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A Widely Used Retinoic Acid Receptor Antagonist Induces Peroxisome Proliferator-Activated Receptor- Activity

Division of Endocrinology, Diabetes, and Metabolism, Department of Medicine and Department of Genetics, and the Institute for Diabetes, Obesity, and Metabolism, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania (M.S., J.C.C., R.J.K., A.N.B., M.A.L); Division of Endocrinology, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania (R.J.K.); and Department of High-Throughput Biology, GlaxoSmithKline, Research Triangle Park, North Carolina (A.N.B.)

Received December 19, 2006; accepted February 8, 2007

	Abstract

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Nuclear receptors (NRs) are transcription factors whose activity is regulated by the binding of small lipophilic ligands, including hormones, vitamins, and metabolites. Pharmacological NR ligands serve as important therapeutic agents; for example, all-trans retinoic acid, an activating ligand for retinoic acid receptor (RAR), is used to treat leukemia. Another RAR ligand, (E)-S,S-dioxide-4-(2-(7-(heptyloxy)-3,4-dihydro-4,4-dimethyl-2H-1-benzothiopyran-6-yl)-1-propenyl)-benzoic acid (Ro 41-5253), is a potent antagonist that has been a useful and purportedly specific probe of RAR function. Here, we report that Ro 41-5253 also activates the peroxisome proliferator-activated receptor (PPAR), a master regulator of adipocyte differentiation and target of widely prescribed antidiabetic thiazolidinediones (TZDs). Ro 41-5253 enhanced differentiation of mouse and human preadipocytes and activated PPAR target genes in mature adipocytes. Like the TZDs, Ro 41-5253 also down-regulated PPAR protein expression in adipocytes. In addition, Ro 41-5253 activated the PPAR-ligand binding domain in transiently transfected HEK293T cells. These effects were not prevented by a potent RAR agonist or by depleting cells of RAR, indicating that PPAR activation was not related to RAR antagonism. Indeed, Ro 41-5253 was able to compete with TZD ligands for binding to PPAR, suggesting that Ro 41-5253 directly affects PPAR activity. These results vividly demonstrate that pharmacological NR ligands may have "off-target" effects on other NRs. Ro 41-5253 is a PPAR agonist as well as an RAR antagonist whose pleiotropic effects on NRs may signify a unique spectrum of biological responses.

To better understand the effects of atRA on adipocyte differentiation, we used Ro 41-5253; originally synthesized by Hoffman LaRoche, it is a specific antagonist for RAR with little affinity for RAR and RAR (Apfel et al., 1992). This compound has been widely used to dissect the role of RAR in atRA-dependent biological processes (Shang et al., 1999; Emionite et al., 2003; Higuchi et al., 2003; Engedal et al., 2004; Lu et al., 2005). We were surprised to find that Ro 41-5253 stimulated the adipogenic differentiation of mouse 3T3-L1 preadipocytes as well as a human preadipocyte cell line. This function of Ro 41-5253 is RAR-independent. In exploring potential RAR-independent mechanisms, we discovered that Ro 41-5253 is, unexpectedly, an agonist ligand for PPAR. This finding suggests, among other things, that biological studies that employed this eccentrically pleiotropic ligand may require re-interpretation.

	Materials and Methods

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Cell Culture and Differentiation. Reagents were obtained from Invitrogen (Carlsbad, CA) unless otherwise noted. 293T and murine 3T3-L1 cells were maintained in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum (U.S. Bio-Technologies Inc., Parkerford, PA), 100 U/ml penicillin, and 100 µg/ml streptomycin. Cells were grown to confluence and induced to differentiate 2 days after confluence with media containing 0.4 µM dexamethasone, 3 µg/ml bovine insulin, and 0.25 mM 3-isobutyl-1-methylxanthine (IBMX; all from Sigma, St. Louis, MO) for 2 days and for additional 2 days in insulin only. In differentiation studies, IBMX was replaced by either pioglitazone or Ro 41-5253 for the first 4 days. Culturing and differentiation of human preadipocytes from the Simpson Golabi Behmel Syndrome (SGBS) were described elsewhere (Kim et al., 2006), and ligands were present for the first 7 days of differentiation. Oil Red-O staining was performed as described previously (Li and Lazar, 2002).

