Phosphatidylinositol 4-kinase IIIß mediates contraction-induced GLUT4 translocation and shows its anti-diabetic action in cardiomyocytes.

In the diabetic heart, long-chain fatty acid (LCFA) uptake is increased at the expense of glucose uptake. This metabolic shift ultimately leads to insulin resistance and a reduced cardiac function. Therefore, signaling kinases that mediate glucose uptake without simultaneously stimulating LCFA uptake could be considered attractive anti-diabetic targets. Phosphatidylinositol-4-kinase-IIIß (PI4KIIIß) is a lipid kinase downstream of protein kinase D1 (PKD1) that mediates Golgi-to-plasma membrane vesicular trafficking in HeLa-cells. In this study, we evaluated whether PI4KIIIß is involved in myocellular GLUT4 translocation induced by contraction or oligomycin (an F1F0-ATP synthase inhibitor that activates contraction-like signaling). Pharmacological targeting, with compound MI14, or genetic silencing of PI4KIIIß inhibited contraction/oligomycin-stimulated GLUT4 translocation and glucose uptake in cardiomyocytes but did not affect CD36 translocation nor LCFA uptake. Addition of the PI4KIIIß enzymatic reaction product phosphatidylinositol-4-phosphate restored oligomycin-stimulated glucose uptake in the presence of MI14. PI4KIIIß activation by PKD1 involves Ser294 phosphorylation and altered its localization with unchanged enzymatic activity. Adenoviral PI4KIIIß overexpression stimulated glucose uptake, but did not activate hypertrophic signaling, indicating that unlike PKD1, PI4KIIIß is selectively involved in GLUT4 translocation. Finally, PI4KIIIß overexpression prevented insulin resistance and contractile dysfunction in lipid-overexposed cardiomyocytes. Together, our studies identify PI4KIIIß as positive and selective regulator of GLUT4 translocation in response to contraction-like signaling, suggesting PI4KIIIß as a promising target to rescue defective glucose uptake in diabetics.

Assessing fatty acid oxidation flux in rodent cardiomyocyte models.

The healthy adult heart primarily relies on fatty acid oxidation (FAO) for energy production but instantaneously adapts its substrate preference in response to physiological or pathological challenges. Accurate FAO measurements are crucial to investigate early metabolic (mal)adaptations. While measurements in intact cardiomyocytes offer greater physiological relevance, current FAO protocols mainly employ cell-free systems and/or require expensive equipment. Here, we present an easy-to-use, inexpensive, and sensitive method to measure, compare and modulate FAO in various cardiomyocyte models. Basal FAO was 2-fold higher in fresh versus cultured adult rat cardiomyocytes (aRCM), while OXPHOS protein levels were maintained. Basal FAO was higher in cultured (3-fold) and fresh (8-fold) aRCM, versus widely used neonatal rat cardiomyocytes (nRCM) and mouse HL1 cardiomyocytes. Moreover, we utilized chemical and pharmacological treatments in order to modulate the FAO flux at different cellular signalling levels. Our data indicate that caution should be taken when studying metabolism in nRCM and HL1 cell models, as these display significantly lower FAO than aRCM. Accurate FAO measurement in cultured aRCM opens new avenues for studying the complex cardiomyocyte metabolic responses to mechanical, nutritional, pharmacological, and genetic manipulations.

Caffeine-stimulated fatty acid oxidation is blunted in CD36 null mice.

