Peroxisome Proliferator-Activated Receptor Delta: A Conserved Director of Lipid Homeostasis through Regulation of the Oxidative Capacity of Muscle. PPAR Res. 2008;2008:172676. [PMID: 18815630 PMCID: PMC2547483 DOI: 10.1155/2008/172676]

Fasting upregulates the monocarboxylate transporter MCT1 at the rat blood-brain barrier through PPAR δ activation

BACKGROUND

The blood-brain barrier (BBB) is pivotal for the maintenance of brain homeostasis and it strictly regulates the cerebral transport of a wide range of endogenous compounds and drugs. While fasting is increasingly recognized as a potential therapeutic intervention in neurology and psychiatry, its impact upon the BBB has not been studied. This study was designed to assess the global impact of fasting upon the repertoire of BBB transporters.

METHODS

We used a combination of in vivo and in vitro experiments to assess the response of the brain endothelium in male rats that were fed ad libitum or fasted for one to three days. Brain endothelial cells were acutely purified and transcriptionaly profiled using RNA-Seq. Isolated brain microvessels were used to assess the protein expression of selected BBB transporters through western blot. The molecular mechanisms involved in the adaptation to fasting were investigated in primary cultured rat brain endothelial cells. MCT1 activity was probed by in situ brain perfusion.

RESULTS

Fasting did not change the expression of the main drug efflux ATP-binding cassette transporters or P-glycoprotein activity at the BBB but modulated a restrictive set of solute carrier transporters. These included the ketone bodies transporter MCT1, which is pivotal for the brain adaptation to fasting. Our findings in vivo suggested that PPAR δ, a major lipid sensor, was selectively activated in brain endothelial cells in response to fasting. This was confirmed in vitro where pharmacological agonists and free fatty acids selectively activated PPAR δ, resulting in the upregulation of MCT1 expression. Moreover, dosing rats with a specific PPAR δ antagonist blocked the upregulation of MCT1 expression and activity induced by fasting.

CONCLUSIONS

Altogether, our study shows that fasting affects a selected set of BBB transporters which does not include the main drug efflux transporters. Moreover, we describe a previously unknown selective adaptive response of the brain vasculature to fasting which involves PPAR δ and is responsible for the up-regulation of MCT1 expression and activity. Our study opens new perspectives for the metabolic manipulation of the BBB in the healthy or diseased brain.

Transcription factors KLF15 and PPARδ cooperatively orchestrate genome-wide regulation of lipid metabolism in skeletal muscle

Skeletal muscle dynamically regulates systemic nutrient homeostasis through transcriptional adaptations to physiological cues. In response to changes in the metabolic environment (e.g., alterations in circulating glucose or lipid levels), networks of transcription factors and coregulators are recruited to specific genomic loci to fine-tune homeostatic gene regulation. Elucidating these mechanisms is of particular interest as these gene regulatory pathways can serve as potential targets to treat metabolic disease. The zinc-finger transcription factor Krüppel-like factor 15 (KLF15) is a critical regulator of metabolic homeostasis; however, its genome-wide distribution in skeletal muscle has not been previously identified. Here, we characterize the KLF15 cistrome in vivo in skeletal muscle and find that the majority of KLF15 binding is localized to distal intergenic regions and associated with genes related to circadian rhythmicity and lipid metabolism. We also identify critical interdependence between KLF15 and the nuclear receptor PPARδ in the regulation of lipid metabolic gene programs. We further demonstrate that KLF15 and PPARδ colocalize genome-wide, physically interact, and are dependent on one another to exert their transcriptional effects on target genes. These findings reveal that skeletal muscle KLF15 plays a critical role in metabolic adaptation through its direct actions on target genes and interactions with other nodal transcription factors such as PPARδ.

Explaining Unsaturated Fatty Acids (UFAs), Especially Polyunsaturated Fatty Acid (PUFA) Content in Subcutaneous Fat of Yaks of Different Sex by Differential Proteome Analysis

Residents on the Tibetan Plateau intake a lot of yak subcutaneous fat by diet. Modern healthy diet ideas demand higher unsaturated fatty acids (UFAs), especially polyunsaturated fatty acid (PUFA) content in meat. Here, the gas chromatography (GC) and tandem mass tag (TMT) proteomic approaches were applied to explore the relationship between the proteomic differences and UFA and PUFA content in the subcutaneous fat of yaks with different sex. Compared with male yaks (MYs), the absolute contents of UFAs, monounsaturated fatty acids (MUFAs) and PUFAs in the subcutaneous fat of female yaks (FYs) were all higher (p < 0.01); the relative content of MUFAs and PUFAs in MY subcutaneous fat was higher, and the value of PUFAs/SFAs was above 0.4, so the MY subcutaneous fat is more healthy for consumers. Further studies showed the transcriptional regulation by peroxisome proliferator-activated receptor delta (PPARD) played a key role in the regulation of UFAs, especially PUFA content in yaks of different sex. In FY subcutaneous fat, the higher abundance of the downstream effector proteins in PPAR signal, including acyl-CoA desaturase (SCD), elongation of very-long-chain fatty acids protein 6 (ELOVL6), lipoprotein lipase (LPL), fatty acid-binding protein (FABP1), very-long-chain (3R)-3-hydroxyacyl-CoA dehydratase 3 (HACD3), long-chain fatty acid CoA ligase 5 (ACSL5) and acyl-CoA-binding protein 2 (ACBP2), promoted the UFAs’ transport and synthesis. The final result was the higher absolute content of c9-C14:1, c9-C18:1, c9,c12-C18:2n-6, c9, c12, c15-C18:3n-3, c5, c8, c11, c14, c17-C20:5n-3, c4, c7, c10, c13, -c16, c19-C22:6n-3, UFAs, MUFAs and PUFAs in FY subcutaneous fat. Further, LPL, FABP1, HACD3, ACSL1 and ACBP2 were the potential biomarkers for PUFA contents in yak subcutaneous fat. This study provides new insights into the molecular mechanisms associated with UFA contents in yak subcutaneous fat.

