GLUCOCORTICOID INHIBITION OF INTERLEUKIN-1-INDUCED INTERLEUKIN-6 PRODUCTION BY HUMAN LUNG FIBROBLASTS - EVIDENCE FOR TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL REGULATORY MECHANISMS

Interleukin (IL)-6 is a pleiotropic cytokine produced by a wide variety of cells including fibroblasts, macrophages, endothelial cells, and T and B lymphocytes. Regulated IL-6 production is an important part of normal biologic homeostasis, and abnormal IL-6 production has been associated with a large number of diseases including asthma and lung allograft rejection. Glucocorticoids are potent anti-inflammatory agents that are widely used to suppress pulmonary inflammation. To further understand the mechanisms underlying this inhibition, we determined whether glucocorticoid compounds regulate human lung fibroblast IL-6 production and characterized the mechanisms of the effects that were noted. These studies demonstrate that glucocorticoids inhibit IL-1-induced IL-6 production in a dose-dependent fashion. A greater than 95% decrease in IL-6 production was seen with 10(-6) and 10(-7) M dexamethasone, prednisolone, and hydrocortisone, and IC50 values for these agents were approximately 5 X 10(-10), 5 X 10(-9), and 10(-8) M, respectively. mRNA analysis demonstrated that these alterations in protein production were associated with proportionate decreases in IL-6 mRNA accumulation, and that this suppression of IL-6 mRNA could be reversed by the glucocorticoid receptor anatagonist RU 486. Nuclear run-on studies demonstrated that glucocorticoids inhibit-IL-1-induced IL-6 gene transcription. However, the magnitude of this effect could not fully account for the potency of the glucocorticoid-induced alterations in IL-6 mRNA accumulation and protein production since 10(-6) M dexamethasone caused only a 50% decrease in IL-1-induced IL-6 gene transcription. mRNA half-life determinations using the transcription inhibitor actinomycin-D provided at least a partial explanation for this discrepancy since they demonstrated that glucocorticoids also selectively destabilize IL-6 mRNA. When viewed in combination, these studies demonstrate that glucocorticoids inhibit lung fibroblast IL-6 production via complex transcriptional and post-transcriptional mechanisms that include the inhibition of IL-6 gene transcription and augmentation of IL-6 mRNA degradation.