Muscle palmitate uptake and binding are saturable and inhibited by antibodies to FABP(PM)

CD36 as a gatekeeper of myocardial lipid metabolism and therapeutic target for metabolic disease

The multifunctional membrane glycoprotein CD36 is expressed in different types of cells and plays a key regulatory role in cellular lipid metabolism, especially in cardiac muscle. CD36 facilitates the cellular uptake of long-chain fatty acids, mediates lipid signaling, and regulates storage and oxidation of lipids in various tissues with active lipid metabolism. CD36 deficiency leads to marked impairments in peripheral lipid metabolism, which consequently impact on the cellular utilization of multiple different fuels because of the integrated nature of metabolism. The functional presence of CD36 at the plasma membrane is regulated by its reversible subcellular recycling from and to endosomes and is under the control of mechanical, hormonal, and nutritional factors. Aberrations in this dynamic role of CD36 are causally associated with various metabolic diseases, in particular insulin resistance, diabetic cardiomyopathy, and cardiac hypertrophy. Recent research in cardiac muscle has disclosed the endosomal proton pump vacuolar-type H+-ATPase (v-ATPase) as a key enzyme regulating subcellular CD36 recycling and being the site of interaction between various substrates to determine cellular substrate preference. In addition, evidence is accumulating that interventions targeting CD36 directly or modulating its subcellular recycling are effective for the treatment of metabolic diseases. In conclusion, subcellular CD36 localization is the major adaptive regulator of cellular uptake and metabolism of long-chain fatty acids and appears a suitable target for metabolic modulation therapy to mend failing hearts.

Relevance of mitochondrial dysfunction in heart disease associated with insulin resistance conditions

Insulin resistance plays a key role in the development and progression of obesity, diabetes, and their complications. Moreover, insulin resistance is considered the principal link between metabolic diseases and cardiovascular diseases. Heart disease associated with insulin resistance is one of the most important consequences of both obesity and diabetes, and it is characterized by impaired cardiac energetics, diastolic dysfunction, and finally heart failure. Mitochondrion plays a key role in cell energy homeostasis and is the main source of reactive oxygen species. Obesity and diabetes are associated with alterations in mitochondrial function and dynamics. Mitochondrial dysfunction is characterized by changes in mitochondrial respiratory chain with reduced ATP production and elevated reactive oxygen species production. These mitochondrial alterations together with inflammation contribute to the development and progression of heart disease under insulin resistance conditions. Finally, numerous miRNAs participate in the regulation of energy substrate metabolism, reactive oxygen species production, and apoptotic pathways within the mitochondria. This notion supports the relevance of interactions between miRNAs and mitochondrial dysfunction in the pathophysiology of metabolic heart disease.

Regulation of direct adipose tissue free fatty acid storage during mixed meal ingestion and high free fatty acid concentration conditions

We found that direct free fatty acid (FFA) storage (fatty acid cycling back into adipose tissue) in leg vs. abdominal subcutaneous fat is related to regional differences in adipose tissue diacylglycerol acyltransferase (DGAT) activity under high-FFA conditions and to differences in adipose tissue acyl-CoA synthetase (ACS)activity under meal ingestion conditions. We also found that direct FFA storage rates in leg fat were significantly less in physically active than sedentary adults. Direct FFA storage into adipocytes relates to body fat distribution. Adipose tissue CD36, ACS, and DGAT may account for some of the between-depot and interindividual variability in FFA storage. These studies were to test whether CD36, ACS, or DGAT might be important for direct palmitate storage under meal ingestion or high-FFA conditions. We measured upper (UBSQ) and lower body subcutaneous (LBSQ) adipose tissue FFA storage rates by infusing palmitate tracers intravenously and performing adipose biopsies under hypoinsulinemic (high-FFA) and mixed-meal conditions. We recruited five postmenopausal women, physically active males (5) and females (5), and sedentary males (5) and females (5). We found that 1) the ratio of UBSQ to LBSQ DGAT activity predicted the ratio of palmitate storage [adjusted R = 0.25, F = 8.0, P = 0.01, 95% CI (0.07, 0.48)] under high-FFA conditions; 2) the ratio of UBSQ to LBSQ ACS activity predicted the ratio of palmitate storage under meal conditions [adjusted R = 0.18, F = 6.3, P = 0.02, 95% CI (0.12, 1.28)]; 3) LBSQ direct palmitate storage rates were significantly less in physically active than sedentary and 4) adipose tissue CD36 protein content, ACS, or DGAT activities did not independently predict palmitate storage rates. We conclude that physically active adults have lesser fatty acid cycling back into adipose tissue and that adipose ACS and DGAT may affect competition between UBSQ and LBSQ adipose for direct palmitate storage.

Acute exercise in mice transiently remodels the hepatic lipidome in an intensity-dependent manner

Background

The content of triacylglycerol (TAG) in the liver is known to rapidly increase after a single bout of exercise followed by recovery to sedentary levels. The response of other hepatic lipids, and acyl chain composition of lipid classes, would provide a deeper understanding of the response of hepatic lipid metabolism to acute exercise.

Methods

Female mice performed a single bout of continuous exercise (CE), high-intensity interval exercise (HIIE), or no exercise (CON). The total content of various lipids in the liver, and fatty acids within lipid classes, were measured in tissues collected 3 h after exercise (Day 1) and the day following exercise (Day 2).

Results

The total concentration of TAG rose on Day 1 after exercise (P < 0.05), with a greater elevation in HIIE than CE (P < 0.05), followed by a decline toward CON levels on Day 2. The total concentration of other measured lipid classes was not significantly altered by exercise. However, n-6 polyunsaturated fatty acid relative abundance in diacylglycerol (DAG) was increased by HIIE (P < 0.05). In CON liver, TAG content was positively correlated with DAG and phosphatidylethanolamine (P < 0.05), while these statistical associations were disrupted in exercised mice on Day 1.

Conclusions

The response of lipid metabolism to exercise involves the coordination of metabolism between various tissues, and the lipid metabolism response to acute exercise places a metabolic burden upon the liver. The present findings describe how the liver copes with this metabolic challenge. The flexibility of the TAG pool size in the liver, and other remodeling of the hepatic lipidome, may be fundamental components of the physiological response to intense exercise.

Myocardium Metabolism in Physiological and Pathophysiological States: Implications of Epicardial Adipose Tissue and Potential Therapeutic Targets

The main energy substrate of adult cardiomyocytes for their contractility are the fatty acids. Its metabolism generates high ATP levels at the expense of high oxygen consumption in the mitochondria. Under low oxygen supply, they can get energy from other substrates, mainly glucose, lactate, ketone bodies, etc., but the mitochondrial dysfunction, in pathological conditions, reduces the oxidative metabolism. In consequence, fatty acids are stored into epicardial fat and its accumulation provokes inflammation, insulin resistance, and oxidative stress, which enhance the myocardium dysfunction. Some therapies focused on improvement the fatty acids entry into mitochondria have failed to demonstrate benefits on cardiovascular disorders. Oppositely, those therapies with effects on epicardial fat volume and inflammation might improve the oxidative metabolism of myocardium and might reduce the cardiovascular disease progression. This review aims at explain (a) the energy substrate adaptation of myocardium in physiological conditions, (b) the reduction of oxidative metabolism in pathological conditions and consequences on epicardial fat accumulation and insulin resistance, and (c) the reduction of cardiovascular outcomes after regulation by some therapies.