The effect of toll-like receptor ligands on energy metabolism and myokine expression and secretion in cultured human skeletal muscle cells

Skeletal muscle plays an important role in glycaemic control and metabolic homeostasis, making it a tissue of interest with respect to type 2 diabetes mellitus. The aim of the present study was to determine if ligands of Toll-like receptors (TLRs) could have an impact on energy metabolism and myokine expression and secretion in cultured human skeletal muscle cells. The myotubes expressed mRNA for TLRs 1–6. TLR3, TLR4, TLR5 and TLR6 ligands (TLRLs) increased glucose metabolism. Furthermore, TLR4L and TLR5L increased oleic acid metabolism. The metabolic effects of TLRLs were not evident until after at least 24 h pre-incubation of the cells and here the metabolic effects were more evident for the metabolism of glucose than oleic acid, with a shift towards effects on oleic acid metabolism after chronic exposure (168 h). However, the stimulatory effect of TLRLs on myokine expression and secretion was detected after only 6 h, where TLR3-6L stimulated secretion of interleukin-6 (IL-6). TLR5L also increased secretion of interleukin-8 (IL-8), while TLR6L also increased secretion of granulocyte–macrophage colony stimulating factor (GM-CSF). Pre-incubation of the myotubes with IL-6 for 24 h increased oleic acid oxidation but had no effect on glucose metabolism. Thus IL-6 did not mimic all the metabolic effects of the TLRLs, implying metabolic effects beyond the actions of this myokine.

Decreased Glucocorticoid Signaling Potentiates Lipid-Induced Inflammation and Contributes to Insulin Resistance in the Skeletal Muscle of Fructose-Fed Male Rats Exposed to Stress

The modern lifestyle brings both excessive fructose consumption and daily exposure to stress which could lead to metabolic disturbances and type 2 diabetes. Muscles are important points of glucose and lipid metabolism, with a crucial role in the maintenance of systemic energy homeostasis. We investigated whether 9-week fructose-enriched diet, with and without exposure to 4-week unpredictable stress, disturbs insulin signaling in the skeletal muscle of male rats and evaluated potential contributory roles of muscle lipid metabolism, glucocorticoid signaling and inflammation. The combination of fructose-enriched diet and stress increased peroxisome proliferator-activated receptors-α and -δ and stimulated lipid uptake, lipolysis and β-oxidation in the muscle of fructose-fed stressed rats. Combination of treatment also decreased systemic insulin sensitivity judged by lower R-QUICKI, and lowered muscle protein content and stimulatory phosphorylations of insulin receptor supstrate-1 and Akt, as well as the level of 11β-hydroxysteroid dehydrogenase type 1 and glucocorticoid receptor. At the same time, increased levels of protein tyrosine phosphatase-1B, nuclear factor-κB, tumor necrosis factor-α, were observed in the muscle of fructose-fed stressed rats. Based on these results, we propose that decreased glucocorticoid signaling in the skeletal muscle can make a setting for lipid-induced inflammation and the development of insulin resistance in fructose-fed stressed rats.

Mitochondrial Utilization of Competing Fuels Is Altered in Insulin Resistant Skeletal Muscle of Non-obese Rats (Goto-Kakizaki)

Aim

Insulin-resistant skeletal muscle is characterized by metabolic inflexibility with associated alterations in substrate selection, mediated by peroxisome-proliferator activated receptor δ (PPARδ). Although it is established that PPARδ contributes to the alteration of energy metabolism, it is not clear whether it plays a role in mitochondrial fuel competition. While nutrient overload may impair metabolic flexibility by fuel congestion within mitochondria, in absence of obesity defects at a mitochondrial level have not yet been excluded. We sought to determine whether reduced PPARδ content in insulin-resistant rat skeletal muscle of a non-obese rat model of T2DM (Goto-Kakizaki, GK) ameliorate the inhibitory effect of fatty acid (i.e., palmitoylcarnitine) on mitochondrial carbohydrate oxidization (i.e., pyruvate) in muscle fibers.

