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

Impact of Intracellular Lipid Binding Proteins on Endogenous and Xenobiotic Ligand Metabolism and Disposition

The family of intracellular lipid binding proteins (iLBPs) is comprised of 16 members of structurally related binding proteins that have ubiquitous tissue expression in humans. iLBPs collectively bind diverse essential endogenous lipids and xenobiotics. iLBPs solubilize and traffic lipophilic ligands through the aqueous milieu of the cell. Their expression is correlated with increased rates of ligand uptake into tissues and altered ligand metabolism. The importance of iLBPs in maintaining lipid homeostasis is well established. Fatty acid binding proteins (FABPs) make up the majority of iLBPs and are expressed in major organs relevant to xenobiotic absorption, distribution, and metabolism. FABPs bind a variety of xenobiotics including nonsteroidal anti-inflammatory drugs, psychoactive cannabinoids, benzodiazepines, antinociceptives, and peroxisome proliferators. FABP function is also associated with metabolic disease, making FABPs currently a target for drug development. Yet the potential contribution of FABP binding to distribution of xenobiotics into tissues and the mechanistic impact iLBPs may have on xenobiotic metabolism are largely undefined. This review examines the tissue-specific expression and functions of iLBPs, the ligand binding characteristics of iLBPs, their known endogenous and xenobiotic ligands, methods for measuring ligand binding, and mechanisms of ligand delivery from iLBPs to membranes and enzymes. Current knowledge of the importance of iLBPs in affecting disposition of xenobiotics is collectively described. SIGNIFICANCE STATEMENT: The data reviewed here show that FABPs bind many drugs and suggest that binding of drugs to FABPs in various tissues will affect drug distribution into tissues. The extensive work and findings with endogenous ligands suggest that FABPs may also alter the metabolism and transport of drugs. This review illustrates the potential significance of this understudied area.

Vitamin A regulates intramuscular adipose tissue and muscle development: promoting high-quality beef production

During growth in cattle, the development of intramuscular adipose tissue and muscle is dependent upon cell hyperplasia (increased number of adipocytes) and hypertrophy (increased size of adipocytes). Based on the results of previous studies, other adipose tissue depots (e.g., perirenal and subcutaneous) develop from the fetal stage primarily as brown adipose tissue. The hyperplastic stage of intramuscular adipose is considered to develop from late pregnancy, but there is no evidence indicating that intramuscular adipose tissue develops initially as brown adipose tissue. Hyperplastic growth of intramuscular adipose continues well into postweaning and is dependent on the timing of the transition to grain-based diets; thereafter, the late-stage development of intramuscular adipose tissue is dominated by hypertrophy. For muscle development, hyperplasia of myoblasts lasts from early (following development of somites in the embryo) to middle pregnancy, after which growth of muscle is the result of hypertrophy of myofibers. Vitamin A is a fat-soluble compound that is required for the normal immunologic function, vision, cellular proliferation, and differentiation. Here we review the roles of vitamin A in intramuscular adipose tissue and muscle development in cattle. Vitamin A regulates both hyperplasia and hypertrophy in in vitro experiments. Vitamin A supplementation at the early stage and restriction at fattening stage generate opposite effects in the beef cattle. Appropriate vitamin A supplementation and restriction strategy increase intramuscular adipose tissue development (i.e., marbling or intramuscular fat) in some in vivo trials. Besides, hyperplasia and hypertrophy of myoblasts/myotubes were affected by vitamin A treatment in in vitro trials. Additionally, some studies reported an interaction between the alcohol dehydrogenase-1C (ADH1C) genotype and vitamin A feed restriction for the development of marbling and/or intramuscular adipose tissue, which was dependent on the timing and level of vitamin A restriction. Therefore, the feed strategy of vitamin A has the visible impact on the marbling and muscle development in the cattle, which will be helpful to promote the quality of the beef.

Single-Cell Analysis Reveals Regional Reprogramming During Adaptation to Massive Small Bowel Resection in Mice

BACKGROUND & AIMS

The small intestine (SI) displays regionality in nutrient and immunological function. Following SI tissue loss (as occurs in short gut syndrome, or SGS), remaining SI must compensate, or "adapt"; the capacity of SI epithelium to reprogram its regional identity has not been described. Here, we apply single-cell resolution analyses to characterize molecular changes underpinning adaptation to SGS.

METHODS

Single-cell RNA sequencing was performed on epithelial cells isolated from distal SI of mice following 50% proximal small bowel resection (SBR) vs sham surgery. Single-cell profiles were clustered based on transcriptional similarity, reconstructing differentiation events from intestinal stem cells (ISCs) through to mature enterocytes. An unsupervised computational approach to score cell identity was used to quantify changes in regional (proximal vs distal) SI identity, validated using immunofluorescence, immunohistochemistry, qPCR, western blotting, and RNA-FISH.

RESULTS

Uniform Manifold Approximation and Projection-based clustering and visualization revealed differentiation trajectories from ISCs to mature enterocytes in sham and SBR. Cell identity scoring demonstrated segregation of enterocytes by regional SI identity: SBR enterocytes assumed more mature proximal identities. This was associated with significant upregulation of lipid metabolism and oxidative stress gene expression, which was validated via orthogonal analyses. Observed upstream transcriptional changes suggest retinoid metabolism and proximal transcription factor Creb3l3 drive proximalization of cell identity in response to SBR.

CONCLUSIONS

Adaptation to proximal SBR involves regional reprogramming of ileal enterocytes toward a proximal identity. Interventions bolstering the endogenous reprogramming capacity of SI enterocytes-conceivably by engaging the retinoid metabolism pathway-merit further investigation, as they may increase enteral feeding tolerance, and obviate intestinal failure, in SGS.

Effects of different fatty acid chain lengths on fatty acid oxidation-related protein expression levels in rat skeletal muscles

Skeletal muscles can adapt to dietary interventions that affect energy metabolism. Dietary intake of medium-chain fatty acids (MCFAs) enhances mitochondrial oxidation of fatty acids (FAO) in type IIa skeletal muscle fibers. However, the effect of MCFAs diet on mitochondrial or cytoplasmic FAO-related protein expression levels in different types of muscle fibers remains unclear. This study aims to examine the effects of a high-fat diet, containing MCFAs, on mitochondrial enzyme activities and heart-type fatty acid-binding protein (H-FABP) levels in different types of skeletal muscle fibers. Five-week-old male Wistar rats were assigned to one of the following three dietary conditions: standard chow (SC, 12% of calories from fat), high-fat MCFA, or high-fat long-chain fatty acids (LCFAs) diet (60% of calories from fat for both). The animals were provided food and water ad libitum for 4 weeks, following which citrate synthase (CS) activity and H-FABP concentration were analyzed. The epididymal fat pads (EFP) were significantly smaller in the MCFA group than in the LCFA group (p < 0.05). MCFA-fed group displayed an increase in CS activity compared with that observed in SC-fed controls in all types of skeletal muscle fibers (triceps, surface portion of gastrocnemius (gasS), deep portion of gastrocnemius (gasD), and soleus; p < 0.05,). H-FABP concentration was significantly higher in the LCFA group than in both the SC-fed and MCFA-fed groups (triceps, gasS, gasD, and soleus; p < 0.05,). However, no significant difference was observed in the H-FABP concentrations between the SC-fed and MCFA-fed groups. The results of this study showed that the MCFA diet can increase the expression of the mitochondrial enzyme CS, but not that of H-FABP, in both fast- and slow-twitch muscle fibers, suggesting that H-FABP expression is dependent on the chain length of fatty acids in the cytoplasm of skeletal muscles cells.

