Tissue distribution of porcine peroxisome proliferator-activated receptor alpha: detection of an alternatively spliced mRNA

Polymorphism in 3' untranslated region of the pig PPARA gene influences its transcript level and is associated with adipose tissue accumulation

The PPARA (peroxisome proliferator-activated receptor-α) gene encodes a nuclear receptor that plays an important role in fatty acid catabolism by transcriptional regulation of genes involved in fatty acid oxidation and can be considered as a candidate gene for fatness traits in the pig. The aim of the study was to search for a functional polymorphism in 3' untranslated region (UTR), their association with production traits, and postnatal PPARA transcript level in 2 skeletal muscles (longissimus and semimembranosus) of 5 commercial pig breeds (Polish Landrace [PL], Polish Large White [PLW], Duroc, Pietrain, and Pulawska). Altogether, 9 novel polymorphisms (8 SNP and 1 indel) were found in the 3' UTR. The in silico analysis revealed 6 putative microRNA target sequences in the analyzed region. The c.*636A>G substitution was widely distributed across breeds and located near the putative target sequence for miR-224. The relative PPARA transcript level was higher (P < 0.05) in LM of AA than in those of GG homozygous animals for SNP c.*636A>G. The luciferase assay revealed that miR-224 probably acts as a negative regulator of the PPARA expression in pig adipocytes (P = 2.9 × 10(-7)), but we did not observe the effect of the A or G alleles on the interaction between miR-224 and its putative target sequence. We hypothesize that the 2 predominant haplotypes, differing at 4 sites (including c.*636A>G), present different architecture of its 3' UTR and it could affect the level of the transcript. The c.*636A>G SNP, analyzed in PL and PLW, was significantly associated with backfat thickness at 3 points (P < 0.05) and intramuscular fat content (P < 0.01) in PL. Suggestive associations were found between 4 SNP (c.*321A>C, c.*324G>C, c.*626T>C, and c.*636A>G) and fatty acid contents in LM and subcutaneous and visceral fat tissue of PL, PLW, Duroc and Pietrain pigs. The PPARA mRNA level was higher in semimembranosus muscle than in LM (P = 8.38 × 10(-12)) in a general comparison and the same trend was found in most breeds (except for PL) and at all tested days of age (60, 90, 120, 150, 180, and 210 d). The effect of breed was highly significant in a general comparison (P = 0.48 × 10(-8)), but there was no common expression pattern in both muscles among different age groups. We conclude that the c.*636A>G SNP in the PPARA gene can be considered in PL breed as a useful genetic marker for adipose tissue accumulation.

Analysis of porcine adipose tissue transcriptome reveals differences in de novo fatty acid synthesis in pigs with divergent muscle fatty acid composition

Background

In pigs, adipose tissue is one of the principal organs involved in the regulation of lipid metabolism. It is particularly involved in the overall fatty acid synthesis with consequences in other lipid-target organs such as muscles and the liver. With this in mind, we have used massive, parallel high-throughput sequencing technologies to characterize the porcine adipose tissue transcriptome architecture in six Iberian x Landrace crossbred pigs showing extreme phenotypes for intramuscular fatty acid composition (three per group).

Results

High-throughput RNA sequencing was used to generate a whole characterization of adipose tissue (backfat) transcriptome. A total of 4,130 putative unannotated protein-coding sequences were identified in the 20% of reads which mapped in intergenic regions. Furthermore, 36% of the unmapped reads were represented by interspersed repeats, SINEs being the most abundant elements. Differential expression analyses identified 396 candidate genes among divergent animals for intramuscular fatty acid composition. Sixty-two percent of these genes (247/396) presented higher expression in the group of pigs with higher content of intramuscular SFA and MUFA, while the remaining 149 showed higher expression in the group with higher content of PUFA. Pathway analysis related these genes to biological functions and canonical pathways controlling lipid and fatty acid metabolisms. In concordance with the phenotypic classification of animals, the major metabolic pathway differentially modulated between groups was de novo lipogenesis, the group with more PUFA being the one that showed lower expression of lipogenic genes.

