PPARalpha governs glycerol metabolism

An Overview of the Role of Peroxisome Proliferator-activated Receptors in Liver Diseases

Peroxisome proliferator-activated receptors (PPARs) are a superfamily of nuclear transcription receptors, consisting of PPARα, PPARγ, and PPARβ/δ, which are highly expressed in the liver. They control and modulate the expression of a large number of genes involved in metabolism and energy homeostasis, oxidative stress, inflammation, and even apoptosis in the liver. Therefore, they have critical roles in the pathophysiology of hepatic diseases. This review provides a general insight into the role of PPARs in liver diseases and some of their agonists in the clinic.

Peroxisome proliferator-activated receptors as targets to treat metabolic diseases: Focus on the adipose tissue, liver, and pancreas

The world is experiencing reflections of the intersection of two pandemics: Obesity and coronavirus disease 2019. The prevalence of obesity has tripled since 1975 worldwide, representing substantial public health costs due to its comorbidities. The adipose tissue is the initial site of obesity impairments. During excessive energy intake, it undergoes hyperplasia and hypertrophy until overt inflammation and insulin resistance turn adipocytes into dysfunctional cells that send lipotoxic signals to other organs. The pancreas is one of the organs most affected by obesity. Once lipotoxicity becomes chronic, there is an increase in insulin secretion by pancreatic beta cells, a surrogate for type 2 diabetes mellitus (T2DM). These alterations threaten the survival of the pancreatic islets, which tend to become dysfunctional, reaching exhaustion in the long term. As for the liver, lipotoxicity favors lipogenesis and impairs beta-oxidation, resulting in hepatic steatosis. This silent disease affects around 30% of the worldwide population and can evolve into end-stage liver disease. Although therapy for hepatic steatosis remains to be defined, peroxisome proliferator-activated receptors (PPARs) activation copes with T2DM management. Peroxisome PPARs are transcription factors found at the intersection of several metabolic pathways, leading to insulin resistance relief, improved thermogenesis, and expressive hepatic steatosis mitigation by increasing mitochondrial beta-oxidation. This review aimed to update the potential of PPAR agonists as targets to treat metabolic diseases, focusing on adipose tissue plasticity and hepatic and pancreatic remodeling.

PPAR Alpha as a Metabolic Modulator of the Liver: Role in the Pathogenesis of Nonalcoholic Steatohepatitis (NASH)

Simple Summary

In the context of liver disease, one of the more growing public health problems is the transition from simple steatosis to non-alcoholic steatohepatitis. Profound metabolic dysregulations linked to inflammation and hepatic injury are features of non-alcoholic steatohepatitis. Since the peroxisomal-proliferator-activated receptor alpha has long been considered one of the key transcriptional factors in hepatic metabolism, its role in the pathogenesis of non-alcoholic steatohepatitis is discussed in this review.

Abstract

The strong relationship between metabolic alterations and non-alcoholic steatohepatitis (NASH) suggests a pathogenic interplay. However, many aspects have not yet been fully clarified. Nowadays, NASH is becoming the main cause of liver-associated morbidity and mortality. Therefore, an effort to understand the mechanisms underlying the pathogenesis of NASH is critical. Among the nuclear receptor transcription factors, peroxisome-proliferator-activated receptor alpha (PPARα) is highly expressed in the liver, where it works as a pivotal transcriptional regulator of the intermediary metabolism. In this context, PPARα’s function in regulating the lipid metabolism is essential for proper liver functioning. Here, we review metabolic liver genes under the control of PPARα and discuss how this aspect can impact the inflammatory condition and pathogenesis of NASH.

Exploring Compounds to be used as Cosmetic Agents that Activate Peroxisome Proliferator-Activated Receptor Alpha

OBJECTIVE

The human epidermis is formed by the proliferation and differentiation of keratinocytes adjacent to the basement membrane. The outermost layer, the stratum corneum, is equipped with a barrier function that prevents water evaporation, and intercellular lipids play an important role in this barrier function. When the barrier is functioning normally, evaporation is prevented; however, when barrier function is impaired, moisture evaporates, resulting in dry and rough skin. Therefore, maintenance of normal barrier function is critical for maintaining normal skin function. Peroxisome proliferator-activated receptor α (PPARα) is mainly involved in lipid metabolism in the liver but is also expressed in the epidermis and is involved in inducing keratinocyte differentiation, promoting lipid production, maintaining barrier function, and suppressing skin inflammation. Hence, compounds that activate PPARα are expected to control skin function. Therefore, we identified PPARα activators from among extracts of natural resources that have been approved for use in humans and analyzed the effects of these extracts on skin function.

METHODS

First, extracts of 474 natural resources were screened using a PPARα activator screening cell line independently constructed in our laboratory. Next, reporter assays were performed using the Gal4-chimera system to evaluate whether these extracts act as ligands for PPARα. We then analyzed their effect on primary normal human epidermal keratinocyte cells by using real-time RT-PCR. Finally, we evaluated PPARα activation effect by the combination of these extracts.

RESULTS

We identified 36 extracts having the effect of activating PPARα. In particular, #419, a Typha angustifolia spike extract, showed concentration-dependent transcriptional activation through PPARα-LBD and was considered to be likely to contain a compound that is a ligand of PPARα. #419 increased the expression of PPARα target genes as well as genes related to skin function in primary cultured human epidermal keratinocytes. Finally, the use of #419 in combination with nine extracts increased PPAR activity more than twice as much as #419 alone treatment.

CONCLUSIONS

These results showed that the reporter cell line could be useful for discovering extracts of natural resources and that the identified Typha angustifolia spike extract could be used in cosmetics that activate PPARα, which expected to improve skin function.

Mice with a deficiency in Peroxisomal Membrane Protein 4 (PXMP4) display mild changes in hepatic lipid metabolism

Peroxisomes play an important role in the metabolism of a variety of biomolecules, including lipids and bile acids. Peroxisomal Membrane Protein 4 (PXMP4) is a ubiquitously expressed peroxisomal membrane protein that is transcriptionally regulated by peroxisome proliferator-activated receptor α (PPARα), but its function is still unknown. To investigate the physiological function of PXMP4, we generated a Pxmp4 knockout (Pxmp4^-/-) mouse model using CRISPR/Cas9-mediated gene editing. Peroxisome function was studied under standard chow-fed conditions and after stimulation of peroxisomal activity using the PPARα ligand fenofibrate or by using phytol, a metabolite of chlorophyll that undergoes peroxisomal oxidation. Pxmp4^-/- mice were viable, fertile, and displayed no changes in peroxisome numbers or morphology under standard conditions. Also, no differences were observed in the plasma levels of products from major peroxisomal pathways, including very long-chain fatty acids (VLCFAs), bile acids (BAs), and BA intermediates di- and trihydroxycholestanoic acid. Although elevated levels of the phytol metabolites phytanic and pristanic acid in Pxmp4^-/- mice pointed towards an impairment in peroxisomal α-oxidation capacity, treatment of Pxmp4^-/- mice with a phytol-enriched diet did not further increase phytanic/pristanic acid levels. Finally, lipidomic analysis revealed that loss of Pxmp4 decreased hepatic levels of the alkyldiacylglycerol class of neutral ether lipids, particularly those containing polyunsaturated fatty acids. Together, our data show that while PXMP4 is not critical for overall peroxisome function under the conditions tested, it may have a role in the metabolism of (ether)lipids.

Aquaglyceroporin Modulators as Emergent Pharmacological Molecules for Human Diseases

Aquaglyceroporins, a sub-class of aquaporins that facilitate the diffusion of water, glycerol and other small uncharged solutes across cell membranes, have been recognized for their important role in human physiology and their involvement in multiple disorders, mostly related to disturbed energy homeostasis. Aquaglyceroporins dysfunction in a variety of pathological conditions highlighted their targeting as novel therapeutic strategies, boosting the search for potent and selective modulators with pharmacological properties. The identification of selective inhibitors with potential clinical applications has been challenging, relying on accurate assays to measure membrane glycerol permeability and validate effective functional blockers. Additionally, biologicals such as hormones and natural compounds have been revealed as alternative strategies to modulate aquaglyceroporins via their gene and protein expression. This review summarizes the current knowledge of aquaglyceroporins’ involvement in several pathologies and the experimental approaches used to evaluate glycerol permeability and aquaglyceroporin modulation. In addition, we provide an update on aquaglyceroporins modulators reported to impact disease, unveiling aquaglyceroporin pharmacological targeting as a promising approach for innovative therapeutics.

Medium-chain fatty acids enhance expression and histone acetylation of genes related to lipid metabolism in insulin-resistant adipocytes

Background

The expressions of genes related to lipid metabolism are decreased in adipocytes with insulin resistance. In this study, we examined the effects of fatty acids on the reduced expressions and histone acetylation of lipid metabolism-related genes in 3T3-L1 adipocytes treated with insulin resistance induced by tumor necrosis factor (TNF)-α.

Methods

Short-, medium-, and long-chain fatty acid were co-administered with TNF-α in 3T3-L1 adipocytes. Then, mRNA expressions and histone acetylation of genes involved in lipid metabolism were determined using mRNA microarrays, qRT-PCR, and chromatin immunoprecipitation assays.

