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Molecular Endocrinology, doi:10.1210/me.2004-0188
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Molecular Endocrinology 19 (5): 1135-1146
Copyright © 2005 by The Endocrine Society

Peroxisome Proliferator-Activated Receptor {alpha} Physically Interacts with CCAAT/Enhancer Binding Protein (C/EBPß) to Inhibit C/EBPß-Responsive {alpha}1-Acid Glycoprotein Gene Expression

Audrey Mouthiers, Anita Baillet, Claudine Deloménie, Dominique Porquet and Najet Mejdoubi-Charef

Laboratoire de Biochimie et de Biologie Cellulaire (A.M., A.B., D.P., N.M.-C.), Equipe d’Accueil de Doctorants 1595, Faculté de Pharmacie, Université Paris XI, France; Plate-forme Transcriptome-Protéome (C.D.), Institut National de la Santé et de la Recherche Médicale, Institut Fédératif de Recherche-75, Faculté de Pharmacie, Université Paris XI, France

Address all correspondence and requests for reprints to: Najet Mejdoubi-Charef, Laboratoire de Biochimie et de Biologie Cellulaire, Equipe d’Accueil de Doctorants 1595, Tour D4 1erétage, Faculté de Pharmacie, 5 rue J. B. Clément, 92296 Chaatenay-Malabry Cedex, France. E-mail: najet.charef{at}cep.u-psud.fr.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
Recently, the role of the peroxisome proliferator-activated receptor {alpha} (PPAR{alpha}) in the hepatic inflammatory response has been associated to the decrease of acute phase protein transcription, although the molecular mechanisms are still to be elucidated. Here, we were interested in the regulation by Wy-14643 (PPAR{alpha} agonist) of {alpha}1-acid glycoprotein (AGP), a positive acute phase protein, after stimulation by Dexamethasone (Dex), a major modulator of the inflammatory response. In cultured rat hepatocytes, we demonstrate that PPAR{alpha} inhibits at the transcriptional level the Dex-induced AGP gene expression. PPAR{alpha} exerts this inhibitory effect by antagonizing the CCAAT/enhancer binding protein (C/EBPß) transcription factor that is involved in Dex-dependent up-regulation of AGP gene expression. Overexpression of C/EBPß alleviates the repressive effect of PPAR{alpha}, thus restoring the Dex-stimulated AGP promoter activity. Furthermore, glutathione-S-transferase GST pull-down and coimmunoprecipitation experiments evidenced, for the first time, a physical interaction between PPAR{alpha} and the C-terminal DNA binding region of C/EBPß, thus preventing it from binding to specific sequence elements of the AGP promoter. Altogether, these results provide an additional molecular mechanism of negative regulation of acute phase protein gene expression by sequestration of the C/EBPß transcription factor by PPAR{alpha} and reveal the high potency of the latter in controlling inflammation.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
THE PEROXISOME PROLIFERATOR-ACTIVATED receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear receptor superfamily. The three isoforms (PPAR{alpha}, PPARß, and PPAR{gamma}) are characterized by their ligand specificities and tissue distributions (1, 2). The first PPAR cDNA cloned was isolated from a mouse liver library and corresponds to the PPAR{alpha} subtype (3), the main isoform expressed in the liver. In rodent hepatocytes, peroxisome proliferators (PPs) cause a dramatic increase in the number and size of peroxisomes, an effect associated with the parallel activation of many genes, the products of which are involved in the metabolism of fatty acids (1). In addition to their hypolipidaemic effects, it has recently been demonstrated that PPAR{alpha} plays a role in the inflammatory response. Several studies have been aimed at delineating the cellular and molecular mechanisms explaining the control of the inflammatory response by PPAR{alpha} (4). Indeed, leukotriene B4 (LTB4), a proinflammatory eicosanoid, binds to PPAR{alpha} and induces the transcription of genes involved in {omega}- and ß-oxidation, which leads to the induction of its own catabolism (5). Thus, the duration of the inflammatory response is prolonged in PPAR{alpha}-deficient mice in response to LTB4 (5). Furthermore, it has been shown that fibrates decrease the plasmatic concentrations of cytokines such as TNF{alpha} (6, 7) and IL-6 (8) and subsequently that PPAR{alpha} acts as a negative regulator of the vascular inflammatory gene response by antagonizing the activity of the transcription factors NF-{kappa}B (nuclear factor {kappa}B) and AP1 (activator protein 1) (9). Finally, it has been shown that PPAR{alpha} exerts its antiinflammatory activities in the liver by repressing the expression of proinflammatory genes such as acute phase proteins (APPs). Indeed, exposure of rodents to PPs leads to the down-regulation of many positive acute phase response genes, including the fibrinogen-ß, {alpha}1-acid glycoprotein (AGP), {alpha}1-antitrypsin, ceruloplasmin, and serum amyloid A (10, 11, 12, 13). In line with these observations, PPAR{alpha} has been shown to be involved in this PP-induced transcriptional repression because the effect is completely abolished in PPAR{alpha} knockout mice (12). Recently, PPAR{alpha} has been shown to repress human fibrinogen gene expression by interference with the CCAAT/enhancer binding protein ß (C/EBPß) pathway through titration of the coactivator GRIP1[glucocorticoid receptor (GR)-interacting protein]/TIF2 (transcriptional intermediary factor) (14).

