Regulation of Peroxisome Proliferator-Activated Receptor α by Protein Kinase C

The transcription factors CREBH, PPARa, and FOXO1 as critical hepatic mediators of diet-induced metabolic dysregulation

The liver is a critical mediator of lipid and/or glucose homeostasis and is a primary organ involved in dynamic changes during feeding and fasting. Additionally, hepatic-centric pathways are prone to dysregulation during pathophysiological states including metabolic syndrome (MetS) and non-alcoholic fatty liver disease. Omics platforms and GWAS have elucidated genes related to increased risk of developing MetS and related disorders, but mutations in these metabolism-related genes are rare and cannot fully explain the increasing prevalence of MetS-related pathologies worldwide. Complex interactions between diet, lifestyle, environmental factors, and genetic predisposition jointly determine inter-individual variability of disease risk. Given the complexity of these interactions, researchers have focused on master regulators of metabolic responses incorporating and mediating the impact of multiple environmental cues. Transcription factors are DNA binding, terminal executors of signaling pathways that modulate the cellular responses to complex metabolic stimuli and are related to the control of hepatic lipid and glucose homeostasis. Among numerous hepatic transcription factors involved in regulating metabolism, three emerge as key players in transducing nutrient sensing, which are dysregulated in MetS-related perturbations in both clinical and preclinical studies: cAMP Responsive Element Binding Protein 3 Like 3 (CREB3L3), Peroxisome Proliferator Activated Receptor Alpha (PPAR), and Forkhead Box O1 (FOXO1). Additionally, these three transcription factors appear to be amenable to dietary and/or nutrient-based therapies, being potential targets of nutritional therapy. In this review we aim to describe the activation, regulation, and impact of these transcription factors in the context of metabolic homeostasis. We also summarize their perspectives in MetS and nutritional therapies.

PKCλ/ι Loss Induces Autophagy, Oxidative Phosphorylation, and NRF2 to Promote Liver Cancer Progression

Oxidative stress plays a critical role in liver tissue damage and in hepatocellular carcinoma (HCC) initiation and progression. However, the mechanisms that regulate autophagy and metabolic reprogramming during reactive oxygen species (ROS) generation, and how ROS promote tumorigenesis, still need to be fully understood. We show that protein kinase C (PKC) λ/ι loss in hepatocytes promotes autophagy and oxidative phosphorylation. This results in ROS generation, which through NRF2 drives HCC through cell-autonomous and non-autonomous mechanisms. Although PKCλ/ι promotes tumorigenesis in oncogene-driven cancer models, emerging evidence demonstrate that it is a tumor suppressor in more complex carcinogenic processes. Consistently, PKCλ/ι levels negatively correlate with HCC histological tumor grade, establishing this kinase as a tumor suppressor in liver cancer.

Molecular Actions of PPARα in Lipid Metabolism and Inflammation

Peroxisome proliferator-activated receptor α (PPARα) is a nuclear receptor of clinical interest as a drug target in various metabolic disorders. PPARα also exhibits marked anti-inflammatory capacities. The first-generation PPARα agonists, the fibrates, have however been hampered by drug-drug interaction issues, statin drop-in, and ill-designed cardiovascular intervention trials. Notwithstanding, understanding the molecular mechanisms by which PPARα works will enable control of its activities as a drug target for metabolic diseases with an underlying inflammatory component. Given its role in reshaping the immune system, the full potential of this nuclear receptor subtype as a versatile drug target with high plasticity becomes increasingly clear, and a novel generation of agonists may pave the way for novel fields of applications.

Pharmacogenomics of Chemically Distinct Classes of Keap1-Nrf2 Activators Identify Common and Unique Gene, Protein, and Pathway Responses In Vivo

