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Published ahead of print on October 20, 2005, doi:10.1165/rcmb.2005-0289OC
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American Journal of Respiratory Cell and Molecular Biology. Vol. 34, pp. 242-246, 2006
© 2006 American Thoracic Society
DOI: 10.1165/rcmb.2005-0289OC

Role of Prostacyclin versus Peroxisome Proliferator-Activated Receptor beta Receptors in Prostacyclin Sensing by Lung Fibroblasts

Ferhana Y. Ali, Karine Egan, Garret A. FitzGerald, Béatrice Desvergne, Walter Wahli, David Bishop-Bailey, Timothy D. Warner and Jane A. Mitchell

Cardiothoracic Pharmacology, Unit of Critical Care Medicine, National Heart and Lung Institute, Imperial College; William Harvey Research Institute, Barts and the London, Queen Mary's School of Medicine and Dentistry, Charterhouse Square, London, United Kingdom; Center for Experimental Therapeutics, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; and Centre for Integrative Genomics, University of Lausanne, Lausanne, Switzerland

Correspondence and requests for reprints should be addressed to Jane A. Mitchell, Cardiothoracic Pharmacology, Unit of Critical Care Medicine, National Heart and Lung Institute, Imperial College School of Medicine, Dovehouse Street, London SW3 6LY, UK. E-mail: j.a.mitchell{at}ic.ac.uk


   Abstract
Top
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Prostacyclin and its mimetics are used therapeutically for the treatment of pulmonary hypertension. These drugs act via cell surface prostacyclin receptors (IP receptors); however, some of them can also activate the nuclear receptor peroxisome proliferator-activated receptor beta (PPARbeta). We examined the possibility that PPARbeta is a therapeutic target for the treatment of pulmonary hypertension. Using the newly approved (for pulmonary hypertension) prostacyclin mimetic treprostinil sodium, reporter gene assays for PPARbeta activation and measurement of lung fibroblast proliferation were analyzed. Treprostinil sodium was found to activate PPARbeta in reporter gene assays and to inhibit proliferation of human lung fibroblasts at concentrations consistent with an effect on PPARs but not on IP receptors. The effects of treprostinil sodium on human lung cell proliferation are mimicked by those of the highly selective PPARbeta ligand GW0742. There are no receptor antagonists for PPARbeta or for IP receptors, but by using lung fibroblasts cultured from mice lacking PPARbeta (PPARbeta–/–) or IP (IP–/–), we demonstrate that the antiproliferative effects of treprostinil sodium are mediated by PPARbeta and not IP in lung fibroblasts. These observations suggest that some of the local, longer-term benefits of treprostinil sodium on reducing the remodeling associated with pulmonary hypertension may be mediated by PPARbeta. This study is the first to identify PPARbeta as a potential therapeutic target for the treatment of pulmonary hypertension, which is important because orally active PPARbeta ligands have been developed for the treatment of dyslipidemia.

Key Words: fibroblast • nuclear receptors • PPARbeta • prostacyclin • pulmonary hypertension


   Introduction
Top
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Pulmonary hypertension is defined clinically as a mean pulmonary artery pressure value exceeding 25 mm Hg at rest or 30 mm Hg during exercise. Pulmonary hypertension is caused when there is an imbalance in the release of vasoactive mediators that favor vasoconstriction, which stimulates the release of growth factors and subsequent vessel wall (remodeling). Remodeling in pulmonary hypertension is characterized by hypertrophy and hyperplasia of the various cells types within the vessel, including fibroblasts, medial smooth muscle cells, and endothelial cells (1). In this respect, pulmonary fibroblasts are important because their migration and differentiation into the vessels contributes significantly to the formation of the neointima. Arteries within the lung become narrow, creating increased resistance and blood pressure and thereby increasing workload for the heart. The prognosis for patients with pulmonary hypertension is usually poor, with survival rates of 68% at 1 yr, 48% at 3 yr, and 34% at 5 yr (2).