C/EBPß is a key transcription factor involved in the induction of genes during acute phase or immune response (15). In response to extracellular stimuli, C/EBPß may form heterodimers with other C/EBP family members or interact with other transcription factors such as members of the NF-{kappa}B family (16), AP1 (17), Sp1 (18), p53 (19) or GR (20). In the case of the AGP gene, the maximal induction by glucocorticoids requires C/EBPß binding elements located downstream and upstream of the glucocorticoid-responsive element (21) and interactions between TIF1ß, GR, and C/EBPß (22). Moreover, the synergistic interaction between cytokines and glucocorticoids has been attributed to a protein-protein interaction between C/EBPß and GR (20).

In this work, we delineate the molecular mechanism of inhibition by PPAR{alpha} of dexamethasone (Dex)-inductive effects on AGP gene expression. We demonstrate that PPAR{alpha} inhibits Dex-induced AGP gene expression at a transcriptional level and that this inhibition originates from a physical interaction between PPAR{alpha} and C/EBPß. This protein-protein interaction described here for the first time prevents C/EBPß binding to AGP promoter and thus results in the repression of C/EBPß-dependent transactivation of AGP gene.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
DISCUSSION
 MATERIALS AND METHODS
 REFERENCES
 
The PP Wy-14643 Down-Regulates Glucocorticoid-Inductive Effects on AGP Gene Expression at a Transcriptional Level
PPs are known to inhibit inflammation by interfering with several major mediators involved in the acute phase response (4). We explored the effects of Wy-14643 on the Dex-signaling pathway because the latter can directly stimulate the expression of APPs and potentiate cytokine effects. We worked on AGP, a positive APP, whose up-regulation mechanism by Dex is well characterized. We first measured by quantitative PCR the effect of Wy-14643 on the Dex-induced AGP mRNA expression in a physiological system such as cultured rat hepatocytes (Fig. 1AGo). As already demonstrated in vitro (10), the addition of Wy-14643 alone has no effect on basal AGP mRNA expression. However, when cells were treated with both Dex and Wy-14643, AGP mRNA expression decreased by 52% (only 3.3-fold induction) compared with Dex alone (7-fold induction). Pretreatment with Wy-14643, before Dex induction, gave similar transcriptional inhibition of AGP gene expression (50%) even though Dex-induced AGP mRNA expression was slightly higher (9-fold induction). As expected from our short-term experimental conditions (less than 48 h in the presence of Wy-14643), and due to the long half-life of AGP (5 days), we could not observe any decrease of the amount of secreted AGP after a Dex and Wy-14643 treatment compared with Dex induction alone (Fig. 1BGo).



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  		Fig. 1. Effect of Wy-14643 on Dex-Induced AGP Gene Expression

A, Rat hepatocytes were either not treated (control) or treated with 100 µM Wy-14643 or 10–6 M Dex, alone or in association, for 48 h (1 ). Rat hepatocytes were either not treated (control), treated by 10–6 M Dex, or pretreated with 100 µM Wy-14643 for 16 h followed or not by a 10–6 M Dex stimulation for additional 24 h (2 ). Total RNA was extracted and AGP mRNA was quantified by quantitative PCR. Values obtained for AGP mRNA transcripts (copy number) were normalized relative to those of GAPDH (copy number). Results are expressed as variations [mean (±SD) of three separate cultures] relative to control values (fold induction). B, Western blot analysis of secreted AGP in culture medium from either 100 µM Wy-14643 and/or 10–6 M Dex-treated or untreated (control) cultured hepatocytes. The supernatant of a constant 2 x 106 cultured hepatocytes was used for each condition. C, Transient transfections into primary cultured rat hepatocytes were performed with the AGP promoter construct [pAGP(–763/+20)luc]. Transfected cells were treated with either 10–6 M Dex or 100 µM Wy-14643 or both for 48 h. Luciferase activities are expressed relative to those of untreated cells. Each value is the mean (±SD) of luciferase activity measurements performed in three or four independent transfection experiments.

 
To test whether Wy-14643 inhibits glucocorticoid-induced AGP gene expression at a transcriptional level, Sprague Dawley rat hepatocytes were transiently transfected with a luciferase expression vector driven by the 763-bp promoter fragment of AGP gene, which contains regulatory elements for basal and glucocorticoid-inducible promoter activity. Transfected cells were then treated with Wy-14643, Dex or both drugs. As shown in Fig. 1CGo, Wy-14643 had no effect on basal promoter activity, whereas it inhibited Dex-induced AGP promoter transcription by 39% (11-fold induction) compared with Dex alone (18-fold induction). These results indicate that the inhibition of glucocorticoid-induced AGP gene expression by Wy-14643 occurs at a transcriptional level.

It is well known that glucocorticoids potentiate the action of cytokines (IL-1, IL-6, TNF{alpha}) on the regulation of AGP gene (23, 24). Hence we investigated by quantitative PCR analysis whether Wy-14643 modulates this potentiating effect. Hepatocytes were stimulated by Wy-14643 and Dex, alone or associated, together with each of the above cytokines. As expected, cytokines used alone have no effect in our experimental conditions on AGP mRNA expression, their inductive effect being only observed in the presence of Dex, especially that of IL-6 (10-fold induction) and IL-6 in association with IL-1 (14-fold induction) (Fig. 2Go). Consistent with an inhibitory effect of Wy-14643 on Dex-mediated stimulation of AGP gene expression, the potentiation of cytokine action by Dex was also counteracted by Wy-14643 treatment. In both Dex and IL-6 or Dex and IL-6 + IL-1 associations, the repressive effect reached 70% (Fig. 2Go). Altogether, these experiments demonstrate that Wy-14643 represses AGP mRNA expression specifically upon glucocorticoid-induced stimulation.



