients than in the nondiabetic CD36-deficient patients. The prevalence of insulin resistance in the nondiabetic CD36-deficient patients was similar to that in the control subjects.

CONCLUSIONS

Human CD36 deficiency is not necessarily responsible for insulin resistance. Lipid abnormalities in CD36 deficiency may partly depend on the presence of diabetes, and increased levels of triglyceride and HDL cholesterol may be due to impaired binding of fatty acids and HDL to CD36 and subsequent clearance.

Characterization of fatty acid oxidation in human muscle mitochondria and myoblasts

The mitochondrial oxidation of fatty acids (FAO) is the main energy-producing pathway in skeletal and cardiac muscle. Starting from standard muscle biopsies (100-200mg), we determined the optimal conditions of mitochondrial oxygen consumption by the FAO pathway, and in parallel we performed the isolation and primary culture of muscle cells to test their cellular FAO capacities. The determinations of maximal beta-oxidation rates in the presence of palmitoyl-CoA or palmitoyl-L-carnitine (mean+/-SEM: 32.5+/-2.0 and 34.1+/-1.3nmol O(2) min(-1) mg(-1) protein, n=16, respectively) provide a screening method of mitochondrial fatty acid transport system and intra-mitochondrial beta-oxidation. We also determined the conditions of tritiated palmitate oxidation by human myoblasts (mean+/-SEM: 6.6+/-0.1nmol 3H fatty acid h(-1) mg(-1) protein, n=8), and show that beta-oxidation defects can be detected in our experiments. Overall, we propose an original laboratory test to investigate FAO in human skeletal muscle and to screen for FAO disorders in myopathies and cardiomyopathies in human.

Cardiac Hypertrophy by Hypertension and Exercise Training Exhibits Different Gene Expression of Enzymes in Energy Metabolism

Hypertension-induced pathological cardiac hypertrophy (hypertensive heart) and exercise training-induced physiological cardiac hypertrophy (athletic heart) have differences in cardiac properties. We hypothesized that gene expression of energy metabolic enzymes differs between these two types of cardiac hypertrophy. To investigate whether mRNA expression of key enzymes in the long-chain fatty acid (FA), glucose, and lactic acid metabolic pathways differs between these two types of cardiac hypertrophy, we used the hearts of spontaneously hypertensive rats (SHR; 19 weeks old) as a model of the hypertensive heart, swim-trained rats (Trained; 19 weeks old, swimming training for 15 weeks) as a model of the athletic heart, and sedentary Wistar-Kyoto rats (Control; 19 weeks old). SHR developed hypertensive cardiac hypertrophy, of which cardiac function was deteriorated, whereas Trained rats developed an athletic heart, of which cardiac function was enhanced. The mRNA expression of CD36, which involved in uptake of long-chain FA, in the heart was almost never detected in the SHR group. Furthermore, the mRNA expression of key enzymes in the long-chain FA metabolic pathway (acyl CoA synthase [ACoAS], carnitine palmitoyl transferase [CPT]-I, CPT-II, and isocitrate dehydrogenase [ISCD]) in the heart was significantly higher in the SHR group compared with the Control group. The mRNA expression of ACoAS, CPT-I, ISCD, and CD36 in the heart did not differ between Trained group and Control group, whereas that of CPT-II in the Trained group was significantly higher compared with the Control group. The mRNA expression of key enzymes (phosphofructokinase and lactate dehydrogenase) in glycolytic metabolic pathway in the heart was markedly higher in the SHR group compared with the Control group, whereas these mRNA expressions did not differ between Trained group and Control group. These findings suggest that the molecular phenotypes in the energy metabolic system differ in hypertension-induced pathological and exercise training-induced physiological cardiac hypertrophy, and these differences may participate in the differences in cardiac function.

Pharmacogenetic evidence that cd36 is a key determinant of the metabolic effects of pioglitazone

Pioglitazone, like other thiazolidinediones, is an insulin-sensitizing agent that activates the peroxisome proliferator-activated receptor gamma and influences the expression of multiple genes involved in carbohydrate and lipid metabolism. However, it is unknown which of these many target genes play primary roles in determining the antidiabetic and hypolipidemic effects of thiazolidinediones. To specifically investigate the role of the Cd36 fatty acid transporter gene in the insulin-sensitizing actions of thiazolidinediones, we studied the metabolic effects of pioglitazone in spontaneously hypertensive rats (SHR) that harbor a deletion mutation in Cd36 in comparison to congenic and transgenic strains of SHR that express wild-type Cd36. In congenic and transgenic SHR with wild-type Cd36, administration of pioglitazone was associated with significantly lower circulating levels of fatty acids, triglycerides, and insulin as well as lower hepatic triglyceride levels and epididymal fat pad weights than in SHR harboring mutant Cd36. Additionally, insulin-stimulated glucose oxidation in isolated soleus muscle was significantly augmented in pioglitazone-fed rats with wild-type Cd36 versus those with mutant Cd36. The Cd36 genotype had no effect on pioglitazone-induced changes in blood pressure. These findings provide direct pharmacogenetic evidence that in the SHR model, Cd36 is a key determinant of the insulin-sensitizing actions of a thiazolidinedione ligand of peroxisome proliferator-activated receptor gamma.

