Rat Peroxisome Proliferator-activated Receptor Research Articles

PPARβ/δ Regulates Paneth Cell Differentiation Via Controlling the Hedgehog Signaling Pathway

All 4 differentiated epithelial cell types found in the intestinal epithelium derive from the intestinal epithelial stem cells present in the crypt unit, in a process whose molecular clues are intensely scrutinized. Peroxisome proliferator-activated receptor beta (PPARbeta) is a nuclear hormone receptor activated by fatty acids and is highly expressed in the digestive tract. However, its function in intestinal epithelium homeostasis is understood poorly. To assess the role of PPARbeta in the small intestinal epithelium, we combined various cellular and molecular approaches in wild-type and PPARbeta-mutant mice. We show that the expression of PPARbeta is particularly remarkable at the bottom of the crypt of the small intestine where Paneth cells reside. These cells, which have an important role in the innate immunity, are strikingly affected in PPARbeta-null mice. We then show that Indian hedgehog (Ihh) is a signal sent by mature Paneth cells to their precursors, negatively regulating their differentiation. Importantly, PPARbeta acts on Paneth cell homeostasis by down-regulating the expression of Ihh, an effect that can be mimicked by cyclopamine, a known inhibitor of the hedgehog signaling pathway. We unraveled the Ihh-dependent regulatory loop that controls mature Paneth cell homeostasis and its modulation by PPARbeta. PPARbeta currently is being assessed as a drug target for metabolic diseases; these results reveal some important clues with respect to the signals controlling epithelial cell fate in the small intestine.

Quantitative structure–activity relationships for inducers of cytochromes P450 and nuclear receptor ligands involved in P450 regulation within the CYP1, CYP2, CYP3 and CYP4 families

The results of quantitative structure–activity relationships (QSARs) are reported for several series of cytochrome P450 inducers, including those which also act as ligands for the various nuclear receptors involved in regulation of the relevant P450 genes, namely, CYP1, CYP2, CYP3 and CYP4. In several examples presented, the QSARs are consistent with homology modelling studies of the nuclear receptor ligand-binding domains (LBDs) based on available crystal structures of the oestrogen and peroxisome proliferator-activated receptors’ LBDs. Good correlations ( R=0.91–0.99) are found between various structural parameters and biological activity (either in the form of P450 induction or ligand-binding affinity) for the Ah receptor (AhR), human estrogen receptor α (hERα), human glucocorticoid receptor (hGR) and the rat peroxisome proliferator-activated receptor α (rPPARα).

Activation of peroxisome proliferator-activated receptor isoforms and inhibition of prostaglandin H 2 synthases by ibuprofen, naproxen, and indomethacin

A series of nonsteroidal anti-inflammatory drugs (NSAIDs) [ S(+)-naproxen, ibuprofen isomers, and indomethacin] were evaluated for their activation of peroxisome proliferator-activated receptor (PPAR) α and γ isoforms in CV-1 cells co-transfected with rat PPAR α and γ, and peroxisome proliferator response element (PPRE)-luciferase reporter gene plasmids, for stimulation of peroxisomal fatty acyl CoA β-oxidase activity in H4IIEC3 cells, and for comparative inhibition of ovine prostaglandin endoperoxide H synthase (PGHS)-1 and PGHS-2 and arachidonic acid-induced human platelet activation. Each drug produced a concentration-dependent activation of the PPAR isoforms and fatty acid β-oxidase activity, inhibition of human arachidonic acid-induced platelet aggregation and serotonin secretion, and inhibition of PGHS-1 and PGHS-2 activities. For PPARα activation in CV-1 and H4IIEC3 cells, and the stimulation of fatty acyl oxidase activity in H4IIEC3 cells, the rank order of stereoselectivity was S(+)- ibuprofen > R(−)-ibuprofen; S(+)-ibuprofen was more potent than indomethacin and naproxen on these parameters. On PPARγ, the rank order was S(+)-naproxen > indomethacin > S(+)-ibuprofen > R(−)-ibuprofen. Each drug inhibited PGHS-1 activity and platelet aggregation with the same rank order of indomethacin > S(+)-ibuprofen > S(+)-naproxen > R(−)-ibuprofen. Notably, the S(+)-isomer of ibuprofen was 32-, 41-, and 96-fold more potent than the R(−)-isomer for the inhibition of PGHS-1 activity, human platelet aggregation, and serotonin secretion, respectively. On PGHS-2, the ibuprofen isomers showed no selectivity, and indomethacin, S(+)-ibuprofen, and S(+)-naproxen were 6-, 27-, and 5-fold more potent as inhibitors of PGHS-1 than PGHS-2 activity. These results demonstrate that the mechanisms of action of NSAIDs on these cell systems are different, and we propose that the pharmacological effects of NSAIDs may be related to both their profile of inhibition of PGHS enzymes and the activation of PPARα and/or PPARγ isoforms.