Methods

Bioenergetic function was characterized in oxidative soleus (S) and glycolytic white gastrocnemius (WG) muscles with measurement of respiration rates in permeabilized fibers in the presence of complex I, II, IV, and fatty acid substrates. Mitochondrial content was measured by citrate synthase (CS) and succinate dehydrogenase activity (SDH). Western blot was used to determine protein expression of PPARδ, PDK isoform 2 and 4.

Results

CS and SDH activity, key markers of mitochondrial content, were reduced by ∼10-30% in diabetic vs. control, and the effect was evident in both oxidative and glycolytic muscles. PPARδ (p < 0.01), PDK2 (p < 0.01), and PDK4 (p = 0.06) protein content was reduced in GK animals compared to Wistar rats (N = 6 per group). Ex vivo respiration rates in permeabilized muscle fibers determined in the presence of complex I, II, IV, and fatty acid substrates, suggested unaltered mitochondrial bioenergetic function in T2DM muscle. Respiration in the presence of pyruvate was higher compared to palmitoylcarnitine in both animal groups and fiber types. Moreover, respiration rates in the presence of both palmitoylcarnitine and pyruvate were reduced by 25 ± 6% (S), 37 ± 6% (WG) and 63 ± 6% (S), 57 ± 8% (WG) compared to pyruvate for both controls and GK, respectively. The inhibitory effect of palmitoylcarnitine on respiration was significantly greater in GK than controls (p < 10^-3).

Conclusion

With competing fuels, the presence of fatty acids diminishes mitochondria ability to utilize carbohydrate derived substrates in insulin-resistant muscle despite reduced PPARδ content.

Genomic and Non-Genomic Mechanisms of Action of Thyroid Hormones and Their Catabolite 3,5-Diiodo-L-Thyronine in Mammals

Since the realization that the cellular homologs of a gene found in the retrovirus that contributes to erythroblastosis in birds (v-erbA), i.e. the proto-oncogene c-erbA encodes the nuclear receptors for thyroid hormones (THs), most of the interest for THs focalized on their ability to control gene transcription. It was found, indeed, that, by regulating gene expression in many tissues, these hormones could mediate critical events both in development and in adult organisms. Among their effects, much attention was given to their ability to increase energy expenditure, and they were early proposed as anti-obesity drugs. However, their clinical use has been strongly challenged by the concomitant onset of toxic effects, especially on the heart. Notably, it has been clearly demonstrated that, besides their direct action on transcription (genomic effects), THs also have non-genomic effects, mediated by cell membrane and/or mitochondrial binding sites, and sometimes triggered by their endogenous catabolites. Among these latter molecules, 3,5-diiodo-L-thyronine (3,5-T2) has been attracting increasing interest because some of its metabolic effects are similar to those induced by T3, but it seems to be safer. The main target of 3,5-T2 appears to be the mitochondria, and it has been hypothesized that, by acting mainly on mitochondrial function and oxidative stress, 3,5-T2 might prevent and revert tissue damages and hepatic steatosis induced by a hyper-lipid diet, while concomitantly reducing the circulating levels of low density lipoproteins (LDL) and triglycerides. Besides a summary concerning general metabolism of THs, as well as their genomic and non-genomic effects, herein we will discuss resistance to THs and the possible mechanisms of action of 3,5-T2, also in relation to its possible clinical use as a drug.

Ovary removal modifies liver message RNA profiles in single Comb White Leghorn chickens

Ovaries produce sex hormones, and ovariectomized animals are often used as models for ovarian dysfunction. The liver is a vital organ involved in metabolism and immunity. In the present study, we conducted experiments to investigate the effects of ovariectomy on transcription and metabolic processes in the liver in chicken. Eight Single Comb White Leghorn (SCWL) female chickens were ovariectomized at 17 wk of age, and 8 intact SCWL females served as controls. At 100 wk of age, all chickens were euthanized. High-throughput transcriptome sequencing was performed on liver RNA obtained from ovariectomized and intact females. A total of 267 differentially expressed genes (DEG) were identified in our study. After analysis using DAVID functional annotation tool, one significant Kyoto Encyclopedia of Genes and Genomes pathway, the phosphatidylinositol signaling pathway, was clustered. Gene Ontology enrichment analysis yielded 46 significant Gene Ontology terms. Among terms describing biological processes, the glycerolipid metabolic and lipid localization processes were dominant. The anabolic genes, PEPCK and GK5, and the catabolic genes, VTG1; VTG2; PLD5; DGKQ; DGKE; and FABP3, were detected in ovariectomized chickens. Differentially expressed genes such as ENSGALG00000000162, IL-1Β, SVOPL, and CA12 implied that livers in ovariectomized chickens were subjected to strong inflammatory reactions, whereas defenses against endogenous materials were compromised. A comprehensive view of gene expression in the liver of ovariectomized chickens would advance our understanding of lipid metabolism, glycometabolism, and their relationships to pathologies induced by absence of the ovary. The identified DEG indicated that ovariectomy disturbed lipid metabolism in the liver and was accompanied by an increase in hepatic gluconeogenesis and reductions in phosphatidic acid synthesis and lipid carrier capacity.