Fatty acid transport across membranes: relevance to nutrition and metabolic pathology

Long-chain fatty acids are an important constituent of the diet and they contribute to a multitude of cellular pathways and functions. Uptake of long-chain fatty acids across plasma membranes is the first step in fatty acid utilization, and recent evidence supports an important regulatory role for this process. Although uptake of fatty acids involves two components, passive diffusion through the lipid bilayer and protein-facilitated transfer, the latter component appears to play the major role in mediating uptake by key tissues. Identification of several proteins as fatty acid transporters, and emerging evidence from genetically altered animal models for some of these proteins, has contributed significant insight towards understanding the limiting role of transport in the regulation of fatty acid utilization. We are also beginning to better appreciate how disturbances in fatty acid utilization influence general metabolism and contribute to metabolic pathology.

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.

Patterns of gene expression in atrophying skeletal muscles: response to food deprivation

During fasting and many systemic diseases, muscle undergoes rapid loss of protein and functional capacity. To define the transcriptional changes triggering muscle atrophy and energy conservation in fasting, we used cDNA microarrays to compare mRNAs from muscles of control and food-deprived mice. Expression of >94% of genes did not change, but interesting patterns emerged among genes that were differentially expressed: 1) mRNAs encoding polyubiquitin, ubiquitin extension proteins, and many (but not all) proteasome subunits increased, which presumably contributes to accelerated protein breakdown; 2) a dramatic increase in mRNA for the ubiquitin ligase, atrogin-1, but not most E3s; 3) a significant suppression of mRNA for myosin binding protein H (but not other myofibrillar proteins) and IGF binding protein 5, which may favor cell protein loss; 4) decreases in mRNAs for several glycolytic enzymes and phosphorylase kinase subunits, and dramatic increases in mRNAs for pyruvate dehydrogenase kinase 4 and glutamine synthase, which should promote glucose sparing and gluconeogenesis. During fasting, metallothionein mRNA increased dramatically, mRNAs for extracellular matrix components fell, and mRNAs that may favor cap-independent mRNA translation rose. Significant changes occurred in mRNAs for many growth-related proteins and transcriptional regulators. These transcriptional changes indicate a complex adaptive program that should favor protein degradation and suppress glucose oxidation in muscle. Similar analysis of muscles atrophying for other causes is allowing us to identify a set of atrophy-specific changes in gene expression.

Effects of obesity and weight loss on the expression of proteins involved in fatty acid metabolism in human adipose tissue

OBJECTIVE

Disturbances in adipocyte lipolysis in obesity may contribute to elevated circulating non-esterified fatty acid (NEFA) concentrations and insulin resistance. In experimental models, NEFA metabolism is influenced by adipocyte proteins such as adipocyte and keratinocyte lipid binding proteins (aP2/ALBP and mal1/KLBP) and fatty acid translocase (CD36). We investigated the effect of obesity and weight loss on the expression of these proteins in human subcutaneous adipose tissue.

STUDY DESIGN AND SUBJECTS

Subcutaneous adipose tissue was obtained from 12 obese (body mass index (BMI) 42.4+/-1.6 kg/m(2)) and 12 lean (23.4+/-0.6 kg/m(2)) subjects. The obese subjects underwent gastric banding and biopsies were taken again after 2 y following a significant weight reduction (BMI 32.8+/-1.4 kg/m(2)). Adipose tissue proteins were quantified by Western blotting.

RESULTS

Differential expression of ALBP, KLBP and CD36 was observed in lean and weight-reduced subjects compared with obese individuals. This resulted in a significantly lower ALBP/KLBP ratio in lean and weight-reduced individuals compared to obese subjects. Furthermore there was a significant influence of gender on this ratio. Moreover, the commonly used internal standard protein actin was expressed significantly higher in lean compared to obese individuals.

CONCLUSION

The relative content of ALBP and KLBP in human adipose tissue changes with obesity, weight loss and gender indicating differential regulation. Differing responses in the expression patterns of adipose tissue proteins capable of binding NEFAs in response to weight changes suggest a potential importance in the development of obesity-associated complications.