Transfections and Luciferase Assay. 3T3-L1 adipocytes and 293T cells were transfected by electroporation (Nucleofector II; Amaxa Biosystems, Gaithersburg, MD). Adipocytes were detached from culture dishes with 0.25% trypsin and 0.5 mg of collagenase/ml in phosphate-buffered saline, washed twice, resuspended in electroporation buffer (solution V; Amaxa Biosystems), mixed with 2 µgof pGl3–3xAOxPPRE plasmid, electroporated, seeded into 12-well plates, and incubated for 24 h with compounds as indicated. 293T cells were electroporated with 2 nmol of nontargeting or human smart-pool RAR oligonucleotides (Dharmacon, Lafayette, CO) and seeded into 24-well plates and used for transactivation assays 24 h later. pGal4-hPPAR-LBD, pGal4-hPPAR/-LBD, and pGal5-TK-pGL3 were transfected in 293T cells using Lipofectamine 2000 (Invitrogen) according to the manufacturer's protocol and incubated for 24h with compounds as indicated. All transfection were normalized to cotransfected pRL-CMV and measured using the dualluciferase reporter assay (Promega, Madison, WI).

Immunoblot Analysis and Antibodies. Protein were isolated and separated in 4 to 20% SDS polyacrylamide gels (Invitrogen) and transferred to polyvinylidene difluoride membrane (Invitrogen). After incubation with the primary antibodies for PPAR (Santa Cruz Biotechnology, Santa Cruz, CA), RAR (Santa Cruz Biotechnology) or the ubiquitously expressed GTPase RAN (BD Biosciences, San Jose, CA), a secondary horseradish-conjugated antibody (Invitrogen) was added, and an enhanced chemiluminescent substrate kit (Amersham, Chalfont St. Giles, UK) was used for detection.

Quantitative Polymerase Chain Reaction. RNA was purified with the RNeasy Mini Kit (QIAGEN GmbH, Hilden, Germany). cDNA was generated using the Sprint Powerscript System (Clontech, Mountain View, CA). Primers and probes for adipose protein 2 (aP2), PPAR₂, and 36B4 for normalization have been described elsewhere (Chui et al., 2005; Schupp et al., 2005). All PCR reactions were carried out using Taqman Universal Polymerase Master Mix (Applied Biosystems, Foster City, CA) and the PRISM 7900 instrument (Applied Biosystems) and were evaluated according to the standard curve method.

Scintillation Proximity Assay for the PPAR-Ligand Binding Domains. The measurement of ligand displacement was performed as described previously (Nichols et al., 1998). The radioligands were [³H]rosiglitazone for PPAR and [³H]GW2433 for PPAR and PPAR/ (Xu et al., 1999).

Statistical Analysis. Representative results of at least three independent experiments are shown. All results are expressed as mean ± S.D. of triplicates. Statistical significance was determined using either the 2-tailed Student's t test or ANOVA, as appropriate, and P < 0.05 was deemed significant.

	Results

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Ro 41-5253 Was an Inducer of Adipocyte Differentiation. Ro 41-5253 was used previously in 3T3-L1 cells to block the inhibitory effect of atRA on differentiation (Kamei et al., 1994). We tested the possibility that this compound has enhancing effects on differentiation itself. 3T3-L1 and human SGBS preadipocytes were therefore induced to differentiate into fat cells by exposing them to the hormonal inducers. Using a differentiation mix devoid of IBMX diminished the grade of adipocyte differentiation (Hamm et al., 2001) (Fig. 1A). The presence of pioglitazone or Ro 41-5253 could rescue the ability of differentiation as shown in increased Oil Red O staining, PPAR₂ mRNA expression in 3T3-L1 cells (Fig. 1A), and PPAR protein expression in human SGBS cells (Fig. 1B). However, pioglitazone was more efficient than Ro 41-5253 in promoting the adipocyte phenotype (Fig. 1, A and B).