Results

We found in microarray and subsequent qRT-PCR analyses that the expression levels of several lipid metabolism-related genes, including Gpd1, Cidec, and Cyp4b1, were reduced by TNF-α treatment and restored by co-treatment with a short-chain fatty acid (C4: butyric acid) and medium-chain fatty acids (C8: caprylic acid and C10: capric acid). The pathway analysis of the microarray showed that capric acid enhanced mRNA levels of genes in the PPAR signaling pathway and adipogenesis genes in the TNF-α-treated adipocytes. Histone acetylation around Cidec and Gpd1 genes were also reduced by TNF-α treatment and recovered by co-administration with short- and medium-chain fatty acids.

General significance

Medium- and short-chain fatty acids induce the expressions of Cidec and Gpd1, which are lipid metabolism-related genes in insulin-resistant adipocytes, by promoting histone acetylation around these genes.

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Expressions of lipid metabolism genes are reduced in insulin-resistant adipocytes.

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Short- and medium-chain fatty acids inhibit lipid metabolism gene downregulation.

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Capric acid enhances expressions of PPAR signaling and adipogenesis genes.

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This mechanism involves recovery of histone acetylation in lipid metabolism genes.

Identification of Gm15441, a Txnip antisense lncRNA, as a critical regulator in liver metabolic homeostasis

Background

The majority of mammalian genome is composed of non-coding regions, where numerous long non-coding RNAs (lncRNAs) are transcribed. Although lncRNAs have been identified to regulate fundamental biological processes, most of their functions remain unknown, especially in metabolic homeostasis. Analysis of our recent genome-wide screen reveals that Gm15441, a thioredoxin-interacting protein (Txnip) antisense lncRNA, is the most robustly induced lncRNA in the fasting mouse liver. Antisense lncRNAs are known to regulate their sense gene expression. Given that Txnip is a critical metabolic regulator of the liver, we aimed to investigate the role of Gm15441 in the regulation of Txnip and liver metabolism.

Methods

We examined the response of Gm15441 and Txnip under in vivo metabolic signals such as fasting and refeeding, and in vitro signals such as insulin and key metabolic transcription factors. We investigated the regulation of Txnip expression by Gm15441 and the underlying mechanism in mouse hepatocytes. Using adenovirus-mediated liver-specific overexpression, we determined whether Gm15441 regulates Txnip in the mouse liver and modulates key aspects of liver metabolism.

Results

We found that the expression levels of Gm15441 and Txnip showed a similar response pattern to metabolic signals in vivo and in vitro, but that their functions were predicted to be opposite. Furthermore, we found that Gm15441 robustly reduced Txnip protein expression in vitro through sequence-specific regulation and translational inhibition. Lastly, we confirmed the Txnip inhibition by Gm15441 in vivo (mice) and found that Gm15441 liver-specific overexpression lowered plasma triglyceride and blood glucose levels and elevated plasma ketone body levels.

Conclusions

Our data demonstrate that Gm15441 is a potent Txnip inhibitor and a critical metabolic regulator in the liver. This study reveals the therapeutic potential of Gm15441 in treating metabolic diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00722-1.

Flavonoid Phloretin Inhibits Adipogenesis and Increases OPG Expression in Adipocytes Derived from Human Bone-Marrow Mesenchymal Stromal-Cells

Phloretin (a flavonoid abundant in apple), has antioxidant, anti-inflammatory, and glucose-transporter inhibitory properties. Thus, it has interesting pharmacological and nutraceutical potential. Bone-marrow mesenchymal stem cells (MSC) have high differentiation capacity, being essential for maintaining homeostasis and regenerative capacity in the organism. Yet, they preferentially differentiate into adipocytes instead of osteoblasts with aging. This has a negative impact on bone turnover, remodeling, and formation. We have evaluated the effects of phloretin on human adipogenesis, analyzing MSC induced to differentiate into adipocytes. Expression of adipogenic genes, as well as genes encoding OPG and RANKL (involved in osteoclastogenesis), protein synthesis, lipid-droplets formation, and apoptosis, were studied. Results showed that 10 and 20 µM phloretin inhibited adipogenesis. This effect was mediated by increasing beta-catenin, as well as increasing apoptosis in adipocytes, at late stages of differentiation. In addition, this chemical increased OPG gene expression and OPG/RANKL ratio in adipocytes. These results suggest that this flavonoid (including phloretin-rich foods) has interesting potential for clinical and regenerative-medicine applications. Thus, such chemicals could be used to counteract obesity and prevent bone-marrow adiposity. That is particularly useful to protect bone mass and treat diseases like osteoporosis, which is an epidemic worldwide.

Utilizing systems biology to reveal cellular responses to peroxisome proliferator-activated receptor γ ligand exposure

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Human (HepG2) cells were exposed to PPARγ ligands to induce systems-level effects.

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Ciglitazone decreases HepG2 cell viability while GW 9662 had no effect.

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Ciglitazone and GW 9662 increase neutral lipids as a function of concentration.

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Cholesterol biosynthesis transcripts are affected by ciglitazone and GW 9662.

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Ciglitazone alters lipid profiles but GW 9662 was similar to vehicle-exposed cells.

Peroxisome proliferator-activated receptor γ (PPARγ) is a nuclear receptor that, upon activation by ligands, heterodimerizes with retinoid X receptor (RXR), binds to PPAR response elements (PPREs), and activates transcription of downstream genes. As PPARγ plays a central role in adipogenesis, fatty acid storage, and glucose metabolism, PPARγ-specific pharmaceuticals (e.g., thiazolidinediones) have been developed to treat Type II diabetes and obesity within human populations. However, to our knowledge, no prior studies have concurrently assessed the effects of PPARγ ligand exposure on genome-wide PPARγ binding as well as effects on the transcriptome and lipidome within human cells at biologically active, non-cytotoxic concentrations. In addition to quantifying concentration-dependent effects of ciglitazone (a reference PPARγ agonist) and GW 9662 (a reference PPARγ antagonist) on human hepatocarcinoma (HepG2) cell viability, PPARγ abundance in situ, and neutral lipids, HepG2 cells were exposed to either vehicle (0.1% DMSO), ciglitazone, or GW 9662 for up to 24 h, and then harvested for 1) chromatin immunoprecipitation-sequencing (ChIP-seq) to identify PPARγ-bound regions across the entire genome, 2) mRNA-sequencing (mRNA-seq) to identify potential impacts on the transcriptome, and 3) lipidomics to identify potential alterations in lipid profiles. Following exposure to ciglitazone and GW 9662, we found that PPARγ levels were not significantly different after 2–8 h of exposure. While ciglitazone and GW 9662 resulted in a concentration-dependent increase in neutral lipids, the magnitude and localization of PPARγ-bound regions across the genome (as identified by ChIP-seq) did not vary by treatment. However, mRNA-seq and lipidomics revealed that exposure of HepG2 cells to ciglitazone and GW 9662 resulted in significant, treatment-specific effects on the transcriptome and lipidome. Overall, our findings suggest that exposure of human cells to PPARγ ligands at biologically active, non-cytotoxic concentrations results in toxicity that may be driven by a combination of both PPARγ-dependent and PPARγ-independent mechanisms.

The Endocannabinoid System and PPARs: Focus on Their Signalling Crosstalk, Action and Transcriptional Regulation

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear receptors including PPARα, PPARγ, and PPARβ/δ, acting as transcription factors to regulate the expression of a plethora of target genes involved in metabolism, immune reaction, cell differentiation, and a variety of other cellular changes and adaptive responses. PPARs are activated by a large number of both endogenous and exogenous lipid molecules, including phyto- and endo-cannabinoids, as well as endocannabinoid-like compounds. In this view, they can be considered an extension of the endocannabinoid system. Besides being directly activated by cannabinoids, PPARs are also indirectly modulated by receptors and enzymes regulating the activity and metabolism of endocannabinoids, and, vice versa, the expression of these receptors and enzymes may be regulated by PPARs. In this review, we provide an overview of the crosstalk between cannabinoids and PPARs, and the importance of their reciprocal regulation and modulation by common ligands, including those belonging to the extended endocannabinoid system (or "endocannabinoidome") in the control of major physiological and pathophysiological functions.

Adipose ABHD6 regulates tolerance to cold and thermogenic programs

Enhanced energy expenditure in brown (BAT) and white adipose tissues (WAT) can be therapeutic against metabolic diseases. We examined the thermogenic role of adipose α/β-hydrolase domain 6 (ABHD6), which hydrolyzes monoacylglycerol (MAG), by employing adipose-specific ABHD6-KO mice. Control and KO mice showed similar phenotypes at room temperature and thermoneutral conditions. However, KO mice were resistant to hypothermia, which can be accounted for by the simultaneously increased lipolysis and lipogenesis of the thermogenic glycerolipid/free fatty acid (GL/FFA) cycle in visceral fat, despite unaltered uncoupling protein 1 expression. Upon cold stress, nuclear 2-MAG levels increased in visceral WAT of the KO mice. Evidence is provided that 2-MAG causes activation of PPARα in white adipocytes, leading to elevated expression and activity of GL/FFA cycle enzymes. In the ABHD6-ablated BAT, glucose and oxidative metabolism were elevated upon cold induction, without changes in GL/FFA cycle and lipid turnover. Moreover, response to in vivo β₃-adrenergic stimulation was comparable between KO and control mice. Our data reveal a MAG/PPARα/GL/FFA cycling metabolic signaling network in visceral adipose tissue, which contributes to cold tolerance, and that adipose ABHD6 is a negative modulator of adaptive thermogenesis.

Visceral adipose adipose α/β-hydrolase domain 6 regulates cold adaptation and acts as a brake for heat production via the regulation of thermogenic glycerolipid/free fatty acid cycling.

