Troglitazone Improves Psoriasis and Normalizes Models of Proliferative

STUDY Troglitazone Improves Psoriasis and Normalizes Models of Proliferative Skin Disease Ligands for Peroxisome Proliferator-Activated Receptor-␥ Inhibit Keratinocyte Proliferation

Charles N. Ellis, MD; James Varani, PhD; Gary J. Fisher, PhD; Mary E. Zeigler, PhD; Harrihar A. Pershadsingh, MD, PhD; Stephen C. Benson, PhD; Yiqing Chi, MD; Theodore W. Kurtz, MD

Background: Psoriasis is often treated with agents that Interventions: Oral troglitazone therapy at various dos- activate nuclear hormone receptors for glucocorticoids, ages in patients with psoriasis; also, use of troglitazone, retinoids, and vitamin D. The peroxisome proliferator- ciglitazone, and 15-deoxy-⌬-12,14-prostaglandin J2 in pso- activated receptor-␥ (PPAR␥) is a related nuclear hor- riasis models. mone receptor that can be activated by its ligands, including the thiazolidinediones. Main Outcome Measures: Investigator-determined clinical results in patients and cell counts and histologi- Objective: To assess whether treatment with tro- cal evidence in models. glitazone, a currently available thiazolidinedione used to treat diabetes mellitus, has an effect on psoriasis in nor- Results: All patients’ psoriasis improved substantially moglycemic patients and whether ligands for PPAR␥ have during troglitazone therapy. Peroxisome proliferator-acti- an effect on models of psoriasis. vated receptor-␥ was expressed in human keratinocytes; ligands for PPAR␥ inhibited the proliferation of normal Design: Open-label administration of troglitazone in pa- and psoriatic human keratinocytes in culture. Troglit- tients with psoriasis and evaluation of drug actions in cel- azone treatment normalized the histological features of lular, organ, and transplant models of psoriasis. psoriatic skin in organ culture and reduced the epidermal hyperplasia of psoriasis in the severe combined immunodeficient mouse and human skin transplant model Setting: University and community hospital outpa- of psoriasis (PϽ.05 compared with untreated controls). tient departments and university laboratories. Conclusions: Peroxisome proliferator-activated recep- Patients: Patients with chronic, stable plaque psoriasis tor-␥ might be a useful intracellular target for the treat- and control subjects. Five patients with psoriasis ment of psoriasis; further study is needed to assess the clini- received troglitazone (none withdrew); 10 different cal value of ligands for PPAR␥, including troglitazone. untreated patients and 10 controls provided tissue samples. Arch Dermatol. 2000;136:609-616

SORIASIS IS a common and of- andvitaminDreceptors)arecommonlyused ten debilitating skin disor- as antipsoriatic agents, each of these treat- der that affects 1% to 2% of ments has limitations, and new therapeutic the US population.1 Disor- options in psoriasis are needed.4 dered differentiation and hy- The thiazolidinediones are a novel Pperproliferation of keratinocytes with in- class of insulin-sensitizing agents used for flammation are the hallmarks of psoriasis; the treatment of type 2 diabetes melli- however, the pathophysiological features tus.6,7 Thiazolidinediones act as ligands for of this disorder are complex and its etiol- the peroxisome proliferator-activated ogy is unknown.2,3 A variety of therapies receptor-␥ (PPAR␥), a member of the nu- are available for psoriasis.4 Nevertheless, clear hormone receptor superfamily that because of problems with adverse effects includes the retinoic acid receptor and the and variability in clinical response, there vitamin D receptor. Ligand activation of is a desire for new treatments. PPAR␥ by thiazolidinediones can inhibit Nuclearhormonereceptorshaveproved proliferation and promote differentiation The affiliations of the authors to be useful targets in the development of an- in a variety of malignant and nonmalig- appear in the acknowledgment tipsoriatic drugs.5 Although ligands for such nant tissues,8-13 although the effects on skin section at the end of the article. receptors(includingglucocorticoid,retinoid, are unknown.

