Peroxisome Proliferator-activated Receptor α (PPARα) Turnover by the Ubiquitin-Proteasome System Controls the Ligand-induced Expression Level of Its Target Genes

Abstract

Peroxisome proliferator activated-receptor α (PPARα) is a ligand-activated transcription factor belonging to the nuclear receptor family. PPARα is implicated in the regulation of lipid and glucose metabolism and in the control of inflammatory response. Recently, it has been demonstrated that a number of nuclear receptors are degraded by the ubiquitin-proteasome pathway. Since PPARα exhibits a circadian expression rhythm and since PPARα is rapidly regulated under certain pathophysiological conditions such as the acute phase inflammatory response, we hypothesized that PPARα protein levels must be under tight control. Here, we studied the mechanisms controlling PPARα protein levels and their consequences on the transcriptional control of PPARα target genes. Using pulse-chase experiments, it is shown that PPARα is a short-lived protein and that addition of its ligands stabilizes this nuclear receptor. By transient cotransfection experiments using expression vectors for PPARα and hemagglutinin-tagged ubiquitin, it is demonstrated that PPARα protein is ubiquitinated and that its ligands decrease the ubiquitination of this nuclear receptor, thus providing a mechanism for the ligand-dependent stabilization observed in pulse-chase experiments. In addition, treatment with MG132, a selective proteasome inhibitor, increases the level of ubiquitinated PPARα and inhibits its degradation in transfected cells. Furthermore, MG132 treatment enhances the level of endogenous PPARα in HepG2 cells. Finally, transient transfection and quantitative reverse transcription-PCR show that inhibition of PPARα degradation increases its transcriptional activation and expression of target genes such as apoA-II and fatty acid transport protein (FATP). Taken together, these data demonstrate that PPARα is degraded by the ubiquitin-proteasome system in a ligand-dependent manner. Regulation of its degradation provides a novel regulatory mechanism of transcriptional activity of this nuclear receptor. Peroxisome proliferator activated-receptor α (PPARα) is a ligand-activated transcription factor belonging to the nuclear receptor family. PPARα is implicated in the regulation of lipid and glucose metabolism and in the control of inflammatory response. Recently, it has been demonstrated that a number of nuclear receptors are degraded by the ubiquitin-proteasome pathway. Since PPARα exhibits a circadian expression rhythm and since PPARα is rapidly regulated under certain pathophysiological conditions such as the acute phase inflammatory response, we hypothesized that PPARα protein levels must be under tight control. Here, we studied the mechanisms controlling PPARα protein levels and their consequences on the transcriptional control of PPARα target genes. Using pulse-chase experiments, it is shown that PPARα is a short-lived protein and that addition of its ligands stabilizes this nuclear receptor. By transient cotransfection experiments using expression vectors for PPARα and hemagglutinin-tagged ubiquitin, it is demonstrated that PPARα protein is ubiquitinated and that its ligands decrease the ubiquitination of this nuclear receptor, thus providing a mechanism for the ligand-dependent stabilization observed in pulse-chase experiments. In addition, treatment with MG132, a selective proteasome inhibitor, increases the level of ubiquitinated PPARα and inhibits its degradation in transfected cells. Furthermore, MG132 treatment enhances the level of endogenous PPARα in HepG2 cells. Finally, transient transfection and quantitative reverse transcription-PCR show that inhibition of PPARα degradation increases its transcriptional activation and expression of target genes such as apoA-II and fatty acid transport protein (FATP). Taken together, these data demonstrate that PPARα is degraded by the ubiquitin-proteasome system in a ligand-dependent manner. Regulation of its degradation provides a novel regulatory mechanism of transcriptional activity of this nuclear receptor. peroxisome proliferator activated-receptor PPAR response elements hemagglutinin apolipoprotein fatty acid transport protein