Modulation of the hepatic fatty acid pool in peroxisomal 3-ketoacyl-CoA thiolase B-null mice exposed to the selective PPARalpha agonist Wy14,643

Mouse Models to Study Peroxisomal Functions and Disorders: Overview, Caveats, and Recommendations

During the last three decades many mouse lines were created or identified that are deficient in one or more peroxisomal functions. Different methodologies were applied to obtain global, hypomorph, cell type selective, inducible, and knockin mice. Whereas some models closely mimic pathologies in patients, others strongly deviate or no human counterpart has been reported. Often, mice, apparently endowed with a stronger transcriptional adaptation, have to be challenged with dietary additions or restrictions in order to trigger phenotypic changes. Depending on the inactivated peroxisomal protein, several approaches can be taken to validate the loss-of-function. Here, an overview is given of the available mouse models and their most important characteristics.

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.

Overexpression of Nudt7 decreases bile acid levels and peroxisomal fatty acid oxidation in the liver

Lipid metabolism requires CoA, an essential cofactor found in multiple subcellular compartments, including the peroxisomes. In the liver, CoA levels are dynamically adjusted between the fed and fasted states. Elevated CoA levels in the fasted state are driven by increased synthesis; however, this also correlates with decreased expression of Nudix hydrolase (Nudt)7, the major CoA-degrading enzyme in the liver. Nudt7 resides in the peroxisomes, and we overexpressed this enzyme in mouse livers to determine its effect on the size and composition of the hepatic CoA pool in the fed and fasted states. Nudt7 overexpression did not change total CoA levels, but decreased the concentration of short-chain acyl-CoAs and choloyl-CoA in fasted livers, when endogenous Nudt7 activity was lowest. The effect on these acyl-CoAs correlated with a significant decrease in the hepatic bile acid content and in the rate of peroxisomal fatty acid oxidation, as estimated by targeted and untargeted metabolomics, combined with the measurement of fatty acid oxidation in intact hepatocytes. Identification of the CoA species and metabolic pathways affected by the overexpression on Nudt7 in vivo supports the conclusion that the nutritionally driven modulation of Nudt7 activity could contribute to the regulation of the peroxisomal CoA pool and peroxisomal lipid metabolism.

Hepatic transcriptome analysis from HFD-fed mice defines a long noncoding RNA regulating cellular cholesterol levels

To elucidate the transcriptomic changes of long noncoding RNAs (lncRNAs) in high-fat diet (HFD)-fed mice, we defined their hepatic transcriptome by RNA sequencing. Aberrant expression of 37 representative lncRNAs and 254 protein-coding RNAs was observed in the livers of HFD-fed mice with insulin resistance compared with the livers from control mice. Of these, 24 lncRNAs and 179 protein-coding RNAs were upregulated, whereas 13 lncRNAs and 75 protein-coding RNAs were downregulated. Functional analyses showed that the aberrantly expressed protein-coding RNAs were enriched in various lipid metabolic processes and in the insulin signaling pathway. Genomic juxtaposition and coexpression patterns identified six pairs of aberrantly expressed lncRNAs and protein-coding genes, consisting of five lncRNAs and five protein-coding genes. Four of these protein-coding genes are targeted genes upregulated by PPARα. As expected, the corresponding lncRNAs were significantly elevated in AML12 cells treated with palmitic acid or the PPARα agonist, WY14643. In Hepa1-6 cells, knockdown of NONMMUG027912 increased the cellular cholesterol level, the expression of cholesterol biosynthesis genes and proteins, and the HMG-CoA reductase activity. This genome-wide profiling of lncRNAs in HFD-fed mice reveals one lncRNA, NONMMUG027912, which is potentially regulated by PPARα and is implicated in the process of cholesterol biosynthesis.

Growth arrest specific 1 (Gas1) and glial cell line-derived neurotrophic factor receptor α1 (Gfrα1), two mouse oocyte glycosylphosphatidylinositol-anchored proteins, are involved in fertilisation

Recently, Juno, the oocyte receptor for Izumo1, a male immunoglobulin, was discovered. Juno is an essential glycosylphosphatidylinositol (GIP)-anchored protein. This result did not exclude the participation of other GIP-anchored proteins in this process. After bibliographic and database searches we selected five GIP-anchored proteins (Cpm, Ephrin-A4, Gas1, Gfra1 and Rgmb) as potential oocyte candidates participating in fertilisation. Western blot and immunofluorescence analyses showed that only three were present on the mouse ovulated oocyte membrane and, of these, only two were clearly involved in the fertilisation process, namely growth arrest specific 1 (Gas1) and glial cell line-derived neurotrophic factor receptor α1 (Gfrα1). This was demonstrated by evaluating oocyte fertilisability after treatment of oocytes with antibodies against the selected proteins, with their respective short interference RNA or both. Gfrα1 and Gas1 seem to be neither redundant nor synergistic. In conclusion, oocyte Gas1 and Gfrα1 are both clearly involved in fertilisation.