The Kelch-like erythroid-associated protein 1 (Keap1)-NF-E2-related factor 2 (Nrf2) signaling pathway is the subject of several clinical trials evaluating the effects of Nrf2 activation on the prevention of cancer and diabetes and the treatment of chronic kidney disease and multiple sclerosis. 3H-1,2-dithiole-3-thione (D3T) and 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole (CDDO-Im) are representative members of two distinct series of Nrf2 chemical activators. Previous reports have described activator-specific effects on Nrf2-dependent gene regulation and physiologic outcomes. Here we used a robust chemical genomics approach to characterize expression profiles between D3T and CDDO-Im in livers from wild-type and Nrf2-null mice. At equally efficacious doses in wild-type mice, 406 genes show common RNA responses to both treatments. These genes enriched the Nrf2-regulated pathways of antioxidant defense and xenobiotic metabolism. In addition, 197 and 745 genes were regulated uniquely in response to either D3T or CDDO-Im, respectively. Functional analysis of the D3T-regulated set showed a significant enrichment of Nrf2-regulated enzymes involved in cholesterol biosynthesis. This result was supported by Nrf2-dependent increases in lanosterol synthase and CYP51 protein expression. CDDO-Im had no effect on cholesterol biosynthesis regardless of the dose tested. However, unlike D3T, CDDO-Im resulted in Nrf2-dependent elevation of peroxisome proliferator α and Kruppel-like factor 13, as well as the coactivator peroxisome proliferator γ coactivator 1β, together indicating regulation of β-oxidation and lipid metabolic pathways. These findings provide novel insights into the pharmacodynamic action of these two activators of Keap1-Nrf2 signaling. Although both compounds modify Keap1 to affect canonical cytoprotective gene expression, additional unique sets of Nrf2-dependent genes were regulated by each agent with enrichment of selective metabolic pathways.

The cationic (calcium and lead) and enzyme conundrum

The environmental toxicant lead (Pb) and the essential element calcium (Ca) play an interactive role in extracellular and intracellular regulatory functions that affect health. Lead's usurping calcium binding sites, as well as its interactions with thiols and phosphates have been suggested to be the basis for adverse effects on many organ systems especially the nervous system. Among regulatory processes controlled by Ca are calmodulin-dependent phosphodiesterase, calmodulin-dependent protein kinases, calmodulin inhibitor sensitive potassium channels, and calmodulin-independent protein kinase C (PKC) activation. This review focused on Pb studies describing the modulation of PKC, which is also regulated by steroids. Steroid hormone regulation may relate to a focal point for the sex differences of Pb and cellular signaling events. Picomolar concentrations of Pb may stimulate partially purified PKC, but higher concentrations inhibit activity. Although knowledge exists regarding Pb and PKC isoforms, especially interaction of Pb with the purified enzyme, there are conflicting reports concerning metal-mediated activation or inhibition of PKC and downstream signaling events. The effect of Pb on PKC in vivo remains elusive. Most reports of Pb and PKC in whole animal and human studies indicated that Pb either inhibits PKC or exerts no significant effect. However, most of the animal studies were performed with males. Recent studies performed with females and males separately revealed that females and males respond to Pb quite differently, and for this reason, it is suggested that future Pb studies of PKC and other biomedical investigations be performed with females and males.

Di-(2-ethylhexyl) phthalate suppresses IL-12p40 production by GM-CSF-dependent macrophages via the PPARα/TNFAIP3/TRAF6 axis after lipopolysaccharide stimulation

Activation of peroxisome proliferator-activated receptor α (PPARα) by di-(2-ethylhexyl) phthalate (DEHP) has an anti-inflammatory effect. This study investigated the potential combined influence of PPARα, tumor necrosis factor α-induced protein 3 (TNFAIP3/A20), and tumor necrosis factor receptor-associated factor 6 (TRAF6) on interleukin (IL)-12p40 production by macrophages exposed to DEHP and stimulated with lipopolysaccharide (LPS). LPS upregulated IL-12p40 expression by granulocyte-macrophage colony-stimulating factor-dependent macrophages (on day 9 of culture), whereas adding DEHP to cultures significantly attenuated the response of IL-12p40 to LPS stimulation. PPARα protein was also reduced by DEHP. Interestingly, transfection of macrophages with small interfering RNA (siRNA) duplexes for PPARα, TNFAIP3/A20, or dual oxidase 2 restored the response of IL-12p40 protein to LPS stimulation in the presence of DEHP. siRNAs for various protein kinase Cs (PKCs) (α, β, γ, or δ) also restored IL-12p40 production by macrophages exposed to LPS and DEHP. While LPS upregulated both IL-12p40 and TNFAIP3/A20 production, adding DEHP to cultures dramatically reduced IL-12p40 and TNFAIP3/A20 levels. Silencing of PKCα reduced TNFAIP3/A20 production, whereas PKCγ siRNA (but not PKCβ or δ siRNA) significantly increased TNFAIP3/A20. TRAF6 was also attenuated by macrophages with DEHP. The PPARα/TNFAIP3/TRAF6 axis may have an important role in the mechanism through which DEHP reduces IL-12p40 production by LPS-stimulated macrophages.