Pulmonary hypertension is treated with a number of therapeutic interventions (1), including administration of the vasodilator prostacyclin. Prostacyclin has a short half-life (2–3 min) and undergoes spontaneous hydrolysis into 6-keto PGF1{alpha} (3); because of this, therapeutic administrations need to be via continuous intravenous infusion. Most recently, the stable prostacyclin mimetic, treprostinil sodium (Remodulin; United Therapeutics Corp., Silver Spring, MD), has been developed for the treatment of pulmonary hypertension. Treprostinil sodium is stable at room temperature and has a neutral pH, which allows subcutaneous infusion.

Prostacyclin and its analogs activated G protein–coupled cell-surface prostacyclin (IP) receptors. Activation of IP receptors stimulates adenylyl cyclase, leading to an increase in cyclic adenosine monophosphate, which inhibits cell proliferation and inflammatory mediator release (4). However, prostacyclin and some mimetics are capable of activating the peroxisome proliferator-activated receptor beta (PPARbeta) (3). PPARs are a family of three nuclear receptors (PPAR{alpha}, PPARbeta/{delta}, and PPAR{gamma}) that have influences on many biological processes, including lipid and energy metabolism (5, 6), epidermal wound repair (7), inflammation responses, and atherosclerotic plaque formation. The literature describes the function of PPAR{gamma} or PPAR{alpha} in physiologic and pathophysiologic responses, but much less is known about the role of PPARbeta in health and disease. Nevertheless, activation of PPARbeta has been found to have anti-inflammatory properties via interactions with the transcriptional repressor BCL-6 (8).

The potential role of IP versus PPARbeta as therapeutic targets in pulmonary hypertension has not previously been addressed. There are no antagonists of IP or PPARbeta receptors. In the current study, we used selective ligands and cells grown from the lungs of mice deficient in IP (9) or PPARbeta (10) to address this question. We show that treprostinil sodium is a PPARbeta ligand and that its antiproliferative effects on lung fibroblasts are mediated by PPARbeta and not by IP receptors.


   MATERIALS AND METHODS
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Origin and Genotyping of Genetically Modified Mice
IP receptor and PPARbeta/{delta} gene-deleted (IP–/– and PPARbeta/{delta}–/–) mice were generated as previously reported (9, 10). Both types of genetically modified mice were back bred for seven generations with C57 BLK 6 mice. To prepare cultures of lung cells, animals were killed, and their lungs were removed. The lungs were placed in sealed sterile culture tubes containing sterile Dulbecco's modified Eagle's medium (DMEM) with penicillin/streptomycin (100 U/ml), L-glutamine (2 mM), nonessential amino acids (1%), and amphotericin B (2.5 µg/ml) and shipped by overnight courier. Lungs taken from C57 BLK 6 mice and treated in the same way were used as controls. The tissue was genotyped by PCR. Details on the methods can be found on the online data supplement.

Murine Cell Culture
Lung fibroblasts are an important cell target for antiproliferative agents in the treatment of pulmonary hypertension. We used primary cultures of human or murine lung fibroblasts to address our questions. Lungs were washed with sterile PBS, chopped, and placed in culture flasks in supplemented DMEM. Tissue was incubated in culture until cells had explanted and achieved more than 80% confluence. Cells were seeded onto separate 96-well plates at 40% (20,000 cells/ml) confluence. After 24 h, the medium was replaced with fresh DMEM without FCS but with BSA (0.1%). Drugs were added 2 h later. After 4 h, proliferation was initiated by the addition of FCS (3%). Cells were incubated in an atmosphere of 5% CO2 in air at 37°C for 72 h, after which time proliferation was quantified using the CyQUANT assay (11).

Primary Cultures of Human Lung Fibroblasts
Human lung fibroblasts (American Type Culture Collection CCL-211) were grown in supplemented DMEM containing 10% FCS. Cells were plated at 20,000 cells/ml into 96-well plates. At 40% confluence, the cells were growth arrested (no FCS) and left for 24 h. Drugs were added and incubated for 4 h, and cells were stimulated with the addition of FCS (3%). Cells were incubated in at 37°C for 5 d, and proliferation was quantified using the CyQUANT assay.