The basal free fatty acid concentration in human saliva is related to salivary lipolytic activity

Fat perception during eating is a complex sensation that involves various sensory modalities, such as texture, aroma and taste. Taste is supported by the discovery of fatty acid receptors in the tongue papillae. Dietary fat is mainly composed of esterified fatty acids, whereas only free fatty acids can bind to taste receptors. Some authors have mentioned the necessity and efficiency of salivary lipolytic activity to hydrolyse the esterified fatty acids present in foods and enable fat perception. Our hypothesis is that salivary lipolytic activity is also involved in regulating the basal level of salivary fatty acids in humans. To test this hypothesis, total fatty acid (TFA) and free fatty acid (FFA) concentrations and selected salivary characteristics (such as lipolytic activity) were analysed in the resting saliva of 54 subjects. The results show differences in the TFA and FFA profiles, with TFA and FFA concentrations of 8.99 and 3.56 µg/mL of saliva, respectively. Interestingly, lipolytic activity had a significant positive correlation with FFA concentration (0.51, p < 0.01). This result highlights a possible physiological role of salivary lipolytic activity in the regulation of the basal FFA concentration. This regulation could be involved in fat taste sensitivity.

7-Ketocholesterol is increased in the plasma of X-ALD patients and induces peroxisomal modifications in microglial cells: Potential roles of 7-ketocholesterol in the pathophysiology of X-ALD

X-linked adrenoleukodystrophy (X-ALD) is a genetic disorder induced by a mutation in the ABCD1 gene, which causes the accumulation of very long-chain fatty acids in tissue and plasma. Oxidative stress may be a hallmark of X-ALD. In the plasma of X-ALD patients with different forms of the disease, characterized by high levels of C24:0 and C26:0, we observed the presence of oxidative stress revealed by decreased levels of GSH, α-tocopherol, and docosahexaenoic acid (DHA). We showed that oxidative stress caused the oxidation of cholesterol and linoleic acid, leading to the formation of cholesterol oxide derivatives oxidized at C7 (7-ketocholesterol (7KC), 7β-hydroxycholesterol (7β-OHC), and 7α-hydroxycholesrol (7α-OHC)) and of 9- and 13-hydroxyoctadecadienoic acids (9-HODE, 13-HODE), respectively. High levels of 7KC, 7β-OHC, 7α-OHC, 9-HODE and 13-HODE were found. As 7KC induces oxidative stress, inflammation and cell death, which could play key roles in the development of X-ALD, the impact of 7KC on the peroxisomal status was determined in microglial BV-2 cells. Indeed, environmental stress factors such as 7KC could exacerbate peroxisomal dysfunctions in microglial cells and thus determine the progression of the disease. 7KC induces oxiapoptophagy in BV-2 cells: overproduction of H₂O₂ and O₂^-, presence of cleaved caspase-3 and PARP, nuclear condensation and/or fragmentation; elevated [LC3-II/LC3-I] ratio, increased p62 levels. 7KC also induces several peroxisomal modifications: decreased Abcd1, Abcd2, Abcd3, Acox1 and/or Mfp2 mRNA and protein levels, increased catalase activity and decreased Acox1-activity. However, the Pex14 level was unchanged. It is suggested that high levels of 7KC in X-ALD patients could foster generalized peroxisomal dysfunction in microglial cells, which could in turn intensify brain damage.

Protective effects of aerobic swimming training on high-fat diet induced nonalcoholic fatty liver disease: regulation of lipid metabolism via PANDER-AKT pathway

