Three members of the human pyruvate dehydrogenase kinase gene family are direct targets of the peroxisome proliferator-activated receptor beta/delta

The nuclear receptors peroxisome proliferator-activated receptors (PPARs) are known for their critical role in the metabolic syndrome. Here, we show that they are direct regulators of the family of pyruvate dehydrogenase kinase (PDK) genes, whose products act as metabolic homeostats in sensing hunger and satiety levels in key metabolic tissues by modulating the activity of the pyruvate dehydrogenase complex. Mis-regulation of this tightly controlled network may lead to hyperglycemia. In human embryonal kidney cells we found the mRNA expression of PDK2, PDK3 and PDK4 to be under direct primary control of PPAR ligands, and in normal mouse kidney tissue Pdk2 and Pdk4 are PPAR targets. Both, treatment of HEK cells with PPARbeta/delta-specific siRNA and the genetic disruption of the Pparbeta/delta gene in mouse fibroblasts resulted in reduced expression of Pdk genes and abolition of induction by PPARbeta/delta ligands. These findings suggest that PPARbeta/delta is a key regulator of PDK genes, in particular the PDK4/Pdk4 gene. In silico analysis of the human PDK genes revealed two candidate PPAR response elements in the PDK2 gene, five in the PDK3 gene and two in the PDK4 gene, but none in the PDK1 gene. For seven of these sites we could demonstrate both PPARbeta/delta ligand responsiveness in context of their chromatin region and simultaneous association of PPARbeta/delta with its functional partner proteins, such as retinoidXreceptor, co-activator and mediator proteins and phosphorylated RNA polymerase II. In conclusion, PDK2, PDK3 and PDK4 are primary PPARbeta/delta target genes in humans underlining the importance of the receptor in the control of metabolism.     

Citral,a component of lemongrass oil,activates PPARα and γ and suppresses COX-2 expression

Lemongrass is a widely used herb as a food flavoring, as a perfume, and for its analgesic and anti-inflammatory purposes; however, the molecular mechanisms of these effects have not been elucidated. Previously, we identified carvacrol from the essential oil of thyme as a suppressor of cyclooxygenase (COX)-2, a key enzyme for prostaglandin synthesis, and also an activator of peroxisome proliferator-activated receptor (PPAR), a molecular target for “lifestyle-related” diseases. In this study, we evaluated the essential oil of lemongrass using our established assays for COX-2 and PPARs. We found that COX-2 promoter activity was suppressed by lemongrass oil in cell-based transfection assays, and we identified citral as a major component in the suppression of COX-2 expression and as an activator of PPARα and γ. PPARγ-dependent suppression of COX-2 promoter activity was observed in response to citral treatment. In human macrophage-like U937 cells, citral suppressed both LPS-induced COX-2 mRNA and protein expression, dose-dependently. Moreover, citral induced the mRNA expression of the PPARα-responsive carnitine palmitoyltransferase 1 gene and the PPARγ-responsive fatty acid binding protein 4 gene, suggesting that citral activates PPARα and γ, and regulates COX-2 expression. These results are important for understanding the anti-inflammatory and anti-lifestyle-related disease properties of lemongrass.     

microRNA-211 promotes invasion and migration of colorectal cancer cells by targeting FABP4 via PPARγ

Fatty acid binding protein 4 (FABP4) is a novel tumor regulator that is abnormally expressed in many human cancers. In our study, upregulated microRNA-211 (miR-211) and reduced FABP4 expression were detected in colorectal cancer (CRC) patients and CRC cells. Mimic miR-211 or anti-miR-211 were transfected to investigate the effects of miR-211 on SW480 cells. The results showed that miR-211 promoted but anti-miR-211 inhibited cell migration, invasion, and epithelial–mesenchymal transition (EMT) of SW480 cells. Luciferase activity was decreased after cotransfection with miR-211 and WT-FABP4-UTR in SW480 cells. And reduced FABP4 protein expression by miR-211 indicated that FABP4 was the targeted gene of miR-211. miR-211 inhibited the activation of peroxisome proliferator-activated receptor (PPAR) γ, whereas overexpression of FABP4 reversed that effect. Finally, FABP4 inhibited the migration, invasion, and EMT of SW480 cells, whereas PPARγ agonist reversed the effects of FABP4. Thus, the miR-211/FABP4/PPARγ axis may be a novel target for CRC therapy.     

