Fig. 3. Glucocorticoids induce MKP-1 gene expression. RBL-2H3 mast cells (A), mouse bone marrow-derived mast cells (B) and NIH-3T3 fibroblasts (C) were treated for the indicated time with dexamethasone (Dex, 0.1 µM) or solvent alone. Total RNA was extracted and subjected to northern blotting using cDNA probes for MKP-1 and GAPDH. (D) COS-7 cells were transiently co-transfected with an MKP-1 promoter firefly luciferase construct (-1716MKP-1luc) and a Renilla luciferase construct as an internal control, together with either wild-type glucocorticoid receptor (GRwt) or two different dimerization/transactivation-deficient GR mutant [A458T or hGR(D4X)] expression vectors. Treatment with dexamethasone (Dex, 0.1 µM) or solvent alone was performed 4 h after the transfection and the cells were harvested 48 h later for luciferase activity measurements. The results are expressed as the level of MKP-1 promoter-driven firefly luciferase expression after correcting for the transfection efficiency by Renilla luciferase measurements (relative luciferase activity), and are presented as the mean ± SD of three independent experiments.