Alterations in oral [1-(14)C] 18:1n-9 distribution in lean wild-type and genetically obese (ob/ob) mice

Obesity may result from altered fatty acid (FA) disposal. Altered FA distribution in obese individuals is poorly understood. Lean wild-type C57BL/6J and obese C57BL/6Job/ob mice received an oral dose of [1-(14)C]18:1n-9 (oleic acid), and the radioactivity in tissues was evaluated at various time points. The (14)C concentration decreased rapidly in gastrointestinal tract but gradually increased and peaked at 96 h in adipose tissue, muscle and skin in lean mice. The (14)C concentration was constant in adipose tissue and muscle of obese mice from 4 h to 168 h. (14)C-label content in adipose tissue was significantly affected by genotype, whereas muscle (14)C-label content was affected by genotype, time and the interaction between genotype and time. There was higher total (14)C retention (47.7%) in obese mice than in lean mice (9.0%) at 168 h (P<0.05). The (14)C concentrations in the soleus and gastrocnemius muscle were higher in obese mice than in lean mice (P<0.05). Perirenal adipose tissue contained the highest (14)C content in lean mice, whereas subcutaneous adipose tissue (SAT) had the highest (14)C content and accounted for the largest proportion of total radioactivity among fat depots in obese mice. More lipid radioactivity was recovered as TAG in SAT from obese mice than from lean mice (P<0.05). Gene expression suggested acyl CoA binding protein and fatty acid binding protein are important for FA distribution in adipose tissue and muscle. The FA distribution in major tissues was altered in ob/ob mice, perhaps contributing to obesity. Understanding the disparity in FA disposal between lean and obese mice may reveal novel targets for the treatment and prevention of obesity.

Pathways of polyunsaturated fatty acid utilization: implications for brain function in neuropsychiatric health and disease

Essential polyunsaturated fatty acids (PUFAs) have profound effects on brain development and function. Abnormalities of PUFA status have been implicated in neuropsychiatric diseases such as major depression, bipolar disorder, schizophrenia, Alzheimer's disease, and attention deficit hyperactivity disorder. Pathophysiologic mechanisms could involve not only suboptimal PUFA intake, but also metabolic and genetic abnormalities, defective hepatic metabolism, and problems with diffusion and transport. This article provides an overview of physiologic factors regulating PUFA utilization, highlighting their relevance to neuropsychiatric disease.

Contraction-induced signaling: evidence of convergent cascades in the regulation of muscle fatty acid metabolism

The regulation of fatty acid utilization during muscle contraction and exercise remains to be fully elucidated. Evidence suggests that the metabolic responses of skeletal muscle induced by the contraction-induced changes in energy demand are mediated by the activation of a multitude of intracellular signaling cascades. This review addresses the roles played by 3 intracellular signaling cascades of interest in the regulation of fatty acid uptake and oxidation in contracting skeletal muscle; namely, the AMP-activated protein kinase (AMPK), calcium/calmodulin-dependent protein kinases (CaMKs), and the extracellular signal-regulated kinase 1 and 2 (ERK1/2) signaling cascades. Data delineating the potential role of AMPK in cross-talk with CaMKII, CaMK kinase (CaMKK), and ERK1/2 are presented. Collectively, data show that in perfused rodent muscle, regulation of fatty acid uptake and oxidation occurs via (i) CaMKII signaling via both AMPK-dependent and -independent cascades, (ii) CaMKK signaling via both AMPK-dependent and -independent cascades, (iii) AMPK signaling in a time- and intensity-dependent manner, and (iv) ERK1/2 signaling in an intensity-dependent manner.

In vivo and in vitro insulin and fasting control of the transmembrane fatty acid transport proteins in Atlantic salmon (Salmo salar)

We have examined the nutritional and insulin regulation of the mRNA expression of transmembrane fatty acid (FA) transporters [FA transport protein-1 (FATP1) and CD36] together with the lipoprotein lipase (LPL), the cytosolic FA carrier FA binding protein (FABP3), and mitochondrial FA-CoA and -carnitine palmitoyl transferase carriers (CPT)1 and -2 in Atlantic salmon tissues and myocyte cell culture. Two weeks of fasting diminished FATP1, CD36, and LPL in adipose tissue, suggesting a reduction in FA uptake, while FABP3 increased in liver, probably enhancing the transport of FA to the mitochondria. Insulin injection decreased FATP1 and CD36 in white and red muscles, while both transporters were upregulated in the adipose tissue in agreement with the role of insulin-inhibiting muscle FA oxidation and stimulating adipose fat stores. Serum deprivation of 48 h in Atlantic salmon myotubes increased FATP1, FABP3, and CPT-2, while CPT-1 was diminished. In myotubes, insulin induced FATP1 expression but decreased CD36, FABP3, and LPL, suggesting that FATP1 could be more involved in the insulin-stimulated FA uptake. Insulin increased the FA uptake in myotubes mediated, at least in part, through the relocation of FATP1 protein to the plasma membrane. Overall, Atlantic salmon FA transporters are regulated by fasting and insulin on in vivo and in vitro models.

Free fatty acid storage in human visceral and subcutaneous adipose tissue: role of adipocyte proteins

OBJECTIVE

Because direct adipose tissue free fatty acid (FFA) storage may contribute to body fat distribution, we measured FFA (palmitate) storage rates and fatty acid (FA) storage enzymes/proteins in omental and abdominal subcutaneous fat.

RESEARCH DESIGN AND METHODS

Elective surgery patients received a bolus of [1-(14)C]palmitate followed by omental and abdominal subcutaneous fat biopsies to measure direct FFA storage. Long chain acyl-CoA synthetase (ACS) and diacylglycerol acyltransferase activities, CD36, fatty acid-binding protein, and fatty acid transport protein 1 were measured.

RESULTS

Palmitate tracer storage (dpm/g adipose lipid) and calculated palmitate storage rates were greater in omental than abdominal subcutaneous fat in women (1.2 ± 0.8 vs. 0.7 ± 0.4 μmol · kg adipose lipid(-1) · min(-1), P = 0.005) and men (0.7 ± 0.2 vs. 0.2 ± 0.1, P < 0.001), and both were greater in women than men (P < 0.0001). Abdominal subcutaneous adipose tissue palmitate storage rates correlated with ACS activity (women: r = 0.66, P = 0.001; men: r = 0.70, P = 0.007); in men, CD36 was also independently related to palmitate storage rates. The content/activity of FA storage enzymes/proteins in omental fat was dramatically lower in those with more visceral fat. In women, only omental palmitate storage rates were correlated (r = 0.54, P = 0.03) with ACS activity.

CONCLUSIONS

Some adipocyte FA storage factors correlate with direct FFA storage, but sex differences in this process in visceral fat do not account for sex differences in visceral fatness. The reduced storage proteins in those with greater visceral fat suggest that the storage factors we measured are not a predominant cause of visceral adipose tissue accumulation.

Age-related expression profile of the SLC27A1 gene in chicken tissues

The solute carrier family 27 (SLC27, also known as fatty acid transport proteins [FATPs]) plays important biological roles in cells. However, there is no report about the expression profile of SLC27 member in chicken. In this study, we quantified the expression of SLC27A1 (FATP1) mRNA in a mountainous black-boned chicken breed (MB) and a commercial meat type chicken breed (S01), to discern the tissue and age-related specific expression pattern and their potential involvement in fat deposition and muscle fatty acid metabolism. Real-time quantitative PCR assays were developed for accurate measurement of SLC27A1 mRNA levels in different tissues from chicken with different ages (0-12 weeks). Expression of SLC27A1 mRNA was detected in all tissues examined. There was a significantly age-related change of the SLC27A1 mRNAs in heart, breast muscle (BMW), leg muscle (LMW), liver, and abdominal fat (AF) tissues (P < 0.05). The breast muscle and leg muscle tissues had the highest expression of SLC27A1 mRNA than the other tissues from the same individual at 0, 2 and 4 weeks. The overall SLC27A1 mRNA level exhibited a "rise-decline" developmental change in all tissues except for breast muscle, subcutaneous fat, and brain. The S01 chicken had a higher expression of the SLC27A1 mRNA in breast muscle, subcutaneous fat, and heart tissues than the MB chicken. Our results showed that the expression of SLC27A1 mRNA in chicken tissues exhibits specific developmental changes and age-related patterns.