PPARs: regulators of metabolism and as therapeutic targets in cardiovascular disease. Part I: PPAR-α

This article provides a comprehensive review about the molecular and metabolic actions of PPAR-α. It describes its structural features, ligand specificity, gene transcription mechanisms, functional characteristics and target genes. In addition, recent progress with the use of loss of function and gain of function mouse models in the discovery of diverse biological functions of PPAR-α, particularly in the vascular system and the status of the development of new single, dual, pan and partial PPAR agonists (PPAR modulators) in the clinical management of metabolic diseases are presented. This review also summarizes the clinical outcomes from a large number of clinical trials aimed at evaluating the atheroprotective actions of current clinically used PPAR-α agonists, fibrates and statin-fibrate combination therapy.

PKCβ: Expanding role in hepatic adaptation of cholesterol homeostasis to dietary fat/cholesterol

Cholesterol homeostasis relies on an intricate network of cellular processes whose deregulation in response to Western type high-fat/cholesterol diets can lead to several life-threatening pathologies. Significant advances have been made in resolving the molecular identity and regulatory function of transcription factors sensitive to fat, cholesterol, or bile acids, but whether body senses the presence of both fat and cholesterol simultaneously is not known. Assessing the impact of a high-fat/cholesterol load, rather than an individual component alone, on cholesterol homeostasis is more physiologically relevant because Western diets deliver both fat and cholesterol at the same time. Moreover, dietary fat and dietary cholesterol are reported to act synergistically to impair liver cholesterol homeostasis. A key insight into the role of protein kinase C-β (PKCβ) in hepatic adaptation to high-fat/cholesterol diets was gained recently through the use of knockout mice. The emerging evidence indicates that PKCβ is an important regulator of cholesterol homeostasis that ensures normal adaptation to high-fat/cholesterol intake. Consistent with this function, high-fat/cholesterol diets induce PKCβ expression and signaling in the intestine and liver, while systemic PKCβ deficiency promotes accumulation of cholesterol in the liver and bile. PKCβ disruption results in profound dysregulation of hepatic cholesterol and bile homeostasis and imparts sensitivity to cholesterol gallstone formation. The available results support involvement of a two-pronged mechanism by which intestine and liver PKCβ signaling converge on liver ERK1/2 to dictate diet-induced cholesterol and bile acid homeostasis. Collectively, PKCβ is an integrator of dietary fat/cholesterol signal and mediates changes to cholesterol homeostasis.

Pro-inflammatory signaling by 24,25-dihydroxyvitamin D3 in HepG2 cells

The vitamin D metabolite 24,25-dihydroxyvitamin D3 (24, 25[OH]2D3) was shown to induce nongenomic signaling pathways in resting zone chondrocytes and other cells involved in bone remodeling. Recently, our laboratory demonstrated that 24,25-[OH]2D3 but not 25-hydroxyvitamin D3, suppresses apolipoprotein A-I (apo A-I) gene expression and high-density lipoprotein (HDL) secretion in hepatocytes. Since 24,25-[OH]2D3 has low affinity for the vitamin D receptor (VDR) and little is known with regard to how 24,25-[OH]2D3 modulates nongenomic signaling in hepatocytes, we investigated the capacity of 24,25-[OH]2D3 to activate various signaling pathways relevant to apo A-I synthesis in HepG2 cells. Treatment with 24,25-[OH]2D3 resulted in decreased peroxisome proliferator-activated receptor alpha (PPARα) expression and retinoid-X-receptor alpha (RXRα) expression. Similarly, treatment of hepatocytes with 50 nM 24,25-[OH]2D3 for 1-3 h induced PKCα activation as well as c-jun-N-terminal kinase 1 (JNK1) activity and extracellular-regulated kinase 1/2 (ERK1/2) activity. These changes in kinase activity correlated with changes in c-jun phosphorylation, an increase in AP-1-dependent transcriptional activity, as well as repression of apo A-I promoter activity. Furthermore, treatment with 24,25-[OH]2D3 increased IL-1β, IL-6, and IL-8 expression by HepG2 cells. These observations suggest that 24,25-[OH]2D3 elicits several novel rapid nongenomic-mediated pro-inflammatory protein kinases targeting AP1 activity, increasing pro-inflammatory cytokine expression, potentially impacting lipid metabolism and hepatic function.