The PPARδ Agonist GW501516 Improves Lipolytic/Lipogenic Balance through CPT1 and PEPCK during the Development of Pre-Implantation Bovine Embryos

The PPARs (peroxisome proliferator-activated receptors) play critical roles in the regulation of lipid and glucose metabolism. PPARδ, a member of the PPARs family, is associated with decreased susceptibility to ectopic lipid deposition and is implicated in the regulation of mitochondrial processes. The current study aimed to determine the role of PPARδ in fatty acid β-oxidation and its influence on PEPCK for the lipogenic/lipolytic balance during in vitro bovine oocyte maturation and embryo development. Activation of PPARδ by GW501516, but not 2-BP, was indicated by intact embryonic PEPCK (cytosolic) and CPT1 expression and the balance between free fatty acids and mitochondrial β-oxidation that reduced ROS and inhibited p-NF-κB nuclear localization. Genes involved in lipolysis, fatty acid oxidation, and apoptosis showed significant differences after the GW501516 treatment relative to the control- and 2-BP-treated embryos. GSK3787 reversed the PPARδ-induced effects by reducing PEPCK and CPT1 expression and the mitochondrial membrane potential, revealing the importance of PPARδ/PEPCK and PPARδ/CPT1 for controlling lipolysis during embryo development. In conclusion, GW501516-activated PPARδ maintained the correlation between lipolysis and lipogenesis by enhancing PEPCK and CPT1 to improve bovine embryo quality.

Comparative population genomics unveils candidate genes for athletic performance in Hanoverians

Equine athletes have a genetic heritage that has been evolved for millions of years, which provides an opportunity to study the genetics of locomotion pattern and performance in mammals. The Hanoverian, a breed originating in Germany, is arguably among the most athletic of horse breeds, as well as possessing a balanced character and beautiful appearance. Here, we compared the whole genomes of Hanoverian with three other horse breeds (Akhal-Teke, Franches-Montagnes, and Standardbred), using the fixation index (Fst) and cross-population composite likelihood ratio (XP-CLR) methods for testing the multi-locus allele frequency differentiation between populations. We identified 299 and 485 positively selected genes using the Fst and XP-CLR methods, respectively. Further functional analyses showed that the ACTA1 gene is potentially involved in athletic performance in the Hanoverian breed, consistent with its role observed in human population. In addition, three other loci on chromosomes 1 and 20 were identified to be potentially involved in equine physical performance. The selected candidate genes identified in this study may be useful in current breeding efforts to develop improved breeds in regard to athletic performance.

Models of non-Alcoholic Fatty Liver Disease and Potential Translational Value: the Effects of 3,5-L-diiodothyronine

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disorder in industrialized countries and is associated with increased risk of cardiovascular, hepatic and metabolic diseases. Molecular mechanisms on the root of the disrupted lipid homeostasis in NAFLD and potential therapeutic strategies can benefit of in vivo and in vitro experimental models of fatty liver. Here, we describe the high fat diet (HFD)-fed rat in vivo model, and two in vitro models, the primary cultured rat fatty hepatocytes or the FaO rat hepatoma fatty cells, mimicking human NAFLD. Liver steatosis was invariably associated with increased number/size of lipid droplets (LDs) and modulation of expression of genes coding for key genes of lipid metabolism such as peroxisome proliferator-activated receptors (Ppars) and perilipins (Plins). In these models, we tested the anti-steatotic effects of 3,5-L-diiodothyronine (T2), a metabolite of thyroid hormones. T2 markedly reduced triglyceride content and LD size acting on mRNA expression of both Ppars and Plins. T2 also stimulated mitochondrial oxidative metabolism of fatty acids. We conclude that in vivo and especially in vitro models of NAFLD are valuable tools to screen a large number of compounds counteracting the deleterious effect of liver steatosis. Because of the high and negative impact of liver steatosis on human health, ongoing experimental studies from our group are unravelling the ultimate translational value of such cellular models of NAFLD.

Peroxisome proliferator-activated receptor-delta agonist ameliorated inflammasome activation in nonalcoholic fatty liver disease

AIM: To evaluate the inflammasome activation and the effect of peroxisome proliferator-activated receptors (PPAR)-δ agonist treatment in nonalcoholic fatty liver disease (NAFLD) models.

METHODS: Male C57BL/6J mice were classified according to control or high fat diet (HFD) with or without PPAR-δ agonist (GW) over period of 12 wk [control, HFD, HFD + lipopolysaccharide (LPS), HFD + LPS + GW group]. HepG2 cells were exposed to palmitic acid (PA) and/or LPS in the absence or presence of GW.

RESULTS: HFD caused glucose intolerance and hepatic steatosis. In mice fed an HFD with LPS, caspase-1 and interleukin (IL)-1β in the liver were significantly increased. Treatment with GW ameliorated the steatosis and inhibited overexpression of pro-inflammatory cytokines. In HepG2 cells, PA and LPS treatment markedly increased mRNA of several nucleotide-binding and oligomerization domain-like receptor family members (NLRP3, NLRP6, and NLRP10), caspase-1 and IL-1β. PA and LPS also exaggerated reactive oxygen species production. All of the above effects of PA and LPS were reduced by GW. GW also enhanced the phosphorylation of AMPK-α.

