Uptake of long chain free fatty acids is selectively up-regulated in adipocytes of Zucker rats with genetic obesity and non-insulin-dependent diabetes mellitus

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.

Thiazolidinediones upregulate impaired fatty acid uptake in skeletal muscle of type 2 diabetic subjects

We examined the regulation of free fatty acid (FFA, palmitate) uptake into skeletal muscle cells of nondiabetic and type 2 diabetic subjects. Palmitate uptake included a protein-mediated component that was inhibited by phloretin. The protein-mediated component of uptake in muscle cells from type 2 diabetic subjects (78 +/- 13 nmol. mg protein-1. min-1) was reduced compared with that in nondiabetic muscle (150 +/- 17, P < 0.01). Acute insulin exposure caused a modest (16 +/- 5%, P < 0.025) but significant increase in protein-mediated uptake in nondiabetic muscle. There was no significant insulin effect in diabetic muscle (+19 +/- 19%, P = not significant). Chronic (4 day) treatment with a series of thiazolidinediones, troglitazone (Tgz), rosiglitazone (Rgz), and pioglitazone (Pio) increased FFA uptake. Only the phloretin-inhibitable component was increased by treatment, which normalized this activity in diabetic muscle cells. Under the same conditions, FFA oxidation was also increased by thiazolidinedione treatment. Increases in FFA uptake and oxidation were associated with upregulation of fatty acid translocase (FAT/CD36) expression. FAT/CD36 protein was increased by Tgz (90 +/- 22% over control), Rgz (146 +/- 42%), and Pio (111 +/- 37%, P < 0.05 for all 3) treatment. Tgz treatment had no effect on fatty acid transporter protein-1 and membrane-associated plasmalemmal fatty acid-binding protein mRNA expression. We conclude that FFA uptake into cultured muscle cells is, in part, protein mediated and acutely insulin responsive. The basal activity of FFA uptake is impaired in type 2 diabetes. In addition, chronic thiazolidinedione treatment increased FFA uptake and oxidation into cultured human skeletal muscle cells in concert with upregulation of FAT/CD36 expression. Increased FFA uptake and oxidation may contribute to lower circulating FFA levels and reduced insulin resistance in skeletal muscle of individuals with type 2 diabetes following thiazolidinedione treatment.

Potential mechanisms and consequences of cardiac triacylglycerol accumulation in insulin-resistant rats

The accumulation of intracellular triacylglycerol (TG) is highly correlated with muscle insulin resistance. However, it is controversial whether the accumulation of TG is the result of increased fatty acid supply, decreased fatty acid oxidation, or both. Because abnormal fatty acid metabolism is a key contributor to the pathogenesis of diabetes-related cardiovascular dysfunction, we examined fatty acid and glucose metabolism in hearts of insulin-resistant JCR:LA-cp rats. Isolated working hearts from insulin-resistant rats had glycolytic rates that were reduced to 50% of lean control levels (P < 0.05). Cardiac TG content was increased by 50% (P < 0.05) in the insulin-resistant rats, but palmitate oxidation rates remained similar between the insulin-resistant and lean control rats. However, plasma fatty acids and TG levels, as well as cardiac fatty acid-binding protein (FABP) expression, were significantly increased in the insulin-resistant rats. AMP-activated protein kinase (AMPK) plays a major role in the regulation of cardiac fatty acid and glucose metabolism. When activated, AMPK increases fatty acid oxidation by inhibiting acetyl-CoA carboxylase (ACC) and reducing malonyl-CoA levels, and it decreases TG content by inhibiting glycerol-3-phosphate acyltransferase (GPAT), the rate-limiting step in TG synthesis. The activation of AMPK also stimulates cardiac glucose uptake and glycolysis. We thus investigated whether a decrease in AMPK activity was responsible for the reduced cardiac glycolysis and increased TG content in the insulin-resistant rats. However, we found no significant difference in AMPK activity. We also found no significant difference in various established downstream targets of AMPK: ACC activity, malonyl-CoA levels, carnitine palmitoyltransferase I activity, or GPAT activity. We conclude that hearts from insulin-resistant JCR:LA-cp rats accumulate substantial TG as a result of increased fatty acid supply rather than from reduced fatty acid oxidation. Furthermore, the accumulation of cardiac TG is associated with a reduction in insulin-stimulated glucose metabolism.

Effects of obesity on lipid profiles in neutered male and female cats

OBJECTIVE

To examine whether obese cats, compared with lean cats, have alterations in lipoprotein metabolism that might lead to a decrease in glucose metabolism and insulin secretion.

ANIMALS

10 lean and 10 obese adults cats (5 neutered males and 5 neutered females each).

PROCEDURE

Intravenous glucose tolerance tests with measurements of serum glucose, insulin, and nonesterified fatty acid (NEFA) concentrations were performed. Lipoprotein fractions were examined in serum by isopycnic density gradient ultracentrifugation.

RESULTS

Obese cats had insulin resistance. Plasma triglyceride and cholesterol concentrations were significantly increased in obese cats, compared with lean cats. Very low density lipoprotein (VLDL) concentrations were increased in obese cats, compared with lean cats; however, the composition of various fractions remained unchanged between obese and lean cats, indicating greater synthesis and catabolism of VLDL in obese cats. Serum high density lipoprotein (HDL) cholesterol concentrations were increased in obese cats, compared with lean cats. Serum NEFA concentrations were only significantly different between obese and lean cats when separated by sex; obese male cats had higher baseline serum NEFA concentrations and greater NEFA suppression in response to insulin, compared with lean male cats.

