Lung fibroblasts are a major source of several cytokines including CC chemokine eotaxin. We aimed to study the regulation of eotaxin-1/CCL11 production by dexamethasone and analyze its molecular mechanisms in human lung fibroblasts. Normal human lung fibroblast cells were exposed to IL-4 (40 ng/ml) and/or dexamethasone (10(-6)-10(-9) M), and eotaxin mRNA expression and production was evaluated. Mechanisms of transcriptional regulation were assessed by Western blotting and dual luciferase assay for eotaxin promoter. The effects of dexamethasone on suppressor of cytokine signaling (SOCS)-1 and eotaxin mRNA expression in the cells transfected with expression vector (pAcGFP1-C1) or short interfering RNA (siRNA) for SOCS-1 were also investigated. Within 24 hours, dexamethasone inhibited IL-4-induced eotaxin mRNA expression and protein production, while eotaxin production was markedly increased at 48 and 72 hours after coincubation with IL-4 and dexamethasone. IL-4-induced eotaxin promoter activity was inhibited by dexamethasone at 8 hours, but enhanced at 48 hours after coincubation. Dexamethasone suppressed SOCS-1 mRNA expression but enhanced IL-4-induced STAT6 phosphorylation at 36 to 48 hours after coincubation. Enhanced expression of eotaxin mRNA by dexamethasone 48 hours after coincubation was completely diminished in the cells transfected with either expression vector or siRNA for SOCS-1. These results indicated that dexamethasone, depending on the exposure duration, can either inhibit or enhance IL-4-induced expression and production of eotaxin in the lung fibroblasts. The mechanisms of later enhanced production may depend on the prolonged transcriptional activity of the eotaxin gene, in part due to inhibition of SOCS-1 expression.