Fatty acid (FFA) transport in cardiomyocytes revealed by imaging unbound FFA is mediated by an FFA pump modulated by the CD36 protein

Free fatty acid (FFA) transport across the cardiomyocyte plasma membrane is essential to proper cardiac function, but the role of membrane proteins and FFA metabolism in FFA transport remains unclear. Metabolism is thought to maintain intracellular FFA at low levels, providing the driving force for FFA transport, but intracellular FFA levels have not been measured directly. We report the first measurements of the intracellular unbound FFA concentrations (FFA(i)) in cardiomyocytes. The fluorescent indicator of FFA, ADIFAB (acrylodan-labeled rat intestinal fatty acid-binding protein), was microinjected into isolated cardiomyocytes from wild type (WT) and FAT/CD36 null C57B1/6 mice. Quantitative imaging of ADIFAB fluorescence revealed the time courses of FFA influx and efflux. For WT mice, rate constants for efflux (∼0.02 s(-1)) were twice influx, and steady state FFA(i) were more than 3-fold larger than extracellular unbound FFA (FFA(o)). The concentration gradient and the initial rate of FFA influx saturated with increasing FFA(o). Similar characteristics were observed for oleate, palmitate, and arachidonate. FAT/CD36 null cells revealed similar characteristics, except that efflux was 2-3-fold slower than WT cells. Rate constants determined with intracellular ADIFAB were confirmed by measurements of intracellular pH. FFA uptake by suspensions of cardiomyocytes determined by monitoring FFA(o) using extracellular ADIFAB confirmed the influx rate constants determined from FFA(i) measurements and demonstrated that rates of FFA transport and etomoxir-sensitive metabolism are regulated independently. We conclude that FFA influx in cardiac myocytes is mediated by a membrane pump whose transport rate constants may be modulated by FAT/CD36.

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

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

CaMKK is an upstream signal of AMP-activated protein kinase in regulation of substrate metabolism in contracting skeletal muscle

Multiple signals have been shown to be involved in regulation of fatty acid (FA) and glucose metabolism in contracting skeletal muscle. This study aimed to determine whether a Ca(2+)-stimulated kinase, CaMKK, is involved in regulation of contraction-induced substrate metabolism and whether it does so in an AMP-activated protein kinase (AMPK)-dependent manner. Rat hindlimbs were perfused at rest (n = 16), with 3 mM caffeine (n = 15), with 2 mM 5-aminoimidazole-4-carboxamide 1-beta-d-ribofuranoside (AICAR; n = 16), or during moderate-intensity muscle contraction (MC; n = 14) and with or without 5 microM STO-609, a CaMKK inhibitor. FA uptake and oxidation increased (P < 0.05) 64% and 71% by caffeine, 42% and 93% by AICAR, and 65% and 143% by MC. STO-609 abolished (P < 0.05) caffeine- and MC-induced FA uptake and oxidation but had no effect with AICAR treatment. Glucose uptake increased (P < 0.05) 104% by caffeine, 85% by AICAR, and 130% by MC, and STO-609 prevented the increase in glucose uptake in caffeine and muscle contraction groups. CaMKKbeta activity increased (P < 0.05) 113% by caffeine treatment and 145% by MC but was not affected by AICAR treatment. STO-609 prevented the caffeine- and MC-induced increase in CaMKKbeta activity. Caffeine, AICAR, and MC increased (P < 0.05) AMPKalpha2 activity by 295%, 11-fold, and 7-fold but did not affect AMPKalpha1 activity. STO-609 decreased (P < 0.05) AMPKalpha2 activity induced by caffeine treatment and MC by 60% and 61% but did not affect AICAR-induced activity. Plasma membrane transport protein content of CD36 and glucose transporter 4 (GLUT4) increased (P < 0.05) with caffeine, AICAR, and MC, and STO-609 prevented caffeine- and MC-induced increases in protein content. These results show the importance of Ca(2+)-dependent signaling via CaMKK activation in the regulation of substrate uptake and FA oxidation in contracting rat skeletal muscle and agree with the notion that CaMKK is an upstream kinase of AMPK in the regulation of substrate metabolism in skeletal muscle.

High-intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle

High-intensity aerobic interval training (HIIT) is a compromise between time-consuming moderate-intensity training and sprint-interval training requiring all-out efforts. However, there are few data regarding the ability of HIIT to increase the capacities of fat and carbohydrate oxidation in skeletal muscle. Using untrained recreationally active individuals, we investigated skeletal muscle and whole-body metabolic adaptations that occurred following 6 weeks of HIIT (~1 h of 10 x 4 min intervals at ~90% of peak oxygen consumption (VO2 peak), separated by 2 min rest, 3 d.week-1). A VO2 peak test, a test to exhaustion (TE) at 90% of pre-training VO2 peak, and a 1 h cycle at 60% of pre-training VO2 peak were performed pre- and post-HIIT. Muscle biopsies were sampled during the TE at rest, after 5 min, and at exhaustion. Training power output increased by 21%, and VO2 peak increased by 9% following HIIT. Muscle adaptations at rest included the following: (i) increased cytochrome c oxidase IV content (18%) and maximal activities of the mitochondrial enzymes citrate synthase (26%), beta-hydroxyacyl-CoA dehydrogenase (29%), aspartate-amino transferase (26%), and pyruvate dehydrogenase (PDH; 21%); (ii) increased FAT/CD36, FABPpm, GLUT 4, and MCT 1 and 4 transport proteins (14%-30%); and (iii) increased glycogen content (59%). Major adaptations during exercise included the following: (i) reduced glycogenolysis, lactate accumulation, and substrate phosphorylation (0-5 min of TE); (ii) unchanged PDH activation (carbohydrate oxidation; 0-5 min of TE); (iii) ~2-fold greater time during the TE; and (iv) increased fat oxidation at 60% of pre-training VO2 peak. This study demonstrated that 18 h of repeated high-intensity exercise sessions over 6 weeks (3 d.week-1) is a powerful method to increase whole-body and skeletal muscle capacities to oxidize fat and carbohydrate in previously untrained individuals.

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.

Protein-mediated fatty acid uptake: regulation by contraction, AMP-activated protein kinase, and endocrine signals

Fatty acid transport into heart and skeletal muscle occurs largely through a highly regulated protein-mediated mechanism involving a number of fatty acid transporters. Chronically altered muscle activity (chronic muscle stimulation, denervation) alters fatty acid transport by altering the expression of fatty acid transporters and (or) their subcellular location. Chronic exposure to leptin downregulates while insulin upregulates fatty acid transport by altering concomitantly the expression of fatty acid transporters. Fatty acid transport can also be regulated within minutes, by muscle contraction, AMP-activated protein kinase activation, leptin, and insulin, through induction of the translocation of fatty acid translocase (FAT)/CD36 from its intracellular depot to the plasma membrane. In insulin-resistant muscle, a permanent relocation of FAT/CD36 to the sarcolemma appears to account for the excess accretion of intracellular lipids that interfere with insulin signaling. Recent work has also shown that FAT/ CD36, but not plasma membrane associated fatty acid binding protein, is involved, along with carnitine palmitoyltransferase, in regulating mitochondrial fatty acid oxidation. Finally, studies in FAT/CD36 null mice indicate that this transporter has a key role in regulating fatty acid metabolism in muscle.

Evidence for concerted action of FAT/CD36 and FABPpm to increase fatty acid transport across the plasma membrane

There is substantial molecular, biochemical and physiologic evidence that long-chain fatty acid transport involves a protein-mediated process. A number of fatty acid transport proteins have been identified, and for unknown reasons, some of these are coexpressed in the same tissues. In muscle and heart FAT/CD36 and FABPpm appear to be key transporters. Both proteins are regulated acutely (within minutes) and chronically (hours to days) by selected physiologic stimuli (insulin, AMP kinase activation). Acute regulation involves the translocation of FAT/CD36 by insulin, muscle contraction and AMP kinase activation, while FABPpm is induced to translocate by muscle contraction and AMP kinase activation, but not by insulin. Protein expression ofFAT/CD36 and FABPpm is regulated by prolonged AMP kinase activation (heart) or increased muscle contraction. Prolonged insulin exposure increases the expression of FAT/CD36 but not FABPpm. Trafficking of fatty acid transporters between an intracellular compartment(s) and the plasma membrane is altered in insulin-resistant skeletal muscle, as some FAT/CD36 is permanently relocated to plasma membrane, thereby contributing to insulin resistance due to the increased influx of fatty acids into muscle cells. Studies in FAT/CD36 null mice have revealed that this transporter is key to regulating the increase in the rate of fatty acid metabolism in heart and skeletal muscle. It appears based on a number of experiments that FAT/CD36 and FABPpm may collaborate to increase the rates of fatty acid transport, as these proteins co-immunoprecipitate.