Insulin stimulates long-chain fatty acid utilization by rat cardiac myocytes through cellular redistribution of FAT/CD36

The existence of an intracellular pool of fatty acid translocase (FAT/CD36), an 88-kDa membrane transporter for long-chain fatty acids (FAs), and the ability of insulin to induce translocation events prompted us to investigate the direct effects of insulin on cellular uptake of FA by the heart. Insulin (0.1 nmol/l and higher) increased FA uptake by isolated rat cardiac myocytes by 1.5-fold. This insulin-induced increase in FA uptake was completely blocked by phloretin, sulfo-N-succinimidylpalmitate (SSP), and wortmannin, indicating the involvement of FAT/CD36 and the dependence on phosphatidylinositol-3 (PI-3) kinase activation. Subcellular fractionation of insulin-stimulated cardiac myocytes demonstrated a 1.5-fold increase in sarcolemmal FAT/CD36 and a 62% decrease in intracellular FAT/CD36 with parallel changes in subcellular distribution of GLUT4. Induction of cellular contractions upon electrostimulation at 4 Hz enhanced cellular FA uptake 1.6-fold, independent of PI-3 kinase. The addition of insulin to 4 Hz-stimulated cells further stimulated FA uptake to 2.3-fold, indicating that there are at least two functionally independent intracellular FAT/CD36 pools, one recruited by insulin and the other mobilized by contractions. In conclusion, we have demonstrated a novel role of insulin in cardiac FA utilization. Malfunctioning of insulin-induced FAT/CD36 translocation may be involved in the development of type 2 diabetic cardiomyopathies.

CD36 genotype and long-chain fatty acid uptake in the heart

Homozygous or compound heterozygous mutation of the CD36 gene (CD36-/-) in humans results in severe defects of the uptake of long-chain fatty acids (LCFAs) in the heart. Because the effect of a single mutation of this gene (CD36+/-) on the LCFA uptake is not known, it was evaluated in 29 subjects with the CD36 wild-type gene (WT) (6 healthy subjects, 10 patients with heart disease), CD36+/- (4 healthy subjects, 5 patients) and CD36-/- (4 patients). The CD36 genotype was identified in the coding region of genomic DNA, and the expression of CD36 protein was examined by flow cytometry after staining with monoclonal anti-CD36 antibody. The LCFA uptake in the heart was assessed as the radioactivity accumulation ratio of heart to mediastinum after intravenous administration of iodine-123 15-(p-iodophenyl)-3-R, S-methylpentadecanoic acid (H/M ratio). The H/M ratios in WT, CD36+/- and CD36-/- were 2.28 +/- 0.10, 1.90 +/- 0.06 and 1.40 +/- 0.11, respectively (p < 0.0001, among groups). The H/M ratio between healthy subjects and patients with heart disease for WT and CD36+/- did not differ significantly (ie, those of WT and CD36+/- in healthy subjects and patients were 2.29 +/- 0.08 vs 2.27 +/- 0.12 and 1.90+/- 0.07 vs 1.89 +/- 0.05, respectively). Not only CD36-/- but also CD36+/- resulted in a significant reduction of the LCFA uptake in the heart independent of heart disease, suggesting genotype dependency and that CD36 might be a fundamental determinant of myocardial LCFA uptake.

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.

Cellular fatty acid uptake is acutely regulated by membrane-associated fatty acid-binding proteins

Cellular long-chain fatty acid uptake is believed to occur largely by protein-mediated transmembrane transport of fatty acids, and also by passive diffusional uptake. It is postulated that the membrane proteins function in trapping of fatty acids from extracellular sources, whereafter their transmembrane translocation occurs by passive diffusion through the lipid bilayer. The key membrane-associated proteins involved are plasma membrane fatty acid-binding protein (FABP(pm)) and fatty acid translocase (FAT/CD36). Their plasma membrane contents are positively correlated with rates of fatty acid uptake. In studies with heart and skeletal muscle we observed that FAT/CD36 is regulated acutely, in that both contraction and insulin can translocate FAT/CD36 from an intracellular depot to the sarcolemma, thereby increasing the rate of fatty acid uptake. In addition, from studies with obese Zucker rats, an established rodent model of obesity and insulin resistance, evidence has been obtained that in heart, muscle and adipose tissue FAT/CD36 is permanently relocated from an intracellular pool to the plasma membrane, resulting in increased fatty acid uptake rates in this condition. These combined observations indicate that protein-mediated fatty acid uptake is a key step in cellular fatty acid utilization, and suggest that malfunctioning of the uptake process could be a critical factor in the pathogenesis of insulin resistance.

Adipocytes recognize and degrade oxidized low density lipoprotein through CD36

CD36 expressed on adipocytes is thought to function as a fatty acid transporter (FAT). Here we report that adipocytes can endocytose and lysosomally degrade OxLDL, mainly mediated by CD36. Mouse 3T3-L1 preadipocytes showed marked increase in uptake and degradation of (125)I-OxLDL during their differentiation to adipocytes. RT-PCR and immunoblot analysis indicated that expression of CD36 but not of scavenger receptor class A or macrosialin is required for the increase in uptake and degradation of (125)I-OxLDL in 3T3-L1 cells. An anti-CD36 antibody inhibited both uptake and degradation activities of (125)I-OxLDL up to 60%. These results strongly suggest that adipocytes may function as phagocytes like macrophages and that CD36 plays a novel role in adipose tissues.

