Profiling of adipokines secreted from human subcutaneous adipose tissue in response to PPAR agonists

The role of PPARs in the regulation of human adipose tissue secretome has received little attention despite its potential importance in the therapeutic actions of PPAR agonists. Here, we have investigated the effect of selective PPARgamma, PPARalpha, and PPARbeta/delta agonists on the production of adipokines by human subcutaneous adipose tissue. Antibody arrays were used to measure secreted factors in media from cultured adipose tissue explants. Sixteen proteins were produced in significant amounts. Activation of PPARs regulated the production of five proteins. Treatments with the three PPAR agonists decreased the secretion of leptin and interleukin-6. PPARalpha and beta/delta agonists markedly enhanced hepatocyte growth factor secretion whereas PPARbeta/delta down-regulated angiogenin and up-regulated TIMP-1 release. Hepatocyte growth factor, interleukin-6, and TIMP-1 are chiefly expressed in cells from the stromal vascular fraction whereas angiogenin is expressed in both adipocytes and cells from the stromal vascular fraction. Our data show that PPAR agonists modulate secretion of bioactive molecules from the different cell types composing human adipose tissue.     

Double gene deletion reveals the lack of cooperation between PPARalpha and PPARbeta in skeletal muscle

The peroxisome proliferator-activated receptors (PPARs) are involved in the regulation of most of the pathways linked to lipid metabolism. PPARalpha and PPARbeta isotypes are known to regulate muscle fatty acid oxidation and a reciprocal compensation of their function has been proposed. Herein, we investigated muscle contractile and metabolic phenotypes in PPARalpha-/-, PPARbeta-/-, and double PPARalpha-/- beta-/- mice. Heart and soleus muscle analyses show that the deletion of PPARalpha induces a decrease of the HAD activity (beta-oxidation) while soleus contractile phenotype remains unchanged. A PPARbeta deletion alone has no effect. However, these mild phenotypes are not due to a reciprocal compensation of PPARbeta and PPARalpha functions since double gene deletion PPARalpha-PPARbeta mostly reproduces the null PPARalpha-mediated reduced beta-oxidation, in addition to a shift from fast to slow fibers. In conclusion, PPARbeta is not required for maintaining skeletal muscle metabolic activity and does not compensate the lack of PPARalpha in PPARalpha null mice.     

Be fit or be sick: peroxisome proliferator-activated receptors are down the road

Investigating metabolism by unveiling the functions of the nuclear receptors peroxisome proliferator-activated receptors (PPARs) in the numerous intricate pathways ensuring energy homeostasis and fitness has been extremely rewarding. Major lines of research were initially determined by the first-characterized crucial roles of PPARalpha in fatty oxidation and of PPARgamma in adipocyte differentiation and lipid storage. Today, the molecular bases of the functional links between glucose, lipid, and protein metabolism, under the important but nonexclusive control of PPARalpha and PPARgamma, are starting to be uncovered. In addition, in the last couple of years evidence has been provided for an important role of PPARbeta (delta) in lipid metabolism. Inevitably, such actors of metabolic homeostasis are implicated in the physiopathology of complex metabolic disorders, such as those constituting the metabolic syndrome, resulting in atherosclerosis and cardiovascular diseases. This review presents a summary of the recent findings on their dual involvement in health and disease.     

PPARbeta/delta activation inhibits angiotensin II-induced collagen type I expression in rat cardiac fibroblasts

Peroxisome proliferator-activated receptors (PPARalpha, beta/delta and gamma) are nuclear receptors and PPARgamma activation was previously reported to inhibit collagen expression in the heart, but whether PPARbeta/delta also regulates collagen expression in the heart remains unclear. In this study, we investigated the effect of PPARbeta/delta activation on angiotensin II (Ang II)-induced collagen type I expression in adult rat cardiac fibroblasts. The results showed that PPARbeta/delta was expressed at the moderate level in cardiac fibroblasts. GW501516, a selective PPARbeta/delta agonist, depressed Ang II-stimulated collagen type I expression and collagen synthesis in cardiac fibroblasts in a concentration-dependent manner. Furthermore, these inhibitory effects of GW501516 were completely reversed by the knockdown of PPARbeta/delta via RNA interference. In summary, we find that PPARbeta/delta is present in cardiac fibroblasts and PPARbeta/delta activation inhibits Ang II-induced collagen type I expression at least in part via decreasing collagen synthesis. PPARbeta/delta may be a promising therapeutic target for myocardial fibrosis.     