This study aimed to investigate the mechanism by which aerobic swimming training prevents high-fat-diet-induced nonalcoholic fatty liver disease (NAFLD). Forty-two male C57BL/6 mice were randomized into normal-diet sedentary (ND; n = 8), ND exercised (n = 8), high-fat diet sedentary (HFD; n = 13), and HFD exercised groups (n = 13). After 2 weeks of training adaptation, the mice were subjected to an aerobic swimming protocol (60 min/day) 5 days/week for 10 weeks. The HFD group exhibited significantly higher mRNA levels of fatty acid transport-, lipogenesis-, and β-oxidation-associated gene expressions than the ND group. PANDER and FOXO1 expressions increased, whereas AKT expression decreased in the HFD group. The aerobic swimming program with the HFD reversed the effects of the HFD on the expressions of thrombospondin-1 receptor, liver fatty acid-binding protein, long-chain fatty-acid elongase-6, Fas cell surface death receptor, and stearoyl-coenzyme A desaturase-1, as well as PANDER, FOXO1, and AKT. In the HFD exercised group, PPARα and AOX expressions were much higher. Our findings suggest that aerobic swimming training can prevent NAFLD via the regulation of fatty acid transport-, lipogenesis-, and β-oxidation-associated genes. In addition, the benefits from aerobic swimming training were achieved partly through the PANDER-AKT-FOXO1 pathway.

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.

Liver x receptor regulates arachidonic acid distribution and eicosanoid release in human macrophages: a key role for lysophosphatidylcholine acyltransferase 3

OBJECTIVE

Liver X receptors (LXRs) are oxysterol-activated nuclear receptors that are highly expressed in macrophages and regulate lipid homeostasis and inflammation. Among putative LXR target genes, lysophosphatidylcholine acyltransferase 3 (LPCAT3) involved in the Lands cycle controls the fatty acid composition at the sn-2 position of glycerophospholipids and, therefore, the availability of fatty acids, such as arachidonic acid (AA), used for eicosanoid synthesis. The aim of our study was to determine whether LXRs could regulate the Lands cycle in human macrophages, to assess the consequences in terms of lipid composition and inflammatory response, and to work out the relative contribution of LPCAT3 to the observed changes.

APPROACH AND RESULTS

Transcriptomic analysis revealed that LPCAT3 was upregulated by LXR agonists in human macrophages. Accordingly, LXR stimulation significantly increased lysophospholipid acyltransferase activity catalyzed by LPCAT3. Lipidomic analysis demonstrated that LXR activation increased the AA content in the polar lipid fraction, specifically in phosphatidylcholines. The LXR-mediated effects on AA distribution were abolished by LPCAT3 silencing, and a redistribution of AA toward the neutral lipid fraction was observed in this context. Finally, we observed that preconditioning of human macrophages by LXR agonist treatment increased the release of arachidonate-derived eicosanoids, such as prostaglandin E2 and thromboxane after lipopolysaccharide stimulation, with a significant attenuation by LPCAT3 silencing.

CONCLUSIONS

Altogether, our data demonstrate that the LXR-mediated induction of LPCAT3 primes human macrophages for subsequent eicosanoid secretion by increasing the pool of AA, which can be mobilized from phospholipids.

Increased amyloid-β peptide-induced memory deficits in phospholipid transfer protein (PLTP) gene knockout mice

Oxidative stress is recognized as one of the earliest and most intense pathological processes in Alzheimer's disease (AD), and the antioxidant vitamin E has been shown to efficiently prevent amyloid plaque formation and neurodegeneration. Plasma phospholipid transfer protein (PLTP) has a major role in vitamin E transfers in vivo, and PLTP deficiency in mice is associated with reduced brain vitamin E levels. To determine the impact of PLTP on amyloid pathology in vivo, we analyzed the vulnerability of PLTP-deficient (PLTP-KO) mice to the toxic effects induced by intracerebroventricular injection of oligomeric amyloid-β 25-35 (Aβ 25-35) peptide, a non-transgenic model of AD. Under basal conditions, PLTP-KO mice showed increased cerebral oxidative stress, increased brain Aβ 1-42 levels, and a lower expression of the synaptic function marker synaptophysin, as compared with wild-type mice. This PLTP-KO phenotype was associated with increased memory impairment 1 week after Aβ25-35 peptide injection. Restoration of brain vitamin E levels in PLTP-KO mice through a chronic dietary supplementation prevented Aβ 25-35-induced memory deficits and reduced cerebral oxidative stress and toxicity. We conclude that PLTP, through its ability to deliver vitamin E to the brain, constitutes an endogenous neuroprotective agent. Increasing PLTP activity may offer a new way to develop neuroprotective therapies.