Atorvastatin treatment induced peroxisome proliferator-activated receptor alpha expression and decreased plasma nonesterified fatty acids and liver triglyceride in fructose-fed rats

We aimed to investigate the effect of atorvastatin (5 and 30 mg/kg/day for 2 weeks) on hepatic lipid metabolism in a well established model of dietary hypertriglyceridemia, the fructose-fed rat. Fructose feeding (10% fructose in drinking water for 2 weeks) induced hepatic lipogenesis and reduced peroxisome proliferator-activated receptor alpha (PPARalpha) expression and fatty acid oxidation. As a result, plasma and liver triglyceride and plasma apolipoprotein B (apoB) levels were increased. Atorvastatin, 5 and 30 mg/kg during 2 weeks, markedly reduced plasma triglyceride, but decreased apoB levels only at the highest dose tested (50%). Triglyceride biosynthetic enzymes and microsomal triglyceride transfer protein were unchanged, whereas liver PPARalpha, acyl-CoA oxidase, and carnitine palmitoyltransferase I mRNA levels (1.9-, 1.25-, and 3.4-fold, respectively) and hepatic fatty acid beta-oxidation activity (1.25-fold) were increased by atorvastatin at 30 mg/kg. Furthermore, hepatic triglyceride content (45%) and plasma nonesterified fatty acids (NEFAs) (49%) were reduced. These results show for the first time that liver triglyceride increase in fructose-fed rats is linked to decreased expression of PPARalpha, which is prevented by atorvastatin treatment. The increase in PPARalpha expression caused by atorvastatin was associated with reduced liver triglyceride and plasma NEFA levels.

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.

Contrasting effects of exercise and NOS inhibition on tissue-specific fatty acid and glucose uptake in mice

Isotopic techniques were used to test the hypothesis that exercise and nitric oxide synthase (NOS) inhibition have distinct effects on tissue-specific fatty acid and glucose uptakes in a conscious, chronically catheterized mouse model. Uptakes were measured using the radioactive tracers (125)I-labeled beta-methyl-p-iodophenylpentadecanoic acid (BMIPP) and deoxy-[2-(3)H]glucose (DG) during treadmill exercise with and without inhibition of NOS. [(125)I]BMIPP uptake at rest differed substantially among tissues with the highest levels in heart. With exercise, [(125)I]BMIPP uptake increased in both heart and skeletal muscles. In sedentary mice, NOS inhibition induced by nitro-L-arginine methyl ester (L-NAME) feeding increased heart and soleus [(125)I]BMIPP uptake. In contrast, exercise, but not L-NAME feeding, resulted in increased heart and skeletal muscle [2-(3)H]DG uptake. Significant interactions were not observed in the effects of combined exercise and L-NAME feeding on [(125)I]BMIPP and [2-(3)H]DG uptakes. In the conscious mouse, exercise and NOS inhibition produce distinct patterns of tissue-specific fatty acid and glucose uptake; NOS is not required for important components of exercise-associated metabolic signaling, or other mechanisms compensate for the absence of this regulatory mechanism.

Changes in FAT/CD36, UCP2, UCP3 and GLUT4 gene expression during lipid infusion in rat skeletal and heart muscle

OBJECTIVE

It has been reported that an increased availability of free fatty acids (NEFA) not only interferes with glucose utilization in insulin-dependent tissues, but may also result in an uncoupling effect of heart metabolism. We aimed therefore to investigate the effect of an increased availability of NEFA on gene expression of proteins involved in transmembrane fatty acid (FAT/CD36) and glucose (GLUT4) transport and of the uncoupling proteins UCP2 and 3 at the heart and skeletal muscle level.

STUDY DESIGN

Euglycemic hyperinsulinemic clamp was performed after 24 h Intralipid(R) plus heparin or saline infusion in lean Zucker rats. Skeletal and heart muscle glucose utilization was calculated by 2-deoxy-[1-(3)H]-D-glucose technique. Quantification of FAT/CD36, GLUT4, UCP2 and UCP3 mRNAs was obtained by Northern blot analysis or RT-PCR.

RESULTS

In Intralipid(R) plus heparin infused animals a significant decrease in insulin-mediated glucose uptake was observed both in the heart (22.62+/-2.04 vs 10.37+/-2.33 ng/mg/min; P<0.01) and in soleus muscle (13.46+/-1.53 vs 6.84+/-2.58 ng/mg/min; P<0.05). FAT/CD36 mRNA was significantly increased in skeletal muscle tissue (+117.4+/-16.3%, P<0.05), while no differences were found at the heart level in respect to saline infused rats. A clear decrease of GLUT4 mRNA was observed in both tissues. The 24 h infusion of fat emulsion resulted in a clear enhancement of UCP2 and UCP3 mRNA levels in the heart (99.5+/-15.3 and 80+/-4%) and in the skeletal muscle (291.5+/-24.7 and 146.9+/-12.7%).