Regulation of inflammatory and lipid metabolism genes by eicosapentaenoic acid-rich oil

Omega-3-PUFAs, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), are associated with prevention of various aspects of metabolic syndrome. In the present studies, the effects of oil rich in EPA on gene expression and activation of nuclear receptors was examined and compared with other ω3-PUFAs. The EPA-rich oil (EO) altered the expression of FA metabolism genes in THP-1 cells, including stearoyl CoA desaturase (SCD) and FA desaturase-1 and -2 (FASDS1 and -2). Other ω3-PUFAs resulted in a similar gene expression response for a subset of genes involved in lipid metabolism and inflammation. In reporter assays, EO activated human peroxisome proliferator-activated receptor α (PPARα) and PPARβ/γ with minimal effects on PPARγ, liver X receptor, retinoid X receptor, farnesoid X receptor, and retinoid acid receptor γ (RARγ); these effects were similar to that observed for purified EPA. When serum from a 6 week clinical intervention with dietary supplements containing olive oil (control), DHA, or two levels of EPA were applied to THP-1 cells, the expression of SCD and FADS2 decreased in the cells treated with serum from the ω3-PUFA-supplemented individuals. Taken together, these studies indicate regulation of gene expression by EO that is consistent with treating aspects of dyslipidemia and inflammation.     

A pan-PPAR ligand induces hepatic fatty acid oxidation in PPARα−/− mice possibly through PGC-1 mediated PPARδ coactivation

Tetradecylthioacetic acid (TTA) is a hypolipidemic modified fatty acid and a peroxisome proliferator-activated receptor (PPAR) ligand. The mechanisms of TTA-mediated effects seem to involve the PPARs, but the effects have not been assigned to any specific PPAR subtype. PPARα−/− mice were employed to study the role of PPARα after TTA treatment. We also performed in vitro transfection assays to obtain mechanistic knowledge of how TTA affected PPAR activation in the presence of PPARγ coactivator (PGC)-1 and steroid receptor coactivators (SRC)-1 and SRC-2, which are associated with energy balance and mitochondrial biogenesis. We show that TTA increases hepatic fatty acid β-oxidation in PPARα−/− mice. TTA acts as a pan-PPAR ligand in vitro, and PGC-1, SRC-1 and SRC-2 have cell type and PPAR-specific effects together with TTA. In the absence of exogenous ligands, SRC-1 did not induce PPAR activity, while PGC-1 was the most potent PPAR coactivator. When the coactivators were overexpressed, pronounced effects of TTA were observed especially for PPARδ and PPARγ. We conclude that PPARα is involved in, but not required for, the hypolipidemic mechanisms of TTA. It appears that the activity of PPARδ, with substantial contribution of nuclear receptor coactivators, PGC-1 in special, is conducive to TTA's mechanism of action.     

Impact of high altitude on the hepatic fatty acid oxidation and synthesis in rats

High altitude (HA) affects energy metabolism. The impact of acute and chronic HA acclimatization on the major metabolic pathways is still controversial. In this study, we aimed to unveil the impact of HA on the key enzymes involved in the fatty acid (FA) metabolism in liver. Rats were exposed to an altitude of 4300 m for 30 days and the expressions of two key proteins involved in FA β-oxidation (carnitine palmitoyl transferase I, CPT-I; and peroxisome proliferator-activated receptor alpha, PPARα), two proteins involved in FA synthesis (acetyl CoA carboxylase-1, ACC-1; and AMP-activated protein kinase, AMPK), as well as the total ketone body in the liver and the plasma FFAs were examined. Rats without HA exposure were used as controls. We observed that the acute exposure of rats to HA (3 days) led to a significant increase in the expressions of CPT-I and PPARα and in the total hepatic ketone body. Longer exposure (15 days) caused a marked decrease in the expression of CPT-I and PPARα. By 30 days after HA exposure, the expression levels of CPT-I and PPARα returned to the control level. The hepatic ACC-1 level showed a significant increase in rats exposed to HA for 1 and 3 days. In contrast, the hepatic level of AMPK showed a significant reduction throughout the experimental period. Plasma FFA concentrations did not show any significant changes following HA exposure. Thus, increased hepatic FA oxidation and synthesis in the early phase of HA exposure may be among the important mechanisms for the rats to respond to the hypoxic stress in order to acclimatize themselves to the stressful environments.     