Epidermal fatty acid-binding protein protects nerve growth factor-differentiated PC12 cells from lipotoxic injury

Epidermal fatty acid-binding protein (E-FABP/FABP5/DA11) binds and transport long-chain fatty acids in the cytoplasm and may play a protecting role during neuronal injury. We examined whether E-FABP protects nerve growth factor-differentiated PC12 cells (NGFDPC12 cells) from lipotoxic injury observed after palmitic acid (C16:0; PAM) overload. NGFDPC12 cells cultures treated with PAM/bovine serum albumin at 0.3 mM/0.15 mM show PAM-induced lipotoxicity (PAM-LTx) and apoptosis. The apoptosis was preceded by a cellular accumulation of reactive oxygen species (ROS) and higher levels of E-FABP. Antioxidants MCI-186 and N-acetyl cysteine prevented E-FABP's induction in expression by PAM-LTx, while tert-butyl hydroperoxide increased ROS and E-FABP expression. Non-metabolized methyl ester of PAM, methyl palmitic acid (mPAM), failed to increase cellular ROS, E-FABP gene expression, or trigger apoptosis. Treatment of NGFDPC12 cultures with siE-FABP showed reduced E-FABP levels correlating with higher accumulation of ROS and cell death after exposure to PAM. In contrast, increasing E-FABP cellular levels by pre-loading the cells with recombinant E-FABP diminished the PAM-induced ROS and cell death. Finally, agonists for PPARβ (GW0742) or PPARγ (GW1929) increased E-FABP expression and enhanced the resistance of NGFDPC12 cells to PAM-LTx. We conclude that E-FABP protects NGFDPC12 cells from lipotoxic injury through mechanisms that involve reduction of ROS.

Epidermal fatty acid-binding protein (E-FABP) may protect nerve cells from the damaging exposure to high levels of free fatty acids (FA). We show that E-FABP can neutralize the effects of reactive oxygen species (ROS) generated by the high levels of FA in the cell and protect PC12 cells from lipotoxic injuries common in Type 2 diabetes neuropathy. Potentially, E-FABP gene up-regulation may be mediated through the NFkB pathway and future studies are needed to further evaluate this proposition.

Fatty acid-binding protein expression in the gastrointestinal tract of calves and cows

Fatty acid-binding protein (FABP) has high affinity for long-chain fatty acids and appears to participate in the metabolism and intracellular transport of lipids. Liver- and intestinal-type FABP (L-FABP and I-FABP, respectively) are expressed in the small intestine. However, in the gastrointestinal tract of ruminants, expression and localization of FABPs are unknown. In this study, we investigated the expression of I-FABP and L-FABP in the gastrointestinal tract of cattle. I- and L-FABP had higher messenger RNA (mRNA) and protein expression levels in the duodenum and jejunum relatively to other gastrointestinal regions in both calves and cows. Furthermore, L-FABP mRNA and protein expression were high in the colon. Both these protein types were confirmed to be in the cytosol of jejunal epithelial cells, where they were found in the villi rather than in the crypts. We concluded that duodenal and jejunal FABPs might be involved in the metabolism of fatty acids mainly in epithelial cells in cattle.

Intestinal absorption of long-chain fatty acids: evidence and uncertainties

Over the two last decades, cloning of proteins responsible for trafficking and metabolic fate of long-chain fatty acids (LCFA) in gut has provided new insights on cellular and molecular mechanisms involved in fat absorption. To this systematic cloning period, functional genomics has succeeded in providing a new set of surprises. Disruption of several genes, thought to play a crucial role in LCFA absorption, did not lead to clear phenotypes. This observation raises the question of the real physiological role of lipid-binding proteins and lipid-metabolizing enzymes expressed in enterocytes. The goal of this review is to analyze present knowledge concerning the main steps of intestinal fat absorption from LCFA uptake to lipoprotein release and to assess their impact on health.

Role of peroxisome-proliferator-activated receptor beta/delta (PPARbeta/delta) in gastrointestinal tract function and disease

PPARbeta/delta (peroxisome-proliferator-activated receptor beta/delta) is one of three PPARs in the nuclear hormone receptor superfamily that are collectively involved in the control of lipid homoeostasis among other functions. PPARbeta/delta not only acts as a ligand-activated transcription factor, but also affects signal transduction by interacting with other transcription factors such as NF-kappaB (nuclear factor kappaB). Constitutive expression of PPARbeta/delta in the gastrointestinal tract is very high compared with other tissues and its potential physiological roles in this tissue include homoeostatic regulation of intestinal cell proliferation/differentiation and modulation of inflammation associated with inflammatory bowel disease and colon cancer. Analysis of mouse epithelial cells in the intestine and colon has clearly demonstrated that ligand activation of PPARbeta/delta induces terminal differentiation. The PPARbeta/delta target genes mediating this effect are currently unknown. Emerging evidence suggests that PPARbeta/delta can suppress inflammatory bowel disease through PPARbeta/delta-dependent and ligand-independent down-regulation of inflammatory signalling. However, the role of PPARbeta/delta in colon carcinogenesis remains controversial, as conflicting evidence suggests that ligand activation of PPARbeta/delta can either potentiate or attenuate this disease. In the present review, we summarize the role of PPARbeta/delta in gastrointestinal physiology and disease with an emphasis on findings in experimental models using both high-affinity ligands and null-mouse models.

Differential gene expression of fatty acid binding proteins during porcine adipogenesis

Four different subtypes of fatty acid binding proteins i.e. liver-type FABP1, heart/muscle-type FABP3, adipocyte-type FABP4 and epithelial/epidermal-type FABP5 are expressed in adipose tissue. However, only the regulatory role of FABP4 in adipogenesis has been thoroughly investigated. To increase the knowledge on possible roles of these FABP subtypes in preadipocyte differentiation, gene expression patterns were examined during adipogenesis in pig (Sus scrofa). FABP1 expression was induced in proliferating cells, whereas FABP3, FABP4 and FABP5 expression increased throughout preadipocyte differentiation. Interestingly, the FABP4 and FABP5 expression increased early in the differentiation, followed by FABP3 later in the differentiation process. This indicates a role of FABP4 and FABP5 in intracellular fatty acid transport during initiation of differentiation, whereas, FABP3 likely is involved in the transport of fatty acids during intermediate stages of adipogenesis. In this study we demonstrate that FABP3, FABP4 and FABP5 expression is correlated with that of the peroxisome proliferator-activated receptors alpha and gamma (PPARA and PPARG). Altogether, this suggests a role of FABP1 during cell proliferation, whereas a coordinated expression of FABP3, FABP4 and FABP5 together with that of PPARA, PPARG1 and PPARG2 might be critical for the metabolic regulation during porcine adipogenesis.