CONCLUSIONS

As a result of the increased availability of NEFA, FAT/CD36 gene expression increases in skeletal muscle, but not at the heart level. The augmented lipid fuel supply is responsible for the depression of insulin-mediated glucose transport and for the increase of UCP2 and 3 gene expression in both skeletal and heart muscle.

Defective fatty acid uptake modulates insulin responsiveness and metabolic responses to diet in CD36-null mice

Deficiency of the membrane protein FAT/CD36 causes a marked defect in fatty acid uptake by various tissues and is genetically linked to insulin resistance in rats and humans. Here, we examined insulin responsiveness of CD36-/- mice. When fed a diet high in complex carbohydrates and low (5%) in fat, these animals cleared glucose faster than the wild-type. In vivo, uptake of 2-fluorodeoxyglucose by muscle was increased severalfold, and in vitro, insulin responsiveness of glycogenesis by the soleus was enhanced. Null mice had lower glycogen levels in muscle and liver, lower muscle triglyceride levels, and increased liver triglyceride content--all findings consistent with increased insulin-sensitivity. However, when the chow diet was switched to one high in fructose, CD36-/- mice but not wild-type mice developed marked glucose intolerance, hyperinsulinemia, and decreased muscle glucose uptake. High-fat diets impaired glucose tolerance equally in both groups, although CD36 deficiency helped moderate insulin-responsive muscle glucose oxidation. In conclusion, CD36 deficiency enhances insulin responsiveness on a high-starch, low-fat diet. It predisposes to insulin resistance induced by high fructose and partially protects from that induced by high-fat diets. In humans, CD36 deficiency may be an important factor in the metabolic adaptation to diet and in susceptibility to some forms of diet-induced pathology.

CD36 mediates the cardiovascular action of growth hormone-releasing peptides in the heart

Growth hormone-releasing peptides (GHRPs) are known as potent growth hormone secretagogues whose actions are mediated by the ghrelin receptor, a G protein-coupled receptor cloned from pituitary libraries. Hexarelin, a hexapeptide of the GHRP family, has reported cardiovascular activity. To identify the molecular target mediating this activity, rat cardiac membranes were labeled with a radioactive photoactivatable derivative of hexarelin and purified using lectin affinity chromatography and preparative gel electrophoresis. A binding protein of M(r) 84 000 was identified. The N-terminal sequence determination of the deglycosylated protein was identical to rat CD36, a multifunctional glycoprotein, which was expressed in cardiomyocytes and microvascular endothelial cells. Activation of CD36 in perfused hearts by hexarelin was shown to elicit an increase in coronary perfusion pressure in a dose-dependent manner. This effect was lacking in hearts from CD36-null mice and hearts from spontaneous hypertensive rats genetically deficient in CD36. The coronary vasoconstrictive response correlated with expression of CD36 as assessed by immunoblotting and covalent binding with hexarelin. These data suggest that CD36 may mediate the coronary vasospasm seen in hypercholesterolemia and atherosclerosis.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

Fat targets for insulin signaling

Adipose cells control flux of fatty acids to peripheral tissues by storing and hydrolyzing triglyceride under hormonal control. New data reveal that insulin may regulate this process in part by promoting membrane trafficking of intracellular fatty acid transporters FATP1 and FATP4 to the plasma membrane.

Chronic leptin administration decreases fatty acid uptake and fatty acid transporters in rat skeletal muscle

Chronic leptin administration reduces triacylglycerol content in skeletal muscle. We hypothesized that chronic leptin treatment, within physiologic limits, would reduce the fatty acid uptake capacity of red and white skeletal muscle due to a reduction in transport protein expression (fatty acid translocase (FAT/CD36) and plasma membrane-associated fatty acid-binding protein (FABPpm)) at the plasma membrane. Female Sprague-Dawley rats were infused for 2 weeks with leptin (0.5 mg/kg/day) using subcutaneously implanted miniosmotic pumps. Control and pair-fed animals received saline-filled implants. Leptin levels were significantly elevated (approximately 4-fold; p < 0.001) in treated animals, whereas pair-fed treated animals had reduced serum leptin levels (approximately -2-fold; p < 0.01) relative to controls. Palmitate transport rates into giant sarcolemmal vesicles were reduced following leptin treatment in both red (-45%) and white (-84%) skeletal muscle compared with control and pair-fed animals (p < 0.05). Leptin treatment reduced FAT mRNA (red, -70%, p < 0.001; white, -48%, p < 0.01) and FAT/CD36 protein expression (red, -32%; p < 0.05) in whole muscle homogenates, whereas FABPpm mRNA and protein expression were unaltered. However, in leptin-treated animals plasma membrane fractions of both FAT/CD36 and FABPpm protein expression were significantly reduced in red (-28 and -34%, respectively) and white (-44 and -56%, respectively) muscles (p < 0.05). Across all experimental treatments and muscles, palmitate uptake by giant sarcolemmal vesicles was highly correlated with the plasma membrane FAT/CD36 protein (r = 0.88, p < 0.01) and plasma membrane FABPpm protein (r = 0.94, p < 0.01). These studies provide the first evidence that protein-mediated long chain fatty acid transport is subject to long term regulation by leptin.