Conclusions

These results will help in the identification of genetic variants at loci that affect fatty acid composition traits. The implications of these results range from the improvement of porcine meat quality traits to the application of the pig as an animal model of human metabolic diseases.

The suckling piglet as an agrimedical model for the study of pediatric nutrition and metabolism

The neonatal pig ranks among the most prominent research models for the study of pediatric nutrition and metabolism. Its precocial development at birth affords ready adaptation to artificial rearing systems, and research using this model spans a wide array of nutrients. Sophisticated in vitro and in vivo methodologies supporting both invasive, reduction-science research as well as whole-animal preclinical investigations have been developed. Potential applications may dually benefit both agricultural and medical sciences (e.g., "agrimedical research"). The broad scope of this review is to outline the fundamental elements of the piglet model and to highlight key aspects of relevance to various macronutrients, including lipids, carbohydrates, proteins/amino acids, and calcium/phosphorus. The review examines similarities between piglets and infants and also piglet idiosyncrasies, concluding that, overall, the piglet represents an adaptable and robust model for pediatric nutrition and metabolism research.

Functional Role of PPARs in Ruminants: Potential Targets for Fine-Tuning Metabolism during Growth and Lactation

Characterization and biological roles of the peroxisome proliferator-activated receptor (PPAR) isotypes are well known in monogastrics, but not in ruminants. However, a wealth of information has accumulated in little more than a decade on ruminant PPARs including isotype tissue distribution, response to synthetic and natural agonists, gene targets, and factors affecting their expression. Functional characterization demonstrated that, as in monogastrics, the PPAR isotypes control expression of genes involved in lipid metabolism, anti-inflammatory response, development, and growth. Contrary to mouse, however, the PPARγ gene network appears to controls milk fat synthesis in lactating ruminants. As in monogastrics, PPAR isotypes in ruminants are activated by long-chain fatty acids, therefore, making them ideal candidates for fine-tuning metabolism in this species via nutrients. In this regard, using information accumulated in ruminants and monogastrics, we propose a model of PPAR isotype-driven biological functions encompassing key tissues during the peripartal period in dairy cattle.

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.

Differential regulation of peroxisome proliferator-activated receptor (PPAR)-alpha1 and truncated PPARalpha2 as an adaptive response to fasting in the control of hepatic peroxisomal fatty acid beta-oxidation in the hibernating mammal

Seasonal obesity and fasting-associated hibernation are the two major metabolic events governing hepatic lipid metabolism in hibernating mammals. In this process, however, the role of the nuclear receptor known as peroxisome proliferator-activated receptor (PPAR)-alpha has not been elucidated yet. Here we show, as in human, that jerboa (Jaculus orientalis) liver expresses both active wild-type PPARalpha (PPARalpha1wt) and truncated PPARalpha forms and that the PPARalpha1wt to truncated PPARalpha2 ratio, which indicates the availability of active PPARalpha1wt, is differentially regulated during fasting-associated hibernation. Functional activation of hepatic jerboa PPARalpha, during prehibernating and hibernating states, was demonstrated by the induction of its target genes, which encode peroxisomal proteins such as acyl-CoA oxidase 1, peroxisomal membrane protein 70, and catalase, accompanied by a concomitant induction of PPARalpha thermogenic coactivator PPARgamma coactivator-1alpha. Interestingly, sustained activation of PPARalpha by its hypolipidemic ligand, ciprofibrate, abrogates the adaptive fasting response of PPARalpha during prehibernation and overinduces its target genes, disrupting the prehibernation fattening process. In striking contrast, during fasting-associated hibernation, jerboas exhibit preferential up-regulation of hepatic peroxisomal fatty acid oxidation instead of the mitochondrial pathway, which is down-regulated. Taken together, our results strongly suggest that PPARalpha is subject to a hibernation-dependent splicing regulation in response to feeding-fasting conditions, which defines the activity of PPARalpha and the activation of its target genes during hibernation bouts of jerboas.