Serine 350 of human pregnane X receptor is crucial for its heterodimerization with retinoid X receptor alpha and transactivation of target genes in vitro and in vivo

The human pregnane X receptor (hPXR), a member of the nuclear receptor superfamily, senses xenobiotics and controls the transcription of genes encoding drug-metabolizing enzymes and transporters. The regulation of hPXR's transcriptional activation of its target genes is important for xenobiotic detoxification and endobiotic metabolism, and hPXR dysregulation can cause various adverse drug effects. Studies have implicated the putative phosphorylation site serine 350 (Ser(350)) in regulating hPXR transcriptional activity, but the mechanism of regulation remains elusive. Here we investigated the transactivation of hPXR target genes in vitro and in vivo by hPXR with a phosphomimetic mutation at Ser(350) (hPXR(S350D)). The S350D phosphomimetic mutation reduced the endogenous expression of cytochrome P450 3A4 (an hPXR target gene) in HepG2 and LS180 cells. Biochemical assays and structural modeling revealed that Ser(350) of hPXR is crucial for formation of the hPXR-retinoid X receptor alpha (RXRα) heterodimer. The S350D mutation abrogated heterodimerization in a ligand-independent manner, impairing hPXR-mediated transactivation. Further, in a novel humanized transgenic mouse model expressing the hPXR(S350D) transgene, we demonstrated that the S350D mutation alone is sufficient to impair hPXR transcriptional activity in mouse liver. This transgenic mouse model provides a unique tool to investigate the regulation and function of hPXR, including its non-genomic function, in vivo. Our finding that phosphorylation regulates hPXR activity has implications for development of novel hPXR antagonists and for safety evaluation during drug development.

PKC-mediated USP phosphorylation at Ser35 modulates 20-hydroxyecdysone signaling in Drosophila

The nuclear receptor complex of the steroid hormone, 20-hydroxyecdysone (20E), is a heterodimer composed of EcR and USP. Our previous studies in Drosophila suggest that PKC modulates 20E signaling by phosphorylating EcR-USP. However, the exact phosphorylation sites in EcR and USP have not been identified. Using LC-MS/MS analysis, we first identified Ser35 of USP as a PKC phosphorylation site. Mutation of USP Ser35 to Ala35 in S2 cells not only eliminated USP phosphorylation, but also attenuated the 20E-induced luciferase activity, mimicking the treatment with a PKC-specific inhibitor chelerythrine chloride in Kc cells. In the larval salivary glands (SG), inhibition of PKC activity with the binary GAL4/UAS system reduced USP phosphorylation and down-regulated the 20E primary-response genes, E75B and Br-C, and RNAi knockdown of Rack1 had stronger inhibitory effects than overexpression of PKCi. Moreover, RNAi knockdown of four PKC isozyme genes expressed in the SG exhibited a variety of inhibitory effects on USP phosphorylation and expression of E75B and Br-C, with the strongest inhibitory effects occurring when aPKC was knocked down by RNAi. Taken together, we conclude that PKC-mediated USP phosphorylation at Ser35 modulates 20E signaling in Drosophila.

Bilobetin ameliorates insulin resistance by PKA-mediated phosphorylation of PPARα in rats fed a high-fat diet

BACKGROUND AND PURPOSE

The amelioration of insulin resistance by bilobetin is closely related to its hypolipidaemic effect. The aim of the present study was to determine the insulin-sensitizing mechanism of bilobetin by elucidating its effect on lipid metabolism.

EXPERIMENTAL APPROACH

Rats fed a high-fat diet were treated with bilobetin for either 4 or 14 days before applying a hyperinsulinaemic-euglycaemic clamp. Triglyceride and fatty acids labelled with radioactive isotopes were used to track the transportation and the fate of lipids in tissues. The activity of lipid metabolism-related enzymes and β-oxidation rate were measured. Western blot was used to investigate the phosphorylation, translocation and expression of PPARα in several tissues and cultured cells. The location of amino acid residues subjected to phosphorylation in PPARα was also studied.

