Abstract
Peroxisome proliferator-activated receptor (PPAR) transcription factors are pharmaceutical drug targets for treating diabetes, atherosclerosis, and inflammatory degenerative diseases. The possible mechanism of interaction between the three PPAR isotypes (α, β/δ, and γ) is not yet clear. However, this is important both for understanding transcription factor regulation and for the development of new drugs. The present study was designed to compare the effects of combinations of synthetic agonists of PPARα [2-[4-[2-[4-cyclohexylbutyl (cyclohexylcarbamoyl)amino]ethyl]phenyl] sulfanyl-2-methylpropanoic acid (GW7647)], PPARβ/δ [4-(3-(2-propyl-3-hydroxy-4-acetyl)phenoxy)propyloxyphenoxy acetic acid, (L-165041)], and PPARγ (rosiglitazone, ciglitazone) on inflammatory gene regulation in rat primary astrocytes. We measured cyclooxygenase-2 (COX-2) expression and prostaglandin E2 synthesis in lipopolysaccharide (LPS)-stimulated cells. PPARα, PPARβ/δ, and PPARγ knockdown models served to delineate the contribution of each PPAR isotype. Thiazolidinediones enhanced the LPS-induced COX-2 expression via PPARγ-dependent pathway, whereas L-165041 and GW7647 had no influence. However, the addition of L-165041 potentiated the effect of PPARγ activation through PPARβ/δ-dependent mechanism. On the contrary, PPARα activation (GW7647) suppressed the effect of the combined L-165041/rosiglitazone application. The mechanism of the interplay arising from combined applications of PPAR agonists involves changes in PPAR expression levels. A PPARβ/δ overexpression model confirmed that PPARβ/δ expression level is the point at which PPARγ and PPARα pathways converge in control of COX-2 gene expression. Thus, we discovered that in primary astrocytes, PPARγ has a positive influence and PPARα has a negative influence on PPARβ/δ expression and activity. A positive/negative-feedback loop is formed by PPARβ/δ-dependent increase in PPARα expression level. These findings elucidate a novel principle of regulation in the signaling by synthetic PPAR agonists that involves modulating the interaction between PPARα,-β/δ, and -γ isoforms on the level of their expression.
Footnotes
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This work was supported in part by the Bundesministerium für Bildung und Forschung [Grants 01ZZ0407 and RUS 04/004] and by the Russian Foundation for Basic Research [Grant 07-04-01160].
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ABBREVIATIONS: PPAR, peroxisome proliferator-activated receptor; COX-2, cyclooxygenase-2; DMEM, Dulbecco's modified Eagle's medium; LPS, lipopolysaccharide; PBS, phosphate-buffered saline; L-165041, 4-(3-(2-propyl-3-hydroxy-4-acetyl)phenoxy)propyloxyphenoxy acetic acid; GW9662, 2-chloro-5-nitrobenzanilide; GW7647, 2-[4-[2-[4-cyclohexylbutyl (cyclohexylcarbamoyl)amino]ethyl]phenyl]sulfanyl-2-methylpropanoic acid; FCS, fetal calf serum; RT-PCR, reverse transcription polymerase chain reaction; PCR, polymerase chain reaction; siRNA, small interfering RNA; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PGE2, prostaglandin E2.
- Accepted May 27, 2009.
- Received March 8, 2009.
- The American Society for Pharmacology and Experimental Therapeutics
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