Dysregulated lipid and fatty acid metabolism link perfluoroalkyl substances exposure and impaired glucose metabolism in young adults

BACKGROUND

Per- and polyfluoroalkyl substances (PFASs) exposure is ubiquitous among the US population and has been linked to adverse health outcomes including cardiometabolic diseases, immune dysregulation and endocrine disruption. However, the metabolic mechanism underlying the adverse health effect of PFASs exposure is unknown.

OBJECTIVE

The aim of this project is to investigate the association between PFASs exposure and altered metabolic pathways linked to increased cardiometabolic risk in young adults.

METHODS

A total of 102 young adults with 82% overweight or obese participants were enrolled from Southern California between 2014 and 2017. Cardiometabolic outcomes were assessed including oral glucose tolerance test (OGTT) measures, body fat and lipid profiles. High-resolution metabolomics was used to quantify plasma exposure levels of three PFAS congeners and intensity profiles of the untargeted metabolome. Fasting concentrations of 45 targeted metabolites involved in fatty acid and lipid metabolism were used to verify untargeted metabolomics findings. Bayesian Kernel Machine Regression (BKMR) was used to examine the associations between PFAS exposure mixture and cardiometabolic outcomes adjusting for covariates. Mummichog pathway enrichment analysis was used to explore PFAS-associated metabolic pathways. Moreover, the effect of PFAS exposure on the metabolic network, including metabolomic profiles and cardiometabolic outcomes, was investigated.

RESULTS

Higher exposure to perfluorooctanoic acid (PFOA) was associated with higher 30-minute glucose levels and glucose area under the curve (AUC) during the OGTT (p < 0.001). PFAS exposure was also associated with altered lipid pathways, which contributed to the metabolic network connecting PFOA and higher glucose levels following the OGTT. Targeted metabolomics analysis indicated that higher PFOA exposure was associated with higher levels of glycerol (p = 0.006), which itself was associated with higher 30-minute glucose (p = 0.006).

CONCLUSIONS

Increased lipolysis and fatty acid oxidation could contribute to the biological mechanisms linking PFAS exposure and impaired glucose metabolism among young adults. Findings of this study warrants future experimental studies and epidemiological studies with larger sample size to replicate.

The development of metabolic endotoxemia is dependent on the type of sweetener and the presence of saturated fat in the diet

Fat and sweeteners contribute to obesity. However, it is unknown whether specific bacteria are selectively modified by different caloric and noncaloric sweeteners with or without a high-fat diet (HFD). Here, we combined extensive host phenotyping and shotgun metagenomics of the gut microbiota to investigate this question. We found that the type of sweetener and its combination with an HFD selectively modified the gut microbiota. Sucralose and steviol glycosides led to the lowest α-diversity of the gut microbiota. Sucralose increased the abundance of B. fragilis in particular, resulting in a decrease in the abundance of occludin and an increase in proinflammatory cytokines, glucose intolerance, fatty acid oxidation and ketone bodies. Sucrose+HFD showed the highest metabolic endotoxemia, weight gain, body fat, total short chain fatty acids (SCFAs), serum TNFα concentration and glucose intolerance. Consumption of sucralose or sucrose resulted in enrichment of the bacterial genes involved in the synthesis of LPS and SCFAs. Notably, brown sugar and honey were associated with the absence of metabolic endotoxemia, increases in bacterial gene diversity and anti-inflammatory markers such as IL-10 and sIgA, the maintenance of glucose tolerance and energy expenditure, similar to the control group, despite the consumption of an HFD. These findings indicate that the type of sweetener and an HFD selectively modify the gut microbiota, bacterial gene enrichment of metabolic pathways involved in LPS and SCFA synthesis, and metabolic endotoxemia associated with different metabolic profiles.

CDKN2A/p16INK4a suppresses hepatic fatty acid oxidation through the AMPKα2-SIRT1-PPARα signaling pathway

In addition to their well-known role in the control of cellular proliferation and cancer, cell cycle regulators are increasingly identified as important metabolic modulators. Several GWAS have identified SNPs near CDKN2A, the locus encoding for p16INK4a (p16), associated with elevated risk for cardiovascular diseases and type-2 diabetes development, two pathologies associated with impaired hepatic lipid metabolism. Although p16 was recently shown to control hepatic glucose homeostasis, it is unknown whether p16 also controls hepatic lipid metabolism. Using a combination of in vivo and in vitro approaches, we found that p16 modulates fasting-induced hepatic fatty acid oxidation (FAO) and lipid droplet accumulation. In primary hepatocytes, p16-deficiency was associated with elevated expression of genes involved in fatty acid catabolism. These transcriptional changes led to increased FAO and were associated with enhanced activation of PPARα through a mechanism requiring the catalytic AMPKα2 subunit and SIRT1, two known activators of PPARα. By contrast, p16 overexpression was associated with triglyceride accumulation and increased lipid droplet numbers in vitro, and decreased ketogenesis and hepatic mitochondrial activity in vivo Finally, gene expression analysis of liver samples from obese patients revealed a negative correlation between CDKN2A expression and PPARA and its target genes. Our findings demonstrate that p16 represses hepatic lipid catabolism during fasting and may thus participate in the preservation of metabolic flexibility.

Hepatic Glycerol Metabolism-Related Genes in Carnivorous Rainbow Trout (Oncorhynchus mykiss): Insights Into Molecular Characteristics, Ontogenesis, and Nutritional Regulation

Glycerol metabolism in rainbow trout is poorly studied even though it is at the interface between lipid and glucose metabolism. Moreover, glycerol can be an important ingredient in new aquafeed formulation to decrease the catabolism of dietary amino acids. Thus, the present study aimed to characterize for the first time the different genes coding for key enzymes and proteins involved in hepatic glycerol metabolism. From the trout genomes, all the paralogous genes coding for glycerol transport (aqp9b), glycerol kinase (gk2a and gk5), glycerol-3-phosphate phosphatase (pgp), and glycerol-3-phosphate dehydrogenase (gpd1a, gpd1b, and gpd1c) were identified. The ontogenesis determined that the capacity to metabolize glycerol begins with the apparition of the liver during the development (stage 22) and are more expressed at the endogenous–exogenous feeding period (stage 35). The postprandial regulation of the expression of these genes in juvenile trout showed that the postprandial peak of expression is between 4 and 24 h after the last meal for many of the genes, demonstrating that glycerol metabolism could be nutritionally regulated at a molecular level. However, surprisingly, no regulation of the mRNA abundance for the glycerol metabolism-related genes by different levels of dietary glycerol (0, 2.5, and 5%) have been detected, showing that hepatic glycerol metabolism is poorly regulated at a molecular level by dietary glycerol in rainbow trout juveniles.

Peroxisome Proliferator-Activated Receptors and Caloric Restriction-Common Pathways Affecting Metabolism, Health, and Longevity

Caloric restriction (CR) is a traditional but scientifically verified approach to promoting health and increasing lifespan. CR exerts its effects through multiple molecular pathways that trigger major metabolic adaptations. It influences key nutrient and energy-sensing pathways including mammalian target of rapamycin, Sirtuin 1, AMP-activated protein kinase, and insulin signaling, ultimately resulting in reductions in basic metabolic rate, inflammation, and oxidative stress, as well as increased autophagy and mitochondrial efficiency. CR shares multiple overlapping pathways with peroxisome proliferator-activated receptors (PPARs), particularly in energy metabolism and inflammation. Consequently, several lines of evidence suggest that PPARs might be indispensable for beneficial outcomes related to CR. In this review, we present the available evidence for the interconnection between CR and PPARs, highlighting their shared pathways and analyzing their interaction. We also discuss the possible contributions of PPARs to the effects of CR on whole organism outcomes.

Peroxisome Proliferator-Activated Receptors and Their Novel Ligands as Candidates for the Treatment of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a major health issue worldwide, frequently associated with obesity and type 2 diabetes. Steatosis is the initial stage of the disease, which is characterized by lipid accumulation in hepatocytes, which can progress to non-alcoholic steatohepatitis (NASH) with inflammation and various levels of fibrosis that further increase the risk of developing cirrhosis and hepatocellular carcinoma. The pathogenesis of NAFLD is influenced by interactions between genetic and environmental factors and involves several biological processes in multiple organs. No effective therapy is currently available for the treatment of NAFLD. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that regulate many functions that are disturbed in NAFLD, including glucose and lipid metabolism, as well as inflammation. Thus, they represent relevant clinical targets for NAFLD. In this review, we describe the determinants and mechanisms underlying the pathogenesis of NAFLD, its progression and complications, as well as the current therapeutic strategies that are employed. We also focus on the complementary and distinct roles of PPAR isotypes in many biological processes and on the effects of first-generation PPAR agonists. Finally, we review novel and safe PPAR agonists with improved efficacy and their potential use in the treatment of NAFLD.

Hepatocyte-specific deletion of Pparα promotes NAFLD in the context of obesity

Peroxisome proliferator activated receptor α (PPARα) acts as a fatty acid sensor to orchestrate the transcription of genes coding for rate-limiting enzymes required for lipid oxidation in hepatocytes. Mice only lacking Pparα in hepatocytes spontaneously develop steatosis without obesity in aging. Steatosis can develop into non alcoholic steatohepatitis (NASH), which may progress to irreversible damage, such as fibrosis and hepatocarcinoma. While NASH appears as a major public health concern worldwide, it remains an unmet medical need. In the current study, we investigated the role of hepatocyte PPARα in a preclinical model of steatosis. For this, we used High Fat Diet (HFD) feeding as a model of obesity in C57BL/6 J male Wild-Type mice (WT), in whole-body Pparα^- deficient mice (Pparα^−/−) and in mice lacking Pparα only in hepatocytes (Pparα^hep−/−). We provide evidence that Pparα deletion in hepatocytes promotes NAFLD and liver inflammation in mice fed a HFD. This enhanced NAFLD susceptibility occurs without development of glucose intolerance. Moreover, our data reveal that non-hepatocytic PPARα activity predominantly contributes to the metabolic response to HFD. Taken together, our data support hepatocyte PPARα as being essential to the prevention of NAFLD and that extra-hepatocyte PPARα activity contributes to whole-body lipid homeostasis.