The PPARalpha activator fenofibrate slows down the progression of the left ventricular dysfunction in porcine tachycardia-induced cardiomyopathy

It has been reported that high intramyocardial peroxisome proliferator-activated receptor alpha (PPARalpha) stimulation or overexpression altered cardiac contractile function in mouse models of cardiac hypertrophy and heart failure. Nevertheless, it has never been demonstrated that clinically relevant doses of drugs stimulating PPARalpha activity such as fenofibrate increase the risk to develop heart failure in humans. To determine if fenofibrate accelerates the development of heart failure in large mammals, we have tested its effects on the progression of left ventricular dysfunction in pacing-induced heart failure in pigs. Fenofibrate treatment blunted reduction in left ventricular ejection fraction, reduced cardiac hypertrophy, and attenuated clinical signs of heart failure. Fenofibrate impeded the increase in atrial natriuretic peptide, brain natriuretic peptide, and endothelin-1 plasma levels. The expression of PPARalpha, fatty acyl-CoA-oxidase, and carnitine palmitoyltransferase-Ibeta was reduced at mRNA levels in the left ventricle from untreated heart failure pigs but maintained near normal values with fenofibrate. Fenofibrate prevented heart failure-induced overexpression of TNFalpha mRNA and enhanced catalase activity in left ventricle compared to placebo. These data suggest that a clinically relevant dose of fenofibrate does not accelerate but slows down heart failure development in the model of pacing-induced heart failure in large mammals.

Clofibrate causes an upregulation of PPAR-α target genes but does not alter expression of SREBP target genes in liver and adipose tissue of pigs

This study investigated the effect of clofibrate treatment on expression of target genes of peroxisome proliferator-activated receptor (PPAR)-α and various genes of the lipid metabolism in liver and adipose tissue of pigs. An experiment with 18 pigs was performed in which pigs were fed either a control diet or the same diet supplemented with 5 g clofibrate/kg for 28 days. Pigs treated with clofibrate had heavier livers, moderately increased mRNA concentrations of various PPAR-α target genes in liver and adipose tissue, a higher concentration of 3-hydroxybutyrate, and markedly lower concentrations of triglycerides and cholesterol in plasma and lipoproteins than control pigs ( P < 0.05). mRNA concentrations of sterol regulatory element-binding proteins (SREBP)-1 and -2, insulin-induced genes ( Insig) -1 and Insig-2, and the SREBP target genes acetyl-CoA carboxylase, 3-methyl-3-hydroxyglutaryl-CoA reductase, and low-density lipoprotein receptor in liver and adipose tissue and mRNA concentrations of apolipoproteins A-I, A-II, and C-III in the liver were not different between both groups of pigs. In conclusion, this study shows that clofibrate treatment activates PPAR-α in liver and adipose tissue and has a strong hypotriglyceridemic and hypocholesterolemic effect in pigs. The finding that mRNA concentrations of some proteins responsible for the hypolipidemic action of fibrates in humans were not altered suggests that there were certain differences in the mode of action compared with humans. It is also shown that PPAR-α activation by clofibrate does not affect hepatic expression of SREBP target genes involved in synthesis of triglycerides and cholesterol homeostasis in liver and adipose tissue of pigs.

