Changes in internal pH caused by movement of fatty acids into and out of clonal pancreatic beta-cells (HIT)

Advanced omics techniques shed light on CD1d-mediated lipid antigen presentation to iNKT cells

Invariant natural killer T cells (iNKT cells) can be activated through binding antigenic lipid/CD1d complexes to their TCR. Antigenic lipids are processed, loaded, and displayed in complex with CD1d by lipid antigen presenting cells (LAPCs). The mechanism of lipid antigen presentation via CD1d is highly conserved with recent work showing adipocytes are LAPCs that, besides having a role in lipid storage, can activate iNKT cells and play an important role in systemic metabolic disease. Recent studies shed light on parameters potentially dictating cytokine output and how obesity-associated metabolic disease may affect such parameters. By following a lipid antigen's journey, we identify five key areas which may dictate cytokine skew: co-stimulation, structural properties of the lipid antigen, stability of lipid antigen/CD1d complexes, intracellular and extracellular pH, and intracellular and extracellular lipid environment. Recent publications indicate that the combination of advanced omics-type approaches and machine learning may be a fruitful way to interconnect these 5 areas, with the ultimate goal to provide new insights for therapeutic exploration.

Strong relationship between cholesterol, low-density lipoprotein receptor, Na+/H+ exchanger, and SARS-COV-2: this association may be the cause of death in the patient with COVID-19

Lipids have a wide variety and vital functions. Lipids play roles in energy metabolism, intracellular and extracellular signal traffic, and transport of fat-soluble vitamins. Also, they form the structure of the cell membrane. SARS-CoV-2 interacts with lipids since its genetic material contains lipid-enveloped ribonucleic acid (RNA). Previous studies have shown that total cholesterol, high-density lipoprotein, and low-density lipoprotein (LDL) levels are lower in patients with severe novel coronavirus disease 2019 (COVID-19) compared to patients with non-severe COVID-19.Na⁺/H⁺ Exchanger (NHE) is an important antiport that keeps the intracellular pH value within physiological limits. When the intracellular pH falls, NHE is activated and pumps H⁺ ions outward. However, prolonged NHE activation causes cell damage and atherosclerosis. Prolonged NHE activation may increase susceptibility to SARS-CoV-2 infection and severity of COVID-19.In COVID-19, increased angiotensin II (Ang II) due to angiotensin-converting enzyme-2 (ACE2) dysfunction stimulates NHE. Lipids are in close association with the NHE pump. Prolonged NHE activity increases the influx of H⁺ ions and free fatty acid (FFA) inward. Ang II also causes increased low-density lipoprotein receptor (LDLR) levels by inhibiting proprotein convertase subtilisin/kexin type 9 (PCSK9). Thus, intracellular atheroma plaque formation is accelerated.Besides, SARS-CoV-2 may replicate more rapidly as intracellular cholesterol increases. SARS-CoV-2 swiftly infects the cell whose intracellular pH decreases with NHE activation and FFA movement. Novel treatment regimens based on NHE and lipids should be explored for the treatment of COVID-19.

SSO and other putative inhibitors of FA transport across membranes by CD36 disrupt intracellular metabolism, but do not affect FA translocation

Membrane-bound proteins have been proposed to mediate the transport of long-chain FA (LCFA) transport through the plasma membrane (PM). These proposals are based largely on reports that PM transport of LCFAs can be blocked by a number of enzymes and purported inhibitors of LCFA transport. Here, using the ratiometric pH indicator (2',7'-bis-(2-carboxyethyl)-5-(and-6-)-carboxyfluorescein and acrylodated intestinal FA-binding protein-based dual fluorescence assays, we investigated the effects of nine inhibitors of the putative FA transporter protein CD36 on the binding and transmembrane movement of LCFAs. We particularly focused on sulfosuccinimidyl oleate (SSO), reported to be a competitive inhibitor of CD36-mediated LCFA transport. Using these assays in adipocytes and inhibitor-treated protein-free lipid vesicles, we demonstrate that rapid LCFA transport across model and biological membranes remains unchanged in the presence of these purported inhibitors. We have previously shown in live cells that CD36 does not accelerate the transport of unesterified LCFAs across the PM. Our present experiments indicated disruption of LCFA metabolism inside the cell within minutes upon treatment with many of the "inhibitors" previously assumed to inhibit LCFA transport across the PM. Furthermore, using confocal microscopy and a specific anti-SSO antibody, we found that numerous intracellular and PM-bound proteins are SSO-modified in addition to CD36. Our results support the hypothesis that LCFAs diffuse rapidly across biological membranes and do not require an active protein transporter for their transmembrane movement.

Effects of Dietary Fatty Acids in Pancreatic Beta Cell Metabolism, Implications in Homeostasis

Fatty acids are involved in several metabolic processes, including the development of metabolic and cardiovascular diseases. In recent years a disease that has received escalated interest is type 2 diabetes (T2D). Many contributing factors including a high-caloric diet rich in dietary saturated fats have been broadly characterized as triggers of T2D. Insulin resistance resulting from a high saturated fat diet leads to alterations in lipid cellular intake and accumulation which generate lipotoxic conditions, a key phenomenon in the metabolism of β-cells. Alternatively, unsaturated fatty acids have been described to show opposite effects in pancreatic β-cells. The purpose of this work is to perform a critical analysis of the complex role of saturated and unsaturated fatty acids in β-cell metabolism. We discuss the diverse effects main dietary fatty acids have upon pancreatic β-cell metabolism as a key factor to maintain homeostasis by focusing in the cellular and molecular mechanisms involved in the development and progression of T2D. For instance, modifications in protein homeostasis as well as the intracellular management of lipid metabolism which are associated with inflammatory pathways. These conditions initiate critical metabolic rearrangements, that in turn have repercussions on insulin β-cell metabolism. This review allows an integral and broad understanding of different functions of fatty acids inside β-cells, being important metabolites for novel therapeutic targets in T2D treatment.