Rat Peroxisome Proliferator-activated Receptors and Brown Adipose Tissue Function during Cold Acclimatization

Brown adipose tissue (BAT) hyperplasia is a fundamental physiological response to cold; it involves an acute phase of mitotic cell growth followed by a prolonged differentiation phase. Peroxisome proliferator-activated receptors (PPARs) are key regulators of fatty acid metabolism and adipocyte differentiation and may therefore mediate important metabolic changes during non-shivering thermogenesis. In the present study we have investigated PPAR mRNA expression in relation to peroxisome proliferation in rat BAT during cold acclimatization. By immunoelectron microscopy we show that the number of peroxisomes per cytoplasmic volume and acyl-CoA oxidase immunolabeling density remained constant (thus increasing in parallel with tissue mass and cell number) during the initial proliferative phase and the acute thermogenic response but increased after 14 days of cold exposure, correlating with terminal differentiation of BAT. A pronounced decrease in BAT PPARalpha and PPARgamma mRNA levels was found within hours of exposure to cold, which was reversed after 14 days, suggesting a role for either or both of these subtypes in the proliferation and induction of peroxisomes and peroxisomal beta-oxidation enzymes. In contrast, PPARdelta mRNA levels increased progressively during cold exposure. Transactivation assays in HIB 1B and HEK-293 cells demonstrated an adrenergic stimulation of peroxisome proliferator response element reporter activity via PPAR, establishing a role for these nuclear receptors in hormonal regulation of gene transcription in BAT.

Hypertension and insulin resistance: role of peroxisome proliferator-activated receptor gamma.

1. Insulin resistance has been highlighted as a common causal factor for hypertension, hyperlipidaemia, diabetes mellitus and obesity, all of which are recognized to occur simultaneously, and a distinct clinical entity is defined as 'multiple risk factor syndrome'. 2. Recently, a new class of antidiabetic agents, thiazolidinediones (TZD) has been developed and has been shown to improve insulin resistance by binding and activating a nuclear receptor, peroxisome proliferator-activated receptor (PPAR) gamma. 3. cDNA of rat PPAR gamma 1 and gamma 2 were cloned and gene regulation of PPAR gamma in rat mature adipocytes was examined. Hydrogen peroxide, an oxygen radical, which is recognized to be the common intracellular signal for multiple risk factors, potently down-regulated PPAR gamma mRNA expression in rat mature adipocytes. 4. Tumour necrosis factor (TNF)-alpha, which is considered to play a role in obesity-induced non-insulin-dependent diabetes mellitus and to augment oxidative stress, also suppressed PPAR gamma expression. 5. Thiazolidinediones dose-dependently recovered TNF-alpha-induced down-regulation of PPAR gamma mRNA expression. 6. The modulation of PPAR gamma expression by TZD can be one mechanism for the improvement of insulin resistance by TZD. 7. Vascular tone and remodelling are controlled by several vasoactive autocrine/paracrine factors produced by endothelial cells in response to several vascular injury stimuli, including hypertension. The PPAR gamma gene transcript was detected in cultured endothelial cells. 8. The administration of TZD stimulated the endothelial secretion of type-C natriuretic peptide, which is one of the natriuretic peptide family and is demonstrated by us to act as a novel endothelium-derived relaxing peptide. 9. Concomitantly, TZD significantly suppressed the secretion of endothelin, a potent endothelium-derived vasoconstricting peptide. 10. Thiazolidinediones can affect vascular tone and growth by modulating the production of endothelium-derived vasoactive substances to influence occurrence and progression of hypertension and atherosclerosis.