Long-chain polyunsaturated fatty acid accretion in brain

Brain is highly enriched in long-chain polyunsaturated fatty acids (PUFAs), particularly arachidonic acid and docosahexaenoic acid, which play important roles in brain structural and biologic functions. Plasma transport, in the form of free fatty acids or esterified FAs in lysophosphatidylcholine and lipoproteins, and de-novo synthesis contribute to brain accretion of long-chain PUFAs. Transport of long-chain PUFAs from plasma may play important roles because of the limited ability of brain to synthesize long-chain PUFAs, in the face of high demand for them. Although several proteins involved in facilitated fatty acid transport (e.g. fatty acid transport protein, fatty acid binding protein and very-long-chain acyl-coenzyme A synthetase) have been found in brain, their roles in fatty acid accumulation in brain are poorly defined. The primary pathways that are involved in long-chain PUFA accumulation in brain may vary according to brain region and developmental stage.

Role of CD36 in membrane transport of long-chain fatty acids

CD36 is a multispecific membrane glycoprotein that has been postulated to have a variety of functions. Evidence generated in isolated cells and in mice and rat models of altered CD36 expression has indicated an important role for CD36 in membrane transport of long-chain fatty acids. The cumulative data indicate that CD36 facilitates a major fraction of fatty acid uptake by muscle and fat, and that CD36 deficiency is associated with a large (60-80%) defect in fatty acid uptake by those tissues. In humans, polymorphisms in the CD36 gene may underlie defective fatty acid metabolism and some forms of heart disease. Herein we review our current understanding of the transport function and regulation of CD36. The realization that the transport step rate limits cellular fatty acid utilization suggests that abnormalities in CD36 expression or function may impact on susceptibility to certain metabolic diseases such as obesity and insulin resistance.

CD36 deficiency induced by antiretroviral therapy

BACKGROUND

The molecular basis of lipodystrophy, a syndrome associated with HIV antiretroviral (ARV) therapy, remains unknown.

OBJECTIVES

To examine whether ARV therapy might inhibit the expression of CD36, which is known to play an important role in fatty acid and glucose metabolism, and if this might contribute to the metabolic alterations associated with lipodystrophy.

DESIGN

The effects of ARV therapy on CD36 levels was examined in vivo in a prospective cohort of individuals treated with ARV therapy and in vitro in assays of human cell lines exposed to ARV drugs.

METHODS

Monocyte CD36 levels were assessed by flow cytometry at baseline and after 7 days of therapy in five healthy volunteers and 10 treatment-naive HIV-1-infected individuals. ARV therapy included protease inhibitors (ritonavir, nelfinavir or lopinavir/ritonavir). In addition, human cell lines (THP-1 and C32) were assessed for CD36 levels pre and post-ritonavir treatment.

RESULTS

Three of four healthy controls (one withdrew because of adverse effects) and 6 of 10 HIV-1-infected individuals had a 50 to > 90% decrease in monocyte CD36 levels after 7 days of therapy. One of ten HIV-infected subjects had a 30% decrease, and the remaining individuals had no change or an increase in CD36 levels. CD36 levels decreased significantly in human cell lines treated with ritonavir but not in those treated with zidovudine.

CONCLUSIONS

ARV therapy resulted in a marked decrease in CD36 in approximately 70% of our participants. Sustained ARV therapy-induced CD36 deficiency may contribute to insulin resistance and other metabolic complications of lipodystrophy.

Fatty acid transport: the roads taken

Efficient uptake and channeling of long-chain fatty acids (LCFAs) are critical cellular functions. Although spontaneous flip-flop of nonionized LCFAs from one leaflet of a bilayer to the other is rapid, evidence is emerging that proteins are important mediators and/or regulators of trafficking of LCFAs into and within cells. Genetic screens have led to the identification of proteins that are required for fatty acid import and utilization in prokaryotic organisms. In addition, functional screens have elucidated proteins that facilitate fatty acid import into mammalian cells. Although the mechanisms by which these proteins mediate LCFA import are not well understood, studies in both prokaryotic and eukaryotic organisms provide compelling evidence that uptake of LCFAs across cellular membranes is coupled to esterification by acyl-CoA synthetases. This review will summarize results of studies of non-protein-mediated and protein-mediated LCFA transport and discuss how these different mechanisms may contribute to cellular metabolism.