CONCLUSION: PPAR-δ agonist reduces fatty acid-induced inflammation and steatosis by suppressing inflammasome activation. Targeting the inflammasome by the PPAR-δ agonist may have therapeutic implication for NAFLD.

Molecular characterization of carnitine palmitoyltransferase IA in Megalobrama amblycephala and effects on its expression of feeding status and dietary lipid and berberine

Carnitine palmitoyltransferase I (CPT I, EC 2.3.1.21) controls the main regulatory step of fatty acid oxidation, and hence studies of its molecular characterization are useful to understand lipid metabolism in cultured fish. Here, a full-length cDNA coding CPT I was cloned from liver of blunt snout bream Megalobrama amblycephala. This cDNA obtained covered 2499bp with an open reading frame of 2181bp encoding 726 amino acids. This CPT I mRNA predominantly expressed in heart and white muscle, while little in eye and spleen. The phylogenetic tree constructed on the basis of sequence alignments among several vertebrate species suggests that this blunt snout bream CPT I sequence belongs to the CPT IA family. In order to investigate the characterization of CPT IA mRNA expression, post-prandial experiment and feeding trial were conducted. The results showed that CPT IA mRNA expression was unchanged from 2 to 12h, and then significantly increased at 24h post-feeding in liver and heart. Berberine, an alkaloid, was identified as a promising lipid-lowering drug. In order to elucidate the effect of berberine on CPT I expression, fish were fed for 8 weeks with three diets (low-fat diet (LFD, 5% fat), high-fat diet (HFD, 15% fat), and berberine-supplemented diet (BSD, 15% fat). The results showed that HFD could decrease the expression of CPT IA and PPARα, while BSD increased those expressions.

Nitrooleic Acid Attenuates Lipid Metabolic Disorders and Liver Steatosis in DOCA-Salt Hypertensive Mice

Nitrooleic acid (OA-NO2) is endogenous ligands for peroxisome proliferator-activated receptors. The present study was aimed at investigating the beneficial effects of OA-NO2 on the lipid metabolism and liver steatosis in deoxycorticosterone acetate- (DOCA-) salt induced hypertensive mice model. Male C57BL/6 mice were divided to receive DOCA-salt plus OA-NO2 or DOCA-salt plus vehicle and another group received neither DOCA-salt nor OA-NO2 (control group). After 3-week treatment with DOCA-salt plus 1% sodium chloride in drinking fluid, the hypertension was noted; however, OA-NO2 had no effect on the hypertension. In DOCA-salt treated mice, the plasma triglyceride and total cholesterol levels were significantly increased compared to control mice, and pretreatment with OA-NO2 significantly reduced these parameters. Further, the histopathology of liver exhibited more lipid distribution together with more serious micro- and macrovesicular steatosis after DOCA-salt treatment and that was consistent with liver tissue triglyceride and nonesterified fatty acids (NEFA) content. The mice pretreated with OA-NO2 showed reduced liver damage accompanied with low liver lipid content. Moreover, the liver TBARS, together with the expressions of gp91phox and p47phox, were parallelly decreased. These findings indicated that OA-NO2 had the protective effect on liver injury against DOCA-salt administration and the beneficial effect could be attributed to its antihyperlipidemic activities.

PPARα-independent actions of omega-3 PUFAs contribute to their beneficial effects on adiposity and glucose homeostasis

Excess dietary lipid generally leads to fat deposition and impaired glucose homeostasis, but consumption of fish oil (FO) alleviates many of these detrimental effects. The beneficial effects of FO are thought to be mediated largely via activation of the nuclear receptor peroxisomal-proliferator-activated receptor α (PPARα) by omega-3 polyunsaturated fatty acids and the resulting upregulation of lipid catabolism. However, pharmacological and genetic PPARα manipulations have yielded variable results. We have compared the metabolic effects of FO supplementation and the synthetic PPARα agonist Wy-14,643 (WY) in mice fed a lard-based high-fat diet. In contrast to FO, WY treatment resulted in little protection against diet-induced obesity and glucose intolerance, despite upregulating many lipid metabolic pathways. These differences were likely due to differential effects on hepatic lipid synthesis, with FO decreasing and WY amplifying hepatic lipid accumulation. Our results highlight that the beneficial metabolic effects of FO are likely mediated through multiple independent pathways.

Nuclear Receptors, RXR, and the Big Bang

Isolation of genes encoding the receptors for steroids, retinoids, vitamin D, and thyroid hormone and their structural and functional analysis revealed an evolutionarily conserved template for nuclear hormone receptors. This discovery sparked identification of numerous genes encoding related proteins, termed orphan receptors. Characterization of these orphan receptors and, in particular, of the retinoid X receptor (RXR) positioned nuclear receptors at the epicenter of the "Big Bang" of molecular endocrinology. This Review provides a personal perspective on nuclear receptors and explores their integrated and coordinated signaling networks that are essential for multicellular life, highlighting the RXR heterodimer and its associated ligands and transcriptional mechanism.

