Induction of cyclooxygenase-2 by platelet-derived growth factor (PDGF) andits inhibition by dexamethasone are independent of NF-kappa B/I kappa B transcription factors

Gucocorticoids are potent inhibitors of cyclooxygenase-2 (prostaglandin G/H synthase-2, COX-2) expression. The focus of this work was to investigate t he molecular mechanisms, by which glucocorticoids interfere with platelet-d erived growth factor (PDGF)-mediated induction of COX-2 with special emphas is on the role of the transcription factors NF-kappa B/I kappa B alpha. In rat renal mesangial cells. PDGF induced a rapid and transient increase o f COX-2 mRNA and protein, which reached maximal levels after 1-2 and 4 h, r espectively. The in vivo half-life of COX-2 mRNA, which was estimated to be less than 1 h, was reduced by dexamethasone. Kinetic studies and COX-2 pro moter activity assays indicated that dexamethasone also interfered with COX -2 transcription. Inhibition of COX-2 induction by dexamethasone was abroga ted by cycloheximide, an inhibitor of translation, indicating dependence on de novo protein synthesis. As a possible mediator of dexamethasone action, the NF-kappa B/I kappa B alpha system of transcription factors was investi gated. Dexamethasone doubled I kappa B alpha protein levels within 1 h anti reduced complex formation of nuclear NF-kappa B proteins with DNA. Newly s ynthesized I kappa B alpha may thus bind to NF-kappa B and interfere with g ene activation. PDGF-induced signalling, however, barely affected the NF-ka ppa B/I kappa B alpha s\i stem: I kappa B alpha protein remained unaltered for 30 min after treatment of mesangial cells with PDGF and was only reduce d by 30% after h. Concomitantly, binding of NF-kappa B proteins to DNA, det ected by electrophoretic mobility shift assays, was slightly increased by 3 0%. Furthermore, stably transfected COX-2 promoter constructs with and with out the NF-kappa B binding site were comparably activated by PDGF (2.5-fold increase of luciferase activity). Taken together, these data indicate that although dexamethasone interferes with the NF-kappa B/I kappa B alpha system of transcription factors, this m echanism is not essential for the inhibition of PDGF-induced COX-2 expressi on.