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©2000 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/30/2021 PATIENTS, MATERIALS, (downstream).These primers amplify a 277–base pair fragment from the N-terminal region of PPAR␥2 (Genbank ac- AND METHODS cession number U63415). The PCR products were ana- lyzed by 2.5% agarose gel electrophoresis. PATIENTS Immunoblotting was performed after sodium dodecyl sulfate–polyacrylamide gel electrophoresis separation of Five patients with moderate to severe psoriasis provided keratinocyte protein extracts using a rabbit anti-PPAR␥ an- informed consent for the new use of troglitazone for the tibody and an enhanced chemiluminescence detection treatment of psoriasis. Ten different untreated patients with system (Amersham Corp, Arlington Heights, Ill). Immu- psoriasis and 10 control subjects provided skin samples af- nofluorescence cytochemical analysis was performed on ter giving informed consent. The University of Michigan methanol-fixed and permeabilized keratinocytes using Health System institutional review board approved the pro- rabbit anti-PPAR␥ antibody and fluorescein isothiocya- cedures. We used skin from any individual in only one of nate–labeled goat antirabbit IgG. the studies described herein. CELL CULTURE AND PROLIFERATION ASSAYS ANALYSIS OF PPAR␥ mRNA AND PROTEIN Studies were performed with keratinocytes (obtained from Human epidermal keratinocytes (Clonetics Inc, San Diego, Clonetics Inc or prepared from biopsy samples of skin of 8 Calif) were grown in monolayer cultures.14 Peroxisome pro- controls and lesional skin of 5 patients with psoriasis) main- liferator-activated receptor-␥ messenger RNA (mRNA) was tained in either keratinocyte basal medium (KBM) or kerat- detected in human keratinocytes by reverse transcriptase– inocyte growth medium (HyClone Laboratories, Logan, polymerase chain reaction (RT-PCR) analysis; PPAR␥ pro- Utah). Each is a low Ca++ (0.15 mmol/L) modification of tein was detected by immunoblotting and immunocyto- MCDB-153 medium; KBM contains no serum or exog- chemical studies. For RT-PCR analysis, RNA was prepared enous growth factors, and keratinocyte growth medium is from quiescent human keratinocytes and reverse tran- supplemented with several growth factors, including hu- scribed to generate complementary DNA, which was am- man recombinant epidermal growth factor (0.1 ng/mL), in- plified by PCR to detect message for PPAR␥1 and PPAR␥2. sulin (2.5 µg/mL), and pituitary extract (2% vol/vol). For pro- The primers used for PCR amplification of PPAR␥1 were liferation assays, cells were plated in 24-well dishes at 2ϫ104 5-ЈCTC GAG GAC ACC GGA GAG-3Ј (upstream) and cells per well. Twenty-four hours after attachment, trogl- 5Ј-GTC ATT TCT GCG GCC ACG-3Ј (downstream). These itazone (Parke-Davis Pharmaceuticals, Ann Arbor, Mich), primers specifically amplify a 50–base pair fragment of the ciglitazone (Upjohn Co, Kalamazoo, Mich), 15-deoxy- 5Ј untranslated region of PPAR␥1 (Genbank accession ⌬-12,14-prostaglandin J2 (Cayman Chemical Co, Ann Ar- number X90563). The primers used to amplify PPAR␥2 bor), or dimethyl sulfoxide vehicle was added at indicated were 5Ј-GGT GAA ACT CTG GGA GAT TCT-3Ј (up- concentrations. Control and treated keratinocytes were in- stream) and 5Ј-TGT AAT CTG CAA CCA CTG GAT-3Ј cubated for 3 to 8 days and then counted. Cell numbers were

In view of the antiproliferative effects of ligands for Patient 2 PPAR␥, we gave troglitazone, a thiazolidinedione mar- keted in the United States, orally to patients with pso- A 42-year-old nondiabetic man had chronic lesions of psoriasis. With this description of 2 normoglycemic pa- riasis over 30% of his body surface. Five months before tients, we now have treated 5 patients with troglitazone presentation he was unresponsive to topical and sys- in an open fashion. We also report our investigations into temic treatment with glucocorticoids. Oral administra- the effects of ligands for PPAR␥ in experimental cellular tion of troglitazone, 200 mg/d for 2 weeks followed by and animal models of psoriasis. 400 mg/d for 10 weeks, resulted in remission of psoriasis (Figure 2). Troglitazone use was continued for 12 RESULTS additional weeks at 200 mg/d. Eight weeks after stop- ping therapy his psoriasis was still in remission. There PATIENT REPORTS were no adverse effects.