Effects of parental dietary linoleic acid on growth performance, antioxidant capacity, and lipid metabolism in domestic pigeons (Columba livia)

The objective of this study was to evaluate the effects of dietary linoleic acid (LA) on growth performance, antioxidant capacity, and lipid metabolism in pigeon squabs by supplementing LA in their parental diets. A completely randomized design that consisted of a control group, 1% dietary LA addition group (LA1%), 2% dietary LA addition group (LA2%), and 4% dietary LA addition group (LA4%) was used. Six squabs from each treatment were randomly sampled at the day of hatch and days 7, 14, and 21 after hatch. The results showed that parental dietary LA had no significant influence (P > 0.05) on body weight (BW) gain or relative organ weights (% of BW) in squabs. The activities of superoxide dismutase, catalase, and glutathione peroxidase in the LA1% were significantly increased (P < 0.05) compared with those in the control group. The malondialdehyde content in the LA1% was significantly lower (P < 0.05) than that in the control group. The levels of serum triglyceride in the LA1% and LA2% were significantly decreased (P < 0.05) compared with those in the control group, whereas the serum high-density lipoprotein cholesterol level in the LA1% and LA2% and the free fatty acid level in the LA4% were significantly higher (P < 0.05) than those of the control group. The activities of lipoprotein lipase, hepatic lipase, and hormone-sensitive lipase in the LA1% were significantly higher (P < 0.05) than those in the control group. The 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in the LA1% and the hormone-sensitive lipase activity in the LA4% were significantly decreased (P < 0.05) compared with those in the control group. The mRNA expression of carnitine palmitoyltransferase 1, acyl-CoA 1, and peroxisome proliferator-activated receptor α was significantly upregulated (P < 0.05) in the LA1% compared with that in the control group. The Oil Red O staining area in the LA1% and LA2% was significantly reduced compared with that in the control group. The results indicated that although supplemental LA had negligible effects on growth and development in pigeon squabs, parental dietary LA at a concentration of 1% could have beneficial effects on maintaining squabs healthy as reflected by improved antioxidant capacity and lipid metabolism.

Transcriptomic profiling of PBDE-exposed HepaRG cells unveils critical lncRNA- PCG pairs involved in intermediary metabolism

Polybrominated diphenyl ethers (PBDEs) were formally used as flame-retardants and are chemically stable, lipophlic persistent organic pollutants which are known to bioaccumulate in humans. Although its toxicities are well characterized, little is known about the changes in transcriptional regulation caused by PBDE exposure. Long non-coding RNAs (lncRNAs) are increasingly recognized as key regulators of transcriptional and translational processes. It is hypothesized that lncRNAs can regulate nearby protein-coding genes (PCGs) and changes in the transcription of lncRNAs may act in cis to perturb gene expression of its neighboring PCGs. The goals of this study were to 1) characterize PCGs and lncRNAs that are differentially regulated from exposure to PBDEs; 2) identify PCG-lncRNA pairs through genome annotation and predictive binding tools; and 3) determine enriched canonical pathways caused by differentially expressed lncRNA-PCGs pairs. HepaRG cells, which are human-derived hepatic cells that accurately represent gene expression profiles of human liver tissue, were exposed to BDE-47 and BDE-99 at a dose of 25 μM for 24 hours. Differentially expressed lncRNA-PCG pairs were identified through DESeq2 and HOMER; significant canonical pathways were determined through Ingenuity Pathway Analysis (IPA). LncTar was used to predict the binding of 19 lncRNA-PCG pairs with known roles in drug-processing pathways. Genome annotation revealed that the majority of the differentially expressed lncRNAs map to PCG introns. PBDEs regulated overlapping pathways with PXR and CAR such as protein ubiqutination pathway and peroxisome proliferator-activated receptor alpha-retinoid X receptor alpha (PPARα-RXRα) activation but also regulate distinctive pathways involved in intermediary metabolism. PBDEs uniquely down-regulated GDP-L-fucose biosynthesis, suggesting its role in modifying important pathways involved in intermediary metabolism such as carbohydrate and lipid metabolism. In conclusion, we provide strong evidence that PBDEs regulate both PCGs and lncRNAs in a PXR/CAR ligand-dependent and independent manner.

Host Transcription Factors in Hepatitis B Virus RNA Synthesis

The hepatitis B virus (HBV) chronically infects over 250 million people worldwide and is one of the leading causes of liver cancer and hepatocellular carcinoma. HBV persistence is due in part to the highly stable HBV minichromosome or HBV covalently closed circular DNA (cccDNA) that resides in the nucleus. As HBV replication requires the help of host transcription factors to replicate, focusing on host protein–HBV genome interactions may reveal insights into new drug targets against cccDNA. The structural details on such complexes, however, remain poorly defined. In this review, the current literature regarding host transcription factors’ interactions with HBV cccDNA is discussed.

Lifestyle and Food Habits Impact on Chronic Diseases: Roles of PPARs

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that exert important functions in mediating the pleiotropic effects of diverse exogenous factors such as physical exercise and food components. Particularly, PPARs act as transcription factors that control the expression of genes implicated in lipid and glucose metabolism, and cellular proliferation and differentiation. In this review, we aim to summarize the recent advancements reported on the effects of lifestyle and food habits on PPAR transcriptional activity in chronic disease.

Effects of Paramylon Extracted from Euglena gracilis EOD-1 on Parameters Related to Metabolic Syndrome in Diet-Induced Obese Mice

Paramylon (PM), a type of β-glucan, functions like dietary fiber, which has been suggested to exert a protective effect against obesity. We evaluated the potential beneficial effects of PM powder on obesity in mice. Male C57BL/6J mice were fed a high-fat diet supplemented with either 2.5 or 5% PM powder, extracted from Euglena gracilis, for 74 days. Growth parameters, abdominal fat content, serum biochemical markers, hepatic lipid accumulation and hepatic mRNA expression were measured. Dietary supplementation with PM resulted in decreased food efficiency ratios and abdominal fat accumulation. Dose-dependent decreases were observed in postprandial glucose levels, serum low-density lipoprotein (LDL)-cholesterol, and serum secretary immunoglobulin A (sIgA) concentrations. PM supplementation increased peroxisome proliferator-activated receptor α (PPARα) mRNA expression in the liver which is suggested to induce β-oxidation through activation of acyl-coenzyme A oxidase (ACOX), carnitine palmitoyltransferase (CPT) and fatty acid transport protein 2 (FATP2) mRNA expression. Changes in fatty acid metabolism may improve lipid and glucose metabolism. In conclusion, a preventive effect against obesity was observed in mice given a PM-enriched diet. The mechanism is suggested to involve a reduction in both serum LDL-cholesterol levels and the accumulation of abdominal fat, in addition to an improvement in postprandial glucose concentration.

Changes in peroxisome proliferator-activated receptor alpha target gene expression in peripheral blood mononuclear cells associated with non-alcoholic fatty liver disease

Objective

To identify differences in the expression of peroxisome proliferator-activated receptor alpha (PPARα) target genes in human peripheral blood mononuclear cells (PBMCs) associated with non-alcoholic fatty liver disease (NAFLD) among Chinese individuals.

Methods

Thirty healthy subjects were selected as the control group (CN), and 43 patients newly diagnosed with NAFLD were subdivided into two groups, non-obese group (NF, n = 21) and obese group (OF, n = 22). Expression of PPARα and its target genes was determined in PBMCs. The levels of liver cell damage markers, total cholesterol (TC), triglyceride (TG), free fatty acids (FFA), glucose, and insulin were determined in serum.

Results

Compared to the CN group, the blood pressure and homeostasis model assessment for insulin resistance (HOMA-IR) were increased in the other groups (P < 0.05), while the systolic blood pressure (SBP) and liver cell damage markers were significantly increased in the OF group (P < 0.05). In the OF group, PPARα target gene expression was 2.03–3.31 times higher than that in the CN group, and a negative correlation was found between PPARα target gene expression and abdominal circumference (AC), body mass index (BMI), diastolic blood pressure (DBP). Additionally, solute carrier family 25 (carnitine/acylcarnitine translocase) member 20 (SLC25A20) and acyl-coenzyme A dehydrogenase 2 long chain (ACADVL) were negatively correlated with HOMA-IR; PPARα, acetyl-coenzyme A dehydrogenase 2 (ACAA2), and carnitine palmitoyltransferase 1A (CPT1A) were positively correlated with HOMA-IR.

Conclusion

There is an up-expression of PPARα target genes in the PBMCs of NAFLD patients, possibly leading to changes in β-oxidation and insulin resistance.