High carbohydrate availability increases LCFA uptake and decreases LCFA oxidation in perfused muscle

To determine whether changes in long-chain fatty acid (LCFA) oxidative metabolism induced by elevated intracellular carbohydrate availability are due to changes in LCFA uptake or in mitochondrial transport capacity, rat hindquarters were perfused with 500 microM palmitate and [1-14C]palmitate or [1-14C]octanoate as well as with either low (LG) or high (HG) carbohydrate availability. Glucose uptake was higher in the HG vs. LG group (23.6 +/- 1.5 vs 4.7 +/- 0.9 micromol x g(-1) x h(-1), P < 0.05). Palmitate delivery was not significantly different between groups and averaged 97.1 +/- 4.6 nmol x min(-1) x g(-1) (P > 0.05). Fractional and total palmitate uptake values were 60% higher (P < 0.05) in the HG (0.125 +/- 0.012 and 7.4 +/- 1.2 nmol x min(-1) x g(-1)) vs. LG (0.079 +/- 0.009 and 11.8 +/- 1.5 nmol x min(-1) x g(-1)) group. Values of percent and total palmitate oxidized were significantly lower (P < 0.05) in the HG (9.1 +/- 1.1% and 1.31 +/- 0.16 nmol x min(-1) x g(-1)) vs. LG (23.4 +/- 5.2% and 0.76 +/- 0.08 nmol x min(-1) x g(-1)) group. Conversely, values of fractional uptake and percent oxidation of octanoate were not significantly different between groups (P > 0.05). Malonyl-CoA levels were inversely correlated with LCFA oxidation (P < 0.05). These results demonstrate that high carbohydrate availability in muscle is associated with a decrease in LCFA oxidation that is not due to a parallel decrease in LCFA uptake; rather, the decrease in LCFA oxidation could be due to malonyl-CoA inhibition of mitochondrial LCFA transport.

Post-transcriptional stimulation of the assembly and secretion of triglyceride-rich apolipoprotein B lipoproteins in a mouse with selective deficiency of brown adipose tissue, obesity, and insulin resistance

A mouse model of insulin resistance and its associated dyslipidemia was generated by crossing mice expressing human apolipoprotein B (apoB) with mice lacking only brown adipose tissue (BATless). On a high fat diet, male apoB/BATless mice became obese, hypercholesterolemic, hypertriglyceridemic, and hyperinsulinemic compared with control apoB mice. Fast performance liquid chromatography revealed increased triglyceride concentrations in intermediate density lipoprotein/low density lipoprotein (LDL) and reduced high density lipoprotein cholesterol concentrations. Inhibition of lipolysis by the drug, tetrahydrolipostatin, demonstrated that very low density lipoprotein-sized particles were initially secreted. Metabolic studies employing Triton WR-1339 and either [(3)H]glycerol or [(3)H]palmitate showed that the hypertriglyceridemia in apoB/BATless mice was due to the increased synthesis and secretion of triglyceride. Furthermore, lipoprotein lipase and hepatic lipase activities were not defective. ApoB was also secreted at increased rates in the apoB/BATless mice. Similar levels of apoB mRNA in apoB and apoB/BATless mice indicated that apoB secretion was regulated post-transcriptionally. LDL receptor mRNA was increased in the apoB/BATless mice, indicating that the observed increase in apoB-lipoprotein secretion was not due to their decreased reuptake. Finally, mRNA levels of the large subunit of microsomal triglyceride transfer protein, a required component for very low density protein assembly, were not different between apoB and apoB/BATless mice. This rodent model should prove useful in exploring mechanisms underlying the regulation of apoB secretion in the context of insulin resistance.

Protection from obesity and insulin resistance in mice overexpressing human apolipoprotein C1

Apolipoprotein (APO) C1 is a 6.6-kDa protein present in plasma and associated with lipoproteins. Using hyperinsulinemic-euglycemic clamp tests, we previously found that in APOC1 transgenic mice, the whole-body insulin-mediated glucose uptake is increased concomitant with a decreased fatty acid uptake. These latter results are confirmed in the present study, showing that APOC1 transgenic mice exhibit a 50% reduction in the uptake of the fatty acid analog 15-(p-iodophenyl)-3-(R,S)-methyl pentadecanoic acid in white adipose tissue stores. We next investigated whether APOC1 overexpression can modulate the initiation and/or development of obesity and insulin resistance. When crossbred on the genetically obese ob/ob background, APOC1 transgenic mice were fully protected from the development of obesity compared with ob/ob only mice, as reflected by a strong reduction in body weight (21 +/- 4 vs. 44 +/- 7 g), total adipose tissue stores (15 +/- 3 vs. 25 +/- 3% body wt), and average adipocyte size (7,689 +/- 624 vs. 15,295 +/- 1,289 microm(2)). Although less pronounced, APOC1 overexpression also reduced body weight on a wild-type background, solely due to a reduction in adipose tissue. Furthermore, despite elevated plasma free fatty acid and triglyceride levels, APOC1 overexpression significantly improved insulin sensitivity in ob/ob mice, as demonstrated by a strong reduction in plasma glucose and insulin levels, as well as a better performance in the glucose tolerance test. In conclusion, a marked reduction in the uptake of fatty acids into adipocytes may underlie the protection from obesity and insulin resistance in transgenic mice overexpressing human APOC1.