Molecular modelling of the rat peroxisome proliferator-activated receptor - α (rPPAR α) by homology with the human retinoic acid X receptor α (hRXR α) and investigation of ligand binding interactions I: QSARs

The construction of a homology model of the ligand binding domain of the rat peroxisome proliferator-activated receptor- α (rPPAR α) based on the crystal structure of the human retinoic acid X receptor- α (hRXR α) is reported. It is demonstrated that many known peroxisome proliferators are able to occupy the putative ligand binding site of the rPPAR α, including clofibric acid, ciprofibrate, nafenopin and related compounds. The log. relative potency of several peroxisome proliferators can be quantitatively related (R=0.99) to their binding affinity and lipophilicity as measured by their distribution coefficients (logD 7.4 values) and other QSARs are discussed in the light of receptor–ligand interactions. The molecular modelling of a representative number of peroxisome proliferators within the putative ligand binding site is consistent with experimental information on relative potency and enantioselectivity.

Distribution of mRNAs Encoding the Peroxisome Proliferator‐Activated Receptor α, β, and γ and the Retinoid X Receptor α, β, and γ in Rat Central Nervous System

We report the isolation, by RT-PCR, of partial cDNAs encoding the rat peroxisome proliferator-activated receptor (PPAR) isoforms PPAR alpha, PPAR beta, and PPAR gamma and the rat retinoid X receptor (RXR) isoforms RXR alpha, RXR beta, and RXR gamma. These cDNAs were used to generate antisense RNA probes to permit analysis, by the highly sensitive and discriminatory RNase protection assay, of the corresponding mRNAs in rat brain regions during development. PPAR alpha, PPAR beta, RXR alpha, and RXR beta mRNAs are ubiquitously present in different brain regions during development, PPAR gamma mRNA is essentially undetectable, and RXR gamma mRNA is principally localised to cortex. We demonstrate, for the first time, the presence of PPAR and RXR mRNAs in primary cultures of neonatal meningeal fibroblasts, cerebellar granule neurons (CGNs), and cortical and cerebellar astrocytes and in primary cultures of adult cortical astrocytes. PPAR alpha, PPAR beta, RXR alpha, and RXR beta mRNAs are present in all cell types, albeit that PPAR alpha and RXR alpha mRNAs are at levels near the limit of detection in CGNs. PPAR gamma mRNA is expressed at low levels in most cell types but is present at levels similar to those of PPAR alpha mRNA in adult astrocytes. RXR gamma mRNA is present either at low levels, or below the level of detection of the assay, for all cell types studied.

Stereoselective effects of chiral clofibric acid analogs on rat peroxisome proliferator-activated receptor alpha (rPPAR alpha) activation and peroxisomal fatty acid beta-oxidation.

Enantiomers of a series of substituted analogs of 2-(4-chlorophenoxy) -acetic acid (CPAA) were synthesized and used to examine the influence of steric and structural parameters on peroxisome proliferation. The effects of these compounds were studied on the activation of the peroxisome proliferator-activated receptor alpha (PPAR alpha) in CV-1 cells using an in vitro co-transfection assay. Selected sets of isomers were tested for their ability to increase peroxisomal fatty acyl-CoA oxidase (ACO) activity in H4IIEC3 (rat Reuber hepatoma) cells. Of the series of 2-substituted analogs studied, the isomers of the nu-propyl and phenyl derivatives of CPAA showed a high degree of stereoselectivity [(S)-isomer >> (R)-isomer]. In general, the potency of the compound to activate the receptor increased with the size of the 2-alkyl substituent. Among the 4-chlorobenzyloxy- and 4-(4'-chlorophenyl)benzyloxy- analogs studied, 2-[4-(4'-chlorophenyl)-benzyloxy]-propanoic acid exhibited a high degree of stereoselectivity in both the biological systems studied [(R) >> (S)]. The congeners of 2-methyl substituted CPAA showed a reverse stereoselectivity (R) > (S)] as compared to the other 2-substituted analogs [(S) > (R)]. Our results indicate that (1) both structural and steric characteristics of CPAA analogs play an important role in the activation of rPPAR alpha and on stimulation of peroxisomal ACO activities, and (2) clofibric acid and analogs exert their peroxisome proliferative effects by interaction with a specific site on a protein. The enantiomers of the 2-nu-propyl and the 2-phenyl CPAA analogs may be useful as mechanistic probes in elucidating the nature of this binding site.

