Effect of dexamethasone on interleukin-1β-(IL-1β)-induced nuclear factor-κB (NF-κB) and κB-dependent transcription in epithelial cells

Abstract

The production of inflammatory mediators by epithelial cells in inflammatory lung diseases may represent an important target for the anti- inflammatory effects of glucocorticoids. Nuclear factor-κB (NF-κB) is a major activator of inflammatory genes and has been proposed as a target for inhibition by glucocorticoids. We have used human pulmonary type-II A549 and airway epithelial BEAS-2B cells to investigate the effect of glucocorticoids on NF-κB regulation and κB-dependent transcription. In A549 cells following interleukin-1β (IL-1β) treatment, there was no effect of dexamethasone on the disappearance of IκBα protein, its subsequent reappearance 90-min later or the rapid induction of IκBα mRNA and transcription rate. Expression of p65 and p50/p105 proteins were also unaffected by dexamethasone. In addition, the rapid IL-1β-induction of NF-κB DNA binding and p65 nuclear localisation was unaffected by short (1-6 hours) dexamethasone pre-treatments. Similarly, BEAS-2B cells showed no effect of dexamethasone on IL-1β-induced NF-κB (p50/p65). Stable transfection of a κB-dependent reporter in A549 cells resulted in an 8-9-fold activation by IL-1β or phorbol ester, that was repressed 30-40% by dexamethasone. However, in these cells, IL-1β induction of inducible nitric oxide synthase, granulocyte-macrophage colony stimulating factor and cyclooxygenase-2 mRNA showed 70-90% repression by dexamethsone. We, therefore, conclude that in these epithelial cells, the repressive effects of glucocorticoids are not mediated by up-regulation of IκBα, decreased p50/p65 gene expression or inhibition of NFκB DNA binding. Furthermore, since the maximal repression of IL-1β or phorbol-ester-induced κB-dependent transcription by dexamethasone was less than 40%, simple inhibition of κB-dependent transcription cannot by itself account for the full repressive effects of glucocorticoids observed in these cells.