PPARδ activation in human myotubes increases mitochondrial fatty acid oxidative capacity and reduces glucose utilization by a switch in substrate preference

The role of peroxisome proliferator-activated receptor δ (PPARδ) activation on global gene expression and mitochondrial fuel utilization were investigated in human myotubes. Only 21 genes were up-regulated and 3 genes were down-regulated after activation by the PPARδ agonist GW501516. Pathway analysis showed up-regulated mitochondrial fatty acid oxidation, TCA cycle and cholesterol biosynthesis. GW501516 increased oleic acid oxidation and mitochondrial oxidative capacity by 2-fold. Glucose uptake and oxidation were reduced, but total substrate oxidation was not affected, indicating a fuel switch from glucose to fatty acid. Cholesterol biosynthesis was increased, but lipid biosynthesis and mitochondrial content were not affected. This study confirmed that the principal effect of PPARδ activation was to increase mitochondrial fatty acid oxidative capacity. Our results further suggest that PPARδ activation reduced glucose utilization through a switch in mitochondrial substrate preference by up-regulating pyruvate dehydrogenase kinase isozyme 4 and genes involved in lipid metabolism and fatty acid oxidation.

PPARs: history and advances

Peroxisome proliferator-activated receptors (PPARs) are members of the steroid hormone receptor superfamily, discovered in 1990. To date, three PPAR subtypes have been identified; PPARα, PPAR β/δ, and PPARγ. These receptors share a high degree of homology but differ in tissue distribution and ligand specificity. PPARs have been implicated in the etiology as well as treatment of several important diseases and pathological conditions such as diabetes, inflammation, senescence-related diseases, regulation of fertility, and various types of cancer. Consequently, significant efforts to discover novel PPAR roles and delineate molecular mechanisms involved in their activation and repression as well as develop safer and more effective PPAR modulators, as therapeutic agents to treat a myriad of diseases and conditions, are underway. This volume of Methods in Molecular Biology contains details of experimental protocols used in researching these receptors.

Effect of 2,4-thiazolidinedione on limousin cattle growth and on muscle and adipose tissue metabolism

The main adipogenic transcription factor PPARγ possesses high affinity to 2,4-TZD, a member of the Thiazolidinedione family of insulin-sensitizing compounds used as adipogenic agents. We evaluated 2,4-TZD's effect on bovine growth and PPAR tissue expression. Seventeen Limousin bulls (18 month-old; 350 kg body weight (BW)) were assigned into 2 treatments: control and 2,4-TZD (8 mg/70 kg BW) and were fed until bulls reached 500 kg BW. They were weighed and their blood was sampled. DNA, RNA, and protein were determined in liver; skeletal muscle; subcutaneous (SC), omental, perirenal adipose tissues (AT) to determine protein synthesis rate and cellular size. Expression of PPAR mRNA was measured in liver and muscle (PPARα, -δ, and -γ) and SC adipose tissue (γ) by real-time PCR. No significant differences were found (P > 0.1) in weight gain, days on feed, and carcass quality. Muscle synthesis was greater in controls (P < 0.05); cell size was larger with 2,4-TZD (P < 0.05). PPARα, -δ, and -γ expressions with 2,4-TZD in liver were lower (P < 0.01) than in muscle. No differences were found for PPARγ mRNA expression in SCAT. The results suggest the potential use of 2,4-TZD in beef cattle diets, because it improves AT differentiation, liver, and muscle fatty acid oxidation that, therefore, might improve energy efficiency.

Skin-targeted inhibition of PPAR β/δ by selective antagonists to treat PPAR β/δ-mediated psoriasis-like skin disease in vivo

We have previously shown that peroxisome proliferator activating receptor ß/δ (PPAR β/δ is overexpressed in psoriasis. PPAR β/δ is not present in adult epidermis of mice. Targeted expression of PPAR β/δ and activation by a selective synthetic agonist is sufficient to induce an inflammatory skin disease resembling psoriasis. Several signalling pathways dysregulated in psoriasis are replicated in this model, suggesting that PPAR β/δ activation contributes to psoriasis pathogenesis. Thus, inhibition of PPAR β/δ might harbour therapeutical potential. Since PPAR β/δ has pleiotropic functions in metabolism, skin-targeted inhibition offer the potential of reducing systemic adverse effects. Here, we report that three selective PPAR β/δ antagonists, GSK0660, compound 3 h, and GSK3787 can be formulated for topical application to the skin and that their skin concentration can be accurately quantified using ultra-high performance liquid chromatography (UPLC)/mass spectrometry. These antagonists show efficacy in our transgenic mouse model in reducing psoriasis-like changes triggered by activation of PPAR β/δ. PPAR β/δ antagonists GSK0660 and compound 3 do not exhibit systemic drug accumulation after prolonged application to the skin, nor do they induce inflammatory or irritant changes. Significantly, the irreversible PPAR β/δ antagonist (GSK3787) retains efficacy when applied topically only three times per week which could be of practical clinical usefulness. Our data suggest that topical inhibition of PPAR β/δ to treat psoriasis may warrant further exploration.