Cardiac inotropy, lusitropy, and Ca2+ handling with major metabolic substrates in rat heart

Fatty acid (FA)-dependent oxidation is the predominant process for energy supply in normal heart. Impaired FA metabolism and metabolic insufficiency underlie the failing of the myocardium. So far, FA metabolism in normal cardiac physiology and heart failure remains undetermined. Here, we evaluate the mechanisms of FA and major metabolic substrates (termed NF) on the contraction, relaxation, and Ca²⁺ handling in rat left ventricular (LV) myocytes. Our results showed that NF significantly increased myocyte contraction and facilitated relaxation. Moreover, NF increased the amplitudes of diastolic and systolic Ca²⁺ transients ([Ca²⁺]_i), abbreviated time constant of [Ca²⁺]_i decay (tau), and prolonged the peak duration of [Ca²⁺]_i. Whole-cell patch-clamp experiments revealed that NF increased Ca²⁺ influx via L-type Ca²⁺ channels (LTCC, I_Ca-integral) and prolonged the action potential duration (APD). Further analysis revealed that NF shifted the relaxation phase of sarcomere lengthening vs. [Ca²⁺]_i trajectory to the right and increased [Ca²⁺]_i for 50 % of sarcomere relengthening (EC₅₀), suggesting myofilament Ca²⁺ desensitization. Butanedione monoxime (BDM), a myosin ATPase inhibitor that reduces myofilament Ca²⁺ sensitivity, abolished the NF-induced enhancement of [Ca²⁺]_i amplitude and the tau of [Ca²⁺]_i decay, indicating the association of myofilament Ca²⁺ desensitization with the changes in [Ca²⁺]_i profile in NF. NF reduced intracellular pH ([pH_i]). Increasing [pH]_i buffer capacity with HCO₃/CO₂ attenuated Δ [pH]_i and reversed myofilament Ca²⁺ desensitization and Ca²⁺ handling in NF. Collectively, greater Ca²⁺ influx through LTCCs and myofilament Ca²⁺ desensitization, via reducing [pH]_i, are likely responsible for the positive inotropic and lusitropic effects of NF. Computer simulation recapitulated the effects of NF.

Effects of human serum albumin complexed with free fatty acids on cell viability and insulin secretion in the hamster pancreatic β-cell line HIT-T15

AIMS

The effects of human serum albumin (HSA) complexed with various free fatty acids (FFAs) on ß-cells have not been studied in detail. In this study, we examined the effects of HSA and its mutants on FFA-induced cell viability changes and insulin secretion from the hamster pancreatic insulinoma cell line, HIT-TI5.

MAIN METHODS

Cells were exposed to different FFAs in the presence of HSA or its mutants and/or bovine serum albumin (BSA) for 24h. Cell viability, apoptosis, insulin secretion, and unbound FFA (FFA(u)) levels were determined.

KEY FINDINGS

In the presence of 0.1mM HSA, palmitate and stearate induced significant cell death at 0.1mM or higher, whereas myristate, palmitoleate, oleate, elaidate, linoleate, linoelaidate, and conjugated linoleate showed minimal changes on cell viability. Furthermore, oleate and linoleate were clearly cytoprotective against palmitate-induced cell death. The apoptosis inhibitors, cyclosporin A (csA) and the caspase inhibitor ZVAD-FMK, did not completely prevent FFA-induced cell death, although ZVAD-FMK blocked apoptosis with no differences in the presence of either HSA or BSA. In addition, insulin secretion from the cells was significantly reduced in the presence of HSA/oleate complexes. We also found differential effects of HSA mutants complexed with FFAs on cell viability.

SIGNIFICANCE

In summary, our results showed that saturated FFAs induced more cell death than unsaturated FFAs. Furthermore, modified HSA/FFA interactions caused by mutations of key amino acids involved in the binding of FFA to HSA resulted in changes in cell viability, suggesting a possible role of HSA polymorphism on FFA-induced changes in cellular functions.

Measuring the adsorption of Fatty acids to phospholipid vesicles by multiple fluorescence probes

Fatty acids (FA) are important nutrients that the body uses to regulate the storage and use of energy resources. The predominant mechanism by which long-chain fatty acids enter cells is still debated widely as it is unclear whether long-chain fatty acids require protein transporters to catalyze their transmembrane movement. We use stopped-flow fluorescence (millisecond time resolution) with three fluorescent probes to monitor different aspects of FA binding to phospholipid vesicles. In addition to acrylodan-labeled fatty acid binding protein, a probe that detects unbound FA in equilibrium with the lipid bilayer, and cis-parinaric acid, which detects the insertion of the FA acyl chain into the membrane, we introduce fluorescein-labeled phosphatidylethanolamine as a new probe to measure the binding of FA anions to the outer membrane leaflet. We combined these three approaches with measurement of intravesicular pH to show very fast FA binding and translocation in the same experiment. We validated quantitative predictions of our flip-flop model by measuring the number of H(+) delivered across the membrane by a single dose of FA with the probe 6-methoxy-N-(3-sulfopropyl) quinolinium. These studies provide a framework and basis for evaluation of the potential roles of proteins in binding and transport of FA in biological membranes.

Use of a pH-sensitive fluorescent probe for measuring intracellular pH of Caco-2 cells

This paper describes the application of a pH-sensitive fluorescent probe [2',7'-bis(2-carboxylethyl)-5(6)-carboxyfluorescein or BCECF] to measure intracellular pH (pH(i)) changes in Caco-2 cells. As a function of BCECF's ionization, the fluorescence was monitored at lambda(ex)=440 and 503nm, and lambda(em)=535nm. Time course studies were conducted with the addition of two weak acid delivery agents, one weak base delivery agent, oleic acid, or tetradecylamine. When applicable, 10microM bovine serum albumin or 10mM ammonium chloride was added into the cell suspension to hinder the pH gradient effect. Adding a weak acid at 2, 10, or 50mM to the cell suspension, the pH(i) dropped substantially from 7.4 to 7.1, 6.9, or 6.7, respectively. The pH(i) then increased gradually over a 10-min period but did not return to its initial value. Conversely, the pH(i) increased instantaneously after the addition of a weak base. When Caco-2 cells were placed in solutions with different bulk pH (7.0, 7.5, and 8.0), the lower the pH in which the cells were exposed, the larger the pH(i) drop occurred with the addition of an acid. The results suggest that these weak acids or bases are transported transcellularly across Caco-2 cells.