Comparative aspects of lipid metabolism: impact on contemporary research and use of animal models

The emerging obesity crisis and consequent concerns for corrective measures and appropriate public policy have stimulated research into causes, prevention, remediation, and health consequences of obesity and associated maladies. Such research areas include eating behavior, appetite control, and food intake regulation as well as the regulation of lipid metabolism, cardiovascular function, endocrine function, and dyslipidemia states utilizing various animal models and cell culture systems. Although the liver has a central role in lipid/fatty acid synthesis and glucose is the precursor for de novo fatty acid synthesis in rodents and humans, in many other species, adipose tissues are the primary sites of lipogenesis. In addition, many species utilize acetic acid as a precursor for fatty acid synthesis. This fundamental difference in the site of fatty acid synthesis and the pattern of consequent lipid trafficking influences overall animal lipid metabolism and the role of regulatory hormones and transcription factors. Researchers utilizing various animal species in targeted biomedical research should be aware of these species differences when interpreting their data. In addition, many animal species are used for food production, recreational, and companion purposes. Understanding the lipid metabolism regulatory mechanisms of such species from a comparative perspective is important for the proper nutrition and health of these animals.

Induction of overlapping genes by fasting and a peroxisome proliferator in pigs: evidence of functional PPARα in nonproliferating species

Peroxisome proliferator-activated receptor α (PPARα), a key regulator of fatty acid oxidation, is essential for adaptation to fasting in rats and mice. However, physiological functions of PPARα in other species, including humans, are controversial. A group of PPARα ligands called peroxisome proliferators (PPs) causes peroxisome proliferation and hepatocarcinogenesis only in rats and mice. To elucidate the role of PPARα in adaptation to fasting in nonproliferating species, we compared gene expressions in pig liver from fasted and clofibric acid (a PP)-fed groups against a control diet-fed group. As in rats and mice, fasting induced genes involved with mitochondrial fatty acid oxidation and ketogenesis in pigs. Those genes were also induced by clofibric acid feeding, indicating that PPARα mediates the induction of these genes. In contrast to rats and mice, little or no induction of genes for peroxisomal or microsomal fatty acid oxidation was observed in clofibric acid-fed pigs. Histology showed no significant hyperplasia or hepatomegaly in the clofibric acid-fed pigs, whereas it showed a reduction of glycogen by clofibric acid, an effect of PPs also observed in rats. Copy number of PPARα mRNA was higher in pigs than in mice and rats, suggesting that peroxisomal proliferation and hyperresponse of several genes to PPs seen only in rats and mice are unrelated to the abundance of PPARα. In conclusion, PPARα is likely to play a central role in adaptation to fasting in pig liver as in rats and mice.

Hepatic beta-oxidation and carnitine palmitoyltransferase I in neonatal pigs after dietary treatments of clofibric acid, isoproterenol, and medium-chain triglycerides

A suckling piglet model was used to study nutritional and pharmacologic means of stimulating hepatic fatty acid beta-oxidation. Newborn pigs were fed milk diets containing either long- or medium-chain triglycerides (LCT or MCT). The long-chain control diet was supplemented further with clofibric acid (0.5%) or isoproterenol (40 ppm), and growth was monitored for 10-12 days. Clofibrate increased rates of hepatic peroxisomal and mitochondrial beta-oxidation of [1-(14)C]-palmitate by 60 and 186%, respectively. Furthermore, malonyl-CoA sensitive carnitine palmitoyltransferase (CPT I) activity increased 64% (P < 0.05) in pigs receiving clofibrate. Increased CPT I activity was not congruent with changes in message, as elevated abundance of CPT I mRNA was not detected (P = 0.16) when assessed by qRT-PCR. Neither rates of beta-oxidation nor CPT activities were affected by dietary MCT or by isoproterenol treatment (P > 0.1). Collectively, these findings indicate that clofibrate effectively induced hepatic CPT activity concomitant with increased fatty acid beta-oxidation.

