Do we taste fat?

Sense of taste informs the body about the quality of ingested foods. Five sub-modalities allowing the perception of sweet, salty, sour, bitter, and umami stimuli are classically depicted. However, the inborn attraction of mammals for fatty foods raises the possibility of an additional orosensory modality devoted to fat perception. For a long time, dietary lipids were thought to be detected only by trigeminal (texture perception), retronasal olfactory, and post-ingestive cues. This minireview analyses recent findings showing that gustation also plays a significant role in dietary lipid perception.

The gene encoding the human ileal bile acid-binding protein (I-BABP) is regulated by peroxisome proliferator-activated receptors.

Peroxisome proliferator-activator receptors (PPAR) are involved in cholesterol homeostasis through the regulation of bile acids synthesis, composition, and reclamation. As ileal bile acid-binding protein (I-BABP) is thought to play a crucial role in the enterohepatic circulation of bile acids, we investigated whether I-BABP gene expression could also be affected by PPAR. Indeed, treatment with the PPARalpha-PPARbeta/delta agonist bezafibrate led to the up-regulation of I-BABP mRNA levels in the human intestine-derived Caco-2 cells. Cotransfections of the reporter-linked human I-BABP promoter (hI-BABP-2769/+44) together with PPAR and RXR expression vectors demonstrated that the fibrate-mediated induction of the I-BABP gene is dependent on PPARalpha or PPARbeta/delta. Using progressive 5' deletions of the hI-BABP promoter and sequence analysis, we identified a putative PPAR-binding site located at the position -198 and -186 upstream of the transcription initiation site. Electrophoretic mobility shift assays showed that the PPAR/RXR heterodimer can specifically bind to this PPRE-like motif. The deletion of the PPRE within the hI-BABP promoter abolished the PPAR-mediated transactivation in transient transfection assays. The regulation of the I-BABP promoter by PPAR appears species-specific, as the mouse I-BABP promoter, which lacks a conserved PPRE, was not responsive to exogenous PPAR expression in the presence of bezafibrate. Our findings show that the I-BABP gene may be a novel target for PPAR in humans and further emphasize the role for PPAR in the control of bile acid homeostasis.

Obesogen effects after perinatal exposure of 4,4'-sulfonyldiphenol (Bisphenol S) in C57BL/6 mice.

Bisphenol A were removed from consumer products and replaced by chemical substitutes such as Bisphenol S (BPS). Based on their structural similarity, BPS may be obesogen like Bisphenol A in mice. Our objective was to determine the impact of BPS on lipid homeostasis in C57Bl/6 mice after perinatal and chronic exposure. Pregnant mice were exposed to BPS via the drinking water (0.2; 1.5; 50µg/kg bw/d). Treatment began at gestational day 0 and continued in offspring up to 23-weeks old. Then, offspring mice were fed with a standard or high fat diet. The body weight, food consumption, fat mass and energy expenditure were measured. A lipid load test was performed to check the postprandial triglyceridemia. Plasma parameters and mRNA gene expression in adipose tissues were also analysed. BPS induced overweight in male mice offspring fed with a HFD at the two highest doses. There was no change in food intake and energy expenditure. The overweight was correlated to the fat mass, hyperinsulinemia and hyperleptinemia. The plasma triglyceride clearance was significantly increased with BPS and tyloxapol(®) (triglyceride clearance inhibitor) reversed this phenomenon. BPS induced alteration in mRNA expression of marker genes involved in adipose tissue homeostasis: hormone sensitive lipase, PPARγ, insulin receptor, SOCS3 and adiponectin. This is the first time that BPS is described as obesogenic at low doses and after perinatal and chronic exposure in male mice. BPS potentiated the obesity induced by a HFD by inducing the lipid storage linked to faster lipid plasma clearance.

Indirect dexamethasone down-regulation of the liver fatty acid-binding protein expression in rat liver.

The effects of glucocorticoids on the regulation of the liver fatty acid-binding protein (L-FABP) were studied in vivo and in primary culture of hepatocytes in rats. No change in L-FABP cytosolic content and mRNA levels occurred after adrenalectomy. By contrast, a twofold decrease in L-FABP expression was found in dexamethasone (Dex) treated rats. In primary culture of rat hepatocytes, insulin did not modify the L-FABP mRNA levels, whereas Dex produced a significant decrease. This down-regulation was independent of specific glucocorticoid receptors, of alteration in the turnover of L-FABP mRNA and did not require a de novo protein synthesis. However, it was totally prevented when 320 microM oleic acid was added in the culture medium. These findings show that the dex-mediated down-regulation of the L-FABP expression found in vivo is not due to a direct endocrine effect, but is likely secondary to changes in cellular lipid metabolism.