Fatty acid transport and metabolism in HepG2 cells

The mechanism(s) of fatty acid uptake by liver cells is not fully understood. We applied new approaches to address long-standing controversies of fatty acid uptake and to distinguish diffusion and protein-based mechanisms. Using HepG2 cells containing an entrapped pH-sensing fluorescence dye, we showed that the addition of oleate (unbound or bound to cyclodextrin) to the external buffer caused a rapid (seconds) and dose-dependent decrease in intracellular pH (pH(in)), indicating diffusion of fatty acids across the plasma membrane. pH(in) returned to its initial value with a time course (in min) that paralleled the metabolism of radiolabeled oleate. Preincubation of cells with the inhibitors phloretin or triacsin C had no effect on the rapid pH(in) drop after the addition of oleate but greatly suppressed pH(in) recovery. Using radiolabeled oleate, we showed that its esterification was almost completely inhibited by phloretin or triacsin C, supporting the correlation between pH(in) recovery and metabolism. We then used a dual-fluorescence assay to study the interaction between HepG2 cells and cis-parinaric acid (PA), a naturally fluorescent but slowly metabolized fatty acid. The fluorescence of PA increased rapidly upon its addition to cells, indicating rapid binding to the plasma membrane; pH(in) decreased rapidly and simultaneously but did not recover within 5 min. Phloretin had no effect on the PA-mediated pH(in) drop or its slow recovery but decreased the absolute fluorescence of membrane-bound PA. Our results show that natural fatty acids rapidly bind to, and diffuse through, the plasma membrane without hindrance by metabolic inhibitors or by an inhibitor of putative membrane-bound fatty acid transporters.

Acyl coenzyme a synthetase regulation: putative role in long-chain acyl coenzyme a partitioning

OBJECTIVE

Long-chain acyl coenzyme A synthetase (ACSL) converts free fatty acids (FFAs) into their metabolizable long-chain acyl coenzyme A (LC-CoA) derivatives that are essential for FFA conversion to CO(2), triglycerides, or complex lipids. ACSL-1 is highly expressed in adipose tissue with broad substrate specificity. We tested the hypothesis that ACSL localization, and resulting local generation of LC-CoA, regulates FFA partitioning.

RESEARCH METHODS AND PROCEDURES

These studies used cell fractionation of rat adipocytes to measure ACSL activity and mass and compared cells from young, mature, fed, fasted, and diabetic rats. Functional studies included measurement of FFA oxidation, complex lipid synthesis, and LC-CoA levels.

RESULTS

High ACSL specific activity was expressed in the mitochondria/nuclei (M/N), high-density microsomes (HDM), low-density microsomes (LDM), and plasma membrane (PM) fractions. We show here that, during fasting, total FFA oxidation increased, and, although total ACSL activity decreased, a greater percentage of activity (43 +/- 1.5%) was associated with the M/N fraction than in the fed state (23 +/- 0.3%). In the fed state, more ACSL activity (34 +/- 0.5%) was associated with the HDM than in the fasted state (25 +/- 0.9%), concurrent with increased triglyceride formation from FFA. Insulin increased LC-CoA and ACSL activity associated with the PM. The changes in ACSL activity in response to insulin were associated with only minor changes in mass as determined by Western blotting.

DISCUSSION

It is hypothesized that ACSL plays an important role in targeting FFA to specific metabolic pathways or acylation sites in the cell, thus acting as an important control mechanism in fuel partitioning. Localization of ACSL at the PM may serve to decrease FFA efflux and trap FFA within the cell as LC-CoA.

Transport of 13C-oleate in adipocytes measured using multi imaging mass spectrometry

The mechanism of long chain free fatty acid (FFA) transport across cell membranes is under active investigation. Here we describe the use of multi imaging mass spectrometry (MIMS) to monitor intracellular concentrations of FFA and provide new insight into FFA transport in cultured adipocytes. Cells were incubated with 13C-oleate:BSA and either dried directly or dried after washing with a medium deprived of 13C-oleate:BSA. Cells were analyzed with MIMS using a scanning primary Cs+ ion beam and 12C-, 13C-, 12C14N-, 13C14N-) (or 12C 15N-) were imaged simultaneously. From these quantitative images the values of the 13C/ 12C ratios were determined in the intracellular lipid droplets, in the cytoplasm and outside the 3T3F442A adipocytes. The results indicate that after incubation with 13C-oleate:BSA the droplet 13C/ 12C ratio was 15 +/- 6%. This value is about 14-fold higher than the 13C/ 12C terrestrial ratio (1.12%). After washing the 13C-oleate:BSA, the droplet 13C/ 12C ratios decreased to 1.6 +/- 0.1%, about 40% greater than the natural abundance. Results for washed cells indicate that relatively little FFA was esterified. The unwashed cell results, together with the value of the lipid water partition coefficient, reveal that intracellular unbound FFA (FFAu) concentrations were on average about 4.5-fold greater than the extracellular FFAu concentrations. These results are consistent with the possibility that FFA may be pumped into adipocytes against their electro-chemical potential. This work demonstrates that MIMS can be used to image and quantitate stable isotope labeled fatty acid in intracellular lipid droplets.

Alterations of fatty acid metabolism and membrane fluidity in peroxisome-defective mutant ZP102 cells

We investigated lipid composition and FA metabolism in Chinese hamster ovary CHO-K1) cells and Pex5-mutated CHO-K1 (ZP102) cells to clarify the biochemical bases of peroxisome biogenesis disorders (PBD). ZP102 cells have defective peroxisomes and exhibit impairments of peroxisomal beta-oxidation of FA and plasmalogen biosynthesis. In addition, we identified FA metabolic alterations in the synthesis of several classes of lipids in ZP102 cells. The concentration of FFA in ZP102 cells was twice that in CHO-K1 cells, but methyl esters and TAG were decreased in ZP102 cells in comparison with control cells. Also, ceramide monohexoside (CMH) concentration with ZP102 cells was significantly increased compared with the control cells. The FA molecular species, particularly the saturated to unsaturated ratios, of individual lipids also differed between the two cell types. The rate of incorporation of [14C]-labeled saturated acids into sphingomyelin (SM) and CMH in ZP102 cells was higher than that in CHO-K1 cells. Lignoceric acid incorporated into cells was predominantly utilized for the synthesis of SM at 24 h after removal of [14C]lignoceric acid from the culture medium. ZP102 cells showed higher fluorescence anisotropy of 1,3,5-diphenylhexatriene, corresponding to lower membrane mobility than in CHO-K1 cells. In particular, alteration of lipid metabolism by a Pex5 mutation enhanced metabolism of saturated FA and sphingolipids. This may be related to the reduced membrane fluidity of ZP102 cells, which has been implicated in the dysfunction of membrane-linked processes in PBD.