C/VDdb: A multi-omics expression profiling database for a knowledge-driven approach in cardiovascular disease (CVD)

The cardiovascular disease (C/VD) database is an integrated and clustered information resource that covers multi-omic studies (microRNA, genomics, proteomics and metabolomics) of cardiovascular-related traits with special emphasis on coronary artery disease (CAD). This resource was built by mining existing literature and public databases and thereafter manual biocuration was performed. To enable integration of omic data from distinct platforms and species, a specific ontology was applied to tie together and harmonise multi-level omic studies based on gene and protein clusters (CluSO) and mapping of orthologous genes (OMAP) across species. CAD continues to be a leading cause of death in the population worldwide, and it is generally thought to be an age-related disease. However, CAD incidence rates are now known to be highly influenced by environmental factors and interactions, in addition to genetic determinants. With the complexity of CAD aetiology, there is a difficulty in research studies to elucidate general elements compared to other cardiovascular diseases. Data from 92 studies, covering 13945 molecular entries (4353 unique molecules) is described, including data descriptors for experimental setup, study design, discovery-validation sample size and associated fold-changes of the differentially expressed molecular features (p-value<0.05). A dedicated interactive web interface, equipped with a multi-parametric search engine, data export and indexing menus are provided for a user-accessible browsing experience. The main aim of this work was the development of a data repository linking clinical information and molecular differential expression in several CVD-related traits from multi-omics studies (genomics, transcriptomics, proteomics and metabolomics). As an example case of how to query and identify data sets within the database framework and concomitantly demonstrate the database utility, we queried CAD-associated studies and performed a systems-level integrative analysis. URL: www.padb.org/cvd

Nuclear Receptor Regulation of Aquaglyceroporins in Metabolic Organs

Nuclear receptors, such as the farnesoid X receptor (FXR) and the peroxisome proliferator-activated receptors gamma and alpha (PPAR-γ, -α), are major metabolic regulators in adipose tissue and the liver, where they govern lipid, glucose, and bile acid homeostasis, as well as inflammatory cascades. Glycerol and free fatty acids are the end products of lipid droplet catabolism driven by PPARs. Aquaporins (AQPs), a family of 13 small transmembrane proteins, facilitate the shuttling of water, urea, and/or glycerol. The peculiar role of AQPs in glycerol transport makes them pivotal targets in lipid metabolism, especially considering their tissue-specific regulation by the nuclear receptors PPARγ and PPARα. Here, we review the role of nuclear receptors in the regulation of glycerol shuttling in liver and adipose tissue through the function and expression of AQPs.

PPARγ activation mitigates glucocorticoid receptor-induced excessive lipolysis in adipocytes via homeostatic crosstalk

Proper balance between lipolysis and lipogenesis in adipocytes determines the release of free fatty acids (FFA) and glycerol, which is crucial for whole body lipid homeostasis. Although, dysregulation of lipid homeostasis contributes to various metabolic complications such as insulin resistance, the regulatory mechanism remains elusive. This study clarified the individual and combined roles for glucocorticoid receptor (GCR) and peroxisome proliferator-activated receptor (PPAR)γ pathways in lipid metabolism of adipocytes. In mature 3T3-L1 adipocytes, GCR activation using dexamethasone upregulated adipose triglyceride lipase (ATGL) and downregulated phosphoenolpyruvate carboxykinase (PEPCK), resulting in enhanced glycerol release into the medium. In contrast, PPARγ ligand pioglitazone modestly upregulated ATGL and hormone sensitive lipase (HSL), but markedly enhanced PEPCK and glycerol kinase (GK), thereby suppressed glycerol release. Dexamethasone showed permissive like effect on PPARγ target genes including perilipin A and aP2, therefore co-administration of dexamethasone and pioglitazone demonstrated synergistic upregulation of these enzymes excepting PEPCK, of which downregulation by dexamethasone was abolished by pioglitazone to the level above control. Thus, the excessive glycerol release was prevented as the net outcome of the co-administration. Consistently, the bodipy stain demonstrated that dexamethasone reduced the amount of cytosolic lipid, which was preserved in co-treated adipocytes. Moreover, silencing of PPARγ suppressed the synergistic effects of co-treatment on the lipolytic and lipogenic genes, and therefore the GCR pathway indeed involves PPARγ. In conclusion, crosstalk between GCR and PPARγ is largely synergistic but counter-regulatory in lipogenic genes, of which enhancement prevents excessive glycerol and possibly FFA release by glucocorticoids into the circulation.

Implications of Aquaglyceroporin 7 in Energy Metabolism

The aquaglyceroporin AQP7 is a pore-forming transmembrane protein that facilitates the transport of glycerol across cell membranes. Glycerol is utilized both in carbohydrate and lipid metabolism. It is primarily stored in white adipose tissue as part of the triglyceride molecules. During states with increased lipolysis, such as fasting and diabetes, glycerol is released from adipose tissue and metabolized in other tissues. AQP7 is expressed in adipose tissue where it facilitates the efflux of glycerol, and AQP7 deficiency has been linked to increased glycerol kinase activity and triglyceride accumulation in adipose tissue, leading to obesity and secondary development of insulin resistance. However, AQP7 is also expressed in a wide range of other tissues, including kidney, muscle, pancreatic β-cells and liver, where AQP7 also holds the potential to influence whole body energy metabolism. The aim of the review is to summarize the current knowledge on AQP7 in adipose tissue, as well as AQP7 expressed in other tissues where AQP7 might play a significant role in modulating whole body energy metabolism.

Level of dietary energy and 2,4-thiazolidinedione alter molecular and systemic biomarkers of inflammation and liver function in Holstein cows

Background

The objective of the study was to evaluate the effect of overfeeding a moderate energy diet and a 2,4-thiazolidinedione (TZD) injection on blood and hepatic tissue biomarkers of lipid metabolism, oxidative stress, and inflammation as it relates to insulin sensitivity.

Results

Fourteen dry non-pregnant cows were fed a control (CON) diet to meet 100% of NRC requirements for 3 wk, after which half of the cows were assigned to a moderate-energy diet (OVE) and half of the cows continued on CON for 6 wk. All cows received an intravenous injection of 4 mg TZD/kg of body weight (BW) daily from 2 wk after initiation of dietary treatments and for 2 additional week. Compared with CON cows and before TZD treatment, the OVE cows had lower concentration of total protein, urea and albumin over time. The concentration of cholesterol and tocopherol was greater after 2 wk of TZD regardless of diet. Before and after TZD, the OVE cows had greater concentrations of AST/GOT, while concentrations of paraoxonase, total protein, globulin, myeloperoxidase, and haptoglobin were lower compared with CON cows. Regardless of diet, TZD administration increased the concentration of ceruloplasmin, ROMt, cholesterol, tocopherol, total protein, globulin, myeloperoxidase and beta-carotene. In contrast, the concentration of haptoglobin decreased at the end of TZD injection regardless of diet. Prior to TZD injection, the mRNA expression of PC, ANGPTL4, FGF21, INSR, ACOX1, and PPARD in liver of OVE cows was lower compared with CON cows. In contrast, the expression of HMGCS2 was greater in OVE compared with CON cows. After 1 wk of TZD administration the expression of IRS1 decreased regardless of diet; whereas, expression of INSR increased after 2 wk of TZD injection. Cows fed OVE had lower overall expression of TNF, INSR, PC, ACOX1, FGF21, and PPARD but greater HMGCS2 expression. These differences were most evident before and after 1 wk of TZD injection, and by 2 wk of TZD differences in expression for most genes disappeared.

Conclusions

Based on molecular and blood data, administration of TZD enhanced some aspects of insulin sensitivity while causing contradictory results in terms of inflammation and oxidative stress. The bovine liver is TZD-responsive and level of dietary energy can modify the effects of TZD. Because insulin sensitizers have been proposed as useful tools to manage dairy cows during the transition period, further studies are required to investigate the potential hepatotoxicity effect of TZD (or similar compounds) in dairy cattle.

Electronic supplementary material

The online version of this article (doi:10.1186/s40104-017-0196-y) contains supplementary material, which is available to authorized users.

Nutrient-sensing nuclear receptors PPARα and FXR control liver energy balance

The nuclear receptors PPARα (encoded by NR1C1) and farnesoid X receptor (FXR, encoded by NR1H4) are activated in the liver in the fasted and fed state, respectively. PPARα activation induces fatty acid oxidation, while FXR controls bile acid homeostasis, but both nuclear receptors also regulate numerous other metabolic pathways relevant to liver energy balance. Here we review evidence that they function coordinately to control key nutrient pathways, including fatty acid oxidation and gluconeogenesis in the fasted state and lipogenesis and glycolysis in the fed state. We have also recently reported that these receptors have mutually antagonistic impacts on autophagy, which is induced by PPARα but suppressed by FXR. Secretion of multiple blood proteins is a major drain on liver energy and nutrient resources, and we present preliminary evidence that the liver secretome may be directly suppressed by PPARα, but induced by FXR. Finally, previous studies demonstrated a striking deficiency in bile acid levels in malnourished mice that is consistent with results in malnourished children. We present evidence that hepatic targets of PPARα and FXR are dysregulated in chronic undernutrition. We conclude that PPARα and FXR function coordinately to integrate liver energy balance.

TRPV1 activation counters diet-induced obesity through sirtuin-1 activation and PRDM-16 deacetylation in brown adipose tissue

Background/Objective

An imbalance between energy intake and expenditure leads to obesity. Increasing metabolism and thermogenesis in brown adipose tissue (BAT) can help in overcoming obesity. Here, we investigated the effect of activation of transient receptor potential vanilloid subfamily 1 (TRPV1) in the upregulation of thermogenic proteins in BAT to counter diet-induced obesity.