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  		Fig. 2. Wy-14643 Counteracts the Dex-Potentiated Cytokine Effects

Rat hepatocytes were cultured for 48 h with or without Dex (10–6 M), Wy-14643 (100 µM), IL-1 (25 ng/ml), IL-6 (25 ng/ml), or TNF{alpha} (25 ng/ml) or in association as indicated. Total RNA was extracted and AGP mRNA was quantified by quantitative PCR. Values obtained for AGP mRNA transcripts (copy number) were normalized relative to those of GAPDH (copy number). Results are expressed as AGP/GAPDH ratio [mean (±SD)] of three separate cultures.

 
Because GR, together with the transcription factor C/EBPß, is involved in the glucocorticoid-mediated enhancement of AGP gene (21, 22), we further investigated the effect of Wy-14643 on GR and C/EBPß mRNA transcription by quantitative PCR analysis. As already described for C/EBPß (25), Dex stimulation strongly increased cell GR and C/EBPß mRNA and protein content, whereas Wy-14643 treatment did not influence this induction (Fig. 3Go). These results show that the inhibitory effect of Wy-14643 on glucocorticoid-mediated enhancement of AGP gene expression is not related to mRNA and protein down-regulation of the glucocorticoid signaling partners.



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  		Fig. 3. Effect of Wy-14643 on C/EBPß and GR mRNA and Protein Expression

A, Rat hepatocytes were either not treated (control), treated by 10–6 M Dex, or pretreated with 100 µM Wy-14643 for 16 h followed or not by a 10–6 M Dex stimulation for additional 24 h. Total RNA was extracted and AGP mRNA was quantified by quantitative PCR. Values obtained for AGP mRNA transcripts (copy number) were normalized relative to those of GAPDH (copy number). Results are expressed as variations [mean (±SD) of three separate cultures] relative to control values (fold induction). B, The C/EBPß and GR protein contents were analyzed by Western blot.

 
Wy-14643-Mediated Decrease of C/EBPß and GRIP1/TIF2 Transactivating Effects on Dex-Induced AGP Promoter Activity Involves PPAR{alpha}
Because Wy-14643 is a specific PPAR{alpha} agonist, we tested whether PPAR{alpha} was actually involved in mediating the inhibitory effects we observed above. Because the overexpression of exogenous transcription factors in cultured hepatocytes usually gives low transfection efficiencies, we carried out the subsequent studies in a heterologous system such as Hela cells that are known to express the GR (23). To mimic the glucocorticoid signaling pathway of hepatocytes, an expression vector coding for C/EBPß was cotransfected with the luciferase gene constructs that contained either a large 763-bp promoter fragment [(–763/–60)pGluc] or the 138-bp fragment including the AGP glucocorticoid-responsive element [(–138/–60)pGluc]. As described in Fig. 4AGo, transfection of C/EBPß enhanced Dex-induced luciferase promoter activity of the (–763/–60)pGluc construct (28.8-fold induction). The overexpression of PPAR{alpha} resulted in a marked decrease of Dex-induced luciferase activity (–50%) that was enhanced in the presence of Wy-14643 (–70%) (Fig. 4AGo). These results demonstrate that the glucocorticoid signaling pathway can be reproduced in Hela cells and that PPAR{alpha} inhibits Dex-induced activation of the AGP gene promoter in this model. As shown in Fig. 4BGo, cotransfection of C/EBPß with the (–138/–60)pGluc remarkably increased the basal (71-fold induction) and Dex-induced (134-fold induction) luciferase activity, as previously described (22). In the absence or presence of Dex, cotransfection of a constant amount of C/EBPß with increasing amounts of PPAR{alpha} led to a dose-dependent inhibition of AGP promoter transactivation. This effect was further amplified in the presence of Wy-14643, whereas the GR protein level did not change (Fig. 4BGo). It is to note that an effect of Wy-14643 alone could be observed in the absence of Dex (data not shown) by contrast to hepatocytes (Fig. 1Go). This discrepancy can be explained by the fact that induction of AGP gene transcription in hepatocytes by endogenous C/EBPß requires an interaction of the latter with a Dex-activated GR, whereas, in Hela cells, AGP promoter transcription occurs (to a much lesser extent) even in the absence of Dex because of the high C/EBPß overexpression level.



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  		Fig. 4. Inhibition of GRIP1- and C/EBPß-Induced AGP Promoter Activity by PPAR{alpha}

A, Hela cells were transfected with the (–763/–60)pGluc plasmid (1 µg) in the presence of pSG5-PPAR{alpha} or CMV-C/EBPß. Cells were either not stimulated (control) or stimulated with 10–6 M Dex and treated or not with 10 µM Wy-14643. B, Hela cells were transfected with the (–138/–60)pGluc plasmid (1 µg) and increasing amounts of pSG5-PPAR{alpha} (0.5x, 1x, and 2x) were added to a constant amount of CMV-C/EBPß (100 ng). Cells were either not stimulated (control) or stimulated with 10–6 M Dex and treated or not with 10 µM Wy-14643. GR protein content was analyzed by Western blot. C, Hela cells were transfected with the (–138/–60)pGluc plasmid (1 µg) in the presence of PPAR{alpha} or GRIP1/TIF2 expression vectors and either not stimulated (control) or stimulated with 10–6 M Dex and treated or not with 10 µM Wy-14643. The ratios of luciferase-to-ß-galactosidase activities were measured in duplicates. The mean ratio values (±SD) obtained from four independent experiments are expressed relative to the pSG5 control ratio.