Identification and characterization of a selective peroxisome proliferator-activated receptor beta/delta (NR1C2) antagonist

The identification of small molecule ligands for the peroxisome proliferator-activated receptors (PPARs) has been instrumental in elucidating their biological roles. In particular, agonists have been the focus of much of the research in the field with relatively few antagonists being described and all of those being selective for PPARalpha or PPARgamma. The comparison of these agonist and antagonist ligands in cellular and animal systems has often led to surprising results and new insights into the biology of the PPARs. The PPARbeta/delta receptor is emerging as an important regulator of energy metabolism, inflammation, and cell growth and differentiation; however, only agonist ligands have been described for this receptor thus far. Here we describe the first report of a PPARbeta/delta small molecule antagonist ligand. This antagonist ligand will be a useful tool for elucidating the biological roles of PPARbeta/delta.     

Natural ligands of PPARgamma: are prostaglandin J(2) derivatives really playing the part?

The peroxisome proliferator-activated receptor (PPAR) family was discovered from an orphan nuclear receptor approach, and thereafter, three subtypes were identified, namely PPARalpha, PPARbeta or PPARgamma and PPARgamma. The two former seem to regulate lipid homeostasis, whereas the latter is involved, among others, in glucose homeostasis and adipocyte differentiation. PPARs were pharmacologically characterised first using peroxisome proliferators such as clofibrates, which demonstrate moderate affinity (efficiency at micromolar concentrations) and low PPARalpha/delta versus PPARgamma specificity. Hence, several laboratories have started the search for potent and subtype-specific natural PPAR activators. In this respect, prostaglandin (PG)-related compounds were identified as good PPARgamma agonists with varying specificity, the most notable PPAR ligand being 15-deoxy-Delta12-14-PGJ2 (15d-PGJ2). Recently, an oxidized phosphatidylcholine was identified as a potent alternative (patho)physiological natural ligand of PPARgamma. In the present review, we discuss the different PPARgamma-dependent and -independent biological effects of the PG PPARgamma ligands and the concern about their low potency in molecular models as compared with thiazolidinediones (TZDs), a family of potent (nanomolar) synthetic PPARgamma ligands. Finally, the oxidized lipids are presented as a novel and interesting alternative for discovering potent PPARgamma activators in order to understand more in details the implications of PPARgamma in various pathophysiological conditions.     

Ethanol specifically decreases peroxisome proliferator activated receptor beta in B12 oligodendrocyte-like cells

Peroxisome proliferator activated receptors (PPARs) are nuclear receptors that control important genes involved in lipid metabolism. Their role in nerve cells is uncertain, although anomalous myelination of the corpus callosum has been described in the PPARbeta-null mouse, and abnormalities of this tissue have been documented in fetal alcohol syndrome in humans. We report here that ethanol treatment of B12 oligodendrocyte-like cells induces a concentration- and time-dependent decrease in the mRNA and protein levels of PPARbeta, with no effect on PPARalpha or PPARgamma. The effect on PPARbeta is seen as an increase in mRNA degradation, as assessed by run-off assays, due to a significant decrease in PPARbeta mRNA half-life, with no observed changes in intracellular localization. Our results suggest a possible link between PPARbeta function and ethanol-induced abnormal myelination in oligodendrocytes.     

Expression of peroxisome proliferator-activated receptors in zebrafish (<Emphasis Type="Italic">Danio rerio</Emphasis>)