11beta-HSD1 inhibition improves triglyceridemia through reduced liver VLDL secretion and partitions lipids toward oxidative tissues

Tissue-specific alterations in 11beta-hydroxysteroid dehydrogenase (HSD) type 1 activity, which amplifies glucocorticoid action, are thought to contribute to some of the metabolic complications of obesity. The present study tested whether hypertriglyceridemia is one such complication by investigating the effects of an 11beta-HSD1 inhibitor (compound A, 3 mgxkg(-1)xday(-1), 21 days) on triglyceride (TG) metabolism in a rat model of diet-induced obesity. The dose of compound A used did not affect food intake or final body weight. Compound A improved fasting triglyceridemia (-42%) through a robust reduction (-41%) in hepatic TG secretion rate, without change in plasma TG clearance rate. Uptake of TG-derived fatty acids was, however, increased in oxidative tissues, including red gastrocnemius (+47%), heart (+39%), and brown adipose tissue (BAT, +46%) at the expense of the liver, with a concomitant increase in plasma membrane fatty acid-binding protein. Lipid oxidation products were increased in red gastrocnemius (+35%) and heart (+33%), as were levels of uncoupling protein 1 mRNA in BAT (+48%), and carnitine palmitoyltransferase 1 activity tended to be increased in some oxidative tissues. These findings demonstrate that pharmacological inhibition of 11beta-HSD1 at a dose that does not affect food intake improves triglyceridemia by reducing hepatic very low density lipoprotein-TG secretion, with a shift in the pattern of TG-derived fatty acid uptake toward oxidative tissues, in which lipid accumulation is prevented by increased lipid oxidation.

Evidence for the regulation of contraction-induced fatty acid oxidation via extracellular signal-regulated kinase 1/2 activation independent of changes in fatty acid uptake

Data show that extracellular signal-regulated kinase 1/2 (ERK1/2) may be involved in the regulation of fatty acid (FA) uptake during muscle contraction via stimulation of CD36 translocation to the plasma membrane. The perfused hind limb model was used to determine (1) the importance of ERK1/2 signaling on contraction-induced FA uptake and (2) the effect of ERK1/2-mediated FA uptake on contraction-induced FA oxidation. We perfused rat hind limbs with 8 mmol/L glucose, 550 micromol/L palmitate, and no insulin at rest in the absence of inhibitor and during moderate-intensity electrical stimulation and dose-dependent pharmacologic inhibition of ERK1/2 using increasing concentrations of PD98059 (P1 = none, P2 = 10 micromol/L, P3 = 20 micromol/L, P4 = 50 micromol/L). Increasing PD98059 concentration resulted in a gradual decrease in contraction-induced ERK1/2 phosphorylation, and this was accompanied by a decrease in contraction-induced FA uptake (concentration required for 50% inhibition [IC(50)] = 15.8 +/- 1.6 mumol/L) and in plasma membrane CD36 content (IC(50) = 8.7 +/- 0.3 micromol/L) (P < .05). Percent FA oxidation was significantly lower in P3 and P4 compared with P1 and P2. Based on IC(50) values, FA oxidation demonstrated a greater sensitivity than FA uptake to changes in ERK1/2 phosphorylation (IC(50) = 5.4 +/- 0.3 micromol/L) (P < .05). A positive correlation was found between FA uptake and plasma membrane CD36 content (R(2) = 0.85, P < .05). Plasma membrane CD36 content, FA uptake, and FA oxidation each shared a positive correlation with ERK1/2 phosphorylation (R(2) = 0.64, 0.66, and 0.71, respectively; P < .05). These results suggest that during moderate-intensity muscle contraction, ERK1/2 phosphorylation is required for translocation of CD36 to the plasma membrane and the subsequent increase in FA uptake. In addition, these data suggest that ERK1/2 signaling may be involved in the regulation of FA oxidation independently of its effects on FA uptake.

Fatty acid binding protein facilitates sarcolemmal fatty acid transport but not mitochondrial oxidation in rat and human skeletal muscle

The transport of long-chain fatty acids (LCFAs) across mitochondrial membranes is regulated by carnitine palmitoyltransferase I (CPTI) activity. However, it appears that additional fatty acid transport proteins, such as fatty acid translocase (FAT)/CD36, influence not only LCFA transport across the plasma membrane, but also LCFA transport into mitochondria. Plasma membrane-associated fatty acid binding protein (FABPpm) is also known to be involved in sacrolemmal LCFA transport, and it is also present on the mitochondria. At this location, it has been identified as mitochondrial aspartate amino transferase (mAspAT), despite being structurally identical to FABPpm. Whether this protein is also involved in mitochondrial LCFA transport and oxidation remains unknown. Therefore, we have examined the ability of FABPpm/mAspAT to alter mitochondrial fatty acid oxidation. Muscle contraction increased (P < 0.05) the mitochondrial FAT/CD36 content in rat (+22%) and human skeletal muscle (+33%). By contrast, muscle contraction did not alter the content of mitochondrial FABPpm/mAspAT protein in either rat or human muscles. Electrotransfecting rat soleus muscles, in vivo, with FABPpm cDNA increased FABPpm protein in whole muscle (+150%; P < 0.05), at the plasma membrane (+117%; P < 0.05) and in mitochondria (+80%; P < 0.05). In these FABPpm-transfected muscles, palmitate transport into giant vesicles was increased by +73% (P < 0.05), and fatty acid oxidation in intact muscle was increased by +18% (P < 0.05). By contrast, despite the marked increase in mitochondrial FABPpm/mAspAT protein content (+80%), the rate of mitochondrial palmitate oxidation was not altered (P > 0.05). However, electrotransfection increased mAspAT activity by +70% (P < 0.05), and the mitochondrial FABPpm/mAspAT protein content was significantly correlated with mAspAT activity (r = 0.75). It is concluded that FABPpm has two distinct functions depending on its subcellular location: (a) it contributes to increasing sarcolemmal LCFA transport while not contributing directly to LCFA transport into mitochondria; and (b) its primary role at the mitochondria level is to transport reducing equivalents into the matrix.

Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women

Our aim was to examine the effects of seven high-intensity aerobic interval training (HIIT) sessions over 2 wk on skeletal muscle fuel content, mitochondrial enzyme activities, fatty acid transport proteins, peak O(2) consumption (Vo(2 peak)), and whole body metabolic, hormonal, and cardiovascular responses to exercise. Eight women (22.1 +/- 0.2 yr old, 65.0 +/- 2.2 kg body wt, 2.36 +/- 0.24 l/min Vo(2 peak)) performed a Vo(2 peak) test and a 60-min cycling trial at approximately 60% Vo(2 peak) before and after training. Each session consisted of ten 4-min bouts at approximately 90% Vo(2 peak) with 2 min of rest between intervals. Training increased Vo(2 peak) by 13%. After HIIT, plasma epinephrine and heart rate were lower during the final 30 min of the 60-min cycling trial at approximately 60% pretraining Vo(2 peak). Exercise whole body fat oxidation increased by 36% (from 15.0 +/- 2.4 to 20.4 +/- 2.5 g) after HIIT. Resting muscle glycogen and triacylglycerol contents were unaffected by HIIT, but net glycogen use was reduced during the posttraining 60-min cycling trial. HIIT significantly increased muscle mitochondrial beta-hydroxyacyl-CoA dehydrogenase (15.44 +/- 1.57 and 20.35 +/- 1.40 mmol.min(-1).kg wet mass(-1) before and after training, respectively) and citrate synthase (24.45 +/- 1.89 and 29.31 +/- 1.64 mmol.min(-1).kg wet mass(-1) before and after training, respectively) maximal activities by 32% and 20%, while cytoplasmic hormone-sensitive lipase protein content was not significantly increased. Total muscle plasma membrane fatty acid-binding protein content increased significantly (25%), whereas fatty acid translocase/CD36 content was unaffected after HIIT. In summary, seven sessions of HIIT over 2 wk induced marked increases in whole body and skeletal muscle capacity for fatty acid oxidation during exercise in moderately active women.