 
Because PPAR{alpha} similarly regulates the luciferase activity of both (–763/–60)pGluc and (–138/–60)pGluc plasmids, we further pursued our investigation on the molecular mechanism of PPAR{alpha} action with the latter luciferase construct that was more sensitive to C/EBPß and Dex treatment.

The above results suggested that C/EBPß could be a target of PPAR{alpha} inhibitory effect on Dex-induced stimulation of the AGP promoter activity. To determine whether other known GR partners could be PPAR{alpha} targets, the (–138/–60)pGluc construct was cotransfected in Hela cells with the expression vectors coding for TIF1ß (a cofactor that interacts with GR and C/EBPß) (22) or GRIP1/TIF2 (a coactivator of GR) (26). These experiments were performed with or without pSG5-PPAR{alpha} cotransfection and luciferase activity was analyzed. TIF1ß exhibited a weak inductive effect on luciferase promoter activity with or without Dex (data not shown). Conversely, GRIP1/TIF2 cofactor strongly induced the luciferase promoter activity (Fig. 4CGo), but only in the presence of Dex (39-fold induction) as expected (27). Furthermore, the cotransfection of PPAR{alpha} totally abolished the GRIP1/TIF2 activating effect with or without Wy-14643.

Taken together, these results indicate that PPAR{alpha} can counteract C/EBPß- and GRIP1/TIF2-induced AGP transactivation.

C/EBPß But Not GRIP1/TIF2 Alleviates the Repressive Effect of PPAR{alpha} on Dex-Induced AGP Gene Transcription
Because PPAR{alpha} decreases C/EBPß and GRIP1/TIF2 transactivating effects, we addressed the question whether an excess of each factor would alleviate the inhibitory effect mediated by PPAR{alpha}. Hela cells were cotransfected with the (–138/–60)pGluc plasmid, a constant quantity of pSG5-PPAR{alpha} and increasing amounts of C/EBPß or GRIP1/TIF2 expression vectors before monitoring luciferase. In contrast with the results obtained by Gervois et al. (14) in the case of the fibrinogen-ß gene, we did not observe any abolishment of PPAR{alpha} inhibitory effect by an excess of GRIP1/TIF2 (results not shown), indicating that, in our model, GRIP1/TIF2 is not the obvious target of PPAR{alpha}. However, increasing amounts of C/EBPß made possible the restoration of Dex-induced AGP promoter activation to the level of the control. A total abolishment of the inhibitory effect of PPAR{alpha} in a ligand-independent manner was achieved for a C/EBPß:PPAR{alpha} ratio of 5:1 (Fig. 5Go). These results suggest that PPAR{alpha} represses Dex-induced AGP expression by titration of C/EBPß, leading us to search for an interaction between the two proteins.



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  		Fig. 5. C/EBPß Alleviates the PPAR{alpha} Inhibitory Effect on Dex-Induced AGP Gene Transcription

Hela cells were cotransfected with the (–138/–60)pGluc plasmid (1 µg) and either pSG5-PPAR{alpha} or CMV-C/EBPß. Increasing amounts of CMV-C/EBPß (100 ng) were added to a constant amount of pSG5-PPAR{alpha} (100 ng). Cells were stimulated with 10–6 M Dex and treated or not with 10 µM Wy-14643. The ratios of luciferase-to-ß-galactosidase activities were measured in duplicates. The mean ratio values (±SD) obtained from four independent experiments are expressed relative to the pSG5 control ratio. PPAR{alpha} and C/EBPß protein levels were analyzed by Western blotting of protein extracts from Hela cells transfected with a constant amount of PPAR{alpha} and increasing amounts of C/EBPß expression vectors.

 
Physical Interaction between C/EBPß and PPAR{alpha}
To test whether C/EBPß and PPAR{alpha} proteins interact, we performed glutathione-S-transferase (GST) pull-down experiments. GST or GST-PPAR{alpha} were incubated with different amounts of [35S]methionine-labeled C/EBPß in the presence or absence of Wy-14643. As shown in Fig. 6AGo, C/EBPß specifically interacts with PPAR{alpha} because 1) no binding could be detected in the presence of empty GST and 2) the interaction was enhanced with increasing amounts of one of the two partners. The densitometric analysis of the autoradiogram further shows that the interaction for a C/EBPß:PPAR{alpha} ratio of 1:1 was enhanced in the presence of Wy-14643 (+20%), as already suggested by previous transfection experiments (see Fig. 4AGo). These results clearly indicate that C/EBPß and PPAR{alpha} coexist in a complex providing a pretty good explanation of our transfection data (Figs. 4Go, A and B, and 5Go).



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  		Fig. 6. Physical Interaction between C/EBPß and PPAR{alpha}

Radiolabeled C/EBPß and LIP proteins were synthesized in vitro and then incubated with Sepharose beads covered with immobilized GST or GST-PPAR{alpha} proteins in the presence or absence of Wy-14643; bound C/EBPß or LIP proteins were eluted, analyzed by SDS-PAGE and visualized by autoradiography as shown in panels A and B, respectively; bound C/EBPß was quantified by densitometry. C, Coimmunoprecipitation was performed on total protein extract prepared from rat liver. Anti-PPAR{alpha} antibody was used to precipitate endogenous PPAR{alpha}, and the immune complexes were revealed with anti-C/EBPß antibody (lane 1). Immunoprecipitation of liver protein extracts with preimmune serum (PI) (lane 2) and of RIPA buffer with anti-PPAR{alpha} (lane 3) were included as negative controls. Total liver protein extract was used as a positive control for the CEBPß detection (lane 4).