Differential expression of peroxisome proliferator-activated receptors alpha and gamma gene in various chicken tissues

The peroxisome proliferator-activated receptors (PPARs) are the members of superfamily of nuclear hormone receptors. A great number of studies in rodent and human have shown that PPARs were involved in the lipids metabolism. The goal of the current study was to investigate the expression pattern of PPAR genes in various tissues of chicken. The tissue samples (heart, liver, spleen, lung, kidney, stomach, intestine, brain, breast muscle and adipose) were collected from six Arber Acres broilers (8 weeks old, male and female birds are half and half). Semi-quantitative RT-PCR and Northern blot were used to characterize the expression of PPAR-alpha and PPAR-gamma genes in the above tissues. By semi-quantitative RT-PCR, the results showed the expression level of PPAR-alpha gene was higher in brain, lung, kidney, heart and intestine, medium in stomach, liver and adipose than in spleen, and it did not express in breast muscle. The expression level of PPAR-gamma gene was higher in adipose, medium in brain and kidney than in spleen, heart, lung, stomach and intestine, but it did not express in liver and breast muscle. Northern blot results showed that PPAR-alpha gene expressed in heart, liver, kidney and stomach, and the intensity of hybridization signal was the stronger in liver and kidney than in other tissues, however, PPAR-gamma gene only expressed in adipose and kidney tissues. The results of this study showed the profile of PPAR gene expression in the chicken was similar to that in rodent, human and pig. However the expression profile of chicken also have its own specific trait, i.e. compared with mammals, PPAR-alpha gene can not be detected in skeletal muscle and PPAR-gamma gene can be stronger expressed in kidney tissues. This work will provide some basic data for the PPAR genes expression and lipids metabolism of birds.

Molecular characterisation of six alternatively spliced variants and a novel promoter in human peroxisome proliferator-activated receptor alpha

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a ligand-activated transcriptional factor that governs many biological processes, including lipid metabolism, inflammation, and atherosclerosis. We demonstrate here the existence of six variants and multiple transcriptional start sites of the 5(') untranslated region (UTR) of hPPARalpha gene, originating from the use of alternative splicing mechanisms and four different promoters. Three new novel exons at the 5(')-untranslated region of human PPARalpha gene were also identified and designated as Exon A, Exon B, and Exon 2b. In addition, 1.2kb promoter fragment which drives the transcription of 2 variants with Exon B (hPPARalpha4 and 6) was successfully cloned and characterised. Sequencing results revealed promoter B did not contain a conservative TATA box within the first 100 nucleotides from transcriptional start site but has several GC-rich regions and putative Sp1 sites. Using luciferase reporter constructs transfected into HepG2 and Hep3B cell lines, promoter B was shown to be functionally active. Basal transcriptional activity was significantly high in the promoter fragment -341/+34, but lower in the region -341/-1147 as compared to the fragment -341/+34, indicating the presence of an element conferring transcriptional activation between positions -341 and +34 or alternatively, the presence of transcriptional repression between positions -341 and -1147 in the promoter B of hPPARalpha.

Tissue distribution of porcine peroxisome proliferator-activated receptor alpha: detection of an alternatively spliced mRNA

Peroxisome proliferator-activated receptor alpha (PPAR alpha) plays a key role in regulating the catabolic pathway of lipids in response to a variety of compounds named peroxisome proliferators (PPs). The cellular responses to PPs differ among mouse/rat and other species and actualize the study in swine, which show close resemblance to human lipid physiology and metabolism. We have isolated the cDNA containing the open reading frame of porcine PPAR alpha whose deduced amino acid sequence revealed an evolutionary distance to mouse/rat that could be implicated in causing the species-dependent response to PPs. Interestingly, an alternatively spliced PPAR alpha mRNA, lacking exon 5, was detected by reverse transcriptase-polymerase chain reaction in several porcine tissues. This deletion alters the reading frame and introduces a premature stop codon of PPAR alpha, presumably giving rise to a C-terminal truncated protein. We have also examined PPAR alpha expression by Northern blot analysis in tissues taken from pigs at three different stages of maturation, including two breeds that differ considerably in body composition and fat deposition. Porcine PPAR alpha was predominantly expressed in kidney and liver in mature individuals. When comparing piglets of a young age, a breed-specific tissue distribution of PPAR alpha mRNA was observed, particularly in liver and heart.