Fatty acid transport and FAT/CD36 are increased in red but not in white skeletal muscle of ZDF rats

An increased rate of fatty acid transport into skeletal muscle has been has been linked to the accumulation of intramuscular lipids and insulin resistance, and red muscles are more susceptible than white muscles in developing fatty acid-mediated insulin resistance. Therefore, we examined in Zucker diabetic fatty (ZDF) rats, relative to lean rats, 1) whether rates of fatty acid transport and transporters (FAT/CD36 and FABPpm) were upregulated in skeletal muscle during the transition from insulin resistance (week 6) to type 2 diabetes (weeks 12 and 24), 2) whether such changes occurred primarily in red skeletal muscle, and 3) whether changes in FAT/CD36 and GLUT4 were correlated. In red muscles of ZDF compared with lean rats, the rates of fatty acid transport were upregulated (+66%) early in life (week 6). Compared with the increase in fatty acid transport in lean red muscle from weeks 12-24 (+57%), the increase in fatty acid transport rate in ZDF red muscle was 50% greater during this same period. In contrast, no differences in fatty acid transport rates were observed in the white muscles of lean and ZDF rats at any time (weeks 6-24). In red muscle only, there was an inverse relationship between FAT/CD36 and GLUT4 protein expression as well as their plasmalemmal content. These studies have shown that, 1) before the onset of diabetes, as well as during diabetes, fatty acid transport and FAT/CD36 expression and plasmalemmal content are upregulated in ZDF rats, but importantly, 2) these changes occurred only in red, not white, muscles of ZDF rats.

Differential effects of contraction and PPAR agonists on the expression of fatty acid transporters in rat skeletal muscle

We have examined over the course of a 1-week period the independent and combined effects of chronically increased muscle contraction and the peroxisome proliferator-activated receptor (PPAR)alpha and PPARgamma activators, Wy 14,643 and rosiglitazone, on the expression and plasmalemmal content of the fatty acid transporters, FAT/CD36 and FABPpm, as well as on the rate of fatty acid transport. In resting muscle, the activation of either PPARalpha or PPARgamma failed to induce the protein expression of FAT/CD36. PPARalpha activation also failed to induce the protein expression of FABPpm. In contrast, PPARgamma activation induced the expression of FABPpm protein (40%; P < 0.05). Chronic muscle contraction increased the protein expression of FAT/CD36 (approximately 50%; P < 0.05), whereas FABPpm was slightly increased (12%; P < 0.05). Neither PPARalpha nor PPARgamma activation altered the contraction-induced expression of FAT/CD36 or FABPpm. Changes in protein expression of FAT/CD36 or FABPpm, induced by either contractions or by administration of rosiglitazone, were largely attributable to increased transcription. The contraction-induced increments in FAT/CD36 were accompanied by parallel increments in plasmalemmal FAT/CD36 and in rates of fatty acid transport (P < 0.05). Up-regulation of FABPpm expression was, however, accompanied by a reduction in plasmalemmal FABPpm, which did not affect the rates of long chain fatty acid (LCFA) transport. These studies have shown that in skeletal muscle (i) neither PPARalpha nor PPARgamma activation alters FAT/CD36 expression, (ii) PPARgamma activation selectively up-regulates FABPpm expression and (iii) contraction-induced up-regulation of LCFA transport does not appear to occur via activation of either PPARalpha or PPARgamma.

NO-1886 (ibrolipim), a lipoprotein lipase-promoting agent, accelerates the expression of UCP3 messenger RNA and ameliorates obesity in ovariectomized rats

The synthetic compound NO-1886 (ibrolipim, [4-(4-bromo-2-cyano-phenylcarbamoyl)-benzyl]-phosphonic acid diethyl ester, CAS 133208-93-2) is a lipoprotein lipase (LPL)-promoting agent that decreases plasma triglycerides, increases high-density lipoprotein cholesterol levels, and prevents fat accumulation in high fat-fed rats. However, the effect of NO-1886 on body weight, fat accumulation, and energy expenditure in ovariectomized (OVX) rats is not clear. The primary aim of this study was to ascertain whether NO-1886 ameliorated obesity in OVX rats and to examine the effects on fatty acid oxidation-related enzymes. NO-1886 decreased accumulation of visceral fat and suppressed the increase in body weight resulting from the ovariectomy. NO-1886 decreased the respiratory quotient and increased expression of the fatty acid translocase messenger RNA (mRNA) in the liver, soleus muscle, and mesenteric fat. NO-1886 also increased the expression of fatty acid-binding protein mRNA in the liver and soleus muscle and the expression of the uncoupling protein 3 (UCP3) mRNA in the heart, soleus muscle, and mesenteric fat, but not in the brown adipose tissue. Furthermore, NO-1886 did not affect UCP1 and UCP2 in brown adipose tissue. Therefore, amelioration of obesity by NO-1886 in OVX rats is possibly because of an the increased expression of fatty acid oxidation-related enzymes and UCP3, both of which are related to fatty acid transfer and fat use. Our study indicates that the LPL-promoting agent NO-1886 may be potentially beneficial in the treatment of obesity and obesity-linked health problems in postmenopausal women.

The Identification of Mouse Sperm-Surface-Associated Proteins and Characterization of Their Ability to Act as Decapacitation Factors1

Mammalian spermatozoa must undergo capacitation before acquiring the ability to fertilize the oocyte. This process is believed to be initiated following the release of surface-associated decapacitation factors that are elaborated by both the epididymis and the male accessory organs. Herein, we report the identification of a number of proteins that are actively released from the surface of mouse spermatozoa during capacitation in vitro. As anticipated, the addition of these factors back to suspensions of mouse spermatozoa was shown to suppress several correlates of the capacitation process. Specifically, they induced a significant, dose-dependent inhibition of the ability of spermatozoa to undergo a progesterone-induced acrosome reaction and to bind to the zona pellucida in vitro. Inhibition of these functions was associated with the suppression of tyrosine phosphorylation in the sperm plasma membrane but had no effect on the phosphorylation of internal proteins in either the sperm head or tail. This inhibitory activity was attributed to a subset of the isolated proteins compromising at least four putative decapacitation factors. These proteins were identified via tandem-mass spectrometry amino acid sequence analysis as plasma membrane fatty acid binding protein, cysteine-rich secretory protein 1 (CRISP1), phosphatidylethanolamine binding protein 1 (PBP), and an unnamed protein product that we have termed decapacitation factor 10 (DF10). Of these proteins, PBP was identified as a primary candidate for a decapacitation factor.

Skeletal Muscle Lipid Metabolism in Exercise and Insulin Resistance

Lipids as fuel for energy provision originate from different sources: albumin-bound long-chain fatty acids (LCFA) in the blood plasma, circulating very-low-density lipoproteins-triacylglycerols (VLDL-TG), fatty acids from triacylglycerol located in the muscle cell (IMTG), and possibly fatty acids liberated from adipose tissue adhering to the muscle cells. The regulation of utilization of the different lipid sources in skeletal muscle during exercise is reviewed, and the influence of diet, training, and gender is discussed. Major points deliberated are the methods utilized to measure uptake and oxidation of LCFA during exercise in humans. The role of the various lipid-binding proteins in transmembrane and cytosolic transport of lipids is considered as well as regulation of lipid entry into the mitochondria, focusing on the putative role of AMP-activated protein kinase (AMPK), acetyl CoA carboxylase (ACC), and carnitine during exercise. The possible contribution to fuel provision during exercise of circulating VLDL-TG as well as the role of IMTG is discussed from a methodological point of view. The contribution of IMTG for energy provision may not be large, covering ∼10% of total energy provision during fasting exercise in male subjects, whereas in females, IMTG may cover a larger proportion of energy delivery. Molecular mechanisms involved in breakdown of IMTG during exercise are also considered focusing on hormone-sensitive lipase (HSL). Finally, the role of lipids in development of insulin resistance in skeletal muscle, including possible molecular mechanisms involved, is discussed.