Comparison of intestinal gene expression in Atlantic cod (Gadus morhua) fed standard fish meal or soybean meal by means of suppression subtractive hybridization and real-time PCR

Gene expression was studied in Atlantic cod fed two different diets, fish meal (FM) and dehulled and extracted soybean meal (SBM). RNA was isolated from the distal part of the mid-intestine of Atlantic cod and suppression subtractive hybridization (SSH) was employed to screen for genes that showed changes in expression in response to the two dietary treatments. We made a cDNA subtracted library, isolated and sequenced 192 clones. Identification of 157 clones was predicted by BLAST. Most of the clones were previously unidentified in cod. Expression of 12 selected clones was further studied by quantitative PCR. Expression of four clones showing similarity to aminopeptidase N, transcobalamin I precursor, cytochrome P450 3A40, and ras-related nuclear protein was significantly up regulated in intestine of cod fed SBM compared to cod fed FM. A trend towards up regulation of a clone with similarity to fatty acid binding protein in SBM-fed cod was also observed. No significant differences in expression were observed for: transmembrane 4 superfamily protein member, polypeptide N-acetylgalactosaminyltransferase, glutathione peroxidase, peroxiredoxin 4, SEC61, F-BOX, and 14-3-3.

9-cis-Retinoic acid (9cRA), a retinoid X receptor (RXR) ligand, exerts immunosuppressive effects on dendritic cells by RXR-dependent activation: inhibition of peroxisome proliferator-activated receptor gamma blocks some of the 9cRA activities, and precludes them to mature phenotype development

At nanomolar range, 9-cis-retinoic acid (9cRA) was able to interfere in the normal differentiation process from human monocyte to immature dendritic cell (DC) and produced a switch in mature DCs to a less stimulatory mode than untreated cells. 9cRA-treated mature DCs secreted high levels of IL-10 with an IL-12 reduced production. The phenotypic alterations unleashed by 9cRA were similar but not identical to other specific retinoid X receptor (RXR) agonists and to those already reported for rosiglitazone, a PPARgamma activator, on DCs. The simultaneous addition of 9cRA and rosiglitazone on DCs displayed additive effects. Moreover, addition to cultures of GW9662, a specific inhibitor of PPARgamma, or the RXR pan-antagonist HX603, blocked these changes. All these results suggest an activation of PPARgamma-RXR and other RXR containing dimers by 9cRA in DCs. Finally, both GW9662 and HX603 by themselves altered the maturation process unleashed by TNFalpha, poly(I:C) or LPS on human DCs further suggesting that the heterodimer PPARgamma-RXR must fulfill a significant role in the physiological maturation process of these cells in addition to the repressing effects reported till now for this nuclear receptor.

Diet-related variation in cellular retinol-binding protein type II gene expression in rat jejunum

Cellular retinol-binding protein type II (CRBPII) is involved in the transport of vitamin A and its metabolism in the small intestine. In the present study, we demonstrated diet-related variations in CRBPII expression in rat jejunum. The CRBPII protein and mRNA levels increased in parallel after the start of feeding period regardless of whether the feeding period was restricted to the hours of darkness or of light. In addition, this variation was observed in the rats fed high-fat diet or low-fat diets, but not in those fed a fat-free diet or in fasted rats. A similar diet-induced variation was seen in the mRNA of liver-type fatty acid-binding protein in rat jejunum. In the transient transfection experiment, unsaturated fatty acid increased rat CRBPII gene promoter activity via the PPARα/retinoid X receptor-α heterodimer. Taken together, these results suggest that the diet-related variation in CRBPII expression in rat jejunum may be brought about by the transcriptional induction of CRBPII gene expression mainly triggered by dietary fatty acids.

Activation of PPARα lowers synthesis and concentration of cholesterol by reduction of nuclear SREBP-2

To elucidate the mechanisms underlying the cholesterol lowering effects of PPARalpha agonists we investigated key regulators of cholesterol synthesis and uptake in rats and in the rat hepatoma cell line Fao after treatment with the PPARalpha agonists clofibrate and WY 14,643, respectively. In rat liver as well as in Fao cells, PPARalpha activation led to a decrease of transcriptionally active nuclear SREBP-2. mRNA concentrations of the key regulators of SREBP processing, Insig-1 in rat liver and Insig-1 and Insig-2a in Fao cells, were increased upon PPARalpha activation. Thus we suggest, that the observed reduction of the amount of nuclear SREBP-2 was due to an inhibition of the processing of the precursor protein. Both, in rat liver and in Fao cells, mRNA concentrations of the SREBP-2 target genes HMG-CoA reductase (EC1.1.1.34) and LDL receptor were reduced after treatment with the PPARalpha agonists. Furthermore, treatment of Fao cells with WY 14,643 reduced cholesterol synthesis. As a result, the amount of total cholesterol in liver, plasma and lipoproteins of clofibrate treated rats and in WY 14,643 treated Fao cells was decreased compared to control animals and cells, respectively. In conclusion, we could show a novel link between PPARalpha and cholesterol metabolism by demonstrating that PPARalpha activation lowers cholesterol concentration by reducing the abundance of nuclear SREBP-2.

Molecular Mechanisms of Cross-talk between Thyroid Hormone and Peroxisome Proliferator Activated Receptors: Focus on the Heart

Thyroid hormone receptors (TR) and peroxisome proliferator activated receptors (PPAR) regulate cardiac metabolism. Numerous studies have examined TR and PPAR function since PPAR was first discovered in the early 1990s, however few have evaluated TR and PPAR interactions. Although ligands for these members of the nuclear steroid receptor family are under evaluation for treatment of congestive heart failure and various metabolic diseases, their interactions have not been investigated in detail in heart. These interactions are remarkably complicated. Nevertheless, their identification and elucidation is extremely important for further development of specific drugs. We review here the fundamental ways TRs and PPARs are regulated and how their cross-talk patterns mediate transcription of their target genes.

Peroxisome proliferator-activated receptor subtype- and cell-type-specific activation of genomic target genes upon adenoviral transgene delivery

Investigations of the molecular events involved in activation of genomic target genes by peroxisome proliferator-activated receptors (PPARs) have been hampered by the inability to establish a clean on/off state of the receptor in living cells. Here we show that the combination of adenoviral delivery and chromatin immunoprecipitation (ChIP) is ideal for dissecting these mechanisms. Adenoviral delivery of PPARs leads to a rapid and synchronous expression of the PPAR subtypes, establishment of transcriptional active complexes at genomic loci, and immediate activation of even silent target genes. We demonstrate that PPARgamma2 possesses considerable ligand-dependent as well as independent transactivation potential and that agonists increase the occupancy of PPARgamma2/retinoid X receptor at PPAR response elements. Intriguingly, by direct comparison of the PPARs (alpha, gamma, and beta/delta), we show that the subtypes have very different abilities to gain access to target sites and that in general the genomic occupancy correlates with the ability to activate the corresponding target gene. In addition, the specificity and potency of activation by PPAR subtypes are highly dependent on the cell type. Thus, PPAR subtype-specific activation of genomic target genes involves an intricate interplay between the properties of the subtype- and cell-type-specific settings at the individual target loci.