KEY RESULTS

Bilobetin ameliorated insulin resistance, increased the hepatic uptake and oxidation of lipids, reduced very-low-density lipoprotein triglyceride secretion and blood triglyceride levels, enhanced the expression and activity of enzymes involved in β-oxidation and attenuated the accumulation of triglycerides and their metabolites in tissues. Bilobetin also increased the phosphorylation, nuclear translocation and activity of PPARα accompanied by elevated cAMP level and PKA activity. Threonine-129-alanine and/or serine-163-alanine mutations on the PPARα genes and PKA inhibitors prevented the effects of bilobetin on PPARα. However, cells overexpressing PKA appeared to stimulate the phosphorylation, nuclear translocation and activity of PPARα.

CONCLUSIONS AND IMPLICATIONS

Bilobetin treatment ameliorates hyperlipidaemia, lipotoxicity and insulin resistance in rats by stimulating PPARα-mediated lipid catabolism. PKA activation is crucial for this process.

Omega-3 fatty acids and metabolic syndrome: effects and emerging mechanisms of action

Epidemiological, human, animal, and cell culture studies show that n-3 fatty acids, especially α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), reduce the risk factors of cardiovascular diseases. EPA and DHA, rather than ALA, have been the focus of research on the n-3 fatty acids, probably due to the relatively inefficient conversion of ALA to EPA and DHA in rodents and humans. This review will assess our current understanding of the effects and potential mechanisms of actions of individual n-3 fatty acids on multiple risk factors of metabolic syndrome. Evidence for pharmacological responses and the mechanism of action of each of the n-3 fatty acid trio will be discussed for the major risk factors of metabolic syndrome, especially adiposity, dyslipidemia, insulin resistance and diabetes, hypertension, oxidative stress, and inflammation. Metabolism of n-3 and n-6 fatty acids as well as the interactions of n-3 fatty acids with nutrients, gene expression, and disease states will be addressed to provide a rationale for the use of n-3 fatty acids to reduce the risk factors of metabolic syndrome.

Control of nuclear receptor activities in metabolism by post-translational modifications

Nuclear receptors (NRs) are molecular transducers of endocrine and dietary signals allowing tissues to adapt their transcriptional responses to endogenous or exogenous cues. These signals act in many cases as specific ligands, converting of NRs into transcriptionally active molecules. This on-off mechanism needs, however, to be finely tuned with respect to the tissue environment and adjusted to the organism needs. These subtle adjustments of NR transcriptional activity are brought about by post-translational modifications (PTMs), which can be, in the case of orphan NRs, the sole regulatory mechanism. The role of PTMs, with a more specific focus on phosphorylation, affecting the functions of NR controlling metabolic events is described in this review.

A preliminary operational classification system for nonmutagenic modes of action for carcinogenesis

This article proposes a system of categories for nonmutagenic modes of action for carcinogenesis. The classification is of modes of action rather than individual carcinogens, because the same compound can affect carcinogenesis in more than one way. Basically, we categorize modes of action as: (1) co-initiation (facilitating the original mutagenic changes in stem and progenitor cells that start the cancer process) (e.g. induction of activating enzymes for other carcinogens); (2) promotion (enhancing the relative growth vs differentiation/death of initiated clones (e.g. inhibition of growth-suppressing cell-cell communication); (3) progression (enhancing the growth, malignancy, or spread of already developed tumors) (e.g. suppression of immune surveillance, hormonally mediated growth stimulation for tumors with appropriate receptors by estrogens); and (4) multiphase (e.g., "epigenetic" silencing of tumor suppressor genes). A priori, agents that act at relatively early stages in the process are expected to manifest greater relative susceptibility in early life, whereas agents that act via later stage modes will tend to show greater susceptibility for exposures later in life.

