The Peroxisomal 3-keto-acyl-CoA thiolase B Gene Expression Is under the Dual Control of PPARα and HNF4α in the Liver

Is the peroxisome proliferator-activated receptor gamma a putative target for epilepsy treatment? Current evidence and future perspectives

The peroxisome proliferator-activated receptor gamma (PPARγ), which belongs to the family of nuclear receptors, has been mainly studied as an important factor in metabolic disorders. However, in recent years the potential role of PPARγ in different neurological diseases has been increasingly investigated. Especially, in the search of therapeutic targets for patients with epilepsy the question of the involvement of PPARγ in seizure control has been raised. Epilepsy is a chronic neurological disorder causing a major impact on the psychological, social, and economic conditions of patients and their families, besides the problems of the disease itself. Considering that the world prevalence of epilepsy ranges between 0.5% - 1.0%, this condition is the fourth for importance among the other neurological disorders, following migraine, stroke, and dementia. Among others, temporal lobe epilepsy (TLE) is the most common form of epilepsy in adult patients. About 65% of individuals who receive antiseizure medications (ASMs) experience seizure independence. For those in whom seizures still recur, investigating PPARγ could lead to the development of novel ASMs. This review focuses on the most important findings from recent investigations about the potential intracellular PPARγ-dependent processes behind different compounds that exhibited anti-seizure effects. Additionally, recent clinical investigations are discussed along with the promising results found for PPARγ agonists and the ketogenic diet (KD) in various rodent models of epilepsy.

Human Peroxisomal 3-Ketoacyl-CoA Thiolase: Tissue Expression and Metabolic Regulation : Human Peroxisomal Thiolase

This paper reports that the human peroxisomal 3-ketoacyl-CoA thiolase expression shows three transcripts: Tr1 (1705 bp), Tr2 (1375 bp) and Tr3 (1782 bp). Their highest expression is observed in the human liver and at a lesser extent in hepatic-derived HepG2 cells. The intestine and blood and endothelial cells show lower expression. The lowest expression is found in adipocytes. The transcript Tr3 appears to be the most abundant. So far, no data have been published regarding the regulation of the human peroxisomal thiolase. After cloning a fragment of the 5' region involved in the regulation of the human thiolase gene, the effects of different treatments have been studied on the thiolase expression in the hepatoma HepG2 human cell line. Biocomputing analysis indicates that (i) a GRE (glucocorticoid response element) is located at -650 bp upstream of the transcription initiation site; (ii) a C/EBPα (CCAAT/enhancer-binding protein) binding site is located at - 1000 bp upstream of the transcription initiation site - and (iii) there is no putative PPRE (peroxisome proliferator-activated receptor response element). In the human HepG2 cells, thiolase expression is upregulated by glucose and downregulated by insulin and sterols, while dexamethasone and fatty acids have no effect. The ciprofibrate, a peroxisome proliferator, leads only to a weak stimulation of the mRNA expression as compared to thiolase B expression in the rat liver.

Inactivation of SREBP-1a Phosphorylation Prevents Fatty Liver Disease in Mice: Identification of Related Signaling Pathways by Gene Expression Profiles in Liver and Proteomes of Peroxisomes

The key lipid metabolism transcription factor sterol regulatory element-binding protein (SREBP)-1a integrates gene regulatory effects of hormones, cytokines, nutrition and metabolites as lipids, glucose, or cholesterol via phosphorylation by different mitogen activated protein kinase (MAPK) cascades. We have previously reported the impact of SREBP-1a phosphorylation on the phenotype in transgenic mouse models with liver-specific overexpression of the N-terminal transcriptional active domain of SREBP-1a (alb-SREBP-1a) or a MAPK phosphorylation site-deficient variant (alb-SREBP-1a∆P; (S63A, S117A, T426V)), respectively. In this report, we investigated the molecular basis of the systemic observations by holistic analyses of gene expression in liver and of proteome patterns in lipid-degrading organelles involved in the pathogenesis of metabolic syndrome, i.e., peroxisomes, using 2D-DIGE and mass spectrometry. The differences in hepatic gene expression and peroxisomal protein patterns were surprisingly small between the control and alb-SREBP-1a mice, although the latter develop a severe phenotype with visceral obesity and fatty liver. In contrast, phosphorylation site-deficient alb-SREBP-1a∆P mice, which are protected from fatty liver disease, showed marked differences in hepatic gene expression and peroxisomal proteome patterns. Further knowledge-based analyses revealed that disruption of SREBP-1a phosphorylation resulted in massive alteration of cellular processes, including signs for loss of targeting lipid pathways.