Measurement of Cell Number
A CyQUANT cell proliferation assay was used to determine cell proliferation. All medium was removed from the cells, which were frozen and stored at –80°C for 1 h. Cells were thawed, lysed, and treated with a fluorescent dye according to manufacturer's instructions. Visual scoring was performed on all experiments before fluorescence assay. Data were observed as a percentage of confluence. MTT assay was also used to confirm cell viability after incubation with ligands. Details can be found in the online data supplement.

Western Blotting
Standard protocols were used to blot for PPARbeta in cells. Details can be found in the online supplement.

PPAR Reporter Gene Assay
HEK293 cells were seeded onto six-well plates in 10% DMEM. At 50% confluence, cells were transfected with the desired plasmids (Dre-1 [p4xDRE-luc] and pcDNA or Dre-1 and PPARbeta [pCMX-mPPARb/d] or PPAR{gamma} [pCMX-mPPARg]; 2 µg total). Plasmids were initially made up to 200 µl of serum-free DMEM. Transfection reagent (NovaFECTOR) (4 µl) was added, and the mixture was incubated at room temperature for 15 min to allow for binding of plasmid to transfection reagent. Just before transfection, cells were washed with PBS, and 800 µl of DMEM was added. The plasmid–reagent complex was added to each corresponding well dropwise and incubated for 3 h at 37°C. Medium was removed, and cells were washed with PBS, trypsinized, and transferred onto 96-well plates at 100% confluence in DMEM containing 10% serum. Cells were incubated overnight before the addition of drugs. Drugs were incubated overnight. The supernatant was removed, and 50 µl of reporter lysis buffer (Promega, Southampton, UK) was added to each well and incubated at room temperature for 15 min. Twenty-five microliters of this sample was used to measure protein (BCA), and 25 µl was used to measure reporter gene activation in a luciferase assay as previously described (12, 13).

Reagents
DMEM, BSA, and Mayer's hematoxylin stain were purchased from Sigma (Dorset, UK); all other cell culture materials and pcDNA were purchased from Invitrogen (Paisley, UK). CyQUANT cell proliferation assay kit was from Cambridge BioScience Ltd. (Cambridge, UK), and 15-deoxy-{Delta}12,14-prostaglandin J2 (15 d-PGJ2) was purchased from Biomol (Plymouth Meeting, PA). GW0742 and treprostinil sodium were kind gifts from Timothy M. Willson (GlaxoSmithKline, Research Triangle Park, NC) and Carl Sterritt (United Therapeutics Corp.), respectively. Rosiglitazone was purchased from Alexis (Nottingham, UK). Rabbit anti-PPARbeta/{delta} antibody was from Abcam (Cambridge, UK), and goat anti-rabbit antibody was from Dako (Glostrup, Denmark). ECL detection reagent and hyperfilm were from Amersham Biosciences (Buckinghamshire, UK). NovaFECTOR was from VennNova (Pompano Beach, FL). P4xDRE-luc reporter gene was a gift from Bert Vogulstein at Johns Hopkins University (Baltimore, MD), pCMX-mPPARbeta plasmid was a gift from Ron Evans at the Salk Institute (San Diego, CA), and pCMX-mPPAR{gamma} was a gift from Christopher Glass at the University of California (San Diego, CA).

Statistics
All results are represented in the text and figures as mean ± SEM. Statistical differences between groups of data were tested using appropriate tests, which are described in the relevant figure legends. A P value of < 0.05 was considered to be statistically significant.