Crosstalk between the thyroid hormone and peroxisome proliferator-activated receptors in regulating peroxisome proliferator-responsive genes

Peroxisome proliferators and thyroid hormones have overlapping metabolic effects and regulate a similar subset of genes involved in maintaining lipid homeostasis. Transcriptional activation by peroxisome proliferators is mediated by peroxisome proliferator-activated receptors (PPARs) that bind to specific peroxisome proliferator-response elements (PPREs) through heterodimerization with retinoid X receptors (RXRs). We examined the effect of thyroid hormone receptor α (TRα) on DNA binding in vitro and transcriptional activation in vivo by rat PPAR. Gel mobility shift assays using in vitro translated receptors demonstrated that TRα was capable of binding on its own and cooperatively with RXRα to the rat acyl-CoA oxidase PPRE and of inhibiting the binding of rat PPAR RXRα heterodimers to this element. This inhibition was the result of competition between TRα and PPAR for limiting amounts of the heterodimerization partner RXRα and for binding to the PPRE. Interestingly, cotransfection of a TRα expression plasmid into mammalian cells resulted in potentiation of the peroxisome proliferator- and PPAR RXRα - dependent transcriptional induction of a reporter gene containing the acyl-CoA oxidase PPRE. TRα therefore appears to cooperate with RXR and PPAR to positively modulate peroxisome proliferator-dependent transactivation in vivo. Our findings suggest that there is crosstalk between the thyroid hormone and peroxisome proliferator signaling pathways in the regulation of peroxisome proliferator-responsive genes.

Differential expression of peroxisome proliferator-activated receptors (PPARs): tissue distribution of PPAR-alpha, -beta, and -gamma in the adult rat.

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily that can be activated by various xenobiotics and natural fatty acids. These transcription factors primarily regulate genes involved in lipid metabolism and also play a role in adipocyte differentiation. We present the expression patterns of the PPAR subtypes in the adult rat, determined by in situ hybridization using specific probes for PPAR-alpha, -beta and -gamma, and by immunohistochemistry using a polyclonal antibody that recognizes the three rat PPAR subtypes. In numerous cell types from either ectodermal, mesodermal, or endodermal origin, PPARs are coexpressed, with relative levels varying between them from one cell type to the other. PPAR-alpha is highly expressed in hepatocytes, cardiomyocytes, enterocytes, and the proximal tubule cells of kidney. PPAR-beta is expressed ubiquitously and often at higher levels than PPAR-alpha and -gamma. PPAR-gamma is expressed predominantly in adipose tissue and the immune system. Our results suggest new potential directions to investigate the functions of the different PPAR subtypes.

Fatty acid activation of peroxisome proliferator-activated receptor (PPAR)

Peroxisome proliferators such as clofibric acid, nafenopin, and WY-14,643 have been shown to activate peroxisome proliferator-activated receptor (PPAR), a member of the steroid nuclear receptor superfamily. We have cloned the cDNA from rat that is homologous to that from mouse, which encodes a 97% similar protein. To search for physiologically occurring activators, we established a transcriptional transactivation assay by stably expressing in CHO cells a chimera of rat PPAR and the human glucocorticoid receptor that activates expression of the placental alkaline phosphatase reporter gene under the control of the mouse mammary tumor virus promoter. 150 microM concentrations of arachidonic or linoleic acid but not of dehydroepiandrosterone, cholesterol, or 25-hydroxy-cholesterol, activated the receptor chimera. In addition, saturated fatty acids induced the reporter gene. Shortening the chain length to n = 6 or introduction of an omega-terminal carboxylic group abolished the activation potential of the fatty acid. To test whether a common PPAR binding metabolite might be formed from free fatty acids we tested the effects of differentially beta-oxidizable fatty acids and inhibitors of fatty acid metabolism. The peroxisomal proliferation-inducing, non-beta-oxidizable, tetradecylthioacetic acid activated PPAR to the same extent as the strong peroxisomal proliferator WY-14,643, whereas the homologous beta-oxidizable tetradecylthiopropionic acid was only as potent as a non-substituted fatty acid. Cyclooxygenase inhibitors, radical scavengers or cytochrome P450 inhibitors did not affect activation of PPAR. In conclusion, beta-oxidation is apparently not required for the formation of the PPAR-activating molecule and this moiety might be a fatty acid, its ester with CoA, or a further derivative of the activated fatty acid prior to beta-oxidation of the acyl-CoA ester.

