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

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 NF‐κB 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.