   RESULTS
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Effects of Treprostinil Sodium, Traditional IP Ligands, and Traditional PPAR Ligands on Human Lung Fibroblast Proliferation
Treprostinil sodium inhibited, in a concentration-dependent manner, the proliferation of human lung fibroblasts induced by a submaximal concentration of FCS (3%). Inhibitory effects became statistically significant at concentrations of 10–7 M and higher (Figure 1). Similarly, the PPARbeta-specific ligand GW0742 inhibited human lung cell proliferation, with two clear concentration-related effects being noted. At low concentrations, GW0742 inhibited proliferation in a concentration-dependent manner, which was maximal at 10–6 M (25 ± 6% inhibition) (Figure 1). At high concentrations (10–4 M), GW0742 induced an additional and dramatic inhibition of cell proliferation. All results were confirmed by visual scoring of cells and by MTT assay, which showed a reduction in cell number and no effects of cell death (data not shown). Western blotting demonstrated that human lung fibroblasts contain PPARbeta (Figure 1). In addition to treprostinil sodium (Figure 1), cicaprost, iloprost, and carbaprostacyclin inhibited proliferation of human lung fibroblasts but at concentrations above those expected to activate IP receptors (10–4 M) (Figure 2). In addition to the PPARbeta ligand GW0742, the putative endogenous PPAR ligand 15 d-PGJ2 and the PPAR{gamma} ligand rosiglitazone inhibited human fibroblast cell proliferation (Figure 2). However, the PPAR{alpha} ligand WY-14643 or the mixed PPAR{alpha}/PPARbeta ligand bezafibrate had no inhibitory effects on cell proliferation (Figure 2).


Figure 1
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  		Figure 1. Importance of IP versus PPARbeta in human lung fibroblast proliferation. (A) Effect of treprostinil sodium and (B) GW0742 on human lung fibroblast proliferation. Proliferation is expressed as percentage of control response induced by 3% FCS. Data are means ± SEM; n = 9. *P < 0.05 by one sample t test. (C) Expression of PPARbeta protein in human lung fibroblasts using cells cultured from two separate donors.

 

Figure 2
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  		Figure 2. Effects of prostacyclin or PPAR ligands on proliferation of human lung fibroblasts. Each drug was used at 10–4 M. Proliferation is inhibition of control response induced by 3% FCS. Data are means ± SEM. n = 9 for treprostinil sodium, GW0742, and 15 d-PGJ2; n = 6 and for rosiglitazone and bezafibrate; and n = 3 determinations for all other mimetics.

 
Role of PPARbeta versus IP Receptors in the Normal Growth and Morphology of Murine Lung Fibroblasts
The genotypes of IP–/– and PPARbeta–/– mice were confirmed by PCR (Figure 3). Lung cells cultured from each type of mouse seemed to be similar in morphology to those from control wild-type (C57 BLK 6) mice, with a typical fibroblast phenotype (Figure 3). Lung fibroblasts cultured from control mice and from PPARbeta–/– mice proliferated in response to FCS similarly, whereas lung fibroblasts from IP–/– mice seemed to proliferate at approximately twice the rate of the control cells. As seen in human lung fibroblasts, the prostacyclin mimetics treprostinil sodium (Figure 4), cicaprost, or carbaprostacyclin (10–4 M; data not shown) inhibited proliferation of murine lung fibroblasts. The inhibitory effects of treprostinil sodium on proliferation were significantly reduced in cells derived from PPARbeta–/– animals (Figure 4). No significant differences were seen in the ability of treprostinil sodium to inhibit proliferation in cells from control versus IP–/– mice (Figure 4). The inhibitory effects of rosiglitazone on lung fibroblast proliferation were unaffected by PPARbeta (Figure 4B) or IP gene deletion (data not shown). Treprostinil sodium had no effect on cell viability at concentrations up to 10–4 M measured by MTT (data not shown).


Figure 3
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  		Figure 3. Phenotype and genotype characterization of murine lung fibroblasts cultured from control wild-type mice or from mice lacking IP or PPARbeta genes. (A) Morphology of murine lung fibroblasts. Cells were grown in a sterile 96-well plate, washed with PBS (without calcium and magnesium), and incubated at room temperature for 10 min with Mayer's hematoxylin stain. Original magnification: x10. (B) Proliferative responses of cells from each type of animal in response to FCS (0–20%). The amount of fluorescence detected is directly proportional to the amount of DNA in each sample. Data are means ± SEM; n = 6 experiments using cells grown from two animals for each cell type. (C) PCR genotyping of genetically modified mice. For genotyping of PPARbeta, PCR reactions were performed using primers allowing an amplification of a 500-bp DNA fragment of the wild-type PPARbeta/{delta} allele and no amplification of the mutant allele or primers allowing the amplification of a 650-bp DNA fragment of the mutant allele and no amplification of the wild-type allele. For genotyping of IP–/– and control wild-type mice, PCR reactions were performed from genomic DNA isolation from mice toes. PCR reactions were performed using primers for the wild-type allele, which result in a 425-bp product.