Sulfonylureas rapidly cross phospholipid bilayer membranes by a free-diffusion mechanism

Because sulfonylureas directly activate the exocytotic machinery, we were interested in the extent to which these compounds penetrate the beta-cell plasma membrane and the underlying molecular mechanism(s). We now provide evidence that sulfonylureas cross phospholipid bilayer membranes rapidly and effectively by a free-diffusion mechanism. Two sulfonylurea compounds investigated by 1H nuclear magnetic resonance spectroscopy, glibenclamide and tolbutamide, were found to incorporate into phospholipid bilayers, with the ionizable sulfonamide exposed to the aqueous interface and its apparent dissociation constant (pKa) increased to approximately 7.0. Diffusion of weak amphiphilic acids across membranes is associated with a measurable change in pH. Thus, by using a fluorescence-based pH assay, we could investigate the diffusion of sulfonylurea compounds across phospholipid bilayer membranes. A fluorescent pH indicator (pyranin or [2',7'-bis (2-carboxyethyl)-5(6)-carboxyfluorescein] [BCECF]) was trapped in egg phosphatidylcholine vesicles. Addition of glibenclamide decreased internal pH (pHin), and addition of albumin reversed this drop by 50%. With the same amount of tolbutamide, the decrease in pHin was much smaller, primarily because of the lower partitioning of tolbutamide into phospholipid bilayers. Using similar protocols, we also demonstrated diffusion by the same mechanism across the beta-cell plasma membrane. Thus, we now provide a molecular mechanism by which sulfonylureas can penetrate the plasma membrane and reach intracellular sites regulating exocytosis.

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.

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

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

Pleiotropic effects of fatty acids on pancreatic beta-cells

Hyperlipidemia is frequently associated with insulin resistance states as found in type 2 diabetes and obesity. Effects of free fatty acids (FFA) on pancreatic beta-cells have long been recognized. Acute exposure of the pancreatic beta-cell to FFA results in an increase of insulin release, whereas a chronic exposure results in desensitization and suppression of secretion. We recently showed that palmitate augments insulin release in the presence of non-stimulatory concentrations of glucose. Reduction of plasma FFA levels in fasted rats or humans severely impairs glucose-induced insulin release. These results imply that physiological plasma levels of FFA are important for beta-cell function. Although, it has been accepted that fatty acid oxidation is necessary for its stimulation of insulin secretion, the possible mechanisms by which fatty acids (FA) affect insulin secretion are discussed in this review. Long-chain acyl-CoA (LC-CoA) controls several aspects of the beta-cell function including activation of certain types of protein kinase C (PKC), modulation of ion channels, protein acylation, ceramide- and/or nitric oxide (NO)-mediated apoptosis, and binding to nuclear transcriptional factors. The present review also describes the possible effects of FA on insulin signaling. We showed for the first time that acute exposure of islets to palmitate upregulates the intracellular insulin-signaling pathway in pancreatic islets. Another aspect considered in this review is the source of FA for pancreatic islets. In addition to be exported to the medium, lipids can be transferred from leukocytes (macrophages) to pancreatic islets in co-culture. This process consists an additional source of FA that may plays a significant role to regulate insulin secretion.

Evidence in favor of a facilitated transport system for FA uptake in cultured L6 cells

In this manuscript we report a study of the transport of FA in L6 muscle cells. Cultured L6 cells took up labeled FA (C10 to C20) as a linear function of time up to 15 min. Thereafter, the rate of uptake gradually declined although it persisted for at least 12 h after the addition of the substrate. Kinetic parameters (Km, Vm, and k(o)) were determined from a fitted Michaelis-Menten-type equation modified by a term for a saturable (linear) component of the measured total uptake. Vm values were different for some of the FA studied, and Km data showed significant differences between saturated and unsaturated FA. The maximal rate of uptake was observed at pH 7.40 for decanoate, palmitate, and eicosatrienoate. Uptake was significantly influenced when the pH of the incubation medium was changed. Experiments designed to study the influence of FA/albumin molar ratio indicated that Vm was dependent on the total (bound and free) concentration of the FA. A concentrative uptake was demonstrated in short-term experiments with an apparent plateau of 20 and 40 microM for palmitate and eicosatrienoate, respectively. A competitive inhibition was also observed between palmitate as substrate and the other FA. From our results we can postulate that the uptake of FA in L6 cells is the sum of passive diffusion plus a saturable component and that the rate of uptake is dependent on one (or more) protein structures, although their precise characteristics and functions remain to be elucidated.

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.

Arachidonic acid-induced H+ and Ca2+ increases in both the cytoplasm and nucleoplasm of rat cerebellar granule cells

1. Arachidonic acid (AA) exerts multiple physiological and pathophysiological effects in the brain. By continuously measuring the intracellular pH (pH(i)) and Ca2+ levels ([Ca2+]i) in primary cultured rat cerebellar granule cells, we have found, for the first time, that 20 min treatment with 10 microM AA resulted in marked increases in Ca2+ and H+ levels in both the cytosol and nucleus. 2. A much higher concentration (40 mM) of another weak acid, propionic acid, was needed to induce a similar change in pH(i). The [Ca2+]i increase was probably caused by AA-induced activation of Ni2+-sensitive cationic channels, but did not involve NMDA channels or the Na+-Ca2+ exchanger. 3. AA-induced acidosis occurs by a different mechanism involving predominantly the passive diffusion of the un-ionized form of AA, rather than a protein carrier, as proposed by Kamp & Hamilton for fatty acids (FAs) in artificial phospholipid bilayers (the 'flip-flop' model). The following results, which are similar to those observed in lipid bilayers, support this conclusion: (1) FAs containing a -COOH group (AA, linoleic acid, alpha-linolenic acid, and docosahexaenoic acid) induced intracellular acidosis, whereas a FA with a -COOCH3 group (AA methyl ester) had little effect on pH(i), (2) a FA amine, tetradecylamine, induced intracellular alkalosis, and (3) the AA-/FA-induced pH(i) changes were reversed by bovine serum albumin. 4. Further evidence in support of a passive diffusion model, rather than a membrane protein carrier, is that: (1) there was a linear relationship between the initial rate of acid flux and the concentration of AA (2-100 microM), (2) acidosis was not inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid, a potent inhibitor of the plasma membrane FA carrier protein, and (3) the involvement of most known H+-related membrane carriers and H+ conductance has been ruled out. 5. Since AA can be released under both physiological and pathophysiological conditions, the possible significance of the AA-evoked increases in H+ and Ca2+ in both the cytoplasm and nucleoplasm is discussed.