LOCALIZATION OF PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR IN MOUSE AND RAT-TISSUES AND DEMONSTRATION OF ITS NUCLEAR TRANSLOCATION IN TRANSFECTED CV-1 CELLS

Hepatocarcinogenesis in rodents induced by nongenotoxic peroxisome proliferators is postulated to be a receptor-mediated process. The peroxisome proliferator-activated receptors (PPAR) are members of the steroid hormone receptor superfamily, which participate in ligand-dependent transcriptional activation of peroxisomal fatty acid beta oxidation enzyme system genes in liver parenchymal cells of rats and mice. In order to study the tissue distribution and cellular localization of PPAR, we raised polyclonal antibodies against PPAR using a recombinant rat PPAR (rPPAR) expressed as a glutathione-S-transferase-rPPAR fusion protein. On immunoblot analysis the antibodies specifically recognized a 55 kDa PPAR protein in rat, mouse and human liver homogenates. Immunoblotting also showed that in the mouse and rat, PPAR is expressed in liver, kidney and heart, and only weakly in brain and testis. Immunohistochemical localization in the rat and mouse revealed that PPAR is highly expressed in perivenular (i.e., those surrounding hepatic vein) hepatocytes and very weakly in the cytoplasm of remaining hepatocytes. In the kidney, PPAR was visualized predominantly in the p(3) segments of proximal convoluted tubular epithelium. CV-1 cells transiently transfected with rPPAR cDNA construct showed predominant cytoplasmic fluorescence; treatment of these cells with ciprofibrate, a peroxisome proliferator, resulted in the nuclear translocation of PPAR signal.

Human and rat peroxisome proliferator activated receptors (PPARs) demonstrate similar tissue distribution but different responsiveness to PPAR activators

We have isolated a human peroxisomal proliferator activated receptor (hPPAR) from a human liver cDNA library. Based on sequence analysis, we have determined that this cDNA encodes the human PPARα. When assayed in a reconstituted hPPAR responsive transcription system in mammalian CV-1 cells, this receptor was shown to be transcriptionally activated by hypolipidemic agents like clofibric acid, and ETYA (5,8,11,14-eicosatetraynoic acid; a synthetic arachidonic acid homolog). When analyzed in CV-1 cells, the rat PPARα was similarly transcriptionally regulated. However, when assayed in a human liver cell line (HepG2) we noticed that ETYA was a more efficient activator of hPPARα than rPPARα. Thus, factors other than the receptor are important in determining the cellular responsiveness to this class of compounds. Interestingly, WY-14,643, another peroxisome proliferator, was a much more potent activator of rPPARα than human PPARα when assayed in both cell lines. This may explain in part why certain fibrates are potent hepatocarcinogens in rodents. Northern analysis indicates that hPPARα and rPPARα are well expressed in heart, kidney and liver. We further demonstrate that hPPARα and human retinoid X receptorα synergistically interact to bind and transactive through a peroxisomal proliferator response element. Thus in a similar cell and promoter context the rat and human PPARs show a differential response to certain activators. Cumulatively these data suggest that differential ligand responsiveness does not provide a complete explanation for the different biological effects exhibited by hypolipidemic drugs when administered to humans and rats.

Association of peroxisome proliferator-activated receptor and Hsp72.

In an effort to understand the relationship between a 72-kDa heat shock protein (Hsp72) and peroxisome proliferator-activated receptors (PPARs), we have characterized their interaction using clofibric acid-Sepharose chromatography and co-immunoprecipitation with antisera raised against either rat PPAR (rPPAR) or Hsp72. First, we observed that both rPPAR and Hsp72 elute in a clofibrate-dependent manner from the clofibric acid-Sepharose matrix. Second, we found that immunoprecipitation of either protein from solution resulted in the precipitation of the other. This result was obtained from rat liver cytosol, from Spodoptera frugiperda (Sf9) insect cells expressing rPPAR, and from reticulocyte lysate rPPAR expression systems. These results suggest that Hsp72 and rPPAR form a complex in vivo and that Hsp72 may play a role in the folding, subcellular localization, and/or signaling pathway of PPARs.