Mutations in Arabidopsis acyl-CoA oxidase genes reveal distinct and overlapping roles in beta-oxidation

Indole-3-butyric acid (IBA) is an endogenous auxin used to enhance rooting during propagation. To better understand the role of IBA, we isolated Arabidopsis IBA-response (ibr) mutants that display enhanced root elongation on inhibitory IBA concentrations but maintain wild-type responses to indole-3-acetic acid, the principle active auxin. A subset of ibr mutants remains sensitive to the stimulatory effects of IBA on lateral root initiation. These mutants are not sucrose dependent during early seedling development, indicating that peroxisomal beta-oxidation of seed storage fatty acids is occurring. We used positional cloning to determine that one mutant is defective in ACX1 and two are defective in ACX3, two of the six Arabidopsis fatty acyl-CoA oxidase (ACX) genes. Characterization of T-DNA insertion mutants defective in the other ACX genes revealed reduced IBA responses in a third gene, ACX4. Activity assays demonstrated that mutants defective in ACX1, ACX3, or ACX4 have reduced fatty acyl-CoA oxidase activity on specific substrates. Moreover, acx1 acx2 double mutants display enhanced IBA resistance and are sucrose dependent during seedling development, whereas acx1 acx3 and acx1 acx5 double mutants display enhanced IBA resistance but remain sucrose independent. The inability of ACX1, ACX3, and ACX4 to fully compensate for one another in IBA-mediated root elongation inhibition and the ability of ACX2 and ACX5 to contribute to IBA response suggests that IBA-response defects in acx mutants may reflect indirect blocks in peroxisomal metabolism and IBA beta-oxidation, rather than direct enzymatic activity of ACX isozymes on IBA-CoA.     

Role of adipocyte lipid-binding protein (ALBP) and acyl-CoA binding protein (ACBP) in PPAR-mediated transactivation

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that are activated by a number of fatty acids and fatty acid derivatives. By contrast, we have recently shown that acyl-CoA esters display PPAR antagonistic properties in vitro. We have also shown that the adipocyte lipid binding protein (ALBP), the keratinocyte lipid binding protein (KLBP) and the acyl-CoA binding protein (ACBP) exhibit a prominent nuclear localization in differentiating 3T3-L1 adipocytes. Similarly, ectopic expression of these proteins in CV-1 cells resulted in a primarily nuclear localization. We therefore speculated that FABPs and ACBP might regulate the availability of PPAR agonists and antagonists by affecting not only their esterification in the cytoplasm but also their transport to and availability in the nucleus. We show here that coexpression of ALBP or ACBP exerts a negative effect on ligand-dependent PPAR transactivation, when tetradecylthioacetic (TTA) is used as ligand but not when the thiazolidinedione BRL49653 is used as ligand. The results presented here do not support the hypothesis that ALBP facilitates the transport of the fatty acid-type ligands to the nucleus, rather ALBP appears to sequester or increase the turn-over of the agonist. Similarly, our results are in keeping with a model in which ACBP increase the metabolism of these ligands.     

Activation of PPARδ counteracts angiotensin II-induced ROS generation by inhibiting rac1 translocation in vascular smooth muscle cells

Abstract

Angiotensin II (Ang II)-mediated modification of the redox milieu of vascular smooth muscle cells (VSMCs) has been implicated in several pathophysiological processes, including cell proliferation, migration and differentiation. In this study, we demonstrate that the peroxisome proliferator-activated receptor (PPAR) δ counteracts Ang II-induced production of reactive oxygen species (ROS) in VSMCs. Activation of PPARδ by GW501516, a specific ligand for PPARδ, significantly reduced Ang II-induced ROS generation in VSMCs. This effect was, however, reversed in the presence of small interfering (si)RNA against PPARδ. The marked increase in ROS levels induced by Ang II was also eliminated by the inhibition of phosphatidylinositol 3-kinase (PI3K) but not of protein kinase C, suggesting the involvement of the PI3K/Akt signalling pathway in this process. Accordingly, ablation of Akt with siRNA further enhanced the inhibitory effects of GW501516 in Ang II-induced superoxide production. Ligand-activated PPARδ also blocked Ang II-induced translocation of Rac1 to the cell membrane, inhibiting the activation of NADPH oxidases and consequently ROS generation. These results indicate that ligand-activated PPARδ plays an important role in the cellular response to oxidative stress by decreasing ROS generated by Ang II in vascular cells.     