Saturated FFAs, palmitic acid and stearic acid, induce apoptosis in human granulosa cells

Obesity is associated with insulin resistance and some reproductive abnormalities. Circulating FFAs are often elevated in obese subjects and are also closely linked to insulin resistance. In this study, we demonstrated that saturated FFAs, such as palmitic acid and stearic acid, markedly suppressed the granulosa cell survival in a time- and dose-dependent manner. Polyunsaturated FFA, arachidonic acid, had no effect on the cell survival, even at supraphysiological concentrations. The suppressive effect of saturated FFAs on cell survival was caused by apoptosis, as evidenced by DNA ladder formation and annexin V-EGFP/propidium iodide staining of the cells. The apoptotic effects of palmitic acid and stearic acid were unrelated to the increase of ceramide generation or nitric oxide production and were also completely blocked by Triacsin C, an inhibitor of acylcoenzyme A synthetase. In addition, acylcoenzyme A, pamitoylcoenzyme A, and stearylcoenzyme A markedly suppressed granulosa cell survival, whereas arachidonoylcoenzyme A had no such effect, and this finding was consistent with the effect of the respective FFA form. Surprisingly, arachidonic acid instead showed a protective effect on palmitic acid- and stearic acid-induced cell apoptosis. A Western blot analysis showed the apoptosis of the granulosa cells induced by palmitic acid to be accompanied by the down-regulation of an apoptosis inhibitor, Bcl-2, and the up-regulation of an apoptosis effector, Bax. These results indicate that saturated FFAs induce apoptosis in human granulosa cells caused by the metabolism of the respective acylcoenzyme A form, and the actual composition of circulating FFAs may thus play a critical role in the apoptotic events of human granulosa cells. These effects of FFAs on granulosa cell survival may be a possible mechanism for reproductive abnormalities, such as amenorrhea, which is frequently observed in obese women.

Fat depot origin affects fatty acid handling in cultured rat and human preadipocytes

Regional differences in free fatty acid (FFA) handling contribute to diseases associated with particular fat distributions. As cultured rat preadipocytes became differentiated, FFA transfer into preadipocytes increased and was more rapid in single perirenal than in epididymal cells matched for lipid content. Uptake by human omental preadipocytes was greater than uptake by abdominal subcutaneous preadipocytes. Adipose-specific fatty acid binding protein (aP2) and keratinocyte lipid binding protein abundance was higher in differentiated rat perirenal than in epididymal preadipocytes. This interdepot difference in preadipocyte aP2 expression was reflected in fat tissue in older animals. Carnitine palmitoyltransferase 1 activity increased during differentiation and was higher in perirenal than in epididymal preadipocytes, particularly the muscle isoform. Long-chain acyl-CoA levels were higher in perirenal than in epididymal preadipocytes and isolated fat cells. These data are consistent with interdepot differences in fatty acid flux ensuing from differences in fatty acid binding proteins and enzymes of fat metabolism. Heterogeneity among depots results, in part, from distinct intrinsic characteristics of adipose cells. Different depots are effectively separate miniorgans.

Glucagon-like peptide 1 stimulates lipolysis in clonal pancreatic beta-cells (HIT)

Glucagon-like peptide 1 (GLP-1) is the most potent physiological incretin for insulin secretion from the pancreatic beta-cell, but its mechanism of action has not been established. It interacts with specific cell-surface receptors, generates cAMP, and thereby activates protein kinase A (PKA). Many changes in pancreatic beta-cell function have been attributed to PKA activation, but the contribution of each one to the secretory response is unknown. We show here for the first time that GLP-1 rapidly released free fatty acids (FFAs) from cellular stores, thereby lowering intracellular pH (pHi) and stimulating FFA oxidation in clonal beta-cells (HIT). Similar changes were observed with forskolin, suggesting that stimulation of lipolysis was a function of PKA activation in beta-cells. Triacsin C, which inhibits the conversion of FFAs to long-chain acyl CoA (LC-CoA), enhanced basal FFA efflux as well as GLP-1-induced acidification and efflux of FFAs from the cell. Increasing the concentration of the lipase inhibitor orlistat progressively and largely diminished the increment in secretion caused by forskolin. However, glucose-stimulated secretion was less inhibited by orlistat and only at the highest concentration tested. Because the acute addition of FFAs also increases glucose-stimulated insulin secretion, these data suggest that the incretin function of GLP-1 may involve a major role for lipolysis in cAMP-mediated potentiation of secretion.

Acute stimulation with long chain acyl-CoA enhances exocytosis in insulin-secreting cells (HIT T-15 and NMRI beta-cells)

Non-insulin-dependent diabetes mellitus is associated with, in addition to impaired insulin release, elevated levels of free fatty acids (FFA) in the blood. Insulin release is stimulated when beta-cells are acutely exposed to FFA, whereas chronic exposure may inhibit glucose-induced insulin secretion. In the present study we investigated the direct effects of long chain acyl-CoA (LC-CoA), the active intracellular form of FFA, on insulin exocytosis. Palmitoyl-CoA stimulated both insulin release from streptolysin-O-permeabilized HIT cells and fusion of secretory granules to the plasma membrane of mouse pancreatic beta-cells, as measured by cell capacitance. The LC-CoA effect was chain length-dependent, requiring chain lengths of at least 14 carbons. LC-CoA needed to be present to stimulate insulin release, and consequently there was no effect following its removal. The stimulatory effect was observed after inhibition of protein kinase activity and in the absence of ATP, even though both kinases and ATP, themselves, modulate exocytosis. The effect of LC-CoA was inhibited by cerulenin, which has been shown to block protein acylation. The data suggest that altered LC-CoA levels, resulting from FFA or glucose metabolism, may act directly on the exocytotic machinery to stimulate insulin release by a mechanism involving LC-CoA protein binding.