Subjects/Methods

We investigated the effect of dietary supplementation of capsaicin (TRPV1 agonist) on the expression of metabolically important thermogenic proteins in BAT of wild type and TRPV1^−/− mice that received either a normal chow or high fat (± capsaicin; TRPV1 activator) diet by immunoblotting. We measured the metabolic activity, respiratory quotient and BAT lipolysis.

Results

CAP antagonized high fat diet (HFD)-induced obesity without decreasing energy intake in mice. HFD suppressed TRPV1 expression and activity in BAT and CAP countered this effect. HFD feeding caused glucose intolerance, hypercholesterolemia and decreased the plasma concentration of glucagon like peptide-1 and CAP countered these effects. HFD suppressed the expression of metabolically important thermogenic genes, ucp-1, bmp8b, sirtuin 1, pgc-1α and prdm-16 in BAT and CAP prevented this effect. CAP increased the phosphorylation of sirtuin 1 and induced an interaction between PPARγ with PRDM-16. Further, CAP treatment, in vitro, decreased the acetylation of PRDM-16, which was antagonized by inhibition of TRPV1 by capsazepine, chelation of intracellular Ca²⁺ by cell permeable BAPTA-AM or the inhibition of SIRT-1 by EX 527. Further, CAP supplementation, post HFD, promoted weight loss and enhanced the respiratory exchange ratio. CAP did not have any effect in TRPV1^−/− mice.

Conclusions

Our data show that activation of TRPV1 in BAT enhances the expression of SIRT-1, which facilitates the deacetylation and interaction of PPARγ and PRDM-16. These data suggest that TRPV1 activation is a novel strategy to counter diet-induced obesity by enhancing metabolism and energy expenditure.

PPARα-dependent increase of mouse urine output by gemfibrozil and fenofibrate

While gemfibrozil and fenofibrate are prescribed for anti-dyslipidemia treatment, a rational basis for the use of these drugs for treatment of dyslipidemia with concurrent metabolic syndrome has not been established. In this study, wild-type and Pparα-null mice were fed gemfibrozil- or fenofibrate-containing diets for 14 days. Urine output (24 h) was monitored, and urine, serum, and liver and kidney tissues were subjected to toxicity assessment. A 2-month challenge followed by a 2-week wash-out was performed for gemfibrozil to determine urine output and the potential toxicity. A therapeutically equivalent dose of gemfibrozil was more effective than fenofibrate in increasing urine output. This regulatory effect was not observed in Pparα-null mice. In contrast, hepatomegaly induced by fenofibrate was more pronounced than that of gemfibrozil. No significant toxicity was observed in liver or kidney in the 2-month treatment with gemfibrozil. These data demonstrated PPARα mediates the increased urine output by fibrates. Considering the relative action on hepatomegaly and the regulatory effect on urine output, gemfibrozil may be the preferable drug to increase urine output. These results revealed a new pharmacodynamic effect of clinically prescribed PPARα agonists and suggested the potential value of gemfibrozil in modification of blood pressure.

Application of network construction to estimate functional changes to insulin receptor substrates 1 and 2 in Huh7 cells following infection with the hepatitis C virus

Hepatitis C virus (HCV) is closely associated with insulin resistance (IS), acting primarily by interfering with insulin signaling pathways, increasing cytokine-mediated (tumor necrosis factor α, interleukin 6) inflammatory responses and enhancing oxidative stress. In the insulin signaling pathways, the insulin receptor substrate (IRS) is one of the key regulatory factors. The present study constructed gene regulatory sub-networks specific for IRS1 and IRS2 in Huh7 cells and HCV-infected Huh7 (HCV-Huh7) cells using linear programming and a decomposition algorithm, and investigated the possible mechanisms underlying the function of IRS1/2 in HCV-induced IS in Huh7 cells. All data were obtained from GSE20948 of the Gene Expression Omnibus database from the National Center for Biotechnology Information. Genes which were significantly differentially expressed between Huh7 and HCV-Huh7 cells were analyzed using the significance analysis of microarray algorithm. The top 50 genes, including IRS1/2, were used as target genes to determine the gene regulatory networks and next the sub-networks of IRS1 and IRS2 in HCV-Huh7 and Huh7 cells using Gene Regulatory Network Inference Tool, an algorithm based on linear programming and the decomposition process. The IRS1/2 sub-networks were divided into upstream/downstream groups and activation/suppression clusters, and were further analyzed using Molecule Annotation System 3.0 and Database for Annotation, Visualization, and Integrated Discovery software, two online platforms for enrichment and clustering analysis and visualization. The results indicated that in Huh7 cells, the downstream network of IRS2 is more complex than that of IRS1, indicating that the insulin metabolism in Huh7 cells may be primarily mediated by IRS2. In HCV-Huh7 cells, the downstream pathway of IRS2 is blocked, suggesting that this may be the underlying mechanism in HCV infection that leads to insulin resistance. The present findings add a further dimension to the understanding of the pathological mechanisms of HCV infection-associated insulin resistance, and provide novel concepts for insulin resistance and glucose metabolism research.

Hepatocyte and Sertoli Cell Aquaporins, Recent Advances and Research Trends

Aquaporins (AQPs) are proteinaceous channels widespread in nature where they allow facilitated permeation of water and uncharged through cellular membranes. AQPs play a number of important roles in both health and disease. This review focuses on the most recent advances and research trends regarding the expression and modulation, as well as physiological and pathophysiological functions of AQPs in hepatocytes and Sertoli cells (SCs). Besides their involvement in bile formation, hepatocyte AQPs are involved in maintaining energy balance acting in hepatic gluconeogenesis and lipid metabolism, and in critical processes such as ammonia detoxification and mitochondrial output of hydrogen peroxide. Roles are played in clinical disorders including fatty liver disease, diabetes, obesity, cholestasis, hepatic cirrhosis and hepatocarcinoma. In the seminiferous tubules, particularly in SCs, AQPs are also widely expressed and seem to be implicated in the various stages of spermatogenesis. Like in hepatocytes, AQPs may be involved in maintaining energy homeostasis in these cells and have a major role in the metabolic cooperation established in the testicular tissue. Altogether, this information represents the mainstay of current and future investigation in an expanding field.

Liver PPARα is crucial for whole-body fatty acid homeostasis and is protective against NAFLD

OBJECTIVE

Peroxisome proliferator-activated receptor α (PPARα) is a nuclear receptor expressed in tissues with high oxidative activity that plays a central role in metabolism. In this work, we investigated the effect of hepatocyte PPARα on non-alcoholic fatty liver disease (NAFLD).

DESIGN

We constructed a novel hepatocyte-specific PPARα knockout (Pparα(hep-/-)) mouse model. Using this novel model, we performed transcriptomic analysis following fenofibrate treatment. Next, we investigated which physiological challenges impact on PPARα. Moreover, we measured the contribution of hepatocytic PPARα activity to whole-body metabolism and fibroblast growth factor 21 production during fasting. Finally, we determined the influence of hepatocyte-specific PPARα deficiency in different models of steatosis and during ageing.

RESULTS

Hepatocyte PPARα deletion impaired fatty acid catabolism, resulting in hepatic lipid accumulation during fasting and in two preclinical models of steatosis. Fasting mice showed acute PPARα-dependent hepatocyte activity during early night, with correspondingly increased circulating free fatty acids, which could be further stimulated by adipocyte lipolysis. Fasting led to mild hypoglycaemia and hypothermia in Pparα(hep-/-) mice when compared with Pparα(-/-) mice implying a role of PPARα activity in non-hepatic tissues. In agreement with this observation, Pparα(-/-) mice became overweight during ageing while Pparα(hep-/-) remained lean. However, like Pparα(-/-) mice, Pparα(hep-/-) fed a standard diet developed hepatic steatosis in ageing.

CONCLUSIONS

Altogether, these findings underscore the potential of hepatocyte PPARα as a drug target for NAFLD.

White-to-brite conversion in human adipocytes promotes metabolic reprogramming towards fatty acid anabolic and catabolic pathways

Objective

Fat depots with thermogenic activity have been identified in humans. In mice, the appearance of thermogenic adipocytes within white adipose depots (so-called brown-in-white i.e., brite or beige adipocytes) protects from obesity and insulin resistance. Brite adipocytes may originate from direct conversion of white adipocytes. The purpose of this work was to characterize the metabolism of human brite adipocytes.

Methods

Human multipotent adipose-derived stem cells were differentiated into white adipocytes and then treated with peroxisome proliferator-activated receptor (PPAR)γ or PPARα agonists between day 14 and day 18. Gene expression profiling was determined using DNA microarrays and RT-qPCR. Variations of mRNA levels were confirmed in differentiated human preadipocytes from primary cultures. Fatty acid and glucose metabolism was investigated using radiolabelled tracers, Western blot analyses and assessment of oxygen consumption. Pyruvate dehydrogenase kinase 4 (PDK4) knockdown was achieved using siRNA. In vivo, wild type and PPARα-null mice were treated with a β₃-adrenergic receptor agonist (CL316,243) to induce appearance of brite adipocytes in white fat depot. Determination of mRNA and protein levels was performed on inguinal white adipose tissue.

Results

PPAR agonists promote a conversion of white adipocytes into cells displaying a brite molecular pattern. This conversion is associated with transcriptional changes leading to major metabolic adaptations. Fatty acid anabolism i.e., fatty acid esterification into triglycerides, and catabolism i.e., lipolysis and fatty acid oxidation, are increased. Glucose utilization is redirected from oxidation towards glycerol-3-phophate production for triglyceride synthesis. This metabolic shift is dependent on the activation of PDK4 through inactivation of the pyruvate dehydrogenase complex. In vivo, PDK4 expression is markedly induced in wild-type mice in response to CL316,243, while this increase is blunted in PPARα-null mice displaying an impaired britening response.