Patient 1 Patients 3-5

A 37-year-old nondiabetic man who had chronic, general- Patients with psoriasis and concomitant diabetes melli- ized plaque psoriasis since age 14 years presented with 80% tus had improvement of both conditions under therapy of his body surface involved with psoriasis (Figure 1). He with troglitazone, 400 to 600 mg/d; these patients have begantakingoraltroglitazone(400mgoncedailyfor5weeks been described previously.20 and 600 mg daily thereafter) without any other form of psoriasis therapy. After 5 weeks of troglitazone therapy, the pa- PPAR␥ IS EXPRESSED IN HUMAN tient showed a marked improvement in all lesions, and by KERATINOCYTES 12 weeks his psoriasis was in nearly complete remission, with involvement of less than 5% of his body surface. This Immunofluorescence analysis of human keratinocytes response has now been sustained for longer than 5 months treated with anti-PPAR␥ antibody revealed an intense sig- with therapy (Figure 1). There were no adverse effects. nal in the nucleus and perinuclear region consistent with

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©2000 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/30/2021 determined in 6 replicates of each experimental condition obtained and divided into portions. One portion from each using the MTS assay 3-(4,5-dimethylthiazol-2-yl)-5-(3- patient was reserved; the remaining tissue was divided and carboxymethoxyphenyl)-2-(4-sulfophenyl)-H-tetrazolium as transplanted onto CB17 SCID mice as previously described.19 described in the CellTiter 96 Aqueous Non-Radioactive Cell Each SCID mouse received 2 pieces of psoriatic skin from Proliferation Assay (Promega Corp, Madison, Wis) or the one of the human patients. The transplanted tissue was al- neutral red dye assay. The effect of thiazolidinediones on the lowed to heal for 4 weeks, after which 11 animals (with 21 growth of human dermal fibroblasts in KBM supplemented pieces of psoriasis; 1 piece did not survive) were fed once with calcium, 1.4 mmol/L, was assessed using fibroblasts pre- daily for 6 weeks with 200 µg of troglitazone dissolved in pared from biopsy specimens of skin of 5 controls. 300 µL of sugar-free gelatin dessert; 3 animals (with 6 pieces of psoriasis) fed plain gelatin dessert served as controls. At STUDIES IN ORGAN CULTURED HUMAN SKIN the end of treatment, the human skin pieces were removed, fixed in 10% buffered formalin, embedded in paraffin, sec- Under serum-free conditions, normal and psoriatic human tioned, and stained with hematoxylin-eosin. Under light mi- skin can be maintained in organ culture for days and abnor- croscopy, we traced the epidermal boundaries and determined mal histological features of psoriasis persist.15-18 Pieces of skin the area of the epidermis (in square micrometers) across 3 from 2 controls and from psoriasis lesions from 2 patients nonoverlapping regions (NIH Image software, version 1.61; were placed into wells of a 96-well dish containing 250 µL National Institutes of Health, Bethesda, Md). The average of culture medium. The culture medium consisted of KBM of the 3 measurements is an index of epidermal hyperpla- supplemented with calcium chloride to a final Ca++ concen- sia of the psoriatic epidermis for each piece of skin. tration of 1.4 mmol/L. The tissue pieces were incubated at 37°C in 5% carbon dioxide and 95% air for 8 days with fresh STATISTICAL ANALYSIS culture medium as previously described.15,16,18 Various concentrations of troglitazone or dimethyl sulfoxide vehicle alone We compared counts of keratinocytes in culture at all con- were added at days 2, 4, and 6. At the end of the incubation centrations for each experimental condition with counts period, tissue pieces were fixed in 10% buffered formalin and of keratinocytes maintained under similar conditions (but embedded in paraffin. Sectioned tissues were stained with without experimental agents added) by analysis of vari- hematoxylin-eosin and evaluated by light microscopy. ance; we calculated the average of the epidermal hyperplasia indices for each patient for each “treatment” assign- STUDIES IN THE SEVERE COMBINED ment in SCID mice (nontransplanted skin, transplanted skin IMMUNODEFICIENT MOUSE AND HUMAN SKIN on untreated mice, and transplanted skin on treated mice) TRANSPLANT MODEL OF PSORIASIS and compared the means across treatment assignments by repeated-measures analysis of variance with the Tukey stu- Human psoriasis skin engrafted onto severe combined im- dentized range for pairwise comparisons (SigmaStat; SSPS munodeficient (SCID) mice maintains its histological char- Inc, Chicago, Ill). Differences were considered statisti- acteristics.19 Lesional psoriasis skin from 3 patients was cally significant at PϽ.05; data are given as mean±SE.