Effect of short- and long-term treatments by a low level of dietary L-carnitine on parameters related to fatty acid oxidation in Wistar rat.

This study was designed to examine whether short- and long-term treatments by a low level of dietary L-carnitine are capable of altering enzyme activities related to fatty acid oxidation in normal Wistar rats. Under controlled feeding, ten days of treatment changed neither body weights nor liver and gastrocnemius weights, but succeeded in reducing the weight of peri-epididymal adipose tissues. Triacylglycerol contents were lowered in liver and ketone body concentrations were found slightly more elevated in blood. In the liver, mitochondrial carnitine palmitoyltransferase I (CPT I) exhibited a slightly higher specific activity and a lower sensitivity to malonyl-CoA inhibition, while peroxisomal fatty acid oxidizing system (PFAOS) was found to be less active. Carnitine supplied for one month reduced the mass of the periepididymal fat tissue, but not those of the other studied organs, and produced a slight but non-significant gain in body weight after ten days of treatment. In the liver, CPTI characteristics were comparable in control and treated groups, while PFAOS activity was less in rats receiving carnitine. Data show that L-carnitine at a low level in the diet exerted two paradoxical effects before and after ten days of treatment. Results are discussed in regard to fatty acid oxidation in mitochondria and peroxisomes, and to the possible altered acyl-CoA/acylcarnitine ratio with increased concentrations of L-carnitine in the liver.

Output of liver fatty acid-binding protein (L-FABP) in bile.

Liver fatty acid-binding protein (L-FABP) is a small cytoplasmic molecule highly expressed in the liver. Since L-FABP exhibits affinities for several biliary components, its presence in bile was explored by Western blotting and competitive ELISA in various mammalian species. A L-FABP-like immunoreactivity was consistently found in both hepatic and gallbladder bile. A close molecular identity between this 14 kDa biliary protein and the purified L-FABP was assessed by immunological analyses and high performance capillary electrophoresis. Pharmacological induction of hepatic L-FABP biosynthesis led to a similar increase in biliary L-FABP levels showing a close relationships between the cytosolic and biliary contents of this protein. Finally, a correlation between the presence of L-FABP in bile and both bile flow and bile acid release was found. These data suggest an output of L-FABP in bile in normal conditions which might be coupled with the physiological release of biliary components.

Development of conjugated linoleic acid (CLA)-mediated lipoatrophic syndrome in the mouse.

Conjugated linoleic acids (CLA) are positional and geometric dienoic isomers of linoleic acid. Dietary CLA supplementation leads to a drop in fat mass in various species, including in humans. The t10,c12-CLA isomer is responsible for this anti-obesity effect. The reduction of fat mass is especially dramatic in the mouse, in which it is associated with severe hyperinsulinemia, insulin resistance and massive liver steatosis. The origin of these adverse side effects and putative chronology of events leading to CLA-mediated lipoatrophic syndrome are presented and discussed in this review.

9-cis-retinoic acid enhances fatty acid-induced expression of the liver fatty acid-binding protein gene.

The role of retinoic acids (RA) on liver fatty acid-binding protein (L-FABP) expression was investigated in the well differentiated FAO rat hepatoma cell line. 9-cis-Retinoic acid (9-cis-RA) specifically enhanced L-FABP mRNA levels in a time- and dose-dependent manner. The higher induction was found 6 h after addition of 10(-6) M 9-cis-RA in the medium. RA also enhanced further both L-FABP mRNA levels and cytosolic L-FABP protein content induced by oleic acid. The retinoid X receptor (RXR) and the peroxisome proliferator-activated receptor (PPAR), which are known to be activated, respectively, by 9-cis-RA and long chain fatty acid (LCFA), co-operated to bind specifically the peroxisome proliferator-responsive element (PPRE) found upstream of the L-FABP gene. Our result suggest that the PPAR-RXR complex is the molecular target by which 9-cis-RA and LCFA regulate the L-FABP gene.

Effect of bile on the intestinal bile-acid binding protein (I-BABP) expression. In vitro and in vivo studies.