Nutritional regulation and role of peroxisome proliferator-activated receptor δ in fatty acid catabolism in skeletal muscle

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors primarily involved in lipid homeostasis. PPARδ displays strong expression in tissues with high lipid metabolism, such as adipose, intestine and muscle. Its role in skeletal muscle remains largely unknown. After a 24-h starvation period, PPARδ mRNA levels are dramatically up-regulated in gastrocnemius muscle of mice and restored to control level upon refeeding. The rise of PPARδ is accompanied by parallel up-regulations of fatty acid translocase/CD36 (FAT/CD36) and heart fatty acid binding protein (H-FABP), while refeeding promotes down-regulation of both genes. To directly access the role of PPARδ in muscle cells, we forced its expression and that of a dominant-negative PPARδ mutant in C2C12 myogenic cells. Differentiated C2C12 cells responds to 2-bromopalmitate or synthetic PPARδ agonist by induction of genes involved in lipid metabolism and increment of fatty acid oxidation. Overexpression of PPARδ enhanced these cellular responses, whereas expression of the dominant-negative mutant exerts opposite effects. These data strongly support a role for PPARδ in the regulation of fatty acid oxidation in skeletal muscle and in adaptive response of this tissue to lipid catabolism.     

Angiotensin II receptor blocker telmisartan enhances running endurance of skeletal muscle through activation of the PPAR‐δ/AMPK pathway

Clinical trials have shown that angiotensin II receptor blockers reduce the new onset of diabetes in hypertensives; however, the underlying mechanisms remain unknown. We investigated the effects of telmisartan on peroxisome proliferator activated receptor γ (PPAR‐δ) and the adenosine monophosphate (AMP)‐activated protein kinase (AMPK) pathway in cultured myotubes, as well as on the running endurance of wild‐type and PPAR‐δ‐deficient mice. Administration of telmisartan up‐regulated levels of PPAR‐δ and phospho‐AMPKα in cultured myotubes. However, PPAR‐δ gene deficiency completely abolished the telmisartan effect on phospho‐AMPKαin vitro. Chronic administration of telmisartan remarkably prevented weight gain, enhanced running endurance and post‐exercise oxygen consumption, and increased slow‐twitch skeletal muscle fibres in wild‐type mice, but these effects were absent in PPAR‐δ‐deficient mice. The mechanism is involved in PPAR‐δ‐mediated stimulation of the AMPK pathway. Compared to the control mice, phospho‐AMPKα level in skeletal muscle was up‐regulated in mice treated with telmisartan. In contrast, phospho‐AMPKα expression in skeletal muscle was unchanged in PPAR‐δ‐deficient mice treated with telmisartan. These findings highlight the ability of telmisartan to improve skeletal muscle function, and they implicate PPAR‐δ as a potential therapeutic target for the prevention of type 2 diabetes.     

Revealing a steroid receptor ligand as a unique PPARγ agonist

Peroxisome proliferator-activated receptor gamma (PPARγ) regulates metabolic homeostasis and is a molecular target for anti-diabetic drugs. We report here the identification of a steroid receptor ligand, RU-486, as an unexpected PPARγ agonist, thereby uncovering a novel signaling route for this steroid drug. Similar to rosiglitazone, RU-486 modulates the expression of key PPARγ target genes and promotes adipocyte differentiation, but with a lower adipogenic activity. Structural and functional studies of receptor-ligand interactions reveal the molecular basis for a unique binding mode for RU-486 in the PPARγ ligand-binding pocket with distinctive properties and epitopes, providing the molecular mechanisms for the discrimination of RU-486 from thiazolidinediones (TZDs) drugs. Our findings together indicate that steroid compounds may represent an alternative approach for designing non-TZD PPARγ ligands in the treatment of insulin resistance.