the expression of an intracellular receptor. Reverse tran- hibited proliferation of normal and psoriatic keratino- scriptase–polymerase chain reaction analysis demon- cytes in a dose-dependent and saturable fashion (data not strated the presence of mRNA for both PPAR␥1 and shown). In supplemented media, addition of the pros- PPAR␥2 isoforms in human keratinocytes; Western blot taglandin metabolite 15-deoxy-⌬-12,14-prostaglandin J2, analysis of protein extracts using an antibody that re- a natural ligand for PPAR␥,22,23 inhibited proliferation of acts with both PPAR␥ isoforms revealed a distinct band cultured keratinocytes in a dose-dependent fashion corresponding to a protein of the expected size for PPAR␥ (PϽ.001) (Figure 3, C). Thus, the inhibition of prolif- of 62 kd. eration in keratinocytes likely represents a general effect of ligands for PPAR␥ and is not unique to thiazoli- LIGANDS FOR PPAR␥ INHIBIT GROWTH dinediones. However, troglitazone treatment did not OF NORMAL AND PSORIATIC affect the growth of human dermal fibroblasts in serum- HUMAN KERATINOCYTES BUT NOT free media at any of the concentrations tested (P=.97) DERMAL FIBROBLASTS (Figure 3, D).

Use of the PPAR␥ ligand troglitazone inhibited in a dose- TROGLITAZONE AMELIORATES dependent manner the growth of normal and psoriatic THE HISTOLOGICAL PHENOTYPE keratinocytes maintained in serum-free media (PϽ.001 OF PSORIASIS WHEN IN ORGAN CULTURE for each) (Figure 3, A). In normal keratinocytes main- OR TRANSPLANTED ONTO SCID MICE tained in supplemented growth media, higher concentrations of troglitazone were required for the antiprolif- After incubation for 8 days, the histological appearance erative effects (PϽ.001) (Figure 3, B). In all cases, of cultured normal skin resembled that of freshly biop- inhibition of keratinocyte proliferation was observed with sied skin, and lesional psoriatic skin in culture contin- concentrations of troglitazone that can be attained in ued to express typical abnormal histological features (as plasma during administration of standard oral doses of seen in previous studies16,18). The abnormal phenotype the drug for the treatment of type 2 diabetes mellitus.21 of the psoriatic skin was qualitatively nearly normal in Use of ciglitazone, another thiazolidinedione, also in- all organ cultures treated with 2.2 µmol of troglitazone;

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D E F

Figure 1. Oral administration of troglitazone improved chronic psoriasis in patient 1. A and D, Before therapy. B and E, After 5 weeks of taking troglitazone, 400 mg/d. C and F, After 4 additional months of taking troglitazone, 600 mg/d.

psoriasis tissue treated with 1.1 µmol of troglitazone 49±2ϫ104 µm2 for psoriatic skin transplanted onto tro- showed similar changes, whereas tissue treated with glitazone-treated SCID mice (PϽ.05 compared with non- 0.2 µmol of troglitazone did not. transplanted and transplanted skin on untreated mice). The typical histological appearance of psoriasis was maintained in lesional psoriatic skin from 3 patients trans- COMMENT planted onto untreated control SCID mice (as validated previously19). In lesional psoriatic skin transplanted onto Ligands for nuclear hormone receptors expressed in skin SCID mice treated with troglitazone orally, overall skin (eg, retinoic acid, vitamin D, and glucocorticoid recep- thickness was reduced, a more normal pattern of epider- tors) have proved to be of considerable value in the treat- mal differentiation occurred, the granular layer (which ment of dermatologic diseases, including psoriasis.4,24 In is decreased or lacking in the untreated psoriatic skin) the present studies, we found that thiazolidinedione li- was present in much of the epidermis, and the inflam- gands for the nuclear hormone receptor PPAR␥ might matory response was reduced (Figure 4). The index of offer a new therapeutic approach to the management of epidermal hyperplasia was 70±6ϫ104 µm2 for psoriatic psoriasis. skin not transplanted onto SCID mice, 74±2ϫ104 µm2 Peroxisome proliferator-activated receptor-␥ was for psoriatic skin transplanted onto untreated SCID mice originally believed to be expressed in just a few tissues, (PϾ.05 compared with nontransplanted skin), and including fat. However, recent studies25,26 in humans have