Enterocytes actively transport bile acids from the ileal lumen to the portal blood. This physiological process greatly contributes to maintaining the bile acid homeostasis. However, little is known about the molecular mechanisms involved in this transport system. The effect of bile on gene expression of the intestinal bile-acid binding protein (I-BABP) expressed in the enterocytes was studied in vivo, using the by-pass method, and in vitro, using organ culture of ileum explants and Caco-2 cell line. The low cytosolic I-BABP concentration and I-BABP mRNA level found in diverted ileum was totally recovered when bile was added in the ileal lumen. Northern blot analysis of the ileal explants revealed a dose-dependent increase in the I-BABP mRNA in the presence of bile. In Caco-2 cells, the I-BABP transcript was dramatically increased in the presence of human bile while it was undetectable in the control cultures. These data offer the first evidence that biliary components regulate the I-BABP gene expressed in the enterocytes.

Regulation of gene expression by fatty acids: special reference to fatty acid-binding protein (FABP).

During the last years, the direct involvement of lipidic nutrients in the regulation of genes has been established. Fatty acids may induce or repress the transcription rate of several genes involved in both lipid and carbohydrate metabolisms. Gene up-regulation has been found in various tissues including liver, adipose tissue and small intestine. It is only triggered by saturated and unsaturated long-chain fatty acids or their CoA-derivatives. In contrast, gene down-regulation appears to be restricted to the liver. This negative effect is exerted only by polyunsaturated fatty acids. Long-chain fatty acids are able to regulate the expression of two different genes oppositely in the same cell type. The molecular mechanism of these fatty acid-mediated effects remains unclear. The involvement of members of the peroxisome proliferator-activated receptor is discussed.

Short term treatment by fenofibrate enhances oxidative activities towards long-chain fatty acids in the liver of lean Zucker rats.

Lean Zucker rats were dosed orally for 1 week with fenofibrate (100 mg/kg/day). Liver weights of treated rats, expressed as per cent of body weight, were increased, while protein, DNA and triacylglycerol contents were not changed to any great extent per gram of liver, but increased when expressed per whole liver. Compared with the control animals, activities of fatty acid oxidase, of the peroxisomal fatty acid-oxidizing system and of catalase were markedly enhanced by fenofibrate, both per gram of liver and per total liver, while urate oxidase activity was slightly depressed when expressed per gram of liver. The activity of cytochrome c oxidase used as a mitochondrial marker was only higher when expressed per total liver. Besides, fenofibrate treatment induced a pronounced increase in the mitochondrial activities of carnitine palmitoyl- and acetyltransferases, of palmitoyl-CoA dehydrogenase and of carnitine-dependent oleate oxidation. Fenofibrate also enhanced significantly the carnitine content in liver and hepatic mitochondria. Malonyl-CoA content per gram of liver was found to be twice as high as in control rats, while the sensitivity of carnitine acyltransferase I to malonyl-CoA inhibition was hardly altered. The drug enhanced the percentage of palmitic acid in lipids of liver, but not in adipose tissues. The present data show that fenofibrate induced greater oxidative activities towards fatty acids, even in the lean animal. This stimulation could be related to the energy used for building new cells. In turn, at the same time of treatment, an enhanced fatty acid synthesis would provide specific fatty acids for the formation of new membranes. This latter effect will eventually disappear and the maintenance of a higher fatty acid oxidation may explain part of the overall hypolipaemic effect of fenofibrate.

Enhancement of activities relative to fatty acid oxidation in the liver of rats depleted of L-carnitine by D-carnitine and a gamma-butyrobetaine hydroxylase inhibitor.

This study was designed to examine whether the depletion of L-carnitine may induce compensatory mechanisms allowing higher fatty acid oxidative activities in liver, particularly with regard to mitochondrial carnitine palmitoyltransferase I activity and peroxisomal fatty acid oxidation. Wistar rats received D-carnitine for 2 days and 3-(2,2,2,-trimethylhydrazinium)propionate (mildronate), a noncompetitive inhibitor of gamma-butyrobetaine hydroxylase, for 10 days. They were starved for 20 hr before being sacrificed. A dramatic reduction in carnitine concentration was observed in heart, skeletal muscles and kidneys, and to a lesser extent, in liver. Triacylglycerol content was found to be significantly more elevated on a gram liver and whole liver basis as well as per mL of blood (but to a lesser extent), while similar concentrations of ketone bodies were found in the blood of D-carnitine/mildronate-treated and control rats. In liver mitochondria, the specific activities of acyl-CoA synthetase and carnitine palmitoyltransferase I were enhanced by the treatment, while peroxisomal fatty acid oxidation was higher per gram of tissue. It is suggested that there may be an enhancement of cellular acyl-CoA concentration, a signal leading to increased liver fatty acid oxidation in acute carnitine deficiency.

