- prostaglandinJ 2-Induced PPAR- Activation That Mediates Dedifferen- tiation But Not Cyclooxygenase-2 Expression in Articular Chondrocytes

Peroxisome proliferator-activated receptor gamma (PPAR- ) is a ligand-activated transcription factor and plays an important role in growth, differentiation, and inflam- mation in different tissues. In this study, we investigated the effects of 15d-PGJ2, a high-affinity ligand of PPAR- , on dedifferentiation and on inflammatory responses such as COX-2 expression and PGE2 production in rabbit articular chondrocytes with a focus on ERK-1/-2, p38 kinase, and PPAR- activation. We report here that 15d-PGJ2 induced dedifferentiation and/or COX-2 expression and subsequent PGE2 production. 15d-PGJ2 treatment stimulated activation of ERK-1/-2, p38 kinase, and PPAR- . Inhibition of ERK-1/-2 with PD98059 recovered 15d-PGJ2-induced dedif- ferentiation and enhanced PPAR- activation, whereas inhibition of p38 kinase with SB203580 potentiated dedifferentiation and partially blocked PPAR- activa- tion. Inhibition of ERK-1/-2 and p38 kinase abolished 15d-PGJ2-induced COX-2 expression and subsequent PGE2 production. Our findings collectively suggest that ERK-1/-2 and p38 kinase oppositely regulate 15d-PGJ2-induced dedifferentiation through a PPAR- -dependent mechanism, whereas COX-2 expression and PGE 2 production is regulated by ERK-1/-2 through a PPAR- -independent mechanism but not p38 kinase in articular chondrocytes. Additionally, these data suggest that targeted modulation of the PPAR- and mitogen-activated protein kinase pathway may offer a novel approach for therapeutic inhibition of joint tissue degradation.