Peroxisome Proliferator-Activated Receptor Alpha Target Genes
TL;DR: An overview of the involvement of PPARα in lipid metabolism and other pathways through a detailed analysis of the different known or putative PPAR α target genes is presented.
Abstract: The peroxisome proliferator-activated receptor alpha (PPARα) is a ligand-activated transcription factor involved in the regulation of a variety of processes, ranging from inflammation and immunity to nutrient metabolism and energy homeostasis. PPARα serves as a molecular target for hypolipidemic fibrates drugs which bind the receptor with high affinity. Furthermore, PPARα binds and is activated by numerous fatty acids and fatty acid-derived compounds. PPARα governs biological processes by altering the expression of a large number of target genes. Accordingly, the specific role of PPARα is directly related to the biological function of its target genes. Here, we present an overview of the involvement of PPARα in lipid metabolism and other pathways through a detailed analysis of the different known or putative PPARα target genes. The emphasis is on gene regulation by PPARα in liver although many of the results likely apply to other organs and tissues as well.
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University of Düsseldorf1, Goethe University Frankfurt2, Vrije Universiteit Brussel3, University of Freiburg4, Leipzig University5, Dow Chemical Company6, University of Tübingen7, Durham University8, University of Turin9, Seoul National University10, University of Jena11, Charité12, University of Valencia13, Heidelberg University14, University of Paris15, French Institute for Research in Computer Science and Automation16, University of Liverpool17, University of Groningen18, Lund University19, University of Manchester20, South Valley University21, University Hospital Regensburg22, Merck KGaA23, Musashino University24, University of Kansas25, Ludwig Maximilian University of Munich26, Fresenius Medical Care27, University of Regensburg28, Merck & Co.29
TL;DR: This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro and how closely hepatoma, stem cell and iPS cell–derived hepatocyte-like-cells resemble real hepatocytes.
Abstract: This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.
1,085 citations
TL;DR: In Inherited defects for most of the FAO enzymes have been identified and characterised and are currently included in neonatal screening programmes and will ultimately lead to better treatment.
Abstract: Over the years, the mitochondrial fatty acid β-oxidation (FAO) pathway has been characterised at the biochemical level as well as the molecular biological level. FAO plays a pivotal role in energy homoeostasis, but it competes with glucose as the primary oxidative substrate. The mechanisms behind this so-called glucose–fatty acid cycle operate at the hormonal, transcriptional and biochemical levels. Inherited defects for most of the FAO enzymes have been identified and characterised and are currently included in neonatal screening programmes. Symptoms range from hypoketotic hypoglycaemia to skeletal and cardiac myopathies. The pathophysiology of these diseases is still not completely understood, hampering optimal treatment. Studies of patients and mouse models will contribute to our understanding of the pathogenesis and will ultimately lead to better treatment.
708 citations
Cites background from "Peroxisome Proliferator-Activated R..."
...Activation of PPARα and PPARβ by (dietary) fatty acids such as oleic acid makes tissues more dependent on FAO, by stimulating cellular fatty acid utilisation pathways including transport, esterification, and oxidation (Mandard et al. 2004)....
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TL;DR: The molecular mechanisms by which hepatic triglyceride homeostasis is achieved under normal conditions are discussed, as well as the metabolic alterations that occur in the setting of insulin resistance and contribute to the pathogenesis of NAFLD.
Abstract: Non-alcoholic fatty liver disease (NAFLD) is characterized by hepatic lipid accumulation in the absence of excess alcohol intake. NAFLD is the most common chronic liver disease, and ongoing research efforts are focused on understanding the underlying pathobiology of hepatic steatosis with the anticipation that these efforts will identify novel therapeutic targets. Under physiological conditions, the low steady-state triglyceride concentrations in the liver are attributable to a precise balance between acquisition by uptake of non-esterified fatty acids from the plasma and by de novo lipogenesis, versus triglyceride disposal by fatty acid oxidation and by the secretion of triglyceride-rich lipoproteins. In NAFLD patients, insulin resistance leads to hepatic steatosis by multiple mechanisms. Greater uptake rates of plasma non-esterified fatty acids are attributable to increased release from an expanded mass of adipose tissue as a consequence of diminished insulin responsiveness. Hyperinsulinemia promotes the transcriptional upregulation of genes that promote de novo lipogenesis in the liver. Increased hepatic lipid accumulation is not offset by fatty acid oxidation or by increased secretion rates of triglyceride-rich lipoproteins. This review discusses the molecular mechanisms by which hepatic triglyceride homeostasis is achieved under normal conditions, as well as the metabolic alterations that occur in the setting of insulin resistance and contribute to the pathogenesis of NAFLD.