Changes of intrinsic membrane potentials induced by flip-flop of long-chain fatty acids

The passive transbilayer movement-flip-flop-was investigated on planar bilayer lipid membranes (BLMs), containing myristic, stearic, or linoleic long-chain fatty acids (FA). In response to a transbilayer pH gradient, a difference in the surface charges between inner and outer leaflets appeared. Because the BLM was formed from FA and neutral lipid, a surface potential difference was originated solely by a concentration difference of the initially equally distributed ionized FA. As revealed by zeta-potential measurements, the corresponding surface potential difference DeltaPhi(s) was at least twice the value expected from a titration of the FA alone. The additional surface charge was attributed to FA flip-flop induced by the transbilayer pH gradient. DeltaPhi(s) was derived from capacitive current measurements carried out with a direct current (dc) bias and was corrected for changes of membrane dipole potential Phi(d). Dual-wavelength ratiometric fluorescence measurements have shown that Phi(d) values of the pure DPhPC bilayers and BLMs containing 40 mol % FA differ by less than 6%. It is concluded that fast FA flip-flop is not restricted to membranes with high curvature. The role of pH gradient as an effective driving force for the regulation of FA uptake is discussed.

Possible mechanism(s) of arachidonic acid-induced intracellular acidosis in rat cardiac myocytes

Arachidonic acid (AA) and other nonesterified fatty acids (FAs) have been shown to exert harmful effects during cardiac ischemia. By continuously measuring intracellular pH (pH(i)) changes in neonatal and adult cardiac myocytes, we have found, for the first time, that 10 micromol/L AA induces a substantial intracellular acidosis (0.3 to 0.4 pH units). We have ruled out the possibilities that the AA-induced acidosis is caused by (1) inhibition or stimulation of the pH(i) regulators, (2) protein kinase C activation or the generation of AA metabolites or free radicals, or (3) activation of NADPH oxidase or an inward H(+) current. The AA-induced acidosis fits to a simple diffusion mechanism, as proposed by Kamp and Hamilton (flip-flop model) for artificial phospholipid bilayers. The important properties found in the cardiac myocyte are that (1) the initial rate of acid flux (J(H)) increases with the AA concentration (2 to 50 micromol/L), (2) FAs with a (-)COOH group (eg, AA, oleic acid, and linoleic acid) induce intracellular acidification, but FAs with a (-)COOCH(3) group (eg, AA methyl ester) have little effect on the pH(i), (3) tetradecylamine (FA amine) induces intracellular alkalosis, and, most importantly, (4) both the AA- and tetradecylamine-induced pH(i) changes can be reversed by 0.3% BSA. Because a low concentration of AA (10 micromol/L) can induce a substantial acidosis, the possible involvement of the FA-evoked acidosis in the negative inotropic effect during cardiac ischemia is discussed. The full text of this article is available at http://www. circresaha.org.

Thioglycolate-elicited rat macrophages exhibit alterations in incorporation and oxidation of fatty acids

Incorporation and oxidation of fatty acids (FA) were investigated in resident and thioglycolate-elicited (TG-elicited) rat macrophages (Mphi). Both cell types presented a time-dependent incorporation of [14C]-labeled palmitic acid (PA), oleic acid (OA), linoleic acid (LA), and arachidonic acid (AA) up to 6 h. The total amount of [14C]-FA incorporated by resident Mphi after 6 h was: AA > PA = LA > OA. TG-elicited cells presented a 50% reduction in the incorporation of LA, PA, and AA, whereas that of OA remained unchanged as compared to resident Mphi. The FA were oxidized by resident Mphi as follows: LA > OA > PA > AA. TG elicitation promoted a reduction of 42% in LA oxidation and a marked increase in AA oxidation (280%). The increased oxidation of AA in TG-elicited cells may account for the lower production of prostaglandins in Mphi under these conditions. The full significance of these findings for Mphi function, however, remains to be examined.

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.

Transport of fatty acids across membranes by the diffusion mechanism

In early research on fatty acid transport, passive diffusion seemed to provide an adequate explanation for movement of fatty acids through the membrane bilayer. This simple hypothesis was later challenged by the discovery of several proteins that appeared to be membrane-related fatty acid transporters. In addition, some biophysical studies suggested that fatty acids moved slowly through the simple model membranes (phospholipid bilayers), which would provide a rationale for protein-assisted transport. Furthermore, it was difficult to rationalize how fatty acids could diffuse passively across the bilayer as anions. Newer studies have shown that fatty acids are present in membranes in the un-ionized as well as the ionized form, and that the un-ionized form can cross a protein-free phospholipid bilayer quickly. This flip-flop mechanism has been validated in cells by intracellular pH measurements. The role of putative fatty acid transport proteins remains to be clarified.

Oleic Acid Increases Cell Surface Expression and Activity of CD11b on Human Neutrophils

Traumatic bone injury frequently results in the release of marrow-derived fatty material into the circulation. This may lead to the syndrome of fat embolism, associated with the generation of free fatty acids, the sequestration of neutrophils in the lungs, and the subsequent development of acute respiratory distress. Neutrophil accumulation in tissues requires their adherence to vascular endothelial cells and involves the β2 integrin, CD11b/CD18 (Mac-1). We now report that the exposure of isolated human neutrophils to oleic acid causes a rapid increase in the cell surface expression and affinity state of CD11b, particularly under acidic conditions that are typical of inflammatory sites. Oleic acid also triggers neutrophil aggregation and neutrophil adherence to both fibrinogen-coated surfaces and confluent cultures of HUVEC. These processes are blocked by CD11b-specific inhibitors, including neutrophil-inhibitory factor and mAbs to CD11b. These observations may help explain the etiology of so-called fat embolism wherein trauma-induced release of fatty material causes pulmonary neutrophil accumulation and the development of acute respiratory distress.

