Dexamethasone inhibition of TGF beta-induced cell growth and type II collagen mRNA expression through ERK-integrated AP-1 activity in cultured rat articular chondrocytes

Intraarticular injection of dexamethasone (DEX) accelerates cartilage degra dation due to the suppression of chondrocyte proliferation and extracellula r matrix formation. The present study first demonstrated the interaction be tween DEX and TGF beta, a potent growth factor for cultured rat articular c hondrocytes (CRAC), and then investigated the molecular mechanism by which DEX counteracts TGF beta-induced chondrocyte proliferation and differentiat ion through the regulation of AP-I activity. DEX reduced serum-deprived and TGF beta-stimulated cell growth and [H-3]-thymidine incorporation of CRAC. DEX also inhibited the expression of (alpha)1 type II collagen with concom itant suppression of the promoter activity. Transfection studies using a re porter vector with AP-1 responsive elements showed that DEX reduced TGF bet a-activated but not basal luciferase activities. Activation of 3TP-luc, ano ther AP-1 responsive element containing reporter was also blocked by DEX. G AL4-Elk1 studies revealed that DEX suppressed TGF beta-induced ERK activati on which led to c-fos gene expression followed by increase in AP-1 complex formation, whereas the Smad pathway was not involved in DEX-dependent negat ive regulation of AP-1 in a reporter assay that requires FAST1-Smad2 for th e activation. DEX also eliminated TGF beta-induced c-fos mRNA expression an d ERK activation in Northern analysis and in vitro kinase assay, respective ly. Further, DNA synthesis and transactivation of type II collagen by TGF b eta were inhibited by PD98059, an inhibitor of MEK. Our results indicate th at DEX suppressed TGF beta-induced chondrocyte proliferation and type II co llagen expression, probably through selective inhibition of ERK integrated AP-1 activation. (C) 2000 OsteoArthritis Research Society international.