Peroxisome Proliferator-Activated Receptor (PPAR)-γ Positively Controls and PPARα Negatively Controls Cyclooxygenase-2 Expression in Rat Brain Astrocytes through a Convergence on PPARβ/δ via Mutual Control of PPAR Expression Levels

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 (alpha, beta/delta, and gamma) 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 PPAR alpha [2-[4-[2-[4-cyclohexylbutyl cyclohexylcarbamoyl)amino]ethyl]phenyl] sulfanyl-2-methylpropanoic acid (GW7647)], PPAR beta/delta [4-(3-(2-propyl-3-hydroxy-4-acetyl)phenoxy) propyloxyphenoxy acetic acid, (L-165041)], and PPAR gamma (rosiglitazone, ciglitazone) on inflammatory gene regulation in rat primary astrocytes. We measured cyclooxygenase-2 (COX-2) expression and prostaglandin E 2 synthesis in lipopolysaccharide (LPS)-stimulated cells. PPAR alpha, PPAR beta/delta, and PPAR gamma knockdown models served to delineate the contribution of each PPAR isotype. Thiazolidinediones enhanced the LPS-induced COX-2 expression via PPAR gamma-dependent pathway, whereas L-165041 and GW7647 had no influence. However, the addition of L-165041 potentiated the effect of PPAR gamma activation through PPAR beta/delta-dependent mechanism. On the contrary, PPAR alpha 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 PPAR beta/delta overexpression model confirmed that PPAR beta/delta expression level is the point at which PPAR gamma and PPAR alpha pathways converge in control of COX-2 gene expression. Thus, we discovered that in primary astrocytes, PPAR gamma has a positive influence and PPAR gamma has a negative influence on PPAR beta/delta expression and activity. A positive/negative-feedback loop is formed by PPAR beta/delta-dependent increase in PPAR alpha expression level. These findings elucidate a novel principle of regulation in the signaling by synthetic PPAR agonists that involves modulating the interaction between PPAR alpha, -beta/delta, and -gamma isoforms on the level of their expression.