Selectivity of fatty acid ligands for PPARalpha which correlates both with binding to cis-element and DNA binding-independent transactivity in Caco-2 cells

It is thought that peroxisome proliferator-activated receptor alpha (PPARalpha) is a major regulator for fatty acid metabolism. Long-chain fatty acids have been shown to induce expression of the genes related to fatty acid metabolism through PPARalpha. However, it is unclear whether the intensity of PPARalpha activation is different among various fatty acids. In this study, we compared various fatty acids in the capability of PPARalpha activation by differential protease sensitivity assay (DPSA), electrophoretic mobility shift assay and GAL4-PPAR chimera reporter assay in intestinal cell line, Caco-2. DPSA revealed that polyunsaturated fatty acids of 18 to 20 carbon groups with 3-5 double bonds strongly induced a PPARalpha conformational change. The ligand-induced changes in the sensitivity to protease corresponded to the enhancement of the binding of PPARalpha-RXRalpha heterodimer to the PPAR-response element (PPRE). The GAL4-PPAR chimera reporter assay revealed that the DNA binding-independent transactivity of PPARalpha was induced by various fatty acids with a wide spectrum of intensity which correlated with the conformational change of PPARalpha. These results suggest that PPARalpha has greater selectivity to certain types of polyunsaturated fatty acids, and that the ligand-induced conformational change of PPARalpha leads to parallel increases in both DNA binding to the PPAR-response element and the DNA binding-independent transactivity.

Coordinate transcriptional repression of liver fatty acid-binding protein and microsomal triglyceride transfer protein blocks hepatic very low density lipoprotein secretion without hepatosteatosis

Unlike the livers of humans and mice, and most hepatoma cells, which accumulate triglycerides when treated with microsomal triglyceride transfer protein (MTP) inhibitors, L35 rat hepatoma cells do not express MTP and cannot secrete very low density lipoprotein (VLDL), yet they do not accumulate triglyceride. In these studies we show that transcriptional co-repression of the two lipid transfer proteins, liver fatty acid-binding protein (L-FABP) and MTP, which cooperatively shunt fatty acids into de novo synthesized glycerolipids and the transfer of lipids into VLDL, respectively, act together to maintain hepatic lipid homeostasis. FAO rat hepatoma cells express L-FABP and MTP and demonstrate the ability to assemble and secrete VLDL. In contrast, L35 cells, derived as a single cell clone from FAO cells, do not express L-FABP or MTP nor do they assemble and secrete VLDL. We used these hepatoma cells to elucidate how a conserved DR1 promoter element present in the promoters of L-FABP and MTP affects transcription, expression, and VLDL production. In FAO cells, the DR1 elements of both L-FABP and MTP promoters are occupied by peroxisome proliferator-activated receptor alpha-retinoid X receptor alpha (RXRalpha), with which PGC-1beta activates transcription. In contrast, in L35 cells the DR1 elements of both L-FABP and MTP promoters are occupied by chicken ovalbumin upstream promoter transcription factor II, and transcription is diminished. The combined findings indicate that peroxisome proliferator-activated receptor alpha-RXRalpha and PGC-1beta coordinately up-regulate L-FABP and MTP expression, by competing with chicken ovalbumin upstream promoter transcription factor II for the DR1 sites in the proximal promoters of each gene. Additional studies show that ablation of L-FABP prevents hepatic steatosis caused by treating mice with an MTP inhibitor. Our findings show that reducing both L-FABP and MTP is an effective means to reduce VLDL secretion without causing hepatic steatosis.

Mechanisms of mutual functional interactions between HNF-4alpha and HNF-1alpha revealed by mutations that cause maturity onset diabetes of the young

Hepatic nuclear factor (HNF)-4alpha and HNF-1alpha are key endodermal transcriptional regulators that physically and functionally interact. HNF-4alpha and HNF-1alpha cooperatively activate genes with binding sites for both factors, whereas suppressive interactions occur at regulatory sequences with a binding site for only one factor. The liver fatty acid binding protein gene (Fabp1) has binding sites for both factors, and chromatin precipitation assays were utilized to demonstrate that HNF-4alpha increased HNF-1alpha Fabp1 promoter occupancy during cooperative transcriptional activation. The HNF4 P2 promoter contains a HNF-1 but not HNF-4 binding site, and HNF-4alpha suppressed HNF-1alpha HNF4 P2 activation and decreased promoter HNF-1alpha occupancy. The apolipoprotein C III (APOC3) promoter contains a HNF-4 but not HNF-1 binding site, and HNF-1alpha suppressed HNF-4alpha APOC3 activation and decreased HNF-4alpha promoter occupancy. Maturity onset diabetes of the young (MODY) as well as defects in hepatic lipid metabolism result from mutations in either HNF-4alpha or HNF-1alpha. We found that MODY missense mutant R127W HNF-4alpha retained wild-type individual Fabp1 activation and bound to HNF-1alpha better than wild-type HNF-4alpha, yet did not cooperate with HNF-1alpha or increase HNF-1alpha Fabp1 promoter occupancy. The R127W mutant was also defective in both suppressing HNF-1alpha activation of HNF4 P2 and decreasing HNF-1alpha promoter occupancy. The HNF-1alpha R131Q MODY mutant also retained wild-type Fabp1 activation and bound to HNF-4alpha as well as the wild type but was defective in both suppressing HNF-4alpha APOC3 activation and decreasing HNF-4alpha promoter occupancy. These results suggest HNF-1alpha-HNF-4alpha functional interactions are accomplished by regulating factor promoter occupancy and that defective factor-factor interactions may contribute to the MODY phenotype.

C/EBP and Cdx family factors regulate liver fatty acid binding protein transgene expression in the small intestinal epithelium

A transgene constructed from the rat liver fatty acid binding protein gene (Fabp1) promoter is active in all murine small intestinal crypt and villus epithelial cells. Coincident Cdx and C/EBP transcription factor binding sites were identified spanning Fabp1 nucleotides -90 to -78. CDX-1, CDX-2, C/EBPalpha, and C/EBPbeta activated the Fabp1 transgene in CaCo-2 cells, and mutagenizing the -78 site prevented activation by these factors. CDX but not C/EBP factors bound to the site in vitro, although C/EBP factors competed with CDX factors for transgene activation. The -78 site adjoins an HNF-1 site, and CDX and C/EBP family factors cooperated with HNF-1alpha but not HNF-1beta to activate the transgene. Furthermore, CDX-1, CDX-2, C/EBPalpha, and C/EBPbeta bound to HNF-1alpha and HNF-1beta. The transgene with a mutagenized -78 site was silenced in vivo specifically in small intestinal crypt epithelial cells but remained active in villus cells. These results demonstrate functional interactions between HNF-1, C/EBP, and CDX family factors and suggest that these interactions may contribute to differential transcriptional regulation in the small intestinal crypt and villus compartments.