Long-chain fatty acid uptake and FAT/CD36 translocation in heart and skeletal muscle

Cellular long-chain fatty acid (LCFA) uptake constitutes a process that is not yet fully understood. LCFA uptake likely involves both passive diffusion and protein-mediated transport. Several lines of evidence support the involvement of a number of plasma membrane-associated proteins, including fatty acid translocase (FAT)/CD36, plasma membrane-bound fatty acid binding protein (FABPpm), and fatty acid transport protein (FATP). In heart and skeletal muscle primary attention has been given to unravel the mechanisms by which FAT/CD36 expression and function are regulated. It appears that both insulin and contractions induce the translocation of intracellular stored FAT/CD36 to the plasma membrane to increase cellular LCFA uptake. This review focuses on this novel mechanism of regulation of LCFA uptake in heart and skeletal muscle in health and disease. The distinct signaling pathways underlying insulin-induced and contraction-induced FAT/CD36 translocation will be discussed and a comparison will be made with the well-defined glucose transport system involving the glucose transporter GLUT4. Finally, it is hypothesized that malfunctioning of recycling of these transporters may lead to intracellular triacylglycerol (TAG) accumulation and cellular insulin resistance. Current data indicate a pivotal role for FAT/CD36 in the regulation of LCFA utilization in heart and skeletal muscle under normal conditions as well as during the altered LCFA utilization observed in obesity and insulin resistance. Hence, FAT/CD36 might provide a useful therapeutic target for the prevention or treatment of insulin resistance.

The subcellular compartmentation of fatty acid transporters is regulated differently by insulin and by AICAR

Cellular fatty acid uptake is facilitated by a number of fatty acid transporters, FAT/CD36, FABPpm and FATP1. It had been presumed that FABPpm, was confined to the plasma membrane and was not regulated. Here, we demonstrate for the first time that FABPpm and FATP1 are also present in intracellular depots in cardiac myocytes. While we confirmed previous work that insulin and AICAR each induced the translocation of FAT/CD36 from an intracellular depot to the PM, only AICAR, but not insulin, induced the translocation of FABPpm. Moreover, neither insulin nor AICAR induced the translocation of FATP1. Importantly, the increased plasmalemmal content of these LCFA transporters was associated with a concomitant increase in the initial rate of palmitate uptake into cardiac myocytes. Specifically, the insulin-stimulated increase in the rate of palmitate uptake (+60%) paralleled the insulin-stimulated increase in plasmalemmal FAT/CD36 (+34%). Similarly, the greater AICAR-stimulated increase in the rate of palmitate uptake (+90%) paralleled the AICAR-induced increase in both plasmalemmal proteins (FAT/CD36 (+40%)+FABPpm (+36%)). Inhibition of palmitate uptake with the specific FAT/CD36 inhibitor SSO indicated that FABPpm interacts with FAT/CD36 at the plasma membrane to facilitate the uptake of palmitate. In conclusion, (1) there appears to be tissue-specific sensitivity to insulin-induced FATP1 translocation, as it has been shown elsewhere that insulin induces FATP1 translocation in 3T3-L1 adipocytes, and (2) clearly, the subcellular distribution of FABPpm, as well as FAT/CD36, is acutely regulated in cardiac myocytes, although FABPpm and FAT/CD36 do not necessarily respond identically to the same stimuli.

Optimizing fat oxidation through exercise and diet

Interventions aimed at increasing fat metabolism could potentially reduce the symptoms of metabolic diseases such as obesity and type 2 diabetes and may have tremendous clinical relevance. Hence, an understanding of the factors that increase or decrease fat oxidation is important. Exercise intensity and duration are important determinants of fat oxidation. Fat oxidation rates increase from low to moderate intensities and then decrease when the intensity becomes high. Maximal rates of fat oxidation have been shown to be reached at intensities between 59% and 64% of maximum oxygen consumption in trained individuals and between 47% and 52% of maximum oxygen consumption in a large sample of the general population. The mode of exercise can also affect fat oxidation, with fat oxidation being higher during running than cycling. Endurance training induces a multitude of adaptations that result in increased fat oxidation. The duration and intensity of exercise training required to induce changes in fat oxidation is currently unknown. Ingestion of carbohydrate in the hours before or on commencement of exercise reduces the rate of fat oxidation significantly compared with fasted conditions, whereas fasting longer than 6 h optimizes fat oxidation. Fat oxidation rates have been shown to decrease after ingestion of high-fat diets, partly as a result of decreased glycogen stores and partly because of adaptations at the muscle level.

Regulation of plasma long-chain fatty acid oxidation in relation to uptake in human skeletal muscle during exercise

In the present study, we investigated possible sites of regulation of long-chain fatty acid (LCFA) oxidation in contracting human skeletal muscle. Leg plasma LCFA kinetics were determined in eight healthy men during bicycling (60 min, 65% peak oxygen uptake) with either high (H-FOX) or low (L-FOX) leg fat oxidation (H-FOX: 1,098 +/- 140; L-FOX: 494 +/- 84 micromol FA/min, P < 0.001), which was achieved by manipulating preexercise muscle glycogen (H-FOX: 197 +/- 21; L-FOX: 504 +/- 25 mmol/kg dry wt, P < 0.001). Several blood metabolites and hormones were kept nearly similar between trials by allocating a preexercise meal and infusing glucose intravenously during exercise. During exercise, leg plasma LCFA fractional extraction was identical between trials (H-FOX: 17.8 +/- 1.6; L-FOX: 18.2 +/- 1.8%, not significant), suggesting similar LCFA transport capacity in muscle. On the contrary, leg plasma LCFA oxidation was 99% higher in H-FOX than in L-FOX (421 +/- 47 vs. 212 +/- 37 micromol/min, P < 0.001). Probably due to the slightly higher (P < 0.01) plasma LCFA concentration in H-FOX than in L-FOX, leg plasma LCFA uptake was nonsignificantly (P = 0.17) higher (25%) in H-FOX than in L-FOX, yet the fraction of plasma LCFA uptake oxidized was 61% higher (P < 0.05) in H-FOX than in L-FOX. Accordingly, the muscle content of several lipid-binding proteins did not differ significantly between trials, although fatty acid translocase/CD36 and caveolin-1 were elevated (P < 0.05) by the high-intensity exercise and dietary manipulation allocated on the day before the experimental trial. The present data suggest that, in contracting human skeletal muscle with different fat oxidation rates achieved by manipulating preexercise glycogen content, transsarcolemmal transport is not limiting plasma LCFA oxidation. Rather, the latter seems to be limited by intracellular regulatory mechanisms.

Triacylglycerol accumulation in human obesity and type 2 diabetes is associated with increased rates of skeletal muscle fatty acid transport and increased sarcolemmal FAT/CD36

We examined whether, in human obesity and type 2 diabetes, long chain fatty acid (LCFA) transport into skeletal muscle is upregulated and contributes to an excess intramuscular triacylglycerol accumulation. In giant sarcolemmal vesicles prepared from human skeletal muscle, LCFA transport rates were upregulated approximately 4-fold and were associated with an increased intramuscular triacylglycerol content in obese individuals and in type 2 diabetics. In these individuals, the increased sarcolemmal LCFA transport rate was not associated with an altered expression of FAT/CD36 or FABPpm. Instead, the increase in the LCFA transport rate was associated with an increase in sarcolemmal FAT/CD36 but not sarcolemmal FABPpm. Rates of fatty acid esterification were increased threefold in isolated human muscle strips obtained from obese subjects, while concomitantly rates of fatty acid oxidation were not altered. Thus, the increased rate of fatty acid transport may contribute to the increased rates of triacylglycerol accumulation in human skeletal muscle. The altered FAT/CD36 trafficking in muscle from obese subjects and type 2 diabetics juxtaposes the known alterations in GLUT4 trafficking, i.e., GLUT4 is known to be retained in its intracellular depots while FAT/CD36 is retained at the sarcolemma. This redistribution of FAT/CD36 to the sarcolemma may contribute to the etiology of insulin resistance in human muscle, and hence, FAT/CD36 provides another potential therapeutic target for the prevention and/or treatment of insulin resistance.