Phosphorylation of farnesoid X receptor by protein kinase C promotes its transcriptional activity

The farnesoid X receptor (FXR, NR1H4) belongs to the nuclear receptor superfamily and is activated by bile acids such as chenodeoxycholic acid, or synthetic ligands such as GW4064. FXR is implicated in the regulation of bile acid, lipid, and carbohydrate metabolism. Posttranslational modifications regulating its activity have not been investigated yet. Here, we demonstrate that calcium-dependent protein kinase C (PKC) inhibition impairs ligand-mediated regulation of FXR target genes. Moreover, in a transactivation assay, we show that FXR transcriptional activity is modulated by PKC. Furthermore, phorbol 12-myristate 13-acetate , a PKC activator, induces the phosphorylation of endogenous FXR in HepG2 cells and PKCalpha phosphorylates in vitro FXR in its DNA-binding domain on S135 and S154. Mutation of S135 and S154 to alanine residues reduces in cell FXR phosphorylation. In contrast to wild-type FXR, mutant FXRS135AS154A displays an impaired PKCalpha-induced transactivation and a decreased ligand-dependent FXR transactivation. Finally, phosphorylation of FXR by PKC promotes the recruitment of peroxisomal proliferator-activated receptor gamma coactivator 1alpha. In conclusion, these findings show that the phosphorylation of FXR induced by PKCalpha directly modulates the ability of agonists to activate FXR.

Aldose reductase regulates hepatic peroxisome proliferator-activated receptor alpha phosphorylation and activity to impact lipid homeostasis

Aldose reductase (AR) is implicated in the development of a number of diabetic complications, but the underlying mechanisms remain to be fully elucidated. We performed this study to determine whether and how AR might influence hepatic peroxisome proliferator-activated receptor alpha (PPARalpha) activity and lipid metabolism. Our results in mouse hepatocyte AML12 cells show that AR overexpression caused strong suppression of PPARalpha/delta activity (74%, p < 0.001) together with significant down-regulation of mRNA expression for acetyl-CoA oxidase and carnitine palmitoyltransferase-1. These suppressive effects were attenuated by the selective AR inhibitor zopolrestat. Furthermore, AR overexpression greatly increased the levels of phosphorylated PPARalpha and ERK1/2. Moreover, AR-induced suppression of PPARalpha activity was attenuated by treatment with an inhibitor for ERK1/2 but not that for phosphoinositide 3-kinase, p38, or JNK. Importantly, similar effects were observed for cells exposed to 25 mm glucose. In streptozotocin-diabetic mice, AR inhibitor treatment or genetic deficiency of AR resulted in significant dephosphorylation of both PPARalpha and ERK1/2. With the dephosphorylation of PPARalpha, hepatic acetyl-CoA oxidase and apolipoprotein C-III mRNA expression was greatly affected and that was associated with substantial reductions in blood triglyceride and nonesterified fatty acid levels. These data indicate that AR plays an important role in the regulation of hepatic PPARalpha phosphorylation and activity and lipid homeostasis. A significant portion of the AR-induced modulation is achieved through ERK1/2 signaling.

Modulation of receptor phosphorylation contributes to activation of peroxisome proliferator activated receptor alpha by dehydroepiandrosterone and other peroxisome proliferators

Dehydroepiandrosterone (DHEA), a C19 human adrenal steroid, activates peroxisome proliferator-activated receptor alpha (PPARalpha) in vivo but does not ligand-activate PPARalpha in transient transfection experiments. We demonstrate that DHEA regulates PPARalpha action by altering both the levels and phosphorylation status of the receptor. Human hepatoma cells (HepG2) were transiently transfected with the expression plasmid encoding PPARalpha and a plasmid containing two copies of fatty acyl coenzyme oxidase (FACO) peroxisome-proliferator activated receptor responsive element consensus oligonucleotide in a luciferase reporter gene. Nafenopin treatment increased reporter gene activity in this system, whereas DHEA treatment did not. Okadaic acid significantly decreased nafenopin-induced reporter activity in a concentration-dependent manner. Okadaic acid treatment of primary rat hepatocytes decreased both DHEA- and nafenopin-induced FACO activity in primary rat hepatocytes. DHEA induced both PPARalpha mRNA and protein levels, as well as PP2A message in primary rat hepatocytes. Western blot analysis showed that the serines at positions 12 and 21 were rapidly dephosphorylated upon treatment with DHEA and nafenopin. Results using specific protein phosphatase inhibitors suggested that protein phosphatase 2A (PP2A) is responsible for DHEA action, and protein phosphatase 1 might be involved in nafenopin induction. Mutation of serines at position 6, 12, and 21 to an uncharged alanine residue significantly increased transcriptional activity, whereas mutation to negative charged aspartate residues (mimicking receptor phosphorylation) decreased transcriptional activity. DHEA action involves induction of PPARalpha mRNA and protein levels as well as increased PPARalpha transcriptional activity through decreasing receptor phosphorylation at serines in the AF1 region.