The contrasting roles of PPARδ and PPARγ in regulating the metabolic switch between oxidation and storage of fats in white adipose tissue

BACKGROUND

The nuclear receptors peroxisome proliferator-activated receptor γ (PPARγ) and peroxisome proliferator-activated receptor δ (PPARδ) play central roles in regulating metabolism in adipose tissue, as well as being targets for the treatment of insulin resistance. While the role of PPARγ in regulating insulin sensitivity has been well defined, research into PPARδ has been limited until recently due to a scarcity of selective PPARδ agonists.

RESULTS

The metabolic effects of PPARγ and PPARδ activation have been examined in vivo in white adipose tissue from ob/ob mice and in vitro in cultured 3T3-L1 adipocytes using (1)H nuclear magnetic resonance spectroscopy and mass spectrometry metabolomics to understand the receptors' contrasting roles. These steady state measurements were supplemented with (13)C-stable isotope substrate labeling to assess fluxes, in addition to respirometry and transcriptomic microarray analysis. The metabolic effects of the receptors were readily distinguished, with PPARγ activation characterized by increased fat storage, synthesis and elongation, while PPARδ activation caused increased fatty acid β-oxidation, tricarboxylic acid cycle rate and oxidation of extracellular branch chain amino acids. Stimulated glycolysis and increased fatty acid desaturation were common pathways for the agonists.

CONCLUSIONS

PPARγ and PPARδ restore insulin sensitivity through varying mechanisms. PPARδ activation increases total oxidative metabolism in white adipose tissue, a tissue not traditionally thought of as oxidative. However, the increased metabolism of branch chain amino acids may provide a mechanism for muscle atrophy, which has been linked to activation of this nuclear receptor. PPARδ has a role as an anti-obesity target and as an anti-diabetic, and hence may target both the cause and consequences of dyslipidemia.

Direct effects of iodothyronines on excess fat storage in rat hepatocytes

BACKGROUND & AIMS

Previous studies have demonstrated that 3,5-L-diiodothyronine (T(2)) is able to prevent lipid accumulation in the liver of rats fed a high-fat diet. Whether this effect is due to a direct action of T(2) on the liver has not been elucidated. In this study, we investigated the ability of T(2) to reduce the excess lipids in isolated hepatocytes treated with fatty acids (FFAs). The effects of T(2) were compared with those elicited by 3,3',5-L-triiodothyronine (T(3)).

METHODS

To mimic the fatty liver condition, primary cultures of rat hepatocytes were overloaded with lipids, by exposure to FFAs ("fatty hepatocytes"), and then treated with T(2) or T(3). Lipid content, morphometry of lipid droplets (LDs), and expression of the adipocyte differentiation-related protein (ADRP) and the peroxisome proliferator-activated receptors (PPAR-α, -γ, -δ) were evaluated. Activities of the lipolytic enzyme acyl CoA oxidase-AOX and the antioxidant enzymes superoxide dismutase-SOD and catalase-CAT were also determined.

RESULTS

FFA-induced lipid accumulation was associated with an increase in both number/size of LDs and expression of ADRP, PPAR-γ, and PPAR-δ/β mRNAs, as well as in the activities of AOX, SOD, and CAT. The addition of T(2) or T(3) to "fatty hepatocytes" resulted in a reduction in: (i) lipid content and LD diameter; (ii) PPAR-γ and PPAR-δ expression; (iii) activities of AOX and antioxidant enzymes.

CONCLUSIONS

These data demonstrate, for the first time, a direct action of both T(2) and T(3) in reducing the excess fat in cultured hepatocytes.

PPARs: Nuclear Receptors Controlled by, and Controlling, Nutrient Handling through Nuclear and Cytosolic Signaling

Peroxisome proliferator-activated receptors (PPARs), which are known to regulate lipid homeostasis, are tightly controlled by nutrient availability, and they control nutrient handling. In this paper, we focus on how nutrients control the expression and action of PPARs and how cellular signaling events regulate the action of PPARs in metabolically active tissues (e.g., liver, skeletal muscle, heart, and white adipose tissue). We address the structure and function of the PPARs, and their interaction with other nuclear receptors, including PPAR cross-talk. We further discuss the roles played by different kinase pathways, including the extracellular signal-regulated kinases/mitogen-activated protein kinase (ERK MAPK), AMP-activated protein kinase (AMPK), Akt/protein kinase B (Akt/PKB), and the NAD+-regulated protein deacetylase SIRT1, serving to control the activity of the PPARs themselves as well as that of a key nutrient-related PPAR coactivator, PPARgamma coactivator-1alpha (PGC-1alpha). We also highlight how currently applied nutrigenomic strategies will increase our understanding on how nutrients regulate metabolic homeostasis through PPAR signaling.