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©2000 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/30/2021 severe chronic psoriasis. It would be unusual for stable, A B chronic plaque psoriasis to undergo such complete and sustained remissions as we observed in these patients either spontaneously or because of a placebo effect, suggesting that a troglitazone treatment–induced effect occurred. The observation that several different ligands for PPAR␥ inhibit proliferation of keratinocytes, together with the important role of keratinocyte hyperproliferation in the pathogenesis of psoriasis, suggests that the antipsoriatic effects of troglitazone might be mediated at least in part by the antiproliferative effects of PPAR␥ activation. Certain fatty acids, such as linoleic acid, are weak activators of PPAR␥ and have also been shown to de- crease the replication rate of cultured keratinocytes.29,30 Moreover, in some patients with psoriasis, dietary supple- C D ments of some fatty acids have been associated with lesion improvement.31 Thiazolidinediones may exert a host of other actions such as blockade of calcium channels and mitogen- activated protein kinase signaling pathways that could also affect cell proliferation and the pathogenesis of psoriasis independent of effects on PPAR␥ activation.12,32,33 However, others have provided compelling evidence that, at least in some cell lines, antiproliferative or apoptotic effects of thiazolidinediones are indeed mediated via PPAR␥.34-36 The inhibition of proliferation of keratinocytes is unlikely to represent simply a toxic effect of the drugs because the growth inhibitory effects are reversible on removal of the drugs from the culture medium Figure 2. Oral administration of troglitazone improved chronic psoriasis in patient 2. A and C, Elbows before therapy. B and D, Elbows after taking (data not shown). In a variety of other normal cell lines troglitazone, 200 mg/d for 2 weeks and 400 mg/d for 6 additional weeks. studied under conditions similar to those used in keratinocytes, we and others11-13,34 showed that the antiproliferative effects of these agents are reversible and are demonstrated the presence of PPAR␥ in a variety of tis- without obvious cytotoxic effects or effects on lactic de- sues. In a recent study,27 PPAR␥ was detected in kera- hydrogenase release, exclusion of trypan blue, or viabil- tinocytes by gel mobility shift and RT-PCR assays, and ity assays of mitochondrial function. herein we demonstrate that human keratinocytes express The mechanisms by which thiazolidinediones affect mRNA and protein for the nuclear hormone receptor the cell cycle or programmed cell death may vary among PPAR␥. different cell types. We did not find an effect of tro- In the present studies, we found that structurally dif- glitazone on fibroblasts. This suggests that troglitazone’s ferent ligands for PPAR␥ inhibit human keratinocyte pro- therapeutic benefit in psoriasis is not the result of inter- liferation, a characteristic feature of psoriasis. We also ference with any putative dermal effect on the epidermis. found that troglitazone, a PPAR␥ ligand used to treat type We do not know whether the antiproliferative effects of 2 diabetes mellitus, ameliorates the abnormal histologi- PPAR␥ ligands in keratinocytes are mediated through the cal phenotype of human psoriatic skin in culture and in same intracellular pathways involved in the growth ef- an animal model in vivo. fects of these agents in breast, prostate, colon, vascular In addition, we demonstrated that oral administra- smooth muscle, and fat cells. tion of troglitazone improved psoriasis in a few pa- The pathophysiological features of psoriasis are com- tients. Although troglitazone treatment has been associ- plex, and the antipsoriatic effects of thiazolidinediones ated with rare cases of hepatic failure, newer PPAR␥ may involve more than just their effects on cell cycle con- ligands, such as pioglitazone hydrochloride and rosigl- trol and proliferation of keratinocytes. Disordered cel- itazone, have not been reported to induce this adverse lular immunity involving inflammatory cytokines (eg, tu- effect and may be better agents to try as psoriasis therapies. mor necrosis factor ␣, interleukin [IL] 1, and IL-6) and The occurrence of psoriasis in patients with diabe- proinflammatory transcription factors (eg, nuclear factor- tes mellitus is not uncommon, and these conditions may ␬B, signal transducer and activator of transcription coexist more often than predicted from the prevalence [STAT], and activator protein-1) have been implicated rates of either disorder alone.28 Three patients with type in the pathogenesis of psoriatic epidermal inflamma- 2 diabetes mellitus and psoriasis had clinically signifi- tion.3,33,37,38 Macrophages may interact with basal kera- cant improvement in their psoriatic lesions when we tinocytes in the pathogenesis of psoriasis, and keratino- treated them with troglitazone.20 cytes and monocytes may elaborate inflammatory We have now observed antipsoriatic effects of tro- cytokines, including IL-1, IL-2, IL-6, and tumor necro- glitazone in 2 normoglycemic patients with moderate to sis factor ␣.39-41