Peroxisome proliferator-activated receptor-alpha is required for the neurotrophic effect of oleic acid in neurons

Oleic acid synthesized by astrocytes behaves as a neurotrophic factor for neurons, up-regulating the molecular markers of axonal and dendritic outgrowth, growth-associated protein 43 and microtubule-associated protein 2. In this work, the nature of the receptor involved in this neurotrophic effect was investigated. As oleic acid has been reported to be a ligand and activator of the peroxisome proliferator-activated receptor (PPAR), we focus on this family of receptors. Our results show that PPARalpha, beta/delta, and gamma are expressed in neurons in culture. However, only the agonists of PPARalpha, Wy14643, GW7647 and oleoylethanolamide, promoted neuronal differentiation, while PPAR beta/delta and gamma agonists did not modify neuronal differentiation. Consequently, we investigated the involvement of PPARalpha (Nr1c1) in oleic acid-induced neuronal differentiation. Our results indicate that oleic acid activates PPARalpha in neurons. In addition, the effect of oleic acid on neuronal morphology, growth-associated protein 43 and microtubule-associated protein 2 expression decreases in neurons after PPARalpha has been silenced by small interfering RNA. Taken together, our results suggest that PPARalpha could be the receptor for oleic acid in neurons, further broadening the range of functions attributed to this family of transcription factors. Although several works have reported that PPARalpha could be involved in neuroprotection, the present work provides the first evidence suggesting a role of PPARalpha in neuronal differentiation.

Modulation of PPAR activity via phosphorylation

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily of transcription factors that respond to specific ligands by altering gene expression in a cell-, developmental- and sex-specific manner. Three subtypes of this receptor have been discovered (PPARalpha, beta and gamma), each apparently evolving to fulfill different biological niches. PPARs control a variety of target genes involved in lipid homeostasis, diabetes and cancer. Similar to other nuclear receptors, the PPARs are phosphoproteins and their transcriptional activity is affected by cross-talk with kinases and phosphatases. Phosphorylation by the mitogen-activated protein kinases (ERK- and p38-MAPK), Protein Kinase A and C (PKA, PKC), AMP Kinase (AMPK) and glycogen synthase kinase-3 (GSK3) affect their activity in a ligand-dependent or -independent manner. The effects of phosphorylation depend on the cellular context, receptor subtype and residue metabolized which can be manifested at several steps in the PPAR activation sequence including ligand affinity, DNA binding, coactivator recruitment and proteasomal degradation. The review will summarize the known PPAR kinases that directly act on these receptors, the sites affected and the result of this modification on receptor activity.

Phosphorylation of a conserved serine in the deoxyribonucleic acid binding domain of nuclear receptors alters intracellular localization

Nuclear receptors (NRs) are a superfamily of transcription factors whose genomic functions are known to be activated by lipophilic ligands, but little is known about how to deactivate them or how to turn on their nongenomic functions. One obvious mechanism is to alter the nuclear localization of the receptors. Here, we show that protein kinase C (PKC) phosphorylates a highly conserved serine (Ser) between the two zinc fingers of the DNA binding domain of orphan receptor hepatocyte nuclear factor 4alpha (HNF4alpha). This Ser (S78) is adjacent to several positively charged residues (Arg or Lys), which we show here are involved in nuclear localization of HNF4alpha and are conserved in nearly all other NRs, along with the Ser/threonine (Thr). A phosphomimetic mutant of HNF4alpha (S78D) reduced DNA binding, transactivation ability, and protein stability. It also impaired nuclear localization, an effect that was greatly enhanced in the MODY1 mutant Q268X. Treatment of the hepatocellular carcinoma cell line HepG2 with PKC activator phorbol 12-myristate 13-acetate also resulted in increased cytoplasmic localization of HNF4alpha as well as decreased endogenous HNF4alpha protein levels in a proteasome-dependent fashion. We also show that PKC phosphorylates the DNA binding domain of other NRs (retinoic acid receptor alpha, retinoid X receptor alpha, and thyroid hormone receptor beta) and that phosphomimetic mutants of the same Ser/Thr result in cytoplasmic localization of retinoid X receptor alpha and peroxisome proliferator-activated receptor alpha. Thus, phosphorylation of this conserved Ser between the two zinc fingers may be a common mechanism for regulating the function of NRs.