CONCLUSIONS AND CLINICAL RELEVANCE

Lipid metabolism changes in obese cats, compared with lean cats. The increase in VLDL turnover in obese cats might contribute to insulin resistance of glucose metabolism, whereas the increase in serum HDL cholesterol concentration might reflect a protective effect against atherosclerosis in obese cats.

Comparative aspects of diabetes mellitus in dogs and cats

Diabetes mellitus is a common disease in cats and dogs. Its incidence is increasing, possibly due to an increase in obesity in both species. Different types of diabetes have been identified in pet animals. The classification of diabetic dogs and cats is modeled after the human classification but especially in the diabetic dogs, many aspects are different. The diabetic cat, however, resembles type 2 diabetic human patients more closely. The clinical presentation, pathophysiology, and histologic findings are described for both dog and cat and possible etiological mechanisms are discussed.

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

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

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

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

Leptin and the adipocyte endocrine system

Although adipose tissue has long been considered to be metabolically passive and primarily responsible for energy storage, recent scientific advances have dramatically altered our understanding of the function of this ubiquitous tissue. The fat cell is a transducer of energy supply for the changing metabolic needs of the body, modulating glucose homeostasis, hypothalamic function, sympathetic output, vascular tone, immune response, and reproduction. Through endocrine/autocrine and paracrine actions, adipocyte-derived molecules defend the body during periods of energy deficit and stress. With the development of obesity, maladaptive responses to adipose excess result in pathologic states of inflammation, coagulopathy, and altered insulin sensitivity.

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

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

The fatty acid translocase (FAT)/CD36 and the glucose transporter GLUT4 are localized in different cellular compartments in rat cardiac muscle

The fatty acid translocase (FAT)/CD36 plays an important role in the acute regulation of fatty acid uptake in muscle tissue. We studied the subcellular distribution of FAT/CD36 in rat cardiac muscle after in vivo insulin stimulation by membrane fractionation and immunoisolation of GLUT4- and FAT/CD36-vesicles. FAT/CD36 was equally present in both plasma and microsomal membranes with no effect of insulin on the cellular distribution, whereas GLUT4 increased 2- to 3-fold in the plasma membrane. FAT/CD36 resides in one intracellular pool, whereas GLUT4 is present in two distinct pools. Immunoadsorption of GLUT4-vesicles indicated that FAT/CD36 is undetectable in these vesicles. Likewise, no GLUT4 could be detected in FAT/CD36-vesicles. These vesicles contain a high amount of Rab11 that remained unaffected after insulin stimulation, whereas Rab11 increased about 3-fold in the GLUT4-vesicles in response to insulin. These data show that GLUT4 and FAT/CD36 do not co-localize in cardiac muscle and that FAT/CD36 is not redistributed in response to insulin in the heart. Rab11 may be involved in endosomal recycling of FAT/CD36, however, insulin-associated Rab11 functions appear to be limited to GLUT4-vesicles.

Human aquaporin adipose (AQPap) gene. Genomic structure, promoter analysis and functional mutation

Aquaporin adipose (AQPap), which we identified from human adipose tissue, is a glycerol channel in adipocyte [Kishida et al. (2000) J. Biol. Chem. 275, 20896-20902]. In the current study, we determined the genomic structure of the human AQPap gene, and identified three AQPap-like genes that resembled (approximately 95%) AQPap, with little expression in human tissues. The AQPap promoter contained a putative peroxisome proliferator response element (PPRE) at -46 to -62, and a putative insulin response element (IRE) at -542/-536. Deletion of the PPRE abolished the pioglitazone-mediated induction of AQPap promoter activity in 3T3-L1 adipocytes. Deletion and single base pair substitution analysis of the IRE abolished the insulin-mediated suppression of the human AQPap gene. Analysis of AQPap sequence in human subjects revealed three missense mutations (R12C, V59L and G264V), and two silent mutations (A103A and G250G). The cRNA injection of the missense mutants into Xenopus oocytes revealed the absence of the activity to transport glycerol and water in the AQPap-G264V protein. In the subject homozygous for AQPap-G264V, exercise-induced increase in plasma glycerol was not observed in spite of the increased plasma noradrenaline. We suggest that AQPap is responsible for the increase of plasma glycerol during exercise in humans.

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

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

Loss of regulation of lipogenesis in the Zucker diabetic rat. II. Changes in stearate and oleate synthesis

De novo lipogenesis and dietary fat uptake are two major sources of fatty acid deposits in fat of obese animals. To determine the relative contribution of fatty acids from these two sources in obesity, we have determined the distribution of c16 and c18 fatty acids of triglycerides in plasma, liver, and epididymal fat pad of Zucker diabetic fatty (ZDF) rats and their lean littermates (ZL) under two isocaloric dietary fat conditions. Lipogenesis was also determined using the deuterated water method. Conversion of palmitate to stearate and stearate to oleate was calculated from the deuterium incorporation by use of the tracer dilution principle. In the ZL rat, lipogenesis was suppressed from 70 to 24%, conversion of palmitate to stearate from 86 to 78%, and conversion of stearate to oleate from 56 to 7% in response to an increase in the dietary fat-to-carbohydrate ratio. The results suggest that suppression of fatty acid synthase and stearoyl-CoA desaturase activities is a normal adaptive mechanism to a high-fat diet. In contrast, de novo lipogenesis, chain elongation, and desaturation were not suppressed by dietary fat in the ZDF rat. The lack of ability to adapt to a high-fat diet resulted in a higher plasma triglyceride concentration and excessive fat accumulation from both diet and de novo synthesis in the ZDF rat.