Differential action of 13-HPODE on PPARalpha downstream genes in rat Fao and human HepG2 hepatoma cell lines

In rats, oxidized fats activate the peroxisome proliferator-activated receptor alpha (PPARalpha), leading to reduced triglyceride concentrations in liver, plasma and very low density lipoproteins. Oxidation products of linoleic acid constitute an important portion of oxidized dietary fats. This study was conducted to check whether the primary lipid peroxidation product of linoleic acid, 13-hydroperoxy-9,11-octadecadienoic acid (13-HPODE), might be involved in the PPARalpha-activating effect of oxidized fats. Therefore, we examined the effect of 13-HPODE on the expression of PPARalpha target genes in the rat Fao and the human HepG2 hepatoma cell lines. In Fao cells, 13-HPODE increased the mRNA concentration of the PPARalpha target genes acyl-CoA oxidase (ACO), cytochrome P450 4A1 and carnitine-palmitoyltransferase 1A (CPT1A). Furthermore, the concentration of cellular and secreted triglycerides was reduced in Fao cells treated with 13-HPODE. Because PPARalpha mRNA was not influenced, we conclude that these effects are due to an activation of PPARalpha by 13-HPODE. In contrast, HepG2 cells seemed to be resistant to PPARalpha activation by 13-HPODE because no remarkable induction of the PPARalpha target genes ACO, CPT1A, mitochondrial HMG-CoA synthase and delta9-desaturase was observed. Consequently, cellular and secreted triglyceride levels were not changed after incubation of HepG2 cells with 13-HPODE. In conclusion, this study shows that 13-HPODE activates PPARalpha in rat Fao but not in human HepG2 hepatoma cells.

The peroxisome proliferator-activated receptor gamma ligand rosiglitazone delays the onset of inflammatory bowel disease in mice with interleukin 10 deficiency

AIMS

To test whether the peroxisome proliferator-activated receptor gamma (PPARgamma) ligand rosiglitazone (Ro) has therapeutic activity in the IL-10(-/-) mouse model of inflammatory bowel disease (IBD), and to identify the cellular targets and molecular mechanisms of Ro action.

METHODS

The progression of spontaneous chronic colitis in IL-10(-/-) mice was compared in 5-week-old mice fed a standard diet with or without Ro for 12 weeks. The possible therapeutic effect of Ro was also tested over a 6-week interval in older IL-10(-/-) mice with established IBD.

RESULTS

Treatment with Ro slowed the onset of spontaneous IBD in IL-10(-/-) mice. Crypt hyperplasia, caused by increased mitotic activity of crypt epithelial cells, was also delayed by Ro. Treatment with Ro significantly decreased expression of interferon gamma (IFNgamma), interleukin 17 (IL-17), tumor necrosis factor alpha, and the inducible nitric oxide synthase mRNA in the colon, whereas expression of IL-12p40 was unchanged. PPARgamma was detected in epithelial cells throughout the crypts and surface. Ro increased expression of PPARgamma protein in these cells, suggesting the existence of a positive feedback loop that would potentiate its action in these cells. Ro also specifically increased expression of a novel PPAR target, aquaporin-8 (AQP8), in differentiated colonic epithelial surface cells, demonstrating that PPARgamma is not only present but also regulates gene expression in these cells in vivo. Finally, Ro was ineffective in improving disease activity in older IL-10(-/-) mice with established IBD.

CONCLUSIONS

PPARgamma is expressed, and the PPARgamma ligand Ro regulates gene expression in colonic epithelial cells. As a single agent, Ro works best for disease prevention in the IL-10(-/-) mouse model for IBD.

Localization and regulation of SR-BI in membrane rafts of HepG2 cells

The scavenger receptor class B, type I (SR-BI) mediates cholesteryl esters (CE) selective uptake from low density lipoprotein (LDL) and high-density lipoprotein (HDL) particles. In a number of tissues expressing caveolin, SR-BI is localized in caveolae. We show using detergent-free sucrose gradients that SR-BI is found in membrane rafts devoid of caveolin-1 in the human hepatoma HepG2 cell. Perturbation of the structure of HepG2 cell membrane rafts with cholesterol oxidase or sphingomyelinase decreased LDL-CE association due to selective uptake by 60%, while HDL3-CE selective uptake was increased 2.3-fold by cholesterol oxidase but was not affected by sphingomyelinase. Sequestration of membrane cholesterol with filipin III decreased LDL-CE selective uptake by 25%, while it had no effect on HDL3-CE selective uptake. Extraction of cell membrane cholesterol with beta-cyclodextrin increased LDL- and HDL3-CE selective uptake by 1.6-fold and 3-fold, respectively. We found that CE-selective uptake from both HDL and LDL occurs by a pathway involving retro-endocytosis in HepG2 cells. An analysis of the effect of SR-BI level on the expression of critical lipid sensor and lipid binding proteins was conducted with stable transformants of HepG2 cell overexpressing SR-BI. We found that liver-type fatty acid binding protein expression level is higher in SR-BI-overexpressing cells and that caveolin-1 and sterol response element binding protein-2 levels are reduced. Thus, in this hepatic cell model, SR-BI is associated with membrane rafts devoid of caveolin and its expression affects intracellular lipid binding and lipid sensor proteins. SR-BI-dependent LDL- and HDL-CE selective uptake are affected differently by the integrity of membrane rafts, but both occur by a retroendocytic pathway in HepG2 cells.

Overlapping transcriptional programs regulated by the nuclear receptors peroxisome proliferator-activated receptor alpha, retinoid X receptor, and liver X receptor in mouse liver

Lipid homeostasis is controlled in part by the nuclear receptors peroxisome proliferator (PP)-activated receptor alpha (PPARalpha) and liver X receptor (LXR) through regulation of genes involved in fatty acid and cholesterol metabolism. Exposure to agonists of retinoid X receptor (RXR), the obligate heterodimer partner of PPARalpha, and LXR results in responses that partially overlap with those of PP. To better understand the gene networks regulated by these nuclear receptors, transcript profiles were generated from the livers of wild-type and PPARalpha-null mice exposed to the RXR pan-agonist 3,7-dimethyl-6S,7S-methano, 7-[1,1,4,4-tetramethyl-1,2,3,4-tetrahydronaphth-7-yl]-2E,4E-heptadienoic acid (AGN194,204) or the PPAR pan-agonist WY-14,643 (WY; pirinixic acid) and compared with the profiles from the livers of wild-type and LXRalpha/LXRbeta-null mice after exposure to the LXR agonist N-(2,2,2-trifluoroethyl)-N-[4-(2,2,2-trifluoro-1-hydroxy-1-trifluoromethylethyl)phenyl] sulfonamide (T0901317). All 218 WY-regulated genes altered in wild-type mice required PPARalpha. Remarkably, approximately 80% of genes regulated by AGN194,204 required PPARalpha including cell-cycle genes, consistent with AGN-induced hepatocyte proliferation having both PPARalpha-dependent and -independent components. Overlaps of approximately 31 to 62% in the transcript profiles of WY, AGN194,204, and T0901317 required PPARalpha and LXRalpha/LXRbeta for statistical significance. Ofthe 50 overlapping genes regulated by T0901317 and WY, all but one were regulated in a similar direction. These results 1) identify new transcriptional targets of PPARalpha and RXR important in regulating lipid metabolism and liver homeostasis, 2) illustrate the importance of PPARalpha in regulation of gene expression by a prototypical PP and by an RXR agonist, and 3) provide support for an axis of PPARalpha-RXR-LXR in which agonists for each nuclear receptor regulate an overlapping set of genes in the mouse liver.