 
To determine which domain of C/EBPß interacts with PPAR{alpha}, GST pull-down experiments were also performed with a truncated C/EBPß isoform (also termed liver-enriched inhibitory protein or LIP) that lacks the 151 amino acids in the N-terminal transactivation region. As shown in Fig. 6BGo, LIP specifically interacts with PPAR{alpha}, indicating that the C-terminal DNA binding domain of C/EBPß is involved for its direct interaction with PPAR{alpha}. To finally assess that the sequestration of C/EBPß by PPAR{alpha} also occurs under physiological conditions, immunoprecipitations were performed with a monoclonal anti-PPAR{alpha} on liver-soluble protein extracts. The presence of C/EBPß in the endogenous immune complexes was detected by Western blot using a polyclonal anti-C/EBPß (Fig. 6CGo), indicating that PPAR{alpha} and C/EBPß coexist in a protein complex in vivo.

PPAR{alpha} Prevents C/EBPß from Binding to Its Target DNA Element
To finally determine whether PPAR{alpha} prevents C/EBPß from binding to its target DNA elements on the AGP gene or whether the PPAR{alpha}-C/EBPß interaction reduces C/EBPß transactivating function, gel shift assays were performed using the specific C/EBPß sequence located within the steroid-responsive unit of the AGP promoter and in vitro-translated C/EBPß and PPAR{alpha}. As shown in Fig. 7Go, C/EBPß bound specifically to its target DNA element; the binding specificity being confirmed by antibody-mediated super-shift. Furthermore the addition of increasing amounts of in vitro-translated PPAR{alpha} abolished the binding of C/EBPß to DNA, confirming that PPAR{alpha} interacts with the DNA binding domain of C/EBPß.



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  		Fig. 7. PPAR{alpha} Prevents C/EBPß from Binding to Its DNA Element

The 32P-labeled probe was incubated without or with unprogrammed reticulocyte lysate (Ret. Lys) or in vitro-translated C/EBPß. The DNA/C/EBPß complex formed is indicated by an arrow. Competition analysis was performed with a 100-fold molar excess of non radiolabeled homologous probe. Supershift experiments were performed by adding anti-C/EBPß antibody as described in Materials and Methods. Gel retardation assays were also performed with a constant amount of C/EBPß and increasing amounts of PPAR{alpha}.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
MATERIALS AND METHODS
 REFERENCES
 
Initially known to alter diverse liver-specific genes involved in fatty acid catabolism pathways, PPAR{alpha} has more recently been shown to have a direct implication in the modulation of the inflammatory response in the liver. Indeed, in rodent livers, PPAR{alpha} down-regulates APPs as observed in vivo for serum amyloid A, ceruloplasmin, haptoglobin, fibrinogen-ß and -{gamma}, {alpha}2-macroglobulin and AGP (11, 12, 13). In vitro studies have been assayed to address the molecular mechanisms of inflammation control by PPAR{alpha} (4). It has been reported that PPAR{alpha} represses basal or IL-6-induced fibrinogen-{alpha} or -ß and serum amyloid A gene expression (14). Similarly, IL-1-induced C-reactive protein (CRP) is down-regulated by PPAR{alpha} activation (28). In this paper, we evidenced that Wy-14643, a specific agonist of PPAR{alpha}, strongly decreases Dex-induced AGP gene expression, the only APP the plasma level of which is not only elevated during acute but also chronic inflammatory states. As observed for CRP, PPAR{alpha} action on AGP expression is dose dependent, occurs at the transcriptional level, and is observed only after stimulation.

Increasing information is emerging on the antiinflammatory properties of PPAR{alpha} in the liver even though the molecular mechanisms involved in this effect are not definitively established. It has been demonstrated that PPAR{alpha} down-regulates the expression of the IL-6 receptor in the liver and subsequently decreases the activation of transcription factors such as signal transducer and activator of transcription 3 and c-Jun involved in the IL-6 signaling pathway (29). Moreover, PPAR{alpha} mediates the repressive effect of fibrates on fibrinogen-ß expression through sequestration of GRIP1/TIF2, a cofactor of C/EBPß (14). In the case of IL-1-induced CRP expression, PPAR{alpha} activators also inhibit gene transcription by reducing the formation of nuclear C/EBPß-p50-NF-{kappa}B complexes (28). Remarkably, we evidenced a novel molecular mechanism of negative gene regulation by PPAR{alpha}, demonstrating, for the first time, a physical, ligand-independent, interaction between PPAR{alpha} and the transcription factor C/EBPß. Both mobility shift assays where PPAR{alpha} impairs the binding of C/EBPß to its target DNA and GST pull-down experiments with the LIP isoform of C/EBPß in which most of the transactivation domain has been truncated suggest a role for the C-terminal DNA binding domain of C/EBPß to directly interact with PPAR{alpha}. Indeed, C/EBPß is known to be a basic region-leucine zipper (bZIP) protein with a leucine zipper domain at the C terminus that is essential for DNA binding and dimer formation (20).