Fatty acid transport: the roads taken

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

Acyl-CoA binding protein expression is fiber type- specific and elevated in muscles from the obese insulin-resistant Zucker rat

Accumulation of acyl-CoA is hypothesized to be involved in development of insulin resistance. Acyl-CoA binds to acyl-CoA binding protein (ACBP) with high affinity, and therefore knowledge about ACBP concentration is important for interpreting acyl-CoA data. In the present study, we used a sandwich enzyme-linked immunosorbent assay to quantify ACBP concentration in different muscle fiber types. Furthermore, ACBP concentration was compared in muscles from lean and obese Zucker rats. Expression of ACBP was highest in the slow-twitch oxidative soleus muscle and lowest in the fast-twitch glycolytic white gastrocnemius (0.46 +/- 0.02 and 0.16 +/- 0.005 microg/mg protein, respectively). Expression of ACBP was soleus > red gastrocnemius > extensor digitorum longus > white gastrocnemius. Similar fiber type differences were found for carnitine palmitoyl transferase (CPT)-1, and a correlation was observed between ACBP and CPT-1. Muscles from obese Zucker rats had twice the triglyceride content, had approximately twice the long-chain acyl CoA content, and were severely insulin resistant. ACBP concentration was approximately 30% higher in all muscles from obese rats. Activities of CPT-1 and 3-hydroxy-acyl-CoA dehydrogenase were increased in muscles from obese rats, whereas citrate synthase activity was similar. In conclusion, ACBP expression is fiber type-specific with the highest concentration in oxidative muscles and the lowest in glycolytic muscles. The 90% increase in the concentration of acyl-CoA in obese Zucker muscle compared with only a 30% increase in the concentration of ACBP supports the hypothesis that an increased concentration of free acyl-CoA is involved in the development of insulin resistance.

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.

Anorexic effect of K+ channel blockade in mesenteric arterial smooth muscle and intestinal epithelial cells

Activity of voltage-gated K+ (Kv) channels controls membrane potential (E(m)). Membrane depolarization due to blockade of K+ channels in mesenteric artery smooth muscle cells (MASMC) should increase cytoplasmic free Ca2+ concentration ([Ca2+]cyt) and cause vasoconstriction, which may subsequently reduce the mesenteric blood flow and inhibit the transportation of absorbed nutrients to the liver and adipose tissue. In this study, we characterized and compared the electrophysiological properties and molecular identities of Kv channels and examined the role of Kv channel function in regulating E(m) in MASMC and intestinal epithelial cells (IEC). MASMC and IEC functionally expressed multiple Kv channel alpha- and beta-subunits (Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv2.1, Kv4.3, and Kv9.3, as well as Kvbeta1.1, Kvbeta2.1, and Kvbeta3), but only MASMC expressed voltage-dependent Ca2+ channels. The current density and the activation and inactivation kinetics of whole cell Kv currents were similar in MASMC and IEC. Extracellular application of 4-aminopyridine (4-AP), a Kv-channel blocker, reduced whole cell Kv currents and caused E(m) depolarization in both MASMC and IEC. The 4-AP-induced E(m) depolarization increased [Ca2+]cyt in MASMC and caused mesenteric vasoconstriction. Furthermore, ingestion of 4-AP significantly reduced the weight gain in rats. These results suggest that MASMC and IEC express multiple Kv channel alpha- and beta-subunits. The function of these Kv channels plays an important role in controlling E(m). The membrane depolarization-mediated increase in [Ca2+]cyt in MASMC and mesenteric vasoconstriction may inhibit transportation of absorbed nutrients via mesenteric circulation and limit weight gain.

Genomic structure and insulin-mediated repression of the aquaporin adipose (AQPap), adipose-specific glycerol channel

Aquaporin adipose (AQPap) is a putative glycerol channel in adipocytes (Kishida, K., Kuriyama, H., Funahashi, T., Shimomura, I., Kihara, S., Ouchi, N., Nishida, M., Nishizawa, H., Matsuda, M., Takahashi, M., Hotta, K., Nakamura, T., Yamashita, S., Tochino, Y., and Matsuzawa, Y. (2000) J. Biol. Chem. 275, 20896-20902). In the current study, we examined the genomic structure of the mouse AQPap gene and its regulation by insulin. The mouse AQPap gene spanned 12 kilobase pairs in chromosome 4 and consisted of 8 exons and 7 introns. The first two exons, designated exon 1 and exon 1', are alternatively spliced to common exon 2, and thus the AQPap gene possessed two potential promoters. The exon 1-derived transcript is dominant in both adipose tissues and adipocytes on the basis of RNase protection assay and promoter analysis. The mRNA increased after fasting and decreased with refeeding. Insulin deficiency generated by streptozotocin enhanced the mRNA in adipose tissue. Insulin down-regulated AQPap mRNA in 3T3-L1 adipocytes. The AQPap promoter contained heptanucleotide sequences, TGTTTTT at -443/-437, similar to the insulin-response element identified previously in the promoters of insulin-repressed genes. Deletion and single base pair substitution analysis of the promoter revealed that these sequences were required for insulin-mediated repression of AQPap gene transcription. The phosphatidylinositol 3-kinase pathway was involved in this inhibition. We conclude that insulin represses the transcription of AQPap gene via insulin response element in its promoter. Sustained up-regulation of AQPap mRNA in adipose tissue in the insulin-resistant condition may disturb glucose homeostasis by increasing plasma glycerol.

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.