680 citations
Cites background from "Peroxisome Proliferator-Activated R..."
...PPARa is a fatty acid-activated nuclear hormone receptor that plays a central role in the transcriptional regulation of lipid and glucose metabolism [49]....
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...Although a common feature of PPARa target genes, PPREs have not been identified in all genes that are regulated by this transcription factor [49]....
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TL;DR: Nrf2 activity is tightly controlled via CRLKeap1 and SCFβ-TrCP by oxidative stress and energy-based signals, allowing it to mediate adaptive responses that restore redox homeostasis and modulate intermediary metabolism.
622 citations
Cites background from "Peroxisome Proliferator-Activated R..."
...Hepatic lipid metabolism is largely regulated by the nuclear receptor PPARa, which controls the expression of genes involved in lipid transport, β-oxidation of fatty acids, ketogenesis, lipogenesis, lipid mobilization, and cholesterol metabolism, some examples of which are shown in Table 3[379]....
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TL;DR: A significant research effort has recently been undertaken to explore the PPARγ-activating potential of a wide range of natural products originating from traditionally used medicinal plants or dietary sources.
497 citations
References
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TL;DR: Results are in accord with two previous trials with different pharmacologic agents and indicate that modification of lipoprotein levels with gemfibrozil reduces the incidence of coronary heart disease in men with dyslipidemia.
Abstract: In a randomized, double-blind five-year trial, we tested the efficacy of simultaneously elevating serum levels of high-density lipoprotein (HDL) cholesterol and lowering levels of non-HDL cholesterol with gemfibrozil in reducing the risk of coronary heart disease in 4081 asymptomatic middle-aged men (40 to 55 years of age) with primary dyslipidemia (non-HDL cholesterol greater than or equal to 200 mg per deciliter [5.2 mmol per liter] in two consecutive pretreatment measurements). One group (2051 men) received 600 mg of gemfibrozil twice daily, and the other (2030 men) received placebo. Gemfibrozil caused a marked increase in HDL cholesterol and persistent reductions in serum levels of total, low-density lipoprotein (LDL), and non-HDL cholesterol and triglycerides. There were minimal changes in serum lipid levels in the placebo group. The cumulative rate of cardiac end points at five years was 27.3 per 1,000 in the gemfibrozil group and 41.4 per 1,000 in the placebo group--a reduction of 34.0 percent in the incidence of coronary heart disease (95 percent confidence interval, 8.2 to 52.6; P less than 0.02; two-tailed test). The decline in incidence in the gemfibrozil group became evident in the second year and continued throughout the study. There was no difference between the groups in the total death rate, nor did the treatment influence the cancer rates. The results are in accord with two previous trials with different pharmacologic agents and indicate that modification of lipoprotein levels with gemfibrozil reduces the incidence of coronary heart disease in men with dyslipidemia.
3,697 citations
TL;DR: A member of the steroid hormone receptor superfamily of ligand-activated transcription factors is cloned that is activated by a diverse class of rodent hepatocarcinogens that causes proliferation of peroxisomes.
Abstract: We have cloned a member of the steroid hormone receptor superfamily of ligand-activated transcription factors. The receptor homologue is activated by a diverse class of rodent hepatocarcinogens that causes proliferation of peroxisomes. Identification of a peroxisome proliferator-activated receptor should help elucidate the mechanism of the hypolipidaemic effect of these hepatocarcinogens and aid evaluation of their potential carcinogenic risk to man.