View larger version (27K): [in this window] [in a new window]   Fig. 1. Ro 41-5253 induces differentiation of mouse and human preadipocytes. 3T3-L1 preadipocytes were treated with the full differentiation mix (Mix) or Mix without IBMX and supplemented with either 500 nM pioglitazone, or 500 nM Ro 41-5253. Differentiation was evaluated by phase contrast microscopy, Oil Red O staining and PPAR2 expression at day 8 after initiation of differentiation (A). Human SGBS preadipocytes were treated for 14 days with Mix without IBMX and supplemented with 500 nM ligands for the first 7 days. Protein levels of PPAR were determined and cells assessed by phase contrast microscopy. *, p < 0.05; **, p < 0.01.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Ro 41-5253 increases PPAR target gene expression in 3T3-L1 adipocytes and down-regulates PPAR protein. Day 8 adipocytes were incubated for 24 h with vehicle, 500 nM pioglitazone, or 500 nM Ro 41-5253. mRNA expression of aP2 (as -fold induction over vehicle treatment) and PPAR protein expression were measured (A and B). *, p < 0.05.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Ro 41-5253 increases endogenous PPAR activity in adipocytes. 3T3-L1 adipocytes were electroporated with either empty vector (pGL3) or a PPAR-response element containing reporter (pGL3–3xAOx-PPRE) and incubated for 24h with vehicle and 500 nM pioglitazone or 500 nM Ro 41-5253 and assayed for luciferase activity. Data represent the -fold induction of luciferase activity over vehicle treatment of the PPRE-containing plasmid. *, p < 0.05; **, p < 0.01.

The RAR antagonist potently activated the PPAR-LBD but with much less efficiency than pioglitazone (Fig. 4A). Although the concentrations necessary for half-maximal activation (EC₅₀) for both ligands are in the same range, the maximal activation of Ro 41-5253 over vehicle-treated cells was less than 30% of the activation of pioglitazone (Table 1). The full activation of the PPAR-LBD induced by 1 µM pioglitazone was consistently attenuated by cotreatment with increasing concentrations of Ro 41-5253 (Fig. 4C). This strongly suggests that pioglitazone and Ro 41-5253 both act via the same PPAR activating mechanism. On the contrary, Ro 41-5253 could not activate the PPAR/-LBD (Fig. 4B). We noticed a slight decrease of the basal PPAR/-LBD activity with high concentrations of Ro 41-5253. Accordingly, there was a reduction in the full PPAR/-LBD activation induced by 20 nM GW 610742 by cotreatment with 10 µMRo 41-5253 to 53% (data not shown).

View larger version (11K): [in this window] [in a new window]   Fig. 4. Ro 41-5253 partially activates the PPAR-LBD but not the PPAR/-LBD and competes with TZD for activation. 293T cells were transiently transfected with pGal4-hPPAR_DEF or pGAL-hPPAR/_DEF and the pGal5-Tk-pGL3 reporter followed by stimulation with pioglitazone or GW 610742 (black squares in A and B) or Ro 41-5253 (red circles) as indicated for 24 h. Data represent the -fold induction of luciferase activity over vehicle treatment for activation of PPAR (A) or PPAR/ (B). Transfected cells were incubated with 1 µM pioglitazone and increasing concentrations of Ro 41-5253 for 24 h and assayed for luciferase activity (C).

View this table: [in this window] [in a new window]   TABLE 1 EC50 and maximal activation of the human PPAR-LBD in transient transfections Dose response curves from the transiently transfected 293T cells with the pGal4-hPPAR_DEF and pGal5-Tk-pGL3 reporter (Fig. 4) were used to calculate values for the EC50. The maximal activation represents the maximal relative activation of Ro 41-5253 in comparison to pioglitazone (=100%).