 

Figure 4
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  		Figure 4. Role of PPARbeta versus IP in the effects of treprostinil sodium on lung cell proliferation. Proliferation responses in cells from control or PPARbeta–/– mice (A) or IP–/– mice (B) were induced by 3% FCS and are expressed as percentage of control. Data are means ± SEM; n = 6 for each data point. *P < 0.05 by two-way ANOVA. Effects of treprostinil sodium (TS) on PPARbeta (C) or PPAR{gamma} (D) Luciferase reporter gene activity in HEK293 cells. Data show fold activation normalized to control relative light units. Data are means ± SEM; n = 3–8.

 
Effects of Treprostinil Sodium of PPARbeta/{delta} versus PPAR{gamma} Receptor Activity
Plasmids for PPARbeta or PPAR{gamma} could not be successfully transfected into primary cultures of pulmonary fibroblasts (human or murine). We therefore performed reporter gene assays in HEK293 cells. Treprostinil sodium (Figure 4) or GW0742 (data not shown) but not rosiglitazone (data not shown) increased PPARbeta in a reporter gene assay. By contrast, treprostinil sodium had no effect on PPAR{gamma} activation (Figure 4).


   DISCUSSION
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
The treatment of pulmonary hypertension is aimed at two pathologies: (1) the vasoconstriction and (2) the profound remodeling that characteristically mediates lung vessel narrowing. Prostacyclin therapy can reduce vasoconstriction in pulmonary hypertension, but its key mode of action is on reducing vascular remodeling. Lung fibroblasts, which infiltrate and proliferate within the blood vessel (14–16), are important target cells for antiproliferative treatments used for pulmonary hypertension. Prostacyclin is thought to relieve some of the symptoms of pulmonary hypertension via an action on cell-surface IP receptors. However, some prostacyclin mimetics are also ligands for the nuclear receptor PPARbeta. In the current study, we demonstrate, for the first time, that PPARbeta is expressed in human lung fibroblasts and that its activation leads to reduced proliferation. Using cells from genetically modified animals, we show that some of the antiproliferative effects of treprostinil sodium, which was recently approved for the treatment of pulmonary hypertension, are via PPARbeta.

We found that the proliferation of human lung fibroblasts was inhibited by a number of prostacyclin mimetics, including the newly introduced treprostinil sodium. In contrast to observations using vascular smooth muscle cells (17), the inhibitory effects of treprostinil sodium (and other mimetics) were seen only at concentrations above those associated with activation of IP receptors (i.e., > 10–7 M) but within the range observed for prostacyclin mimetics on PPARbeta receptors (i.e., > 10–6 M) (18). These data suggest that although IP receptors may mediate the vasodilator properties of prostacyclin and its mimetics, it is not the receptor target that mediates inhibition of fibroblasts growth. We found that human lung fibroblasts expressed detectable levels of PPARbeta and that proliferation of these cells was inhibited by the highly selective PPARbeta ligand GW0742. Proliferation of these cells was also inhibited by PPAR{gamma}, but not PPAR{alpha}, ligands. These observations suggest that treprostinil sodium, like some other prostacyclin mimetics, could be a ligand for PPARbeta. This was confirmed in the current study using our standard reporter gene assay (12). By contrast, using similar reporter gene assays, we show that treprostinil sodium did not activate PPAR{gamma}. Because PPAR{alpha} ligands did not limit proliferation in these cells, it seems clear that, in the micromolar range, treprostinil sodium inhibits lung fibroblast proliferation by an action on PPARbeta.