Dulbecco's modified Eagle's medium fetal calf serum radioimmuno- precipitation assay thymidine kinase 643, 4-chloro-6-(2,3-xylidino)-2-pyrimidinyl-thio)acetic acid The peroxisome proliferator-activated receptors (PPARs)1 are members of the nuclear receptor superfamily that act as ligand-dependent transcription factors. PPARα is highly expressed in liver, skeletal and cardiac muscle, and proximal tubular epithelium of kidney. A significant expression of PPARα has also been shown in endothelial cells, smooth muscle cells, and cells involved in the inflammatory process (1Chinetti G. Fruchart J.C. Staels B. Inflamm. Res. 2000; 49: 1-9Crossref Scopus (762) Google Scholar). The ligands of PPARα are natural fatty acids and derivatives such as leukotriene B4 and 8-S-hydroxyeicosatetraenoic acid or oxidized phospholipids from oxidized low density lipoprotein. The fibrates hypolipidemic drugs are synthetic PPARα ligands (1Chinetti G. Fruchart J.C. Staels B. Inflamm. Res. 2000; 49: 1-9Crossref Scopus (762) Google Scholar). PPARα plays a role in intracellular fatty acid metabolism and in triglyceride metabolism by regulating genes involved in the transport and degradation of fatty acids in mitochondria and peroxisomes (2Schoonjans K. Staels B. Auwerx J. J. Lipid Res. 1996; 37: 907-925Abstract Full Text PDF PubMed Google Scholar). PPARα is also implicated in the metabolism of lipids and lipoproteins. As a result, PPARα activation decreases the hepatic very low density lipoprotein secretion and plasma triglyceride levels (3Fruchart J.C. Duriez P. Staels B. Curr. Opin. Lipidol. 1999; 10: 245-257Crossref PubMed Scopus (405) Google Scholar). Furthermore, PPARα agonists increase plasma concentration of high density lipoprotein particles by regulating the transcription of the major high density lipoprotein apolipoproteins (apo) in liver including apoA-II (4Vu-Dac N. Schoonjans K. Kosykh V. Dallongeville J. Fruchart J.C. Staels B. Auwerx J. J. Clin. Invest. 1995; 96: 741-750Crossref PubMed Scopus (364) Google Scholar). PPARα regulates gene expression by binding, as a heterodimer with the retinoid X receptor, to specific DNA sequences, called PPAR response elements (PPRE), resulting in the transcriptional activation of target genes (5Tugwood J.D. Issemann I. Anderson R.G. Bundell K.R. McPheat W.L. Green S. EMBO J. 1992; 11: 433-439Crossref PubMed Scopus (804) Google Scholar). More recently, PPARα has also been shown to play a negative role in the inflammatory response by interfering negatively with the AP-1 and NF-κB signalization pathway (6Delerive P. Fruchart J.C. Staels B. J. Endocrinol. 2001; 3: 453-459Crossref Scopus (662) Google Scholar). Physiological responses to nuclear receptor ligands not only depend on the potency of the ligand but also on the expression levels of the nuclear receptors in a given tissue. Regulation of nuclear receptor expression occurs at both protein and mRNA levels. For instance, expression of certain nuclear receptors is repressed in the acute phase inflammatory response (7Beigneux A.P. Moser A.H. Shigenaga J.K. Grunfeld C. Feingold K.R. J. Biol. Chem. 2000; 275: 16390-16399Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar). This effect is very rapid, which suggests a tight control not only of nuclear receptor mRNA but also of protein levels, likely via a control of the degradation and stability of the nuclear receptors. Indeed, several nuclear receptors, such as the retinoid X receptor α, the retinoic acid receptor γ (8Boudjelal M. Wang Z. Voorhees J.J. Fisher G.J. Cancer Res. 2000; 8: 2247-2252Google Scholar, 9Kopf E. Plassat J.L. Vivat V. de The H. Chambon P. Rochette-Egly C. J. Biol. Chem. 2000; 275: 33280-33288Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar), the retinoic acid receptor α (9Kopf E. Plassat J.L. Vivat V. de The H. Chambon P. Rochette-Egly C. J. Biol. Chem. 2000; 275: 33280-33288Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar), the thyroid hormone receptor (10Dace A. Zhao L. Park K.S. Furuno T. Takamura N. Nakanishi M. West B.L. Hanover J.A. Cheng S. Proc. Natl. Acad. Sci. U. S. A. 2000; 16: 8985-8990Crossref Scopus (130) Google Scholar), and PPARγ (11Hauser S. Adelmant G. Sarraf P. Wright H.M. Mueller E. Spiegelman B.M. J. Biol. Chem. 2000; 275: 18527-18533Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar), have been shown to be degraded by the ubiquitin-proteasome system. This degradation pathway is implicated in the regulation of many short-lived proteins involved in essential functions of the cells, including cell cycle control, transcription regulation, and signal transduction (12Mimnaugh E.G. Bonvini P. Neckers L. Electrophoresis. 