Statin induction of liver fatty acid-binding protein (L-FABP) gene expression is peroxisome proliferator-activated receptor-alpha-dependent

Statins are drugs widely used in humans to treat hypercholesterolemia. Statins act by inhibiting cholesterol synthesis resulting in the activation of the transcription factor sterol-responsive element-binding protein-2 that controls the expression of genes involved in cholesterol homeostasis. Statin therapy also decreases plasma triglyceride and non-esterified fatty acid levels, but the mechanism behind this effect remains more elusive. Liver fatty acid-binding protein (L-FABP) plays a role in the influx of long-chain fatty acids into hepatocytes. Here we show that L-FABP is a target for statins. In rat hepatocytes, simvastatin treatment induced L-FABP mRNA levels in a dose-dependent manner. Moreover, L-FABP promoter activity was induced by statin treatment. Progressive 5'-deletion analysis revealed that the peroxisome proliferator-activated receptor (PPAR)-responsive element located at position -67/-55 was responsible for the statin-mediated transactivation of the rat L-FABP promoter. Moreover, treatment with simvastatin and the PPARalpha agonist Wy14,649 resulted in a synergistic induction of L-FABP expression (mRNA and protein) in rat Fao hepatoma cells. This effect was also observed in vivo in wild-type mice but not in PPARalpha-null animals demonstrating the direct implication of PPARalpha in L-FABP regulation by statin treatment. Statin treatment resulted in a rise in PPARalpha mRNA levels both in vitro and in vivo and activated the mouse PPARalpha promoter in a reporter assay. Altogether, these data demonstrate that L-FABP expression is up-regulated by statins through a mechanism involving PPARalpha. Moreover, PPARalpha might be a statin target gene. These effects might contribute to the triglyceride/non-esterified fatty acid-lowering properties of statins.

New insights into the fatty acid-binding protein (FABP) family in the small intestine

The fatty acid-binding protein (FABP) superfamily is constituted by 14-15 kDa soluble proteins which bind with a high affinity either long-chain fatty acids (LCFAs), bile acids (BAs) or retinoids. In the small intestine, three different FABP isoforms exhibiting a high affinity for LCFAs and/or BAs are expressed: the intestinal and the liver-type (I-FABP and L-FABP) and the ileal bile acid-binding protein (I-BABP). Despite of extensive investigations, their respective physiological function(s) are not clearly established. In contrast to the I-FABP, L-FABP and I-BABP share several common structural features (shape, size and volume of the hydrophobic pocket). Moreover, L-FABP and I-BABP genes are also specifically regulated by their respective preferential ligands through a very similar molecular mechanism. Although, they exhibit differences in their binding specificities and location along the small intestine supporting a specialization, it is likely that L-FABP and I-BABP genes exert the same type of basic function(s) in the enterocyte, in contrast to I-FABP.

In vivo studies of liver-type fatty acid binding protein (L-FABP) gene expression in liver of transgenic zebrafish (Danio rerio)

Mammalian liver fatty acid binding protein (L-FABP) is a small cytosolic protein in various tissues including liver, small intestine and kidney and is thought to play a crucial role in intracellular fatty acid trafficking and metabolism. To better understand its tissue-specific regulation during zebrafish hepatogenesis, we isolated 5'-flanking sequences of the zebrafish L-FABP gene and used a green fluorescent protein (GFP) transgenic strategy to generate liver-specific transgenic zebrafish. The 2.8-kb 5'-flanking sequence of zebrafish L-FABP gene was sufficient to direct GFP expression in liver primordia, first observed in 2 dpf embryos and then continuously to the adult stage. This pattern of transgenic expression is consistent with the expression pattern of the endogenous gene. F2 inheritance rates of 42-51% in all the seven transgenic lines were consistent with the ratio of Mendelian segregation. Further, hhex and zXbp-1 morphants displayed a visible liver defect, which suggests that it is possible to establish an in vivo system for screening genes required for liver development.

Enhancement of the aquaporin adipose gene expression by a peroxisome proliferator-activated receptor gamma

The current study demonstrates that aquaporin adipose (AQPap), an adipose-specific glycerol channel (Kishida, K., Kuriyama, H., Funahashi, T., Shimomura, I., Kihara, S., Ouchi, N., Nishida, M., Nishizawa, H., Matsuda, M., Takahashi, M., Hotta, K., Nakamura, T., Yamashita, S., Tochino, Y., and Matsuzawa, Y. (2000) J. Biol. Chem. 275, 20896-20902), is a target gene of peroxisome proliferator-activated receptor (PPAR) gamma. The AQPap mRNA amounts increased following the induction of PPARgamma in the differentiation of 3T3-L1 adipocytes. The AQPap mRNA in the adipose tissue increased when mice were treated with pioglitazone (PGZ), a synthetic PPARgamma ligand, and decreased in PPARgamma(+/-) heterozygous knockout mice. In 3T3-L1 adipocytes, PGZ augmented the AQPap mRNA expression and its promoter activity. Serial deletion of the promoter revealed the putative peroxisome proliferator-activated receptor response element (PPRE) at -93/-77. In 3T3-L1 preadipocytes, the expression of PPARgamma by transfection and PGZ activated the luciferase activity of the promoter containing the PPRE, whereas the PPRE-deleted mutant was not affected. The gel mobility shift assay showed the direct binding of PPARgamma-retinoid X receptor alpha complex to the PPRE. DeltaPPARgamma, which we generated as the dominant negative PPARgamma lacking the activation function-2 domain, suppressed the promoter activity in 3T3-L1 cells, dose-dependently. We conclude that AQPap is a novel adipose-specific target gene of PPARgamma through the binding of PPARgamma-retinoid X receptor complex to the PPRE region in its promoter.

Target genes of peroxisome proliferator-activated receptor gamma in colorectal cancer cells

Activation of the nuclear hormone peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits cell growth and promotes differentiation in a broad spectrum of epithelial derived tumor cell lines. Here we utilized microarray technology to identify PPARgamma gene targets in intestinal epithelial cells. For each gene, the induction or repression was seen with two structurally distinct PPARgamma agonists, and the change in expression could be blocked by co-treatment with a specific PPARgamma antagonist. A majority of the genes could be regulated independently by a retinoid X receptor specific agonist. Genes implicated in lipid transport or storage (adipophilin and liver fatty acid-binding protein) were also activated by agonists of PPAR subtypes alpha and/or delta. In contrast, PPARgamma-selective targets included genes linked to growth regulatory pathways (regenerating gene IA), colon epithelial cell maturation (GOB-4 and keratin 20), and immune modulation (neutrophil-gelatinase-associated lipocalin). Additionally, three different genes of the carcinoembryonic antigen family were induced by PPARgamma. Cultured cells treated with PPARgamma ligands demonstrated an increase in Ca(2+)-independent, carcinoembryonic antigen-dependent homotypic aggregation, suggesting a potential role for PPARgamma in regulating intercellular adhesion. Collectively, these results will help define the mechanisms by which PPARgamma regulates intestinal epithelial cell biology.