Effects of Endogenous PPAR Agonist Nitro-Oleic Acid on Metabolic Syndrome in Obese Zucker Rats

Nitroalkene derivatives of nitro-oleic acid (OA-NO₂) are endogenous lipid products with novel signaling properties, particularly the activation of PPARs. The goal of this proposal was to examine the therapeutic potential of this OA-NO₂ in treatment of obesity and obesity-related conditions in obese Zucker rats. The animals were randomly divided to receive OA-NO₂, oleic acid (OA), both at 7.5 μg/kg/d, or vehicle ethanol via osmotic mini-pumps for 2 weeks. Following OA-NO₂ treatment, food intake was decreased as early as the first day and this effect appeared to persist throughout the experimental period. At day 14, body weight gain was significantly reduced by OA-NO₂ treatment. This treatment significantly reduced plasma triglyceride and almost normalized plasma free fatty acid and significantly increased plasma high-density lipid (HDL). The plasma TBARS and proteinuria were paralelly decreased. In contrast, none of these parameters were affected by OA treatment. After 14 days of OA-NO₂ treatment, hematocrit, a surrogate of fluid retention associated with PPARγ agonists, remained unchanged. Together, these data demonstrated that OA-NO₂ may offer an effective and safe therapeutic intervention for obesity and obesity-related conditions.

Transcriptional targets in adipocyte biology

The global burden of metabolic disease demands that we develop new therapeutic strategies. Many of these approaches may center on manipulating the behavior of adipocytes, which contribute directly and indirectly to a host of disease processes including obesity and type 2 diabetes. One way to achieve this goal will be to alter key transcriptional pathways in fat cells, such as those regulating glucose uptake, lipid handling, or adipokine secretion. In this review, we look at what is known about how adipocytes govern their physiology at the gene expression level, and discuss novel ways that we can accelerate our understanding of this area.

Mimetics of hormetic agents: stress-resistance triggers

Mimetics of hormetic agents offer a novel approach to adjust dose to minimize the risk of toxic response, and maximize the benefit of induction of at least partial physiological conditioning. Nature selected and preserved those organisms and triggers that promote tolerance to stress. The induced tolerance can serve to resist that challenge and can repair previous age, disease, and trauma damage as well to provide a more youthful response to other stresses. The associated physiological conditioning may include youthful restoration of DNA repair, resistance to oxidizing pollutants, protein structure and function repair, improved immunity, tissue remodeling, adjustments in central and peripheral nervous systems, and altered metabolism. By elucidating common pathways activated by hormetic agent's mimetics, new strategies for intervention in aging, disease, and trauma emerge. Intervention potential in cancer, diabetes, age-related diseases, infectious diseases, cardiovascular diseases, and Alzheimer's disease are possible. Some hormetic mimetics exist in pathways in primitive organisms and are active or latent in humans. Peptides, oligonucleotides, and hormones are among the mimetics that activate latent resistance to radiation, physical endurance, strength, and immunity to physiological condition tolerance to stress. Co-activators may be required for expression of the desired physiological conditioning health and rejuvenation benefits.

Improved insulin sensitivity and islet function after PPARdelta activation in diabetic db/db mice

The peroxisome proliferator-activated receptors (PPARs) are transcription factors belonging to the nuclear receptor superfamily. Several reports have shown that PPARdelta is involved in lipid metabolism, increasing fat oxidation and depleting lipid accumulation. Whether PPARdelta is involved in the regulation of glucose metabolism is not completely understood. In this study, we examined effects of long-term PPARdelta activation on glycemic control, islet function and insulin sensitivity in diabetic db/db mice. Male db/db mice were administered orally once daily with a selective and partial PPARdelta agonist (NNC 61-5920, 30 mg/kg) for eight weeks; control mice received vehicle. Fasting and non-fasting plasma glucose were reduced, reflected in reduced hemoglobinA(1c) (3.6+/-1.6% vs. 5.4+/-1.8 in db/db controls, P<0.05) and furthermore, the AUC(glucose) after oral glucose (3g/kg) was reduced by 67% (P<0.05) after long-term PPARdelta activation. Following intravenous glucose (1g/kg), glucose tolerance was improved after PPARdelta activation (K(G) 1.3+/-0.6 vs. -0.05+/-0.7 %/min, P=0.048). Insulin sensitivity, measured as the glucose clearance after intravenous injection of glucose (1g/kg) and insulin (0.75 or 1.0 U/kg), during inhibition of endogenous insulin secretion by diazoxide (25mg/kg), was improved (K(G) 2.9+/-0.6 vs. 1.3+/-0.3 %/min in controls, P<0.05) despite lower insulin levels. Furthermore, islets isolated from PPARdelta agonist treated mice demonstrated improved glucose responsiveness as well as improved cellular topography. In conclusion, PPARdelta agonism alleviates insulin resistance and improves islet function and topography, resulting in improved glycemia in diabetic db/db mice. This suggests that activation of PPARdelta improves glucose metabolism and may therefore potentially be target for treatment of type 2 diabetes.