Overexpression of membrane-associated fatty acid binding protein (FABPpm) in vivo increases fatty acid sarcolemmal transport and metabolism

Fatty acid translocase (FAT/CD36) is a key fatty acid transporter in skeletal muscle. However, the effects on fatty acid transport by another putative fatty acid transporter, plasma membrane-associated fatty acid binding protein (FABPpm), have not been determined in mammalian tissue. We examined the functional effects of overexpressing FABPpm on the rates of 1) palmitate transport across the sarcolemma and 2) palmitate metabolism in skeletal muscle. One muscle (soleus) was transfected with pTracer containing FABPpm cDNA. The contralateral muscle served as control. After injecting the FABPpm cDNA, muscles were electroporated. FABPpm overexpression was directly related to the quantity of DNA administered. Electrotransfection (200 μg/muscle) rapidly induced FABPpm protein overexpression ( day 1, +92%, P < 0.05), which was further increased during the next few days ( days 3–7; range +142% to +160%, P < 0.05). Sarcolemmal FABPpm was comparably increased ( day 7, +173%, P < 0.05). Neither FAT/CD36 expression nor sarcolemmal FAT/CD36 content was altered. FABPpm overexpression increased the rates of palmitate transport (+79%, P < 0.05). Rates of palmitate incorporation into phospholipids were also increased +36%, as were the rates of palmitate oxidation (+20%). Rates of palmitate incorporation into triacylglycerol depots were not altered. These studies demonstrate that in mammalian tissue FABPpm overexpression increased the rates of palmitate transport across the sarcolemma, an effect that is independent of any changes in FAT/CD36. However, since the overexpression of plasmalemmal FABPpm (+173%) exceeded the effects on the rates of palmitate transport and metabolism, it appears that the overexpression of FABPpm alone is not sufficient to induce completely parallel increments in palmitate transport and metabolism. This suggests that other mechanisms are required to realize the full potential offered by FABPpm overexpression.

Excess body fat in men decreases plasma fatty acid availability and oxidation during endurance exercise

The effect of relative body fat mass on exercise-induced stimulation of lipolysis and fatty acid oxidation was evaluated in 15 untrained men (5 lean, 5 overweight, and 5 obese with body mass indexes of 21 +/- 1, 27 +/- 1, and 34 +/- 1 kg/m2, respectively, and %body fat ranging from 12 to 32%). Palmitate and glycerol kinetics and substrate oxidation were assessed during 90 min of cycling at 50% peak aerobic capacity (VO2 peak) by use of stable isotope-labeled tracer infusion and indirect calorimetry. An inverse relationship was found between %body fat and exercise-induced increase in glycerol appearance rate relative to fat mass (r2 = 0.74; P < 0.01). The increase in total fatty acid uptake during exercise [(micromol/kg fat-free mass) x 90 min] was approximately 50% smaller in obese (181 +/- 70; P < 0.05) and approximately 35% smaller in overweight (230 +/- 71; P < 0.05) than in lean (354 +/- 34) men. The percentage of total fatty acid oxidation derived from systemic plasma fatty acids decreased with increasing body fat, from 49 +/- 3% in lean to 39 +/- 4% in obese men (P < 0.05); conversely, the percentage of nonsystemic fatty acids, presumably derived from intramuscular and possibly plasma triglycerides, increased with increasing body fat (P < 0.05). We conclude that the lipolytic response to exercise decreases with increasing adiposity. The blunted increase in lipolytic rate in overweight and obese men compared with lean men limits the availability of plasma fatty acids as a fuel during exercise. However, the rate of total fat oxidation was similar in all groups because of a compensatory increase in the oxidation of nonsystemic fatty acids.

Palmitate movement across red and white muscle membranes of rainbow trout

We examined the movement of [3H]palmitate across giant sarcolemmal vesicles prepared from red and white muscle of rainbow trout (Oncorhynchus mykiss). Red and white muscle fatty acid carriers have similar affinities for palmitate (apparent Km = 26 +/- 6 and 33 +/- 8 nM, respectively); however, red muscle has a higher maximal uptake compared with white muscle (Vmax = 476 +/- 41 vs. 229 +/- 23 pmol.mg protein-1.s-1, respectively). Phloretin (250 microM) inhibited palmitate influx in red and white muscle vesicles by approximately 40%, HgCl2 (2.5 mM) inhibited palmitate uptake by 20-30%, and the anion-exchange inhibitor DIDS (250 microM) inhibited palmitate influx in red and white muscle vesicles by approximately 15 and 30%, respectively. Western blot analysis of red and white muscle vesicles did not detect a mammalian-type fatty acid transporter (FAT); however, preincubation of vesicles with sulfo-N-succinimidyloleate, a specific inhibitor of FAT in rats, reduced palmitate uptake in red and white muscle vesicles by approximately 15 and 25%, respectively. A mammalian-type plasma membrane fatty acid-binding protein was identified in trout muscle using Western blotting, but the protein differed in size between red and white muscle. At low concentrations of free palmitate (2.5 nM), addition of high concentrations (111 microM total) of oleate (18:0) caused approximately 50% reduction in palmitate uptake by red and white muscle vesicles, but high concentrations (100 microM) of octanoate (8:0) caused no inhibition of uptake. Five days of aerobic swimming at approximately 2 body lengths/s and 9 days of chronic cortisol elevation in vivo, both of which stimulate lipid metabolism, had no effect on the rate of palmitate movement in red or white muscle vesicles.

Aging is associated with elevated muscle triglyceride content and increased insulin-stimulated fatty acid uptake

The purpose of the present study was to examine the utilization of fatty acids (FA) and muscle substrates by skeletal muscle in young, middle-aged, and old adult rats under hyperglycemic and hyperinsulinemic conditions. Male Fischer 344 x Brown Norway rats aged 5, 15, or 24 mo underwent hindlimb perfusion with a medium of 20 mM glucose, 1 mM palmitate, 1,000 microU/ml insulin, [1-14C]palmitate, and [3-3H]glucose. Glucose uptake and palmitate delivery were similar among age groups. Palmitate uptake and oxidation as well as muscle protein concentration of fatty acid translocase (FAT/CD36) and plasma membrane fatty acid-binding protein (FABPPM) were significantly increased (P < or = 0.05) in 24- vs. 5- and 15-mo-old animals. Compared with 5- and 15-mo-old animals, pre- and postperfusion muscle triglyceride (TG) levels were significantly (P < 0.05) elevated 72-145% in red and 112-129% in white muscles of 24-mo-old animals. Palmitate uptake was associated with total preperfusion TG concentration (r2 = 0.27, P < 0.05) and total TG synthesis rate (r2 = 0.68, P < 0.05). These results indicate that, under insulin-stimulated conditions, FA uptake is significantly increased in old animals, which is associated with increased rates of TG synthesis and may contribute to the accumulation of TG in muscle of old animals.

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.

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.