Peroxisomes in cardiomyocytes and the peroxisome / peroxisome proliferator-activated receptor-loop

It is well established that the heart is strongly dependent on fatty acid metabolism. In cardiomyocytes there are two distinct sites for the β-oxidisation of fatty acids: the mitochondrion and the peroxisome. Although the metabolism of these two organelles is believed to be tightly coupled, the nature of this relationship has not been fully investigated. Recent research has established the significant contribution of mitochondrial function to cardiac ATP production under normal and pathological conditions. In contrast, limited information is available on peroxisomal function in the heart. This is despite these organelles harbouring metabolic pathways that are potentially cardioprotective, and findings that patients with peroxisomal diseases, such as adult Refsum’s disease, can develop heart failure. In this article, we provide a comprehensive overview on the current knowledge of peroxisomes and the regulation of lipid metabolism by PPARs in cardiomyocytes. We also present new experimental evidence on the differential expression of peroxisome-related genes in the heart chambers and demonstrate that even a mild peroxisomal biogenesis defect (Pex11α-/- ) can induce profound alterations in the cardiomyocyte’s peroxisomal compartment and related gene expression, including the concomitant deregulation of specific PPARs. The possible impact of peroxisomal dysfunction in the heart is discussed and a model for the modulation of myocardial metabolism via a peroxisome/PPAR-loop is proposed.

PPARα Downregulates Hepatic Glutaminase Expression in Mice Fed Diets with Different Protein:Carbohydrate Ratios

BACKGROUND

Glutamine is catabolized in the liver by glutaminase 2 (GLS2). Evidence suggests that peroxisome proliferator-activated receptor α (PPARα) represses the expression of several amino acid-catabolizing enzymes, but for Gls2 this is unknown.

OBJECTIVE

The aim of the study was to assess whether PPARα regulates Gls2 expression.

METHODS

For 8 d, 7-9-wk-old male C57BL/6 wild-type (WT) and Ppara-null mice weighing 23.4 ± 0.5 g were fed diets with different dietary protein:carbohydrate (DP:DCH) ratios (6%:77%, 20%:63%, or 50%:33%). Liver samples were obtained after 16 h of feed deprivation or 3 h of refeeding, and microarrays were performed. Hepatic glutaminase expression was measured by quantitative polymerase chain reaction and Western blotting. Cotransfection analyses in hepatocellular carcinoma cell line (HepG2) cells with PPARα and hepatocyte nuclear factor 4α (HNF4α) expression vectors were performed.

RESULTS

The microarray results showed that Gls2 was the only upregulated gene in WT mice, but not in the Ppara-null mice. In the feed-deprived WT mice, the Gls2 mRNA and protein abundances in the 50%:33% group were 2.5- and 1.1-fold greater (P < 0.05), respectively, than those in the 20%:63% group, which were 2.3- and 0.4-fold greater than those in the 6%:77% group (P < 0.01). Gls2 mRNA expression in the 6%:77% group of feed-deprived Ppara-null mice was 33-fold greater than that in the same group of WT mice (P < 0.0001). GLS2 protein abundance in HepG2 cells was 78% greater than that in the controls (P < 0.0001) after HNF4α overexpression, and it was 99% greater after transfection with a short hairpin targeting PPARα.