1999; 20: 418-428Crossref PubMed Scopus (77) Google Scholar). The proteins degraded by this pathway are covalently modified by fixation of an 8-kDa polypeptide, called ubiquitin, on lysine residues in a three-step process. In the first step, ubiquitin is activated by a ubiquitin-activating enzyme (E1). Then, the activated ubiquitin is transferred to a ubiquitin carrier protein (E2). Finally, ubiquitin-protein isopeptide ligase (E3) catalyzes the covalent bond of ubiquitin to the target protein. Following this process, multiubiquitinated proteins are rapidly degraded by the 26 S proteasome (13Hodges M. Tissot C. Freemont P.S. Curr. Biol. 1998; 8: R749-R752Abstract Full Text Full Text PDF PubMed Google Scholar). We demonstrated previously that PPARα mRNA and protein levels follow a circadian rhythm (14Lemberger T. Saladin R. Vazquez M. Assimacopoulos F. Staels B. Desvergne B. Wahli W. Auwerx J. J. Biol. Chem. 1996; 271: 1764-1769Abstract Full Text Full Text PDF PubMed Scopus (282) Google Scholar). More recently, it was reported that PPARα mRNA is rapidly down-regulated in the acute phase inflammatory response (7Beigneux A.P. Moser A.H. Shigenaga J.K. Grunfeld C. Feingold K.R. J. Biol. Chem. 2000; 275: 16390-16399Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar). Since these responses imply a rapid regulation also at the levels of PPARα protein, the present study was designed to test whether PPARα protein levels are controlled by the ubiquitin-proteasome degradation pathway. Our results demonstrate that PPARα is an unstable protein that is rapidly degraded and that ligand activation stabilizes this nuclear receptor. Moreover, we show that the degradation of PPARα involves the ubiquitin-proteasome pathway and that its stabilization observed in the presence of the ligand is due to a decrease of PPARα ubiquitination. Inhibition of the proteasome increases the amount of PPARα protein and consequently the transcriptional activation of PPARα-dependent promoters. These results indicate that the proteasome plays an important role in the regulation of PPARα protein level, a mechanism contributing to the magnitude of ligand response. Dulbecco's modified Eagle's medium (DMEM) without methionine was purchased from ICN (Orsay, France), and DMEM and fetal calf serum (FCS) were purchased from Invitrogen. [35S]methionine was obtained from PerkinElmer Life Sciences. Wy 14,643 was from Chemsyn Science Laboratories (Lenexa, KS); fenofibric acid was from Laboratoires Fournier (Dijon, France); ciprofibrate was from Sanofi (Aramon, France); and GW7647 was kindly provided by Dr. Peter J. Brown (GlaxoSmithKline) (15Brown P.J. Stuart L.W. Hurley K.P. Lewis M.C. Winegar D.A. Wilson J.G. Wilkison W.O. Ittoop O.R. Willson T.M. Bioorg. Med. Chem. Lett. 2001; 11: 1225-1227Crossref PubMed Scopus (159) Google Scholar) and cerivastatin was provided by Bayer (Wuppertal, Germany). MG132 was from Calbiochem. ExGen 500 was obtained from Euromedex (Souffelweyersheim, France). The anti-PPARα antibody (H98) and the monoclonal antibody against the HA epitope was obtained from Santa Cruz Biotechnology, Inc. (Le Perray en Yvelines, France), and the secondary antibody against rabbit IgG was purchased from Sigma. COS 7 cells were grown in DMEM medium containing 10% FCS. Cells were transfected with 2 μg of PPARα expression vector (pSG5hPPARα) (16Staels B. Koenig W. Habib A. Merval R. Lebret M. Pineda Torra I. Delerive P. Fadel A. Chinetti G. Fruchart J.C. Najib J. Maclouf J. Tedgui A. Nature. 