Fatty acid transport protein-1 mRNA expression in skeletal muscle and in adipose tissue in humans

Fatty acid transporter protein (FATP)-1 mRNA expression was investigated in skeletal muscle and in subcutaneous abdominal adipose tissue of 17 healthy lean, 13 nondiabetic obese, and 16 obese type 2 diabetic subjects. In muscle, FATP-1 mRNA levels were higher in lean women than in lean men (2.2 +/- 0.1 vs. 0.6 +/- 0.2 amol/microg total RNA, P < 0.01). FATP-1 mRNA expression was decreased in skeletal muscle in obese women both in nondiabetic and in type 2 diabetic patients (P < 0.02 vs. lean women in both groups), and in all women there was a negative correlation with basal FATP-1 mRNA level and body mass index (r = -0.74, P < 0.02). In men, FATP-1 mRNA was expressed at similar levels in the three groups both in skeletal muscle (0.6 +/- 0.2, 0.6 +/- 0.2, and 0.8 +/- 0.2 amol/microg total RNA in lean, obese, and type 2 diabetic male subjects) and in adipose tissue (0.9 +/- 0.2 amol/microg total RNA in the 3 groups). Insulin infusion (3 h) reduced FATP-1 mRNA levels in muscle in lean women but not in lean men. Insulin did not affect FATP-1 mRNA expression in skeletal muscle in obese nondiabetic or in type 2 diabetic subjects nor in subcutaneous adipose tissue in any of the three groups. These data show a gender-related difference in the expression of the fatty acid transporter FATP-1 in skeletal muscle of lean individuals and suggest that changes in FATP-1 expression may not contribute to a large extent to the alterations in fatty acid uptake in obesity and/or type 2 diabetes.

Acute regulation of fatty acid uptake involves the cellular redistribution of fatty acid translocase

We used muscle contraction, which increases fatty acid oxidation, as a model to determine whether fatty acid transport is acutely regulated by fatty acid translocase (FAT/CD36). Palmitate uptake by giant vesicles, obtained from skeletal muscle, was increased by muscle contraction. Kinetic studies indicated that muscle contraction increased V(max), but K(m) remained unaltered. Sulfo-N-succinimidyl oleate, a specific inhibitor of FAT/CD36, fully blocked the contraction-induced increase in palmitate uptake. In giant vesicles from contracting muscles, plasma membrane FAT/CD36 was also increased in parallel with the increase in long chain fatty acid uptake. Further studies showed that like GLUT-4, FAT/CD36 is located in both the plasma membrane and intracellularly (endosomally). With muscle contraction, FAT/CD36 at the surface of the muscle was increased, while concomitantly, FAT/CD36 in the intracellular pool was reduced. Similar responses were observed for GLUT-4. We conclude that fatty acid uptake is subject to short term regulation by muscle contraction and involves the translocation of FAT/CD36 from intracellular stores to the sarcolemma, analogous to the regulation of glucose uptake by GLUT-4.

Tissue-specific actions of antidiabetic thiazolidinediones on the reduced fatty acid oxidation in skeletal muscle and liver of Zucker diabetic fatty rats

Fatty acid overload has been proposed as a cause of decreased responsiveness in the major insulin target tissues of the body such as muscle and liver tissue. We therefore investigated fatty acid oxidation in soleus muscle and liver isolated from Zucker diabetic fatty (ZDF) rats treated with thiazolidinediones, a new class of antidiabetic agents. 14CO2 production from [14C]palmitic (C16:0) acid was lower in the soleus muscle and liver of ZDF rats versus lean rats (P < .05). When administered orally to ZDF rats for 2 weeks, the thiazolidinediones troglitazone (300 mg/kg) and KRP-297 (10 mg/kg) increased palmitic acid oxidation in the soleus muscle of ZDF rats (P < .05). KRP-297, but not troglitazone, increased palmitic acid oxidation in the liver of ZDF rats (P < .05), and both troglitazone and KRP-297 inhibited triglyceride accumulation in the skeletal muscle of ZDF rats. Hepatic triglyceride accumulation in ZDF rats was inhibited by KRP-297, but not by troglitazone. A reduction of fatty acid oxidation in the liver of ZDF rats and an increase in response to KRP-297 were observed only when C16:0 and C18:0 fatty acids, not C8:0, were used as substrates. Thus, there were defects in fatty acid catabolic activity and triglyceride accumulation in the soleus muscle and liver of ZDF rats. These results indicate that KRP-297 has advantages over troglitazone in the amelioration of these lipid metabolic abnormalities in insulin resistance associated with obesity.

Intestinal fatty acid binding protein may favor differential apical fatty acid binding in the intestine

The intestinal mucosa metabolizes fatty acids differently when presented to the lumenal or basolateral membrane. Expression of both liver and intestinal fatty acid binding proteins (L- and I-FABPs) uniquely in the enterocyte offers a possible explanation of this phenomenon. An organ explant system was used to analyze the relative binding of fatty acids to each protein. More fatty acid was bound to L-FABP than to I-FABPs (28% vs. 6% of cytosolic radioactivity), no matter on which side the fatty acid was added. However, a 2-3-fold increase in fatty acid binding to the intestinal paralog was noted after apical addition of palmitic or oleic acid in mucosa from chow fed rats. When oleic acid was added apically, a 1.4-fold increase in binding to I-FABP was observed in mucosa derived from chronically fat fed rats, consistent with the previously observed 50% increase in the content of that protein. Immunocytochemical localization of both FABPs in vivo demonstrated an apical cytoplasmic localization in the fasting state, and redistribution to the entire cytoplasm after fat feeding. These data are consistent with the hypothesis that I-FABP may contribute to the metabolic compartmentalization of apically presented fatty acids in the intestine.