Ro 41-5253 Activated the PPAR-LBD Independent of RAR. We next addressed the question of whether RAR was involved in the specific PPAR-activating property of Ro 41-5253 using a pharmacological approach. 293T cells were transfected with the Gal4-RAR-LBD and the corresponding reporter and titrated for activating/repressing concentrations of Ro 41-5253, and the RAR agonist AM-580. The strong repression elicited by 500 nM Ro 41-5253 was completely abolished by coincubation with 5 µM AM-580 (Fig. 5A, compare the Ro 41-5253 and the Ro 41-5253 + AM-580 repression). The same concentration of the RAR agonist AM-580 had little effect of the pioglitazone—as well as Ro 41-5253—induced activation of the PPAR-LBD (Fig. 5B), showing that RAR antagonism is not necessary for the activation of the PPAR-LBD by Ro 41-5253.

View larger version (16K): [in this window] [in a new window]   Fig. 5. Ro 41-5253 activates the PPAR-LBD in presence of the RAR agonist AM-580. 293T cells were transiently transfected with the pGal4-hRAR_DEF (A) or pGal4-hPPAR_DEF (B) and pGal5-Tk-pGL3 reporter. Cells were treated with 500 nM Ro 41-5253, 5 µM AM-580 (a RAR agonist), or both for 24 h. Data represent the -fold repression over vehicle-treated cells (A). Activation the PPAR-LBD was measured after 24-h incubation with 0.03, 0.3, and 3 µM concentrations of either pioglitazone or Ro 41-5253 in the presence of vehicle or 5 µM AM-580. Data represent the -fold induction over vehicle treated cells (B). **, p < 0.01.

To provide further evidence for the RAR-independent mechanism, we depleted RAR in 293T cells (Fig. 6A, compare protein levels of cells electroporated with either siControl or siRAR). This depletion did not prevent the activation of the PPAR-LBD by Ro 41-5253, proving that RAR is not involved in the PPAR activation (Fig. 5B). On the contrary, it slightly increased the efficiency by which Ro 41-5253 activated the PPAR-LBD.

View larger version (22K): [in this window] [in a new window]   Fig. 6. Ro 41-5253 activates the PPAR-LBD in cells depleted of RAR. Reduction of RAR protein levels by electroporating siRNA control or siRNA RAR in 293T cells (A). After 24 h, cells were transfected with pGal4-hPPAR_DEF and pGal5-Tk-pGL3 and incubated for another 24 h with 3 µM Ro 41-5253. Data are shown as -fold induction over vehicle-treated cells (B). *, p < 0.05.

Ro 41-5253 Competes with Specific Ligands for Direct Binding to PPAR. Because RAR was nonrelevant for the PPAR activation, we questioned whether Ro 41–5253 directly interacts with the PPAR protein. We therefore measured the competition with specific radiolabeled ligands to human PPAR-LBDs by a scintillation proximity assay and calculated the concentrations for half-maximal displacement. Ro 41-5253 was able to bind to PPAR and PPAR/ with IC₅₀ values in the low micromolar range (Table 2).

	Discussion

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In this study, we have shown that Ro 41-5253 is a strong inducer of adipogenesis in mouse and human preadipocytes. We provide evidence that Ro 41-5253 can bind and activate PPAR, the master regulator of adipogenesis. On the other hand, Ro 41-5253 could bind PPAR/ but was not able to activate the PPAR/-LBD Gal4 fusion protein. Using pharmacological and biochemical interventions, we can exclude the involvement of RAR in the PPAR activating property of Ro 41-5253. Thus, Ro 41-5253 is not only an antagonist for RAR but also an agonist for PPAR.

Although the potency of Ro 41-5253 was comparable with pioglitazone (Table 1), its efficiency of promoting lipid accumulation, target gene expression, and activation of the PPAR-LBD was consistently lower than by using pioglitazone, a full agonist (Figs. 2A and 4A). Although Ro 41-5253 seems to bind the PPAR-LBD in a manner similar to that of TZDs, TZD-induced activation could be diminished with increasing concentrations of Ro 41-5253, which is the traditional definition of partial agonist behavior. The exact mechanism for this partial agonism of PPAR by Ro 41-5253 may involve selective or reduced interaction with NR coactivators as has been shown for other partial agonists, such as N-(9-Fluorenylmethoxycarbonyl)-L-leucine (Rocchi et al., 2001), MCC-555 (Reginato et al., 1998) and certain angiotensin receptor blockers (Schupp et al., 2005).