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60 % of Control 40

A B 0 Control 0.02 0.10 0.20 1.10 2.20 Control 5 10 20 40 Concentration, µmol/L Concentration, µmol/L

120

100

60 % of Control 40

C D 0 Control 124812 Control 0.02 0.10 0.20 1.10 2.20 Concentration, µmol/L Concentration, µmol/L

Figure 3. Ligands for peroxisome proliferator-activated receptor-␥ inhibit the proliferation of normal and psoriatic human keratinocytes but not human dermal fibroblasts. For each concentration, 6 replicates were performed; the mean±SE of the percentage of the mean control value is shown. A, Dose-response inhibition of proliferation by troglitazone treatment in normal and psoriatic keratinocytes maintained in serum-free media. B, Dose-response inhibition of growth by

troglitazone treatment in normal keratinocytes maintained in supplemented media. C, Dose-response inhibition of growth by 15-deoxy-⌬-12,14-prostaglandin J2 in normal keratinocytes maintained in supplemented media. D, Absence of dose-response inhibition of growth by troglitazone treatment in normal human dermal fibroblasts maintained in serum-free media. For A-C, PϽ.001; for D, P=.97 by analysis of variance.

A B

Figure 4. Oral administration of troglitazone ameliorates the abnormal histological phenotype of human psoriatic skin transplanted onto severe combined immunodeficient (SCID) mice (hematoxylin-eosin, original magnification ϫ40). A, Human psoriatic lesional skin transplanted onto an untreated SCID mouse for 6 weeks. B, Human psoriatic lesional skin transplanted onto a SCID mouse that was treated with troglitazone, 200 µg orally for 6 weeks.

Studies42,43 demonstrating anti-inflammatory edly up-regulated in activated macrophages, and effects of PPAR␥ ligands raise the possibility that the ligands for PPAR␥ can inhibit the activities of nuclear antipsoriatic effects of thiazolidinediones might also factor-␬B, STAT, and activator protein-1, and down- be mediated through inhibition of skin inflammation. regulate inflammatory cytokines such as tumor necro- Peroxisome proliferator-activated receptor-␥ is mark- sis factor ␣.42,43