3,370 citations
"Peroxisome Proliferator-Activated R..." refers background in this paper
...PPARα was first discovered in the early 1990s and since then has been identified as the master regulator of hepatic lipid metabolism [8]....
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TL;DR: Gemfibrozil therapy resulted in a significant reduction in the risk of major cardiovascular events in patients with coronary disease whose primary lipid abnormality was a low HDL cholesterol level, suggesting that the rate of coronary events is reduced by raising HDL cholesterol levels and lowering levels of triglycerides without lowering LDL cholesterol levels.
Abstract: Background Although it is generally accepted that lowering elevated serum levels of low-density lipoprotein (LDL) cholesterol in patients with coronary heart disease is beneficial, there are few data to guide decisions about therapy for patients whose primary lipid abnormality is a low level of high-density lipoprotein (HDL) cholesterol. Methods We conducted a double-blind trial comparing gemfibrozil (1200 mg per day) with placebo in 2531 men with coronary heart disease, an HDL cholesterol level of 40 mg per deciliter (1.0 mmol per liter) or less, and an LDL cholesterol level of 140 mg per deciliter (3.6 mmol per liter) or less. The primary study outcome was nonfatal myocardial infarction or death from coronary causes. Results The median follow-up was 5.1 years. At one year, the mean HDL cholesterol level was 6 percent higher, the mean triglyceride level was 31 percent lower, and the mean total cholesterol level was 4 percent lower in the gemfibrozil group than in the placebo group. LDL cholesterol levels...
3,327 citations
TL;DR: This work has shown that direct expression of PPAR mRNAs in the absence of a specific carrier gene results in down-regulation in the activity of other PPARs, and these properties are consistent with those of a “spatially aggregating substance”.
Abstract: I. Introduction II. Molecular Aspects A. PPAR isotypes: identity, genomic organization and chromosomal localization B. DNA binding properties C. PPAR ligand-binding properties D. Alternative pathways for PPAR activation E. PPAR-mediated transactivation properties III. Physiological Aspects A. Differential expression of PPAR mRNAs B. PPAR target genes and functions in fatty acid metabolism C. PPARs and control of inflammatory responses D. PPARs and atherosclerosis E. PPARs and the development of the fetal epidermal permeability barrier F. PPARs, carcinogenesis, and control of the cell cycle IV. Conclusions
3,028 citations
"Peroxisome Proliferator-Activated R..." refers background in this paper
...PPREs are characterized by a common core sequence consisting of a direct repeat of the consensus sequence AGGTCA interspaced by a single nucleotide [1, 4]....
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TL;DR: The current state of knowledge regarding the molecular mechanisms of PPAR action and the involvement of the PPARs in the etiology and treatment of several chronic diseases is presented.
Abstract: ▪ Abstract The peroxisome proliferator-activated receptors (PPARs) are a group of three nuclear receptor isoforms, PPARγ, PPARα, and PPARδ, encoded by different genes. PPARs are ligand-regulated transcription factors that control gene expression by binding to specific response elements (PPREs) within promoters. PPARs bind as heterodimers with a retinoid X receptor and, upon binding agonist, interact with cofactors such that the rate of transcription initiation is increased. The PPARs play a critical physiological role as lipid sensors and regulators of lipid metabolism. Fatty acids and eicosanoids have been identified as natural ligands for the PPARs. More potent synthetic PPAR ligands, including the fibrates and thiazolidinediones, have proven effective in the treatment of dyslipidemia and diabetes. Use of such ligands has allowed researchers to unveil many potential roles for the PPARs in pathological states including atherosclerosis, inflammation, cancer, infertility, and demyelination. Here, we presen...
2,421 citations
"Peroxisome Proliferator-Activated R..." refers background in this paper
...[11] J....
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...Synthetic agonists of PPARα lower plasma triglycerides and raise plasma highdensity lipoprotein (HDL) levels and are thus used clinically in the treatment of dyslipidemia [2, 9–11]....
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...The most important class of synthetic PPARα ligands is the fibrates, including gemfibrozil, bezafibrate, clofibrate, fenofibrate, and Wy14643 [2, 9–11, 46]....
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