AIM: The increase in skeletal muscle fatty acid metabolism during exercise has been associated with the release of calcium. We examined whether this increase in fatty acid oxidation was attributable to a calcium-induced translocation of the fatty acid transporter CD36 to the sarcolemma, thereby providing an enhanced influx of fatty acids to increase their oxidation. METHODS: Calcium release was triggered by caffeine (3 mm) to examine fatty acid oxidation in intact soleus muscles of WT and CD36-KO mice, while fatty acid transport and mitochondrial fatty acid oxidation were examined in giant vesicles and isolated mitochondria, respectively, from caffeine-perfused hindlimb muscles of WT and CD36-KO mice. Western blotting was used to examine calcium-induced signalling. RESULTS: In WT, caffeine stimulated muscle palmitate oxidation (+136%), but this was blunted in CD36-KO mice (-70%). Dantrolene inhibited (WT) or abolished (CD36-KO) caffeine-induced palmitate oxidation. In muscle, caffeine-stimulated palmitate oxidation was not attributable to altered mitochondrial palmitate oxidation. Instead, in WT, caffeine increased palmitate transport (+55%) and the translocation of fatty acid transporters CD36, FABPpm, FATP1 and FATP4 (26-70%) to the sarcolemma. In CD36-KO mice, caffeine-stimulated FABPpm, and FATP1 and 4 translocations were normal, but palmitate transport was blunted (-70%), comparable to the reductions in muscle palmitate oxidation. Caffeine did not alter the calcium-/calmodulin-dependent protein kinase II phosphorylation but did increase the phosphorylation of AMPK and acetyl-CoA carboxylase comparably in WT and CD36-KO. CONCLUSION: These studies indicate that sarcolemmal CD36-mediated fatty acid transport is a primary mediator of the calcium-induced increase in muscle fatty acid oxidation.

Requirement for distinct vesicle-associated membrane proteins in insulin- and AMP-activated protein kinase (AMPK)-induced translocation of GLUT4 and CD36 in cultured cardiomyocytes.

AIMS/HYPOTHESIS: Upon stimulation of insulin signalling or contraction-induced AMP-activated protein kinase (AMPK) activation, the glucose transporter GLUT4 and the long-chain fatty acid (LCFA) transporter CD36 similarly translocate from intracellular compartments to the plasma membrane of cardiomyocytes to increase uptake of glucose and LCFA, respectively. This similarity in regulation of GLUT4 traffic and CD36 traffic suggests that the same families of trafficking proteins, including vesicle-associated membrane proteins (VAMPs), are involved in both processes. While several VAMPs have been implicated in GLUT4 traffic, nothing is known about the putative function of VAMPs in CD36 traffic. Therefore, we compared the involvement of the myocardially produced VAMP isoforms in insulin- or contraction-induced GLUT4 and CD36 translocation. METHODS: Five VAMP isoforms were silenced in HL-1 cardiomyocytes. The cells were treated with insulin or the contraction-like AMPK activator oligomycin or were electrically stimulated to contract. Subsequently, GLUT4 and CD36 translocation as well as substrate uptake were measured. RESULTS: Three VAMPs were demonstrated to be necessary for both GLUT4 and CD36 translocation, either specifically in insulin-treated cells (VAMP2, VAMP5) or in oligomycin/contraction-treated cells (VAMP3). In addition, there are VAMPs specifically involved in either GLUT4 traffic (VAMP7 mediates basal GLUT4 retention) or CD36 traffic (VAMP4 mediates insulin- and oligomycin/contraction-induced CD36 translocation). CONCLUSIONS/INTERPRETATION: The involvement of distinct VAMP isoforms in both GLUT4 and CD36 translocation indicates that CD36 translocation, just like GLUT4 translocation, is a vesicle-mediated process dependent on soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex formation. The ability of other VAMPs to discriminate between GLUT4 and CD36 translocation allows the notion that myocardial substrate preference can be modulated by these VAMPs.

Increased levels of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1alpha) improve lipid utilisation, insulin signalling and glucose transport in skeletal muscle of lean and insulin-resistant obese Zucker rats.