The fatty acid transport protein (FATP1) is a very long chain acyl-CoA synthetase

The primary sequence of the murine fatty acid transport protein (FATP1) is very similar to the multigene family of very long chain (C20-C26) acyl-CoA synthetases. To determine if FATP1 is a long chain acyl coenzyme A synthetase, FATP1-Myc/His fusion protein was expressed in COS1 cells, and its enzymatic activity was analyzed. In addition, mutations were generated in two domains conserved in acyl-CoA synthetases: a 6- amino acid substitution into the putative active site (amino acids 249-254) generating mutant M1 and a 59-amino acid deletion into a conserved C-terminal domain (amino acids 464-523) generating mutant M2. Immunolocalization revealed that the FATP1-Myc/His forms were distributed between the COS1 cell plasma membrane and intracellular membranes. COS1 cells expressing wild type FATP1-Myc/His exhibited a 3-fold increase in the ratio of lignoceroyl-CoA synthetase activity (C24:0) to palmitoyl-CoA synthetase activity (C16:0), characteristic of very long chain acyl-CoA synthetases, whereas both mutant M1 and M2 were catalytically inactive. Detergent-solubilized FATP1-Myc/His was partially purified using nickel-based affinity chromatography and demonstrated a 10-fold increase in very long chain acyl-CoA specific activity (C24:0/C16:0). These results indicate that FATP1 is a very long chain acyl-CoA synthetase and suggest that a potential mechanism for facilitating mammalian fatty acid uptake is via esterification coupled influx.

Holo-sterol carrier protein-2. (13)C NMR investigation of cholesterol and fatty acid binding sites

Although sterol carrier protein-2 (SCP-2) stimulates sterol transfer in vitro, almost nothing is known regarding the identity of the putative cholesterol binding site. Furthermore, the interrelationship(s) between this SCP-2 ligand binding site and the recently reported SCP-2 long chain fatty acid (LCFA) and long chain fatty acyl-CoA (LCFA-CoA) binding site(s) remains to be established. In the present work, two SCP-2 ligand binding sites were identified. First, both [4-(13)C]cholesterol and 22-(N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (NBD-cholesterol) binding assays were consistent with a single cholesterol binding site in SCP-2. This ligand binding site had high affinity for NBD-cholesterol, K(d) = 4.15 +/- 0.71 nM. (13)C NMR-labeled ligand competition studies demonstrated that the SCP-2 high affinity cholesterol binding site also bound LCFA or LCFA-CoA. However, only the LCFA-CoA was able to effectively displace the SCP-2-bound [4-(13)C]cholesterol. Thus, the ligand affinities at this SCP-2 binding site were in the relative order cholesterol = LCFA-CoA > LCFA. Second, (13)C NMR studies demonstrated the presence of another ligand binding site on SCP-2 that bound either LCFA or LCFA-CoA but not cholesterol. Photon correlation spectroscopy was consistent with SCP-2 being monomeric in both liganded and unliganded states. In summary, both (13)C NMR and fluorescence techniques demonstrated for the first time that SCP-2 had a single high affinity binding site that bound cholesterol, LCFA, or LCFA-CoA. Furthermore, results with (13)C NMR supported the presence of a second SCP-2 ligand binding site that bound either LCFA or LCFA-CoA but not cholesterol. These data contribute to our understanding of a role for SCP-2 in both cellular cholesterol and LCFA metabolism.

Selective up-regulation of fatty acid uptake by adipocytes characterizes both genetic and diet-induced obesity in rodents

Long chain fatty acid transport is selectively up-regulated in adipocytes of Zucker fatty rats, diverting fatty acids from sites of oxidation toward storage in adipose tissue. To determine whether this is a general feature of obesity, we studied [(3)H]oleate uptake by adipocytes and hepatocytes from 1) homozygous male obese (ob), diabetic (db), fat (fat), and tubby (tub) mice and from 2) male Harlan Sprague-Dawley rats fed for 7 weeks a diet containing 55% of calories from fat. V(max) and K(m) were compared with controls of the appropriate background strain (C57BL/6J or C57BLKS) or diet (13% of calories from fat). V(max) for adipocyte fatty acid uptake was increased 5-6-fold in ob, db, fat, and tub mice versus controls (p < 0.001), whereas no differences were seen in the corresponding hepatocytes. Similar changes occurred in fat-fed rats. Of three membrane fatty acid transporters expressed in adipocytes, plasma membrane fatty acid-binding protein mRNA was increased 9-11-fold in ob and db, which lack a competent leptin/leptin receptor system, but was not increased in fat and tub, i.e. in strains with normal leptin signaling capability; fatty acid translocase mRNA was increased 2.2-6.5-fold in tub, ob, and fat adipocytes, but not in db adipocytes; and only marginal changes in fatty acid transport protein 1 mRNA were found in any of the mutant strains. Adipocyte fatty acid uptake is generally increased in murine obesity models, but up-regulation of individual transporters depends on the specific pathophysiology. Leptin may normally down-regulate expression of plasma membrane fatty acid binding protein.

Impact of 1 wk of diabetes on the regulation of myocardial carbohydrate and fatty acid oxidation

The aim of this study was to investigate the effect of increasing exogenous palmitate concentration on carbohydrate and palmitate oxidation in hearts from control and 1-wk diabetic rats. Hearts were perfused with glucose, [3-(13)C]lactate, and [U-(13)C]palmitate. Substrate oxidation rates were determined by combining (13)C-NMR glutamate isotopomer analysis of tissue extracts with measurements of oxygen consumption. Carbohydrate oxidation was markedly depressed after diabetes in the presence of low (0.1 mM) but not high (1.0 mM) palmitate concentration. Increasing exogenous palmitate concentration 10-fold resulted in a 7-fold increase in the contribution of palmitate to energy production in controls but only a 30% increase in the diabetic group. Consequently, at 0.1 mM palmitate, the rate of fatty acid oxidation was higher in the diabetic group than in controls; however, at 1.0 mM fatty acid oxidation, it was significantly depressed. Therefore, after 1 wk of diabetes, the major differences in carbohydrate and fatty acid metabolism occur primarily at low rather than high exogenous palmitate concentration.