Differential involvement of peroxisome-proliferator-activated receptors alpha and delta in fibrate and fatty-acid-mediated inductions of the gene encoding liver fatty-acid-binding protein in the liver and the small intestine

Liver fatty-acid-binding protein (L-FABP) is a cytoplasmic polypeptide that binds with strong affinity especially to long-chain fatty acids (LCFAs). It is highly expressed in both the liver and small intestine, where it is thought to have an essential role in the control of the cellular fatty acid (FA) flux. Because expression of the gene encoding L-FABP is increased by both fibrate hypolipidaemic drugs and LCFAs, it seems to be under the control of transcription factors, termed peroxisome-proliferator-activated receptors (PPARs), activated by fibrate or FAs. However, the precise molecular mechanism by which these regulations take place remain to be fully substantiated. Using transfection assays, we found that the different PPAR subtypes (alpha, gamma and delta) are able to mediate the up-regulation by FAs of the gene encoding L-FABP in vitro. Through analysis of LCFA- and fibrate-mediated effects on L-FABP mRNA levels in wild-type and PPARalpha-null mice, we have found that PPARalpha in the intestine does not constitute a dominant regulator of L-FABP gene expression, in contrast with what is known in the liver. Only the PPARdelta/alpha agonist GW2433 is able to up-regulate the gene encoding L-FABP in the intestine of PPARalpha-null mice. These findings demonstrate that PPARdelta can act as a fibrate/FA-activated receptor in tissues in which it is highly expressed and that L-FABP is a PPARdelta target gene in the small intestine. We propose that PPARdelta contributes to metabolic adaptation of the small intestine to changes in the lipid content of the diet.

Differential effects of rexinoids and thiazolidinediones on metabolic gene expression in diabetic rodents

Both retinoid X receptor (RXR)-selective agonists (rexinoids) and thiazolidinediones (TZDs), PPAR (peroxisome proliferator-activated receptor)-gamma-specific ligands, produce insulin sensitization in diabetic rodents. In vitro studies have demonstrated that TZDs mediate their effects via the RXR/PPAR-gamma complex. To determine whether rexinoids lower hyperglycemia by activating the RXR/PPAR-gamma heterodimer in vivo, we compared the effects of a rexinoid (LG100268) and a TZD (rosiglitazone) on gene expression in white adipose tissue, skeletal muscle, and liver of Zucker diabetic fatty rats (ZDFs). In adipose tissue, rosiglitazone decreased tumor necrosis factor-alpha (TNF-alpha) mRNA and induced glucose transporter 4 (GLUT4), muscle carnitine palmitoyl-transferase (MCPT), stearoyl CoA desaturase (SCD1), and fatty acid translocase (CD36). In contrast, LG100268 increased TNF-alpha and had no effect or suppressed the expression of GLUT4, MCPT, SCD1, and CD36. In liver, the rexinoid increased MCPT, SCD1, and CD36 mRNAs, whereas rosiglitazone induced only a small increase in CD36. In skeletal muscle, rosiglitazone and LG100268 have similar effects; both increased SCD1 and CD36 mRNAs. The differences in the pattern of genes induced by the rexinoids and the TZDs in diabetic animals found in these studies suggests that these compounds may have independent and tissue-specific effects on metabolic control in vivo.

The expression of PPAR-associated genes is modulated through postnatal development of PPAR subtypes in the small intestine

In this study, we found that the mRNA level of peroxisome proliferator-activated receptor (PPAR) alpha, but not of PPARdelta, was elevated in the jejunum during the postnatal development of the rat. Moreover, we found that the expressions of PPAR-dependent genes, such as acyl-CoA oxidase, L-FABP, and I-FABP, were also increased during the postnatal development of the small intestine. Electrophoretic mobility shift assay revealed that both the PPARalpha-9-cis-retinoic acid receptor alpha (RXRalpha) heterodimer and the PPARdelta-RXRalpha heterodimer bound to the peroxisome proliferator response element (PPRE) of acyl-CoA oxidase and L-FABP genes. The binding of the PPARalpha-RXRalpha heterodimer to the PPREs of the various genes was enhanced by the addition of PPARalpha, with a concomitant reduction of the binding of PPARdelta-RXRalpha to the PPREs. Furthermore, the binding activity of PPARalpha-RXRalpha, but not PPARdelta-RXRalpha, to the PPREs was enhanced by the addition of a PPAR ligand, WY14,643. The GAL4-PPAR-chimera reporter assay showed that WY14,643 transactivated the reporter gene through action of PPARalpha, but not through PPARdelta, in Caco-2 cells. Furthermore, oral administration of a PPAR ligand, clofibrate, during 3 consecutive days of the weanling period caused a parallel increase in the mRNA levels of these PPAR-dependent genes. These results suggest that acyl-CoA oxidase, L-FABP and the other PPAR-dependent genes in the small intestine may be coordinately modulated during postnatal development by the disproportional expression of PPARalpha over PPARdelta.

Lipocalins and cancer

Lipocalins are mainly extracellular carriers of lipophilic molecules, though exceptions with properties like prostaglandin synthesis and protease inhibition are observed for specific lipocalins. The interest concerning lipocalins in cancer has so far been focussed to the variations in concentration and the modification of lipocalin expression in distinct cancer forms. In addition, lipocalins have been assigned a role in cell regulation. The influence of the extracellular lipocalins on intracellular cell regulation events is not fully understood, but several of the lipocalin ligands are also well-known agents in cell differentiation and proliferation. Lipophilic ligands can, after lipocalin-mediated transport to the cell surface, penetrate the cell membrane and interact with proteins in the cytosol and/or the nucleus. The signaling routes of the lipocalin ligands, retinoids and fatty acids are presented and discussed. Tumor growth in tissue is restricted by extracellular protease/protease inhibitor interactions. Several lipocalins also have protease inhibitory properties and possess the ability to interact with tumor specific proteases, revealing another pathway for lipocalins to interact with cancer cells.