CONCLUSIONS

In Ppara-null mice, Gls2 mRNA expression was greater than in WT mice, regardless of the DP:DCH ratio. In HepG2 cells overexpressing HNF4α, Gls2 expression increased, an effect repressed by overexpression of PPARα. This suggests that Gls2 depends on the PPARα/HNF4α counterregulatory transcriptional control.

PPAR-alpha and PPAR-beta expression changes in the hippocampus of rats undergoing global cerebral ischemia/reperfusion due to PPAR-gamma status

Background

Peroxisome proliferator-activated receptors (PPARs, including alpha, beta and gamma subtypes) and their agonists have a protective role in treatment of central nervous system (CNS) diseases. The present study was designed to investigate the expression changes of PPAR-alpha, -beta, -gamma and NF-kappa B in the hippocampus of rats with global cerebral ischemia/reperfusion injury (GCIRI) after treatment with agonists or antagonists of PPAR-gamma.

Methods

A rat GCIRI model was established by occlusion of bilateral common carotid arteries and cervical vena retransfusion. GW9662 (5 μg), a selective PPAR- gamma antagonist, was intraventricularly injected at 0.5 h before GCIR; Rosiglitazone (0.8, 2.4 and 7.2 mg/kg), a selective PPAR- gamma agonist, was injected intraperitoneally at 1 h before GCIRI. The expression changes of PPAR-alpha, -beta and -gamma at mRNA and protein levels were detected by RT-PCR and western blotting. The changes of spatial learning and memory (SLM) functions were assessed by using a Morris water maze; the pathohistological changes of hippocampal neurons were evaluated by hematoxylin-eosin (HE) staining; the contents of IL-1, IL-6, IL-10 and TNF-alpha, and the NF- kappa B expression were measured by enzyme-linked immunosorbent assay (ELISA) and immunohistochemical staining. The superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were also detected.

Results

The SLM function and hippocampal neurons were significantly impaired after the occurrence of GCIRI. The MDA, IL-1, IL-6, IL-10, TNF-alpha content and expression of PPARs increased significantly, but the SOD activity and NF-kappa B expression were weakened in the hippocampus. Rosiglitazone treatment significantly protected rats from SLM function impairment and neuron death, and resulted in higher expressions of SOD activity and NF-kappa B, but lower contents of MDA and inflammatory factors. After treatment with rosiglitazone or GW9662, no significant change in PPAR-alpha or -beta expression was detected.

Conclusions

Rosiglitazone, a PPAR-gamma agonist, plays a protective role in hippocampal neuron damage of GCIRI rats by inhibiting the oxidative stress response and inflammation. The activation or antagonism of PPAR-gamma did not affect the expression of PPAR-alpha or -beta, indicating that the three subtypes of PPARs act in independent pathways in the CNS.

Disturbances in cholesterol, bile acid and glucose metabolism in peroxisomal 3-ketoacylCoA thiolase B deficient mice fed diets containing high or low fat contents