AIMS/HYPOTHESIS: Reductions in peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1alpha) levels have been associated with the skeletal muscle insulin resistance. However, in vivo, the therapeutic potential of PGC-1alpha has met with failure, as supra-physiological overexpression of PGC-1alpha induced insulin resistance, due to fatty acid translocase (FAT)-mediated lipid accumulation. Based on physiological and metabolic considerations, we hypothesised that a modest increase in PGC-1alpha levels would limit FAT upregulation and improve lipid metabolism and insulin sensitivity, although these effects may differ in lean and insulin-resistant muscle. METHODS: Pgc-1alpha was transfected into lean and obese Zucker rat muscles. Two weeks later we examined mitochondrial biogenesis, intramuscular lipids (triacylglycerol, diacylglycerol, ceramide), GLUT4 and FAT levels, insulin-stimulated glucose transport and signalling protein phosphorylation (thymoma viral proto-oncogene 2 [Akt2], Akt substrate of 160 kDa [AS160]), and fatty acid oxidation in subsarcolemmal and intermyofibrillar mitochondria. RESULTS: Electrotransfection yielded physiologically relevant increases in Pgc-1alpha (also known as Ppargc1a) mRNA and protein ( approximately 25%) in lean and obese muscle. This induced mitochondrial biogenesis, and increased FAT and GLUT4 levels, insulin-stimulated glucose transport, and Akt2 and AS160 phosphorylation in lean and obese animals, while bioactive intramuscular lipids were only reduced in obese muscle. Concurrently, PGC-1alpha increased palmitate oxidation in subsarcolemmal, but not in intermyofibrillar mitochondria, in both groups. In obese compared with lean animals, the PGC-1alpha-induced improvement in insulin-stimulated glucose transport was smaller, but intramuscular lipid reduction was greater. CONCLUSIONS/INTERPRETATIONS: Increases in PGC-1alpha levels, similar to those that can be induced by physiological stimuli, altered intramuscular lipids and improved fatty acid oxidation, insulin signalling and insulin-stimulated glucose transport, albeit to different extents in lean and insulin-resistant muscle. These positive effects are probably attributable to limiting the PGC-1alpha-induced increase in FAT, thereby preventing bioactive lipid accumulation as has occurred in transgenic PGC-1alpha animals.

Pyruvate dehydrogenase kinase 4 expression is synergistically induced by AMP-activated protein kinase and fatty acids.

Organs are flexible as to which substrates they will use to maintain energy homeostasis. Under well-fed conditions, glucose is a preferred substrate for oxidation. During fasting, fatty acid oxidation will become a more important energy source. Glucose oxidation is decreased by fatty acids, a process in which the pyruvate dehydrogenase complex (PDH) and its regulator pyruvate dehydrogenase kinase 4 (PDK4) play important roles. It is currently unknown how energy status influences PDH activity. We show that AMP-activated protein kinase (AMPK) activation by hypoxia and AICAR treatment combined with fatty acid administration synergistically induce PDK4 expression. We provide evidence that AMPK activation modulates ligand-dependent activation of peroxisome proliferator-activated receptor. Finally, we show that this synergistic induction of PDK4 decreases cellular glucose oxidation. In conclusion, AMPK and fatty acids play a direct role in fuel selection in response to cellular energy status in order to spare glucose.

Contribution of FAT/CD36 to the regulation of skeletal muscle fatty acid oxidation: an overview.

Long chain fatty acids (LCFAs) are an important substrate for ATP production within the skeletal muscle. The process of LCFA delivery from adipose tissue to muscle mitochondria involves many regulatory steps. Recently, it has been recognized that LCFA oxidation is not only dependent on LCFA delivery to the muscle, but also on regulatory steps within the muscle. Increasing selected fatty acid binding proteins/transporters on the plasma membrane facilitates a very rapid LCFA increase into the muscle, independent of any changes in LCFA delivery to the muscle. Such a mechanism of LCFA transporter translocation is activated by muscle contraction. Intramuscular triacylglycerols may also be hydrolysed to provide fatty acids for mitochondrial oxidation, particularly during exercise, when hormone-sensitive lipase and other enzymes are activated. Mitochondrial LCFA entry is also highly regulated. This however does not involve only the malonyl CoA carnitine palmitoyltransferase-I (CPTI) axis. Exercise-induced fatty acid entry into mitochondria is also regulated by at least one of the proteins (FAT/CD36) that also regulates plasma membrane fatty acid transport. Among individuals, differences in mitochondrial fatty acid oxidation appear to be correlated with the content of mitochondrial CPTI and FAT/CD36. This paper provides a brief overview of mechanisms that regulate LCFA uptake and oxidation in skeletal muscle during exercise and in obesity. We focus largely on our own work on FAT/CD36, which contributes to regulating, in a coordinated fashion, LCFA uptake across the plasma membrane and the mitochondrial membrane. Very little is known about the roles of FATP1-6 on fatty acid transport in skeletal muscle.