Muscle contractile activity increases fatty acid metabolism and transport and FAT/CD36

We have examined whether 1) fatty acid (FA) uptake, 2) FA transporter expression, and 3) FA metabolism are increased when the oxidative capacity of skeletal muscle is increased. The oxidative capacities of red and white tibialis anterior and extensor digitorum longus muscles were increased via chronic stimulation (10 Hz, 24 h/day for 7 days). The contralateral muscles served as controls. After 7 days of increased muscle activity 1) palmitate uptake by giant sarcolemmal vesicles was increased twofold (P < 0.05), 2) the expression of FA translocase (FAT)/CD36 was increased at both the mRNA (3.2- to 10-fold) and protein (3.4-fold) levels, and 3) palmitate oxidation and esterification into triacylglycerols and phospholipids were increased 1.5-, 2.7-, and 1.7-fold, respectively (P < 0.05). These data show that when the oxidative capacity of muscle is increased, there is a parallel increase in the rate of FA transport and FA transporters at the sarcolemmal membrane, which is associated with the enhanced expression of the membrane transporter FAT/CD36.

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

It has been suggested that specific membrane-associated and cytoplasmic proteins cooperate in the uptake of long-chain fatty acids by cardiac and skeletal muscle cells. A prerequisite for this hypothesis would be the co-occurrence of these proteins in muscle. Thus, we studied the possible co-expression in rat muscles of the genes coding for the integral membrane proteins fatty acid transport protein (FATP) and fatty acid translocase (FAT), the membrane-associated plasmalemmal fatty acid-binding protein (FABPpm) and the cytoplasmic heart-type fatty acid-binding protein (H-FABPc). The transcripts of the four proteins were assessed in heart and skeletal muscles of adult Wistar rats, in isolated cells and cell lines from rat heart and also in rat heart during development and upon streptozotocin-induced diabetes. All four genes showed high expression levels in heart, somewhat lower in red skeletal muscle (soleus) and appreciably lower in white skeletal muscle (extensor digitorum longus). FATP, FAT and H-FABPc showed a 3- to 5-fold increase in mRNA expression during maturational growth of the heart, while the FABPpm expression remained virtually constant. In the heart, streptozotocin-diabetes induced a slight, but statistically not significant, increase in the expression of all four genes. In conclusion, this study shows the co-expression of FATP, FAT, FABPpm and H-FABPc in rat muscles. This finding supports the possible cooperation of these proteins in the uptake of long-chain fatty acids by muscle cells.

Muscular long-chain fatty acid content during graded exercise in humans

We measured the content of long-chain fatty acids (LCFA) in biopsies obtained from the vastus lateralis muscle in humans at rest and after different exercise intensities. Nine volunteers exercised at 65% of maximal oxygen uptake (VO2 max) for 40 min and at 90% of VO2 max for another 15 min on a Krogh bicycle ergometer. LCFA measured in muscle tissue averaged 76 +/- 5 nmol/g wet wt at rest, decreased significantly after exercise at 65% VO2 max to 48 +/- 4 nmol/g wet wt, and increased to 68 +/- 5 nmol/g wet wt (P < 0.05) after high-intensity exercise. The calculated myocyte LCFA content at rest amounted to 69 +/- 5 nmol/g wet wt, decreased by 43% (P < 0.05) after exercise at 65% of VO2 max, and subsequently increased by 54% after exercise at 90% of VO2 max (P < 0.05) compared with the values obtained at the lower workload. The blood plasma LCFA concentration during the low-intensity exercise (366 +/- 23 nmol/ml) was similar to the values obtained at rest (372 +/- 31 nmol/ml) but decreased significantly during the high-intensity workload (249 +/- 49 nmol/ml). From these data it is proposed that 1) in human skeletal muscle, metabolism rather than cellular availability of LCFA governs the rate of LCFA utilization at rest and during exercise, and 2) consequently reduction in muscle LCFA oxidation during high-intensity exercise (e.g., 90% VO2 max) is due primarily to a decrease in mitochondrial LCFA oxidation rate rather than an insufficient cellular availability of LCFA.

A region of the cellobiohydrolase I promoter from the filamentous fungus Trichoderma reesei mediates glucose repression in Saccharomyces cerevisiae, dependent on mitochondrial activity

The upstream activating region that controls cellulose-induced expression of the glucose-repressible cellobiohydrolase I gene (UARcb1) of the filamentous fungus Trichoderma reesei is shown to mediate transcription and glucose repression of a reporter gene in Saccharomyces cerevisiae, a unicellular microorganism that lacks the genes required for the utilization of cellulose. Glucose-controlled transcription mediated by UARcb1 requires the products of the genes SNF1 and SSN6, a protein kinase and a repressor, respectively, that regulate glucose-repressible yeast genes. Previously, it has been shown that mitochondrial function is implicated in cellobiohydrolase I gene expression in T. reesei and this sensitivity to the metabolic state of the mitochondria was shown to be transcriptionally controlled by the 5'-flanking sequence of the cbh1 gene [Abrahão-Neto et al. (1995) Biochemistry 34, 10456-10462]. Remarkably, transcription of the reporter gene controlled by UARcb1 in S. cerevisiae also showed a requirement for active mitochondria, suggesting that a common mechanism involving mitochondrial activity controls glucose-repressible genes in both microorganisms.