A systematic analytical chemistry/cell assay approach to isolate activators of orphan nuclear receptors from biological extracts: characterization of peroxisome proliferator-activated receptor activators in plasma

Using a novel combination of analytical chemical and molecular biological techniques, lipophilic components of human plasma separated according to their physico-chemical properties were screened for their ability to activate the rat peroxisome proliferator-activated receptor (rPPAR). Activation of an rPPAR/glucocorticoid receptor chimera stably expressed in CHO cells by fractions in the initial screening guided further subfractionation. Characterization of an active subfraction by gas chromatography alone and in combination with mass spectrometry (GC-MS), indicated the presence of free fatty acids. Individual active components in this mixture were isolated by a final fractionation using high performance liquid chromatography (HPLC). GC-MS analyses of HPLC fractions able to activate the chimeric receptor identified palmitic acid, oleic acid, linoleic acid, and arachidonic acid as endogenous activators of rPPAR. No other activators were identified. This approach is able to specifically extract and identify endogenous activators of PPAR from a complex biological extract and as such may be valuable in the identification of activators of other orphan receptors in the steroid hormone receptor superfamily.

Diverse peroxisome proliferator-activated receptors bind to the peroxisome proliferator-responsive elements of the rat hydratase/dehydrogenase and fatty acyl-CoA oxidase genes but differentially induce expression.

The ability of peroxisome proliferator-activated receptors (PPARs) to induce expression of a reporter gene linked to a peroxisome proliferator-responsive element (PPRE) from either the rat enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase gene or acyl-CoA oxidase [acyl-CoA:oxygen 2-oxidoreductase, EC 1.3.3.6] gene was examined by transient transfection assays in COS cells. Mouse and rat PPARs, as well as Xenopus PPAR alpha (xPPAR alpha) could induce expression of a reporter gene linked to the hydratase/dehydrogenase PPRE in the presence of the peroxisome proliferators ciprofibrate or Wy-14,643, whereas xPPAR beta and xPPAR gamma were ineffective. A similar induction of expression of a reporter gene linked to the acyl-CoA oxidase PPRE was observed with all PPARs except xPPAR beta. Extracts from cells transfected with PPAR-encoding genes contained factors that bound to both PPREs. In vitro synthesized PPARs could interact weakly with both PPREs; however, binding of each PPAR to both PPREs was significantly increased by the addition of COS cell nuclear extracts, demonstrating that efficient PPAR/DNA binding requires auxiliary cofactors. One cofactor was identified as the 9-cis-retinoic acid receptor, RXR alpha (retinoid X receptor alpha). Cooperative DNA binding and heteromerization between RXR alpha and each of the PPARs could be seen with both PPREs. Our results demonstrate that PPAR/PPRE binding and cooperativity with RXR alpha (and other cofactors) are obligatory but not necessarily sufficient for peroxisome proliferator-dependent transcription induction and that distinct PPREs can selectively mediate induction by particular PPARs.

Fatty acids activate a chimera of the clofibric acid-activated receptor and the glucocorticoid receptor.

Peroxisome proliferators such as clofibric acid, nafenopin, and WY-14,643 have been shown to activate PPAR (peroxisome proliferator-activated receptor), a member of the steroid nuclear receptor superfamily. We have cloned the cDNA from the rat that is homologous to that from the mouse [Issemann, I. & Green, S. (1990) Nature (London) 347, 645-650], which encodes a 97% similar protein with a particularly well-conserved putative ligand-binding domain. To search for physiologically occurring activators, we established a transcriptional transactivation assay by stably expressing in CHO cells a chimera of rat PPAR and the human glucocorticoid receptor that activates expression of the placental alkaline phosphatase reporter gene under the control of the mouse mammary tumor virus promoter. Testing of compounds related to lipid metabolism or peroxisomal proliferation revealed that 150 microM concentrations of arachidonic or linoleic acid but not of dehydroepiandrosterone, cholesterol, or 25-hydroxy-cholesterol, activate the receptor chimera. In addition, saturated fatty acids induce the reporter gene. Shortening the chain length to n = 6 or introduction of an omega-terminal carboxylic group abolished the activation potential of the fatty acid. In conclusion, the present results indicate that fatty acids can regulate gene expression mediated by a member of the steroid nuclear receptor superfamily.