1998; 393: 790-793Crossref PubMed Scopus (1056) Google Scholar) using ExGen 500 according to the manufacturer's protocol. Twenty-four h after transfection, COS 7 cells were deprived from methionine for 30 min, labeled with [35S]methionine for 1 h, and after washes, treated with 50 μm Wy 14,643 or Me2SO (vehicle) in DMEM medium containing 2% for the of Cells were in 2 containing and 1 The were at for 30 at was by the in the by precipitation of the proteins with acid at for 30 The proteins were on a under with was to the and the was on a PPARα was from 7 using μg of an antibody against acids of of at under of protein were for 30 at under The were by for 1 at and were in 1 of 1 of 1 of 2 and 1 of was to the the proteins were by and by COS 7 cells were grown in DMEM containing 10% FCS. Cells were transfected for h with 2 μg of expression vectors for PPARα (pSG5hPPARα) and for ubiquitin M. Full Text PDF PubMed Scopus Google Scholar) using ExGen h, cells were treated with for h in the of fetal calf Then, cells were on in containing and 1 The were at for 30 at and the protein of the were using the France). of PPARα protein was on μg of protein as The proteins were by using a monoclonal antibody against the HA The was with the according to the manufacturer's protocol. HepG2 cells were treated with μm MG132 for 2 or h in the of serum and and Then, cells were in DMEM medium containing 10% and 10% in and at cells were for at and in of 1 at min, in of with and for at and the was The containing the was with of containing and of were at for 30 in 50 of containing and for at and the to the nuclear was The concentration of protein in the was using the μg of nuclear were by using the anti-PPARα The was with the HepG2 cells were in Cells were transfected with of (4Vu-Dac N. Schoonjans K. Kosykh V. Dallongeville J. Fruchart J.C. Staels B. Auwerx J. J. Clin. Invest. 1995; 96: 741-750Crossref PubMed Scopus (364) Google Scholar) or the control and 50 of control vectors using ExGen h, cells were treated with μm MG132 for 2 or h to 50 μm Wy 14,643 or Me2SO for were and and were as previously (16Staels B. Koenig W. Habib A. Merval R. Lebret M. Pineda Torra I. Delerive P. Fadel A. Chinetti G. Fruchart J.C. Najib J. Maclouf J. Tedgui A. Nature. 1998; 393: 790-793Crossref PubMed Scopus (1056) Google Scholar). HepG2 cells were grown in DMEM medium containing 10% with 1 and Cells were treated with μm MG132 for 2 h to 50 14,643 or Me2SO for was using according to the manufacturer's protocol. μg of was using and reverse levels were by quantitative using the DNA Green on the system and as for and and as for and As control, mRNA was using and For the of the was to have a of at the by a with from HepG2 cells. were in in as the cycle number in which the the J. Res. 1996; PubMed Scopus Google Scholar, Res. 1996; PubMed Scopus Google Scholar), were for and apoA-II and to the of using the target study the stability of PPARα protein, pulse-chase experiments were in COS 7 cells transfected with the expression PPARα protein was after a of and In the of PPARα protein is rapidly degraded in cells 2 h of the of PPARα obtained was that PPARα is a short-lived protein. Since ligand activation has been shown to the stability of nuclear receptor proteins Z. A.P. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: PubMed Scopus Google Scholar, H. J. 1998; PubMed Scopus Google Scholar, J.L. 2000; PubMed Scopus Google Scholar, H. A. M. T. P. Life Sci. 1998; PubMed Scopus Google Scholar, J.C. A. M. J.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), the effect of treatment with Wy a PPARα was 2 and h of a amount of PPARα protein was observed in the presence of Wy 14,643 in the presence of Me2SO that Wy 14,643 a stabilization of PPARα protein. a was to PPARα protein stability in the presence and of ligand 1 This that PPARα a of 1 h in the of ligand and of 2 h in the presence of Wy with the cells, degradation of PPARα protein was observed in the presence of Wy this effect of the ligand was observed only the first h of activation 1 These data indicate that PPARα protein is rapidly degraded in cells and that ligand activation stabilizes this protein in a manner. demonstrate the of the proteasome in the degradation of a pulse-chase was in COS 7 cells transfected with the PPARα expression vector and treated or not with PPARα protein was after and In with the data the results obtained show that PPARα protein is rapidly degraded in the cells. 2 h of a very low amount of PPARα proteins was which after h 1 In the presence of MG132, a stabilization of the protein was 2 h of the of PPARα protein was not 1 Since HepG2 cells significant of endogenous PPARα protein G. H. C. V. P. Fruchart J.C. J. C. Staels B. J. Clin. Invest. 