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©2000 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/30/2021 In our SCID mouse model, the inflammatory infil- Reprints: Charles N. Ellis, MD, Department of Der- trate in transplanted psoriatic skin was reduced during matology, University of Michigan Medical School, 1910 A. oral troglitazone therapy. Nevertheless, this may not have Alfred Taubman Center, 1500 E Medical Center Dr, Ann been a primary mechanism of the salutary effect of tro- Arbor, MI 48109-0314. glitazone on the skin transplant; our studies were not de- signed to determine whether ligands for PPAR␥ affect the REFERENCES immunologic determinants of psoriasis. Future studies of the anti-inflammatory effects of thiazolidinediones are 1. Sander HM, Morris LF, Phillips CM, Harrison PE, Menter A. The annual cost of warranted. psoriasis. J Am Acad Dermatol. 1993;28:422-425. For ligands of PPAR␥ to affect the transcription of 2. McKay IA, Leigh IM. 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PPAR-␥: adipogenic regulator and thiazolidinedione receptor. some proliferator response elements in DNA, leading to Diabetes. 1998;47:507-514. a change in protein transcription. 8. Mueller E, Sarraf P, Tontonoz P, et al. Terminal differentiation of human breast The cooperative interaction of PPAR␥ and RXR at cancer through PPAR ␥. Mol Cell. 1998;1:465-470. the molecular level suggests that ligands for each might 9. Kubota T, Koshizuka K, Williamson EA, et al. Ligand for peroxisome proliferator- activated receptor ␥ (troglitazone) has potent antitumor effect against human share several therapeutic effects. Indeed, retinoids have prostate cancer both in vitro and in vivo. Cancer Res. 1998;58:3344-3352. efficacy in dermatologic disorders, certain forms of can- 10. Sarraf P, Mueller E, Jones D, et al. Differentiation and reversal of malignant changes cer, and type 2 diabetes mellitus.47,48 In a similar fash- in colon cancer through PPAR ␥. 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Varani J, Nickoloff BMR, Mitra R, Dixit V, Voorhees JJ. All-trans retinoic acid Since the manuscript was accepted for publication, stimulates growth of adult human keratinocytes cultured in growth factor defi- others49,50 have reported that ligands of PPAR␥ inhibit cient medium, inhibits production of thrombospondin and fibronectin and re- T-cell activation and the production of IL-2. These find- duces adhesion. J Invest Dermatol. 1989;93:449-454. ings support an immunomodulatory mechanism of 15. Varani J, Fligiel SE, Schuger L, et al. Effects of all-trans retinoic acid and Ca++ on human skin in organ culture. Am J Pathol. 1993;142:189-198. action for ligands of PPAR␥ in the treatment of psoriasis. 16. Varani J, Kang S, Stoll S, Elder JT. Human psoriatic skin in organ culture: com- parison with normal skin exposed to exogenous growth factors and effects on an antibody to the EGF receptor. Pathobiology. 1998;66:253-259. 17. Varani J, Perone P, Griffiths CE, Inman DR, Fligiel SE, Voorhees JJ. All-trans reti- In March 2000, the manufacturer of troglitazone volun- noic acid (RA) stimulates events in organ-cultured human skin that underlie re- pair: adult skin from sun-protected and sun-exposed sites responds in an iden- tarily discontinued its sale in the United States because tical manner to RA while neonatal foreskin responds differently. J Clin Invest. of reports of liver toxicity. 1994;94:1747-1756. 18. Varani J. Preservation of human skin structure and function in organ culture. Accepted for publication September 30, 1999. Histol Histopathol. 1998;13:775-783. 19. Nickoloff BJ, Kunkel SL, Burdick M, Strieter RM. Severe combined immunode- From the Departments of Dermatology (Drs Ellis and ficiency mouse and human psoriatic skin chimeras: validation of a new animal Fisher) and Pathology (Drs Varani, Zeigler, and Chi), Uni- model. 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A prostaglandin J2 metabolite binds peroxisome proliferator-activated receptor ␥ and consultants for Bethesda Pharmaceuticals, Mill Valley, Calif. promotes adipocyte differentiation. Cell. 1995;83:813-819. Drs Pershadsingh and Kurtz own stock in and are principals 24. Stern RS. Psoriasis. Lancet. 1997;350:349-353. in Bethesda Pharmaceuticals. 25. Elbrecht A, Chen YL, Cullinan CA, et al. Molecular cloning, expression and char- This work was supported by grant R41 AR44767 from acterization of human peroxisome proliferator activated receptors ␥1 and ␥2. the National Institutes of Health, Bethesda, Md, and by Biochem Biophys Res Comm. 1996;224:431-437. 26. Mukherjee R, Jow L, Croston GE, Paterniti JR Jr. Identification characterization, Bethesda Pharmaceuticals. and tissue distribution of human peroxisome proliferator-activated receptor (PPAR) We thank Navin M. Amin, MD, for assistance and Ted isoforms PPAR␥2 versus PPAR␥1 and activation with retinoid X receptor ago- A. 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