Cardiac contractile dysfunction in insulin-resistant rats fed a high-fat diet is associated with elevated CD36-mediated fatty acid uptake and esterification.

AIMS/HYPOTHESIS: Changes in cardiac substrate utilisation leading to altered energy metabolism may underlie the development of diabetic cardiomyopathy. We studied cardiomyocyte substrate uptake and utilisation and the role of the fatty acid translocase CD36 in relation to in vivo cardiac function in rats fed a high-fat diet (HFD). METHODS: Rats were exposed to an HFD or a low-fat diet (LFD). In vivo cardiac function was monitored by echocardiography. Substrate uptake and utilisation were determined in isolated cardiomyocytes. RESULTS: Feeding an HFD for 8 weeks induced left ventricular dilation in the systolic phase and decreased fractional shortening and the ejection fraction. Insulin-stimulated glucose uptake and proline-rich Akt substrate 40 phosphorylation were 41% (p < 0.001) and 45% (p < 0.05) lower, respectively, in cardiomyocytes from rats on the HFD. However, long-chain fatty acid (LCFA) uptake was 1.4-fold increased (p < 0.001) and LCFA esterification into triacylglycerols and phospholipids was increased 1.4- and 1.5-fold, respectively (both p < 0.05), in cardiomyocytes from HFD compared with LFD hearts. In the presence of the CD36 inhibitor sulfo-N-succinimidyloleate, LCFA uptake and esterification were similar in LFD and HFD cardiomyocytes. In HFD hearts CD36 was relocated to the sarcolemma, and basal phosphorylation of a mediator of CD36-trafficking, i.e. protein kinase B (PKB/Akt), was increased. CONCLUSIONS/INTERPRETATION: Feeding rats an HFD induced cardiac contractile dysfunction, which was accompanied by the relocation of CD36 to the sarcolemma, and elevated basal levels of phosphorylated PKB/Akt. The permanent presence of CD36 at the sarcolemma resulted in enhanced rates of LCFA uptake and myocardial triacylglycerol accumulation, and may contribute to the development of insulin resistance and diabetic cardiomyopathy.

Skeletal muscle fatty acid transporter protein expression in type 2 diabetes patients compared with overweight, sedentary men and age-matched, endurance-trained cyclists.

AIM: Membrane fatty acid transporters can modulate the balance between fatty acid uptake and subsequent storage and/or oxidation in muscle tissue. As such, skeletal muscle fatty acid transporter protein expression could play an important role in the etiology of insulin resistance and/or type 2 diabetes. METHODS: In the present study, fatty acid translocase (FAT/CD36), plasma membrane-bound fatty acid-binding protein (FABPpm) and fatty acid transport protein 1 (FATP1) mRNA and protein expression were assessed in muscle tissue obtained from 10 sedentary, overweight type 2 diabetes patients (60 +/- 2 years), 10 sedentary, weight-matched normoglycemic controls (60 +/- 2 years) and 10 age-matched, endurance trained cyclists (57 +/- 1 years). RESULTS: Both FAT/CD36 and FATP1 mRNA and protein expression did not differ between groups. In contrast, FABPpm mRNA and protein expression were approx. 30-40% higher in the trained men compared with the diabetes patients (P < 0.01) and sedentary controls (P < 0.05). CONCLUSIONS: Skeletal muscle FAT/CD36, FABPpm and FATP1 mRNA and protein expression are not up- or downregulated in a sedentary and/or insulin resistant state. In contrast, FABPpm expression is upregulated in the endurance trained state and likely instrumental to allow greater fatty acid oxidation rates.