The peroxisomal 3-ketoacyl-CoA thiolase B (ThB) catalyzes the thiolytic cleavage of straight chain 3-ketoacyl-CoAs. Up to now, the ability of ThB to interfere with lipid metabolism was studied in mice fed a laboratory chow enriched or not with the synthetic agonist Wy14,643, a pharmacological activator of the nuclear hormone receptor PPARα. The aim of the present study was therefore to determine whether ThB could play a role in obesity and lipid metabolism when mice are chronically fed a synthetic High Fat Diet (HFD) or a Low Fat Diet (LFD) as a control diet. To investigate this possibility, wild-type (WT) mice and mice deficient for Thb (Thb(-/-)) were subjected to either a synthetic LFD or a HFD for 25 weeks, and their responses were compared. First, when fed a normal regulatory laboratory chow, Thb(-/-) mice displayed growth retardation as well as a severe reduction in the plasma level of Growth Hormone (GH) and Insulin Growth Factor-I (IGF-I), suggesting alterations in the GH/IGF-1 pathway. When fed the synthetic diets, the corrected energy intake to body mass was significantly higher in Thb(-/-) mice, yet those mice were protected from HFD-induced adiposity. Importantly, Thb(-/-) mice also suffered from hypoglycemia, exhibited reduction in liver glycogen stores and circulating insulin levels under the LFD and the HFD. Thb deficiency was also associated with higher levels of plasma HDL (High Density Lipoproteins) cholesterol and increased liver content of cholesterol under both the LFD and the HFD. As shown by the plasma lathosterol to cholesterol ratio, a surrogate marker for cholesterol biosynthesis, whole body cholesterol de novo synthesis was increased in Thb(-/-) mice. By comparing liver RNA from WT mice and Thb(-/-) mice using oligonucleotide microarray and RT-qPCR, a coordinated decrease in the expression of critical cholesterol synthesizing genes and an increased expression of genes involved in bile acid synthesis (Cyp7a1, Cyp17a1, Akr1d1) were observed in Thb(-/-) mice. In parallel, the elevation of the lathosterol to cholesterol ratio as well as the increased expression of cholesterol synthesizing genes were observed in the kidney of Thb(-/-) mice fed the LFD and the HFD. Overall, the data indicate that ThB is not fully interchangeable with the thiolase A isoform. The present study also reveals that modulating the expression of the peroxisomal ThB enzyme can largely reverberate not only throughout fatty acid metabolism but also cholesterol, bile acid and glucose metabolism.

Nuclear control of the inflammatory response in mammals by peroxisome proliferator-activated receptors

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that play pivotal roles in the regulation of a very large number of biological processes including inflammation. Using specific examples, this paper focuses on the interplay between PPARs and innate immunity/inflammation and, when possible, compares it among species. We focus on recent discoveries establishing how inflammation and PPARs interact in the context of obesity-induced inflammation and type 2 diabetes, mostly in mouse and humans. We illustrate that PPAR γ ability to alleviate obesity-associated inflammation raises an interesting pharmacologic potential. In the light of recent findings, the protective role of PPAR α and PPAR β / δ against the hepatic inflammatory response is also addressed. While PPARs agonists are well-established agents that can treat numerous inflammatory issues in rodents and humans, surprisingly very little has been described in other species. We therefore also review the implication of PPARs in inflammatory bowel disease; acute-phase response; and central, cardiac, and endothelial inflammation and compare it along different species (mainly mouse, rat, human, and pig). In the light of the data available in the literature, there is no doubt that more studies concerning the impact of PPAR ligands in livestock should be undertaken because it may finally raise unconsidered health and sanitary benefits.

Upregulation of Scavenger Receptor BI by Hepatic Nuclear Factor 4α through a Peroxisome Proliferator-Activated Receptor γ-Dependent Mechanism in Liver

Hepatic nuclear factor 4α (HNF4α) modulates the transcriptional activation of numerous metabolic genes in liver. In this study, gene-array analysis revealed that HNF4α overexpression increased peroxisome proliferator-activated receptorγ (PPARγ) greatly in cultured rat primary hepatocytes. PPAR-response-element-driven reporter gene expression could be elevated by HNF4α. Bioinformatics analysis revealed a high-affinity HNF4α binding site in the human PPARγ2 promoter and in vitro experiments showed that this promoter could be transactivated by HNF4α. The presence of HNF4α on the promoter was then confirmed by ChIP assay. In vivo, hepatic overexpression of HNF4α decreased cholesterol levels both in plasma and liver and several hepatic genes related to cholesterol metabolism, including scavenger receptor BI (SR-BI), were upregulated. The upregulation of SR-BI by HNF4α could be inhibited by a PPARγ antagonist in vitro. In conclusion, HNF4α regulates cholesterol metabolism in rat by modulating the expression of SR-BI in the liver, in which the upregulation of PPARγ was involved.