PPARs and the Placenta

The discovery of the peroxisome proliferator-activated receptors (PPARs) in 1990s provided new insights in understanding the mechanisms involved in the control of energy homeostasis and in cell differentiation, proliferation, apoptosis and the inflammatory process. The PPARs became thus an exciting therapeutic target for diabetes, metabolic syndrome, atherosclerosis, and cancer. Unexpectedly, genetic studies performed in mice established that PPARgamma are essential for placental development. After a brief description of structural and functional features of PPARs, we will summarize in this review the most recent results concerning expression and the role of PPARs in placenta and of PPARgamma in human trophoblastic cells in particular.

peroxisome proliferator-activated receptor alpha activation-mediated regulation of endothelin-1 production via nitric oxide and protein kinase C signaling pathways in piglet cerebral microvascular endothelial cell culture

Elevated endothelin (ET)-1 has been implicated in cerebrovascular complications following brain trauma characterized by dysregulation of endothelial nitric oxide synthase (eNOS), protein kinase C (PKC), and cerebral function. Recently, vascular expression of PPARalpha has been observed and suggested to improve vascular dysfunction. We speculate that activation of PPARalpha in cerebral microvessels can improve cerebral dysfunction following trauma, and we tested the hypothesis that activation of cerebral endothelial peroxisome proliferator-activated receptor (PPAR)alpha will attenuate ET-1 production via a mechanism involving nitric oxide (NO) and PKC. Phorbol 12-myristate 13-acetate (PMA) (1 microM), bradykinin (BK, 1 microM), angiotensin II (AII, 1 microM), or hemoglobin (Hem, 10 microM) increased ET-1 levels by 24-, 11.4-, 3.6-, or 1.3-fold increasing ET-1 levels from 0.36 +/- 0.08 to 8.6 +/- 0.8, 4.1 +/- 0.7, 1.30 +/- 0.1, or 0.47 +/- 0.03 fmol/microg protein (p < 0.05), respectively. Clofibrate (10 microM) reduced basal ET-1 from 0.36 +/- 0.08 (control) to 0.03 +/- 0.01 and blunted vasoactive agent-induced increase to 0.12 +/- 0.07 (PMA), 0.6 +/- 0.04 (BK), 0.25 +/- 0.03 (AII), or 0.12 +/- 0.03 (Hem) fM/microg protein (p < 0.05). L-arginine methyl ester (100 microM) inhibited clofibrate-induced reduction in basal ET-1 production. Clofibrate increased PPARalpha expression, accompanied by increased NO production and eNOS expression. PKC inhibition by calphostin C (10 microM) blocked these effects, whereas activation by PMA reduced basal PPARalpha expression. Thus, PPARalpha activation attenuated ET-1 production by agents that mediate brain injury through mechanisms that probably result from PPARalpha-induced increase in eNOS expression/NO production and complex PKC signaling pathways. Therefore, PPARalpha activators can be appropriate therapeutic agents to alleviate cerebrovascular dysfunction following cerebral vasospasm.

From molecular action to physiological outputs: peroxisome proliferator-activated receptors are nuclear receptors at the crossroads of key cellular functions

Peroxisome proliferator-activated receptors (PPARs) compose a family of three nuclear receptors which act as lipid sensors to modulate gene expression. As such, PPARs are implicated in major metabolic and inflammatory regulations with far-reaching medical consequences, as well as in important processes controlling cellular fate. Throughout this review, we focus on the cellular functions of these receptors. The molecular mechanisms through which PPARs regulate transcription are thoroughly addressed with particular emphasis on the latest results on corepressor and coactivator action. Their implication in cellular metabolism and in the control of the balance between cell proliferation, differentiation and survival is then reviewed. Finally, we discuss how the integration of various intra-cellular signaling pathways allows PPARs to participate to whole-body homeostasis by mediating regulatory crosstalks between organs.