2001; PubMed Scopus Google Scholar), the of MG132 on endogenous PPARα level was using a specific PPARα demonstrated that treatment of HepG2 cells with MG132 2 in a significant increase of endogenous PPARα proteins levels These results indicate that both and expressed PPARα proteins are degraded by the ubiquitin-proteasome pathway. A number of nuclear receptors are degraded by the ubiquitin-proteasome pathway (8Boudjelal M. Wang Z. Voorhees J.J. Fisher G.J. Cancer Res. 2000; 8: 2247-2252Google Scholar, 9Kopf E. Plassat J.L. Vivat V. de The H. Chambon P. Rochette-Egly C. J. Biol. Chem. 2000; 275: 33280-33288Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, A. Zhao L. Park K.S. Furuno T. Takamura N. Nakanishi M. West B.L. Hanover J.A. Cheng S. Proc. Natl. Acad. Sci. U. S. A. 2000; 16: 8985-8990Crossref Scopus (130) Google Scholar, S. Adelmant G. Sarraf P. Wright H.M. Mueller E. Spiegelman B.M. J. Biol. Chem. 2000; 275: 18527-18533Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar). whether PPARα is also degraded by this COS 7 cells were with expression vectors for PPARα and an HA treatment with Wy 14,643 or for h, PPARα proteins were and the ubiquitinated PPARα proteins were with an The results from this show that PPARα is ubiquitinated The high of the ubiquitinated PPARα proteins suggests the presence of ubiquitination of Wy 14,643 the amount of ubiquitinated PPARα protein data that are in with the stabilization effect of the ligand obtained in the pulse-chase whether the ubiquitinated PPARα protein is degraded by the COS 7 cells transfected with an expression vector for PPARα were treated with the proteasome inhibitor, MG132, which inhibits the degradation of proteins by the 26 S proteasome In the presence of this inhibitor, an increase of ubiquitinated PPARα protein was observed in cells treated or not with Wy 14,643 and These results show that ubiquitinated PPARα protein is degraded by the that the effect of Wy 14,643 on PPARα ubiquitination is a the of PPARα including fibrates and the highly specific PPARα were In addition, the of which is not a PPARα ligand but which is to this nuclear receptor by its G. H. C. V. P. Fruchart J.C. J. C. Staels B. J. Clin. Invest. 2001; PubMed Scopus Google Scholar), was The results obtained show that the decrease in PPARα ubiquitination was observed only with the PPARα ligands but not with cerivastatin These data demonstrate that PPARα is ubiquitinated and that ligand activation decreases PPARα protein with PPAR α ligands decreases the ubiquitination of PPARα 7 cells were transfected with the ubiquitin and expression Cells were treated with or or the ligands for PPARα proteins were by proteins were with a monoclonal antibody against the HA epitope As control for transfection μg of cell were by using an antibody against PPARα the of inhibition of PPARα degradation on its transcriptional HepG2 cells were transfected with a vector containing the of the apoA-II gene or with the control vector and treated or not with MG132 for 2 and h to activation with the Wy 14,643 In cells transfected with the of the activity was by the treatment with Wy 14,643 and MG132 In the cells transfected with the a activation of activity was observed in the presence of Wy 14,643 as with of HepG2 cells with MG132 both the and transcriptional activation of the This effect was observed the cells were for 2 h with MG132 and was after h whether this stabilization of PPARα protein by MG132 in in PPARα target gene HepG2 cells were treated or not with MG132 for 2 h to activation with Wy and the expression level of PPARα target apoA-II (4Vu-Dac N. Schoonjans K. Kosykh V. Dallongeville J. Fruchart J.C. Staels B. Auwerx J. J. Clin. Invest. 1995; 96: 741-750Crossref PubMed Scopus (364) Google Scholar) and D.A. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), was by quantitative with MG132 Wy expression of both the apoA-II and genes. These data show that MG132 stabilization of PPARα protein levels results in an expression of PPARα target genes in HepG2 cells. cells control gene expression by regulating the expression and activity of transcription factor The of these transcriptional are at in protein The first in this process, the ubiquitination of which are degraded by the 26 S proteasome is a highly regulated process to the of transcription factor activity (13Hodges M. Tissot C. Freemont P.S. Curr. Biol. 1998; 8: R749-R752Abstract Full Text Full Text PDF PubMed Google Scholar). In this it is shown that the ubiquitin-proteasome pathway the degradation of PPARα protein and as such the concentration of PPARα in from pulse-chase experiments demonstrate that PPARα protein has a which is by of the receptor, results that and data that this was in M. T. J. H. T. Res. 2001; PubMed Scopus Google Scholar). In addition, we show that PPARα protein stabilization by its ligand is with a in ubiquitination of PPARα protein. Furthermore, a highly specific of the MG132, PPARα protein and the resulting expression of PPARα to a transcriptional activation of a gene by a In addition, treatment of HepG2 cells with MG132 results in an expression of endogenous genes. Recently, it was demonstrated that expression of PPARα in HepG2 cells by PPARα results in an expression of endogenous PPARα target genes U. J. Biol. Chem. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar), of PPARα expression results in expression of PPARα target genes T. J. H. Biol. 1995; PubMed Scopus Google Scholar), that PPARα protein levels are a of the response to its The results from this that PPARα protein expression and activity are regulated at the level of thus a novel mechanism of control of PPARα only a number of have PPARα protein regulation, the concentration of PPARα has been shown to be under a number of PPARα expression has been shown to with a circadian rhythm in liver (14Lemberger T. Saladin R. Vazquez M. Assimacopoulos F. Staels B. Desvergne B. Wahli W. Auwerx J. J. Biol. Chem. 1996; 271: 1764-1769Abstract Full Text Full Text PDF PubMed Scopus (282) Google Scholar). The of PPARα mRNA is by a of PPARα protein. This rapid of PPARα protein levels that the of the protein be to its levels to decrease a of Our results from pulse-chase experiments demonstrate that PPARα protein is 1 h due to its rapid degradation by the which provides a mechanism contributing to the rapid circadian of this protein. Furthermore, data indicate that regulation of PPARα protein level by controlling its degradation the expression of PPARα target genes in response to its For the expression of the apoA-II and PPARα target genes (4Vu-Dac N. Schoonjans K. Kosykh V. Dallongeville J. Fruchart J.C. Staels B. Auwerx J. J. Clin. Invest. 1995; 96: 741-750Crossref PubMed Scopus (364) Google Scholar, D.A. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), by MG132 treatment is in a ligand-dependent manner. in addition to PPARα control by the level of its gene transcription as as by the potency of the the magnitude of the response to PPARα activation is also regulated at the level of its be of to that PPARα degradation and as such the PPARα pathway. study that ligand activation PPARα from ubiquitination and degradation in a rapid but transient manner. demonstrated that the and receptors are also degraded by the proteasome and that this is after ligand Z. A.P. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: PubMed Scopus Google Scholar, H. J. 1998; PubMed Scopus Google Scholar). receptor protein levels are down-regulated after treatment J.L. 2000; PubMed Scopus Google Scholar, H. A. M. T. P. Life Sci. 1998; PubMed Scopus Google Scholar). results indicate a of the receptor J.C. A. M. J.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). receptor has a of h, decreases its to In to the receptors, PPARα has a very and ligand activation its We a in which the ubiquitin-proteasome pathway to the regulation of and magnitude of the response to PPARα The with its ligand the ubiquitination of the PPARα protein and consequently its This effect which be due to a rapid of the ligand in liver cells to a ligand-dependent of that the degradation of the PPARα protein. Indeed, it has been shown previously that the of certain nuclear receptors a of the proteasome in a E. C. Vivat B. H. Chambon P. R. EMBO J. 1996; PubMed Scopus Google Scholar), which in the of the transcriptional activation by the receptor in a manner. is to the mechanisms the stabilization of PPARα protein by its We Dr. for providing the ubiquitin expression We the of