To date, an interaction between PPAR{alpha} and a transcription factor has only been shown for AP1 or NF-{kappa}B on proinflammatory genes in extrahepatic tissues (9, 30). The titration of the transcription factor C/EBPß involved in AGP stimulation by Dex treatment explains the inhibitory effect of PPAR{alpha} that we observed on Dex-induced AGP promoter activity. Indeed, an excess of C/EBPß alleviates the repressive effect of PPAR{alpha} on Dex-stimulated AGP transcription. Interestingly, this PPAR{alpha} control of a Dex-induced APP expression through sequestration of the transcription factor C/EBPß may also be relevant to other signaling pathways involving C/EBPß.

This novel molecular mechanism of inhibition of the C/EBPß signaling pathway by PPAR{alpha} through a physical interaction is likely to contribute to the overall antiinflammatory properties of PPAR{alpha} agonists. Indeed, it can complete already known molecular mechanisms implicated in the inhibition of cytokine-induced APPs by PPAR{alpha}. In the case of IL-6-induced fibrinogen-ß gene, PPAR{alpha} has been shown to exert its repressive effect via titration of a cofactor of C/EBPß, GRIP1/TIF2 (14); in addition, from our data, one can also hypothesize that PPAR{alpha} might directly act through sequestration of C/EBPß. Furthermore, the fact that Wy-14643 also prevents Dex-potentiated IL-1 and IL-6 action on AGP gene expression as well as on several other acute phase genes (14) suggests once again that at least part of the inhibitory effects of PPAR{alpha} are mediated through the sequestration of C/EBPß, a common transcription factor of both IL-1 or IL-6 and Dex signaling pathways. Similarly, regarding the repression of IL-1-induced CRP, PPAR{alpha} could sequester the C/EBPß protein, one partner of the complex p50NF-{kappa}B-C/EBPß, in addition to the decrease of formation of nuclear complexes by up-regulation of inhibitor {kappa}B (i{kappa}B) expression in vitro and strong reduction of basal C/EBPß and p50NF-{kappa}B protein expression levels in vivo, as described previously (28). Thus, PPAR{alpha} interference with the C/EBPß signaling pathway would reveal the powerful potency of PPAR{alpha} in controlling inflammation in the liver.

It has been demonstrated in hepatocytes that PPAR{alpha} expression is controlled in vivo and in vitro by glucocorticoids (31, 32), proinflammatory modulators that can directly stimulate or potentiate cytokine action on APP gene expression, the regulation of most of which involves the transcription factor C/EBPß (33). In the case of C/EBPß-responsive genes, a feedback mechanism could be envisioned in which the up-regulation of PPAR{alpha} expression by glucocorticoids would in turn lead to the down-regulation of Dex and Dex-potentiated cytokine action on APP gene expression through the sequestration of the transcription factor C/EBPß. Such a feedback mechanism could thus provide a means for hepatic cells to control the duration of an inflammatory response.

Altogether, our findings identify a novel target for negative gene regulation by PPAR{alpha}. They also reinforce the idea that PPAR{alpha} is a major actor in the modulation of the inflammatory response in the liver and finally, emphasize the therapeutic potential of PPAR{alpha} ligands in controlling some pathological aspects of the inflammatory response.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
REFERENCES
 
Chemicals
Dex (Soludecadron, 4 mg/ml) was from Merck Sharp & Dohme Chibret (Puy-en-Velay, France). Wy-14643 was from Calbiochem (Merck Biosciences, Fontenay-sous-bois, France). Isopropyl ß-D-thiogalactosyde was from Sigma (St Quentin Fallavier, France). Cytokines were from PromoKine, Bioscience Alive (Heidelberg, Germany).

Cell Culture
Hepatocytes from Sprague Dawley rats (190–250 g) were isolated by collagenase method as previously described (23). Cells (2 x 106) were plated in 3 ml of William’s E culture medium containing 10% fetal calf serum, 2 mM L-glutamine, 10 mM sodium pyruvate, 30 nM selenium, 8 µM niacinamide, 100 U/ml penicillin, 100 µg/ml streptomycin, and 250 ng/ml fungizone as described previously (24). The culture medium was changed daily, and chemicals (10–6 M Dex and/or 100 µM Wy-14643) were added to the culture medium. Twenty-four hours after the last treatment, total cellular RNA was extracted for quantitative PCR.

Hela cells were maintained in DMEM supplemented with 10% fetal calf serum, penicillin at 100 U/ml, streptomycin at 100 µg/ml, and gentamycin sulfate at 0.25 µg/ml and grown at 37 C in 5% CO2.

RNA Extraction, cDNA Synthesis, and Quantitative PCR (QPCR)
Total RNA was isolated from a pellet of cultured hepatocytes using the Trizol total RNA isolation reagent (Invitrogen Life Technologies, Carlsbad, CA) as previously described (24). First-strand cDNA was generated by reverse transcription of 2 µg of total RNA using oligo(deoxythimidine)12–15 primer and Superscript III reverse transcriptase (Invitrogen Life Technologies) according to the manufacturer’s instructions, in a total reaction volume of 20 µl. Reverse and forward oligonucleotide primers, specific to the chosen candidate and housekeeping genes, were designed using Primer3 software (http://frodo.wi.mit.edu/cgi-bin/primer3/primer3_www.cgi). Table 1Go lists the sequence of oligonucleotide primers that were used. Real-time PCR was performed in a LightCycler (Roche Diagnostics, Meylan, France) thermal cycler. Each cDNA was performed in a 10-µl volume, using the Fast Start DNA MasterPLUS SYBR Green I master mix (Roche Diagnostics), with 300-nM final concentrations of each primer. Dissociation curves were generated after each QPCR run to ensure that a single, specific product was amplified. For establishing the calibration curves, a cDNA fragment of AGP and GR genes was amplified using conventional PCR, generating standard fragments of 448 and 870 bp in length, respectively. These fragments were purified using the High Pure PCR Product Purification Kit (Roche Diagnostics), quantified spectrophotometrically and sequenced (MWG Biotech, Courtaboeuf, France). Purified plasmid sequences were used as standards for C/EBPß and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). All PCR efficiencies (E), calculated from the slopes of the calibration curves according to the equation E = [10(–1/slope)] – 1, were above 90%. QPCR data for each gene were normalized to the GAPDH mRNA content of each cDNA. Mean values ± SD from three separate experiments are presented.


