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

Abstract Intraarticular injection of dexamethasone (DEX) accelerates cartilage degradation due to the suppression of chondrocyte proliferation and extracellular matrix formation. The present study first demonstrated the interaction between DEX and TGFβ, a potent growth factor for cultured rat articular chondrocytes (CRAC), and then investigated the molecular mechanism by which DEX counteracts TGFβ-induced chondrocyte proliferation and differentiation through the regulation of AP-1 activity. DEX reduced serum-deprived and TGFβ-stimulated cell growth and [ 3 H]-thymidine incorporation of CRAC. DEX also inhibited the expression of (α)1 type II collagen with concomitant suppression of the promoter activity. Transfection studies using a reporter vector with AP-1 responsive elements showed that DEX reduced TGFβ-activated but not basal luciferase activities. Activation of 3TP-luc, another AP-1 responsive element containing reporter was also blocked by DEX. GAL4-Elk1 studies revealed that DEX suppressed TGFβ-induced ERK activation 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 negative regulation of AP-1 in a reporter assay that requires FAST1-Smad2 for the activation. DEX also eliminated TGFβ-induced c-fos mRNA expression and ERK activation in Northern analysis and in vitro kinase assay, respectively. Further, DNA synthesis and transactivation of type II collagen by TGFβ were inhibited by PD98059, an inhibitor of MEK. Our results indicate that DEX suppressed TGFβ-induced chondrocyte proliferation and type II collagen expression, probably through selective inhibition of ERK integrated AP-1 activation.