Expression and regulation of mRNA for putative fatty acid transport related proteins and fatty acyl CoA synthase in murine epidermis and cultured human keratinocytes

The epidermis has a requirement for fatty acids in order to synthesize cellular membranes and the extracellular lipid lamellar membranes in the stratum corneum. Despite high endogenous production of fatty acids the transport of exogenous essential fatty acids into the epidermis is an absolute requirement. Fatty acid uptake by keratinocytes has been shown to be mediated by a transport system. In this study we determined in murine epidermis and human cultured keratinocytes the expression of three putative fatty acid transport related proteins and fatty acyl CoA synthase, an enzyme that facilitates the uptake of fatty acids by promoting their metabolism. In cultured human keratinocytes, mRNA for fatty acid transport protein (FATP), plasma membrane fatty acid binding protein (FABP-pm), and fatty acyl CoA synthase (FACS) were detectable. Differentiation, induced by high calcium, did not affect FATP mRNA levels, but resulted in an approximately 50% increase in FACS mRNA, while decreasing FABP-pm mRNA by 50%. Fatty acid translocase (FAT) mRNA was not detected in cultured human keratinocytes. In murine epidermis, FATP, FABP-pm, FACS, and FAT mRNA were all present. Barrier disruption by either tape stripping or acetone treatment increased FAT mRNA levels by approximately 2-fold without affecting FATP, FABP-pm, or FACS. Occlusion with an impermeable membrane immediately after barrier disruption completely blocked the increase in FAT mRNA levels, indicating that this increase is related to barrier disruption rather than a nonspecific injury effect. In summary, this study demonstrates that several putative fatty acid transport related proteins as well as fatty acyl CoA synthase are expressed in keratinocytes and epidermis, and that the expression of these proteins may be regulated by differentiation and/ or barrier disruption.

Skeletal muscle fatty acid transport and transporters

Long-chain fatty acids (LCFAs) are an important energy source for many tissues. The dogma that LCFAs are freely diffusible has been challenged. It is now known that LCFAs are transported into many tissues. Our studies have shown that LCFAs are also transported into skeletal muscle and into the heart. In recent years a number of putative fatty acid transport proteins have been identified. These are known as plasma membrane fatty acid binding protein (FABPpm, 43 kDa), fatty acid translocase (FAT, 88 kDa) and fatty acid transporter protein (FATP, 63 kDa). All three proteins are present in skeletal muscle and in the heart. The existence of an LCFA transport system in muscle may be essential 1) to facilitate the rapid and regulatable transport of LCFA to meet the metabolic requirements of working muscles and 2) to cope with an increase in circulating LCFAs in some pathological conditions (e.g. diabetes). There is now some evidence that metabolic changes and chronically increased muscle activity can increase the transport of LCFAs and increase the expression of putative LCFA transporters.

Training and fatty acid metabolism

Older studies in humans seem to suggest a correlation between plasma long chain fatty acid (LCFA) turnover and oxidation on the one hand and plasma LCFA concentration on the other hand during submaximal exercise. However, recent studies in man, in which higher concentrations of plasma LCFA have been reached during prolonged submaximal exercise, have revealed a levelling off in net uptake in spite of increasing plasma LCFA concentrations. Furthermore, this relationship between plasma LCFA concentration and plasma LCFA uptake and oxidation was altered by endurance training such that levelling off was not apparent in the trained state. These recent findings in man give support to the notion from other cell types that transport of fatty acids from the vascular compartment to the cytosolic space in the muscle cell is not only due to simple diffusion, but is predominantly carrier-mediated. During prolonged submaximal knee-extension exercise it has been demonstrated that the total oxidation of LCFA was approximately 60% higher in trained compared to nontrained subjects. The training-induced adaptations responsible for this increased utilization of plasma fatty acids by the muscle could be located at several steps from the mobilization of fatty acids to skeletal muscle metabolism in the mitochondria. To what extent triacylglycerol located in the muscle cell contribute to the overall lipid utilisation during exercise is still not clear. However, due to underestimation of the contribution of plasma LCFA and fatty acids liberated from the circulating VLDL-triacylglycerols to the overall fatty acid oxidation during exercise there is increasing understanding that muscle triacylglycerol contributes to a lesser extent as fuel during exercise in man than mostly stated.

Characterization of the murine fatty acid transport protein gene and its insulin response sequence

Fatty acid transport protein (FATP) was identified by expression cloning strategies (Schaffer, J. E., and Lodish, H. F. (1994) Cell 79, 427-436) and shown by transfection analysis to catalyze the transfer of long-chain fatty acids across the plasma membrane of cells. It is expressed highly in tissues exhibiting rapid fatty acid metabolism such as skeletal muscle, heart, and adipose. FATP mRNA levels are down-regulated by insulin in cultured 3T3-L1 adipocytes and up-regulated by nutrient depletion in murine adipose tissue (Man, M. Z., Hui, T. Y., Schaffer, J. E., Lodish, H. F., and Bernlohr, D. A. (1996) Mol. Endocrinol. 10, 1021-1028). To determine the molecular mechanism of insulin regulation of FATP transcription, we have isolated the murine FATP gene and its 5'-flanking sequences. The FATP gene spans approximately 16 kilobases and contains 13 exons, of which exon 2 is alternatively spliced. S1 nuclease and RNase protection assays revealed the presence of multiple transcription start sites; the DNA sequence upstream of the predominant transcription start sites lacks a typical TATA box. By transient transfection assays in 3T3-L1 adipocytes, the inhibitory action of insulin on FATP transcription was localized to a cis-acting element with the sequence 5'-TGTTTTC-3' from -1347 to -1353. This sequence is very similar to the insulin response sequence found in the regulatory region of other genes negatively regulated by insulin such as those encoding phosphoenolpyruvate carboxykinase, tyrosine aminotransferase, and insulin-like growth factor-binding protein 1. Fluorescence in situ hybridization analysis revealed that the murine FATP gene is localized to chromosome 8, band 8B3.3. Interestingly, this region of chromosome 8 contains a cluster of three other genes important for fatty acid homeostasis, lipoprotein lipase, the mitochondrial uncoupling protein 1 (UCP1) and sterol regulatory element-binding protein 1. These results characterize the murine FATP gene and its insulin responsiveness as well as present a framework for future studies of its role in lipid metabolism, obesity, and type II diabetes mellitus.