There are no selective antagonists of PPARbeta or IP receptors. Therefore, we used lung fibroblasts cultured from mice deficient in either receptor to further identify the role of each in proliferation. As seen with human cells, proliferative responses in murine lung fibroblasts cultured from control animals were inhibited by treprostinil sodium and other IP receptor ligands. Similarly, the inhibitory effects were seen only at high concentrations, suggesting that IP receptors were not involved in the effects of treprostinil sodium on lung cell proliferation. This hypothesis was substantiated by our findings that the inhibitory effects of treprostinil sodium were unaffected by IP receptor deletion. However, the inhibitory effects of treprostinil sodium were reduced in cells derived from PPARbeta–/– animals. These observations strongly suggest that the prostacyclin mimetics can inhibit lung fibroblast proliferation independently of IP receptors and via activation of PPARbeta, in accordance with previous observations using other cell types (17, 19). Deletion of the IP receptor resulted in an increased amount of proliferation in the lung fibroblasts compared with wild-type cells, indicating that the IP receptor may serve a role in controlling the proliferation of lung fibroblasts. However, the increased rate of IP–/– cells was not deterred in the presence of indomethacin, which blocks prostacyclin production via an action on cyclo-oxygenase-1 or cyclo-oxygenase-2 (20). This suggests that the increased proliferation is not dependent on an interaction of the receptor with prostacyclin per se, but rather is due to some intrinsic activity or to an unrelated phenotypic difference in these cells.

Prostacyclin and related therapy is the gold standard for the treatment of pulmonary hypertension. Many of the benefits of prostacyclin in this disease are associated with the arrest and reversal of the profound remodeling that occurs in the pulmonary vascular wall. The observations presented in this article suggest that prostacyclin mimetics limit lung fibroblast proliferation via an action of PPARbeta receptors. However, the therapeutic concentrations of these drugs in the treatment of pulmonary hypertension are in the nanomolar (IP receptor) range. The effects of prostacyclin mimetics on blood pressure (via IP receptor pathways) limits the amount of drug that can be delivered safely. Prostacyclin therapy has another important limitation: It must be give by injection or infusion. The data presented in this article suggest that some of the effects of prostacyclin-like therapies may occur independently of the classical IP receptor and at the level of PPARbeta activation. These observations suggest that PPARbeta may be a novel therapeutic target for the treatment of pulmonary hypertension, which is important because orally active agonists of PPARbeta have already been synthesized and are in development for the treatment of dyslipidemia.


   Acknowledgments
 
The authors thank Jessica Wray for her assistance with the reporter gene assays.


   Footnotes
 
This work was funded by the Research Advisory Board of St. Bartholomew's and the Royal London Charitable Foundation, and by the British Heart Foundation.

This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org.

Conflict of Interest Statement: F.Y.A. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. K.E. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. G.A.F. has received $6,000 from Merck, $13,000 from GlaxoSmithKline, $15,000 from Nicox, $7,000 from Altana, $5,000 from Fujisawa, and $6,500 from CV Therapeutics over the past two years. He serves on the advisory boards for Arcoxia and the MEDAL study, for which he has received $11,000. B.D. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. W.W. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. D.B.-B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. T.D.W. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. J.A.M. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

Originally Published in Press as DOI: 10.1165/rcmb.2005-0289OC on October 20, 2005

Received in original form July 26, 2005

Accepted in final form September 19, 2005


   References
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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  3. Lim H, Dey SK. A novel pathway of prostacyclin signaling-hanging out with nuclear receptors. Endocrinology 2002;143:3207–3210.[Abstract/Free Full Text]
  4. Wort SJ, Mitchell JA, Woods M, Evans TW, Warner TD. The prostacyclin-mimetic cicaprost inhibits endogenous endothelin-1 release from human pulmonary artery smooth muscle cells. J Cardiovasc Pharmacol 2000;36(Suppl 1)S410–S413.
  5. Berger J, Moller DE. The mechanisms of action of PPARs. Annu Rev Med 2002;53:409–435.[CrossRef][Medline]
  6. Desvergne B, Wahli W. Peroxisome proliferator-activated receptors: nuclear control of metabolism. Endocr Rev 1999;20:649–688.[Abstract/Free Full Text]
  7. Wahli W. Peroxisome proliferator-activated receptors (PPARs): from metabolic control to epidermal wound healing. Swiss Med Wkly 2002;132:83–91.[Medline]
  8. Lee C-H, Chawla A, Urbiztondo N, Liao D, Boisvert WA, Evans RM. transcriptional repression of atherogenic inflammation: modulation by PPARdelta. Science 2003;302:453–457.[Abstract/Free Full Text]
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