Effect of dietary n-3 and n-6 polyunsaturated fatty acids on lipid-metabolizing enzymes in obese rat liver.

This study was designed to examine whether n-3 and n-6 polyunsaturated fatty acids at a very low dietary level (about 0.2%) would alter liver activities in respect to fatty acid oxidation. Obese Zucker rats were used because of their low level of fatty acid oxidation, which would make increases easier to detect. Zucker rats were fed diets containing different oil mixtures (5%, w/w) with the same ratio of n-6/n-3 fatty acids supplied either as fish oil or arachidonic acid concentrate. Decreased hepatic triacylglycerol levels were observed only with the diet containing fish oil. In mitochondrial outer membranes, which support carnitine palmitoyltransferase I activity, cholesterol content was similar for all diets, while the percentage of 22:6n-3 and 20:4n-6 in phospholipids was enhanced about by 6 and 3% with the diets containing fish oil and arachidonic acid, respectively. With the fish oil diet, the only difference found in activities related to fatty acid oxidation was the lower sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition. With the diet containing arachidonic acid, peroxisomal fatty acid oxidation and carnitine palmitoyltransferase I activity were markedly depressed. Compared with the control diet, the diets enriched in fish oil and in arachidonic acid gave rise to a higher specific activity of aryl-ester hydrolase in microsomal fractions. We suggest that slight changes in composition of n-3 or n-6 polyunsaturated fatty acids in mitochondrial outer membranes may alter carnitine palmitoyltransferase I activity.

Decreased expression of Intestinal I- and L-FABP levels in rare human genetic lipid malabsorption syndromes.

We investigated, for the first time, the expression of I- and L-FABP in two very rare hereditary lipid malabsorption syndromes as compared with normal subjects. Abetalipoproteinemia (ABL) and Anderson's disease (AD) are characterized by an inability to export alimentary lipids as chylomicrons that result in fat loading of enterocytes. Duodeno-jejunal biopsies were obtained from 14 fasted normal subjects, and from four patients with ABL and from six with AD. Intestinal FABP expression was investigated by immuno-histochemistry, western blot, ELISA and Northern blot analysis. In contrast to normal subjects, the cellular immunostaining for both FABPs was clearly decreased in patients, as the enterocytes became fat-laden. In patients with ABL, the intestinal contents of I- (60.7 +/- 13.38 ng/mg protein) and L-FABP (750.3 +/- 121.3 ng/mg protein) are significantly reduced (50 and 35%, P < 0.05, respectively) as compared to normal subjects (I-135.3 +/- 11.1 ng, L-1211 +/- 110 ng/mg protein). In AD, the patients also exhibited decreased expression (50%, P < 0.05; I-59 +/- 11.88 ng, L-618.2 +/- 104.6 ng/mg protein). Decreased FABP expression was not associated with decreased mRNA levels. The results suggest that enterocytes might regulate intracellular FABP content in response to intracellular fatty acids, which we speculate may act as lipid sensors to prevent their intracellular transport.

Hyperinsulinaemia triggered by dietary conjugated linoleic acid is associated with a decrease in leptin and adiponectin plasma levels and pancreatic beta cell hyperplasia in the mouse.

AIMS/HYPOTHESIS: Dietary supplementation with conjugated linoleic acids (CLA) has a fat-reducing effect in various species, but induces severe hyperinsulinaemia and hepatic steatosis in the mouse. This study aimed to determine the causes of the deleterious effects of CLA on insulin homeostasis. METHODS: The chronology of adipose and liver weight, hepatic triglyceride accumulation and selected blood parameters, including lipids, insulin, leptin and adiponectin, was determined in C57BL/6J female mice fed a 1% isomeric mixture of CLA for various periods of time ranging from 2 to 28 days. Insulin secretion was measured in 1-h static incubations of pancreatic islets, and pancreas morphometric parameters were determined in mice fed CLA for 28 days. RESULTS: Plasma levels of leptin and adiponectin sharply decreased after 2 days of CLA feeding, although adipose tissue mass only decreased after day 6. Hyperinsulinaemia developed at day 6 and consistently worsened up to day 28, in parallel with increases in hepatic lipid content. Islets from CLA-fed mice displayed three- to four-fold increased rates of glucose-stimulated insulin secretion, both in the absence and presence of isobutyl methylxanthine or carbachol. The increased insulin-releasing capacity of islets from CLA-fed mice was explained by an increase in beta cell mass and number. CONCLUSIONS/INTERPRETATION: The data suggest that CLA supplementation induces a profound reduction of leptinaemia and adiponectinaemia, followed by hyperinsulinaemia due to the increased secretory capacity of pancreatic islets, leading, in turn, to liver steatosis. These observations cast doubt on the safety of dietary supplements containing CLA.

