Glucocorticoid regulation of GM-CSF in bronchial epithelial cells

Abstract

Inflammation plays a central role in the pathogenesis of asthma. Glucocorticoids are first line antiinflammatory therapy in the treatment of asthma and are effective inhibitors of inflammatory cytokines. Clinical data demonstrate that granulocyte-macrophage colony-stimulating factor (GM-CSF) production by airway epithelial cells may be an important target of inhaled glucocorticoid therapy. In this study, the regulatory mechanisms of GM-CSF expression by interleukin-1β (IL-1β) and the synthetic glucocorticoid dexamethasone (DEX) were examined in the BEAS-2B human bronchial epithelial cell line. It is hypothesized that glucocorticoids inhibit GM-CSF production in these cells through transcriptional mechanisms involving induction of the NF-κB inhibitory protein, IκB-α. Treatment of the BEAS-2B cells with IL-1β induced GM-CSF protein and mRNA levels, and further investigation showed this induction was mediated through transcriptional mechanisms. DEX treatment of BEAS-2B cells inhibited IL-1β-induced GM-CSF protein and mRNA production. GM-CSF mRNA was rapidly degraded in these cells, and DEX treatment did not significantly affect this decay rate. These data suggest that dexamethasone repression of GM-CSF expression is mediated predominantly through transcriptional mechanisms. This study then examined expression of IκB-α in the BEAS-2B cells as a possible mechanism of glucocorticoid repression of GM-CSF. IκB-α RNA levels were minimally induced by DEX in these cells, but this did not result in concurrent induction of IκB-α protein. Additional analysis showed that DEX treatment of BEAS-2B cells did not prevent nuclear translocation of the NF-κB subunit p65, or IL-1β-induced degradation of IκB-α protein. From these data, this study concludes that induction of IκB-α is not a significant mechanism of glucocorticoid-mediated repression of GM-CSF in the BEAS-2B cells.