No known endogenous ligand for NRs has both RAR-antagonizing and PPAR-activating properties. However, it is intriguing to think of the existence of endogenous pleiotropic ligands, considering the functional antagonism of RAR and PPAR for instance during adipogenesis (Chawla and Lazar, 1994; Xue et al., 1996). Furthermore, Ro 41-5253, chemically derived from atRA, has no obvious similarity with any synthetic PPAR activator. On the other hand, it shares structural elements with arachidonic metabolites, such as prostaglandin J₂, that have been shown to activate PPAR (Forman et al., 1995; Kliewer et al., 1995; Yu et al., 1995).

Finally, the unexpectedly pleiotropic effects of Ro 41-5253 that we have uncovered indicate that the caution must be applied to the interpretation of effects elicited by Ro 41-5253 and previously attributed to specific RAR antagonism. This caution pertains not only to studies of adipogenesis (Kamei et al., 1994) but also to many other cell types. For example, several studies have used Ro 41-5253 as an ostensibly specific probe of RAR function in breast cancer (Shang et al., 1999; Schneider et al., 2000; Emionite et al., 2004; Lu et al., 2005; Toma et al., 2005), where PPAR is expressed at significant levels, not only in cell lines such as MCF-7, MDA-MB-231. or ZR-75.1 (Kilgore et al., 1997; Nwankwo and Robbins, 2001; James et al., 2003) but also in primary and metastatic breast adenocarcinomas (Kilgore et al., 1997; Mueller et al., 1998). Ro 41-5253 was able to bind / in the low micromolar range, which adds to the concerns about overinterpretation of its effects as RAR-specific in biological systems. Unfortunately, to our knowledge, no other RAR antagonist is commercially available.

	Acknowledgements

 
We thank Dr. M. Wabitsch (Pediatric Endocrinology, University of Ulm, Germany) for the SGBS preadipocyte cell line.

	Footnotes

 
This work was supported by National Institutes of Health grant R01-DK49780 (to M.A.L). M.S. was supported by Mentored Fellowship award from the American Diabetes Association, and J.C.C was supported by National Institute of Diabetes and Digestive and Kidney Diseases training grants 5-F32-DK070405-03 and 5-R01-DK045780-12.

Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.

doi:10.1124/mol.106.033662.

ABBREVIATIONS: PPAR, peroxisome proliferator-activated receptor; RAR, retinoic acid receptor; TZD, thiazolidinedione; RXR, retinoid X receptor; NR, nuclear receptors; atRA, all-trans retinoid acid; Ro 41-5253, (E)-S,S-dioxide-4-(2-(7-(heptyloxy)-3,4-dihydro-4,4-dimethyl-2H-1-benzothiopyran-6-yl)-1-propenyl)-benzoic acid; IBMX, 3-isobutyl-1-methylxanthine; SGBS, Simpson Golabi Behmel Syndrome; LBD, ligand binding domain; aP2, adipose protein 2; GW2433, 2-(4-{3-[1-[2-(2-chloro-6-fluoro-phenyl)-ethyl]-3-(2,3-dichloro-phenyl)-ureido]-propyl}-phenoxy)-2-methyl-propionic acid; PPRE, peroxisome proliferator-activated receptor response element; AM-580, 4-[(5,5,8,8-tetramethyl6,7-dihydronaphthalene-2-carbonyl)amino]benzoic acid.

Address correspondence to: Dr. Mitchell A. Lazar, Division of Endocrinology, Diabetes, and Metabolism, University of Pennsylvania School of Medicine, 611 CRB, 415 Ci Blvd., Philadelphia, PA 19104-6149. E-mail: lazar{at}mail.med.upenn.edu

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