Pathway of alpha-linolenic acid through the mitochondrial outer membrane in the rat liver and influence on the rate of oxidation. Comparison with linoleic and oleic acids.

The movement of alpha-linolenic acid (C18:3, n-3) through the mitochondrial outer membrane to oxidation sites was studied in rat liver and compared with the movement of linoleic acid (C18:2, n-6) and oleic acid (C18:1, n-9). All differ in the degree of unsaturation, but have the same chain length and the same position of the first double bond when counted from the carboxyl end. The following results were obtained. (1) The overall beta-oxidation in total mitochondria was in the order C18:3, n-3 greater than C18:2, n-6 greater than C18:1, n-9, independent of the amount of albumin in the medium. (2) The rate of formation of acylcarnitine from acyl-CoA was higher with oleoyl-CoA than with linoleoyl-CoA, and remained very low with alpha-linolenoyl-CoA for all concentrations studied. (3) When the formation of acylcarnitines originated from fatty acids (as potassium salts) in a medium containing CoA and ATP, the conversion of alpha-linolenate was greater than that of linoleate, which in turn was greater than that of oleate. (4) Use of a more purified mitochondrial fraction, practically devoid of peroxisomes, did not modify the results obtained with alpha-linolenate. (5) alpha-Linolenoyl-CoA did not inhibit oxidation of labelled alpha-linolenate, whereas the other acyl-CoAs did. (6) Transfer to carnitine of all three fatty acids (as potassium salts) by carnitine palmitoyltransferase-I (CPT-I) was similarly inhibited by increasing concentrations of malonyl-CoA. (7) On using a fraction containing mitochondrial outer membranes, the formation of acylcarnitines from potassium salts of fatty acids was qualitatively and quantitatively similar to that found with whole mitochondria. (8) Our observations show that alpha-linolenoyl-CoA synthesized other than in the mitochondria cannot be used to any great extent by the mitochondria due to its configuration. However when added as the unactivated form, alpha-linolenate appears to be very quickly oxidized, but should first be activated by acyl-CoA synthetase in the mitochondrion itself. Then it is rapidly channelled to CPT-I. These enzymic sites are probably close together in the mitochondrial outer membrane. The different behaviour of the alpha-linolenic group compared with the other acyl groups in the studied pathway can be explained by a different spatial arrangement due to the number and position of the double bonds.

Evidence for transcriptional induction of the liver fatty-acid-binding-protein gene by bezafibrate in the small intestine.

The effect of bezafibrate on cytosolic fatty-acid-binding-protein (FABPc) production along the small intestine has been investigated in mice. This drug increased the intestinal fatty-acid-binding-protein (I-FABPc) and liver fatty-acid-binding-protein (L-FABPc) mRNA levels in the duodenum. The extents of induction in the duodenum and in the liver are similar. However, the degree of stimulation gradually decreases along the length of the gut, no effect being found in the ileum. An efficient absorption of this drug as early as the proximal part of the small intestine may explain this phenomenon. The L-FABPc gene is silent in terminal ileum of mice, but a direct infusion of bezafibrate into the ileum switches it on. We used this original model to follow the time course of induction of the L-FABPc gene by bezafibrate. L-FABPc mRNA was first detected 4 h after fibrate infusion, reached a maximum level at 16 h and subsequently decreased at 24 h. This induction was totally blocked by cycloheximide. Sunflower oil also caused small increases in the L-FABPc mRNA levels. The transcriptional origin of the induction triggered both by bezafibrate and sunflower oil was demonstrated by run-on assays. These data indicate that (a) the transcription of the L-FABPc gene is induced by bezafibrate via de novo protein synthesis and (b) components of sunflower oil can transcriptionally activate the L-FABPc gene. Our results also demonstrate that the mouse terminal ileum is a useful system for studying the regulation of L-FABPc gene expression both in vivo and in vitro.