Conclusions

Conversion of human white fat cells into brite adipocytes results in a major metabolic reprogramming inducing fatty acid anabolic and catabolic pathways. PDK4 redirects glucose from oxidation towards triglyceride synthesis and favors the use of fatty acids as energy source for uncoupling mitochondria.

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PPARγ and α agonists induce conversion of human white into brite adipocytes.

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Fatty acid anabolism and catabolism are activated in human brite adipocytes.

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Glucose use in brite adipocytes is redirected from oxidation to glyceroneogenesis.

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PDK4 induction is responsible for the shift from glucose to fatty acid oxidation.

Argan oil prevents down-regulation induced by endotoxin on liver fatty acid oxidation and gluconeogenesis and on peroxisome proliferator-activated receptor gamma coactivator-1α, (PGC-1α), peroxisome proliferator-activated receptor α (PPARα) and estrogen related receptor α (ERRα)

In patients with sepsis, liver metabolism and its capacity to provide other organs with energetic substrates are impaired. This and many other pathophysiological changes seen in human patients are reproduced in mice injected with purified endotoxin (lipopolysaccharide, LPS). In the present study, down-regulation of genes involved in hepatic fatty acid oxidation (FAOx) and gluconeogenesis in mice exposed to LPS was challenged by nutritional intervention with Argan oil. Mice given a standard chow supplemented or not with either 6% (w/w) Argan oil (AO) or 6% (w/w) olive oil (OO) prior to exposure to LPS were explored for liver gene expressions assessed by mRNA transcript levels and/or enzyme activities. AO (or OO) food supplementation reveals that, in LPS-treated mice, hepatic expression of genes involved in FAOx and gluconeogenesis was preserved. This preventive protection might be related to the recovery of the gene expressions of nuclear receptors peroxisome proliferator-activated receptor α (PPARα) and estrogen related receptor α (ERRα) and their coactivator peroxisome proliferator-activated receptor gamma coactivator-1α, (PGC-1α). These preventive mechanisms conveyed by AO against LPS-induced metabolic dysregulation might add new therapeutic potentialities in the management of human sepsis.

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Argan oil prevents LPS-treated mice from liver dysregulation of FAOx and gluconeogenesis.

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Argan oil improves hepatic expression of PPARα and ERRα, and their coactivators PGC-1α and Lipin-1.

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New preventive mechanisms conveyed by Argan oil against LPS-induced metabolic dysregulation.

Analysis of aquaporin expression in liver with a focus on hepatocytes

A deeper understanding of aquaporins (AQPs) expression and transcriptional regulation will provide useful information for liver pathophysiology. We established a complete AQPs mRNA expression profile in human and mouse liver, as well as protein localization of expressed AQPs. Additionally, the modulation of AQPs mRNA levels in response to various agents was determined in human HuH7 cells and in primary culture of mouse hepatocytes. AQP1, AQP3, AQP7, AQP8, and AQP9 mRNA and protein expressions were detected in human liver, while only AQP6 and AQP11 mRNAs were detected. We reported for the first time the localization of AQP3 in Kupffer cells, AQP7 in hepatocytes and endothelial cells, and AQP9 in cholangiocytes. In addition, we confirmed the localization of AQP1 in endothelial cells, and of AQP8 and AQP9 in hepatocytes. On HuH7 cells, we reported the presence of AQP4 mRNA, confirmed the presence of AQP3, AQP7, and AQP11 mRNAs, but not of AQP8 mRNA. On primary culture of murine hepatocytes, AQP1 and AQP7 mRNAs were identified, while the presence of AQP3, AQP8, AQP9, and AQP11 mRNAs was confirmed. At the protein level, murine endothelial liver cells expressed AQP1 and AQP9, while hepatocytes expressed AQP3, AQP7, AQP8, and AQP9, and macrophages expressed AQP3. Dexamethasone, forskolin, AICAR, rosiglitazone, octanoylated, and non-octanoylated ghrelin regulated some AQP expression in primary culture of murine hepatocytes and human HuH7 cells. Additional studies will be required to further assess the role of AQPs expression in human and murine liver and understand the transcriptional regulation of AQPs in hepatocytes under pathophysiological conditions.

Nutrigenomics: A controversy

Nutrigenomics is an emerging science which investigates a certain area of nutrition that uses molecular tools to search access and understand the several responses obtained through a certain diet applied between individual and population groups. The increased need for the use of personalised nutrition in patients is increasing and research is being made on its possible effects. However, research on nutrigenomics and in particular, obesity is still ongoing. Following a current metanalysis on thirty-eight nutrigenomics genes, it seems that a definite association between the genes usually examined in nutrigenomics testing and several diet-related diseases is lacking, even though there is a limited number of studies associating them. In 2014, literature search results in a great number of studies on several polymorphisms. This heterogeneity could only show the way towards new research aims. Nutrigenomics was born due to the need to move from Epidemiology and Physiology to Molecular Biology and Genetics. Currently, there are steps that need to be considered in order for nutrigenomics to be applied: the genes, the gene/protein network, and the strategy towards the determination of the nutrients' influence on gene/protein expression. It is certainly an interesting evolving science with many areas to be investigated further and from different perspectives, as it involves ethics, medicine, genetics and nutrition.

Changes in gene expression in PBMCs profiles of PPARα target genes in obese and non-obese individuals during fasting

BACKGROUND

The prevalence of obesity has risen dramatically and the World Health Organization estimates that 700 million people will be obese worldwide by 2015. Approximately, 50% of the Brazilian population above 20 years of age is overweight, and 16% is obese.

AIM

This study aimed to evaluate the differences in the expression of PPARα target genes in human peripheral blood mononuclear cells (PBMCs) and free fatty acids (FFA) in obese and non-obese individuals after 24 h of fasting. We first presented evidence that Brazilian people exhibit expression changes in PPARα target genes in PBMCs under fasting conditions.

METHODS

Q-PCR was utilized to assess the mRNA expression levels of target genes.

RESULTS

In both groups, the FFA concentrations increased significantly after 24 h of fasting. The basal FFA mean concentration was two-fold higher in the obese group compared with the non-obese group. After fasting, all genes evaluated in this study showed increased expression levels compared with basal expression in both groups.

CONCLUSION

However, our results reveal no differences in gene expression between the obese and non-obese, more studies are necessary to precisely delineate the associated mechanisms, particularly those that include groups with different degrees of obesity and patients with diabetes mellitus type 2 because the expression of the main genes that are involved in β-oxidation and glucose level maintenance are affected by these factors.

Impairments of hepatic gluconeogenesis and ketogenesis in PPARα-deficient neonatal mice

Peroxisome proliferator activated receptor-α (PPARα) is a master transcriptional regulator of hepatic metabolism and mediates the adaptive response to fasting. Here, we demonstrate the roles for PPARα in hepatic metabolic adaptations to birth. Like fasting, nutrient supply is abruptly altered at birth when a transplacental source of carbohydrates is replaced by a high-fat, low-carbohydrate milk diet. PPARα-knockout (KO) neonatal mice exhibit relative hypoglycemia due to impaired conversion of glycerol to glucose. Although hepatic expression of fatty acyl-CoA dehydrogenases is imparied in PPARα neonates, these animals exhibit normal blood acylcarnitine profiles. Furthermore, quantitative metabolic fate mapping of the medium-chain fatty acid [(13)C]octanoate in neonatal mouse livers revealed normal contribution of this fatty acid to the hepatic TCA cycle. Interestingly, octanoate-derived carbon labeled glucose uniquely in livers of PPARα-KO neonates. Relative hypoketonemia in newborn PPARα-KO animals could be mechanistically linked to a 50% decrease in de novo hepatic ketogenesis from labeled octanoate. Decreased ketogenesis was associated with diminished mRNA and protein abundance of the fate-committing ketogenic enzyme mitochondrial 3-hydroxymethylglutaryl-CoA synthase (HMGCS2) and decreased protein abundance of the ketogenic enzyme β-hydroxybutyrate dehydrogenase 1 (BDH1). Finally, hepatic triglyceride and free fatty acid concentrations were increased 6.9- and 2.7-fold, respectively, in suckling PPARα-KO neonates. Together, these findings indicate a primary defect of gluconeogenesis from glycerol and an important role for PPARα-dependent ketogenesis in the disposal of hepatic fatty acids during the neonatal period.

Validation of a genomics-based hypothetical adverse outcome pathway: 2,4-dinitrotoluene perturbs PPAR signaling thus impairing energy metabolism and exercise endurance

2,4-dinitrotoluene (2,4-DNT) is a nitroaromatic used in industrial dyes and explosives manufacturing processes that is found as a contaminant in the environment. Previous studies have implicated antagonism of PPARα signaling as a principal process affected by 2,4-DNT. Here, we test the hypothesis that 2,4-DNT-induced perturbations in PPARα signaling and resultant downstream deficits in energy metabolism, especially from lipids, cause organism-level impacts on exercise endurance. PPAR nuclear activation bioassays demonstrated inhibition of PPARα signaling by 2,4-DNT whereas PPARγ signaling increased. PPARα (-/-) and wild-type (WT) female mice were exposed for 14 days to vehicle or 2,4-DNT (134 mg/kg/day) and performed a forced swim to exhaustion 1 day after the last dose. 2,4-DNT significantly decreased body weights and swim times in WTs, but effects were significantly mitigated in PPARα (-/-) mice. 2,4-DNT decreased transcript expression for genes downstream in the PPARα signaling pathway, principally genes involved in fatty acid transport. Results indicate that PPARγ signaling increased resulting in enhanced cycling of lipid and carbohydrate substrates into glycolytic/gluconeogenic pathways favoring energy production versus storage in 2,4-DNT-exposed WT and PPARα (-/-) mice. PPARα (-/-) mice appear to have compensated for the loss of PPARα by shifting energy metabolism to PPARα-independent pathways resulting in lower sensitivity to 2,4-DNT when compared with WT mice. Our results validate 2,4-DNT-induced perturbation of PPARα signaling as the molecular initiating event for impaired energy metabolism, weight loss, and decreased exercise performance.