Recent advances in membrane microdomains: rafts, caveolae, and intracellular cholesterol trafficking

Cellular cholesterol homeostasis is a balance of influx, catabolism and synthesis, and efflux. Unlike vascular lipoprotein cholesterol transport, intracellular cholesterol trafficking is only beginning to be resolved. Exogenous cholesterol and cholesterol ester enter cells via the low-density lipoprotein (LDL) receptor/lysosomal and less so by nonvesicular, high-density lipoprotein (HDL) receptor/caveolar pathways. However, the mechanism(s) whereby cholesterol enters the lysosomal membrane, translocates, and transfers out of the lysosome to the cell interior are unknown. Likewise, the steps whereby cholesterol enters the cytofacial leaflet of the plasma membrane caveolae, rapidly translocates, leaves the exofacial leaflet, and transfers to extracellular HDL are unclear. Increasing evidence obtained with model and isolated cell membranes, transfected cells, genetic mutants, and gene-ablated mice suggests that proteins such as caveolin, sterol carrier protein-2 (SCP-2), Niemann-Pick C1 protein, steroidogenic acute regulatory protein (StAR), and other intracellular proteins mediate intracellular cholesterol transfer. While these proteins bind cholesterol and/or interact with cholesterol-rich membrane microdomains (e.g., caveolae, rafts, and annuli), their relative contributions to direct molecular versus vesicular cholesterol transfer remain to be resolved. The formation, regulation, and role of membrane microdomains in regulating cholesterol uptake/efflux and trafficking are unclear. Some cholesterol-binding proteins exert opposing effects on cellular cholesterol uptake/efflux, transfer of cholesterol out of the lysosomal membrane, and/or intracellular cholesterol trafficking to select membranous organelles. Resolving these cholesterol pathways and the role of membrane cholesterol microdomains is essential to our understanding not only of processes that affect cholesterol metabolism, but also of the abnormal regulation that may lead to disease (diabetes, obesity, atherosclerosis, neutral lipid storage, Niemann-Pick C, congenital lipoid adrenal hyperplasia, etc.).

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.

The gene-nutrient-gene loop

It is increasingly apparent that the genetic influence on the development and severity of a particular phenotype (e.g. diabetes, hyperlipidemia, hypertension, and coronary heart disease) can be strongly modulated by diet. In turn, the response of the phenotype to dietary intervention is determined by the individual genotype. The reviews in this issue provide striking examples of recent progress related to the molecular basis of nutrient-gene interactions. As our understanding of these interactions improves, we should be better equipped to identify individuals at risk of specific pathologies and make a better assessment of the risk involved. Nutritional support could then be tailored to the individual genotype to favour beneficial phenotypic expression or to suppress that leading to pathology and disease.

Defective fatty acid uptake in the spontaneously hypertensive rat is a primary determinant of altered glucose metabolism, hyperinsulinemia, and myocardial hypertrophy

Genetic linkage studies implicated deficiency of CD36, a membrane fatty acid (FA) transporter, in the hypertriglyceridemia and hyperinsulinemia of the spontaneously hypertensive rat (SHR). In this study we determined whether loss of CD36 function in FA uptake is a primary determinant of the SHR phenotype. In vivo, tissue distribution of iodinated, poorly oxidized beta-methyliodophenyl pentadecanoic acid (BMIPP) was examined 2 h after its intravenous injection. Fatty acid transport was also measured in vitro over 20 to 120 s in isolated adipocytes and cardiomyocytes obtained from SHR and from a congenic line (SHRchr4) that incorporates a piece of chromosome 4 containing wild-type CD36. SHR heart and adipose tissue exhibited defects in FA uptake and in conversion of diglycerides to triglycerides that are similar to those observed in the CD36 null mouse. However, a key difference in SHR tissues is that fatty acid oxidation is much more severely impaired than fatty acid esterification, which may underlie the 4-5-fold accumulation of free BMIPP measured in SHR muscle. Studies with isolated adipocytes and cardiomyocytes directly confirmed both the defect in FA transport and the fact that it is underestimated by BMIPP. Heart, oxidative muscle, and adipose tissue in the SHR exhibited a large increase in glucose uptake measured in vivo using [(18)F]fluorodeoxyglucose. Supplementation of the diet with short-chain fatty acids, which do not require CD36-facilitated transport, eliminated the increase in glucose uptake, the hyperinsulinemia, and the heart hypertrophy in the SHR. This indicated that lack of metabolic energy consequent to deficient FA uptake is the primary defect responsible for these abnormalities. Hypertension was not alleviated by the supplemented diet suggesting it is unrelated to fuel supply and any contribution of CD36 deficiency to this trait may be more complex to determine. It may be worth exploring whether short-chain FA supplementation can reverse some of the deleterious effects of CD36 deficiency in humans, which may include hypertrophic cardiomyopathy.