Binding of [3H]palmitate to BSA

Determination of the BSA-palmitate high-affinity binding constant (Ka) traditionally relied on the heptane-water partitioning technique. We used this technique to calculate Ka for the BSA-[3H]palmitate complex, to determine if Ka was independent of protein concentration, and to determine if the unbound [3H]palmitate concentration is constant at different BSA concentrations using constant BSA-to-palmitate molar ratios (range 1:1 to 1:4). After extensive extraction of non-[3H]palmitate radiolabeled substances, the heptane-to-buffer partition ratio, in the absence of BSA, was 702 +/- 19 (mean +/- SD, n = 6). This value was much lower than the predicted value of 1,376 and was highly dependent on which phase (organic or aqueous) initially contained the [3H]palmitic acid. The data were consistent with the notion of self-association of [3H]palmitate in the aqueous phase. Ka for the BSA-[3H]palmitate complex was determined to be similar (2.2 +/- 0.1) x 10(8) M-1 (mean +/- SD, P > 0.05) at all BSA concentrations studied. At each BSA-to-palmitate molar ratio, the equilibrium unbound ligand concentration was constant only at low BSA concentrations (<10 microM) and at low BSA-to-palmitate molar ratios (i.e., 1:1 and 1:2). At higher BSA concentrations and molar ratios, the unbound ligand concentration increased with an increase in protein concentration. Hepatocyte uptake using the manufacturer-supplied radiolabeled product was significantly higher than with the purified product, suggesting that a non-[3H]palmitate radiolabel is also a substrate for the uptake process.

Cellular regulation of prostaglandin H synthase catalysis

Prostanoids are a group of potent bioactive lipids produced by oxygenation of arachidonate or one of several related polyunsaturated fatty acids. Cellular prostaglandin biosynthesis is tightly regulated, with a large part of the control exerted at the level of cyclooxygenase catalysis by prostaglandin H synthase (PGHS). The two known isoforms of PGHS have been assigned distinct pathophysiological functions, and their cyclooxygenase activities are subject to differential cellular control. This review considers the contributions to cellular catalytic control of the two PGHS isoforms by intracellular compartmentation, accessory proteins, arachidonate levels, and availability of hydroperoxide activator.

Transport of long-chain native fatty acids across human erythrocyte ghost membranes

Evidence from a number of laboratories suggests that membrane proteins may meditate the transport of physiologic fatty acids (FA) across cell membranes. However, actual transport of unbound free fatty acids (unbound FFA) from the aqueous phase on one side of a cell membrane to the aqueous phase on the other side has not been measured previously. In this study, we have used the fluorescent probe of unbound FFA, ADIFAB, to monitor the time course of FA movement from the outer to the inner aqueous compartments, and from the lipid membrane to the outer aqueous compartment of red cell ghosts. These two measurements, together with measurements of the lipid/aqueous partition coefficients, allowed the determination of the rate constants for binding (kon), flip-flop (kff), and dissociation (koff) for the transport of long-chain natural FA across red cell ghosts. Measurements done using palmitate, oleate, and linoleate at temperatures between 20 and 37 degreesC revealed that the overall transport times ranged from about 0.5 to more than 10 s, depending upon FA type and temperature. Analysis of these time courses yielded kff values between 0.3 and 3.0 s-1, and these values were consistent with those obtained using ghosts containing pyranine to detect intracellular acidification by the translocating FA. The measured koff values ranged from about 0.3 to 5 s-1, while the rate of binding, for the ghost concentrations used in this study (>50 microM phospholipid), exceed both kff and koff. Thus, long-chain FA transport across red cell ghost membranes is rate-limited by a combination of flip-flop and dissociation rates. Binding of FA to ghost membranes was well described by simple, nonsaturable, aqueous/membrane partition, and that partition appears to be governed by the aqueous solubility of the FA. Transport rates did not reveal any evidence of saturation and were not affected by a variety of protein-specific reagents. These FA binding and transport characteristics are similar to those observed previously for lipid vesicles, although the rate constants are generally about 2-3 fold larger for ghosts as compared to the lipid vesicles. We suggest, therefore, that FA transport across red cell ghosts is reasonably well described by transport across the lipid phase of the membrane.

Biological complexity is under the 'strange attraction' of non-esterified fatty acids

It is now clear from numerous data that non-esterified fatty acids (NEFAs) can act without any metabolic modification as second messengers or modulators of the complex signalling network which is characteristic of mammals. This network can respond differently to adapt the organism to the various endogenous and exogenous environmental situations. NEFAs have a wide range of molecular structures, and thus can exert different specific modulatory actions on this signalling network, such as amplification, inhibition or signal redirection. We have chosen the term 'strange attractions' to describe these signalling modulations by analogy with the 'strange attractions' concept introduced in deterministic chaos theory. NEFAs can modulate the functions of mammals at all levels of organization (molecular, cellular, tissue, organ, etc).

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

To examine whether fatty acid transport is abnormal in obesity, the kinetics of [3H]oleate uptake by hepatocytes, cardiac myocytes, and adipocytes from adult male Wistar (+/+), Zucker lean (fa/+) and fatty (fa/fa), and Zucker diabetic fatty (ZDF) rats were studied. A tissue-specific increase in oleate uptake was found in fa/fa and ZDF adipocytes, in which the Vmax was increased 9-fold (p < 0.005) and 13-fold (p < 0.001), respectively. This increase greatly exceeded the 2-fold increase in the surface area of adipocytes from obese animals, and did not result from trans-stimulation secondary to increased lipolysis. Adipocyte tumor necrosis factor-alpha mRNA levels, assayed by Northern hybridization, increased in the order +/+ < fa/fa < ZDF. Oleate uptake was also studied in adipocytes from 20-24-day-old male +/+, fa/+, and fa/fa weanlings. These animals were not obese, and had equivalent plasma fatty acid and glucose levels. Tumor necrosis factor-alpha mRNA levels in +/+ and fa/fa cells also were similar. Nevertheless, Vmax was increased 2.9-fold (p < 0.005) in fa/fa compared +/+ cells. These studies indicate 1) that regulation of fatty acid uptake is tissue-specific and 2) that up-regulation of adipocyte fatty acid uptake is an early event in Zucker fa/fa rats. These findings are independent of the role of any particular fatty acid transporter. Adipocyte mRNA levels of three putative transporters, mitochondrial aspartate aminotransferase, fatty acid translocase, and fatty acid transporting protein (FATP) were also determined; mitochondrial aspartate aminotransferase and FATP mRNAs correlated strongly with fatty acid uptake.