Effect of gender on lipid kinetics during endurance exercise of moderate intensity in untrained subjects

We evaluated lipid metabolism during 90 min of moderate-intensity (50% VO(2) peak) cycle ergometer exercise in five men and five women who were matched on adiposity (24 +/- 2 and 25 +/- 1% body fat, respectively) and aerobic fitness (VO(2) peak: 49 +/- 2 and 47 +/- 1 ml x kg fat-free mass(-1) x min(-1), respectively). Substrate oxidation and lipid kinetics were measured by using indirect calorimetry and [(13)C]palmitate and [(2)H(5)]glycerol tracer infusion. The total increase in glycerol and free fatty acid (FFA) rate of appearance (R(a)) in plasma during exercise (area under the curve above baseline) was approximately 65% greater in women than in men (glycerol R(a): 317 +/- 40 and 195 +/- 33 micromol/kg, respectively; FFA R(a): 652 +/- 46 and 453 +/- 70 micromol/kg, respectively; both P < 0.05). Total fatty acid oxidation was similar in men and women, but the relative contribution of plasma FFA to total fatty acid oxidation was higher in women (76 +/- 5%) than in men (46 +/- 5%; P < 0.05). We conclude that lipolysis of adipose tissue triglycerides during moderate-intensity exercise is greater in women than in men, who are matched on adiposity and fitness. The increase in plasma fatty acid availability leads to a greater rate of plasma FFA tissue uptake and oxidation in women than in men. However, total fat oxidation is the same in both groups because of a reciprocal decrease in the oxidation rate of fatty acids derived from nonplasma sources, presumably intramuscular and possibly plasma triglycerides, in women.

Fasting activates the gene expression of UCP3 independent of genes necessary for lipid transport and oxidation in skeletal muscle

Fasting triggers a complex array of adaptive metabolic and hormonal responses including an augmentation in the capacity for mitochondrial fatty acid (FA) oxidation in skeletal muscle. This study hypothesized that this adaptive response is mediated by increased mRNA of key genes central to the regulation of fat oxidation in human skeletal muscle. Fasting dramatically increased UCP3 gene expression, by 5-fold at 15 h and 10-fold at 40 h. However the expression of key genes responsible for the uptake, transport, oxidation, and re-esterification of FA remained unchanged following 15 and 40 h of fasting. Likewise there was no change in the mRNA abundance of transcription factors. This suggests a unique role for UCP3 in the regulation of FA homeostasis during fasting as adaptation to 40 h of fasting does not require alterations in the expression of other genes necessary for lipid metabolism.

Impaired fatty acid oxidation in muscle of aging rats perfused under basal conditions

The purpose of the present study was to examine the utilization of fatty acids (FA) and muscle substrates by skeletal muscle in young, middle-aged, and old adult rats under conditions of euglycemia with low insulin levels. Male Fischer 344 x Brown Norway rats aged 5, 15, or 24 mo underwent hindlimb perfusion with a medium of 8 mM glucose, 1 mM palmitate, 25 microU/ml insulin, [1-(14)C]palmitate, and [3-(3)H]glucose. Glucose and palmitate uptake were similar among age groups. The percent and total palmitate oxidized (nmol.min(-1).g(-1)) were 30-36 and 41-49% lower (P < 0.05) in 15-mo- and 24-mo-old than in 5-mo-old animals. Compared with 5-mo- and 15-mo-old animals, pre- and postperfusion muscle triglyceride (TG) levels were significantly (P < 0.05) elevated 91-305% in red and 118-219% in white muscles of 24-mo-old animals. Fatty acid-binding protein content was 40-64% higher (P < 0.05) in 24-mo- than in 5-mo- or 15-mo-old animals. In red muscle, hormone-sensitive lipase (HSL) content was 28% lower (P < 0.05) in 24-mo- than in 5-mo-old animals. These results indicate that, under euglycemic conditions in the presence of low insulin levels, the reduction in FA disposal to oxidation and the decrease in HSL content may contribute to the accumulation of TG in muscle of old animals.

Brief food restriction increases FA oxidation and glycogen synthesis under insulin-stimulated conditions

To determine the effects of brief food restriction on fatty acid (FA) metabolism, hindlimbs of F344/BN rats fed either ad libitum (AL) or food restricted (FR) to 60% of baseline food intake for 28 days were perfused under hyperglycemic-hyperinsulinemic conditions (20 mM glucose, 1 mM palmitate, 1,000 microU/ml insulin, [3-(3)H]glucose, and [1-(14)C]palmitate). Basal glucose and insulin levels were significantly lower (P < 0.05) in FR vs. AL rats. Palmitate uptake (34.3 +/- 2.7 vs. 24.5 +/- 3.1 nmol/g/min) and oxidation (3.8 +/- 0.2 vs. 2.7 +/- 0.3 nmol.g(-1).min(-1)) were significantly higher (P < 0.05) in FR vs. AL rats, respectively. Glucose uptake was increased in FR rats and was accompanied by significant increases in red and white gastrocnemius glycogen synthesis, indicating an improvement in insulin sensitivity. Although muscle triglyceride (TG) levels were not significantly different between groups, glucose uptake and total preperfusion TG concentration were negatively correlated (r(2) = 0.27, P < 0.05). In conclusion, our results show that under hyperglycemic-hyperinsulinemic conditions, brief FR resulted in an increase in FA oxidative disposal that may contribute to the improvement in insulin sensitivity.

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.

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

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

Coordinately regulated expression of FAT/CD36 and FACS1 in rat skeletal muscle

Protein-mediated fatty acid uptake and intracellular fatty acid activation are key steps in fatty acid metabolism in muscle. We have examined (a) the abundance of fatty acid translocase (FAT/CD36) mRNA (a fatty acid transporter) and long-chain acyl CoA synthetase (FACS1) mRNA in metabolically heterogeneous muscles (soleus (SOL), red (RG) and white gastrocnemius (WG)), and (b) whether FAT/CD36 and FACS1 mRNAs were coordinately up-regulated in red (RTA) and white tibialis muscles (WTA) that had been chronically stimulated for varying periods of time (0.25, 1, 6 and 24 h/day) for 7 days. FAT/CD36 mRNA and FACS1 mRNA abundance were scaled with (a) the oxidative capacity of muscle (SOL > RG > WG) (p < 0.05), (b) the rates of fatty acid oxidation in red and white muscles, and (c) fatty acid uptake by sarcolemmal vesicles, derived from red and white muscles. In chronically stimulated muscles (RTA and WTA), FAT/CD36 mRNA and FACS1 mRNA were up-regulated in relation to the quantity of muscle contractile activity (p < 0.05). FAT/CD36 mRNA and FACS1 mRNA up-regulation was highly correlated (r = 0.98). The coordinated expression of FAT/CD36 and FACS is likely a functional adaptive response to facilitate a greater rate of fatty acid activation in response to a greater rate of fatty acid transport, either among different types of muscles or in muscles in which capacity for fatty acid metabolism has been enhanced.

Increased fatty acid uptake and altered fatty acid metabolism in insulin-resistant muscle of obese Zucker rats

Altered muscle fatty acid (FA) metabolism may contribute to the presence of muscle insulin resistance in the genetically obese Zucker rat. To determine whether FA uptake and disposal are altered in insulin-resistant muscle, we measured palmitate uptake, oxidation, and incorporation into di- and triglycerides in isolated rat hindquarters, as well as muscle plasma membrane fatty acid-binding protein (FABP(PM)) content of lean (n = 16, fa/+) and obese (n = 15, fa/fa) Zucker rats (12 weeks of age). Hindquarters were perfused with 7 mmol/l glucose, 1,000 micromol/l albumin-bound palmitate, and albumin-bound [1-(14)C]palmitate at rest (no insulin). Glucose uptake was 42% lower in the obese than in the lean rats and indicated the presence of muscle insulin resistance. Fractional and total rates of palmitate uptake were 42 and 74% higher in the obese than in the lean rats and were associated with higher muscle FABP(PM) content (r(2) = 0.69, P < 0.05). The percentage of palmitate oxidized was not significantly different between groups. FA disposal to storage was altered according to fiber type. When compared with lean rats, the rate of triglyceride synthesis in red muscle was 158% higher in obese rats, and the rate of palmitate incorporation into diglycerides in white muscle was 93% higher in obese rats. Pre- and postperfusion muscle triglyceride levels were higher in both red and white muscles of the obese rats. These results show that increased FA uptake and altered FA disposal to storage may contribute to the development of muscle insulin resistance in obese Zucker rats.