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  		Table 1. Primer Sets for Quantitative Real-Time PCR Used for mRNA Quantitation

 
Expression Vectors and Plasmid DNA Constructs
The pSG5 and pSV-ß-galactosidase control vector plasmids were from Stratagene (Amsterdam, The Netherlands) and Promega (Charbonnières, France), respectively.

The pSG5-PPAR{alpha} expression plasmid, which contains the complete mouse PPAR{alpha} coding sequence, and the pSG5-TIF1ß one, which encodes for the mouse TIF1ß, were generous gifts from Pr. N. Latruffe (Laboratoire de Biologie Moléculaire et Cellulaire, University of Bourgogne, Dijon, France) and Dr. P. Chambon (Institut National de la Santé et de la Recherche Médicale, University Louis Pasteur, Strasbourg, France), respectively.

CMV-C/EBPß and CMV-LIP were gifts of Pr. U. Schibler (University of Geneva, Geneva, Switzerland) and pSG5-GRIP1/TIF2 was a gift of Pr. M. R. Stallcup (University of Southern California, Los Angeles, CA).

pGEX-PPAR{alpha} was constructed by inserting the BamHI PPAR{alpha} cDNA fragment from pSG5-PPAR{alpha} into similarly digested pGEX-4T-1 (Amersham Biosciences, Orsay, France).

To generate pAGP(–763/+20)luc, the –763/+20 fragment of the rat AGP gene promoter was excised by SmaI/XhoI from pAGPcat (33) and inserted into the corresponding sites of the pGL3 Basic vector (Promega).

The –763/–60 fragment from pAGPcat was subcloned into pGluc plasmid, which contains the minimal ß-globin promoter. (–138/–60)pGluc was obtained by creating a HindIII restriction site at position –138 into the (–763/–60)pGluc using the QuikChange Site-Directed Mutagenesis kit (Stratagene); the HindIII/BamHI fragment thus obtained was then subcloned into the HindIII/BamHI cloning sites of pGluc.

Transient Transfection and Luciferase Reporter Assay
Freshly isolated hepatocytes were transfected by the electroporation method as previously described (34). After transfection, cells were incubated for 48 h with the indicated compounds (10–6 M Dex and/or 100 µM Wy-14643) in medium containing 10% fetal calf serum.

For Hela cells, transfections were carried out using FuGene 6 reagent (Roche Diagnostics). Cells were plated at a density of 2 x 105 in six-well dishes 1 d before transfection. Cotransfections were typically performed using 1 µg of (–138,-60)pGluc and different amounts of expression plasmids plus 0.5 µg of internal control (pSV-ßgal); the total amount of DNA was maintained constant with pSG5. DNA solutions were combined at a 1:3 ratio with FuGene 6 reagent (1 µg of DNA/3 µl of FuGene). Cells were overloaded with this mixture in a final volume of 1 ml of medium for overnight. Wy-14643 (10 µM) or/and Dex (10–6 M) were then added to fresh medium for 24 h.

Cell extracts were prepared, and luciferase activity was measured using a MicroLumatPlus LB 96V (Berthold Technologies, Thoiry, France) and a luciferase activity kit (Promega). ß-Galactosidase activity was determined for 100 µg of extract to normalize for transfection efficiency.

Western Blotting
After separation by SDS-PAGE using 9% or 12% polyacrylamide gels, proteins were electroblotted onto a polyvinylidene difluoride membrane. After blocking, membranes were probed with either anti-GR (Santa Cruz, Biotechnology), anti-C/EBPß (sc-150, Santa Cruz Biotechnology), anti-PPAR{alpha} (MA1-822, Ozyme, Saint-Quentin en Yuelines, France) or anti-AGP polyclonal antibodies. Purified goat antirabbit or goat antimouse antibodies were used to detect primary antibodies. The immune complexes were visualized by an enhanced chemiluminescence assay (ECL, NEN Life Science Products Inc., Boston, MA).

In Vitro Translation
PPAR{alpha}, C/EBPß, [35S]methionine-labeled C/EBPß, and [35S]methionine-labeled LIP proteins were transcribed and translated in vitro using the TNT T7 coupled reticulocyte lysate system (Promega) according to the manufacturer’s instructions.

GST Fusion Protein Binding Assay
GST or GST-PPAR{alpha} fusion protein were produced in BL21 Escherichia coli after induction with 0.5 mM isopropyl ß-D-thiogalactosyde for 2 h. Cells were harvested by centrifugation and resuspended into PBS. The bacteria were lysed by mild sonication at 4 C in PBS. Triton X-100 was added to a final concentration of 1%, followed by gentle mixing for 30 min at 4 C. The supernatant was gently mixed with PBS-washed glutathione-Sepharose 4B beads (Amersham) at room temperature for 30 min. GST proteins bound to beads were collected by centrifugation at 500 x g, followed by three successive washes with PBS.