Inhibitory effects of fatty acids on glucose-regulated B-cell function: association with increased islet triglyceride stores and altered effect of fatty acid oxidation on glucose metabolism

Long-term exposure to fatty acids (FA) inhibits B-cell function. We tested whether the inhibitory effects are associated with increased islet triglycerides (TG). Rat pancreatic islets were cultured for 48 hours in RPMI 1640 medium with 10% fetal calf serum (FCS) and 11 mmol/L glucose in the presence or absence of the long-chain FA, palmitate. Palmitate (0.125 mmol/L) exposure successively increased islet TG 70% after 6 hours and 200% after 48 hours of culture. The dose-response for palmitate was similar for the increase in TG and inhibition of glucose-induced insulin secretion. Reversal of elevated islet TG in RPMI medium (after 48 hours of palmitate) was 29% after 6 hours and 84% after 24 hours. A more rapid decline of TG was observed in Krebs-Ringer bicarbonate (KRB) medium in the absence of nutrients. This decline was totally prevented by 1 mumol/L of the carnitine palmitoyl transferase-I (CPT-I) inhibitor, etomoxir. Etomoxir enhanced glucose-induced insulin secretion from palmitate-cultured islets; however, this effect was lost when TG were normalized. Under conditions when oxidation of FA from islet TG stores was blocked with etomoxir, we tested the effects of octanoate, the oxidation of which is not blocked by etomoxir. Oxidation of [1-14C]octanoate from islets precultured with palmitate (48 hours) did not differ from that in control islets. Conversely, after palmitate, octanoate inhibited glucose oxidation (14CO2 production from [U-14C]glucose, 613 +/- 41 pmol/10 islets/90 min v 1,129 +/- 87 after control conditions, P < .01). In conclusion, (1) palmitate induces increases in islet TG that are associated with inhibition of B-cell function, and (2) long-term exposure to palmitate also induces an inhibitory effect of FA oxidation on glucose metabolism that is independent of TG.

A polymorphism in the human intestinal fatty acid binding protein alters fatty acid transport across Caco-2 cells

The human intestinal fatty acid binding protein (IFABP) binds long-chain fatty acids in vitro, but its intracellular function has remained speculative. A polymorphism in the gene that encodes IFABP results in an alanine (Ala54) to threonine (Thr54) substitution at codon 54 that alters the in vitro binding affinity of the protein for long-chain fatty acids. To identify potential functional variability between Ala54 and Thr54 IFABP, we established permanently transfected Caco-2 cell lines that express either Ala54 or Thr54 IFABP. We found that Caco-2 cells expressing Thr54 IFABP transport long-chain fatty acids and secrete triglycerides to a greater degree than Caco-2 cells expressing Ala54 IFABP. These results provide the first demonstration that IFABP participates in the intracellular transport of long-chain fatty acids. In addition, the observed increase in transport of fatty acids across cells expressing Thr54 IFABP suggests a plausible physiologic mechanism for our prior observation that Pima Indians with a Thr54 IFABP genotype have increased post-absorptive lipid oxidation rates and are more insulin-resistant than Pimas with a Ala54 IFABP genotype.

Comparison of hydroperoxide initiator requirements for the cyclooxygenase activities of prostaglandin H synthase-1 and -2

Two isoforms of prostaglandin H synthase have been described: isoform-1 (PGHS-1), which is ascribed a role in basal or housekeeping prostaglandin synthesis; and isoform-2 (PGHS-2), which has been found to be strongly inducible in many tissues and has been associated with inflammatory processes. Recent observations have indicated that cyclooxygenase catalysis by the two isoforms can be differentially regulated when both are present simultaneously (Reddy, S. T., and Herschman, H. R. (1994) J. Biol. Chem. 269, 15473-15480). The requirement of the cyclooxygenase for hydroperoxide initiator has been proposed as an important limit on cellular prostaglandin synthesis (Marshall, P. J., Kulmacz, R. J., and Lands, W. E. M. (1987) J. Biol. Chem. 262, 3510-3517). To compare the levels of hydroperoxide required for cyclooxygenase initiation in the two PGHS isoforms, we have examined the ability of a hydroperoxide scavenger, glutathione peroxidase, to suppress the cyclooxygenase activity of purified preparations of human PGHS-2, ovine PGHS-2, and ovine PGHS-1. Half-maximal prostaglandin synthetic activity was found to require a much lower hydroperoxide level with human PGHS-2 (2.3 nM) and ovine PGHS-2 (2.2 nM) than with ovine PGHS-1 (21 nM). Similar results were obtained when cyclooxygenase activity was monitored by chromatographic analyses of radiolabeled arachidonate metabolites or with oxygen electrode measurements. Mixing four parts of ovine PGHS-1 with one part of human PGHS-2 did not markedly change the sensitivity of the overall cyclooxygenase activity to inhibition by glutathione peroxidase, indicating that the PGHS-1 activity was not easily initiated by PGHS-2 activity in the same vessel. Effective catalysis by PGHS-2 can thus proceed at hydroperoxide levels too low to sustain appreciable catalysis by PGHS-1. This difference in catalytic characteristics provides a biochemical mechanism for differential control of prostaglandin synthesis by the two PGHS isoforms, even when both are present in the same intracellular compartment.

Membrane permeation and intracellular trafficking of long chain fatty acids: insights from Escherichia coli and 3T3-L1 adipocytes

Long chain fatty acids are important substrates for energy production and lipid synthesis in prokaryotes and eukaryotes. Their cellular uptake represents an important first step leading to metabolism. This step is induced in Escherichia coli by growth in medium containing long chain fatty acids and in murine 3T3-L1 cells during differentiation to adipocytes. Consequently, these have been used extensively as model systems to study the cellular uptake of long chain fatty acids. Here, we present an overview of our current understanding of long chain fatty acid uptake in these cells. It consists of several distinct steps, mediated by a combination of biochemical and physico-chemical processes, and is driven by conversion of long chain fatty acids to acyl-CoA by acyl-CoA synthetase. An understanding of long chain fatty acid uptake may provide valuable insights into the roles of fatty acids in the regulation of cell signalling cascades, in the regulation of a variety of metabolic and transport processes, and in a variety of mammalian pathogenic conditions such as obesity and diabetes.