The fatty acid transporter FAT/CD36 is upregulated in subcutaneous and visceral adipose tissues in human obesity and type 2 diabetes.

BACKGROUND: Long-chain fatty acids (LCFAs) cross the plasma membrane via a protein-mediated mechanism involving one or more LCFA-binding proteins. Among these, FAT/CD36 has been identified as key LCFA transporter in the heart and skeletal muscle, where it is regulated acutely and chronically by insulin. In skeletal muscle, FAT/CD36 expression and/or subcellular distribution is altered in obesity and type 2 diabetes. There is limited information as to whether the expression of this protein is also altered in subcutaneous and/or visceral adipose tissue depots in human obesity or type 2 diabetes. OBJECTIVES: To compare (a) the expression of FAT/CD36 in subcutaneous and visceral adipose tissue depots in lean, overweight, and obese individuals and in type 2 diabetics, (b) to determine whether the protein expression of FAT/CD36 in these depots is associated with the severity of insulin resistance (type 2 diabetes>obese>overweight/lean) and (c) whether FAT/CD36 protein expression in these adipose tissue depots is associated with alterations in circulating substrates and hormones. SUBJECTS: Subjects who were undergoing abdominal surgery and who were lean (n=10; three men, seven women), overweight (n=10; three men, seven women) or obese (n=7; one man, six women), or who had been diagnosed with type 2 diabetes (n=5; one man, four women) participated in this study. MEASUREMENTS: Subcutaneous and visceral adipose tissue samples, as well as blood samples, were obtained from the subjects while under general anesthesia. Adipose tissue samples were analyzed for FAT/CD36 using Western blotting. Serum samples were analyzed for glucose, insulin, FFA and leptin. BMI was also calculated. RESULTS: Subcutaneous adipose tissue FAT/CD36 expression was upregulated by +58, +76 and +150% in overweight, obese and type 2 diabetics, respectively. Relative to subcutaneous adipose tissue, visceral adipose tissue FAT/CD36 expression was upregulated in lean (+52%) and overweight subjects (+30%). In contrast, in obese subjects and type 2 diabetics, no difference in FAT/CD36 protein expression was observed between their subcutaneous and visceral adipose tissue depots (P>0.05). The subcutaneous adipose tissue FAT/CD36 expression (R=0.85) and the visceral adipose tissue FAT/CD36 expression (R=0.77) were associated with alteration in BMI and circulating glucose and insulin. CONCLUSIONS: Subcutaneous adipose tissue FAT/CD36 expression is upregulated in obesity and type 2 diabetes. As FAT/CD36 expression is not different in lean, overweight and obese subjects, and was only increased in type 2 diabetics, it appears that visceral adipose tissue FAT/CD36 may respond in a less dynamic manner to metabolic disturbances than subcutaneous adipose tissue FAT/CD36.

Divergent effects of rosiglitazone on protein-mediated fatty acid uptake in adipose and in muscle tissues of Zucker rats.

Thiazolidinediones (TZDs) increase tissue insulin sensitivity in diabetes. Here, we hypothesize that, in adipose tissue, skeletal muscle, and heart, alterations in protein-mediated FA uptake are involved in the effect of TZDs. As a model, we used obese Zucker rats, orally treated for 16 days with 5 mg rosiglitazone (Rgz)/kg body mass/day. In adipose tissue from Rgz-treated rats, FA uptake capacity increased by 2.0-fold, coinciding with increased total contents of fatty acid translocase (FAT/CD36; 2.3-fold) and fatty acid transport protein 1 (1.7-fold) but not of plasmalemmal fatty acid binding protein, whereas only the plasmalemmal content of FAT/CD36 was changed (increase of 1.7-fold). The increase in FA uptake capacity of adipose tissue was associated with a decline in plasma FA and triacylglycerols (TAGs), suggesting that Rgz treatment enhanced plasma FA extraction by adipocytes. In obese hearts, Rgz treatment had no effect on the FA transport system, yet the total TAG content decreased, suggesting enhanced insulin sensitivity. Also, in skeletal muscle, the FA transport system was not changed. However, the TAG content remained unaltered in skeletal muscle, which coincided with increased cytoplasmic adipose-type FABP content, suggesting that increased extramyocellular TAGs mask the decline of intracellular TAG in muscle. In conclusion, our study implicates FAT/CD36 in the mechanism by which Rgz increases tissue insulin sensitivity.