In vitro protein-protein interaction assay (GST pull-down) was carried out by incubating 10 µl of GST-PPAR{alpha} beads with 10 or 20 µl of in vitro synthesized [35S]methionine-labeled protein in the presence or not of 100 µM Wy-14643 in a total volume of 150 µl of incubation buffer [20 mM HEPES (pH 7.8), 100 mM KCl, 10 mM MgCl2, 10% glycerol, 0.1% Nonidet P-40, 0.1% Triton X-100, 0.1% BSA, 1 mM dithiothreitol, 1 µg/ml of aprotinin, leupeptin, and pepstatin]. The mixture is gently rotated for 90 min at 4 C. After centrifugation, the beads were washed four times with incubation buffer without BSA, resuspended in 30 µl of 1x Laemmli buffer, boiled for 5 min, and centrifuged. The supernatant was loaded onto a SDS-PAGE. After drying the gel, bound proteins were visualized after autoradiography and subjected to densitometry after digitization on a personal computer.

In Vivo Protein-Protein Interaction Assay (Coimmunoprecipitation)
A freshly isolated piece of rat liver of about 1 g was homogenized in ice-cold RIPA buffer [50 mM Tris-HCl (pH 7.4), 1% Nonidet P-40, 0.25% Na-deoxycholate, 150 mM NaCl, 1 mM EDTA, 1 mM phenylmethylsulfonyl fluoride, 1 mM Na3VO4, 1 mM NaF and 1 µg/ml of aprotinin, leupeptin, and pepstatin] and centrifuged at 10,000 x g for 10 min to recover soluble proteins. Liver extracts were then incubated with 5 µg of primary mouse anti-PPAR{alpha} antibody (MA1–822, Affinity Bioreagents) or mouse preimmune serum at 4 C for 16 h. Antigen-antibody complexes were immunoprecipitated by adding 100 µl of magnetic protein A Dynabeads (Dynal, Compiègne, France) and rotated at 4 C for 2 h. Complexes were then washed four times with 1 ml of ice-cold PBS containing 0.1% SDS. Protein samples were boiled in Laemmli electrophoresis buffer and subjected to SDS-PAGE for Western blot analysis with a rabbit polyclonal anti-C/EBPß antibody (sc-150, Santa Cruz Biotechnology).

Gel Retardation Assays
The specific sequence for C/EBPß corresponds to the C/EBP interaction sites within the glucocorticoid regulatory unit of the AGP promoter (–94 to –64). The probe was prepared by annealing the sense strand oligonucleotide (5'-GATCCTGGTGAGATTGTGCCACAGCTCTGCA-3') with the corresponding antisense strand oligonucleotide, and then 5' end-labeled using T4 polynucleotide kinase and {gamma} 32P ATP (3000 Ci/mmol, Amersham France SA, Les Ulis, France).

In vitro-synthesized C/EBPß was preincubated with or without in vitro-synthesized PPAR{alpha} in a total volume of 20 µl of buffer [20 mM HEPES (pH 7.9), 50 mM KCl, 1 mM dithiothreitol, 1 mM EDTA, 10% glycerol] with 1 µg of poly-deoxyinosine-deoxycytosine with or without 2 µg of anti-C/EBPß antibody for 10 min on ice. After 20 min of incubation at room temperature with 1 ng end-labeled oligonucleotide, DNA-protein complexes were separated by electrophoresis in 5% polyacrylamide gels in TBE 0.5x buffer (45 mM Tris borate, 1 mM EDTA) at 4 C at 200 V for 3 h.


   ACKNOWLEDGMENTS
 
We are grateful to Dr. P. Chambon (Institut de Gènétique et de Biologie Moléculaire et Cellulaire-Laboratoire de Gènétique Moléculaire des Eurcaryotes Unité 184, Université Louis Pasteur, Strasbourg, France) and Pr. N. Latruffe (Laboratoire de Biologie Moléculaire et Cellulaire, Université de Bourgogne, Dijon, France) for providing the pSG5-TIF1ß and the pGluc, pSG5 and pSG5-PPAR{alpha} plasmids, respectively. We also thank Pr. M. R. Stallcup (University of Southern California) and Pr. U. Schibler (University of Geneva, Geneva, Switzerland) for providing the pSG5-GRIP1/TIF2 and the CMV-C/EBPß and CMV-LIP plasmids, respectively.


   FOOTNOTES
 
This work was supported by a grant from the Institut de Recherche International Servier and by the Fondation pour la Recherche Médicale (to A.M.).

First Published Online January 20, 2005

Abbreviations: AGP, {alpha}1-Acid glycoprotein; AP1, activator protein 1; APP, acute phase protein; C/EBP, CCAAT/enhancer binding protein; CRP, C-reactive protein; Dex, dexamethasone; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GR, glucocorticoid receptor; GRIP, GR-interacting protein; GST, glutathione-S-transferase; LIP, liver-enriched inhibitory protein; NF{kappa}B, nuclear factor {kappa}B; PP, peroxisome proliferator; PPAR, PP-activated receptor; QPCR, quantitative PCR; TIF, transcriptional intermediary factor.

Received for publication May 4, 2004. Accepted for publication January 12, 2005.


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Nuclear Receptors:   PPARα  |  GR
Coregulators:   GRIP1
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