Peroxisomes increase in size and number in responsive animals ranging from mammals to marine mussels and fish species when treated with certain compounds named peroxisome proliferators. This phenomenon, known as peroxisome proliferation, is mediated by nuclear receptors termed peroxisome proliferator-activated receptors (PPARs). Three PPAR subtypes have been described (alpha, beta, and gamma) and in mammals PPARalpha is mainly expressed in tissues that catabolize fatty acids, PPARbeta is ubiquitously distributed, and PPARgamma is mainly expressed in the adipose tissue and immune system. The aim of this study was to analyze the tissue distribution of different PPAR subtypes in zebrafish Danio rerio using commercially available antibodies against PPARalpha, PPARbeta, and PPARgamma. In western blots, specific bands were detected at about 58 kDa for PPARalpha and PPARbeta. For PPARgamma the band was detected at 56 kDa. Similar results were obtained in mouse liver homogenates used as positive control, indicating the specificity of the antibodies. Immunohistochemistry was performed in paraformaldehyde-fixed tissue using either microwave or microwave plus trypsin pretreatment for antigen retrieval. In zebrafish, PPARalpha was expressed mainly in liver parenchymal cells, proximal tubules of kidney, enterocytes, and pancreas. PPARbeta showed a widespread distribution and was expressed in the liver, proximal and distal tubules and glomeruli of the kidney, pancreas, enterocytes and smooth muscle of the intestine, skin epithelium, lymphocytes, and male and female gonads. PPARgamma expression was weak in pancreatic cells, intestine, and gonads for both pretreatments. Most of the signal detected was cytoplasmic; only in the cases of PPARalpha and PPARbeta was some nuclear labeling detected in the liver. In mouse tissues, the distribution of PPAR subtypes was similar to that described previously for rats. Our results demonstrate that all three distinct PPAR subtypes are present in zebrafish. The tissue and cellular distribution of PPAR subtypes in zebrafish resembled partly that described before in mammals. Further studies are needed to decipher the functions of PPAR subtypes in zebrafish and other aquatic organisms and particularly their role in regulation of metabolic responses to xenobiotic exposure.     

Identification and expression analysis of peroxisome proliferator-activated receptors cDNA in a reptile, the leopard gecko (Eublepharis macularius)

Despite the physiological and evolutionary significance of lipid metabolism in amniotes, the molecular mechanisms involved have been unclear in reptiles. To elucidate this, we investigated peroxisome proliferators-activated receptors (PPARs) in the leopard gecko (Eublepharis macularius). PPARs belong to a nuclear hormone-receptor family mainly involved in lipid metabolism. Although PPARs have been widely studied in mammals, little information about them is yet available from reptiles. We identified in the leopard gecko partial cDNA sequences of PPARalpha and beta, and full sequences of two isoforms of PPARgamma. This is the first report of reptilian PPARgamma mRNA isoforms. We also evaluated the organ distribution of expression of these genes by using RT-PCR and competitive PCR. The expression level of PPARalpha mRNA was highest in the large intestine, and moderate in the liver and kidney. The expression level of PPARbeta mRNA was highest in the kidney and large intestine, and moderate in the liver. Similarly to the expression of human PPARgamma isoforms, PPARgammaa was expressed ubiquitously, whereas the expression of PPARgammab was restricted. The highest levels of their expression, however, were observed in the large intestine, rather than in the adipose tissue as in mammals. Taken together, these results showed that the profile of PPARbeta mRNA expression in the leopard gecko is similar to that in mammals, and that those of PPAR alpha and gamma are species specific. This may reflect adaptation to annual changes in lipid storage due to seasonal food availability.     

Identification and organ expression of peroxisome proliferator activated receptors in brown trout (Salmo trutta f. fario)

Although widely studied in mammals, little information about fish peroxisome proliferator activated receptors (PPARs) is yet available. As a baseline for future studies, the three PPAR isotypes were identified in brown trout (Salmo trutta f. fario) and their organ distribution pattern was established. The cDNA fragments encoding PPARs alpha, beta and gamma were amplified by PCR, and the deduced sequences of the correspondent peptides were compared with other species sequences. Both the 183 amino acid sequence from PPARalpha and the 103 amino acid sequence from PPARbeta shared high levels of homology with the correspondent peptides of other fishes and terrestrial vertebrates, whereas PPARgamma 108 amino acid sequence showed much less similarity with non-fish PPARgamma. According to both semi-quantitative RT-PCR and real-time RT-PCR, PPARalpha mRNA predominates in white muscle, heart and liver and PPARbeta is more expressed in testis, heart, liver, white muscle and trunk kidney. PPARgamma was only detected in trunk kidney and liver by real-time RT-PCR and also in spleen by semi-quantitative RT-PCR. PPARbeta seems to be the most strongly expressed isotype, whereas PPARgamma shows a much weaker global expression.