Increased FAT (fatty acid translocase)/CD36-mediated long-chain fatty acid uptake in cardiac myocytes from obese Zucker rats.

Disturbed cardiac lipid homoeostasis in obesity is regarded as a key player in the development of cardiovascular diseases. In this study, we show that FAT (fatty acid translocase)/CD36-mediated LCFA (long-chain fatty acid) uptake in cardiac myocytes from young adult obese Zucker rats is markedly increased, but insensitive to insulin. Basal and insulin-induced glucose uptake rates in these myocytes are not changed, suggesting that during the development from obesity to hyperglycaemic Type II diabetes, alterations in cardiac LCFA uptake precede alterations in cardiac glucose uptake.

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.

Cytoplasmic fatty acid-binding protein facilitates fatty acid utilization by skeletal muscle.

The intracellular transport of long-chain fatty acids in muscle cells is facilitated to a great extent by heart-type cytoplasmic fatty acid-binding protein (H-FABP). By virtue of the marked affinity of this 14.5-kDa protein for fatty acids, H-FABP dramatically increases their concentration in the aqueous cytoplasm by non-covalent binding, thereby facilitating both the transition of fatty acids from membranes to the aqueous space and their diffusional transport from membranes (e.g. sarcolemma) to other cellular compartments (e.g. mitochondria). Striking features are the relative abundance of H-FABP in muscle, especially in oxidative muscle fibres, and the modulation of the muscular H-FABP content in concert with the modulation of other proteins and enzymes involved in fatty acid handling and utilization. Newer studies with mice carrying a homozygous or heterozygous deletion of the H-FABP gene show that, in comparison with wild-type mice, hindlimb muscles from heterozygous animals have a markedly lowered (-66%) H-FABP content but unaltered palmitate uptake rate, while in hindlimb muscles from homozygous animals (no H-FABP present) palmitate uptake was reduced by 45%. These findings indicate that H-FABP is present in relative excess and plays a substantial, but merely permissive role in fatty acid uptake by skeletal muscles.

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.

Giant membrane vesicles as a model to study cellular substrate uptake dissected from metabolism.

In order to use giant vesicles for substrate uptake studies in metabolically important tissues, we characterized giant vesicles isolated from heart, liver, skeletal muscle and adipose tissue. We investigated which cell types and which plasma membrane regions are involved in giant vesicle formation and we examined the presence of transporters for metabolic substrates. Analysis of giant vesicles with markers specific for distinct cell types and distinct domains of the plasma membrane reveals that the plasma membrane of parenchymal cells, but not endothelial cells, are the source of the vesicle membranes. In addition, plasma membrane regions enriched in caveolae and involved in docking of recycling vesicles from the endosomal compartment are retained in giant vesicles, indicating that KCl-induced alterations in recycling processes are involved in giant vesicle formation. Giant vesicles contain vesicular lumen consisting of the soluble constituents of the cytoplasm including, fatty-acid binding proteins. Furthermore, giant vesicles isolated from heart, liver, skeletal muscle and adipose tissue are similar in size (10-15 microm) and shape and do not contain subcellular organelles, providing the advantage that substrate fluxes in the different organs can be studied independently of the surface/volume ratio but most importantly in the absence of intracellular metabolism.

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.

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.