Peroxisome proliferator-activated receptor-α signaling in hepatocarcinogenesis

Peroxisomes are subcellular organelles that are found in the cytoplasm of most animal cells. They perform diverse metabolic functions, including H2O2-derived respiration, β-oxidation of fatty acids, and cholesterol metabolism. Peroxisome proliferators are a large class of structurally dissimilar industrial and pharmaceutical chemicals that were originally identified as inducers of both the size and the number of peroxisomes in rat and mouse livers or hepatocytes in vitro. Exposure to peroxisome proliferators leads to a stereotypical orchestration of adaptations consisting of hepatocellular hypertrophy and hyperplasia, and transcriptional induction of fatty acid metabolizing enzymes regulated in parallel with peroxisome proliferation. Chronic exposure to peroxisome proliferators causes liver tumors in both male and female mice and rats. Evidence indicates a pivotal role for a subset of nuclear receptor superfamily members, called peroxisome proliferator-activated receptors (PPARs), in mediating energy metabolism. Upon activation, PPARs regulate the expression of genes involved in lipid metabolism and peroxisome proliferation, as well as genes involved in cell growth. In this review, we describe the molecular mode of action of PPAR transcription factors, including ligand binding, interaction with specific DNA response elements, transcriptional activation, and cross talk with other signaling pathways. We discuss the evidence that suggests that PPARα and transcriptional coactivator Med1/PBP, a key subunit of the Mediator complex play a central role in mediating hepatic steatosis to hepatocarcinogenesis. Disproportionate increases in H2O2-generating enzymes generates excess reactive oxygen species resulting in sustained oxidative stress and progressive endoplasmic reticulum (ER) stress with activation of unfolded protein response signaling. Thus, these major contributors coupled with hepatocellular proliferation are the key players of peroxisome proliferators-induced hepatocarcinogenesis.

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

PPARα and HNF4α are nuclear receptors that control gene transcription by direct binding to specific nucleotide sequences. Using transgenic mice deficient for either PPARα or HNF4α, we show that the expression of the peroxisomal 3-keto-acyl-CoA thiolase B (Thb) is under the dependence of these two transcription factors. Transactivation and gel shift experiments identified a novel PPAR response element within intron 3 of the Thb gene, by which PPARα but not HNF4α transactivates. Intriguingly, we found that HNF4α enhanced PPARα/RXRα transactivation from TB PPRE3 in a DNA-binding independent manner. Coimmunoprecipitation assays supported the hypothesis that HNF4α was physically interacting with RXRα. RT-PCR performed with RNA from liver-specific HNF4α-null mice confirmed the involvement of HNF4α in the PPARα-regulated induction of Thb by Wy14,643. Overall, we conclude that HNF4α enhances the PPARα-mediated activation of Thb gene expression in part through interaction with the obligate PPARα partner, RXRα.

A role for the peroxisomal 3-ketoacyl-CoA thiolase B enzyme in the control of PPARα-mediated upregulation of SREBP-2 target genes in the liver

Peroxisomal 3-ketoacyl-CoA thiolase B (Thb) catalyzes the final step in the peroxisomal β-oxidation of straight-chain acyl-CoAs and is under the transcription control of the nuclear hormone receptor PPARα. PPARα binds to and is activated by the synthetic compound Wy14,643 (Wy). Here, we show that the magnitude of Wy-mediated induction of peroxisomal β-oxidation of radiolabeled (1-(14)C) palmitate was significantly reduced in mice deficient for Thb. In contrast, mitochondrial β-oxidation was unaltered in Thb(-/-) mice. Given that Wy-treatment induced Acox1 and MFP-1/-2 activity at a similar level in both genotypes, we concluded that the thiolase step alone was responsible for the reduced peroxisomal β-oxidation of fatty acids. Electron microscopic analysis and cytochemical localization of catalase indicated that peroxisome proliferation in the liver after Wy-treatment was normal in Thb(-/-) mice. Intriguingly, micro-array analysis revealed that mRNA levels of genes encoding cholesterol biosynthesis enzymes were upregulated by Wy in Wild-Type (WT) mice but not in Thb(-/-) mice, which was confirmed at the protein level for the selected genes. The non-induction of genes encoding cholesterol biosynthesis enzymes by Wy in Thb(-/-) mice appeared to be unrelated to defective SREBP-2 or PPARα signaling. No difference was observed in the plasma lathosterol/cholesterol ratio (a marker for de novo cholesterol biosynthesis) between Wy-treated WT and Thb(-/-) mice, suggesting functional compensation. Overall, we conclude that ThA and SCPx/SCP2 thiolases cannot fully compensate for the absence of ThB. In addition, our data indicate that ThB is involved in the regulation of genes encoding cholesterol biosynthesis enzymes in the liver, suggesting that the peroxisome could be a promising candidate for the correction of cholesterol imbalance in dyslipidemia.