Peroxisome proliferator-activated receptor alpha-mediated pathways are altered in hepatocyte-specific retinoid X receptor alpha-deficient mice

Retinoid x receptor alpha (RXRalpha) serves as an active partner of peroxisome proliferator-activated receptor (PPARalpha). In order to dissect the functional role of RXRalpha and PPARalpha in PPARalpha-mediated pathways, the hepatocyte RXRalpha-deficient mice have been challenged with physiological and pharmacological stresses, fasting and Wy14,643, respectively. The data demonstrate that RXRalpha and PPARalpha deficiency are different in several aspects. At the basal untreated level, RXRalpha deficiency resulted in marked induction of apolipoprotein A-I and C-III (apoA-I and apoC-III) mRNA levels and serum cholesterol and triglyceride levels, which was not found in PPARalpha-null mice. Fasting-induced PPARalpha activation was drastically prevented in the absence of hepatocyte RXRalpha. Wy14,643-mediated pleiotropic effects were also altered due to the absence of hepatocyte RXRalpha. Hepatocyte RXRalpha deficiency did not change the basal acyl-CoA oxidase, medium chain acyl-CoA dehydrogenase, and malic enzyme mRNA levels. However, the inducibility of those genes by Wy14,643 was markedly reduced in the mutant mouse livers. In contrast, the basal cytochrome P450 4A1, liver fatty acid-binding protein, and apoA-I and apoC-III mRNA levels were significantly altered in the mutant mouse livers, but the regulatory effect of Wy14,643 on expression of those genes remained the same. Wy14,643-induced hepatomegaly was partially inhibited in hepatocyte RXRalpha-deficient mice. Wy14,643-induced hepatocyte peroxisome proliferation was preserved in the absence of hepatocyte RXRalpha. These data suggested that in comparison to PPARalpha, hepatocyte RXRalpha has its unique role in lipid homeostasis and that the effect of RXRalpha, -beta, and -gamma is redundant in certain aspects.

High-fat diets affect the expression of nuclear retinoic acid receptor in rat liver

The purpose of this study was to differentiate between the effects of the amount and the type of dietary lipids on the expression of the retinoic acid receptor (RAR), but also the peroxisome proliferator-activated receptor (PPAR) and the receptor of the 9-cis retinoic acid (retinoid X receptor (RXR)) in rat liver. Six groups of eight rats (5-weeks old) were fed during 4 weeks on the following diets: control 50 g vegetable oil/kg, high-fat diet 250 g vegetable oil/kg. These oils were either coconut oil (rich in saturated fatty acids) or olive oil (rich in monounsaturated fatty acids) or safflower oil (rich in polyunsaturated fatty acids, mainly as n-6). The three high-fat diets induced a significant decrease of the maximal binding capacity of RAR and of the abundance of RAR beta mRNA. Simultaneously, an increased expression of PPAR alpha mRNA was observed while no significant difference on abundance of RXR alpha mRNA was observed. The mechanisms involved are probably multiple, but one hypothesis is that a modification of the equilibrium between the nuclear receptors, resulting from an increased expression of PPAR, induces a decreased expression of RAR in rat liver.

Induction of the fatty acid transport protein 1 and acyl-CoA synthase genes by dimer-selective rexinoids suggests that the peroxisome proliferator-activated receptor-retinoid X receptor heterodimer is their molecular target

The intracellular fatty acid content of insulin-sensitive target tissues determines in part their insulin sensitivity. Uptake of fatty acids into cells is a controlled process determined in part by a regulated import/export system that is controlled at least by two key groups of proteins, i.e. the fatty acid transport protein (FATP) and acyl-CoA synthetase (ACS), which facilitate, respectively, the transport of fatty acids across the cell membrane and catalyze their esterification to prevent their efflux. Previously it was shown that the expression of the FATP-1 and ACS genes was controlled by insulin and by peroxisome proliferator-activated receptor (PPAR) agonists in liver or in adipose tissue. The aim of this investigation was to determine the effects of retinoic acid derivatives on the expression of FATP-1 and ACS. In several cultured cell lines, it was shown that the expression of both the FATP-1 and ACS mRNAs was specifically induced at the transcriptional level by selective retinoid X receptor (RXR) but not by retinoic acid receptor (RAR) ligands. This effect was most pronounced in hepatoma cell lines. A similar induction of FATP-1 and ACS mRNA levels was also observed in vivo in Zucker diabetic fatty rats treated with the RXR agonist, LGD1069 (4-[1-(3,5,5,8,8-pentamethyl-5,6,7, 8-tetrahydro-2-naphthyl)ethenyl]benzoic acid). Through the use of heterodimer-selective compounds, it was demonstrated that the modulatory effect of these rexinoids on FATP-1 and ACS gene expression was mediated through activation of RXR in the context of the PPAR-RXR heterodimer. The observation that both RXR and PPAR agonists can stimulate the transcription of genes implicated in lipid metabolism, suggest that rexinoids may also act as lipid-modifying agents and support a role of the permissive PPAR-RXR heterodimer in the control of insulin sensitivity.

Phosphorylation of peroxisome proliferator-activated receptor alpha in rat Fao cells and stimulation by ciprofibrate

The basic mechanism(s) by which peroxisome proliferators activate peroxisome proliferator-activated receptors (PPARs) is (are) not yet fully understood. Given the diversity of peroxisome proliferators, several hypotheses of activation have been proposed. Among them is the notion that peroxisome proliferators could activate PPARs by changing their phosphorylation status. In fact, it is well known that several members of the nuclear hormone receptor superfamily are regulated by phosphorylation. In this report, we show that the rat Fao hepatic-derived cell line, known to respond to peroxisome proliferators, exhibited a high content of PPARalpha. Alkaline phosphatase treatment of Fao cell lysate as well as immunoprecipitation of PPARalpha from cells prelabeled with [32P] orthophosphate clearly showed that PPARalpha is indeed a phosphoprotein in vivo. Moreover, treatment of rat Fao cells with ciprofibrate, a peroxisome proliferator, increased the phosphorylation level of the PPARalpha. In addition, treatment of Fao cells with phosphatase inhibitors (okadaic acid and sodium orthovanadate) decreased the activity of ciprofibrate-induced peroxisomal acyl-coenzyme A oxidase, an enzyme encoded by a PPARalpha target gene. Our results suggest that the gene expression controlled by peroxisome proliferators could be mediated in part by a modulation of the PPARalpha effect via a modification of the phosphorylation level of this receptor.

Peroxisome proliferator-activated receptor alpha mediates the adaptive response to fasting

Prolonged deprivation of food induces dramatic changes in mammalian metabolism, including the release of large amounts of fatty acids from the adipose tissue, followed by their oxidation in the liver. The nuclear receptor known as peroxisome proliferator-activated receptor alpha (PPARalpha) was found to play a role in regulating mitochondrial and peroxisomal fatty acid oxidation, suggesting that PPARalpha may be involved in the transcriptional response to fasting. To investigate this possibility, PPARalpha-null mice were subjected to a high fat diet or to fasting, and their responses were compared with those of wild-type mice. PPARalpha-null mice chronically fed a high fat diet showed a massive accumulation of lipid in their livers. A similar phenotype was noted in PPARalpha-null mice fasted for 24 hours, who also displayed severe hypoglycemia, hypoketonemia, hypothermia, and elevated plasma free fatty acid levels, indicating a dramatic inhibition of fatty acid uptake and oxidation. It is shown that to accommodate the increased requirement for hepatic fatty acid oxidation, PPARalpha mRNA is induced during fasting in wild-type mice. The data indicate that PPARalpha plays a pivotal role in the management of energy stores during fasting. By modulating gene expression, PPARalpha stimulates hepatic fatty acid oxidation to supply substrates that can be metabolized by other tissues.