Palmitate transport and fatty acid transporters in red and white muscles

We performed studies 1) to investigate the kinetics of palmitate transport into giant sarcolemmal vesicles, 2) to determine whether the transport capacity is greater in red muscles than in white muscles, and 3) to determine whether putative long-chain fatty acid (LCFA) transporters are more abundant in red than in white muscles. For these studies we used giant sarcolemmal vesicles, which contained cytoplasmic fatty acid binding protein (FABPc), an intravesicular fatty acid sink. Intravesicular FABPc concentrations were sufficiently high so as not to limit the uptake of palmitate under conditions of maximal palmitate uptake (i.e., 4.5-fold excess in white and 31.3-fold excess in red muscle vesicles). All of the palmitate taken up was recovered as unesterified palmitate. Palmitate uptake was reduced by phloretin (-50%), sulfo-N-succinimidyl oleate (-43%), anti-plasma membrane-bound FABP (FABPpm, -30%), trypsin (-45%), and when incubation temperature was lowered to 0 degrees C (-70%). Palmitate uptake was also reduced by excess oleate (-65%), but not by excess octanoate or by glucose. Kinetic studies showed that maximal transport was 1.8-fold greater in red vesicles than in white vesicles. The Michaelis-Menten constant in both types of vesicles was approximately 6 nM. Fatty acid transport protein mRNA and fatty acid translocase (FAT) mRNA were about fivefold greater in red muscles than in white muscles. FAT/CD36 and FABPpm proteins in red vesicles or in homogenates were greater than in white vesicles or homogenates (P < 0.05). These studies provide the first evidence of a protein-mediated LCFA transport system in skeletal muscle. In this tissue, palmitate transport rates are greater in red than in white muscles because more LCFA transporters are available.

Gustatory mechanisms for the detection of fat

Recent evidence suggests that free fatty acids may be one of the important stimuli used by taste receptor cells for the detection of fat. Consistent with this interpretation, the proteins necessary for the release and transport of lipophilic fatty acids are found in the oral cavity, and taste cells have recently been shown to contain fatty-acid-sensitive ion channels and transport molecules for the uptake of fatty acids.

Detection and cellular localization of plasma membrane-associated and cytoplasmic fatty acid-binding proteins in human placenta

The aim of this study was to investigate location and the types of membrane-associated and cytoplasmic fatty acid-binding proteins in human placental trophoblasts using monospecific polyclonal antibodies. Western blot analysis demonstrated the presence of multiple membrane and cytoplasmic fatty acid transport/binding proteins in human placenta. In addition to previously reported placental membrane fatty acid-binding (p-FABPpm, 40 kDa), fatty acid translocase (FAT, 88 kDa) and fatty acid transport protein (FATP, 62 kDa) were detected in both microvillous and basal membranes of the human placenta. Among the cytoplasmic proteins, heart (H) and liver (L) type FABP were detected in the cytosol of the human placental primary trophoblasts as well as in human placental choriocarcinoma (BeWo) cells. The immunoreactivity of epidermal type (E)-FABP was not detected in trophoblasts or BeWo cells despite its presence in human placental cytosol. Location of FAT and FATP on the both sides of the bipolar placental cells may favour transport of free fatty acids (FFA) pool in both directions i.e. from the mother to the fetus and vice versa. However, p-FABPpm, because of its exclusive location on the microvillous membranes, may favour the unidirectional flow of maternal plasma long-chain polyunsaturated fatty acids present in the FFA pool to the fetus, due to binding specificity for these fatty acids. Although the roles of these proteins in placental fatty acid uptake and metabolism are yet to be understood fully, their complex interaction may be involved in the uptake of maternal FFA by the placenta for delivery to the fetus.

Prostaglandins promote and block adipogenesis through opposing effects on peroxisome proliferator-activated receptor gamma

Fat cell differentiation is a critical aspect of obesity and diabetes. Dietary fatty acids are converted to arachidonic acid, which serves as precursor of prostaglandins (PGs). PGJ2 derivatives function as activating ligands for peroxisome proliferator-activated receptor gamma (PPAR gamma), a nuclear hormone receptor that is central to adipogenic determination. We report here that PGF2 alpha blocks adipogenesis through activation of mitogen-activated protein kinase, resulting in inhibitory phosphorylation of PPAR gamma. Both mitogen-activated protein kinase activation and PPAR gamma phosphorylation are required for the anti-adipogenic effects of PGF2 alpha. Thus, PG signals generated at a cell surface receptor regulate the program of gene expression required for adipogenesis by modulating the activity of a nuclear hormone receptor that is directly activated by other PG signals. The balance between PGF2 alpha and PGJ2 signaling may thus be central to the development of obesity and diabetes.