Localization and regulation of the putative membrane fatty-acid transporter (FAT) in the small intestine. Comparison with fatty acid-binding proteins (FABP).

The expression of the putative membrane fatty-acid transporter (FAT) was investigated in the small intestine. The FAT mRNA level was higher in the jejunum than in the duodenum and was lower in the ileum, as observed for cytosolic fatty-acid-binding proteins (FABP) expressed in this tissue. No FAT transcript was found in the stomach or colon. FAT mRNA was constitutively expressed in the epithelial cells located in the upper two thirds of villi, while it was undetectable in the crypt cells and submucosal cells. In jejunal mucosa, immunochemical studies showed that FAT protein was limited to the brush border of enterocytes. No fluorescence was found in the goblet cells. To determine whether FAT responded to changes in fat intake, as reported for FABP, the effect of two high-fat diets, which essentially contained either medium-chain fatty acids or long-chain fatty acids (sunflower-oil diet), was investigated. The sunflower-oil diet greatly increased FAT mRNA abundance throughout the small intestine. In contrast, a weak effect of medium-chain fatty acids was observed only in the jejunum. As found for FABP expression, treatment with the hypolipidemic drug bezafibrate affected FAT expression. These data demonstrate that FAT and FABP are co-expressed in enterocytes, as has been shown in adipocytes, myocytes and mammary cells. The data suggest that these membrane and cytosolic proteins might have complementary functions during dietary-fat absorption.

Involvement of microsomal vesicles in part of the sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition in mitochondrial fractions of rat liver.

Liver mitochondrial fractions as normally isolated contain only 10-20% of total mitochondria and may not be representative of the whole mitochondrial population. This study was designed to evaluate the dependence of the sensitivity of carnitine palmitoyl-transferase I (CPT I) to malonyl-CoA inhibition in mitochondrial fractions that are not normally studied. Four fractions prepared from rat liver were found to be contaminated to different extents by microsome vesicles, on the basis of marker-enzyme activities and micrographic data. Purification of mitochondrial fractions on a Percoll gradient decreased to some extent the microsomal contamination, which was due in part to the existence of close bonds between microsomes and the outer membranes of mitochondria. A greater degree of contamination of mitochondrial fractions by microsomes was correlated with a greater sensitivity of CPT I to malonyl-CoA inhibition. Attempts were made to enhance the sensitivity of CPT I to malonyl-CoA with the use of microsomes. Measurements performed by adding mitochondria and microsomes in the same CPT I assay failed to demonstrate any significant enhancement of malonyl-CoA inhibition. However, addition of ATP to a mixture of mitochondria and microsomes was shown to trigger the binding of both particles, as assessed by enzymic and micrographic data, and to increase the sensitivity of CPT I to malonyl-CoA inhibition. These results demonstrated that the binding of microsomes to mitochondria, unlike the simple mixing of both particles, was capable of altering the sensitivity of CPT I to malonyl-CoA. The data also suggest that this process could be of physiological importance, owing to the frequency of contiguous zones between mitochondria and endoplasmic reticulum observed in sections of intact liver cells.

Up-regulation of the expression of the gene for liver fatty acid-binding protein by long-chain fatty acids.

The role of fatty acids in the expression of the gene for liver fatty acid-binding protein (L-FABP) was investigated in the well-differentiated FAO rat hepatoma cell line. Cells were maintained in serum-free medium containing 40 microM BSA/320 microM oleate. Western blot analysis showed that oleate triggered an approx. 4-fold increase in the cytosolic L-FABP level in 16 h. Oleate specifically stimulated L-FABP mRNA in time-dependent and dose-dependent manners with a maximum 7-fold increase at 16 h in FAO cells. Preincubation of FAO cells with cycloheximide prevented the oleate-mediated induction of L-FABP mRNA, showing that protein synthesis was required for the action of fatty acids. Run-on transcription assays demonstrated that the control of L-FABP gene expression by oleate was, at least in part, transcriptional. Palmitic acid, oleic acid, linoleic acid, linolenic acid and arachidonic acid were similarly potent whereas octanoic acid was inefficient. This regulation was also found in normal hepatocytes. Therefore long-chain fatty acids are strong inducers of L-FABP gene expression. FAO cells constitute a useful tool for studying the underlying mechanism of fatty acid action.