Integrated physiology and systems biology of PPARα

The Peroxisome Proliferator Activated Receptor alpha (PPARα) is a transcription factor that plays a major role in metabolic regulation. This review addresses the functional role of PPARα in intermediary metabolism and provides a detailed overview of metabolic genes targeted by PPARα, with a focus on liver. A distinction is made between the impact of PPARα on metabolism upon physiological, pharmacological, and nutritional activation. Low and high throughput gene expression analyses have allowed the creation of a comprehensive map illustrating the role of PPARα as master regulator of lipid metabolism via regulation of numerous genes. The map puts PPARα at the center of a regulatory hub impacting fatty acid uptake, fatty acid activation, intracellular fatty acid binding, mitochondrial and peroxisomal fatty acid oxidation, ketogenesis, triglyceride turnover, lipid droplet biology, gluconeogenesis, and bile synthesis/secretion. In addition, PPARα governs the expression of several secreted proteins that exert local and endocrine functions.

Metabolite and transcriptome analysis during fasting suggest a role for the p53-Ddit4 axis in major metabolic tissues

BACKGROUND

Fasting induces specific molecular and metabolic adaptions in most organisms. In biomedical research fasting is used in metabolic studies to synchronize nutritional states of study subjects. Because there is a lack of standardization for this procedure, we need a deeper understanding of the dynamics and the molecular mechanisms in fasting.

RESULTS

We investigated the dynamic changes of liver gene expression and serum parameters of mice at several time points during a 48 hour fasting experiment and then focused on the global gene expression changes in epididymal white adipose tissue (WAT) as well as on pathways common to WAT, liver, and skeletal muscle. This approach produced several intriguing insights: (i) rather than a sequential activation of biochemical pathways in fasted liver, as current knowledge dictates, our data indicates a concerted parallel response; (ii) this first characterization of the transcriptome signature of WAT of fasted mice reveals a remarkable activation of components of the transcription apparatus; (iii) most importantly, our bioinformatic analyses indicate p53 as central node in the regulation of fasting in major metabolic tissues; and (iv) forced expression of Ddit4, a fasting-regulated p53 target gene, is sufficient to augment lipolysis in cultured adipocytes.

CONCLUSIONS

In summary, this combination of focused and global profiling approaches provides a comprehensive molecular characterization of the processes operating during fasting in mice and suggests a role for p53, and its downstream target Ddit4, as novel components in the transcriptional response to food deprivation.

Molecular docking of Glyceroneogenesis pathway intermediates with Peroxisome Proliferator- Activated Receptor-Alpha (PPAR-α)

Peroxisome proliferator-activated receptor alpha (PPAR-α) belongs to the nuclear receptor superfamily of proteins. It is one of the principle regulators of metabolism and lipid homeostasis whose malfunction leads to complications including obesity and type 2 diabetes. In the adipose tissue, glyceroneogenesis is a unique pathway through which pyruvate is converted into glycerol-3- phosphate (G3P) in a multistep process. Previous findings demonstrated that glyceroneogenesis regulates triacylglycerol synthesis and adipogenesis. This led us to hypothesize that one of the pathway intermediate is physiologically relevant PPAR-α ligand. In the present study using in silico docking, we proved that glycerate, dihydroxy acetone phosphate, glyceraldehyde-3-phosphate, and G3P are key glyceroneogenesis pathway intermediates which bind to PPAR-α. They bind PPAR-α with comparable binding energy and docking score to that of (2s)-2-ethoxy-3-[4-(2-{4-[(methylsulfonyl)oxy]phenyl}ethoxy)phenyl]propanoic acid(AZ-2), a synthetic high affinity ligand of PPAR-α. These intermediates could be studied further as potential physiologically relevant activators of PPAR-α in vitro and in vivo.

Diabetes and infections-hepatitis C: is there type 2 diabetes excess in hepatitis C infection?

Individual epidemiologic studies as well as the pooled analysis of observational studies have indicated the association between type 2 diabetes (T2D) and hepatitis C virus infection (HCV). Whether HCV infection is the cause of diabetes or diabetic patients are more prone to get HCV infection is still in question. The objective of the present review was to provide answers to this issue, based on available evidence from epidemiologic, molecular, experimental and therapeutic studies. Our current understanding of how chronic HCV infection could induce T2D is incomplete, but it seems twofold based on both direct and indirect roles of the virus. HCV may directly induce insulin resistance (IR) through its proteins. HCV core protein was shown to stimulate suppressor of cytokine signaling, resulting in ubiquitination and degradation of tyrosine kinase phosphorylated insulin receptor substrates (IRS1/2) in proteasomes. HCV-nonstructural protein could increase protein phosphatase 2A which has been shown to inactivate the key enzyme Akt by dephosphorylating it. Insulin signaling defects in hepatic IRS-1 tyrosine phosphorylation and PI3-kinase association/activation may contribute to IR, which leads to the development of T2D in patients with HCV infection. The peroxisome proliferator-activated receptors (PPARs) are also implicated. PPARα/γ, together with their obligate partner RXR, are the main nuclear receptors expressed in the liver. PPARα upregulates glycerol-3-phosphate dehydrogenase, glycerol kinase, and glycerol transport proteins, which allows for glucose synthesis during fasting states. Decreased activity of PPARs could attribute to HCV-induced IR. Immune-mediated mechanisms may be involved in the indirect role of HCV in inducing IR. It is speculated that TNF-alpha plays a major role in the pathogenesis of IR through lowering IRS1/2. Furthermore, HCV infection- triggered ER stress could lead to the activation of PP2A, which inhibits both Akt and the AMP-activated kinase, the regulators of gluconeogenesis. In summary, we illustrate that HCV infection is accompanied by multiple defects in the upstream insulin signaling pathway in the liver that may contribute to the observed prevalence of IR and diabetes. Future studies are needed to resolve this issue.

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.

Hepatic glucose production pathways after three days of a high-fat diet

OBJECTIVE

A three-day high-fat diet induces hepatic steatosis and hepatic insulin resistance in rats without altering fasting plasma glucose concentration or the rate of glucose production. However, as the nutrient profile available to the liver is substantially altered by a high-fat diet, we hypothesized that the relative fluxes supporting hepatic glucose production would be altered.

MATERIALS/METHODS

To test this hypothesis, we used multiple tracers ([3,4-(13)C(2)]glucose, (2)H(2)O, and [U-(13)C(3)]propionate) followed by NMR analysis of blood glucose to quantify net glucose production and the contributions of glycogen and key gluconeogenesis precursors in 4-5-h fasted rats.

RESULTS

NMR analysis demonstrated that the majority of blood glucose was derived from glycogen and the citric acid cycle, while a smaller fraction of glucose was derived from glycerol in both controls and high-fat-fed animals. High-fat feeding was associated with a two-fold increase in plasma glycerol concentration and an increase in the contribution (both fractional and absolute) of glycerol-gluconeogenesis. The increase in gluconeogenesis from glycerol tended to be balanced by a decrease in glycogenolysis. The absolute fluxes associated with the citric acid cycle including gluconeogenesis from the cycle intermediates, pyruvate cycling and the citric acid cycle flux itself, were not altered by this short high-fat diet.

CONCLUSIONS

A short term high-fat diet altered the specific pathways for hepatic glucose production without influencing the overall rate of glucose production or flux in the citric acid cycle.

Major facilitator superfamily domain-containing protein 2a (MFSD2A) has roles in body growth, motor function, and lipid metabolism

The metabolic adaptations to fasting in the liver are largely controlled by the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARα), where PPARα upregulates genes encoding the biochemical pathway for β-oxidation of fatty acids and ketogenesis. As part of an effort to identify and characterize nutritionally regulated genes that play physiological roles in the adaptation to fasting, we identified Major facilitator superfamily domain-containing protein 2a (Mfsd2a) as a fasting-induced gene regulated by both PPARα and glucagon signaling in the liver. MFSD2A is a cell-surface protein homologous to bacterial sodium-melibiose transporters. Hepatic expression and turnover of MFSD2A is acutely regulated by fasting/refeeding, but expression in the brain is constitutive. Relative to wildtype mice, gene-targeted Mfsd2a knockout mice are smaller, leaner, and have decreased serum, liver and brown adipose triglycerides. Mfsd2a knockout mice have normal liver lipid metabolism but increased whole body energy expenditure, likely due to increased β-oxidation in brown adipose tissue and significantly increased voluntary movement, but surprisingly exhibited a form of ataxia. Together, these results indicate that MFSD2A is a nutritionally regulated gene that plays myriad roles in body growth and development, motor function, and lipid metabolism. Moreover, these data suggest that the ligand(s) that are transported by MFSD2A play important roles in these physiological processes and await future identification.